Second Edition, Revised and ExDanded
edited by
ANI1 K. BHOWMICK Rubber Technology Centre Indian Institute of Technology Kharagpur, India
HOWARD L. STEPHENS The University of Akron Akron, Ohio
M A R C E L
95 D E K K E R
MARCEL DEKKER, INC.
NEWYORK BASEL
ISBN: 0-8247-0383-9 This book is printed on acid-free paper.
Headquarters Marcel Dekker, Inc. 270 Madison Avenue, New York, NY l0016 tel: 2 12-696-9000; fax: 2 12-685-4540 Eastern Hemisphere Distribution Marcel Dekker AG Hutgasse 4, Postfach 812. CH-4001 Basel, Switzcrland tell 4 1 -6 1-7-61-8482; fax: 4 1-6 1-26 1-8896 World Wide Web tlttp://www.dekker.com The publisher offers discounts on this book when ordered i n bulk quantitics. For more information, write to Special SaleslProfessional Marketing at thc headquartcrs addrcss abovc.
Copyright 0 2001 by Marcel Dekker, Inc. All Rights Reserved. Neithcr this book nor any part may be reproduced or transmitted i n any form or by any mcans. electronic or mechanical, including photocopying, microfilming, and recording. or by any information storagc and retricval systcm. without permission in writing from the publisher. Current printing (last digit): 1 0 9 8 7 6 5 4 3 2 1
PRINTED IN THE UNITED STATES OF AMERICA
TO
Jatindra Mohan Bhowmick Hem Prova Bhowmick Kundakali Bhowmick Asmit Bhowmick Marian Stephens
This Page Intentionally Left Blank
Preface
Exactly 10 years have glidedby since the first edition of Hardbook of Elastomers was published. Wherever we have traveled, we have heard good words about the book. It has been found to be useful for teaching, research, and business purposes. The overwhelming response from around the world prompted us to undertake a second edition. Considering the success of the first edition, the style of the book has not been changed. New chapters have been included and materials no longer in vogue have been deleted. Most of the chapters from the first edition have been updated with new information and technology, and only a few have been retained in their original form because no significant new developments have occurred. Readers’ suggestions have been incorporated in many places. We wish to thank all the authors for their fine contributions and sharing of their expertise. We are grateful to many rubber companies, polymer institutes, and research and development organizations around the world for valuable suggestions and assistance. We acknowledge our indebtedness to our family members, especially to Asmit Bhowmick, Dr. S. K. Biswas, and Ms. K. Biswas, for their patient understanding. Last,but not least, we are thankful to Russell Dekker, Chief Publishing Officer, and EricStannard, Production Editor, of Marcel Dekker, Inc., for their wholehearted support and guidance. We hope that the second edition of Handbook of Elastomers will be even more useful to our readers. Ani1 K. Bhownick Howard L. Stephens
V
This Page Intentionally Left Blank
Contents ...
111
.vi
1. Guayule Rubber D. Mclrltyre, Ho~turclL. Stephens, W. W. Schlorwnn, Jr., crr~clAni1 K . Bhon*nlick
1
2. H e l w Natural Rubber A. H. Eng m t l E. L. Or1g
29
3.
Modified Natural Rubber Crispirl S. L. Brrkrr
61
4.
Chemical Modification of Synthetic Elastomers Dorltrltl N . Sch~rl:m t l A ~ ~ I ~ I I I0. L IPatil II~II
109
5. Liquid Rubber Douglcrs C. Ed1cwrd.s
133
6. Powdered Rubber Colirl W. EIWS
167
7. Rubber-Rubber Blends: Part I C. Miclwel Roltrrltl
197
8.
Rubber-Rubber Blends: Part 11. New Developments C. Micl~nelU o l c r d
227
9.
Short Fiber-Filled Rubber Composites Lloyd A. Goc.tt1L.r arlcl Willimu F. Cole
241
10. Thermoplastic Elastomeric Rubber-Plastic Blends Aubert Y. C o t m
265
11. Thermoplastic Styrenic Block Copolymers Geojfrey Holrlerl m r l C11rrrles U. Wilder
321 vii
viii
Contents
12. PolyesterThermoplasticElastomers:Part Rorleric P. Quirk unci Qizkuo Zhuo
I
353
13. PolyesterThermoplasticElastomers:Part H . M. J . C. Creerwers
I1
367
14. Thermoplastic Polyurethane Elastomers CIzarles S. Schollenberger
387
15. Thermoplastic Polyamide Elastomers Anil K. Bhowrnick
417
16. Ionomeric Thermoplastic Elastomers Kurnal K. Kar and Ani1 K. Bhowrnick
433
17. Miscellaneous Thermoplastic Elastomers Anil K. BhoMmick
479
18. Halogen-Containing Elastomers Daniel L. Hertz, Jr.
515
19. Tetrafluoroethylene-Propylene Rubber Gen Kojirna and Masayuki Saito
547
20. Carboxylated Rubber John R. Dunrl
561
21. Polyphosphazene Elastomers D. Frederick Lohr and Harold R. Penton
591
22. Advances in Silicone Rubber Technology: Part I, 1944-1986 Keith E. Polrnarzteer
605
23. Advances in Silicone Rubber Technology: Part 11, 1987-Present Jerome M. Klosowski
649
24. Acrylic-Based Elastomers Piero Anrlreussi und Arturo Carrano
659
25. Poly(propy1ene oxide) Elastomers Dotninic A. Berta ancl Edwin J. Vandenberg
683
26. Polyalkenylenes Adolf Drusler
697
27. Polytetrahydrofuran P. Dreyfuss
723
Contents
ix
28. Crosslinked Polyethylene Bllarot Dave'
735
29. Millable Polyurethane Elastomers Klalrss Knoerr and Uwe HofSlnann
753
30. Cast Polyurethane Elastomers Klaw Reeker
765
31. Polynorbornene Rubber Ani1 K. Bhonwick, C. Stein, and H o ~ ~ r L. r d Stephens
775
32. Nitrile and Hydrogenated Nitrile Rubber Saclzio Huycrslli
785
33. Diene-Based Elastomers Jcrdit E. Plrskas
817
34. Recycling of Rubber Willianl H.Klingerwnith and Krishna C. Bararntwl
835
35. EPDM Rubber Technology Richard Karpeles m r l Anthony V. Grossi
845
36. Isobutylene-Based Elastomers Neil F. Nert-rnnrt and James V. Flrsco
877
I n dex
909
This Page Intentionally Left Blank
Contributors
Piero Andreussi EniChem, Milan. Italy Crispin S. L. Baker Tun Abdul Razak Research Centre, Brickendonbury. Hertford, England Krishna C. Baranwal AkronRubberDevelopmentLaboratory. DominicA.Berta
Inc., Akron, Ohio
Basell R&D Center, Elkton, Maryland
Ani1 K. Bhowmick India
RubberTechnology Centre, IndianInstitute of Technology.Kharagpur.
Arturo Carrano EniChem, Milan. Italy William F. Cole FlexsysAmerica L.P.. Akron. Ohio Aubert Y. Coran The Institute of Polymer Engineering, The University of Akron, Akron. Ohio H. M. J. C. Creemers DSM Engineering Plastics
BV, Sittard. The Netherlands
Bharat DavC ECC Productd3M Co.. Chelmsford.Massachusetts AdolfDraxler"
Degussa-Hnls AG. Marl, Germany
P. Dreyfuss Consultant.Midland,Michigan John R. Dunn J. R. Consulting, Sarnin. Ontario, Canada Douglas C. Edwards" PolysarLimited. Sarnia, Ontario, Canada A. H. Eng Rubber Research Institute of Malaysia, Malaysian Rubber Board. Kuala Lumpur. Malaysia Colin W. Evans? Consultant.Gateshead.England James V. Fusco" ExxonChemical Co., Baytown, Texas LloydA. Goettler$ Solutia. Inc.. Pensacola.Florida Anthony V. Grossi Crompton Corporation/UniroyalChemical Company. Inc.,Middlebury. Connecticut
xi
xii
Contributors
Sachio Hayashi
NipponZeonCo.,
Daniel L. Hertz, Jr. Uwe Hoffmann
Ltd., Tokyo. Japan
Seals Eastern, Inc., Red Bank, New Jersey
Rhein Chelnie Rheinau GmbH, Mannheim, Germany
Geoffrey Holden
HoldenPolymer Consulting, Incorporated,Prescott,Arizona
Kamal K. K a r
Rubber Technology Centre, Indian Institute of Technology, Kharagpur, India
Richard Karpeles Crompton CorporationNniroyal Connecticut William H. Klingensmith
AkronConsultingCo.,Akron,
Jerome M. Klosowski Dow Corning Klaus Knoerr Gen Kojima
ChemicalCompany.Inc.,Naugatuck, Ohio
Corporation,Midland,Michigan
Rhein Chemie Rheinau GmbH, Mannheim, Germany Asahi Glass Co., Ltd., Yokohama, Japan
D. Frederick Lohr* The D. McIntyre The
Firestone Tire and Rubber Company, Akron, Ohio
University of Akron, Akron, Ohio
Neil F. Newman" Exxon Chemical Co., E. L. Ong Rubber Malaysia
Research Institute of Malaysia, Malaysian Rubber Board, Kuala
Abhimanyu 0. Patil Jersey Harold R. Penton
ExxonMobilResearch
and Engineering Company, Annandale,New
Enterprises, Lady Lake, Florida
University of Western Ontario, London, Ontario, Canada
Judit E. Puskas Roderic P. Quirk Akron, Ohio
Maurice Morton Institute
of Polymer Science, The University of Akron,
Bayer AG, Leverkusen, Germany
C. Michael Roland Masayuki Saito
Lumpur,
EthylCorporation,BatonRouge,Louisiana
Keith E. Polmanteer Consultant, KEP
Klaus Recker
Baytown, Texas
NavalResearchLaboratory,
Washington, D.C.
Asahi Glass Co., Ltd.,Yokohama, Japan
W. W. Schloman, Jr. The University of Akron, Akron, Ohio Charles S. Schollenberger* PolyurethaneSpecialistand
Consultant, Hudson, Ohio
Donald N. Schulz ExxonMobil Research and Engineering Company, Annandale, New Jersey C. Stein* CdF Chimie
S.A.,Paris,France
Howard L. Stephens The Edwin J. Vandenberg Charles R. Wilder* Qizhuo Zhuo Ohio
* Retired
University of Akron, Akron, Ohio
Arizona State University,
Tempe, Arizona
PhillipsPetroleum Company, Bartlesville, Oklahoma
Maurice Morton Institute of Polymer Science, The University of Akron, Akron,
HANDBOOK OF ELASTOMERS
This Page Intentionally Left Blank
Guayule Rubber D. Mclntyre, Howard L. Stephens, and W. W. Schloman, Jr. The University of Akron, Akron, Ohio
Ani1 K. Bhowmick Rubber Technology Centre, Indian lnstitute of Technology, Kharagpur, India
1. INTRODUCTION 1.l
Commercial Natural Rubbers
Although thousands of plant species contain rubberin small amounts,only a few speciesgenerate enough rubber to make them commercially attractive. In fact. only H e l m Drcrsiliensis and Pertheniun~argentaturn (guayule) have been used commercially in modern times and are likely to be used in the immediate future. Therefore. in this review these two natural rubbers will be called hevea rubber and guayule rubber. Other natural rubbers will be included in the discussion to giveabetterperspective on the nature of naturalrubbersandtheirprospects for future development. While guayule rubber is discussed here, the developments in the area of hevea rubber are treated in the next chapter.
1.2 Previous Reviews and Present Perspectives There have beenseveralreviews of hevea and guayule rubbers. The internationalscientific conferences on guayulerubber are particularlyinterestingandrelevantbuthavefrequently included overwhelming amounts of agronomic and economic material (McGinnies and Haase, 1975; Campos-Lopez. 1978;Gregg et al., 1983). Thelater conferences (Guayule Rubber Society, 1983, 1984, 1985) only have abstracts and are therefore less helpful for details of research. The International Symposium on Natural Rubber, 1980 (on hevea, guayule, and other rubbers), has some interesting comparisons of the two rubbers in its discussions. Several early authorative review articles on the whole subject of the viability and value of guayule rubber have been published (U.S. National Academy of Sciences, 1977; Campos-Lopez, 1978). A more easily accessible article on guayule rubber also appeared (Eagle, 1981), but it comprises an extensive literature review without either a critical evaluation of the underlying inconsistenciesof different workers or an attempted synthesis of the science and technology of guayule rubber. More recently, the Guayule Administrative Management Committeeand the U.S. Departmentof Agriculture (USDA) Cooperative State Research Service preparedacollectivereview of basicand applied research on guayule, including biochemistry (Benedict, 1991), processing (Wagner and Schloman. 1991), and rubber and coproduct utilization (Schloman and Wagner, 1991). 1
Mclntyre, et al.
2
In this small chapter we hope to present a current critical, scientific, and technological point of view. The remarks on the agronomicand economic aspects of the subject are tenuous and are included briefly only to suggest the challenge and uncertainty involved in discussing this important aspect of guayule rubber.
2. 2.1
STRUCTURE AND BIOGENESIS OF GUAYULE RUBBER Plant Morphology and Biogenesis of Rubber
Rubber is found in plant ductsinas the omnipresent milkweed or the tropical Heuea brasiliensis tree. The free-flowing milk from the tapped or broken duct contains the rubber as emulsified spheres in the cell fluids. Rubber is also found in single cells (parenchymal of cells) the ubiquitious goldenrod or guayule plants. Since there is no long duct to tap or break, each cell must be broken open by strong mechanical forces. Common childhood experience with goldenrod and milkweed neatly illustrates the technological problem in producing rubber from guayule and hevea. Milkweed. like hevea, immediately gives the sticky rubber-containing milk, while goldenrod, like guayule, does not appear to conta rubber. Fortunately, the Indians of northern Mexico and the southwestern United States found that by chewing the guayule plant they could spit out coagulated rubber. The extraction technology thus is more complicated for guayule than for hevea. Figure 1 shows an electron micrographof a cross sectionof a stem of the guayule plant. Note the dark cellular regions where the rubber is contained. The parenchymal cell, 30 p m X 30 pm X 30 pm, is full of spherical rubber droplets, as shown in Figure 2. Although the size of the mbber droplets appears to be approximately 1 pm, there are many droplets as small as
Fig. 1 Cross section of a stem of the guayule plant.
Guayule
Rubber
3
li t+
i.2‘$1 .’?I
‘f
Fig. 2 The parenchymal cell is full of spherical rubber droplets.
In Figure 1, note the large holes designated resin canals. Like hevea rubber, the guayule rubber in the parenchymal cells has very little dissolved resin. However, the cutting of adjacent resin canals and parenchymal cells during processing allowstotal dissolution of the resin into the rubber. Thus, “recovered” guayule rubber contains resins that must be extracted before the rubber behaves physically and chemically like hevea rubber. Rubber is contained not only in the main trunk and stems, but also in the roots in large amounts and in the leaves in small amounts. No one yet knowswhy plants make rubber, although the finding of rubber in monocotyledons in these laboratories suggest that the evolutionary development of rubber in plants was much more widespread and casual than previously believed. A simple differential solubility staining technique coupled with modem gel permeation chromatography (MacArthur and McIntyre, 1983) has allowed field screening of the morphology of rubber storage and quick retrievalof samples for rubber molecular weightand composition analysis even in plants with parenchymal rubber like guayule. The rubber biosynthesis occurs through the pathway of secondary metabolism like the steroids and uses isopentenyl pyrophosphate (IDP) (Archer etal., 1963; h e g u i n , 1978; Benedict, 1983; Benedict, 1991; Cornish al., et 1994tGornish and Siler, 1995). A solubilized preparation of the enzyme complex responsible for IDP incorporation is capable of catalyzing the formation of polymer with a peak molecular weight of 1 X lo5 (Benedict et al., 1989; Cornish and Backhaus, 1990). The most abundant of these proteins, designated the rubber particle protein by Backhaus et al. (1991), has an apparent molecular weightof 5.2 X lo4 (Siler and Cornish, 1994a). Enzyme activity has been correlated to various environmental factors, including day length, ambient temperature, and water stress (Appleton and van Standen, 1989; Reddy and Das, 1988). As a consequence, the average molecular weight and molecular weight distribution of the rubber polymer varies with cultivation and harvest history (Schloman etal., 1986; Backhaus and Nakayama, 1988; Ji et al., 1993; Sidhu et al., 1993; Angulo-Sanchez et al., 1995).
4
Mclntyre, et al.
2.2 Molecular Structure of Rubber
The complete molecular structure of any polymer would include many physical and chemical parameters. However, today the high polymeric properties of an uncross-linked polymer can be grossly defined in terms of the chemical composition and length of the chain, provided that the repeat structure is relatively simple. In the case of guayule rubber, the composition is relatively simple and very regular and the molecular weight is high and has a fairly broad distribution. In these regards guayule rubber is very much like hevea rubber. However. the nonrubber contaminants are quite dissimilar. Consequently the chemical degradation, stress crystallization, green strength, curing times. and ultimate physical properties can be very different in batches of hevea rubber and guayule rubber that have not been under quality-control processing conditions, (Campos-Lopez, 1978; Winkler et al.. 1978a; Porter and Stephens, 1979; Bhowmick et al., 1984b). Chemical Structure The chemical structure of both guayule and hevea rubber is entirely cis- 1,4-polyisoprene (Campos-Lopez and Palacios, 1976). There appear to be no high-molecular weight polymers produced that are not totally cis or totally truris, although work by Tanaka (private communication) has suggested that there is a primer triplet at the initiation that is different: trans-trmstrans for hevea rubber and trans-trans-cis for guayule rubber. However. for the purposes of rubber usage, a triplet represents only a 0.03% difference in composition and does not significantly affect any high polymeric properties of flow, elasticity, or crystallization. Molecular Weight The molecular weight distributions (MWD) of guayule and hevea rubber are roughly comparable (Canipos-Lopez, 19781, although the age of the guayule plants influences the average molecular weight. Very young guayule plants produce a lower molecular weight rubber (Hager et al.. 1979). Table 1 shows the average molecular weights found by gel permeation chromatography (GPC) on freshly extracted rubber from 4-year-old and 6-month-old guayule plants (McIntyre et al., 1984). Figure 3a shows the GPC traces of rubber from mature plants. and Figure 3b. those from 6-month-old plants. Thus. from the studies of both guayule rubber and hevea rubber as well as rubber from numerous other rubber-bearing plants, there appear to be two molecular weights of natural rubber. There is a low molecular weight peak of
a
E
U U
ID
T
Table 1 Molecular Weights of Guayule Rubber Whole Polymers from GPC and [q] Sample" ~~
~
~
Commercial bale Deresinated crumb Native (Mex.). wild ( 4 7 7 ) Native (Mex.). wild (7/77) Native (Cal.), #A481 15 Native (Cal.). AT5229 6-Mo Plant. hand-chopped 6-Mo Plant. Waring Blender "
~~
0.57 0.92 1.00
1.10 2.10 1%08 0.16 0.4 1
All plants are 1 years old except last two 6-month plants
I .70 2.20 2.00 2.70 3.63 2.65 1.61 2.5 I
3.10 4.00 3.40 1.80 5.80 5.06 5.41 8.35
2.98 2.39 2.00 2.45 1.74 2.45 10.38 6.17
1.82 1 .31
1.70 1.78 1.59 1.91 3.36 3.33
4.5 1 4.93 5.53 5.59 9.01
-
1.85 2.09 2.45 2.48 3.71 -
-
-
-
6
Mclntyre, et al.
Mexlcan
Oeresmated Crumb
3.2
3.6
RI
2.8
3.03.4
3.8
LO
4.2
4.2
4.4
Elution volume ( c t s 1
Extracted with Worlng Blender
R I
2.8
3.03.4
3.2
3.6
3.8
4.0
L6
Elution volume (ctsl Fig. 3 GPC chromatograms of
(a) whole
polymers of guayule and (b) 6-month-old guayule plants.
increases with molecularweight as reported by Angulo-Sanchez ( 1 98 l ) , because extensive studies on polystyrene show no change. As a consequence, the molecular weights would be even higher. Therefore gel problems ornonrubber contaminants must be unduly affecting the molecular weight determination by light scattering.
Gel Cotiterit Hevea rubber usually has a high gel content unless special precautions are taken in selecting the rubber-producingtrees,instantly dissolving thetappedrubber in protectedsolvents.and shortly thereafter making measurements. Gel may be sepaated into macrogel and microgel by gross filtration and sedimentation techniques. Guayule rubber from the processing plant has gel. Thus, most guayule used in physical property tests has approxmately 10% gel. Also, the percentage of gel increases with long-term storage. It is believed that the gel content increases in the final rubber-drying step during rubber processing. Small amounts of guayule rubber carefully extracted in the laboratory from fresh shoots and examined shortly thereafter are free of gel. However. guayule rubber from shrubs
Guayule Rubber
7
that have been left out to dry before the rubber is extracted shows considerable gel. The conclusions are that ( I ) the gelling reaction is postpolymeric in the ducts or cells and ( 2 ) the drying step either in pilot plant processing or in shrub drying causes large amounts of gel in guayule rubber. Whereas hevea rubber has the protein fraction implicated i n the gelling reaction and also has 20 epoxide and amino groups on each of the polymericchains, guayule rubberfreshly extracted in the laboratory appears to have no epoxide or amine groups beyond experimental en-or of the analytical methods. Of course, large bales of guayule may be oxygenated in the final drying step of the extraction. Therefore, an understanding of the rubber constituents in guayule rubber must always be kept in mind because of the high reactivity of the double bond in rubber that can lead to gels or to chain scission (Black et al., 1986).
2.3
Physical Properties
The glass-transition temperature (Tg), crystallization, and heat capacities (C,,) of guayule and hevea rubber are similar. The crystallization rate of guayule rubber is, however, slightly faster because of the greater linearity of guayule rubber macromolecules (Arid Lands Studies, 1979). Stress-induced crystallization of guayule rubber is much lower than that of hevea rubber, as observed in elasticity, WAXS, and birefringence measurements (Bhowmick et al., 1986b). Thermal and thermoxidative stabilities of these two rubbers are comparable (Bhowmicket al.. 1987). Activation energy of chain scission in both rubbers is 239 kJ/mol. Guayule rubber that has been isolated by solvent extraction does not storage harden like hevea rubber (Angulo-Sanchez et al., 1981). In contrast, antioxidant-stabilized guayule rubber can undergo significant losses in bulk viscosity due to anaerobic thermolysis (Schlornan et al., 1996a). Green strength of guayule rubberis intermediate between thatof hevea rubber and synthetic cis- 1.4-polyisoprene. The green strength could be improved by control of resin residues (Bhowmick et al., 1984b). Thiscould also be increased by using chemical promoters (Ramos de Valle and Montelongo, 1978). The presence of residual plant tissues with particle size >45 km is detrimental for natural rubber properties. Even 0.5%of these causes 30-40% reduction in tensile and fatigue-to-failure properties (Bhowmick etal., 1986a). Stress-strain isotherms athigh elongation, birefringence. and wide-angle x-ray diffraction have also been studied for guayule rubber blends.
2.4
Nonrubber Constituents
The nonrubber constituents in both guayule and hevea rubbers are primarily resins, residual plant tissue, and minerals. However, the amounts and types of constituents and their effects on physical and chemicalproperties can be quite different. For example,the principal residual plant tissue in hevea rubber is protein, which increases green strength, whereas the principal plant tissue in guayule rubber is ligneous and cellulosic cell wall material, which in ltrrge Jing~nenfs impairs ultimate physical properties. The mineral content of guayule is very low in all known guayule rubber extraction processes and is particularly low in processes that avoid water separation steps. Therefore there will be no further discussion here of rubber contaminants. The resins from the plant often remainin the rubber. Figures 3and 4 show the GPC curves of the tetrahydrofuran extractables from the totally ground-up branches of a guayule bush. The GPC trace of the high molecular weight rubber portion is given in Figure 3, while that of the resin is shown in Figure 4. The resin peak can be resolved into many small peaks by highpressure liquid chromatography or more extensive GPC.However, even in routine analyses the
a
Mclntyre, et al.
Fig. 4 GPC chromatogram of resin from fresh guayule shrub.
peaks can be separated into two main peaks: a low molecular weight peak approximating monoto triterpenes ( A ) and a high molecular weight peak approximating triglyceride molecular weight (B). Both the A and B curves can be conceived as resolvable into two major peaks. For the processing of guayule rubber it is imperative to control the total amount of A and B and specific compounds in A or B that change therubberpropertiesdrastically. Forexample. both the triglyceride and terpenoids plasticize lubber, yet some unsaturated fatty acid components increase thermal and oxidative degradation (Bhowmick et al., 1985). Again. large plant tissue fragments give inferior strength properties (Bhowmick et al.. 1984a). Apparently both the fine dirt ( c 4 5 p n ) and coarse dirt (B45 pm) contain a mesh of finer plant tissue tied together by rubber gel. The typical values allowed in commercial guayule rubber are approximately 0.2% for the plant tissue (dirt) and 3-4% for resin. The components of harvested guayule shrubs are given in Table 2.
Table 2 Components of Harvested Guayule Shrubs. c/c Moisture Rubber Resins Bagassc Leaves Cork Water-solubles Dirt and rocks
4-60 8-26' 5-25" so-SS" 15-20' 1-31' 10-12''
Variable
Guayule Rubber 2.5
9
Guayule RubberLatex
In its native form. guayule rubber exists in plant tissue as a latex (Hager et al., 1979; Goss et al., 1984; Backhaus. 1985: Backhaus et al.. 1991). The averagesize of rubber particles in latex from mature (3-year-old) plants is about 0.45-0.50 km. As isolated. guayule latex has a dry rubber content of about 40% (Schloman et al., 1996b). The coagulated rubber phase contains 8- 10%resin, although it is not clear whether these particular secondary metabolities are actually present in the rubberparticlesthemselvesprior to processing.Fromreported bulk viscosity measurements, it appears that latex rubber is indistinguishable from unfractionatedpolymer isolated by solvent extraction techniques (Schloman et al., 1987). Among the complement of proteins in guayule latex are a glycoprotein with a molecular weight of 5.2 X 10‘. possibly a subunit of a large rubber transferase complex, and two smaller proteins with molecular weights of 0.9 X IO‘ and 1.5 X 10‘ (Siler and Cornish, 1994a). Unlike Hevea latex (Slater, 1904).guayule latex does not elicit an immediate or Type I hypersensitivity reaction in human test subjects (Schrank et al., 1993). This is due to the absence of allergenic proteins analogous to those in Hevea latex (Siler and Cornish, 1994a; Siler et al., 1996). These allergens have molecular weights from 0.2 to 10 X 10‘ and include both soluble and particlebound proteins (Czuppon et al.. 1993; Beezhold et al., 1994). As a consequence, removal of soluble components by conventional washing techniques cannot render hevea latex freeof allergens. Extracts of finished goods such as latex gloves contain these same allergens (Hamilton et al., 1994).
3. 3.1
RUBBER-SEPARATION PROCESS General Methods of Processing Guayule Rubber
There are three major components to be separated in the bush: rubber, plant tissue, and resins. Fortunately. the plant tissue is not soluble in simple organic solvents or water. Also, there are solvents that will dissolve resin and not rubber (e.g., acetone), denoted S,,.,, and solvents that will dissolve both rubber and resins(e.g.. tetrahydrofuran and chlorinated hydrocarbons). denoted Thenthere are nonsolvents for rubber. NS,.,,,,, and for resins, NS,.,,. The individual diffusion coefficients for resin diffusion fl-orn pure rubber have been measured (Budiman and Mclntyre. 1984). In all processes. it is necessary to rupture the cellular structure of bark and woody tissue to gain access to guayule rubber, whether in the form of a latex or a coagulated bulk polymer. At the same time it is necessary to separate the rubber from various nonpolymeric secondary metabolities (resin). Commercially acceptable processing must also accommodate the inherent variability in the composition of the rubber in cultivated shrub. An additional consideration is the need to accommodate posthat-vest degradation of the rubber, either in intact shrub or in ground plant material (Black et al., 1986: Dierig et al., 1991 ). For the production of bulk rubber, three methods have been evaluated on a pilot scale: flotation. sequential extraction, and simultaneous extraction. In flotation processing (Fig. 5 ) . a dilutecausticsolution is used to coagulaterubber in the form of resinous “worms”(large aggregates of rubber). which are skinmed off and deresinated by washing with a polar organic solvent. typically acetone. Flotation process rubberwas produced in 1976- 1980 at a pilot facility in Saltillo.Mexico.operated by the Centre de lnvestigacion en Quimica Aplicada (CIQA) (Campos-Lopez et al., 1978; Foster and Compos-Lopez, 1982: Motomochi. 1983). Used successfully in the manufacture of aircraft. automobile, and truck tires, the rubber had acceptably high
Mclntyre, et
10
al.
aqueous alkali
Flotation
SHRUB
_____
RESINOUS RUBBER WORMS
Resin Extraction rubber non-solvent
1
RESIN SOLUTION rubber sol"ent
DERESiNATED RUBBER WORMS
- ---
~
BAGASSE
+
AQUEOUS EFFLUENT
Removal *""""""""""-""."
Filtration
Solvent Removal
RUBBER
Fig. 5 Flotation processing.
Mooneyviscosities but did not meet the FederalEmergency Management Agency (FEMA) specification for acetone extractables (4.0% maximum) (Wagner and Schloman, 1991). Sequential extraction involves the initial deresination of ground shrub with a polar solvent such as acetone. The resin-free plant tissue is then extracted with a nonpolar solvent such as hexane to remove rubber (Fig. 6). Flash evaporation of solvent with steam injection is necessary, with the water being removed from the rubber slurry viaa combination of dewatering operations of thermal or mechanical origin. Sequential extraction was evaluated using an oilseed extractor
Resin
SHRUB
- "1
I
Extraction
Extraction
WO?:K:SUE
,___""____
RUBBER
CEMENT
Filtration
1 RESIN SOLUTION
Solvent Removal
t RUBBER Fig. 6 Sequentialextraction.
RESIN
11
Guayule Rubber
rubber-resin solvent
SHRUB
,
_____________
Size Reduction
____._"_
Extraction RUBBER-RESIN MISCELLA
t
rubber non-solvent
SWOLLEN RUBBER
~
_______________
Filtration RESIN + LOW-MW RUBBER
Removal
RUBBER
BAGASSE
Fig. 7 Simultaneousextraction.
operated in a semi-batch mode at the Northern Regional Research Center of the USDA (Hamerstrand and Montgomery. 1984). Product quality was never reported. Simultaneous extraction (Fig. 7) addresses the weaknesses of both flotation and sequential extraction; the need for efficient deresination, theneed to control the bulk viscosity of the rubber, and theneed to minimizesolventstream contamination. In simultaneousextraction,ground shrub is extracted with a monophase solvent or solvent mixture (toluene, perchloroethylene, pentane, or pentane-acetone azeotrope) capable of removing both rubber and resin as a dilute solution or miscella (Cole et al.. 1987; Kay and Gutierrez, 1987; Wagner andParma,1988; Wagner et al., 1991). High molecular weight polymer is then coagulated from the miscella by addition of a polar solvent such as acetone, ethanol, or methanol (Beinor and Cole, 1986). The process used to isolate high molecular weight guayule rubber is associated with the formation of a resinous by-product containing, among other components. low molecular weight guayule rubber. The resin is dissolved in acetone. Further treatment with 90% ethanol results in the precipitation of low molecular weight guayule rubber. An alternative separation process involves the use of xylene with subsequent rubber precipitation by addition of ethanol. The latter process ensures high-purity, grey colored, tack-free rubber having a molecular weight of 40,000-50.000. The organic-soluble resins, separated into several fractions, the water-soluble resinous portion, and bagasse, a woody pulp containing lignins and cellulosics, are used for derivatization and development of value added materials. For the production of latex (Fig. 8),fresh shrub is cut and milled in water or other aqueous medium (Jones, 1948; Cornish, 1996). The dilute rubber dispersion that results is clarified and concentrated by centrifugation, creaming, or some combination of the two (Jones, 1949; Cornish, 1996; Schloman et al., 1996b). Overall process efficiency can be increased by recovering and using thedilutelatex serum in the dispersion step. Laticeswith 35-50% solidshavebeen produced in this way. While no continuous pilot-scale evaluations have been carried out, Jones (1 948) estimated that it may be possible to disperse 85% of the rubber available in the shrub and recover 90% of the dispersed rubber as latex product. Processing the residual plant tissue after latex removal has not been reported to date. Presumably, somevariation of solvent extraction could be applied to recover resin and undispersed rubber.
ification
12
Mclntyre, et al.
SHRUB
Concentration
Washing DILUTE RUBBER 1 4 SUSPENSION
_."_""_."_""_."".....""..~..."""".., '
RESINOUS BAGASSE
v LATEX CONCENTRATE Fig. 8 Production of guayule latcx.
3.2
AcceptanceSpecifications
The rubber specifications currently used for accepting guayule rubber are basically those of hevea rubber, with extra care being given to the amount of resin or its practical consequences: flow properties due to plasticization by resin and degradative properties due to the enhancement of lubber degradation by resins. The acceptance specification for bulk guayule rubber (FEMA. 1984) is based on the ASTM specification for grade 20 natural rubber (Table 3). A modified specification (Cole et al., 1991) was established for the rubber produced by BridgestoneFirestone, Inc., at its Sacaton, Arizona. pilot processing facility. No specification has been established for guayule latex.
Table 3 Specifications: Raw Guayule Rubber Compared
with Grade 20 Hevea Rubber
Guayule rubbcr FEMA
Modified Grade FEMA
20 NR ASTM
Property 5 0 . 2 0 50.20 Dirt, % 5 1.25 Ash, 9 50.80 Volatilc tnattcr. % 50.60 Nitrogen. % 50.008 Copper. 50.00 % 50.002 50.002 Manganese, 54.0 Acetone cxtract. 'P 230 Wallocc plasticity. P,, 240 Plasticity rctention, PR1 (Z-
'' N o spccificntlon. " Corrected fur antloxldant content
50.20 5 1.25 51
.so 1
50.008 54.0'' 2 30 240
5 I .00
50.80 50.60 50.008 15 I
235 240
13
Guayule Rubber Table 4 FeedstockShrub:Usablc Rubber and MeanPlasticity of Product Batches
Shrub cultivntor(s) AZ-IO1 (Sacaton) USDA composite (Sacaton) USDA composite (Marana) USDA composite (Salinas)
3.3
3.4
l.O 3.5 2
11.3-
38 -t 3 37 f 3 37 f 2 45 f 3
Recent Production History
The largest contemporary processing operation has been the 1987- 1990 production compaign carried out at the Sacaton. Arizona, prototype facility operated by Bridgestone/Firestone. Inc. (Cole et a l . , 1991). Extraction of rubber and resin employed acetone and pentane or hexane. Coagulation and fractionation were effected by the addition of acetone. Total rubber production amounted to 8.7 1 long tons. Atotal of 3.26 long tons of rubber meeting the TSR20 specification was delivered to various contractors for the production and evaluation of military aircraft tires. A total of 2.06 long tons meeting the FEMA specification was delivered to a contractor for the production of light truck tires. Feedstock for rubber production included cultivated shrub maintained at sites other than Sacaton, including Salinas, California, and Marana. Arizona. Table 4 provides a comparison of the rubber quality from these locations. A general observation was that the rubber product frequently had ash levels in excess of the ASTM maximum specification of 1.00%. It was determined that the most likely source of the ash was inorganic material contained within the shrub itself, rather than soil and dust on the surface of the plant material. Volatiles were also frequently higher than the ASTM maximum specification of 0.80%. Unlike hevea rubber volatiles, usually water or low nmlecular weight fatty acids, guayule rubber volatiles consist of entrained terpenes. Evaluations at Bridgestone/Firestone confirmed the suitability of higher-ash and higher-volatiles rubber for truck tire compounds (Valaitis et al.. 1992). Those batches meeting the ASTM specifications for ash and volatiles were selected for aircraft tire fabrication.
4.
COMPOUNDING AND PROCESSING OF GUAYULE RUBBER FOR ENDPRODUCT USES
Guayule rubber, (GA) has been used in both the resinous and deresinated form since the early 1900s. However, due to the high level (ca. 25%)of guayule resin components in guayule rubber, it was found that both the raw rubber and its vulcanizates were prone to rapid and accelerated oxidative degradation. This effect was probably due to the resin fatty acid compounds (i.e.,linoleic and linolenic) in the rubber, which are effective in promoting oxidative degradation of natural and other unsaturated rubbers (Bhowmick et al., 1985). Studies conducted with present-day forms of deresinated guayule rubber have shown that the amount of oxidative degradation is greatly reduced when the resin content is in the range of 5 phr or lower.
Mclntyre, et al.
14
4.1
Processing of Guayule Rubber and the Effect of Guayule Resin on Processing Characteristics
A comparative study of transient and steady-state shear viscosity, stress relaxation, and elonga-
tional stretching of hevea rubber, guayule rubber, and two synthetic polyisoprenes (IR) has been reported by Montes and White( 1982). For example, steady-state viscosity of the gum elastomers is shown in Figure 9. Hevea rubber has the highest viscosity and maximum relaxation time because of the presence of large amounts of gel and higher levels of long-chain branching. The effect of gel on natural rubber (NR) properties has been studied by means of novel experiments (Bhowmick et al.. 1 9 8 6 ~ )Gel . and nonrubber constituents have marked effects on the extrusion properties of guayulerubber (Montes andPonce-Velez, 1982). At 14OoC, gel increasesthe viscosity at low shear rates. The mixing of natural, guayule, and isoprene rubbers with EPC, FEF, and HAF black at different concentrations (30,50,and 70 phr)at 60 and 80°C was reported. The black incorporation time, optimum mixing time, and energy are lower for GR and IR than for NR (Ponce and Ramirez, 1981 ). Since the earlier studies conducted with GR did not indicate the resin content of the rubber, the poor properties reported for vulcanizates containing GR were due to the excessive amounts of resin present in the raw rubber. The current practice of removing or reducing the amount of resin to 5 phr has greatly improved the quality of the rubber and its vulcanizates. The plasticizing effect caused by the resins has been explained by Winkler and Stephens (1978), whose experiments involved various amounts of guayule resin in raw rubber mixes. In addition. the resin was effective i n reducing the gel content of both NR and GR, thus acting as an efficient chemical plasticizer. Thechanges occurring in plasticitymeasurements and gel content with the addition of resins as shown by mill mixing are given in Table 5. The reduction in molecular weight (lower plasticity values) is shown in Figure 10, where the reductions in extrusion time utilizing a Monsanto capillary rheometer for a natural rubber (SMR-5), CR. and styrene-butadiene rubber are compared. It is quite apparent that in all three elastomerstheaddition of guayule resin waseffective in reducingmolecularweightand is probably why GR processes more readilywith lower powerconsumption andfaster incorporation of fillers than NR. This effect has been reported by many authors, and perhaps the resin also functions as a homogenizing agent when fillers are used in rubber mixes. The data given in Table 6 illustrate the rapid breakdown of GR when compared with the other forms of NR. Ramirez and Ponce (1978) also reported this effect when they measured the power consunlption of pale crepe, smoked sheet, and guayule rubber (with added resin) when mixed at
?
O
HR
-
F.5 m 0
"
L
3
1
I
I
I
I
l
-4
-3
-2
-1
0
1
2
Fig. 9 Steady-state viscosity shear rate bchavior of cis-l ,4-polyisoprenesat 100°C.
Guayule Rubber
VI*
0 0 0 a em .- .an-
/ ]m
l-
.-i
V
h
v
m
'D
W ¶
3
.-S
h
e
m
h
0
U-
15
0
16
Mclntyre, et al.
Table 5 Effect of Milling on WallacePlasticity"
Resin (phr) passes
Mill
1
2
5
10
SMR-5 0
58
-
5
56
10
50 35 21 12
S4 41 49 27 33 16 19 10
25 50 100
-
43
49 43 29 17
45
9
9
23 14 9
-
-
-
44 38 37 25
33
15
16
13
10
9
36
GR 0 5 10 25 50 100 "
-
46 54 40 28
42 37 22 26
18 11
7
33 29 19 12 7
N o gel ohserved below the solid line.
80°C for set time periods. Their results, given in Table 7, also confirm that the addition of resin does reduce the power needed when compared to NR, especially when 6% resin was added. This produced a 24% reduction in power consumption under the conditions used. Consequently, guayule resin may find a market as a processing aid.
4.2
Rate and State of Vulcanization
The preparation of suitable vulcanizates with GR has always been questionable, since therubber collected from different sources was not thoroughly deresinated to the same extent or as well characterized as the elastomer prepared in the Mexican pilot plant. Using this elastomer,Winkler and Stephens (1978) conducted studies showing how vulcanization systems can be modified to produce adequate properties compared to NR. The results of these studies, indicating the compounds used and the vulcanization systems studied, are given in Tables 8 and 9 and illustrated in Figures 1 1 - 14. The ASTM and efficient vulcanization recipes were selected to determine whether or not the GR had the same vulcanization characteristics and physical properties as a high-quality NR. in this case SMR 5L.
Table 6 EffectofMilling
on MolecularWeight (g/mol)
Guayule rubber Four mill passes Ten mill passes
49 I ,000 254,000
Smoked sheet Pale crepe
472,000 382,000
614,000 321,000
So1trc.r: U.S. Department of Agriculture Technlcal Bullctin No. 1327: Research on Guayule. 1942- 1959.
17
Guayule Rubber Table 7 EnergyConsumptionin10-MinuteMixing Type of rubber Mixing speed (RPM) #1Pale crepe
Energy used
106.80 106.24 99.42 89.97 80.88 70.24 112.17 105.25 94.05 86.85 73.02 69.74 105.44
70 60 50
#l Smoked sheet
40 30 20 70 60 50
Guayule
Guayule
+ 2% resin
+ 6% resin
(watts)
40 30 20 70 60 50 40 30 20 70
100.00
90.88 82.56 71.60 72.56 90.32 87.62 82.53 73.06 68.43 53.12
60 50
40 30 20
Table 8 CompoundRecipes ASTM Semi-eff. Efficient vulcanization stock Black stock Gum Elastomer (NR, GR, or IR) Carbon black, N 330 Zinc oxide Stearic acid Polymerized 1.2-dihydro 2,2,4-trimethylquinoline Sulfur N-t-Butylbenzothiazolyl-2sulfenamide N-CycIohexyl-2benzothiazolylsulfenamide Tetramethylthiuram disulfide Total
vulcanization
100
100
100
0
35 5 2
0
5 2 -
2.25 0.70
109.95
3.5 2.5
-
2.0
2.25 0.70
-
144.95
0.25
100
0 3.5 2.5 2.0 1.2 -
2.2
0.8
1 .o
0.4
1 1 1.45
110.40
Mclntyre, et al.
18
Table 9 CuremeterVulcanizationData (140°C) ASTM
stock Gum Initial torque, dN-m Minimum torque, dN-m Maximum torque, dN-m t,(2), min t,(90), min Cure rate, d N - d m i n Reversion time, min
Efficient vulcanization vulcanization
Black stock
Semi-eff.
GR
NR
GR
NR
GR
NR
GR
NR
2.00
2.56 2.12 20.6 5 24.5 3.4 85
5.65 4.20 28.3
6.20 4.50 39.6 7 19 6.0 60
2.83 2.12 7.49
3.96 2.68 18.5
4.52 2.46 14.5
IO
8 21 .S
4.52 3.16 22.3 7 12 6.8 > 120
1.00
15.5 IO
31 I .7 94
11
25 2.8 55
24.5 1.1
3.4
> 120
> 120
10
16 4.8 35
TIME (minl
a-
ASTM BLACK STOCK I l C O ' C )
NR
GR
(B 0 0
I
1
I
20
I
40
I
ao
TIME (minl Fig. 11 Vulcanization characteristics of natural and guayule rubbers (a) in an ASTM gum stock and (b) in an ASTM black stock.
Guayule Rubber
19
1
I E V RECIPES GUM STOCKS (140°C) E
-
NR
Z
GR SEMI
U
W' 1 0 3,
GR
0. 0
1
l
40
80
120
TIME (min) Fig. 12 Vulcanization characteristics of natural and guayule rubbers with efficient vulcanization recipes.
A S T M G R BLOCKSTOCK (14Oy)
E
0
20
60
40
TIME (min) Fig. 13 Effect of recipe variations on the vulcanization rate of guayule rubber (ASTM black stock).
I E V RECIPES GUM GR
1
STOCKS (14Oy)
€ 1
z
+1SA
*2SA
t-
g
0 1 0
0
L 20
s
!
E
40
3
60
Time (min) Fig. 14 Effect of the addition of stearic acid on the vulcanization rate of guayule rubber.
Mclntyre, et al.
20
It is apparent that with the ASTM recipe, neither the gum nor the black GR stocks gave vulcanizationrates or torquevalues comparableto those of the NR stocks.However, both elastomers gave similarly shaped cure curves, indicating that the degree of cross-linking in the GR stocks was lower than that with NR at the same cure time. The two recipes represented as efficient and semi-efficientcure systems showeda definite increase in vulcanization rate with lengthy plateaus, that is, less than 2% reversion. The semiefficientrecipewasmoreefficient in producing a rapidvulcanizationtime with these gum stocks. Neither of the GR stocks was as tightly cross-linked as the NR stocks. The ASTM recipe utilizing GR when modified by using 4 phr of stearic acid in place of 2 phr and 1 phr of accelerator (TBBS) in place of 0.7 phr did not affect the scorch properties of the stock too greatly, but the additional accelerator noticeably reduced the 90% cure time. This indicates that in recipes of this type the cure rate of GR is more affected by the addition of accelerator than by the use of stearic acid, which functions as an accelerator-activator. It is known that GR generally requires additional stearic acid to obtain reasonable cure rates and physical properties. The efficient vulcanization recipe was used to show this effect with this well-characterized GR. It is apparent from the data that only 1 phr of additional stearic acid was necessary to increase the torque to about double that of the original recipe. Addition of 2 phr did not give results greatly different from those obtained with 1 phr.
4.3 Physical Properties of Vulcanitates Although studies by Spence and Boone (19271, Hauser and Le Beau (1943a.b. 1944), Morris et al. (1943, 1944), and Clark et al. (1945, 1956a,b) had examined the effects of various compounding ingredients and vulcanization techniques, none of these researchers worked with a well-characterized GR. Hopefully, with the use of modem technology and GPC for structure characterization, present-day studies may prove that extraction techniques are capable of producing a “standard” GR. Utilizing the ASTM recipe, physical properties were obtained both on unaged and aged black samples (14 days at 70°C) (Table 10). The 90% cure time was used for the sample preparation. Basically, the GR stock containing carbon black gave lower modulus, tensile strength, rebound, hardness, and tear strength than SMR. However, with the exception of tear strength, the properties would be sufficient for most industrial compounds. The differences in properties may have been due to the lower cross-link density obtained with the GR compound.
Table 10 VulcanizateProperties”(ConventionalRecipe
Cure time, t,(90), min Stress at 300% elongation, MPa Tensile strength, MPa Elongation, % Set at break, % Bashore rebound, 70 Shore A hardness Tear strength, kN/m Molecular weight between cross links, M,
A, 140°C)
GR
NR
25 7.24 ( + 120) 25.14 ( - 12) 635 ( -43) 14 40 54 (0) 31.15 13,000
19 12.21 ( + 100) 27.93 ( - 15) 490 ( - 39) 13 48 60 (0) 76.65 9,500
‘‘ Percent change after aging for 14 days at 70°C is gwen in parentheses.
Guayule Rubber
21
Table 11 VulcanizateProperties(EfficientVulcanizationRecipes,
140°C)
Efficient vulcanization GR
Cure time, t,(90), rnin Stress at 500% elongation, MPa 3.62 Tensile strength, MPa 620 Elongation, 8 34 Shore A hardness Bashore rebound, % Molecular weight between cross links, M,
24.5 1.38 9.48 690 31 59 16,800
12
Semi-eff. Vulcanization
NR
GR
21.5 5.00 1 1S21.21 5 680
16 1.72 11.38 720 32
37 62 13,900
NR
60
64
1 5,000
1 1,500
On aging, the percent changes in tensile properties were equivalent, indicating that GR was sufficiently stabilized to age at the same rate as SMR. The properties obtained with the gum stocks, using the efficient vulcanization systems (Table 1 l ) , showed the same trend. However, the tensile strength for GR was improved using the semi-efficient recipe. Again, without adjustments for increasing the fatty acid content of GR, the cross-link densities of the guayule compounds were lower, indicating a slower cure rate and lower tensile values. Processing and compounding studies were conducted on IR, NR. and GR in tank track padrecipes (Touchet, 1987). The guayulerubberwasproduced by TexasA&M University using a simultaneous extraction process. In general, the stock containing guayule rubber was indistinguishable from the NR stocks. While tensile strengths did not vary much among the various compounds, the guayule stock had higher 200% modulus and heat resistance. Aged flex fatigue was lower than that of either IR or NR stocks. The compounding properties of guayule latex reported by Scholam et al. ( 1 996b) are consistent with earlier work (Jones, 1948), indicating that the latex produces slowcuring, lowmodulus films. Unaged guayule films had a 500% modulus of 1.4 MPa after 60-minute prevulcanization at 65°C followed by 20 minutes at 104°C. In contrast, an equivalent Hevea film recipe had a 500% modulus of 5.1 MPa. Suitably aged to accommodate the slower cure rate, guayule film had a tensile strength ( 1 9 MPa) comparable to that of films produced from hevea lattices (2 1-22 MPa). The modulusof unaged guayule films canalso be increased by longer prevulcanization times and increased levels of zinc oxide and accelerator. The vulcanization properties of guayule latex are at least in part a consequence of having lower viscosity rubber [MLI + 4 (100°C) 5 691 and higher resin levels ( 2 8 % ) than hevea latex.
4.4
End Uses
Studies conducted utilizing ASTM and efficient vulcanization recipesfor determining vulcanization characteristics and physical properties of guayule rubber indicate that this elastomer can be utilized as a direct substitute for hevea natural rubber. Neglecting recipe modifications, guayule rubber does give physical properties similar to those obtained with natural rubber vulcanizates. If a “technical specified type” of guayule is commercially feasible, this form of rubber can become a direct substitute for all types of hevea rubber. For example, tests conducted by the U.S. Navy and other governmental agencies have found that guayule functionswell when used in aircraft tires and othermechanical goods. Blends with other synthetic rubbers and grafted copolymers could be used for many applications.
22
Mclntyre, et al.
Tires fabricated from rubber produced at the BridgestoneSirestone prototype processing facility were tested from July 1993 through December 1995 at the U.S. Army Yuma Proving Ground (Lucas, 1996). Two test sets were evaluated: one compounded with a 50 :50 blend of guayuleandhevearubbersand a secondwith 100% guayule rubber. Guayulestocks were substituted in all parts of the tires where hevea rubber would normally be used. The test tires were mounted on light trucks designated as commercial utility cargo vehicles. Tire performance was compared with three control, orbaseline, sets of commercial light truck tires. Guayule tires met or exceeded the performance of controls in ride handling, stability, evasive maneuver, and braking. The multiseason on-road endurance capabilities of the guayule tires were comparable to those of the baseline tires over 10,000 miles. Cured dipped film prepared from guayule latex yielded 0.16 mg of leachable protein per gram, 44% of the yield from a hevea film (Schloman et al., 1996b). Enzyme-linked immunosorbent assays of guayule film extracts confirmed the absence of immunogenic proteins. Guayule latex would not elicit a systematic Type I allergic reaction in individuals sensitized to hevea latex. Processing of guayule shrub provides mainly five coproduct fractions including high molecular weight rubber, low molecular weight rubber, organic-soluble resin, water-soluble resin, and bagasse. Although the guayule generates high-quality and high molecular weight natural rubber with properties comparable to Heveu brusiliensis, cultivation, harvesting, and processing costs are high. As a result, development of value-added materials from the coproduct fractions of guayule is necessary. The alkene character of low molecular weight guayule rubber affords the opportunity for chlorination, epoxidations. maleinization, cyclization, hydrogenation, and many other reactions. Thames et al. (1994) reported the chlorination of low molecular weight guayule rubber. They developed 100% solid coatings that cured rapidly with UV light, thus offering energy savings, ease of handling, and wide formulation lattitude. The addition of reactive groups to the polymer backbone of chlorinated rubber resulted in the formationof environmentally compliant coatings. Thus, chlorinated hydroxylated rubber was developed. This rubber (2.7 weight% hydroxyl) with polyol was reacted with isocyanate in a hydroxyllisocyanate ratio of 1.0/1.05- 1.10. The coating cured at room temperature confirmed toughness, high gloss, and resistance to water, organic solvents, and chemicals, as given in Table 12 (Thames et al. 1994). Additionally, chlorinated hydroxylated rubber was reacted with acrylol chloride to produce acrylated chlorinated rubber, a binder for use in wood fillers and clear finishes. Epoxidized rubber and maleinized rubber were also reported by the same authors. The resins seemed tooffer the most promiseof financial return in connection with producing deresinated rubber (Arid Lands studies, 1979). Guayule resin may find use as a peptizing agent for rubber, aiding to breakdown gel or high molecular weight fractions in the rubber, which are broken by mechanical shear. Work hasalso been carried out atCenter deInvestigacion en QuimicaAplicada to developvarnishes and adhesives. Possible other uses are pigment dispersors and tackifers for rubber. The use of guayule coproducts as extender/plasticizer for epoxy resin coatings was also investigated (Thames and Kaleem, 1991). The coatings were applied onto aluminum and cold rolled steel substrates. When compared with the unmodified epoxy coating, the formulations containing 10% guayule resins performed equallywell on treated metal substrates. In contrast, the films formed on nonheated steel and aluminum panels are strippable. These resins contains triglycerides of fatty acids, which when incorporated into epoxy formulations impart flexibility and plasticizationto the resultant coatings. Guayule resins have also been evaluated as a wood protectant against termites, fungi, and barnacles (Thames and Poole, 1992). Based on weight, bagasse is the largest by-product. In a typical harvested guayule shrub, bagasse constitutes 50-55% as compared to 8-26% of rubber and 5-15% of resin. Its importance
23
Guayule Rubber Table 12 Properties ofTwoComponent Polyurethane Coatings with Chlorinated
Hydroxylated Low Molecular Weight Guayule Rubber Property
Wet thickness Drying time touch to Set free Dust free Tack Solid ess Pencil strength, Tensile Wpsi Elongation at break n-lb direct, Impact Adhesion D-3359) (ASTM rub) MEK (double 8-hour spot tests Water NHJOH 10% NaOH 20% HZSOJ 5 = No effect; 4 = stain only: 3 lifted film; 1 = failure.
2 mils 15 min min 105 min 65.40 H
3.90 1270
120 5B 200
5 5
5 5 =
blistering; 2 =
as a source of fuel for guayule processing is indisputable. Direct combustion of bagasse gives a fuel value of 18200 kJ/kg. A gas containing olefins, hydrogen, and carbon monoxide is formed (Thames and Poole, 1992). Bagasse and guayule leaves mixed in a particular ratio and subjected to a pressure of 8000 pounds per square inch (psi) and temperature of 90- 110°C indicated possible use of these materials for insulation or wallboard (Arid Lands-Studies, 1979). Bagasse can also be used to provide cellulosic materials and is a source of fermentable sugars or fibers. Various cellulosic derivatives like cellulosic acetate, cellulose nitrates,and regenerated cellulosics have been prepared. It may also find use in paper, cardboard, pressed board manufacturing, or some other lower quality uses. Leaves constitute an excellent soil amendment, especially when composted. After parboiling, they can be compressed into a building board.
5. IMPLICATIONS OF COMMERCIAL GUAYULE PRODUCTION 5.1
Present
In its most recent manifestation, guayule research has involved cooperation among government agencies, academic institutions, and private industry. Various programs have emphasized integrating the science and economics of agricultural production, processing, and product development. Extensive testing has validated guayule rubber as a substitute for hevea rubber in tire applications (Bailey, 1995). Ongoing research indicates that guayule latex is a promising material for use in medical products. While a significant amount of the rubber and coproduct resin from
24
Mclntyre, et al.
the Bridgestone/Firestone production campaign still remains, the pilot facility that produced the material has been mothballed.
5.2 Future Guayule hasbeen characterized timeand again as a new industrial crop destined for commercialization. The Office of Technology Assessment of the U.S. Congress (1991) has concluded that economic, not technical, constraints will have the most profound impact on such development. Guayule rubber must be competitive withhevea andsynthetics in terms of cost as well as quality and performance. Weihe and Nivert (1983) predicted commercial viability for producing bulk rubber if the yield of rubber per acre exceeded 1000 lb, if the price of rubber reached the record highs of the 1970s andif guayule resin had a value greater than that of pine resin. More recently, Foster et al. (199 1) concluded that guayulegrowers would have toyield over 1300 lb of high-molecular weight rubber per acre per year to break even. Alternatively, the price of rubber would have to exceed $1 .00 per pound. The necessary increases in agricultural production will have to come from ongoing efforts to identify high-biomass, high-yield cultivars, as well as efforts to develop techniques for low-cost direct seeding. Chemistry, such as the use of amine bioregulators to stimulate biomass and rubber production, has had less of an impact in this area thanconventional plant breeding. In the Foster analysis, profitability was more affectedby the value of the resin and bagasse (residual woody tissue) rather than of the rubber. Process development may ultimately focus on products for niche markets rather than for the tire industry. Guayule rubber’s future could be in high-value applications such as the production of hypoallergenic dipped goods and industrial products. What seems of even more importance for the future of natural rubbers and fuels is the hope that intensive and continuing studies of guayule rubber biosynthesis and guayule rubber extraction in a commercially successful venture would lead to a host of new discoveries in bioregulation and plant extraction processes. If these trained scientists and new discoveries were then harnessed for future world polymer production and hydrocarbon fuels, the exploitation of land that is not useful for food production could be kept for the production of hydrocarbons to the benefit of human kind.
REFERENCES Angulo-Sanchez, J. L., Jimcnez-Valdez, L., and Campos-Lopez, E. (1981), J. Appl. Polvm. Sci. 26:lSl I . Angulo-Sanchez. J. L., Neira-Velazquez, G., and Jasso de Rodriguez, D.(1995). I t d . Crops Prod. 4 :113. Appleton, M. R., and van Standen, J. (1989), J. PIrrr~t.Physiol. 134524. Archer,B.L.,Bamard, D., Cockbain, E. G.,Dickenson, P. B.,andMcMullen, A. T. (1963), in The Chernistp U I I Physics ~ .f Rubber-like Substarms (L. Baternan, Ed.), Wiley. New York, Ch. 3. AridLandsStudies(1979), Report-A SociotechnicalSurvey of GuayuleRubberCommercialization, Office of Arid Lands Studies, University of Arizona, Tucson, Arizona and Midwest Research Instltute, Kansas City, MO, April 1979. Arreguin, B. (1978). in Gucryule: Reencuentro er] el Desierto (E. Campos-Lopez, Ed.), CONACYT, Mexico. p. 9s. Backhaus, R. A. (198S), Bot. Grrz. 144391, Backhaus, R.A., Cornish, K., Chen, S.-F., Huang, D.-S., and Bess, V. H. (1991).P / t ~ ~ t o c ~ ~ r 30:2493. rr~~i.~tr~ Backhaus, R. A.. and Nakayama, F. S. (1988). Rubher Chem Techno/. 61:78. Bailey, C. A. (1995), paper presented at the 1995 Spring National Meeting of the AIChE. Houston, TX.
Guayule Rubber
25
Beezhold. D. H., Sussman. G. L., Kostyl, D. A., and Chang, N. S. (1994), Clin. Exp. lmmurwl. 98:408. Beinor, R. T., and Cole. W. M. (1986). U.S. Pat. 4,623,713 (The Firestone Tire & Rubber Co., assignee). Benedict. C. R. (1983), in Biosyrlthesis of lsoprerroid Corrlpoutds, Vol. 2 (J. W.Porter and S. L. Spurgeon, Eds.), Wiley-Intersciencc, New York, pp. 355-369. Benedict, C. R., Madhavan, S., Greenblatt, G. A., Venkatachalam, K. V., and Foster, M. A. (1989), Plcu~t Physiol. 92:8 16. Benedict, C. R. (1991), in Guqwle Nuturd Rubber (J. W. Whitworth and E. E. Whitehead, Eds.), GAMCUSDAKSRS. TucsoI1, AZ, pp 93-106. Bhowmick. A. K., Rampalli. S.. Kasemsuwan, S., and McIntyre, D. (1984a). Proc. Fifth Arlrl. Cor!f:Gltnyuie R ~ d ~ hSocie/y. t~r Washington, DC. June 17-21, p. 94. Bhowmick, A. K., Manzur. A., and McIntyre, D. (l984b), unpublished observations. Bhowmick, A. K., Rampalli, S., and McIntyre, D. (1985). J. Appl. Polyrtl. Sci.. 30:2367. Bhowmick. A. K., Kasemsuwan, S., Oroz, M. A., Patt, J., Secger, R., MacArthur, A., and McIntyre, D. ( 1986a), Kc~rrtschrrk Gurwni Kurlststofle 39:1075. Bhowmlck, A. K., Kuo, C. C., Manzur, A., MacArthur, A., and McIntyre, D. ( 1986b), J. Mr~crorr~ol. Sei.Plrys. E d 25:283. Bhowmick, A. K., Cho, J., MacArthur, A., and McIntyre, D. (1986c), Polyrrler 2 7 1889. Bhowmick, A. K., Rampalli, S., Gallagher, K., and McIntyre, D. (1987), J. Appl. Polym. S r i 33:l 125. Black, L. T., Swanson, C. L., and Hamerstrand, G. E. (1986), Rubber Cllerr~.Teckr~ol.5Y: 123. Budiman, S., Chu. E., Secger, R., and McIntyre, D. (1981). Rubber World 184:26. Budiman. S.. and McIntyre, D. (1984), Rubber Cherrl. Techol., 57:352. 370. Campos-Lopez, E., and Angulo-Sanchez, J. L. (1976). J. P o l y r ~ Sci.. . Purl A - l 14:649. Campos-Lopez. E., and Palacios, J. (1976). J. Po/.vrr~.Sei.. Ptrrt A - l 14:1561. Campos-Lopez, E., Ed. ( 1978), Gurryle: retwc'uentro er1 el cle>.sierto,Proc. Intern. Conf. Guayuleat Saltillo, Coahuila. Mexico, August 1-5, 1977. Campos-Lopez, E., Neavez-Cnmacho, E., and Garcia, R. M. (1978). in Gucryule reerlcuerltro en el rlesierto (E. Campos-Lopez. Ed.). CONACYT, Mexico, p. 375. Clark, F. E., and Place, W. F. L. (1945). I r ~ d i c cRubber World 112:67. Clark, F. E., and Place, W. F. L. (1946a), l t l d i r i Rubber World, I /5:370. Clark. F. E.. and Place. W. F. L. (1946b), h r l . h g . C h m . 38:1026. Cole. W. M.. Fenskc, S. L., Serbin, D. J., Malani, S. R., Clark, F. J., and Beattie, J. L. ( 1987). U.S. Pat. 4,681,929 (The Firestone Tire & Ruber Co., assignee). Cole. W. M., Hilton, A. S.. Schloman. W. W., Jr., Compton, J. B., Dembek, J. A., Jr. (1991). Guayule Rubber Projcct: Final Report Prepared under Contract No 53-3142-7-6005, pp. 68-182. Cornish, K., and Backhaus. R. A. (1990), Ph~toche~rt~i.stt~v 29:3809. Cornish, K., Siler, D. J., and Grosjean, 0.-K. K. (l994), Pk~ltoc'hmzistt~\~ 35:1425. Cornish, K., and Siler, D. J. (l99S), J. Plrrlt. Physiol. 147:301. Cornish, K. ( 1996), U S . Pat. 5,580,942 (Secretary of Agriculture, assignee). Czuppon, A. B., Chen, Z.. Rennert, S., Engelke. T.. Meyer, H. E., Heber, M,, and Baur, X. (1993). J. Allergy Clir~.,Irr~rrrur~ol.92:690. Dierig, D. A., Thompson, A. E., and Ray, D. T. (1991). R u l h r Chertz. Techrlol. 64:211. Eagle, F. E. (1981 ), Rubber Cherr~.Tec1111ol. 54:662. Federal Emcrgency Management A g e ~ ~ ( 1984), y National Stockpile Purchase Specification. Rubber-Parthenium (Guayule), U.S. Department of Commerce Report P-48C-R. Foster. K. E., and Campos-Lopcz, E. (1982), Technology Assessment of the Commercialization of Mexican Guayule, Final Report, July, NSF Grant No. PRA 880 7458, and CONACYT, p. 73. Foster. K. E., Weight, N. G., and Fansler, S. F. (1991), Gutryule Natured R~rhberC~rt7rr1ercj~~/i~cr/i(1r~; A Scttle-Up Fetr.sibili/y Study. OALS, Tucson. AZ. Goss, R. A., Benedict, R. A., Keithly, J. H., Nessler, C. L., and Stipanovic, R. D. (1984). Plorlt Physiol. 74534. Gregg, E. C., Tipton,J. L.. and Huang, H. T., Eds. ( 1983). Proceedings of the Third International Guayule Confcrencc, Pasadena. CA. Guayule Rubber Society ( 1983). Fourth Annual Conference, Riverside, CA; (1984). Fifth Annual Conference, Washington, DC; ( 1985) Sixth Annual Conference, Tucson, AZ.
26
Mclntyre, et al.
Hager, T., MacArthur, A., McIntyre, D., and Seeger, R. (1979), Rubber Clfern. Techr~ol.52:693. Hamerstrand, G. E., and Montgomery, R. R. (1984), Rubber Chen~.Tecknol. 57344. Hamilton, R. C., Charous, B. L., Adkinson, N. F., Jr., and Yunginger, J. W. (1994), J. Lab. Clin. Med. 123594. Hauser, E. A., and Le Beau, D. S. (1943a), India Rubber World 106:447. Hauser, E. A., and Le Beau, D. S. (1943b), India Rubber World 107568. Hauser, E. A., and Le Beau, D. S. (1944), India Rubber World 108:37, 44. Ji, W., Benedict, C. R., and Foster, M. A. (1993), Plant. Physiol. 103:535. Jones, E. P. (1948). Ind. Eng. Chenl. 0 8 6 4 . Jones, E. P. (1949), U.S. Pat. 2,475,141 (Secretary of Agriculture, assignee). Kay, E. L., and Gutierrez, R. (1987), U.S. Pat. 4,684,715 (The Firestone Tire & Rubber Co., assignee). Lucas, W. (1996), SummaryTest Report for the Evaluation of the M1028, Commerclal Utility Cargo Vehicle, TECOM Project No 1-EG-095-000-028, YPG No 96-055. MacArthur, A., and McIntyre, D. (1983), in Proc. Third Intern. Guayule Cor$, Guayule Rubber Soc. (E. C. Gregg, J. L. Tipton, and H. T. Huang, Eds.), p. 309. McGinnies, W. G., and Haase, E. F., Eds. (1975), Proc. Intern. Conj On the Urilizatiorr of Guayule, Tucson, AZ. McIntyre, D. ( 1978), in Guayule: reencuentro C I I el desiertn (E. Campos-Lopez, Ed.), CONACYT,Mexico, p. 251. McIntyre, D., Shih, A.L., Savoca, J., Seeger, R.,and MacArthur, A. (1984),in Size Exclusion Clmrncttography, ACS Publ. 245 (T. Provder, Ed.), American Chemical Society, Washington, DC, p. 227. Montes, S. A., and White, J. L. (1982), Rubber Chern. Techno/. SS:1354. Montes, S. A., and Ponce-Velez, M. A. (1982), Rubber Chern. Teclrnol. 55:l. Morris, R. E., Barrett, A. E., Lew, W. B., and Werkenthin, T. A. (1943), 111dict Rubber World, pp. 109, 150, 192, 252. Moms, R. E., Barrett, A. E., Lew, W. B., and Werkenthin, T. A. (1944), India Rubber World, pp. 57, 63. Motomochi, B. (1983), Proc. Third Intern. Guayule Con$ Guayule Rubber Soc. (E. L. Gregg, J. L. Tipton, and H. T. Huang, Eds.), p. 89. d 5S:203. Nakayama, F. S., Cornish, K., and Schloman, W. W., Jr. (1995), J. Arid h r ~ Studies Office of Technology Assessment (1991), Agricultural Materials os lndusrricrl Rarcl Materictls, OTA-F476, USGPO, Washington, pp. 1-10, Ponce, M. A., and Ramirez, N. R. (1981). Rubber CIre~n.Techrlol. 5 4 2 1 I . Porter, L. S., and Stephens. H. L. (1979). Rubber Clrerrr. Techrlol. 52:361. Ramos de Valle, L. F., and Montelongo, M. (1978), Rubber Clfern. Tecllrrol. 51:863. Ramirez, R. R., and Ponce, M. A. (19781, in Guayule: Corlsejo N d n c r l de Cierlcirr Y Technologin. CIQA, Salitillo, Mexico. Reddy, A. R., and Das, V. S. R. (1988), J. Plant Physiol. 133:152. Schloman, W. W., Jr., Carrot, D. I., Jr., and Ray, D. T. (1986), J. Agric. Food Cherrl. 34683. Schloman, W. W., Jr., Ray, D. T., and Coates, W. (1987). paper presented at the 7th Annual Meeting of the Guayule Rubber Society, Annapolis, MD. Schloman, W. W., Jr., and Wagner, J. P. (1991). in Guuvule Naturcrl Rubber (J. W. Whitworth and E. E. Whitehead, Eds.), GAMC-USDNCSRS, Tucson. AZ. pp. 287-310. Schloman, W. W., Jr.. McIntyre, D., Hilton, A. S., and Beinor, R. T. (1996a). J. Appl. Polym. Sei. 60: 1015. Schloman, W. W.. Jr., W Y Z ~ O SF., ! ~ ,McIntyre, D., Cornish, K., and Siler, D. I. (1996b), Rubber Chert!. Technol. 69:215. Schrank, P. J., Carey, A. B., Simon, R. A., Ward, B., and Cornish, K. (1993), J. Allergy Clin. I ~ I I I I I U I I O ~ . 91:385. Sidhu, 0. P,,Ratti, N., and Behl, H. M. (1993). J. Agric. Food Chern. 41:1368. Siler, D. J., and Cornish, K. (1994a), Phytocl~ernistr~ 36:623. Siler, D. J., and Cornish, K. (1994b). I n d . Crops Prod. 2:307. Siler, D. J., Cornish, K., and Hamilton, R. G. (1996), J . Allergy Clin. ImfurroI. 98:895. 94:139. Slater, J. E. (1994) J. Allergy Clin. In~n~unol.
Guayule Rubber
27
Spence, D. and Boone, C. E. (1927), NBS Tech. Rep. No. 353. Subramanian, A. (1972). Rubber Chem. Technol. 48:346. Tanaka, H. (1985), private communication. Thames, S. F. and Kaleem, K. (1991), Biosource Technol. 35:185. Thames, S. F. and Poole, P. W. (1992), Polymeric materials from agriculture commodities, in ACS Symposium series 476: Emerging Technologies f o r Materials and Chernicals from Biomass. pp. 274-297. Thames, S. F., Poole, P. W., He, Z. A., and Copeland, J. K. (1994), Synthesis, characterization, derivation and application of guayule coproducts, in ACS Symposium series 575; Polyners fromAgricultural Coproducts. pp. 223-239. Touchet, P. (1987), in Elastomers andRubber Technology: Sagamore Army MaterialsConfererlce Proceedings, Vol. 32 (R. E. Singler and C. A. Byrne, Eds.), USGPO, Washington, pp. 535-545. U.S. National Academy of Sciences (1977), Guayule, An AlternativeSource of Natural Rubber, U.S. Natl. Acad. Sci., Washington, DC. Valaitis, J. K., Kern, W. J., Schloman, W. W. Jr., and Hilton, A. S. (1992), in New Industrial Crops crnd Products: Proceedings of the First International Conferenceon New Industrial Crops and Products, Riverside, Cali$, 1990 (H. H. Naqvi, A. Estilai, and I. P. Ting, Eds.), AAIC, Riverside, CA, pp. 131-134. Wagner, J. P., Engler, C. R., Parma, D. G., and Lusas, E. W. (1988), Po1ym.-Plast. Technol. Eng. 2 7 155. Wagner, J. P., and Parma, D. G. (1988), Po1ym.-Plast. Technol. Eng. 27335. Wagner, J. P., and Parma, D. G. (1989), Polvm.-Plast. Technol. Eng. 28:753. Wagner, J. P,, and Parma, D. G., and Benedict, C. R. (1991). Po1ym.-Plast. Technol. Eng. 30:473. Wagner, J. P,, and Schloman, W. W., Jr. (1991), in Guayule Natural Rubber (J. W. Whitworth and E. E. Whitehead, Eds.), GAMC-USDAKSRS, Tucson, AZ, pp. 261-286. Weihe, D. L., and Nivert, J. J. (1983), in Proc. Third Intern. Guayule Conk (E. C. Gregg, J. L. Tipton, and H. T. Huang, eds.), Guayule Rubber Society, Riverside, CA, p. 115. Winkler, D. S., Schostarez, H., and Stephens, H. L. (1978a), in Guayule Consejo Nacional de Ciencia Y Technologic/. CIQA, Saltillo, Mexico, Ch. 15. Winkler, D. S., and Stephens, H. L. (1978b), in Guuyule, Consejo Nacional de Ciencia Y Technologia, Ch. 17.
This Page Intentionally Left Blank
Hevea Natural Rubber A.
H. Eng and E. L. Ong
Rubber Research lnstitute of Malaysia, Malaysian Rubber Board, Kuala Lurnpur, Malaysia
1. INTRODUCTION 1.l
Naturally Occurring Polyisoprene
Natural cis- 1.4-polyisoprene occurs in over 2000 species of higher plants, the most well known of which is natural rubber from Helva brasiliensis. Other plants, such as guayule (Partheniuru argentaturn),Russian dandelion (T~rrasacu~~z kok-scrghyz), goldenrod (Solidago rdtissir~~a), Jelutong (Dyera retusa), and fungal genera such as Lnctarius tvolerrlus have also been known to producecis-1,4-polyisoprene.Only a relatively few plantspeciessuchas balata (Marrru.scq)s balata) and Gutta percha ( P ~ l a q ~ r i gutta) ~ r r ~ ~produce gutta or trrrns- 1,4-polyisoprene. Chicle (Achras sapota) is known to produce a mixture of tram- 1,4-polyisoprene and cis- 1.4-polyisoprene in a ratio of about 1 : 4 (Schlesinger and Leeper, 1951; Stavely et al., 1961. Archer and Audley,1973). Despite the various possible sources of naturally occurring rubbers. natural rubber from Het9ea brasilierlsis remains the most widely used. Much effort has been made to replace natural rubber with synthetic analogs in various applications, such as the discovery of the Ziegler-Natta catalyst for theproduction of syntheticcis-polyisoprene in 1956. This, however, has never been achieved, and today natural rubber accounts for about 40% of the total rubber consumed worldwide.Withthegrowingconcern for thehugequantities of toxicwastegeneratedand energy consumed by the synthetic rubber industries, the consumption of natural rubber (Wan A. Rahaman, 1994; Jones, 1994), a more environment friendly and sustainable raw material, is expected to increase in the years to come.
2. STRUCTURE AND BIOGENESIS OF NATURAL RUBBER 2.1 Natural RubberLatex Cis- 1,4-polyisoprene rubber occurs in the H. brmi1iensi.s tree as minute particles, which form the dispersion phase of a milky fluid or latex. Latex vessels have been found in all organs of the tree. However. their density varies. with the lowest occurring in the wood and highest in the secondary phloem. where the vessels occur as a series of rings concentric with the cambium (Gomez and Moir, 1979). It is this part of the rubber tree that produces the latex initials. The 29
Eng and Ong
30
sequence of these rings indicates the developmental stages of these vessels, ranging from the youngest laticifers in the ring next to the cambium to senescent and disintegrating vessels in the outer bark. Upon tapping, or with a puncture made into the phloem with a sharp object, many vessels are severed and the latex that flows out is a mixture of the latices from vessels of different developmental stages. When atree of anyone clone or seeding is first tapped, it produces an unstable latex with a high dry rubber content (DRC). As the tapping is continued with a regular tapping system, the latex stability increases and the DRC falls to a steady level, which can vary between25 and 45%, depending on the nature of the planting material. Changes in DRC can also be brought about by other factors, such as the tapping system, seasonal variation, and yield stimulation (Wiltshire, 1934;M o m s and Sekhar, 1959;Resing, 1955;Abraham et al., 1971). For example, a full spiral cut gives a lower DRC than a half spiral cut, and alternate daily tapping results in a higher DRC than daily tapping. Higher DRCis also observedfor latex obtained from high-panel tapping compared with that from low-panel tapping (Heusser and Holder, 1931).Furthermore, intensive tapping or drastic tapping causes marked decreases in DRC. 2.2
Rubber Particles
The rubber particles range in size from about 50 A to about 30,000A (3 pm). Exceptionally, particles up to 5 or 6 pm in diameter are found. In young trees and potted plants the particles are spherical (Fig. l), but in mature trees the larger particles are often pear-shaped (Fig.2). The origin of the pear shape is mysterious; it is frequent in certain clones (e.g., Tjir 1 and PR 107) and rare in others (e.g., Pil. B84* and RRIM 526). The pear shape is visible under the light
Fig. 1 Osmium tetra-oxide stained rubber particles from young tree. (Magnification:
X 30,000)
Hevea Natural
Fig. 2 Osmium tetra-oxide stained rubber particles from mature trees. (Magnification:
31
X25,OOO)
microscope and was noted as long ago as 1911: the early literature is briefly reviewed by Southorn (1968). Attempts to measure the particle size distribution by light microscopy gave misleading results since many of theparticles lie beyond the limit of resolution. Usingelectron microscopy, van den Tempe1 (1952) found a maximum inthe size frequency curve at about loo0 A; in fact, the most numerous particle species is too small to be seen by light microscope. A subsequent study of latex fromemature trees of clone RRIM 600 also showed a unimodal curve with a maximum at 1000 A and a long tail in the large particle size range (Fig. 3). A multimodal distribution was found in latices from young potted plants. It has been calculated that a rubber particle with a diameter of 1000 A would contain several hundred moleculesof the hydrocarbon. The hydrocarbon is surrounded by a surface film of protein and lipids, including phospholipids. About 40% of the membrane proteins of the rubber particles were found to be proteolipids. They were hydrophobic proteins containing 70% nonpolar amino acids and were closely associated with phospholipids and glycolipids. Triglycerides, sterols, sterol esters, tocotrienols, and other lipids are also associated with the rubber particles. Their precise location is not known, but it has been suggested that the sterol esters are located inside the particles rather that at the surface. The surface film or envelope is visible in sections of osmium-stained rubber particles (Fig. 4) and is approximately 100 A thick. In permanganate-fixed preparationsthe hydrocarbon is oxidized and ashell remains, which may represent the original envelope (Fig. 5). The envelope carries a negative charge and confers colloidal stability on the rubber particles.
Eng and Ong
32
LATEX PARTICLE SIZE DISTRIBUTION (mean of 3 tappings)
10m - m
0
1
2000
4000
6000 PARTICLE SIZE IN 8 UNITS
a a m I I 8000
Fig. 3 Rubber particle size distribution in mature trees of clone RRlM 600.
Fig. 4 Section throughrubber partcles in mature latex vassel. (Magnification: X45,OOO)
Hewea Natural
Fig. 5 Section through rubber particles from latex fixed with permanganate. (Magnification:
2.3
33
X 9,OOO)
Biogenesis of NaturalRubber
Although research work on natural rubber has been carried out for more than a century, the fundamental question of whyplants produce rubber remains to be conclusively answered.Some researchers suggested that natural rubberis a by-productof the tree (Archer and Audley, 1981), while others reported that it is a stored energyfor the plant (Fournier and Tuong, 1961). These suggestions, however, have not been verified. Recently, different a proposal-that natural rubber acts as a radical scavenger (Tangpakdee and Tanaka, 1998b)"was made on the basis of the presence of oxidative degraded rubber sample found in untapped rubber trees. However, the presence of oxidative degraded rubber in the untapped rubber tree is not totally unexpected, because it is well known that natural rubber can gradually oxidize through radical process in latex state during long-term storage inside or outside the tree (Bloomfield, 1951). Therefore, the finding does not necessarily imply its role in the plant. The rubber biosynthesis starts from trans, trans-famesyl pyrophosphate or its derivative as the initiating species followed by addition of isopentenyl pyrophosphatein the cis configuration to form a two-trans and poly-cisisoprene structure (Eng et al., 1994b;Tanaka et al., 1996). The termination step probably involves the formation of a phospholipid complex (Eng et al., 1994a). 2.4 Molecular Structure of Natural Rubber Chemical Structure On the basis of NMR studies, the fundamental structure of natural rubber has been confirmed to be as follows:
Eng
34
CH3
l
l
R-CH2
H
H&
and
Ong
H
where R and CL are believed to be protein or amino acid and phospholipid, respectively (Eng et al., 1992, Tanaka et al., 1996, Tangpakdee and Tanaka, 1998a). The n-value is in the range of 600-3000 (Eng et al., 1994a). Abnormal Groups Apartfromthisbasicstructure,small amounts of nonisoprenegroups,whichareknownas abnormal groups, havebeen reported to be present on the main-chain molecule. These abnormal groups are very low in concentration, but they exert a strong influence on the properties of the polymer that distinguishes it from the synthetic analog. The abnormal groups reported to be on themain-chainmoleculeinclude epoxide (Burfield,1974), ester (Tanaka, 1984), aldehyde (Sekhar, 1960. Subramaniam, 1977), and lactone (Gregg and Macey, 1973). The presence of epoxide groups was suspected when a reduction of rubber molecular weight was observed after treating the hydrolyzed rubber with periodic acid (Burfield and Can, 1977). However, recent I3C-NMR studies confirmed that natural rubber contains no significant amount of such groups (Enget al., 1998a,b). Thepresence of ester groups in commercial natural rubber was first reported by Gregg and Macey (1 973). However, they attributed the infrared band at 1738 cm" in the spectra of commercial rubber to the presence of lactone groups. It was later confirmedthat the ester groups areassociated with fatty acids, which could be removed by transesterification with sodium methoxide (Tanaka, 1984). The fatty acids have been postulated to be located at the branching point of the rubber (Eng, 1994; Tangpakdee and Tanaka. 1998a). The existence of aldehyde groups was proposed because rubber-hydrazone was found when natural rubber was treated with 2.4-dinitrophenylhydrazine (Subramaniam, 1977). More recently, both aldehyde and ester groups were found to have a similar distribution in fractionated natural rubbers of different molecular weights (Eng et al., 1997). The concentration of these groups decreasedwithdecreasingmolecularweight of therubber,suggestingthataldehyde groups are not derived from oxidative degradation of the rubber. A drastic reduction in the aldehyde content was foundwhen the bonded fatty acids were removedfrom the rubber, indicating that aldehyde groups could be derived from oxidative degradation of olefinic groups in unsaturated fatty acids bonded to the rubber molecule (Eng et al., 1997).
Gel and Branching Natural rubber isolated from fresh field latex immediately after collection and dried at room temperature normally contains small amounts of rubber insoluble in rubber solvent, known as the gel phase. The gel content of commercial rubbers and rubbers from commercial latices can be as high as 70%. It is also a matter of common knowledge that the gel content varied with source, type of rubber. and with the polarity of the solvent used (Allen and Bristow, 1963). The process of gelationinlatex is acceleratedunderalkaline storage conditions(Gorton.1974). Addition of alcohol or acids could help to dissolve the rubber in the solvent (Bloomfield, 195 l).
Hevea Natural
35
The gel phase also containshigher nitrogen and mineral contentsthan the sol phase (Grechanovskii et al., 1987). This led to the postulation that rubber chains in the gel phase are linked up by proteins via hydrogen bonding. This is further supported by the observation that the gel phase in therubber from high-ammonialatexdecreased from 42.5 to 2.2%afterdeproteinization (Ichikawa et al., 1993). The gel fraction became solubilized after it was treated with sodium methoxide. The number-average molecular weight of the rubber chain that makes up the gel phase was found to be in the range of 5.5-8.3 X IOs (Tangpakdee and Tanaka, 1997). Based on these observations, it was suggested that branching and gel phase of natural rubber consist of two types crosslinks, i.e., one through association with protein at the initiating end and the other through phosphoric ester at the terminal end. The existence of branching in natural rubber is indicated by the higher Huggins constant value, K’ (Eng, 1994; Tangpakdee and Tanaka, 1998a) than in the linear polymer. The degree of branching in natural rubber has been quantitatively estimated using GPC viscometry and was found to increase with increasing molecular weight in the range of 1-6 branches per rubber molecule (Angulo-Sanchez and Caballero-mata, 1981; Fuller and Fulton, 1990). A similar result was also obtainedusing I3C-NMR (Eng et al., 1993).Whenextrapolated to zerodegree of branching, it was estimated that natural rubber molecules with molecular weight of 0.65 X 10’-1 X lo5 have no branching, i.e., are linear (Angulo-Sanchez and Caballeromata, 1981). Storage Hardening The progressive increase in Mooney viscosity of natural rubber on prolonged storage under ambient conditions has long been recognized (De Vries, 1927). This phenomenon is known as storage hardening of natural rubber. The increase in the viscosity upon storage is not a desirable property of natural rubber as raw material because this means change a in its processing behavior. However, thetechnologicalaspect of this has been overcome (Sekhar,1964),andconstant viscosity grade rubbers (CV grade) are now available on the market. On the other hand, the mechanism of storage hardening has yet to be conclusively explained (Burfield. 1986, 1989). It is generally agreed that the process involves certain cross-linking reactions of abnormal groups, most probably aldehyde groups in natural rubber (Sekhar, 1962; Subramaniam, 1976; Burfield, 1987). Although other reactions involving epoxide groups have also been postulated (Burfield, 1974; Burfield and Gan, 1977), the failure to detect the abnormal groups in the recent studies weakens this argument (Eng et al., 1998a,b). The characteristics of the process are: ( 1 ) hardening is accelerated under low-humidity conditions (Wood, 1952), (2) the process requires amino acids or proteins (Gregory and Tan, 1976), and(3) it can be inhibited by the additionof monocarbonyl reagent such ashydroxylamine, dimedone, or semicarbazide (Sekhar, 1961). Although storage hardening leads to the formation of gel in dry rubber. the process may involve a mechanism different from that of gelation of natural rubber in latex, because the former is accelerated under low-humidity conditions, whereas the latter proceeds under aqueous conditions (Burfield, 1989). Studies of natural rubber under accelerated storage hardening conditions revealed that the bimodal molecular weight distribution rubber gradually changed to unimodal, where the peak in the low molecular weight region slowly shifted to high molecular weight region (Li et al., 1997). Storage hardening was found to increase the plasticity retention index of natural rubber (Morris, 1991) and contributes to the high green strength of the elastomer (Fernandoand Perera, 1987). Molecular Weight and Molecular Weight Distribution Many factors such as clonal origin, the age of the rubber tree, weather, frequency of tapping, method of rubber isolation, and treatmentof the rubber sample beforeanalysis (e.g.,mastication,
36
I
lo4
Eng and Ong
I
lo5
I
I
lo6
lo7 Molecular welght
Fig. 6 Molecular weight distribution of naturalrubber.
heating) have been known to affect the molecular weight (MW) and molecular weight distribution (MWD) of natural rubber (NR). The effects of clonal variation on MW and MWD of natural rubber from fresh latex have been investigated by gel permeation chromatography (GPC) (Subramaniam, 1976). The MW of natural rubber has been found to be of either a distinctly bimodal distribution, wherethe peak height in the low molecular weight region is nearly equal or half of that in the high molecular weight region, or a unimodal distribution, with a shoulder i n the low MW region as shown in Figure 6. The MW is normally in the range of 104-107 with high MW and low MW peaks centered at 10" and lo5, respectively. The polydispersity of MW. MJM,,. is therefore wide, usually in the region of 2.5-10. Study of the MW of rubber obtained from rubber trees of different ages revealed that young rubber trees also produce rubber with bimodal distribution (Tangpakdee et al., 1996). However, in this case, the height of the low MW peak is greater than that of the high MW peak. As the age of the tree increases, the intensity of the peak at low MW decreases while that at high MW increases. The positions of both peaks remain unchanged despite the variation in tree age, indicating that the bimodal distribution is due tothe biosynthesis process in the rubber tree. A similar observation was alsoreported by Hager et al. (1979)in their studies on guayule rubber. The MWD of NR is also influenced by the frequency of tapping. If a mature tree is tapped for the first time, the rubber contains asmuch as 80% gel (Sekhar. 1962) and the soluble fraction contains mainly oxidizedrubber of low MW(Bloomfield,1951;Tangpakdee and Tanaka, 1998b). Because the tree is frequently being tapped, the gel content decreases and the MW in the soluble fraction increases accordingly (Sekhar, 1962). Mastication has long been known to be a way of breaking down the high MW fractions of natural rubber. Therefore. masticatedNR normally has a unimodalMWD. Heating at elevated temperature can causeoxidative degradation of the rubber double bond. The removal of moisture by heating the rubber at reduced pressure, on the other hand, accelerates the storage hardening process. Therefore, sample treatment of natural rubber can influence the MWD of NR. The actual molecular weight of NR is expected to be much higher than that obtained from GPC analysis because even rubber isolated from freshly tapped latex contains some high molecular weight insoluble microgel. which is normally filtered and discarded in the sample "
Hevea Natural Rubber
37
preparation. The development of the thermal field flow fractionation (ThFFF) technique has allowed the MWD of whole NR to be analyzed at a resolution higher than GPC without removing the microgel. In fact, the gel content could be estimated by analyzing the filtered and unfiltered samples with ThFFF (Leeand Molnar, 1995). Analysis of a commercial NR revealed that rubbers witha MW of 10' to 3 X lo7 arestar-shaped or branchedmolecules.Above 3 X lo', the rubbers are mostly in the form of microgel particles (Fulton and Groves, 1997). However, the analysis of gel fraction with ThFFF is complicated by the phenomenon of steric inversion, where larger microgel particles may be co-eluted with the soluble low molecular weight species at the beginning of the ThFFF separation.
2.5
Nonrubbers
Heveo latex as obtained from the tree consists not only of rubber hydrocarbon particles but also of nonrubbersubstances,includinglipids, proteins. carbohydrates,acids. amines, and some inorganic constituents. It is generally known that some of these nonrubbers can affectthe properties of latex concentrates and bulk rubber derived from the field latex. Most of the nonrubber compounds in natural rubber are either trapped, tenaciously held, or co-precipitated with the rubber during coagulation due to their poor solubility in the aqueous medium or strong entanglement with the rubber molecule. A typical composition of natural rubber is given in Table 1.
Lipids Natural rubber lipids, conlprised of neutral lipids, phospholipids, and glycolipids, make up the largest proportion of the nonrubber components. Water-insoluble lipids are expected to remain inthe dry rubberafternormallatex-processingconditions.Atypicallipid of wholenatural rubber consists of 54% neutrallipids, 33% glycolipids, and 14% phospholipids (Hasma and Subramanium, 1986; Ho et al., 1976). The amount of lipids isolated from rubber particles was found to vary from 1.3% for clone PR225 to 3.4% for clone PB 28/59. In contrast, the phospholipid and glycolipid contents do not vary significantly (Hasma, 1987, 1991). Neutral lipids are composed of more than 14 substances, including sterols, sterol esters, free fatty acids, fatty acid esters, wax esters, monoglycerides, diglycerides. triglycerides. and phenolic compounds. The distribution of these substances varied according to rubber clone. In the case of clone RRIM 501, 63% of the neutral lipids were triglycerides, of which 98% were
Table 1 Composition of Natural Rubber Percentage Component 93.1 hydrocarbon Rubber Lipids Proteins Carbohydrates Ash Others
by weight 3.4 2.2 0.2
38
Eng and Ong
Table 2 Composition of FreeFatty Acids in Natural Rubber ~
Acid
Caproic Myristic Palmitic Palmioleic Stearic Oleic Linoleic Linolen~c Furanoic
~
~~
Percentage by weight Trace 0.01 0.08 0.01 0.16 0.12 0.29 0.03 0.09
found to be furanoid or 10,13-epoxy-l l-n~ethyloctadeca-l0,12-dienoic acid (Hasma and Subramaniam. 1978). A typical free fatty acid composition of natural rubber is given in Table 2. Other acids include inorganic acids, such as hydrochloric, glycero-phosphoric, and phosphoric, and volatile organic acids. such as succinic, malic, acetic latic, and propionic. Three free tocotrienols (a-,6-. y-) and two tocotrienol esters (S-. y-), three sterols and their derivatives, three types of fatty alcohol acetates, have been identified in neutral lipids of natural rubber. The total level of tocotrienol in dry natural rubber is about 0.1% w/w rubber (Dunphy et al., 1965). Morimoto (1985) analyzed the acetone extract of commercial rubber and found four types of tocotrienols (a-,p-, 6-, y-) and three types of free fatty acids (stearic. arachic, behenic) in the sample. Fatty acids were also found in the rubber molecule at the chain terminal. which could be isolated by treating it with sodium methoxide. Glycolipids of natural rubber consist of esterified steryl glycoside (ESG), monogalactosyl diglyceride (MGDG), steryl glucoside (SG), and digalactosyl diglyceride (DGDG). The fatty acids of ESG, MGDG, and DGDG consist mainly of stearic oleic and linoleic acids (Hasma and Subramaniam, 1986). Phospholipids in H e w a rubber consist of phosphatidylcholine (or lecithin) as the main component (Altman and Kraay, 1940) and phosphatidyl ethanolamine, phosphatidyl inositol, and metal phosphatides.The acyl components of these phospholipids are mainly palmatic, stearic, oleic, and linoleic acids (Hasma and Subramaniam, 1986). Processing of natural rubber latex into dry rubber changes the composition of the lipids, especially polar lipids. and different processing methods result in different lipid compositions. In the presence of ammonia, free fatty acids can form soaps in latex. More than 95% of the higher fatty acids (HFA) soaps were found to be associated with the rubber phase of the latex. About 80-90% of the HFA soaps are in their free forms, but only 50% of the furanoic acid is in its free form (Jurado and Mayhan, 1986). Free fatty acids such as stearic and linolenic are activators of sulfur vulcanization. Free fatty acids. which serve as crystallization nuclei, have also been found to accelerate cold crystallization of natural rubber by a factor of about 5 at a temperature where the rate of crystallization is highest (i.e., - 25°C) (Gent. 1954). It was also found that free fatty acids and unsaturated methyl esters could accelerate the oxidation rate and chain scissionof deproteinized natural rubber(Arnold and Evans, 1991).Free tocotrienols arising from neutrallipids are themostimportant natural antioxidants of naturalrubber. The 0.1% tocotrienols (0.8% in lipid fractionof latex) together with phenoliccompounds fromthe unsapon-
Hevea Natural
39
ifiable fraction of tocotrienol esters are responsible for preventing the autoxidation of the raw rubber (Morimoto, 1985; Hasma and Alias, 1990).
Proteins, Amino Acids, and Other- Nitrogenous Substances Fresh latex contains about 2% of proteins, about 25% of which are absorbed on the rubber particle, 25% on the bottom fraction, and 50% on the serum fraction of centrifuged latex (Tata, 1980). The proteins include acidic/anionic and basic/cationic with isoelectrical point in the pH region of 3.5-9.5 and molecular weights of < l 4 to >l00 kDa (Hasma and Amir, 1997). The largest component of proteins in the serum is a-globulin (Archer and Cockbain, 1955). It is soluble in salt and can be coagulated by heat. Its isoelectrical pH of 4.55, which is similar to thepHvaluewherefreshlatex is coagulated, suggests that it is one of theproteins that is absorbed on the rubber particles and partly responsible for the colloidal stability of the latex (Archer et al., 1969). In the case of bottom fraction, 70% of the protein contentis hevein (Archer, 1960; Tata, 1976), a water-soluble, anionic protein with a molecular weight of 5 kDa. In addition, minorproteinssuch as hevaminesA and B (Archer,1976), somehighbasicproteins,and enzymes (Archer et al., 1963) are present. The presence of rubber elongation factor, a protein bound to rubber particles, has also been reported (Dennis and Light, 1989). The preparation of latex concentrate results in a loss of the amount of proteins and changes the composition of proteins,particularly very acidic (pH 3.5-4.6) andbasic (pH 8.0-9.5) proteins (Hasma and Amir, 1997). The level of these proteins was found to diminish on increasing storage period of the latex, so much so that 2- to 3-month-old latex contained mainly acidic proteins (pH 4.6-6.0). The proteins in high-ammonia latex concentrate have also been studied (Hasma, 1992). Substantial amounts of proteins were found to be strongly bound to the rubber particle, and they were only extractable by detergent or organic solvents. Compounding the high-ammonia latex that was less thana month old was found to affect the basic serum proteins, especiallyin the presence of zinc oxide. Heating the compounded latex at 70°C for 2 hours rendered the basic proteins and the very acidic proteins undetectable (Hasma and Amir, 1997). Free amino acids constitute about 0.1% of the latex, of which 80% is found in the serum fraction. The main amino acids in latex are glutamic acid and its amide, alanine, and aspartic acids (Ng, 1960; Brzozowska et al., 1974). Other nitrogenous components in the latex include methylamine, ethanolamine, tetramethylenediamine,pentamethylenediamine,stachydrine,trigonelline,nucleic acids, nucleotides (McMullen, 1960, 1962), and lipids such as lecithin and alkaloids (Archer, 1976). Proteins absorbed on the surface of latex particles have been shown (Resing, 1955; Yip, 1978) to play an important role in the stability of Hevea latex. A certain influence of seasonal changes on this parameter was also observed by Resing (1955) in a study of older clones. The presence of proteins in natural rubber increases its moisture content as well as water uptake of the dry rubber when immersed into water (Muniandy et al., 1988). Proteins have been found to increase the modulus, creep, and stress relaxation of natural rubber. The development of low-nitrogen natural rubber is desirable for engineering applications, requiring low creep and stress relaxation (Fuller et al., 1988). Morimoto ( I 985) reported that proteins or amino acids played a role in protecting natural rubber against thermal oxidative degradation. However, Hasma and Alias (1990) reported otherwise. Since these compounds have been shown to promote storage hardening of natural rubber, it is not surprising to observe a retention of tensile strength at elevated temperature. Atman (1 948) observed the accelerating effects of choline and ethanolamine on the vulcanization of rubber. Ethanolamine and arginine increase the torque modulus and reduce the scorch time and cure rate.
40
Eng and Ong
Table 3 Mineral inthe Ash Component of Natural Rubber Component MnO, CuO Chlorine, carbonic acid
so3 MgO Na20 CaO K20
P20s
Percentage by weight 0.00 1 0.7 1.4 6.2 8.9 16.4 23.3 43.0
Ash A typical composition of minerals in ash of natural rubber is given in Table 3 (Archer, 1963). These components vary according to the methods of latex coagulation. Approximately one third of the phosphorus in latex is found in the rubber hydrocarbon phase and the rest in the rubber hydrocarbon and nonrubber phases. Magnesium. on the other hand, is found mainly in the nonrubber fractions (C- and B-sera) (Yip and Chin, 1977).Both phosphorus and magnesium have been shown to affect latex stability. While phosphorus compounds exert a stabilizing effect, magnesium,in the form of the divalent cation, is destabilizing. It has been shown that although the ratio of phosphorus to magnesium is colrelated with the stability of latex to a certain extent, it is by no means the only influencing factor (Yip and Subramaniam, 1984).Nevertheless, in the processing of latex concentrate, diammonium hydrogen phosphate is sometimes added to precipitate the undesirable free magnesium ions to ensure better stability of the concentrate produced. Copper, manganese,and iron are the well-knownpro-oxidants of natural rubber,with copper being the most active (Barnard et al., 1963;Bateman and Sekhar, 1966).The normal method of determining copper content in natural rubber does not relate well to the PR1 value of the rubber, as observed by Alias and Chan (1980).This has been attributed to the inability of the analytical method to determine the actual amount of free copper from the total copper (Hasma and Alias, 1990) since only free copper can act as a catalyst in a thermal oxidation process of natural rubber (Shelton, 1972).The copper in natural rubber could be reduced by soaking the rubber in phosphoric acid or thiourea. Copper in fresh latex might complex with proteins and amino acids, and it will not impart any deleterious effects on the aging of natural rubber. However, when proteins and amino acid-copper complexes are attacked by microbial activities. free copper is released. This probably explains the general susceptibility of autocoagulatedrubber to thermaloxidativedegradation as comparedto normalacid-coagulated rubber (Hasma and Alias, 1990).
Inositols and Curbohydrates The most abundant polyolin latex is quebrachitol(1-~-2-O-methyl-( - )-chiro-inositol). It constitutes about 1% of fresh natural rubber latex (Rhodes and Wiltshire, 1931).M- and /-inositols have also been reported to be present in latex (Archer et al., 1963).The major glucid in the latex is sucrose. Small amounts of glucose, galacose, frutose, raffinose, and two pentoses have also beenidentified(Smith, 1953, 1954; Lowe, 1960;Tupy andResing, 1968).Quebrachitol has been reported to be a potential starting material for the synthesis of certain natural bioactive materials (Lau, 1996).
Hevea Natural Rubber
41
Volatile MatterWater is the major component in the volatile matter. Other volatile acids such as formic, acetic, and propionic have also been reported (Crafts et al., 1990). The water adsorption of natural rubber is due to thepresence of hydrophilicimpurities, mainly inorganicsaltsandproteins (Burfield et al., 1989). High volatile matter content can promote mold growth and causes undesirable odor of the rubber (Nadarajah et al., 1987).
3.
NATURAL RUBBERPROCESSING
The premium product of a rubber tree is latex. The by-product of tapping process is cuplump, which is actually the latex drip collected at alternate days after the collection of latex. Along with the cuplump. small amounts of treelace also combine with the cuplump. Under normal conditions latex contributes to about 80% of the output, while the cuplump andtreelace amounts to about 20%. Thus, the raw output for natural rubber processing can generally be classified as latex (liquid) and cuplump (solid). The types and grades of natural rubber processed depend greatly on the raw material input.
3.1 Classification of Rubber Processing Natural rubber is normally processed into either latexor dry rubber, depending on its application. Rubber products such as dipped goods, foam, and thread produced from latex, whereas other products (e.g., tires) are made from dry rubber. Different types and grades of commercial natural rubber are available in the market, and they summarized in Figure 7.
3.2 Technically Specified Natural Rubber The demand for technically specified rubber (TSP) in the form of block rubber has been overwhelming. Thus, most of the NR-producing countries have been converting their conventional rubber processing to TSR. The preparation of NR in block form has given a tremendous boost to the success of technical specifications for NR. Technical specification allows fordiversity of rubber-producing units of widely varying sizesto conform to important technical parameters, consistencyin quality, minimum space for storage area. cleanliness. and ease of handling. 3.3
Production of Block Rubber
The production of block rubber is basically the conversion of wet raw rubber into granular form by fast and continuous processing techniques. In its final form the dried crumb or the granule is compacted into blocks of solid rubber. Hence this presentation is known as block rubber. In processing field latex, thefollowing operations are involved: reception, bulking, chemical addition, coagulation, milling, size reduction, drying, baling, testing, grading, and packing. A combination of machinery such as crusher, crepers, hammermill, and shredders is used. Technicallyspecified NR in theblock form latex is as lightcoloredrubberstandard Malaysian rubber (SMR L), constant viscosity (SMR CV). SMRL production essentially focuses on color, with the addition of sodium metabisulfite at 0.04% dry rubber content (DRC). In CV production addition of hydroxylamine neutral sulfate at 0.15% DRC is necessary. The coagulation of field latex is done at field DRC.
42
Eng and Ong
NR
Latex
grade)
Latex Concentrate Conventional (=,LA) RSS, ADS, Pale Crepe
1
TSR S M R 10, SMR 20
Fig. 7
Cuplump (Field
TSR Specialty rubber S M R L, SP, MG, SMRCV DPNR
Speciality TSR S M R 10 CV, S M R 20 CV SMR GP (House grades)
General natural rubber types and grades
The popular grades of TSR produced from fieldgrade or combination in the form of block rubber is SMR 10 and SMR 20, SMR GP, SMR 10 CV, and SMR 20 CV. Selection and blending are necessary before further processing. The other steps involved are precleaning, initial size reduction, crepeing, intermediate size reduction, crepeing, and final size reduction. A combination of machinerylikeslabcutter,granulator,prebreaker,crepers,extrude,andshredder is generally used. The individual processors select different types and numbers of machines for the samepurpose with the idea of increasing productivity and meeting the required specifications. Processing methods have reduced processing timesto less than 24 hours. Besides technical specifications, deep bed drying at a temperature of 100-120°C has been a vital change. This allows the drying to be completed in less than 4 hours. The dried rubber biscuit of crumb is weighed and baled using a hydraulic press. 3.4 Specification The introduction of technical specifications was an important step in the development of the NR-processing industry. The specification parameters and their limits are changing features in thescheme. Advancements in the rubber product-manufacturingindustryandthe need for continuous improvement in the raw natural rubber-processing sector has necessitated the reexamination of the existing specification parameters and introduction of new grades and parameters, which may truly reflecttheprocessibilityandtechnologicalproperties ofNR (see Tables 4 and 5).
Hevea Natural Rubber
a c n
P
43
44
Eng and O n g
m
U
i
F
Hevea Natural Rubber
45
3.5 Conventional Types The conventional types of NR include the ribbed smoked sheet (RSS), air-dried sheet (ADS), and crepe rubbers. Among the conventional grades the production of RSS was popular until the emergence of block rubber-processing techniques. The rubber smoked sheet is the oldest method of processing latex grade rubber. This is still made in small or medium-sized rubber estates where the infrastructure for transportation of latex is lacking. Theprocessing of RSS passes through the stages of latex collection. bulking, reduction of field latex DRC to about 12.5%, coagulation, overnight maturation, milling the next day, and smoking (drying). The millingoperation allowsthe latex coagulumto besheetedaftermaturation. The sheeting process actually squeezes out serum present in the coagulum and reduces its thickness to about 3.0 mm by passingthe coagulum throughaset of four smooth rollersandfinally grooved rolls, lending a ribbed design to the final sheet. The ribbed pattern assists in increasing the surface area and improves the drying performance. The RSS is dried in a smokehouse, whereas the ADS is dried in a hot air chamber. A tunnel smokehouse is operatedatatemperature of 45-63°C. The dried sheets are visually examined andgraded.Visualgrading is seen as asetback for thetechnicalspecification of rubber. Visual grading is based on the recommendations provided by the International Rubber Quality and Packing Committee. Since there are no technical specifications, rubber is graded on the degree of dryness, contamination, virgin rubber. blisters, bubbles, oxidized rubber, transparency, color, tackiness, and mold growth.
3.6 Avoiding the Problems of Odor During Rubber Processing Cis-polyisoprene is both colorless and odorless. It is the 5% or so of nonrubbers that gives bale natural rubber, particularly field-grade material. its color and the characteristic smell, which those experienced in the rubber industry immediately recognize and generally accept. These nonrubbers, the inherent products of latex biosynthesis, also give natural rubber many of the characteristic advantages in processing and aging that have enabled it to resist successfully the challenge of synthetic polyisoprene, the development of which was once predicted to sound the death knell of the natural product. Nevertheless, it must be admitted that the odor given off during the processing of natural rubber may cause offense to both workers and people living close to factories where processing takes place. Methods of coagulation and subsequent conversion of the coagulum into bale rubber affect the smell of natural rubber. There appears to be no direct correlation between grade of rubber and odor, but field-grade material tends to have a stronger smell than rubber prepared by the deliberate, controlled coagulation of latex. The main constituents of the gases arelow molecular weight volatile fatty acids (Table 6), which can be effectively removed by water scrubbers with efficiencies of 92-99%. The drying process reduces much of the volatile fatty acid content of SMR, some 99% of the 0.8% (8000 ppm) of volatile matter allowableunder the SMR specification being moisture. However, other nonrubbers that constitute about 6% of the total weight of bale rubber comprise a range of chemical species,as described earlier. Whenthese nonrubbers are subjected to high-temperature processing conditions, such as mastication, the proteins break down to give amine derivatives, which can smell quite unpleasant. Certain fatty acids are also pungent, and when combined with amines, the resulting mixture produces an odor that can cause complaint. High-temperature mastication is recognized as one of the predominant factors causing workers
Eng and Ong
46
Table 6 Volatile Fatty Acids in DrierExhaustGases from SMR Factories
Concentration range (ppm)" ~~
Factory acidFatty
A
Acetic Isobutyric Isovaleric Valeric
459- 1438 40 1-707 726- 1239 793- 1471
~~~~~
Factory B 41 1-725 343-922 798- 1526 867-20 10
For elght (factory A ) and five (factory B ) sampling perlods of 2 hours.
"
in the rubber-manufacturing industry and membersof the public who live in the vicinityof rubber factories to express concern regarding the odor given off during the manufacturing process. Odor can be determined by olfactometry. This technique uses the human nose to detect very low concentrations of compounds that cause odor. Assessment of odor is very much a subjective matter. Olfactometry uses a panel containing several people who give a yes or no decision as to whether an odor can be detected or not; a positive result is obtained at the point at which 50% of panel members can detect an odor after its dilution with odor-free
4.
LATEXCONCENTRATE
Latex is the base raw material for the production of dipped goods, carpet backing, thread, and adhesives. Thelatex concentrate is the preferred raw material for the aboveproduct manufacturing due to its characteristics as concentrate latex. The DRC of field latex as obtained from the tree ranges from 30 to 40% by weight. This DRC is normally raised to approximately 60%. Differentprocesseshave been developed to
Hevea Natural Rubber
47
concentrate latex, but only centrifuging is widely practiced in the industry. A small proportion of creamed latex is also produced by selected processors.
4.1
Centrifugation
Centrifugation of natural rubber latex is the most important commercial methodof concentration. This process consists essentially of passing field latex through a high-speed machine, which separates it into a concentrated fraction of about 60% dry rubber and a skim fraction of low rubber content. Latex is regarded as a suspension of very minute particles of rubber (specific gravity 0.9 1 ) i n an aqueous serum (specific gravity 1.02). The theory of centrifugal concentration of latex is the same as that of creaming. In this centrifugation process. the separation of field latex into latex concentrate and skim is effected by means of centrifugal force rather than by gravitation. During this process, the latex is subjected to a centrifugal force many times greater than gravity. resulting in suspended rubber particles being separated into a concentrate at the center of the bowl and the serum at the rim. Currently, different types of centrifuge machines are commercially available. However, the basic design of these machines is similar. It consists of a rotating bowl i n which a set of conical metallic separator discs are enclosed.
4.2
Latex Concentrate Properties
Speciferrtion m r l Typiccrl Properties A number of quality parameters of natural rubber latex concentrate are assessed under the IS0 specification 2004 (Table 7). The latex concentrate is either marketed as high ammonia ( H A ) or low ammonia (LA), with a secondarypreservation.Most of theparameters of the latex concentrate do not change duringstorage.However.threeimportantcharacteristics,namely
Table 7 I S 0 2004. Specifics for Centrifuged Natural Rubber Latex Concentrate method Parameter
latex
Total solids content, min: 8 Dry rubber content, min; 8 Nonrubber solids, max:2.09 Alkalinity. as ammonia.0.29 on latex wcight; % (max) Mechanical stability time, min; S Coagulum content, max: % Volatile fatty acid numbcr.0.20 max Potassium hydroxide number, max Coppcr content, max; mglkg solids Mangancsc content, max: mg/kg solids Sludgc content, max; 8r Color Odor
HA latcx
LA
61.5
61 .S
60.0 2.0 0.6 (min) 650.0 0.05 0.20 1 .0 8.0 8.0 0. I
60 .0
I S 0 test no. I24 I26 -
125
650.0 0.05 1 .0 8.0 8.0 0. I No blue or grey after No putrefactive odor with boric acid
35 706 506 127 1654 1655 2005 neutralization
48
Eng and Ong
Table 8 Typical Values of Latex Concentrate (HA)
Parameter
Range
Mean ~
TSC
DRC KOH No. MST NH?
VFA PH
6 1.45 59.91 0.5759 1078 0.74 0.0249 10.61
~~~~~
61 .O-3-61.82 59.53-60.18 0.4188-0.71 11 915-1250 0.67-0.80 0.0151-0.0358 10.46-10.77
volatile fatty acid (VFA) number, potassium hydroxide (KOH) number, and mechanical stability time (MST), are often affected by handling of latex. seasonal effects, and pumping. Typical properties of latex concentrate are given in Table 8. KOH and VFA Numbers KOH number is a measure of the content of anions in latex, including volatile fatty acids, higher fatty acids, phosphates, carbonates, and bicarbonates. Most of the volatile fatty acids are the products of bacterial action on latex due to inadequate preservation and are normally measured separately by the VFA number, which is expressed as number of grams of potassium hydroxide equivalent to the volatile fatty acid containedin 100 g of total solids. The presence of carbonates and bicarbonates may be due to the absorption of carbon dioxide during exposure of the latex to air. The other anionsare inherent components of the latex system, the contents of which may vary from clone to clone. The KOH number is therefore defined as the number of grams of potassium hydroxide equivalent to the acid radicals combined with ammonia containing 100 g of total solids. A high KOH number and a high VFA number indicate inadequate preservation of latex.
Mechaniccl1 Stability Time This is a measure of the colloidal stability of latex concentrate. It is assessed by a measure of the resistance of the latex particles to irreversible flocculation or coagulation when subjected to mechanical stirring. The mechanical stability time (MST) of a freshly prepared latex concentrate is normally low. During storage at ambient temperature, it increases very rapidly for 3-4 weeks, after which the rise becomes more gradual for the next 1 or 2 months. The initial rapid increase in MST has been correlated to the increasing content of higher fatty acid soaps with time arising from the hydrolysis of some lipids (mainly phospholipids and glycolipids) on the surface of the rubber particles (Chen and Ng, 1984). Because most of these soaps are absorbed on the surface of the latex particles, this gives rise to higher surface electrical charge density, greater particle repulsion forces, and hence higher MST. The subsequent slow rise in MST is due to exposure of latex to oxygen in air (Collier, 1955). According to the specification limit, a minimum MST of 650 seconds is required, below which the latex is considered unstable. Various factors associated with the inherent properties of the latex system couldbe responsible for the low latex stability (Yip, E. and Gomez, 1980). These factors include the lower concentration of higher fatty acid soaps, lower contents of proteins and saponifiable lipids on therubberparticlesurface,andexcessivequantity of inorganiccations in the serum phase (Philpott and Westgarth, 1953). In order to identify the influence of the individual factors, a
Hevea Natural
49
separate and systematic study is required. Parentage of clones also appears to play a role in latex stability. Anioniclong-chainfattyacidsoaps, when added to latexconcentrates,areknown to increase their mechanical stabilities (Resing, 1955;Cockbain and Philpott, 1963). This is mainly due to the adsorption of the surface-active soap molecules on the rubber particle surface. Latices of low stability are often associated with high KOH numbers which usually have high VFA numbers (due to inadequate preservation of latex). A relationship between MST and KOH has not been obtained, as shown by the statistical analysis of data that indicated that the correlation between thesetwo parameters was insignificant.This lack of correlation could probably be partly explainedby the fact that not all the acidic anions asmeasured by the KOH number exert destabilizing effects on latex particles. Those of the higher fatty acids, for example, when adsorbed onto the rubber surface, can in fact confer latex stability. Also, as reported by Yip and Subramaniam (1984) and mentioned briefly earlier, the MST is greatly influenced by a number of other inherent latex properties, which show variability between clones.
5. 5.1
PHYSICAL PROPERTIES OF RAW NATURAL RUBBER Cold Crystallization of Natural Rubber
Natural rubber has been known to undergo crystallization at subambient temperature, with the fastest rate at around -25°C. The rate was found to decrease after the nonrubbers had been removed by acetoneextraction, but it returnedback to theoriginallevel when stearicacid was added to the extracted rubber (Gent, 1954). These effects were also observed in the cold crystallization studies of deproteinized natural rubber and synthetic cis- 1,4-polyisoprene (Burfield, 1984). However, the rate of crystallization of the synthetic rubber containing stearic acid was found to be lower than that of natural rubber. It was later found that natural rubber contains both saturated and unsaturated fatty acids (Tanaka et al., 1992). The mixture of these two types of fatty acids had a synergistic effect on the acceleration of the rate of crystallization of natural rubber (Kawahara, 1996). In addition, the presence of bonded and unbonded fatty acids wasalso found to accelerate the crystallization of natural rubber (Nishiyama, 1996). 5.2 Stress-Induced Crystallization As indicated earlier, the structure of natural rubber consists of a string of long and uninterrupted cis-1,4isoprene units. Because of the stereoregularity of thepolymer chain, natural rubber crystallizes readily on stretching, e.g., in a tensile testing unit, with the equilibrium melting temperature being raised due to the decreased entropy of the amorphous polymer induced by the stretching process. This phenomenon of stress-induced crystallization results in the polymer with greatly increased modulus andis an important feature to be considered in polymer rheology and processing. Smit and Vegt (1969) and Folt et al. (1969, 1971) reported that unvulcanized natural rubber readily crystallizes under thecombined pressure and orientation forcesin capillary extrusion, even over thetemperaturerangecovering commercial processingandfabricating operations. The occurrence of the stress-reduced crystalization in capillary extrusion resulted in an anomalous increase in melt viscosity as the rate of shear is increased. This anomalous viscosity increase hasalso been observedforother polymers. In an investigation on the rheological behavior of natural rubber grades using a capillary rheometer, it was reported (Ong and Lim, 1982a, b) that the onsetof stress-induced crystallization resultedin a break in the flow (viscosity-
50
Eng and Ong
0 Sheet rubber e SMRCV
~~~
1
~
~
10
100
1000
SHEAR RATE (S") Fig. 8 Plotof log viscosity versus log shear rate for various natural rubber gradcs.
shear rate) curve, subsequent to which the viscosity remained relatively constant or increased with increasing shear rate (Fig. 8). Concomitantly, the extrudate surface gave a grossly distorted appearance. The onset of stress-induced crystallization occurs at a critical stress (Fig. 9), which is dependent on temperature and die geometry. At an extrusion temperature of 100°C using a die of diameter 1.5 mm and a lengtwdiameter ratio of 20, the critical stress is estimated to be about 0.35 MPa (Ong and Lim, 1982a, b). It was found that different grades of natural rubber require different shear rates to reach thesame criticalshearstress and, based on theirtendency to stress-crystallize,theease of processing of the various grades of natural rubber can be ranked (Ong and Lim, 1982a. b). The critical shear stress occurs at about 54, 144, and 216 S" for RSS-I, SMR-CV, and SMR-GP, respectively. This meansthat the SMR-GP can be taken up to higher shear rates before processing difficulties arising from stress-induced crystallization occur, which in factory terms means that the output rate can be increased. Increasing the extrusion temperature or masticating a rubber was found to allow greater shear rates before the critical shear stress value was reached (Folt et al., 1971; Ong and Lim, 1982a, b; Lim and Ong 1984) It was also reported that rubbers of similar Mooney viscosities can give different flow curves, especially at high-shear rate regions (Fig. IO). The flow behavior of natural rubber is influenced by the drying temperature and mechanical history (Ong and Lim, 1982a, b). This, therefore, confirms the inadequacy of the Mooney viscosity value in describing and predicting the rheological and processing behavior of unvulcanized natural rubber.
5.3 Friction and Slip of Unvulcanized Rubber In fabrication processes involvingrubber, processing difficulties encounteredin the rubber industry include difficulties in forming a band on a two-roll mill, ribbingof sheets during calendering,
51
Hevea Natural Rubber
10”
Sheet rubber
r
‘.$
SMRCV
104
1
2
3
4
5
6
7 0 9
S H E A R STRESS (MPa) x 0.1 Fig. 9 Plot of log viscosity versus log shear rate for a sheet rubber and SMR CV samples.
10
I
I
I
l
1
SMR GP 0 50°C
0 100°C
- 10 a
Ex
\
._
U)
8
5 UJ
3
l0
\ 1c
I
I
I
0.2
I
1
05
Log Shear Stress (MPa)
Fig. 10 Viscosity-shear stress relationship for
SMR-GP at different temperatures.
Ong
and
52
Eng
loss of output during extrusion, and variation in the surface features of the extrudates. Because during processing rubber comes into intimate contact with a hard surface, usually metal, these processing difficulties are therefore partly related to the question of whether the rubber grips or slips on a surface during the various processes. Den Otter (1975, 1979) reported that slip at the wall occurs with unfilled rubbers and flow at lot temperatures shows a stick-slip nlechanism that changed to normal viscous flow at higher temperatures. A systematic study of the friction behavior of unvulcanized natural rubber indicated (Ong and Roberts, 1983a,b) that in common with vulcanized rubber (Schallamach, 1952), the surface friction of raw rubber was both load and rate dependent. In contrast with vulcanized rubber,during low rate sliding (less than 1 mms"), a spherical contact surface of raw rubber becomes deformed into a protruding ridge, and no Schallamach waves (Schallamach, 1953) are generated in the contact zone. At higher sliding rates (10-100 mms") there was only slight scuffing of the raw rubber surface and a tendency to stickslip motion with an occasional hint of Schallamach waves, all of which suggests that in the shorter time scalethe raw rubber was behaving in a more elastic manner.The sliding frictionexperiments also showed that the transition from ridge formation to stickslip motion occurred at a higher speed for a masticated rubber. Studies on the rheological behavior of raw natural rubber using a capillary rheometer have shown that with increasing shear rate a change from continuous to stickslip motion occurs (Fig. 11) (Ong and Lim, 1982a, b). Generally, the surface morphology of the raw rubber shows the following transitions with increasing sliding rate: bulk drawing accompanied by material transfer onto track, ridge formation, and slight scuffing under stickslip motion. Material transfer was less distinct for unmasticated rubbers. These changes in the morphology with sliding speed parallel the transitions of the extrudate's appearance with rates of extrusion. This suggeststhat the surface appearanceof extrudates is partly influenced by the frictional sliding of rubber against the metal surface of the die. A study of the surface morphology of rubbed hemispheres also shows that friction forces can produce tear cracks in raw rubber. Such cracks may relate, for example, tothe tiny multiple
'sticklip' 72 s-l
''
'conhnuous'
3.6S"
Hevea Natural Rubber
53
Fig. 12 Changes in extrudate appearancewithshear rates.
ridges and surface cracks sometimes found on highly masticated rubber after die extrusion at a very low rate (Fig. 12). The friction-induced rate of crack growth into the rubber bulk was found to be related to a fundamental material-property-the tear energy of that grade of rubber (Schallamach, 1953). This is independent of specimen geometry and may relate widely to situations encountered in the industrial processing behavior of unvulcanized rubber (Ong and Roberts, 1984). The friction of unvulcanized rubber decreased with surface roughness of the substrate at room temperature. An exception was highly masticated rubber, which showed greatest friction on aslightlyroughtrack, the trendbeing exaggerated atlong times of contactdwell. This presumably reflects their ability to flow and come into a high degree of intimate contact with a rough track, resulting in higher adhesion (Fuller and Roberts, 1981). This ability is likely to improve with increasing temperature. Ithas been observed (Turnerand Moore, 1980) that roughening the rotor in a Mooney-type viscometer can increase the shear stress at a temperature of 100°C. Some exploratory studies with a capillary rheometer suggest that there is a region of intermediate extrusion rate over which the friction of a die with a roughened internal wall can be markedly greater than that for a smooth die. Theabsolute rates at which this occurs depends on rubber type, and all indications are that it also depends on temperature. At high extrusion rates when complete slip between die wall and rubber occurs, there is no noticeable difference between smooth and rough dies. Apparently some level of friction between wall and rubber is required to show the roughness effect. These die extrusion observations might be rationalized in terms of the ability of a rubber to flow at a particular temperature to produce a high area of interfacial contact with the rough wall in the contact dwell-time available. It is clear that some valuable insight into the complicated processing behavior of rubber can be gained from a study of its friction characteristics. 5.4
Stress Relaxation of Unvulcanized NR
During any of the rubber-processing steps from raw material to finished product, the rubber undergoes deformation resulting in creep, stress relaxation, recovery, and flow. An understanding
Eng and Ong
54
of these phenomena is therefore relevant to rubber processability as a whole. For example, the elastic properties of molten polymers or rubbers govern a wide variety of important processing phenomena such as die swell, nerve, green strength, shrinkage, etc. Yet there has not been much progress in the techniques of measuring the elastic properties of unvulcanized rubbers. When performed manually, extrudate (or die) swell measurements are tedious and slow; moreover, for unvulcanized rubbers such measurements are rendered difficult by surface roughness or melt fracture. Stress relaxation, which is at least in part determined by material elasticity, provides a contribution in this respect. When viscoelastic materials are deformed, the stresses set up decrease with time. This is the phenomenon of stress relaxation; it can constitute a major disadvantage of rubber springs in engineering applications. In vulcanized natural rubber the stress relaxation behavior normally consists of two distinct phases: the shorter-term physical relaxation and the chemical relaxation that predominates at longer times. The former is associated with molecular rearrangement or reorientation, and the relaxation of stress is usually a linear function of log time. The latter is concerned with molecular chain scission and reformation, and the decrease of stress is more a linear function of time. In unvulcanized rubber no chemical cross links are present, and the relaxation of stress results from the molecular rearrangement of a network of physical cross links or entanglements. Flow may also result from the slippage of molecular chains passing each other. In stress relaxation tests of unvulcanized, unfilled polymers, two general simplifying principles have been found to hold (Djiauw and Gent, 1974), namely that (a the times tTI, tT7 for a particular value of stress to be attained during relaxation at temperatures T I , T 2are found to be related in accordance with the WLF time-temperature equivalence principle (William et al, 1955), and (b) for extensions up to about 200%, the degree of extension generally does not affect the form of the observed stress relaxation at any temperature but merely alters the stress scale by a time- and temperature-independent multiplying factor.The tensile stress during relaxation can thus be treated as the product of two independent terms-a function of the time and the temperature incorporating all of the relaxation effects and a nonlinear strain function. Stress relaxation, creep, recovery, and set in unvulcanized polynlers may be analyzed by means of the two-network hypothesis (Andrews et al.. 1946) if certain plausible assumptions are made about the entangled molecular network that gives rise to elastic effects in these materials. These assumptions are: Stress relaxation, on the usual experimental time scale, is attributed to disentangling of those molecular chains that formed a transitory elastic network at the time the deformation was imposed. 2 . The fractionalstressremaining after a time is assumed to be proportional to the fractional number of those entanglements remaining. 3. The total number of entanglements is assumed to be constant, new ones being formed at the same rate at which old ones disappear. 1.
The steady state is not affected by moderate deformations. As a result, a new relaxed network is formed in the stretched state, the number of chains comprisingthe new network at any instant being equal to those lost from the original one.
6.
CONCLUSION
With the growing concern about global warming and environmental pollution, natural rubber, which is a “green,” sustainable, renewable and biodegradable material, is expected to be more
Hevea Natural
55
widely used. replacingitssyntheticanaloginwhereverpossible. The rubber trees annually consume about 363 million kg of carbon dioxide from the atmosphere and release an equivalent a111oullt of much-neededoxygen.Thishelpstoalleviatethe greenhouse effectandglobal warming.
REFERENCES Abraham, P. D., Blencowe, J. W., Chua, S. E., Gomez, J. B., Molr, G. F. J., Paklnnnthan. S. W.. Sekhar, B. C., Sothorn, W. A., and Wycharley, P. R. (1971), Novel stimulants and procedures in the exploitation of phosphoric acid (Ethephon) and acetylene with various tapping systems, J. Rubh. Res. blst. Mnlayrr 23535. Alios, O., and Chan B. L. (1980), Effcct of pH coagulat~on and sulfuric acid as a coagulant 011 natural rubber properties. J . Rubb. Res. Irrst. Mrrlrrysirr 28: 109. Allen, P. W., and Brlstow, G. M. (1963). The gel phase 111 NR. J. AppL Po/vrif.Sei. 7603. Andrews, R. D., Tobolsky. A. V., and Hanson, E. E. (1946). J. A/)/>/.Ph.~.s. /7:352. Angulo-Sanchez, J. L., and Caballero-mata, P. ( 198 1 ), Long chain blanchingin natural H n w rubbcr-determination by gel-permeatlon chromatography. Rubb. Clrerrt. T d ~ r ~ o54:34. l. Archer, B. L., and Cockbam, E. G. ( 1955). The proteins of H e \ w hrur.si/ietr.si.slatex, 2. Isoaltion of the a-globulin of frcsh latex serum, Biocherrr. J. 61:508. Archer. B. L.( 1960), Theprotcins ofHr~vcrbrrr.si/ier~.si.s latex. 4.Isolation and charncterizntlon of crystalline hcvcin. Biochcw. J. 7.5236. Archer, B. L., Barnard, D., Cockbain. E. G., Dickenson, P. B.. and McMullen, A. I. (1963), Structure, composition and biochemistry of Hevea latex. I n The Chemistn m u / Physics rfRuhhur t r r d RIIhbet" Like Substnrlcw (L. Bateman. cd.), Maclaren, London, p. 41. Archer, B. L., Audley, B. G., McSwceney, G. P,, and Tan, C. H. (1969). Studies on compositio~~ of latex serum and bottom fraction particles. J. Rubb. Res. Imt. M N / N21560. ~ Archer, B. L.. and Audley, B. G. (1973). Rubber, gutta percha and chicle, 111 Ph~tclcher~~i.s/r?. Vol. 2 (L.P. Miller, cd.) Van Nostrand Relnhold, New York, p. 310. brrt.si/ier~.si.slatex. P/y/oc/wrrli.stry Archer. B. L. ( 1976), Hevamlne. A crystalline basic protein from H r ~ w 1.5997. Archer. B. L., and Audley, B. G. (1981), Rubber biosynthesis: More picces in the puzzle. Rubb. Dedop. 34:96. Arnold, A. R.. and Evans. P. (1991), Role of fatty a c d s I n autoxldation of deproteinized natural rubber. J . Nut. Rubb. Res. 6:75. Atman, R. F. A. and Kraay, G. M. (1940), Separation and identification of lecithins from H e w r latex. Kuhh. Cherrl. Trchol. I3:750. Atman. R. F. A. ( 1948). Natural vulcanization accelerators in Hewo latex. I r d . Eng. Cllerrl. 40:241. Barnard. D., Bateman, L., Cunncn, J. I., and Smith. J. F. (1963). Oxidation of olefins and sulfides. In Cherui.s/r:vt r r d Ph~sic~,s c~f'R~thhera r d R~rhh~r-likc~ Sub.stwcc.s (L. Bateman, ed.) Maclaren and Sons, London. p. 593. Bateman. L., and Sekhar, B. C. (1966). Significance of PR1 in raw and vulcanised natural rubber. J . Rubb. Res. Illst. Mrrlrrytr 19: 13.3. Bloomfield, G. F. (195 I ), Characteristics of rubber in latex of untapped trees and in branches of tress in regular tapping. J . Rlthb. Res. h r . M d q w Corrlru. 273: 1 IO. Brzozowska, J., Hanower, P,, and Chezeau, P. (1974). Free amino aclds of the Hc\*en brctsi/irr~.si.slatex. E.~perirr~titr. 30:894. Burfield, D. R.(1974). Epoxy groups responsible for storage hardeningin natural rubber. Ntrrure, 24929. Burfield. D. R., and Can, S. N. (1977). Non-oxidative crosslinkingreaction in natural rubber. 11. Radiotracer study of the reactions of amino acids in rubber latex. J. Po/xrn. Sei. Po/yrr~.C/Ieru. Ed. 15:2721. Burfield, D. R. (1984). Compnrative d.s.c. studies of the crystallization of natural rubber and its synthetlc analogues. Po/yrrtrr 25:1824.
56
and
Eng
Ong
Burfield, D. R. (1986), Storage hardening of natural rubber. Examination of current mechanistic proposal. J. Nat. Rubb. Res. 1:202. Burfield, D. R., and Tan, T. C. (1987). Storage hardening of NR: Interrelationship between plasticity and number of crosslinlung groups Polym. Cornm. 28: 190. Burfield, D. R. (1989), Some recent results on the storage hardening of natural rubber. In Development ofP1astic.s and Rubber Product Industries(J. C. Rajaro, Ed.), Rubber Research Institute of Malaysia, Kuala Lumpur, p. 204. Burfield, D. R., Eng, A. H., and Loi, P. S. T. (1989), Investigation of an indirect GC method for the determination of water content of rubbers. Br. Polym. J . 21:77. Chen, S. F., and Ng, C. S. (1984). The natural higher fatty acid soaps in natural rubber latex and their effect on the mechanical stability of the latex. Ruhb. Chem. Technol, 57243. Cockbain, E. G., and Philpott, M. W. (1963), Colloidal properties of latex. In Clzemistry and Physics of Rubber and Rubber-like Substances (L. Bateman, Ed.), Maclaren and Sons Ltd., London, p. 73. Collier, H. M. (1955), Effect of storage conditions on the properties of latex. Trans. Inst. Ruhh. Ind. 31: 166. Crafts, R. C., Davey, J. E., McSweeney, G. P,,and Stephens, I. S. (1990). Comparison of FFA values with individually determined organic and inorganic acid constituents in the extracts of Malaysian rubbers. J . Nut. Rubb. Res. 5:275. Dennis, M. S., and Light, D. R. (1989), Rubber elongation factor from Hevea brasiliensis. Identification, characterization, and role in rubber biosynthesis. J. B i d . Cllem., 264:18608. den Otter, J. L. (1975). Rheological measurements on two crosslinked, unfilled synthetic rubbers. Rheol. Actcc 14:329. den Otter, J. L. (1979). IOIh Akron Polymer Conference, Akron University. De Vries, 0 . (1927), Coagulation, structure and plasticity of crude rubber. Trans. Inst. Rubber lnd.3:284. Djiauw, L. K., and Gent, A. N. (1974), Elasticity of uncrosslinked rubbers. J. Polym. Sci.: Sytnp. (48): 159. Dunphy, P. J., Whittle, K. J., Pennock, J. F., and Morton, R. A. (1965), Identification and estimation of tocotrienols in Hevea latex. Nature 20752 1. Eng, A. H., Tanaka, Y., and Can, S. N. (1992), FTIR studies on amino groups in purified Heveu rubber. J. Nut. Rubb. Res. 7152. Eng, A. H., Ejiri, S., Kawahara, S., and Tanaka, Y. (1993), Structural characteristics of natural rubber. Proc. Int. Ruhh. Technol. Con& Kuala Lutpur, p. 386. Eng, A. H. (1994). Structural characterization of natural rubber. Ph.D. thesis, Tokyo University of Agriculture and Technology. Eng, A. H., Ejiri, S., Kawahara, S., and Tanaka, Y. (1994a), Structural characterization of natural rubber: Role of ester groups. J. Appl. Polym. Sci. Appl. Pol.vm. Sytnp. 53:5. Eng, A. H., Kawahara, S., and Tanaka, Y. (1994b) Trans isoprene units in natural rubber. Rubb. Chem. Technol. 6 7 159. Eng, A. H., Tangpakdee, J., Kawahara, S., and Tanaka, Y. (1997), Distribution and origin of abnormal groups in natural rubber. J. Nut. Rubb. Res. 12:ll. Eng, A. H., Tangpakdee, J., Kawahara, S., and Tanaka, Y. (19984, Epoxide groups in natural rubber. I. "C-NMR study of oxidative degraded rubbers. J . Rubb. Res. 1:67. Eng, A. H., Kodama, S., Tangpakdee, J., Kawahara, S., and Tanaka, Y. (1998b). Epoxide groups in natural rubber. 11. "C-NMR study of undegraded natural rubbers. J . Rubb. Res. 1:199. Fernando, W. S. E., and Perera, M. C. S. (1987). Some observations on mastication and green strength of Hevea rubber. Kautsch. Gurnmi. Kunst. 40:1149. Folt, V. L. (1969), The effects of mechanical degradation on rheological properties of elastomers. Ruhb. Chenl. Technol. 42: 1294. Folt, V. L., Smith, R. W.,and Wilkes, C. L. (1971), Crystallization of cis polyisoprene in capillary rheometer. I. Rubb. Chem. Technol. 44: I , 12, 26. Fournier, P,, and Tuong, C. C. (1961), The biosynthesis of rubber. Rubb. Chenz. Technol. 34:1229. Fuller, K. N. G., Grgory, M. J., Harries, J. A., Muhr, A. H., Roberts, A. D., and Stevenson, A. (19881, Engineering use of natural rubber. In Naturul Rubber Science atld Technology(A. D. Roberts, Ed.), Oxford University Press, p. 892.
Hevea Natural Rubber
57
Fuller, K. N. G., and Fulton, W. S. (1990), The influence of molecular weight distribution and branching on relaxation behavior of uncrosslinked natural rubber. Polymer 31:609. Fuller, K. N. G., and Roberts, A. D. (1981), Rubber rolling on rough surfaces. J. Phvs. D: Appl. Pkys. l4:22 1. Fulton, W. S., and Groves, S. A. (1997), Determination of the molecular architecture of synthetic and natural rubber by the use of thermal field flow fractionation and multi-angle laser light scattering. J. Nrtt. Ruhh. Res. 12:154. Gcnt, A. N. (1954). Crystallization in natural rubber. 11. The influence of impurities. Trans. Inst. Ruhb. Ind. 30: 139. Gomez, J. B., and Moir, G. F. J. ( 1979), The Ultracytology of Lcttex Vessels in Heverr brnsi1im.si.s.Malaysian Rubber Research and Development Board: Kuala Lumpur. Gorton, A. D. T. (l974), The viscosity of rubber in natural rubber latex concentrate and its modification. Ruhh. Ind. 1 11. Grechanovskii, V. A., Dmitrieva, I. P. and Zaitsev, N.B. ( 1987), Separation and preliminary characterization of protein component from commercial varieties of Hewcl rubber. Inter. Polym. Sei. Techno/.14: TII. Gregg, E. C. Jr., and Macey, J. H. (1973), The relationshlp of properties of synthetlc polyisoprene and natural rubber in the factory. The effects of non-rubber constituents of natural rubber. Ruhh. Cltern. Technol. 4647. Gregory, M. J., and Tan A. S. (1976), Some observations on storage hardening of natural rubber. Proc. h t . Rubb. Con$ K u d o Lunnpur 1975 4 2 8 . Hager, T., MacArthur, A., McIntyre, D., and Segger, R. (1979), Chemistry and structure of natural rubber. Rubb. Chern. Tecknol. 52:693. Hasma, H., and Subramaniam, A. (1978), Theoccurrence of furanoid fatty acid in Hewn hrasiliensis latex. Lipids 13:905. Hasma, H., and Subramanium, A. (1986), Composition of lipids in latex of Hevea brasiliensis clone RRIM 501. J. Nut. Rubb. Res. 1:30. Hasma, H. (1987). Proteolipids of natural rubber particles. J. Nnt. Rubb. Res. 2: 129. Hasma, H., and Alias 0. (1990), Role of some non-rubber constituents on thermal oxidative ageing of natural rubber. J. Nat. Rubb. Res. 5 : 1. Hasma, H. (1991), Lipids associated with the rubber particles and their role in mechanical stability of latex concentrates. J. Nar. Rubh. Res. 6:105. Hasma, H. (1992), Proteins of natural rubber latex concentrate. J. Nut. Rubb. Res. 7 102. Hasma, H., and Amir, M. Y. (1997), Changes to NR latex proteins on processing the latex to its products. J.Nrtt. Rubh. Res. 12:21. Ho, C. C., Subramaniam, A., and Yong, W. M. (1976), Lipids associated with rubber particles in Heven latex. Proc. Int. R u l h Cor$ K u d o Lurnpur 1975 2:441. Heusser, C., and Holder, H. J. V. S. (1931), Tapping results with the new double cut tapping system on He\,ett buddings in 1930. Arclg Rubber Cult. 15:246. Ichikawa, N., Eng, A. H., and Tanaka, Y. (1993). Properties of deproteinized natural rubber. proc. h r . Rubb. Tecltr~ol.Con$ Cor$ Kuala Luntpur, 101. Jones, K. P. (1994). Natural rubber as a green commodity. Part 11. Ruhb. Del,. 4737. Jurado, C. W., and Mayhan, K. G. (1986), Natural higher fatty acid soapsin natural rubber latex concentrate and correlations to other latex variables. Ruhb. Cherrt. Techrwl. 59:84. Kawahara, S., Nishiyama, N., Kakubo, T., and Tanaka, Y. (1996), Origin of characteristic properties of natural rubber-synergistic effect of fatty acids on crystallization of cis-1,4-polyisoprene: I. Saturated and unsaturated fatty acids. Ruhh. Chern. Tecknol. 69:600. Lau, C. M. (1996),Quebrnc~llitol-A Carhollvdrnte Extrctct from Hevea Latex. Rubber Research Institute of Malaysia, Kuala Lumpur. Lee, S., and Molnar, A. ( 1995). Determination of molecular weight and gel content of natural rubber using thermal field-flow fractionation. Mncrotnolecules 28:6354. Li, S. D., Yu, H. P,, Peng, Z., and Li, P. S. (1997), Study of variation of structure and properties of natural rubber during accelerated storage. Proc. In!. Rubb. Conf K t t n l ~Lumpur 1997, 569.
58
Eng and Ong
Lim, C. L., Ong, E. L., and Lim, H. S. (1984), SMRGP-Some rheological characteristics. 11. Comparison with other natural rubber grades. J . Ruhh. Res. I r ~ s t .Mrrlery.sirr 32: 144. h t . Rubb. I n d . 36202. Lowe. J. S. (1960) Substrate for VFA formation in natural rubber. Tro~r~s. McMullen, A. I. (1960), Nucleotides of Hcvecr 17rcrsilierl.si.s latex. The pyrophosphate components. Eiockint. Biophys. Acta 41:341. McMullen, A. I. ( 1962). Particulate ribonucleo-protein components of Hew1 brmilierlsi.s latex. Biochinl. J. 85:491. Morimoto, M. ( 1985), Effects of non-rubber ingredients in natural rubber on ageing properties. Proc. I n t . Ruhh. Cor$ I985 2:6 1. Morris, J. E., and Sekhar, B. C. (1959). Recent developments in the production and processing of natural rubber in Malaya, Proc. D I I . Ruhb. Cor$ 1959, Wcrshir~gtor~ D.C.. p. 277. Morris, M. D. ( 1991), Contribution of storage hardening to plasticity retention index test for natural rubber. J. Ncrt. Rubb. Res. 6:96. Muniandy. K., Southern, E., and Thomas, A. G . ( I 988). Diffusion of liquids and solids in rubbers. In Ncrturcrl Rubber Science trrtd Techr~ology(A. D. Roberts, Ed.), Oxford University Press, p. 820. Nadarajah, M,, Veerabangsa. M. T., De Silva. G. A., Senarntne, S., and Perera, D. R. C. (1987), Control of volatilemattercontenttoproduceconsistentquality natural rubber. Proc. Ruhbercorl '87, p. 3N1. Ng, T. S. (1960). Isolation, identification of the free amino acids in fresh unammoniated Havecc latex. Proc. Nut. Ruhb. Res. COI$ K u d c Lunlpur, 1960. p. 809. Nishiyama, N., Kawahara, S., Kakubo, T., Eng, A. H., and Tanaka, Y. (1996), Origin of characteristic properties of natural rubber-synergistic effectof fatty acids on crystallizationof cis-1,4-polyisoprene: 11. Mixed and esterified fatty acids in natural rubber. R U M . Chenz. T e c h ~ ~ n69608. l. Ong, E. L., and Lim, C. L. (1982a), Rheological properties and processing behaviour of NR: Effect of drying temperatures. Proceedirlg ofthe P o l w e r Ser~~ir~rrr, Kucrltr Lu~nprrrN o v e ~ ~ ~ bVol. e r , I, p. 237. Ong, E. L., and Roberts, A. D. (1984), Friction tearing of raw rubber. J. Phys. D: Appl. Pkys. 171961. Ong, E. L., and Lim, C. L. (1982b). Rheological properties of raw and black filled natural rubber stocks. J. Rubh. Res. 111.st.Mrrlrr.v.sicr 30:9 1 Ong, E. L., and Roberts, A. D. (1983a), Experiments on friction of raw natural rubber. J. Ruhh. Res. I n s t . Mal(cysitr 31:236. Ong, E. L., and Roberts, A. D. (1983b), 6th Australasian Polymer Technology Convention, Canberra. latex, J. Rubb. Philpott, W. M,, and Westgarth, D. R. (1953). Stability and mineral compositlon of H e ~ w Res. I I I S I .Mdrryn. 14:133. Resing, W. L. (1955), Variability of Heveo latex. A r c h Rubb. Cult. 32:75. Rhodes, E., and Wiltshire, J. L. (1931), Quebrachito1"a possible by-product from latex. J . Rubb. Res. I m t . M~lcrysitr3: 160. Schallamach. A. ( 1952), The velocity and temperature dependence of rubber friction. Proc. Phys. Soc. Lord. B65:657. Schallamach. A. ( 1953). The load dependence of rubber friction. Proc. Phps. Soc. Lnr~d.B66:386. Schlesinger, W., and Leeper, H. M. (1951), C h d e cis- and trrrr~sfrom a single plant species. I f d . Engng. Clren~.43398. Sekhar, B. C. (1960), Degradation and crosslinkmg of polyisoprene in Hevecr brcr.si/imsi.s latex dunng processing and storage. J. Polym. Sci. 48: 133. Sekhar, B. C. (1961 ), Inhibition of hardening in natural rubber. Proc. Ncct. Rubb. Cor$ Kuttla Lurrzpur 1960, p. 512. Sekhar, B. C. ( 1962). Abnormal groups in rubber and microgel. Proc. 4th Rub/?. T t ~ h z o l Conf . Lor~don, p. 460. Sekhar. B. C. (1964), Improvements in or relating to the stabilization of natural rubber. Er. Pat. 965.757. Shelton, J. R. (1972). Review of basic oxidation process in elastomer. Rub/>. Chem ToJcl~~~ol. 45:359. Smit, P. P. A.. and van der Vegt. A. K. (1968), The influcnce of shear-induced crystallization on the flow behavlour of unvulcanized rubbers. Proc. 5th IIII. Rubb. Conf:, 1967. Maclaren & Sons, London, p. 99. Smith, R. H. (1953). Phosphatides of Hewor latex. J. Ruhb. Res. I n s t . Mcrlo.wr 14:169.
Hevea Natural Rubber
59
Smith, R. H. (1954). The phosphatides of the latex of Hevea brasiliensis. 3. Carbohydrate and polyhydroxy constituents. Biocher~f. J . 57: 140. Southom, W. A. (1961). Microscopy of HCIJPLI latex. Proc. Nut. Rubb Res Cor$ Kuala Lumpur 1960. Stavely. F. W., Biddison, P. H., Forster, M. J., Dawson, H. G.. and Binder, J. L. (1961), The structure of various natural rubbers. Rubb. Cher~t.Techrtol. 34:423. Subramaniam, A. (1976), Molecular weight andother properties of natural rubber. A study of clonal variations. Proc. Itft. RLIM.Corlf: Kunln Lurrtpur 4:3. Subramaniam, A. ( 1977), Estimation of aldehyde groupsin natural rubber with 2,4-dinotrophenylhydrazine. J. Rubb. Res. I n s t . Mulnysitr 2 5 6 1. Tanaka, Y. (1984), Structural characterization of naturally occurring cis-polyisoprenes. ACS Symp Ser. No. 247, NMR ~ r r f r lMrrcron~o/ectt/e,s(J. C. Randall, ed.), American Chemical Society, Washington, DC, p. 233. Tanaka. Y., Eng, A. H., and Ejiri, S . (1992), Structural characterizationof natural rubber. Proc. Ncrt. Rubh. Res. Conf Rubber Research Institute of Malaya, Kuala Lumpur. p. 147. Tanaka, Y., Eng. A. H., Ohya, N., Nishiyama. N., Tangpakdee, J.. Kawahara, S., and Wititsuwannakul, R. (1996), Initiationof rubberbiosynthesis In Helver brcrsilierfsis: Characterization of initiating species by structural analysis. Phytocllertlisfr?;41:1501. Tangpakdee, J., Tanaka. Y., Wititsuwannakul, R., and Chareonthiphakorn,N.(1996), Possible mechanisms controlling molecular weight of rubbers in Helve( brctsilierfsis. Phytochenfi.st,p 42:353. Tangpakdee. J.. and Tanaka, Y. (1997), Characterization of sol and gel in He18etr natural rubber. Rubb. Cheltf. Techno/.70:707. Tangpakdee, J., and Tanaka, Y. (l998a) Branching in natural rubber. J. Rubb. Res.. I: 14. Tangpakdee, J., and Tanaka, Y. (1998b), Why rubber trees produce polyisoprene-Role of NR in the Herw tree. J. Rubb. Res. 1:77. Tata, S. J. (l976), Hevein: Its isolation. purification and some structural aspects. Proc. I r f t . Rfrhh. Cor!/: Kucrltr Lurupur, I975 2:499. Tata, S. J. ( 1980), Distributionof proteins between the fraction of Hevea latex separated by ultracentrifugation. J. Ruhh. Res. Inst. Mrrlcglsirr 28:27. Tupy, J., and Resing, W. L. (1968). Anaerobic in latex of Helvn brrrsiliensis substrate and limitingfactors. Bid. Plrtrff.10:72. Turner D. M., and Moore. M. D. ( 1980), Effect of pressure and metal surface on the flow behaviour of rubber. Conf. On Practical Rheology in Polymer Processing, Loughborough, U.K. (PRI/BSR, London), p. 6.1. van den Tempel. M. (1952), Electron microscopy of rubber glObllkS in Heverr latex. 7'rrtrt.s.I.R.I. 28303. Wan A., Rahaman, W. Y. (1994), Natural rubber as a green commodity-Part I. Rubb. Dev. 47: 13. William, M. L., Landel, R. F.. and Feery, J. D. (1955), The temperature dependenceof relaxation mechanisms in amorphous polymer and other glass forming liquids. J. Am. C l ~ e nSoc. ~ . 773701 Wiltshire. J. L. (1934), Variationsin the compositlon of latex from clone and seedling rubber. RRIM Bull. No. S. Wood, R. I. (1952), Mooney vlscosity changes in freshly prepared raw natural rubber. J . Rubb. Res. 1 m t . Mctkcyct 14:20. Yip. E., and Chin, H. C. (1977), Latex flow studies X. Distribution of metallic ions between phase of Hellm latex and effect of yield stimulation on the distribution. J. Rrtbb. Re,s. / u t . Mulltysirr 25:3 I . Yip, E. (1978). A study of fresh latex properties of Hewer hrcrsilierrsi.7 in relation to the mechnnism of latex vessel plugging. Ph.D. thesis, University of Gent. Belgium. Yip, E., and Gomez, J. B. (1980). Fractors influencing the colloidal stability of fresh clonal Hetvcr latices a s determined by the aerosol OT test. J. Ruhh. Res. I m r . Mdoysict 28% Yip, E.. and Subramaniam, A. (1984), Characterisationof variability in properties of clonal latex concentrates and rubbers. Proc. I I I ~R. d h Cor$ 1984. Color~tbo,Sri L m k o 2547.
This Page Intentionally Left Blank
3 Modified Natural Rubber Crispin S. L. Baker Tun Abdul Razak Research Centre, Brickendonbury, Hertford, England
1. INTRODUCTION Natural rubber has been modified in many different ways since as early as 1801, although the first commercial form was not manufactured until about 1915. The term nzodified rubber can refer to any degree of chemical modification from a very small mole percent for the purposes of introducing bound antioxidant functions, crosslinking, bonding, etc., without introducing any changes to the basic physical properties, through to the reaction of a significant number of the repeat units, say 20-50 mol%, which does result in changes in the physical properties of the rubber. Higher levels of modification tend to alter the nature of the polymer from a rubber to a more plastic-like or resinous material. Because natural rubber (NR) hasa fixed cis-polyisoprene structure and cannot have its polymerization process tailored like that of the synthetic rubber industry to provide suitable pendent groups, many of the first type of modified rubbers have been prepared with a variety of groups for a whole cross section of purposes, for example, the addition of thiols and related compounds to improve low-temperature properties and for crosslinking (Cunneen and Shipley, 1959; Cunneen et al., 1960), epoxidation for the reaction of the oxirane ring (Colclough et al., 1961; Colclough, 1962), addition of maleic anhydride and maleimides (Pinazzi et al., 1960, 1962), the “ene” addition reaction (Cain et al., 1968; Baker et al., 1970), and many others since. The number of thesetypes of modified NR would be sufficient to fill this handbook by themselves, and therefore this chapter has been restricted to the second and third types of modified natural rubber mentioned above. Even so, taking into account those that have not been commercially successful, the number of forms of modified natural rubber examined has still been quite large. The modified forms to be considered will includehydrogenatedNR;chlorinated NR; hydrohalogenated NR; cyclized NR; resin-modified NR; poly(methy1 methacrylate)-grafted NR; superior processing rubber ( S P P A grades); ENPCAF-modified NR; polystyrene-grafted NR; epoxidized NR and hydrogenated epoxidized NR. In addition, for completeness, liquid NR and thermoplastic derivatives of NR (TPNR and TPENR) will be included. The former is hardly a modification in the true sense, but rather a degradation, whereas the latter are physical blends of NR or ENR with polypropylene resulting in new materials from NR. Since all these forms of NR are so totally different, are prepared by unrelated routes, and result in rubbers for use in their own individual applications, it is best thatthey be taken on their own and each onediscussed on its own merits. The order used is mostly chronological, with the liquid NR and thermoplastic 61
Baker
62
NR reviewed after the more genuine modified forms. The first four forms will be discussed only brietly, since they are in part historical but are included for the sake of completeness.
2.
HYDROGENATEDNATURAL RUBBER
Hydrogenated NR is so far only a scientific curiosity, since it has not been produced commercially and probably never will be on account of the experimental problems associated with its preparation. The early processes required either highly purified rubber in dilute solution or the use of very high temperatures that caused degradation. In addition. hydrogenation requires its catalyst to come into contact with each double bond for the reaction. and thus relatively large amounts of catalyst are essential if the hydrogenation is to be completed in a reasonable period of time. In addition. all traces of catalyst must be removed to avoid rapid oxidation of the product. Very early work by Pummerer and Burkard ( 1922) and Harries ( 1923) produced hydrorubber ( C 5 H l o ) xfrom dilute solutions of purified rubber using platinum black as catalyst. Only very small amounts were obtained. but these experiments showed that the elasticproperties were retained alongwith a relatively high molecular weight.Otherroutes. followed by Staudinger and coworkers, employed high temperatureand pressure (Staudingeret al., 1930),0.1%solutions in methyl cyclohexane and a nickel catalyst (Staudinger and Feisst, 1930). and long reaction times with a very high nickel catalyst content at lower temperatures (Staudinger and Leupold, 1934). It is interesting that when both gutta-percha and balata, the transforms of NR, are hydrogenated by similar methods, the result is hydrorubber with essentially the same properties. since thehydrogenation stepremoves the stereospecificity. One of the moredetailedmethods of preparation of hydrogenated NR was patented in 1946. Here a relatively high concentration of pale crepe rubber in cyclohexane (2%) was caused to react with hydrogen at 30-35 atm over a nickel-kieselguhr catalyst in an autoclave at 200-220°C for 12 hours. The fully hydrogenated product was then obtained from the reaction mixture by the removal of catalyst by flocculation with glacial acetic acid, followed by concentration and precipitation with ethyl alcohol. More recently. Burfield et al. (1985) investigated homogeneous two-component catalyst systems based on a variety of nickel and cobalt compoundsin combination with triisobutylaluminum.They report that complete hydrogenation can be achieved in solutionafter1hourat 28°C using this system with nickel 2-ethylhexanoate. However, the more economical route of hydrogenation of NR in the form of dry sheet is considerably slower. Only 70% hydrogenation can be achieved after 22 hours at 50-70°C. The major problem here is the addition of the airsensitive aluminum alkyl to the rubber preblended on a mill with the nickel catalyst.The present spraying technique provides inadequate reduction of the nickel owing to its slow diffusion into the rubber. If mixing under inert conditions could be realistically achieved. a practical route to hydrogenated rubber may have been found. Hydrogenated rubber is colorless and transparent. It is a plastic-elastic waxy solid with the peculiar characteristic of pulling out into fine threads when stretched. Unfortunately. the limitedscale of its preparation so far hasprecludedthemeasurement of any technological properties. It is known that fully hydrogenated NR is unaffected by the chemical reagents that normally attack unmodified natural rubber owing to the removal of the unsaturation. However, this also results init being difficultto vulcanize in the conventional manner. Potential applications have been considered to be in the cable industry, which would utilize its insulation properties, and possibly in adhesives. 3.
CHLORINATEDNATURALRUBBER
Chlorinated rubber was one of the first forms of modified NR to find any real commercial application. During chlorination, substitution and cyclization reactions also occur whether the
Modified Natural
63
modificationtakesplace in solution. in latex, or in solidrubber. In thereaction of gaseous chlorine with rubber in carbon tetrachloride at 80"C, three distinct stages appear to take place, according to Bloomfield (1943) and Kraus and Reynolds (1950):
-CloH13CI,
+ 2 Cl*+
-ClfiIIC17
+ 2HCl
In the first stage, with up to one substitution per CS unit, most of the cyclization appears to take place, possibly by a carbonium-ion mechanism (Kraus and Reynolds, 1950; Bloomfield. 1961). At this point the product contains 35% chlorine and is still rubbery. It is not particularly stable toward heat, but attempts have been made to use it in oil-resistant compositions (Woods, 1949). After the second stage. the product contains 57% chlorine, and no unsaturated groups remain. It is now a crumb-like material with no rubbery properties. Further chlorination results in a final product containing 65.5% chlorine, in close agreement with the empirical formula
Chlorinated rubber containing approximately 65% chlorine used to be available commercially under the trade names Alloprene (ICI), Parlon (Hercules), and Pergut (Bayer). Removal of the last traces of the solvent used in its manufacture proved extremely difficult, andthus low-density foams could easily be obtained by heating the commercial products in a press and opening the mold when hot (Schidrowitz and Redfarn, 1935). It is a pale cream or off-white thermoplastic powder, which is nonflammable and highly resistant to chemical attack. For this reason its main use has been for chemical- and heat-resistant paints and coatings, where the article in question requiresprotection from acorrosive environment.The chlorinatedrubber is dispersed in a suitable solvent and mixed with appropriate plasticizers and pigments. Such paints usually contain about IO- 12% chlorinated rubber and can be applied by brush or spray.In addition to being used to protect wood, steel, cement, etc., from environmental attack, these paints have been applied to chemical equipment as they also possess a degree of abrasion resistance. Indeed, they have even been recommended for traffic paint on roads. Chlorinated NR has also been used in adhesives. printing inks, paper coatings, and textile finishes. The advent, however, of chlorinated synthetic rubber, especially polychloroprene, has resulted in the demise of chlorinated NR, which has now ceased to be available commercially.
4.
HYDROHALOGENATEDNATURALRUBBER
Natural rubber can be modified with hydrogen chloride, bromide, iodide, or fluoride to give rubber hydrochloride. rubber hydrobromide. rubber hydroiodide, or rubber hydrofluoride, respectively. However. the latter three areof theoretical interest only, since the reaction with hydrogen chloride is the cheapest and by far the most feasible; the hydrobromide product is unstable, the HI addition has received only limited attention, and HF is highly toxic. The attraction of such a modification is the same as that of halogenation itself, to reduce the chemical reactivity of rubber by addition to its double bonds. The structure of rubber hydrochloride has been established by Bunn and Garner ( 1942), who confirmed that the addition of hydrogen chloride to polyisoprene obeys Markownikoff's rule. with the chlorineatom attaching itself to the carbon possessing the least number of hydrogen atoms.
64
-
CH,-C.CH,=CH-CH,
-
Baker
+ HCI
” CH,-C.CH3.CI-CH2-CH,
However, the original reaction was carried out in 1900 by Weber, whopassed hydrogen chloride into a solution of rubber in chloroform. He obtained a hard white product whose analysis was close to (C5HUCl),,. Later,in 1913, Harries carriedout similarexperiments using purified rubber, andhisproductcontained 33.5% chlorine, very close to thetheoretical 33.9%. In practice, hydrochlorinated rubber may be made either in solution or from latex. When prepared by the solution method, raw NR is milled to reduce its molecular weight, dissolved in benzene, and allowed to react with dry hydrogen chloride for6 hours under pressure at 10°C. If the hydrochloride is required in solution, then it is sufficient to simply remove the excess hydrogen chloride by air-blowing andor neutralization. If the solid form is required, then it can be obtained by precipitation or steam distillation. A chlorine content of between 28 and 30% is desirable to ensure that the product is neither too tacky (if lower) nor too brittle (if higher). Of the other routes to its preparation, mostly on sheet rubber, only the reaction in the latex form can be considered a viable alternative. However, as explained by Van Veersen (1948), NR latex is normally stabilized with ammonia and has to be acid-stabilized before the reaction can take place. Hydrogen chloride can then be passed in until a 32% chlorine content has been achieved. The product is coagulated by the addition of sodium chloride or alcohol and then washed and dried. Rubber hydrochloride itself is a highly crystalline material that undergoes molecular orientation on stretching and “melts” at temperatures above 115°C. Chemically, it is fairly inert, since it is unaffected by dilute acids and bases at room temperature. It is also incompatible with unmodified NR. In spite of this, one of its uses has been as a bonding agent for rubber to metal. when a dispersion of rubber hydrochloride, sulfur, plasticizer, and stabilizer provide excellent in-vulcanization bonding. Its largest use, however, was as a cast transparent film marketed under the trade name Pliofilm. Its exceptionally low permeability to water vapor combined with its elasticity and high tear strength made it particularly suitable for the packaging industry, especially for food. In 1950, about 3500 tons were produced. However, the packaging market was gradually eroded by the advent of plastic film, and rubber hydrochloride is no longer commercially available. The additions of the other halogen acids to NR have also been carried out but have been only of scientificinterest.Again,Harries (1913) andHinrichsenet al. (1913) reportedthat hydrogen bromide maybe added tonatural rubber to give the expected hydrobromide (C5HoBr),,. The reaction proceeds similarly to that with hydrogen chloride to give a yellowish powder, which is unstable owing to the lability of its halogen atom. The hydrogen iodide addition was also studied by Henrichsen et al. (1913) but was not carried beyond the (C I ~ H I ~ Istage. ),, The reaction of natural rubber with hydrogen fluoride hasbeen described by Tom (1956). In this case the addition reaction is in competition with one of cyclization, and many of the earlier workers in fact obtained only cyclized material. Tom investigated the reaction carefully, and by suitable choice of solvent and temperature he reduced the cyclization by a considerable extent to yield a product in which 65-70% of the double bonds had been reacted with hydrofluoric acid. When compounded with 50 parts black and cured witha normal sulfur/sulfenamide system, this rubber hydrofluoride gave a vulcanizate whose properties were still rubbery (tensile strength 23 MPaand elongation at break 430%) but showed significantly less swelling in hydrocarbonsolvents. It also possessed very low gas permeability,which. at only 3% of that of unmodified NR, was lower thanthat of butyl rubber. The material became brittle atlow tempera-
Modified Natural Rubber
65
+
tures with a minimum rebound at 10°C (see Fig. 9 later). However, the toxicity of hydrogen fluoride, combined withitssignificantlygreatercost compared to hydrogenchloride,never allowed hydrofluorination to be a commercial viability.
5. CYCLIZEDNATURALRUBBER Cyclized rubber is probably the oldest known modification of natural rubber, dating back to Leonhardi in 1791. He discovered that when treated with sulfuric acid NR became hard and brittle, but of course he was unaware of the nature of the reaction taking place. Similar observations were made by Harries in 1919 and Kirchhof in 1920. Later, in 1927, Fisher extended the treatment to includeorganicsulfonic acids, sulfonyl chlorides, andsulfates, and theseearly materials were used as rubber-to-metal bonding agents. At about the same time, Bruson et al. (1927) discovered that cyclized rubber could alsobe obtained by reaction of rubber with stannic chloride to give a thermoplastic form. There seems little doubt that internal cyclization is the mechanism taking place during the modification of natural rubber firstput forward by van Veersen (1950) and Gordon (1951a,b) and shownin Figure 1. This protonation reaction explains theknown features of the modification: unchanged empirical formula (C,H,), but with unsaturation reduced to 57% of the original NR, acid-catalyzed type of reaction, loss of elastic behavior, and, above all, increased density. Essentially, two main groups of cyclized rubber were made commercially. There were those prepared by the sulfuric acid (or its derivatives) route and those by the chlorostannic acid route. The formerfell into two classes according the to extent of degradation during manufacture. Class 1 were brown to black resinous substances varying from tough balata-type materials to brittle shellac types according to the time and temperature of cyclization. Aromatic sulfonic acids or their chlorides were added to the rubber on a mill or internal mixer. which was then
I
Transfer
Fig. 1 Mechanism for the cyclization of natural rubber, according to M. Gordon. (From Bloomfield and Stokes, 1956.)
Baker
66
heated in shallow pans according to the extent of cyclization required, typically 30 minutes at 125-145°C to 4 hours at 125°C. This process gave cyclized rubber designated Thermoprene and Fenolac (Goodrich), which were used as adhesives, bonding agents, and reinforcing resins. Class 2 were light-colored transparent resins of comparatively low molecular weight prepared commercially for many years by carrying out the cyclization in the presence of an excess of phenol containing a little sulfuric or phosphoric acid. With this method, substantial degradation occurs, resulting in a low molecular weight product. This class of cyclized rubber is readily soluble in many solvents and is compatible with arange of resins, oils, and plasticizers.Commercial materials were Plastoprene (Plastanol Ltd.) andAlpex 4505 (Societa Italiana Resine), which were used in printing inks and surface coatings, especially where resistance to chemicals was required. The second group were produced with chlorostannic acid as cyclizing agent. They could be prepared either from dry rubber by milling in 10% of the hydrated chlorostannic acid for 2-5 hours at 130- 150°C. again depending onthe degree of cyclization required. or by refluxing a solution of masticated rubber in benzene with the chlorostannic acid, decomposing the acid, and evaporating off the solvent. These cyclized rubbers had high physical strength and were thermoplastic and fairly inert chemically. Plioform, marketed by Goodyear, was a thermoplastic molding material. It could be compounded with the normal ingredients of rubber mixes, albeit at rather high mill temperatures, often up to 15OoC, owing to the toughness of the material. Molding took place under pressure at 125-155"C, but cooling to 40°C was necessary before opening the mold. Pliolite, also by Goodyear, was a special-purpose type for protective and decorative moldings. Premasticated and dissolved in suitable solvents, it was mixed with plasticizers, pigments, and drying oils to give paints that after drying and stoving provided excellent resistance to acid, alkali. and organic solvents. Another formof cyclized rubber developedby Bloomfield and Stokes(1956) was produced by reaction in the latex stage and co-coagulation 50:50 with unmodified latex to produce a cyclized rubber masterbatch (CRMB), since one of the more promising uses of cyclized rubber appeared to be a reinforcing resin for natural rubber itself. This is exemplified in Figure 2, where hard compounds can be prepared from cyclized rubber with densities significantly lower than those obtained with conventional light fillers. There areadditional benefits in tensile properties and modulus(Fig. 3)that make it particularly attractive for shoe-soling applications. The pure gum vulcanizate had a hardness by itself of 90 IRHD, and cyclized rubber could easily be blended with unmodified NR in any proportion to give vulcanizates of lower hardness. However, attempts to prepare partially cyclized rubber of cyclization equivalent to a mixture of the two proved not to give the same properties and was also impracticable since it had a tendency to oxidize rapidly on storage. Cyclized rubber Inasterbatch was supplied commercially as Cyclite (Durham Raw Materials) and Cyclatex (Hubron Rubber). Several formulations were available for shoe soling, hard moldings, and nonblack heavy-duty industrial rollers. Table 1 indicates the kinds of compounds used and their hardness.
6.
RESIN-MODIFIED NATURAL RUBBER
Resin-modified NR is not a modified natural rubber in the true sense in that it is not another form of NR that is available as a raw material. It is the result of a reaction of NR with phenolformaldehyde resins during vulcanization to produce a chemically modified NR vulcanizate. It is the first type to be discussed that is still in active use and is likely to be for the foreseeable
67
Modified Natural Rubber
Density (mg/mm3) 2.0 j-
Chma clay
1.5 -
1.0 -
Cyclized rubber
I
40
50
I
I
I
60 70 80 Hardness (IRHD)
I
1
90
100
Fig. 2 Density-hardness relationshipforcyclized Bloomfield and Stokes, 1956.)
rubbercompared to NR with various fillers. (From
Modulus at 100% extension (MPa)
/
10-
a-
/
Preclpitated calclum carbonate
6-
Cyclized
rubber
Calcium silicate
42OL
40
I
50
I
I
I
60 70 80 Hardness (IRHD)
I
90
I
100
Fig. 3 Modulus-hardness relationship for cyclized rubber compared Bloomfield and Stokes, 1956.)
to NR wlth various fillcrs. (From
68
Baker
Table 1 CyclizedRubberMaterbatchFormulations Shoe soling
Hard molding
Cyclized rubber MB Natural rubber Aluminum silicate Zinc oxide Stearic acid Yellow ocher Red ocher Antioxidants" CBS Sulfur
45 55 60
5 2 3 0.2 1.8 0.7 2.5
Cure, 8 min at 153°C Hardness, IRHD
93
Cyclized rubber MB MPC black China clay Zinc oxide Stearic acid Antioxidant" TMTD MBT Sulfur Cure, 5 min at 162°C Hardness, IRHD
100 15 15 5
2 0.5 1 1 1.5
99
'' Nonox HFN 1.0, Agerite Whlte 0.8. Nonox EXN 0.5. Sorrrcv: NRPRA Technical Information Sheet No. 8, 1962. Welwyn Garden City, Umted Kingdom. "
future due to its unique properties. If only for this reason, it deserves a place in this chapter on modified natural rubber. The direct addition of fully condensed phenolic resins to natural rubber for reinforcement purposes is impracticable because of their incompatibility with rubber. The method adopted, therefore, is to carry out the condensation of the resin in the presence of the rubber. In addition, because of their thermoplastic nature in the uncured state. these resins act as processing aids during mixing and yet at the same time reinforce the final product. It seems almost certain that chemical reaction between the resin and rubber occurs during the condensation of the phenol with formaldehyde, as saligenin has been shown by Cunneen et al., (1943) to combine with rubber to form a chromane ring as indicated below:
-CH2 -CH,
CH,-
CH,-
Thus these resins result in a chemical modification of the NR vulcanizate, with the extent of modification dependent upon the rubber-to-resin ratio employed. Reinforcement can be varied from providing a soft vulcanizate through to hard ebonitetype materials, thus extending the potential field of applications for natural rubber, although resin modification can, of course, also be appliedto synthetic rubbers. NR vulcanizates reinforced
69
Modified Natural Rubber
Mooney plasticity
100
r
I
HAF black
60
20 40 80 60 Reinforcement (parts phr)
100
Fig. 4 Effect of Cellobond reinforcement on processing compared to black. Cellobond H.831 is now Cellobond J1115H from Blagden Chemicals. (From British Resin Products Ltd., “Cellobond Rubber Reinforcing Resins,” Technical Manual No. 11, 1964.)
with such resins, for example, Cellobond* (BP Chemicals Ltd), are reported in a British Resin Products technical manual (1964) to exhibit high resilience, excellent flex life, low compression set, and good aging. Compared to similar loadings of reinforcing blacks, such as N330, phenolic resins do not increase Mooney viscosity, as shown in Figure 4, and therefore process more easily. In addition,theresins can permit an increasedfillerloading by actingasextenders, provide a high degree of tack, andpromote adhesion to metal and textiles.The reinforcing effect of these resins can be seen in Table 2, where increase of resin from zero to 25 parts raises the hardness from 58 to 70 IRHD. Natural rubber’s resistance to oil, gasoline, and other solvents is known to be poor, especially compared to “oil-resistant’’ materials such as polychloroprene and nitrile rubbers. The phenolic resin modification of NR vulcanizates can at least improve this undesirable feature, albeit not placing it in the same class as the truly oil-resisting materials. This is shown in Table 3, where gasoline absorption is reduced from 218% with no resin to 71 % with 100 parts resin. This therefore allows natural rubber to be used in applications where there might be a risk of exposure to such oil and solvents. Formulations containing phenolic resins also show improved adhesion to cotton and nylonfabric andare used for friction, topping, and spreadingcompounds. Finally, suitable adjustments of black/resin concentrations can provide semihard or pseudoebonite types of vulcanizates. In the case of the former, the resin can allow the use of high levels of black without the processing problems normally associated with this, whilein the latter the use of the resin improves impactstrength, raises the softening pointto allow easy machining, and reduces the long cure time necessary for true ebonites. Applications of these ebonite types
* Cellobond is a registered trademark of the British Petroleum Company plc.
Baker
70
Table 2 The Reinforcing Effect of Resins Formulation Natural rubber N330 black Reinforcing resin" Zinc oxide Sulfur Stearic acid CBS Properties" Hardness, IRHD Tensile strength, MPa Elongation a t break, c/o Modulus at 3008, MPa
100 25 5 5 3 1 0.85
100
25 IO 5 3 l 0.85
100 25 15 5 3 1 0.85
100
25 0 5 3 I 0.85
0.85
25 25 5 3 I 0.85
55 24.1 5 20 7.9
57 25.2 525 8.8
62 25.8 480 14.1
65 22.4 400 14.8
68 20.4 375 15.2
70 19.3 365 16.6
100
25 20 5 3 1
100
Ccllohond H83 1, now Cellobond J I 1 15H. cunng for 20 nun at 141°C. Sorrrce: Brltish Resin Products Ltd., "Ccllohond Rubber Rcnhrcing Resm." Technical Manual No. 1 I . 1964.
'I
" After
Table 3 Gasoline and Oil Resistance Formulation Natural rubber N330 black Reinforcing resin" Zinc oxide Sulfur Stearic acid MBT Properties" Gasoline absorption (7 days at 25°C). % Oil absorption i7 days at 20°C), IC
of Resin-Reinforced Natural Rubber
100
100
25 5 3
25 25 5 3
1
1
1.25
1.25
100
25 50 5 3 1 1.25
100
25 75 5 3 1
1.25
IO0 25 l00 5 3 1 1.2s
218
120
118
98
71
337
146
145
144
149
Cellobond H83 l , now Ccllohond J 1 1 15H. After curlng for 29 min at 141°C. Source: Brltish Rcsln Products Ltd., "Cellobond Rubber Remforclng Resins." Technical Manual No. 11. 1964.
"
"
Modified Natural Rubber
71
include ducting. rigid pipes, containers, casters, panels, moldings, and extrusions where conventional ebonite is unsuitable owing to its low impact strength and/or softening point.
7. POLY(METHYLMETHACRYLATE)-GRAFTEDNATURAL RUBBER Polymer-modified forms of natural rubber can bemade by the polymerization of vinyl monomers either in rubber solution or in latex (Allen and Merrett, 1956; Allen et al., 1960; Allen, 1963). The fact that the polymerization takes place in a rubber environment does not necessarily mean that interaction with the polyisoprene molecules takes place. However, model studies with dihydromyrcene have indicated that reaction does occur with polymerizing styrene (Scanlan, 19541, methyl methacrylate (Allen et al., 1955; Allen and McSweeney, 1958), vinyl acetate (Allen et al., 1955) and acrylonitrile (Allen and McSweeney, 1958) provided that appropriate initiators (e.g., peroxides. hydroperoxides) are used. The general structure of final rubber copolymer graft can be indicated as follows:
- R u b b e r 7 -
Poly(methy1 methacrylate)-modified natural rubberhas been marketed sincethe mid- 1950s under the trade name Heveaplus. Originally there were two types conceived according to the BRPRA technical bulletin (1954): Heveaplus MM, which was a simple mixture of poly(methy1 methacrylate) and natural rubber prepared by combining NR latex concentrate with the appropriate amount of poly(methy1 methacrylate) dispersion to give the required rubber/polymer ratio, followed by co-coagulation, washing, and drying; and Heveaplus MC, which was the grafted form prepared by reacting NR latex with the required amount of methyl methacrylate monomer with r-butyl hydroperoxide as the polymerization initiator. After stilring in tetraethylene pentarnine (as the other half of the redox system), the reaction is essentially complete in 2 hours. The product is coagulated by adding boiling water containing 0.1% calcium choride and then sheeted out and dried. Thefirst of these types never appeared to be of commercial interest, but the grafted type has proved usefulon several counts and indeed is still in production in Malaysia. Various levelsof modification can be achieved according to the proportionof grafted poly(methy1 methacrylate), and this alters themodulus of the rubber as indicated in Figure 5. In this instance, the compounds studied were obtained by diluting Heveaplus MC23 (23% methyl methacrylate concentration), but essentially the sameresults would be foundfrom Heveaplus MGmade solely to the concentration shown. The main advantages of the poly(methy1 methacrylate)-grafted NR lie in an ability to produce self-reinforced vulcanizates and in adhesive applications. Originally developed during a time when the price of natural rubber was high relative to that of methyl methacrylate, Heveaplus MG was thought to have potential in tire treads as a means of producing a hard rubber that should be more easily processable than vulcanizates filled with carbon black to the equivalent hardness. Unfortunately, the abrasion resistance was found to be inadequate, and hence this use
72
Baker
Modulus at 100% extension (MPa)
0.5 I I I I I 0 5 10 15 20 Methyl methacrylate concentration("/. of polymer) Fig. 5 Increase of modulus with methyl methacrylate concentration. (From BRPRA Technical Bulletin No. 1, 1954.)
was never developed. Later developments in the automotive field created a market for hard flexible materials based on natural rubber in applications such as sight shields (filler panels), soft fronts, rear ends, rubbing strips, and bumpers, and here blends of Heveaplus MG49 (49% methyl methacrylate) with NR can be used (Wheelans, 1977). Table 4 indicates the degree of hardness and the properties that can be achieved when a reinforcing black is also included. Three grades of Heveaplus MG are commercially available: MG30 based on a nominal 30% methyl methacrylate content, MG40 based on 40%, and MG49 based on 49%. In the case of Heveaplus MG49, approximately 80% is graft polymer, 10% free poly(methy1 methacrylate), and 10% free polyisoprene.Ascanbeseen in Table 4, themodified rubber can be readily blended in any proportion with NR, filled with conventional carbon blacks, and vulcanized with normal sulfur curing systems. Alternatively it can be used undiluted and unfilled, when it still exhibits a remarkably high stiffness as shown in Table 5. Present-day production amounts to about 50 tons/yr for Heveaplus MG49 and about 250 tons/yr for MG30. Its major use, however, is in adhesives, rather than in hard flexible materials. It was established at a very early stage that poly(methy1 methacrylate)-grafted NR could give exceptionally good bond strengths for NR onto PVC, and these are particularly useful to the shoe industry.The modified rubbercontains within its molecular structureboth polar poly(methy1 methacrylate) and nonpolar polyisoprene components, thus making it an attractive material for bonding unlike surfaces, normallyincompatible owing to their polar and nonpolar characteristics. It can be used either as a solution of the grafted polymer or preferably as an in situ graft of natural rubber with methyl methacrylate monomer in a suitable solution by benzoyl peroxide catalyst. Bevan and Bloomfield (1 963) found that there is a substantial increase in bond strength obtained between NR and PVC oncethe level of poly(methy1 methacrylate) is in excess of 35% (Fig. 6). The use of poly(methy1 methacrylate)-grafted NR solution for directvulcanization soling of an NR compound to PVC is therefore attractive owing to these high peel strengths. In addition, these bonding solutions give consistently good results, are equally efficient at any
HD
73
Modified Natural Rubber Table 4 Properties of Blends of Hcaveaplus MC49 with Natural Rubber Formulation Natural rubber Heaveaplus MC49 N326 Black Zinc oxide Stearic acid Antioxidant" Sulfur CBS PVI"
50 50 50 5 2 1 2.5 0.5 0.6
70 30 50 5 2 1 2.5 0.5 0.4
Properties'' Hardness, IRHD Elongation at break, % Stored energy, MJ/m Resilience, Dunlop, at 23"C, %
78 430 52 49
89 270 28 42
2.2-Methylene-bis-(4-methyl-h-1-butyphenol). Prevulvantzation Inhibitor. N-cyclohexylthiophthalimlde. ' After curing 2 min at 180°C. Source: Wheelans, I977
l'
"
Table 5 Typical Basic Properties of Undiluted Unfilled 49% Poly(rnethy1 methacrylate)-Grafted NR Formulation Heaveaplus MC49 Zinc oxide Stearic acid Sulfur CBS
100
5 2 2.5 0.7
Properties" Hardness, strength, Tensile MPa Elongation at break, 5% Young's modulus in flexure, MPa At 20°C At 70°C At 100°C Vicat point, softening
"C
96 17 215 20.0 12.4 9.7 109
After curing for 20 min at IS3"C. Source: MRPRA Natural Rubber T e c h n d Information Sheet D101. 1982. Hertford, Umted Kindgom. 'I
30 70 50 5 2 1
2.5 0.5 0.6 98 130 IO 38
Baker
74
180" peel strength (N/mm)
10 20 30 40 50 Methyl methacrylate concentration ("10) Fig. 6 NR-PVC bond strength versus MG level. (From Bevan and Bloomfield, 1963.)
level of grafted methacrylate in excess of 35%, and are unaffected by curing temperature and time. Solvents used are usually toluene or MEK, and a fine, free-flowing powdered form of Heveaplus MG49 has been made especially for theadhesives market to assist in rapid dissolution. However, the in situ method of preparation of poly(methy1 methacrylate)-grafted NR solution makes the availability of a commercial material not essential for this use, albeit the in-house production is likely to be more costly and involves additional expertise.
8. SUPERIOR PROCESSINGRUBBER Superior processing rubbers, which are intimate mixtures of crosslinked and uncrosslinked materials, can also be regarded as modified forms of natural rubber, since up to 80% of the rubber may be in the vulcanized phase. Also developed in the mid-1950s (Baker, 1956; Sekhar and Drake, 1958; BakerandFoden.1960; Baker and Stokes, 1960),thisform ofNR stillhasa sizable market of some 3000 tons/yr. The vulcanized phase is produced from prevulcanized latex. The process described by the Rubber Research Institute of Malaya (1957) involves the addition of a stable suspension in water of the chemicals necessary for vulcanization (sulfur, zinc oxide, zinc diethyldithiocarbamate, and mercaptobenzthiazole) to field latex, followed by their reaction over 2-3 hours at about 85°C.The vulcanized latex is then blended with diluted field latex in the proportions required to obtain the given percentageof vulcanized phase, coagulated,anddried in thenormalway.Morerecently,grades in which the vulcanizedphase is based on sulfur,zincdiethyldithiocarbamate,andtetramethylthiuramdisulfidehavebecome available. Superior processing rubber prepared by this method has been shown (Baker, 1956) by electron microscopy to consist of particles of vulcanized rubber dispersed in a matrix of normal crepe, each particle consisting of an aggregate of rubber molecules crosslinked by polysulfides. The superior processing properties of this rubber are attributed to the persistence of this system throughout the stages of rubber manufacture.
75
Modified Natural Rubber Table 6 Grades of Superior Processing Rubber Available Vulcanized phase Grade
OilRaw phase rubber
SP20 SP40 SP50 PA51 PA80
20 40 50 80 80
80 60 50 20 20
0 0 0 40 0
source: MRpRA Natural Rubber Technical Information Sheet D47. 1979. Hertford. United Kingdom.
Five main grades of superior processing rubber are available; as can be seen in Table 6, the numeral in the grade coding denotes the nominal percentage of vulcanized phase in the mixture. Grades containing up to 50% vulcanized phase are coded SP for superior processing. while those above 50% are masterbatches and are coded PA for process aid. These superior processing rubbers contain no diluents orfillers except for PA57, which is a 40-phr oil extended version of PA80 and thus contains 57% crosslinked rubber, 14% unmodified rubber, and 29% oil. As implied by their name, these superior processing rubbers provide distinct manufacturing advantages. Their two-phase structure increases the stiffness and Mooney viscosity of rubber mixes (both natural and synthetic) and at the same time improves flow behavior in extrusions, etc. The result is faster production rates, as shown in Table 7, with various loadings of calcium carbonate. In contrast to some other processing aids, SP rubbers do not affect the properties of the final vulcanizates, since, of course, the processing aid is modified natural rubber itself. This is exemplified in Table 8, which also showsthe need for adjusting the vulcanizing system to allow for the reduced amount of unvulcanized rubber in the formulation. The SP rubbers give advantages in calendering, extrusion. molding, reworking,pan curing, and continuous vulcanization processes. The crosslinked phase imparts much-improved shape retention on extrusion and eliminates porosity in high-temperature curing. During calendering these rubbers result in better surfacefinish, improved gauge control, lessshrinkage, and reduced temperature sensitivity. They can reduce air-trappingproblems in molding andgive less softening of mixes on reworking of compound. For pan curing, they eliminate collapse or sag problems and reduce cloth and water markings. Finally, they have particular value in the liquid curing medium (LCM) process, where the compound is extruded into liquid salt at temperatures up to
Table 7
Viscosities and Extrusion Throughput of SP20 Compared to NR Mooney viscosity
(phr)carbonate Calcium
0 20 50
54
Throughput (g/min)
NR
SP20
NR
SP20
30 38 40
35
200 320 420
1000 940 600
55
Source: MRPRA Natural Rubber Techn~calInformation Sheet D47, 1979. Hertford, United Kingdom.
Baker
76
Table 8
Retention of Properties on Addition of S P Rubber
Formulation Natural rubber 80% crosslinkcd NR (PA80) Zinc oxide Stearic acid MBTS“ DPG” Sulfur Properties“ Hardness, IRHD Tensile strength, MPa Elongation at break, Q Modulus at 300%. MPa l’
100 0 3 I 1 0.1
3 44 26 735 1.7
80 20 3 1
0.92 0.09 2.76 44 28 745 1.7
60 40 3 l 0.84 0.08 2.52 45 28 750 1.7
40 60 3 1
0.76 0.08 2.28 45 26 740 1.8
20 80 3 1 0.68 0.07 2.02
46 26 750 1.6
0 100
3 1 0.6 0.06 1.8 44 25 735 2.1
2.2-Dibenzothiazyl disulfide.
” N,N-Diphenylguanidinc.
‘ Following cure for 20 nun at 148°C. Source: MRPRA Natural Rubber Technical Information Sheet D47.
1979, Hertford, Unlted Kingdom.
200°C. In this continuous vulcanization process, the normal practice is to extrude the rubber at as high a temperature as possible to ensure rapid vulcanization on entering the salt bath, thus reducing the chance of collapse or change of profile and maximizing production output. This, however. results in a soft extrudate liable to collapse and porosity problems due to trapped air or moisture. Replacement of 50% of the rubber with 80% crosslinked natural rubber (PA801 eliminates these problems and at the same time reduces die swell. These modified forms of natural rubber may also be blended with synthetic rubbers to give similar processing advantages. 9.
ENPCAF-MODIFIED NATURAL RUBBER
To some extent,ethyl N-phenylcarbamoylazoformate (ENPCAF)-modifiednaturalrubber is only a scientific curiosity in the same vein as hydrogenated NR. However, its preparation gave an early indication as to how the physical properties of NR might bechanged if it were chemically modified by the systematic addition of a known number of polar pendent groups. During the mid-l960s, attention turned to the “ene” addition as a clean, thermal cycloaddition reaction to modify NR, shown in general folm in Figure 7.
77
Modified Natural Rubber Table 9 Second-Order Rate Constants for Addition of Azo
Compounds to 2-Methylpent-2-ene compound Azo
k2 (liter mol" s"
Ph.CO.N=N.CO.Ph EtO.CO.N=N.CO.Oet Ph.NH.CO.N=N.CO.NH.Ph Ph.CO.N=N.CO.Et Bu.NH.CO.N=N.CO.Oet Ph.NH.CO.N=N.CO.Oet (ENPCAF) Source:
X
IO')
2.20 3.95 5.53 19.5 109
Porter, 1978.
In the first instance, X = Y was a nitroso group adapted by Cain et al. (1968) for the introduction of rubber-bound antioxidant functions. The addition of p-nitrosodiphenylamine during black mixing of NR resulted in a network-bound p-phenylenediamine. Another aspect of this reaction utilized novel urethanes for the generation of nitrosophenols in rubber and their addition to give pendent aminophenol groups for crosslinking with diisocyanates (Baker et al., 1970). Attention was then turned to the azo-ene addition reaction, which was well known to be highly efficient, since it had been used in attempts by Flory and coworkers (1949) and Van der Hoff and Buckler (1967) toprovide quantitative correlation between the modulus and the degree of crosslinking of olefinic rubbers. However, the rate of reaction is relatively slow with trialkyl ethylenic olefins like NR, and so the effect of structural alterations of the azo compound on rates of reaction with 2-methylpent-2-ene as a model was examined by Knight et al. (1974). Table 9 (Porter, 1978) shows the second-order rate constants found for the addition of these various azo compounds at 100°C in benzene. This work (Knight et al. 1974) indicated that ENPCAF might have a practically useful rate of addition to NR, and its use as a means of chemically modifying lvbber itself has been studied by Barnard et al. (1975). Themodification can be carried outin dry rubber in an internal mixer or on a mill in about 7 minutes at 1IO"C, or at the latex stage in a few hours at 33°C. The addition reaction results in the formation of hydrazoester pendent groups as shown in Figure 8. ENPCAF-modified natural rubber can be vulcanized with sulfur systems in the normal way, although at modification levels above 2 mol% substantial increases in sulfur/accelerator
H-N
\N.CO.NH.Ph Fig. 8 Reaction of ENPCAF with NR. (From Porter, 1978.)
78
Baker
Resilience
A
-50
-2575
050
25
Temperature ("C) Fig. 9 Changeintemperature of minimumrebound with increase of ENPCAFmodification.(From MRPRA Annual Report, 1975.)
concentrations are found to be required, and therefore peroxidecuring ispreferred. The effect of ENPCAF modification of natural rubber upto 1S mol% has been examined. and some interesting changes in propertiesoccurasaresult of introducing such highly polargroups.Evenata concentration of only 1 mol% they interfere with the alignment of the polymer chain to retard the rate of crystallization. The modification also increases the glass transition temperature, thus altering the temperature of minimum rebound as shown in Figure 9. In practice this means that the modified rubbers are highlydamped compared toNR itself. Furthermore, changes havebeen found in gas permeability and solvent resistance properties, where NR is notably inferior to butyl and nitrile rubbers, respectively. Laboratory values of gas permeability and linear swelling found for various mole percentages of ENPCAF-modified natural rubbers are shown in Table IO.
Table 10 Permeability and Solvent Swelling of ENPCAF-Modified NR Modification (mol%)
0 5 10 15
Permeability constant (NI) (cm' S-' atm" X IO')
Modification (mol%)
Linear swelling"
3.50 2.04 0.98 0.52
0 3.65 5.12 6.75
1.51 1.37 1.31 1.27
(1/1,,)
I,, = Initial length; I = length after swelling to equilibrium m 60-80" petroleum ether at 20°C. Source: Bamard et al.. 1975. "
Modified Natural Rubber
79
There is a significant reduction in gas permeability, since the equivalent figure quoted by Van Amerongen ( 1 947) for a medium nitrile rubber would be 0.5 cm2 S" atm" X I O x at 17°C and that of butyl rubber 0.11. Solvent swelling was also improved, and modification with 10.75 mol% of p-nitro-ENPCAF was reported to reduce linear swelling still further to 1.12. However, ENPCAF-modified natural rubber never became a practical proposition. partly because of the high costs that would have been involved for the levelsof modification required for a significant change in properties, and partly because of undesirable side effects introduced by this particular reagent. These were an unacceptablyhighcompression set andatendency for asubstantial increase i n modulus during oxidativeaging. Excluding cost, these two latter features alonewould have been sufficient to preclude commercialization, but the work did indicate that chemical modification of NR to a significant mole percentage could alter some of its physical properties where it was at a disadvantage compared to certain speciality synthetic rubbers. Other uses have been made of the ENPCAF addition. since its aromatic ring provides a straightforward means of introducing a variety of functional groups to be attached to NR. Examples of these (Porter, 1978) are cinnamate (-CH==€H-COOEt) for photosensitized crosslinking and trialkoxysilyl [-CH2--CH2-Si(OCH3)3] for coupling to silica. However, as stated in the introduction, these do not constitute a modified form of natural rubber and will therefore not be discussed further.
10. POLYSTYRENE-GRAFTED NATURAL RUBBER The advent of synthetic thermoplastic rubbers in the early 1970s produced a new challenge-to modify natural rubber to introduce this feature. In the case of the synthetic polymers, there is ample opportunity to control chemical structure during manufacture, and thus the styrene-diene block copolymers were designed to give thermoplasticity. Such elastomers possess the elastic properties of permanently crosslinked rubbers and yet can be melted and injection molded in a similar manner to thermoplastics. The most common type is the A-B-A block copolymer from styrene and butadiene. The butadiene can first be polymerized followed by the polymerization of styrene onto both ends, or alternatively the styrene ends can be polymerized first followed by butadiene and then the two coupled together. Both routes result in a rubbery polybutadiene central unit with hard polystyrene ends. Thepolystyrene segments form glassy regions at normal temperaturesactingascrosslinksbetween the softelastomericpolybutadiene chains, which providetherubberproperties. Natural rubber from thetree, with its highmolecular weight polyisoprene structure, cannot be used in such reactions, and other means of producing such thermolabile crosslinks must be used. Several workers (Falk et al., 1973; Sigwalt et al., 1976; Sundet et al., 1976; Foss et al., 1976) have reported that suitably constructed graft copolymers with elastomeric backbones and hard side chains can behave as thermoplastic rubbers, and this route has been adopted by Campbell et al. (198 1 ) for natural rubber. The previous preparative methods of grafting on a hard segment by polymerization onto backbone initiation sitesare in practice not applicable to NR by cationic and anionic mechanisms owing to the presence of rubber constituents. The old methods using free radical polymerization for poly(methy1 methacrylate)-grafted NR are also unsuitable, since they cannot provide the necessary control of molecular architecture to develop thermoplastic rubbers. As a result, novel a procedure was developed of preparing the side chains of the graft copolymer as a separate synthesis and attaching the resulting prepolynler to NR via a reactive end group. The addition reaction used was again of the thermal "ene" type, as describedby Hoffmann (1969), sincethis does not requireacatalystand all thedoublebonds of thepolyisoprene backboneareavailable for grafting. The startingpoint for Campbell et al. (1978b) wasthe preparation of a monofunctional hydroxy polystyrene, whichwas converted to the azodicarboxy-
80
Baker
0 N - NH-C-OEt N=N-C-OEt
0- Polymer Fig. 10 Addition of azodicarboxylate-tipped polystyrene to NR. (From Campbell et al.,
I981 .)
late functional polystyrene. This was found to react with NR in cyclohexane solution at 60°C over a period of days to yield the modified NR as shown in Figure IO. However, to have any practicalutility,thismodification,designed to giveathermoplastic comb graft of thetype schematically indicated in Figure 1 1, must be capable of being carried outin the dry rubber phase. It has been found that thiscan indeedbe achieved, provided that two well-defined conditions are satisfied. First, the temperature of the mixing must exceed the softening temperature of the azotipped polystyrene, and second, the mix must be subjected to high shear for at least part of the mixing cycle. Given that these conditions are met, grafting occurs over about I O minutes at 90°C or above. This is indicated by an increase in mixing torque as shown in Figure 12. When the same mixing operation was carriedout under the same conditions with untipped polystyrene, the torque increase in the 4- to 8-minutes region was not observed. Campbell and coworkers haveestimated the grafting efficiencyof the modification reaction by GPC analysis for bound and freepolystyrene. For the purposesof establishing if the nonrubber constituents of natural rubber affect the efficiency, a chemically clean synthetic polyisoprene
Unmodified NR chain
+
+ Reactive prepolymer I
Fig. 11 Formation of comb-grafted natural rubber by its reaction with an azo-tipped prepolymer. (From Campbell et al., I981 .)
81
Modified Natural Rubber
Backbone:
NR (SMRSL).
Azo Polyktyrene): mn 8,200, 40% w/w.
l
2
4 6 8 1 Mixing Time, (min.).
0
Fig. 12 Graftcopolymerformation in an internal mixer. (From Campbell, 1981.)
was also examined. The comparativegrafting efficiencies are shown in Table 1 I . The grafting efficiency for the modification by dry mixing of naturalrubber is clearlylessthanthat of the synthetic polyisoprene, especially at the lower polystyrene concentrations. However, the nonrubber constituents do not seriously inhibit the process at high polystyrene levels, and combgrafted natural rubber has been prepared in the laboratory on a scale of up to 1 kg. These graft copolymers obtained by the dry mixing of NR withtheprepolymers are translucent thermoplastic materials soluble in the solvents that dissolve polystyrene and polyisoprene. At normal ambient temperature they are tough and flexible, and yet they can be compression molded at 150°C into sheets for test purposes. In spite of their lower grafting efficiencies, the copolymers from natural rubber exhibit higher tensile strengths than those from synthetic polyisoprene with values that vary according to polystyrene content (Fig. 13). In addition to this dependence of tensile strength on total polystyrene, it has been observed that the tensile properties are affected by the molecular weight of the polystyrene chains. Thus, the highest tensile strengths were obtained when polystyrene molecular weights were between 7000 and 8000. This variation of tensile properties with molecular weight of the polystyrene segments has no direct equivalent in the styrene-butadiene A-B-A block copolymers, since the polystyrene molecular weightcannot be altered in these without altering their basic composition. These comb-grafted copolymers therefore indicate that natural rubber can be modified to producethermoplasticelastomers of knowncompositionthatbehavesimilarly tothe A-B-
Table 11 Grafting Efficiencies for the Dry-Mix Modification
of Natural Rubber and Synthetic
Polyisoprene Polystyrene (wt%) 25 Natural rubber
Synthetic polyisoprene Source: Campbell et al., 1981.
30
35
40
45
50
ss
33
40
45
48
l7
76
l5
53 l8
5.3
77
60 16
20 29 12
76
Baker
82
Tensile strength (MPa)
20
30 40 50 Polystyrene content ("h wlw)
Fig. 13 Variation of tcnsile strength with total polystyrene content for graft copolymers prepared from azodicarboxylate-functionalpolystyrene and NR or polyisoprene. (From Campbell et al., 1981.)
A-type block copolymers. However, they are structurally different and therefore show some differences in physical property performance. Thermoplastic natural rubber can also be achieved by simple blending with polyolefins; this route is discussed separately in Section 14.
11. EPOXIDIZED NATURAL RUBBER The trialkylethylenic double bonds of natural rubber are well known to react very readily with peracids to yield epoxide groups. Indeed, the very cheapness of modifying natural rubber in the latex stage with hydrogen peroxidelformic acid has led to this process being extensively studied (see Greenspan, 1964, and Gonsovskaya et al., 1971) since its early discovery by Pummerer and Burkard in 1922. In spite of their apparent promise, these reactions often led to materials containing products of subsequent ring-opening reactions (Bloomfield and Farmer, 1934; Ng and Gan, 1981) and thus proved of little interest. Even the introduction of epoxide groups into natural rubber at a low level for crosslinking by Colclough in 1962 proved to be a failure owing to their lack of reactivity. More recently, however, it has been suggested by Burfield and Gan ( l 975) that storage hardening of natural rubber might be caused by the ring opening of epoxide groups by amino groups giving subsequent crosslinking. These observations prompted Gelling ( 1 985) totake a more carefullook at the epoxidation reaction. He established that theacidconcentrationand the temperature of theepoxidation reaction control the extent of secondary ring opening, and this in turn substantially affects the properties of the materials obtained. Thus, the ring-opening reaction to give furan formation can be avoided in all cases, except for the 100 mol% conversion when a white thermoplastic powder termed "furanized NR" is obtained. This has a broad Tg of around lOO"C, can be injection molded, and resembles polystyrene in its impact and stress-strain properties. However, it is not a rubber, and it would be too costly to compete in the plastics field. More promising
-"
83
Modified Natural Rubber
peracetic acid
NR
ENR
- 0
-
-
0
OH X
Fig. 14 Epoxidation of NR and furan formation. (From B a k u et al., 1985.)
are the 50 and 25 mol% epoxidized materials that have been termed ENR-50 and ENR-25. It was found that two distinct types of products can be isolated in the peracetic acid modification of natural rubber. At low acid concentrations and moderate temperatures, epoxidized natural rubber (ENR in Fig. 14) was obtainedas the sole product. However,if high reagent concentrations and temperatures are used, then the furanized materials are produced. The latter give leathery, limp vulcanizates with poor tensile properties and are of little technological interest. Epoxidized natural rubber, on the other hand, is an exciting material whose vulcanizate properties can be considered to more closely resemble those of some of the speciality synthetic rubbers than those of NR itself. In theory, any level of epoxidation can be achieved, but to date only up to 50 mol% has been carried outcommercially,although 70 and 75 mol%materials have been made in the laboratory. The epoxidation has been shown by "C-NMR to be totally random, with one, two, and three adjacent epoxide groups present at levels consistent with those predicted statistically. Vulcanization of ENR can be carried out with the normal sulfur formulations used for natural rubber, but it has been observed that conventional cure systems based on 2.5 phr sulfur and 0.5 phr sulfenamide result in vulcanizates with poor aging characteristicscompared to those of unmodified NR. This is reported by Gelling and Momson (1985) to be caused by a totally different aging mechanism, in which sulfur acids from the oxidation of sulfides attack epoxide groups to give crosslinks. This is consistent with the observed features of ENR aging-ring opening of epoxide groups with subsequent increase in Tg and increase of modulus. The use of semi-efficient or efficient vulcanization typesof cure system is therefore recommended, when aging is then found to be not too dissimilar from NR itself vulcanized with the same formulations. The most interesting effects of this modification of natural rubber, however, lie in the physical properties of vulcanizates. The earlier changes indicated by the ENPCAF modification (see Section 9) are fully exploited in ENR, where three precise epoxide levels of 50, 25, and 10 mol% have been studied in detail by Baker et al. (1985). First, as the level of epoxidation is increased, the glass transition temperature is raised by approximately 1°C per mole percent epoxidation. and this results in a very substantial drop in room-temperature resilience for ENR-
a4
Baker
Table 12 Physical Properties of ENR Vulcanizates Compared to NR' Black filled NRENR-50 Tensile strength, Mpa Elongation at break, % Modulus at 300%, Mpa Hardness, IRHD Resilience at 23"C, % Cresent tear, N/mm Compression set, 1 day at 70"C, % DIN abrasion, % ARI Akron abrasion, v01 loss mm'/1000 rev Heat build-up, "C Ring fatigue, 0- loo%, kcs Aged 3 days at 100°C, % retention in properties Tensile strength Elongation at break Modulus at 300% Hardness change
28 560 13 65 61 150
25 100
0.05 18
88 86 79 128 +4
ENR-25 27 530 66 44 71 21 79 0.03 20 77
25 520 12 68 26 57 37 65 0.03 27 237
86 83 128 +4
89 73 149 +2
13
Formulations as follows: Rubber, 100 (NR was SMR-L. ENR-25 and ENR-S0 was Epoxyprene 25 and Epoxyprene 50 from Kumpulan Guthrie Berhad); zinc oxlde, S; stearic acid, 2; antioxidant (poly-2,2.4-tr1n~cthyl-1,2-dihydroquinoline), 1 ; MOR. 1.7; sulfur, 1; black (N330). SO; and Dutrex 737MB. 4. Source: Kumpulan Guthrie Berhad Epoxyprene booklet Document No. ENR-01-5.
l'
50. This is evident in Table 12, which shows the physical properties of black-filled ENR-50 and ENR-25 vulcanizates compared to NR. As epoxidation levels increase, room-temperature resilience decreases, making ENR a highly damped rubber. However, at elevated temperatures above their Tg, the epoxidized natural rubbers revert to the high resilience of NR itself, and this is the reason for their still exhibiting good heat build-up characteristics. It should also be noted that in spite of the modification, epoxidized natural rubber up to 50 mol% is still a strain-crystallizingrubber and thus exhibits hightensilestrength. X-ray diffraction studies by Davies et al. (1983) have shown that ENR does indeed undergo strain crystallization. The stereospecific epoxidation reaction retains the cis- 1,4 configuration of the polymer, since the relatively small oxygen atom can fit intothe NR crystal lattice without strain. However, above 50 mol%, a distinct reduction in the degree of crystallinity has been observed. Again like ENPCAF modification, the epoxidation process improves the oil and solvent resistance of NR. This is shown in Figure 15 for ENR-25 and ENR-50 against NR, polychloroprene, and nitrile rubber. It can be seen that in ASTM oils Nos. 1, 2, and 3 after 70 hours at 100°C, ENR-25 does exhibitbetter oil resistance thanNR, andthis might be suitablefor applications where the components are only splashed with oil in service. ENR-50, however, can be seen to be much more oil resistant. It is better than polychloroprene in ASTM No. 1 and 2 oils (blend of mineral oils and neutralized naphthenic distillate, respectively) and virtually the same as a mediumnitrilerubberwith 34% acrylonitrilecontent. In ASTM No. 3oil(neutralized naphthenic distillate with lower flashpoint), ENR-50 is as good as polychloroprene but not quite as oil resistant as the medium nitrile rubber. Even so, ENR-50 has a remarkable oil resistance for an "NR-based" elastomer and can be used in place of nitrile rubber in many applications such as oil suction hose, seals, oil-well pipe protectors, and so on.
85
Modified Natural Rubber
Volume swelling %
Fig. 15 Oil resistance of ENR-25 and ENR-50 compared to NR, CR. and NBR after 70 hours at 100°C in ASTM No. 1,2, and 3 oils. (From Kumpulan Guthrie Berhad Epoxyprene booklet, Document No. ENR03-1 .)
Another property that the epoxidation process alters is gas permeability, and there have been a number of studies of this in view of its potential for tire liners. Table 13 shows the comparative air permeabilities of various rubbers. It can be seen from the table that ENR-50 has an order of air permeability similar to that of butyl rubber. While it has been shown that in fact 70 mol% epoxidation is required to match butyl rubber per se, in practice 50 mol%
Table 13 Comparative Air Permeabilities of Rubbers at 30°C Rubber
Air permeability
Natural rubber ENR-25 ENR-50 SBW500 Butyl Nitrile ACN) (34%
100
Kumpulan Guthrie Berhad booklet Document No. ENR-M- 1.
Epoxyprene
Source:
8 48
6 4
Baker
86
Table 14 Comparison of Black and Silica-Reinforced ENR Vulcanizates ENR-25 Silica"Blackd Black" IRHD MPa
Hardness, 67 Modulus at 300%. MPa strength, Tensile Elongation at break, '7c 405 Abrasion, Akron. v01 loss, mm7/500 rev DIN, mm Goodrich heat build-up from 100°C. "C "
69 68 12.4 25.4 435 14
272 7
ENR-5 Silica" 12.8 21 .o 15 250 7
73 13.5 24.5 500 11 278 23
12.6 22.4 435 14 289 19
50 parts phr black (N330) or 50 parts phr Silica (Hi-Si1 2 3 3 ) .
Sourc~c Bakcr et al.. 1985.
modification would be adequate, since the halobutyl rubbers used in tire liners are normally blended with up to 30% natural rubber to improve adhesion onto the carcass. It is worth a brief mention here that ENR-50 has been found to give no better adhesion to natural rather than butyl rubber, since at this extent of modification the nature of the polymer has changed from that of natural rubber. Similarly, like nitrile rubber, ENR-50 is not compatible with NR. Another feature imparted to NR by epoxidation is an inherent reinforcement with silica (without coupling agent). It has been reported that both 25 and 50 mol% epoxidized natural rubber exhibit the same properties when reinforced with silica as with a highly reinforcingblack. such as N330, as shown in Table 14. Hardness, modulus. and tensile properties are essentially the same for both black and silica fillers and, more important for tire applications, abrasion and heat build-up characteristics are also similar. Interaction of the epoxide groups with the silica surface is believed to be responsible for this reinforcement. The modern trend for tiremakers is towardsimprovement in wet traction and rolling resistance.Many of thesyntheticrubbermanufacturers in recentyearshavemodifiedtheir polymerization process to develop new polymers to provide these properties; for example,Shell, with their Cariflex S1215, Huels (Nordsiek, 1984), and Bayer with their vinyl-BR (Marwede et al., 1993). However, it has been shown by Morton and Krol (1982) that most of the general purpose rubbers fall on a wet grip versus rolling resistance line, as shown in Figure 16. Thus, mostattempts to improve wet grip by, for example, oil extensionaredetrimental to rolling resistance (fuel consumption) and vice versa. Some of the new synthetic rubbers, however, due to careful design of their molecular architecture, have been reported to fall off the Morton and Krol line and provide improvements in both wet grip and rolling resistance. One featureof these changes is an alteration in glass transition temperature of the rubber, and thus the ability to alter the Tg of natural rubber by the epoxide modification route was thought to make it suitable for a tire tread rubber. In addition, silica reinforcement generally provides good rolling resistance characteristics, and therefore the inherent reinforcement with silica could be added to the equation. The results of the wet traction performance of radial passenger tires retreadedwith various epoxidized natural rubber formulations hasbeen published by Baker et al.(1985). and the overall rankings are given in Table 15. This indicated that the epoxide modification process gave a substantial improvement in wet traction over NR and that ENR-25 with silica, or black and silica, reinforcement provides betterwet grip than even oil-extended SBR. The same compounds
87
Modified Natural Rubber
Wet grip SHELL S 1 2 1 5 8
8 8 0 i i e x t e n d e d SBR
8
8 8
8 8
8
8
8 8 8
natural rubber 888m(isoprenerubber 88 8 highvinyl butadienerubber 8 8 8 8 8 8 8 8
8 butadiene rubber
I
I L""C
a0
I
120
resistance Rolling
I I L,
I
100
I
95
1
100 Fuel consumption
105
Fig. 16 Morton and Krol diagram of wet grip vcrsus rolling resistance. (From Morton and Krol. 1982.)
were examined for their rolling resistance characteristicson a rolling wheel test rig; those findings are shown in Figure 17. In this method of presenting the data, the lower the rating, the lower the rolling resistance, and hence the NR control with its inherently low rolling resistance is substantially below that of the oil-extended SBR. The ENR-25silica- and black silica-reinforced compounds offer even further improvement over OESBR, and this, combined with their wet grip performance, places then1 in the upper left-hand segmentof a Mortonand Krol-type diagram (Fig. 18). The potential for 25 mol'% epoxidized rubbers in tires is therefore quite high, although thewearindices of the silica-reinforcedrubberswerefound to be relatively poor. Since in
Table 15 Ovcrall Ranklngs of Wet Grlp of Various ENR Formulations Compared t o Reference Compounds Best
Poorest
ENR-25 ENR-25 ENR-25 ENR-30 ENR-25 OESBR ENR-20 NWENR-50 ( 80/20) NR
Silica 15/35 BlacWsilica 35/15 BlacWsilica Black Black Black Black Black Black
88
Baker
Fig. 17 Rolling resistance ratings for steel radial tires retreaded with various rubbers. (From Baker et al., 1985.)
Wet skid rating (pebble from 40kph)
lI
ENR-25 15/35
110 105
ENR-25 35/15 BlackiSilica
. 0
0
ENR-25 Black
.
e
OESBR
#
A
0 -
*. NR Black
Rolling resistance rating
Fig. 18 Wet grip rating (pebble) versus rolling resistance for steel radial tires retreaded with various rubbers. (From Baker et al., 1985.)
89
Modified Natural Rubber DIN abrasion index 130r 120OESBR 90 BR35 0
11010090-
0OESBR
80 -
'
70 100 ENR-25 BR
I
20
90 ENR-25 10 BR
I
I
80 ENR-25
70 ENR-25 30 BR
ENR-251BR ratio
Fig. 19 Effect on DIN abrasion of addition ofBR to ENR-25. (From Baker et al., 1987.)
practice most tire formulations contain a proportion of polybutadiene to improve wear, addition of BR was investigated (Baker et al., 1987). In the laboratory, replacement of 10, 20, and 30 parts of ENR-25 with BR (Fig. 19) showed an increase in the DIN abrasion index, suggesting that tire wear performance should be improved. This led to blends of ENR-25 with both BR and BR/NR being evaluated in tires for wet grip, rolling resistance, and wear. The results of the Morton and Krol-type plot (Fig. 20) were reasonably understandable, giving trends for the most part in line with expectations. However, the wear data on the same tires used for these plots were wholly inconsistent and showed no promise whatsoever. It was concluded that there would almost certainly be a compatibility problem along with a likelihood of the silica becoming preferentially located in the ENR-25. This would have led to the anomalous results which could not be explained in the light of the laboratory data in Figure 19. Epoxidized natural rubber has now been established as a commercial material and is used in a variety of applications where its unique combination of properties can be used to their full advantage. It is important that it is regarded as a new polymer, rather than another form of NR, its properties being so different from the latter. In recent years there has been a spate of papers on ENR, many of which have been on blends of ENR with a whole variety of other elastomers, e.g., Varughese (1990), Brown (1991), Alex (1991), Roychoudhury (1992), Byung(1993), Ramesh (1993), Roland (1994), Bandyopadhyay (1995), and Groves (1998). While such work has continued with solid ENR, there has been considerable interest also in ENR latex, which clearly has potential for oil-resistant gloves, balloons, and a variety of adhesive applications. The problem has always been one of finding a suitable concentration technique since conventional latex concentration processes are not applicable to ENR. Thedifference in specific gravity between the rubber and serum in ENR latex is too small for the normal centrifuging or creaming methods, while its heat sensitivity eliminates theuse of evaporation techniques. However. Nambiar (1993) has established that the ultrafiltration process is a practical means of concentrating ENR latex, achieving a 60-6596 total solids level. While such a material is suitable for the adhesives industry, it must be remembered that the presence of the nonionic surfactant in ENR
90
Baker
Wet skid rating (Concrete from 40kph)
60ENRi20NRi20BR
115 -
BlWSi
0
0
J
0 0
110 -
M
70ENRi30BR
Elk6
0
70ENR/30BR '
105 -
0 0
0
0
Black
m
00
60ENRilONRi30BR BlWSi
0
8
0
H
0
80ENRi20BR (I BlWSi 00 NR
100 -
0
I 90
I
I
100 Rolling resistance rating
110
0
120
Fig. 20 Wet grip rating (concrete) versus rolling resistance for steel radial tires retreaded with various ENR-25 blends. (From Baker et al., 1987.)
latex is not amenable to normal coagulant dipping processes, and therefore its widespread use in the latex industry is still to come.
12. HYDROGENATEDEPOXIDIZEDNATURALRUBBER The chemical modification of diene elastomers in general has attracted attention in recent years because of their ease of reaction and likely retention of their characteristic physical properties. Thus, we have seen the adventof hydrogenated nitrile rubber with good aging and heat resistance combined with its original oil-resistant properties. It is not surprising therefore that there have been a number of workers examining the feasibility of making hydrogenated ENR to prepare a similar oil-resistant polymer with much improved heat and oxidation aging behavior. Roy ( 1993) and Bhattacharjee (1993) have hydrogenated both ENR-25 and ENR-50 with a palladium acetate catalyst at 50-60°C. The degree of hydrogenation of ENR-25 was 80% and that of ENR-50, 75%. With NR itself almost 85% hydrogenation was achieved. Hence it is clear that the degree of hydrogenationobtainable is reduced by the level of epoxidation. It was observed also that there was no changein the epoxide content of the polynler after hydrogenation confirming that the palladium acetate reduces the carbon-carbon double bonds selectively in the presence of the epoxy groups. Both ENR-50 and its hydrogenated counterpart were aged at 70°C for 48 hours. The resulting IR spectra of the products showed that the increase in carbonyl peak at 1733 cm" wasmuchlesswithhydrogenatedmaterial,indicatingitsmuchgreater oxidative stability. The gel content, green strength, modulus at 300% extension, and elongation at break were also measured for the modified ENR-50. It was found that the gel content of the ENR and hydrogenated ENR were virtually the same (60% and 62%), indicating that the hydrogenation does not alter the gel content. However, there was a significant difference in gel
Modified Natural Rubber
91
Table 16 Physical Properties of HydrogenatedENR (Semi-EV cure)
40-H-ENR-55 36-H-ENR-57 Epoxidatlon Hydrogenation first first Modulus at 100% extension, MPa Modulus at 300% extension, 1.68 MPa strength, Tensile MPa Elongation at break, %
0.69 12.1
640
0.84 1.62 1.8
335
content between the two rubbers when they were aged. The ENR-50 showed a considerable increase in gel content on aging (from 62 to 77%), while the hydrogenated ENR only showed a small increase(from 60 to 66%). This suggested a lowamount of oxygenated group formation, due to the reduced level of unsaturation, in the hydrogenated ENR. The hydrogenation was also found to reduce green strength (from 0.65 to 0.57 MPa) and modulus at 300% extension (from 0.26 to 0.22 MPa), which was purported to be the consequence of increased chain flexibility due to the reduction in double-bond content of the hydrogenated ENR. Anotherworkerin the field of hydrogenated ENR wasMente (see Baker, 1992). He realized that there are two routes to make the hydrogenated material. Starting from NR. such a new polymer can be produced by either preparing the epoxidized rubber first followed by hydrogenation or vice versa. Mente attempted both routes and obtained some preliminary properties of both products as shown in Table 16. The “36-H-ENR-57” was made by preparing ENR-57 by the normal epoxidation process and then carrying out the hydrogenation to 36 mol%. This material showed reasonably good properties bearing in mind that the hydrogenation step was extremely difficult and only small amounts were produced. It also showed a typical hysteresis temperature plot as shown in Figure 21. However, when hydrogenated ENR was prepared by hydrogenation first followed by the epoxidationreaction to produce “40-H-ENR-55,” or 40 mol%hydrogenatedand 55 mol% epoxidized. the material was totally different with poor tensile strength as shown in Table 16 and a very broad hysteresishemperature profile (Fig. 21). This was believed to be caused by the hydrogenation step first causing hydrogenated “blocks,” while the other sample (which was epoxidized randomly first) behaved normally. It seems unlikely from these various studies of hydrogenated ENR that it has any particular merit, and, bearing in mind the complexities of its preparation, it would not appear to warrant any commercial exploitation.
13. LIQUID NATURALRUBBER Liquid NR is not a modified form of natural rubber in the same sense as those discussed so far. The early materials, known as depolymerized natural rubber, were dark semi-liquid sticky substances made by various peptized/breakdown processes carried out over a period of time at hightemperatures in internal or Z-blademixers. In general they weremessymaterials,but they had the virtue of being pourable with their accompanying advantages, especially for the manufacture of prototypes. More recently there has been a great resurgence of interest in liquid rubbers, made by cleaner more sophisticated. chemical routes, to act as vulcanizable process aids. These are discussed in the second part of this section.
92
Baker
Tan 6
’e
3 + + + + +
36-H-ENR-57
1.5
-
+
0
0
0 0
+.
0
+
+
+
+a
+
L+
0 0 0 0 0
0 0
1.0 -
0.5 -
0
-40
-20
0 Temperalure In Celsius
20
40
36-H-ENR-57 Epoxldation first, Hydrogenation second 40-H-ENR-55Hydrogenation first. Epoxidalionsecond
Fig. 21 Hystercsis temperature profile of H-ENRs compared to ENR-50. (From Baker, 1992.)
The original “fluid” rubber, as firstpatented by Stevens in 1933, was simplyasoft composition made by adding a high proportion of mineral oil, or low-melting mineral jelly, to rubber so as to render it pourable at room temperature. Addition of vulcanizing ingredients was then carried out to enable the mixture to be vulcanized or set. The advantage of the material was that it could be poured into a mold, allowed to spread into position, and then be vulcanized to a hard or soft “rubber” depending upon the concentration of sulfur and/or accelerators used. The application for which this fluid rubber was first envisaged was printers’ rollers, but later uses were in textile machinery components and mold making. Liquid rubber prepared by a depolymerization route was first described in 1944 by Hardman et al. in their patent concerning the method of making articles such as drums, cans, tanks, and pipes by casting depolymerized rubber. Another process for making fluid natural rubber. or Rubbone, was reported by Pike (1953). Rubbone was prepared by mechanically working a softened rubber, 20-30 Mooney, containing chemical plasticizers. This was carried out in a Zblade mixer heated to 120-140°C. After 6-8 hours, the viscosity was sufficiently reduced so that it flowed at the temperature of manufacture, and yet was still rubbery at room temperature. This material had a viscosity of 170,000-200,000 poises. Rubbone was a viscous liquid, so vulcanizingingredients and fillersweredispersed in oilandblended in theZ-blademixer. Amongthemanyapplications of thisearly commercial liquidrubberwereprinters’rollers, textile machinery parts, and prototype components for engineering uses. Present methods of making liquid rubberby the simple depolymerization process generally employ a peptizer and long mixing times in an internal mixer at about 120°C. Depending upon conditions, liquid rubbers of various viscosities or molecular weights are obtained. Some exam-
und
93
Modified Natural Rubber Table 17 Range of Depolymerized Natural Rubber Available TYPe
Viscosity (P)
Lorival R5 Lorival R25 Lorival R200 Hardman DPR-35 Hardman DPR-40 Hardman DPR-75 Hardman DPR-400 Hardman DPR-01
5,000 at 23°C 25,000 at 23°C 200,000 at 23°C 300-360 at 38°C 360-550 at 38°C 550-950 at 38°C 3000-5000 at 38°C 120 at 150°C
"
l'
Molecular weight 13,500" 23,000 52,000" 38,000" 40,000" 45,000" 80,000" 155,000"
Approxlrnntely. M,, .
ples are given in Table 17. These liquid rubbers based on depolymerized rubber are preferably compounded in a Z-blade mixer, the vulcanizing ingredients and fillers being added before the mineral oil. Alternatively, the dry powders may be dispersed in a portion of the oil in a ball mill, and the dispersionadded tothe liquid rubber in the Z-blade mixer followedby the remaining oil. These liquid natural rubbers can be used to prepare vulcanizates of very low hardness. but it should be noted that such vulcanizates have physical properties much inferior to those made from conventional dry rubber. This is because of their very low molecular weight and because there is no chemical means yet devised to rebuild the broken molecules during vulcanization. By suitable compounding, liquid NR can also be used to make ebonites that do have reasonable properties. A problem associated with these castable rubbers is one of porosity due to entrapped air. The usual practice to avoid this is to subject the final fluid compound to a vacuum before use. Two typical formulations for these depolymerized natural rubbers are shown in Table 18.
Table 18 Depolymerized NR Formulations General-purpose Depolymerized NR Light mineral oil Zinc oxide Stearic acid Calcium carbonate MBT Diphenylguanidine TMTD Antioxidant Retarder Sulfur Pourability pomt Cure Hardness. Shore A
100 175 5 2.5 200 1 0.5 0.25 2 0.75 5 Room temp. 2 hr at 100°C 22
Depolymerized NR Light mineral oil Calcium oxlde drying Stearic acid dust Hard rubber Acccleratof' Sulfur
100
agent
75 25 2 1s 4 50
40°C 4 hr at 140°C ca. 98
('Butyraldehyde-aniline condensate (Vulcafor BA). Sorrrcu: NRPRA Technwl Information Sheet No. 26. 1963 Welwyn Garden City, United Kingdom.
94
Baker
The pourability of depolymerized natural rubber makes it applicable to all casting operations. Molds cheaply constructed from sheet metal, glass, plastic, plaster, etc., can be used, and its lack of shrinkage ensures very faithful reproductions. The combination of such cheap molds with liquid NR provides a very economic route to the manufacture of large prototype moldings, which otherwise would have been prohibitively expensive. Two part compounds have alsobeen designed which are mixed immediately prior to use and then vulcanize at room temperature. These are particularly attractive for sealant and electrical applications. Ebonite compounds have been used for grinding wheels and hard rubber rolls, and to set paint brush bristles. In recent years there has been a renewed interest in liquid natural rubber (LNR) for a totally new application-as a vulcanizable process aid. Low nlolecular weight oils are used universally in the tire and general rubber goods industries as process aids. However, in a number of applications these oils result in an undesirable loss in final vulcanizate properties, such as hardness in tire bead compounds, which are difficult to process without the use of an oil. The concept of “reactive process aids.” which improve processing and then co-vulcanize into the rubber network, hasbeen established by the synthetic rubber industry. but a natural rubber-based equivalent would be considerably more cost-effective. Clearly the early liquid rubbers discussed above are totally unsuitable for such sophisticated use, but the advent of a new cleaner and morecarefullycontrolledcatalyticoxidation process to prepare LNR (IRCA. 1989) seemed likely to produce a more suitable material. This new LNR. named Irprene. was made by a redox reaction of natural rubber latex with phenylhydrazine and was scaledup to pilot plant production under a UNIDO contract in the Republic of Cote D’Ivoire. As well as a vulcanizable process aid. this LNR was also considered for the surface coating of powdered rubber chemicals (to eliminate dustand to enhance distributionand dispersion) and as aroute to avulcanizable phenylenediamine for a network-bound antidegradant system. In general terms, it is conmonknowledge that to act as a vulcanizable process aid, molecular weights of the order of 30,000 appear to be preferable. If the molecular weight is too low, say an M,, of around 9000. then the LNR does not achieve co-vulcanization becausethe molecular weight between crosslinks is roughly about 7000. However, if the molecular weight is too high. say around an M,, of 50.000. then the effectiveness as a process aid is much reduced. According to the report of the UNIDO workshop, the IRCA “Itprene C” liquid NR had too low a molecular weight (ca. 10,000) to act as a useful vulcanizable process aid. The studies at MRPRA (1989) into ENR production gave a spin-off reaction to prepare LNR when it was found that the addition of certain chemicals to destroy the hydrogen peroxide remaining at the end of the epoxidation process led to a substantial reduction in the molecular weight of the lubber. Further investigation revealedthat the extent of molecular weight reduction could be closelycontrolledand LNRs of varyingmolecularweightswerestudied.Table 19 shows how liquid natural rubber of molecular weight -33.000 can act as a vulcanizable process aid. There is therefore little doubt from this table that LNR of the correct molecular weight can act as a vulcanizable process aid. While the Mooney viscosity at the first stage was not quite as low as the process oil control. the compound was perfectly processable and the final vulcanizate showed enhancement in properties in terms of hardness, modulus, resilience. and abrasion resistance. Such new liquid natural rubbers therefore do have commercial potential. They are much cleaner, have controlled molecular weight, and can be used for the established uses of LNR as well as vulcanizable process aids.
14. THERMOPLASTIC NATURAL RUBBER As mentioned in Section 10. thermoplastic natural rubber can be obtained by the comb grafting of polystyrene onto the polyisoprene backbone. Such a route. designed to meet the challenge
95
Modified Natural Rubber Table 19 Vulcanizate Properties with LNR in Place of ProcessOil NR
Process oil LNR (M,, 33.000) 67 (ML + 4),69100°C First-stage mix viscosity Hardness, IRHD Modulus at loo%, MPa 15.0 13.6 Modulus at 300%. MPa 30.6 31.0 Tensile strength, MPa 575 Elongation at break, 8 545 73 Resilience, Dunlop 71 at 23°C. Q Abrasion, DIN index
100
100
90
15
10
62
65
15
60 59 I .9
9.8 29.5 615 67 84
2.5
90
2.8
91
of the synthetic A-B-A block copolymers, is unlikely to be economically feasible, but it has yielded interesting modified rubbers whose properties could be varied by changing the molecular weight of the polystyrene chains. No reader could fail to notice the growth of the thermoplastic elastomers over the last decade, and not surprisinglythermoplasticnaturalrubber (TPNR) has grown up with them. Although not a true modification of NR per se.it has to be includedsince it is clearly another form of NR that justifiably lies within the scope of this chapter. There have been many thermoplastic elastomers (TPEs) based on blends of synthetic rubbers with polyolefins. Some of the early materials by Uniroyal, Dupont, Exxon. ISR, etc. have been takenover by events and/or reorganization of supplier. The big names now are Santoprene and Geolast by Advanced Elastomer Systems, Telcarby Teknor-Apex. and Sarlink by DSM among others. In these blends the microcrystalline regions of the polyolefin provide the stiffness and reinforcement, and there is some crosslinking of the rubber phase during their preparationthat does not impair their thermoplastic behavior (Fischer, 1974; Duncan, 1977; Coran and Pate1 1981). Blends of natural rubber (NR) with polypropylene (PP) and/or polyethylene (PE) were first prepared by Campbell et al. (1978) in conventional internal mixers. It was found that it was essential to raise the temperature of the mix to above the melting pointof the polypropylene (165- 175°C) early in the mixing cycle. On account of this. dump temperatures were in the order of 180-200°C. While still hot, the batch was passed through a cool mill for sheeting out and then cut into strips or granulated for feeding to an injection-molding machine. Obviously, materials over a very wide range of stiffness can be obtainedby simply varying the proportions of rubber to polyolefin as shown in Figure 22. Here, flexural modulus at 23°C can be seen to alter from 50 to 1250 MPa as the ratio of polypropylene is increased from 25 to 100%. However. generally there are two types of thermoplastic NR: those of a semi-rigid nature with a high polyolefin content and those that are soft with a high NR content. The effect of partial crosslinking of the rubbery phase with dicumyl peroxide in soft blends is indicated in Figure 23. Note that the concentration of peroxide in soft blends is expressed here in parts per hundred on the rubber alone, since only the rubber becomes crosslinked. Experiments have indicated that the polyolefin remains soluble in boiling xylene,thus confirming its lackof grafting onto the rubber. The effect of this crosslinking is clearly greater on those blends containing a high proportion of natural rubber. With less than 0.4 part dicumyl peroxideper 100 parts rubber, injection-molded sheets of the 85/15 blend were soft and sticky, with distortion owing tononuniform mold shrinkage.With0.6-0.8 part dicumyl peroxide,muchimprovedmoldingswere obtained. The blends with higher concentration of polypropylene have less distortion, but the
96
Baker
Flexural modulus (MPa)
1500r
"
20
40
80 100
60 PP (%)
Fig. 22 Effect of polypropylene content on flexural modulus at 23°C of unfilled NR-PP blends. (From Elliott, 198 1.)
dicumyl peroxide addition provided an improvement in surface finish. Peroxide contents greater than 0.8 phr result in crumbing during mixing and inferior melt flow characteristics. Hard thermoplastic elastomers are used in the automotive industry. These are injection molded and require both rigidity and impact strength. Low-temperatureimpact strength is important in theiruse,andthebenefit of addition of rubbercan be seen in Figure 24. The lowtemperature impact strength is dependent on several factors:
1. The mean particle diameter of the dispersed rubber must be less than 1 pm. 2. The type of polypropylene plays a significant role in that the copolymer grade gives improved impact strength over the homopolymer blend with NR (Fig. 24). 3. A small amount of high-density polyethylene increases the impact strength of these blends.
Hardness (IRHD) a
a
v
*
65/35 NRlPP blends
20
0
0.2
0.4 0.6 0.8 Dicumyl peroxide (parts phr)
1.o
Fig. 23 Effect of dicumyl peroxide concentration on hardness of 85/15 and 65/35 NR-PP blends. (From Campbell et a l . , 1978.)
97
Modified Natural Rubber
Impact strength (J)
0 0
..
I
I
/ J
J
l I I I J
I
PPIHomopolyrner,,' c
., I
v
-40
-20
0 20 Temperature ("C)
40
I
60
Fig. 24 Effectof addition of NR to PP on low-temperature impact strength. (From Elliott, 1986.)
It has also been shown (Tinker, 1984) that a further improvement in impact properties can be achieved by the introduction of a low degree of crosslinking in the NR phase. Rather than peroxide, it has been found that m-phenylenebismaleimide (HVA-2) is particularly suitable for this purpose. Its addition at 0.5 part per hundred of polymer renders the natural rubber blend superior in Izod impact strengthto the EPDM-polypropyleneblend, especially in the temperature range of practicalimportance. This is demonstrated in Figure 25. The hard naturalrubberpolyolefin blends therefore have potential in the automotive field, with two advantages over the true thermoplastic rubbers: they are made from low-cost materials and they have low densities. The latter property is of particular interest to automobile manufacturers, since modem designers endeavor to keep car weights as low as possible to minimize fuel consumption. With respect to the soft natural rubber-polyolefin blends, thedynamic crosslinking process increases hardness, modulus, strength, and elasticityof the material andalso eliminates tackiness otherwise observed after injection molding. However, the use of organic peroxides described above initiates oxidation of the polymers, with consequent poor resistance to sunlight and heat aging. Furthermore, the addition of antioxidants, stabilizers, and UV inhibitors has been found to have little effect. Alternative crosslinking systems such as sulfur EV systems or sulfur donors provide improved resistance to surface degradation in sunlight, but render inactive UV inhibitors that are essential to polypropylene and its blends. The preferred crosslinking agent is m-phenylenebismaleimide, the same as that used to improve the low-temperature impact strength of the hard natural rubber-polypropylene blends. Figures 26 and 27 show the effect of the addition of HVA-2 on tensile and tear strength, gel content, and tension set. As well as increasing crosslink density (gel content), the crosslinking improves tensile and tear properties and substantially reduces tension set, an important factor for many applications of these soft blends. Modern natural rubber-based TPEs arecommercially viable (Tinker, 1989)and have been taken upby several manufacturers. Asfor any blendof this type,the objective in the development of TPNR since the early work of Campbell et al. (1978a) has been to combine the desirable properties of the two polymers to the best possible advantage (i.e., the thermoplasticity of the polyolefin and the rubbery properties of NR. Tinker has optimized this, established the role of partial dynamic vulcanization, and studied the phase morphology of the blends. The latter is a
98
Baker
lzod impact strength (Jlm)
0’ -50
I
/lTPNR /
1
-10 Temperature (“C)
-30
I
10
Fig. 25 Effect of addition of HVA-2 to TPNR on low-temperature Izod impact strength. (From Elliott and Tinker, 1985.)
Tensile strength, MPa
Tear strength, Nmm’l 50 40
30 20 10 0
0
0.5
1.o
1.5
HVA-2, pphp
Fig. 26 Effect of HVA-2 addition on tensile and tear strength of an 80/20 NR-PP blend. (From Elliott and Tinker, 1985.)
99
Modified Natural Rubber set,
Tension 50
Gel content, %
%
- 100
-
- 80
-
60
Tenslon set
0
l .o
0.5
1.5
HVA-2, pphp Fig. 27 Effect of HVA-2 addition on tension set and gel content of an 80/20 NR-PP blend. (From Elliott and Tinker, 1985.)
very important consideration for such blends. It has been found that simple blends of NR and PP prepared in an internal mixer have a continuous PP phase at PP contents as low as 20%. The NR phase is disperse at NR contents below about 40% but is continuous in between 40: 60 and 50: 50 NR:PP. The existence of co-continuous phases in blendshavingNRcontent of 50-80% has been unequivocally establishedby extraction and microscopy.The process of partial dynanlic vulcanization in improving the blend properties has been shown (Figs. 26 and 27). As well as high tear strength and low compression set, the latest TPNRs also have excellent stressstrain characteristics, not normally observed with, for example, EPDM/PPthermoplastic elastomers. Typical properties of dynamically vulcanized NR (or DVNR) are shown in Table 20. One aspect that must be noted is the difficulty in quoting single values for properties due to anisotropyarising from processhistory. However, readersmustnotethisdependence of properties on the degree of orientation induced in processing and its direction relative to testing is common to all TPEs and not restricted to TPNR. An example of the effect of test direction is shown in Table 2 1. Thus in Table 20 it is not surprising to see that TPNR material “5001” from British Vita gave different values when tested in two separate laboratories. It is important to point this out as in this respectTPEs behave differently to normal rubbers and properties should not necessarily be taken at their immediate face value. Having said that, TPNR in general may be regarded as having high strength properties and low set compared to otherrubber/polyoletin blends. TPNR has also been formulated to have a better resistance to heat aging than general-purpose rubbers, as shown in Table 22. Further, the continuous polypropylene phase protects the NR from ozone cracking, while the unsaturation of the NR is still available for reaction with primersto provide a suitable surface for bonding or painting. Other workers have also studied TPNR within the expanding technology of thermoplastic elastomers. Payne et al.(1990) have reported that the combinationof natural rubber and polypropylene may at first sight seem to be an unlikely combination for a TPE since “NR is a very
100
Baker
Table 20 Typical Properties of Modem TPNRs Teknor Apex "Telcar DVNR"
Hardness, Shore A Modulus at 100%, MPa Tensile strength, MPa Elongation at break, c/o Tear strength Die C, N/mm Die B, N/mm Tension set, % Compression set: ld23"C 3d23"C I d/7OoC 3d170"C ld/lOo"C 3 d 100°C 'I
British Vita "Vitacom DVNR"
Z1 l89
Z1188
500 1
56 1.7 4.0 290
90 6.9 15.9 455
so
21
63
-
-
30
44
-
-
-
-
-
48
40 43 47
58 S4
-
-
59
42 50
2.2 5.0 270
Table 21 Anisotropy in Injection-Molded 70 Shore A TPNR Test direction With flow
Across flow
6.3 8.9 175
3.3 11.1 425
Table 22 Resistance of 70 Shore A TPNR to Heat Aging 125°C
14 days
3 days
96 93 94
80 102
% retention of
Modulus at 100% Tensile strength Elongation at break
100 92 98 98
90 7.2 13.5 350
50
-
Properties measured at MRPRA
7 days
5001"
3.1 6.5 285
30
61
100°C
70 4.2 9.8 320
900 1
22 19 9
Solrrce: Tinker et al., 1989.
Modulus at loo%, MPa Tensile strength, MPa Elongation at break, %
1 700
26 36
Modified Natural Rubber
101
Table 23 Effect of Reprocessing Thermoplastic Natural Rubber
Percentage retention recycle 1st recycle 2nd recycle 3rd recycle 4th recycle 5th Modulus at 100% Tensile strength 107 a t break Elongation
94 I05 104
92
103
84 90 100 103 112 109
87 99 108
highmolecularweightelastomer,haspooroilresistanceandexhibitsonlyfairstability to elevated temperature and UV light.” In practice they observed a good balance of mechanical properties, heat, and fluid resistance. It was found that TPNR had a Tan 6 close to NR at room temperature and a higher Tan 6 at 100°C. When compared to an EPDM-based TPE, TPNR had more hysteresis. TPNR thus exhibited a behavior more like an NR thermoset than an EPDMPP blend. At -4O”C, the creep resistance of TPNR was observed to be significantly better than NR, while at 100°C the opposite was found to be true. Compared to an EPDM-based TPE, TPNR had a better creep resistance at room temperature but not at elevated temperature. In elastic recovery properties, TPNR was consistently superior to its EPDM-based counterpart. In a static load test, such as the GGF standard for glazing seals. TPNR showed almost a 70% recovery after heat treatment at 55°C for 14 days and 50% compression, compared to only just over 55% for the E P D M P P TPE. Further, in a dynamic test for keypad membranes, TPNR outlasted EPDMPP by virtually a factor of 10 while retaining a high force level throughout its life. In terms of resistance to ozone and UV light, natural rubber is not known for being one of the best performers. However, when blended with polypropylene to form TPNR,its properties are much enhanced. In weather resistance testing (Xenon Arc), TPNR was found in several ways to be better than even a commercial EPDM glazing seal compound. Results reported by Payne et al. indicate considerably better color retention and more stable hardness and modulus than EPDM, and even the ozone resistance was a significant improvement on NR itself. Evidence that TPNR is a true thermoplastic elastomer and can be recycled without any significant loss of its overall properties is indicated in Table 23. It is clear therefore that TPNR can be recycled at least two or three times and still retain 90% or more of its modulus with no loss in its tensile properties.This means,like the other TPEs,that all scrap can be safely recycled and any production wastage is reduced to a minimum. However, this is not to say that after a number of years in service TPNR will necessarily be reusable in the same application, since other changes might have taken place. It is likely that recycling after service might well result in its use in a less demanding application, but this will be true of all TPEs. An overall summary of TPNR’s potential has been given by Bowtell (1988), who reviews TPNR’s properties and applications. There can be little doubt that as the fastest-growing sector of the rubber market, TPEs as a whole have a bright future with the current world demand for the recycling of products at theend of their service life. TPNR has a rightful place in this market as a cost-effective TPE derived from a “green” material.
15. THERMOPLASTICEPOXIDIZEDNATURALRUBBER Section 11 of this chapter described epoxidized natural rubber(ENR) asa new,and now commercially available, modified form of NR. TPNR, the thermoplastic derivative of NR, has also just
Baker
102
Table 24 Physical Properties of TPENR 1
TPENR Type Hardness, Shore A Modulus at loo%, MPa Tensile strength. MPa Elongation at break, % Tear strength, Die C, N/mm Compression set, % 1 day at 23°C 1 day at 100°C 7 day at 100°C 3 day at 120°C Volume swelling after 3 days at 125°C. c/o ASTM oil No. 2 ASTM oil No. 3
65 3.7 6.5 240 23
TPENR 1
75 5.0 8.8 260 26 31
TPENR 12
TPENR
8s 6.2 9 .G 25s 36
72 3.7 7.1 290
-
24 36 44 49
28 39 49 55
33 56 49 5s
0 14
1
17
2 17
-
-
5s
Sorrrw: Patel and Tinker. 1997.
been discussed. It is not surprising thereforethat a thermoplasticform of ENR has been developed (Patel et al., 1993). which has been termed thermoplastic epoxidized natural rubber, or TPENR. This material, dynamically vulcanized like TPNR during its preparation, was expected to be an oil-resistant TPE because of the previously discussed oil resistance of ENR itself. In addition, the better resistance to aging observed with TPNR over NR should be further improved by the use of ENR in the blend. In fact this transpires to be even better than the originators anticipated. Processing behavior of TPEs is an important feature of such blends because it controls both the quality of the product and its properties. Patel et al. showed that the flow performance of TPENR is similar over a range of ENWPP compositions. Although TPENR is not found to be such a close approximation to a power law fluid as TPNR (Tinker, 1989). it does share the strong dependence of viscosity on shear rate and low dependence of viscosity on processing temperature as shown by other dynamically vulcanized TPEs. Thusthe processing conditions for TPENR are similar to those for TPNR.Melt temperatures shouldbe in the order of 190-200°C. It has been found that high melt temperatures do not aid processing, but high shear rates induced by high injection speeds are effective. TPENR is in a stateof continuous development with, to date, two gradesor types evaluated, which have been referred to as TPENR 1 and TPENR 2. Table 24 shows their typical physical properties. (Note: Where applicable the figures quoted are averages of values measured for test pieces cut parallel and perpendicular to the flow in the mold.) In general, the tensile and tear properties are fairly typical of dynamically vulcanized TPEs, although tensile and tear strengths are slightly below those found for TPNR at the same hardness. The compression set values are also good, bearing in mind that this is a thermoplastic elastomer, although again it should be noted that this property too is dependent on the process history. If the orientationwithinthematerialinduced by processing is allowed to relax by compression molding melt, lower values are found (Patel et al., 1993). Resistance to oil swelling after 3 days at 125°C is directly comparable to a well-compounded nitrile rubber vulcanizate with a 34% acrylonitrile content. Perhaps the most remarkable feature found in TPENR is its aging performance, bearing in mind that the material is derived from a modified natural rubber still containing a relatively
Modified Natural Rubber Table 25
103
Aging Propertics of TPENR 2 (% retention)
Medium
Air No.
Air
Air
Aging conditions
15d/l25"C
15d/135"C
3 d 150°C
Modulus at 100%
111 101 101
126 100
106 88
80
93
Tcnsile strength Elongation at break
ASTM
3 oil
3d/l25"C 90 83 78
high number of double bonds. Indeed, the heat aging of TPENR is so good that less severe conditions are necessary for comparison to the nitrile rubber vulcanizate referred to above in swelling tests. Thus after 3 days at 125°C when there is virtually no change in TPENR properties, the nitrile vulcanizate undergoes a twofold increase in modulus and a 35% loss in elongation at break. While TPENR 1 has very good aging (Patel et al., 1993), TPENR 2 (Patel and Tinker, 1997) has even better heat aging resistance as shown in Table 25. TPENR 2 is an advanced form of the original TPENR 1 brought about by minor changes to its constitution. Table 24 shows that its initial physical properties are essentially unchanged. but its aging is significantly better. Even when the severity of aging is increased to 15 days at 135"C, the retention of properties is remarkably good. Since 15 days is regarded as too long a test time, ageing at 150°C over 3 days has been adopted as a more realistic accelerated test. Retention of properties at this temperature was still found to be very good, which is the upper limit that can be used before distortion of the test pieces occurs due to the melting point of the plastic component. Retention of properties when immersed in oil at 125°C is also remarkably good. Figure 28 puts TPENR in the context of other elastomers, and its position with respect to TPNR when service temperature is plotted against volume swelling in ASTM No. 3 Oil. This latest form of TPENR is comparable to butyl rubber and EPDM rubber in terms of service temperature, with infinitely better oil resistance. It is better in all respects than polychloroprene rubber and most polyurethanes and ages better than nitrile rubber. TPENR is therefore the latest in the line of rubbers obtained by modifying NR. The combination of good oil resistance and excellent heat resistance is unparalleled by any other rubber derived from natural rubber over the years, and the material must find extensive use in the automotive industry, where recyclable rubbers such as this are now becoming so desirable.
15. SUMMARY As initially the only rubber available, it is perhaps not surprising that attempts at modification of natural rubber began over 75 years ago (e.g., Pummerer, 1922). These first modifications were indeed interesting and sometimes useful, but lack of adequate scientific knowledge of the polymer structure combined with none of the present-day identificationequipment often resulted in impure materials and consequent poor properties(e.g., early ENR). Most of the first commercial forms of modified NR have now been superseded by especially tailored, and better, synthetic materials. Of the older forms, only MG rubbers survive, and this only in a very small volume. Present-day science and technology has led to new liquid natural rubber, epoxidized natural rubber, and thermoplastic elastomers derived from natural rubber or modified natural rubbers. These latest modifications were thoroughly thought outat the outset, with specific targets aimed
104
Baker
Dry heat service temperature, “C Fluoroelastomer
Polyaclylate Hypalon Epichlorohydrin
150 --
Butyl EPDM
TPENR
TPNR
NBR
CR
PU
loot
SBR NR
100
50 I 150
I
I
50 Volume swelling in ASTM No.3 oil, %
I
0
Fig. 28 Position of TPENR in the ASTM D2000 classification scheme. (From Patel and Tinker, 1997.)
for, i.e., vulcanizable process aid,oil resistance/damping/improved gas permeability, and thermoplasticity, respectively. This ability to adopt natural rubber to compete with various modern synthetic elastomers is aremarkableachievement,especially since they are derived from a natural resource material. In this respect, there have been some recent “natural” attempts to modify NR by enzymes. Unpublished work by Lehrle and Gray (1991-1997) at the School of Chemistry at the University of Birmingham has indicated that some very minor modification of NR is possible with rat liver cytochrome PJso,while horseradish peroxidase would only act on dihydromyrcene and not with the larger squalene modelfor NR. These areonly very preliminary observations, but they bring us a step nearer to the possibility in the long term of using enzymes to modify NR and production of a variety of totally “green” elastomers.
ACKNOWLEDGMENTS
I wish to express thanks to all my many colleagues at MRPRA/TARRC who have assisted in the preparation of this chapter, both originally and in its revised form. My special thanks are due to G. M. Bristow for checking the authenticity of the early modified rubbers, K. P. Jones for assistance on references, and the specialists on the various types of modified natural rubber; P. W. Allen on methacrylate grafts, D. S. Campbell on ENPCAF modification and polystyrene comb grafts, I. R. Gelling on ENR, and A. J. Tinker on TPNR and TPENR. REFERENCES Alex, R., De, P. P., and De, S. K. (1991). Polyrner 32:2345, 2546. Allen, P. W., Merrett, F. M,, and Scanlon, J. (1955), Trans. F u r u d q Soc. 5/:95.
Modified Natural Rubber
105
Allen, P. W., and Merrett, F. M. (1956), J. Polym. Sci. 22:193. Allen, P. W., and McSweeney, G. P. (l958), Trans. Faraday Soc., 54:715. Allen, P. W., Bell, C. L. M,, and Cockbain, E. G. (1960). Rubber Chem. Technol. 33(3):825. Allen, P. W. (1963), in The Cltemistry and Physics of Rubber-like Substances (L. Bateman, ed.) Maclxen, London, Chap. 5. Baker, C. S. L., Barnard, D., and Porter, M. R. (1970), Rubber Cltem. Technol. 43:501. Baker, C. S. L., Gelling, I. R., and Newell, R. (1985), Rubber Chern. Technol. 58:67. Baker, C. S. L., Gelling, I. R.,and Wallace, I. R. (1987). ACS Rubber Division Meeting, Cleveland, October 6-9. Baker, C. S. L. (1992), Proceedings of the International Rubber Conference, Beijing. Baker, H. C. (1956). Trans. Inst. Rubber Ind. 32:77. Baker, H. C., and Foden, R. M. (1960). Rubber Chem. Techno/. 33:810. Baker, H. C., and Stokes, S. C. (1960), Proc. Natural Rubber Res. Con&, Kuala Lumpur, p. 587. Bandyopadhyay, S., De, P. P,, Tripathy, D. K., and De, S. K. (1995). Polynzer 36: 1979. Barnard, D., Dawes, K., and Mente, P. G. (1975), Proc. Intern. Rubber Cor$, Kuala Lumpur, p. 215. Bevan, A. R., and Bloomfield, G. F. (1963), Rubber Developments 16:42. Bhattacharjee, S., Bhowmick, A. N., and Avasthi, B. N. (1993), Polymer 345168. Bloomfield, G. F., and Farmer, E. H. (1934), J. Soc. Chem. Ind. 53:121T. Bloomfield, G. F. (1943). J. Chem. Soc., p. 289. Bloomfield, G. F., and Stokes, S. G. (1956), Trtms. Inst. Rubber lnd. 32:172. Bloomfield, G. F. (1961). in The Applied Science of Rubber (W. J. S. Naunton, ed.), Edward Arnold, London, Part I , Ch. 11. Bowtell, M. (1988). Elastomerics 120(2):6. British Resin Products Ltd. (1964), Tech. Manual No. 11, The Westerham Press. BRPRA (1952), Rubber Technical Developments Ltd., London, Broadsheet. BRPRA (1954), Tech. Bull. No. I , Heveaplus M, London. Brown, P,,and Tinker, A. J. (1991), J. Nat. Rubb. Res. 6:87. Bruson, H. A., Sebrell, L. B., and Calvert, W. C. (1927), Ind. Eng. Clwm. 19:1035. Bunn, C. W., and Garner, E. V. (1942), J. Chem. Soc., p. 654. Burfield, D. R.. and Gan, S. N. (1975), J. Pol.ym. Sci. 13:2725. Burfield, D. R., Lim, K. L., Seow, P. K., and Loo, C. T.(1985). Proc. Intern. Rubber Conf., Kuala Lumpur, Vol. 2, p. 47. Byung Kyu Kim, and In Hwan Kim (1993). Polynz. P/OSI.Technol. Eng. 32:167. Cain, M. E., Knight, G. T., Lewis, P. M,, and Saville, B. (1968). J. Rubber Res. Inst. Malaya 22(3):289. Campbell, D. S., Elliott, D. J., and Wheelans, M. A. (1978a). NR Technol. 9:21. Campbell, D. S., Loeber, D. E., and Tinker, A. J. (1978b). Polymer, 19:1106. Campbell, D. S. (1981), Proceedings of UNIDO-Sponsored Symposium on Powdered Liquid and Thermoplastic Natural Rubber, Phuket, p. 57. Campbell, D. S., Mente, P. G., and Tinker, A. J. (1981), Knur. Gummi Kunst. 34:636. Colclough, T., Cunneen, J. I., and Moore, C. G. (1961). Tetrahedron 15:187. Colclough, T. (l962), Trans. Inst. Rubber Ind. 38(1):TII. Coran, A. Y., and Patel, R. (1981), Rubber Chenz. Techno/. 53:141. Cunneen, J. I., Farmer, E. H., and Koch, H. P. (1943). J. Chem. Soc., p. 472. Cunneen, J. I., and Shipley, F. W. (1959), J. Polym. Sci. 36:77. Cunneen, J. I., Moore, C. G., and Shephard, B. R. (1960), J. Appl. Polym. Sci. 3(7):11. Davies, C. K. L., Wolfe, S. V., Gelling, I. R., and Thomas, A. G . (1983), Polymer 24: 107. Duncan, D.J., (1977), Improvements relating to olefin polymer compositions, Br. Pat. 1,489,108. Elliott, D. J. (198 l ) , Proceedings of UNIDO-SponsoredSymposiurnon Powdered Liquid and Thern~oplastic Naturd Rubber, Phuket. p. 23. Elliott, D. J. and Tinker, A. J. (1985), Proc. Intern. Rubber Con&, Kuala Lumpur, Vol. 2, p. 633. Elliott, D. J. (1986), Proceedings of DGF Meeting, Odense. Falk, J. C., Schlott, R. J., Hoeg, D. F., and Pendleton, J. F. (1973), Rubber Chem. Technol. 46: 1044. Fischer, W. K. (1974), Dynamically partially cured thermoplastic blend of mono olefin copolymer rubber and polyolefin plastic, U.S. Pat. 3,806,558.
Baker
106
Fisher, H. L. (1927). Incl. Eng. Chem. 19:1325. Flory, P. J., Rabjohn, N., and Shaffer, M. C. (1949). J. Polyn. Sci. 4:225. Foss, R. P,, Jacobson, H. W., Cripps, H. N., and Sharkey, W. H. ( I 976). M~rcromolecu1e.s9373. Gelling, I. R. (1985). Rubber Cllem. T d l n o l . .58:86. Gelling, I. R. and Morrison, J. (1985). Rubber Chern. Techno/. .58:243. Gonsovskaya, T. B., Ponomarev, F. G.. Poluektor, P. T., and Khvatova, L. K. (1971), Kauch. Rezitla 30: 23. Gordon, M. (l95la), Proc. Roy Soc. London. 204A:569. Gordon, M. (1951b), Ind. Eng. Chern. 33:386. Greenspan, F. P. (1964), in Clrm~icnlReactions of Polymers (E. M. Fettes, ed.), Interscience, New York, Ch. IIE. Groves, S. A. (1998) BIends of Natural Rubber. Novel Techniques for Blending with Speciality Polymers (A. J. Tinker and K. P. Jones, ed) Chapman and Hall. Hardrnan, H. V., and Hardman, K. V. (1944). Method of making depolymerized rubber articles, U.S. Pat. 2,349,549. Harries, C. D. (1913). Ber. 46:733. Harries, C. D. ( I 919). Untersuchungen uher die Naturlichen u r d Kunstlicl~en Kautsehukarten,Julius Springer, Berlin, p. 6. Harries. C. D. (l923), Ber. 56:1048. Hinrichsen, F. W., Quensell, H., and Kindscher, E. (1913). Ber. 46:1283. Hoffman, H. M. R. (1969), A~zgew.Chetn. Int. Ed. 8:556. IRCA ( 1989), Proceedings of UNIDO Sponsored Workshop: Development of the Applications of Liquid Ncrturrrl Rubber. Abidjan 4-8 December. Kirchhof, F. (1920), Kolloidzeitschr$t 27:3 1 I . Knight, G. T., Loadrnan, M. J. R., Saville, B., and Wildgoose, J. (1974), Chem Comrnun.. p. 194. Kraus, G., and Reynolds, W. B. (1950), J. Am. C h m . Soc. 725621. Leonhardi, J.G. (1971), in Systenmtic S u n q j of Rubber Chernistr?. Chem. Catalogue Co., New York, 1923. Manvede, G. W. and Stollfuss, B. (Leverkusen) and Surnner, A. J. M. (Antwerp), (1993), Kaut. Gunmi Kunst. 46:380. Morton, G. F., and Krol, L. H. (1982). American Chemical Society, Rubber Division Meeting, Chicago, 1982; abstract in Rubb. Chern. Techno/. 56:508. MRPRA (1989). Annual report. Narnbiar, J. (1993) in Natural Rubber: Current Developments in Product Manufacture and Applicrrtions (Abdul Aziz bin S. A. Kadir, ed.), Rubber Research Institute of Malaysia, p. 11 1. Ng, S. C., and Can, L. H. (1981), Eur. Po/yn. J. /7:1073. Nordsiek, K. A. (1984), American Chemical Society, Rubber Division meeting, Indianapolis, May 1984, Paper 48; abstract in Rubb. Cllern. Techno/. 88:201. Patel, J., Riddiford, C. L., and Tinker, A. J. (1993), in Naturcrl Rubber: Current Developments in Product Mmufircture and Applications (Abdul Aziz bin S. A. Kadir, ed.), RubberResearchInstitute of Malaysia, p. 395. Patel, J., and Tinker, A. J. (1997), Proceedings ofAustralasian Plastics & Rubber Institute, 10th National Conference, Leura, 12- 15 October. Payne, M. T., Wang, D. S. T., Patel, R. P., and Sasa, M. M. (l990), ACSRubberDivisionmeeting, October 9-12, Paper No. 34. Pike, M. (1953). Rubber Dev. 470. Pinazzi, C., Danjard, J. C., and Pautrat, R. (1960). Proc. Natural Rubber Res. Col$, Kuala Lumpur, p. 55 1. Pinazzi, C., Cheritat, R., and Pautret, R. (l962), Rev. Gen. Caoutchouc 3412): 1951. Porter, M. ( 1 9 7 8 ~Plast. Rubber: Materials Appl. 3:32. Pummerer, R., and Burkard, P. A. (1922). Ber. 55:3458. Ramesh, P., and De, S. K. (1993), Polymer 344893. Roberts, K. C., and Wilson, J. (1946). Improvements in and relating to the preparation of hydrogenated rubber. Br. Pat. 577,472. _,
Modified Natural Rubber
107
Roland, C. M., Santangelo, P. G., Baram, Z., and Runt, J. (1994). Macron~olecules275382. Roychoudhury, A., De, P. P,, Bhowmick, A. K., and De, S. K. (1992), Polytner 33:4737. Roy, S., Bhattacharjee, S., and Gupta, B. R. (1993). J. Appl. Po1.w. Sci. 49:375. Rubber Research Institute of Malaya (1957), Planters Bull. 32:83. Scanlan, J. ( 1954). Trcrns. Fciruday Soc. 50:756. Schidrowitz, P,, and Redfarn, C. A. (1933, J. Soc. Clzem. Ind. 54:263T. Sekhar, B. C., and Drake, J. (19%). J. Rubher Res. Inst. Malnva 15:216. Sigwalt, P,, Polton, A., and Miskovic, M. (1976). J. Pol.vn~.Sci., Polytn. S y n ~ p 56:13. . Staudinger, H., and Feisst, W. (1930), Helv. Chinl. Acta 13:1361. Staudinger, H., Geiger, E., Hubber, E., Schaal, W., and Schwalback, A. (1930). Helv. Clzim. Actcc 13:1334, 1339. Staudinger, H., and Leupold, E. 0. (1934). Ber. 67304. Stevens, H. P,, Clayton, and Stevens (1933). Improvements in or relating to the manufacture of printers’ rollers or like printers’ devices, Br. Pat. 390,820. Sundet, A., Thamm, R. C., Meyer, J. M., Buck, W. H., Caywood, S. W., Subramanian, P. M,, and Anderson, B. C. ( 1976), Macronzolecules 9:37 1. Tinker, A. J. (1984). Polyn~.Co~nrnun.25:325. Tinker, A. J., Icenogle, R. D., and Whittle, I. (1989). Rubber World 199(6):25. Tom, D. H. E. (1956). J. Polyn. Sci. 20:381. Van Amerongen, G. J. ( 1 947), Rubber Chem. Technol. 20:494. Van der Hoff, B. M. E., and Buckler, E. J. (1967), J. kfrrcromol. Sci. AI:747. van Veersen, G. J. ( 1948), Proc. Second Ruhher Techno/. Col$. London, p. 87. van Veersen, G. J. (1950), Rec. Trav. Chinz. 69:1365. Varughese, S., Tripathy, D., and De, S. K. (1990) Kaut. Gun71ni Kunst. 43:871. Weber, C. 0. (1900), Ber. 33:779. Wheelans, M. A. (1977), N R Technol. 8:69. Woods, D. E. (1949). J. Soc. Ckern. Ind. 68:343.
This Page Intentionally Left Blank
Chemical Modification of Synthetic Elastomers Donald N. Schulz and Abhimanyu 0.Patil ExxonMobil Research and Engineering Company, Annandale, New Jersey
1. INTRODUCTION The chemical modification of synthetic elastomers is a useful method for altering and optimizing the physical and mechanical properties of rubbers. This topic has been the subject of a number of reviews(Fettes,1964;Brydonand Cameron, 1975;Pinazziet al., 1975;Brydson,1978; Ceresa, 1978; Cameron, 1980; Luxton, 1981; Schulz et al., 1982; Marechal, 1989;Halasaet al., 1994). Also of relevance is a recent monograph on the synthesis of functional polymers (Patil et al., 1998). This chapter surveys recent developments in the functionalization or structural modification of synthetic elastomers. The modification of natural rubber is treated elsewhere in this volume (see Chapter 3). Also, crosslinking and grafting processeslproducts will be excluded. Recent progress in the modification of synthetic rubbers has been observed along several fronts:synthesis,characterization,structurepropertyunderstanding,andapplications. In the synthetic area, new catalysts (e.g., phase transfer) and methods (e.g., selective modifications of block copolymers) have beenreported.Reactions on elastomers have also been used to aid polymer characterization. Moreover, structure-property understanding has beenrefined to appreciate subtle microstructure, tacticity,and sequence effects. Finally, new applications for modified synthetic rubbers have emerged.
2.
HYDROGENATION
Hydrogenation of synthetic elastomers is an excellent example of a chemical modification process that both modifies the physical properties of rubbers and facilitates molecular level understanding. Changes in physical properties result from changes in polymer T,,, and T, values. Moreover, hydrogenated products are typically more stable than their unsaturated precursors. Since hydrogenated elastomers also tend to have less complicated spectrathen their unhydrogenated analogs, they can also serve as useful model systems. The hydrogenation of polymers has recently been reviewed (Schulz, 1986). 109
Schulz and Patil
110
Table 1 Comparison ofProperties of Hydrogenated Polybutadiene and Low-Density Polyethylene ~~~~
Property
~
~
Hydrogenated polybutadiene Low-density polyethylene
Tensile strength, Mpa Elongation, % Stiffness modulus. Mpa Brittle point, "C Impact strength ( h o d ) Refractivc index
16.2 750 103.4 c-73 Does not break 1S O
13.1 600 137.7 < -73 Does not break 1.51
Solrrcr: Adapted from Jones et al.. 1953
2.1
Products and Processes
The effectsof hydrogenation on the physical properties of polymers is illustrated by a hydrogenation of elastomeric 1,4-polybutadiene (1,4-BR) to a crystalline polyethylene:
H2 -(-CHl-CH=CH-CH2-)-+
-(- CH~-CH~-CHZ-CH~-)-
A comparison of the properties of hydrogenated 1,4-polybutadiene with conventional polyethylene is shown in Table I . If the polybutadiene contains moderate amounts of 1,2 units (1,2-BR), the resultant material will be an elastomer, poly(ethy1ene-co-butylene):
H2 -(-CH?-CH =CH-CH2-)- -(-CH2-CH-)-+
-(- CHZ-CH~-CH~-CH~-)-
I
CHxCH2 -(-CHz-CH-)-
I C2Hs Other unsaturated homopolymers that have recently been hydrogenated include 1,4-polyisoprene ( I ,4-IR), isotactic 1,4-poly(2-methylpentadiene), and syndiotactic 1,2-polybutadiene to give poly(ethy1ene-alt-propylene). hemitacticpolypropylene,andsyndiotacticpoly(]-butene), respectively (Farina et al., 1982; Makino et al., 1982; Zhongde et al., 1985; Schulz 1986). Hydrogenation of copolymers (especially block copolymers) has received considerable attention lately as a means of extending the performance range of copolymers (Tikhomirov, 1968; Jones, 1971; Shaw, 1971; De la Mare and Shaw, 1972; Holden, 1973; Falk et al., 1973; Gilles, 1974; Halasa, 1975, 1981; Krause, 1975; Laramee et al., 1975; Noshay and McGrath, 1977; Maczygemba, 1977; Riess, 1977; Sinaiski et al., 1977; Naylor and Zelinski, 1978; Zotteri andGiuliani,1978;McGrathand Wang, 1980: Mohajeret al., 1980a,1980b, 1981; Xieand Ma, 1985; Bhattacharjee et al. 1993a,b; De Sarkar et al.. 1997). The hydrogenated copolymers are generally of two types: hydrogenated diene-diene copolymers (Table 2) and hydrogenated diene-aromatic copolymers (Table3). Of particular interest are the hydrogenated B I . ~ B 1(where .2 B l , J signifies a 1,4-polybutadiene block and B1,?a 1.2-polybutadiene block) and the SB,& [where S = polystyrene and BM" = medium (30-60%)1,2-vinyl polybutadiene] copolymers. The hydrogenatedB ,.4Bcopolymersshow exceptionalstress-strainproperties for diblock materials, because the hydrogenation produces crystalline polyethylene blocks, which trap the rubbery domains, making a pseudo-network (Halasa, 1981). In turn, hydrogenation of S B M ~ S
Chemical Modification of Synthetic Elastomers
,
111
112
Schulz and Patil
Chemical Modification of Synthetic Elastomers
113
yields a triblock polymer with improved thermal andoxidative stability by virtue of its poly(ethy1ene-co-butylene) center block (Holden, 1973; Noshay and McGrath, 1977). The hydrogenation of functional polymers presents special problems because many functional groups can coordinate with and deactivate organometallic hydrogenation catalysts and/or suffer reduction themselves. Such difficulties are minimized by using chemical or noncatalytic hydrogenation (see below) orby utilizing protecting groups to mask troublesome functionalities (Table 4) (De la Mare, 1973; Neumann et al.. 1975; Inomata et al., 1976, 1978; Karairanova et al., 1976, 1978; Kuz’mina et al., 1976; Azuma and MacKnight, 1977; Chamberlin et al., 1979; Rahrig and MacKnight, 1979, 1980; Yokota and Hirabayashi, 1981 ; Schulz, 1986). For example, diimide has been used successfully to hydrogenateunsaturatedpolymerscontaining “CN, -S03Na, and -PO(OCH3)2 groups. Also, BF3 has been used as a protecting group for amino functionalities, as in the case of poly(butadiene-co-N-vinylpyridine). However, late transition metals tend to be more tolerantof polar functionalities. For example. Bhowmick and coworkers have advantageously used Pd(OAc)? (Bhattacharjee et al., 1990, 1992) for the hydrogenation of the olefins in nitrite rubber without concomitant hydrogenation of the nitrile group. They also hydrogenated epoxidized NR selectively for the olefin segment (Bhattacharjee et al., 1993b). The conversion of unsaturated polymers to saturated ones can be effectedby both catalytic (homogeneous, heterogeneous) and noncatalytic (diimide, hydroboration) methods. There are even reports of hydrogenation of diene polymers in the absence of gaseous hydrogen, using Pd catalyzed H-transfer (Costello et al., 1996). Experimental concerns are yield, selectivity, and side reactions. Homogeneous (soluble) hydrogenation catalysts are usually of the coordination type, i.e., organotransition metals such as cobalt or nickel, plus a reducing agent (e.g.. RjAI or n-BuLi). Sometimes an unsaturated hydrocarbon (e.g., cyclohexene) is added as an activator. The activator should be a stabilizer for active metal-H, by virtue of its ability to coordinate to the metal. The general order of catalytic activities for members of the transition metal series is second row > first row > third row (Yoshimato et al.. 1969; Schulz et al., 1982). The general order of unsaturated elastomer reactivities to homogeneous hydrogenation is:
>
1,2-BR
l ,4-BR
c i S-l ,4-BR -
trans-l ,4-BR
1,4-IR
The selectivity of 1,2 backbones over 1,4 backbones isfurther increased if homogeneous rhodium catalysts, such as (PPh2) RhCl or (PPh3),RhHC0, are used (Kang, 1976). Besides the selectivities of microstructures and catalysts, there are sequence distribution effects in homogeneous catalytic hydrogenation. Forexample, Chamberlin et al. (1981) presented DSC evidence that the homogeneous hydrogenation (CO,Ni R3AI) of 1,2 polymers is statistically random while the hydrogenation of 1,4 polymers is more blocky.
+
Table 4
Hydrogenation of Functional Diene Polymers
Functionality
Backbone"
-OH
Polybutadiene
-COOH -COOCHx
SBR, NBR Pol ~(butadiene-alt-methyl methacrylate) SBR, NBR, IR PP PP PP NR, PIP, 1,4-BR
-CH -SO?Na -PO(OCH3)2 NO2
Poly(butadiene-b-vinyl pyridine or poly(isoprene-b-vinyl pyridine)
Method Homogeneous hydrogenation, e.g., Ni or Co/triethyl aluminum (or Ru, Rh catalyses); heterogeneous catalysis (Raney Ni) Rh catalysis Pt black Rh catalysis [NH=NH] [NH=NH] [NH=NH] Raney Ni, Zn-AcOH
NiEtlAl catalysis
Features Hydrogenated mixed 1,4- and 1.2poly-butadienes as heat-resistant poly-urethane prepolymers
Partial reduction of NOz groups; complete hydrogenation of double bonds BF3 and C12AIEt used to complex vinyl pyridine to increase rate of hydrogenation; BF3 released by NH,OH
BR = Polybutadiene: SBR = poly(styrene-co-l .J-butadiene): NBR = poly(acry1onitrile-co- 1 ..l-butadiene): PP = polypentenamer: IR = polyisoprene. Soitrcv: Adapted from Schulz et al.. 1982 "
3 r
E N nl
3 P
n
Chemical Modification of Synthetic Elastomers
115
Heterogeneous (insoluble) hydrogenationcatalysts are either of thelow-activity (e.g.. nickel oxide) or high-activity (e.g., palladium on calcium carbonate or platinum black) type. The low-activity catalysts require high reaction temperatures and/or pressures and often lead to degraded products. However, the more active catalysts, needing milder conditions, can yield products of high structural purity (Rachapudy et al., 1979). Costello et al. (1996) described a heterogeneous hydrogenation process for polydienes that uses hydrogen transfer agents rather than molecular H?. The preferred hydrogen transfer agent is formic acid;the catalyst used is Pd supported on carbon.With these reagents, practically quantitative saturation is reached for 1,4 and I .2 polydienes. Selective hydrogenation of the polybutadiene block in styrene/diene block polymers has also been observed. Noncatalytic hydrogenations of olefinic polymers have been effected by hydroboration [Eq. (3)] (Ramp et al.. 1962; Pinazzi et al., 1977a, 1977b) or diimide reductions [Eqs. (4) and ( 5 ) ](Harwood et al.. 1973; Mango and Lenz, 1973; Sanui et al., 1973; Nang et al.. 1976; Chen. 1977; Ast et al., 1979; Shahab and Basheer, 1979; Wang et al., 1983).
1. BH3 -C=C-
-CHz-CHz-
(3)
2. HOA,
A ArSOzH (4) + [NH=NH]
ArS02NHNH2+ [NH=NH] +
-GC-
+
-CH*-CHz-
The latter is the more popular laboratory method,
+ N2
(5)
with the order of polymer reactivity
being
l ,2-BR
c i S - l,4-BR -
t r a n s - l ,4-BR
Unfortunately. hydrogenation by diimide not only saturates the double bonds but also promotes cis-trms isomerization in 1,4-polybutadiene (Chen. 1977; Shahab and Basheer, 1979). Attachment of hydrizide fragments from p-toluenesulfonyl hydroazide is another side reaction, requiring additives to suppress it (Wang et al., 1983; Hahn, 1992). Moreover, depolymerization and cyclization have been observed in the diimide reduction of cis-1.4-IR (Nang et al., 1976).
2.2
Characterization and Understanding
Besides being used to generate new products, hydrogenation has aided polymer structure proof. For example, hydrogenation has greatly facilitatedthe NMR analysis of the sequence distribution in polydiene polymers (Randall, 1977, 1978; Harwood, 1982; Makino et al., 1982). Copolymers of butadiene and styrene have I O possible dyads and 40 possible triads, exclusive of tacticity effects. These many possibilities result from the presence of three polybutadiene isomers (cis,
Schulz and Patil
116
trans. and 1,2)as well aspolystyreneunits. After hydrogenation, cisltrms distinctions and olefinic resonances are eliminated. Thus, sequence studies are greatly simplified. Hydrogenated polymers have also been used as model compounds. For example, Mays et al. (1984) used hydrogenated anionically prepared elastomers as models for predicting the unperturbed dimensions of polymer chains. Rachapudy et al. (1979) and Grdessley and Raju (1984) have used such models to examine the effects of long-chainbranching on the melt rheology of polyethylenes.
3. CYCLIZATION The cyclization of synthetic elastomers has been extensively studied and the subject of many reviews (Golub. 1969,1970,1978; Cunneen and Porter, 1973; Hampel and Hawley, 1973;Zachoval et al.,1973; Schulz etal., 1982). Such cyclizations canbe achieved by cationic. thermal. photolytic, and radiation-induced methods. This section will focus on cationic cyclization. Cationic cyclization can be catalyzed by mineral acids (e.g., H2S04), Lewis acids (e.g., AICl3, SnC14, TiC14, BF3, FeC13), and organic acids (e.g., p-toluene sulfonic acid). The Lewis acid systems (e.g., A1C13) can optionally contain a cocatalyst such as PhCHlC1 or C13CCOzH (Abdel-Razik, 1988). The structures of the cyclized products depend upon the structure of the starting elastomers. Thus, 1,4-IR leads to mono-, bi-, tri-, and other fused ring systems. Equation (6) shows some representative product structures: I, 11, and 111.
"H2C
\ /
/c\
HZC
CH3
/
CH
CH2
\
CH2
Chemical Modification of Synthetic Elastomers
117
Cyclized 1,4-IR exists predominantly as a tricyclic structure (Golub, 1970; Zachoval et al., 1973). Cis-trans isomerization of the original double bonds in 1,4-IR is not a concomitant side reaction in the Tic4-catalyzed process (Golub, 1969). Cyclized 1,4-IR has higher density, refractive index, and softening pointbutlowerintrinsicviscositythan conventional 1,4-IR. Cyclized cis-l ,4 polyisoprene has found application as negative photoresists (Chitale, 1994). On the other hand, 3,4-IR cyclizes under the influence of cationic catalysts to partialladder or double-chain polymers [IV; Eq. (7)] (Golub, 1978; Schulz et al., 1982). The degree of cyclization is generally low (i.e., 1-6) (Golub, 1978).
( W
In turn, the degree of cyclization (cyclicity) for 1,4-BR depends upon, the catalyst used. HzS04-catalyzed cyclization shows relatively high cyclicities of -25-45 (Kossler et al., 1967); TiC14-catalyzed cis-1,4-BR shows a low cyclicity of -3 (Shagov et al., 1969). Cyclization of 1,2-BR results in predominantly monocyclic (or isolated six-membered) rings with one methyl group formed for every two vinyl groups reacted (Carbonaro and Greco, 1966; von Raven and Heusinger, 1974). The order of reactivity of unsaturated elastomers to cationic cyclization is (Carbonaro and Greco, 1966; von Raven and Heusinger, 1974; Schulz et al., 1982):
1,4-IR
1,4-BR
1,4-BR 1,2-BR As a consequence of the greater reactivity of 1,4-IR, La1 et al. (1 982) selectively modified the polyisoprene blocks of I-B-I triblock polymers. The selectively cyclized products (IcBIc) with 30-40% polyisoprene are thermoplastic elastomers, whereas the polymers containing 65% iso-
Schulz and Patil
118
prenearesuitable for coatings or films. The IcBIc triblockpolymersshowsuperiortensile strength versus temperature responses compared with conventional SBS thermoplastic elastomers.
4.
ISOMERIZATION
Not only is isomerization a common side reaction in many chemical modifications of polymers, but it is also alegitimate way to modifypolymerproperties in its own right.For example, isomerizing a mixed microstructure, 1,4-polybutadiene, raises the trans content with a consequent increase in polymer crystallinity. Cis-trans isomerization of olefinic elastomers can be achieved by photolytic, irradiative, thermal, or catalytic methods (Golub, 1969, 1970; Brydon and Cameron, 1975; Schulz et al., 1982). Catalysts include NO,, SO?, and compounds that generate Br' RS' radicals. The latter isomerizations involve an "on-off" mechanism [(Eq. (g)] (Cunneen and Porter. 1973; Schulz et al., 1982). The isomerized polymers have been analyzed by 13C-NMR (Harwood. 1973).
R \
c=c
+
/H
R X'
-
\
*
"-+ - H$
c iS -
R \
-
H2C
L -c
/
/
CH2 -
+
\
H
X'
/"
c- c-x
/'
\
\
/
R
CH2
-
CH2-
f-" / e - HZC
c-c-
\
X H
trans
5.
HALOGENATION AND HYDROHALOGENATION
Halogenation is one of the most commercially important chemical modifications of elastomers, especially the halogenation of butyl rubber. Halogenated butyl rubber retains the desirable features of regular butyl rubber (i.e., low gas permeability, high ozone and weather resistance, high hysteresis and chemical resistance). However, in addition to these characteristics, halogenated butyl rubber exhibits the advantages of faster cures with reduced curative levels, better cure compatibility with other rubbers (e.g., NR, SBR, NBR), and better cured adhesion to itself and other elastomers. Halobutyl rubbers have found use in tire (e.g., inner liners, white side walls) and nontire (e.g., liningsandcoverings,belting,hose, gaskets) applications (Condon, 1978; Blackshw,1978). The chlorination of isoprene butyl (IIR) proceeds to give more than 90% of an allyic halide substitution product (Baldwin et al., 1961). Bromination is less regioselective but gives the corresponding bromo derivative [Eq. (9)] (Van Tongerloo and Vukov. 1980).
119
Chemical Modification of Synthetic Elastomers
Kunz (1983) described the halogenation of copolymers of isobutylene and 2,3-dimethyl1.3-butadiene. The chlorination of these polymers follows substitutive stoichiornetry. However, bromination reacts by addition rather than substitution. In addition. Kawaguchi et al. (1985) has brominatedcis- 1.4-BR to form head-to-head poly(viny1 bromide). The partially brominated product has a structure of random or blocky sequences of butadiene units depending upon the bromination solvent used. Hydrohalogenation of unsaturated elastomers has also been widely studied. For example, amorphous cis- 1.4-polybutadiene, 1,4-polyisoprene, and 3,4-polyisoprene have been hydrochlorinated (Golub and Heller. 1964a, 1964b; Heiling et al., 1976). Hydrohalogenation of polybutadiene has been advantageously accomplished by phase-transfer catalysis (Adams et al., 1978). Hydrochlorination of 1.4-polyisoprene in chloroform yields a soluble product that can be plasticized to give a tough flexible film useful in packaging (Cunneen and Porter, 1973). Recently. Bruzzone and Carbonaro (1985) hydrochlorinated transtactic polydiolefins, for example, ~ ~ U I I S 1,4-polybutadiene, trans- 1.4-polyisoprene, and isotactictrans-1 ,4-polypiperylene. All of the precursor polymers are high-melting nonelastomeric materials prior to modification. After hydrochlorination, several of the adducts (e.g., hydrochlorinated trans- 1,4-BR and truns- 1,4 mix 1,2butadiene-piperylene copolymers) become semicrystalline or amorphous elastomers capable of reversible stress-induced crystallization. 6.
EPOXIDATION
Epoxidation is another well-known modification of unsaturated elastomers: RC03H
- CH =CH -
A
0 l \ -CH -CH-
Preacids (Ivan and Kennedy, 1990) or W-based redox phase-transfer catalysts and Yang. 1994) are typical catalysts for this process. The order of backbone reactivity in epoxidation is (Pinazzi et al., 1975):
(Crivello
.
Epoxidation of unsaturated rubber results in useful changes in the properties of synthetic
Schulz and Patil
120
elastomers. For example, epoxidized 1,2-polybutadiene shows improved adhesive strength, heat stability, and solvent resistance (Minoura et al., 1979). In turn, epoxidized EPDM has found use as a viscosity index improver in motor oils (Pappas and Kresge, 1973). Epoxidized SBS copolymers have been shown to have interesting stress relaxation behavior (Wu et al., 1994). Epoxidized rubbers are also ready sites for furtherderivatization (Campbell, 1973; Brosse et al., 1979; Schulz et al., 1980; Soutif and Brosse, 1984):
0
/ \ /
\
OH
>”
I
1
I
I 0-C-R II
-c-
c-
(1 1)
0-
H0
OH
\
I
/
-
C\
I
c-
-c-
‘
OH
LHR
7. ENE REACTIONS The ene reaction is a cycloaddition process involving acompound with a double bond (enophile) and an olefin containing at least one allylic hydrogen (ene):
The mechanism of this process can be free radical or concerted, depending upon the presence or absence of free radical sources as well as the reactivity of the reagents. Enophiles of low reactivity, such as maleic anhydride, usually require free radical catalysts and/or high temperatures. The latter processes are often accompanied by side reactions, e.g., chain scission or gel formation. In contrast, azo enophiles are highly reactive species allowing polymer modification with little or no backbone chain damage. The reactivity of azo-ene reagents with unsaturated elastomers is:
0
>>>
0 II II PhNH-C-N=N-C-OEt
0
0
II
II
>> t-BuNH-C-N=N-C-OEt)
0
R (V)
0
>
II
0
II
0 I1
PhC-N=N-C-OEt PhNHC-N-N-C-NHPh
0
II
0
2
0
I1 II ROC-N=N-COR
Chemical Modification of Synthetic Elastomers
121
The cyclicanalogs [e.g., triazoline diones (V)] areorders of magnitudemorereactivethan the acyclic analogs (Porter. 1977). The cyclic azo-ene modification of diene rubbers has been extensively studied and was reviewed by Butler (1980). The acyclic ene [e.g., azoesters (VI)] modification of olefinic elastomers has been comprehensively studied by Schulz et al. (1980).They found that the order of BR backbone reactivities toward azo esters is:
Moreover, the positions of substitution on the 1,4 backbones are the allylic carbon atoms, with a consequentshifting of the double bond along the chain. In turn,modification of the 1,2 backbone results in isomerization of the vinyl units to internal double bonds: -(-CH~-CHZCH-CH~-)-+ ROOC-N=N-COOR+
-(CH=CH-CHr-CH-)-
I
N-COOR
I
(1 3)
H-N-COOR -(-CH2 CH2-)-+ \ l CH
ROOC-N=N-COOR+
-(-CH2 CH2-)\ l CH
II
I
CH=CH2
(14)
CH-CH2-N-COOR
l
H-N-COOR Spectroscopic, thermal, and dynamic mechanical data all suggest a random modification for the 1,2 backbones and a nonrandom (“blocky”) placement for the 1,4 backbone. The isopropyl azodicarboxylate-modified 1,bpolybutadienes showexceptional green strength (Fig. 1 ) and tack (Fig. 2) properties. In fact, the green strength and tack of these amorphous elastomers approach thatof natural rubber, a strain-crystallizable elastomer (Spiewak et al., 1981). The mechanism for such property enhancements for these modified polybutadienes is a matter of controversy. Some groups believe that it is caused by strain-induced phase separation (Hamed et al.. 1983; Hamed and Shieh, 1984). while others feel that it arises from polar and H-bonded interactions (Roland and Bohm, 1984; Roland et al., 1985). Azo-ene synthons have also been developed for the modification and grafting of NR [Eq. (14)] (Porter, 1977: Campbell et al., 1978, 1979). Presumably, these reagents could also be used to modify synthetic polydienes [Eq. (15)1.
p
-
I
y-N-NHCOOR
0 y= -NH+,
-NH+-X;
-NH-R-S1 (OR)*;
E
- 0 R-polystyrene
Schulz and Patil
122
4.0 r STRESS ( MPo)
3.0 -
2.0 -
14.6 o/o
200
400
600
800
1000 1200
O h ELONGATION Fig. 1 Grecn strcss-strain curves for of 1.4-(cis,trrr~1s)-polybutadicne tmts modified with various lcvels (mol%) of isopropyl azodicarboxylate. (From Spiewak, 198 1.)
Free radical ene modificationsof elastomers have been accomplished by maleic anhydride (Jois and Harrison, 1996) or maleates (Sen et al., 1991).
8. IONOMERICMODIFICATION Ionomeric nlodification of elastomers involves the introduction of low levels (510 nlol%) of ionicfunctionality (e.g., COO-. SOi-) into therubberybackbone. These ionic groups can
TACK ( k Nf m )
1.0
0.8
-
0.6
-
0.4
-
0.2 -
0
5
IO
l5
MOLE % MODIFIED
20 IAD
Fig. 2 Wind-up tack a s a function of mole pcrcent o f repcat 1.3-(c.is,tr.rr~1s)-polyhutadiene repeat units that are modified by isopropyl azodicarboxylate. (From Spicwak. 1981.)
Chemical Modification of Synthetic Elastomers
123
Table 5 Typical PropertyRange of Ionic Elastomer Compounds Property
range
Shore A hardness 1008 modulus. Mpa Tensile strength, Mpa Elongation, 8 Tear strength, N.m Specific gravity Compression set (room temp.) Brittle point, “C Processtng temperature, “C
Typlcal
45-90 1.2-6.9 3.4- l 7 350-900 15-38 0.95-1.95 30-35 -56-46 150-260
associate to form ion-rich aggregates in a nonpolar rubbery matrix. Such ionic domains have a profound effect on the physical and nlechanical properties of the elastomers. Elastomeric ionomers have been the subject of a review by MacKnight and Lundberg (1984). Carboxylate functionalities are usually introduced by direct polymerization. The exception is the hydrocarboxylation of polybutadienes with Pd catalysts (Lapidus, 1989: Narayanan. 1992, 1993). On the other hand, sulfonate groups are more commonly introducedbypostpolymerization modification. For example, EPDM is preferably sulfonated by either complexed forms of SO3 (Cantor, 1972) oracetyl sulfate (Makowski et al.. 1980; Thaler, 1982, 1983). The actual structure of the sulfonated product is sometimes complex and depends upon the diene (e.g., ethylidene norbornene or dicyclopentadiene) used in the EPDM (Thaler, 1983). The sulfonated materials are usually neutralized with metal salts to enhance their properties and stability. The properties of the sulfonated elastomeric ionomers depend upon the crystallinity of the backbones, the level and type of ionic functionality. the degree of neturalization. and the type of cation(e.g., ammonium salt. monovalentmetal,divalent metal). Sometimes suitable polaradditives(e.g..zinc stearate) areadded to facilitate melt processing (MacKnight and Lundberg, 1984).Uniroyal’s ionic elastomer (Uniroyal, 1982) is a commercially available sulfonated ionomer that can be formulated into a wide variety of applications (adhesives, impact modifiers, footwear, garden hose, etc.). Typical ranges of physical properties available from “ionic elastomer” compounds are shown in Table 5. Polyurethane (PU) rubbers have also been modified to form anion. cation, and zwitterion ionomers (Speckhard et al., 1984). Ionomeric modification of polyether polyurethanes tends to improve the degree of phase separation and hard-segment cohesion, with a consequent improvement in mechanical properties compared with unmodified polyurethanes.
9.
CARBENE AND SULFENYL CHLORIDE ADDITION
Carbene addition to polydiene rubbers increases the refractive index, viscosity, flow activation energy, and T, of these polymers. The process is facilitated by the use of quaternary ammonium salts as phase-transfer catalysts. The latter compounds bring the aqueous reagents into contact with the organic phase containing polymer (Pande and Joshi. 1974; Barantsevich et al.. 1978; Konietzny and Biethan, 1978; Sang and Tab, 1979):
Schulz and Patil
124
-(-CHr-CH=CH-CH2-k + HCCl3 + -(-CHr-CH-CH-CH?-)\ l C l \
c1 c1
Carbenation has also been used as a method for modifying block polymers. For example, partially hydrogenated radial block copolymers of 1,4-isoprene and medium vinyl polybutadiene have been carbenated by dichlorophenylthio- and phenylchlorocarbene. The polyisoprene blocks are the sitesfor carbenation (La1 et al., 1982). The alkaline hydrolysis of chloroform has resulted in dichlorocarbene-modified SBR, which has showngood mechanical properties, excellentflame and solvent resistance, and good thermal stability (Ramesan and Alex, 1998). Sulfenyl chloride addition also raises the T, of unsaturated rubbers:
c1
I
ArSCl + -(-CH2-CH=CH-CH2-)---(-CH2-CH-CH-CH2-)-
I
SAr Furthermore, it has been found that p-toluene sulfonyl halide adds in a blocky fashion to unsaturated rubbers while o-nitrophenyl sulfenyl chloride adds in a random fashion (Cameron and Muir, 1976; Buchan et al., 1978; Buchan and Cameron, 1979).
10. HYDROSILYLATION The introduction of silyl groups to rubbery backbones [Eq. (18)] is of interest because such modified elastomers show improved adhesion to fillers (Witte et al., 1975) and better heat (or oxygen) resistance (Ito and Kirnitaka, 1977). HSiCI3 -CH=CH-
>
HPtCI,
-CH-CH-
I
I
H
Sic13
(18)
The order of reactivity of unsaturated rubbers with HSiC13. catalyzed by hydroplatinic acid, is (Pinazzi et al., 1975):
The vinyl groups of 1.2-polybutadiene have been hydrosilated with dichloro(methy1) silane and subsequently grafted with living anionic chains (Cameron and Qureshi. 1981; Hempernius et al. 1997; Xenidou and Hadjichristidis, 1998).
Chemical Modification of Synthetic Elastomers
125
11. BINDING ANTIOXIDANTS One of the more useful chemical modifications of rubbers is the binding of antioxidant molecules to the polymer chains. This can be achieved by chemistry similar to that previously described, e.g., ene and carbene reactions, as well as thiol-type additions (Kuczkowski and Gillick, 1984; Ceansescuet al., 1985) [Eqs. (19)-(21)]. The chemical attachment of antioxidants not only minimizes migration of the antioxidants but enhances their effectiveness. The polymer-bound antioxidants improve thermal stability and heat resistance (Al-Malaika, 1990; Arshady, 1997). Patil and Datta (1994, 1997) reported polyaniline and fullerene grafted EPDMs with improved thermoxidative stability.
C l ,CH3
Schulz and Patil
126
12. MISCELLANEOUSCHEMICALMODIFICATIONS Other modifications of synthetic elastomers worthy of note include hydroboration (Pinazzi et al., 1975). hydroformylation (Sanui et al., 1974; Bhattacharjee, et al. 1992), nitration (Kostov et al., 1980). and nitromercuration (Chien et a l . , 1980).For example, hydroboration with dialkylboranes leads to intermediates from which other functionalities can be derived:
>-,
/W--CH=CH-W
R2BH
\
/
/
\
p-c-c-
+<-c ' I COOH
-
OH
The order or reactivity of unsaturated elastomers to dialkylboranes, such as 9-borabicyclononane. is (Pinazzi et al.. 1975):
I n turn, hydroformylation involves the rhodium-catalyzed reaction of CO/H, with unsaturated polymers. The hydrofornlylated products can be further converted to aldoximes by using hydroxylamine hydrochloride or further dehydrated with P,05 to form cyano polymers: CO/H2
-CHXH -
- CH - CH2-
> -
Cat
I
.
CH0
L -CH-CH2 -
I
HONH2HCI
- CH - CH2-
P205
l
<-
CN
C=NOH
Moreover. nitromercuration of diene polymers is an excellent example of the use of phasetransfer catalysis i n the modification of diene elastomers: NO2 HgCI2MaN02
- CHxCH -
> & N Y , CHzClz
I -CH-CH-
I
Hg
Chemical Modification of Synthetic Elastomers
127
13. SURFACE MODIFICATIONS Polydienes have also been surface modified in solution. For example, syndiotactic 1.2 polybutadiene has been reacted on thesurface by photolyticaddition of thiols,as well astreatment with aqueous permanganate. Such surface treatment leads to changes in wettability (Carey and Ferguson, 1994).
14. SUMMARY This chapter surveys the chemical modification of unsaturated synthetic elastomers. Emphasis is placed on the use of chemical methods to introduce reactive functionalities along synthetic elastomer backbones. Special attentionis drawn to modifications of intrinsic scientific or technological interest. On the scienceside, new processes (e.g., phase-transfer catalysis) and new understandings (e.g., random vs. blocky modifications; modificationas an aid to characterization) are featured. On the technological side, commercially important modified synthetic elastomers (e.g., halobutyl) are covered.
REFERENCES Abdel-Razik, E. A. (1988), Po/yrrwr- 29(9):1704. Adams, H. E., Boutsicaris, S. P,, and Hnlasa, A. F. (1978). U.S. Pat. 4,129,699. AI-Malaika, S . (1990). CHEMTECH 20(6):366. Arshady, R. ( 1997). Desk Rqkr.rcwc.ac$F~rrlctiorlrr/ Po/ymers: Synthesis r r r d Applictrtiorrs. ACS. Washington, DC, p. 62 1. Ast, W.. Zott, C., and Kerber, R. ( 1 9 7 9 ~Mukrorrrol. Cl~err~. /80:3IS. Azuma, C., and MncKnight, W. J. (1977), J. Po/ym. Sei. Po/yrrr. Cherr~.Ed. 15:547. Baldwin. F. P,. Buckley, D. J., Kuntz. I., and Robinso, S. B. (1961), Ruhher P/nstic.s A,ge 42:SOO. Barantsevich, E. N., Breslen, L. S., Rabinerson, E. L., and Kalans, A. E. (1978), V w o k o r n o l Sordir~Ser. A 20: 1289. Bhattacharjee, S., Bhowmlck, A. K., and Avasthi, B. N.(1990~1).J. A/>/>/.P o / w . Sci. 41:1357. Bhattacharjee, S., Bhowmick, A. K., and Avasthi. B. N.(1992), J. Po/yrr~.Sei.. Purt A, Po/yrrr. Clreru. 30: 471. Bhattacharjee. S., Bhowmlck. A. K.. and Avasthi, B. N.(l990b), J. Po/yrrr. Prrrt A, Po/ynr. Chern. 30: 1961. Bhattacharjee, S.. Rajagopalan, P,, Bhowmlck, A. K. (1993a), J. A/>/>/.Po/yr. Sei. -16:1971. Bhattacharjee, S., Bhowmick, A. K., and Avasthi, B. N.(1993b). Pnlyrrrer 34:24. Blackshaw, G. C. (1978). in The Vmtlerbilf Rubher Htrrrr/l>ook (R. 0 . Babbit. Ed.), R. T. Vanderbilt Co., Norwalk, CT, pp. 102-127. Brosse. J. C., Soutif, J. C., and Pinazzi. C. ( 1 9 7 9 ~Mtrkrormrl. Chem. 180:2109. Bruzzone, M., and Carbonaro, A. ( 198S), J. P d y m Sei. Po/ym. Ckeno. Ed. 2.3: 139. Brydon. A., and Cameron, G. G. (1975). in Progress irr Pn/ymer Scicwce. Vol. 4 (A. D. Jenkins, Ed.). Pergamon Press, Oxford, England, Ch. 4. Brydson, J. A. (1978), Ruhher Cherrlistry, Applied Science Publishers, London, England. Ch. 7. Buchan. G. M,, Cameron, G. G.. and Christi, S. A. A. (1978). Mnkrortlol. Chew. 179: 1409. Buchan. G. M,, and Cameron, G. G. (1979). J. Cltern. Soc. Perkirt Trms. I, p. 783. Butler. G. B. (1980), I d . Eng. Cheru. Prod. Res. Dotr. 19512. Cameron. G. G., and Muir. R. B. (1976). J. Po/ynr. Sei. Po/yrrl. Lett. Ed. 14661. Cameron, G. G. (1980). in Mtrcrorr~olecrrlrrr-Cl~ernistry,Vol. 1 (A. D. Jenkins and J. F. Kennedy, Eds.), Royal Society of Chemistry. London, Ch. 16.
128
Schulz and Patil
Camcron, G. G.. and Qureshi, M. Y. (1981), Mocrotnol. Chern. Rcrpirl C ~ l ~ J l l l l U 2:237. lf. Campbell, D. S. (1973), Br. Polvn~.J. 5 5 5 . Campbell, D. S.. Loeber, D. E., and Tinker. A. J. (1978). Polynrer 1 9 1 107. Campbell, D. S.,Loeber, D. E., and Tinker, A. J. (1979), Polymer 20:393. Cantor, N. H. (1972), U.S. Pat. 3,642,728. Carbonaro, A., and Greco, A. (l966), C l ~ e nI~n .d . (Milan) 48:363: Cl~ern.Ahstr. 65:5S36h (1966). Carey, D. H., and Ferguson, G. S. (1994). Mrrcrornolecule.s 277263. Ceansescu, E., Bittman, S., Fieroui, V., Badea, E. G.. Gruber, E., Ciupltoui, A., and Apostol, V. (1985). J . Mtrkrornol. Sci.-Chern. A-22:525. Ceresa, R. J. ( 1978), in Science m d Techol ogy of Ruhher (R. Eirich, Ed.), Academic Press, New York, Ch. 11. Chamberlin, Y., Golc, J., Pascault, J. P,, Durand, J. P,, and Dawans, F. (1979), Makrcrr~rol. Cllern. 180: 2309. Chamberlin, Y.. Pascoult, J. P,, Razzowk, H. and Cheradame, H. (198 I ) , M ~ k r o ~ nCl~ern. d. Rapid Cr~~nmun. 2323. Chen, H. Y. (1977), J. P d y n ~ Sei. . Polyrn. Left. Ed. /5:271. Sei. Chien, J. C. W., Kohara, T.. Lillya, C. P., Saruhbi, T., Su, B. H., and Miller, R. S. (1980). J. Polyr~~. Polym. Chenr. Ed. 1812723. Chitale, V., Rajan, C. R., Ponrathnam, S., Pandey, U. C., and Sai, K. S. K. (l994), IndicI Po/yrrr. Sei. 2: 8 IO. Condon, J. R. (1978), in The Vmdrrhilt Ruhlwr Hrrndhook (R. 0 . Babbit. Ed.), R. T. Vanderbilt Co., Norwalk CT. pp. 133-136. Costello, C. A., Wright, P. J., Schulz, D. N., and Sissano, J. A. (1996), U.S. Pat. 5,399,632. Crivello, J. V., and Yang, B. ( 1994), J. Mncrornol. Sci. Pure Appl. Chern. A31(5):517. Cunnecn, 3. I., and Porter, M. (1973), in Encyclqwditr of Polyrner Science m r l Technology, Vol. 12 (H. Mark, Ed.), Wiley. New York, pp. 304-327. De I n Mare, H. E., and Shaw, A. W. (1972), U.S. Pat. 3,670,054. De In Mare. H. E. (1973). U.S. Pat. 3,766,300. DC Saukar, M., De, P. P,, and Bhowmick, A. K. ( 1997). J. Appl. Po/ynr. Sci. 66:1 1 S 1. Falk, J. C., Hoeg, D. F., Schlott, R. J., and Pendlcton, J. F. (1973). J . Mrrkrornol. Sci. Chern. 71669. 15:1451. Farina, M,. Silvestro, G. D., and Sozzani, P. (1982), Mtrcro~~~olrcrtle.~ Fettcs, E. M. (Ed.) (1964). Clrernicrrl Reccctions of Po/ymer.s, New York. Gerberding, K., Seilcr, E., and Stein, D. (1977). Ger. Put. 2,610,068. Gilles, G. A. (1974), U.S. Pat. 3,792.127. Golub, M. A., and Helier, J. (1964a). J. Poly~n.Sci. Polyrn. Lett. Ed. 2523. Golub, M. A., and Heller, J. (1964b). J. Poly~n.Sci. Poly~n.Lett Ed. 2:723. Golub, M. A. ( 1969), in P o l p w Chert7istry of Syrrfhetic Ehs/onlers, Part I1 (J. P. Kennedy and E. G. M. Tornquist, Eds.), Intcrscience, New York, Ch. IOB. Golub, M. A. ( 1970). in The Chenlistry of Alkenes, Vol. 2 (Z. Zabicky, Ed.), Wiley-Interscience. London, Ch. 9. Golub, M. A. (1978). Ruhbrr Clwrn. T e r h o l . 51:677. . 71:77. Graesslcy. W. W.. and Raju. V. R. (1984). J. Polwr. Sei. P o l y ~ n S. ~ n p Ed. Hahn, S . F. (1992), J. Polvnr. Sci. A.: Poly~n.Clrenr. Ed. 30:397. Halasa, A. F. (1975). Ger. Pat. 2,457,645. Halasa, A. F. ( 1981), Rubber Chern. Techno/. 154:627. Halasa. A . F., Massie, J. M., and Cerasa, R. J. (1994). in Science crnd T e c h o l n g y of Rubher, 2nd ed. (Mark, J. E., Erman. B., Eirich, F. R., Eds.) Academic Press, New York, pp. 513-544. Hamed, G. R., Shieh, C. H., and Schulz, D. N. (1983), Polw?. Bull. (Berlin) 9 5 2 5 . Hamed. G. R., and Shieh, C. H. (1984), Ruhhrr Chern. Technol. 57:227. Hampcl, C. A., and Hawley. G. G. (Eds.), The Encylopedirr cfCltenli.str:~,32nd ed., Van Nostrand, New York, 1973. p. 981. Harwood, H. J., Russell, D. B., Vertkc, J. J., and Zymons. J. (1973). Mtrkromol. Clfern. 163: 1. Harwood, H. J. (1982). Rubber Chem Technol. 55:769.
Chemical Modification of Synthetic Elastomers
129
Heiling. P,, Wewerka, D., and Hummel, K. ( 1976). Ktrutsch. Gurlurli Krrrrslsl. 29238. Hempernius, M. A., Michelberger, W., and Moller, M. ( 1997), Mrccrorr~ol~~cu1r.s 30:5602. Holden, G. (1973). in Block t r t d Grrft Co/)o!\Irrlc~ri.atiorr,Vol. 1 (R. J. Ceresa, Ed.). Wilcy, New York, p. 133. Inomatn, J., Michijima, S., Kasahara, K., Hino, S., Igarashi, S.. Takamiya, N., and Tano, T. (1976), Jpn. Pat. 7,661,593; Cheru. Ahstr: 85:95 154 (1976). Inomatn, J., Hino, S., Igareshl, R., and Takamiya, N. (1978). Jpn. Pat. 7,826.890; Chetu. Ahstr. W75880 (1978). Ito, K., and Kimitaka, K. (1977). Jpn. Pat. 7,777,194; Cherrz. Ahstr. 87:169043. Ivan, B., and Kennedy, J. P. (1990). J. Po!\?rl. Sei. Pur/ A: Po/yrm C / w m 28:89. Jois, Y. H. R., and Harrison, J. B. (1996), J. Mrrcrortrol. Sei. C36(3):433. Jones, R. C. (1971). U S . Pat. 3,629,371. Jones, R. V., Maberly, C. W., and Reynolds, W. B., ( 1 9 5 3 ~6 1 d . Eng. Cherv. 45:I 121. Kang, J. W. (1976). U.S. Pat. 3,993,885. Karairanova, M,, Mladenov, I.. Karpchanska, K., and Alesksier, D. ( 1 9 7 6 ~Kkirn. lrd. (Sf!firr) 48:462; Chetrl. Abstr. 86: 1077 14 (1977). ~ N S12. Karairanova, M., Mladenov. I., and Kosev. P. (1978). God. Vi.ss11. Khim-Tekrd. O r s f . B I ~ ~ ~Brtlg. Pt. 2:71; Chetrl. Ahstr. YO: 105286 (1979). Kawaguchi, H., Loeffler. P., and 0. Vogd (1985), Po1ytrler 261257. Konictzny, A., and Biethan, U.( 1 9 7 8 ,~4 q . Mrrkrorrd. C1jerrz. 74:61. Kossler, I., Vodehnal, J., Stolka, M,, Kalal, J.. and Hartlora, E. ( 1967), J. Polym. Sci. C 16:13 1 1. Kostov, G., Slavcheva, J., and Mladenov, I., (1980). Mdworrlol. C11cw1.181:2587. Krause, R. L. (1975). U.S. Pat. 3,898,206. Kuczkowski, J. A., and Gillick, J. G. (1984). Ruhher C1terrf.Techr~ol.57621. Kunz, I. (1983), in Motlificcrtior~ofPo/yrrwr.s (C. Carraher, Jr., and J. A. Moore, Eds.), Plenum Press, New York, pp. 53-63. Kuz’mlna, V. Y., Tikhomlrov, B. I., Belor, I. B.. and Kakus, A. E. (1976), IT. Vys.sh. Ucherhr. Znvled. Khirrr. Khirrr. Techno/.19: 1439; Cheru. Abstr. 86:75054 ( 1977). Lal, J.. Smith, R. P., and Patterson, D. B. ( 1982). Proc. IUPAC 28th M~crorttolrcrrltrrSyrr~p.,University of Massachusetts, Amherst, MA, July 12-16. Lapidus, A. L., Grobovenko, S. Y., Goryachevskaya, A. V.. Berman, E. L.. and Goryachevskaya, A. V. (1989). I.s\~. Aknd. Nnuk. SSSR. Srr. Khirll. 8:1390. Laramee, A., Goursot, P.. and Prud’homme, J. ( 1975). M r r k r o r d C1reru. 1763079. Luxton, A. R. ( 1981), Ruhlwr Clwrrz. T e c h d . 54:596. McGrath, J. E., and Wang, I. C. (1980). Am. Clreru. Soc. D V . P o / w . Cherrr. Pdytrr. Prepr. 21( 1):69. MacKnight, W. I., and Lundberg, R. D. (1984), Rubber Chert?.Techtrol. 57:652. Makowski, H. S., Lundberg, R. D., and Bock, J. (1980), U S . Pat. 4, 184,988. Makino, K., Ikeyema, M,. Tekeuchi, Y., and Tanaka, Y. (1982), P o / w e r 23:287. Mango, L. A., and Lenz, R. W. (1973). Mnkrorrd. Cllerrl. 163: 13. Mays, J., Hadjichristidis, N., and Fetters, L. J. (1984). Mncronu)/ecu/es172723. Marechal, E. (1989), in Cornpre/Ietzsi\v Po/vmrr Science.: The S~~r1111esi.s. CI~~rrrrctrri~otiorl. Recrctior~s~ r l d App1ic~rti0jz.sof Po/yrrlers, Vol. 6 (Allen, G., Bevington, J. C., Eds.), Pergamon Press, Oxford, England, p. I . Minoura, Y., Yamashita, S., Yamaguchi, H., Khojiya, S., Yamada, K., Muko, M,, and Nishimura, T. ( 1979), N i p p o t f Gunmi Kyokochi 52517; C11etr1.Ahsrr. 91: 158,799 (1979). Maczygemba, G. (1977). U.S. Pat. 4,163,765. . Mohajer, Y., Wilkes, G. L., Martin, M,, Wang, I. C.. and McGrath, J. E. ( l980a), ( 1980b). A ~ IClwnl. Soc. D;\-. Po/yrrr. C1rertf.Po/ym. Prepr. 2 / ( 1):43; 21(2):191. Mohajer, Y., Wilkes, G. L., Wang, I. C., and McGrath. J. E. (1981), Am. Cltrm. Soc. D;\,. Polyrtl. CIIem. Po/ytrl. P r e p . 22(2): 138. Morton, M., Lee, N. C., and Tcrill, E. R. ( 1981). Atn. Chrm. Dit,. Po/ym. Clzertr. Po/yru. Prepr. 22(2):136. Nang, T., Katabe. Y., and Minoura, Y. (1976), Po/.vrtzer 1 7 117. Narayanan. P., Kaye, B., and Cole-Hamilton, D. J. ( 1992). J. Mater. C h m . 2: 1149.
Schulz and Patil
130
Nnrayanm, P,. Iraqi. A., and Cole-Hamilton. D. J. (1993), J. Muter. Cherr~.3:19. Naylor, F. E., and Zelinski. R. P. (1978), U.S. Pat. 4,107,236. Neumann, R., Sanui, K., and MacKnight, W. J. (1975), Mtr~rorrro/rcu/e.sH:665. Noshay, A.. and McGrath, J. E. ( 1977), Block C o ~ ~ ) l y r r l e r - ~ t ~ rtrr~d r , ~CriticcrI i c ~ ~ ~ Slrnvy, Academic Press, NCWYork, pp. 232-237. Pandc, L. M,. and Joshi, G. C. (1974). India Pat. 132,396; Cherr~.Ahstr. 82:32205 (1975). Pappas. J. J., and Kresge, E. J. (1973). Gcr. Pat. 2,313,874. Patil, A . O., and Datta. S. ( 1994), Arr~.Cl~errr.Soc. Dit,. Po/ynr. Cl~enr.P o / y n ~Prepr. . 35( 1 ):716. Patil, A. O., and Datta, S. (1997), Aw. Clwrrr. Soc. Dit,. Po/yrtr. Cllerrr. Po/yrr~.Prrpr. 31(2):456. Patil. A. O., Schulz, D. N., and Novak, B. M,, Eds. ( 1998), Fmctiorzd Po/yrlzer.s: mod err^ Syrrthetic Met/~od.sL r r d N o t ~ Strlcctures. l Am. Chem. Soc.. Washington, DC. Pinozzi, C., Brosse, J. C.. Pleurdeau, A., and Reyx, D. (1975), A/)/>/,Po/yrn. Syrnp. 2673. Pinazzi, C., Vnssort, J.. and Rcyx, D. (1977a), E w . Po/yrn. J . 13:395. Pinazzi, C., Guillaumc, P,, and Rcyx, D. (1977b), Eur. Po/yrrr. J. /3:707. Porter. M. (1977), Eur. Rubher J. 159: 13. Rachapudy, H., Smlth, G. G., Raju, V. R., and Grasslcy, W. W. ( 1979). J. Po!vru. Sci. Po/yrn. Pl7y.s. Ed. 17:1211.
Rahrig, D., and MacKnight, W. J. (1979), Mrrcrorr~o/~.c~r/e.s 12:195. Rahng, D., and MacKnlght. W. J. ( 1980),/ o m ill Po/yrwr.s. Am. Chem. Soc., Washington, DC, pp. 91-1 18. Rameson. M. T., and Alex, R. ( 1998), J. App/. Po/yrrr. Sei. 68: 153. Ramp, F. L., Dcuitt, E. J., and Trapasso, L. E. (1962). J. Org. Clrcw~.27:4368. Randall, J. C. (1977), J. Po/ynr. Sci. Po/yrlr. P h y ~Ed. /5:1451. Randall. J. C. ( 1978), J. Po/yrr~.Sci. Po/yrrr. Phys. Ed. 162667. Riess, G. ( 1977). Alrg,.Mtrkrortrol. Cllerrr. 60/6/:2 1. Roland, C. M., and Bohm, G. G. A. (1984) J. App/. Po/yrr~.Sei. 29:3803. Roland, C. M,, Bohm. G. G. A., and Sadhukhan. P. (1985). J. App/. P o / w r . Sci. 30:2021. Sang, S., and Tab, M. (1979), J. RuDhrr Res. lust. Mrrlrrysitr 26:431; Cllerrl. Ahstr. 91:212357 (1979). Sanui, K., MacKnight, W. I., and Lenz. R. W. (1973). J. Polym. Sei. Pdyrrr. Let!. Ed. /1:427. Sanui. K., MacKnight, W. J., and Lcnz, R. W. (1974). M~ccrorrlo/Pc.u/e.s7952. Schultz, W. J., Etter. M. C., Pocius, A. V., and Smlth, S. ( 1980), J . Am. C h m . Soc. 102:7982. Schulz, D. N., Spicwak, J. W., Valatis, J. K.. Mochcl, V. D., and Barzan, M. L. (1980), Mtrcru,rrro/rc.u/e.s 13:1367. Schulz, D. N., Tumer. S. R., and Golub, M. A. (1982), Rddwr Cl~errr.Tech,~o/.55:809. Schulz, D. N. (l986), i n E~~c:\~c/o/~er/i~r of'Po/~'rr~rr-Scierrce frrlrlErlgi/leerirlR.Vol. 6 (H. F. Mark, N. Bikales, C. C. Overbeger, and G. Mengcs, Eds.), John Wiley & Sons, New York, pp. 807-817. ~ Sen. A. K.. Mukherjee, B.. Bhattacharyya, A. S., DC P. P,, and Bhowmick, A. K. (199 1 ), A I JMrrkrnrrrol. Cllc~rfl.101:3206. Shagov, V. S., Yakubichik, A. I., and Podosokorskyaya, V. N. (1969), P o / v n ~Sei. . USSR 11:1092. Shahab, Y. A., and Basheer, R. A. (1979). J . Po/yrr~.Sei. Po/vrrl. Chern. Ed. 17910. Shaw. A. W.. (1971), U.S. Pat. 3,634,549; C ' / ~ m r . Ahstr. 76:128453 (1972). Sinaiski, G. M,, Sorokina, N. L., Mikheer. A. l., Fraumovich, L. D., and Kovalev, N. F. (1977). ProrrriSt sirrt Kmrclr. 1 : 8 ; Cherrr. Asbtr. 87244576 ( 1977). Soutif. J. C., and Brosse, J. C. (1984). Mrrkrnrr~ol.Cherrr. 185:839. Spcckhard, T. A., Hwang, K. K. S., Yang, C. Z., Lampan, W. R., and Cooper. S. L. ( 1984), J. M~rkromnl. S ~ i - l ' h y . ~B231 . 175. Spicwak, J. W.. Bryant, L., and Schulz. D. N. (198 1 ), J. Appl. P o / w ~Sei. . 26:4336. Thaler, W. A. ( 1982). J. Po/ytu. Sei. P o / w . Clwrrr. Ed. 202375. Thaler, W. A. ( 1983). M~rc~mrrlo/eclt/r.s 16523. Tikhomirov, B. I. ( 1968), Po/yr?r. Sei. USSR 10:2760. Uniroynl ( 1982), Ionic Elastomer, Uniroyal Technical Information Bulletin, Uniroyal, Middlebury, CT. Van Tongcrloo, A.. and Vukov, R. (l979), Proc. O r / . Rubber Cor$:79; Che/tr. Abstr. 02:130306 (1980). ~ . Po/yrrr. C'hrrn. Ed. 12:2255. Von Ravcn. A.. and Heusinger, H.( 1974). 1. P o / v r ~Sei. Wang, H., Bethea, T. W.. and Harwood, H.J. (1993) Mtrcrorm/ecu/r.s 26(4):715.
Chemical Modification of Synthetic Elastomers
131
Wnng, I. C., Minton, R. J., and McGrath, J. E. ( 1 983), Am. Cllrnl. Soc. Div. f o / w r . Clrum. Polyrrr. P r e p . 24(2):28. Wu, G.. Hsiue, G., and Yang, J. (1994), Moter.:C/ww. Phys. 3929. Witte, J., Gucnter, L.. and Pampus, P. (1973, Ger. Pat. 2,344,734; C / w m Ahstr. 83:44498 (1975). Xenidou. M,, and Hadjichristidis. N.(1998). Mac.rorl?o/ecu/e.s31:464.5. Xic, H., and Ma, L. (198.5). J. Mrrcrord. Sci.-C/zrnl. A22:1333. Yokota, K., and Hirabayashi, T. (1981), f o / w . J. 13:813. Yoshimato, T., Kaneko, S., Narumya, T., and Yoshi, H. (1969). S. Afr. Pat. 6,807,486; C/wm. Ahstr. 71: 92454 (1963). Zachoval, J., Kubat, J., Krepclka, J., Veruovi, B., and Mistr, A. (1973), SI,. V y . s k . Chrrtr. Trc.hr~ol.. Prrrze. Org. Chem. Trcl~rzol.Clh':37; Cl~c~rlr. Abstr. 79:137805 (1973). Zhongde, X., Mays, J., Xueln, C., Hadjichristidis, N., Schilling, F., Bur, H., Pearson, D. S., and Fettcrs. L. J. (1985), Mrrcron1olec14lr.s18:2.560. Zotteri, L., and Giuliani, G. P. (1978). P o / w r r 1 9 4 7 6 .
This Page Intentionally Left Blank
Liquid Rubber Douglas C. Edwards* Polysar Limited, Sarnra, Ontario, Canada
1. INTRODUCTION Conventional vulcanized elastomers, whether naturalor synthetic, consist of very long molecules connected into a continuous network by means of occasional crosslinks. High extensibility is possible because the chains are sufficiently flexible and mobile, at temperatures encountered during use, to accommodate large imposed strains without suffering local chain failure. The retractive force following deformation is essentially entropic in origin and depends on the freedom of the chains to undergo very facile thermal motion with respect to one another. Hence elastomers are based on polymeric molecules that at ambient temperatures are far above their glass transition temperatures and are also amorphous (in the unstrained state at least) and relatively free of highly polar or bulky side groups. In practice. the molecular weight between crosslinks requiredto provide a suitable balance between high extensibility, elastic recovery, and strength properties is in the order of 10,000. Consequently, the molecular weight of the polymer must necessarily be high. for otherwise the proportion of dangling chain ends, which cannot contribute to either strength or elasticity, will become excessive. In the case of butyl rubber gum vulcanizates, for example, high strength requires nlolecular weights of at least 100,000 (Flory. 1946). These fundamental considerations are of central importance when considering liquid polymers. The term “liquid” implies easy pourability at ordinary temperatures, and in general this means a molecular weight in the region below about 5000. The constraints therefore impose a requirement that the liquid polymer chains be linked end to end, during cure,so as to be capable of forming a finished network with few free ends and with appropriate average chain lengths between the crosslinking sites. The liquid polymer must carry reactive groups at the chain ends to provide a mechanism for the occurrence of chain extension. This is illustrated schen~atically in Figure 1. The term “telechelic” [from the Greek words telos (end) and chele (claw)] was proposed by Uraneck et al. (1960) to describe such terminally reactive polymers and is now in general use. This chapter will consider only telechelic polymers, since other liquid polymers are not capable of providing strong elastomeric products. In discussing network constraints, we have considered one of the two fundamental requirements necessary for high-performance elastomeric materials. The second fundamental factor is reinforcement. As a generalization, reinforcement involves the presence of a second, harder phase within the continuous elastomeric matrix, the interfacial regions providing a locus for
133
Edwards
134
A
0
Fig. 1 Schematiccomparison of conventional ( A ) and liquid elastomer (B) networks.
stress dissipation under conditions of deformation that would otherwise result in the initiation of catastrophic failure. In conventional vulcanized elastomers, reinforcement is provided by the incorporation of very finely divided, high-surface-areaparticulatefillers. The carbon blacks represent by far the most generally used class of materials. The presence of carbon black raises the tensile strengthof amorphous (non-stress-crystallizing) elastomers by a factor of 10 or more, with concurrent large increases in modulus, tear strength, and abrasion resistance. Reinforcement is therefore essential to the service performance of most rubber goods. Thedetailed mechanisms of carbon black reinforcement remain somewhat controversial, a principal issue being the extent to which chemical bonding across the interface is involved. Valuable reviews of this complex subject have been provided by several authors, including Dannenberg ( 1 985) and Kraus (1977). Reinforcement by non-black finely divided fillers, notably fumed or precipitated silicas, is also extensively practiced. This subject has been reviewed by Wagner (1976). The presence of very small, hard regions within a continuous elastomeric matrix can be established by other mechanisms. In particular. the polymer chains may contain blocks (long sequences of monomer units or alternating pairs) that are inherently glassy or crystalline at ambient temperatures. These blocks may coalesce with others in neighboring chains to form glassy, or crystalline, “domains” within the elastomeric matrix. In thermoplastic elastomers of the ABA type, in which A represents a “hard” block and B an elastomeric block, the domains formed by coalescence of the A components serve both as reinforcement sites and as effective crosslinks to establish and maintain the shape of the unstressed product. In this case, a hightemperatureprocessing step is necessary to permitformation of the “hard” domains while cooling in the desired shape. The role of this type of mechanism, and of reinforcement effects in general, must be considered as one of the fundamental elements affecting the performance of liquid-based elastomeric systems. This chapter first outlines the history and current status of the principal classes of liquid elastomer systems that have achieved commercial importance.Some of the fundamental requirements are then discussed that would be necessaryfor hydrocarbon-based liquid-polymer technol-
135
Liquid Rubber
ogy to displace conventional elastomers in major segments of the rubber industry. Finally, some of the additional themes in telechelic polymer research, past and present, are reviewed.
2. 2.1
CLASSES OF COMMERCIALLY ESTABLISHED LIQUID ELASTOMERS Polysulfides
Polysulfides were the first synthetic elastomers to be manufactured commerciallyin North America, being introduced in the late 1920s. Subsequent developmentsin polysulfide technology led, during the 194Os, to the first family of liquid telechelic elastomers. The route to these products is an example of one of the several generic techniques for telechelic polymer synthesis, namely, the scission of a preformed long-chain polymer into shorter chains by a mechanism that results in reactive groups at the severed chain ends. The basic chemistry of the polysulfides has been reviewed by Bertozzi (1968). The initial discovery of polysulfide elastomers was made in 1920 by J. C. Patrick, who treated ethylene dichloride with sodium polysulfide during a study of possible routes to ethylene glycol (see Whitby, 1954). Patrick’s interest in the unexpected product, a rubbery gum, was the first step on the path to the polysulfide industry. An initial patent was granted several years later (Patrick and Mnookin, 1927). The reaction of organic dihalides with sodium polysulfide leads to linear condensation polymers:
nCI-R-Cl
+
nNazS.
- - (RS.),-
+
2nNaCI
(1)
In practice, a substantial excess of sodium polysulfide is used in order to maintain reactive and groups. Propagation would otherwise terminate becauseof the competing reaction of hydroxyl ions to yield -RS,R’OH. The excess sodium polysulfide functions by solubilizing the inert terminals into the aqueous phase:
- RS.R’OH
+
NaS.Na
- - RS.Na
+
NaS.R’OH
(2)
The two principal variables are the nature of the R group and the value of X (referred to as the “rank” of the polysulfide). Early products employed ethylene dichloride, but these products were found to be inferior in odor and in low-temperature flexibility to polymers based on bis-2-chloroethyl ether. In this case, however. the process produces substantial quantities of asix-memberedether-thioethercyclicby-product. The preferred monomer is now bis(2chloroethyl) formal, which is obtained economically by the reaction of ethylene chlorohydrin with formaldehyde:
2CICHzCH2OH * HCHO
- CICHZCH?OCH~OCH~CHZCI * Hz0
(3)
Small proportions of a trifunctional monomer, 1,2,3-trichloropropane, may be introduced to provide branched polymers. The preparation of liquid telechelic polymers by the scission of pre-polymerized polysulfides was introduced much later (Patrick and Ferguson, 1945). The parent polymer is produced by addition of monomers (difunctional and trifunctional chlorides) to excess aqueous sodium polysulfide in the presence of colloidal magnesium hydroxide and a small amount of surfactant. The high molecular weight polymer forms as a latex. This is then treated with sodium sulfite and sodium sulfhydrate. Polysulfidic linkages in the polymer chains are reduced to disulfide,
Edwards
136
and these are cleaved to the desired extent to provide terminal mercaptan or -RSNa groups as indicated by the following equations:
-RSSSR-
+
-RSSR-
+
NaSSR-
+
- -RSSR-
Na2S03
NaSH
- -RSH
-
+
+
NazS203
NaSSR-
Na2S03 NaSR-
+
Na2S20s
Finally, the mixture is acidified. This destabilizes the latexand also converts theremaining “RSNa terminal groups in the product to “RSH. During the subsequent washing and recovery of the product, molecular weight randomization occurs by interchange reactions between the mercaptan and disulfide groups, as illustrated schematically by the following equilibrium:
-RSSR-
+
R’SH
+ -RSSR’-
+
-RSH
(7)
The curing of the telechelic liquid polymer to form an elastic network requires both chain extension and crosslinking mechanisms (Fig. l ) . As a generalization, the terminal reactivity of liquid elastomers is always used for chain extension and is usually involved in crosslinking reactions as well. The crosslinkingstep, however, does not in principledepend on terminal reactivity. The liquid polymer may be synthesized with branched structures already in place. Crosslinking reactions different from those used for chain extension may also be applied to reactive sites along the polymer backbone. In the case of polysulfides, branched structures are present in the liquid product due to the presence of a trichloride during the synthesis of the parent polymer. The terminal mercaptan groups maybe reacted with tri-or polyfunctional agents to produce crosslink sites if desired, but in principle this is not necessary for network formation. The chain extension is normally carried out by oxidation reactions to form disulfide linkages. Useful oxidizing agents include metal peroxidesandorganicoxidizing agents such as peroxides or p-quinone dioxime. The following reactions are representative:
2 -RSH
+
6 -RSH 2 -RSH
PbOz HON
+
- -RSSR-
PbO
0.0. ,, -
CaOz
+
+
H20
3 -RSSR-
- -RSSR-
+
Ca(OH)2
+
,
H~O N ,N . 2
(8) +
2HzO
(9)
(10)
Lead dioxide is commonly used in formulations to be cured at room temperature in compounds in which a dark color is acceptable and the toxicity of lead is not an impediment. MnO,, CaO,, and Z n 0 2 are also used. Curing is generally accelerated by alkaline additives-amines, inorganic bases-and may be retarded by the addition of acidic materials such as stearic acid. A small amount of moisture accelerates the metal peroxide cures. When using calcium peroxide, Seegman et al. (1961) observed that dry compositions are stable in storage for several months but cure slowly upon exposure to ambient humidity. One-component sealant compositions are formulated on this basis. Doughty (1962) incorporated barium hydroxide into compositions of this type as a desiccant during storage and as a cure accelerator following exposure. The moisture cure of one-component polysulfide sealants can be accelerated by the use of organic additives. One such system (Doughty and Christman, 1967) employedan N.N’-dialkyl amide in the presence of dibutylbutyl phosphonate. A superior system (Doughty and Christman, 1969), claimed to be capable of curing to a depth of about 6 mm in one day at 27°C and 80% relative humidity. is achieved using a mixture of ( I ) an oxidizing metal oxide such as calcium
137
Liquid Rubber Table 1 Properties of Liquid PolysulfidePolymers LP-3 1 Viscosity (25°C). P 950- 1550 Mercaptan content, % 1.0-15 Average mol. wt. 8000 Pour polnt, “C IO Crosslinklng agent. o/o 0.5 Specific gravlty (25°C) 1.31 Avg. Viscosity (4°C). P 7400 Avg. Viscosity (65”C), P 140 Stress-strain Properties Tensile strength. Mpn 2.50 300% modulus, Mpa 2.07 Elongation. % 600 48 Hardness. Shore A Rec~pe:LP-31. LP-32. LP-2, LP-l2 Base Compound: Liquid polymer 100 30 N774 black Curing Paste: PbOT 7.8 HP-40 (plasticner) 4.8 Stearic acid 0.1 Alumina (ALzO?) 0.2 Cire: 2 hr at 70” in closed mold, postcure 20 hr at 2 3 T , 50% RH after unmolding.
LP-2
LP-33
410-52s 1 .S-2.0 4000 7 2.0 l .29 3800 65
410-52s 1.5-2.0 4000 7 0.5 I .29 3800 65
2.82 2.41
2.07 1.45 930 50
5I O S0
45
Black
LP- 12 410-525 1
4000 7 0.2 1.29 3800 65
LP-3
LP-33
9.4- 14.4 5.9-7.7 1000 - 26 2.0 1.27 90 I .S
15-20 5.0-6.5 1000 - 23
0.5 1.27 165
2.1
2.07 2.07 1.38 1.03 900 275 48 Recipe: LP-33 and LP-3
2.59 1.38 700 34
Base Compound Liquld polymer N990 p-Quinonedioxirne (GMF) Diphenylguanldine (DPG) 4. Magnesium oxde Sulfur
Io0 20 6.67 0.67 15 0.50
Cure: 20 hr at 77°C In mold, postcure mlnimum 2 hr at 23°C and 50% RH after unmolding.
peroxide, (2) metalhydroxides such as Ba(OH)? andor oxides such as BaO, and (3) a sulfonamide of thetype RS02NHR’, in whichR is an aryl group and R’ is an alkyl or hydrogen. The properties of liquid polysulfide polymers presently manufactured in North America are illustrated in Table 1. The strengthpropertiesattained with compositions of thistype are low compared withthose of conventionalreinforcedelastomers. Their utility depends on the physical form of the unvulcanized compounds (fluid or low-viscosity paste) together with adhesion to substrates and the development of elastic properties following cure. Resistance of oilsandsolvents,togetherwithgoodweatheringproperties(resistance toozone and moisture) and a fairly broad range of service temperatures, from about - 50 to about 120°C, renderthemsuitable for a variety of applications. These includevarioustypes of sealants used in theaircraft,automotive,building construction, marine,and,particularly,insulated glassindustries.Fluid compositions areused as electricalpotting compounds and as rocket propellantbinders.Polysulfidescan also be used to imparttoughness to epoxyresins, with whichtheycanbeco-cured:
+
Edwards
138
2.2
Silicones
The silicone elastomers are discussed in detail in another chapter, and hence the present section will be limited to a brief account of that part of the technology that relates to telechelic liquids. The basic polymerization chemistry is an exampleof a second generic mechanism for establishing terminal reactivity, namely, systems in which the initiation and termination steps inherently produce distinctive and groups. Silicone elastomers,in the formof high molecular weight gums, were introduced commercially in the 1940s, General Electric and Dow Corning being the dominant suppliers in North America. The polymersare based. in general. on dichlorodimethylsilane as thepredominant monomer. Polymerization occurs viahydrolysis,followed by condensation according to the following simplified scheme:
n(CH3)zSIC1z nHIO +
- H 0 I(CH&SiOlnH
+
2nHCI
(12)
Early use of the silicone polymers did not involve the terminal silanol sites, and hence the chain ends were normally blocked with inactivegroups such as -Si(CH3)3. Thehigh molecular weight polymers were compounded conventionally,in rubber process equipment, andvulcanized by the use of peroxides (see Bueche, 1955). During the 1950s, room-temperature vulcanizing (RTV) silicone compositions were introduced for use in sealant applications. Initially these were limited to two-component systems supplied as liquids or pastesthat,followingmixingandapplication, would develop elastic properties at room temperature. In the late 1950s, more sophisticated one-component systems were developed in which atmospheric moisture catalyzes the cure (see Ceyzeriat,1957; Brunner. 1959; Brown and Hyde, 1960; Nitzsche and Wick, 1960). The following generalized reactions are representative:
-
-RSi(OR')3 HzO -RSi(OR')20H R'OH 2 "RSI(OR')~OH -RSI(OR')~-O-SI(OR')ZR+
-
(13)
+
+
Hz0
(14)
Crosslinking occurs by the same nlechanism,eachhydrolyticscission of the =Si " S i O H sites that will condensetoform =SiUSi= crosslinks with elimination of a molecule of water, The curing reaction thus propagates rapidly through the material following initial exposure to external moisture. The physical strength properties of sealant compositions derived from liquid silicones are low and similar to those obtained with polysulfides. The sealants are outstanding in adhesive properties, high- and low-temperature flexibility, color stability, and immunity to the effects of sunlight and weather. In addition to use in sealants, liquid silicone polymers are used principally in the fabrication of tlexible molds and in formed-in-place seals and gaskets. It is appropriate in this section to take note of the use of telechelic silicone polymers in fundamental studies of network structure. and the effects of network structure on physical properties. In principle, at least, telechelic polymers provide a means to prepare model networks in which the molecular weight between crosslinks and the number of chains that arejoined together at the crosslink sites can be manipulated as independent variables. Networks free of dangling ends are also possible in principle. These potentialities are of interest with respect to physical properties that might be attainablein optimized liquid-polymer systems. They are also of interest in fundamental studies relating to the nature of rubber elasticity. The achievement of truly model networks requires, of course, a very high degree of perfection in the materials used and in the methods of fabrication. Specifically, one must have polymers that are precisely difunctional; chain extension and crosslinking mechanisms that involve no side reactions; a perfect stoichio-
-"-c= bondsproducing
Liquid Rubber
139
metric match between the network-forming agents and the terminal reactive sites; and, most difficult of all, a crosslinking reaction with a “yield” of 100%. These are demanding requirements that may or may not be achievable; the subject will be discussed again in the context of terminally functional liquid polybutadienes. Silicone polymers have been used extensively for model network studies by J. E. Mark and coworkers at the University of Cincinnati. In these studies, silanol-terminated silicone polymers of known molecular weight were crosslinked using a tetrafunctional agent (tetraethylorthosilicate) in the presence of a catalyst (stannous 2-ethylhexanoate);
The reactions were carried out at room temperature for 2 days. vacuum being applied to remove the ethanol. Thenetworks were then extractedto remove material (a few weight percent) not incorporated in the network and attributed, in part at least (Llorente et al., 1981), to inert cyclic polydimethylsiloxane molecules, which are normally present in silicone polymers. In related studies, vinyl-terminated silicone polymers were crosslinked with agents of the type = SiH, using a platinum catalyst, for example: 4 -RSi(Ch)zCH=CH2
+
Si[OSi(CH3)2H j4
- Si[OSi(CH3)2-(CH2)2-Si(CH3)~R-~4
The use of polymers of the type [OSi(CH3)H-],, as crosslinking agents permits the preparation of networks with nlultifunctional crosslinking sites. Reaction time is 2 days at 70°C or 1 day at 95°C in the presence of a small amount of chloroplatinic acid (Llorente and Mark, 1980). While the strength properties of unreinforced polydimethylsiloxane networks are always very low by the standards of ordinary rubber technology. the relative behavior observed in the various model network systems is of considerable practical and theoretical interest. The ultimate strength obtained from networks prepared by endlinking is higher, for a given network density, than that obtained by randomcrosslinking(peroxide or radiation curing) of high molecular weight polymers (Anrady et al.. 1981). A second, more surprising observation is that superior strength properties can be obtained by the purposeful introduction of a population of relatively very short chains. Thus. a mixture of 90 mol% short chains (M,, 220) and I O mol% long chains (M,, 18,500), gave a substantially higher tensile strength at room temperature than other networks tested (Llorente et al., 1981). This result is interpreted (Mark, 1984) as arefutation of the “weakest link” theory of network rupture, whereby local failure is initiated when the shortest chains reach their full extension. This theory contains the assumption of “affine” displacement of network junctions (i.e., relative movement of junction sites proportional to that of the overall deformation). The present observations indicate that very “nonaffine” behavior occurs at high extensions, the network simply reapportioning strains among the longer and shorter chains until no further reapportionment is possible. Recent work by thisschool has touched upon the otherfundamentalrequirement for strength in amorphous elastomers-reinforcement. This study has involved a novel method of introducing particulate fillers into an elastomer network, namely, in situ silica formation via hydrolysis of tetraorthosilicate that has been swollen into the finished network. The hydrolysis may be carried out by immersion i n an aqueous solution of an amine (Ning et al., 1984) or simply by exposure to ambient humidity (Jiang and Mark, 1984).
Edwards
140
The particles form as spheres with an average diameter in the region of 25 nm. Since the particles are precipitatedinto a preformednetwork, they do not form clusters or reticulate structuressuch as arenormallyfound in silica-reinforced elastomers but remain as discrete spheres. Strength is increased severalfold compared to the unreinforced network.
2.3 Polyurethanes While the fundamentals of the polyurethane industry involve chain extension and crosslinking of liquid telechelic polymers, the complexity and scale of the technology has grown to an order of magnitude far beyond that represented by the other materials discussed in this chapter. The outlines of polyurethane technology will be sketched brietly, with particular reference to reinforcement mechanisms and to the relationship between urethane liquid elastomer technology and that of conventional rubber. For broader insights, the reader is referred to textbooks and general references (Saunders and Frisch, 1978; Ulrich, 1983). Early research on polyurethanes was conducted in Germany in the 1930s (Bayer, 1947), and commercial products were first introduced in the 1940s. The liquid precursors are either polyester or polyether diols. Polyesters were introduced initially, being prepared by, for example, the condensation of adipic acid with an excess of 1,4-butane diolto give the terminally functional polymer:
nHOOCRCOOH + mHOR'OH
- H/OR'OC(O)RC(O)-]),OR'OH
+
nHzO
(IS)
The polyethers were introduced in the 1950s and are made by polymerization of cyclic ethers (e.g., ethylene oxide, propylene oxide, tetrahydrofuran). with appropriate catalysts:
I nCH-CH2 \ /
0
OH4
aqu.
I HO(CH"CH2"O).H
The reaction of diisocyanates with diols produces chain extension:
2HOROH + OCNR'NCO
HOROC(0)-NHR'NHC(0)OROH
(20)
The -NHC(O)Ogroup is referred to as urethane. hence the generic term "polyurethanes." Crosslinking may be built into the polyether or polyester precursors by various mechanisms. The facile reaction of isocyanates with reactive hydrogen species leads to a very broad spectrum of polymeric materials and processes.With respect to liquid-polymer processes relating to rubber, this is best exemplified by the chemistry and technology of flexible foams. These are usually made by thereaction of a branchedpolyether diol withtoluenediisocyanatein the presence of water together with catalysts, silicone surfactants, and. if desired, a fluorocarbon blowingagent for low-densityproducts.Foaming is produced by thegeneration of carbon dioxide:
"ROC(0)NHR'NCO + Hz0 The primaryanlineterminal formation of urea moieties:
- -ROC(O)NHR'NHz
+
CO2
group is highlyreactivewithisocyanates,leading
(21 1 to the
Liquid
Rubber
141
Further reaction of isocyanate groups results in branching via formation of “biuret” linkages:
-NHC(O)NH-
+
+‘NCO
- “NHC(0)N-I
c=o I NH
I R” Crosslinking is thus introduced both in the preparation of the prepolymer and in the reactions of the terminal groups. Multifunctional isocyanates may also be used to increase crosslinking. This process lends itself ideally to continuousfoam rubber production; hencesoft polyurethane foams for furniture and mattresses have long since largely replaced rubber latex foam products. The inherently higher raw material costs are more than offset by process efficiency in this type of product. To return to the fundamentals of network formation and reinforcement, in the case of a polyurethane made from a long-chain triol and diisocyanate (i.e., a network of highly flexible chains with only occasional network junctions), the extensibility is high but the strength properties are very modest. In other words, strength is low in polyurethanes, as in other elastomeric networks, in the absence of a reinforcement mechanism. High strength properties are achieved in polyurethanes by the inclusion of both “hard” and “soft” segments in the polymer chains. The “hard” segments contain high proportions of diisocyanates reacted stoichiometrically with low molecular weight diols or diamines(such as butane diol or 3,3’-dichloro-4,4’-diamnodiphenylmethane, respectively) to give relatively inflexible chain sequences having regular structures and a strong capability for hydrogen bonding. These hard segments form domains within the continuous softer elastomeric matrix, providing reinforcement. The detailed nature of the hard domain structure is complex andvariable. depending on both chemical composition and process history. The morphology and temperature behavior of the domains isinfluenced by an interplay of glass transition temperatures, crystallization effects, and hydrogen bonding. A concise review of this subject has been provided by Redman (1978). While a detailed discussionof polyurethane technology is beyond the scope of this chapter, mention should be made of the possible development of polyurethane processes for automotive tire manufacture. If fundamental property and cost factors should permit the liquid fabricationof tires of a quality comparable to today’shighly engineered radials, then a gradual but nevertheless revolutionary change in the tire industry would be anticipated. This subject gained prominence in 1970, when publications were issued by the Firestone Tire and Rubber Company (Alliger et al., 1971) on experimental cast tires for automobiles. In subsequent years, much publicity has been generated by LIM International S.A., formerly PolyairMaschinenbau GmbH(Marshall,1982). Ideally.fromthestandpoint of production economics such tires would comprise a single polyurethane composition injected into a mold andcuredrapidly,thefinishedtire containing no mechanicalreinforcementotherthanthe beadwires. During the past 15 years or so, research and development efforts on liquid injection molded or cast tires have continued throughout the industrial world, as evidenced by a very extensive patent literature. The fundamental challenges are associated with certain thermal and mechanical properties that are characteristic of polyurethanes. The urethane linkageitself is thermally reversible, and hence failure of the primary polyurethane chains will occur at sufficiently elevated temperatures. The reinforcementmechanism is also thermallysensitive in principle, since it depends on the presence of domains that must remain above their glass transition temperatures
142
Edwards
and/ormeltingpointsduringservice.Gradual creep underload, even at relatively moderate temperatures, is also characteristic of this class of materials. These factors have resulted in experimental tires that exhibit gradual “growth” under the combined influences of inflation pressure and heat and that can suffer catastrophic damage to the tread region when subjected to panic stops or other circumstances that produce local overheating. Recent efforts have therefore been directed toward more complex tire structures, in which the polyurethane components are integrated with other functional elements. For example, Rossi (1982) described a tire in which a soft polyurethane tread portion is first spin cast and partially cured and then a reinforcing belt is stapled to the inner surface. A harder polyurethane carcass is now spin cast so as to enclose the reinforcing belt between the tread and carcass components. Rau and Just (1983) described the introduction of a fabric reinforcement connecting the beads before the beadhbric assembly is positioned over the inner core of the mold. The net result is to produce a finished geometry of cord reinforcement somewhat similar in principle to that of a belted radial tire. Cesar et al. (1980) described the use of metal hoops running from bead to bead, together with a provision for positioning a crown reinforcement (belt) between the hoops and the tread region, such that the reinforcingmembers are positioned precisely withinthe liquid or paste material prior to cure. Schmidt (1978) described a three-step operation in which the carcass is first molded, then a cord reinforcement is wound around the circumference so as to provide structural support in the manner of a belt, and the tread component is then added by casting or injection molding. Bead-to-bead winding to provide radial reinforcement has also been described (Schmidt and Kubica, 1978). Many other combinations of fabricationprocessesandmaterials.includingthe use of conventional rubber vulcanizates to perform the tread function (,road holding and wear resistance), have been and are being considered. The ultimateresults of theseeffortscannot. of course, be forecast with certainty. From present perspectives, however, it seems most unlikely that a single isotropic composition can provide a balance of properties capable of matching the performance standards of current high-quality tires. If this is true, then the ideal of fabricating a tire from a single liquid-rubber composition in a single molding operation is not attainable. The direction of technology development in this area hasbeen toward multicomponent structures and increasingly complex fabrication techniques. This trend is in opposition to the economic advantage that liquid-processing methods might otherwise provide. The question of whether or not an economic compromise can be reached is unlikely to be answered in the near future.
2.4 Terminally Reactive Butadlene-Based Polymers Terminally Hylroxvlrtecl Polybutadienes
The preparation of terminally hydroxylated polybutadienes can be accomplished by a number of procedures. Hsieh (1959) described a preparation based on “living” anionic polymerization techniques, this being anotherof the generic methods for telechelic polymer synthesis. A difunctional organolithium catalyst in an inert, dry hydrocarbon solvent medium is employed. The number of polymer chains formed is equal to the number of initiating molecules. If all of the monomer is charged initially, so that all chains begin growing at essentially the same time, the resulting polymer chain lengths are all nearly equal. Polymerization proceeds until all the monomer is exhausted, the chain ends remaining reactive toward the addition of new monomer, if it is introduced. This behavior accounts for the term “living” polymerization, as originally introduced by Szwarc et al. (1956). As applied to butadiene, this may be represented as follows:
Liquid Rubber
LIRLI
+
143
2nCH2=CH"CH=CH2 LjlCH2-CH=CH-CH21.RICH2-cH=cH-cH2~"L~
-
(24)
To form the terminal hydroxyl groups, ethylene oxide is added, followed by acid hydrolysis:
-RLi
+
CH2-CH2 \ / 0
- -R-CH2-CH2-OLi
-. HX
-R"CH2"CH20H
+
LiX (25)
A product of this type was developed by the Phillips Petroleum Company under the trade name Butarez HT. Because of the narrow molecular weight distributions providedby this type of polymerization, the terminally functional products are of interest with respect to model network studies. Morton and Rubic (1977), working with polyisoprene diols crosslinked with p p ' , p"-triphenylmethanetriisocyanate, concluded that strength properties are enhanced by having uniform chain lengths between crosslink sites and that a maximum in gum tensile strength for such networks is reached at molecular weights in the region of 6000. A less elegant, but more economical, route to terminally hydroxylated polybutadiene arose during the late 1950s and early 1960s. beginning with an investigation of the free radical polymerization of butadiene in solution using hydrogen peroxide as the initiator (Burke et al.. 1959). The object of the early work appears to have been simply the preparation of liquid butadiene polymers or copolymers by a clean and fundamentally economic solution process. The reaction mediumwasa common solvent for the monomer and aqueous hydrogenperoxidecatalyst, isopropanol being suitablefor this purpose. Polymerizations were conducted for periods of about 2 hours at temperatures in the region of 1 20"C, yielding essentially water-white. odorless liquid products. Although the technical literature contains little basic information on polymerizations of this type. the predominant mechanisms are essentially as follows:
2HO* HO. + CH2=CH-CH=CH2 2HOR-• HOR ROH
H202
(26) (27) (28) This reaction scheme is, of course, an oversimplification. Although termination by combination [ Eq. (28)] is normally predominant in solution free radical polymerizations of butadiene. chain transfer to solvent. initiator, polymer, and monomer all occur as side reactions. The decomposition of hydrogen peroxide to HOO- and H' is also a significant side reaction. although the indicated path [Eq. (26)] is favored by high temperature (Pinazzi et al.. 1973). French workers have studied this reaction as applied to several monomers and have provided chain transfer data (Brosse et al., 1978).The products are found to contain low molecular weight oligomers (monofunctional polymers) as well as high polymers with two or more hydroxyl groups. In addition to hydroxy end groups, stnall amounts of other oxygenatedspecies, including aldehydes, ketones, carboxylic acids, hydroperoxides, and epoxides, have been detected (Brosse et al., 1982). The commercial development of this technology was pursued in the United States by the Sinclair Oil Corporation and subsequently by A R C 0 Chemical Company. Reaction of the hydroxy-functional polymer, containing an average of 2.1-2.2 hydroxyl groups per chain, with isocyanates was demonstrated in the early 1960s (Verdol and Ryan, 1966).Chain extension via condensation with formaldehyde was also described (Isaacson and Young, 1966). As with all terminally reactive polymers, subsequent reactions may be carried out to change the nature of the end groups. Thus, terminally hydroxylated polybutadiene may be esterified, for example, with an acrylic acid. to give acrylate end groups that can take part efficiently in subsequent free
-.
-
HO"CH2"CH=CH"CH2*
144
Edwards
radical polymerizations (Ryan and Thompson, 1968). Terminally halogenated products, curable with amines at room temperature, can be synthesized via reaction with excess dry hydrogen halide (Edwards and Wunder, 1968). Japanese workers have conducted extensive studies on the curing and reinforcement of this type of polymer. Curing with esters of phosphoric acid was explored as a possible route to enhance flameresistance (Minoura et al.. 1977).The cure is catalyzed by 2,3,6-tris(dimethylaminomethy1)phenol (DMP-30):
R' in this instance is phenyl. The cure occurs at elevated temperatures, e.g., in several hours at 130°C. The crosslinks decompose by hydrolysis upon exposure to water. A thorough investigation of the effects of reinforcing and nonreinforcing fillers has also been reported (Yamashita et al., 1978). Good strength properties were observed at optimum levels of diphenylmethane diisocyanate (about 16.5 phr) using N330 black or precipitated silica. Coarse fillers gave less reinforcement, analogous to conventional rubber. Curing of hydroxyl-terminated polymers by metal chelation has been described (Kambara andAotani,1970). The terminal group is converted by postreaction to -C(O)CH?C(0)"CH3. On heating with metal alkoxides (e.g., aluminum, magnesium, calcium, titanium), chain extension and crosslinking occur rapidly in one step. At the present time, two grades of hydroxytelechelic polybutadiene are produced by the A R C 0 Chemical Companyunder the tradenames Poly bd R-45HT and R-45M. These polymers have number-average molecular weights of about 2800 and hydroxyl functionalities (moles OH per mole polymer) of about 2.3 and 2.5, respectively. As with other terminally hydroxylated polymers, the curing chemistry of practical value is that of the polyurethane industry. Simple chain extension and crosslinkingwithdiisocyanatesproducessoftvulcanizateswithlittle strength. This type of system can be useful in electrical potting compounds and in imparting elastic properties to asphalt compositions used as caulks,mastics, or jointsealers in the construction industry. Soft vulcanizates with good low-temperature properties are also suitable for use as rocket propellant binders. To achieve high strength properties from initially pourable compositions, provision must be made for theincorporation of "hard" domains in the vulcanizate. Thisis illustrated in Table 2, which refers to the use of increasing amounts of a polyfunctional liquid isocynanate in conjunction with a suitable low molecular weight diol, N,N-bis-(2-hydroxypropyl) aniline. A progressive increase in modulus and strength properties, to levels in the order of 20 times those of the simple network, may be noted. Polyurethanesbased on polybutadiene diols are notable for excellentlow-temperature properties, compatibility with low-cost hydrocarbon oils, anddegree a of resistance to hydrolysis claimed to be much superiorto that of polyether-based or (particularly) polyester-based polyurethanes. These properties lead to applications in electrical insulation, waterproofing membranes or coatings, and liquid-castable general-purpose rubber goods. Terminally Reactive Liquid Nitriles
The use of a functional free radical initiator in solution polymerizations of butadiene and butadiene copolymers, particularly butadiene-acrylonitrile, has been developed commercially by the B. F. Goodrich Company. Carboxyl-terminated polymers may be prepared using azodicyanovaleric acid (Siebert, 1964):
Liquid Rubber
145
Table 2 Properties of ARCO Poly bd R-45 HT Urethanc Compositions
Formulations Poly bd R-4SHT, Isonol 100 Catalyst T- 12" drops CAO- 14' g Isonate 143L," g Equivalent ratio, Poly bd/Isonol 100 Vulcanizate properties" Tensile strength, Mpa 200% modulus, Mpa Elongation, c/o Hardness, Shore A Shore D "
I
2
3
100
100
100
4 0. I O 12.76
1.2
4
5
6
2.22 4 0.10 15.45
4.45 4 0.10 19.14
IO0 8.89 4 0. I O 25.53
100 1 1.85
4 0. I O 29.78
100 17.78 4 0.10 38.29
411
211
1/1
314
6.2 2.6 238 7s
8.2 5.3 244 82
2.6I .7 -
101
151
53
56
1.3 195 62
7
8
100
1 00
26.67 4 0. I O S I .05
35.56 4 0.10 63.81
I I2
I /3
114
13.9 10.3 300
18.6 14.7 325
24.0 19.4 297
-
43
-
51
-
53
N,N-bis-(2-Hydroxypropyl) aniline (UpJohn Co.).
'' Dibutyltin dilaurate.
' Antioxidant (Sherex Chemical Co.). '' Polyfunctional liquid isocyanate (Upjohn Co.). '' Press cure 30 min at 80°C. Postcurc 64 hr at 49°C. S O I ~ - CCourtesy ~: of ARCO Chemical Co., Philadelphia. Pennsylvania.
This is analogous to the use of hydrogen peroxide as described above, and the same generalizations with respect to chain transfer side reactions are applicable here. The reaction is carried out in f-butyl alcohol, this solvent being chosen because of a relatively low degree of chain transfer to solvent. Reaction temperature is typically 80°C. The terminal functionality may be converted to hydroxyl by postreaction with ethylene oxide in the presence of a catalytic amount of a tertiary aliphathic amine (Siebert, 196th):
Alternatively, the carboxyl groups may be reacted with an excess of a diol, such as 1.4butane diol, in the presence of an acid catalyst such as p-toluene sulfonic acid (Siebert. 1968b):
146
Edwards
-R--COOH + H O - - R ' - o H R " ~ ~ H - R - ~ ( ~ ) -+ ~H~~' O ~~ (34) Vinyl-terminated polymers having highly reactive double bonds may be obtained by reaction of the terminally carboxylated polymers with glycidyl acrylate or methacrylate in the presence of a tertiary amine catalyst. This reaction may be conducted in acetone or toluene solution at temperatures above 90°C (Skillcorn, 1972):
-RCOOH
CHz-CH-O-C(O)-CH=CH2
+
\
/
.O'
R',N 4
-R-C(0)-OCHz-CH(OH)-OC(O)-C~=~~z
(35)
The preparation of amine-terminated polymers, again fromthe carboxyl-terminated precursors, is described by Riew ( 1 976). This is preferably carried out using an amine with mixed functionality and containing no more than one primary amine site. The preferential reaction of the primary amine minimizes chain extension:
-RCOOH
n
+
NHzR'N
NH
W
- -RC(O)-ONHR'N
q
U
H
+ H20
(36)
This basic technology has led to a family of commercial products. These include carboxylterminated polybutadiene (CTB), suitable for use as a rocket propellant binder, as well as nitrile copolymers with carboxyl (CTBN), hydroxyl (HTBN), vinyl (VTBN), or amine (ATBN)terminal groups. The nitrile contents range from approximately I O to 25% by weight. The carboxyl- and amine-terminated products have been developed principally as modifiers for epoxy resin structural adhesives, coatings, and composites (Siebert, 1984). In the case of the carboxyl functionality, the properties of the epoxy products are dependent on the type of curing system and the sequence of reactions in a complex manner, since the carboxyl groups may react with amine catalysts as well as with the epoxy resin monomers. In some studies this has been simplified by prereacting the carboxyl-terminated polymer with a diepoxide, such as the commonly used diglycidyl ether of bisphenol A, so as to form a prepolymer with epoxy and groups:
- RCOOH
+
CH~-CH"R'-CH"CH~ \ / \ /
As the epoxy cure proceeds, a rubber-rich component separates out as discrete domains in the order of a few micrometers in diameter, these domains being grafted to the continuous epoxy resin matrix.This type of morphology represents a general methodfor imparting increased impactstrengthandtoughness to resinousmaterialswithoutexcessivelosses in modulus or tensile strength. Table 3 shows data for a simplified system of this kind, chosen to illustrate the principles involved. In this case the ingredients were mixed and cured together without a prereaction step. As the proportion of rubbery component increases, a phase inversion occurs in the region of 5060 elastomer/epoxy. At higher rubber contents the elastomeric phase is continuous. This combination of materials therefore provides a spectrum of compositions ranging from rubbertoughened plastics to resin-reinforced castable elastomers. A comprehensive key to the literature on epoxy formulations for adhesives has been provided by Drake and Siebert (1984). The amine-terminated polymers have alsobeen developed primarily for use in conjunction with epoxy resins (Riew, 1981). In this case the amine functionality is reactive with epoxies at
147
Liquid Rubber Table 3
Properties of CBTNEpoxy Resin Compositions 1
Formulations DGEBA" Hycar CTBN 1300x8'' Piperidine Physical properties' Tensile strcngth. Mpa Tensile modulus, Gpa Elongation, 76 Fracture surface energy, kJ/m' Gardner impact, J Heat distortion temp., "C '' Diglycidyl ether of hisphenol A.
100 -
5 65.8 2.8 4.8 0.18
6 80
2
3
4
5
100
100 10
100 15
100
S 5
5
S
62.8 2.5 4.6 2.63 8 76
58.4 2.3 6.2 3.33 8 74
51.4 2.1
8.9 4.73 8 71
20 5 47.2 2.2 12.0 3.33 25 69
-
" CAN
content 18 wt%, functionality 1 .X, M,, 3600. ' Cure 16 hrat 120°C. Sorrrce: Courtesy of the B. F. Goodrlch Co.. Akron. Ohlo
room temperature. The reaction products, unlike the case of carboxyl-terminated polymers, do not contain ester groups and are therefore more stable toward hydrolysis. Once again a variety of compositions, rangingfromtoughenedplastics to reinforced elastomers, is possible. The toughened epoxy compositions are particularly useful as adhesives. The vinyl-terminated polymers are designed principally for use in unsaturated polyester glass-reinforced bulk molding, or sheet molding. compounds. Thehighly reactive terminal double bond can participate efficiently in the styrene-polyester polymerization process that occurs during cure. The nitrile polymer appears in the product as rubber-rich spherical domains. The domain sizes are relatively large (10- 15 pm); nevertheless, improvements in resistance to internal cracking during flexure, and in impact damage resistance, are observed, with little sacrifice in strength or modulus properties (McGarry et al., 1977). In general,theterminallyfunctionalnitrileliquid polymers have evolvedprimarilyas additives, to confer toughness to thermosetting resin compositions, with which they are chemically compatible.
3. MODEL STUDIES USING TERMINALLY FUNCTIONAL POLYBUTADIENE A question of both practical and theoretical interest is whether or not it is possible to achieve conventionally reinforced elastomeric networkscomparable in physical propertiesto high-quality general-purpose rubber vulcanizates, startingfrom liquid telechelic precursors. The processing of natural rubberand of the various high molecular weight synthetics requires very heavy machinery because of the viscosities involved. Indeed,one can only marvelat the temerity of those pioneers, such as Thomas Hancock, who undertook the bulk mixing of a substance as intractable as raw natural rubber. Conceptually. the mixing and shaping of many rubber articles could be carried out much more economically, at least with respect to the capital cost of process equipment, if low-viscosity materials could be used.
Edwards
148
As discussed above, the question resolves itself into two fundamental issues: ( I ) whether it is possible to achieve full network development, that is, networks with as few, or fewer,loose ends than those of conventional elastomers, fromliquid precursors; and ( 2 ) whether it is possible to achieve a full degree of reinforcement, using carbon black in particular, without the very high shearing forces that are brought to bear on the carbon agglomerates during conventional rubber mixing processes. At firstglancemanyrubbertechnologistsmight expect both of these questions to be answerable in the affirmative. However, demonstrationsof high strength i n conventionally reinforced liquid-derived vulcanizates have been rare, and high strength does not in fact appear to beattainablewithmany of thebutadiene-basedliquid polymers that have been tested. The difficulty of achieving full network development in many systems led, in the case of one detailed study, to a conclusion that network development cannot proceed much beyond the gel point becausetheunreacted groups are prevented by theexistingnetwork from encountering one another within a reasonable time (French et al., 1970). With respect to carbon black reinforcement, many mechanisms for possible chemical reactions at the carbon surface have been postulated. It could well be supposed that reinforcement might not develop fully in the absence of the elevatedtemperatures, high-energy mastication processes, and free radical curing procedures involved in conventional rubber manufacture. These questions have been studied using, as the model polymer, a liquid polybutadiene having terminal allylic bromide groups (Edwards, 1975). This polymer was prepared by a free radical emulsion polymerization process (Buckler et al., 1965), using carbon tetrabromide as a chain transfer agent.The normal effect of efficient chain transfer agents in emulsion polymerizations is to produce monofunctional oligomers, that is, short chainswith nonidentical end groups. With carbon tetrabromide, the telomerization occurs during the early part of the reaction, such that the carbon tetrabromide is consumed during approximately the first 10% conversion of the butadiene to form telomers of the type Br(Bd),,,CBr3, in which m is a small number and Bd represents the butenylene radical. The low molecular weight telomer then functions as a chain transfer agent throughout the remainder of the polymerization, which may be carried to essentially complete conversion of the butadiene (Beaton, 1971):
-
-R* + CBrJ(Bd),Br -R& + *CBrz(Bd),Br Br(Bd),CBrZ* + nCHz=CH-CH=CH2 Br(Bd)mCBrplCHp-CH=CH-cHz~n.
(39)
Br(Bd),CBrzI CH~-CH=CH--CHZ-~~* + Br(Bd),CBr3 -Br( Bd)mCBrnlCH~-CH=CH-CHzIn& + Br(Bd),CBrZ* (product)
(40)
-
(38)
The resulting polymer has a reactive (allylic) bromide at both ends of the chain and a relativelynonreactive(nonallylic)dibrornide site locatednear oneend. In practicethere is also a minor amount of telomer remaining in the product, as well as nonpolynleric emulsion polymerization residues. To make suitable model polymers, these materials may be removed by dissolving the crude product in a hydrocarbon solvent, centriguging, and precipitating the telechelic polymer from the clear solution with excess acetone. The allylic bromide can be reacted with a tertiary amine to form the quaternary salt:
-RBr
+
NR',
- -RNR'3@Bro
(41)
For network studies, a tetrafunctional curing agent was prepared by methylation of triethylene tetramine. This agent was observed, as expected, to produce insoluble networks.
Liquid
Rubber
149
Regarding network formation, one should be able to demonstrateconsistency between the physical and chemical characteristics of the polymer, as measured by independent methods, and the stoichiometricbehavior of thepolymer-curativesystem. In other words,the amount of curative observed to give the highest state of crosslinking should be stoichiometrically equivalent to theconcentration of reactive sites based on molecular weight measurements and difunctionality. In the present instance this was done using five polymer samples having different molecular weights. The number-average molecular weights of the polymers were determined by a combination of ultracentrifugation and gel permeation chromatography (GPC). The ultracentriguge measurements provided a value for the weight-average molecular weight in each case. Thesevalues were used to calibrate the GPC data so as to provide the optimum fit between calculated and observed molecular weight values. The polymer samples, after mixing with various loadings of methylated triethylene tetramine (MTETA) and curing for 7 days at room temperature were swollen to equilibrium in benzene for determination of network density. The values of network density were calculated by the method of Flory and Rehner (Flory, 1950) using a value of 0.39 (determined by stressstrain measurements on representative swollen specimens) for the solvent-polymer interaction parameter and assuming tetrafunctional crosslinks. This reaction proceeds at a moderate rate at room temperature and does not involve side reactions or by-products. It is therefore experimentally convenient for model studies. In considering model systems, one must be careful to ensure that the expected, or desired, chemical processes actually occur. One cannot safely assume that the polymer is truly difunctional and that the reactions involved in chain extension and crosslinking proceedto completion. In order to have confidence in the model, one must observe physical behavior that conforms very closely to what is predicted. The simplest case is that of chain extension using a difunctional reagent. The change from a liquid to a soluble, but high molecular weight, solid polymer is unequivocal proof of effective difunctionality in bothcomponents andof suitable reaction conditions. No amount of indirect information based on the quantitative measurement of functional groups and number-average molecular weights can provide a degree of confidence to compare with the demonstration of practical chain extension. In the present instance a sample of the polymer was mixed, in an early experiment, with a difunctional tertiary amine prepared by the methylation of 1,6-hexanediamine. The viscous liquid polymer became an elastic solid within a few hours at room temperature. After 3 days, a portion was placed in a solvent and was observed to dissolve completely. The intrinsic viscosity was measured as 1.8 dL/g (in toluene at 30°C), representing a molecular weight in the same order of magnitude as that of conventional elastomers. Effective chain extension was thus unequivocally demonstrated. With respect to polymer characterization, the absolute value of network density is not significant, since it is not known at this stage to what extent the curing reaction has approached completion. What is important is the location of the maximum in network density. since this should correspond stoichiometrically to the reactive sites in the polymer and thus provide a chemical measure of molecular weight. The data resulting from this comparison are given in Table 4. The columns of interest are thefinaltwo, in whichthenumber-averagemolecularweightsderivedindependently from physical measurements and chemical measurements (curingbehavior) are compared. The agreement is seen to be very satisfactory, although the physical measurements are consistently higher by a few percent. This would suggest a true polymer functionality slightly in excess of 2. Table 4 also includes a column showingthe highest observed value of M, (number-average molecular weight between crosslinks) following the 7-day cures.If the crosslinking reaction had proceeded to 100% “yield,” these values shouldequal thenumber-average molecular weights of
Edwards
150
Table 4 PolymerCharacterizationData
Min. ViscositySp. gr. Polymer (25°C) A
0.92
B
0.93 0.94 0.95 0.96
C D E
M I,
M,
(25"C), P
M,M,
40,000 14,000 3,100 730 300
3.1 3.0 2.4
140
2.1
Chem. (7Phys. d, 25°C) 14,000
2.2
27,000
9,200
1 3,000 10,000 5.000
6,300 5.360 3,860
13,000 8,900 5,800 5,000 3,600
the parent polymers. The actual values are much higher, indicating that the cure was far from complete under these conditions. Since the extent to which full cure is achievable is a major factor in the consideration of model polymer systems, further experiments were conducted using a product of intermediate molecular weight (polymer C ) . Figure 2 shows V, (volume fraction of polymer in the swollen vulcanizate at equilibrium) as a function of curative loading and time of cure at 25°C. The samples were kept under nitrogen, small portions being removed for swelling determinations at the indicated time intervals. The maximum state of crosslinking occurred at 2.0 phr of the
.20
-
.18-
-a
16
-
14
-
c .12a, N
C
g
.lo-
Y
>' .08 .06
-
'O4I .02 I
1
1.6
1.8
1
I
2.0 2.2 MTETA (phr)
l
1
2.4
2.6
Fig. 2 Vulcanization of polymer C gumcompounds at 25°C.
Liquid Rubber
151
X
curative. Figure 3 shows a plot of calculated M, values against time for the sample containing the optimum concentration of crosslinker. The value of M, is seen to approach M,,, the numberaverage molecular weight of the polymer, after approximately 12 weeks at 25°C. In separate measurements it was shown that the crosslinking reaction proceeds in two successive rate stages, a rapid rate up to the gel point (about 3 hr at room temperature) and a much slower rate during the approach to full cure. This is attributed to the inhibition of chain movements following the initial formation of a continuous network. It is tempting to conclude from this experience that the Flory-Rehner expression, with a single value for the interaction parameter, is quantitatively exact as applied to real systems and that the polymer and curing chemistry used in this model study were both quantitatively perfect. However, there is circularity in any such statement,because the observed agreementcould arise, as well. from fortuitous inexactness on both sides of the equation. Nevertheless. it is proper to conclude that the formation of a network very similar to that of conventional rubber vulcanizates is possible by this type of process and that the slowing of cure following the gel point, while very pronounced, does not prevent a subsequent approach to full network development. To determine whether carbon black reinforcement comparable to conventional rubber can be realized, mixtures containing N 1 19 black were prepared. These mixtures were made at room temperature using a three-roll laboratory paint mill, several passes being required. The product of such a process is no longer a pourable liquid but rather a smooth, glossy paste that may be spread or pumped very readily and that retains its shape for an indefinite period under small gravitational stresses. In rheological terms, the behavior is pseudoplastic; viscosity decreases as shear rate increases. Detailed data on liquid polymer systems of this kind have been provided (Rivin and True, 1973). At very high shear rates. viscosity is similar to that of the unfilled liquid. Thixotropy is also observed. The development of tensile strength and modulus upon curing at room temperature is illustrated in Figure4. The tensile strength reaches valuesin the region of 20 MPa, with modulus values above 10 MPa. These strengthvaluesarehigh for apolybutadiene, as evidenced by comparison withconventionalcis-polybutadieneandmixed-structure(butyllithium-polymerized) linear polybutadianes, mixed with the same loading of carbon black and vulcanized by conventional procedures (Edwards, 1975).
152
Edwards
1
1
1
L
2
4
6
8
Tlme at 25°C (Weeks)
Fig. 4 Development of stress-strainproperties carbonate, 5: MTETA, 2.
a t 25°C. Polymer C, 100: N119 black, 40: basic lead
The samestudy included tests using finelydivided esterified silica as the reinforcing agent. As shown in Figure 5, high strength properties are attainable in this case as well. Functionalization of the esterified silica with tertiary amine groups, so as to vary the degree of polymer-filler bonding from zero to highvalues, was includedinthiswork. The results showed thatlow degrees of bonding provide an increase in modulus with little effect on tensile strength, while high degrees of bonding result in excessivelosses in bothextensibilityandstrength. These combined observations are of interest with respect to postulated mechanisms for the reinforcement of rubber by particulate fillers. Carbon black appears to be fully effective in the absence of any history of heating above room temperature, high-energy mastication during mixing, or
15
c
0
1
l
I
1
1
10
20
30
40
50
Flller (phr)
Fig. 5 Reinforcing behavior of esterified silica. Cure 48 hr at 60°C. 2.4 phr MTETA.
Liquid Rubber
153
heat or free radical processes during cure. Fumedsilica, rendered as chemically inert as possible by esterification, also provides high degrees of reinforcement. Thus, chemical bonding to the filler surface, while probably desirablein small amounts forthe optimization of technical properties, is not fundamental to the reinforcement of rubber. With respect to liquid polymer systems intended for use with conventional reinforcing agents, the combined results show unequivocally that the two essential requirements-a high degree of network development together with reinforcementby carbon black-are fundamentally attainable.
4.
PRACTICAL CONSIDERATIONS AFFECTING THE DEVELOPMENT OF TELECHELIC POLYMERS AS GENERAL-PURPOSE ELASTOMERS
As we have seen, the only successful displacement of a conventional elastomer by a telechelic polymer system in a high-volume application has been that of natural rubber latex foam by polyurethane foam.Since both foamprocesses employ liquids (the naturalrubberbeing in latex form initially),displacement of solidrubber is qualified even in thisinstance.Having demonstrated that fundamental factors donot preclude the liquid fabrication of materials closely analogous to conventional rubber, it is appropriate now to consider other factors that bear upon technical and economic progress in this direction. A first consideration is the need for a "universal" curing technology comparable to the sulfur curing of natural and synthetic elastomers. Many practical compositions, particularly in the tire sector, involve mixtures of elastomers for optimized cost and property balances, and strong adhesion between components of differing composition. Compatible,although not necessarily identical, curing systemsare needed in all such compositearticles. This suggests a requirement that various types of butadiene, butadiene-styrene, isoprene, ethylene-propylene, and isobutylene polymers, as a minimum, must be made available at low cost in liquid telechelic form, probably with a common functionality such as hydroxyl. A universal chain extension system, if not diisocyanate chemistry itself. should have some of the same features: economy of scale, efficient and rate-adjustable reactivity, and a reaction mechanism that results in chain extension without the production of objectionableby-products.Athermalstabilitybetter than that of urethanelinkages is also desirable, as well as freedom from toxicological or environmental concerns. A second requirement is for economical mixing processes. This has been studied extensively by workers at the Rubber and Plastics Research Association (RAPRA) of Great Britain. Various procedures using conventional bulk mixers (Z-blade, Brabender) were tested in early trials (Pyne, 1970) but did not produce dispersions equivalent to those obtained with a threeroll paint mill. Further experience with paint mill mixing (Daniel et al., 1972) suggested that the energy requirements for dispersion of carbon black might not be less than those associated with conventional rubber. Concern was also expressed regarding both the physical properties attainable with the hydroxyl-, carboxyl-, and bromine-terminated polymers investigated and the projected costs of these products as compared to conventional rubber. A continuous-mixing device was developed by RAPRA to facilitate the dispersion of carbon black in liquid polymers (Humpidge et al., 1972. 1973). This provided for a bulk premixing stage to disperse the carbon black pellets followed by an intensive mixing stage employing principles similar to those of paint milling to produce the final dispersion. A review of this subject several years later (Lee et al., 1978) indicated that the mixing power required. using the RAPRA mixer, is similar to that of conventional rubber mixing and that the limiting factor is the work required to break down the agglomerates of carbon black.
Edwards
154
Subsequent processing, on the other hand. is greatly simplified in operations such as injection molding, “since the forces required to make the compounded pastes flow are some 30 times smaller than those required for solidrubber compounds.” Injection molding of a tread compound over a conventional tire carcass, without distoriton, was demonstrated.The samereview stressed the practical difficulties involved in achieving degreesof chain extension necessary for satisfactory product performance, as well as the high costs of existing liquid polymers. An alternative method of achieving carbon black dispersions in liquid polybutadienes has been described by Japanese workers (Inomata et al., 1975). The carbon black is dispersed in water using a conventional high-speed homogenizer. Upon mixing of this dispersion with the liquid polymer. transfer of the dispersed carbon black into the organic medium occurs immediately. The water, containing very little residual black, is drained off. and the polymer dispersion is then dried. Physicalpropertiesatleastasgood as thoseobtained by paintmillingwere achieved. It was also shown (Yamawakiet al.. 1974) that very highly loaded black concentrates, suitable for blending with additional polymer to provide the desired final loading, canbe prepared by this general method. Satisfactory carbon black dispersions arealso attainable using conventional internal mixers by first mixing very highly loaded concentrates in which the viscosity is sufficient to permit the necessary input of work and then diluting with fresh polymer (Masuko et al., 1974). High vulcanizate strength properties, comparable to conventional SBR tread compounds, have been claimed using a fabrication process whereby the chain extension and crosslinking steps are conductedseparately and sequentially (de Zarauz, 1975). The compositionswere based on terminally hydrocylated polymers (butadiene-styrene) compoundedwith both a diisocyanate and a conventional sulfur curing system. The mixture was heated successively at two temperatures, the first (ca. 100°C) sufficient for chain extension to occur and the second (ca. 160°C) suitable for sulfur crosslinking. The use of entirely separate mechanisms for chain extension and crosslinking is an interesting approach. providing enhanced flexibility in fabrication techniques and finished network design. There has been relatively little activity in the field of liquid polymers for general-purpose rubber displacement during the 1980s. Although fundamental feasibility has been demonstrated, and many of the basic incentives remain as they were, the practical and economic impediments are formidable. There wasa strong commitment to further research and development along these lines in the former SovietUnion (Fedjukin, 1984). but in general it appearslikely from the perspectives of today that most liquid elastomer work will continue to be focused on relatively high-price. small-volume specialty applications.
5. ADDITIONAL THEMES IN TELECHELIC ELASTOMER RESEARCH AND DEVELOPMENT This section surveys other elements of research that have already led. or may lead i n future, to practical developments in the liquid elastomer field. For convenience the subject matter is discussed under the general classes of preparative chemistry involved.
5.1
FreeRadicalPolymerizations
The preparation of carboxyl-terminated polybutadienes by a solution free radical process was developed during the 1960s (Berenbaum et al., 1961; Hoffman and Gobran, 1973). I n this case the preferred initiator is glutaric acid peroxide. The polymerization may be carried out in acetone solution at temperatures between about 75 and 130°C.
155
Liquid Rubber
A
IHOOC-(CHZ)J-CO)~ II
0 HOOC-(CH2)3COO'
2 HOOC-(CHZ)JCOO*
HOOC"(CH2)3*
+
CO2
(43)
Reaction (43) proceeds to the extent of about 70%. Thepolymerization is initiated predominantly by HOOC-(CH2)I. radicals. The key to effective difunctionality, as in all butadienesolutionfreeradicalprocesses of thiskind, is that termination is predominantly by mutual termination of growing chains. This process was developed by Thiokol Chemical Corporation to produce a polymer. HC-434. having a molecular weightof about 3600 anddesigned primarily for use in rocket propellant compositions vulcanized under mild conditions using tris-(2-methyl aziridiny1)phosphine oxide as the curative. This system was favored for many years owing to compatibility with the propellant mixture and adequate adhesion to the propellant components and the casing as well as excellent low-temperature properties. Reinforcement with carbon black and cure with an epoxy resin were shown to result in fairly high strength properties (Hoffman and Gobran, 1973). The use of such symmetrical free radical initiators, of the types ROOR or RN=NR, has been studied extensively by numerous workers since about 1960. Polymerizations using 4,4'azo-bis-(4-cyano-r?-pentanol) and 4,4'-azo-bis-(4-cyanovaleric acid) were discussed in a series of papers by Samuel F. Reed. Jr., during the period 1971- 1973. Hydroxyl-terminated polymers based on butadiene, isoprene, and chloroprene were prepared that had molecular weights in the order of a few thousands and functionalities (by chemical and molecular weight analysis) in the region of 2. Chloroprene produced higher molecular weights and higher yields, under nominally equivalent conditions, than isopreneor butadiene (Reed, 197 1). Copolymers of these three monomers with p-chlorostyrene, using both carboxy and hydroxy functional azo initiators, were prepared i n dioxane solution (Reed, 1972a). Copolymers of butadiene with chloroethyl acrylate and chloroethyl methacrylate, with both types of initiator, were also described (Reed, 1973). Russian workers have reported the preparationof bromine-terminated polybutadiene using 4,4'-azo-bis(4-cyano-l-bromo-n-pentane)in acetone solution (Barantsevich et al., 1973). The liquid products are curable with polyfunctional amines. Terminally carboxylated polybutadiene may be prepared in an aqueous emulsion system using cyclohexanone peroxide in the presence of ferrous ion (Allen, 1963):
H0
0-0
OH
00
-
Fe"
2 H0
0.4 HOC(CH2)s'
0
II
0
(44)
In a typical polymerization, water, benzene, ferrous sulfate, and an emulsifier are mixed and then cooled to 0°C. Cyclohexanone peroxide in THF is added dropwise during a 3-hour period. Molecular weight is controlled by the rate of addition of the initiator. Products having mixed carboxyl and hydroxyl functionality wereobserved under different reaction conditions (Quinn, 1968) with the same initiator. Preparation of butadiene with nonallylic terminal hydroxyl groups may also be carried out by solution free radical polymerization using r-butyl hydroxyethyl peroxide as the initiator (Gaylord, 1973):
Edwards
156
Initiation is believed to be predominantly by the HOCH,CH20. radical. To obtain satisfactory products, the reaction must be carriedout in a solvent, isopropanol or xylene being suitable. Typical conditions are 2 hours at 120°C. Mutual termination provides the difunctional product. Functionalities above 2 are found when the polymerization is conducted in a solvent, but only about 0.6 in the absence of a solvent. Terminally carboxylated copolymers of vinylidene fluoride and perfluoropropylene have been made using a peroxide initiator of the type
in which R , is a perfluoroalkane group. The initiator is soluble in the mixed monomers, and polymerization is conducted atabout 25-30°C. In practice the initiator may beprepared in aqueous solution in the same reaction vessel via reaction of hydrogen peroxide with acid chloride precursor in the presence of sodium hydroxide. The peroxide then transfers to the fluorocarbon phase, where polymerization occurs. The product, a viscous liquid, is curable to a tough rubber when mixed with pentaerythritol and heated (Rice and Sandberg, 1965). The copolymerization of butadiene with ferrocenyl methacrylate (Baldwin and Reed, 1969) or vinyl ferrocene (Reed, 1972b) may becarriedout in solution using azo-bis(2-methyl-5hydroxyvaleronitrile) as initiator.Yields of about 50% are observed in 72 hours at 67°C in dioxane. The introduction of iron into the liquid polymer is claimed to be useful in solid rocket propellant binders, the iron catalyzing increased burning ratesin the case of ammonium perchlorate-aluminum powder propellant. Another class of free radical polymerization processes involves the use of chain transfer agents of the type RSSR in emulsion systems. The chain transfer process occurs as follows: -*
+
RSSR
- -SR
+
RS'
(46)
If R contains a functional gorup, and if the polymerization conditions are such that a very high molecular weight product wouldbe formed in the absence of a chain transfer agent, the resulting product is difunctional for practical purposes. A study of the chain transfer coefficients of various disulfides was conducted during the 1950s (&stanza et al., 1955). The use of diisopropyl xanthogen disulfide in emulsion polymerizations of butadiene, followed by hydrolysis to yield terminal mercaptan groups. developed shortly thereafter (Byrd, 1958):
s
s
S
S
The resulting terminal groups were hydrolyzed with KOH in this instance. No11 and McCarthy ( I 966) polymerized butadiene (100 pbw) in an emulsion system with diisopropyl xanthogen disulfide (8 pbw) to 75% conversion and then heatedthe product at ISWC to produce terminal mercaptan groups by pyrolysis:
Liquid Rubber
157
Branching of the polymer via addition of thiol to double bonds occurs as a side reaction. This process may be refined by using small amounts of emulsifier and a persulfate initiation system, such that an acceptable product is obtained by direct drying of the latex to yield the finished product (Csontos, 1972). Mercaptan-terminated SBR polymers of high molecular weight provide excellent tensile properties when conventionally compounded and cured with a peroxide (Uraneck et al., 1969). In this case the conversion of the xanthate end groups was carried out in the latex by hydrolysis, using ammonia plus ethylenediamine at 70°C. Russian workers (Fokina et al., 1971) have provided data on the molecular weights and molecular weight distributions of liquid emulsion polymers as a function of conversion and of the concentration of diisopropyl xanthogen disulfide. For their purposes the end groups were hydrolyzed in benzeneethanol with excess ammonia. Another example of this general class of polymerizations is the photo-polymerization of chloroprene in emulsion with 4-hydroxybutyl xanthogen disulfide as the chain transfer agent (Takeshita.1974).
s s
S I1 II HO(CH2)rCSSC(CH2)40H-SC(CHa)rOH
S
”
-.
II
II
+
+
HO(CH2)LS.
(49)
Ultraviolet irradiation of the stirred latex, using a mercury lamp, for example, for16 hours at room temperature, yielded a liquid product capable of chain extension to an elastic solid with a diisocyanate. Since polychloroprene tends to crystallize on standing, pourability may not be retained. Preparation of aminotelechelic polymers in aqueous solution using a TiCI3/NH20Hredox system has been reported (Rubio et al., 198 1). NH,.radicals are formed,initiating polymerization. For the case of methyl acrylate, functionalities close to 2 were reported for polymers prepared under carefully defined conditions. The system is complicated by precipitation of the growing polymer from the aqueous initiation medium. The preparation of carboxy-terminated hydrocarbon polymers by the electrolysis of dicarboxylic acids has been claimed (Mersereau. 1969). This may occur by successive combinations of free radicals generated at the anode (Kolbe reaction):
RCO$
g RCOO- -R.
+
C02
This method is claimed to be capable of producing, for example, dicarboxy poly(isobuty1eneco-methylene) from dimethylglutaric acid, or dicarboxy polyethylene from linear alkyl diacids.
5.2 Cationic Polymerizations Pioneering work of a very comprehensive nature in the field of telechelic polyisobutylenes made by cationic polymerization has been reported by J. P. Kennedy and coworkers at the University of Akron (Kennedy, 1984). This involves the use of cocatalysts that serve both as initiators and chain transfer agents (“inifers”). The further terms “unifer,” “binifer.” and “trinifer” have been coined to describe agents with the corresponding functionalities. The case of a difunctional agent (binifer) may be taken as illustrative (Kennedy and Smith, 1980). The polymerization may conveniently by conducted in dry methylene chloride at tempera-
Edwards
158
tures from about - 30 to - 70°C. A preferred binifer isl'-CI(CHi)~C"C~,HJ"C(CH~),C1, used in conjunction with BC13. The essential steps in the reaction may be represented as follows:
-
ClRCl + ClRoBClP Cl&CL'+ C H ~ = C ( C H J )ClR[ ~ CH~C(CHJ)~-]~%C~~~ C~R~CHZC(CHJ)~-~~OBCIP+ ClRCl CIR~CHZC(CHJ)Z-]~CI + C I&C14' C~R[CHZC(CHJ)Z-]~CI + BC13 BCI~OR~CH~C(CHJ)Z-]~CI
-
-
(51)
(52) (53) (54)
Propagation and chain transfer then proceed. as before, to produce the product Cb"-R"CI. in which both chain ends are equal. Termination without chain transfer produces the same terminal structure for the case:
-CH~C(CH,)ZOBCI~~-CH2C(CH&CI
+
BC13
(55)
It is critical to the process that the rateof chain transfer to monomer be negligible compared to the rate of chain transfer to binifer. The aboveconditions satisfy this requirementto the extent that essentially difunctional products are obtained. Given the terminal unsaturation and the chemical inertness of the poly-isobutylene chain. numerous other terminal functionalities including amine, vinyl, phenol. hydroxyl, and epoxy can be prepared by appropriatesyntheticmethods (Kennedy, 1984). Dehydrochlorination to provide terminal unsaturation, followed by sulfonation with acetyl sulfate. produces terminal sulfonic acid groups of the structure:
-C"CHZSOJH II CHz When applied to a trifunctional "star" polymer derived from a "trinifer," the effect is to produce an "endless" thermoplastic rubber via association of the sulfonic acid end groups or their metal salts. The physical properties of materials such as thermoplastic elastomers are
fairlygood (Bagrodia andWilkes. 1985). The generalstudy of moleculardesigns, based on these elegant caticnic polymerization techniques. is a subject of intensive ongoing research by Kennedy and coworkers. An unusual cationic polymerization of isobutylene, using a molecular sieve (Linde Type 5A) to provide a polymer with terminal unsaturation, has been claimed in a patient specification (Miller, 1969). Here it is speculated that the sieve functions as an initiator by hydride removal due to incompletely neutralized calcium or aluminate ions on the surface:
The second terminal unsaturated group is presumed to arise by normal termination processes. A conversion of 34% was observed in 4 hours at 0°C when 25 g of the dried sieve was stirred with 62 mL of isobutylene. A functionality of 2. l , based on NMR analysis and ozonolysis (no molecular weight decrease). was indicated. Another unusual cationic process involving a dibromide chain transfer agent has been described (Ver Strate and Baldwin. 1977). These authors polymerized 4-methylpentene-l with aluminum chloride at - 60°C in the presence of I ,3-dibromo-3-methylbutane. A liquid product having, by analysis, approximately two bromine a t o m per chain, was obtained. Dehydrobromination followed by ozonolysis gave results consistent with terminal functionality. The unsatu-
Liquid Rubber
159
5.3 Condensation Polymerizations This section takes note of a few condensation reactions that differ generically from those, such as polysulfides and polyesters, already discussed. Terminallyunsaturated monomers canbe condensed with hydrogensulfide to prepare thiol- or vinyl-terminated products (Erickson, 1965). For example.
0 II
0
It CHz=CHCO(CH2)rOCCH=CH2
-
+
H2S
0 0 (57) II II HSICHZCH~CO(CH~)~OCCH~CH~S--]~H
The reaction may be carried out in pyridine solution at room temperature with diisopropylamine as a catalyst. Vinyl-terminated polymers are prepared by using reduced amounts of hydrogen sulfide, about 95% of stoichiometric equivalence. Dithiols may be copolymerized with vinyl acetylene under ultraviolet irradiation to produce thiol-terminated alternating polymers (Oswald. 1970): HRSH (excess)
+
CHGC-CH,
- HSIRSCH-CH-S”),RSH I
CH3 This reaction may be carried out in the absence of solvent at about 15°C. With excess methyl acetylene, terminally unsaturated polymers are prepared. Polyoxyalkylene diols may be condensed with a mixture ofmercapto-alkanoic and thiodialkanoic acids to produce terminal mercaptan liquid polymers(Jones and Marrs, 1972), represented schematically as follows:
n HOOCRSCOOH 2 HOOCR’SH -HSR’C(O)O~C(O)RSRC(O)O-O-]~C(O)R’SH H20 (59) A product of this type (PM polymer) was introduced for use in sealant compositions by the Phillips Petroleum Company. A process based on the addition reaction of dithiols with terminally unsaturated precursors has been described (Singh et al., 1981). the following steps being representative: nHO-OH
+
+
+
nB H S ( C H Z ) Z S ( C H ~ ) (excess) ~SH +
- polymeric dithiol
+
2H20 (62)
Edwards
160
Reaction (60) may be conducted at about 50°C with a free radical catalyst. Reaction (61) may be carried out under nitrogen at 150- 180°C with triphenyl phosphite as a catalyst. The final polymer (62) may be prepared in about 16 hours at 70°C in the presence of t-butyl perbenzoate. Afamily of terminallyreactivepolyetherhhioetherliquids has been introduced by Products Research and Development Corporation. The terminal groups may be either thiol or hydroxyl. The products are used principally as sealants for insulated glass. The hydroxyl-terminated type is claimed to provide polyurethanes with excellent solvent and water resistance.
5.4
Ring-Opening Polymerizations
The polymerization of propylene oxide, tetrahydrofuran, etc., to prepare diols is one of the established routes to polyurethane building blocks,as already discussed. A terminally unsaturated liquid polymer of epichlorohydrin has been disclosed more recently (Hsu, 1979).Polymerization of epichlorohydrin in methylene chloride solution at 50°C. with (C2Hs)30PF(, as catalyst, in the presence of hydroxyethyl acrylate or methaclylate as “chain transfer” agents, is stated to produce liquid polymers with terminal acrylate or methacrylate groups suitable for modification of unsaturated polyester compositions or for photopolymerization.
5.5
Chain Cleavage
The cleavage of high molecular weight polymers to terminally functional liquids was the first generic process for telechelic polymer formation, as described earlier for polysulfides. Among other possible processes of this general type, most attention has been given to the cleavage of unsaturated elastomers by ozonolysis. In principle this is particularly applicable to polymers, such as butyl rubber, in which there are occasional unsaturated linkages in an otherwise inert polymer chain. The chain ends of the liquid products of ozonolysis will vary in composition depending onthe starting materials and the detailsof the ozonization conditions. As a generalization, the endgroups may be mixtures of oxygenatedspecies that can be fullyoxidized(to carboxyl) or reduced (to hydroxyl) by appropriate procedures. Butyl rubber (isobutylene-co-isoprene) may be conveniently ozonized in cold hexane solution using an ozonized oxygen stream. The liquid product, redissolved in diethyl eher, may be treated with LiAIH., at room temperature to produce a liquid polyisobutylene diol curable with diisocyanates (Jones and Marvel. 1964: Manton and Brock, 1965). Nagakawa and Rudy ( 1966)copolymerized isobutylene with small amounts of butadiene or isoprene. The butadiene copolymer was ozonizedin carbon tetrachloride solution and subsequently treated with excess fuming nitric acid to complete the oxidation process. In another example using the isoprene copolymer, the final oxidation was carried out in dioxane solution with sodium hypobromite. The terminally carboxylated products were curable with tris-(2-methyl aziridinyl) phosphine oxide. A useful comonomer for isobutylene in this type of system is 1,3-pentadiene (piperylene). This monomer copolymerizes more readily than butadiene and provides a symmetrical double bond that simplifies the subsequent generation of closely similar and groups. Ozonization to ternlinally carboxylated polyisobutylene followed by modification with ethylene imine to form terminal amine or hydroxyl has been described (Minckler and Watchung, 1970):
Liquid Rubber -RCOOH
161
+
CHz"CH2 "RCO(CH2)2NHz \ / II
N
0
A "+
-RCNH(CH2)20H II
0
(64)
H
In this case the ozonolysis may be conducted in hexane solution at about 4°C for about 5 hours in the presence of pyridine. The solution may be heated to complete the cleavage reaction. The ethylene iminepostmodification may be conducted in hexane/THF solution and heated to 100°C in order to complete reaction (64). The product is curable with diisocyanates. Ethylene-propylene terpolymers with suitable diolefins can be used in a similar manner. For example, tenninally hydroxylated liquid ethylene-propylene has been described by Greene and Soh1 (1973). A terpolymer containing butadiene may be ozonized in a mixture of carbon tetrachloride and ethanol at a temperature below 0°C. Under these low-temperature conditions the reactions are believed to be as follows:
Both of these end groupsmay then be reduced to -CHlOH by addition of sodium borohydride in ethanol. maintaining the temperature at about - 10°C. The recovered liquid polymer product is curable with diisocyanates. Ozonolysis of saturated ethylene/propylene rubber in carbon tetrachloride at 25°C using ozonized air has been claimed to yield liquid polymers having primarily carboxyl end groups (Meyer, 1973). Rhein and Ingham (1975) prepared unsaturated ethylene/propylene lubber elastomers by first brominating the polymer and then dehydrobrominating. The unsaturated product was then cleaved by ozonolysis (ozonized oxygenin carbon tetrachloride or heptane solution). The ozonized liquid products were reduced to terminally hydroxylated derivatives. with various reducing agents being used. Ozone degradation of saturated polymers (polyisobutylene, ethylene-propylene, amorphouspolypropylene) was also observed in this work, and the reduced products exhibited a considerable degree of curability with diisocyanates. Analysis indicated the presence of some hydroxyl functionality along the polymer chains aswell as at the chain ends. Themaximum proportion of polymer isolubilized during the curing process was in the region of 90% in these cases. Cleavage procedures other than ozonolysis may also be noted. Isoprene and, less readily, butadiene-styrene can be copolymerized with sulfur in emulsion. The polymeric products. containing di- and polysulfide linkages in the backbone. can be reductively cleaved while still in emulsion (Costanza, 1963a). using zinc dust together with solvents. followed by hydrochloric acid. Alternatively, the dried polymer can be swollen with a solvent and treated with LiAIHJ (Costanza, 1963b). Costanza reacted the product of polyisoprene cleavage with epichlorohydrin to provide terminal groups of the structure:
-SCH~-CH"CHZ \ /
0
This provided curability with diamines in a manner considered suitable for potting compounds. Postmodification with glycidyl acrylate. for the same purpose, was also demonstrated. High molecular weight polyethers may be cleaved into liquid diols by hydrolysis (Reegen and Frisch, 1964). In the case of stereoregular poly-propylene oxide, the product is claimed to
162
Edwards
be superior to conventional poly-propylene oxide diols as a polyurethane building block. The degradation may be carried out in water plus isopropanol under reflux conditions (e.g., 80°C for 40 hr) in the presence of sulfuric acid.
5.6 Miscellaneous Curing Reactions The scope for postmodification of end groups and for the exploration of novel curing systems is, of course, very broad. It is appropriate here to take note of a few examples that have not otherwise been touched upon. Terminally hydroxylated polymers suchas polyether diols have been converted to terminal primary amines by reaction with ammonia (Palmquist and Jones, 1966). Such a product was designed to form an elastic,adhesivebinder to provide a reflectivesurfacecomposition on bicycle tire sidewalls, by curing with an epoxy resin. Curing of carboxylated liquid nitriles with metal oxides has also been studied (Matsuda and Minoura. 1979). The gum vulcanizates are generally low in tensile strength, about l MPa or less. A 2-to- 1 MO/carboxyl ratio is superior to a 1-to- 1 ratio, and hence the preferred mechanism appears to be pairing or clustering of terminal ionic sites of the type -RC(O)OMOH. Such products are reprocessable at elevated temperatures. When polymers havingterminalacrylate groups, such asdiacrylate or dimethacrylate esters of polyethylene or polypropyleneglycols,aremixed with an organicperoxideanda tertiary amine. they remain stable in the presence of oxygen (air). but they polymerize rapidly when oxygen is excluded, for example, by applying the mixture to a threaded bolt and then installing the bolt. This principle is well established as a means of locking bolts into position more firmly than is possible without the polymerized bonding agent (see, e.g., Bosworth et al., 1963). The reaction of di- or polyfunctional thiols with terminally unsaturated polymers under gamma or electron beam irradiation has been studied extensively (Kehr and Wazolek, 1970). These authors prepared terminally unsaturated products from available precursors by numerous methods, including. for example. the reaction of isocyanate-terminated prepolymers with allyl alcohol. Cure of a liquid film with,for example,pentaerythritol-tetrakis-(P-mercaptopropionate) occurs in a few seconds during passage under an electron beam.
6. CONCLUDING REMARKS From a research standpoint, the subject of terminally reactive liquid elastomers is both broad and deep, embracing all classes of polymerization technology and, in the manipulation of end groups and design of practical curing and reinforcement systems, an extremely wide spectrum of synthetic possibilities. This chapter. which is necessarily a very brief treatment of the field, has outlined the technology of commercially useful products and has given some attention to the question of whether future liquid polymer technology may lead to a significant displacement of high molecular weight elastomers in conventionally reinforced vulcanizates. Since it has been amply demonstrated that there is no fundamental impediment to such a development. one must entertain the possibility that it may occur in the course of time. However, such a change in the fundamentals of the rubber industry would seem to require some form of technology unification that would permit the economic manufacture,compounding, and vulcanization of a broad variety of materials analogous to the general-purpose and specialty elastomers that we now use in high molecular weight form. From today’s perspective, such a development does not seem likely. The refinement andfurther development oftelechelic liquids for specialty purposeswill continue,
Liquid Rubber
163
however, to be one of the most interesting and challenging areas of research in the elastomers field.
ACKNOWLEDGMENT I wish to thank my colleagues for their assistancc and Polysar Limited for permission to publish.
REFERENCES Allen, H. C. (1963). US. Pat. 3,440,292 (issued 1969, Sec. of the A m y , U.S.A.). Alliger, G., Smith, W. A., and Smith. F. M. ( l97 l). Rubber World f64(3):5 I . [Also R1thber World f 61(5): 29 ( 1970)]. Anrady, A. L., Llorente, M. A., Sharaf, M. A., Rahalkar, R. R., and Mark, J. E. (1981). J . App/. Pniynl. Sci. 26: 1829. Bagrodia, S., and Wilkes, G . L. (1985). ACS Spring Meeting. Rubber Division, Los Angeles, Paper No. 3. Baldwin, M. G., and Reed, S. F. (1969), U.S. Pat. 3,847,882 (issued 1974, Sec. of thc Army, U.S.A.). Barantsevich, E. N., Pronin, B. N., Bclov, I. B., Kalaus, A. E., Beresneva, N. K.B., and Troitsky, A. P. (1973), Ger. Pat. 2.444.65 I (issued 1975). Bayer, 0. (1947), Allgew.. Cltern. AS9:257. Beaton, J. ( 197 1 ), Br. Polym. J. 3:129. Berenbaum, M. B., Bullcnko, R. H., Gobran, R. H., and Hofman, R. F. (1961 ), U S . Pat. 3,235,589 (issued 1966, Thiokol Chem. Corp.). Bcrtozzi, E. R. (1968), Rubber Cllenl. Teckrtol. 41: 114. Bosworth, P,, Yysc, B., and Swire, J. (1963), Brit. Pat. 1,090,753 (issued 1963, Borden Chem. Co., U.K.). Brosse. J.-C., Legcay, G.. Lenaln, J.-C., Bonnier, M,, and Pinazzi, C. (1978). McIkromol. Cller~l.f79:79. Brosse. J.-C., Bonnier, M,, and Legeay, G. (1982), Makrormd. Clleru. fK3:303. Brown. L. B.,and Hyde, J. F. (1960), U.S. Pat. 3,170,894 (issued 1965. Dow Corning). Brunner, L. B. (1959). U.S. Pat. 3,077,466 (issued 1963, Dow Coming). Buckler, E. J., Edwards, D. C., Wunder, R. H.. and Beaton, J. (1965), U.S.Pat. 3,506,742 (issued 1970, Polymer Corp.). Buechc, A. M. (1955). RuOher Cham. Technol. 28:865. Burke, 0. W., Kizer, J. A. A., and Davis, P. (1959), Can. Pat. 772,708 (issued 1967). See also U.S.Pat. 3,673,168 (issued 1972). Byrd, N. R. ( 1958), U S . Pat. 3,047,544 (issued 1962, Goodyear). Cesar, J. P., Gouttbessis, J., and Schneider, A. (1980). U S . Pat. 4,476,908 (issued Oct. 1984. Michelin). Ceyzeriat, L. (1957), US. Pat. 3,133,891 (issued 1964, Rhone-Poulcnc). Costanza, A. J., Coleman, R. W., Pierson, R. M,, Marvel, C. S.. and King. C. (1955). J. Poiym. Sei. 27: 3 19. Costanza. A. J. (1963a), U S . Pat. 3,338,874 (issued 1967, Goodyear). Costanza, A. J. (1963b), U.S. Pat. 3,338,875 (issued 1967, Goodyear). Csontos, A. A. (1972), Can. Pat. 954,998 (issued 1974, B. F. Goodrich Co.). Daniel, T. J., Needham, A., and Pyne, J. R. (1972), Trcrrls. Inst. Rubber I d . 6:253. Dannenberg, E. M. (1985). Progr. Rubber P1o.stic.s Techrtol. I: 13. de Zarauz, Y. (1975), Can. Pat. 1,021,889 (issued 1977, Michelin). Doughty, J. I. (1962), U.S. Pat. 3,912,696 (issued 1975, 3M). Doughty, J. I., and Chnstman, P. G. (1967), U.S.Pat. 3,645,956 (issued 1972, 3M). Doughty, J. I., and Christman, P. G. (1969). U.S. Pat. 3,654,241 (isycd 1972, 3M). Drakc, R. S., and Siebert, A. R. (1984). in Adltesive Chemistry (Leing-Huang Lee, Ed.), Plenum, New York, p. 643.
164
Edwards
Edwards, D. C., and Wunder, H. R. (1968). U.S. Pat. 3,551,402 (issued 1970, Polymer Corp.). Edwards. D. C. (1975). Rubber Chern. Tec/rrzo/.48:202. Erickson. J. G. (1965). U.S. Pat. 3,415,764 (issued 1968, 3M). Fedjulun, D. L. (1984). Proc. Infer.Rubber Cor$, Moscow, 1984. Plenary lecture: Prospects of Processing Developments in the Production of Mechanical Rubber Goods. Flory, P. J. (1946), I r z d Eng. Chenr. 38(4):417. Flory, P. J. (1950). J. Clrent Phvs. 18:108. Fokina. T. A., Apukhtina, N. P.. Klebanskii, A. L., Paviova, L. V., and Pikhtengol’ts, V. S. (1971), Po/yrrz. Sei. U.S.S.R. 13:2216. Frcnch, D. M,, Strecker, R. A. H., and Tompa, A. S. (1970), J. App/. Po/yrr~.S i . 14599. G:lylord, N. G. (1973), U.S. Pat. 3,959,244 (issued 1976, Dart Industries). Greene, R. N., and Sohl, E. (1973), U.S. Pat. 3,857,8263 (issued 1974, DuPont). Hoffman, R. F., and Gobran, R. H. (l973), Rubber Chern. Techrzol. 46: 139. Hsieh, H. L. (1959), U.S. Pat. 3,175,997 (issued 1965, Phillips). Hsu, C. C. (1979), U S . Pat. 4,256,910 (issued 1981, B. F. Goodrich Co.). Humpidge, R. T., Morrell, S. H., and Nelms, R. P. (1972), Rubber World 66:47. 46: 148. Humpidge, R. T., Mathews, D., Morrell, S. H., and Pyne, J. R. (1973), Rubber Clzerrz. Tec/zr~o/. Inomata, J.. Michishima, S., Hino, S.. and Igarashi, S. (1975), Proc. Inter. Rubber Cor$. Tokyo (Oct. 1975). Society of the Rubber Industry, Japan, p. 277. Isaacson, H. V., and Young, D. W. (1966). U.S. Pat. 3,392,118 (issued 1968, Sinclair Research Inc.). Jiang, C. Y., and Mark, J. E. (1984). Colloid Polwz. Sei. 262758. Jones, E. B., and Marvel, C. S. (1964), J. Pdvrn. Sei., A2:5313. Jones, F. B., and Marrs, 0. L. (1972), U S . Pat. 3,817,936 (issued 1974, Phillips). Co.). Kambara, S., and Aotani, S. (1970). Can. Pat. 908,897 (issucd 1972, Japan Synthetic Rubber Kehr, C. L., and Wazolek, W. R. (1970). U.S. Pat. 3,725,228 (issued 1973, W. R. Gracc & Co.). Kennedy, J. P,.and Smith, R. A. (1960). J . Pdyrn. Sei. P d y t z . Ckern. Ed. 18:1523. Kennedy, J. P. ( 1984), J . Ap/d. Polyrtz. Sei. Appl. Polyrzz. S>wp. 39:21. Kraus, G. (1977), Arrgew. Mdwortrol. Chern. 60/61:215. Lee, T. C. P,, Morrcll, S. H., and Willoughby, B. G. (1978). Kcrutsch. Gurrzrrzi Kurrsr. 3/:723. Llorentc, M. A., and Mark, J. E. (1980), Mrtcrornolecules 13:681. Llorentc, M. A., Anrady, A. L., and Mark, J. E. ( 198 l), J. Polym. Sei. Po/ym. Ph>x Ed. f9:621. McGarry, F. J., Rowe. E. H., and Riew, C. K. (1977), Section 16-C, 32nd Ann. Tech. Conf., Reinforced Plastics/Composites Institute, The Society of the Plastics Industry Inc., New York. Manton, J. E., and Brock, D. J. (1965), Can. Pat. 792,805 (issued 1968, Polymer Corp.). Mark, J. E. (1984), ACS Spring Meeting, Rubber Division, Indianapolis, Paper No. IO. Marshall, S. ( 1982), Rubber Plrstic News, Dec. 20, p. 6. Masuko, T., Yanagida. O., and Yamamoto, S. (1974), U.S. Pat. 4,098,715 (issued 1978, Mitsubishi Chem. Ind. Ltd.). Matsuda, H., and Minoura. Y. (1979), J . A p p / . Po/yrtr. Sei. 24:81 I . Mersereau. J. M. (1969), U.S. Pat. 3,616,313 (issued 1971, Uniroyal Inc.). Meyer, J. M. (197.7). U.S. Pat. 3,910,990 (issued 1974, DuPont). Miller, J. A. (1969), U.S. Pat. 3,634,383 (issued 1972, NASA). Mincklcr, L. S . , and Watchung, N. J. (1970), U.S. Pat. 3,678,013 (issued 1972, Esso). Minoura, Y.. Yamashita, S., Kojiya, S., Okamoto, H., Yamaguchi, H., Matsuo, T., Sakata, M., and Okada, M. ( 1977), Irzt. Polvnz. Sei. Techrznl. 4( 1 l):T/l4. Morton, M,, and Rubio, D. C. (1977), International Rubber Conference, Brighton, U.S., Paper No. 15. Nagakawa, T. W., and Rudy, T. P. (1966). U.S. Pat. 3,427,351 (issued 1969, United Aircraft Corp.). . Sei. 293209. Ning, N. P,. Tang, M. Y., Jiang, C. Y., and Mark, J. E. (1984), J. A p / ~ l Po!\arz. Nitzsche, S., and Wick, M. (1960), U.S. Pat. 3,065,194 (issued 1962, Wacker-Chcmie G.m.H.). Noll. R. F., and McCathy, W. J. (1966), Brit. Pat. 1.139,655 (issued 1969, B. F. Goodrich Co.). Oswald, A. A. (1970), U.S. Pat. 3,717,618 (issucd 1973, Esso Research and Enginecring). Palmquist, P. V., and Jones, N. (1966). U.S. Pat. 3,449,201 (issued 1969, 3M). Patrick, J. C., and Mnookin. N. M. (1927). Brit. Pat. 302,270 (issued 1930).
Liquid Rubber
165
Patrick, J. C.. and Ferguson, H.R. (l94S), U.S. Pat. 2,466,963 (issued 1949, Thiokol Corp.). Pinazzi, C., Legeay, G., and Brosse, J.-C. (1973). J. P o / y n ~Sei. . Syn~p.42:1 1. Pyne, J. R. ( 1970). J. Inst. Ruhher I u d . 4:207; also Ruhber C / I ~ Techr~ol. ~I. 44350 (1971 ). Quinn, E. J. (1968); U.S. Pat. 3,668,243 (issued 1972, Uniroyal Inc.). Rau, H. J., and Just, G. (1983), U.S. Pat. 4,453,993 (issued 1984, Bayer). Redman,R. P. (l978), in Deve/opnwrzt.s in Po/yurethctrres, Vol. 1 (J. M. Buist, Ed.), Applied Science Publishers, London, Ch. 3. Reed, S. F. (1971), J. P()/yttz. Sci. A - l 92029. Reed, S . F. (1972~1). U.S. Pat. 3,813,306 (issued 1974, Sec. of the Army, U.S.A.). Reed, S . F. (1972b), J. P d y m Sei. P d y m . Chen~.Ed. A - / , /0:2025. Reed, S. F. (1973). J. Po/yrn. Sci. Polytn. Chern. Ed. 11:1435. Reegen, S. L., and Frisch, K. C. (1964). U.S. Pat. 3,395,185 (issued 1968, Wyandotte Chem. Corp.). Rhein. R. A.. and Ingham, J. D. (1975). Polyrrler 16:799. Rice, D. E., and Sandberg, C. L. (1965), U.S. Pat. 3,438,953 (issued 1969, 3M). Riew. C. K. (1976), U.S. Pat. 4,133,957 (issued 1979, B. F. Goodrich). Riew, C. K. (1981), Ruhher Cherr~.Techrlol. 81:374. (See also Hycar Reactive Liquid Polymcrs, Tech. Bull. AB-9 and AB-16, B. F. Goodrich Chemical Group, 1983.) Rivin, D., and True, R. G. (1973). Ruhher Clrem. Tec1utnl. 46:161. Rossi, R. K. (1982), Eur. Pat. App1.,091,391 A (Goodyear). Rubio. S., Scrre, B., Slcdz. J., Schue, F., and Chapelet-Letourneux, G. (1981). P d y r w r 22:519. Ryan, P. W.. and Thompson, R. E. (1968), U.S. Pat. 3,652,520 (issued 1972, ARCO). Saunders, J. H., and Frisch, K. C.(1978), Po/yrrrr,t/ltrrlrs: Chemistry N I I ~ Techrlology. R. E. Krieger, Mclbournc, Florida. Schmidt, 0. (1978). U S . Pat. 4,259,129 (issued 1981, LIM International). Schmidt, O., and Kubica, W. (1978), U.S. Pat. 4,277,295 (issued 1981. LIM Internationnl). Seegman. I. P., Morris, L., and Mallard, P. A. (1961), U S . Pat. 3,225,017 (issued 1965, Products Research Co.). Siebert, A. R. (1964), U.S. Pat. 3,285,949 (issued 1956. B. F. Goodrich). Siebert, A. R. (1968a). U.S. Pat. 3,551,471 (issued 1970, B. F. Goodrich). Siebert. A. R. (1968b). U.S. Pat. 3,551,472 (issucd 1970, B. F. Goodrich). Siebert, A. R. (1984). ACS Adv. Chem. Series. No. 208 p. 179. Singh, H., Hutt. J. W., and Williams, M. E. (1981). U.S. Pat. 4,366,307 (issued 1982, products Research and Chem. Corp.). Skillcorn, D. E. (1972), U.S. Pat. 3,910,992 (issued 1975, B. F. Goodrich). Szwarc, M,, Levy, M., and Milkovich, R. (1956), J. Am, C/~enl.Soc. 78:2656. Takeshita, T. (1974). U.S. Pat. 3,900,379 (issued 1975, DuPont). Ulrich, H. (1983). in K i r k - O f h e r Erlc)lc./opdirr c$ C/wrrliccl/ Tec/rr~o/(,gy3rd d . , Vol. 23, Wiley, New York, p. 576. Uraneck, C. A.. Hsieh. H. L., and Buck, 0. G. (1960). J. po/vn~. Se;. 46:535. SC;. 1 3 : 149. Urancck, C. A.. Hsich, H. L., and Sonnenfeld, R. J. (1969), J. App/. Vcrdol, J. A., and Ryan, P. W. (1966). U S . Pat. 3,427,366 (issued 1969, Sinclair Rcsearch Inc.). Vcr Strate, G., and Baldwin, F. P. (1977), U.S. Pat. 4,278,822 (issued 1981, Exxon). Wagner, M. P. ( 1976), Ruhber C h e r ~T~ d. u w l . 49703. Whitby, G. S. (1954), Syrlthelic. Ru/~ber,Wiley, New York, p. 892. Yamashita, S.. Minoura, Y., Okamoto, H., Nukui, T., and Monmoto, E. (1978). h t . Po/ynr. Sei. Tec/lrlo/. 5(7):T/100. Yarnawakl, S., Masuko, T., Yanagida, 0.. :tnd Yamarnoto, S. (1974): Jpn. Pat. 75,1 10,443 (published 1975, Mitsubishi Chem. Ind.). I'O/~W~,
This Page Intentionally Left Blank
Powdered Rubber Colin W. Evans* Consultant, Gateshead,England
1. INTRODUCTION Powdered lubber is, as its name implies, rubber in powdered form. Strictly speaking, the particle size is approximately 1 mm, although, as discussed later, the technologyalso includes particulate rubber up to approximately 10 mm particle size. Since volume production using powdered and particulate rubbers has existed since the mid-l970s, the following outstanding advantages over olderbale processing methods have been confirmed:
1. 2. 3. 4. 5. 6. 7. 8.
9. 10. 11.
12. 13.
Bale cutting can be eliminated. Much shorter mixing cycles are possible, with either internal mixers or the openmill route. Less power is consumed. Less plant maintenance is needed per kilogram mixed. Processing plants need not be so capital-intensive. Better ultimate dispersion is possible with powdered rubber. More rapidly accelerated compounds can be mixed. Considerably less heat memoryis retained when some of the more difficultpolymers such as polychloroprenes are processed, and hence there is less scorch. Blends can be fed directly to the extruders and molding presses, thus eliminating the internal mixer and/or open-milling operations. There is no need for massive premastication and masterbatching of many rubbers. Mixing and dump temperatures are considerably lower. Because of ease of mechanizing, factory controls are simpler. Better and cleaner environmental conditions can be maintained.
The only serious negative aspect of the use of powdered rubbers is that of the grinding premium, but even this is being reduced now that the use of the powdered material is growing, and in any case the savings of labor and energy compared with existing technology more than justifies its use. It would be very unwise not to consider even partial usage of powdered rubber in presentday mill-room areas.
* Deceased. 167
168
2.
Evans
CONVENTIONALMIXING
For approximately 150 years, the rubber industry has mixed and compounded rubber in very robust and heavy equipment. Plastic polymers, on the otherhand, need much lighter engineering equipment. It is strange that theconventionalandtraditionalmethod of processingwithinrubber factory mill rooms, over many decades, has been to (a) thoroughly masticate, or rather “knock to pieces,” the structure of the polymers, eitheron mills or in internal mixers, then (b) incorporate fillers, softeners, antioxidants, accelerators, curatives, and many other ingredients, in an endeavor to correct the damagealready done. and then(c) build the ruptured chains together by crosslinking. The energy consumed in doing this is out of all proportion to the actual needs, and it is not really surprising that many of thestrangeprocessingvariablesandproblems that occur without warning in the factory, and then disappear just as quickly, can still remain unsolved mysteries. When powdered elastomers became available. it was decided to study whether products with satisfactory properties could be made on equipmentsimilar to that used to process plastics (E. I. duPont, 1972). A comparison of existingrubber-processing techniques with powder systems will therefore be initially discussed. Mixers used for rubber compounds can be categorized into three general types: 1. Open mill 2 . Internal,e.g.,Banbury or Intermix 3. Continuous, high-speed
The cooling of the mixers is of paramount importance, and much development work has been performed by the makers of mixers to make the current faster mixing cycles possible.
2.1
Open-MillMixers
Internal mixers have high outputs, but for hoses, open-mill mixing is still practiced and will continue to be, because of the small production runs of certain products. Also, very necessary, good dispersion and freedom from contamination cannot be guaranteed with internal mixers. The open-mill mixer masticates the polymers until an even and smooth band is formed around the frontroller. The fillers and oils areadded alternately, followed by any small additions and finally the vulcanizing materials. During the whole operation, cutting and blending by hand rolling are carried out. As the powders drop into the mill tray, they are swept to the front by the operator and added back into the mill nip. The mill tray is usually slightly sloping to help the operator, and a vibratory mechanism that continuously returns powder to the operator, thus saving physical effort, is very useful. The best dispersion and blending of compounds containing mixed polymers is obtained by breaking down each polymer individually and then blending them together while still hot. The addition of the fillers, etc., then follows in the usual way. In order to help the breakdown, special peptizers may be added during mastication,and in the case of polychloroprenes, retarders and other processing aids may be added very early in the mixing cycle. For the processing of butyl rubbers, it is desirable to have the mills so positioned as to be able to work safely on the back roll due to the affinity of this polymer for the faster roll. In large user factories, it is possible tokeep butylrubberconfined to itsownline,making it practicable to adjust the mill gear ratios so that the rubber is banded around the front roller.
Powdered Rubber
169
2.2 Internal Mixers With internal mixers, the ideal setup is to have twomills in the chain. The first is used to remove the heat from the compound rapidly prior to the addition of the vulcanizing ingredients on the second mill. This also has the advantage of keeping the internal mixer free of curing materials, with far less tendency to scorch problems. The high initial capital cost of such a system is more than justified by the ease of subsequent processing. Indeed, many factories are installed in this manner. As with open-mill mixing cycles, the general rule is to masticate in the mixer, and when the polymer has reached the desired state the additives are mixed in and dispersed. The batch is then dropped and passed through the first mill several times, and then the curing agents are added on the second mill. However, mixing procedures, whetheron open-mill or internal mixers, are peculiar to particular factories.
Processing Techniques The following techniques are widely used: Direct Mixing Process. The compound is mixed and is then fed directly to an extruder or calender. This processrequirestheaccurateplanning of the mixer cycle andsubsequent operations and has fairly widespread use. It is necessary to have tight quality controlof curing and dispersion. Theinitial mastication is extremely important because of subsequent “nerviness,” as there is no maturing time in the cycle. Indirect Mixing Process. In this cycle, the compound is mixed, slabbed off, and stored. The curing materials may be added before slabbing or after maturing in storage, depending on the particular compound. This system and open-mill mixers give the best compound for processing. Premastication. In certain instances, particularly in compounds where there is a fairly high hydrocarbon content, it is necessary to premasticate the polymers, slab off and cool, and then mix in the normal way with this premasticated material. Alternatively, if the compound contains a fairly high filler content, either black or mineral, a masterbatch may first be mixed, slabbed, and cooled and then final-mixed. A masterbatch is a mixture of polymer and filler, with the filler content as high as 50%. Oil Extension. In the case of naturalrubber compounds, it is possible to oil-extent without undue loss of subsequent processing or physical properties by selecting a suitable oil and preblending it with the requisite carbon black. The “carboil” so produced is added to the internal mixer with the rubber, right at the beginning of the cycle,and the wholeis then masticated together. This techniquepreventsunduechainscission due to mastication,andtheresultant plasticity so obtained is very satisfactory. The addition of dihydrazine sulfate to the compound also helps subsequent processing of such oil-extended natural rubber compounds (Evans,1979). Dump Mixing. When the compound contains a fairlyhighproportion of filler, it is sometimes difficult to get the rotors of the mixer to “bite,” and in such cases it is normal to literally dump the wholeof the ingredients (rubbers and fillers) into the mixer together and then carry on with the cycle. Upside-Down Mixing. This technique consists of adding the powders to the mixer first, followed by the polymer. This not only produces a satisfactory mixed material but also makes it possible to mix certain difficult polychloroprene recipes, which hitherto had to be mixed on open mills because of scorch and/or sticking problems. Seeding. This is another extremelyusefulmixingtechnique (Nye, 1943). Developed during World War I1 because of shortages of natural rubber and other hydrocarbons, it consists of adding a small portion of the previous batch of the same material, and allowing the new
Evans
170
batch to “seed” on it during mixing. This is particularly effective in mixing compounds of extremely low hydrocarbon content. By selecting the correct mixing procedure, coupled with accelerator and curing systems, it is now possible to speed operations by processing the whole batch in the mixer. Additionally, by masterbatching certain of the more difficult accelerators, it is also possible to add these and blend away rapidly without precure, right at the end of the internal mixing sequence. BNtch Size
Wear on both the rotors and shells of the mixers must be compensated for by increasing the batch weights slightly from time to time. Otherwise various mixing difficulties, such as poor dispersion and mastication, occur. Cycles Two techniques are used in an internal mixer. One is to mix to a fixed time cycle and ignore the ultimate temperature reached, and the other is to mix to a fixed temperature rise on mastication and ignore the total time. Both of these systems economically produce uniform material. and in fact it may be necessary to operate both systems (in different machines, of course) to suit the particular recipe. It is current practice to mix as nearly continuously as is practicable. Furthermore, the use of high-speed rotors is increasing. This considerably reduces the mixing time but increases the temperature.Because of this,a lot of work has been carriedout on efficientwater-cooling systems and on methods for the rapid discharge of the mixed compound, such as the use of drop doors. Synthetic Materids The use of magnesium oxide in stick form in chloroprene rubber (CR) recipes helps dispersion at the critical stage of mixing, with a reduction in mixing time and temperature rise (Evans, 1979). This reduces scorch tendencies. Scorchis the term used to describe incipient vulcanization of a rubber compound. CR is a polymer that because of its heat memory develops at each stage of processing an additive and accumulative heat history. Thus, in highly loaded compounds, the basic scorch properties are aggravated. Because powdered rubber compounds, as has been shown, both mix and process at lower temperatures, this accumulative heat buildup is considerably reduced, and hence safer processing characteristics are conferred to the mix.
2.3
Continuous and Semicontinuous Mixers
Before the mid-1960s there had been very little change in the methods or the equipment used within the mixingrooms of the rubber industry for over 100 years. Over the past decade, systems approaching the ultimate goal of continuous mixing (Ellwood, 1978) have been introduced, with varying degrees of success, as many are much more costly than established methods. Because of the various polymers used, the mixing equipment must be very strong and robust to endure the very high loads and stresses developed during processing. The future pattern of continuous mixing could, however, depend very much on the success of the resurrection of the so-called newer technologies of liquid rubber processing and also of powdered rubbers (Morrall, 1973). Over the years, these technologies have had limited success,
Powdered Rubber
171
mainly due to deficiencies in materials, methods, and processing machinery, but they currently stand a much greater chance of success. The rubber industry has always been faced with the physical difficulty of breaking down the material before the fillers and other ingredients can be incorporated and blended. There is a continual search for an easy and cheaper way of achieving this. Latex technology (Murphy and Twiss, 1930) hasbeen examined, as it appears to be a comparatively easy way of obtaining dispersion and good mixing with the minimum of shear and hence lower power consumption. The difficulties caused by the presence of water led to the use of solvents, with the attendant problems of fire and toxicity. Finally, rubber was liquefied by melting (Morrell, 1978), thus producing depolymerized rubber, with a viscosity of about 500 P at 20°C. In the 1930s, fillers were added to this material on rubber mills, and excellent electrical insulation material was produced. More recently, low molecular weight SBR materials have been produced, with viscosities of around 500 P at 25°C (Mees, 1985). The present difficulty is still to obtain dispersion. of the carbon black in particular, while at the some time retaining a pourable material. Another filler is nylon fiber, around 6 mm in length, but in both cases so far the flex resistance is inferior to that of solid rubber. Fulthermore, while it is possible to disperse in Z-blade mixers, the black in particular remains in large aggregates even after prolonged mixing.
3.
POWDEREDPOLYMERTECHNOLOGY
The plastics industry has been using thermoplastic polymers in powder form for at least 40 years. The first of these in any quantity was poly(viny1 chloride) (PVC), which was available as a white, freeflowing resin of approximately 1 mm particle size. It is therefore surprising that this technology has only very recently become of interest to the rubber industry. despite the fact that powdered nitrile rubber has been available for some 20 years. This was first produced by B. F. Goodrich (Goshorn et al.. 1969; Whittington and Woods, 197 1; Woods and Krasky, 1973; Woods andWhittington,1973; Woods et al.,1973;Whittington, 1974) in theUnited States, basically for blending with PVC as a dry and solid plasticizer. Much of the current work with nitrile materials in powder form has been attributed to and published by Goodrich. In addition to Goodrich, the Bayer Company in Germany (Bayer A.G.), has been very active not only with nitriles but also with polychloroprene rubbers, and a complete range of this latter polymer is available in powder form. Nitrile polymers are now also available in powder form from the majority of the major manufacturers. Many of these manufacturers incorporate the “powdering” operation into their mainline streams duringtheproduction of the basicpolymer. It is also possible to produce powder from the existing and finishedbales. and this is currently being carried out in the United Kingdom (Eagles, 1973;Whally,1973; Honday, 1974;B. P. Chemicals, 1985; Wood, 1985). Indeed, not only nitriles, but also powdered natural rubber, polychloroprene, SBR, EPDM, and most other common polymers can equally as easily be made by this method. The process uses primarygranulation,followed by reduction to a true powder form in attritionmillsandthe minimum addition of antitack and partitioning agents to prevent sticking in storage. Other routes to prevent sticking and partitioning include the use of freeze- and spraydrying and freeze-grinding, precipitation, and coagulation in-line. Of course, the method used depends very much on the particular polymer and the proprietary method of the producer concerned. It is therefore sufficient to say that the operation is carried in the presenceof a partitioning agent, which could be a talc, whiting, carbon black, starch and starch xanthate, or other similar
Evans
172
material. The amount of partitioning agent used depends on the polymer and on the method of powder production. At the 1974 National Rubber Conference in Munich, it was revealed that powdered masterbatch from the polymerization plant was now possible, using virtually any elastomer, with any filler, in any proportion (Nordsiek, 1974). This is in addition to the development of nitrile/ SRF black masterbatch as revealed at the I.R.I. National Rubber Conference in May 1974 at Black-pool (Evans, 1974). In the field of natural rubber, the latter was on display at the 1972 I.R.I. International Rubber Meeting at Brighton (Pike, 1972) and has since been used in bulk with satisfactory results. Additionally, at least one other large natural rubber producer is currently investigating the economics of launching a granulated natural rubber on the market (Thompson, 1973). Very recently, powdered reclaim has been made available in pilot quantities. The earliest references to powdered natural rubber are in Dunlop patents (1929).
3.1
Effect of Type and Quantity of Partitioner
A partitioning agent is required for powdered elastomers i n order to make them free flowing
and also capable of being transported and stored in stacked bags without compacting. Five partitioning agents were examined, using NBR, CR, natural rubber (NR), and SBR rubbers with quantities of 2 112, 5, 7 1/2, and 10% partitioner. These were magnesium silicate,
Table 1 Effect of Type and Quantity of Partitioner Versus Time" Magncsium silicate
Calcium silicate Silica Calcium carbonate Starch
Time (mo) Time (mo) Polymer
NBR
Percent partitioner
2 '/. S 7
CR
IO 2 '/. S 7 '/. 10
NR
2 '/. S 7 10
SBR
2
S 7% IO
I
3
2 1
2 1 1
1
I 1 I 1 1
2 I I 1 1 I 1 1
(mo)
6 3 2 2 I 2
1
2 I 1
I 3 2 2 1 2 I 2 I
1
3 1
1 1 1 1 1
3 2 2 2 3
2 I 1 I 1
2
1 3 I
2 I 1 1 2 1
1 1
1 1 1
6
1
1 3 2 2 I 2 2
1
1
1
1
1
3
6
1
3 1 2 3 3 2 1 2 2 3 2 1 1 2 3 1 1 1 1 2 2 1 1 2 3 1 1 1 1 3 I 1 1 1 3 1 1 I 1 3 3 2 3 3 3 2 1 2 2 3 2 2 2 2 3 2 1 1 1 3 3 1 2 3 3 3 1 1 3 3 2 1 1 2 3 1 1 1 2 3
3
Time (mo)
6
1 -
-
"
3
-
-
-
-
-
-
-
"
"
-
-
"
-
-
"
"
"
3 3 3 2 2 3 3 3 2 3 3 3 3 3 3 3 3
6
3 3 3 -
Key: 1. Free flowlng: 2, slight corupacted. hut easily broken down wlth light finger pressure; 3, compacted. not easily broken down. Source: Evans and Thesis. I98 1.
l'
173
Powdered Rubber Table 2 Effect of Magnesium Silicate at 25’&5, 75”. and 10% Levels with NBR Powder
92
1
2 3 4
Average Std. dev.
I12 I09 13
130 131 126 122 127 4
135 132 123 128 130 5
133 125 125 119
126 6
101 103 107 114 106 6
150
I60 170 I59 160 8
200 200 215 195 203 9
185 190
215 205 195
170
190
185
201 II
183 9
155 155 165 150 156 6
calcium silicate, silica, calcium carbonate. and starch, in the particle size range pass 200 mesh. The visual effect of compacting and free-flow properties was examined in 25 kg bags of material after transportation and storage.Stacking of thebagswas fourdeep, andthematerialwas examined in the bag at the base of each stack of post pallets (Table l ) . Transportation included examination of truck container loads. Table 1 shows the immediate unsuitability of starchand calcium carbonate,andthese materials were therefore discarded. Thesuitability of the silicate materials is clearly shown, and the effect of quantities between 2% and 10% shows that the best results are obtained at approximately 5% concentration, before a fall in physical properties is discernible (Tables 2-10). At the end of these tests one 18-ton container of CR partitioned with 5% magnesium silicate was sent by boat to South America and returned. This was to test transportation through the tropics. It was found to be satisfactory on return. At this early stage of the work, it should be mentioned that there is a definite pattern shown of greater consistency attained by the use of powdered materials compared with the bale rubber controls, and furthermore in general there are higher tensile and elongation properties (both at the same time) with the powdered rubber recipes. The achievement of simultaneous improvement in tensile and elongationproperties is amostunusualphenomenon in rubbercompounding technology and can only be attributed to the fact that better dispersion coupled with less chain scission of the polymers (due to mastication) must be present. This is a major advantage of powdered technology, as subsequent results will show.
Table 3 Effect of Magnesium Silicate at 2 k . 5 , 7%, and 10% Levels with CRPowder at break (c/o)
Tensile strength (kglcm’) Elongation Control CR 1
2 3 4
Average Std. dev.
1 l4 1 10 100
I25 112 10
2’/2%
5%
7Y1%
10%
125
114 114 121 106 114 6
110
102 93 102 106
110
114 116 116 6
120 114 102 112 8
Control 5%2Y2%
101
220 248 235 236 235
6
11
240 235 230 242 237 5
235 235 235 235 235 0
75”8
10%
220 225 220 210 219 6
200 200 205 190 199 6
Evans
174
1 2 3 4 Average Std. dev.
7s 62 87 87 88 II
9s 95 9s 9s 9s 0
1 2 3 4 Average Std. dev.
92 124 I06 112
125 12s 132 120 126
109 13
S
95 95 91 96 2
90 84 96 88 90 S
80 84 78 80 81 3
S40 S70 ss0 S78 560
122 12s 126 125 125 2
I10 120 122 129 120 8
105 102 104 101 103 2
150 160 170 1 S9 160 8
101
18
S70 S60 S70 S80 S70 8
S70 S70 S70 S70 S70 0
S40 SS0 S60 S45 S49 9
S35
190 190
190 195 195 195 194
180 175 180 180 179
3
3
IS0 150 160 160 1.55 6
190
200 193 S
S40
S35 S30 S35 4
Table 6 Effect of Calcium Silicate at 2%, S, 7 k , and 10% Levels with CR Powder Tensile strength (kgkm') CR
Control
2Y2%
SQ
7Y2%
Elongation at hreak (%!) Control
2'/2%
5%
7Y2%
10%
108 105 10.5 106
220 248 23s 236 235 11
230 230 235 240 234 6
235 235 230 245 236 6
190 200 200
2
240 245 240 235 240 4
10%
~
I 2 3
1 l4 1IO 100
IIS
118
115 115
115 11.5
4
12s I12 IO
114
112
115 0
115
Average Std. dev.
2
114 114
118 118 115 2
104
180 193 10
0
Powdered Rubber Table 7
175
Effect of Calcium Silicatc at 2Y2, S, 7Y2, and 10% Lcvcls with NR-SBR Powdcr Blend ~_______
Tensile strcngth (kglcm') NR-SBR 1
2 3 4 Averagc Std. dev.
Elongation at break (76)
Control
2Y2%
5%
7
%$10%
Control
2Y'%
S 9
7s I02 87 87 88 II
97 96 92 94 95 2
95 95 98 90 95 3
89 89 94 91 91 2
78 82 76 83 80 3
S40 S70 ss0 578 560
560 580 600 S80 S80 16
S80 580 S70 570 S75 0
18
7?'"/. 570 S70 S90 560 S73 13
IO%
S60 560
SS0 S60 558 S
Table 8 Effcct of Amorphous Silica at 2Y2, S, 7Y2, and 10% Levels with NBR Powder Tensile strcngth NBR
Control
2Y?%,
(kglcm')
5%
7Y>%
break Elongation at 10%
Control
(%)
5%
2Y2c/r
~~
124
I 2 3 4 Average Std. dev.
7Y2%
10%
~~
92 106 I12 109
13
140 122 130 125 130 7
130
12.5
1 50
100
141
130
120 130 9
10.5 110
125 4
104 4
I60 I70 1S9 160 8
195184 195 195 192 190 176 198 180 195 190 195
7
2
145 178
1.55 150
179 179 3
IS0 150
4
Table 9 Effect of Amorphous Silica at 2%, S, 7Y2, and 10% Levels with CR Powder Tcnsilc strength (kglcm')
2Y2%
Control CR
Elongation at break 5%
7Y29
10%
Control
2Y2%
~~~
1 2 3 4 Averxge Std. dcv.
114 1IO
117 116
116 116
100
115 110 115
114 l15 108
I25 112 IO
3
115 I
114
114
110 100
118
114
116 116 2
108
6
(76)
5%
7Y2%
10%
245 240 240 240 241 3
225 230 230 230 4
200 205 220 210 209 0
~~
235 220 248 23s 236 235 11
240 240 240 250 243 S
Evans
176
Table 10 Effect of Amorphous Silica
Tensile strength NR-SBR Control ~
87
I 2 3 4
Average Std. dev.
2 ! 4 5, 7 k . and 10% Levels with NR-SBR Powder Blend
at
Elongation a t break
(kgkm')
(%)
2%%
5%
71/:c/(
10%
Control
2v2%
58
7'/,%
10%
98 98 108 104 102 5
102
99 S40 98 104 102
90 90 X7 94 90 3
570 S50 578 560
580 600 600 590 593
570 575 570 580 574
18
10
490 595 590 590 591 3
550 545 550 530 544 9
~~
75 I02 87 88 II
106 102 100
103 3
101 3
S
3.2 Measurement of Dispersion
In any rubber compound, it is essential that good dispersionof all of the compounding ingredients be achieved, in order to optimize the properties of the particular recipe. The normally accepted methods for decidingthecorrect degree of dispersion in a compounded recipeincludethe measurement of tensile strength (TS) and elongation at break (EB) at various states of cure. The plateau in the plot of the tensile product(TS X EB) gives the optimum state of vulcanization. Modulus and, less frequently, tear strength, hardness. and compression set may be used as added criteria. These tests give a reasonably accurate measure but unfortunately take rather a long time to carry out and therefore are not ideally suited as routine quality control checks in the high-speed context of powder blending and mixing techniques. Other methods are those developed by Dannenberg ( 1 970) and Leigh-Dugmore (1 948) using visual and transmitted light standards to compare the torn surfaces. These are very good and quite accurate, although once again requiring time and a fair degree of skill of assessment during the fixing of the rating. Electrical conductivity and other microscopic techniques have also been developed (Leigh-Dugmore, 1948), but for obvious reasons very few routine control laboratories have these facilities. A rapid and accurate system is thus required to determine the degree of mixing in production batches. It was decided to use as a basis the TSO test. In its original form this was a test for state of cure although it is not in the current specification (ASTM D 1329-79). I n 1941 the T50 test was used by Callenders Cables (now B.I.C.C.) for routine determination of the state of cure of natural rubber used in electrical cables. Considerable work on other polymers (e.g., Smith, 1978) is cited in the ASTM specifications. The T50 value is the temperature at which 50% recovery of the specimen has taken place after a cured dumbbell sample has been stretchedby 1 OO%, frozen in a batch of acetone-alcoholCO? mixture, and allowed to warm up. This is a very rapid means of determining the state of cure of natural rubber compositions and many other polymers, including SBR, NBR, and CR. As TSO is a measure of extensibility, it will be influenced by the amount of crosslinking, just as in glassy polymers the T, is affected by introduction of crosslinks (Gordon, 1963). The T50 value may therefore be used either as a measure of extensibility or to determine the state of cure. T50 values are clearly discernible, and the reproducibility is good. In using this technique, it has been shown in practice (irrespective of the measurement method of dispersion) that if the smallest ingredients by weight in the compound-namely, the curatives, activators, and accelerators (organic and inorganic)-are correctly dispersed, then the state of cure will be correct. If, therefore, the small items are well dispersed, it may be assumed that the bulk fillers, including the blacks and inorganic materials, are equally well dispersed.
Powdered Rubber
177
Three basic compounds of NBR, CR, and NWSBR were used. Carefully check-weighed batchesweremixedunder ideal andsupervisedconditions by internal-mixerandopen-mill techniques (Evans, 1969, 1979). The controls were mixed from bale and powder, and the T50 values were determined and compared with Cabot rating values. Good agreement was achieved.
3.3
Preblend Mixing
Just as with PVC and other thermoplastics, the preblending and compounding process for powdered rubber mixing consists of blending together all the ingredients in the recipe in an intensive rapid mixer.Suitablemachinesforthisoperationincludethosemanufactured by Werner & Fleiderer, Fielder, Papenmeier, Henschel, Lodige, and Drosna (Evans, 1974). These machines have been designed to enable intensive mixing to be carried out to gain the full effect of the mixing action. The equipment is jacketed so that additional heating or cooling may be used as necessary, depending upon the polymer and the recipe. Aerodynamically shaped mixing impellers achieve thorough dispersion and rapid frictional heating by causing powder particles to collide with each other in a turbulent flow pattern above the impeller blades and outof contact with themetal surfaces. The machines are emptiedquickly, usually by means of pneumaticallyoperateddischargevalvesanda chute to mills,internal mixers. storage containers, or other units depending upon the next process stage. With powdered rubber formulations, however, since recorded discharge temperatures are below 50"C, it is not necessary to use an aftercooler, as is the case with thermoplastic recipes. Neither has it been found necessary to carry out any major recipe changes, as are used for baled rubbers. when using the powdered system, but upside-down mixing techniques-i.e.. all ingredients in the mixer first, with the powdered polymer on top-give the best results. The following system has been used throughout this work, using a 300 L capacity Fielder mixer:
0 speed. slow Start, 1 min Switch to high speed. 2% rnin Switch back to slow speed. 3 min Add etc.oils, 3% rnin Continue slow speed. at 4 min stop. Such a cycle produces a free-flowing powdered compound that is suitable for final mixing on a roll mill or in an internal mixer or for direct powder processing. Generally speaking, a batch weight of between 45 and 90 kg is ideal for the 300 mixers, and between these limits. uniform mixes are produced batch after batch. Whenunderloaded, i.e., less than 45 kgbatches,there is insufficientmaterial to fully engage the impellers; and if overloaded, i.e., in excess of 90 kg batches, overflowing occurs. In both cases there is inadequate mixing and consequently poor dispersion. Throughout this work, the preblended powdered recipes are compared with the relevant baled-rubber control recipe mixed in an internal mixer by conventional rubber-processing techniques (Evans, 1974). Because of the considerably higher throughputs that are possible using powdered rubber preblend techniques. it became very obvious at the outset that more rapid testing control techniques were necessary. It is for this reason that the modified T50 test was developed and used for dispersion measurement.
Evans
178
4.
4.1
EXTRUSION, INJECTION MOLDING, AND TRANSFER MOLDING OF POWDEREDRUBBERPREBLENDEDCOMPOUNDS Extrusion
A study was designed to eliminate the necessity of using baled elastomer, mixed either on an open rubber mill or an internal mixer prior to extrusion (Evans, 1969). Preblend powder compounds were prepared in the Fielder intensive rapid mixer, and comparisons were made with similar control bale rubber compounds after extrusion. Several extruder manufacturers (Farrel-Bridge Ltd.,Troester A. G., Werner and Pfleiderer. Francis-Shaw Ltd., Stewart-Belling Inc.) have already developed machines capable of direct processing. The feeding may be either via feed hoppers, with or without vibration depending on the polymer and formulation, by suction (Meyer Mashinen, Howe-Richardson), or by tube conveyor system (Floveyer Ltd.). depending on conditions. lnitial work here commenced by direct feeding a 47’ in. PVC scroll extruder at 25 rpm ( 15 : 1 ) with Fielder-blended CR premix. This proved to be a complete failure in that tremendous heat was generated and premature cure and thermal degradation took place. At first this was considered to be due to the back pressure, which had been deliberately induced by the fitting of both a baffle plate and gauze behind the spider and dies. Upon removing the dies and screw and then very carefully extracting the powder premix. it was seen that at only approximately 25 mm behind the diehad any consolidation occurred, the remainder still being in powder form. The continued turning of the screw had therefore produced the scorch conditions. despite cooling water on both screw and barrel. This unsuccessful experiment and the subsequentscrewexaminationneverthelessproduced the solution to the problems. It was observed that consolidation of the premix, but not fusion, had occurred in the immediate die area. If. therefore, consolidation was achieved by compaction of the powdered premix by a “pill-making” technique, and if the pellets so made were fed to the extruder, it was considered that direct extrusion should then be possible. Indeed, this proved to be the case. and a very good and smooth extrudate was produced. The experiment was repeated several times,not only with CR preblend but also with NBR and NWSBR mixtures, and the same good extrudates were obtained. Processing temperatures were cooler than when conventional compounds wereused, and both TS0 and other physical tests (takenfromthe extrudate) showed that good dispersion properties had been achieved. Having established the fact that, with consolidated and pelletized preblend, direct extrusion is possible, an improvement in consolidation technique was considered to be necessary on a continuous basis. It was ascertained that the bulk density of the preblend is approximately three times the relative density, so a compaction factor of 3 was decided upon for the continuous compactor. This initially was located above the extruder throat. The feed hopper was filled with preblend material via vacuum transfer equipment; the preblend was then continuously compacted and fed directly into the extruder. However, there were processing difficulties in that the preblend did not feed evenly and consistently into the compactor due to “bridging,” and hence voids were being formed within the preblend in the feed hopper. This could not be eliminated even by using vibration techniques on the feed hopper. Erratic feed to the extruder was present, and this in turn produced surging in the extruder and uneven extrudate dimensions. It was therefore decided to operatethe hopperkompactor as a separate unit a shortdistanceawayfromthe extruder and fit it with a twin-die setup to produce continuous strips, which in turn could be detected visually immediately prior to feeding to the extruder.
179
Powdered Rubber F E E D HOPPER
7 J
I POWDER PREBLEND
SCREW CONVEYOR COMPACTION ZONE
Fig. 1 Horizontalcompactor.
Despite improvement in processing techniques, the bridging was not completely eliminated, and, in addition, rather more heat than was considered desirable in producing the strips became apparent. Compactor die temperatures on occasion in excess of I 10°C were developed. It wasthereforedecided to water cool the die and lowercompactionzones.thusachieving satisfactory temperatures of 45-55°C within the material. I n view of the continued bridging tendencies, it was also decided to change from compaction by vertical format to horizontal compaction. This achieved the necessary satisfactory processing system, with all other properties remaining good (see Fig. 1 ). A completely new concept i n mixedextruder design (Fig. 2) has also been investigated. This machine has the capability of continuously mixing and extruding the powder preblend. (This work was carried out on a prototype machine by courtesy of Farrel-Bridge Laboratories. Rochdale.) The results showed that satisfactoly physical properties were developed during the extrusion of the NBR. CR, and NR/SBR compacted preblends and at a compaction ratio of 3/ 1 (bulk density/relative density). However, the ultimate development is to direct extrude from preblend without compaction. Discussion with Farrel-Bridge enabled them to develop a mixing extruder. which they designated MVX. Work also carried out at Farrel-Bridge (Fig. 2) showed that by theuse of this MVX machine it was possible to direct mix and extrude direct from preblend without the compaction operation. The results were equally satisfactory and comparable with those already reported and again show that results using powderedmaterialaremoreconsistent than the baled rubber control.
4.2
InjectionandTransferMolding
Early attempts to mold preblended compounds direct from the Fielder mixer were unsuccessful, even in ordinary compression molds, until compacting by pelletization had taken place. This was caused by entrapped air and spillage of powder due to movement when closing the press.
180
Evans
Fig. 2 Farrel Bridge MVX mixing and venting extruder. This machine operates in four stages, indicated by circled numbers: ( I ) Feed-a compacting, pressurizing, feeding device, with adjustable pressure. (2) Mix-a high-shear twin-rotor mixing section with separate dc driver motor. The chambers and rotors are designed to give intimate shearing, smearing, and blending to all particles within the mixing chamber. care being taken to avoid “dead spots” and short-circuit paths to ensure uniformity of output. ( 3 ) Vent-a venting and transfer port located at the rear of the screw arranged to vacuum vent the mixed material before it passes through to the extrusion screw. (4) Extrude-a precision high-pressure extruder with separate dc drive. This section can be fitted with different screws to suit polymer viscositics. but one screw is able to pump a wide range of polymers. All four distinct operations are automatically synchronized and are controlled by the speed of the extruder screw. (From Farrel Bridge, Ltd., Rochdale, Lancashirc, United Kingdom.)
a satisfactory molding was However, when pills (pellets) were fed to the compression mold, obtained. The work was carried out with NBR and CR control compounds. By continuing the exercise along the same technical course as for direct extrusion, continuous compaction, it is also possible to feed the continuous strips via a screw linked to either the injection or transfer molding press. Further work in the design of machinery includes a mixer screw linked to an injection mold (Werner and Pfleiderer A. G., 1975; Dehnen, 1985). By the use of a mixer screw coupled to an injection mold, an experiment completed at Bayer, West Germany, showed that the NBR recipe in powder preblend form could be mixed and injection molded direct. The complete time was 2 minutes, cure temperature 175”C, shot weight 100 g.
Powdered
Rubber
181
The technology of injection and transfer molding, with respect to compounding, hasparallels in extrusion technology. The results of compression molding show that with the use of compaction techniques the compound properties developed from preblends are similar to those shown for compacted powdered preblend extrusion. This parallel produces similar properties when using compacted strips in transfer and injection molds.
5. 5.1
EFFECT OF POWDER TECHNOLOGY ON MIXING CYCLE TIMES, POWER CONSUMPTION, AND PLANT MAINTENANCE COSTS CycleTimes
An internally mixed compound, by directprocess,takesapproximately 10 minutes to mix, depending upon the formulation and the polymer. Some high-plasticity hose compounds take 12 minutes to mix (in two passes) by the twostage or masterbatch technique (Evans, 1979). This applies in particular to theNBRcontrolrecipe,which must bemixed in two stages by the internal mixer route because of its high plasticity and high cured hardness. It was therefore decided to examine the effect of replacing the polymer in bale form by an equal weight in powder form, all the other ingredients and the batch weight remaining the same. The controls forthe CR and SBR-NR blend can be mixed satisfactorily without two-stage techniques, and therefore only single-stage compounding was used for them. The NBR preblend wasfinal-mixed by open-milltechniques (Evans,1979) in I O minutes,whereas the normal technique of bale mastication requires 60 minutes for final open-mill mixing. In the case of high-plasticity NBR compounds, it is necessary to use a two-stage mixing technique. It was found that it is possible not only to eliminate this two-stage operation but also to produce compounds in one mixing stage either with a preblend or with powder. A reduction in mixing cycle time in theinternalmixer of between9and I O minutes or an increasein production of between 400 and 600% is thus obtained. With CR and NR/SBR rubbers twostage mixing is not necessary, but increases in production of between 333 and 500% are shown to be possible. In open-mill mixing of NBR, a production increase of 600% is possible. In all cases, dispersion is excellent, and yet again the greater reproducibility of results and improved physical vulcanizate characteristics are very evident. The use of shorter mixingcycles with particulates has been discussed. In a little moredetail, a normal industrial products typeof rubber compound, by direct process, takes approximately 10 minutes to mix in, say, a No. 3D Banbury, depending upon formulation and polymer. Some mixes can take up to 12 minutes and even longer if two-stage or masterbatch processes have to be used. As a general statement, not only can masterbatching be eliminated by the use of particulate rubbers, but the mixing cycle can be drastically reduced. Satisfactory mixes using preblended particulates can be dropped from the mixer in a mixing time of 2 minutes and in as little time as 3 minutes using nonpreblended material and with very satisfactory physical and processing properties (Tables 1l and 12). It can thus be seen that internal mixer outputs can be increased at least threefold by the direct mixing of powdered polymer weight-for-weight with bales polymer, or at least fourfold with preblends of the whole mix via the intensive mixer initially. The physical properties are also atleast equal to, or an improvement on, those of the controls.Physicalproperties are improved in both TS and EB, both at the same time. This isa common occurrencewith powdered rubber compounds and has been confirmed by many workers (Goshorn et al., 1969;Whittington and Woods, 1971; Woods and Krosky, 1973; Woods and Whittington, 1973: Whitlington et al., 1974; Woods, 1976; Evans, 1978a,b; Smith, 1978).
182
Evans
Table 11 Effcct of Powdcred Elastomer (NBR) on MixingCycle Timcs lntcrnnl mixet
I 2 3 4 Avg. Std. dcv. "
-
-15 0.07
92 124 106 112 109 13
1.50 I60 170 159 160 8
- 15 - 15 -1.5
-15 -1.5 0
118
17.5
I l6 120 118 118
180 18.5 180 180
2
4
- 15 -1.5 -15 -15 -15
0
I15
170
-
180 180 180
-
-
-
1 l7
-
-
-
-
-
-
117 116 116 1
175 6
-
-
See Table 2.
5.2
Energy Savings
The fact that more batches can be mixed in u n i t time, as shown by the work dealing with the effect of powdered rubber on mixing cycle times, proves that an energy saving is achieved by the use of powdered rubber recipes. It has also been shown that there is a significant reduction in the maximum current used, and this is also reflected in lower recorded dump temperatures, becauselessenergyhasbeen expended and hencelesswork done. This is a very desirable phenomenon. particularly in the case of CR rubbers, which have a heat memory. Lower scorch tendencies are therefore imparted to the compound. Work carried out on laboratory-scale equipment (Whittington et al., 1974) showed that energy savings were possible, and this was also reviewed by Doak (1974). Thiswork investigated the possibility of saving energy on factory-scale, mass production equipment. Electric current and compound temperatures were recorded using the bale control and powdered polymer i n the same recipe. with both internal mixer and open-mill mixing techniques (Evans. 1979).
Table 12 Effcct of Powdcred Elastomer (CR) on Mixing Cycle Times Internal mixer
I 2 3 4 Avg. Std. dev.
- 22.5
0.06 22.5 0.06
114
II O I 10 125 112 IO
220 248 235 236 23s II
-22.5 - 22.5 - 22.5 - 22.5 -22.5 0
112 I 13 I13 I13 113 0
235 240 240 240 239 3
-22.5 - 22.5 - 22.5 - 22.5 -22.5 0
1 14
245 23s 240 240 240
0
4
114 114 114 1 14
See Table 2 - 22.5 0.06
1 14 1 IO 100
125 112 IO
220 248 235 236 23s II
Powdered Rubber
183
Traditionally, the rubber industry has used very heavy and robust mixing equipment in the mill-room areas. This has been necessary because of the need, right from the early origins of the industry, to masticate the natural rubber to the right viscosity prior to the addition of the other compounding ingredients. Obviously, there has therefore been a very high energy usage. Very regretfully. this energy has always been there irrespective of cost, although until the early 1970s it was usually relatively inexpensive. The costof the energy used within a mixedcompound has in general been a very small percentage of the total mixed cost, but the time. although long overdue. is now very opportune for examination of this cost, not only because of the present high price of electricity but also because of the certainty of future shortages and the real need for energy conservation. Without doubt, considerable energy saving lies in the use of powdered and particulate rubbers in the manufacture of hose and cable (Evans, 1978a,b, 1980a,b). The use of polymers i n this form has been rather slow to gain acceptance but is now gaining momentum worldwide, especially in Europe. It has already been shown (Evans, 1978a,b) that, because of the smaller particle size of particulate rubbers. more batchesof compound canbe mixed per hourusing less energy. Perhaps one of the most graphic demonstrations of the value of powdered rubber technology was seen in England during the energy crisis in 1974, when a 3-day week had to be operated with an allowance of 65% of normal power requirements. By the use of powdered rubber rather than baled rubber. 95% of the normal 5-day bale output was achieved without exceeding the 65% energy restriction in 3 days of operation. Another very advantageous. but perhaps not too obvious, property of using particulates via internal mixers is linked very closely with the fact that less power consumption is needed because of the physical form of the polymer as presented to the mixing machines. As a result of this, lower dump temperatures from the internal mixers are achieved. and hence there are fewer fumes on discharge. This, then. is an aid to better environmental conditions. another very important current topic in the industry. It is occasionally argued that the energy savings achieved is less than claimed because of the grinding operation used in some cases forobtaining the particulate material, which obviously has to be taken into account. However, there are manufacturing routes available that do not involve the drying and baling and subsequent grinding of the coagulum at the later stage but rather use to advantage the small particle size already present. This currently does not apply to all polymers. but it is confidently anticipated that in the future this route will be widely followed. So-called friable bales. which are an intermediate stage between full bale and particulates. are already available. and these are also energy savers in the mixing processes. Closely allied with cleanliness. energy savings. and the coagulum route is the so-called polyblack process (B. P. Chemicals Ltd., 1985). which involves the introduction of carbon black in wet form to NBR latex, thus producing a very clean black masterbatch in friable and clean crumb form.which is then capable of processing either through the internal mixer route or other direct particulate route. It can thus be seen that grinding is not always necessary, but if it has to be used. then a much smaller sized particle starting point than a bale is possible, with a very obvious reduction in grinding energy. It is the final grinding operation, i.e., from particulate to true powder (1 mm). that adds time, cost, and energy to the “powdering,” so unless a direct process route using true powder form is to be operated, particulates up to 10 mm in size should be considered in conventional internal mixing equipment. Furthermore, one big advantage of one new machine (MVX. Farrel-Bridge Ltd.. shown i n Fig. 2) (seealso Evans, 1978a.b; Smith. 1978)is that it is capable of directprocessing
Evans
184 Table 13 OperatiodEnergy Usage (Bale) Operation Bale cutting Banbury, stage 1 Banbury, stage 2 Open mill Cracker mill Warm-up mill Hot feed extruder Cold feed extruder Calendering
kWhikg
0.32 0.97 0.97 9.02 0.52 0.52 0.39 0.64 0.77
preblended particulate polymerin many instances, with considerably better processing characteristics of the mixed stock and also improved physical properties when vulcanized. Returning to the use of polyblack NBR, apart from cleanliness, it is possible in the case of hose compounds and those used elsewhere to completely eliminate the two-stage mixing operation, which has previously been essential because of the basic hardness of the stocks with regard to viscosity, and thus there is a considerable energy-saving potential here, once again with improved physical properties. Work within the mill-room areas has shown considerable energy savings achieved by the use of powdered and particulate rubber (up to approximately 10 mm). Typical energy usages and savings are shown in Tables 13- 16 for each operation in bale and particulate form and in typical extrusion routes. It must, of course, be realized that many of these results have been obtained from very high Mooney hydraulic and other hose compounds, and hence some of the energy values quoted could well be higher thanin other branches of the rubber industry. Irrespective of the type of compound used, the comparisons of like with like are valid and show givings by the use of particulates. Tables of each processing operation should be compiled for both bale and particulate over. say, at least one typical week’s production and the total kilowatthours of electricity used set
Table 14 OperatiodEnergy Usage (Powder or Particulatey Operation
2.26
Intensive mixing Compacting Milling Blend 0.52 (Banbury) Blend (mill) MVX Direct extrusion Direct injection molding l‘
1-6 mm.
kWhkg 0.06 0.13 0.06
0.64 0.90 0.32
185
Powdered Rubber Table 15 Energy Usage Bale route
ing
Bale
Banbury, stage (1) Banbury, stage (2) Mill 1 Mill 2, strip Cold feed extruder Total
kWh/kg 0.32 0.97 0.97 0.52 0.52 0.64 3.94
against the total kilograms of compound processed. It is then a simple matter to obtain a very accurate kilowatthour/kilogram reading for each operation and under the conditions prevailing in each particular factory and operation.
5.3
PlantMaintenanceCosts
As has been mentioned, it is a fact that more batches may be mixed in unit time; as a simple example, it is possible to eliminate at least one shift from the three normally operated and still achieve the same volume of output. Thus there is an immediate maintenance saving of 33%, plus many other fringe benefits. All mill room operations should be closely examined to see where savings of time, etc., can be achieved, with the obvious ultimate savings in plant maintenance costs per unit output.
6. CONTINUOUSPRODUCTION This system was developed from the successful introduction of the Fielder preblend via the internal-mixer route or by the use of the horizontal compactor unit. It consists of automatically weighing the recipe and transporting this through the Floveyor (Floveyer Ltd.) into a hopper holding tone situated immediately above the Fielder mixer. The batch is then transferred to the Fielder, mixed, and emptied into a large holding container (1 ton capacity). When filled, this container is transferred to and immediately above the compactor unit, and the compacted strip
Table 16 EnergyComparisons (Powder) PowderParticulate route
feed
Intensive mixing 0.06 Banbury Mill I Mill 2 MVX Cold 0.64 Total
Banbury 0.06 0.52 0.52 0.52
MVX Strip
MVX Direct 0.06
-
-
-
-
-
0.64
0.64
0.64 -
-
-
1.34
0.70
186
Evans
is then fed by belt conveyor to one end of an open mill, with the nip setting slightly out of parallel. This allowsthe compound, asit is masticated. to blend along the mill. prior to continuous strip cuttingand water cooling. The strip is then stored and matured. priorto feeding theextruders. Compound continuously produced by this method gives excellent results. Results show that exceptional reproducibility was obtained when several tons of NBR and CR preblends were processed by the continuous production system. Theseresults are even more remarkable when compared with the conventionally mixed controls and with the rheographs taken from normal baled Banbury batches.
7. POLYMER BLENDS (NBR-SBR) It is everyday practice in the rubber industry to blend various elastomers by both open-mill and internal mixer techniques. However, it is sometinles impossible to obtain satisfactory dispersion. Such a caseoccurs in the hose industry with regard to NBR-SBR hydraulic hose lining compound. where it is necessary to produce high-plasticity, high-hardness materials. NBR-SBR blends are generally used to allow controlled oil swelling in hydraulic oils, which enables assembled hoses to remain coupled. Because of different viscosities between the two polymers. good dispersion cannot be guaranteed even with two-stage internal mixing or open-mill mixing, and this involves very time-consuming and expensive processing techniques in the mill room. It was therefore decided to use the powder preblend mixing technique in the Fielder blender using the NBR control recipe and replacing 25,50, and 75% of NBR, respectively, by SBR. Theresults achieved were satisfactory. Despite the practical mixing difficulty experienced with baled NBR and SBR blends, the results of various blends of NBR and SBR powders quite clearly show that excellent dispersion has been obtained and that new and consistent T50 values have been achieved.
8. ADHESIVESANDDOUGHS These materials are manufactured by thoroughly masticating the polymer prior to final mixing. and then sheeting the compounded mix to approximately 1 mm thick. The sheet is then placed in a rectangular metal box and covered with the appropriate solvent for that polymer. Layers of polymer and solvent are alternated until the bin is filled. and a lid is then placed in position. This operation is known as laying down. The solvent is then allowed to swell the compound for approximately 96 hours. after which the swollen mass is cut with a spade and transferred to a Z-blade mixer for final mixing with more solvent until the desired consistency is achieved. An adhesive containing 20% dry solids by weight is used within the hydraulic hose industry. Experiments using powder preblend with solvent showed that the laying-down procedure could be eliminated and the pl-eblend could be placed at once with the solvent in the Z-blade mixer and completed to a 20V~solids solution within 5 hours. The commercial solvents used for adhesive and doughs are toluol for NBR and CR. and for NR-SBR. The effect of toluol on the NBR control and powdered preblend is shown in Figure 3 . The results obtained show that it is possible not only to eliminate the very time-consuming and power-wasteful operation of laying down but also to achieve a perfectly satisfactory 20% dry solids adhesive in 5 hours instead of 96 hours by using the NBR preblend. This is of great practical importance in the adhesives and rubber-spreading and rubber proofing industries.
187
Powdered Rubber
Fig. 3
9.
Dissolved solids versustimeforpowder
and bale
ENVIRONMENTALCONSIDERATIONS
The mixing and mill-room areas of rubber factories are dusty, and this is not desirable. Indeed, the 1976 U.K. Health and Safety at Work Act sets maximum threshold limits for the various materials used i n rubber formulations. The use of powdered rubber preblends enables all the materials to be bulk handled in a closed system (Fig. 4). thus making it possible for extremely clean and dust-free working conditions. The definition of the separating point between powdered and granulated rubber is I mm (British Standards Inst., London 2955). (1976). The rubber powder may not in itself be a dust explosion hazard. but some of the ingredients. such as finely divided sulfur. are potential hazards if mishandled (Davies, 1976). The guiding principle must be that at no time may a hazardous material be dispersed as an explosive dust cloud in the mixer. This can be avoided by ensuring that inert components are dispersed first. thus rendering inert the atmosphere inside the mixer. Many chemical products, including sulfur, are available in forms that have been treated to render them free-flowing and non-dust-forming. A restriction on the use of potentially hazardous materials to dust-free forms only is a useful extra safeguard in powder processing, but not an alternative. There is some hazard increase in storage over baled rubbers, but this increase is significant only in unsprinkled premises. The use of powdered rubber in a totally enclosed metal storage and conveying system(Fig. 4) should give a reduction in hazard over conventional storage, handling, and processing of baled rubber. Adequate exhaust systems are therefore necessary. Such systems carry no additional insurance premiums. A laboratory test for the assessment of dust in solid rubber chemicals has been developed by Hill and Robinson (1978). This gives an indication of the dust hazard as a possible nuisance,
Evans
188
FIELDER HANDLING SYSTEM
Fig. 4 Fielder closed powder-handlingsystem.
not only as a potential explosion propagator but also in general factory environmental cleanliness and improved working conditions. Other work carried out in Holland (Goshorn and Ciago, 1974) has shown that it was not possible to produce an explosion in eitherastationarydust-airmixture or flowingdust-air mixtures or at various powder concentrations. Early work by Manley and Hampson (1975)with regard to the flammability of vulcanized NBR has shown that the use of dry-blended powdered NBR gives a material with lower flammability. both before and after immersion in various hydraulic fluids, than does the use of conventional baled product. 10.
ECONOMICS OF POWDERED RUBBER SYSTEMS
The cost of a powdered rubber system depends upon the actual process used and the premium charged for the particular polymer. Furthermore, as the process flow lines in Figure 5 show, the actual product process determines which operation can be eliminated. This in turn depends upon the plant and operation process employed. Costs related to production rates have been discussed by Schultz (1973), and figures have been produced by Woods and Whittington ( 1973). derived from a conversion cost equation. These items have also been reviewed by Doak (1974).
189
Powdered Rubber Weighing of ingredients Weighing
of ingredients
i x e rI n t e n s i v em i x e ri n t e n s i v e
and/or Banbury Mill Powdered compound direct
Preparation
J Calendering
1 Extruding/molding/adhcsives (a)
Fig. 5 (a) Normal or powderroutc equipment.
i Extrusion Molding Continuous vulcanization Adhesives (b)
via convcntionalequipment. (b) Powderroutebypassing
heavy
When deciding upon which operation can be omitted from a process, each factory location and layout must be examined to decide which points are relevant to its own particular mode of operation, and the various advantages must be set against the powdered premium cost of the polymer. This latter cost has been progressively reduced as commercial use of the powder has increased. It should be remembered, however, that this premium applies only to the elastomer content of the recipe, unlike the custom mixing premium, which is charged on every kilogram of compound produced. “Custom mixing” is the term used in the rubber industry when the compound is bought from a supplier whose only business is to mix material to a customer’s recipe. Powders are cheaper than bales: 1. Where normal bale custom-mixed material is being used (when powder is mixed inhouse). 2. Where internalmixing is already at capacityand extra mixingequipment is being considered. 3. Where masterbatching is used and either two-, three-.or four-stage mixing is practiced. since one or more stages can be eliminated. 4. Where it is necessary to carry out operations subsequent to mixing, e.g., strip preparation for extruders. These operations may be eliminated, and the strips cut directly from the mixing mill.
In addition, the use of powder permits the use of internal, rather than open-mill. or allows openmill mixers to be operated at higher outputs.
Evans
190
The use of particulate rubber obviously means that bale cutting is no longer required. More and more equipment is being used in the nonure section to transform bales to powder or particulate at the beginning of the in-house factory process. The premium point for powder is approximately at break-even when the elimination of some processing operations leads to a reduction in manpower and when eliminating operator shifts by increasing internal mixer cycle speeds and also by increasing open mill mixer cycle speeds. The elimination of conventional mixing methods (e.g., internal mixers or open-mill techniques) makes savings possible in: 1. Direct feed to cold-feed extruders by powder compressing or compaction 2.Directfeed to transferandotherinjection-moldingmachines 3. The use of pills forcompressionmolding 4. The rapid manufacture of adhesives 5. The rapid preparation of doughs for spreading and proofing operations
Other,lessapparent.areas processing include:
of savings that have been shown to be present in factory
1. Energy. 2. Maintenance, due to (a) extra output in the same time but without high-speed rotor techniques, (b) elimination of operator shifts. 3. Less waste by production of technically improved compounds, because of (a) better dispersion and hence less batch-to-batch variation. (b) consistently lower dump temperatures and hence less scorch tendency, (c) less powder loss in closed systems. 4. Dilution of the compound is possible because of improved physical properties of the powdered vulcanizates. 5. Blending of polymers is more consistent; dilution again is possible. 6. Some blends are possible that are not commercially viable using other techniques.
Environmental conditions and cleansing
costs are also improved because:
l . Lower dump temperatures produce fewer mixing fumes and less contamination. 2. The mill-room area is much cleaner because of the enclosed powder-handling systems.
In an attempt to quantify the potential savings of powdered rubber technology, it has been found necessary to generalize, rather than be specific. because of the diversity of existing processes and the wide differences in premiums charged for the powdering operation. For example, powdered reclaim rubber carries no premium, but powdered natural rubber can be as high as E120 per ton. Also. depending upon the supplier, NBR premiums are in the range of E20 to 70 per ton. However, there are several common factors peculiar to any one factory: 1. Laborcost 2. Fixed costs related to capital,rent, etc. 3. Variable costs includingutilities, etc. 4. Material costs
Material costs are included in common factors because the same recipe, on a weight-for-weight basis, is used for either the bale or the powder route. and the difference in cost therefore becomes a straight addition of the powdering premium onto the hydrocarbon content of the recipe. Thus, taking the first three costs listed as the basic costs. it is possible to derive an equation:
191
Powdered Rubber
Conversion costkg =
labor cost fixed kg
If production mixing rates are included,
Conversion cost/kg =
costs kg
+
costs + variable kg
the equation becomes
+
labor cost/hr fixed costshr mixing rate, k g h r
costs + variable kg
(,This equation assumes that the variable costs are relatively unaffected by production rates. which is commercially true.) In typicalrubberprocesses.twoitemsareknown: (I)the manninglevelsand ( 2 ) the capital cost of the equipment being used. At this point. it is necessary to prepare line drawings of the plant and equipment being used for the particular operation, up to the vulcanization stage. From this, the manning levels for each piece of manufacturing equipment can be summed, and also the capital cost of all the equipment in use can be calculated. Individual plant costs for the various types of equipment are also required. Plant Cost Iterlls
Banbury 3D size Banbury 1 ID size 60" mill ( 2 per Banbury) 84" mill ( 2 per Banbury) 60" three-bowl calender 120 mm cold feed extruder Intensive rapid mixer Compactor unit Floveyor MVX machine
+
mills Banbury Calender Extruder line Intensive mixer, compactor, etc. Therefore, from the last equation and introducing manning levels and capital costs, equation becomes
Conversion c o d k g =
+
manpower fixed costs mixing rate, k g h r
the derived
cost + variable kg
In order to quantify the manpower andfixed costs. together with the variable costs. the following items must be considered for the actual factory location being studied. These will vary from factory to factory, from company to company, and from area to area.
192
Evans
Typical Output Rates (Internal Mixer) Single-stage Banbury mixed,
10-min cycle
6 6
Size 3D: Size 1 1D:
X X
150 Ib = 900 Ibkr = 409 k g h r 450 lb = 2700 l b k r = 1227 k g k r
Two-stages Banbury mixed Size 3D
only:
= 205 k g h r
450 lbhr
Factors Wage rate Overheads Depreciation Maintenance Variable costs (energy, etc.)
W X
Y z C
hour) per (pence (2w, 100%) i.e., hour)per (pence hour)per (pence hour)per (pence
These factors should be converted to unit time, i.e., cost per hour, and the values substituted in the equation Conversion cost/kg =
M + F + V
R
where M = manpower ( W X number of workers) F = fixed costs(x y z) V = variable costs (e) R = mixingrate (kghr)
+ +
For this exercise, at Dunlop (Gateshead), the calculations were based upon depreciation of the plant over 10 years and used the 1976 actual processing costs as follows:
maintenance room Mill E59357 E35,247 energy roomMill 3D output (kg/hr) rate 410
If the Bunbury routeenergy comparisons are compared with thosederived by the use of powdered rubber recipes, it is shown that considerable savings have been achieved, and at today's energy cost levels the savings are even higher. When the powder premium is taken into consideration, the technology is comnlercially viable. up to a powder premium of approximately E60 per ton at a hydrocarbon content level of 50%, dependent, of course. on the Banbury size in use. Furthermore. by "in-house grinding," initial costings indicate thatthe grinding premium will be virtually eliminated when the percentage of partitioning agent is taken into account. This has been confimled by Ellwood (1981). The costs have also been compared with those possible from a Banbury 1 ID, although it is not always desirable to use this larger machine for all recipes for technical reasons such as temperature in hose compounds.
Powdered Rubber
193
If theeconomics ofplant conversion were ignored. thereis nodoubt that particulate rubbers could be used at once, and in all branches of the rubber industry, as a major source of energy savings. Some day, if energy becomes so precious that it musf De c o n s e n t d then once again particulate rubbers will be part of the answer to compound mixing. In the meantime, however, the premium cost of powdered/particulate rubber cannot be ignored, and it is therefore essential that all aspects of the operating process be closely examined to determine which existing stages can be eliminated or up. To establish this. flowcharts should be drawn of the current methods in use and compared with the system that could be used with the powder route (Evans, 1978a. b). If this exercise is carried out for each particular process, and the savings (if any) set against the powder/particulate premium of the polymer. it is then quickly apparent which route should be taken. In the majority of cases, the scales will be tipped in favor of particulates. Wheelans (1981) confirmed the viability of powderedparticulate rubber technology and stated that “financial advantages are critically dependent upon whether the processing advantages exceed the premium on powdered rubber or the costs of granulating.” However, work by Ellwood (198 la) showedthat by the use of in-house granulation, in line with an MVX machine, the cost of particulating the rubber is virtually eliminated or is only a nominal amount in the overall context, and this should open the door to the greater and wider usage of particulate technology within the rubber industry and additionally if those areas where advantage costs are only marginal.
11. CONCLUSIONS The use of powdered and particulate rubber has made it possible to completely eliminate the bale-cutting operation in the mill room of some hose and cableplants, as well as to saveconsiderable amounts of energy. Much greater production rates are possible from existing mixing machines. and two-stage mixingcan be eliminated, particularly for high-viscosity NBR compounds. As less energy is used, the processing temperatures are much cooler, and this not only reduces scorch tendencies but also makes it possible to use more rapidly accelerated compounds and thus increase the speed of vulcanizing cycles in some areas. Also. with CR rubbers there is considerablylessheatmemoryretained by thepolymer.producingmoreeasilyprocessable compounds. Direct extrusion and injection molding of the preblend has been shown to be possible. thus making possible the elimination of heavy and high-capital-cost machines such as internal mixers. This also makes the labor cost factor of the compound much more attractive. It has also been shown that technically improved compounds are produced by the powder route. particularly in thedispersioncharacteristics.Indeed,much closer reproducibility is achieved and better physical propertiesshown, such asincreased tensile strengths and elongation at break. The ASTM (T50 modified) test has been adapted as a process and quality control test for dispersion, and thus production control has been made easier. Because of the easier handling of the polymer in powder systems, there is less process loss. This not only reduces the wasteof materials and energy but also improves the environment, which remains much cleaner than when the old conventional systems are used. Factors delaying the introduction of powderedrubberaretherestrictedavailability of some polymers and the premium charged for manufacturing the powder. These two points are related because unless thedemand is created the premiumwill not be reduced. andif the premium is not reduced, sales will be restricted (Hanmond, 1977).
194
Evans
Finally, with the work currently being done by the rubber machinery manufacturers with continuous mixing equipment and mixer screws for extruders and injection-molding machines, the prospects look very good for powdered rubber, and there is no reason why the forecast should not be ultimately improved.
REFERENCES Dannenherg, E. M. (1980). Cabot Torn Rating Chart. Cabot Corporation, Inc., Cambridge. Mass. Davics, R. (1986), Fire and Explosion Hazards in the U.K. Rubber Industry, Insurance Technical Bureau. Ref. R106, January issue. Doak, N. (1974), Eur. Rubber J., October, p. 60, Dunlop Ltd. (1929), Brit. Pats. 327,451 and 338,975. E. I. Du Pont de Ncmours. Inc. (l972), Wilmington, DE, Delphi Study. Future Rubber Processing. Eaglcs, A. E. (1973). Mernbers J., Rubber and Plastics Res. Assoc., Shawbury, Salop, U.K., April. Ellwood, H. (198 l a ) , paper F6, RUBBERCON, Harrogatc, U.K. Ellwood. H. (1981h).Farrel-Bridge Ltd. (198 l), MVX Machine, Sales Leaflet MVX R79, Rochdale,Lancs.. U.K. Evans, C.W.(1969), Europeancompoundingtechnlques, presented at RubberDivision,A.C.S.,Los Angclcs, CA. Evans, C. W. (1974), IRI National Confcrencc, Blackpool, U.K. Evans, C. W.(1 978a). Powdered u r d Pnrticultrte Rubber Technology, Applied Sclence Publishers, London. Evans, C. W. (1978b). Powdered rubber technology, Int. Rubber Conf.. Kiev, USSR. Evans, C. W. (1979). in Hose Tecknoloe,: 2nd ed. (Stiffener, D.S.C., Ed.), Applied Science Publisher, London, p. 45. Evans, C. W. (1980a). The effect of powdered rubber processing in energy, SGF May Meeting, Swcden. Evans, C. W. (1980b), Encrgy Symposium, ACS Rubber Div., Spring Meeting, Las Vegas, NV. Farrel-Bridge Ltd. (1974). Rochdale, Lanc., U.K., literature. Fernyhough, 1. (1985). B. P. Chemicals Ltd., Barry Glamorgan, Walcs, private communication. Francis Shaw Ltd. (1974), Clayton, Manchester, manufacturer. Gordon, M. (1963), H i g h Po/vrners, 2nd ed.. Iliffe, London, p. 38. Goshorn, T. R., and Ciago, N. V. (1974). Amhem, Holland, private communication. Goshorn, T. R., Jorgenson, A. H., and Woods, M. E. ( 1969), Rubber World, January, p. 66. Hammond, R. (1977), Delphi study update, E. I. Du Pont de Nemours Inc., Wilmington, DG. Hill, P., and Robinson, J. C. (1978), IC1 Organics Division, Blackley MKR. Holiday. G. J. (1974), G. J. Holiday (Plastics) Ltd., Tclford, Salop, U.K. Howe Richardson (1974), Howe Richardson Ltd., England, System Designers. Farrel Bridge Ltd. (l974), Internal Mixers, Farrel Bridge Ltd., Rochdale, Lancs.. and Francis Shaw Ltd., Clayton, Manchcster, U.K. Lehnen, J. P. (1973), Bayer A. G., Levcrkusen, West Germany, prlvate communication. Lehncn, J. P. (1985). Bayer A. G., Leverkusen, West Germany, private communication. Manley, R. T., and Hampson. F. W. (1975). J. Appl. Po/ym. Sci. 19:2347. Mees, F.. (1983, Firestone Tire and Rubber Co., Inc., Akron, Ohio, private communication. Morrell, S. M. (1973),Rubber and Plastm Research Association, Shawbury, Salop.U.K.. private communication. Murphy, E. A.. and Twiss, D. S. (1930), Dunlop Rubber Co., Ltd., Brit. Pat. 327,451. Nye, H. (1943), British Insulated Calenders Cables Ltd.. Leigh, Lanc., U.K., private communications. Nordsiek, K. (1974), National Conference, Munich, FDR Powdered Filler Masterbatches. Pike, M. (1972). Harrison Crossfield Ltd., London, private communication. Schultz, S. (1973). Gurnrni Ahest. Kunst. ?6:258. Smith, L. P. (1978). European mixing techniques, Eltr.storneric.s. Thompson, C. W. (1973), Guthrie Estates Ltd., London, private communication. Whally, V., and Morrell, S. M. (1973, RAPRA, MernhPrs J., February.
Powdered Rubber
195
Whcclans, M. A. (1981 ). paper F.5, RUBBERCON 8, Harrogate. U.K. Whittmgton. W. H.. Woods, M. E,, and Holman, P. R. (1974). paper presented at the Rubber ACS Meeting, Toronto. Whitting, W. H., Woods, M. E.. and Holman, P. R. (1974). paper presented at the Rubber ACS Meeting, Toronto. Wood, N. (1985). Goodycar Chemicals Inc., Paris, France, private communication. Woods. A. E. (1976). Southern Rubber Group Meeting, Houston, TX, February. Woods, M. E., and Krosky, R. P. (1973). R d h r r Age, April, p. 33. Woods, M. E., Morsek, R. J., and Whittington, W. A. (1973), Rubber World. June, p. 42. Woods, M. E., and Whittington, W. A. (1973), paper presented at the Rubber Division, ACS Meeting, Detroit. MI.
This Page Intentionally Left Blank
/ Rubber-Rubber Blends: Part
I
C. Michael Roland Naval Research Laboratory, Washington, D.C.
1. INTRODUCTION
This chapter discusses the thermodynamic and processing considerations involved in preparing rubber-rubber blends and reviews recent developments in the analysis and performance of such materials. Since it has been recognized for some time that the prospects are limitedfor continued synthesis of new polymeric materials with practical utility, efforts to developblends for diverse applications have continued to burgeon. Reviews of rubber blends were published by Corish and Powell in 1974 and Roland in 1989. The focus in this chapter is on more recent advances. With more than half of domestic rubber consumption going into tires. and considering that all majortire components with theexception of the tread ply can andhavebeensuccessfully formulated using rubber blends, it is obviousthat tires represent the major applicationof rubberrubber blends. The properties of blends, however, are described here in general terms, with specific applications undoubtedly suggesting themselves.
2.
MORPHOLOGY
2.1 Thermodynamics of Polymer Compatibility
The morphology of a blend is a function of the nature of the blend components (both their mutual compatibility and the rheological properties of the rubbers) and of the method employed, to produce the blend. It is necessary to distinguish between compatible blends and ones that are truly miscible. The former are homogeneous on a macroscopic scale but miscible only in a technological sense; that is, the rubbers can be mixed together and vulcanized to give a useful product. Truly miscible rubber blends, on the other hand, are those in which the free energy of mixing is negative so that the morphology is homogeneous on a segmental level; that is, the molecular coils interpenetrate. Miscible rubber blends are very rare because of the high molecular weight of elastomers. According to Robeson ( 19821, only about200 casesof miscible blends have been reported among all polymer mixtures, includingeven block copolymers and polyelectrolyte complexes. Thereason for this can be seen from consideration of the requirements for miscibility (McMaster, 1973; Patterson and Robard, 1978). The change in free energy upon mixing must be negative, AG
=
AH - TAS
(1) 197
Roland
198
and, in order that small composition fluctuations do not lead to spinodal decomposition. the second derivative of the free energy change with respect to some measure of composition must be greater than zero. The entropic term, TAS, includes both the usual combinatorial entropy of mixing. which depends upon the number of molecules in the system and is therefore small for the molecular weights associated with rubbery polymers, anda negative (dernixing) contribution. which results from the loss in volume upon mixing of two polymers (Patterson, 1969). This reduction in volume is due to the differences in the free volume of the polymers prior to their blending. The contraction increases the free energy of mixing by reducing the available space and thus the number of ways in which the polymer segments can be arranged. This negative contribution to the entropy of mixing provides a driving force for polymer-polymer immiscibility. As a result. miscible rubber-rubber blends are expected only if the free energy change is made negative by virtue of an exothermic heat of mixing, AH. This statement is strictly true only for polymers of infinite molecular weight (Roland. 1987).Absent some specific interaction between blend components (e.g., hydrogenbonding).heats of mixing of polymerpairs are endothermic. and accordingly literature surveys reveal few examples of miscible rubber-rubber blends (Krause. 1972). An interesting exception to this can be found in the case ofcopolymers. Blends containing these are sometimes found to be miscible even when their corresponding homopolymer blends are not. Although there are no specific interactions in these cases, Paul and Barlow (1984) have suggested that a net mixing exotherm can exist due to dilution of the more unfavorable unlike monomer-monomer contacts when at least one of the components of a blend is a copolymer.
2.2
Miscible Rubber Blends
Examples of rubber-rubberblendsreported to be miscibleincludestyrene-butadienerubber (SBRs) of differentstyrenelevels (Livingston and Rongone, 1967).nitrile-butadienerubber (NBRs) of different acrylonitrile contents (Bartenev and Kongarov. 1963). SBR and butadiene rubber (BR) (Marsh et al., 1968). and natural rubber (NR) with vinyl BR (Ueda et al., 1985; Trask and Roland. 1988). It is not surprising that a copolymer would in some cases be miscible with another copolymerof slightly different composition. Moreover,the seemingly contradictory findings regarding the homogeneity of some blends (Walters and Keyte, 1965; Yoshimula and Fujimoto, 1969) are likely attributable to differences in microstructure of the various materials employed in thesestudies.Anotable example is the blend of NR withBR. While the 1.4polybutadiene-NR blend is heterogeneous (Marsh et al., 1967),highvinyl BR andisoprene rubber (IR) are miscible. as evidenced by their spontaneous interdiffusion (Roland, 1987). This is due to a fortuitons rear equivalence of the polarizabilities of the respective chain subunits (Tomlin and Roland. 1992). Although the incidence of miscible rubber-rubber blends is not high. there are advantages to such a morphology, including greater mechanical integrity than multiphase systems and an enhancement of tensile properties due to the contraction in volume and correspondingly greater number of chains per unit cross-sectionalarea(Friedet al.. 1979). Also, the presence of a morphology that is i n thermodynamic equilibrium can minimize its dependence on mix conditions, along with minimizing the likelihood of changes in the morphology during postmixing operations. In addition to equilibrium considerations regarding rubber blend morphology. it must be recognized that the slow diffusion of macromolecules makes possible the preparation of blend vulcanizates in which the morphology is. however, by no means at equilibrium. Three possibilities exist:
Rubber-Rubber Blends I
199
The polymers are miscible but exhibit heterogeneity because the mixing process was of insufficient duration to allow the attainment of equilibrium prior to crosslinking. Since few elastomer blends exhibit miscibility, this circumstance is perhaps only of academic interest. 2 . The elastomer pair is not miscible, but vigorousmechanicalmixing overcomes the small potential barrier to unlike segmental interactions and so gives rise to a morphology that is homogeneous, at least down to the level accessible by standard analytical technique. Althoughdiffusion of elastomer molecules,particularly at the elevated temperatures of mechanical mixing, would permit demixing of chains over a scale greater than the coil size (ca. 100 A), the inhibition of these reptative motions either from bonding with adjacent filler particlesor due tothe presence of long-chain branching might result in a stable, albeit nonequilibrium. morphology. 3 . The temperature at which blending or curing is carried out may be greater than the lower critical solution temperature (LCST) of the polymer pair. The phase separation of miscible polymers at elevated temperatures results principally from an increase in the entropy loss associated with the volume changes accompanying mixing (Sanchez, 1982). (The combinatorial entropy contribution, which favors mixing, also increases with temperature but, as discussed above, this is negligible for high molecular weight materials.) 1.
Polymer blends that are miscible will usually exhibit LCST behavior. Although observations of upper critical solution temperatures have been reported [for example, in an SBR-BR blend by Inoue et al. (l985)], Sanchez (1982) has argued that, absent attractive interactions of a specific magnitude. UCST is not possible i n high molecular weight materials.
2.3 Multiphase Rubber Blends Although in the great majority of elastomer blends thecomponents are not molecularly dispersed, they may be referred to as compatible if some technically advantageous combination or compromise of properties can be realized from the blend. It would perhaps be more appropriate to reserve the term “compatible” to describe systems that do not spontaneously demix on a macroscopic scale, but, in fact, the retarded diffusion of macromolecules makes this unlikely. even in polymer blends in which the unlike interactions are strongly repulsive. The morphology of these “compatible” rubberblends is dependent upon the mixingprocedureandrheological properties of the blend components as well as thermodynamic considerations. This structurecan be adispersion of onecomponent in a continuous matrix of the other, or thephases can be co-continuous. CO-continuity implies that an interpenetrating polymer network (IPN) exists. Although an IPN can be intentionally produced during synthesis (Sperling and Friedman, 1969)or by controlledmixingtechniques [e.g., latex blendingas described by FrischandKlempner (1970)], with conventionalrubber-mixingtechniques equal-volume fractions and equalviscosities of the components will favor co-continuity (Avergopoulos et al., 1976;Gergenet al.,1985), ascanbe seen in Figure I . While from thestructure of IPNs one canexpect that propertiessuch as modulus will be additive with regard to thecomponents’moduli,the unique morphology of IPNs andtheirpotential for exceptionalultimatepropertieshave led to expandedresearchactivitiesaimed at exploiting their commercial utility (Sperling, 1981). Although only a few of the patented IPN materials involverubber-rubbermixtures (Clark, 1970; Lohr and Kang, 1975;Falcetta et al.,1975), it seems likely that the morphology of many of therubberblendscurrently in commercial use consists of interpenetratingpolymernetworks.
200
Roland
\
8’ 64TORQUE RAT IO EPDM PBD
2I .8 I . 6 - EPDM .4 -- CONT.
0
0.25
0.50
W E I G H FT R A C T I O N
0.75
1 .o
PBD
Fig. 1 Phase continuity of EPDM-BR blendsas a function of compositionandtherelativeBrabender mixing torques of the components. (From Nelson et al., 1977.)
A blend morphology wherein one component is dispersed within a continuum of the other has received by far the greatest attention in the scientific literature. The continuous phase in these materials is invariably found to be the rubber of lower viscosity, provided it is present at a sufficiently high concentration (Avgeropoulos et al., 1976). Thisobservation is at least plausible, sincethe more fluidcomponent can readily encapsulate the more viscous phase.The capacity for a component tobe highly extended without fracturing underthe conditions of mixing probably increases the likelihood of that component existing as a continuous phase. From consideration of the interfacial tension between two phases, it has been argued that the phase with the larger normal stress function will form the dispersed particles (Van Oene, 1972). Confirmation of this prediction is lacking, although measurement of normal stresses, especially at high shear rates, is difficult. During the mixing of rubber blends, the dispersed domains are deformed during passage through the high-shear regions of the mixing vessel and, under the proper circumstances. will fracture to produce smaller particles. Simultaneously, these flowing particles collide and often coalesce toform largerdispersed domains. The blendmorphologyobtainedrepresentsthis competition between dispersion of the rubber particlesand their flow-induced coalescence (Tokita, 1977; Roland and Bohm, 1983). Attempts to predict the morphology of rubber blends from consideration of the competition between breakup and coalescence have beenmade by assuming an energy criterion for particle fracture (Tokita, 1977; Bohm, 1980). In fact, breakup is more related to the stress level exerted on the particle by the flowing matrix and how effectively this stress can sustain particle deformation. The number of particles produced upon breakup also is a strong function of the stress level as well as depending upon the relative viscosities of the components (Grace, 1982). Experimental studies of particle deformation and breakup invariably focus on single drops in a dilute suspension. In the concentrated systems usually employed in more practical situations, particles are surrounded by their neighbors. This shielding makes it more difficult to disperse domains in practice than results from the more idealized situations studied in the laboratory would suggest. Characterizationof the material properties is, moreover, usually based on low-strain andor steady-state data, whereasthe fracture of fluid particles clearly
Rubber-Rubber Blends I
201
involves high deformations and transient behavior. For these reasons, along with the complexity of the factors governing thecoalescence process, the theoretical predictionof blend morphologies is a formidable task. Manystudies,bothexperimental(Bentley, 1985) andtheoretical(Barthes-Bieseland Acrivos, 1973), have focused on thedispersion of fluid particlesin a fluid medium. The minimum stress necessary to break upa suspended droplet hasbeen shown tobe lowest when the viscosities of the two phases approach one another under the prevailing conditions of temperature and deformation rate (Nelson, et al., 1977). Although shear flow dominates in the mixers utilized in the rubber industry today, extensional flow fields are more effective for the dispersion of particles (Roland and Nguyen, 1988). This is due to their continual stretching of the droplets, whereas the vorticity inherentin shear flow causes rotation of suspended particles. Consequently the particles alternately experience extension and compression (Torza et al., 1972; Pipkin and Tanner,1977). The flow-induced coalescence of the dispersed domains requires their collision, removal of the intervening film through its drainage and fracture, and finally molecular interdiffusion between the droplets. Coalescence has been found to be very extensive both when the viscosity of thesuspendedparticles is much lower thanthat of the continuous phase (Rolandet al., 1986) and when the viscosities are comparable (Fig. 2). As would be expected, a more viscous continuous phase reduces the rate of coalescence, although, interestingly, Roland and Bohm (1984) found that an increased rate of shearing increased the fraction of interparticle collisions that resulted in coalescence. Thenet result of flow-induced coalescence in sheared rubber blends is that the ultimate particle sizeis thereby limited. It has long been recognized that a continuation of the mixing process will, after some point, no longer result in a finer dispersion (Rehner and Wei, 1969). This is generally due to the attainment of a steady-state competition between the particle breakup and coalescence processes, although the ability of the high-shear regions of the mixing vessel to further fracture the dispersed particles can also be a limiting factor.
INVARIANT x
~
0.70
20
40
60
-
MI L L PASSES Fig. 2 The reduction in small-angle neutron scattering invariant accompanying mill mixing of a BR-CR blend. The decrease in scattering intensity results from the loss of isotopic purity in the BR domains due to their flow-induced coalescence. The arrow indicates the value corresponding to complete homogenization of the particles after extensive multiple coalescence. (From Roland and Bohm, 1984.)
Roland
202
3. ANALYTICALMETHODS FOR BLENDCHARACTERIZATION Various techniquesexist for study of the thermodynamics of polymer-polymerblends,and considerable attention has been focused on predicting or assessing miscibility. The great bulk of polymer blends possess a heterogeneous morphology by virtue of their immiscibility. Since the multiphase structure producedin many practical applications is not necessarily representative of any equilibriumcondition, it is often usefulto determine the nature of the morphology obtained under particular conditions. Various analytical techniques available for the characterization of blends are described in this section, with an emphasis given to newer developments in the field. 3.1
Electron Microscopy
The most straightforward method of examining the structure of multiphase polymeric systems is direct observation in the electron microscope (EM). The principal difficulty is in ensuring that sufficient contrast exists when the electron densities of the rubber components are similar. When a difference in unsaturation exists, staining techniques (e.g., with Os04) have long been successfully employed. Of particular interest for elastomer blends is the ebonite method (Smith and Andries, 1974), in which the preferential reaction of one of the rubber phases with sulfur and zinc effects a large increase in its electron density. Advantage can also be taken of the differential capacity for swelling in a particular solvent in order to obtain phase contrast. As described by Marsh et al. (1967). the blend sample is immersed in the solvent, stretched, and subsequently observed in the electron microscope after evaporation of the solvent. The phase that was more swollen will have been more thinned out upon stretching. To avoid the distortion in zone sizes and shapes encountered with the differential swelling method, advantage can be taken of the differing susceptibilities to pyrolysis of the rubbers in a blend (Hess and Chirico, 1977).Differentialpyrolysisselectivelyremoves one of the rubbers,causingits domains to becomemoretransmissive in theelectronmicroscope.Rolandetal. (1985) havedescribed several approaches for obtaining electron micrographs of the transient structure that may arise in multicomponent rubbers as a result of deformation. Recent advances in digital image analysis have facilitated the rapid obtaining of particlesize data from electron micrographs (Sax and Ottino, 1985). The micrographic image is converted by a video camera into an array in which each element represents the corresponding optical density of a small section of the original image. Spatial resolution can be as fine as about I O pm. An associated computer extracts from this array the desired particle-size distribution and statistics. With resolution limits as low as a few angstroms (Kruse, 1973), in principle the electron microscope can be used to probe rubber-rubber blends of the finest dispersion. In practice, the need to obtain thin sections and the problem of contrast limit its range of usefulness.
3.2 SolutionBehavior Since the retarded diffusion of polymers in the solid state makes it difficult to attain a condition of true thermodynamic equilibrium, the behavior of polymer mixtures in solution has often been utilized in efforts to assess miscibility. Phase separation of a polymer pair in a common solvent is usually indicative of their immiscibility, although when a sufficiently large difference exists in therespectivepolymer-solventinteractionparameters, phase separation can occur in solutions of miscible polymers (Patterson, 1982). It is even more common to find instances of misciblesolutionsinvolvingpolymerpairs that in the absence of solventexistas a phaseseparated blend. Asan example, Braun and Rehage(1 985) found that blends of BR and polypen-
Rubber-Rubber Blends I
203
tenamer form a miscible solutionin toluene, while electron micrographs clearly indicated heterogeneity in films cast from this solution. As discussed earlier, a negative enthalpy of mixing is usually requiredfor polymer-polymer miscibility. An indication of the magnitude of this interaction, but not its sign, can be extracted from determination of solubility parameters, 6, of the respective blend components. Defined as the square root of the cohesive energy density of the polymer, 6 can be obtained from measurement of the interaction parameter, xI2, which is the ratio of the noncombinatorial free energy of interaction in the polymer-solvent system to the available thermal energy (Hildebrand et al., 1970),
where V is the molar volume, and the empirical binary coefficient I I 2 is often taken to be zero. Various methods are available to measure the polymer-solvent interaction parameter (Orwoll, 1977), includingprediction from intrinsicviscosities(Kok and Rudin, 1982). The polymerpolymer interaction parameter. can then be deduced from the respective solubility parameters for the two polymers, using
where
It can be shown that a negative value of x 2 3 can only result from a nonnegligible value of (Paul and Barlow, 1984). It isobvious that the used in thismethodmustcorrectlyinclude the various contributions (dispersion forces. hydrogen bonding, etc.) to the interaction of the polymer pair, which may require xI2determinations in a variety of solvents. A more useful approach to obtaining polymer-polymer interaction parameters from polymer-solvent interactions is the useof gas-liquid chromatography (Su and Patterson, 1977). Retention volumes of gas-phase components on solid phases composed of the polymeric materials of interest (the respective pure components and their mixtures) provide a measurement of xli.The polymer-polymer interaction parameter can then be calculated according to
where +2 and +3 are the volume fractions of the polymers in the mixture. A related approach is the determination of from the uptake of vapor by the polymer blend (Kwei et al., 1974). While determination of polymer-polymer interaction parameters can be of value as an indication of potential miscibility or in comparing the relative compatibilityof a series of blends, it must be recognized that it is of limited utility in describing the structure of blends encountered in practical processing operations, where decidedly nonequilibrium conditions may prevail. Shutilin (l982), for example, found that solubility parameter differences did not correlate with the apparent relative compatibility of various blends of NR, BR, and SBR.
3.3 Glass Transitions A popular methodof adducing thedegree of homogeneity in polymer blendsis from measurement of the temperatures of transition from rubbery to glassy behavior. This canbe accomplished with
204
Roland
a variety of methods, including dilatometry, nuclear magnetic resonance, dielectric response, differential scanning calorimetry, differential thermal analysis, radiothermalluminescence, and dynamic mechanicalmeasurements. The observation of distincttransitionscorresponding to each component of the blend indicates that a multiphase structure exists. The appearance of a single transition cannot, however, be taken as unambiguousevidence ofmiscibility.If the respective T,s are close (ca. 10"C), they may appear as a single, broad transition. The appearance of one or more transitions in dynamic mechanical testing can also be affected by the frequency employed. Ramosand Cohen ( 1977). for example, observed a single glass transitionin heterogeneous BR-IR blends at 1 10 Hz, while at a lower test frequency both transitions were in evidence. Fried ( 1983) has discussed improving the resolution in thermal analysis by selectively annealing one of the components orevaluating derivatives of the usual heat capacity measurements. When the domain size is sufficiently small [Kaplan (1976) suggests 150 Al. the thermal or mechanical response becomes no longer sensitive bo heterogeneity. For example, when the domains of a NR-cis-polypentenamer were 50- I00 A i n diameter, torsion pendulum measurements revealed only a single. intermediate glass transition (Braun and Rehage, 1985). There is also evidence suggesting that when two rubbers are close i n structure, such as NR and BR (Bauer and Dudley, 1977) orBR and SBR (Inoue et al., l985), a broad or interconnected interface region, developed particularly upon vulcanization, can cause disappearance of the expected distinct glass transitions. The exact temperature of the individual glass transitions i n a multiphase blend will be shifted somewhat from the values for the pure components. This is a mechanical effect and does not necessarily indicate anything about phase interactions (Dickie, 1979). Changes i n the magnitude of the damping have been used to assess the state of cure of the phases in a rubber blend (Husonet al., 1984). Sinceobtaining a satisfactory network structure in both components of a rubber blend can be difficult when there are differences in the extent of unsaturation of the rubbers. this information can be of practical value. There have been recent developments in the use of pulsed NMR to investigate the heterogeneity of blends (Nishi, 1978; Miller et al.. 1990; Roland et al., 1993). Sincethe spatial resolution is limited by spin diffusion distances, this techniqueis comparable to more conventional methods for the detection of small domains. Since the motion of a carbon atom in a polymer chain will be more rapid when the segments are in a rubbery environment than when the surroundings are rigid. measurement of spin-spin relaxation times at temperatures intermediate to the respective T,s can be used as a probe of the extent of interfacial mixing.
3.4
Elastic Scattering
The irradiation of matter usually gives rise to scattering of a portion of the incident intensity, where both the energy and propagation vector of the scattered waves may differ from that of theincidentradiation.Whiletheenergydependence of the scattering is related to dynamic processes in the scatteringmedium,theangledependence of theelasticscatteringprovides morphological information. Elastic light, x-ray, and neutron scatteringall result from heterogeneities in the structure of the irradiated material. In a homogeneous system, thermal fluctuations in density and composition are responsible for the scattering. with the extrapolated zero-angle intensityprovidingameasure of polymer-polymerinteractionparameters (Wendorff, 1980; Murrayet al., 1985). In multiphasepolymerblends, the angledependence of thescattering reflects the size and spatial distribution of the phases. and so it can be usefully applied to the study of the morphology of rubber blends. There is an extensive literature devoted to methods of analyzing scattering data from nlultiphase systems [see, for example, Glatter and Kratky (1982) and Higgins and Stein (1978)l. Thedifferences among light, x-ray, andneutron scattering have to do with the nature of the material heterogeneities as well as with their size.
Rubber-Rubber Blends I
205
The scattering of visible light, whose wavelengths are thousands of angstroms, results from inducedpolarization in themolecular electron cloud. Contrast,therefore,results from variations in refractive index of the sample over distances of a few thousand angstroms and greater, with larger heterogeneities amplifying the scattering at the smaller angles. The application of light scatteringto solid-state polymer blendsis not as popular as its use in the characterization of dilute polymer solutions, due to difficulties in the former with multiple scattering and sample transparency. In the area of elastomer blends, much of the light-scattering work has focused on block copolymers, including, for example, blends of styrene-isoprene copolymers with polyisoprene (Moritani et al., 1970). Light scattering has also been used to monitor the broadening of the interface in BR-SBR blends during vulcanization (Inoue et al., 1985). The wavelengths of x-rays (e.g., 1 S 4 A for Cu K,) are a thousand-fold less than that of visible radiation, so that whereas light scattering arises from coupled electrons as described in terms of a molecular polarizability, the scattering of x-rays is simply related to the number density of electrons. The variations in electron density that give rise to this x-ray scattering can extend spatially from a few to roughly a thousand angstroms, beyond which the scattering angles become toosmall to be experimentally accessible. The application of x-ray scattering to polymer blends is well established and often complements light-scattering results. This is particularly true in two-phase systems in which the particle sizes extend over a broad range. In Figure 3 is displayed the particle-size distribution determined from small-angle x-ray measurements from a BR-CR blend, along with results obtained from analysis of electron micrographs (Roland and Bohm, 1984). Although thereare limitations associated with extracting particle-size distributions both from micrographs and from scattering curves. it can be seen that the different techniques are in reasonable agreement. A major advancement in the investigation of polymer blends has been the development of small-angle neutron-scattering techniques. The wavelengths of the neutrons used in these experiments range from 2 to 20 A, so the sizes of structures probed are comparable to those in
A
TEM
- CURVE
FITTING INTEGRAL TRANSFORM
0
300
600,
900
I200
RADIUS (A)
Fig. 3 BR particle size distribution in a well-mixed CR blend as inferred from (A)analysis of electron micrographs, ( 0 )Fourier inversionof small-angle x-ray scattering data, and( - ) the log-normal distribution function whose adjustable paramcters gave a best fit to the experimental x-ray scattering curves. (From Roland and Bohm, 1984.)
206
Roland
small-angle x-ray studies. The scattering contrast, however, is associated with short-range neutron-nucleus interaction. This interaction is different for different elements aswell as for different isotopes. The great advantage of neutron scattering is that by isotopic substitution, particularly of deuterium for hydrogen, scattering heterogeneities canbe selectively introduced into a sample without appreciable change in the thermodynamic properties of the material. A fraction of the chains of one component of a polymer blend can be labeled in this manner. so that not only can domain sizes in the blend be measured (as with small-angle x-ray scattering), but the dimensions of the polymer chains of the labeled component can also be determined. There has to date been limited application of small-angle neutron scattering to rubberrubber blends. Kirste and Lehnen (1 976) measured the increase in coil size of high molecular weight,polydimethylsiloxanewhen it was blended with lower molecular weight PDMS. Roland and Bohm (1984) used neutron scattering to follow the coalescence of BR domains dispersed in a CR matrix. It seems likely that significant advances in analysis of the structure of multiphase rubber systems can be realized from further application of neutron-scattering techniques. Although the intimate mixing associated with miscible rubber blends might be expected to preclude formation of a crystalline lattice by any crystallizable component of the blend, the principal effect of miscible blending is on the crystallization rate. Since, for example, it has been reported (Ghijsels, 1977) that the heat of fusion of BR reflects the extent of its blending with SBR, investigation of crystallinity in certain rubber blends may prove useful. Riga (1978) has demonstrated how the angle and breadth of the amorphous halo in the wide-angle diffraction pattern from various polymers can be used to distinguish homopolymer blends from the corresponding copolymers. The effect of blending on the crystallization of natural rubber has been studied by Zenel and Roland ( 1992a) and by Tomlin and Roland (1993).
4.
PREPARATION OF RUBBERBLENDS
The objective ofan industrialmixingprocess is production of materialhavingthedesired properties. For multiphase polymer systems, this does not necessarily correspond to any equilibrium or steady-state morphology, although such a structure could probablybe most reproducibly generated. Only when the benefits in terms of a rubber’s performance warrant the additional expenditure of effort is any “ultimate” mode of dispersion likely to be pursued. Moreover. many physical properties of rubber blends are insensitiveto the details of the two-phase structure. Rubber-rubber blends can be prepared by a variety of methods, including during synthesis. by latex or solution blending, and by conventional mechanical mixing. The discussion in the following sections is limited to mechanical mixing, which represents the most widely practiced procedure. Descriptions of other methods of preparing blends can be found in the literature. with references contained in the review articles of Corish and Powell (1974) and McDonel et al. (1978). The intluenceof mixing conditions on the distribution of the various compounding ingredients of a rubber formulationwill also be described herein.The effect of blend morphology on properties will be taken up in Section 5. which is concerned with the individual properties of rubber blends.
4.1
Mixing Equipment
The principal functions of the mixing operation in preparing rubber-rubber blends are essentially identical to those associated with incorporating fillers, curatives,etc., intoa rubber stock; distriburilqr nli.ving, in which the composition of the blend is made uniform throughout the batch, and disprrsi\’e m i x i q , in which the initially macroscopic components are broken up into finer do-
Rubber-Rubber Blends I
207
mains. While the throughput advantages of continuous mixing are well recognized. commercial rubber operations usually employ batch mixing. This is in large part due to the need to supply a continuous mixer with particulates or free-flowing materials, whereas rubberis usually obtained in large bales. Accordingly, the extruders used in the rubber industry are most often employed in an auxiliarycapacity to improvedistributivemixing and to shape therubber.Dispersive mixing is carried out in internal mixers such as the Farrel Banbury or, for small batches, often on open-roll mills. Although developments in the design of rubber mixers were not expressly carried out to improverubber-rubber blending. the optimal distribution and dispersion of carbon black and the other ingredients of a rubber formulation are controlled by the same mechanisms pertinent to the blending of rubbers. In the design of internal mixers these developments include increases in the number of wings, or nogs, on the mixer rotors to increase the quantity of highshear areas inside the mixer, and allowing the rotors to intermesh (Fig. 4) so that dispersive shearing occurs not only between the rotor tip and the walls, but also between the rotors themselves (Freakley, 1985). Dispersive mixing requires high stresses; accordingly, it is often important to efficiently cool the rubber in an internal mixer in order to maintain a high viscosity. Of course, precise temperature control in general requires the capability for efficient removal of heat. The cooling
(b)
Fig. 4
Internalmixer with (a) tangential and (b) intermeshing rotors.
Roland
208
systems of internal mixers have evolved over the years. The most effective method is to drill passages that allow flow of the cooling water close to the inner surfaces of the mixing chamber. Along with their use in the transfer and forming of rubber stocks, extruders can also contribute to distributive mixing. Increased demands for the screw and barrel of the extruder to not only convey the material to a die but also mix it have led to modifications in screw design. The principal aim is to promote homogenization of the mix by the exchange of material between planes. Representative approaches include the use of pins in the barrel to divert and divide the flow of material (Freakley, 1985) and the use of the cavity transfer mixer, whichhas hemispherical cavities in thescrewand barrel to promote exchange of material (Hindmarch and Gale, 1983). The resulting diversion and reorientation of the flow stream improves the distributive blending efficiency of the extruder. In recognition of the efficiencies of automated production systems, there has been continued interest in the rubber industry in developing continuous mixing methods. The plastics industry has further evolved toward this goal, and advancementsin that industry may facilitate development of the continuous processing of rubber. The problem associated with feeding acontinuous rubber mixer has been mentioned. There is the additional constraint of obtaining acceptable dispersion. which, with regard to filler-rubber, if not rubber-rubber, mixing, is a most demanding requirement. In order for an extruder to perform the dispersive functions of an internal mixer, special mixing screws must be employed(Plastics Compounding, 1981, 1984). Thesecan contain dams so that the material is forced through tight clearances with the wall and is thereby subjected to large shearing stresses. Twin-screw extruders can also be used where shearing forces are developed by employing various mixing elements. Other continuous mixers include the Farrel continuous mixer,which is an extruder containingBanbury-typerotors to provideintensive mixing: the Berstorff planetary gear extruder, in which the primary screw is surrounded by an array of planetary spindles; the Buss-Condux extruder. which enlploys a single rotating and reciprocating screw along with teeth running axially inside the barrel: and the Sterling Transfermix, where the variationin the rotor diameter along its length causes material to be transferred from the rotor to concentric stator rings attached to the barrel. Although the various designs lead to good distributive mixing and acceptable dispersive mixing,thelatter is beingobtainedalmoststrictlythrough shear flow. For example, if one compares the carbon black dispersion obtained in various mixers as a function of the energy of mixing, it can be seen that various designs all fall on the same general curve (Fig. 5). In the Mooney viscometer this is known to be shear flow, indicating that the stresses developed in the various mixers are evidently arising from the same type of flow field. Although it is one of the most common, shear flow is by no means the most effective dispersive flow field. In general, a flow can have both rotational and extensional components. The strength of a flow field can be described by consideration of the ratio of the magnitude of the strain tensor to that of the vorticity tensor (Fuller and Leal, 1981). This ratio is given by the quantity I + P I - P
where S = - 1 corresponds to pure rotation and (3 = 0 ;S simple shear flow. It has long been recognized that dispersion is most effectively accomplished with pure straining (P = 1) flow fields. Nevertheless, withthe exception of the drawing off of material from a calender or extruder and the converging of tlow paths into adie orthrough the nipof a roll mill or mixer, commercial mixingreliespredominantly on shearing flows andthe development of shearingstresses to achieve dispersion.
209
Rubber-Rubber Blends I
+ m
“ . . . . J ” -
6oo
200 400 JOULES PER GRAM
Fig. 5 Carbon black dispersion index measured for an SBR stock with 63 phr N339 a s a function of the energy of mixing using the ( ) Mooney viscometer, (a)Brabender Plasticorder, ( 0 )Buss-Condux reciprocating screw kneader, and ( A ) Berstorff planetary gear extruder. (Courtesy of Ulmer et al.. 1985.)
+
4.2
Distribution of Compounding Ingredients
A major difficultyin obtaining acceptable vulcanizate propertiesfrom a blend is that of developing a satisfactory network structure in each of the rubber phases. Due to the higher solubility of sulfur in unsaturated elastomers (Van Amerongen, 1964), along with the greater affinity of many accelerators for more polar rubbers(Gardiner, 1968),the crosslink densityof the respective rubbers in the blend can differ significantly, leading to less than optimal physical properties. Leblanc (1982) has suggested that initially thecurvatives will locate within thecontinuous phase. In fact, the curatives probably make first contact with the lower-viscosity phase, since it tends to occupy the outer regions of the flowing rubber mass, where the shear rates during mixing are highest. Of course, this lower-viscosity component does tend to become the continuous phase, as discussed above. Generally, the details of the mixing scheme can have some effect on the initial distribution of curatives. The curatives will then diffuse into the elastomer component in whichtheirsolubility is highest. Since the levels of sulfurandacceleratortypically employed are below the solubility limits, diffusion of curatives into a given phase is usually not prevented by saturation of that phase; hence, this curative migration often results in a cure imbalance. In addition, if the rate of vulcanization varies considerably between the elastomers of the blend, depletion of the curatives in the faster-curing component can cause continued curative migration into this phase (Bhowmick and De, 1980).Leblanc (1982) demonstrated that preblending of curatives into the respective elastomers at their optimal concentrations prior to blending of the rubbers can improve the blend crosslink distribution, although the usual practice is to incorporate the curative last in order to avoid prevulcanization (scorch) problems. Altera-
Roland
210
tions in blend physical properties can sometimes be realized from the use of very short, hightemperature cure cycles (Bhowmickand De, 1980).In these instances the distribution of curatives obtained during the mixing stage will be more critical, since rapid reaction of the curatives can reduce the extent of curative migration. Approaches to overcome these imbalances in curative distribution include the chemical modification of accelerators so that the respective solubilities in the components of a rubber blend will be more equal (Mastromatteo et al.. 1971) and the direct attachment of curatives to the elastomer with which the solubility is lower (Hashimoto et al., 1976).Obviously the problems encountered with curative imbalance are particularly significant when the components of a blend are most dissimilar, such as blends of EPDM or butyl rubber with dienes or nitrile rubber. Along with the desirability of having a balanced crosslink density in the various rubber phases, there also exists the requirement that for mechanical integrity the phases must bechemically bound to one another; that is, linkages must exist across the phase boundaries. The extent of interfacial crosslinks is sensitive to both the rate of vulcanization and the specific cure systems employed (Woods and Mass, 1975). Hashimoto and coworkers ( 1970) have touted polysulfidic linkages as most useful for interfacial crosslinking of EPDM-SBR blends, while Zapp (1973) found that interfacialbonding in chlorobutyl-BRmixturescorrelatedwith a high degree of monosulfidic linkages. Related to the subject of interfacial bonding is the fact that if a third, small-molecule component that is soluble with both polymers is presentintheir blend, the interface will be richer than the bulk phases in this component. This results from a reduction in energetically unfavorable interactions between dissimilar polymer chains at the interface by virtue of the presence of the third component (Hefland and Tagami, 1972). In principle, this affect could promote accumulation of curative at the interface. A somewhat analogous method of promoting interfacial connectivity is by incorporation into the blend of a block or graft copolymer containing segments identical to, orat least miscible with, each of the rubber phases (Paul, 1978). Provided the block lengths are sufficiently long, the copolymer additive will preferentially locate at the interface in a configuration whereby it is intimately mingled with each phase. Because of their greater ability to favorably configure themselves,block copolymers aresuperior to grafts in this regard.Withthis approach, the interfacial bonds correspond to the covalent backbone of the copolymer molecule, the respective segments of which are dissolved in theseparatephases.While the use of thesepolymeric compatibilizers is largely confined to blends of rubber with glassy (or crystalline)materials, the application to rubber-rubber blends may be feasible when the added cost of the block or graft copolymer is warranted by improvement in ultimate properties. Paddock (1973) has patented such a system based on EPDM and SBR. The distribution of fillers and various processing aids in a multicomponent rubber stock can also be nonuniform, with a resulting influence on properties. Extensive investigations in this area have demonstrated the preferential takeup of carbon black by certain rubbers, with carbon black affinity decreasing in the order (Callen et al., 1971 ). BR > SBR
> NR > EPDM > IIR
During mechanical mixing of carbon black with unsaturated elastomers, sufficient interaction, primarily chemisorption, occurs to prevent any subsequent transfer of the black. If the method of mixing is less vigorous (e.g., solution blending) or involves more saturated rubbers, the carbon black can transfer to phases with which it is more compatible. As discussed in more detail below, the nonuniform distribution of carbon black can influence various properties. Although the significance is perhaps not as great, the oils,resins,andvariousrubber chemicals used in a rubber compound can have differing affinities for the phases of a blend. Both their nonuniform distribution and postmixing migration have been observed.
21 1
Rubber-Rubber Blends I
5.
PROPERTIES OF RUBBERBLENDS
Other than obtaining a lower-cost material, the only motivation for blending rubbersis to improve performance. either the ease with which parts can be manufactured from the stock or the properties the part exhibits in end use. This section discusses certainaspects of the behavior of polymer blends and how they may be intluenced by the details of the blend morphology.
5.1
Rheology
Rubbers are often blended to obtain a better-processing material. This improvement may consist of lowering the stock viscosity or producing a material that is less prone to fracture or crumbling when subjected to flow. Normal stress functions and the related phenomena of die swell and shrinkage can also be altered by blending. Qualitatively. the expectation is that the processing behavior of the blend will be intermediate between that of the components. I n fact. however, polymer blends can often display anomalous rheological properties. The viscosity of a blend may exhibit a minimum and/or maximum as a function of the composition. In this regard, the behavior of rubber blends has been found to follow no simple trends (Fig. 6). A blend viscosity greater than the mean of the components' viscosities can result from the reduction in free volume accompanying miscible polymer mixing. It has been claimed that in a multiphase polymer blend there is an increased fractional free volume. which serves to reduce the viscosity (Lipatov et al., 1981). while the additional energy dissipated into dispersed particles when the continuous phase is sheared can contribute to an elevation in the resistance
30 S EC"
70C
600 ui m A
15 SEC"
I
W
g 50C 2 4OC I l
I
20
I
I
40
I
WT.
I
60 %
I
I
80
I
I
100
BR
Fig. 6 Capillary extrusion force a s a function of BR-NR blcnd composition at different nominal shear ratcs. (From Folt and Smith, 1973.)
212
Roland
50-
U E M U R A 8. TAKAYANAGI
30 -
0.0
0.2
0.4
0.6
0.8
COMPOSITION
1.0
Fig. 7 Viscosity as a function of blend composition calculated according
tO
Eqs. (7)-( IO).
to flow (Chaffey and Mason, 1966). Theoretical treatments are available that attempt to predict the viscosity of blends. Various derived expressions are given below and plotted in Figure 7. From Uemura and Takayanagi (1966):
From Hashin (1 964):
From Heitmiller et al. (1964):
In these expressions for the blend viscosity, q, the subscripts 1 and 2 refer to the respective phases of volume fraction or weight fraction W, and IJ. is Poisson’s ratio. Colby (1 989) observed distinct terminal relaxations in the mechanical spectra of miscible polymer blends. The components’ entanglement molecular weights, and hence the magnitude of the plateau modulus, can be influenced by blending (Roland, 1988; Roovers and Toporowski, 1992; Arendtet al., 1994). An important effect in miscible blends is theso-callednematic interaction, whereby orientation of neighboring chains induces orientation of a given chain(Sotta et al., 1987; Erman et al., 1988; Kornfield et al., 1989; Zemel and Roland, 1992b).
+
Rubber-Rubber Blends I
213
The morphology of a blend can rearrange to better accommodate theapplied stresses. This principle of minimum energy dissipation(Everage, 1973) underlies the often-encountered sheathcore configuration. Since in the vicinity of a wall of the containing vessel the velocity gradients tend to be highest, while at the core of a flowing polymeric material (through a pipe, on a roll mill, etc.) there is often plug flow, the lower-viscosity component will tend to accumulate toward the surface of the polymer mass. The result is a blend viscosity that can in the limit be as low as that of thelower-viscosity component. Van Oene (1978) has suggested that theinternal circulation occurring in the dispersed particles of a sheared blend may also contribute toviscosity minima, the secondary flowgiving rise to “drag reduction.” Incorporation of only a few percent of EPDM was found to significantly reduce the viscosity of a fluoroelastomer, and vice versa (Shih, 1976). This was attributed to the plating out of the minor component onto the wall of the viscometer, giving rise to interfacial slippage. A similar phenomenon has been observed in SBR containing less than 2% by volume of polydimethylsiloxene (Roland and Nguyan, 1988). The lubricity of thelatter effects a reduction in the apparentviscosity of theblend during processing. A complicating factor in predicting the rheology of industrially interesting rubber blends is the presence of a third phase of inextensible filler. A nonuniform distribution of carbon black, for example, canmodifytheprocessingbehavior of the individual components to differing degrees. In particular, saturated rubbers will not suffer the viscosity elevation experienced by unsaturated rubbers, with which the carbon black preferentially reacts. In a study in which both components had a high affinity for carbon black, Lee (1981) found the viscosity of BR-SBR blends to be independent of the location of the carbon black in the blend. This distribution did, however, alter the elastic properties. Generally speaking, the preferential incorporation of carbon black into one of the phases will cause a change in their relative viscosities. Such a viscosity differential can be expected to promote a sheath-coreconfiguration. As was noted,thiscanlead to a lower thanexpected blend viscosity. The expectations with respect to die swell and related phenomena are not as straightforward, since the coremay suffer the same magnitude of extensional flow as the sheath, unlike the case of shear deformation. Sircar et al. (1974) found that the die swell could be correlated to some extent with the location of the carbon black. Optimal reduction in die swell could in some instances be brought about by incorporating the black into the gum rubber whose die swell was found to be most reduced by the presence of filler.
5.2
Modulus
Two factors to be considered in estimating the modulus of an elastomerblend are the individual component moduli and the nature of the blend morphology. One influence on the stress-strain response of the components is the distribution of curatives and the resulting blend network structure. Factors governing this distribution are discussed in an earlier section. Another important aspect is the glass transition temperatures of the elastomers comprising the blend. When a blend is subjected to deformation at a temperature intermediate between the respective component T,s, it is generally found that the stress atlow strains is close tothat expected for the stiffer component while the extensibility nevertheless approaches that of the softer material (Newman, 1973). This isthe increased toughness underlying the rubber modification of plastics. The upper and lower bounds on blend modulus are given by parallel coupling,
Roland
214
and series coupling,
respectively (Nielsen, 1974). Various theoretical formulations [see, for example. Chen (1973)] predict intermediate behavior.The moduli of a seriesof EPDM-BR blends were foundby Nelson and coworkers (1977) to be well described by Eq. ( I 1 ). The crosslinking in that case was accomplished by electron beam irradiation to avoid complications with curative distribution. In general, the extent of radiationcrosslinking in agiven component will not be significantly affected by the specific natureof the other component,at least for blends of the typical hydrocarbon rubbers. In principle, however, the penetrating power and crosslinking efficiency of the radiation can be influenced by the electron density and atomicnumbers of the blend constituents [see Bohm and Tveekrem (1982)]. The nature of the blend morphology can also affect the stress-strain response of the blend. This is most marked when the blends are miscible. Kleiner et al. (1979) have reported that in such a situation the blend modulus can exceed the upper bound given by the parallel model. They have modified Eq. (1 1) by introducing an empirical interaction term, t 1 2 .
where El, is themodulusmeasured blend modulus is given by
for a blend i n which
$l
=
$?
=
0.5. Accordingly,the
This increasedmodulus can be attributed in part to thereduction in volume accompanying miscible blending and the consequent greater density of chains per cross-sectional area. When heterogeneous morphology exists in rubber-tubber blends, the effect of the details of this structure are not particularly significant. While the expectationwould be for the continuous phase tohave more influence, the stress-strain responseof EPDM-BR blends, at high concentrations of both elastomers at least, was found to be unaffected by change in the BR domains from continuous to discrete (Nelson et al., 1977). When one component of the blend is present as discrete particles, in blends of NR with SBR or BR (Walters and Keyte, 1965) and EPDM with BR (Weissert and Avgeropoulos, 1977). there was no effect of domain size on modulus. The stress-opticalcoefficient,which is a modulus characterizing the microscopicdeformation whereas the more usual mechanical modulus refers to the macroscopic strains, has been likewise found to be insensitive to phase separation, although its magnitude can be related to the degree of phase interaction (Bauer, 1982). In elastomer blends reinforced with carbon black, the distribution of filler can profoundly influence the modulus. Displayed in Figure 8 is the dynamic shear modulus measured for an elastomer blend in which the carbon black distribution was systematically varied. It can be seen that, particularly at the lower strains where the carbon black network structure dominates the stiffness properties, an increase in the nonuniformity of this filler distribution results in a lower stock modulus. In terms of the models described by Eqs. ( I 1 ) and (12), this indicates that the transfer of a portion of the carbon black from one phase would lower its modulus proportionally more than the increase in modulus of the phase with the higher carbon black concentration. The effect of carbon black distribution on modulus is thus related to the nonlinear dependence of modulus on carbon black loading.
215
Rubber-Rubber Blends I
54G' (MPa)
32-
01
Io-2
I
1
-
IO" IO0 S T R A I N ?A)
IO'
Fig. 8 Dynamic shcar modulus measured for an SBR with 105 phr carbonblack in which 0 , 20, and 30%. respectively, of the polymer was added as gum rubber during a second stage after incorpornt~onof thc carbon black into the initial ruhbcr. (From Nguycn, 198 1.)
5.3
Transport Properties
The transport properties of polymer blends are of interest for the practical application of blends in air retention, vapor resistance. permselectivity. etc.,as well as for the insight into the morphology of the blend that can be gained from study of the penetration of small molecules into the structure. The passage of vapor through a rubbery material entails dissolution of the gas into the rubber, molecular diffusion, and subsequent evaporation of the gas from the other side of the specimen. The kinetics of this process can usually be described as Fickian. whereby the concentration of the vapor in this rubber is proportional to the external pressure (Henry's law) and the flux of the gas isproportional to its concentration gradient (Fick's law). Thepermeability coefficient P is thereby expressed as the product of the proportionality constants. P = KD
(15)
where K is the Henry's law solubility coefficient and D is the diffusion constant. The diffusion constant can vary with penetrant concentration, while at higher pressures K may become pressure-dependent. In miscible polymer blends the permeability is often found to be described by an empirical relation.
Miscible blends (Nielsen. 1978) as well as certain of those with a heterogeneous structure [for example, rubber-modified polyethylene. as reported by Pieski (1960)] can in some instances exhibit synergistic permeability behavior. Most rubber-mbber blends are heterogeneous. with a permeability that is intermediate between that of the components. Attempts to model the transport properties of blends use some formulation of parallel and series models as their basis. If the continuous phase is the more permeable,a parallel configuration representsthe limiting behavior, with the dispersed phase effecting a more tortuous pathof the penetrant. The series model serves
Roland
216
as the limiting case when the dispersed phase exhibits the greater permeability. Robeson et al. (1978) have combined these extremes to obtain an expression for the permeability given by
P = X;,P,
P1 P2
[
+ XhP2
1
+ 2P, - +?(PI - P?) + 2P, + +?(PI - P?) P1 + 2P3 - 2+,(P, - P l j j P , + 2P2 + +I(P? - PI)
[
where X, represents the fraction of the composition in which component I is the continuous phase, and X h corresponds to a continuous phase of component 2. The description of such cocontinuity is limited by the restriction that
It can be seen that a composition range in which the permeability data are described by Eq. (17) with XA = X” can be taken as an indication of phase inversion. Expressions such as Eq. (17) are based on considerations of the dispersed phase as spherical in shape. More extended structures, particularly those lying in a stacked or lamellar configuration, can lead to reduced permeability due to the more tortuous path that must be taken by penetrants. The most extensive study of the permeability of rubber blends was the early work of Barrier (1955). Some of his data for blends of natural rubber with selected synthetic elastomers are depicted in Figure 9. While the preceding discussion was concerned primarily with mass transport properties (permeability and sorption), the electrical conductivity of rubbers is a transport property that also has some practicalimportance. In elastomers, conductive or semiconductiveproperties
1
20
1
40 WT
I
60
80
rbo
% NR
Fig. 9 The relative air permeability of various elastomer blends, with that of NR taken a s 100. (From the data of Barrier, 1955.)
217
Rubber-Rubber Blends I
CllR
0 2 VOL
0.E
I .o
FRACTION, 2nd POLYMER
Fig. 10 Volumc clcctrical conductivity of blends of CR with (A)chlorinated butyl, (0) nitrilc. and (0) natural rubber. (From Sircar, 198 1.)
result from the presence of carbon black. Indeed, electrical conductivity is a well-established probe of the extent of microdispersion of the carbon black. Blending of elastomers with different affinities for black provides an opportunity to control the state of aggregation and connectivity and thereby influence the electrical conductivity. Displayed in Figure 10 is the electrical conductivity measured by Sircar (1981) for blends of CR with various rubbers. It can be observed that conductivities can be achieved in these blends that exceed those of the pure components. This is due to increased agglomeration of carbon black in these immiscible blends. Carbon black tends to redistribute when mixed into blends, particularly when it has a low affinity for one of the phases. This can result in an accumulation of carbon black at the interface (Marsh et al., 1968) and consequently higher electrical conductivity. Blends of rubbers with similar affinity for carbon black (e.g., SBR-NR) do not exhibit this synergism.
5.4 Adhesion and Tack Acceptable levels of both the (cocure) adhesion and the autoadhesion (or tack) of rubber stocks can often be obtained only through the blending of rubbers. The fact that many synthetic elastomers (e.g., SBR, BR, EPDM)are very deficient in this latter property results in their often being used in combination with natural rubber. The autoadhesion of a blend is unlike many properties in that it is strictly a surface phenomenon; separation of two layers of green stock will typically deformmaterialonlyaboutamillimeter into thebulk of thelayers.Accordingly,thetack performance of ablendreflectsonlythecompositionatthesurface. This providesforthe possibility of obtaining an elevated level of tack without necessarily requiring the use of a large amount of high-tack rubber in the blend. Whether this is realized in practice depends upon the method by which the blend is prepared as well as the rheological characteristics of the blend constituents. Morrissey (1971) measured the tack of a series of blends of NR with various synthetic rubbers and found that the level of tack paralleled the NR content. Hamed (1960) similarly
218
Roland
Table 1 Autoadhesion of NR-SBR Blends Composition (phr)
70 40
NR
SBR
IBMA-SBR"
60 30 30 20
40
-
-
70
-
40
30 40 30
Autoadhesion (Jlrn')
1400 300 I 050 670 0
reported that blends of NR and SBR exhibited autoadhesion that increased with NR concentration; however. a maximum was observed when the NR was 80% of the total polymer. This synergism was attributed to the optimal green strength of such a composition. The high density of chain entanglements i n SBR in combination with the crystallization at high strain of the NR results in a material of greatest energy to rupture. In general, of course. green strength will affect autoadhesion only when the latter is limited by the energy required for rupture. More usually the ability of the plied surfaces to fuse together is the controllingvariable in tack measurements(RolandandBohm, 1985). When BR wasblended with agraft copolymer of isopropylazodicarboxylate and BR. which possesses very high autoadhesion, high tack in the blend stocks was obtained only when the copolymer rubber constituted the continuous phase (Roland et al.. 1985). The behavior described in these examples suggests surface compositions that must at least approximately reflect that of the bulk. When the components of a blend differ widely in viscosity, this may no longer be the case. It is worth noting in this regard that the autoadhesion of the blend is not necessarily an indication of the autoadhesion of the pure components. For example. displayed in Table 1 is the tack measured for a variety of blends of NR. SBR. and a terpolymer consisting of styrene, butadiene. and 3 mol% of N-isobutoxymethylacrylamide (IBMA). When only a small fraction of NR is present in blends with SBR alone, the tack is low. Replacing a portion of the SBR with the IBMA-SBR terpolymer effects a large increase in autoadhesion; nevertheless, blends of SBR and IBMA-SBR without natural rubber have negligible autoadhesion. The IBMA-SBR itself is devoid of tack, yet its presence in SBR-NR blends promotes high autoadhesion. This seeming paradox results fromlack of correspondence between the surface and bulk composition in blends containing the three elastomers. During mixing at elevated temperatures. IBMA-SBR undergoes a condensation reaction leading to coupling of the IBMA moieties. This crosslinking markedly increases the viscosity of the SBR phase. so that during milling to produce the test sheets it locates in the core of the sheets. where the deformationis largely plug flow. The natural rubber constitutes most of the surface phase, so that its high autoadhesive capacity is most fully taken advantage of by incorporating the IBMA-SBR into the blend. While natural rubber is usually selected to impart autoadhesion to a blend, because of both its superior performance in this regard and its general utility and low cost, other elastomers can. of course. be employed for this purpose. Barager (1983) has reported that i n blends with chlorobutyl rubber. for example. polychloroprene produces a larger increase in tack than does natural rubber. Bettercocureadhesion Inay also be obtained by the blending of rubbers.A common example is the adhering of highly unsaturated rubbers to stocks of low unsaturation. Maksimova
Blends
Rubber-Rubber
I
219
and Shvarts (1984) reported that good adhesion could be obtained between blends of IR. BR, and chlorinatedbutyl rubber and blends of EPDM and butyl rubber only if the levelof chlorinated butyl rubber exceeded 75%. A reduction in the level of chlorinated butyl rubber and an increase in polyisoprene, on the other hand, gave, as expected, superior adhesion to SBR. Thenlagnitude of the adhesion in allcases could be influenced by the nature of the cure system. Barager (1983) found that the adhesion of epichlorohydrin rubber to unsaturated rubbers could be accomplished by blendingtheepichlorohydrinwith25-50phr of polychloroprene.Althoughfewstudies investigate the surface composition. it undoubtedly has a large role in determining the adhesion obtained with rubber blend stocks. This adhesion for pure rubber stocks has been shown to depend upon the number and length of the interphase, or interconnecting, strands (Bhowmick and Gent, 1984).
5.5 Hysteresis Low hysteresis may be obtained in a rubber stock by a variety of methods (e.g., reduced carbon black loading or higher crosslink density). which, however. are accompanied by a sacrifice of other aspects of performance, in particular the ultimate properties. Blending of elastomers, on the other hand, affords a means to achieve lower hysteresis with a more tolerable compromise of other properties. The hysteresis of a blend is often found to be lower than the weighted average of the components. This effect is most notable in filled systems in which the carbon black distribution is nonuniform. The phase with the lower carbon black loading will have both reduced modulus and lower hysteresis. Particularly when this softer phase is the continuous phase, a low blend hysteresis is observed (Hess and Chirico, 1977). The origin of this effect can be examined by introducing a nonuniform distribution of carbon black into a single-component stock by delaying addition of a portion of the polymer until after the carbon black has been well dispersed in the initial portion (Nguyen, 198 1 ). The hysteresis measured on a series of compounds prepared by thismethod is displayed in Figure 11, alongwiththeappropriately weighted sum of thehysteresisgenerated in twostocks with the full carbon black loading
0 20r
t 0
IO 20 RUBBER V O L . W O ) ADDED I N 2 n d STAGE
Fig. 11 The ratio of the dynamic loss and elastic moduli of an SBR with 105 phr carbon black, in which a portion of thc rubber was added afterthe black was mixed into the stock. The solid curve wascalculated by assuming lincar additivity of thc hystcrcsis measured for the corresponding filled and gum compounds. (From Nguyen. 198 1.)
220
Roland
0.201
l
I
20
0 VOL
C A R BBOLNA C K
J
40
(N-339)
Fig. 12 Thc phase anglc measurcd for SBR ;IS ;I function of carbon black loading. At the higher filler levels the mix quality becomes progrcssivcly poorer. (From Nguyen, l98 1.)
with without carbon black, respectively. It can be seen that most of the hysteresis reduction accompanyinganonuniformdistribution of carbonblackcan be attributed to thenonlinear relationship between hysteresis and carbon black loading, particularly at very high loadings (see Fig. 12). Another development in the area of hysteresis reduction is the finding of Keller (1973) that the addition of a small amount of chlorobutyl rubber (which itself is relatively hysteretic) to NR-BR blends resulted in tire treads with lower rolling resistance. Afragon et al. (1980)have patented a specific blend ratio of these rubbers to constitute a tread stock in which the decrease in hysteresis is not accompanied by the expected reduction in wet traction. Evidently the blend of a resilient rubberwith the halobutyl rubber produces a material with the appropriate frequency dependence of its energy loss, that is. high resilience at the low deformation rates associated with rolling and high hysteresis at the higher rates accompanying skidding on wet surfaces. Hirakawa and Ahagon ( 1982) have also reported that by introducing a nonuniform distribution of carbon black into these blends. further reductions in rolling loss can be attained.
5.6
Failure Properties
An improvement in tear strength. cut growth resistance, fatigue life. and ozone cracking resistance can all be realized from the blending of elastomers. including attainment of a level of performance that exceeds that of either pure component. Synergism in the strength properties of miscible blends might be expected from a potentially greaterchain density, but heterogeneous blends can likewise exhibit markedly superior ultimate properties. An importantaspect of rubber-rubberblends is the nature of theinterphasebonding. Although even the presence of voids (or a completely unbound dispersed phase) can toughen a material by reducing the stress concentration through blunting of the crack tip. the mechanical integrity of an intercrosslinked morphology will usually lead to superior performance. In blends of SBR and chlorobutyl, for example, a threefold increase in fatigue life was obtained by the introduction of interphase crosslinking (Bauer, 1982). Similarly, providing for interfacial coupling improves the tensile strength of EPDM-silicone rubber blends (Mitchell, 1985). Therelative
Rubber-Rubber Blends I
221
magnitude of the interfacial bonding compared tothe cohesive strengthof the rubbers themselves can influence the blend performance. Hamed (1982) has demonstrated that when the bonding is weak enough to promote deviation in the direction of crack propagation, blends of EPDM and BR exhibit greater tear strength than either pure component. When, on the other hand, the interfacial bonding is sufficiently strong, the crack is not deviated but proceeds through the particle. In this case the tear strength of the blend is found to be intermediate between those of the individual rubbers. Blends of cis-l. 4-BR and syndiotactic 1.2-BR prepared in a proprietary fashion are reputed to have exceptional resistance to tearing and cracking. There is no chemical bonding between the polybutadienes, and it has been suggested that an interpenetration of the phases is the origin of the property improvements (Buckler et al., 1982). When scrap rubber is blended with another elastomer, it is usually found that tensile strength and fatigue life are poor, due primarily to an absence of interphase crosslinking. When additional carbon black is incorporated into the blend, however. this interfacial adhesion increases along with substantial improvement in ultimate properties (Phadke et al., 1984). In multiphase rubber blends, the ultimate properties will to a large extent reflect those of the continuous phase. Low-temperature fatigue performancein particular will be radically altered by a phase inversionif the temperature is intermediate between the component T,s. The continuous phase must be elastic if the blend is to remain flexible. In general, a stronger continuous phase will give rise to a stronger blend. For example, when a nonuniform distribution of carbon black is present,greatertearresistance is found for NR-BR and NR-SBR blends when the reinforcingfiller is depositedprincipally in the continuous phase (Hess andChirico,1977). Similar effects on the cut growth resistance of rubber blends have been reported (Lee. 1982). When co-continuity exists in NR blends. the expectation is that greater strength will be obtained when the reinforcing filleris present in the second component, since the ability of NR to crystallize upon extension confers a measure of self-reinforcement that is lacking in rubbers such as BR and SBR. Blending of unsaturated rubbers with, for example, EPDM is a long-recognized method of obtainingresistance to ozone crackingwithoutthe use of staining or expensive antiozonants. Matthew ( 1984) has reported that in heterogeneous blends of this type a balanced distribution of filler is necessary for the greatest ozone resistance. The tendency for the continuous phase of a blend to be the lower-viscosity material, and moreoverforthe high deformations existing on the surface of aflowingmass to cause the surface to be richer in the lower-viscosity component, can influence the performance of a blend subjected to flexure. The component of the blend vulcanizate that was less viscous during the processing will be subjected to the large surface strains, while the more viscous rubber will be located more in proximity to the neutral axis. where strains are minimal. By controlling the relative viscosities of the component rubbers. improvements in tlex performance can in this manner be realized. It was pointed out in Section 5.2 that when the test temperature is intermediate between the T,s of the blend components, the modulus is largely influenced by the glassy phase. while the ultimate elongation may remain close to that of the rubbery material. A similar increase in toughness. or fracture energy. can be achieved with miscible blends of high and low molecular weight elastomers in which the network is achieved by end-linking the chains. These can be identical chemically. For example, when low. molecular weight(<1000 g/mol) PDMSis blended with a PDMS of number-average molecular weight equal to 18,500. Llorente et al. ( 1 98 1 ) found that the energy under the stress-strain curve for end-linked networks is markedly increased (Fig. 13). This can be achieved by virtue of the limited extensibility of the short chains providing for high drawing stresses, while the high elongation associated with high molecular weight is retained. In some fashion the presence of the long chains restrains the fracture of short chains from propagating into ultimate failure.
222
Roland
0.81
06 m E E
U
m
0
b " "
0
80
2
100
M O L % SHORT CHAIN5
Fig. 13 Energytotensilebreak of PDMS end-linked bimodal neta networks composed 18,500 numberof 18,500 222, (0) I100 average molecular weight chains mixed with short chains of M,, equal to) . ( ;A)1100 660. and (A) g/mol. respectively. (From Llorente et a l . , 1981.)
ACKNOWLEDGMENTS This work was supported by the Office of Naval Research.
REFERENCES Ahagon, A., Misawa, M,. Miyasaka, K., and Hirakawa, H. (1980). Brit. Pat. Appl. GB 2,046.276 (to The Yokohama Rubber Co.). Arendt, B. H., Kannnn. R. M,, Zcwail, M,. Kornficld, J. A.,,gnd Smith, S. D. (1994). Rheol. Acttr 33:322. Avgeropoulos, G. N.. Wcissert, F. C., Biddison, P. H., and Bohm, G. G. A. (l976), R h h e r Chern. Trc11r1ol. 4993. Barager. H. J. (1983). Rubber Div. Meeting, ACS, Houston, TX. Paper No. 78. Barrier. J. (1955). Rul?ber Cherrz. Teclrrwl. 2 8 3 14. Bartenev. G. M,. and Kongarov. G. S. ( 1963), Ruhbrr Cllcwr. Trchol. 36:668. Barthes-Biesel, D.. and Acrivos, A. (1973), J . Fluid Mech. 6 / :1. Bauer. R. F.. and Dudley, E. A. (1977), Rlthhor Cherrr. Tec.hr~ol.50:35. Bauer, R. F. ( 1982). P o l w ~ Er~g. . Sci. 22: 130. Bentley. B. J. (1985). Ph.D. thesis, California Institute of Technology, Pasadena. Bhowmick. A. K., and De. S. K. ( 1980), Rubher C/~err~. Trchr~ol.53:960. Bhowmick. A. K., and Gent, A. N. (1984). Ruhher C/wrrI. Trchrlol. 57216. Bohm, G. G. A. ( 1980).Conf. on Eng. Sci. Foundation, Asilomar. CA. Bohm, G. G. A., and Tveekrem. J. 0. (1982). Rublx~rC/retrl. Trchrrnl. 55:575. Braun, H. G., and Rehagc, G. (1985), ArIgcl\t'. Mrrcrotr~nl,Cherrl. 131:107. Buckler, E. J.. Shackleton, J., and Walker, J. (1982). Eltrsforrrerirs 1417. Cnllan, J. E.. Hess. W. M,, and Scott, C. E. ( 1 9 7 1 Ruhher ~ Cherrr. Techr~ol.44:814. Chaffey. C. E.. and Mason, S. G. (1966), J . Colloid b ~ t r r f k Sci. t ~ 2/:254. Chen, Y. T. (197.3). J. P d y ~ Sci, , Polyrt. P11y.v. Ed. /1:2013.
Rubber-Rubber Blends I
223
Clark, H. A. (1970). U.S. Pat. 3,527,842. Colby, R. H. (1989). P o / w e r 30:1275. Corish, P. J., and Powell, B. D. W. (1974). Rubher Cherr~.T f ? C / l f l ~ J /47:481. . Dickie, R. A. (1979), Po/ytn. Eng. Sei. 191042. Erman. B., Queslel, J.-P. and Monnerie, L. (1988), P o / w e r 2Y: 13. Everage, A. E. (1973), Trnrls. Soc. Rhcwl. 17629. Falcette. J. J., Fnends, G. D., and Niu, G. C. C. (1975), Ger. Pat. 2,518,904. ~. 46:l 193. Folt, V. L., and Smith, R. W. ( 1 9 7 3 ~Rubber C h e r ~Techr~ol. Freakley, P. K. (1985). Rubber Processir~gc m / Prorluction Corltrol. Plenum Press, Ncw York. Frled, J. R., MacKnight, W. J., and Karasz, F. E. (1979). J. Appl. Phys. 50:6052. Fried, J. R. ( 1983). in D~~\v/oprrlerl/s i r l P o / w e r Chartrc./erizcrtiorl. Vol. 4 (J. V. Dawkins, Ed.), Applied Science Pub., New York, p. 39. Frisch, H. L., and Klempner, D. ( 1970). Rubber Ckern. Techno/.43:883. Fuller. G. G., and Leal, L. G. (1981), J. Po!vrrr. Sei. PO/VII.P h y . Ed. 1Y:557. Gardiner. J. R. (1968), Rubber Cltern. T e c h r d . 41: 13 12. Gergen, W. P., Davison, S., and Lutz, R. G. (1985), Rubber Div. Meeting, ACS, Los Angclcs, paper No. 2. Ghijsels, A. ( I 977). R~rbherChew. T e c h o l . 50:278. Clatter. 0.. and Kratky, 0. ( 1982). Smd/-Ary$e X-Ray Sccrtterirl,g, Academic Press, Ncw York. Grace, H. P. (1982), Clrem. ErIg. Corwn. 14225. Hamed, G. R. (1980), Rubber Division Meeting, ACS, Detroit, paper No. 38. Hamcd, G. R. (1982). Rubber Cl~err~. Techol. 55:15 1. Hashimoto. K., Harada. T., Ando, I., and Okubo, N. (1970), J. Soc. Rubber I n d . Jctpclrl 43:659. Hashimoto, K., Miura, M,, Takagi, S., and Okamoto, H. (1976), / u t . Po/yrr~.Sei. T e c h ) / .3(7):84. Hashin. Z. (1964), in Secwrld Order &flec./.s i r l Elrtsticih, P/ct.stici/y c w / Fluid Dyrrctrrlics (M. Reincr and D. Abir, Eds.), Macmillan, New York. Hetland, E., and Tagami, Y. (1972), J. Chum. Phys. 563592. Heitmiller, R. F.. Maar, R. Z., and Zabusky, H. H. (1964), J. Appl. P O / ~ / ISci. I . 8:873. Hess, W. M,. and Chirico. V. E. (l977), Rubber Chern. Techlo/. 50:301. Higgins. J. S., and Stein, R. S. (1978), J. App/. Crv.s/. 11:346. Hildebrand, J. H., Prausnitz, J. M,, and Scott, R. L. (1970), Regltlnr nrld Re/r/ed So/uf;orl,s, van Nostrand, New York, p. 98. Hindmarch, R. S., and Gale, G. M. (1983), R d h 2 r Cherll. TC.C/Itl(J/. 56:344. Hirakawa. M,, and Ahagon, A. ( 1982), Tire Sci. Techr~o/. /0:16. Huson, M. G., McGill, W. J., and Swart. P. J. ( 1984), J. P o / m Sci. Po/?w. Lett. Ed. 22:143. Inoue, T., Shomura. F. Ougizawa, T., and Miyasaka. K. (1985), Rubber Div. Meeting, ACS, Los AIlgeles, paper No. 14. Jarry, J.-P., and Monnerie, L. (1979). M~tcrorrlolec~tles 12:316. Kaplan, D. S. (1976). J. App/. Po/vrrr. Sei. 20:2615. . Keller. R. C. (1973). Tire Sei. T ~ c h n o /1:190. Kirste. R. G., and Lehncn. B. R. (1976). M t r k r o r d . Cherrl. 177:1 137. Kleiner, L. W., Karasz, F. E., and MacKnight, W. J. (1979). Po/yrrl. G 1 g . Sei. 19:519. Kok, C. M., and Rudin, A. (1982), J. A/>/)/.P o / m Sei. 27:353. Kornfield, J. A., Fuller, G. G., and Pearson, D. S. (1989), Mocrorr1nlre14le.~ 22:1334. &:use, S. ( 1972). J, Mocrorlld. Sci.-Rev. M r r u o r w l . Ckerll. C7:25 1. Kruse, J. (1973), Rubber C h e r ~ Techr~ol. . -16:653. 7667. Kwei. T. K., Nishi, T., and Roberts, R. F. (1974), Mac.rorrlo/ec~~/e,s Leblanc, J. L. ( 1982). Pkrst. Rubber: Process A/)/>/.2:361. Lee, B. L. (1981 ), P o l y . E q . Sei. 2/:294. Lce, B. L. (1982), Thirteenth Akron Polymcr Conference, Akron, OH. Lipatov. y. S., Shumsky. V. F., Gorbatenk, A. N., Panov, Y. N.. and Bolotnikova. L. S. (1981), J. App/. P d y r 1 . Sei. 26:499. Livingston, D. I., and Rongonc, R. L. (1967). Proc. Int. Rubber Conf., Brighton. England, paper No. 22.
224
Roland
Llorente, M. A., Andrady, A. L., and Mark, J. E. (1981), J . Polvrrl. Sei. Polyrrl. Phys. Ed. 18:621. Lohr, D. F., and Kang. J. K. (1975). US. Pat. 3,928,282. McDonel, E. T., Baranwal, K. C., and Andries, J. C. (1978),in Polyrrler Blends (D. R. Paul and S. Newman, Eds.), Academic Press, New York. p. 263. McMastcr. L. P. ( 1973). Mrrcrorrlolecules 6:760. Maksimova, N. S . , and Shvarts, A. G. (1984), I n [ . Polym. Sei. Techr~ol.I I ( 11):20. Marsh, P. A., Voet, A., and Price, L. 0. (1967), Rubber Cl~errl.Techr~ol.40:359. Marsh, P. A., Voet, A., Price, L. O., and Mullens, T. J. (1968), Rubber Clwttt. Techr~ol.41:344. Mastromatteo, R. P,, Mitchell, J. M,, and Brett, T. J. ( 1971 ), Rubber Cheru. Techrlol. 44: 106.5. Matthew, N. M. (1984), J. P o l y m Sci. Polvm. Lert. Ed. 22: 13.5. Millcr, J. B., McGrath. K. J., Roland, C. M,, Trask, C. A., and Garroway, A. N. (1990), Mctcrorrlolecules 23:4.543. Mitchell, J. M. (198.5), Ruhher Plast. Neltrs, June 3, p. 18. Moritani, M., Inoue, T., Motegi, M., and Knwai, H. (1970), Mctcrorlloleculrs 3:433. Morrissey, R. T. (1971), Rubber Cherr~.Techr~ol.44:1029. Murray, C. T., Gilmcr, J. W., and Stein, R. S. (198.5). M~!crorr~olrculr.s /8:996. Nelson, C. J., Avgcropoulos, G. N., Weissert, F. C., and Bohm, G. G.A. (1977), A I I ~ ~ H M ,r r. c m r r d . Cl~ern. 60161:49. Newman, S. (1973). Po/ym. P l m t . Techno/.E r ~ g 2:67. . Nguyen, M. N. (1981). unpublished results. Nielson, L. E. ( 1 974). Mechctrlicd Properties of Polyrwrs and Cor,lpo.si/e.s, Marcel Dekker, New York. Nielsen. L. E. (1978), Predictir~gt h e Properties ofMixrures. Marcel Dekker, New York. Nishi, T. (1978), Rubber Cherrl. Techrlol. 51:107.5. Orwoll, R. A. (1977). Ruhbrr Clrerrl. Trcllrrol. 50:4.51. Paddock. C. F. (1973). U.S. Pat. 3,758,435. Patterson, D. ( 1969). Mtrcrnrtloleculrs 2:672. Patterson, D., and Robard, A. (1978), M~tonrrlolrculrsI1:690. Patterson, D. ( 1982), Polvrrr. B I S . Sci. 22:64. Paul, D. R. ( 1978), i n Polymer Bletlrls, Vol. 11 (D. R. Paul and S. Newman. Eds.). Academic Press, New York. Paul, D. R., and Barlow, J. W. (1984). Polyrtwr 25:487. Phadke, A. A . , Chakraborty, S. K., and De. S. K. (1984). Ruhber Clrerrl. Techr~nl.5 7 19. Pieski, E. T. (1960). in Po/yther~e(A. Renfrew and P. Morgan, Eds.), Wiley-Interscience, New York, p. 379. Pipkin. A. C., and Tanner, R. I. ( 1977). A m . Rev. Fluid Mech. Y: 13. Plastics Cornpoundirl,g ( 198 l ), 4:20. P/it.stic.s Corrl/’ourldirl
Rubber-Rubber Blends I
225
Roland, C. M,, Miller, J. B., and McGrath, K. J. ( 1993). Mtrcrorlrole~clrles26:4967. Roovers, J., and Toporowski, P. M. (1992), Mtrcrorrlolc~t~rtlr.s 25:3454. (K. Sole. Ed.), Harwood Academic, Sanchez, I. C. (1982), in P o l y e r Comptrfi/i/it,vt r r d Irleorrf/~~rlihili!\. Ncw York, p. S9. Sax, J. E., and Ottino, J. M. (1985), P o l w e r 26: 1073 Shih, C. K. (1976), P o l w . Erlg. Sei. 16:742. Shutilin. Y. F. (1982). I n t . Polvrr~..Pi. Teclfrrol.9(4):45. Sircar, A. K., Lamond, T. G., and Pintcr, P. E. ( 1974). Ruhhrr Clwrrl. Techrfol.47:48. Sircar, A. K. (l981), Rd?bc~rChrrrr. Ttdrrml. 54:820. Smith. R. W.. and Andrics, J. C. ( 1974). Ruhher Cherrl. Tec.hrfol.47:84. Sotta. P,, Deloche, B., Hcrz, J., Lapp, A., Durand, D.. and Rnbadeux. J.-C. ( 1987), Mnc~,o,rlolrcult.s20: 2769. Sperling, L. H., and Friedman. D. W. (1969), J. Polyrrl. Sei. A-2 7:425. Sperling, L. H. ( 198 1 ), Irlrc~r~~c,rl~frtrtirl~~ Polyrrwr N e ~ ~ ~ trrd r k Reltrtetl s Mtrtrritrls. Plcnum Press, Ncw York. 10:708. Su, C. S., and Patterson. D. ( 1977). M~rc~rnrrrolrcrt/c,.s Tokita, N. ( 1977), Rlrhher Chrnr. Trchrlol. 50:293. Tomlin, D. W., and Roland, C. M. (1992). Mtrcrnrlrolrclrles 25:2994. Tomlin, D. W.. and Roland, C. M. (1993), P o l w c v 342665. Torza, S., Cox, R. G., and Mason, S. G. (1972), J. Colloid. Inter:frrcr Sei. 3(3:39S. Trask, C. A.. and Roland. C. M. ( 1988), Po!\.rrw Corrlru. 29.132. Ucda, A.. Watanabe, H., and Akita, S. ( 1985), International Ruhbcr Confcrcncc, Kyoto, Japan. paper No. 16A09. Uemura, S., and Takayanagi. M. (1966). J. Appl. Polyrrf. Sci. 1O:l 13. Ulmer. J. D.. N~UYCII, M. N., and Nelson, C. J. (1985). unpublishcd rcsults. Van Amcrongcn, G. J. (1964). Rrthhrr Clwrrl. Trchrlol. 37: 1065. Van Oenc, H. (1972). J. Colloid Irlrrrjke Sci. 40:448. Van Oene, H. (1978), Polsrrlrr Blerrtls. Vol. l (P. Ncwman, Ed.). Academic Press, Ncw York, p. 295. Walters, M. H.. and Keyte, D. N. ( 1965), Rlrhher Cl~cw.T(,c.hnol. 38:62. Wendorff, J. H. ( 1980). J. P o l w . P i . Polyrrr. Phys. Ed. 18:439. Weisscrt. F. C., and Avgeropoulos, G. N. (1977), J . E/tr.storrwr.s Pltrsr. Y:102. Woods, M. E., d Mass, T. R. (1975). At/\*. Chrrr~.Srr. 142:386. Yoshimura, N., and Fujimoto, K. (1969), Rubbrr Clwrrl. T d l r r o l . 42:1009. Zapp, R. L. ( 1973). Ruhlwr Chrrn. Tc>c.hrfol.46:25 I , Zemel. I. S., and Roland, C. M. ( 1992a). Polyrrwr 33:.1427. Zemcl. I. S.. and Roland, C. M. ( 1992b), Polyrrwr 33:4S22.
This Page Intentionally Left Blank
Rubber-Rubber Blends: Part H, New Developments C. Michael Roland Naval Research Laboratory, Washington, D.C.
1. INTRODUCTION Polymer blends continue to elicit great interest. Since Chapter 7 was first published, a number of reviews of polymerblendshaveappeared (Roland, 1989;KohudicandFinlayson,1989; Utracki, 1989; Coleman, et al., 1991; Miles and Rostami, 1992), including a couple that focus on phase-separated systems (Sperling, 1981; Utracki and Weiss. 1989; Hess, et al., 1993). This chapterreports developments in thefield since 1986,with an emphasis on selectedaspects relevant to rubbery materials.
2. 2.1
ANALYTICAL TECHNIQUES Thermal Analysis
Differential scanning calorimetry (DSC) has a long history in the study of blends (for a recent review, see Sircar, 1997). Close-lying glass transitions can be better resolved using derivative DSc, with the peak areas reported to reflect the concentration of the components (Sirear et al., 1993). Derivative DSC has been applied to interpenetrating networks (Mishra et al., 1994) and to the comparison of physical and miscible blends (Hourston et al.. 1995). A newer technique is the thermally stimulated current (TSC) method, which relies on quenching a polarized sample, then monitoring the release of current upon subsequent warning. TSC has been used to study rubber blends (Ibar, 1993), yielding results similar to DSc.
2.2
Solid-state NMR
Guo ( 1996) recently reviewed the application of high-resolution, solid-state nuclear magnetic resonance (NMR) for investigating polymer blends. There exist a variety of solid-state NMR methods. I3C-NMR has been applied to the identification of the components in phase-separated rubber blends by Gross and Kelm (1985, 1987) and used to to quantify the component ratio in SBWEPDM blends by van der VeldenandKelm (1990). Miscibility in EPDM andatactic polypropylene blends, up to 45% of latter, was demonstrated using 'jC-NMR by Da Silva and Tavares (1 996). 227
Roland
228
Miller et al. (1990) pioneered the use of cross-polarization, magic angle spin (CP-MAS) I3C-NMR to resolve the component dynamics in blends. They applied the method to blends of 1,2-Br andIR. Chunget al. (1994a) used two-dimensional deuteron NMRto resolve thedynamics in similar blends.
2.3 '29Xenon-NMR The inertness of xenon and the dependence of its chemical shift on local environment make ""Xe-NMR an attractive technique for probing blend morphology (Walton, 1994). This is particularly true for rubber blends, because above T,, Xe readily diffuses between sites, which collapses the inhomogeneously broadened NMR into a narrow line.The homogeneous morphology of a miscible blend gives rise to a single peak in the xenon NMR spectrum. On the other hand, aphase-separated blend exhibits two resonances,unlessthephases are so small ["OS p m according to Walton et al., (1992)l that xenon can exchange among them on the NMR timescale (- miliseconds). The chemical shift of xenon has been interpreted in terms of van der Waals interactions by Miller et al. (1993) and free volumeby Cheung and Chu (1992). Most importantly for blends, it does not depend on the glass transition temperature. This means that blends of nearly equal T, can be usefully studied, whereas many other, more common, techniques would be inapplicable. This approach has been employed by Walton et al. (1992) to demonstrate that a blend of polychloroprene and 25% epoxidized polyisoprene, for both of which T, = - 34°C forms a miscible mixture (Fig. 1). Walton et al. (1993) followed the mutual interdiffusion of 34% cis-1A-polyisoprene and 66% 1,4-polybutadiene ( M , = 23,600) in a blend that had been cooled below the lower critical
EPlZ5
,
?
?
220
210
200
190
I R0
Chemical Shift (ppm) Fig. 1 NMR spectrum of ""Xe dissolved in blends of polyisoprene and polychloroprene (top), POlYiSOprene and 25% epoxidized polyisoprene (bottom), and polychloroprene and 25% epoxidized polyisoprene (middle). Onlythe latter is homogeneous onthe segmental level, as evidcncedby the single NMR resonance. (From Walton et al., 1992.)
229
Rubber-Rubber Blends: Part II, New Developments
46 hours aftcr heatlng
220
215
210
205
200
195
190
Chemical Shift (ppm) Fig. 2 ‘”’Xe-NMR spectra of xenon dissolved in themiscibleblend of 3 4 8 cis-1.4-polyisoprene and 66% 1.4-polybutadiene (bottom), at various times after phase separation induced by heating above the LCST (middle spectra), and corresponding to complete phase separation (top). Complete remixing of the components has not yet been observed, although after just 2 days the blend has completely homogenized according to DSc. (From Wnlton et al., 1993.)
solution temperature (LCST) following spinodal decomposition (Fig. 2). Notwithstanding the persistence of :I phase-separated structure as directly evidenced by the NMR data,DSC measurements no longer sensed the presence of distinct phases. At least for this particular blend, xenon NMR is a higher-resolution probe of phase morphology than the more conventional DSC technique.
2.4
MechanicalBehavior
Empirical modeling of stredstrain data from phase-separated rubber blends, based on the assumption of parallel coupling (additive stresses), has been proposed by Bauer and Crossland (1988). The approach fails for filled rubber (Bauer and Crossland, 1990). It has been shown that secondary plateau modulus and the relaxation time of phase-separated blends reflect deformation of thedispersedphase(Palierne,1990;Graeblinget al.. 1993). Such datacanallow determination of the interfacial tension in blends. For miscible blends, the situation is more complex. One complication comes from the ‘hematic interaction,”whereby an orientedmatrixinducesalignment of chains that would otherwise relax to isotropy (Faivre and Monnerie, 1985; Sotta and Deloche, 1987; Saito et al., 1988; Doi et al.. 1989: Cifra et al.. 1991). In addition to affectingtheorientation of blend components, this effect has consequences on network elasticity (Dubault et al., 1987: Erman et
Roland
230
al.. 1990). Blends of NR and 1.2-polybutadiene exhibit this nematic interaction, as shown from infrared dichroism experiments (Zemel and Roland, 1992a; Arendt et al.. 1997). The segmental relaxation in miscible rubber blends were followed using combined mechanical and dielectric spectroscopy (Alegria et al.. 1994; Ngai and Roland, 1995). The more polar polybutadiene dominates the latter, but interestingly the mechanical loss peak was due almost entirely to the NR component. By combining stress and optical measurements, thecomponent terminal dynamics in similar blends can be resolved (Arendt et al., 1994; Zawada et al.. 1904).
2.5
MiscellaneousTechniques
Electron microscopy has longbeen used to study the morphology of polymer blends. A technique for executing scanning transmission electron microscopy using standard SEM equipment has been described by Cudby and Gilbey(1995). This complements tranmission electron microscopy, although at lower resolution. Dias and Galuska (1996) used static secondary ion time-of-flight mass spectroscopy to image rubber blends. Curatives and their distribution can also be identified by mapping specific curative ions onto the polymer phase information. Ultrasound nleasurements have been used t o characterize the modulus and viscoelasticity of polymers. For miscible blends, the ultrasonic velocity is expected to be the average of that of the pure components; thus, incompatibility canbe identified (Sidkey et al. 199 I ) . Additionally. Sidkey et al. ( 1 992) showed that incompatible blends with poor interphase adhesion exhibit large attenuation coefficients. The composition of SBWBR blends has been deduced by Amraee et al. ( 1996) from thermogravimetric data through the use of calibration curves.
3.
MULTIPHASEELASTOMERS
3.1 CurativeandFillerDistribution Achieving sufficient reinforcement of both phases of a blend is necessary to optimize physical properties. Hess ( 199 1 ) reviewed methods of characterizing the dispersion of fillers in polymers. Since incorporation of filler reduces height of the loss tangent peak, its relative lowering in blendscan be used to estimatefillerdistribution (Maiti et al., 1992). An obvious means to control filler distribution is through the mode of addition of carbon black or silica (Bhaumik et al., 1988: Varughese and Tripathy. 1993). Obtaining a uniform degree of crosslinking in rubber blends is made difficult by curative migration. Precuring the more saturated component prior to blending has been shown by Suma et al. (1993) to alleviate the crosslink misapportionment. yielding better physical properties. Cocuring specifically of XNBR and ACM (polyacrylic rubber) can be optimized when vulcanizing with PbO by using either ethylene thiourea or mercaptobenzothiazole (Chowdhury et al.. 1995). Tinker ( 1 995) reviewed the various methods for assessing the distribution of crosslinks in blends. The use of high field (300 MHz) NMR to determine crosslink distribution in rubber blendshas been described (Brown etal.. 1992: Ellulet al.,1995).This is an extension of earlier work of Loadman and Tinker (1989)using 90 MHz proton NMR, which correlated peak broadening with crosslink density. Higher field strength NMR n~easuretnentsreveal that the chemical shift is slightly crosslink density dependent. This must be taken into consideration when interpreting spectra with overlapping peaks.
Rubber-Rubber Blends: Part II, New Developments
231
3.2 Spontaneous Crosslinking Thermal oxidative crosslinking is generally regarded to be a problem, leading to embrittlement of an elastomer, thus limitingits service life. On the other hand, De and coworkers have generated a plethora of studies in which they exploit this effect to obtain interphase crosslinking in blends without added curatives. They mold various rubber blends for an hour or more at temperatures as high as 200°C. Examples include blends of CR and XNBR (Mukhopadhyay and De, 1991, 1992), ENR and zinc-sulfornated EPDM (Manoj et al., 1993, 1994), polyacrylic acid and ENR (Mallick et al., 1993). and chlorinated NR and carboxylated NBR (Ramesh and De, 1992).
3.3
Compatibilizing Agents
Improved compatibility. which refers to smaller, more interconnected phases and better material properties,can be achievedthroughthe use of compatibihzingagentsor through chemical modification of the components. This approach has been attempted for various rubber blends. Acrylamide-grafted PDMS shows greatercompatibility with sulfonated EPDM (Kole et al., 1994a) or EPDM modified with maleic anhydride (Kole et al., 1996). Jansen et al., (1996) used mercapto modification of EVA to improve the physical properties of blends with NR. Some efforts at compatibiiizing EPDM and PDMShave been described (Kole et al.. 1993, 1994b). Similar attempts for EPDM blended with NR by Coran (1 988) and BR by Go and Ha (1996) have been reported.Thiophosphoryldisulfidehas been used asacouplingagent for XNBR blended with SBR blends (Biswas et al., 1995) and NR (Naskar et al., 1994).
3.4
Physical Properties
The cut growth and fracture behavior of NR/BR (50/50) blends were found by Hamed et al. ( 1996) to be qualitatively similar to that of pure NR due to the overriding effect of strain crystallization. Lee and Moet ( 1993)found that microcracking and crack tip roughening contributed to the toughness of this blend system. The effect of structure on the properties of blends of segmented polyurethanes (which are multiblock copolymers of repeating hard and soft segments) was investigated by Takigawa et al. (1996). Matsumoto (1996) swelled blendsof NBR and SBR with lithium salt solution. yielding films with high ionic conductivity. In addition to their incompatibility, PDMS and EPDM have different aging behavior. For example, hydrolysis causes PDMS to degrade, whereas EPDM crosslinks. Kole et al. ( 1 9 9 4 ~ ) compared the relative hydrolytic stability of these two rubbers and their blend. Bhowmick et al. (1995) observed that PDMS diffuses to the surface during aging of this blend.
4. 4.1
MISCIBLE BLENDS Rheology
The effect of chain architecture on miscibility has been investigated by a number of groups. Kuo et al. (1994) reported that linear PMPS and cyclic PDMS form a more miscible (lower UCST) blend than the corresponding lineadlinear mixture. Similarly. higher LCST were measuredforvariousbranched/linearblends compared to IinearAinear ones(Faust etal.,1989; Russell et al., 1990). On the other hand, Chai et al. (1995) found that long branches can reduce miscibility. This can affect the phase morphology developing during gelation of blends (Clarke et al.. 1995).
Roland
232
Garas and Kosmas (1994) demonstrated that architectural effects are reduced for larger molecular weight components. For lowmolecular weights, there canbe a significant contribution to the thermodynamics from end groups, as seen in PDMS isotope blends by Beaucage et al. (1996) and PDMSPMPS blends by Kuo and Clarson (1992, 1993). The rheology of mixtures of components havingdifferentarchitectures have received considerable attention. For cyclic PDMS (1 2-40 skeletal bonds per ring) blended with unentangled linear PDMS, Orrah et al. (1988) unsurprisingly found that the viscosities, which were all Newtonian, deviated from a simple additivity rule. Higher molecular weight linear PDMS chains, on theother hand, appear to “thread” cyclic PDMS, forming a new type of entanglement (Cosgrove et al., 1996). Dielectric spectroscopy of polyisoprene i n blends has received considerably attention. The normal mode of PI is dielectrically active (type A chain); thus, its terminal motion can be easily resolved in blends with a component having no dipole moment parallel to the chain backbone (type B polymer). Much of this work is directed at examining theoretical predictions concerning the low-frequency dynamics of entangled chains. Watanabeet al. (1991) measured the dielectric spectrum ofPI probe chains in 1,4-BR of varying molecular weight. Both the shape of PI’S relaxation function and its dependence on the molecular weight of the BR were at odds with reptationtheory. This same system has been studied by Adachi and coworkers (1995), who found that as the molecular weight of the host BR increased, the relaxation function broadened. This result is also contrary to the prediction of reptation concerning constraint release effects (Schroeder and Roland, 1999).
4.2
PIIPVE
Since it is mostfruitful to investigatesystemsinwhich the structureand dynamics are not complicated by specific (chemical) interactions, among the many miscible polymer mixtures, one of themostinteresting is the blend of polyisoprene (PIP) (or natural rubber)and1,2polybutadiene (poly(vinylethy1ene) (PVE). First reported by Bartenev and Kongarov (1 963) in the Russian literature, miscibility was subsequently demonstrated to occur over the entire composition range of this van der Waals mixture (Cohen, 1982; Roland, 1987; Trask and Roland, 1988, 1989; Kawahara et al., 1989). Consistent with this result, NMR indicated homogeneity of the blend on the molecular level (Miller et al.. 1990; Bahani et al., 1995). From small angle neutron scattering measurements, Tomlin and Roland ( 1992) determined the interaction parameter, which provides a measure of all noncombinatorial entropy contributions to the mixing free energy, to be negative, despite the absence of specific interactions. The origin for the negative enthalpy was revealed by NMR measurements of the crossrelaxation rate TG’, from which it was concluded that the distance separating the carbon and proton nuclei was reduced by blending (Roland et al.. 1993). This means that the critical interatomic distances, on which the van der Waals energy is strongly dependent (approximately to the 6th power), are reduced upon blending from that of the pure components. This NMR result is not easily reconciled with wide angle x-ray scattering data of Halasa et al. (199 I ) , from which it was deduced that the interchain spacing is greater in the blend. Kawahara et al. (1994) reported this blend to have a negative excess mixing volume. “Dynamic heterogeneity,” wherein the components of a miscible blend exhibit verydifferent dynamics notwithstanding the homogeneous morphology, was first discovered in PIP/PVE blends (Miller et al., 1990; Roland and Ngai, 1992a) (Fig. 3). Through combined mechanical and dielectric spectroscopies, the individual relaxation times for the PVE and PIP components were actually determined and shown to be consistent with a blend model (Alegria et al., 1994). These resultsweresubsequently confirmed by two-dimensionalNMRmeasurements on the same system (Chung et al., 1994b; Ngai and Roland, 1995).
x,
Rubber-Rubber Blends: Part 11, New Developments
233
Fig. 3 The vinyl region of the "C-NMR spectrum of 32% polyisoprene/68% polyvinylethylene at 20K intervals. At temperatures at which the motions are on the NMR time scale, the resonances broaden. This occurs for the polyisoprene at lower temperatures than for PVE. (From Miller et al., 1990.)
Roland
234
Natural rubber’s status as the most important rubber commercially is due at least in part to its propensity for strain-induced crystallization. In miscible blends with PVE, both the extent of crystallization, as well as the crystallization rate, of NR are suppressed (Zemel and Roland, I992b). More anomalouswas the observation that the magnitude of the crystallization suppression in the NR was independent of PVE concentration over a range of 5-50% PVE. Furthermore, whereas a-lamellae predominate forpure NRcrystallization,blending with highmolecular weight PVE causedpreferential development of theP-lamellar form. This alteration of the crystallization behavior is dueto the ability of the P-lamellae to more readily incorporate noncrystallizing entities into the fold plane at the crystal surface; thus, PVE chains trapped between the lamella of the crystallizing PIP can be accommodated. In addition, wide-angle x-ray diffraction measurements showed that PVE is also entrapped within the crystalline lattice itself, causing an expansion of the polyisoprene unit cell (Tomlin and Roland, 1993).
4.3
Blends of ENR
Epoxidized natural rubber (ENR) is a random copolymer of 2-methyl-l ,2-epoxy- 1,4-butanediyl and 2-methyl- 1 -butenylene. Due to attractive interaction between its oxiranegroup and chlorine, ENR is miscible with variouschlorinatedpolymers(Margaritis etal., 1987;Margaritisand Kalfoglou, 1987; Kallitsis and Kalfoglou. 1987). As shown in Fig. 4, it can also compatibilize immiscible blends of halogen-containing polymers (Koklas, et al., 1991). Miscibility of ENR with polychloroprenewasfirstdemonstratedthroughspontaneous interdiffusion experiments (Nagode and Roland, 1991; Roland et al., 1994). Theeffect of fillers on this blend has also been described (Alex et al., 1991: Bandyopodhyay et al., 1995).
PVC
Fig. 4 Phase diagram of ternary blends of polyvinyl chloride, 48% chlorinated polyethylene, and epoxidized natural rubber (coordinates in volume fraction):( 0 )immiscible compositions;(0) miscible compositions. Lines a, b, and c represent PVC/CPE ratios of 1:3, 1: I , and 3.1 respectively. (From Koklas et al., 1991.)
Rubber-Rubber Blends: Part 11, New Developments
/
10J//
4.2
235
/
'
l
I
I
L
1000 / T (K-') Fig. 5 The temperature dependence of the segmental relaxation times for 25% epoxidized natural rubber, polychloroprene,and two blends. When enriched with ENR, the latter exhibit faster relaxation than either neat component. (From Roland et al., 1994.)
At 25% epoxidation, ENR and CR have the same glass transition temperature. While their miscibility could still be affirmed by xenon NMR (as shown in Fig. l ) , the segmental relaxation behavior of the blends is anomalous (Roland et al., 1994). Blends containinga high concentration of ENR have shorter relaxation times(by up to an order of magnitude) than the neatcomponents (Walton et al., 1992). This means that segmental relaxation speeds up upon mixing (Fig. S), and the T, of the blend is less than that of the neat components. Such behavior is due to the decrease in intermolecular cooperativity in the mixtures relativeto the neat components. Weaker intermolecular cooperativity arises from the large positive volume change upon mixing in the blend and the consequent increased unoccupied volume. 4.4
Epichlorohydrin Blends
Iz9Xe-NMR spectroscopy was employed by McGrath and Roland (1994) to demonstrate that polyepichlorohydrin and poly(vinylmethy1ether) (PVME), which have virtually equal glass transition temperatures, form thermodynamically miscible blends. I3C-NMR results corroborated the thermodynamic miscibility. The segmental dynamics for this blend exhibited an anomaly opposite to that of ENWCR. As reported by Alegria et al. (1993, the blend relaxes slower, and has a higher glass transition temperature, than the neat components.
Roland
236
McHugh et al. (I991 ) studied blends of CR with various microstructure ECH rubbers. The proximity of the glass transition temperatures makes assessment of miscibility difficult. However, from calorimetry and dynamic mechanical measurements, the thermodynamic incompatibility of all compositions was demonstrated.
5.
RELAXATION MODELS FOR BLENDS
The dynamics of polymer blends has aroused considerable interest, mainly becauseof the possibility of observing interesting new physics. Three different theoretical models have been proposed, all successful at least in part in describing the relaxation behavior of blends.
5.1
Fischer and Zetsche
This model (Katana et al., 1993, 1995; Zetsche and Fischer, 1994) focuses on the effect of local composition on the glass transition temperature and relaxationdynamics of segments comprising a given local environment. The respective components are not assumed to have intrinsically different mobilities. However, since the higher T, component will on average be associated with high T, domains (and vice versa for the lower T, component), the mean relaxation time of the components can differ, consistent with experiments. The characteristic broadening of the relaxation function towards lower frequency seen in blends at lower temperatures can be reproduced using this model. Applying the model to PIP/PVE blends, Alvarez et al. (1997) showed that the model qualitatively reproduces the spectral shape. However, the model’s results were found by Chung et al. ( 1 994a) to be quantitatively at oddswith the relaxation time distributions determined independently by deuterium NMR. This failing was ascribed to the absence in the model of intrinsic mobility differences between the components.
5.2
Roland and Ngai
A model was proposed (Roland and Ngai, 1991a, 1992a) based on the idea that both intrinsic mobility differences of the constituents and the distribution of local environments in a blend arising from concentration fluctuations cause dynamic heterogeneity. This approach uses the coupling scheme (Ngai, et al.. 1986) to describe the individual component relaxation, so that theshape of the relaxationfunction and thetemperaturedependence of therelaxationtime are governed by intermolecular cooperativity. Because the intermolecular coupling depends on chemical structure (Roland and Ngai, 199 1b; Roland, 1992. 1994; Ngai and Roland, 1993; Ngai et al., 1993), the components of a blend usually have different intermolecular coupling even when in the same local environment. This causes a divergence in their relaxation times and temperature dependencies (i.e., the model predicts thermorheological complexity). The difficulty with applying this model to describe blend dynamics is twofold: ( 1) the determination of the manner in which local environment alters intermolecular cooperativity and (2) the mannerin which the contributionsof various local environments sum to yield the observed macroscopic behavior. To calculate the bulk mechanical response, two extremes can be envisaged-homogeneous stress and homogeneous strain among all local domains. In reality, the mechanical interaction betweenlocal environments is more complicated. For the dielectric response, the simple linear summation of the local responses may be appropriate, and in fact analysis of dielectric data employing this assumption has been carried out for PVME/PS (Roland and Ngai, 1992b), TMPC/PS (Roland et al.. 1992), and PIP/PVE blends (Alegria et al., 1994).
Rubber-Rubber Blends: Part II, New Developments
237
5.3 Jones A. A. Jones and coworkers (Jones et al., 1991) have developedamodelbased on a FloryHuggins-type lattice. Nearest-neighbor contacts are enumerated and their influence on local dynamics assessed. The model has been applied to both polymer blends (Inglefield et al., 1992) and to polymer/diluent mixtures (Zhang et al., 1991; Cauley et al., 1991; Liu et al., 1990).The analysis yields information concerning local populations and their mobilities. The utility of such information depends on the validity of alatticeapproach to chain dynamics, as well as the sensitivity of the calculated results to the assumed population distributions.
ACKNOWLEDGMENT This work was supported by the Office of Naval Research.
REFERENCES Adachi, K., Wada, T., Kawamoto, T., and Kotaka, T. (1993, Mrrcrornolecules, 28:3588. Alegria, A., Colmenero, J., Ngai, K. L., and Roland, C. M. (1994), Mncrornolecule.~27:4486. Alegria, A., Elizetxea, C., Cendoya, I., and Colmenero, J. (199.5), Mrrcrotnolecules 28:8819. Alex, R., De, P. P., and De, S. K. ( l99 1 ), Pol.vnrer 32:2345. Alvarez, F., Alegria, A., and Colmencro, J. (1997), Mncrornolecules 30597. Amraec, I. A., Katbab, A. A., and Aghafarajollah, S. (1990). Rubber Cltent. Techno/. 69:130. Arendt, B. H., Kannan, R. M., Zewail, M., Kornfield, J. A., and Smith, S. D. (1994), R/wo/.Acto 33:322. Arendt, B. H., Krishnamoorti, R., Kornfield, J. A., and Smith, S. D. (1997). Mc/cronro/ecules 30:1127. Bahani, M., Laupretre, F., and Monnerie, L. ( 1993, J. P o l y . Sei. Polynr. Pkys. Ed. 33:167. Bandyopadhyay, S., De, P. P., Tripathy, D. K., and De, S. K. (199% Polymer 36: 1979. Bartencv. G. M., and Kongarov, G. S., (1963). Rubber Cltern. Techno/.36:668. Bauer, R. F., and Crossland, A. H. (1988), Rubber Chent. Technol. 61:.585. Bauer, R. F., and Crossland, A. H. (199O), Rubber Chent. Technol. 63:779. Beaucage, G., Sukumaran, S. Clarson, S. J., Kent, M. S., and Schaefer, D. W. (1996), Macrotnolecu/e.s 29:8349. Bhaumik T. K.,Bhattacharya A. K.,Gupta B. R., and Bhowmick A. K. (1988). Plnsrics Rubber-Proc Appl IO:1OS. Bhowmick, A. K., Konar, J., Kole, S., and Narayanan, S. ( 1 995), J. Appl. Po/ynr. Sci. 57:63 I . Biswas, T., Naskar, N., and Basu, D. K. (1995), J. Appl. Polym. Sci. 58:981. Brown, P. S., Loadman, M. J. R., and Tinker, A. J. (1992), Rubber Cltern. Techrtol. 65:744. Cauley, B. J., Cipriana,C.,Ellis,K.,Roy, A. K.,Jones, A. A. Inglefield, P. T., McKinley, B. J., and Kambour, R. P. ( 199 1), Mrzcrotnolecules 24:403. Chai, Z., Sun, R., Li, S., and Karasz, F. E. (1995), M~rcrontolecu/e.s28:2297. Cheung, T. T. P. and Chu, P. J. ( 1 992), J. Phys. Chern. 96:955 1. Chowdhury, P,, Chakravorti, M. C., and Das, C. K. (1995). J. Appl. Polvrn. Sei. 55:841. Chung, G.-C., Kornfield, J. A., and Smith, S . D. (1994a), Mtrcronrolecules 27964. Chung, G.-C., Kornfield, J. A., and Smith, S. D. (1994b), Macromolecules 275729. Cifra, P,, Karasz, F. E., and MacKnight, W. J. (1991 ), J. Polvnz. Sci. Polynr. Phys. Ed. 29: 1.389. Clarke, N., McLeish, T. C. B., and Jenkins, S. D. (1993, Macrornolecules 28:4650. Cohen, R. E. (1982), A h . Chenr. Series 193:489. Coleman, M. M,,Graf, J. F., and Painter, P. C. (1991), spec if;^ Inrerrrction.s rtnd the Mj.ycj/i/igofPo/vnler Blends, Technomic Publ., Lancaster, PA. Corm, A. Y . (1988), Rubber Chetn. Technol. 61:281. Cosgrove, T., Turner, M. J., and Griffiths, P. C. (1996), Polwner 37:1535.
238
Roland
Cudby, P. E. F., and Gilbcy, B. A. (1995). Rubher C / ~ e r rTechrzol. ~. 68342. Da Silva, N.M., and Tavares, M. I. B. (1996), J . App/. Po/ym. Sei. 60563. Dias. A. J., and Galuska, A. A. (1996), Rubber Cllerrl. Tedlrml. 69615. Doi, M., Pearson, D.. Kornfield, J.. and Fuller, G. (1989). Mocrordecules 22:1488. Dubault. A., Delochc, B., and Herz, J. (1987), M~ccromolecrdes20:2096. Ellul, M. D., Patel, J., and Tinker, A. J. (1995). Rubber- Cl~errl.Trchr~ol.68573. Erman, B., Bahar, l., Kloczkowski, A., and Mark, J. E. (1990), Mrecronzolecules 23:5335. Faivre, J. P., Jasse, B., and Monnerie, L. (1985), Po/yrner 26:879. Faust, A. B., Sremcich, P. S., Gilmer, J. W., and Mays, J. W. (1989). Mrcc,rorlzo/rcu/e.s22: 1250. Garas, G., and Kosrnas, M. ( 1994), Mrrcronlolecules 276671. Go, J.-H.. and Ha, C.-S. (1996). J. A/)/>/.P/>/yttl.S C ~621509. . 26320. Graebling, D., Muller, R., and Palierne. J. F. (1993), Mrec~rorr~oluculrs Gross, D., and Kelm, J. ( 1985), Kmrtsch. Gurrzrrl. Kurzstst. 38: 1089. Gross, D., and Kclm, J. ( 1987). Ktrutsch. GItmrrz. Kurlstst. 40:13. Guo, M. (1996), Trerlrls Po/yrn. Sci. 4238. Halasa, A. F.. Wathen, G. D., Hsu, W. L., Matrana, B. A., and Massie, J. M. (l99l), 1.A J J ~Pdynz. /. Sci. 43: 183. Hamed, G. R., Kim, H. J., and Gent, A. N. (1996), Rubher Cllrrrz. Techno/. 69807. Hess. W. M. (1991). Rubher Chenl. Techol. 64:386. Hess, W. M,, Herd. C. R., and Vegvari, P. C. (1993). Ruhber C/~etrz.Teckrzol. 66329. Hourston, D. J., Song. M,. Pollock, H. M,, and Hammiche, A. ( 1995), Proc. North A m Therm. Allal. Soc. Cor!/:24: 109. Ibar, J. P. ( 1993), Furdntnerztnls of Tlzrrrnd Stirwltrted Currcvlt crud Relnstrtiorz Mrcp Amc/y.si.s, SLP Press, New Canaan, CT. Inglefield, P. T., Jones, A. A., Chin, Y. H., and Zhang, C. (1992), Po/.vrzwr Preprirzts 33( 1):76. Jansen, P,, Comes, A. S., and Soarcs, B. G. (1996). J. A/)/>/.Po/yrrz. Sci. 61591. Jones, A. A., Inglefield, P. T., LIU,Y., Roy, A. K. and Cauley. B. J. (1991). J. Non-Cnst.Solids 131-133: 556. Kallitsis, J. K., and Kalfoglou, N. K. ( 1987). Arzge,t-. Mrekrornol. C/~er?z. 148:103. Katana, G., Kremer, F., Fischer, E. W.. and Plaetschkc, R. (1993), Macrorlzolecules 263075. Katana. G., Fischer, E. W., Hack, T. Abetz, V., and Kremer. F. (1995), Mocrorrlolec.uIe.s28:2714. Kawahara, S., Akiyama, S., and Ueda, A. (1989). Po/yrner J . 2/:221. Kawahara. S . K., Sato, K., and Akiyama. S. (1994), J. Po/ym. Sci. Po/yrrz. Pkvs. Ed. 32: 15. Kohudic, M. A., and Finlayson, K., Eds. ( I 989). Arhwzces irl Po/wler Blerlds trrld A//o.v.s Techrlology. Technomic Publ., Lancaster, PA. Koklas, S. N., Sotiropoulou, D. D., Kallitsis, J. K., and Kalfoglou, N. K. (1991). Po/yrlzer 32:66. Kole, S., Bhattacharya, A., Tripathy, D. K., and Bhowmick A. K. (1993). J. App/. Po/ytrz. Sci. 48:529. Kole, S., Roy, S., and Bhowrnick, A. K. (l994a), Po/vrlzer 353423. Kole, S., Santra, R., and Bhowmick, A. K. (1994h), Rubher Clzem. Techrd. 6 7 119. Kole, S., Srivastava, S. K., Tripathy. D. K., and Bhowmick A. K. (l994c), J. AI>/>/.Po/yrlz. Sei. 54:1329. Kole, S., Roy, S., and Bhowmick, A. K. (1996), Po/ynzer 37:625. Kuo, C. M,, and Clarson, S. J. (1992). Mtrcrornolecules 25:2192. Kuo, C. M,, and Clarson, S. J. (1993), Eur. P o / v r ~J.~ .29661. Kuo, C. M,. Clarson, S. J., and Semlyen, J. A. (1994), Po/ynzer 35:4623. Lee, M.-P,, and Moet. A. ( 1993). Rubher Cher~r.Techrlol. 66:304. Liu, Y.,Roy, A. K., Jones, A. A., Inglefield, P. T., and Ogden, P. (1990), M[rcrorno/ecu/es23:968. Loadrnan, M. J. R., and Tinker, A. J. (1989), Rubber Cllerrl. Teehol. 62:234. Maiti, S., De, S. K., and Bhowmick, A. K. ( 1992). Ruhl>er Chern. Techno/.65:293. Mallick, A., Tripathy, D. K., and De, S . K. (1993), J. AppI. Po/yrrl. Sei. 50:1627. Manoj, N. R.. De, S. K., De, P. P,, and Peiffer, D. G. (l993), Po/vrner 34:2128. Manoj, N.R., De, P. P,, De, S. K., and Peiffer, D. G. (l994), J . A/IJI/.fo/.vrtz. Sei. 53361. Margaritis, A. G., Kallitsis, J. K., and Kalfoglou, N. K. (1987). Polynzer 28:2122. Margaritis, A. G., and Kalfoglou, N. K. (1987), Po/yrrzer 28:497.
Rubber-Rubber Blends: Part 11, New Developments
239
Matsumoto, M. (1996). Po/yrr~er37525; 36:3243. McGrath, K. J.. and Roland, C. M. (1994), Ruhher Cherrl. Techr~ol.67:629. McHugh, E. A., Schneider, N., Wilusz, E., and Brennick. J. (1991), Ruhher Plnsf. N e ~ November x 2, p. 18.
Miles, I. S., and Rostami, S.. Eds. (1993). Mulricornporwrrt Po/yrrrrr Sy.strrrf.s. Longman Group, London. United Kingdom. Miller, J. B., McGrath. K. J., Roland. C. M,, Trask. C. A., and Garroway, A. N. (1990), Mnc.rorflo/rcu/e.s 3:4543. Miller, J. B., Walton, J. H., and Roland, C. M. ( 1993). Mrrc.rorrto/c.clt/e.s26:5602. Mishra. V., Murphy, C. M,. and Sperling, L. H. (1994), J. Apppl. Po/yrrf. Sei. 53:1425. Mukhopadhyay, S., and De, S. K. (1991), J. App/. Po/yrn. Sei. 45:2283. Mukhopadhyay, S., and De. S. K. (1992), J. AppI. Po/yrrl. Sei. 45:181. Nagode, J. B., and Roland, C. M. (1991). Po/yrrler 32:505. Naskar, N.. Biswas. T.. and Basu, D. K. (1994), J. App/. P o / p . Sci. 52: 1007. Ngai. K. L., Rendell, R. W., Rajagopal, A. K., and Teitler. S. ( 1986). A m . N Y Acml. Sci. 484: 150. Ngai, K. L., and Roland, C. M. ( 1993). M~rcr-orrfolrc,rr/rs 266824. Ngai, K. L., Roland. C. M,. and Yee, A. F. ( 1993). Ruhher Cllcwl. Techrwl. 66:817. Ngai, K. L., and Roland, C. M. (1995), Mmrondecules 28:4033. Orrah, D. J.. Semlyen. J. A., and Ross-Murphy, S. B. (1988), f o / w t e r 291455. Palierne, J.-F. (1990), Rheol. A m 29:204. Ramcsh, P.. and De, S. K. ( 1992), Rrthhrr Cherrl. Trchr~ol.65:24. Roland. C. M. (1987). Mrrc,rorrlo/ecu/es 2012557. Roland, C. M. (1989). Ruhber Cllenl. Techol. 62:456. Roland, C. M.. and Ngai, K. L. ( 1991~1). Mtrcrorrro/ecu/t.s 242261. Roland, C. M,, and Ngai, K. L. (1991b), Mtrcrorrloleculrs 245315. Roland, C. M. ( 1992). Mtrcrornolecdrs 25:703 I . Roland, C. M,, and Ngai, K. L. ( 1992a), J. Rheol. 36: 1691. Roland, C. M,, and Ngai, K. L. (1992b), M~cror11olecule.s25:363. Roland, C. M., Ngai, K. L., O'Reilly, J. M,, and Sedita, J. S. (1992), M~ccrorrfo/relr/e.~ 25:3906. Roland, C. M,, Miller. J. B., and McGrath. K. J. (1993). Mtrc.rorrlo/ec,u/e.s26:4967. Roland, C. M. ( I 994). M~c.rorr~o/rcu/e.s 27:4242. Roland, C. M., Santangelo, P. G.. Baram, Z., and Runt, J. (1994), Mncrorrto/ecu/e,s275382. Russell, T. P,, Peters, L. J., Clark, J. C., Bauer, B. J.. and Han, C. C. (1990), M~rc~,onl(j/rcu/r.s 231654. Saito, H., Takahashi, M., and Inouc, T. (1988), J. Po/ym. Se;. Po/yrlr. phys. Ed. 26: 176 1, Schroeder, M. J., and Roland, C. M. (1999). Mocronro/ecu/rs 32:2000. Sidkey, M. A., El Fattah. A. M. A.. Yehia. A. A., and El All, N. S. A. (1991), J . App/. Po/yr?l. S(.;. 43: 1441. Sidkey, M. A., El Fattah, A. M. A., and El All, N. S. A. (1992). J. AppI. fO/ytrl. SC;. 4658 1. Sircar, A. K., Galaska, M. L., Linder. S . M., Kumar, B. Chartoff. R. P,, and Scanion, L. G. (1993). p r O ( ~ . Norfll Am. Therrr~.A r d SOC. Cor$ 2 2 3 19. Sircar. A. K. (1997). in Thrrrrfnl C/~crrcrcferi:~rli~~r~ ofPo/ytrleric Mcrteriols, 2nd ed., Academic press, New York. Chapter S . Sotta, P., and Deloche, B. ( 1987), Mncrorlloleelilps 20:2769. SPcrling, L.-I. ( 1981 ), /rlferper~elr[rfirl~~ fo/yr?ter Ne1work.s c m / Relrtrerl Matrrirrls. Plenum Press, New York. S u m , N.. Joseph, R., and George, K. E. ( 1993). J. App/. Po/yrtl. Sei. 49549. Takigawa, T.. Oodate, M., Urayama, K., and Masuda, T. (1996), J. &p/. P o / . v ~ ~Sei. I . 59: 1563. Tinker, A. J. (1995). Rubber Cherrt. Techno/. 68:461. Tomlin. D. W., and Roland, C. M. (1992). M~rcrorrlolecules252994. Tomlin, D. W., and Roland, C. M. (1993). P o / p e r 34:2665. Trask, C. A., and Roland, C. M. (1988), Po/y. CorttrrI. 29332. Trask, C. A., and Roland, C. M. (1989). Mrrcrorrfolee1rle.s22:256. Utracki, L. A. (1989), Po/yrrwr A//ovs m d Bler~ds.Hanscr Publ.. New York.
240
Roland
Utracki, L. A., and Weiss, R. A., Eds. (1989). Multiphclsr Po/ynlers: B/ertcl.s nr~d/onnn~er.s.American Chem. Society, Washington, D C . 63:215. van der Velden, G. P. M,, and Kelm, J. ( 1990). RuDher Cltenl. Tech~~ol. Varughese, S., and Tripathy, D. K. (1993), J . AppL Polym. Sei. 49:1115. Walton, J. H., Miller, J. B., and Roland, C. M. (1992). J. Po/y. Sei. Po/y. Phys. Ed. 30:527. Walton, J. H., Miller, J. B., Roland, C. M,, and Nagode, J. B., (1993), Macronto/ecu/es 26:4052. Walton, J. H. (1994), Po/.vnl. P o l ~ r t C ~ .o y m i r e s 2:35. Watanabe, H.. Yamnzaki, M,, Yoshida, H., and Kotaka, T. (1991), Mcrcwr1;olecule.y 24:5372. Zawada, J. A., Fuller, G. G., Colby, R. H., and Fetters, L. J. (1994), M~cro~tto/ecu/es 27:686l. Zernel, I. S., and Roland, C. M. (l992b), Po/.vrtler 33:3427. Zemel. I. S., and Roland, C. M. (1992a), Po/yrrter 33:4522. Zetsche, A., and Fischer, E. W. ( I 994). Actcl Po!\w. 45: 168. Zhang, C., Wang, P,. Jones, A. A., and Inglefield, P. T., (1991 ), Mctcrorltolecule.7 24:338.
Short Fiber-Filled Rubber Composites Lloyd A. Goettler* Solutia Inc., Pensacola, Florida
William F. Cole Flexsys America L. P,, Akron, Ohio
1. ABSTRACT This chapter covers thereinforcement of softrubbermatriceswithshortcellulosefibers. It focuses on the relationships of processing to structure and consequently to its effect on mechanicalproperties. The threemostimportantstructuralparameters comprisingfiber orientation, length, and degreeof dispersion are considered.The first of these, fiber orientation, is determined predominantly by the geometry and flow in the forming operation used to produce the final part. On theotherhand,the othertwoderive from an interaction of the materialwiththe mixing process. Following some introductory remarks and a review of recent literature on short fiber-rubber composites, emphasis will be given to the effects of compounding on fiber length and dispersion and the resulting mechanical properties.
2.
INTRODUCTIONTORUBBERCOMPOSITES
The use of continuous cords for rubber reinforcement imparts high strength and stiffness in tension but produces little effect in compression and flexure. In many applications, e.g., tires, both continuous and discontinuous reinforcements may be required. They may either be incorporated in different components of the part or be combined together as a hybrid composite. Another type of hybrid structure comprises the combination of short fibers with some type of particulatefiller or reinforcement. In the rubber industry,carbonblack is routinely employed to upgrade rubber properties. The small submicrometer size of these particles, even in aggregated form, in comparison to most shortfiberdimensions, allowthem to function independently of the presence of the larger discontinuous fiber reinforcement. Thus, short fiber composite mechanics simply considers them to alter the properties of the matrix rubber. The real benefit to the short fiber reinforcement of any polymer, including rubber, lies in processing economics. Short fiber-rubber composites can be handled on conventional rubberprocessing equipment, in sharp contrast to formsof continuous reinforcement (cord, fabric, etc.),
* Current rrffilicctiort: The University of Akron, Akron,
Ohio 241
Goettler and Cole
242
which require specialized techniques for their incorporation and composite article. 2.1
the subsequent forming of the
Short FiberReinforcement
Short fibers are incorporated into plastics and lubber either for cost reduction or to provide unique mechanical properties. The design requirements favorable to short fiber reinforcement of any polymer, including rubber stock, include: Increased stiffness and. to some extent, strength Reduced elongation to failure Improved resistance to crack growth and tearing (at low concentration) Modification of dynamic, abrasion, and fatigue properties Increased hardness with minimum increase in viscosity Several review papers in a recent book edited by De and White ( 1996) cover the design and application of composites comprising rubberymatricesreinforced with short fibers such as poly(ethy1ene terephthalate) and nylon. The glass fibers used historically in the plastics industry find only limiteduse in elastomers because of unique processing and mechanical requirements associated with these low modulus matrices. A longer fiber length (actually a higher aspect ratio, the length of the fiber divided by its transverse dimension) is required to generate the same degree of stress transfer into the fiber memberof the composite. At the same time, however, the mixing actionneeded to disperse the fibers causes breakage of brittle reinforcements such as fiberglass. This limitation has been circumvented in the plastics industry by newer types of fiber-reinforced molding compounds in which the glass or graphite fibers are impregnated with the resin in special operations, such as pultrusion, which are able to produce excellent dispersion without the high energy input characteristic of mixing processes. Unfortunately,such products are not available to the rubber industry, which still largelyrelies on compounding operations to incorporate raw ingredientsinto the rubber formulation to be molded or extruded. Thus. a different approach is needed to provide a high degree of reinforcement by short fibers in rubber. Fortunately, the low modulus of the rubber matrix (usually less than 7 MPa) also sets a lower requirement on the modulus of the reinforcement. There are a number of short or discontinuous textile fibers (natural and chopped) with Young’s moduli in the range of 7-20 GPa which, when properly treated for bonding to the rubber matrix, will yield composite properties suitable for application i n certain tire conlponents and industrial rubber parts (Foldi. 1992). Some higher modulus organic fibers, such as the aramids with modulus values above 100 GPa, are also suitable. although they do kink, but do not break. during rubber-processing operations (Czarnecki and White, 1980). However, due to stress transfer considerations, such excessively high fiber properties generally do not translate to improved performance in rubbery composites. Ibarra and Chamorro ( 1 989) discuss the dynamic and mechanical properties of EPDM matrices reinforced with carbon and polyester fibers. In a later work (Ibarra and Chamorro, 199 1). they include the swelling behavior of polyester-rubber composites in hydrocarbon media. Reinforcement is seen to increase stiffness, resulting in reduced swelling but increased heat generation by viscous dissipation of energy, especially when measured i n the direction of predominant fiber orientation. Stress relaxation is reduced while thedynamic glass transition temperature is raised by fiber reinforcement. The electrical as well as the mechanical properties of SBR reinforced with short polyester fibers has been studied by Ismail and Ghoneim ( 1 999). The presence of an HRH (HiSil-resorcinol-hexa) bonding system improves electrical permittivity without affecting dielectric loss at the same time as it improves mechanical performance. Both the fiber and adhesion components were also found to enhance the aging resistance of the rubber. Akhtar et al. (1986) find that
Short Fiber-Filled Rubber Composites
243
short silk reinforced natural rubber blends obey the Griffith theory of fracture as it relates to the effect of crack length on tensile strength. A 30 phr loading of nylon 6 fiber was found by Senapati et al. ( 1988) to impart optimum properties to natural rubber composites. Anisotropy increased with increasing fiber length. while ultimate elongation decreased. Hysteresis became more severe and air aging improved. I n terms ofapplications, Bayly et al. (1988) describe air-permeable rubber sheets reinforced with about 25%' 12-mtn-long glass fiber. Thefatigue resistance of natural rubber reinforced with short fibers for tire applications was found by Kwon et a l . (1990) to be optimal at 6 phr of a 0.2-in. length fiber. Fatigue resistance was found to decrease as the fiber modulus increased, perhaps due to higher heat generation at elevated temperature. The service life and perfornlance of poly(viny1 chloride) (PVC) can also be upgraded by short fiber reinforcement similar to that used i n rubber compounds. A typical formulation for such a reinforced PVC hose would contain polymer. plasticizer. filler. and stabilizer in addition to the short fiber reinforcement. In many respects these hoses can be processed like and behave like similar articles based on thermoset or thermoplastic elastomers. Their extrusion using a special orienting die and the resulting performance characteristics have been discussed by Goettler (1983).
2.2
Cellulose Fiber Reinforcement
Unregenerated cellulose fibers have dimensional and physical characteristics that provide the best combination of processing and reinforcing properties to rubber composites. These fibers have an ideal aspect ratio of 100-200 for the discontinuous reinforcement of polymers. disperse evenly during mixing without extensive breakage, and are easily oriented during processing. They can also be chemically bonded to a number of different polymer matrix types. resulting in a high level of reinforcement in comparison to other types of short fibers. A cellulose fiber derived from hardwood is ribbon-shaped, typically measuring 8- 12 p m in cross section by about 1-2 n m in length. Its elastic modulus lies between about 10.000 and 30,000 MPa, depending on moisture content, with tensile strength in the range of 450-650 MPa. This chapter focuses on the use of cellulose fibers for rubber reinforcement and covers the phenomena pertinent to the development of mechanical perfornlance in this class of soft materials. The growing literature on applications i n hard plastic composites. a s well as that in which the cellulose is present only as a ground wood flour filler, are not addressed. A broad review of the use of cellulose fibers to reinforce elastomeric and other low modulus polymers such as PVC was given by Goettler i n the earlier editionof this book (1988). The current chapter will build on that work whilegivingspecialtreatment to severalareas,includingthe fiber dispersion process and the dependence of composite properties on the resulting degree of fiber dispersion. The degree of fiber dispersion i n the short fiber composite is critical to strength development. Some fibrous reinforcernents are treated with agents to promote dispersion during norn1aI mixing operations and to effect bonding to the tnatrix rubber under the heat of vulcanization. For example, cellulose fibers were coated by Gatenholm et al. (1993) with a rubbery plasticized PVC to lubricateand protect the fibers during compounding into other polymersystems. I t would be desirable for the coating composition to be compatible, if not miscible. with the final composite matrix polymer. The effectsof fiber length and concentration on the viscosity of short sisal fiber reinforced SBR rubber were studied by Kumar et al. ( 1998) using a capillary rheometer over a range o f shear rate and temperature. Sapieha et al. (1989) address the thermal degradation of cellulosecontaining composites during processing. Strong anisotropy was observed by Yano et al. (1992) in natural rubber-cellulose fiber composites due to a high degree of fiber orientation during
244
Goettler and Cole
processing. The critical volume fraction for longitudinal tensile strength was observed at 12.5%, while transverse strength decreased and modulus increased monotonically with increasing fiber concentration. While dynamic modulus increased, Yano et al. (1990) found the intensity of the tan 6 peak diminished in the presence of cellulose reinforcing fibers. Bhagawan et al. (1987) treat stress relaxation in short jute fiber-reinforced nitrile rubber compositions. Plots of relaxation modulus as a function of time show that bonding of cellulose reinforcing fibers tothe rubber matrix increases the rate of stress relaxation (Flink and Stenberg, 1990a). Even untreated cellulose fibers (unbleached kraft pulp fibers) are found by Flink and Stenberg (1989)to have some degreeof adhesion to a natural rubber matrix. according to electron microscopy and mechanical property measurements. The properties of both sulfur and peroxide vulcanized natural rubber composites containing short cellulose fiber reinforcements studied by Flink et al. (1988) indicated the occurrence of interfacial bonding even in the absence of an adhesive additive. Felix et al. review the surface properties of cellulose fibers in regard to their interactions with polymeric matrices ( 1993). Some approaches directed at strengthening acidbase interactions are found to increase elastic moduli by 10-20%. along with similar increases in ductility, which suggests that failure in tensionkompression is due to weakness of the interfacial region. Bonding can be enhanced by grafting the cellulose fibers with allyl acrylate and methacrylate (Flink and Stenberg, 1990b). A patent by Persson and Raanby ( 1990) describesnatural and syntheticrubbersreinforced by cellulose fibers grafted with methacrylates to produce high strength and modulus. Ahlblad et al. (1994) report butadiene or divinylbenzene grafting to the surface of cellulose fiber by plasma treatment to result in improved mechanical properties. In a companion paper, reduced moisture sensitivity is induced in cellulose fiber-natural rubber composites by irradiating the cellulosefibers in thepresence of butadiene or N-hydroxymethylacrylamide (Ahlblad et al. 1996). Mercerization through immersion in a sodium hydroxide solution increases modulus and strength of an isoprene rubber composite with bleached kraft cellulose fiber. while benzylation of the fibers was found to be detrimental due to loss of bonding (Westerlind et a l . 1987). In another work,Ismailet al. (l997a) reportincreasedmechanicalproperties in natural rubber composites reinforced with oil palm fiber that has been surface modified with 10% aqueous sodium hydroxide solution. The higher properties are attributed to improved adhesion with the matrix. More recently (Ismail and Hasliza, 1999), these studies have been further extended to encompass various bonding agents in natural rubber composites comprisingoil palm wood flour. Mixing torque increased with both the fiber and bonding agent concentrations. The latter also increased properties such as tensile strength and modulus, tear strength and hardness, presumably due to the observed improvement in interface strength. Variousbonding agents havebeenevaluated on oil palm fiber as a rubber composite reinforcement by Ismail et al. (1997b. 1 9 9 7 ~ )The . resulting interfacial strengthening increases mechanical properties while prolonging curing time. The HRHsystem was found to be superior in natural rubber composites. Fracture modes in cellulose fiber composites with recycled tire rubberwereinvestigated by Song andHwang (1997). Good-to-excellentadhesioncould be obtained with MD1 bonding agent, resulting in fiber fracture in the highly densified layers. Goettler and Swiderski (1992)reviewed the applications of cellulose fiber-rubber composites, including hoses, tires, belting, roofing, seals, and diaphragms. Typical rubber matrices for thesetypes of applications,includingsiliconerubber,areaddressed.Zadorecki and Michell (1989) gave a general review of the use of cellulose i n various forms to reinforce polymers of differenttypes,includingelastomers.Hoyes and Grabowski (1988) claimed fiber reinforced elastomeric sheets comprisinghammer-milled wood pulp for the manufacture of gaskets. Subramanian et al. (1997) examined applications of cellulose fiber reinforcement in NR and EPDM
Short Fiber-Filled Rubber Composites
245
Fig. 1 Fiber orientation by flow: (a) shear flow; (b) converging flow; (c) diverging flow.
rubber using both sulfur and peroxide cure systems forapplications in belting, hose, and engine mounts. Emphasis was given to the effects of aging. Short pineapple leaf fiber was found by Bhattacharyya et al. (1986) to impart typical reinforced properties to NR, such as increased tear resistance viscosity, hardness, and compression set with reduced elongation and mill shrinkage. Adhesion was provided by compounding in the HRH bonding system comprising resorcinol, hexamethylenetetramine, and silica. Arumugam et al. (1989) studied coconut fiber-reinforced rubber composites.
2.3.
Flow Orientation of Short Fiber Composites
The rigidity of short fiber-rubber composites, like that of plastic composites, depends strongly upon the degree and direction of orientation of the reinforcing fibers. During processing, the fibers will orient under the joint influences of shear and extensional flow fields, as shown in Fig. 1. Shear tends to align the fibers more closely with the direction of the flow. Because in Poiseuille flow highly concentrated suspensions of moderately high aspect ratio fibers may tend toward plug flow, with at least a significantly blunted velocity profile, the shear region may be relegated to a thin layer near the walls (Fig. la). In the strong flows depicted in Fig. lb, c, on the other hand, the fibers align highly parallel to the direction of the positive axial velocity gradient (i.e., parallel to the direction of stretching). Simple shear flow is characterized by a neutraldirection in which the dimension (or velocity) of the flowremainsconstant. There would be little if any change in fiber orientation relative to that direction. For example, a twodimensional flow element could be stretched in one direction and contracted in the other, while keeping thickness constant. The fibers should not turn into or out of the thickness direction. On the other hand, suppose that a short-fiber composite melt is flowing through an extrusion die. If the width and thickness of the die were simultaneously changed in such a manner that the cross-sectional area of each fluid element was alwayskept constant, then the flow would neither accelerate nor decelerate, i.e., the flow dimension would be effectively constant. In this case, fibers initially oriented parallel to the flow direction by the upstream geometry should stay in that configuration, despite the changes in the shape of the flow channel (Goettler et al. 1979, 1982). An extensive literature has developed on the prediction of fiber orientation during flow, which can be directly applied to fabrication operations. The directionality of the discontinuous
246
Goettler and Cole
fibers has a strong effect on the mechanical performance of the resulting parts. For exanlple, Yamamoto and Matsouka (1996). among others. address the prediction of microstructure and rheological properties of rigid fiber suspensions in sinlple shear and two-dimensional diverging flow fields. Rubber reinforced with shorttreated cellulose fiber displays anisotropic mechanical properties which are dependent on the direction and extent of fiber alignment. The hydrodynamic forces generated by the flow in processing operations, such as extrusion, always produce some degree of fiber orientation. A special tubing die has been developed (Goettler et al. 1979) to control the orientations of the short fibers in the extrudate to meet the higher tensile stresses in the circumferential direction in the hose wall. The forming and reinforcing operations of hose manufacture may thus be combined into a single extrusion step by using this die with short fibers in place of cord reinforcement. I n a conventional hose die, the reduction i n cross-sectional area between the head of the extruder and the die orifice, like the tlow of Fig. I h, i n combination with shear forces, results in an unwanted axial fiber orientation that tends to stiffen the hose. However, the special die has a restriction of the flow at some intermediate point of small diameter. followedby an outward expansion (usually taken at constant channel thickness) up to the size of the extruded hose. The area increase experienced by the flow in the expanding portion of the die causes a decrease in the flow velocity, which turns the fibers away from the tlow direction,as shown in Fig. IC. The material stretches circumferentially around the mandrel. resulting in the desirable predominant hoop orientation of the reinforcing fibers. The primary design parameter governingfiber orientation. and hence the performance of the extruded hose, is the ratio of the diameter of the outlet annulus of the die to the minimum diameter at the point of constriction. The design-performancecharacteristics of these dies and the resulting extruded hoses have been described i n detail (Goettler and Lambright, 1977; Goettler et al., 1979). I t has been found that the burst strength of the hose is adequately defined by the ratio of the mean hose diameter to wall thickness, the tensile strength of the reinforced rubber stock measured with all of the fibers aligned closely into a single direction (as could be produced by mill rolling). and the diameter ratio of the extrusion die. Otherparameters exert only secondary effects.The reinforcement of the elastomer with short fibers also serves to restrict deformation or growth of the hose as well as to increase its burst strength. Hose growth can be related back to the elastic parameters of the composite stock (modulus and Poisson ratios) through relationships given by Li et al. (1978). Ausias et al. ( 1 996) offer an improvement over the fiber-orienting hose extrusion die presented by Goettler, et al. ( 1979) and discussedin the earlier edition of this book by increasing the orientation of fibers lying close to the surface into the hoop direction. Injecting lubricant onto the inner walls of the diverging die has the effect of eliminating shear deformation, so the fiber orientation throughout the wall thickness is affected solely by the decelerating extensional flow field. Burst strength of extruded tubes is significantly increased. I n the case of calendering or milling on a roll mill, the orienting effects of shear and extension combine to orient the fibers strongly into the tnachine or tlow direction. Thus it is possible to construct a highly anisotropic sheet with all of the fibers predominantly aligned into the machine direction that can be used as a standard for measuring the orthotropic elasticity constants E , , and E??. as well as the corresponding ultimate properties. However, in using this technique for sample preparation, it is critical to not allow the composite melt to band on the mill rolls. The circulating tlow in the resultant bank at the roll inlet will destroy all of the fiber orientation established during the previous pass through the nip. Rather it is necessary to move the sheet repeatedly through the rollsin a series of single passes, always keeping the predominant fiber direction parallel to the machine direction. It may be folded in half between passes. or the
247
Short Fiber-Filled Rubber Composites
S
C
c
1-
I
I
20 Fiber Orientation Angle, 8
(4
(b)
I
40
Fiber Orientation Angle,
I
I
I
60
1
80
8
Fig. 2 ( a ) Effects of fiber orientationon the modulus of a rubber composlte. ( h ) Effects of fiber orientation on the strength o f a rubber composite.
nip nlay be progressively reduced on successive passes. About five foldings. corresponding to draw ratio of 2' or 32 : l , is usually sufficient to attain the asymptoticdegree of fiber orientation of about -t 10". A higher degree of orientation is not possible due to the disruptive effects of the shear component of the flow. The equations for particle rotation in shear tlows (Goldsmith and Mason, 1967) showa periodic motion that. although it on average directs the particles along the streamlines. destroys any higher degree of alignment. Both strength and modulus of the composite material (along with associated properties such as creep resistance) are highest in the direction parallel to the fiber orientation, as shown in Fig. 2 for cellulose fiber-rubber composites and in more detail for both single plies and laminates by Coran et al. ( 1976). It should be noted that compression modulus may show the opposite relationship due to the Poisson effect. Tensile elongation also shows an inverse relationship. Dimensional change under the import of a swelling solvent behaves similarly. Tears propagate more easily parallel to the fibers than across them. While any of these properties may be utilized to assess the degree and direction of fiber alignment. the relationshipsamong them arecomplex. For example, the ratio of Young's modulus between the parallel and transverse directions in a composite is not numerically equal to any simple factor of the inverse ratio of extensions under a bi- or tridirectional stress field (Li et al..1978).
I:
3. METHODS FOR THE ANALYSIS OF FIBER ORIENTATION 3.1
Modulus
Since the stiffness of the fibers exceeds that of the rubber matrix by a factor of at least 1000. the composite modulus is a sensitive measure of fiber orientation. Through the mechanics of linear elasticity, the composite Young's modulus may be quantitatively related to the angle the fibers make with the stress vector. Hence, this parameter, which is the slope of the tangent to the tensile stress-strain curve at zero stress, is the preferred measurement. However. the common
Goettler and Cole
248
modulus measurements made at high strain in the rubber industry should also be suitable, though less rigorous. When the fiber orientation is not uniaxial (all fibers parallel), any mechanical property measurement is an average overthe fiber orientation distribution. Since the relationships between the stiffness contribution of each fiber and its corresponding angle are nonlinear, however, the average modulus isnot uniquely related to the average angle. A detailed computation involving the measurement of the entire angular distribution is required to determine this relationship (Goettler, 1970).However, the direction of highest stiffness will be coincident withthe direction of principal orientation. Furthermore, the magnitude of the modulus relative to that of a highly aligned composite with the same concentration of the same fibers of the same length can be taken as a measure of the spread in the orientation about this position. The rate of drop in the measured modulus in directions away from the mean is then a measure of the angular distribution of the fibers, although, as stated above, this relationship is nonlinear and undefined except for the case of perfectly aligned (uniaxial) fibers.
3.2 SolventSwelling It has been shown (Coran et al., 1971) that the swell in any dimension of a fiber-reinforced rubber, a = (1 - l,,) / l,,, where l,, is the original dimension, varies oppositely to the Young’s modulus of the composite in that direction. Thus, relative degrees of fiber orientation in two or more directions can easily be obtained independently of any mechanical property measurement. However, as described above, Poisson effects must be taken into account (Li et al., 1978). This technique is applicable to complex pieces in which the fiber orientation is threedimensional. Unlike sheets, suchpieces are difficult to test mechanically. Toluene hasbeen used successfully in these swelling measurements.
3.3 UltimateTensileProperties Tensile strength and ultimate elongation also vary with fiber orientation, but not to the same extent as the Young’s modulus. Table 1 shows the ratiosof measurements made in two perpendicular directions on a series of samples with increasing degrees of fiber alignment. As can be seen, the modulus is the most sensitive measurement. Tensile strength is the least sensitive of the mechanical measurements in respect to fiber orientation.
Table 1 Property Anisotropy Ratios
1.1
3.3 5.0 3.8 E
5.8 14.6
1.1 1.9
2.1 2.3 2.1
1 .o 2.0 3.1
7.0
IS Young’s modulus, T IS tensile strength and e is elongation to failure. while subscr~pts 1 and 2 indicate directionality. Sample 1 is nearly random. Note that the elongation showsan inverse relationship to the other two propertles.
Short Fiber-Filled Rubber Composites
249
3.4 Tearing When the fiber orientations are narrowly distributed about a single direction in a sheet. a tear will propagate parallel to the fiber direction. The uniformity of the tearing is directly proportional to the uniformity of the fiber orientation. The tear can be started from a small scissor cut at the edge of the sample. Tearing is more difficult when the fibers are not well aligned. at low fiber concentration (<50phr fiber). orwhen the fiber-rubber bonding is poor, due to extensive rubber flow in front of the crack tip. The direction of tear, which can be measured experimentally, is an indication of the mean fiber direction.
3.5
Morphological Analysis
These tests may be either qualitative or quantitative, depending upon the sophistication of the image analysis employed. They usually involve visual observationof a composite surface through light or electron microscopy. The type of orientation observed. i.e., macroscopic or individual fiber, depends upon the level of magnification utilized. Angle measurements may be made by computerized image analysis on photomicrographs for a truly quantitative evaluation, or a more qualitative analysis may be more easily produced. In either case, a major step is the preparation of a suitable surface for the analysis. Peeled S I I I ~ ~ I C P . ~
A sheet of green (uncured) rubber composite can be sandwiched and molded between an aluminum plate and some rubber stock. The ends of the plate should not be bonded to the composite, so it can be easily pulled away i n a tensile testing machine. When the rubber composite tears, the fiber orientation pattern will be visible in the exposed surface. The tear usually propagates through the composite layer, rather than at the adhesive joint or at the interlaminar boundary.
Tensile Fvrrctlrw Srrrfkrs If the fiber-rubber bonding is poor. as. for example. when bonding agent is omitted from the composite formulation, long fibers canbe seen to protrude from the tensile fracture surfacesince pull-out occurs before the breaking stress of the fibers is reached. A microscopic observation in two directions that are normal to each other and parallel to the plane of fracture will indicate the fiber orientation pattern. This can be done either with a stereomicroscope at about 20-30 X magnification or under an ordinary light microscope at 50- 100 X . In the latter case, due to a narrower depth of field. it is convenient to limit the tensile fracture to a plane by cutting partway through the sample with a sharp knife or razorblade prior to tensioning. Examination of a tensile fracture surface under a scanning electron microscope over a range of magnifications will also allow an evaluation of fiber-orientation patterns on both the macro- and micro- scale. Cut s1rt:filcr.s
If a soft composite specimen whose fibers lie nearly in a plane is cut with a sharp blade and thesurface so generated is illuminated with astronglightobliquelydirectedapproximately nornlal to the principal fiber-orientation direction, the fiber-orientation pattern will be indicated by the highlights produced by ridges in the surface. These ridges correspond to fibers or groups of fibers that may be still covered with matrix material. Viewing should be through an optical microscope under SO-2OOX magnification. A better representation is obtained if the cut can be made to contain the principal fiber direction. Cuts normal to the fiberaxis in a uniaxial
250
Goettler and Cole
sample yield no information. When the fiber orientation is three-dimension~II, twonormal cuts, each containing a principal fiber direction. should be made. If the illumination is directed obliquely to the cut surface but parallel rather than normal to the fiber direction in that surface. no discernible orientation pattern will be visible. For this reason, illumination should be multidirectional when the fiber directions are widely distributed i n the plane. The use of a circular (ring) fluorescent or flash lamp is recommended. Harder samples may be microtonled to prepare a thin section for transmission microscopy. Softer matrices will tear when cut unless they are frozen in liquid nitrogen. Not only can the planar orientation pattern be established by image analysis, but the angleof inclination of circular fibers can be determinedfrom the distortion of theircrosssectionwhenasufficientlyhigh magnification is used (>XI0 x ). S1tdler1 Sllfficc~.s
When composite moldings are swollen with a suitable solvent. outlines of the fibers become more visible in the molded surfaces, whichwould otherwise show noindication of fiber direction. Procedures with the microscope and illumination are largely the same as described above. This technique has not been found to be ;IS valuable on cut, rather than molded. surfaces.
4. MIXING EFFECTS Short fiber-rubber composites are particularly attractive i n comparison to continuous cord constructions because they can be prepared as part of the sanle economicalcompounding operations. such a s batch mixing in a Banbury or other internal mixer, that are conlnlonly used in the rubber industry. While discontinuous fibers can be added directly to a rubber mix in neat form. a finer state of final dispersion can be achieved if the fibers are first wetted out by forming a highly concentrated mixture with a coating containing a resinous or polymeric binder. according to the scheme shown in Fig. 3. The resulting ease of dispersionandresistance to breakagefor these "treated" fibers depends critically on the rheology of the coating formulation. The viscosity of the coating can be adjusted through its formulation. This coating could be related i n conlposition to the final compositematrixrubber.though it should be softer so that it can easily be deformedand
Short Fiber-Filled Rubber Composites
251
Table 2 FactorsAffecting Dispcrsion and Breakage of Short Fibers During Compounding
Trcntcd fibcr charactcristlcs: Fiber wetout Coating-rubbcr compatibility Coating viscoelasticity (vs. that of the composite rubber matrix) Level of mixmg stresses: Rubber viscosity:
Formulation Temperature Strain rate Cumulative mixing strain: Efficiency of the mixing device Mixing time or number of passes through the mixing dcvice
assimilated when this “concentrate” is later let down in the final composition during the compounding operation. Alternately, it could comprise an entirely different chemical system that is adequately compatible with the matrix rubber of the composite. The effects of the compounding operation on the final state of fiber length and dispersion include the physical characteristics of both the fiber concentrate and the rubber matrix, as well as the parametersof the mixing process. Eachof the above in turn comprises several determining factors. In the case of the fiber concentrate, these are the degree of individual fiber wetout and separation, the viscoelasticity of the coating material and its compatibility with the composite matrix rubber. The viscoelasticity of the latter also plays a role by determining the magnitude of the stresses developed in the compounding process that will serve to disperse the individual fibers from the concentrate. It in turn is dependent upon the stock formulation and the process temperature. The dispersivestresslevel is also determined by the strainrate of the mixing process. while the total strain imparted during compounding that governs the distribution of the dispersed fibers increases with the speed and time of mixing. Finally, the deformations imparted to the mix ingredients are determined by the efficiency of the mixing process. which in turn depends on the process conditions and on the mixer geometry, including the degree of fill of the chamber volume. The factors affecting the compounding of short fibers into a polymer matrix during mixing operations are listed in Table 2. These conceptsare now applied specifically to rubber reinforced with short cellulose fibers.
4.1
Procedures
Rheology Viscosity measurements of the rubber masterbatches and coating compositions were performed on a capillary rheometer. The graph in Fig. 4 shows viscosity and die swell curves for the two rubber masterbatches used to evaluate fiber dispersion characteristics. measured at 100°C. The high-unsaturation “F” masterbatch is based on NWSBR. while the low-unsaturation ”N” comprises a blend of EPDM elastomers, N-550 carbon black, and paraffinic oil (Table 3). The stocks were not premasticated prior to their introduction into the capillary rheometer barrel. and entrance corrections were not applied. The higher elasticity of the “F” masterbatch
Goettler and Cole
252
’
0.1 10D
APPARENT SHEAR RATE,
-- F, IlX%III *F
t
I
DIE SML
-
IlS
I
(
I
,
1000
+---U VISCISIN R E D I SYBl
Fig. 4 Rheology of rubbermasterbatches.
(as indicated by its die swell) would generate a larger entrance pressure loss, thus reducing its true viscosity values into the range of the “N” stock. In mixing operations, the initial 90 ML 1 +4(100”C) Mooney viscosity of the “F” masterbatch was reduced to about 64 and that of the “N” masterbatch from 68 to 60 during two and one minute(s), respectively, of mastication in the Banbury mixer prior to addition of the treated fibers. Mixing
Standard rubber-mixing procedures can be employed. However, the presence of the fiber reinforcements does cause a higher rate of heat generation. To avoid scorching the batch, it may therefore be necessary to reduce the batch size by about 10% and run the mixer at a lower speed. With certain formulations, e.g., EPDM rubber compounds, an upside-down or sandwich procedure may be advisable. All of the fillers, oil, and powder ingredients are first added to the mixer, followed by half of the rubber, the fiber, and then the remainder of the rubber. In a single-pass mix, the curatives and any necessary bonding ingredients would be added a couple of minutes before dumping the batch. Cl1crrrrcteri;atiorI of Dispersion The degree of dispersion can be characterized by counting undispersed clumps of fiber over a unit area of surface cut into the resulting rubber composite sheet according to the procedure detailed in Table 4.
Table 3 MasterbatchFormulations High-unsaturation masterbatch
NR SBR HAF carbon black Zinc oxide Stearic acid Antiozonant
“F” 50 rubber 50 55 oxide 3 1 2 161
Low-unsaturation masterbatch
“N”
EPDM N-S50 carbon black Paraffinic oil Zinc Stearic acid
100
I22 76 S 1
304
Short Fiber-Filled Rubber Composites
253
Table 4 TestMethod forFiberDispersion 1. Compound curatives and bonding agent into the fiber-reinforced rubber stock. 2. Sheet the stock out using an even-speed roll mill to 6.3 mm thickness. 3. Vulcanize the sheet. 4. Cut 13-25 mm wide strips perpendicular to the fiber direction with a sharp knife. 5 . View the cut edges under a microscope at about 12X magnification. 6. Identify undisperscd fiber clumps by observing differences in their texture, softness. color, and appearance of fibers. 7. Count the number of such clumps over a length of about 1-2 m. 8. Convertthemeasurementto a clumpdensity by dividing the clump count by the product of sheet thickness and cut length.
4.2.
FiberLengthResults
With cellulose fibers there should be no substantial fiber breakage or degradation during normal processing cycles. However, if the coating material used for treatment is too stiff, the resulting fiber damage incurred during mixing may reduce the mechanical properties of the final rubbercellulose composite. On the other hand, a softer treatment coating having lower viscosity and viscoelasticity results in higher composite properties both because of less fiber damage (primarily affecting stiffness, or modulus) and also from the ensuing better dispersion of the fibers in the rubber composite matrix (primarily affecting the strength of the composite). Any means for reducing the coating viscosity can be utilized to produce the desired softer coating consistency. For example. a composite in a typical neoprenebelt compound produced from a concentrate of 85% cellulose fiber in polyethylene yielded a tensile strength at yield of only 6.9 MPa. a strength at break of 12.4 MPa and a Young’s modulus of only 16.6 MPa. A higher modulus of 20.7 MPa achieved by using a softer rubber composition as the fiber coatingindicated that fiber breakage was occurring during the more intense mixing required to disperse the polyethylenetreated fibers. Figure 5 shows the general dependence of composite stiffness on the length of cellulose fibers after their dispersion into a rubber matrix. These data, as well as those dealing with coating consistency in the following section pertain to compositesof hardwood pulp in the high-unsaturation black “F” masterbatch. Mixing was done for 2 minutes at low speed in a “BR” Banbury with 68% fill factor following a 2minute breakdown of the masterbatch.
4.3
Degree of DispersionResults
The mixing conditions, composite matrix characteristics, and fiber coating are all considered to be important determinants of the degree of fiber dispersion achieved in a controlled mixing cycle.Rapidand complete dispersion of the reinforcingfiber is important both foroptimal property development, as shown in Figure 6,as well as for acceptable appearance of the resulting composite material.
Mixing Protocol The Banbury fill factor and mixing conditions can affect the state of fiber dispersion as well as the already considered degree of fiber damage, which in turn alter the mechanical properties of the vulcanized composite.
Goettler and Cole
254
100
90
ao U
70 60
50
a 40
30 20 10
0
0.2
0.4
0.6
0.8
1
1.2
1.4
AVERAGE FIBER LENGTH. mm
Fig. 5 Sensitivity of cornpositc stiffness to fiber length.
5 4.8
4.5 4.4
4.2 4
3.8
3.6 3.4
3.2
n
3
2.8
2.6 2.1 2.3. n
,
a
I
" "
6
I
6
DISPERSION RATIMG, CLUMPS PER SQ. CM.
Fig. 6 Scnsitivity of cornpositc strcngth to fiber dispersion.
I
8
255
Short Fiber-Filled Rubber Composites Table 5 Formulations and Properties
Sample no. 1
2
3
4
5
HMMM“
8 I .x3 16.15 0.27 0.54 0.22 0.54 0.46
81.83 16.15 0.27 0.54 0.22 0.54 0.4s
81.83 16.15 0.27 0.54 0.22 0.54 0.46
81.83 16.15 0.27 0.54 0.22 0.54 0.46
8 1.x3 16.15 0.27 0.54 0.22 0.54 0.46
Total p u t s
100.00
100.00
100.00
100.00
100.00
1.49
1.30
1.12
I .49
I .49
“N” rubber MB Treated cellulose fiber PerkacP MBTS Pcrkacit@ ZDBC Perkad@TMTM
Sulfur
Total charge, kg Fill factor. % Mixing schcrne
A
Power input. KWH Dump T, “C
Rheometer (3 deg., 3 cpm, 160°C): R max. in-lb. R min, in-lb. t l w 9 mln tu,,, n u n tl, min
Undispcrsed particle count,
,270 I35 97
12 55 23 3.9 0.67
A
,264 132 91 8 49 22 3.8
0.5 I
78
78
59
68
78
A
B
C
,264
,249
,191 118 95 7 58 25 4.1 0.037
l35
97 8.5 S5
24 3.3
I32/ 1 16” 92 7 53 23 4.6
0.37
0.37
212 16.62 16
206 15.17 17
cIumps/cm’
Tcnsile properties: cured oriented stock parallel to fibers, 25 m d m i n : 210 Young’s modulus, Mpa 14.55 Ultimate stgth. Mpa 17 Ultimate clong., 8 “
”
I90 15.17 16
193 17.52 19
HMMM: Hexnnlethoxymethylnlelnm~ne.60% actwe. Ist/2nd stage dump temperatures.
The size of the charge was progressively increased with a single-pass mixing procedure calling for the sequential addition of treated cellulose fiber and curatives to premasticated type “N” masterbatch (Table 5 ) . The mixing procedure was also modified at the highest level of fill factor to represent different protocols of single pass and double pass mixing (Table 6). The treated fibers experienced a total of 2 minutes mixing time at the slowest rotor speed setting in all cases. The cooling water was always turned on. All stocks were vulcanized for 60 minutes at 160°C. Hardnesswasrelativelyconstant at 85-88 on the Shore “A” scale. A methylene acceptor component for the bonding system was present in the fiber coating. The higherrheometertorque R,,,,,, of Sample 1 relative to 2 and 3 suggests incipient crosslinking due to a higher dump temperature resulting from higher heat generation in the larger batch size. The longer scorch delay (t2) of Sample 5 relative to Sample 2 reflects the
256
Goettler and Cole
Table 6 Keyto MixingProcedures A: Delayed addition of treated cellulose fiber to a masterbatch Add “N” MB, mix1min Add fiber, mix 45 S, sweep, mix 30 S Add curatives, mix 15 S, sweep, mix 30 S, dump B: Sandwich mix Add 1/2 “N” MB, treated cellulose fiber, curatives, 1/2 “N” MB Mix 2 min, dump C: Two-pass mix Add “N” MB, mix 1 min Add treated cellulose fiber, mix 30 S, sweep, mix 30 S , dump Cool, reload 1/2 first pass, curatives, 112 first pass Mix 1 min, dump
lesser heat history of the curatives added in a second pass, and vice versa for Sample4. Bonding between the fibers and the rubber was observed to be good in all cases. Sample 3 with the smallest charge size (lowest fill factor) displays the highest strength and elongation to fail in combination with a relatively low modulus. It is well known that the stiffness of short fiber composites is strongly dependent upon the aspect ratios of the fiber, whereas ultimate properties are more sensitive to stress concentrators that result from nonuniform fiber distribution. Consequently, the mechanical strengthdata corroborate the direct visual observations of fiber dispersion, both showing that smaller fill factors provide better mixing. The pertinent data from Table 5 are plotted in Fig. 7. This result is in general agreement with the notion that mixing action is improved when the Banbury chamber is less full as long as there is sufficient charge to keep pressure against the ram. It is interesting to note that the two-pass mix of Sample 5 yields no benefit to strength, although the measured fiber dispersion is actually somewhat improved over the delayed addition scheme of Sample 1 at the same fill factor. Consequently, it should only be used to prevent premature scorch when very high heat generation is expected tobe a problem. The better dispersion is attributed to the higher viscosity of the colder masterbatch at the start of the second pass
Fig. 7 Effect of degree of fill on mixing efficiency and resulting propcrties.
sites
Rubber Short Fiber-Filled
257
mix,which may also result in greaterfiber damage, thusnegatinganybeneficial effect on properties. The single-charge sandwich mixing schedule of Sample 4 generates an equivalent level of dispersion to the less economical two-pass mix, with generally high properties. In fact, it produces at least a 10% higher tensile strength in comparison to conventional one- or two-pass mixes in which the rubber is premasticated before adding the fibers. The good dispersion of the fibers in this mix is attributed to the high initial viscosity of the rubber phase in the absence of premastication. Fiber damage is also low. resulting in a high modulus, because the fibers are still somewhat bundled when subjected to the resulting high initial shear stresses. Thus, single-charge, single-pass mixing without premasticationof the rubber and a smaller charge size are indicated for improved mixing. It remains to test the interaction of the preferred sandwich or upside-down mixing procedures with the smaller charge size to achieve an even higher degree of fiber dispersion. Mixing Time
The dependence of fiber dispersion on the mixing parameters of time or power input is illustrated by compounding 60 phr of a cellulose fiber concentrate into the type “N” rubber masterbatch in abatchmixer. A 1200 gchargefilledthe “BR” sizeBanbury to 63% of itsvolumetric capacity. Mixing at slow speed generated an energy input that, measured in KWH. was linear with and numerically equal to I O times the mixing time in minutes, corresponding to a power input of 600 KW. Figure 8 shows a power law dependence between the degree of fiber dispersion and the mixing time or energy input. More specifically. the density of residual clumps of undispersed fiber decreases with about the - 2 power of the mixing time or energy input. The two different symbols in the figure represent two separate determinations that fall nicely on the same correlation. Thus, the longer mixingis carried out or the greater the energy generated through increasing the mixing speed or the viscosity of the rubber stock. the better will be the dispersion of the reinforcing fibers. Rubber- Mcrtrix Viscosity The effect of the rubber matrix rheology on the ease of dispersing treated cellulose fibers is determined by incorporating differing amounts of process oil in the masterbatch formulations, in order to vary their viscosity (Table 7). These stocks were prepared by mixing for 5 minutes at slow speed in a #OO Banbury mixer using about a 3600 g upside-down charge. They were later compounded with 16.2 wt% fiber concentrate in mixes of 0.75, 1.5. and 3.0 minutes duration to determinethe effect of masterbatch viscosity on the dispersion of the fiber. The charge size of the final composite mixes was 1.50 kg, producing a 7 1% fill factor. Increasing the stock viscosity increases power input, which has two beneficial effects on the mixing process: total energy input builds faster with time, and higher stresses are generated to moreeasilydispersethefibers from their highly concentratedinitial state into the final composite. First we see in Fig. 9 that the degree of fiber dispersion continues to correlate well with total energy input, even when the masterbatch viscosity is varied by a factor of 3. Figure 10 breaks out the stock viscosity as a separate parameter when these same data are instead plotted againstthe mix time. It shows that increasingstockviscosity improves dispersion (i.e., reduces residual fiber clump density) for a fixed mixing cycle time. These results are next replotted in Fig. 1 1 against viscosity on the abscissa with mixing time as the curve parameter. In this format. it is possible to extrapolate the lines for each mixing
Goettler and Cole
258
Fig. 8 Equivalcnce of mixing time and power effects on dispersion.
Table 7 Masterbatch Formulatlons for
Variation of Viscosity
A rubber
oxide
EPDM FEF (N-550) blackcarbon Paraffinic oil Zinc Stearic acid
ML 1 + 4 (100°C)
B
304
100100 l12 I00 5 1 328
61.8
42.0
100
112 76 S 1 -
C
D
100 112 125 5
112 150
1 -
1 -
29.6
20.2
353
5
378
259
Short Fiber-Filled Rubber Composites
01
L
0 01
01
1
Total Energy Input. KWH
Fig. 9 Dispersion correlation with mixing
energy independent of stock viscoslty.
ribor DIPP~csiDn.cIumQs/ln.* .l .l.
I
f
.2
.3
I .I
I I I I l l .5 .6 .7.0.91.
I 2
Fig. 10 Viscoslty effects onfiberdispersion
I 3
I I
I
I 1 1 1 1
S
6 7 0 9 1 0
in a mixing cycle.
I 1111
1
I
I
I
2
3
4
5 6 7 ~ 9 1 0 0
260
Goettler and Cole
14 mln. 10
20"
4
0 30 -
MIA-4U0@C) money Vl8co*lty
of Maasterbatch
Fig. 11 Alternate representation of viscosity effects.
. time to their intersection with the x-axis in order to determine the stock viscosity that would berequired to completelydispersethefibersintheallottedmixingtime, i.e., to have zero remaining fiber clumps. Such an extrapolation is effected in Fig. 11 for the data corresponding to a 3-minute mix. This critical value of the stock viscosity can then be correlated with the mix time. The results of this correlation, presented in Fig. 12, show a quadratic relationship between the critical stock viscosity and mixing time that applies for the materials and conditions of this study. The numerical values will differ for other fibers,fiber concentrates, and mixing situations, while the quadratic form of the relationship should be expected to remain valid.
4.4
Theory of Viscosity Effects on Dispersion
The theory of Taylor (1932, 1934) for drop break-up during mixing can be applied to the dispersion of coated (treated) fibers from a concentrate by assuming the fiber concentrate toact like a droplet suspended in a shear field. Accordingly, the break-up of the 'droplet" of concentrate, corresponding to dispersion of the fibers, should be driven by some measureof the variation in viscosity between the fiber coating material and the rubber matrix of the composite. Indeed, Taylor predicts the critical capillary number (drop diameter X shear strednterfacial tension) for break-up to depend onthe viscosity ratio of the phases. Thus, the beneficial effectof a higher masterbatch viscosity could potentially be matched by a lower viscosity of the fiber coating material. The latter effect would not be as strong, however, because the level of shear stress generation under the shear rate of the mixing action that appearsin the definition of the capillary number also depends directly on the viscosity of the matrix phase. Nevertheless, as Fig. 13 demonstrates. reduction in viscosity of the fiber coatingcan significantlyimprove fiberdispersion (reduce the count of undispersed fibers).
tes
Rubber Short Fiber-Filled
261
Fig. 12 Critical stock viscosities required for complete fiber dispersion.
These data pertain to dispersion in the type “F” premasticated rubber masterbatch after mixing for 2 minutes at low speed in a size “BR” Banbury mixer with a 68% fill factor. The viscosity of the coating material represented on the abscissa in Fig. 13 was measured at 100°C and 1000 S” shear rate using a capillary rheometer equipped with a 30:l 0.75-mm orifice.
Conclusions on Dispersion
4.5
Dispersion of the fibers in the final composite improves with increases in the viscosity of the rubber stock (to generate higher dispersive stresses) and the length of the mixing time. The
100
1.01
Fiber Coatlng V l ~ ~ ~ (KPaS) ity
Fig. 13 Effect of coating viscosity on dispersion of the fiber
0.1
concentrate.
262
Goettler and Cole
effects of these separate variables on the degree of dispersion can be taken into account by the single parameter of total mixing power input. Fiber dispersioncan also be improved by reducing the viscosity of the concentrate comprising fibers pretreated with a coating. aswould be predicted by Taylor’s theory of drop break-up. Better dispersion of the short reinforcing fibers benefits the final composite through higher mechanical properties, especially modulus and strength. as well as by improving the surfxe appearance of fabricated articles. The degree of fiber dispersion decreases as the fill factor is increased in the mixer, due to less efficient mixing conditions. Upside-down and two-pass mixing protocols provide improved dispersion over the standard mix, in which the treated fiber is introduced after the masterbatch is already being masticated. There appears to be a good correlation between the degree of fiber dispersion and the tensile strength of the vulcanized composite for all of the single-stage mixes. For unknown reasons, the two-pass mix produces a lower tensile strength at the same state of fiber dispersion. Perhaps fiber breakage occurs under the high stresses generated in the cold composite stock at the start of the second mixing cycle. A power law relationship has been found between the density of undispersed clumps of treated cellulose fiber and the mixing time into a rubber masterbatch in an internal mixer. Breakup of the fiber clumpis believed to be controlling. Further studies identified the factors affecting fiber dispersion and suggested modifications to the rubber and fiber concentrate viscosities for its improvement. By mixing the same treated fiber into various masterbatchesof different Mooney viscosity, it has been shown that the dispersion depends upon the magnitudeof the shear stresses generated and not merely on the length of the mixing cycle. The linear dependence found for dispersion count on viscosity predicts a critical viscosity level required in the rubber for completedispersion to be achieved. This critical viscosity in turn is found to depend upon the allowed mixing time. with which it shows aquadraticrelationship. Thus, higherviscosities would be required to achieve a mix in shorter cycle times. For example, a mix requiring 2-2.5 minutes with a stock of 80 Mooney would require about 3.5 minutes if the viscosity were only 60 Mooney. It has also been shown that in stocks of different viscosity, fiber dispersion correlates well with the total energy input according to a power law relationship. and not with the mix time alone. Although the work described above was performed with wood cellulose fibers. similar dispersion phenomena and criteria for dispersibility of treated fibers would be expectedto apply as well to any discontinuous fiber, such as chopped rayon, nylon. or acrylic textile fiber.
5. SUMMARY Addition of short fibers to a rubber formulation can be facilitated by first treating them to form a concentrate with a coating of low viscosity. However. it may be more cost effectiveto purchase pretreated fiber stock. such as the commercial Santoweb@-treated cellulose fiber soldby Flexsys America L. P. (Akron. Ohio). The dispersion of treatedfibers during theirmixinginto typical rubber compounds to produce short fiber-rubber composites depends on the viscosities of both phases. The degree of fiber dispersion correlates with total energy consumed in the mixing process for variations due to changes in both the mixing time and the viscosity of the rubber stock. A higher degree of fiber dispersion improves both aesthetics as well as composite tensile strength. Care must be taken to avoid fiber breakage during the dispersion process if high composite stiffness is desired. The final properties of the composite also depend heavily upon the degree and direction of fiber orientation imparted during the fabrication step, as well as onthe strength of the chemical
Short Fiber-Filled Rubber Composites
263
bonding at the interface between the reinforcing fiber and the rubber matrix. Recent developments in understanding and controlling these parametershave been identified in the open literature.
REFERENCES Ahlblad. G.. Kron, A.. and Stcnbcrg, B. (1994). Polyrn. Irft. 33( 1 ): 103. Ahlblad. G., Reitberger. T., Stenberg, B., and Danielsson, P. (1996). Polyrx /ut 39(3):261. Akhtar, S., Bhowmick. A. K., De, P. P., and DC, S. K. (1986), J. Mrrter. Sci. 2/(12):4179-84. Arumugam, N., Selvy. K., Tamarc,Rao, K., Venkata, and Rajalingam. P. (1989). J. Appl. P o l y n . Sci. 37(9):2645. Ausias, G., Jarrin., J.. and Vincent, M. (1996). Conrposifrs Sei. Trchrlol. 563719. Bayly, A. E.. Biggs. 1. S., and Radvan, B. (1988), Eur. Pat. 283195. Bhagawan, S. S., Tripathy, D. K., and De. S. K. (1987), J. A/?/>/.Po/yrll Sei. 33(5):1623-40. Bhattacharyya, T. B., Biswas. A. K., Chatterjcc, J., and Pramanick, D. (1986), Pltrst. Ruhber Proc. A/>/>/. 6(2): 119. Corm. A. Y., Boustany. K., and Hamcd, P. ( 1971), J. AppI. Po/xr?l. Sei. /5:247 I . Corm, A. Y., Hamed, P,, and Goettler, L. A. (1976), Rubhrr Cl~ern.Trchrwl. 56:619. Czamccki, L., and White. J. L. (1980), J. App/. P o / w t . Sci. 25:1217. De. K., and White, J. R., Eds. (1996), Slrorf Fiher-Polwer Cornpositex, Woodhead Publishers Ltd., Cambridge. UK. Felix, J. M,, Gatcnholm, P,, and Schreiber. H. P. (1993). Polyrrl. Coruposites /4:449. Flink. P,. and Stcnberg., B. (1989). Br. Po/wz. .l. 2/(3):259-267. Flink, P., and Stenberg., B. (1990a), Br. P o l w . J. 22(3):193-199. Flink, P,. and Stcnberg., B. (1990b), Br. P o l w . J. 22(2):147-153. Flink, P,, Westerlind. B., Rigdahl, M,. and Stenberg, B. (1988). J. Appl. Po/w7. Sci. 35(8):2155-2164. Foldi, A. ( 1992). i n Co~r~posite Applicwtiorls: The Role of M ~ t r i x ,Fiber c r r d htrrfirw T. Vigo, and B. Kinzig, Eds., VCH Publishcrs, New York, pp. 133-177. Gatcnholm. P., Bertilsson, H.. and Mathiasson, A. (1993), J. Appl. P o l w . Sci. 4Y(2):197. Gocttler. L. A. ( 1970). 25th SPI Rcinforced Plastics/Composites Div. Technical Conference, Feb. 1970. Section 14A. Goettler, L. A. ( 1983). P o l w . Cor~rposites4(4):249. Goettler. L. A. ( I988), Short fiber-rubber composites, in Htrndhook ~fEIustorner-s.(A. K. Bhowmick, and H. I. Stephens. Eds.), Marcel Dekker, Inc., New York. Goettlcr, L. A.. and Lambright. A. J. (1977). U.S. Pat. 4,056.591 (to Monsanto Company). Goettlcr, L. A., Lcib, R. I.. and Lambright. A. J. (1979). Rubher Chem Tec/rm/.52:838. Gocttler, L. A.. Sezna, J. A., and DiMauro, P. J. (l982), Rrthher World /K7(1):33. Goettler. L. A., and Swidcrski, Z. ( I 992), in Composite Applicdorrs: The Role of Mcrtrir. F i h t ~t r t l d /rlte~fi/cc,. (T. Vigo, arld B., Kinzig, Eds.). VCH Publishers, New York. pp. 333-363. Goldsmith. H. L., and Mason, S. G. (1967). Thc microrhcology of dispersions, in Rheo/o
264
Cole
Goettler and
Kwon, Y. D., Beringer, C. W., Feldstein, M. A., and Prevorsek, D. 0. (1990), Rubber World 202(2): 29-33. Li, P. C., Goettler, L. A., and Hamed, P. (1978). J. Elast. Plast. 1059. Persson, J. E., and Raanby, B. (1990). World Pat. 900341 I . Sapieha, S., Pupo, J. F., and Schreiber, H. P. (1989). J. Appl. Polym. Sci. 37(1):233. Senapati, A. K.,Nando, G. B., and Pradhan, B. (1988), Int. J. Polym. Mater. 12(2):73-92. Song, X. M,, and Hwang, J.-Y. (1997), Wood Fiber Sci. 29(2):131-141. Subramanian, V., Seshadri, K. R., and Ganapathy, S. (1997), Rubber India 49(3):9-16. Taylor, G. I. (1932). Proc. Rqyal Soc. A138:4l. Taylor, G. I. (1934), Proc. Royal Soc. A146501. Westerlind, B.,Hirose, S., Yano, S., Hatekayama, H., and Ribdahl, M. (1987), Int. J. Polym. Mater. 11(4): 333. Yamamoto, S., and Matsouka, T. (1996). Polym. Eng. Sci. 36:2396. Yano, S., Hirose, S., Hatakeyama, H., Westerlind, B., and Rigdahl, M.(1990), J. Appl. Polym. Sci. 40(5-6): 657-667. Yano, S., Stenberg, B., and Flink, P. (1992), Nihon Reoroji Gakkaishi, J. Soc. Rheol. Japan 20(3):132. Zadorecki, P,, and Michell, A. J. (1989). Polym. Compos 10(2):69-77.
10 Thermoplastic Elastomeric Rubber-Plastic Blends Aubert Y. Coran The Institute of Polymer Engineering, The University of Akron, Akron, O h i o
1. INTRODUCTION At least in principle, a very large number of rubber-plastic blends are possible (Corish and Powell, 1974; Dunn. 1976). However,in this chapter we will be concerned with those compositions, which are softor flexible and whichhave rubber-like elasticity, in that they retract forcibly from large deformations. Such compositions contain larger proportions of rubber than do the impact-resistant, rubber-toughened plastics (Bucknall, 1977),which will not be considered here. Also, we shall not cover blends, which contain thermoplastic rubber block copolymers. Such blends have been reviewed by Kresge (1 984) and are extensively covered in this book. Blends of vulcanizable rubber containing various amounts of resins, which can act as reinforcing or stiffening agents. also were not considered here. Elastomeric rubber-plastic blends have becometechnologically interesting for use as thermoplastic elastomers (Kresge 1978, 1984; Morris, 1979). They can have many of the properties of rubbers, but they can be processable as thermoplastics (O’Connor and Fath, 19811. They do not need to be vulcanized during fabrication into end-use parts. Thus, they offer a substantial economic advantage with respect to the fabrication of finished-part production. Due to their unique fabricability, such materials have added value as raw materials, provided that they also have good properties as elastomers. Although the number of possible elastomeric blend compositions is quite large, relatively few of themare of technologicalimportance. This is largelybecause of thefact that most polymers have been incompatible with one another, at least in the thermodynamic sense (Paul and Barlow, 1979). If the polymers of a blend are thermodynamically compatible, then their blend could exist as a single phase, with mixing having been accomplished on the molecular scale. In such a case, the properties of the resulting blend would tend to be averages of the properties of the two “pure” phases. For example, blending a rubber with a thermodynamically compatible plastic would give a composition with a glass transition temperature between those of therubberandplastic(Olabisi et al., 1979). Frequently,such an averageglasstransition temperature is near room temperature. Thus, a rubber and plastic might be blended to give a highly damped, almost “leathery” material. This, of course, would not be a desirable result. 265
Coran
266
On the other hand, if the rubber and plastic are not thermodynamically compatible, then the blend would containtwophases,withtwoglasstransitiontemperatures.However,such blends frequently contain large particles of one of the polymers only loosely bonded (if at all) to the other (matrix)polymer (Coran and Patel, 1983b). Thelarge, essentially unbonded particles generally act as stress-concentrating flaws. In some cases it is possible to obtain very small droplets of onepolymerdispersed in theother during mixing; however, later,aftermixing. coarsening of the droplets can occur by coalescence during some phase of processing the blend into a fabricated part (Stehling et al., 1981). For many technological end-use applications, the ideal elastomeric rubber-plastic blend would comprise finely divided rubber particles dispersed in a relatively small amount of plastic. The rubberparticles shouldbe crosslinked to promote elasticity (the abilityofthe blend composition to retract forcibly from a large deformation). The favorable morphology should remain during the fabrication of the material into parts. It should be pointed out that, in the “ideal” case proposed above. many of the desired properties could arise as a result of the polymers being t h e ~ r n o ~ ~ n a r n i cincompatible. aI/~ The low glass transition temperature of the rubber phase (not “averaged up” by the hard phase material) would be maintained because of the relative purity of the rubber phase; yet the high crystallinity or high glass transition temperature of the hard plastic phase could be retained for structural integrity over a useful temperature range. In this chapter we will first discuss the elastomers and plastics that have been used in rubber-plastic blends. Then, having noted some of the properties of the rubbers and plastics, we will consider the properties of blends prepared by simple melt blending. Two methods of improving the properties of rubber-plastic blends will then be explored: ( 1 ) dynamic vulcanization (CoranandPatel,1980a, 1980b, 1981b, 1983a; Coran et al., 1982a,1982b; Walker and Rader, 1988; Payne and Rader, 1993), the process of crosslinking the rubber phase during its melt-mixing with the plastic material, and ( 2 ) technological compatibilization by addition (or in situ formation) of small amounts of block copolymers, which contain blocks of each of the polymers to becompatibilized (Coran et al., 1983b, 1985). Both of thesemethods were, of course,devised i n an effort to produce compositions that approachtheabove-stated“ideal material.” Finally, processing and end-use applications will be discussed.
1.l
Compatibility
Before we proceed further, let us consider what is meant by “compatibility.” We refer to two types of compatibility: thermodynamic and technological (Olabisi et al., 1979; Paul and Barlow, 1979). If polymers are thermodynamically compatible, i.e., miscible, as stated above. their intimate mixture exists as a single phase. For this to occur, the following condition mustbe satisfied: AG,,, = AH,,, - TAS,,, 5 0
(1 1
where AG,,,,AH,,,, and AS,,,are, respectively, the free energy, enthalpy, andentropy of mixing. Unlike the case of monomeric materials, the entropy of mixing of polymers is very low. Thus, to be certain that the free energy of mixing would be zero or less, it would be best that the enthalpy of mixing, AH,,,, be negative (i.e., that mixing be exothermic). It would be required that unlike polymer molecules associate with one another more strongly than do like polymer molecules. In other words, specific interactions between unlike polymer molecules would be required, or it would be required that molecules of unlike polymers approach one another more closely than do molecules of like polymers.As a result, polymers are rarely mutually thermodynamically compatible.
Thermoplastic Elastomeric Rubber-Plastic Blends
267
If two polymersaresaid to be tecknologically conyatiblr, it merely means that their blendsaretechnologicallyuseful.Technologicalcompatibilization,then, is anyprocess that improves the properties of a blend to make it more useful. Blends of grossly thermodynamically incompatible polymers are generally useless. Compatibilization techniques for improving such mixtures may be mechanical or chemical in nature. Such techniques generally do not make the mixtures become miscible, i.e.. compatible i n the thermodynamic sense. As a rule, blends of the more nearly thermodynamically compatible polymers give the better compositions, simply by melt blending, without the application of any compatibilization techniques. This is probably because smaller droplets are generally produced during mixing and the adhesion between the phases (Wragg et al., 1981) is better.
2.
RUBBERS AND PLASTICS USED IN BLENDS
Before examining the properties of rubber-plastic blends, it is appropriate to consider the characteristics of the rubber and plastic blend components. This is, of course, because of the fact that the properties of a blend depend, in part, on the characteristics of its component parts. The great differences between the mechanical properties of the rubbers and plastics is shown in Fig. 1, which gives stress-strain curves of typical plastics and rubbers. The curves for polypropylene and high-density polyethylene are typical of crystalline plastics, the curve for polystyrene illustrates glassy materials, while thecurves fornatural rubber and for BR are typical of crystallizable and noncrystallizable unvulcanized rubbers. The stress-strain curve for a meltmixed blend of polypropylene with BR is also shown. Elastic properties of a blend, such as Young’s modulus or shear modulus, are a function of the elastic properties of the blend components and the phase morphology of the blend. The strength-related or ultimate mechanical properties of blends (such as tensile strength, ultimate elongation, fatigue life, etc.), however, are only partly determined by the mechanical properties of the components. Also important are interactions between the polymers at their interfaces. The interfacesbetweenphases of polymerblendsare sometimes called“interphases” since some localized intermolecular mixing or interdiffusion can occur in the case of polymer blends, which approach thermodynamic compatibility. In other words, a small domain of a mixed phase (interphase) can exist between the “pure” phases. This molecular interdiffusion can increase the adhesion between the phases or reduce the effective interfacial tension. Increased adhesion or interdiffusion between the phases would be expected to confer improved ultimate propelties upon the blends. The decreased interfacial tension would be expected to give more extensive subdivision of particles (by the formation of smaller droplets) during melt-mixing. The smaller particles, as we will see later, also can give rise to improved blend properties (Coran and Patel, 19804. The properties of rubber-plastic blends have been correlated with the properties and chardcteristics listed below. Values are given in Table 1. 2.1
DynamicShear Modulus
This property, G*, was taken as a measure of stiffness. It was measured by means of a torsion pendulum with specimens whose dimensions were selected to give test frequencies between 0.5 and 2 Hz. This property was selected rather than Young’s modulus because of convenience. When considering such widely varying materials as rubbers and hard plastics, it is difficult to find a convenient test condition (rate of loading) appropriate for both rubbers and plastics. The
268
Coran
70
60
POLYSTYRENE 50
40
cp
a
z
uj 30
POLYPROPYLENE (PP)
c/) W
U l" 20
cn
l0 5 r
1
H. D. POLYETHYLENE PP-BR BLEND NATURAL RUBBER (NR)
0
20
40
60
200
400
600
800 1000 1200
STRAIN, Yo Fig. 1 Stress-straincurves for variouspolymericmaterials.
shear moduli of the hard andsoftphases (along with rubber-plasticproportions)have correlated with shear moduli of blends (Coran and Patel, 198 1a).
been
2.2 Tensile Strength of the Hard-Phase Material This property, uFl,was considered because it represents a limit for the strength of the rubberplasticblend (Coran et al., 1982a).Yieldstress was used as tensilestrength for crystalline materials rather than the stress at break, which occurs only after necking and drawing. (Generally, the rubbery blends do not exhibit drawing-necking behavior.) Thevalues in Table 1 were determined in the same way as for the rubber-plastic blends, by using molded samples that had been equilibrated against laboratory air.For nylon many of the literature values relateto dried samples and are therefore somewhat higher than the values shown here.
2.3 Crystallinity The weight fractions of crystallinity, W,. of many of the plastics are also given in Table I . The values i n the table are approximations based on the densities of the materials. The reasons for
269
Thermoplastic Elastomeric Rubber-Plastic Blends Table 1 ApproximatePolymerCharacteristics
Polypropylene (PP) Polyethylene (PE) Polystyrene (PS) ABS SAN Polymethylmethacrylate (PMMA) Polytetramethylene terephthalate (PTMT) Nylon-6, 9 (PA) Polycarbonate (PC)
30.0 31.7 42 58" 58 61.8 53.3 46 66.7 -
IIR
EPDM Poly-trans-pentenarner rubber (PTPR) IR (NR) BR
SBR Ethylene-vinylacetaterubber (EVA) ACM Chlorinated polyethylene (CPE) CR NBR
520 760 1170 926 1330 -
909 510 860 0.46 0.97 -
0.32 0.17 0.52 0.93 -
-
0.99
28 29 33 38 38 39 39 39 42 27 28 31 31 32 33 34 37 37 38 39
-
570 460 417 454 416 460 342 778 356 350 290
0.63 0.70 0.00 0.00 0.00 0.00 0.3 1 0.25 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
'' Determined by torswn pendulum at about 1 Hz. Rubbers were not vulcanized. h -yL IS critical surface tension for wetting, mN/m. I ' N, IS critical molecular length for entanglement of rubber molecules, number of chain atoms. "W, is wt. Fractlon of crystallinity. for ABS was consdered to be the same as for SAN. ABS was consldered as SAN containing BR particles: thus The somewhat increased rubber concentration (over 60 wt.%) should have only a small effect on ultimate properties.
considering crystallinity were empirical; however, interesting correlations between hard-phase crystallinity and certain blend properties have been obtained (Coran et al. 1982a).
2.4
InterfacialTension
Wu ( 1 885, 1987) studied the effects of interfacial tension on the dimensions of rubber particles dispersedinpolar polymers suchasnylonandpolyester.Hesuggested that the size of the dispersed rubber particle is directly proportional to the interfacial tension. Elemendorp (1 985, 1986) andElmans ( 1 989) investigated the effectsof viscoelastic properties and interfacial tension on the stability of the phase morphology of polyethylene-polystyrene blends. They concluded that gravity-induced coalescence is due to the high interfacial mobilityin molten polymer blends. Tsai and Min (1997) determined the interfacial tensionof fluoroelastomer (FKM)-polycarbonate (PC) blends in comparison with blends of ethylene-propylene diene rubber (EPDM) and PC. Chen and White (1993)determined the interfacial tensions of a series of high-density polyethylene blends with polystyrene (PS), nylon-6, PET, and 2,6-dimethyl-p-phenylene oxide (PPO). They found that dispersed-phase dimensions increase with interfacial tension, as Wu (1987) proposed. Phase-dimension growth (coalescence) rates increased with interfacial tension values. In order to reduce the interfacial tension, various block or graft copolymers (Patterson et al., 1971; Barentsen et al., 1974; Endo et al., 1986; Wu, 1987; Chen et al., 1993) and maleic
Coran
270
anhydride grafted polymers (Coran and Pates 1982, 1983a; Cimino et al., 1984) have been added to immiscible blends to produce blends of improved fineness of dispersion wherein the growth of phase domains was retarded.
2.5
Critical Surface Tension for Wetting
This parameter, yc, has been used as an estimate of polymer surface energy. It was introduced by Zisman (1964). It was estimated by determining contact angles of various liquids against a given polymer surface. The contact angles were plotted as functions of the surface tension of test liquids. The surface tension of liquid corresponding to an extrapolated contact angle of zero was taken as the critical surface tension for wetting (or spreading). At one time, it was believed that yc was approximately the surface tension y, of solid polymer. At any rate, we felt that the difference between the critical surface tension for wetting (for the rubber and the plastic) might be a rough estimate of the interfacial tension between the rubber and plastic during melt mixing. Interfacial tension is a factor that determines, at least in part, the droplet size of one liquid dispersed in another (Mikami et al., 1975). Lower surface tensions give smaller droplets, which might result in smaller particles of one polymer dispersed in the other after mixing and cooling. The interfacial tension between two immiscible monomeric liquids is approximated by the difference between the two surface tensions. Unfortunately, thisis not the case for polymers. However, there is a hypothetical surface tension, y x , which is characteristic of each polymer listed in Wu’s review (1978) of interfacial tension between molten polymers. If the value of yA for one polymer is subtracted from that of the other, the interfacial tension is estimated fairly reliably. The hypothetical values yx correlate well with yc. It is also interesting that the critical surface tension for wetting correlates with solubility parameter and that differences between solubility parameters (al- IT,)of the polymers of a two-phase system correlate with interfacial tension. Indeed, Helfand and Sapse (1975) have given a theoretical basis for this. From all of this we conclude that Ayc, the difference between critical surface tensions for wetting of each of two polymers, may be at least a qualirari\v estimate of the interfacial tension y , ? . The lower the difference Ay, (sometimes called the surface energy mismatch). the smaller should be the particles of one molten polymer dispersed in the other. Also, a low surface energy mismatch should give better wetting, better interfacial adhesion. and increased diffusion of the polymers across the interface. Though convenient, Aycis not as good an estimate of ylz as is the value calculated by the harmonic mean or geometric mean equations, which require estimates of the polar and dispersion portions of the surface tension of each of the components (Wu, 1978). However, the data needed for such calculations are often not available. Some of the values of y, listed in Table 1 were taken from the literature (Crocker, 1969). Those values not available were estimated on the basis of the correlation between solubility parameter a and y,.
2.6
Critical Entanglement Spacing
Critical entanglement spacing, N,, is defined as the number of polymer chain atoms that corresponds to a molecular weight sufficiently large for entanglements to occur between molecules of undiluted polymer. It has been measured as the molecular weight where the slope of a plot of log viscosity versus log molecular weightchanges from1 .O to 3.4, the change being associated with intermolecular entanglements. The reason for considering entanglement spacing as a parameter to correlate with blend properties was empirical. It was found that dynamically vulcanized elastomers that have IOW values of N, gave the higher-quality blends with plastics (Coran et al., 1982a). Although this
Thermoplastic Elastomeric Rubber-Plastic Blends
RUBBER DOMAIN
INTIERFACE
271
CRYST. PLASTIC DOMAIN
Fig. 2 A schematic visualization of entanglements promoting adhesion across the interface betwecn the molecules of the different polymers. After vulcanizationand after coolingof the composition, entanglementderived locked-in loops improve interfacial adhesion.
was an empirical observation, one might speculate why such elastomers give the best blends (with respect to ultimate properties). It has been observed that when polymers are blended together, fibrous structures appear, which then break up intopolymer droplets(Avgeropoulos et al., 1976; Hamed, 1982). We believe it likely that polymers whose molecules are more entangled might be drawn into finer "fibers" during the early phase of mixing to give emulsions of polymer droplets of smaller size. Of course, after dynamicvulcanization these droplets would become very small vulcanized rubber particles. Another explanation could be that a tendency for entanglements to promote adhesion if some of the entanglement occurred across the interface between the molecules of the different polymers. After vulcanization and after cooling of the composition, such entanglement-derived locked-in loops should improve interfacial adhesion. A schematic visualization of this is shown in Fig. 2. Values of N,obtained under the same conditions for all of the elastomers arenot available in the literature. However, it is possible to calculate values of N, from the chemical structure of the elastomer molecules by using a modified methodof Aharoni ( 1 977). Thecalculated values appear in Table 1.
2.7
Melt Viscosity
Melt blending is most efficient when the viscosities of the phases are the same (Avgeropoulos. et al., 1976).It should be noted that the viscosities must be measured under the same conditions as mixing for the above to be true. Such measurements are difficult, if not impossible. It is not easy to measure the shear ratesand measure the viscosities on the phase domains duringmixing. However, there is an approximate solution to this problem. Mixing torque values for molten rubbers and plastics can be measured in a small laboratory internal mixer such as a Brabender@ or Haake@ internal mixer; thesame volume of molten material must be used, and the temperature
Coran
272
must be the same as the anticipated melt-mixing temperature. The relative mixing torque mismatch between the rubber and plastic can be taken as the viscosity mismatch and correlated with blend characteristics.
3. THE PREPARATION OF RUBBER-PLASTIC BLENDS Polymer blends, in general, have been prepared commerciallyby melt-mixing, solution blending, and latex-mixing (Gesner, 1959). Rubber-plastic blends have generally been prepared by meltmixing techniques. Melt-mixing is an easy and economical way of blending different polymers to avoids problems of contamination,solvent or waterremoval, etc. Melt-mixing has been accomplished by various mixing devices, including two-roll mills, internal mixers, and various types of twin-screw extruders. Compounding rubbers with additives on an open mill has been used often in rubber industry. However, mixing on an open roll mill, in air, at elevated temperatures induces oxidative degradation in many cases. It is also difficult to handle the low-viscosity plastics at high temperatures. Internal mixers and various twin-screw extruders are more efficient and give less oxidative degradation. An internal mixer is a batch mixer, whereas a twin-screw extruder is a continuous mixer. For the purposes of this chapter, emphasis will be on laboratory melt-mixing techniques, whichsimulate what canbe done in afactory, but on a smallscale.Severaltechniques for preparing rubber-plastic blends will be considered. First we will consider blends prepared by merely mixingmoltenthermoplasticresin(theplastic phase) withunvulcanized or slightly vulcanized rubber at temperatures above the melting point of the plastic resin. We will then consider blends prepared by dynamic vulcanization, the process of vulcanizing the rubber during its melt-mixing with plastic. Also considered, in a later section, will be dynamic vulcanizates wherein the rubber and plastic phases had been compatibilized by the incorporation or in situ formation of small amounts of rubber-plastic block or graft copolymers. The procedures given below are based on the use of either a small Brabender mixer with cam-type rotors or a Haake Rheomix internal mixerequipped with cam-type rotors, and optimum batch sizes were between 55 and 75 g.
3.1
Blends Prepared by Simple Melt Mixing
Simple blends of rubber and plastic are preparedby mixing the ingredients for about 2-5 minutes at stock temperatures above the melt or softeningtemperatures of the plastic in an internal mixer at speeds of 40-80 rpm. Theappropriate mixing temperature depends onthe nature of the plastic or resinous material. The temperatures must be high enough that plastic material is molten and flows easily enough for mixing. At the same time, temperatures should be as low as practically possible in order to prevent the occurrence of oxidative degradation. It was expedient to press a fresh hot batch between the platens of a cold press or to pass the batch through the nip of a cold roll mill. Typical melt (stock) temperatures are given in Table 2 for individual blends. The mixed materials can be chopped or pelletized for the use in extrusion, injection molding, hotcalendaring, etc. Test specimens can be easily prepared in the laboratory by compression molding using platen temperatures somewhat higher than the mixing melt temperatures. Good test sheets can usually be obtained by molding at a pressure of about 1.5 MPa between aluminum foil well treated for release. After about 1-2 minutes of heating in the hot press, the moldings are cooled under pressure.
273
Thermoplastic Elastomeric Rubber-Plastic Blends Table 2 Mixing and Molding Temperatures Plastic Polypropylene (PP," Polyethylene (PE)" Polystyrene (PS)" Acrylonitrile-butadiene-styrenepolymer (ABS)" Polystyrene-co-acrylonitrile(SAN)" Polymethyl methracrylate (PMMA)' Poly-tetramethylene terephthalate (PTMT)' Nylon-6, 9 (PA)" Polycarbonate (PC)'
Stock mixing temp. ("C)
Molding temp. ("C)
185-190 160-180 170- 190
210 210 210 210 210 210 250 220 250
170-190 170-190 170-180
220-230 210 200-220
'' Profax@ 6723. " Marlex@ EHM6006. ' Lustrexm HHlOl or Dylark@ 232. Lustran@ 740 or Lustran 246. " Lustran@ DNS2. DNS7. or DN77. I Lucitew 147. Tenitem 6P20A. " Vydyne@ 60H. ' Melron* M40F.
''
The above mixing procedure can be adapted to a commercial-scale mixer. After being chopped, the Banbury "crumb" can be extruded, pelletized, and injection-molded, hot-calendered. etc.
3.2 Blends of Slightly Vulcanized Rubber with Plastics Fisher (1973) claimed improved compositions of EPDM blends with polyolefin resins such as polypropylene (PP). In such blends, the rubber can be slightly crosslinked by the action of an organic peroxide. The slightly vulcanized rubber is then melt-mixed with polyolefin resin. The mixing temperature and time are selected to ensure good melt flow long enough to maximize the intimacy of the mixture. The partial vulcanization of the rubber greatly improves (reduces) the permanent set of blend compositions. A disadvantage of the process of vulcanizing rubber before mixing it with polyolefin is that the compositions generally contain rather large rubber particles. Nevertheless, the process has yielded useful commercial products.
3.3 The Preparation of Blends by Dynamic Vulcanization Dynamic vulcanization, asit is practiced today, is the process of vulcanizing rubber in an intimate blend with a plastic while the blend is being mixed, e.g., in a mixer. An earlier process, called "dynamic vulcanization," was developed by Gessler in 1962. He prepared rubber-plastic blends that contained minor proportions of vulcanized rubber, and the crosslinking reactions started before the formation of an intimate blend. In a patent issued in 1973, Fisher (1963) applied his version of dynamic vulcanization to the preparation of compositions containingvarying amounts of partially vulcanized rubber. An organic peroxide was used to crosslink the rubber in the presence of the resin PP. The PP was greatly damaged by the action of the peroxide. Corm et
Coran
274
al. (1978, 1980a) found that high-strength elastomeric compositions of EPDM and PP could be prepared by dynamic vulcanization provided that peroxide curatives were avoided. If enough plastic phase is present i n the molten state, then the compositions are processable as thermoplastics. It is important that the rubber andplastic be well mixed before the onset of vulcanization and that the mixing should continue during and for some time after the completion of vulcanization. Plasticizers or extender oils can be used to expand the volume of the rubbery phase. In the molten state, the plasticizer expands the volume of both the rubber and the plastic phases. If the plastic material is a crystalline polymer such as PP, then upon cooling the crystallization of the material forces the plasticizer out of the plastic phase into the rubbery phase. Thus, the plasticizer acts as a processing aid at melt temperatures and as a softener at temperatures of use. This technology has ledto a significant number of new thermoplastic elastomeric products commercialized during the last half of the 1980s (Abdou-Sabet and Patel, 1991). It is important to note that the con~mercializationof the dynamic vulcanizationtechnologywasfacilitated by the discovery of preferred compositions based on Lewis acid-catalyzed methylol-phenolic dynamic vulcanization systems for the new thermoplastic elastomers (Abdou-Sabet and Fath, 1982). The dynamic vulcanization process has been applied to many rubber-plastic combinations. A specific procedure is suggested to ensure the ideal elastomeric rubber-plastic blend, which comprise finely divided rubber particles dispersed in a relatively small amount of plastic. The procedure is as follows: rubber and plastic are first melt-mixed in the same way as for simple blends. Then, after sufficient melt-mixing to form a well-mixed blend, vulcanizing agents (curatives, crosslinkers) areadded.Vulcanization then occurs whilemixingcontinues. The more rapid the rate of vulcanization, the more rapid the mixing must be to ensure good fabricability of the final blendcomposition. It is convenient to followtheprogress of vulcanization by monitoring mixing torque or power consumption. After the mixing torque or power curve goes through a maximum, mixing can be continued somewhat longer to improve the fabricability of the blend.Duringthistime,mixingtorque or power may become somewhat reduced. After discharge from the mixer, the blend is handled in much the same way as the above-mentioned simple blends.
4. 4.1
PHASE MORPHOLOGY Phase Morphology Developed During Mixing
Micro-scale morphology is a major determinant of the properties of heterogeneous polymer blends. Nielsen (1974) has pointed out that, in systems of block polymers and in polyblends prepared by melt mixing, phase morphology changes as a function of composition. A given immiscible polymer blend has a characteristic multiphase structure. In each case, this is determined by polymer selection, blending conditions, rheological properties of polymeric components, etc., as well as by the proportions of polymeric components mentioned above. If a blend of two polymers prepared by molten-state mixing contains small portionsof either rubber or plastic (less than about 30% by volume), one observes particles of the polymer, which is present in the lower concentration. If the volume concentrations of the components are similar. then co-continuous phases can be observed (sheets, rods, or random co-continuous-type structures). The dimensions of the dispersed phase in the plastic-rubber and rubber-plastic blends are in the range of 1 p m to about 200 k m (Wu. 1985, 1987; Jordhamo et al., 1986; Bhowmick and Inoue, 1993; Chung and Coran, 1997). Mixing is most efficient when the viscosities of the phase components aresimilar, and the smallest particles are obtainedwhen the interfacial tension
Blends Rubber-Plastic Elastomeric Thermoplastic CR
275 EPDM
NBR
CHLOROBUTYL
25 %
50 %
75 %
LM PHASE CONTRAST
40
Fig. 3 Photomicrographsof pure gum blends with natural rubber. (From Callan et al., 1971.)
between the phase components is the smallest. However, wealso note that very smallparticles are obtained when one of the components, having a lower viscosity than the other, is present at a low concentration. The microscopy of blends of rubbers has been studied extensively (Callar et al., 1971; Avgeropoulos et al., 1976; Hamed, 1982);excellent photomicrographs of such blends are available in the literature. Rubber-rubber blends are good models for polymer-polymer blends in general. This is because theycan be mixed at lower temperatures, they can be crosslinked (e.g., by radiation) to stabilize the phase morphology againstchanges due to agglomeration,annealing, etc., and they can be stained (e.g., by the action of osmium tetroxide) for electron microscopy opacity. Figure 3 illustrates the relationship between polymer proportions and morphology. As stated above, intermediate or nearly equal-volume proportions of polymers can give a variety of phase morphologies. This was also illustrated by the work of Avgeropoulos et al. (1976), in which the phase morphology of a 50/50 blend of EPDM and BR was determined only by the mixing condition. In that case, either polymer could be dispersed or the phases could be cocontinuous, depending upon the mixing temperature.The change in morphology was attributed to changes in thedifference between the viscosities of the EPDM and BR, each of which varied as a different function of temperature. In the composition mixed at 112"C, the EPDM was the most viscous, thus it became the dispersed phase because of the tendency of the least viscous or more fluid phase to encapsulate the viscous phase, thereby minimizing the energy required for mixing. In the50/50 EPDMBR composition mixed at 20"C, the BR was mostviscous under the conditions of mixing; thus it became the dispersed phase. In the composition mixed at 55"C, both the EPDM and the BR phase were continuous.
276
Coran
Fig. 4 Effect of mixing time on the phase morphology of BrabendeP-mixed EPDM/BR blends: (a) 5 min (3300X), (b) 15 min (3300X), (c) 30 min (10,OOOX). (From Hamed, 1982.)
Other characteristics of phase morphology are particle size and shape. Avgeropoulos et al. (1976) and Hamed (1982) have studied the effects of phase morphology of melt-mixed elastomer blends as a function of time of slow mixing. Early inthe mixing, the dispersed phase appears as large elongated structures that become drawn into fibrous domains. Upon further mixing, the elongated droplets are broken into smaller spherical particles or droplets. This is illustrated in Fig.4. There are some similarities between thisscheme of events and that observed by Scott and Macosco (1991) with respect to blends of thermoplastic resins. Thus, mixing time is a determinant of particle size and shape. It hasalso been reported that mixing torque and mixingtorque mismatch are determinants of particle size. When mixing torques (measured under the conditions of mixing) are well matched and when they are high, then very small domain sizes are obtained (Avgeropoulos et al., 1976). The equality of viscosities of the two phases during mixing maximizes the transfer of mixing stresses between the phases, while increases in viscosity give increased stresses at a given speed of mixing (rpm of rotors). The maximized distribution of mixing stresses (due to viscosity matching) and generation of increased stresses (due to higher viscosities) would be
Thermoplastic Elastomeric Rubber-Plastic Blends
277
expected to givemore extensivebreak-up of thedispersedphase. It is true that the above considerationsrelate to rubber-rubberblendsratherthan to rubber-plasticblends.However, studies of the microscopy (Danesi and Porter, 1978; Kalfoglou, 1983a,b) of blends of rubbers with plastics have given similar patterns of morphological behavior. Similar conclusions have been reached with respect to plastic-plastic blends (Grace, 1982). The effects of viscosity ratio appear to be general, indeed similar, though less dramatic effects were noted vis-a-vis nonpolymeric Newtonian fluids (Taylor, 1932, 1934). In addition to the effects of changes in the concentrations of the components, particle size of the dispersed phase is greatly affected by interactions between the phases. Large particles can be the result of a gross difference between the surface energiesof the two phases. It should be noted that the surface energies of the phases are more nearly matched when the polymers are more nearly compatible in the thermodynamic sense. Starting from a low level, as the concentration of a component is increased, a critical concentration is reached wherein a co-continuous structure is achieved where phase inversion occurs. Avgeropoulos et al. (1976) showed mixing torque ratios (similar to viscosity ratios q , / q,,,)versus morphology of EPDM-BR blends as a function of composition. A conclusion from this work is represented by Fig. 5. PaulandBarlow (1980) and Jordhamoet al. (1986)
Only phase B continuous
Only phase A continuous
L
I
0
0.1
0.2
0.3
0.4
0.6
0.6
0.7
I
I
0.8
0.9
1
Volume fraction of phase A Fig. 5 The effects of viscosity ratio and concentrations of components on the type of blend phase morphology.
278
Coran
Table 3 Charactcristics of Polymers Used in this Study
Polymer
Dcnsity
9.8 EPDM- 1 EPDM-2 22.9 EPDM-3 NBR- 1 NUR-2 28.6 PP CPET
0.86 0.86 0.86 0.99 0.99 0.9 1.18
Torque Ethylene content
44.1
(?h)
Bound AN
(?h)
58 50 60
18.4
29.1-35.9 29.1-35.9
10.8 9.5
proposed an expression relating phase continuity to the ratio of the viscosity ratio and volume fraction ratio: ( q l l q 2 )X ( & / + I )
continuity
phase
22
1, phase
5
I , phase (2b) 1 continuous 1. dual
continuous (2a) (2c)
where phase 2 is continuous when the lefthand side is greater than unity, phase 1 is continuous if the expression is less than unity, and dual phase continuity (co-continuity) arises when the quantity is approximately unity. They noted that this model for phase continuity and inversion conditions in mechanical blends is limited to low shear rates. Chung and Coran (1997) studied the effects of polymer selection, composition,viscosity. mixing history, etc. on the phase morphology of rubber-plastic blends. Polymers for that study (Table 3) were selected for three reasons: To obtain compositions based on nonpolarhonpolar, polar/polar, polarhonpolar, and nonpolar/polarrubber-plasticblends to determine effects of changes in interfacial tension 2. To determine effects due to viscositymismatch 3. To determine effects due to changes in polymer concentrations 1.
Thus, one selected PP as a nonpolar plastic, EPDM as a nonpolar rubber, copolyester-ether thermoplastic elastomer (CPET) as a polar plastic. and nitrile rubber (NBR) as a polar rubber. The copolyester thermoplastic elastomer was considered here as a plastic. The polymers were melt-blendedat 190°C by using a small-scale laboratory mixer, with the volume of each mix kept constant. Mixing torques and temperatures were recorded as functions of mixing time. After mixing, a batch was removed from the mixer and pressed to a thickness of about 3-4 mm between cold platens of a hydraulic press. It was then Compression-molded at 190°C. Small samples were taken for electron microscopic examination directly from the mixer after the batch was cold-pressed and afterthe composition was compression-molded. In the case of sampling directly from the mix. a procedure was devised to cool a small sample very rapidly to minimize coalescence. Phase morphologies of the blend samples were observed by SEM. The structural dimensions and morphology types are given in Table 4. ln general, the more nearly matched are the polarities, the finer is the texture of the phase morphology in the quick-quenched samples. Also, the coarsening during compression molding was greatest when the polarities were mismatched. The more divergent the viscosities (or mixing torques) of the blend components, the coarser is
Thermoplastic Elastomeric Rubber-Plastic Blends
279
Table 4 Typical Phase Domain Dimensions and Types" Fast
Blends EPDM- IIPP
EPDM-2lPP
EPDM- llPP
NBR- IIPP NBR-?/PP NBR- IICPET
NBR-2lCPET
EPDM- I ICPET EPDM-2/CPET
Blend ratio, WP (vol.%) 80120 60140 40160 20180 80120 60140 40160 20180 80120 60140 40160 20180 56.61434 11.8182.2 56.6143.4 11.8182.2 63.1136.3 33.9156.1 22.7171.3 63.61363 43.9156.1 32.7174.3 66.9133.I 25.2114.8 66,9133. I 25.2174.8
quenched I minute aftcr mas. torque ~
0.2 SR -
1.0 SR
1.5 R 0.4 PR -
8.0 PR 7.0 PR 8.0 PWSR 7.0 PR 6.0 PR 8.0 PR -
5.0 L 1.5 PR 1.5 PR 8.0 L 1.6 PR 1.4 PR 2.0 RP 4.0 PR -
10.0 PR
Fast quenched 4 mmutes aftcr
AfterAfter
cool
pressing max. torque
1.3 c 0.5 SRIPR 0.2 PR 0.3 PP 0.5 PWC 0.4 PR 0.3 PR 0.4 PP 1.5 PR 0.7 PR 0.6 PR 5.5 PR 4.7 PR 4.5 L 6.0 PR 3.0 c 1 .0 PR 0.6 PR 4.0 c 1.2 PR 0.8 PR 2.0 PP 4.0 PR 2.5 PR 5.0 PR
compression
molding -
0.1 SR 1.0 L
0.7 SR 0 . 3 SR -
1.5 SR 0.6 SR 0.5 SR 4.0 L 3.5 SR 5.0 L -
3.5 L 1.2 SR 1.1 PR 4.5 c 1.5 c 1.2 PR 2.0 L 6.0 PR -
2.2 PP 3.3 PP 2.0 PR 0.9 PR 1.1 PP 2.2 c 1.8 PR 1.5 PR 0.7 PP 2.0 c 1.8 PR 0.9 PR 10.0 c 8.5 PR 8.0 c 6.0 PR 5.0 LIC 3.5 PR 1.5 PR 5.0 CIL 3.5 PR 1.5 PR 10 PP 7.0 PR 5.0 PP -
" The nuruhers are typ~calnunmum dimenslons o f :I phase domam. and the letters after thc numhers indicate the following: C. random co-continuous phases: L, Iamnar co-contInuow PR, particulate rubber dom;ms dispersed m continuous crystalline plastic: PP. particulate crystalline plast~cdispersed In rubber nlatrlx: RP. crystalline plast~crods dispersed In rubber; SK. rubber sheets dispersed In crystalline plast~cmatrix.
the morphological texture. However, the particle size of polymer blends more strongly depends on the interfacial tension (polaritymatch) than the viscosity ratio. Viscosity ratio requirementsfor small phase-domain dimensions are much less critical than expected. especiallywhen interfacial tension is low. The type of morphology varies both with the concentrations of the components and with the viscosity or mixing torque ratio of the two phases. The high-concentration component tends to be the matrix phase in a dispersed-particle morphology (e.g., in the 80120 and 20/80 rubberplastic volume ratio blends). In the case of the 60/40 or 40/60 rubber-plastic blends. there can be a tendency towards the formation of co-continuous blends. I n the cases studied, the rubber was generally the more viscous phase and could be particulate even in the case of 60/40 rubberplastic blends.
280
Coran
Whentherubberandplastic phases arenearlymatchedwithrespect to polarity(e.g., EPDM/PP blends), the development of the phase morphology is very fast. This is especially true as the viscosities of the polymer components approach one another. When the hot batchis cold-pressed, then a striatedor laminar phase morphology is formed and texturaldimensions aregreatly increased. Duringcompression molding, the laminar structure transforms itself into a random co-continuous structure of vastly increased textural dimensions, i.e., having an extensively coarsened structure. The coarsening is greatest when polarities are most divergent and when the viscosities of the polymers are lowest. Laokijcharoen and Coran (1996) studied the kineticsof the evolution of phase morphology during molten-state mixing of natural rubber and high-density polyethylene. NR (viscosity of 60 & 5 Mooney units) and HDPE (melt index 5.5 g/10 min),in 30/70,50/50, and 60/40 volume ratios, were melt-mixed at 160°C in a laboratory mixer at 10. 20, 40, 60. and 80 rpm. After 2 minutes (to melt the polyethylene) the rubber was added, and as soon as the rubber was taken into the mixer the ram was dropped simultaneously with the start of the clock. Samples were collected every minuteby using a cold, speciallydesigned tool, with littlechance forcoalescence. Samples takenaftereveryminute of mixingwere cryo-microtomed.The microtomed surfaces of samples were treated by Os04 vapor for selective staining of the rubber phase. The treated specimens were observed by SEM. Photomicrographs of all of the blend specimens were analyzed to find the average diametersof the dispersed NR particles. Number-average diameters, DN, and weight-average diameters, Dw, were found. Under all circumstances, the only type of phase morphology observed was dispersed NR particles in a polyethylene matrix. It appeared that during mixing of the blends, the mass of rubber quickly ruptured to form rather large pieces, which behaved as agglomerates of bundles of entangled rubber molecules, which then eroded to form the small particles that may have been small bundles of entangled rubber molecules that became the ultimate particles of the dispersion. Thisled us to a mathematical treatment, which wasnot unlike the treatment of the dispersion of filler particles in rubber by erosion of the larger agglomerates applied by Rwei and Manas-Zloczower (1990). The presumed erosion that occurred in this N R P E specific polyblend system was described as a function of total strain, t* (the accumulated number of mixer-rotor revolutions at time t, i.e., t X rpm), and rubber concentration, +R:
which integrates to
where m and n are constants, each having a value of about 2. The longer the mixing time, the finer is the particle size. After 1 minute of mixing at 40 rpm, large pieces of rubber were observed and a few very small pieces of NR were dispersed in the HDPE matrix. The amount of small-sized particles increased, and the sizes of the large particles appeared to decrease with time. It was surprising that the mixing speed and mixing torque were not important; only the total number of mixer revolutions determined the degree of dispersion in the cases studied. Compatibilization of natural rubber-polyolefin thermoplastic elastomeric blends by phase modificationhasbeendiscussed (Choudhury and Bhowmick, 1989).Adhesionbetweenthe components plays a key role in determining the morphology. The role of chemical compatibilizers in the rnorphology of the blendsof hydrogenated nitrile rubberand nylon has been established with the help of light scattering, SEM, and TEM studies (Bhowmick et al., 1993).
Thermoplastic Elastomeric Rubber-Plastic Blends
4.2
281
Morphology Developed During the Final Fabrication Processes
The n1icro-scale phase morphology of rubber-plastic blends can change greatly during the final fabrication processes. Processes such as injection-molding, extrusion, and calendaring require the moltell blend compositions to tlow through channels. orifices. etc. The flowpatterns induced by such processes are generally highly directional throughout rather large sectors of a given molded part. Thus. by a combination of flow-induced alignment and agglomeration, relatively largefibrous or even sheet-likestructurescan form; for example, sheathkore structurescan form during extrusion and injection molding (Min et al., 1984; Ghiam and White, 1991). Occasionally. such structures are desired; however. they generally act as large tlaws, which serve only to concentrate the stresses and causepremature failure. For example, if a layer of unvulcanized rubber forms in a part produced from a rubber-plastic blend, then under stress the part will be especially liable to failure in the weak, unvulcanized rubber layer. Frequently such laminar tlaws show up as weak knit-lines in injection moldings. They form at the confluences of streams flowing from the various mold gates. Another source of morphological change after mixing is simply the coarsening of the rubber-plastic emulsion due to melt agglomeration a s reported by Stehling et al. (198 1). If a formed part is allowed to remain in the melt for an extended period with little or no occurrence of tlow, then the particle sizes can increase as touching or colliding particles agglomerate. This phenomenon has also been observed occasionally in the plastic-plastic blends, which have large differences in interfacial tension. even during extrusion and injection molding (Barentsen et al. 1974, Liang et al.. 1983: White and Min, 1985). During extrusion or injection molding. the inner regions of the polymer melt in the dies or molds have quiescent zones. There. the dispersed phase tends to coalesce and generate large particle sizes: this caneven causephase inversion (Stehling, 1981; White and Min, 1985). Again, the result is the formation of large structures, which can act as tlaws. Dynamic vulcanization, the process of crosslinking the rubbery phase during the mixing process (Coran and Patel, 1980a), can greatly improve morphology stability, and thus, it can be used to avoid the problems just mentioned. Dynamic vulcanization can improve the quality of finished parts since the vulcanized rubber particles formed during the dynamic vulcanization process greatly resist agglomeration. The process also forces the noncrosslinked plastic phase to be continuous. Since the ultimate properties are greatly dependent upon the properties of a continuous phase. and since the strength of the plastic (hard) phase material is generally greater than that of the rubbery phase. dynamic vulcanization greatly improves the ultimate properties of rubber-plastic blends. This will be covered in greater detail in a later section. Other morphological changes that can result from the product fabrication after the mixing process have been reviewed by Kresge ( 1978. 1984). In the case of rubber-plastic blends, such a s those of EPDM rubber with polypropylene. a strong tendency for the surfaces of moldings to be rich in the plastic phase has been observed. Generally, in such cases the plastic phase has the lower viscosity under the conditions of molding. and thus it tends to encapsulate the bulk of the molding. Another effect pointed out by Kresge (1984) has to do with the morphology of crystallites in the crystalline plastics. In such cases, extreme fineness of the blend texture can greatly limit or even prevent spherulitic growth. As a result of this. in the case of an intensely mixed 70/30 EPM-polypropylene blend, only nonspherulitic smectic and monoclinic crystallites could be microscopically observed. A one-phase compositionis formed in a blend of a noncrystalline plastic with a thermodynamically compatible rubber. I n such a case. phase morphology cannotbe observed; the propelties of the individual components are not expressed a s such, because only average properties are obtained. This occurrence can be used to test for the existence of phase boundaries without the
Coran
282
use of a microscope. For example, theglasstransitiontemperatures T,, of a blend can be measured by scanning calorimetry. or by the measurement of a mechanical loss peak temperature, etc. If a l l the T, values of all the polymeric components are observed. there must exist a phase corresponding to each polynleric component. If only the average T, value (the average of those of the polymeric components) is observed, then only one phase exists in the blend. In some cases. both average and single-component T, values are observed. In such cases, there may be phases consisting essentially of the "pure" polymer components as well as phases that comprise molecularly mixed polymeric components. Relatively small broadening shifting of the rubber and plastic glass transition temperatures towards one another can indicate that there is a limited but significant amount of phase mixing (Olabisi and Farnam, 1979).
5. 5.1
PROPERTIES OF UNVULCANIZED RUBBER-PLASTIC BLENDS General Considerations
The properties of heterogeneous rubber-plastic blends are determined by a number of factors. many of which have been mentioned in previous sections. The main factors are: 1. The material properties of the rubber and plastic phases 2. The I-ubbedplasticproportions 3. The phasenlorphology 4. The interactionsandproperties at the interfaces (or interphases)
This is illustrated in part by Fig. 6. which is an idealized plot of hysteresis (e.g., tan S) and shear modulus against temperature.At very low temperatures (below the glass transition temperatures of both the rubber and plastic phases). the cornposition is very stiff. (At such temperatures, compositions are also generally brittle.) As the temperature is increased. the modulus changes relatively little until the glass transition temperature of the rubber phase is approached. Then the shear modulus drops rapidly with increasing temperature. Hysteresis increases rapidly to a maximum that corresponds roughly to the temperature at which the modulus decreases most rapidly with temperature. This temperature.which corresponds to an inflectionpoint in the modulus-tempelature curve and a peak in the hysteresis-temperature curve. is the glass transition temperature of the rubber phase. As the temperature is further increased, another range of temperatures is reached in which modulus (or hysteresis) changesrelatively little with temperature. This plateau is the "rubbery region" of thenlodulus-temperature curve. It embraces therange of temperatures at which the composition has elastomeric properties. The modulus level in this region of the modulustemperature curve is determined largely by rubber/plastic proportions (being lower when higher proportions of rubber are used). As the temperature is further increased, another temperature range is reached wherein the modulus again decreases (andhysteresis increases) rapidly as a function of the temperature (Fig. 6). This occurs as the glass transition temperature of the plastic is reached. If the plastic phase is completely glassy (as opposed to being at least partially crystalline). then the composition loses essentially all of its stiffness as the glass transition temperature is exceeded. If the plastic or hard phase is at least partly crystalline, then as the temperature is furtherincreased.the rubbery plateau is extended after a tnodest drop i n the modulus of the composition that occurs in the region of the glass transition of the plastic phase. Note that there is also 11 hysteresis nxlxinlum associated with the glass transition temperature of the plastic phase. Then with further temperature increase. the crystalline plastic phase melts. with an increase in hysteresis and total loss of stiffness of the composition.
283
Thermoplastic Elastomeric Rubber-Plastic Blends
Z
U
o -l
S o f t domain Tm Ts
c
TEMPERATURE
-
Hard domain Tg
.c
c
Fig. 6 Dynnmlc mechanlcnl properks 21s a function of temperature.
The effects of changes in the rubber/plastic proportions on stress-strain curves are illustrated by Fig. 7. Here, melt-mixed blends of EPDM and polypropylene are used as examples. In the case of pure polypropylene, the typical necking and yielding behavior is observed. As the rubber is added to the crystalline plastic-phase material (polypropylene), the tendency for neckingdecreasesand the maximumstress (either yield stress or breakstress,whichever is higher) decreases. In the case of the example blends of polypropylene and EPDM, compositions that have useful strength and other properties can be obtained. This is because the rubber and plastic have similar surface energies and similar polarities, e.g.. as measured by the Hildebrand solubility parameter. This is consistent with the expectation of a very low interfacial tension between the phases. However, most rubber-plastic combinations givecompositions whose ultimate properties (strength or ultimate elongation) are very poor. In such cases, the rubber and plastic are grossly incompatible in the thermodynamic sense, and they have vastly different surface energies. The result is the generation of poorly bonded large domains during melt mixing. These can act as stress-concentrating flaws. In addition to being poor. the ultimate properties of such blends are both difficult to reproduce and unpredictable. On the other hand. low-strain elastic properties
Coran
284
PROPERTY VALUE 1000
stress at break, MPa
0.1
’ 0
I
20
l
I
60 EPDM CONC., %
40
l
80
100
Fig. 7 Mechanical properties of (Brabender melt-mixed) Epsyn@ 70A EPDMProfaxw 6723 polypropyleneblends,showing stress at yield my. stress at break mH, ultimate clongationandYoung’smodulus E.
such as Young’s modulus or shear modulus are predictable with a fair degree of confidence. The interfacial interaction could be improved by specific interaction between the components. The influence of interaction between nylon and acrylic rubber on the mechanical and dynamic mechanical properties of the blends has been demonstrated. Using this concept, Jha and Bhowmick (1997) reported a series of unique thermoplastic elastomers from the reaction blending of the rubber and the plastics. 5.2
Predicting Elastic Moduli of Melt-Mixed Rubber-Plastic Blends
Manymethodshavebeen used to predict the elasticproperties of heterogeneousmaterials (Smallwood, 1944; Guth, 1945; Mooney, 1951;Kerner, 1956: Hashinand Shtrikman,1963;
Thermoplastic Elastomeric Rubber-Plastic Blends
285
Takayanagi etal.1963;Tsai,1968;Davies,1971; Coran and Patel,1976). They have been applied to filled resins, filled rubbers, fiber-reinforced resins, block polymers, and polyblends. Some of these methods were basedon a modulus-viscosity analogyusing Einstein’s ( 1905) relationship for the viscosity of rigid spheres in a fluid. Kerner (1956) devised a theory based on the effect of hydrostatic stresses on a spherical particle imbedded in and well bonded to its matrix. Kerner’s equations were recast as the Halpin-Tsai equations, which accommodated nonsphericalparticulategeometry (Tsai,1968).To use any of thesemodels,precisephase geometry must be known. A more general approach has been given by Takayanagi et al. (1963). In this case a twophase material was treated as a mixture of series and parallel elements. However, in using this approach, fitting parameters relatingto model geometry must be evaluated for eachrubberlplastic volume proportion. Thus, this approach is not useful for predicting properties of compositions as a function of hardhoft proportions. (The Takayanagi approach is excellent for predicting the properties of a particular composition as a function of temperature, the property-temperature profiles of the component materials being known.) In this section, an approach to the prediction of moduli of compositions of systems wherein the phase morphology is a function of hard or soft volume fraction is described (Coran and Patel, 1976, 1981a). It is generally accepted that the modulus of a blend must be between the parallel-model upper bound, Mu, and the series-model lower bound, ML. given by the equations MO = VHMH + VsMs and MI. = (VH/MH
+ Vs/Ms)- ’
where MH and MS are the moduli of the pure hard (plastic) and soft (rubber) phases, and VH and Vs are the volume fractions of the hard and soft phases, respectively. The modulus M of the two-phase composition can then be written
f“
M =
-
MI-)
+
Ml~
(7)
where f can vary between zero and unity. The value o f f is a function of phase morphology. If only the soft phase were continuous, f would be low; if only the hard phase were continuous, f would be closer to 1.0. Likewise, the case of co-continuous phases would be characterized by an intermediate value o f f . The value o f f is expected to most rapidly change with VFI orVs where V I Iis near to the transition or phase inversion concentration. If this concentration is 0.5, one can write: dfldVII = 6 v ~ v s
(8)
which integrates to (9)
f
3vH’
-
2V[I3
since Vs is 1 - V H . A more general case, wherein the transition can VF1 = 0.5, is
f = VFl”(nVs
+
occur other than where
1)
(10)
If n is 2.00, the case of Eq. (9) is realized. Combining Eq. (10) with Eq. (7), we obtain M
=
VH”(nVS
+
l)(MLI - mi^)
+ MI.
1)
(1
Thus, M lies between ML and MLjand is expressed as a function of only one fitting parameter,
Coran
286 1000 r
G *.
0
0.2
0.4
0.6
0.8
1.0
ZH
Fig. 8 Dynamic shear moduli of melt-mixed butadiene rubber-polypropylenc blends. (From Coran and Patel. 198 l a . )
n. The parameter n, then, must contain aspects of phase morphology. The change in f with respect to VH is greatest when VI, = (n - I ) / n ; thus, the value (n - l)/n could be viewed as the volume fraction of hard-phase material that corresponds to a phase inversion or transition. A value of n = 2.00 corresponds to a transition concentration of VH = 0.50. In a comparison between theory and experimental results by Coran and Patel (198 la), melt-mixed blends of 35 rubber-plastic combinations were prepared and compression molded.In each case 3- I O compositions of various rubbedplastic proportions were prepared. The dynamic modulus Gd' was measured at room temperature with a torsion pendulum at frequencies between about 0.5 and 2 Hz. The dynamic shear modulus for each of the blend component polymers was also measured. From the dynamic shear moduli of the component polymers and from the volume fractions of the phases, upper- and lower-bound moduli G;, and G,. were obtained from Eqs. ( 5 ) and (6). In the case of each composition. a value of n was selected to give the best fit of the data according to Eq. (1 1 ). Examples of this are given in Fig. 8 and 9. which demonstrate cases of the best and poorest agreement between the data and Eq. ( 1 1). In the case of the poorest agreement. the standard error of estimating G* from Eq. ( 1 1 ) was 63%. This is not as poor as it may seem, since the modulus data range over three decades.
G *,
0.1 0
0.2
0.6
0.4
0.8
1.0
#H
Fig. 9 Dynamlc shear moduli of mclt-mixed butyl rubbcr-polypropylcnc blends. (From Corm and Patcl, 1981a.)
287
Thermoplastic Elastomeric Rubber-Plastic Blends 10
0.1
G*lG*,
0 01
0 001
00
01
02
03
04
05
0.6
07
08
09
10
0"
Fig. 10 The range of experimental data, cxprcssed relative to the hard-phase moduli, i.e.. normalized thereto. (From Corm and Patel, 198la.)
Overall, the standard error was 29% with 109 degrees of freedom for error. All the determined values of G* are plotted as relative moduli, G*/G*II,against V" in Fig. 10, between upper- and lower-bound values. The plotted values generally fall in a relatively narrow range compared to the entire field between the upper and lower bounds. The values of n varied between 1.6 and 4.5, the average value being 3.1 8. It is interesting that more than half of the values were in the range 3.0-3.6. The values of n were correlated with properties of the rubbers and plastics, and the following was concluded: 1. An increase in the viscosity or mixing torque of the rubber phase generally tends to decrease its continuity: it favors the encapsulation of rubber particles by a plasticphase matrix. 2 . An increase in the surface energy mismatch appears to increase the tendency toward upper-bound behavior (to give higher moduli); it favors plastic-phase continuity. 3. High degrees of crystallinity of the hard-phase material also generally tend to favor its continuity. Another conclusion was that elastic modulican indeed be estimated as a function of the properties and concentrations of the blend components.
5.3
Properties of Some Useful Elastomeric Rubber-Plastic Blends
A great number of unvulcanized rubber-plastic blends have been prepared, many of which have beenreviewed(CorishandPowell.1974; Dunn, 1976,Kresge,1978;Morris,1979;Kresge, 1984). In this section we will review the properties of a few of these blends, with emphasis on the properties of those that have had some commercial significance. The examples used here will not generally relate to the proprietary blends in commercial use; rather. the examples will be of blends whose compositions are disclosed in the technical, scientific, or patent literature.
Coran
288
NBR-PVC Blends The first useful rubber-plastic blends were those of NBR and PVC (Badum, 1942; Emmett, 1944). In certain concentration ranges (around50/50),the unvulcanized nitrile rubberacts mainly as a nonvolatile plasticizer (Olabisi et al., 1979b). However, the miscibilityof NBR-PVC blends has been controversial. For example, Matsuo et al. (1969) presented a series of electron microscope photographs,whichclearly shows the two-phasemorphology of NBR-PVC (100/15) blends with NBRs having varying nitrile contents. The PVC phase, with a few extremely small inclusions of nitrile rubber, was dispersed in nitrile matrix. However, Takayanagi et al. (1962) and Zakrzewski (1973) found evidence for partial miscibility based on the measurements of dynamic mechanical properties and the density as functions of temperature. Studies of PVC blends with various rubbers were reviewed by Gotoh in 1970. Based on properties of blends of PVC with various rubbers, it was concluded that, in the blend with NBR, the nitrile groups interact with chloride substituents of the PVC. The result of this was reduced particle size of the dispersed rubber phase and improved mechanical properties. In compositions containinglarge proportions of PVC, the incorporation of rubber provides impact resistance. In vulcanized rubber versions. which generally contain major proportions of rubber, the PVC functions largely to improve ozone resistance of the rubber and to provide increased stiffness. Compositions containing intermediate concentrations of unvulcanized NBR and PVC behave as plasticized vinylsthat contain permanent plasticizers. They have excellent room-temperature properties. This is probably due to the fact that PVC and NBR are at least partially miscible (Dunn, 1976; Olabisi et al., 1979b). However, it should be noted that such compositions are not useful at even moderately elevated temperatures. This is due to the fact that the T, of PVC is only about 85"C, and since PVC is only very slightly crystalline, the temperature-use range cannot be effectively extended above T, toward a crystalline melting temperature. Blends M i t h Crystalline P n l y o l e j h Blends of polyolefin plastic with EPDM rubber are the most studied of this group. The stressstrain properties of this type of blend depend largely on which type of crystalline polyolefin is used. This is illustrated by Table 5 (Kresge, 1984). Here, either polypropylene, high-density
Table 5 Properties of EPDM-Polyolefin Blends Blend EPDM: parts 30 Polypropylene,' parts Low-density polyethylene," parts High-density polyethylene," parts Physical properties Tensile strength (MPa) Elongation at break ((h) Elongation set at break (%)
80 20
IO
-
-
8.3 220 28
10.5 150 30
60 40
80
60
80
60
-
-
-
-
-
20
30
-
-
-
-
20
40
13.9 80 30
5.8 290 35
8.5 210 25
10.2 130 33
8.0 190
30
Banbury mixer. about 7 mm. maxlnlum temperature about 200°C. Amorphous. hlgh molecular welght ethylene-propylene-dicyclopentadiene( - 5 wt%) tern~onon~er ' p = 0.903 g/cm', melt Index = 4.0 g/10 nun at 230°C. " p = 0.919 g/cm', melt Index = 2.0 6/10 mln at 190°C. " p = 0.956 g/cm', melt index = 0.3 gll0 lnin at 190°C.
"
"
289
Thermoplastic Elastomeric Rubber-Plastic Blends Table 6 Properties of SemicrystallineEPDM-PolyolefinBlcnds Blend" EPDM, parts EPDM crystallinity (wt%) 2.7 20parts High-density polyethylene," Low-density polyethylene,' parts Physical properties Tensile strength (MPa) Elongation at break (%)
80 12.9 20
80 80 2.7
80 12.9
-
20
-
20
5.4 940
14.5 120
15.0 730
-
-
7.6 880
Mill mixed at 150°C " p = 0.95 g/cm'. ' p = 0.97 g/cml.
"
polyethylene. or low-densitypolyethylene was blended with an amorphous.highmolecular weight ethylene-propylene-dicyclopentadiene(ca. 5% of the diene) terpolymer EPDM. Increases in the amount of crystalline phase are associated with increases in tensile strength and modulus but decreases in ultimate elongation. It is also noted that the polyolefinsof the highest crystallinity (polypropylene and high-density polyethylene) give the highest tensile strengths. In blends with polyethylene, if the EPDM rubber phase is semicrystalline (due to relatively long ethylene-monomer sequences), compositions of improved strength result. This is illustrated by Table 6, which also contains data from Kresge (1984). Similar results were obtained by Lindsay et al. (1979).In fact these authors report that blends can be obtainedwith high-ethylene EPDM and low-density polyethylene that have tensile strengths higher thanthe tensile strengths of either component polymer. Lindsay et al. (1979) have also made DSC measurements that show that the crystallization temperature of low-density polyethylene is decreased significantly by the presence of the EPDM. They believe that this indicates partial miscibility (or thermodynamic compatibility) between melts of low-density polyethylene and high-ethyleneEPDM. Their DSC cooling scans give evidence that upon cooling. the polyethylene crystallites nucleate the crystallization of high-ethylene molecular segments of the EPDM. (See also the example in the patent issued to Carman et a l . in 1977). Morris (1977) found that improvements can also be realized by using EPDM of sufficiently high viscosity. The properties of rubbery EPDM-polyolefin blends are much better than those of most of the rubbery rubber-plastic blends produced. This is illustrated by the survey of data given in Table 7 (A. Y. Coran and R. Patel, unpublished). Here it can be noted that the strongest blends are obtained when the plastic phase is crystalline and when the surface energies (or polarities) are most nearly matched.
6.
PROPERTIES OF BLENDS PREPARED BY DYNAMIC VULCANIZATION
Vulcanized blends of rubbers with thermoplastic resins have been knownfor a long time. Reznikov et al. reported the tensile strength of various NBR-PVC blends with and without vulcanization in 1952. They used recipes of sulfur and peroxide curing agents. Both types of vulcanizates showed improvements in tensile strength in comparison to the NBR-PVC blends without vulcanization. In all of these cases, the rubber was vulcanized in the conventional way: strrticul/y, e.g.. in a mold, rather than d y m n i c a / / y , e.g., in a mixer, compounding extruder, on a roll mill, etc.
Coran
290
Table 7 True Strcss at Break of Selected Melt-Mixed Rubber-Plastic Blends" True
Plastic Rubber IIR EPDM NR NBR EPDM NBR NBR IIR EPDM BR IIR NBR IIR
break
stress at
Polypropylene Polypropylene Polypropylene Polypropylene Polyethylene Polyethylene Poly(tetramethylene terephthalatc) Polystyrene Polystyrene Polystyrene SAN SAN PMMA
(MPa)
26 26 16 23 27 13 27 2.3 3.7 4.1 5.0 10.9 3.6
'' The compositions were 60/40 and SO/S0 rubber-plastlc welght ratio. True stress at break IS the product of ultimate strength and ultmate extenswn ratlo.
As stated earlier. dynamic vulcanization is the process of vulcanizing rubber during its intimate melt mixing with a nonvulcanizing thermoplastic polymer. Small rubber droplets are vulcanized during melt processing to give a particulate vulcanized rubber phase of stable domain morphology. Dynamic vulcanization produces thermoplastic vulcanizate (TPV) compositions that have the following improvements in comparison to similar, but unvulcanized blends: Reduced permanent set Improved ultimate mechanical properties Improved fatigue resistance Greater resistance to attack by fluids, e.g., hot oils Improved high-temperature utility Greater stability of phase morphology in the melt Greater melt strength Improved reliability of thermoplastic fabricability Reduced die swell for control of extruded profiles Dynamicvulcanization can provide compositions that are morerubber-like in their end-use performance characteristics. Nevertheless, these same thermoplastic vulcanizate compositions can be rapidly processed as thermoplastics. Compositions of greatly improved permanent set can be producedby only slight or partial vulcanization of therubber. Such slightlyvulcanized compositions can be produced by the partial vulcanization of the rubber before its mixture with plastic or by dynamic vulcanization (Fischer. 1973) (during mixing with plastic). However. the other improvements (at least in the case of EPDM-polyolefin compositions) can be obtained only by dynamic vulcanization, in which the rubber is technologically fully vulcanized. 6.1
EPDM-PolyolefinPlasticThermoplasticVulcanizates
The dynamicvulcanization of blends of EPDM rubber with polypropylene and with polyethylene havebeendescribed (Coran and Patel,1980a).Mechanicalproperties.hardness,tensionset
291
Thermoplastic Elastomeric Rubber-Plastic Blends Table 8 Propcrties of Unfilled ThermoplasticCompositions Resln at Hardtype"1parts partlcleCrosslinkMethod per Compo100 of parts sltlon of Sulfur prcparation number (phr) rubber (phr)
ion
I 2 3 4 5 6 7 8 9 10 II 12 13
PPI8 14 15 16
17 18 I9 20
PP166.7 PPl66.7 PPl66.7 PP166.7 PPl66.7 PP166.7 PPl66.7 PP166.7 PP166.7 PPl66.7 PPl33.3 PP142.9 PP153.8 1.8 PPI122 PPI233 None"1O PPi'/x PE166.7 PEl66.7
2.00 2.00 2.00 2.00 2.00 1.00 0.50 0.35 0.13
0.00 1.00 2.00 2.00 2.00 2.00 5.00 2.00 0.00 2.00 0.00
density (moles x 105/nL)
size (km) d,, d, 72 750 39 290 17 96 5.4 30 -1-2
D D D D D D S
16.4 16.4 16.4 16.4 16.4 12.3 7.8 5.4 1.0 0.0 12.3 16.4 16.4 16.4 16.4 14.5 16.4
-
11
-
-
-
D
16.4 0.0
-
71 35 21
S S S S D D D D D -
-
ness, Young's Shore modulus
D 43 41 41 42 42 40 39 40 35 22 29 34 36 43 48 59
-
~
-
~
values. and other parameters associated with unfilled general recipe of the compositions was as follows: resin
(Epsyn EPDM 70A) Polyolefin Zinc oxide Stearic acid Sulfur TMTD (tetramethylthiuram disulfide) MBTS (2-benzothiazolyl disulfide)
(MP;I) 97 102
I 0s 103 S8 60 61 56 S7 72 13 22 32 82 I62 435 2.3 854 51 46
Stress
Elon-
Tension I O O T at Tensile strain strength (MPa) (MPa)
break (%l
8.4 8.4 8.4 8.4 8.0 7.2 6.3 6.7 6.0 4.8 3.9 5.6 7.6 8.5 11.3
13.6 I .S 19.2 7.2 4.1
8.6 9.8 13.9 19.1 24.3 18.2 15.0 15.8 9.1 4.9 12.8 17.9 2s. I 24.6 27.5 28.8 2.0 28.5 14.8 3.5
165 215 380 480 530 490 S00 510 407 I90 490 470 460 SS0
560 580 1SO 530 440 240
set
(8) 22 22 20 16
17 19 19 27 66 7 9 12 19 31 46 1 -
18 24
compositions are given in Table 8. The
l00 X 5 1 Y Y12 Y14
where X, the number of parts by weight of polyolefin resin, and Y, the amount of sulfur. were varied. Not all of the compositions of Table 8 were prepared by dynamic vulcanization, however. In the first four compositions. for comparison purposes, the rubber was first press-cured and then ground, by tight roll-milling, to various particle sizes. The ground rubber particles were then mixed with molten polypropylene. Particle sizes were determinedby optical microscopy. For the measurement of the crosslink density of the rubber. samples of the rubber alone were press-cured under conditions selected to simulatetheconditions of dynamic vulcanization.Crosslink densities of the press-cured samples were then determined on the basis of solvent-swelling measurements by using the Flory-
Coran
292
STRESS, MPa
25 I
0
100
200
300
400
500
600
STRAIN, 40 Fig. 11 The effect of vulcanized-rubber particle sizeonmechanicalproperties
(x dcnotes failure)
Rehner equation (Flory, 1953b). Mechanical properties and hardnesses were measured by the usual means. An EPDM-PP dynamic vulcanizate always exhibits a dispersed phase morphology. with the vulcanized rubber comprising the dispersed phase. This type of morphology is independent of the elastomer-thermoplastic ratio or the molecular weights. viscosity, etc., of the constituent polymers. By contrast, a variety of morphologies can be obtained for simple unvulcanized blends depending on polymer ratios, molecular weights of the polymers, etc., or the mixing conditions (Abdou-Sabet and Patel, 1990). The effect of the rubber particle size is shown in Fig. 1 1. This is a composite stress-strain curve constructed from data associated with composition 1-5 of Table 8. Each x denotes a stress and strain at rupture or failure. From Fig. 1 1, then, ultimate strength, ultimate elongation. and energy to break (area under the curve from the origin to an appropriate x) are apparent. The averagerubberparticlesizeassociatedwith each x is noted. The ultimatepropertiesare an inverse function of rubber particle diameter. Major effects of changes in crosslink density are given in Fig. 12. Only a small amount of crosslink formation is required for a large improvement in tension set. Tensile strength improves rather continuouslyas the crosslink densityof the rubber phase increases. but the compositions remain fabricable as thermoplastics even at high rubber crosslink densities. However, only small changes in the stiffness of the compositions occur with great changes in the extent of cure. Young’s modulus can even &creme slightly as a result of vulcanization. The improved strength could arisefrom a more favorable microdomain morphologyof the compression-molded samples used in the determinationof mechanical properties. Another reasonfor the improvement
.
293
Thermoplastic Elastomeric Rubber-Plastic Blends TENSION SET,
20
15
MPa
'Yo
UTS,
+
1'5
10
5
0 0
62
4
168
14 10
12
CROSSLINK DENSITY, "MOLES"X106/rnL
Fig. 12 The effects of crosslink density on tensile strength and tension set. (From Coran and Patel, 1980a.)
in ultimate properties may be a reduction in the tendency for intraparticulate cavitation (Gent and Lindley, 1958; Gent and Wang, 1991) as the Young's modulus of the particles increases with increasing vulcanization. Such cavitation can initiate fracture or failure (Itnoue, personal communication). As the ratio of polypropylene resin to rubber increases, the compositions become less like the rubber and more like the plastic (Fig. 13). Modulus, tension set, and tensile strength increase. The shapeof the plot of strength against the proportion of polypropylene is interesting. Strengths are low until at least 30 parts of polyolefin resin per 100 parts of rubber are used. Then, as the proportion of resin is further increased, strength rapidly increases until about SO phr of the polyolefin is used. Further increases in the amount of resin can increasethe strength only slightly. Young's modulus values are plotted in Fig. 14. The points are experimental, and the solid line is calculated from Eqs. (S), (6), and (1 1). In the case of this work, the parameter n [Eq. (1 l)] has a value of about 3.1. This contrasts with a value of 2.0 for unvulcanized blends of EPDM and polypropylene. A change in micromorphology resulting from dynamicvulcanization is thus indicated. Dynamic modulus as a function of temperature is shown in Fig. 15. The composition is the same as No. 5 of Table 8. The dynamic shear moduli were determined by using a torsion pendulum (Nielsen, 1951). One effect of dynamic vulcanization is to prevent a complete loss of elasticity and strength at the melting point of the resin polypropylene. If the composition contains a high rubber concentration, the vulcanized composition continues toexhibit sufficient strength for the torsion-pendulum modulus measurements even after the plastic-phase melting point has been surpassed.
Coran
294
30 I
jrooo
I
0 0
I
I
I
100 150 200 CONCENTRATION OF POLYPROPYLENE, PHR
50
1 250
Fig. 13 The effects of polypropylene concentration of EPDM/polypropylene t h c r m o p l a s t i c - v u l ~ ~ ~ ~ ~ i ~ ~ ~ t e properties. (From Corm and Patel. 1980a.)
Another effect of dynamic vulcanization, indicated in Fig. 15, is a snlaller decrease in modulus as the temperature is increased. The unique performance of the thermoplastic vulcanizates even above their melting points indicates a variety of high-temperature applications. The data given for compositions 19 and 20 of Table 8 show that EPDM-polyethylene blends are also greatly improved by dynamic vulcanization. However, on the basis of comparisons between compositions of polyethylene and polypropylene. one can conclude that the best compositions are prepared from polypropylene. Table 9 demonstrates the effects of carbon-black loading and oil extension. The compositions are variations of No. 15 of Table 8. The effectof the filler is to strengthen the composition somewhat and to give some stiffening (with respect to hardness and stress at 100% strain, but not in respect to the“zero-strain”Young’s modulus). Both oil extension and carbon black loading can give compositions of lower cost but excellent quality. From data such as those in Table 9 and from measurements of hot-oil resistance, fatigue life. etc., it has been concluded that oil-extended, filled compositions can be prepared that are thermoplastic but perform i n very much the same way as conventional rubber vulcanizates. Recently,PuydakandHazelton (1988) prepared TPVsfrom rubber-rubber-plastictriblends. I n most of these compositions the best choice for the plastic phase was polypropylene. One of the elastomers is crosslinked under dynamic vulcanization conditions during moltenstate mixing with the second elastomer and the plastic. These TPVs offer some advantages over two-component TPVs (Table 10). The EPDM-PP TPVs have good low-temperature brittleness resistance; nevertheless. the low-temperature flexibility can still be improved by the addition of long-chain aliphatic ester plasticizers (Ellul, 1997).
Thermoplastic Elastomeric Rubber-Plastic Blends
295
VH
Fig. 14 Young's modulus a s a function of the fraction of polypropylene. (From Coran and Patel, 1980a.)
6.2
NBR-Nylon Thermoplastic Elastomeric Compositions
As in the case of the EPDM-polypropylene compositions, NBR-nylon compositions were prepared by melt-mixing the polymers andother componentsin a (Brabender) laboratory-size mixer (Coran and Patel. 1980b). The temperatures for mixing and molding varied with the melting point of the nylon used in each composition. The nitrile rubbers were grouped into twocategories: those that are self-curing at elevated mixing temperatures in the absence of curative and those that are resistant to self-curing. (Selfcuring at high temperatures is a result of thermal-oxidative instability.) To determine whether or not a rubber is self-curing, a sample of the rubber canbe mixed at225°C (oil-bath temperature). Self-curingrubbers gel and crumble(scorch) within 2-8 minutes,whereasnon-self-curing rubbers can be mixed for 20 minutes without crumbling.
The Effect o j Cumti\w The effect of curatives is complicated by the fact that some nitrile rubbers tend to self-cure at the temperature of mixing. This is particularly true when high-melting nylons are used. In such cases, the effect of adding curative is minimized, since the properties of the composition are apparently improved by the crosslinking of the rubber, which occurs just from mixing. A much greater curative effect is observed with the non-self-curing rubber, but the best properties are obtained with the self-curing type of NBR. Surprisingly, in the case of dimethylol phenolic curative treatment, high-strength blends are obtained evenwhen the gel content of the NBR is as low as 50%. A portion of the dimethylol
Coran
296
SHEAR MODULUS, MPa l000
100
l0
1
0.1 -100
-60
0
150 50
100
200
260
TEMPERATURE, DEG. C
Fig. 15 The effects of temperature on the stiffness of EPDM-polypropylene blends. (From Corm and Patel, 1980a.)
Table 9 Effect of Carbon Black and Extender Oil" Carbon black (Phr) 0.0 80.0 0.0 80.0 80.0 "
Extender oil (!-W (MPa)
Tensile strength (MPa)
0.0 0.0 80.0 80.0 160.0
27.5 31.0 15.2 23.0 15.2
Stress at 10% strain 11.3 14.3 6.4 7.2 4.8
Carbon black is N 327, and oil is Sunpar"' 2280.
Young's modulus (MP4
Elongation at break
162 120 47 23
560 410 550 S30 490
11.5
Shore hardness
Tension set (%h)
48D SID 29D 33D 14A
31 30 19 16 13
trength,
297
Thermoplastic Elastomeric Rubber-Plastic Blends Table 10 Synergism on Combining Elastomers TPE property
in TPEs
Neoprene(CR1
Tensile 220elongation. r/r Ultimate Hardness, Shore A ASTM #3 Oil Swell (70 h @ 100°C). % Tear strcngth, KNlm Compression set (22 h @ 100°C). %
4.7 170 58 21
Butyl(I1R)
CR
7.1
7.5
60
12.3 66
+ IIR(I: I )
75
66 44
19.3 47
24.5 52
phenolic compound isbelieved to react with the nylon to give chainextension or a small amount of crosslinking. This could increase the viscosity of the molten nylon to where it is more like that of the rubber and, thus, mixing, homogenization, and rubber particle size reduction are greatly enhanced. In addition, some of the crosslinking can be between molecules of the nylon and those of the rubber to formnylon-NBR graft molecules. This can also induce better homogenization and interfacial adhesion. The effect of the curatives on tension set is widely variable. This is suggested by the data of Table 1 1 , where the various curatives give a wide range of set values. This is in cowtrust to what was obser1wi with EPDM-polypropylene cotnpositiot~s.In that case, curatives invariably reduce set values. The reason for this variation of the effect of curative on tension set is not understood, but it could relate to the extent to which the curatives promote molecular linkages between the nylon and rubber, rather than crosslink the rubber.
The Effect of Rubber Clznractetistics Rubbers of differing viscosities, differing nitrile contents, and differing tendencies towards selfcuringwerestudied. Thereappearsto be nosimple relationshipbetween the strength of a
Table 11 Cured NBR/NylonBlends"withDifferent ~~
Stress Tensile strength type Curative None (control)2.5 Accelerated sulfur" Activated bismaleimide" Peroxide"
Types of Curatives
~
W " )
3.1 8.3 8.5 7.9
at 100% strain (MPa) 7.4 3.7 6.1
Elongation at break set (c/o)
290 160 310 220
True Tension Shore D (9'0) (MPa) hardness 72 21.715 51 31
17 35 28 32
stress at break 12.3 34.9 25.3
Blends comprlse 40 parts of nylon 6.6-6.10 terpolymer (mp 160°C) and 60 parts Chemlgum@N36S non-self-curing NBR 39% AN). " Accelerated sulfur system contains 5 parts ZnO, 0.5 parts stearic acid. 2 parts tetramethylthiuram disulfide. 1 part morpholinothiohenzothlazole, and 0.2 parts of sulfur per 100 parts of ruhher. ' Activated hismaleamide is 3 parts of ,t~-phenylenehismaleanlideand 0.75 part of 2.2-h1shenzoth1nzolyldisulfide per 100 parts of ruhher. I' Peroxide IS 0 3 part of 2,S-dimethyl-2,S-bis(t-butylperoxy)hexane(90% active), LupersoP L-101.
"
298
Coran
25
20 ASTM NO. 3 OIL SWELL, %
15
IO
5 \ \
\
0' IO
1
l
.
I
1
I
20
30
ANCONTENT
40
50
60
OF RUBBER. %
Fig. 16 The effect of acrylonitrile ( A N ) content on hot (150°C) oil resistancc.
(0) Self-curing rubbcr;
( 0 )non-self-curing rubber. (From Coran and Patel, 1980b.)
composition and the characteristics of its rubber phase. However, as mentioned above, selfcuring rubbers tend to give the best strengths, and the effect of curative addition is greatest for the non-self-curing rubbers. The effect of nitrile content on hot oil resistance is similar to that observed in the usual NBR vulcanizates (high AN content gives low oil swelling). This is true for both the self-curing and non-self-curing types of NBR. Oil swell data forall of the compositions are plotted in Fig. 16.
The EfSects of NBR-Nylon Proportions These effects are similar to those obtained for EPDM-polyoletin compositions.Increases in the amount of rubber in the compositions reduce stiffness and strength but increase resistance to permanent set. Also. extensibility can be increased somewhat. If more than 50% of the composition is rubber, elastomer compositions (having tension set values less than 50%) are obtained. However. excessive amounts of rubber can result in poor fabricability.
The Ejfect of Plasticizers Plasticizers can be addedto compositionsof NBR and nylon. The effect is to soften the compositions and improve fabricability. Surprisingly, the melting point of the nylon phase can increase or decrease, as can the crystallinity. The expected effect is to lower the melting point (Wagner and Flory, 1952; Flory, 1953). However, another expected effect is to decrease the viscosity of the nylon phase. The decrease in viscosity can promote crystallization from the melt and enable
299
Thermoplastic Elastomeric Rubber-Plastic Blends Table 12 The Effect of Plasticizers on the Properties of Nylon-NBR Compositions"
Plasticizers (compatible phase") Methyl phthalyl ethyl glycolate (R) Butyl phthalyl butyl glycolate (R) C&, trialkyl trimelitate (R) Dioctyl phthalate (R) Dibutyl sebacate ( R ) N-Ethyl-o- and p-toluene sulfonamide (N) 2-Propyl-4.4'bisphenol (N) Nonylphenol (N)
Change in tensile strength (Q)
Change in Change in melting Change pt" in ("C) elongation at break hardness (76) ( D units)Final Peak
- 25
+ 15
-21
-31
- 28
- 35
- 26 - 34
-
38
- 23
+l
+l
-10
-3
-1
--96
-2
-5 10
+l +l
-1
-1
-
Change in crystallinity' (%) -
18
-4
0
-1
-4 -6 - 12
0 -II - 12
+6
+ 12
0
+ 21
-7
-8
- 12
- 22
0
+31
-1
-3
-6
-S
' I Reclpe: 50 parts nylon 6-6.6-6.10 polymer (Zytele 63). S0 parts NBR (Hycar" 1092-80). 10 parts plasticlzer. 2.5 parts ZnO. I .0 part TMTD, and 0.50 part bisbenzothiazolyl disulfide. Propertles without plasticlzer: UTS = 1 1.6 MPa: UE = 260%: ShoreDhardness = 45; meltlngpoint = 152°C (peak) or 168°C (final);heat of fusion(proportionalto crystallinlty) = I 1.6J/g of nylon. " The symbols N or R Indicate primary compatibility with nylon or rubber. respectively. ' Melting points and relative crystallinity were deternuned by differential calormetry (Perkin-Elmer Differential Scanning Calorimeter D S c - IB. 10"C/min, 20-mg sample).
formation of more nearly perfect crystals. In some cases these two effects tend to cancel. Plasticizer can either increase or decrease ultimate elongation. Tensile strength generally decreases with the incorporation of plasticizer. Effects of plasticization are indicated by the data in Table 12. Note that plasticizers, which are more compatible with the nylon phase, tend to give compositions of better mechanical integrity. The E8ects of Filler
Small amounts of clay have little effect onhardness, stiffness, or strength, although extensibility is reduced. Young's modulus actually decreases. This is similar to what was reported for blackfilled compositions containing EPDM and polypropylene. Again, it is thought that the filler is in the rubber phase and has the effect of both stiffening the rubber and increasing the volume of its phase. These effects are opposites and largely cancel each other out. Another effect of filler is to severely reduce the thermoplasticity and therefore to reduce the expected fabricability. In order to obtain the full benefit of filler, plasticizers can be used to regain both thermoplasticity and extensibility. Overall Assessment of NBR-Nvlon Thennoplastic Elastomeric Conyositions Assessment is complicated by the large number of variations possible. There are a number of types of nylon (polyamide) resins with a wide range of melting points. polarities, etc. Also, the
Coran
300
variations in the types of NBR are great (nitrile content, viscosity, susceptibility to self-curing, etc.). In addition, the effects of the different curing systems vary widely. Nevertheless, some conclusions can be drawn. A variety of nylons of differing melting points andNBRs of differing acrylonitrile contents can be used in broad ranges of proportions. The compositions can be further altered by the incorporation of fillers and plasticizers. Thus, a number of types of NBR-nylon-based elastomeric materials, fabricable as thermoplastics and exhibiting good strength and excellent hot oil resistance, can be produced with a range of hardnesses.
6.3
Other Thermoplastic Vulcanizates: Correlation Between Blend Properties and the Characteristics of the Blend Components
A large numberof rubber-plastic combinations havebeen used in the preparationof thermoplastic vulcanizates by dynamic vulcanization. In one study, such compositions were compared in a systematic way (Coran et al., 1982a). Compositions were prepared as before. by vulcanization of blends during melt-mixing. A variety of curative systems were used. being selected, in the case of each composition, on the basis of at least some optimizational experimentation. Types of' curative systems used were are as follows: Dimethylolphenolic (P) Bismaleimide (M) Bismaleimide-MBTS (M-M) Bismaleimide-peroxide (M-0) Organic peroxide (0) Organic peroxide-coagent (0-C) Accelerated sulfur (S) Soap-sulfur or soap-sulfur donor (SO-S) In addition, thermal-oxidative stabilizers were used when appropriate. All of the compositions contained rubber and plastic in a weight ratio of 60/40. This ratio was chosen for screening of rubber-plastic combinations because of the fact that, when good compositions were obtained at the 60/40 rubber-plastic concentration ratio, they were soft enough and elastic enough (tension set less than 50%) to be considered elastomeric. The compositions studied are listed in Table 13, in which the types of curing systems are identified by the parenthetic symbols given in the above list of curing systems. Ultimate tensile strength. ultimate elongation, and tension set values, which were obtained for the compositions, are given in Tables 14, 15, and 16. The values of tensile strength, u ~ , which are in parentheses for CPE-PTMT, CR-PTMT, and CR-PAare in doubt since the rubbers (CPE and CR) are insufficiently stable to withstand processing at the high melt temperatures for PTMT (polyester) andPA (high-melting nylon). The value for EVA-PTMT is in doubt because of the instability of the peroxide curative, which wasno doubt spent before its complete mixture with the molten blend. For similar reasons, other tensile strength values may be IOW; however, forthe purposes of this work only the parenthetic values were removed from consideration in attempts to correlate the measured properties of the rubber-plastic compositions with characteristics of the rubber and plastic components. Of course, the stability of a component in the presence of the others, in a systemof conditions depending thereon, is in itself a characteristic expected to correlatewith blend properties.In fact, onemight say thatc1 rubber is techrlologically ir~cornpatibleM-ith N plastic if the plastic rwst be processed at a temperature above wlzich the rubber is stable.
Thermoplastic Elastomeric Rubber-Plastic Blends
301
Table 13 Curatives in Rubber-Plastic Blends" RubhedPlastic PP IIR EPDM PTPR NR BR SBR EVA ACM CPE CR NBR
PS
PE
ABS PMMA SAN
S
P S P P M P 0
P S S M-M M M-M M-0
P M-0 P M-M M M M-0
so-S
so-S
so-S
so-S
M-0 S M-0
0 S M-M
0 M-M 0
0 M-M 0
S P M-M M M-M M-0
PBT
PA
PC
P M-0 M M-M M M 0
P M-0 M M-M M M 0
P M-0 P M-M M M 0
S M-0 P M-M M M-M M-0
P P S M-M M M M-0
so-S
so-S
so-S
so-S
so-S
0 M-M 0
0 S M
0 M
0 M-M M
-
0 M-M
Each compositlon is identified corresponding t o B rubber-plastlc (row-column) combinatlon. There are I I rubbers are (rows) and 9 plastlcs (columns), which give 9 x 1 I = 99 combinations.Abbreviationsforplasticsandrubbers given in Table I , Curative symbols are identified in the text.
"
In the case of the ultimate elongation values, uE,given in Table 15, the parenthetic values are in doubt for the reasons stated above, and again such values were not used in correlations between blend properties and characteristics of the components. Many of the tension set values, E,, are missing from Table 16 because the measurement is impossible with poor compositions, which cannot be stretched to an elongation of at least 100%.Other values are missing because the work was done early in the program, before tension set was routinely measured.The tension set value of 17% obtained for ACM-PC (acrylate rubber-polycarbonate resin) appears excessively low (high elastic recovery). Young's modulus for this composition is also very low (1.9 MPa). It would appear that rubber is the only continuous phase, yet the composition is moldable as a thermoplastic. This could be explained if either the rubber did not cure in the presence of molten polycarbonate resin or the molten polycarbonate decomposed in the presence of the
Table 14 Ultimate Tensile Strength, uIj,of 60/40 Rubber-Plastic Thermoplastic Vulcanizates RubberlPlasticPE IIR EPDM PTPR NR BR SBR EVA ACM CPE CR NBR l'
PP 21.6 24.3 22.7 26.4 20.8 21.7 17.8 4.0 12.3 13.0 17.0
14.9 16.4 12.1 18.2 19.3 17.1 18.9 4.2 10.5 13.8 17.6
PS
ABS
SAN
0.9 7.9 6.9 6.2 1 1.6 15.8 12.7 11.4 14.0 15.5 7.7
1.7 3.2 11.0 5.8 9.9 10.8 9.6 9.4 13.7 12.8 13.6
4.3 5.6 13.4 8.4 8.3 8.1 12.9 7.7 17.9 12.5 25.8
Values are in Mpa; see footnote to Table 13.
PBT PMMA 5.4 6.0 4.7 1.8 3.5 5.7 9.3 6.2 17.0 8.9 10.8
1.4 12.2 12.1 10.9 12.8 21.7 (3.4) 14.6 (13.0) (13.5) 19.3
PA
PC
4.0 7.7 10.8 5.7 16.3 14.6 10.9 16.1 17.3 (3.2) 21.5
1.3 15.7 2.5 6.7 2.1 7.3 9.6 5.2 20.8 14.7 18.2
Coran
302
Table 15 Elongation at Break.
E,$,
of 60/40 Rubber-Plastic Thermoplastic Vulcanizates"
Rubber/Plastic
PP
PE
PS
IIR EPDM PTPR NR BR SBR EVA ACM CPE CR NBR
380 530 210 390 258 428 3 l9 18 3 l4 141 204
312 612 280 360 229 240 349 20 22 1 390 190
3 69 35 85 73 89 166 20 140 67 20
"
ABS
SAN
18
7 5 10 14
18 15 S6 64 70 I02 I44 197 96 164
12
12 109 135 18
IS1 7 I96
PBT PMMA
6 6 IO 4258 5
IS S9 21 I46
S S6
PA
I56 102 47 62 52 20 102 (126) (159) 65 350
34 30 60 121 1
160 I63 160 (6) 320
PC 161 66 5 21 5 19 81 I40 135 91 I30
Values are in %. See footnote to Table 13.
acrylate polymer, possibly by transesterification. This would be another type of technological incompatibility. The stress at break was correlatedwith the component characteristics as a relative ultimate tensile strength ulJcrl,,where ul, is the strength of the hard phase (plastic) material, believed to be a limiting factor. The effects on relative tensile strength uR/uH,ultimate elongation and tension setE, are plotted according to regression equations in Fig. 17. In each case a property is plotted as a function of one of the three characterizing parameters ( h y s H ,N,. or W,) with the other two variables held constant, each at a desirable level. If we accept Fig. 17 as an overall view of the effects, certain conclusions can be drawn:
I.
An increase in the crystallinity of the plastic material component improves both mechanical integrity and elastic recovery.
Table 16 Tension Set, E\., of 60/40 Rubber-Plastic Thermoplastic Vulcanizates" ~~
~
~
Rubber/PlasticPE
PP
PS
ABS
SAN
PMMA
PBT
PA
PC
~~
IIR EPDM PTPR NR BR SBR EVA ACM CPE CR NBR
28 27
36
23 16 20 24 27 30 36
-
-
-
-
70
-
-
-
S5 33 31
58 37
91 -
-
55
'' Values are in 96. See footnote to Table 13.
41 40
26 S6 S9 (-)
25
44
17 85
303
Thermoplastic Elastomeric Rubber-Plastic Blends
0
15
L mN/m (N, W
350. 0.6)
a00
200
0
0.6
NC
WC
CHAIN ATOMS (Ay, W,
0,
0.6)
CRYST. FRACT. (
AX N,
-
0,
350)
Fig. 17 Theeffects of pure-componentcharacteristics ontheproperties of thermoplastic vulcanizate strength, ell is the ultimateelongation,andis thetension compositions. uII/uIIis therelativetensile set. Ay,,, is the difference between the critical surface tension for wetting yc, of the rubber and that of the plastic; N, is the critical molecular length for entanglement; W, is the weight fraction of crystallinity of the hard-phase material. (From Corm et d . , 1982~1.)
2. Rubbers of higher entanglement densities (lower N,) give compositions of greater mechanical integrity. 3. Compositions in which the surface energies of the rubber and plastic phases are closely matched are strong and extensible. As stated previously, we feel that the matching of surface energies tendsto give lowerinterfacial tensions. resulting in smaller rubber particles that act as smaller stress-concentmtor flaws. thus the high strength and extensibility associated with lower values of A~s1.l. Thus, based on a few characteristics of pure rubbers and plastics, rubber-plastic combinations can be selected with a good probability of success to give thermoplastic vulcanizates (by dynamic vulcanization) of good mechanical integrity and elastic recovery.The best compositions are prepared when the surfaceenergies of the rubber and plastic materials are matched,when the entanglement molecular lengthof the rubber is low, and when the plastic material is crystalline. It is required that neither the plastic nor the rubber decomposes in the presence of the other at temperatures required for melt-mixing. Also, a curing system is required that is appropriate for the rubber under the conditions of melt-mixing. It is interesting that high-diene rubbers as well as saturated rubbers can give excellent TPVs. Thermoplastic elastomers based on blends of polyolefins with diene rubbers such as BR,
Coran
304
Table 17 Mechanical Properties of Different Natural Rubber-PP-Based TPEs Property
Method
Hardness, Shore Tensile strength, Mpa Stress at 100% strain. MPa Ultimate elongation. 96 Tension set, 8 Teilr strength, KN/m Compression set (22 h) at 23"C, 96 at loooc,% Brittle point, "C Ozone resistance at 40°C. 100 PPM of 0 3 " Specific gravity
D2240 D412 D412 D412 D412 D624 D395 Method B
"
D746 D518
60A 5.0 2. I 300 IO 22 24 30 - 50 IO
D297
I .04
IOA 7.6 3.7 380 16 29 26 32
- so IO
I .04
50D 20.8
90A 11.4
6.5 400 35 65 32 38 - 45 IO
10.5
620 50 98 45 63 - 35 IO
1.02
0.99
Ozone rating of I O indicates no cracks after specified time.
NR,NBR,SBR, etc.have been described by Corm and Patel (1978, 1 9 8 0 ~ . 1 9 8 1 ~These ). compositions have fairly goodinitial tensile properties, and their thermal stabilities are somewhat better than those of thermoset diene rubbers. Much commercial development work has centered around TPVs based uponNR-PP blends. Campbell et al. (1978) prepared compositions of partially vulcanized natural rubber. Payne et al. (1990) reported improved mechanical properties of fully vulcanized NR-PP-based TPVs of different hardnesses. They found that, unlike thermoset natural rubber, these TPVs have very goodresistance to crackinginduced by ozone (Table17). The low-temperaturebrittlepoint increases as the natural rubbercontent decreases. These compositionshave fairly good retention of tensile properties in hot air at 100°C for up to one month (Table 18). 6.4 Technological Compatibilization of NBR-Polyolefin Blends by RubberPlastic Graft Formation
The large particles that result from the use of grossly thermodynamically incompatible polymers give blend compositions of lowmechanicalintegrity. These are sometimes describedas "cheesy."However,the use of compatibilizerscangreatlyimprove the properties of these blends. Such compatibilizers are block copolymers whose molecules contain blocks which are the same as (or similar to) those of thepolymers of the blend (Gaylord, 1975;Olabisiand Farnam, 1979; Coran and Patel, 1983b). Were it not for their gross mutual incompatibility (in the thermodynamic sense), a combination of a polyolefin resin with NBR might be a good choice of materials from which to prepare oil-resistant thermoplastic elastomeric compositions by dynamic vulcanization. Early work with these materials demonstrated only marginal success in obtaining good mechanical properties for such compositions (Coran and Patel, 1978). This was likely due to the large surface energy difference between the two types of polymers. Mutual wetting between the polymers appeared incomplete; relatively large particles of cured rubber dispersed in polyolefin resin formed during mixing and dynamic vulcanization. An approach to
Blends Rubber-Plastic Elastomeric Thermoplastic
305
Table 18 Effecxt of Hot Air Aging on % Retention of Tensile Properties of NR-PP-Based TPEs TPE Hardness 60A
70A
90A
50D
Aging time at 100°C (days)
1
7
15
30
99 104 98
91 65 110 87 90 110 91 103 93 95 109 93
80 80 126
40 68 85 43 80 56 66 99 60 66 103 70
Property Tensile strength Stress at 1 0 0 % strain Ultimate elongation Tensile strength Stress at 100%strain Ultimate elongation Tensile strength Stress at 100%strain Ultimate elongation Tensile strength Stress at 100%strain Ultimate elongation
76 86
100 100
98 103 107 93 92 80 101 102108 93
86
113 104
91
technological compatibilization, in addition to dynamic vulcanization, was thus sought (Coran and Patel, 1982. 1983b). It is now generally accepted that a block copolymer can compatibilize mixtures of the “parent” homopolymers. The block copolymers act as macromolecular surfactants to promote and stabilize the emulsion of the molten homopolymers (Gaylord, 1975; Paul, 1978; Olabisis et al, 1979). Figure 18 is an idealized representation of this. It has been found that a dimethyl01 phenolic compound (i.e.,“phenolic resin” curative) can be used to technologically compatibilizea mixture of polyolefin NBR. and This compatibilization could be the result of the formation of a block copolymer of the type shown in Fig. 18. Such a compatibilizing block copolymer could be formed by the reaction scheme on page 306:
INTERFACE
DOMAIN II
Fig. 18 Idealizedcompatibilizingblockcopolymer.
306
Coran
R dimethylol-phenolic compound
R methide quinone
Poly-propylene
phenolic.modified
This scheme requiresthepresence of olefinicunsaturation in thepolypropylenemolecules. Indeed, in the type of polypropylene available at the time of this work, there was on average one double bond per polyolefinmolecule. The scheme is similar to that proposedlong ago for the phenolic, resin curing of diene rubbers (Vander Meer, 1943; Thelamon, 1963; Giller, 1966). In practice, the polyolefin resin is treated with about 1-4 parts of a phenolic curative (e.g., SP1045) per 100 parts of polyolefin resin (e.g., polypropylene) in the presence of 0.1-0.5 parts of a Lewis acid (e.g., SnCI2) atatemperature of about 180-190°C. The phenolic-modified polyolefin is then melt-mixed with NBR for a sufficient time for compatibilization to occur, with the formation of a blend of improvedhomogenization.Then with continuingmixing, curative for the rubber is added. (This can be additional dimethylol phenolic resin curative.) If the NBR contains a small amount (ca. 5 % ) of an amine-terminated liquid NBR (e.g., ATBN 1300X 16"BF Goodrich Co.), then the properties of the compatibilized blend are even more improved. The formation of polymer-polymer grafts can be accomplished by a number of other chemical means in addition to the above use of dimethylol phenolic derivatives. In some cases the results are even better. Such a case is the use of maleic-modified polypropylene to form the block-polymeric compatibilizing agent by reaction with the amine-terminated liquid NBR. In this case, polypropylene is modified by the action of either maleic acid or maleic anhydride in the presence of decomposing organic peroxide (Minoura, et al. 1969; Ide and Hasegawa, 1974). During the process the molecular weight of the polypropylene becomes greatly reduced as the molecules thereof acquire pendant succinic anhydride groups:
307
Thermoplastic Elastomeric Rubber-Plastic Blends
maleic-modified polypropylene
or
COOH If part of the polypropylene in a NBR-polypropylene composition is maleic-modified. and if part of the NBR is amine-terminated. then compatibilizing amounts of NBR-polypropylene block copolymers form in situ during melt-mixing:
compatibilizing block copolymer
Onlyasmallamount of compatibilizing block copolymer is needed to obtainasubstantial improvement in the properties of a blend. The data in Table 19 relate to compositions in which 10% of the polypropylene is maleic-modified (by the action of 5 parts of maleic anhydride i n the presence of 0.87 parts of L- 10 1 peroxide per 100 parts of polypropylene at 180- 190°C). Varying amounts of the NBR are replaced by amine-terminated liquid NBR. Since the tnaleicmodified polypropylene is generally in stoichiometric excess, it can be assunled that essentially all of the amine-terminated rubber is grafted to some of the polypropylene. After each compatibilized blend was prepared, it was dynamically vulcanized and subjected to the usual treatment for molding and testing. The data in Table 19 indicate that improved blend properties are obtained when as little as 0.16% of the rubber is grafted to polypropylene. Also, after as much as about 2-3% of the rubber is grafted to the polyolefin, additional graft formation gives no further improvement. It should be noted that the mechanical properties of the compatibilized dynamically vulcanized blends of NBR and polypropylene can be about as good as those of dynamically vulcanized EPDM-PP blends.
Hot Oil Resistmce crrlcl Brittle Poirlt The hot oil resistance and brittle point of a compatibilized NBR-PP thermoplastic vulcanizate prepared by dynamic vulcanization is given by Table 20 (Stock 1). Although the hot oil resistance is excellent. low-temperature performance is somewhat lacking. The low-temperature brittle point can be reduced with a minimum sacrifice of hot oil resistance by blending the compatibilized NBR-polypropylene composition with a commercially
Coran
308 Table 19 Properties of Compatibilized-Blend Dynamic Vulcanizates as Rubber Grafted to Plastic
Recipe" Polypropylene" Maleic-modified polypropylene'. NB R/' NBR Masterbatch" SP- 1045' SnCI2.2HzO ATBN' as % of NBR ( % rubber grafted to polypropylene)' Properties MPa Tensile strength, Stress at 100% strain, MPa Young's modulus E, MPn Elongation at break, E ~ % , Tension set, E , ~ ,% True stress at break, U((*, MPa Breaking energys, J/cm3 Improvement in breaking energy due to compatibilization, %
U,$.
1
2
3
50
45 5 50
45
-
50 -
-
3.75 0.50 0.00
3.75 0.50 0.00
8.8 -
209 19
12.0 12.0 200
-
110 -
10 33 1.2
25 II
-
-
5 49.22 0.78 3.75 0.50 0.16
12.4 12.1 212 170 45 18.4 67
4 45 5 46.88 3.12 3.75 0.50 0.62
15.2 12.0 223 290 40 59 34.6 215
;I
Function of thc Amount of
5 45 5 43.75 6.25 3.75 0.5 1.25
22.0 12.3 185 400 40 l10 54.9 399
6
45 5 37.5 12.5 3.75 0.50 2.5
25.5 12.3 I 88 440 40 I38 64. I 483
7 45 5 25 25 3.75 0.50 S
25.7 12.5 1X4 430 42 136 61.7 46 1
8 45 5 50
3.75 0.50 10
26.7 12.0 237 540 45 17 I 86.5 686
Parts by weight. "Polypropylene is Profax" 6723; NBR is Hycarw 1092-80 nitrile rubber; ATBN is Hycar@ ATBN 1300 x 16 amine-termlnated liquid nitrile rubber. " See text. "90% by weight of Hycar" 1092-80. 10% by welght of Hyca'B ATBN 1300 x I6 liquld nitrile ruhber. " S P 1045 is a dimethylolphenolic vulcanizing agent. A quantitative reaction is assumed between ATBN and maleic-modified polypropylene (assumed to he prcscnt In excess). p Breaking energy values were obtained from the stress-strain curves.
"
available thernloplastic elastomer based on polypropylene and vulcanized EPDM. The twothermoplasticelastomer compositionsare mutuallycompatible, since they are both based on a continuous phase of polypropylene. The results given in Table 20 indicate that the blend exhibits average mechanical properties, but surprisingly better than the average hot oil resistance.
Overall Assesstnent o j Cotnpatibilized NBR-Po!vl,rol,ylerle Tlwrt,wplastic Ml1cuni:ate.s The results of this work suggest a practical route to hot-oil-resistant thermoplastic elastomers based on NBR and a polyolefin resin, although these two types of polymer are normally grossly incompatible with each other. The mechanical properties of such blends can approach those of dynamically vulcanized EPDM-polypropylene blends, and, in addition, excellent hot oil resistance can be achieved. The NBR-based compositions may not have good enough resistance to low-temperature embrittlement for some applications. but compositions based on EPDM and polypropylene can be blended with the NBR-polypropylene composition to improve the brittle point without causing severe losses of other attributes.
309
Thermoplastic Elastomeric Rubber-Plastic Blends Table 20 CompatibilibcdNBR-PolypropylcncComposition Polypropylene Composition 3 wt% NBR/Polypropylcnc composition" wt% EPDM/Polypropylene composition" Properties Tensile strength, MPa Strcss at 100% strain, MPa Elongation at break, % 87 Hardness, A scale Tension set. 8 True stress at break. MPa ASTM No. 3 oil volume swelling, %' Brittle point, "C
1
100
0 22.6 11.2
585 93 48 155 22 - 24
Blended with EPDM2 50 50 15.9 7.9 5 IO 23 97 32.5 - 47
0 100
8.6 4.4 415 68 IO 44.3 62.5 <-60
Recipe for the NBR-polypropylene composition: Profax@ 6723 polypropylene. 45 (parts by weight); malcic-modified polypropylene. S; Hycar" 1092-80 NBR/Hycar@ ATBN 1300 x 16 liquid NBR (90:IO). 50;SP-1045 phenolic curativc. 3.75; SnC12.?H20.0.50; Naugard'" 495 stabilizer (added after vulcan~zation). I .O. " Cummerclally availablc Smtoprene* 201-73 (Advanced Elastomers Systems). ' Hot-oil swelling. 70 h at 100°C.
6.5
Blends of Thermoplastic Vulcanizates Based on Dissimilar Plastics
In this section we describe thermoplastic elastomer compositions comprising technologically compatibilized blends of TPVs based on nylon and polypropylene. Both the nylon- and the polypropylene-based TPVs were previously prepared by dynamic vulcanization. The compositions were technologically compatibilizedby the presenceof chemically modified polypropylene, which presumably reacted with small portions of the nylon to form compatibilizing amounts of nylon-polypropylene graft-linked copolymer (Coran and Patel, 1982, 1985). Componentsof the blends of TPVs are described below. EPDM-Polypr.opylene TPV A 50150 blend of EPDM and polypropylene was dynamically vulcanized by 5 parts (per 100 parts of combined polymers) of SP 1045 dimethylolphenolic curative in the presence of 1 part of SnC12.2H20at 180- 190°C. The composition was designated EPDM-PP TPV.
NBR-NVlotI TPV
A 65/35 blend of NBR and nylon 6,6-6 copolymer (melting point, 2 13°C) was treated with 1.3 parts (per 100 parts of combined polymers) of SP 1045 phenolic curative during its mixing at 215°C. (FlectoP H antidegradant, added to the melt after dynamic vulcanization, was used at a level of 2 parts per 100 parts of polymer.) The composition was designated NBR-NY TPV. EthylerwAcrylic R~rhher--NylorrTPV
A 45/55 blend of Vamacm rubber (containing an additional 23 phr of filler and stabilizer) and the nylon 6,6-6 copolymer (melting point 2 13°C) was dynamically vulcanized by the action of
310
Coran
1. I parts of magnesium oxide (per I00 parts of combined polymers) at 235°C. The product was designated EA-NY TPV.
Polyurethme Rubber-Nylon TPV A 50/50 blend of a millable, vulcanizable polyurethane rubber (Adiprene C ) and nylon 6. 6-6. 6-10 (melting point I63"C-ZyteP 63) was dynamically vulcanized at 180°C by the action of 1 .0 171-phenylenebismaleimide in the presence of 0.5 parts of L- 101 organic peroxide per 100 parts of combined polymers. The product was designated PU-NY TPV. Epichlorohydrin Rubber-Nylorl TPV A 50l.50 blend of Hydrinm 400 rubber and nylon 6. 6-6, 6-10 (melting point 163"C"Zytel 63) was dynamically vulcanized at 170-180°C by the action of 1.67 parts of zinc stearate. 1.0 part of bisbenzothiazole disulfide (MBTS), and 0.4 part of sulfur per 100 parts of combined polymers, i n the presence of 1 .0 part of FlectoP H antidegradant. The product was designated ECH-NY TPV. M d e i c Acid-Modified Polypropylerw This functionalized polymer was prepared by melt-mixing 100 parts of polypropylene with 5 parts of maleic acid in the presence of L-101 peroxide (added after the acid and polymer were well mixed) at 180°C for about 3 minutes. The product was designated MA-mod. PP. Mi.vture.7
of Tlwrrlloplcrstic Vulcmizates
Blends of thevariousthermoplasticvulcanizates, with andwithoutthechemicallymodified functionalized polypropylene, were prepared by melt-mixing the ingredients in a Brabender or Haake Rheomix internal mixer at temperature about 10°C above the melting point of the nylon used in each case. Properties of molded sheets of the mixtures of thermoplastic vulcanizates are given in Table 21. Thesetest results indicate that nylon-based TPVs containinga variety of different types of rubbers can be used in the compatibilized blends. A substantial improvement in properties is obtained in each case. [Note the comparison between the controlstocks containingadded unmodified polypropylene (odd-numbered stocks) and the experimental stocks containing the maleic acid-modified polypropylene (even-numbered stocks).] It is believed that the maleicacid-modifiedpolypropylenemoleculescontainpendant succinic anhydride groups, which react with amine groups in the nylon to form compatibilizing amounts of a graft-linked nylon-polypropylene block copolymer (Ide and Hasegawa. 1974). It should be noted that a numberof other chemical modifications of polypropylene have alsogiven results similar to those obtained with maleic acid-modified polypropylene (Coran and Patel. 1982,1985). From this work one can conclude that compositions that have excellent mechanical properties can be prepared by melt-mixing thermoplastic \ulcanizates (previously prepared by dynamic vulcanization). Excellent mixed-TPV compositions can be obtained even though the rubbers and plastics are mutually grossly incompatible with respect to thermodynamic considerations. In such cases, however, it is necessary that a compatibilizing agent be present in the mixture to promote the interaction between the thermoplastic materials. The rubber associated with one of the thermoplastic components can differ greatly from the rubber associated with another thermoplastic component. Thus a composition can be produced that has good nlechanical properties and that can contain both differing thermoplastic
311
Thermoplastic Elastomeric Rubber-Plastic Blends Table 21
Compatibilization of Polypropylene-NylollCompositions" I
2
3
4
S
6
S0
S0
S0
SO
SO
SO
S0
S0 SO
SO SO
S0
16.1
26.9
14.0
16.5
132 170 40 43
I47 3I O 39 I10
9.2 173 73 -
16
19.6 11.8
I S7 210 52 73
11.9
9.3 206 230 40 39
22.0 12.0 199 340
S0 97
resins and differing rubbers. As a result, the possible combinations of components for TPV compositions has been greatly expanded.
7. TECHNOLOGICAL APPLICATIONS The rubber-plastic blends discussedin this chapter are generally intended foruse as thermoplastic elastomers. These are materials that have many of the properties of conventional vulcanized (thermoset) rubbers but are processable and can be fabricated into parts by the rapid techniques used for thermoplastic materials. Thermoplastic processing is far more economically attractive than traditional n1ultistep rubber processing. In the case of conventional thermoset rubber processing. the producer of rubber articles purchases gum rubber. fillers. extender oils or plasticizers, curatives. antidegradants. etc.: these ingredients must thenbe mixed and uniformly dispersed. Aftera stock is mixed. it is then shaped by extrusion, calendering. etc. The crudely shaped preform is then vulcanized in its final shape. e.g.. in a mold contained by a press and heated for vulcanization, which can take a long periodof time. Mold tlash or overflow, as well as rejected parts, arenot reprocessable without expensive reclaiming to break down the crosslinked rubber network. On the other hand. a part produced from a thermoplastic elastomer is shaped or lnolded into its final shape in a single step. Also mold flash and rejected parts can be simply ground and reused. Detailed comparisons have shown time and again that thermoplastic processing is more economical than thermoset rubber processing (O'Connor and Fath, 1981. 1982). The lower cost of thermoplastic processing is the nlotivational spirit for the developlnent of thermoplastic elastomers. However. in the past. failure in the achievement of truly rubberlike properties i n many cases has impededthe acceptance of thermoplastic-elastomer technology.
312
Coran
The first thermoplastic elastomers were polyurethanes developed in the 1930s in Germany and introduced commercially in the United States in the late 1950s (Schollenberger et al., 1958). These materials gained rapid acceptance because of their high-performance characteristics: solvent resistance, abrasion resistance, high tear strength, good load-bearing capacity, and lowtemperature flexibility. Continued growth in the use of these materials at the expense of vulcanizedrubberusage is unlikelybecausetheirhighprice cannot beoffset by thereduction in processing costs resulting from thermoplastic processing (Auchter, 1981). Styrenic triblock (polystyrene end-segment-rubbery center-segment-polystyrene endsegment) copolymers (Holden and Milkovich, 1965; Zelinski, 1966) were the second type of thermoplastic elastomer to be commercialized (in the early 1960s). These low-cost elastomers have achieved the highest volumeof use thus far forthermoplastic elastomers. This wasbecause of their acceptance in the footwear and adhesives markets (Auchter, 1981). Copolyester thermoplastic elastomers [poly(oxy- 1A-butylene)-poly- 1,4-butylene terephthalate alternating block copolymers] were introduced in the early 1970s. They are high-performance materials with excellent solvent resistance. abrasion resistance, ultimate properties, fatigueresistance, and very lowhysteresis. Theiracceptancehas beenlimitedonly by their relatively high price and high hardness (O’Connor and Fath, 1981). Polyamide-based TPEs have also been commercialized (Nelb and Chen, 1996). These materials have good high temperature resistance.Some of them are usedas liquid-water barriers. which readily transport water vapor, e.g.. in ski jacket linings. Thermoplastic olefinics (TPOs) were also introduced in the early 1970s. These are blends of EPDM and polyolefin plastic (usually polypropylene), such as those discussed earlier in this chapter. The rubber was slightly or not-at-all crosslinked (Morris. 1979). These blend compositions replaced thermoplastic urethane elastomers and even vulcanized rubbers in some exterior automotive components. Theyreplaced PVC in wire and cable applications and thermosetrubber in mechanical goods applications. though to a limited extent. The polyurethane and copolyester types of thermoplastic elastomers have good performance properties, but they are relatively expensive materials. The styrenics and TPOs are relatively inexpensive, but they are poor in solvent or fluid resistance. They have poor set and strength properties even at moderately elevated temperatures. The more recently commercialized compositions based on polypropylene and completely vulcanized EPDM (the thermoplastic vulcanizates preparedby dynamic vulcanization. described earlier herein) have many of the excellent properties of the polyurethane and copolyester type thermoplastic elastomers and even improved set andfatigue properties. Comparisons between the properties of commercial grades of completely vulcanized EPDM-polypropylene thermoplastic vulcanizate materials and those of conventional vulcanized specialty rubbers (CR, EPDM, and CSM) have been made by O’Connor and Fath (1982). A comparison of selected properties Of various thermoplastic elastomers and those of conventional elastomers is given in Table 22.
7.1 Processing-FabricationTechnology:MeltRheology The processing of a rubber-plastic blend composition into a finished part is a function of the melt rheology of the composition (Ouadi et al., 1996), and the melt rheology is a function of temperature and shear rate. The first investigation of the rheological properties of a TPV was performed by Goettler et al. (1982). They found that TPVs flow like filled polymer melts, i.e, extrudate swell (die swell) is low, viscosity obeys a power law. etc. However, they showed that even at very low shear rates. a Newtonian plateau could not be achieved. This would suggest yield behavior. Han and White (1995) obtained similar results on commercial EPDM-PP TPV samples. Araki and White (1997) showed the existanceof a critical stress for flow (yield stress).
313
Thermoplastic Elastomeric Rubber-Plastic Blends Table 22 Properties of Various Types of Elastomer Compositions
Ester-Ether copolymer
Completely vulcanized
Partially vulcanized EPDM/PP EPDM/PP Neoprene'" thermoplastic
Pronertv ~~
77 Hardness. A scale rength, Tensile Mpa Ultimate elongation, YC Volume swelling % in ASTM No. 3 oil (74 h a t IOO'C) Compressionmethod set (ASTM 22 h a t IOO'C), c/r Upper use temperature. "C TP Type of processing"
80
80
6.6 200
9.7
92 25.5
400 50
400 35
450 30
70
39
35
33
100
125
I10
125
-/
B,
9.7
Processing is also a function of the strength of the molten material under the strain due to its processing, i.e.. a function of its resistance to melt fracture. Melt fracture, under the conditions of extrusion. can give rise to very poor surface textures or even functionally useless parts. The melt rheology of a rubber-plastic blend composition is related to that of the plastic material. This is illustrated by Fig. 19. At high shear rates the viscosity-shear rate profiles are similar for the blend and the plastic material perse. However, at very low shear rates the viscosity of the blend can be very high. In the case of highly rubber-loaded thermoplastic vulcanizates (prepared by dynamic vulcanization). the viscosity can approach infinity. where the shear rate is zero. Thus, under the conditions of melt extrusion, the molten material undergoes rapid flow in the die. Then, as the material passes out of the die, the rate of deformation drops to zero, and since the viscosity approaches infinity (presumably due to cured rubber particle-particle interference). little or no die swell is observed. The dimensions of extrusion profiles of such materials are thus easily controlled. From Fig. 19 it appears that rubber-plastic blends are highly shear-rate sensitive in respect to melt viscosity. The temperature sensitivity of the viscosity of a thermoplastic elastomeric (highly rubber-loaded) rubber-plastic blend is illustrated by Fig. 20, which relates to an elastomeric EPDM-polypropylene blend. The viscosity of this type of blend is relatively temperatureinsensitive. (In the case of another composition based on another type of thermoplastic phase material, the viscosity may indeed be more temperature-sensitive.) Figures 19 and 20 indicate that in processing certain rubber-plastic blends by flow techniques. such as extrusion and injection molding, shear rates should be kept high enough to facilitate adequate tlow. The high melt viscosity of these products can be advantageous. The high viscosity can providehigh melt integrity or "green strength" and permittheretention of shapes of parts produced by extrusion or blow molding. The high melt viscosity and low die swell are also helpful in calendering sheet and film products. For injection-molded parts, fast injection rates (under high pressure) give lower viscosities due to the high shear rate. This facilitates rapid and complete mold filling. Then. after the mold is filled. the viscosity increases greatly due to shear
314
Coran
LOG VISCOSITY
RUBBER-PLASTIC BLEND
l
L LOG SHEAR RATE
-
Fig. 19 The relationship between viscosity and shear rate for a plastic and for its blend with
II
rubber.
rate reduction (to zero). This increased viscosity. which can approach infinity, enables more rapid extraction of the part from the mold. The overall effect is a faster irljection molding cycle. In addition. the low-temperature sensitivity of the viscosity of such acomposition givesa “broad temperature window” for processing. Typical injection molding and extrusion conditions for an EPDM-polypropylene blend are given in Tables 23 and 24. In addition to the above (injection molding. calendering, extrusion. and blow nlolding). foaming, thermoforming, and compression molding of olefinic rubber-plastic blends have been reviewed (Morris. 1979). However, it should be noted that processing conditions vary widely with equipment mold designs.specific blend compositions.etc. The best conditionsfor the production of ;I given part in a given factory must be found by experimentation.
7.2 End-UseApplications A large number of thermoplastic elastomeric rubber-plastic blends having good properties can now be prepared. This was not thought possible until relatively recently. Because of the fact that only very few polymer-polymer combinations are thermodynamically compatible, it was originally thought that only a few rubber-plastic blend combinations could be useful. As stated earlier in thischapter. new techniques of technologicalcompatibilization, such as dynamic
315
Thermoplastic Elastomeric Rubber-Plastic Blends
'
1,000 20
200
2,000
SHEAR RATE, RECIP. (SECONDS) Fig. 20 The effect of temperature on the typical thermoplastic vulcanizate.
EPDM-polypropylene viscosity-shear rate relatlonship for a
vulcanizationandconlpatibilization by blockcopolymeraddition or in situ formation,have expanded the number of rubber-plastic combinations that can produce useful blends. As a result of this, it is probable that only the beginning of the list of commercial end-use applications is presently visible. And that list of thermoplastic elastomeric blend applications is largely limited to those of the EPDM-polypropylene type.
Table 23 Injection-MoldingConditlons for EPDMPolypropylene-Based Thermoplastlc Elastomeric Blends Rear-zone barrel temperature, "C Center-zone barrel temperature, "C Nozzle temperature. "C Mold temperature, "C Iqcction pressure, Mpa Hold pressure, Mpa Back pressure. Mpa Screw speed, rpm Injection speed Injection time, S Hold time. S Total cycle time. S
180-220 205-220 205-220 20-65 35-140 30- I I O 0.7-3.5 25-75 Moderate to fast 5-25 15-75 20- 100
316
Coran
Extrusion Conditions forEPDMPolypropylene-Based Elastomeric Blends
Table 24
Rear-zone barrel temperature, "C Center-zone barrel temperature, "C 190-220 Front-zone temperature, "C temperature. Adapter "C temperature.Die "C temperature, Melt "C Screw speed, rpm
175-210 175-210 200-225 205-22s 205-235 10- 150
Uses of Blerlrls Coutolining Umw1cani:ed or. Only Slightly Cr.osslirtkrtl Olejlfirlic Rubber. A major application of EPDM-polypropylene blends of this type has been in exterior automotive body parts such as filler panels, bumper covers, fender extensions, lower fascias. flexible front andrearpanels, corner panels, and sightshields (Morris, 1979). The blendshavereplaced vulcanized EPDM rubber and thermoplastic polyurethane thermoplastic elastomers. Much of this market has been styling-related and has been subject to the whims of designer-consumer tastes. Other, more functional exterior autonlotive applicationssuch as gaskets and weather stripping have been tried. Specific examples include door guards, body and bumper strips, hood gaskets, trunk gaskets, taillight gaskets, and windshield gaskets. One reason for these applications is the excellent weather resistance of these materials. Interior automotive parts that have been made with the partially vulcanizedor unvulcanized EPDM-polypropylene blends include steering wheels, connector strips, grommets, seals. bushings, seatbelt housings, horn pads, flexible trim, and certain decorative parts. Under-the-hood applications have included sparkplug boots, electrical connectors, hose, and tubing. all of which must be stable against the effects of long-term elevated temperatures. Wireand cable applicationsarealarge potential marketfortheseblends. They have been used mainly as insulation and jacketing materials. They are competitive with crosslinked polyethylene. which is widely used in power and communications cable. The good resistance to thermal-oxidative degradation, as well as excellent electrical insulating properties are driving forces suggesting the wire and cable (and other electrical insulation-type) applications. Uses of Tllenno~~~lrrstit. Vulcar1i:ates Contairlirlg Corllpletely Vulccrni:ed Rlrhher Particles The end-use applications of EPDM-polypropylene blends have been greatly expanded by vulcanizing the rubber phaseto a high stateofcure. This is done commercially by dynanlic vulcanization. The potential of this type of material has expanded because of the inlprovements (in conlparison to the essentially unvulcanized compositions discussed just above) with respect to permanent set, ultimate mechanical properties, fatigue resistance, resistance to attack by fluids, high-temperature mechanical integrity. stabilityof phase morphology in the melt (during processing), melt strength, and thermoplastic fabricability. Thus these materials are more rubber-like in respect to their performance characteristics;yet they are more rapidly fabricable as thermoplastics. Potential and proven applications of the TPVs are as follows: Mecllrmicrrl rr~bber.goods Lrpp1iccrtiorl.s: caster wheels, convoluted bellows. flexible diaphragms. gaskets. seals, extruded profiles. tubing. mounts. bumpers. housings, air-
Thermoplastic Elastomeric Rubber-Plastic Blends
317
bag doors, glazing seals, valves, shields, suction cups, torque couplings, vibration isolators, plugs, connectors, caps,rollers, oil-well injection lines, handles, and grips. Under-the-hood urrtornoti~e applicutions: air conditioninghose covers, fuel line hose covers, vacuumtubing.vacuumconnectors,bodyplugs,seals,bushings, grommets, electrical components, convolutedbellows,steering gear boots,emissiontubing, protective sleeves, shock isolators, and air ducts. Zndustriul hose applications: hydraulic (wire braid), agricultural spray, paint spray, plant air-water, industrial tubing, and mine hose. Electrical applications: plugs, strain relief, wire and cable insulation and jacketing, bushings, enclosures, connectors, and terminal ends. Other applicutions: liners for food and beverage closures and syringe pistons. Currently, there a high level of product-developmendintroduction activity sustained by the industry. Efforts are being directed towards the development of thermoplastic elastomeric blend compositions that are more resistant to hot oil, compositions that are more resistant to high temperatures, and, eventually, compositions that are more resistant to both hot oil and higher temperatures. There is also activity in the area of food-related and medical applications.
REFERENCES Abdou-Sabct, S., and Fath, M. A. (1982). U.S. Pat. 4,311,628. Abdou-Sabet, S., and Patel, R. P. ( 1990), paper prcsented at American Chemical Socicty, Rubber Division, Washington, DC, October 9- 12. Abdou-Sabet, S., and Patel, R. P. (199 I ) , Rfthher CIIPI~I. Techrlol. 64:769. Aharoni, S . M. (1977), J. AI>/)/.Po/yrr~.Sci. 21:1323. Araki and White ( 1997), Fluid Mecl~crnics.In press. Auchter, J. F. (1981). papcr presented at a meeting of the Rubber Division, ACS, October. Avgeropoulos. G. N., Weissert, F. C.. Biddison, P. H., and Boehm, G. G.A. (1976). Ruhher Clzern. Techno/. 49:93. Badum (1942), U.S. Pat. 2,297,194. Barentsen, W. M,, Heikens, D. and Piet, P. (1974). P o / y r ~ e r15:l 19. Bhowmick, A. K., and Inoue, T. (1993), J. Ap,u/. Polyru. Sci. 49: 1893. Bhowmick, A. K.. Chibn, T., and hove. T. (1993), J. App/, Po/yru. Sci. 50:2055. Bucknall, C. B. (1977), Torcgkerled Pltrstics. Applied Sciencc Publishcrs, London. Callan, J. E., Hess, W. M., and Scott, C. E. ( l 9 7 l ) , Rubber Chern. Techrlol. 44:814. Campbell, D. S., Elliot, D. J., and Wheelans, M. A. (1978), N R Techno/.9:21. Carman, C. J., Batiuk. M., and Herman. R. M. (1977), U.S. Pat. 4,046,840, Sept. 6. Chen, C. C., and White, J. L. (1993). P d y m En,?. Sci. 33:923. Choudhury, N. R., and Bhowmick, A. K. (1989). J. App/. P o / w . Sci. 38:1091. Chung, O., and Corm, A. Y. (1996). a paper presented and amecting of thc RubberDivision,ACS. Octobcr 8- 1 1. Chung, 0.. and Corm, A. Y. ( 1997). Rrrhlwr Cllerr~.Techno/. 70:781 Cimmino. S.. Dorazio. L., Greco, R., Magho, G., Malinconico, M., Mancarella, C., Martuscelli, E., Palumbo, R., and Ragosta. G. ( 1984). Po/ym. Enc?. Sci. 24:48. Coran, A. Y.. and Patel, R. ( 1976), J. App/, P o / y n . Sci. 20:3005. Corm, A. Y., and Patel, R. (1978). U.S. Pat. 4,104,210, Aug. 1. Coran, A. Y., and Patel, R. ( 1 9 8 0 ~ )U.S. . Pat. 4,183,876, Jan. 15. Coran, A. Y., and Patel, R. ( 1980a), Ruhher C/tem. Techno/. 53:141. Coran, A. Y., and Patel, R. ( 1980b). Rubher Chctn. T d ~ n o l53:78 . 1. Coran, A. Y., and Patel. R. ( 198 I a). Rlrhhrr Chcwl. Tecltnol. 54:9 1. Corm, A. Y., and Patcl, R. ( l981 b), Rrthhrr Chcm Techno/.54892.
318
Coran
Coran, A. Y., and Patel, R. ( 1 9 8 1 ~ ) U.S. . Pat. 4,271.049, June 2. Coran. A . Y.. and Patcl. R. ( l982), U.S. Pat. 4.355,139. Oct. 19. Coran. A . Y., and Patcl. R. (1983a). Rttbbrr Clzrrrz. T e h o l . 56910. C o r m A . Y., and Patel. R. (198%). Rubher Chrnt. Techzol. 56:1045. Cora% A. Y.,a11d Pntcl. R. (1985). Rubher Chrrrz. Trchrrol. 58:xxx. Coran. A. Y., Das, B.. and Patcl, R. P. (1978). U.S. Pat. 4,130,535. Dec. 19. Coran, A. Y., Patel, R., and Williams, D. ( 1982a). Ruhber CIzprrt. Trchrzol. 55:116. Cora% A . Y., Patcl, R., and Williams, D. (1982b). Rtthher Churn. T e c h d . 55: 1063. Coran. A. Y., Patcl. R., and Williams-Hcadd. D. (1985). R~thDerCl~rrrr.Teclrrwl. 58: Corish. P. J.. and Powell, B. D. (1974), Ruhhrr Chrrrz. Trcltrtol. 4748 I . Crocker, G. I. ( 1969), Rttbhrr C h m . Tdzrzol. 42:30. Danesi. S.. and Porter, R. ( 1978), Po/yr)wr /9:448. Davies, W. E. A. (197 l), J. Phys. ( D ) 4318. Dunn, J. R. ( 1976), R~thhrrC l ~ e rTrchrrol. ~. 49978. Einstcin, A.. (1905), Arw. P I y . /9:549. Elemans. P. H. ( 1989). Modelling of the processing of incompatible polymcr blends, dissertation, Eindhoven University. Elcmendorp. J. J. (1985), Po/wrz. O r g . Sei. 25: 104 I . Elcmendorp, J. J. ( 1986). A study on polymer blending morphology, dissertation. Eindhoven Univcrsity. Ellul, M. ( 1998). Rrrhher Chrrrz. T r c h o l . 71:244. Emmctt, R. A. (1944). h d . D I ~ Cllern. . 36:730. Endo. S.. Min. K.. White, J. L., and Kyu, T. (1986), Po/yrz~.B I R . Sci. 26:45. Fischcr. W. K. (1973). U.S. Pat. 3,758.643, Sept. 1 I . Flory. P. J. ( 1953). Prirzciples ofPo/yrrzrr C/wmi.s/ry,Corncll University Prcss, Ithaca, NY, pp. 568, 576. Gnylord. N.G. ( 1975), Ad~v.Chrrrz. Srr. /42:76. Gesncr. B. D. ( 1969), Errc:\c/oprdict ofPo!\vrt. Sci. ttrttl Trchrzol.. Vol. 10 (Mark. H. F., and Gaylord, N. G. Eds.). Intcrscicncc Publishcrs, Wiley 6t Sons. Inc., New York, p. 694. Gent. A. N., and Lindley. P. B. (19%). Pmc. R. Soc. (Lorldnrt) A 2493195. Gent. A. N.. and Wong. C. (199 I ) , J. M~trer.Sci. 26:3392. Gcssler. A. M. (1962). U.S. Pat. 3,037,954, June 5 . Ghinm. F.. and Whitc, J. L. (1991). Polyrrz. Eng. Sci. 3/:76. Giller, A. ( 1966), Krtut. Gurwri K m s t s t . 19:188. Goettler, L. A., Richwine. J. R., and Willc, F. J. ( 1982). Ruhhcv Chenr. T c c h r d . 55:1448. Gotoh, K. (1970), in Pdyrrrrr Blerrds. Nikkan Kogyo Shinbun-sha, Tokyo. p. 109. Grace, H. P. (1982). Chcwz. Ertg. Corrzrrzwz. 14:225. Guth, E. J. (1945). J . AppI. Phys. 16:20. Hamcd, G. R. (1982), Rtthhrr C/rrrrz. Trclzrrol. 55:15 I . Han. P. K.. and Whitc, J. L. (l99S), Ruhlwr C/zrrrz. Tc.c./trzo/.68:728. and Shtrikmm, S. J. (1963), Mrch. Phys. Solids / I : 127. Hashin, Z.. Hclfand. E., and Sapse, A. M. (1975). J. Cltrrrr. Phys. 62: 1327. Holden, G.. and Milkovich. R. (1965). U.S. Pat. 3.265.766. Ide. F. and Hascgnwa, A. ( 1974). J . Appl. Pdyrrz. Sci. 18:963. Jha. A., and Bhowmick, A. K. (1997). Rubhcv Clzcwz. T e c h o l . 70:798. Jordhamo, G. M,, Manson, J. A., and Sperling, L. H. ( 1986), Po/.vrzz. Gig. Sei. 26517. Kolfoglou, N. K. ( 1983a). J. Mtrcrorrznl. Sci. -Phys. B22:343. Kalfoglou. N. K. (1983b). J. Mttcrorrtol. Sei. -Phy.s. B22:363. Kerncr, E. H. (1956), Proc. Phys. Soc. 69808. Kresgc, E. N.( 1978), Po/yrrwr BImtls, Vol. 2 (Paul, D. R. and Newmnn, S. Eds.), Acadcmlc Press, New York. p. 293. Kresge. E. N. ( 1984). J . A ~ / J / Po/yrrz. . Sei.: Appl. Pdyrrr. Syrrtp. 3Y:37. Laokijcharoen and Coran ( 1996),a paper presented and a meeting of the Rubber Division, ACS, Louisville. KY. October 8- 1 I . Liang, B. R., White, J. L., Spruiel, J. E., and Goswam, B. C. (1983). J. AppI. Pdyrrt. Sei. 28:201 I .
Thermoplastic Elastomeric Rubber-Plastic Blends
319
Lindsay, G. A., Singleton, C. J., Carman, C. J.. and Smith, R. W. (1979), Multiph~sePolyrt~ers.(Cooper, S. L. and Estcs, G. M., Eds.), American Chemical Society, Washington, DC, p. 367. Matsuo, M,, Nozaki, C., and Jyo, T. (1969). P o l y n ~ B. 7 g . Sei. 9(3):197. 2: 1 12. Mikami, T., Cox, R. G., and Mason, S. G. (1975), I n / . J. Mulfiphcrse Min. K., White, J. L., and Fellcrs, J. F. ( 1984), Polyrn. Ens. Sei. 24: 1327. Minoura. M. Veda, Mizunuma, S., and Oba, M. (l969), J. Appl. P o l w l . Sci. 13:1625. Mooney, M. J. ( 195 1 ). J. Colloid Sci. 6:162. Morris, H. L. ( 1 9 7 7 ~U.S. Pat. 4,031,169, June 21. Morris. H.L.(1979). in Hmdbonk nf Tl~er~nr~pl~rstic Eltrstorner.~(Walker, B. M,, Ed.), van Nostrand Rclnhold, New York, p. 5. Neb, R. G., and Chen. A. T. (1996), in Tlrermnplmtic~Ek~~.ston~ers. 2nd ed. (Holden. G.,Lcgg, N. R., Quirk, R., and Schroeder. H. E., Eds.), Hanser, New York. p. 191. Nielsen, L. E. (1974), Rl~rol.Act(/. /3:86. Nielson, L. E. (195 I), Rev. Sei. Instr. 22:690. O’Connor, G. E., and Fath, M. A. (1981), Ruhber Wnrld, December. O’Connor, G. E., and Fath, M. A. ( 1982), R~thherWorld, January. Polvrners. (Cooper. S. L., and Estes, G. M., Eds.), Olabisi, 0.. and Famam, A. G. (1979). Mrdfi[~l7~1.s~ American Chemical Soc., Washington, DC, p. 559. Olabisi, O., Robeson, L. M,, and Shaw, M. T. (1979).Pol~lnlrr-Pol~nler Miscibili~.Academic Press, New York, pp. 277, 321. Ouhadi, T., Shen, K. S., and Abdou-Sabet, S . (1996), SPE ANTEC Tech. Papers 42:3376. Patterson, H. T., Hu, K. H., and Grindstaff, T. H. ( 1971 ), J. Polyn7. Sci. C (34):3I. Paul, D. R. (1978). Polyrl7er Blends. Vol. 2 (Paul, D. R., and Newman, S.. Eds.), Academic Press, New York, p. 35. Polvmers (Cooper S . L. and Estcs, G. M,, Eds.), American Paul. D. R. and Barlow, J. W. (1979),M~rltiphr~se Chemical Society, Washington, DC, p. 315. Paul, D. R., and Barlow, J. W. (1980). J . Mac.ron~ol.Sei.-Rev. Mocrornol. Chem. C18:109. Payne, M. P,, Wang, D. S. T., Patel, R. P,, and Sasn, M. M. (1990). a paper presented at a Kubber Division, ACS Meeting, Washington, DC, Oct. 10-12. Paync, M. T.. and Rader, C. P. (1992). in Elrrstorr~erT e c h o l o g y Harldbook, CRC Prcss, Boca Raton, FL, Chap. 14. Puydak, R. C. and Hazelton D. R. (1988), P h i . Ens. 4437. Rwei, S. P,, and Manas-Zloczower (1990), P n l w l . E q . Sci. 30( 12):701. Schollenberger. C. S., Scott, H., and Moore, G. R. (1958). Rubher World 137549. Polym. Bull. 26:341. Scott, C. E., and Macosko, C. W. ( 1 9 9 1 ~ Smallwood. H. M. (1944). J. Appl. Pl7y.s. /5:758. Stchling, F. C., Huff, T., Speed, C. S.. and Wissler, G. (1981), J . Appl. Polvn~.Sei. 26:2693. Takayanagi, M. (1962), P/tr.sric.s 13:I . Takayanagi, M., Harlma, H., and Iwala, Y. (1963). Mern. Soc. B I ~K.y u s l ~ uUr7iv. 23( 1):l. Taylor, G. I. (1932). Proc.. Roy. Snc. A138:41. Taylor, G. I. (1934). Proc. Roy. Soc. A146:501. Thelamon, C. (1963). Rubber Clwrr~.Techrzol. 36:268. Tsai, H. Y., and Min. K. (1997), Polyrl~.Ens. Sci. Tsai, W. S . ( 1 9 5 8 ~Formulasfor the clasticproperties of fibers-reinforcedcomposites, A. D. 834851. June. Vander Meer. S. (1943), Rev. Gerf. Cnourch. P l m . 20:230. Wagner, H. L., and Flory. P. J. (1952). J. Am. C l ~ e mSoc. . 74:195. of Tlwrtrlnp/a.sticElnston~ers, 2nd ed., Van Walker,B. M,, and Rader, C. P. Eds. (1988), Hrrr~dl~ook Nostrand Reinhold, New York. White, J. L.. and Min. K. (1985). in Po/.vrrrer Bletuls m7d Mixtures (Walsh, D. J.,Higgins. J. S., and Maconnachie, A., eds.). NATO AS I Series No. 89, Martinus Nijhoff. Wragg, R. T.. Yardley, J. F., and Nightengale A. F. (1981), Kaursch. G u n ~ n Kur~stst. ~i 34:657. F l o ~ l
320
Coran
Wu, S. (1978), in Po/yrner Blends. Vol. 1 (Paul D. R., and Newman, S.. eds.), Academic Press, New York, p. 244. Wu, S. (1985), Polymer 26:1855. Wu, S. (1987), Polyrtz. Eng. Sci. 2 7 3 3 5 . Zakrzewsk, G . A. (1973), Polymer 14347. Zelinski, R. P. (1966), U.S. Pat. 3,287,333. Zisman, W. A. ( 1964). Advnrrcrs in Clrernistry Ser. 43: 1.
Thermoplastic Styrenic Block Copolymers Geoffrey Holden Holden Polymer Consulting, Incorporated, Prescott, Arizona
Charles R. Wilder* Phillips Petroleunl Company, Bartlesville, Oklahorna
1. INTRODUCTION Synthetic rubber has a long and illustrious history. However, it wasn’t until World War I1 that synthetic rubbers became the“workhorses” of the industry that they are today. With the shortage of natural rubber it became a necessity for the rubber industry to develop a substitute. With a combined effort that reached across company boundaries, emulsion-polymerized styrene-butadiene rubber in today’s form was created. This copolymer,which was to be known as emulsion SBR, became the basic polymer for the tire industry as well as many other segments of the rubber industry. Butadiene and styrene have been combined in many ways to make rubbery copolymers. The emulsion SBR copolymers had a random distribution of the two monomers. Modification of emulsion SBR is mainly possible through changes in molecular weight and butadienektyrene ratios. Other changes in the polymer were possible through the use of different polymerization temperatures,initiatorsystems.andfinishingtechniques. In more recent years,styrene-butadiene-based copolymers have been produced in solution polymerization systems that have allowedmuchmore control.Someexamples aremicrostructure (cis, frans, and vinyl ratios), molecular configuration, molecular weight distribution, and combination of comonomers. The discovery and development of the new polymerization systems have led to greater versatility in the preparation of elastomers. These systems havebeen applied to several monomers. principally styrene. butadiene, and isoprene. Developments worthy of note are selectively structured polyisoprenes (synthetic natural rubber and balata), polybutadienes (polymers of high cis, trans, and vinyl content), and random copolymers containing styrene. Probably the most interesting polymers to come fromthe technology of solution polymerization are the styrenic block copolymers as described by Haws ( 1974). Legge (1985). and Holden and Legge ( 1996). Polybutadiene-polystyrenetapered diblock copolymers havebeen produced since the early 1960s and have been described by Crouch and Short ( 1961) and Railsback et al. (1964). These block copolymers havebeen used to blend with other polymers to provide blends with improved
322
Holden and Wilder
-
-
Polystyrene Domain Elastomer Mid Segment
Fig. 1 Morphology of styrenic block copolymers.
properties, including combinations with rubbery polymers and plastics. They also found use as thesolepolymer in vulcanizedrubber compounds. The applications of thesetaperedblock copolymers have been described by Kraus and Railsback (1974). Other interesting block copolymers are thosewith multiple blocksof polystyrene connected by segments that are essentially rubbery in nature. Polymers of this type exhibit high strength and elastomeric characteristics without theuse of vulcanization chemicals-i.e., they are thermoplastic elastomers. Their thermoplastic nature can be explained by a simple example. Consider an S-E-S structure. with the S segments blocks of polystyrene separated from each other by an elastomeric segment designated asE. If the elastomer is the main constituent, the polymer should have a morphology similar to that shown in Fig. 1. Here, the polystyrene end segments form separate spherical regions, i.e., domains, dispersed in a continuous elastomer phase. Most of the copolymer molecules have their polystyrene end segments in different domains. At room temperature, these polystyrene domains are hard and act as physical crosslinks, tying the elastomeric midsegments together in a three-dimensional network. In some ways. this is similar to the network formed by vulcanizing conventional rubbers using sulfur crosslinks. The difference is that in thermoplastic elastomers. the domains lose their strength when the material is heated or dissolved in solvents. This allows the polymer or its solution to flow. When the material is cooled down orthe solvent is evaporated, the domains harden and the network regainsits original integrity. Branched block copolymers with the structure (S-E),,x (where x is a junction point with a functionality of n) have similar properties. However, block copolymers with only one hard segment (e.g.. S-E or E-S-E) have quite different properties. The elastomer phase cannot form a continuous interlinked network since only one end of each elastomer segment is attached to the hard domains. These polymers are not thermoplastic elastomers, but are weaker materials similar to wnwlcunized synthetic rubbers. The structure of styrenic block copolymers, without associationinto domains, is illustrated in Fig. 2. Figure 3 illustratesthese samepolymers withthestyreneblocksassociated into domains.
-
Thermoplastic Styrenic Block Copolymers
L I NEAR
RAD I AL
f
323
POLYSTYRENE
(TRI-CHAIN)
(TETRA-CHAIN)
Fig. 2 Linearandradial
LINEAR
5
polystyrene-polybutadiene block copolymers.
S-8-S
W
Fig. 3 Structure of linearandradial the polystyrene blocks.
polystyrene-polybutadietle blockcopolymersafternssocintion
of
propylene chloride)
Holden and Wilder
324
Table 1 Comparison of Thermoplastic Elastomers with Conventional Plastics and Rubbers
Thermosetting
T
Rigid Phenol-formaldehyde Urea-formaldehyde High-density polyethylene Flexible Highly filled and/or highly vulcanized Poly(viny1
Rubbery
rubbers
Vulcanized rubbers (NR, SBR, IR, etc.)
Low-density polyethylene EVA Plasticized PVC Thermoplastic Elastomers
The use of styrenic block copolymers has significantly increased since they were first produced about 35 years ago. An article by Reisch ( 1996) estimated their worldwide annual consumptionatabout 500,000 metric tons per year in1995, and thiswasexpected to rise to about 700,000 metric tons per year by 2000. In commercial applications. four elastomeric midsegments have been used-polybutadiene, polyisoprene, poly(ethy1ene-butylene), and poly(ethylene-propylene). The correspondingblock copolymers will be referred to as S-B-S. S-I-S, S-EB-S, and S-EP-S. The properties of thermoplastic elastomers in relation to other polymers are summarized in Table 1 . This table classifies all polymers using two characteristics-how they are processed (as thermosets or as thermoplastics) and the physical properties (rigid, flexible, or rubbery) of the final product. All commercial polymers used for molding, extrusion, etc.. fit into one of the six resulting classifications-the thermoplasticelastomersarethenewest. Their outstanding advantage can be summarized in a single phrase: they allow rubberlike articles to be produced using the rapid processing techniques developedby the thermoplastics industry. They have many of the physical properties of rubbers, e.g., softness, flexibility, and resilience. However. they achieve their properties by a physical process (solidification) compared tothe chemical process (crosslinking) in vulcanized rubbers. Two bookshavecovered this subject in detail. T/~errnoplasticElastonwrs (Holdenet al., 1996) concentrates mostly on the scientific aspects of these polymers, while Hunclhok of Thermoplastic Elrrstornrra (Walker and Rader, 1988) concentrates on their end uses.
2.
HISTORY
Development of homogeneous anionic polymerization systems in the 1950s provided new capabilities for controlling the course of polymerization, which in turn changed the synthetic rubber industry. Articles by Szwarc in Natlrre (1956) and the Jour-rlul cfthe Arr~ericanCherl~icrrlSociety (Szwarc etal., 1956) described the discoverythat the polymerization of styrene in tetrahydrofuran initiated by sodium naphthalene complex produced what he called a “living” polymer. After the initial volumeof styrene had been consumed, the red color of the polystyryl ion still remained. indicating that the reaction had not been terminated. The living polystyrene added not only additional styrene nlonomer to increase molecular weight. but other monomers as well.
Thermoplastic Styrenic Block Copolymers
325
Szwarc recognized the potential for producingblock copolymers by thismethod and succeeded in preparing block copolymers of styrene and isoprene using a sodium naphthalene complex. which produces a dianionic species:
3SSSSSSSS-
+
Isoprene Monomer
e- IIIIIIIIIIII-SSSSS-Illlllllllll
Lithium metal and lithium alkyls also initiate polymerization of styrene and dienes but produce mono-anions (Stravely, 1956; Foreman, 1969). Porter (1957) showed that S-B-S block copolymers could be produced using lithium alkyl initiator. Later Holden and Milkovich ( 1962) produced both S-B-S and S-I-S using lithium alkyl initiator and showed that the products were thermoplastic elastomers. Morton and Ells( 1962) found that during polymerization of butadiene and styrene in benzene, the values of the reactivity ratios were such that butadiene polymerizes preferentially. The addition of butadiene to a butadiene end is more likely than the addition of styrene to a butadiene end. Data from a Phillips Petroleum Company patent (1958) describing the polymerizations of styrene. butadiene. and a 25/75 styrene-butadiene mixture in cyclohexane show how the amount of boundstyrenevaries with conversion (Fig. 4). Very little styrene monomer is polymerized into the polymer chain during the early stages of polymerization of a styrene-butadiene mixture. As the butadiene monomer is depleted, more styrene monomer is added to the polymer chain, and a section of the molecule becomes tapered. The tapered section, which contains both butadiene and styrene units, is short. Polymers prepared in these systems differ little from pure block copolymers polymerized by sequential polymerization of two monomers. The first commercial production of styrenic block copolymers began in 1963. when Phillips Petroleum Company introduced solution-polymerized tapered S-B materials under the Solprene trade name. As noted above, these were not thermoplastic elastomers but were used in blends and conventional vulcanizates. In 1965 Shell Chemical Company introduced a series of S-B-S and S-I-S block copolymers. These were thermoplastic elastomers and initially had the trademark Thermolastic. which was later changed to Kraton. Some time later Phillips produced branched (S-B)x,, and (S-I)x,,block copolymers that were also thermoplastic elastomers.
BOUND SNRENE. X 24
-
20
-
16 -
12 8 4
J
I ( I I ( J I I l ( l 10 20 30 40 50 60 70 80 90 100 CONVERSION, X Fig. 4 Bound (i.e.. polymcrizcd) styrene versus conversm durlngpolymcrizntlon of of styrenc and butndicnc monomcrs.
;I
25/75 mixture
Holden and Wilder
326
A chapter by Holden and Leggein a recent book (Holden et al.. 1996) and a paper presented by Legge (1985) at a meeting of the ACS Rubber Division describe much of the history of the development of Shell's thermoplastic elastomers.
3.
MANUFACTURE
Styrenic block copolymers are usually produced commercially in polymerization systems that employ a butyllithium initiator in a hydrocarbon solvent. Since the reaction involves anionic species, there are no termination reactions from interaction of polymer chains as there are in free radical systems. Initiation is very rapid, and since the polymer chains grow at the same rate. the molecular weightof the polymer(M,,)is determined by the amount of initiator employed:
M" = weiaht of monomer moles of initiator If oxygen, moisture, and other reactive nlaterials are rigorously excluded, the individual polymer anions persist indefinitely. After polymerization is complete, the reaction product can be deactivated by the addition of a protonating species such as alcohol. Solvent is removed by steam-stripping or devolatilizing in an extruder or other equipment. The finished products are marketed as crumb, bale, pellets, or powder.
3.1
S-B and S-l Diblock Copolymers
The simple diblock copolymers can be produced by two routes, either by sequential polymerization or by mixing monomers having different reaction rates. In the case of S-B diblock copolymers: Seque~~tial polstnerixztior~:
Styrene monomer SSSSSSSS-
+ initiator + SSSSSSSS-
+ butadiene monomer + SSSSSSSBBBBBBBBB-
Diflerentiul reaction rates:
Styrene monomer BBBBBBSB-
+ butadiene monomer + initiator +- BBBBBBSB-
+ styrene monomer + BBBBBBSBSSSSS-
Sequential polymerization produces pure block copolymers. since the second monomer is added only after the firstone has been completely polymerized. Tapered block copolymers are produced by polymerization of a mixture of monomers that have differential reaction rates. Processes to produce tapered block copolymers have been described in both Phillips (1058) and Firestone (1966) patents.
3.2
S-B-S and S-I-S Triblock Copolymers
Triblock copolymers of the S-B-S or S-I-S type are produced commercially by sequential polymerization. by coupling of S-B or S-I anions. or by multifunctional initiators. In the case of S-B-S triblock copolymers:
327
Thermoplastic Styrenic Block Copolymers
S c c / r / l ~ r / l r t rP/ o l ~ ~ / r r c ~ / ~ r , t r r i c , r r
Styrene monomer
+ initiator
SSSSSSS-
+ D
SSSSSS- + butadiene monomer ”+ SSSSSSBBBBBBBBBSSSSSSBBBBBBBB’ + styrene monomer + SSSSSSBBBBBBBBBSSSSSS-
-
I
1,2 I -CH,-CH=CH-CH2-CH,-CH1,4
‘‘
I
I
CH
II CH,
Polvbutadiene
E
‘
B
I
---> -CH2-CH,-CH2-CH2-CH,-CHl 1
I I Polv(ethv1ene-butvlene)
Holden and Wilder
328
In the same way, S-EP-S block copolymers can be produced by hydrogenating S-I-S precursors (Bhattacharya et al., 1993) and have similar properties to S-EB-S analogs. A more complete review of anionic polymerization is given in a book by Hsieh and Quirk ( 1996). Styrenic block copolymers are commercially available as diblock copolymers, triblock copolymers, diblockhriblock mixtures, and branched copolymers. They aremarketed as (l) pure (nonextended or neat), (2) oil-extended. or (3) fully compounded products. Trade namesof some manufacturers are given in Table 2 .
4.
CLASSIFICATIONANDSTRUCTURE
In styrenic block copolymers, several structural variations
4.1
are possible.
MolecularWeight
Compared to similar homopolymers. the melt viscosities of styrenic block copolymers are very high and unusually sensitiveto molecular weight. Both these effects are caused by the persistence of the two-phase domain structure in the melt and the extra energy required to disrupt it during flow. For these reasons the pure styrenic block copolymers are often difficult to process. and in practical use compounded products are always used (see later).
4.2
PolystyreneSegmentContent
The hardness of these styrenic block copolymers depends on the ratio of the hard polystyrene phase to the softer elastomer phase and so can be varied within quite wide limits. As the ratio of the S to B segments in an S-B-S block copolymer is increased, the phase morphology changes from a dispersion of spheres of S in a continuous phase of B to a dispersion of rods of S in a continuous phase of B. As the proportion of S increases still further, a lamellar or “sandwich” structure is formed in which both S and B are continuous (see Fig. S ) . If the proportion of S is increased still further, the effect is reversed in that S now becomes disperse and B continuous. As the polystyrene phase predominates, the block copolymergets harder and stifferuntil eventually it becomes a clear flexible thermoplastic (e.g., Phillips K-Resin).
4.3
Diblock Content
Many styrenic block copolymers contain significant amounts of diblock. This is usually the result of incomplete coupling during production. The diblock makes the product softer, weaker, and less viscous. For some purposes (mostly adhesives and sealants) this diblock content is desirable, and materials with up to 80% diblock are produced commercially.
4.4
ElastomerSegments
Analogous S-B-S, S-1-S, S-EB-S. and S-EP-S block copolymers have somewhat different properties (Table 3). The differences in therelativestiffness of thesepolynlersarerelated to the difference in the degree of entanglements i n the three types of elastomer segment.Poly(ethy1enebutylene) and poly(ethy1ene-propylene) are very similar and will be considered together. They are the most highly entangled and so have the most effective crosslinks per unit volume of polymer, thus giving them the highest modulus. In contrast, polyisoprene is the least entangled and so S-I-S block copolymers arethe softest of the three types.All these differences are reflected
Thermoplastic Styrenic Block Copolymers
”
a
a W.
W
8 m 8-mmmm 8mm 8 m m m m
x
C
E
L
U
m
m
e,
I
.-U
c,
C
x m
5 .-
n
x
F
329
330
Holden and Wilder
A
Spheres
A Cylinders
A. B Lamellae
B
B
Cylinders
Spheres
Decreasing B-Content Fig. 5 Morphology changes with composition in A-B-A block copolymers.
in the end uses. S-B-S polymers are often used to make lower-cost products where stability is not critical (e.g., footwear).S-I-S analogs are softerand stickier and are mostly used in adhesives. S-EB-S and S-EP-S copolymers are the hardest of the three and themost resistant to degradation. Thus they are used where high stability is required (e.g., autoniotive parts and wire insulation).
5.
PHYSICALPROPERTIES
Properties of styrenicblock copolymers result from the number.length. and type of block segments. When two incompatible homopolymers are mixed, phase separation occurs, with one polymer usually becoming the dispersed phase and the other the continuous phase. We see a similar effect i n styrenic block copolymers. This presence of the two-phase structure in styrenediene copolymers has been verified by electron microscope studies reported by many workers, including Vanzo (1966). Hendus et al. (1967), Bradford and Vanzo (196X), Meier (l969), and Bi andFetters (1975). Very small areas, or domains, of polystyrenedispersed in arubbery matrix can be seen. The domains are so small that they do not refract light; hence the block copolymers are transparent, although a blend of the same homopolymers would be translucent or opaque. This two-phase system gives a structure with the polystyrene segments associating into hard "domains" connected by flexible elastomer chains. In triblock copolymers, this results in an elastomeric network being formed without the necessity of forming crosslinks by conventional sulfur/accelerator curing systems. Styrenic triblock copolymers havehigh tensile strength, resilience, and coefficient of friction. When heated, the polystyrene domains soften and the polymer will flow under pressure. Similarly, in a suitable solvent (e.g., toluene) both the end and center segments dissolve to give low viscosity solutions. When the solvent is evaporated the network reforms and the polymer regains its original properties.
Table 3 Comparison of S-B-S. S-I-S. S-EB-S, and S-EP-S Block Copolymers Relative stiffness Relative S-B-S S-I-S S-EB-S and S-EP-S
cost
1
1
0.5 2
1.3 2 to 2.5
Stability Degradation Moderatc Moderate Excellent
product Crosslinking Chain scission Chain scission
Thermoplastic Styrenic Block Copolymers
Besides high strength,elasticity.resilience,andthermoplasticity.thestyrenic copolymers have other advantages:
331
triblock
They are elastomeric. Their scrap can be recycled. They have high tensile strength at moderate temperature. They can be prepared or compounded to give a wide range of product properties. They possess a high coefficient of friction. Vulcanization is not needed, and so there are no vulcanization residues. They have no color. They are thermoplastic. They have good low-temperature properties. Their die swell is low. Although the styrenic block copolymers have all the advantages listed above, there are limitations. Tensile strengthand hardness decrease as temperature increases, with strength and hardness becoming low as the softening point of the polystyrene phase is approached.
5.1 TensileStrength The morphology of styrenic block copolymers accounts for ( I ) the decidedly different tensile properties of S-B copolymers compared to those of S-B-S analogs and (2) themoresubtle differences between the properties of the radial (S-B),,x copolymers and those of allalogous S-B-S linear copolymers. S-B copolymers are clusters of molecules formed by association of the polystyrene segments with polybutadiene chains extending from a central polystyrene domain. One end of each polybutadiene chain is firmly attached to the polystyrene domain. However, the remaining part of each polybutadiene chain is attached to the other chains only by weak attractive forces and by chain entanglements. Thus, uncured S-B copolymers have little tensile strength and must be cured to provide the properties usually associated with rubbery products. In contrast to the S-B copolymers, the S-B-S copolymersof either the linear or radial type exhibit tensile properties in the uncured state that are typical of good-quality vulcanized rubber. The unusually high strength of these polymers results from the extensive network created as the polystyrene domains tie down both ends of the polybutadiene chains. In addition to the domains acting as effective crosslinks. permanent chainentanglements areformed in the rubbery portion of the block copolymer. In the case of radial (S-B),,xblock copolymers. the site at which the coupling agent connects the individual polymer segments is also an effective crosslink. An increase in molecular weight resultsin an increase in the service temperatureof styrenic block copolymers. An example of this is for S-B-S block copolymers, shown in Fig. 6. Morton et al. (1969) reported that the stress-strain properties of S-B-S block copolymers containing 20-40 wt% polystyrene and with molecular weights in the range of 60,000-150,000 show little effect of molecular weight when compared at constant composition. Holden et al. (1968, 1969) reported similar results on three S-B-S copolymers with about 27% polystyrene content and molecular weights ranging from 73,000 to 150,000. In contrast. Zelinski and Childers (1 968) found that at higher molecular weights, increasing molecular weight causes an increase in tensilestrength. These apparentcontradictions may beresolved asfollows. Below some critical molecular weight for the polystyrene segments.an increase in molecular weight improves tensileproperties.However, once thiscriticalmolecularweight has been exceeded, further increases have negligible effects. Styrenic blockcopolymers having the best balanceof rubber-like properties usually contain 15-40 wt% polystyrene. Below this range the tensile strength is reduced. Above this range, the
332
Holden and Wilder
TENSILE STRENGTH, M Pa
MOLECULAR
30
20
l0
27 49 TEST TEMPERATURE.
60 C
Fig. 6 The influcncc of molccular weight and temperature on thc tensile strength of S-B-S block copolymers.
block copolymers have yield pointsandexhibitcolddrawproperties.Blockcopolynlers of the S-EB-S typehavebeendescribedandcharacterized by Gergen(1985, 1996). One area of particularinterest is the comparison of tensilepropertiesexhibited by the S-B-S and SEB-S block copolymers. Of the two, the S-EB-S exhibits higher modulus, which is attributable to its higher association energy, greater degree of elastomer chain entanglement, and absence of substantialinterfacevolume. The elongation of the S-EB-S block copolymer is lower than that of the S-B-S equivalentbecausethe S-EB-S hasthesmaller contour length. The rate of tensile loss with increase in temperature is much less for the S-EB-Scopolymer than for the S-B-S copolymer. This difference in the temperature-tensile strength relationship allows the S-EB-S block copolymer to be used forapplications in which similarS-B-S block copolymers would not besuitable.
5.2 Hardness Hardness of styrenic block copolymers isstrongly dependent onpolystyrene content. Thiseffect is shown in Table 4 for a series of S-B-S block copolymers.
5.3 Melt Flow Melt flow of S-B and S-B-S block copolymers has been studied by Kraus and Gruver (1967) and Holden et al. (1968, 1969). They reported the melt viscosities of block copolymers to be much higher than those of either the corresponding homopolymers orthe corresponding random copolymers. The higher viscosities are attributed to the persistence of the network in the melt. Van der Bie and Vlig (1969) reported that the effect of temperature on melt viscosity is much more pronounced at low shear rates. Processing under low-shear conditions should be done at
333
Thermoplastic Styrenic Block Copolymers Table 4 Effectof Styrene Content ontheHardnessof Copolymers Molecular Shore weight wt% Styrene
S-B-S Block A hardness
13 21.5 39 53 65 80
93,000 7 3,000 84,000 72,000 73,000 83,000
41 63 89 92 96 98
the upper end of the temperature range. High-shear processing, such as injection molding, can be done over a wider temperature range.
5.4 Stress Softening Repeated extension of conventionally crosslinked or cured elastomers leads to lower stresses at the same extension. This stress softening is also observed with S-B-S block copolymers. Zelinski and Childers (1968), Childers and Kraus (1967), and Holden et al. (1968, 1969) all reported that S-B-S block copolymers with styrene content above 30% exhibit yield points and stress softening. With increasing styrene content, interconnections between domains increase, and the yield point occurs as this network is broken. In block copolymers that do not exhibit yield points, stress softeningoccurs when polystyrene segments are pulled out of the domains. Stress softening of S-B-S block copolymers is reversible; if the sample isrelaxed or annealed at elevated temperatures, the original stress-strain properties are regained.
5.5 Glass Transition Temperature Styrene-butadiene random copolymers exhibit single glass transition temperatures intermediate between those of polystyrene and polybutadiene. In contrast, block copolymers of polybutadiene and polystyrene with sufficiently long blocks exhibit two glass transition temperatures that are close to the glass transition temperatures of the individual homopolymers (see Fig. 7). 5.6
Environmental Resistance
Environmental resistance of polymers can be subdividedinto three classes: resistanceto degradation produced by oxygen, ozone, and UV light. In styrenic block copolymers, resistance to all three types of attack mostly depends onthe elastomer segment. Thus,for those that haveunsaturated midblocks, their stability is similar to that of polybutadiene or polyisoprene. In contrast, those with poly(ethy1ene-butylene)or poly(ethy1ene-propylene)midblocks show the better stability associated with EPR or EPDM rubbers. Hindered phenols in combination with thiodipropionate esters are usefulantioxidants.Zincdibutyl dithiocarbamate improves high-temperature properties. Ozone resistance of the unsaturated styrenic block copolymers can be improved by compounding with ozone-resistant polymers such as EVA or LDPE. UV resistance of styrenic block copolymers can be improved by addition of stabilizers such as benzotriazoles andor by compounding with pigments such as carbon black or titanium dioxide. Application of these technologies will be discussed in Section 6.
334
Holden and Wilder
r ses " "
\
l60
200
\
"" 200
240
S6R
3 20
360
L
1
Ternperohre, K'
Fig. 7 Viscous damping of an S-B-S block copolymcr andan SBR random copolymer.
5.7 AbrasionandFlexResistance The abrasion and flex resistance of the compounded styrenic block copolymers is sufficient for many applications. An example of a suitable use would be in shoe soles. A balance of resistance and processability can be obtained by the incorporation of compounding ingredients such as polystyrene, oils, and filler. Flex resistance is better at lower temperatures; this is a complete contrast to most competitive materials. In compounds based on S-B-S and (S-B),x block copolymers, those with higher molecular weight polystyrene segments show the best flex resistance.
5.8 Coefficient of Friction The frictional properties of compounds based on S-B-S and (S-B),,x block copolymers are in the same range as vulcanized rubbers such as natural rubber and emulsion SBR. Their values are about 50% higher than those of flexible plastics of similar hardness (Holden, 1973).
5.9 Shear Resistance in Adhesives Higher styrene content, higher molecular weight, and branching all improve the resistance of the styrenic block copolymers used in adhesives to failure under shear. Higher molecular weights and styrene contents increase viscosity; however, branching (at constant arm length) has little effect on this property. At similar formulation viscosity there is an increase in shear resistance as the structure changes from linear (di-branched) to tri-branched to tetra-branched. This effect was shown by Man's et al. (1973), and more details are given in Table 5.
5.10
PeelStrength in AdhesiveFormulations
Peel strengths follow the same general trends as shear strengths. The best peel strengths are obtained from styrenicblock copolymers with relativelyhighmolecularweightpolystyrene
335
Thermoplastic Styrenic Block Copolymers Table 5 S-B-S and (S-B),,XBlock Polymers at Equal Formulation Viscosity (7000 Butadiene-
Styrene Copolymers) Formulation resistance viscosity Shear Molecular structure weight Chain ~~
(Pa5)
(hours to fail a t 90°C)
84,000 136,000
1 .S2 1.S8
IX2.000
I .x2
I .o 2.4 2.8
~~
Linear Trichain Tetrachain
segments. However, this increase in molecular weight or styrene content may be limited by the desired tack and viscosity.
6. APPLICATIONS Likemostconventionalvulcanized rubbers-and unlikemost thermoplastics-the styrenic block copolymers arenever used commercially as purematerials. To achievetheparticular requirements for each end use, they are compounded with other polymers, oils, resins, fillers, etc. In almost all cases, the final products contain less than 50% of the block copolymer. Thus, a study of their end uses is in effect a study of how they are blended to achieve the properties needed for the particular application. Before discussing the end usesin detail, it is important to consider how the added materials are distributed with respect to the two phases in the block copolymer. For any additive, there are four possibilities. It can go intothe elastomerphase. I n this case theadditiveincreasestherelative volume of the elastomer phaseand so makes the product softer. The addition can also change the glass transition temperature of the elastomer phase. This in turn affects such properties as tack and low-temperature flexibility. 2. It can go into the polystyrene phase. In this case the additive increases the relative volume of the polystyrene phase and makes the product harder. The glass transition temperature of the additive should be similar to or greater than that of polystyrene (100°C). If not, its addition will reduce the high-temperature performance of the final product. 3. It canform a separatephase.Unless the molecularweight of theadditive is much less than that of either type of segment in the block copolymer, this is the most likely outcome. Thus, only oils and low molecular weight resins are compatible with either of the existing two phases. Higher molecular weight(i. e., polymeric)materials generally form a separate third phase. This polymeric third phase is usually co-continuous with the block copolymer. Therefore, it confers someof its own properties(e.g., higher upper service temperature, improved solvent resistance) on the final blend. 4. It cango into both phases.Additives that do so areusuallyavoidedbecause they reduce the degree of separation of the two phases and so weaken the product.
1.
Blending with such a widerange of materials gives the styrenic block copolymers an exceptional variety of end uses, three of which are major:
I . Formed goods, i.e., replacements for vulcanizedrubberarticles 2. Adhesives,sealants,andcoatings 3. Polymer andasphaltmodification
Wilder
336 Table 6 CompoundingStyrenicBlockCopolymers
Effect on Oil/Solvent resistance Processability Hardness Component Oils
Polystyrene Polypropylene
Cost
Decreases Increases Increases
Improves Improves Improves especially
-
Improves
Decreases Decreases Decreases
-
Improves resistance
with S-EB-S
Slight increase Slight Filler
Other
-
density Increases Decreases
improvement
6.1 Formed Goods (Replacements for Vulcanized Rubber) This end use has been described in several books (Halper and Holden, 1988; Holden, 1996.)The products are usually manufactured by machinery used to process conventional thermoplastics. Examples are injection molding, blow molding, blown film, and profile extrusions. S-I-S block copolymers are not used very much in this application. However, there are many applications for compounded products based on S-B-S, (S-B),,x, S-EB-S, and S-EP-S block copolymers. A list of some compounding ingredients and their effects on the properties of the products is given in Table 6. Products that are as soft as 5 Shore A have been described by Deisler (1991). Others are as hard as 55 Shore D. Large amounts of compounding ingredients can be added. The final products often contain as little as 25% of the styrenic block copolymers. From an economic point of view, this is most important. For example, it enables compounds based on S-EB-S block copolymers to compete with those basedon polypropylene/EPDM or EPR blends, although the pure S-EB-S copolymer is much more expensive than the polypropylene. the EPDM, or the EPR. Polystyrene is often used as a compounding ingredient for S-B-Sand (S-B),,xblock copolymers. It actsasaprocessingaid and makes the productsstiffer.Napthenicmineraloils are excellent processing aids but make the products softer. Inert fillers such as calcium carbonate, talc. and clays are often used in these compounds. They haveonly a small effect on most physical properties but reduce costs. In some casesup to 200 parts (per 100 parts of the block copolymer) are used in compounds intended for footwear applications. Reinforcing fillers such as carbon black are not suitable for this application. Large quantities of such fillers make the final product stiff and difficult to process. Resins can adjust hardness and increase melt flow and adhesion. A combination of resins can be used to achieve a desired balance of properties but may reduce the rubbery feel of the product. The influence of resins in a variety of applications has been described by Haws and Wright (19761, Halper and Holden (1988), and Holden (1996). Addition of process oils to thermoplastic S-B-S block copolymers generally increases melt flow and flex life. Conversely, tensile strength, hardness. and abrasion resistance are reduced. Naphthenic oils have a minimum influence on tensile strength and hardness. In contrast, ester plasticizers and aromatic oils soften the polystyrene block of the S-B-S block copolymers and greatly reduce properties, and so they are not used. Only low levels of paraffinic oils should be
Thermoplastic Styrenic Block Copolymers
337
used, as these oils are less compatible with the polybutadiene phase and tend to bleed from the compound. S-EB-S and S-EP-S block copolymers can be similarly compounded. In this case polypropylene is the preferred polymeric additive, actingin two different waysto improve the properties of the compounds. First, it improves processability. Second, when the compounds are processed under highshear, the polypropylene and the block copolymedoil mixture form two co-continuous phases.Polypropylene is insolubleandhasahighcrystalmeltingpoint ( - 165°C). The COcontinuous polypropylene phase significantly improves both the solvent resistance and upper service temperatureof these compounds. Another advantageof S-EB-S and S-EP-S block copolymers is that because of their lower midsegment solubility parameter, they are very compatible with paraffinic mineral oils. Large amounts of these oils can be added without bleedout. and this allows very soft compounds to be produced. Napthenic oils are less compatible with the EB or EP center segments, and oils with high aromatic contents are not used. Blends of these block copolymers with mineral oils and polypropylene are transparent. This is probably because therefractive index of an S-EB-Soil or S-EP-S/oilmixturealmostexactlymatches that of crystalline polypropylene. Blends with silicone oils are used in some medical applications. Thesame inert fillers used in theS-B-S-based compoundsare also usedwiththe S-EB-S and S-EP-S analogs. In addition, barium or strontium sulfate fillers give very dense compounds for sound-deadening applications. In another development, fire retardants can be added. These compounds will qualify under many current regulations. Compounds based on styrenic block copolymers must be protected against oxidative degradation and, in some cases, against sunlight. Hindered phenols are effective antioxidants and are often used in combinationwiththiodipropionatesynergists.Benzotriazolesare effective UV stabilizers and are often used in combination with hindered amines. If the product does not have to be clear, titanium dioxide or carbon black pigments give very effective protection against sunlight. Phase modification of S-EB-S block copolymer by different additives and its effect on morphology, mechanical, and dynamic mechanical properties have been reported by Ghosh et al. (19%). Compounding techniques are relatively simple and standard. There is one important generalization-the processing equipment should be heated to a temperature at least 20°C above either the melting point of the polymeric additive or the glass transition temperature of the polystyrene segments, whichever is greater. The use of cold mills, etc. can result in polymer breakdown. Not only is this unnecessary, but it is also detrimental to the properties of the final product. S-B-S, S-EB-S, and S-EP-S block copolymers with high molecular weight polystyrene segments and/or high styrene contents are very difficult to process as pure polymers. Versions with oil contents from about 25% to about 5 0 8 are commercially available. These are easier to process, and of course more oil can be added during mixing. Extruders, internal mixers, open roll mills, or dry-blendingmethodscan be used to mix styrenicblock copolymers with the various compounding ingredients. Extruder. Mixing Unfilled or lightly filled compounds can be made on a single screwextruder fitted with a mixing screw. The length/diameter ratio should be at least 24: I . If large amounts of fillers or fire retardants are to be added, these can be dispersed on either a twin screw extruder or a closed intensive mixer that discharges into an extruder. BmDuty Mixirlg Internal mixers such as Banburys permit fast and efficient processing. A preheated mixer chamber will aid incorporation if large amounts of filler or oil are to be added. If large amounts of
Holden and Wilder
338
oil are to be used, it should be added in increments. Addition of the first portion of oil early in the mixing cycle may be beneficial to flux the ingredients. If a high-melting polymer such as polystyrene or propylene is to be added, it should be melted before the oil is added. To obtain good fluxingandpropermixingandblending,temperaturecontrol is important. The mixer discharge temperature should not exceed 160°C for S-B-S-based compounds and 200°C for S-EB-S-based compounds. A suggested Banbury mixing cycle for a high fillerhigh oil S-B-S compound, using a mixer preheated to 65-9S°C, is as follows:
0 min: Add all ingredients except oil.* 2-3 min: Scrape and add one fourth of the oil. When thepower load increases, add another increment of oil, and continue to add oil in this manner until all the oil is dispersed. 145-160°C: Discharge and sheet off on a roll mill with roll temperatures of 85-105°C. Trialbatches or small-scaleproductionrunscan be madeusingasmallbatch-typeclosed intensive mixer. Mixing times of about 5 minutes are usually adequate. After this mixing, the hot product is passed to a heated two-roll mill. When it has banded, it is cut off, allowed to cool, and then granulated. The granulator blades must be sharp and clearances minimized.
Open Mill Mi.xing Mixing on an open roll mill requires roll temperatures of about 100°C for S-B-S-based compounds and 170°C for S-EB-S compounds containing polypropylene. Smooth banding will not take place if roll temperatures are too cool, while the compound will stick to the rolls if they are too warm.
Dry Blending Dry blending is defined as the mechanical mixing of the components of a compound to give an even dispersion throughout the mixture but without causing the mixing temperature to rise above the melting temperature of any of the ingredients. This mixing procedure works best if the materials to be blended have about the same particle size. It is particularly desirable to have all polymeric material about the same particle size. Absorbed oil tends to cover the surface of the polymer particles and should bind the coating of fines, stabilizers, and powdered resins. The advantages of dry blending in comparison to other mixing techniques are that it: 1. Eliminatesinternalmixers 2 . Eliminates pelletizing by feeding the dry blend directlyto theplastic fabrication equipment 3. Reducesmixingtime 4. Reduces mixingcost
This technique requires that all the polymers in the blend are free-flowing powders. The polymers can bepurchased in this form, orthey can be groundon-site i n commercially available equipment. A typical dry-blend mix cycle for a Welex 8M mixer with a water-cooled jacket is: 0 min: Add all ingredients, except any resins melting below mixer speed of 90 Hz (1500 rpm).
110°C. Mix for 30
~
* For very high oil and filler levels,
a small amount of oil may be added at 0 min to wet the filler.
S
with
Thermoplastic Styrenic Block Copolymers
339
0.5 min:Reducespeed to 20 Hz (1200 rpm) and begin gradual oil addition. An extra mixing time of 45-60 s is suggested for 40-50 phr of oil. 1.5 min: Mix at 90 Hz ( 1 500 rpm) until the compound appears to be free-flowing. If powdered resins are to be added, add them after the mixture is free-flowing and mix an additional 30 S at 33-42 Hz (2000--2500 rpm). 4-6 min: Discharge the batch. The temperature of the compound should be less than 65°C to prevent softening and agglomeration of the ingredients. Pelleti:ir~gr r t ~ Chopping l
Compounded stock that is to be used i n plastics-processing equipment must be in a form that will feed to the machines. Dry-blended compounds are satisfactory in the form in which they are discharged from the dry blender. Compounded stock mixed in an internal mixer is either pelletized or chopped for ease of feeding into the processingequipment. Onemethod of pelletizing is to strip-feed the product into an extruder and then extrude through a multistrand die. After extrusion, the compound is fed to a pelletizer. This can be either a strand-cuttingor an underwater face-cutting system. If the first type is used, it is important to remember that rubbery compounds must be cut rather than shattered. Thus, the blades of the cutter should be sharp and the clearance between the fixed and the rotating blades should be minimized. With this type of pelletizer. the strands must be thoroughly cooled before they enter the cutter. A chilled water bath can be used to increase production rates. The processing and property advantages that give the compounds their value are: Ease of molding Temperature resistance Low cost Low density Softness Paintability Bonds to other polymers These compounds usually have relatively high surface friction. Ejection of the molded parts can be difficult. especially with softer products. Use of a release agent or a suitable coating on the mold makes for easier ejection. If possible the mold should be designed so that the ejection of the part can be air-assisted. Tapering the sides of the mold is also helpful, as isthe use of stripper rings. Small-diameter ejection pins should not be used, since they tend to deform the molded part rather than eject it. Ground scrap from molding is reusable and can be blended with virgin product. Grinding is quite easy if the conditions required for successful pelletization are met, i.e., if the grinder blades are kept sharp and the clearances minimized. Many end users prefer precompounded products, and numerous specialized grades have been developed. Some examples are products designed to make milk tubing. shoe soles, sound deadening parts, wire insulation, and tlexible automotive parts (Holden and Speer, 1988). After priming, the parts can be coated with flexible paints. If a compound contains a homopolymer (e.g., polystyrene or polypropylene), it will adhere well when insert-molded or extruded against this homopolymer oragainst other compoundscontaining it. This allowsthe production of parts having a rigid structure supporting a soft, flexible outer surface, as described by Holden and Sun (1991). The physical properties of compounded thermoplastic elastomers based 011 both styrenic block copolymers are affected by both the processing conditions and the processing equipment. Thus, it is most important to make test samples under conditions and on equipment similar to
Wilder340
and
Holden
Table 7 TypicalOperatingConditions for Injection Molding S-B-S-
01
S-EB-S-Based Compounds on Reciprocating Screw Machincs ~~
Mold type Cavities Shot weight, oz. Cylindcr temperatures,"F ("C) Feed zone Center zone Front zone Nozzle Mold temperature, "F ("C) Injection pressure. PSI" High Low Injection time, sec Injection rate Hold time, sec Clamp time, sec Screw rpm Back pressure, psi Cycle time, sec "
~~~
S-B-S-based compounds
S-EB-S-bascd compounds
2 Plate I 2
2 Plate I
2
1 IS (80) 350 (17s) 380 (195) 390 (200) 1s (25)
700
so0
700 S00
3
1
Moderate
Fast
S0
S 7 40 50
20
1s
S IO 30
These pressures are typ~calof those used wlth the more viscous compounds
those that will be used in production. Misleading results will be obtained if, for example, prototype parts or test pieces are compression molded when the actual products will be made by extrusion or injection molding. Processing conditions for these compounds have been discussed in some detail in Shell Chemical Company Technical Bulletin SC:455-96 (1996). Generally, compounds based on SB-S block copolymers are processed under conditions suitable for polystyrene. Those based on S-EB-S block copolymers are processed under conditions suitable for polypropylene. Typical conditions for injecting molding these compounds on reciprocating screw machines are given in Table 7. Various compounds have been developed for the production of blown and extruded (slot cast) film, including heat-shrinkable films. These were discussed in Shell Chemical Company Technical Bulletin SC: 1 105-90 (1990). These films are based on both S-B-S and S-EB-S block copolymers and can be very soft and tlexible. They also exhibit low hysteresis and low tensile set. A significant advantage is that they can be used in contact with skin or with certain foods. One unusual application is the use of solutions of S-EB-S/oil blends to replace natural rubber latex in the manufacture of dipped articles such as surgeon's gloves. This is described in a patent granted to Buddenhagen et al. ( 1 992). These blends have two advantages over natural rubber. First, they are more resistant to attack by oxygen or ozone. Second, natural rubber latex contains proteins that can produce dangerous allergic reactions in some people. These proteins do not occur in the extremely pure S-EB-S block copolymer.
341
Thermoplastic Styrenic Block Copolymers Table 8 Resins Ued to Formulate Adhesives, Sealants, etc., from Styrenic Block Copolymers Resin
compatibility
Polymerized CS resins (synthetic polyterpenes) Hvdromnated rosin esters Saturated hydrocarbon resins Naphthenic oils Paraffinic oils Low molecular weight polybutenes Aromatic I
-
Segment
I B EB I. B EB EB S
I = Compatible with polyisoprene segments; B = compatible with polybutadiene segments: EB = compatible with poly(ethy1ene-butylene) or poly(ethy1ene-propylene) segments; S = compatible with polystyrene segments.
6.2.
Adhesives,Sealants,andCoatings
These are very important applications for styrenic block copolymers and probably the fastest growing. They are often used in solution. In this case, some of their advantages are: 1. 2. 3. 4. 5.
Polymersare directly soluble-no millingnecessary. Solutiontimes are shorter-greater output. Solutionsare more uniform-better control. Solution viscosity is lower-higher solids level, lower solvent cost. Solutions are stable-longer shelflife.
Again, the products are always compounded, and the subject has been extensively covered by St. Clair (1982),Harlan et al. (1989), and Shell Chemical Company TechnicalBulletin SC: 19892 ( 1 992). As previously mentioned, the effectsof the various compounding ingredients depend on the region of the phase structure with which they associate. Since fourelastomers (B, I, EB, and EP) are used in these block copolymers, each has particular resins andor oils with which it is most compatible. Table 8 gives details of the various resins and oils suitable for use with each elastomer as well as with the polystyrene phase. Ingredients that go into both phases are not used, since they make the phases in the block copolymer more compatible with each other. This makes the product weaker. Polymers that form a separate phase (e.g., polypropylene) are used in some hot melt applications (see below). These polymers stiffen the products and improve upper service temperature. Fillers can also be added to reduce cost. The products can be applied eitherfrom solutions or as hot melts. The high tensile strength and ready solubility of styrenic block copolymers are important advantages in solvent cements and mastics. The existence of two separate and essentially incompatible segments in the same molecule should be taken into account when the styrenic block copolymers are used in solution. Aromatic solvents dissolve both the elastomer and the polystyrene. Thus they are good solvents for styrenic block copolymers. Aliphatic solvents dissolve only the elastomer blocks and hence will dissolve only those block copolymers whose styrene content is low. Blends of aliphatic and aromatic or polar solvents can be used. The necessary amount of polar solvent depends on the molecular weight and polystyrene content of the copolymer. In general, suitable solvents have solubility parameters between those of polystyrene and the elastomer (B, I, EB, or EP)
342
Holden and Wilder
Table 9 SolubilityParameters of Polymers and Solvents rl-Pentnne"." rl-Hcxane"." Poly(ethylcne-propylene) Poly(ethy1ene-butylene) Polyisoprcne Cyclohexane Polybut ;I d'lene Methyl isobutyl ketone Amyl acetate Toluene Xylcne Ethyl acetate Methyl ethyl ketone" Polystyrene Benzene 1,1,2.2-Tetl.achlorocthat~e Acetone"," lsopropyl alcohol""
7.0 7.4 7.7 7.8 8.I 8.2 8.4 8.4 8.5 8.9 9.0 9.1 9.I 9.I 9.2 9.7 10.0 11.9
(see Table 9). The molecular weights of these block copolymers are relatively low (typically 15O.OOO), and so the solutions can be made at high solids content. Some details of the solution behavior of styrenic block copolymers are given in Shell Chemical Company Technical Bulletin SC:72-85 (1985). Styrenic block copolymers have made possible the development of rubber-based hot-melt adhesives. This developmenthas been most importantto formulators faced with increased solvent cost and government restrictions 011 the release of solvents into the atmosphere. Low molecular weight additives such as hydrocarbon oils, resins, and polyisobutylenes may be used to lower the melt viscosity. They usually improve tack but may have a harmful effect on peel and shear strengths. In hot-melt applications the molten resins and/or oils may be regarded as taking the place of the solvents. Application rates of hot-melt products are usually faster than those of solventbased analogs. This is because the time for a product to cool is much less than the time for a solvent to evaporate. Some of the advantages and disadvantages of hot-melt and solvent-based applications are summarized in Table IO. The high temperatures necessary for mixing hot-melt adhesives require that the mixture be protectedfromoxidation during processing.Ablanket of carbon dioxide or nitrogen, i n combination with antioxidants, is useful in preventing degradation. S-I-S block copolymers are often preferred over S-B-S equivalents for hot-melt adhesives, because they are more tacky and are less likely to form gel during processing. Partial replacement of S-I-S with S-B in hot-melt adhesive formulations can give lower-cost compounds. These block copolymer blends retain good adhesive properties and adequate stability. Presslrre-Seilsiti\!e Adhesives
This is probably the largest single end use for styrenic block copolymers. These adhesives are usually applied as hot melts. Solvent application is also possible and takes advantage of the low
ore
343
Thermoplastic Styrenic Block Copolymers Table 10 Solvent-BascdversusHot-MeltApplication
Disadvantages ~~
Advantages
~
basedSolvent
Low Fl:mmability viscosity Toxicity High solids equipment Simple
Air pollution Drying time
Safcty Hot melt No pollution Fast set-up
Dcgradation
solution viscosity of these copolymers. The end uses include various kinds of tapes and labels as well as adhesive fasteners such as diaper tabs. The mechanism by which the resins and the styrenic block copolymers combine to give tacky products has been described by Kraus et al. (1977),Chu and Class (1985), and Halper and Holden (1988).According to this theory. the resins have two functions. First, they mix with the elastomer phase in the styrenic block copolymer and so soften the product. This softening allows the adhesive to conform to the substrate. This is considered as the “bonding” stage of adhesion and is relatively slow. The second stage is the removal of the adhesive from the substrate. This is the “disbonding” stage of adhesion and is much faster. Here the function of the resins is to adjust the glass transition temperature of the elastomer phase (i.e., the mixtureof the resins withthe midsegment of the styrenic block copolymer). This causes the adhesive to stiffen up and so resist removal from the substrate. Tack is maximized when the calculated glass transition temperature for the elastomer phase is about - 15°C. This is a good starting point for making trial formulations intended for room-temperature service. Since a soft product is necessary to form the adhesive bond. softer styrenic block copolymers are used to formulate pressure-sensitive adhesives. These soft styrenic block copolymers usually have low polystyrene contents and may contain significant amounts of diblock (i.e., S-I, S-B. S-EB. or S-EP). The diblock is non-load-bearing. It. and the resins in the elastomer phase. weaken and soften the adhesive. The weakening can be tolerated as long as it does not cause cohesive failure of the adhesive during service. Adhesiveshavebeendescribed by Erikson (1986) that can be crosslinkedafterbeing applied to the tape. This improves the solvent resistance of the adhesive. which is important for applications such ;IS masking tapes. AsserllDIy Aclllesives
In this application S-B-S and S-EB-S block copolymers are preferred. Again. hot-melt application is more usual than application from solution. Tack is not important (it may even be undesirable). and so harder products are satisfactory. These adhesives are usually formulated to contain two types of resins. One type of resin is compatible with the polystyrene phase. and the other (and possibly oil) is compatible with the elastomer phase. The relative proportions of these resins determines the softness of the adhesive. Thetotal amount added determines the viscosity of the final product. Constrlrctiorl Aclhe.si\v>s There is a growing use of mastics in construction projects for adhering wall panels and plywood subflooring. The S-B-S block copolymers areparticularly well suited for this application because
344
Holden and Wilder
Table 11 ConstructionAdhesive,Typical Formulation*
styrene S-B-S 30% polymer, Aliphatic resin, softening 105°C point Styrenic resin, softening point 140°C carbonateCalcium filler tioxidant Phenolic Low molecular weight epoxy resin Toluene VM&P Super naphtha
* Parts by
100 100 100 350 2 3.5 80 350
weight.
of their high uncured strength. These mastics often contain a high proportion of resin and clay. Resins with low residual unsaturation are suggested for better aging properties. Solvents are mostly naphtha with enough toluene added to dissolve the copolymer. A typical construction adhesive formulation is shown in Table 11.
Sealants This application is dominated by S-EB-S block copolymers. Both hot-melt and solvent-based applications are important. Hot-melt sealants are often applied by robotics. They canbe processed as foamed products. Often they are used as formed-in-place gaskets. In contrast. the solventbased products are mostly used in the building industry, where they are applied on-site. They can be used both in the initial construction and in subsequent maintenance and repair. Diblock copolymers are often part of both hot-melt and solvent-based sealants. They either reduce the viscosities of the hot-melt products or allow the solvent-based products to be formulated at higher solids content. The diblock copolymers can also reduce the strength of the sealant to the point where it fails cohesively during peel. This is a requirement in many sealant specifications. Again, both types of resins are often used, together with oils. If clarity is not a requirement; large amounts of fillers such as calcium carbonates can be added. Suggested startingformulations for both hot melt and solvent-based sealants have been published by Holden ( 1982) and Holden and Chin (1986). Coatings The most important application for coatingsbased on styrenic block copolymers is the chemical milling of metals. A protective film is first applied to the whole surface of the metal sheet. This is then selectively taken off the areas from which metal is to be removed. The assembly is then immersed in an etchant bath, which dissolves away the unprotected metal. The two metals most commonly processed in this way are aluminum andtitanium. Aluminum isetched under alkaline conditions. It can be protected by coatings based on S-B-S block copolymers. Titanium,however. is etched by strongly oxidizing acids that attack S-B-S block copolymers. Therefore. coatings based on S-EB-S block copolymers must be used for this application. Both types of protective coatings are probably formulated with the usual resins, fillers, etc. Details of the compositions have not been published. Oil Gels Styrenic block copolymers are very compatible with mineral oils. Blends with as little as 5% of an S-EB-S block copolymer (the remainder being 90% mineral oil and 5% wax) have been
Thermoplastic Styrenic Block Copolymers
345
described by Mitchell and Sabia (1980). These are used in cable-filling compounds, which fill the voids in “bundled” telephone cables and prevent water seepage. Other potential applications i n toys, hand-exercising grips, etc. are covered in patents granted to Chen (1983, 1986, 1993). In Shell Chemical Company Technical Bulletin SC: 1102-89 (1989), diblock copolymers such as S-EB and S-EP are suggested for use as gelling and antibleed agents in greases.
6.3
Blends with Thermoplastics, Thermosets, or Other Polymeric Materials
The styrenic block copolymers are technologically compatible with a surprisingly wide range of other polymeric materials. They give blends with improved properties when compared to the originalpolymers.Impactstrength usually is the most obvious improvement.Othersinclude tear strength. stress crack resistance. low temperature flexibility, and elongation. Thermoplastic and thermoset polymers can be modified in this way, as can asphalts and waxes. B1etd.Yn-it11 Tller7llol,l~lstic.r
Styrenic block copolymers have several advantages in this application. The other elastomers that can be blended with thermoplastics (e.g., SBR, EPDM, and EPR) can normally be used only in the unvulcanized state-the vulcanized products cannot be dispersed.Since unvulcanized elastomers are soft and weak, they reduce the strength of the blends. Therefore, only limited amounts can be added. In contrast. styrenic block copolymers are much stronger, even though they are unvulcanized. and so unlimited amounts can be added without reducing the strength of the product. Blending is usually carried out in the processing equipment (injection molders, extruders. etc.). Thisis an easy process if a styrenic block copolymer with low viscosity is used. The styrenic block copolymer forms a separate phase and so does not change the T, or T,,, of the thermoplastic into which it is blended. Thus, these blends retain the upper service temperature of the original thermoplastic. Shell Chemical Company Technical Bulletin SC: 165-93 (1993) describesthe use of styrenic block copolymers to modifythreelarge-volumethermoplastics-polystyrene, polypropylene, and polyethylene (both high and low density). Because of their lower price, S-B-S block copolymers are most commonly used with these thermoplastics. I n polystyrene there are two important applications: one is restoring the impact resistance that is lost when flame retardants are mixed into high impact polystyrene, the other upgrading highimpact polystyrene to a super high-impact product (Fig. 8). Polypropylene has very poorimpactresistanceat low temperatures. This also canbe improved by adding styrenic block copolymers. Impact improvement of any polymer usually results in a loss of clarity. This is because the added elastomeric polymer forms a separate phase with a different refractive index. However, blends of S-EB-S and S-EP-S with polypropylene are about as transparent as pure polypropylene, probably because of a match in refractive indices. As described in a patent granted to Holden and Hansen (l990), blends of LLDPE and S-EB-S with polypropylene also retain the clarity of pure polypropylene and show improved impact resistance (Fig. 9). Blends with polyethylene are mostly used to make blown film, where they have improved impact resistance and tear strength. Both S-B-S and S-EB-S block copolymers are blended with poly(phenyiene oxide) to improve its impactresistance.Shell Chemical Company Technical Bulletin SC: 1432-93 ( 1993) describesthe use of maleated S-EB-S block copolymers as impact modifiers for more polar thermoplastics such as polyamides. Another application is the use of styrenic block copolymers to make useful blends from otherwise incompatible thermoplastics. For example, polystyrene is completely incompatible
Holden and Wilder
346
t Impact Strength
_____)
25
5
Rubbery Volume Fraction, % Fig. 8 Impact resistance of blends of an S-B-S copolymer with high-impact polystyrene.
with polyethyleneor polypropylene. Blends of this type form a two-phase system with virtually no adhesion between the phases. Thus, when articles made from them are stressed, cracks easily develop along the phase boundaries and the productsfail at low elongations. Addition of a low molecular weightS-B-S or S-EB-S block-copolymerconverts the blendsto more ductile materials, as described by Paul (1996.) Similar results were reported on blends of poly(pheny1ene oxide) with polypropylene compatibilized using S-EB-S and S-EP. Also, polyamides were compatibilized with polyolefins by the use of functionalized S-EB-S block copolymers. In another example, polystyrene was compatibilized with ABS by the addition of S-B-S or S-EB-S. This allowed mixed scrap from coextruded sheet to be recycled (see Table 12). Blends with Thermosets Sheet molding compounds (SMC) are thermoset compositionscontaining unsaturated polyesters, styrene monomer, chopped fiberglass, and fillers. They are cured to give rigid parts that are often used in automobile exteriors. Special types of styrenic block copolymers have been developed as modifiers for these compositionsand are described in Shell Chemical Company Technical Bulletin SC: 1216-91 (1991). They give the final products improved surface appearance and better impact resistance. In an entirely different application, Arkles (1983) describes the useofS-EB-S block copolymers in blends withsilicone rubbers. These contain either vinyl or silicon hydride functional groups. The silicone rubbers containing the vinyl groups are pelletized separately from those containing the silicon hydride groups. When melted and mixed together in the processing equipment, the two groups react underthe influence of a platinum catalyst. This gives an interpenetrating network of the vulcanized silicone rubber and thestyrenic block copolymer.The products are useful in medical applications.
347
Thermoplastic Styrenic Block Copolymers 1000
140
130
100 Gardner Impact, inIb a t -10°C
120
Flexural
Modulus, psi x l o 3 110
10
100
1
90
0
5
10
'la S-EB-S / LLDPE
Fig. 9 Impactresistanceandstiffnessofa copolymer polypropylene.
15
in Blend 50/50 mixture of S-EB-S/LLDPE blended with Random
Asphalt Blends The styrenic block copolymer content of these blends is usually less than 20%. Even as little as 3% can significantly change the properties of asphalts. The styrenic block copolymers make theblendsmoretlexible(especially at low temperatures)andincreasetheirsofteningpoint. They decrease the penetration and reduce the tendency to flow at high service temperatures, such as those encountered in roofing and paving applications. They also increase the stiffness, tensile strength. ductility, and elastic recovery of the final products. Melt viscosities remain low, and so the blends are still easy to apply. The effects vary with the amount of styrenic block copolymeradded. At low concentrations,thisstyrenicblock copolymeris dispersed in the asphalt.Astheblock copolymer concentration is increased to about 5%, an interconnected
Table 12 Impact Strength of HIPS/ABSBlends Composition* HIPS ABS S-B-S S-EB-S impact strength Dart impact (ft-lb/in.)
100
-
90
-
100
10
-
-
-
180
<20
80
82
82 9 9
9 -
290
230
348
Holden and Wilder
copolymer network is formed. At this point the nature of the blend changes from an asphalt modified by a copolymer to a copolymer extended with an asphalt. Piazza et a l . (1980). Kraus (1982), Kelly and Bresson (1982). Goodrich (1988). and Bouldin et al. (1991) described various applications. These included road surface dressings such as chip seals (that hold the aggregate in place when a road is resurfaced), slurry seals, asphalt concrete (a mixture of asphalt and aggregate used in road surfaces). road crack sealants, roofing, and other waterproofing applications. Due to the relatively low cost of asphalt, it is widely used in paving and roofing, as a water barrier in construction, as a joint sealer i n road construction and maintenance, in mastics, in automobile undercoating, and i n waterproofing materials. Its disadvantages in many of these applications are ( 1 ) brittleness at low temperatures, (2) excessive flow at high temperatures, and (3) IOW elasticity. Incorporation of styrenic block copolymers into asphalt greatly extends its serviceability. The use of S-B-S and S-EB-S block copolymers appears to be particularly attractive in high-quality membranes. In this application the carrier base may be nonwoven fabric and the asphalt may contain 10-14% of these block copolymers. In all these applications it is important to choose the correct grade of asphalt. Asphalts are complex mixtures of many different materials ranging from oils to high molecular weight organic compounds. For simplicity, they are usually considered to be of two principal fractions: maltenes and asphaltenes. Van Beem and Brasser (1973) found that those with low asphaltenes content and/or high aromaticity in the rualtene fraction usually gave the best results. The maltenes (the lower molecular weight components) serve as solvent for the higher molecular weight asphaltenes. Kraus and Rollmann (1980) reported that asphalt modified with S-B-S exhibits a two-phase structure at room temperature. The block copolymer. swollen by the maltene fraction of the asphalt, formsa network extending throughout the system. The other phase is the asphaltenes.and this contains little or no rubber. At ordinarytemperatures.the styrenicblock copolymer network (see Fig. 1 ) remains intact. This provides the elastomeric characteristics displayed by the modified asphalt. At higher service temperatures. this network increases the viscosity of the modified asphalt, and flow is greatly reduced. However, at processing and application temperatures. viscosities of the modified asphalt blends are only slightly higher than that of the asphalt alone. The styrenic block copolynlers are incorporated into the asphalt by stirring them in powdered or ground form into the molten asphalt at a temperature of 180-205°C. High-shear mixers are useful but not essential. Typically, the time of incorporation of the copolymer into the asphalt is 20 minutes to 1 hour. The hot melts are quite stable. However, after 3-5 hours at the elevated temperatures. some degradation of S-B-S-based blends occurs. The degradation is evident by large increases in melt viscosity. If the hot mixture n u s t be stored. the telnperature should be reduced. a blanket of inert gas employed. and the use of high-shear mixers avoided. Because of their lowercost, S-B-Sblock copolymers are usually chosen for this application. S-EB-S block copolymers are also sometimes used because of their better UV. oxidative, and thermal stability.
Wcrx Blends Shell Chemical Company Technical Bulletin SC: 1043-90 (1990) describes the use of styrenic block copolymers in blends with waxes.S-l-S and S-B-S block copolymers have limited compatibility and usually require the addition of resins. S-EB-S block copolymers are more compatible. The products are used to give flexible coatings for paper products and can be applied by curtain coaters.
Thermoplastic Styrenic Block Copolymers
349
Use irr LU~WOils Viscosity index modifiers based on S-EP have been very successful. Detailsof their formulation and performance are given in Shell Chemical Company Technical Bulletins SC:239-80 ( 1980). SC: 1064-89 ( 1989). SC: 1065-89 (1989), and SC: 1189-90R (1990).
REFERENCES Arkles, B. C. (1983). Medical Device and Diagnostic Industry, 5(1 I ) , 66 (1983). Arkles, B. C. (1985). U.S. Pat. 4,500,688 (to Petrarch Systems). Bhattacharya, S.. Rajagopnlon, P,, Avasthi. B. N.. and Bhowmick, A. K. (1993), J . A/?/)/.Polyrrl. Sci. 39: 1971. Bi. L. K., and Fetters, L. J. ( 1975). Mrrcrorrrolrc~rtIrs 8:98. Bouldin, M. G.. Collins, J. H., and Berker, A. ( 1991). Ruhbrr ChrrII. Teclrrwl. 64:577. Bradford. E. B., and Vanzo. E. ( 1968), J . Polyrrr. Sci. Pt. A - l 6: 1661. Buddenhagen. D. A., Lcgge, N. R., and Zscheuschler, G. (1992),U.S. Pat. 5,l 12,900 (to Tnctyl Technologies. Inc.). Childers, C. W., and Kraus, G. ( 1967). R d ) l w r Clrurrl. Tec,/rrwl.40:l 183. Chen, J. Y. (1983), U.S. Pat. 4,369,284 (to Applied Elastomerics, Inc.). Chcn, J . Y. (1986). U.S. Pat. 4,618,213 (to Applied Elastomerics, Inc.). Chcn, J. Y. (1993), U.S. Pat. 5.262.468 (to Applied Elastomerics. Inc.). Chu, S. G.. and Class. J. (1985). ./. Appl. Po!\'. Sci. 30:805. Crouch, W. W., and Short, J. N. (1961), RuDbcr Pltrst. A,ye 42:276. Deisler, R. J.. ( 199 I ), paper presented at the 4th Internotional Conference on Thermoplastic Elastomer Markets and Products sponsored by Schotland Business Research, Orlando, FL. Erikson, J. R. ( 1986), Ad/rr.si~v.sA,yr 29(4):22. Firestone Rubber Company ( 1966), Neth. Pat. 67,13383. Foreman,L. E. ( lY69). in Po/yrrrrr C/wrrli.stt:v of' Syrrthetic Eltrstorrwrs, Part I1 (J. P. Kennedy and E. Tornquist, cds.). John Wilcy & Sons, New York, p. 497. Gergen, W. P. (1985). 124th Meeting of the ACS Rubber Division. Houston, TX, paper No. 57. Gergcn. W. P,, Lutz. R. G., nnd Dnvison, S. ( 1996). in T/wrrrwpltrs/ic E/u.stotrwr.s--A Corry?rr/rerr.sil,e Retinr,. 2nd ed. (G. Holden, N. R. Legge, R. P. Quirk, and H. E. Schrocder, Eds.) Hnnser &L Hanserl Gardner, Munich, Ch. I I . Ghosh S.. Khastgir. D., nnd Bhowmick, A. K. (1998). J . Appl. Po/yrrl. Sei. 67:2015. Ghosh, S.. Bhowmick. A. K.. Roychoudhury, N. R, Holden. G. (2000), J. App/. fo/ym Sci., 76. Goodrich. J. L. (1988). Asphalt and polymer-modified asphalt properties related to the performance of asphalt concrete mixes. Asphalt Paving Technologist Proc. AAPT 57. Halper. W. M,. and Holden. G. ( 1988). in H r r d l m o k r!J'T/~c~rrrlo)~)/rr.s/ic. Eltr.srortwr.s, 2nd ed. (B. M. Walker and C. E. Rnder, Eds.). Vnn Nostrand Rcinhold, New York, Ch. 2. Harlan, J. T., Pctershagen, L. A.. E w m , E. E., Jr., and Davis, G. A. (1989). in H a r d l , o o k c$Ar/l~r.si~~r.s. 3rd ed. (I. Skeist, Ed.), Van Nostrand Reinhold, New York, Ch. 13. Haws, J. R. ( 1974). I n Nrn, I/rt/u.strirr/ Polytrlrrs, ACS Symp. Ser., No. 4, p. I . Haws. J. R., and Wright, R. F. ( 1976). in Hnrldhook o / T/~errrrop/trsticElrrsrorrrers (B. M. Walker. cd.), Van Nostrand Rcinhold. New York, Ch. 3 . Hendus. H.. Illcrs, K. H.. and Roptc. E. (1967). Kolloid Z.Z. Polyrrwrr 216-217:l IO. Holden, G.. and Milkovich. R. (1964). U.S. Pat. 3,265,765 (to Shell Oil Co.), filed 1962. Holden,G.,Bishop,E. T., and Legge. N. R. (1968), Proc. IrIterrdorr~rl R~rhhrrCor!/trerlcr, 1967. MocLaren and Sons, London. p. 287. Holden. G.. Bishop, E. T., and Legge. N. R. (1969). J . Po/yrrr. Sci.. PI. C 2637. Holden, G. ( 1973). in Block r t ~ r dGrr!ji Copo/yrr~c~r.s (R. J. Cercsa, Ed.), Buttcrworth, Washington. DC, Ch. 6. Holden, G. ( 1982). paper presented at the Adhesives and Sealants Council Seminar, Chicago 11.
350
Holden and Wilder
Holden, G., and Chin. S. S.( 1986), paper presented at thc Adhesives and Sealants Conference, Washington, DC. Holden, G., and Speer. K. H. (l%%), Automotive Polvrtt. Desigrt 8(3):15. Holden, G., and Hansen. D. R. (1990). U.S. Pat. 4,904,731 (to Shell Oil Co.). Holden, G., and Sun, X.-Y. (1991 ), papcr presented at the 4th International Conference on Thermoplastic Elastomer Markets and Products sponsored by Schotl;lnd Business Research, Orlando, FL. Holden, G., and Legge, N. R. (1996), in Thermoplastic Elostorners. 2nd ed. (G. Holden, N. R. Legge, R. P. Quirk, and H. E. Schroeder, Eds.), Hanscr & Hanser/Gardncr, Munich, Ch. 3. Holden, G. (1996), in Therrmplastic El~r.sfomers-A Corttpreltrrt,si\v Rr1ietr.. 2nd ed. (G. Holden, N. R. Legge, R. P. Quirk, and H. E. Schroeder, Eds.), Hanser & Hanser/Gardncr, Munich Ch. 16. Holden, G., Lcggc, N. R., Quirk, R. P,. and Schroeder. H. E., Eds. (1996), Tlrermqdtrstic E/trsrorrwrs"A Corrr/,rrllerlsi~~r Review. 2nd ed., Hanser & Hanscr/Gardner. Munich. Hsieh, H. L., and Quirk. R. P. ( 1996), Anionic Po!\,rlrt.rizcrtiorr: Prirtciples c m / Prctcticctl Applic~rtior~s, Marcel Dckker, Inc., New York. Kelly, D. E., and Bresson, C. R. (1982). 121st Meeting of the ACS Rubber Division, Philadelphia, paper No. 45. Kraus, G. (i982), Rtrhlwr Clterrl. Techrwl. 5 3 4 ) : 1389. Kraus, G., and Gruver, J. T. ( 1 9 6 7 ~J. Appl. Polyrr. Sci. 1/:212 I. Klaus, G.. and Railsbxk, H. E. (1974), i n Kecertf A c l ~ w ~ cirr~Polyvrrr s Bler~cls,Grufis c r d Blocks (L. H. Spcrling, Ed.). Plcnurn, New York, p. 245. s Kraus, G., Joncs, F. B., Marrs, 0. L., and Rollmann, K. W. (1977), J. A t l l t e s i ~ ~8:235. Kraus, G.. Rollman, K. W., and Gray, R. A. ( 1979). J . Adlresiort 10:221. Kraus, G.. and Rollman, K. W. (1980), Int. Rubber Conf., Nurnberg, Germany, Sept. 24-26. Legge, N. R. ( 1985), Symposium on Thermoplastic Elastomers, 127th meeting of the ACS Rubber Division, Los Angeles, paper No. 73. Legge, N. R., Holden, G.. and Schroeder, H. E., Eds. ( 1987), Tlrrrrr~oplrr.s/icEltr.sfor~rc~rs--ACor~r/~rehertsi~~e R r ~ i e ~ Hanscr r,, and Oxford University Press. MunicWNew York. Marrs. 0. L., Zelinski, R. P., and Doss, R. C. (1973), 104th Meeting of the ACS Rubbcr Division, Denver, paper No. 19. Meier. D. J. (1969). J. P o l y n . Sci. Pt. C 26:8l. Mitchcl. D. M,, and Sabia. R. (1980). Procerrlirlg of the 29th Ittfertt(rfiowl Wire r t r d Ctrblc Syrrlpo.siurr?. p. 15. Morton, M,, nnd Ells. F. R. (1962). J. Polv~rt.Sei. 61:25. Morton, M,. McGrath. J. E.. and Juliano, P. C. (1969), J. Polyrrr. S c i C26:99. P;~ul,D. R.. (1996), t n Tlterw~pltrsticEltrsrormw-A Corrrprehrmiw R e ~ i e w 2nd . ed (G. Holden. N. R. Leggc, R. P. Quirk, and H. E. Schroeder, Eds.), Hanscr &L Hanser/Gardncr. Munich, Ch. 15c. Phillips Petroleum Company (1958). Br. Pat. 888.624. Piazza, S.. Arcozzi, A.. and Verga, C. ( 1980), R~tbherChertl. Trcllrtol. 53994. Portcr. L. M. (1964) U.S. Pat. 3, 149,182. (to Shell Oil Co.). filed 1957. Railshack, H. E., Baird, C. C., Haws, J. R., and Wheat R. C. ( 1964). Ruhht~rAge 94:583. Reisch, M. S . (lY96), C rrrlrl E Ne\r*s7 4 3 2 ) :10. Shell Technical Bullctin (1980). SC:239-80, Shell Chemical CO.. Houston. TX. Shell Technical Bulletin (1985). SC:72-85, Shell Chemtcal Co.. Houston, TX. Shell Technical Bulletin (1989), SC: 1064-89, Shell Chcmical CO., Houstol1, TX. Shell Technical Bulletin (1989). SC:1065-89, Shell Chemical CO., Houston. TX. Shell Technical Bullctin (1989), SC:1102-89, Shell Chcmical CO., Houston, TX. Shell Technical Bulletin ( 1990). SC:1043-90. Shell Chemical Co.. Houston. TX. Shell Tcchnicnl Bulletin ( 1990), SC: 1105-90. Shell Chemical Co., Houston, TX. Shell Technical Bulletin ( 1990). SC:I 189-90R. Shell Chemical Co., Houston, TX. Shcll Technical Bulletin ( 1991), SC:1216-91, Shell Chcmlcal Co., Houston, TX. Shell Technical Bullctin ( 1992). SC:198-92, Shell Chcmical Co., Houston, TX. Shcll Technical Bulletin ( 1993). SC:l65-93, Shcll Chemical Co.. Houston. TX. Shell Technical Bulletin ( 1993). SC:1432-93. Shell Chemical Co., Houston. TX.
Thermoplastic Styrenic Block Copolymers
351
Shell Technical Bulletin (1996), SC:445-96, Shell Chemical Co., Houston, TX. St. Clair, D. J. (1982), Rubber Cherrz. Techrzol. 55:208. Stravely, F. W. (1956), I d . Eng. C/lenz. 48:778. Szwarc, M. ( 1 9 5 6 ~Nature 178: 1168. Szwarc, M,. Levy, M,, and Milkovich, R. (1956), J. Am. Chem Soc. 78:2656. Tung, L. H., and Lo, G. Y. S. (1994), Mncrorrzolecules 27:2219. Tung, L. H., Lo, G. Y. S., and Beyer, D. E. (1980). U.S. Pat. 4,196,154 (to Dow Chemical Co.). Tung, L. H., Lo, G. Y.-S., Rakshys, J. W., and Beyer, D. E. (1980), U.S. Pat. 4.201.729 (to Dow Chemical Co.). Van Beem. E. J., and Brasser, P. (1973), J. h s t . Perroleur~~ 59:91. Van der Bie, G. J., and Vlig, M. (1969), Mtrtrr. Plast. Elrrst. 35(2):184. Vanzo, E. (1966). J. Polyrn. Sci. A-4:1727. Walker, B. M,, and Rader,C. E., Eds. (1988). Htrrltl1x)ok of Thernzoplustic E1trston1er.s. 2nd ed., Van Nostrand Reinhold, New York. Zelinski, R. P,, and Childers, C. W. (1968), Ruhhrr Cl~em.Techrlol. 41:161. Zelinski, R. P. (1961), U.S.Pat. 2,975,160 (to Phillips Petroleum Company). Zelinski, R. P., and Hsieh, H. L. (1963), U.S. Pat. 3,281,383 (to Phillips Petroleum Company). Zelinski, R. P. (1966). U.S. Pat. 3,251,905 (to Phillips Petroleum Company). Zelinski, R. P., and Hsieh, H. L. (1966), U.S. Pat. 3,078,254 (to Phillips Petroleum Company).
This Page Intentionally Left Blank
12 Polyester Thermoplastic Elastomers: Part I Roderic P. Quirk and Qizhuo Zhuo Maurice Morton Institute of Polymer Science, The University OiAkron, Akron, Ohio
1. INTRODUCTION Thernloplastic elastomers are polymers that have the physical properties of elastomers (e.g.. long range, reversible extensibility) but, in contrast to conventional elastomers (rubbers). can be processed as thermoplastics (Holden, 1996; Allport. 1973; Noshay. 1977). Conventional elastomers must be covalently crosslinked into a three-dimensional network structure (e.g.. via peroxide curing or sulfur vulcanization) to provide useful physical properties (Morton. 1987). Consequently. these elastomers behave a s thermosetting materials and cannot be processed or reprocessed after crosslinking. Thermoplastic elastomers rely on a physical crosslinking process (i.e.. intermolecularforces of attraction) to formathree-dimensionalnetworkstructureand prevent viscoelastic flow. At sufficiently high temperatures, the physical forces of attraction forming the network can be disrupted, which allows the polymer to soften and flow. Some of the advantages of thermoplastic elastomers are that they can be fabricated using conventional thermoplastic processes (e.g.. injection molding. extrusion. and blowmolding) and that the scrap can generally be recycled. Structurally.thermoplasticelastomersaremultiphase copolymers composed of two or more covalently bonded chain segments or blocks that are thermodynamically incompatible. The physical. reversible crosslinking resultsfrom phase separation of the incompatible segments. One ofthe phases is composed of blocks characterized by an accessible glass transition temperature or melting point that is above the useful temperature range of the thermoplastic elastomer; this phase is called the "hard" phase. Another phase consists of blocks characterized by a glass transitiontemperature that is well belowtheusefultemperaturerange of thethermoplastic elastomer; thisphase is referred to asthe "soft" phase. The essentialstructural unit for a thermoplastic elastomer is A-B-A, where A is the hard. thermoplastic phase segment and B is the soft, elastomeric phase segment. This structural requirement for a thermoplastic elastomer can be satisfied by several types of molecular architecture. For example, one type is a simple AB-A triblock copolymer such as poly(styrene-b-diene-b-styrene). Another type is an elastomeric backbone chain grafted with a random distribution of pendant, hard-block segments. Finally. segmentedblock copolymers with the generalstructures (A-B),, represent another structural variation. One family of thermoplastic elastomers with the segmented block copolymer structure is based on polyester hard-block segments. The polyester blocks in these polymers generally have 353
Quirk and Zhuo
354
regular structures that can phase separateinto crystalline domains that maximize the intermolecular forces of attraction between the hard-phase block segments. These crystalline domains provide physicalcrosslinkingforthe amorphous, elastomericsegments. These materialsretain their integrity at temperatures approaching the melting temperature of the crystallites and will flow at higher temperatures under shear for thermoplastic processing. Although a wide variety of segmented thermoplastic polyester structures is possible, those based on phthalate ester hardphase segments and poly(ether) diol soft segments are commercially available(HytreP. Dupont; Amitel", Azko; EcdeP, Eastman; Elitel'"', Elana; Gaflex@ then Riteflex@, Hoechst Celanese; Lomod@,General Electric; [email protected]; Pibiflex@,EniChem; Zeospan@, NipponZeon) (Lloyd, 1982; Khan, 1986; Adams et al., 1996) and have been most carefully examined. Other telechelics have been proposedassoft segments in combination with polyester as the hard segment. These alternative soft segments are usually aimed at remedying some weak properties of the polyether soft segmentsuch as degradation underthe influence of hot air and UV radiation (van Berkel et al., 1997).Typical examples include dimerized C I Sand C'? fatty acids (Hoeschele, 1973; Manuel and Gaymans, 1993) and telechelic a,o-dianhydride polyisobutylene (Walch and Gaymans, 1994). A water-swellable thermoplastic polyether-esterelastomer using poly(ethy1ene oxide) as the main soft segment and polyester as the hard segment was also described (Greene, 1992).
2.
BASICSTRUCTURE
Most polyester-based thermoplastic elastomers are prepared by polycondensation reactions of a poly(ether) diol with a mixture of a phthalate ester and a low molecular weight diol (Hoeschele and Witsiepe, 1973; Witsiepe, 1973; Hoeschele, 1974; Adams et al. 1996). Thegeneral structure of such a polyester thermoplastic elastomer is as follows (structure A): _f
segment)
CO-C~H4-CO-O(CH~),O jaCO-C,H4-CO-O-[-CH-(CH,)y-O]+
I
(hard
(soft segment)
Thus, these polymers can be classified as segmented polyester-polyether block copolymers that have a random distribution of hard and soft segments with random-length sequences (Adams etal..1996). The hard,crystallizablesegmentsconsist of eitherethylenephthalate (x = 2, structure A) orbutylene phthalate (x = 4. structure A) units with ortho, rnrta, orparcc substitution in the aromatic dicarboxylic acid unit (Noshay and McGrath 1977). In addition, other dicarboxylic acids. such as 2,6-naphthalenedicarboxylicacid, 1~~eta-terphenyl-4,4'-dicarboxylic acid, and sebacic acid. can be used (Noshay and McGrath. 1977). The soft elastomeric segments consist of phthalate esters of long-chain poly(alky1ene oxide) diols. Theuseful polymeric diols include poly(oxyethy1ene) diol (R = H, y = I , structure A), poly(oxypropy1ene) diol (R = CH3. y = l), and poly(oxytetramethy1ene) diol (R = H. y = 3 ) (Wolfe. 1977). Even within this somewhat limited range of starting materials, a variety of polymers with a considerable range of properties can be prepared by varying the molecular weight of the polymeric diol (z. structure A), the isomeric composition of the phthalate ester ( o , m , or p , structure A), and the relative volume fraction of hard- and soft-phase segments (m versus n, structure A). Polymers with properties ranging from soft elastomers to hard elastoplastics can be obtained ( A d a m et al.. 1996).
355
Polyester Thermoplastic Elastomers Table 1 Monomer and CopolymerCodeSymbols
Ethylene glycol X-methylene diol (e.g., 3G is trimethylene diol) trum- I . I -Cyclohexanedimethanol
3.3
XG t-CD
c,t-CD BD
cis. t r w s - 1,4-Cyclohcxancditnethat~ol 1..l-Bcnzencdimetl~at~ol
T
Terephthalate
I
Isophthalate
P ND
Phthnlatc 2.(,-Naphthnlencdicarboxylate
TO
rtletcl-TcrphenyI-4.4-dicarboxylnte
10
Scbncate Poly(tctrnmethyleleether) diol of XXXX number-averape tnolccular
PTMEG(XXXX)
weight
PEG PPG 60% 4GT.PTMG( I000)T
Poly(ethylcne ether) diol Poly(propylene ether) diol 60% by wctghttetramethyleneterephthalate. terephthalate copolymer
40%- PTMG( 1000)
The polyester-b-polyether segmented block copolymers of various compositions can be uniquely designated by using a system of nlonomer and copolymer symbols. shown in Table 1 (Wolfe. 1083). Atypicalpolyether-/>-polyester would be designated as 4GT/PTMG( 1000)T. The number and letters i n front of the slash mark refer to the hard-segment ester units, and those after the slash mark refer to the soft-segment polyether-ester units. Thus. 4GT symbolizes tetramethylene terephthalate ester units, and PTMEG( 1000)T symbolizesthe poly(tetramethylene ether) diol with molecular weight of 1000 g/mol esterified to terephthalate.
3. SYNTHESIS,MANUFACTURE, AND CHEMISTRY 3.1
SynthesisandManufacture
Segmented polyester-polyether block copolymers are prepared economically by melt transesterification of a mixture of a phthalate ester, a low molecular weight diol. and a poly(alky1ene ether) diol (Wolfe. 1983). A more costly solution process utilizes diacid chlorides in place of diesters (Wolfe, 1983). The solution process can be used in preparations that exhibited thermal instability or phase separation in the melt due to incompatibility of the copolymerizing segments. A typical synthesis would involve the interaction of dimethyl terephthalate, an excess of butanediol(overall 50% excess of hydroxyl functionality), and poly(oxytetramet1~ylene)diol (molecular weight = 1 0 0 0 g/mol). as shown in Scheme 1. These materials are slowly heated to 160°C i n the presence of small amounts of antioxidant (e.g.. N,N'-di-/~-naphthyl-pphenylenediamine)(Witsiepe.1973)and atransesterificationcatalyst [e.g..Mg(OAc)7-Ti(OC4H,,)41 (Wolfe. 1073; Hoeschele. 1973). resultingin the gradual distillation of methanol. Further heating to 250°C over a l-hour period completes the distillation of methanol. At this stage the mixture consists of prepolymers. which can be further condensed to high molecular weight product. This final stage is completed by reducing the pressure to less than 1 torr to distill off the excess I ,4butanediol while maintaining the temperature at approximately 250°C; higher temperatures tend to promote degradation reactions, which decrease rather than increase the product molecular
Quirk and Zhuo
356
weight (Witsiepe. 1973). The extent of the polymerization reaction can be monitored by measuring the torque (power) required to stir the reaction mass at a low constant rate using a torsional viscometer, which gives a measure of the viscosity of the reaction mixture (Wolfe. 1977). When the viscosity reaches a plateau or starts to decrease, the polymerization is terminated by simply cooling the reactor (Cella, 1977).
160-200°C
.1
Mg(OAc)2-Ti(OCqH9)4
PREPOLYMER + CH30H
*0-@02(CH2)@HCO
m
f
~ c 0 2 ~ c H 2 c H 2 c H 2 C H 2 0 ~
+ HOCH2CH2CH2CH20H Scheme 1 The exact time required to obtain high molecular weight product depends on the temperature,pressure,andcondition of agitation. The finalmolecular weight is determined by the balance of couplinganddegradationreactions. If the short-chaindiol is 1.4-butanediol. the predominant degradation reaction involves formation of carboxyl end groups by elimination of tetrahydrofuran from 4-hydroxybutyl ester endgroups. Based on information in the patent literature. it is anticipated that analogous processes will be used for the commercial production of these materials. Typical values for the number-average molecular weight of the resulting copolymer, designated 4GT/PTMG( 1000)T, are approximately 25,000-30,000 (Witsiepe. 1972). This corresponds to an intrinsic viscosity of 1.5 dL/g and a melt viscosity of 20,000 poise at a shear rate of IO sec". The molecular weight distribution is reported to be geometric (Witsiepe. 1973). Recently it was reported that the presence of a co-catalyst such a s tetrapropyl zirconate may permit more rapid finishing with less degradation (Hoeschele et al.. 1992; Adams. et al. 1996). Addition of phenolic antioxidants to minimize oxidation degradation was reported (Witsiepe. 1973; Hoeschele, 1980; McCready, 1085).A continuous process has also been described (Takanawo et al., 1994). A new development is the production of polyether-esters based on terephthalic acid instead of dimethyl terephthalate. When terephthalic acid is used, the amount of tetrahydrofuran formed through acid-catalyzed dehydrationof 1,4-butanediol is high (-2S%) if no precautions are taken. The formation of tetrahydrofuran can be reduced by using a combination of tetrabutyl titanate anda tin compound suchasmonobutylhydroxytin oxide ascatalyst (Taniura et al.. 1990). Another procedure for effectively using terephthalic acid as starting material and minimizing tetrahydrofuran formation is to use a mixture of short-chain and long-chain diols (Tamura et al.. 1990). Addition of certain salts (e.g., sodium phosphate and potassium terephthalate) can reduce the amount of tetrahydrofuran formed to approximately 15% (Chang et al.. 1995). The product molecular weight and viscosity can be increased at lower degrees of conversion by
Polyester Thermoplastic Elastomers
357
including small amounts of multifunctional branching agents such as trimellitic anhydride, pentaerythritol, or branched poly(a1kylene ether) diols in the basic polymer recipe (Adams et al., 1996).
3.2
Block Length Distributions
An important structural parameter affecting the propertiesof poly(ether-ester) block copolymers is the block frequency distribution. The average block sequence length can be calculated for these condensation polymers using statistical methods based on the mole percentage of each component as described by Frensdorff ( 197 I ). For example. a typical composition of 57.5 wt% tetramethylene terephthalate and 42.5 wt% poly(oxytetramethy1ene) diol terephthalate contains 87.4 mol% of the former and 12.6 mol% of the latter (Cella. 1977). Cella (1977) has shown that for this composition, assuming that equilibrium conditions exist during the synthesis, the calculated average tetramethylene terephthalate block length is approximately eight repeat units, and that approximately 40 wt% of the polymer consists of hard segments of 10 or more contiguousrepeatunits.Using I3C-NMR. Higashiyamaand coworkers (Higashiyama et al., 1992) found only a trace of ether-terephthalate-ether linkages i n a 60 wt% 4GT/PTMG copolymer, but they found 16 wt% of this type of linkage in a polymer having 24 wt% 4GT, which suggests that the probability of forming a long sequence of soft segments in 4GT/PTMG i n lower 4GT contents is more likely than in higher 4GT content. The average block length depends not only on the composition but alsoon the molecular weight of the poly(ether)diol; increasingthe polyether molecular weight from 1000 to 2000 g/mol while holding other composition variables constant will increase the average tetramethylene terephthalate block lengthfrom 8 to 14. Longer average sequence lengths for the hard-segment blocks would be expected to increase the crystallinity and modulus of the resulting thermoplastic elastomers.
3.3 Solubility and Chemical Resistance In accordance with the semicrystalline nature of these poly(ether-ester) block copolymers, these materials are insoluble below their melting point in most solvents (Witsiepe, 1973). Thus, they are unusuallyresistant to oilsandsolvents, and thischaracteristicincreases with increasing crystalline content (Whitlock, 1973). Theclasses of solvents to which these thermoplastic elastomers are resistant include aromatic and aliphatic hydrocarbons, phosphate esters. and chlorinated hydraulic fluids, a s well as polar solvents suchas alcohols. ketones, esters. amines. and nitromethane (Whitlock. 1973). Although they have excellent resistance to nonpolar materials such as oilsandhydraulicfluids at elevatedtemperatures.theirresistance to polarfluids is poorat temperatures of 70°C or above (Wells. 1979). These poly(ether-ester) block copolymers are resistant to salt solutions, dilutebases. and mineral acids. They are relatively stableto hydrolysis, as indicated by their tensile half-life of 250 days at 150°F (Whitlock. 1973). In addition, they are very resistant to oxidation. Useful solventsfor these polyether-/>-polyestersincludephenolsand some chlorinated hydrocarbons such as chloroform and 1.1.2.3-tetrachloroethane (Witsiepe, 1973). m-Cresol is reported to be a useful solvent for measurement of dilute-solution viscosity, osmotic pressure determinations, and size exclusion chromatographic analyses (Witsiepe. 1973). No significant gel fraction is reported for these materials based on their complete solubility in m-cresol. None of the properties evaluated for them shows any indication of significant long-chain branching (Witsiepe. 1973).
Quirk and Zhuo
358
3.4 Blends Some properties of blends of 4GT/PTMG with pure 4GT homopolymer and with poly(viny1 chloride) have been reported (Wells, 1979). Blends of softer poly(ester-ether) copolymers (33 and 58 wt% 4GT) with poly(buty1ene terephthalate) (4GT) exhibited more flexibility at low temperatures but were stiffer at room temperature thanthe corresponding 4GT/PTMG copolymer with the same composition. Blends of 4GT/PTMG (33% 4GT) with flexible compositions of poly(viny1 chloride) exhibited considerable improvement in tensile and tear strength as well as better low-temperature flexibility and impact strength (Crawford and Witsiepe. 1973; Thomas et al., 1987). Blends of polyester with a dissimilar polymer such as polypropylene (Blakely and Seymour. 1992), polyacetal (Gergen, 1989),and alternating co-olefin copolymer (Gergen, 1989) were also reported to improve processing and toughen the base polymer.
4. 4.1
MORPHOLOGY Electron Microscopy, X-Ray Diffraction, and Light Scattering
The morphologies of PTMG/4GT-type polymers have been examined by transmission electron microscopy, x-ray diffraction,andsmall-anglelightscattering(Cella.1973; Seymour et al.. 1975; Lilaonitkul and Cooper, 1977). All of this experimental evidence is in accord with the expected two-phase morphological structurefor segmented poly(ester-ether) copolymers. A simple model for these copolymers is presented in Figure l , showing both crystalline and amorphous domains. The most direct evidence for the two-phase morphology of these copolymers has been obtained by electron microscopy. For example. a transmission electron micrograph of a thin film of PTMG( 1000)/4GTcopolymer is shown in Figure 2 (Cella. 1973). The polymerfilm was cast froma 1o/o solution of the polymer in 1.1,2,2-tetrachloroethaneand stained with phosphotungstic acid, which is preferentially absorbed by the elastomeric phase. Thus. thealight regions correspond to the hard-segment domains. These domains are approximately I00 A in thickness and up to several thousand A in length. Wide-angle x-ray diffraction patterns for a drawn fiber of the copolymer are shown i n Figure 3 (Cella.1973). By comparison with the diffractionpattern for the homopolymer of poly(tetramethy1ene terephthalate), 4GT, it was concluded that the hard segments in the thermoplastic elastomer crystallize in the same way as 4GT. More detailed morphology characterization has been obtained from small-angle light scattering (SALS) of these copolymers (Seymour et al., 1975; Lilaonitkul and Cooper, 1977). Although the data from electron microscopy and x-ray diffraction were interpreted in terms of a two-phase structure of randomly oriented lamellar hard-segment domains (Cella, 1973).the lowangle light-scattering patterns are consistentwith a spherulitic morphology(Seymour et al., 1975; Lilaonitkul and Cooper, 1977). Typical scattering patterns are shown in Figure 4 (Lilaonitkul and Cooper, 1977). It is important to note that these patterns. characteristic of well-developed spherulitic structures, were observed across a broad range of composition. In a l l cases the 4GT hard blocks crystallize into a lamellar structure that forms the skeleton ofthe spherulitic structure. and it is proposed that the interradial regions contain amorphous polyester and the amorphous PTMG segments (seeSec.4.2)(Seymour et al., 1975;LilaonitkulandCooper. 1977).The morphological models capable of producing the three different spherulitic superstructures (see Fig. 4 ) are shown in Figure 5 (Lilaonitkul and Cooper, 1977). It is noteworthy that all of the different spherulitic morphologies displayed by the block copolymers have also been observed for the poly(tetran1ethylene terephthalate) (4GT) homopolymer.
Polyester Thermoplastic Elastomers
359
Fig. 1 Schematic diagramof the morphology of poly(ether-b-ester) thermoplastic elastomers: (l. tetramethyleneterphthalatehard-phasesegment; (m),poly(oxytetramethy1ene)diolterephthalate soft segment. (From Cella, 1977.)
The explanation for the fractionation of the hard segments between the crystalline and amorphous phases isa matter of some controversy. Wegner et al. (1978) postulated a nucleation process in which only hard segments with the most frequent hard segment length are able to crystallize while both shorter and longer sequences were rejected to the amorphous region. Wegner’s model excludedthe possibility that chainfolding will occur. However,other evidence strongly suggests that chain-folded lamellae can develop. The first evidence of chain folding in 4GT/PTMG was provided by Seymour and coworkers (1975). More recently, based on the small-angle neutron-scattering studies of the chain conformations of hard and soft segments as well as the whole chain in bulk polymer samples, Cooper, Miller, and coworkers (Cooper and Miller, 1985; Miller et al.. 1985; Cooper et al., 1988) proposed that the polyester hard segments can chain fold to adjacent cells at room temperature witha repeat distance of three chain units. There is now a general agreement in the literature that the overall morphology of a polyetherpolyester is that of a two-phase system consisting of a pure 4GT crystalline phase and an amorphous phase. However, clean phase separation between the polyether and the polyester segments does not occur (Adams et al., 1996). The amorphous phase contains a substantially homogenous mixture of polyether soft segments and 4GT hard segments rejected from the crystalline phase (Adams et al., 1996).
360
,
Quirk and Zhuo
Fig. 2 Transmission electron micrograph of a poly(ether-h-ester) film cast from 1,l ,2,2-tetrachloroethane solution and stained with phosphotungstic acid. (From Cella, 1973.)
4.2 Thermal Analysis Further evidence for the two-phase nature of these segmented poly(ether-ester)copolymers can be deduced fromthermal analysis of these materials. Theyexhibit a glasstransition temperature for the elastomeric phase and a melting endotherm for the crystalline domains. Typical differential scanning calorimetricthermograms for two poly(ether-ester)copolymers are shown in Figure 6. Representative values ofT,, T,, and calculated percent crystallinity are summarized inTable 2 (Lilaonitkul and Cooper, 1977). The T, for the homopolymerof poly(oxytetramethy1ene)diol is - 88°C (Yoshida et al., 1973). The observed glass transition temperature for the poly(etherester) copolymers depends on the weight fraction of the hard segments in the polymer. The Gordon-Taylor equation (Gordon and Taylor, 1952):
accurately modelsthe T, behavior of these samples provided that the crystallinepolyester component is not included in the definition of the hard-segment composition. The Gordon-Taylor equation also predicts the glass transition temperature for compatible copolymers, where k is the ratio of the difference in thermal expansioncoefficientsof the two polymers T, andis the glass transition temperature of a copolymer consisting of weight fractions W I and W2 of monomers 1 and 2, which have homopolymer glass transition temperatures Tgl and Tg2. respectively. It has been proposed that part of this dependence on composition results from incomplete phase separation; thus, the compositionof the amorphous phase wouldinclude a moleculardispersion of some of the hard segments within the elastomeric phase (Cella, 1973; Seymour et al., 1975). However, this interpretation has been questioned by Lilaonitkul and Cooper (1977) based on the large difference in calculated solubility parameters for the two polymers:8.4 for poly(tetramethylene oxide) and 10.4 for poly(tetramethy1ene terephthalate), respectively. Other factors to
stic
Polyester
361
(b)
Fig. 3 X-raydiffractionpatterns of (a) drawn fiber of a poly(ether-b-ester); (b) drawn specimen of poly(tetramethy1ene terephthalate). (From Cella, 1973.)
be considered are the expected decrease in the length of unrestricted elastomeric chain segments, the increased number of crystalline tie points, and the greater reinforcement of the amorphous phase by the on-average longer polyester segments (Cella, 1973; Seymour et al., 1975). As expected, increasing hard-segment content increases the observed melting point, which approaches the value for the 4GT homopolymer (222°C) (Lilaonitkul and Cooper, 1977). This increase in meltingpoint is accompaniedby an increase in the percentcrystallinity. The percent crystallinity was calculated from the observed value of the heat of fusion, AHf, determined from the area under the melting endotherm curve, assuming that 3 1.4 kJ/mol corresponds to 100% crystallinity (Lilaonikul and Cooper, 1977). A number of studies have shown that copolymer melting points increase with increasing polyether block molecular weights. This has been explained by assuming that increasing the polyether chain length at constant weight percent 4GT forces the 4GT units to form longer blocks with higher melting points (Wolfe, 1983). It would also be expected that the compatibilityof the two types of segments woulddecrease with increasing molecular weight (Meier, 1969). While there is general agreement on the morphology of 4GT/PTMG cooled rapidly to ambient temperature, the morphology of materials annealed at elevated temperature is more controversial and is the subject of ongoing research. The long period spacing,L, defined as the average distance between crystalline domains including the thickness of the crystal plus the amorphous region between crystals, has been found to increase regularly withincreasing temper-
362
Quirk and Zhuo
(Cl
Fig. 4 Small-angle light-scattering patterns of spherulitic structures in PTMEG/4GT segmented copolymers: (a) Type I; (b) Type II; (c) Type 111. (From Lilaonitkul and Cooper, 1977.)
(C)
(dl
Fig. 5 Quadrants of schematic spherulite models: (a) Type I, lamellar principal axis at 45" to radical direction; (b) Type I, lamellar principal axis in radial direction, molecular chains tilted at 45" to lamellar surface; (c) Type11, lamellar principal axis in radial direction; (d) Type 11, lamellar principal axis perpendicular to radial direction. (From Lilaonitkul and Cooper, 1977.)
363
Polyester Thermoplastic Elastomers
I -100
r.
-50
I
A
150 Temperature, ' c
Fig. 6 Typicaldifferentialscanningcalorimetryscans 58% 4GT; B, 33% 4GT. (From Cella, 1977.)
1
I
200
250
for several poly(ether-b-ester) copolymers: A,
Table 2 Thermal Characterization of 4GT/PTMEG(1000) Poly(ether-ester) Copolymcrs
content. "C 33.3
GT wt% DSC T,, T,,,, "C Crystallinity, 28.6 r/c22.9
33
11.5 11.5
50 - 59 214 189
57
63
55 209 I96
-51 200
-
76 -
33 40.7
84
-9 42.8
DSc. differential scannmg colormetry: GT refers to ethylene terephthalate content (see Table 1 for definition of GT).
364
Quirk and Zhuo
atureduringtheannealingprocess at elevatedtemperature or on crystallizationatelevated temperature. along with an increase in the volume of crystalline phase in the sample. Early workers believed that this increase in L was largely due to thickening of the crystalline lamellae (Buck et al., 1974). Bandara and Droescher (1983) confirmed the increase in L and found that the melting point was also increased after annealing. They suggested that the increase in melting point after annealing is mainly due to refinement of the crystal structure and the new growth in the lateral direction of the polymer crystal without an increase of lamellar thickness in the chain direction. More recently, Apostolov and Fakirov (Apostolov and Fakirov. 1992; Fakirov et al.. 1990. 1992), working with 4GT/PEOT, confirmed the increase in both L and crystallinity but found by other means that the lamellar thickness is relatively unaffected by the annealing process. They also showed that the increase in L is proportional to the square root of the molecular weight of the long-chain diol and thus to itsunperturbed end-to-end distance. This suggeststhat the increase i n L is in the amorphous phase rather than in the crystalline phase.
REFERENCES Adams. R. K., Hocschcle. G. K.. and Witsiepe, W. K. (1996), Thermoplastic polycther ester elastomers, in T/~errr~o)plrr.stic Elostorrler.7, 2nd ed. (Holden. G., Leggc, N. R., Quirk, R., and Schrocder, H. E., eds.), Hanscr/Gardner Publications, Inc., Cincinnati, OH. p. 192. Allport, D.. and Janes. W. H., Eds. (1973), Block Copo/ymer.s, Halsted Press, New York. Apostolov. A. A.. and Fakirov. S. (1992). J. Mtrcrorrwl. Sci. Phy. R3/(3):329. Bandara. U,. and Droescher. M. (1983), Colloid Polym 26/:26. Blakcly. D. M,, and Seymour. R. W. (l992), U.S. Pat. 5.1 18,760 (to Eastman Kodak). ( 1986), Br. Plastics K u h h r r 6:7 Buck. W. H., Cella, R. J., Gladding. E. K., and Wolfe, J. R. (1974). J. P o l w . Sci. S w p . 48347. Cella. R. J. (1973), J . P d y u . Sc;. Symp. 42:727. Cella, R . J. ( 1977). i n D ~ c y o p e t l i uof’Poly~rlerScier~c.ec7rrd Techr~ology,Suppl. Vol. 2, Wilcy, New York, p. 485. . 35:190-194. Chang, S.-J., Chang, F.-C. and Tsai, H.-B. (1995), Polyrrl. B I ~ Sci. Cooper, S. L., and Miller, J. A. ( 1985), Ruhher Cllrm. Techno/.58:899. Cooper. S. L., Miller, J. A., and Homan, J. G. ( 1988), J . A/)/)/.Clyst. 21:692. Crawford. R. W.. and Witsiepe. W. K. (l973), U.S. P a t . 3,7 18,715 (to DuPont). Du Pont ( 1986).De.rip~Htrrldhok, Hytrel Bull. E-52083. E. 1. du Pont de Nemours & Company, Wilmington, DE. Du Pont ( 1986). Hytrel Will Clltrrlge the Wuy You 7 h i d Ahour Kuhhe,; Hytrel Bull. E-53634, E. I. du Pont de Nemours & Company, Wilmington, DE. Du Pont (1986). Grrlerd Guide to Products m t i Propcrric~s,Hytrel Bull. E-80913, E. I. du Pont de Ncmours & Company, Wilmington, DE. Fakirov. S., Apostolov, A. A., and Fakirov, C. (1992). h / . J. f o l y r m v i c Mtrter. 18:s1. Fakirov. S., Apostolov, A. A., Boescke, P,, and Zachtnann, H. G. (1990). J. Mtrcrotlwl. Sei. Phys. 829(4): 379. Frensdorff. H. K. ( 197 1 ), Mtrc,rorl~olecules4369. Gergen, W. P. (1989). U.S. Pat. 4,818,798 (to Shell Oil Co. USA). Gordon. M,. and Taylor. J. S . ( 1952). J. App/. Cheru. 2:493. Grcene, R. N. (1992), U.S. Pat. 5,116,937 (to DuPont). Higashiynma, A., Yamamoto. Y., Chujo, R., and Wu, M. (1992), Polyrrz. J. 24:1345. Hoeschele, G. K. (1973). Gcr. Pat. 2,263,046 (to DuPont). Hoeschele, G. K. (1974), Chinrirc 28544. . Hoeschele, G. K., and Witsiepe. W. K. ( 1973). Aqew. Mtckrotnol. C / I ~29(30):267. Hoeschele, G. K. (1980). U S . Pat. 4,185,003 (to DuPont). Hoeschele. G. K., McGirk, R. H., and Health. R. (1992). U S . Pat. 5,120.822 (to DuPont).
Polyester Thermoplastic Elastomers
365
Holden, G.. Lcgge, N. R., Quirk, R., and Schroeder, H. E., Eds. ( 1 9 9 6 ~Tlwrrloplastic Eltrstor?zer.s, 2nd ed., HanserEardner Publications, Inc, Cincinnati, OH, p. 192. Khan, F. A. (1986), Br. Plnst. Rubber 9:32. I. Lilaonitkul, A., and Cooper. S. L. ( 1977), Rubber Cllern. Tecl111ol50: Lloyd, I. R. (1982), in Developrnerlts ill Rubber Techrdogy9Vol. 3, Tller~nopltrsticElnstorners (A. Whelan and K. S. Lee. Eds.), Applied Science Pub., London, England, p. 183. Manuel H. J. and Gaymans, R. J., (1993). Polymer, 34(3),636-641. McCready, R. J. (1985), U.S. Pat. 4,544.734 (to General Electric). Meier, D. J. (1969). J. folyrrz. Sei., Pt. C 26:8 I . Miler, J. A., McKenna, J. M,, Pruckmayr, G., Epperson, J. E., and Cooper, S. L. (1985), Macrorrlolecules 18:1727. Morton. M,, Ed. (1987), Rubber Techrlology, 3rd ed., Van Nostrand Reinhold, New York. Noshay, A., and McGrath, J. E. ( 1977). Block Copolwlers. Overview mtl Critictrl Survey Academic Press, New York. Ryan, J. D. ( 198 1), Org. Cont. Plnst. Cllerll. 44387. Seymour, R. W.. Overton, J. R., and Corley, L. S. (1975). Macromolecules 8:331. Takanawo, Y., Okino, I., and Nakatani, Y. (1994), U.S. Pat. 5,331,066 (to Kanagafuchi Kagaku Kogyo Kabushiki Kaisha) Tamura, S., Matsuki, T., Kuwata, J., and Ishii, H. (l990), Jpn. Pat. 02,269.1 18 (to DuPont-Toray). Thomas, S., Gupta, B. R., and De. S. K. (1987). J. Vir1.d. Techrd. 9(2):71. van Berkel, R. W.M,. Borggreve, R. J. M,,van der Sluijs, C. L., and Buning, G. H. W. (l997),in Handbook of T1~e~rr~o~~~ltr.sric.s. (0.Olabisi, Ed.), Marcel Dekker, New York, p. 397. Wegner, G., Fujii, T., Meyer, W., and Lieser, G. (1978). Angew. Makronlol. Cllem. 74:295. Walch, E., and Gaymans, R. J. (l994), Polymer .?5(3):636-641. E1a.storrler.s(B. M.Walker, Ed.), Van Nostrand Reinhold, Wells, S. C. (1979), in Htrrlt~l?ookofTllerrrlo/,la.stic Ncw York, p. 103. , 2:42. Whclan. T., and Goff. J. (1985). Br. P l t ~ s t i cRuhberirld. Whitlock. K. H. (1973), Ned. Rubberirztl. 34:l. Witsiepe, W. K. (1972), Poly171.Prepr. Am. Cllern. Soc., Div. Polym. Cllem. 13(1):588. Witsiepe, W. K. (1973), in Polyrrlerizutiorl Re.rrctior1.sand New Polymers (Adv. Chem. Ser., No. 129) (N. Platzer, Ed.), American Chemical Society, Washington, DC, p. 39. Wolfe, J. R., Jr. (1973), U.S. Pat. 3,775,373 (to DuPont). Wolfe, J. R., Jr. (1977), Rubber Cller?l.Techrlol. 50:688. Wolfe, J. R., Jr. (1983), in Block Copo1.vnrer.s. Science trnd Techrwlogg (D. J. Meier, Ed.), MM1 Press Symp. Ser., Harwood Academic Pub., MM1 Press, New York, p. 145. Yoshida, S., Suga, H., and Seki, S. (1973). Polyrrl. J. 5:25.
This Page Intentionally Left Blank
13 Polyester Thermoplastic Elastomers: Part II H. M. J. C. Creemers DSM Engineering Plastics SV, Sittard, The Netherlands
1. COMMERCIALCOPOLYESTERELASTOMERICGRADES 1.l
Introduction
Polyester thermoplastic elastomers (TPE-Es or COPEs) have been on the market for about 20 years. The total market volume produced in 1994 was around 35 kton, of which Western Europe accommodated for - l 1 kton, the United States 20 kton, and the rest of the world, including Japan, -4 kton. The expected growth figures (average annual increase) were for Europe 7% and for the United States 3%. The biggest producer of COPEs is the company that developed and first marketed the product. du Pont. which produced Hytrel. Other producers. more or less in order of available production capacity. are:
Company
Product
du Pont
Hytrel Arnitel Ritetlex Pibiflex Pclprene Ecdcl Kopel Skypel
DSM Hoechst-Cclanese Enichem Toyobo Enstman Chcm. Kolon Sunkyong Ind.
The most important market segments for COPE are automotive, electrical (mainly wit-e and cable), hoses and tubes, mechanical goods, and footwear. A new market segment for these products is films with breathability properties. COPEs are available to the market mostly in pellet form. For all commercial COPEs the normal hardness range is between 40 and 72 Shore D. There are a few exceptions for special applications with lower (Hytrel 3078. Shore D30) and higher hardnesses (Hytrel 8238. 82D = 104R Rockwell hardness). COPEs or TPE-Es are situated at the higher hardness end of the TPE family, as are copolyamides (COPAs) and polyester-based thermoplastic polyurethane elastomers (TPUs). The
367
Creemers
368
-res
Temperature range
L
-70 to 16OoC -
-
-50 to 12OoC .
-
-60 to 12OoC
-50 to 110°C -40 to 80°C
I
I fuel r a s l s t a n o e . - Oil pain+abllity,creep,
COPE / TPE-E
mechanlcalpmpdes
Abrasion. X-linkable,
I
ESTER-TYPE TPU ETHER-TYPE TPU
smulding Hydrolysis, chemlcal
~ W H E R AMIMS PA 11/12] mechanlcal properties Low cost
'
High Rexlblllty. X-linkable, low
ti
Cost
ShoreA 40 50 60 70 80
Shore D
30
40
50
60
70
80
90
Fig. 1 Range of hardness per TPE family.
lower-hardnessrange (Shore A 40-80) consists of styrenic block copolymers (SBCs or S(E)BS), thermoplastic polyolefins (TPOs), and thermoplastic vulcanisates (TPVs) (Fig. 1). Figure 1 shows the common available hardness ranges of the different TPEs versus the temperature ranges at which the materials can be used. The important features of the TPEs are also given in this figure. Another method of comparing the different TPEs is shown in Fig. 2, where so-called continuous use temperature is plotted against the Vicat,indicating temperature stability under load, an important characteristicfor demanding engineering applications. Clearly
Fig. 2 TPE types: thennomechanical stability.
Polyester Thermoplastic Elastomers II
369
Soft-segmcnt properties
Hard-segment properties
Flex fatigue Low-temperaturc properties Impact strength Hydrolysis resistance
Mechanical strength UV/Ozone Oxidation resistance Oil/Chemical resistance
Effects of increasing hardness
Effccts of decreasing hardness
Mechanical strength UV and oxidation resistancc ChemicallOil resistance Permeability (lower)
Hydrolysis resistancc Flcxibility Impact strength Low-temperature properties
In theirnaturalstate, segmented poly(ether-ester)materials are opaque, creamywhite solids. Ecdel materials from Eastman have high clarity when processed under the correct conditions. These block polymers have a different polyester block PCT (basedon cyclohexane dirnethanol) rather than PBT. Teijin has developed a product called Nouvelan. in which the hard segment is based on polybutylene naphthalate (PBN) rather than terephthalate (4300 series) and the soft segment on an aromatic polyether (4100 series). Teijin is claiming for these materials better hydrolysis and thermal resistance (4300 series) and better chemical resistance and vibration damping (4100 series), respectively. However, most of the TPE-Es or COPES have PBT as the hard segment and PTMEG. PPG, or PEG as soft segments. 1.2 The Hytrel Family The different grades of Hytrel are designated using four digits, the first two of which represent the durometer D hardness. The third digit has no official significance, but relates to viscosity. The fourth digit represents the type of antioxidant (0-5, discoloring; 6-9, nondiscoloring). There are two main groups of products depending on the type of soft segment used: Standard grades.The mosteconomicalgradesofferthebestbalance of costand performance. These grades range in Shore D hardness from 35 to 82. 2. High-performance grades. These provide an extra measure of performance and service life in applications where properties such as abrasion resistance and tear strength are critical. They range in Shore D hardness from 40 to 72. 1.
In addition, Hytrel has some special grades, e.g., flame-retardant. blow molding, improved heat aging, and high fuel permeation-resistant. DuPont has developed some special grades for film applications (HTR 817 1, HTR 8206).
370
Creemers
1.3 The Arnitel Family Polyester thermoplastic elastomers manufactured by DSM Engineering Plastics B.V. aredistributed under the trade name Arnitel. There are three groups of Arnitels based on different soft segment formulations. Arnitel E is based on PTMEG, Arnitel P on modified PPG, and Arnitel U is a polyester-ester elastomer specially developed for cable applications. The following example of an Arnitel code illustrates the meaningsof the different constituents: EM400-G6 where: E = Arnitel type (E, P, or U) M = L, low viscosity; M, medium viscosity; B, high viscosity for blow molding application 40 = hardness (Shore D) 0 = commercialserialnumber G = additives: G, glass fiber-reinforced; V, flame-retardant; L. UV stabilized; M, mineral-filled; H, heat stabilized 6 = amount of filler or reinforcement ( X 5 % )
2.
ENGINEERING PROPERTIES
We will elucidate the engineering properties of TPE-Es using the properties of the different Arnitel grades. Arnitel combines the performance characteristics of elastomers with the processing features of a thermoplastic. Its noteworthy properties include: High load-bearing capability Excellent flexural fatigue endurance Good thermal stability High impact strength even at low temperature Good resistance to chemicals and weathering High tear and abrasion resistance High moisture vapor permeability Ease of processing A list of standard and specialized properties IS0 14910-2.
2.1
of COPEs is given in the International Standard
Properties of COPEs
The mechanical and electrical properties of the Arnitel product portfolio, as an example of a typical COPE range, is shown in Table 2. The chemical resistance and chemical properties of some softer representatives of the Arnitel COPE range is given in Tables 3 and 4.
2.2
Processing of COPEs
Material Handling COPEs like Arnitel are supplied predriedin moisture-proof bags at a moisture content sufficiently low to permit immediate processing for most applications.When exposed to air,Arnitel granules
371
Polyester Thermoplastic Elastomers II Table 1 Comparison of Properties of DifferentArnitcls Property profile
Stability oxidative
uv hydrolytic Low-temperature Impact properties Tear strength Chemical resistnncc Oil resistnncc Wear
Arnitcl E
Arnitcl P
Arnitel U
+
+ + +++ +++ + + +
+++ +++ + + +++ ++ +++
++ ++t +++ +++ ++ +
+++
+
+++
+ . good: + + . very good: + + + . exccllenr.
absorb moisture. At the high temperatures encountered during processing. even small quantities of absorbed moisture (e.g.. 0.02%) in the Arnitel granules can cause degradation during processing. This can result in varying molecular weights leading to a decrease in mechanical performance. For this reason, it is important to limit the moisture content of the granules as much as possible. The following precautions should be t’‘I k en: room to adaptslowly to the Allowmaterial that has been stored in arelativelycold temperature in the processing room. Do not open the packages until the machine is heated and ready for production. Always feed the entire contents of one or more bags into the hopper and close the hooper tightly immediately. Do not refill the hopper until there is room for the entire contents of a bag. Always try to refill the hopper to the top. Ensure that the hopper is not larger than necessary i n order to limit residence time of the material. Granules that have been exposed to ambiant air for too long must be assumed to have picked up moisture. These granules can be dried in a circulation oven with hot, predried air or in a rotary vacuum drier. The recommended drying conditions are as shown i n Table 5. Materials dried in this way will reabsorb moisture quickly during cooling. Therefore one of the following procedures must be adopted: Leave the hot. dried granules to cool in a sealed moisture- and airtight package. After cooling these to room temperature, these granules can be processed in a similar way as directly delivered Arnitel. If sealing equipment is not available. the hot and dry granules should be transferredimmediately to the hopper and the lid of the hopper closed tightly. If the temperature of the granules does not go below XO’C, then the amount of moisture absorbed will not be excessive. Rapid cooling of the granules can be prevented by insulating the hopper or by using a hopper dryer set to IOO’C.
Injectiorl Molding I n principle, Arnitel can be processed on all standard injection-molding machines plasticization. A plunger machine is not recommendable.
with screw
372
Creemers
Table 2 Properties forDifferent AmitelGrades Polyether cstcrs Properties
Units
Physiccrl properties Relative density Melting point Coefficient of linear thermal exp. Deflection temperature under load Vicat softening temperature: at 10 N at 50 N Moisture absorption: equilibrium in air equilibrium in water Flammability Mechanical properties Tensile modulus Tensile stress: at 5% elongation at 10% elongation at 50% elongation Tensile strength Elongation at break Izod impact strength: unnotched, at + 2 3 T unnotched, at - 30°C notched, at 23°C notched, at - 30°C Hardness Shore D
+
Electricrrl properties Dielectric strength Volume resistivity Surface resistivity Dielectric constant (E' ): at 50 Hz at 1 MHz Dissipation factor (tan 8): at 50 Hz at 1 MHz Tracking resistance: CTI CTI( M) Source: From Ref. 4.
EM400 EM460 EL550 EL630 EL740 PL380 1.12 195 220
1.16 I85 160
1 .20
"C pm/m.K
202 I 50
1.23 213 140
I .27 22 1 I 10
1.16 I97 150
"C
-
-
1 10
I IS
I20
-
"C "C
130
1 50
50
I80 85
200 1 15
205 150
145
-
r/C -
0.30 0.75 HB
0.30 0.70 HB
0.20 0.65 HB
0.20 0.60 HB
0.45 0.60 HB
0.40 7.0 HB
MPa
5s
1 10
220
375
900
60
MPa MPa MPa MPa
4.0 5 .4 8.4 17 700
7. I 9.0 11.4 21 800
13.2 15.7 16.6 32 600
20.2 23 22.0 40 600
26.9 33.5 26.8 4s 360
3.5 5.2 8.5 15 450
NB NB NB NB 38
NB NB NB NB 45
NB NB NB 20 55
NB NB NB 4 63
NB 200 9 4 74
NB NB NB NB 38
-
76
8
kJ/m' kJ/m' H/m' kJ/m' -
-
-
-
-
-
-
-
IO'" 10''
10"
10'4
10'1'
IO'"
I1
S x 10" > 10'5
10'4
10"'
10'"
IO'"
-
4.1 4.0
-
-
4.0
3.8 3.4
__
4.4
3.3
4.7 4.4
x 10' x 10"
10 170
-
-
350
400
3.8 350
300
310 350
-
600 600
600 600
600 600
600 600
600 600
600 600
MV/m bl.cm
-
ermoplastic Polyester
373
It
Polyester esters
PL720 PL580
UM55 UL740 UL.550
UM55 1
1-V
methods Test
1.23 218 I10
1.28 223 90
1.25 200 160
I.27 217 40
1.28 205
1.26 195
IS0 R 1183 ASTM D21 l7
-
-
-
120
80
120
-
-
205 95
210 I 55
I80 80
205 I 50
-
-
-
-
0.40 2.6 HB
0.25 0.85 HB
0.25 0.75 HB
0.15
0.40
0.16
0.35 HB
-
-
-
-
u194
27 5
920
I90
1.150
310
I80
I S 0 527 IS0 527
17.0 19.5 21 .o 22 300
24.9 30.9 28.2 37 300
13.4 15.8 17.8 35 500
31.1 36.0 27.7 38 350
8.2 13.9 17.6 30 600
7.0 11.0 14.0 31 500
NB NB NB 25 58
NB NB 17
NB NB NB 4.0 55
NB NB 13 6 75
-
-
-
-
-
-
55
55
14 3 x 101"
-
10'4
10'4
10
72 -
-
-
> 1016 > 10'"
10'4 10'4
> 10Ih >l014
-
I S 0 75 I S 0 306
-
1.8
,
14.6 X IO'" -
4.0
-
-
-
3.3
3.9
3.0
5.03 4.02
5.43 4.45
400
-
-
-
1 50
110
350
500
200
470
590
600 600
600 575
600 600
600 450
-
-
-
-
I S 0 306/A IS0 306/B
IS0 527 IS0 527 IS0 180 I S 0 180/1C I S 0 180/1C IS0180/1A IS0 180/1A I S 0 868 IEC 243 IEC 93 IEC 93 IEC 247
IEC 247
IEC112
Creemers
374
Table 3 Chemical Rcslstance of Polycstcr Thermoplastic Elastomcrs of 40 and 46 Shorc D Hardness
Chemical rcsistancc (6 weeks at 23°C)
5% Acetic acid 100% Acctic acid 10% Sodium hydroxyde 50% Sodium hydroxyde Sca watcr 25% SO2 solution 30% Sulfuric acid Acetone Ethanol Ethylacetate Mcthanal Tetrachloromethane Xylenc Gasolinelmcthanol (85/15) Keroscnc Gasolinc Two-stokc gasolinc Isooctalle Isooctane/toluene (70/30) Antar LVC Antar LVR 30 ASTM oil No. 1 ASTM oil No. 3 Avcat Dcrd 2498-7 Crude oil Dicso KN 10323 Donax HB break fluid Esso turho oil 2380 Esso turho oil 2389 Skydrol LD Skydrol 500 B
Weight incrcase, Tensile strength, 9 % rctcntion
E h g a t i o n at break, % retention
EM 400
EM 460
EM 400
EM 460
EM 400
EM 460
I .6 130 - 0.06 - 0.02 0.26 1.S - 0.04 28 17 47 13 170 86 54 16 33
1.4 68
-
__
-
-
31 IIO I 10 95 98 97 72
57 I10 I IS
53
77
100 100
10s 100
83
97 97 95 88
-
-
-
64 -
79
81
-
-
91 -
54 66
60 70 7s
75 80 81
-
-
-
-
94 I00 94 81 96 I 00 98 99 97 96 105
0.01 - 0.0 1
0.30 1.4 0.0 I 19
9.5 28 -
97 48 35 8.5
37 48 57
9s -
100
19
-
h5 -
-
86
-
5 16
100
105
-
-
83 77
-
-
__
1.6
0.5 I 6.6 9.5 10 11.5 13 4.5 7.2 18 20
105
92 73 I 00 I 10 I 05 95
I00 89 80
-
9 27
15
17 19
20 -
13 18
32 37
-
-
100 94
98 92 96 95 88
-
80
-
99 92 78 77
105
93 90 94 87
IO0 90 88
95 97 85 83
Notes
96 -
100 95 ~
100 98
96 90
Cylinder. For optimal processing of Arnitel. the residence time of the material in the cylinder diameter should be such that the product weight is within a range of approximately 40-70% of the maximum shot capacity. The heating elements should have sufficient heating capacity and the temperature should be accurately controlled to avoid large melt temperature fluctuations. Generally, but i n particular for glass-filled grades, a high injection rate is required for a good product quality. Screw. The geometry of the screw determines the transport behavior and the degree of plasticization of the granules. Standard three-zone screws with an L/D ratio from 17 to 23 and a thread depth ratio of approximately 1 :2 yield excellent results. Conical progressive screws (as used for PVC) are not suitable. To avoid backflow of the melt during injection and holding pressure, the screw should be equipped with a nonreturn valve.
Polyester Thermoplastic Elastomers II Table 4
375
Chemlcal Properties of Polyester Thermoplastic Elastomers of 40 and 46 Shore D Hardness r/c
Weight increase Chemical resistance at 100°C for 6 weeks
EM 400
Water Karafol BU, I cm'/litcr Neo Disher AX, 0.5% ASTM oil No. 1 ASTM oil No. 2 ASTM oil No. 3 Crude oil Esso turbo oil 23x0 Esso turbo oil 2389 Skydrol LD Skydrol S00 B At vnrious temperatures and immersion times Sulfuric ~ d 30% . Sulfuric acid. 45% Water pH5 Water pH8 ATF oil Kexosenc Klubcr Oppnnol Optilnol Peanut oil Soybean oil Sunflower oil
EM 460
Tensile strength, Elongation 8 retention EM 400
EM 460
at break, 76 rctcntion EM 400
7s
72
-
-
-
EM 460
-
100 86 96 94 88 -l -I
7s S0 66 65
I os S2 43 S8
-
-
-
81
1os
97
100
I 0s
-
90
91 1 10
-
93 93 92
-
86 -
70 80 68 70 -I
I os 97 9s 95
102 17 -
-
-
-
Notes
1 1 11 11
wk/70"C wW70"C wW90"C wW90"C -
4 wW60"C 3 W!i/lI0"C 1 wW85"C 3 ww1 10°C 6 wW60"C 6 wWh0"C 6 wW60"C
Nozzle. Arnitel is preferably processed on decompression-controlled machines with an open nozzle. With ;I short nozzle anda wide bore (3 mm or more), frictional heating and pressure losses are thus minimized. Particularly with glass fiber-reinforced and tlame-retardant grades, injection molding problems can thus be avoided. Nozzles that can be closed (hydraulically, if possible) may be used as well, provided they are equipped with an effective. precision-controlled nozzle heating system. It is advisable to withdraw the nozzle from the mold after the injectiotdholding pressure phase to prevent it from cooling down LIIKIUIY. Hopper. The hopper should be equiped with a tightly closing lid. which should be kept closed during processing to keep the granules dry and free from dust. SOM. Aspects of Mold Design Good mold design is essentialforoptimalinjectionmolding and. consequently, for ahighquality product. In designing molds for the processing COPES like Arnitel, the following points should be observed.
376
Creemers
Table 5 Recommended Drying Conditions for Different Arnitel Grades Depending on Hardness Amitel grades Base grade
Drying conditions
Hardness, shore D
Time (hr)
Temperature ("C)
28 38 S8 12 40 46
3 3 6 6
120 120 120 120
10
100 100 1I O 110 110
P P P
P E E E
ss
E E E
58 63 14
U U
ss 14
IO 10
8 8 6 3 6
120 I20 120
Gating Systems. All familiar gating systems-cone, pinpoint, tunnel, film, fan,and ring gates-may be used. Externally heated hot runner and semi-hot runner systems also qualify, but they require efficient heating and very accurate temperature control to avoid freezing or overheating of the material. Gate Locations. Gate locations should be chosen with care to minimize deformation or warpage of the product due to anisotropic shrinkage behavior; this particularly holds for glassfiber-reinforced Arnitel grades. The gate should preferably be located on the thickest section of the product and in such a position that the product fills as evenly as possible. Dimensions of Runners and Gates. The cross section of the runners should preferably be trapezoidal. Recommended runner and gate dimensions for various wall thickness are given in Table 6 and Figure 3. For products with a wall thickness exceeding 3-5 mm a full sprue gate with a diameter of approximately three fourths of the largest wall thickness is to be preferred. A short sprue cone with a taper of at least 1" 30 is recommended. Venting. Special attention should be given to effective mold venting. Venting is effected by vents (dimensions 1.5 X 0.02 mm) provided in the mold faces or via existing small channels such as around ejector pins and cores. Vents should be located in the mold at the end of the flow paths.
-
Table 6 Recommended Runner and Gate Dimensions Versus Wall Thickness Wall thickness (mm)
0.7- 1.2 1.2-3.0 3.0-5.0 >5.0
Gate diametedlength (mm) 0.7- 1.0/0.8- 1 0.8-2.0/0.8-1 1.5-3.YO.9- 1 3.5-6.0/0.8- I
377
Polyester Thermoplastic Elastomers II
. .
I o
1
2
3
4
5
6
Fig. 3 Recommended runner dimenuons.
Ejection. Molded products are removed from the mold by means of ejector pins, plates, or rings. The designand number of ejectors isdictated by product design and stiffness. Ejection must not cause damage or deformation. In view of Arnitel’s flexibility, particularly the softer types. the part of the product i n contact with the ejector should be under uniform load. It follows that a fairly large ejector face is required. Cooling. The cooling system is an important part of the mold and needs to be configured with scrupulous care. To prevent warpage and long cycle times, the product must be cooled down rapidly and uniformly. hjection Molclitlg Corditions
Cylinder Temperatures. In accordance with theirrespectivemeltingpoints,Arnitel grades are processed in the 220-260°C range (harder grades at higher temperatures; soft grades at low temperatures). A rising temperature profile will normally yield the best results. Because it depends on injection molding grade, type of machine, and product to be injection-molded, there is no generally applicable optimum temperature. The residence time of the material in the cylinder is an important processing parameter. To avoid thermal degradation of the melt as a result of prolonged residence times, it is best to observe the lower limit of the recommended temperature range. Too high melt temperaturesshould be avoidedbecausethermaldegradationadversely affectsmechanicalproperties. Cylinder temperaturesettingsshouldbe in the range of 200-250°C.
378
Creemers
Mold Temperature. Forthin-walledproductsa mold temperature of 50°C is recommended. Thick-walled products can be moldedat 20°C. Higher mold temperatures improve flow but add to cycle time. Injection Pressure. Injection pressure is primarily determined by the wall thickness of the product.the tlow pathlength,andtheflowbehavior of the injectionmolding grade. In general. Arnitel grades have excellent flow properties. Injection pressure should be sufficiently high to obtain uniform mold filling. Injection Rate. To avoid premature freezing during injection. fast filling of the mold is recommended. A relatively high injection rate is usually possible. but a moderate rate may be necessary in certain conditions. Holding Pressure and Time of Follow-Up Pressure. Shortly before the mold is completely filled, the injection pressure is usually stepped down to the holding pressure. which in most cases is 40-70% lower than the injection pressure. During the holding pressure phase, the volume shrinkage of the cooling melt is compensated. Holding pressure should therefore be set high enough to prevent sink marks. Excessively high holding pressures should be avoided since they may cause residual stresses in the product or visible burning. Holding pressure should be sustaineduntil the gate freezes.The appropriate time of followup pressure is best determined by weighing the product. Sink marks of shrinkage voids indicate that the time of follow-up pressure has been too short. Time of follow-up pressure should be prolonged proportionately as wall thickness and gate dimension increase. Back Pressure and Screw Speed. In general. back pressure and screw speed should be set as low as possible to avoid excessive heat generation through friction and the reduction of glass fiber length in reinforced grades. Back pressure promotes homogenity of the melt. It should be set just high enough to ensure that the melt is free of air bubbles. that the screw plasticizes evenly, and that the product weight is constant. In practice. a back pressure of approximately 3-6 bar has been found to be sufficient. The screw speed should be such that plasticizing time remains just within the cooling time. Low screw speeds (30-100 rpm) will limit both heat generation through shear and the reduction of glass fiber length. Clamping Pressure. Theclamping pressure is matched to the injectionpressureand the projected surface of the product. Arnitel’s low relative viscosity and good flow properties render a high clamping pressure necessary to prevent flashing forination. Metering. The screw metering rate should be so controlled that. during holding pressure. there remains a sufficiently large buffer of molten material in front of the screw to serve as afterfilling material. A small buffer of 2-5 m m is recommended, since a large buffer might lead to loss of pressure and to prolonged residence of the melt in the cylinder. CoolingKycle Time. The cycle time is primarily determined by the injectiothfter pressure time and the cooling time. The nucleating agent. crystallization accelerator, and glass fiber reinforcement. if any. bring aboutrapidcrystallization of Arnitel grades.Thecooling time depends on wall thickness and shape of the product. For this reason the cycle time varies from approximately 9 seconds for products with a wall thickness from 0.8- I .5 n m to approximately 40 seconds for products with a wall thickness of 5-6 mm. Mold Shr-inkqe Mold shrinkage of Arnitel moldings is intluenced by many factors. such a s product design, wall thickness. mold temperature. cross section of gating. type of gate. dwell pressure. and holding time. Wall thickness and mold temperature have the greatest intluence. Mold shrinkage increases with wall thickness and mold temperature. It decreases with higher dwell pressure and longer
Polyester Thermoplastic Elastomers It
379
holding time. The difference between shrinkage in the flow direction and shrinkage across the tlow direction is relatively low for Arnitel.
Extrusion Arnitel is as easy to extrude as other polymers. Good results are obtained with most conventional single screw extruders. Extruder Barrel. Extruder barrels that are suitable for the use of PA, PVC. and polyolefins are usually also suitable for Arnitel. Barrels with axial grooves and intense cooling of the intake zone are not suitable for the extrusion of Arnitel. If an intake zone with relatively short and shallow grooves (< 1 mm) is used. good results can be obtained only if intensive cooling is avoided. Screw Design. For Arnitel extrusion, good results are obtained with conventional single screw extruders equipped with a three-zone screw. Length-to-diameter ratios of 24 or higher provide the best melt quality. The clearance between screw tlights and barrel should be small: 0.08-0.10 mm for extruders up to 45 mm in screw diameter 0.1-0.15 mm for larger extruders
The compression ratios should be between 2.4 and 3.2, as determined by the depth of the feed section divided by the depth of the metering section. The depth of the channel in both the feed and metering sections is important; if the feed channel is too deep and not sufficiently long, particularly with largediameterscrews,poor feeding and loss of output can result. If the metering channel is too deep, insufficient pressure will be built up, resulting in lower output, particularly with low viscous grades. A metering channel which is tooshallow can result in overheating of the melt. due tohigh shear, particularly at high viscous types. Many factors have to be taken into consideration in selecting the correct screw design. To give some idea of screw design successfullyused i n the extrusion of Arnitel, the characteristic design parameters and approximate values are listed in Table 7. It should be added that certain designs of barrier screws have been found to be effective in extruding Amitel. Power Requirements. The twomainfunctions of an extruder are (1 ) to produce a homogeneous melt from solid material and ( 2 ) to convey material from the feedhopper to the die so that a constant and stable flow of material is delivered to the die opening at a constant pressure. The energy required for melting is supplied by two sources: the heater bands and the electric motor driving the screw. Because Arnitel has a high heat capacity and a high heat of
Table 7 CharacteristicDeslgnParameters Screw length Pitch Extruder diameter mm Length of section: D section Feed Compression sectlon Meterlng Dsection Channel depth: mm sectionFeed Metering sectmn mm
for Arnitels
24-27 1D
D
30
45
60
90
7-10 4-6
7-10 4-6 8-11
7-10 4-6
7-10 4-6
8-11
8-11
2.5 6.5
3.5 8
4 IO
8-11
2 5
Creemers
380
Table 8 RecommendedProcessingTemperaturesfor
Arnitcl Grades
Melt flow indcx Processing temperatures at: grades Arnitel P-X5975 (28D) PM380 PM580 EM400 EM460 EM550 EM630 EM740 UM55 1 UM552 UM552-V UM740
Melting point ("C) 207 218 218 195 185
202 213 22 1 200 195 200 217
220°C (dg/min) (dg/min) (dg/min)
230°C
240°C
13 6
25 38 7
18
7 18
6 7
Maximum Minimum ("C) 220 225 230 205 195 215 225 230 210 210 210 230
("C) 255 260 265 240 240 260 260 260 260 260 260 260
fusion, high engine power is needed to attain the high temperature required for extrusion. One of the two heating zones situated directly downstream from the hopper should have a power of 4-5 Wkm'. For the remaining zone a power of 1.5 Wkm' is sufficient. For extruder start-up, an engine power of approximately 0.3 kW per kg output is required, after which an engine power of 0.15-0.2 kW per kg output is sufficient. Heating elements and thermocouples should be installed at strategic positions to avoid overheating of the melt. All positions that are not directly heated should be properly insulated to avoid cold spots, which result in an undesirable flow and varying material properties. To ensure that all residual polymer is thoroughly melted before start-up, a brakerplate (head clamp) is usually used. This is an area where a large amount of heat can be lost to the surrounding air; thus, the heater design in this zone is crucial. Processing Temperatures. Depending ongrade, processingtemperaturesfor Arnitel range from 200 to 250°C. The optimum temperature profile depends largely on the grade and its application. Some guidelines are given. along with other important information, in Table 8. The actual temperature of the molten polymer in the extruder should be between the given maximum and minimum temperatures. Temporary Shutdown. When the extrusion process is interrupted for less than 30 minutes. no specialmeasures are required. The normaltemperaturesettingscan be maintained. When the production is resumed. the extruder should be purged until the residual material has been replaced. Use of Regrind. The excellent heat stability of the Arnitel melt permitsthe use of regrind provided the material was properly processed during the initial extrusion. Depending on the demands to be met in service by the products. up to 20% regrind can be used. It is recommended that the scrap be chopped into granules approximately the same size as the original pellets. The regrind must be blended with virgin polymer and dried to ensure uniform quality. Afrertreatnrent
Coating. Arnitel is easily coated provided that no silicone-containing mold release agents or other products with an adverse effect on adhesion were used during the injection-molding
Polyester Thermoplastic Elastomers II
381
process. No special adhesion promotors are necessary. Paint system suppliers will be glad to tell you whether the tlexibility of the coating you have selected matches the hardness-or rather stiffness-of the Arnitel grade used. Metallizing. The vacuum-metallizing method is the best metallizing procedure for Arnitel. In connection with the low flexibility of the metal film, it is best not to use soft Arnitel grades and always to run a test first. Printing. Arnitel is easy to print.Polyesterprintingfilm,whichpermitsthe use of standard equipment, is a relatively simple method. offering a wide choice of coatings adapted to thespecific properties of the end product. So-called laser printingis also possible with Arnitel. Scfety Aspects Arnitel is anonreactive,nonvolatile,harmlessrawmaterial. No specialsafetymeasures are necessary during storage (but exposure to moisture must of course be avoided for quality reasons). Upon opening of the package and during the processing of unreinforced or mineral reinforced grades, a slight odor will be perceptible near the injection-molding machines, which will usually be discharged by the room ventilation system customary for production areas. Although no special precautions are necessary,the use of safety glasses near the processing machines is recommended. If any part of the skin shouldcome into contact with molten polymer, at once cool the affected part in running cold water and keep it submerged for several minutes at least. Local fume extraction is recommended in the processing of Arnitel, particularly if the recommended temperatures and/or residence times are exceeded, since there is always the risk of hazardous volatile components escaping that are highly flammable (e.g., tetrahydrofurane) and could irritatethe respiratory tract. Localfume extraction is indispensable in cases ofoverheating, such as during a stop with a filled cylinder or during the cleaning of machine parts in an oven.
3. APPLICATIONS Commercial available COPEs generally range in hardness from 40 to 80. shore D. This means that the applications are very diversified. Materials with a hardness of Shore D > 60 find their applications in the field of impact-modified engineering plastic (e.g.. connectors and optical fiber-coatings). A hardness range between 40 and 60 Shore D (the most important part) finds its applications in the rubberlike applications such as (CVJ) boots and bellows, tubes and hoses, airbag covers, etc. The few materials with a hardness lower than 40 are finding applications in the softtouch area. likekeypadsandgrips,and in the filmandnonwovenarea. The most important properties for each of the segments is given in Table 9. Another division can be made in injection molding, blow molding (CVJ boots) and extrusion applications. Typical examples of extrusion applications for COPEs are: Tube and hoses: Hydraulic hoses Convoluted tubes Profile and belting: Conveyer belts Automotive belts HardSoft combinations for water-tight sealings
Creemers
382
Table 9 Important Properties for Diffcrent Hardness Segments of Thermoplastic Elastomcrs ~
Elastic fibers and film
~
~~~~
Rubhcr-like
Impact-modified EP
<40 El.‘Istlclty .’ ’
40-60 Melt-processable
>60 Impact rcsistance
Vapor tr;msmission
Low modulus Low temp. propertics Cont. use temperature Chemical resistancc Flex fatigue
Cont. use tcmpcrnturc HDT Chemical resistancc Proccssability Crecp
Wire and cable: For halogen-free Arnitel U’s automotive cables. class T3 (CUT 125°C) and T4 (CUT 150°C) Telecommunication cables Coiled wire jacketing Fibers and nonwovens Various textile applications Monofilament: Textile applications Flexible woven tubing Film and coating: Durable and disposable laminates with textile and/or nonwovens Medical applications such as surgical drapes and gowns. wound coverings. and transdern u l patches (most Arnitel grades are USP VI approved) Construction (e.g., roof and wall membranes) Food packaging (several Arnitel grades have FDA and BGA approvals) Diaper backsheet Divided over the hardness range. these applications can be classified as shown in Table 10. Looking at the usage in tonnage, automotive applications are oneofthe biggest segments (25%). Typical examples are boots, bellows, airducts. airbag covers, body plugs, and shock absorption applications. Another 25% of the applications may be generalized as hosesand tubes, like painting hoses, hydraulic hoses, umbilicals. convoluted tubes, mandrels, etc. About 12.5% of the applications is found in the sport and footwear area, e.g.. cushions, shoe studs, binders. windsurf mast socks. ski-boot parts. etc. The rest is divided over wire and cable insulation. film, nonwovens, and miscellaneous applications like special brushes. watch streps. soft touch grips. etc.
3.1 SomeDetails of COPE Applications Autornoti\v> Applicrttior1.s
I n the automotive market the trend is moving away from thermoset rubbers like EPDM. chlorinated PE. etc. to TPEs. This replacement is not only based on performance, but are also a result of the introduction of new processing techniques allowing more complicated shapes. There are new possibilities of using rigid/soft conlbinations in one processstep. In the past. when formulat-
on grade
Polyester Thermoplastic Elastomers II
383
Arnitcl WireBase ProfileTube Hardness. grade
P P P P E E
E E E E U U
Shore D
and hose
and belting
and Fiber and cnblc nonwovcns
Film Monofilnmcnts
and sheet
28 38 S8 72 40 46
ss S8 63 74 55 74
ing air ducts, thermoset EPDM was used in the flexible parts, while polyamide was used in the rigid parts. In the case of dirty ducts. combinations PBT and COPE are preferred and are under study (oil resistance). CVJ (constant velocity joint) boots in frontwheel-driven cars are another example of an interestingmarket.where i n thiscase COPEs arereplacingthermosetelastomersbased on performance (temperature resistance. cold and hot; abrasion resistance, grease resistance). The use of more aggressive greases by the automotive industry is bringing COPEs in a good position for this application. This trend started in the United States and continued into Europe. In Japan this changeover is still small, mainly due to the big price difference i n favor of rubbers. Airbag covers is a new, safety-driven. application. Originally only installed on the drivers side. today airbags areinstalled on both frontsides and even in the back of the car. Thepolymeric materials used in the manufacture of airbag covers are TPVs. thermoplastic copolyesters. and SBS materials. Intercompetition between the different TPEs is, in fact. happening. Depending on the requirements. a choice among the TPEs has to be made. Soji-Tolrcl?
Because of their rubber-like behavior. TPEs in general are well suited for so-called soft-touch applications like watchstraps and grips for tooling. For price performance reasons TPOs and TPVs perform very well. Film Applicatiotls
COPEs are certainly not only aiming a t replacement or vulcanized rubber application. One of the characteristic examples is the use of TPE-E in elastic and breathable film. The polyether segment (the soft segment) gives COPE a certain permeability for vapor. The longer the soft polyether segment, the higher the moisture vapor transmission. Also. the kind of polyether used in the soft segment has an intluence. The lower the number of carbon atoms between the ether groups. the higher transmission for moisture vapor. Polypropylene glycol (soft segment in Armi-
Creemers
384
tel P) is better in this respect than polytetramethylene glycol (in Amitel E). COPE films are elastic and water-repellant, but moisture vapor transmission can be adjusted. This makes COPE film very well suited for application in sport clothes, in general in applicationswherebody comfort is important.
3.2 Arnitel Approvals, Recognitions, and Registrations Arnitel has been approved. recognized, or registered by several institutes and organizations. These approvals are necessary to operate successfully in today's markets.
General TOSCA (Toxic Substances Act: USA). Arnitel E and P are Tosca registrated (include. DSL-Canada) under the following CAS numbers: Arnitel P CAS nr 648 11-37-6; Arnitel E CAS nr. 37282-12-5. For Arnitel U, a so-called polymer exemption document has been filed (P-case nr. Y-95-92). This registration is important for imports into the United States. UL Recognition. Underwriters Laboratories Inc. (UL), an independent, noncommercial institute in the United States, tests materials according to standards which they partly develop themselves. The test results, which provide as independent opinion about a product for customers, are listed on a yellow card, company by company. and collected once a year in a yellow book. UL recognition is especially important in the E and E application area. The Arnitel materials given in Table I 1 have UL recognition.
Food and Medical Approvals EEC Food Approval. Food approval is necessary for end products in contact with foods. For Europe therequirements are laid down in national and supra-nationallaws (EEC Commission
Table 11 AnlitelMaterials with UL Recognition
Grade
Color
Min. Thk mm
RTI "C EL UL94
PL460-S 130 130 BK 130 94V-0 1.60 PL720-S All 1.S0 94V-0 UL 550 All 0.75 160 94HB 120160 94HB 1.50 150 120 160 94HB 3.00 All UM550 0.75 16094HB 1.50 94HB 16094HB 3.00 UM55 l All 0.75 160 94HB 150120160 94HB 1.50 3.00 160 94HB UMS52 All150 120 0.75 16094HB 1.50120160 94HB 3.00 94HB UMSS2-V NC 1.50 94V-2
HA1 CTI D495 HVTR
HWI WO1 W1 -
-
-
-
-
1
-
0 0 1 0 0
-
-
0
S
0
1 0 0 1
-
-
-
I S0
4 3 2 4 3 2 4 3 2 4 3 2
0 0
S0
-
-
130 130130
120
160
IS0 IS0
120 120 120 120
150 150 150 IS0
120
150 150
160
50
120 SO
-
-
0
5
0
-
-
-
-
-
-
S
0 -
-
-
0
5
0
-
-
-
0 -
-
Polyester Thermoplastic Elastomers II
385
Directive 90/128/EEC plus corrections and amendments). For the EEC, Arnitel E and P are in compliance with the regulationsfordryfoodstuffsand for nonfattyfoodapplicationsup to 12 1°C. FDA Food Approval. Arnitel E grades are also in compliance with the code of Federal Regulation,issued by the Food and DrugAdministration (FDA) 21 CFR 177.2600(rubber articles for repeated use) in the United States-so-called FDA approval. Arnitel E may be used in contact with foods containing not more than 8% alcohol and limited in its use in contact with food at temperatures not exceeding 150°F. Arnitel E is also approved under 177.1590 (polyester elastomers), but is restricted to dry food with nonfat or oil at the surface. 3A Sanitary Standard-US Department of Agriculture. Arnitel grades EM460 and EM550 are in compliance with the requirements of the 3A Sanitary Standard for multiple use Plastic Materials used as Product Contact Surface for Diary Equipment. This approval is important for the application of COPES in transport belts for dairy products. U.S. Pharmacopoeia Class VI. Arnitel EM400,EM460,EM550, PM580.and P28D have been tested and approved according to U.S. Pharmacopoeia (USP) ClassVI. This approval is necessary for medical applications.
REFERENCES Amitel, TPE-E Typical Properties of Thermoplastic elastomer grades (DSM), Edition 05/93. Arnitel Guidelines for the injection moulding of thermoplastic elastomer TPE-E (Ed. 2/97). Arnitcl Guidelines for thc extrusion of polyester elastomers (TPE-E) (Ed. 3/97). Creemers, H. M. J. C. ( 1996), Thermoplastische Elastomere (TPE). Ku/uf.sfoflc,86: 1845- 185 1. Dupont-Hytrel Engineering Thermoplastic Elastomers. General Guide to products and properties. Edition 03.96 (H 23287). Guldemond, C. P. (1996). Thermoplastic Elastomers Market Review, lecture a t Thcrrnoplastic Elostomer Course at University of Twenic, The Netherlands.
This Page Intentionally Left Blank
14 Thermoplastic Polyurethane Elastomers Charles S. Schollenberger* Polyurethane Specialist and Consultant, H ~ d s o n ,Ohio
1. BACKGROUND Rubberlike elastonler systems that have achieved commercial importance through the years are largely aI11oIphol1s. gumlike masses of polymer chains with low glass transition tenlperatures (T,) whose interchiin attractive forces are relatively weak and delocalized (randomly distributed) along the polymer chains. To be useful for mechanical applications. such polymer chains must be joined together in lateral fashion by covalent chemical crosslinks (CC) in the process known a s vulcanization or curing. 0. Bayer and his chemists. first at the Main Scientific Laboratory of I. G. Farbenindustrie beginningabout 1937, lateratFarbenfabrikenBayerandthen other laboratoriesworldwide, subscribed to and applied the classical CC vulcanization concept in explaining and exploiting the excellent mechanical properties of the extant elastomeric polyurethanes (Bayer, 1947; Bayer et a l . . 1950; Bayer and Muller. l960), including the castable liquid types. However, it was subsequently recognisedthat very high-level, useful nlechanical properties can be obtained in amorphous, low-T, polyurethane elastomer systems that are essentially linearly structured, thermoplastic. soluble, and therefore devoid of chemical crosslinks (Schollenberger et al.. 1958, 1962). This phenomenon was attributed to tiepoints among the linear polyurethane chains that are reversible with heat or solvation, and the term “virtual crosslink” (VC), i.e., “crosslinked in effect but not in fact.” was applied to such tiepoints (Schollenberger et al., 1958, 1962). Virtual crosslinks are a consequence of polymer chain structure. and the recognition and exploitation of this concept (Schollenberger et al., 1958. 1962; Schollenberger. 1959) resulted in the invention, development, and commercialization of thermoplastic polyurethane elastomers (TPUs), the first members of a unique polymer class, thermoplastic elastomers, which has since assumed major commercial importance (Dreyfuss et al., l98 1 : Hepburn, 1982). Published views bearing on the nature of virtual crosslinks in TPUs include hydrogen bonding among urethane-group hydrogen atoms and carbonyl groups (Boyarchuk et al., 1965: Cooper and Tobolsky, 1966; Nakayama et a l . , 1969). urethaneaggregationtendency due to hydrogen bonding (Boyarchuk et al., 1965). and the enhancement of these phenomena in urethane-rich TPU chainsegments (Cooper and Tobolsky,1966), whichproducesheterophase polymer morphology.
387
Schollenberger
388
2.
INTRODUCTION
Thermoplastic polyurethane elastomers largely possessthe same outstanding mechanical properties as other types of solid polyurethane elastomers. such as the castable and injectable liquids and millable gums. These property levels usually enable less material (TPU) to be used in a given application than would be the case with a nonurethane material, and this feature gives TPU an edge in many instances. Relative to other important types of polyurethane, the plastic, highly polar nature of TPU allows the use of a different set of processing methods that prove to be the stock-in-trade of the plastics fabricating industry. These include milling, Banbury mixing, calendering, extruding, molding [injection, compression, transfer, centrifugal (powder). blow], and solution processing. Further assets of TPU are its ability to be fabricated into finished elastomer products without the need for curing (crosslinking) them, which makes it possible to reprocess TPU scrap formed during product manufacture. In contrast to mostliquid-processingpolyurethane systems, TPU can be compounded extensively with additives and loaded heavily with pigments. But the need for compounding, e.g., plasticization with its problems of migration, evaporation, etc., or pigment reinforcement is often precluded by the ability to tailor the desired mechanical and chemical properties into TPUs during their chemical synthesis. This has considerable value from an aesthetic standpoint, often enabling transparency, light color. etc., and also from a physiological standpoint. because of the superior purity that can be attained, which might not be achieved with conventional elastomers that require compounding. With so many favorable features, it is little wonder that TPUs have become so attractive to polymer producers and so well received by plastics fabricators. It is estimated that the worldwide production and use of TPU in 1985 reached a level of 40 million (Mobay Chemical Co. 1985 estimate) to 55 million pounds (B. F. Goodrich 1985 estimate). Fromatechnicalstandpoint, the linear,tractablenature of TPUshas allowedtheir investigation by numerousaccomplishedscientistsinternationally. in solution, in the melt, and inthesolid form.This hasgreatlyaided in theirscientificstudy, thereby helping to providea clearer understanding of thebehavior of TPUs and of a l l polyurethanesand thermoplasticelastomers.
3. SCOPE AND CONTENT OF CHAPTER This chapter deals with several aspects of thermoplastic polyurethane elastomers, but there is much more published information on the subject than could be cited and discussed here. What has been attempted is to provide a path through the wealth of information on TPUs, leading from their inception, progressively through the science and technology that have attended their development, to the uses and markets for these remarkable materials. In the process. the topics covered include TPU chemistry: reaction components and structure effects; polymerization processes; chain structure. organization, and behavior; morphology and thermal responses; molecular weight effects; chemical crosslinking effects; environmental stability (to hydrolysis, ultraviolet and gamma radiation, and microbes);compounding. processing, and applications; commercial polymers; and market volume. Hopefully, the path will be considered to have been sure and direct. If rather narrow.
389
Thermoplastic Polyurethane Elastomers
4.
CHEMISTRY
Polyurethane isocyanate chemistryis a very broad fieldthat embraces a large numberof chemical reactions, including the many reactions of isocyanates with active hydrogen compounds, with other isocyanate groups. with other unsaturated compounds, etc. (Saunders and Frisch, 1962). Only the more important isocyanate reactions in TPU formation are covered here. Thesynthesis of TPU components is not discussed. 4.1
Urethane Group Formation
The basic chemical reaction involved in making any type of polyurethane, including TPU, is urethane group formation. Thisis almost always accomplished by causing an organic isocyanate group (-N===C"V) to react with an alcoholic hydroxyl group ("OH), as seen in Eq. (1). RNCO Isocyanate
+
R'OH Alcohol
- RNHCOOR' .- Urethane
+
AH Heat
As seen in the equation urethane formation is a reversible equilibrium reaction.The equilibrium state liesfar tothe right sideof the equationat normal temperatures, andat room temperature there is negligible urethane dissociation. However, the dissociation increases with increasing temperature, and the converse is also true: TPU melt polymerization studies in a small torque rheometer have made this clear (Schollenberger et al., 1982) (see Fig. I). Studies also show that the thermal stability of urethanes varies considerably depending on their structure. For example, those from tertiary alcohols decompose readily at temperatures as low as 50°C. In contrast, urethanes from primary and secondary alcoholsmay undergo changes only slowly at 150-200°C. The presence of other reactants and of catalysts can also influence the stability of urethanes (Saunders and Frisch, 1962).
4.2
Urea Group Formation
Equation (2) shows the isocyanate group reacting with the carboxyl the urea group.
Isocyanate
+ 2R'C02H
Acid
-
RNH-CO-NHR Disubstituted urea
+
group (<02H)
(R'CO).$ Acid
anhydride
+ CO2
to form
(2)
Urea group formation in TPU via the isocyanate-carboxyl reaction is of low probability due to the deliberate paucityofcarboxyl groupsin the system and the polymerization method commonly used to manufacture TPU. Equation (3) shows the isocyanate group reacting with water to form the urea group. 2RNCO Isocyanate
+
HOH Water
~
RNH-CO-NHR Substituted urea
+
Urea formation in TPU via the isocyanate-water reaction is also of low probability due to the deliberate exclusion of water from the system in TPU manufacture. There are several reasons to exclude water in this operation. First, as seen in Eq. (3), water behaves as a difunctional reactant with isocyanate. putting it in the category of a polyurethane monomer. Second. its low molecular weight of 1 X enables a small amount of adventitious water to exert a major effect on
Schollenberger
390
the nature of the product in TPU polymerization. And third, the water-isocyanate reaction generates carbon dioxide gas. whose bubbles may not be desirable in the TPU product.
4.3 Allophanate Group Formation Equation (4) shows the isocyanate group reacting with the urethane group to form the allophanate group. R'N-COOR"
I RNCO Isocyanate
+
R'NHCOOR" Urethane
CO
I
NHR Allophanate
Allophanate formation is seen to be an equilibrium reaction, and the allophanate group a thermallylabilestructure. At temperatures of 106°C andabove. its dissociation to thestarting isocyanate and urethane groups is reportedly considerable (Kogon, 1959). Allophanate group formation produces chain branches andor crosslinks in TPU that affect polymer characteristics and processing options, e.g., solution applications. For this reason it is generally avoided. However, the allophanate group is sometimes deliberately introduced into TPU to provide chemically crosslinked TPU products (Mobay Chemical Co.. 1968).
4.4
Biuret Group Formation
Equation (5)shows the isocyanate group reacting with the urea group to form the bluret group. R'N-CO--NHR' RNCO Isocyanate
+
R'NH-CO-NHR' Urea
I I
NHR Biuret
Biuret formation is yet another thermally reversible reaction that can be encountered i n TPU synthesis, e.g., via Eq. (2)and/or Eq. ( 3 ) .It is a reaction deliberately arrangedin (chemically crosslinked) polyurethane castings but avoided i n TPU for the same reasons that allophanate linkings are avoided. The dissociation of biurets is appreciable at 120°C (Kogon, 1958).
5.
REACTION COMPONENTS AND STRUCTURE EFFECTS
TPU elastomer formation requires difunctional reactants to enable the building of long linear chains. The types of components used to prepare TPU are listed by class in Table 1. Almost
Table 1 TPU Componcnts
Gcneral Component Diisocynnatc M~roglycol Chain extender
structure
OCN-R-NCO HO-R"OH HO-R"-OH
Thermoplastic Polyurethane Elastomers
391
all TPU componentsare liquids at roonl temperature or low-melting solids for process advantages such as conveyance, metering, and mixing. The specific natures of R. R’, and R” in Table 1 are apparent in the typical examples of Tables 2-4, which follow, and hold wherever they appear in subsequentequations,suchas those dealing with polyurethane chain formation or chemical structure.
5.1 Diisocyanates The diisocyanate component is a relatively small molecule of molecular weight - 150-250. I n TPU its function is twofold. First. it acts as a coupling agent for the macroglycol component to produce urethane-sparse TPU “soft segments,” +CONHR-NH-COq-Rf-O+x
TPU soft segment
asacouplingagent segments.”
for thechain-extender component to produceurethane-rich
fCONHR-NH-CO-O-R”-Ot
TPU “hard
Y
TPU hard segment
and as a coupling agent for the “soft” and “hard” segments to build TPU chains. ~CONH-R-NHCO-O-R”O~~CONH-R~NHCO~O-Rf”O~n
TPU chain
The second function of the diisocyanate isits structural contribution to TPUphysical properties, which is of greatest consequence in the hard segments. Although there are a multitude of diisocyanates to choose from for polyurethane preparation (Sietken, 1948: Sayigh et al.. 1972). the most useful ones for TPU formation are those with cyclic. compact, symmetricalnuclei. They producehard segments andurethane groups that apparently associate and pack well with one another, thereby enhancing TPU physical properties (Schollenberger. 1969;Seafriedet al.. 1975).Aliphaticdiisocyanates yield morecolor-stable TPUs than aromatic diisocyanates do (Schollenberger and Stewart, 1972, 1973b). Because of the structural requirements, relatively few of the available diisocyanates have been used to make TPU. Table 2 lists some that are suitable.
5.2
Macroglycol
The TPU rnacroglycol component is a relatively large molecule of molecular weight 500-4000 (usually 1000-2000) whose requisite difunctionality permits the formationof long, strong, linear TPU chains. It comprises the bulk of derived TPU elastomers, often 50-80 wt%, and so has considerable influence on TPU physical and chemical properties (Schollenberger, 1969). Chain structural features such as uniformity and lack of appendages favor the association and packing of macroglycol chains in TPU and thus higher physical property levels. Conversely, rnacroglycol chain irregularity, appendages, etc., favor more amorphous TPUs with lower physical property levels.
Schollenberger
392
Table 2 SomeCommercialDiisocyanatesSuitableforTPUElastomcrs Abbreviation namc
chemical
MD1
Common
Structure
OCNDH
Methylene-bis(4-phenyl isocyanate) (diphenylmethane-p,/>,’-diisocyanate)”
H IMethylene ?MD1
bis(4-cyclohexyl isocyanate)”
pPDI
p-Phenylene diisocyanate“
NDI
1 ,SNaphthylene diisocyanatc“
pXDldiisocyanate“
p-Xylylene
OCN~CHz-@~cO
OCN QNCO
oc OCNCHG
’‘ Aromatic. h c
Cycloaliphatic Aliphatic.
Thermoplastic polyurethanes based on polyether glycols are considerably more hydrolysis- and fungus-resistant than thosebased on polyesterglycols (excepting polycarbonate glycols). Table 3 lists the types of macroglycols that could be used to make TPU elastomers and provides some specific examples.
5.3
Chain Extenders
The chain-extender component of TPU elastomers is again a relatively small difunctional molecule, usually a glycol, of molecular weight 100-350, about the same size as the diisocyanate component. In the TPU polymerization,thechainextender and somediisocyanatereact to produce urethane-rich hard segments that comprise regions of strong hydrogen bonding in the TPU polymer chains. The chain extenders used in TPU also make a pronounced structural contribution to polymer physical properties (Schollenberger, 1969). Some of the same generalizations as were discussed for the diisocyanate component apply to the chain extender; the chain extender should produce hard segments and urethane groups that associate and pack well. Short, open-chain structures free of appendages favor this, while chain extenders having compact, symmetrical, cyclic nuclei produce harder, higher-modulus TPU. Table 4 lists the types, with examples, of some TPU chain extenders that have achieved commercial importance.
-
6.
POLYMERIZATION PROCESSES
Basically,thereare two polymerisationmethods by which TPU may be prepared:thetwostep (“prepolymer”) process and the one-step (“one-shot”) process. The former involves the
393
Thermoplastic Polyurethane Elastomers Table 3 Some Commcrcial Macroxlycols Used to Make TPU Elastomers Abbreviation PTAd
Common chemical name
Structure
HOf(CH2~OCO(CH2tqC00]-(CH2~OH
Poly(tctramethy1ene adipatc) glycol"
n
€-cnprolactollc) PCL glycol" PHC
Poly(hexamethy1cnecarbonate) glycol'
PTMO
Poly(oxytetran1ethylcnc) glycol'l
PPG
1,2-oxypropylenc) Poly( glycol''
Table 4 Some Commercial Chain Extenders Structure
Abbreviation namc
chemical
Useful in TPU Elastomers
Common
I ,4-BDO
1.4-Butanediol"
CHDM
1.4-Cyclohcxanedimethanol" p-Xylylenc glycol'
HQEE
I ,4-Bis(2-hydroxycthoxy) benzene''
Schollenberger
394
preparation of a low molecularweight,linear.isocyanate-terminatedprepolymer[Eq. followed by its chain extension to a high molecular weight linear polynler (Eq. (&)l. nOCN-R-NCO Diisocyanate
+
(6a)l
~-
HO-R'-OH Macroglycol
OCN-R-(NHCO-O-R'-OCONH-R-)NCO
+
(n
- 2)OCN-R-NCO
Prepolymer Unreacted diisocyanate
l
(n
- 1)HO"R""OH Chain extender
ftNHCO"O-R"O-CONHR~~NHCOO-R"-OCONHR)-+
n-l x
In the first step [Eq. (6a)], the dry macroglycol and (excess) diisocyanate react in urethane link formation [Eq. ( l ) ] to produce isocyanate-terminated linear chains, which remain relatively low i n molecularweightand melt viscosity,thusfacilitatingsubsequentmixing with chain extender. The diisocyanate-macroglycol chain segments (singly underlined) comprise the ut-ethane-sparse soft segments in the TPU chains. In the second step [Eq. (6b)], added dry chain extender reacts with prepolytner terminal isocyanate groups in further urethane link formation to couple the prepolymer molecules and produce a high-molecular-weight TPU elastomer. If the prepolymer of Eq. (6a) contains free diisocyanate component, which can be achieved by raising the diisocyanate/macroglycol molar ratioto >2/1 i n the initial charge orby supplemental diisocyanate addition to the preformed prepolymer, a quasi-prepolymer results, whose reaction with equivalent omounts of chain extender [Eq. (6b)l produces TPU elastomers that are harder and of higher modulus due to their increased content of urethane-rich hard segment. In Eq. (6b). when the isocyanate content of the prepolytner and the hydroxyl content of the chain extender are charged to be equivalent. or when the latter is a bit greater. linear TPU results. In effect, the process has produced linear polyurethane chains that can be segmented, i.e.. of the +AB+,, structure, to any desired degree. But if somewhat less than an equivalent amount of the chain extender is charged with the prepolymer. then the excess isocyanate of the prepolymer can ultimately react with TPU-chain urethane groups in forming allophanate links [Eq. (4)1, providing branches on the TPU chains or chemical crosslinks among them. Such regulation of component stoichiometry allows the production of chemically crosslinked TPU if desired, e.g.. like Mobay's Texin. In the "one-shot" (single-step, random-melt-polymerization condition) process all of the TPU components are mixed together at one time. Here the alternating soft (singly underlined) and hard (doubly underlined) segments joined end-to-end through urethane linkages [Eq. (7)]. nOCN-R-NCO Diisocyanate
+
HO-R'-OH Macroglycol Chain
+
(n
- 1)HO-R'"- OH extender
As in Eq. (6b), the stoichiometric balance of isocyanatehydroxyl groups charged determines whethertheproduct will be alinearlystructured TPU (NCO/OH 5 1.0) as pictured. or an allophanate-branched and/or crosslinked TPU (NCO/OH > 1 .0).
Thermoplastic Polyurethane Elastomers
395
Just as the prepolylner process. TPU hardness and modulus can be increased by inci-easing the diisocyanate and the (balancing) chain-extender levels charged in the polynlerization, which, i n effect, produces more and longer diisocyanate-chain extender hard segments in the polymer. ,411 example of the one-shot batch process for preparing TPU is provided in the fOllOWing. A mixture of 1.70 mol of dry poly(tetramethy1ene adipate) glycol (M,,, 849; acid no.. 0.89) and 1.22 mol of dry 1.4-butanediol is mechanically stirred at I 10°C with a spiral ribbon stirrer in a 4-liter resin kettle heated with a Glas-col mantle. After about 10 min of such mixing, 2.92 mol of diphenylmethane-~,p’-diisocyanateis added cleanly in one portion. Vigorous stirring is continued for 1 min, and the reaction mixture is then poured into a lubricated I-gal can, which is promptly closed with a friction-fitting lid. The sealed can is placed in a 140°C oven for 3.5 hr to complete the polymerization and then cooled, and the snappy elastomeric TPU product is removed. It has 85 degrees Shore A hardness, mills satisfactorily at 225°F on a plastics mill, dissolves in N,N-dimethylformamide,and has excellentphysicalproperties (Schollenberger, 1959). Other details of TPU preparation and continuous commercial processes have been published (Saunders and Piggot, 1965; Bartel et al., 1971; Rausch and McClellan, 1972; Quiring et al., 198 I ) . The two latter references involve the use of a twin-screw extruder-reactor. An elegant way to study TPU melt polymerization is to conduct it in a small torque rheometer.where it is possible to followthereaction full termwhilecontinuouslyrelating; polymerizate viscosity to reaction temperature and time. Such studies of the one-shot polymerization process have been done using a C.W. Brabender Plasticorder torque rheometer as reactor (Schollenberger et al.. 1982). Polymerization variables investigated included the effects on the course and degree of polymerization of reaction temperature and time, macroglycol acid number, antioxidant. TPU composition, catalyst, shortstop, and reactant imbalance. Figure 1 shows the plot of typical results from this study when identical recipes were polymerized at 202°C (Polymers 6 and 7) and at ”222°C (Polymer 8).
7. CHAINSTRUCTURE,ORGANIZATION, ANDBEHAVIOR The urethane linkages in TPU chainsprovide ample opportunity for interchain hydrogen bonding. This occurs to different degrees depending on the groups involved, for example, between the urethane hydrogen atoms of one chain and the urethane and ester carbonyl groups or the ether oxygen atoms of adjacent chains (Boyarchuket al., 1965; Seymour et al., 1970). In polyurethane elastomer chains, urethane-urethane association through hydrogen bonding,with attendant ordering (aggregation) of the urethane units thereby (Boyarchuk et al., 1965), and the same greatly intensified process in segmented polyurethane elastomer chains (Cooper and Tobolsky, 1966) would seem to account for the elastic nature of TPU. The foregoing interpretations can be drawn together and stated as the “virtual crosslink” principle, which underlies the behavior of TPU: TPU elastomerpolymerization produces linear, segmented, polymer primary chains comprising alternating urethane-sparse, low-T, soft segments (diisocyanate-macroglycol linear reaction products) (A) and urethane-rich. high-T, hard segments (diisocyanate chain-extender linear reaction products) (B), which are connected end to end through urethane linkages. TPU chains thus have the structure (AB),,. These chains exhibit the customary, rather delocalized, weak van der Waals association forces for each other, and they are long enough to tangle with each other. But in addition, due to their high concentration of urethane groups, hydrogen bonding is particularly strong among the TPU-chain hard segments, and the association of aromatic T electrons when present supple-
396
Schollenberger
0
I
2
3
4
5
6
7
8
9
1 0 1 1 1 2 1 3 1 4 1 5 1 6
REACTION TIME(mlnutes) Fig. 1 Antioxidant-stabilized thermoplastic urethane melt polymerization viscosity-time-temperature relations. (O), Polymer 6; (X), Polymer 7, (A), Polymer 8.
1I
A
or SOLVENT
lb)
Fig. 2 TPU e1;lstomer chainstructure,organization,andbehavior:(a), network of polymer primary chains; (h), polymer primary chains.
virtually crosslinkedextended
Thermoplastic Polyurethane Elastomers
397
ments this attraction. Consequently, the hard segments associate to form aggregates (domains) in thesoft-segmentmatrix,which also becomesmoreself-associatedintheprocess. These phenomena give rise to detectable heterophase morphology in TPU. The aggregated hard segments accordingly tie the TPU chainstogether at localized points in lateral fashion and extend them linearly, producing a giant networkof chains andthus elasticity in TPU. However, although these crosslinks and extension links are effective under practical use conditions, they are reversible with heat and with solvation. This permits thermoforming and solution applications of TPU. As a consequence. this type of TPU linkage has been called a“virtualcrosslink” (VC)(Schollenberger et al., 1958,1962)andthenetworkproduced a “virtual network,” for they are crosslinks and networks “in effect but not i n fact.” The foregoing types of TPU chain structure, organization, and behavior are depicted schematically in Figure 2.
8. MORPHOLOGYANDTHERMALRESPONSES The return to the VC state on cooling after VC thermal disruption in a heated TPU sample has been shown to be a morphology-related, time-dependent, phase-separation phenomenon that is concurrent with and explains the redevelopment of mechanical properties on cooling and aging the heated sample (Schollenberger andDinbergs, 1975, 1978; Wilkeset al. 1975). Comparisons of VC network thermal disruption in VC versus CC-VC TPU (covalent-crosslinked TPU) show that the covalent crosslinks of the latter system impede but do not prevent the redevelopment of virtual crosslinks on cooling and aging (Schollenberger and Dinbergs, 1975, 1978; Ophir and Wilkes,1979). The hard-segment aggregates (domains) in TPU have been studied from several standpoints. In unmanipulated (unoriented. unannealed) samples of many useful TPU compositions, the hard segments do not registerwell, if atall, as crystallineregions in wide-angle x-ray diffraction studies (Cooperand Tobolsky, 1966; Wilkesand Yusek, 1973; Wilkes and Wildnauer, 1975; Wilkes et al., 1975). Inadequate crystallite size has been proposed to explain this or the difficulty in achieving a good crystalline lattice due to the impeding action of hydrogen bonding on segment mobility (Schollenbergerand Dinbergs.1975,1978;Schollenberger, 1979). The term “paracrystalline” (Hosemann andBagchi.1962; Hosemann, 1975)has been applied to thesemicrystallinestate of aggregated hard segments in unmanipulated,N-monosubstituted polyurethane elastomer samples (Bonart, 1968). In any case, small-angle x-ray scattering measurements clearly show the presence, size, and separation of hard-segment aggregates in relaxed TPU samples (Bonart, 1968; Bonart and Muller. 1974; Wilkes and Emerson, 1976; Ophir and Wilkes 1979, 1980). Thermal analysis by differential scanning calorimetry (DSC) shows their presence, thermal disruption (melting), and reaggregation (crystallization) on cooling, as can be seen in Figure 3 (Schollenberger, 1979). The TPU sample tested (at lO”C/min) in Figure 3 wasinjectionmoldedfrom a highurethane-content injection-molding polymer that contains MDI/PTAd soft segments and MDI/l, 4-BD0 hard segments. On heating note the -30.5”C T, value for the soft segment at C and hard-segment thermal disruption (melting) at 175.5-204.5”C between D and H, with a heat of fusion, A H,-,of 3.20 mcal/mg. Then on cooling from 250°C note the hard-segment reaggregation (crystallization) exotherm at 145.0-105.0”C between J and L, with a heat of crystallization, A H,. of 3.1 1 mcal/mg. The annealing and orientation of TPU samplescan develop and improve their crystallinity, making it easier to detect and to study both soft- and hard-segment domains by wide-angle xray diffraction. Such treatment (manipulation) has led to much useful information about the
Schollenberger
398
C -30.5"
I\ I
SAMPLE TEMPERATURE, "c
-
Fig. 3 DSC thermogram of an injection-moldingTPU.
crystalline character of the domains, including hard-segment packing. hydrogen bonding.conformation, density. and size (Blackwell and Lee, 1983; Born et al., 1984). Both the structure and the sizeof hard segments are factors in their effectiveness as virtual crosslinks in TPU. Good component symmetry, compactness, lack of appendages, rigidity, and long segmentsarefavorablefactors(Schollenberger, 1969; Seefriedet al., 1975). But it has been shown that hydrogen bonding is, i n fact, r w t a prerequisite for effective hard segments by the demonstration that certain N-disubstituted poly(ether-urethane)TPUs whose urethane groups bear no hydrogen, precluding hydrogen bonding, nevertheless exhibit elasticity and good strength (Hanell, 1969). In this type of TPU, whose structure is shown i n Figure 4, the hard segments
X
Fig. 4
Non-hydrogeu-bondiugTPUstructure.(FromHarrell,
1969.)
Thermoplastic Polyurethane Elastomers
399
again associate to form aggregates (VCs) in the soft-segment matrix, but these hard-segment aggregateshaveconsiderablecrystallinity (Harrell, 1969; Samuels andWilkes, 197 I , 1973; Allegreza et al.. 1974). possibly due to the absence of hydrogen bonding to impede segment ordering within their aggregates. Studies have indicated that hard segments of only one unit length (MW 228) can produce effective virtual crosslinks in TPU (Harrell. 1969; and Sanwels and Wilkes, 1973). So now we see that crystallinity alone, as well as hydrogen bonding, can produce virtual crosslinks. In addition. the VCprinciple has been extended to includeglassy (amorphous) hardsegment aggregates (VCs), as in the segmented, triblock A-B-A (e.g., polystyrene-polybutadienepolystyrene) type of thermoplastic elastomers and ionomers where the VCs are ionic (electrostatic).
9. MOLECULARWEIGHT EFFECTS Due tothe linearity of their primarychains (seeFig. 2). TPU molecular weights can be determined and related to physical properties (Schollenberger andDinbergs. 1973, 1974. 1979a.b; Scholleaberger 1979). This was done using gel permeation chrornotography for a series of TPUs (M, 48,000-367,000) all made according to the same recipe from MDI, PTAd,and 1,4-BDO (SchollenbergerandDinbergs, 1979a.b. Schollenberger 1979).The interestingresults of the study show that in many cases the change (enhancement or impairment) in TPU characteristics and properties with increasing polymer weight-average molecular weight(M,) continued to a certain M, level (in the range 100,000-200.000) and then tended to level off at an inflection molecular weight (IMW). The physical explanations advanced to account for IMW were the achievementof polymer chain average lengths that (1 ) favor a "balled" (less extended)chain configuration. (2) produce a virtual network of chains (due to increase in VC sites and entanglement opportunities per chain) that is unresponsive to furtherchainlengthincrease,and (3) have a chainend (free volume) content whose further reduction by molecular weight increase does not affect polymer internal (segment) mobility (and thus polymer morphology development) or polymer density. Results showed that as TPU weight-average molecular weight increased, the following polymer characteristics and properties increased (approximate IMWvalues givenin parentheses): specific gravity ( I 80.000), processing temperature by dynamic extrusion rheometer T2 value (200,000). T, value ( 1 60,000). tensile strength ( 1 45.000), abrasion resistance (Taber) and lowtemperature modulus Gehman Tloo( 1 12,000). and Clash-Berg T , ( 134,000). In addition, as TPUweight-average molecular weight (M,) increased, the following polymer characteristics and properties decreased: melt index (200,000), hysteresis (125,000). extension set ( 130,000). stress relaxation and flex life (deMattia) ( 1 80,000. with a maximum flex life at 100,000). TPU hardness varied little with M,, and neither hardness nor 300% modulus showed systematic dependence on MW. In contrast with the number-average molecular weight (M,,) versus intrinsic viscosity[q]relationships noted in an earlier study (Schollenberger and Dinbergs, 1973, 1974). the foregoing study showed no IMW in the MWversus [q]plot, but only increasing [q]with increasing Mw. Figure 5 shows the stress-strain plot for the TPU series wherein the individual polymers had the following M, values: A, 47,900; B. 60, 100; C. 100,200; D, 1 17,100; E. 182,800; F, 190,500; G, 248,700; H. 289,100; 1.35 1,200:J, 366,800 (Schollenberger and Dinbergs, 1979a,b). In Figure 5, the stress-strain curves steepen with increasing M, for polymers A-J. Sample tensile strength increases with MWto a maximum level in the group E-H. An increase in overall
-
400
Schollenberger
polymermoduluswithincreasing MW is quite obvious in Figure 5 . reflecting the effect of increasingly moreextensive chain networks, which alsoaccounts forthe progressively decreasing ultimate elongation of the polymer samples with increasing chain length MW.These relationships are attributed to the greater number of chain entanglements and the greater number of virtual crosslinks per chain in the longer TPU chains, both contributing to a more extensive and thus tougher and less extensible network of polymer chains. 10. CHEMICAL CROSSLINKING EFFECTS It would not seem that the covalent crosslinking (CC) of TPU would be of interest for these materials, so touted for their advantages as therrnoplastic elastomers. But the usefulness of TPU
401
Thermoplastic Polyurethane Elastomers
--CH,&
-CHPeroxidL -CH-CH-
~.
-CH
2
-
Methylenepositions in polyurethane chains chains
.
-CH_ _ _ _ L '
Freeradical positions in polyurethane chains
I
Carbon-carbon crosslinked polyurethane
Fig. 6 Covalent chemical crosslinking of TPU by organic peroxide.
can sometimes be increased by superposing a CC network on the existing VC network. This Can be done via free radical sources such as organic peroxides, which can be incorporated into ordinary TPU on a rubber or plasticsfriction roll mill (Schollenberger andDinbergs, 1975. 1978) or during TPU polymerization (Schollenberger and Dinbergs, 1981). Here peroxide decomposition tenlperatures must be compatible with TPU processing temperatures. A peroxide found useful for theseapproaches is a,a'-(bis-t-buty1peroxy)diisopropylbenzene supplied as Percadox 14 (Akzo Chemie; half-life at 100°C. 1000 hr; at 160°C, 0.32 hr) (Schollenberger and Dinbergs, 198 I). Solution mixing of TPU with peroxides is another option, allowing gentler mixing conditions. The peroxide-TPU compound, which may also contain a co-agent and other additives. is stable until the peroxide is activated, e.g., thermally, as during extrusion or injection molding, to provide polyurethane parts having both thermally reversible VC and thermally stable CC networks. The free radical crosslinking of TPU is considered to involve the generation of free radical positions in the polyurethane chains and the coupling of these positions to form carbon-to-carbon covalent chemical crosslinks as depicted in Figure 6. Sites suggested as likely points for free radical formation in polyurethane chains are the a-methylene positions in the adipyl moieties of adipate-based polymers (Urs, 1962: Weisfeld et al., 1962) and the methylene group between the two phenyl rings in the diurethane bridge structure of MDI-based polymers (Bayer and Muller, 1960). Thus, one can capitalize on the familiar high-speed thermoplastic processability of TPU while producing polyurethane products with enhanced properties, including greater resistance to solvation effects, stress relaxation, heat distortion, permanent set, etc. The CC-VC TPUsalso show higher modulus values but reduced extensibility, tear strength, low-temperature flexibility, and flex life-all expected consequences of a higher degree of polymer chain crosslinking. Figure 7 shows the stress relaxation behavior at 25 and 100°C of a (1.30) MDVPTAd (MW 1000)/1,4-BDOTPU in the uncured (V) and peroxide-cured (C) states.All, samples were preconditioned relaxed at test temperature for 10 min just before testing. At 25"C, the VC polyurethane (V) showed 50% stress relaxation in 1 hr versus 35% for the CC-VC polyurethane (C), showing the relativelyweak virtual crosslinking in this lowurethane-content TPU and the positive effect of covalent crosslinking. AtIOO'C, the VC polyurethane (V) melted and does not appear in Figure 7, whereas, the CC-VC polyurethane (C), withitsvirtualcrosslinksthermallydispelledattesttemperature but its thermally stable covalent crosslinks still intact and effective, showed only 15% stress relaxation. Virtual crosslink networkdevelopment in aged VC and CC-VC samples has been compared in stress relaxation tests (Schollenberger and Dinbergs, 1975,1978) and by other methods (stressstrain, wide-angle x-ray diffraction. differential scanning calorimetry) (Ophir and Wilkes, 1979).
402
Schollenberger
loo
-
90
-
a W
z
e 80v)
a a g ro-I
U
z
25"
2 a
z 600
S 50 -
V = VC POLY URETHANE C = %-VC POLYURETHANE
Fig. 7
\
X
25"
Stress relaxation (continuous, 20% extension).
Results show that virtual crosslink development progressed during long-term aging in both VC and CC-VC samples, but it was impeded by covalent crosslinking in the CC-VC TPU.
11. ENVIRONMENTALSTABILITY AND STABILIZATION Polyurethanes, including TPUs, although highly useful and serviceable materials,do havelimitations. Some of these are related to their instability in certain environments such as in the presence of water, ultraviolet radiation, microorganisms, and nitrogen dioxide. Here they occasionally must be stabilized with additives or by polymer chemical structural changes. On the other hand, polyurethanes show excellent resistance to certain other environments such as gamma radiation, ozone, andhydrocarbon oils and fuels. Atthis point TPU environmental stability and stabilization will be reviewed. 11.l
Hydrolysis
When carboxylate polyester-based TPUs such as those based on poly(tetramethy1ene adipate) glycol (see Table 3 ) are prepared, a few of the chains may terminate in unreacted carboxyl groups (-C02H) introduced with the macroglycol component. And when carboxylate polyesterbased TPUs hydrolyze, chain cleavage occurs at the carboxylate ("COO-) linkage, producing some new chains that bear terminal carboxyl groups. The carboxyl groups from both sources autocatalyze further TPU hydrolysis and accelerate this degradation process.
403
Thermoplastic Polyurethane Elastomers
\lW0
1,m0
0
I
I
I
I
2
3
I 4 IMMERSION TIME
I 5
n
6
IN 709: W4TER (WEEKS)
I
7
I
e
,0%
0-
9
Fig. 8 Effcct of added polycarhodilmidc on TPU hydrolysis stability.
Polycarbodiimides of the structure
n prove to be very effective hydrolysisstabilizers for poly(ester-urethane)s.Theyfunction by reacting with carboxyl groups asthey are generated, neutralizingthem and sinlultaneously mending cleaved chains in the hydrolyzing polymer. Figure 8 shows the effectiveness of 2.0 phr of a polycarbodiimide, Stabaxol P (Mobay, Bayer). in stabilizing an ordinary TPU against hydrolysis in 70°C water (Schollenberger and Stewart, 1971a, b. 1973a). Polyepoxides are also used as polyurethane hydrolysis stabilizers due to their ability to react with and neutralize carboxyl groups and to thereby also concurrently mend cleaved TPU chains (Kaiserman and Singh. 1976). Lactone polyester-based TPUs such as those made from poly(ecapro1actone) glycol (see Table 3 ) behave much like the carboxylate polyester-based TPUs with respect to hydrolysis stability and stabilization.In fact, the macroglycolester linkage in PCL is the carboxylate linkage, but the macroglycol is made by lactone polymerization rather than by dicarboxylic acid-glycol condensation. Carbonate polyester-based TPUs such as those based on. poly(hexan1ethylene carbonate) glycol (see Table 3 ) have excellent hydrolysis resistance (Muller, 1970). Hydrolysis of their ester linkage, the carbonate link ( - ~ O - O - ) , produces terminal hydroxyl groups on the cleaved chains and carbon dioxide gas. but no carboxyl ends to autocatalyze further hydrolysis (Schollenberger and Stewart, 19733).
Schollenberger
404
Polyether-based TPUs, such as those based on poly(oxytetran1ethylene) glycol (see Table 3), have outstandinghydrolysisstability, since the ether linkage of themacroglycol is very difficult to hydrolyze. In fact, the urethane linkage becomes the most susceptible linkage in regard to hydrolysis in poly(ether-urethane)s. Another significant factor in determining TPU hydrolysis stability is the degree of hydrophobic character and thepermeability to water of the TPU chains (Muller,1970; Schollenberger and Stewart 1971a. b; Schollenberger and Stewart, 1973; Dieter et al., 1974). Thus, the more hydrophobic (due, e.g., tohigh hardness) the TPU is, the less water it will absorb and the more hydrolysis resistant it will be.
11.2 Ultraviolet Radiation (Schollenberger and Dinbergs, 1961; Schollenberger; and Stewart, 1972, 1973b, 1976a,b) Perhaps the greatest factor in the terrestrial weathering of TPU is ultraviolet radiation in the wavelength range 330-410 nm. This energy, from incident solar radiation, initiates an autoxidative degradation processin TPU that can chemically crosslinkthe chains extensively, embrittling and insolubilizing the TPU, particularly aromatic urethane TPUs. In addition, aromatic urethane TPUs develop a pronounced yellow-to-brown color in the process, whereas aliphatic urethane TPUs are color-stable. The UV degradation process is believed to generate quinone-imide structures in aromatic urethanes having proquinoid structures [Eq. (g)] that can be followed by active hydrogen moiety addition to the quinone-imide that would crosslink TPU chains.
Quinone
Urethane
Imide +
H,O
Autoxidation of Urethane to Quinone-Imide A different view holds that the aromatic urethane UV degradation process involves photoFries rearrangements of the type seen in Eq. (9) to explain discoloring and crosslinking of TPU chains.
+o
NH -CO,R
___,
CO2 R
-Aminobenzoate o-ArninobenzoateUrethane then 2NH2
Amino group
0 2 --
-N=N-Azo group
+
H 0 2
Equations (8) and (9) are not possible in aliphatic urethane TPUs, which are nonquinoid structures. Both mechanisms are compatible with the fact that aliphatic urethane TPUs show better UV stability than aromatic urethane TPUs. There is little doubt that some hydroperoxidation of the TPU chains also occurs in the UV-initiated autoxidation process regardless of the structure of their urethane groups. In view
Thermoplastic Polyurethane Elastomers
405
of the nature of the TPU UV degradation process, it is not surprising that a combination of a UV absorber, 2-(2’-hydroxy-3’,5’-di-r-amylphenyl)benzotriazole(Tinuvin 328, Ciba-Geigy), and an antioxidant, tetrakis[methylene-3-(3’,5’-di-r-butyl-4’-hydroxyphenyl) propionatelmethane (Irganox I O I O , Ciba-Geigy), proves to be a particularly effective UV stabilization system for TPU (Schollenberger and Stewart, 1976a,b). In addition, low levels of carbon black alone, acting as a UV screen, are extremely effective UV stabilizers for TPU (Schollenberger and Dinbergs, 1961). 11.3 Microbial Attack
Some polyurethanes, including TPUs, are subject to microbiological attack (B. Pat., 1958, 1972; DiPinto, 1963; Kanaval et al., 1966; Darby and Kaplan, 1968; Kaplan et al., 1968; Elmer, 1970; Huang, 198 1 ). Both poly(ester-urethane)s and poly(ether-urethane)s will support microbiological growth, but only the poly(ester-urethanels are degraded by it. Fungi are commonly involved, and damagetakes the form of surface cracks (“fungus cracks”) that penetrate the sample. These cracks usually eventually proliferate and become obvious to the unaided eye in stressed areas (bends, folds. etc.) of infected samples. They areusually not attended by polymer discoloration. in contrast to microbial attack involving soil exposure, which often discolors the TPU. Factors favoring microbiological attack on TPU are those that favor microbial infection and growth. These include outdoor exposures above, on, or below ground to damp, warm, dark environments. If these conditions are met indoors, microbiological attack can also occur there. Various chemical compounds that find use as microbicides in polyurethanes, including TPU. include Cunilate DOP (copper-8-quinolinolate) and Fungitol 11 [N-(trichloromethylthio)phthalimide], both from Nuodex Products Division, Heyden Newport Chemical Corp.; Vinyzene SB-I [ 10, IO’-bis(phenoxyarsine) from Ventron Div., MortonlThiokol Chemical Corp.]; TMTD (tetramethylthiuram disulfide); TBTO [bistri(n-butyltin) oxide];chlorosulfonyl (sulfinyl pyridine) compounds; and carbodiimides. Troysan Polyphase AF-I (3-iodo-2-propynyl butyl carbamate, Troy Chemical Corp.) might be especially suited for use in polyurethanes, including TPU, due to its urethane structure.
11.4 Gamma Radiation Early studies (Harrington, 1957, 1958, 1959; Schollenberger et al., 1960) comparing the gammaradiation resistance of many types of high molecular weight materials showed polyurethanes to be among the most radiation-resistant of polymers and polyurethanes the most radiation-resistant elastomer tested (Bauman). The tendency is for polyurethanes, including TPU, to both chaincleave and (predominantly) crosslink during gamma radiation, but the latter appreciably less than other elastomers, so original properties are better retained. As a consequences. TPUs have found use in applications involving nuclear radiation. 11.5 Other
Polyurethanes, including TPUs, undergo thermal autoxidation (Singh et al.. 1966; Fabris, 1976). This was clearly seen in oxygen uptake experiments with a poly(ester-urethane) TPU at 130°C, which also showed the antioxidant DPPD (diphenyl-p-phenylenediamine) to be highly effective in suppressing oxygen absorption. Surprisingly, EPC carbon black was not far behind in effectiveness (Schollenberger and Dinbergs, 196 1 ). Poly(ether-urethane)s thermally autoxidize more readily than poly(ester-urethane)s.
406
Schollenberger
Polyurethanesundergosimultaneouscrosslinkingandchainscission when exposedto nitrogen dioxide (NO?),with the latter predominant in long exposures. The degradation is accelerated by air in mixtures with NO?. Losses in polyurethane strength accompany the degradation (Jellinek and Wang. 1973; Jellinek et al.. 1974). Tests on atypicalpoly(ester-urethane) TPU showed it to haveexcellentresistance to degradation by ozone (Schollenberger and Dinbergs, 1961 ). The polar nature of polyurethanes, including TPU and especially poly(ester-urethane)s, renders them notably resistant to aliphatic hydrocarbon oils and fuels (Schollenberger and Dinbergs. 1975, 1978).
12. COMPOUNDING Compounding (Schollenberger, 1969; Schollenberger and Esarove. 197 I ) is practiced to stabilize, aid in the processing of, extend, reinforce, or plasticize TPU. Stabilizatiotl was just discussed: it involves only minor amounts of additives ( 1 -5 phr), as does processing. However, the other cases usually involve large amounts of additives (10-100 phr). TPU compounds can be mixed on plastics or rubber mills, in a Banbury mixer, in situ during TPU polymerization, or in solution. Additives should be dry, and the best are neutral, stable materials that are not reactive with the forming or finished TPU either when they are added or later. The TPU polymer itself should be dry for the mixing operation. TPU processing aids are added as release agents for calendering and molding operations and to increase extrusion rates and reduce blocking tendencies in extrusion compounds. They includelubricantssuch as powderedpolyolefins(e.g., low molecularweight polyethylene), synthetic waxes (e.g.. Ross Wax stearamide types), and natural waxes (e.g., bees wax). Extendersareadded to TPU primarily to lower thecost of articlesmade from TPU. Secondary effects usually include increased compound hardness, modulus, and tear strength, and sometimes improved processability. On the other hand, tensile strength usually declines: at the 25-phr level, extenders may reduce TPU tensile strength by 20-309’0. Extenders useful in TPU include carbon black and silica (preferred because they reduce tensile strength the least), whiting, clay. and others. Mixtures with other polymers are possible and provide some interesting and useful blends. Examples of blendable polymers are vinyl polymers (e.g., PVC), vinyl copolymers, copolymer nylons, ABS, polycarbonates, polyolefins (limited amounts), and a wide range of elastomers. The effect on TPU properties is dependent on the blending polymer. In general, the hardness. modulus, and elongation move toward that of the blending polymer with increasing amounts of it. The effect on tensile strength depends on the degree of compatibility of the blending polymer with TPU. Table 5 shows some property changes i n a typical TPU [MDIPT Ad (MW 1000)/1. 4BD01 due to the addition of 100 parts of a vinyl polymer, a phenoxy, and a nitrocellulose per 100 parts of the TPU. The TPU andblendingpolymerwere mixed in methyl ethylketone solution, and test pieces were cut from 3- to 5-mil-thick film cast from the solution. The term “reinforcing agent” originated in the rubber industry, where gum rubbers have low strengthbeforeand after cure (covalent chemical crosslinking) but very good strength, abrasion resistance, tear strength, etc.. after mixing with the preferred reinforcing agent, carbon black, and cure. Such a type and degree of improvement is not seen in TPUs, which, due to virtual crosslinking, are very strong to begin with and are not further strengthened in the established sense by adding “reinforcing agents,” which act more like fillers in TPU.
407
Thermoplastic Polyurethane Elastomers Table 5 Properties of Somc TPU-Plastic Polymcr Blends 300% Modulus
Tcnsile strength Addcd plastic
psi
MPn
None Vinyl“ Phenoxy” Nitroccllulosc‘
5300 6200 7300 8100
36.5 42.7 50.3 55.8
Elong. ( 8 )
730 350 400
< 100
Graves tear strength
psi
MPa
pli
kN/m
450 5 100 6400
3.1
35.2 44.1
-
-
200 400 570 620
3.5.0 70.0 99.8 108.6
Table 6 shows the effect of several levels of EPC carbon black on some properties of a typical TPU [MDIPTAd (MW 1000)/1. 4-BDO]. Mixing was done on a rubber mill. and test samples were 75-mil-thick microdumbbells (Schollenberger, 1969). The preferred method of regulating TPU properties (e.g.. of reinforcing or softening them) is to tailor the polymerchemical structure during TPUsynthesis. This has been termed “chemical compounding.” As discussed earlier, there are numerous ways to do this, including the adjustment of urethane content upward for harder. higher-modulus. tougher polymers and downward for softer. lower-modulus. more soluble polymers. The use of longer. less regular macroglycols is another route to softer TPUs. Dibenzoates, phthalates. and organic phosphates have been used to plasticize TPU. but in the writer’s experience the phosphates tried have degraded any polyurethane. including TPU. in which they have been incorporated. It can be appreciated that plasticization (solvation) of TPU could weaken virtual crosslinks and the virtual network in the polymer. This, together with the migratory. fugitive nature of many plasticizers. recommends a skeptical attitude with respect to plasticization as ;I means of softening TPUs in the place of “chemical compounding.”
13. PROCESSING Thermoplastic polyurethanes can be processed by many methods that are familiar to the plastics and rubber industries. addingto the attractiveness and acceptance of these materials. They include
Table 6 Effect of EPC Carbon Black on TPU Propcrties ~
300% Modulus
Black Tensile strength
0 5 10 25 50
~~
Shore hardness (deg.)
Elot~g.
COIIC.
(phr)
Graves strength tear
psi
MPa
(%v)
5200
35.9 35.2 36.9 22.8 22.4
630 630 630 470 320
5190
5350 3300 3250
psi
MPa
pli
1000 1300 1700 2600 3200
6.9 9.0 11.7 18.6 22.0
420 470 550 640 760
kN/m
55 74 82 96 73 112 133
A
88 90 92 95 98
C
59 62 85
Schollenberger
408
extrusion, molding [compression, injection, blow, transfer, centrifugal (powders)], calendering, coating (extrusion, melt, transfer, solution), film blowing, sealing (heat, solvent), etc. Many widely used plastic materials including TPUs absorb some moisture from the air. and, although supplied dry as granules or pellets, TPUs and their compounds are best dried at 105°C for 2 hr in a tray or a dehumidifying hopper-dryer before processing from open bags (B. F. Goodrich. 1984). The abbreviated discussion that follows is intended to give the reader some feel for the conditions employed in processing typical commercialTPUs and is limited to extrusion. injection molding. and calendering.
13.1Extrusion
(B. F. Goodrich,1984)
The effective extruder barrel length should be at least 24 times its internal diameter (L/D = 24/1), but higher L/D ratioshavebeenusedsuccessfully. Screwdesign shouldincorporate the following principles: single-stage, constant-taper construction; 3/1 compression ratio; long transition section (30-40% of screw length); long meter section (40-50% of screw length); hard chrome. pinhole-free, polished surface; cored for temperature control; screw-barrel linear clearance, 0.003-0.005 inch. In the extrusion process the proper melt temperature will range between 165 and 200°C depending upon the particular equipment used, the compound selection, and other processing parameters. Harder TPU compounds will require higher processing temperatures than the softer compounds.
13.2 Injection Molding
(B. F. Goodrich, 1985a)
Although all types of machines have been used successfully, a reciprocating screw machine is preferred for injection molding Estane TPU because of the speed, control. and melt uniformity it provides. A shot weight of 60-75% of barrel capacity is recommended, but shot sizes as low as 30-35% can be accommodated withadjustments. The besttype of screw is a"generalpurpose" screw with a compression ratio of211 to 311 and a 60" included angle tip with an antibackflow mechanism (ball-check or sliding-ring type). It should rotate at about 40-50 rpm. A straight, open nozzle with a reverse-taper tipis recommended. Any type of mold that incorporates good thermoplastic design principlesis satisfactory for Estane TPU.Two-plate, three-plate, and hot runner molds have all been used successfully for a variety of large and small parts. The mold must also be adequately cored for cooling to 10-44°C to provide optimum cycles. The recommended stock temperature for injection molding is 192-210°C but varies with the Estane TPU. Temperatures to 232°C are satisfactory for molding large parts. It is recommended that injection-molded TPU parts be held for 48 hr before exposure to elevated temperature (such as the 122°C that would be encountered in a paint-curing oven) to relieve molding stress and avoid heat distortion of the parts. Table 7 summarizes injection-molding information for a typical poly(ester-urethane) TPU, Estane 58206, which has a Shore A hardness of 85 degrees (B. F. Goodrich. 1985a).
13.3 Calendering TPU can be calendered (Schollenberger, 1969; B. F. Goodrich. 1964) in much the same way as other thermoplastic polymers adapted to this form of processing. In general they are readied for calendering by mastication in an internal mixer (e.g., a Banbury mixer) or on a plastics or rubber mill at stock temperature of 140-170°C. The high frictional heat generated during TPU mastication enables production mixing equipment to be set lower, at 100- 120°C. When lubri-
-
409
Thermoplastic Polyurethane Elastomers
Table 7 Injection-MoldingConditionsforEstane
BUITI temp., "C Rear Mid Front Nozzle Melt temp.. "C Mold temp., "C Fill rate Screw rate. rpm Back pressure, psi/MPa Injection pressure, psi/MPa Molding pressure, psi/MPa Mold shrinkage," in./in. or cm/cm
58206 TPU 177 188 199
204 203 10-32 Slow to modcrate 20-50 50/0.345 3000-8000/20.7-55.1 2000-5000/13.8-34.5 0.0 I2
cant levels above 1% are used. somewhat higher equipment temperatures are required. Calender roll temperaturesare usually 120- 150°C depending on sheet thicknessand compounding ingredients. Calendered sheet in the thickness range 3-60 mils (0.008-0.152 cm) has been produced. Close control of roll and stock temperatures is necessary to calender the thinner gauges, while steady feed and stock temperatures are important in getting uniform heavy gauge sheets. Table 8 lists the conditions that were used to calender a typical TPU in an "inverted L" calender configuration.
-
14. APPLICATIONS
The ease of processing TPUs by numerous familiar methods; their outstanding physical properties. which enable effective use in thin cross sections; the aesthetics they allow in products; their persisting novelty; and their proven value and acceptance have all combined to generate a great number of applications for TPUs, and the number continues to grow. The followinglist gives some idea of the versatility of TPUs in their applications.
Table 8 ConditionsforCalendering TPU Compound Stock tempcraturc. "C Banbury mixer (at drop) Preparation mill Calender temperature, "C Offset roll Top roll Middle Bottom roll
;I
Typical
168-177 17 I 124 130
135 140
41 0
Schollenberger
Arrtorwti\*e: Fascia, sight shields, bumpers. wing nuts Petrochemicrrl: “Pigs,” liners, jackets. hose, vapor barriers (storage tanks) Grrrphic r u t s : Printing rolls Apl~c~rel: Coats. jackets. rainwear, handbags Footnwr: High-fashion boots, shoe uppers, soles. heels. heel plates, ski boots Sports: Golf ball covers. golf club and racquet grips. footballs and soccer balls. rollerskate wheels. skateboard wheels, life jackets. life rafts Medicrrl: Catheters, prostheses,bloodbags.tubing.surgicaldrapes.disposablegloves. suction bulbs, hypothermia pads. lapidous pads (light, thin. inflatable. anti-bed-sore pads) Hygiene: Disposable diapers (elastic leg bands) Agriculture: Cattle ear tags Furniture: Upholstery Aerosprce: Deicer boots, erosion protection (rain. hail. sand). fuel tanks, window gaskets (satellites). aircraft interior parts (arm rests. floor track inserts), weather ballons. logging ballons lr~t1~rstrir~Iyrotl~rct.s: Chute liners. pipe liners, conveyor belts (including food-grade),cleats. powertransmissionbelts,seals.gaskets, hose (jackets and liners for garden and fire hoses) tubing. rolls. film, sheet. profile extrusions. sewer rehabilitation sleeves, adhesives (solution), adhesives (film forfusing emblems togarments, wear patches), caulks.paintsandcoatings, pipe thread caps, silent gears, flexiblebacking pads (sanders). water sprinklers (cutoffunit). oil pouches, pipeline wraps. bellows(spring, gasoline pump nozzle),shopping carts (wheels,bearing housing). washing machines (ball hinge). tarpaulins. coated paper, coated metal and foil E/Pctronic.s: Magnetic tape binders. w/c jackets, retractile cords. cablejackets (audio. lowtemperature, underwater, underground. power). fiber optics inner and outer jackets. computers, communications. connectors, plugs Energy: Cable jackets (mining, geophysical, nuclear radiation) TrNnsl’ortcrtion: Covers. dunnage bags, OTR vehicles (bushings. bearings) Militc1r;y: Gas masks. capes. footwear. drop bags (potable water).collapsible storage tanks Marine.: Ship repair (retrofit conduit)
Table 9 Commercial TPU Suppliers United States Company
Albis
Foreign Product
Company
B. F. Goodrich Estane Dcsmopan Bayer Dow BASF Mohay Texin Polyurethane Nippon Ind. American Cyanamid Cyanaprenc, Dainippon Cytor Ink CYC Chem. K. J. Quiun Q-ThaIIe Nippon Elastollan Ind. Morthane Morton-Thiokol Plastlcs (UK) Hookcr
Product Elastollan Paroprene
Pandex Elastollan Jectothane
P D
v)
F D 0 Y
Table 10 TPU Product Comparison Chart BFG estane Polyester Property
ASTM method
Units
58206
Shore hardness Specific gravity Ult. tensile str.
D2240 D 792 D 412
deg g/cm3 MPa
85A 1.20 45
-
psi
Tensile stress At 100% elong.
D 412
At 300% elong. Ult. elongation Compression set" (22 hr at 70°C) Flex modulus at 23°C Vicat soft. temp. (method B) Taber abrasion CS 17 wheel, I -kg load HI8 wheel, I-kg load H22 wheel, I-kg load Tear resist., die C Mold shrinkageh Price (list, T. 1. qty)
-
-
-
-
D 412 D 395 D 790 -
D 1525
Dl044 D 624
-
-
MPa psi MPa psi 9c
7c
MPa psi "C "F mgll OOO cycles
-
kN/m pli idin. $/lb
6500 -
8.5 800 10 1500 550 64
-
85 I85 3 36 -
86 500 0.012 2.13
f
4
Polyether
58137 70D 1.23 40 5800
-
22 3200 33 4800 440 SO 33 1 48,000 149 300
12
119
-
228 1.300 0.005 2.11
58300 80A 1.13
32
58810
6400
-
9
-
76 169 I
(D
m 9ON42D 1.15 44
4600 4.8 700 6.9 1000 700 78
D 3
53 $
1300 17 2400 590 64
111
232
-
-
36 -
70 400 0.016 2.70
70
88 500 0.014 2.75
2 .A
Table 10 continued Dow (Upjohn) Pellethane Polyester
2 102-8SA
Property ~
~~
~~~
Mobay texin Pol yether
2355-65D
-
2
ro
Polyester
2 103-80A
2103-90A
480A
80A ? 5 1.13 41 6000
9OAi47D 1.14 43 6250
86A 1.20 40 5800
6 800 12 I675 550 25-40
I1 1530 24 3430 475 25-40
-
-
902B
~
87A k 5
Shore hardness Specific gravity Ult. tensile str.
63D k 4 1.22 41 5900
1.18
43 6300
Tensile stress At 100% elong.
22 3200 31 4500 450 50 269 39,000
8 1100
At 3 0 0 4 elong.
15
2100 600 30
Ult. elongation Compression set:' (22 hr at 70°C) Flex modulus at 23°C Vicat soft. temp. (method B) Taber abrasion CS17 wheel, I-kg load HI8 wheel, 1-kg load H22 wheel. 1-kg load Tear resist., die C
-
5 700
37 5400
11
-
1600 520
55 55 80oO 91
-
Mold shrinkageh Price (list. T/i. qty)
I96 2.7 -
15 245
50 88 500
I400
-
-
2.42
20 83 475 2.53
2.04
~
.' BFG samples unannealed: Dow (Upjohn) and Mobay samples annealed 16 hr at 240°F. Mold shrinkage determined on 0.125 x 3 x 6 molded plaques, Actual shrinkage will vary with part size and design Part thickness. in. 80A 55D
' Mold shrinkage values for Pellethane:
0.125
0.250 >0.250
0.01 1-0.015 0.015-0.020 0.020-0.030
'0.120-in. wall thickness
0.008-0.01 1
0.010-0015 0.0 15-0.020
10 95 540 -
2.50
75D 1.21 41-48 6000-7000
94 535 0.008" 2.19
150- 175 1068 155,000 I82 360
-
2.19
Thermoplastic Polyurethane Elastomers
413
15. COMMERCIAL POLYMERS ANDTHEIR PROPERTIES There are several commercial suppliers of thermoplastic polyurethanes. A fairly complete list of these appears in Table 9. Table 10 lists the physical properties of several typical commercial TPU polymers to provide the reader with some idea of the rangeof property levels available. However, there are many other intermediate polymers whose properties lie between the extremes appearing in Table
10.
REFERENCES Allegreza, A. E., Jr.. Seymour, R. W., Ng. H. N., and Cooper, S. L. (1974). P o / w e r /5:433. Bartel, G. F., Klnwittcr, M,, and Denker, E. (197 I ). U.S. Pat. 3,620,680, to Die Kunststoffburo Orzabruck, Dr. Rester, Nov. 16. Baumnn, R. G., unpublished, B. F. Goodrich Res. Ctr., Brecksvillc, OH. Bayer. 0. (1947),.Arl,qew.Chrrr~.59(9):257. Bayer, 0.. and Mullcr, E. (1960), A r ~ ~ q Chrrrz. e ~ ~ . 72(24):934. Bayer, 0..Muller, E., Peterscn, S., Piepenbrink, H. F., and Windcmuth, E. (1950), Ruhher Cllrrrl. T r c h o l . 23(4):812. Blackwell, J.. and Lce. C. D. (1983), J. Po/yrn. Sci.-Phqs. 212169. Bonart, R. (1968),.J. Mrrcmrrrol. Sci.-Phys. B2( 1):l 15. Bonart, R., and Muller, E. H. (197.4). J . Mlrcrorlrol. Sci.-P/ry.s. B/0:345. Born, L., Crone, J.. Hespe, H., Mullcr, E. H., and Wolf, K. H. (1984), J. PO/WI.Sci.. Phys. Ed. 22: 163. Boyarchuk, Y. M,,Rappaport, L. Y., Nikitin, V. N., and Apukhtina. N. P. (1965), Po/yrr~.Sei. USSR 7: 859. British Patcnt (1958). 707.575, to Farbenfahriken Bayer, July 2. British Patent (1972). 1,274,145, to Mobay Chemical Co., May 10. Cooper, S. L., and Tobolsky, A. V. ( 1966), J. A/>/)/.PO/JWI.Sei. /0:1837. Darby, R. T., and Kaplan, A. M.( 1968), App/. Micro/io/. f6(16):900. Dieter. J. A.. Frisch, K . C.. Shanafelt. G. K., and Devanney, M. T. (1974). Ku/&r Atgo /06(7):49. DiPinto, J. G. (1963), Tech. Bull., Fungus Resistance of Adiprene L-100, E. I. Dupont de Nemours and Co., Mar. 22. Dreyfuss. P,. Fctters. L. J., and Hansen, D. R. ( 1961), RLt/>berCkurtI. Techrlol. 5 4 1): 18 1, Elmer, 0. C. (1970). U.S. P a t . 3,513.433, to the General Tire and Rubber Co.. Sept. 29. Fabris, H. J. ( 1976), Adv. Urethrrrle Sci. Trchrlol. 4 8 9 . B. F. Goodrich (1964). Tcch. Bull.. Estane Sheet and Film (Nov.). B. F. Goodrich. B. F. Goodrich (1984). Estnnc Tech, Bull. ES-20, B. F. Goodrich Chem. Group. B. F. Goodrich (1985a), Estane Tech. Bull. ES-1 I , B. F. Goodrich Chcm. Group. B. F. Goodrich (1985b). Estane Tech. Bull., Product Comparison Data, B. F. Goodrich Chem. Group. B. F. Goodrich (198Sc), Estane Markcting Group private communication, (September. B. F. Goodrich Chem. Group. Harrell, L. L.. Jr. (1969), Mrrcrorrlo/rcrr/r.s2(6):607. Harrington, R. (1957), R d h r Age (N.Y.) 82(3):461. Harrington. R. (1958). Ruhher Age (N.Y.) 82(6):1003. Harrington, R. (1959). Ruhbrr Age (N. Y.) 85(6):963. Hepburn, C. ( 1982). Polytrrrthrrrlr E/tr,storrwrs. Applied Science Pub., London. Hoseman, R. ( 1975),J. Po/yrrl. Sei. S w p . 50:265. Hosemm R., and Bagchi, S. N. (1962). Direct Ar~cr/y,si,sof'D(ff'r~tior1 /?>l Mtrtter, Narth-Holland publ., Amsterdam. Huang. S . J. (1981). Biodegradation of polyurcthancs. Lecture, Polymer Conf. Ser., UIliv, Detroit, JuIlc, . Cherrl. Ed. 1/:3227. Jellinek, H. H. G., and Wang. T. J. Y. (1973). J. Po/yrrt. S C ~Po/SVI.
Schollenberger
414
Jellinek. H. H. G., Martin, F., and Wegner, H. (1974), J. Appl. Polym. Sei. /(1(6):1773. Knisernwn. S., and Singh, A. (1976), Br. Pat. 1,425,529, to the American Cyanamid Co., Feb. 18. Kanaval. G. A., Koons, P. A., and Lauer, R. E. ( 1966), Rlt/Awr Chprrr. Trc/v~o/. 39(4):1338. Kaplan, A. M., Darby, R. T.. Greenberger, M., and Rogers, M. R. (1968), L h v / o p . / d . Mj(.r.o/>jo/.9:201. Kogon, I. C. ( 1958), J. Org. Cherrr. 23:1594. Kogon. 1. C. (1959). J. Org. Chrm. 2493. Mobya Chem. Co. ( 1968). An engineering handbook for texin urethane elastolnertc matertals. technical bulletin. Mobay Chem. CO. (1985). Textn Marketing Group, private communication. September. Muller, E. (1970). Arrgrw. Mnkrorrlol. Chrrrl. /4(203):7.5. Nakayama. K., 1110. T.. and Matsaburo, I. (1969). J. Mocron~ol.Sci. Chrrn. A 3 ( 5 ) :1005. Ophir. Z. H., and Wilkes, G. L. (1979). in M~tlriphctsrPolyrnrrs S. L. Cooper and G. M. Estes, Eds.), American Chemical Society, Washington. DC, p. 53. Ophir. Z. H., and Wilkes. G. L. (1980), J. Po/w. Sei. Polym. P h y . Ed. /8:1469. Quiring. B., Niedcrdellmann. G., Goyert, W., and Wagner, H. (1981). U.S. Pat. 4,245,081, to Baycr A. G.. Jan. 13. Rausch. K. W.. Jr., and McClellan, T. R. (l972), U.S. Pat. 3,642,964. to the Upjohn Co., Feb. 1.5. Snmuels, S. L.. and Wilkes, G. L. ( 1971a). Polyrr~.Prep., D;\,, Polyr~r.C / ~ I Am. . , C / ~ r mSoc., , 12, No. 2694. Samuels, S. L., and Wilkes, G. L. (1971). J. Polyrr~.Sci. BY:761. Samuels. S. L.. and Wilkes, G. L. ( 1973). J. Poly~rr.Sci. Syrrrp. 43: 149. Saundcrs, J. H., and Frisch, K. C. (1962). Poly~rrrthcrrles:Cltrrristry c t r d Trclrrwlo,qy. Part I, Chrrr~istry. Wiley-Intersciellce, New York. Saunders. J . H.. and Piggot, K. A. (1965). U.S. Pat. 3,214,411, to the Mobay Chemical Co., Oct. 26. Saytgh. A. A. R.. Ulrtch, H., and Fanissey, W. J. (1972). in Corlderlsrttiorl M o r t o r r w ~ s(J. K. Stillc, Ed.), Wiley, New York, Ch. 5. Schollenbergcr. C. S. (1959). U.S. Pat. 2,871,218, to the B.F. Goodrtch Co., Jan. 27. Schollenberger, C. S. (1969), in Polyurrrhrrrw T c d ~ r ~ o l o g(P. y F. Bruins, Ed.), Wiley-Interscience, New York. p. 197. Schollenbcrger. C . S. (1979). in Mlrltiphcrse Polyrwrs (S. L. Cooper and G. M. Estes, Eds.). American Chemical Society, Washington, DC. Ch. S . Schollenhcrger. C. S., and Dinbergs, K. (1961), SPE Trtrus. l(l):3l. Schollenbergcr. C. S., and Dinbergs, K. (1973), J . EI~~stoplrr.st.5:222. Schollenhcrger, C. S., and Dinbergs, K.(1974), U r r t l ~ ~Sci. r ~ cT r c h o l . k36. Schollcnberger. C. S.. and Dinbergs, K. ( 1975). J. Ektst. P h t . 7:6.5. Schollenberger, C. S.. and Dinbergs, K. (1978), Ad\,. Urrthtrr~eSei. Trchrwl. 6:60. Schollcnherger. C. S . , and Dinbergs, K. (1979a), J. Elast. Pltrsr. 1 1 5 8 . Schollenbcrger, C. S.. and Dinbergs, K. (1979b). A d , . U W I I I L ISci. I I ~ Trc/rrrol. 7 1. Schollenberger. C. S.. and Dinbergs, K. (1981). U.S. Pat. 4,255.552, to the B.F. Goodrich Co., Mar. IO. Schollenhcrger. C. S.. and Esarovc, D. (1971), 111 Tlw Scirrrcr crrrrl Trchrrolo~yyof Polyrrrrr Films, Vol. I1 (0.J. Sweetmg, Ed.). Wiley-Interscience, New York, p. 487. Schollenhergcr. C . S., Scott, H., and Moore. G. R. (IO%), Kublwr World 137(4):549. Schollcnhergcr. C. S.. Pappas, L. G., Park, J. C.. and Vickroy, V. V., Jr. (1960), Krrbhrr World /42(6): Ad19.
81.
Schollenbergcr. Schollenhergcr. Schollenberger. Schollenbergcr, Schollenbcrger. Schollenherger, Schollenberger, Schollenherger. Schollenhcrger.
C. S,, Scott. H.. and Moore, G. R. ( 1962), Rrtbhrr Cllcw. Trchrfol.35(3):742. C. S., Dinbergs, K.. and Stewart, F. D. (1982). Krthhrr Chew. Trchrrol. 54( 1 ): 137. C. S . , and Stewart, F. D. (197 la), J. E/o.sfop/rr.st. 3:28. C. S., and Stewart, F. D. ( 1971h), Arbs. UrrfIfctwSci. Trchrlol. 1:65. C. S.. and Stewart. F. D. (1972), J. Eltrstopltrst. 4294. C. S.. and Stewart. F. D. ( 1973a), A ~ ~ y c wMerkrorrlol. '. Clwru. 29/-?0:413. C. S., and Stewart, F. D. ( 1973h), Ad\,. U r r f h r r Sci. i"cc/~~~ol. 2:71. C. S., and Stewart, F. D. (1976a). J. Elrrst. Plrrst. 8:1 I . C. S., and Stewart. F. D. ( 1976b). Adv. Urrthctrw Sci. Trc.l~r~ol. 4:68.
Thermoplastic Polyurethane Elastomers
41 5
Seefried, C. G., Jr., Koleske, J. V., and Critchfield, F. E. (1975). J. A/>/>/.Po/yrr~.Sci. (Cherrr.) 19( 12): 3185. Seymour, R. W., Estes, G. M,, and Cooper, S. L. (1970), Mmrorrrol. 3(5):579. Siefken. W. ( 1948), Arrrr. C/rem. 562(2):75. Singh, A., Weissbcin. L.. and Mollica, J. C. ( 1966). Rrthher A , p (Dcc.):77. Urs, S. V. (1962), O r d . B r g . Prod. Res. Dr~vlop./ ( 3 ) :199. Weisfcld. L. B.. Little. J. R., and Wolstenholme, W. E. (1962), J. Po/yrrr. Sci. 56:455. Wilkcs. C. E., and Yusek, C. E. (1973). J. Mrrcrorrrol. Sci.-Phys. B7( 1 ):157. Wilkcs, G. L., and Emerson, J. A. (1976). J. A/)/)/. Phys. 47(10):4261. Wilkes, G. L., and Wildnauer, R. (1973, J. A/>/>/.Phys. 46:4148. Wilkes, G. L., Bagrodin, S., Humphrics, W.. and Wildncucr, R. ( 1975). J . Po/yrrr. Sci. Po/yrrr. let^. Ed. /3:321.
This Page Intentionally Left Blank
Thermoplastic Polyamide Elastomers
Thermoplastic polyamide elastomers consist of a regular linear chain of rigid polyamide segments interspaced with flexible polyether segments. The general formula may be written as
HO-
Ls
C -P A - C - 0 - P E - 0
II 0
1
In
-H
where PA representsthe polyamide segmentsand PE thepolyether segments. These are basically segmented block copolymers having the general structure (AB),,. The hard segments may be based on partially aromatic polyamide or aliphatic polyamide. Polyester amide (PEA) andpolyether ester amide (PEEA) are prepared from the aromatic polyamide, while in the polyether block amide (PEBAX or VESTAMID)the hard segments are derived from aliphatic polyamide. In both PEEA andPEBA, thesoftsegments of aliphaticpolyethersarelinked to the hard segments by an ester group. All thermoplastic polyamides are characterized by excellent toughness and tlexibility at low temperature in the absence of plasticizer, low density, high dimensional stability. ease of conventionalthermoplasticprocessing, good chemical resistance. good environmentalstress cracking resistance, good elastic recovery, and good dynamic properties. The properties could be varied by variation of the block length and nature of the block (Bornschegel et a l . . 1985; Legge et al.. 1987). In this chapter, the polyether block amide (PEBAX or VESTAMID) will be discussed first.
1. POLYETHER BLOCK AMIDE
1.1 Grades Vestamid grades (CREANOVA Engg. Plastics) are represented by E40-S3, E47-S3. E47-S1, E55-S3. E62-S3, E62-S1, X4442. X7375, EX9200, E50-R2, etc. All these commercial grades are designated by a series of letters and numbers: ( 1 ) E and two digits: Nylon-12 elastomer in which the two digits indicate Shore D hardness: (2) S and digit: heat-stabilized S 1 and heat and light-stabilized S3; (3) R and digit: permanently antistatic grade in which the digit indicates the average surface resistivity to the power of 10. E(X) indicates products under development. 417
418
Bhowmick
Similarly, the following PEBAX from ATOCHEM are available: 2533SN01, 3533SN01. 4033SN01, 5533SN01, 6333SNO1,7033SN01, 2533SD01, 3533SD01,4033SD01, 4033SN70, 5533SN70. 401 1MAOO1, 5562MAOl. MX1502. MX1503, MX1057, MX1058,MX1059, MX1060, MX1084. The first two digits indicate Shore D hardness, the second two digits are used for the series, S and N represent applications (S for all uses, M for molding, and E for extrusion) and stabilization (A for no additive, N for UV stabilized, D for UV. stabilized and mold release additive, and T for heat stabilized), respectively. The last two digits represent fillers and formula code, respectively.
1.2
Preparation of Polyamide Elastomers
Several research groups studied the reaction of dicarboxylic polyamide blocks polyether blocks
with diamine
HOOC-PA-COOH+H2N-PE-NH2+H 0 €1
n
or diamine polyamide blocks with dicarboxylic polyether blocks in the molten state (ICI, 1965, Morsanto, 1969; Toray. 197 1).
H ? N - P A - N H z + H O O C -P E - C O O H + HN - P A - N - C - P E - CO H
‘A :1 li
An
The reaction of poly(oxyethy1ene) a,w-bis chloroformate with adipyl chloride and piperazine in solution to give a polyamide polyether block copolymer with urethane linkages between the blocks was reported (DuPont, 1968). Studies on the polyesterification reaction in the melt between a dicarboxylic polyamide and a polyether diol have been discussed (BASF, 1968; Unilever Emery, 1973). Thefirst patent covered the reaction of a dicarboxylic acid polyamide based on caprolactum and poly(oxyethylene) dihydroxy polyether blocks at 250°C with paratoluene sulfonic acid as a catalyst. The second patent described the reaction of a C-36 fatty acid dimer and a diamine with a polyoxyethylene dihydroxy polyether blocks without catalyst at 250°C. These two products were low in molecularweightand were used as waxy additives in textile fiber formulations to provide antistatic properties.The use of a particular catalyst family,Ti (OR)4,discovered by ATOCHEM, was the appropriate way to produce a high molecular weight polyamide polyether block copolymer with ester linkages. This catalyst appears to modify the con~patibilitybetween the diacid polyamideanddihydroxypolyethersegmentsandallowspolymerization in a homogeneous phase, which was not possible with any other catalysts. A study of the kinetics of polyesterification was done by Deleens, and the first patent was applied for in 1974 (Deleens et al., 1974). A wide studyof the combinations between different polyamide blocks and polyether blocks was then made. In addition, research on catalysts, polymerization processes, formulations, and applications was also undertaken (Deleens et al., 1976. 1977). Dicarboxylic polyamide blocks were produced by the reaction of polyamide precursors with a dicarboxylic acid chain limiter.
419
Thermoplastic Polyamide Elastomers
The reaction was achieved at high temperature under pressure. The molecular weight of the polyamide block was controlled by the amount of chain limiter. The polyamide precursors can be selected from the following: Amino acids (aminoundecanoic acid. aminododecanoic acid, etc.) Lactams (caprolactam. lauryllactam, etc.) Dicarboxylic acids (adipic acid, azeloic acid. dodecanedioic acid. etc.) The polyether blocks are produced by two different reactions: Anionic polymerization of ethylene oxide and propylene oxide for polyoxyethylene dihydroxy and polyoxypropylene dihydroxy polyether blocks Cationic polymerization of tetrahydrofuran for polyoxytetramethylene dihydsoxy polyether blocks
1.3 Structure The structure of thermoplastic polyamide elastomer compriseslinear and regularchains of polyamide segments and flexible polyether segments. The chemical structure (see above) can be varied by manipulating the following: the nature of the polyamide blocks. the nature of the polyether blocks. the length of the polyamide and the polyether blocks, and the mass relationship between the polyamide and the polyether blocks. Table 1 gives an idea of the block length of thermoplastic polyamide elastomer obtained from nylon- 12 and poly(tetramethy1ene ether) glycol. Since polyamide and polyether segments are not miscible, Vestamid or PEBAX presents a biphasic structure where each segment offers its own properties to the polymer. Figure 1 shows the atomic force microscopy (AFM) structure of a typical polymer. White regions or "hills" are polyamide blocks whose typical dimensions are 400-800 nm. The nature of the polyamide
Table 1 M,, of PTHF and PA Segments, Hardness. and Melting Point for Differcnt Segmented Polynmldes"
Samples
DSC melting point ("C )
Shore D hardness
174 171
68 62
MI174Ol~ MII4I,XI MIiZOO M1115110
M H I IIHI "I,
MSl40Il Ms2cnn, .' In the structure shown
166 1 S6 IS1 IS2 1S0 I S3
PTHF segment
PA scgment
M,,
X
M,,
35 30
I400 2000
14 14 14 14 14 9 19
7400
S3
1000 1000 1000 1000 1000 650
ss 47
40
28
4100
2300 I S00 1 100 1200 1100 I100
YI +
Y2
37 21 11 7
S 6
S S
I n the Introductlon. k = 4 (polytetrahydrofuran): I = II (laurol:rcturn); m = 11 (dodecanoic acid): 11 = 7 - 10: x Indicates the length of the soft block. while y I + y2 Indicates the length of the hard block. The total molecular wclght of the samples estmated by the terminal group tltration was between 31.000 and 76.000. h Here Mkl denotes v:lrl;ltlon of hard block molecular welght at a constant soft segment molecular welght of 1000. But M Sdenotes varlatlon of soft segmentmolecular welght at a constant hard block molecular welght of I 100. The numbers such a s 7400 or OS0 occurring In the prefix of M , , or M S denotes the corresponding h x d block or soft segment molecular welght.
420
Bhowmick
Fig. 1 Atomic force micrographs of typical Vestarmd polymers. (a) M H ~ polymer; ~w (b) MHI100 polymer; (c) Mssso polymer; and (d) Mszm polymer.
block influences the melting point of the polymer, the specific gravity, and the chemical resistance. The nature of the polyether block influences the glass transition temperature, the hydrophilic properties, and the antistatic properties. 1.4 Structure-PropertyRelationship Effects of block lengthof the hard and thesoft segments of the polyamide thermoplastic elastomers are shown below. Figure 2 illustrates the variation of tan8 with temperature for a series
421
Thermoplastic Polyamide Elastomers
8
l
I
-150
-100
I
-50
I
I
I
0
50
100
150
50
100
150
T E M P E R A T U R E ('C )
( 4
I 10.75
t
t
-.. -M H 1500 -+-
-1 50 (b)
M H l100
-1 00
- 50
0
TEMPERATURE('C)
Fig. 2 Dynamic mechanical properties of PEBA of varying block lengths: modulus curve. (From Ghosh et al., 1998a.)
(a) tan6 curve; (b) storage
Bhowmick
422
of polymers having the hard segment molecular weights of 7400, 4100, 2300, and 1100 at a constant soft segment molecularweight (Mn = 1000) (Ghosh etal..1998). Three distinct dispersion regions labeled as a, fi, and y descending from higher to lower temperatures are observed. For MH7JO0 polymer (see Table 1 for designation), these transitions occur at 27, - 65, and - 1 13°C. respectively. The a transition reflects the onset of motion of large chain segments caused by the breaking of intermolecular bridging in the amorphous region of the hard block. The (3 dispersion is due to the combined relaxation of the soft amorphous polyether segments and local segmental motionof the amide groups in theamorphous region, which arenot hydrogen bonded to the other amide groups. They transition occurs because of the cooperative movement of the methylene groups. As the hard block molecular weight decreases at a constant soft block molecular weight, the a peak gradually shifts towards lower temperature and tan6 peak height increases. The fi peak also becomes lessconspicuous. They peak at - 1 13°C does not change. E’ decreases asthe molecular weightof the hard block decreases, especially in the high-temperature region. Similarly, in the series Msbso, MslJo(), and Mslo()opolymers (Table l ) , the soft segment n~olecularweight increases at a constant hard block molecular weight of 1 100. The a and p peaks merge together to give one single transition occurring at about - 32, - 47, and - 47°C respectively. for these three polymers. Log E’ at 25°C has the highest value of 9.52 in the case of Mshso polymers. E’ decreases with increase in molecular weight of the soft segment. Figure3 depicts the variation of stress-strainpropertieswith the change in molecular weight of the hard and the soft segments. The tensile strength, 300% modulus, and Young’s modulus decrease with the decrease in molecular weight of the hard block. The values for Mill 1 0 0 polymer with lowest hard blockmolecularweightare 22.9, 11.5. and 32.9 MPa, whichare
ELONGATION
(O/O)
Fig. 3 Stress-strnin properties of thermoplastic polyarnidc clastomers. (From Ghost1 ct al., I998n.)
423
Thermoplastic Polyamide Elastomers
-3 ll -200
Fig. 4
I
I
I
450
-200
-50
I
I
I
0 50 100 TEMPERATURE ("C)
l
150
l
1
200
Dielectric properties of PEBA of varying block lengths. (From Ghosh et
250
d.,
1998b.)
similar to those of the vulcanized rubbers. The hard polyamide blocks are responsible for the good mechanical strength of the polymer, and their decrease in molecular weight lowers the mechanical properties. The crystallinity also decreases with decreasing hard block molecular weight. In theseries of polymers MS(,5o,MsI4(H),and Mszooo.thepolymerhasthelowest soft segment molecular weight and exhibits the highest tensile strength (32.3 MPa). The 300% nlodulus decreases with increase in soft segment molecular weight and attains a minimum value of 7.3 MPa for Ms2000polymer with highest soft segment molecular weight.The M,174lH, polymer having the highest hard block molecular weight exhibits the highest hysteresis loss, hysteresis loss ratio, and set values of 22.1 X IO" J/m3, 0.95 and 78%, respectively. With the decrease in molecular weight of hard block, these parameters decrease and attain minimum values in the case of MI[, polymer. Mshso polymer with the lowest soft segment molecular weight exhibits the highest set of 69%. The effects of molecular weight variation of the blocks on dielectric properties have been studied (Ghosh et al., 1998b). The decrease in molecular weight of the hard block lowers the a- and@-transitiontemperaturesatconstantsoftsegmentmolecularweight (Fig. 4). The a transition gradually becomes less conspicuous for low hard block molecular weight polymers. The y-transition temperature remains unaltered. Values of activation energy for the a,@,and y transitions decrease with decrease in the hard block molecular weight. The activation energy records the highest value for the a transition. However, the activation energy shows marginal
424
C
y$ ZE E
l
l
A
l z gA z
A
I ILgg
N
c c c
4 4 4
C
C
C
8 4 4 4
- c c c
$3riDD
044D m c c c
0444 Q c c c
Bhowmick
Thermoplastic Polyamide Elastomers 425
Bhowmick
426
increase with increaseinsoftsegmentmolecularweightforthe p transition. The dielectric constant of the polyether block amide polymers decreases with decrease in hard block molecular weight at 1000 Hz at 100°C. An increase in soft segment molecular weight from 1400 to 2000 lowers the dielectric constant value. The shear viscosity of the thermoplastic polyamide elastomers decreases with increasing shear stress. An increase in soft segment molecular weight or a decrease in hard block molecular weight decreases the shear viscosity in general. The die swell at a fixed temperature and shear rate decreases with decrease in hard block molecular weight and increases with increase in soft segment molecular weight (Ghosh et al., 1999). The adhesion strength between two aluminum sheets joined by polyether block amide increases with either decrease in hard block molecular weight at a constant soft segment molecular weight of 1000 or with increase in soft segment molecular weight at a constant hard block molecular weight of 1100 (Ghosh et al., 2000). Thermal aging of the joints has a significant effect on their adhesion strength. As the aging time is increased, the joint strength increases.
1.5
Properties
The properties of variousVestamidgradesare shown in Table 2 (Creanova Engg. Plastics, 1998). The density may vary from 1 .0 1 to 1.14 and is among the lowest of any thermoplastic elastomers. It has low water absorption, which gives dimensional stability and consistent mechanical andelectricalproperties.Depending on the grade, the meltingpointvaries from 120 to 210°C. The vitreous transition phase is always approximately -60°C. As shown in Fig. 5, the polyether block amide bridges the gap between thermoplastic and rubbers on Shore hardness. which can vary from 60A to 75D. although certain grades are not marketed at present. The hardness varies relatively little with temperature between -40 and 80°C. It retains almost ail of its flexibility at low temperatures and maintains its excellent mechanical properties down to -60°C. The behavior under stress varies as a function of the polyether content, as discussed
POLYETHER BLOCK AMIDES I
1
I
I I
THERMOPLASTIC POLYURETHANE
I
1
I POLYETHER BLOCK I
I
I I
I
,
Thermoplastic Polyamide Elastomers
427
earlier. The polyether content gives the product its flexibility and its more or less elastomeric character. The strengthatlowdeformation is high-higher than that of most thermoplastic elastomerswiththe same hardness.allowingthickness to be reduced in many applications. Thermoplastic elastomeric polyamide has good resistance to tearing and abrasion.The resistance increases with the hardness of the grade. Compared to most other thermoplastic elastomers, its abrasion resistance is high even in contact with highly abrasive media, making it suitable for athletic footwear. The hysteresis values of PEBA polymers are lower than most thermoplastic elastomers or vulcanized rubber with equivalent hardness. The high resistance to cyclic flexing, even at low temperatures, is one reason why it is chosen for the soles of football and ski boots, transmission belts, gear trains, etc. The thermal and thermo-mechanical properties depend on the length of the blocks and the type of polyamide. For example, the melting point of PEBAX grades 2533-401 1 varies from 133 to 204"C, with a latent heat of fusion between 1.2 and 6.3 cal/g (Elf Atochem, 1998). Similarly, thermal conductivity of the same grades lie between 0.26 and 0.29 W/m"C at 30°C. The linear coefficient of expansion is of the order of 20-25.1 X lo5 cm/cm/"C in the temperature range between 30 and 0°C. Deformation temperature under load indicates that at constant stress, more flexible grades distort at lower temperatures. The volume resistivity varies from 10'j to IO3 L! cm'/cm, as shown in Table 2. It offers excellent resistance to tracking. The standard VDE 0303Part 6 test indicates that this elastomer leads to no corrosion, even under extremely humid conditions. Antistatic and semiconductive materials can be produced from this elastomer by introducing carbon black. The resistance to aging in dry heat depends on the grade. The rigid grades withstand dry heat better than the flexible grades. The rigid grades also have better UV resistance than the flexible grades. Gas permeability decreases with increase in the hardness. Typical PEBAX 6333 and 2533 grades of 120 km thickness have gas permeability values of 3 1 and 150 (oxygen), 420 and 2600 (carbon dioxide), 5 and 170 (nitrogen), and 46 and 235 (helium), respectively (all units in 10- l o cm3 mm/cm'.s.cm of Hg) at 23°C. Certain grades can be made permeable to water vapor. Table 3 reports the resistance to various solvents and chemicals.
1.6 Processing PEBA can be processed using the following techniques: Injection molding Extrusion Thermoforming Coating
No hazardous degradation products are generated by processing of PEBA resins (Elf Atochem, 1998). However, special precautions must be taken in drying the materials. PEBA grades are generally supplied as pellets in moisture-proof packaging ready for use. The drying conditions are 4 hours at 80°C (PEBA with Shore D 2 40) or 6 hours at 70°C (PEBA with Shore D < 40) in a propelled air oven, dehydrated hot air oven, or vacuum oven. The rheological properties, as discussed earlier, are important for understanding processing. Melt flow index, which corresponds to the quantity of material at 235°C that can flow in 10 minutes through a 2 mm line when 1 kg of mass is applied, gives a measure of the viscosity. MFI values lying between 5.5 and 12 g/10 minutes have been reported for PEBAX grades. Melt viscosity, which is a function of shearing speed and temperature, indicates that a high molding temperature increases flow capability. Unlike melt flow index and melt viscosity, tlow lengths may be obtained by injecting PEBA via an Archimedes or reciprocating screw into a mold with rectangularsection.Duringmolding, the dimensions of thegating may be determined. For
428
Bhowmick
Table 3 Resistance of PEBAX to Solvents ~~
Test conditions days
PEBAX grade"
(J)
("C)
7033
6333 4033
5533
10% acidsulfuric 10% caustic soda
l l 7
A A A A A A A A A A A A A B A A B A A A A A A A A A C B A B B B
A
chloride SO% zinc Boiling "Skip" detergent (30 g/L) Caustic potash (34" Baume) Lockeed H55 ASTM No 1 oil ASTM No I oil ASTM No 3 oil ASTM No 3 oil Ethanol Propanol Butanol Isooctane 4-Star petrol M l5 fuel Kerosene Paraffin Fuel B Fuel B Fuel C Benzene Acetone glycol Ethylene Methylethylketone chloride Methylene Trichloroethylene Perchloroethylene FREON 1 I FREON R 22 FREON R 502
23 23 23
A A A A A A B A A A A A B B A B B A A A B B A A A A C B B B C B
Chemical
water
l l 7
l 3
l 3
l l 7 7 7
l 2
l 7 7 2 2
l l l 7
l 7 7
l l l
100
95 l0 121 IO0 121 100
121 23 23 23 23 23 50 23 23 23
so 50 23 23 23 23 23 23 23 23 45 45
A A A A A A A A A A A A B A A B A A A A A A A A A C B A B B B
A A A A A A C A A C B A B C A B C B A B B B B A A B C C B B C C
3533
2533
A A A A A A C B C C C B C C A C C B B C C C C A A C C C C C C C
A A A A A A C B C C C C C C A C C C B C C C C A A C C C C C C C
A = little or n o effcct: B = moderate cffect; C = severe effect. (Reprinted f r o m Elf Atochem wlth permission.)
example, in the case of large components, the size of the gate may be increased to broaden the range of temperature and pressure available and to facilitate processing. Itljectiotl Molding
The equipment used for polyamideinjectionmolding is suitableforPEBAprocessing.For flexible grades, mold designs suitable for polyurethanes (particularly the cavity feed) may be used. The choice of injection temperature depends primarily on the length of time the material stays in the sleeve. If the cycle is short, a high injection temperature should be used. Depending on the grade, recommended temperature varies from 160 to 280°C. In some cases, an injection temperature of 300°C is reported (Elf Atochem, 1998). Higher temperatureresults in lower
Thermoplastic Polyamide Elastomers
429
pressures within the component during cooling. Injection speeds are chosen according to the lowest flow sections, which cause the highest shearing speeds. PEBA should be molded in cold molds (20-40°C). which help in the release from molds. The right molding temperature controls the component’s finish. dimensional stability, and shrinkage. An injection pressure in the range of 500-800 bars for nonreinforced grades has been recommended. Holding pressure is applied when the mold cavity is full to compensate for material shrinkage during cooling. Extrusion
PEBA can be extruded using the same types of machines and screws used for polyamide. The screws used for PVC or PE can be used for flexible grades. The recommended temperature for extrusion of PEBAX 2533-7033 lies between 160 and 230”C, depending on the grade. Assembly
There are a few ways of assembling PEBA to other materials: Insert molding Bonding Welding The process parameters canbe adjusted by optimizing adhesion between the material and inserts. The oven-molding technique is used for athletic shoe soles (Elf Atochem, 1998). Other Methods
Blown film can be made from PEBA. Blown extrusion film (25 Km thick) has been made on a Kaufmann line (D = 64 mm, L/D = 28, die = 150 mm, width = 500 mm) at a temperature of 160- 180°C and a screw speed of 30 rpm, drawing speed of 20 m/min and blow rate of 2. I , and a die temperature of 175°C. Cast film of 25 p m thickness has been made on the ERWEPA extruder (D = 90 mm, L/D = 32) at 96 rpm at 200-250°C at a speed of 100 m/min. Similarly, PEBA can be coated onto other substrates in the same machine (D = 60 mm) at a lower speed andscrewrpm. PEBA can be recycled by incorporating IO- 15% of the materialinto new granules of the same grade. Compounding with mineral or organic pigments or incorporation of liquid dyes or masterbatches has also been reported.
1.7 Applications The special properties of PEBA make it suitable for a range of applications, for example, in sports, fashion, medical, automotive, electrical and electronic industries, household appliances, machine tools, agriculture, toys, etc. (Fig. 6). The properties of breathable films from polyether block amides are shown in Table 4.
2.
PEEA AND PEABLOCK COPOLYMERS
PEEA and PEA block copolymers have been synthesized by the condensation of aromatic diisocyanate, [4,4’-methylene bis(phenylisocyanate),(MDI)] with dicarboxylic acids and a carboxylic acid-terminated polyester or polyether prepolymer with a Mn of 500-5000 (Chen et al., 1978). The homogeneous polymerization is carried out at elevated temperature in a polar solvent that is nonreactive with isocynates. The dicarboxylic acid serves as the hard segment chain extender
430
Bhowmick . . .
-
, .
.
.””~ ‘j-
“6
Fig. 6 Typical applications of thermoplastic polyamide elastomer: (a) sports shoes; (b) toys. (From Elf Atochem with permission.)
and forms the amide hard segment with the MDI. The carboxylic acid-terminated prepolymer forms the soft segment matrix-ester for PEA and ether for PEEA polymers. The amide content of the elastomer and hence crystallinity can be changed by varying the amounts and types of dicarboxylic acid chain extenders in the formulation or by changing the molecular weight of the polyester or polyether soft segments. As discussed earlier, physical properties of PEA and PEEA segmented block copolymers are influenced bythe chemical composition of the hard and soft segments andtheir respective length (Deleenset al., 1987).Generally,hard segmentchemical composition affects the polymer melting point, degree of phase separation, and mechanical strength. Similarly,the soft segment chemical compositioninfluences hydrolytic stability, chemical and solvent resistance, thermooxidative stability, and low-temperatureflexibility. The molec-
Thermoplastic Polyamide Elastomers Table 4
431
Pronerties of Breathable Film from PEBAX
Hardness, shore D Melting point. "C Density USP Class VI Water absorption, r/r EquiIibriun1"20"C, 65% RH 24 hr i n water MVTR, g/m2/24hr (permeability to water) ASTM E96 BW (38°C/S0% RH) 12 pm 25 pm 50 p m ASTM E96 E (38"C/90%RH) 12 p m 25 p m 50 pm
MX 1205
MV 1041
MV 3000
MV 1074
40 I47 I .01 Yes
60 170
35 1 58
40 1sx
1.04
I .02
Yes
Yes
1.07 Yes
0.5 I .2
0.9 12
I .0 28
48
3,000 1,800 1,400
l 8.000 12.000 7,000
28,000 22.000 18,000
30.000 35.000 2 l ,000
3.000 1,800 1.200
3.s00 2,700 1,800
4.500 3,300 2,200
4.800 4.300 3.600
1 ..I
ular weight of the hard segment influences also polymer melting point, thermal stability. and low-temperature flexibility. These thermoplasticelastomers (TPEs) have highermechanicalpropertiesand moduli than those of many otherTPEs in the same hardnessrange.They also retain thesetensile properties at higher temperature. PEEA and PEA elastomers are very resistant to long-term dry heat agingeven at 150°C. The abrasion resistance, tear strength, and compression set are excellent and comparable with other segmented block copolymers like TPVs. Theyexhibit good insulating and adhesive properties. High-temperature tensile properties. dry heat aging, humid aging, chemical and solvent resistance, tear strength. abrasion resistance, compression set. flex properties, adhesive, weatherability, electrical properties, processing characteristics. and applications have been discussed in detail by Nelb et al. (1987). They can be melt processed on injection molding. blow molding, and extrusion equipment like other TPEs.
ACKNOWLEDGMENTS The author is thankful to Dr. J . Lohmar of Creanova Spezial Chemie GmbH, Marl, Germany, and Dr. Y. Aubert of Elf Atochem, Paris. France. for providing the technical literature on the subject.
REFERENCES Bornschlepl, E., Goldbach, G.. and Meyer, K. (1985). Prog. Co//oir/.P o / y w v Sci. 7/: I 19. BASF, (1968), U.K. Pat. 1 , l 10,394. Chen, A. T., Farrissey Jr., W. J., and Nelb, R. G. ( 1 978), U.S. Pat. 4,129.7 IS (The Dow Chemical Company). Creanova Spezial Chemic GmbH (1978), Germany, technical literature.
432
Bhowmick
Deleens. G., Foy. P,, and Jungblut, C. (1974), French Pat. 2,273,021. Deelens. G., Guerin, B., and Poulain, C. (1976), French Pat. 2,359,879. Deleens, G.. Ferlampin, J., and Gonnet, M. (1977), French Pat. 2,401.947. Deleens. G., Foy, P., and Marechal. E. (1977), Eur. Po/. J. /3:337,343,351. Deleens. G. ( 1987) in T/fer~,lol,/os/ic. E/o.storl/er.s-A Conqm/wrl.siw R e ~ i e (N. ~ . R. Legge. G. Holden. and H. E. Schroeder, Eds.). Hanser Pub., Munich. p. 215. Du Pont de Nemours (1968). U.K. Pat. 1,098,475, Elf Atochem ( 1998). Paris, France, technical literature. Ghosh. S.. Kahstgir, D., and Bhowmick, A. K. (l998a), P o / w w r 39:3967. Ghosh, S., Khastgir, D., and Bhowmick, A. K. (1998b), Po/yruer P o / y w r . Cortrpos. 5:323. Ghosh, S., Khastgir, D.. Bhattacharyyu, A. K., and Bhowmick, A. K. ( 1999). J. A/I/J/.Po/ytl7. Sci. 7/:1739. Ghosh, S., Khastgir, D., and Bhowmick. A. K. (2000),J . Adlwsiorl Sci. Techno/. /4:529. IC1 (1965), U.K. Pat. 1,108,812. Legge N. R., Holden G., and Schroeder H. E.,Eds., ( 1987), T/fcf.ff/ol~/~/.s/I'(. E/o.storrwrs-A CorlqJreherl.si,~e Rr\iew, Hanser Pub., Munich, 1987. Monsanto, (1969), U.S. Pat. 3,454,534. Nelb, R. G., Chen. A. T., and Onder, K. ( 1987). in Thermop/rt.stic Elo.stnmer.s-A C ~ ~ ~ f f / ~ r e / fRe e~ f~. si iml ,~~ , (N. R. Leggc. G. Holden, and H. E. Schroeder, Eds.). Hanser Pub., Munich, p. 197. Toray (1971). French Pat. 1.603,901. Unilever Emery French Pat. (1973). 2,178,205.
16 lonomeric Thermoplastic Elastomers Kamal K. Kar and Ani1 K. Bhowmick Rubber Technology Centre, Indian Institute of Technology, Kharagpur, lndia
1. INTRODUCTION Ionic polymersare a specialclass of polymeric materials havinga hydrocarbon backbone containing pendant acid groups. These are then neutralized partially or fully to form salts. Members of one family of these ionic polymers where the salt group content is very high (i.e., where every monomer repeatunit has a pendant salt groups) aregenerally water soluble. Suchpolymers are called polyelectrolytes. Other classes of ionic polymers in which the polymers backbone is highly polar contain fewionic pendant groups and also behave as polyelectrolytes. The combination of low ionic content and low-polarity backbone results in ionomers that show high extensibility and low permanent set: ionic elastomers. Ionic thermoplastic elastomers are another class of ionic elastomers in which the properties of vulcanized rubber are combined with the ease of processing of thermoplastics. These polymers contain upto 10 mol% ionic groups. However, they have a number of practical advantages over conventional rubbers: 1. Theyrequire no vulcanizationand little compounding. 2. They are amenable to methods of thermoplastic processing like blow molding, thermoforming, injection molding, heat welding. etc., unsuitable for conventional rubber; they have short mixing and processing cycles and low energy consumption. 3. Theirscrapcan be recycled. 4. Their properties can be easily manipulated by changing the ratio of components.
The disadvantages associated with ionic thermoplastic elastomers are that they soften/melt at elevatedtemperatureand showcreep behavior on extendeduse. They deteriorate in the presence of water, which is not the case forthermoplastic elastomers. A typical ionic thermoplastic elastomer is shown in Fig. I . The ratio of n/m is usually on the order of 10-100. The presence of this level of salt groups combined chemically with a nonpolar backbone with covalent bonds has a dramatic influence on polymer properties and applications. The properties of ionic thermoplastic elastomers depend on the ionic interactions of the polymer backbone with the pendant groups. The degree of ionic interaction depends on the following variables: 1. Types of polymers, i.e., elastomer or plastic 2 . Chemical structure of thepolymers 3. Level of ionicfunctionality (ionic content) (0-1076)
433
434
Kar and Bhowmick
F”
ICH3C H 3 I fCf-12
142-ffCH2-C X L - v q - - C H 2 - C H 2
V C H 2 C
% L= HCH3
\ “ C H 3
$03- N
‘*
Fig. 1 Schematic representation of an ionic thermoplastic elastomer (sodium sulfanated EPDM).
4. Degree of neutralization (0-100%) 5. Types of ionic moiety. i.e., carboxylate, sulfonate, phosphate, etc. 6. Types of cation, i.e., monovalent, divalent, multivalent, amine, etc. With this range of variables, the spectrum of polymer properties and applications in extremely broad, which bridgesthe gap betweenconventionalrubbervulcanizatesandthermoplastics. Ionic polymers have been discussed in detail in several reviews and books (Holliday, 1975; Eisenberg, 1980; MacKnight andEarnest, 198 1 ; Chakraborty et al. 1982; MacKnight and Lundberg, 1984,1987;Rees,1987;TantandWilkes.1988;Mauritz,1988;FitzgeraldandWeiss, 1988;Lundberg,1989;Haraand Sauer, 1994), butthere has beenlittlediscussion on ionic thermoplastic elastomers. This chapter will briefly discuss the salient features of various ionic thermoplastic elastomers.
2.
CLASSIFICATION OF IONICTHERMOPLASTICELASTOMERS
At the present time there is no standard method for the classification of ionic thermoplastic elastomers (ITPEs). They can be grouped on the basis of the following variables: Type of cations Nature of ionic groups Method of preparation Example of commercial and experimental ITPEs are given in Table 1
3. SYNTHESIS OF IONICTHERMOPLASTICELASTOMERS There are two techniques for the preparation of ITPEs. The first is copolymerization of a low level of functionalized monomer with an olefinic unsaturated monomer (Farbenindustric, 1932; Semon, 1946;Bryant,1970;Ress,1966,1968; Longworth, 1975).Directfunctionalization of apreformedpolymer is asecondroute(Gilbert,1965;Makowskietal.. 197th; Saidt et al., 1980a;Makowski and Lundberg,1978b,1980a;Fitzgerald andWeiss, 1988).Carboxylated elastomers are prepared by free radical polymerization, solution polymerization, emulsion polymerization, and grafting procedures on preformed polymers. Typically 6% carboxylic monomer is incorporated into the polymer to maintain elastomeric properties. But the vast majority of commercially available carboxylated elastomers are synthesized by emulsion polymerization in acidic medium because the free acid copolymerizes much more rapidly than the neutral salt and the low solubility of the monomer salt in the hydrocarbon phase prevents significant monomer
435
lonomeric Thermoplastic Elastomers Table 1 Comtncrcial and Experimental ITPEs Polymcr systctn
T K I ~n;mc K Applications Mnnufncturer
Commercial Sulfonatcd ethylene propylcnc dicnc tcrpolymer Goodrich Telechelic polyhutadicnc Goodrich Butadicne acrylic acid copolytncr
Ionic elnstomcr Hycar Hycar
Uniroyal
surlyn
dl1 Pont
Ethylcnc mcthacrylic acid copolymer Experimentd Quatcmary phosphonium sulfonated ethylene propyletlc dicnc tetpolymcr Meleatcd cthylenc propylcnc dicnc tcrpolytner Tekchelic polyisohutylcnc sulfonated ionomer Thioglycolated polypentenamer Phosphonatcd polypentenamer Carboxylatcd polypentenamer Sulfonated polypcntcnamcr Polyurethane ionomcr Sulfonated styrene (cthylcnc-cobutylene) styrcnc trihlock copolymcr Poly (butadiene-co-sodium styrene sulfonate) 2-Butyl styrcnc sulfonate isoprene copolymcr Copolymer of ethylcnc vinyl acetate and organic acid Blcnd of zinc salts of malcated EPDM and ethylcne methacrylic acid copolymer
Thermoplastic clastomcr Spccinlty uscs High green strcngth elastomer Modified thermoplastic Model ionomer
Modcl ionotner Model ionomer Model ionomer Model ionomer Model 1onon1cr Model ionomer Model ionomcr Model ionomer
Model ionomct Model ionomc~ Elvax
du Pont
Model ionomcr Modcl ionomct
incorporation. The polymerization is carried out at 30-50°C. A typical formulation for preparation ofcarboxylated elastomeris given as 100 parts butadiene (or combination with other monomers), 100 parts deionized water, 5 parts nlethacrylic acid, 1 part sodium alkyl aryl polyether sulfate, and 0.4 part potassium persulfate (Jenkins and Duck, 1975). The resulting elastomers are neutralized to the desired degree by metal hydroxide, metal acetates or sinlilar salts. On the other hand. the direct functionalization of a preformed polymer is carried out i n a homogeneous solution, using a sulfonating agent, (i.e., acetyl sulfate). I n the next section, the salient features of synthesis of various ITPEs are discussed.
3.1
Metal Sulfonated EPDM
The preparation of metal sulfonated EPDMs consists of two steps: sulfonation of EPDM and neutralization of the free acid form of sulfonated EPDM (Makowski et.al., 1980).Typical diene
Kar and Bhowmick
436
Table 2 Specifications ofStartingEPDMs ~
-
~~~
viscosity
content
sourcc
(wt%)
(100°C)
(wt%)
Commercial 40 Extruder 20 breakdown Extruder breakdown Direct 17 synthesis Direct synthesis Commercial
52 52
Designation V-2504 CR-2504 CR-709A E-S5 E-70
~
Ethylene ENB content
Mooncy
S
68
20
55 70 70
20
5 7.5 4.4 4.7
-
-
S o ~ r r w :Makowski et al.. 1980
monomers that can be sulfonated include5-ethylidene-2-norbornene (ENB), endo-dicyclopentadiene (DCPD), and 1,4 hexadiene. The unique features of these diene monomers are that only one of the two unsaturated bonds is consumed during polymerization. The specifications of EPDMs used are presented in Table 2. These materials vary primarily with respect to Mooney viscosity, a measure of molecular weight. and are fully amorphous. The EPDM designated as V-2504 is a commercial elastomer. CR-2504 is thermal and shear degraded by an extruder. A higher ethylene content EPDM, CR709A, is generated similarly through extruder. The EPDMs designated as E-55 and E-70 are synthesized directly. All of the EPDMs contain ENB as the third monomer. The free acid form of sulfonated EPDM is prepared from EPDM by using sulfonating agent in a manner similar to that described by O’Farrell and Serniuk ( 1974) and Makowski et al. (1978b). First, EPDM is dissolved in an aliphatic hydrocarbon. namely hexane or heptane, to make a concentration of 50-100 g/L. The sulfonating agent (O’Farrell and Serniuk. 1974; and Gilbert, 1965) or SO3 complex of triethyl phosphate dioxane and tetrahydrofuran (Canter, 1972) is added at room temperature. After 30 minutes a chain-terminating agent (i.e., isopropanol or alcohol) is added to terminatesulfonationreaction. At thispoint, 2-2’ methylene-bis (4methyl-6-tert butylphenol) antioxidant is added. The polymeric sulfonic acidis separated through steam stripping of the resultant solution. The resultant polymeric mass is washed with water in a Waring blender. The crumb is filtered and then dewatered by banding on a rubber mill at about 50°C to a water level of 2-5% or is dried in a laboratory aeromatic fluid bed dryer at 70°C to a water level of 0.2%. Sulfur content is determined by direct sulfur analysis (ASTMD-1552). The acid content of the sulfonated EPDM is determined through the titration of a solution of the sample in 95 mL toluene and 5 mL methanol with 0.1 N NaOH. A95% conversion of sulfuric acid to polymer sulfonic acid is typical (Fig. 2). Neutralization of sulfonated EPDM is carried out in one of two ways: neutralization of theisolatedpolymericacid or directneutralization, i.e., after sulfonation. In thefirstcase. polymer is redissolved in a mixture of toluene/methanol or hexane/alcohol to make a concentration of 50- 100 g/L. A solution of metal acetate in water or water/methanol is added to neutralize thepolymer. After 30 minutes of agitation,theneutralizedsulfonated EPDM is isolated by solvent flashing in boiling water. The wet polymer is dried on a rubber mill at 100°C or in a laboratory aeromatic fluid bed dryer at 100°C. In the second procedure (direct neutralization), after sulfonation but before drying, the resultant solution is treated with an alcohol or wateralcohol solution or an aqueous solution of metal hydroxide or acetate. The neutralized polymer is isolated and dried by the same procedure.
437
lonomeric Thermoplastic Elastomers
N CH c 0 3
f C H 2 -CH 2
F
CH2-CHCH3t
c -CH
n
j’
3
I
Fig. 2 Synthesis of sodium sulfonated EPDM.
The sulfonation of EPDM is alsocarried out in a single screw extruder (Saidt et al., 1980b). It is prepared by injecting acetic anhydride with sulfuric acid into the barrel along withan EPDM. The barrel temperature is 100°C. The residence time varies from 5 to 10 minutes depending on the degree of conversion. The resulting acid is neutralized by the addition of metal stearate, which is pumped directly into the product stream as it exits the extruder. The advantages of using an extruder are shorter processing time and the absence of a solvent. 3.2 Quaternary Phosphonium Sulfonated EPDM Brenner and Oswald (1980) have synthesized quaternary phosphonium sulfonated EPDM containing 1 mol% sulfonation over a wide range of chain length in the counterions. The free acid form of sulfonated EPDM is prepared from commerical EPDM by using acetyl sulfate in a manner similar to that described in the section on metal sulfonated EPDM. The acid form of sulfonated EPDM is dissolved in a mixture of 95% toluene and 5% methanol. The resulting solution is neutralized fully with solution of various quaternary hydroxide at room temperature. The neutralized sulfonated polymer is recovered from solution by either steam stripping or by precipitation in methanol. Samples are dried in vacuum for 3 days at 50°C.
3.3
Zinc Maleated EPDM
Zinc maleated EPDM is another ITPE (Uniroyal Technical Information Bulletin, 1982). There are two grades of maleated EPDM. One has a low ethylene content (ethylene/propylene ratio 55/45) and the other a high ethylene content (ethylene/propylene ratio 75/25). Zinc maleated EPDM is prepared by the addition of zinc oxide into maleated EPDM. The mixing processes
438
Kar and Bhowmick
depend on the ethylene content of nlaleated EPDM. The mixing of zinc oxide into maleated EPDM of low ethylene content is done on an open two-roll mill. The mixing of zinc oxide into maleated EPDM containing of high ethylene content is done in a Brabender plasticorder at a temperature of 70°C and at a rotor speed of 60 r p m .
3.4 Telechelic Carboxylated Elastomers The synthesis of telechelic carboxylated elastomers by free radical-initiated polymerization and anionic polymerization has been reported by Reed (1971) and Schulz et al. (1981 ). The first routeprovidespolymers of broadermolecularweightdistribution.Reactiontemperature is 70- 130°C. This process is utilized for copolymerization of 1.3-butadiene and acrylonitrile. The typical free radical initiator is 4.4‘ azobis (4-cyano-pentanoic acid). The selection of solvent is important to minimize chain transfer to the solvent. Typically t-butyl alcohol is preferred. Tetrahydrofuran and acetone are also used. The liquid polymers are recovered by solvent stripping. The neutralization of these carboxyl-terminated polymers are done by suitable alkoxide in toluene under vacuum (Broze et al.. 1981). The secondroute offers polynlers of relativelynarrowmolecularweightdistribution: 1500-6000. But in this case a large amount of organometallic compound is required. These ionomers are prepared in the following three molecular architectures: linear monofunctional.lineardifunctional,andthree-armstartrifunctional (Kennedy and Storey, 1982; Bagrodia et al., 1983a, 1983b, 1985; Mohajer et al.. 1984; Tant et al., 1985), which are obtained by the use of unifier (monocumyl chloride/BCI3),binifer (p-dicumyl chloride/BCI3),and trinifer (tricumyl chloride/BC13) initiator. respectively. Sulfonation is canied out i n hexane solution at room temperature.
3.5 2-Butyl Styrene Sulfonate Isoprene Copolymer 2-Butyl styrene sulfonate isoprene copolymer is prepared by emulsion copolymerization (Saidt andLenz.1980).Asolution of 1.6 mL of polyoxyethylene,0.25 mL of 14% solution of 1dodecanethiol in benzene. 0.25 mL of diisopropyl benzene hydroperoxide, and 1 mL of 2-butyl styrene sulfonate in 25 mL of isoprene is added to a mixture containing 25 mL of activator solution. 0.15 g of sodium pyrophosphate decahydrate, and 5 mL of distilled water. Polymerization reaction is conducted at 25°C for 10 hours at 9 psi pure nitrogen atmosphere. The polymerization reaction is terminated at the desired conversion by the addition of a solution containing 0.22 g of 2.6-di-t-butyl-4-methyl phenol (antioxidant), 0.015g of hydroquinone (inhibitor), 1.86 mL of methanol, 4.7 mL of distilled water, and 0.094 g of emulsifier. The ester copolymers are hydrolyzed to ionomers by the dropwise addition of a 10% solution of ester copolymerin toluene to 15 mL of 5 N solution of NaOH in methanol.
3.6 Poly(butadiene-co-sodium styrene sulfonate)
Poly(butadiene-co-sodiumstyrene sulfonate) is prepared by free radical copolymerization (Weiss et al., 1980). Butadiene monomer is injected into a solution containing sodium styrene sulfonate. sodium lauryl sulfate (surfactants). triethylene tetramine (redox initiator), dodecanethiol (chain transfer agent), sodiumpyrophosphate decahydrate (buffer). and water.At the end of the reaction, a methanolic solution of hydroquinone (inhibitor) and 2,2’-methylene-bis (4-methyl-6-t-butyl phenol) (antioxidant)is added to the reaction mixtures. The polymer is precipitated in methanol and washed with water.
lonomeric Thermoplastic Elastomers
3.7
439
Poly(ethy1ene-co-methacrylic acid) lonomer
The poly(ethy1ene-co-methacrylic acid) ionomersare marketed by du Pont as surlyn. The copolymer is prepared by free radical polymerization (Rees, 1966. 1968; Langworth. 1975). Typically 3-6 mol% methacrylic acidis incorporated into the polymer. The acid copolymers are neutralized on a two-roll mill at 150-200°C by the addition of NaOH or other bases. 3.8
Substituted Polypentenamer lonomers
Sanui et al. ( 1974a). Azuma and MacKnight ( 1978), Rahrig and MacKnight(1980a). and Tanaka and MacKnight (1979) have synthesized several new ionomers based on polypentenamer. These new ionomers are thioglycolated polypentenamer, phosphonated polypentenamer. carboxylated polypentenamer, and sulfonated polypentenamer. All ionomers are prepared from polypentenamer, which is a linear elastomer having very few or no vinyl groups and a relatively narrow molecular weight distribution (M,v/M,, = 1 .g). Thioglycolate, phosphonate, sulfonate. and carboxylate groups are incorporated up to 19 mol% based on the polypentenamer (i.e.. 3.8/100 backbone carbon). Ester form of thioglycolated polypentenamers (PPS) is prepared by adding methyl thioglycolate to the double bonds of polypentenamers via a free radical reaction (Sanui et al., 1974a). Hydrolysis reactions are conducted to prepare the corresponding acid (PPSH). Various salts are prepared by neutralization with various bases. Phosphonateside groups areincorporatedinto the polypentenamer by the freeradical addition of dimethyl phosphite to synthesize dimethyl ester of phosphonated polypentenamer (PP-PO) (Azumaand MacKnight, 1978). Thecorresponding acid derivative(PPPOH) is prepared by bubbling HCI gas through a dilute solutionof the ester. The sodium salt. PP-PONa.is prepared by treatment with methanolic sodium hydroxide solution. Carboxylic acid groups are introduced into the polypentenamer chain by a carbene addition of ethyl diazoacetate to give ethyl ester form (PP-COOEt) of carboxylated polypentenamer. Acidandsaltderivatives are prepared by hydrolysisandtreatmentwithmethanolicsodium hydroxide solution. Sulfonatedpolypentenamer is synthesized by reactinga complex of sulfur trioxide to diethylphosphatewithpolypentenamer. The reactionmixture is precipitated into ;L sodium hydroxide solution. which converts the polymer directly to the sodium salt. PPS03Na. 3.9
Sulfonated Styrene(ethy1ene-co-butylene)Styrene Triblock lonomer
Weiss et al. (1990) have synthesized sodium and zinc salt of sulfonated styrene (ethylene-cobutylene) styrene triblock ionomer. The starting material is a hydrogenated triblock copolymer of styrene and butadiene with a rubber midblock and polystyrene end blocks. After hydrogenation, the midblock is converted to a random copolymer of ethylene and butylene. the ethylene segments arising from 1,4 addition of butadiene and butylene groups from 1,2 addition. Ethyl sulfate is used to sulfonate the block copolymer in 1.2-dichloroethane solution at a temperature of 50°C using the procedure developed by Makowski et a l . (1975). The sulfonic acid form of the functionalized polymer is recovered by steam stripping. The neutralization reaction is carried out in toluene or toluene/methanol solution using the appropriate metal hydroxide or acetate.
3.10 Polyurethane lonomer Polyurethaneionomersareobtained from polyurethanethermoplastic elastomers, which are composed of short. alternating blocks of hard and soft segments. Polyurethane is prepared from
Kar and Bhowmick
440
a prepolymer obtained from glycol (poly(oxytetramethy1ene glycol) and diisocyanate (methylene-bis (4-phenyl isocyanate)) and a diisocyanate in the presence of a tertiary amine containing diol extender (N-methyldiethanolamine).Then polyurethane is transformed into azwitterionomer by dissolving it in dimethyl acetamide and addingan appropriate amount of y-propane sulfone (Hwang et al., 1981). A ring opening reaction occurs on the sulfone, resulting in the formation of a quaternary ammonium ion closely linked to a sulfonyl anion. Miller et al. (1983) have converted this zwitterionomer to an ionomer by reacting it with metal acetate.
3.1 1 Ionic Thermoplastic Elastomeric Blends and Some Recent lonomers Ionic thermoplastic elastomeric blends prepared by blending of rubbers and plastics in the right proportions have not received wide attention. Duvdevani et al. (1982) have prepared a ionic thermoplastic elastomeric blends from sulfonated EPDM and PP. Another blend was developed from sulfonated EPDM and poly(styrene-co-4-vinyl pyridine) by Peiffer et al. (1986). Blends based on polyurethane ionomers with polyacrylonitrile have been studied by Oh et al. (1994). Datta et al. (1996) have prepared a ionic thermoplastic elastomer by blending of zinc maleated EPDM and zinc saltof ethylene methacrylic acid copolymer. Blends are prepared ina Brabender Plasticorder at 170°C at a rotor speed of 60 rpm. Fitzerald and Weiss (1988) and Hara and Sauer (1998) have reviewed blends of ionomers. DeSarkar et al. (2000) prepared ionomeric hydrogenated styrene-butadiene rubber, and Chakraborty etal. (2000) reported ionic polychloroprene rubber.
4.
STRUCTURE
ITPEs contain boundion and free counterion. The bound ion is covalently bonded to the polymer network, as a carboxyl group in butadiene acrylic acid anda sulfonyl group in sulfonated EPDM. In contrast to bound ion, the counterion is free to move. But its actual mobility depends on the strength of the ionic bond, the nature of the polymer backbone, the temperature,and the presence of other additives in the polymer matrix. The ion pairs without a hydrocarbon layer aggregate to form multiples. A single ion pair can be represented as a small multiplet, and the size of the multiplet depends on its geometry:ionpairs,triplets,quartets, etc. The maximum size of a multiplet is eight ion pairs. The multiplet is completely coated by a hydrocarbon layer.Therefore, it is impossible for another multiplet to come closer than the distance of the thickness of a hydrocarbon chain. A loose association of multiplets is called a cluster. A cluster consists a central core having a multiplet of maximum size surrounded at a distance by other multiples of various sizes. This association is favored by the electrostatic interactions between multiplets and opposed by forces arising from the elastic nature of the backbone chains. Composition of clusters strongly depends onthe polarity of polymer matrix, ionic functionality, and temperature. At low ion content, multiples are favored only in a low polarity matrix. Cluster formation is favored with increasing ion content. With an increase in matrix polarity. a higher content of ionic groups isrequired in order to favor cluster formation. Asthe polarity of the matrix increases, the degree of ionic functionality required for cluster formation increases substantially. Several questions arise as to the state of aggregation of the ionic bonds: 1. What is the critical concentration for the formationof multiplets and clusters in ITPEs? 2. Are the multiplets and clusters uniformly distributed in space? 3. If they are multiplets or clusters, how large are they?
lonomeric Thermoplastic Elastomers
441
Various analytical techniques and a numberof models have been developed to answer the above questions. The analytical methods of characterizing the structure of ionomers include infrared spectroscopy,far-infraredspectroscopy.Ramanspectroscopy. Mossbaur spectroscopy, x-ray scattering. small-angle x-ray scattering, electron spin resonance spectroscopy, fluorescencespectroscopy,transmissionelectronmicroscopy, dynamic mechanicalthermalanalysis.dielectric thermal analysis. differential scanning calorimetry, etc. A number of different models i.e., these of BonottoandBonner (1968:~ 1968b),Eisenberg (l970), LongworthandVaughan (196821, 1968b), Holliday (1975). Marx et al. ( 1973a), Binsbergen and Kroon (1973), MacKnight et al. (1974), Meyer andPineri (1978). andYarussoand Cooper (1983), have been developed to determine the structure of ionomers. The following factors are involved in each model (Holliday, 1975): Upon cluster formation. work is undertaken to stretch the segments of polymer chain between ionic groups fromthe distance corresponding to random dispersed multiplets to the distance corresponding to higher clusters, which will be further apart. 2. Electrostaticenergy is released when multipletsaggregate. 3. The cluster is not infinitely stable, and above sometemperature, Tc, it will decompose. At this temperature. the electrostatic and elastic forces balance each other. 4. Some ring formation will take place between sequential ion pairs incorporated in the same cluster. 1.
No single model. however, can adequately describe the wide range of ionomer structures. Most systems are intermediate between the homogeneous aggregate and the phase-separated cluster, depending on backbone polarity and ionic functionality. The first attempt to develop a quantitative theory to answer the above questions was made by Eisenberg. Among the various models. those of Eisenberg, MacKnight et al.. and Yarusso and Cooper are worth noting.
5. ATTEMPTS TO DEVELOP QUANTITATIVE THEORIES FOR THE STRUCTURE OF IONIC THERMOPLASTIC ELASTOMERS 5.1
TheEisenbergModel (1970)
A molecular energy based theory of microphase formation in ionomers has been proposed by Eisenberg (1970). He chose a salt solution in a media of low dielectric constant. His model is based on eight assumptions: 1.
2.
3.
4. 5.
6.
The cation and anion are separated from each other by a distance corresponding to their ionic radius. Steric properties are used to calculate the largest number of ion pairs, which can group together without the presence of any intervening hydrocarbon. Energetic considerations are invoked to argue the formation of larger entities, which are composedof ion pairs separated from eachother by a hydrocarbon skin consisting of a portion of the backbone to which the acid groups are chemically attached. Work is performed to stretch the polymer chains during the formation of a cluster. The chain lengths between ionic groups are Gaussian in nature. The driving force for an ion pair aggregation or multiplet-multiplet condensation is an electrostatic interaction. Electrostaticenergy is released when multipletsaggregate toform acluster. The amount of released energy depends on the geometry of clusters and the dielectric constant of the medium.
442
Kar and Bhowmick
7. One-half of all the sequential ion pairsincorporated within the same clusteryields "rings." 8. The clusters will break down at some critical temperature. Tc., at which the rubberelastic forces and electrostatic forces are in equilibrium. These assumptions lead to the followingexpressions for concentration of ion pairs per cm' (C). number of ion pairs within a stable cluster (n), radius of multiplets (rIl1). averageintercluster distance (r,). and work required to separate an ion pair into dissociated ions for single charged ions (W):
(1)
r,,,
h p
= -
S,,
where. C, p. N. M,. n, MC., 1. k. TV, h'. h,)'. k'. K, eo, e, r. up. S,,,. rill. W. MO, n,,. and r, are the concentration of bound ions per cm', macroscopic polymer density. Avagadro's number. average molecular weight of the chain between ion groups. number of ion pairs within a stable cluster. molecular weight between ionic groups, length of the C-C bond, Boltzmann's constant. critical temperature above which the cluster becomes unstable, mean square end-to-enddistance of the free chain. mean square end-to-end distance for a corresponding freely joined chain. fraction of the electrostatic energy released upon formation of an ion pair from isolated ions for the particular ionic aggregate geometry. dielectric constant of the medium, permittivity of free space. electronic charge. center-to-center distance of positive and negative charges in a contact ion pair. volume of the ion pair. area of the hydrocarbon chain i n contact with the surface of the multipletsphere.radius of thetnultiplet.workrequired to separate an ion pair into dissociated ions. molecular weight per chain repeat unit, number of ion pairs per multiplet. and average intercluster distance, respectively. Quantitative calculations of Eisenberg's model are difficult. Values of n. r,. and rlllare reported for various cluster geometries. For a sodium salt of ethylene methacrylic acid copolymer containing 4.5 mol% acid, model predictedr, ranged from 44 to 95 However. the experirnental measured r, value is around 83 On this basis. the model is adjudged reasonable.
A.
A.
5.2 The MacKnight, Taggart, and Stein Model (Core-Shell model) (1974) MacKnight et al. (1974) have elucidated an entirely different origin for the ionic peak in ionomers. This model is based on the radial distribution function (RDF) and the analysis of lowangle x-ray scattering peak using the theories of Porod and Guinier. The RDF is the Fourier transform of the angular dependence of the scattered x-ray intensities. The intensity I (in electron units) of x-ray scattered by the amorphous medium is given by: Ill1
Ill1
443
lonomeric Thermoplastic Elastomers
where
Here, 28. h, N I , ml, n,. fi, IillC, rI, pi, ( 7 ) and el, ( m ) are scattering angle. wavelength of the radiation. total number of structural units contributing to the scattering intensity, number of different kinds of atoms present in the structural unit, number of i-type atoms in the structural units, the atomic scattering factorof the i-type atoms, Comptonscattering factor of i-type atoms, distance, the number of j-type atoms per unit volume at a distance r from a given i-type atom, and the average number of j-type atoms per unit volume, respectively. RDF is represented as:
5
4 m l [ D ( r 1 )- Do] =
-
Si(S) sin (Sri) dS
where
cc Ill1
111,
x
Si(S) = IT
r l sin Sr,
1 - 1
ni ni KiKj[pij(r,)- pi,(m)]drl
,-I
Here, Si(S), K,, K,. and D(rl)are the interference intensity, effective electron numbers for a t o m of type i(which is equal to the atomic number of the i-type atom), effective electron numbers for atoms of type j (which is equal to the atomic number of j-type atom), and superposition of RDFs for each kind of atom, respectively. MacKnight et al. (1974) adapted the experimental RDF difference curve to a hard sphere model in which number of particles per sphere and radius of the sphere are varied to give the best fit. A best fit is obtained for a cluster of radius 8 A and number of particles per cluster 48. The two peaks between 3 and 9 A in the RDF are due to the internal cluster structure. The peaks in RDF arise from the arrangement of multiplets within the cluster. RDF analysis indicates that ionic peak does not arise from interference between scattering centers, as assumed by both Marx et al. (1973) and Binsbergen and G o o n (1973). The size of the cluster radius is well beyond Eiserberg's multiplet limit. MacKnight et al. (1974) have predicted the radius of the cluster to be in the order of 3 to 4 in the dry state and somewhat larger than this in the wet state from the Porod analysis based on Eq. (IO) and other assumptions; i.e., the discrete phase is composed of monodisperse spheres and the volume fraction of the dispersed phase is 0.05 based on RDF analysis.
A
where Jc
Q
SfUS.)d S ;
= 0
2 sin H S . = ___ h
+
Here S/Vl, 4, and +?, and are interfacial area per unit volumeof the dispersed phase, volume fraction of two phases, and scattering invariant. respectively. However, comparable radius. i.e., 8-10 A. is observed from Guinier analysis. in which volume fraction of clusters (+l) is calculated from Eq. (13) (MacKnight et al.. 1974):
444
Kar and Bhowrnick
SS'(I' - 1)dS $1
=
l, x
S+S'I.' dS
where I' is the calculating intensity for the Guinier approximation. I' is obtained from Eq. (14) (MacKnight et. al., 1974):
US
k )
= I(O)exp[ - (4/3)aS;R']
(14)
where I(0) is the extrapolated scattering intensity at zero angle and R is the radius of gyration. These deviations in results are due to the particle size dispersity, nonuniform electron density of phases, possible lackof sharp phase boundary, and the presence of interference effects between particles. Based on this analysis, MacKnight et al. ( 1974) have proposed a shell-core model and a lamellar shell-core model for clusters. In the shell-core model, a cluster of 8- 10 A in radius is shielded from surrounding matrix ions by a shell of hydrocarbon chain. The surrounding matrix of ions, which cannot approach the cluster more closely than the outside of the hydrocarbon shell. is attracted to the cluster by the electrostatic force in the case of coordinate metal ions. The distance between the cluster and the matrix ions is on the order of 20 A, which is the origin of the ionic peak. 5.3 The Yarusso and Cooper (Modified Hard-Sphere Model) (1983)
The simple hard-sphere model involvesthe assumption of spherical particles that have nointeraction other than impenetrability. This model is incapable of predicting the observed intensity upturn near zero angle, the large difference between the calculated values, and the experimental values of functional groups in aggregates and provides a poor fit to the experimentaldata. Yarussoand Cooper (1983) proposed a model called the modifiedhard-spheremodel. This model is based on the following assumptions: The closest distance between the aggregate is 2 RCA, where RCAis greater than R (radius of the aggregate). 2. Each ionic aggregate is coated by alayer of hydrocarbonmaterialwhoseelectron density is same as the matrix.
I.
Yarusso and Cooper (1983) calculated the intensity of x-ray scattered, I(S) by the equation:
where
(b(SR) = 3
sin SR - SR cos SR (SR).'
Here, I,. V, p,, and E areintensity of x-ray scattered by thesingle electron, volume of the sample, difference of electron density between the spheres and the matrix. and constant very
lonomeric Thermoplastic Elastomers
close to one, respectively. The model provides a better existing model.
445
fit to the experimental data than any
6. PROPERTIES The presence of metal carboxylate ormetal sulfonate groups or other groups pendent in a polymer chain has a strong effect on polymer properties such as mechanical properties, glass transition temperature. the rubbery modulus above the glass transition temperature, dynamic mechanical behaviour. relaxation behavior, melt rheology. dielectric properties, thermal properties. electrical properties. optical properties, polymer solution behavior. etc. These properties depend on the morphology of ionomers. The morphology of ionomers is characterized by electron microscopy, infrared spectroscopy. Raman spectroscopy, Massbauer spectroscopy, nuclear magnetic resonance spectroscopy, x-ray diffraction, nuclear scattering, and electron spin resonance spectroscopy. A discussion and a review on ionomer properties are given by Holliday (1975), Eisenberg (1980), MacKnight and Earnest (1981). Rees (1987). MacKnight and Lundberg (1987), Tant and Wilkes ( 1988), Mauritz ( 1988). Fitzgerald and Weiss (1988), Lundberg ( 1989). and Hara and Sauer ( 1 988). In this section, the effects of ion content, counterion type, degree ofneutralization, aging and thermal treatment, plasticizer and other additives. and blend composition on various properties of ionic thermoplastic elastomers are discussed. 6.1
Infrared Spectroscopy
Infrared, Fourier transform infrared, and far-infrared spectroscopies are widely used to investigateion-ioninteraction and domainformation in carboxylatedandsulfonatedelastomersat variouslevels of neutralization with differentcations (Rees. 1964;MacKnightet al.. 196th; Otoaka and Kwei, 1968b: Uemura et al., 197 I : Tsatsas et al., 1971; Eisenberg and King, 1977; Brozoski et al., 1984a, 1984b; Agarwal et al., 1987; Coleman et al. 1990). Infrared investigation has been conducted on a sulfonated EPDM system. where the sulfonate group is neutralized with various monovalent and divalent cations to understand ion-ion interactions. Table 3 lists the vibrational bands observed in metal sulfonated EPDM (Agarwal et al., 1987). The base EPDM polymer shows bands due to atactic polypropylene and amorphous ethylene units. Two bands are observed in the region from 1750 to 1700 cm", at 1725 cm-' and 1715-' cm-' for Li',Ba'+,Mg",Zn", and Pb'+, and at 1720 cm" and 1700 cm-' for the NH4+salt. These bands are assigned to acetic acid, which is liberated upon reaction of the metal acetate with the sulfonic acid. The Zn'+.Pb" and NH4+ salts do not show any band in the region of 1650 to 1550 cm-', whereas Li' and ME'+ show at 1620 cm" due to cation-bound water of hydration, and Ba'" shows at 1580 cm" due to free water. The band at -1250 cm" is due to a -CH2 wagging motion. The intensity of this band is dependent on the polar end group and the conformation of the -CH2 group. The band at 1 100 cm-' is due to the longchain fatty acid. The band at 731 cm-' is characteristic of crystalline polyethylene segments containingthe tmrrr7s sequence of themethylenegroup. The1050 cm" band is due to the symmetrical stretching of the -SO3- group. Theband at 610-615 cm-' is due to C-S stretching of the polymer -SO>- band. The bands appear at 1 190. 1 155, and I 162 cm-' for Li+, Mg'+ and NH4'. indicatingasymmetricalattachment to the SO3- chain. In thecase of B$+. the interaction with the sulfonate ion appears to beasymmetrical,sincewell-definedbands are observed at 1 192 and 1 155 cm-'. There is some splitting in the case of Zn" and Pb'+, indicating an asymmetrical bonding. The sharp band at 731 cm" for Ba". a small amount in Zn'+. and
Kar and Bhowmick
446
Table 3 Vibrational Bands (cm"
)
1730 1 720
I728
1725
I725
I720
1718 1620 -
-
17 18 -
1700 -
I255
I155 1200 (S11) 1 15 0
1255 I 175 (Sh) I150 (Sh)
I255
-
1135 1115
I135
1728 1718 1 580 1255 I192 I155 I135 1115 I 100 1052 1030 73 I 61 0
1620 -
125s I190 113s 1118
I100 1065 103x 73 I 615 SorrrcY.
of Sulfonated EPDM Salts
-
1155 I135 1115 1 100 1050
1030 73 I 608
1115 1100
I 055 1030 731 (Sh) 60X
1450
I100 1045 (Sh) 1020 73 I 605
1162 -
111s 1100
1052 1025 7 31 610
AgLlrwal et a i , . 1987.
various degrees intheothersaltssuggest that ionicaggregationdue to thesulfonatecation interaction brings aboutan order rearrangement of the polymer chains, forming extended regions in which the methylene groups are in a f r a m configuration. Infrared spectra of ethylene-methacrylic acid containing 4. I mol% methacrylic acid and its ionomer over a range of neutralization from 0 to 78% at room temperature (MacKnight et al.. 1968a) have been investigated. The 2650 cm" band is characteristics of hydrogen bonding (hydrogen-bondedhydroxyl). Thereis unionizedcarbonyl at 1700 cm" and asynmetrical stretching of the carboxylate ion at 1560 cm". The 1560 cm" band increases with degree of neutralization.Investigation of the temperature dependence of the relativeintensities of the bands at 1700 cm" (hydrogen-bonded carbonylstretchingvibration)and 3540 cm" (free hydroxyl stretching vibration) evident in the acid copolymer gives a dissociation constant for the carboxyl-dimer association. The dissociation constant, K,,. is defined as:
K''
=
["COOH]' [(-COOH)2]
These infraredstudies have been extended to use the infrareddichroism to characterize the structural features. which are responsible for (Y relaxation (MacKnight et al.. 1968a: Uemura et al.. 197 1 ). The infrared dichroism is related to the orientation of a molecular chain by:
where ,f: is the molecular orientation function of the ith molecular segment. defined as: ,f: = [3 < cos2 0> ave (21)- 1]/2
D, is the dichroism. defined as
where A , I and A , are the absorbances for radiation polarized parallel and perpendicular to the
lonomeric Thermoplastic Elastomers
447
stretching direction. and C, is a constant related to the angle between the stretching axis and the transition moment. The orientation function (Uemura et al., 1971) and dichroic ratio (MacKnight et al.. 196th) have been calculated for various bands characteristic of the hydrocarbon segments (1470 cm". CH, bending: 720 cm". CHI rocking).unionizedacid (1700 cm". hydrogen-bonded C = O), and ionized carboxyl (1560 cm". carboxylate ion). The 720 cm" band shows large perpendicular dichroism and little of the hydrogen-bonded carbonyl (1700 cm") and carboxylate (1560 cm"). But the orientation functions of the above bands are found to increase with the degree of stretching. Infrared spectra of ethylene methacrylic ionomers are strongly dependent on annealing conditions. the presence of moisture. as well as coordinating tendency of metal ion (Brozoski et al., 1984a. 1984b; Coleman et al.. 1990). Far infrared spectroscopy is also applied to investigate domain formation in ionic polymer. The spectra covers a range of 33-800 cm". A well-defined band in the region below 600 cm" is observed in all salts. which is not present in acid forms of the copolymer. This band shifts from 450 ? S cm-' for the Li ionomer to 230 k 5 cm" for Na". 180 k 3 cm" for K'. and 135 ? 3 c n - ' for CS'. This band is attributed to the cation motion in the anionic fields of the polymers. The intensity of the peak is related to the cation properties andis assigned to perturbed skeletal motions of a neighboring polymer segment.
6.2 NuclearMagneticResonance Nuclear magnetic resonance is widely used in the study of relaxation phenomena. the extent of aggregation of metal ions. and phase transition of ionomers. Read et al. ( 1969) have measured relaxation time of copolymer derived from ethylene methacrylic acid copolymer containing 4.1 mol% methacrylic acid and its 53% ionized sodium salt at a radiofrequency of 30 MHz. T I , the spin-lattice relaxation time, and TIC.the spin-lattice relaxation time in the rotating frame, are rates at which the nuclear spins exchange energy with other modes of motion under certain conditions and are measured as a function of temperature in the study of phase transition. The n1inima in the TI curve at 0 and - 100°C are identified as y and 6 relaxation. The TI, data is represented by two separate relaxation times a t each temperature. which are tentatively assigned to nuclei in the amorphous and crystalline regions of the polymer. Four minima are observed in each of the TI, curves. These are assigned as a,p. y. and 6 relaxations. Similar behavior is observed for TI and TI, in unionized copolymer. The broad-line NMR technique gives some idea about the extent of aggregation of the metal ions in ionomer. Otocka and Davis (1969) examined NMR linewidth of ethylene acrylic acid copolymer (4.9 mole% acrylic acid) and its lithium ionotner (fully neutralized) over;I range of temperatures measured by both proton and lithium-7 magnetic resonance spectroscopy. In all cases. NMR line width narrows into two stagesidentified by y and p transition. The difference of the line narrowing observed in these two techniques indicatesthat nuclei are not well dispersed throughout the matrix. They are segregated to some extent. 6.3 X-RayDiffraction Wide-angle and small-angle x-ray scattering results elucidate the state of ionic aggregation in ionicpolymer.Wilson et al. (1968) compares diffraction scans of branchedpolyethylene.a copolymer of ethylene, and methacrylic acid. and its ionomer prepared by fully neutralizing with sodium. Polyethylene-like crystallinity is observed in all three samples. arising from the orthorhombic polyethylene unit. This is characterized by the presence of 1 10 and 200 peaks. The percent crystallinity is calculated from the ratio of the areas of 1 10 and 200 peaks to the
Kar and Bhowmick
448
total area. The acid copolymer and ionomer show less crystallinity than the parent polyethylene. The ionomer contains a new peak at approximately 20 = 4", referred to as the ionic peak. This is a common feature of all ionomers regardless of the presence or absence of backbone crystallinity and the natureof the backbone. In addition to this, the ionic peak has the following characteristics (MacKnight et al., 1974):
1. The ionic peak is observed above a certain ion content. 2. The ionic peak appears in all ionomers regardless of the nature of the cations being present with lithium,including all alkalimetals,as well asheavymetals,divalent cation, trivalent cation, quaternary ammonium ion, etc. 3. The nature of thecation influences thelocation of the ionic peak as well asthe magnitude of ionic peak. The ionic peak is observed at a low angle for cesiumcation compared to lithium cation at the same ion concentration. Similarly, the magnitude of the ionic peak of cesium is several thousand-fold greater than that of lithium. 4. The ionic peak is relatively insensitive to temperature. Its peak persistsat a temperature of even 300°C (Wilson et al., 1968). 5. The ionic peak shows no evidence of orientation in cold draw samples. 6. The ionic peak is moved to a lower angle or destroyed when the ionomer is saturated with water. The scattering profile in the vicinityof the ionic peakin the water-saturated ionomer is different from that of the parent acid copolymer. 7. The magnitude and position of the ionic peak depend on the amount of acid present in the parent copolymer and on the degree of neutralization (Wilson et al.. 1968). The low-angle x-ray scattering measurementsof a series of cesium ionomer obtained from ethylene methacrylic acid copolymer have been extended to a level of 28 = 0.01 radian (Delf and MacKnight, 1969). Theresults were also comparedwith low-density polyethylene. A steady decrease in intensity with increasing Bragg angle is observed in low-density polyethylene and the copolymer of ethynene-methacrylic acid. But the ionomer shows a strong peak at 28 = 0.02 radian. corresponding to a periodicity of 83A. This is due to the presence of aggregates containing cesium ions. Thispeak is insensitive to annealing. Thecorresponding lithium ionomer does not show any peak, which is due to the poor scattering of lithium. An extensive study of x-ray scattering has been done on ionomer prepared from butadiene methacrylic acid copolymer and ethylene methacrylic acid copolymer (Marx et al.. 1973a; Marx and Cooper, 1973).Variabilityincludestheeffect of plasticizersincludingwater,methanol. formic acid, acetic acid and methacrylic acid, degree of neutralization and acid content (up to 7 mol%). The results are interpreted according to the following equation: dBrkIgI! = c(V'
f-1)"3
(23)
where dH,k,gg, V', c, and .f" are the spacing correspondingto the measured Braggangle, constant, volume per carboxyl group (calculated from composition), and number of carboxyl groups per scattering site, respectively. The values of f - ' are increased from 2 for copolymers containing 2 mol% acid to 3 for composition between 3 and 5 mol% and 4 for compositions of 5-7 mol%. The number is not dependent on degree of neutralization. They do not observe the low angle peak as observed by Delf and MacKnight (1969). They conclude that the existence o f a secondary low angle peak outside the main peak in the wide angle is indicative of regularity in the spacing between the scattering centres.
Neutron Scattering Small-angle neutron scattering(SANS) hasgreat importance in the investigation of polymer morphology. Oneofthe most impressive accomplishmentis the measurementofdimensions ofa single
lonomeric Thermoplastic Elastomers
449
chain in bulk or the dimensions of ionic clusters. Several SANS studieshave been done on both deuterium-labeled and unlabeled ionomers (Mayer and Pineri. 1978; Roche et al., 1980; Earnest et al.. 1982). Contrast is achieved by adding measured amount o f D 2 0 to the samples. There is no evidence of a scatteringmaximum in thecase of adry sample. But the SANS peak becomesdetectable when small amounts of D,O are added to the sample. This can be explained by the measurements of neutron contrast factors shownin Table 4 calculated from the following equation:
where b, and b2 arethescatteringlengthspermolecular units and V I and v2 are the molar volumes of these molecular units. Table 4 reveals that the differences between neutron scattering contrast factorsof polypentenamer chains and cesium sulfonategroupsaresmall. But there is asubstantialdifference between D 2 0 and polypentenamer. Similar scattering curves are obtained for the 5.5% and 12% ionomers. The Bragg spacing of the SANS ionic peak observed at low D 2 0 concentration is the same as the SAXS peak for dry samples of cesium sulfonated polypentenamer. The SANS ionic peak moves to a low angle with increasing D 2 0 concentration in the sample (above a D20/S03- ratio of 6).
6.4
MossbauerSpectroscopy
MOssbauer spectroscopy is applied to several families of ionomers, such as the ferric salt of poly(butadiene-CO-styrene-CO-4-vinylpyridine)(Meyer and Pineri, 1978) and Nafion. to identify the various types of ionic aggregates. The appearance of a hyperfine spectrum at the expense of the doublet D1 is characteristic of magnetically ordered clustered complexes of radius 30 A with supermagnetic behavior. The supermagnetic behavior is confirmed by the existence of a residual thermal magnetization. At a temperature of -245°C there is only doublet Dl 1 and hyperfine spectrum (SH). The second doublet (D1 l ) , i.e.. the second component of the spectrum, has three characteristics: 1. The large quadruple splitting indicates a very asymmetrical environment. 2 . It appears at a temperature of - 33°C. near to the glass transition temperature, 3. The evolution of D l 1 with an applied magnetic field is characteristic of a zero spin.
Table 4 Scattering Lengths and Neutron Scattcrinp Contrast Factors (K,,) for Chemical Units in lonomer Chcmical unit -(CH?)j-CHSH-(CH?)j-CHS-
l
S0~"CS' -(CH:)j-CHSHD20 "CH: CH:D20
b , X 10'' (cm)
0.004 I 0.0 165
0.004 1 0.096 - 0.0071 0.096
K,,
X
10" (cm-2) 1S 4
84 106
Kar and Bhowmick
450
These lead to theconclusion that these complexes are dimerized with an antiferromagnetic coupling. At a temperature of -269°C. there appears a third doublet (Dl 11) in addition to second doublet and hyperfine spectrum.
6.5 Electron Spin Resonance Spectroscopy Electronspinresonancespectroscopy is used to studytheinteractionsbetween cations and ionomers. The spectrum of electronspinresonance is sensitive to the local environment of paramagnetic ions and depends on the interactions between the electrons and nuclear spins of ions and ligands. An electron spin resonance spectrum of butadiene methacrylic acid iononer neutralized with zinc (95%) and copper (5%) and containing 9% acid group at 25°C shows that when copper is used the degeneracy of five 3d levels is removed in the presence of a crystal field (Pineri et al., 1975). The spectrum is a characteristic of isolated Cu2+.It clearly shows the presence of hyperfine structure. The Lande factors, g, and g , and hyperfine interaction parameters. A;, and A;, are 2.056. 2.282. 146 Oe, and 23 Oe, respectively. This is attributed to the presence of the RCOO- Cu'+ -0OCR group.
,,
6.6
Raman Spectroscopy
Raman spectroscopy is also ernployed on a series of ionomers based on ethyl acrylate-co-sodium acrylate and poly(styrene-co-p-carboxy styrene) to characterize the degree of ionic aggregation (Neppel et al., 1979a, 1979b. 1981).
6.7 Electron Microscopy Various experimental studies have shown that strong structural changes occur in a variety of systemsuponneutralization of polymeracids.Electronmicroscopy is the best technique to demonstrate the presence of ionic regions in ionomer systems. Electron microscopy studies of sulfonated EPDM havebeen done by Handlin et al. (1981). Sulfonated EPDM contains doublebonds. The bondsare strained by osmium tetraoxide. Strained sections show the presence of ionic domains. Most of these ionic domains are spherical in shape and less than 3 nm in diameter. An examination of iononlers from butadiene methacrylic acid copolymer was done by electron microscopy (Marx et al., 1971). A grainy appearance was observed in the ionomers but not the free acid. The granular size was found to be considerably smaller than the ethylene ionomers. varying from 13 to 26 W. The distribution and size of ionic clusters in a butadiene-styrene-4-vinyl pyridine terpolymer-crosslinked by coordination of the pyridine groups with ferric chloride have been examined by electron microscopy (Meyer and Pineri, 1978). Many heterogeneities of high electron density were visible. No such heterogeneities are observed in the uncoordinated polymers. Large domains are due to the super position of smaller ones. Diameters vary from 50 to 1000 A. Transmission electron microscopy and surface replication electron microscopy have been carried out on ethylene methacrylic acid ionomer to elucidate the ionomer morphology (Davis et al., 1968; Langworth, 1975; Handlin et al., 1981). In the acid copolymer, the lamellas typical of crystalline morphology of polyethylene are clearly observed.These lamellas are further organized into spherulitic structures. But the ionomer shows no evidenceof such structures exhibiting two major features.
lonomeric Thermoplastic Elastomers
451
I.
Spherical regions approximately 1 k m in diameter appear: secondary electron imaging of a gold coated film shows that these are surface features, which are not present in the highly transparent bulk polymer. 2. Irregular electron dense features of about 2-20 nm proposed by Davis et al. ( 1968) are ionic domains. These are randomly distributed throughout the film. Another study showed thatthe acid form exhibits spherulitic morphology and the rubidium salt shows no such spherulitic structure, rather presents an irregular granular structureof diameter about 100
A.
7. MECHANICALPROPERTIES The high tensile strength of ITPEs compared to base polymers is attributed to their ability to relieve local stresses by an ion exchange mechanism. ITPEs in general show low permanent set even at considerable levels of stress relaxation and creep. The creep andstress relaxation behaviors are explained by the exchange mechanisms between time-dependent crosslinks. However, the creep recovery indicates that some of the crosslinking sites are very stable. These stable crosslinking sites are expected to be larger aggregates of clusters of ionic groups and to be stable at high temperatures. Mechanical properties of metal sulfonated EPDMs have been systematically studied by Amassetal. (1972), Rees and Reinhardt (1976). Makowski et al. (1980), andKurianetal. ( 1995). The effect of sulfonate content on tensile strength for zinc sulfonated EPDM at room temperature is shown in Fig. 3 (Makowski et al.. 1980). Tensile strength begins to develop at about IO- 15 mEq of sulfonate per l00 g of polymer. Remarkable tensile strength is obtained at 30 mEq of sulfonate per I00 g of polymer (equivalent to 1 mol% sulfonate) and attributed to the association of the ionic groups. Figure 3 also shows that tensile strength depends on the base polymer. Highest tensile strength is observed in E-70 (see Sec. 3). This is directly attributed to the combined effects of the ethylene crystallinity and ionic association. E-55 and CR-709-A behave similarly. The CR-2504, which is a fully amorphous copolymer, deviates from the behavior of other polymers. This can be explained on the basis of a less even distribution of sulfonate groups within the polymer backbone. Comparative data for mechanical properties and rheological properties for nine different cations are given in Table 5 (Makowski et al.. 1980). The rheological properties will be discussed in detail. The high degree of ionic association of these various metal sulfonated EPDMs are clear in their low elongation. These low elongations decrease the tensile strength. But zinc and lead systems show high tensile strength and elongation. The mechanical properties of sulfonated EPDMs dependon the ethylene content (Makowski et al., 1980). The tensile strength increases appreciably with increase in ion content in all series of ionomers. It also increases with the ethylenecontent of the ionic thermoplasticelastomer at any given ion content. Makowski and Lundberg (1980b) studied the plasticization effect of a large number of metal stearates and stearic acid on mechanical properties of various metal sulfonated EPDMs. The tensile strength of zinc sulfonated EPDM showslittle change with stearic acidconcentration. On the other hand, barium and magnesium sulfonates exhibit improvement in tensile properties. Although stearic acid is beneficial to tensile strength at room temperature, it shows a deleterious effect at high temperature, even at 70°C. The effects of different kinds of metal stearate (i.e., zinc, barium, and magnesium) on tensile properties of barium. magnesium. and zinc sulfonated EPDMs are given in Table 6 (Makowski and Lundberg, 1980b).
Kar and Bhowmick
7000 ._. 6000
[ E-70
W a "
."R 5000Y
I
5 r E &OOOIY
0 ( CR-709-A )
Li
!3000' v)
Z W
I[R-2504)
l-
20001000 a-
0
10 20 30 40 50 60 SULFONATE CONTENT, meq./100 POLYMER
Fig. 3 Effect of sulfonatecontent and EPDM on tensile strength at 25°C (metalcation:zinc).(From Makowski ct al., 1980.)
Table 5 Effect of Cations on Physical and Rheological Properties" 70°C
(poise Metal
Hg Mg
ca
CO
Li Ba Na Ph Zn
Melt Apparent viscosityh shear at X 10')
fracture rate (sec" )
-
-
55.0 53.2 52.3 51.5 50.8 50.6 32.8 12.0
<0.88 C0.88 <0.88 <0.88 <0.88 <0.88 88 147
25°C Melt index Tensile Tensile psl, strength Elongation strength Elongation g/10 (psi) min) (%) (psi)
( 190°C, 478
Disintegrated 0 0 0 0 0 0 0.1 0.75
-
-
-
320 410 1180 760 340 960 1680 1480
70 90 290 320 70 350 480 400
150 I70 450 250 150 270 320 270
(%l 40
40 160 130 30 1 10 350 450
Base polymer: CR-2504. Sulfonate content: 31 mEq/100 EPDM. Dissolved 1 0 0 g of frec acid in 1 0 0 0 mL hexaneIS0 mL Isopropanol: neutralized wlth 90 mEq acctatc In 25 m L water. h At 200°C and 0.88 sec". Sortrcrt Makowski et al.. 1980. .I
,
lonomeric Thermoplastic Elastomers
70°C
453
25°C
Tensile
Tensile Elongation strength strength Metal sulfonate
110
Ba Ba Ba 340 Ba Ba
ME ME
ME ME
ME Zn Zn Zn Zn
Elongation (70)
Melt Index 250 ( 190"C, psi, g/10 min)
Plasticizer
(psi)
(v?)
None BaSt, 230 MgStz ZnSt, StCOOH None Bast, MgSt, ZnSt, StCOOH None BaSt,
450
100
40
40
0
-
-
-
-
-
770 2050 1290 980 870 870 3.530 2040 1480 1920 2570 3040
470 590 0 210 140310 205 520 560 400 410 490 460
610 40 370 320 300 840 40 270 960 690 1 l50
410 >800 75
ZnSt,
(psi)
85 470 >800 450 370 370 750
0.0 1 0.6 1.0 0.02 0.02 I .9 4.0 0.2 0.04 0.05 8.2
Base polymer: CR-2.501. Sulfonate content: 33 mEq/100 g polymer. Source: Makowskl and Lundbcrg. 1980b. "
Little improvement in tensile properties is observed with barium and magnesium stearates. On the other hand, the best tensile property is observed from a zinc stearate plasticized zinc sulfonated EPDM. The effect of zinc stearate concentration on the tensile properties of zinc, barium, and magnesiumsulfonated EPDMs at roomtemperature and 70°C wasreported by Makowski and Lundberg (1980b). Substantial improvements were observed for every cation. But zincsulfonated EPDM shows the best properties.Zincsulfonated EPDM showshigher tensile strength compared to barium and magnesium EPDMs. Retention of tensile properties is maximunl for zinc due to its stability. Brenner and Oswald ( 1980) studied systematically the influenceof the structure of quaternary phosphonium counterious on physical properties and melt flow rate of sulfonated EPDM containing I mol% sulfonation. The physical properties and melt flow rates are given in Table 7 (Brenner and Oswald, 1980). Table 7 demonstrates the effect of chain length of n-alkyl substituents, number of higher alkyl substituents, and mono- and divalent quaternary ionson physical properties. Tensile properties decrease with increasing length of n-alkyl substituents and with an increasing number of higher alkyl substituents on the charged central atom. The addition of long chain further diminishes the available space near the central atom. This results in a much higher degree of steric hindrance in the region near the central atom as well as making it difficult for anions to come close. Of course. the magnitude of the effects of adding long chains depends strongly on the length of the short chain originally present in the polymer and on the length of the added long chain.Increasinglength of theoriginal short chain decreases the effect of theadded chain lengths. The physical properties of the ionomer depend not only on the structure of the single counterion but also on the interaction between counterion-anion ion pairs. The anion from a
Kar and Bhowmick
454
Table 7 Physical and Rheological Quatcrnary Phosphonium Ions
Propcrties of Sulfonated EPDM" Neutralized with Various
Tcnsile
modulus strengthh Quatcrnary Samplc no. I 2 3 4 5 6 7 X 9 10 11
12 13 14
15 16
phosphomum ions
Initial
(kPa) (kPa) S40 300 290 260 300 290 300 520 390 I80 550 310 210 I85 I60 700
1700 I100 970 940 1000 I000 1000 I 500 1250 700 1500 1100 YO0
750 560 1700
flow Mclt rate (150"C, 12.5 kg of I 10 min)
0.09 0.01 0.03 0.22 0.01 0.08 0.0 I 0.02 0.2s
0.003
first ion pair, in attempting close approach to the cation of a second ion pair. must contain not only the substituents of that quaternary ion but also the anion i n the second ion pair. At the same time, the second anionis trying to minimize its distance from its own associated counterion and thereby is also dragging its covalently attached polymer backbone chain close to the central charged atom. In addition, the first anion must contain the substituents of its quaternary counterion in the first ion pair. to which it is closely held. as well as with its appended polymer backbone chain. which it must drag alone. Thus, physical properties of ionomers depend on the strength of the interaction of the ion pairs.Similarly, the relative strength of the ionomer samples depends on the monovalent and divalent counterions. But it is very difficult tomake any generalizations. Aging at ambient temperature near the glass transition temperature for longer periods of time or thermal treatment at elevated temperatureshave little effect on the mechanical properties of metal sulfonated EPDMs (1 mol%) (Duvdevani et al., 1986). The modulus of an ionomer containing 50 phr zinc stearate increases about 35% at an aging time of 100 hours. Only the stress-strain response is slightly altered at 230 hours of aging. The copolymerization of methacrylic acid with 1,3-butadiene gives a tougher, less elastic elastomer. more thermoplastic in nature than base polymer (Brown, 1963; Jenkins and Duck, 1975). The advantages of carboxyl incorporation are increased hardness, higher green strength. better milling properties. good resistance to hydrocarbon solvents, higher temperature limit of elasticity. and superior adhesion properties. The main disadvantages are a greater tendency to swell or react in aqueous ammoniacal or alkaline solutions and increased oxidative attack leading to degradation and crosslinking. These propertiesincrease with increasingmethacrylicacid content, as shown by an increase in tensile strength. The rubbery properties of the copolymer lessen above 40% acid content. Theincorporation of a suitable divalent metal oxide in a butadieue-methacrylic acid copolymer containing less
lonomeric Thermoplastic Elastomers
455
than 40% methacrylic acid leads to a rubber of very high green strength. This divalent metal oxide neutralizesthe acid group andproducesastableneutralsalt, -OOC-Zn"-COO-. But neutralization of this carboxylated rubber with monovalent salts provides a weaker networkthan that obtained with divalent salts. For example, the tensile strength and elongation at break of butadiene-methacrylicacid copolymer having 10% methacrylicacid change to 11.7 M N h ' and 900% from 0.7 MN/m' and 1600% after treatment with NaOH followed by heating and to 41.4 MN/m' and 400% after zinc oxide treatment (Jenkins and Duck, 1975).It can be crosslinked by salt of diamine. salt of polyamine. polyalcohol, polyepoxy resin, and carbodiimides. Polyalcohol and polyepoxy are used for covalent crosslinking, but the physical properties are inferior compared to sulfur vulcanizates. It shows very poor compression set and high stress relaxation at elevated temperature. However, a combination of epoxy and metal oxide gives excellent hightemperature properties. As for the raw polymer. tensile strength of the cured polymer increases with acid content i n the copolymer (Jenkins and Duck, 1975). The thermoplastic nature of butadiene-methacrylic acid copolymers of varying acid content neutralized by lithium have been observed by Otocka and Eirich ( 1968). The effect of zinc oxide concentration on butadiene-acrylonitrile-methacrylic acid copolymer (56:33: I O ) has been studied by Brown ( 1957). Theoretically the amount of metal oxide or salt necessary to obtain the best properties should be proportional to the amount of carboxyl groups present in the polymer. But in actual practice. twice the calculated amount of metal oxide is required to obtain optimum mechanical properties. Only a portion of the oxide is used in effective chemical crosslinking; the rest remains as a mixture of some free oxide, some -0OCM-OH unlinked metal bonds, some intramolecular bonds, and some -00C-Zn2'-C00- bonds (Brown, 1975). The disadvantagesof oxide-cured butadiene-acrylonitrile-methacrylic acid ionomer are poor compression set. high stress relaxation, low flex resistance, low hysteresis loss, very low fluidity at high temperature. tendency to scorch during compounding, and poor mold flow of the compounded rubber. These properties are a result of the lack of stable crosslinking. The tendency to scorch during compounding can be avoided by using retarder (e.g.. phthalic anhydride) or by late addition of metal oxide in the mixing cycle. The mechanical properties of butadiene-styrene-methacrylic acid copolymer containing I .5 wt% carboxyl group with various divalent metal oxides and hydroxides have been discussed by Dolgoplosk et a l . (1959). They found that metal hydroxides give better physical properties than metal oxides. except magnesium oxide. Neutralization of these carboxylated rubbers with monovalentsaltsgivesaweakernetworkthan that obtainedfromdivalentsalts. This weak network is destroyed above 100°C. The lack of stablecrosslinks i n the metal oxide-cured vulcanizates is evident in the stress relaxation experiment. The unmodifiedtelechelicpolyisobutylene (PIB) polymer is aviscous liquid atroom temperature. while the sulfonated telechelic polyisobutylene ionomers (SPIB)exhibit properties of typical ITPEs. SPIB shows low permanent set, low hysteresis loss. and high tensile strength. It can be extended upto 1000% (Bagrodia et al., 1983a).Mechanical properties of SPIB ionomers depend onthe molecular weight.SPIB ionomershaving number average molecular weightabove 1 1,000 show good tensile strength andhigh elongation at break.The enhancementof mechanical properties with increased molecular weight is attributed to the formation of a end-linked pseudonetwork by the association of physical crosslinks of the ionic groups at the chain ends (Bagrodia et al.. 1983a). Unmodified SPIB doesnot show any strain-induced crystallization.However, above 600% elongation. a sharp diffraction pattern is observed by wide-angle x-ray diffraction in Ca-SPIB ionomer. The presence of terminal ionic groups and long-range coulombic interactions are responsible for maintaining molecular orientation with strain (Mohajer et al., 1982; Bagrodia et al., 1982). The strain-induced crystallites assist in maintaining low permanent set.
Kar and Bhowmick
456 Table 8 Influence of Cations on Ethylene Methacrylic Acid Ionomer Properties
Condition Anion Cross-linking agent (Wt%) Property Melt index ( I O g/min) Yield point (MPa) Elongation (%) Ultimate tensile strength (MPa) Stiffness (MPa) Visual transparency
Acid
Na'
Li+
BaZ+
Mg'
-
CHjO4.80
OH2.80
OH9.60
CH3COO8.40
5.80
0.03
0.12
0.19
0.12
6.10 553.00 23.40
13.20 330.00 35.80
13.10 317.00 33.90
13.40 370.00 33.90
15.00 326.00 40.40
13.20 3 13.00 29.70
7.10 347.00 22.00
68.90
190.30 Clear
206.80 Clear
223.40 Clear
164.10 Clear
208.00 Clear
103.40 Clear
-
Hazy
t
Zn"
AI3+
CH3COO12.80
CH3COO14.00
0.09
0.25
Soltrce: Rees. 1966.
Ca-SPIB ionomer shows higherelongation than K-SPIBionomer.Mechanicalproperties of SPIB ionomers neutralized with lanthanide series elements (cerium,lanthanum, etc.) are comparable with potassium and calcium ionomers and depend on cation valence andexcess neutralizing agent (Tant et al., 1986). The influence of various metal cations on mechanical properties of ethylene methacrylic acid ionomer is given in Table 8 (Rees, 1966). Monovalent,divalent,andpolyvalent cations enhance physicalproperties.Mechanical properties depend on the degree of neutralization. Table 9 demonstrates the effect of degree of neutralization on mechanicalpropertiesand melt index of ethylene-acrylicacid copolymer neutralized with various alkali metals (Rees, 1987). The modulus increases with degree of neutralization up to 30% and then decreases. Stiffness increases with neutralization to a plateau at about 40%. and tensile strength increases up to 77% neutralization for most cations (Rees, 1987). The effectsof aging in nitrogen atmosphere at ambient temperature on mechanical properties of ethylene-methacrylic acid ionomers neutralized by zinc and a combination of zinc and organic amine, such as 1,3-bis(amino methyl) cyclohexane (BAC), have been investigated by Hirasawa et al. (1989). Stiffness increases with aging time. The increase in stiffness is high in the case of fully neutralized ionomer. After 38 days of aging, the percent increase in stiffness is about 28% for the acid, 56% for the 0.6 zinc ionomer, 81 % for the 0.6 zinc and 0.4 BAC ionomer, and 130% for the 0.2 zinc and 0.97BAC ionomer. In another study of ethylene-methacrylic acid ionomers neutralized with various typesof counterions at various degree ofneutralization,asignificantincrease in both modulus and yield stress was observed (Hirasawa et al., 1991). But the effect of ageing on large strain properties is different. There was little change in the value of tensile strength and elongation at break in the prolonged aging time. The stiffness and yield point of diamine salts of ethylene methacrylic acid ionomer are increased substantially, but the tensile strength and melt viscosity remain constant ( R e a , 1987). A relationship between diamine chain length, acid content, and these properties has been established (Rees, 1987). Short diamines are effective only at high acid levels, while long diamines such as decamethylene diamine are unsuitable in all acid-containing polymers. But the diamine ionomers of ethylene methacrylic acid are not commercially viable due to oxidative instability.
Table 9 Properties of Ethylene-Acrylic Acid Copolymer Salts 4 Neutralized Starting material, 14.84 acrylic acid wt Sodium salt
Potassium salt
Lithium salt
" Infrared measurements Source: Rees. 1987.
0
12.0 30.0 47.5 66.0 8.0 25.0 51.0 63.0 12.0 28.5 52.5 67.5
Melt index, 43.25 psi (dg min-') 67 12.2 3.9 1 .o 0.3 16.30 4.50 2.70 0.57 18.7 5.2 1.4 0.2
Melt index, 432.5 psi (dg min-') 2.570 256.0 92.0 30.0 7.6 360 110 49 15 442.0 116.0 38.0 5.4
Secant modulus 14 extension (psi)
Tensile strength (ultimate) (psi)
Elongation at break (%)
Density (g/cc)
7000
2150
470
0.949
33,400 48,600 42,500 39,700 29,300 52,600 49,500 44,800 26.300 48,900 48,500 36,900
3,150 4,000 4,600 4.800 3,050 3.700 4,450 5,000 3,150 3,850 4, I00 4.600
420 330 310 280 470 410 370 390 410 350 260 250
0.9568 0.9586 0.9603 0.9633 0.9588 0.9626 0.9684 0.9750 0.95 16 0.95 10 0.9493 0.9446
Kar and Bhowmick
458
Table 10 Mechanical Properties of Ionomers Derived from Terpolymer Precursors Composition by weight Methacrylic index Melt Tensile modulus Strength Elongation Cation acid acetate Ethylene Vinyl 70 70 70 65 65 65
20 2s
10
-
10
Nai Mg?'
10 10 IO 10
Na' Mg' '
(g110 min)
(MPa) (MPa)
9.00 1.20 0.04 12.50 1.70 0.007
13.80 56.00 37.44 12.90 18.50 37.40
(%)
10.3 43.4 45.0 9.6 30.9 37. I
530 410 280 610 410 300
The mechanical properties of a large number of ionomers derived fromethylene dicarboxylic acid have also been reported by Rees (1987). Tensile strength increases with neutralization when the acids contain adjacent carboxyls. The polymers do not increase significantly in modulus on neutralization.althoughthetensilestrengthincreases.Withnonadjacent carboxyls, as in itaconic acid. the expected modulus increase is observed. Rees (1987) has reported a series of ionomers from ethylene vinyl acetate methacrylic acidterpolymersynthesized by Wolff. A very interestingproperty, good tensilestrength, is obtained, as shown in Table IO. A report from E.I. du Pont de Nemours shows another interesting type of ionomer derived from terpolymers of ethylene, vinyl acetate, and an organic acid. These were supplied with the trade name Elvax and have the advantages of superior oil and grease resistance and improved adhesion to polar substances. Their important properties are shown i n Table 1 I . The effect of mixed peroxide and metal oxide vulcanizates on the mechanical properties of carboxylic nitrile elastomer have been discussedby Brown (1963). Compression set drastically decreases and modulus increases in salt and peroxide crosslinking vulcanizates. On the other hand, tensile strength and tear strength dramatically decrease. The best properties are obtained from mixed peroxide and metal oxide vulcanizates compared to pure metal oxide vulcanizates. The effect of a mixed crosslinking system in carboxylated nitrile rubber has been reviewed by Chakraborty et al. (1982).
8.
DYNAMICMECHANICALPROPERTIES
The dynamic mechanical properties of ionomer systems provide definite evidence that the salt forms of these systems are dramatically different from the acid forms and the parent polymers. Specifically,theneutralized forms of thesematerialsdisplayarubbery plateau in modulus temperature curves that is not present in the base polymers. In this section, the storage modulus, loss modulus, loss tangent, stress relaxation behavior, and glass transition temperature of individual ionomers are highlighted. The data of storage modulus for three different monovalent cations, i.e., lithium. cesium, and ammonium, and one bivalent cation, i.e., barium, at different temperatures was reported by
lonomeric Thermoplastic Elastomers
459
Table 11 TypicalProperties of Ethylcne Vinyl Acetate h o m e r . '
Mclt indcx" Vinyl acetate, % Acid number' Density ( ASTM D I 505 ). 23°C kg/m.' (g/cm.') Tensile strength (ASTM D 1708)," MPa (psi) Elongation at break, (ASTM D 1708)," ?+ Elastic (tcnsilc) modulus (ASTM D 1708),'1~'MPa (psi) Hardncss, Shore A-2 (Durometer, 10 sec, ASTM D 2240) Softening point, ring and ball, (ASTM E 28), "C ("F) Cloud point in paraffin wax,' "C ("F) ~~
Elvax 4260
Elvax 4310
Elvax 4320
Elvax 4355
5.0-7.0 27.0-29.0 4-8 955 (0.955)
420-580 24.0-26.0 4-8 945 (0.945)
125-175 24.0-26.0 4-8 947 (0.947)
5.0-7.0 24.0-26.0 4-8 952 (0.952)
19 (2700)
2.0 (300)
5.2 (750)
19 (2800)
IO00 10 (1500)
600 6.2 (900)
900 8.3 ( 1200)
1000 I4 (2000)
80
68
72
83
158 (316)
83 (181)
91 (195)
151 (304)
99 (210)
88 ( 190)
88 (190)
88 (190)
~~
These data are presented as a general description of properties and are not Intended to he used for design spccificatlons. l' dg/min (ASTM D 1238, modified). Milligrams potasslum hydroxlcie per gram polymer. <' Samples die cutfrom pressed films: guage dimcns~ons2.23 cm X 0.47 c111 X 0.13 cm (0.876 In. X 0.187 in. X 0,050 in.) crosshead speed 5.1 cm (2 in.)/min. Elongation b a e d on sample length of l . Y l cm (0.75 In.). '' Modulus calculated as 111 ASTM D 638. ' 10% Elvox In fully refined paraffin wax 146 AMP. Source: Htrrdlxwk of Eltrsrorwrs. ( 1988): E. 1. du Pont de Nrnlours & Co.. Ltd. I'
Agarwal et al. (1980). The degree of sulfonation is almost identical. i.e., 0.7 mol%. The storage modulus data are multiplied by the factorT(fl due to the temperature dependence of the modulus in accordance with the kinetic theory of rubber elasticity. The results show that the barium, lithium, and cesium sulfonated EPDMs form a very stable and tight network. The barium salt has a higher modulus than the lithium and cesium salts. The results indicate that the strength of ionic associations in the barium salt is higher than that of cesium and lithium salts. On the other hand, theammoniumsulfonated EPDM formsamuchweakerpseudonetwork. In this material, a very short rubbery plateau is observed up to room temperature. Beyondroom temperature,themodulus drops morerapidly. The ionic group interactions in divalentcations,i.e., barium, magnesium, and lead, are very strong (Agarwal et al., 1980). But the lead sulfonate forms a weaker network than the other two sulfonates. The modulus drops above 100°C. The degree of sulfonation has a strong effect on storage modulus. A drop of the modulus of base EPDM about 3 orders of magnitude is observed in the neighborhood of 50°C. The modulus decreases with increasingtemperature, and above 100°C the sample rapidlyapproachesthe viscous flow region. A well-defined rubbery plateau regionis developed when a sulfonate group is incorporated into the base EPDM polymer. About 0.5 mol% sulfonate enables the polymer structure to remain intact up to a temperature of 150°C. The increase in sulfonate content results in an increase in the magnitude of the rubbery plateau region. The modulus of the samples having a higher degree of sulfonation decreases comparatively slowly with increasing temperature. At the higher sulfonation levels and the intermediate temperature region, storage modulus increases moderately with increasing temperature. This can be explained by the theory of rubber elasticity.
Kar and Bhowmick
460
This type of behavior is typically observed in covalently crosslinked elastomers of moderate to high crosslink densities. The plasticizer has a strong effect on the storage modulus of sulfonated EPDMs. The modulus of 0.6 mol% zinc sulfonated EPDM drops rapidly and approaches the flow region at 100°C at about 20% zinc stearate concentration. This is attributed to the fact that either the zincstearate eliminates the ionic group association or theionic domains become mobile and permit the flow of polymer molecules. Rubbery modulus also increases in both low and high sulfonate systemswith increasing plasticizer loadings.This can be explained as follows. As the plasticizer loadings are increased, regions rich in ionic groups are solvated and increase in size. This contributesto an increase in modulus. Paeglis and O’Shea (1988) studied the effect of various polar plasticizers on zinc sulfonated EPDM having 25 mEq of sulfonation per 100 g of rubber. The blend formulation is 100 phr zinc sulfonated EPDM, 67 phr paraffinic oil, and 22 phr various polar plasticizers,i.e.. oleamide, stearamide, N , A”-ethylene bis(stearamide). They established the relationship between the melting of the ionolyzer and the drop in the storage modulus. A large number of modulus-frequency curves over a range of temperature from - 70 to 25°C is generated in order to understand the primary relaxation behavior (Agarwal et al., 1980). The various curves have similar shapes. There is no unusual behavior. The curves are shifted to the frequency axis to generate master curve at a reference temperature of -40°C. The WLF shift factors are usedto generate master curves.The master curves forthe lowest and highest zinc sulfonated EPDM were compared with base EPDM. The overall shape of the master curves was similar and the behavior of the material was thermorheologically simple, revealing that the viscoelastic relaxation processin zinc sulfonated EPDMs is due to the nonionic phase. The phase rich i n ionic groups does not contribute to the primary relaxation process except for slowing them down. At very low frequencies, a rubberyplateau is observed, indicating the existence of crosslinking through ionic association. The magnitude of the rubbery modulus increases with increasing sulfonate content. The glassy moduli of zinc sulfonated EPDM are a function of ionic group concentration. The highest glassy modulus is observed at the highest sulfonate level and is due to the increased interaction between the polymer backbone. Relaxation time distributions havealso been calculated from the following equation (Ferry, 1980):
where E’(w), H(T).W, and T are frequency dependence storage modulus, distributionof relaxation time, frequency. and relaxation time, respectively. A broadening of relaxation times and a decrease of rate of relaxation with increasing sulfonatecontent has been observed. This couldlead to theconclusion that an increase in ionic group concentration decreases thediffusionalprocess due to increasedintermolecular interactions and thereby restricts configurational motions. Loss tangent temperature curves for various zinc sulfonatedEPDMs including base EPDM show only one major peak corresponding to the primary transition (Agarwal et al., 1980).This temperature is designated by Tg. Thewidth or height of the main transition peak does not change with increasing sulfonate group concentration. This suggests that there is no significant change in the nonionic phase of these polymers. There is a secondary transition in the vicinity of room temperature. This peak indicates some sort of relaxation phenomena occurring in the regions rich in ionic groups. The Tgof EPDM is not strongly influenced by the incorporation of sulfonate groups. An increase of about 8°C in Tg occurs by incorporation of 1.4 mol% zinc sulfonate groups into EPDM polymer.
-
lonomeric Thermoplastic Elastomers
461
However, Tg is not affected by barium. magnesium. lead, lithium, Cesium, Or ammonium salts at the sulfonatioll level of 0.7 mol%. All the samples have more or less the same Tg of about - 5 0 " ~ The . Tg increases by 3°C with the incorporation of 10 parts zinc Stearate into the zinc sulfonated EPDM containing 0.6 mol% of sulfonate group. Doubling the amount of plasticizerincreases the Tg another 5°C. Similareffectshave been observed at highersulfonate content. The height of therubbery plateau increases with increasing ion content in butadience lnethaclylic acid copolymer and its lithium salts (Otocka and Eirich, 1968). The distribution of relaxation times calculated from stress relaxationdata showsa maximum associated with relaxations when the 1nodulus begins to drop strongly after the plateau. This maximum is related to another additional mechanism of relaxation (Tobolsky etal., 1968). Thebehavior of shift factors from stress relaxation experiments indicates the presence of a second relaxation mechanism. At low temperature, the relaxation process operates by diffusive mechanisms associated with Tg. However. at higher temperature, the relaxation process becomes dominant by another process, which is dependent on relaxations in the ionic phase. Stress relaxation studies of rubber-based ionomers made by Otocka and Eirich (1968), Tobolsky et al. (1968), and Meyer and Pineri (1978) indicate that there is no breakdown of time-temperature superposition principle within the limited range of scale. Three relaxations are observed at - 133, - 58, and - 3"C, designated 7 , p. and p'. respectively (Otocka and Eirich. 1968; Pineri et al., 1975; Meyer and Pineri, 1978). The p' relaxation arises from large-scale micro-Brownian chain motionsin the amorphous phase, which is crosslinked by intermolecular hydrogen bonding of the carboxylic acid groups. The p' peak disappears upon neutralization by zinc or copper, and a new peak at 67°C designated by a, arises. This a peak is assigned to motions within the ionic phase. But the p peak remains at the same temperature. Tg increases very little with increasing degree of neutralization. The butadiene-acrylonitrile-methacrylic acid copolymer has a sharp decrease in modulus with increasing temperature compared to the zinc salt of butadiene-acrylonitrile-methacrylic acid terpolymer. indicating a low level of acid group interaction (Tobolsky et al., 1968). The vulcanizates of butadiene-acrylonitrile-methacrylicacidcured by metal oxide haveahigh modulus at ambient temperature that slowly decreases with increasing temperature. The rate of stressrelaxation of oxide-curedbutadiene-acrylonitrile-methacrylicacid copolymercorresponding to relatively labile crosslinks or movement of whole ionic clusters is high compared to sulfur-cured vulcanizates containing stable covalent crosslinks (Tobolsky et al., 1968). Three types of relaxation are observed i n partially neutralized (<30%) butadiene-styrene-vinyl pyridene ionomer (Pineri et. al.. 1975; Meyer and Pineri, 1978). The p relaxation that is associated with Tg occurs at - 56°C. The second relaxation. labeled by a , is observed at 7°C after the start of the rubbery plateau. At 57"C, the third relaxation, labeled as a', is observed after the start of viscous flow region. The a' relaxation does not increase at higher percent neutralization; however, the a' peak broadens and shifts to higher temperatures. The a relaxation is assigned to the breaking and reforming of isolated ion pair-ion pair association. The U' relaxation arises from the motions of large aggregates or clusters. The pendent group concentration, nature of pendent group (i.e., acid, ester, or salt), and type of pendent group (i.e., thioglycolate, phosphonate. carboxylate, or sulfonate) have strong effects on dynamic mechanical properties of substituted polypentenamer ionomers (Sanui et al., 1974a. 1974b; Sanui and MacKnight, 1976;Azuma and MacKnight, 1978; Rahrig, 1978; Tanaka andMacKnight,1979;RahrigandMacKnight,1980a,1980b). Two relaxations are evident, labeledas p and 7 , in order of decreasingtemperatureforester, acid, andsodiumsalts of thioglycolated polypentenamers having 5.5 mol% thioglycolate (Sanui and MacKnight, 1976). The p relaxationcorresponding to the glass transition temperature decreases by10°C going from ester to salt. The relatively high modulus level maintained above p relaxation argues in
462
Kar and Bhowmick
favor of a reinforcing effectdue to ionic domains i n addition to the aggregated salt groups acting as a multifunctional crosslinks. The p relaxation is shifted to higher temperature and appears to be quite broad for cesium of phosphonylated polypentenamer ( I O mol%) compared to ester and acid derivatives (Rahrig, 1978). On the other hand, the maximum temperature, i.e., Tg. is nearly the same for ester, acid, and salt derivatives of phosphonylated polypentenamer. But in general, Tg increases with an increase in pendent group (MacKnight and Earnest, l981 ). The rate of increase of Tg with pendent groups follows in the order: thioglycolate = carboxylate < sulfonatephosphonate. This is dueto the two reasons. The firstreason is related to the composition of copolymer. The bulky phosphonate group with its two cations is expected to increase Tg more effectively than the smaller carboxylate group occurring in the thioglycolate and carboxylate derivatives. The second reason is related to the ability of the various salts to segregate into separate ionic phases. There is strong evidence for cluster formation as a separate ionic phase in thioglycolate and sulfonate salts. while weak evidence existsfor cluster formation in phosphonate salts. As a result, the concentration of thioglycolate and sulfonate salts decreases in the hydrocarbon phase at low concentration and decreased Tg than phosphonate, where the salt groups are homogeneously dispersed in the hydrocarbon phase. The nature of the pendent group has little effect on Tg at a constant concentration. For 5% thioglycolate derivatives, Tg increases from - 94°C for the ester to - 92°C for the acid and - 91°C for the salt, whereas for 5% carboxyl derivatives. Tg is within experimental error. The 10% thioglycolates show Tgs of - 89, - 87, and - 87°C for the acid, ester,and salt. respectively. The phosphonate groups differ in Tg by 3""C at 10% concentration. The presence of ions in the polar hard phase improves the dynamic mechanical properties of polyurethaneionomer by coulombic interactionsbetweenionicspecies(Eisenberget al., 1982). Polyurethane ionomers are functionalized by a variety of metal neutralized anionomers. The plateau modulus and softening temperature increase with increasing charge on the cation. This is due to the increase in ionic crosslinking. Increasing the number of zwitterionic groups lowers the Tg. This behavior is explained by the decreasing compatibility between the polar hard segments and the nonpolar hard segments. As a result, the soft segments segregate into a another phase and decrease Tg. The influence of molecularweight,cation. and excess neutralizingagent on dynamic mechanical properties has been investigated for sulfonated polyisobutylene telechelic ionomers (SPIB) (Bagrodiaet al., 1985). The unmodified hydrocarbon polymer flows at room temperature, while the sulfonated polymer exhibits a small rubbery plateau above the Tg of the polymer. The rubbery plateau region is extended to about 100°C upon neutralization due to the strong interactions of ionic groups. With increasing neutralization. the softening temperatureof ionomer shifts to higher temperature. However. the initial modulus does not change significantly with excess neutralizing agent. The dynamic mechanicalproperties of ethylenemethacrylicacid copolymer andtheir ionomers have been extensivelyinvestigatedandthesubject of many reviews (Rees and Vaughan, 1965; MacKnight et al., 1967, 1968a, 1968b; Ward and Tobolsky, 1967; Longworth andVaughan, 196%; OtockaandKwei,1968a,1968b.1968c, 1969; Mckenna et al., 1969; Kajiyuma et al., 1970). In general, the response of both the parent acid copolymers and their salts are equal to that of the low-density polyethylene. At the lowest temperature ( - 120°C) there is a large loss peak labeled as y relaxation. The y relaxations of the copolymer and its ionomers are resolved into two peaks (Mckenna 1969). One is labeled as yc. which occurs at the lowest temperature and is proportional to the degree of crystallinity. The other one is labeled as yti,which is proportional to the amount of amorphous materials. But the degree of crystallinity of the copolymer and itssalt is very low even at 7 mol% methacrylic acid. Therefore, y relaxation
lonomeric Thermoplastic Elastomers
463
is best described by the crankshaft motion of short segments of hydrocarbon chain. Another new relaxation, labeled as p'. appears at a temperature between0 and 50°C in the acid copolymer. The magnitude and sharpness of this new relaxation depend on the comonomer composition, degree of neutralization, and thermal history of the sample. The p' relaxation increases with increasing acid content. This peak is assigned to micro-Brownian segmental motion. which is related to the Tg. The Tg of polyethylene is - 20°C. The higher temperature compared to the Tg of polyethylene is attributed to the crosslinking effect of dimerized carboxyl group. Another relaxation, designated by p, appears at a lower temperature. between -20 and 0°C. near the Tg of polyethylene. The effectof neutralization of sodium salt of annealed ethylene-methacrylic acid copolymers on p and p' peaks shows that the p' peak disappears completely and that p relaxation shifts to the lower temperature with increasing degree of neutralization. The p peak is, therefore, assigned to a relaxation occurring in the amorphous phase of polyethylene. But the location of p relaxation in polyethylene is strongly dependenton the degree ofcrystallinity(Earnest and MacKnight, 1977).The decreasing p-relaxation temperature with increasing neutralization in annealed ethylene-methacrylic acid copolymer is ascribed to the better phase separation and less ionic material at the amorphous phase. But the p peak of quenched ethylene methacrylic acid remains at the same temperature. It does not depend on the degree of neutralization. This result is attributed to incomplete phase separation ratherthan to lowercrystallinity. Contradictory results, i.e., a regular increase in the P-relaxation temperatureas a function of increasing ion content. has been reportedby Otocka and Kwei ( 1 9 6 t h 1968b, 1968c, 1969). Theyconcluded that the separate ionic phase does not exist. Another new relaxation, labeled as p, is observed at high temperature in the acidcopolymer containinglow amounts of acrylic acid. This is related to the CY transition of polyethylene. At higher concentration the CY peak does not arise. The relaxation sepectrunl of ethylene-methacrylic acid copolymer and its salt over a wide range of temperatures has been calculated (Sakamoto et al., 1970). The unionized copolymer shows onlyflowregion,whereastheionizedpolymer shows arubbery plateau region. The rubbery plateau region decreases with increasing temperature. These results indicatethe existence of ionic aggregates above the crystalline melting point. The ionic aggregates are mobile to move from one domain to another during flow. This flow becomes easier with increasing temperature and more difficult with increasing frequency. The rubbery region is shifted to shorter time with increasing frequency due to the loosening of the ionic structure. The calcium salts show similar behavior.Longworth (1975) calculatedtheparameters of the Voigt elements of aseries of ethylene methacrylic acid ionomers having various degree of neutralization as:
T: =
Jiqi
(27)
where J i , T:, qi. and J m are compilance, retardation time, internal viscosity, and total elastic compilance, respectively. The basepolymer contains 5.4mol%methacrylicacid. The most striking feature is the enormous increase in retardation time from < 100 to 10' seconds. This increase is due to the increase in viscosity from 10" to 10'' poise. At the same time. there is very little increase in elastic component with increasing neutralization up to loo%, which then decreases further. This clearlyindicates the presence of strongionicaggregates.Wardand Tobolsky ( I 967) have studied the stress relaxation behavior of a series of ethylene methacrylic acid ionomer and superimposed the stress relaxation curves. They have observed the dependence of shift factors on temperature due to the presence of crystallinity in the polymer backbone and an additional relaxation mechanism arising from the salt groups.
Kar and Bhowmick
464
9.
MELT RHEOLOGY
This section discusses the effectsof molecular weight, molecular weight distribution, concentration of counterion, types of counterion, composition of ionomer, hydrogen bonding, sulfonylsulfonyl association, and carboxyl-carboxyl association on rheological properties of individual ionomers at low and high shear stresses. The influence of sulfonatecontent on melt viscosity of four different zincsulfonated EPDMs having thesame Mooney viscosity, different ethylene/propylenecomposition, and different sulfonate distribution is reported by Makowski et al. (1980). The melt viscosity is substantially enhanced over that of base EPDM at the zinc sulfonate level of 20 mEq/100 g polymer (or 0.6 mol%). A marked increase in melt viscosity is observed at a sulfonate content of 40 mEq/l00 g polymer. The difference of melt viscosity in this family of polymers is due to the difference in molecularweight,molecularweight distribution, andunsaturationdistribution. Three materials, i.e., E-70, E-55, CR-709A (see Table 2), show similar behavior but a difference in magnitude. The differences are attributed largely to differences in molecular weight. The deviaticn of the CR-2504 system from the other three systems is interpreted as a result of an uneven distribution of sulfonate groups along the CR-2504 backbone. The sulfonate groups are used less effectively in network formation. Melt viscosity of ionomers depends on the type of counterion (Makowski et al., 1980). The melt viscosity data for nine different cations were given in Table 5. The metal sulfonated EPDMsare preparedfrom CR-2504 sulfonicacid by neutralization with thecorresponding acetate. Mercury shows very high viscosity. Six cations other, i.e.. magnesium. calcium, cobalt, lithium, barium, and sodium, show high and identical viscosity. All of these materials are melt fractured at the shear rate of viscosity measurement, 0.88 sec". This lack of distinction in cation type is most likely attributable to the departure of these highly viscous materials from laminar flow. A considerable lower viscosity is achieved with lead and zinc cations. These data suggest that sulfonate ionomers based on EPDM showan exceptional degree of ionic interaction. The degree of neutralization affects the melt viscosity properties of sulfonated EPDMs (Makowski et al., 1980). Varying amounts of metal acetate are added to the polymeric acid containing 33 mEq of sulfonic acid per 100 g of sulfonated polymer and isolated through solvent flashing. Zinc acetate concentration decreases melt viscosity by a factor of 2 . The decrease of melt viscosity with increasing zinc acetate is explained by the plasticization of the ionic associations by the zinc acetate, therebylowering theapparent molecular weight. It is alsoobserved that disulfonates -S03-Zn-S03are converted to monosulfonated -S03-Zn-OOC--CH3 at higher zinc acetate concentration. A substantial change in melt viscosity is also observed in other ionomer systems. A comparative study has beenconducted of the melt viscosities of zinc sulfonated EPDMs that are identical in all respects except with respect to Mooney viscosity or molecular weight (Makowski et al., 1980). V-2504 was used for this study, having a Mooney viscosity about 40. It was degraded to a material having Mooney viscosity of 20. The materials were compared at two sulfonate levels. A significant decrease in melt flow was observed when the Mooney viscosity was approximately doubled from 20 to 40. By doubling of sulfonate content from a level of approximately 22 mEq/lOO g polymer, a more substantial reduction in melt flow was observed. It is expected that much higher viscosities will result in a considerable reduction in melt flow. However, the tensile properties of the zinc sulfonated EPDM are not affected over the 20-40 Mooney range. This alsoindicates that it is possible to reduce Mooney viscosity withoutsubstantial loss of mechanical properties. But more substantial increases of melt viscosities are obtained with further increases in Mooney viscosity.
lonomeric Thermoplastic Elastomers
465
The melt flow rateof metal sulfonated EPDMs dependson the structure of the counterions (Brenner and Oswald, 1980). Thevalues are discussed with respect to the structure of quaternary phosphonium counterions. Variability includes the chain length of n-alkyl substituents, number of higher alkyl substituents, and mono- and divalent quaternary ions (Table 7). The melt flow rate increases with increasing length of n-alkyl substituents and with an increasing number of higher alkyl substituents on the charged central atom of the quaternary ion. The rate of increase of melt flow rate strongly depends on the substituents with branched or other bulky structures attached near the central charged atom. The melt flow rate of metal sulfonated EPDMs depends on the concentration and type of plasticizer (Makowski and Lundberg, 1980b). Lead, zinc, and ammonium improve significantly the melt flow properties, whereas other cations and stearate show little effect. Melt flow rate depends on the concentration of plasticizers. The effect of stearic acid upon the melt flow index of zinc, barium, and magnesium sulfonated EPDMs has also been demonstrated. The zinc salt responds more readily to the plasticization, followed by the magnesium salt and then the barium salt. This order of melt flow behavior corresponds to the order of the strength of the ionic association: barium sulfonate > magnesium sulfonate > zinc sulfonate. The molecular architecture, types of cation, excess neutralizing agent, andionic plasticizer affect the rheological propertiesof SPIB ionomers (Bagrodia et al., 1984, 1986). Monofunctional ionomer does not form a network structure, although the viscosity varies by a factor of about 2 depending on the cation used. Monofunctional ionomer shows lower viscosity, compared to di- and trifunctional ionomers. Viscositydecreases with addition of a small amountof monofunctional ionomer to the trifunctional star ionomer. In all cases, the viscosity of the zinc ionomer is lower than that of potassium and calcium ionomers. The use of excess neutralizing agent increases melt viscosity. The excess neutralizing agent is believed to be preferentially incorporated into ionic domains rather than distributed in the hydrocarbon phase. Theincrease in viscosity is prominent for the lower molecular weight ionomer. Increasing molecular weight results in an increase in viscosity. Ionic plasticizers effectively reduce the melt viscosity through solvation of ionic network. The melt rheology of low molecular weight carboxyl-terminated polybutadiene ionomers neutralized with mono- and divalent cations hasbeen investigated by Cooper (1958)and Otocka et al. (1969). Otocka et al. (1969) observed that the end group association occurs both in monoand divalent cations. Analysis of data for the acid-terminated polymers in terms of apparent molecular weight is inadequate to determine the degree of hydrogen-bonded dimer formation. The evidence suggests that the rate of interchange between hydrogen-bonded species is faster than the time scale of the experiment. Viscosity is a function of apparent molecular weight in the case of partially neutralized polymers. The low shear viscosities of ethylene methacrylic acid copolymer depend on copolymer composition (Longworth. 1975). Two mol% methacrylic acid increases the viscosity by about half,whereasasimilarpercent of sodium salt increases viscosity about20-fold. This great increase in viscosity is a measure of the powerful intermolecular attraction between the ionized residues.Longworth (1975) reportedthecorrelationbetweenthis viscosity (q,,).molecular weight (K), and methacrylic acid content (MAA) as: log q,,= 3.23 log
M,,
+ 0.033 (% MAA)
- 15.5
(28)
Viscosity increases with increasing molecular weight of copolymer and percentage of methacrylic acid content. Hydrogen bonding influences the viscosity of ionomer (Blyler and Hass. 1969). Theactivation energy for the viscous flow increases from 12 kcal/mol (polyethylene) to 17 kcal/mol (copolymer) when the amount of methacrylic acid is 8 mol%. The increased activa-
466
Fig. 4
Kar and Bhowrnick
Schematic representation of thecarboxyl dimer.
tion energy for viscous flowof copolymer overpolyethylene is thought to be due tothe presence of carboxyl dimer (Fig. 4). At higher stresses, the crosslinks are no longer effective, and there is a decrease in activation energy. Values of activation energy for viscous flow (AE,,,,,) are calculated from theviscosities of polymers over a range of temperature from 120 to180°C and from the followingequation:
AE,.,,,
=
d In(29) qo / d (l/T)
where T is temperature in kelvin. The activation energies of copolymers and fully neutralized sodiumionomers (Longworth, 1975) are similar to the reported values (Blyler and Hass, 1969; Sakamoto et al., 1970). This observation leads to the conclusion that the temperature dependence of the viscosity of these materials is influenced by the changes in free volume rather than by changes in specific chemical interactions. These data are treated by reduced variables, i.e., reduced viscosity. Viscosities of ionomer show different behavior at high shear rates. Viscosity decreases with increasing shear rate as (Longworth, 1975): Log(rldrlo) = Q I ( h
m’
(r
2 1 / ~ ~ )
(30)
where qO,qt,Q I ,and T~ are zero stress viscosity, shear rate-dependent non-newtonian viscosity at shear rate r, unknown parameter, and dimension of time respectively. As observed earlier, there is a strong increase in the low shear rate viscosity with increasing degree of neutralization. But the effect is much less at high shear rates.
10. ELECTRICALPROPERTIES The electrical properties of ionomers offerconvincing evidence that salt forms of these systems are dramatically different from the parent polymers. It identifies the various relaxation peaks and leads to conclusions similar to those made from dynamic mechanical analysis. A systematic studyof the electrical propertiesof ethylene methacrylic acid and its ionomer was done by several researchers (Bonotto and Purcell, 1965; Longworth and Vaughan, 1968a; Read et al., 1969; Phillips and MacKnight, 1970) over the range of acid contents, i.e., from 0 to 8.3 mol%,andfordifferenttypes of cation,i.e.,lithium. sodium, and calcium. The acid copolymer containing 4.2 mol% methacrylic acid is characterized by a p’-relaxation temperature of about 30°C. This p’ relaxation decreases, and new relaxations, i.e., p and a,appear with increasing degrees of neutralization. The p relaxation increases in magnitude but decreases in temperature,and a relaxationincreases both in magnitude and temperaturewithincreasing degree of neutralization. Similar observations are observed for sodium and calcium salts. Both
lonomeric Thermoplastic Elastomers
467
relaxations depend on the frequency of the measurement. The dielectric constant increases with an increase in degree of neutralization and decrease in frequency. Sodium ionomer shows a higher dielectric constant than calcium ionomer. However. thereis very little change in dielectric constant for acid copolymers with change of acid content (Longworth and Vaughan. 1968a). This retlects the fact that the carboxyl groups are associated as dimers at room temperature and the carboxyl-carboxyl dimer is electrically neutral. Activation energies for various relaxation processes are obtained from the plot of frequency of maximum loss versus reciprocal temperature. These variousrelaxationprocessesareanalogous to themechanicalrelaxationprocess described in the dynamic mechanical section. The a relaxation is absent in the unneutralized acid copolymer and increases in magnitude and temperaturewith increasing degree of neutralization. Reed et al. (1969) concluded on the basis of high activation energy that this relaxation is due to the ionic areas and retlects the breakup of large ionic regions into smaller clusters. They cannot explain the same activation energy of cx relaxation of sodium and the lithium ionomers Another explanation for cx relaxation is that it is due to the Maxwell Wagner effect (Smyth, 1955). It is observed in a medium consisting of droplets of a material of high dielectric constant dispersed in amedium of low dielectricconstant. The dielectric (3’ relaxation is similar in behavior to mechanical (3 relaxation with respectto strength and location andchanges i n composition. This relaxation is ascribed to the motion of amorphous hydrocarbon chains bonded by electrically neutral carboxyl dimers. Dielectric p relaxation differs from mechanical relaxation with respect to lower activation energy ( 1 5 kcal/mol vs. 35 kcal/mole). It shows behavior similar to mechanical relaxation in terms of dependence of degree of neutralization and location of temperature. The mechanical relaxation is ascribed to the motion of the amorphous segments. But the amorphous segments are electrically neutral. Therefore, dielectric relaxation is due to the motion of a few free acid or isolated salt groups attached to the amorphous segments. The dielectric y relaxation correlates well with the mechanical y;, relaxation process. resulting from the motion of short segments of hydrocarbon chain excluding polar groups and lamella crystals.
11. THERMALPROPERTIES Thermal properties of ionomers of ethylene-methacrylic acid having 4.1 mol% methaclylic acid and degreeof neutralization varying from0 to 80% determined by differential scanning calorimetry have been critically examined by several researchers (Rees and Vaughan, 1965: MacKnight 1967, 1968a; Otocka and Kwei. 1968a. 1968b; Marx and Cooper, 1973a). Themelting temperature designatedby T,,, and the peak temperature of the endotherm designated by T i (except 78% ionization) are independent of the degree of neutralization. The peak temperature of exotherm, designated by Ti, heat of fusion, designated by A H , , and weight percentage of crystallinity (except 0% neutralization) decrease with degree of neutralization. Similar trends are also observed in the ionomers having different acid contents (Otocka and Kwei, 196Xa, 1968b). The results are explained by Flory’s equation as
where T,,,.T,,1(),R, AH,,, N’are the melting point of copolymer, melting point of homopolymer, universal gas constant, heat of fusion of homopolymer crystals, and fraction of crystallizable units, respectively. Carboxyl group and salt group are included for calculation of N’. Linear plot of l/T,,, versus log N’ gives a value of 750 cal/mol for heat of fusion and three different
468
Kar and Bhowmick
intercepts for UT,”for acid. sodium, and magnesium ionomers. Thisis explained by the change of either the fold surface energy of the lamella crystal or the lamella thickness. In addition to these variables. the temperature of annealing influences the peak temperatureof melting endotherm, melting enthalpy, volume fraction of ionomer crystallinity, and density (Marx and Cooper, 1973; Marx et al.. 1973a).
12. OPTICALPROPERTIES Optical properties help in understanding the structural features of ionomers (Stein, 1964; Kajiyama et al., 1970; Prud’homme and Stein, 1971. 1973). This is based on the measurement of birefringence. The static strain coefficient. K,, is calculated from the slope of linear plot of birefringence versus elongation. The dynamic strain coefficient, K*, is separated into two parts, i.e., real andimaginary. The birefringence does not depend on timeandtemperature. K, of copolymer decreases drastically with temperature, whereas the ionomer exhibits a maximum at 42°C. This maxima temperature does not depend on the nature of cation (Kajiyama et al., 1970). The temperature of the peak varies in the followingorder: dynamic loss tangent peak > dynamic birefringence peak > static birefringence peak. These results are due to the two processes. As the transition temperature is approached,there is an increase in orientation of both the hydrocarbon- and ion-containing segments, corresponding to a softening of the ion-containing segments. It is alsoconsidered that the acid groups function as quasi-crosslinks either by hydrogen bonding or by electrostatic attraction. The strain optical coefficient is proportional to the number of these crosslinks. The quasi-crosslinks decrease with increasing temperature. As a result, the value of K, decreases with temperature. The shape and intensity of scattered light in static and dynamic light scattering measurementelucidatethe shape andsize of thescatteringparticles (Prud’homme andStein,1971, 1973). It shows spherulitic structures in quenched copolymer containing 4.1 mol% methacrylic acid, anisotropic rods in quenched sodium ionomer (55% neutralized), and spherulitic structure in annealed sodium ionomer (anneling at 91°C for 18 hr). Thesize of the spherulites is 2.3 p m for acid copolymer and 4.1 p m for annealed sodium ionomer. Furtherinformation about spherulitic structure is obtained by plotting the values of in-phase and out-of-phase components of the scattered light against temperature in dynamic light scattering experiments.
13. BLENDS OF IONICTHERMOPLASTICELASTOMERS Ionic thermoplastic elastomeric blends of sulfonated EPDM and polypropylene (PP) have been investigated over a broad range of compositions (Duvdevani et al., 1982). Comparisonof tensile strength, melt viscosity, and morphology with blends of EPDM and PP are reported. The tensile strength and ultimate elongation at break% of blends of sulfonated EPDM and PP are higher than those of blends of PP and EPDM. The high tensile strength is attributed to the association of the ionic groups in sulfonated EPDM and PP. Another ionic thermoplastic elastomeric blendof transition metal salt of sulfonated EPDM (zinc and copper) and poly(styrene-CO-4-vinyl pyridene) was reported by Peiffer et al. (1986). The blend showed higher melt viscosity. In the blend, a complex is formed between ionomers and vinyl pyridene groups of the copolymer. The complex is involved in a I : I stoichiometry of sulfur to nitrogen and is confirmed from infrared spectroscopy and melt viscosity results. The influence of blend composition of trifunctional sulfonated polyisobutylene telechelic ionomers with monofunctional sulfonated polyisobutylene telechelic ionomer on mechanical
lonomeric Thermoplastic Elastomers
469
properties has been reported by Bagrodia et al. (1 987). Both ionomers have the same molecular weight. Tensile strength decreases with an increase in compositionof monofunctional sulfonated polyisobutylenetelechelicionomer.Initial modulus andelongationatbreak% also decrease. Bagrodia et al. (1987) also studied mechanical properties of blends of trifunctional sulfonated polyisobutylenetelechelic ionomers havingdifferentmolecularweights,i.e., from8,300to 34,000. The tensile strength of ionomer having a molecular weight of 8,300 is 1 MPa. When 60% of the 8,300 molecular weight ionomer is blended with 34,000 molecular weight ionomer, tensile strength increases to 4 MPa, which is identical to the value of 34,000 molecular weight ionomer. The stress-strain properties are reported for ionomeric blends of sulfonated butyl rubber(SBR) with PP, high-density polyethylene (HDPE), and styrene-butadiene-styrene blockcopolymer (SBS) (Xie et al., 1991). The synergetic effects on tensile strength are observed in the blends of S-BR/SBS and S-BR/PP. These interesting results are attributedto the formation of interpenetrating networks (IPN). In the S-BR/SBS blend, the crosslinks are provided by the ionic interactions in the ionomer componentand by the hard, glassy polystyrene domains in the SBS component. But in the S-BR/PP blends, the crystallites of PP provide the crosslinks. In addition to this, there is some degreeof affinity between the two phases.This affinity is due tothe presence of common methyl groups in the butyl rubber component and in the PP component. This leads to good adhesion between the phases. Another ionic thermoplastic elastomeric blendis the blend of zinc oxide-neutralized maleated EPDM and zinc salt of ethylene-methacrylic acid (Datta et. al., 1996). A typical 60/40 mEPDM/Zn-EMA blend has the following properties: 100% modulus 5.53 MPa, tensile strength 12. I O MPa, elongation at break 220%. and tension set at 100% elongation 29%. The properties of the blends at25°C are greater than those predictedby the additivity rule. Dynamic mechanical properties and infrared spectroscopic studies reveal that the ionic bonds are formed at the interfaces of m-EPDM and Zn-EMA, which facilitates formation of the technologically compatible blend. A blend of zinc salt of maleated high-density polyethylene and maleated EPDM rubber is another ionic thermoplastic elastomer (Antony and De, 1999).
14.
PROCESSING OF IONICTHERMOPLASTICELASTOMERS
Processing of ionic thermoplastic elastomers is similar to that of other thermoplastic elastomers. Typical extrusion conditions are as follows: Barrel temperature, ("C) Rear Front Die temperature, Melt ("C)
160 350 160-168
All products of ionic thermoplastic elastomers are injection-moldable. A combination shear and high temperature is required.
of high
15. APPLICATIONS The wide spectrum of properties, the simple fabricating technique, and the attractive cost are the important factors that have led to many uses of ionic thermoplastic elastomers. The formulation and compounding of ionic elastomers are similarto that of traditional rubber compounding,
Kar and Bhowmick
470 Table 12 Typical CompoundingIngredientsfor Range
of phr
Ingredients Ionolyzer Process oil Fillcr Other polymers Processing acid Antioxdant
Ionic ThermoplasticElastomers Examples
5-35 25-200 25-250 10-125 2- 1 0 0.2-2.0
Zinc stearate. zinc acetate, stearamide Paraffinicnaphthenic oil, oil Carbon black, silica, clay. calcium carbonate. metal oxides Polyethylene, polypropylene Waxes, lubricants Naugard 445, Irganox 1010
Table 13 Typical Property Range of Ionic Thermoplastic Elastomers ~~
~
Property
Range
Hardness, Shore A 100% modulus, MPa Tensile strength, MPa Elongation, 9 Tear strength, MPa Specific gravity Compression set, 76 Brittle point, "C Processlng tempcrature, "C
45-90 1.2-6.9 3.4-17.2 350-900 0.9-2.3 0.95- I .95 30-35 -57 to -46 93-260
Source: Paeglis and O'Shea. 1988.
but they have several distinct differences. Table 12 lists some typical compounding ingredients that may be used for ionic thermoplastic elastomers (Paeglis and O'Shea, 1988). A wide variety of products can be made by combining materials such as ionolyzer and filleralong with othercompoundingingredientsbased on severalionicelastomeric groups. Typical physical properties of ionic thermoplastic elastomers are given i n Table 13 (Paeglis and O'Shea, 1988).
Table 14 SulfonatedEPDMElastomericSheetingFormulation Description
Ingredients
30
IE 2590" Lubrazinc W sunpar 2280 N 110 N 550black Marlex 6060 Naugard 44.5 .l
Polymer Zinc stearate Paraffinic process oil Reinforcing black Reinforcing polyethylene High-density diphenylamine Modified
phr I 00 25
IS 60 15
antioxidant
Sulfon;lted EPDM containing 2.5 mEq of sulfonation per 100 g of rubher.
Source: Paeglis and O'Shea. 1988
I
471
lonomeric Thermoplastic Elastomers
Sulfonated EPDM ionomers are used as thermoplastic elastomers and are marketed by several leading industries.Two grades of sulfonated EPDM are available in powder form (Uniroyal Technical Information Bulletin, 1982). Theycan be compounded with fillers, rubber processing oils, and selected polymers to meet a wide range of properties. A typical sheeting formulation of sulfonated EPDM is given in Table 14 (Paeglis and O'Shea. 1988). Table 15 shows the physical properties of these compounds (Paeglis and O'Shea. 1988). Their performance is claimed to be better than that of black filled EPDM vulcanizates specified by the Rubber Manufacturer Manufacturer Association (RMA). Sulfonated EPDM is also used in making shoe soles. garden hoses, and calendered sheets. The use of zinc sulfonated EPDM as a waterproof heat-sealable roofing membrane of high tear strength was described in a patent to Uniroyal (Paeglis. 1984). The excellent characteristics of ethylene-methacrylic acid-based ionomer marketed by du Pont as Surlyn are high toughness, abrasion resistance. adhesion, clarity, melt strength during processing. etc. (Table 16).These outstanding characteristics find applications in packaging film including composite structures employed as heat-seal layers, vacuum packaging for processed meats. skin packaging for electronic and hardware items, paper and foil coatings of multiwall bags, golf ball covers, roller skate wheels, bowling-pin coatings.etc. ( M a c b i g h t and Lundberg, 1987) (Fig. 5).
Table 15 Physical Properties of Black Filled Sulfonated EPDM Compounds
Physical properttes
Ionic elastomer ASTM requirements methodminimum compound
Color Thickness. mm D 412 Specific gravlty D 297 A ShoreHardness, strength, Tensile Elongation at break. % 412 D set, Tensile % at 50% elongation D 412 Tear. kN/m (Die C ) 68.4 D 624 absorptton Water c/r ( I60 hr at 70°C) D 471 Field seam strength. KN/m (Lap scam-T peel) 794 C Heat aging resistance 28 days at 1IO"C MPa Tensile. 412 Elongation at brcnk, c/c D 412 Tear, KN/m (Die C ) 42. D 624 Dimensional stability, c/cdays 7 at 116°C D 1204 Ozone reststance (166 hr at 40°C. 100 pphr) D 1149 Temperature dependence brittle polnt. "C D 746 Propertics at 50°C Tensile, MPn D 412 Elongation, % 790 D 412 Properties at 75°C Tensile, MPn D 412 Elongation. 0 D 412 Sorrrcrt Paeglis a n d O'Shea. 1'9x8.
Black 15 1.07 73 13.5 S80 5
RMA black filled EPDM Black
> 10
*
1.18 0.03 60 k 10 >9.7 >300
< 10
5.2-8.8
>21 .9 +2 -
12. I 460 I
>8.3 >?l0 >31.9
- 1.8
-c2 No crack
f
2.3
No crack - 56
>-ss
4.1
-
4. I 850
-
Kar and Bhowmick Table 16 SURLYN@Ionorner ResinsPropertyCornparisonst
lonomeric Thermoplastic Elastomers
473
Kar and Bhowmick
474
Fig. 5 a) Winter sports accessories; b) buoys of foamed Surlyn; c) bumper guard of mjection-molded foamed Surlyn; d) automotive trim and decorative parts; e) bulletproof glass that includes a transparent layer of modified Surlyn; f) surlyn golf ball. (Source: Surlyn Ionomer Resins, E.I. du Pont de Nemours 8c Co., Ltd. 1999.)
Table 17 Princlpal Properties and Uses of Ionic Ethylene Vinyl Acetate Ionomers Gradea Elvax 4260 Elvax 4310 Elvax 4320 Elvax 4355
Princlpal properties and uses High molecular weight resin for use in hot-melt systems where improved adhesion to polar, nonporous substrates is required; in coatings, provides superior hot tack, improved grease resistance and optimum barrier properties Low molecular weight resin deslgned to provide improved grease resistance and adhesion in low-viscosity systems; permits maximum Elvax content at a given viscosity in solvent or hot-melt systems Intermediate molecular weight resm, higher in viscosity than Elvax 4310 and intermediate in performance between Elvax 4310 and 4355; can be combined (as can Elvax 4310) with Elvax 4355 or 4260 to optimize performance at a desired viscosity level High molecular weight resin preferred in high-hot-tack systems: most effective of the 4300 series resins in imparting toughness, flexibility, and seal strength to blends wlth wax
All grades contain 200-800 ppm butylated hydroxytoluene. Source: Handbook of Elastomers. 1988.
lonomeric Thermoplastic Elastomers
475
Severalpatentshavesuggested new usesforionicthermoplasticelastomers,which vary fromasphaltmodification to impactmodification of engineeringthermoplastics. The asphalt ionomers prepared by modification of asphalt are usefulas road pavingmaterials that retain a very highfraction of theirstrengthunder wet conditions(Ciplijauskas et al., 1980). Grades andimportantpropertiesanduses of ionic ethylene vinylacetateionomers are given in Table 17.
ACKNOWLEDGMENT The technical data taken from the product literature of various companies and contained herein are guides to the use of their product. The advice is based on tests and information believed to be reliable, but users should not rely upon it absolutely for specific applications. It is given and accepted at the user’s risk, and confirmation of its validity and suitability in particular cases should be obtained independently. The companies make no guarantee of the results and assume no obligation or liability in connection with their advice. This publication is not to be taken as a license to operate under or a recommendation to infringe any patent. The author acknowledges theassistancereceived from his studentsintypesettingthe manuscript.
REFERENCES Agarwal, P. K., Makowski. H. S., and Lundberg, R. D. (1980). Moor~~rnolre~~lrs 13:1679. Agarwal, P. K., Dutta, P. K.. and Lundberg, R. D. (1987), Polwrer 28:1467. Amass, A. J., Duck, E. W., Hawkins, J. R., and Locke, J. M. ( 1972), Eur. Polyrn. J. 8:781. Antony. P,, and De, S. K. (1999). P o l w e r 40:1487. Azuma, C., and MacKnight, W. J. (1978). J. Polyru. Sei.. Polv. Chenl. 15547. Bagrodia, S., Mohajcr, Y., Wilkes, G. L., Storey, R. F., and Kennedy, J. P. (1982), Polyrx Bull. 8:281. Bagrodia. S., Mohajcr, Y., Wilkes. G. L.. Storey, R. F., and Kennedy, J. P. (19831). Polytn. Bull. 9:174. Bagrodia, S., Wilkes, G. L., and Kennedy. J. P. (1983b). J. Rkeol. 28:474. Bagrodia, S.. Pisipati, R., Wilkes, G. L., Storey, R. F., and Kennedy, J. P. (1984), J. Ai~pl.P d y t u . Sci. 293065. Bagrodia, S. R.. Wilkcs, G. L., and Kennedy, J. P. (1983, J. Appl. P o l w . Sei. 30:2179. Bagrodia. S., Wilkes, G. L., and Kennedy, J. P. (1986), Po/.vr,l. E q . Sei. 26:662. Bagrodia. S., Tant, M. R., Wilkes. G. L., and Kennedy, J. P. (1987). P o l w e r 28:2207. Blyler, L. L.,and Hass, T. W. ( 1969), J. Appl. Polwn. Sci /3:2721. Bonotto, S., and Bonner, E. F. (1968aj, MNcrorlfolec1rle.s 1514. Bonotto, S.. and Bonner, E. F. (196Sb), A m C h e m Soc. Polyrn. Prep 9537. Bonotto, S., and Purcell, C. L. (1965), Modern Pktsrics 42: 135. Binsbergen, F. L., and b o o n . G. F. (l973), MLlc.r-onlolec./~le.s 6 :145. Brenner, D., and Oswald, A. A., (1980), in 1orf.s i r l Pnlyrwrs (Eiserberg, A., Ed.), Am. Chem. Soc. Adv. Chem. Ser. 187. Washington, D.C.. p. 53. Brown, H. P. ( 1957), Huhher Chern. Techno/.30:1347. Brown. H. P. ( 1963j, RuhDrr Cher17. T e c h o l . 36:93 1. Broze, G.. Jcrome, R., Teyssie. P,, and Gallot, B. ( 198 I). J. Polyrn. Sci.. Po/yrn. LW. 19:415. Brozoski. B. A., Coleman, M. M,, and Painter, P. C. (1984a). MLtcrorllolecrtles 17230. Brozoski, B. A., Painter. P. C., and Colemnn, M. M. (1984b), Mrrcror,folecu/e.s171591. Bryant, C. L. (l970), J. bit. Rubher I m t . 4:202. Canter, N. H. (1972). U.S. Pat. 3.642,728. Chakrahorty, D.. Mal. D.. Konar. J., and Bhowmick, A. K. (2000). J. Elnsromer. Plnstics 32:xxx.
476
Kar and Bhowmick
Chakrahorty, S., Bhowmick, A. K., and De, S. K. (1982), J . Macror~rol.Sei. Re\,. C2/(2):313. Ciplijauskas, L.. Piggott, M. R., and Woodhams, R. T. (1980). in loris i r l Po/vrrrer.s (Eisenherg, A., Ed.), Am. Chern. Soc. Adv. Chem. Ser. 187, Washington, D.C., p. 171. Cokernnn, M. M,, Lee, J. Y.. and Painter, P. C. (1990). Mrrcrorr~olecrr1e.s.23:2339. Cooper. W. ( 1958). J . Polyrn. Sei. 28: 195. Datta, S., De. P. P,. and De, S. K. (1996), J. AppI. Pdyrrr. Sci. 61: 1839. Davis, H. A., Longworth. R., and Vaughan, D. J. (1968). Am. Chem. Soc. Div., Po/wr?. Cherrr. Po/wu. Prrpr. Y:515. Delf, B. W.. and MacKnight. W. J. (1969), Mncrorrrolrcu/rs 2:309. De Sarkar, M.. De, P. P,. and Bhowmick, A. K. (2000). P d y r w r 41:907. Dolgoplosk, B. A., Tinyakova. E. l., Reikh, V. N.. Zhuravlevn. T. G.. and Belonovskaya. G. P. (1959). Rrthher Chenr. Tdrrrol. 32:321. Duvdevanl, l., Agarwal, P. K., and Lundherg. R. D. ( 1982). Po/yrtl. EIIS.Sei. 22:322. Duvdevani. I., Lundberg, R. D., Wood-Cordova, C., and Wilkes. G. L. (1986).Arrr. C h w . Soc. Syrrr. Ser. 302, Washington, D.C., p. 184. Earnest, T. R., Jr.. and MacKnlght, W. J. (1977), Mrrc.rnrrlo/rcrr/es,/0:206. Earnest. T. R.. Jr.. Higgins. J. S.. and MacKnlght, W. J. (1982). MNcrOrt1~)IPcuIB.s15:1390. Eisenherg. A. ( 1970). Mrrcrorrrolaer4lrs 3 : 147. Eisenberg. A., and King. M. ( 1977). lor1 Corrtclirrirr*q Po/vrrlrr.s. Academic Press Inc., New York. Eisenberg, A., Ed.. (1980). lorw irr Po/yrrwr.s. Am. Chem. Soc. Div. Adv. Chem. Scr. 187,Washington. D.C. Eisenherg. A.. Smith, P,, and Zhou. L. L. (1982). Po/yrrr. Gig. Sci. 22: l 1 17. Farhenindustric, I. G . (1932). French Pat. 842,186. Ferry, J. D. (1980). The Vi.scoe/r~stic. Properfirs ofPo/yrrrrr.s, 3rd cd., John Wiley and Sons, New York. Fitzgerald, J. J.. and Weiss. R. A. (l988), J. Mcrcrorrd Sei. R a ? . Mcrcrorrlol Clrerrr. Phys. C28( 1):99. Gilbert. E. E. (196.5). S r l / j i u t r r / i o r r o r d Rrlrrtrd Rrtrctiorls. Interscience, New York Handlin. D. L.. MacKnight, W. J.. and Thomas. E. L.. ( 1981). Mrrcrorrlo/~.crr/e.s/4:795. Hara. M,, and Sauer, J. A. ( 1994). J . Mtrcronlol. Sei. Re,,. Mtrcrorrlol. Chrrrr. PIITS. C.M(3):325. Hara. M., and Sauer, J. A. ( 1998), J . Mocwrrrol. Sei. Re,,. Mrrcrorrrol. Chrrtr. Phys. C38(2):327. Hirasawa, E., Yamamoto, Y.. Tndano, K., and Yano, S. (1989). Mtrc,~orrlo/rc,rt/c.s22:2776. Hilosnwa. E.. Yamamoto, Y.. Tadano. K., and Yano. S. (1991). J. AI)/>/. Po/vrrr. Sci. 4235 I . Holliday. L., Ed. (1975). h i e Polyrrwrs, Applied Sciencc Publishers Ltd.. London. Hwang. K. K. S.. Yang. C. Z., and Cooper, S . L. (1981). Po/yrrr. B r g . Sci 21:1027. Jenkins, D. K. and Duck, E. W. (1975). in lorzic Po1yrrrrr.s (Holliday. L.. Ed.), Applied Sclence Publishers Ltd.. London. p. 173. Kajiyuma. T., Stein. R. S., and MacKnight, W. J. (1970). J . A/>/>/.Phys. 4/:4361. Kennedy. J. P,, and Storey, R. F. ( 1982). Arrt. Clrerrr. Soc. L ) ~ I , . Org. Cocrf. App/. Po1yrrr. Sei. 946:182. Kurinn. T.. De, P. P,, Khastgir, D.. Tripnthy, D. K., De. S. K.. and Peiffer. D. G. ( 1995), Po/yrrrer 26: 3875. Longworth, R., and Vaughan, D. J. (1968~1).Nrflrre 218:85. Longwol-th. R.. and Vaughan. D. J. ( 1968h). Am. C h m . Soc. Po/yrrr. PrqJr. 952.5. Longworth, R. (197.5). in l o r l i c Po/xrrrrr,s(Holliday, L., Ed.), Applied Sclence Publishers Ltd., London, p. 69. Lundhcrg. R. D. ( 1989), in Erlcyc/o/wt/in of Polyrrler Scirr~cerrrltl Brgirreerirr,y, 2nd ed. (Mark, H. F., Bikdes. N. M,. Overherger. C. G.. and Menges, G., Eds.), John Wiley and Sons Inc.. New York, Vol. 8. p. 383 MacKnlght W. J.. McKennn, L. W., and Read, B. E. ( 1967). J . / \ p p / . Phys. 38:4208. MacKnight W. J., Kajiyama, T.. and McKenna, L. W. (1968a), Po/yrrl. E I I ~Sei. . 8:267. MacKnight W. J.. McKenna. L. W., Read, B. E.. and Stcin, R. S. (1968h). J. Phys. Cherrr. 7 2 1 112. MacKnight W. J.. Taggart, W. P,, and Stein, R. S. (1974). J. P d y r l . Sci. Po/yrrr. Syrrrp. 45:I 13. MacKnight W. J., and Earncst, T. R.. Jr. (1981). J. P d y r r . S(,;.. Mtrcrorrrol. Re,.. /6:41. MncKnight, W. J., and Lundherg, R. D. (1984), Rltbber ChrrrI. Techrro/ 57:652. MacKnight. W. J.. and Lundberg, R. D. (1987). in T/wrrrrop/cr.sficElrrsforrrers (Lcgge. N. R.. Holden, G., and Schroeder. H. E., Eds.), Hanser Publishers. New York, p. 245.
lonomeric Thermoplastic Elastomers
477
Makowski, H. S., Lundberg, R. D., and Singhal, G. S . (1975). U.S. Pat. 3,870.841. Makowski, H. S., Lundberg, R. D., Westerman, L., and Bock, J. (1978a), Art!. Chert!. Soc. D ~ I ?Polw!. . Clwrrl. Po/,vrr~.Pre13. /9:292. Makowski, H. S. and Lundbcrg, R. D. (1978). Am. Cherrl. Soc. Di11. Polyrrr. Chertl. Polyrtl. Prep. IY:304. Makowski, H. S., Lundberg, R. D., Westerman, L., and Bock, J. (1978b), Arn. Cherrr. Soc. Div. Polyrr~. Clrern. Polyrtl. Prep. I9:2. Makowski, H. S.. and Lundberg, R. D. ( 1980a), U.S. Pat. 4,184,988. Makowski, H. S., and Lundberg, R. D. (1980b). in Iorrs ill Polyrtlers (Eisenberg, A., Ed), Am. Chem. Soc. Adv. Chem. Ser. 187, Washington, D.C., Ch. 3: p. 37. Makowski. H. S., Lundberg, R. D., Westerman, L., and Bock, J. (1980), in 1or1.sirr Polyrrrers (Eisenberg, A., Ed.), Am. Chem. Soc. Adv. Chem. Ser. 187, Washington, D. C., p. 3. Marx, C. L., Koutsky, J. A., and Cooper, S. L. ( 197 1 ), J . Po/ynz. Sci.. Polvrtl. Lett. 9: 167. Marx, C. L., Caulfield, D. F., and Cooper, S. L. (1973a), Mtrcrorrloleclrles 6:344. Man, C. L., and Cooper, S. L.. (1973), Mtrcrortrol. C / ~ e r r ~ 168:339. ., Marx. C. L.. Coulficld, D. F., and Cooper, S. L. (1973b). Polyrrr. Prep. 14(2):890. Mauritz, K. A. ( 1988). Mrrcrotnol. Sci. Re\!. Mrrcrortwl. Cherr~.Phys. C28( 1):6S. McKenna, L. W., Kajiyama, T., and MacKnight, W. J. (1969), M~rc~ror~lmlec~tles 258. Mcyer, C. T., and Pineri, M. ( 1978), J. Pdyrrr. Sei. Po/yrrl. Plrys. 16569. Miller. J. A., Hwang. K. K. S., and Cooper, S. L. (1983), J. Mtrcrorrlol. Sei. Phys. 22321. Mohajer, Y.. Tyagi, D., Wilkes, G. L., Storey. R. F., and Kennedy, J. P. (1982). Polvrtl. Bull. 8:47. Mohajer, Y., Bagrodia, S., Wilkes, G. L., Storey, R. F., and Kennedy, J. P. (l984), J. App/. Polyrn. Sei. 29: 1943. Neppel, A., Butler, I. S., and Eisenberg, A. (1979a), Mncrorrlolecules 12:948. Neppel, A., Butler, 1. S., and Eisenherg, A. (1979b), J. Po/yrtl. Sei. Polyrr~.Phys. /7:2145. Neppel, A., Butler, I. S., Brockman, N., and Eisenberg, A. (1981), J. Mtrcrorrrol. Sei. Phys. 19:61. O’Farrell, C. P., and Scrniuk, G . E. (1974), U.S. Pat. 3,836,511. Oh, Y. S., Lee, Y. M.. and Kim, B. K. (1994), J. Mucrorrlol. Sci.. Polym. P h y . 33(2):243. Otocka. E. P,, and Kwci, T. K. (1968a), MrrcrorrlolecLt/e.s 1:244. Otocka, E. P., and Kwei. T. K. (1968b), Macrortlolrcrtles 1:401. Otocka, E. P., and Kwei, T. K. (1968c), P d y m . Prep. 9:583. Otocka, E. P,, and Eirich, F. R. (1968), J. Polyrtr. Sci. P o / w . Pllys. 6:921. Otocka, E. P., and Davis, D. D. (1969). Mrccrorrrolrer~lr,s2:437. Otocka, E. P,, and Kwei, T. K. ( 1969), M~rerortlolt,cule.s2 :110. Otocka, E. P,, Hcllman. M. Y.. and Blyler, L. L. (1969), J. Appl. phys. 40:4221. Pacglis. A. U. ( 1984). U.S. Pat. 4.480.062, Paeglis, A. U,, and O’Shca, F. X. ( 1988). Huhher Cl~errr.Tec/u~o/. 61:223. Peiffer, D. G., Duvdevani, I.. Aganval, P. K., and Lundberg, R. D. (1986). J. Po/yrrl. Se;., p(dyrjl, Lett. 24:58 1. Phillips, P. J., and MacKnight. W. J. (1970). J. Polyrrl. Sei., po!\?lr. phy.~.8:727. Pineri, M.. Meyer, C.. and Bournet, A. (1975). J. Polyrr~.Sci.. Polwtr. Plry.s. 13:188 I . Pinen. M.. Meyer, C., Levclut, A. M., and Lambert, M. ( 1975). J. Polyrll. SC;.. f o l w . ~ h y . 12: ~ . I IS. Prud’homme, R. E.. and Stein, R. S. (1971), Mtrc,ror~ro/ecu/r.s 4:668. Prud’homme, R. E., and Stein, R. S. (1973), J . Polyrrl. Sei. p o / ~ ~p l~~ly.. ~11:1347. . Rahrlg, D. B. (1978), Ph.D. thcsis, University of Massachusetts, Boston. Rahrig, D., and MacKnight, W. J. ( 1980a). in loris it1 Po/y/ner.y (Eisenberg, A., Ed.), Am. Chcm, Soc. Div. Adv. Chem. Ser. 187, Washington, D C , p. 77. Rnhrig, D., and MacKnlght. W. J. (1980b). in 1orl.s i r ~Polytrlers (Eisenbcrg, A., Ed.), Am, Chem, Soc. Div. Adv. Chem. Scr. 187. Washington, D.C.. p. 91. Read, B. E., Carter, E. A., Connor. T. M,. and MacKnight, W. J. (1969), B. po/ym. J . 11123. Reed, S . E. (1971), J. Polyru. Sei., Po!\atl. Cllem. 92147. RCCS,R. W. (1964). Mod. Pltrst. 42:209. . Rccs, R. W.. and Vaughan, D. J. (1965). Am. Chtwr. SOC. poly^. C/~enr.P o l y ~ P. W ~ 6:287. Rees. R. W. (1966). U.S. Pat. 3,264.272.
478
Kar and Bhowmick
Rees, R. W. ( 1968). U.S. Pat. 3,404,134. Rees, R. W., and Reinhardt. H. G. (1976), U.S. Pat. 3,997,487. Rees. R. W. (l987), in T/lerrrlo~/)/r~.sfic~ E/o.sfnrrler.s: A C[~rrf/~rr/rerf.sit,e Retien. (Legge, N. R., Holden, G.. and Schroeder, H. E., Eds.), Hanser Publishers, New York, p. 231. Rochc, E. J., Stein, R. S., and MacKnight, W. J. (1980), J. Po/vrtl. Sei.. Po/yrtl. Plrys. 18:1035. Saidt. B., Lundberg, R. D.. and Lenz, R. W. (1980a). Po/yrrr. Eug. Sci. 20:530. Saidt, B., Lundberg, R. D., and Lenz, R. W. (1980b), Po/yrrr. Eng. Sei. 20324. Saidt, B., and Lenz, R. W., (1980), J. Pdvrrr. Sci. Po/yrrl. Cherrl. 18:3273. Sakamoto, K., MacKnight, W. J., and Porter, R. S. (1970). J. Polyrr~.Sei.. P o / w r . Phys. 8:277. Sanui. K., Lenz, R. W., and MacKnight, W. J. (1974a). J. f d y m Sei.. Po/yrn. C11cm 12:1965. Sanui, K., MacKnight, W. J., and Lenz, R. W. (1974b). M~rc~rorr~o/eerr/c.s 7:101. Sanui, K., and MncKnight, W. J. (1976), Br. Po/yrrr. J. 8:22. Schulz, D. N.,Sanda, J. C., and Willoughby, B. G . ( 1981), A w . C/rrrrf.Sot. Syrrlp. Ser. 1663427. Semon, W. L. (1946). U.S. Pat. 2,395,017. Srnyth, C. P. ( 1955). Dir/c)cfricB e h i o u r n r d Srrrrcrurr. McCraw-Hill, New York, p. 73. Stein, R. S. (1964). in Newer Mrfhods ofPo/vrrtrr Clrcrrtrcreriztrtiorf (Ke, B., Ed.), Intcrscience, New York. Tanaka. H., and MacKnight, W. J. (1979). J. Po/yrrf.Sei.. f o / v r r l . Cllrrrf. 172975. Tant. G., Wilkes, G. L., and Kennedy, J. P. (1985). P o / w . Prep. 26( 1):32. Tant, M. R., Wilkcs, G. L.. Read, M. D., and Kennedy. J. P. (1986). J . Po/vrrr. Sei.. P d y r t f . Left. 24619. Tant, M. R., and Wilkes, G. L. (1988), J. Mtrcwrrrol. Sei. Ret,. Mrrcrorrrol. Cherrl. P/r.v.s. C2N( 1 ): 1. Tobolsky, A. V., Lyons, P. F.. and Hatta, N.(1968), Mocrorrlo/rtrt/e.s 1:SlS. Tsatsas. A. T., Rced, J. W., and Risen, W. M,, Jr. ( 197 1), J. Chenr. Phys. 55:3260. Uemura. Y., Stein, R. S., and MacKnight, W. J. (1971), Mocrorrrolecu1e.s J:490. Uniroyal Technical Information Bulletin. Naugatuck, Connecticut ( 1982). l o r l i c Elasforrrrrs. Ward. T. C.. and Tobolsky, A. V. ( 1967). J. Appl. P o l w ~ Sci. . 11:2403. Weiss. R. A., Lundberg, R. D.. and Werner, A. ( 1980), J. Po/yrrr. Sci.. Polyrrl. Chertr. 18:3427. Weiss, R. A.. Sen, A.. Pottlck, L. A., and Wills, C. L. (1990), Po/vttr. Cotrftrfm.31:220. Wilson, F. C.. Longworth.R., and Vaughan, D. J. (1968), Am. Cllerrr. Soc. Dit,. Po!\)rrr. Clrrrrr. Po/yrfr. Pre/). 9 : m . Xie, H., Xu. J., and Zhou. S. (1991). Po/yrlrrr 32:95. Yarusso. D. J.. and Cooper, S. L. (1983). Matrorrlo/ec,rr/e.s16:1X7 1.
17 Miscellaneous Thermoplastic Elastomers
In the previous chapters. various features of important thermoplastic elastomers have been described. This chapter deals briefly with the manufacture, properties, and applications of a few thermoplastic elastomers not discussed earlier and recent developments in this field.
1. THERMOPLASTIC 1,P-POLYBUTADIENE 1.l
Synthesis
1.3-butadiene is polymerized in the presence of CoHal,-ligand-AIR3-H,0 in solution to yield 1.2-polybutadiene (Takeuchi et al., 1974):
Polymerization
CH2 = CH - CH = CH2
-(- CH2! - CH- )-
l
CH I
'
CH2 1,2-Polybutadiene This has 9 1% 1,2 and 970 cis- 1.4 structure. Of the 1,2 units, 5 1-66% are in the syndiotactic form and 34-49% are in heterotactic triads. Syndiotactic 1,2 content could be increased to as high as 99.7% using a Co(acac)3-A IEt3-CS2catalyst system (Ashitaka et al.. 1983b). Themelting point of this material is 208°C. Thermoplastic 1.2-polybutadiene (1,2-PBD) is manufactured by Japan Synthetic Rubber (JSR), using its own technicalknow-how, under the designation JSR RB. This companysupplies materials with three degrees of crystallinity. namely 15. 25, and 29%. having crystalline melting point between 80 and 90°C. The molecular weight of this material is several hundred thousand, and the heterogeneity index is 1.7-2.6. The polymer does not have any long-chain branching.
1.2 Properties The basic properties of thermoplastic 1.2-polybutadiene are given in Table 1. The tensile strength and modulus of a typical polymer of 25% crystallinity are 10.5 MPa and 6 MPa. respectively. 479
480
Bhowrnick
Table 1 PhysicalPropertiesof1.2-Polybutadiene
Measured valued Properties
JSR RB830 RB820 RB810 methods Testing
Density, kg/m3 Crystallinity, c/o Microstructure 1,2-unit content. %
Refractive index, n''?s MFI (melt flow index) (150"C, 2160 g), g/10 min Thermal properties Vical softening pt., "C Melting pt.. "C" Brittle pt., "C Tensile properties 300% modulus, MPa Tensile strength, MPa Elongation, 8 Hardness, degrees Shore D JIS A Izod impact, kg m/m Light transmittance, %Haze, %'
JSR
Density-gradient tube method Density-gradient tube method Infrared ray spectrum (Morero method) ASTMD542 ASTM D1238
ASTM D1525 (DSC method) JIS K 6301 JIS K6301
ASTM D1706 JIS K6301 ASTM D256 JISK6714 JISK6714
JSR 90 1 -1s
90
906 -25 92
1.513 3
39 75 - 40
1.515
3
S2 80 -
37
909
-29
93 1.517 3
66 90 - 35
4.0 6.5 750
6.0 10.5 700
8.0 13.5 670
32 79
40 91
41 95
h
91 2.6
h
89
3.4
h
x2 8.0
I' Endothermic peak temperature according to the differentla1 scannmg calorimeter mcthod. (Speed of temperature rise: 3O"C/min.) broken. c The figures apply to 2-mm-thick sheet injectlon molded with cylinder temperature. 150°C and mold temperature 30°C. Source: JapanSynthetic Rubber C o . Ltd.. 1986. " Not
The elongation at break is about 700%. The hardness can go as high as 40 Shore D, and the melt flow index at 150°C is 3 g/10 minutes. The flexural moduli are 47 N/mm' and 560 N/ m?. respectively. The tear strength has been found to be about 100 N/mm (A. K. Bhowmick, unpublished). All these properties dependon the amount of crystallinity. For example,an increase in crystallinity enhances the tensile strength of this elastomer. Stress-strain propertiesof different grades of 1,Zpolybutadiene are reported in Figure 1. Elastic modulus versus temperature curves of JSR RB are shown in Figure 2. andthermalpropertiesaregiven in Figure 3. The glass transition temperature (Tg) varies from - 17 to - 30°C. It shows a wide endotherm, indicating the melting of crystals. This behavior is remarkably different from the melting of polyethylene (PE) andpolypropylene. The electricalproperties,given in Table 2, aresimilar to those of polyethylene except for tans. The chemical resistance of JSR RB depends on the amount of crystallinity. It is stable against dilute solutions of strong acids and alkalis. It is easily soluble in aromatic hydrocarbons, halogenated hydrocarbons. and carbon disulfide, and particularly soluble in aliphatic hydrocarbons. It is insoluble in ketones and alcohols. As shown in Figure 4. the rheological behavior of 1.2-polybutadiene is similar to that of low-density polyethylene. The temperature dependence of flowability of JSR RB 20 is shown
481
Miscellaneous Thermoplastic Elastomers
( p l a s t i c i z e r 50parts)
Ethylene- vinykcetotr copolymer (vinylacetate 11%)
150
'
Low density polyelhylrne ( MI = L )
-0 100
300
500
700
Strain ( ' A ) Fig. 1 Stress (Ikgf/cm'
a
50
0
=
0.098 MPa)-strain curve of 1.2-PBD (specimen: I-mm-thick pressed sheet).
100
50
Temperature
( O C
1
Fig. 2 Temperature vs. elastic modulus curve
of JSR RB.
482
I
Bhowmick
1
-60
I
,
-
Tmz SPY
0 50 100 Temperature (OC 1
Fig. 3 DSC curves of JSR RB. Speed of temperature rise, 20"C/min.
in Figure 5. Since polybutadiene is susceptible to oxidative attack and crosslinking,it is desirable to have the fabrication temperature below 160°C. Hence, extrusion and blow molding are done in the temperature range of 130- 160°C.Typical conditions for blow film forming and injection molding are given in Tables 3 and 4, respectively. Since 1.2-polybutadiene has double bonds,it will be sulfur-vulcanizable. The double bonds are on the side chain rather than on the backbone, and hence it has good weathering and ozone resistance very similar to that of EPDM. However, it degrades with theaction of chemical agents, heat, and UV light.
Table 2 Electrical Properties of JSR RB820 Property Volume resistivity (60% R.H., 20°C). strength, Dielectric KV/rnrnh Dielectric constant (E)' 60 Hz IO" Hz tan 6' 60 Hz ( X IOv3) 10" Hz ( X 10-j)
JSR RB820'
Soft PVC
JIS K691 1 JIS C21 11
2 X 10'' 46
10-30
18-28
JIS K691 1
2.6 2.6 2.5 4.5
5.0-9.0 3.3-4.5 80- l50 40- l40
2.2-2.4 2.2-2.4
methodTesting
il-cm
JIS K691 1
PE >1oIh
.' The figures for JSR RB820 apply t o 2-mm-thlck pressed sheets, except for the dielectrlc strength figures, whlch apply to 1 -mm-thick pressed sheets.
At 25°C. 40% reletlvc humidity. At 20°C. 60% relative humldity. Sultrcr: Japan Synthetic Rubber Co. Ltd. c
Miscellaneous Thermoplastic Elastomers
"---l Shear stress
( dyn/cm21
Fig. 4 Flow curvc of JSR RB 820. Temperature. 150°C.
Fig. 5 Temperature dcpendence of flowability of JSR RB 820.
483
Bhowmick
484
Table 3 Examples of JSR RB820 Blown Film Forming Material
JSR RB820
Extruding machine Screw type L/D Compression ratio Die bore diameter Die lip clearance Preset temperature
c1 c2 c3 C., AD D Screw revolution Resin pressure Resin temperature Discharge rate Blowup ratio Haul-off speed Film thickness
50 mm @ Metering type 28 2.0 75 mm 0.7 mm 120°C 145°C 145°C 150°C 150°C 150°C 48 rpm 15.5 MPa 150°C 31 k g h 4.8 15 d m i n 18 km
Source: Japan Synthetic Rubber Co. Ltd.
Table 4 Typical Injection-Molding Conditions of 1,2-PBD Conditions
Item ~~
~~~
Prescnt temperature C , (rear) C, (center) C3 (front) Nozzle Injection pressure Injection time" Cooling time" Back pressure Injection speed Mold temperature
130°C 150°C 160- 170°C 150- 160°C 60- 100 MPa 2-8 secb 80-120 sec 0.5- 1.5 MPa As rapid as possible As low as possible
.' Time depends on the injectloll speed. should be 2 sec when an accumulator is used. Time IS about 120 sec when water at normal temperature is used and 80-90 sec when cooling water of about 5°C is used. Source: Japan Synthetlc Rubber Co. Ltd.
" Time L
485
Miscellaneous Thermoplastic Elastomers Table 5 Recipes for BlendingJSRRB with ci.s-l,4-PolybutadieneRubber
BR (JSR BR 01) JSR RB 820 HAF carbon black oil Proccssing Zinc 3 Stearic acid (Nocrac Antioxidant 810 NA) accelerator Curing Cz:' Sulfur Total
163.55
1
2
3
4
100
70 30
60 40 50
50
5
5
-
50 5
50 5
3 2
2
1
1
0.8 1.75 163.55
0.8 1.75
2 1 0.8 1.75 163.55
50 50
2 1
0.8 1.75 163.55
Blending of l,2-PBD with natural rubber, 1,4-PBD, andPE hasbeen reported. An NR-1,2PBD blend improves the aging resistance and flex-crack growth resistance of NR (Japan Synthetic Rubber, 1984). Similarly, 1.2-PBD can improve the rolling resistance, heat buildup, and other properties of tire tread compounds(Wang et al., 1984). Ablend of 50% SBS block copolymer and 50% I,2-PBD has been suggested for thermoplastic elastomer tape (Japan Synthetic Rubber, 1985). The properties of blends of cis-l,4-PBD and 1,ZPBD arereported in Tables 5 and 6. Carbon black fillers do not have any reinforcing activity, unlike conventional rubber vulcanizates. A peroxide curing agent does not give much improvement in properties (Bhagawan, 1987). On the other hand. CaC03 modified with 1,2-PBD has been found to be apotential reinforcing filler for SBR (Kiji, 1983). It is interesting to mention briefly another material that is a rubber (not a thermoplastic elastomer) and contains syndiotactic 1.2-PBD in the structure-UBEPOL VCR (Ashitaka et al.. 1983a,b; Hoshino et al., 1985). For example, Ubepol VCR 412 has 12% of syndiotactic 1,2PBD, 86% cis- 1,4, and 2% hans- 1,4 structure; and Ubepol VCR 309 has 9% syndiotactic I ,2. Vinyl groups are combined to the main chain in the 1,2 syndiotactic conformation. It exists in BR in the form of fine fibrils (about 0.1-0.2 p m in diameter and 2-4 p m long). It has excellent processability, especially in high-shear-rate regions. and low die swell (-l%), characteristics of self-reinforced rubber. The property enhancement in all these cases is due to the cocrystallization of syndiotactic 1,2-BR with cis-1.4-BR (D. McIntyre, private communication). This results in high green strength, modulus ("14 MPa at 300%). and tensile strength (19 MPa); good flex crack resistance (crack growth from 2 to 15 mm takes about 60,000 cycles). abrasion resistance (PIC0 abrasion index 270), rebound (50%), and tear strength (6 MPa); and low heat buildup (23°C). The above vulcanizate contains rubber, 100; ZnO, 5 ; stearic acid, 2; HAF black, 50; process oil, IO; antioxidant, I .O; sulfur, 1.5; and accelerator, 1. This material is finding applications in tires, belts. and other rubber goods. 1.3 Applications
Applications of JSR RB depend on itsproperties. A few examples of the numerous applications and features of syndiotactic 1,2-polybutadiene are given in Table 7. An important application described by Ashitake et al. (1983a.b) is the preparation of carbon fibers and graphite fibers.
Bhowmick
406
Table 6 Physical Properties of JSR RB/cis- 1.4-PBR Blends 1
BR OI/RB 820 ratio Properties /frmtl corr~pour~~ls Measurcd at room temperaturc: 50% modulus, kglcm' 100% modulus. kg/cm' Tensile strcngth, kglcm' Elongation, % Measured at 40°C 50% modulus. kglcm' I00% modulus, kg/cm' Measured at 40°C Tensile strength, kg/cm' Elongation, %Measured at 80°C 50%-modulus, kg/cm' 100% modulus, kg/cm' Tensile strcngth, kg/cm' Elongation, % Extrusion processahility" Extrusion speed, cm'/mtn Die shrinkage, % Die swell. % Evaluation of shape ~ropc,rtiesof ~~u/crrr~i:crtesl' 300% modulus. kg/cm' Tensilc strength, kg/cm' Elongation, %Hardness (JIS A ) Ozone rcsist;ulcc' After 3 hr 10 hr 24 hr 48 11s 96 hr
100/0
2 70130
3 60/40
4 50/50
10.4 14.5 20.9 320
15.7 21.2 33.4 380
20.7 27.8 43.4 320
I .8 1.8
6.2 9.3
9.6 15.3
13.1 19.5
I .9 810
13.5 260
23.0 240
31.2 270
1.3
3.3 4.5 5.2 180
4.5 7.8 10.5 150
9.0 9.8 12.0
220 47.3 89.8 15 (4.4.3.4)
-
2.1 2.1 4.4 1810
1.3 1.4 410
209 57.2 133 13 (3.4.3.3) 98 207 500 64 c-2 c-3 C-4 C-4 c-5
143 I92 410 76
B-2 B-3 B-4 B-4 B-S
153
197 390 82 A- 1 A-2 A-2
A-4 A-5
130 -
-
162 199 390 85 A- I A- 1 A- 1 A- 1 A- 1
I' Evaluation of shape 16 potnts is the full mark; 4 pomts are glven to each of (1) swelling of sectloll (2), continuity o f edge. (3) surface skin, and (4)shape of corner. Extrusion conditions: barrel temperature. 70°C: die temperature. I1O"C; screw revolution rate. 2 I r.p.m. " Cured at 145°C for 30 n m . Measurement conditions: temperature,40°C: ozone concentration. 50 pphm; elongation rate. 209. Evaluation methods: number of cracks: A. ;Lfew; B, many: C. Innumerable. Size and depth of cracks: 1, cannot he detected by nnked eye but visible thrc~ugha 10-fold magnifying glass: 2. can be detected wlth naked eye: 3. relatively large cracks of less than 1 nm depth: 4. large cracks of 1-3 n m depth: S, crack5 of 3 m111 or more L
487
Miscellaneous Thermoplastic Elastomers Table 7
Applications and Features of I ,2-Polybutadiene
Application Applic~rtiort(1s rr Tlrc~r-rrtol~lo.stit. Kesirt Films:stretch film, laminatcd film, shrinkablc film
Footwear: unit solcs, innersoles, and outersolcs by injection molding
Tubesand hoses:liquidfoodtransfertubes, tubes for medical practice Other: blow moldings, injection moldings, rcsin modifier Applictrfior~C I S CI KuhherSponges:microccllular,hard,semihard, soft, crcpc-tone
High-hardnessrubbergoods:footwear. solid tires, industrial goods, dock fendcrs. sporting goods. sundrics Injection-cured goods: footwcar, solid tires. industrial goods, rubber ~ I O V C S
Rubbcr modifier: various rubber goods
Other: transparent cured rubber goods
Ofher-Applicrrtiorts Adhcsive (hot melt type): adhesive for various woven or unwoven cloth. paper. leather, and wooden board Reactionaccelerator: crosslinkingaccelerator for polyolefins Photosensitive polymer: printing plates, photosensitive paint, etching-resistant material Thcrnmsetting resin: electrical insulation material Othcrs: fibers,modifier forcomposite resins, photodegradable polymer Sorwce: Japan Synthetic Rubber Co. Ltd.
Features Safcty for food packaging transparency, self-tack, pliability, shrinkability at low temperaturc, puncture strength resistance, hcat sealability at low tempcraturc, gas permeability Light weight, hardness, rubbery feeling. no deformation, snappincss,reproducibility of mold pattern, coating performance, adheslvc properties, crack resistance Safety for food transfcr and medical use. transparency, flexibility Flexibility, safety for food packaging
One-step vulcanization. wide range of curing conditions, high loading, elasticity, snappiness, no deformation. weatherability. ozonc resistancc. heat resistance, tear resistancc, coating pct-formance,adhesive propertics, skid reststance, abrasion resistance Elongation, tensile strength, hardness, snappiness. good flowability, casy vulcanization, weatherability,ozoneresistance, heat resistance, skid resistance, abrasion resistance Flowability, injection molding processability. easy vulcanization, weatherability, ozonc resistancc, heat resistance, skid resistance, abrasion, resistance, snappiness Greenstrength,flowability,extrudability, injcction molding processability, weathcrability, ozone resistance, heat resistance, snappiness Transparency, safety for food application, weatherability, heat resistance Low melting point, flowability
Easy crosslinking, reduction in use of crosslinking agents Photo sensitivity (photocuring),flowability,low solution viscosity Chemical resistance,heat resistance,electrical properties
4aa
Bhowmick
These are produced by combination of cyclization and crosslinking by AIBr3 in benzene, dehydrogenation with the help of molten sulfur at 275”C, and heat treatment. The precursor fibers are made by melt spinning. The yield of carbon is higher than that from polyacrylonitrile. The carbon fiber has a modulus around 1420 t/cm’ and tenacity of 16.6 &m’, while the graphite fiber has a modulus of 4010 t/cm’ and tenacity of 20.0 t/cm’.
2.
TRANSl,4 POLYISOPRENE
2.1 Structure Trans-l ,4-polyisoprene (rruns-PIP) is an isomer of natural rubber. Balata and gutta-percha are the two natural polymers consisting of trans- 1,4 units. This has 90.6% trans content, 7.7% cis, and 1.7% 3,4 structure. M , is of the order of 3.2 X IOs g/g-mol. The thermoplastic behavior of the material is due to the presence of crystallities that have a melting point of 63°C.
2.2 Properties Trans-polyisoprene crystallizes rapidly at temperature below 63°C and shows very high hardness and tensile strength. These properties are in contrast to the properties of natural rubber, which shows slow crystallization behavior at room temperature. The important properties of raw synthetic truns-polyisoprene are high tensile strength (35 MPa). modulus (19 MPa), elongation at break (500%),tear strength (20 kN/m), and hardness (70 Shore D). It has a low coefficient of expansion (0.008PC). Withproper orientation, films of rrrtns-PIP may achieve a strength of about 70 MPa. Truns-PIP may be compression molded, injection molded, extruded, and calendered by rubber and plastic equipment. With proper modification, it becomes a thermoplastic elastomer (A. K. Bhowmick, unpublished). Since it has double bonds, could it be sulfur-vulcanized. Vulcanization imparts higher heat and chemical resistance to the material. With the same curing system, it has lower crosslinking efficiency than NR (V, = 0.146 vs. 0.175 for natural rubber). It also has a higher cure rate, higher scorch time, and lower optimum cure time than natural rubber (Bhowmick et al., 1986). Blends with natural rubber improve high-temperature properties of NR.
2.3 Applications Some typicalapplicationsare golfball covers,orthopedicdevices,heat-sealablecoating,toe inserts for safety shoes, coating and caulking compounds, and pressure-sensitive adhesives.
3.
ETHYLENE-VINYLACETATE
Polyethylene is a crystalline material. The modification of polyethylene with some comonomers (e.g., vinyl acetate and ethyl acrylate) reduces the crystallinity of polyethylene, and the resulting products have many characteristics of thermoplastic elastomers. The crystalline phase acts as a hard block in these materials. In fact, a range of materials from thermoplastics elastomers to rubbers can be produced by varying the amount of vinyl acetate. Only ethylene-vinyl acetate copolymers (EVA) will be described in this chapter. The properties of ethylene-ethyl acrylate (EEA) aresimilar in many respects to those of EVA when compared at equal comonomer level. However, a few differences do exist.
489
Miscellaneous Thermoplastic Elastomers
3.1Manufacture
of EVA
EVA is manufactured by continuous bulk polymerization and solution polymerization methods. The details are given by Gilby (1982). There are three processes for the manufacture of EVA: a high-pressure process for 0-45% VA; a low-pressure process (inemulsion) for55-100% VA; and a medium-pressure process (in solution) for 30-100% VA.
3.2 Structure The molecular structure of EVA copolymer is as
H
( c
l l
H
H
I
""- C")
l
H
H
H
I
I
"-c --- c I H
H-C
-
~
0
H -C
-
fOllOWS:
I
F0 I I
H
"H
-H -
N
Various grades of EVA resins are commercially available. The properties of the resin will depend on molecular weight and vinyl acetate content. Generally, thermoplastic resins have 9.5-33% VA. Products with 40-45% VA are rubbery and are marketed by Bayer (1999) and DuPont Dow( 1999).
3.3 Properties The typical physical properties of EVA copolymers are given in Table 8. The melt index decreases with the decrease in vinyl acetate content, whereas tensile strength, elastic modulus, and hardness increase. The enhanced properties are related to the higher amount of crystallinity. The effects of vinyl acetate content on crystallinity are shown in Figure 6. The modification of polyethylene by vinyl acetate increases flexibility, toughness, and clarity. It has excellenttoughnessat low temperature with a brittlenesstemperaturebelow - 100°C. Most of the properties of EVA are comparable to those of low-density polyethylene. EVA materials have good environmental stress-crack resistance (ESCR). EEA materials are excellent in this respect. Changes in dielectric loss and stiffness modulus with VA content are shown in Figures 7 and 8. EVA copolymers are soluble in aliphatic, aromatic, and chlorinated solvents. They are more soluble than LDPE. The same is true of EEA. Their resistance to strong alkalis, brine solutions, detergents, and other nonoxidizing media are better than those of LDPE. Because of their oxidizing tendency, an upper limit of 230°C is recommended during processing. Their
Table 8 Typical Physical Properties of Ethylene-Vinyl Acetate Copolymers" ~~~
~
~
~
Shipping specification Melt index'
% Vinyl
Elvax 40-W
2 48-66
3 39.0-42.0
Elvax 150
38.0-48.0
32.0-34.0
Elvax 210
365-440
27.2-28.8
Elvax 220
134-168
27.2-28.8
Elvax 230
100-120
27.2-28.8
Elvax 240
38.0-48.0
27.2-28.8
Gradeh 1
Elvax 250
22.0-28.0
acetate
27.2-28.8
Elvax 260
5.3-6.7
27.2-28.8
Elvax 265
2.6-3.4
27.2-28.8
EIvax 3 I0
365-440
24.3-25.7
Elvax 350
17.3-20.9
24.3-25.7
Elvax 360
1.7-2.3
24.3-25.7
Density (23°C) kg/m3 (g/cm3); ASTM D 1505
4 965 (0.965) 957 (0.957) 95 1 (0.951) 95 1 (0.951) 950 (0.950) 95 1 (0.95I ) 95 1 (0.951) 955 (0.955) 955 (0.955) 948 (0.948) 948 (0.948) 950 (0.950)
Hardness, Shore A-2 durorneter (10 sec); ASTM D 2240
Tensile strength, MPa (psi); ASTM D 1708"
Elongation at break, c70; ASTM D 1708"
Elastic (tensile) modulus, MPa (psi); ASTM D 1708"
5 4.8-6.2 (750-900) 6.9-8.3 ( 1000-1200) 2.8 (400) 5.5 (800) 5.9 (850) 9.7
6 10001300 900-1 I 0 0
7 3.0 (450) 10.0 ( 1400) 12 (1700) 16 (2300) 16 (2300)
8 40
18
73
1400) 11
(16001 24 (3500) 29 (4200) 3.3 (475) 14
800-1000 800-1OOo 800-1000
800- 1OOO
62 69 70
( 2600)
800- 1000 800-lo00
800- 1000 800-1000
800- I000
c2000)
26 (3800)
65
800-1OOo
19 (2800) 26 (3800) 28 (4100) 16 (2300) 25 (3600) 35 ( 5 100)
75 80 83
70
Softening point, ring and ball, "C ( O F ) ; ASTM E 28
9 104 (220) 110 (230) 82 ( 180) 88 (190) 96 (205) 1110
(230) 127 (260) 154 (3 10) 171 (340) 88
Cloud point in paraffin wax', "C (OF)
Elvax 4 10
455-550
17.5-18.5
Elvax 420
136-165
17.5- 18.5
Elvax 450
6.7-9.3
17.0-19.0
Elvax 460
2.2-2.8
17.5-18.5
Elvax 470
0.6-0.8
17.0-19.0
Elvax 550
6.7-9.3
14.0-16.0
Elvax 560
2.1-2.9
14.0-16.0
Elvax 650
6.7-9.3
11.o-13.0
Elvax 660
2.1-2.9
I I .O- 13.0
Elvax 670
0.2-0.4
1 1 .O- 13.0
Elvax 750
6.3-7.7
8.0-10.0
Elvax 760
1.8-2.2
8.8-9.8
Elvax 770
0.6- 1 .O
8.5-10.5
934 (0.934) 937 (0.937) 940 (0.940) 94 1 (0.941) 940 (0.940) 935 (0.935) 940 (0.940) 933 (0.933) 940 (0.940) 940 (0.940) 930 (0.930) 930 (0.930) 930 (0.930)
4.7 (675) 8.6 ( 1250)
600-900
18
600-900
(2550) 23 (3300) 26 (3800) 18
(2600) 22 (3200) 17 (2500) 21 (3000) 26 (3800) 15
(2200) 21 (3000) 22 (3200)
600-900
600-900 600-900 800-900 800-900 750-850 750-850
33 (4800) 42 (6 I 00) 51 (7400) 52 (7500) 63 (9100) 64 (9300) 74 ( 10700) 85 (12300) 91 ( 13200)
750-850
100
80 84 90 90 92 93 93 94 94 94
( 14500)
110
600-750
95
( 1600@
600-750 600-750
140 (20000) 160 (23200)
96 96
I' These data are presented as a general description of properties and are not intended to be used for design specifications. All grades contain 200-800 ppm butylated hydroxytoluene. d g h m (ASTM D 1238. modified). Samples die-cut from pressed films: guage dimensions 2.23 cm X 0 47 cm X 0 13 cm (0.876 in. X 0.187 in on sample length of 1.91 cm (0.75 in.). '' Modulus calculated as in ASTM D 638 ' 10% Elvax i n fully refined paraffin wax 146 AMP Sowre: E I du Pont de Nemours & Co. Ltd.
X
0.050 in.): crosshead speed 5. I cm ( 2 in )/min. Elongation based
492
Bhowmick
0 Weight percent VA in copolymer ( 'A V A 1 Fig. 6 Effect of vinyl acetate content on crystallinity.
useful life, however, is equal to that of LDPE. The crystalline melting point of EVA is lower than that of LDPE. and hence the upper service temperature is about 65°C for low-vinyl acetate grades. The processability of EVA is very similar to that of LDPE. Hence, it can be injection molded, extruded, or blow molded like other thermoplastics. Typical injection-molding conditions are givenin Table 9. A plot of flow versus injection pressureis given in Figure 9. Shrinkage is also dependent on the vinyl acetate content (Fig. 10). Although they can be processed without a crosslinking agent, dynamic crosslinking does not markedly change the processability of EVA resins (Thomas et al., 1986). At a VA content of >40%, the product becomes rubbery, and hence crosslinking is necessary. Typicalmechanicalproperties of one insulation compound based on EVA rubberare
0.08,
I
PercentageVA (%l Fig. 7
Dielectric loss level associated with vinyl acetate content (room temperature data).
493
Miscellaneous Thermoplastic Elastomers
0
S
l 0 15 20 25 VA content ( O/O 1
30
3
Fig. 8 Effect of vinyl acetate content on stiffness modulus.
Table 9 Suggested Startup Conditions for Elvax Resins in Injection Molding Cylinder temperature, "C ("F) Rear Center Front Nozzle Maximum melt temperature, "C ("F) Screwbnck pressure Mold temperature? "C ("F) Cycle, sec Injection Booster Cure Injection pressure Fill rate
120 (250)" 150 (300) 175 (350) 150-200 (300-400) 220 (425) 0 I O (50) 15'
0-5 I
Max. without flash' Slow
Raise temperature at thls location for parts of cycles taxing plasticizing capacity of machine. h Rase to improveflowand surtice finlsh. Lower for fastcycle.lower shrinkage. good ejection. c Varies with part thickness; reduce Injection time a s possible. Follow by reduclng cure. 'I Approximately 90 sec per 6.35 mm (0.250 in.) of thickness. Adjust t u t i l l cavlty. Second-stage pressure may be lower for holding.
"
494
Bhowmick I
MACHINE : l L 2 - 9 ( 5 - 8 2 ) R c c i p r o m b ; n g s c r ~
MOLD TEMP. : 10°C ( SOOF) MELT TEMP.: 196OC ( 3 8 5 O F )
MOLD SIZE : 2.5Lmm w12.7mm
(0,Iin.xO.Sin.)
*O
1.016 mm r12.7mm (0.04 i n x 0.5 in.)
t Injection pressure MPa ( p s i 1
Fig. 9 Snake flow (injection flow number).
reported in Table 10 for comparison. EVA (700HV) with a vinyl acetate content of 68.5-71.5 is also available from Bayer. The swelling and also the change in physical properties of the vulcanizate after immersion in the medium clearly depends on the VA content (Fig. 11). The swelling increases with the aromatic content of the oil.
3.4
Applications Genercrl-purpose extrusion, injection and blow-n1oIcling appliccrtions: Freezer door gaskets, boots, artificial ski trails, ice cube trays, etc.; convoluted tube for swimming pool suction cleaner and domestic and industrial vacuum cleaners; anasthesia face mask,anasthesiahoses,contactlensholders,babybottlenipples,etc.,furniture webbing, toys, etc Film applications: Fresh meat packaging,frozen poultry packaging.horticultural film, bags for chemicals, stretch wrap film, clean film, disposable surgical gloves Shoe inclustry: Reinforcement for shoe heels and tips. soles, microcellular products, etc. Arrror?~otiwappliccrtiorzs: Injection-moldable crosslinked foam (tires for baby carriages andshoppingcarts, life jackets, fishingfloats, etc.); blends with tack resins and waxes in hot-melt adhesive for general packaging, book-binding, carpet construction, edge-veneering of furniture, pressure-sensitive adhesive, shoes, bricolage; blends with carbon black for obtaining semiconductive compounds and as a noise barrier
495
Miscellaneous Thermoplastic Elastomers
0.017 \\
0.015 0.013
Injection molded plaques, 7 6 m x 152mm (3in x 6 i n 1
-
.-c \
.E 0.011 E
E \
E E
0
0.009-
0.001
-
.x
.-Lc r
0.005 0.003
0.001 I 0
l
1
I
I
I
1
25 20 30 5 10 1 5 Vinyl acetate content, w t '10
l 35
Fig. 10 Shrinkage of Elvax resins.
between the engine and passenger compartment of cars; blends with bitumens for achieving high- and low-temperature properties, decreasing the penetration of the bitumens and increasing the flexibility (typical applications in bridge decking. roofing sheets. pipe lapping. etc.); as powder for making containers. traffic cones. balls by rotational moldings
4.
EVA BLENDS
Thermoplastic elastomers from blends of polyethylene and ethylene vinyl acetate copolymer have been prepared by electron beam technology (Chattopadhyay et al. unpublished). The disadvantages associated with dynamic vulcanization in the mixer can be eliminated by using this technology and continuous uniform curing even in complicated products, can be achieved. Table 1 1 shows the permanent set and reprocessibility of different films from these blends. The reprocessibility has been carried out by remixing for 4 minutes at 130°C and remolding for 2 minutes at 150°C. Even under these processing cycles, the changes in properties are not significant for
Bhowmick
496
Table 10 Mechanical Properties Before and After Hot-Air Aging, Low-Temperature Behavior, Hot-Set Test of Insulation Compound Based on Levapren 400'
and
Property Tensile strength, MPa Elongation at break. r/c (TI(x), MPa u?Olt>MPa Tension set at 150%) elongation, % Hardness (Shore A ) At 20°C At 70°C Hot-set test, VDE 0472 $615, at 150, 200, and 250°C. c/r Glass trans. temp. (DIN 53 513), "C Brittleness point (DIN Draft 53 546). "C Clash and Berg test (DIN 53 447), "C Gehman test T l o (ASTM-D 1053), (50-mm test chamber), "C
11.0 260 7.7 10.0
f4 +4
-2 - 35 -
-2 0 -
-I
+
-
4s 81
62 5 - 20 - 26 -21.5 - 22
" 0 . Initial value: 1 , after hot-air aging, 20 days at 150°C: 2, after hot-air agmg, 20 days at 170°C: 3. after agmg in an oxygen bomb, 20 days at 70°C; 4. after aging in an u r bomb. 20 days at 127°C. Sowcc~:H. Bugel. Bayer AG. 1978.
!
2 4 h / 7 0 ° C Diesel oil
24h/100°C A S T M oil no..3
40
20 0 35
40
45
50
55 VA
60
65
('/e)
Fig. 11 Effect of VA content on the swelling in oil.
70
75
80
497
Miscellaneous Thermoplastic Elastomers Table 11 Permanent Set and Reprocessibility Studics on Original
P.S. Sample code
(%)
PEVA55000 €'EVA55020 PEVA5502 1 PEVA55023 PEVA55025 PEVA64020 PEVA46020 PEVA.55050
38 31 28 27 24 33 23 27
I" cyclc
Different Films
2°C' cycle
3"' cycle
4'" cycle
S" cycle
T.S. E.B. T.S. E.B. T.S. E.B. T.S. E.B. T.S. E.B. T.S. E.B. (MPa) (%) (MPa) (76) (MPa) (76) (MPa) (%) (MPa) (%) (MPa) ( 7 6 ) 4.2 5.3 6.3 6.6 6.3 6.7 5.1 6.7
340 410 480 370 280 430 590 450
4.1 5.4 6.1 6.4 5.8 6.7 5.0 6.3
300 440 420 260 230 410 530 220
4.0 5.2 5.8 5.1 5.2 6.4 5.1 6.0
280 420 390 230 210 400 420 170
4.0 5.1 5.6 5.1 5.1 6.4 4.9 5.6
250 400 360 210 190 400 410 1 10
3.9 5.1 5.3 4.9 4.8 6.2 4.6 5.3
230 380 320 180 170 390 400 100
3.7 5.0 5.3 4.8 4.7 6.1 4.5 5.2
230 370 310 160 170 360 400 90
T.S.. tensile strength; PS..permanent set; E.B.. elongation break
most of these blends. The authors have also prepared heat-shrinkable film from these blends. Heat shrinkage increases and the amnesia rating decreases with increase in percent crystallinity (Chattopadhyay et al., 1999). It may be mentioned here that electron beam crosslinking is gaining importance in vulcanization of rubber because of several advantages (Bhowmick and Mangarai, 1994; Sen Majumder and Bhowmick, 1998; Banik et al, 1999). Bhowmick et al. also prepared rubber-plastics combinations from modified plastics and rubber, forexample, ethylene vinyl acetate/ethylenemethacrylate-epoxidizednaturalrubber blend, silicone rubber-ethylene methyl acrylate blend, silicone rubber-polyethylene blend etc. It was noted that only a few combinations gave thermoplastic elastomeric character (Santra et al., 1993; Kole et al., 1995; Kannan et al., 1995. Kali Ray et al., 1997).
5. ETHYLENE-OCTENE COPOLYMER Engage polyolefin elastomer is an ethylene octene copolymer prepared by using INSITE catalyst and process technology that allows extraordinary control of polymer structure, properties, and rheology (DuPont Dow, 1999). The ethylene-to-octene ratio could be varied to produce a range of polymeric materials from rubber to thermoplastic elastomer. For example, the polymers with 25, 19, and 14% octene content have DSC melting points of 55, 76, and 95"C, respectively. These copolymerspresent a broad range of solid-state structures from highly crystalline lamellar morphologies to fringed micellar morphologies of low crystallinity. The physical properties, as discussed below, are controlled by the structure of the copolymer. It bridges the gap between the plastics and the rubber. Engage is currently being manufactured and supplied by DuPont Dow Elastomers, USA. The grades available are shown in Table 12, as are thephysical properties. As usual, the density and the hardness increase with increase in the ethylene content. Mooney viscosity decreases with increase in melt index. Recently,Bhowmick et al. (unpublished) determined thepropertiesandthermoplastic elastomeric behavior of a series of Engage copolymers and their blends. The results are given in Table 13. The copolymer with low octene content (Engage 8440) displays highest tensile strength, elongation at break. modulus, and permanentset, while Engage 8 150 exhibits excellent permanent set, tensile strength, and modulus. Blending of Engage 8150 with LDPE in a ratio
Bhowmick
498
Table 12 Different Grades of Engage and Their Properties
Engage grade
Density (gicm') ASTM D-792
8180 8150 8100 8200 8400" 8452 841 1" 8003 8585 840 1 '' 8440 8480 8450 8550 8402" 8540 8445 8403"
0.863 0.868 0.870 0.870 0.870 0.875 0.980 0.885 0.885 0.885 0.897 0.902 0.902 0.902 0.902 0.908 0.9 10 0.9 I3
~~
Mooney visocosity ML 121°C 1 + 4 at ASTM D-l646
index flow Melt (dgimin) ASTM D-1238
~
Hardness, Shore A peak ASTM D-2240
DSC melting ("C) rate IO'Clmin
~~
35 35 23 8 1.S
66 75 75 75 72 79 76 86 86 85 92 9s 94 94 94 94 94 96
0.5 0.5 1 .0 5.0 30 3.0 18 1 .0 2.5 30 I .6 I .0 3.0 4.3 30 I .0 3.5
11
3 22 12 1.5
16 18 10
7 I .S 18 8
30
I .S
49 55 60 60 60 67 78
76 76 76 95 100
98 98 100
103 103 107
These grades contam a slip additlve for injection molding application.
Table 13 Results of Different Blends with Engage ~
Blend Engage 8 150 Engage 840 1 Engage 8440 EVA 12 Engage 8 1SO/LDPE 70 : 30 Eng8150/LDPE 70: 30, 0.6phr DCP Engage 8 1SO/EVA 1270 : 30 Eng840I/EVA12 70:30 Eng8440/EVA 12 70 : 30 Eng8 1SO/EVA 12 70 : 30, 0.6phr DCP Engage 8 150/PP 70: 30 Eng8150/PP 70: 30,0.6phrDCP Engage 8 1 50iEVA45 70 : 30 Engage 8401iEVA45, 70:30 Engage 8440iEVA45, 70: 30 Engage 8150/EVA45 70: 30, lphr DCP
T.S. MPa
E.B. (%I)
100% Modulus
200% Modulus
16.5 13.2 30.7 18.6 16.8 19.4 20.3 13.1 28.7 18.9 8.0 16.9
99 1 972 1012 613 918 943 999 798 933 783 246 418 92 1 947 794 352
2.3 1 4.03 6.00 7.12 3.56 3.67 3.14 4.57 6.09 4.07 6.3 9.39 I .63 2.56 2.88 2.73
3. 10 4.52 6.58 7.65 4.1 I 4.28 3.92 5.04 6.66 5 .45 7.6 11.74 2.21 2.95 4.53 4.76
11.8
8.6 17.3 7.2
300% Modulus
Permanent set (%)
3.79
6 16 24 30
5.1 1
8.02 9.35 4.86
5. 15 4.7 I 5.77 8.07 6.82 -
13.82 2.79 3.42 5.5 1 6.53
11
9 10 18
24 9 21 26 9 14 21 5
Miscellaneous Thermoplastic Elastomers
499
of 70: 30 improves the modulus at the sacrifice of set and elongation at break (Table 13). All the above properties are, however. further improved with the addition of 0.6 phr dicunlyl peroxide as a crosslinker i n the dynamic vulcanization process. An interesting observation from these studies is the development of polypropylene-based thermoplastic elastomer from Engage X 1 SO. As shown in Table 13. a tensile strengthof 16.9 MPa, elogation at break of41 8%. 300%modulus of 13.82 MPa. and a permanentset of 20% have been achieved. These properties are comparable to EPDM/PP blends. The authors have also studied the effect of blending EVA 12 and EVA 45. It is observed that at a blend ratio of 70: 30 Engage:EVA12. the modulus increases with the ethylene content of Engage at the expense of permanent set. EVA45 blend at the same blend ratiodisplayssimilarbehavior,althoughthemechanicalpropertiesareinferior. It hasbeen reported that Engage8450 and 8550 without modification have thermoplastic elastomeric character (DuPont Dow, 1999). Engage provides excellent heat aging, compression set. and weather resistance properties compared to many rubbers and thermoplastics. These can be crosslinked by peroxide, silane, or irradiation. The properties could also be manipulated by using fillers and other compounding ingredients. The suggested applications are general purpose elastomers, flexible molded goods. footwear (shoe soles). extruded profiles, thermoplastic elastomers. wire and cable, etc.
6.
ETHYLENE-STYRENEINTERPOLYMERS
Copolynlerizaiton of ethylene and styrene by the INSITE technology from Dow generates a new family of ethylene-styrene interpolymers (Table 14). Polymers with up to SO wt% styrene are semicrystalline. The stress-strain behavior of the low-crystallinity polymers a t ambient tem-
Table 14 Properties of Ethylene-StyreneInterpolymers
DesigStyrene Density nation (mol%)
ES 16 ES24 ES27 ES28 ES30 ES44 ESS3 ESS8 ES62 ES63 ES69 ES72 ES73 ES74
4.7 7.7 0.9355 9.2 0.9464 22.9 9.5 10.2 17.4 22.9 27.2 31.2 31.3 37.7 41.8 41.9 43.6
(glcc)
0.9420 0.944s
0.9454 0.9486 0.9678 0.98 10 0.9937 0.99OI 1.0123 1.0186 1.0207 1.0214
Crystnllinity ( V )(DSC) (based on copolymer) tans)
37.5 26.6 17.4 19.6 S .0 n/a nln nla nla nln nla nln nla
Activation T, ("C) energy (DMTA, Fracture of glass I Hz, transition (kJ1mol)
12.7 0.0 - 3.7 - 4.0 - 1.9 - 9.4 - 2.4 S.9 11.4 11.1
22.7 33.8 32.6 33.0
nla nla n/a
282 nln 256 308 318 nla 340 362 nla
373 376
Modulus"
(MPa) 52.8 26.4 25.0 10.5 25.4 3.2 2.5 2.7 3.9 3.9 10.6 741 311 724
2.2 1.3 2.6 1.4 1.4 1.1 ? 0.3 f 0.3 f 0.0 f 0.4 ? 2.5 t 83 f 16 f 201
f f f t f f
stress:' (MPa)
31.7 & 2.3 33.3 k 2.4 29.9 f 3.0 32.8 k I .6 32.0 -t 1.2 10.9 f 0.3 a . 7 a . 7 5.9 f 0.1 2.9 f 0 . I 21.2 f 0.3 22.8 f 0.4 14.4 f 0.4 25.8 t 2.5
Fracture strain" ([x-)
666 t 34 517 2 28 453 t 23 564 2 17 468 t 14 576 t 2 >2000 >2000 ss9 f 9 107 f 39 412 +. 4 292 f S 263 f 3 268 f 32
Bhowmick
500
perature exhibits elastomeric characteristics with low initial modulus, a gradual increase in the slope of the stress-strain curve at higher strain, and large instantaneous recovery (Chen et al., 1998). The origin of the elastomeric behavior is ascribed to the entangled network of flexible chains tied to the fringed micellar crystals. The polymers with more than 80 wt% styrene are amorphous. Table14 also reports the propertiesof the ethylene-styrene interpolymers. Similarly, ethylene-butylene copolymers may also be prepared.
7. EPDM-BASEDBLENDS Thermoplastic elastomeric blends of ethylene-propylene rubber and polypropylene have been reported by several workers. Excellent propertisof the type of elastomers emergefrom the partly similar chemical structure of both the polymers. Following thesame argument,soft thermoplastic elastomers have been prepared by blending polyethylene and ethylene propylene diene rubber (Sen et al., 1990). Studies of the crosslinking and crystallization behavior reveal that at higher EPDM content the heat of fusion, percent crystallinity, melting point, and entropy change of fusion are reduced. Crosslinking of the blends further reduces the above properties. Gel fraction or degree of crosslinking increases with the increase in EPDM/PE ratio. The same authors also prepared thermoplastic elastomers by silane grafting of PE and EPR (Sen et al., 1993). It has been reported that the dynamic elastic modulus at any temperature is always higher for silane crosslinked systems compared to that of DCP ones. At equivalent degree of crosslinking, the network produced by silane crosslinking is more resistant to deformation under dynamic shear than that produced by DCP crosslinking. A few mechanical properties are shown in Table 15. Although the tensile strength and elongation at break are slightly inferior to the peroxide crosslinked systems at the same gel fraction, high-temperature properties, especially the hot elongation, are superior for the silanemodified systems. Stability against oxidative degradation and air aging is also markedly higher for silane crosslinked materials due to the higher bond energy of Si-0-Si linkage and threedimensional network formation.
8.
BLENDS OF POLYETHYLENE AND NATURAL RUBBER
Natural rubber producing countries prepared thermoplastic elastomers by blending natural-rubber and high melting plastics, i.e. polypropylene, similar to Santoprene. Elliott ( 1990) reviewed the subject. However, the advantages of compatability of EPDM-PP blend do not exist in NRPP blend. Hence, molecular engineering is to be done in order to achieve similar properties. Concurrently soft thermoplastic elastomers from natural rubber-polyethylene was developed. Table 16 reports the mechanicalproperties of such thermoplastic elastomers (Elliott, 1990). RoyChoudhury and Bhowmick (1989, 1990) demonstrated that significant improvement of the properties could be done by judicious choice of physicalkhemical conlpatibilizer. For example, Table 17 indicates that chlorinated polyethylene or ethylenepropylene rubber. which is structurallysimilar to polyethylene and amorphous-likerubbers, improves the adhesion and hence the mechanical properties. Chemical interaction further enhances the properties. Subsequently, Bhowmick ( 1 999) reported a series of thermoplastic elastomers from modified polyethylenes. It was also highlighted in one of the reports that morphology plays a dominant role in determining the properties. For example, thermoplastic elastomers with similar properties could be obtained from recycled rubber and plastics, although the properties of the individual recycled systems are inferior to the neat polymer (Nevatia et al., 1995: Naskar et al., 1999).
Table 15 Properties of the Various Silane Crosslinked PE and EPDM Systems
Sample
Sample reference
Tensile properties (measured at ambient temperature, 30°C)
Tensile properties after annealing (measured at ambient temperature, 30°C)
High-temperature tensile properties (measured at 130°C)
Tensile strength (MPa)
Elongation at break
Tensile strength (MPa)
Elongation at break
Tensile strength (MPa)
13.3
40 305
(%)
1 2 3
PE PE,S XLPE,S" (gel fraction 45%)
13.3 13.3 15.0
340 330 325
4 5 6
XLPE,Sh (gel fraction 65%) XLPE' (gel fraction 45%) XLPE" (gel fraction 65%)
16.4 18.0 20.0
310 550 530
7
XL(PE-EPR),S' (gel fraction 75%) XL (PE-EPR)? (gel fraction 75%) EPR XLEPR,S' (gel fraction 75%) XLEPR~(gel fraction 75%)
12.0
8
9 10 11
(%)
Elongation at break (%)
Hot elongation
Hot set
(%I
(%)
(measured at 200°C)
(measured at 200°C)
-
-
-
0.9
I30
0.3
55
350
1.2
90
I30 (failed after 10 min) 50 Failed after 1 min 30 (failed after 7 min) 15
15.0
600
0.8
100
25
Nil
12.5 10.0 11.0
900 220 600
-
55 80
-
-
~
a. b, e, and g = silane crosslinked: c, d. f. and h = DCP crosslinked.
-
14.5
16.3 16.0 17.0
220 530 460
0.4
-
2.0 1.4
-
15
20
Failed after 10 min
Ni 1 Failed after 1 min Nil (failed after 7 min) Nil
Ni 1 Nil
Bhowmick
502 Table 16 Typical Properties of Intermcdiatc to Hard Grade TPNR Flexural modulus (MPa) Yield stress (MPa) Tensile strength (MPa) Elongation 700 at brcak650 (VC) 550 1zod impact strength a t - 30°C I mm notch (Jm" ) 0.25 mm notch (Jm")
9.
300 8.5 20 550
400 10.5 33 23
>h40
>640 300
450
700 16 30
900 19
>640 400
250 90
BLENDS OF POLYETHYLENE AND HYDROGENATED STYRENE BUTADIENE RUBBER
Recently, De Sarkar et al. (1999) reported a series of new thernloplastic elastomers from blends of hydrogenated styrene-butadiene rubber (HSBR) and polyethylene. The blends show twophase laminated structure, althoughother measurements indicate that there is some compatibility between HSBR and PE. The transition temperatures for polyethylene are observed at 64, -6, and - 108°C and those for HSBR at - 18 and - 106°C. The thermoplastic elastomers from the blends register a reduced tan6 peak height at - 18°C for HSBR. On incorporation of HSBR, a-transition temperature for polyethylene (64°C) is shifted towards higher temperature and the tan6 peak valuesare also raised. E' valuesareintermediatebetween PE andHSBR for the thermoplastic elastomer in this temperature range. From the x-ray analysis. it has been informed that interplanar distance of PE increases with the addition of PE, indicating appreciable migration of HSBR into the interchain space of PE. The tensile strength, elongation at break. modulus. set, and hysteresis loss are comparable to conventional rubber and are excellent (Table 18).The thermoplastic elastomers have been shown to be reprocessible. Interestingly, when LDPE is replaced by high-density polyethylene or HSBR by SBR, the mechanical properties deteriorate,
Table 17 Mechanical Properties of thcNR/PE Mixes with Compatihilizer
no.
Mix A(NR70/PE30) A' (NR70/PE27/PEm3) B(NR70/CPE20/PE30) B' (NR70/CPE20/PE27/PEm3) C(NR70/EPDM20/PE30)
C'(NR70/EPDM20/PE27/PEm3) D(NR70/S-EPDM20/PE30) D'(NR70/SEPDM20/PE27/PEm3) E(NR70/ENR20/PE30) E'(NR70/ENR20/PE27/PEm3)5.21
20% modulus (MPa)
Tensile strength, U\, (MPa)
Elongation at break.
0.54 0.90 0.42 0.74 0.64 I .01 l .02 I.ox 0.86 2.68
3.60 3.94 4.61 4.73 3.40 4.42 3.77 4.12 3.86
560 485 490 666 600 684 577 634 627 470
(%)
PEm: M;llclc modified PE: S-EPDM: sulfonated EPDM: ENR: epoxidizcd NR. U,,: Tensile strength of the hard phase. " Sample failed under testing c o n d ~ t ~ o n .
(T~,/(T~~"
0.39s 0.46 0.5 1 0.58 0.37 0.52 0.40 0.50 0.42 0.64
Hysteresis loss W, ( X 10' Nm) 141 90 I07 101 b
15 79 7s 86 2
503
Miscellaneous Thermoplastic Elastomers
80120 10010 HSBWLDPE 7.0 5.7 Tensilc strcngth (MPa) 730 960 Elongation at break ( % ) 13.9 12. I Work to brcnk (kJ1m') Modulus (MPa) 2.7 I .S 100% 3.7 2.3 300% 2.0 6.3 Set at 1008 elongation (P) 3.04 6.14 Hysteresis loss (J/cm3)
70130 X.4 735 15.5
60140 8.6 600 15.8
3.3 4.4 7.5 7.43
3.8 5.0 10.0 9.73
50150 9.0 580 15.8 4.7 5.8 12.5 1 I .S5
40160 10.5 560 16.8 5.7 7.0 15.0 13.58
30170 11.4 55s 19.3 6.2 7.5 17.5 14.13
20180 13.2 510 21.5 7.1 8.8 22.5 17.26
01 1 0 0 14.4 500 23.2 8.6 10.2 45.0 34.37
which are explained on the basis of the change of morphology. crystallization. and interaction between the components (Fig. 12). 10. MELT PROCESSABLE RUBBER
Alcryn'"' melt processable rubber is a family of midperformance elastomers that do not require vulcanization. Alcryn'" provides the economics of melt processing on both plastics equipment
/I m
Degree of crystdlinity ( O / O )
Fig. 12 Crystallinity and mechanical properttes of SBWLDPE. HSBWLDPE. and HSBWHDPE blcnds.
Bhowmick
504
and modified rubber equipmentwithout sacrificing the key properties of cured rubber. Alcryn"' is a thermoplastic elastomer based on a partially crosslinked chlorinated olefin. Polyvinylidene chloride acts as the hard segment, while crosslinked polyvinyl acetate copolymer acts as the soft phase. This rubber is presently sold by Advanced Polymer Alloys ( 1 999).
10.1
Grades
The Alcryn'" product line is grouped into four series- 1000,2000,3000, and4000-differing in hardness and mechanical properties (ALCRYN Product Literature). For example, Alcryn"' series 1000 has general plastic processing characteristics and excellent weather resistance and is not suitable for injection molding, for which series 2000 has been designed. Series 2000 has high flow and is excellent for complex extrusion and is not suitable for calendering operation. Previously DuPont used to manufacture these (in commercial-scale equipment)under the trade names ALCRYN R1201 B-60A, R1201 B-70A, R1201 B-80A.
10.2 Properties Typical properties of three grades of ALCRYN melt processable rubber are given in Table 19. The tensile strength varies between 12.1 and 13.4 MPa, while the elongation at break lies in the rangeof 210-320%. The properties tested at 23°C after 168 hours of oven exposure at 121°C do not reflect significant changes. In fact, ALCRYN has better heat aging resistance than most nitrile rubbers. It is positioned as a 125°C heat-resistant rubber.
Table 19 Typical Properties of ALCRYN Melt Processable Rubber ASTM Method ~~
~
R1201B 60A
R1201B 70A
R1201B 80A
~
Hardness Shore A durometer Tensile properties Tensile strength, MPa 100% Modulus, MPd Elongation at break, 9% Tear strength, Graves die C KN/m Impact brittleness temperature, "C Stiffness temperature Clash berg. 69 MPa, "C Properties tested at 100°C Tensile strength, MPa 100% Modulus, MPa Elongation at break, % Hardness Shore A durometer Properties tested at 23°C after oven exposure (168 hrs at 121°C) Tensile strength, MPa 100% Modulus, MPa Elongation at break, % Hardness, Shorc A durometer
D 2240
62
69
78
D 412 D 412 D 412 D 1004 D 1790 D 1043
12.1 3.1 325 21.9 - 46 - 29
13.1 4.5 295 23.6 - 44 - 26
13.4 1.2 210 21 .0 - 42 - 23
D 412 D 412 D 412 D 2240
3.7 1.4 295 38
3.9 1.6 215 43
4.1 2.3 200 52
D 412 D 412 D 412 D 2240
12.3 4.1 315 63
14.9 6.0 285 70
13.1 9.4 150 76
505
Miscellaneous Thermoplastic Elastomers
The stress-strain curve of ALCRYN'K' is similar to rubber, showing that it has the same flexibility as rubber (Fig. 13). It recovers elastically from 100% strain, and there is no yielding in the stress-strain curve. Table 20 compares stiffness, yield strain, and ultimate strength of various nlaterials. The rubbery nature of ALCRYNIK' is indicated by its response to tension, compression. and tlexure. The tension set at 100% elongation at 23°C is about IO%, while, the compression set at 100°C for 22 hours is about 50% and the flex modulus is in the range of 7.5-9.5 MPa. The compression set values (22 hours at 100°C) of TPV, nitrile (ACN - 33%) and polychloroprene rubber are 38, 27, and 54% respectively. ALCRYN'R' is also resistant to creep in tension even after 1000K at 23"C, is comparable to rubber, and is superior to TPV. Table 21 shows the oil resistant properties of typical grades of ALCRYN'R'. There is slight increase in properties on immersion of ALCRYN'K' in ASTM oil #l. ALCRYN"' is ranked as better than neoprene and similar to nitrile rubber. Its rating is 125°C in oil. For example, volume change in ASTM oil #I is - 9 for ALCRYN'R', while the same for nitrile, neoprene, and TPVs are -4. - 14, and 30, respectively. Fuel resistance (Fuel B) is also better than many TPVs. ALCRYN"' offers excellent resistance to weathering-superior to nitrile and neoprene. It is relatively easy to flame retard.
Table 20 TPEs Compared with Other Materials
in YieldStiffncss Material 70A Rubber 70A AlcrynB 70A TPV Nylon Steel
(%I)
1.500
1,500 6,000 175.000 30 x I06
> 100 > 100 3.5 3 0.1
strength Ultimate 1,500 1,500 1,700 12,000 160,000
(psi)
506
Bhowmick
Table 21 Oil Resistance of ALCRYN Melt ProcessableRubber: Typical Properties
ASTM Oil No. 1 Immersion 7 days at 100°C Tensile strength. MPn 100'2 Modulus. MPa Elonption at break, % Hardness. Shore A durometer Volume change. 'A ASTM Oil No. 3 Immersion 7 days at 100°C Tensile strength. MPa 100% Modulus. MPa Elongation a t break. Hardness Shore A durometer Volume change. (2 Orlginal Properties Tensile strength, MPa 100% Modulus. MPn Elongatloll at break. r/r Hardness, Shore A durometer
R1201 B 60A
R1201 B 70A
R1201 B 80A
13.9
15.4
4.0 335 66 - IO
5.9 280 73 -9
15.9 9.2 205 84
11.2 3.0 290 53
12.3 6.5
+ 10
10.7 4.6 205 60 +II
12.1 3.7 325 62
13. I 4.5 205 60
13.4 7.2 210 78
- 11
180
68 +7
10.3 Processing ALCRYN'K' is a unique plastic to process because it never melts. but it does soften enough above 300°F that high shear will reduce its viscosity drastically. The molding grades have no crystalline melting point and are essentially amorphous. ALCRYNIR' can be injection molded. extruded. and blow molded in faster. higher-productivity cycles than rubber. Conventional plastic equipment. particularly that used for polyvinyl chloride, is satisfactory. Rubber equipment may need to be modified.
10.4 Applications ALCRYN'"' offers high value i n use for many applications now served by vulcanized rubbers, thermoplastic elastomers, and flexible thermoplastics. The applications can be divided into the following groups: ( 1 ) automotive, ( 2 ) architectural, ( 3 ) industrial. (4) wire and cables. and (5) tools and appliances. Some of the applications suggested include gasoline cap seal. antifreeze testing lid. truck bumpers. door latch. truck hub seal. taillight housing, sidelight window lace, in-line fuse housing. fuel tank gaskets. seatbelt duct. copper belt cores. gasoline splash guard, reducing couplings, bulkhead seal. splash goggles. instrument gauge enclosures. full face mask. pipe couplings and adapter. window weather stripping. tool handle cores, suction cups, palm sander. friction rubber. tool grips. and flashlight handle.
11. HIGH-TEMPERATURETHERMOPLASTICELASTOMERS FROM RUBBER PLASTIC BLENDS Most of the thermoplastic elastomers prepared so far from rubber-plastic blends have poor hightemperatureproperties. For example, commercialblendsbased on EPDM-PP, NBR-PP, and
Miscellaneous Thermoplastic Elastomers
507
Alcryn melt processable rubber have a maximum operating temperature of 150°C. Thermoplastic elastomers could be made by using plastic of high melting point, thereby enhancingthe operating temperature.However, many high-meltingplastics like polyamidespresent some interesting processing problems, somewhat like those involved in perfluorocarbon resins. Also, many rubbers like silicone, which can withstand high temperature, do not have an appropriate match of hard segments. The problems are still greater when chemical and oil resistance are demanded from such TPEs. This section will highlight a few recent developments in this area. Jha and Bhowmick (1997a) reported the preparation of thermoplastic elastomeric reactive blends of nylon-6 and acrylate rubber, their characterization, and the influence of interaction between nylon-6 and ACM on the mechanical and dynamic mechanical properties, rheology, swelling, and thermal degradation of the blends. It has been observed that during melt blending of nylon-6 and ACM in a Brabender Plasticorder at 220°C the mixing torque increases after the initial softening period, indicating the occurrence of interfacial reaction between nylon-6 and ACM at the processing temperature (Fig. 14). The increment of torque value is maximal atthecomposition of 55/45 (w/w) nylon-6/ACM,suggesting that the maximumamount of reaction occurs at that proportion. The solubility measurement of the blends in formic acid (a solvent for nylon phase) has revealed a maximum amount of graft formation near 50/50 (w/w) ratio. The dynamically vulcanized blends display higher amounts of graft formation relative to unvulcanized blends. IR spectroscopic analysis has shown a reduction in the intensities of the peaks corresponding to epoxy groups of ACM as well as carboxylic acid end groups of nylon6, suggesting a chemical reaction between the two at the processing condition. NMR analysis of 40/60 (w/w) nylon-6/ACM blend has suggested 75% consumption of epoxy groups of ACM during the reaction (Jha and Bhowmick, 1999a). Based upon IR and NMR analyses, a probable mechanism of reaction between nylon-6 and ACM at the processing temperature has been pro-
60 50/50 BLEND (DV)
Fig. 14 Torque-time chart at 220°C for nylon,ACM (-) andnylon:ACM (S0:SO w/w) blendboth unvulcanized and dynamically vulcanized (---) with 0.5 phr HMDC in a Brabander Plastlcorder.
508
Bhowrnick
t
Fig. 15 Dynamic mcchanical spectra of ny1on:ACM (40:60 w/w)blcnd a t various molding times.
posed. In the dynamic mechanical analysis. it has been observed that the tan 6,,,;,, (maximum value of tan6 at the transition) as well as Tg (glass transition temperature) of the bulk rubber phasearedecreasedwith the level of interactionbetween the twophases, followed by the appearance of a secondary transition at a higher temperature region (1 7-22°C) for 40/60 (w/w) nylon-6/ACM blend due to the formation of graftedACM chains(Fig. 15). In thecase of dynamically vulcanized blends. the tan6 peaks corresponding to the rubber phase are broadened and shifted to slightly higher temperatures ascompared tothe blends withoutdynamic vulcanization. With increasing level of interaction, the storage modulus of 40/60 (w/w) nylon-6/ACM blend at room temperature (25°C) increases from 1.26 X to 4.36 X MPa. The observed storage moduli of the blends at 50°C are close to those obtained from Kerner’s hard matrix-soft filler model, suggesting the formation of nylon-6 as the continuous matrix, which has beenfurtherconfirmed from themorphologystudies(Fig. 16). The interactionbetween nylon-6 and ACM also increases the Young’s modulus of 40/60 (w/w) blend from 20 MPa to 37 MPa and the hardness of the blend from 35 to 48 Shore D. Also, the tensile strength and the elongation at break increase appreciably with the level of the reaction (Table 22). Dynamic vulcanization of the blends results in a slight reduction in hardness and Young’s modulus, but the tensile strength and the elongation at break increase significantly.
ermoplasticMiscellaneous
509
NYLON-6 MATRIX Fig. 16 Morphology of ny1on:ACM (50:50 w/w)blend.
The blends are pseudoplastic in nature and an increase in shear rate decreases the viscosity and increases the extrudate swell of the blends (Jha and Bhowmick, 1997b). The viscosity of the blends displays positive deviation from the average values, suggesting the reactive nature of the blend components. The viscosity increases with increasing degree of crosslinking of the rubber phase. In the case of dynamically vulcanized blends, the interparticle interaction is low, and hence the effect of concentration of the rubber phase on the viscosity is found to be low compared to those without dynamic vulcanization. The activation energy of the melt flow of the dynamically vulcanized 40/60 (w/w) nylond/ACM blend varies in the range of 8-15 kcaV
Table 22 Mechanical Properties of Nylon-6/ACM Blends With and Without Dynamic Vulcanizationn Weight percent
(“/.l 40 45 50 55 60
of plastic Tensile strength Elongation at break Young’s modulus Hardness (ma) (%) (10)b 12 13 (13) 12 (15) 19 (16) 17 (20)
(122) 96
90 (142) 92 (140) 120 (116) 100 (150)
48
(30)
Wa) 37 41(35) 45 (43) 62 (56) 71 (68)
Vulcanned with 0.5 phr of W C . Value In parentheses Indicates the properties corresponding to the dynamically vulcanned blends.
(Shore D)
(44) 50 (47) 55 (51) 57 (54) 60 (57)
Bhowmick
51 0
mol and decreases with increasing shear rate. The morphology study of the extrudate suggests rupture of the ACM phase at a high shear rate in the case of uncrosslinked blends. whereas the morphology of the dynamically vulcanized blends is stable against shear stress. The blends are found to be reprocessable at 240°C without any appreciable degradation of either phase, which suggests its applicability as a thermoplastic elastomer. The swelling behavior of nylon-6/ACM blends in various solvents and oil and the effect of blend ratio, dynamic vulcanization of the ACM phase, and the interaction between the two phases on the extent of swelling of the blends in different solvents and oils have been examined (Jha and Bhowmick, 1998). The swelling of the dispersed ACM particles in nylon-6 matrix is greatly constrained compared to the free swellingof crosslinked ACM rubber i n the same solvent. This is due to the constraints imposed by the least swellable continuous phase (i.e., hard nylon6 matrix) and also due to formation of a reduced mobility zone in the ACM phase by grafting reaction at the interface. To evaluate the constraints imposed by the nylon-6 matrix alone. a simple thermodynamic model based on the modified Flory-Huggins equation has been applied to this system, which could explain the data over a certain region. With increasing the extent of reaction between the two phases. both the rate and the extent of swelling of 40/60 (w/w) nylon-6/ACM blend decrease progressively. Also, the increase in the crosslink density of the rubber phase substantially improves the solvent resistance of the blends. The fuel resistance of the 40/60 (w/w) nyIon-6/ACM (dynamically vulcanized) blend at 25°C and oil resistance (in ASTM oil #3) at 150°C are found to be excellent. The mechanical properties of the thermoplastic elastomeric 40/60 (w/w) blend (dynamically cured) do not deteriorate to a significant extent when the samples are aged at different temperatures ( 150-200°C) and times ( 1 -7 days) (Fig. 17). This implies excellent heat-resistant properties of the blends. The DMTA results of the aged sample suggest that during aging. the
16
170
Ageing a t 1 5 O O C
6 0
1
2
3
1,
5
6
7
Number o f d a y s o f ageing Fig. 17 Aging behavior of ny1on:ACM thermoplastic vulcmizates at 150°C.
Miscellaneous Thermoplastic Elastomers
51 1
bonds between nylon-6 and ACM break down predominantly. The FTIR studies of the aged samples indicate the formation of imide linkages on nylon4 chains through a thermal oxidation process. The results of the investigation on the effects of various fillers and plasticizers on the key performance of thermoplastic elastomeric blends based on nylon-6 and ACM have been described (Jha and Bhowmick, 1999b). It is concluded that the addition of carbon black and clay reduces the extent of reaction between nylon-6 and acrylate rubber. while silica interacts with ACM chainsthrough covalent bond formation. which increasesthe overall polymer-filler interaction in the blends. The viscosity of the filled blends is found to be higher than that of the control, unfilled blend.However.addition of ester plasticizerlowerstheviscosityandimproves the processability. The fillers do not change the glass transition temperature of the ACM phase, but the Tg of the nylon4 phase is reduced in the filled blends, probably due to a decrease in its percent crystallinity. However, a substantial improvement in the damping properties of the blends in the service temperature rmge (25-175°C) is revealed from the DMTA results. Mechanical properties of the blends are greatly improved with the addition of a lower amount of carbon black (i.e., 10-20 phr) and a higher percentage of silica (30 phr). The extensibility of the blends is increased by 50% with the addition of silica due to higher polymer-filler interaction. Also, the elastic recovery of the blend is improved in the case of filled samples; the improvement in the overall integrity of the blends is probably due to the formation of a co-continuous morphology, which is evident from the optical micrographs of the blends. However. the volume swell in ASTM oil #3 at 150°C of the blends is well below IO%, which suggests its excellent hot oil resistance. An attempthas also been made to enhance the thermalstabilityandhigh-temperature resistance of EPDM/PP TPEby incorporating nylon, whichf o r m a continuous matrix (Venkataswanly and Payne, 1990). The same group has prepared a variety of rubber-plastic conlpositions that impart thermoplastic elastomeric properties (Venkataswamy, 1998). It has been reported (see Chapter 10) that the TPVs prepared from the blends of nitrile rubber (NBR/PP) provide very good heat and oil resistance with improved mechanical properties by thepresence of small amount of in situ formed NBR-PP block copolynler duringmeltblending. It has also been observed that the compatibilized NBR/PP TPVs can be mixed with EPDM/PP TPVsto give compositions of both excellent mechanical properties andoil resistance. Coran and Patel ( 1982) reported on TPVs based on reactive blends ofpolyamide and chlorinated polyethylene (CPE), which show excellent mechanical properties and hot oil resistance (Table 23). This was attributed to the chemical bond formation between the rubber and plastic molecules during melt-blending. Technological conlpatibilization of nylon/NBR through the formation of nylon-NBR graft copolymer by the use of phenolic resin curative hasalso been shown to improve both mechanical integrity and hot oil resistance of the above blends. Structure development and reactive processing of nylon/HNBR blends in the presence of compatibilizers have been thoroughly discussed (Bhowmick and Inoue. 1993). It has been demonstrated that mixing time and temperature, addition and amount of vulcanizing agent, and nature of compatibilizer influence the particle size of the dispersed domains. The blends of EPDM and polybutylene terephthalate (PBT) have been studied recently as thermoplastic elastomers, both with and without dynamic vulcanization. by Moffett and Dekkers( 1992). In situ formation of EPDM-PBT block copolymer by the addition of maleated EPDM was necessary to reduce high surface energy betweenEPDM and PBT. The structure of the blends made from epoxy functionalized EPDM and PBT controls the mechanicaland dynamic mechanicalproperties(Vongpanishet al., 1994). Patel (19%) highlightedhigh-temperaturestable,lowsolventswelling TPVs comprising po1y:lmide resin and crosslinked acrylate rubber. Jha et al. ( 1997b) reported TPEs prepared by reactive blending of polyethyleneterephthalate (PET) andacrylaterubber (ACM) throughtrans-esterification
Bhowmick
512
Table 23 Properties of ThermoplasticVulcanizates 6 number
Stock5
4
3
Ingredients, parts by wt. Nylon 6, 6-6, 6-10' Nylon 6-9" CPE rubber' MgOd Lead stearate, stabilizer Epoxide stabilizer" rmPhenylenebismaleimide' Trimethylolpropane triacrylate' 2,5-Dimethyl-2,5-bis(t-butyl-peroxy)hexane (90% active)" Temperatures Mixer oil bath temp., "C Molding temp., "C Properties U", MPa u I l H ) r ME1 8.5 E". o/r E,, p/o
Hardness, D scale 70-hour volume swell, 125"C, r ASTM No. 3 oil, 7
49 26
1
2 40
-
60
60
-
-
I .2 -
0.60.6
160 180 14.7 8.2 340 45 40 38
40
-
21.8 310 46
40
40
-
-
60
-
-
-
-
60 6 1.2 3
-
-
4.8 -
6
-
1.2
1.2
2.4
160 180
180 210
19.5 13.817.917.2 10.0 6.5 270 350 45 45 46 35 -
-
1.2 0.6
-
3 -
-
-
180 210
210 250
12.4 280 35 56
100
-
-
40 60 6 l .2 3
40 60
59 50 42
1.2 0.6
210 250 17.3 15.9 160 59 59 23
zytel8 63(du-Pont): 506 nylon 6. 3 1 6 nylon 6-6, 10% nylon 6-10 terpolymer. VydyneB 60H (Monsanto). CPE CM 0342 (Dow). " MagliteB D: (Merck). DrapexO 6.8, epoxlde stabilizer (Argus). ' HVA-2 curatlve (du Pont). 8 SR351 (Sartomer). " L-101 (LucidoI). h
reaction between the blend components. Thermoplastic elastomers made from PBT/ACM prepared by the same authors show a tensile strength of 4 MPa and hardness of 35 Shore D (Jha and Bhowmick, 1999e).
ACKNOWLEDGMENT The technical data taken from the product literature of various companies and contained herein are guides to the use of their product. The advice is based on tests and information believed to be reliable, but users should not rely upon it absolutely for specific applications. It is given and accepted at the user's risk, and confirmation of its validity and suitability in particular cases should be obtained independently. The companies make noguarantee of the results and assume no obligation or liability in connection with their advice. This publication is not to be taken as a license to operate under or a recommendation to infringe any patent. Theauthoracknowledges the assistancereceived from his students in typesettingthe manuscript.
Miscellaneous Thermoplastic Elastomers
513
REFERENCES Advanced Polymer Alloys, Technical Literature on ALCRYN'"', 1999. Ashitaka, H., Kusuki, Y.. Asano, Y . .Yamanioto, S., Ueno. H., and Nagasoka, A. (19833). J. Po/yrrl. Sci. Po/yrr?. Cken1. Et/. 21: 1 1 1 1. Ashitaka, H., Ishikawa,H., Ueno, H., and Nagasakn, A. (198%). J. Po/yrrl. Sci. Po/yrr~.Clrrrrl. Ed. 21: 1863. Banik, I., Dutta, S., Chaki, T. K., and Bhowrnick, A. K. (1999). Po/yrrler 0 4 4 7 . Bayer A. G. (1993). M m l t n l j b r the Rlrhher Irldustr?. T. Kempermann, S. Koch and J. Summer. Eds. Bayer Publications, Leverkusen, Germany. Bayer A. G. (1999), Technical Literature, Leverkusen, Germany. Bhagawan S. S. (1987). Ph.D. thesis, Indian Institute of Technology, Kharagpur. Bhowmick. A. K., (1999) Polymer Processing Society Meeting, Bangkok, Thailand. Bhowmick. A. K., and Inoue, T. (1993). J. App/. P o / w . Sei. 491893. Bhowmick. A. K., and Mangarai, D. (1994), in Ruhher Prot/ucts Mttrrufitcturirl,q T d l r l o l o g y (A. K. Bhowmick, M. M. Hall, and H. Benarey, Eds.), Marcel Dekker, New York. Bhowmick, A. K., Kuo. C. C., Manzur, A., MacArthur. A., and McIntyre, D. ( 1986). J. M t r c w r r ~ o l .Sei.P h y . B25(3):283. Chattapadhyay, S., Chak, T. K., and Bhowmick A. K. ( 1999). Rnditrt. P h y . Cl~errl.( i n press). Chen, H., Guest, M. J., Chum, S., Hiltner. A., and Baer, E. ( 1998), J. A/)/)/. P o / y r ~Sci. . 70: 109. Coran, A. Y.,and Patel, R. (1982), U.S. Pat.4,355,139 (Oct. 19). De Sarkar, M,, De, P. P,, and Bhowmick, A. K. (1999), P o / w ~ e 40:1201. r DuPont Dow Elastomers, (1999). Product information literature. Freeport, Texas. Elliott, D. J. (199O), in Therrrqdtrstic E/tr.storrwr.s ,frorrl Rlrhher-P/osric.s Bler~tls( S . K. De and A. K. Bhowmick. Eds.), Ellis Horwood, London. Rlthhrrs (A. Whelan Gilby, G. W. (1982). in De\v/oprrlerlt.s irl Ruhher TechrIo/o,qv.Vol. 3, T/~errr~o/,/rr.stic. and K. S. Lee, Eds.), Applied Science Pub., New York. Hoshino, S., Yamamoto, S.. and Asano, Y. (1985). Int. Seminar Elastomers, Itoh. Shizuoka, Japan. Oct. 20-22. Japan Synthetic Rubber Co. Ltd. (1984). Jpn. Pat. 58,168.640: Chrrrl. Abstr. 100:176, 245 f. Japan Synthetic Rubber Co. Ltd. (1985), Jpn. Pat. 59,155,478; Chrrrl. Ahstr. 102:800-905. Japan Synthetic Rubber Co. Ltd., (1999) technical literature. Tokyo, Japan. Jha, A., and Bhowmick, A. K. ( 1997a), RLhber Chcwr. Techr~oI.70:798. Jha, A., and Bhowmick, A. K. (1997b). Po/ynwr 17:4337. Jha, A., and Bhowmick, A. K. (1998). J. A p p / . Po/yrrr. Sci. 6P23.7 I . Jha, A., Bhowmick, A. K., Fujitsuku, R. and Inoue, T. ( 1999a). J. Adh. Sei. T ~ I ~ I 13, o / 649. . Jha, A., and Bhowmick, A. K. (1999b). J. AppL Po/yrn. Sei. 74, 1490. Jha, A., and Bhowmick, A. K. ( 1 9 9 9 ~ )J.. App/. Po/yrn. Sei. ( i n press). Jha, A., Bhnttacharya, A. K., and Bhowmick, A. K. (1997), Polymer Networks and Blends, 7. 177. Kali Ray, A., Jha, A., and Bhowmick, A. K. (1997). J. Eltrstnrrwrs P/tr.stic.s 29:201. Kannan, S., Mathew, N. M,, Nando. G. B., and Bhowmick, A. K., (1995). P/tr.stic,y R ~ t / ) / wCorllp~.y;tesr Proc. AppL 24: 149. Kiji, J. ( 1983), Ar~gew.Mncrornol. Chrrrl. I f 1:53. Kole, S., Santra, R., Samantaray, B. K., Tripathy, D.K.. Nando, G. B., and Bhowmick, A. K. (199.5). Po/yrner Net,t*orks Blerlds 5:I 5 I . Moffett, A. J., and Dekkcrs M. E. (1992). Po/yrrwr E r ~ gSei. 32:l. Naskar, A. K., Bhowmick, A. K., and De, S. K. (1999). Po/yrrl. G I ~ Sei. . (in press). Nevatia, P., Banerjee. T. S., Dutta, B., and Bhowmick, A. K. (1995). Report on thermoplastic elastomers from blends of waste rubber-waste plastics. Patel, R. (1998), U.S. Pat. 5,591,798. Roy Choudhwory, N. R., and Bhowmick, A. K. (1989), J. AppL Po/yrr~.Sei. 38:1091. Roy Choudhwory, N. R., and Bhowmick, A. K. (1990), J. Atlh. Sei. 32: 1. Santra, R., Nando, G. B., and Bhowmick, A. K. (1993). J. AppI. P o / y I . Sci. 49: 1 145.
514
Bhowmick
Sen Majumdar, P., and Bhowmick, A. K. (1998), Rcrditrrion Ph.v.sics Cl~cwz.53:63. Sen, A. K., Mukherji, B., Bhattacharya, A. S., Sanghi, L. K., De. P. P,, and Bhowmick, A. K. (1990), Tlwnmchem. Acttr 157:45. Sen, A. K., Bhattacharya. A. S., De, P. P,, and Bhowmlck, A. K. (1993). J. 7'herr). A d . 39887. Takcuchi, Y., Sekimoto, X., and Abe, M. (1974), ACS Div. Org. Coatings-Plast. Chem. Paper 34:122. Thomas, S., Kunakose, B., Gupta, B. R., and De, S. K. (1986). PIcrst. Rubber Pruc. Appl. 6 9 3 . Venkataswamy, K. (1998), Symposlum on Dynamic Vulcanixates, Polymeric Materials Science and Engineerlng Div., fall ACS meeting, Boston. Venkataswamy, K.. and Payne, M. T. (1990). Symposium on TPE, ANTEC, Dallas, Texas. Vongpanish, P,, Bhowmick, A. K., and Inoue, T. (1994). P1rt.sfic.s Rubbur Conll'"sites-Proc.. Appl. 21: 109. Wang, S., Zhang, X., and Zhao, D. (1984). Hecheng Xiarlgiicro Gongye 7215; Che~n.Absfr. I 0 I : l 12213.
18 Halogen-Containing Elastomers Daniel L. Hertz, Jr. Seals Eastern, Inc., Red Bank, New )ersey
1. INTRODUCTION This chapter tells the story of halogen-containing elastomers. As synthetic rubbers, they range from non-oil-resistant to the most oil-resistant elastomers that can be developed. Consider first the breadth of the category: Halobutyl elastomers Chlorinated polyethylene Chlorosulfonated polyethylene Epichlorohydrin elastomers Polychloroprene (neoprene) elastomers Fluorosilicone elastomers Hydrofluorocarbon elastomers Pertluoroelastomers Now consider some of the unique properties theseelastomers offer asa result of halogen substitution: Versatility of crosslinking chemistry due to ease of halogen displacement Increased oil resistance Improved solvent resistance Enhanced thermal stability Increased flame retardancy or nonflammability 1.l
HalogenReactivity
The halogens (group VI1 elements) are reactive in the order of fluorine (F2) > chlorine (Cl?) > bromine (Br?) > iodine ( l ? ) .The high reactivity is due to their basic electronic structure: they are one electron shyof an argononlic (inert) species. The alkanehlkeneseries have hydrogens that are readily abstracted and replaced by halogens, the process initiated by heat or ultraviolet light. The relative ease of hydrogen replacement follows the classic route: 3" > 2" > 1" > CH3-H. 515
516
Hertz, Jr.
Since the halogens are electronegative elements, they create polar bonds, which in turn create polar molecules.
1.2
Polar Molecules
Polar molecules (structures distinguished by the presence of halogen, oxygen, or nitrogen atoms), in sufficient quantity, create oil (hydrocarbon) resistance. Hydrocarbons are essentially nonpolar and are therefore repelled by polar molecules.
1.3
Polar Bond Strength
Bond strengths, or, more correctly, bond dissociation energies (BDE as measured by (AH) (H/ m) for methane and substituted methane, are revealing: ~~
CH3-H CHj-F CHj"C1 CH3-Br CH>-I
435.14 45 I .87 35 1.46 292.88 234.30
The difference in bond dissociationenergies establishesbasic parameters for crosslink site availability and elastomer thermal stability.
1.4
Crosslinking Mechanisms
The relative ease of a halogen displacement (elimination) through various reactions utilized to create a crosslink site. Typical examples follow.
is often
Polychloroprene Typically, in chloroprene polymerization, about 1 '/. mol% of the monomer rearranges to form a 1,2addition with the chlorine atom in an allylicposition. This is a very easy chlorine to displace, creating a crosslink site. Cltlorobutyl When isobutylene-isoprene rubber is chlorinated, the isoprene backbone segment rearranges, forming several structures. predominantly a pendant 1.2 addition with the chlorine in the allylic position. This cure site is essentially the same one utilized for crosslinking polychloroprene. Cltlorasulfmated Polyethylene A tertiary amine readily reacts with the sulfonyl chloride group. This creates a reactive sulfene, ultimately leaving a double-bond site suitable for crosslinking.
Epichlorohylrin Elastomers The chlorine onthe chloromethyl group is readily displaced by a strong base, typicallya diamine, creating a reactive site. The diamine serves a dual role: it is an electron donor (nucleophile) to displace the chlorine and subsequently serves as a difunctional crosslink.
Halogen-Containing Elastomers
517
Hydrofluorocarbon Elastomers There are twodistinct halogen-elimination reactions that subsequently develop crosslinking sites in hydrofluorocarbon elastomers:
1. E2 mechanism: the simultaneous departure of a hydrogen and the adjacent fluorine atominitiated by a base. This is theprobableroute for creatingcrosslinksitesin vinylidine fluoride (VF2)-containing elastomers. 2. E l mechanism:normally operates without a base. DuPont patentsindicatebromine or iodine-containing C? and C., fluoro-olefinswereexcellent cure sitemonomers. These sites readily undergo attackby peroxide-generated radicals, which in turn create free radical sites on the elastomeric backbone or chain terminal groups. In the case of the perfluoroelastomer, the monomer appears tobe a perfluorophenoxyvinyl ether.
2.
HALOBUTYL ELASTOMERS
2.1 Introduction The history, chemistry, and compounding of halobutyl elastomers are described in a separate chapter, so we will only briefly comment on the vulcanization process.
2.2 Vulcanization Halobutyl elastomers can be vulcanized with a wide variety of curing systems. As mentioned in the introductory discussion, the bond dissociation energy (BDE) is lower for bromine, so bromobutyl tends to cure more rapidly. Timar and Edwards (1979) describe a curing study using retention of tensile product as a practical approximation of the retained cohesive energy. Common to all cure systems is zinc oxide, which readily dehalogenates the halogen, creating a zinc halide (a strong Lewis acid). After dehalogenation, the crosslinking can be achieved by any number of crosslinkers, for example, Zinc oxide (alone) Quinone complex Dithiocarbamate and magnesium oxide Thiuram Thiuram and magnesium oxide Phenol-formaldehyde resin (SP- 1045) Peroxide-bismaleimide Bismaleimide These systems were consideredof interest for high-temperature service. For less rigorous applications, crosslinking combinations using sulfur or sulfur donorscan be considered. Two typically used for covulcanization are sulfur-MBTS-TMTD and sulfur-CBS-TMTD.For zinc oxide-free cure systems, consider using a diamine, such as hexamethylenediamine carbamate. 3. CHLORINATED POLYETHYLENE ELASTOMERS
3.1 Introduction MaynardandJohnson (1963) note that the development of severalsaturatedpolymerswith elastomeric character prior to 1941 freed us from assuming that therubberystatecould be
Hertz, Jr.
518
Table 1 Producers of ChlorinatedPolycthylcne
name Trade Company Country
ostaprenO Hocchst Hoechst Germany Dow United States Osaka Japan owa Japan Hungary
Location
Plaquemine, CPEO
tons/yr mctric Capacity
LA Arnagnsaki
N.A.
6,000 40,000' 2,400
N.A.
duplicated only with the natural rubber structure. Their references (McQueen 1940; McAlevey etal.. 1947; Brookset al., 1953) aretheearliest ones to chemicalderivatives of the newly discovered polyethylene. Dow Chemical Co. is currently the only domestic producer offering two grades of 36 and 42 wt% chlorine. World production facilities for chlorinated polyethylene production are listed in Table 1. Label capacity for all plants worldwide appears to be far in excess of current consumption (author's estimate).
3.2 Applications Chlorinatedpolyethylenesare used in extruded goods such as hose (tubing and covers) for hydraulic oil and hydrocarbonserviceandoil-resistant cable jacketing; ascalenderedstock. liners, and membranes; and for molded mechanical goods, including seals. The growthpotentialcould be large, if only due to acurrently low usagepercentage compared to other elastomers.
3.3 Nomenclature and Basic Properties Nomenclature and basic properties of chlorinated polyethylene are summarized
in Table 2.
3.4 Characteristics of Chlorinated Polyethylene Chlorinated polyethylene is available in two grades based on weight-percent chlorine: (1) 36% chlorine, 35 and 80 Mooney viscosity, and (2) 42% chlorine, 80 Mooney viscosity. The higher Mooney materials allow higher plasticizer and/or filler loadings for extruded and calendered goods. The chlorination process, as described by Sollberger and Carpenter (1974), is initiated by free radical chlorination of Ziegler-process high-density polyethylene in an aqueous slurry. In the chlorination process, the larger-diameter chlorine atoms randomly replace the far smaller hydrogen atoms. The size discrepancy and random location aspect of the chlorine substitutions destroy the effectiveness of the dispersion (London) forces that originally created the highly crystalline structure. Chlorinated polyethylene is now essentially a high glass transition (T,) structure with a strong dipole force at each chlorine-carbon bond. On drying, the resulting product is a powder (due to dipole forces), making it unique as an uncompounded elastomer. The incorporation of a thermodynamicallysolublepolarplasticizer destroys the dipole attractionbetween the interchains, and the chlorinated polyethylene powder becomes a truly rubbery elastomer.
519
Halogen-Containing Elastomers Table 2 Nomenclature and Properties of Chlorinated
Polyethylene Elastomers Common name ASTMname ASTM D 14 18 designation IUPAC trivial name IUPAC structure-based name SAEJ200/ASTM D2000 Spccific gravity Durometer rangc Tensile strength (max), MPa Elongation (max), 5% Glass transition temp. (T,),“ K
Chlorinated polyethylene Chloro-polyethylene CM Not applicable Not applicable CE 1.16-1.25 50-YO
20 350 26 I
Considering that the basicraw material, polyethylene, is a series of ethylene (C,) structures, Oswald and Kuber (1963) present an interesting observation: chlorinated polyethylene can (and should) be considered as a terpolymer consisting of ( I ) ethylene, (2) vinyl chloride, and (3) 1. l-dichloroethylene (vinylidene chloride). Keep these three “monomers” in mind, as we will dwell on their individual contributions in terms of crosslinking, aging, and fluid resistance during subsequent discussions. Increasing the weight-percent chlorine from 36 to 42 decreasesthe swell in lighter hydrocarbons but does not enhance upper temperature limits (Normand and Johnson, 1975). Normand and Johnson( 1 975) alsoreview theeffects of various other weight-percent chlorinated polyethylenes. Sollberger and Carpenter (1974)and Dow Chemical literature should beread for additional background on the effects of varying the weight-percent of chlorine in polyethylene. Heat resistance and low-temperature tlexibility are best reviewed separately i n light of the elastomer being treated as a terpolymer. Heat resistance, i.e., ISO’C, is advertised as a strong selling point by the domestic supplier. Consider that part of the polymer chain is a number of vinyl chloride segments. The welldocumented heat instability of PVC is clearly established; Blanchard (1973) uses compounding guidelines with plasticizersandheatstabilizerstypically used forPVCcompounding.Guy and Sollberger ( 1970) describe a method of deliberately introducing backbone unsaturation by treatment at high temperature with zinc oxide. Guy is utilizing the zinc oxide to dehalogenate backbone sites that have chlorine atoms on adjacent carbons (geminal dihalides). Guy also notes the possibility of “hazard due to occasional rapid decomposition.” Dehalogenation at the vinyl chloride sites is activated by zinc and magnesium and, to a lesser extent, by copper, iron, and nickel. Low-temperature properties are improved by plasticizers. DOS (di-2-ethylhexyl sebacate) gives the greatest improvement in flexibility, with a trialkyl trimellitate offering the best overall balance. Epoxidized soybean oil would be the plasticizer of choice for the highest temperature stability. Chlorinated polyethylene elastomers should be used only in nonelectrolytes (hydrocarbon service). The ease of dehydrochlorination at the I , 1 -dichloroethylene site in a basic environment should always be considered. Crosslinking of chlorinated polyethylene can be achieved at each of the three specific monomers:
Hertz, Jr.
520
Ethylene site: A peroxide is used to abstract a hydrogen and a coagent to effect the crosslink (triazine, bismaleimide, or dimethacrylate). 2. Vinyl chloride site: Organic accelerator combinations of an amine-thiadiazole serve as a crosslink.The chlorine is best scavenged by a lead complex orepoxidzed soybean oil plasticizer. Avoid zinc and magnesium oxides. 3. 1, I-Dichloroethylene site: Same as mechanism 2, except that themechanism is a dehydrohalogenation-a reaction specific to a geminal dihalide. 1.
3.5
Compound Technology
Typical chlorinated polyethylene model recipes might consist of the ingredients listed below. PeroxideCure Chlorinated polyethylene Filler (reinforcing agent) Plasticizer (peroxide compatible) Stabilizer (halogen acceptor) Peroxide Coagent Antioxidant (peroxide compatible) 2. Amine-ThiadiazoleCure Chlorinated polyethylene Filler (reinforcing agent) Plasticizer Stabilizer (halogen acceptor) Amine-type accelerator (nucleophile) Dimercaptothiadiazole derivative (crosslinker) 1.
Mixing Mixing can be done on both open-mill and internal mixers. The high-Mooney elastomers have rapid heat-buildup tendencies and should be treated accordingly. Process Aids Process aids, both proprietary and generic, are widely available. Typically, materials such as paraffin, low molecular weight polyethylene, and waxes are commonly utilized. More complex and expensive process aids are offered; however, there is no outstanding candidate. Fillers Fillerstypically are furnace-typeblacksbecause of theirantioxidanttendencies (Blanchard, 1973). The finer particle sizes, N500-N300, are reported by Rose and Coffey ( 1 982) to offer the best balance of properties. Plasticizers
Plasticizers, as noted earlier, are a necessity. A good understanding of plasticizer structure and plasticizer theory is key to successful compounding. The book by Sears and Darby (1982) is particularly recommended because of the PVCsimilarity of chlorinated polyethylene. Plasticizers such as those based on epoxidized soybean oil serve a dual function: plasticizing and halogen
Halogen-Containing Elastomers
3.6
521
ChlorinatedPolyethyleneElastomerBlends
Blends of chlorinated polyethylene with other elastomers are noted in various reports. Considering the use of PVC-elastomer blends and the time needed to develop them, it is not likely that this application will create a major market.
4.
4.1
CHLOROSULFONATEDPOLYETHYLENE(CHLOROSULFONYLPOLYETHYLENE) ELASTOMERS Introduction
Maynard and Johnson (1963) developa very broad background on theearly history and development of chlorosulfonated polyethylene, first introduced by DuPont in 1955. The development was intended(like chlorinated polyethylene) to utilize theadvantage of the saturated polyethylene backbone and concurrently develop a more universal group of crosslinking sites. Dupuis ( l 982), in his excellent review. points out the basic differences between chlorosulfonated polyethylene (CSM) and chlorinated polyethylene (CM). Using polyethylene as a raw material, the DuPont process begins by solvating the polyethylene in chlorinated solvents and treating with chlorine and sulfur dioxide. The chlorosulfonation process creates two modifications (Keeley, 1959): The chlorine atoms break up the regularity of the polyethylene chain structure so that crystallization is no longer possible, thus imparting an elastomeric character to the polymer. 2 . The sulfonyl chloride groups provide sites for crosslinking. 1.
By manufacturingviaasolutionprocess,theheterogeneityassociated with theslurry process (used for chlorinated polyethylene) is avoided and homogeneous polymer results. World production facilities for chlorosulfonated polyethylene are listed in Table 3.
4.2
Applications
Chlorosulfonated polyethylene, with its inherent oil and heat resistance at relatively low cost, has a broad range of volume applications. Automotive tubing and electrical insulation are major markets. Others include construction, specifically single-membrane roofing for service more rigorous thanEPDM can handle; pond liner membranes for water and liquid storage;and mechan-
Table 3 ProductionFacilities for ChlorosulfonatedPolyethylene Trade Company Country United States Japan United Kingdom Source: IISRP, 1997.
tons/yr DuPont Dow Tosoh DuPont Dow
HypalonB TOSO-CSMB HypalonB
Beaumont, TX Shinnanyo Londonderry. Northern Ireland
Capacity metric 34,500 2.000 18,100
522
Hertz, Jr.
Table 4 Nomenclature and Properties of Chlorosulfonated Polyethylene Elastomers Common name ASTM name ASTM D 1418 designation IUPAC trivial name IUPAC structure-based name SAEJ200/ASTM D2000 Specific gravity Durometer range Tensile strength (max), MPa Elongation (max), % Glass transition temp. (T,)," K
Chlorosulfonated polyethylene Chloro-sulfonyl-polyethylene CSM Not applicable Not applicable CE 1.08-1.27 45-95 28 500 274
.' From Brandrup and Immergut. 1975.
ical molded goods, includingsealsand gaskets. Coloredelectrical cable jacketing is avery visible application. Growth potential appears to be in line with the U.S. gross national product (GNP). The absence of major competition indicates a stable and modest growth pattern.
4.3
Nomenclature and Basic Properties
The nomenclatureandbasicproperties Table 4.
4.4
of chlorosulfonatedpolyethylene
are summarizedin
Characteristics of Chlorosulfonated Polyethylene
The elastomeric chlorosulfonated polyethylenes can be made from branched, free radical (lowdensity), or linear(Ziegler process) polyethylenes. This affordsagroup of elastomerswith viscosities ranging from very low to very high suitable for paint coatings to highly abrasionresistant hose covers. Dupuis (1982) interrelates viscosity, weight-percent chlorine, and type (branched, linear) of the various Hypalonsm. Chlorine content can range from 24 to 438, with the optimal content of 3 0 8 for branched (Hypalona 20)and 35% for linear (HypalonB40) chlorosulfonated polyethylene. Chlorosulfonation, as noted earlier, is performed by solvating the polyethylenein a mixture of carbon tetrachloride and chloroform. Chlorine and sulfur dioxideare added, and the process, a chain reaction (Jones, 1964), is initiated by a free radical generator or ultraviolet light (Reed reaction). The effect of increasing weight-percent chlorine is predictable; note the volume increase in ASTM No. 3 oil, aged 70 hr at 121°C (Dupuis, 1982):
% Chlorlne Volume 5% increase
CSM (24%) 86%
+
CSM (35%) 38%
+
CSM (43%) 13%
+
Halogen-Containing
523
The upper temperature limit based on 1000-hr serviceability is 135°C. Dupuis (1982) cautions that this value is very dependent on compound quality, i.e., filler level, plasticizer volatility, and crosslink stability. Low-temperature performance is characterized as usually good, but the “rubbery response” or rebound would by nature always be sluggish (time-dependent). Fluid resistance is indicated by oil swelling (see tabulation above). Swell characteristics in nonelectrolytes are typicallysimilar to polychloroprene,exceptthathydrocarbon swell is dependent on weight-percent chlorine, as previously noted. Service in aqueous and nonaqueous electrolytes should be viewed with caution. The presence of metal chlorides as a by-product of crosslinking leads to potential problems as outlined by Briggs et al. ( 1 963). Crosslinking chemistry for chlorosulfonated polyethylenes is understandably very broad. In addition to the mechanisms outlined in Section 3, the chlorosulfonyl group introduces another dimension. An article by Devlin and Folk ( 1 984) has very interesting observations based on spectroscopic techniques for those seriously interestedin crosslinking chemistry. Peroxide cures, reviewed by Honsberg (1983), represent the latest concepts in crosslinking chlorosulfonated polyethylene elastomers. Maximum thermal stability as measured by stress relaxation appears to be represented by bismaleimide crosslinks. Haaf and Johnson (197 1 ) have written a very comprehensive paper covering this technology, which should be reviewed by those concerned with long-term stress-relaxation consequences.
4.5 Compound Technology Chlorosulfonated polyethylene elastomers can be compounded with a broad variety of filler, plasticizers,andcrosslinkingsystems. The very highmolecularweightlinearpolyethylenes utilized for HypalonB 45 allow formulations that require no crosslinking. Such compositions might be usedfor cable jacketing, roofing membranes, andpond liners. Examples of two formulations from Dupuis ( 1982) are detailed below.
1. Thiuram-SulfurCure Vulcanization Chlorosulfonated polyethylene (Hypalon” Filler (clay) Plasticizer (aromatic oil) oxide)(magnesium Stabilizer (pentaerythritol) Stabilizer Process aid (paraffin) Accelerator (TMTD) Crosslinker (sulfur)
2.
100.0 80.0 25.0 4.0 3.0 3.0 2.0 1.o
45)
100.0 50.0 35.0 4.0
UncuredApplications
Chlorosulfonated polyethylene (HypalonB Filler Pigment (titanium dioxide) acceptor) (halogen Stabilizer Processing aids glycol Polyethylene Stearamide
carbonate) (calcium
40)
2.0 I .o
Hertz, Jr.
524
Mixirzg
Mixing can be done on both open-mill and internal mixers. The high-Mooney elastomers have rapid heat-buildup tendencies and should be treated accordingly.
Plocess Aids Process aids, both proprietary and generic, are widely available. Typically, materials such as paraffin, low molecular weight polyethylene, and waxes are utilized. More complex and expensive process aids are offered; however, there is no outstanding candidate.
Fillrrs Filler effects in chlorosulfonated polyethylene are similar to those in polychloroprene. The high molecular weight of the elastomer categorizes it as a strain-crystallizing elastomer, so the essential reinforcing effects of fine particle carbon blacks are not required. Nonblack formulations are very popular. both for aesthetic appeal and for electrical properties. The range of nonblack fillers that can be utilized should be selected based on suppliers’ recommendations (see also Briggs et al., 1963).
P1nstici:er.s Plasticizers are utilized i n almost all formulations, and the guidelines presented in Section 3 are a good starting point. Epoxidized soybean oil plasticizers are utilized in white or colored stocks. Maynard and Johnson (1963) note that, as in PVC, these plasticizers react with the HCI byproduct to create a water-insoluble, high molecular weight chlorohydrin. Aromatic hydrocarbon plasticizers are widely used in nonperoxide cures. Diester plasticizers (phthalates. adipates, and sebacates) are plasticizers of choice for peroxidecures. They are also used for developing improved low-temperature properties.
4.6
Chlorosulfonated Polyethylene Blends
Blends of chlorosulfonated polyethylene with both chloroprene and nitrile rubber have been noted in suppliers’ literature. In each case, improved resistance to ozone attack was the goal. As with any blend ofelastomers, the result is often difficult to reproduce due to phase incompatibility and varying cure rates between the two elastomers.
5. 5.1
EPICHLOROHYDRIN ELASTOMERSCHEMISTRYAND MARKETS Introduction
Vandenberg ( 1 983) discusses the development ofepichlorohydrin elastomers in a unique context. In theprocess of developing crystallinepolymersfrompolarmonomers,Vandenbergnoted that n particular catalyst polymerized epichlorohydrin to a rubbely, predominantly amorphous polymer. The amorphouspolyepichlorohydrin (a polyalkylene oxide) was categorized as a homopolymer (CO). Subsequent work resulted in the development of a 1/1 copolymer of epichlorohydrin and ethylene oxide (ECO) that was also primarily amorphous with only small amounts of
525
Halogen-Containing Elastomers Table 5 Production Facilities for Epichlorohydrin Elastomer Country United States Japan
Company
Trade name
Location
Capacity metric tondyr
Zeon Chemicals Nippon Zeon Osaka Soda
HydrinB GechronB EpichlomerB
Hattiesburg, MS Tokuyama Kurashiki
9,900 1.500 1,200"
'' IISRP estimate. Source: IISRP. 1997
crystallinity. The moderate cost and good oil and gasoline resistance of these two products, coupled with their excellent low-temperature flexibility, made them particularly attractiveto the automotive industry. Hercules subsequently licensed the technology to B. F. Goodrich in the early 1960s and recently (1 986) has decided to exit the business, with Zeon Chemical now the domestic supplier. World production facilities for epichlorohydrin elastomer production are listed in Table 5.
5.2 Applications
The combination of fuel resistance, air aging, broad temperature range, and cost has assured a largemarket in the automobile industry.Hose,tubing, seals gaskets,andcoatedfabrics are major applications. Rubber-covered rolls, oil-field specialties, and industrial products are noted by Kyllingstad (1982) as substantial markets. The very good low-temperature properties have led to special military applications such as oxygen mask hose and large gaskets for fuel-transfer systems. Growth potential appears to track U.S. gross national product (GNP) statistics, with automotive production a key indicator. The exit of Hercules as a major supplier appears to be in line with a slow-to-average growth situation.
5.3
Nomenclature and Basic Properties
Nomenclature and basic properties are summarized in Table 6.
5.4 Characteristics of Epichlorohydrin Elastomers Epichlorohydrin elastomers are available as both a homopolymer [poly(epichlorohydrin)] and a copolymer [poly(epichlorohydrin-co-ethyleneoxide)]. There is also a terpolymer (Oetzel and Scheer, 1978), essentially a copolymer with a cure-site monomer to allow greater freedom in crosslinking chemistry. Within the various classes, thereare a range of molecular weights (Mooney values) available for specific compounding. Monorners for Polymerizntion
Monomers employedin epichlorohydrin elastomer production are characterized as cyclic ethers. Three examples are ( I ) epichlorohydrin (chloromethyl oxirane):
Hertz, Jr.
526 Table 6 Nomenclature and PropertiesofEpichlorohydrinElastomers
Common name ASTM name
Epichlorohydrin polymer Polychloromethyl oxirane
ASTM D141 8 designation IUPAC triv~alname SAEJ200/ASTM 02000 Specific gravity Durometer range Tensile strength ( m a x ) , MPa Elongation (max). 7!Gloss transition temp. (TS),"K
CO Poly (epichlorohydrin) CH 1.36 30-40
Epichlorohydrin copolymer Ethylene oxide (oxirane) and chloromethyl oxirane ECO Poly (epichlorohydrin-co-ethylenc oxide) CE 1.27 40-90
18
17
350 2s I
400 22 I
0
l\ CH2 -CH-CH2 C l (2) ethylene oxide (oxirane):
0
l\ CH2 - CH2 and (3) allyl glycidyl ether (cure-site monomer):
0
l \ CH, -CH-CH -0-CH 2
2
-CH=CH
2
Polymerization of the monomer(s) is viaacationicsolutionprocess using amodified aluminum alkyl-water catalyst. Molecular weights of the copolymer were very high, and it was necessary to deliberatelylowerthem to aid in processing (Vandenberg, 1983). There is no branching or gel formation during the process. The homopolymer (CO) is simply described as a saturated polyether with the polar aspect created by the chloromethyl side group. The 38% chlorine content develops the fuel resistance and promotes flame retardancy. The copolymer (ECO) has a lower chlorine content (about 26%). It has improved low-temperature flexibility contributed by the ethylene oxide monomer and higher fuel swell due to lower chlorine content. As noted earlier,the 1/1 copolymer (ECO) hadexcellentlow-temperatureproperties, with a glass transition temperature (T,) below -40°C compared to the homopolymer (CO) value of - 20°C. The terpolymer cure-site monomer allows a broad application of peroxide, peroxide/ coagent, and sulfur-cure mechanisms. Heat Resistunw and Tlwr-rml Stability The ease of halogen displacement by a nucleophile and heat is a recurring topic in this chapter. The very polar chloromethyl group common to the CO and ECO elastomers not only creates
Halogen-Containing Elastomers
527
the basicoil resistance but is also the crosslinking sitein these elastomers. Duringthe crosslinking process it assumed that there is one crosslink for every 130-200 constitutional repeating units (CRU). This would leave the bulk of the chlorine atoms on the chloromethyl groups vulnerable to dehalogenation. It was initially assumed that dehalogenation and the subsequent formation of HCI caused the rapid degradation of early formulations. Yatnada et al. (1973) and Nakamura et al. (1974) provedconclusively that aging occurs in twosteps: ( 1 ) oxidativedegradation initiating at a beta hydrogen following the thermal decomposition mechanismfor alkylene oxides (Dulog, 1966),and (2) subsequent formation of a hydroperoxide, creating a chloroketone structure that decomposes, yielding HCI. The mechanism(s)of protection for long-term heat stability dictate both an antioxidant (step 1 ) and an HCI acceptor (step 2). Typicalantioxidantsare metal dithiocarbamates: nickel dibutyldithiocarbatnate (NBC), nickel diisobutyldithiocarbamate (NIBC), and nickel dimethyldithiocarbamate (NMC). Typical HCI acceptorsare red lead oxide andmagnesium oxide. Zinc oxide andzinc stearate should be avoided. as they become strong Lewis acids and promote rapid elastomer breakdown. Low-temperature properties of the copolymer (ECO) are particularly good, as was previously noted. The ether (oxygen)linkage in the backbone is highly mobile, much like the siloxy linkage in the silicone rubber backbone. Long-term aging characteristics are a function of operating temperature. The antioxidant and HCI acceptor ingredients are essentially sacrificial. On depletion of these agents. there is typically a reversion to lower molecular weight materials. Fluid resistance values in terms of the automotive environment are available from the suppliers. whose bulletins are very encouraging with respect to long-term utilization of CO and ECO elastomers. A major problem is with the sour (peroxidized) gasoline, which causes an attack similar to that described above as step 2 of the aging process. Specific compounding, to be described below, can minimize sour gasoline attack but cannot completely stop it (Mori and Nakamura, 1984). Aqueous and nonaqueous electrolytes should be avoided, as they promote nucleophilic attack on the chloromethyl group. causing rapid breakdown. The crosslinking of CO and ECO elastomers can proceed by several mechanisms, each utilizing nucleophilic displacement of the chlorine from the chloromethyl group using: Ethylenethiourea (nucleophile and crosslinker) Red lead oxide (acid acceptor) 2. Amine accelerator (nucleophile) Thiadiazole complex (crosslinker) Barium carbonate (acid acceptor) Magnesium oxide (acid acceptor) 3. Diphenyl guanidine(nucleophile) 2,4,6-Trimercapto-s-triazine (crosslinker) Magnesium oxide (acid acceptor) l.
The terpolymer may be crosslinked through the reactive backbone by the following cure systems:
double bond that is pendant to the
Peroxidekoagent, peroxide Sulfur and organic accelerators 2,4,6-Trimercapto-s-triazine and organic accelerators (Mori and Nakamura, 1984) The various crosslinking mechanisms are discussed in detail by Mori and Nakamura Kyllingstad (1982), Oetzel and Scheer, (1978), and Nakamura et al. (1974).
( 1984).
528
5.5
Hertz, Jr.
Compound Technology
Some typical CO and ECO formulations utilizing the different cure mechanisms follow: 1.
For-mulcttion N Homopolymer (CO) 100.0 Process aid I .0 Red lead oxide (Pb304) 5.0 Filler 40.0 Ethylene thiourea 1.2 NBC 1 .o
Copolymer (ECO) Process aid Calcium carbonate (shelf-life improver) Filler 2.4.6-Trimercapto-S-triazine Cyclohexylthiophthalimide(cure retarder) NBC
100.0 2.0 5.0 40.0 0.9 I .0 I .0
Terpolymer (ECO) 100.0 Process aid 2.0 Magnesium oxide 3.0 Calcium carbonate 5.0 Filler 40.0 Peroxide-DBPH 2.0 Trimethylolpropane-trimethacrylate 3.0
Mixirlg
Mixing can be can-ied out on both open-mill and internal mixers. The high-Mooney elastomers have rapid heat-buildup tendencies and should be treated accordingly. Process Aids
Process aids are critical for mill release. An incorrect selection can interfere with cure rate or promote rapid aging. The elastomer suppliershould be consulted for the latest recommendation based on the type of elastomer, cure system, and service requirements. Fillers
Fillers are typically furnace-type blacks with the reinforcing effects predictable (Kyllingstad, 1982). N500 types are used in plasticizer-free formulations. N700 types are used to offset the
Halogen-Containing
5.6
529
Epichlorohydrin Elastomer Blends
Blends with other elastomers do not appear in the literature. There is little evidence that blend would offer any improvements over the properties offered by either elastomer.
6.
6.1
POLYCHLOROPRENE RUBBER Introduction
Collins (1973),in his Charles GoodyearMedal Address, has a very brief but concise story about the discovery of polychloroprene. As with many famous discoveries. it was preceded by a series of events that were pieced together by several dedicated perceptive scientists. in turn creating the product. As background, theexperiences of World War 1 and the rapid growthof the automobile led many to the belief that a synthetic replacement for natural rubber was imperative. The basic diene structure of natural rubber was accurately proposed by W. A. Tilden in 1892 (Fisher, 1957). Duplication of the diene molecule was assumed to be a necessity as the basis of any truly elastic synthetic rubber, with the early work using butadiene (a gas). The intractability of the rubber developed from sodium metal-catalyzed butadiene discouraged the process, and other reactive dienes were sought out. Work by Father Nieuwland at Notre Dame on divinylacetylene. reported in 1925. was noted by Dr. E. K. Bolton, DuPont's research director. Arnold Collins. whose background was in coatings, joined the group as a resins expert. During separation of the various isomer fractions by distillation in April of 1930, it was found that one spontaneously polymerized. This fraction was analyzed as a monovinylacetylene that had become chlorinated by hydrogen chloride (partof the catalyst composition). Larger quantities weremade by emulsion polymerization techniques developed by Ira Williams, and formal announcement of the discovery was made on November 2, 1931, at an ACS Rubber Division meeting. After factory trials in 193 1. the polychloroprene rubber was offered for sale in 1932 under the trade name DuPrene. Worldproductionfacilities for polychloropreneelastomerarelisted in Table 7. Label capacity for all plants worldwide is estimated to be in excess of 502,000 metric tons.
6.2 Applications Markets forpolychloroprene, as evidenced by production capacity, arefar ranging. Typical highvolume applications are industrial and automotive hose, construction, vee-belts, tires, molded goods,footwear, caulking andglazing, conveyer belts. wireand cable insulation. andadhe-
Hertz, Jr.
530
Table 7 ProductionFacilitiesforPolychloropreneElastomer ~~
~~
Country ~
United States France Germany Japan
People's Republic of China United Kingdom CIS
Company ~
~
Location tondyr Capacity metric ~~
discontinued 1998 DuPont Dow EniChem Bayer AG Denki Kagaku Kogyo DuPont-Showa Denko TOSOH
DuPont Dow V I 0 Raznoimport
Louisville Champagnier Dormagen Omi Kawasaki Shinnanyo Changshou Datong Quingdao Londonderry, Ireland Erevan
136,000 40,000 68,000 48,000 20,000 20,000 10,000' 5,000' 5,000' 33,000 40,000'
.' IISRP estlmate. Source: X R P . 1997.
sives. The distribution of applications is well balanced over six primary industries (Graham, 1982):
Automotive Construction Machinery Apparel Appliances industrialMisc.
208 15% 15%
15% 5Yo 30%
Growth potential appears to track gross national product (GNP) statistics, as indicated by plant label capacity, and is not much greater for 1997 figures than for 1986. 6.3
Nomenclature and Basic Properties
The nomenclature and basic properties of polychloroprene elastomers are summarized in Table 8. 6.4
Characteristics of Polychloroprene Elastomers
After the discoveryof chloroprene rubber, poly(1 -chloro- 1 -butenylene), various potential routes to develop the monomer were considered (Johnson, 1976). The acetylene route was originally utilized, and butadiene subsequently became the monomer precursor of choice for safety and other considerations. As with any four-carbon (C.,) structure, the chlorination created a mixture of isomers and analogs that required separation; Johnson (1976) should be referred to for this discussion. Basically,there are two distinctclasses of polychloropreneelastomers: copolymers of
531
Halogen-Containing Elastomers Table 8 Nomenclaturc and Properties of Polychloroprene
Elastomers Common name
ASTMname ASTM D I41 8 designation IUPAC trivial name IUPAC structure-based names SAEJ200/ASTM D2000 Specific gravity Durometer range Tensile strength (mnx). MPa Elongation (max), c/r Glass transition temp. (T3,),‘l K
Neoprene Chloroprene CR Poly(ch1oroprene) Poly( 1-chloro- I-butcnylene) BC. BE I .?S
30-95 22 600
233
From Brandrup and lmlnergut 1975
chloroprene and sulfur (G series) and Polymers and copolymers of chloroprene nlonomers (W series) Monomers and Polynxrixttion The complexity of monomers, isomers, analogs, and comonomers utilized in the production of polychloroprene elastomers, although relevant,are beyond the purview of this chapter. Johnson ( 1 976) and Hargreaves ( 1 968) are recommended as background reading. Polymerization of the monomer(s) is by a free radical emulsion process. The need for molecular-weight control was identified early, and the copolymerization with sulfur allowed cleavable sites in the chain for molecular-weight control (G series). Subsequent developments i n emulsion polymerization led to the development of the sulfurless W series, with mercaptans utilized for molecular-weight control serving as chain transfer agents. A typical emulsion polymerization recipe for a sulfur-modified (G series) polychloroprene is detailed by Gintz (1968). The effect of copolymerization of the various monomers along with their concentration and sequencing can lead to many specific, useful properties. Some examples are:
G Series: The sulfur actually copolymerizes in the backbone chain in multiple sequences of sulfur ranging from two to six atoms. The sulfur linkages, from a mechanical viewpoint, are highly mobile but thermally very weak. This gives the G series outstanding flexibility but poor resistance under high stress-strain relaxationconditions. W Series: These have far better aging characteristicsdue to the absence of sulfur-backbone linkages and aremore suitable for nondynamic applications (see Murray and Thompson, 1963. and Johnson, 1976). Elastomer microstructure is unique. Consider the following. Polyisoprene derives many properties from the high(99% + ) degree of backbone uniformity of head-tail cis units. Polychloroprene is also highly regular in structure but consists primarily of trans units. Considering natural rubber, if all units were tram, the material would be categorized as gutta-percha or balata, a very stiff, thermoplastic rubber with a higher specific gravity than natural rubber. (The higher specific gravity results from the more uniform structure of the chain, which allows closer “packing.”) Although polychloroprene is predominantly trans, there is sufficient cis to disturb the backbone symmetry and maintain a rubbery state (Murray and Thompson, 1963). The ability to develop increasing and decreasing cis-trans relationships by polymerization is the basis for polychloroprene-base adhesives.
532
Hertz, Jr.
Heat resistance of polychloroprene is superior to that of polyisoprene, but the vulnerability of the backbone double bond cannot be obviated. Conventional approaches using antioxidants are a necessity, and suppliers should be consulted the for latest concepts ofantioxidant protection. Murray and Thompson (1963) deal specifically with compounding for heat resistance, noting that the W series are superior to the G series for such service. Polychloroprenes seem suitable for long-tern1 heat resistance in applications below 100°C. Low-temperature properties of polychloroprene have to be considered in two different aspects: crystallization andoperating close to the glass-transition temperatureof polychloroprene (Murray and Thompson, 1963). The high percentage of tram units creates the inherent potential of crystallization (100-fold increase in stiffness). The reaction is time-dependent and appears most commonly in the vicinity of - 12°C; it is completely reversible on warming. The polar nature of polychloroprene allows the use of a broad spectrum of plasticizers. Again, the reader should review the specific low-temperature properties desired (impact, torsion, or tension) and utilize the supplier’s technical support. Long-term aging characteristics are excellent for polychloroprene elastomers that have been compounded specifically for such applications. The chlorine atom not only tends to shield the trans double bond from ozoneattack but also servesto make polychloroprene rubber incapable of supporting combustion. Serviceabilityis directly relatedto correct selectionof age-resisters (antioxidants,antiozonants),plasticizerstability,andfiller-volumerelationships.Highly extended formulations that use large ratios of plasticizers and fillers should not be considered candidates for such service. Fluid resistance should be considered in terms of nonelectrolytes (nonpolar fluids) and aqueouslnonaqueous electrolytes (polar fluids). Nonelectrolytesare typically hydrocarbons ranging from gases to liquids to solids. Lower molecular weight aliphatic hydrocarbons cause high swell, decreasing as molecular weight increases. Aromatic hydrocarbons should be avoided. Lubricating oils cause intermediate swelling as a function of their aniline points. Halogenated hydrocarbons cause excessive swell and should be avoided. Fluid service guides from suppliers should be consulted if the electrolytes are specific cases. Water has a strong affinity for metal oxides utilized in the cure mechanism as well as for fillers (Briggs et al., 1963; Murray and Thompson, 1963). Specificformulations aresuggested if polychloroprene properties are required in such service. Crosslinking in polychloroprene rubber is a function of the particular type. Common to both the W and G series is the specific crosslink site, a 1,2 addition created by a rearrangement of the chloroprene molecule. As a result of the 1,2 polymerization, 1.5% of the chlorine is in an allylic position and is easily displaced by a nucleophile. The controlled number of crosslink sites (1,2 additions) prevents overcuring, a valuable feature when a molded component is subjected to a mechanically demanding application (Murray and Thompson, 1963). Curingproceeds through a somewhat controversial mechanism (Hargreaves, 1968),but there is reasonable agreement that crosslink sites are created by the allylic chlorine displacement. G series elastomers may be crosslinked by using zinc oxide and magnesium oxide alone. The basic environment probably displaces the chlorine, which reacts with the zinc oxide to become zinc chloride, a powerful Lewis acid. The magnesium oxide reacts with waterto become a hydroxide. A PO~YSUIfidic segment in the elastomer backbone matures to a lower order of rank, donating sulfur to serve as a crosslink. W series mechanisms (homopolymers) typically utilize an organic accelerator, normally a difunctional amine (nucleophile), which activates the reaction in the presence of heat. The accelerator (diamine) becomes the crosslink, creating 2 moles of water (scavenged by the magnesium oxide) for each crosslink generated. The range of crosslinking systems is quite extensive (Johnson, 1976).
Halogen-Containing Elastomers
6.5
533
Compound Technology
Typical polychloroprene formulations for the two classes are listed below. 1.
G Series Polychloroprene 100.0 oxide Zinc 5 .O Magnesium oxide 4.0 Antioxidant 2.0 Process aid 0.5 Carbon black 0-200 0-70 Plasticizer
2.
W Series Polychloroprene Zinc oxide Magnesium oxide Antioxidant Process aid Carbon black Plasticizer Organic accelerator
100.0 5 .O
4.0 2.0 0.5 0-200 0-70 0.25- 1 .O
Mixing Mixing can be done on both open-mill and internal mixers. The high-Mooney elastomers have rapid heat-buildup tendencies and should be treated accordingly. Process Aids Process aids, both proprietary and generic, are widely available. Typically, materials such as paraffin, low molecular weight polyethylene, and waxes are utilized. More complex andexpensive process aids are offered; however, there is no one unique product. Fillers
Fillers typicallyare the N990 thermal blacks.These blacks offer reasonable reinforcing characteristics with the economy of higher loadings and lower attendant hardness increases. Furnace blacks ranging from N 100 to N700 typesare utilized to developspecific propertiesfor mechanical applications. Nonblack fillers should be used with great caution if the environment is humid or wet. Briggs et al. (1 963) should be reviewed before considering nonblack fillers. In addition, aqueous testing should be performed at the pH anticipated in actual service. Some nonblack fillers and compounding ingredients are very specific in some instances to pH variation. Nonblack fillers, although widely used, should be considered in light of the application. Suppliers’ literature is of great value and should be studied. Plasticizers Plasticizers are utilized in almost all formulations. Aromatic oils (forhigh levels) and naphthenic oils (for low and medium levels) are most commonly used. There are distinct solubility limits
Hertz, Jr.
534
for hydrocarbon oils in polychloroprene elastomers, and suppliers’ recommendations should be used as guidelines.
Polychloroprene Rubber Blends
6.6
Blends ofpolychloroprene with other elastomers have potential problems due tophase incompatibility. The literature illustrates the use of elastomers such as SBR, nitrile (NBR), and butyl in blends for specific applications.There doesnot appear tobe any indication that elastomer blends offer overall improvement to either material.
7.
FLUOROSILICONE ELASTOMERS
7.1
Chemistry and Markets History
The addition of fluorine into a polyalkylsilsiloxane (silicone) system created an elastomer with a high degree of solvent resistance and excellent low-temperature capabilities. LS-53, originally developed by Dow Corning Corporationunderagovernmentcontract.wasthefirst of the tluorosilicone family. Pierce (1970) briefly describes its chemistry and commercial uses along with high- and low-temperature response. The original LS-53 appears to have been named LS (lowswell). 53 (1953) based on a paper by Pierce (1953). Currentlytherearethreemajor suppliers: Dow Coming Corp., General Electric Company, and Shinetsu Chemical Co. World production facilities for tluorosilicone rubber are listed in Table 9. Label capacity for all producers worldwide is estimated to be in the range of 650-700 metric tons per year (author’s estimate).
7.2 Applications The original market, primarily O-ring seals, has expanded greatly. Current high-volume applications also include shaft seals and gaskets, molded goods, duct hose, and covers. Other volume applications include wire and cable, insulation, electrical connector inserts, and North Slope oilfield service. The long-termfluorosiliconegrowthpotentialappears to be in excess of 10% a year. Increasingly aggressive automotive fuels and higher engine operating temperatures are creating major markets for fluorosilicones. 7.3 Nomenclature and Basic Properties The nomenclature and basic properties IO.
of fluorosilicone elastomers are summarized
Table 9 Production Facilities for Fluorosilicone Rubber Country United States Japan
Company
Trade name
Plant location
Dow Corning General Electric Shinetsu
LS FSE FE
Midland, MI Waterford, NY Isobe
in Table
535
Halogen-Containing Elastomers Table 10 Nomenclature and Properties of FluorosiliconeElastomers ~~
~
Common name ASTM name ASTM D1118 designatton IUPAC trivial name IUPAC structure-based mtne SAEJ200/ASTM D2000 Specific gravity Durometer grange Tensile strength ( n u x ) . MPa Elongation (max). %, Glass transttion temp. (T?)." K
Fluorosilicone Fluorosilicone FVMQ Poly(methyltrifluoropropylsiloxnne) Poly(oxymethyl-3,3.3-triflu~~ropropylsilylet~e~ FK 1.4 40-80 10 400 < 193
.' From Brandrup ;und Immcrgut. 1975.
7.4 Characteristics of Fluorosilicone Elastomers Fluorosilicone elastomers as specified by MIL-R-25988 are available ;IS ( 1 ) general-purpose. (2) high-strength general-purpose. and (3) high-modulus. increased temperature resistant. Razzano and Simpson (1976) describetheprocessforpreparation of arange of tluorosilicone copolymers. Typically, tluorosilicone elastomers are a copolymer of 90 mol% tritluoropropylsiloxy and I O mol% din1ethylsiloxy monomers.
Mononwrs ar~dPo1yrrreri:atiou Monomers currently utilized for commercial tluorosilicone elastomer production are cyclic alkyl trifluoropropyl trisiloxane, (CFICH,CH,SiCH30)3, cyclicdimethyltrisiloxane, (CH3 SiCH30)3,and cure-site monomers. A specific cure site is developed by incorporating 0.2 mol% of methylvinyl siloxane. The highly reactive vinyl site allows a wide latitude in the peroxide crosslinker selection. The basic hydrocarbon resistance is imparted by the polar tritluoro (-CF.<) group capping the propyl (C3) branch. Pierce ( 1970) notes that this approach to develop a polar elastomer wasnecessitated by the weakness of the thermallystablesilicon-fluorine bond to hydrolysis (cleavage by water). Thernlalandhydrolyticstabilitywas not achieved until the fluorine atoms were i n a gamnw position (three carbon atoms removed from the silicon atom). and hence the propyl group was considered. Longer branching leaves the CH bonds subject to oxidation. Polymerizationreactionsaretypicallyperformed in asolutionphase using an aprotic solvent such as tetrahydrofuran (Razzano and Simpson. 1976). Fluorosilicone content of the copolymer can range from 40 to 90 mol%. Cost and service (swelling) considerations dictate the copolymer ratios. The basic copolymers appear to polymerize with little or no branching as evidenced by low gel contents. A range of molecular weights ( a s measured by Mooney viscosity) aretypicallyavailable as part of the supplier's product specifications. A better molecular weight-viscosity relationship can be rationalized by considering the average number of constitutional repeating units (CRUs) or monomer segments in a particular tluorosilicone. I n terms of IUPAC nomenclature, the basic CRUs for fluorosilicone elastomers are an (SiO) structure. Using this consideration, the degree of polymerization (DP) of 6000 reported by Pierce and Kim (1973) would indicate that the tluorosilicones are typically high molecular weight elastomers.
Hertz, Jr.
536
Heat Resistance and Thermal Stability These terms, often used interchangeably, are defined for this discussion as follows: Heat resistance: The maximum temperatureatwhich a given elastomer is capable of operating for an extended period of time while still maintaining usable properties. Tlzernzal stability: The maximumtemperature an elastomer can withstand before a chemical change occurs. In terms of heat resistance, the fluorosilicone elastomers are capable of thousands of hours of service life at 175°C. At 200"C, about half of the tensile strength is reported lost after 26 weeks (Pierce and Kim, 1973). Useful service lifeis often strongly influencedby design and application. In terms of physical properties, the high molecular weight and minimum attraction between adjacent atoms contributes substantially to both the room temperature and elevated-temperature strength of fluorosilicones (Gant and Cabey, 1976). A typical fluoroelastomer loses 70% of its tensile strength at IOO'C, while fluorosilicone elastomers loseonly 50%. Thermal stability varies according to compounding technique. Degradation is accompanied by both chain scission and depolymerization. Thomas (1968) developsa broad picture utilizing stress relaxation studies of several fluorosilicone elastomers crosslinked with different peroxides to demonstrate the problems caused by peroxide residues. Prior work by Luyendijk (1966) using various fluorosilicone rubbers with different fillers and stabilizers is of interest and should be reviewed. Low-Temperature Properties The (SiO) structure offers a high degree of backbone mobility as evidenced by the very good low-temperature properties of fluorosilicone elastomers. Pierce ( 1970) illustrates the excellent low-temperature flexure using the Gehman test (ASTM D1053). Aging Characteristics Long-term aging characteristics are outstanding for all fluorosilicone elastomers with no agecontrol considerations for seals and molded components. After installation. age controls might be incorporated to monitor stress-strain relaxation effects in seals, hoses, and gaskets. Fluid Resistance Fluid resistance should be considered in terms of nonelectrolytes (nonpolar fluids)and aqueous/ nonaqueous electrolytes (polar fluids). Nonelectrolytes are typically hydrocarbons ranging from gases toliquids to solids. Lower molecular weight aliphatic hydrocarbons have moderate interactions with all fluorosilicones. Aromatic hydrocarbons (benzene, toluene, xylene, etc.) interact to a greater extent. Aqueous and nonaqueous electrolytes, aldehydes, ketones, and esters generally have varying degrees of interaction with fluorosilicone elastomers, and such applications should be subjected to long-range testing before consideration. Most suppliers have extensive fluid resistance guides, which should be reviewed for final elastomer selection. Crosslinking Fluorosilicone Elastomers Fluorosilicone elastomers can be crosslinked by any of the basic classes of peroxides (Table 1 l). The rationale for each class of peroxides is briefly outlined below. Dialkyl and diaralkyl peroxides are specifically reactive to vinyl-crosslinking sites. Their half-life temperatures (activation) are higher and allow greater processing safetyat higher curing temperatures.
537
Halogen-Containing Elastomers Table 11 CrosslinkingFluorosiliconsElastomers Class produced radical Freeperoxide Typical
Diaralkyl Dinlkyl
Diaroyl Alkyl aroyl
Dicumyl peroxide Di-/-butyl peroxide Benzoyl peroxide r-Butyl perbenzoate
Cumyloxy /-Butoxy Benzyloxy Benzyloxy, 1-Butoxy
Diaroyl peroxides are not only reactive to vinyl crosslinking sites but also create additional crosslink sites. Half-life temperatures are far lower and allow HAV (hotair vulcanization) cures. Alkyl aroyl peroxides are reactive at temperatures intermediate between those of dialkyl and diaroyl peroxides. Free radicals produced are reactive at both the specific vinyl crosslinking site and nonspecific sites. They are suitable for HAV-type cures. Each of the peroxide groups has possible disadvantages as reported by Thomas (1968). Dialkyl radicals react with oxygen and cause elastomerchain scission. Benzyloxy radicals leave acidic by-products, causing poor aging in confined applications. It is advisable to review the specific manufacturing process with the tluorosilicone supplier for their recommendation.
7.5 Compound Technology Typically, formulations are available to meet specific requirements and specifications for commercial applications. Base compounds of 40 and 80 durometer hardness are available. Blending various ratios allows the molder to developspecific hardness compounds. Reinforced gum stocks are also available for in-house specification compounding. Misirlg
Blending and curative additioncan be done on both open-mill and internal mixers. Baker-Perkins mixers are typically used by suppliers to make original formulations. Process Aids Process aids are typically proprietary. Their functions range from blocking filler surface activity to serving as tlow improvers.
Fi1Ic~r.s Fillers (reinforcing) are typically silicas (silicon dioxide) because of their stability and their compatibility with the elastomeric silicon-oxygen backbone. These silica fillers cover the entire process-of-origin range, i.e.. naturally occurring, or obtained by silicon tetrachloride hydrolysis, arc furnace, or precipitation. Surface area ranges from 0.54 to 400 m’/q (Dunnom and Wagner 1981 ), with average particle size ranging from 10 to 600 nm (Wagner, 1981). This broad range of particle size and surface area gives the compounder substantial flexibility in developing or controlling properties such as modulus, abrasion resistance. tear resistance, and processability. Lutz et al. (1985) gave an excellent three-part review on performance of wet-process (precipitated) silicas in silicone and fluorosilicone elastomers. Siloxane and silazane coupling agents are also discussed i n these articles. Other commonly used fillers are calcium carbonate, iron
Hertz, Jr.
538 oxide, titanium oxide. and zinc oxide. Conventional dimethylsilicone oils and used in small percentages to aid i n mill release and mold release.
gums have been
Plrrsrici:er:s
Plasticizers are generally fluorosilicone oilsof various viscosities. The efficiency of a plasticizer (which must be thermodynamically soluble) is a function of the molecular weight; the lower the value. the higher the efficiency. This is also true of the plasticizer's volatility: increasing molecular weight, decreasing volatility. Postcuring tends to volatilize some low molecular weight plasticizers during the postcure process. 7.6
Fluorosiiicone Elastomer Blends
Blends with other silicone elastomers should be assumed to have possible phase incompatibility with the poladnonpolar nature of the components.
8. 8.1
FLUORINE-CONTAINING ELASTOMERS introduction
Fluoroolefin history is well developed by Fearn ( 1972) in his chapter "Polymerization of Fluoroolefins." Fearn notes the pioneering work by Swarts ( 1892) and how the Swarts reaction allowed the development of many fluoroalkanes and tluoroalkenes. Work by Midgely ( 1930)and coworkers utilizing the Swarts reaction led to the commercial development of halohydrocarbon gases suitable as refrigerants. This large potential market encouraged cost-efficient production techniques. creating relatively low-cost precursors for most of the fluorooleffns. Chlorotrifluoroethyletle (CTFE) was the first tluoroolefin of commercial importance. Early work by M. W. Kellogg. sponsored by the U.S.ArmyQuartermaster Corps (Monternlorso. 196 1 ). led to the development of a rubbery tluoropolymer. acopolymer consisting ofchlorotrifluoroethylene and vinylidene fluoride(Kel-FO 3700 and 5500). Processing and molding difficulties limited the usage. and Kellogg's tluoropolymer business was subsequently purchased by the 3M Company. The actual birth of the tluoroelastomer industry as we know it today might be considered 1956 (Rugg et a l . . 1956). with patents issued to DuPont (Rexford, 1962) and 3M Company (Lo, 1962) for a copolymer of vinylidene fluoride and hexatluoropropylene. marketed as Viton@-A and Fluorel@ 2140. From that date on, there has been a rapid development of additional di-. ter-. and tetrapolymers.eachhavingsufficiently unique properties to assuretheireconomic viability. World production facilities fortluoroelastomer production are listed in Table 12. Label capacity for a l l plants worldwide is estimated to be approaching 10,000 metric tons per year.
8.2 Applications The original market. primarily O-ring seals, has expanded greatly. Current high-volume applications also include shaft seals and gaskets. molded goods, powerplant flue-duct expansion joints. and hose linings andcovers. Other volume applications include wire and cable. chemical-resistant coatings, damping applications. and oil-field-related applications. The long-term tluoroelastomer growth potential appears in excess of 10% a year. Increasingly aggressive automotive fuels and lubricants have developed major markets for tluoroelastomers.
Halogen-Containing Table 12 Production Facilities forFluoroelastomers name Company Country Trade
location Plant
Deepwater, United VitonB Dow DuPont ow DuPont States AL FluorelB/Kel-F@ Decatur,Dyneon yneon Belgium BAtochem France Marengo Italy Spinetta TechnoflonB Ausimont Asahi Japan Daikin Kogyo Kawasaki VitonO DuPont/Showa Denko KK DuPont Netherlands
M
539
.I
Capacity tons/yr metric
3,000
NJ
n/:l
2,500" Pierre Benite
Dai-El@
2.000 2.000 1,00Oh 700 da
Osaka
IISRP estirnatc.
Increased 1998. Source: IISRP. 1997 "
8.3
Nomenclature and Basic Properties
The nomenclature and typical properties Table 13. 8.4
of a copolymer and a terpolymer are summarized in
Characteristics of Fluorine-Containing Elastomers
Fluoroelastomers are available in three compositionally distinct categories basedon polymerization of two or more specific types of monomers:
Table 13 Nomenclature and Properties of Fluoroelastomers Fluoroelastomer (copolymer) Fluoroelastomer name Common (terpolymer) ASTM name ASTM D 14 18 designatlon IUPAC trivial name
Fluoro rubber FKM Poly(viny1idene fluorideco-hexafluoropropylene)
IUPAC structure
Poly (( 1,l-difluoroethy1ene)co-difluoromethyleneco-perfluoropropylene)
bascd names
Fluoro rubber FKM Poly(viny1idene fluoridetetrafluoroethylencco-hexafluoropropylene) Poly (( I,l-difluoroethy1cne)co-perfluoropropylene)
ASTM D 1418 category Specific gravity Durometer range Tensile strength (max), MPa Elongation (max), % 255, temp. (TS);' K Glass transition
FKM 1.S6 60-95 20 250 255
1.88-1.90 65-95 20 250 210
Hertz, Jr.
540
Nonsubstituted (hydrocarbon) alkene monomers Partially substituted (hydrofluoro) alkene monomers Fully substituted (perfluoro) alkene monomers Mo~~or~~env c111rlPoIwer-i:ation Monomers currently utilized for commercial fluoroelastomer production are (Apotheker,
82):
Norlsubstitlrtetl trlkenes: Ethylene (E) (CH? = CH-,) Propylene (P) (CH-, = CHCH3) Purtitrlly slrbstituterl trlkene: Vinylidene tluoride (VF?) (CH2 = CF-,) Flrlly s~rhstituterlcrlketws: Tetrafluoroethylene (TFE) (CF? = CF?) Hexatluoropropylene (HFP) (CF? = CFCF3) Pertluoro (methyl vinyl ether) (PMVE) (CF? = CFOCF3) Curt>-sitt’1~1o11o1~1c~r.s (CSM): Two- or four-carbon monomers with terminal iodine or bromine
The elastomers are predominantlyproduced by a continuous emulsionpolymerization process due to substantial donor-acceptor differences between partially substituted alkenes and perfluoroalkenes. This is both the safest and the most cost-effective process. but it is limiting in both monomer selection and batch size. Solution and suspension polymerization processes are utilized for some specialty tluoroelastomers. Mooney numbers are typically controlled by blendinglattices of knownvalues.Mooneynumbersunfortunatelyaredetermined by a low-shear-rate instrument. They do not reflect the rheological properties under higher shear rates in typical factory operations such as extrusion, transfer,or injection molding. Erratic shrinkage values after postcure are often indicative of a disproportionate amount of low molecular weight components. Fluoroelastonlers containing vinylidene fluoride (VFZ) constitute the bulk of fluoroelastomer production (Ranney 1971). Typical di-. ter-. and tetra- combinations are:
VFJ-HFP General purpose, good halnnce of overall properties VF2-PVME-CSM Better low-temperature performance; peroxide cured VFZ-TFE-HFP Higher heat resistance and improved solvent resistance VFJ-TFE-HFP-CSMHigher heatresistance, hest solventresistance of theVF?-contnining fluoroclastomcrs; peroxide-cured and highercost; peroxide-cured VF,-TFE-PMVE-CSMBetterlow-temperatureperformance higher swell in hydrocarbon P-TFE‘ Service in aqueous and nonaqueous electrolytes, liquids, higher glass transition tcmperature; pcroxidc-cured E-TFE-PMVE-CSMService in aqueous and nonaqueouselectrolytes,lower swell in hydrocnrhon (Moore, 1986) liquids, lower glass transition temperature. higher cost; peroxide-cured and best chemical and solvent resistance; suitablcin TFE-PMVE-CSM”Highestheatresistance aqueous and nonaqueous electrolytes; highest cost (Knlh et d . 1973) Categorized a s FEPM per ASTM D141 8. Catrgorized a s FFKM per ASTM D141X. All VF1-contamlng elnstomcrs are In the FKM category.
”
”
541
Halogen-Containing Elastomers Table 14 Weight-Percent Fluorme and H/F Ratios for Sclectcd Elastomers Type
%F
H/F ratio
VF2-HFP VF2-TFE-HFP VFZ-TFE-HFP-CSM E-TFE-PMVE-CSM P-TFE TEE-PMVE-CSM
66 68 69.5 62/68 S4 73.9
0.64 OS9 0.44 O.S0/0.36 (est.) 1 .oo -
Perfluoroelastonlers (elastomers containing only perfluoroalkenes) may also be polymerized by solution and suspension polymerization processes. The effect of varying monomer types, concentration, and sequencing can be substantial, as illustrated by the following tabulation, which lists the overall properties for each category of tluoroelastomer. Generally stated, increasing weight-percent of fluorine in VF,-type elastomers improves solvent and heat resistance while diminishing low-temperature performance. Supplier specificationsoftendefine the elastomer in terms of weight-percentfluorineas 66, 68, and 69.570. Considering the hydrogen-fluorine atomic Inass ratio of 1/19, reconsider these percentages in terms of hydrogen-fluorine number percents (Table 14). With the exception of TFE-PMVECSM, which contains no hydrogen, all have substantial percentagesof hydrogen atoms available. The basic elastomers appear to polymerize with little or no branching as evidenced by low gel contents. A range of molecular weights (as measured by Mooney viscosity) are typically available as part of the supplier’s product specifications. Molecular weight values, when available, should be considered with the historical perspective of hydrocarbon elastomers. Consider that we are substituting a fluorine atomic mass of 19 for a hydrogen atomic mass of 1. A better molecular weight versus viscosity relationship can be rationalized by considering the average number of constitutional repeating units (CRU) or monomer segments in a particular tluoroelastomer. In terms of IUPAC nomenclature, the basic CRUs for fluoroelastomers are all C? structures. Using this consideration. the CRU-viscosity relationship would approximate:
Low Mooney Medium Mooncy High Mooncy
750 CRUs 1 100 CRUs 2200 CRUs
Heat Resistance n t d Tlwrwral Stability
In terms of heat resistance, all of the fluoroelastomers are capable of thousands of hours of service life at 200°C. The TFE-PMVE-CSM elastomer has been reported by its manufacturer to be capable of service a s high as 340°C. Useful service life is often strongly intluenced by design andapplication. In terms of physicalproperties.thestrongionicattractionbetween adjacent hydrogen and fluorine atoms contributes substantially to the room-temperature strength
542
Hertz, Jr.
of fluoroelastomers. A typical fluoroelastomer loses 70% of its tensile strength at 100°C. while a hydrocarbon elastomer (EPDM) loses only 50%. Thermalstabilityvaries within the differentfluoroelastomers. The vinylidene tluoride-containing elastomers are slightly less stable due tothermally induced dehydrofluorination. These elastomers should not be used in a totally confined environment. Improvementsin crosslink stability have been made over the past 15 years. but the thermal stability of all crosslinking systems appears to be lower than that of the corresponding elastomer. bas^ Resistnrlcr
We live i n a world of aqueous and nonaqueous electrolytes; this is the domain of electrochemistry. Any chemical reaction that can occur with a monomer will also occur with its polymeric state. as all chemistry is surface chemistry. The first stage of the electrochemical process is either a direct or indirect electron transfer that leads to the creation of a cation or anion. In the case of an elastomer it is more typically the anion. This is then followed by an electrophilic attack if protons are present or alternatively a nucleophilic attack. The stabilization process of lubricants. engine coolants. and stray electric currents are all sources of electrons-hence the vulnerability of partially substituted hydrocarbons and the bisphenol (ether) crosslink.Harwood ( 1983) states in the event of random backbone cleavage. “it is only necessary to break 1% of the bonds of a polymer of any size to reduce its degree of polymerization to 100.” In light of this statement. it is logical to assume that not only the vinylidene l-luoride (VF?) is vulnerable. but also the bisphenol crosslink. The conventional FKM fluorcelastomers should not be considered for such applications for long-term service. This is a quantum mechanical effect so the maximum stability in such environments would be developed by elastomers consisting of nonsubstituted and fully substituted monomers, e.g., ethylene, propylene, TFE, HFP.
Lo~~,-T~Jnli,ercltilre Properties
The very process of substituting a hydrogen atom (electropositive) with a halogen (electronegative) reduces low-temperature properties. The same strong ionic attraction noted previously also tends to cause attraction between adjacent polymer chains and thus reduces physical volume. This is verified by considering the permeability of nitrogen (N?) in a fluoroelastomer versus an EPDM rubber. about 30 times greater for EPDM. The basis for low-temperature classification of fluoroelastomers should be viewed with great caution. Standard ASTM Tests D746, D1053. and D1329 often appear to be contradictory, since they measure impact. torsion, and tension, respectively. Ferry and Kramer (1978) developed a more logical basis for time-temperature response through application of the Williams-LandelFerry (WLF) equation. The relationship between Shore A/IRHD hardness and shear modulus G shown by Gent (1978) for a static condition is readily broadened to dynamic conditions by applying the WLF equation. A detailed description is beyond the scope of this chapter. The reader should be aware that creep. stress relaxation. dynamic loss tangent. hysteresis, and heat generation are all interrelated properties.
Aging Chnrvcteristics
Long-term aging characteristics are outstanding for all fluoroelastomers, with no age-control considerations needed for seals and molded components. After installation. age controls might be incorporated to monitor stress-strain relaxation effects in seals, hoses. and gaskets.
Halogen-Containing Elastomers
543
Fluid resistance should be considered in terms of nonelectrolytes (nonpolar fluids) and aqueoldnonaqueous electrolytes (polar fluids). Nonelectrolytes are typically hydrocarbons ranging from gases to liquids to solids. Lower molecular weight aliphatic hydrocarbons have little interaction with any vinylidene fluoride-containing tluoroelastomer. Aromatic hydrocarbons (benzene. toluene, xylene, etc.) interact to a greater extent. Increasingthe percent fluorine contentdevelops elastomerswith decreasing swell. Gasohol@,a nonpolar-polarmixture,hasstronginteractions that decrease substantiallywith increasing fluorine content. Aqueous and nonaqueous electrolytes, aldehydes, ketones, and esters generally have varying degrees of interaction with vinylidene fluoride-containing fluoroelastomers, so nonvinylidene fluoride types of fluoroelastomers might be considered. Most suppliers have extensive fluid resistance guides, which should be reviewed for final elastomer selection.
Crosslirlkirlg Fluoloelrrstor,ler.soelmto111ers Fluoroelastomerscanbecrosslinked using mainly threedifferentcrosslinktypes: diamines, dihydroxy aromatic (bisphenol) compounds (ionic). and triazines (peroxide-initiated). The diamine anddihydroxycrosslinksitesaregenerated in situ in vinylidenefluoride-containing fluoroelastomers by a dehydrofluorination-E2 mechanism (nucleophilic). Thetriazine crosslink is activated by the peroxy free radical-El mechanism (electrophilic) at a specific site. Diamine crosslinks are extensively reviewed by Paciorek (1972). Thechemistry generates 2 moles of water for each crosslink created. and water is combined with metal oxides serving as scavengers. Postcuring for an extended period of time at a high temperature removes the balance of the water. The reaction is reversible. so aqueous environmentsshould be avoided. The diamine-cured fluoroelastomers are still popular for rubber-metal bonding and some dynamic applications. The high compression set and stress relaxation of diamine crosslinks led to the next generation of crosslinking chemistry using bisphenol compounds. Bisphenol crosslinks, commercialized in the mid- 1970s, were an immediate success. The dramatically improved compression-set resistance, greatly improved processing safety, and reasonable hydrolytic stability were properties sorely needed. Schmiegel (1977, 1979) reviews the chemistry extensively in great detail. Again. the vinylidene fluoride segment is necessary for development of the crosslinking site. Triazine (peroxide) crosslinks occur at specific sites offered by the cure-site monomer (CSM) described earlier. The curing mechanism was originally developed for the VF?-PMVE (VitonB GLT) elastomer. Conventional ionic crosslinking chemistry (diamine. bisphenol) activated a perfluoroalkoxy elimination (nucleophilic) reaction of the - O W 3 group from the PMVE monomer.Thiswas apparentlypromoted by thevinylidene tluoride proximity. Since this -0CF3 group gave low-temperature enhancement to the elastomer, such a side reaction was undesirable. To avoid the problem, a crosslink site activated by an electrophilic (as opposed to a nucleophilic) reaction was a necessity. Apotheker et al. (1982) have reviewed the chemistry of this crosslinking mechanismin sufficient detail. The bisphenol-AFcrosslink is more thernlally stable that the triazine in dry heat (250°C vs. 230°C). In an aqueous/nonaqueous application the bisphenolcrosslink (ionic) is unpredictableas compared to thetriazine crosslink. Recently, Banick and Bhowmick (1998. 1999) described the electron beam crosslinking of fluoroelastomers and its effect on various properties.
8.5 Compound Technology Typicalfluoroelastonler formulations utilizing the differentcuremechanismsconsist following (all quantities listed in parts per hundred parts rubber. phr):
of the
544
Hertz, Jr.
l.
Vinylidene Fluoride Types-DiamineCure Diamine crosslinker Inorganic base: MgO, lead salts Filler: carbon black
2.
Vinylidene Fluoride Types-Bisphenol Bisphenol crosslinker Quatcrnary onium accelerator Inorganic bases C ~ (OH): I Acid acceptor: MgO Filler: carbon black
3.
1-3 15 15-60
Cure-SiteMonomer-Triazine
Cure
1-3 0.25- 1 3-6 3-6 15-60
(Peroxide) Cure
Organic peroxide 2-6 Inorganic base: PbO (optional) 3-6 Filler: carbon black 15-60
Mixing Mixing can be done on both open-mill and internal mixers. The high Mooney elastomers have rapid heat-buildup tendencies and should be treated accordingly. Process Aids
Process aids, both proprietary and generic, are widely available. Typically, materials such as paraffin, low molecular weight polyethylene, and waxes are commonly utilized. More complex and expensive process aids (fluorinated analogs) are offered; however, there is no outstanding candidate. Fillers
Fillers typically are the N990 thermal blacks. These blacks offer reasonable reinforcing characteristics, with the economy of higher loadings and lower attendant hardness increases. Furnace blacks ranging from NlOO to N700 types are used to develop specific properties for oil-field applications.Nonblackfillersshouldbeutilizedwithgreatcautionbecause of the rigorous environments fluoroelastomers are subject to. Briggs et al. (1963) should be reviewed before considering nonblack fillers. In addition, aqueous testing should be performed at the pH anticipated in actual service. Some nonblack fillers and compounding ingredients are very specific in some instances to pH variation resulting in unusual swelling. Plusticixrs
Plasticizers are generally ineffective due to their lack of permanence. The efficiency of a plasticizer (which must be thermodynamically soluble) is a function of the molecular weight; the
Halogen-Containing Elastomers
545
lower the value, the higherthe efficiency. This is also true of the plasticizer’s volatility: increasing molecular weight. decreasing volatility. The 200°C postcure tends to volatilize most plasticizers during the postcure process.
8.6
Fluorine-Containing Elastomer Blends
Blends of other elastomers havenot proven successful due tophase incompatibility. Polyacrylate elastomers have occasionally been utilized to reduce compound costs in diamine cured formulations. These blends offer no improvements in heat or fluid resistance, and, more important, they decrease critical performance. Reviews by Arnold et al. (1973) and compounding techniques by Stivers ( 1978) are recommended for additional reading.
REFERENCES Apotheker, D., Finlay. J. B., Krusic, P. J., and Logothctis, A. L. (1982), Ruhhrr Clrerrt. Twhrrol. 55:1005. Arnold, R. G., Barney, A. L., and Thompson, D. C. ( 1973), Ruhhrr Chcrrr. T o c h o l . 46:619. Baldwin, F. P,, and Thomas, R. M. ( 1960),U.S. Pat. 2,964,489. Banik, I., and Bhowmlck, A. K. ( 1998), J. Appl. Polynl. Sei. 69:2071. Banik, I., Dutta. S., Chaki, T., and Bhowmick, A. K. (1999), Pn/yrrrc.r 40:447. Banik, I., and Bhowmick, A. K. ( 1999), Rrrditrtiorl Phys. Clwrrl. 54:135. Blanchard, R. R. (1973). Rubbcr Division. ACS Mecting, Detroit. May 1-4, paper 49. Brandrup, J., and Immcrgut, E. H., Eds. (1975), Polyrrrer H a r d h o o k , 2nd ed., Wilcy. New York. Briggs, G. J., Edwards, D. C., and Storey. E. B. ( 1963). Rlt/?her. Clrrrrr. Techrlol. 36:621. Brooks, R. E., Strain, D. E., and McAlevy. A. (1953), lrlrlia Ruhlwr World 127:791. Collins, A. M. (1973), Rubber Clrm. Trclrrlol. 46:G48. Dcvlin, E. F., and Folk, T. L. (1984), Ruhher Clretrr. Tcchrrol. 5 7 1098. Dulog, A. (1966). Mrrcrorrrol. Clretrr. YI:50. Dunnom, D. D., and Wagner, M. P. (1981 ), ASTM S/~rr/~/~rr[/i~~rt;orl Nrnx, November. pp. 10-14. Dupuis, 1. C. (1982), New York Rubber Group, Education Program, April 28. Fearn. J. E. (1972). in ~luoro/)~~/yrrrer.s (L. A. Wall, Ed.), Wiley-lntersciencc. New York, pp. 1-32. Ferry, J. D., and Kramer, 0. ( 1978). in Scirrrcc. t r r r d T d l r l o l o g y t ? f R r r / h ~(F. r R. Eirich. Ed.). Acndcmlc Press. New York, pp. 179-221. Fctterman. M. Q. ( 1973). Rubber Cllerrr. Tecllrlol. 46:927. Fisher. H. L. (1957), Clrerrristr:v of’Nrct~rrtrlm d Syrrtlrrtic Ruhhzrs. Rheinhold. New York, pp. 1 - 13. Fusco, J. V., and Hous, P. ( l987), in Rrrhher Trchrrolop. 3rd ed., (M. Morton, Ed.), Van Nostrand Rheinhold. New York, pp. 284-3 IO. Cant, G. A. L., and Cabey, M. A. (1976), Rubber Division, ACS Mcetlng, San Francisco. Oct. 5-8, paper 18.
Gent. A. N. (1978), in Sc.ierlce c m / Techrrology of Rrrhher (F, R. Eirich. Ed.), Academic Press, New York, pp. 1-21. Gintz, F. P. (1968), in Virlyl rrrd Allied P o / w ~ ) r .(P. s D. Ritchie, Ed.), lliffe Books Ltd.. London, p, 241. Graham, J. W. (1982), New York Rubber. Education Program, Mor. 3. Guy, A. R., alld Sollbergcr, L. E. (1970). Rubber Division. ACS Meeting. Washington, DC. May 5-8, paper 6. Haaf, F., and Johnson, P. R. ( I97 1), Rubher Clrrrrr. Techol. 44: 1410. Hargreaves. c . A. (1968), in Po/Yt?wrC/wrrli.vtt;vc~fSvr~lhelic~ Rrthbc~r(J. P. Kennedy and E. G. M. Tornqqvist. Eds.), Wiley-Interscience, New York, pp. 727-252. 1 I , 4 p. 29 I . Harwood, H. T. ( 1983). in J. Tcst. & EIY~/.-ASTM Honsbcrg, W. (1983). Rubber Division. ACS Meeting, Houston, TX, Oct. 25. paper 3. IISRP ( 1986), Wor/d\tlide Rubher Sttrti.stics (/YX6), International Institute of Synthetic Rubber Producers, Houston, TX. 77063.
546
Hertz, Jr.
Johnson, P. R. (1976). Rubber Chenl. Techno/. 49:650. Jones, G. D. ( 1 9 6 4 ~in Clwnicol Recrctiorzs qf Polvmers (E. M. Fettes, Ed.), Interscience, New York, pp. 263-266. Kalb, G. H., Quarles, R. W., Jr., and Graff, R. S. (1973), in App/ied Po/ynwr Syttrposiurn, No. 22 (M. A. Golub and J. A. Parker. Eds.), Wiley, New York, pp. 127-142. y Morton, Ed.), Rheinhold, New York, Keeley, F. W. (1959), in Introclucfiorr to Rubber T e c h r ~ ~ l o g(M. Ch.14. Kyllingstad. V. L. (1982), New York Rubber Group, Education Program, Apr. 21. Lo, E. S. ( 1 9 6 2 ~U.S. Pat. 3.023,187. Lutz, M. A., Polmanteer, K. E., and Chapman, H. L. (1985), Rubber C h m . Techru~l.58:939. Luyendijk. N. (1966), Tech. Report AFML-TR-65-230 (NTIS AD 630907). McAlevy, A., Strain, D. E.. and Chance, F. S. (1947), U S . Pat. 2,416,061. McQueen. D. M. (1940). U.S. Pat. 2,212,786. Maynard, J. T., and Johnson, P. R. (1963), Rubber Chern. Techno/. 36:963. Midgley, T., Jr. (l930), lnd. &g. Chenr. 22:542. Montermoso, J. C. ( 1961), Rubber Cllem. Techml. 34: 1521. Moore, A. L. (1986). Int. Symp. Centenary of Discovery of Fluorine,Paris,France,Aug. 28, 1986. E/crsrorrreric.s. Sept 1986, pp. 14-18. Mori, K., and Nakamura, Y. (1984), Rubber Che~n.Techno/.57:665. Murray. R. M., and Thompson, D. D. (1963). The Neopretres, E. I. DuPont de Nemours & Co., Inc., Wilmington, Del. Nakamura, Y., Mori, K., and Oka, S. ( 1974), J. Rubber I d . Normand. R. J., and Johnson, J. B. (1975), Rubber Division, ACS Meeting, New Orleans, Oct. 7-10, paper 5. Oetzel, J. T., and Scheer, E. N. (1978), Rubber Division ACS Meeting. Quebec, May 2-5, paper 41. Oswald, H. J., and Kuber, E. T. (1963), SPE Tech Papers, paper 1, Vol. 17. Paciorek, K. J. L. (1972). in Fluorr~polymers(L. A. Wall, Ed.), Wiley Interscience, New York, pp. 291-305. Pierce, 0. R. (1953), J. Am. Cherrz. Soc. 75:6324. Pierce, 0 . R. ( 1970). J. App/. Po/ym. Sci. App/. Poly. Svrnp. 14:7. Pierce, 0. R., and Kim, K. Y. (1973), J. Appl. Po/.vrn. Sei. Appl. Pml.ynr. Sytnp. 22:103. Ranney, M. W. (1971). Chemical Process Review No. 51, Noyes Data Corporation, Park Ridge, NJ, pp. 60-105. Razzano, J. S., and Simpson, V. G. (1976). U.S. Pat. 3,974,120. Rexford, D. R. (1962). U S . Pat. 3,051,677. Rose, J. C., and Coffey, R. J. (1982), Rubber Division, ACS Meeting, Philadelphia, May 4-7, paper 51. Rugg, J. S., Dixon, S., and Rexford, D. R.(1956), AmericanChemicalSociety,September,Fluorine Chemicals Symposium, Paper 1 1 1. Schmiegel, W. W. (1977), Deutsch Kautschuk und Gummi Gesellschaft, April 28-29, Paper 415. Schmiegel, W. W. (1979), A ~ y e r v Mrlkrornol. . C/~enz.7617739. Sears, J. K., and Darby. J. R. (l982), The T e c h d o g y of P/n.sricizers, Wiley-Interscience, New York. Sollberger, L. E., and Carpenter, C. B. (1974). Rubber Division, ACS Meeting, Toronto, May 7-10, paper 9. Stivers, D. A. (1978),in The VnrltferbiltRubber Handbook (R. 0. Babbit, Ed.), R. T. Vanderbilt Company, Norwalk, CT, p. 244. Swarts, F. (1982), Bull. Accrrl. Roy. Bell 24(3):474. Sylvest, R. T., Barnes, C., Warren, N. E., and Worsley, W. R. (1981), Rubber Division, ACS Meeting, Cleveland, Oct. 13- 16, paper 15. Thomas. D. K. (1968),Tech. Rep.68234, Royal Aircraft EstablishmentFarnborough Hauts (NTISAD68645 1 ). Timar. J., and Edwards, W. S. (l979), Rubber Clrenl. Teclrrd. 52:319. Vandenberg, E. J. (1983), Clrrnrfeclt, August, p. 474. Wagner, M. P. (1981), Elnstornerics, August, p. 40. Yamada, M., Arai, S.. and Masuda, Y. (1973), Nippon Gornu Kvoknislri 46:404.
19 Tetrafluoroethylene-Propylene Rubber Gen Kojima and Masayuki Saito Asahi Class Co., Ltd., Yokohama, Japan
1. INTRODUCTION Since the advent of the vinylidenefluoride-hexafluoropropylene-basedelastomer (FKM) in the 1950s. a variety of fluoroelastomers have been developed and commercialized. In 1975, a new fluoroelastomer family, based on an alternating copolymer of tetrafluoroethylene and propylene, was introduced by Asahi Glass Co., Ltd., under the trade name of AFLAS. The elastomeris unique in that it offers (1) excellent heat resistance with maximum continuous service temperature of about 230°C and above, ( 2 ) distinguished chemical resistance with no or little deterioration even in contact with strong acids and bases at high temperature, and ( 3 ) high electrical resistivity of the order of 10's-10'6 fl cm. The elastomer has been distributed worldwideand is used in awide variety of industrial fields whererubberpartsmeetharsh environments. Reflecting the recent trend of increasing automobile engine power the temperature of the engine becomes higher and high-performance engine oils, which are heavily formulated with amine-based additives, have come to be used. In such a field, elastomer parts are required to have more heat resistance and engine oil resistance even if a fluoroelastomer is used (Grootaert et al., 1990). Therefore. the tetrafluoroethylene-propylene elastomer has been attracting more attention as a material that meets such harsh conditions. On the other hand, the tetrafluoroethylene-propylene elastomer has been finding new applications in the wire and cable industries as an elastomericinsulatingmaterialwiththehighestheatresistance. The tetrafluoroethylenepropylene elastomer is now mainly used in the automotive industry as an oil seal and in the wire and cable industries as insulation jacketing.
2.
MANUFACTURING
The elastomer is manufactured either by copolymerization of tetrafluoroethylene (TFE) and propylene (P) or copolymerization of TFE, P, and vinylidenefluoride (VdF), optionally followed by proprietary treatments to improve the vulcanizability of the polymer. Tetrafluoroethylene,propylene,andvinylidenefluoridearecopolymerized in aqueous emulsions under pressure and at low temperature by use of a specific redox initiator to produce a high molecular weight polymer (Kojima and Hisasue, 1981). 547
548
Kojima and Saito
Polymerization behavior is shown in Fig. 1, which illustrates the strong tendency of two monomers to form analternating copolymer (Kojima and Tabata, 1972). Fineman-Ross calculation gives the monomer reactivity ratios of TFE and P as about 0.01 and 0.1, respectively. These very small reactivity ratios are attributed to their similar small Q values and much different e values (Q.rFE= 0.049. eTFE= 1.220, Q,,= 0.002, e,, = -0.780). The copolymerization kinetics implies the following polymerization mechanism (Kojima and Tabata. 1972; Kojima and Hisasue, 1981): Initiation * C 2 F 4 S 0-4
Polymn.
s2082-
c 2F4 A s o : -
[Fe2+
-
EDTA] CH2(OH)S03-
+ s o 4 k ; ~ e 3 +- E D T A ~ : H ~ ( O H I S O ;
Propagation
kFP 2F 4 *
-C
2
F
4
+
C3H6 "
+ C2F4
-~k~~
-C2F4'
kPF
H + C2F4 3 6
C-
3H 6
C
-c
F
2 4
kPP H + C3H6 -C 3 6
-C
k~~ -
kPF
= 0.01
H
*
kPP - 0 . 1 --
kFP
3.
POLYMERSTRUCTURE AND FUNDAMENTALPROPERTIES
As described in Section 2, the polymer essentially consists of the highly alternating sequence of TFE and P monomers. Figure 2 illustrates typical '"F and ' H NMR spectra. The '"F NMR spectrum shows four sets of AB quartets of equal intensity, whichare ascribed to the tluoromethylene groups of the isolated tetrafluoroethylene affected by the methyl group of propylene. and the 'H NMR spectrum shows a sharp resonance, which is ascribed to the methyl group of the isolatedpropyleneunit.Fromthese data, themicrostructure is deduced as shown in Fig. 3, where the highly alternating monomer sequence and random methyl configuration are the main characteristics (Kojima et al., 1976).
3 6
549
Tetrafluoroethylene-Propylene Rubber
'80O l
!
0
20
l
l
40
I
l
l
60
1
1
80
I
l00
T F E i n monomers (mol%)
Fig. 1 Copolymerizationcurve of TFE-P system. (From Kojima and Tabata, 1972.)
Table 1 liststhefundamentalproperties of thepolymer. The very highdecomposition temperature and chemical resistance in spite of the relatively low fluorine content are worthy of notice. These facts indicate that the vulnerable propylene unit is almost completely protected by the vicinal TFE units (Kojima and Kojima, 1977). Figure 4 illustrates the solubility of the dipolymer in various solvents. The polymer is soluble in THF and swollen by some solvents of relatively low polarity. Though the glass transition temperature of the dipolymer is -3°C and that of the terpolymer is - 13"C, vulcanizates do not become brittle even at -40°C.
4.
COMMERCIAL POLYMER TYPES AND GRADES
The commercial polymers are classified into two types: the TFE-P dipolymer type (AFLAS 100 and 150 grades) and the TFE-P-VdF terpolymer type (AFLAS 200 and AFLAS SX grades). AFLAS 200 is characterized by the improved low-temperature properties and versatile vulcanizability, and AFLAS SX is characterized by the improved processability for vulcanization, demoldability, and metal bonding while maintaining most of the high heat and chemical resistance and electrical resistivity the dipolymer. Table 2 lists the polymer grades now available, which are mainly classified according to Mooney viscosity. As shown in the table, dipolymer is mostly used in the wire and cable and automotive industries, while terpolymer is often favored for automotive use in terms of the processability.
5. COMPOUNDING AND VULCANIZATION The elastomer can be compounded easily by conventional means, such as Banbury and open mill mixers. It can also be fabricated into a variety of articles by means of press, mold, extruder, and calender.
Kojima and Saito
550
~
l
l
-110
-120
1
l
-90
-100
l
-130
(a)
I
7.00
l
8.00
I
9.00
-c*( PPm)
(b) Fig. 2 ( a ) Typical "'F NMR spectrum of TFE-P copolymer. (From Kojima et al.. 1976.) (h) Typical ' H NMR spectrum of TFE-P copolymer. (TFE/P molar ratlo in copolymer: 55/45) (From Kojima et al.. 1976.)
551
Tetrafluoroethylene-Propylene Rubber
F F H H F F H H F F H C H 3 F F H H F F H C H 3
-c-c-c-c-c-c-c-c-c-c-~-c- c-c-c-c-c-c-c-cF
F
H
F
CH3F
H
CH3F
F
H
H
F
F
H
CH3F
F
H
H
Fig. 3 Microstructure of the copolymer. (From Kojima and Wachi, 1985.)
Table 1 Fundamental Properties of AFLAS Polymers Specific gravity Fluorinc content, % Glass transition tempcrature, "C Brittle point, "C Decomposition temperature, "C Solubility
1.52- I .60 55-60% - 3 to -13°C below - 40°C 380-420°C Soluble in THF
So~rrcr:Kojilna and Kojima, 1977; Kojima and Wachi, 1985
4 3
n
3 -
c,
c
al E 0
E al
2 .
7
0 p. .c
n
1 '
6
a
10
12
14
16
S o l u b i 1 it y p a r a m e t e r Fig. 4 Solvent affinity of the AFLAS 100-type copolymer. Immersion condition, room temperature for 48 hr. Dashed line. contour of volume increases of 20%; solid line, contour of volume increase of 100%: solid circle, polymer soluble in the solvent. Numbers correspond to the following solvents: I , HCONHCH,: 2, CH3OH; 3, HOC,H,OH; 4, (CH3)lSO; 5, CH,NO,; 6, CZHSOH; 7, HCON(CH,),; 8, CH3CN; 9,/1C,H,OH; 10, CHZCHCN; 11, CH?COCH,COCH,; 12, C,H,N; 13. (CH,)ZCO; 14, CS,; IS,CH,(C,H,)CO; 16, CH,COOC2HS, 17, C,H,O; 18, C,HCI,; 19, ChH,; 20, C6H,CH3; 21, CH,(i-C,H,l)CO; 22, CH3CC13; 23, C - C ~ H 24, I ~ ;(i-C3H7):0; 25, n-C7HI,.(From Kojima and Kojima, 1977.)
Table 2 Commercial Polymer Types and Grades TFElP Grade
#150L
#150E
#150P
H00H
#1OOS
#150CS
#I 50C
#sX
#2OOS
K!!ooP
1.55 57 35
I .55 57 60
1.55 57 95
1.55 57 110‘
1.55 57 160*
1.55 57 130
1. 55 57 1
oo*
I .52 55 75
1.60 60 85
I .60 60 90
-3
-3
-3
-3
-3
-3
-3
-5
- 13
- 13
Specific gravity Fluorine content, c/c Mooney viscosity MLI + lO(1WC) Glass transition temperature, “C Color
<
Dark brown
Vulcanization
I
Peroxide
Characteristics and applications
TFWPNdF
Lining
Extrusion
General
Source: Kojima and Kojima. 1977: Kojima and Wachi, 1985; Miwa et
c---White -----+ +
+-
High strength -+
al.. 1996.
- +------
EB
------+
Extrusion
Transparent (yellowish) Peroxide
Transparent Light (yellowish) brown Peroxide, Peroxide, bisphenol bisphenol General ------+
553
Tetrafluoroethylene-Propylene Rubber Table 3 StandardCompoundingFormulationandMechanicalProperties
Formulation (by weight) Polymer Peroxide" Triallyl isocyanurate Sodium stearate Calcium hydroxide Magnesium oxide Promoter A" Promoter B" Bisphenol-AF MT carbon black Vulcanization Press cure, "Clmin Post cure, " C h Mechanical properties Tensile strength, Mpa Ultimate elongation, c/o Modulus at 100% elongation, Mpa Hardness (Shore A) Compression set, %, 70 hr at 200°C
#SX
#200S(I)
#100S
#15OP
#150E
100
100
100
IO0
2
2
1 5 1
1
100 1
100 I
5 1
#20OP
#200S(2) #200S(3) 100
IO0 -
3
-
I 3
-
-
-
-
6 3
-
-
6 3
-
S 1 -
S 1 -
-
-
-
-
-
3
-
-
-
-
1
0.5
-
-
-
-
-
25
2s
25
2s
2s
25
25
1.7 2s
170120 20014
170120 20014
110120 230124
170120 170120 170120 20014 20014 20014
170120 170120 20014 20014
3 0.5 0.5
3 0.5 0.5
1 -
17.5 320 4.5
18.1 260 4.4
14.6 300 4.0
19.8 200 9.1
18.8 260 7.9
16.8 320 2.9
19.9 190 S .9
17.6 220 8.0
71 26
72 29
71 29
78 25
78 25
63 35
68 25
73 25
~~
1,3-Bis-(t-huty1peroxy)diisopropyl benzene. h Specific vulcanization promoter.
Table 3 lists the standard compounding formulation and vulcanization conditions for each grade of AFLAS. AFLAS 100, 150 and AFLAS SX grades are vulcanized by organic peroxides with the aid of an appropriate co-agent, such as triallyl isocyanurate (TAIC), under normal conditions, preferably followed by postcure. AFLAS 200P and 200s can be vulcanized either by organic peroxides or by nucleophiles, such as bisphenol-AF andhexamethylenediaminederivatives.Figure 5 shows theproposed mechanism for the peroxide vulcanization of the dipolymer (Kojima and Wachi. 1978), and Figure 6 shows that for bisphenol vulcanization of the terpolymer. The proposed mechanism of the peroxide vulcanization of AFLAS SX and AFLAS 200s is shown in Fig. 7, which implies the possible in situ formation of double bond provided by the vinylidene fluoride unit in the polymer (Miwa et al., 1996).This vulcanization system does not need such specific cure site monomer such as bromine (Apotheker and Krusic, 1980) or iodine (Tatemoto and Morita, 1982). AFLAS 200s can be also cured without TAIC ascontrasted to otherperoxide vulcanizable elastomers. Non-TAIC formulated AFLAS 200s shows improved demoldability, and furthermore, hardness of AFLAS 200s vulcanizate can be easily controlled in the range from 60 to 80 Shore A according to the level of TAIC when the co-agent is formulated. Figure 8 illustrates the vulcanization curves of AFLAS 200s observed by means of an oscillating disk rheometer (ODR) when bisphenol-AF and peroxide are used as vulcanization
Kojima and Saito
554
1
CH2CH =CH2
Fig. 5 Proposed mechanism of peroxide vulcanization. (From Kojima and Wachi, 1978.)
CHZ-CFZ(Polymer)
-CH
= CF-
b
+
-HF I -r
Onium Salt / Acid Acceptor
(R,N+X-)
- - -O-@&)-
H o@ Q --H o
N+R4
CF3 CF3
(Bisphenol-AF)
"*
-c"c-
-c-cFig. 6 Proposed mechanism of bisphenol-AF vulcanization of AFLAS 200.
555
Tetrafluoroethylene-Propylene Rubber
CH2-CF2-
yfc
= CF-
-CH
(Polymer)
- HF
4
Onium Salt / Acid Acceptor
RORO-OR (Peroxide)
-c-c(TbC)n b
-c-c-
I
R I
-c-c-
(Closslinked Polymer)
Fig. 7 Proposedmechanism of peroxide vulcanization of AFLAS SX and 200s. (From Miwa, et al., 1996)
E
Y
0
6
AFLAS 150P
12
18
24
Vulcanization Time (min)
Fig. 8 Typical ODR curves of AFLAS SX and 150P vulcanized by peroxide. Measured at 170°C. with a microdie, no preheat, an oscillator frequency of 100 cpm and 3" arc.
556
Kojima and Saito
Bisphenol
h
0
6
12
18
24
Vulcanization Time (min) Fig. 9 Typical ODR curves of AFLAS 200s vulcanized by bisphenol-AF andperoxide.Measuredat 170"C, with a microdie, no preheat, an oscillator frequency of 100 cpm and 3" arc.
agents, respectively, and Fig. 9 illustrates the vulcanization curves of AFLAS SX and AFLAS 150 when compounded according to the formulation described in Table 3.
6. VULCANIZATEPROPERTIESANDAPPLICATIONS Table 3 also lists the fundamental properties of the standard vulcanizate. (All of the properties hereinafter described are of the standard vulcanizates, unless otherwise specified.) The tensile properties, hardness, elastic recovery, and other properties are observed to be sufficient for a variety of practical use. Vulcanizates exhibit excellent thermal stability, suggesting that continuous service temperatures can reach 230°C or more as shown in Fig. IO. Tables 4 and 5 compare the resistance of AFLAS 150 and 200 against liquid organic and inorganic chemicals in terms of their volume swell when they are immersed therein. AFLAS 150 shows slightly better resistance to inorganic chemicals and polar solvents, whereas AFLAS 200 is more resistant to nonpolar solvents. Figure 11 exhibits distinguished resistance of AFLAS vulcanizates to a high-performance engine oil which is heavily formulated with amine-based additives as antioxidant. These data show that AFLAS is much more resistant against such extremely harsh conditions in oil seal applications for the recent heavy-duty automotive than FKM type fluoroelastomer. Bonding of elastomer to metal is a significant process in oil seal application. AFLAS SX and 200 can be bonded to various metals and other materials by use of a primer specifically developed by Asahi Glass. Table 6 illustrates the electrical properties of the vulcanizate. AFLAS hasas high electrical resistivity as do silicone and EPDM elastomers, ranging from IO" to 10" Q cm. The dipolymer is more suitable for electrical insulating applications owing to the higher resistivity. Table 7 shows that the dipolymer can be also vulcanized by electron beam irradiation (EB cure). Among the commercial grades, AFLAS 150C and 150CS give excellent physical proper-
557
Tetrafluoroethylene-Propylene Rubber
320
.
300
-
h
V 0
280 260
Y
W
L
240
1
c, 4
:\ -
. 200 -
L
220
W D.
E W
c
180 l
I
I
I l l ]
2 I
I
I
l
1
2
3
6
l
month
12 1
2
10
3
year
Fig. 10 Continuous serviceable period of time at high temperature, as estimated with the time of 50% retention of tensile properties. (From Kojima and Kojima, 1977; Kojima and Wachi, 1985.)
Table 4 Oil and Solvent Resistance (Volume Change after Immersion) Immersion ("C X day)
Oil/solvent ~
~
~~~
Volume change (%) #l 50
ROO
~~
Fuel B Gasoline Gasoline (80%) + methanol (20%) Lubricant oil (ASTM No. 3) Engine lubricant oil (Castle S-3) Long-life coolant (Castle LLC 50%) Methanol Benzene Acetone Perchloroethylene Carbon tetrachloride Ethyl acetate Sourcet Kojima and Kojima,
R.T. X 40 X 40 X 175 X 150 X 180 x 40 X 40 X 40 X 40 X 40 X 40 X
1977; Kojima and Wachi
7 3 3 3 7 3 3 3 3 3 3 3
1985
58 50 46 17.1 5.1
15.3 1.6 40 50 95 86 88
24 28 44 10.2 2.8 17.6 6.2 41 103 25 31 166
Kojima and Saito
558
Table 5
InorganicChemicalResistance(VolumeChangeAfterImmersion)
Chemical
#l 50
#200
3 0 8.7
15.4 0.7
-
24.8
100 x 3 100 x 3 12.6 150 X 3 180 x 3 29.3x 3 200
Sulfuric acid (96%) Sodium hydroxide (SO%) Steam
Source:
Volume change (%)
Immersion ("C X day)
16.6
Kojirna and Kojima. 1977: Kojima andWachl,198.5.
120
l
100
80
"""
-AITAS 1SOP -D. ARAS SX
- -
."
-S
-FKM terpolymer
I \
60
40 20 J
0"
"
0
200
400
800
600
1000
Immersion Time (hr)
Fig. 11 Retention of thetensilestrengthandelongationofTFE-Pelastomersand a FKM terpolymer (fluorine content 69 wt%, peroxide vulcanizable) when immersed in SG-class engine oil (Castle Clean SG).
Table 6 ElectricalProperties" Property Volume resistivity, Cl cm Dielectrical constant at 1 kHz Dissipation factor at 1 kHz Dielectric breakdown, kV/mm
#I S0
#200
3 x 10"' 6.0
4 x 10'5 5.9 3.3 x 10" 16
5 x 10"
23
I' Vulcanized by peroxlde and TAIC. with the nonfiller compound. Measured 23°C. Source:
Kojinla and Kojima.1977:KojimaandWachi.1985.
at
Tetrafluoroethylene-Propylene Rubber Table 7 Fundamental Propertiesofthc Grade gravitySpecific Hardness (Shore A ) Modulus at 100% elongation, Mpa Tensile strength. Mpa Ultimatc clongation. % Volume resistivity at 23°C. 0 cm Dielectric constant at 1kHz, 23°C Dielectric breakdown at 23"C, kVlmm
559
Vulcanizate by EB Cure" #I 00s 1 .ss so
I .S 18 330 >IO'" 2.8
25
#l S0E 1.ss
39 1.1 9 460 > IO'"
2.8 23
#I
soc 1 .ss
S2 I .S 19 400 >IO"'
2.8 24
#I
socs 1 .ss S1
I .4 17 360 >IO'"
7.8 23
ties and electricalpropertieswithout any ingredients. such asvulcanizingagents,co-agents. promoters. and fillers. From these data. the TFE-P elastomer is observed to be the most heat-resistant elastomer among the elastomeric insulating materials. Blends of AFLAS with other elastomers likeacrylic elastomer provide the customer with an economically reasonable elastomer. which has a well-balanced combination of heat and oil resistance at lower cost. Blends of AFLAS with thermoplastic resins are of interest in terms of polymer alloys. which are expected to have intermediate properties between elastomer and thermoplastic resin. Polyethylene and ethylene-tetrafluoroethylene copolymers are especially of interest since they are easily blended with AFLAS dipolymer in a wide range of blend ratio by means of heatprocessing mixers, such as extruders and kneaders. These blends can be extruded like a thermoplastic resin and cured by EB irradiation to yield a soft elastic resin. As described above, TFE-P elastomer has been successively developed to be provided with versatile types and gradesas well as various application technologies.all of which now allow the elastomer a worldwide established market appreciation as a distinguished high-performance elastomer to be used in variety of applications where harsh conditions and special requirements rule out the use of other elastomers.
REFERENCES Apothekcr, D., and Krusic, P. J. (1980).U.S. Pat. 4,214.060 (to E. I. Du Pont de Nemours and Company). Grootaert, W. M,, Kolb. R. E., and Worm, A. T. ( 1990). Ruhher C / w m 7 e h o l . 6 3 5 16. Kojima, G.. and Hisasuc, M. (1981),M w ~ o ~ ~ MC/rertr. J / . 182: 1429. Kojima, G.. and Kojima, H. ( 1977), R ~ h h e rCherrr. T e h w l . 50(2):403. Kojima. G., and Tabata, Y.(1972), J . Mrccrorlrol. Sci.-C/~eru.A6(3):417. Kojima. G . . and Wachi, H. ( 1978). K u b b ~ ~Cherr~. r T e c h o l . 5/(5):940. Kojima, G., and Wachi, H. (IOXS),Int. Rubber Conf., Kyoto, Oct. 15-18, Paper No. 16A18. Kojima, G., Wachi. H.. Ishigre, K.. and Tabata. Y.( 1976), J. Polyrr. Sci. Po/yru. Ed. 1 4 6 ) : I3 17. Miwa, T., Kaneko. T.. and Saito, M. (1996). The 9th Seminar on Elastomers, Kobe. Dec. S-6, Paper No. A-X. Tatemoto. M,, and Morita, S. (1982). U.S. Pat. 4,361,678 (to Daikin Industries. Ltd.).
This Page Intentionally Left Blank
20 Carboxylated Rubber John R. Dunn 1. R. consulting,
Sarnia, Ontario, Canada
1. INTRODUCTION
2.
HISTORICAL
The early work on carboxylated elastomers was authoritatively reviewed by H. P. Brown (1957). Subsequently, several further reviews have been presented (Bryant, 1970; Jenkins and Duck, 1975; Longworth, 1983; MacKnight and Lundberg, 1983; Shaheen and Grimm, 1985). Jones and Smith (1985) compared the properties of carboxylated hydrogenated NBR with those of carboxylated NBR. The firstpreparation of a carboxylic elastomer, acopolymer of butadiene and acrylic acid, was recorded in a French patent published in June 1933 and assigned to I. G. Farbenindustrie (1933). In the ensuing years several more patents were issued describing the introduction of carboxyl groups by emulsion polymerization, including one assigned to B. F. Goodrich Co. by Semon (1946) that describes the preparation of CO-and terpolymers. This includes butadiene and isoprene among the dienes and acrylonitrile among the third monomers. As Brown (1957) notes, nothing remarkable was recorded about the vulcanizate properties of these polymers. It was claimed that solvent resistance was better than that of the corresponding uncarboxylated polymers. The first commercial carboxylated elastomer, a butadiene-styrene-acrylic acid terpolymer latex, was introduced byB. F. Goodrich in 1949 under the trade name HYCAR 1571 (Jenkins and Duck, 1975). Brown (1957) realized the role of carboxyl groups in crosslinking reactions in 1950, and a series of patentsresulted. In September 1954,Brown and Duke (1954) reportedthat two carboxylated NBR latices were commercially available. They noted that high strengths could be obtained in vulcanizates of gum stocks and latex films. Equivalent cures could be obtained 561
562
Dunn
without sulfur by the use of polyvalent metal oxides or salts. At the time, dry rubbers were not available. Even in 1975. Jenkins and Duck ( 1975) noted that latices were used in far greater volume than dry rubbers because of the tendency for compounds containing metallic oxides to scorch. The use of coated zinc oxide and of zinc peroxide masterbatch, or of modified polymers, to reduce scorch is described in the papers by Bryant ( 1970) and Shaheen and Grimm (1985). This topic will be discussed in a later section. As a result of improvements in scorch resistance, carboxylated NBR is now being used in avariety of applications,which will be discussed subsequently. While emulsion polymerization is the most common route to carboxylated elastomers, the modification of noncarboxylic elastomers to introduce carboxyl groups has also been studied and is described in the earlier reviews. So far as is known, none of the commercially available carboxylated elastomers are produced by polymer modification.
3. 3.1
PREPARATION OF CARBOXYLIC RUBBERS Emulsion Polymerization
The majority of carboxylated elastomers are produced by emulsion polymerization at temperatures ranging from about 50 to 60°C. Many emulsifying agents have been investigated and are suitable, but the use of an acidic system is regarded as essential. Marvel et al. (1952) showed that butadiene could not be copolymerized with methacrylic acid in the Mutual GR-S recipe, presumably because the acids were converted to water-soluble salts. Coagulation must also be carried out using reagents that ensure that the carboxyl group remains acidic,andacids, or blends of salts and acids, fulfill this function satisfactorily. Jalics ( I 984) claims that emulsion copolymerization of half-esters of carboxylic acids may be carried out in basic media. The halfester moiety is said to renderthecarboxyl compound hydrophobic. The ability to produce carboxylated elastomers in a basic medium would indeed offer an advantage, since polymerization in an acidic medium requires the use of stainless steel vessels and piping. The preparation of carboxylated elastomers by emulsion polymerization hasbeen described in some detail by Jenkins and Duck (1975). They suggest the following as a typical recipe:
persulfate
0.2 ulfate
Monomcrs Sodium alkylarylpolyether sulfate Potassium Water
100.0 1.0
0.3 188.0
As in many emulsion polymerization recipes, tertiary dodecyl mercaptan is recommended as a modifier to control molecular weight. HCI (or methanolic HCI) is suggested as coagulant. Brown and Gibbs (1955) quoted the following as a typical recipe:
Monomers Dodeeylaminc (90% neutralized with HCI) Aluminum chloride Potassium Sulfole Water
100.0 5.0 0.2
200.0
Carboxylated
Rubber
563
The recommended polymerization conditions were 6-25 hours at 30-50°C for 75-90% conversion. This is a somewhat lower temperature than the 60°C quoted in earlier patents (Brown. 1957). Polymerprocessabilityandpropertiesareaffected to someextent by polymerization temperature. The amount of carboxylic acids in the polymer and their distribution depend on the type of acid, the ratio of monomers charged, whether or not they are charged incrementally. and the degree of conversion to polymer. The efficiency of incorporating the acid has been said to depend on its relative solubility in the hydrocarbon and water phases. Acrylic acid is more soluble in the aqueous phase, and only half of that charged is polymerized. Methacrylic acid is about five times as soluble in the hydrocarbon than in the water phase. Hence it is efficiently incorporated into the polymer. Marvel et al. (1955) described the preparation of copolynlers of butadiene with 15-20 parts of acrylic acid. They used asodiumalkyl aryl polyethersulfate (Triton X-301) as ~ l n emulsifier in order tobroaden the range of monomer composition. They used azobisisobutyronitrile as initiator andrl-decyl mercaptan as the modifier. Dolgoplosket al.(195Ya) used decomposition of isopropyl benzene hydroperoxide byFe’+ salts, in the presence of dihydroxymaleic acid, to initiate polymerization at 5°C of various monomers (includingisoprene) with methacrylic acid. The preparation of crosslinked carboxylated NBR and SBR containing methacrylic acid. a chloroethyl methacrylate, and111- andp-diisopropylbenzene and crosslinker has been described by Ivanova and coworkers (1970). Polymerization was carried out at 30°C at pH 3.0. Despite the activity on polymerization of carboxylated rubbers in emulsion i n the 195Os, the topic has remained of interest, and the terpolymerization of butadiene, acrylonitrile, and methacrylic acid was investigated further by Jerman and Janovic (1984). Polymerization was carried out at 50°C using potassium persulfate as initiator and sodium dioctylsulfosuccinate as emulsifier. When polymerizations were stopped at low conversions, the experimental and theoretical copolymerization data for the terpolymers were in good agreement. Although there was no point of true azeotropic composition, where the composition of the polymer at all degrees of conversion corresponded with the monomer composition, a “pseudo-azeotropic” region was recognized. Okubo et al. ( 1 987) described the localization of carboxyl groups at the particle surface during the polymerization of styrenehutyl acrylate/methacrylic acid. A terpolymer“seed” emulsion was prepared at low pH. and then polymerizationof styrene andbutyl acrylate was continued at high pH. The resulting particles in the emulsion had carboxyl groups located predominantly at the surface. Kalinina et al. (1996) claimed that copolymerization of methacrylic acid with butadiene, styrene, and/or acrylonitrile resulted in a localization of carboxyl groups in the surface layers if conversion of the main monomers was high at the moment of addition of the methacrylic acid. Introduction of methacrylicacid at 56% conversion of the main monomers resulted in improved resistance of the latices to mechanical effects. When methacrylic acid was replaced by acrylic acid, the degree of conversion at which the acid was introduced did not appreciably affect properties. Okubo (1990) patented a process for producing hollow carboxylated latex particles.
3.2 Preparation by Polymer Modification Carboxylated elastomers have also been produced by grafting unsaturated acids onto polymers. but the products of such reaction do not appear to have become commercially significant. Prior to 1950 such reactions had been conducted primarily with natural rubber, using either grafting in solution or grafting by mechanically induced reaction on the mill. This work includes reaction of maleic anhydride with rubber and its subsequent hydrolysis to carboxyl groups. Cuneen et al.
Dunn
564
( 1 960) described the graftingof thioglycolic esters ontonatural rubber latexusing hydroperoxide
initiators and the subsequent formation of the acid by hydrolysis. Jenkins and Duck ( 1975) described the grafting of thioglycolic acid onto polybutadiene. A 6% solution of polybutadiene, treated with benzoyl peroxide and thioglycolic acid for 24 hours at 50°C, was said to give 85% incorporation of the acid. The peroxide constituent could also be produced in situ by blowing air through a mixture of a thiol acid and rubber in toluene. The same authors describe grafting p-mercaptopropionic acid to polybutadiene on atwo-roll mill by milling in thepresence of benzoyl peroxide for 38 minutes at 38°C. Sanui et al. ( I 974) introduced from > 1 to about 16 mol% of thioglycolic acid derivatives into polypentenamer. They used free radical addition reactions and subsequently hydrogenated the polymers.Tanaka and MacKnight (1979) prepared carboxylated three-membered ring derivatives of a polypentenamer by carbene addition of ethyl diazoacetate using a copper catalyst. Again the products were fully hydrogenated.This method of preparation introduced 5-10 mol% of the three-membered rings. The carboxyl groups couldbe hydrolyzed to the acid or neutralized to form salts. Reaction conditions could be chosen to prevent backbone degradation and crosslinking during modification. The preparation of cyclized polydiene rubbers containing pendant photosensitive p-unsaturatedcarboxylicacids,suitable for useasphotoresists, has been described by Azuma et al. (1980). In a typical reaction, 10 g of cinnamic or acrylic acid was added with stirring to 0.5 g of polydiene in 25 cm3of chlorobenzene containing 0.12 g ofp-toluenesulfonic acid. The reaction was terminated by addition of triethylamine. The process for polymerization of ethylene propylene elastomers is not suitable for the preparation of carboxylated EPM or EPDM. The comment made by Jenkins and Duck ( l 975) that it has proved difficult to graft carboxyl-containing reagents onto these rubbers is still true. They drew attention to the observations of Gaylord et al. (l972), who injected a live styrene maleic anhydride copolymer into an extruder containing ethylene propylene rubber. The living copolymer reacted with the rubber, and a carboxylated terpolymer was formed on hydrolysis. Joshi (1979) has described the chlorocarboxylation of polyethylene. EPDM. and other rubbers. generally at l 10°C, withchlorineandmaleic anhydride (M) andsimilarmaterials to givea carboxylated elastomer. The key reactions were given as
+ Cl.
P-H
-
P.
+ HCI
is polymer and P . is polymer radical.
where P-H P.
-
+ c12
P"CI (chlorination)
+
+ c1
1;ISl
P. M-P-M. (carboxyl grafting with maleic anhydride and
-
P-M.
+P
P-".
+ P-".
hl
P-M (propagation) rw
its propagation)
+ P.
+
P = M P - M (termination by disproportionation)
After reaction, the elastomer solution may be emulsified with excess water and then steamdistilled. The solvent may thus be recovered without hydrolysis. EPDM rubbers containing ethylidene norbomene as termonomer have been llletal~atedin solution at tertiary hydrogen centers in the termonomer (Amass et al., 1972). Treatment of the
Carboxylated Rubber
565
alkali metal salt with CO2 was said to generate the carboxylate salt of the rubber. Metallation of EPDM or of butadiene- or isoprene-based rubbers was reviewed by Schulz et al. (1982). They note that when a lithiated polymeris exposed to carbon dioxide to form “cOOLi(CO0H) groups, an intractable gel forms almost immediately.
4. COMPOSITION OF CARBOXYLATED EMULSION POLYfvlERS The principal monomers used in commercially available carboxylated rubbers made in emulsion are acrylonitrile or styrene and/or butadiene or isoprene. The acrylic-type carboxylic acids with which they are copolymerized include acrylic acid, methacrylic acid, sorbic acid, P-acryloxYpropionic acid, &acrylic acid, 2-ethyl-3-propylacrylic acid, vinylacrylic acid, Cinnamic acid, maleic acid, and fumaric acid. Brown and Gibbs (1955) found that at 4% 01’ less Conversion, copolymers of butadiene and acrylic acid charged at levels of 5.3, 10, and 20% gave 1.6, 4.2, and 1 1.3%, respectively, of acrylic acid in the copolymer. This was attributed to IOW solubility of acrylic acid in the hydrocarbon phase. On the other hand, charges of 5 and 10% by weight of the hydrocarbon soluble methacrylic acid resulted in 10.4 and 20%. respectively, Of methacrylic acid in the copolymer. The hydrocarboninsolublesorbicacidcharged at 6and 10% resulted in 3.8 and 4.596, respectively, of sorbic acid in the copolymer. In a system containing butadiene and acrylonitrile in a ratio of 55/45, replacement of acrylonitrile by various quantities of methacrylic acid resulted in the expected composition at both low and high conversion. The distribution of carboxyl groups was seen to be fairly unfirom in these polymers. This was also seen at butadiene/acrylonitrile ratiosof 67/33. At low acrylonitrile levels,for example,butadiene/ acrylonitrile, 86.1/13.9, the system behaved more like butadiene. When acrylonitrile was replaced by 0.1 equivalent of methaclylic acid, the acid content of the polymer was 0.23 equivalent at low conversion and 0.1 equivalent at high conversion, i.e., the polymer was nonuniform at IOW acrylonitrile levels and would need incremental addition of monomers to assure homogeneity. Dolgoplosk et al. (195%) showed that copolymerization of isoprene and methacrylic acid (like that of butadiene) resulted in nonuniform acid distribution, with the majority of the acid being incorporated at low degrees of conversion. Dolgoplosk et al. (19594 noted that relative amount of 1,2 and 3,4 1,4-cis and 1,4-trrrns structures in copolymers of isoprene or butadiene with methacrylic acid were not appreciably different from the amounts of these structures in the emulsion homopolymers of isoprene or butadiene. At low levels of acid (up to 2%) the Tg was not affected, but there was some increase in Tg when the acid concentration was raised to 3% or more. Jerlnan and Janovic (1984) noted thatthe Tg of butadiene-acrylonitrile-methacrylicacidterpolymersincreasedmarkedlywith increasing acid content in the range from 19.5 to 66.2%. The Tg in this range increased from 19 to 224°C. Brown ( 1963) noted that at carboxyl levels exceeding 0.1 equivalent per hundred parts of rubber (ephr),raw polymer green strength develops because of hydrogen bonding. A butadienemethacrylic acid polymer with 0.377 ephr carboxyl had a tensile strength of 21 MPa at 365% ultimate elongation, while at0.54 ephr, tensile strength was 61.2 MPa at 25% ultimate elongation. As might be expected, this strength is lost as the temperature is raised.
5. VULCANIZATION OF CARBOXYLATED RUBBERS 5.1
Metal Oxide Crosslinking
Brown and Duke ( 1954) pointed out that one of the outstanding differences between carboxylic and regular nitrile rubbers is very high green strength in pure gum recipes. Furthermore, the
Dunn
566
highstrength may be achieved using zinc oxide asthesolecrosslinkingagent. As carboxyl content increased from 0.01 to 0.05 ephr, the tensile strength of the zinc oxide-cured stock was found to increase from 3.4 to 25.6 MPa. Whereasthe level of zinc oxide i n conventional rubber vulcanizates is relatively unimportant, provided that more than 1.0 phr is present, the level of zinc oxide in carboxylated elastomers profoundly influences the properties. Brown andGibbs ( 1955) cited asevidence of interaction between zincoxide and carboxylic groups the dependence of tensile development on both zinc oxide and carboxyl content, the disappearance of zinc oxide, as seen by x-ray examination, and the liberation of palmitic acid equivalent to the carboxylic content when the elastomer reacted with zinc palmitate. When neartheoretical quantities of zinc oxide were used. the gum stocks were almost transparent, but at higher levels the vulcanizates were opaque. About twice the theoretical amount of zinc oxide was found to be required for development of optimum stress-strain properties. Brown (1963), in a comprehensive review of the crosslinking of carboxylic elastomers, points out that in its initial reaction with a carboxylic elastomer the zinc ion may form a bond between carboxyl groups on the same chain as well as those on different chains and also that the zinc might form a basic salt such as --COO.ZnOH, which would not constitute a crosslink. Sat0 ( 1983) examined the zinc oxide crosslinking of carboxylated SBR containing 0.616 gram equivalent of acid per kilogram of polymer. Both tensile and 300% modulus increased withzinc oxide level andthenleveled off at aloading of 6-7 phr in theabsence of other curatives or fillers. Ultimate elongation leveled off at about 5 phr zinc oxide. In this phase, 3.5 phr zinc oxide would have been sufficient to neutralize all the acid. The classical chemistry of ionic bonding would require one zinc ion to bind two acid groups thus: 2R.COOH
+ ZnO
-
(R.C00)2Zn
+ H70
Sat0 pointed out that if ion clusters were responsiblefor the crosslinksin carboxylated elastomers, as had been suggested by Tobolsky et al. (1968). the basic salt R.COOZnOH would also contribute to crosslinking. The concept of ion clusters is discussed further in Section 5.2 Although the crosslinked XSBR did not need carbon black to develop tensile strength, reinforcing fillers were required to give a practical level of 300% modulus. The increase in Young’smodulus i n thepresence of carbon black is much moremarked in XSBR, which contains zinc oxide, than in regular SBR. Thiswas attributed to an enhanced effect of the surface characteristics of black in XSBR. The ionic bonding i n XSBR was found to lose its effectiveness at elevated temperatures. Tensile strength decreased rapidly above 60°C. and at 100°C only 5 % of the tensile strength remained. Similar observations were reported by Brown (1963). Brown (1963) noted that even monovalent ions enhance the tensile strengthof carboxylated elastomers. A butadiene-methacrylic acid copolymer containing 0.1 18 ephr carboxyl exhibited a tensile strength of X . 7 MPa, but if treated with aqueous sodium hydroxide, it had a microtensile strength of 11.7 MPa at an ultimate elongation of 900%. This would appear to be consistent with the theory of ion clusters. The same polymer crosslinked with zinc oxide had a tensile strength of 41.4 MPa at an ultimate elongation of 400%. Brown (1963) discussed the crosslinking of carboxylated elastomers with many polyvalent metal ions derived from metal oxides, hydroxides. salts of acids weaker than acetic, and salts readily eliminated from the crosslinking site. Zinc, lead, calcium, magnesium,barium, cadmium, and aluminum are amongthe ions employed in the crosslink. Calcium saltsused include silicate, sulfide. and hypochlorite; zinc salts include those of 2-mercapto-thiazoline and 2-mercaptothiazole. Lead has been used as the monohydrated tribasic lead salt of maleic acid. tin as dibutyltin oxide, and beryllium as the 2-ethylhexanote. The weak acid salts have enabled the incorporation of metals such as aluminum nickel, chromium.manganese, and tin. The highest tensile strengths
Carboxylated Rubber
567
and elongations were recorded for crosslinking by zinc and lead oxides. Zinc salts gave much poorer properties in the same carboxylated elastomer. Dolgoplosket al. (1959b) studiedtheproperties of butadiene-styrene-methacrylicacid terpolymer with 1.5 wt%
5.2
Ionic Clusters in Metal Oxide Crosslinks
The concept of ionic clusters in metal oxide crosslinking of carboxylated elastomers was introduced by Tobolsky et al. ( 1968) and reviewed in detail by Jenkins and Duck (1975). It was conceived that hard ionic clusters are dispersed throughout an amorphous rubbery matrix and serve as a reinforcing “filler” and quasi-crosslink. It seems unlikely that the volume of clusters is sufficient for them to provide reinforcement. The formation of clusters was believed to arise from the unfavorable situation of ionic salts dissolved in a hydrocarbon medium. Even in the absence of oxides, clusters are produced by the aggregation of un-ionized acid groups. As in the case of block polymers, elevated temperature tends to break up the cluster and remove the reinforcement. The presence of ionic clusters in ionorners. defined as polymers having a hydrocarbon backbone containing pendant acid groups, which are neutralized partially or completely to form salts, was discussed in depth by MacKnight and Earnest ( 1981) and by Bazuin and Eisenberg (1981). Eisenberg (1970) postulated the existence of “multiplets,” each of which is a group of ion pairs with no hydrocarbon content, and “clusters.” which are a loose association of ”multiplets.” The minimum-size multiplet would be a single ion pair. and the maximum generally eight ion pairs. The multiplet would be completely coated with a hydrocarbon skin.
568
Dunn
Pineri et al. (1974) discussed the evidence for ionic clusters in salt-neutralized butadienemethacrylic acid copolymers. In a telechelic liquid polymer with 2% acid, small-angle x-ray scattering indicated multiplets of mean radius OS6 nm at mean distances of 7 nm. In the case of the copper salts, these structures comprised two Cu2+ and four R.COO ions with two H 2 0 or R.COOH molecules. In high molecular weight polymers with 9% acid, larger clusters were evident. Meyer and Pineri (1976. 1978) studied ion clustering in butadiene-styrene-4-vinylpyridine terpolymer crosslinked by coordination with nickel chloride or iron(II1) chloride. Electron microscopy, small-angle x-ray (SAXS), small-angle neutron scattering (SANS), andMossbauer spectroscopy all indicated clustering. Electron microscopy indicated clustersof diameters 5- 100 nm. with the majority being under 10 nm. In the iron complexes (Meyer and Pineri, 1978), size distributionmeasurement by thismethodwas not regarded as reliable.Overall.theirstudy indicated that dimers represented 20-40% of the iron, and clusters represented 40-60% of the iron complexes. Mossbauerand small-angle x-ray scattering indicated that 90% of the clustered complexes were under 3 nm in diameter, and on average there were 30 complexes per cluster. Sat0 (1 983) reported that a transmission electron micrograph of XSBR reacted with 3 phr zinc oxide indicated particles of fairly uniform size distribution and about S nm diameter. Bazuin and Elsenberg (198 1) rationalized the properties of metal oxide-cured carboxylated polymer as follows: These rubber based ionomers possess anunusual combination of properties-high initial modulus, high elongation at break and low permanent set. The low permanent set can be explained by the presence of a small concentration of highly stable crosslinks, namely multiplets. The high elongation atbreak can be attributed to the relaxation by ionic bond interchange of the strained network elements which would initiate rupture in permanently crosslink systems. Finally, the presence of clusters which act as reinforcing filler (but which can fall apart under lower strain values than multiplets) can account for the high initial modulus.
Mandal et al. (1993) found by dynamic mechanical studies that, while sulfur vulcanized XNBR showed a single transition in the - 7 to - 3°C range, zinc oxide or zinc oxide-sulfur vulcanized XNBR had a second transition around 55°C. This second peak rose with increased loading of silica filler. This was attributed to the formation of ionomers from XNBR during zinc oxide crosslinking and the stabilization of these ionomers by silica filler. Mandal et al. (1995) confirmed, from the variation in properties such as storage modulus (E”)and loss tangent (tan6), with temperature that there were two transitions in zinc oxide-cured XNBR. At low temperature this was interpreted as the glass-rubber transition (Tg), while the high-temperature transition was attributed to the formation of ionic clusters. Again, reinforcing silica filler made the high temperature transition more prominent and high filler loading produced an increase in transition temperature.
5.3
Metal Oxide-Sulfur Crosslinking
It was pointed out in Section4.1 that metal oxide crosslinking of carboxylatedelastomers enhances tensile strength, modulus, and hardness, but that the metal carboxylate crosslink is labile and subject to high compression set and loss of strength at elevated temperatures. Butadiene-based carboxylated elastomers are also curable with conventional sulfur systems. Since most sulfur cure systems contain zinc oxide, a “mixed” crosslinking system is produced using normal cure systems. Such crosslinking systems. which have been described by Chakraborty and coworkers ( 198 1 b), result in improved compression set with some loss of tensile strength. Premature cure (scorch) presents a problem in “mixed” systems as in metal oxide cures, and this will be discussed further in Section 6.
Carboxylated Rubber
569
Brown (1957) noted that a carboxylated butadiene-acrylonitrile copolymer having a carboxyl content of 0.1 ephr had a tensile strength of 54 MPa at 475% ultimate elongation when cured with zincoxide andphthalic anhydride. Whenthis vulcanizate was subjected to a Peacheytype vulcanization with SO2and H2S,the tensile strengthdropped to17 MPa at 395% elongation. Brown (1963) illustrated the effect of “mixed” crosslinks using a 55/35/10 butadiene-acrylonitrile-methacrylic acid terpolymer. The cure with zinc oxide alone was characterized by high tensile strength and poor stress retention. Crosslinking with sulfur and zinc dimethyl dithiocarbamate produced good stress retention but low tensile strength. When the crosslinking systems were combined, the properties of metal salt crosslinks were manifested at short cure times. The properties became more typical of sulfur cures at long cure cycles. Jenkins and Duck (1975) reported similar behavior for a butadiene-cinnamic acid copolymer. Bhowmick and De (1980) studied the effectof curing temperature on the technical properties of NBR and XNBR. They noted the slower cure rate of XNBR compounds, which was attributed to restriction of motion by steric hindrance and molecular interaction (Tsekhanskii. 1973). They also noted that Zn0,-sulfur vulcanization systems were less scorchy than ZnO? alone. In a mixed crosslink system. vulcanizate properties were little affected by the ratio of sulfur to accelerator.NBRvulcanizates in a comparable recipewereshown to have lower crosslink density than corresponding vulcanizates of XNBR. Beekman and Hastbacka ( 1 986) discussed the effect of replacing half of the zinc oxide in a zinc oxide-sulfur cure of XNBR by magnesium oxide orbasic magnesium carbonate. They also replaced all of the zinc oxide by magnesium hydroxide. Scorch time was reduced when zinc oxide was partiallyreplaced by magnesium oxide and compression set was increased. However, tensile properties were improvedsomewhat and the Pic0 abrasion index was increased. Biswas and Basu (1996) examined cure synergism in XNBR vulcanization by thiophosphoryl disulfides in presence of amine disulfide/thiazole accelerators. Combination of thiophosphoryl disulfides with N-oxydiethylene 2-benzothiazole sulfenamide (OBTS) produced the highest mutual activity regarding physical properties such as higher tensile strength and lower oil swell. Agingresistance also benefited from the in situ formation of zincdialkyldithiophosphates. The number of sulfidic crosslinks produced by reaction between carboxylic acid groups and thiophosphoryl disulfides was said to control both network structure and vulcanizate physical properties.
5.4
Metal Oxide-Peroxide Crosslinking
Peroxides and metal oxides crosslink carboxylated elastomers independently. When they are used in combination, compromise properties are found. Brown ( 1957) notes that use of peroxide in conjunction with metal oxide cures to reduce permanent set is preferable to sulfur vulcanization, since the effect on tensile strength is less deleterious. Brown (1963) indicated that superimposition of metal oxide curing on peroxide curing increased modulus, tear, and hardness and improved retention of properties at elevated temperature and in ASTM oil. The peroxide imparted improved compression set using butadiene-acrylonitrile-methacrylic acid terpolymer (70/20/10). Brown (1963) cites studies by Miller et al. (19%) on carboxylated nitrile rubber crosslinked with ( 1 ) a constant amount of dicumyl peroxide and varying amounts of calcium hydroxide and ( 2 ) a constant amount of calcium hydroxide and varying amounts of dicumyl peroxide. The compression set was reduced as the relative amount of dicumyl peroxide was increased, but tensile and tear strength decreased at the same time. Jenkins and Duck (1975) stated that the physical properties of peroxide-cured ethylene propylene rubber were improved by the addition of coagents such as maleic or fumaric acid, especially in the presence of metallic oxide. It was suggested that the acid components were
Dunn
570
grafted to the copolymer during vulcanization and the oxide then formed metallocarboxylate crosslinks. Chakraborty ( 1983) examined the effect of combining peroxide, sulfur. and metal oxide crosslinking. He concluded that the carboxyl group reacted with metal oxide at an early stage during cure. while sulfur vulcanization occurred at a later stage. If insufficient metal oxide was present, the sulfur vulcanization was sped up, perhaps because the free carboxyl groups helped solubilize zinc ions. Addition of peroxide to a metal oxide sped up cure. but addition of sulfur and/or accelerator reduced cure rate. Tensile strength was found to be the same in all mixes, but tear strength depended on the content of sulfur crosslinks. The higher the proportion of sulfur crosslinks. the better the tear resistance. Flex resistance deteriorated with increasing metal carboxylate content.
5.5
Epoxy Crosslinks
Brown (1963) reviewed the crosslinking of carboxylic elastomers by epoxy compounds. The resulting properties are similar to those of systems crosslinked with zinc oxide and sulfur, and the vulcanizntes are reinforced by carbon black. When large quantities of epoxy resin are used. the resin may serve as a reinforcing agent in place of carbon black. A combination of epoxy resin and metal oxide crosslinking was said to be superior to that produced by metal oxide alone. Stl-ess-strain properties and abrasion resistance produced by this cure system were excellent. This combinationwasinferior to zincoxide-tetramethylthiuramdisulfide in hot air and oil resistance. Hayes (1960) reported unusually high tensile strengths and ultimate elongations for a combination of salt and epoxy ester crosslinks in a butadiene-methacrylic ester copolymer. Comparable data for metal oxide-sulfur crosslinks were not shown. Magnesium or barium oxides gave stronger vulcanizates than zinc oxide under these conditions. Blackshaw ( 1981) found that when epoxidized hydrocarbyl compounds (such as epoxidized linseed oil) were incorporated into carboxylated butadiene-acrylonitrile copolymer they served a s plasticizers during processing but were capable of acting as satisfactory vulcanizing agents. The presence of materials such as epoxidized soybean oil enhanced tensile strength and abrasion resistance and reduced compression set. Chakraborty and De (1982a) studied the effect of crosslinking a highly carboxylated NBR with an epoxy resin (the diglycidyl ether of bisphenol A). More than 3 phr resin was required for satisfactory cure. Increasing levels of resin resulted in an increase i n crosslink density, a decrease in compound Mooney. and a decrease in scorch time. The optimum level of epoxy resin was found to be 7.5 phr, since scorch time decreased rapidly at higher levels.
5.6 Other Crosslinking Systems Crosslinking by esterification has also been achieved using polyols (Brown, 1963). Theinteraction of butadiene-styrene-methacrylic acid (70/30/4) latex with polyethylene glycol in the presence of orthophosphoric acid was investigated by Skomyakova et al. (1961). Films obtained on heating at high pH at150°C had tensile strengths of4.4 MPa if orthophosphoric acid was present. In the absence of acid, tensile strength was about 3.3 MPa whether ethylene glycol was present or not. Strength increased with pH, which was attributed to the reinforcing effect of a complex between glycol and alkali. Since strength increases somewhat with pH even in the absence of glycol. this explanation does not appear to be sufficient. Brown (1963) described the use of polyamides as crosslinking agents for carboxylated elastomers along and in combination with metal oxides. sulfur. or peroxide systems. Carboxylic elastomers will react with hexamethylenediamine at 80°C to give salt-crosslinked vulcanizates
Carboxylated
Rubber
571
with high tensile strength and high compression set. On heating above 125°C tensile strength decreases and compression set improves. This wasthought to be because ammonium salt crosslinks are replaced by permanent amide crosslinks. Since amides react at low temperatures. they present a serious scorch problem in dry rubbers. Springer (1983) claimed that when up to 50 phr of a particulate metal reinforcing agent, comprising a nickel-chromium alloy with smaller dispersed particles of titanium, was included in a sulfur vulcanizate of carboxylated NBR. the metal particles became bound. This was said to result in higher gloss, density. toughness, and abrasion resistancein drill pipe protectors made from the compound.
6.
SCORCH AND BIN STORAGE STABILITY OF CARBOXYLIC ELASTOMERS
6.1 Compounds Containing Metal Oxides Brown and Duke (1954) stated that carboxylicrubbers mixed withzinc oxide cure at room temperature on standing. They also warned that zinc oxide stocks were scorchy on the mill. Organic acids and anhydrides were said to prevent room-temperature cure and improve processability and tensile strength of the vulcanizates. They recommended that zinc oxide be added near the end of the mixing cycle. Brown and Gibbs (1955) added silica, boric acid, and amines to the list of “controllers” of scorch and highlighted succinic anhydrides as particularlyeffective cure retardants in carboxylated NBR. They proposed the following reactions as possibly being responsible for improved scorch and storage stability: Devulcanization followed by reformation of crosslinks in a more uniform distribution among elastomer chains 2. Reaction with polymer - C O O . M . O H to yield polymer “COO.M.0CC.R’ 3. Salt formationwithunreactedzinc oxide 1.
The fact that zinc stearate and zinc phthalatedo not crosslink XNBR as effectively as zinc oxide was cited as supporting evidence. Brown (1963) recommended that the amount of “controller” used should be chemically equivalent to at least one-fifth the carboxyl content of the polymer and preferably from one quarter to one full equivalent. According to Zakharov and Shadricheva (1963), maleic anhydride completely prevented sulfur and metal oxide scorching in XSBR containing zinc and magnesium oxides, sulfur, and TMTD. The cure rate was also retarded. The use of magnesium stearate in place of magnesium oxide was said to reduce scorch with little effect on vulcanization and some improvement in vulcanizate properties. More recent observations include that of Jenkins and Duck (1975) that butadiene-methyl methacrylate copolymer may be vulcanized by barium hydroxide octahydrate. Scorch is reduced because the active bariumoxide must be formed in situ. Rigbi(1985) has observed that zirconium oxide compounds are less scorchy than those containing zinc oxide and physical properties are better. 6.2
Scorch Control Using Coated Zinc Oxides
Hallenbeck ( 1973) sought to obtain scorch control by isolating the carboxyl groups from zinc oxide during the milling. processing, and storage stages and releasing the zinc oxide at curing
Dunn
572
temperatures. He obtained improved scorch control in carboxylated NBR and polybutadiene by coating the zinc oxide with zinc sulfide or zinc phosphate. Scorch could also be controlled by adding metal alkoxide to the carboxylated elastomer before the other compounding ingredients were added. The amount of coating or alkoxide required for best results was found to depend on the type of carboxylic elastomer. With zinc sulfide coating, the Mooney at 100°C steadily decreased as the percentage of ZnS increased. Increasing the percentageof zinc phosphate above 2.65% had no effect. With alkoxides, there was an optimum level above which Mooney values rose again. Zinc sulfide not only increased the scorch time of fresh compounds at 125"C, it also increased the scorch time of 30-day stored compounds even more. The use of blocking agents did not seriously affect physical properties, although there was a gradual loss of modulus and tensile strength and an increase in ultimate elongation in most compounds.
6.3 Scorch Control Using Zinc Peroxide Masterbatch Bryant (1970) discussed the use of zinc peroxide masterbatch as a means of introducing salt crosslinks into carboxylated NBR with reduced scorch problems. It was made clear that it is unwise to make zinc peroxide-rubber masterbatches by milling or internal mixing. The zinc peroxide may decompose at the temperatures generated by frictional heat in these operations, with the possible ignition of the rubber. Rheometer data indicated that zinc peroxide reduced scorch and rate of cure while promoting a high ultimate state of cure. While coated zinc oxide also reduced scorch and rate of cure, it significantly reduced the ultimate state of cure. Zinc peroxide increased scorch time at125°C almost as well as coated zincoxide and affected modulus and tensile strength less. Regarding the stability of methyl ethylketonesolutions of XNBR in thepresence of crosslinking agents, zinc peroxide was less prone to increase viscosity than even coarse zinc oxide. Coated zinc oxide was still better but reduced the tensile strength of films more than did the zinc peroxide. Weir and Gunter (1978) noted that a 50/50 masterbatch of zinc peroxide in medium acrylonitrile content NBR gave twice the scorch time available with untreated zinc oxide. It also gave good bin stability and excellent physical properties.
6.4
Effect of Humidity on Cure Rate and Bin Stability
Bryant (1970) pointed out that humidity commonly has a serious effect on the scorch rate of rubbers; nitrile rubbers, whether carboxylated or not, are no exception. The scorch sensitivity of a compound in humid conditions depends on the accelerator. The commonly used MBTS is quitesensitive to humidity,as is TMTM. TMTD is not affected by humidity,nor is DPG, although DPG compounds become more scorchy as the stock ages. Bryant's data indicate that scorch time in the presence of zinc peroxide after 3 days of storage is reduced more as humidity is increased, but at all humidity levels scorch time is longer in the presence of zinc peroxide than with zinc oxide. Bryant's observations indicate that it would be advisable to use TMTD rather than MBTS or TMTM as accelerators for zinc oxide-sulfur vulcanization of XNBR.
6.5 Modification of Carboxylated Elastomers to Improve Scorch Safety Grimm (1983a) discussed the improvement of scorch resistance of carboxylated elastomers with succinic anhydride derivatives distributed throughout the rubber beforeor aftercoagulation. The derivatives suggested included alkenyl succinic anhydrides, alkyl succinic anhydrides, and their corresponding dicarboxylic acids. Alkienyl succinic acid was shown to give a time to five-point
Carboxylated Rubber
573
Mooney rise at 121°C in carboxylated NBR of 39 minutes as compared with 0.6 minute when succinic anhydride was used to control scorch in a zinc oxide-sulfurcure system. Grinm ( 1983b) claimed improved scorch resistance in carboxylated rubber when oligomerized fatty acid was distributed throughout the rubber. Oligomerized fatty acid comprised predominantly of trimer acids was specifically claimed. This process for improving scorch resistance was said to have little effect on cure rate. Grimm (1984) claimed that adipic acid. distributed throughout a carboxylated rubber before or after coagulation, improved scorch resistance.Time tofive-point Mooney rise at 121°C in carboxylated NBR was said to be 31 minutes. Grimm (1985) claimed. as scorch inhibitors of carboxylated rubbers, succinic acid derivative salts: 0 ll
R - C H - C - 0
where R is an alkyl or alkenyl moiety with 8-25 carbon atoms and M is a cation with n being I or 2 . Shaheen and Grimm (1985) discussed the propertiesof a carboxylated NBRthat contained scorch controllers added during the manufacturing process. These controllers permitted the use of small-particle-size surface-treated zinc oxide as the source of salt crosslinks with improved scorch safety and extended shelf life. It was pointed out that the coated zinc oxide was less expensive than zinc peroxide masterbatch. The surface treatment was said to have a tendency to keep moisture from reacting with the zinc oxide, but it was noted that precautions should be taken to guard against the catalytic effect of moisture. These included the use of dry fillers and elevated mixing temperatures. Presumably, bin storage under humid conditions would remain a problem. Starmer (1985) discussed yet another modifier, alkyl or alkenyl monocitrate, which might be dispersed through carboxylated NBR. Scorch and cure times were said to lie between those of “uninhibited” XNBR and those of compositions described by Grimm (19834. These compositions were said to deteriorate less rapidly than those based on succinic anhydride derivatives when stored under conditionsof high humidity. Monostearyl citrate in the presence of zinc oxide was specifically cited. Sat0 (1985) found that scorch resistance incompounds containing zinc oxide was improved by the addition of alkali metal salts of C I 2 - C l 8alkanoic acids, e.g., sodium stearate.
7. COMPOUNDING INGREDIENTS FOR CARBOXYLATED ELASTOMERS Normally. the same compounding ingredients are used in carboxylated elastomers as in their uncarboxylated counterparts. The discussion here will be confined largely to compoundingingredients that behave differentlyin carboxylated elastomers orthat have been the subject of detailed study.
7.1
Fillers
Weir and Burkey ( 198 1 ) noted that medium and semireinforcing carbon blacks such as N550. N650, and N774 give an excellent balance of compound viscosity and vulcanizate hardness.
574
Dunn
tensile strength, abrasion resistance, and cut growth on flexing. The type of black used has little effect on hardness and tensile strength. Highly reinforcing blacks are generally not required. since their only advantage is to increase the already high abrasion resistance at the expense of processability and cut growth resistance. The response to white fillers was said to be similar to that of uncarboxylated NBR, with precipitated silica giving the highest level of strength and abrasion resistance but an excessively high compound viscosity. Shaheen and Grinm (1985) compared precipitated hydrated amorphous silica and fumed colloidal silica as fillers in carboxylated NBR.The fumed silica produced astiffer stock before and aftercuring, with higher modulus. which resulted in higher Pic0 abrasion resistance andmuch lower water swellthan were imparted by precipitated silica. Increasing particle sizein the precipitated silica alsoaffected the properties. The large-particlesilicaproduced compounds that weremorescorchyandcuredfasterand resulted i n vulcanizates with lower hardness. modulus, and tensile strength; better compression set; poorer tear and abrasion resistance; and higher water swell. Chakraborty and De ( 1982b) found that coupling agents such as y-mercapto-propyl triethoxy silane. which would promote polymer-silica interactionin NR or an epoxy resin (diglycidyl ether-bisphenol A), which might be expected to interact with both carboxyland the silanol group of the silica. failed to enhance the properties of carboxylated nitrile rubber. They concluded that the carboxyl group was itself capable of interacting with silanol groups on the silica. It was noted that silica is more reinforcing than clay. Chakraborty and De (19x3) showed that in XNBR. carbon black obeyed the equation of Kraus ( 1 963):
where V,,) is the volumefraction of rubber in the unfilled (gum) vulcanizateand Vli. is the volume fraction in the filled vulcanizate, is the volume fraction of filler, and m is equivalent to 3c( 1 - V”’,,,)) V,,) - 1. where c is a parameter dependent on the filler but not on 4 or V,,,). The polymer-black interaction depended on the cure system in the following manner:
+
+
Ionic > ionic-sulfur (mixed) > sulfur > peroxide At low carbon black levels. tensile strength was found to depend on the cure system. At 50 phr black. tensile strength was similar for all four systems. At this loading. the strength was thought to be guided mostly by the slippage of polymer chains over filler surfaces. Bandyopadhyay et a l . (1995). using Monsanto rheometric, dynamic mechanical, and swelling techniques, determined that oxidized carbon black reacted with XNBR at elevated temperatures. The reinforcing ability of oxidized black exceeded that of its unoxidized counterpart. The promotion of interaction between surface oxidized carbon black and XNBR by (3aminopropylethoxy) silane was discussed by Bandyopadhyay et al. (1996a). Amine groups on the silane were said to interact with carboxylic acid groups on the XNBR. while ethoxy groups on the silane were believed to interact with hydroxy groups on the carbon black. The influence of silica content, silane coupling, and elastomer content on the physical and heat resistance properties of XNB vulcanizates was exanlinedby Byers et al. (1988). Bandyopadhyay et al. ( 1996b) studied the interaction between XNBR and precipitated silica and found that primary bonds were formed. This bonding was assistedby the addition of(3-aminopropylethoxy) silane at the expense of filler-filler networks. Dispersion was improved and crosslinking of the rubber matrix was promoted. Chakraborty et al. (1982)found that jute fiber markedly increased Young’s modulus both in the longitudinal and transverse directions and increased tensile strength in the longitudinal direction. However. it greatly decreased ultimate elongation and somewhat decreased tensile strength i n the transverse direction with a decrease in eiongation. At 40 parts jute. increasing
Carboxylated Rubber
575
silica level had little influence on physical properties. Anisotropy became marked at and above 40 phr jute.
7.2 Antioxidants Weir and Burkey (198 1) warned that strongly basic amines suchas 2-mercaptoimidazole should be avoided in carboxylated nitrile rubber because they can enter into crosslinking reactions with the carboxyl groups. Otherwise, the same protective systems may be used as in NBR.
7.3 Plasticizers and Extrusion Aids Weir and Burkey (1981) suggested dioctylphthalate as an economical plasticizer giving a good balance of properties. They recommended a polyester (Paraplex (3-50) or a saturated aliphatic ester of pentaerythritol (Hercoflex 600) forgood performance at elevated temperaturesand butyl carbitol formal (TP90B) orether thioether for low-temperature performance. Shaheen and Grimm (1985) recommended a polyester glutarate (Plasthall P7092) for high-temperature performance and nonextractability. For low-temperature performance, they recommended butyl carbitol formal andalso a diesterof triethylene glycol (PlasticizerSC). Both of the low-temperature plasticizers reduced hardness efficiently, and both imparted low water swell. Shaheen and Grimm (, 1985) examined the effect of stearic acid on XNBR. It was said to reduce compound viscosity significantly more than other lubricants. High levels of stearic acid prevented mill sticking but decreased scorch safety, perhaps through interaction with the surface treatment on the zinc oxide. At low levels of stearic acid, increasing the level increased the cure rate, with some sacrifice in tensile strength and Pic0 abrasion. TE80, polyethylene, and wax all led to very slow extrusion rates. Weir and Burkey ( 1981) recommended the use of TE80 or a fatty ester mixture on silica(Structol WB212) if mill, mold release,calender. or extrusion problems were encountered. They pointed out that since uncured XNBR is more thermoplastic than uncured NBR, the processing behavior of its compounds is generally superior. Weir and Gunter (1978) pointed out that the processability of XNBR compounds is sometimes marred by a tendency to adhere to metal. This could be overcome by increasing the level of stearic acid, by addition of TE80, or, best of all. by using low molecular weight polyethylene.
7.4 Tackifiers The data of Shaheen and Grimm (1985) indicate that not all materials recognized as tackifiers for rubber compounds improve autoadhesion (“tackiness”) or adhesion to metal (“stickiness”). Autoadhesion, but not metal adhesion, was improved significantly by a cumarone-indene resin (Cumar P-25). Both properties were improved by Hercotac A. A phenol-formaldehyde resin (SP1068) was said also to improve metal adhesion, although this was not obvious from the data presented. Cumar P-25 and Hercotac A both reduced modulus and tensile strength. increased ultimate elongation, and had no effect on heat aging. The phenol-formaldehyde resin (SP1068) did not affect tensile strength but reduced modulus and increased ultimate elongation and adversely affected heat aging.
8.
PHYSICAL PROPERTIES
8.1 Tensile Strength The high gum strength of salt and mixed crosslink vulcanizates of carboxylated elastomers and its origin have been discussed in Section S . Weir and Gunter (1978) record a tensile strength
576
Dunn
of 36 MPa for a medium-high carboxyl content, medium-acrylonitrile XNBR crosslinked with zinc peroxide andsulfur-TMTM. The compoundcontained only 5 phr silica as filler. Sat0 (1983) recorded a tensile strength of 30 MPa for XSBR compounded only with 8 phr of zinc oxide. When 50 parts N660 black was also present in the compound, the tensile strength remained at about 30 MPa, while modulus at 300% elongationwassubstantiallyincreased and ultimate elongation dropped from about 500% to slightly over 300%. Shaheen and Grimm (1985) comparedthe properties of regular and carboxylated NBR in a sulfur-TMTD-vulcanized system filled with 40 phr N660 black. The presence of carboxyl increased tensile strength from 18.2 to 25.5 MPa. Blends of the two polymers exhibited intermediate tensile strength. Weir and Gunter ( 1978) showed similar data for carboxylated and regular NBR and also showed that the difference in tensile strength was still more marked at elevated temperature. At 125°C the presence of carboxyl raised the tensile strength from 7 to 14 MPa. Chakraborty and De ( 1983) comparedthe properties of zinc peroxide, zinc peroxide/sulfurCBS. sulfur-CBS. and peroxide cures of carboxylated NBR at levels of 0-50 parts N550 black. In the absence of black, tensile strength was greatest (8.8 MPa) in the zinc peroxide-cured system and was under 2.0 MPa in the absence of zinc peroxide. At 50 parts black, the tensile strength remained greatest (21.66 MPa) in the zinc peroxide-cured system but exceeded 20 MPa in all systems. The reinforcingeffect of the salt crosslinks was overshadowed by the interaction between carboxylated elastomer and black.
8.2 Abrasion Resistance Weir and Gunter found that the NBS abrasion index improved over 10-fold, from 105 to 1200, when XNBR wascompared in the same compoundwith NBR of the some viscosity and acrylonitrile content. Shaheen and Grimm (1985) also indicated that XNBR had from 2 to I O times the abrasion resistance of NBR. Since XNBR is used “downhole” in oil wells, Matheson (1988) studied the abrasion resistance of XNBR at elevated temperatures. Chakraborty et al. (1981a) studied the abrasion resistance of N550 black-filled, mediumhigh carboxylated NBR crosslinked with different systems. The lowest abrasion loss (test method unspecified) was observed in a compound vulcanized with sulfur and CBS with no metal oxide present. A similar system with zinc peroxide masterbatch present gave slightly poorer abrasion resistance but had higher tensile strength. A carboxylated NBR with less carboxylation showed appreciably greater abrasionloss. It appears fromthis thatenhanced abrasion resistance is related to carboxylation but is not necessarily directly related to salt crosslinking or to tensile strength. Chakraborty and De (1982b) recorded abrasion losses of the same order (0.3 cm3/100 rev) for silica-filled XNBR as had been recorded for the N.550-filled compounds. Jonesand Smith (1995) described the abrasion resistance of hydrogenated XNBR as “unsurpassed.” Since abrasion resistance is regarded as the most important single attribute of carboxylated elastomers. further studies of the parameters that control it would be of great value.
8.3 Tear Strength Weir and Gunter (1978) noted an increase in both Die B andDie C tear strength when carboxylated NBR was compared with regular NBR. Chakraborty and De (1983) showedthat tear strength of carboxylated nitrile rubber in the absence of filler ranged from 7.92 kN/m for a peroxide cure to 17.25 kN/m for a zinc oxide-sulfur cure. In the presence of 50 phr N550 black, tear strength was greatly increased and ranged from 42.25 kN/m for the peroxide cure to 52.05 kN/
Carboxylated Rubber
577
Table 1 TearStrength of Unfilled and Black-FilledCarboxylated NBR
A XNBR
Stearic acid NS50 black Sulfur CBS
ZnOl masterbatch Optimum cure, min at 150°C Tear strength (Die C). kN/m
100 2 2.4 0.8 -
68 7.54
B
C
95 2
95
-
-
2.4 0.8 10.0 S2 13.74
-
2
D 100 2 40 2.4
0.8 10.0 180 55 13.21 27.55 32.89 -
E 95
2 40 2.4 0.8 IO.0 33
F 95 2 40
10.0 52 18.99
Solcrw: Chakraborty et al.. 19821.
m for a sulfur cure with no zinc oxide present. Chakraborty et al. (l%?), using unfilled and N550-filled compounds, recorded the data presented in Table I . The mechanism of tear was studied with the aid of a scanning electron microscope. In the gum mixes, tear was found to proceed through a stick-slip process. The number of tear lines was greater in mix A, which may have been responsible for its lower tear strength. Addition of carbon black to the zinc oxide-free sulfur system (compare D and A) resulted in knotty tear and a fourfold increase in tear strength. Addition of carbon black to the systems containing zinc oxide led to smooth tear and a lesser increase in tear strength. Chakraborty and De (1982b) examined the effect of 0-50 parts of silica or 0-46 parts of clay on XNBR crosslinked with Z n 0 2 masterbatchanda sulfur-MBTS curesystem. Silica increased the tear strength from 12.2 to 47.6 kN/m. Clay increased it from 12.2 to 30.4 kN/m. For gum vulcanizates, scanning electron microscopy again showed smooth lines with ripplings on the surface. The torn surface was rough in the presence of silica, and fracture was seen to follow the debonding of silica aggregates.The rough surface oftorn clay-filled XNB vulcanizates indicated good adhesion of clay to polymer. Such interaction of clay and polymer is not seen in regular NBR. Chakraborty et al. (1982b) found tear strengths around 50 kN/m for XNBR reinforced with jute fiber and silica, regardless of the cure system.
8.4 Dynamic Properties Sat0 (1983) found that carboxylated SBR crosslinked with zinc oxide exhibited a distinct second maximumintheplot of mechanical loss tangent (tans) againsttemperature. The firstpeak occurred during glass transition. The second peak was peculiar to carboxylated rubbers and occurred only when the bound acid was neutralized to form ionic crosslinks. Sato and Blackshaw (1985) studied this phenomenon further using carboxylated NBR. The shape of the second peaks and the temperatureat which they appeared varied with the metal ion used, but not in a systematic manner. The second peak was associated with the ionic clusters formed on interaction of XNBR with metallic oxide. In the case of zinc oxide. the peak height was a maximum at 5 phr. Since regular NBR, sulfur- or peroxide-cured, in the presence of zinc dimethacrylate did not show a second peak in tans, the second peak i n XNBR was associated with the formation of basic salt --COO.Zn.OH rather than the salt bridge ”COO.Zn.00C-. It could also be taken as evidence of the formation of ionic clusters by nletallic oxides in XNBR, which zinc dimethacrylate might not produce in regular NBR.
578
Dunn
Strong interaction between carbon black and the ionic clusters was indicated by a shift of both tan6 peaks to higher temperatures. Strong interaction was foundwith calcium carbonate, but not with hard clay or silica. The degree of interaction might be related to the basicity of the filler.
8.5 Compression Set and Stress Relaxation Brown ( 1963) showedthat in a zinc oxide-crosslinked gum vulcanizate of XNBR, compression set after 70 hr at 100°C was 95%, but if a mixed dicymyl peroxide-zinc oxide system was used, the lability of the salt crosslink was overcome, and set after 70 hr at 100°C was only 7%. In a mixed zinc peroxide masterbatch-sulfur-TMTM system, XNBR and regular NBR showed 65 and 69% set. respectively. Room-temperature stress relaxation at 100% extension of XNBR and with various crosslinking systems was studiedby Chakraborty and coworkers (1981a).In line with the observations on compression set recorded above, they found that a sulfur-cured vulcanizate containing 2.0 phr zincperoxidemasterbatchshowed little stressrelaxation.Whenthezincperoxidemasterbatch level was increased to 10 phr, relaxation became more marked. When the sulfur was omitted or zinc oxide was used fin place of zinc peroxide, relaxation became still more marked. 8.6 Oxidative Aging Resistance Very little systematic examination has been made of the oxidative aging resistance of carboxylated elastomers. Dolgoplosk et al. (1959a) indicated that in compounds filled with 30 parts channel black, carboxylated polybutadiene. polyiroprene. and SBR exhibited better retention of ultimate elongation and tensile strength after 144hours at 100°C than corresponding uncarboxylated polymers. Chakraborty (1983) showed good retention of properties of XNBR cured with several “mixed” systems containing dicumyl peroxide. His aging conditions, 24 hr at 100”C, wererathermild.Bhowmickand De (1980) hadindicated that zincperoxide-cured,highly carboxylated, medium-acrylonitrile NBR had poor retention of elongation and tensile strength after as little as 36 hours at 100°C. Retention was much better for zinc peroxide-sulfur cure systems. When NBR and XNBR were compared in the same zinc oxide-sulfur system, the regular NBR was found to have the better resistanceto oxidative aging. A similar observation was made by Shaheen and Grimm (1985). They concluded that XNBR continued to crosslink by reaction of zinc oxide with the carboxyl group during aging. It is also possible that the carboxyl group increases the susceptibility to oxidative aging. This could be resolved by comparing zinc oxide-free vulcanizates (e.g., peroxide cures) of NBR and XNBR. Jones and Smith (1995) found that hydrogenated XNBR had better heat aging resistance than XNBR, but poorer heat aging resistance than HNBR. This was attributed, in part, to the fact that the hydrogenated XNBR was sulfur cured. The observation confirms that some loss of aging resistance is seen when rubbers are carboxylated.
8.7 Ozone Resistance Brown and Gibbs (1955)attributed the apparently superiorozone resistance of zinc oxide vulcanizates of carboxylated polybutadiene to a high rate of stress relaxation. Weir and Burkey (198 1) pointed out that ozone and tlame resistance of XNBR can be improved by flux blending with poly(viny1 chloride) (PVC) at ahightemperature.Increasing PVC contentincreases ozone
Carboxylated Rubber
579
resistance andalso hardness, tear strength, and resistance to flame, but low-temperature flexibility decreases.
8.8
Hydrogen Sulfide Resistance
Resistance to hydrogen sulfide is important for elastomersused in certain oil-field environments. Shaheen and Grimm (1985) noted that XNBR outperformed regular NBR in H2S resistance. After %hour exposure to 1.38 MPa H,S at I2 1 "C. a conventional NBR vulcanizate lost 84% of its ultimate elongation. while an XNBR vulcanizate lost only 32%. Pfisterer and coworkers (1983) examined the behavior of vulcanizatesexposed to a 33/17/50 mixture of H2S/C02/ CH4 at 1.72 MPa plressure for 24 hours at 200°C. An N990 black-filled compound of highly carboxylated medium-nitrile NBR showed an increase in modulus at 100% elongation of only 20%. which compared favorably with a 100% modulus increase in a tluoroelastomer compound of 5%. Regular NBR compounds showed modulus increases of over 100%. Elongation losses recorded in this study were 4 0 4 5 % for the best XNBR and for the best NBR compounds.
9.
9.1
BLENDS OF CARBOXYLATED ELASTOMERS WITH OTHER POLYMERS Blends of Carboxylated and RegularNBR
Coulthard et al. (1976) examined the blending of NBR with XNBR as a route to improving tensilestrengthandabrasionresistancewithout loss of oil resistance. They sought mininlal increase in cost and compression set and minimal loss of scorch resistance and low-temperature flexibility. Using sulfur-zinc peroxide or sulfur-zinc oxide systems resulted in poor properties because of differing cure rates in the two rubbers. When sulfur donor-low sulfur or peroxide cures were used. cure rates were comparable in NBR and XNBR. The cure rates of the blends were also comparable, and physical properties of the blends, as well as abrasion resistance and compression set. were intermediate between those of the parent properties as expected. Bhowmick and De (1980) examined the effect of replacing 10 and 20 phr of regular NBR with XNBR in a low-sulfur/accelerator recipe. The minor quantities of XNBR increased 300% modulus but decreased tensile strength and abrasion resistance and increased compression set markedly. They noted that their results did not agree with those of Coulthard and coworkers ( 1976) andsuggested that this might be becausethe vulcanization characteristics of XNBR were not identical with those of the NBR in the recipe used.
9.2 Blends of Carboxylated NBR with Polybutadiene Weir and Gunter (1 978) noted that blending of 75 parts XNBR with 20 parts polybutadiene and using 10 parts zinc oxide masterbatch as curative improved the low-temperature flexibility of XNBR. The brittle point fell from - 40 to - 48°C in the presence of the polybutadiene. Furthermore, the already extraordinary Akron abrasion index of 273 was increased to 39 1. There was some loss in tensile and tear strength and. not surprisingly. an increase in swell in ASTM No. 3 oil from 1 1 to 37%. 9.3 Blends of Carboxylated NBR with Epichlorohydrin Rubber Nakata et al. (1976) noted that carboxylated nitrile rubber and epichlorohydrin rubber were crosslinked directly in the presence of DBU ( 1,8-diazabicyclo[5.4.O]undec-7-ene). This produced
Dunn
580
an ester linkage, and the resulting vulcanizate had much lower Compression set than conventional blend vulcanizates.
9.4
Blends of Carboxylated NBR with PVC
Brown (1957) stated that blends of carboxylated NBR with PVC were more easily prepared than those with regular NBR. He claimed that the XNBR-PVC blends extruded more smoothly, remained nonbrittle down to lower temperatures, and exhibited greater abrasion resistance and thermoplasticity. Schwarz ( 1980a) pointed out that the tendencyfor XNBR to crosslink during fluxing with PVC had been an obstacle to the production of XNBR-PVC. He said that proprietary blends containing nonstaining. nondiscoloring stabilizers were on the market. The XNBR-PVC compounds. whether black- or white-filled, extruded faster and swelled less than their NBR-PVC counterparts. although at low PVC levels the NBR-based extrudates were slightly smoother. Tear and tensile strength and especially abrasion resistance were increased when XNBR replaced NBR. XNBR blends were stiffer then NBR vulcanizates at low temperatures, and, as expected, increasing PVC decreased low-temperature flexibility. Some complex compression set results were noted. including lower set for black-filled XNBR-PVC counterparts. Up to a 70/30 XNBRI PVC ratio, dynamic ozone resistance was better than for NBR-PVC. At higher XNBR levels. dynamic ozone resistance was poorer because of the high modulus of the blend. Schwarz ( 1982) studiedXNBR-PVC blends for use in light-colored extra heavy-dutycable jackets. He noted a substantial increase in tensile and tear strength and an appreciably lower brittle point forthe XNBR-PVC blends compared to those ofregular NBR-PVC.At low teniperatures the XNBR-PVC compounds are very stiff, but they are not brittle. Replacing dioctylphthalate plasticizer by dioctylsebacate resulted in improved tensile strength and abrasion resistance. better low-temperature flexibility, and better aging resistance. Many of these attributes are consistent with lower plasticizer volatility. The presence of 0.25 part sulfur as coagent in a peroxidecuredsystemincreasedscorchsafety and raisedultimateelongationbeforeand after aging. Sulfur cannotbe included in light-colored lead press-cured compounds, sincelead sulfide. which is black, would rapidly be formed. It was noted that these compounds (XNBR-PVC blends) could be readily vulcanized in the electron beam. Shaheen and Grimm (1985)referred to the care that must be taken in fluxing XNBR-PVC blends.Theywarnedagainst using stabilizers that containzincions and suggestedcalcium stearate at 2 phr. They statedthat stabilizers containing zinc would promote scorch by formation of zinc salts with carboxyl groups. One part of stearic acid added during fluxing was said to improve flow properties. Schwxz (1983) compared XNBR and NBR-PVC blends for soft roll cover compounds. Overall. XNBR-based blends gave the highest physical properties with satisfactory processing. XNBR gave the best hexane swell resistance at a ratio of 100/60/120 XNBR/PVC/DOP. Bridgestone Tire (1984) examined blends in which both the PVC and the NBR were carboxylated. Tensilestrength was increasedfrom 29 to 45 MPa if nlagnesium oxide was present. A high fracture energy of 6 kN-m/m was claimed.
9.5
Blends of Carboxylated NBR with Regular NBR and PVC
Weir andBurkey (1981) showed that the substitution of some PVC and/or NBR in regular NBR-PVC blends with XNBR increased stiffness, tensile strength, tear strength, and abrasion resistance. Blackshaw (1984) indicated that replacing a 70/30 NBWPVC blend by a 30/40/30
Carboxylated Rubber
9.6
581
Blends of Carboxylated NBR with Chlorosulfonated Polyethylene
Since both chlorosulfonated polyethylene(CSM) and XNBR canbe crosslinked with magnesium oxide, Schwarz (1980b) postulated that magnesium oxide would covulcanize blends of these polymers. He found that modulus and tensilestrength of CSM increase when blendedwith XNBR and cured with magnesium oxide at room temperature. XNBR greatly increased oil and fuel resistance of CSM membrane compounds, which was foreseen as an advantage of the blend. Unfortunately, blending reduced the tear resistance and aging resistance of the compounds.
9.7
Blends of Carboxylated NBR with Chlorobutyl Rubber
BhattacharyyaandDe (1991) found that an unfilledmill-mixedblend of XNBRand CIIR, withoutaddedcuratives, was renderedinsoluble in chloroform aftermolding 60 minutesat 180°C. The physical properties of this “self-vulcanized” unfilled system were not spectacular. and no data were presented for blends containing fillers. The rubbers in this blend were shown to be immiscible.
9.8
Blends of Carboxylated NBR with Polychloroprene
Mukhopadhyay and De (1991) studied a mill mixed blend of XNBR and CR. After molding 60 minutes at 180”C, the blend was shown to be covulcanized. The covulcanization was confirmed by Monsanto Rheometry. solvent swelling and FT-IR analysis. The extent of crosslinking increased with increasing CR content. Mukhopadhyay and De (1992) discussed the properties of unfilled and ISAF black-filled mill-mixed 5050 blends of XNBR and CR, molded 60 minutes at 190°C. The products were insoluble in chloroform. DSC and dynamic mechanical analysis indicated that the rubbers were immiscible. The authors claimed that their system required less energy for processing since it contained no rubber chemicals. 9.9
Blends of Carboxylated NBR with Modified Natural Rubber
Rdmesh and De (1992) found that mill-mixed blends of XNBR and carboxylated NR were crosslinked during molding for 60 minutes at 160°C. Dynamic mechanical measurements indicated that the components of theseblendswerecompatible. The blendsweresaid to have excellent processing safety, good abrasion resistance, and excellent oil resistance, but the balance of physical properties as reported does not appear to provide useful practical vulcanizates at any blend ratio. Subsequently, Ramesh andDe (1993) examined XNBRas a compatibilizer for chlorinated NR and epoxidized NR, which are partially miscible. The quantity of XNBR required was found to depend on the blend composition. The physical properties of the blend were not discussed.
9.10
Blends of Carboxylated NBR with Polyacrylic Rubber
Roy and Das (1995) studied blends of XNBR with ACM. Processibility of the ACM was improved by blending and physical properties were also improved. IR spectra indicated that covul-
Dunn
582
canization had taken place on heating the blend. Preheating the blend of virgin polymers improved vulcanizate properties and delayed high-temperature degradation. Chowdhury et al. (1995) examined the covulcanization of 5050 blends of XNBR and ACM i n presence of lead oxide and2-n~ercaptobenzothiazole(MBT) or ethylene thiourea (ETU). They concluded that lead oxide enhancedthe effectiveness of MBT orETU as curatives, through the formation of lead sulfide and metal ion bridges. MBT produced a greater degree of cure than ETU, probably through the formation of monosulfide crosslinks.
9.11
Blends of Carboxylated NBR with Polyolefins
Coran and Patel (1 983) included carboxylated NBR in a study of compatibilized blends of NBR and polyolefins. The polyolefin used was triethylene tetramine-modified, maleic acid-modified propylene masterbatch blended 50/50 with XNBR. Unfortunately, no other rubber in the study appeared to have been blended with the same modified polypropylene. In any event, as might be expected. the blend containing XNBR had high tensile strength (19.2 MPa)and high modulus at 100% extension. and it also had relatively high ultimate elongation (250%).
9.12
Blends of Carboxylated elastomers with Polyamides
Several recent patents describethe use of carboxylated elastomers to toughen polyamides. Roura (1982) claimed that a blend comprising exhibited 6.8 kN-m/m notched Izod impact strength at -40°C compared with 1.4 kN-ndm for a control not containing Nylon 6.
Nylon 66 Nylon 6 Ethylcne-propylcne-1,4-hexadicnc-2,5-norbornadiene[sic] copolymer-fumaric acid adduct (1.5-2.0% acid) Ethylcnc-propylenecopolymer 1.4-hexadiene
45 36
IO 9
Mitsui Petrochemical (1984) studied blends comprising polyamide ( 1 00 parts), an ethylene propylene rubber (80/20 ethylene/propylene) grafted with 0.5 phr maleic anhydride (20 parts), and a l-butene-ethylene copolymer grafted with maleic anhydride (20 parts). The product was claimed to have excellent low-temperature impact resistance and water resistance. Bridgestone Tire (1983) described 70/30 blends of Nylon 12 and carboxylated NBR that cured with zincoxide to give test bars with54 MPa tensile strength and420% ultimate elongation. Without zinc oxide, the tensile strength was 21 MPa and ultimate elongation 190%. A 30/70 blend had 43 MPa tensile strength at 430% ultimate elongation. Toyoda Gosei (1984) claimed excellent ozone resistance for a composition comprising 30-60 parts polyamide and 70-40 parts carboxylated NBR of carboxyl content 20.5 with the rubber crosslinked 2 7 wt% during mixing in a Brabender Plastograph at 190°C.
9.13
Blends of Carboxylated EPDM with Polyethylene Terephthalate
Unitika Limited (1983) claimed the notched Izod impact strength of polyethylene terephthalate tripled when it was blended in 70/30 ratio with carboxylated EPDM. The latter was produced by blending an ethylene propylenerubber containing72% ethylenetogetherwithendo-bi-
Carboxylated Rubber
583
cyclo[2.2.1]-5-heptene-2.3-dicarboxylic acid anhydride and t-butyl peroxide in an extruder at 200°C in the ratio 1000/3/1.
10. APPLICATIONS FOR CARBOXYLATEDELASTOMERS 10.1 Uses of Carboxylated Latices The uses of carboxylated latices were summarizedby Brown and Duke ( 1954)under the headings Dipped Goods, Leather Finishing, Paper Industry, and Textile Industry. These headings cover most of the areas of application for which carboxylated elastomer latices are still being used. Dipped Goods Brown (1957)noted that because of the reactivity of carboxyl groups with themselves and with metal ions, wet films of excellent strength have been obtained from both uncompounded and compounded latex. These may be strengthened by hot-air curing. It was pointed out that sodium aluminate could be used as a water-soluble curing agent. On the other hand, polyvalent ions (e.g., Ca’+) could be derivedfrom the coagulant dip solution. The products listed included rubber gloves, protective clothing. pen sacs, bottle liners, and diaphragms. If carboxylated NBR was used, these products were resistant to facts and oils. Jenkins and Duck (1975)point out, with respect to advantages of carboxylated latices, that curing through the carboxyl permits sulfurless curing to give colorless, odorless products, and room-temperature curing.
Leather Finishing The use ofcarboxylated latices, particularly XNBR latices,in leather finishing was said to impart a drier hand, better penetration of the finish into the leather, and improved abrasion resistance to the leather. Brown (1957)also noted improved chemical resistance, flexibility, aging resistance, and impermeability to liquids and gases. Jenkins and Duck (1975)described the use of carboxylated latex for the reconstruction of scrap leather into a useful form. Pryer Coating
Brown and Duke ( 1 954)pointed out that a major advantage of carboxylated latices was their ability to penetrate thick sheets of paperboard because of their low viscosity and good wetting. Such impregnated paper boards were useful as oil-resistant seals, gaskets,and packings. Specialty papers saturated with water-soluble phenolic resins showed fivefold improvement in resistance to repeated folding if the resin was accompanied by an equal amount of nitrile rubber. Replacement of regular by carboxylated nitrile doubled the fold endurance. Jenkins and Duck (1975) discussed applications in paper coating at some length. Patents are still being issued in this field. Abakina et al. (1982)claimed a cold-weldable packaging paperwith a coating comprising33-42 wt% carboxylated butadiene rubber, 33-42 wt% polyisoprene latex, and 34-16 wt% filler. Textile Applications Jenkins and Duck (1975)discussed carboxylated latices as binders for nonwoven fabrics. They stated that the requirementsfor an ideal binder are high strength; location only atfiber crossover points; high adhesion; enhanced elastic recovery; and crease, light, and dry cleaning resistance. Carboxylated elastomers have greater polymer fiber bond strength than that of noncarboxylated
584
Dunn
elastomers because of their polarity and dramatically increased resistance to washing and dly cleaning. Diene polymers were saidto give thebest drape and handle, whilethe more polar polymers gave better adhesion to fiber, and acrylates gave the best cleaning resistance. Jenkins and Duck (1975) also discussed the use of carboxylated latices (mainly SBR) in foam and carpeting. The white fine-cell foam has been used in clothing inner linings, car liners, bathing suits, shoe insoles, and cushion coatings. Carboxylated latex gives good “quick grab” and high tuft anchorageintufted carpets, anda layer of foamlatexcanprovide a built-in underlay. Patents continue to appear on the use ofcarboxylated latices in textile applications. Burnett and Miller ( 198 1 ) described conveyor belting prepared from woven polyester fabric, a layer of polyester fibers entangled by needling,and a carboxylated NBR latex binder. The Taberabrasion weight loss of the belt was 0.7% after 1000 cycles. PPG Industries (1982) claimed carboxylated SBR latex as the principal component of an elastomeric coating to improve the weaving and handling qualities of glass fibers. Black and Garrett (1984) used a blend of ammonia, oxzidized polyolefin, SBR latex, organosilane, and water as a size to apply to glass fibers for improved reinforcement of polypropylene. Aclhesives
Jenkins and Duck (1 975) note that carboxylated latices are used for adhering more polar and more porous surfaces such as paper, wood, and leather, since they have the ability to wet or combine with water-wetted surfaces. Gevorkyan et al. (1981) describe the use of carboxylated polychloroprene for high-adhesion bonding of shoe soles to natural or synthetic leather uppers. The polymer was prepared by copolymerization of chloroprene with methacrylic acid and its esters. Hisaki and Suzuki ( 1989) discussed the use of carboxylated vinylpyridine-butadiene-styrene latex to adherepolyethylene terephthalate (PET) tire cord to rubber. Tire life was prolonged as compared to the life of tires containing vinylpyridine latex binder and the carboxylated latex was said to be used in truck and bus tires. The authors suggested that carboxylic groups reacted with accelerators (amines) and prevented them from migrating through the adhesive layer. They also proposed that the network structureof carboxylic and pyridyl radicals obstructed penetration of accelerators through the adhesive layer.
10.2 Uses of Solid Carboxylated Elastomers Adhesiws
The use of solid carboxylated elastomers in adhesives was one of the early applications of these materials. Such applications have been discussed by Jenkins and Duck (1975) and, in considerable detail, by Gross and Weber (1977). The solidcarboxylated elastomers aredissolved in solvents appropriate for the corresponding noncarboxylated polymers. If emulsifier residues are to be avoided, it is advantageous to polymerize directly in rubber to nonmetallic adherends, such as tire cord or glass, and metal to metal. In rubber-to-metal bonding, the carboxyl group reacts with the metal while the elastomer chains covulcanizewith the rubber substrate. Carboxylic polyacrylates are affectivelaminatingadhesivesandpressure-sensitiveadhesives,while carboxylated polychloroprenes are useful contact adhesives. A patent (Nitto Electrical Industrial Co. Ltd., 1980) describes uncured or partially crosslinked 70/30 blends of epoxy resin and carboxylated NBR extruded as a hollow cylinder to hold electrical elements and adherethem to a printed circuit boardby heating at 150°C for 30 minutes.
Carboxylated Rubber
585
Seals m c l 0-Rings Bryant ( 1 970) noted the usefulness of carboxylated NBR in shaft seals because of its abrasion resistance. Weir and Burkey (1981) reiterated the value of using XNBR in shaft seals and lip seals that must seal rotating or reciprocating shafts. They also noted applications in seals for the floating roofs on petrochemical tanks, where the seal must resist wearing and cutting as it travels up anddown the vessel wall. The developmentof hard vulcanizates from easy-processing compounds based on carboxylated NBR has led to its use in O-rings and packings. In some high-pressure applications, XNBR parts have been found to be supplementary to, or competitive with, elastomeric urethane. Oil-Field Aypliccrtions The list of oil-field applications for carboxylated NBR given by Shaheen and Grimm (1985) includes ram-type blowout preventers, drill pipe protectors, pumpdown pistors, mud pump pistons, pump impellers, drill bit seals, and rotary drilling hose. The applications utilize the high strength and abrasion resistance of NBR and the resistance of its vulcanizates to extrusion under pressure. Weir and Gunter (1978) noted a 10-fold increase in life span of XNBR high-pressure packings compared with NBR packings. Minirlg Applicatiows Blackshaw ( 1 984) discussed the use of carboxylated NBR in ball mill linings. Rubber is preferable to steel because of reduced noise level, power consumption, and cost. NR and SBR are not suitable when oils are used in the separation process. XNBR-based linings were described as providing the necessary oil resistance and increased service life in this application. It was also noted that XNBR-PVC blends were useful in skirt boards. chute linings, screens. and mining cable jackets. Roll Cornpouncls Bryant (1970) described the use of carboxylated NBR containing epoxy resin in black- and white-filled roll compounds. The use of a low-Mooney rubber was advocated in the presence of white fillers. Weir and Burkey (1981) cite applications in textile. steel, and paper mill rolls, a l l because of high hardness with easy processing and good wear resistance. They also note applications i n pipe, glass, and cargo conveyor rolls and i n copying machine rolls. Blackshaw (1984) noted considerable interest in the use of XNBR in rice hulling rolls i n the Philippines, India, and the United States. Surprisingly, the use of XNBR in rice hulling rolls was patented several years later (Anon., 1990).
Dunn
586
Schwarz (1986) compared the use of NBR, XNBR, and their blends with other polymers in various types of roll covering. XNBR was said to impart improved abrasion resistance and a slight improvement in compression set without loss of oil resistance. Footbrvar.
Bryant (1970) described the use of XNBR in soling because of its resistance to abrasion and oil. Weir and Burkey ( 198 1 ) listed footwear applications in industrial and military soling, military boot heels. and toplifts. In wear trials, XNBR toplifts have been found to be equivalent to cast urethane at lower cost. In a patent (Milkovich. 1983), the use in soling of an elastomeric starblock copolymer containing carboxyl groups is described. Themelt index was appreciably higher than for a compound containing a copolymer without carboxyl groups.
Hose TrrDrs c r r d Co~ars Weir and Burkey ( 1978) noted considerable interest in hydraulic hose tubes based on carboxylated NBR because of Ease of extrusion of high-hardness compounds and the possibility of achieving high hardness with low levels of softer blacks 2. Resistance to abrasion caused by hose tube impulsing in and out of the wire braid 3. Ability to withstandhighhydraulicpressures 4. Resistance to cuttingaction of couplingbarbs 5. Possibility of improvedadhesion of tube to wirebraidbecause of the polarity of XNBR 1.
It was noted that the lower heat resistance and higher compression set of XNBR compounds, compared with those of NBR, was a disadvantage. In dredging hose liners, NBR was found to be superior to NR i n abrasion and cut resistance. The possibility of using XNBR was also discussed. preferably in blends with PVC, in covers for rock drilling hose, mining hose. large floating oil discharge hose, fuel hose, and fire hose, all of which see rough treatment in service. Miscrl1arworr.v Applictrtions
Blackshaw ( 1983) described the use of carboxylated nitrile rubber containing a photosensitizer in printing plates. A negative of the pattern to be printed is drawn on a screen placed over the rubber. which is exposed to UV light. The exposed rubber is crosslinked, and the unexposed rubber is dissolved away with alkali. In a platent (Nippon Oil Seal Industry Co. Ltd.. 1981), it was claimed that a cured diaphragm formed from black-filled NBR had considerably improved resistance to a toluene-isooctane blend if it contained 18.2 phr of a masterbatch containing carboxylated NBR with 20 phr black and 50 phr aromatic polyamide fibers. Maldonado et al. (1979) described the use of a crosslinked polyblock copolymer of styrene-carboxylated polybutadiene or polyisoprene as a modifier for asphalt. The modified product had better high- and low-temperature properties and elastic properties than compositions without carboxylated moieties or without sulfur. Sorokina et al. (1983) described a leather substitute stable to methanol and gas condensaates at or above - 50°C. This was produced by coating a mixture of butadiene-methacrylic acidrubberand EPDM on rayon fabricandvulcanizing it. Gordonet al. (1982) used 10% maleimidized polybutadiene mixed with epoxy resin and iron oxide and combined it with a
Carboxylated Rubber
587
mixture containing low-viscosity coal tarand tris-(dimethylaminomethyl) phenol. The twocornponents combined to form a material capable of coating surfaces that are difficult to coat. Thus, the high strength and abrasion resistance and adhesion to metal and other surfaces of carboxylated polymers continue to find new applications under demanding conditions.
11. CONCLUSION Since their commercial introduction in the early 1950s, carboxylated dry rubbers and latices produced by emulsion polymerization havefound many applications relatedto their high strength and abrasion resistance and to the reactivity of the carboxyl groups. A surge of growth in the use of thesepolymersfollowedthediscovery of ways to improve the scorchresistance of carboxylated rubber compounds. Further growth may be expected when carboxylated rubbers with scorchresistanceand compound storagestability fully equivalent to the corresponding uncarboxylated rubbers become available. Hitherto, most of the commercial use of carboxylated rubbers has involved emulsionpolymerized materials. Carboxylated derivatives of other rubbers have been reported, and uses are expected to emerge for these in plastics modification or as elastomers in their own right.
REFERENCES Abakina, G. N., Boiko, D. M,. Boichenko, E. I., Khazanovich, 1. G., Solov'eva. U. S., and Golovina, V. G. (1982). Russ. Pat. 1,090,778 (Oct. I I , 1982). Amass, A. J., Duck, E. W., Hawkins, J. R., and Lockc, J. M. ( 1 972). Eur. Polynr. J. 8:78 1 . Anon. (1990). U.S. Patent 4,897,440 (January 23). Asahi Chemical Industry Col. Ltd. (l983), J p . Kokoi Tokkyo Koho, JP S8 40,343 (JP. 83 40,343) (Mar. 9, 1983); Chern. Abstr. 9 9 8 9 4 7 2 ~ . Aubrey, D. W., and Ginosatis, S. (1981), J. Adlws. 12(3):189. Azuma, C., Sanui, K., and Ogata, N. ( 1980), J . Appl. P o / y r t ~Sci. . 25: 1273. Bandopadhyay, S., De, P. P,, Tripathy. D. K., and DC S . K. (1995). J. Applied poly^^ Sei. 58:719. Bandyopadhyay, S., De, P. P,, Tripathy, D. K., and De, S. K. (1996a). J. AppI. Po/yrr1. Sui. 6 / :1813. Bandyopadhyay, S.. De, P. P,, Tripathy, D. K,, and De, S. K. (1996b), Ru/>herChettl. TerllIlol. 69:637. Bazuin, C. G., and Eisenberg, A. ( l 98 I ). Inti. B I R . Chem Prod. Res. Dev. 20:27 1, Beekman, G., and Hastbacka, M. (1986). Rubber Plastics News (December l):l4. Bhattacharyya, T., and De, S . K. (1991). Eur. Po/wt~.J. 2 7 1065. Bhowmick, A. K., and De, S. K. (1980), Rubber Cllrrrl. Techrzol. 53: 107. Biswas, T., and Basu, D. K. (1996), J. Appl. Po/yrtl. Sci. 60:1349. Black, D. E.. and Garrett, D. W. (1984), U.S. Pat. 4,448,917 (to Owens-Coming Fibcrglas Corp.. May 15, 1984). Blackshaw, G. C. (1981). U.S. Pat. 4.271,052 (to Polysar Ltd., June 2, 1981). Blackshaw, G. C. (1984), Asitrrl Rubber Plnst. 1(5):22. Bridgestone Tire Co. Ltd. (1983), J / m . Kokni Tokkyo Koho. JP 58 213,044 (83 213,044) (Dec. 10, 11983): Cllerr~.Abstr. 100: 14032n. Bridgestone Tire Co. Ltd. (l984), Jpn. Koktri Tokkyo Koho. JP S9 68,360 (84 68,360) (Apr. 18, 1984); Chrm. Abstr. 10/:172381v. Brodskii, G. I., Sakhnovskii, N. L., Rezfnikovskii, M. M,,and Evstratov, V. F. (1960). So\*.Ruh/>erTpc/1rzo/. 1Y(8):22. Brown, H. P. (19S7), Rubber Chern. Techrlol. 30: 1347. Brown, H. P. ( 1963), Ruhber Clletn. T e c h r d . 36:93 l . Brown, H. P,, and Duke, N. G. (1954). Rubber World 130:784.
588
Dunn
Browt~.H. P., and Gibbs. C. F. ( 1955). 1 1 1 d O t g . Chem 4 7 1006; RU/J/WI-CIwru. Techrud. 28:937. Bryant. C. 1,. (1970). J.I.H.I. 4:202. Buiko. G. N.,Sakhnovskii, N. L.. Estratov, V. F., Smimova. L. A., Levitiua, G. A., and Katkov, V. I. ( I961 ). Sol,. Rubber Techr~ol.20(3):8. Burnett. E. J.. and Miller, R. A. (1981). Pat. Specif. (Aust.) AU 514,720 (to Albany International Corp., Feb. 19, l98 I ); Cl~err~. Abstr. 96: 1055 l9m. Byers. J . T., Tultz, J. P,. and Hewitt, N.L. (1988), Rubber Division, ACS Meeting, Dallas, April 1988, II, 6/:725. Paper No. 43; abstract ( 1988), Rubher C / I ~ ~Techr~ol. Chakrabotty. S. K. (1983). K c l u t s c h . Gurrlrrli KurI.st,st. .36:461. Chakraborty, S. K.. and De, S. K. ( 1982a), J . App/. P o / w . Sei. 274561, Chakraborty, S . K.. and De. S. K. ( 1982b). Rubher. Cherll. T e c h t d . 55:990. Chakraborty. S. K.. and De, S. K. (1983), P o / w e r 24:1055. Chnkrnborty, S. K.. Bhowmick, A. K., and De, S . K. (198 I a), J. App/. P O / W J .Sci. 26:401 I . Chakraborty, S. K., Bhowmick, A. K., and De. S. K. (1981 h), J. Mtrcrort~ol.Sci. Re\!. Mncrorr~ol.Chertl. C2/(2):313. Chakraborty, S. K., Bhowmick, A. K., De, S. K., and Dhindaw, B. K. (l982a), RuOher Chern. Teclu~ol. 55:4 I . T e c h o l . 55:1286. Chakrnborty. S. K.. Setua, D. K., and De, S. K. (1982b), Rubber C/~err~. Chowdhury, P., Chnkravorty, C. and Das. C. K . (1995). J. AppL Po/yrI. Sci. 55:841. Cooper, W. ( 1958). .l. f o / v r r l . Sei. 28: 195. Corm. A. Y., and Patel, R. (1983). Rubber Cllerrt. Techr~ol.56: 1045. Coulthard, D. C., Ritchie, K., and Walker, J. (1976). Rubber Division Meeting,ACS,SanFrancisco, October 1976; abstract ( 1977) Kuhher Cl~onz.T e c h d . 50:434. Cuneen. J. 1.. Moore, C. G.. and Shephard. B. R. (1960). J. Appl. fo/yru. Sei. 3(7):11. Dolgoplosk, B. A.. Tinyakova, E. I.. Reikh, V. N., Zhuravela. T. G., and Belonovskaya, G. P. (1959a). K l 1 h / J c J t ’ chert^. T d l n o l . 32:322. Dolgoplosk, B. A.. Reikh. V. N., Tinyakova. E. I.. Kalaus, B., Koryushenko, Z. A., and Sladkevich, E. II. 32:328. G. ( I 95%). R~tbherC / I ~ ~Techr~ol. Eisenberg. A. ( 1970), Mtrc,rorrlo/rcrr/c.s3:147. Faulkncr. R., and Meineckc. E. A. ( 1982). Proc.. Irlt. Cor!/ R C L I ~ PI .r o c ~ s sP o / p . : 3 I . Gaylord, N. G.. Takahaski. A., Kikuchi, S., and Guzzi, K. A. (1972). J. Po/w1. Sei., Po/vrr~.Lett. 10:95. Gevorkyan, A. V.. Egiyan, E. S., Andreasyan. G. A., Vinogradova, L. A., and Protas, Yu, V. (1981), Ko:h.-Ohv~. Prorrl. St. 2 / ( 1 1 ):25; Cl~ern.Ahstr 96:21099m. Gordon. K. R., Stannard, I. C., and Kelsey, J. R. (1982). Eur. Pat. Appl. EP 49,098 (Apr. 7, 1982). Grimm, D. C. (1983a). U.S. Pat. 4.415,690 (to The Goodyear Tire and Rubber Co., Nov. 15, 1983). Grimm. D. C. (I983b). Eur. Pat. Appl. 094,894 A1 (to The Goodyear Tire and Rubber Co., Nov.23, 1983). Grimm. D. C. (1984). U.S. Pat. 4.435,535 (to The Goodyear Tire and Rubber Co.. Mar. 6, 1984). Grimm, D. C. (1985). U.S. Pat. 4,520,156 (to The Goodyear Tire and Rubber Co., May 28, 1985). Gross, M. E., and Weber. C. D. (1977). in H c r r ~ d l ~ o oo/Adhe.sive.s k (I. Skest, Ed.) Van Nostrand Reinhold, New York. Ch. 20 Guscv, Y. K., Moiseev, D. L., Sigov, 0. V., and Romanova, A. B. ( 1981 )], Kc~uclr.Rezit~nI / : 19; Chertt. Ahstr. 96:36589v. Hallenbeck, V. L. ( 1973), Ruhber Chertl. Techrlol. 46:78. Hayes, R. A. (1960). J . Cherlr. Big. Dnftr 5( 1):63. Hisaki. H.. and Suzuki, S . (1989), Rubher Work/ 201 ( N o v . ) : ~ ~ . 1. G. Farbenindxustrie Akticngesellschnft (1933)d, addition to French Pat. 701,102 (June 21, 1983). Ivanovn. G. P,.Zimin, E. V.,Peresypkina, S. R., Suchkova, N. G., andStarovitova, E. I. (1979). I u t . Po/yrrl. Sei. Techrol. 6( I 1 ):T/36. Jalics, G. (1984). U.S. Pat. 4,446,282 (to The Goodyear Tire and Rubber Co., May I , 1984). Jenkins. D. K., and Duck. E. W. (1975), in lorric Po/wrzrr.s (L. Holliday, Ed.), Wiley, New York, Ch. 3. Jcrman. M. B., and Janovic. Z. (1984), J . Mcrc~rorr~oI. Sci. Cl~err~. A2/(6/7):887. Jones, M. S., and Smith, C. T. ( 1995). Ruhber P/tr.stic.s Nert3.s (June 5):41.
Carboxylated Rubber
589
Joshi, R. M. (1979). J. App/. Polyrn. Sei. 24:595. K;llinina, I. E., and Dement’eva E. V. (1996). K R Krruchuk i Rezirlr 5:3 (Russian); abstract (1996), fjlt. Po/yrll. Sci. m d Trchr~ol.23: 12. Kalinina, I. E., Dement’eva E. V., Averko-Antonovich, L. A., and Averko-Antonovich I. Yu (1996). I l l ? . Po1vm. Sci. m d Trchr~ol.23( 1 ):T/49. Kraus, G. J. (1963). J. Appl. P o / y r ~Sei. . 7:595. Kaun, T. H., and Sommer, J. G. (1981), Ruhbrr C ~ I P ITechrlo/. II. 54:1 124. Longworth, R. ( 198.1), in De\v/opnlrrlt.s irr fonic Po/vrurr.s, Vol. 1 (A. 0. Wilson and H. J. Prosser, Eds.) Applied Science, London, Ch. 3. MacKnight, W. J., and Earnest, T. R. Jr. ( 1981), J. Po/yrn. Sei. Mcrcroruol. R n ! . 16:41. April 1983. MacKnight, W. J., and Lundberg, R. D. (1983), RubberDivision,ACSMeetingToronto, Paper No. 31; (1984), Rubber Cheru. Techrd. 57:652. Maldonado, P., Eber, D., and Phung Trung Kiet (1979), U.S. Pat. 4,330,449, (Nov. 15, 1979). Mandal, U. K., Tripathy, D. K.. and De, S. K. (1991 ), J. App1. Po1ynt. Sei. 43:2283. Mandal, U. K., Tripathy, D. K., and De, S. K. (1993),Rubber Division, ACS Mceting, Denver, May 1993, Paper No. 8; abstract ( 1993). Rubber Cherm Trchrrol. 66:680. Mandal, U. K., Tripathy, D. K., and De, S. K. ( l99S), J. Apppl. Polyrrl. Sci. 55: I 185. Marvel, C. S., Fukuto, T. R., Berry, J. W., Taft, W. K., and Labbe, B. G. (1952), J. Po/Vrrl. Sci. 8599. Marvel, C. S., Potts, R., Economy, J., Scott, G. P,, Taft, W. K., and Labbe, B. G. (1955), / d . Eug. Chern. 47:2221. Matheson, D.R. (1988), Rubber Division, ACS Meeting, Dallas, April 1988, Paper No. 5 ; abstract (1988), Ruhber Clwnl. T c d u d . 61:7 18. Meyer, C. T., and Pineri. M. (1976), P o / w e r 1 7 3 8 2 . Meyer, C. T., and Pineri, M. (1978), J. Po1yl. Sei. Po1w. P h y . 16:569. Milkovich, R. (1983), U.S. Pat. 4,409,357 (to Atlantic Richfield Co., Oct. 11, 1983). Millcr, V. A., Gienger, E. B., Brown, R. R., and Zimmerman, C. A. (1955) Armed ForcesTechnical Information Agency Publications AD92179. Mitsui Petrochemical Industries Ltd. (1984), Jpn. Kokai Tokkyo Koho JP 59 13 1,642 (84 13 1,642) (July 28, 1984). Mukhopadhyay, S., and De, S. K. (1991), J. Appl. Po/yru. Sei. 43:2283. Mukhopadhyay, S., and De, S . K. ( 1992). J. AppI. Po1yrn. Sci. 45: 181. Natata, T., Hashimoto, A., Bunnomori. Y.. and Yamada, N. (1976). Nipporl Corm Kyoknishi 49(6):449. Nippon Oil Seal Industry Co. Ltd. (1981 ), Jpn Kokai Tokkyo Koho JP 56 157,441 (81 157,441 ) (Dec. 4, 1981); Cherrl. Ahstr. Y6:105,694k. Nitto Electrical Industrial Co. Ltd. ( 1980). Jpn Kokai Tokkyo Koho JP 56 135,579 (8 I 135,579) (Mar. 29, 1980); Cheru. Ahstr. Y6:53249k. Okubo. M,, Kanaida, K., and Matsumoto, T. ( 1987), J . App/. Po/yrrI. Sei. 33 : 151 1. Okubo, M. (1990), U S . Pat. 910,229 (to Nippon Zeon Co., March 20, 1990). Pfisterer, H. A., Dunn, J. R., and Vukov, R. ( 1983). Ruhhrr Chenl. Tec/mo/. 56:418. Pineri. M,, Meyer, C., Lcvelut, A. M,, and Lambcrt, M. ( 1974). J. Po/ym. Sci. P o / y . P1lF.s. 12: 1 15. Pitt, J. J., Pearce, P. J., Rosewarne, T. W., Davidson, R. G., Ennis, B. C.. and Morris, C. E. M. (1982), J. Mrccroruol. Sei. Cllnrl. A17(2):227. PPG Industries Inc. (1982), Neth. Pat. Appl. NL80 06,676 (July I , 1982). Ramesh, P., and De, S. K. (1992). Ruhbrr Chenz. Techrlol. 65:24. Ramesh, P,, and De, S. K. ( 1993), J. Elrrstorners Plnstics 25: 106. Rigbi, Z. (1985), private communication. Roura, M. J. (1982), U.S. Pat. 4,346,194 (to Dupont de Nemours, E.I. and Co., Aug. 24. 1982). Sanui, K., Lenz, R. W., and MacKnight, W. J. (1974). J. Po/ym. Sei. C11rrr1.12:1965. Sato, K. (1983), Rubher Cllern. Trchrd. 56(5):942. Sato, K . ( 1983, U.S. Pat. 4,5253 17 (to Polysar Ltd., June 25, 1985). Sato, K., and Blackshaw, G. C. (1985). Int. Rubber Conf., Kyoto, Japan, October 1985, paper 16D14. Schulz, D. N., Turner, S. R., and Golub, M. A. (1982), Rubber C h m . Techrzol. 55:809. Schwarz, H.F. ( 1980a). E/rr.sforwric.s112: 17.
590
Dunn
Schwarz. H. F. (1980b). Rubbcr Division, ACS Meeting, Las Vegas, May 1980: abstract (1980), R d h r Clrerr~.T r c h o l . 53: 1261. Schwarz, H. F. ( 1982), Ruhbrr World 187:22. Schwarz. H. F. (1983). Rubber Division, ACS Mccting, Houston October 1983: abstract (l984), Ruhber C / I ~ I Trchr~ol. II. 57:4 12. Schwarz. H. F. (1986). Rubbcr Division, ACS Mccting. Ncw York, April 1986. Paper No. 88; abstract ( 1986), Ruhbrr Chern. Trchrwl. 59687. Semon, W. L. (1946). U.S. Pat. 2,395,017 (to B. F. Goodrich Co., Feb. 19, 1946). Shaheen, F. G., and Grimm, D. C. (1985). Rubber Division, ACS Educational Symp., Los Angeles, April 1985. Singha Roy, S. K., and Das, C. K. (1995). J . Eltrstor~lrrsPlrsrics 27:239. /. Skornyokovn, T. A., Monastryskaya, M. S., and Pavlov, S . A. (1961), Sot,. Ruhhrr T ~ / I o 20:4. Sorokina, V. A.. Kitaev, V.P,, Gruzdcva, L., and Khanukaeva,Yu. (1983). Kozh.-Obuvn. Prom. S t . 1983(6):7; Cllern. Ahsrr. YY:7 1 8 0 3 ~ . Springer, V. E. (1983). U.S. Pat. 4.419.479 (to Regal International Inc.). Startncr, P. H. (1085), U.S. Pat. 4,529,766 (to B. F. Goodrich Co., July 16, 1985). Tanaka, H., and MacKnight, W. J. ( 1979). J. k'olw~. Sei. Chen?. 172975. Tobolsky, A. V., Lyons, P. F., and Hata, N. (1968). M~rc,rorl?o/ec,u/r.s 1:514. Toyoda Gosei (1984). Jpn. Kokai Tokkyo Koho JP 59 49,246 (49,246) (Marh. 21. 1984); Clwrl. Ahstr. /01:74145b. Tsekhanskii. R. S. (1973). Uch. .&/p. Y ~ r o s k iCos. . Perlo,qog. I n s t . 122:36. Unitika Ltd. (1983), Jpn. Kokal Tokkyo Koho JP 58 38,747 (83 38,747, Mar. 7, 1983); Cl~rrn.Ahstr. YY: I06308r. Weir. R. J., and Burkey, R. C. (1981), RtrhDer Plrtst. News (Feb. 16):30. Weir. R. J., and Gunter, W. D. (1978), Eur. Polvm. J. 14:20. Zhakarov, N. D. (1963), Ruhher Cl~rr?~. T e c h o l . 36:568. Zhakarov, N. D., and Shadricheva, T. A. (1963, Ruhber Cl~em.T e c h o l . 36:576.
Polyphosphazene Elastomers D. Frederick Lohr* The Firestone Tire and Rubber Conlparly, Akron, Ohio
Harold R. Penton Ethyl Corporation, Baton Rouge, Louisiana
1. INTRODUCTION Phosphonitrilic chloride elastomers, or polyphosphazenes, represent the first new class of semiinorganic elastomers to be developed and successfully commercialized since silicone rubber. and the full synthetic and application potentials of this class of polymers are only beginning to be exploited. Characterized by a polymeric backbone of repeating pentavalent phosphorus and nitrogen atoms with pendant organic groups attached to the phosphorus, the properties of the polyphosphazene elastomers can be modified over a broad range by proper selection of the organic side groups. Polyphosphazenes are unique in the field of polymer synthesis, since, in theory, an infinite number of polymers having various properties may be derived from a common polymeric intermediate, polydichlorophosphazene, designated by the structure (PNCl,),,, by replacing the chlorines with different organic nucleophiles. If a polydichlorophosphazene precursor is reacted with the sodium salts of trifluoroethanol and a mixed fluorotelomer alcohol, a polyfluoroalkoxyphosphazene elastomer. referred to hereafter by the generic designation FZ (as per ASTM D141 g), is obtained that has a unique set of properties including a wide service-temperature range ( - 65 to 175"C), fuel andoil resistance, and excellent flexfatigue anddamping characteristics. Replacement of the chlorine atoms of the polydichlorophosphazene with phenoxy and p-ethylphenoxy groups yields a polyaryloxyphosphazene, hereafter referred to by the generic designation PZ (as per ASTM D I4 1 g), a nonhalogenated, flame-resistant, low-smoke-producingelastomer suitable for use as wire and cable insulation and insulating foams. OCH2CF3
OCoHs
I
I
[P = NI,,
IP = NI,,
I
I
OCH2(CF2),CF?H (x = 1,3,5,7,...) FZ Elastomer
OCOH.~",HS elastomer PZ
* Retired 591
Lohr and Penton
592
2. 2.1
POLYPHOSPHAZENE CHEMISTRY Historical Development
The development of polyphosphazenes can be traced back to when Liebig (1834) and later Gladstone and Holmes( 1 864, 1865) discoveredthat the reaction betweenammonia and phosphorus pentachloride yielded hexachlorocyclotriphosphazene, a cyclic trimer having the structure (NPCl,)3. Near the end of the nineteenth century, Stokes (1 897) reported the first synthesis of polydichlorophosphazene,which he described as “inorganic rubber,” by the thermal polymerization of hexachlorocyclotriphosphazene. Polydichlorophosphazene,or chloropolymer as it is often called, is itself strong and very elastic, but upon exposure to atmospheric moisture it loses its rubbery qualities due to hydrolysis, subsequent cross-linking, and ultimate breakdown of the polymer to form phosphoric acid, ammonia, and hydrochloric acid. In addition to its hydrolytic instability, polydichlorophosphazene was generally obtained from the polymerizationof hexachlorocyclotriphosphazene as a gelled, insoluble polymerdue to excessivebranching and crosslinking as the cyclic trimer’s conversion to polymer exceeded 50%. The problems associated with the synthesis and handling of polyphosphazenes were a major barrier to the developmentof this class of polymers until Allcock and Kugel(1965, 1966) found that polydichlorophosphazene could be obtained as a soluble, uncrosslinked polymer if conversions of the cyclic trimer to polymer were limited to less than 50%. Subsequent replacement of the chlorines under anhydrous conditions by the solution reaction of the chloropolymer with sodium alkoxidesor aryloxidesyielded organosubstituted polyphosphazenes that were both thermally and hydrolytically stable. Although much of Allcock’s early work in this area involved the synthesis of homopolymers (i.e., polyphosphazenes having like substituents on the phosphorus), which were characterized as semicrystalline thermoplastics, other researchers (Rose, 1968) began investigating the preparation and properties of phosphazene copolymers (i.e., polyphosphazenes having two or more different substitutents attached to the phosphorus). This work led to the development and refinement of FZ elastomers by Rose (1 970), Roseand Reynard (1972), andKyker et al. (1976). This was followed by the development of PZ elastomers by Reynard and Rose(1 974) and Cheng (1978).
2.2
Preparation of Polyphosphazene Elastomers
Preparation of the polydichlorophosphazeneintermediate has been the subject of numerous studies (Allcock,1972; Hagnauer, 198 l ) . The conventional route to this chloropolymer is a hightemperature ( >250”C) melt polymerization of highly purified hexachlorocyclotriphosphazene trimer. Careful selection of time and temperature is necessary to avoid the formation of a crosslinked matrix. More recent studies have demonstrated the effectiveness of various acids and organometallic compounds as catalysts for the preparation of the chloropolymer (Hagnauer, 198 1 ). The catalyzed polymerizations can generally be carried out at temperatures less than 250°C. and conversions to gel-free chloropolymer are typically 70% and higher. Polydichlorophosphazene obtained from the uncatalyzed thermal polymerization is characterized by high weight-average molecular weights (Mw,2 X 10”) and a fairly wide molecular weight distribution ( M J M , , , 5-10). Catalyzed polymerizations have been foundto yield chloropolymers with slightly lower M , but significantly narrower molecular weight distribution (MJM,,, <5). Recently, alternative routes for the preparation of polydichlorophosphazene that do not involve the ring-opening polymerization of trimer have been reported (Hornbaker and Li, 1980; De Jaeger et al., 1983). These routes are based on a condensation polymerization and may prove
Polyphosphazene Elastomers
m N
L
X
II
4
c?
m
m
Ir.
0
U
N
Ls
W
4
N Ls
593
d,
ii
Lohr and Penton
594
Fig. 2 Random structure of polyorganophosphazenes having different substituents.
to be of technologicalimportance for the preparation of low to moderatemolecularweight polyphosphazenes. The true value of polydichlorophosphazene lies in its use as an intermediate for the synthesis of a wide variety of polyorganophosphazenes, of which the FZ and PZ elastomers are prime examples (Fig. 1 ). The nucleophilic displacement of the chlorines by alkoxides and aryloxides generally falls into the category of an SN2-type displacement; hence, the substitution reaction is affected by the nucleophilicity and steric characteristics of the attacking nucleophile. Steric effects are particularly noticeable whenbulky nucleophiles are employed,and higher substitution temperatures are required to obtain complete replacement of the chlorines (Allcock and Kugel, 1966). After the substitution is complete, the by-product sodium chloride is removed from the polyphosphazene by water washing, and the polymer is recovered by conventional techniques. In general, the substitution of polydichlorophosphazene with mixed substituents of similar reactivity yields polyorganophosphazenes having a random distribution of the pendant groups along the polymer backbone. Thus. structure such as [NP(OR)(OR')],, are only an average representation. A more accurate depiction of the structure is shown in Figure 2. The nature and size of the substitutents attached to the phosphorus play a dominant role in determining the thermal and physical properties of the polyorganophosphazenes (Table 1). Although most of the phosphazene homopolymers tend to be semicrystalline, the copolymers may be either semi-crystalline or amorphous. Copolymers having pendant groups significantly different in size or nature are amorphous, whereas copolymers having similar-size substituents such as [NP(OC~,H4-p-CI)(OC6H4-p"C2Hs)]or [NP(OC6H5 )(OC6H4--pOCH3)1 can be crystalline (Dieck and Golfarb, 1977; Beres et al., 1979). This wide range of properties of the polyphosphazenes, based on the same polymer chain, is unique in polymer chemistry.
3.
COMMERCIAL DEVELOPMENT OF POLYPHOSPHAZENE ELASTOMERS
Although considerable scientific work remains to be carried out to better define the chemistry and structural properties of the polyorganophosphazenes, the technological importance of this
Table 1 Properties of Typical Polyorganophosphazenes Polymer [NP(OCHG3)~1,, fNP(OCf7H5),1t, [NP(OC~HJ-P--C~HS)~~,~
T, ("C) - 66
[NP(OCH2CF~)(OCH2(CF2),CF~H)I,1
-5 43 - 63
INP(OC~HS)(OC~HJ-P-C~HS)l,,
-
18
T,,, ("C)
Physical form at 25°C
245 thermoplasticFlexible >350 Flexible thermoplastic Rigid thermoplastic >350 elastomer Amorphous Amorphous elastomer
Polyphosphazene Elastomers
595
class of polymers was early recognized by Firestone Tire and Rubber Company. which led to Firestone’s commercial development of FZ elastomers in the mid-1970s under the PNF trademark (Kyker and Antkowiak, 1974) aswell as the semicommercial development of PZ elastomers under the trademark APN. The Ethyl Corporation, which has been actively involved i n polyphosphazene research for anumber of years,obtained exclusive worldwiderights to Firestone’spolyphosphazene technology and patents in 1983. Ethyl Corporation is now actively developing both FZ and PZ elastomers for a wide variety of applications areas and began commercial production of these elastomers. trade-named EYPEL polyphosphazenes, in 1985.
4.
4.1
POLYFLUOROALKOXYPHOSPHAZENE ELASTOMERPROPERTIES AND APPLICATIONS
Polyfluoroalkoxyphosphazene ElastomerProperties
When polydichlorophosphazene is substituted with tritluoroethoxide and a mixture of thorotelomer alkoxides, an amorphous FZ polymer is obtained. A slightamount of an unsaturated substituent is also attached to the polymer backbone to aid in curing. The FZ polymer is a soft gum rubber that behaves in a manner similar to nitrile rubbers. FZ polymers respond to compounding ingredients in the same manner as expected for other elastomers. The basic compounding variables that can be utilized to obtain optimum properties from FZ polymers for various applications are shown in Table 2. A peroxide curative is preferred.although sulfurhccelerator andradiation curing have been demonstrated. Filler systems can consist of a combination of silicas, clays, and carbon blacks. For optimum heat resistance. the silicas and clays should be silane treated to mask acidic surface species. Heat resistance may be enhanced through use of various organometallic stabilizers. The mold flow and processability of FZ conlpoundsare excellent.Mill and calender release can be enhanced through addition of small quantities of silicone and fluorosilicone rubbers and/ or polytetrafluoroethylene powders. Silane coupling agents and coagents such as triallylisocyanurate give the technologist latitude in compound design. Metal oxides can be used as potential acid acceptors and to improve metal adhesion. Selected metal oxides may also be used to yield compounds with higher modulus values. A basic formulation is shown in Table 3. Typical dry rubber processing equipment is used; all of the ingredients except the peroxide curative are added to an internal mixer such as a Banbury. Mold release of FZ elastomers is excellent when silicone emulsion or soap is used as release agents. Good adhesion to metal is achieved through use of conventional metal surface
Table 2 Basic Compounding Curt. system: Peroxide,sulfur, radiation Fi//ers: Silane-treatedclays and silicas and carbon blacks Sttrhilizer: Selectedorganometallics Procmsirlg crid: Siliconeorfluorosiliconerubber Mod$ers: Silanecouping agents, coagents Mettrltrrllresion: Commercial proprietary agents
Lohr and Penton
596 Table 3 BasicFormulation
PNF-200 Filler Metal oxide Modifiers Stabilizer Peroxide
100 parts
30-60 parts 2.0- 10.0 parts
Variable Variable 0.4-2.0 parts
preparations and primers such as Chemlok 607 and FC 5 150. Adhesion to reinforcing yarns and fabrics is also excellent. The range of vulcanizate properties that can be obtained is shown in Table 4. Physical properties of three typical FZ formulations are shown in Table 5. The PNF-265-004 (EYPELF 6504) formulation features an all carbon black filler system and has found use in a variety of applications in which low-temperature flexibility, flex-fatigue resistance, and general chemical resistance are of prime importance. The PNF-270-003 (EYPEL-F 7003)formulation is a mineral-filled formulation used in a variety of O-ring and other sealing applications, both static and dynamic. Silica-filled FZ compounds offer slightly improved fuel and heat resistance over carbon black-filled compounds. All mineral-filled FZ compounds are color-coded green for easy identification. The PNF-270001 (EYPEL-F 7001) formulation contains a combination of mineral and carbon black fillers and wasdesigned for optimum processability in calenderingoperationsand for flexfatigue
Table 4 TypicalPropertiesof
FZ Elastomers
Physical
point,
Density, g/mL Tensile strength, MPa (psi) 1 0 0 % modulus, MPa (psi) Elongation, YO Compression set (70 hr at 150"C), O/c Hardness, Durometer (Shore A) TR- 10, "C Brittle "C Temperature range, Tear resistance, Die B, kN/m (psi) Bonding Excellent fabrics to and metals characteristics Excellent damping Vibration in dynamic applications Excellent lifeFlex Weatherability Fungus resistant Flame resistant Resistant to broad range of fluids Liquid oxygen compatible Sortrce: Penton. 1986.
1.75-1.85 6.9-13.8 (1000-2000) 2.8- 13.8 (400-2000) 15-250
15-25 35-90 - 56 - 68
-65 to + l 7 5
44to
(to 250)
Excellent
597
Polyphosphazene Elastomers Table 5 Physical Properties of TypicalFZCompounds EYPEL-F (PNF) Physical property Durometer, Shore A Modulus at 100% elong., MPa Tensile strength, MPa Elongation at break, 70 Compression set (70 hr at 149"C), 70 Specific gravity Mooney viscosity, ML4 at 100°C Crescent tear (Die B). kN/m
6504 (265-004) 7003 65 4.1 7.6 200 50 I .74 66 27.0
(270-003) 70 9.0 10.3 I20 20 I .85 45 19.3
7001 (270-001) l0 6.7 9.3 l 40 30 I .77 45 17.5
resistance. In general,mineral-filled FZ compounds offer somewhat betterheatresistance, compression set. and fuel and oil resistance than carbon black compounds, which have superior mill processability and flex-fatigue resistance. Table 6 shows the volume swell in toluene of the PNF-270-003 (EYPEL-F 7003) formulation compared to the swell of other specialty elastomers. The data were obtained on O-rings purchased from commercial sources. The FZ compound has the lowest volume swell of any of the specialty elastomers tested. The exceptionally low volume swell i n toluene is an indication of the excellent resistance of FZ elastomers to hydrocarbon fuels, fluids. and lubricants. The changesin physical properties of O-rings of the PNF-270-003 (EYPEL-F7003) formulation after immersion for 166hoursatelevatedtemperatures in variousfuels are shown in Table 7. Good to excellent retention of properties is noted for all fuels. Sinlilar data obtained after immersionof FZ elastomers in various lubricants for I66 hours at 150°C are shown in Table S. FZ elastomers have excellent resistance to most lubricants, and even the harsh jet engine lubricants, such as MIL-L-7808G, have only a moderate deleterious effect on FZ properties. Data obtained on PNF-270-003 (EYPEL-F 7003) O-rings after immersion in several hydraulic fluids are shown in Table 9. Excellent resistance to the hydrocarbon-based fluids was
Table 6 at 25°C
Equilibrium Volume Swell
of Elastomers i n Toluene
Elastomer Polyfluoroalkoxyphosphazene, FZ Fluorosilicone Fluorocarbon Silicone Polyepichlorohydrin EthylendAcrylate Polyacrylate
14.9 19.5 25.5 142.2 150.8 246.3 432.0
Lohr and Penton
598
Table 7 Physical Properties of PNF-270-003 (Eypel-F 7003) Aftcr Immersion for 166 Hours at Indicated Temperature in Various Fuels
Fuel ASTM fuel C Aviation 100 JP4 Arctic diescl ASTM fuel D/ethanol"
19 IS 9 10 20
100
82 1I O I S0 78
- 12 - 12
- 24
- 14
28 - 12 - 24 - 22
-9 14 9 -7
-
-4 - 13 - 26
-
Table 8 Physical Propcrties of PNF-270-003 (Eypcl-F 7003) Immersion in Various Lubricants ( 166 hr at 150°C) Volumc swell Lubricant ~
(%) ~~~
~
ASTM No. 3 oil Tcxaco Havoline IOW-40 Stauffer Blend 7700 Anderol 774 MIL-L-7808G MIL-L23699 2lithium-based Grease,
Microhardncss change (points)
Tcnsile strcngth change
Ehngation change
(%)
(%)
-2 -3 14 - I I - 20 0
- 16 - 18 - 46 - 38
-
~~
2 0 29 22 IS II
7
38 36 - 22
14 -7 -21 - l4
-
-1
0 - I4
Table 9 Physical Properties of PNF-270-003 (Eypel-F 7003) Aftcr Immersion in Hydraulic Fluids change
Elongation Microhardncss strength Volume Tensile change swcll
Fluid"
(%)
MIL-H-5606B ( I ) MIL-H-83282A ( 1 ) Wagner 2 1 B (2) Dexrotl I1 ATF ( 1 ) GM Power Stccring ( 1 ) Skydrol S00B 4 (3) Pydraul 1 ISE (2)
4 2 38 2 3 1S4 8
change (points) -3 -2 - 34 l -2 -9
-S
(%)
(9)
-4
-7 0
-S -21
-21
- 85
-4 0 - 63 0
9 -7
0
599
Polyphosphazene Elastomers
STRAIN = 7 0 %
FREQUENCY = I Hz
3r 21
LOG G ' , G " I+
MPa x 2 9 8 / T )
I
ot
-21" -73
l
G"
,
. -1
-33
..
,
" 1
7 TEMP
~
1
,
47
Ipr"-l-
87
-1
127
("C)
Fig. 3 Viscoelastic functions of PNF-280-003 (EYPEL-F 8003).
noted. WhileFZcompounds have good resistance to thearylphosphate ester fluidssuch as Pydraul 1 15E, FZ elastomersare not recommended for the alkyltypessuch as Skydrol 500. FZ elastomers can be designed to give excellent servicein vibration isolation mounts where requirements include damping over a broad temperature range in combination with resistance to fuels, oils. and hydraulic fluids. The viscoelastic properties of one compound that has been used in such applications are shown in Figure 3. The peak in the loss tangent curve occurs at about -45°C. The response is flat from 25 to 150°C. the upper limit of the test. The data are shown for a frequency of 1 Hz. but the curves for frequencies of 0.4-2.5 Hz formed a very narrow envelope.
4.2 Applications of F2 Elastomers In recent years FZ elastomers have found increasing use in demanding applications, particularly in the aerospace. military, petrochemical. and gas pipeline areas. Parts weighing up to 30 kg are in commercial service, and FZ elastomerseals solved critical sealing problems in current army battle tanks. Sealsfor this application madefrom other elastomers failed repeatedly. allowing dust ingestion and causing costly engine damage. Several manufacturers of jet engine fuel control units have specified FZ elastomerO-rings and diaphragmsfor use with JP-4 fuel at servicetemperature from - 5 5 to 160°C. In this application. FZ elastomers offer low-temperature flexibility superior to tluorocarbon and much improved physical properties relative to fluorosilicone. The low-temperature flexibility. tlex-fatigue resistance. resistance to hydrocarbons, and good damping properties of FZ elastomers allowed the new design of a large pressure surge suppressor now in use on liquid propylene pipelines. This new surge suppressor system 'l I1ows higher operating pressures and consequently greater throughput. These applications represent only a few of those developed or under investigation for FZ elastomers. The unique properties of FZ elastomers assures them an important role in the specialty polymer industry.
Lohr and Penton
600
5.
POLYARYLOXYPHOSPHAZENEELASTOMERPROPERTIES AND APPLICATIONS
The PZ elastomers obtained from the substitution of polydichlorophosphazene with approximately equimolar amountsof phenoxide andp-ethylphenoxide (Fig. I ) are of commercial interest due to their excellent fire resistance without the incorporation of halogens attached to thepolymer or as an additive (Lawson and Cheng, 1978). Depending on theapplicationrequirements,a small amount of an unsaturated cure site may also be incorporated onto the polymer backbone to adjust cure rates and tensile properties of the elastomer. The PZ elastomeris a soft gum rubber that can be compounded and processed on conventional dry rubber equipment. Filler systems consist of silicas, clays. carbon black, and alumina trihydrate either alone or in combination. The resulting compound can be cured with peroxides, sulfur, or radiation to give an elastomer having a service temperature range between - 20 and 125°C. If necessary, slight modification of the polymer structure will allow service at temperatures below -20°C. A comparison of the service temperature range of PZ with several other flame-resistant elastomers is shown in Figure 4. Uncompounded PZ polymers are self-extinguishing in air, having an LO1 of 28. Compoundingtechniquesraise the LO1 to 44, and upon exposure to flame. PZ cotnpounds char rather than drip or flow. The toxicity of combustion products from PZ vulcanizates has been studied extensively by Alarie et al. (1980). This work showed that PZ combustion products were considerably less toxic than those obtained from other commonly used fire-resistant polymers (Fig. 5).
l
jl Servicc temperature rangc for flame-resist~ntelastomers. CR = Chloroprenc: CSM fonatcdpolyethylene; ECO = cpichlorohydrin; PZ = polyaryloxyphosphazene. Fig. 4
=
chlorosul-
Polyphosphazene Elastomers
601
10
9 1 8
4
0 WOOD
P2 POLYTEFLON UREA. PVC COM. FORMALDEHYDE URETHANE POUNDS
Fig. 5 Rclative toxicity of combustion products from various fire-resistant elastomcrs and plastics.
602
Lohr and Penton
Table 10 Properties of PZ Elastomer Wire and Cablc Insulation and Jacketing Physical property Tensile strength. MPa (psi) Elongation, % Tear resistance (Die B), kN/m (ppi) Halogen. c/r ahsorptwn. Water mp/cm' (Shore Durometer Hardness, A) Heat aging (168 hr at 158°C) Tensile retention. % Elongation retcntion, % LOI, (ANBS smokc dcnsity Flaming, Nonflaming. DM const., Dielectric 10 Hz Volume reslstivity. 3.2ohm-cm
Valuc 9.0- 1 I .0( 1300- 1600) 200-400 13(75) 0.05 3.25 83 1 10
45
44 200-225 65-75 3.9 x 10'4
Sorrtw; Books et a l . . 1984.
PZ elastomers have been found to have very desirable characteristics for electrical wire and cable insulation and jacketing when fire safety is a concern (Peterson. 1979). Recently, researchers at Ethyl Corporation(Books et al., 1984)have developed new PZ compoundshaving excellent physical and electrical properties (Table IO). PZ elastomers also display significant resistance to attack by many functional fluids andcompare very favorably to other flame-resistant elastomers (Fig. 6).
-10 I
ASTM 011 #l
ASTMASTM 011 #2
Fig. 6 Oil resistancc of PZ elastomers: change in volume after 24 hours at 100°C.
011 #3
Polyphosphazene Elastomers
603
Table 11 Properties of PZ ElastomerClosed-CellFoams Property F/mrIrrIdi/it.v Limiting 46 D-2863) (ASTM NBS smoke density (ASTM E-662) Nonflaming Flaming Mrckrrrliccrl D-1622), Density (ASTM kglm' (lblft') Noise reduction coefficient C-423) (ASTM Compressive resistance (MIL-IS280H,46.1 (psi) l), MPa Tensile strength (MIL-lS28OH,4.6.1 I ), 0.12(17) MPa (psi) D-1667).set (ASTM Yo Compression Thermalconductivity (ASTM C-S 18), watts/m.K (BTU-idhr. ft','F, bsorption, Water g/m' (Iblft')
Typical
oxygen
2s 49 6.4(4.0) 0.30 0.013(2)
75°Fmean)
26.0 0.04 I(0.283) 73(O.O15)
Sour-cet Penton. 1986.
The PZelastomers are also being developed for use as flame-resistant, closed-cell insulating foam. Typical properties of the closed-cell foams are shown in Table 11 (Penton, 1986).
6. SUMMARY Althoughpolyphosphazenetechnology is nearly 100 years old, it hasonly been in thelast few yearsthatthistechnologyhasreachedcommercialization. These new polyphosphazene elastomers offer the materials engineer and the elastomer scientist novel materials with unique properties. The FZ elastomers offer excellent flex-fatigue resistance, damping properties, and chemical andtluid resistance anda broad service-temperature range.These high-performance materials are being increasingly used in demanding applications where other elastomers have failed. The PZ elastomers have a unique combination of properties including outstanding flame resistance, and in a burning situation they rapidly char, do not contribute to flame spread, and produce very low levels of smoke.
REFERENCES Alarie, Y. C., Lleu, P. J.. and Magill. J. H. (1980), J. Fire Flc~rr~rr~cthilify 11:63. Allcock, H. R. (1972). P k o . s / ' h o , u s - N i ~ r ~Corrrpourzrls. ~~~r/~ Academic Press, New York, p. 303. Allcock, H. R., and Kugel. R. L. (1965). J . Am. Chern. Soc. 874216. Allcock, H. R., and Kugel, R. L. (1966), Inorg. Cherrl. 5:1709. Bcckman, J. A.. and Lohr. D. F., PNF phosphonitrilic fluoroelastomcr: Propcrties and applications, 120th Mtg., Rubber Div., Am. Chem. Soc., Oct. 13-16, 1981. Cleveland. OH. Beres, J. J., Schncider. N. S., Desper. C. R., and Singlcr, R. E. ( 1 9 7 9 ~MNcrorrloleculrs 12:S66. Books, J. T., Indykc, D. M,, and Mucnchmger, W. 0. (1984), Int. Wire Cable Symp. Proc., Cherry Hill, NJ, pp. 1-3. Cheng, T. C. (l978), U.S. Pat. 4,116,785.
604
Lohr and Penton
DC Jaeger, R., Helioui, M,, Puskaric, E. (1983), U.S. Pat. 4,377.558. Dieck, R. L., and Goldfarb. L. (1977). J. Po/yrn. Sei. Chern. Ed. 15361. Gladstone. J. H., and Holmes, J. D. (1864), J. Chern. Soc. Lorldon 17225. Gladstone. J. H., and Holmes, J. D. (1865), Bull. Soc. Chirrz. Fr. (2),3:113. Hagnauer, G. L. (1981), J. Muerorno/. Sci.-Cllerrl. A16:385. Hornbnker, E. D.,and Li, H. M. (1980), U.S. Pat. 4,198,381. Kyker. G. S., and Antkowiak, T. A. (1974). Rubber Chern. Techrlol. 4732. Kyker, G. S.. Antkowiak, T. A., and Halasa. A. F. (1976), U.S. Pat. 3,970,533. Lawson, D. F., and Cheng, T. C. (1978), Fire Res. 1:223. Liebig. J. ( I 834), A m Chenl. l / :139. Penton, H.R. (1986). Kcwfsch G~trnrniKurlstst 3Y:301. Peterson. T. C. (1979). Polyphosphazene wire and cable insulation. Naval Sea Systems Command, Contract No. N00024-78-C-3644. Horizons Research, Inc., Cleveland, Ohio. Reynard, K. A., and Rose, S . H. (1979). U.S. Pat. 3,856,712. Rose. S . H. (1968), J. P o l y . Sei. B6:837. Rose, S. H. (1970). U S . Pat. 3,515,588. Rose, S. H., and Reynard, K. A. (1972), U.S. Pat. 3,702,833. Stokes, H. N . ( 1897), Am. C h m . J . 19:782.
22 Advances in Silicone Rubber Technology: Part I, 1944-1986 Keith E. Polmanteer Consultant, KEP Enterprises, Lady Lake, Florida
1. INTRODUCTION This chapter will discuss the key advances in the technology of silicone rubber since it first became commercially availablein 1944 fromboth the Dow CorningCorporation and the General Electric Company. Advances in siliconerubberhavebeenbroughtabout by theneed to satisfypractical marketplace requirements not fulfilled by other elastomers and,certainly not least in importance, by the scientific challenge to improve technology. Fundamentally involved in the use of silicone rubber in fulfillment of market needs is the combination of inherent properties that silicone rubbersexhibit. These propertiesincludeexcellent weath.er andthermalstability,ozoneand oxidation resistance, good electrical properties, extreme low-temperature flexibility (e.g., very low temperatures for T,, T,, and T,,,),low activation energy for viscousflow, high gas permeability, good release from organic materials,good solvent and oil resistance, physiologic a1 Inertness (compatible with body tissue), andcurability by a variety of methodsat both elevatedand ambient temperatures. In addition, polymerization versatility in the preparation of silicone polymers allows the easy inclusion of sufficient amounts of different organic groups to extend the list of properties achievable with elastomers prepared from these altered silicone polymers. For example, 3,3,3-trifluoropropyl groups in polytrif-luoropropylmethylsiloxane contribute excellent oil and fuel resistance. Inclusionof phenyl groups (e.g., greater than50 mol%) provides excellent radiation resistance. Vinyl groups can be positioned at random or at selected locations in the polymer chain toalter the crosslink network in vulcanization via vinyl group reactions. This latter ability is of paramount importance in changing the resultant property profile (e.g., significant improvement by reducing compression set). The inherent characteristics of silicone polymers that are primarily responsible for their unique combination of properties are strong chain bonds, backbone chain flexibility, ease of rotation of organic groups, low inter- and intramolecular forces, and inorganic/organic makeup. Advances in silicone rubber could not be discussed had it not been for the very interesting pioneering research on silicon. McGregor (1954a) provides an excellent historical synopsis of the early chemistry leading upto the development of silicone rubber and othersilicone products. McGregor (1954b) and Meals and Lewis (1959a,b) describe earlycommercialactivities of, '
605
606
R
-
Polmanteer
:y3CH3
1
-
0
-
CH3 SI - R ,
WHERE
R
C A N BE ORGAN AN
IC,
H Y D R O X Y L , OR ALKOXY GROUP a
X
Fig. 1 Polydimethylsiloxane used in silicone rubber.
especially, the first two U S . manufacturers of silicone rubber, Dow Corning and General Electric. The silicone polymers used in making silicone rubber compositions include inorganic and organic portions. The polymer backbone consists of alternate silicon and oxygen atoms. Each silicon chain atom has two organic groups attached to it. The chain-end silicon atoms have a third group (organic. hydroxyl, or alkoxy) to satisfy silicon’s fourth valence. The most used silicone elastomer polymer consists principally of methyl group substitution with or without less than 0.5 mol% of the methyl radicals replaced with vinyl groups and can be represented (small amount of vinyl groups omitted) as in Figure 1. The inclusion of organic groups other than methyl will be further discussed in relation to advances in silicone rubber technology.
2.
2.1
GENERAL SILICONE RUBBERTECHNOLOGY Introductory Remarks
The discussion in this section will focus primarily on heat-vulcanized high-consistency silicone rubber technology and technology that is common to the three principal types of silicone rubber that now exist. These threetypesincludehigh-consistency compositions, room-temperature vulcanizing (RTV) compositions, and liquid silicone rubber (LSR) compositions. High-consistency elastomers led the way into the marketplace. Both RTVs and LSRs will be dealth with in Section 3.
2.2
Manufacturing and Polymerization
Figure 1 shows the all-11lethyl-substituted silicone polynler polydimethylsiloxane. Polymers of either this type or one containing a small amount (e.g., less than 0.5 mol%) of vinyl substitution for methyl groups are the most widely used polymers i n silicone elastomer technology. The methyl group substitution is the most easily accomplished and can be made from quartz. coke, chlorine. and water. A flow diagram for the manufacture of polydimethylsiloxane is given in Figure 2. The coke is used to reduce silicon in quartz to metallic silicon, which can then be reacted with methyl chloride to form a mixture of silanes. C1,-JCH3),,Si, where n can be 0, 1. 2. 3, or 4. The dimethyldichlorosilane is separated from the mixture of silanes by distillation for the subsequent hydrolysis step to form dimethyl-substituted cyclic siloxanes, [(CHj),SiO]., for use in preparing polymers. Theaverage number of siloxane units in the polymer is controlled to best satisfy the properties desired from a given composition. The polymers used in highconsistency compositions usually contain from 3.000 to 10,000 siloxane repeating units, while polynlers used in RTV and LSR compositions contain between 5 0 and 2.000 siloxane repeating units. The normal procedure for preparing high molecular weight polydimethylsiloxane is to control the hydrolysis of the dimethyldichlorosilane to form cyclic siloxane intermediates. which
607
Advances in Silicone Rubber Technology, Part I, 1944-1986
CH30H
I 2c
Catalyst
2H2
t
t
SI02
A
HCI
I 2CH3CI
t
Catalyst
Fig. 2 Polydimethylsiloxanemanufacturing.(FromPolmanteer, 1981.)
are then polymerized via a ring-opening reaction. A wide variety of catalysts may be used to polymerize the cyclic siloxanes. These catalysts include proton acids, Lewis acids, acid clays, and many bases. The principal cyclic used in preparing high polymers is the tetramer, which may be polymerized with alkaline catalysts as shownin Figure 3. This is anequilibrium reaction such that a certain ratio between linear and cyclic species is maintained at equilibrium. A study by Carmichael and Winger ( 1 965) revealed that the total cyclic content including all cyclics through D , ,, where D = (CH,<)$3iO,is 12.5-14 wt%. They also determinedthat the equilibrium amount of cyclics is independent of the specific catalyst, whether it be acidic or basic. This is true despite the fact that the reactivity and reaction rates of the Si-U-Si linkage are different for different species. For example, the relative reactivity toward acids is:
D3 > MM > MDM > MDzM > DJ and the corresponding reactivity for the bases is
D3 > DJ > MD2M > MDM > MM where D = (CH3),Si0 and M = (CH3)3Si0112. Carmichael and Heffel(1965a) showed (see Table 1) that the amount of cyclics at equilibrium is dependent onthe molecular weight of the polymer being made.In this case the molecular weight has been controlled by the quantity of triorganosubstituted silicon end blocking added
608
Polmanteer
90% KOH At 14OoC NaOH A t 17OoC
d CH3-
\
\Si I
10%
I
I I
I
99% L " " " " " " " 1
x=3000-5000
KOH A t 170°C NaOH A t 25OoC
Fig. 3 Polymerization of cyclic dirnethylsiloxanes. (From Polrnanteer. 1981.)
to the polymerization reaction. Alkaline-or acid-catalyzed polymerization of octamethylcyclosiloxane is an equilibration reaction, and reactions other than ring opening occur. For example, Carmichael and Heffel (1965b) showedthat the catalyst is concurrently opening cyclics, cutting into polymer chains (interchain reactions), and cutting back (intrachain reactions) on itself to split out cyclic species. Figure 4 shows the effect of time on these concomitant reactions in altering the molecular weight distribution of a low molecular weight polydimethylsiloxane. The molecular weight distributions shown in Figure 4 were followed during an acid clay-catalyzed reaction of DJ and MM at 80°C. The relatively large amount of MDJM and MDxMpresent after 0.5 hour indicates that D., initially enters the linear chains as a unit, followed by reorganization with increased time. As found in other polymer systems, copolymers of siloxanes play an important role in the technology of silicone elastomers. The organic radicals attached to silicon have a marked effect on the final propertiesof the elastomer. Hence,it is often advantageous to combine several kinds of organic radicals in the same polymer to achieve specific combinations of properties. For example, 5-10% bulky radicals, such as phenyl, are used in the form of phenylmethylsiloxane or diphenylsiloxane to break up the regularity of polydimethylsiloxane such that crystallization is
Table 1 Effect of Molecular Weight on Equilibrium Cyclics ~
Nurnher-average molecular weight
Total cyclics (wt%)
~
S21
3.80
557
4.70 4.86
598 V03 1121 1 x IO"
7.69 8.92 12.80
609
Advances in Silicone Rubber Technology, PartI, 1944-1986
0
1
3 NUMBER
5
7
9
11
13
15
CIF SI ATOMS IN MOLECULE, ASMD,M
Fig. 4 Comparison of equilibrium and kinctically controlled distributions. (From Polmanteer. 1981.)
inhibited. Vinylmethylsiloxane units are also included in low concentration to improvevulcanization efficiency with peroxides. This improved vulcanization efficiency makes it possible to use only one fourth to one half as much peroxide as is required in a vinyl-free polymer. Much improved or lower compression set valuesare achieved with vinyl group-containing elastomers. Low compression set values reflect an improved ability to sustain a seal under high-temperature conditions, which is one of the requirements for O-rings and gaskets. Trifluoropropylmethylsiloxane-and cyanoalkyl-containing units are incorporated into copolymers to obtain improved resistance to solvents and oils.Siloxane copolymers are normally prepared by copolymerizing mixtures of the desired cyclics. such as [(CH3)2SiO14, (C6HSCH3SiO)4,and [CH3(CH2=CH)SiOI4. The random inclusion of an organic biradical in place of oxygen between silicon atoms has a chain-stiffening effect that serves the purposeof upsetting the regular polysiloxane structure and hence will inhibit crystallization in the same way as the inclusion of bulky side groups. The mechanical properties of elastomers prepared from these randomcopolymers arenot significantly different from polydimethylsiloxane-based elastomers. It is general practice to identify elastomers prepared from silicone polymers according to ASTM designation D-1418-81 as MQ. VMQ. PVMQ, and FVMQ. The Q defines a silicone polymer of alternate silicon and oxygen chain atoms. The organic groups attached to the silicon atoms are designated by letter. A polymer labeled MQ indicates a chain with two methyl groups attached to each silicon atom. If other organic groups are substituted in place of some of the
Polmanteer
610
methyl groups. their initials precede the and F, 3,3,3-trifluoropropyl groups).
MQ (e.g., V indicates vinyl groups, P. phenyl groups,
2.3 Reinforcement It is necessary to reinforce the siloxane polymers used in elastomer applications. This is the case since the typical linear diorganosubstituted polysiloxanes used in commercial silicone elastomers are amorphous. flowable polymers at room temperature and have only approximately 0.34 MPa tensilestrengthwhencrosslinked. It is indeedfortunatethat the strengthcan be increased by a factor as high as 40 (to the 13.8 MPa range) by the addition of special silica fillers. This is certainlyphenomenal,since a comparativefactor of only 10 is realizedwith amorphous organic rubber polymers such as SBR, NBR, andpolybutadiene (Na polymer).Most commerciallyavailablesiliconeelastomershavetensilestrength values ranging from 5.6 to about 10.5 MPa. The exceptional increase in tensile strength of silicone polymer by silica compared to the more modest increases observed for amorphous organic rubber polymers is chiefly influenced by strongpolymer-fillerbonding and the sizableresponse of tensilestrengthincreasewith structure. This latter point is clearly demonstrated in Figure 5 by the work of Polmanteer and Lentz ( 1975).The strong bonding of the silica filler to the polymer is related to the combination of chemical and physical bonds. The physical bonds are strong and include both van der Waals
20
15
vi In lu
10
RE
0
200
400
600
800
% STRAIN
Fig. 5 Stress-strain curves to rupture for silicone elastomers filled with low- and high-structure silica. The ultimate particlc size for both silicas was 5.8 nm, and the pore volumes were 0.6 and 5.8 dm3/kg. (From Polmanteer, I98 I . )
Advances in Silicone Rubber Technology, Part I, 1944-1986 61
1
forces and hydrogen bonding of silica silanols (present even if silica is treated by reacting with an organosiloxane) with polymer silanols and the polydimethylsiloxane chain oxygen atoms. The story of reinforcement is very complicated and would require an entire volume to discuss all its ramifications. In a joint review article, Warrick and coworkers (1979) summarize the results from many referenceson silica reinforcement of silicone elastomers. These references deal with many aspects of polymer-filler interaction. It is concluded that a substantial level of polymer-silica bonding can and does occur. The exact nature of the bonding forces is dependent on the filler surface energy, including the accessibility of the silica silanol groups. In the case of hydrophilic silica surfaces, the bonding was found to be related predominantly to physical forces (hydrogen bonding and van der Waals forces)but not to the exclusion of covalent bonding upon vulcanization. In the case of hydrophobic silica surfaces, bonding was essentially totally physical in nature prior to vulcanization. However, upon vulcanization, covalent polymer-filler bondsareformed,as i n the previousinstance. The polymer-fillerbonding in thesesilicone elastomer systems is probably the reason that silicas exhibit the highest known reinforcement increase, by mechanisms described by Eirich (1972). Fillers important in high-consistency silicone rubber technology include avariety of materials that are inherently thernlally stable so as to not detract from the thermal stability of the silicone polymers. Among the more frequently used fillers are high-surface-area amorphous silicas, diatomaceous earth, quartz, zinc oxide, titanium dioxide, iron oxide. calcium carbonate, and many other heat-stable metal oxides. Among the commercially available fillers, amorphous silica in the 150-400 &/g surface area range provides the best reinforcement. A plasticizer described by Konkle et al. ( 1959) such as
1
CH3
is required in conlbination with these high-surface-area silicas to prevent undesirable polymerfiller interaction prior to vulcanization. When a suitable plasticizer is not used, softening of the stock on a mill is extremelydifficult. The other fillersmentioned above areextending-type fillers and do not provide the high level of polymer-filler interaction and reinforcement that the high-surface-area amorphous silicas provide. Carbonblack, an importantreinforcingfiller for organicpolymers. does not reinforce silicone polymers as well a s silica. and it also detracts from the thermal stability. It is therefore used only in applications where a low-resistivity silicone elastomer is desired. Most commercial high-consistency silicone elastomers contain a combination of several fillers to give ;I specified combination of properties for a particular type of application. For example, iron oxide and other metal oxides are often used in small quantities to improve such properties as thermal stability and compression set or to impart a particular color to the elastomer. Extending fillers are often used to improve the processing properties and reduce the cost of the elastomer. Typically, lowerconsistency silicone rubber compositions such as those used for RTVs and LSRs contain lower levels of fillers than high-consistency rubber compositions.
2.4
Vulcanization Methods
As in the case of other elastomers, the polymer molecules must be chemically linked together to establish three-dimensional stability. These chemical links (called crosslinks) are separated
612
Polmanteer
from each other by a few hundred chain segments normally in the range of 100-400 repeating units. In addition to these chemical crosslinks, there are chain entanglements that behave as effective crosslinks, at least in short-time experiments. When fillers are includedin the elastomer, many complications as to the nature of the total effective crosslinks are introduced; these are discussed by Polmanteer and Helmer (1965) and Polmanteer and Lentz (1975). The chemical routes to be described for the vulcanization of silicone elastomers will be confined to the primary technologically important systems: (1) elevated temperature cures and ( 2 ) room-temperature vulcanizing (RTV) mechanisms.
Elevated Terrpernrur-e Cures
Peroxides Bork and Roush( 1964) discussa number of different peroxides used commercially to vulcanize silicone elastomers. They differ in their reactivity with the organic radicals attached to silicon, and their decomposition productsalso differ.For example, benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, and t-butyl perbenzoate are each reactive enough to react with both methyl and vinyl groups, and their decomposition products include organic acids that in some situations are detrimental.On the other hand, such peroxides as di-t-butyl peroxide,dicumyl peroxide, and 2,5-bis( t-butyl peroxy)-2,5-dimethylhexane havemuchgreaterreactivitywith vinyl groups than with methyl groups and are more selective in their reactivity. In addition, they do not form organic acids upon decomposition but rather form ketones. which are less detrimentalandeasier to remove. The peroxides decompose to generatefreeradicals when heated. Although reactivity of the specific peroxide is very important in determining the reactions it will undergo, this will not be considered in the discussion on the possible crosslinking reactions. Table 2 lists the possible free radical crosslinking reactions that can take place during the course of peroxidevulcanization in methyl-andvinyl-containingsilicone polymers. It is of interest to note the difference between the reaction of a free radical with a vinyl group and with a methyl group as shown in the first two reactions in Table 2. In the case of the vinyl group the free radical adds to the vinyl group, while in the case of the methyl group a hydrogen atom is abstracted. leaving a methyl free radical attached to silicon. The products of the free radical reactions in boldface type indicate that crosslinksare formed in these particular reactions. Notice that there are several typesof crosslinks; that is, a different number of carbon atoms are involved in the crosslinks. The effects of vinyl content in the polymer and the concentration of t-butyl perbenzoate on the effective degree of polymerization between crosslinks, S,,are shown in Figure 6. It is apparent from this figure that the inclusion of vinyl in the polymer improves crosslinking efficiency until the vinyl groups have been used up. An important point is that this occurs at much lower concentrations of peroxide than in the case of an all-methyl-substituted polymer. This means that less peroxide is required for a good cure and less of the peroxide decomposition products will be formed to affect hydrolysis and siloxane rearrangement reactions. As mentioned earlier. the inclusion of small amounts of vinyl groups provides elastomers with better compression set properties. Hydrosilation Crosslinking canalso be accomplished with ahydrosilationreaction whereby a Si-H group will add to a vinyl group. A platinum catalyst such as chloroplatinic acid is normally used in this reaction (Fig. 7).A low molecular weight silicone molecule having several Si-H groups in it is mixed with a silicone polymer having severalvinyl groups attached to silicon atoms. The platinum catalyst causes the necessary addition reactions to occur, thus forming a crosslinked polymer network. These reactions are accelerated by heat but also occur readily at room temperature. Hence, this crosslinking method can be used either at elevated
613
Advances in Silicone Rubber Technology, Part I, 1944-1 986 Table 2 Possible Free Radical Crosslinking Reactions" H H H H H H A. (a) R + =Si-C=C +=Si-C-CR or "si"c-C* R H * H H H (b) R * + =Si-CH + R-H =Si-C* H H H H B. ( a ) I or 11 =Si-CH + (1)-H or (1I)"H =Si-C* H H (b) I or I1 ESi-CH + H H H
+
+
+
(1)
(11)
(11)
H
+
Esi-C-C-!jiK H H
(111)
+
(c) 111 = R H H H H H eSi-c=C + ==Si-C-C-C-sie H - H H H H H H (b) IV + "Si-CH =Si-C-C-C-Si= H H H H D. I + I1 (all combinations) + crosslink
C. (a) 11
+
"+
+ I1
Boldface Indicates crosslink formation [e.g.. B(b). C ( a ) .C(h). Dl Source: Polmanteer. 198 I . 'l
temperatures or at room temperature. Since this is an addition reaction, no volatiles are generated when the crosslinks are formed. This hydrosilation reaction is particularly important and widely used in LSR compositions.
Room-Temperature Vulcanization Room-temperature vulcanization is normally used withlow-consistencysiliconeelastomers. Being of low consistency, the material can be easily extruded from a tube or other container and then cured by mechanisms to be discussed. Two-Part Systems The hydrosilationmethod just described can be used to crosslink silicone elastomers (both low and high consistency) at room temperature. Since this system cures readily, even at room temperature, when the three necessary components, SiH-containing crosslinker, Si-Vi-containing polymer, and platinum catalyst, are mixed together, it is referred to as a two-part system. In other words. one part contains two of the components. and the second part contains the third necessary component. When vulcanization is desired, the two parts are mixed together, and after a cure inhibition period (to allow sufficient working time), vulcanization takes place. Hence, systems involving the mixingof two componentsfollowed by vulcanization are referred to as two-part cure systems. Condensation reactions can be used in two-part RTV systems. The reactions normally involve a functionalchain end and a polyfunctional compound. For example, a convenient method is to react a silanol chain end with a silicate such as polyethylsilicate (see reaction
614
Polmanteer
2.0
MOLE % V i
0 0 W
0.0714
1.5
l-
a
A 0.1429
O
N
z W
m U W a
V
0.333
J
>
l-
3 m
1.0
l-
OI
Lu ll-
z W
V U n W
0.5
C 0
200
400
-
600
800
DPC
Fig. 6 Effect ofvinyl Polmanteer, 1981.)
content in polymer and concentration oft-butyl
Fig. 7 Hydrosilation rcaction. (From Polmanteer, 1981.)
perbenzoate on DP,. (From
615
Advances in Silicone Rubber Technology, Part I, 1944-1986
H0
-
]-I!
iy3-[0
OSI CH3 - OH
+
ETO
CH3 X Y
x
=
100
-
l
200
Y.3-10
H WHERE:
ET
=
-
H
C R O S S L I N K E D RUBBER
+
H
c -c
H
H
ETOH
Fig. 8 Condensation crosslinking chemistry. (From Polmanteer,
1981.)
shown in Fig. 8). This cure system was filed with the U S . Patent Office in 1954, but the patent was not issued to Polmanteer until 1960 (Polmanteer, 1960). This is a simple condensation reaction catalyzed with fatty acid salts of tin, lead, cobalt, etc., giving a siloxane linkage and ethanol. Thus, the silicate, being polyfunctional, can react with several chain ends to tie the network together. The catalyst necessary for this reaction is normally not added until it is desired to have the crosslinking start; hence this system is referred to as a two-part system. Cure does not require atmospheric moisture,hence deepsection cures are possible.However, it is preferable for the alcohol by-product formed to rapidly diffuse out of the system so as not to retard the crosslinking reactions via silanol-ethanol reactions. One-Part Systems One-part curesystems canbe placed in one packageandremain non-crosslinked. When the material is exposed to a reactant, normally from the atmosphere, a sequence of reactions take place leading to the formation of crosslinks. Water in the atmosphere is the most common reactant of one-part RTVs. One-part cure systems that depend on atmospheric water to start the sequenceof reactions leading to crosslink formation have several points in common. Atmospheric water first reacts with hydrolyzable groups attached to silicon (Fig. 9). Once hydrolysis begins, the silanol formed can condense with either another silanol group or a hydrolyzable group on silicon, thus forming a S i U S i linkage and a condensation byproduct(Fig. IO). In mostinstancestheseby-products are acidic or basic enough to actas catalysts for the crosslinking (Si+Si-forming) reaction. These one-part systems must be packaged in water-impernleable containers to prevent crosslinking within the package. The two
S i O Y + HOH + S i O H
+ HOY
where O Y = hydrolyzable groups
Fig. 9 Hydrolysisreaction.(FromPolrnanteer,1981.)
616
Polmanteer
HOSE 3 i O H + or -+ S i - O - S i G YO S 6
+
HOH or HOY
Fig. 10 Condensation reaction. (From Polmanteer,1981.)
Name Acyloxy
Group
,OC-R (e.g.. R = CH3) 0
Oxime
-ON=C,/R 1 (e.g.. R 1 . R 2 = alkyl)
Alkoxy
-OR (e.g.. R =
Amlno
-NH,-, R3-P,where p = 1,2.3 R = alkyl or cycloalkyl
R2
CH,,etc.)
Fig. 11 Examples of hydrolyzable groups attached to silicon. (From Polmanteer, 1981 .)
generalized reactions just described can be applied to a wide variety of specific hydrolyzable groups. Examples of suitable hydrolyzable groups are shown in Figure 11. The reactivity of hydrolyzable groups attached to silicon is influenced by several factors such as the number of OY groups per silicon, number and kind of R groups also attached to silicon, and the substituents on the hydrolyzable group itself. For example, oximes (OX) are shown in Figure 12. Similar effects are observed with other hydrolyzable groups. To formulate a one-part sealant, it is normally best to attach the hydrolyzable groups to both ends of each polymer molecule. Using acetoxy groups as an example, under anhydrous conditions an excess of methyltriacetoxysilane can be mixed with a dihydroxy-ended polydimethylsiloxane to give a methyldiacetoxysilane group on each end of the polymer molecules. Fillers can alsobe added and thematerial packaged in watertightcontainers (Fig. 13). Depending on the specific hydrolyzable group incorporated, catalysts can also be added to assist in the condensation reactions. Although a specific catalyst may be applicable to only a certain type of RTV, examples of some of the catalysts include fatty acid salts of metals such as tin and lead, amines, amine salts, titanates, and aluminates.
2.5
Formulating Methods
Typical silicone rubber compositions are prepared by adding reinforcing silica filler, extending fillers,pigments, special additives (e.g., heatstabilityimprovers, flame retardants,handling property improvers, etc.), and a vulcanizing agent. If the surface of the reinforcing silica filler has not been modifiedby pretreatment to reduce polymer-filler interaction,a hydroxyl-containing low molecular weight silicone plasticizer may be used to prevent or minimize polymer filler interaction, thus allowing good shelf-aging characteristics (e.g., no crepehardening). The plasti-
617
Advances in Silicone Rubber Technology, Part I, 1944-1986
--
2.
.-
.->
U
m Q,
Si
VISI (OX),
(0x14
[r
RSi (OX),
PhSl (OX),
2
R2Si (OX)?
MeSl (OX),
B w
R,Si (OX),
0
C .v) m
V
EIS1 (OX), V
l
-SION=C(Me)2 =StON=CMeEl ~SION=C (El)?
Fig. 12 Order of reactivity of oximes. (From Polmanteer, 1981.)
cizer is usually dispersed in the polymer prior to addition of the reinforcing filler. With the exception of plasticizer.thematerials are added to thesiliconepolymer in the order listed above. Fillers should be added in small enough increments to avoid the formation of large filler agglomerates, and enough mixing time should be used to obtain good uniform dispersion of ingredients. This must be determined by the formulator and depends on the specific mixing equipment and the quantity of the composition being formulated. The type of mixing equipment suitable for formulating high-consistency silicone rubber includes two-roll mills, Banbury mixers, dough mixers, and continuous mixers. In the case of thetwo-rollmills,the roll speed ratioshould be in therange of 1.2/1 to 1.4/1. The lowerconsistency RTVs and LSRs are formulated using equipment capable of mixing and dispensing low- to medium-viscosity materials. Because of the highly competitive market and the manufacturing complexity of formulating these low-consistency RTV and LSR products,the compounding and packaging of these products is proprietary with the basic suppliers.
H
+ CH3Si(OCCH3)3 (excess)+ 0
CH3 (CH3C0)2Si-0
+ C H J S ~ ( O C C H J+) ~
0
0
0
Trace HOCCH
0 ( 2 ) Add Filler Pa*ge
(3)
R T V Compound
R T V Compound (packaged)
+ HOH +Crosslinked Elastomer + CHJCOH t 0
Fig. 13 Example of preparingaone-part
RTV. (From Polmanteer, 1981.)
618
2.6
Polmanteer Fabrication Methods
It is good practice to freshen a silicone rubber stock prior to fabricating into the desired product form for use. However, there are many stocks that are designed to not require freshening prior to fabrication. Freshening is done by remilling on a two-roll mill. Silicone rubber has good flow propertiesunderpressureeven at ambienttemperatures. This property is responsible for its success in manyconventionalfabricationmethods.For example, thesefabricationmethods include compression molding, transfer molding, blow molding, extrusion, calendering, dispersion coating on fabrics and organic polymerfilms, andmandrel wrapping techniques. Fabrication details are extensive, and space does not permit a detailed discussion here. However, Lynch (1978), in his book on silicone rubber fabrication, describes details of various fabrication methods as they pertain to silicone rubber.The chemistry of the various vulcanizing systemswas discussed earlier. but nothing was said about the mechanics of accomplishing vulcanization. This subject is paramount for a well-fabricated rubber part. For heat-vulcanized compositions, the method of applying heat to the fabricated shape becomes an important factor that must be integrated intothefabrication system. In moldingoperations such as compression,transfer, blow, and injection molding, both heat and pressure are applied via the heated walls of the mold. Extrusionsarecuredusinga variety of alternativemethodsincluding CV (continuous steam vulcanization), HAV (continuous hot-air vulcanization), and FBV (continuous fluid-bed vulcanization). The method used is chosen based on the applicability to the total fabrication process and the incremental cost added to the product being produced. Dispersioncoating procedures use a dispersion coating tower, which usually has three separate heating zones. The first zone has thetemperature set to drive off solvent below the vulcanization temperature, the second has its temperature setto accomplish vulcanization, and the third hasits set to driveoff vulcanization by-product volatiles. Mandrel wrapped fabricated parts are usually vulcanized in a steam autoclave.
2.7 General Properties and Uses The unusual combination of properties exhibited by silicone rubber has provided the basis for significant growth in the marketplace, with many new uses continually being unveiled since its 1944 introduction. The unique combination of properties were listed in the second paragraph of Section 1, and the inherent characteristics of silicone polymers that are primarily responsible for their properties was given in the third paragraph. By way of emphasis, these inherent characteristics include strong chain bonds, backbone chain flexibility, ease of rotation of the organic side chain groups, low inter- and intramolecular forces, and inorganic/organic makeup.
Polyn~erRheological Properties Polmanteer (1981) showed that the type of organic groups attached to silicon have a marked effect on rheological properties. For example, in Figure 14 the viscosity as a function of shear stress for silicone homopolymers is shown. This type of plot depicts the relative newtonian character of the homopolymers (e.g., Newtonian character increases with decrease in slope-a Newtonian fluid has a slope of zero). In this homopolymer series, Newtoniancharacter increases in the order of increase in size of the second group attached to silicon (e.g. methyl, propyl, trifluoropropylmethyl, and phenyl). The first group attached to each silicon atom is a methyl group. It may also be seen in Figure 15 that viscosity increases with the size of the second group for a given degree of polymerization, E. The linear plot of viscosity versus temperature in Figure 16 shows the dramatic effect of the particular R groups on the viscosity-temperature relationships. Figure16 clearly demonstrates
Advances in Silicone Rubber Technology, Part I, 1944-1986
619
Fig. 14 Apparent viscosity as a function of shear stress. (From Polmanteer, 1981.)
the small effect of temperature on the viscosity of polydimethylsiloxane, which is the most commonly used polymer in silicone rubber. Energy of activation for viscous flow values, E,,,,,, numerically define thesensitivity of viscosity to temperature. The measured E,,,,, values in kilojoules per mole for the homopolymers were 14.2 for polydimethylsiloxane, 18.0 for polymethylpropylsiloxane, 33.0 for poly(1nethyl-3,3,3-trifluoropropyl)siloxane,and 49.8 for polymethylphenylsiloxane. The molecular weight range of homopolymers studied did not appear to change the E,.,,, values. The values for the energy of activation for viscous flow give a good index for the level of inter- and intramolecular attraction forces. Low-Tentperuture Properties One of the salient characteristics of silicone rubber that make it stand out among all types of elastomers is its ability to remain flexible at very low temperatures. The factors that cause an
620
Polmanteer
1 05
1o4
10)
10’ m
(F, PrMeSiO)
v (PrMeSiO)
(Me,SiO) 10
0
1
2
DP, x
1
1
1
l
3
4
5
6
103
Fig. 15 Log Newtonian viscosity as a function of degree of polymerization. (From Polmanteer, 198I .)
elastomer to become stiff at some particular temperature are crystallization, nearnessto theglass transition, or a superposition of the two phenomena. Both of these phenomena can be affected by changing the makeup of the polymer. The crystallization temperature, T,, can be lowered and/or eliminated by the random inclusion of bulky side groups. If these bulky groups possess alargerintergroup or intermolecularforceconstant than methyl groups,theglasstransition temperature, T,, will be increased by an amount that depends on the molar concentration of these bulky groups. Polmanteer and Hunter (1959), using a Gehman cold-flex apparatus, showed how the random inclusionof phenylmethylsiloxane mer units changed the equilibrium stiffening temperature (see Fig. 17). The influence of the same polymer compositions on T, is shown in Figure 18. T, for MQ- and VMQ-based silicone rubber compositions is - 123°C while for the PVMQ extreme low-temperature compositions, it is slightly higher at about - 114°C because of the increase in intermolecular forces. The incipient crystallization temperature for MQ- and
621
Advances in Silicone Rubber Technology, Part I, 1944-1986
.96 .88
.eo .?Q .64
.56 Y)
4
0-
z -
.4a
0
c
.40
32 .24
.l€
.08
t 60 40
+
-T
1
70
50 "C
80
- TEMPERATURE
Fig. 16 Newtonian viscosity as n function of temperature. (From Polmanteer, 1981.)
VMQ-based silicone rubber varies somewhat with experimental test methods and compositional variables but is approximately - 54°C. This is lower than the equilibrium crystallization temperature, which approaches the T,,, value (-40°C).
Liquid Media Resistance Solubilityparameter, a,, valuesgive a good indication of whether an elastomer containing sizable amounts of a given siloxane unit will be resistant to a particular solvent or oil. For example, if the 6, of the solvent or oil is close to that of the elastomer, considerable swell is anticipated. The 6, values for some of the moreimportant siloxane units are givenin
622
Polmanteer 0
-2c V W
5
-40
k
a
EK
-60
5lU
zZ
-80
W
U-
-100
lv)
-120
I I 10 3020
-140
I
I
I 50
40
I
I
60
70
I 80
l 90
1
M % PHENYLMETHYLSILOXANE
Fig. 17 Stiffening temperature as a function of phenylmethylsiloxane molar content. (From Polmanteer. 1981.)
0
40
20
M
60
80
100
% PHENYLMETHYLSILOXANE
Fig. 18 Effect ofphenylmethylsiloxane content on glass transition temperature. (From PoImantecr, 198 1 .)
623
Advances in Silicone Rubber Technology, Part I, 1944-1986 Table 3 Solubility Parameters of Scvcrnl Siloxane Unlts Unit
6,
(CH1)2Si0 (CH3)?Si(/)./)’-ChH,)Si(CH1)~0 (CH3 NCCH2CHI(CH3)Si0 F3CCH2CH2(CH3)Si0
7.5 8.9 9.0 9.0 9.6
Source;
Polmnntcer. 198 I .
Table 3. Chemical-. solvent-, and oil-resistance data are given in Table 4.
for VMQ and FVMQ silicone elastomers
Su$ace Energy Properties The surface energy of a polymeric surface provides an excellent guide as to the ability of other materials to adhere to the surface. An organic material with a surface energy or surface tension higher than that of the substrate surface will not adhere to that surface. Silicone elastomers have lower surface energy values than most organic materials, including most foods. Consequently, the goodreleaseproperties of silicone elastomers allowtheir use in many food-processing
Table 4 Chemical and Oil Resistance of Silicone Rubber Volume change (9) Chemical or mcdia
WMU)
Acid (7 days at 24°C) 10% Hydrochloric Hydrogen chloride 10% Sulfuric 10% Nitric Alkali (7 days at 24°C) 10% Sodium hydroxide 50% Sodium hydroxide Solvent (24 hr at 24°C) Acetone Ethyl alcohol Xylene JP 4 fuel Butyl acetate Oil ASTM No. 3 (7 days at 149°C) Turbo Oil 15 (Mil L-7808)(1 day at 177°C) Dimethylsiloxane, 500 CS.(14 days at 205°C)
Fluorosilicone rubber (FVMQ) +l +8 Nil +l
Nil
+ 180 + 180 +S
+ 20 + 10 >IS0 +6
+8 Nil
Siliconc mbher
f 3
+ IS +S +8 Nil +B
+ IS +B
>IS0
> 150 > IS0 + 2 0 to +S0 30 Swells, deteriorates
Polmanteer
624
Table 5 Surface Energy Values of Polymers Polymer Polydimethylsiloxanc Polytrifluoropropylmethylsiloxanc Polyphenylmethylsiloxane
Polystyrene Poly(viny1 chloride) Polyethylene Poly(viny1 alcohol) Poly(viny1idene chloride) Polyacrylamide Polyacrylate Poly(ethy1ene terephthalate) Poly(methy1 methacrylate) Polytctrafluorocthylene Polyhexafluoropropylcne Polytrifluorocthylene Poly(viny1idene fluoride) Wool
Starch Cellulose (regenerated) Amylose
21 -22 21-22 26 33-35 39 31 37 40 35-40 35 43 33-44 18.5 16.2-17.1 22 25 45 39 44 37
operations and in the manufacture of thermoplastic polymeric end products such as films. Many applications utilizing the low surface energy of silicone elastomers also take advantage of their other properties such as good high-temperature resistance. The surface energy values given in Table 5 demonstrate how various polymers compare. It can be noted that the only polymers having lower surface energy than polydimethylsiloxane are some of the highly fluorinated polymers such aspolytetrafluoroethylene. The low surface energy of polydimethyl-siloxane is maintained even when reinforced with silica. The low surface energy of silicone elastomers suggests applications as both abhesives and adhesives, depending on how they are used with materials having other surface energy values.
Physiological Inertness Properties The use of silicone elastomers as body prostheses began in about 1956. with the result that these elastomers are generallyconsideredphysiologically inert. However,eachapplicationhasits associated boundary conditions that determine the results for a specific use. One of the very successful implant uses is the replacement of diseased finger joints with silicone rubber joints.
Pertneability Properties Siliconeelastomers are based on polymers having low intermolecularforcesandrelatively unhindered single bonds that link the alternate silicon and oxygen backbone chain atoms together. These facts combine to provide a polymer of a higher than normal amount of free volume and a high degree of chain mobility. These characteristics explain why it behaves like a rather open
625
Advances in Silicone Rubber Technology, Part I, 1944-1986 Table 6 ContinuousExposureTemperature as a Function of Service Life Temperature ("C)
15,000 7,500 2,000 100-300 0.25-0.50
150
200 260 316 37 1 Solrrcet
Service lifc (hr)
Courtesy o f Dow Cornmg Corporation. Mid-
land. MI.
screen to gases. For example, a silicone elastomer based on polydimethylsiloxane is 25 and 429 times more permeable to oxygen than natural rubber and butyl rubber. respectively.
Therrwal StabiliQ Properties Silicone rubber has long been recognized as the rubber of choice for high-temperature service uses. For example, the data in Table 6 are for samples formulated to enhance high-temperature stability. The samples were exposed at the indicated temperature continuously until the elongation decreased to 50%, with the time necessary for this to occur considered the service life under the oven-aging conditions. Actual service conditions vary with time and make accelerated test results difficult to use directly in predicting actual life expectancy. Fortunately. the continuous exposure service times are usually very conservative, with actual service times being longer in those caseswhere service conditionsconsist of high-temperature air. When the high temperatures accompany contaminants such as oils, sulfur oxides, and nitrogen oxides. the service life can be shortened.
Electrical Properties The electrical properties of silicone rubber are very good in general but can be varied by the type and amount of compounding ingredients used in the composition. Since electrical properties can be varied by compounding ingredients, special electrical application compounds have been developed for such applications as wire and cable insulation and rubber insulating tapes. Table 7 lists the range of electrical properties typical for silicone rubber.
Table 7 ElectricalProperties Test Electric strength (1/4-in. electrodes, rapid rise on specimens 1/16 in. thick), V/mil Dielectric Dissipation Volume resistivity, ohm-cm Insulation ohms resistance,
Results of 500 V/sec,
450-600 2.9-3.6 0.0005-0.2 8 x 10'3-2 x 10'5 1
x
10'2-1
x
10'3
Polmanteer
626
Mechanical Pwyerties and Uses The applications for silicone elastomers cover a multiplicity of areas. Both the number of uses for silicone elastomers and the volume used are increasing at a good rate as a result of rapid advancements in all technology areas. As a consequence of these rapid changes in technology, many new silicone elastomer applications have been born that require a high level of performance under severe conditions. In every instance the reason for selecting a silicone elastomeris related to its uniqueproperties, which allow it to be functional in theindicatedapplication,while organic elastomers fail for various reasons such as poor thermal stability, poor low-temperature flexibility, poor ozone resistance, low gas transmissibility, poor weatherability,or lack of physiological inertness. Examples of the types of commercial silicone elastomers available from theDow Corning Corporation are listed in Tables 8 and 9 with their property profiles and some of their major uses. Similar products are also available from other manufacturers. These elastomers are sold both in a form containing the vulcanizing agent and in an uncatalyzed form. The uncatalyzed compounds have improved shelf stability and permit more flexibility in fabrication methods by appropriate selection of special-purpose peroxides. The test specimens used for property measurement were preparedby vulcanizing and oven-curingthat rubber according to the specific recommendations for each particular stock. The general-purpose elastomers listed in Table 8 satisfy applications in a wide variety of areas. The extreme high temperature classification represents a group of elastomers specially designed for long life at high temperatures. These materials normally remain flexible for up to about 300 hours at 3 15°C. The high strength classification elastomers are based on phenylcontaining low-temperature polymers. These polymers were the first to be designed to give high strength and Die B tear values as well as improved low-temperature properties. Most of the wire and cable compounds are unique in that the raw stock does not require mill freshening before being fabricated, and they are supplied in hat form for direct continuous feeding to an extruder. The elastomers typical of those in the extreme low temperature classification in Table 9 are lower in strength than those in the high strength classification. However, they exhibit somewhat more resilience, which is important in some applications. Rubber in the low compression set classification give, exceptionally good compression set, even up to 250°C as shown. These materials. although showing lower Die Btear values, are more resilient and havemore resistance to tear initiation than some of the higher-tear compounds. Elastomersclassified as fuel, oil, and solvent resistant are based upon 3,3,3-trifluoropropylmethyl-substituted polumers. Compounds in the no postcure classification may be used in applications after vulcanization and do not require the usual oven postcure. These compounds are more reversion resistant than many of the materials represented in the previous classifications. It should also be pointed out that the compressionsetvalues are in many cases asgoodas or better than thoseformany of the compounds that require oven postcures.
3. 3.1
ADVANCES IN SILICONERUBBER Introductory Remarks
Advances in silicone rubber have been continuous since the first commercial silicone rubber becameavailable in 1944. They includeadvances in all aspects of siliconerubbersuchas technology,fabrication,andproductmarketing. In addition, new useshavedeveloped,and economics have continually improved to benefit the silicone rubber supplier. the fabricator, and the end user.
Advances in Silicone Rubber Technology, Part I, 1944-1986 627
628
Polmanteer
In some instances, various types of advances have combined and are jointly responsible for the growth in the silicone rubber market. The limited number of examples singled out for discussion here represent the key advances in silicone rubber. but are a small percentage of the actual total. This is reflected in the thousands of patents that have been issued worldwide in this field as well as the very large number of technical and commercial papers published. It is impossible to mentionherealltheindividualsand companies who have contributed to the progress in this field. This section will list advances in chronological order as first choice. However, in some instances it will be more convenient to keepthe type of advance suchas "new synthetic silicas" under one heading and simply list the year of pertinent events. Uses listed in this section are not all-inclusive, but instead are meant as representative examples.
3.2 Tensile Strength The first commercial advances in silicone rubber were primarily related to improvements in tensile strength. Hunter (1964) and later Warrick ( 1976) discussed these i n some detail. The stepwise increase in tensile strength is depicted in Figure 19 from Warrick (1976) in his Charles Goodyear Medal address of that year. Brietly, the principal contributions responsible for the tensile strength advances were polymerization improvements starting from polymer gels to high molecular weight linear polymers and copolymers, improved reinforcement with small-particlesize amorphous precipitated and/or fume silicas compared to the originally used larger-particlesize metal oxide fillers (e.g.. TiOz, ZnO, etc.), development of methods to retard or eliminate "crepe hardening" of amorphous silica-reinforced compositions (e.g., low molecular weight hydroxyl-containing silicone fluids-work in 1950 by Konkle, McHard, and Polmanteer with patent issued much later in 1959), and introduction of vinyl groups to the polymer first reported by Marsden (1948) but not commercially used until the early 1950s.
629
Advances in Silicone Rubber Technology, Part I, 1944-1986
16
Tensi 1 e MPa
I
12
a
I 44
48
52
56
60
64
YEAR
Fig. 19 Tensilc strengthimprovemcnt step curvc. (From Warrick, 1976.)
Figure 19 provides a historical review of improvements in tensile strength that produced satisfactory range for many applications. Once a desirable tensile strength range was achieved, attention was focusedon other properties needing improvement, suchas tear strength, toughness, and flammability resistance.
21
3.3 Room-TemperatureVulcanization The first commercial RTVs were sold in 1954 and used the crosslinking chemistry shown in Figure 8. During subsequent years many specific new leaving groups were identified with the chemistry depicted in a generalized manner in Figures 9- 13. Hydrosilation chemistry, discussed in Section 2.4. is also occasionally used in RTV products. The hydrosilationreaction is an addition reaction rather than the condensation reaction used in other RTVs and is often used where by-products cannot be tolerated. Advances in RTVs include significant improvements in their mechanical property profile such that the mechanical propertiesof special high-strength and tough RTV products are comparable to those of high-consistency silicone rubber. Interested readers should contact silicone rubber manufacturers for a listing of specific products and attendant properties. FMQ-based RTVs were developed by Dow Corning Corporation in the late 1950s and exhibit resistance to many solvents and oils in addition to having other properties typical of RTVs. RTVs in general embrace an interesting combination of properties that make them very desirable for a very large number of applications. These special properties include ( I ) a consistency range from flowable liquids to soft pastes; (2) excellent thermal stability characteristic of high-consistency silicone rubber; (3) good adherence to most surfaces without the application of pressure; (4) The ability to replicate fine detail such as record grooves, newspaper pictures and print, wood grain, and leather detail; (5) curability without heat, as the name RTV implies; and (6) all the properties of silicone rubber discussed in Section 1 of this chapter. The above combination of properties make silicone RTVs particularly well suited to construction industry uses such as sealants, adhesives. or protective coatings with masonry, metal, wood, plastic, and glass substrates for outdoor and/or indoor locations. RTVs have found applications in seals for
630
Polmanteer
automotive, appliance, and lighting areas.A particularly sizable use is as fot-med-in-place gaskets (FIPG), wherein a thin bead of a single-component RTV silicone adhesive sealant is applied to one of the surfaces, which is then pressed with the mating surface to provide an excellent seal within a few minutes.The characteristics of RTVs makethem very good mold-making materials for such things as furniture, art objects, and many other items. RTV applications are limited only by the imagination and innovative ability of those using them.
3.4 Copolymers (VMQ; PMQ and PVMQ; FMQ and FVMQ) The first patent that teaches the use of vinyl groups i n silicone polymers was issued to Marsden ( 1948) and assigned to General Electric. As so often is the case, work was also being done in competitive silicone-manufacturing companies. The use of vinyl groups was very important, since it significantly improved the peroxide crosslinking efficiency shown in Figure 6 (Sec. 2.4). One of the benefits was the much improved compression set of VMQ compared to that of MQ silicone rubber (e.g., from >20 to
Advances in Silicone Rubber Technology, Part I, 1944-1986
631
these deficiencies and could be used to make molded, extruded, or calender-coated parts. This product displays high tear strength (about 43.8 kN/m), excellent tensile strength (8.75 MPa), high elongation (about 500%). “lively” dynamic properties. and increased adhesion to most metals. Likepreviousfluorosiliconerubbers. it possessesgoodresistance to fuelsandoils, remains rubbery at service temperatures from - 57 to 232°C. and has good dielectric properties. with a volume resistivity of about 3.8 X 10” ohm-cm.
3.5 New Silicas It was mentioned earlier that fumed silica provides better reinforcement than the metal oxide fillers such a s titaniun1 dioxide and zinc oxide that were first used. Warrick (1976) described his group’s1948experiments with fumed silica in MQ and obtainedtensilestrengthsthree times better than the early Ineta1 oxide-filled silicone rubber. By 1949 silicone rubber products reinforced with precipitated silica were introduced. although the amorphous precipitated silica did not reinforce the silicone polymers as well as fumed silica. However. by l951 the betterreinforcing fumed silica was being used in some products. A significant advancement was made in the discovery of the virtues of amorphous precipitated and fumed silicas in silicone polymer. There was an incentive to synthesize a silica having an organosiloxane surface, along with particle size Llnd structure conducive to even better reinforcement. As aresult. an active research program was directed toward this goal. After several patent applications were filed. a comn1ercial product. Silastic 9 16, was introduced in 1956. This product provided tensile strength values well above thoseof other silicone rubber productsat the time.It also provided excellent vibrationdamping properties and was used extensively by the Lord Manufacturing Company in shock mounts to protect both delicate and heavy equipment. A disadvantage was the higher price, as the silica was more expensive than the commercially available precipitated and fumed silicas. Because of this price problem and advancing technology using fumed silicas. the product was discontinued in 1974. The next silica advance in the silicone rubber area was briefly described in a presentation at the 1983 ACS meeting in Washington. DC, by Polmanteer and Falender (1984). At the 127th Rubber Division of ACS meeting in 1985. coauthors Lutz, Chapman. and Polmanteer i n three companionpapersdescribed in detail the synthesis,reinforcementperformance, and use in optical applications of this wet-process hydrophobic (WPH) silica developed especially for use in silicone elastomers. The synthesis of this WPH silica is shown in Figure 20. This WPH silica process provides a technology that allows control of surface area, structure, and surface treatment. These are the primary factors that determine the extent to which a silica can reinforce silicone elastomers. Thus,it is possible to tailor the silica toachieve specific desirable elastomerproperty profiles. The reinforcing capability of WPH silica is greater than that of pyrogenic silica. as shown i n Table 10. The unique method of passivating the silica by introducing the treating agent prior to formation of the silica eliminates the need for separate processing steps to passivate the silica and produces well-treated silica. a s evidenced by exceptional shelf stability of silicone bases containing WPH silica. One of the salient features of WPH silica is that it has a narrow distribution of particle sizes with the largest dimensions between 5 0 and 100 nm. which is substantially smaller than the shortest wavelength (400 nm) of the visible light spectrum. Because of the small particle size. visible light is not seriously dispersed as it passes through WPH-reinforced silicone rubber. With “clean room” formulating conditions to elininate foreign matter contamination. optically clear highly reinforced silicone rubber can be prepared. It is not necessary to match the refractive indices of the WPH silica and the silicone polymer.
Polmanteer
632 H20
+
CH30H
I
+ NH3
e H y d r o p h o b e Agent
\L Hydrophilic Silica Alcogel*
L Hydrophobic Sillca Alcogel*
* A l c o g e l = a l c o h o l w e t t e d gel Fig. 20 Synthesis of wet process hydrophobic (WPH) silica. (From Lutz ct al.. 1985.)
During recent years the applications and potential applications investigated for optically clear silicone elastomers have included transparent protective masks. corneal prostheses such as intraocular lenses and contact lenses, medical tubing, flexible light guides, and clear flexible industrial tubing for hot liquid transfer. The fact that this WPH silica technology is capable of simultaneously providing a substantial level of reinforcement along with optical clarity that can bemaintained over a broadertemperature range thanpreviouslypossiblewithisorefractive compositions should open the door to new applications in the future.
3.6 Water-Base Silicone Rubber The advance involving the preparation and use of water-base silicone rubber evolved from early work of Findlay and Weyenberg ( 1 966), who discovered how to form stable silicone emulsions
Table 10 Comparison of Rcinforccmcnt of WPH Silica and Pyrogenic Silica
Filler' MS-75 MS-75 MS-75 MS-75 MS-75 WPH WPH WPH WPH W PH
Filler loading (phr) (MPa) 30 40 50 60 70 30 40 50 60 70
Plast. (mm) 1.S7 1 .57
2.62 3.38 4.14 2.44 2.74 3.20 4.67 5.61
Modulus at 100% elong.
Tensile strength ( MPa)
0.69 0.83 1.07 1.24 1.65 0.97 1.10 I .24 1.S2 I .79
7.3 1 8.24 9.34 9.28 8.55 7.3 1 9.93 12.62 14.00 14.13
Elong. break (8)
Strength at break (MPa)
540 480 S20 47s 40s 460 480 S30 540
46.78 47.79 57.92 53.92 43.18 40.93 57.59 79.49 89.58 89.05
at
S30
Advances in Silicone Rubber Technology, Part I, 1944-1986
633
via emulsion polymerization. The work of Johnson et al. (1980) provided practical routes to reinforced silicone latex. The intriguing scientific and engineering features such as processing ease associated with polymer-in-water emulsions led to a significant amount of effort aimed at silicone rubber. Saam et al. (1981) described the mechanisms of how silicone rubber is formed and crosslinked by these water-base silicone systems. Ingredients incorporated during and after polymerization included dimethylsiloxane oligomers, ionic surfactants and catalysts, organotin compounds, colloidal silica, and water.In one sense. the colloidal silicaserves the same function as amorphous fumed silica does in more conventional silicone rubber technology: it increases modulus and ultimate strength and improves swelling resistance. Products based on this technology provide thin films from aqueous media. The films possess excellent exterior durability and adhere to most common substrates without primer. The films are truly elastomeric and retain their flexibility over a wide range of temperatures. Typical tensile and elongation values are 3 MPa and770%, respectively. From a practical standpoint, the water-base silicone rubber provides material of lower viscosity than other formsof silicone rubber andis free fromhazards associated with handling solvent dispersions. Because of processing advantages as well as the easy cleanup features commonly associated with latex paints, the materials are being considered for commercialization for applications such as construction coatings, water containment, and several other areas. To avoid misapplication of thesematerials, it shouldbepointedoutthatwater immersion of these films increases acid and base attack, reduces the volume resistivity from about 8 X IO" ohm-cmto 8 X 1 0 ' ohm-cm as curedfilms,and slightly reduces thermal endurance.
3.7
High-Tear Strength, Tough Silicone Rubber
There are applications that require elastomers possessing a combination of high strength, high tear, and good resilience. It may be noted that this combination of properties does not exist for any of the classifications of silicone elastomers in Tables 8 and 9. To satisfy this need, Dow Corning Corporation research and development personnel developed and introduced in 1966 a new family of products represented by the third set of data given in Table l 1. This commercialization advance evolved from research efforts initiated by Polmanteer (1953) and demonstrates the long time lag that often exists between idea of first work and actual commercialization. The approach involved altering the type of crosslink distribution within the network primarily by blending small amounts of high-vinyl-content polymer molecules with low-vinyl-content polymers. Vulcanizationcreated local highconcentrations of crosslinks betweenrelativelylong
Table 11 Abrasion and Tear Propagation Resistance Profile Hardness strength, durometer (Shore A) 50 50 50
Bnshorestrength"
.' ASTM D624-54. Die B. " A S T M Dlh30-61. ASTM D813-59. Soro-ce: Polmanteer. 1970. L
Abrasionh
propagation"
(8) (cycles/1.27 cm) (revJ0.254 cm)
(kN/m) MPa 6.90 10.34 9.66
of Silicone Rubber versus Physical Property
17.50 33.2s 35.00
58 29 4x
1S5
300 1600
120 x20 150.000
Polmanteer
634
intervals of polymer network chains in these polymer blends. This increased energy loss mechanisms shy of rupture and hence provided improved tensile and tear properties. This technical approach has since appeared in U.S. patents to Wada and Ito (1972) and Bobear (1972). The data in Table 11 may be compared with the earlier high-strength but low-resilience elastomers given in the second set of data in Table 11. The high-resilience stocks listed previously lacked the high tensile strength and high tear strength (first set of data in Table 11). It should be noted that thecombination of hightensileandtear strengths and high resiliencegavesignificant improvements in resistance against abrasion and tear propagation. These improvementsin resistance to abrasion and to tear propagation clearly demonstrate the significant improvement in toughness. Extensions and variations of this high-tear strength, tough rubber technology have continued to the present by most of the manufacturers of silicone rubber.
3.8 Miscellaneous Advancements in Other Special Properties Other advancements include (1) methods of acid neutralization from peroxide decomposition or oil decomposition, (2) methods of improving flame retardency, (3) methods of improving handling and processing properties, (4) methods of improving heat stability, (5) methods of incorporating mold release additive, and (6) a method of improving tensile strength in compositions that contain extending fillers, etc. In these areas, inventors with the major silicone manufacturers accrued many patents, covering the period of about 1945 to 1986.
3.9
Liquid Silicone Rubber
In theintroductoryremarks(Sec. 3.1), it was mentioned that progress sometimes embraces more than one type of advance. LSR is a good example, since it represents a combination of technological, fabrication, and new marketing advances. LSRs became commercially available in the 1976-77 time period. Presently there are three suppliers in the United States: Dow Corning Corporation, General Electric Company, and Shin Etsu from Japan. LSRs are 100% solids, are pourable or pumpable in consistency, and have a two-component cure system that vulcanizes rapidly (in a few seconds) at elevated temperatures via the hydrosilation chenlistry described earlier to provide silicone elastomers having properties comparable to many of the conventional high-consistency silicone products. Using automatic meter-mix equipment, the two liquid parts and any additives or pigments are mixed and pumped to the mold or extrusion die. They require less total processing energy than the conventional products. For example, about one third as much total energy is consumed per spark plug boot (four-cavity injection mold) using LSR as with high-consistency silicone rubber in a 40-cavity compression mold. Because of these attributes, for large production volumes the final cost per small rubber part made of LSR can be less than similar parts made of organic rubbers that cost one tenth as much per pound as LSR. In addition to being readily injection molded, LSRs may be coated on wire, cable, or reinforcing fabrics. For details of specific products and properties, the manufacturers should be contacted.
3.1 0 Silicone Rubber Compounding System
In 1977 DowCorning Corporation made a significant advancementin the field of silicone rubber by introducing the Silastic Compounding System (SCS). SCS included both new marketing and
Advances in Silicone Rubber Technology, Part I, 1944-1986
635
technology that benefited the basic producer,the fabricator, and the end user. The basic producer benefited with economies of scale, resulting in lower material costs and faster delivery. It provided compounding flexibility that enabled the fabricator to develop proprietary silicone elastomers by blending bases and using various additives and modifiers. Silicone rubber prices have been reduced by as much as 75%. SCS consists of a limited numberof bases (32 in 1987). each containing polymer. reinforcing filler, and a stabilizing plasticizer to prevent crepe hardening. These bases are available with special properties such as no postcure, high strength, high tear strength, flexibility at very low temperatures. and low compression set. according to their intended application, such as generalpurpose or wire and cable insulation. A number of modifiers are also included as part of the SCS concept. These modifiers may be added by the fabricator to neutralize acids, retard flame, harden or soften the stock. improve green strength, improve heat stability, provide internal mold release, and improvetensilestrength when extendingfillers areadded.ThisSCS approach replaced the many specific silicone rubber stocks whose number had increased through the years with many used only in small volume. In early 1983 the General Electric Company provided their version of a silicone compounding system under the Silplus brand.
3.11CostIPerformance The sizable decrease in the value of the U.S. dollar between 1944 and 1985 is well known. Despite this trend. the actual cost (in U.S. dollars) of silicone rubber decreased. Warrick (1976) presented a figure showing a comparison of the wholesale price indices of synthetic organic rubberandsiliconerubber,whichclearly showed the much lowerpriceindices for silicone rubber over time. Since prices vary with many economic factors, suppliers of silicone rubber should be contacted for current costs. The other sideof the picture is performance control affecting the life of the rubber, which directly intluences replacement costs over time. The performance of silicone rubber is far superior to that of other elastomers in most rigorous environments, and hence it lasts much longer. This has become increasingly important with the increased emphasis on safety and reliability. This all leads to the realization that cost/performance advancements of silicone rubber truly should be considered important.
3.12 Silicone Rubber for Medical Uses Many physicians. university medical researchers, and others connected with the medical profession began in the early 1950s to contact Dow Corning Corporation for information about and samples of silicone rubber for use in their medical research. In 1956 Dr. R. R. McGregor was given the assignmentof responding to the many requests.By 1959, the Center forAid to Medical Research was formed and operated as a free service with Dr. McGregor as the director. The requests and expenses forthis service continued to escalate to the extent that a Medical Business was formed in 1962 to defer the special fabrication costs. Silicone rubber fabricated into many parts for use within the body and outside as in blood pumps has truly improved the quality of life for many people. Mantle (1985) described a new Silastic OB obstetrical cup, which should eventually eliminate the use of forceps in delivering infants. Examples of the use of silicone rubber in a variety of applications were described by Frisch ( 1984) and are given in Figures 2 1-36.
636
Polmanteer
Fig. 21 A silicone elastomer hydrocephalus shunt. This type of shunt is used to drain cerebrospinal fluid from the ventricle of the brain to either the vascular system or the pentoneal cavity. The first hydrocephalus shunt was developed by Holter in 1955. The shunt shown here contains a dual flushing chamber to ensure continual function of the shunt and is designed to drain cerebral spinal fluid from theventricle of the brain to the peritoneal cavity. (From Frisch, 1984).
Fig. 22 Positioning of the hydrocephalus shunt in a child's body. The entire shunt is implanted subdermally. The tip of the shunt is inserted into the ventricle of the brain through a hole made in the skull, while the drainage catheter is placed in the peritoneal cavity through a small incision in the pentoneal lining. An extra length of the peritoneal catheter is generally left so that the child can grow without dislodging the catheter from the pentoneal cavity. (From Frisch. 1984.)
Advances Silicone in
Rubber Technology, Part I, 1944-1986 637
Fig. 23 Flexible hinge finger jointimplants designed by Alfred B. Swanson, M.D., for use in reconstruction of diseased or destroyed finger joints. The high flexural durability of these implants is derived from the design of the load-distributing hinge and the flexural fatigue resistance of medical grade high-performance silicone elastomer. (From Fnsch, 1984.)
Fig. 24 Surgical placement of the flexible hinge finger joint implant. The metacarpal head is removed to create an appropriate joint space, and the intramedullary canals are then prepared to accept the implant stems. When the implant is placed in position, the stems fit securely in the intramedullary canals with the flexible hinge permitting 90" active motion. Joint space is maintained by transfer of the compressive forces of joint motion across the implant to cortical bone. Careful attention to reconstructions of tendons, ligaments, and joint capsules and postoperative therapy are very important in this procedure. (From Frisch, 1984.)
638
Polmanteer
Fig. 25 Typical appearance of a hand deformed by rheumatoid arthritis and a candidate for reconstruction by implant resection arthroplasty. Ulnar deviation and subluxatlon in the metatarsophalangeal joints and deformity of the thumb are evident. (From Frisch, 1984.)
Fig. 26 Appearance of the hand shown in Figure 25 after reconstruction. The hand now has essentially a normal appearance and is pain-free, mobile, and functional. (From Frisch, 1984.)
Advances in Silicone Rubber Technology,
Part I, 1944-1986 639
Fig. 27 Chn implants molded from medical grade silicone elastomer to increase the projection of the mandible. (From Frisch, 1984.)
I.
Fig. 28 Preoperative appearance of a patient who believed her quality of life would be improved by a chin augmentation. (From Frisch, 1984.)
640
Polmanteer
Fig. 29 Postoperative appearance of the same patient shown in Figure 28. (From Frisch, 1984.)
Fig. 30 An ear implant molded from medical grade in ear reconstruction. (From Frisch, 1984.)
silicone elastomer and used as artificial cartilage
Advances in Silicone Rubber Technology,Part I, 1944-1986
641
Fig. 31 Preoperative.appearanceof a child with a missing ear. (From Frisch, 1984.)
Fig. 32 Postoperativeappearance of the same child shown in Figure31 following ear reconstruction with thesilicone elastomer implant.His own subcutaneous tissue and skin were shapedaround the silicone framework during the process of ear reconstruction. (From Frisch, 1984.)
Polmanteer
642
Fig. 33 Preoperative appearance of a patient who has undergone a unilateral mastectomy for carcinoma of the breast. (From Frisch, 1984.)
i"
Fig. 34 Appearance of the patient shown in Figure 33 following reconstruction of a breast shape with a silicone-gel type mammary implant. The nipple may be reconstructed by a split thickness skin graft from the remaining nipple, or the color can be established by tattooing. (From Frisch, 1984.)
Advances in Silicone Rubber Technology,Part I, 1944-1986
643
Fig. 35 Preoperative appearanceof an adult female patient who has not developed normal female breast contour. (From Frisch, 1984.)
Fig. 36 Postoperative appearanceof the patient shown in Figure 35 following breast reconstruction with silicone-gel mammary implants. (From Frisch, 1984.)
644
Polmanteer
Fig. 37 Miscellaneous hosesand other silicone components after being in use for 60,000 miles ona test car. The silicone maintainedits flexibility and “like new” appearance. (Courtesy of Dow Coming Corp., Midland, Michigan.)
3.13 Examples of Uses Figures 37-47 provide examples of high-consistency silicone rubber uses (Figs.37-43), silicone RTVs (Figs. 44-46), and an interesting and increasingly important application (Fig. 47) for LSR as a keyboard pad in the Sinclair Spectrum home computer.Sinclair Research Ltd. specified the Silastic liquid silicone rubber formulation after testing thekeypads for over 5 million cycles. The keys stayed lively and “responsive” to a degree unmatched by any organic polymer. The liquid rubber also molds easily and rapidly with low scrap rates. The keyboard assemblyconsists of silver circuitry on two sheets of Mylar plastic, kept apart by a plastic interleaf with holesat the switch points. The silicone rubber padfits over the plastic sandwich, giving aresilient touch as keys are pressed to bring the contacts together. Liquid silicone rubbers are used inthe manufacture of many interesting small parts where large volumes make them economical compared with organic rubber as well as providing the typical good performance of silicone rubber in hostile environments.
Advances in Silicone Rubber Technology, Part I, 1944-1986 645
Fig. 39 Turbochargers create severe conditions for hoses. This silicone rubber turbochanger duct is on heavy-duty equipment. (Courtesy of Dow Coming Corp., Midland, Michigan.)
Fig. 40 Injection molded on steel inserts, beads of silicone rubber provide good seals on head gaskets, rocker cover gaskets, and intake manifold gaskets. The temperature resistance and low compression set of the silicone allow many of these gaskets to be reused. (Courtesy of Dow Coming Crop., Midland, Michigan.)
Fig. 41 Transmission shaft seals were one of the first applications of fabricated silicone rubber automotive parts. Used by a number of automotive manufacturers, the seals retain thelr elasticity despite exposure to heat and cold and contact with oils and greases at high shaft speeds. The front and rear crankshaft seal of a V-8 engine is shown here. (Courtesy of Dow Corning Corp., Midland, Michigan.)
Polmanteer
646
I r-4
~~
l I
d 4 l
1
~~
~
~
-
"-
Fig. 42 This silicone rubber axle joint boot proved Its ability to survive tough exposure to 150°C temperatures when tested in taxicabs for over 100,OOO miles. The boot developed for "X"-body cars holds grease within the joint while protecting it from contaminatlon.
Fig. 43 With this mushroom-type flap valve, the pump cavity in Ford carburetors was reduced by 50%. The fluorosilicone (FVMQ) valve will remain flexible at all workmg temperatures and is not affected by gasoline. (Courtesy of Dow Coming Corp., Midland, Michgan.)
Fig. 44 Silicone sealant ( R V )applied around truck axle Joints virtually eliminates leaks despite the high temperatures and vibrations of actual road conditions. (Courtesy of Dow Coming Corp., Midland, Michigan.)
Fig. 45 Silicone sealant (RTV)used as a formed-in-place gasket eliminates the need for an inventory of preformed gaskets. This particular application also illustrates how simply the sealant can be applied to a vertical surface. (Courtesy of Dow Coming Corp., Midland, Michigan.)
k Fig. 46 Sealing Xplorer motor homes was difficult because the sealant (RTV) had to adhere to fiberglass, metal, wood, and glass to maintain good seals between these dissimilar materials. Silicone sealant (RTV) was a perfect solution. (Courtesy of Dow Coming Corp., Midland, Michigan.)
Fig. 47 Silicone rubber keys for the Sinclair computer are marked by silk-screen printing. The keypads are also useful for many special-purpose computers such as those used in process controls, telephones, and even vending machines. (Courtesy of Dow Coming Europe, Brussels, Belgium, and Dow Coming Corporation, Midland, Michigan.)
648
Polmanteer
ACKNOWLEDGMENTS
I would like to extend my appreciation to Dow Corning Corporation, Midland. Michigan, and especially F. W. Gordon Fearon, for allowing the writing of this chapter and providing aid and encouragement duringthe lengthy process.The valuable assistance of Glenda McCombin typing and overseeing the reproduction of the manuscript is much appreciated. I also thank several Dow Corning Corporation persons for helpful discussions: R. M. Fraleigh. E. E. Warrick, M. C. Murray, C . M. Monroe, L. D. Fiedler, E. E. Frisch. M. E. Thelen, C. A. Romig, and R. G. Dean. Finally, I thank my wife, Donna, for her patience, since the writing was done during home hours, which required her important support. REFERENCES Bobear, W. J. (1972) U.S. Pat. 3,660,345 (to General Electric Co.), May 2. Bark, P. G., and Roush, C. W. (1964),in Vulccrrzizcrtiow ofElcrstorlwrs (G. Akger and 1. J. Sjothum, Eds.), Reinhold, New York, Ch. 1 1. Carmichael. J. B., and Heffel, J. (1965a), J. Plzqs. Chew. 69:22 18. Carmichael, J. B., and Heffel, J. (196%). J. Pkvs. Clzert~.69:2213. Carmichael, J. B., and Winger R. (1965), J. Polyrtz. Sci. A3:971. Elrich, F. R. (1972), Mechcrrziccrl B e h v i o r of Mcztericrls, Proc. 1st Int. Conf.. Vol. 3., 405. Findlay, D. E., and Weyenberg, D. R. (1966) (to Dow Corning Corp.), U.S. 3,294,725. Fischer. D. J., Chaffee, R. G., and Flegel, V. (1960), Rrtbhrr Age 8759. Frisch, E. E. (1984), in Polynleric Mnteriuls c/tzd Artjficicrl Orgcrrzs, 0097-6156/84/0256-0063 $09.75/0, 01984 American Chemical Society. Hunter, M. J. (1964), Kmtsch. Gurnrtli Kunstst. Y:498. Johnson, R. D., Saam, J. C., and Schmidt, C . M. (1980) (to Dow Corning Corp.), U.S. Pat. 4,221,688. Konkle, G. M,, McHard, J. A., and Polmantecr K. E. (1959). U.S. Pat. 2,890,188 (Junc 9, 1959). (Appl. Dec. 17, 1951) (to Dow Corning Corp). Lutz, M. A., Chapman. H. L., and Polmantecr, K. E. (1985), Rubher Chertr. Techrlol. W 7 5 l , 765, 777. of Silicorw Rubber Fcrhriccrtion, Van Nostrand Reinhold, Ncw York. Lynch, W. (1978), Htrt~dt~ook l Uses, McGraw-Hill, New York, pp. 1-26. McGregor, R. R. (1954a), Silicones ~ n c Their McGregor, R. R. (1954b). Silicones n r ~ dTheir Uses, McGraw-Hill, New York, pp. 26-30. McGregor, R. R. (1954c), Silicones mtd Their Uses. McGraw-Hill, Ncw York, pp. 149-186. Mantle, L. E. (1985), Elastornerics (February), p. 25. Marsden, J. G. (1948) U.S. Pat. 2,445,794 (to General Electric Co.). Meals, R. N., and Lewis, F. M. (1959a). Silicones. Rcinhold, New York, pp. 5-10, Meals, R. N., and Lewis, F. M. (l959b), Silicorzes, Rcinhold. New York, pp. 34-63. Polmanteer, K. E. (1953), initial high tear, tough silicone rubber research, unpublished. Polmanteer, K. E. (1960). (continuation-in-part of application filed Feb. 18, 1954, issued Mar. 8, 1960) U.S. Pat. 2,927,907 (to Dow Corning Corp.). Polmanteer, K. E. (1970), J. Elastoplastics 2: 165. Polmanteer, K. E. ( 198 1 ), Rubber Chetn. Teclznol. 54:105 1. Polmanteer, K. E.. and Falender, J. R. (1984). 0097-6156/84/0260-01 17, 01084 AmericanChemical Society, pp. 117-141. Polmanteer, K. E., and Helmer, J. D. (1965), Rubber C/~enz.Techrrol. 38:I 23. Polmanteer, K. E., and Hunter, M. J. (1959), J . Appl. Polyn. Sei. 1:3. Polmanteer, K. E., and Lentz. C. W. (1975). Rubher Chertl. Tecl~nol.48:795. Saam, J. C., Graiver, D., and Baile, M. (1981). Ruhher. C/wm Tc.chrzol. 53:976. Talcott, T. D., Brown, E. D., and Holbrook, G. W. (1957), American Chemical Society Meeting, New York,Sept.13. Wada, T., and Ito, K. (1972), U.S. Pat. 3,652,475 (to Shin Etsu). Warrick, E. L. (1976). Rubher C/ret?z.Tedrrlol. 49:909. Warrick, E. L., Pierce, 0. R., Polmanteer, K. E., and Saam, J. C. (1979). R ~ r h h ~Chet~l. r T e c h d . 52:437.
23 Advances in Silicone Rubber Technology: Part II, 1987-Present Jerome M. Klosowski Dow Corning Corporation, M i d h d , Michigan
Chapter 22 of this volume discusses the primary advances in silicone chemistry to the present except for the most recent advances in low-consistency technology. This chapter will present a brief overview of the current thinking in the low-consistency area. Polmanteer in Chapter 22 indicated the history of the start of the low-consistency area. Indeed the silicone sealants of the early 1960s worked very well and were very durable in their applications. They did things that other sealants didn’t do. In the appliance area they sealed against heat and cold and maintained adhesion. Many sealed against water intrusion or sealed steam irons, and some were used in full exposure to the elements in construction applications. Many of the same sealants are used in the same applications today. There have been some major changes and some minor ones to some sealants. Some of the minor changes involve heat stability and adhesion. The use of metal salts to increase heat stability shows some benefits, but the major advances in heat stability resulted from cleaner materials. Polymers had typically contained residual catalysts of the original polymerization. Eliminating these trace catalysts from the systems added to theheat stability of the systems and thus extended the useful high-temperature range in some sealants consistently to the 225°C range from the typical 200°C range. The use of tetrabutylphosphonium oxide resulted in a catalyst-free polymer. Filler changes will be discussed later, but it is significant to note here that some of the new fillers introduced impurities that are catalytic to the thermal degradation, so some newer sealants are less thermally stable than their earlier counterparts and some of the present sealants have thermal stability down in the 135-150°C range. Thus. it is important to indicate the thermal stability needed when specifying a silicone for a particular application, for not all silicones are the same and there are some wide variations in their stability. Note that even at 135°C (the lowest heat stability of a silicone known to this author) the silicones are still 50°C more stable than their primary nonsilicone counterparts in many of the sealant areas. Another change involves the expanded use of nonsilica fillers. Many of the older, less expensive sealants used 10-15% fumed silica as the only filler, and such sealants for general use typically had low tear strength. This was especially problematic in the construction area, where sealants in joints could “unzip” once a small tear was initiated. The tear problem could not be solved using conventional thinking in which addition of more reinforcing filler is used to toughen the system. More reinforcing filler or fibrous fillers typically were added to increase tear resistance, but in addition to adding tear resistance they also added to tensile strength and 649
650
Klosowski
most typically increased the modulus of elasticity as well. Fibrous fillers also add significantly to the cost. A construction sealant should have low modulus of elasticity along with high elongation. It does not need high tear strength, but it does need a nonpropagating tear, which consequently gives higher tear values. A change in technology was needed, which came with the nonsilica fillers. The construction industry is very price sensitive and, except for somespecialty areas, will not pay the higher pricesfor more tear-resistant products.Thus, when General Electric introduced the first commercially accepted, calcium carbonate-filled silicone sealant system, it was quite quickly accepted, even though it was a two-part, mix-on-the-job sealant. This new sealant had a nonpropagating tear. The calcium carbonate did not bond to the polymer or get involved in the crosslinking system. Each particle served as a center for energy dissipation as the crack front tried to proceed. This gavethe much needed nonpropagating tear at a low cost. In addition, this new sealant was of low modulus and high elongation and also introduced a new cure-the diethylhydroxylamine cure chemistry. The industry had already seen some nonacetic cures with an alcohol and oxime introduced earlier, but these didn’t catch on outside the specialty areas, since the formulas in which they were usedhad conventional reinforcement andsaw conventional performance. Thus the oxime and alcohol leaving groups were used on cementatious and other acid-sensitivematerials but inlimited amounts since theyadded no new dimensions to the science and application. The new hydroxylamine cure targeted the huge, low-cost construction market, and it did have new chemistries and did offer something the other sealants did not-the nonpropagating tear ata low cost. It was nonacidic and thus wasacceptable on most construction substrates. This caused a large stir in the market. The hydroxylamine cure was a particularly interesting and innovative cure chemistry in that it used methylhydrogen cyclic tetramer as the other reactant. It is worthwhile to pause for a moment and examine this unique chemistry. To get low modulus of elasticity one needed to have low crosslink density. To have high elongation one needed long polymer chains. To get the higher tear resistance one needed to have a way of dissipating the energy at the crack front, which is accomplished with filler particles that don’t react with the polymer and crosslinker. Therefore, one really did need the high quantity of nonreacting filler. The idea is to have low crosslink density, long polymer chains, and high filler loading all in the same system. At first this sounds like an impossible riddle because when long, viscous polymers are mixed with large quantities of nonreinforcing filler, the systemgets toothick to be easily extrudedfrom a cartridge or pail. It is too thick to mix in a pail. The difficulty is getting easily extruded materials that contain high molecular weight polymers and lots of filler. Further, the idea of low crosslink density typically means less than complete crosslinking of the chains. With complete crosslinking, the chain entanglement crosslinkswill occur andbe effective and limit the polymer mobility and rubber elongation. The thought of using insufficient crosslinker to achieve only a partially crosslinked system is generally not possible since sealants need an excess of crosslinker to react with ambient moisture permeating the cartridges and pails and not have the sealant react in the cartridge or pail but only after it is extruded. It seemed impossible. The solution to the riddle is to use small, low-viscosity polymers to mix with the filler for easy mixing and good extrudability and then grow the polymers by chain extension once they are extruded from the cartridge and then crosslink: in essence, to have chain extension as part of the curing process. That solves the extrudability problem from mixing viscous polymers with large amounts of filler. The problem of the limited crosslinking mustthen be addressed. The solution seems pretty simple, but in fact it is not. The problem is to getchain extension to proceed at a rate comparable to the crosslinking so that there is just the appropriate amount of chain extension andan appropriate amount of crosslinking. To have more than needed chain extension would give a poorly
Advances in Silicone Rubber Technology II
651
crosslinked, punky cured rubber sealant. To have crosslinking prevail instead of chain extension would give primarily a highly crosslinked elastomer of small chain length. In essence there must be the right proportion of crosslinking and chain extension taking place during the cure to give a lightly crosslinked polymer of long chain length. The solution was to partially endcap the methyl hydrogen cyclic tetramer with an olefinlike octene. On the four available sites just enough olefin is used so that there is a significant amount with two of the four sites per cyclic chain capped and lesser amounts with one or no sites capped. This is done with the traditional platinum-catalyzed addition reaction (see Chapter 22). With the capping complete, the diethylhydroxylamine is added slowly to the above reaction product. Now all the unreacted silylhydride siteswill react and the resultwill be cyclic siloxanes, with some having two pendent hydroxylamine groups and others having three or four pendent groups:
+
?BH,,
+
CBH,,
+
3
+
Et,NOH
Me,
is,
ONEt,
0 I
iiOMe
$ONEt,
O-#
/
ONEt,
he
The true elegance of this is not that there is a mixture of difunctional with tri- and tetrafunctional
652
Klosowski
crosslinker, although that mixture is fundamental, but that all the groups on the silicone crosslinker and chain extender are equally reactive. The problem with most condensation systems (neucleophilic substitution) involving di-, tri-, and tetrafunctional materials using monomeric crosslinkers is that the reactivity of the silanes changes as the number of electron-withdrawing groups differ on the silane. Thus, a silane with two reactive groups and two nonreactive alkyls would typically have a reactivity perhaps I O times slower than a silane with one alkyl group and three electron-withdrawing groups. which is perhaps 10 times slower to react that1 a silane with four active leaving groups. Thus,simply mixing difunctional silanes with tri- and tetrafunctional materials does not accomplish chain extension in the presence of the crosslinkers since the crosslinking and chain extension materials often have totally different reaction rates. In this cyclic mixture all of the attached hydroylamine groups on the siloxane have the same electrical environment. and thus all have comparable reactivity. Thus. the difunctional material reacts at the same rate as the tri- and tetrafunctional materials. Chain extension proceeds at the same rate as crosslinking. The compounder experiments to find the proper ratio of the materials in the mixture to give the desired crosslink density. and the job is done. The properties of the new sealant with chain extensionduring cure to produce low modulus andhighelongation in alow-consistency,easilyextruded material startedarevolution. The adhesion to concrete and other hard-to-adhere-to surfaces also seemed to be enhanced. While this system did not truly have better adhesion in the purest sense of the word, it did stick better. This apparent increase in adhesion is due to the low stresses put on the bond line because of the lower modulus of elasticity. With a low modulus sealant, the movement of building joints put low stresses on bonds and thus the sealant adhered and remained adhered not because the bonds were stronger but because the stresses were less. The only tlaw in this technology was in the shelf life. The crosslinker and chain extender stayed as indicated in the earlier equations if the system stayed dry. If, however, water was introduced from the air through an imperfect seal in the package containing the chain extender and crosslinker mixture. there would be an instant hydrolysis of one of the groups in the system. This does two things. It introduces free hydroxyl amine. which is a strong base and can cleave the cyclic chain extender. When this happen one end becomes a silanol and the other end has the hydroxyl amine attached. The difunctional material becomes tetrafunctional and thus the chain extender is now a crosslinker. Further. the original silanol generated can condense with a functional site. In essence a little water in the system can cause a great deal of havoc. Thus, even as a two-part system the modulus of elasticity. as a function of time, was a bit variable and a one-package system was not possible. However. the hydroxylamine functional cylic system did scare the industry, and there was a swift reaction from Dow Corning, which responded witha system that used n-methylacetamide for two of the leaving groups on a silicon and two alkyls a s the other groups. The acetamide groups with their excellent delocalization of electrons make superb leaving groups and were first explored i n crosslinkers and difunctional monomers by Rhone Polunc, but never in a sealant with a chain extender and crosslinker of equal reactivity. Since the amide leaving group is quite neutral, a one-package sealant was possiblethat was even more shelf stable than the two-package system with the hydroxylamine leaving group. Thus, a one-package, ready-to-use product was introduced that cured by a mixture of chain extension and crosslinking. The sealantrevolutionstarted in 1960 with theintroduction of siliconesandurethane sealants and was revitalized several times. If the revolution of the 1970s in silicone sealants involved chain extension, then the revolution of the 1980s involved adhesion and the neutral cures and that continued into the 1990s. The largest volume of silicone sealant in use today is still acetoxy cures. a s was the original commercial one-package sealant invented in the 1950s and marketed in the 1960s. It worked and lasted a long time;some sealantinstalled in the early 1960s is still performing.When
Advances in Silicone Rubber Technology II
653
something works SO well it is difficult to convince people there is something better for them to use. But in fact, relative to adhesion and general usability. the newer sealants are better. The neutral cures with oxime, amide, alcohol, or amine leaving groups can be used equally well on aluminum, glass, concrete. galvanized metal, and a variety of other surfaces, some ofwhich are problems for the acetic acid cures. More currently the move is to narrow the broad variety of cures to predominantly alcohol cures. This allows for even more general use of the common sealants, adding tothe listof compatible surfaces copper, brass, and the large markets of sensitive electronics. The alcohols also have a relatively inoffensive smell and are low in toxicity. Thus, the last revolution in sealants involves narrowing the cures to the various alcohols as leaving groups. Alcohol cures have been used since the early 1960s in the electronics industry because of their compatibility with copper and similar metals. However, the systems were difficult to deal with in terms of cure rate. shelf life, and adhesion. Advances in science have helped in all these areas, and now alcohol systems are starting to dominate the field. Since alcohol is a lowcost leaving group, it would seem natural that when the technology allowed it to be equal or better in performance to the other leaving groups, it would start to dominate. Some obvious advantages to the alcohol leaving groups are cost, neutrality. and shrink. The first two are quite obvious, but in shrink, it is simply a case of equal molar amounts of crosslinker weighing less with alcohol crosslinkers than with other crosslinkers. Thus, with cure there is a lesser amount of material released. The biggestproblems with the alcohol cure were in adhesion. Mostresearchers don't know why theadhesion in alcoholsystemswas so difficult to obtainandsustain.From those who have persevered in these studies, their alcohol systems now provide some of the most reliable adhesion in the industry. None of the answers to adhesion are recorded as public informationexcept forthe disclosure of the use of some coupling agents with the crosslinkingsystems. The list of couplingagentsreported to be useful is as longasthelist of couplingagentsitself. Of these,themost commonly used are(Et0)3SiCH2CH,CH,NH2, (Me0)3SiCH2CH2CH?NHCH2CH2NH,. (MeO)3SiCH2CH2CH20CH2CHCHCH20, and [(Me0)3SiCH2CH,CH2NCOl3. These are the mainstays of the silicone industry. Because of the importance tothe industry of alcohol cures, it is of interest to explorethese a little further. The alcohol systems come in essentially three forms. The tin-catalyzed two-part systems, similar to theoriginalsealantPolmanteeraddressed in the 1950s, involveda tincatalyzed part and a titanate-catalyzed part. These use the whole range of crosslinkers from orthosilicates to MeSi(OR)3, where R is methyl or ethyl, and even occasionally someone will throw a difunctional silane into the stew. In all cases in alcohol systems with a difunctional, it is used to attenuate the crosslink density but it is not a chain extender. The R2CHSi(OR), materials are extremely slow to react with silanol and thus don't participate well in chain extension. Because tin catalysts are strong rearrangement catalysts, in this system it is critical to have a one-part sealant package that is water-free and alcohol-free:
Klosowski
654
Thus, the one-package alcohol curing sealant system with a tin catalyst will also use very dry components and dry the filler before use. Those making these systems will prereact the polymer ends and anything else that can be prereacted so that the alkoxyfunctional crosslinker i n the system does not see a lot of water or silanol and thus does not produce a lot of alcohol in the manufacutering of the sealant or in the package. In fact these systems also use alcohol/water scavangers so the system stays alcohol/water-free until it is used and the cure is started: Me3SiNHSiMe3 Me3SiNH2
-+ + ROH
+ ROH
Me3SiNHSiMe3
Me3SiOR
-
HOH
Me3SiNH2 + HOH
Me3SiNH2
+ ROSiMe3
+ NH3
Me3SiNH2 + HOSiMe3
Me3SiOH
+ NH3
-
The titanate-catalyzed alcohol cures are also not straightforward: -SiMe(OMe):
+ HOSiMe?O-
TiCat.
-SiMe?OSiMe?O-
+
HOMe
The above equation is not technically correct, since the titanate catalyst is also a reactant. In fact, the first reactionto take place is not the alkoxysilane-silanol reaction but the titanate-silanol reaction in a system that starts with silanol polymers. This makes the system very thick and unmanageable in all but the most sturdy mixers of high shear. Thus, most people start this system by endcapping the polymer with alkoxysilane. This is effective but adds to the cost. With the above problems in mind it is easy to understand why the two-part tin-catalyzed systems are by far the easiest to deal with and have been useful since their discovery i n the 1950s. In the two-package system one keeps the bulk of the polymer in one package and the catalyst and crosslinker in the other. Thus one does not have to deal with extensive reversion in thepackage or any of theotherchemistriesmentionedabove. Cure is either by silanolsilylmethoxy condensation or silanol-silanol condensation: -SiMe(OMe)-,
or
+ HOSiMe20-
-
TiCat.
TiCal.
-SiMe(OMe)OSiMe?O-
+ HOMe
+ HOH -SiMe(OMe)OH + MeHO -SiMe(OMe)OH + HOSiMe(0Me)O-SiMe(OMe)OSiMe?O- + HOH -SiMe(OMe)2
Ti('a~.
Anothersystemfrequently used on a worldwidebasis is the oxime cure. The most common leaving group in this series is ethylmethylketoxime. The most common crosslinker is MeSi(ON=CEtMe)3, withsignificant use of ViSi(ON==CEtMe), wherefast cure is needed and Si(ON=CEtMe), where even faster cure is wanted. Substitution of the more electron withdrawing vinyl in place of methyl on the crosslinker makes it sufficiently reactive that crosslinking may not require a catalyst. The tetrafunctional crosslinker is extremely fast. and it too does not need a catalyst. Sometimes, however, catalysts are used with the fast systems just to give a bit of surface dryness when cured in high humidities. The most commonly used oxime crosslinker is MeSi(ON=CEtMe)3, which is always used with a tin catalyst. This system uses more tin catalyst than the faster oxime functional crosslinkers since the catalyst is needed for both cure rate and drying the surface in that system, while the faster crosslinkers need the catalyst only for drying the surface. A popular system is to use combinations of crosslinkers with different mixtures allowing for different cure rates and different crosslink densities. In oxime cure chemistry there is a whole bag of functional silanes to use including difunctional materials. The difunctionals are used to decrease crosslink density. as is phenyltrieth-
Advances in Silicone Rubber Technology II
655
ylmethylketoxinlosilane. Probably because of steric requirements, the phenyl silane seems to react twice and not three times and thus is a modulus modifier more than a crosslinker. None of the standard oxime functional silanes function in true chain extension. The advantage of the oxime system and the reason it is often used is that sealants using this system are easy to manufacture. It is the system of choice for the small compounder with limited mixingequipment who wantsto make avariety of sealants of different curerates. different moduli of elasticity, and different tensile and elongation properties. Allied Signal supplies most of the industry with the oxime crosslinkers and offer more than 10 in their catalog. They also give adviceandhelp with the compounding so the sealantdesignercanreadilyachievethe desired performance profile on simple equipment. One disadvantage is that the oxime leaving group is relatively heavy, thus an equal molar amount of crosslinker to that used in an alcohol or acetic acid system will weight considerably more and when the leaving during the cure will cause more shrink. MeSi(OOCH3),, often used at 4.5% in a sealant, produces approximately 3.6% shrink. MeSi(OMe)3equimolar to the acetoxy crosslinker equals 2.8% in sealant and has 1.8% shrink. MeSi(ON=CEtMe)3 equimolar to acetoxy equals 6.2% in a sealant and produces 5.3% shrink. Oxime crosslinkers cost more per pound than acetoxy or alcohol crosslinker. and more pounds are neededperbatch. This all adds up to ahighercost for the sealant-a distinct disadvantage. Even so, the oxime crosslinker is noncorrosive to many materials and easy to use. These features make it a highly used system. The intent of this chapter was to indicate what was new in the low-consistency silicone rubber area. One cannot address this topic without addressing the mainstay of the business. The largest selling sealant in the industry is still the acetoxy cure.
-0 SiMe(OOCMe)2
+ HOSiMe20-
-
-OSi(OOCMe)OSiMe20-
+ HOOCMe
While there is nothing really new in acetoxy sealants, the news is that it is still dominant after almost 40 years in commercial production. The reason for its dominance is that it is relatively easy to work with and uses an inexpensive crosslinker, and thus a rather low-cost. one-package. ready-to-use sealant can be produced. It can be made by big and small producers alike using variety of equipment. It has a long shelf life and a very long history of very good maintenance of performance after years of service. A typical formula is as follows:
12% fumed silica (cyclic-treated) 3.5% MeSi(OOCH3), 0.1% Bu2Sn(OOCH3).3 84% silanol-ended polydimethylsiloxane or some adhesion promoter or In addition to the above ingredients there might be a pigment fungicide or additive to achieve some other specific characteristic not normally present in the system. While the acetoxy silicone sealant is corrosive to some surfaces, it has many applications. There have been few if any changes in the acetoxy sealant over the years, but there could have been since the technology of the carboxylic acid cures has progressed. For some the working time of these systems was too short, most having less than 5 minutes before a ski11 formed during the cure. This can be changed by changing either the alkyl group on the crosslinker to a more bulky one, the carboxylic acid to a larger one, or both. Changing the methyl group to an ethyl group increases the skin time or working time to 10-15 minutes and is more userfriendly in many applications. Changing the leaving group from acetic acid to propionic acid changes the cure rate again and MeSi(OOCC,HS)3 will typically have a skin again i n 10-15 minutes and give more working time than the conventional sealant. The obvious next cross-
656
Klosowski
linker-EtSi(00CC2H5)3-will extend the working time even further. Changing the leaving group to 2-ethylhexanoic acid extends it even further. Fillers could also be changed in the acetoxy system from the fumed silica used in the above formula. But to do that would probably sacrifice the clear nature of the product. Clarity is important is some markets, and the idea of producing one product that is clear and pigmentable appeals to most manufacturers. For some specialty applications there are crosslinkers with amine leaving groups. These are smelly, corrosive to some surfaces, and a bit more expensive and thus little used. For some applications a crosslinker with an acetone leaving group is used. It is a good system but is less reactive than the methanol leaving group system, and because the crosslinker is so difficult to make, it is costly. Some sealant systems use the addition cure system (see Chapter 22). It is sufficient to say here that the addition cures are still primarily two-package systems and that the development of adhesion to wide varieties of substrates is difficult with addition cures. Since the 1960s the bulk of the patents in the sealants area have involved the development of adhesion. Adhesion is the most difficult aspect of the sealants and silicone sealants business. Most of the work involves adding the proper adhesion promoters and combination of adhesion promoters to give adhesion to a wide variety of substrates. Adding adhesion promotersto sealants means that less priming is needed, and the ability to use a sealant without a primer is a strong competitive advantage. Many patents have been issued, but much is still proprietary. It is sufficient for this review to say that the most typical adhesion promoters are the silane coupling agents, and the compounder must match the silane to the job needed. It is important to know the application and the exact surfaces that will be adhered and the conditions the sealant area will encounter. Many materials will give dry adhesion, but adhesion that withstands some stress while the sealant is in the weather or in a hot liquid for days, months. or years is quite another matter. Future developments will also involve adhesion because thecustomer wants easier applications and less labor involved, which translates to less than perfect cleaning. Future reviews will list sealants that adhere through oil and dirt films on the surfaces. Presently no such sealant product exists. Also of importance are recent advances in aesthetic areas. Since the start of the sealant business, silicone sealants in outdoor exposures got dirty. All sealants got dirty, but the nonsilicone sealants would deteriorate in the sunlight. Their surfaces chalk, and the dirt and chalk blow away in the wind. With silicones it is a bit different. Silicone sealants don't deteriorate in the sunlight. Dirt lands on silicones just like on organic sealants but with no deterioration-the dirt stays. Thus, silicone-sealed buildings would often have dirtier joints than nonsilicone cones. Silicone sealants last many times longer in outdoor joints than their organic competitors, but Some specifiers are reluctant to use them. Many of the newer silicones stay cleaner than the older ones, and buildings stay cleaner with many of the new formulas. There even existstoday a commercial silicone sealant that stays quite clean. Presently this clean sealant costs a little more. and thus its use is restricted. This new productfromDow Corninghasthe industry scrambling, much like GE's introduction of the chain extension system in the 1970s. It is almost certain that the other principal manufacturers will solve the riddle of the clean silicone sealant for outdoor use. The durability of the silicone sealant has kept its market share continously growing. The new technologies will make that growth continue. Silicone sealants last longerthan other sealants for all the same reasons that silicone rubber is SO very durable. Silicone sealants resist sun and rain. They handle higher temperatures than other sealants, and they retain their elasticity at lower temperatures better than other sealants. Silicone sealants can be formulated to a lower modulus and higher elongation than any of their
Advances in Silicone Rubber Technology II
657
present organic competition. Thus,they put very little stress on joints. Silicone sealants can also be formulated to high modulus and low movement and almost any degree of movement and elasticity in between. It is this combination of properties and extremely wide formulation range that makes the future for silicone sealants very bright. Silicone sealants are used as adhesives that hold facades on buildings. In structural glazing it is only the silicone sealant in the back and on the edge of the panels that holds the sheets of glass, aluminum, and stone on the sides of many high-rise and low-rise buildings. The use of silicone sealants as adhesives is also very important in the appliance and automotive industries. The combinedadhesive/sealant applications for silicones will continue to spread. Wherever lowtemperature flex, high-temperature stability. long duration in the weather, and flexible adhesion is needed, silicones can be found, now and well into the future.
This Page Intentionally Left Blank
24 Acrylic-Based Elastomers Piero Andreussi and Arturo Carrano EniChem, Milan, Italy
1. INTRODUCTION Acrylic elastomers have been called ACM and “specialty rubbers” by the American Society for Testingand Materials (ASTM) (1985). Their unique characteristic, i.e., the absence of unsaturated sites in the polymer backbone, is not present in so-called general purpose rubbers. As a consequence of this aspect, their properties include resistance to high temperature (>300”F, or 150”C), ozone, oxygen,ultraviolet light, inertness to sulfur-bearing oils and greases, and dimensionalstabilityinaliphatichydrocarbons. All theseproperties make acrylicelastomers very useful for service in the applications of the modern automotive, industry in particular in transmission seals, rear axle seals, oil cooler hoses, and generally in under-the-hood applications. Early interest in acrylate polymers rose with the growing wish in Germany before World War I to develop a viable artificial rubber. Rohm (1912) reported the first attempts to utilize an acrylic polymer for commercial purposes by heating it for 2 hours at 40°C (104°F) i n the presence of sulfur. I. G. Farbenindustrie (1930) and Schnabel (1940) claimed an increase of rubberlike properties adding to acrylic polymer mixtures of tannic acid, ferric acetate, copper powder, and antimony potassium tartrate. Nowak and Hofmeier ( 1938) and Nowak (1938) rejected Rohm’s vulcanization system using sulfur and claimed the use of a surface catalyst in the curing reaction; in the reported example, they reported using of carbon black. Earlier, Mark andFickentscher (1934) studied copolymers with allylacrylate,while I. G. Farbenindustrie (1938) used vinyl P-methylcrotonate in an attempt to create vulcanization sites in the elastomer. Neher (1936) emphasized the soft and elastic peculiarities of poly(n-alkyl ac1ylate)s. Due to the presence of an active a-hydrogen and an ester group on the same carbon atom of the saturated backbone of acrylic polymers, these groups seem to be potential sites for crosslinking reactions. However, before these sites were explored (see Sec. 7.1 ), 1,3-butadiene and related dienes were copolymerized with acrylic esters to give acrylate-modified poly( 1,3butadiene)sratherthan1,3-butadiene-modifiedpolyacrylateelastomers.Ziegler (1938)described several modified poly(l ,3-butadiene)s in which the vinyl monomers were acrylateesters, acrylonitrile, or styrene.. Fisher et al. (1944) started the first systematic efforts to prepare polyacrylate elastomers containing crosslinking sites derivedfrom butadiene and butadiene/acrylonitrile(2- 13.5%)comonomers. Comonomerslike allyl-lactate-maleate and isoprene werealso investigated. The resulting elastomers, called Lactoprene, were cured with sulfur, accelerators, and other compounding agents of the rubber industry. 659
660
Andreussi and Carrano
The vulcanized samples had rubberlike properties. including: Tensile strength of 1840 psi (1 2.7 MPa) Elongation at break from 200 to 800% Hardness from 32 to 78 points Shore A Brittle point from - 6 to - 19°C Earlier, Trommadorf (1937) reported softening points for methyl, ethyl, and n-butyl acrylates, respectively, of 8, - 20, and - 40°C. The next stage was the development and the commercialization of acrylic elastomers. Mast one of Fisher’s group in the early 1940s at the Eastern Regional Research Laboratory of the U.S. Department of Agriculture, recalled the introduction of Lactoprene EV (W. C. Most, personal communication). Samples were distributed to the rubber companies in Akron, Ohio, but interest in it was very poor. B. F. Goodrich was the first to develop andmarket Hycar PA in 1948. Goodyear marketed PAR in the early 195Os, but just for a short period. In 1963 American Cyanamid and Thiokol Chemical Corporation entered the market. At this time the producers are ZeonChemicals (United States), Nippon Mektron (Japan), NOK (Japan), JSR (Japan), TOA Paint Co. (Japan, Taiwan), and EniChem (Europe).
+
2.
BASICSTRUCTURE
The basic structure of acrylic elastomers is a carbon-carbon backbone with a pendant carbalkoxy group and an a-hydrogen attached to alternate carbon atoms in the polymer chain:
Although the principal acrylic elastomer is poly(ethy1 acrylate), its glass transition temperature (Tg) of - 15°C may reduce its use in some applications. Rehberg and Fisher (1 944) recognized the possibility to control some physical properties like low-temperature flexibility by varying the structure of the alkyl group. The brittle point of different poly(n-alkyl-acrylate)s, where R varied from C l to Clc,,decreased from 3°C for C , to - 65°C for CSand then increasedto 35°C for C,(,. Rehberg etal. (1944) stated that the branching of the alkyl group R raised the brittle point except in the anomalous cases of secondary and branched-chainundecanolandtetradecanol.Tertiaryalcoholswere not investigatedbecause they did not undergo alcoholysis reactions with methyl acrylate. Studies on modifying the R group began when investigators recognized that a reduction of low-temperaturebrittlenessresulted in asignificantincrease in polymerswell in various solvents. In the 1950s and 1960s considerable work was done on alkoxyalkyl R group polymers, especially when their discovery ledto new advances in lowering the brittle point withoutconsiderable loss of polymer swelling resistance in selected solvents. Mowry et al. (1954) studied the polymer of 2-(2-cyanoethoxy) ethyl acrylate, which had an ultimate tensile strength of 144 psi ( 1 MPa) and total elongation of 800%. Harris and Wilt (1958) drastically improved the ultimate tensile strength of this polymer by adding acrylic acid (0. I %) as comonomer. Tucker (1967) used self-curing copolymers of ethoxyalkyl acrylates and N-ethoxyalkyl acrylamides to improve oil resistance and low-temperature properties. In a similarway. Jorgensen
Acrylic-Based Elastomers
661
and coworkers ( 1967) used N-nlethylol acrylamide. Aloia and Kaizerman ( 1968) achieved improvements developing a terpolymer of n-butyl acrylate (76 parts), 2-cyanoethyl acrylate (19 parts), and vinyl-chloroacetate (5 parts). Gobran and Bernstein (1969) used n-butyl acrylate (58 parts),methylthioethylacrylate (38 parts),chlorovinylacetate (4 parts), and a trace of allyl methacrylate to obtain a brittleness temperature of -42°C and an ASTM No. 3 oil swelling of 30%. The search for a balance between strength, low-temperature properties and swelling resistance in solvents began to decline asthe number of suitable monomers failed to meet new cost/ benefit criteria. Jorgensen (1970a) and Chalmers ( I970a) both reported variations on this subject using third monomers such as vinyl chloroacetate, allyl chloroacetate, glycidyl acrylate, and glycidyl methacrylate. Starmer ( 1972) used both halogen and carboxyl sites to achieve the desired scorchkure rate balance. Falk et al. (1984) produced elastomers that could be processed by extrusion and injection molding. Chang (1993) has patented a new no-postcure technology for low-chlorine acrylic polymers with suitable packages.
3. METHODS OF PRODUCTION Interest in new polymers and new monomers began to wave in the mid- 1960s because of a shift in U S . government funding, because of industry concerns about the high cost of new product development. and because of the requirements of U.S. government regulations to control worker and public exposure to new chemicals. 3.1
Monomers
The main backbone monomers used in acrylic elastomers are ethyl, n-butyl, 2-methoxyethyl, and 2-ethoxyethyl acrylates. Brandrup and lmmergut (1 975) published their properties as including propagation and termination constants, inhibitors, transfer constants, catalyst for polymerization, and reactivity rat,i,os. Riddle ( 1 954) provided a comprehensive review of acrylic esters from the perspective of Rohm and Haas Co., an early pioneer in this field. The sourceof acrylate monomers hashistorically been coal, carbohydrates,and petroleum, thus giving them ;1broad raw material base. Several methods were explored in the early 1930s before commercial interest in acrylate monomers developed. Bauer (1928) first dehydrohalogenated (3-chloropropionic esters. The sameresearcher (1 93 1 ) hydrolyzed and dehydrated ethylene cyanohydrine toacrylic acid followedby esterification to thedesired acrylate, after which (1933) he dehydrated ethyl f3 hydroxypropionate with sulfuric acid on silica gel. Acetylene from coal served as the next raw material base for acrylates, particularly in Germany before World War 11. Callomon and Kline (1945) translated Walter Reppe’s original work on the reaction of acetylene, carbon monoxide, and alcohol at atmospheric pressure in the presence of nickel carbonyl, Ni(CO).,, catalyst. Reppe (1953)published a more complete review on his work. The interest in Reppe’s work continued.Bhattacharyya and Sen (1964) dida systematic study using various salts of iron, cobalt, and nickel as catalysts in pressure reactors. Hardman and Miller (1965) produced acrylonitrile from acetylene and hydrogen cyanide followed in a continuous process by hydrolysis and esterification to give the acrylate ester. More recently, Shulyakovskii et al. (1 982) produced n-butyl acrylate by the carbalkoxylation of acetylene using a palladium iodide catalyst. Hoberg et al. (1984) similarly carboxylated acetylene with carbon dioxide using nickel complexes to give acrylic acid. Carbohydrates also provided a substantial raw material base for acrylateesters. There was a strong interest in using milk by-products and surplus products from agriculture. Smith et al.
662
Andreussi and Carrano
(1942) used lacticacidprepared from lactose in wasteproducts to give methyl a-acetoxypropionate, which pyrolyzed to methyl acrylate. Ratchford and Fisher ( I 945) produced methyl acrylate in high yeld by optimizing the pyrolysis reaction. P-Propiolactone made from ketene and formaldeyde became, for some time, a source of alkyl acrylates by reaction with alcohols. Fisher (1958) reported a new continuous process for acrylates produced from P-propiolactone. Petroleum displaced the foregoing materials as a primary raw material base for acrylate esters.Propylenebecamethematerial of choice.BrownandNewman (1965) firstoxidized propylene to acrolein and then to acrylic acid using a cobalt molybdate catalyst. Grasselli and Suresh ( 1981) oxidized acrolein with molecular oxygen in the presence of methanol over an uranium-tungsten-molybdenum oxide catalyst. Aoshima and Murofushi ( 198 1 ) oxidized propylene directly with molecular oxygen in the presence of methanol over a heavy metal catalyst to give both ester and acrolein as products. Jaeger and Germain (1982) used a two-stage reactor to produce acrylic acid by the oxidation of propylene followed by nearly quantitative conversion to methyl acrylate in the second stage. The production of alkyl acrylates by esterification of polymerization-prone aclylic acid has received some attention. Sat0 et al. (1983a) passed acrylic acid and methanol over a strong acid ion exchange resin. Chase and Wilkinson (1984) found ethylene and sulfuric acid to give ethyl acrylate in high yield if a wiped film evaporator was used to reduce polymer formation. Dougherty et al. (1985) also used the wiped film evaporator but added manganese or cerium salts as catalysts. Villieras and Rambaud (1984) produced ethyl acrylate in high yield by the Wittig-Homer reaction of phosphonic esters with alkaline aqueous formaldehyde.
3.2 Polymers Poly(acry1ate)s can betheoreticallyproduced by means of traditionalprocesseseven if the characteristics desired in commercial elastomeric products are poblematic with the use of bulk polymerization as well asthesolutionpolymerizationbecause of theviscous cement being difficult to handle. These two methods, along with photochemical and radiation initiation, are not used in commercial production. However, control of bulk polymerization using mercaptobenzothiazole as a retarder has been investigated by Dwivedi and Mitra (1 984). Solution polymerization has recently received further attention.Solvent effects on propagation and termination rate constants for the radical polymerization of butyl acrylate were investigated by Kamichietal. ( 1982). New initiatorshavebeenreported:Rasmussen (1982) used potassium peroxydisulfate and 18-Crown-6in aqueous acetone; Sat0 et al. ( 1983a) found dimethylaniline N-oxide plus some chlorosilanes to be effective in acetonitrile andalso (1983a) investigated triphenylboron in acetonitrile; Kairn (1 984) used cyclohexanone in dioxane. Ionic polymerization was reported by Fumkawa (1962), but it remained a bench-scale method. Emulsionpolymerizationbecamethe chosen method for theproduction of acrylic elastomers. Fisheret al. (1944) begantheseriousinvestigation of theLactoprenefamily of acrylaterubbers. His group studied mainly ethylacrylateandaseries of copolymers with allyl lactate maleate, allyl maleate. and n-butyl acrylate. Mast and coworkers (l945a) extended thedevelopment on thispolymerizationtechnique.Howertonetal. (1952)expanded the bench-scale preparation of acrylic elastomers to a small pilot plant. A typical formula would beasfollows:
663
Acrylic-Based Elastomers
Material Dclonizcd water Ethyl acrylate Triton Tergitol paste No 4“ Ammonium persulfatc
Parts by weight
200 100 720 (alkyl aryl ether sulfonate). 0.5-1 (% of monomer) 1-2 ((X- monomer) 0.005 ( % of monomer)
Anionic surfactants allow coagulation of the latex by hot salt solution. Therecipe contains no antioxidant because polyacrylates have a saturated backbone chain and do not react with oxygen during storage. Laboratory studies have demonstrated certain modifications of emulsion polymerization. Bonamy and coworkers (1982) developed an efficient system for the ultraviolet initiation of polymerization using a photoinitiator. In the industl-ial processes.oxygenactsas an inhibitor of freeradicalpolymerization. However. Reddy et al. ( 1982) studied oxygen as a promoter using Cu” as the catalyst for an ascorbic acid-oxygen redox reaction. thus decreasing time and loss of conventional initiators in the early polymerization stages.Osada and Takase ( 1983) applied plasma initiation to produce high molecular weight polymers. Kinetics of polymerization have recently received attention.Snuparek and Kleckova ( 1984) found that particle nucleation and growth proceed as a competitive process. Fitch et al. (1984) studied the rates of particle formation and growth during the earliest stages of polymerization as a function of surfactant concentration by measuring Rayleigh scattering intensities.
4.
COMMERCIAL POLYMER TYPES
4.1 Background The history of the commercialdevelopnlent of acrylic elastomers is well documented. The Department of Agriculture’s Eastern Regional Research Laboratory made the first investigations of Lactoprene EV in the early 1940s. opening the door to commercial development by B. F. Goodrich in 1948 of Hycar PA and PA-2 1 (now Hycar 402 I ). I n the 1960s. American Cyanamid introduced Cyanacryl. Today acrylic polymers continue to be produced i n Japan. Europe. and the United States. 4.2
Today’s ACM Elastomers
The polymer backbone is mainly based on copolymers of ethyl. butyl. and methoxyethyl acrylates with a variety of mononlers of a proprietary nature that provide cure sites and modify the balance among heat resistance, low-temperature properties, and oil resistance. The suppliers of ACM elastomers provide detailed indications for the properties. compounding, and potential applications of their specific polymers. The polymers are listed by supplier in alphabetical order in Tables 1 and 2.
4.3 Market Growth Greek ( 1984) reported that industry analysts forecast steady growthahead. An analysis by Gabris ( 1991) estimated world consumption at about 7500 metric tons. A recent study by CEH-SRI
Andreussi and Carrano
664 Table 1 Types,Properties, and Applications of ACM Elastomers Mooney TY Pe EniChem (Europe)" Europrene AR 153 EP AR 156 LTR AR 157 LTR R L C AR 45 AR 1803 AR 2503 AR 2903
Reactive site
T,
( I +4)I0O0C
Epoxy/Acid Epoxy/Acid Epoxy/Acid
- 15
30 - 38 - 15 - 24 -31 - 36
44 32 36 42 37 35 30
Chorine NPC grades NPC grades NPC grades
Nippon Zeon Co., LTD (USA)h Hytemp 4051 Chorine/Carboxyl 405 1EP Chorinc/Carboxyl 4051CC Chorine/Carboxyl 4451CC Chorine/Carboxyl 4052 Chorine/Carboxyl 4052EP ChorinelCarboxyl 4053EP Chorine/Carboxyl 4054 ChorincKarboxyl 4454 Chorine/Carboxyl AR 71 Chlorine AR 715 Chlorine AR 72 LF Chlorine 4001 Hydroxyl
4004 4014 Nippon Zeon Co., LTD (USA)h Nipol AR 32 EPOXY AR 42 W Epoxy AR 5 1 EPOXY EPOXY AR 53 L AR 54 EPOXY AR 72 LS Chlorine
-
Applications
Good proccssnbility Low-temperature properties Low-temperature properties Similar to AR grades Similar to AR grades Similar to AR grades Similar t o AR grades Similar to AR grades Similar to AR grades Similar to AR grades
15
35
-21 - 26
35 25
15 - l5 - 15 - 15 - 28 - 28 - 38 - 37
45-60 35-50 25-40 25-40 35-40 20-35 25-40 22-37 22-37 42-57 33-48 27-32 40-60 22-37 22-37
High heat and oil resistance, mcdium low-temperature properties, low compression set Hot oil res. and cold flex. Oil res. and cold flex. Optimum cold flex. and heat res.. mcdium oil res.
- 26.5 - 26.5 - 14
35-45 27-40 SO-60
Standard low-temperature res. Low temp. resistance Heat resistance
32 - 37 - 28
31-37 22-35 30-36
Very low brtttlc point Low temperature and easy proc.
-
-
-
37 15
- 19 - 26
14 38 - 38
-
-
Crosslinkable by isocynnate For adhesives For adhesives
see EniChem Bulletin. h
1995. See Nippon Zeon Bulletin. 1993.
(1996) reported world consumption to be 10,300 metric tons. The same study reported that with recovery of the U.S. automobile industry. consumption of solid acrylic elastomers should increase 5-7% peryear,reaching 4300-4500 metrictons by 2000. Demand in WesternEuropewas expected to increase at a rate of about 2-3% per year. reaching a level of 2300-2400 metric tons by 2000.
665
Acrylic-Based Elastomers Table 2 OtherACMElastomers
Reactive
Type
site
T,
NOK (Japan) Noxtite A- 1095 PA-2 12 PA-301 PA-3 PA-7885 PA-402
Mooney ( 1 +4)100"C
40-60 30-50 35-45 30-40
Applications
Fast curing Low-temperature applications Improved extruding properties Improved low temp. res. fab.)
Japan Synthetic Rubber (Japan) Arex 51 60 40
1 10
120 210 220 310 320 Toa Paints (Japan) Toa Acron AR 60 I 601x1 601X2 1 80 1 715 740 840 825 860
43 32 30
- 15 - 1s - 15 - 15 - 25 - 35 - 40 - 40
-50
25-35 20-30 45-55 50-60 35-45 35-45 40-50 35-45 30-40
Standard heat res. Low ML grade of 601 High ML grade of 601 St. grade of fast curing Semi-cold resistance Std. cold resistance Fast cure of cold res. grade Easy proc. grade of 840 Most cold res. grade
In the Japanese market. growth of acrylic elastomers was positively affected by the trend toward use of higher-performance materials in automotive applications. Acrylic elastomers were expected to replace nitrile and silicone rubbers. Japanese automobile production has leveled out since 1990, and the demand for acrylic elastomers has been stagnant, slipping to a low point in 1994. Consumption was estimated to grow at a moderate rate through 1999.
4.4
Health and Safety
The various monomers and chemicals used in the production of acrylic elastomers havedifferent grades of toxicity. It is important to examine the material safety data sheets available from the suppliers of all monomers and other chemicals used in polymerization processes. The highlyexothermicpolymerizationreactionmakesbulkpolymerizationpotentially hazardous. Adequate cooling must be provided even in emulsion or suspension processes. Raw materials do not represent a health threat. Residual monomer would be detected by its odor. Materialsafetydatasheetsfor all compounding ingredientsareavailable from the suppliers and should be consulted.
Andreussi and Carrano
666
Studies by Dillingham et al. (1983) highlighted the long-term chronic toxicity potential of acrylate esters. However. in rats exposed to n-butyl acrylate, Engelhardt and Klimisch (1983) found no chromosome damage. Merkle and Klimisch (1983) detected no teratogenic effects. Waegemaekers andBensink (1984) found no mutagenicactivity using theAmestest on 27 aliphatic acrylates. Van der Walle and Bensink (1982) were unable to find a universal screening allergen among 26 acrylate monomers to detect acrylic monomer sensitization. Fourteen accelerators were testedfor mutagenicity by You et al. (1982), who found positive resultsonlywith TMTM,TMTD, andzincdiethyldithiocarbamate. The remaining 1 1 were inactive under all conditions.
5. COMPOUNDINGTECHNIQUES The development of acrylic elastomer products requires the incorporation of various chemical ingredients into a “compound” based on identity and quantity shown in a “recipe.” All ingredients are based on a total of 100 parts rubber and listed as parts per hundred parts rubber. Stephens (1973) classified the typical components of a recipe other than elastomer into nine classes. These classes will be applied to acrylic compounds with data taken from B. F. Goodrich Bulletin HPA-l (B. F. Goodrich, 1985). Processing uids are chemicals used to modify the compound during mixing so as to achieve certain properties during extrusion, calendering, or molding. Stearic acid is commonly used at levels of 1-3 phr, usually in combination with commercially available processing aids such as TE-80. Struktol WB 222. and Vanfre AP-2 at the 1-4 phr levels. These three processing aids provide both external and internal lubrication. Vzrlcm?izatiot?uger?tscause crosslinking reactions that are necessary to improve the physical properties of the final rubber product. Cure site has an important impact on the choice of curing system. Therefore,the supplier of elastomers is the best adviser. Becauseof the anomalous cure behavior of acrylic compounds, the Monsanto Rheometer does not accurately predict cure behavior. Bases accelerate the cure. and acids retard the cure. In a soap-sulfur system (loll), sodium or potassium stearate acts as the curing agent and sulfur acts as the accelerator. Holly and coworkers (1965) report the use of tetramethylthiuram disulfide as the source of sulfur. Recently EniChem (1995) introduced TCY and a no-postcure agent called Ezcure to cure low-chlorine (0.03%) acrylic rubbers without postcuring. Zeon Hytemp and EniChem epoxy acid types can be vulcanized by means of low aliphatic chain amnlonium salts or (epoxy-acid grades) with orthotolylguanidine. Accelercrfors increase vulcanization rate. In the case of acrylic elastomers with active cure sites. the following accelerators have found use:
TYPC
Trade
Example
name
~~
Urea disulfide Tetrabutylthiuram Thiuram Dithiocarbamate
N.N-Dimethyl-N’-(3,4-dichlorophenyl) urea Zinc dicthyldithiocarbamate Ethyl
Karmcx Diuron Butyl Tuads Zirnate
Acti\wtors form complexeswith accelerators and further increasethe rate of vulcanization. They are alkaline and include magnesium oxide, lead stearate, dibasic lead phosphite, and salts of amines with weak acids.
Acrylic-Based Elastomers
667
Retarders decrease the rate of vulcanization. Acid chemicals such asstearic acid and zinc stearate perform this function. Age resisters slow down the deterioration reactions of rubber with oxygen, ozone, light, heat, or radiation. Antioxidants are usually not required because of the saturated backbone of the elastomer. Dry-heatagingresistance is improvedslightly by the use of particularsolid diphenylamine antioxidants. Oil-aging properties are not improved because of extraction of the antioxidant by the oil. Fillers reinforce or modify physical properties, may produce desirable processing properties, and usually reduce the compound’s cost. Carbon blacks, particularly FEF blacks, provide superior processing and balance of physical properties with heat resistance. Particle size and structure relateto changesin properties similarto those withother elastomers. Synthetic graphites provide surface lubricity and abrasion resistance. Neutral or basic mineral fillers such as silica andaluminum silicate are used where colored compounds or electrically resistant compounds are desired. Softeners may aid mixing, increase tack. or replace some of the rubber without loss of physical properties. In the case of acrylic elastomers, softeners or plasticizers (5- I O phr) improve low-temperature properties at the sacrifice of oil-swell resistance. Low volatility and extractability enhance low-temperature characteristics. Polyesters or ether esters are sold as commercial products for this purpose. Miscellaneolrs ingredients serve to provide a specific propertynot produced by the foregoing compounding ingredients. They include flame retardants. blowing agents. colorants, odorants, etc. Compounding and testing demonstrate whether or not the desired property has been achieved at the expense of others.
6.
PROCESSINGCHARACTERISTICS
The suppliers of acrylic elastomers provide detailed processing information with their commercial products. In general, acrylic elastomers can be mixed in a Banbury or on an open mill, extruded, and calendered. They can be molded by compression, transfer. or injection methods and open steam-cured. B. F. Goodrich (1985) provides a comprehensive overview of processing.
6.1
Mixing
Acrylic elastomers are process sensitive and thus require very rigid mixing control. Banbury (internal) mixing is preferred because of its efficiency and to avoid the release problems associated with open roll mixing. Since acrylic elastomers are soft and thermoplastic, they lose shear resistance at a rate that requires efficient cooling and early addition of all reinforcing pigments to maximize dispersion. The compounds are scorch-sensitive yet slow-curing, thus requiring postcure for finished products. Increased viscosityfrom premature crosslinking that breaks down with subsequent mixing may mask the presence of a scorch problem. Poor vulcanizate physical properties may then be the final signal of scorch when remedy is foreclosed. To control scorch and assure product uniformity and quality, the Mooney viscometer and Monsanto rheometer along with vulcanizate test methods should be used. The Mooney viscometer relates viscosity and scorch at processing temperatures.The Monsantorheometer relates cure behavior at a givencure temperature. These data, when correlated with final physical properties, allow the rheometer to be used as a quality control instrument for mixing. Banbury mixing normally employs two-pass mixing (or one-pass with nonscorchy compounds). A typical mix starts at slow speed with full cooling water. For the masterbatch. after
Carrano 668
and
Andreussi
polymer charge, in 0.5 min add filler, stearic acid, and antioxidant; at 3 min add process aid and plasticizer; at 5 min bump and sweep: at 6 min dump at 125-1S0°C, and dip-cool. The finish mix involves one-half each of master and curative master, mixing for 1-3 rnin, dumping at 107°C maximum. and dip-cooling. Millmixingstarts on room-temperaturerollswith full coolingwater.Bandpolymer; achieve rolling bank by adjusting nip ofrolls; add filler, stearic acid, and antioxidant, maintaining rolling bank; add process aid and plasicizer; dip-cool; add curatives and dip cool. Mixing time may vary from 20 to 60 min. At the conclusion of either Banbury or mill mixing, the stocks should be given six cuts from each side plus six refining passes on the mill before stripping. 6.2
Extrusion and Calendering
Acrylic compounds show variable extrusion and calendering behavior. In general, problems do not arise for intermediate process purposes. For finished goods, special compounding adjustments may be needed. These may include the use of high structure-reinforcing pigments, additional lubricants, and process aids. Nonblack reinforcing pigments may not be satisfactory for finished goods because of excess nerve. Mill freshening before extrusionor calendering is recommended. Typical processingconditions for extrusion are screw or ram type, barrel temperature 60-82"C, and die temperature 82-107°C. For three-roll calender conditions are top, 60-71°C; middle, 71-82°C; and bottom, 82- 107°C. 6.3
Storage
Raw acrylic elastonlers are stable for one year or more at room temperature when protected from moisture as originally packaged. Shelf life of finished compounds varies from 1 to 4 weeks. depending on the activity of the cure system. Refrigerated storage and low humidity extend the shelf life. Whatever the case, mill freshening is recommended before further processing.
6.4 Vulcanization and Postcure Cure cycles vary with mass and conformation. Some typical times and temperatures are:
Compression mold Transfer mold Inyxtion mold Open Ste;lIll
2 min at 190°C to 1 min at 204°C 8 mm a t 163°C to 4 min a t 177°C 90 sec at 163°C to 45 sec at 204°C 30-60 min a t 163°C
Postcuring involves 4-8 hours at 177°C in a circulating air oven: higher temperatures reduce the time. but >204"C is not recommended.
7. VULCANIZATION METHODS 7.1 Poly(ethy1 acrylate)
Cures involving the backbone polymer via the a-hydrogen or carbalkoxy group did not achieve commercialsignificancealthoughtheywereattempted.Mastetal. (1944) cured poly(ethy1
Acrylic-Based Elastomers
669
acrylate) with benzoyl peroxide and quinone dioxime supposing the a-hydrogen to be the crosslink site. Semegen andWakelin (1952) used sodiummetasilicate and lead oxide to achieve crosslinking. Schulz and Bovey (1956) produced crosslinks by electron radiation but not at the position of the a-hydrogen in the polymer backbone. Breslow (1966) investigated the use of tetramethylene-bisazidoformateasapotential curing agent.
7.2
Acrylate Copolymers
The first commercial acrylic elastomer, Hycar PA-2 I , required liquid polyamine cure systems or solid diamines. Later elastomers had sufficient reactivity to get soap (sodium stearate) and sulfur common curesystems.Most cure systemsarebasicand then they areaccelerated by bases and retarded by acids. Starmer and Wolf ( 1 985) summarized commonly used cure systems for acrylic elastomers in the United Sattes. In addition, McMonagle and coworkers (1983)demonstrated the effectivenessof trithiocyanuric acid in eliminating postcure necessity.Okumoto ( 1 983) and Matsuo ( 1983) have presented reviews on acrylic elastomers vulcanization in the Japanese literature. Unscrtur-ateclCurt. Sites
Vulcanization methods depend on the cure sites available for crosslinking reactions. Butadiene and isoprene were used by Fisher et al. (1944), but less volatile agents were incorporated in the backbone by Mast et al. (1944) and by Mast and Fischer (1949). Peroxide cures of Hycar 212 1-38 were studied by Mendelsohn and Minter ( 1964). producing polymers with better low-temperature properties. Chuiko et al. (1976) studied the curing kinetics of peroxidate systems. More complex monomers were investigated. Japan Synthetic Rubber Co. (1982) reported that S-( l-alkenyl)-2-norbomene gave excellent physical properties when cured with a soap-sulfur system. Nippon Mektron K. K. K. (1982) incorporated 2-vinyl-2-oxazoline to give a fast-curing anticorrosive polymer.
R\
c1
Most commercially produced acrylic elastomers today contain chlorine cure sites. The first of such elastomers containing 2-chloroethyl vinyl ether was Lactoprene EV, described by Mast et al. ( 1947). B. F. Goodrich introduced a similar polymer, Hycar PA (later PA-21 and 4021), in 1948. Mast and Fisher ( 1948) explored a number of chlorine-containing monomers. 2-Chloroethyl vinyl ether does not readily copolymerize with ethyl acrylate, and the chlorine is only moderately active i n the vulcanization reaction. Liquid polyamines such as triethylenetetramine were required, but they caused the polymer to stick to the mill rolls. Solid blocked diamines improved the milling performance. Other monomers were sought with more reactive chlorine. Kaizerman (1965) used vinyl chloroacetate to provideneoprene-likeactivity and cured the polymerwiththiazolidinethiones.Morris (1970) reported the use of chloroacetoxymethyl-5norbomene. and Jorgensen ( 197 I ) investigated allyl chloroacetates. Cure systems were also explored to match the new polymers. Holly et al. (1965) used a soap-sulfur system with Cyanacryl. introduced by American Cyanamid in 1962. Behrens (1978)
Andreussi and Carrano
670
reported a bis-maleimide-sulfur system. Tritiocyanuric acids were used by Ermidis (1971), by McMonagle et al. ( 1983), who claimed elimination of postcure, and by Nippon Mektron K. K. (1983). Mori and Nakamura (1984) found triazine dithiols to be useful in elastomer blends. Hinderer (1985) showed a curing system using dimercaptothiodiazole and dithiocarbamate. The lack of reactivity in copolymerization with acrylate monomers andvinyl chloroacetate led researchers to seek a balance between chlorine activity and copolymerization ability. Two types of monomers succesfully achieved this balance of properties: the first was an acrylate or methacrylate having an active chlorine in the alcoholic portion of the molecule (in 1945 Mast et al. were the first to use 2-chloroethyl acrylate. 3-chloropropyl acrylate. and their bromine counterparts, and i n 1970 Chalmers used a reaction product of chloroacetic acid and glycidyl methacrylate); the second type of enhanced reactivity monomers were derivatives of styrene (in 1973 De Marco and Tucker achieved improved processing safety and increased scorch time by the incorporation of chioromethyl styrene and various commonly used chlorine containing monomers. and in 1976 Ebina et al. used vinylbenzyl chloroacetate). I n 1993 EniChem patented a new technology to make low-chlorine types not requiring a postcure treatment.
R I
Epoxy groups provide very versatile cure sites that respond to a variety of cure systems. Simms ( 196 1 ) showed that allyl glycidyl ether was no more reactive in copolymerization with acrylates than 2-chloroethyl vinyl ether. but that glycidyl methacrylate had improved reactivity. In the preparation of the copolymer. dodecyl mercaptan was used as a molecular weight regulator. Ebina et al. (1975) cured the allyl glycidyl ether with diethyl-thiourea as an additive. Denki Kngaku Kogyo K. K. ( 1984) described a complex curing system consisting of dicumyl peroxide, triallyl ixocyanurate. trimethyl thiourea, and phenothiazine, again using glycidyl methacrylate copolymer. Thissystem gave products with improved compression setand longer Mooeny scorch time. Dah1 et al. ( 1982) determined the reactivity of glycidyl methacrylate with n-butyl, isobutyl, and 2-ethylexyl acrylates.
COOH These nlonomers are often reaction products of ethenic carboxylic acids and epoxides. Saxon and Lestienne ( 1964)utilized the hydroxyl in the carboxy groups of a methacrylic acidcopolymer to react with the curing agent hexakis (methoxymethyl) melamine with the eliminationof methanol to form methylene ester linkages. Chen et al. (1985) prepared hydroxypropyl acrylate, but he did not report its copolymerization.
Acrylic-Based Elastomers
671
Other Cure Sites Other cure site monomers have received attention. Fisher et al. (1944) studied allyl maleate and allyl lactate maleate. N-Methoxymethyl acrylamide provided elastomers with self-curing properties and the cure acceleration by acids rather than bases, according to Tucker and Essig ( 1969). The copolymer of ethyl acrylate and methyl acrylamidomethoxyacetate was reported by American Cyanamid (1984). Curing occurred with stearic acid and hexanlethylenediamine carbamate. Allyl acetoacetate was used by Uchyama Kogyo Kaisha, Ltd. (1984) to prepare a copolymer that was cured with equal parts of tetramethyl thiuram disulfide and dibenzothiazyl disulfide followed by a postcure period. The same company (198s) described a copolymer of ethylacrylateandtetrahydrofurfuryl monomer withexcellentscorchresistanceandstorage stability. Mlrltiplo Cure Sites
The postcure of acrylic elastomers, required to obtain low compression set, may be reduced or eliminated by incorporating both achlorineandacarboxymonomer in the polymerchain. Morris ( 1975) conducted a comprehensive study on this combination of reactive monomers. Chloromethyl styrene and methacrylic acid were used by Ebina (1979). with the cure catalyzed by benzyltriphenylphosphonium chloride. Vinyl benzyl chloride and methacrylic acid provided polymers studied by Jablonski ( 1983). who cured them with bis(4-(a,a-dimethylbenzyl) phenyl) amine and by Nippon Mektron K. K. (1982). Gianetti et al. (1983) studied Elaprim AR 153 (today known as Europrene AR 153 by EniChem). a copolymer with epoxy and free carboxyl sites using quaternary ammonium salts to catalyze crosslinking. He found catalyst activity enhanced by low basicity and high steric hindrance. A copolymer with allyl glycidyl ether and methacrylic acid was cured with alkyltrimethylammonium bromide by Nippon Zeon Co.. Ltd. ( 1984). Three curesites in acrylicelastomershavebeenstudied. Cantalupo et al. (1980) used various dual combination of allyl glycidyl ether, glycidyl methacrylate, vinyl chloroacetate.allyl chloroacetate, and chloroethyl vinyl ether, all with acrylic and methacrylic acids.
8.
PHYSICAL PROPERTIES
Both raw and cured polymer property data for commercially available acrylic elastomers are readily obtainable by their suppliers. This review will focus mainly on those properties related to a generalized elastomer in an effort to point the direction to a specific application. 8.1
Uncured Elastomer
Raw. uncured acrylic elastomers are noncrystalline and amorphous with a specific gravity of I . IO- 1.1 S. They are greenish or off-white and somewhat soft and tacky. A Mooney viscosity ( M L , , -I at 100°C) of 25-60 is related to molecular weight of the polymer. They are commercially available a s solid slabs. ground crumbs. and. more recently, free-flowing powder. Early work by Craemer ( 1940) is summarized in Table 3 as a background for mechanical properties. The low-temperature properties of uncured elastomers have received much attention because of product requirements and the inverse relationship between low-temperature properties of the longer-chain alcohol polyacrylates and oil swell.
672
Andreussi and Carrano
Table 3 HistoricalMechanicalPropcrtles Polyacrylntc Brittle point Methyl Ethyl 2000 rr-ButylTacky
3 Softer - 24 - 44
("C)
of AcrylicElastomers
Obscrved properties Elongation rubbery Tough, 228
(%)
6895750 1800
Tensile strength
(Wa)
( 1000)h
(33) 21 ( 3 )
Second-order transition temperature. T,, controlled by the ester alkyl group, marks the onset of segmental mobility in the polymer chain. Fox (1956) proposed that T, of a copolymer was intermediate between the T,'s observed for the homopolymers based on the weight fraction of each in the coplymer. Krause et al. ( 1965) synthesized 25 acrylic homopolymers and determined their thermal expansion coefficients and dilatometric glass transition temperature, T,. The determination of T, by dilatometry (change in volume) or change in refractive index with temperature is time-consuming. For more approximate requirements, the brittle point (ASTM D746) and Gehman freeze point (ASTM D-1053-54T)are useful. The latter is similar to torsional modulus. More recently T, has been easely determined by means of either differential scanning calorimetry (DSC) ordynamic-mechanical spectra, like rheometers. where phase angle (or tans) maximun~peak estimates T,. Mangaraj and coworkers ( 1963) confirmed that T, and cohesive energy density decreased with increase in alcoholic chain length. Correlation between elastomeric structure and T,, in order to meet processing requirements andother elastomeric properties, was studied by Chadwick (1982). Similarly, Suvorova et al. (1982) looked at the effect of ethoxyethyl acrylate on butyl acrylate elastomer, and Lin (1985) manipulated comonomer ratios to optimize flexibility at low temperature. However, when alkoxy comonomers areused to achieve improved low-temperature properties coupled with acceptable oil resistance, there is a decline in strength. DeMarco (1979) highlights the compounding techniques used to improve strength in cured elastomers. Dynamic relaxation behavior in the glassy state is related to low-temperature properties. Kolarik (1982) fo.und two secondaryrelaxationsbelow T, that wereassigned to side-chain motions: p. due to partial rotation of the - COOR groups. and -y, due to internal rotation within the group R and p(yif a diluent is introduced. SteckandBartoe ( 1964) made dielectricnleasuramentsat 60 Hz at -40 to 200°C to discover a peaks in all acrylic polymers and p peaks in some of them. Mikhailov (1964) related dielectric properties over a wide range of temperature and frequency to structure of polymer and dipole relaxation processes. Bur (1985) found that poly(methy1 acrylate) had significant loss of microwave frequencies ( IO0 MHz to 1 0 0 Ghz), which was attributedto dipolar absorption dispersion. Segmental motion in elastomer chains and entanglement contribution to rubber elasticity were discussed by Queslel and Mark (1984). Mark's review on rubber elasticity ( 1 98 1 ) provides a basic discussion on elasticity theory. Saeki et al. (1983) obtained segment fraction activity coefficients using a piezoelectric vapor sorption apparatus. Skinner (1983) introduced a new one-dimensional kinetic lsing model to study the cooperativedynamics of linear chain molecules such as poly(ethy1 acrylate). Aharoni (1983) claimed that empirical relationships for tlexible polymers were i n excellent agreement with recent theoretical expectations. Wang and Lowry
Acrylic-Based Elastomers
673
(1985) introduced an excimer fluorescence technique in an attempt to clarify the concept of internal viscosity related to segmental motions of pyrene-labeled poly(methy1 acrylate). Mechanical rupture of polymer chains on an open mill was demonstrated by Ina et al. (1970). Liang et al. (1983) were able to crosslink poly(/?-butyl acrylate) in a flash photolysis study using electron spin resonance.
8.2 Cured Elastomer The physical properties of cured elastomers are well described in the suppliers' literature. For example, Table 4 summarizes the properties of an EniChem product. In addition to these nleasured properties. cured elastomers are judged to have excellent ozone resistance and conductive electrical properties, good weather resistance, fair tear strength. abrasion resistance. insulating electrical properties. resistance to nitrogen penneability and radiation, low resilience. and poor flame resistance. Since 1948. low-temperature usefulness and cure technology have greatly improved. The brittle points of cured elastomers are similar to those of the uncured lower members of the acrylic series of polymers. The glass transition temperature. T,. is commonly determined by differentialthermalanalysys.Likewise.torsionalstiffness (ASTM D- 1053) andretraction (ASTM D- 1329) are related to low-temperature performance. Very early in the development of acrylic elastomers, Mast and Fisher( 1949)discovered that after aging at 149°C (300°F). plasticizers lowered the brittle point at the expense of tensile strength. modulus. and resilience. Mast et al. ( 1947) also noted that extended cures (24 hr at 148"C, or 298°F) caused reversion. Several cited authors have commented on efforts to improve strength, abrasion resistance. and gasoline resistance. Complete elimination of the need for postcure is also a desired improvement. Suppliers of raw elastomers provide comprehensive dataon methods for achieving satisfactory physical properties in end use products. EniChem ( 1997) is an example ofthe compounding assistence made available in product literature. Kandler et al. ( 1973) related crosslink density. whichis proportional to curative concentration. and stress relaxation in the cured elastomer. Van der Waal (1985) studied the relation of nonpolarity index and wear characteristics. Yeo and coworkers ( 198 1 ) investigated mutifunctional crosslinkereffects on the elasticity of poly( 11-butyl acrylate). Early investigations on radiation resistance by Harrington (1958) ranked ACM next to SBR and determined that ~0111pounding ingredients had a significant positive effect. A number of relevant reviews appeared i n the 1980s. Lauretti et al. (1984) reported on a family of polymeric elastomers that were oil and high temperature resistant. as did Ebina ( 1985)
Table 4
Oil-ResistantElastomerGuide
Service temp.. max.. "C Continuous service temp.. max.. (2000 hrs in air)"C Service flexibility temp.. min.. "C Specific Hardness rnnge (durometer,Shore A ) . 45-80 points strength. Tensile max.. MPa Elongation. max., % Comprcssion set (70 hrs at 150°C).'1 Sorrrw: EniChem. 1997.
17s 1so - 40 1.10
IS 400 15-60
Carrano 674
and
Andreussi
and Asai (1 985). Arkhangelskaya (1985) compared acrylate, nitrile, and epichlorhydrine elastomers.
9. APPLICATIONS Acrylic elastomers with saturated backbone and pendant carbalkoxy groups have very good resistance to heat. automotive fluids and oils, including those containing sulfur used in hypoid extreme-pressure gear lubricants, and to sunlight and ozone. Compounding and curing are required for most applications. However, uncured polymers may be used as binders or as blending ingredients in plastics. Acrylics compete with other important oil-resistant elastomers suchas butadiene-acrylonitrile (NBR), chloroprene (CR), chlorosulfonated polyethylene (CSM), epichloridrine (CO-ECO), silicone (MQ), fluorosilicone (FMQ), fluorocarbon (FKM), and hydrogenated butadiene-acrylonitrile (HNBR). Acrylic heat resistance is superior to that of NBR, CR, CSM, and CO-ECObut is inferior to that of FMQ, FKM, MQ, and HNBR. With respect of these latter polymers, ACM performance is comparable orsuperior when the combination of heat and fluid resistance is taken in account.
9.1
Automotive
The automotive industry provides the major marketfor acrylic elastomers: principal uses include critical seals for automatic transmission, pinion, valve stem. and crank shaft. Various gaskets, diaphragms, hoses, and packings provide other opportunities for superior performance. Several of these applications were reported by Andreussi et al. (1997), based on the resistance of acrylic elastomers to automatic transmission fluids, SH classification motor oils, and spindle bearing greases use in front wheel drive assemblies. These fluids are hydrocarbon based with additives for detergency, dispersion, corrosion resistance, and viscosity stability. These additives include succinamides or phosphonates. which may cause loss of compound modulus and tensile strength under conditions of high-temperature exposure to the parent fluids. Changes in gasoline fuels toward aromaticsto increase octane ratings of unleaded gasoline and the use of alcohols and n-butyl methyl etherto replace tetraethyl lead used in regular gasoline refocused attention on hydrocarbon-resistant elastomers. Nersasian (1982) found tluoroelastomers (VT-R5362) with high fluorine content superior to ACM in gasohol, pure ethanol, or sour (peroxidized) gasoline. Low-temperature flexibility of FKM remains to be improved. Abu-Isa (1983) used his “solubility parameter” concept to explain the effect of these fuels on a variety of elastomers. Automotive hose has been the subject of two reviews with somewhat different outlooks for ACM. Dunn and Vara (1983) included industrial hose and the properties of ACM in ASTM No. 3 oil, but they did not stress the importance of ACM in the hose market. Inagami et al. ( 1983) reported that 25% of the demand for ACM is in auto hose. They pointed out the need for low compression set in high-pressure hose, which dictates the use of epoxide cure monomers instead of those with active halogen. They discussed the sacrifice in physical properties and heat resistance to meet - 40°C temperature service requirements. Brewster and Megna (1984) commented that ACM continued to meet oil and heat-resistance requirements. The applications of ACM in the automotive industry were recently reviewed by Hashimoto et al. (1998). Both Zeon (1994) and EniChem (1997)reported several reviews on ACM characteristics and applications, mainly in the automotive field.
Acrylic-Based Elastomers
9.2
675
Adhesives
Gross and Weber (1977)provided a comprehensive review on carboxylic polymers in adhesives. Hutchinson (1978) discussed elastomer and adhesive selection in forming elastomer-to-metal bonds andthe cause for failure of this bond. Atable of elastomer properties for acrylic elastomers is provided. Aubrey and Ginostatis (1981) found that the introduction of carboxylic acid end groups into a poly(n-butyl acrylate) adhesive greatly increased the bond strength of elastomer to glass or elastomer to cellulose. Dynic Corp. (1983) used a poly(ethy1 acrylate) in a foamed composition to bind nonwoven fabric. The effect of various model follows on the adhesive strength of poly(n-butyl acrylate) tapes was investigated by Bhowmick (1989).
9.3 Blends B. F. Goodrich (1985) reported that acrylic elastomers exhibit excellent physical and cure compatibility with epichloridryne (CO-ECO) and diamine curable fluorocarbon FKM) elastomers. Stanescu (1980) claimed improved cold resistanceof a blendof CO-ECO andacrylic elastomers. Stanescu and Iovitoiu (1980) also claimed that FKM improved flow during processing if there was about 30% acrylate rubber. Yet Vincinte Baez (1983) reported that 70 parts of Viton B and 30 parts of acrylic rubber were not compatible. More recently, Nogushi et al. presented an acrylic-fluorurate blend ( 1997). Lonseal Corp. (198 1) proposed the use of acrylic rubber (40 parts), PVC (100 parts), and polyester plasticizer in a film for automotive tops. Teijin Chemicals Ltd. (1985)blended acrylic elastomers with an aromatic polysulfone to obtain an high-impact substrate that could be NiCu plated. Today's trend, however, is to use ACM without blending other elastomers.
9.4
Cement and Mortar
Bothaqueousdispersion and latex have been used. Lynn (1983) reportedthat the aqueous dispersion of acrylic polynlers in hydraulic cement increased the modulus of rupture, which allowed use in roof tiles or shingles. Ohama and Shiroishida (1983) found that mortars containing polyacrylate emulsion lost physical properties as the test temperature increased from -50 to 150°C, expecially at higher polymerkement ratios.
9.5
Medical
Acrylic elastomers suffer sensitivity to water and expecially to alkaline solutions. As a consequence there is little medical use reported.LeeperandWright (1983) reported that acrylic esters had been displaced by silicone for maxillofacial reconstruction. The regulation aspects of elastomers use in medicine have been also discussed. 9.6 Plastic Impact Improvers Plastics may suffer embrittlement at lower operating temperatures. Acrylic elastomers are used to improve the impact resistanceof certain plastics and in some casesin thermoplastic elastomer compositions as an elastomeric additive. Shah and Temin ( 198 1 ) used a 95/5 copolymer of ethyl acrylate and acrylic acid with poly(viny1 pyrolidone)toimprove themodulus of thethermoplasticacrylicelastomer.Gift (1983) reported the production of acrylate impact-resistant moldable thermoplastic.
676
Andreussi and Carrano
Epoxy resins of the diglycidyl ether of bisphenol A type (Epon 828 by Shell) showed improved impact resistance by the use of poly( n-butyl acrylate) terminated by carboxyl groups, according to Gazitand Bell (1983).KirshinbaumandBell (1983) describedprocedures for increasing the compatibility of acrylic elastomers with epoxy resins. Poly(viny1 chloride) with 7.5 parts of acrylic elastomer (HIA 28) had improvedlowtemperature impact, as reported by Plas-Tech Corp. (1983). Polycarbonates with high impact toughness contributed by Acryloid KM 330 were described by Witmann ( 1984). ACM-modified nylon has been also investigated by Biggi and Gobbi (1995), where a description of impact resistance improvement by means of epoxy acid acrylic elastomer is provided.
9.7 Rocket Propellant Binders Acrylic elastomers burn readily and completely. leaving no ash, which makes them useful for rocket propellant binders or explosive mixtures. On the other hand. if fire resistance is desired, one must incorporate into the compound alumina and antimony oxide along with phosphatetype protective agents. Culverhouse (1984) discussed both burning mechanism and additives, while Fabris and Sommer (1973) covered flame retardancy of polymeric materials.
9.8 Roof Mastics This is a relatively recent application of acrylic elastomers. Frankel et al. (1982) described an acrylic mastic that was resistant to ultraviolet light and spryable. Kern (1984) reviewed a new generation of acrylics developed specifically for roof mastics. They must be properly formulated with flame retardants to meet the fire rating requirements.
9.9
Sealants and Caulks
The chemical and physical properties of acrylic elastomers fit well with various applications in sealants and caulks. For example. Grabmuller (1982) reported only slight degradation after 8 years of exposure. Nippon Carbide Industries Co., Inc. (1983) patented tack-free, soil-resistant, and weather-resistant sealants useful over a temperature range of - 90 to 200°C. Solar collectors are a hostile environment for sealants, caulks, and gaskets. Mendelson et al. (1983) found that ACM gaskets were satisfactory at intermediate temperatures. In the case of caulks. Hycar 4054 performed well in all tests except ultimate elongation, which failed a bit. Hycar 4054 produced a clear, transparent. continuous film on the glazing component. These films produced little effect on the light transmittance even over time (Luck and Mendelsohn, 1983). Instrumental techniques found use in control and development of sealants, binders, and adhesives. Willoughby (1982) reported the use of a vibrating needle curometer for this purpose.
9.10
Shaft Seals
Friction coefficient and type of carbon filler affect seal life. Yurovskii et al. (1982)claimed that the friction coefficient shouldbe equal to or less than 20.5 to ensure good service life irrespective of whether one was dealing with acrylic.fluorocarbon. or siliconerubbers.Yurovskiiet al. (1983) reported the use of carbon fiber to replace graphite to increase wear resistance in shaft seals.
Acrylic-Based Elastomers
677
Zeon (1994) and EniChem (1997) also reported ACM compressive stress-relaxation measurement results, which is a more suitable method than compression set to investigate gasket and seal performance under stress in service conditions.
REFERENCES Abu-]sa, I. A. ( 1983). R~tbberCheru. Teclrriol. 56( 1 ): 169. Aharoni. S . M. (1983). M~cc~rnr~lolrculc,.s f6(1 l):1722. Aloia, R. R., and Kaizerman, S. (1968). U.S. Pat. 3,397.193 (to American Cyanamid Co.), August 13. American Cyanamtd Co. (1984). Jpn. Pat. 59,100,114, June 9. Andreussi, P,, Baldovino, C., Carrnno,A., Da Lio, J., and Barnes, C. (EniChem Elastolneri), paper presented at 1997 SAE International Congress & Exposition, Detroit. Aoshima. A.. and Murofushi, T. (1981). Fr. Pat. 2,477,534 (to Asaki Chemical Industry Co.). Arkhangellskaya, B. N. (1985), K m c h Rezirltr 4:40. Asai, H. ( 1985), Nippor~Goruu Kyokaishi 5H(3):133. ASTM D 1418-85 (1985). Annual Book of ASTM Standards, Vol. 901, American Society for Testing and Materials, Philadelphia, p. 356. Aubrey, D. W.. and Ginostntis, S. ( 1981), J. Arllwsiort 12(3):189. Baucr. W. (1928). Ger. Pat. 546,141 (to Rohm & Haas A.-G.), July 31. Bauer, W. (1931), U.S. Pat. 1,829,208 (to Rohm & Haas A.-G.), October 27. Bauer, W. (1933), U.S. Pot. 1,890,277 (to Rohm & Haas A.-G.), December 6. Beatty, J. R., and Studebnker. M. L. (1975), Rubber Age f07(8):22. Behrens, R. A. (1978). U.S. Pat. 4,092.303 (to American Cyanamid Co.), May 30. Bhattacharyya, S. K.. and Sen. A. K. ( 1964), h / . Er1g. Cherrr. Proc. Des. Dev. $2): 169. Bhowmick A. K. (1989), J. Atlll. Sci. Techr~ol.3(5):371. Biggi, A.. and Gobbi, C., (1995), U.S. Pat. 5,478,889 (to EniChem). Bonamy, A., Founssier, J. P,, and Lougnot, D. T. (1982), J. Polytn. Sci. Polym. Lett. Ed. 20(6):315. Brandrup, J.. and Immcrgut, E. H. (Eds.) (1975), Polvrrwr Hm~dbook.2nd ed., Wiley, New York, p. II47. Breslow, D. S . (1966). U.S. Pat. 3,284,421 (to Hercules, Inc.). Brewster. T. I., and Mcgna. I. S. ( 1984), Autorrrotiw Elrrstonlcrs & Desi
678
Andreussi and Carrano
Dillinghm. E. 0.. Lawrence. W. H.. Autian, J.. and Schmalz. G. (1983). J. Biorrlecl. Mater. Res. / 7 ( 6 ) : 945. Dougherty, E. F., Scatcs. M. O., and Paul, J. L. (IOXS), U.S. Pat. 4.507.495 (to Cclanese Corp.). March 26. Dutp, J. R., and Vara, R. G. (1983). Rubber C/rerr~.Techlo/. 56(3)557. Dwlvedi. U. V.. and Mitra, B. C. ( 1984). J . App/. Polyrrl, Sci. 2% l2):4199, Dynic Corp. (1983). Jpn. Pat. 58.13.662, March 13. Ebina, Y. (1979), Jpn. Pat. 79.83.047 (to Nippon Oil Seal Industry Co.Ltd.). July 2. Ebinn, Y. (1985). N i p p m Corm Kyokrrlislri SK(3):195. Ebina. Y.. Iramatsu. F., Suzuki. Y., and Kojima, Y. (1975).Jpn. Pat. 75,61.487 (to Nippon Oil Sed 1nd.Co.. Ltd.). May 27. Ebina. Y.. Susukl. Y., and Kurihora. H. ( 1976). Jpn. Pat. 7658.443 (to Nippon Oil Seal Industry Co.. Ltd.), May S . Enpelhardt. G.. and Klimisch. H. J. (1983). A t r l t h r r . AppL T o r i d . 3(6):640. Ernmidis. N. P. (1971), U.S. Pat. 3,622.547 (to Amcricnn Cyanamid), Nov. 13. Fabris. H. J., and Sommer, J. G. (1973). in Fltrrrw Ret~rr~tltrrrcyr!f' Polyrrleric Mrrreritrls. Vol. 7- (W. C. Kuryla and A. J. Papa. Eds.). Marcel Dckkcr. New York. p. 135. Fnlk. J . C., Stnmatios. M.. and Van Beck, D. A., Jr. ( 1984). U S . Pat. 4,473,679 (to Borg Warner Chcm.Inc.). Sept. 25. Fisher. G. I. (1958). U.S. Pat. 2,844,672 (to Cclancse Corp.). July 22. Fisher. C. H., Mast, W. C., Rehberg, C. E., and Smith. L. T. (1944). [ r r d Err,?. Clrc~rr.36:1032. Fitch. R. M.. Palmgrcn. T. H., Aoyagi, T., and Zuckov. H. ( 1984), Ar~,gor.Mtrkrorrrol. Cllett1. /23//24: 261. Fox. T. G.. Jr. (1956), Bltll. Anr. Phys. Soc. 1(3):123. Frankel, L. S.. Perry, D. A., and Lavellc. J. A. (1982).Krsirr Rei,. 32( 1 ):IO. Furuknwa. J. (1962), Po/yrrrcr 3:487. Gabris, T. (1985). RuAher Pltrst. News. Nov. 25, p. 6 . Gnzit, S., and Bell. J. P. (1983). ACS S v r r p Ser. 221:69. Ginnetti. E.. Mazzorchi. R., Fiore, L., and Crespi. E. ( 1983). Rrrl~herChorr. T e c h o l . 56( 1):21. Gift. W. H. (1983). U.S. Pat. 4,384,077 (to Plaskolite. Inc.). May 17. Goblan. R. H.. nnd Bernstein. P. (1969), U.S. Pat. 3,450,681 (to Thiokol), June 17. B. F. Goodrich Co. ( 1985). Product Bull-HPA-I. Grabmuller. V. (1982). Sttrvivo 60( 12):494. (Chew. Ahstr. YY:6898q (1984)) Grasselli. R. K., and Surcsh. D. D. (1981). Can. Pat. l.107.753 [to Stnndnrd Oil CO (Ohio)]. Aug. 25. Greek. B. F. (1984). Clrcw. Err,?. N c J l t x . April 30. p. 35. Gross, M. E., and Weber. C. D. ( 1977). i n H t t r d h o o k oj'Atl/re.si\v.s (I. Skcist, Ed.). Van Nostrand Rcinhold, New York. p. 33 I . Hardman, H. F.. and Miller. A. F. (1965). Cherrr. E I I , ~Ne1t.s . 43(39):62. Harrington. R. (1958). Rrrhh~rAge (N. Y.) 82:1003. Harris. J. 0.. and Wilt, M. H. (1958). U.S. Pat. 2.839.51 I (to Monsanto), June 17. Hashimoto. K., Mncda, A., Hosoya, K.. and Todani. Y. ( 1998). Rrthher Cheru. Techrrol. 71:449. Hinderer. R. F. (1985). U.S. Pat. 4,524,185 (to Fedcr:~l-Mogd Corp.),June 18. Hoberg. H.. Schnefer, D., Burlkhart. G.. Kruegcr,C., and Romao. M. J. ( 1984). J. Orgttr1orrwt. C ' k r r r . 266(2):203. Holly, H. W., Mihal, F. F.. and Starcr. I. ( 1965). Rrthher Age Y6:565. Howcrton. W. W., Dictz. T. J.. Snydcr. A. D.. and Alden. G. E. (1952). Rrrhhc~rAge 72(3):353. Hutchison. J . D. ( 1978). E/~r.storr~eric~s 110(4):35. I. G. Farbcnindustrie, A.-G. (1930) Br. Pat. 358.534. May 6 . 1. G. Farbcnindustrte. A.-G. (1931). Fr. Pat. 710.901. Feb. 4. I. G. Farbemndustric. A.-G. (1938),Br. Pat. 496.276. Nov. 28. IISRP ( 1985). W o r ~ l r h i t l eR~rhhcrStrttistir's. International Institute of Synthctic Rubber Producers. Inc. Houston, TX, p. 70. I n a . T.. Kurosawa. T.. Komatsu. T.. and Taktota, T. (1970). N ~ / J ~ Gorrrrc O I I Kyokrrishi 43:107.
Acrylic-Based Elastomers
679
Inagami. M,, Saito. Y., and Hashimoto. K. ( 1983), Rd)her Cllcw. Tr>chrlo/.56(3):677. Jablonski, D. E. (1983a), U.S. Pat. 4,399.262 (to B. F. Goodrich Co.), Aug. 16. Jablonskt, D. E. (l983b), U.S. Pat. 4,405.758 (to B. F. Goodrich Co.), Sept. 20. Jaeger. P,, and Germain. J. E. ( 1982). Hull. Soc.. Clrirrl. Fr. l l112 (Pt. I ):395. Japan Synthetic Rubber Co.. Ltd. (1982). Jpn. Pat. 58,138,749, Feb. 12. Jorgenscn. A. H., Jr. ( 1 9 7 0 ~ )U.S. . Pat. 3,488,331 (to B. F. Goodrich Co.), Jan. 6. Jorgc~~sct~. A. H., Jr. (1970b), U.S. Pat. 3,525,721 (to B. F. Goodrich Co.), Aug. 25. Jorgenscn, A. H.. Jr. ( 197 1 ), U S . Pat. 3,624.058 (to B. F. Goodrich Co.), Nov. 80. Jorgcnsen. A. H., Jr., Starmcr, P. H.. and Stuesse, J. F. ( 1967). U.S. Pat. 3,315,012 (to B. F. Goodrich Co.), April 18. Kaim. A. (1984), J. Po/ynf. Sci. P o l w . Le//. Ed. 22(4):203. Kaizcrman, S . (1965), U.S. Pat. 3,201.373 (to American Cyanamid Co.), Aug. 15. Kamachi, A. I., Fujii. M., Ninomtya, S., Katsuki. S., and Nozakura, S. (1982),J. Po/ynl. Sci. Po/yrrt. Cherrl. , Ed. 20(6):1489. Kandler, Von J.. Pcshuk, G., and Woss. H.-P. ( 1973),A I I ~ ~ Mtrkrorrlol. .. Cherrl. 2Y/30:241. Kern, W. A. (1984). M o d . Ptrirrr Cot//. 74(IO): 164. 166, 170. Kirshenbaum, S. I.. and Bell, J. P. (1983). P o l w . Mrrter. Sei. B l g . 49398. Kolarik. J. ( 1982), Arb. P o l y t l . Sci. 4 4 6 ) :136. Krause, S., Gormley. N. R., Shettcr, J. A.. and Watanabe, W. H. (1965), .l. P o / w . Sci. A3:3573. Lnuretti, E., Mezzera, F., Santarelli, G., and Spelta, A. L. (1984), I r l d Gorrlrrltr 28( 10):49, 64 (Italian). Leeper, H. M,, and Wright, R. M. ( 1983). Ruhher Clleru. T d l r l o l . 56(3):523. Liang, R. H.. Tsay, F. D., Kim, S. S.. and Gupta, A. ( 1983), Po/yrll. Muter. Sci. Err
680
Andreussi and Carrano
Nersasinn, A. (1982). /nd.-Aw. /04(95):18. (Chem. A h w . 98:10863~,1983) Nippon Carbide Industries Co., Inc. (1983). Jpn. Pat. 58,176,289, Oct. 15. Nippon Mektron K. K. (1982a), Jpn. Pat. 57,l 15,443, July 17. Nippon Mektron K. K. (1982b), Jpn. Pat. 57,170,912, Oct. 21. Nippon Mektron K. K. (1983), Jpn. Pat. 58,180,539, Oct. 22. Nippon Zeon Co., Ltd. (1984). Jpn. Pat. 59,168,013, Sept. 21. Noguchi, T., Oka, M., Matsumoto, K., Kishine, M., and Minamino, E. (1997), presented at SAE International Congress & Exhibition, Detroit. Nowak, P. (1938), U.S. Pat. 2,105,361 (to General Electric Co.). Jan. I . Nowak, P,, and Hofmeier, H. (1938), U.S. Pat. 2,105,362 (to General Electric Co.). Jan. 1 1 . Ohama, Y., and Shiroishida, K. (1983), Proc. J p . Congr. Mctter. Res. 26:195 (English). Okumoto, T. (1983). Toyodrr Gosei Giho 25(3):85. Osada, Y., and Takase, M. (1983), J . Polym. Sci. Polym. Lett. Ed. 21(8):643. Plas-Tech Corp. (1983), Jpn. Pat. 58,217,541. Dec. 17. Queslel, J. P.. and Mark, J. E. (1984), Ach. Po/.vrrl. Sei. 65:135. Rasmussen, J. K. (1982), U.S. Pat. 4,426,049 (to 3M Co.), April 20. Ratchford, W. P,, and Fisher, C. H. 1945, I d . Eng. Clleru. 37:382. Reddy, G. G., Nagabkushanam, J., Ras K. V., and Santappa, M. (l982), Bt. Po/ym. J. /4(3):89. Rehberg, C. E., Faucette, W. A., and Fisher, C. H. ( 1944). J. Am. Clmz. Soc. 66: 1723. Rehberg, C. E., and Fisher, C. H. ( 1944), J. Am. Cllenz. Soc.. 66: 1203. Reppe, W. (1953). Justus Liebigs A t m Clwrn. 582: 1. Riddle. E. H. (1954). Monon~ericAcrylic Esters, Reinhold, New York. Rohm, 0. ( 1912), Ger. Pat. 262,707, Jan. 3 1. Rohm, 0. (1913), Br. Pat. 613, Jan. 9. Rohm, 0. (1914), U.S. Pat. 1,121,134, Dec. 15. Rubber Red Book (1983), Communication Channels, Inc., Atlanta, CH, p. 554. Ruhher World Blue Book (1985). Lippincott & Peto, Inc., Akron, OH, p. 397. Saeki, S., Holste, J. C., and Bonner, D. C. (1983), J. Po/ytu. Sei. Polwn. Pkys. Ed. 2/( 30):2049. Sato, T., Fukumura, N., and Otsu, T. (1983a), M ~ k r o r n o l Chenl. . /84(2):43I . Sato, T., Nakashima, S., and Baba, M. (1983b). Fr. Pat. 2,509,294 (to Nippon Shokubai Kagaku Kogyo Co., Ltd.), Jan. 14. Sato, T., Yamada, K., Yasuda, T., and Otsu, T. (1983c), Mnkron~ol.C11em.184(3):519. Saxon, R., and Lestienne, F. C. (1964), J . Appl. Polyvz. Sei. 8:475. Schnabel, E. (1940). U.S. Pat. 2,219,661 (to Resistoflex Corp.), Oct. 29. Schulz. A. R., and Bovey. F. A. (1956), J. P o / w . Sei. 22:485. Semegen, S. T., and Wakelin, J. H. (l9S2), Rubber Age 7/57: Rubber Cheru. Techno/. 25582. Shah, K. R., and Temin, S . C. (1981). U.S. Pat. 4,306,039 (to the Kendall Co.), Dec. IS. Shulyakovskii, G. M,, Temkin, 0. N., Pavlyuk, V. V., and Kharkin, A. A. (1982). Nuelcofil’nqe Rectkts. Ktrrhonil’r~ykhSoedin. p. 103. (Chern. Abstr. /01:90380e, 1984) Simms, J. A. ( 1961), J. Appl. Po/vm. Sei. 5:58. Skiest Laboratories (1985),Specinlty polymer.^, Multi-Client MarketingStudy of l00 Po/ytver.s,Livingston. N.J. Skinner, J. L. (1983). J. Chem. Pkys. 79(4):1955. . Chern. 34:473. Smith, L. T., Fisher, C. H., Ratchford, W. P,, and Fein, M. L. (1942), b ~ d Eng. Smithers Scientific Services ( 1985). European rubber industry report (includes Soviet Union), Akron, OH. Snuparek, J., Jr., and Kleckova, Z. (1984). J. Appl. Polymer Sei. 29( 1 ): I . Sony Corp. (l984), Jpn. Pat. 59,213,196, Dec. 3. Stanescu, C. S. ( 1980), Romanian Pat. 7 1,71I , Aug. 8. Stanescu, C. S., and Iovitoiu, N. (1980). Ronianian Pat. 71,712. Aug. 15. Starmer, P. H. (1972), U S . Pat. 3,697,490 (to B. F. Goodrich Co.), Oct. IO. Starmer. P. H., and Wolf, F. R. (l98S), Encyclopedin of Polynwr Scier~cem r l E n g i r ~ c w i / ~2nd g . ed., Vol. 1 (H. Mark et al., Eds.), Wiley, New York, p. 316. Steck, N. S., and Bartoe, W. F. ( 1964), SPE Trolls. 4( 1):M.
Acrylic-Based Elastomers
681
Stephens, H. L. (1973), Ruhher Technology (M. Morton, Ed.), Van Nostrand Reinhold. New York, p. 19. Suvorov:t, E. A., Fomicheva, M. M,, and Grinblat, M. P. ( 1982). Kouch. R(J:irrn ( / M .Po/ym. Sci. Tec.hrto/.) 7:4. (Clletn. Abstr. 97: l 1 1089h). Teijin Chemicals, Ltd. (1985), Jpn. Pat. 60, 19,527, Jan. 31. Trommadorf, E. (1937). KurrststofSv 27:75. Tucker, H. A. (1967), U.S. Pat. 3,326.868 (to B. F. Goodrich Co.), June 20. Tucker, H. A., and Essig, H. J. (1969), U.S. Pat. 3,445,403 (to B. F. Goodrich Co.), May 20. Uchiyama Kogyo Kaisha, Ltd. (1984), Jpn. Pat. 59,136.303, Aug. 4. Uchiyama Kogyo Kaisha, Ltd. ( I 9 8 3 , Jpn. Pat. 60.76.5 19, May 1. Van der Wad, R. H. (1985). Techo/ogie.s of Speci~rltyH i g h Prrfonrrcrrm~E/rt,sto~rrer.s,Technomic Pub. Co., Lancaster. PA. Van der Waal G. (1985), S w t h . Lubr. 1(4):280. Van der Walle, H. B., and Bensink, T. (1982). Corlftrct Derrrlotitis 8(6):376. Villieras. J.. and Rimbaud, R. I. (1984), Swr/re.sis 5:406. Vincinte Baez, F. (1983). Hul Mex. Pltrsr. 39(446):16. Waegemaekers, T. H. J. M., and Bensink, M. P. M. (l984), Murcrgrrr. Res. /37(213):95. Walter, G . (1976). Rubber Chem. Techno/.49:775. Wang, F. W., and Lowry, R. E. (1985). P o / y e r 26:1046. Willoughby, B. G. (1982), Rubber World 187(3):26. Witmann, M. W. (l984), Ger. Pat. 3,325,056 (to Mobay Chelnical Corp.), Jan. 26. Yeo, J. K., Sperling, L. H., and Thomas, D. A. (198 l ) , J. Appl. P o / w . Sc;. 26( 12):3977. You, X., Zhou, Y., and Hu, Y. (1982), Hunrljirlg Kesue 3(6):39. (Cllenl. Ahstr. 98:84705t. 1983) Yurovskii, V. S., Komornitskii-Kuznetsov, V. K., Panyushkina. N. M,, Savitskaya, L. V., and Perfil’cva, N. v . (1982). Proc. Int. Meet. Int. Exhib. Sealing Tcchnol.. 7th 3:997 (Russian). Yurovskii, V. S., Kharitonovich, T. N., and Zubova, M. V. (1983). K m . d ~ RrZjrltr . 5:25. Ziegler, K. (1938), Rubber Cherrl. Techno/. 11:501.
This Page Intentionally Left Blank
Poly(propy1ene oxide) Elastomers Dominic A. Berta Basell R&D Center, Elkton, Maryland Edwin J.Vandenberg Arizona State University, Tempe, Arizona
1. INTRODUCTION For many years, polyethers, suchas [--CH2CH(R)O--],,, were considered to be excellent candidates for elastomers, in the expectation that the oxygen atom would contribute greatly to chain flexibility and thus enhance elastomeric behavior (Price, 1961). Poly(propy1ene oxide), with its low cohesive energy between chains, should be a superior elastomer, but the known methods of polymerizingpropylene oxide gave onlylowmolecularweightliquidpolymers, not the desired high molecular weight polymer. Such liquid propylene oxide polymers could be made largely hydroxyl-ended and then converted to a rubbery polyether urethane by reaction with dior polyisocyanate. These polyurethane-type propylene oxide rubbers are widely used for lowdensity foams and other fabricated articles in which a propylene oxide-based polyol is chainextended andcrosslinked during fabricationwith di- or polyisocyanates(Price, 1958). This chapter discusses only polyether elastomers that are of sufficiently high molecular weight to be processed and fabricated by conventional rubber equipment and that are then crosslinked (i.e., vulcanized) in a separate step. The preparation of such polyether elastomers required the development of new catalyst systems, specifically coordination catalysts, for polymerizing propylene oxide and other epoxides. In 1957, some especially effective coordination catalysts for polymerizing epoxies to high polymers were discovered (Vandenberg, 1960). Some of these new catalysts were the reaction products of organic compounds of aluminum, zinc, and magnesium with water. Unusually versatile was the combination of organoaluminum compounds with water and acetylacetone. These new catalysts led to the discovery and development of two classes of commercial polyether elastomers: (1) the epichlorohydrin (ECH) elastomers (Williset al., 1965),e.g., the ECH homopolymer, abbreviated CO by ASTM (ASTM D141 8), and the ECH ethylene oxide (ECH-EO) copolymer (abbreviated ECO); and (2) thepropylene oxide (PO) elastomers(Vandenberg, 1973a). The epichlorohydrin elastomers were first introducedin 1965 under the trademark Hydin 683
Vandenberg 684
and
Berta
and later under the trademark Herelor. Presently, these elastomers are being made in quantities of thousands of metric tons per year. In the early work on epoxide polymerization, high molecular weight, largely amorphous propylene oxide-unsaturated epoxide copolytners were made. and their potential value as improved elastomers was recognized (Vandenberg and Robinson, 1975). Subsequently, properties of similar propylene oxide-unsaturated epoxide copolymer elastomers were reported (Gruber et al., 1964). This new type of polyether elastomer (Vandenberg, 197313) became commercially available under the trademarkParel (GPO) in 1972. Parel elastomer is a sulfur-curable copolymer of propylene oxide and allyl glycidyl ether (AGE). This polyether elastomer is finding use in applications where good dynamic properties and flexibility at extremely low temperatures are important. Performance is similar to that of natural rubber, but with the added advantage of good resistance to aging at high temperature [ 275°F ( 135"C)] and to ozoneand moderate resistance to oils and fuels. Parel is finding use in such applications as motor mounts, bushings, insulators. and belts.
2. STRUCTURE OF GPO Commercial poly(propy1ene oxide) elastomer (GPO) hasthe basic structure shown in Figure 1 and is a copolymer with an unsaturated epoxide, i.e.. 6 wt% of AGE. The propylene oxide units contain an asymmetricalcarbonatomandthuscanexist in randomstereosequence (atactic polymer) or in stereoregular sequences, such as in isotactic sequence where all the monomer units in a given chain have the same configuration. I n addition, the units can be arranged in head-to-head, tail-to-tail. and, asis normally anticipated, head-to-tail polymerizationwith propylene oxide (Steller, 1975). Presumably, the commercial product contains substantial amounts of each of these sequences and, by design, only small amounts of crystallinity. The presence of the unsaturated conlonomer units also serves to reduce chain regularity and hence crystallinity. The propylene oxide provides the backbone and the bulk of the performance properties related to this elastomer. while the allyl glycidyl ether provides unsaturation to act as a site for crosslinking with sulfur and sulfur-donor systems.
PROPY A OLXELI D N YEL C L YECTI D HY EL R
H H
-cH
HCH I c - 0 H
-
H
I H HCH I
0 I HCH
I
CH I1 HCH Fig. 1 Chcmical structure of GPO.
H
c-c-0-
Poly(propy1ene oxide) Elastomers Table 1 FundamentalProperties ofGPOPolymcr ~
y
Specific I .OO
Ash, 76
wt%
18-22
Glass transition temp. "F Unsaturation, Service temperature Low, "F ("C) High, "F ("C) ODR Viscosity
("C)
- 103 (-75)
6
-78 (-60) 136 (275)
GPO also has a low density, which is advantageous in providing good strength at lower weight. The glass transition temperature is low [ - 103°F ( - 75"C)], which provides vulcanizate flexibility well below - 58°F ( - 50°C). Fundamental polymer properties are given in Table 1. Experimental products with the same composition as Parel, but which are largely isotactic and largely head-to-tail, are crystallizableelastomers with some property advantages. particularly when tested without a reinforcing filler.
3.
METHODS OF PRODUCTION OF GPO
PO is copolymerized with AGE in a solution polymerization in an aliphatic, aromatic, or chlorinated hydrocarbon, in the presence of coordination catalysts such as the aluminum alkyl-wateracetylacetone (Vandenberg, 1969),diethyl zinc-water (Sakata et al., 19601, and complex cyanide catalysts (Herold and Wittbecker, 1974). The copolymer has the same composition as the monomer charge. Thus, complete conversion and a uniform copolymer are obtained. Thecommercial product, which is mostly amorphous, contains about 6% AGE. Molecular weight is controlled by variouscarbonium ion precursors(FilarandVandenberg.1967). GPO is soluble in most organic solvents. After appropriate treatment to inactivate or remove the catalyst. and after the addition of phenolic antioxidant and/or other oxidation stabilizers, the polymer is isolated in dry form from solution by evaporation or steam stripping. Conditions of polymerization and isolation have to ensure a product essentially free of monomers and solvent. GPO can be prepared in latex form by emulsifying a solution of the polymer and then stripping off the solvent (Vandenberg, 1972). However, these elastomers are not commercially available in this form. Bulk polymerization is also feasible because of the perfect copolymerization; i.e.. the reactivity ratio is 1. Specific approaches to slurry-type polymerizations in isobutane diluenthave been reported in thepatentliterature(Shibatami and Magata.1973; Schlatzer, 1976). Partly crystalline copolymers containing isotactic PO sequences obtained with a variety of catalysts are reported in the literature (Teyssie et al., 1975; Vandenberg, 1976). Theproducts from the Et,Zn . H 2 0 catalyst (Cooper et al., 1968) and from the bimetallic rn-oxo-alkoxide catalyst of Asgan and Teyssie (Vandenberg, 1986) have been examined in detail.
4.
MOLECULAR WEIGHT AND SOLUTION BEHAVIOR OF GPO
Specific methodsfor determiningthe molecular weightof Parel elastomer have not been reported. A dilute solution viscosity method (Shambelan, 1959) used for GPO is applicable: i.e.. [q]=
Berta and Vandenberg
686
0.81 X IO"'M,"", where [q]is the intrinsic viscosity in benzene at 25°C and M,, is the weightaverage molecular weight. Solution property data have also been reported (Allen et al., 1964) for GPO.Based on intrinsic viscosity data and light scattering in hexane on fractions of polymer varying in degree of crystallinity, they show the following relationships: For hexane at 4 6 ° C [q] = 1.97 X IO-'
M;'.'
For benzene at 25°C. [q] = 1. l 2 X
M\v
The unperturbed dimensions of GPO were studied from intrinsic viscosity and light scattering of fractions in a theta solvent (isooctane) at 50°C (Allen et al., 1967). The value of (r&I)"' was 8.0 nm, where r0 is the unperturbed mean square end-to-end distance of the polymer coils. Due to its high molecular weight. theMooney viscosity of GPO may be difficult to determine because of the viscometer configuration and the nature of the polymer. GPO tends to slip in the Mooney viscometer during measurement and thus gives erroneously low values. It has been our experiencethat more meaningful numbers canbe obtained by using an oscillating disk rheometer (ODK) to measure polymer viscosity. The ODK is run at 212°F (100°C) with an oscillating arc of k 3 " and a rate of 3 cycles/min. The ODR viscosity reading is quick and accurate.
5. CROSSLINKSYSTEM FOR GPO GPO is specifically made to respond to cure systems that crosslink through unsaturation. The pendant allyl groups provide the unsaturated crosslinked sites. Two types of crosslinks are most commonly used to provide crosslinking via unsaturation: sulfur and peroxides. Unfortunately, peroxides can also cause chain scission. When there are methyl groups on the backbone, chain-scission reactions usually dominate. As a result. GPO cannot be effectively crosslinked with peroxides. Fortunately, sulfur-cure packages have been developed that give excellent unaged and aged physical properties. GPO responds to sulfur cures much like natural rubber and, like natural rubber, does not have outstanding compression set. The recommended crosslinking system for GPO is given in Table 2. In this system, the sulfur is the crosslinker, while the zinc oxide, 2-mercaptobenzothiazole (MBK), and tetramethylthiuram monosulfide (TMTM) are accelerators that provide a good balance of cure rate and scorch safety. Adjustments can be made to this basic package to provide the desired cure state, cure rate, and scorch safety, dependingon specific application need. For instance, a lower sulfur level will provide a lower cure state, while a reduced level of MBT will provide better scorch safety but a slightly slower cure rate.
Table 2 RecommendedCurePackageforGPO
oxide
Zinc TMTM (tetramcthylthiuram monosulfide) MBT (2-mercaptobenzothiazole) Sulfur
S .o 1 .S
I .5 1.2s
o.
687
Poly(propy1ene oxide) Elastomers
6. VULCANIZATE PROPERTIES OF GPO Vulcanizates based on GPO provide a combinationof good low-temperature tlexibility and good high-temperature serviceability in addition to moderate fuel and oil resistance. A comparison of GPO to natural rubber (NR) and chloroprene (CR) is given i n Table 3. The physical properties such as modulus, elongation, and hardness are essentially equivalent. Tensile properties are not as high for GPOas for either NR or CR, since both the latterelastomers stress crystallize, while GPO does not. As mentioned previously, the mobile sulfur crosslink does not allow good compression set resistance with either the GPO or the NR vulcanizates. GPO vulcanizates also exhibit good aging performance and moderate fuel resistance. Table 4 gives the property change upon aging a vulcanizate at 302°F (150°C) for 70 hours. The tluid resistance is shown in Table 5. Low-temperature flexibility is outstanding even without oilsor plasticizer. Mooney scorch is adequate. and specific gravity is low. A typical cure curve for GPO (Fig. 2) shows that the cure gives both good processing time and a rapid cure rate.
Table 3 Comparison of GPO with NR and CR GPO
Formulation Polymer 100 HAF Stearic 0.5 acid 2.0 NBC ZnO MgO Antioxidant AMAX 1 ETU TMTM MBT Sulfur 2.5 P h y i c ~ r properties l 100% modulus, psi (MPa) 300% modulus. psi (MPa) strength, Tensile psi (MPa) Elongation, 8 points Shore A hardness. Compression set, at 70 hr 212°F (IOOT), 9 Torsional rigidity ("C)"F(t,,,), Mooney scorch at 250°F ( 121"C) 36 Minimum viscosity 38 Min 3-pt to rise 12 16 26 Min 5-pt to rise Mln to I O pt rise 1.41 Specific 1.18 NBC = Nickeldibutyldithmcarhamatc:
ETU
=
100 40 1.0 1 .S 5.0
NR 40
CR 40 -
__
5.0
5.0 4.0 2.0
-
-
1 .o 1.0
-
-
-
-
I .S 1 .S 1.25
-
-
-
-
530 (3.6) 300 1850 (12.8) 3370 2280 2225 ( 15.3) 33703785 300360 520 68 61 61 60 -68 ( - 5 5 ) (-38) -35 (-48) -54 45 9 12 14 1.25
ethylenethlourea.
0.5
-
590 (4;1 ) (2i.2) (2;.2)
72 32
18 >30
22
Berta and Vandenberg
688
Table 4 Aged Properties of Vulcanizate of GPO Physical property
aged 70 hr at 302°F (150°C)
Original Air-oven
550 (3.7) 1190 (8.4) 1930 (13.6) 370 70
100% modulus, psi (MPa) 20010 modulus, psi (MPa) Tensilc strength, psi (MPa) Elongation, % Shore A hardness, points
660 (4.6) 1450 (10.2) 1580 (11.1) 210 73
Table 5 Fluid Resistance of GPO Vulcanizate (70-hr Immersion Test) Volume swell (gravitometer) (%l Fluid fuel
73°F (23°C)
302°F ( 150°C)
12 69 104
-
-
39
Water ASTM A ASTM fuel B ASTM No. 1 oil
-
100 32OoF(16O0C)
:
80
1
36OoF(182OC)
60
40
20
0
5
io
15
20
25
30
35
TIME. MINUTES
Fig. 2 ODR curecurvefor
GPO.
40
45
50
55
60
689
Poly(propy1ene oxide) Elastomers
7. COMPOUNDING OF GPO GPO is off-white in color and is supplied in bale form. It can be stored under ambient conditions for many months without significant property changes. The recommended mill mixingprocedure is to cool the mill rolls to about75-100°F (25-38°C) before adding the GPO. At first the GPO tends to crumble, but after 3-4 minutes of mixing it will begin to band: filler and other compounding ingredients can be added at this point. A processing aid added with the polymer initially usually reduces banding time. The recommendedBanbury mixing procedureis to have the mixer temperature low initially for best carbon black dispersion. A temperature of about 70-100°F (24-38°C) is usually sufficient. The mixer is kept cool until all the filler has been added. Plasticizers and oils can be added after all the black has been incorporated. The curative is added on the mill or in a second Banbury pass, after the compound has been allowed to cool. The curative can be added in the first pass of mixing only if the stock temperature does not exceed 200°F (93°C) and provisions for rapid cooling after dumping the stock are provided. Some compositions using GPO have low greenstrength,and mill stripping may be a problem. Adding a few parts of medium-viscosity ethylene propylene diene terpolymer (EPDM) or bromobutyl rubber (BIIR) usually helps.
8. CARBON BLACKS AND PLASTICIZERS IN GPO GPO, like other elastomers, responds to carbon black reinforcement. In the case of GPO, the interaction with carbon black is not strong, and thus it will accommodate levels up to 100 parts per hundred (phr). Figures 3-5 show the effect of carbon black on Mooney viscosity, tensile strength, and hardness, respectively.
190
1
170 150 130
-
11090
-
70 l
l
0
20
l
I
40 60 CARBON BLACK LEVEL, phr
I
80
Fig. 3 Effect of carbon black on Mooney viscosity of GPO.
l 0
690
Berta and Vandenberg
2600 2400 2200
-
2000 -
-
A i:l\
1600 1400 -
1200 MT
1000 -
600 -
'YIO
0
l
I
20
40
80
60
l O
CARBON BUCK LEVEL, phr
Fig. 4 Effect of carbon black on tensile strength of GPO.
90
W L r n 00
70
60
50
I
0
20
1
I
1
40 60 CARBON BUCK
SO phr
Fig. 5 Effect of carbon black on Shore A hardness of GPO.
lOC
691
Poly(propy1ene oxide) Elastomers Table 6 Effect of Aromatic Oil on Properties of Lightly Loadcd GPO Composition Formulation
Parts by weight
GPO SRF Stearic acid NBC ZnO TMTM MBT Sulfur Aromatic oil
100 30 1 .0 1.S S .0 I .S I .S I .2s As indicated
Aromatic oil level Physical properties
20
.,
Compound viscosity ML, + at 265°F ( I 30°C) 100% modulus, psi (MPa) 200% modulus. psi (MPa) 300% modulus, psi (MPa) strength, Tensile psi (MPa) Elongation, 9 800 points Shorc A hardness,
30
10 45 190 (1.3) 420 (2.9) 780 (5.4) 17702440 ( 12.2) S30 47
35 130 (0.9) 250 290( 1.7) 510 (3.5) (16.8) 42
25 90 (0.6)
(I3 ) 350 (2.4) 22x0 (15.7) 910 37
Although GPO will accommodate plasticizer, it is not ordinarily necessary, since GPO compounds exhibit excellent low-temperature flexibility and performance. However, when it is desirable to lower compound viscosity or hardness, oils and plasticizers can be used. Naphthenicandaromaticoilsarecompatible with GPO up to levels of about 30 phr. Higher levels may cause processing problems. Paraffinic oils should not be used, because they are not compatible with the more polar GPO rubber. Table 6 shows a typical GPO compound and the effect of aromatic oil on properties. Ester plasticizers such as dioctyl phthalate (DOP) can be used with GPO, but plasticizers with unsaturation, such as butyl oleate, interfere with the cure and result in poor vulcanizate properties. Table 7 shows this interference in a dramatic way.
9.
DYNAMIC PROPERTIES OF GPO
The most useful properties of GPO are its outstanding dynamic and hysteresis properties.Hysteresis is measured by determining the heat buildup with a Goodrich flexonleter (ASTM D 623, Method A ) . Table 8 shows some typical GPO formulations and the hysteresis (heat buildup) along with a control CR compound. It is of particular interest that Hi-Sil, a silica filler, shows the best hysteresis properties with little sacrifice in physical properties. Table 9 compares a formulation containing Hi-Si1 233 with one containing carbon black. It can be seen that the hysteresis properties are outstanding. In addition to GPO’s outstanding room-temperature hysteresis, it exhibits good dynamic properties over a wide temperature range. Figure 6 shows the dynamic modulus of a typical
Vandenberg 692
and
Berta
Table 7 Various Plasticizers and Their Effect on Properties of GPO Compositions Formulation
Parts by weight
GPO HAF MT Stearic acid NBC ZnO TMTM MBT Sulfur Plasticizer
100
50 100 1 .o
I .5 5.0 1.5 1.5 1.25 As indicated Plasticizer
Physical properties
250
1 0 0 % modulus, psi (MPa) 200% modulus, psi (MPa) 300% modulus, psi (MPa) Tensile strength, psi (MPa) Elongation, 8470 Shore A hardness, points
None
Butyl oleate
DOP
Aromatic oil
1130 (7.8) -
I 0 0 (0.7) 140 (0.9) 190 ( I .3) 260 ( I .8)
770 (5.3) 1300 (9.0) 1480 (10.2)
680 (4.7) 1140 (7.9)
1610 (11.1) 180 84
42
71
Table 8 Comparison of Hysteresis Properties of GPO and CR
GPO Fornlulation Polymer HAF Stearic acid NBC ZnO MgO Antioxidant ETU TMTM MBT Sulfur Physical properries Heat buildup, "F ("C) 100% modulus, psi (MPa) 200% modulus, psi (MPa) Tensile strength, psi (MPa) Elongation, 8 Shore A hardness, points
IO0 40 1 .o 1.o
5.0
CR 100
40 1.o -
-
4.0 2.0
-
0.5 -
1.5 1.5 1.25
79 (26.1) 390 (2.7) 1650 ( 1 1.4) 1905 (13.1) 355 65
127 (52.8) 585 (4.0) 1980 (13.6) 2140 (14.7) 320 67
-
1330 (9.2) 70
693
Poly(propy1ene oxide) Elastomers Table 9 GPO FormulatedforLowerHysteresis Carbon black Fonnuloriorl Polymer Filler Stearic acid
NBC ZnO Aromatic oil TMTM MBT Sulfur Physicwl properties Heat buildup. "F ("C) 100% modulus, psi (MPa) 200% modulus, psi (MPa) Tensile strength, psi (MPa) Elongation, 76 Shore A hardncss. points
HI-SIL 233 100 40 1 .0
100 40 I .0 1 .0 5 .0 3.0 1.5 1.5 I .25 64 ( 17.8) 230 (1.6) 1010 (7.0) I905 ( I 3.4) 500 51
-
3.0 1.5 1.5 1.25 44 (6.7) 250 (1.7) 1650 ( I 1.3) 2140 (14.7) 550 64
Fig. 6 Comparison of dynamic properties of GPO and NR.
Berta and Vandenberg
694
loooooa 1000000 (W
-100-80 -60 -40 -20
0 20 40 60 80 100 l20 WO TEMPERATURE, DEC. C
Fig. 7 Effect of aging on properties of GPO.
-100-60-60
-40-20 0 20 40 60 80 100 l20 l40 TEMPERATURE, DEG. C
Fig. 8 Effcct of aging on dynamic properties of natural rubber.
Poly(propy1ene oxide) Elastomers
695
GPO compoundand compares it to a CR compound. Theflatness of the curve for GPObetween -40 and 284°F ( -40 and 140°C) is obvious, especially in contrast to the behavior of a natural rubber compound. The GOP dynamic properties are maintained after aging 7 days at 302°F (150°C) (see Fig. 7). Again, this compares to a natural rubber compound, which changes significantly after aging even one day at 257°F (125°C) (see Fig. 8).
10. APPLICATIONS FOR GPO The most important applicationfor GPO is in motor mounts becauseof its unusual heat resistance combined with its good rubber properties. NR cannot withstand the high under-the-hood temperature as well. Addition of GPO to tread rubbers improves fatigue life and resistance to heat and atrnospheric aging (Boguslovskaya et al., 1976). Studies have been reported on elastic fibers made from a vulcanized polymer of propylene oxide (Dunlop, 1966).Alloys of GPO with polystyrene prepared by polymerizing styrene in the presence of these elastomers have been found to give betterweather-resistant,high-impact compositions than commercialhigh-impactpolystyrene (Oetzel,1975).
11. CONCLUSION GPO is a high-performance elastomer that provides an excellent balance of low- and hightemperature properties with moderate hydrocarbon resistance. GPO is especially useful when good dynamic properties are desired over a wide temperature range. It can be cured with sulfur much like natural rubber to give good physical properties. Because of GPO’s low T,, plasticizers are not necessary to achieve good low-temperature properties, but oils and plasticizers may be added to reduce viscosity. In addition, GPO can be blended with other polymers such as BIIR or EPDM to improve processing with little effect on dynamic properties. Because of its balance of performance, GPO should be considered as a replacement for natural rubber in heat-resistant. dynamic automotive components.
REFERENCES Allcn, G.. Booth, C., and Jones, M. N. (1964). P o / w e r 5:195. Allen, G., Booth, C., and Price. C (1967). Po/yrrler #:397. Boguslovskaya. K. V., Kolobenin, V. N., Reikh, V. N., and Zimin, E. V. (1976). f n t . Po/yrn. Sci. Tech/lo/. 3:Ti93. Cooper, W., Pope, G. A., and Vaughn. G. (1968). Eur. P o / m . J. 4:207. Dunlop (1966), Fr. Pat. 1,440, 449, May 27. Filar, L. J., and Vandenberg, F. J. ( 1967), U.S. Pat. 3.3 13,743 (to Hercules Inc.). Apr. 1 1. Gmber, E. E., Meyer, D. A., Swart, G. H., and Weinstock, K. V. (1964), fnd. Cl~enl.Product Resin Del: 3 : 194. Herold, R. J., and Witthcckcr, E. L. (1974), Mtrcrortlolecrtlnr S ~ n t h e s i sVol. 5 , Wiley, New York, p. 9. Oetzcl, J. T. (1975), Ruhher World f72:55, April. Price, C. C . (1958), U.S. Pat. 2,866,774 (to University of Notre Dame), Dec. 30. Price, C . C. (1961), Cl~etrlist38:131. Sakata, R., Tsurnta, T., Saegusa, T., and Furukawa, J. (l960), Macronlo/. Chem. 40:64.
Vandenberg 696
and
Berta
Schlatzer, R. K. U.S. Pat. 3,957,697 (to B. F. Goodrich Co.), May 18. Shambelan, C. (1959), Ph.D. thesis, University of Pennsylvania. Shibatami, K., and Magata, S. (1973). U.S. Pat. 3,776,863 (to Kuraray Company, Ltd.), Dec. 4. Steller, K. K. (1975), in Polyethers (E. J. Vandenberg, Ed.), American Chemical Society, Washington, DC, p. 136. Teyssie, P., Oerhadi, T., and Broul, J. P. (1975). in Mc~crotnoleculur Science (C. E. H. Bawn, Ed.), Butterworth, London, p. 216. Vandenberg, E. J. (1960), J. Polym. Sci. 47:486. Vandenberg, E. J. (1969), J. Polytn. Sci. 7525. Vandenberg, E. J. (1972). U.S. Pat. 3,639,267 (to Hercules Inc.). February. Vandenberg, E. J. (1973a), U.S. Pat. 3,728,320 (to Hercules Inc.), Apr. 17. Vandenberg, E. J. (1973b), U.S. Pat. 3,728,321 (to Hercules Inc.), Apr. 17. Vandenberg, E. J. (1976). Pure Appl. Clwm. 48:295. Vandenberg, E. J. (1986). J. Polym. Sci. Chenz. Ed. 241423. Vandenberg, E. J., and Robinson, A. P. (1975). in Pol.vether.7 (E. J. Vandenberg, Ed.), American Chemical Society. Washington, DC,p. 101. Willis, W. D., Amberg, L. 0..Robinson, A. E., and Vandenberg, E. J. (1965). Rubbar WorM 153:88.
26 Polyalkenylenes Adolf Draxler* Degussa-Huls AG, Marl, Germany
1.
INTRODUCTION
The catalyticdisproportionation of olefins,publishedabout 1960 and laterreferred to as a metathesis reaction, is based on the principle shown in Eq. (1). In this equilibrium reaction, an intermolecular rearrangement of the bonding system takes place initially. The catalyst systems suitable for this purpose are complex and contain,in addition to activating components, a compound of a transition element as an essential component. Molybdenum, tungsten, and rhenium have been recognized as particularly active components.
R1 - H C = C H -
R2
+
R3-HC=CH-R'
R1
-
-CH
CH
- R2
c-
R3
II
- CCHH
+
II
- R'
The metathesis reaction was soon used forthe polymerization of cycloolefins to give novel unsaturated polymers. As shown in Eq. (l), the double bond is retained in the reaction. The polymerization of olefins containing 4- 15 carbon atoms and exhibiting certain ring strains has been published. Cyclohexene, whosering is energetically balanced, cannot be polymerized. [The metathetic fonnation of oligomers of cyclohexene at very low temperatures has been observed (Patton and McCarthy,1985).]The polymerization takes placeas an equilibrium reaction according to Eq. (2):
where x may be 2. 3, 5. 6, 7 . .
* Retired. 697
698
Draxler
In the course of metathesis polymerization, further reaction equilibria arise, depending on the monomer. the catalyst composition, and the reaction conditions. These relate to The oligomer content The molecular weight distribution The macrostructure of the polymers, especially the contents of cyclic and acyclic polymer forms The microstructure, in particular the proportion of cis and trans double bonds. Some cycloolefins have been polymerizedby the metathesis reaction: Cyclobutene was converted to 1,4-polybutadiene (Dall’Asta et al., 1962); cyclopentene to polypentenylene (Eleuterio, 1957; Natta et al., 1964; Calderon et al.. 1967a,b; Scott et al., 1969; Gunther et al., 1970; Haas et al., 1970: Graulich etal., 1972; Ofstead and Calderon, 1972; Kupper andStreck, 1973, 1974: Arlie et al., 1974: Graulich, 1974; Witte and Hoffman, 1978); cycloctene to polyoctenylene (Natta et al., 1966; Calderon et al., 1967b; Wasserman et al., 1970; Holtrup et al., 1974; Streck, 1979; Hocker et al.. 1980); cyclododecene to polydodecenylene (Scott et al., 1969; Wasserman et a l . , 1970; Holtrup et al., 1974; Riebel, 1976; Hocker et al., 1980); norbornene. a typical bicyclic olefin to polynorbornene (Truett et al., 1960; Michelotti and Keaveney, 1965; Porri etal.. 1974; Ivin et a l . . 1977; Reif, 1983); and cyclopentadecene topolypentadecenylene (Reif, 1978). Other compoundsthat have been polymerized include cycloolefins containing heteroatoms, cycloolefins possessing substituents, and polyunsaturated cycloolefins. A large numberof possible methods for the copolymerization of different cycloolefins have also been studied; see the review by Ivin and Saegusa (1984). So far only the polymers of norbornene and cyclooctene have become industrially important. Both these polymers are unsaturated and are therefore sulfur vulcanizable rubbers. The polyoctenylenes are distinguished in particular by theirwide variety of technicalfunctional properties and possible uses, while the polynorbornenes possess an extremely high molecular weight. great extendability with oil, and accordingly a large capacity to take up filler.
2.
BASIC STRUCTURES AND COMMERCIAL TYPES OF POLYALKENYLENES
The polymerization of cyclic olefins by metathesis is capable in principle of giving both linear polymers and macrocyclic oligomers or polymers. The possibility of macrocyclic polymers and catenane like compounds of some of these was mentioned by some authors (Wasserman et al., 1970) at an early stage. This applies in particular to polyoctenylenes. The formation of macrocyclic compounds is virtually completely absent in polynorbornenes, the equilibrium of the polymerization reaction in this case lying on the side of the linear polymers (Reif, 1983). These macrorings are difficult to detect analytically, since even mass spectrometry methods are at best capable of distinguishing these rings from the linear polymers of the same molecular weight only up to a degree of polymerization of 15. As the international literature shows, it has long been assumed that the polyoctenylenes too consisted of a cyclic oligomer component( 10- 15%) and a linear polymercomponent (85-90%). However, technological test results with polyoctenylenes suggested the existenceof a cyclic polymercomponent (Draxler, 1980, 1983a). Subsequent controlled analytical investigations provided scientific evidence its existence (Holtrupand Hammel, 1985; Usami et al., 1985). Over the years, various authors have produced various hypotheses to explain the mechanism of the metathesis processes, including the polymerizationofcycloolefins. Theinterpretation generally accepted today as being the most probable is a reaction mechanism attributable to Herisson and Chauvin ( 1970). Its characteristics are the intermediate formation of a meta lacarbene and the subsequent ring opening shown in Eq. (3). This is a plausible reaction scheme for the formation of linear polymers, which takes place as a chain reaction. As the availability of
Polyalkenylenes
699
mononlers decreases, the probability of an intramolecular metathesis step increases with detachment of an oligomeric ring [back-biting process, see Eq. (4)]. R. CH = [ W ]
i
+
i
R . CH
II
[Wl---CHT (CH,),
II
)I
+
=
CH = [ W )
RCH=CH-(CHz)x-CH=CH-(CH2)x-
j
+
j
etc
C:c2:> The ligands still present on the 6-valentWatomhave been omitted in theschematic illustration. This would explain the formation of the exclusively cyclic oligomers of the polyalkenylenes. If acyclic monoolefins are added to the catalyst, they act as molecular weight modifiers that also influence the proportionof cyclic macromolecules. In these reactions, the modifiers are consumed and undergo addition at the ends of the newly formed chains (Dall'Asta, 1974); see Eqs. ( 5 ) and (6). When the reptation movement of the linear polymer (deGennes, 1971)and the associated, relatively restricted "action radius" of the oscillating active chain ends carrying the metallacarbene groups are taken into account.
m
Cleavage of macrocycles
R1
+ CH= CH
I I
R2
CHR2 RICH
" " " " "
"
+ R,CH=CH.R2
cham
Degradatron by linear sclsslon
Draxler
700
it is obvious that the intramolecular metathesis leads to the formation of relatively small, i.e.. oligomeric, rings. The formation of macrocrings in the polymer range, which must be assumed in the case of polyoctenylene, is more difficult to explain, a possible route being ring closure by means of a metathesis reaction between two linear polymers with active ends. If a
I mear
CH2-CHy"H2-CH =CH -CH2-CH2-CH2
CYCllC
CH
CH
I1
II
CH
CH
linear polymer is just at the point of passing through a cyclic molecule, a catenane is formed. The reptation of linear polymers in the longitudinal direction has been studied for some years. In the case of cyclic molecules, this movement must necessarily take place in the peripheral direction. This ensuresthat the macrorings are open topermit linear polymers to "pass through" in this way. The structural details and mechanisms of formation in this field are difficult to explain and will remain a subject of research for polymer chemists and polymer physicists for a fairly long time. Regarding the theory of catenane formation, see also Jacobson (1984). Polyoctenylenes have a bimodal molecular weight distribution curve. GPC diagrams (Fig. 1) have an oligomer fraction in the first maximum that can be extracted with isopropanol or acetone up to a molecular weight of about 1100. With increasing mean molecular weight, these
100-
I (M)/%.
iiw
-
80-
M, = J
60-
LO-
Molec. weight Fig. 1 Vestcnamer 8012, not extracted.
= 120000
x
6100 117m11g
701
Polyalkenylenes
9 IPA-Ext
1
x
[%l
X
15-
10-
5X
Fig. 2 Isopropanol extract versus viscosity number J of polyoctenylenes.
extraction values (Fig. 2) therefore also increase as a result of smaller amounts of modifier. The scatter of the values is due to different catalysts and reaction procedures. Another peculiarity of the polyalkenylenes is the way in which their crystallization properties vary with their microstructure. The polyoctenylenes obtained always possess double bonds that have cis and trans configurations. The cisltruns ratio is substantiallyconstant over all
-20 100
1
10 90
1
Z’O
$0 70
LO
50 50
60
710
30
80
9‘0 10
*
I d 0 YO t r a n s 0 % CIS
Fig. 3 Polyoctenylene melting temperature (T,,,) versus cis-rrms proportion.
Draxler
702
molecular weight fractions of a polyoctenylene. Both the cis double bonds and the truns double bonds (the latter to a much greater extent than the former) in sequential order form crystallites that have defined melting temperatures (T,ll).By extrapolation, it is possible to calculate the T,,, for both 100% cis- and 100% trans polyoctenylenes (Gianotti and Capizzi, 1970; Gianotti et al.. 1976); the relevant values are about 38 and 79"C, respectively. The lowest T,,, of - 8°C was determined by Giannotti for a polyoctenylene containing about 35% of the trans component and 65% of the cis component. Even this product had crystalline properties (Fig. 3). Figure 4 shows the DSC graph of a polyocetenylene containing 80% truns double bonds (Vestenamer 80 12). Norbornene polymers crystallize only when they contain a predominant amount of the cis component. This is due to a syndiotactic monomer arrangement in the chain, which, occurs only for the cis configuration of the double bond. Truns polynorbornene is amorphous (Ivin and Saegusa,1984). High crystallinity. as obtained in the case of polyoctenylene where the trum content is greater than 7.570, leads to high hardness and relatively high tensile strength of the uncrosslinked polymer at room temperature. Its stress-strain diagram resembles that of a thermoplastic, for example. a polyethylene. The molecular structure of the polyalkenylenes prepared by metathesis shows an exact head-to-tail addition of the monomers. Polyoctenylenes therefore have an ideal polymethylene structure with a double bond in the chain exactly after every eighth carbon atom, regardless of whether the polymer is linear or cyclic. This explains their high chain mobility and their low glass transition temperature (T,) (see Fig. 5). Depending on the amount of modifier, very different molecular weights (M,, ) can be obtained. A four-digit combination whose first two digits indicate the tmns content in percent and whose last two digits denote the J value (mL/g) times 10" as an integer has proven useful for designating the various polyoctenylenes. Depending on whether the t t m s content or the cis content predominates in the polyoctenylene. the substances are referred to as a TOR or a COR (trans- or cis-polyoctenylene rubber). HUIS AG in Marl (Germany) currently produces two relatively low polymer TOR grades (see Table 1). Both grades are supplied in the form of granules. They have an extremely low Mooney value (ML, , at 100°C = -5). The polyoctenylenes have a molecular structure that makes them relatively stable to UV radiation and oxygen (Draxler, 1983a). When stored in hot air at 100"C, both Vestenamer grades remain virtually unchanged after as long as 24 hours. even without the addition of a stabilizer. However, the commercial product always contains a small amount of a phenolic nonstaining stabilizer approved foruse with foodstuffs. The stability to hot air also implies high resistance to degradation (Draxler, 1980). Polynorbornenes have a different molecular structure.
.,
CH
CH =
CH 7
CH,-
/
CH2
As a result of the metathetic polymerization. C5 rings (1,3-cyclopentylene groups) remain in the polymer chain:
703
Polyalkenylenes
t 0
Fig. 4
L O 80
20
60 Tempero!ure,OC
DSCgraph for Vestenamer 8012.
J '
LOO [mLlgI
300 -
200-
100-
0
io
2'0
io
io
5'0
io
io
Fig. 5 Dependence of T, on viscosity number J and
io
e
do rbo sc
I T ~ O I Scontent
trans
In polyoctenylenes.
Table 1 Basic Parameters of VestenamerTypes Crystallinity Trans numbercontent Vestenamer 8012 Vestenamer 6213
T,,, ("C)
22°C
(%)
Viscosity J (mL/g)
80 62
120 130
55 28
30 8
(%)
M
\I
100.000 110,000
T, ("C) - 75 - 80
Gel content (%)
<2 <2
704
Draxler
S y n d i o t a c t i c S t r u c t u r e o f cis-Polynorbornene (crystallinic)
IsotacticStructureoftrans-Polynorbornene(amorphous)
-
U
-
"_
The molecular weight (Mw) is extremely high and is given as 2 X 10". There are virtually no oligomers and no polymeric macrocycles. The C5rings in the chain have a pronounced damping effect and are responsible for the very high T, value of 35°C. However, the type and amount of extender oil have a considerable effect on the T, value. The high truns content of 75-80% results in this case in an amorphous polymer structure. The usual grades are: Norsorex 150 AR, extended with 150 phr of aromatic oil Norsorex 150 NA, extended with 150 phr of naphthenic oil Both grades are supplied in bale form. The pure oil-free Norsorex is available as a powder. All Norsorex grades contain a nonstaining stabilizer. The followinganalyticalmethodsaresuitablefor determining the technicallyrelevant structural parameters of polyalkenylenes: GPC analysis for determiningthe molecular weight distribution and the mean molecular weights M,, and M,, using a suitable standard 2. Spectrographic analyses, such as IR absorptiometry. for determining the proportions of cis and trans double bonds 3. DSC analysis for determining the T,,,, fusion enthalpy, and crystallinity 4.Torsionalanalysis fordetermining T, 5. Viscometry (Ostwald viscometer) for determining the viscosity number J (solution of 5 g of polymer in 1 L of toluene) I.
3.
PRODUCTION OF POLYALKENYLENES
The time from the initial work on the polymerization of cyclic olefins by a ring-opening metathesis reaction to the industrial production of these polymers was quite long. Eleuterio (1957) and Truett etal. (1960) described the catalyticpolymerization of cycloolefins. This early work initially was only of theoretical importance. Natta discovered the polymerization of cyclopentene in 1964 and that of cyclooctene in 1965 (Natta et al., 1964, 1966) using tungsten chloride or molybdenum chloride and triethylaluminum as cocatalysts. This made it possible to consider industrial production of these polymers. A decisive improvement in the polymerization conditions from the thermal and economic points of view was made by Calderon with the WCl(4 EtAICI,/EtOH catalyst system (Calderon, et al., 1967a), from which countless variations and modifications were subsequently produced. All metathesis reactions are extremely demanding with regard to anhydrous conditions and the chemical purity of the substances involved. In the production of polyalkenylenes, it is important that the monomer be obtainable economically. Norbornene is prepared from cyclopentadieneand ethylene by aDiels-Alder synthesis. Cyclooctene is obtainable from butadiene by a process of Huls AG. In this process,
Polyalkenylenes
705
the butadiene is converted to cyclooctadiene by catalytic dimerization, and the product is then selectively hydrogenated to give the monomer. At present. there are only two production sites worldwide for metathesis rubbers. High molecular weight polynorbornenes with a high t r m s content have been produced under the trade name Norsorex since 1976 by Cdf Chinlie in Carling, France. Since 1980, Huls AG in Marl, Germany, has been producing low-viscosity polyoctenylenes with various trrrrzs contents under the trademark Vestenamer. Current production capacity is 3000 tonslyear at least. Recently (1985) Hercules Inc., in the United States, produced another polyalkenylene product. which must be referred to as polydicyclopentadiene. This is a thermosetting polymer. The monomeris polymerized and crosslinked simultaneously in an RIM process using a metathesis catalyst. The brand name of the product is Metton.
4.
COMPOUNDING AND PROCESSING CHARACTERISTICS
For the industrial compounding and processing of polyalkenylenes, the same equipmentand the same crosslinking agents areusedas for conventionalrubbers. Some differences,especially between the compounding and processing of polyoctenylenes and the other rubbers, result from the differences in the polymer parameters molecular weight. macrostructure, and microstructure. The low-viscosity polyoctenylenes available today as Vestenamer 8012 and 62 : 3 are without exception used as blends with other rubbers. Blending is always substitutive and never additive. The TOR content is usually 10-30% of the total polymer. Where blends with diene rubbers are vulcanized with sulfur, it should be noted that TOR has a relatively low degree of unsaturation and therefore vulcanizes slowly, resembling, for example, the “slow” SBR grades. During compounding, it may therefore be necessary to adapt the accelerator when blending with more rapidly vulcanizing rubbers, such as NR, IR, and BR. As a rule, the sulfur content can be retained. Because of their low Mooney viscosity (ML, +, at 100°C = 5), thetwoVestenamer grades. when used as blend components in the processing of the individual rubbers, initially exhibit rheological behavior very similar to that of liquid rubbers, e.g., liquid IR-50, Hycar 1312. Neoprene FB, and various liquid polybutadienes. This implies a reduction in the filler incorporation time during mixing; a reduction in the injection time in the injection-molding and transfer-molding processes; improved flow behavior in all shaping processes, such as calendering, extrusion,and molding; reduced die swell;and better surfacesmoothness of the extrudates and mill sheets (Draxler, 1980). Furthermore, the energy consumption in all these processes is reduced (Draxler, 1980). Frequently, too, less heat is evolved. These effects are not surprising and can be predicted on the basis of the viscosity. A different type of behavior is encountered regarding the distribution of poorly dispersible fillers, such as highly active carbon black. in particular conductive carbon black (Table 2). The polyoctenylenes produce a surprising improvement in the dispersion, which is detectable, for example, from the substantially higher conductivity. In these cases. liquid rubbers of the type mentioned always have the opposite effect. The polyoctenylenes are also capable of improving the dispersion of rubber blends with poorlycompatible components. Suchblendsare, in particular.blends of polarrubbers with nonpolar ones or of emulsion rubbers with solution rubbers. e.g.. NBR-EPDM, NR-BR. IREM-SBR, and many others. In general. even a small amount. e.g., 5- 1O%, of TOR is sufficient to produce an effect. During blending on open mills and during calendering of such blends,
Draxler Table 2 ConductiveCompounds A
B
C
D
100
80 20
80
80
Formtrltrtiorl
EPM
Vestenamer 80 I2 Vestenamer 62 13 Liquid IR-50 ZnO Stearic acid Carbon black N550 black (Ketjen black Conductive carbon 25 EC)25 25 Parafunic oil 20 process Emuls. aid (Aflux 42) Peroxide (40%) 6.5 6.5 TAC Antioxidant
-
-
-
5
-
I
1
1
1
40
40
40
40
20 6
20
25 20 6 l 1,S 1,5
20
-
-
5
5
6 6.5
1
1 1,S
1 ,S
-
20 5
6 1 1S
Elrctricwl rosistcom
cm) Press-molded specimen 10 min a t 180°C b. Extruded tape, steam curedhin at 210°C"
(Cl X ;I.
"
16.1 41
8 25.1
46
6.5 26.3
62
And tempered 2 hr In h o t air at 70°C.
the generally considerable differences i! sheet smoothness due to the dispersing effect of the polyoctenylenes are directly visible (Draxler. 1983a) (see Fig. 6). The macrostructure of the polyoctenylenes is also responsible for the fact that calendered sheets and extrudates possess particularly constant dimensions even when a forced procedure is used in the production process. For example, it is possible to draw extremely thin calendered sheets without defects such as holes or edge defects. The anisotropic shrinkage of calendered sheets (calendering effect) is reduced partly or even eliminated by the molecular structure and to an even greater extent by the crystalline forces of high-rrons polyoctenylene. None of these phenomena are exhibited by the conventional liquid rubbers (linear polymers). The same applies. of course, to thesubstantialimprovement in the greenstrength of the blendsbelowthemeltingtemperature T,,,, owing to crystalline TOR. Blending with IO%, based on thepolymercontent, of Vestenamer 8012 results in a substantialincrease in thegreenstrength, in some cases virtually a doubling of thisstrength. The process is reversible and can be repeated as desired by passingthroughthemeltingpoint in both directions without a time delay. The rapid recrystallization, which is very important technologically. distinguishes TORfrom allothercrystallinerubbers. The considerableincrease in viscosity of the green blends at room temperature, which is associated with the crystallinity, can be avoided if desired by using the less crystalline grade. Vestenamer 6213, in this case, of course. the improvement in the green strengthhas to be sacrificed.However, all other processingcharacteristics are fullyretained. Another effect, which may also be regarded as a technological indication of the presence of a cyclic polymer fractionin the polyoctenylene, manifests itself in the shape stability of green blends at temperatures above T,,,. i.e., in the melting range of the polyoctenylene. It is known that during the vulcanization of extrudates. such as profiles and tubes. there is a danger of the
Polyalkenylenes 707
Draxler
708
extrudate becoming deformed under its own weight in an undesirable way before the beginning of the crosslinking reaction at the vulcanization temperatures in steam, hot air, or another heating medium. Compounds not containing any vulcanization additives, when blendedwith low-viscosity polyoctenylene, did in fact show reduced viscosity ( M 4 at 100°C) of the rubber compounds, but their shap,e stability at high temperatures, such as 100 or 130°C. did not decrease but rather increased (Draxler, 1980, 1983a). Crystalline effects arecompletely absent at these temperatures. Up to a proportion of about 30%. based on rubber polymer, of Vestenamer 8012 or 6213 in the blend, the stability increases and the Mooney viscosity decreases continuously; above this proportion, the shape stability also decreases sharply (Fig. 7). When a TOR having a viscosity number J of 200 (corresponding to aMooneyviscosity ML, of 20 at 100°C) is used,the improvement is effective even up to a proportion of 40% in the blend. This effect is observed in all rubbers used as a component in blends withTOR. On the other hand,when the compounds are deformed rapidly on roll mills, internal mixers, calenders, orextruders, or even in a Mooney apparatus. the only effect observed at these temperatures is a clear reduction in the viscosity. This phenomenon can be explained only in terms of a loose, but bulky structure of the polymer blends that yields under the effect of fairly high shear rates or fairly high shearing forces but substantially withstands its own weight in the form of the particular extrudate. Such structures
Green shape stability. l hr a t 100°C
4
8012
0
30 l00 0 100
0
1
1
I
90 10 96 L
80
70
20 92 8
30 88 12
m p t s 6UNA EM 1500 40 VESTENAMER8012 8L D t s BUNA EM 1500 16 Polyol 130
709
Polyalkenylenes
Batch temperatures. 110°C Batch ML(141, 100°C. 18. TOR 8012 J- 120 Pol.-ML(l*L). 100°C. 5
1L2OC
60. TOR 8020 J -200 20
1L 2OC
75. TOR 7030 J-300 58
ILBOC 78.
SBR 1507 J= l80
30
Fig. 8 Power consumption (kW); 2-L Laboratory mixer (GK2); TOR 8012, 8020,7030, and SBR 1507. Water temperature: 50°C. Batch recipe: rubber 100; ZnO, 5; stearic acid, 1; N330, 50.
must be associated with a polymeric macrocyclic polyoctenylene fraction. In all probability, large rings or catenanes become entangled with the linear polymers. In experiments using extracted oligomers up to a M, of 2000 obtained from TOR by means of solvent/precipitating agent systems and consisting only of small ring molecules, such a heat distortion resistance effect could not be achieved. If these entanglements of various polymer forms are fixed by crosslinking, remarkable phenomena are encountered again, these phenomena being reflected in the elastic properties of the vulcanizates. as will be shown in Section 5. Another effect of these bulky molecular structures in the nonvulcanized state is also obvious during filler incorporation by the polyoctenylenes in an internal mixer or on roll mills when the molecular weights and viscosity numbers, respectively, of the polymers are gradually increased. Figure 8 shows the power consumption graphs for a 2-L laboratory mixer, recorded forvariousrubbers. The figure shows that thepowerconsumption increasessharplywith the J value. It is noteworthy that TOR 8020, which has a J value of 200 and a Mooney viscosity ML, (at 100°C) of 20, results in higherpeakkilowattconsumptionthan SBR 1507 with its Mooneyvalue of 30. Thistooconfirms the supposition that theproportion of polymericmacrocyclics (and not alone that of oligomeric macrocyclics)increases with decreasingaddition of modifier during polymerization. This structuraleffect explains why Dall’Astareporteddifficulties in processing COR withRSVvaluessubstantiallyabove 2 ( = J substantiallygreaterthan 200) inconnection withthe incorporation of carbonblack on roll mills;onlywhen very hightemperatures (up to 120°C) wereusedwas it possible to overcome thesedifficultiesandobtainacceptableincorporationanddispersion of carbon black (Dall’Asta, 1974). Without a doubt, the polymeric macrostructures of TOR are also responsible for the relationship between the viscosity number J and the Mooney viscosity M L , +,, at 100°C. In this connection, it is known that the proportions that apply to the polyoctenylenes differ from those that apply to linear polymers. Furthermore, in the case of polyoctenylenes, these proportions vary with catalyst composition and polymerization conditions. A tabular presentation of the main effects of the polymer parameters of polyoctenylenes on the properties of unvulcanized rubber compounds follows:
Draxler
710
Posltive Low molecular weight Improvement in the flow properties,swelling less during extrusion and calendering Macrostructure (macrocycles) Improvementin the shape stability even at highvulcanizationtemperatures. improveddimensionalconstancy, reduction in the calendering effect,better dispersion of fillers that are difficult to disperse, better dispersion of polymers of poor compatihility, and reduction in mill sticking Crystallinity High green strength below the T,,,, veryhighshapestability belowthe T,,,, pronounced reduction in the calendering effect at room temperature
5.
Negative
-
Reduction in tack
High compound viscosity below T,,,
VULCANIZATION METHODS AND PHYSICAL PROPERTIES OF VULCANIZED RUBBERS
As stated earlier. the unsaturated character of the polyalkenylenes permits crosslinking with sulfur, making them true elastomers. In principle, vulcanization can also be carried out using other crosslinking agents, such as peroxides, phenol resins, and quinone dioximes. The polyoctenylenes are used exclusively as blending rubbers, generally in low proportions.Nevertheless, even relativelysmall amounts of polyoctenylene are capable of having considerable effects in the vulcanizates, especially with regard to those properties associated with elastic behavior, such as hysteresis, heat buildup. abrasion resistance, and permanent set after dynamic loading. This is all the more remarkable since the M,. of the polyoctenylenes is relatively very low and furthermore even small proportions in the blend, such as 5-10% based on the polymer. frequently have a substantial effect. The macrostructure of the polyoctenylenes is no doubt also responsible for this very striking behavior. As expected, liquid linear polymers having a comparable M,. tend to exhibit the opposite effects with regard to these properties. Macrocyclics of the polymer fraction apparently undergo covulcanization with the linear chains of both their own polymer and the other component of the blend to form a particularly resilient network. In this respect. too, catenanes may play a special role. It is certain that only polymeric ringshavingamolecularweight within acertainrangearetechnologicallyeffective in this manner,therelevantrangebeingbetween 1.000 and 50.000 or 100,000. Any largercyclic molecules that may be present would exhibit behavior very similar to that of linear polymers. This also applies to the properties in the nonvulcanized state (described in Section 2 ) . On the other hand, the macrocyclic oligomers play virtually no role in the vulcanization process and can be partially extracted from the vulcanizate. for example,with toluene. They must be regarded as asort of extender. In contrast to the mineral oil extenders,theyhave a relativelysmall plasticizing effect in the crude polymer, and a smaller elasticity-reducing function in the vulcanizate owing to the lack of terminal groups. In addition to the good elastic properties, due to their molecular structure, vulcanized polyoctenylenes have very low T, values and relatively high gas diffusion values. which are somewhat higher than in the case of natural rubber (Fig. 9). In the vulcanized compounds, highlycrystallinepolyoctenylenes, such asVestenamer 80 12, have a certain reinforcing effect onthe Shore hardnessand stress-strain of the vulcanizates. The hardening achieved with Vestenamer 8012 corresponds to about one third of that achieved with the same amount of a styrene resin batch composed of 6 0 8 of 85/15 styrenehutadiene
711
Polyalkenylenes
cm3
for 1 mm test. spectmen
m 2 ‘days ’ bar (difference of pressure)
5000 4 500
4000
3500 3000 2500 2000 1 500
1000
500 1 2 3 4 5 6 7 8 19 2 3 4 5 6 7 8 19 2 3 4 5 6 7 8 9 20oc 5OoC 70°C
Fig. 9 Pcrmcation to air (DIN 53536), unblended rubber compounds containing 50 phr N 774, nooil: all compounds are sulfur-vulcanized. I , NR; 2, SBR 1500; 3, BR (Buna CB IO): 4, 1R (Natsyn 2200); 5 , NBR (Perbunan N 2810); 6, CIIR (Exxon HT 1068); 7, EPDM (Bum AP 341): 8, Vestenamcr 8012; 9,
Vestcnamer 62 13.
resin and 40% of SBR 1507 (e.g., Duranit B from Huls AG). However, the two products have opposite effects on elastic properties. Furthermore, the content of cyclic polyoctenylenes results in a certain reduction in the elongation at break for all vulcanized compounds. EV and semi-EV systems can also be used withTORblends. However, thelowercrosslinkingdensity due to thesesystemsleads to a somewhat higher residual crystallinity in the vulcanized TOR, a feature that is always observed in soft rubber grades and depends on the concentration of crosslinking agent. Very high sulfur concentrations, for example, in ebonites, eliminatesthe residual crystallinity. The residual crystallinity manifests itself in a somewhat greater temperature dependence of the hardness and the stress-strain in the temperature range below the T,,,. It is. clearly recognizable from the shape of the shear modulus curve in this temperature range (Draxler, 1980). In the highly crosslinked ebonites, it is therefore not possible to increase the hardness by blending with highly crystalline TOR, although polyoctenylenes can be used to produce a substantial reduction in the T,, which is known to reach very high values in the case of ebonites. It may be said that TOR produces an elastification effect in ebonites too. As already mentioned, the good elastic properties of a vulcanizate of extremely low molecular weight are very striking. In order to study the phenomenon in more detail, a number of gumstock blends with increasing sulfur content were prepared, and both linear polymers and polyoctenylene test products of various molecular weights were used. The different degrees of unsaturation of the polymer chains of individual rubbers were also taken into account in the sulfur doses. The lowest sulfur concentration (n = 1 ) used was 1/100 mol of sulfur, based on the molecular weight of the rubber polymer associated with one double bond present in the chain ( = “mole olefin”). Therefore, for n = 1, follow various phr values of sulfur for the individual rubbers, for example n = 0.59 phr S for BR, n = 0.29 phr S for TOR n = 0.47 phr S for IR. n = 0.49
Draxler
712
% Resilience (22OC) Llnear polymers
4
l
3
5
7
9 10 1 1
15
13
% Resilience (22OC) Polyoctenylenes
90-
4845 8020 801 2 6213
Reclpe: 100 Rubber
5 ZnO 1 Stear ac.
70
S var
60
CBS/TMTMS var.
6213 8012
L
n = M O L S x 10-2pro Mol Olefin
1
3
5
7
9
10 1 1
13
15
Fig. 10 Resilience (%) of gumstockvulcanizateswithincreasingcrosslinkingdensity(vulc.temp 160°C).
=
713
Polyalkenylenes Table 3 Toluene Extracts (%) of Gumstock Vulcanizates with Increasing Crosslinking Density 2
n = l
4
6
8
IO
Lineor- polytners
IR SBR1507 SBR1502 SBR1.572
3.1 10.6 10.1 10.9
3.0 9.7 9.6 10.5
2.4 7.2 8.3 9.9
2.3 8.0 8.0 9.4
I .7 7.4 7.8 8.7
1.7 7.1 7.2 8.2
Polvocterlylerzes Vestenamer 80 12 Vestenamer 62 13 TOR 8020 COR 4645
19.0 21 .o 12.9 16.6
5.1 9.8 11.0 8.7 12.0 12.6
6.1 7.4 6.8 12.6
5.7 6.1
4.1 4.8 5.2 11.6
4.0 4.8 5.2 10.9
phr S for SBR 1500, and n = 0.30 phr S for 1,2-BR with 50% of vinyl groups. AS n increases, the resilience of all rubbers increases and reaches maximum values in the region of n = about 10. This behavior, which is known per se, is interesting because the resilience of the linear polymers and that of the polyoctenylenes form two different families of substantially parallel curves. Only in the case of IR does the reversion effect not completely guarantee that the curves will be parallel. The resilience curves of the polyoctenylenes have a substantially steeper slope than those of the linear polymers. Apart from the sulfur concentration itself,a structural network factor must also affect the elastic properties of the polyoctenylenes. Of course, the reduction in the number of end groups brought about by the presence of the cyclic components also plays a considerable role. Comparison of SBR 1507, 1502, and 1572 also shows that the reduction in the number of therminal groups as a result of increasing mean molecular weights has an effect on the resilience. However, in the case of the SBR elastomers, the parallel nature of the curves isnot altered but their level is increased with increasing M,,,. Sulfur concentrations below n = 1 did not appear very reasonable in the individual cases. As the toluene extracts of the vulcanizates show, larger amounts of extract are obtained at values as low as n = 1-2, indicating incomplete crosslinking. Relatively high amounts of extract are attributable to the content of organicacids in the case of morehighlyvulcanized EM-SBR, and to thevarious oligomer fractions in the case of the polyoctenylenes. Figure 10 shows the families of resilience curves, while Table 3 shows the toluene extract values. The specific gravities of the gumstock vulcanizates (Table 4) provide information about the dependence of the residual crystallinity of TOR on the degree of crosslinking. The specific gravity of the crystalline phase in pure TOR is 1.006, and that of the amorphous phase is 0.872 (22°C). While BR exhibits a steady increase in the specific gravity with increasing crosslinking
Table 4 Specific Gravity of Gumstock Vulcanizates with Increasing Crosslinking Density" n = l
3
5
7
9
II
13
15
17
I9
21
~~
BR Vestenamer 8012 Vestenamer 6213 TOR 8020
0.958 0.925 0.924 0.948
0.963 0.924 0.920 0.941
0.968 0.923 0.923 0.935
0.979 0.922 0.924 0.927
0.986 0.926 0.930 0.926
0.988 0.926 0.931 0.933
-
-
-
-
-
0.932 0.934 0.933
0.937 0.935 0.940
0.940 0.937 0.943
0.942
0.945 0.946 0.952
0.044
0.943
714
Draxler
density, the values for the polyoctenylenes of various crystallinities clearly show the antagonistic relationship between crystallinity (physical crosslinking) and vulcanization (chemical crosslinking). With progressing sulfur vulcanization, the crystallinity is gradually decreased, as far as the line in the individual columns. Thecrystallinity is most pronounced in fairly high molecular weight TOR 8020, which must have longer tmns sequences than 80 12, and least pronounced in the 62 13 grade. which possesses little crystallinity. This table of values also gives an impression of the very pronounced crystallinity of metathesis polymers. In many. if not in all, cases,polyoctenylenes may have a special function at the interfacial area during the covulcanization of various types of rubbers. As already mentioned in Section 4, polyoctenylenes, when added to certain rubber blends as a third component, are capable of effecting better dispersion. This results in improved physical datafor the vulcanizates. When this phenomenon is investigated further, it is found that in a number of such blends polyoctenylene substantially improves not only the dispersity but also the covulcanization in the interfacial area of the blend components in question. The covulcanization of two rubbers can be tested simply by vulcanizing mill sheets of compounds of both rubbers together in the press and determining the bond strengthof this composite in newtons per millimeter(peeling test). Somerubber blends, e.g., NR-BR, possess inadequate dispersity and excellent covulcanization. A blend containing 5- 10% polyoctenylene can improve the dispersity and thereforealso the qualityof the vulcanized blend.Polyoctenylenes are even more effective in blends, such as NBR-EPDM, whichare difficult to disperse and difficult to covulcanize. As the peeling test shows, addition of polyoctenylenes is effective only in NBR, not in EPDM. Similarly, other rubbers, such as NR, SBR,CR, CM, CSM, CIIR, or BIIR, can also achieve improved covulcanization with EPM or EPDM by means of polyoctenylenes (Table 5) (Draxler, 1983b). This applies in principle to both sulfur
Table 5 Covulcanizntion of EP(D)M with Various Rubbers-Pccling Adhesion Strength i n Interfacial Area"
Peeling
components
Covulcanization
EPDMEPM EPDM (S vulc.) EPM (pcrox. v.) EPDM (S) EPM (perox.) EPDM (S) EPM (pcrox.) EPM (perox.) EPDM (S) EPM (pcrox.)
Test,
Other NR (S vulc.) 80 NR/20 TOR (S) NR (pcrox. v.) 80 NW20 TOR (pcrox.) CR (ZnO, THU) 80 CR/20 TOR (ZnO, THU) CR (perox.) 80 CW20 TOR (perox.) CllR (ZnO, TMTDS) 80 ClIR120 TOR (ZnO. TMTDS) CIIR (pcrox.) 80 CIIR/20 TOR (perox.) CM (perox.) 80 CM/20 TOR (pcrox.) NBR ( S ) 80 NBR/20 TOR (S) NBR (perox.) 80 NBW20 TOR (perox.)
All compounds contam 50 phr N 550
strength (N/mm) I .6 4.0 4.5 9.0 I .0 3.8 3.7 7.8 4.5 9.3 4.8 6.0 2.0 4.9 0.2 4.1 2.9 13.2
Polyalkenylenes
715
I
Fig. 11 TEM picture of blend of 40 NBR,40 EPDM, 20 TOR.
Draxler
716
and peroxide crosslinking systems. The improved covulcanization in the interfacial area has nothing to do with the oligomer content of the polyoctenylenes. If polyoctenylenes rendered oligomer-free by extraction are used, the adhesion values obtained are exactly the same as those achieved with the nonextracted samples. Furthermore, as demonstrated above, the oligomers are not capable of participating in the vulcanization process. Instead, the effect in question must be a surface effectin the interfacial area itself, a certain proportionof the polyoctenylene remaining in this area and not being dispersed as domains in the blend component. This requires a certain restriction in the compatibility with the individual rubbers during the blending process. Completely dispersed polyoctenylene is known to form very small domains and is then certainly no longer capable of being effective in the interfacial area. Photographs of various polyoctenylene-containing three-component blends taken with the transmission electron microscope show in some casesthat such a polyoctenylene concentration inthe phase boundaries can in factoccur (see Fig. 1 1 ) (Lohmar, 1985). Where polyoctenylene is sufficiently covulcanizable with both components; e.g., with NBR and with EPDM, an improved peeling strength results. It is to be assumed that in this case entanglements of fairly large macrorings or their catenanes are capable of forming such accumulations at the phase surface, even though these accumulations may be extremely thin (approximately 0.1 km). They are presumably also the reason for the dispersing effects of the polyoctenylenes in poorly compatible rubber blends. It is therefore not surprising that experiments with linear polymer liquid rubbers are not capable of producing any improvement at all in the bond strengthof such composites or of displaying any dispersing andcovulcanking effects i n poorly compatible rubber blends, regardless of the chemical constitution of the liquid rubbers. With butyl rubber; however, polyoctenylene does not produce any improvement in adhesion to other rubbers, since virtually no covulcanization is possible between IIR and polyoctenylenes.
Table 6 NR-Carcass Compounds-Dynamic
Properties and Adhesion to Synthetic Fibers
Formultrtiorl
NR Vestenamer 8012 ZnO RS Stearic acid 40Carbon black N 330 Antioxidant Sulfur insoluble TBBS MBTS
100 -
95 5
92.5 7.5
5
5
5
2
2
2
40 2 3
2 40 2 3
3 -
0.05
0.1
Proprrtirs
I.
Goodrich-Flcxometer ( I O O T , load 46.4 kp) (a) Vulc. Temp. 140°C AT aftcr 60 min. "C Permanent wt. @/r ( b ) Vulc. temp. 160°C AT after 60 min, "C Permanent set. 8 2. Adhesion to woven fabric of synthetic fibers, RFL dipped ( a ) Nylon. N/cm 210 (b) Polyester. N/cm
26 14.2
22
Destroyed Destroyed
31
26 11.9
150
160
35 12.6 19.8
200
14.6
150
210
Polyalkenylenes
717
On the other hand, the vulcanization adhesion between sulfur-vulcanized EPDM blends may also be markedly reduced by blending with polyoctenylene. Extremely highly unsaturated EPDM grades having a high vulcanization rate are least affected by this restriction. This should be noted in splice vulcanizing of the profiles in question. The predominantlyadvantageouseffect of polyoctenyleneblends on the vulcanization adhesion of rubbers is also observed to a certain extent i n the adhesion of rubber to fabrics of fullysynthetic fibers (polyesters, polyamides),provided that thefabricsaretreated with the conventional latex dip (Table6, adhesion of a natural rubbercompound topolyester and polyamide fabrics). Rubber metal adhesion can be achieved with the conventional adhesive systems, as shown in Table 7. Restrictionsariseonly in the case of very highVestenamer contents in sulfurvulcanized EPDM compounds. The polyoctenylenes have little effect on the aging properties of the blend vulcanizates. In general, the stability of diene rubber vulcanizates to hot airis slightly improved by polyoctenylenes, whereas the stability of EPDM to hot air at temperatures of 150°C and higher is somewhat adversely affected. Adverse effects on aging are also observed in the case of special polymers, such as silicone rubbers and fluorinated rubbers, and in resin cured butyl rubber. The swelling properties of the polyoctenylenes in various liquid media are very similar to those of the EPDM grades. Greater resistance is shown to polar liquids. Polyoctenylenes can therefore be used to reduce the water swelling of polar rubbers such as nitrile rubber. They also decrease the water-vapor diffusionof these rubbers. The stability of silicone rubber to hydrolysis by steam is also improved. The followingtabulation shows the main effects of the polymer parameters of polyoctenylene on the properties of vulcanizates.
Negative
Positive
Low molecular weight 2. Macrostructure Advantageous effect on elastic behavior; (macrocycles) reduction in heat buildup and permanent set, particularly after dynamic loading; improvement of covulcanization for many rubbers that are difficult to vulcanize with one another; reduction in vulcanizate shrinkage; greater dimensional constancy of finished articles 3. Crystallinity Moderate increase in hardness and stressstrain 1.
Moderate deterioration in physical properties Reduction in elongation at break and in some cases also in tensile strength and tear resistance
Moderate increase in compression set at room temperature
6. APPLICATIONS Usually, polyoctenylenes are used to produce more than one effect. This can be explained in terms of the individual polymer parameters, as described in Sections 4 and 5. The fields of use of the polyoctenylenes include virtually all areas of production in the rubber industry apartfrom the latex sector.Their processing properties and hysteresis chardcteristics are the main reason for their use in tire compounds. Of relevance to the subject of processing
Draxler
718
Table 7 Rubber-to-Steel Bonding of Sulfur-Cured Polyoctenylene Blend Compounds" 1. SBRCompounds Btw SBR IS00 BR Vcstenamer 80 12 N 774 N 330 High-arom. oil Bnrldir~g.strerlcqth Method A.h MPa Chcmosil 220/211 Chcmosil 23 112 1 I Mcthod B," daN/in. Chemosil 220/2 1 1 Chemosil 23 1/21 1 Rubber-roll bonding, daN/in. Chemosil 23 1/21 1 Chemosil 602/211 2. NRCompounds Btrse NR Vestenamcr 80 12 N 660 Naphthenic oil Borldirtg strerlgth Method A.h MPa Chemosil 220/2 1 1 Chemosil 23 1/2I 1 Mcthod B." daN/in. Chemosil 220/21 1 Chcmosil 23 1/2I 1 3. EPDMCompounds Bnse EPDM Vestenamcr 80 12 N 774 Emuls. procaid Borlding .sfrwgtlz Rubber roll bonding, daN/in. Chemosil 23 112 11 Chemosil 602/2 1 1
~'All speclmens showed h
70 30
70
-
30 40 40 6
40 40 6
-
so -
so 40 40 6
9.9 10.0
10.9
62 65
65 69
62 62
44 44
43 43
47 46
100 -
22 3
90 IO 22 3
80 20 22 3
12.1 10.7
11.6 9.8
11.2 9.3
42 39
40 38
29 28
100
90
80 20 90
60 40 90
S
5
S
21 29
29 31
10 IO
9.4 9.9
-
100% separation, within the rubber layer Source; Courtesy of Henkel KGaA. Dusseldorf. Germany.
11.1
Polyalkenylenes
719
in this case are not only the flow properties of compounds with a high filler content but also the dimensional constancy during forced procedures on the calender or extruder. This applies to hard plies, apex, carcasses, side walls, etc. Shape stability of the green tire compounds as a result of increased green strength during the assembling process is utilized for inner and outer plies. In all these applications, the important factoris that the use of polyoctenylene also reduces the hysteresis effect in these tire components. Tables 6 and S show the effect of thepolyoctenylenes on the properties of the green compounds and vulcanizates, in particular the dynamic behavior and those properties that are important for steelcord adhesion. The tire sector has become one of the most important field of use of the polyoctenylenes. In the nontire sector, the polyoctenylenes were introduced at an early stage. The most important areas of use are profiles, moldings, hoses, calendered articles, and roll coverings. Here too. the polyoctenylenes are used in specific cases because of both advantages in terms of processing and certain vulcanizate properties. The production of extremely hard rubber profiles, whichto datehas been a serious technical and economic problem area, profited to a great extent from polyoctenylene. The mixing process of the compounds and in particular extrusion were substantially facilitated, the constancy of quality and dimensions and the surface finish of the profiles were improved hitherto impossibly high resilience coupled with Shore hardnesses of 90-95 were achieved, andnew profile designs, such as plug profiles without metal reinforcement in the hard profile foot, became possible. This development affected mainly EPDM profiles, but also profiles made of SBR, NR, NBR, CR, CM, CSM, etc., as well as profiles made of blends of these rubbers. These profiles are generally used in automobile construction as well as in engineering and in the building industry. The molded article sector also made increasing use of the above possibilities afforded by the polyoctenylenes. Shorter injection times and a reduction in the number of rejects due to flow defects are achieved in the injection and transfer molding processes. Vestenamer grades can even be used advantageously in the conventional compression molding process,for example, where large vulcanization molds with long flow paths are used, for low specific compression pressures, and wherenmlticavitymolds are used. In theproduction of moldedarticles,the Vestenamer grades are blended with a very wide variety of rubbers. Construction elements of NR that are subjected to dynamic loads at fairly high temperatures prove to be substantially more resistant to reversion and dynamic heating when blended with 20-30% Vestenamer (Draxler,1980,1983a). Moldings that are permitted to contain only very small amounts of extractables, which is afrequentrequirement in the case of NBR and EPDM, constitute another field of use for Vestenamers. Examples are moldings that are in contact with hydraulic or brake fluids in closed systems. The same applies to hoses (brake hoses), sealing profiles, calendered articles, rubber roll coverings, etc., where comparable requirements apply. Vestenamer is frequently used in precision moldings in which the dimensional tolerances are very small. The reasons for using polyoctenylene in the hose sector are in principle the Same as the reasons for its use in the profile sector. However,its use in this sector canin many cases simplify the production technology for reinforced hoses. Usually, the wire reinforcement of these hoses is braided onto the unvulcanized inner tube. To prevent the latter from becoming deformed, it is frozen with liquid nitrogen. A number of manufacturers have dispensed with the freezing process for various types of hose by changing over to an inner tube based on a blend with Vestenamer 8012. The high deformation resistance permits braiding to be carried out at room temperature without deformation of the inner tube. With Vestenamer blends, it has also become possible to improve the design of reinforced hoses. Polyoctenylenes are also used in compounds for hose covers, particularly in the case of profiled hose surfaces (line patterns, script, etc.).
720
Draxler
Table 8 Evaluation of Polyoctenylene in RFS Steelcord Carcass" Compounds B Base F ( ~ r ~ ~ u / n / i o r z NR BR Vestcnamer 801 2 High silica, (VN 3) N 326 Coumarone resin Sulfur Other
s
days
A
80 20
65 20
15
15 15
45 4 3.5
45 8 3.5
-h
-l>
H Test N Adhesion level, 863 793 APR (max. = 9) 8.3 8.2 Water diffusion in the rubber (100% RH, 40°C) 0.85 0.85 H 2 0 in original, % Increase of H 2 0 (%) after 1 day 0.24 0.29 10 days 0.14 0.88 20 days 1.28 1.Ol 40 days 1.71 I .44 80 days 2.38 2.03 160 2.25 Humidity test on CPT samples, cord adhesion (N) of three cords 0 day 195 220 195 I O days 130 20 l 60 205 245 40 days 140 80 days 103 I35 Salt spray (5% NaCI) on CPT samples, cord adhesion (N) of three cords 220 0 day 1 95 I wcek 180 280 4 weeks 130 200 Monsanto fatigue test. Number of cycles to failure X 10" 68% elongation X 1358 3049 1.66 S 1.27 89% elongation I364 X 934 1.44 S 1.10 101% elongation X 132 1230 1.40 ss 1.15 126% elongation X 295 322 S 1.30 1.73 .' Steelcord No. 3472. 7 X 4 X 0.175 + I . h Equal amounts of antloxidants. formaldehyde and resorcm donors and accelerators Source: Courtesy of Bekaert. Zwevegem. Belgium.
Polyalkenylenes
721
There is also alarge variety of blend components forpolyoctenylenes in the hose sector, including polyblends based on NBR-PVC. It is envisaged that the improved covulcanization of various rubbers will be utilized in the hose sector. There are also good reasons for using polyoctenylenes in calendered rubber compounds, as shown in Sections 3 and 5. Other improvements in the quality of the articles include the easier penetration in the fabric during lamination of fabrics with hard rubber compounds, as a result of the reduction in viscosity (heavy tarpaulins, air beds, etc.). Vestenamer is also used for reducing the roll sticking of CR compounds. which strongly tend to scorchduringprewarming or on the calender. The inclusion of 5-1095 of TOR is sufficient for this purpose. Furthermore, hardening of CR and CSM as a result of hot-air aging is reduced by adding TOR. In no case does polyoctenylene have an adverse effect on the ozone resistance. The Vestenamergrades are therefore used in fabric coatings that areresistant to chemicals and are waterproof, in container linings, printing blankets, protective aprons. etc. In special cases, it may also be possible to provide better solutions to covulcanization problems in laminates. The same applies to the adhesion to polyester fabrics. When blended with TOR, ebonite linings of NR, SBR, or NBR exhibit higher elongation values and notched impact strengths as well as greater resistance to ozone and to chlorine. The Martens value (softening temperature) of ebonites is not reduced by TOR. These properties are also of interest for the production of ebonite moldings. The producers of rubber coatings for rollers for various industrial purposes began using TOR at a remarkably early stage. In particular, Vestenamer blends made it possible to process NBR grades much more uniformly and easily. The reduction in the rejection rate is particularly advantageous in the roll sector. For reasons relating to oil swelling, an upper limit of 15-20% of TOR is generally maintained in NBR blends. If necessary, Hycar 1312 may also be used in addition to Vestenamer. TOR is used in rubber coatings for textile and printing rollers, as well as on rollers for the paper and steel industries. An interesting possibility is the use of compounds containing larger amounts (up to40%) of TOR to provide rubber coatings on fairly small, very hard rollers by injection molding. This applies in particular to rice-husking rollers and to typewriter rollers. Because of the compression set at room temperature, only the less crystallineVestenamer 62 13 grade is suitable for the latter application.
7 . SUMMARY The metathetic polymerization of cyclic olefins led to new unsaturated polymers that are used as vulcanizable rubbers. In particular, the polymerization of cyclooctene can in principle be used to produce not only linear polymers but also cyclic ones and is very relevant in rubber technology. All knowledge to date indicates that the oligomeric macrocyclics also formed are to be regarded as extenders comparable with mineral oils but have no other technical function. The polymer fraction of the polyoctenylenes was long regarded as being purely linear. Since the postulation of the presence of a macrocyclic fraction within the polymer in 1980 on the basis of the interpretation of particular technological effects, initial positive analytical results in this very difficult and inaccessiblearea of polymerchemistryandphysicsare now also available. It is very probable that the polymeric macrocyclics contain substantial proportions of catenanes. With regard to rubber technology. the macrostructures and the molecular weight of the polyoctenylenes, as well as their crystallinity based on their microstructure, are of particular importance. These three polymer parameters are each associated with advantageous and dis-
722
Draxler
advantageous trends as far as the rubber processor is concerned. However, the possibilities of utilizing the advantageous effects with a suitable choice of polymer and suitable compounding are so fascinatingand varied that the polyoctenylenes are used even todayincontinuously increasing amounts in all areas of production in the rubber industry with the exception of latex processing. This applies in particular to tire production.
REFERENCES Arlie, J. P., Chauvin, Y., Commereuc, D., and Soufflet. J. P. (1974), Makrornol. Cltern. /75:861. Calderon, N., Chen, H. Y., and Scott, K. W. (1967a), Tetrahedron Lett., 3327. Calderon, N., Ofstead, E. A., and Judy, W. A. (1967b), J. Polyn~.Sci. A-/(5):2209. Dall’Asta, G. (1974). Rubber Chern. Tecknol. 4751 1. Dall’Asta, G., Mazzanti, G., Natta, G., and Porri, L. (l962), Makrornol. Chem. 56:224. de,Gennes, P. G. (1971). J. C h m . Phys. 55572. Draxler, A. (1980), presented at the ikt 80, Nuremberg. Reprint (1981) in Kautsch. Gunznti Kunst.st. 3 4 ( 3 ) : ._ 185. Reprint (1983) in Elastomerics //5(2):16. Draxler, A. ( 1 983a). Kuutsck. Gunmi Kunstst. ,36(12):1037. Draxler, A. (1983b), U.S. Pat. 4,551,392 (to Huls AG). Eleuterio, M. C. (1957), U.S. Pat. 3,074,918 (to du Pont). Gianotti, G., and Capizzi, A. (1970), Eur. Polwn. J. 6:743. Gianotti, G., Capizzi, A., and Del Giudice, L. (1976), Rubber Cllem. Techno/. 49:170. Graulich, W. (1974). Chirnia 28:534. Gr,aulich, W., Swodenk, W., and Theisen, D. (l972), Hydrocarbon Proc., 71 ... Gunther, P., Haas, F., Marwede, G., Nutzel, K., Oberkirch, W., Pampus, C. Schon, N., and Witte, J. (1970), J. AngeM,. Markrorn. Cltern. /4:87. Haas, P,,Nutzel, K., Pampus, G. and Theisen, D. (1970), Rubber Cltern. Technol. 43: 11 16. Herisson, J. L., and Chauvin, Y. (1970), Makronrol. Cllern. /4/:16l. Hocker, H., Reimann, W., Reif, L., and Riebel, K. (1980). J. Mol. Cutcrl. 8:191. Holtrup, W., and Hammel, R. (1985), Che~n.Z. 7/8:267. Holtrup, W., Kupper, W., Meckstroth, W., Meyer, H. H., and Nordsiek, K. H.(1974), Der Lichthogen /75:86 1. Ivin, K. J., Lavertin, D. T., and Rooney, J. (1977). Mukromol. Chern. 178:1545. Ivin, K. J., and Saegusa, T. (l984), Ring Opening Polymerization, Elsevier, New York. Jacobson, H. (1984), Mucrornolecules 17705. Kupper, F. W., and Streck, R. (1973). J. Orgunomet. Chern. 55:C75. Kupper, F. W., and Streck, R. (1974), Makrornol. Chetn. /75:2005. Lohmar, J. (1985). presented at the ikt 85, Stuttgart. Michelotti, F. W., and Keaveney, W. P. (1965), J. Polyn. Sci. A3:895. Natta, G.. Dall’Asta, G., and Mazzanti, G. (1964), Angew. Chem 3:723. Natta, G.. Dall’Asta, G., Bassi, J., and Carella, G. (1966). Mukrond. Cltern. 91237. Ofstead, E. A., and Calderon, N. (1972), Mukrornol. Chem. 15421. Patton, P. A., and McCarthy, T. J. (1985), presented at the ACS Meeting, Miami, Florida. Porri, L., Rossi, R., Diversi, P,, and Lucherini, A. (1974), Makronwl. Cltern. /75:3097. Reif, L. (1978), diploma thesis, University of Mainz, Germany. Reif, L. (1983), thesis, University of Bayreuth, Germany. Riebel, K. (l976), thesis, University of Mainz, Germany. Scott, K. W., Calderon, N., Ofstead, E. A., Judy, W. A., and Ward, J. P. (1969), A h . Chem. Ser. 91:399. Streck, R. (1979), cl ten^ Z. 99397. Truett, W. L., Johnson, D. R., Robinson, J. M,, and Montague, B. A. (1960) J. Am. Cltenl. Soc. 82:2337. Usami, T., Gotoh, Y., and Takayama, S. (1985), Eur. Po/ynz. J. 21:885. Wasserman, E., Ben Efraim, D. A., and Wolovsky, R. (l970), J. Am. Chem. Soc. 92:2132, 3286. Witte, J., and Hoffmann, M. (1978), Makrornol. Chern. /79:641.
Polytetrahydrofuran P. Dreyfuss Consultant, Midland, Michigan
1. INTRODUCTION Polytetrahydrofuran (PTHF) is an elastomer with the repeat unit [--CHlCH2CHlCH20-],,. It cannot be prepared usefully by the free radical methods commonly used to prepare many commercial polymers. Instead, cationic ring-opening polymerization methods must be used (Dreyfuss, 1982; Inoue and Aida, 1984; Penczek et al., 1985; Dreyfuss et al., 1989; Pruckmayr et al., 1996). PTHF wasfirstprepared in the late 1930s by Meelwein (1 939) and coworkers, who continued their polymerization studies for several decades thereafter (Meerwein et al., 1960). Worldwide interest in PTHF polymers and copolymers started after WorldWar 11. It was demonstrated that the high molecular weight material can have properties comparable to those of a good general-purpose elastomer, but its production is too expensive to competein such applications. (In 1996, for example, general-purpose rubbers were selling for about $1 per kilogram, while PTHF cost $3.50-4.20 per kilogram.) Fortuitously,the nature of the polymerization makes it easy to produce PTHF in the form of low molecular weight glycols, which are well suited for the preparation of polyurethanes and polyester thermoplastic elastomers with outstanding enough properties to justify the relatively high cost of the specialty elastomer. In these applications PTHF is valued as a precursor leading to products with outstanding hydrolytic stability at elevated temperatures, high fungal resistance, superior abrasion resistance, excellent resiliency, and very desirable dynamic properties. In 1996 PTHF was produced as a glycol at a rate in excess of I10,OOO metric tons per year worldwide. Current producers, in order of overall world production capacity. include du Pont,BASF, Q 0 Chemicals (asubsidiary of Great Lakes Chemicals), Hodagaya, Mitsubishi, Sanyo, and Asahi. Both du Pont and BASF announced plans for construction of new PTHF facilities to come on stream in 1997 or shortly thereafter (Pruckmayr, et al. 1996). Polytetrahydrofuran is the name mostoftenused,and it will be usedthroughoutthis chapter. Alternative names used elsewhere include poly(tetramethy1ene oxide), polyoxytetramethylene, and polytetrahydrofurane. Chenzicul Abstrucrs lists PTHF under the entries “furan, tetrahydro, polymer, homopolymer,” (C4H,0), (24979-97-3), and “poly(oxy-l,4-butanediyl),” (C4HxO),, (26913-43-9). Polymers with known end groups have still other entries. For example, the a,w-glycolis listedas “poly(oxy- 1,bbutanediyl,a-hydro-o-hydroxy,”(C4HxO),,H10 (25 190-06-1). Alternative names for the glycol are poly(tetramethy1ene ether) glycol, poly( 1,4723
Dreyfuss
724
oxybutylene)glycol, and a-hydro-w-hydroxypoly(oxytetramethy1ene).All names refer to PTHF with the same repeat unit and the same linear backbone.
2.
POLYMER PREPARATION
The synthesis of PTHF has been exhaustively reviewed (e.g., Saegusa and Kobayashi, 1973; Dreyfuss, 1982; Inoue and Aida, 1984; Pruckmayr et al. 1996). Only the most salient features of its preparation will be discussed here. Readers are referred to the earlier reviews and the references contained therein for further detail. PTHF is prepared from tetrahydrofuran (THF), the only important source of the polymer, by ring-opening polymerization of its tertiary oxoniumion [Eq. (1 )l. The first stepin the polymerization is thegeneration of a stable tertiaryoxoniumion.Pure,dryreagents in a dry, inert atmosphere or high-vacuum conditions are essential for a controlled polymerization (Plesch, 1963; Dreyfuss, 1982).
initiation)
THF THF
C
+0-R X'
3
>> ROfCH2CH2CH2CH2)0+ a '
M
X'
4
/
l
depropa .gation depropagation propagation
propagation
,-"'\
R0
2.1
Counterions
A complex counterion must be associated with the oxoniumion to prevent its immediatecollapse. With a simple anion like Cl-, for example, an alkyl halide and an alkyl ether are formed:
725
Polytetrahydrofuran
C
>
+OR Cl’
ROCH~CH~CHZCH~C~
The most stable counterions are complex ions such as PF6-, AsF6-, and SbF6-. SbC16supports polymerization,butthechlorineligandcanreactwith the oxonium ion,liberating SbCIS, which is able to reinitiate THF polymerization. BF4- also supports polymerization, but it, too, can react slowly with THF. Since BF3 alone, exceptin very large concentrations, cannot reinitiate THF polymerization, termination of polymerization occurs. S03CF3-, S03F-,C104-, and Nafion Resin ion (NfS03-) also stabilize oxonium ions very effectively, but polymerization rates are reduced when these counterions are used because the growingoxonium ion equilibrates with a nearly unreactive ester:
R-O(”J
2.2
CF3S03’
RO( CH2)40S02CF3
Temperature
The temperature of polymerization must also be carefully selected. Rates of initiation and propagation become impractically slow much below 0°C. In addition, the polymerization of THF is an equilibrium process with a significant rate of depropagation. Conversion to polymer falls off rapidly as the temperature approaches 83 ? 2 ° C the ceiling temperature for THFpolymerization (Dreyfuss and Dreyfuss, 1966; Dreyfuss, 1982).
2.3 Kinetics The details of the kinetics of the polymerization of THF havebeen worked out (Penczek, 1979; Dreyfuss, 1982). The principal conclusions are With “stable” complex anions, such as PF6-, SbF6-, and AsF(,-, the polymerization is “living” under normal polymerization conditions. This means that the polymerizations are free from significant spontaneous termination and transfer reactions, polymers with narrow molecular weight distributions can be prepared, and sequential block copolymerizations are possible. When initiation is fast, kinetics of polymerization in bulk can be closely approximated by the expression:
where k, is the specific rate constant of propagation, t is time, I, is the initiator concentration at t = 0, and [M,,], [M,], and [M,] are the monomer concentrations at time = 0, equilibrium, and t, respectively. k,, at 25°C is about 0.046 L/mol/sec in the presence of stable “complex” anions in 8.0 M monomer in CC14 solvent. Withless stable conlplex counterions like SbCI6- or BF4-, atmosttemperaturesthe influence of transfer and termination reactions, respectively, must be taken into account (Vofsi and Tobolsky, 1965). THF does not behave ideally in solution,and the equilibrium monomer concentration varies with both solvent and temperature. Kinetics of THF polymerization in solution fit Eq. (4) provided the equilibrium monomer concentration is determined for the conditions used.
726
Dreyfuss
With anions that can form esters from the growing oxonium ions [Eq. (3)], the kinetics of propagation are dominated by the rate of propagation of the macro ions. With any given anion, the proportion of macro ion compared to macro esters varies with the solvent-monomer mixture and must be determined independently before a kinetic analysis can be made. The macro esters can be considered to be in a state of “temporary termination.” When the proportion of macro ions is known and initiation is sufficiently fast, Eq. (4) is satisfied.
2.4
Initiation
The initiator determines the head group of a THF polymer, except in cases where the head group is capable of furtherreaction with another species in thesystem(e.g.,hydroxyl or tertiary oxonium ion head groups). The chemistry of the initiation of THF polymerization has been described in detail (Dreyfuss 1982, 1983). Several methods of generation to tertiary oxonium ion are known, including the following: 1. Direct alkylation or acylation of the oxygen by exchange or addition 2 . In situ generation of the oxonium ion from a variety of combinations suchas epichlorohydrin (ECH) or reactive halide and Lewis acid (e.g., ECH + BF3, CH3CH2CI FeC13), or reactive halide and metal salt (e.g., CH3COCI AgSbF(,), or sometimes from Lewis acid alone (e.g., PFS but not BF3) 3. Direct addition of strong protonic acid (e.g., HS03F) or of in situ generated strong protonic acid (by e.g.,PhN2+PF6-, Ph2Br+PF(,-)followed by addition to THFto form an intermediate oxonium ion, which is slowly converted to a tertiary oxonium ion in the next reaction with THF
+
2.5
+
Transfer
Transfer to counterion has already been described. Transfer reactions can also occur with added small molecules such as ethers, formals, anhydrides, and acid chlorides. Reaction with small molecules is an effective way to control polymer molecular weight. For example, if a strong acidsuch as NfS03H is used in combinationwithacetic anhydride, adiacetate of PTHF, CH3CO2CH?CH~CH~CH?-(OCH7CH2CH~CH~),,”OCH,CH~CH~CH7_O2CCH3, is formed. Subsequent hydrolysis or alcoholysis of such a diacetate is oneway of producing a low molecular weight PTHF glycol (du Pont, 1979). It is a much more reliable method than initiation with strong protonic acid followed by termination with water, because hydroxyl end groups that are present during polymerization can interact with the growing oxonium ion to form unwanted high molecular weight PTHF (Pruckmayr and Wu, 1978b). The oxygen atomsin the polymer backbone can also react with the growing oxonium ion. The principal result of such reactions is a much more rapid progression toward a normal molecular weight distribution than would be expected if only propagation and depropagation reactions occurred. A minor result is the formation of up to 3% of product in the form of macrocyclics (McKenna et al., 1977; Pruckmayr and Wu, 1978a). 2.6
Termination
It is necessary to terminate most THF polymerizations in order to produce a stable product with controlled end groups. (Otherwise the PTHF might depolymerize on drying at elevated temperatures and/or end groups will form from something adventitiously present.) The beauty of being able to terminate PTHF polymerizations at will is that almost any desired end group
Polytetrahydrofuran
727
can be introduced. As stated above, termination with water leadsto hydroxyl end groups;termination with polystyryl anion gives block copolymer: and termination with any number of UVor NMR-active groups (e.g.. phenoxide, triphenylphosphine, I3C-labeled methoxide) leads to products in which the number of active end groups at any time in the polymerization can be quantitatively determined (Dreyfuss. 1982).
2.7
Commercial Polymers
Only low molecular weightPTHF in the form of either glycols or glycol derivatives (isocyanates) is produced on a large scale commercially. The major producers were listed in the introduction. Du Pont, the largest producer, markets ~ I Y C O I worldwide S under the tradename Terethane,while BASF and Q 0 Chemicals market them as PolyPTHF and Polymeg, respectively. Commercial PTHF isocyanates are called Adiprenes by du Pont and Vibrathanes by Uniroyal. 3M marketed PTHF amines for a time, but the product did nothave a large market and has been discontinued. Several large-scale processes for the preparation of the glycols initiate the polymerization of THF with HS03F at low temperatures. The exothermic polymerization that ensues results in PTHF chains with sulfate ester end groups (Pruckmayr and Wu, 1978b):
THF
FSOj
In the subsequent product workup, the sulfates are hydrolyzed. Water extraction removes both the acid and most water-soluble short polyether chains, and the polydispersity is reduced from the theoreticalvalue of 2 to 1.6 or less, depending on theoverallmolecularweight. After neutralization and vacuum-drying, the product is filtered hot and loaded into drums or tank cars under nitrogen. The glycols are hygroscopic and must be stored in tightly closed containers and subsequently transferred under nitrogen (Pruckmayr et al., 1996). (Moisture present in the glycols would participate in later reactions and could detrimentally alter the properties of the polyurethanes and polyesters prepared from the glycols at a later stage.)PTHF is sensitive to oxidation, and 300- 1000 ppm of 2.6-di-tert-4-hydroxytolueneis normally added commercially to prevent peroxide formation. THF polymerization can be initiated by many strongly acidic substances. but not many of them produce the required bifunctional polyether glycol with a minimum of by-products. Many polymerization processes have been patented, but in 1996 only a few of them appear to be developed or under development forcommercial production (Pruckmayr et al., 1996).Examples of such large-scale processes include acetic anhydride with either acid montmorrillonite clay or strongperfluorosulfonic acid resin initiators.The product in these processes is a diacetate, which must be subjected to alkaline hydrolysis or hydrogenolysis. Another exampleis the Asahi Chemical Industries process in which PTHF glycol is produced directly by using heteropoly acids such as phosphomolybdic or phosphotungstic acid in the presence of a small amount of water.
3.
PHYSICAL PROPERTIES
Some typical properties of PTHF and its glycol are given in Tables 1 and 2 , respectively. More detailed discussion of the properties can be found in Dreyfuss ( I 982). in Dreyfuss et al. (1994), and in the references therein, especially in the product literature.
728
Dreyfuss
Table 1 Selected Properties of PTHF Property Melting temperature, "C Glass transition temperature, "C Density Amorphous (25"C), g/cm3 Crystalline 300% modulus, Mpa High to low mol. wt. Tensile strength, MPa High mol. wt. High to low mol. wt. Cured Elongation at break, % High mol. wt. High to low mol. wt. Cured Modulus of elasticity, Mpa Shore A hardness Thermal expansion coefficient, deg" Compressibility, kPa" Internal pressure, Mpa AC,, (at Tg), J/(mol . K ) Rapidly cooled Annealed Coefficient of expansion (dV,/dT), cm3/(& . K) Refractive index (at 20°C) dddc, mL/g (THF, A = 546 km)' Dielectric constant k, (at 25°C) Solubility parameter, (J/cm')"'
Value
43, 58-60' - 86 0.975 1.07- 1.08 1.6- 14.3" 29.0 27.6-4 1.4~' 16.8-38.3" 820 300-600;' 400-740h 97.0 95 4-7 x 1 0 - 4 4-10 X IO" 28 1 19.4 15.8 7.3 x 10-4 1.48 0.064 5.0 17.3-17.6
'' Depends o n molecular welght. h c
Depends on curlng system. dn/dc values for other solvents can be found In Dreyfuss (1982). Dreyfuss and
Dreyfuss (1982) and in the onglnal references contamed thereln. Source; Dreyfuss, 1982.
Infrared, Raman, NMR, and x-ray spectroscopy (Bunn and Holmes, 1958; Imada et al., 1965; Vainstein etal., 1969) have established that PTHF isa linear polymer with a planar zigzag conformation. Its melting temperature is above room temperature but well below the boiling point of water, and polymers of all molecular weights crystallize readily at or below room temperature. The glass transition temperature of PTHF is low, - 86°C. High molecular weight (>100,000 &/mol) polymers have properties comparable to those of general purpose rubbers. while very high molecular weight (> 1,000.000 g/mol) polymer behaves like a plastic. When molten, these high molecular weight polymers have good tack and good green strength. When crystallized, low molecular weight (650-5000 g/mol) PTHF is a tack-free wax. Manufacturers of the glycol settheir own specifications for hydroxyl number, melt viscosity, water content,color, melting point, etc. The PTHFglycols are strictly difunctional andallow
rathane
729
Polytetrahydrofuran Table 2 Selected Properties of Terathane Polyether Glycols
Property Hydroxyl Viscosity at 40"C, rnPa . S (1450 = cP) 28-40 Melting point, "C Color, APHA Refractive index, 1 1 , ~ ~ ' 1.464 Heat of fusion. kJ/kg Water content, wt% Ash, wt% Iron, pprn Peroxide, as HzOl, ppm
Flash point, TOC, "C
107-118
9501.465
260-320 25-33 < 10 1,46390.4 <0.0 15 <0.001
53-59
< 10
- 109 <0.0 15 <0.001
> 163
> 163
<5
the number average molecular weight to be calculated from the hydroxyl number according to the formula: 7
Molecular weight = 56.1 X 1000 X hydroxy; number
Most of the PTHF glycols used commercially have molecular weights between 1000 and 2000 g/mol. Glycols with molecular weights above 5000 g/mol have no significant applications at the present time. Some analytical and test methods for PTHF glycols have been described by Pruckmayr et al. (1996). The most important general test methods have become ASTM test methods and are periodically updated by the Polyurethane Raw Materials Analysis Committee (PURMAC) of the Society of the Plastics Industry (SPI) (SPI, 1992). High molecular weight PTHF has been analyzed by most standard polymer techniques and requires no special handling (Dreyfuss, 1982). However. notethat the normally used polystyrene or poly(methy1methacrylate) GPC standards may give erroneous results, and FTHF standards must be used for calibration of GPC and HPLC columns. Solvents for PTHF include the monomer. aromatic and chlorinated hydrocarbons, esters, ketones,nitroparaffins,andliquidsulfurdioxide(Howardet al., 1972; Dreyfuss,1982). The low molecular weight glycols are also soluble in alcohol, diethyl ether. and water. Aliphatic hydrocarbons are nonsolvents except for very low molecular weight polymers, which are slightly soluble in hydrocarbons. PTHF is not regulated as a hazardous material by the U S . Department of Transportation. It can be handled as a nontoxic waste.
4. 4.1
USES Polyurethanes
The most important commercial polymers containing PTHF polyurethanes. are Standard technology is applicable. Properly chosen reactants can be mixed simultaneously in a one-step synthesis
Dreyfuss
730
to give readily processable elastomers with good properties that need no further vulcanization. For example. a polymer prepared from 1 mole of PTHF glycol of 3020 molecular weight, 1 mole of 1,4-butanediol, and 2.0 moles of diphenylmethane-pp-diisocyanate (MDI) produced a thermoplastic polymer with 65 Shore A hardness, 44.1 MPa ultimate tensile strength, 650% elongation at break, 6.2 MPa 300% modulus, 51 kN/m angle tear, and 0 g weight loss in the Taber abrasion resistance apparatus (Quaker Oats Co., Bull. 208). More commonly, PTHF with isocyanate end groups is synthesized first or obtained commercially. The polyurethane synthesis is completed by adding diamines, polyols. or moisture. One recipe.whichincluded 100 parts of Adiprene L-200 (a polymer with 7.35 wt% NCO) and 23.3 parts 4,4’-methylene-Dis-(2-chloroaniline) gave a polyurethane with 58 durometer D hardness,20.7 MPa 100%modulus. 57.2 MPa ultimatetensilestrength, 320% elongation at break. 23.6 Kn/m tear strength (ASTM D-470), 370% of standard NBS abrasion index, and a brittleness temperature of about -70°C (Du Pont. AP-210.1). In this recipe the concentration of diamine curing agent was about 95% of the theoretical stoichiometric amount required to react with the isocyanate groups. The excess isocyanate thus reacted with ureas to form a biuret structure. Properties of the polyurethane vary. as would be expected, with the chemical structure of the starting materials. the amount of chain-extending and curing agents,and the curing temperature. Hardness, 100% modulus, ultimate tensile strength. tear strength, abrasion resistance, and brittleness temperature increase with percent NCO content, while percent elongation at break decreases with percent NCO content. Compared to polyurethanes based on other soft segments, polyurethanes containing PTHF soft segments have excellent hydrolytic stability, high abrasion resistance, and excellent elastomeric properties, such as low hysteresis, high rebound, high flexibility, and high impact resistance, even at low temperatures. The polyurethanes are strong. tough, durable, and have low compression set and high water vapor permeability. These excellent properties offset the higher price of this specialty material and its uses continue to expand. The largest polyurethane end-use area (in 1993 about 50% of the PTHF glycol market) is in spandex fibers for apparel. Thermoplastic polyurethanes(TPUs) and castable polyurethanes (CPUs) constitute the next largest use area (in 1993 about 15 and 14% of the market, respectively). TPUs and CPUs are used in wheels, high-speed rolls, automotive parts, bushings, specialty hose, cable sheathing and coating, and pipe liners. When manufactured as clear. colorless film, polyurethanes from PTHFglycols find use in security glazing as glass laminates for aircraft and in bulletproof glass. Elastomers based on PTHF glycols have excellent microbial and fungal resistance. This combined withtheirhydrolyticstability makes them useful as jacketing material for buried cables. Good biocompatability has led to medical applications such as catheter tubing. Properly purified products are approved by the U.S. Food and Drug Administration for use in products that come into direct contact with food (FDA Regulations 121.2550 and 121.2562).
4.2
Polyesters
Commercial polyesters from PTHF are block copolymeric thermoplastic elastomers. These are products of an aromatic carboxylic acid. PTHF glycol, and a monomeric diol. Melt polymerization procedures are used with starting materials like dimethyl terephthalate. PTHF glycols of 1000 molecularweight,andvariousmonomericglycols,especially 1.4-butanedio1, ethylene glycol (Thompson, 1972; Brown and Witsiepe, 1972: Witsiepe. 1973; Wolfe, 1977). and 1.4-
Polytetrahydrofuran
731
cyclohexanedimethanol (Wolfe, 1979). Du Pont markets Hytrel, a polymer based on 55% soft segment and 45% hard segment derived from 1,4-butanediol and dimethyl terephthalate:
Hard segment
S o f t segment
Eastman Chemicals markets Ecdel, a similar polyester basedon a different diol. Representative properties for a copolymer containing 57% of poly(tetramethy1-terephthalate) and 43% PTHF glycol terephthalate are 13.4 Mpa 100% modulus, 47.2 Mpa ultimate tensile strength, 660% elongation at break, 365% permanentset,and 62 Kn/m tearstrength (Wolfe, 1977). Studies varying monomeric glycol, the molecular weight of PTHF glycol, and the concentration of the PTHF glycol have been carried out (Wolfe, 1977. 1979). The best overall balance of properties from the 1.4-butanediol, the 1000 molecular weight PTHF glycol, and terephthalic acid is the polymer with 45% hard segment and 55% soft segment. Other proportions and other monomeric glycols are better with different aromatic esters (Wolfe, 1979). The polyesters from PTHF glycol are newer than the polyurethanes, and their uses are still evolving. Potential markets include hydraulic hose tubes and covers, chemical hose, lowpressure tires, specially belting, snowmobile tracks, cable jacketing, seals, and coatings. In 1993 polyesters accounted for about 10% of the total PTHF glycol production. 4.3
Copolymers with Other Monomers
PTHF will copolymerize with a varietyof other cyclic ethers and formals (Dreyfuss and Dreyfuss, 1966; Dreyfuss,1982;DreyfussandDreyfuss,1982).None of the copolymers hasachieved commercial importance. In 198 I , Manser et al. reported the synthesis of copolyglycols of THF with oxetanes, four-membered ring cyclic ethers in which one or more of the hydrogen atoms in the 3-position have been replaced by electron-deficient groups like - C H 2 N 3 , " N O ? ,and -CH20CH2C(CH3)(N02)2. Their use as precursors to energetic polymers that can be used for the preparation of propellants and other explosives have been suggested (Manser. 1983; Hardenstene et al., 1985). One or another of these copolymers may achieve commercial significance. 4.4
Block, Graft, and Star Copolymers
A host of copolymers in this class have been prepared (Dreyfuss, 1982; Pruckmayr etal., 1996). They include block copolymers from ecaprolactamand a PTHF glycol, block copolymers from PTHF, and other cationically polymerizable polyheterocyclics including 3,3-bis-(chloromethy1)oxetane (Saegusaetal., 1970), 7-oxabicyclo[2,2,l]heptane,dioxolane,pivalolactone,and ethylene oxide; and block copolymers from polystyrene and PTHF including AB, ABA, and (AB),, copolymers, where A and B can be either polystyrene or PTHF(e.g., Burgess et al., 1976, 1977a,b; Richards et al., 1978). Still other copolymers have been prepared with blocks from
Dreyfuss
732
azetidine (Goethals and Hosteaux, 1991). from oxazolines (Kobayashi et al., 1990), and from polyether sulfones (Zhao et al.,1992). among others. One-, two-. three-, and four-arm stars have been prepared with PTHF arms (Lehmannet al., 1975). Graft copolymers with PTHF branches have been prepared from a variety of hydrocarbon backbones (Dreyfuss and Kennedy, 1976; Franta et a l . , 1976). Graft copolymers with poly(viny1 chloride) branches have been prepared from PTHF backbones (Sakomura et al., 1972). Also, PTHF has been grafted from silane polymers such as poly(phenylmethylsily1ene) (Hrkach et al.. 1988). Availability of these various block and graft PTHF copolymers has led to their use as compatibilizers in blending studies (Wu and Jong. 1993). Theblock copolymer with cyclic imino ether is recommended as a nonionic polymer surfactant (Kobayashi et al., 1990). No major commercial application of these types of copolymers exists in 2000.
REFERENCES Brown, M.. and Witsiepe. W. K. (l972), Ruhher Age (March), p. 35. Bunn. C. W., and Holmes, D. R. (1958), Discuss. Ftrrrrrlrry Soc. 2495. Burgess, F. J., Cunliffe, A. V., MacCallum, J. R., and Richards, D. H. (1977a), Po/yrner lR719. Burgess. F. J.. Cunliffe, A. V., MacCallum, J. R., and Richards, D. H. (1977b). P o / w w r 18:726. Dreyfuss. M. P,. and Dreyfuss, P. (1966). J . Polyrrl. Sci. A-1(4):2179. Dreyfuss, P. (1982). Po/y(tetrtr/r~r/r~fitrtrrl). Gordon and Breach, New York. Dreyfuss. P. (1983). in Irliticctiorl ofPo/srlrrrizcctio/l (ACS Symp. Scr., Vol. 212, F. E. Bailey, Jr., Ed.), p. 1 15, Washington, D.C. Drcyfuss. P,, and Dreyfuss, M. P. (1967), Arh. Po/ym. Sci. 4328. Dreyfuss. P,, Dreyfuss, M. P,, and Pruckmayr, G. ( 1989). in E~rcvc/q,ec/itrof'P o/>werScience r t r d Engineeriug, Vol. 16 (J. I. Kroschwitz, Ed.), Wiley, New York, p. 649. Drcyfuss. P., and Kennedy, J. P. (1976), J. Po/vrm Sri. Po/yn7. Syrup. 56: 129. du Pont (1979). E. I. du Pont de Nemours and Co. Belg. Pat. 868,726. Jan. 4. du Pont ( 1980). D/rPor/tGlvcols: Terrrcol P d y e l h r G/ycd.s t r r d 1,4-Buttrrwiiol: Properties. U.se.s.Storrrp. t r d Hrrrdlirtg, E. 1. du Pont de Ncmours and Co., Wilmington, DE. du Pont. E. 1. du Pont de Nemours and Co., Elastomers Chemicals Dcpt., Types of "Adiprene L" and "Adiprene LW," AP 210.1 (RI), Wilmington. DE. Du Pont Industrial Chemicals Dept., Storage and Handling of Tcracol Polyether Glycol. Wilmington. DE. Smp. 56: 139. Franta. E., Reibcl. L., Lchmann, J.. and Penczek, S. (1976). J. fo/ym. Sci. P(J/.v~II. Gocthnls, E. J., and Hostcaux, F. P. F. (1991). Eur. Pat. 454.226 (to Stamicarbon BV), Oct. 30. Hardenstcnc, K. E.. Murphy, C. J., Jones, R. B., Sperling, L. H., and Manser. G. E. (1985), J. App/ . Po!\'nz. sei. 30:205 l . Howard, N., Huglin. M. B., and Richards, R. W. (1972), J . App/. Po/yrrt. Sci. 16:1525. Hrkach, J.. Ruehl. K., and Matyjaszcwski, K. ( 1988), ACS Polyrrr. Prcyr. 29(2):112. I m a d a , K., Miyakawa, T., Chatani, Y., Tadakoro, H., and Murahashi. S. (1965). Mtrkrorrlol. Chem. 83: 113.
Inoue, S.. and Aida, T. (1984). in Ring-Opening Po/yr,~eri:cctior~. Vol. I (K. J. Ivin. and T. Sacgusa. Eds.). Elsevicr, New York. p. 185. Kobayashi, S., Uyama, H., Ihara, E., and Saegusn, T. (1990). M a e ~ ~ ~ r ~ ~ n / e 23: c l t /1586. r.s Lehmann. J., Reibel, L., Franta, E.. Dobrogoszcz, W., m d Pcnczek, S. (1975). Abstr.,FirstInt. Symp. Polymerization Hcterocyclcs (Ring-Opening), Warsaw-Jablonna, Poland, p. 141. McKenna, J. M,. Wu, T. K., and Pruckmayr, G. (1977). Mrrcrorrlo/rc,lr/c.s 10:877. Manscr, G. E. (1983). U.S. Pat. 4,393,199 (to SRI International), July 12. Matsuda, K.. Tmako. Y., and Sakai, T. (l976), J. A/?/>/.Po/yrtt. Sei. 20:2821. Meerwein, H. (1939). Germ. Pat. 741,476, June 21. Mecrwein, H., Dclfs, D., and Morshel. H. (1960), Allgew. C/zerr/. 72927. Penczek. S. ( 1979). Mtrkrorlwl CheIrr. . S u p / d . 3:17.
Polytetrahydrofuran
733
Penczek, S., Kubisa, P,. and Matyjaszewski, K. (1985), A&. Po/vnr. Sci. 68/65? Plesch, P. H. (1963), in The Clletnistr?. of Cationic Po/werizntion (P. H. Plesch, Ed.), Macmillan, New York, p. 673. Pruckmayr, G., and Wu, T. K., (1978a), Macromolecules 1f:265. Pruckmayr, G., and Wu, T. K., (1978b), Mucronlo/ecu/e.s 11:662. Pruckmayr, G., Dreyfuss, P,, and Dreyfuss, M. P. (1996), in Kirk-Othmer Elzcyclopedicr of Clwnical Tecknology, 4th ed., Vol. 19 (J. I. Kroschwitz, Ed.), Wiley, New York, p. 743. Quaker Oats Co., Chemical Division, Q 0 Polymeg, Bull. 207, Chicago, IL. Quaker Oats Co., Chemical Division, Liquid TDI Castcrble Elastomers, Q 0 Po/ynreg. Bull. 208, Chicago. IL. r ~ ~ ~ rElrrstnrmm, ~~~e Form No. 10131/ Quaker Oats Co. (1976), P o / y r t q Po/wl.sfor H i , ~ h - P e ~ ( ~Uretkorw 8, Chemicals Division, Chicago, IL. Richards, D. H., Kingston, S. B., and Souel, T. (1978). P o / y n w 1968. Saegusa, T., and Kobayashi, S . (1973), Progr. Po/yrn. Sci. (Jnpnrl) 6 :107. Saegusa, T., Matsumoto, S., and Hashimoto, Y. (1970), Mucrorno/ecxles 3:377. Sakomura, T., Yoshida, T., Fujita. Y., and Shinbara, H. (1972). U.S. Pat. 3,696,173 (to Toyo Soda Manufacturing Co., Ltd.), Oct. 3. Society of the Plastics Industry ( 1992), Test Mefhods,forPo/yurrtkarw Rrrw Mrrterids. 2nd ed., New York. Thompson, D. C. (1972). du Pont frzrmvrrior~4( 1):13. Vainstein, E. F., Kushnerov, M. Ya., Popov, A. A., and Entelis, S. G. ( 1969) Polyn~.Sci. USSR / I : 1820. Vofsi, D., and Tobolsky, A. V. ( 1965), J. Po/ym. Sci. A.M26l. Witsicpe, W. K. (1973). Adv. Clrerll. Ser. 129:39. Wolfe, J. R., Jr. (1977). Rubber Chem Techno/. 50:688. Wolfe, J. R., Jr. (l979), Adv. Chenl. Ser. 176:129. Wu, W. L., and Jong, L. (1993). P o / w w r 34:2357. Zhao, L., et al. ( 1992), P o / y n ~Muter. . Sci. E q . 66308.
This Page Intentionally Left Blank
28 Crosslinked Polyethylene Bharat Dave €CC Products/3M Co., Chelmsiord, Massachusetts
1. INTRODUCTION Normally, polyethylene is a thermoplastic material. Production of polyethylene is carried out by polymerization of ethylene gas. The twomost common methods for production of polyethylene are (a) the high-pressure process and (b) the low-pressure process. The properties of polyethylene are controlled by the type of process employed tomanufacture the thermoplastic material. The molecular structure of polyethylene resin is dependent upon polymerization variables, i.e.. temperature pressure, catalyst type, modifiers, and reactor design used in the manufacturing process. Physical properties vary from hard to soft. rigid to flexible, and tough to weak. Optical properties vary from clear to opaque and glossy to dull. Flow rates during processing range from extremely high to extremely low (Kresser 1969). Low-censity polyethylene (LDPE) was first produced by a high-pressure process. In 1950, a low-pressure process was developed to produce high-density polyethylene (HDPE). In the late 1970s, a low-pressure process was also developed to produce linear low-density polyethylene (LLDPE). whose structure and properties are somewhere between those of LDPE and HDPE. Generally a l l types of polyethylene resin have some degree of crystallinity. Density of unmodified polyethylene is dependent upon the degree of crystallinity. Melt index or melt flow behavior is dependent upon the molecularweight and molecularweightdistribution of the polymer. These twoproperties are mostcommonly used to characterize and categorize polyethylene resins. Polyethylene resins have been modified by copolymerizing ethylene with various monomers. e.g., vinyl acetateethylacrylate,and methyl acrylate. to producethedesiredphysical properties. Almost a l l of the modifiers contribute to reduction of crystallinity, and most reduce hardness and increase flexibility. The discovery of crosslinking of natural rubber by Charles Goodyear and other scientists during the years 1938-1941 opened doors to the practicalimportance of crosslinkingmany polymeric materials including polyethylenes. Similarly, in 1948, Dole showed the way to industrial application of crosslinking of polyethylene by high-energy rays and established that stressstrain properties of crosslinked polyethylene were more like those of a crosslinked elastomer (see Dole, 1985). He also showed that crosslinked polyethylene could not be cold-drawn due to formation of C< bonds between long polyethylene chains. 735
Dave
736 Table 1 Consumption (in millions of pounds) of Low- and High-Denslty Polyethylene in 1985 and 1997 (North American Markct Segment Volumes)
1997 Wire and cahle (XLPE, LDPE, LLDPE. and copolymers) Molded goods (LDPE, HDPE, XLPE) Stretch film (shrink film) Rotornolding (XLPE, LDPE, LLDPE, HDPE) Other (blending) (XLPE) Total
1985 292 92 390 290
15
500 200 750 600
30 1079
Polyethylene can be crosslinked by a free radical mechanism. This is accomplished by chemical means or by using high-energy irradiation. such as an electron beam or gamma source. Thermoplastic or uncrosslinked polyethylene has excellent properties at ambient temperature; however. at elevated temperatures (Raff and Allison, 1956; Amberg, 1964). it Becomes tluid Dissolves or swells in numerous solvents Becomes weak and brittle Undergoes environmental stress cracking By crosslinking, all of these deficiencies areovercome toa large extent without affecting inherent thermoplastic properties: for example. toughness, flexibility, impact resistance, and chemical resistance are improved. Most XLPE to date is used in wire and cable coating applications, heat-recoverable tubing, tapes, repairsheaths, etc. Crosslinkedpolyethylene compounds areavailable for injection. compression, and rotational molding applications. Crosslinked LDPE foams areused in automotive padding and industrial packaging. Most shrink-wrap film for packaging both frozen and fresh meat, poultry. and produce is an XLPE product. North American market segments of volume of polyethylene consumption in 1985 and 1997 is shown in Table 1.
2.
BASIC STRUCTURE
Polyethylene is manufactured using a basic vinyl type of polymerization reaction-an polymerization based on vinyl unsaturation. Simply stated.
addition
where R , and R2 are hydrogens. Ethylene can be polymerized alone or copolymerized by the same reaction mechanism. Monomers and comonomers influence the nature and properties of the polymer or copolymer produced. Common comonotners used with ethylene include the following (Kresser. 1969):
737
Crosslinked Polyethylene l-Hexene
H
H
Vinyl Acetate
H
H
I
I
I I c=c l 1 O H l
O=C"CHI
? H I I c=c
Acrylate Methyl or Ethyl R = CH3 or C2Hs
I A
O=C-R Acrylic A c ~ d
H
H
,
.
I
I
c=c
I H
O=C-OH
Polyethyleneand its copolymers can be crosslinked by one of several methods. most notably by peroxide, electron-beam irradiation,or gamma radiation. All methods involve abstraction of hydrogen from the main chain, creating a site for crosslinking.
2.1
Peroxide Crosslinking
In the presence of peroxide, e.g., dicumyl peroxide. and under heat and pressure, polyethylene polymersand copolymers can be crosslinked(RaffandAllison, 1056: Amberg, 1064). The reaction scheme is as follows:
CH3
CH3
Initiation CH3
CH3
I 0-c-0-0-c-0 1 l l CH3 Dicumyl Peroxide
.
AH
I I
20-c-0'
Cumyl Peroxy Radical
Propagation
-(CH2-CH,-CH2-)x
+ 0-C-0' I
CH3 CHI " +
-(CH?"CH-CH?-)
+
l I
0-C-OH
CH, Dicumyl Alcohol
738
Dave
Addition --(CH,-CH-CH,-)x --(cH~--CH-CH,-)~
-1 Crosslinked PE (XLPE)
2.2
Radiation Crosslinking
An electron beam (electron bombardment) or gamma energy source can be used to activate bonds and can cause them to split into free radicals. Again. the mechanism involves abstraction of hydrogen from the polyethylene main chain, creating a free radicalthat combines with another free radical to form a crosslink. In this case, hydrogen is given out as a by-product. The reaction scheme is as follows: Initiation -(CH~"CH2"CH~")x
AE
Propagation H' --(CH~-CH,-CH~-)~
+
Addition (-CHZ"CH-CH2-)x
--(cH~--CH-CH,-)~
"(CHI-CH"CH2")x Free Radical
-
+
-(CH?--CH--CH~--)~ Free Radical
H'
+
H?
-1 Crosslinked PE (XLPE)
This mechanism propagates as long as free radicals are available to combine with each other. Radiation crosslinking (Raft' and Allison, 1956 Amberg. 1963: Dole et al., 1979; Dole, 1985) is carried out at ambient temperatures and in semicrystalline polymers; as with polyethylene, crosslinking takes place only in the amorphous regions. During the crosslinking process by electron beam irradiation, energy applied toward splitting off of C-H bonds is partially used up in raising the temperatureof the polymer during the crosslinking of products. In dynamic conditions. there is approximately ;I 4°F rise i n temperature per megarad dose over ambient temperature. Prorads or free radical initiators are often used in commercial processes to speed up the crosslinking process for economic reasons.
2.3
Moisture Cure forXLPE
Another method for chemically crosslinking polyethylene is to graft alkoxysilyl groups onto a polyethylene chain and then catalyticallycondense grafted PE chainsin the presence of moisture (Bullen, 1983; Currat, 1983) toproduce XLPE. Thereaction scheme formoisture cure of polyethylene is ;IS follows:
739
Crosslinked Polyethylene m + RO - - vAwHv m +
ROH
PE WAMA
PE
+
CH =CH--Si(OR)3 2
2$-CH2-CH-Si(OR) 2 3
-CH-Si(OR)3
3 ( eH20 x c e s s )- j - C H 2 - C H
2$-"H2-CH2-Si(OR)20H > OR
l "CH
A $ - C H Z
OR
XLPE
+ CH30H
Catalytic
condensation OR l
-Ssi-O--Si-Cfi
2 1
2-Si(OR)20H
I
2
OR CH -2
"1I2O
< 'S
>
3. COMPOUNDINGAND MIXING OF POLYETHYLENE In a review article on compounding crosslinked polyethylene, Carlson ( 1960) pointed out that low-density polyethylene polymers and copolymers have the lowest crystallization temperature and thus they are readily processed in rubber-processing equipment. In early 1940, laboratory experiments on crosslinking polyethylene with peroxides had been carried out: however. a practical method was developed only in the mid-1950s. A. Gilbert and F. Precopio of General Electric successfullyused dicumyl peroxide to crosslink polyethylene. Improved heat stability and better volatility of dicumyl peroxide enabled it to be dispersed into polyethylene above its melting temperature in a practical processing operation such as mixing, molding, and extrusion without decomposition of peroxide. The fabricated product can be successfully cured at temperatures above the peroxide decomposition temperature to fully crosslink the polymer (Martens, 1978). Low-densitypolyethylene,ethylene vinyl acetate (EVA), and ethylene ethylacrylate (EEA) copolymers are the base polymersused most extensively i n compounding XLPE products. e.g., wire and cable insulations. Polyethylene (homopolymer) exhibitssuperior electrical properties and is used for higher voltage cables, e.g.. 15 kV and above. However,its molecular weight and melt index must be selected for efficient crosslinking. Lower-voltage cables use copolymers. In XLPE, the choice of base polymer, antioxidant. fillers. and crosslinking agents can have a great effect on end-product properties. The selection of compounding ingredients is very importantandmakes it possible to design a compound with specificproperties for theend product.
740
Dave
3.1 Crosslinking Agents Processing and propertiesof XLPE compoundsdepend on the type and level of peroxide selected. Some peroxides offer processing advantages: for example, higher processing temperatures can be used without decomposing the peroxide during mixing and other processing operations such as molding and extrusion. The level of peroxide affects the degree of crosslinking. In general, the higher the peroxide level, the greater the crosslink density, provided it is cured at the same temperature, pressure, and time. Reactive monomers are used as coagents to improve the rate and degree of crosslinking. Commonly used additives are triallyl cyanurate (TAC), triallyl isocyanurate (TAIC), and trimethylolpropane trimethacrylate (TMPTMA), and a wide selection of similar acrylic monomers. For crosslinking polyethylene by electron beam irradiation or gamma rays. It is not necessary to use free radical initiators. However, for economic reasons and faster processing. free radical initiators, e.g., TAC, TAIC and TMPTMA, are used. 3.2
Antioxidants
Antioxidants are important additivesin crosslinked polyethylene. These additives provide stabilization against oxidative degradation, especially at elevated temperatures. Selection of antioxidant type and level is very important in designing compounds, anda balance between antioxidant and crosslinking agent must be made to achieve the optimum processing and product characteristics. Without antioxidants, the polymer would rapidly degrade, leading to embrittlement of the compound. Antioxidants have a significant effect on crosslinking efficiency. Since saturated polymers such as polyethylene are more difficult to crosslink than unsaturated rubbers, interference by reactive ingredients is of greater importance. Therefore, it is essential to use antioxidants that are relatively less reactive as free radical acceptors and preferably ones that are eitherneutral or alkaline. 3.3
Fillers
Polyethylene has a much higher tolerance for particulate fillers when it is crosslinked than in the uncrosslinked state. This property allows the use of fillers for improvement of mechanical, chemical, and,in some cases,electrical properties of XLPE products. Relatively, nonreinforcing fillers have been found useful in XLPE. In most applications for XLPE, good extrudability and electrical properties are required. For black filler formulations, nonacidic medium thermal black or furnace blacks are used, especiallyin peroxide cures, where they do not interfere with peroxide crosslinking. In wire and cable applications for black insulations, low loading of carbon black is preferred, since the dielectric loss is directlyproportional to carbon blackloading.Black insulations are used only for 600 V to 1 kV cables. This voltage range is usually unshielded, and carbon black provides good protection from UV radiation. Semiconducting XLPE wire and cable shielding conlpounds use high-structure blacks, e.g., acetylene black or furnace black. Such blacks provide the necessary carbon-to-carbon linkage within the polymer matrixto impart semiconductive properties. Inorganic mineral fillers such as clay, calciumcarbonate, silica, talcs, and hydrated alumina have been used in combination or alone and with carbon black to improve the properties of XLPE. The development of ultimate product properties depends on the level and type of filler. Such fillers improve thermomechanical properties, e.g., deformation resistance and cut through resistance at elevated temperatures. Mineral fillers improve dielectric strengthin cables. Considerable work has been done on nonblack pigments for use with XLPE. Peroxide curing in the
Crosslinked Polyethylene
741
presence of mineral fillers is more difficult than with carbon black. Reinforcement of the polymer can be enhanced by modifying the filler/polymer interface through the use of filler treatments that result in a modification of the bond between filler and polymer. Silicone compoundshaving alkoxysilicone groups and a reactive organic group, e.g., vinyl structure, may react with the polymer during crosslinking at the filler surface. Thisresults in a strongchemical bond across the mineral filler/polymer interface.This surface modification rendersthe filler surface hydrophobic, resulting in lowmoisturepickupandstable wet electricalpropertiesundervoltagestress at elevated temperature. Crosslinkable polyethylene formulations may be readily handled in conventional rubberprocessing equipment. The mixing of formulations containing peroxides is primarily done in two steps. The first step is the thorough mixing and dispersion of all the ingredients except the peroxide. This is done at temperatures well abovethepolymermeltingpointandperoxide decomposition temperatures. The second step is the mixing and dispersion of peroxide in the compound. This is done by keeping the mixing temperature below the peroxide (dicumyl peroxide) decomposition temperature but above the polymer melt temperature (230-270°F). Mixing of XLPE formulations is carried out in an internal or intensive-type mixer. e.g., a Banbury mixer. In order to achieve uniform dispersion of all ingredients in the finished compound, it is essential that the mixing equipment be equipped withappropriateandaccurate temperature control instrumentation,e.g.. process water or steam inlet-outlet temperature monitor and stock temperature-monitoring device. The most commonly used process for mixing XLPE compounds is shown in the flow diagram of Figure 1. It is essential that the mixing temperature be controlled so as not to decompose the peroxide or other additives during the mixing and forming process. In the case of radiation-crosslinkable compounds, a one-pass mix can be used. since such formulations do not requireperoxides. However, quite often reactiveliquid nlonorners are addedto such compounds, and it is necessary to keep the mixing temperature below the flash point temperature of the monomers. Mixing temperature range for radiation-crosslinkable polyethylene compounds is 280-32OoF, depending on the properties of the monomers used.
L L P0LYME.S
2?P?
0>!
TWO R C L L MILL
Y ? i ? +F F?
e(
CCQ!L.
3 Ic E?.
31CE3
-Jc COr.!?nU:;C
Fig. 1 Proccss flow diagram for compounding and mixing XLPE compounds (*e.g.. a Banbury mixer, ;I registered tradcmnrk of Farrcll Corp.).
Banbury being
Dave
742
Moisture-cured polyethylene (Currat, 1983; Bullen. 1983) compounds require the blending of two components prior to forming the product. Presently, two basic processes are available for moisture-cured crosslinked polyethylene products. Sioplas is a two-step process in which two masterbatch compounds are prepared separately by mixing in extruders or internal mixers. This two-part system is then melt blended and extruded in one process to manufacture the end product. Monosil, on the other hand, is a one step process in which all the ingredients are mixed in an extruder and the product is extruded. A detailed description of these two processes is given in Section 4.4.
4. 4.1
PROCESSING Product-Forming Processes
Major cases of XLPE compounds are in wire and cable industry applications. Almost 85-90% of products made from XLPE compounds are extruded products. Other types of forming processes include injection molding, compression molding. ratational molding, etc.
4.2
Continuous Vulcanization
All applications of wire and cable insulations, sheaths, tapes. tubes, etc., are extruded products. This process involves melt extrusion of XLPE compoundsbelow the decomposition temperature of crosslinking additives, mainly dicumyl peroxide. In the case of peroxide-cured compounds, curing of the product is done in-line under high-pressure steam in a process called continuous vulcanization (CV). In this process the product is directly extruded from the forming die into a high steam pressure chamber or tube. A process flow diagram for steam CV is presented in Figure 2. Several other processes of continuous vulcanization have also been made commercial. In some. steam is replaced by unpressurized or pressurized liquid heat transfer medium, e.g., molten salts (eutectic mixture) or high-temperature heat transfer water-soluble oils. e.g., Ucon oils. In another case, the steam or liquid is replaced by hot dry nitrogen gas under pressure. All of these processes are very efficient in producing a variety of large-diameter heavy-duty cables where uniform crosslinked insulation or jacket is required. Since the curing of peroxide-based XLPE is both time and temperature dependent. the heat transfer medium under pressure ensures that the XLPE insulation or jacket will be cured uniformly throughout the cross section of the cable. The rate or line speed will depend on the size of the cable.
Fig. 2 Centinuous vulcanization of XLPE wirc and cable coatings.
743
Crosslinked Polyethylene
4.3
Radiation Crosslinking
In the case of crosslinking by electron beam irradiation. the extrusion and crosslinkingor curing are done in two separate steps. Product is first extruded or formed by any one of the forming processes and is then crosslinked in a separate operation during which it is exposed to electron beam irradiation. Typically, extruded products like wire and cable, tubings, tapes and sheaths are extruded in long lengths. In a separate setup, these extruded products are irradiated by being passed under an electron beam, often several times. to achieve the required crosslink density. A typical set up for electron beam irradiation system is shown in Figure 3.
V-
CABLE & PIPE
W I R E & TUBING
PLASTIC F I L M
Fig. 3 Electron beamcrosslinking of XLPE products: wire andcable,tubing, stock, web, etc. (From Becker et al., 1979).
plasticand rubber flat
744
Dave
The development of crosslink density in the radiation-crosslinked products depends the following processing parameters:
on
Dose Beam energy Beam current Beam power, i.e., beam energy X beam current Material characteristics controlling the degree of crosslinking are Material response to irradiation Material density Material thickness The relationship between thickness in thousands and megavolts of power determines the useful electron beam penetration characteristics. This relationship is graphed in Figure 4.
Fig. 4
Electron beam penetration curve. (From Morganstern,
1974.)
745
Crosslinked Polyethylene
4.4
Moisture Cure Processes
There are two processes available for moisture curing of XLPE compounds: the Sioplas process and the Monosil process. The chemistry of crosslinking in both cases is identical. They differ only in the process itself.
Sioplas Process The Sioplas process is executed in two stages (Fig. 5): 1. Polyethylene is mixed with an organic silicone derivative called silane in the presence of an antioxidant and a peroxide. Chemical interactionoccurs at some elevated temper-
I
I
LPPE PS30XIPJt
I
I
* CROSSLINKABLE CZAFTED CCI.I?CIU.'i!l
cA:,:.lT
::/ 3
S I O P L A SM O I S T . C U R A B L SH I X T U R S (BLEND)
I<
l
M O I S T U R EC U R E (POSTFORMING)
F POiR? OX CI IZNSCS , I.E., 5 X ~ 3 U S I O N OR S Z A P I P I C OF F I N A LP R O D U C T , M O L 3 I N CE,T C .
I
V
F I N A LC U R E DP R O D U C T
Fig. 5 Flow diagram of the Sioplas process (two-step) for XLPE. (Sioplas is a registered trademark of Dow Corning Company.)
746
Dave
Fig. 6 Flow diagramof the Monosil process (one-step) for XLPE. (Monosil is a registered trademark of Maillcfer SA.)
ature, e.g., 300-320°F. The purpose of this step is to graft a free silane radical onto the polyethylene chain. The grafting process takes place in a single-screw or twinscrew extruder. The resulting product must be kept dry. In a similar process, a masterbatch of polyethylene and the catalyst dibutyltin laurate (DBTL) is also prepared, dried, and stored separately. 2 . The twoproducts produced i n stage 1 are then mixed in a particular proportion,usually 95 partsgraftedpolyethylene to 5 partscatalystmasterbatch,andextruded in the shape of the desired final product. Finally, the formed product is immersed in hot water for crosslinking.
Monosil PI-mess The Monosil process is somewhat different in that all the ingredients are metered into the singleor twin-screw extruder and extruded in the final product form (Fig. 6). This process differs from the Sioplas process in the method of blending the different components to produce the final product. The curing of the final product is carried out in the same fashion as in the Sioplas process.
4.5
Rotational Molding
Rotational molding is used for producing hollow, seamless products of various shapes and sizes. This type of process is suitable for producing iterns from crosslinkable polyethylene compounds. Rotational molding does not involve the high temperatures and pressures normally needed for injection molding. The metering of compounds i n the mold is not as critical. The basic process is simple (Kraus, 1986). Compounded or blended XLPE compound is placed in the mold, which is then rotated in a planetary fashion while being gradually heated.
Crosslinked Polyethylene
747
The heating is carried out at a givenrate so that the polymer melts atthe mold surface. Gradually, as the heating cycle progresses, the polymer is melted uniformly and forms a homogeneous layer of uniform thickness. Eventually the melted polymer coats the entire mold surface, and as the temperature is raised the XLPE compound starts crosslinking and ceases to flow. When crosslinking is complete. thecooling cycle begins, and the end product is cooled to room temperature by air or water spray. The molded part is then removed from the mold. Process control is very important in this type of molding, and the end-product properties depend largely on good process control. Several rotational molding machines now in operation have microprocessor controls to ensure large production runs and product uniformity. Typical rotational molded products include (a) agricultural sprayers. (b) storage tanks, (c) automotive dashboards, (d) door liners, (e) gear shift covers, (f) advertising display racks. and many similar applications.
4.6 Compression and Injection Molding There arelimitedapplications of XLPE compounds produced i n compressionandinjection moldingprocesses. In both processes,the material mustfirst be melted below the peroxide decompositiontemperature to fill the mold.Then curing takesplace in the mold when the temperature is raised above the decomposition temperature of peroxide. The molding cycle consists of four sequential steps: 1 . Meltingthecompound 2 . Fillingthe mold 3. Curing in the mold 4. Cooling in the mold
Moldedproducts from radiation-crosslinkable compounds have been produced for the electrical and electronic industries, e.g., cable and protection caps, splice boots, and termination boots. In this process products are first transfer or compression molded and then crosslinked by irradiation. This is done with batch processing.
5.
PHYSICAL PROPERTIES OF CROSSLINKED POLYETHYLENE
As discussed in Section 1, the physical properties of XLPE differ quite markedly from those of noncrosslinked or thermoplastic polyethylene. At elevated temperatures, thermoplastic polyethylene flows under pressure, undergoes oxidative degradation, becomes weak and brittle, and also undergoes environmental stress cracking. In addition, thermoplastic polyethylene dissolves or swells in numerous common solvents, especially at elevated temperatures. All of these deficiencies are somewhat corrected by crosslinking polyethylene by the various methods described earlier. Essentially, crosslinked polyethylene behaves more like a rubber than a thermoplastic. Figure 7 shows stress-strain curves for both crosslinked and uncrosslinked polyethylene. These curves show that crosslinked polyethylene has properties between those of the original thermoplastic material and a vulcanized elastomer at room temperature. Thermoplastic polyethylene has a typical yield stress point that is sharply defined by a shoulder in the stress-strain curve. Typically, the stress does not increase rapidly beyond the yield point as it did prior to reaching the yield point. In the case of XLPE, the yield point is suppressed, and the stress-strain curve resembles that of vulcanized elastomer at room temperature. At elevated temperatures. e.g., 300"F, the stress strain behavior is very similar to rubber at that temperature. Yield point is almost nonexistent. and recovery after stretching is as complete as that of an elastomer at the
748
Dave
i'
-
"
I;/ STRESS
Fig. 7 Comparision of stress-strain curvcs for LDPE. XLPE,and a vulcanized elastomer.(From Amberg, 1964.)
same temperature. This characteristic of crosslinked polyethylene lends itself to applications at elevated temperatures without being affected by compression forces towhich it may be subjected, e.g.. in wire and cable applications. Table 2 shows comparison of properties of noncrosslinked polyethylene, crosslinked polyethylene, and vulcanized elastomer (a neoprene rubber).
5.1
Effect of Fillers in XLPE
Polyethylene when crosslinked has a much higher tolerance for particulate fillers than in the uncrosslinkedstate. This property allows the use of fillers for improvement of mechanical, chemical, and, in some cases, electrical properties of XLPE products. Generally, nonreinforcing fillers have been found most useful in XLPE. In most applications, the typesof fillers used require good processability in extrusion. The most common carbon blacks found suitable for use in XLPE are the medium thermal (MT) type. Table 3 shows the effect of carbon black (MT) loading on properties of crosslinked polyethylene. In the rangeof 0- 100 phr carbon black filler in XLPE, room-temperature tensile strengthis quite constant; however, it increases rapidly at 200 and 300 phr. Elongation at room temperature
749
Crosslinked Polyethylene Table 2 Comparison of Physical Properties of Non-XLPE, XLPE, and Vulcanized Rubber (Neoprene) Type of resistance
Neoprene-type non-XLPE rubber XLPE PE
Ozone Corona Environmental stress cracking Heat Cold bend Abrasion Moisture Dielectric breakdown Power factor
Very good Fair Marginal Poor Very good Very good Excellent Excellent Excellent
Very good Fair Good Very good Very good Excellent Very good Good Good
Good Poor Very good Very good Good Far Good Poor
Source; Amberg, 1964.
decreases linearly with increase in carbon black loading, andShore D hardness increases linearly with carbon black loading. In nonblack fillers, calcined clays (treated and untreated) have been found to be most suitable for use in electrical applications. Sincethe fillers interfere with polyethylene vulcanization, especially by peroxide, it is important to select the proper grade of filler. Incorporation of fillers imparts such characteristics as deformation resistance, corona resistance or conductivity and flame and abrasion resistance. In addition, incorporation of fillers also gives higher ultimate tensile strength over a wide temperature range than unfilled XLPE. Table 4 shows a comparison of physical properties of crosslinked polyethylene unfilled and filled with treated clay. It is evident from Table 4 that increasing the level of peroxide in unfilled XLPE simply increases the state of cure, resulting in increased deformation resistance without affecting other properties. However, with the addition of treated clay (formulations 3 and 4), the demand for peroxide increases and significant improvement in tensile strength and deformation resistance is realized.
Table 3
Dependence of Physical Properties on Carbon Black Content
Base fornzulrrtion, plrr Low-density polyethylene (LDPE) Carbon black, medium thermal (MT) Antioxidant (hydroquinoline type) Dicumyl peroxide Plzyicctl properties ur room temp." Tensile strength MPa psi Elongation at break, YO Shore D hardness
I00 0 0.5 2.0
100 50 0.5 2.0
100 100 0.5 2.0
100 200 0.5 2.0
100 300 0.5 2.0
15.98 2350 730 49
15.58 2290 350 58
15.65 2300 140 63
18.57 2730 35 70
19.32 2840 20 76
'' Compression-molded slabs at 320°F for 20 min. Source: Amberg. 1964.
Dave
750
Table 4 Comparison of Physical Properties of XLPE Without and With Treated Calcined Clays 4
3
Low density polyethylene (LDPE) Treated clay Antioxidant (hydroquinoline type) Dicumyl peroxide Proprrtirs Density, mg/m3 Tensile strength MPa psi Elongation at break, % Dcforrnation ( 150°C). %-
1
2
l00 0
100
1 00
0
60
I 00 60
1.25
0.5 2.5
0.5 1.25
0.5 2.5
0.92
0.92
0.92
0.92
18.14 500
14.69 2130 220
19.17 2780 290
29
45
19
0.5
18.14 2670 550 51
2610
S o w r e : Martens. 1978.
6.
APPLICATIONS OF CROSSLINKED POLYETHYLENE
The major uses for crosslinked polyethylene compounds have been in the electrical, electronic, and telecommunications industries. Peroxide-crosslinked filled and unfilled formulations continue to account for the largest volume of all XLPE used. The following is a partial listing of the applications of XLPE compounds: Low to medium voltage wire and cable insulations, e.g., service drop, service entrance, both unfilled and mineral-filled Semiconductive and conductive materials for the electrical and electronic industries, e.g., high-structure carbon black-filled for high-voltage conductive sheathing materials, EM1 and RFI shielding products, and positive temperature coefficient products, and electronic keypads Low-voltage automotive, appliance, and motor lead wiring Electromotive control cables Control cables for power generating stations, both fossil and nuclear Heat-recoverable tapes, tubes, sheaths, and molded shapes Rigid XLPE pipes for agricultural use, irrigation, etc. Rotational molded parts for automotive, industrial, and commercial uses XLPE foam products for automotive padding, packaging, etc. Shrink-wrap film for packaging of frozen and fresh foods-meat products, vegetables, etc. Molded products for electronic and electrical connectors, low-voltage end caps, boots, etc. Medical products, e.g., unfilled XLPE angiography tubes, catheter tubes, etc., cold packs for burn victims Telecommunication, TV cable and wiring Telephone cable repair kits, etc. Production of TPR and TPO products Mining cable repair sheaths
Crosslinked Polyethylene
751
7 . SUMMARY The technologyandapplication of crosslinkedpolyethylene (XLPE) formulationshasbeen around for 40 to 60 years. However, many new technical advances have been made recently. New and improved products continue to appear in the marketplace. Due to their unique structure/property relationship, XLPE compounds have been found in a variety of applications. Developments in production technology continue to have a tremendous impact on XLPE compounds and technology. One such areais XLPE products from linear low-density polyethylene (LLDPE) and very low-densitypolyethylene (VLDPE). The interestingcombination of properties in such new base LDPE resins leads to new products when these are crosslinked. The latest influx of thermoplastic elastomer or rubber (TPE or TPR)and such blended products is based on the XLPE technology. In summary, it is appropriate to say that XLPE technology continues to grow into new and exciting products and that it will continue to have a significant impact in the future.
REFERENCES Amberg, L. 0. (1964). in Vulcnr~izntior~ of Elastonlers (G. Alligcr and I. J. Sjothun, Eds.). Reinhold, New York. Rubher Hcrrldlmok, R. T. Vanderbilt Co., Norwalk, CT. Babbit, R. O., Ed. (l978), The V~trlrlerl~ilt Becker, R. C., Bly, J. H., and Cleland, J. P. (1979), Adv. Rcrrlirrt. Process.. Rcrdicrt. Plrqs. Clwm. 14(3-6): 252. Bullen, D. J. (1983), The Silnne Crosslirlkirrg Bellmiour qf Low, Derzsih Polqethqlerze Ctrhle Cornpourds. BXL Plastics, Ltd. (B. P. Chemicals), Stirlingshire, U.K. Carlson, B. C. (1960), Rubher World, p. 91. Currat, C. ( 1983),Silme Crosslinked / ~ ~ . s u l r t i o ~ ~Medium , f r , r Voltrrgr P o ~ Cd~les. w Maillefer S. A. Report, Maillefer S . A., Hadley, Mass. Dole, M. ( 1983, ACS Proc., Miami, FL. Dole, M,, Gupta, C., and Gvozdic, N. (1979), Ad\,. Rtrditrt. Process., Rrufiot. Phys. Clzenl. 14(3-6):71 1. Kraus, T. J. (1986), in Modern Plnstics Er~cycloprdirr,1985-86. McGraw-Hill, New York. Kresser, T. 0. (1969). Polqolyfirl Plastics, Van Nostrand-Reinhold, New York. Maillefer ( 1983), Mormsil-An EcomnIical Neb13 P rocess,for the h~sulatiorrofLn\tf c t r ~ dMerliur~rVollrrge Crrhlcs rvitlz Crosslirlked Polqethqlerw. Tech. Rep., Maillefer S. A., Ecublens, Switzerland. Martens, S. C. ( 1 9 7 8 ~in The Vrrrulerbilt Ruhhrr Hcr1zdhok (R. 0. Babbit, Ed.). R. T. Vanderbilt Co., Norwalk, CT., pp. 208-318. Morlern Plrrstics (1985). Special report-material 1985. McGraw-Hill, New York. Morganstem, K. H. ( 1974), R ~ r l i ~ r t i oTime r ~ Hrrs A r r i ~ w / j i )Pltr.stic.s r nrtd Ruhher. Rep. FC 74-540. Society of Manufacturing Engineers, Dearborn, MI. Raff, R. A., and Allison, J. R. (1956), Polqrthglrrw (ACS High Polymer Ser.), Interscicnce, New York.
This Page Intentionally Left Blank
Millable Polyurethane Elastomers Klaus Knoerr and Uwe Hoffmann Rhein Chernie Rheinau GmbH, Mannheirn, Germany
1. INTRODUCTION
Conventional grades of rubber are based on unsaturated linear hydrocarbon chains crosslinked with sulfur bridges.These rubbers may also be crosslinked by other mechanisms using peroxide. These different crosslinking systems may generally be used without having to modify the polymer structure of the rubber. Polyurethane rubber is crosslinked not only by peroxide and sulfur systems but also, as is typical in polyurethane chemistry, by isocyanates. The various crosslinking systems require different raw material formulations, which are precisely tailored to the crosslinking chemistry. The production process is thus of great significance. Increasing levels of automation in production operations entail corresponding improvements in compounding and molding processes. Polyurethane rubbers have for many years been considered difficult to process. Polyurethane rubberis produced in a virtually continuous process, so ensuring great consistency in product properties, which cannot be achieved with conventional batch processes. A reduction in the Mooney viscosity and modifications to the supply form of the product have ensured that urethane compounds may be produced by any process commonly used in the rubber industry. Furthermore, even complicated articles may now be produced by injection molding, a low-cost process with great future potential.
2. CHEMICAL STRUCTURE ANDMORPHOLOGY
Polyurethane elastomers aredividedinto three groups of products: (a) castableliquids, (b) thermoplastic resins, and (c) millable gums (Table 1). We will focus here on millable polyurethane rubber. The properties of urethane rubber are largely determined by itschemical composition, which is thus of great significance. On the basis of the pioneering work by 0. Bayer, polyurethane elastomers are generally produced by polyaddition of diisocyanates with microdiols ( = chain extenders) and macrodiols ( = polyols) (Table 2). This processmeansthat an extraordinarily wide range of structural variants and thus of material properties may be obtained. Polyurethane elastomers are classed as segmented copolymers consisting of a sequence of highly flexible, long-chain structural units, or soft segments, and rigid hard segment blocks, 753
754
Knoerr and Hoffmann
Table 1 Typical Properties of UrethaneElastomers
Property
Thermoplastic elastomers gum Millable elastomerCast
Hardness, Shore Tensile str, MPa Elongation, % Modulus 300 MPa 10-22 Tear strength Graves Nlmm C-set", 24 WRT 8 C-set", 24 W100"C 20-45 % Abrasion resistanceh
A65-D70
A45-95
31-48 350-650 4-20 7-12 10-25 100 25-55
20-39 300-600 5-25 5-15 25-60
A82-D60 34-43 400-500 7-20 25-45 20-30 100 30-60
DIN S35 17: " DIN S35 16 abraslon loss In cmm
which are incorporated between the soft segments. The soft segments, formally derived from the reaction between the isocyanate and macrodiol, interalia dominate the elasticity of the elastomer, while the hard segment blocks (formed from the reaction of the isocyanate and microdiol) largely determine the mechanical strength of the rubber.
2.1
Macrodiols
Selection of the macrodiol has fundamental consequences for product properties. Polyetherbased grades thus exhibit excellent resistance to hydrolysis by hot water, acids, or bases as the ether bonds are relatively resistant to nucleophilic attack. Ester-based grades are primarily characterized by outstanding mechanical strength and oil resistance. The former may be viewed as a self-reinforcing effect, while the excellent oil resistance is determined by the elevated polarity of the ester grades.
Table 2 Urethane-Basic
Chemistry
Macrodiol Ester diol Adipic acid butanediol + ethylene glycol diethylene glycol propanediol hexanediol + methylpropanediol ether or + neopentyl glycol together with mixtures of the stated diols b. Etherdiol Polytetramethylene oxide
Diisocyanate
a.
+ + + +
Ethylene glycol Butanediol
1,4-Bis-(P-hydroxyethoxy) benzene Glycerol monoallyl ether Trirnethylolpropane rnonoallyl
Diphenylmethane diisocyanate (MD0 Tolylene diisocyanate (TDI)
755
Millable Polyurethane Elastomers Table 3 PolvurethaneGrades Chemical basis
Grade
Crosslinking system Peroxide SulfurPeroxide SulfurPeroxide Isocyanate Isocyanate
EsterMD1 EsteriTDI EtherMDI EsteriTDI EtherRDI
Polyester (AU-P) Polyester (AU-S) Polyether (EU-S) Polyester (AU-I) Polyether (EU-I)
2.2 Diisocyanates The nature of the isocyanate useddetermines not only color-fastness and resistanceto hydrolysis and elevated temperatures, but above all the crosslinking behavior of polyurethane rubbers. In urethane rubber, diphenylmethane diisocyanate (MDI) has proved to be a suitable reaction partner for the reactive intermediates formed during peroxide curing because it is capable of forming stabilized diphenylmethane radicals. Tolylene diisocyanate (TDI) is distinctly less reactive than MD1 and is thus less suitable for peroxide curing.
2.3
Microdiols
The nature and quantity of the short-chain diols used increase or reduce the thermoplastic and crosslinking properties of the rubber. Glycerol monoallyl ether, for example,has a double bond, which can crosslink with sulfur. This microdiol is thus used as a cure-site monomer in sulfurvulcanizable millable polyurethane. Of course, glycerol monoallyl ether also allows peroxide curing. However, saturated rubbers and MDI-based grades are generally preferred for peroxide curing of Urepan.
3.REVIEW
OF GRADES AND CLASSIFICATION
In general there are two basic types of polyurethane: polyester-urethane (AU) and polyetherurethane (EU). AU urethane has good resistance against mineral oil. gasoline. and grease but shows a slightly reduced hydrolysis resistance. EU urethane is more resistant against hydrolysis but shows slightly lower resistance against oils (Table 3 ) . The grade of millable urethane rubber suitable for a particular application may be selected on the basis of the following criteria (Table 4):
Table 4 Decision Matrix for the Urethane Rubber Grades
Hardness Hydrolysis resistance Oil resistance Heat resistance Wear characteristics 0 = adequate;
+
AU-P
AU-S
EU-S
AU-I
EU-I
45-85 ShA
45-85 ShA 0
45-85 ShA
70 ShA-50 ShD 0
70 ShA-50 ShD
= good:
0
++ + + ++
= excellent.
++ 0 ++
++ 0 0
++
++ 0 ++
+
+ 0
++
756
Knoerr and Hoffmann
Achievable hardness Hydrolysis resistance Oil resistance Wear characteristics Heat resistance All grades of polyurethane rubber exhibit excellent resistanceto oxygen and ozone. Theycontain no extractable constituents, so their composition does not alter, even afterimmersion in solvents.
4. 4.1
COMPOSITION OF POLYURETHANE RUBBER COMPOUNDS Peroxide-Cured Polyester (AU-P)
The properties of peroxide-curable urethane rubber are greatly influenced by the nature and quantity of the selected filler. A quantity of at least 5 phr is advisable so that the compounds may be processed without forming bubbles. Hardness values of up to 85 Shore A (Sh A) may be straightforwardly achieved with active carbon blacks and silicas. Plasticizers are required in order to achievehardnessvalues of below 60 Sh A. However, due to itselevatedpolarity, Urepan has poor compatibility with most mineral oil-based plasticisers so that polar adipic acid polyesters, such as Ultramoll grades, are generally used. Urepan may, in principle, be crosslinked with any commercially available peroxides (Table 5). Coagents such as triallyl cyanurate are generally necessary to increase the compression set. Thanks to the saturated polymer backbone, antioxidants are not required to provide protection against ozone and oxygen. Bayer does,however, add 1.5 phr of Staboxol P, a highly effective polycarbodiimide-based antihydrolysis agent during the production of AU-P in order to counteract hydrolytic degradation of the vulcanisate. Other manufacturers do not use this approach.
4.2
Sulfur-Vulcanized Polyester and Polyether (AU-S and EU-S)
Of the various options for sulfur vulcanization the most favorable approach has been found to be the use of mercapto accelerators in conjunction with sulfur. Both MBT and MBTS have a positiveeffect on the degree of crosslinking,butthey have opposing effects on the rate of vulcanization: MBT accelerates, while MBTS retards. Zinc chloride/thiazole compounds (e.g., Rhenocure AUR) and zinc stearate are used as co-activators. In sulfur-vulcanized polyurethane
Table 5 Typical Peroxide-Curable Urepan Rubber Base Compounds
Urepan 640 G" Urepan 641 G" Urepan 0332 G" FEF-black N 550 5 -Curnylperoxide 50% Triallylcyanurate 70% Msr. Rhein Chemie.
I (AU-P)
2 (AU-P)
3 (AU-P)
101.5 -
-
-
101.5
-
-
-
20 5
20 5 I
101.5 20
1
5 1
757
Millable Polyurethane Elastomers Table 6 Sulfur-Vulcanizable Urepan Black Base Compounds 4 (AU-S) Urepan 0359 G" Urepan 50 EL 06 G" Zinc stearate HAF-black N 330 MBTS MBT Zn-chloridelMBTS complex Sulfur 'l
5 (EU-S)
100 100
0.5 30 4 2
0.5 30 4
2
1 1.5
1
1.5
Msr. Rhein Chemie
rubber, the latter simultaneously acts as a processing promoter (Table 6). Essentially the same comments as for peroxide-curable urethane rubber apply here, too, with regard to usable fillers, plasticizers, and additives.
4.3
Isocyanate-CrosslinkedPolyester (AU-I)
The constituents of the compound are mixed in the sequence stated in Table 7. Stearic acid is used in order to suppress unwanted tackiness. AU-I vulcanisates generally contain neither fillers nor plasticizers. Silicas are selected as the filler to produce light-colored vulcanisates, while HAF black is used for black final products. A dimeric tolylene diisocyanate (e.g., Desmodur TT) is used as the vulcanizing agent. Crosslinking is here primarily brought about by the chemical reaction of the diisocyanate with the nucleophilic hydroxyl end groups of the urethane rubber. The minimum quantity required is 8-10 wt% relative to 100 parts of rubber. Elevated hardness values may only be achieved by adding a special chain extender. Adding 1,4-bis-(P-hydroxyethoxy)benzene (e.g., Crosslinker 30/10), entails an increase in the quantity of isocyanate. Dimeric Isocyanate and 1,4-bis-(P-hydroxyethoxy)benzeneare used in a fixed quantity ratio. This reagent enters into the crosslinking reaction, forming hard segments. 1,4-Bis-(P-hydroxyethoxy)benzene furthermore provides new crosslink sites by allophanate ester formation. Lead carbamate (e.g., Desmorapid DA) is used as a supplementary accelerator in quantities of 0.1-0.3 phr.
Table 7 Isocyanate Crosslinked Polyurethane Rubber-Based Compounds
6 (AU-I) Urepan 600" Stearic acid
1,4-Bis-(P-Hydroxyethoxybenzene Dimeric isocyanate Lead carbamate
'' Msr. Rhein Chernie.
100
0.5 10
0.3
l (AU-I)
8 (AU-I)
100
100
0.5 l 20 0.3
0.5 10
21 0.3
758
Knoerr and Hoffmann
5. COMPOUNDINGANDVULCANIZATION 5.1 Compounding
Compounds may be produced both on the open mill and in the kneader. The preferred type of compounder is ultimatelydetermined by thecrosslinking system or thesupply form of the rubber. Urethane-rubber compounds arepreferably produced in a completely cooled kneader. The compounding sequence and times are determined on the basis of the same criteria as for other rubbers. If the correct peroxide is selected, compounding may even be performed in a single stage without any problems. The discharge temperature should not exceed approximately 100°C in order to avoid unwanted tackiness. This latter phenomenon may also be caused by residues from compounding other rubbers. Thorough cleaning of the compounding units with conventional cleaning batches is thus essential. The sulfur-vulcanizable grades are also ideally suited to processing in an internal mixer due to their granular supply form. Their tendency towards tackiness at elevated temperature is slightly more marked than for peroxide-curable urethane. The discharge temperature should thus be below 80°C. Isocyanate crosslinked polyurethane rubber is generally processed on a completely cooled open mill. As soon as a continuous sheet has formed, stearic acid is first incorporated, then the hydrolysis protection and fillers (where required). The final constituents of the compound to be added are the crosslinking chemicals in the following order: crosslinking agent, lead carbamate, isocyanate. The temperature of the compound should not exceed 65°C. A rapid compounding cycle is desirable. The constituents of the compound are most effectively absorbed if the compound is processed as a small roll. Dependinguponproductionandstorage conditions, the storage life of the compounds is between 1-3 days. The compounds should be stored as cool an environment as possible and be protected from excessive atmospheric humidity.
5.2 Vulcanization
For peroxide-curable urethane rubber, the vulcanization temperature is primarily determined by the selected peroxide, while temperatures of 160°C should not be exceeded for sulfurvulcanization. Isocyanate-crosslinked polyurethane typically crosslinked at temperatures between 130and 140°C.
5.3
Molding
Molded articles may be produced from all grades of polyurethane rubber by compression, transfer, and injection molding techniques, extrusion(for the peroxide-cured types only witha barrier against oxygen, like LCM for continous vulcanization or a steam barrier-Mayla folie-for the autoclave process or calendering). However, the lowest possible temperatures (1 30- 140°C) shouldbe used especially for isocyanate-crosslinkableUrepan in order reliably to prevent scorching.
5.4
Physical Properties of Polyurethane Vulcanisates
Articles made from millable polyurethane rubber have many characteristic properties including excellent tensile strength, modulus, elongation at break and abrasion values. Peroxide-curable
759
Millable Polyurethane Elastomers Table 8 PhysicalProperties of Peroxide-CurableUrethaneRubber 1 (AU-P) 69 Hardness (ShA) to DIN 53505 strength Tensile (MPa) to DIN 53504 37 Elongation at break (%) to DIN 53504 420 4.8 100% modulus (MPa) to DIN 53504 300% modulus DIN(MPa) to 53504 28.1 29.2 45Resilience (%) 39 to DIN 53512 Tear resistance propagation (Nlmm) 15.4 12.9 16.2 Graves to DIN 53515 Abrasion loss (5%) to DIN 53516 64 Compression set % 24 h. 70°C to DIN 53517 3
33 390
28 (AU-P)
3 (AU-P)
69 30 320 4.4
70
60 5
65 3
4.6 28 55
polyurethane-rubber exhibits outstandingphysicalproperties (Table 8). In addition to good mechanical values, sulfur-vulcanizable urethane rubber primarily possesses very good abrasion resistance (Table 9). Even at extremely high hardness, AU-I exhibits excellent rubber properties (Table lo), vulcanisates as hard as 95 Sh A having mechanical strength, including elongation at break, of the same high level as natural rubber.
6. APPLICATIONS OF POLYURETHANE-RUBBER WITH REGARD TO ITS PROPERTIES 6.1
Thermal Stability
Maximum thermal stability is of great significance, especially in the automotive industry. In order to limit noise emissions, enginesare often fully enclosed.Such measures result in elevated engine compartment temperatures, which any rubber materials located there must be capable of withstanding. As with all elastomers, the high temperature limit for continuous use of polyurethane rubber must be viewed in the light of the test method used.
Table 9 Physical Properties of Sulfur-Vulcanizable Polyurethane Rubber
Hardness (ShA) to DIN 53505 Tensile strength (MPa) to DIN 53504 Elongation at break (%) to DIN 53504 100% modulus (MPa) to DIN 53504 300% modulus (MPa) to DIN 53504 Resilience (5%) to DIN 53512 Tear propagation resistance (N/mm) Graves to DIN 53515 Abrasion loss (%) to DIN 53516 Compression set % 24 h, 70°C to DIN 53517
4 (AU-S)
5 (EU-S)
70 32 620 5.4 14.6 39 30
74 26 420 6.0 18.3 48 14
42 49
30 41
Hoffmann 760
and
Knoerr
Table 10 Physical Properties of Isocyanate-Cured PU Rubber Compounds
3
1
Hardness (ShA)71 to DIN 53505 Tensile strength ("a) 28 to DIN 24 53504 break Elongation at (%) to 560 DIN 600680 53504 100% modulus ( m a ) to DIN 53504 7.5 5.3 2.8 300% (MPa) modulus to DIN 53504 11.2 10.1 7.6 Resilience (%) 54 to DIN 53512 resistance Tear propagation (N/mm) 30 Graves to DIN 53515 Abrasion loss (%) to 18 DIN 53516 Compression set % 72 h. 70°C to DIN 6545 53517 35
2 90
95
32.8
44 47 32
42 53
30
Peroxide-curable polyurethane rubber can withstand continuous service in hot air at temperatures ofup to approximately 125"C, with some grades up to 140°C, sulfur-vulcanizable of up to approximately100°C, and isocyante-cured polyurepolyurethane rubber at temperatures thane rubber at temperatures of up to approximately70°C. Peroxide-curable grades of urethane rubber may even be exposed to peak temperatures of up to 150°C (Fig. 1). 6.2 Low-TemperatureCharacteristics Like all synthetic rubbers, millable polyurethane rubber also becomes stiffer as temperatures is distinctly less marked. The glass fall. However, in comparison with other PUR, this tendency
Tensile strength(MPa)
5 01 t 40
1
7d, 150"
"
J
original
150°C
Fig. 1 Tensile strength after hot air aging for 7 days at 150°C.
761
Millable
transition temperature of millable urethanerubber is approximately - 30°C. It should, however, be borne in mind that, even below its glass transition temperature, urethane rubber does not become brittle, but merely loses some of its resilience and hardens. Depending upon the load, test method, and grade, embrittlement does not occur until temperatures as low as -70°C. 6.3
GasPermeation
Polyurethane rubber vulcanizates are distinguished by very low gas permeability, comparable with that of butyl rubber. Peroxide-cured urethane in particular performs particularly well in this respect. 6.4
Resistance to Chemicals
Speciality elastomers such as millable polyurethanerubber are not judged merely on their physical properties. It is precisely their resistance to chemicals and environmental influences that makes them of interest for many applications. The degradation of rubber properties by ozone is the result of attack on the double bonds of the vulcanisates. Peroxide- and isocyanate-crosslinkablepolyurethane rubber products have a saturated polymer backbone and are thus virtually unaffected by ozone. Long-term exposure to severe hydrolytic stress may damage ester-based polyurethane rubber. Resistance may be substantiallyincreased by adding a hydrolysis protection like Stabaxol P or Rhenogran P-50. Vulcanisates protectedin this manner may be exposed to water at 70°C for at least one year. Thanks to its polyether structure, polyether-based urethane rubber may be stored in hydrolyzing media even longer. One significant property of polyurethane rubberis its excellent oil resistance; for example, virtually no swelling is observed in ASTM oils 1 and 2. Even when exposed to the strongly swelling ASTM oil 3 at temperatures approaching the continuous service temperature, the urethane rubber vulcanisateis virtually undamaged (Figs. 2,3). Swellingin petrol, diesel, or biodiesel
*O
(ShA) I 1 UHardness Tensile strength
:I l 20 0
origil
72hl125 "C
Fig. 2 Swelling in ASTM oil no. 3.
(MW
0 Elongation(%) X10 UVolume swelling (%)
762
Knoerr and Hoffmann
80
H Hardness (Sh A) Tensile strength (MPa)
60
0 Elongation x 10 (%) Volume swelling (%)
40
ASTM fuel C 20
0
original
72hI 25°C
-
Fig. 3 Swelling in ASTM fuel C.
is also slight, even in comparison with more costly materials, and products made from Urepan have even been found to withstand contact with aromatic or chlorinated solvents.
7. APPLICATIONS
Urepan elastomers are used in many branches of industry, including mechanical engineering, the textiles industry, petroleum industry, transport, and motor vehicle construction. Typical applications are roller coverings, elastic suspension components, liners, couplings, seals, squeegees, ceramic matrixes, lifting slings, and pump stators. Shock absorber membranes madefrom peroxide-cured Urepan are used in self-regulating and hydropneumatic automotive damping systems. In addition to excellent dynamic load-bearing capacity, essential properties include excellent oil resistance and lowgas permeability. Urepanis the material ofchoice in this application and has proved itself over decades of use. Excellent resilience on exposure to pressure is the essential characteristic for seal applications. Peroxide-cured Urepan is used in such applications at temperatures up to 125"C"special grades up to 140°C. Thanks to its relatively low mechanicalloss factor, peroxide-cured Urepanis ideally suited for use as damping componentsfor high-frequency vibration in vehicle constructionand mechanical engineering. Pump stators for eccentric pumps must exhibit excellent wear resistance to abrasive media, such as sandwatermixtures or fresh concrete. Sulfur-vulcanizedUrepan fulfills these requirements. Thanks to its excellent resistance to chemicals combined with good tear propagation resistance, Urepanfulfils the requirements of roller manufacturers for the printing, paper, and steelprocessing industries. The ceramics industry uses rubber material combining elevated abrasion resistance with good hardness for embossing its products. Isocyanate-crosslinked Urepan has been usedas a material for rollers, wear protectionparts, molded articles, and stampsfor ceramic tails for some decades.
Millable Polyurethane Elastomers
763
REFERENCES Hepburn, C. (1982). PolvuredIane Elastonlers. Applied Science, New York. Hepburn, C. (1995),Rubber Compounding Iqredienrs-Need, Theory and Innovation, Part I, Vulcannizing Systems, antidegradants and particulate fillers for general purpose, Communications of RAPRA, GB. Hoffman, U. (1997), in Ullrmmr~Cl~enlicalEncyclopedia. Hoffman, U. 1997) Re-TK 1 Urepan a Polyurethane Rubber, Mannheim, Germany. Kallert, W. (1966), Kuursch. G u t m i Kurmrsr. 19:363. Kallert, W. (1968), J. [RI 2:26. Kleimann, H. (l986), Rubber World 4 : 175. Knoerr, K. (1996), Processability of urethane rubber, paper given at East-West Rubber Conference, Budapest, Hungary. Knoerr, K. (1997). Molding of polyurethane rubber, paper given at 15 1 American Chemical Society Rubber meeting, Anaheim, CA. Ozaki, S. (1972), Chem Rev. 5:457. Urepan Processing Guidelines (1999), Bayer AG Study.
This Page Intentionally Left Blank
"
30 Cast Polyurethane Elastomers Klaus Recker Bayer AG, leverkusen, Germany
1. INTRODUCTION Polyurethane (PU) parts canbe produced by different technologies, e.g., reaction injection molding, injection molding, compression molding, casting into open molds, and rotational casting without molds. Casting into open molds is the oldest method of producing solid polyurethane elastomers. The history of polyurethanes started in the late 1930s, when Otto Bayer (Bayer. 1947) invented the polyaddition reaction of diisocyanates (1) and polyols (2). If the functionality of the poly01 is two, a linear molecule containing urethane groups (3) is formed: HO-R'OH (2) '
"
'
R "
o
+ OCN-R-NCO (1)
+ HO-R'OH + OCN-R-NCO (2)
..
+ . . . -+
(1)
..
(3) The formation of polyurethane elastomers with acceptable mechanical properties requires the reaction of a diisocyanate with both a high and a low molecular weight diol. The short-chain diol forms the hard segment, because its amount relative to the soft segment, built from the long-chain diol and the diisocyanate, directly influences the hardness-related properties of the elastomer. Basically,there is no chemical bonding between the linear molecular chains. However, the interchain interaction between the hard segments leads to rather strong hydrogen bonds that can be regarded as "physical" crosslinks. In 1990 the worldwide consumption of polyurethanes was in the range of 5 MM tons (Nybakken, 1996). High-performance PU cast elastomers serve aniche market with a worldwide consumption of "only" 57,000 tons in 1993.
2.
RAW MATERIALS
2.1
Diisocyanates
The most important diisocyanates for the production of polyurethane cast elastomers are the aromatic diisocyanates 4,4'-diisocyanatodiphenylmethane (MDI) (4) and 2,4-toluene diisocya765
Recker
766
nate (5). usually blended with 2,6-toluene diisocyanate (TDI) (6) (Atwater et al., 1994). They are used as monomeric components or prereacted with polyols to form storage-stable prepolymers with an isocyanate content of between 3 and 10%. It is common to lower the melting point of monomeric MDI, which is approximately 38"C, by special reactions in order to facilitate processing. By thismodification to quasi-prepolymers, the isocyanatecontent changes from 33.6% to, e.g., 23%, andthe diisocyanates canbe processed more easily because they are liquid at room temperature.
Compared to MDI. monomeric TDI has a relatively high vapor pressure at room temperature. Even TDI prepolymers with a low isocyanate content contain a relatively high amount of monomeric TDI. For industrial hygiene reasons, the raw material suppliers offer TDI prepolymerscontainingless than 0.5% monomeric TDI (Dieterich and Schmelzer, 1994).They are manufactured by reacting the polyol with a large excess of TDI, which is finally removed by a thin-layer evaporation process in the factory. The different reactivity of the isocyanate groups in the TDI molecule has two effects: the 18 times more reactivepara position predominantly reacts with the hydroxyl groups of the polyol. This results in a narrower molecular weight distribution; i.e., the viscosity of the prepolymers is lower than a comparable MD1 prepolymer. Due to its sterically hindered position, the remaining isocyanate group in the ortho position reacts much more slowly with the chain extender. Both effects make processing easier. 1,5-Naphthalene diisocyanate (NDI) (7) is the oldest diisocyanate used in high-performance applications. Its high reactivity and high melting point (127.5"C) do not allow its use in the manufacture of storage-stable prepolymers. The processor has to prepare the prepolymer inhouse prior to the casting step (see below). NCO
NCO
NCO
NCO
NCO
Further aromatic diisocyanates of some importance are p-phenylene diisocyanate (PPDI) (8) and 3.3'-dimethyl-4.4' biphenyl diisocyanate (TODI) (9). Exanlples of aliphatic diisocyanates are bis(4-isocyanatocyclohexyl)methane, cyclohexane- 1.6-diisocyanate (CHDI), and l-isocyanato-3.3.5-trimethyl-5-isocyanatomethylcyclohexane (IPDI).
2.2
Polyols
The molecular weight of the polyols necessary for producing rubber-like products ranges from 1000 to 3000. The standard types have a molecular weight of 2000. Hydroxyl-terminated polyesters and polyethers are the most important backbones for the synthesis of PU elastomers.
767
Cast Polyurethane Elastomers
Hydroxyl-terminated polyesters are made from adipic acid and an excess of glycol such as ethylene glycol, butanediol-1,4, hexanediol-1.6, and neopentyl glycol. Mixtures of these are used if particular properties, such as the low-temperature flexibility of the elastomer, are to be influenced. Mixing different polyesters reduces the tendency of the soft segment to crystallize. Polycaprolactones are produced by polymerization of E-caprolactone. Aliphatic carbonate esters are manufactured by transesterification of diethyl or diphenyl carbonate and glycols. Because the polyester polyols are either rather highly viscous liquids or even solids at room temperature, most of these systems are hot-cured systems. Hydroxyl-terminated polyethers are produced by a ring-opening reaction of propylene oxide started by polyfunctional alcohols or amines. They are called polyoxypropylene glycols (PPG ethers, C3 ethers). Their comparatively low viscosityat room temperature allows the incorporation of mineral fillers in considerable quantitiesfor economicreasons but also in order to achievespecial effects (reduceshrinkage, increase stiffness). In order to enhancethe reactivity of the polyethers, ethylene oxide can be used for end-capping of the PPG chain. C4 ethers are produced by polymerization of tetrahydrofurane (polytetramethylene glycols, PTMEG). It is a common practice to blend polyols with molecular weightsof 1000 and 2000 to reduce the danger of crystallization of the soft segment (cold-hardening). C., ether-based systems are also hotcured systems due to the high viscosity of the polyol.
2.3
Chain ExtendedAdditives
The chain extenders are very important with regard to the mechanical and dynamic properties of the cast elastomers. Short-chain glycols and aromatic diamines are used for the production of solid elastomers. If cellular elastomers are to be manufactured, the common foaming agent is a water-detergent blend. The standard chain extender for MDI-, NDI-, PPDI-,and TODI-based systems is butanediol-1,4. The hard segments formed by this glycol are well crystallized. This leads to better phase separation between the hard and soft segments. If elastomers of a hardness higher than 90 Shore A are to be produced, the reactivityof butanediol- 1,4 can be reducedby small amounts of acidic ingredients. In order to shorten the demolding time of softer castings, which may be produced by blending the butanediol with trimethylol-propane (TMP), activated chain extenders are commercially available. Due to its high melting point of 104"C, hydroquinone bis(2-hydroxyethyl)ether (HQEE) is not easy to process, but the general properties can be adjusted to a much higher level. Aromatic amine curatives are mainly used as chain extenders for TDI-based systems. The high reactivity of the diamines fits in with the low reactivity of the free para position of tl TDI molecule in prepolymers. The reactivity of the diamines is influenced
Cl
by thesubstituents.Electron-attractingsubstituentslikechlorineandcarboxylgroups lower the reactivity of the amine group, resulting in a longer pot life after mixing the components. Commercially, the most important diamine is methylene-bis-orthochloro aniline (IO) (MOCA, MBOCA, MBCA). The debate about the possibly carcinogenic potential of this amine curative has led to a search for alternatives. Compared to MBOCA, the 2-methylpropyl-4-chloro-3,5-
Recker
768
diaminobenzoate (1 l), with two electron-attracting groups in the molecule. offers an even longer pot life coupled with quick demolding of the cast item. Toluylene diamines substituted with ethyl groups (DETDA) have a high reactivity that requires machine processing. They are also used in elastomericsystems, which are sprayed and rotationally cast. 3,5-Dimethylthiotoluylenediamine, commercially available as an 80: 20 blend of the 2,4- and 2,6 isomers that is liquid at room temperature, shows a reactivity between that of DETDA and MBOCA, MCDEA is another diamine introduced into the market as an alternative to MBOCA. If glycols or triols like TMP and TIPA (triisopropanolamine) areused as chain extenders for TDI-based systems, very soft elastomers can be produced for special applications.
3. PROCESSING/MOLDING Polyurethane cast elastomers can be produced by two methods: the prepolymer and the “oneshot” process. Basically, both ways allow machine and hand mixing, if the reactivity of the components is not too high (Awater et al., 1994). 3.1
Prepolymer Process
In order to produce an isocyanate-terminated prepolymer, the poly01 is reacted with an excess of diisocyanate at elevated temperatures. If the polyols have a water content of less than 0.1%, it is necessary to first dehydrate them. The temperature of prepolymerization depends on the kind of polyol and the reactivity and the melting point of the diisocyanate. MD1 can easily be liquefied, whereas the high melting pointof NDI (1 27°C) makes theaddition of the solid isocyanate to the polyol more practicable. System houses offer molders different prepolymers for a wide range of hardnesses. Due tothe high reactivity and the high melting pointof NDI, prepolymers based on this isocyanate have to be produced by the molders themselves. Prior to adding the chain extender, the prepolymer should be degassed for a short time in order to prevent bubble formation in the molding. For hand mixing, the prepolymer is filled into a casting pot and the necessaryamount of chain extender isadded while stirring the prepolymer with a propeller stirrer for approximately 30 seconds. Machine mixing is the method of choice if larger series of moldings are being produced. Low-pressure casting machines with gear pumps and agitator mixheads are widely used. Hot cast systemsneed mold temperatures of 80- 120°C, according tothe recommendations of the raw material suppliers. Metal molds require the use of release agents. If metal inserts are placed in the molds, the application of bonding agents is necessary to ensure good adhesion of the PU elastomer tothe metal surface.This isparticularly important for the production of wheels and rollers with metal hubs and cores. After demolding, the cast items have to be postcured in an oven for8- 12 hours. The final mechanical properties are achieved after storing the moldings for 2-4 weeks at ambient temperatures. Polyurethane elastomers based on PPG ethers are easier to process because their viscosity at room temperature is low. The molds need not be heated, and postcuring is not necessary. 3.2
One-Shot Process
The one-shot processing of polyurethanes means that the pure monomeric or slightly modified isocyanate. the polyol, and the chain extender are simultaneously mixed together. This applies to both for hot cast and cold-cured systems. Both hand and machine mixing are possible. Blends of glycols and polyester polyols have a limited storage stability, particularly at higher tempera-
dard
Cast
769
tures and in the presence of catalysts. Therefore, preferably three-component machines with a separate vessel for the chain extender are used. Due to the lower viscosity of the diisocyanates, one-shot processing is basically much easier than working with prepolymers, and thin-walled items with more complicated shapes can be cast without incorporating air bubbles. MD1 quasiprepolymers, however, with their rather high content of monomeric MD1 sometimes tend to crystallize. The MD1 forms dimers very quickly and steadily,whichprecipitateand can no longer be liquefied on heating, which affects the mechanical properties of the elastomers. Sometimes both methods of processing are combined forpractical reasons. If softer elastomers are required,the viscosity of the prepolymertends to be very highdue to the lower isocyanatecontent and an undesiredincrease in the softsegmentmolecularweight. In such cases, it is more practicable to take a prepolymer with a higher isocyanate content and hence a lower viscosity and to “dilute” the chain extender with a certain amount of the polyol, which is the basis of the prepolymer or even a different one. This processing method is familiar to processors as the splitting process. The reactivity difference between the short-chain crosslinker and the long-chainpoly01 may require the use of catalysts in order toachieve reasonable demolding times and good mechanical properties. It has been reported (Ruprecht et al., 1992) that highly reactive systems with a pot life of a few seconds can be cast directly onto the surface of rotating bodies without using molds. The viscosity of the reacting system increases so rapidly that none of the reaction mix runs off.
4.
PHYSICAL ANDCHEMICAL PROPERTIES
The physical and chemical properties of polyurethane elastomers are determined by the nature of the soft and the hard segments, their amounts relative to each other, and their segregation during the formation of the polymer (Awater et al.,1994).Important physical properties include the glass transition point and heat distortion temperatures, the mechanical loss factor or damping value, stiffness, tensile strength, and wear resistance (Table 1 ). Chemical properties mainly refer to the resistance to organic and inorganic solvents and tluids.
Table 1 Properties of NDI Elastomers Based on DifferentPolyol Backbones
NDI Elastomers based on
DIN Property Shore A/D hardness Tensile strength, MPa Elongation at break, c/o Tear strength, kN/m Rebound resilience, c/o Abrasion loss, mm3 Compression set at 70°C. lo ‘l
c
53505 53504 53504 535 15 535 12 53516 53517
100 p.b.w. ethylene butylene adipate M,, 2000 18 p.b.w. NDI 2 p.b.w. butanediol-1.4 100 p.b.w. polytetramethylene glycol M,, 2000 18 p.b.w. NDI 2 p.b.w. butanediol- 1.4
EBA“
EA^
PTMG‘
HA“
84/32 46 655 33 60 32 20
82/30 55 675 48 54 38 18
90138 24 500 23 68 35 20
94/44 49 420 38 60 36 24
”
“
100 p.b.w. ethylene adipate M,, 2000 18 p.b.w. NDI 2 p.b.w. butanediol-1.4 100 p.b.w. hcxylene ndipate M,, 1400 21 p.b.w.NDI 2 p.h.w. butancdiol- 1.4
Recker
770
The higher the melting point of the hard segment-which depends on the melting points of the glycol or diamine and the diisocyanate-the better the segregation from the soft phase and the lower the damping values over the temperature range. Increasing the amount of hard segment relative to the soft segment increases the Young’s modulus, resulting in a higher stiffness, higher tensile and tear strength, and less compression deflection, which means a higher static load-bearing capacity.The hardnessranges from 10 Shore A to 70 Shore D; the corresponding Young’s modulus varies from 10 to 700 MPa. Mechanical properties like rebound resilience, elongation at break, and low-temperature behavior are primarily determined by the soft segment. Polyester-based elastomers generally show a higher tensile strength, bettertear propagation, and better wear resistance than polyetherbased ones. Polyester urethanes also offer the advantageof better resistance to mineral oils and grease, many solvents, oxygen, ozone, and UV radiation. Polyester urethanes do not contain large amounts of extractable components. Therefore, they do not change their chemical composition when exposed toorganic fluids. Polar chlorinated hydrocarbons like trichloroethylene and other polar solvents have a negative influence in that they may cause excessive swelling, which reduces the hardness-related properties. Aromatic hydrocarbons also cause the elastomer to swell,but to a lesser degree than polyether urethanes. Increasing the amount of hard segment relative to the soft segment may lead to better behavior, i.e., lower swelling, but this tendency cannot be generalized for every polyester urethane. The better phase segregation with C4-ether-based elastomers leads to higher hardness and rebound resilience. The high load-bearing capacity and low damping values of NDI-and PPDIbased elastomers make them ideal for dynamic applications (see below). The nature of the soft segment to a large extent determines the chemical properties of the PU elastomer. The hydrolytic and microbial stability of a polyester urethane depends on the constitution of the polyester. Polyadipates made from hexanediol are less sensitive than those made from ethyleneglycol. Polyurethanes based on 1,6-hexanediol polyarbonate show extremely good resistance to hydrolysis, but the processing is some times difficult. A common measure
103
100
-50
0
50
100
150
200
Temperature (“C) Fig. 1 Influence of the hard segment on the shear modulus: (1) ethylene adipate M, 2000 1 mol)/NDI (4 mol)/butanediol-1,4 (2.6 mol); (2) ethylene adipate M, 2000 1 mo1)MDI (2 mol)/butanediol-1,4 (0.85 (0.85 mol); (3) ethylene adipate M,. 2000 1 mol)/TDI (2 mol)/4-chloro-3,5-diaminobenzoate-isobutylester mol); (4) ethylene adipate M, 2000 1 mol)/MDI (4 mol)/butanediol-1,4 (2.6 mol).
Cast Polyurethane Elastomers
771
to prolong the lifetime of a polyester urethane elastomer is the use of hydrolysis stabilizers like mono- and polycarbodiimides. Special additives may be used to enhance the resistance to microbial attack. Figure l shows the influence of the hard segment on the shear modulus of elastomers based on different diisocyanates. Basically, polyether urethanes have good elasticity and flexibility. They are less wear and UV resistant than polyester-based ones and have higher swelling rates when exposed to aggressive fluids. Their key advantage, however, is their excellent hydrolytic and microbial stability. If the application requires, polyether-based urethanes can be protected with stabilizers to prevent degradation by UV irradiation and oxidation. It has been shown (Barksby and Allen, 1993) that PPG-ether urethanes with mechanical properties close to C4-ether urethanes can be obtained if the amount of monols is reduced.
5. APPLICATIONS The versatility of polyurethane clastomers has led to their acceptance in numerous industry sectors (Franke et al., 1994). The selection of applications listed below cannot, of course. be complete.
5.1
Mechanical Engineering and Plant Process Equipment
Nonpneumatic tires and wheels are without doubtthe most important applicationfor cast urethane elastomers. The key advantages over rubber are the significantly higher stiffness at the same hardness, their excellent abrasion resistance, and lower rolling resistance. Polyester urethanes based on NDI with their lower damping and compression set values offer the highest loadbearing capacity, even at elevated temperatures. Typical examples of these applications are tires for forklift trucks, guide wheels for elevators, and guide and supporting rollers for conveyor belts. Figure 2 shows the load-bearing capacity of wheels as a function of hardness, modulus, and wheel diameter per width of the cover surface at a speed of 7.5 km/h (4.65 mph). Another important application is roller covers. Hardnesses from 10 to 60 Shore A are required for rotational pressure and pigment-transfer rollers in printing machines. TDVpolyester and C4-ether systemsare mainly used because of theirgoodresistance to varioussolvents. Higher hardnesses are obligatory for operation in the steel, textile, and paper industries. Typical examples are tension and supporting rollers, driving and squeezing rollers, reel spools, and guide rolls. Both hot and cold curing systems are utilized. A classical application field for high-performance urethanes today is the textile industry. Rotor bearings for spinning machines revolve up to 80,000 times a minute. NDI-polyester systems can withstand this really severe dynamic load and assure a long service life. Seydel rollers are used for stretch-breaking continuous fiber tow into slivers for spinning. Elasticcoupling elements are preferablymade from NDI-polyesterandTDI-C4-ether systems. They are able to transmit power from the drive source and reduce vibrations. The high stiffness of the elastomers allowsthe use of unreinforced moldings and the transmission of large forces with small parts. A critical aspect of silk screening is how well the squeegee blade can resist swelling when exposed to inks and harsh solvents. Adipates are the poly01 backbone of choice for this application; NDI, MDI, and PPDI are mainly used as diisocyanates.
Recker
2250 I
I
2000 1750 1500 1250
1000
.
74n
I
I
"V
120
140
160
180
200
Diameter (mm)
Fig, 2 Load-hearing capacity of wheels covered with an NDUpolyester urethane, cover thickness 20 mm. Shore D hardness: (a) 62, (b) 56, (c) 50, (d) 40. Young's modulus: (a) 400 MPa, (b) 300 MPa, (c) 200 MPa, and (d) 100 MPa.
5.3 Mining Industry The high abrasion resistance of hot-cast PU elastomers is the key property for making screens for the classification of granular mass materials. such as gravel, ore, and coal. Hydrocyclones are used for clarifying and thickening suspensions of abrasive slurries. The high stiffness of NDI-polyester urethanes allows for unreinforced self-supporting designs. 5.4
Oil Industry
In order to maintain an efficient flow of oil in pipelines. solid deposits frequently have to be removed from the insidewalls. The minimalswelling in petroleumandtheexcellentwear resistance are good reasons to produce pipeline pigs from polyester urethanes, mainly based on MDI.
Cast Polyurethane Elastomers
773
machine. Outstanding advantages of PU elastomers in this field are long life expectancy, low maintenance cost, good weather resistance, and the possibility of tailoring sport-related properties. Stoneware pipes are used in the drainage industry. The sealings can be cast directly onto the ceramic surface of the pipes. The good resistanceto household andmany industrial wastes and the low creep of the polyether urethanes assures goodlong-term performance in this application. Formwork mats are needed for the production of relief pattern concrete. Their elasticity and flexibility and good wear resistance allow manifold molding/demolding procedures. The alkalinity of the concrete makes the use of polyether urethanes necessary. Table edgingwith PU elastomersnot only helpsto prevent injuries, but also allowscreation of an individual design by using colored lightfast systems. PU elastomers are also used as a binder of abrasive fillers for the production of grinding disks.
REFERENCES Awater, A., Franke, J., Hentschel, K.-H., Prolingheuer, E. C., and Ruprecht, H.-D. (1994), Po/vurethane Hmclbook, Carl Hanser Verlag, Munich, pp. 390, 392, 400. Awater, A., Franke, J., Hentschel, K.-H., Prolingheuer, E. C., and Ruprecht, H.-D. (1994b), Po/yurethme Handhook. Carl Hanser Verlag Munich Vienna New York, p. 392. Awater, A., Franke, J., Hentschel, K.-H., Prolingheuer, E. C. and Ruprecht, H.-D. (1994c), Polyurethane Handbook, Carl Hanser Verlag Munich Vienna New York, p. 400. Barksby, N., and Allen, G. L. (1993). Low mono1 polyols and their effects in urethane systems, Proceedings of the Polyurethane World Congress, Vancouver, B.C., Canada, p. 445. Bayer, 0. (1947). Angew. Chem. 59(9):275. Dieterich, D., and Schmelzer, H. G. (1994), in Po/yurethane Handbook, Carl Hanser Verlag, New York, p. 26. Franke, J., Hentschel, K.-H., Hoppe, H.-G., Hoscheid, R., Ruprecht,H.-D. and Stelte, B. (1994), in Polyurethane H m d h o k , Carl Hanser Verlag, New York, p. 438. Nybakken, G. (1996). Elastomeric castable polyurethane: A review of markets and recent technical developments, Proceedings of Utech ‘96, The Hague, Netherlands, paper 39, p. 1. Ruprecht, H.-D., Recker, K., and Grimm, W. (1992). Kurlsts. Ger. P / ~ . s t82( . 10):44.
This Page Intentionally Left Blank
31 Polynorbornene Rubber Ani1 K. Bhowmick Rubber Technology Centre, Indian Institute of Technology, Kharagpur, India
C. Stein* CdF Chirnre S.A., Paris, France
Howard
L. Stephens
The University of Akron, Akron, Ohio
1. INTRODUCTION One of the interesting developments of the 1980s was polynorbomene, the aromatic equivalent of a polyalkenamer. The synthesis of this rubber has been described by Draxler (Chapter 26, this volume). The monomernorbornene (for bicyclo [2.2. I]heptene-2),produced by the addition of ethylene to cyclopentadiene by the Diels-Alder reaction, is polymerized by the ring-opening metathesis mechanism as shown in Figure 1. Le Delliou (l977), and Ohm and Stein (1982) claim that both cis and rruns structures could be obtained by varying the catalyst system. This polymer has the ASTM designation PNR and is commercially known as Norsorex. It is always plasticized with a naphthenic or aromatic oil or an ester plasticizer to lower its T, of 35°C down to -60°C. It has an apparent density of 0.35, and its molecular weight (M,) is above 2 million. It has a very low ash content (<0.2%) and low volatile matter ( < O S % ) . It is obtained in powder form (grain size 0.05-0.80 mm). Norsorex is protected by anonstaining,nontoxic,andnonmutagenicantioxidant. The polymer itself is nonmutagenic and nontoxic (LD 50 on rats is 11 g k g ) .
2.
STANDARDGRADES OF NORSOREX
Norsorex N is the pure polynorbornene powder of particle size 50.80 mm. Other grades such as Norsorex 150 NA Norsorex 150 AR. Norsorex 80 NA, and Norsorex 80 AR are available. 150 NA means 150 phr of naphthenic oil. Similarly, AR stands for aromatic oil. Norsorex 150 NA/25 EP, a blend of polynorbornene with EPDM rubber, is also made commercially. Nippon Zeon Co., Ltd. has the following grades: NSX-ISNA, NSX-I5NB, NSX-ISNC, NSX-ISND, NSX-ISAR, and NSX-20NA. Elf-Atochem, France has been manufacturing Norsorex under the toll production agreement with Nippon Zeon Co., Ltd.
* Retired, 775
Bhowmick et al.
776
0 /
\
POLYM.
Fig. 1 Synthesis of Norsorex.
3. PROCESSING OF NORSOREX All standard rubber-processing techniques can be applied to Norsorex compounds. In addition, Norsorex obtained as a free-flowing powder may be compounded by the dry-blending techniques normally used for PVC. Compression, transfer, and injection molding at a temperature between 140 and 200°C have been reported. Calendering between 80 and 100°C is recommended. Extrusion of Norsorex compounds is also mentioned. The temperature of the feed section in the extruder is adjusted to 40-60°C and the nozzle temperature to 90- 1 10°C. Recommendedtemperatures for extrusion of 20 Shore A black compound with a vacuum extruder (17, D 0 90) are as follows: feeding, 40°C: body 1, 40°C: vacuum, 50°C: body 11, 70°C; head, 80°C; and die, 80-90°C. The extrudates can be cured by autoclave, salt bath (LCM), or microwave (UHF). The bath temperature in LCM should not exceed 220"C, and optimum cure time is 30-45 seconds. All the conditions for processing depend on the nature of the curing system and the amounts of fillers. Norsorexpowdercan also be mixed atroom temperature with a veryhighamount of aromatic ornaphthenic plasticizer (200-500 parts). Such compounds canbeused as room temperature castable and curable elastic mass. This process is known as the Norsofluid process. The influence of oil and fillers on Mooney viscosity is described in Figures 2 and 3. The viscosity decreases with increase in oil content. SAF black-filled compounds show the highest Mooney viscosity. Norsorex can take very large amounts of oil and fillers. Storage stability of compounded stocks depends mainly on the curing system employed. For example, sulfur-CBS (cyclohexylbenzthiazyl sulfenamide) systems give quite a long scorch time, while scorch times of compounds containing dithiocarbamate ultra-accelerators are shorter. Peroxide systems are safe. There is a tendency for Norsorex-based compounds to revert during vulcanization at high temperature. Vulcanized Norsorex that has reverted shows deterioration in properties such as tensile strength and hardness. Hence it is advisable to adjust the curing system for the Norsorexbased compound to the processing technique used (compression, injection, etc.).
777
Polynorbornene Rubber
l
I
I
50
100
I 150
I 200 phr o i l
Fig. 2 Mooney viscosity of Norsorex-oil compounds. (Courtesy of CdF Chimie, France.)
4.
COMPOUNDING OF NORSOREX AND PROPERTIES OF VULCANIZED COMPOUNDS
The wide range of applications for Norsorex is due to its versatility in compounding and its physical properties. Norsorex compounds can be compounded to give a wide hardness range, from 10 to 80 Shore A.It has excellent mechanical strength, elongation150-700%, low compression set, good low-temperature properties, heat resistance up to 90°C, good ozone resistance (when blended with EPDM), and excellent water resistance. Compounding of Norsorex is very similar to that of other synthetic rubbers. For example, atypicalrecipecontainsrubber,zinc oxide, stearic acid, fillers, oil, andcuring agents. As mentioned before, unlike other synthetic rubbers, it can take a large quantity of oil and fillers. Blending with other elastomers (EPDM, NBR, CSM, CR, NR,IR, SBR, and BR, etc.) has also been reported in the literature. Most of the products use sulfur and CBS. An efficient vulcanization system using high CBS and low sulfur is recommended. The use of other accelerators such as tetramethyl thiuram disulfide (TMTD), dithiodimorpholine (DTDM), ethylenethiourea (ETU), and tellurium diethyl dithiocarbamate (TeDEDC) has also been mentioned. Fillers like HAF, FEF, GPF, MT, clay, whiting, and Ti02 have been used to impart various properties. The effect of these fillers on tensile strength is, however, different from that in conventional rubbers. Some typical formulations along with the properties are given in Table 1. It has been shown that for a given hardness
778
Bhowmick et al.
100
50
0
I 50
I
I
I
I
100
150
200
250
1
300 phr Filler
Fig. 3 Influence of quality and type of filler on the Mooney viscosity of Plasticized Norsorex. (Courtesy of CdF Chimie, France.)
the tensile strength does not much improve with the addition of reinforcing filler, although the modulus and hardness increase. A low-hardness compound (15-20 Shore A) with very good mechanical properties can be produced with the use of norbornene rubber. Hardness of 80 Shore A and above also could be produced by using thermosetting phenolic resins. Aging properties of compounds in Table 1 are reported in Table 2. Compounds containing semireinforcing fillers are better in aging resistance than those with reinforcing fillers. Since Norsorex compounds contain plasticizer, choice of plasticizer is very important for better aging resistance. High viscosity aromatic oils show the lowest changes in hardness and the smallest weight losses after aging. Peroxide systems are preferred to high-sulfur conventional systems in agingresistance.Antioxidantused for storagestability of Norsorex is usually enough to produce age-resistant vulcanizates. Norsorex, however, has limited resistance to ozone. This could be improved by adding 20-30 phr of EPDM. Theblend vulcanizate shows a slightdrop in mechnical properties. Combination of microcrystalline wax (1.5 phr) and substituted paraphenylenediamine (6 phr) is also effective but staining. Time for appearance of cracks for typical protected Norsorex compound is 320 hours under static test and 190 hours under dynamic test as against 2 hours under both tests for the unprotected compound at 50 pphm ozone concentration. Resistance of Norsorex compounds to differentoils is shown in Table 3. In general,
779
Polynorbornene Rubber Table 1 Influence of Various Fillers on Mechanical Properties of Norsorex Vulcanizates Fonnulation Norsorex ZnO Stearic acid Fillers Low viscosity aromatic oil Paraffinic oil CBS
100
5 1
200 180
20 5 1.5
Sulfur Properties
HAF N 330
FEF N 550
GPF N 660
MT N 990
Mooney viscosity (100°C. ML, + J ) Optimum curing time ( 1 55°C). min Tensile strength, MN/m' Elongation at break, o/n Modulus 1 OO%, MN/m' Modulus 300%, MN/m' Hardness, Shore A Tear resistance, kN/m Rebound resilience at 20°C c/o Nonbrittle temp., "C Compression set (22 hr. at 70°C), o/n Density
95 9 10.5 330 2.5 18.5 55 30 18 - 38 14 1.20
85 11 17.0 350 2.5 15.0 53 32 26 - 38 14 1.20
80 12 15.5 380 2.2 14.0 50 35 32 - 38 12 1.20
45 16 15.0
~~~
510
0.7 6.0 32 27 55 - 38 12 1.20
Clay
Whiting
40 18 14.5 520 0.8 3.8 33 24 55 - 38 26 1.30
39 16 12.0 580 0.5 0.9 25 11 59 - 38 15
1.32
~~~
Source: Courtesy of CdF Chimle. Paris. France
theaggressiveness of petroleum-basedhydrocarbonstowardvulcanizatesincreases with the aromaticity of the oil. UnvulcanizedNorsorex is soluble or subject to considerableswelling in aromaticand chlorinated solvents and cyclohexane andcyclohexanone. Swelling is less in estersand lactones, and there is none in alcohols. Factices seem to reduce the swelling in esters and ketones. Vulcanizates of Norsorex are attacked by concentrated nitric and sulfuric acids but can withstand concentrated hydrochloric acid. They have good resistance to boiling water and fair resistance to detergents. Above the dynamic transition temperatures, the damping coefficient and the moduli E' and E" decrease only very slightly with increase of temperature. The plasticizers and fillers
Table 2 Variation of Mechanical Properties After Aging 7 Days at 70°C in a Ventilated Oven" Property
HAF
FEF
GPF
MT
Clay
Tensile change, lo Elongation change, % Hardness, points
- IO - 15
-8 - l5 +5
5 - 15 +6
-3
-5
-5
-6 +3
- 10
- 15
+3
+5
~'For formulation. see Table
+5 1.
Source: Courtesy of CdF Chime, France.
Whiting
780
Bhowmick et al.
Table 3 OilResistance Formulation, phr Norsorex Zinc oxide Stearic acid HAF black Aromatic oil Paraffinic oil CBS Sulfur
100
5 1
220 180 20 5 1.5
Vnriation irr properties (after immersion for 3 days at 70°C in ASTM oils)
ASTM No. Tensile strength, % Elongation at break, p/o Hardness (Shore A), pt Volume, %,
2
ASTM No.
1
+5
+2 25 t 9 +l
- 27
-9
3
- 35 - 25 - 19
-
+ l9
ASTM No.
+ 40
Volurtw chtrrzge (after immersion for 3 days at 20°C in various hydrocarbons)
Gasoline Gasoline
) Fuel B Fuel A Fuel -
+ 30%
ISYO
+ 45%
+ 80%
+ 65%
Sortrce: Courtesy of CdF Chimie. Paris, France.
Table 4 Influence of Type and Level of Carbon Black on Dynamic Properties of the Vulcanizates HAF
GPF Formulutiorz Norsorex ZnO Stearic acid Sevacarb MT black G P F black H A F black LV aromatic oil" CBS Sulfur Properties Hardness, Shore A Compression set (22 hr at 70°C), % Dynamic transition temp. at 125 Hz, "C At 20°C. 125 Hz E'. MN/m' 6 At 80°C, 125 Hz E'. MNlm' 6 ~
MT 100
5
100 5
1
1
100
250
l00 5 1 -
-
-
100
105 5 I .S
190 5 1.5
140
44 7.5 -6 5.36 0.27 4.99 0.06
43 16.5 - 19 8.17 0.23 4.79 0.15
-
~~
Iranolin 20 SR BP. Source: Courtesy o f CdF Chime, Parts. France.
-
S 1.5 42 8.5 - 12 6.33 0.2 1 5.35 0.07
IO0 5 I -
250 -
250 5 1.S
46 20 - 27 8.70 0.28 5.23 0.16
IO0 5
100
S
1 -
1 -
100 1so
250 280
5 1.5
42 8.6 - 13 7.00 0.25 5.11 0.08
5 1 .S
S2 28 - 32 18.40 0.39 9.07 0.23
Polynorbornene Rubber
781
affect the dynamic transition temperature. Forexample influence of fillers on dynamicproperties is shown in Table 4. The larger the amount of fillers and the higher the structural index of the filler, the higher the damping coefficientof the vulcanized clastomer. Figure4 shows the dynamic properties of polynorbornene as compared to two elastomers commonly used for shock absorption,namely,chlorobutyl and naturalrubber.It shows a remarkableuniformity in dynamic properties with increasesin temperature. Norsorex-based compounds have a quite constant damping coefficient over a range of frequency 0-500 Hz. Properties at low temperature depend on the choice of plasticizer. A blend of high viscosity aromatic oil (80%) and paraffinic oil (20%) generally provides good cold resistance. The best results are obtained with low viscosity naphthenic oils or alkylbenzols, which avoid staining problems. Bonding of polynorbornene with metals is usually done during curing and with the help of standard bonding agents (Chemosil 255 or Chemosil 210/220).
freq. 125 H Z
,"
F
\
\
/ I
10 N@wton
\
\ \
\ NORSOREX-MT\ NR
*.
\ \
0
- 50
l
1
"
1
1
" 0
'
I
I
50
,
,
,
,
I
100
T 'C
Fig. 4 Influence of temperature on the damping coefficient of chlorobutyl, natural, and Norsorex rubber. (Courtesy of CdF Chimie, France.)
Bhowmick et al.
782
5. APPLICATIONS OF NORSOREX The applications of Norsorex depend on the properties discussed earlier. Properties 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Wide range of hardness (from 10 to 70 Shore A) Excellent mechanical strength (tensile strength 8-25 MPa) Elongation 150-700% Good low temperature properties ( - 45°C) Low compression set Good heat resistance up to 90°C Excellent water resistance Good ozone resistance Moderate oil resistance Wide range of dynamic properties Easy processing by extrusion and injection Very good suitability for calendering Excellent adhesion to metals
Applications
1. Very Soft Solid Parts (10-45 Shore A) Autornotive use: bumpers, lock seals, grommets, lamp seals, convoluted boots, hood, door trunk and sun roof seals, dashboard and air-conditioning packings Marine use: Hatch seals, porthole and hatchway seals Appliances: Mixer and robot pads, electric motor packing, and seals Electrical: Connectors, seals Construction: Seals for water pipes, sewer pipes, manholes, windows, and doors Footware: Cushion insoles, heel pads, orthopedic insoles Roll colterings: Rolls used in printing, wood, paper, leather, nut-shelling industries Graphic arts: Photocopier rollers, self inking stamps and rollers For example, the properties of two soft rubber compounds, of Shore A hardness 20 and 40, respectively, are given in Table 5. These compounds are injection-moldable. The following injection-molding conditions for a REP B-53 K press may be followed:
Temperature, "C Screw Pot Mold Time, sec Injection Keeping of pressure Vulcanization
125 70-90 190
2 3 50
Compression or transfer molding can also be done with soft compounds. Use of 1.5 phr sulfur and 6 phr CBS is recommended.
783
Polynorbornene Rubber Table 5 Properties of Soft Norsorex Rubber Compounds A
B
100
100
20 220
175
Forrnuln/ion
Norsorex EPDM Naphthenic oil Stearic acid Zinc oxide MT black TMTD DMDPTD DTDM Te DEDC DOTG ETU Vulkalent E Sulfur Proper/ies (cured at optimum cure time) Hardness (Shore A) Tensile strength, MPa Elongation at break, % 300% Modulus, MPa Compression set (22 hr at 70°C)
20
1
1
5
5 200
100
1.5 1.5 1.5 0.8 0.5 0.5
-
1.S 1.5
0.8 -
1.o 0.5
20 10.0 550 2.1
40 13.5 430 9.0
15
9
2. Shock and Vibration D Arnping Applications Automotive use: Engine and transmission mounts, solenoid mounts, etc.: hood and lift gate bumpers Marine use: Shock rings, dock fenders, engine mounts Railways: Coach body mounts, ballast and railway underlay Construction: Air conditioner dampers, elevator bumpers, floor insulators Sports: Ski inserts, shoe inserts, etc. Others: Loudspeakermembrane seals,turntable damping parts,isolation of electrical boxes, seismic equipment pads 3. Other Applications Other applicationsincludebinding of fillers in abrasivepowders used in flexible or semirigid grinding wheels: friction materials used in brake shoe linings and pads; castable compounds; modification of fluid properties of oils, paints, and solvents; modification of dynamic properties of other elastomers:modification of thermoplasticsand thermosets; oil spill cleanup on ground, rivers, and oceans; waste treatment; etc. New applications appear every year as processors and users learn more about this elastomer.
REFERENCES De Delliou, P. (1977), Preprints of the international Rubber Conference, Brighton, England. Ohm, R., and Stein, C. (1982), in Kirk-Ohner Encvclopedia of Chenzicnl Technology. 3rd ed., Vol. 18, (H. F. Mark, D. F. Othmer, C. G. Overberger, and G. T. Seaborg, Eds.), Wiley, New York, p. 436. Nippon Zeon Co., Ltd. (2000), Technical literature, Tokyo, Japan.
This Page Intentionally Left Blank
32 Nitrile and Hydrogenated Nitrile Rubber Sachio Hayashi Nippon Zeon Co., Ltd., Tokyo, lapan
1. INTRODUCTION Acrylonitrile-butadiene rubber is a synthetic rubberof copolymerized acrylonitrile and butadiene. Called nitrile rubber, it is abbreviated NBR and is one of typical oil-resistant elastomers ( 1-3). Nitrile rubber, which is selectively hydrogenated. is called hydrogenated nitrile rubber (HNBR) (4). NBR was born through the study of copolymerization of butadiene and other monomers using a free radical catalyst from IG in Germany. In 1930, it was found that a synthetic rubber based on butadiene and acrylonitrile showed good properties against oil and fuels, and it was named “Buna N” (5). In Japan, NBR has been manufactured since 1959. HNBR was developed to improve its heat and weathering resistance. In 1984 Nippon Zeon started commercial production of HNBR using their own technology (6). There are many applications of NBR and HNBR such as fuel hoses, oil hoses, oil seals, packing, diaphragms, printing rolls, blankets, brake shoes, adhesives, belts, safety shoes, etc.
2.
MANUFACTURINGMETHOD OF NBR
NBR has been manufactured by an emulsion polymerization system the same as SBR. To start emulsion polymerization,butadiene and acrylonitrile monomer arecharged into thepolymerization vessel and mixed with deionized or softened water and emulsifier, and then catalysts and control agents are added. For cold polymerization, redox catalystsystems are used whichconsist of inorganic peroxides such as hydrogen peroxide or potassium persulfate, or organic hydroperoxide such as cumene hydroperoxide, di isopropyl benzene hydroperoxide or paramenthane hydroperoxide, and a reducing agent such as ferric salt or tetraethylenepentamine. A pH control agent such as sodium phosphate is also used. When the appropriate conversion from 60 to 90% is reached, the polymerization reaction is shortstopped by hydroquinone or carbamates (7,8) (Table 1). After removing and recovering nonreacted monomers from the latexby heating, reduction of pressure, and/or steam distillation, stabilizers are added to maintain the polymers’ storage stability. The latex is coagulated by calcium chloride, aluminum sulfate, sodium chloride with 785
786
Hayashi
Table 1 ExamplePolymerizationRecipe
of NBR
Butadiene Acrylonitrile Water Sodium oleate KOH KC1 Condenscd sodium naphthalene sulfonate and formaldehyde EDTA.Na4.4H20 t Dodecyl mercaptan FeSOJ SFS (NaS01.CH20H.2H20) p-Menthanehydroperoxide
acrylonitrile butadiene
n -
polymerization reactor
shon stopper
antioxidant
W
monomer
recovery
61 33 230 5 0.05 0.30 0.20 0.02 0.38 0.01 0.05 0.04
ntry
Hydrogenated Nitrile and
787
Nitrile Rubber
sulfuric acid or polymeric coagulants. The product is filtered, washed, and dried into its final form (Fig. 1). In the case of manufacturing NBR without metallic iron or with a relatively small amount of metallic iron, emulsion, solution, suspension, or bulk polymerization can be used (9).
3.
MANUFACTURING METHOD OF HNBR
Selective hydrogenation of C==€ double bonds while maintaining the cyano group in the side chain is one of theeffectivemethods to improve heatresistance of NBR.Afterdissolving NBR in an appropriate solvent, catalysts and hydrogen are added to produce HNBR through a hydrogenation reaction; the catalyst and solvent are recovered, and the product is coagulated and dried (loa). Studies of homogeneous catalytic hydrogenation of NBR to prepare HNBR have been reported in the literature (lob, 1Oc).
4.
GRADES OF NBRANDHNBR
The manufacturers of NBR are listed in the elastomer manual from IISRP(1 1) (Table 2). There are no unified grade numbers for NBR and HNBR as there are for emulsion SBR. Numbering of these elastomers is doneby the supplier. NBRs areclassified by acrylonitrile content, Mooney viscosity, polymerization temperature, stabilizer, third monomer, and appearance of product. In
Table 2 NBRProducers Producer Nippon Zeon Co., Ltd. Zeon Chemicals Europe, Ltd Zeon Chemicals Incorporated Bayer AG Bayer Polymers Bayer Rubber Inc. Buna GmbH DSM Copolymer, Inc DSM Elastomers Europe B.V. EniChem Elastomeri S.r.1. Goodyear Tire & Rubber CO Goodyear Chemicals Europe Industrias Negromex, S.A. de C.V. Japan Synthetic Rubber Korea Kumho Petrochemical Co. Krasnoyarsk SR Plant Co. Nitriflex S A Industria e Comercio PASA S.A Synthetics & Chemicals Ltd. Takeda Chemical Industries Uniroyal Chemical Co., Inc. Zaklady Chemiczne Oswieclm
Japan UK USA Germany France Canada Germany USA Netherlands Italy USA France Mexlco Japan Korea Russia Brazil Argentina India Japan USA Poland
Nipol Breon, Nipol Nipol Perbunan N Krynac Krynac Buna NYsYn NYsYn Europrene N Chemigum Chernigum Emulprene JSR Kosyn Nitriflex N Arnipol Chemaprene Croslene Paracril Ker
788
Hayashi
Table 3 NBR Classification Classification Acrylonitrile content
(%)
Low nitrile Medium nitrile Medium-high nitrile High nitrile Ultra-high nitrile
24 maximum 25-30 31-35 36-42 43 minimum
Source: Ref. 2
addition, HNBRs are classified also by degree of hydrogenation. Degree of hydrogenation indicates iodine value and degree of unsaturation. 4.1
Acrylonitrile Content
Grades of NBR with acrylonitrile content of 15-5396 are available on the market. There is no standardization of acrylonitrile content. However, NBRs are generally categorized as “low,’’ “medium,” “medium high,” “high,” and “ultra high” in acrylonitrile content as shown in Table 3. Oil resistance is determined by acrylonitrile content in NBR. Increasing acrylonitrile contentimproves oil resistance,but causes poorer cold flexibility. Thus there is a trade-off relationshipbetweenoilresistance and coldflexibility. Glass transitiontemperature (Tg) of NBR is estimated by the following equation (3): Tg(”C)
- 85
+ .1.4A
(1)
where A denotes the acrylonitrile content (%). 4.2
Polymerization Temperature
Polymerization temperatures from 5 to 50°C are used for NBR. NBR reacted at higher than 25°C is called “hot NBR,” which provides high tensile and cohesive strength. Generally, hightemperaturepolymerizationgivesa faster reaction, lower degree of polymerization, higher branching, and higher gel, which makes for poorer processability. NBR polymerized at lower than 25°C is called “cold NBR””general1y reacted at 10°C maximum-which features better mixing, extrusion, calendering, processability, and slightly lower physical properties compared with hot NBR. More than 80% of the grades of NBR are manufactured by cold polymerization. 4.3
Mooney Viscosity of NBR and HNBR
NBR is manufactured as a liquid having 3000 molecular weight and as a solid having up to 10” molecular weight andfrom 25 to 140 Mooney viscosity. A product with a high Mooney viscosity provides high tensile strength and low compression set, and is used for high-pressure applications. When combined with high levels of plasticizer, the compounds are useful for fuel devices and ink rolls. NBR compounds with low Mooney viscositiesshow relatively low mechanical properties, but better flowability. Therefore they are suitable for injection molding, calendering, and extrusion with only small amounts of plasticizer.
789
Nitrile and HydrogenatedNitrile Rubber
4.4
Conversion of Polymerization
Conversion of polymerization is decided by the economics of production and requirements for polymer properties. Generally high conversion improves productivity because of the cost of recovering monomers. But high conversion tends to promote self-crosslinking and branching in the polymer, which causes higher swelling in an extrusion process. Conversionof polymerization is commonly from 60 to 90% commercially. Bound acrylonitrile content in NBR is influenced not only by the charge ratio of butadiene and acrylonitrile, but also by final conversion. Because the reaction rates of the two monomers differ. polymer composition is different at different conversions. Figure 2 shows bound acrylonitrile and butadiene content in polymers being formed at different conversions during polymerization. The so-called azeotropic mixtureis at 37% acrylonitrile content, which produces thesame composition of acrylonitrile and butadiene at anyconversion. 4.5
Stabilizers
Stabilizers are added to maintain Mooney viscosity during storage of raw NBR. Seventy-five percent of NBRs contain hindered-phenol a n d o r phosphonium derivatives as nonstaining stabilizers.Slightlystaining NBR contains amine stabilizers such as alkyldiphenylamine,which provide better bin stability and heat resistance. The use of NBR with staining stabilizers is declining. 4.6
Terpolymers
NBR terpolymers arewidely known. Typical terpolymer is called XNBR, which contains carboxylic acid in a side chain from terpolymerized acrylic or methacrylic acid. XNBR features excellent tensile strength and abrasion resistance. Zinc peroxide or surface-treated zinc oxides are used as cure activators of XNBR to improve scorch times (12, 13).
h
h
8
8
v
v
Y
8 E: 8
Y
Q)
3."
Prepared Acrylonitrile content (%)
0
75
( 5 ) 28
(1) 60
l
C
l
l
0
10 20 30
l
Y
l
l
40
50
l
l
l
l
l
60 70 80 90 1 0 0
Conversion (%) Fig. 2 Polymer composition generated at each moment in polymerization (polymerization temperature 5°C).
Hayashi
790
NBIR acrylonitrile-butadiene-isopreneterpolymer is available commercially, which features relatively higher tensile strength and elongation especially for light color compounds. For improved heat resistance of NBR, bound antioxidant NBR with copolymerized amino or phenol function group in the polymer (14) and NBAR acrylonitrile-butadiene-acrylic ester are also on the market (15-17). Self-crosslinked NBR is manufactured by terpolymerizing with multifunctional monomers such as divinyl benzene, ethylene-glycol dimethacrylate, and so on. Its purpose is to improve the dimensional stability of NBR compounds during the extrusion and calendering processes. Usually self-crosslinked NBRs are blended with conventional NBR because of their inferior physical properties. They are also applied as a nonextractable plasticizer, softener, and impact modifier in phenolic resin, epoxy resin, polyvinyl chloride, ABS, and so on. Liquid NBRs terminated or functionalized by hydroxide, amino, orcarboxylic acid groups are also available as hardenersor modifiers for epoxy resin or asplasticizers for NBR to improve solvent crack resistance.
4.7
Physical Form of Products
Standard NBR is a bale or sheet. Crumb type is also used for solution applications. to easily dissolve in solvents for adhesives. PowderNBR is mainly used for blending with thermoplastics such as PVC, EVA, ABS, AS, and phenolic resin. There are two ways to manufacture powder NBR-by mechanically grinding from bale NBR, and by spray-drying directly from NBR latex.
4.8
Polymer Blend and Carbon Wet Masterbatch
NBR is a good oil-resistant elastomer but is not good for ozone resistance because of the double bonds in the polymer backbone.One solution to improve ozoneresistance of NBR is by blending with PVC. which has good compatibility ( 1 8). Seventy parts of NBR blended with 30 parts of PVC is commonly used. There are two methods used to disperse PVC in NBR. One is to latex blend. co-coagulate, and dry. The other is to mechanically blend by means of a Banbury mixer or kneader. The formerprovides better properties because of the fine dispersion of PVC in NBR (1%.
NBR wet carbon black masterbatch is available and is manufactured by co-coagulating NBR latex with acarbonblackdispersion (20). Bale forms of NBRpreblended with DOP plasticizer and liquid NBRs are also available for easy processing and mixing ( I l ) .
4.9
After Reaction of NBR
NBR has double bonds in the main chain so that itschemicalstability,heatresistance,and ozone resistance are limited. To improve the heat resistance of NBR there are a number of ideas to develop highly saturated nitrile elastomers (HSN). Acrylonitrile-ethylene copolymer or terpolymer (NEM) was studied. However, no commercial gradesare on the market. Selective hydrogenation of NBR was developed and commercialized by holding the cyano group in the polymer to maintain oil resistance and hydrogenating C S double bonds, which are the weak point for heat resistance. These polymers were initially abbreviated HSN or NEM, but they are now called HNBR to designate “H” for hydrogenation. Physical properties and processability of HNBR are intluencedby the acrylonitrile content and Mooney viscosity. Additionally. the degree of unsaturation in the polymer is a key property of HNBR, indicated by its iodine value. Manufacturers and grades of HNBR are listed in Tables 4 and 5 (21-25). There are HNBR commercial grades with Mooney viscosity values from 57
791
Nitrile and Hydrogenated Nitrile Rubber Table 4 Location
HNBR ProductionCapacity
Producer
(MTlyr)
yama, Zeon Nippon 1600 Bayer Polysar 1500 Zeon Chemicals
Texas Texas
Source: Refs. 1 I . 24
Table 5 HNBR CommercialGrades Grade Acrylonitrile content 0020 A4555 C4550 1010 1020 1907 B3850 2000 2000L 2010H 2010 2010L 2020 2020L 2030L 1706 1707 1746 1747 1767 31 10 41 10 PBZl23' zsc2295"
(%)
Mooney viscosity" Iodine valueh Unsaturation
49 45 45 44 44 38 38 36 36 36 36 36 36 36 36 34 34 34 34 34 25 17 44 36
ML ( I +4)at 100°C. g. Double bond content. " Trade name: Zeon-Zetpol; Bayer:-Therban, Tornac Not reported. Maxlmum value. F PVC blend. " Zinc-methacrylate alloy. Trade name: Zeoforte. ' ML ( I + 4 ) at 125°C. Source: Refs. 2 1-25. " g/lOO L
L'
'
66 90 90 85 78 80 85 85 65 135 85 58 78 58 58 60 75 60 70 70 85 90 48' 95
(%)"
23
10 1'
10
25
5.5 4 10 1'
2 4 4 II 11 11 28 28 57
e c
15 15 25 28
1 1
4 4 4 10
10 20 1' 1' 4 4 5.5 7 6 10 10
Hayashi
792
to 137 and acrylonitrile content from 17 to 50 wt%. Fully hydrogenated grades are defined as 99% hydrogenation. in which unsaturation is 1% or less, with indicated iodine values up to 4. Partially hydrogenated grades with iodine values between 10 and 28 are widely used. HNBR blended with PVC is used for gasohol, oxidized fuel, or dynamic ozone resistance is required. HNBR blended with zinc methacrylate provides high tensile strength (up to 50 MPa) and is used for high abrasion resistance (26-28).
5.
FORMULATION AND PROCESSING
Compounding technology for NBR resembles NR or SBR except for plasticizers. NBR can be vulcanized by sulfur or peroxide. 5.1
Compounding of NBR
Fine particle carbon black, high structure carbon black, and fine particle silica are useful to make high-strength and abrasion-resistant compounds. In the case of high loadings, soft carbon black, calcium carbonate, clay, and talc are added. Nonstaining antioxidants can be selected from phenols, phosphites, andfor hydroquinoline. To improve heat resistance, a combination of radical catchers such as an amine antioxidant with a hydroperoxide decomposer (i.e., such as imidazole salt) is more useful. Nonextractable antioxidants such as phenylenediamine and metal salts of dithiocarbamate are effective for fuel application. Antiozonants are generally selected from paraffin wax and paraphenylene diamine. In the case of peroxide cure systems, antioxidants and antiozonants can be strong retarders so one must be careful to add a minimum level. Ozone resistance can be obtained through blending techniques with other polymers such as PVC or EPDM. NBR blended with PVC from 15 to SO parts is commonly used not only to
Acrylonitrile content (%)
Fig. 3 Absorption of plasticizer in NBR.
Nitrile and Hydrogenated Nitrile Rubber
793
Table 6 Plasticizer Absorption into HNBR Plasticizer DMP DEP DBP DOP DIDP DOA DOS TOP TCP DBEEA EBO Soltrcu:
Dimethyl phthalate Diethyl phthalate Dibutyl phthalate Dioctyl phthalate Diisodecyl phthalate Dioctyl adipate Dioctyl cebacate Trioctyl phosphate Tricresil phosphate Dibutoxyethoxy ethyl adipate Epoxidized soybean oil
HNBR (AN36%) (phr)
HNBR (AN44%,) ($0
235 254 246 1os S8 36 20 27 215
285 27 1 213 28 12 11 4 4
so
33 4
17
188
Ref. 3 1.
improve ozone resistance, but also oil resistance. Thirty parts is a usual level of PVC. EPDM blends can improve ozone resistance.However, their compatibility and differences in cure speed have to be considered (29). Plasticizer is added to NBR and HNBR to reduce viscosity, control volume swell in fuel and lubricant. and improve low-temperature properties. Compatibility of plasticizer relates to acrylonitrile content of NBR and HNBR. Thesolubility of plasticizer in NBR can be estimated from the solubility parameter (SP value). Selection of plasticizers for NBR is quite similar to that of PVC. To avoid a bleeding problem, which will occur from excess plasticizer in NBR, it is very useful to know the maximum absorption level of a plasticizer in NBR. This can be measured by immersion of cured NBR in plasticizer as shown in Figure 3 and Table 6 (30, 31). Low molecular weight or liquid NBR can provide a nonvolatile, nonextractable, and nonmigration formulation. Curing systems forNBR generally use sulfur, a sulfur donor,or a peroxide. Typical systems are shown in Figure 4 (6, 15). Use of TMTD is limited. TMTD is one of the most popular accelerators to provide fast cure. During vulcanization, TMTD reacts with zinc oxide and forms the zinc salt of dithiocarbamate, which is an antioxidant. However. it is limited in solubility in NBR, and blooming will occur if exceeded. 5.2
Compounding of HNBR
HNBR reacts to filler and plasticizer loadings in almost the same way as NBR, except for the higher physical properties obtained. Mixing, milling, calendering, and molding characteristics are similar to those of NBR or EPDM. As with these polymers, either a peroxide or sulfur-cure system may be employed. For polymers with greater than96% saturation, a peroxide-cure system would be required to provide a good balance of properties. The level of peroxide needed is 4-15 phr, combined with 1-20 phr of either trimethylol propane triacrylate, triallyl isocyanurate, or N,K-m-phenylenedimaleimide. Sulfur-cure or sulfur-donor-cure systems have been commonly used for the 90% hydrogenated grades of HNBR (32-34). 6. STRUCTURE AND PROPERTIES OF NBRANDHNBR Table 7 shows an example of raw polymer characteristics foran NBR having a Mooney viscosity of 50 andacrylonitrilecontent of 35% (18). The characteristicsincludeboundacrylonitrile
Hayashi
794
4 DCP40% 3 MBTS I .5 ZnMW
l
t
0.3 sulfur
t
compressions e 1 cos1 reduction
Heat resistance
t
t
Heal resistance
/ 0.5 TETD
cure
peroxide
r/
4 1
Heat mislance scorch 4 MBTS
1.5 sulfur
fast cure
I
1.5 MBT 1.5 ZnMDC 1.5 sulfur
scorch &bloom
bloom
0.4 TMTD
0.5 sulfur
Fig. 4 Typical NBR curing system.
Table 7 Raw Polymer Characteristics of Medium High NBR ~
~
1. Copolymerization component: Bound acrylonitrile content, average Composition distribution, tolerance 2. Microstructure of butadiene unit (13C-NMR): tmns- 1,4 cis- 1,4 1.2 3. Frequency (13C-NMR): BBB BBA ABA AAA AAB BAB 4. Molecular weight and molecular weight distribution: lql 30°C MEK Mn X 10' (osmometer) MW X 10' (GPC) MwMn (GPC) 5 . Branch x x lo5 Source: Ref. 18
35% 11% 83% 8Yo 9% 20% 53% 27% 4% 6% 90% l .36 69 226 4.1
3.7
dand
Nitrile
795
content, chemical composition distribution, microstructure of the butadiene unit, and branching. 6.1
sequencing,
Structure and Analytical Methods
Qualitative analysis of NBR is measured by infrared spectrum or pyrolysis gas chromatography (PGC). Quantitative analysis of bound acrylonitrile content is determined from nitrogen andor PGC. Microstructure and sequencing analysis requires NMR. Compound formulation of cured NBR can be estimated by thermogravimetric analysis (TGA). The first step in analyzing raw polymer NBRis to removesmall volumes of polymerization ingredients and antioxidant by Soxhlet extraction in methanol solvent or by immersion at high temperature. Solubility of NBR differs with acrylonitrile content. NBR or HNBR is soluble in benzene, toluene, tetrahydrofuran, and acetone and insoluble in water, alcohols, hexane, and so on. Combinations of good solvent and poor solvent can be used to clean up the polymer by thereprecipitationmethod. Figure 5 shows an infraredabsorptionspectrum of NBR, which demonstrates typical peaks at 970 and 910cm" based on truns-l,4-butadiene and 1,2-butadiene and a peakof 2240 cm" determined by stretch and shrinkage vibrations of C-N. Acrylonitrile content can be measured by the ratio of the peak value at 2244 to 970 cm", but generally the Kjeldahl method of nitrogen determination is more precise. An infrared absorption spectrum of HNBR is shown in Figure 6. In comparison to NBR, the peak at 720 cm" from methylene chains increases. Table 8 shows the infrared spectrum of NBR decomposed by pyrolysis (35). The iodine value indicates the degree of hydrogenation of HNBR. It is reported that the degree of hydrogenation can be obtained from NMR and infrared spectrum (36, 37). Numberaverage molecular weight (Mn) is obtained by osmometer and so on; weight-average molecular weight (MW) is measured by the sedimentation equilibrium method, light-scattering method, etc. Viscosity-average molecular weight is traditionally calculated by the equation shown in Table 9, after measuring the intrinsic viscosity. Molecular weight distribution is measured by gel permeation chromatography compared to standard polystyrene. Gel and insoluble material are measured by paper, mesh, or glass filter.
100
90
EO 70
t60 C
2
'B
50 40
l-
30
20 IO
0 4OOO
3200
240
19co
1700
l500
1300
1100
900
700
Wave number (cm')
Fig. 5 Infrared spectrum of NBR (acrylonitrile content 37%, iodine value 280).
796
Hayashi
100
90-
0 4ooo
I
I
1
1
I
I
I
I
I
3200
2400
1900
1700
1500
1300
1100
900
700
Wavenumber (cm')
Fig. 6 Infrared spectrum of HNBR (acrylonitrile content 36%, iodine value 28).
Table 8 Infrared Spectrum of Decomposed NBR by Pyrolysis Wave number (cm")
Group
2260 small 1645- 1603 broad 1450 small 1380 1073 we,& 1045 weak 990 965 small 908 small 890 weak
-
RHCKHZ R H C K H R ' trans R(CH)CSH?
Source: Ref. 35
Table 9 (Y and K Values ofNBR for Estimation of Molecular Weight-from Intrinsic Viscosity [q]" Solvent Toluene Benzene Chloroform Acetone "[q]= k M . Source: Ref. S
(Y
K
0.64 0.55 0.68 0.64
4.9 x 10-4 1.3 X lo-' 5.4 x 10-4 5.0 X lo-'
797
Nitrile and HydrogenatedNitrile Rubber Table 10 NBR Raw Polymer Properties Property Specific gravity Linear expansion coefficient ( X
Value
"C' )
Specific heat (cal/g deg) Adiabatic compressibility ( X 10- " cm'ldyn) Thermal conductivity (kcal/m.h.deg) Glass transition temperature ("C)
H 0.999', MH 0.978', M 0.968' H 150, MH 170, M 175 H 220, M 230, L 240" [below Tg H 73, M 70, L 801 H 0.471 H 35 H 0.220, MH 0.215, M 0.215 H -22, MH -38, M -46, L -56 frequency 1000 cyclelmin
100 10
1 6.6-8.2 X 10' 10~~-10~1 5-6 7-12 20 H 10.36, MH 9.64, M 9.38, L 8.7
Speed of sound ( c d s e c ) Volume resistivity ( O c m ) Power factor (%) Dielectric constant (1000 hertz) Breaking voltage (kVlmm) Solubility parameter
H -20 -36 -45 -48 -39 -24 -26 -27
M
L
-41 -55 -45
-51
H, High nitrile:h MH, medium hlgh nitrile;' M. medium nitrile;d L, low nltrile. Source: Refs. 1. 5 , 11, 38-47.
6.2
Basic Properties
Basic characteristics of NBR are demonstrated in Table 10 (1, 5, 11, 38-47). With increasing acrylonitrile content, glass transition and brittleness temperatures rise as shown in Table 11 (5). Typical properties of NBR are influenced by acrylonitrile content. Other factors are molecular
Table 11 Glass Transition Temperature, Tg, and Brittleness Temperature, BT, of NBR Acrylonitrile (%) 0 20 22 26 29 30 31 33 37 39 40 52 Sortrce: Ref. 5
Tg ("C)
BT ("C)
-
- 80 - 55 - 49.5 - 47 - 46 - 38 - 33 - 26.5 - 23 - 16.5
- 56 - 52 - 52 -
46
- 41 - 43
-37
- -39
-
-26
34
- -33
- 22 - 16
Hayashi
798
Table 12 Factor, Properties, and Characteristics of NBR Factor/property Higher acrylonitrile content: Oil resistance Abrasion resistance Gas impermeability Cold flexibility Rebound Density Hardness Tensile modulus Tensile strength Compression set Heat resistance Heat built up Flowability Extrusion die swell Higher molecular weighthigher Mooney viscosity: Tensile strength Compression set Extrusion die swell Wider molecular distribution Filler loading Extrusion die swell Compared with conventional NBR Terpolymerized crosslinking monomer: Extrusion die swell Compatibility with plasticizer Tensile strength Elongation Terpolymerized with carboxylic acid: Abrasion resistance Friction coefficient Tensile strength Scorch Flowability Blended with PVC Weathering resistance Oil resistance Hardness Cold flexibility Extrusion die swell Higher polymerization temperature Cohesive strength Tensile strength Extrusion die swell powder and particulated compared with conventional bale: Solubility Storage stability Source: Refs. 1-3, 5 , 7, 15.
Characteristics Better; lower volume change Better; less abrasion loss Better; less permeation volume Inferior; higher brittleness temperature Inferior; lower rebound Higher Slightly higher Higher Slightly better; higher Slightly inferior; higher Slightly better; lower properties change Inferior; higher heat built up Slightly better; higher flow rate Slightly better; lower die swell Slightly better; higher Slightly better; lower compression set Slightly better; lower die swell Better Inferior; slightly higher swell
Better; lower and smooth surface Better; higher loading Slightly inferior; lower Lower; lower Better; lower abrasion loss Higher Better; higher Inferior; shorter scorch time Inferior; lower flow rate Better; longer initial time of Ozone crack observed Better; lower volume swell Higher Inferior; higher brittleness temperature Better: lower and smooth surface Better: higher Better; higher Inferior; higher Better; faster solving time Inferior; easily blocking
799
Nitrile and Hydrogenated Nitrile Rubber Table 13 Factor,Properties. and Characteristics of HNBR Factor/propcrty
Characteristics
Hydrogenated NBR compared with NBR: Oxidized fuel resistance Hydrogen sulfide resistance High temperature properties Heat resistance Weathering rcsistance Brittleness temperature Tensile strength Abrasion resistancc Oil resistancc Heat built up Physical relaxation Higher hydrogenation ratio: Better; resistance Heat Weather resistance ; Slightlyrelaxation Physical Tear resistance Abrasion resistance resistanceOil Heat built up
Bctter; lower properties change after immersion Better; lower properties change after immcrsion Better; higher tensile strength and modulus Better; lower properties change after agings Better; longer initial timc of ozonc crack obscrved Bctter; lower Bctter; higher Better; lower abrasion loss Slightly inferior; highcr volume change Slightly inferior; highcr Slightly inferior; higher lower properties changc after agings initial time ofobserved crack ozone higher Better; higher Bctter; abrasion lower loss Slightlychange inferior; volume higher inferior;Slightly higher Better; longer
Sorrrcv: Refs. 89-91,
weight, molecular weight distribution, polymerization temperature, third monomer, and blending with other materials, which influence physical properties as shown in Table 12 (1-3, 5. 7, 15). Table 13 ( 3 2 . 47-50) demonstrates HNBR properties compared with NBR. The glass transitiontemperature (Tg) of HNBR is slightlyhigher than conventional NBR and differs depending upon manufacturing method, catalyst, and microstructure as shown in Figure 7 and
EfAN -30
Tean /+
-50 -
-70
-
l
10
20
30
I 40
Bound acrylonitrile content (%) Fig. 7 Glass transition temperature of NBR, HNBR. and ethylene-acry!onitrilc copolymer (32, 46-50). E/AN; ethylene-acrylonitrile copolymer; Therban, Tean and Zetpol; HNBR.
800
Hayashi
50 45 41 37
34
28
I
I IO'
10'
IO1 Iodine value (g/IOOg)
Fig. 8 The effect of the degree of hydrogenation on Tg of HNBR.
Figure 8 shows the relationship between the Tg of HNBR with acrylonitrile from 18 to 50% and iodine value (51, 52). The Tg of HNBR is strongly influenced by its iodine value. The Tg of NBR before hydrogenationis decided by its acrylonitrile content, but the Tg of HNBR changes during hydrogenation. Below 34% acrylonitrile content, the Tg passes a minimum point and gradually increases through the hydrogenation reaction. Above 34%, the opposite occurs. The Tg of HNBRs on the market is shown in Table 14 (21, 53).
6.3 Unvulcanized NBR Characteristics Characteristics of unvulcanized NBR are estimated by Mooneyviscosity,molecularweight distribution, branching, and gel. Lowering Mooney viscosity improves the flowability and mixing processability. For the same polymer,theMooneyviscosity of thepolymer doesn't always match the Mooney viscosity of the compound because of differences in the molecular weight distribution and branching. When the polymer has a wide molecular weight distribution, the Mooney viscosity of the compound tends to be low.
Table 14 GlassTransitionTemperature Polymer Acrylonitrile content, %, Iodine value, gll00 g Mooney viscosity, ML - ( 1 4). 100°C Glass transition temperature, "C
+
Sowcr: Ret's. 21, 53.
H 1020
MH 2020
3644 24
36 28 78 -31
78
-24
MH 2010
- MH
M
2000
3110
L 4110
36
25
17
11
15 4
15
85
95 -35
85 -29
- 28 -45
90
801
Nitrile and Hydrogenated Nitrile Rubber
6.4 Characteristics of Vulcanized NBR, HNBR Fuel resistance is the most important characteristic of NBR, and it depends on the acrylonitrile content. The effect of higher acrylonitrile content is summarized in Table 15 and leads to the following characteristics:
I. 2. 3. 4. 5.
High tensilestrength,modulus,andhardness Improved fuel resistance (lower volume change in oil or fuels) Poorercoldflexibility Low rebound and poor heat build-up resistance under vibration Highcompressionset
HNBR has inlproved heat resistance and weatherabilitycompared toNBR. HNBR characteristics depend not only on acrylonitrile content but also on hydrogenation degree. Hydrogenation degree is measured by iodine value. Lower iodine value leads to the following characteristics, as shown in Figure 9:
Table 15 PhysicalProperties of NBR
Raw polymer properties: Acrylonitrile content, % Mooney viscosity, ML - ( 1 + 4) Uncured properties: Oscillating disk rhcometer at 160°C T-S, min T-95, min Maximum torquc, dN/m Cured 20 mm at 160°C: Physical properties Tensile strength, MPa Elongation, % 100% Modulus, MPa Hardness, JIS A Tear strength, die C, kN/m Volume change, p/c JIS #l oil. 70 h at 120°C JIS #3 oil, 70 h at 120°C Fuel B, 48 h at 40°C Cold flexibility, German tortional stiffness T-S, "C T-10, "C T- 100, "C Cured 30 min a t 160°C: Rebound, % Goodrich Flexometcr Heat built up. "C Compression set, %, 70 h at 120°C Picco abrasion index
DN401
DN302H
1042
1043
DN003
18
28 80
33 77
41 78
so
82
4 7 39
4 8 42
4
4
10
10
34
4s
4 18 37
16.0 370 2.5 64 42
380 2.9 68 43
19.2 430 2.9 68 47
19.5 440 3.6 72 48
19.6 440 4.1 74 S6
-S
-5 +S
+4
18.0
-2
-4
78
+ 27
+ 43
+ 10
+71
+ 16 + 30
+ 23
+ IS
- 44 - 46 - S2
- 26 - 28 - 34
- 24 - 2s - 29
- 13 - 14 - 19
-S
61
S4
43
23
7
+ 27
+31 16 95
+ 30
+ 32
+ 32
18 100
20 113
26 I S9
+ss
16 89
-6 13
-
Hayashi
802
Compnssion Sec**
(W
50
Cold Flexibility*** Gehman's T-IO
(c)
Oil Resistance**** Volume Change (S)
-30
-25
+30
Tcsr CondiIions: * 1 6 8 hours (11 1sO't: (ASTM D-573) ** 168 hours a1 (ASTM D395 B)
***
****
lsOc
(ASTMD-1053) 70 houn a1 ISOC ( A m D-471)
Fig. 9 Properties vs. iodine value of HNBR (AN 37).
1. Improvedheatresistance 2. High compression set with sulfur cure but no effect with peroxide cure 3. Poor cold flexibility in case of less than 10 iodine value 4. Slightly larger volume change in ASTM #3 oil (54)
Table 16 shows the volume change of rubbers by solvent. Few rubbers have low-volume changes to all oils, solvents, and liquids. Generally speaking, oil resistance means resistance to oils made from petroleum, such as lubricating oil and engine oil. NBR has a low-volume change in these. The solubility parameter (SP value) is used as an index of volume change. NBR hardly swells in gasoline or ASTM oil whose SP value is different from the NBR SP value but swells significantly in MEK whose SP value is close to NBR SP value. Another index for volume change is the aniline point. The lower the aniline point, the larger the volume change (1, 5 , 7, 16, 55). Figure I O shows a balance between oil resistance and cold flexibility. Generally, the more improved oil resistance, the poorer the cold flexibility. Compared at the same volume change, the brittleness temperature of HNBR is lower than that of NBR (56). Fuel resistance of NBR and HNBR depends on acrylonitrile content. NBR and HNBR have been tried for use in fuel containing ethanol or methyl-t-butylether (5758). But the volume change of NBR in fuel containing 15-20% methanol is too large (Fig. 11). Mineral lubricating oil contains addeddispersant, detergent, antioxidant, extreme pressure additive, antiwear agent, and so on. Lubricating oil life is lengthened by these additives, so improved resistance to lubricating oil additives is needed. HNBR has excellent lubricating oil additives resistance and higher tensile strength than other rubbers when they are measured after swelling in oil, as shown in Figure 12 and Table 17 (28, 59). High acrylonitrile content NBR
803
Nitrile and Hydrogenated Nitrile Rubber Table 16 Volume Change of Rubbers by Solvents (%) Temp NBR NBR NBR 38% ("C)33% 28%
Solvent Gasoline ASTM #l oil ASTM #3 oil Diesel oil Olive oil Lard Formaldehyde Ethanol Glycol Diethyl ether Methyl ethyl ketone Trichloromethane Tetrachloromethane Benzene Aniline Phenol Cyclohexanol Silicon oil Distilled water Sea water
I h
NR
10 5.5 3 12 -2
15 -1
50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 100 50
-
10 20 -2 0.5 10 20 0.5 50 250 290 1 10 250 360 450 50 - 1.5 10 2
I
11
3
55 5 65 70 27 30 25 7 2 95 150 380 330 300 125 85 40
6 -2 0.5 5 -2
1
10 20 0.5 30 250 230 75 200 380 470 40 -2
CR
1.5
10 18 0.5 20 250 230 55 160 420 510 25 - 2.5 12 3
I
-1 12 5
60 -
-
2
50-
-
8
f
>"
100 1 10
6 3 0.5 170 85 420 420 350 15
35 55 -2 10 27
IIR
VMQ
CSM
I40 12 130 150 50 50 7 -5 0.5 135 80 400 400 350 30 60 35 - 2.5 25 7
240 20 120 250 10 10 0.5 2 - 0.2 90 15 300 275 150 10 3 7 - 0.5 5 0.5
260 4 40 150 4 4
85 4 65 120 40 45 1.2 5 0.5 85 150 600 350 430 70 80 20 - 0.5 4 0.5
I
8
v
250 60 200 250
SBR
40 -
30O -80
I
I
-20 BrittlenessTemperature -40
I
0
('C )
Test Conditions: *Fuel Cfor 48 hours at 40c Fig. 10 Volume change vs. brittleness temperature:
(e),
HNBR; (+), NBR; (@), ECO.
1 15 1
270 150 300 300 240 7 10 25 30 2 0.5
Hayashi
804
Methanol /Fuel C ratio (%) Fig. 11 Alcohol contained fuel resistance of FKM (+), FMVQ (B), and NBR (A).
has improved gas impermeability,as shown inTable 18 (1,34,60). Table 19 (61) shows solvent impermeability of various rubbers. Figure 13 shows strain-stress curves of NBR measured at various temperatures. At a temperature under the glass transition, the curve is like a resinthat has a yield point.At temperatures over the glass transition, it has a typical rubber-like S curve. At hightemperatures, tensile strength and elongation are low (1, 62).
Fig. 12 Lubricating oil additive resistance tensile strength at break after 168 hours immersed in ASTM #2 oil, including various additives, at 150°C.
nd
805
Nitrile Table 17 Lubricating Oil Additives Used in the Evaluation No. A- 1 B- 1
Type of additive -
c-1 2 3 4 D- 1 2 3 4 E- 1 2 F- 1 G- 1 2 H- 1 2 3 4 5 I- 1 2 3 4
Dispersant
Detergent
Antioxidant Viscosity index improver Antiwear agent
Extreme pressure additive
Additive package
Main chemical composition
Concentration"
Original tensile strength ASTM #2 oil with no additive Polyalkenyl succinimide Polyalkenyl succinimideborate Polyalkenyl succinic ester Polyalkenyl succinimide/succinic ester Calcium sulfonate-Basicity 24 Calcium sulfonate-Basicity 300 Magnesium sulfonate-Basicity 400 Calcium phenate-Basicity 205 Primary dialkyl zinc dithiophosphate Secondary dialkyl zinc dithiophosphate Polyalkyl methacrylate Olefin sulfide for gear oil Olefin sulfide (Technical grade) Dialkyl phosphoric ester Dialkyl phosphoric ester Zinc dibutyl dithiocarbamate Molybdenum compound Lead naphthenate Package #l for gear oil Package #2 for gear oil Package for automobile engine oil Package for ATF (Dexiron IID) oil
-
10 IO 10 10 IO IO IO 10 5 5 10
IO IO 10
2 1
0.3 IO 10 IO 10
10
g/100 cc of ASTM #L oil.
Table 18 Gas Permeability" ofNBR and Other Rubbers Rubber
Temp. ("C)
NR BR SBR NBR 27 % NBR 39% CR
IIR
T I O cm'/sec/atm
25 50 25 50 25 50 25 50 25 50 25 50 25 50 25
H-2
0-2
N-2
37 91 32 77 31 74 12 34 5.4 17
18 47 14 36 13 35 2.9 10.5 0.7
6.1 19
10
28 5.5 17 1.2
3.5 3 IO
1 4 6.2
4.9 14
4.8 14
0.8 3.6 0.18 1.1
0.9 3.5 0.25 1.3
CO-2 IO0 22 1 105 200 94 195 23 68 5 .l 22 19 56 3.9 14 2.4
C H 4
He
24 52 -
16 43 2.4 10 -
2.5 IO -
17 42 9.3 23 5.2 14 6 -
6.4 17
Hayashi
806
Table 19 Effect of Temperature on Various Rubbers Solubility of liquid in rubber (rnLlrnL) Test liquid ("C)
and temp. Permeability (kghm')
Specific permeability days (kg.m/h.rn')14
1 day
STYRENE RUBBER Di-isobutylene 25.0 54.4 82.2 SR-6 25.0 54.4 82.2 Methyl ethyl ketone 25 .O 54.4 82.2 Benzene 25.0 54.4 82.2 Ethyl acetate 25.0 54.4 82.2 Methyl alcohol 25.0 54.4 82.2 Carbon tetrachloride 25.0 54.4 82.2
9.12E 1.36E 1.60E
+ 00
+ 01 + 01 3.51E + 01 5.59E + 01 7.18E + 01 l.lOE + 01 1.95E + 01 3.12E 01
+
6.98E - 01 1.00E + 00 1.17E + 00
I .42 1S O 1.67
1.38 1.48 2.28
+ 00
+ 00 + 00
2.40 2.37 2.47
2.37 2.40 3.82
8.15E - 01 1.36E + 00 2.24E + 00
0.88 0.99 1.10
0.85 0.98 1.19
2.568 3.97E 5.458
5.358 9.04E 1.02E
+ 01
3.938 6.04E 7.71E
+ 00
+ 00 + 00
3.05 2.9 1 2.89
2.88 3.02 3.63
1.21E 2.538 3S7E
+ 01
+ 01
+ 01
9.38E - 01 1.95E + 00 2.72E + 00
0.93 1.02 1.11
0.88 1.oo 1.29
5.85E - 02 6.47E - 01 1.30E 00
4.31E - 03 4.62E - 02 9.73E - 02
0.02 0.05
0.05 0.16
-
-
+ 00 + 00 + 00
2.36 3.33 3.36
3.20 3.37 4.73
+ 01 + 02
+
3.34E 5.258 7.45E
+ 01 + 01
+ 01
2.38E 3.97E 5.70E
PARACRIL 18 Di-isobutylene 25.0 54.4 82.2 SR-6 25.0 54.4 82.2 Methyl ethyl ketone 25.0 54.4 82.2 Benzene 25.0 54.4 82.2
2.24E - 01 9.31E - 01 2.61E + 00
2.398 - 02 6.79E - 02 1.96E - 01
0.30 0.43 0.49
0.34 0.69 0.44
7.12E 1.69E 2.62E
+ 00 + 01 + 01
5.40E - 01 1.22E + 00 1.91E + 00
1.07 I .09 1.20
1.03
3.75E 5.73E 8.30E
+ 01 + 01 + 01
6.888 1.09E 1.52E
+ 00 + 01 + 01
2.32 1.89 2.02
2.04 2.07 2.74
4.41E 5.73E 1.02E
+ 01 + 01
8.01E 1.22E 1.84E
+ 00 + 01
2.43 2.40 2.36
2.41 2.44 2.64
+ 02
+ 01
?
1.29
kghm’)
807
Nitrile and Hydrogenated Nitrile Rubber Table 19 Continued
Solubility of liquid in rubber (mL/mL) Test liquid and temp. (“C) Ethyl acetate 25.0 54.4 82.2 Methyl alcohol 25.0 54.4 82.2 Carbon tetrachloride 25.0 54.4 82.2
Permeability
2.34E 3.62E 4.488
+ 01 + 01 + 01
2.668 - 01 1.96E 00 7.97E 00
+ + 1.08E + 01 2.03E + 01 2.63E + 01
Specific permeability (kg,m/h,m’)
1 day
14 days
1.65 1S 9 I .60
1.62 1.65 1.81
4.978 - 02 3.58E - 01 1.43E 00
0.14 0.18
0.14 0.15
-
-
+ 00 + 00 + 00
1.72 1.65 1.63
l .66 1.64 1.86
4.49E 6.78E 8.46E
+ 00 + 00 + 00 +
2.04E 3.94E 4.84E
PARACRIL 35 Di-isobutylene 25.0 54.4 82.2 SR-6 25.0 54.4 82.2 Methyl ethyl ketone 25.0 54.4 82.2 Benzene 25.0 54.4 82.2 Ethyl acetate 25.0 54.4 82.2 Methyl alcohol 25.0 54.4 82.2 Carbon tetrachloride 25.0 54.4 82.2
Negligible 6.48E - 02 9.66E - 01
Negligible 1.29E - 02 1.71E - 01
0.07 0.22 0.27
0.18 0.20 0.23
2.58E 4.63E 6.59E
+ 00 + 00 + 00
4.91E - 01 8.72E - 01 1.19E 00
0.68 0.70
0.66 0.70
-
-
3.92E 5.48E 7.07E
+ 01
+ 00 + 01 + 01 6.56E + 00 8.53E + 00 1.11E + 01
2.39 2.42 2.61
2.5 1 2.70 3.38
2.22 2.16 2.14
2.15 2.18 2.41
1.73 1.63 1.61
1.69 1.70 1.92
9.66E - 02 1.74E - 01 3.59E - 01
0.18 0.23
0.17 0.22
2.40E - 01 4.08E - 01 7.55E - 01
1.06 1.06 1.09
1.03 1.04 1.11
0.02 0.05 0.08
0.04 0.06
+ 01 + 01 3.55E + 01 1.51E + 01 5.87E + 01 2.21E + 01 3.03E + 01 3.85E + 01 1.31E + 00 2.31E + 00 4.92E + 00 3.41E + 00 5.75E + 00 1.06E + 01
+
7.56E 1.03E 1.31E
1.64E 2.17E 2.77E
+ 00 + 00 + 00
THIOKOL Di-isobutylene 25.0 54.4 82.2
Negligible Negligible Negligible
Negligible Negligible Negligible
.l
(continued)
Hayashi
808
Table 19 Continued Solubility of liquid in rubber (mL/mL) Test liquid and temp. Permeability (“C) SR-6 25.0 54.4 82.2 Methyl ethyl ketone 25.0 54.4 82.2 Benzene 25.0 54.4 82.2 Ethyl acetate 25.0 54.4 82.2 Methyl alcohol 25.0 54.4 82.2 Carbon tetrachloride 25.0 54.4 82.2
(kghd)
Specific permeability (kgmhm’)
1 day
14 days
Negligible 4.87E - 01 1.31E 00
Negligible 5.14E - 02 1.04E - 01
0.16 0.22 0.27
0.19 0.22 0.37
+ 00 + 01
3.32E - 01 5.378 - 01 8.88E - 01
0.60 0.65 0.77
0.61 0.70 0.77
8.60E + 00 1.60E + 01 2.92E 01
6.50E - 01 1.20E + 00 2.17E 00
1.22 1.29 1.46
1.25 1.41 2.3 1
+ 00 + 00
+ 00
1.64E - 01 4.25E - 01 6.27E - 01
0.48 0.50 0.53
0.48 0.52 0.58
Negligible 4.45E - 01 2.14E 00
Negligible 3.26E - 02 1.55E - 01
0.06 0.09
0.07 0.09
-
-
6.93E - 01 1.95E 00 3.56E 00
1.25E - 01 3.94E - 01 7.14E - 01
0.36 0.64 0.75
0.54 0.65
+ 4.40F + 00 5.28E 1.22E
+
2.11E 5.76E 8.25E
+
+ +
+
1
NEOPRENE Di-isobutylene 25.0 54.4 82.2 SR-6 25.0 54.4 82.2 Methyl ethyl ketone 25.0 54.4 82.2 Benzene 25.0 54.4 82.2
Negligible l.lOE + 00 2.23E + 00
Negligible 2.39E - 01 4.68E - 01
0. I O 0.48 0.57
0.25 0.47 0.57
7.48E + 00 1.25E + 01 1.72E 01
1.66E 2.78E 3.33E
+ 00 + 00
1S 4 1.84 2.06
1S 9 1.90 2.41
1.39 1.41 1.71
1.42 1.52 2.02
2.94 3.59 3.61
2.98 3.75 4.12
1.06E 1.55E 2.31E
+ 01 + 01 + 01
+ 00 2.44E + 00 3.91E + 00 5.33E + 00
2.61E 4.79E 6.19E
+ 01 + 01 + 01
5.85E + 00 1.03E + 01 1.41E + 01
+
809
Nitrile and Hydrogenated Nitrile Rubber Table 19 Continued ~
~~~
~
Solubility of liquid in rubber (mL/mL) Test liquid and temp.
Permeability (kghm’ )
(“C)
Ethyl acetate 25.0 54.4 82.2 Methyl alcohol 25 .O 54.4 82.2 Carbon tetrachloride 25.0 54.4 82.2
Specific permeability (kg.m/hm’)
1 day
14 days
+ 00 + 01
+ 00 + 00 + 00
I .25
+ 01
1.31E 3.26E 2.85E
1.15 1.24
1.20 1.16 1.27
1.21E - 01
2.87E
-
0.02 0.09
0.04 0.26
-
-
3.09 3.20 3.24
3.44 3.69 3.41
5.628 1.43E 1.27E
h
h
h
h
+
1.75E 01 2.40E + 01 h
3.62E 5.13E
02
+ 00
+ 00
h
Samples dissolved. be measured due to leakage.
” Could not
- 30°C
40
0°C
0
100
200
300
Elongation (%) Fig. 13 NBR stress-strain curves at various temperature.
400 600
500
Hayashi
810
ACM Peroxide Cured HNBR Sulfur Cured H N B R NBR CR 10’
10’
I 0‘
Service Life* (hour) *Service life based on the time to lose 80% of elongationafter aging in air.
Fig. 14 Servicetemperature and service life.
There are some indexes for the heat resistance, tensile strength, elongation, the product of strength and elongation, and compression setof rubber. These aredifferent for different kinds of rubber product (18, 63, 64). For example, we used the temperature at which the change in elongation is 80% after 1000 hours. For CR cured by ethylene thiourea (ETU), the temperature change is 100°C. NBR cured by sulfur is 106°C. HNBR cured by sulfur is 126°C. HNBR cured by peroxide is 150°C. ACM cured by ammonium benzoate is 159°C as shown in Figure 14 (65).
7. APPLICATIONS OF NBRANDHNBR NBR, XNBR and HNBR are widely used in industrial products for their oil resistance, solvent resistance, and chemical resistance. Their function is sealing and delivering oil, fuel, water, and chemicals in the automotive, aerospace, chemical, food,machinery, oil-drilling, marine, railroad, textile, and printing industries (2, 7). 7.1
AutomotiveUses
NBR and HNBR are mainly used for automobiles. Automotive elastomersare classified by type of heat resistance and class of oil resistance (66,67). Figure 15 demonstrates the elastomers classified by the Society of Rubber Specification Committee (CARS)in the Society of Automotive Engineers (SAE), adding some candidate elastomers that might be registered in the near future (22,66-71). NBR is classified BF, BG, BK at 100°C application and CH at 125°C. HNBR is classified as a DH and will be proposed to be DK.
7.2 Hoses and Tubes NBR is typically used as the liner in hoses reinforced by fabrics or steel wire and covered by metal, textile,or weather-resistant rubbers. NBRis used for fuel,transmission, brake, and steering
811
Nitrile and Hydrogenated Nitrile Rubber Volume Swell(%)in ASTM # 3 Oil ~ ~ ' 1 4120 0 l o o 80 60 40 30 20 IO I
I
I
I
I
I
I
I
I
I
J -
n-
e
'G
x
2
-
FE-
U)
3 c4
c-
Sm
B A
-
mMQ
- 225 - 200 - l75
ACM HSN
D -
-.a
- 250
VMQ
EPDM GPCO
€CO
CR
NBR
I1R
- 275
FKM
-
- 125
- loo -
SBR NR I
I
I
I
A
B
C
D
I
E
I
F
I
1
G
H
I
I
p
I
70
A
K
Oil Resistance (Class)
* No Rquircmcnt Fig. 15 Classification on SAE J 200.
hoses for automobiles. In industrial applications, hydraulic hose; delivery hose for crude oil, heavy oil, fuels, acid,and alkali; and dairy hoses are also made of NBR. HNBR hasbeen adopted for fuel hose, as shown in Figure 16 (72). HBNR has also been used for power steering and air conditioning hoses for longer life (59. 73, 74). 7.3 Seals,Packing,and O-rings
Various elastomers are used for sealing of oils, water, fuel, and chemicals. One of the main applications of NBR and HNBR is in seals and gasketsbecause of theirresistance to oils, lubricants, and greases (2, 7, 58). Typical specifications for seals and O-rings in Japan are listed on JIS B2401-1 grades A and B, B2402 grade B of seals and JIS K6380 of industrial packing BI, BII, BIII materials (75-77). HNBR is used in high-pressure and heat-resistance applications (9). 7.4
Rolls
NBR covered rolls are widely used in the paper, dyeing, textile, fabric, leather, steel, printing, chemical, and polymer processing industries (7). NBR rolls are resistant to oils, surface-active agents, dyes, ink, solvent, acid, alkali, and so on. Polyurethane rubber is widely used for highload roller applications.However, HNBRand HNBR blended with zinc methacrylate are recently being used in heavy-duty roller applications such as paper, steel, and textile rollers because of their long life at high temperature (28, 59). 7.5 Belts NBR and XNBR are widely used in conveyor belts carrying ore, coke, sand, and oil sand and in flat belts for conveying paper moneyor train tickets. NBR blended with PVC is often employed for textile and food applications requiring ozone resistance (7). Synchronous belts, which trans-
812
Hayashi
Immersion Time (day)
Conditions Immersion Media : Fuel B containing 1% lauroyl peroxide. It was replaced twice a week. Immersion Temperature : 60%
Fig. 16 Oxidized fuel resistance: (O), HNBR 1020; (+), NBR; ( X ) , ECO.
Ho
t 0
5
10
I5
20
Vehicle Running Distance (1000km) Fig. 17 Belt running test.
25
30
813
Nitrile and Hydrogenated Nitrile Rubber
Tensile Strength in Liquid Phase
HNBR 1020
Conditions Gas:
T
W
-A-
EPDM
-A-
FKM-GFFKM-E -INBR
0
I
I
I
24
72
I68
XNBR
-+-
-*-
HP CO:
5% 20% "%I
Liquid:
oil
9-55 4% AmineB 1% 150C 6.9MPa
HX)
Exposure Time(hours)
Fig. 18 Sour cnvironmcnt test for tensile strength
in liquid phase.
mit power from the crankshaft to camshaft in automobiles, are mainly made of HNBR having well-balanced properties of high modulus at high temperature, abrasion resistance. flexibility, and oil resistance, as shown in Figure 17 (74. 78. 79). 7.6
MiscellaneousApplications
Nitrile rubbers including NBR, XNBR. and HNBR are widely used for products such as pump valves, diaphragms, bushings, grommets, adhesives (80, 81), brake shoes (82) clutch facings sponges, isolation insulators. (83) cables (84) and so on. Oil-drilling devices such as blowout preventors, downhole packers, drill pipe protectors, snake pumps, accumulators, pump stators, and rotary drilling hoses are made of conventional NBR. HNBR is applied in products that require resistance to hydrogen sulfide, steam, methane gas at high pressure, corrosion inhibition in deep wells, and extrusion resistance.Figure 18 shows that HNBR demonstrates good resistance to sulfidemixtures(85-87). Examples of HNBRlatexapplications are gaskets.paints and adhesives to metal, ceramics, and textiles (80, 88).
REFERENCES 1. Komuro K. (1973), Gorrru Kogpo Bi/rrm, 3rd ed., The Society of Rubber Industry. 2. Komuro, K. Todani, Y. and Matsukawa, J. (1976), Gouseigorrlu Kctkougijutsu Zensyo Nitorirugnrw
Taiseisya. 3. Scil. D. A. and Wolf, F. R. ( 1987), in Rubher Tc.c/rr~o/ogy. 3rd ed. (M. Morton, Ed.), Van Nostrand Reinhold. Ncw York; p. 22. 4. ASTM D-22 Sub-Commlttee Mceting, 1990. S . Hofman, W. ( 1964), Kuhher Chern. Techlo/. 3 7 1. 6. JPX8-35641B, 88-35642B, 86-41922B.
814
Hayashi
7. Furuyo. M. and Komuro, K. (1963, Application c u d Processirrg of NER. 1963. 8. Uchida, I. (1972), J. Soc. Rubber Ind. J p . 45:1057. 9. Sugli, N. (1990), J. Soc. Rubber Ir~d.Jpn. (Nihorl Cornu Kyokaishi) 63322. lOa.Kubo, Y . Proc. 1111.Rubber Conf. Kyoto, Japan, Oct. 15-18 1985, p. 32. I0b.Bhattacharya. S . Avasthi B. N. and Bhowmick, A. K. (l992), J. Polwrl. Sci.-Po/vrn. Clzern. 32471. 10c.Bhattacharya. S. Avasthi, B. N. and Bhowmick, A. K. (1991), h r l . B i g . Chenl. 30:1086. 1 1. International Institute of Synthetic Rubber Producers (1992). TISRP Elustorrwr Marzucrl. 12. Urabe, S. (1988), Polymer Digest ( T o k y ) 40:65. 13. Weir, J. and Burkey, R. C. (1989), Rubber Plasr. News. Feb. 16. 14. Meyer, G. E. Kavchol, R. W. and Naples, F. J. (1993), Rubber Clwrr~.Tuchrlol. 46:106. 15. Kotani, T. and Teramoto, T. (1980), J. Soc. Ruhher I t d . J p . 53:350. 16. Mori, Y. and Nishihata, S. (1985). J. Soc. Rubber I r d . Jpn. 58:158. 17. Asai, H. (1985). J. Soc. Rubber- I r d . J p . 58:133 (1985). 18. Inoue, T. and Takemura, Y. (1979), in Kouseirm Errrsutorrw r ~ oK r d ~ r t u(J. Furukawa, Ed.), Taiseisya, p. 167. 19. Abrams, W. J. (1962), Rubber Age 91:255. 20. Zeon Chcmicals Europe, Technical Report. 21. Ishihara, M. Toya, T. Hayashi, S. and Oyama M. (lO91), TheSociety of RubberIndustry,Japan, Erasutoma Touronkai. Dec. 5 . 22. Hashimoto, K. (l992), The Society of Rubber Industry, Japan, Haiteku Semina, Tokyo, Feb. 18. 23. Zetpol Brochure, Nippon Zeon. 24. Bayer Japan. Technical Report. 25. Zeoforte Brochure. Nippon Zeon. 26. Klingender, R. C., Oyama, M. and Saito, Y. (1989). paper no. 62 presented at a meeting of the Rubber Division, American Chemical Society, Mexlco City. Mexico, May 9-12, 1989; abstract in R14bher Cherrz. Techrlol. 62:77. 27a.Klingender, R. C. (1990). Rubber Roller Group Meeting, Ncw Orleans, LA, Jan. 31. 27b.Thawamani, P. and Amie, A. Bhowmick, L. (1992). Ruhher Clfern. Techrzol. 65:31. 28. Nishimura, K. (1991). Pol$le 2851. 29. Dunn, J. R. Coulthard, D. C. and Pfisterer H. A. (1978). Rubher Cl~err~. Techrlol. 51:38. 30. Fukuda, H. Nippon Zeon Technical Report. 3 1 , Zetpol Technical Note, Nippon Zeon Technical Report. 32. Hashimoto, K. and Todani, Y (l988), in Hcrrrdhook qfE/cl.storrwr.s (Bhowmick. A. K. and Stephens, H. L. Eds.), Mercel Decker Inc., New York. 33. Oyama, M. and Watanabe. N. (1984), Polvtrz. Frierds 22:202. 34. Zetpol Technical Report, Nippon Zeon. 35. Takeuchi, T. and Murase. H. (1965), Koukrrshi 68:2505. 36. Kondo. A. Ohtani, H. Kasuge, Y. Tsuge, S. Kubo, T. Asada, N. Inuki, H. and Yoshioka, A. (1988). Mcrcrornolecules 21:29 1. 37. Marchall, A. J. Jobe, D. T. and Taylor, C. (1990). Rubber 63:244. 38. Wood, L. A. et a l . (1943), Rubber 16:244. 39. Rebizova, V. G. Bartenev, G. M. and Kosenkova, A. S . (1966), Sol'. Ruhher Tdzrzol. 25( 11):20. 40. Bekkedhl. N. and Scott, R. B. ( 1943). Ruhher Clleru. Techrlo/. 16:310. 41. Copeland, L. E. (1983). J. Appl. Plys. 19445. 42. Tagcr, A. A. et a l . (1988). Colloid J. USSR 17373. 43. Moson, P. (l9S9), Trms. Frtmcloy Soc. 55:146. 44. Marris, R. E. James, R. R. and Guyton, C. W. (1956). Ruhhur Age 78:725. 45. Nishizawa. H. (1973), J. Soc. Rubber I t d . Jpu. 46:688. 46. Scheehan, C. J. and Bisso, A. L. (1966), Ruhher Clfern. Teclmol. 39149. 47. Ushold. R. E. and Finlay, J. B. (1974). Appl. Polvn~.Svrrfp. 25:205. 48. Oppclt, 0. Schuster, H. Thormer, J. and Brdcn, R. (1977), DOS, 2539132. 49. Weinstcin. A. H. (1984), Ruhber 57203. 50. Hashimoto. K. Watanabe, Oyama, M. and Todani, Y. (1985), paper presented at a meeting of the Rubber Division. American Chemical Society, Los Angeles, CA, April 24.
Nitrile and Hydrogenated Nitrile Rubber
815
Hayashi, S. Sakakida, H. Oyama, M. and Nakagawa, T. (1991), Rubber 64:534. Kubo, Y. (1990), S e k i y t Gtrkknishi 33:193. Nippon Zeon, Technical Report. Hashimoto, K. Watanabc. N. Oyama, M. and Todani, (1984). Y. Swedish Institution of Rubber Tcchnology Gothenburg, Sweden, May 17- 18. 5 5 . Maeda, M. (1960). Po/wr. Friends 3:383. 56. Kubo, Y. (1986), J. Soc Rubber 6 1 d . Jprr. 69:442. 57. Nippon Zeon (1981). Taiyugomu no Atarashiitenkai, Kobunshi Gijutsu Kenkyukai, March 18. 58. Matsuda, J. (l985), J. Soc. Ruhhrr Ind. J ~ I58: . 148. 59. Klingender, R. C. Watanabc, N. Hashimoto, K. and Oyama, M. (l989), paper no. 38 presented at a meeting of the Rubber Division, American Chemical Society, Cincinatti, Ohio, Oct. 18-21, 1988; abstract in Rubber Cherll. Techrlol. 62:165. 60. Amcrongen, G. J. ( 1950), J. Polvnl. Sci. 5:307. 61. Mueller, W. J. (1957), Rrthher Agr 81:982. 62. Ecker,R. (1960), Kmttsch. Gummi. 297. 63. Walter, G. (1976). Rubber 49:775. 64. Maeda, A. and Hashimoto, K. (1981). Taiyugomu no Taikyusci, Kobunshi Gijutsu Kcnkyukai. 65. Hashimoto, K. Watanabe, N. Oyama, M. and Todant, Y. (1984). Swedish Rubber Conf., May 21. 66. SAEHandbook, 1990. 67. HS K 6403, 1981. Y. (1988),SAE Tech. Paper880026. 68. Hashimoto, K. Oyama, M. Watanabe, N. Komatsu, K. and Todani, 69. Sugimoto, T. (1986). Polvjile 23:33. 70. Macda, A. Hashimoto, K. and Inagami, M. (1987). SAE Tech. Paper 870194. 71. Eggers, R. E. (1991). Rubber World 204(3):24. 72. Fukushima, H. and Oyama, M. (1983), Polyrrl. Frier~ds2/:487. . 10):650. 73. Tsuzuki, Y. (1989). J. Soc. Rubber I n d . J ~ H62( 74. JP84- 184235A. 75. JIS B 2401. 76. JIS B 2402. 77. JIS K 6380. 78. Hashimoto, K. Oyama, M. Watanabe, N. and Todani, Y. (1986). paper No. 5 presented at a meeting of the Rubber Division. American Chemical Society, Clevcland Ohto, October 1-4. 1985; abstract in Rubher Cllerrr. Techno/.59:16 1. 79. Bradford, W. G. and Klingender, R. C. (1991). Elostomerics 123(8):10. 80. Nakao, K. (1987), Polj$/e 42:16. 81. Sato. M. (1987). J. Soc. Rubber I d . Jprl. 60:69. 82. Bahnhousc, J. (1990). Zcon Chem. Inc., unpublished. 83. Iwasaki, Y. Ohyama, T. and Hashimoto, K. (1992), paper no. 30 presentcd at ;I meeting of the Rubber Division, Amcrican Chemical Society, Lounsville, KY, May 19-22. 1992; abstract in Rubber Clwnl. Tec.lrrlo/.65:856. 84. Thormcr, J. Mirza, J. and Buding, H. (1984), PR1 Ruhhrr Cor$, Eirr1~inghm,UK, March 12-15. 85. Hashimoto, K, Watanabe, N. and Yoshioka, A. (1983), paper presented at a meeting of the Rubber Division, American Chemical Society, Houston, TX, October 25-29. 86. Watanabe,N.Kyker, G. S. andHashimoto,K. (1989), paperno. 3 presented at ;I meetingof the Rubber Division, Amcrican Chemical Society, Dallas. TX, April 19-22, abstract in Rubber Cllenl. Techno/.61:717. 87. Klingender, R. C. Hashimoto, K. Kubo, Y. Oyama, M. Todani, Y. and Watanabe, N. (]%g), Energy Rubber Group, Dallas, TX. 88. Kubo, Y. Mori, 0. Ohura, K. and Hisaki, H. (1990). paper no. 29 presented at a meeting of the Rubber Division. American Chcmical Society, Washington DC, Oct. 9-12. 89. Komuro, K. and Fukushima, H. (1983), Eng. Mttter. 31: 104. 90. Todani, Y. and Fukushima, H. (1988), Intern. Cornbust. Etlgirle 23:25. 91. Mirza, J. Leibbrandt, F. and Thoermer, J. (1987). SAE Tech. Paper 870193.
5 1. 52. 53. 54.
This Page Intentionally Left Blank
33 Diene-Based Elastomers judit E. Puskas University of Western Ontario, London, Ontario, Canada
1. INTRODUCTION Diene-based elastomers comprise the majority of the over 15 million tons of total elastomer consumption in the world. Approximately 35% of the total global elastomer consumption is natural rubber (NR), while the remaining 65% is synthetic rubber (SR). The major application of elastomers is in car tires-60% of the global rubber consumption represent. tire manufacturing. The building block of natural rubber is isoprene, a conjugated diene:
H
H
The driving force for the development of SR was the supply shortage World Wars. The first SR was derived from 2,3-dimethyl-butadiene:
CH,
I
of NR during the two
H
I
The building blockor monomer,polymerized. becomes theelastomer. The elastomer willbecome a crosslinked rubber by incorporating the polymer chains into a network structure. In this socalled curing or vulcanizing process the double bonds of the polymer chain serve as curing sites. In diene-based elastomers each repeat unit has a potential curing site; in diene-containing copolymer elastomers such as styrene-butadiene, the diene sequences will provide the potential crosslinkingsites. The crosslinkdensity will be determined by the fraction of thesites that actually participated in the crosslinking process. In the literature the terms “elastomer” and “rubber” are often used interchangeably. The first tires produced commerciallyfrom poly(2,3-butadiene) were used by the Emperor Wilhelm I1 of Germany on his car. The properties of this rubber did not measure up to NR (for 817
Puskas
818
instance. the maximum speed the emperor could drive his car was 40 k m h , due to excessive heat generation), which further inspired the search for a possible synthetic replacement of NR. During World War 11, SBR (styrene-butadiene copolymer rubber) wasdeveloped. Today, there are more than 20 different kinds of elastomers, most developed after World War 11. It is interesting, however, that no human-made elastomer canmatch the combination of properties NR gives us, and the exact chemical structure of NR is still unknown. The fieldof diene-based elastomers isvery large. Several reviews and encyclopedia chapters have been published on this subject. The science and technology of elastomers and tire manufacturing are perceived to be mature, with incremental developments. However, we have witnessed major changes in the past decade ortwo. These changeswere driven by environmental concerns and restrictions (e.g., green tire, emission control in the polymerization processes), demands for tire performance improvements, and, last but not least, advances in polymer and catalyst synthesis (Quirk et al., 1996; Taube and Sylvester, 1996a). The versatility of the living anionic polymerization process (Szwarc, 1956; Hshieh and Quirk, 1996) continues to open up new synthetic routes to custom-made polymers. The development of new living polymerization processes [cationic (Kennedy et al., 1990; Kaszas et al., 1990, 1992) and radical (Georges et al., 1993, 1994)] opens up entirely new avenues for elastomer synthesis. This chapter will give an overview and update on the latest developments in the field of diene-based elastomers. The high-volume commodity elastomers-butadiene (BR), styrene-butadiene (SBR), and isoprene (IR) rubbers-will be discussed in more detail. Butyl (IIR) will be covered briefly. EPDM and NBR rubbers and block copolymers (polybutadiene-polystyrene,polyisoprene-polystyrene, and polyisobutylene-polystyrene)will be covered in separate chapters.
2.
2.1
DIENE-BASEDELASTOMERS Butadiene Rubbers
Butadiene rubbers are made of 1,3-butadiene, a conjugated diene:
H
I
I
H
When this diene is polymerized, a polymer containing double bonds in its main chain is formed. The configuration of the double bonds and the substituents attached to the double bond can be cis-1,4 (A), trans-1,4 (B), and vinyl (or 1,2 incorporation, C): H H H H
\ / H
/c=c
A
\
-C
!/
\ / H
/c=c\
H
C H\q H
B
-c
/"H H
C
Diene-Based Elastomers
a19
The vinyl incorporation, C, can be atactic, with random D and L spatial configuration, or with regulated spatial configuration. The isotactic structure has the vinyl groups all in either D or L configuration,whilethesyndiotacticstructurehasalternating D and L configuration. The microstructure has a profound effect on the physical properties of the polymer. The manufacturing conditions will determine the microstructure of the BR (Stephens, 1989). High-cis, hightrans, and syndiotactic polybutadiene are produced by Ziegler-Natta (ionic-coordination) polymerization, while radical and anionic polymerization producemixed microstructures. In general, polybutadienes are linear polymers, but recent advances in analytical techniques have revealed that some polybutadienes, believed to be linear, contain branched fractions. A perfectly linear, perfectly1,4-polybutadienewasproduced in the laboratory by acyclic diene metathesis (ADMET) polymerization (Ne1 et al., 1989). Branching is considered to be an advantage; longchain branching is believed to improve processability (Kozlov et al., 1991). High-cis Polybutadienes High-cis polybutadiene is a very important commodity elastomer, mostly used in tire manufacturing. It is also used as the rubber component in the manufacture of high impact polystyrene (HIPS) and technical goods (including golf balls). The glass transition temperature, T,, of cis1,4-polybutadiene is low and varies with the microstructure. High-cis polybutadiene contains about 93-98% cis-1,4 structures, with T, reported in the - 103 to - 109°C range, depending on the trans-1,4 and vinyl content (Laflair, 1993). Other reported values are - 106°C for pure cis (Bahary et al., 1967), - 102°C for “high” (not specified)-cis-1,4 (Trick, 1960), and -95°C for 98-9976 cis-1,4-polybutadiene (Baccaredda, et al., 1960).High-cis polybutadiene has good low-temperature properties, high resilience and low hysteresis, and good tear strength and abrasionresistance. It crystallizesuponstretching,givingthepolymerhightensilestrength; the melting point, T,,,, of the crystalline domains is 2°C (Stephens, 1989). While the tensile properties of this elastomer are good, it has a lower “green strength” (strength of the unvulcanized elastomer) and tack than NR. Depending on the manufacturing process, high-cis polybutadienes contain more or less branched fractions. The occurrence of branching has been verified by size exclusion chromatography (SEC) coupled with viscometry and sedimentation (Kozlov et al., 1991). While it is not easy to identify the presence of small branched fractions or the degree and nature of branching, the effects of branching can easily be observed by dynamic mechanical and stress relaxation tests. It is suggested that branched polymers areeasier to process, but linear polymers havebetter mechanical properties. Long-chain branching reduces cold flow or “creep” characteristic of highlylinearpolymerssuch as high-cis polybutadienes. The occurrence of branching during polymerization and the structure-property relationships in branched polymers are not fully understood, and presently there is a great interest in researching this area (Gnanou, 1996; Fetters et al., 1996). High-cis polybutadiene is produced by Ziegler-Natta catalysis in a solution process. Commercially important catalysts are based on cobalt, titanium, nickel, and neodymium (lanthanide) compounds (Laflair, 1993). Cobalt-based catalysts produce polymers with varying degrees of branching and a medium molecular weight distribution. The titanium-catalyzed polymers also have medium molecular weight distribution and a lower degree of branching. The nickel-catalyzed elastomers have a broad distribution and higher degree of branching. It was reported that at high conversion nickel-catalyzed polybutadienes exhibit bimodal molecular weight distribution (Schroeder et al., 1992). The neodymium-catalyzed polymers have very broad distribution and are highly linear. These polymers have a good balance of properties, with the exception of high cold flow and poor extrusion.
820
Puskas
High-trans Polybutudienes High-trans polybutadiene has been of relatively little practical importance due to its high crystallinity. However, it has been used as a blend with NR, polyisoprene, and styrene-butadiene rubber (SBR) fortire building (Haynes, 1988). It has two melting transitions, with valuesdepending on the trans content (97 and 145°C for 99-100% and 50 and 175°C for 90-99%) (Dreyfuss, 1996). The T, was reported to be - 107°C for 100% trans (Bahary et al., 1967) and - 83°C for 94% trans (Dainton et al., 1962), indicating good low-temperature properties. It is produced by Ziegler-Natta polymerization; the stereoregularity will be influenced by the reaction conditions such as the nature of the ligand a n d o r the solvent. The mechanism of stereoregulation was discussed recently in a study comparing allyl-nickel and allyl-lanthanide catalysts (Taube et al., 1995). A recent patent disclosed a copper-based catalyst for the synthesis of truns-1,4polybutadiene (Seki et al., 1996). Vinyl Polybutadietzes The Ziegler-Natta-type high-cis and high-trans polybutadienes contain a small amount of vinyl structures (0-5%), while emulsion BR can be produced with 7-25% vinyl content (Stephens, 1989).The vinyl content of polybutadienes produced by anionic polymerization canvary between 10 and nearly loo%, demonstratingthe versatility of this process (Hsieh and Quirk, 1996).The vinyl content has a profound effect on the properties of the polymer; for instance, as the vinyl content is changing between 0 and 100%. the T, of the polymer changes between - 100 and 5°C (HalasaandMassie, 1993). Alow-vinyl (IO-30%) polybutadienewith a T, of -70 to - 85°C has good wearand fuel economy but poor traction. while a high-vinyl (80-95%) polymer with a T, of - 10 to - 30°C has good traction and fuel economy but poor wear properties. The optimum was found at about 70% vinyl with a T, of - 40°C (Halasa and Massie, 1993). Recent developments in anionic polymerization techniques put new life into vinyl polybutadienes. For instance. the versatility of this process allows the controlled termination of the chain ends. The end groups were shown to have a profound impact in tire performance by influencing the fillerpolymer interaction in spite of their small concentration in the polymer chain. For instance, the introduction of bis(4,4’-dimethylamino)benzophenone and N-methyl-Zpyrrolidone end goups improved tire performance (Nagata etal., 1987; Kawanaka, 1989).The importanceof end group functionalization further increased with the introduction of silica and silica-carbon black “composite fillers” (“green tire”), in which the improvement of filler-polymer interaction is crucial (Wang and Wolff. 1991; Okel and Waddell, 1994). Functional groups can also be introduced by the use of functional anionic initiators; developments in this area were reviewed recently (Quirk et al., 1996). Anionic polybutadienes are essentially linear with narrow molecular weight distribution ( M J M , , = 1 in batch; M,/M,, = 2 in a continuous process), which results in poor processability. This can be improved by introducing branching into the polymer chain (Tsutsumi et al., 1990; Sierra et al., 1995). Linking agents such as tin tetrachloride can be used to make star polymers; in addition, the tin was shown to improve carbon black-polymer interaction, thereby improving tire performance. Low-Vinyl Polybutadienes In this section polymers with around 10-25% vinyl content will be discussed. These elastomers are produced with anionic (living) or emulsionpolymerization and are also called low- or medium-cis polybutadienes. A typical anionic polybutadiene has about 31% cis, 53% trans, and 10% vinyl content, with a reported T, of -93°C (Laflair, 1993; Brydson, 1988). This rubber is mostly used to produce HIPS; it dissolves readily in the styrene and produces rubber domains of the right size distribution to arrest crack propagation in polystyrene. The viscosity of the polybutadiene dissolved in the styrene is very important and can be regulated by controlling the molecular weight and the molecular weight distribution
Diene-Based Elastomers
821
or introducing long-chain branching into the polybutadiene. It was shown that narrow molecular weight distribution gives a better core-shell structure in high-gloss HIPS; interestingly, the use of star-branched polybutadienes reportedly ledto nonuniform structures (Toyoshima et al., 1997). Branching also reduces the coldflow of this elastomer and improves processability.The standard anionic polybutadiene has poor processability and wear. The anionic living polymerization of butadiene is attractive because it is well understood (Hsieh and Quirk, 1996). This process has no termination or other side reactions. and the molecular weight of the polymer can easily be controlled by the initiator/monomer ratio or by deliberately added terminating agents (Puskas, 1993). It is a very versatile process that can produce tailor-made structures, including branched and end-functionalized polymers (Fetters et al., 1996; Quirk et al., 1996; Fathi et al., 1996). The interest in these specialty elastomers is steadily growing. A typical emulsion butadiene has about 17% vinyl, 70% trans-, and 13% cis-1,4 structures (de Decker et al., 1965). Due to its mixed microstructure, the T, of this polymer was reported to be - 80°C. It also has a higher gel content than other BRs. As a rule, the emulsion polymerization must be terminated at about 60% conversion to avoid excessive gel formation. Emulsion low (medium)-vinyl polybutadiene is used for tire manufacturing and rubber-toughening of plastics. Tire-grade emulsion butadienes usually are not available commercially-tire manufacturers produce these rubbers for captive usage (Obrecht, 1993). The most important use of emulsion low(medium)-vinyl polybutadiene is the production of ABS (styrene-acrylonitrile toughened with butadiene rubber) (Dinges, 1979; Echte, 1989) and other composite materials. Medium-Vinyl Polybutadienes Medium-vinylpolybutadienescontainabout35-55% vinyl structuresand have T, values in therange of -70 to - 50°C (Haynes, 1988). These elastomers were found to have properties similar to SBR; good wear and wet-skid properties, low hysteresis and rolling resistance. They are also produced by anionic polymerization. but in the presence of polar additives. The versatility of the anionic living processallows the production of tailor-made medium-vinyl polybutadienes. Medium-vinyl polybutadiene is used in tire manufacturing. alone or as a blend (Laflair, 1993; Halasa and Massie, 1993). High-Vinyl Polybutadienes Atactic. High-vinylatactic (amorphous) polybutadiene with nearly 100% vinyl content was first produced by anionic polymerization in the presence of polaradditives (Halasa et al., 1981).High-vinylatacticpolybutadienesgeneratedinterest recently due totheir low hysteresis and good rolling resistance together with improved wet grip. Optimum properties were foundaround 70%vinyl content, with a reportedT, of - 40°C (Halasa and Massie 1993). Theseelastomers can also be tailor-made to producebranched or other architectures, further improving their physical properties and processability. High-vinyl polybutadienes are used in winter tires and tires with lower rolling resistance. Syndioractic. This vinyl polybutadiene is produced by Ziegler-Natta polymerization and was thefirst example of a syndiotacticpolymer (Natta and Corradini, 1956). The physical properties of this elastomer are determined by the degree of crystallinity and molecular weight. The melting point of syndiotacticpolybutadiene is reported as 156°C (Stephens,1989), but melting points as high as 220°C were reported (Ashitaka et al., 1983; Halasa and Massie, 1993) and - 28°C (Laflair, 1993) were reported. Polymers with various melting points canbe prepared using different polymerization conditions(Halasa andMassie, 1993; Dreyfuss, 1996). The industrially moreimportanttypes have high (T,,, = 190-260°C) or low (T,,, = 70-90°C)melting transitions. Commercially, syndiotactic polybutadiene is produced with 15-30% crystallinity so it can be processed (Laflair, 1993). It is used as a highly permeable film in food packaging. Isotactic. Isotactic 1,2-polybutadiene was produced by Ziegler-Natta polymerization and has a melting point of 126°C. Natta (1965) described the properties of 99% isotactic polybutadiene. This polymer has not generated commercial or scientific interest.
Puskas
822
2.2
Isoprene Rubbers
Isoprene is a methyl-substituted butadiene. When polymerized, it can also form cis-(A) and trans-1,4 (B) enchainment. However, because of the methyl substituent. the 1,2-(C) and 3,4(D) enchainments are different, so polyisoprene has more microstructure variation than polybutadiene:
-C
/c=c\
C -
B
A H
H
l
l
H3C\;
C "
-C / S H '
H C
D
The 3,4structure (D) is a dialkyl-substituted olefin, therefore based on olefin chemistry(Morrison and Boyd, 1992) it would be expected tobe thermodynamically more stable than the monoalkylsubstituted 1,2 structure (C).Indeed, vinyl polyisoprenes mostly have 3,4 enchainment (Morton, 1983). Similarly to that discussed for polybutadiene, the microstructure has a profound effect on the properties of the polymer. Comparison of natural and synthetic cis-polyisoprenes shows, however, that microstructure is not the only determining factor. Natural rubber is almost 100% cis-polyisoprene, but synthetic high-cis-polyisoprene has inferior properties (e.g., green strength and tack) (Schoenburg et al., 1977; Chakravarty et al.). The reason for this is still unknown. The steady availability of NR since World War I1 severely hindered developments in synthetic polyisoprene production in the developed countries. However, the former Eastern block countries developed a substantial high-cis-polyisoprene capacity,as the lack of convertible currency necessary to buy NR forced them to substitute IR for NR. Since the fall of the Berlin wall these countries are marketing synthetic polyisoprene as a NR substitute. However, in a free-market economy the future of high-cis-polyisoprene production is questionable. Vinyl polyisoprenes have very little commercial importance; they are used mostly as processing aides for NR. In vinyl polyisoprenes, the spatial arrangement of vinyl groups can be atactic, syndiotactic, or isotactic.
Diene-Based Elastomers
823
High-cis Polyisoprenes NR is almost 100%cis-polyisoprene. Polyisoprene with very high cis content (97%) (Van, 1966) can be produced by titanium-based Ziegler-Natta polymerization, but anionic polymerization can also producepolymers with up to 96%cis content (with 4% 3,4-vinyl enchainment) (Morton, 1983). The so-called “low-cis” anionicpolyisoprene still has about 92% cis content (Duck and Locke, 1977), but its properties are worse than the high-cis Ziegler-Natta IRs. High-cispolyisoprene can also be obtained by neodymium-based catalysts (Laflair, 1993). High-trans Polyisoprenes High-trans polyisoprene is also produced by Ziegler-Natta polymerization. It is a crystalline polymer with low meltingpoint. Its properties are similar to natural 100%trans-l ,4-polyisoprene (guttapercha or balata). It is used in golfball covers and splinting/prosthetic devices. Its high price limits its use as specialty polymer. Vinyl Polyisoprenes These polymers have 3,4 enchainment, probably due to the higher stability compared to the 1,2 structure. They can be amorphous orcrystalline. The preparation of amorphous 3,4-polyisoprene by anionic or Ziegler-Natta polymerizations have been reported (Ziegler, 1936; Duch andGrant, 1964; Natta et al., 1964; Halasa et al., 1981). In the presence of polar solvents, anionic polymerization can yield almost 100% 3,4-vinyl-polyisoprenes,with some 1,2content possible (Morton, 1983). Crystalline 3,4-polyisoprene was first prepared using organometallic catalysts (Sun and Wang, 1988; Qiu et al., 1989). Thestructure and properties of vinyl polyisoprenes with 70-81 % 3,4 structure and 30-19% cis content were investigated in more detail (Brock and Hakathorn, 1972). These polymers had no 1,2 content, and long sequences of head-to-tail units were said to be responsible for its crystallization. The T, of these polymers was measured to be around 8”C, and they exhibited several melting transitions.It was speculated that the spatial arrangement of the vinyl groups is syndiotactic (Halasa and Hsu, 1996). Pure isotactic vinyl (3,4)-polyisoprene has not been prepared yet, possibly due to the steric hinderance of the methyl side groups.
3.
DIENE-CONTAINING COPOLYMER ELASTOMERS
The most important representatives of this group are the styrene-butadiene rubbers (SBR). On a volume basis, these are the most important synthetic elastomers. They are mostly used in tire manufacturing and the production of industrial rubber goods. Otherimportant diene-containing copolymers are poly(styrene-co-isoprene-co-butadiene)terpolymer (SIBR) and poly(isobuty1ene-co-isoprene) (butyl rubber, IIR). The following section will discuss the above-mentioned copolymers. The specialty rubbers poly(butadiene- 1,3-co-isoprene) and poly(butadiene- 1&COpentadiene- 1,3) will be mentioned briefly. Poly(acrylonitri1e-co-butadiene-1,3)(nitrile rubber, NBR) will be covered in a separate chapter.
3.1 Styrene-ButadieneRubbers SBR was first made in 1929 by an emulsion polymerization process. The resulting material (Buna-S) was inferior compared to NR, but the development of this commercial process was a fundamental milestone in the history of rubber. Thesolution process was developedin the 1950s.
Puskas
824
The structure of these polymers can be characterized by the styrene content, by the microstructure of the polybutadiene segments (cis,trans, and vinyl), and by the sequence distribution of the styrene and butadiene units (random or blocky). A typical emulsion SBR (E-SBR) has 23.5% styrene, 14% cis-1,4-, 67% trans-1,4-, and 19% vinyl (1,2-) polybutadiene, with random distribution. Note that the vinyl, cis, and trans contents add up to 100%; another convention used in the literature reports styrene + cis + trans vinyl = 100% (Niziolek, 1997). E-SBR is prepared by radical initiation in a “hot” (50°C) or a “cold” (5”C, redox initiation) process. This elastomer has a broad molecular weight distribution and a high degree of branching, so it processes better than solution SBR. Theemulsifier residues increase the hysteresis of the rubber (KernandFutarama, 1987) but improveprocessabilityandtearstrength. Some E-SBRs are crosslinked by introducing divinylbenzene into the process. Emulsion SBR represents 40% of the market today becauseof its excellent balance of properties and cost performance. It is mainly used in replacement tire manufacturing (Lambert, 1993; Niziolek, 1997). In contrast, solution SBR is used mainly in new tires. The solution process uses anionic initiators (living polymerization) and is very versatile; the composition and microstructure of the polymer can be controlled, functional and end groups can be introduced, and the molecular architecture can be varied. The styrene sequence distribution can be varied from completely random to blocky (Zelinski and Childers, 1968;Hsieh andGaze, 1970;Antkowiaket al., 1972;Butonand Futamara, 1974, Tanaka et al., 1983; Bywater, 1985). In nonpolar solvents the butadiene polymerizes to high conversion before the styrene starts to react, leading to a blocky structure; the typical vinyl content under these conditions is 10%. In polar solvents or in the presence of polar compounds called “randomizers,” butadiene and styrene polymerize with nearly equal rates, yielding random sequence distribution. These randomizers also increase the vinyl content, which can go up to 90% (Oberster et al., 1973). For random SBR, with increasing vinyl content and increasing styrene content the T, increases, which negatively impacts the low-temperature properties of the rubber. However, high-performance tires are made with SBR with a styrene content as high as 40% and vinyl content up to 70% (Nentwig,1993). As mentioned in Section 2.1, the versatility of the living anionic polymerization allows the controlled termination of the chain ends. The end groups were shown to have profound impact on tire performance by influencing the fillerpolymer interaction in spite of their small concentration in the polymer chain (Nagata et al., 1987; Kawanaka, 1989). The importance of this filler-polymer interaction is magnified in the case of silica and silica-carbon black composite fillers (“green tire”)(Wang and Wolff, 1991; Wang et. al., 1997: Okel and Waddell, 1994). Functional groups can also be introduced by the use of functional anionic initiators; developments in this area were reviewed recently (Quirk et al., 1996). Similarly to anionic polybutadienes, solution SBRs are essentially linear with narrow molecular weight distribution (Mw/M,, = I in batch; M,/M,, = 2 in a continuous process). The processability of SBRs can also improved by branching (Tsutsumi et al., 1990; Sierra et al., 1995). Tin tetrachloride used as a linking agent to produce branched SBRs led to improved carbon black-polymer interaction and tireperformance, similar to that discussed in the polybutadiene section. Solution SBR technology continues to improve, and because of the incredible versatility of the anionic living polymerization process, the skyis the limit. Continuous original tire performance improvements will drive further developments. The cost factor will probably maintain E-SBR’s position as the workhorse of the replacement tire industry.
+
3.2 Poly(styrene-co-isoprene-co-butadiene)Terpolymer The development of SIBR, also called “integral rubber,” was based on the wide modal loss factor concept (Nordsiek and Kiepert, 1985). This can be achieved by using blends such as polybutadiene. natural rubber, or synthetic polyisoprene and SBR, but immiscibility in these
Diene-Based Elastomers
825
blends leads to problems and the effects of blend morphology on the physical properties of the final cured rubber are not fully understood. Halasa and coworkers (Halasa, 1997) pioneered the development of SIBR, a terpolymer prepared by anionic polymerization. By taking advantage of the versatility of living anionic polymerization and reaction engineering principles, these researchers achieved simultaneous controlof polymer composition, microstructure, and sequence distribution. The polymer chain is segmented, with different T,s for the individual segments. Thus the position, height, and breadth of the loss factor-temperature correlation can be varied, resulting in complex viscoelastic properties and optimum rolling resistance/wet grip/wear resistance combinations. This is a truly magnificent example of macromolecular engineering, allowing tailor-made elastomers for specific tire applications. In addition, morphologies previously reported only in SBS or SIS plastic-elastic block copolymers were obtained in the new SIBRs. This opened up new avenues in investigating the relationships between phase morphology and the physical properties of cured rubber. The commercial production of SIBR started in 1991 (Nentwig. 1993; Marwede et al.. 1993). 3.3
Isobutylene-IsopreneRubbers
Butylrubber (IIR) is the copolymer of isobutylene with 1-3% isoprene. The isopreneunits supply the curing sites. The first butyl-type rubber contained butadiene instead of isoprene, and it was the first example of “low-functionality” rubber (Thomas and Sparks, 1944; Thomas. 1969). According to this principle, only a small number of curing sites is necessary to obtain good physical properties, as opposed to the traditional diene rubbers with a curing site for each repeat unit. The isoprene is incorporated ina rruns-l,4 configuration. A small fraction (0.1-0.3%) of other structures, which were thought to be 1,2 units (Chu and Vukov, 1985), were proven to be branching points (White etal., 1995). The presence of branchinghimodal molecular weight distribution was claimed to improve the processability of the rubber (Duvdevani, 1989; Puskas and Kaszas 1993). IIR has the lowest permeabilityto air ormoisture of all elastomers, combined with high damping and good oxidative and chemical resistance (Kennedy, 1975). It is used in tire-relatedapplications (inner tubes,tire-curingbladders,innerliner blends) and as abase polymer for halobutyl production (Duffy and Wilson, 1993; Kaszas et al., 1996). The market share of IIR itself is decreasing, with little incentive for product development. With the advent of living carbocationic polymerization, block copolymers of isobutylene and styrene and styrene derivatives-butyl-like polymers that do not need chemical crosslinking-were made for the firsttime (Kennedy et al., 1990;PuskasandKaszas, 1996). The development of these new materials is still only on a pilot scale (Kaszas et al., 1992). The livingcopolymerization of isobutylene with isoprene and 2,4-dimethyl-1,3-pentadienehas been achieved (Kaszas et al., 1992).
3.4
SpecialtyDiene-ContainingCopolymerElastomers
The specialty butadiene-isoprene and butadiene-piperylene copolymers were discussed in the earlier edition of this book (Haynes, 1988). They are used in specialty tire applications.
4.
RECENTDEVELOPMENTS IN DIENE-CONTAININGELASTOMERS
The information published during the period 1995-1997 concerning catalyst and initiator development is almost overwhelming. The former Soviet Union and China seem to be very active, especially in catalyst and process development.
826
4.1
Puskas
Emulsion Polymerization
Not much activity was found in this area. A new copper(I1)-enolate redox initiating system for BR and SBR production was reported, with the option for termination by various functional groups (Harwood and Goodrich, 1995). Another report discussed the use of aqueous emulsion of organicperoxides for E-SBR (Tauraet al., 1997).Amethod of short-stopping with the suppression of nitrosamine formation was reported (Maestri and Lo, 1995). An interesting report discusses the suspension polymerization of dienes with transition metal catalysts in diluents of high specific gravity (Kimura et al., 1996).
4.2
SolutionPolymerization
New Catalysts and Initiators
Ziegler-Natta and Related Catalysts Publicationandpatentingactivity in thisarea testifies to renewed interest in this field. The following new and modified catalysts were found in the literature: Ni-based catalystwith increased activity (Wang and Wang, 1997);Ni naphthenatealuminum triisobutyl-borontrifluoride etherate complex (Zhu, 1996); Ni-acetylacetonate with methylaluminoxane as cocatalysts (Endo etal., 1996); new dicationic Ni complex (Ni-phosphine) (Engel Gerbase et al., 1996); Ni-naphthenate with triisobutyl aluminum and aluminum chloride (Xu et al., 1995); highly active allyl Ni-based catalysts for cis-1,4-polyisoprene (Novak and Deming, 1995); dibromo-bis(tripheny1phosphine)cobalt-magnesium chloride-dimethoxydiphenylsilane-triethylaluminum catalysts in the presence of ethylene (Takeuchi et al., 1995); triisobutylaluminum-dichloroiodotitanium,to which isopropylbenzene hydroperoxide is added prior to the deactivation of the catalyst (Grachev et al., 1996); polynuclear Nd-AI dimetallic complex (Dong et al., 1995, 1997); titanium(1V)-halide/triethylaluminum neodymium or praseodyrnium complex (Aksenov 19951); neodymium versatate with diisobutylalane, trichlorotriethyldialuminum alcohols, and aluminoxanes(Sylvester and Vernaleken, 1995); halogenated complexes of lanthanides, neodymium toluene ethylchloroaluminum (Garbassi et al., 1995); neodymium tributylphosphate/triisobutylaluminum(Iovu et al., 1997); neodymium-triisobutylaluminumwith a complexing agent (lovu, 1997a); isopropyl alcohol solution of rare earth chlorides with the reaction product of water and triisobutylaluminum in the presence of piperylene (Bodrova et al., 1995), catalyst formed by the oxidative addition of hydrocarbons to lanthanides (Dolgoplosk, 1996); and neodymiumbutoxide/triisobutylaluminum (Biagini et al., 1995a).It was reported that the neodyn~ium-phosphonate/triisobutyIaluminum/ethylaluminumsesquichloride system yields quasiliving polymerization of isoprene (Liu et. al., 1995). Other interestingcatalyst systems includealkyl-iron complexes(Xiaet al., 1997a,b; 1996a); a molybdenum-aluminum colloid (Xia et. al., 1996b); transition metals (e.g., vanadium) with aluminoxane (Igai et al., 1997); toluene solution of triisobutyl aluminum with aluminum chloride (Panfilova et al., 1996); molybdenum trichloride dioctanoate with diisobutyl (methylphenoxy)aluminum (Xia et. al., 1 9 9 6 ~ )and ; a tungsten/aluminum alkyl catalyst for SBR (Song et al., 1995). These catalysts produce cis configurations. For trans configurations an irradiationadsorbing copper cyclohexanedicarboxylate catalyst was reported (Seki et al., 1996). Metallocene Catalysts Dienes are believed to bepoisonsformetallocene-based catalysts. However, the following metallocene or similar catalysts were found in the literature for diene polymerization: monocyclopentadienyl vanadium, niobium, tantalum with aluminoxane and borate cocatalysts(Tsujimoto et al., 1996); metalloceneA4AOcomplexes for the polymerization of isoprene and butadiene (Endo, 1997); lanthanoceneh4AO for butadiene, isoprene, and styrene(Cuiet al., 1997); group IV metal (zirconium,titanium) complexes with MAO for low-temperature butadiene and isoprene polymerizations (Longo et al., 1996); mono- and bis-
+
Diene-Based Elastomers
827
cyclopentadienyl compounds of vanadium and titanium with MAO (Ricci et al., 1996, 1996a); and the metallocene TiBzMAO for butadiene and isoprene polymerizations (Huang and Tion, 1996). The copolymerization of isobutylene with isoprene was reported with the metallocenelike cp-TiMe3/B(Cc,FS)3catalyst system (Barsan and Baird, 1995). Supported Catalysts There is great interest in supported (both inorganic and polymersupported) heterogeneous catalysis. For diene polymerization, the following work was reported during the review period: MgC1,-supported cobalt-based catalyst with trimethylaluminum (Takeuchi et al., 1996); poly(acry1amide-styrene)-supported metal complexes (e.g., neodymiumtrichloride) (Zheng et al. 1996; Zheng, 1997);poly(styrene-4-vinylpyridine)-supportedneodymium (Li et al., 1997); and poly(styrene-2-methylsu1finyl)ethyl methacrylate-supported rare earth catalysts (Li et al., 1995). An interesting report discusses diene polymerizations catalyzed by neodymium salts supported on homogeneous fullerenes (C60/70) (Chen et al., 1995). The area of supported Ziegler-Natta and lanthanide catalysts for diene polymerizations was reviewed (Ran, 1996; Yu and Li, 1996). The copolymerization of isobutylene with isoprene using poly(biphenylaminoethy1styrene)-supported TiC14/Et2AlClwas reported, butlowmolecularweight polymer formed (Ran, 1996). Anionic (Living) Initiators There is great activity in the field of functionalized and other specialized anionic initiators. The following reports were of interest: (tert-butyldimethyl siloxy) alkyllithium-protected initiator (Sutton and Schwindeman, 1997); composite modifiers for anionic living polymerizations (Pan et al., 1997); potassium salts of ethoxylated alcohols (Yudin et al., 1996); dipiperidinoethane-based initiators (Wang et al., 1996); a new class of ether-functionalized initiators (e.g., dimethylethoxi propyllithium) producing star-branched and heterotelechelic polymers (Schwindeman et al., 1997, 1997a); butyldimethylsilyloxypropyllithium functionalized initiator producing star polymers by coupling with silicon tetrachloride (Letchford et al., 1997); multifunctional anionic initiators from the reaction of divinylsilanes with alkyllithiums (Chamberlain et al., 1997); new ether modifiers (tetrahydrofurfuryl-based) for isoprene polymerization (Halasa and Hsu, 1996a); functionalized amine (e.g., dimethylpropylamine) initiators (Engel et al., 1996); anionic soluble organosodium catalyst (Arest-Yakubovich et al., 1995); hydrocarbon-soluble initiators for SBR polymerizations (Kitamura et al., 1997); hydrocarbon-soluble lithioamine, lithium amide, amino-substituted aryllithium, and trimethyltetrahydroazepine hexamethyleneimine lithium initiators for diene rubber production with reduced hysteresis (Lawson, 1996, 1996a, 1996b, 1997); multifunctional organic alkali metal initiators (Zhang et al., 1997); lithium cycloamino-magnesiate initiator for diene rubbers with reduced hysteresis (Antkowiak and Hall, 1996); divinylbenzene-butyllithium adduct for multifunctional initiation (Lutz et al., 1996); and triorganotin lithium initiator (Hergenrother et al., 1995). In this lastworkSn-NMR evidence was found for 1,2 initiation (Hergenrother etal., 1995a). Reports on star polymers include silane-coupled SBR stars for silica tires (Toba et al., 1997) and star polymers by efficient anionic living coupling (Ono et al., 1995). The status of lithium polymers in China has been reviewed (Li et al., 1995; Sun, 1997). Process Research and Development The number of publications in process research and development indicate a renewed interest in this area. The reports listed cover the following aspects: effect of process conditions on the structure and properties of cis- and trans- 1,4 polydienes (Cai, 1995); catalyst-monomer interaction in cobalt naphtenoate/isobutylaluminum chloride/water catalyst system in the low temperature polymerization of cis-1,4-polybutadiene (Smimova et al., 1996, 1997); the effect of water in Ni-naphtenate/isobutylaluminum chloride-catalyzed diene polymerizations (Xu et al., 1997); the effect of using a preformed cobalt salt/alkylaluminum chloride in low-temperature diene
Puskas
828
polymerization (Glebova et al., 1996); effect of additives (octanol, butylacetate) in Ni-boron trifluoride etherate-catalyzed polymerizations (Zhu, 1996a); the effect of modifiers in CO-catalyzed polymerizations (Krivoshein et al., 1995); control in CO-naphtenoate-diisobutylaluminum chloride-water-catalyzed low temperature diene polymerization (Smirnova et al., 1996); cis1,4 BR with less gel (Suzuki et al., 1997);the effect of titanium trichloride purity anddeactivation problems in diene polymerizations (Costa et al., 1997); the nature of active centers and pecularities in CO-catalyzeddiene polymerizations (Sharacv, 1996); the effect of solvent in aluminoxanebased neodymium-carboxylate-catalyzed polymerizations (Wilson, 1996); the effect of Nd/ halide ratio and halide type in Nd-catalyzed butadiene polymerizations (Wilson and Jenkins, 1995); titanium tetrachloride-alkylaluminum chloride-catalyzed process (Aksenov, 1995b); the effect of alkylaluminums and alkylaluminum chlorides on Nd-catalyzed polymerizations (Wilson, 1995); kinetics, molecularweight distribution, andchainend structure in Nd-catalyzed polybutadiene (Nickaf et al., 1995); the effect of diisopropyl xantogenathe disulfide/catalyst ratio andprocess control in Ziegler-Natta-catalyzed continuous isoprene polymerizations (Abramzon et al., 1995); polymerization kinetics in Nd-di(isopropoxy) chloride/triethylaluminum-catalyzed diene polymerization (Cai et al., 1995); effect of parameters on Nd-tris(2ethylhexanoate)/diisobutylaluminum chloride/triisobutylaluminum-catalyzed diene polymerizations (Gehrke et al., 1996): effect of reaction time on the molecular weight distribution of Ndcatalyzed diene polymerization (Oehme et al., 1997); Nd-based catalyst preparation (Biagini et al., 1995); kinetics of lanthanide-catalyzed isoprene polymerization (Dimonie et al., 1995); the efficiency of titanium-magnesium/triisobutylaluminum catalyst in diene polymerization (Mushina et al., 1996). Direct extraction is discussed as an improved preparation of Nd-naphthenate (Yang et al., 1996). An interesting report discusses a process for cis-1,4-polyisoprene production using a multizone tubular prereactor (Minsker et al., 1996). In anionic polymerization technology, less process research isreported. The process control is discussed in continuous anionic polymerization (Konovalenko et al., 1996). The effect of feed distribution in anionic continuous diene copolymerization with divinylbenzene is discussed (Aksenov et al., 1995b). The effect of polar additives (ethoxylated sodium or aluminum alcoholates) on thecontinuous anionic copolymerization of a butadiene-styrene with divinylbenzene is reported (Shalganova et al., 1996). Termination and long-chain branching was modeled in the anionic copolymerization of butadiene with divinylbenzene (Fathi et al., 1996). It is interesting to mention the development of a butyl process using liquid CO? (Baade et al., 1996). reduced fouling in the butyl process (Baade et al., 1996a), and an olefin metathesis process for degelling diene polymerization reactors (Oziomek, 1995). 4.3
Gas-Phase Polymerization
One of the most important developments of the past few years was the gas-phase polymerization of diene elastomers. The gas phase, rare earth allyl/aluminoxane-catalyzed polymerization of butadieneyielded a high-cis product (Taube et al., 1996). In a recent report, a neodymium catalyst was used in the gas-phase laboratory polymerization of polybutadiene (Sun et al., 1997). For gas-phase butadiene polymerization, silica gel-supported rare earth alcoholates (Reichert et al., 1996) and activated charcoal-supported Nd-catalysts (Buysch et al., 1995) were reported.
ACKNOWLEDGMENTS The author would like to acknowledge the contribution of Rosemary O’Donnell and her colleagues (Technology Department, Rubber Division, Bayer Inc., Canada) for the literature search.
Diene-Based Elastomers
829
REFERENCES Abramzon, I. M.,Borcjko, Y. I., Vasilev, V. A., Drozdov, V. A., Zajdullin, A. A., and Isakov,N.D. (1995), Russian Pat. RU2036509 (Nizhnekamskneftekhim). Aksenov, V. I., Golovina, N. A., Murachev, V. B., Zolotarev, V. L., Khlustikov, V. I., Gol’berg, 1. P., Ryakhovskii, V. S. (1995). Russian Pat. RU 2028309. Aksenov. V. I., Golovina, N. A., Birikhin, V. S., Murachev, V. B.,Gol’berg, I. P.,Byrikhin, A. S., Zaboristov, V. N., Ryakhovskii, V. S., and Khlustikov, V. I. (1995a), Russian Pat. RU 2028308. Aksenov, V. I., Stepanova, E. V., Zolotarev, V. L.. Ryakhovskii, V. S., Zaboristov, V. N., Khlustikov, V. I., and Gol’berg, I. P. (1995b), Russian Pat. RU 20283 IO. Antkowiak, T. A., Oberster, A. E., Halasa, A. F., and Tate, D. P. (1972), J. Po/vnz. Sei. Po/.vrr~.Cherrz. 10: 1319. Antkowiak, T. A., and Hall, J. E. (1996), European Pat. EP 747405 (Bridgestone Co.). Arest-Yakubovich, A. A., Pakuro, N. I., Zolotareva, I. V., Kris’talnyi, E., and Basova, R. V. (1995), Po/ynf. h r . 37165. Ashitaka, H., Junda, K., and Ueno, H. (1983), J. P o / w z . Sei. Po!\nz. Clzem. 21:1951, 1973, 1989. Baade, W., Langstein. G., and Puskas, J. E. (1996), U S . Pat. 5,539.065. Baade, W., Heinrich, R., Langstein, G., Muelder, T., and Puskas, J. E. (1996a), U.S. Pat. 5,543,479. Baccaredda, M., and Butta, E. ( 1960), Clzin~.lnd. (Milan) 42:978. Bahary, W. S.. Sapper, D. I., and Lane, J. H. (1967), Rubber Cllent Techno/. 40:1529. Barsan, F., and Baird. M. C. (1995), J. C h e r ~Soc.. . Cltent. Conmun. 95:1065. Bazhenov, Y. P,, Kutuzov, P. I., Rakhimov, R. K., and Klepikova, V. I. (1996), Russian Pat. RU 2061546. Biagini. P., Lugli, G., Abis, L., and Millini. R. (1995), New J. Cherrz. 19:713. Biagini, P,, Lugli, G., Garbassi, F., and Andreussi, P. (1995a), European Pat. EP 667357 (Enichem.). Bodrova, V. S., Kormer, V. A., Piskareva, E. P,, Polctaeva, I. A., Shelokhneva, L. F., Brandstetter, F., Gauscpohl. H., and Thiele, R. (1995). German Pat. DE 4327495 (BASF). Brock, M. J., and Hakathorn, M.J. (1972), Rubber Cherrz. Techrzol. 45: 1343. Brydson, J. A. (1988) Rubher?. M~terirtl.7c m / Their Corlzpourtd.7, Elsevier Applied Sei., London. Buton, T. C., and Futamara, S. ( 1974), Rubber Age 10633. Buysch,H.-J.,Mendoza-Frohn, C., Notheis, U,, and Sylvester, G. (1995), GermanPat. DE 19505355 (Bayer AG). Bywater, S. (1983, in Encycloperh of Po/vrner Science nrzd Bzgirteering (H. F. Mark, Ed.), Wiley, New York. Cai, H. (1995). Hec/zert*qXinr~gjitroGongye 18:144. Cai. X., Gong, Z., Wang, F., Zhu, X., and Li, X. (1995), Gnofenzi Xuebno 6:653. Chakravatty, S. N., Mustafi, S. K., and Mukherjee, A. K. (l989), Rubber News (July 28):33. Chamberlain, L. R., Hoxmeier, R. J., and DeFriend, P. A. (1997). U S . Pat. 5605991 (Shell Oil). Chen, D., Zhao, C., Piao, G.. Xu, L., Zhong, C.. Tang, X., Yang, H.. Li, M,, and Zou, G. (1995). Hedzerzg Xinngjiczo G o r t g y /8:244. Chu, C. Y., and Vukov, R. (1985), M~tcrornnlecules18:1423. Costa, T. H. S., Costa, M. A. S., de Santa Maria, L. C., Coutinho, F. M. B., and Pires, N. M.T. (1997), Quint. No11n 20( 1):93. Cui, I., Jin, Y., Sun, J., Li, K., Ba, X., and Teng, H. (1997). Hechrrzg Xicrrzgjicto Gorlgye 20:79. Dainton, F. S., Evans, D. M,,Hoare, F. E., and Melia, T. P. (1962), P o / y r ~ e3:297. r de Decker, H., McCall, C., and Bahary, W. (1965), Rubber Plast. Age 46(3):286. Dimonie, M,, Hubca. G., Badea, E., Simionescu, E., Iovu, H., Vasile, I., and Stan, S. (1995), Re\,. Rourrl. Chirn. 40:83. Dinges, K. ( I 979). KctutscIt. Gurrzrm’ Kurlstst. 32( 10):748. Dolgosplosk, B. A., Tinyakoya, E. I., Yakovlev, V. A., and Gol’shtein, S. B. (1996), Vysokomol. Soedin. Ser.A. Ser.B. 38:442. Dong. W., Yang, J.. Pang, S., and Huang, B. (1995). Y i n g y l r l g Hunxue 12:l. Dong, W., Yang, J., Shan. C., Pang, S., and Huang, B. (1997), Cuikuct Cielm) 18(3):234.
830
Puskas
Dreyfuss, P. (1996). The Polyr?~ericMtrterirrls Encyclopeditr (J. Salamon, Ed.), CRC Press, Inc., Boca Raton, FL. Duch, M. W., and Grant, D. M. (1964), Mrrkrornol. Chenl. 3:165. Duck, E. W., and Locke, J. M. (1977), The Stereo Rubbers (W. M. Saltman, Ed.), Wiley, New York. Duffy, J., and Wilson, G. J. (l993), in Ullrrlann 'S Encyc./opedicr V23, Synthetic Rubher. VCH Publishers, Inc., Weinheim. Duvdevani, I., Gursky, L., and Gardner, I. J. (l989), Paper #22, ACS Rubbcr Division Meeting, Mexico City. Echte, A. (1989), Ad\!. Chern. Ser. 222: 15. Endo, K., Uchida, Y., Matsuda, Y.(1996). Mcrcromol. Chem. phys. 197(11):3515. Endo, K. (19971, Nippon GOIll/4 Kiokrrishi 70:69. Engel, J. F., Kamienski, C. W., Schwindcman, J. A., Hall, R. W., Morrison,R. C., and Dover, B.T. (l996), U.S. Pat. 5527753 (FMC Corp.). Engel Gerbase, A., De Souza, R. F., De Souza Vichi, E. J., and Da Cunha,F. V. (1996).Angew. Me/kronlo/. Chenl. 23933. Fathi, H., Hamielce, A. E., and Davison, E. J. (1996), P o l y ~ n .Recrct. Eng. 4(4):243. Fetters, L. J., Crest, G. S., Huang, J. S., and Richter, D. ( 1 996), A h . Chenl. Phys. XCIV67. Garbassi, F., Biagini, P,, Andreussi, P,, and Lugli, G. (1993, European Pat. Appl. 637589 (Enichem). Gehrke, K., Krueger, G., Gebauer, U,, and Lechner, M. D. ( 1996), Kcrutsch. Curmi Kunstsf. 49( 1 1 ):760. Georges, M. K. Veregin, R. P. N., Kazmaier, P. M,, and Hamer, G. K. (1993). Mrrcrornolecules 26:2987. Georges, M. K. Veregin, R. P. N., Kazmaicr, P. M., and Hamer, G. K. (1994), U.S. Pat. 5,322,912. Glebova, N. N., Dolgoplosk, B. A., Markevich, 1. N., Tinyakova, E. I.. Sharaev, 0. K., Yakovlev, V. A., Byrikhina, N. N., Golberg, 1. P,, and Zaboristov, V. N. (1996). Russian Pat. RU2059654. Gnanou, Y. (1996), J.M.S.-Rev. Mcrcromol. Cllern. Phys. C36:77. Grachev, G. M., Drozdova, E. V., Krol, V. A., Kudryavtsev, L. D., Rylkov, A. A., Sidorov, S. I., Chcrkasina, l. M,, Chirskij, F. l., and Yakunin, V. A. (1996), Russian Pat. SU 1244928. Halasa, A. F. (1997), Rubber Chenz. Techno/. 70:295. Halasa, A. F., andMassic,J.M. (1993), Kirk-Othrner Encyc~lopedicrof' Chern. Technol., 4"' ed.,John Wiley & Sons, New York. Halasa, A., and Hsu, W. L. (1996), The Po/yneric Mclfericds Encyclopedirr (J. Salamone, Ed.), CRC Press, Inc., Boca Raton, FL. Halasa, A. F., and Hsu, W. L. (1996a). Polvn~.Prepr. 37:676. Halasa, A. F., Lohr, D. F., and Hall, J. E. (l981), J. Polym. Sei. P o l w . Chenl. IY:1347. Hanvood, H. J., and Goodrich, S. D. (1995), U.S. Pat. 5405913 (U. Akron). Haynes, A. C. (1988), in Hcrnrlhook qfElastonlers. New Developments nnel Technology (A. K. Bhowmick and H. L. Stephens, Eds.) Marcel Dekker, New York. Hergcnrother, W. L., Smith, W. N., Muratore, A. J.. Single, J. C., and Nemeth, M . T. (1995), European Pat. EP 682035 (Bridgestone Co., Cyprus Foote Mineral Co.). T. W., and Oziomek,J. (1995a), J. Polvrn. Hergcnrother, W. L., Doshak, J. M,, Brumbaugh, D. R., Bethea, Sei., Part A: Polynl. Cllenl. 33:143. Hrgis, I. G., Livigni, R. A., and Agganval, S. L. (1987), in Deve1opnent.s in Rubber Teckrlology, 4 (A. Whelan, K. S. Lee, Eds.), Elsevicr Applied Sci., London. Hsieh, H. L., and Gaze, (1970), Rubber Chern. Technol. 43:22. Hsieh, H. L., and Quirk, R. P. (1996), Anionic Pn!\Inlerizrrrior/.s: Principles trnd Prrrcficd Applictrtions, Marcel Dekker, New York. Huang, B., and Tian, J. (1996), in The Polylneric Mrrterictls Encyclopedicr (J. Salamone, ed.), CRC Press, Inc., and Boca Raton, FL. Igai, S., Imaoka, K., and Kai, J. (1997), Jpn. Pat. JP 97194526; 9777818 (Ube Industries). Iovu, H., Hubca, G., Simionescu, E., Badea, E. G., and Dimonie, M. (1997), Angew. Mrrkrornol. Chem 24959. Iovu, H., Hubca, G., Simionescu, E., Badea, E. G., and Dimonie, M. (1997a), Mater. Plcr.st. 34:5. Kaszas, G. (1992), Polynl. Mater. Sei. Eng. 67:325. Kaszas, G., Puskas, J. E., Chen, C. C., and Kennedy, J. P. (1990). Macromolecules 23:3909.
Diene-Based Elastomers
831
Kaszas, G.. Puskas, J. E., and Kennedy, J. P. ( 1992), Mwrorrloleculrs 25: 1775. Kaszas, G.. Puskas, J. E., and Baadc, W. (1996), in The Po/yr,leric Materinls Em-yc~lopedirr(J. Salamon, Ed.), CRC Press. Inc., Boca Raton, FL. Kawanaka, T., Watanabc, H., Noguchi, K., and Akita, S. (1989), Jap. Pat. JP 8879003 (Nippon Zeon Co. Ltd.). Kennedy, J. P. (1975), Crrtionic Po/yrneriztrtiorl of0lqfirt.s: A Critical h r w l t o r y . Wilcy Interscience, New York. Kennedy, J. P,, Puskas, J. E., Kaszas, G., and Hagcr, W. G. (1990). U.S. Pat. 4,946,899. Kcrn. B., and Futarama, S . (1987), ACS Rubber Divisior~Meetirlg. Moutrecrl. Konovalcnko, N. A., Kolomytsev, I. A., Khitrova, R. A., Yudin, V. P,, Scmcnova, N. M., Tikhomirov, G. S., Dorofccv, V. I., Rachinskij, A. V., Rylkov, A. A., and Rupyshcv, V. G. (1996). Russian Pat. SU 1631984. Kimura, 0..Sakagami, S., Motoyuki. K., and Baba, Y. (l996), Jpn. Pat. JP 96134132. Kitamura, T., Lawson, D. F., Morita, K., and Ozawa, Y. (1997), U.S. Pat. 5674798 (Bridgcstone Co.). Kozlov, V. G., Sigaycva, N. N., Monakov, Y. B., Volodina, V. P,, Savelycva, I. G., Korogodova, N. A., and Filatova, F. S. (1991 ), Po/.vnl. Sci. 33(1 1):2296. Krivoshcin, V. V., Konovalcnko, N. A., Tikhomirova, I. N., Molodyka, A. V., Privalov, V. A., Rachinskij, A. V., and Volkov, L. A. (1995), Russian Pat. RU 2043367. Laflair, R. T. (1993), in Ullntcrrtn 'S E t q d o p e t l i t r V23. Swthetic Rubber, VCH Publishers. Inc. Wcinheim. Lambert. J-P. (1993), in Ullmnrlrl '.S Encycloprdin V23. Synthetic Rubber, VCH Publishers, Inc. Wcinheim. Lawson, D. F., Hall, J. E., and Ozawa, Y. (1996), European Pat. EP 741 148 (Bridgcstone Co.). Lawson, D. F., Brumbaugh, D. R., Stayer, M. L., Schrcffler, J. R., Antkowiak, T. A., Saffles, T. A., Morita, K., Ozawa, Y., and Nakayama, S. (1996a). Polynl. Prepr. 37728. Lawson, D. F., Antkowiak, T. A., Stayer, M. L.,Schreffler, J. R., and Komatsu, H. (1996b), U.S. Pat. 5523371 (Bridgcstone Co.). Lawson, D. F., Hall, J. E., and Ozawa, Y. (1997). U.S. Pat. 5643848 (Bridgcstone Co.). Letchford, R. J., Kamicnski, C. W., and Schwindeman, J. A. (1997), World Pat. WO 9705180 (FMC Corp.). Li, X., Yu, G., and Li, Y. (l997), Yirlgiotlg Huosue 14( 1):80. Li, Y., Li, Y., Zhu, Y., and Yu, G. (1995) Hecheng Xiunu'ino G o q y 18:344. Li, Y., Zhang, S., Liu, Q., and Hong, D. (1995a), Heclrerlg Xiorl@o Gortgge 18:266. Liu, L., Zheng, Y., and Gong, Z. (1993, H e c h e q Xiartgjino Gongye 20: 179. Longo, P,, Oliva, P,, Proto. A., and Zambclli, A. (1996), Gcczz. Chim Ita/. 126377. Lutz, P,, Rcmpp, P,, Rein, D., Heim, P., Nicol, P., and Vuillemin, B. ( l996), European Pat. EP 698617 (Elf Atochem). Maestri. P,, and Lo, P. A. (1995). European Pat. EP 656371 (Enichem.). Manvcdc, G. W., Stollfuss, B., and Summer, A. J. M. (1993), Ktrutsch. Gunmi Kur~stst.46:380. Minsker, K. S., Prochukhan, Y. A., Biglova, R. Z., Rakhimov, R. K., Bazhenov, Y. P,. Minsker, S. K., Nasyrov, I. S., Kutuzov, P. I., Kolcsov, S. V., and Vizhnyacv, V. I. ( 1996), Russian Pat. RU 2059657. Morrison, R. T., and Boyd, R. N. (1992), 0 r g m i c Chenli.stry. 6"' ed.. Prenticc Hall, Englewood Cliffs, NJ. Inititrtiorl ofPo/?~r,lrrizrrfiorl(F. E. Bailey, Ed.), ACS, Washington, Morton, M., and Rupert, J. M. ( 1 9 8 3 ~ DC. Morton, M. (1983), Alliorlic Pn/ymel.izatior~:Prir~cip/e.sm d Practice, Academic Press, New York. Mushina, E. A., Gavrilenko, I. F., Borodina, I. A., Tinyakova, E. I., Antipov, E. M,, Bondarenko, G. N., Podol'skii, Y. Y., Frolov, V. M., Gabutdinov, M. S., and Krentscl, B. A. (1996), Vvsokorrlol.Sorciirl. Ser.A. Ser.B. 38:453. Nagata, N., Kobatakc, H.. Watanabc, A., Ucda, A., and Yoshioka, A. (1987). Rubber Cllenl. Techrtol. 60: 837. Natta, G., and Corradini, P. (1956). J. Po/ymer Sci. 20:251. Natta, G., Porri, L., and Carbonaro, A. (1964), Mnkrontol. Chern. 7 7 126. Natta, G. ( I 965). Science 147269.
832
Puskas
Nel, J. G., Wagener, K. B., and Boncella, J. M. (1989), Po/ymer Preprints 30(2):130. Nentwig, W. (1993). Ulltncrrfrf'S Encycloperli~rV23. Synthetic Rubher, VCH Publishers. Inc., Weinheim. Nickaf, J. B., Burford, R. P., and Chaplin, R. P. (1995), J. Po/ynz. Sci.. Part A: Po/vrn. Chern. 33:1125. Niziolek, A. (1997), Paper #P, ACS Rubber Division Meeting, Anaheim, CA. Nordsiek, K. H., and Kiepert, K. M. (1985). Ktrursch. Gunuui Kurzstst. 38: 178. Novak, B., and Deming, T. J. (1995), U.S. Pat. 539581 I (U. California). Oberster, A. E., Bouton, C., and Valaitis, J. K. (l973), Angew. Mcrkromol. Clzern. 29/30:291. Obrecht, W. (1993), in Ullrrztrtzn 'S Encyc~lopuediaV23. Synthetic R14hber; VCH Publishers, Inc., Weinheim. Oehme, A., Gebauer, U,, Gehrke, K., and Lechner, M. D. (1997). Ktrut.wh. Gumnli Kumtst. 50(2):82. Okel, T. A., and Waddell. W. H. (1994). Ruhher C l m l . Techlo/. 67:217. Ono, T., Kanamori, T., Ito, K., Betsusho, K., and Kubota, K. (1995). Jpn. Pat. JP 95196729 (Japan Synth. Rubber Co. Ltd.). Oziomek,J.,Hergenrother, W. L.,Hamm, D. R., and Bouton, T.C.(1995). US Pat. US 5446102 (Bridgestone Co.) Pan, Z., Yang, .l., and Gu, M. (1997). Heckerzg Xicrngjitro Gongye 20(2): 105. Panfilova, Z. P., Korneev, N. N., Govorov, N. N., Tomashevskij, M. V., Zolotarev, V. L., Kropacheva, E. N., Smirnova, L. V. (1996). Russian Pat. RU 2057756. Puskas, J. E. (1993), Makronzol. Clrerrl., T e o n S i n d . 2:141. Puskas, J. E., and Kaszas, G. (1993), U.S. Pat. 5,194,538 (Bayer AG). Puskas, J. E., and KZIW~S, G. (1996), Rubber C/rem. Techno/. 69444. Qiu, Z. W., Chen, X., Sun B., Zhoul, Z., and Wan, F. ( 1989). J. Macrorrzol. Sci., Chenl. A25: 127. Quirk, R. P,, Jang, S. H., and Kim, J. (1996). Rubher Chem. Techrwl. 69444. Ran. R. (1996). The Po/yrrreric Motrricrls Encylopedicr (J. Salamone, Ed.). CRC Press, Inc., Boca Raton FL. Ran, R.,Pittman, C. U. (l996), Met-Corzttrirzirfg Po/ym. Muter. (Proc. Int. Symp., C. U. Pittman Ed.), Plenum, New York, p. 241. Reichert, K.-H., Marquardt, P,, Eberstein, C. Garmatter, B., and Sylvester. G. (1996), World Patent WO 963 1 543 (Bayer AG). Ricci. G., Bosisio, C., and Porri, L. (1996). Mtrcnmol. Rapid Convmrn. 17( 11 ):78 I . Ricci, G., Panagia, A., and Porri, L. (1996a), P o l y r w r 37363. Saengcr, J., Tefehne, C., Lay, R., and Gronski. W. (1966), P o / w . h / / . 36:19. Schoenburg, E., Marsh, H., Walters, S., and Saltman. W. (1977). Rubber Cllern. Techno/. 52527. Schroedcr, K., Gehrke, K., Schmitz, G., and Lechner, M. D. (l992), Makrornol. Chetrz., Rapid Cotrztrturz. 1357 1. Schwindeman, J. A., Kamienski, C. W., and Morrison, R. C. ( 1 9 9 7 ~U.S. Pat. 5654371 (FMC Corp.). Schwindeman, J. A., Letchford, R. J. Kamienski, C. W., and Quirk, R. P. (1997a), World Pat. WO 9705174 (FMC Corp.). Seki, K., Fujiwara, M., Kamaike, K., Mori, K., and Kajiwara, A. (1996), Jpn. Pat. JP 9621781 1. Shalganova, V. G., Yudin, V. P., Semenova, N. M,,Mistyukova, L. N., Markova, Z. N., Zudina, N. N., Stankevich, V. V., and Ivanova, T. P. (1996), Soviet Pat. SU 788672. Sharaev, 0. K., Glebova,N. N., Markevich. I. N., Bondarenko, G. N., and Tinyakova, E. I. (l996), Vysokormd. Soedin. Ser. A. Ser. B. 38(3):447. Sierra, C. A., Galan, C., Gomez Fatou, J. M., and Ruiz Santa Quiteria, V. (1995), Ruhher Chertr. Techrzol. 68:259. Smirnova, L. V.. Tikhomirova, I. N., Kropacheva, F. N., and Zolotarev, V. L. (1996), Vysokortfol. Soediff. Ser. A. Se,: B. 38(3):458. Smirnova, L. V., Saraev, V. V., Cherkasov, V. K., Tikhomirova, I. N., and Kropacheva, E. N. (1997), R14.s.s. J. Koorcl. Chenf. 23(5):332. Song, J., Fan, H., Chen, D., Zhong, C., and Tang, X. (1995), Heckerzg Xirrngjirro G o r f p e 18:233. Stephens, H. L. (1989), P o / ~ v wHorltlbnok, r j r l f ed. (J. Bandrupt and E. H. Immergut, Eds.), John Wiley & Sons, New York. Sun, J., Eberstein, C., and Reichert, K.-H. (1997), J . Appl. Polynf. Sci. 64:203. Sun, Y. (1997), Hechertg Xictngjicro Gongye 2054.
Diene-Based Elastomers
833
Sun, Q., and Wang, F. (1988) Acta Po/ymericLr Sinica 2: 145. Sutton, D. E., and Schwindeman, J. A. (1997), Po/vrner Mater. Sci. Eng. 76:6. Suzuki, K., Maeda, K., Kamei, T., and Nakayima, T. (1997), Jpn. Pat. JP 9752912 (Ube Industries, Ltd.). Szwarc, M. (1956). Nuture 178:1 168. Sylvester, G., and Vernaleken, H. ( I 995), Eur. Pat. Appl. 647657 (Bayer AG). Takeuchi, M,, Shiono, T., and Soga, K. (1996), Mncromol. Ckrn. Phvs. 197729. Takeuchi, M., Shiono, T., and Soga, K. (1995). Mrrcrornol. R q ~ i dC o n m 16:169. Tanaka, Y., Sato, H., Nakafutamai, Y., and Kashiwazaki, Y. (1983), Macrornolecules /6:1925. Taube, R., and Sylvester, G. (l996), Apppl. Homogeneous Crttnl. Orgcrrtonlet. Con~p.(B. Comils. W. A. Herrmann eds.), WCH, Weinheim, Germany. Taube, R., Windisch, H., and Maiwald, S. (1995), Mokromd. Cl~em.M~c-ron~ol. Syrnp. 89393. Taube, R., Maiwald, S., Ruehmer, T., Windisch, H., Giesemann, J., and Sylvester, G. (1996). World Pat. WO 963 1544 (Bayer AG). Taura, K., Nishikawa, T., and Ishigaki, H. (1997). Jpn. Pat. JP 97143214 (Nippon Oil&Fats Co., Ltd. Thomas, R. M. (1969). Rubber Cl~em.T e c h . 42: 901 I . Thomas, R. M,, and Sparks, W. J. (1944), US Pat. US2,323,073 (JASCO, Inc.). Toba. T., Kiyomi, T., and Kitamura, T. (1997). Jpn. Pat. JP 9787426 (Bridgestone Co.). Toyoshima, T., Matsuda, H., and Onishi, H. ( 1 9 9 7 ~Paper 14, ACS Rubber Division Meeting. Cleveland, OH. Trick, G. S . (1960), J. A p p / . Po/vtn. Sci. 3:253. Tsujimoto, N., Suzuki, M., Tsukahara, K., Ikai, S.. Imaoka, K., Kai, Y., and Yamashita, J. (1996), Jpn. Pat. JP 9683863 (Ube Industries, Ltd.). Tsutsumi, F., Sakakibara, M,, and Oshima, N. (1990), Rubber Chern. Techno/.63%. Van, G. J. ( I 966), Ad\-. Chern. Ser. 52: 136. Wang, M. J., and Wolff, S. (1991). Rubber Chetn. Techno/. 64:559. Wang, M.J., Patterson, W. J., and Brown, T. A. (1997).Paper 25, ACS Rubber Division Meeting, Anaheim, CA. Wang, C., and Wang, G. (1997) Quingckro Huugorrg Xucyuan Xuebrro 18(1):67. Wang, Y., Yang, Y., Li, G., Liu, H., and Gu, M. (1996), Shioyu Hurrgong 25(6):385. White, J. L., Schaffer, T. D., Ruff, C. J., and Cross, J. P. (1995), Mncrornolecules 28:3290. Wilson, D. J. (1996), Polym. h ! . 3923.5. Wilson, D. J. (1995). J. Poly. Sci., Port A. Po/ytn. Chen~.33:2505. Wilson, D. J., and Jenkins, D. K. (1993, Po/vtn. Bull. 34:257. Xia, S., GC, X., Li, K., and Xia, S. (1997a), Cuihurr Xuebm /8(5):406. Xia, S., Xia, S., Zang, P,, Zhao, Q. (1997b), Quir~gr/uoHrccrgong Xueyunn Xuebno /8( 1 ): 1. Xia, S., Xiong, P. Bi, Y., Xia, S. (1996a), Q u i n g d m Hurrgong Xueyunn Xuebno /7(4):314. Xia, S., Xia, S., Zhao, Q. and Zang, P. (1996b), Quingdm H~trrgongXurvucrn Xrtebrro /7(3):205. Xia, S., Xia, S., Zhao, Q., and Zang, P. (1996c), Quingdao Huagon,q Xueyunn Xuebao /7(8):1297. Xu, L., Li, S., Chen, D., Zhong, C., and Tang, X. (1993, Quingdao Hucrgortg Xuevuon Xuebao 16(3): 297. Xu, L., Yu, L., Zhong, C., and Tang, X. (1997), Hecheng Xicrnlrjiao Gongye 20( 1):4 I . Yang, J., Pang, S., and Gong, Z. (1996). Yin,q.vong Hunxue 13(4):117. Yudin, V. P., Tikhomirov, G. S., Fil, V. G., Semenova, N. M,, Kartasheva, G. P,, Grachev, G. M., Rylkov, A. A., Usov, F. N., and Klementev, V. A. ( 1996), Russian Pat. SU 1352891, Yu, G., and Li, Y. (1996), The Polvmeric Materids Encyclopedicr (J. Salamone, Ed.), CRC Press, Inc., Boca Raton, FL. Zelinski, R., and Childers, C. W. (l968), Rubber Clletn. Techno/. 4 / :161. Zhang, X., Jin, G., and Zhao, S. (1997), World Pat. WO 9714722 (China Petrochem. Co.). Zheng, W., Li, Y., Yu, G., and Li, X. (1997), Heclreng Xinr~gjicroGongye 20:17. Zheng, W., Li, Y., Yu, G., and Li, X. (1996), Fertzi Cuihun 10:178. Ziegler, K. (1936), Angevc?. Chetn. 0 4 9 9 . Zhu, H., Zhong, C., Chen, D., Tang, X. (1996a), Shivou Hucrgong 25( l0):693. Zhu, H., Zhong, C., Chen, D., Tang, X. (1996b), Heclteng Xirrnlrjiao G o n g y 19(2):84.
This Page Intentionally Left Blank
Recycling of Rubber William
H. Klingensmith
Akron Consulting Co., Akron, Ohio
Krishna C. Baranwal Akron Rubber Development Laboratory, Inc., Akron, Ohio
1. INTRODUCTION
The interest in recycling of rubber has increased in the last decade. This has been driven by the concern about the effect of scrap tires and rubber products on the environment. Many tire companies, trade associations, government agencies, and private recycling firms have expanded their efforts in solving the problem. In the United States an estimated 270 million tires a year are scrapped. It is estimated that over 800 million used tires are stockpiled in various tire piles all over the United States. Large piles are reported in Ohio, California, and Texas. In addition, it is estimated that over 350 million pounds of rubber are scrapped from the production of nontire goods in the form of runners, trim, and pads. Many landfills are closing to scrap rubbers. Numerous small, medium, and large rubber companies are trying to find a way to deal with their scrap and are looking for ways to reuse it.
2. TIRE-DERIVEDFUEL It was estimated that over 172 million (64%)of the total number of tires would be used as tirederived fuel (TDF) in the United States in 1997, representing the single largest use of scrap tires in the US. The data in Table 1 summarize the total utilization of scrap tires in the United States for 1997 (1). TDF is expected to continue to grow as more and more cement plants, power-generation facilities, and pulp and paper plants continue to expand their use of scrap tires for fuel. The purpose of this chapter is to review the technical uses of scrap tires; the discussion of TDF is beyond its scope. A more thorough review of TDF can be found in Ref. 1.
3. OVERVIEW OF SCRAP TIRE USE With the efforts of so many companies and recycling firms trying to develop new uses for scrap tires, the results have been many new andexpanded uses. Table 2 summarizes the market demand for size-reduced rubber. 835
836Baranwal
and
Klingensmith
Table 1 Reported and Estimated Market Demand for Scrap Tires by Market Segment (millions of tires, except totals) Market segment
1998"
Tire-derived fuel Cement klins Pulp and paper mills Utility boilers Dedicated tire to energy Industrial boilers Resource recovery facilities Lime kilns Copper smelters Iron cupola foundries Total fuel Products Size-reduced rubber CISIP products Civil engineering Pyrolysis Agricultural Export Miscellaneous uses Totals Annual generation Scrap tire markets as % of total generation
I996
1997" 45.5 35 29.5 15 20.5 6
53 37 32 15
23 8 2
1
0 0 152.5 12.5 8 IO 0 2.5 15
I .5 202,000,000 266,003,000 75.9
1 1
I72 15 8 14 Unknown 2.5 15 1.5 228,000,000" 270,000,000" 84
Estimated. Source: Ret'. 1.
Table 2 U.S. Market Demand (lb.) for Size-Reduced Rubber from Tires (Ibs) 1998" Pneumatic tires Friction materials Molded and extruded goods Rubberplastic-bound products Athletic and recreation Asphalt products Total Estimated. Source: Ref. 2.
1996 48 million 8 million 18 million
140 million 8.5 million 524 million
24 million 168 million 400 million
50 million 200 million 582.5 million
58 39 36 IO 25 IO 3 1 4 186 18 8 18 Unknown 2.5 15 1.5 249,000,000" 275,000,O0Ot 90
ntaineer Mercury
Recycling of Rubber
4.
a37
AUTOMOTIVEINDUSTRY’SRECYCLINGEFFORTS
The automotive industry is a major user of rubber products. As such, it also is a major generator of used and scrap rubber parts. Through the Vehicle Recycling Partnership, a precompetitive cooperative effortbetween Ford, General Motors, and Chrysler, work is being done toestablish the most efficient way to dismantle and reuse automotive components. Eachof the automotive companies is also pursuing the use of recycled rubber in the components they purchase. Ford has been noticeably active in their tire efforts and are working closely with the tire companies to reach a goal of 10% recycle content. A recent announcement by Ford reports that 1.2 million Continental General tires on the F-series trucks containing recycled rubber will be in service. In addition, 100,000 recycled content tires from Michelin are projected for the Ford Windstar. The recycled rubber content is reported at 5%, mostly in the tread area (3). In the nontire area, the following are reported as using recycled rubber:
Ford Ford
Lincoln Navigator
Air reflectors Fender insulators
GM
Not disclosed (report over 1 10 parts) Not disclosed
Baffles Sound barriers
Chrysler
deflectors Water Splash seals
Many of the nontire components containing recycled rubber are in static applications. Much work is underway looking at using recycled rubber in more dynamic applications such as body seals, bushings, and gaskets. The goal is to utilize 25% postconsumer scrap in the automotive nontire components and 10% recycle content in tires (R. Pett personal communication). Dr. Robert Pett of Ford reports that the automotive industry’s goals are to have no more than 15% of the vehicles retired from service go to landfills by 2002, and this is to be reduced to 5% by 2015 (3). The automotive andtransportation industry is the biggest consumer of rubber goods, using an estimated 70-75% of all rubber articles produced. Experiments to use ground scrap tires in roads have proven useful in some areas. The product is known as crumb rubber modifier (CRM). California,Arizona, Texas, and Florida use CRM in their asphalt roads. Effortsto makethis a national standard through legislation were tried in 1991. A bill called the Intermodal Surface Transportation Efficiency Act (ISTEA) required incorporating 20 pounds per ton of CRM to portions of roads being paved. Because of the increased cost involved, many states resisted, and in 1995 the act was repealed. 5.
RECYCLINGMETHODS
At the present time the major methods of producing recycled rubber are reclaiming, ambient grinding, cryogenic grinding, and wet or solution grinding. Reclaiming of silicone and butyl rubbers is common, and the resulting recycled products are useful for cost reduction and improved processing when added to virgin compounds.
5.1
Reclaiming
In the past large volumes of reclaimed tires and tubes were used in the manufacture of tires and mechanical goods. Reclaiming is done by grinding the used rubber articles into small pieces
Klingensmith and Baranwal
838
and mixing it with a reclaiming agent, usually a thiol derivative, or exposing the material to steam digestion. The reclaim rubber can be added to rubber compounds at concentrations of 5-35%. The reclaim rubber improves processingandextrusionquality.However, it lowers tensile strength, tear, abrasion resistance, and green strength. The need for high green strength, abrasion resistance, and consistency in the manufacture of radial tires will lead to the decline and eventual elimination of reclaim rubber from themanufacture of tires. It is still used in small quantities in footwear, mats, solid tires, and low-end mechanical goods.
5.2 Ambient Versus Cryogenic Grinding Vulcanized scrap rubber is first reduced to a 2“ X 2“ or I ” X 1” chip. This can then be further reduced using ambient ground mills or frozen and “smashed” or ground into fine particles while frozen using cryogenic grinding. Below is a brief comparison of the two methods. Ambient Grinding
The ambient process often uses a conventional high-powered rubber mill set at close nip, and the vulcanized rubber is seared and ground into a small particles. Itis common to produce10-30 mesh materials using this relatively inexpensive method to produce relatively large crumbs. Typical yields are 2000-2200 pounds per hour for 10-20 mesh and 1200 pounds per hour for 30-40 mesh. The finer the desired particle, the longer the rubber is let run on or in the mill. In addition multiple grinds can be used to reduce the particle size. The lower practical limit for the process is the production of 40 mesh material. Any fiber and extraneous material must be removed using air separation or an air table. Steel is separated using a magnetic separator. The resulting material is fairly clean. The process produces a material with an irregular jagged particle shape. In addition, the process generates a significant amount of heat in the rubber during processing. Excess heat can degrade the rubber, and, if not cooled properly, combustion can occur upon storage. Cryogenic Grinding
Cryogenic grinding usually starts with chips or a fine crumb. This is cooled using a chiller. The rubber is put through a mill white frozen. This is often a paddle type mill. The Best Practice on Cryogenic Grinding covers this process in detail. The final product is a range of particle sizes, which are sorted and either used as is or passed on and further size reduction performed, e.g., using a wet-grind method. A typical process generates 4000-6000 pounds perhour. Typical sizes are 60-100 mesh. The cryogenic process produces fairly smooth fracture surfaces. Little or no heat is generated in the process. This results in almost no degradation of the rubber. In addition the most
Table 3 AmbientVersusCryogenicallyGroundRubbers
Physical property Ambient Cryogenic ground ground gravity
Specific Particle shape Fiber content Steel content cost
Irregular
Regular
0.5% 0.1%
Nil
Comparable
Nil Comparable
Recycling of Rubber
839
Table 4 Particle Size Distribution for Two 60MeshGround
Rubbers Amount retained Ambient 10-12
30 mesh 40 mesh
60 mesh 80 mesh 100 mesh Pan
(%)
2
15 60-75 15
5 5-10
Cryogenic
(5%)
2 35-40
35-40
20 2- 10
significant feature of the process is that almost all fiber or steel is liberated from the rubber, resulting in a high yield of usable product and little loss of rubber. The price of liquid nitrogen has come down significantly recently, and cryogenically ground rubber can compete on a large scale with ambient ground products (4). Table 3 compares the properties and benefits of ambient and cryogenically ground rubbers. Table 4 shows the particle size distribution for two typical 60 mesh ground rubbers. One was prepared ambient and the other cryogenically. Table 5 shows the properties of rubber compounds containing ambientground rubber and cryogenically ground rubber.
6.
SOLUTION OR WET GRINDING
Micromills or micromilling, also called the wet process, is apatentedgrindingprocess for ultrafine grinding. It reduces particle sizeby grinding in a liquid medium, usually water. Grinding is performed between two closely spaced grinding wheels. A certificate of analysis for a typical lot of wet-ground rubber, Ultrafine 80 from Rouse Rubber, is shown in Figure 1. A review of scrap tire processing was published by Astafan in Tire Technology Intemational '95 ( 5 ) . In addition, microwave (6), ultrasonic (7), chemical devulcanization (8) microbial degradation, and mechanical shear have been used to produce recycled rubber. Ultrasonic and chemical devulcanization are discussed in detail in Tire Technology International '96 (9). 7.
SURFACE TREATMENT AND ADDITIVES FOR PRODUCING RECYCLED RUBBER
Numerous methodshavebeenand are being used to modifythesurface or composition of recycled rubber to make it more compatible or useful. These include halogenation liquid polymers (10): thermoplastic polymers, homogenizing agents (1 l), and wetting agents. These are too extensive to cover here, but they are reviewed in detail in the Clean Washington Center's (CWC) Best Practices Manual (4). Many other processes are also being studied to enhance the value of ground scrap rubber.
8. TESTINGSTANDARDS Recently we summarized the quality, testing, and handling issues for dealing with scrap tires and rubber for the CWC Best Practices Manual (4). In December 1996 ASTM published two
840
Klingensmith and Baranwal
Table 5 Properties of Different Rubber Compounds The following data show the effects of a 20 mesh, ambient-ground rubber compounded into an SBR 1502 compound. The ground rubber was evaluated at 17, 33, and SO% levels. The compound recipe is as follows: Ingredient
Level, phr
SBR1502 Zinc oxide Stearic acid TMQ N660 Carbon black Aromatic oil Sulfur MBTS TMTD
100.0
5.0 1.o 2.0 90.0 50.0 2.0 1.o 0.5
The 20 mesh crumb was added at 17, 33, and 50%. The properties of the materials are as follows: 0% Ground Mooney Viscosity, MU Rheometer max torque, Ibf inch tc90, min. Tensile strength, psi Ultimate elongation, %
40 59 2.5 1470 330
17% Ground 34
61 47 2.4 1IS0 270330
33% Ground
50% Ground
91 33
111
1.8
870 300
2.0 560
The following data shows the effect of concentration and particle size of a cryogenically ground rubber on an EPDM compound: CryOfine ground rubber used at 10% levels (except control at 0%) Control Tensile strength, psi 1290 Ultimate elongation, % 100% modulus, psi 300% modulus, psi 1220 Hardness, Shore A Die C tear, ppi 175
1410 410 535 1180 1220 73 193
40 Mesh 330 490 1230 70
60 Mesh
80 Mesh
100 Mesh
1430 340 530 1230 70 173
1470
400 490
1440 380 480
70 171
71 172
CryOfine ground rubber used at 2070 levels (except control at 0%) Control Tensile strength, psi 1230 Ultimate elongation, % 100% modulus, psi 300% modulus, psi 1220 Hardness, Shore A Die C tear, ppi
1410 1410 410 535 1180 13 181193
40 Mesh 1460 320 450 72 165 178
60 Mesh 1360 390
so0 1200 1300 70 163
80 Mesh
100 Mesh
390 460
390 460 1 l60 68
69
841
Recycling of Rubber
Rouse Rubber Industries, Inc. P.O. BOX 820369 VICKSBURG, MS 39182-0369 TELEPHONE: 601-636-7141 FAX: 601-636-1181
CUSTOMER: R: ORDER HIPPED: DATE SHIPPED: WEIGHT
2113/98 I O Ib
Stock:
GF-80
SIEVE ANALYSIS: SCREEN: 30M 40M 60M 80M 1OOM 200M PAN
% PASSING
% RETAINED
0 0 0 5 6 44 45
0
MOISTURE:
0.67
TGA ANALYSIS: ACETONE EXTRACTION: ASH: CARBONBLACK RHC:
13.59 7.963 30.21 48.24
Sample. W-80
0
0 6 6 39 49
TGA
Slzt. 6 . 7 3 3 0 mg Method: rubber. t e s t s
F i l e . O.\TA\rGA\GATA\RUNO219.g01 Operator: TGA TA Team Hun O a t e : 20-Feb-9a 01.03
n
13.39% a c e t u m c n t r a c : (0.9149mg)
80
In
-I
-==7 f l/ I l\
4 8 . 2 4 % RHC (3.2481119)
I
I
30.26 carbon black 17 O d m o \
'
l-
L " L~ IPJb:,.I, lI . ' r 00
10
20
30
40
Time (mln)
Fig. 1 Certificate of analysis.
50
(0.5361mg) 60
7 0 -10
Universal V 1 108 TA I n s t r u m e n t s
Klingensmith and Baranwal
842
documents: ASTM D-5603-96 (12) and ASTM D-5644-96 (13). In late 1997 the Chicago Board of Trade (CBOT) also published a document that includes definitions of terms and particle size specifications of recycled rubber for buying and selling materials (14). Thus, there are specifications available for recycled rubber that vendors and customers should use to ensure material quality.
9.
MATERIALSTORAGE
Recycling of rubber from whole tires into chipsand crumbs can generate heat up to 220-240°F. At these temperatures and in the presence of oxygen, spontaneous combustion can occur. Also, the presence of iron catalyzes oxidation of natural rubber. To minimize any such problem, material should be cooled with air or water before storage or shipping. Make sure the material temperature is below 200°F. Avoid iron, store at ambient temperature, and not in metal sheds or warehouses at both vendor’s and customer’s location.
10. MOISTURECONTENT
The current accepted level of moisture is 1% (ASTM D-5603-96). Typically, however, it is less than 1%. Too much moisture can cause caking andmay inhibit free flowin processing. Anticaking agents suchas calcium carbonate can be used. Moisture build-upcan lead to acidic conditions giving slower cure rates in compounds. Therefore, recycled crumb rubber should be stored in a cool and dry place. Moisture content is determined by heating a weighed amount of sample at 125°C for one hour, cooling and weighing again. The difference in sample weight is the heat loss (ASTM D1509-95).
11.
BULKDENSITY
Because of the particulate nature of crumb rubber, it is rather difficult to measure specific gravity of crumb rubber. Bulk density measurement may be more appropriate. There is no bulk density specification for crumb rubber; however, ASTM D-15 13 for carbon black can be used. Our recommendation is that bulk density be part of material specification and a range of values be agreed upon between vendor and customer. Another way of determining specific gravity of crumb rubber may be done by making solid sheets or pieces by passing crumb rubber through a tight mill nip and measuring density of compressed pieces.
12. SAMPLINGAND QA TESTING
In crumb manufacturing plants, two samples, each about 125 g, are taken from each skid (about 1000 kg) at the timeof bagging. At the customer’s site, two samples per truckload are tested for percent moisture and bulk density.For ash,carbon black, acetone extract, and rubber hydrocarbon content, testing is doneoncea day per shipmentaccording to ASTMD-5603-96.Specific frequency and sampling procedures may be agreed upon between vendor and the customer. For ambient ground materials, moisture content and bulk density measurements are made on every
Recycling of Rubber
a43
skid because of possible moisture content variations in feedstock. Vendors should send material specification conformance data along with shipment to customers.
13. CHEMICAL ANALYSIS AND MATERIAL SPECIFICATIONS
ASTM D-5603-96 lists specificationsfor acetone extractable, ash, moisture, carbon black, natural rubber, and rubber hydrocarbon contents for recycled rubber. As mentioned in this document, these chemical tests are done according to ASTM D-297. This specification also lists maximum metal content of 0.1% and fiber content of 0.5% in whole tire crumb. Fiber and metal contents in tread buffing should be zero. In production of recycled rubber steel wire pieces are separated by magnetic separator. Fibers are separated by use of vibrating screen table and “vacuuming” off the “fabric balls” from the top of the screen. To determineiron content, a preweighedamount of recycled rubber is spread on a nonmagnetic flat surface. A small magnet is used to go over the material, which should pick up steel pieces. The weight of the material thus picked up is obtained. However, for very small particles of crumb rubber (c100 mesh), the magnet may pick up recycled rubber particles as well. In that case atomic absorption (AA) should be used to determine amounts of iron.
14. PARTICLE SIZE AND DISTRIBUTION
Particle size and distribution of recycled cured rubber particulates are determined by the RoTap method as described in ASTM 5644-96. Six sieves are used in this mechanical shaker. The first two screens are defined in the above document for 10, 20, 30, 40, 60, 80, and 100 mesh particle size designations (see Table 6). The remaining four screens are to be decided upon by the vendor and customer. About 100 g of crumb rubber are weighed and put on the top pan with a cover and five other screens. After a fixed time of running the shaker, materials in each panareweighedandplotted as afunction of screensizegivingparticlesizedistributions. Vibrators and sieves areavailable from most scientific suppliers. This technique works well for
Table 6 Recycled Rubber Product Deslgnation Nominal product designation
Example ASTM D 5603 designation”
Zero screen (km)
Percent retained on zero screen
10 Mesh 20 Mesh 30 Mesh 40 Mesh 60 Mesh 80 Mesh 100 Mesh
Class 10-X Class 20-X Class 30-X Class 40-X Class 60-X Class 80-X Class 100-X
2360 (8 mesh) 1180 (16 mesh) 850 (20 mesh) 600 (30 mesh) 300 (50 mesh) 250 (60 mesh) 180 (80 mesh)
0 0 0 0 0 0 0
Size designation screen (km)
2000 (10 mesh) 850 (20 mesh) 600 (30 mesh) 425 (40 mesh) 250 (60 mesh) 180 (80 mesh) 150 (100 mesh)
Maximum percent retained on designation screen
5 5 10 10 10 10
10
“When specifylng materials replace the X with the proper parent materlal grade deslgnation code. For example Class 30-2 would indicate a 600 k m (30 mesh) product made from Grade 2 material. car. truck, and bus tread rubber. Class 100-6 would indicate a 150 k m (100 mesh) product made from Grade S material, nontlre rubber.
a44
Klingensmith and Baranwal
coarser particles (>S0 mesh). For 80 mesh and finer, small balls are formed as a result of particle agglomeration on screens giving higher “apparent” particle sizes than they really are. Several other techniques for determining finer particle sizes are being evaluated by Dr. Baranwal in our laboratory. We have developed an ultrasonic technique where a small quantity of crumb rubber is put in a nonsolvent liquid exposed to low levels of ultrasonic energy. The resulting dispersion is put on a glass slide and dried. Using an image analysis software program, particle size distribution is obtained. Our experience is that this technique works well even with small particles, i.e., up to 1-2 pm. Some of the other commercially available techniques are those from Coulter Corporation (Miami, Florida), Malvern Instruments Limited (Southborough, Massachusetts), Particle Sizing Systems (Langhorn, Pennsylvania), and Elcan Industries, Inc. (New Rochelle, New York).
REFERENCES 1. Scrap Tire Use / Disposal Study (1996), Scrap Tire Management Council. 2. Scrap Tire Recovery, An Analysis of Alternatives. ( 1 9 9 8 ~published by Goodyear Tire and Rubber Company. Akron, Ohio. s, 23, 1998. pp. 22-23. 3. Ruhher rrnd Plostics N ~ M ~February 4. Best Prrrcticrs it1 Scrtrp Tires & R&!wr Recycliug. Clean Washington Center, June 1997. 5 . Scrap Tire Processing in the US, Tire Technology International 95, by Charles Astafan, 1995. 6. U.S. Pat. 4,104,205. Microwave Devulcanization of Rubber, August 1, 1978. 7. U.S. Pat. 5,258,413. Ultrasonic Devulcanization, November 1993. 8. European Pat. Application EP 0690 901 A l , Application No. 95301399.2, filed 03.03.1995. 9. Ultrasonic Devulcanization of Tire Compounds and De Link Concept Tire Technology International 1996, pp. 82-84, 87-88. IO. U.S. Pat. 4,481,335 Rubber Compositions and Methods, issued to Fred Stark, Nov. 6, 1984. 1 I . U.S. Pat. 5.510,419, Polymer Modified Surface, issued to Burgoyne. Fisher, and July, April 23, 1996. 12. ASTM D-5603-96. Standard Classification for Rubber Compounding Materials-Recycled Vulcanizatc Particulate Rubber. 13.ASTMD-5644-96,StandardTestMethodforRubberCompounding Materials-Determination of Particle Size Distribution of Recycled Vulcanizate Particulate Rubber. 14. Chicago Board of Trade, Crumb Rubber (Tire or Non-Tire), CrumbRubber Grades Definitions, 1997.
EPDM Rubber Technology Richard Karpeles Crompton Corporation/Uniroyd/ Chemical Company, Inc., Naugatuck, Connecticut
Anthony V. Crossi Crompton Corporation/Uniroya/ Chemical Company, Inc., Middlebury, Connecticut
1. INTRODUCTION 1.l
Nomenclature and Structure
EPDM is the designation agreedupon by ASTM and IISRP for ethylene-propylene rubber, where “E“ and “P” stand for ethyleneand propylene, respectively. “D” designates the nonconjugated diene that provides a site of unsaturation for sulfur vulcanization, and “M” refers to the polymethylene saturated backbone of the polymer, i.e., CH2-(CH2),,-CH2(Fig. 1). Several good reviews on EPDM rubber have been published by Baldwin and Ver Strate (1972). Ver Strate ( 1986), and Allen and Easterbrook (1987).
1.2 EPDM Weather Resistance The saturated backbone of EPDM is the main structural feature that provides this rubber with its excellent weather and chemical resistance. Sites of unsaturation in a polymer are the primary point of attack for oxidants. Oxidative cleavage of double bonds contained in the backbone of a polymer. such as a diene rubber, will reduce polymer molecular weight and result in loss of desirable physical properties such as tensile, modulus, tear, oil swell, etc. In EPDM the sites of unsaturationarependant to thebackbone,andoxidation of the double bond will not affect ultimatephysicalpropertiessignificantly.Commercial EPDM containsbetweenzeroand IO wt% nonconjugated diene and therefore contains significantly less unsaturation than styrenebutadiene rubber, butadiene rubber, polyisoprene, etc.
1.3 Interaction with Oils and Carbon Black EPDM is a nonpolar substrate. It has excellent compatibility with aromatic, naphthenic, and paraffinic mineral oils. BecauseEPDM’s level of unsaturation is lowerthan thatof diene rubbers, EPDM interactslessstrongly with carbonblackthan diene rubbers. This results in a very 845
846
Karpeles and Grossi
CH 3 Fig. 1 Structurc of EPDM containing ENB termonomcr.
reasonable compounded Mooney viscosity and good physical and mechanical properties, even at very high filler loading.This makes EPDM an economical rubber for many common applications.
1.4
HighMolecularWeightEPDM
For years EPDM was available only as a high molecular weight rubber ranging from 300,000 to greater than 1,000.000 daltons as measured by get permeation chromatography (GPC) using polystyrene (PS) standards. All molecular weights referenced in this chapter will be PS equivalent. GPC of high molecular weight EPDM is carried out at 150°C using ortho-dichlorobenzene or trichlorobenzene as solvent. Care must be taken when comparing data in patents and open literature, where both polystyrene and polyethylene equivalent molecular weights are reported. For linear polymers in orrho-dichlorobenzene, the polyethylene equivalent molecular weights are a factor of approximately 2 less than polystyrene equivalent molecular weights. This factor decreases as branching increases. High molecular weight EPDM is used primarily in construction and automotive markets. Included in constructionapplicationsaresingle-ply roof sheeting, doorandwindowseals, spacers, wire and cable sheathing and insulation. and hoses and seals for water systems. Diverse automotive application areas include radiator hoses. sponge and dense weather seals, wire and cable sheathing and insulation, thermoplastic elastomer and thermoplastic vulcanizate bumpers and interior surfaces, engine oil thickeners and dispersant-thickeners, tire inner tubes, tire sidewalls, seals (gaskets and O-rings), air ducts, bellows, shock mounts, and belts. Although used primarily in construction and automotive market areas. EPDM also finds application in diverse markets, such as medical devices, oil well and pipeline applications, rolls, sporting equipment, impact modification of engineering plastics by functionalized EPDM. etc. Jebens and Kaufman (1996) have published a marketing research report on ethylene-propylene elastomers. Recent advances in process engineering have now extended the molecular weight range of the EPDM family to cover a continuum of molecular weights, including traditional high molecular weight products, intermediate molecular weight products ranging between 80.000 and 300,000 daltons.andlowmolecularweightmaterialswithmolecularweightsup to 80,000 daltons.
1.5
Low MolecularWeightEPDM
In addition to many of the traditionalrubberapplications. EPDM products of intermediate molecular weight provide advantages in molded and extruded applications, as components of
EPDM Rubber Technology
thermoplastic elastomers, as lube oil thickeners, and as reactive plasticizers weight rubber compounds. 1.6
847
for high molecular
Very Low Molecular Weight EPDM
Lowmolecularweightproductsrangingfrom liquid oligomers to polymers with molecular weights up to 80,000 daltons are alsocommercially available in both copolymer and terpolymer grades from Uniroyal Chemical under the tradenameTrileneBand copolymer grades from Mitsuiunder the trade name Lucant@. These productsaremadewithbothmetallocene and traditional Ziegler-Natta catalysis. The GPC of very low molecular weight EPDM is run at low temperature (e.g., 35°C) in tetrahydrofuran. These unique very low molecular weight products find use as reactive plasticizers, encapsulants, viscosity modifiers, synthetic oil components, adhesives, and sealants.
2. 2.1
EPDM PHYSICAL FORM Bales
EPDM is most commonly supplied in solid rectangular bales, which can weigh between 25 and 34 kg (55 to 75 Ib), and can vary in density from 0.5 to 0.86 gkc. Bales are individually wrapped in an ethylene vinylacetate or a polyethylene film. The film-wrapped bales are packaged in a multitude of ways, including reusable and nonreusable cardboard containers on wood pallets, reusable aluminum containers,high-densitypolyethylenestretchwrapping on woodpallets, wood boxes. etc. To obtain faster mixing, bales can be supplied in a low-density, “friable” form. These friable bales break apart more easily in internal mixers allowingfor faster carbon black dispersion Friable bales are usually reserved for EPDM grades with high ethylene content. because they contain enough crystallinity and green strength for bale shape retention. Polymers with low ethylene content are supplied in dense bales because their lack of crystallinity and low green strength will cause the polymer to flow and coalesce. resulting in nonuniform bale densification.
2.2
Pellets
A small number of product grades are supplied in pellet form. These products are sold into the
wire and cable and thermoplastics markets where continuous extruder feeding is required. Pelletized products also tend to have a high ethylene content to avoid coalescence of the pellets into a solid mass. Even with a high ethylene content, EPDM pellets must be treated with an antiagglomeration agent, such a s polyethylene dust, to keep them free-tlowing.
2.3
Crumb Rubber
A limited number of manufacturers offer granulated crumb rubber as an alternative product form but this product form is very difficult to store. Due to the irregular shape of the granulate, the granulated EPDM coalesces quickly and densifies.
2.4 Latex No EPDM manufacturers offer product in a latex form. EPDM latex stability tends to be poor, and upon storage the solidswill rise. This process, called creaming, can be reversed by agitation.
Karpeles and Grossi
848
As withall latexes, EPDM latex is susceptible to biological attack. EPDM latex finds applications in coatings and in blends with other latexes.
2.5
Extender Oils
EPDM grades at the high molecular weight end of the spectrum (i.e., >600,000 daltons) are routinely extended with 50- 100 parts of mineral oil per 100 parts of rubber to lower the raw polymer’s viscosity. The reduction in viscosity of high molecular weight EPDM grades resulting from oil extension benefits both the manufacturer, by providing easier polymer finishing in the back end of the manufacturing process, and the end user, by providing improved processing (easier and/or faster mixing). Many different types of extender oils are used, but there is a clear trend toward the use of paraffinic oils (especially white oils) and away from use of aromatic oils (due to toxicological concerns) and naphthenic oils, which are usually darker in color than the paraffinic oils. Additional benefits to the use of white oils are improved stability toward exposure to both sunlight and tluorescent light and improved raw polymer color for colored (non-carbon black containing) end-use applications.
3. EPDMHEATSTABILITY EPDM is manufactured containing phenolic antioxidants to ensurestorage stability. The antioxidants are nonstaining for automotive applications in proximity to painted surfaces. When stored in a cool, dark environment, EPDM should have a long shelf life. Exposure to heat (Baranova et al. 1970),light, or chemical agentswill reduce the polymer’s shelf life. In general, raw EPDM rubber is stable at elevated temperatures up to 150°C for short periods of time. During the heat-induced EPDM degradation process, both chain scission and crosslinking occur (Saha Deuri and Bhowmick, 1987). This dynamicprocess can be studied using rheological measurements such as complexviscosity and tangent delta (the ratio of the viscous to the elastic modulus). The value of tangent delta is very sensitive to small increases in branching, which simultaneously decreases the viscous modulus and increases the elastic modulus. Tangent delta also reveals the increase in the viscous component due to chain scission. With phenolic antioxidants, it can be observed that early in the heat aging process, tangent delta does not change significantly, but there is evidence that chain scission is occurring by the tackiness of the polymer surface. It must therefore be concluded that during this time period, chain scission and crosslinking are occurring simultaneously. Later in the degradation process, however, crosslinking dominates and the viscosity rises and tangent delta drops. Addition of phosphite co-antioxidant appears to shift the balance between the degradation pathwaysto favor crosslinking, eliminating the early tackiness noted when using phenolics alone. Figure 2 shows the effect of heat aging at 121°C on EPDM’s viscosity and tangent delta. The presence of transition metal impurities greatly affects the stability of EPDM. Vanadium, iron, and other transition metals are pro-oxidants that catalyze degradation.
4. 4.1
EPDMLIGHTSTABILITY Sunlight
All EPDM is susceptible to degradation due to exposure to sunlight. Although EPDM grades containing ENB termonomer exhibit the greatest sensitivity to sunlight, DCPD- and 1,4-IID-containing terpolymers and ethylene-propylene copolymers also exhibitsensitivity to sunlight expo-
849
EPDM Rubber Technology
1,01E*06
8,iOEtOS
5
.-b VI
0 0
p5 6.10Et05
.U vi
5s c-!
0""
4'i0E+05
-p
F
U
2,10E*05
1,WEt04
0
50
100
200
150
3W
250
350
4W
450
500
Hours at 125OC
Fig. 2 Dynamic viscosity and tangent delta of heat-aged EPDM.
sure. Lightexposure results in formation of hydroperoxides (De Paoli, 1988; De Paoli and Duek, 1990; De Paoli et al., 1990; Chmela et al., 1996) that ultimately cause surface gelation via crosslinking. When the raw polymer additionally contains extender oils, sensitivity to sunlight (i.e., the potential for gelation)is even greater. Tangent delta, again,is a useful tool for determining the presence of surface gelation (see Fig. 3).
1
-
75 phr Oil Exlended High ENB
VI
.
Grade
I% 0,95
1 V-
5
0~9
:
X 0.85
W
-
8
W
d .
Non-all Extended Hlgh ENB
2-
0,8
Grade
0,75
___~
~-
0,7 0
1
2
3
4
Hours of Sunlight Exposure
Fig. 3 Effcct of sunlight exposure on tangent delta of EPDM.
5
6
7
Karpeles and Grossi
850 1.2 Non-Oil Extended High EN6 Grade
II
75 phr 011Extended Hgh ENB Grade
0 0
5
10
15
20
25
Days of Fluorescent Light Exposure
Fig. 4 Fluorescent light exposure effect on EPDM tangent delta
4.2
FluorescentLight
Surface gelation also occurs due to fluorescent light exposure of oil-free and oil-extended grades of EPDM containing ENB (see Figs. 4.5). The rate of surface gelation occurs in the following order: ENB grades containing aromatic extender oils > ENB grades containing naphthenic oils > ENB grades containing paraffinic nonwhite oils > ENB grades containing paraffinic white oils > oil-free ENB grades. In general, DCPD- and 1,4 HD-containing polymers and ethylenepropylene copolymers donot exhibit sensitivity to fluorescent light, even when oils are present.
45
Days Exposure to Fluorescent Light
Fig. 5 Branching Gel formation of light-exposed EPDM.
EPDM Rubber Technology
851
5. EPDMSTRUCTURE,COMPOSITION, AND PROPERTIES As discussed previously, the versatility of EPDM rubber arises from: 1. Its unique combination of weather and heat resistance due to its saturated polymethylene backbone 2 . Its reasonable compounded cost, because its high molecular weight allows for high extendibility with inexpensive oils and fillers 3. The structural diversity that can be designed into the polymer by manufacturers.
Controllable structural propertiesinclude molecular weight (MW), molecular weight distribution (MWD), diene type and content, level of branching, ethylene/propylene monomer ratio, monomer distribution along the polymerchain, and the homogeneityor heterogeneity of different polymer chains. EPDM structural properties are influenced by a variety of factors in the polymerization process, which will be discussed in the following section. 6. THE EFFECT OF ETHYLENE AND PROPYLENE CONTENT ON EPDM
PROPERTIES The character of EPDM changes greatly based on the ratio of ethylene to propylene in the polymer. An ideally alternating ethylene-propylene copolymer would contain 40 wt% ethylene (50 mol%) and 60 wt% propylene (50 mol%). Polymers of this composition are amorphous. Commercial polymers, however, generally contain between50 and 80 wt%ethylene. The boundary values of ethylene were chosen for practical reasons. Above 75% ethylene, EPDM is extremely hard and difficult to mix in internal mixers. Below 50% ethylene, traditional vanadiumbased Ziegler-Natta catalysts have difficulty incorporating propylene at an acceptable commercial production rate. These catalysts exhibit significantly higher reactivity toward ethylene than toward propylene. Manufacture of propylene-rich EPDM grades is therefore slower and more costly. As additional carbon atoms are added to the length of the alpha-olefin chain, the ZieglerNatta catalyst’s reactivity toward the alpha-olefin decreases. The effect of polymer structure on low-temperature properties (Kontos and Slichter, 1962; Martini and Milani, 1986; Avella et al. 1987; Mahlke,1987)and the relationshipbetweenglasstransitiontemperatureandpolymer composition (Baldwin and Ver Strate, 1972) have been reported. 6.1
Effect of Ethylene Content on Crystallinity
Low Ethylene Content Polymers with ethylene contents at the low end of the commercial range, i.e., containing 50-55 wt% ethylene, are totally amorphous and exhibit no ethylenecrystallinity above their low glass transition temperature (Fig. 6) as observed by differential scanning calorimetry (DSc). They are soft and pliable and have excellent low-temperature flexibility and compression set, but they cannot accept high levels of fillers. Between 56 and 62% ethylene, EPDM containslonger and/or morefrequentethylene sequences and exhibits a below room temperature melt transition as seen by DSc. This lowtemperature crystallinity only influences the most demanding low-temperature applications. Interinediate Ethylene Content Products with intermediate ethylene content, (63-67%) will contain both below room temperature crystallinity and a small amount of higher-temperature crystallinity between 40 and 60°C.
Karpeles and Grossi
852
45
40
-
35
€/P = 75/25 L
E
g
a
25
EIP = 68/32
L 20 CI
g
15
€/P = 51/49
I 10
5 0
m
-61 -71 -81 -91
-51
4 1-11 -21 -31
-1 19 9
29
39
49
59
69
79
89
99
1W138 128 119
148
Temperature ("C) Fig. 6 DSC thcrmograrns of EPDM with varying ethylene/propylene ratios.
High Ethylene Content High-ethylene polymers (6840%) exhibit high green strength, high vulcanizate tensile strength and toughness at room temperature, and they can accept high filler loading. These products, however. will have inferior low-temperature properties due to significant crystalline melt transitions, both below room temperature and at 40-60°C. EPDM produced by conventional vanadium-based Ziegler-Natta catalysis generally does not have any melt transitions above 60°C and does not contain any lamellar crystals indicative of true polyethylene crystallinity. Polymers produced with titanium-based catalysts can contain melt transitions above 60°C.
6.2 AnalyticalTechniques The monomer composition of EPDM is measured by Fourier transform infrared spectroscopy (FTIR) using a transparent thin film of rubber (Noordermeer, 1996). The test methodology is described in ASTM D3900 (1994). The infrared instrument is calibrated using standard EPM polymers whose composition has been determined by nuclear magnetic resonance (NMR) as described by DiMartino and Kelchtermans (1995). The EPDM industry has standardized this test method, but not the reporting format. Reporting options include: wt% Ethylene 100%) 2. wt%Ethylene 3 . mol%Ethylene
+
1.
wt% Propylene = 100% (thediene content is additional to the
+ wt%Propylene + wt% Diene = 100% + mol%Propylene = 100% (thediene content is additional to the
100%)
4. mol% Ethylene
+ mol%Propylene + mol%Diene
=
100%
When comparing EPDM grades made by multiple manufacturers, the method of compositional reporting must be considered.
853
EPDM Rubber Technology
3 ENB
DCPD
1,4-HD
Fig. 7 Commercialdienes used for EPDM.
7. DIENES The choice of diene heavily influencesEPDM properties and structure. Three dienes are currently usedinthemanufacture ofEPDM: 5-ethylidene-2-norbornene (ENB), dicyclopentadiene (DCPD), and 1P-hexadiene (HD). Their structures are shown in Figure 7. Cyclic dienes suchas ENB and DCPD influence thelow-temperature properties of EPDM by increasing the glass transition temperature (Tg) due to their rigid structure, but they also reducecrystallinity by breakingup ethylenesequences. VerStrate (1972) reported that Tg increases 0.8"C for every wt% ENBin the polymer, up to a maximum of 10%.Linear, nonconjugated dienes similarly reduce ethylene crystallinity by breaking up long ethylene sequences and increase Tg to a lesser extent than cyclic dienes.
7.1 ENB ENB termonomer is the most widely used by EPDM manufacturers because ENB is a fastcuring diene with a sulfur cure system due to the six allylic hydrogens on carbon atoms adjacent to the olefinic bond (Baldwin et al., 1970). The allylic hydrogens are the sites of attack of the cure system. ENB is a bicyclic, nonconjugated diene that incorporates effectively into EPDM during the polymerization process.This occursbecause the double bond contained in the bicyclo[2.2.l.]heptene (norbornene) portion of the ENB structure places a strain on the ring system. This strain is eliminated when the monomer is incorporated into the polymer backbone and the double bond is eliminated. The second, noncyclic double bond is available for crosslinking. Use of ENB termonomer results in a product with low to moderate long-chain branching. Branching arises during thepolymerizationprocess from cationicsidereaction of ENB's double bond outside of the ring structure. This branching reaction is easily controlled to achieve specific levels of branching, which provides the desired processability.
7.2 DCPD EPDM grades containing DCPD termonomer are not as popular because DCPD is slower curing than ENB. It contains only three allylic hydrogens on carbon atomsadjacent to the double bond that can participate in vulcanization. DCPD contains two cyclic double bonds, and like ENB, the double bond at the 2 position of the bicyclo[2.2.1 .]heptene portion of the molecule incorporates easily into the polymer backbone. The second double bond in the attached five-member ring is available for vulcanization. Unlike ENB, however, the second double bond in DCPD can participate in the Ziegler-Natta polymerization, resulting in a highly branched polymer with
a54
Karpeles and Grossi
broad MWD. DCPD that has participated in branching is not available for crosslinking, further reducing the polymer’s cure rate. Moreover, polymers with broad MWD are generally slower curing.
7.3 1,4-HD
1,4-HD is a linear nonconjugated diene. It has one terminal and one internal double bond. The terminal double bond is incorporated into the polymer backbone, and the internal double bond is available for vulcanization. Although the internal double bond has five hydrogens on adjacent carbons, it is much slower curing than ENB. One possible explanation reported in the patent literature is that up to 25% of the 1,4-HD isomerizes to yield a saturated cyclic structure (U.S. 3,467,633, 1969)that cannot take part in sulfur-based crosslinking. 1,4-HDtermonomer is essentially a long-chain alpha-olefin and therefore is less reactive towards the Ziegler Natta catalyst. It also does not have the elimination of ring strain as a driving force for reaction like ENB or DCPD, 1,4-HD inherently provides a linear polymer structure with narrow MWD because the internal double bond is inactive toward either Ziegler Nattd catalysts or acid-catalyzed cationic branching.
7.4 AnalyticalTechniques Diene content can be determined by either high-temperature refractive index (HTRI), FTIR of thick polymer films, or NMR spectroscopy. A standardized test method has been adopted based on athickfilminfraredtechnique (Noordermeer,1996). Although not practical for routine quality control testing, NMR spectroscopy is employed to certify the reference standards used to createtheFTIRcalibrationcurves. The FTIRtechniqueprovidessignificantlyimproved (lower) standard deviation than the HTRI test.
7.5
Branching
Branching in EPDM can be measured indirectly by the “branching index,” the logarithm of the ratio of the zero-shear viscosity of an EPDM to that of a linear copolymer having the same intrinsic viscosity (Beardsley and Tomlinson, 1990). Tangent delta, the ratio of the viscous to elasticmodulus, is also an indicator of branching (BeardsleyandHo, 1984), but itis also influenced by changes in MWD. In general, the lower the tangent delta, the greater the branching/ MWD. Branching greatly influences the viscosity of compounded EPDM. Branched or broad MWD polymers are more non-Newtonian and hence are lower in compounded viscosity. The ratio of the raw polymer’s Mooney viscosity to the compounded Mooney viscosity provides information on the relative branching of different grades of EPDM rubber with the same raw Mooney viscosity. A lower ratio indicates a lower level of branching.
8.
RHEOLOGICAL PROPERTIES AS RELATED TO EPDM STRUCTURE
EPDM structure impacts on rheological properties like tangent delta and dynamic viscosity. Dynamic testing is very sensitive to small differences in structure. This is illustrated in Table
855
EPDM Rubber Technology Table 1 Polymer Properties for Four Widely Differing EPDM Polymers Polymer A
Polymer B
Polymer C
63 53/47 0 4.3 I .9 2.2
62 57/43 2.0 ENB 5.0 1.7
60 52/48 2.0 ENB 5.7 I .7 3.3
+
MLl 4 at 125°C E P , Wt. Ratio Diene content, o/o MW ( X lo-'), (PS equiv.) Mn ( X MwlMn 3.0
Polymer D
66 56/44 3.0 DCPD 4.6 l .2 3.7
1 by a series of four polymers (A-D) varying greatly in branching and MWD (Beardsley and Wortman, 1997). 8.1
DynamicViscosity vs. Frequency
Polymer A is a linear ethylene-propylene copolymerwith narrow MWD. Polymers B and C are ENB terpolymers with an intermediate level of branching but are differentiated by Polymer C's slightly broader molecular weight distribution. Polymer D is a highly branched DCPD terpolymer. All four polymers have Mooney viscosities (ML1 + 4 at 125°C) between 60 and 66. Data in Figure 8, gathered on an RPA-2000 at 1OO"C, show the dynamic viscosity over a range of frequencies for each polymer. The data illustrate the effect of structural differences on dynamic mechanical properties. At low shear rates, the polymers are ranked according to their level of branching. Thus, linear Polymer A has the lowest viscosity and branched Polymer D has the highest viscosity at the lowest shear rate. The curves cross overnear the Mooney viscosity shear rate. This is expected since the polymers have similar Mooney viscosities. At the highest shear rates, the polymers are ranked in order of their number average molecular weights.
+A
(linear EPM)
-C
( 2.0% ENB, 3.3 Mw/Mn)
D ( 3.0% DCPD, 3.7Mw/Mn) 0
1
1
2
5
10
21
52
105
209
Frequency in radianslsecond
Fig. 8 Dynamic viscosity vs. frequency for EPDM polymers varying in branching and MWD
Karpeles and Grossi
856
-x-
A (linear EPM)
-C-
B ( 2% ENB, 3.0 MwlMn) ( 2% €NB. 3.3 MwlMn)
-+C
D ( 3% DCPD. 3.7 MwlMn)
9 .
0.0
' 0
1
10
100
1000
Frequency radiandsecond
Fig. 9 Tangentdelta vs frequency for EPDM.
The effect of MWD is shown by comparison of Polymers B and C. Due to its broader MWD. Polymer C has a slightly lower viscosity at higher shear rates than Polymer B. 8.2 TangentDeltaVersusFrequency
Figure 9 shows the relationship between tangent delta and frequency for the four polymers. Tangent delta differentiates the polymers more dramatically than the dynamic viscosity. The linear Polymer A has a very high tangent delta at low shear rates, which indicates that polymer flow during storage will occur without rigid packaging. The branched Polymer D has the lowest tangentdelta from the lowest frequency to the crossover point,indicating a high degree of elasticity. As with dynamic viscosity, the tangent delta value is heavily influenced by MW at high shear rates. Polymer A, which has the highest MW, has the lowest tangent delta, and Polymer D, which has the lowest MW, has the highest tangent delta at the highest shear rates. Polymer C has a broader molecular weight distribution than Polymer B. It shows a lower tangent delta at lowshear rates thanPolymer B, indicating increased elasticitydue to the presence of a higher MW fraction.
9.
EPDM MANUFACTURING PROCESSES
First commercialized in 1962, the nameplate worldwide production capacity for EPDMcurrently stands at 920,000 metric tons per year. Table 2 lists the major worldwide manufacturers of EPDM and their respective product trade names. Solution-based manufacturing processes are utilized for approximately 85% of this capacity. The remaining 15% of the EPDM production capacity utilizes a slurry phase polymerization
857
EPDM Rubber Technology Table 2 MajorWorldwideManufacturers of EPDM
Company
Trade name
Uniroyal Chemical Exxon DSM (Copolymer, Nitriflex, DSM-Idemitsu) DuPont-Dow Enichem B ay er
Royalene Vistalon
Sumitomo Mitsui
Esprcnc Mitsui EPDM JSREPDM Herlene Kumo EPDM
JSR Herdillia Unimers Kumo
Keltan Nordel
Dutral Buna
process.Most of thisexistingcapacity is based on Ziegler-Nattachemistry. New capacity, totaling 90,000 MTA, was recently brought onstream by DuPont-Dow based on a metallocene solution process. Future capacity of 90,000 MTA has been announced by Union Carbide based on a gas phase process. Process descriptionsof solution and slurry phase manufacturing facilities have been extensively analyzed and reviewed. An excellent report of early patents, economics, and process flow diagrams for solution and slurry phase processes was provided in SRI Report 4B (1981). The information was updated in SRI Report 4C (1990) to include an assessment of the gas phase EPDM process in comparison to the solution and slurry processes.
9.1
The Solution Process
In the traditional solution process (U.S. 8,341,503, 1967; SRI Report 4B, 1981: SRI Report 4C, 1990) chilled monomers and solvent, vanadium catalyst, aluminum cocatalyst, and polymerization modifiers are fed into the polymerization reactor. Chilling monomers and solvent aids in removing heat from the exothermicpolymerization process. The reaction is carried out between 40 and 80°C. Temperatures above 80°C are not utilized due to the temperature instability of vanadium-based Ziegler-Natta catalysts. When a vanadium catalyst species comes in contact with an aluminum cocatalyst, the vanadium catalyst is reduced from its original vanadium (IV) or (V) oxidation state to the vanadium (111) oxidation state, which is the active oxidation state for EPDMpolymerization. The developingpolymer chains aresoluble in the hydrocarbon solvent and forma“cement.” This polymer cement can vary from4 to 15% “solids”(polymer) depending on the molecular weight of the polymer and the temperature of the polymerization system. After polymerization, the reaction is terminated, monomers are removed, and the cement is washed to remove metals left over from the catalyst system. Solvent is then removed via one of two approaches:
1. Steam flocculation, to give an aqueous slurry of polymer crumb. The wet crumb is dewatered, dried, baled, weighed, checked for metal impurities, film-wrapped, and packaged. 2 . Direct solvent evaporation by mechanical means, providing dry polymer that is extruded and pelletized or baled. Similarly, the EPDM is then weighed, checked for metal impurities, and packaged.
Grossi 858
9.2TheSlurry
and
Karpeles
Process
In the slurry process(SRI Report 4B, l981 ;Galli et al., 1985; SRI Report 4C, 1990) the polymerization occurs in liquidpropylenemonomer. Thedeveloping polymer is not soluble in the polymerization medium. A casolvent is used to swell the polymer particles to aid in washing out the catalyst residues. After polymerization, the reaction is terminated and the co-solvent and the propylene are removed via steam flocculation to give an aqueous slurry of polymer crumb. The back end of the slurry EPDM plant looks very much like that of a solution plant. Simplified slurry processes have been developed for the production of EPM and are described by Galli et al. (1985).
9.3TheGasPhaseProcess Process development work toward the commercial application of gas phase processes for EPDM is currently being carried out. No commercial terpolymer products are currently manufactured via a gas phase polymerization process. U.S. patent 4,7 10.538 (1987) describes a process in which chilled gaseous monomers, catalyst, and “inert filler” are injected into a fluidized bed reactor. The growing polymer particles, by design, become coared with inert filler to prevent particle agglomeration. The coated particles are removed from the reactor, and unreacted monomer is removed with a purge stream of hot inert gas or steam (U.S. 5,05 1,546, 1991). The particulate is then packaged.
10. ZIEGLER-NATA CATALYSTS AND COCATALYSTS 10.1Ziegler-NattaCatalysts The subject of Ziegler-Natta catalysis has been extensively reviewed by Boor ( 1 979), Kaminsky and Sinn (1980), and Chandrasekhar et al. (1988). Most manufacturers utilize vanadium-based Ziegler-Natta catalysts for the production of EPDM rubber (Brett et al., 1971; Baldwin and Ver Strate, 1972). Titanium (Ti)-based catalysts are only utilized for the production of EPR (Galli et al., 1985), because they are not effective in incorporating dienes into the polymer. Moreover, titanium catalysts tend to produce more crystalline polymer than their vanadium counterparts. Vanadium catalysts cited in the literature include vanadium oxytrichloride (V0Cl3), vanadium tetrachloride (VC14), vanadium acetylacetonate (V[AcAcI3), and vanadates (VOCI,[OR]3.,). The above catalysts are used in conjunction with aluminum based cocatalysts to form the active Ziegler-Natta catalyst species. No simple correlation can be made between the vanadium catalyst type and the polymer structure produced.The molecular weight of the resultant polymer is directly dependent on the amount of catalyst used in the polymerization, i.e., using higher quantities of catalyst will lower MW, and conversely, use of lower levels of catalyst will raise MW. MW and MWD are measured by a size exclusion chromatography technique known as gel permeation chromatography (GPC).
10.2 Cocatalysts Aluminum (AI) cocatalysts are required to activate (reduce) the vanadium (IV) or vanadium (V) catalyst to vanadium (111), the active state for EPDM polymerization. Common aluminum cocatalysts include diethylaluminum chloride (DEAC) [(CH3-CH2),AICI], ethylaluminum sesquichloride(EASC) [(CH3-CH2)3A12C13],andethylaluminum dichloride(EADC) [(CH3CH?)AICI?].
859
EPDM Rubber Technology
10.3 CocatalystKatalyst Ratio Effect on Polymer Structure The ratio of AI to V used in the polymerization system controls the solubility of the catalyst system. Changes in catalyst solubility affects monomer incorporation, branching, and molecular weight distribution (Brett et al., 1971). At high A I N ratio, the catalyst is homogeneous, soluble, and “single-sited.’’ Soluble catalyst provides random monomer distribution, a narrow molecular weight distribution,lower levels of branching, and a faster curerate. For a givenethylene content, the crystallinity of a polymer made at a high ratio will be lower than one made at a low ratio. EPDM made by this technology is often excellent in mixing but poor in mill processing. At low A I N ratio, the catalystis made up of both soluble and insoluble components and is multisited. The multisite catalyst produces a broad molecular weight distribution polymer with a more heterogeneous intramolecular monomer and diene composition, a slower cure rate, and a lower cure state. Polymer chain composition varies with the MW of each individual chain. EPDM polymers madewith lowA W ratio exhibit excellentmill processing due totheir broad molecular weight distribution.
10.4 Cocatalyst Effect on Branching and Rheological Properties Polymer structure is influenced heavily by combining the catalyst of choice with a variety of cocatalysts.Cocatalysts vary in theiracidity (Baldwin and Ver Strate, 1972) based on their chlorine-to-aluminum ratio. The higher the CVAI ratio, the higher the acidity. For the above three cocatalysts, the acidity and the Cl/AI ratio decrease in the following order: EADC (CI/Al = 2) > EASC (CI/Al = 1.5) > DEAC (CI/AI = l ) ) . Use of more basiccocatalystslike DEAC provide a linear but heterogeneous polymer structure with blocky ethylene sequences and polymer chain compositions that vary with MW. Use of a very acidic cocatalyst such as EADC with ENB-based terpolymers canproduce amorphouspolymers and high levels of branching (Baldwin and Ver Strate, 1972). Thus, the cocatalyst can be a valuable tool in modifying EPDM structure, i.e., branching, to fit the customer’s processing needs. EPDM manufacturers choose cocatalyst and catalyst types to tailor the product’s structure. Table 3, showstheproperties of threepolymersthat are similarinterms of Mooney viscosity and polymercomposition (Beardsley and Wortman,1997). The polymerswere prepared with three different cocatalyst systems, each designed to impart varying levels of branching in the polymer through changes in the catalyst system’s acidity.
Table 3 Polymer Properties for Three Similar EPDM Polymers Prepared with Catalysts of Varying Acidity Polymer E
Polymer F
High 67 60140 8.5 4.3 1.6 2.6
Intermediate 67
Polymer G ~~
Relative catalyst acidity MLl 4 at 125°C E P Wt. Ratio60140 ENB content, c/o MW ( X lo-’) (PS equiv.) Mn ( X IO-’) Mw/Mn2.5
+
8.7 4.2 1.6
Low 65 60140 8.7 4.3 1.9 2.3
Karpeles and Grossi
860
-E
(Acidic Calalysl)
"... F (Intendlate)
(LeastAadic)
-G
01
10
1
FrequencyIn radiandsecond
Fig. 10 Dynamic viscosity vs. frequency for different catalyst systems of varying acidity.
Figure 10 illustrates the dynamic viscosity vs. frequency profiles obtainedon a rheometrics dynamic spectrometer at 150°C. The behavior of linear Polymer G, prepared with the more basiccatalystsystem,closelyresembles that of linear Polymer A discussed in theprevious section. Branched Polymer E, prepared with the acidic catalyst system, correlates well with the behavior of branched Polymer D. The catalyst of intermediate acidity produces polymer whose
10
1
Frequency in radiandsecond
Fig. 11 Tangent delta vs. frequency for different catalyst systems.
100
EPDM Rubber Technology
861
viscosity falls between that of Polymers E and G, produced by the acidic and low-acidity catalysts. Figure 1 1 shows the tangent delta versus frequency results for Polymers E through G. The tangent delta curve of Polymer G, prepared with the more basic catalyst system, again correlates well with that of linear Polymer A. Similarly the tangent delta of branched Polymer E prepared with the most acidic catalyst system, resembles the resultsfor Polymer D.As expected, Polymer E, made by the catalyst with intermediate acidity, falls between the two extremes. Dynamic mechanical testing indicates very clearly that more acidic catalyst systems produce increased polymer branching. Since EPDM producers employ different catalyst and cocatalyst systems, their polymers tend to vary slightly in terms of branching level. The variation is, however, normally not as severe as between the three polymers presented in this section.
11. POLYMERIZATIONADDITIVES
Polymerization additives are utilized to control the polymerization and the polymer structure. The patent literature describes the use of chain-transfer agents, bases, activators (oxidants), and branching agents. 11.l
Chain-TransferAgents
The most common chain-transfer agent used to control MW and MWD is hydrogen (Condit et al., 1963; U S . 3,051,690, 1962). Hydrogen acts selectively to reduce or eliminate thepolymer’s high molecular weight fractionand narrow the MWDby terminating the polymer chain’s growth. Use of diethylzinc in place of hydrogen has been reported (Belgium 720,059, 1969). 11.2 Bases The addition of bases, such as ammonia, pyridine, ethers, etc.,serve to eliminate cationic branching ( U S . 3,242,149. 1966) causedby catalyst acidity. Bases can be used in lieu of using a more basic co-catalyst system.This technique will produce linear products with a concurrent molecular weight distribution broadening. 11.3 Activators(Oxidants)
Catalyst activatorscan be utilized with vanadium-based Ziegler-Natta catalysts to “reactivate,” or oxidize, vanadium in the vanadium(I1) oxidation state, which is inactive for EPDM polymerization, to vanadium(III), the active EPDM polymerization oxidation state. Activators include chlorinated hydrocarbons (U.S. 3,349,064, 1967; British 1,020,808, 1966; U S . 4,181,790, 1980; U.S. 4,36 1,686, 1982), such as hexachlorocyclopentadiene,ethyl trichloroacetate, chlorinated malonates, butyl perchloro-crotonate, etc., and nitro compounds ( U S . 3,441,546, 1969). The use of these agents results in increased catalyst productivity (measured by pounds EPDM produced per pound of catalyst). 11.4
BranchingAgents
Nonconjugated olefin branching agents (Christman and Keim, 1968) can be used to create long chain branches and tobroaden MWD. Branching occurs because both doublebonds in the agent
Karpeles and Grossi
862
are very active toward polymerization. Examples of branching agents include 1,5-hexadiene, 1,7-octadiene, vinyl norbornene, and methylene norbornene. With ENB-containing polymers, cationic initiators (Kautt and Kuehne, 1984) can be utilized to branch the polymer, but this technique is less controllable than use of the above dienes. Branching agents are especially necessary for EPDM based on 1.4-HD termonomer, which would otherwise be very linear and have a narrow MWD. Moreover, when metallocene systems are utilized with ENB, DCPD, or 1.4-HD, the resulting polymers are inherently linear, with a narrow MWD. Toobtain the desired rheological properties, some quantity of branching is required.
12. METALLOCENE CATALYST AND COCATALYST SYSTEMS 12.1 Catalysts Metallocene catalysts (Kashiwa et al., 1992; Hamielec and Soares, 1995; Huang and Rempel, 1995) are structurally distinct from Ziegler-Natta catalysts because they contain either one or two five-carbon aromatic cyclopentadienyl (Cp) rings coordinated to a Group 4btransition metal such as titanium, zirconium, or hafnium. The Cp rings can be simple structures with a hydrogen attached to each carbon or intricate structures substituted with complex organic groups. The two Cp rings can be bridged (connected), most commonly by carbon or silicon, or unbridged. All metallocenes are single-site catalysts. Each catalyst molecule has a specific structure, which results in only one type of active center. Single-site catalysts produce inherently linear polymers with a most probable polydispersity (MWD) of 2.0.
12.2 Cocatalysts Metallocene catalysts require unique cocatalysts for activation and initiation of polymerization. Two classes of cocatalysts are used: (1) MAO(methylaluminoxane), a reactionproduct of trimethyl aluminum and water, and (2) boranes, such as tris perfluorophenyl borane, trityl- and tetrakis-perfluorophenyl borate, and dimethylphenylamino tetrakis perfluorophenyl borate. Manymetallocenestructures are capable of producing EPM or lowmolecularweight EPDM, but onlyalimitednumber of metallocenestructures are capable of producinghigh molecular weight EPDM. Thereis significant interest in metallocene systems for EPDMbecause of the catalysts’ stability at higher temperatures, their very high productivity, and their high reactivity toward ethylene, propylene, and higher alpha-olefins.
13.MODIFIEDEPDM EPDM and EPM can be modified with a variety of monomers or inorganic agents. The primary uses for modified EPDM are as dispersant viscosity modifiersfor lubricants; in impact modification of plastics,suchaspolypropylene,polyethylene, polyamide, polycarbonate,PET, PBT, PVC, ABS, and SAN; and as compatibilizers for polarhonpolar polymer blends.
1. Chlorinationandbromination:Chlorinationhas been heavilyinvestigatedasa way to impart oil resistance to EPDM. Mitsui (U.S. 4,764,562, 1988), Showa Denko (EP 0268457, 1988), JSR, and Sumitomo Chemical have patent coverage in this area. Production costs for chlorination are a major hurdle to commercialization. Bromination is reported to provide faster cure and higher tensile strength in cured EPDM (Hashim et al., 1995; Kohjiya et al., 1995).
EPDM Rubber Technology
863
2.Maleation:Maleicanhydride-modified EPDM(Greco etal.1987; U.S. 4,010.223, U.S. 4,506,056, 1995); has found wide use in the lubricants industry for the manufacture of dispersant-grade viscosity modifiers. The reactive anhydride moiety serves as an anchor for many polar materials. A second application areais the impact modification of engineered plastics such as Nylon 6, Nylon 66, PET, and PBT. Commercial products based on maleic anhydride modification are available from Uniroyal Cheniical under the trade name RoyaltuP (Constable et al., 1997) and from Exxon under the trade name [email protected] of dibutyl maleate in place of malcic anhydride has also been reported (Sen et al., 1991). 3. N-Vinylpyrrolidone and C-vinylpyridine:Modification of EPDM with unsaturated pyridines and pyrrolidones are reported (U.S. 4.146,489, 1979) for the manufacture of dispersant-grade viscosity modifiers. 4. Vinyl silanes: No commercial products are available based on organosilane-modified EPDMs, but their preparation has been patented (U.S. 4,340.689. 1982). 5. Sulfonation,chlorosulfonation. and ionomers: The preparation.characterization.and uses of sulfonated and chlorosulfonated EPDM have been reviewed (Earnest and MacKnight, 1980; Lundberg and MacKnight. 1984). The use of zinc-sulfonated EPDM has been reported for creating ionic thermoplastic elastomers. (De et al., 1994; Bhattacharya et al.. 1995). 6. Antioxidants:Graftingantioxidants (Scott, 1987, 1989; Devore andHahnfeld, 1993) onto EPDMprovides a nonmigrating, nonextractable antioxidantfor lubricant applications and applications in contact with tluids. 7.Styrene-acrylonitrile:Styrene-acrylonitrile (SAN)grafts on EPDMare used for weather resistant applications and plastics impact modification (U.S. 3,538,190, 1970; U.S. 3,538,191, 1970; U.S. 3,657,395, 1972; U.S. 3,671,608, 1972; U.S. 3,683,050. 1972; U.S. 3376,727, 1975; Hamann et al.. 1989; Motomatsu, 1989). SAN grafted material is commercially available from Uniroyal Chemical under the Royaltuf@ trade name.
14. INDUSTRIAL APPLICATIONS AND
USE OF EPDM
14.1 Introduction Industrial applications and uses of EPDM were reviewed by Grossi and Karpeles (1996). The proper selection of polymer and curatives for the various applications utilizing EPDM rubber is critical. It is important that the choice of polymer and compound components be consistent with the processing requirements and desired final product properties (Chodha. 1994). Several characteristics of EPDM rubber make it the polymer of choice for a variety of applications (Cheremisinoff, 1992; Suryanarayanan, 1992; Umeda, 1995). Table 4 sunlrnarizes many of the attributes inherentin EPDM polymers, and Table 5 summarizes many of the features of EPDM polymers that result from their structural characteristics. EPDM polymers are very nonpolar, and, unlike many other rubbers, EPDM has a saturated polymer backbone. Yet, while the backbone of EPDM rubber is saturated, these polymers contain diene termonomers such as ENB, DCPD, or 1,4-HD. allowing for curing using sulfur or sulfur donors. EPDM polymers can also be cured by peroxides (Endstra and Wreesman, 1993; Hellendorn, 1995a. 1995b) when sulfur cure is not acceptable. The polymer is an excellent choice for outdoor applications when good ozone and weather resistance is needed. In severe environments where excellent heat and oxidation resistance is needed, such as in under-the-hood automotive applications where high temperatures are common, EPDM rubber is a very good choice. EPDM polymers also have
864
Karpeles and Grossi
Table 4 Attributes of EPDM Saturated polymer backbone Diene termonomer for sulfur curing Versatility in polymer structure possible Ethylene/Propylene ratio Diene type and amount Molecular weight Molecular weight distribution Branched/Linear structure Economical cost
excellent low-temperature flexibility and high resiliency. They also have very good resistance to water and aqueous solutions and other polar fluids. The lack of inherent polarity (discussed in detail in Section I) also provides for excellent nonconductive electrical properties. Other structural features can be varied, which can have a great effect on the polymer’s properties, which in turn can effect serviceability, processability. and cure characteristics. For example, the ratio of ethylene to propylene, the diene type and amount, the MW and the MWD, and the branching(or linearity) of the polymer areall characteristics that can be varied. Increasing the ratio of ethylene topropylene can improve the modulus or cold green strengthof the polymer by introducingmore“crystallinity.”However,low-temperatureproperties are sacrificed in high-ethylene polymers. lncreasing the MW of a polymer can improve the hot and cold green strength (Stella, 1994) and allow for higher filler loading, but it can make the compound more difficult to mix unless processing aids are used. High molecular weight polymers also provide increased vulcanizate tensile strength. The molecular weight distribution can be varied to affect processing-narrow to allow for fast extrusion, or broad to improve milling and calendering. The diene type and amount can affect the cure rate of the polymer and can also have an effect on properties such as compression set and aging. It is the proper balance of these characteristics that must be taken into consideration when choosing a polymer for a particular application. Examples of how altering the polymer structure can effect properties are shown in Table 6. Some of the common elastomeric applications for EPDM rubber are summarized in Table 7. These include (but are not limited to) hose, automotive weather seals, roof sheeting, wire and cable, plastic modification, tires and tubes, gaskets and seals, and diaphragms. A few of these will be discussed in more detail, providing some general guidelines for polymer and cure system selection.
Table 5 Features of EPDMPolymers Ozone and weather resistance Heat and oxidation resistance Polar fluid resistance Water and aqueous solution resistance Low-temperature flexibility High resilience Excellent electrical properties
EPDM Rubber Technology
865
Table 6 Polymer Structure Versus Effect on Property High MW
Narrow MWD
Broad MWD High ethylene content
Increasing diene content
Increased green strength Potential for poorer mixing Increased vulcanizate tensilehear strength Poor calenderability Slower extrusion (higher temperature required) Higher loading possible Increased extrusion rate Smooth extrusion surface Lower die swell Improved low-temperature properties Increased cure rate and state of cure Faster mixing Higher green strength Improved mill handling and calendering Increased cold green strength More difficult to mix Increased tensile strength of vulcanizate Higher filler/oil loading possible Poorer low-temperature properties Higher hardness Decreased scorch safety Faster rate of cure Improved compression set of vulcanizate Increased modulus of vulcanizate Decreased elongation and heat aging of vulcanizate
14.2 Hose EPDM isused in many types of automotive hose, including automotive heater and radiator hose (Keller and Mills, 1991), air-conditioning hose, air-emission hose, crankcase vent hose, brake hose, and tubing. It is also commonly used for nonautomotive applications such as hydraulic hose tube andcover, utility hose, inlet and drain hosesfor appliances, garden hose, and industrial air and water hose. In choosing a polymer for hose, service conditions and processing must be considered. In general, most hose is extruded and subsequently reinforced, so polymers that provide good
Table 7 Some Uses of EPDM Rubber Hose Sponge and weather seals Sheeting/Roofing Wire and cable Plastic modification Seals and gaskets Diaphragms Tires and tubes
and Grossi
866 Table 8 Hose: General Polymer, Processing, and Cure Requirements ~~
Automotive coolant hose
Appliance, industrial, and garden hose
Polyrner Requirements Lower ethylene content for improved low-temperature properties Medium ENB for fast cure High molecular weight for green strength Narrow MWD for fast extrusion Cure Requirements Peroxide for improved heat aging Process Extrusion
Pnlvnwr Requirer?~ents
High ethylene for high filler/oil loading and green strength High molecular weight for freen strength and high filler/oil loading Medium ENB for fast cure Cure Requirenuwts Sulfur/Sulfur donor Process Extrusion
cold and warm green strength and good extrudability are preferred. High-ethylene polymers (cold green strength) and high molecular weight polymers (warm green strength) provide good shape retention, while a high ethylene content and narrow molecular weight distribution allow for fast extrusion. These polymers, with high molecular weight and high ethylene content, also allow for higher filler (improved cost) and oil loading (improved cost and mixing). Medium ENB polymers utilizing a sulfur cure are generally favored to provide fast cure and good heat aging characteristics. Heat aging can be improved as needed by using a sulfur donor system or a peroxide cure system. Compounds for appliance, industrial, and garden hose applications are generally highly loaded for cost-effectiveness. They use high molecular weight, high-ethylene polymers and a sulfur/sulfur donor cure system. On the other hand, automotive coolant hose typically has more stringent physical property and aging requirements. Filler and oil loadings are lowerto accommodate those requirements. Polymers with lower ethylene content are used to meet special lowtemperature and compression set requirements, and peroxide cures are used for stringent heataging requirements. Table 8 summarizes the general selection criteria for hose applications. Tables 9, IO, and 11 provide examples of typical formulations for a heater hose, radiator hose, and industrial garden hose, respectively.
Table 9 HeaterHose Formulation EPDM
100
EA' = 15/25 ENB = 5 (medium) ML, I 4 at 125°C = 70 MWD = narrow Activator Reinforcement Process aid, extender Antioxidant Improve ratehate of cure Curekrosslinkforimproved heataging
+
Zinc oxide Carbon black N-650 Paraffinic oil Styrenated diphenyl amine TMPT (trimethylolpropane triacrylate) Peroxide
5
160 120 1 2
IO
867
EPDM Rubber Technology
Table 10 RadiatorHoseFormulation ~
~~
EPDM 1
50
E/P = 75/25 ENB = 5 (medium) ML I 4 at 125°C = 70 MWD = narrow E/P = 56/44 ENB = 5 (medium) ML I 4 at 125°C = 75 MWD = medium Activator Reinforcing Less reinforcing Process aid, extender Antioxidant Antioxidant Ultrafast accelerator Sulfur donor/accelerator Sulfur donor/accelerator Accelerator Cure/crosslinking
+
EPDM 2
50
+
Zinc oxide Carbon black, N 650 Carbon black, N 762 Paraffin oil Styrenated diphenyl amine ZMTI (zinc-2-mercaptotoluinedazole) ZMDC (zinc dimethyl dithiocarbamate) TMTD (tetramethyl thiuram disulfide) DTDM (dithiodimorpholine) ZBDC (zinc dibutyldithiocarbamate) Sulfur
5 13 85 12
I .5 I .5 3 3 2 3 0.5
14.3 Closed Cell Sponge Weather Seals Closed cell automotive sponge weather seals are processed by extrusion and typically cured using a microwave oven, hot air oven, liquid salt bath, or fluidized bed (Burbank et al., 1995). Curing is very fast, typicallytaking 1.5-3.5 minutesat 150-200°C(300-40OoF), depending on the equipment used. For microwave curing, in the precure phase of curing (no expansion) approximately 30% of total cure must occur in the first 30 seconds. Enough modulus must be developed prior to the decomposition of the chemical blowing agent so as to provide sufficient
Table 11 GardenHoseFormulation EPDM
100
EIP = 75/25 ENB = 5 (medium) ML I 4 at 125°C = 70 MWD = narrow Activator Activator, process aid Reinforcing black Low reinforcing filler Plasticizer Process aid Process aid Accelerator Sulfur donor/accelerator Accelerator Cure, vulcanization
+
Zinc oxide Stearic acid Carbon black, N-650 Clay Naphthenic oil Polyethylene glycol Paraffin wax Dibenzthiazyl disulfide TMTD (tetramethylthiuram disulfide) ZMDC (zinc dimethyldithiocarbamate) Sulfur
5 I 260 200 2 10 2 5 I .0 1.S 1.0
2.0
868
Karpeles and Grossi
Table 12 Closed-CellSpongeWeatherSeals:GeneralPolymer, Processing. and Cure Requirements Polymer requirernerlts High ENB (>8%) for ultra-fast cure High molecular weight andor high ethylene for green strength Narrow molecular weight distribution for fadsmooth extrusions Cure reqrrirernerzts Sulfur cure Ultra-fast accelerators, e.g., thiazoles, dithiocarbamates. and thiurams Carbon black required for microwave receptivity Chemical blowing agent for gas formation Processing Extrusion Microwave cure Hot-air cure LCM (salt bath) cure Fluidized beds
cell wall strength to contain the gas pressure developed i n the second phase of the cure. In the expansion phaseof curing, alsolasting about30 seconds, the chemical blowing agent decomposes completely and the rapid cure continues until the extrudate reaches close to 100% of cure. In the final phase, the cure is completed, volatile decomposition products are driven off and the normalization of the sponge occurs. The total average cure time for this technique is about 1.5 minutes. Because very fast curing is required, high-ENB polymers (>8%) are used. The choice of other polymer characteristics, such as percent ethylene, molecular weight, and molecular weight distribution. are determined by the final required sponge properties and the method of curing to be used. Forexample, high molecular weight polymersor high-ethylene polymers are generally used for LCM curing in order to provide better green strength to control stretching and shape distortion. Curing of closed-cell sponge is typically done with sulfur and the use of accelerators such as thiazoles, dithiocarbamates, and thiurams to provide the required ultrafast curing. Sufficient heat will not be generated for microwave curing EPDM unless receptive promoting ingredients such as carbon black are added, since EPDM is very nonpolar. The most common types of chemical blowing agents used are azodicarbonamides, which decomposeto give off large amounts of nitrogen gas.Table 12 summarizes thegeneralselectioncriteriaforclosed-cell sponge weather seals. Table 13 provides an example of a typical microwave-cured, closed-cell, extruded sponge weather seal formulation.
14.4 Wire and Cable EPDM is used to produce wire andcable forboth low-voltage and mediundhigh-voltage applications. Someexamples of eachtypeareshown in Table 14. Thechoice of EPDMdepends upon the application and differs considerably between low-voltage and mediundhigh-voltage applications. Low-voltage wire and cable compound is generally highly filled. For this reason. highethylene, high molecular weight polymers, with a narrow MWD. that provide for good green
869
EPDM Rubber Technology
Table 13 Closed-Cell Extruded Sponge WeatherSeals,MicrowaveCureFormulation I00
Zinc oxide Stearic acid Carbon black. N-660 Carbon black, N-550 Whiting Paraffinic oil Brown Facticc Accelercrfiort Mercnptohenzyl thi;mle TEDC (tellcnuln diethyldithiocarbamate) ZBDC (zinc dihutyldithiocarbamatc) Dipentamethylene thiutxm tetrasulfide Sulfur 9 p m ADZ (azodicnrbonamide)
4 1 1 IO 20 Filler 30 80
IS I .S 0.7 1.5 I .2 2 l .S
E/P = 57/43 ENB = 8.5 (high) MWD = medium Activatorcure/CBA) (for Activator. process aid Lower reinforcing Reinforcing and nucleating agent Plasticizer Extender, resiliency Ultra-fast accelerator Ultra-fast accelcrator Ultra-fast accelerator, sulfur donor Cure/Crosslink Gas fonnation
strength and extendibility are typically used. Wire and cable is processed by extrusion, so oils and fillers are added to aid in processing to compensate for the narrow MWD of the polymer. Because of the more stringent electrical requirements in mediudhigh-voltage wire and cable applications. much lower compound loading with oils and fillers can be used. For this reason. low molecular weight polymers with broad MWD that aid in processing are generally used. Polymers with high ethylenecontent are often used where high electricalbreakdown strength is needed, such as i n single conductor cables. Low-ethylene, amorphous polymers are used for multiple conductor cables where less distortion is tolerated. Both low-voltage and mediutdhigh-voltage cable require low levels of ionic, nonpolar constituents for wet electrical stability. All are peroxide cured by CV steam, salt bath, or hot dry gas under pressure at temperatures between 175 and 200°C (350-400°F). Since sulfur curing is not acceptable for wire and cable. the level and type of termonomer in the polymer is not important. Table IS sunlrnarizes the general selection criteria for wire and cable. Tables 16 and 17 provide examples of two types of wire and cable formulations.
Table 14 Wire and Cnblc Applications Low-voltage applications High-voltage applications UL flexible cord Submersible pump cables Appliance wire Automotive ignition cable Insulation Track resistawe insulators Welding cable insulation
Industrial powcr cable Utility power cables URD cable
Karpeles and Grossi
870 Table 15 Wire and Cable: General Polymer, Processing,
and Cure Requirements
h-voltage applications Low-voltage Polymer requircvnents High MW (high Mooney) High ethylene Narrow MWD Low ionic polar constituents Highly fillcd for low cost
Process requirenlents
Extrusion Oilslfillers to aid processing
Curing requirements Peroxide 350°C CV steam, salt bath, or hot dry gas
14.5
Polymer requirements Low MW (low Mooney) High ethylene for single conductor cable
Low ethylene for multiple conductor cable Broad MWD Low ionic polar constituents Low filler for better electrical properties Process requirements Extrusion Broad molecular weight distribution polymer to aid processing Curing requirernents Peroxide 350°C CV steam, salt bath, or hot dry gas
Roof Sheeting
EPDM compounds for sheeting are mixed in internal mixers using fast mix cycles and subsequently calendered into several plieson multiroll calenders.Curing istypically done in autoclaves with cure times of 4-6 hours at temperatures of 130-160°C (270-320°F). Important cured and uncured properties must be taken into consideration when choosing the proper EPDM polymer (Gish, 1995). Important uncured properties include good tack, good greenstrength, and good mixingandcalenderingcharacteristics.Importantcuredproperties include moderately high tensile strength and high tear strength, excellent weathering resistance and heat aging, bondability, and good low-temperature properties.Typically, low-ENB polymers
Table 16 FlexibleCordLow-VoltageFormulation EPDM
100
Calcined clay TMQ (polymerized 1,2-dihydro-2,2,4trimethylquinoline) Paraffinic oil Paraffin wax Vinyl
200
Zinc oxide TMPT (trimethylolpropane triacrylate) Peroxide
1
70 5
5 2 7.5
EIP = 75/25 ML 1 4 at 125°C = 60 MWD = narrow Filler Antioxidant
+
Process aid Process aid Coupling agent, improves interaction of polymedfiller Acid acceptor CO-agent, peroxide activator crosslinking Cure,
871
EPDM Rubber Technology Table 17 Medium-Voltage Insulation 100
EPDM
Lead oxide Zinc oxide Silane-treated clay Vinyl silane TMQ (polymerized 1,2-dihydro-2,2,4trimethylquinoline) Peroxide
EIP = 75/25 ML 1 4 at125 = 25 MWD = broad 5 scavenger Ion Acid 5 acceptor 60 Filler, treated to enhance electrical properties 1.5 Coupling agent, polymer/filler 2 Antioxidant 3
+
Curelcrosslinking
with medium ethylene content are used to provide the good green strength, tensile and tear strength, and resistance to weathering and heat aging. A moderately broad molecular weight distribution provides a combination of good mixing and calendering of the compound while also providing good overall cured properties. Sheeting compounds are typically sulfur cured and contain carbon blacks for reinforcement. High-viscosity, low-volatility oils are used to aid in the processing of the compound. Table 18 summarizes the general selection criteria for sheeting compounds. Table 19 provides a typical roof-sheeting compound. 14.6
Mechanical Goods and Other Applications
EPDM can be used in a varietyof molded and extruded mechanical goods. For example, EPDM is an excellent choice for automotive brake components where good ozone resistance and heat resistance, low stress relaxation, and resistance to nonmineral oil hydraulic fluids are required. In addition, EPDM is used in conveyor belting (both skim and cover), bridge bearing pads, dock fenders and bumpers, window gaskets, grommets, bushings, and seals. EPDM has
Table 18 SheetinglRoofing:GeneralPolymer,Processing,and Cure Requirements Polvrnrr requirenwzt.7 Low ENB for good weathering resistance Medium ethylene for green strength Moderately broad molecular weight distribution for good mixing, calendering, and overall cured properties Cure requirer~wnts Sulfur cure Autoclave 130-160°C (270-320°F) 4-6 hours Processing Internal mixers Calenders High-viscosity, low-volatility oils to aid processing
872
Karpeles and Grossi
Table 19 RoofSheetingFormulation
EPDM Zinc oxidc Stearic acid Paraffinic oil Carbon black (C.&.. N650) TBBS (N-ferf-butyl-benzothiazolesulfenamide) TMTD (tetramethylthiuramdisulfide) TETD (tetraethylthiuramdisulfide) Sulfur
EIP = 60140 ENB = 2 (low) MWD = broad 5 1 95 I25 2
0.5 0.5 0.7
Activator Activator, process aid p1.dstlclzcr .’: Reinforcement Primary accelerator accelcrator Secondnry accelcrator Secondary Vu1c:unization
also been found to be effective in gaskets and seals for water systems requiring good chloramine resistance.
14.7 Very Low Molecular Weight EPDM Reactive Plasticizers Very low molecular weight liquid EPDM is used as a processing aid that can react into the polymer matrix during peroxide or sulfur cure, rendering it nonextractable and nonvolatile as opposed to processing oils (Cesareet al., 1987; Cesare, 1995). Thisuse of low molecular weight EPDM hasadvantages in molding.extrusion,andcalenderingoperationsand is particularly important in applicationswhere the part comes in contact with fluids.which can extract a conventional process aid and cause premature failure. In severe applications. these “reactive plasticizers” are nonvolatile and offer advantages i n heat aging compared to conventional process oils. Some examples ofwhere these types of EPDM are used for processing andpel-fomxulce advantages include automotive brake cups and hose ( U S . 5,445.191, 1995). automotive heater hose, industrial hose, adhesives. sealants. and waterproofing membranes.
15. CONCLUSIONS EPDM polymers are used for a multitude of applications due to the many unique features of this class of polymers. The exact typeof EPDM used and cure systememployed for any particular application will depend on the processing requirements and service requirements of the enduse product. Many structural characteristics can be varied in the EPDM polymer to accommodate the many varying requirements. Specialty-grade EPDM can be used in combination with conventional EPDM or other polymers for enhanced properties in demanding applications.
ACKNOWLEDGMENT The authorswould like to acknowledge Dr. Ken Beardsley and Mr. Gerard Rioux for the rheological measurements and Dr. Ali Mohammadi for the thermal analysesused in this chapter (Uniroyal Chemical Company. Polymer Physics Laboratory. Naugatuck CT): Mr. Bill Wortman. Mr. Joe Longo. Mr. Manfred Stegmeier, and Mr. Dan Janczak for conducting lab polymerizations and
EPDM Rubber Technology
873
aging experiments (Uniroyal Chemical Company, Royalene R&D, Naugatuck CT): and Mr. Vern Vanis. Mr. Thomas Jablonowski, Mr. Donald Tredmnick, Mr. Arturo Maldonado, and Mr. Charanjit Chodha (UniroyalChemical Company, Royalene Technical Sales Service, Naugatuck, CT) for their input and assistance in preparing the applications portion of this chapter.
ABBREVIATIONS ABS ADZ A1 ASTM BR CBA
c1 CP CV DCPD DEAC DPA DSC DTDM EADC EASC ENB E/P EPDM EPM EPR
FTIR GPC 1,4-HD IR IISRP MAO MBT MBTS ML Mn MW MwIMn MW MWD PBT PET PEG PPM PS PVC
acrylonitrile butadiene styrene azodicarbonamide aluminum Association of Standards and Testing Materials butadiene rubber chemical blowing agent chlorine cyclopentadienyl ring continuous vulcanization dicyclopentadiene diethyl aluminum chloride diphenylamine differential scanning calorimetry dithiodimorpholine ethyl aluminum dichloride ethyl aluminum sesquichloride ethylidene norbornene ethylene/propylene ratio ethylene-propylene-dienerubber ethylene-propylene rubber ethylene-propylene rubber Fourier transform infrared spectroscopy gel permeation chromatography 1.4-hexad’lene polyisoprene International Institute of Synthetic Rubber Producers methylaluminoxane 2-mercaptobenzothiazole benzothiazyl disulfide Mooney viscosityflarge rotor number-average molecular weight weight-average molecular weight molecular weight distribution molecular weight molecular weight distribution polybutylene terephthalate polyethylene terephthalate polyethylene glycol parts per million polystyrene polyvinyl chloride
874
RIS SAN SBR TBBS TEDC TETD TMPT TMQ TMTD TPE TPV Ti UL URD V ZBDC ZMDC ZMTI
Karpeles and Grossi
radians per second styrene acrylonitrile styrene butadiene rubber N-tert-butyl-benzothiazolesulfenamide tellerium diethyldithiocarbamate tetraethylthiuram disulfide trimethylopropane triacrylate polymerized 1,2-dihydro-2,2,4-trimetylquinoline tetramethylthiuram disulfide thermoplastic elastomer thermoplastic vulcanizate titanium Underwriters Laboratories underground residential distribution cable vanadium zinc dibutyldithiocarbamate zinc dimethyldithiocarbamate zinc 2-mercaptotoluimidazole
REFERENCES Allen. R., and Easterbrook, E. K. (1987), in Rubber Teckrlology, 3rd ed. (Morton, M,, Ed.), Van Nostrand Rcinhold, New York, pp. 260-283. ASTM D3900 (1994), Standard Test Methods for Rubber. Avella, M,. Greco, P,, and Malinconico, M. (1987). in Po[yrners at Low Ternpercrturr, Proceedings of the confercnce held in London, pp. 5/1-5/10. Baldwin, F. P,, and Ver Strate, G. (1972), Rubber Chetn. T e c h r d 15(3):709-881. Baldwin, F. P,. Borzel, P,. Cohen. C. A., Makowski, H. S. and Van de Castle, J. F. (1970). Rtrhher C l l m ~ . T<’c/1rlol.43(3):548-552. Baranova. A. S.. Maslova, I. P., and Ptotrovskii, K. B. (1970). Srrl I.s.slr/. Eft. Khum. Polirrl. Mtrter. (4), 179-94. Beardsley, K. P,, and Wortman, W. A. (1997). Dynamic mechanical testing as a measure of the viscoelastic consistency of EPDM produced with vanadium-based catalysts, Presentation to the NJ: Regional Rubber and Plastics Exposition, Mahwah, NJ. Bcardsley, K. P., and Ho, C. C. (1984), J. Elustorrlers Pltrstics 16:20-35. Beardslcy, K. P., and Tomlinson, R. W. (1990), Rubber Cherrl. Techrlol.. 63(4):540-553. Belgium 720.059 to Uniroyal (1969). Bhattacharya, A. K., De, P. P., DC, S. K., Kerian, T., and Tripathy, D. K. (1995), Plast., Rubber Cort1po.s P ~ o c A/>/>I. . 24(5):285-292. Boor, J. ( 1979), Zirlgler-Nrrtfa Crrtrr/wt.s m d Po/yrrlrrizatiorls. Academic Press, New York. Brett, T. J., Easterbrook, E. K., Lovelcss, F. C., and Matthcws, D. N. (1971).XXIII Infernntiorml Congress of Pure & Applied Cl~er~isfrv: Macromolecular preprints (Boston), IUPAC. British 1,020,808, 1966 to Hercules Powder Company. Burbank. F., Fredinnick, D., and Tyler, R. (1993, Continuous mixing of EPDM automotive weatherseals, Presented at Meeting of the Rubber Division, American Chemical Society. Cleveland, OH. Cesane, F., Matthews, D. N., and Paeglis, A. (1987), Rubber Plastic Ne\vs, Dec. 28. Cesane, F. (1995), Use of liquid EPDM in natural rubber to improve static ozone resistance and as a reactiveplasticizer i n otherelastomers,Presentedto the AssociationFrancaisdesIngenieurs et Cadres de Caoutchouc et des Plastiques, Paris, France. Chandrasekhar, V., Stvaram, S., and Srinivasan, P. R. (l988), Rcccnt dcvelopments in Ziegler-Natta catalysts for olefin polymerization and their processes. Ir~dicrr~ J. T e c h t d , 2653-82.
EPDM Rubber Technology
875
Cheremismoff, N.P. (1992), Spotlight on EPDM Elastomers,Polv~n.Plasf. Techno/. Eng. 31(7,8):714-744. Chmela, S., Lacoste, J. Pilnebowski, J. F., and Tessedre, G. (1996). Polym. Degmd. St& 53(2):207-215. Chodha, C. (1994). FPDM technology and application, Presented at the SPOGS Seminar, Czech Republic. Christmar, D. I., and Kem G. I. (1968), Mrrcmnlolecules 1(4):358-363. Condit, P,, Frics, B., and Hottmar, A. (1963), J. Pol.vrner Sci. Part C 4:109-126. Constable, R., Roberts, D., and Flinruvengada, S . (1997), Advances In maleated polyolefins for plastic applications, Polynwr Eng. Sci. 37(8):1421- 1426. De, P. P,, De, S. K., Manoj, N. R., and Peiffer, D. G. (1994). J. Appl. Polyrner Sci. 53(3):361-370. De Paoh, Marco-A.( 1988), The photo-oxidation of EPDM rubber Part l-Kinetics of oxygen consumption, Polymer Drgrccd. Stubility, 2/:277-283. De Paoh, Marco-A, and Duek, F. R. ( 1990), The photo-oxidation of EPDM rubber: Part 111-Mechanistic aspects and stabilization, Polvrner Degrrtd. Stability 30:283-292. De Paoli, Marco-A, Duek, E. R., Guzzo, M,, Juliano, V. F., and Kascheres, C. (1990). Thephoto-oxidation of EPDM rubber Part 11-The photo-initiation process, Polvnler Degmd. Stability 28:235-248. Devore, D. D., and Hahnield, J. N. (1993), Polyn. Degrnd. Stclh. 39(2):241-249. DiMartino, S., and Kelchtermans, M. (1995), J. Appl. Poly. Sci. 56(13):1781-1787. Sci. Macromol. Rev., 16:41-122. Earnest, T. R., and MacKnight, W. J. (1980), J. Pol~w?er Endstra, W. C., and Wreesman, C. T. (1993), Peroxidecrosslinking of EPDM rubbers, in Elmtorner Technology Handbook, (Cheremasnoff N. P., ed.) CRC Press, Boca Raton, FL, pp. 495-518. Showa, E EP026845 (1988) Denko, Chlorinated EP in aqueous suspension. Galii, P,, Milani, U,, and Seaghoth, F. (1985). USRP 26th Annual Mtg. San Francisco, CA. Gish, B. (1995). Performance classification system for polymer based roofing membranes. Presented at a Meeting of the Rubber Division, American Chemical Society, Philadelphia, Paper No. 36, p. 3 1. Greco, R., Maglio, C., and Musko, P. V. (1987), J. Appl. Polymer Sci. 33:2513-2527. Grossa, A. V., and Karpetes, R. (1996). An introduction to the synthesis, structure, properties and uses of EPDM rubber.Presented at a Meetingof the Rubber Divislon, American Chemical Society, Montreal, Queber, Canada, Paper No. K. Hamann, B., Klodt, R. D., and Runge, J. (1989), Plnste Kcrut. 36(6):188-193. Hamelec, A., and Soares, J. ( 1995). Polytn. Reclcr. Eng., 3(2):, p. 13 1. Hashim, A. S., Ikeda, Y., Kohjiya, S. and Yoon, J. (1995), Rub. Chem. Tech. 68(5), pp. 824-35. Hellendom. R (1995a), Peroxide crosslinking of EPDM rubbers. I. Rubber blend preparation. In, Polyrn. Sci. Tech~lol.22(3):95-103. Hellendom, R. (1995b), Peroxide crosslinking of EPDM rubbers. 11. Processing of stocks, Plasp Kuuc. 32(3):70-73. Huang, J., and Rempel, G. (1995), Prog. Polynl. Sci. 20:459. Jebens, A., and Kautman, S. (1996), CEH Marketing Research Report: Ethylene Propylene Elastonwr.y. SRI International, Menlo Park, CA. Kameisky, W., and Sinn, H. (1984), Advcrnces Orgccnonwtallic C/leln. 18:99-149. Kashiwa, N., Kioka, M,, and Tsutsu, T. (1992), Karninsky catalysts, Petrotech. 15(2):138-142. Kabu, J., and Kuehne, J. K. (1984), Ktrutsch Curnmi Kunstst. 37(2):101-104. Kelier, R. C., and Mills, T. A. (1991). Evolution of ethylene-propylene radiator Prose technology. Kaol, N.Gu~nrniKunst 4 4 I l): 1032-1038. Kohjiya, S., Tsukahara, Y., and Yoon, J. (1995). Pol~vrn.Plrrst. Tech. Eng. 34(4):581-98. Kontos, I. G. and Slichler, W. P. (1962). Relaxationphenomena in homopolymers and copolymers of ethylene and propylene. J. Polymer Sci. 61:61-68. Lundberg, R., and MacKnight, W. I. (1984), Plastomerlc monomers. Rub. Chetn. Tech. 57(3):652-653. Mahlke, D. ( 1987). Verhalten von EPDM bei Tiefen Temperatwen. Kmtsch. Gumrni Kurutst 40:93 1-934. Martini, E., and Milani, F. (1986), Correlation between structure and low temperature properties of EPM and EPDM elastomers, Int. Rubber Conference 86: Proceedings. pp. 198-235. Motomatsu, K. (1989), Putasuchikkusu En 334):164-168. Noordermeer, J. W. M. (1996), Standardization of EPDM characterization testsfor quality-control purposes, Kuutsch. Gurnnli Kunstsf. 49(7/8), pp. 52 1-53 1. Saha Deuri, A., and Bhowmick, A. K. (1987), J. Appl. Polyrn. Sci. 342205.
Karpeles and Grossi
876
Scott, G. (1987), Po/vn1. Degrud. Stab. 19( 1):43-50. Scott, G. (1989),Modification of polyolefins by antioxidants and stabilizers, Mtrkrort~ol chert^. Mtrcrorr~ol. S ~ I ~ I28159-7 P. I. Sen, A. K., Mukherjee, B., Bhattacharya, A. S., De P. P., and Bhowmick, A. K. (l99l),Angewandie Chenl. 191:15.
Stella, G. (1994), The molecular weight determines the performance of EPDM rubber. I d Cornu. 38(9): 14-21. SRI Report 4B. Ethylene-Propylene Perpolymer Rubber, 198I . SRI Report 4C, Ethylene-Propylene Copolymer and Terpolymer Rubbers, 1990. Suryanarayanan, B. (1992). Recent developments in EPDM elastomer technology. Chern. Eng. Work1 27(12):141-145. Umeda, I. (1995), Recent trends of the application of ethylene-propylene elastomers (EPDM) and their future development, Porirna Duijesurn 47(6):47-57. U.S. 3,051,690 to Hercu!es Powder Company (1962). U.S. 3,242,149 to Montecation (1966). U.S. 3,341,503 to Uniroyal (1967). U.S. 3,349,064 to Hercules Powder Company (1967). U.S. 3,441,546 to Uniroyal (1969). U.S. 3,467,633 to DuPont (1969). U.S. 3,538,190 (1970); U.S., 3,538,191 (1970); U.S. 3,657,395 (1971); to CoPolymer Corp., process to make AES U.S. 3,671,608 (1972); U S . 3,683,050 (1972); U.S. 3,876,727 (1975); to CoPolymer C o y , AES product. U.S. 4,010,223 to DuPont, Maleic anhydride modified polymers. U S . 4,146,489 to Rohm and Haas (1979), bolyolciin graft copolymers. U.S. 4,181,790 to Huels (1980). U.S. 4,340,689 to Copolymer Corp. (l982), method of grafting EPM and EPDM. U.S. 4,361,686 to Huels (1982). U.S. 4,506,056 to Gaylord Research Institute ( 1995), Maleic anhydride modified polymers and process for preparation thereof. U.S. 4,710,538 to Union Carbide (1987), process for the production of sticky polymer. U.S. 4,764,562 to Mitsui, chlorinated EPDM in CC14 solution (1988). U.S. 5,05 1,546 to Union Carbide (1991), process for removing dienes from EPDM resins U.S. 5,445,191 to General Motors Corporation (1995). Ver Strale, G. (1986). GlcvclopediLI of Polvn~rrScience m t l Engir~eerir~g. Vol. 6. John Wiley & Sons. New York, pp. 522-564.
36 Isobutylene-Based Elastomers Neil F. Newman* and James V. Fusco* Exxon Chemical Co., Baytown, Texas
1. INTRODUCTION AND HISTORY Isobutylene-based elastomers of commercial importance include homopolymers, copolymersof isobutylene with isoprene, and halogenated isobutylene-isoprene and isobutylene-p-methylstyrene copolymers. They owe their commercial success to a unique set of properties, including exceptionallylowpermeability to gases, excellent vibration damping, andgood-to-excellent heat, chemical, ozone, and oxidation resistance. Low molecular weight homopolymersof isobutylene were first prepared in 1873 by Gorianov and Butlerov (I). In the early 1930s, the German company I. G. Farben produced a higher molecular weight homopolymer using boron trifluoride catalyst and low temperatures. Standard Oil Development Company (now Exxon Research and Engineering Co.) quickly advanced this work with the production of rubbery homopolymers via still lower temperatures and purified ingredients. Further development at Standard Oil, principally conducted by R. M. Thomas and W. J. Sparks, focused on copolymerizing isobutylene with dienes in an effort to make a vulcanizable elastomer. Their work achieved fruition in 1937 (2-6) with the invention ofbutyl rubber. copolymers of circa 97% isobutylene with up to 3% of isoprene. Butyl rubber represented the first of the limited-functionality elastomers. The low level of olefinic functionality gave this elastomer chemical,heat, and ozone resistance much superior to that of the highly unsaturated general purpose rubbers. Commercialization of butyl rubber was accelerated by the events of World War 11, and the polymer was first manufactured in a large-scale plant in 1943. Butyl rubber achieved its major success as the elastomer of choice for tire inner tubes, a position it retains today. Halogenation of butyl rubber was first performed commercially at B. F. Goodrich (7-1 1 ) in the mid-1950s. Their product. which was made via bulk batch bromination of butyl rubber. proved difficult to manufacture and was withdrawn from the market in 1969. Solution halogenation processes,whicharemuchmorecontrollable.were developed during thistimeperiod. Chlorobutyl was developedby Exxon Chemical Company(12- 15) and introduced commercially in 1961. Solution Bromobutyl was commercialized by Polysar (now Bayer) ( 16) in 197 1 and by Exxon Chemical Company in 1980.
* Retired. 877
Newman and Fusco
878
Halogenation increases the reactivity of cure sites and introduces additional pathways for vulcanization. Accordingly. halobutyls are more covulcanizable with general purpose rubbers than is butyl rubber. This cure activity, combined with low permeability to gases, led to the development of tubeless tires employing halobutyl innerliners.This remains the largestcommercial application for halobutyls. Butyl and halobutylshavingimprovedprocessing and low die swell were introduced commercially by Exxon Chemical Company (17, 18) in 1989. These processability improvements are achieved via the addition of a branching agent to the polymerization so that a portion of the polymer is present as highly branched star-like structures. Hence, the products have been labeled “star-branched (halo) butyls.” The newest member of the isobutylene-based elastomer family is the brominated copolymer of isobutylene with pam-methylstyrene. This product has a fully saturated backbone and so is completely resistant to ozone and highly heat stable, while retainingthe desirable attributes of the other isobutylene-based elastomers. The bromine is present as benzylic bromide, a versatile and reactive functional group that covulcanizes well with general purpose rubbers and lends itself to further polymer modifications. This rubber, trademarked as “Exxpro Elastomers,” was commercialized by Exxon Chemical Company in 1995 (19, 20).
2. 2.1
HYDROCARBONISOBUTYLENE-BASEDELASTOMERS Polymerization Chemistry
The isobutylene-based elastomers are formed via a cationic polymerization mechanism ( 2 1, 2 2 ) initiated by Lewis acids, optionally activated with Bronsted acids or alkyl halides. Typical Lewis acids include BF3, AlC13, and (C2H5)A1C12.Typical activators include HCI. H 2 0 , and f-butyl chloride. Representative initiation reactions for Lewis acid/Bronsted acid are: AICI,
+ HCI
. dAIC16
AICI, H+ + CH,=C(CH,),
H’
“--,
CH,-C’AICI,‘
I
CH3
Initiation reactions are followed by a chain of propagation reactions in which monomer unitsadd to the carbenium ion end of the growingpolymer. These reactionsare very fast, highly exothermic, and affected by the reaction temperature. solvent polarity. and nature of the counterion.
CH,
CH3
Propagation proceeds until either a chain-transfer or termination reaction occurs. Chain transfer occurs if the carbenium ion of the chain reacts with another species, the chain-transfer agent. in a way that terminates the growing chain and starts a new one. Isobutylene itself acts as a chain-transfer agent via proton transfer to generate an olefinic chain end and initiate a new chain.
Isobutylene-Based Elastomers
879
The averagemolecularweight of thepolymer chains depends on therelativerates of propagation and chain transfer.The activation energy of chain transfer is generally much greater than that of propagation, so molecular weight (MW) is highly dependent on temperature (23, 24), and lower temperatures give higher molecular weights.In commercial practice, temperatures below approximately - 90°C are required to produce polymers of MW suitable for typical rubber applications. Termination reactions leadto discontinuance of the propagation without immediate generation of a new chain. This can be caused by reaction of the carbenium ion with nucleophiles, including the anion of the propagating ion pair or adventitious electron-rich species, usually oxygenates, which may be present as impurities. A representative example of the latter is:
--CH2-C+
I
AICI;
+
ROH
+ Polymer-OR +
H'AICI,
CH3
where R = H or alkyl. For the star-branched butyls, a styrene-butadiene-styrene block copolymer is added to the polymerization to serve as a termination agent for multiple propagating chains. Termination occurs when the growing chains react with olefinic functions on the polybutadiene blocks (17).
2.2
Polymer Structure
Polyisohrtylene Polyisobutylene is a linear amorphous polymer. At high molecular weight, it can crystallize when extended. The crystallites have a helical conformation with a repeating length of eight monomer units (25). It is the only member of the isobutylene-based elastomer family to so crystallize, dueto the absence of comonomer.The gem-dimethylfunctionality on alternate chain carbon atoms causes sufficient crowding to force the bond angles away from the normal tetrahedral 109.5"to approximately 123" (26,27). Polyisobutylene has a glass transition temperature of approximately - 70°C (28). The molecular weights of commercial polyisobutylenes range from approximately 30,000 to approximately 5 million. They usually have one olefinic chain end due to chain transfer with an isobutylene molecule. High-MW polyisobutylenes generally have the narrowest molecular weight distribution (MWD) of the isobutylene-based elastomers. with M,/M,, slightly above 2.0.
Butyl Rlrbher Isoprene is incorporated into butyl rubber via 1,4-polymerization, with the polymer chain in the trarls configuration (29):
Newman and Fusco
880
For commercial grades of butyl rubber, the ratio of n :m ranges approximately from 40 : 1 to 200 :1. Because the isoprene content is low and the reactivity ratio for the monomers is near unity (4), the unsaturation is randomly distributed along the chain. Commercialelastomers have molecular weights of approximately 150,000 (M,) and 450,000 ( M w ) . The MWD, as MwN,, is typically 2.5-5. Star-Branched Butyl
The star-branched butyls have a bimodal MWD. One mode consists of normal linear chains, as described above. The second mode consists of branches of normal chains connected through the SBS polymeric branching agent added to the polymerization. Because of this bimodality, the polymer andits compounds give improved processingrelative to conventional butyl rubber. The amount of branching agent is chosen to give the best balanceof processability and properties. In commercial star-branched butyl, approximately 87% of the weight is in normal chains and approximately 13% in the star polymer. Figure 1 illustrates the overall structure, and Figure 2 shows MWD curves for conventional and star-branched products of similar bulk (Mooney) viscosity.
Poly(isobuty1ene-co-para-methylstyrene) Pura-methylstyrene (PMS) copolymerizes through its vinyl functionality:
This hydrocarbon polymer, sometimes called XP-50, is not sold commercially but is brominated at the benzylic positionto give the commercial bromo-(isobutylene-co-paru-methylstyrene)elas-
CH2&,
,CH3
+
?H3
CHyC-CH-CHz
CH3
Isobutylene
+
Styrene Isoprene I
Cationic Polymerlzatlon
BUTYL CHAINS
- 87% Fig. 1 Structure of star-branched butyl (SBB).
\ Block Copolymer
Isobutylene-Based Elastomers
881
Fig. 2 Molecular weight distribution of SB BIIR vs. BIIR.
tomers denoted as BIMS by the IISRP. Commercial products have ratios of x:y ranging from 16: 1 to 40: 1. Carbon- 13 NMR studies have shown that the p-methylstyrene preferentially polymerizes with itself. For example, a polymer with x: y of 37 was found (30) to have about half of the PMSasindividualmersflanked by isobutylenes, i.e., -BSB-, andabout half as diads, i.e.,-BSSB-units. A random copolymer at this low a concentration of PMS would have virtually all of the mers as individual units. This implies that the reactivity-ratio product for these two monomers is significantly greater than unity. Poly(isobuty1ene-co-para-methylstyrene)hasmolecularweightssimilar to thosegiven above for butyl rubber. Its MWD tends to be narrower and is typically in the range of 2.2-3.5.
2.3 PolymerProperties Physical Properties
The most conlmercially important propertiesof the isobutylene-based elastomers are low permeability to gases and high mechanical damping. These properties. as well as their high density for hydrocarbon elastomers of 0.92 g/cc, are rooted in the gem-dimethyl groups on alternate carbon atoms of the long polyisobutylene chain segments. Thisfunctionality causes densepacking along the chain and low chain mobility. This combination has been directly linked to the unique permeability characteristics of these elastomers (3 I ) . Comparative diffusivities of several gases for butyl and natural rubbers are shown in Table 1 (32). The practical manifestation of low permeability is the much better air retention of butyl rubber innertubes relative to NR tubes. This is clearly shown in Table 2 (33) for inner tubes on cars driven at 97 k d h for 161 k d d a y . The high damping ability of isobutylene-based elastomers is caused by the crowding due to the gem-dimethyl groups. This crowding imposes rotational restrictions and high internal friction because of the energy dissipated in moving the gem-dimethylgroups around each other's interference. High internal friction increases the loss modulus, G", because a high proportion of imposedenergy is expended nonrecoverablyintothemovement of the dimethylgroups. Correspondingly, the storage modulus,G', is decreased because only alow proportion of imposed energy goes into elastically recoverable distortions of the polymer backbone. (See Ref. 33 for
ber
Newman and Fusco
882
Table 1 Diffusivity of Gases in ButylandNaturalRubbers (25°C cm% X 10")
Butyl
Gas Helium Hydrogen Oxygen Nitrogen dioxideCarbon
21.6 10.2 1S 8 1.10
5.93 1.52
0.08 1 0.045 0.058
1.10
further discussion on this subject.) The high damping of butyl rubbers leads to their use in shock-, vibration-, and sound-absorption applications. Compared to other hydrocarbon elastomers, the isobutylene family has a unique combination of high molecular weight between entanglements and low tendency to crystallize on extension (35) resulting ina low plateau modulus.Therefore, they have relatively low uncured strength but high tack and self-adhesion, leading to commercial application in adhesives, caulks, and sealants. Uncured strength, at short time scales, is markedly improved for the star-branched butyls in which entanglements have been sharply increased via the long-chain branches of the star molecules. Chetnical Properties
Isobutylene-based elastomers show solubility characteristics consistent with amorphous hydrocarbon polymers. They are very soluble in hydrocarbon solvents, with alicyclics being the most solvating, paraffinics next, and aromatics least. They are also soluble in chlorohydrocarbons and tetrahydrofuran. They are essentially insoluble in polar liquids such as acetone, ether, dioxane, and lower alcohols. The relationship between intrinsic viscosity[q]and molecular weightfor linear isobutylene polymers in diisobutyleneat 20°C follows (36, 37), and data have beentabulated for other solvents (38): (MW > ca. lo4)
[q] = (3.6 X 10-4)(MW0.64)
(MW < ca. 10")
[q] = (1.78 X 10")(MWO.JhS)
The olefinic groups in butyl rubber undergo reactions typicalof hindered olefins such as halogenation and oxidation. The first of these is the route to commercial halobutyls. Oxidation is slow and is inhibited in commercial products by addition of antioxidants such as hindered phenols (39). Oxidation of the butyls results in degradation of molecular weight.
Table 2 Air Loss of Inner Tubes in Driving
Tests (Wa) Air pressure loss
Original elastomer tube pressure Inner 3.4 Butyl rubber
Natural rubber
1 week 193 193
28
1 month 13.8 114
Isobutylene-Based Elastomers
883
The presence of olefinic functionality and the associated allylic hydrogen atoms allow butyl rubber to be vulcanized with curatives similar to those used in general purpose rubbers. Higher levels of ultra-accelerators, such as thiurams or dithiocarbamates, are usually required because of the low functionality levelin butyls compared to otherhydrocarbon elastomers. Other types of crosslinking agents used for butyl rubber includep-quinone dioxime and polymethylolphenol resins (40). Cures of the latter type give carbon-carbon crosslinks and, therefore, the most heat-stable butyl vulcanizates.
2.4
Manufacturing
The bulk of the world production of isobutylene-based elastomers is made in a low-temperature slurry process using methyl chloride as the polymerization medium and aluminum chloride as initiator. One Russian plant uses a solution process (41) with an alkane solvent andan alkylaluminum halide initiator. Figure 3 shows an overview of the slurry process for the complete family of isobutylenebased elastomers. The hydrocarbon members of the family (PIB, IIR, star-branched IIR, and the precursor of BIMS) pass directly from the polymerization sectionof the plant to the finishing section without going through the halogenation process. The heart of the polymerization section is the reactor itself.This reactor is of sophisticated design because it must operate at - 100°C to - 90°C while removing the considerable heat of reaction-820 J/gm. This is accomplished via a vertical reactor configuration, which combines the technology of stirred reactors with that of shell-and-tube heat exchangers. Refrigeration is accomplished by boiling ethylene in the shell portion of the reactor interior. Polymerization is conducted in the tubes. In practice, an axialflowmixeratthebottom of thereactor is used to circulatethe polymerizing mixture through the tubes. A mixture of purified monomers dissolved in methyl chloride is introduced to the bottom of the reactor. Catalyst, also dissolved in methyl choride is added in a separate stream, as is the SBS branching agent when star-branched products are desired. Turbulentflow of these solutions gives very rapid contact and the polymerizing mixture is forced upward through the tubes and eventually overflows the reactor. The growing polymer chains are insoluble in methyl chloride at the reactor temperatures and quickly form a milky
Butyl. Vx, XP-50 MeCl Monomers Catalyst @ MeCl
Hexane (Exxpro)
Antloxldant Hexane
or
DISSOLVER XP-50
Fig. 3 Manufacturingoverview.
"2'
Newman and Fusco
884
slurry of fine particles. The concentration is kept sufficiently lowso that the suspension is always maintained at low bulk viscosity and turbulent flow. The suspension does not agglomerate within the bulk because the rubber particles arewell below their glass-transition temperature. However, this process is prone to slow fouling at the tube surfaces. This is handled in commercial plants through the installation of multiple reactors. Reactors may then be sequenced through periods of polymerization and periods of cleanout while maintaining continuous plant production. Monomer conversion to polymer is restricted to obtainpolymer of satisfactorilyhigh MW. In practice, isoprene conversion is typically 45-85% and isobutylene conversion 75-95%, depending on the desired composition of butyl rubber (42). When PMS is the comonomer, its conversion is similar to that of isobutylene. Consequently, significant amounts of monomers, as well as methyl chloride, need to be separated from the polymer. This is done by contacting the slurry with either hot hexane for butyl that is to be halogenated or hot water for polymers that are to be finished nonhalogenated. The hexane contacting is known as the solvent-replacement process because it directly replaces one solvent (methyl chloride) with another, hexane, without an intermediate finishing step. This is the process used for most of the world supply of halobutyls. although some is manufactured by finishing the butyl and redissolving it in hexane. The halogenation portion of the process is discussed later in this chapter. For polymers that are finished unhalogenated, the polymer slurry, containing 30% rubber, overflows the polymerization reactor and is transferred to a stirred flash tank containing water at 55-70°C. The slurry contacts steam and hot water in the transfer nozzle. Slurry aid. typically zinc stearate or calcium stearate, is added at that point to control crumb size and prevent agglomeration of the crumb particles.Antioxidant may also beadded.Most of the methyl chloride andresidual monomers flash overhead at this stage. The remaining solvent and monomers are removed in a second stripper stage maintained under vacuum. The combined overhead streams are dried, separated, and recycled. The polymer (-10% rubber in water) is now ready to be dried. This is typically accomplished in a three-stage finishing procedure. Firstthe aqueous slurry is transferred onto a screen, through which most of the water passes while the wet polymer crumb, containing 50% water, is retained. Second, the wet crumb is passed through a squeezing device, such as an extruder. which presses most of the entrained water from the rubber and reduces the moisture content to 5-1096. Third, the wet rubber is put through a heated drying extruder, which has a peak temperature of 150-200°C. This high temperature. combined with a sudden release of the internal pressure at the extruder outlet, flashes off almost all of the remaining water. The dried rubber is cooled and conveyed, typically in a fluidized bed or airvey line, to the final finishing steps. For products that will be sold commercially, these consist of baling, wrapping, and boxing. Dense bales are formed by pressing carefully metered weights of rubber, typically 34 kg. into blocks.The bales are wrapped with polyethylene or EVA film and assembled into multibale boxes for the bulk of commercial sales.
2.5
Applications
Polyisobutylrrle
Polyisobutylene (PIB) cannot be vulcanized because it is a paraffinic elastomer. Its commercial uses include: Impact-strength blending with polyolefin plastics Uncured sheeting (e.g., roofing membranes)
885
Isobutylene-Based Elastomers Table 3 Butyl RubberApplications:MarketShare sales rubber Application Butyl
(%)
71 8 7
Inner tubes Adhesives and sealants Tire curing bladders Shock absorbers Valves Cable insulation Health care Other
6 2
2 2 2
Adhesives, caulks, and sealants Wax modifier Pipe wrap and electrical tapes Chewing gum base Fluids modification and drag reduction Worldwide sales of PIB is approximately 20 ktonnes/year Butyl Rubber
Low permeability to air is key to the worldwide market for butyl and halobutyl, whose principal application is air retention in tires.Totalannualworldwidesales of butylrubberare -250 ktonnedyear, primarily as inner tubes for tires (Table 3). Table 4 gives formulationsfor typical butyl inner tubes. The standard compound is recommended for bias ply and heavy-duty tubes. The low-modulus compound, designed for radial tire applications and severe operating conditions, offers excellent splice life under demanding conditions. The bicycle tube offers improved processability via higher levels of high-structure black, while maintaining properties compatible with the lower demands of these small vehicles.
Table 4 ButylInner TubeFormulations (phr) Standard Butyl 268 N550 (FEF) black N660 (GPF) black N762 (SRF) black Paraffinic oil Escorez 1102 Stearic acid Zinc oxide Sulfur TMTDS MBT ZDEDC
100
Low Bicycle modulus IO0 10
100 40
50 22
30 22 3
70 25 1 -5 2 1
0.5
I 5 1.25 1.S 0.5
1
5 1.5 1.5 0.5 0.75
Newman and Fusco
886
Butyl rubber is the polymer of choice for tire-curing bladders because of its good heataging resistance and low permeability to curing media. For this very severe high-temperature duty, compounds aregenerally cured through a polymethylolphenol resin. This leads to carboncarbon crosslinks and the most thermally stable butyl vulcanizates. A typical curing bladder formulation is 100 butyl 268, 5 neoprene W, 50 N330 black, 5 castor oil, 5 zinc oxide, 10 polymethylolphenol resin. Neoprene is included to provide reactive chlorine, which combines with the zinc oxide to activate the resin cure. The excellent vibration and shock-damping of butyl leads to its use in applications such as automotive suspension bumpers (43) and body mounts. A typical body-mount compound is 100butyl268, 40 N330 black,15 N-990 black, 20 paraffinic oil, 5 zinc oxide, 1 sulfur,2 CdDEDC. 0.5 MBTS.
3. HALOBUTYLS Halogenatedelastomericpolymers have beenknown for sometime in thesyntheticrubber industry. The nature of these products, their synthesis, and properties are adequately covered in the chapter in this volume on halogenated elastomers. The halogenation of isobutylenehsoprene copolymers (previously described), commonly referred to as “halobutyls,” is detailedhere, since their compositions contain small amounts of halogen ( 5 2 . 2 wt%) to give extraordinary reactivity to enhance vulcanization latitude and rates. This enhancement provides co-vulcanization capability with high-unsaturation rubbers. Butyl rubbers, in general, cannot otherwise be co-vulcanized very easily in practical systems with the general purpose rubbers. Halogenation with elemental chlorine andor bromine at approximately a 1 : 1 molar ratio (moles of halide to moles of unsaturation) results in the cure and co-cure enhancements while preserving the many unique attributes of the basic butyl molecule.
3.1
Halogenation Reactions and Process Description
The synthesis of the halogenated butyl involves the reaction of a solution of butyl in an alkane (e.g.,hexane or pentane) in the “dark” withelementalhalogens at processtemperatures of 40-60°C. The target is to produce a halogenated butyl in which no more than one halogen per unsaturation site is introduced. The final product weight-percent halogen specification for the major halobutyl commercial grades can be summarized as follows: Chlorobutyl: 1.1-1.3 wt% Cl for a 1.9-2.0 mole YO unsaturation Bromobutyl: 1 .8-2.2 wt% Br for a 1.6- 1.7 mole % unsaturation The reaction with chlorine under the above conditions is very fast, essentially completed in 15 seconds or less, even at the low molar concentration of reactants. The bromine reaction is much slower-about five times slower than chlorination. In both systems, thorough mixing is a prerequisite to meet the synthesis targets.These reactions leading to the primary specification products have been well documented (44-49) (Fig. 4). The relatively fast reactions occur via an ionic mechanism.The halogen molecules are polarized at the olefinic sites undergoing heterolytic scission and consequent reaction. The resulting halogenated derivatives are shown in Figure 5. The predominant allylic halide structures are structure I1 (80-90%) and structure 111 (10-20%). Minor amounts (<2%) comprise the remaining structures. During the halogenation reaction, a smallamount of polymerdegradationcan occur. Structuredistributioncanbealtered. The relative reactivities of the various structures have been detailed by Gardner and Fusco (50).
Isobutylene-Based Elastomers
887
Fig. 4 Halogenation of butyl rubber.
Fig. 5 Polymerfunctionality in halobutyl.
The actual process flow diagram for halogenation conducted in an alkane solvent (e.g., hexane or pentane) is shown in Figure 6. The hydrogen halide generated during the halogenation step must be neutralized with dilute aqueous caustic solution. The aqueous phase, following neutralization, is separated and removed.The halogenated cement isthen stabilized withcalcium stearate and antioxidant is added to protect the halogenated productduring the polymer recovery and finishing steps, much along the same lines as for regular butyl recovery. In the case of bromobutyl, calcium stearate in combination with epoxidized vegetable oilis required for stabilization, since it tends to be less stable and more reactiveduring the finishing process. Commercial grades of halobutyls vary in halogen type and content and molecular weights. Major producers of halogenated butyls are Exxon Chemical Co. and Bayer.
3.2 Halobutyl Properties The chemical property attributes for the halobutyls are primarily related to the reactivity of the corresponding C-Cl and C-Br bonds in the resulting polymer allylic structure. The benefits of greater cure latitude, kinetics and co-vulcanization capability, with general purpose elastomers, expanded the use of butyl-type polymers into inner inners for tubeless bias ply tires i n the early 1960s and later in the 1960s into tubeless radial tires.
888
Newman and Fusco Solution Storage
Halogenatlon Contactors
Neutrallzation Contactors
m
m
/
00
From Solution Preparation
t
-
Hexane Recycle
Stripper
Flash Drums
Settler
Fig. 6 Schematic flow plan of halogenated butyl rubber.
The main difference between chloro- and bromobutyl resides is the activity of the halogencarbon bond. Bromobutyl advantages over chlorobutyl are: Generally faster cure rate Higher cure efficiency-higher crosslink density per mole of halogen Somewhat greater vulcanization versatility Better co-vulcanizability with highly unsaturated rubbers Chlorobutyl, on the other hand,maintainsmost of thebenefitsattributed to enhanced cure activity. but it generally provides greater scorch safety and better stability in processing and i n storage over bromobutyl.
3.3 Compounding Halobutyls Carbon Black Carbon blacks affect the compound properties of halobutyl similarly to their affect on the compound properties of other rubbers: particle size and structure determine the reinforcing power of the carbon black and hence the final properties of the halobutyl compounds. Increasing reinforcing strength, for example, from GPF (N660) to FEF (N.550) to HAF (N347), raises the compound viscosity,hardness,andcuredmodulusandtensile strength.
Isobutylene-Based
889
Cured modulus increases with the carbon black level up to 80 phr. Tensile strength goes through a maximum at 50-60 phr carbon black level. Mineral Fillers Mineral fillers vary not only in particle size but also i n chemical composition. As a result, both cure behavior and physical properties of a bromobutyl compound are affected by the mineral filler used, although to a lesser extent than chlorobutyl compounds. Generally the common mineral fillers may be used with halobutyl, but highly alkaline ingredients and hygroscopic fillers should be avoided.
C b y s are semireinforcing. Acidic clays give very fast cures. therefore, extra scorch retarders may be needed. Calcined clay is the preferred filler for pharmaceutical stopper compounds based on halobutyl. Tulc is semireinforcing in halobutyl. without a major effect on cure Hydratecl silicas even at moderate levels cause compound stiffness and slower cure rate, so their use should be restricted. One way to enhance the interaction between polymer and silicates, and hence to improve compound properties, is to add small (1 phr) amounts of silanes. Particularly useful silanes are the mercapto and amino derivatives. Plasticirers Petroleum-based process oils are the most commonly used plasticizers for halobutyl. They improve mixing and processing, soften stocks, improve flexibility at low temperatures, and reduce cost. ParaffinicDJapthenic oils are preferred for compatibility reasons. Other useful plasticizers are paraffin waxes and low molecular weight polyethylene. Adipates and sebacates improve flexibility at very low temperatures. Process Aids Struktol 40 MS, Promix 400. and mineral rubber not only improve theprocessing characteristics of halobutyl compounds by improving filler dispersion, theyalso enhancecompatibility between halobutyl and highly unsaturated rubbers. Tackifying resins should be selected with care. Phenol-formaldehyde resins, even those where the reactive methylol groups have been deactivated, react with halobutyl, especially bromobutyl, causing a decrease in scorch time, while partially aromatic resins, such as Koresin, have an intermediate effect on scorch in bromobutyl. It should be noted that zinc stearate, which can also be formed via the zinc oxide and stearic acid reaction. is a strong dehydrohalogenation agent and a cure catalyst for halobutyl. Similar effects will be observed with other organic acids, such as oleic acid or naphthenic acid. Alkalinestearates, on the other hand (e.g., calcium stearate) have aretardingaction on the halobutyl cure. Amine-type antioxidants/antiozonants,such as Flectol H. mercaptobenzimidazole,and especially p-phenylenediamines, will react with halobutyl. They should be added with the curatives, not in the masterbatch. Phenol derivative antioxidants are generally preferred.
3.4
Processing Halobutyls
Mixing halobutyl compounds is generally accomplished in two stages similar to regular butyl. Calenderings, extrusions, and moldings are also generally similar to regular butyl. However,
Newman and Fusco
890
because of the general activity of allylic halogen in halobutyl, greater care must be taken in temperaturecontrol. Mix temperaturesshould not exceed 150°C. Calendering andextrusion temperatures should be kept below 90°C. Zinc oxide is a curative for halobutyl and must not be added in the masterbatching mixing step-it should be added to the mixed stock, generally in the second-pass mix with the other curatives at lower temperatures (<100"C) to minimize scorching reactions or premature vulcanization.
3.5
HalobutylVulcanization
A variety of vulcanization techniques are available with the halobutyls by virtue of the presence of olefinic unsaturation and reactive halogen. Many of these cure systems are unavailable to elastomers having only the unsaturation site. Conventional sulfur accelerator cure systems are possible and useful in halobutyl. The following are two general vulcanization systems unique to the allylic halide reactivity:
Zinc Oxide Cure and Mod8cations Zinc oxide can function as the sole curing agent forhalobutyl. The reaction is generally accelerated in the presenceof controlled amounts of stearic acid. Mostof the chlorinehromineoriginally present in the polymer can be extracted as the zinc halide saltafter vulcanization. The postulated mechanism shown in Figure 7 using the bromide derivativeof halobutyl is based on the formation of stable carbon-carbon crosslinks through a cationic polymerization route (5 l ) . The necessary initiation amounts of the zinc halide are probably formed through the thermal dissociation of a portion of the allylic halide to yield the corresponding hydrogen halide. The subsequent reaction of the acid halide with zinc oxide then provides the zinc halide catalyst. While the ZnO crosslinking reaction is relatively slow. the inclusionof thiurams, dithiocarbamates, thioureas, or reactive phenolic resins can greatly enhance the cure efficiency, yielding faster cure rates along with high states of cure. In the cure with ZnO alone. only a limited number, (1 0% for chlorobutyl and 20% of the bromobutyl) of the original reactive sites are utilized in the actual formation of carbon-carbon crosslinks. Other side reactions can occur, as
-C-C-CH-CHZPropagation HzC CH2
I@
ICH&-CH-CHz-
I C
HzC
I
II
II
+ -CHz-C-CH-CHz + -C-C-CH-CHZ-
I
-CHz-C-CH-CX
CH2
I
I
I
"A
HzC -CHZ-C-CH-C-
@i (ZnStzW
Fig. 7 Zinc oxide cure chemistry.
Or
-
(ZnStzX) -
Isobutylene-Based Elastomers
891
shown in Figure 7. Side reactions involve the formation of conjugated dienes with the ZnO reaction. The conjugated diene, while not crosslinked with ZnO alone, is postulated to take part in subsequent crosslinking reactions in the presence of multifunctional dienophiles, such as m phenylene bismaleimide (HVA-2) via a Diels-Alder reactionto yield higher crosslinked systems. Esterification side reactions can also occur between the halogen and zinc stearate. The reaction does not contribute to crosslinking and can be minimized by careful control of reactive ingredients. Vulcanization Through Bis-Alkylation
This crosslinking reaction is best illustrated by crosslinking with primary amines. There are numerous extensions of this bis-alkylation vulcanization reaction,and, in general, any functional molecule having two active hydrogens can, under the proper catalytic conditions, crosslink the polymer (e.g., dihydroxy aromatics and mercaptans). Cure S y s t e m Specific to Brornobutyl
Some cure systems are functional in bromobutyl and not with chlorobutyl, reflecting the relative difference in the C-Br versus C-Cl reactivities. For example, bromobutyl can be crosslinked with free radical-type cure systems, (e.g., peroxides). Also, bromobutyl can be cured in the total absence of zinc (generally not possible with chlorobutyl). This is particularly useful in some pharmaceutical applications. A summary of the most common halobutyl vulcanization systems is contained in Figure 8, highlighting characteristics and related applications for use. Scorch control can be effected through the use of MgO; type and concentrations depend on the compound and cure system used as well as the application requirements. 3.6 Applications for Halobutyls Applications for halobutyls usually take advantage of the desirable properties generic to butyl polymers, such as oxidation and ozone resistance, low permeability to gases and moisture. and high damping or hysteretic characteristics. These, together with the enhanced cure versatility andrate,highlyheat-stablecrosslinkedsystems,andcovulcanizability with generalpurpose rubbers, make the halobutyls applicable to a variety of tire and nontire applications. Tire Applications
The combination of property attributes outlined above makes halobutyls particularly attractive for use in a number of tire applications. They include Tubeless tire innerliners Tire sidewall components Black sidewalls (BSW) White sidewalls (WSW) Cover strips (CS) Staining barriers (SB) Heat-resistant inner tubes These are illustrated in the tire schematic shown in Figure 9. Innerliners The combination of excellent flex resistance, high heat resistance, low permeability, and thecapability of covulcanizabilitywithhighlyunsaturatedrubbers make the
Newman and Fusco
892 c
Cure System
Curatives, phr
Zinc Oxlde
ZnO-3
Dithiocarbamate
ZnO - 3 ZDEDC - 1.5 MgO -0.1 - 0.5 ZnO-3 TMTDS - l MBTS - 2 ZnO-3 Sulfur - 0.5 MBTS - 1.5 MgO - 0.1 - 0.5
Thiuranfhiazole
Sulfur/MBTS
BO-3
Sulfur/MElTS
Cure Rate
corch Safety
Characterlstlcs
Applications
l
Food Very and health care Sensitive to compounding Moderate Product quality control ingredients
Good Very Fast
Fair
Good
Low compressionset Heat resistant
Mechanical goods Injection molding
Heat resistant Fast
General purpose Mechamcal goods
Very Good
Moderate
Co-vulcanizationwith highly unsaturatedrubbers (GPR)
Tire innerliners Blends with GPR
Very Good
Fast
Co-vulcanizationwith GPR
Tire innerliners Blends with GPR
Very Good
Very Fast
Heat resistant
Heat resistance
Good
Fast
Co-vulcanizationwith GPR
BromobutyVGPWEPM triblends for black sidewalls
Good
Fast
Sulfur free
Good
Fast
Sulfur free
Clay filled compounds Pharmaceuticals Clay filled compounds Pharmaceuticals
Sulfur - 0.4 MBTS - 2.0 TMTDS - 0.25 MgO - 0.1 - 0.5 TMTDS - 1 Santocure MOR - 1.5 SulfudSulfenamlde
Sulfur - 0.6 Santocure
NS - 0.9
TMTDS - 0.15 ZnO - 3 SP 1045-2 ZnO - 5 SP 1045 - 5
Resm(low levels) Resm (high levels) Peroxide
DiCup 40C- 2 HVA-2 - 1
Fair
Room temperature
ZnO - 5 SnC12 - 2 ZnCl2 - 2
Very Scorchy
Very Fast
Good
Fast at high temperature. Moderate at
Pb02 - 5 DPTU - 4
100°C
Scorchv -
1
Sulfur - 2 ZDEDC-2
1
Good
Fig. 8 Summary of halobutylcuresystems.
l
1
Fast
I
Moderate
Heat resistant Moderate
High temperature and steam resistance
Should be premixed in Vistanex
Room temperature cunng sheeting, tank lining Hot water cure
l I comDresslon Low set
I
I
IZincaDolications free Speclal pharmaceuticals 1 Highly silica filled compounds
I
893
Isobutylene-Based Elastomers
Sidewall
Fig. 9 Steel beltedradial passenger tire.
halobutyls particularly attractive for use in inner liners for tubeless tires (52, 53). Typical chlorobutyl and bromobutyl formulations are shown in the Tables 5 and 6. Generally, for low-cost regular tubeless passenger tires, blends with GPR rubbers can be used. In more demanding applications (e.g., radial passenger and truck tubeless tires). 100% halobutyl compositions are preferred for maximum resistance to air and moisture permeability flex, and heat resistance.While the bromobutyl compounds offer faster cure rates andgood cured adhesion, the chlorobutyl compounds can give satisfactory performanceand may be preferred for their better stability in plant processing. Permeability of air and moisture for halobutyls compared to other GPR rubbers are shown in Table 7. The halobutyls, by virtue of their extremely low permeability, are uniquely suited to the inner liner function, minimizing air diffusion from the air chamber and minimizing intracarcass pressure and oxidative degradation in the tire laminated structure. This provides greater performance and durability, even under severe service conditions.
Table 5 HalobutylInner Liners for Passenger Tires Chlorobutyl 1066 Bromobutyl 2244 SBR-1712 NR#20 N330 HAF BIK Whiting Flexon 876 oil Stearic acid MgO (Maglite D) Zinc oxide Vultac 5 MBTS TMTDS Santocure NS Sulfur
60 -
20.5 25 40 40 IO 1 0.5 3
0.9 1.o 0.25 -
60 20.5 25 40 40 IO 1 0.5 3 -
0.2 1.5 0.5
894
Newman and Fusco
Table 6 HalobutylInner Linersfor Radial and Truck Tubeless Tires Chlorobutyl 1066 Bromobutyl 2244 N660 GPF BIK Flexon 876 Stearic acid MgO (Maglitc D) Struktol 40MS Phenol-formaldehyde resin Zinc oxide MBTS Sulfur
100 -
100
60 8 2 0. 15 7 4 3 1 .S 0.5
60 12 2 0.5 7 3 I .5 0.5
Table 7 Relative Air and Water Vapor Permeation Rates at 65°C Relative permeatlon rates Polymer'
Air
Moisture
100% NR 100% SBR 60% Chlorobutyl 60% Bromobutyl 100% Chlorobutyl 100% Bromobutyl
8.3 6.8
11.0
3.1 3.1 1.0 1.0
3.0 3.0 1.0 1.0
13.3
~'Typical inner liner compound.
Table 8 ChlorobutylWhiteSidewalls Chlorobutyl 1066 Vistalon 6506 (EPDM) SMR-5, NR Titanox 1000 NuCap 290 Mistron Vapor Talc Sunolite 240 W24 Ultramarine Blue SP-1077 Resin Zinc oxide Vultac 5 MBTS Sulfur
60 20 20 25 32.5 34 3 0.2 4 5 1.3 1.0 0.5
895
Isobutylene-Based Elastomers Table 9 ChlorobutylPharmaccuticalClosures Chlorobutyl 1066 MgO (Maglite D) Whitetex No. 2 Low molecular weight polyethylene Stearic acid Zinc ZDEDC Phenolic curing Resin (SP- 1045)
100
0.25 90 5 1
Cure with sulfur accelerator Cure 3.0 I .5
.o
with no sulfur 3.0
-
-
2.0
White Sidewalls The performance of tire sidewalls is critical from both an appearance and a durability point of view. Tire sidewalls must have long-term flex fatigue, weathering and ozone resistance, good whiteness retention, and nonstaining and serviceable adhesion to adjacent tire components. A typical high-quality halobutyl formulation for radial tire white sidewalls is shown in Table 8. Nontire Applications
The butyl-like properties of the halobutyls, together withcure rate and latitude advantages, make them especially suited for a variety of other nontire applications. These include: Automotive dynamic parts Belting and hose components for high-temperature service Chemical-resistant tank linings Pharmaceutical applications Ball bladders Adhesives and sealants The halobutyls and, to an extent, the butyls are extensively used in the pharmaceutical closure application because of the inherent butyl-like properties, together with: Chemical/Biological inertness “Clean” polymer compositions Suitable nontoxic vulcanization systems Resistance to heat, ozone, and UV light Typical chlorobutyl and bromobutylcompounds forpharmaceutical closures are contained in Tables 9 and 10. Otherapplicationstake advantage of the heat, dynamic properties,andenhancedcure activity of thehalobutyls. The ability to blendwith other elastomers provides even greater latitude in synergistic property achievement with the blending elastomer.
4.
BROMINATED ISOBUTYLENWpMETHYLSTYRENE ELASTOMERS
A new generation of isobutylene-based elastomers based on the copolymerization of isobutylene
with p-methylstyrene was developed by the Exxon Chemical CO (56-58). Brominated copolymers have been prepared over a wide range of monomer ratios, bromine content, and molecular
Newman and Fusco
896
Table 10 BromobutylPharmaceuticalClosures
Zn and Bromobutyl 2244 Whitetex Mistron Vapor Talc Cab-0-Si1 Stearic acid Low molecular weight polyethylene Wax White oil Zinc oxide SP1045 Resin DlAK No. 1
S Free 100
Sulfur-free 100
60
-
-
30 30
-
I 3 2 S 1
1
4 2 S 3
4 -
weights. These products are commercially available from Exxon Chemical Co. under the trademark ExxproT” Bromination is directed, in a controlled manner, to the para-methyl group of the styrenic comonomer in the copolymer, providing a reactive benzylic bromide functionality, which is key to this elastomer’s vulcanization and modification versatility. The backbone of BIMS elastomersis completely saturated, providing complete ozone and oxidation resistance, while the benzylic bromide functionality provides increased latitude for vulcanization and covulcanization with the highly unsaturated general purpose elastomers. This new family of elastomers preserves the basic properties of the butyl and halobutyl elastomers and by virtue of their unique structure also offers the ozone and heat resistance of the EPDM elastomers.
4.1
Process Description, Bromination, and Structure
The polymerization of isobutylene with p-methylstyrene (PMS)is a classic example of a cationically catalyzed reaction. Schematically this reaction was previously shown. The typical carbocationic polymerization is conducted in methyl chloride at - 100°C using a Lewis acid catalyst, e.g., AlClj or alkyl aluminum halides. Since the relative reaction rates of the monomers are very nearly the same, theresultant copolymer composition is similar to the feed monomer composition. To enhance the vulcanization reactivity of the IB/PMS copolymer, selective bromination to the p-methyl groups is necessary. In addition to being a very favorable monomer for copolymerization with isobutylene, PMS provides the very convenient site for introducing bromide functionality. A solution of the polymer in pentane is produced, and bromination is subsequently conducted in solution via a freeradical initiation “dark” reaction using bromine with free radical initiators. This reaction is schematically shown in Figure IO. Polymer recovery after solution neutralization is similar to the halobutyls. wherein small amounts of calcium stearate stabilizer are added priorto solvent removal with steam and waterin a flashdrum. Thepolymer is separated from the slurry and ultimately extruder-dried and baled. The completereaction process is outlined in Figure 1 1. A wide range of PMS contents and bromide concentrations is possible in BIMS products. The glass transition temperaturesof the BIMS polymers changevery little at PMS concentrations of < 15 wt% PMS. A plot of glass transition temperature versus PMS contentis shown in Figure
897
Isobutylene-Based Elastomers
CH
Pentane, lnltlalor
BlMS
CH 2 Br
Fig. 10 Bromination of the isobutylene/[]-methylstyrenecopolymer.
12. Essentially all of the BIMS grades are at the 5-15 wt% PMS levels and 1-2 wt% bromine or 0.5-1.2 mol% benzylic bromide. As noted in the structure shown in Figure 10, not all of the PMS is brominated. The ratio of PMS bromo-methyl groups to total PMS in the polymer is variable in different grades. BIMS elastomers are commercially produced by the Exxon Chemical Co.
4.2
Overview of BlMS Properties
The BIMS elastomers’ fully saturated, predominately polyisobutylene structureand high molecular weight provide high physical strength, excellent resistance to gas and moisture permeability, chemical inertness, good vibrational damping, and excellent resistance to heat and atmospheric aging. Besides these outstanding properties, BIMS elastomers also provide:
Free-radical Initiator and
CatalystBromine
MeCl
iC4=
MeCl
Pentane Pentane NEUTRALIZATION
Water Fig. 11 BIMS process block diagram.
Water Steam
898
Newrnan and Fusco 120 100 80
60
+
40 20 0 -20 -40 -60 -80
0
20
40
60
80
100
Wt. % PMS
Fig. 12 Tg of p-methyl styrenehsobutylene copolymer.
Enhanced cure activity by virtue of thereactivebenzylicbromidefunctionality. Even though halogen contents may be lower than the halobutyls, cure activity is as good as if not better than the halobutyls. Covulcanizability with high unsaturation general purpose rubbers. Facile functional modification thru the benzyl bromide site to introduce other functional groups suchas dithiocarbamate, acrylate, carboxy, andhydroxy derivatives. In addition, polymers containingfunctional end groups can be grafted onto thebenzyl bromide polymer site. Because of BIMS elastomers’ excellent chemical resistance, oxidative and thermal degradation is extremelyslow, as illustrated in high-temperature Brabender internalmixingtests conducted at 180°C. Degradation of butyl polymer after60 minutes and crosslinking of halobutyl polymers after 10-25 minutes resulted under these test conditions BIMS polymer showed little or no degradation or crosslinking even after 120 minutes. Cure response of the BIMS after this heat aging was relatively unchanged.BIMS elastomers superior thermal stability should translate into less sensitive mixing and processing, while maintaining good vulcanization capability. A comparison of BIMS vulcanizate properties compared to other elastomers is contained in Figure 13. Like other isobutylene-based polymers, BIMS polymers are soluble in nonpolar liquids. They arereadily soluble in alkanes andcycloalkanes, less so in benzene, and insolublein methyl ethyl ketone. The heat-aging properties of BIMS vulcanizates exhibit better retentionof tensile strength, elongation, tear strength, and flex resistancethan the halobutyls. Compared tothe EPDM peroxide-cured systems, BIMS gives essentially equivalent tensile strength retention but significantly better retention of elongation and compression set resistance. The BIMS elastomer’s saturated backbone together with its stable carbon-to-carbon crosslinking systems provides these exceptional heat-aging characteristics. Light-colored BIMS compounds have been shown to be less sensitive to UV attack than other isobutylene polymers by virtue of its paramethylstyrene comonomer. 4.3
Compounding and Processing BIMS Elastomers
Filler and plasticizer response of BIMS elastomer compounds are very similar to those of other isobutylene-based elastomers, particularly the halobutyls. Loading capacity/property relation-
Isobutylene-Based Elastomers
Attribute
!l3
BlMS
M
Properties Physical Ozone Resistance Heat stability
Flex Resistance Damping
High
Low Permeabiilty Covulcanlzatlon Compresslon Set (aged)
B B B B B pJ B
B B
o pJ
m
Resistance Chemical Solvent and Oil Resistance polar
-
B
- hydrocarbon Legend
0 B Good
pj Medium
Fig. 13 BIMS attributes vs. those of other rubbers.
ships are much the same as halobutyl systems. Special attention should be given to mineralfilled BIMS compounds. In general, the acidic or basic nature of the mineral fillers are less of an influence on the cure of BIMS compounds.Silica fillers, however, tend to be very retarding, as they are in halobutyls. Tackifier resins (2-5 phr), such as the non-heat-reactive phenol-formaldehyde types, are most effective in BIMS compounds. Hydrocarbon resins are also useful in enhancing tack. Generally higher concentrations are necessary since they are less effective than the phenolic resins. The major advantage of the hydrocarbon resinsis that they are less discoloring in mineralfilled light-colored compounds. Stearic acid is most commonly used as a processing aid and release agent in rubber compounding. In BIMS elastomer systems,however, the stearic acid and zinc stearates are active curatives and should be added with the curatives. The alkaline earth metal stearates are often used as release agents in rubber compounding. In the BIMS systems, they can have a drastic retarding effect on cure response. The use of processing aids such as Struktol40MS and Promix 400 are especially helpful in BIMS compounding and processing, as was discussed in Section 3. Mixing BIMS compounds in internal mixers and in mill mixing are very similar to the halobutyls. Again, special consideration should be givento the ZnO and zinc stearate additions during the mix cycle, since they are curatives. They should be added not during the first stage
Newman and Fusco
900 Alkylation Routes With lewis Acid Catalysis
@
-@
CH&
Lewis Acid ___)
@
CH,@ MBr
A I
t
t
i
Aromatic
Olefinic
Basic Heteroatoms
Nucleophilic Substitution With Bifunctional Species ViaOtherDerivatives
Fig. 14 BIMS vulcanizationchemistry.
of the mix cycle, but rather with the curatives generally at a lower mix temperature in the second pass. Extrusion. calendering, and molding operations are similar to those of the halobutyls.
4.4 Vulcanization of BIMS Elastomers The vulcanization of BIMS elastomers has been shown to proceed via the electrophilic addition of the benzylic bromide throughan alkylation reaction to the styrenic comonomer i n the polymer chain (59-62). The zinc salts catalyze the crosslinking reaction to form carbon-to-carbon bonds as depicted in Figures 14 and 15. The Lewis acid salt formation of the benzylic bromide via the zinc ion (present as mixed halide from the oxide or stearate)undergoes subsequent alkylation reactions to aromatics or addition reactions with olefinic groups providing a mechanism for
Isobutylene-Based Elastomers
901
crossalkylution of the BIMS with the unsatuation of general purpose rubbers (e.g., NR. SBR, or BR). The Lewis acid salt speciesof BlMS is also capable ofnucleophilicsubstitution reactions with bifunctional organic compounds, again capable of yielding the crosslinking of the BIMS. The benzylic bromide concentration strongly affects network density (cure state). Times to optimum cure are dependent on the benzylic bromide concentration and the specific curative combination employed. For some acceleration systems. such as those containing dithiocarbamates (e.g.. ZDEDC). variation in benzylic bromide has only a moderate effect on cure. The temperature coefficient of vulcanization for carbon black-filled BIMS compounds is about 1.6 per 10°C. Severalsulfilr-containillgacceleratorscommon to halobutyl vulcanizationsystemsare equally effective i n combinations with the ZnO-stearic acid system of BIMS. The following combinations have been defined with corresponding cure behavior and utility characteristics. Su/firr trrltl Tllirr:olo.s
The combination of a mercaptobenzothia~,oleand sulfur with zincoxide-stearicacidyields good aging and tlex resistance. Typically the levels of curatives are 2.0 phr each of zinc oxide and MBTS and I .O phr of each of the stearic acid and sulfur. Thiazoles used alone tend to be scorchy. Sulfur and sulfenamide are generally inferior in cure rate. Tlliurnt~~s trrlti Ditllioc,tlrl,arrlrrtt..s Ultra-accelerators tend to yield very fast but scorchy cures i n BlMS compounds. The control of scorch can be difficult. Thiuram hexasulfide (Tetrone A)is a very effective curative for BIMS combining the the thiuram activity with the sulfur donor characteristics of the hexasulfide.
Alkyltrtiorl Cwcrti\?rs Alkylation reaction mechanisms are basicto the ZnO-stearic acid cure i n BIMS. Thepolymethylo1 phenol resins (heat reactive), phenol disulfides. diphenyl porcr-phenylenediamine (DPPD), and Permalux are very effective curing agents for BIMS. In this regard they resemble the cure behavior of the halobutyls, although reduced levels are equally effective. Combinations of resin with sulfur. MBTS. and TEG are generally recommended to control cure rate and scorch. This curative combination is very effective for high heat and compression set resistance. Effective scorch retarders include calcium stearate. MgO, and in some cases triethylene glycol, for mostof the cure systemsdefined. Concentrations of scorch retarders must be carefully controlled for the balance of scorch safety and cure rate. Zinc hydroxycarbonate with MgAIOH(C03) and with MgO/Mg(OH)? is especially effective in providing scorch safety with minimal cure rate interference.Curative combinations most appropriate for specific property achievement in different applications will be illustrated in the next section. 4.5
Applications for BlMS Elastomers
The unique combination of properties available with the BIMS elastomer system provides the basis for a number of industry applications in both tire and nontire areas. The cornbination of butyl-likeattributestogetherwiththe complete ozone,oxidation.andheatresistance of the EPDM elastonlers is not available in any other synthetic rubber system.
Tire Cor~rpor~twt.~ A number of applications in tire components have been developed utilizing the BIMS property attributes. These extend the use of isobutylene-based elastomers i n the tire area (63). including:
-
902
Exxpro 96-4
BR 1207 SMR-20
N330
Flexon 641 Escore2 11 02 SP-1068 2.00 Resin Sbuktol40MS
I!&MwmQ
1st Staae
I
I
Zinc Oxide Sulfur
30 :1 :1 ;! Acid Stearic
L
50.00 41.67 8.33 40.00 12-17
1. Mb: Poiymers let lo warm lo 75'C. 2. R w n add 0590% of Carbon Black and mix lo 130'C. 3. Add Oils + Reelmand res1 of Carbon Black 4. MIX to 180.C. Dump and mol. 5. b e NO until& alds WIh &c Soaps or Zinc Steamlee. 6. Mixer at 6 0 %M Sdea and Rotors -07% Door. 7. Stock P m m m 01 ,255 MPa (37 PSI). 0. ROIM n p speed of 32 Mslers/ minute.
5-0 4.00
163.00
I
EbmLsmQ
Remill Stage
0.32
Newman and Fusco
163.00 0.75 0.80 0.60 0.50
Rnal Slag0 01 'Remnr Mix l.SaLoading 01 2nd Pass and CuraliveS. 2. Flnal Mix lo 100-105'C then dump and mol ASAP 3. NO antitack alds WlIh Zlnc Soaps or Zinc Stearates. 4. Mixer el 60% on M e s h Rotor 87% Doar . 5. Stock ProSWre 01.255 MPa (37PSI).
-
I
Fig. 16 Recommended BIMS: tire sidewall formula and mix procedure.
Tire sidewalls-improved ozone and flex resistance in blends without the use of discoloring antidegradants. Inner liners-in severe service OTR and largetrucktires for improvedheatandflex resistance along with air and moisture impermeability properties Curing bladders-for improved heat and flex resistance Blend treads-to deliver improved traction at low temperatures and reduced rolling resistance with minimum abrasion loss. A sidewall formulation is contained in Figure 16 (64, 65). Highlighted are the optimal grade definition and polymer blend ratio togetherwith a cure system and mix procedure for best performance. BIMS compositionsoffer improved sidewall longevityand improved improved tire appearance compared to regular compositions with GPR rubbers using discoloring antidegradants. Curing bladder formulations are contained in Table 11. Compound 2 is more reversion resistant and has less tendency to grow in size during usage. Inner liner compositions with BIMS are similar to the halobutyl examples with a cure system comprised of stearic acid, 1.O phr; zinc oxide, 2.0 phr; sulfur, 1 .O phr; and MBTS, 2.0 phr. This liner composition has improved heat and flex resistance for severe service in OTR and large truck tubeless tires. Nontire Applications
BIMS compositions are finding application in a variety of mechanical and extruded parts, such as: Automotive dynamic mechanical molded goods-Improved age resistance while preserving butyl like dynamic properties (66) Heat-resistant hoses and belting-improved heat and age resistance and blending capability with other elastomers (67)
903
Isobutylene-Based Elastomers Table 11 BIMS Curing Bladder Formulations ~~
2
1
Compound no.
100 55 2 7 0.75 3 l 0.5 1.2 1.1
100
BIMS (ExxproQ 3035) N330 Carbon black Paraffin wax Castor oil Perkalink 900 Zinc oxide SP- 1045 Stearic acid MBTS DHT4A2 Sulfur
55 2 7 -
3 7 0.5 1.2 1.1 0.75
Pharmaceutical applications-“clean polymer” compositions with sulfur-free cures. Adhesives and mastics-good adhesive qualities with excellent age resistance (68) Air springs-improved flex resistance with good impermeability properties
BIMS elastomer functionality and molecular parameters suitable for different applications are summarized in Figure 17. 4.6
Other Functional Modifications of BIMS
The benzylic bromide functionality in BIMS provides a wide range of nucleophilic substitution reactions. These reactions enable conversions of all or part of the benzylic bromide to other desirable functionalities for specific purposes and also enables the preparation of a multitude
2-
1.81.6c
6 2
1.4-Recommended for most 100% BIMS .2-
8
applications.
f
1
/
I % m 0.6
for
Recommended
0.4 applications.
0.2 01
3
4
5
6
7
8
9
PMS, Weight percent
Fig. 17 BIMS parameters (ex Mooney viscosity).
l 0 1 1 1 2 1 3
904
Newman and Fusco
Graft Copolymer
Polymers
c 3
PPO. PI. PB.SBR
P S , PP.
Bmmphmom.Clnnmate M d Acrylalcs Coating. Adheshna
TPE.
Comptibiliura. Impact Modifier
CHzBr lonomers
Polar Groupt~
.
OH.COOH
Calionw Ammnnwn. Phosphonium
FunctionalEder
Anionic. Sulfonales
Fig. 18 Latitude in polymer modification.
of grafted copolymers. Other functional modifications include esters and ethers, carboxylation and hydroxyl functional derivatives, ionomers, and a variety of alternative grafted polymer compositions (69, 70). The range of functional and grafting latitude capable with the BIMS is illustrated in Figure 18. The development of these functionalized derivatives of BIMS extends the cure and chemical reactivity of theisobutylene-basedpolymers. The acrylate andmethacrylate derivatives substitute the active olefin ester for the benzyl bromide, providing a vulcanizable base that can be essentially halogen-free. An internally stabilized system has been developed through the benzophenone modification of the acrylate derivatives. Free radical-driven curing reactions such as peroxides with the acrylate derivative have been shown to give fast cures and effective crosslink densities. Radiation cure with electron beam sources proceed equally efficiently and effectively to give useful and practical cured products. These modifications hold the promise of extending the latitude and utility of the isobutylene-based elastomers.
COMPOUNDING INGREDIENTS
Trade Name or Abbreviatlon
CompositlonDescription
Supplier
Cab-0-Si1 CdDEDC Diak No. 1 DiCup 40C DHT 4A2 DPTU Escorez 1102 Flexon 641 Flexon 876 HVA-2
Fumed silica Cadmium diethyldithiocarbamate Hexamethylene diamne carbamate Dicumyl peroxide (40% Active) MgAlOH carbonate Diphenylthiourea Petroleum-based resin Naphthenic mineral oil Paraffinic mmeral oil N,N'-Phenylenebis maleimide
Cabot R.T. Vanderbilt Co. DuPont Hercules Kyowa Chem. Co. Several suppliers Exxon Chem. Co. Exxon Co. (USA) Exxon Co. (USA) DuPont
isobutylene-Based Elastomers
Trade Name or Abbreviation
Composition/Description Supplier
MBT MBTS Mistron Vapor Talc NuCap 290
Mercaptobcnzothiazole Mercapto bcnzothiazyl disulfide Ultra-fine magnesium silicate Mcrcapto silane functional hydrated-aluminum silicate 1.3-bis(Citracollirnidomethyl) benzene Alkylated naphthenic/aromatic hydrocarbon resin Alkylphcnol polysulfide 2-(Morpholinothio) benzothiazole N-tc~rt-Butyl-2-bcnzothioz~lesulfenamide Alkyl phenol formaldehyde resin Unreactive I)hcnol/formaldehyde resin Alkylated naphthenic/arornatic hydrocarbon resin Petroleum wax Tricthylenc glycol Tctramethyl thiuram disulfide Alkylphenol disulfide Calcined clay Zinc diethyl dithiocnrhamatc
Pcrkalink 900 Promix 400 Rylcx 301 1 Santocure MOR Santocure NS SP- 104s SP-1068, SP-1077 Struktol 40MS Sunolitc 240 TEG TMTDS Vultac 5 Whitetex No. 2 ZDEDC
905
Several suppliers Several suppliers Cyprus Ind. Min. CO J.M. Huber Co. Flcxsys America Flow Polymcrs. Inc. Ferro Corp. Flexsys America Flexsys America Schencctady Chem Schenectady Chem Struktol Co. Sun Oil Co. Scveral suppliers Several suppliers Pennwnlt Corp. Freeport Kaolin Several suppliers
REFERENCES 1. Butlerov and Gorianov, ( 1873) A r m 160:146. 2. Thomas, R. M,. Lightbown, I. E., Sparks, W. J., Frohlich, P. K., and Murphree, E. V. hui. E q . Chern. 32, 1283 ( 1940). 3. Thomas. R. M., and Slotterheck, 0. C. (1941). U.S. Pat. 2,243.658 (May 27). 4. Thomas, R. M., and Sparks. W. J. ( 1944), U.S. Pat. 2,356,128 (Aug. 2). 5 . Sparks, W. J.. and Thomas, R. M. (1944). U.S. Pat. 2,356.129 (Aug. 2, 1944). 6. Thomas,R. M,, and Sparks. W. J. (1963). Sylthr/ic Klthhrr, Chapter 24, Whitby, G. S.. ed. John Wiley and Sons. New York. 7. Crawford. R. A., and Morrisey, R. T.(l953), U.S. Pot. 3,631,984 (March 17). 8. Morriscy. R. T. ( 1955). Hlthhrr World l.?h':725. 9. Morrisey, R. T. ( 1955). 1ntl E r q . Cherrl. 4 7 1562. IO. Hnllenbcck. V. L. (1957). U.S. Pat. 2,803,448 (Aug 27). I I . Morrisey, R. T. (1957). U.S. Pat. 2,816,098 (Dec. 10). 12. Baldwin, F. P.. and Thomas, R. M. ( 1960). U.S. Pat. 2.964,489 (Dec. 13). 13. Baldwin, F. P,. Bucklcy, D. J.. Kuntz, I., and Robison, S . B. ( I Y O I ) , KuObrr Plustics Age 42:500. 14. Tcgge. B. R., Baldwin, F. P,, and Serniuk, G. E. (1962). U.S. Pat. 3,023,191 (Feb 27). 15. Johnson. J. A., Jr., and Luallin, E. D. (1966), U.S.Patent 3,275,349 (June 21). 16. Walker, J.. Jones. R. H., and Feniak. G. (1973). Philadelphia Rubber Group Technical Meeting. Oct. 22. 17. Wang. H. C., Powers. K. W.. and Fusco. J. V. ( l9X9), ACS Rubber Division Meeting, Mexico City, May. 1x. Duvdevani, I.. Gursky, L., and Gardncr, 1. J. (19x9). ACS Rubber Division Meeting. Mexico City, May, 19. Wang. H. C.. and Powers, K. W. (l992), Elastorrwrics /4:22. 20. Krusc. D. F., and Fusco. J. V. ( 1992). Eltr.s/orrwricx /4:24.
906
and
Newman
Fusco
21. Plesch, P. H., and Gandini, A. (1966), The Chenlistr?,of Polyrnerizcrtion Processes. Monograph 20, Society of the Chemical Industry, London. 22. Kennedy, J. P., and Marechal, M. (1982). CLrrlJocationic Polyrnerization, John Wiley and Sons, Inc., New York. 23. Kenncdy, J. P., and Thomas, R. M. (1962),Polvmerization c m i Polvco~~de~~sarior~ Processes, Advances in Chemistry Scries No. 34, American Chemical Society, Washington, DC, p. 1 1 1. 24. Tomqvist in, E. G. M., Kennedy, J. P,, and Tomqvist, E. G. M. eds. (1968), Polymer Chemistry of Synthetic Elrstorners, Part l, John Wiley and Sons, Inc., New York, p. 326. 2s. Fullcr, C. S., Frosch, C. J., and Pape, H. R. (1940). J. A m Ckenl. Soc. 62:1950. 26. Boyd, R. H., and Breitling, S. M. (1972). Mucrornolecules 5:l. 16:1317. 27. Suter, U. W., Saiz, E., and Flory, P. J. (1983), M~rcro~nolecules 28. Wood, L. A. ( 1976). Rubber Chem. Trchnnl. 49: 189. 29. Che, H. Y., and Ficld, J. E. (1957), J. P o l y ~ n Sci. . (Part B)5:501. 30. Cheng, D. M., and Wang, H. C. (1991), ACS Rubber Division Meeting, Toronto, May. 31. Krishna Pant, P. V., and Boyd, R. H. (1993), Mncronlolecules 26:679. 32. Stannett, V. ( 1968), in D$fu.sion i n Polymers (J. Crank and G. S. Park, Eds.), Academic Press, Inc., Orlando, FI. 33. Fusco, J. V., and Hous, P. (1987), in Rubber Technology. 3rd ed. (M. Morton, Ed.), Van Nostrand Reinhold Company, New York, p. 290. 34. Ferry, J. D. (1980), Viscoelastic Properties of Polvrners, 3rd ed.. John Wiley and Sons, Inc., New York, pp. 277, 341, 606, 608. 3s. Dias. A. J. (1996). in Polvrneric Moterials Encyclopedia. Vol. S (J. C. Salomone, Ed.), CRC Press, New York, pp. 3488-3489. 36. Fox, T., and Flory, P. J. (1949). J . Phvs. Colloid Chern. 53:197. 37. Matsumoto, T., Niskoka, N., and Fujita, H. (1972), J. Poly. Sci. Prrrt A2 10:23. 38. Brandrup, J., and Immergut, H. eds. (1975). Polv~nerHandbook, Vol. 9, 2nd ed., John Wiley and Sons, New York. 39. Yu, J. F. S., Zakin, J. L., and Patterson, G. K. (1979), J. A ~ J JPoly. / . Sci. 23:2493. 40. Smith, W. C. (1964). Vulcmiz(rtion ofElrrstorners. (G. Alligcr and I. J. Sjothun, Eds.) Reinhold, New York. 41. Scherbakova, N. V. (1977). Canadian Pat. 1,019.09S. 42. Kresge, E. N.. and Wang, H. C. (1993), Kirk-Othnler Encyclopedicr of Cherniccrl Technology 4th ed. Vol. 8, Wiley, Ncw York, p. 941. 43. Kruse, D. F., and Edwards, R. C. (1968), SAE Trrtns. 7 7 1868. 44, Walkcr, J., Jones, R. H., and Feniak, G. (1972), Philadelphia Rubber Group Technical Meeting, Oct. 22. 4s Baldwin, F. P., and Kuntz, I. (1960), Kirk-Othrner Encyclopedia of Cherniccrl Technology, 2nd Suppl. (A. Standen, ed.), Interscience. Ncw York, p. 716. 46 McNeil, I G. (1963). Polynwr 4:lS. 47 Van Tongerloo, A., and Vukov, R. (1979), Proc. Int. Rubber Conf., Venice, p. 70. 48. Vukov, R. (1984). Rubher Chern. Technol. 57:275. 49. Baldwin, F. P,. and Schatz, R. (1979). Encyclopedirr of Chern. Tech., 3d ed., John Wiley and Sons, New York, Vol. 8, p. 470. so. Gardncr. 1. J., and Fusco, J. V. (1987). Paper presentation, ACS Rubber Div., Montreal, Canada, May 29. SI. Baldwin, F. P,, Buckley, D. J., Kuntz, I., and Robison, S. B. (1961 ), Rubber Plastics Age 42500. 52. Fusco, J. V., and Dudley, R. H. (l960), paper #27, New compounds for air barriers in tubeless tires, presented to Rubbcr Div. of ACS, New York, Sept. 53. Fusco, J. V., and Dudley, R. H. (1961), Rubber World 14457. 54. Coddington, D. M. (1980),Tire inflation pressureloss, its causes and effects, presented to 5th Australian Rubber Tech. Convention, Camberra, Australia, Oct. 1-3. ~~ h n . Tech. 52:909. 5s. Coddington. D. M. (1979). R u I J I IC 56. Wang. H.-C.. and Powers, K. W. (1991), Rubber Division, ACS Meeting, Toronto, Ontario, Canada, May 2 1-24.
Isobutylene-Based Elastomers
907
57. Powers, K. W., and Wang, H.-C. (1989). European Pat. Publication, No. 0344021 (Nov. 29). 58. Wang, H.-C., and Powers. K. W. (1992), Elnstornerics (Jan. and Feb.). J. V., and Hous, P. (1992). Crosslinking of brominated p59. Gardner,I. J., Wang,H.-C.,Fusco, methylstyrene/Isobutylene copolymers in blends with general purpose rubbers, presented to Rubber Div., Am. Chem Soc. Meeting, Louisville, Kentucky, May 19-22. 60. Bielski, R., Frechet, J. M. J., Fusco, J. V., Powers, K. W., and Wang, H.-C. (1993), J. Po/wner k i . , Parr A: Poly. Chem. 31:755. 61. Frechet, J. M. J., Bielski, R.. Wang, H.-C., Fusco, J. V., and Powers, K. W. (1993), Rubber Cllem. Tech. 66:98. 62. Eckman, R., Gardner, I. J., and Wang, H.-C. ( 1 9 9 3 ~Rubber Chern. Tech. 66 ( I ) . 63. Fusco, J. V., and Young, D. G. (1990), Isobutylene based polymers in tires-status and future trcnds, Presented to ACS Rubber Division, Washington, DC, October. 64. Tisler, A. L., McElrath, K. O., Tracey, D. S., and Tse. M.-F. (1997). New grades of BIMS for nonstaining tire sidewalls, Presented at Rubber Div. of ACS Meeting, Cleveland, Ohio, October. 65. Waddell, W. H. (1997). Tire sidewall surface discoloration-renew, Presented to the Rubber Div. of the ACS, Cleveland, Ohio, Oct. 21-24. 66. McElrath, K. O., Measmer, M. B., and Yamashita, S. (1996), Dynamic properties of elastomer blends, presented to the Rubber Div. of the ACS, Montreal, Canada, May 4-8. 67. Costemalle,B.,Fusco, J. V., and Kruse, D. F. (1993).A new elastomerforthe mining industry, 18-21, Presented at the Rubber Div. ACS Meeting, Denver Colorado, May 68. McElrath, K. O., and Robertson, M. H. (1995), Heat resistant isobutylene copolymers, ArI/~rsive.sAge (Sept.):28. 69. Wang, H.-C., Fusco, J. V., and Hous, P. (1994), Acrylate ester modification of isobutylene and paramethylstyrene copolymers, Rubber World (Oct.). 70. Merrill, N. A., Powers, K. W., and Wang, H.-C. (1992), Functionalized p-methylstyrene/isobutylene copolymers, Polymer Preprint 33:962.
This Page Intentionally Left Blank
Index
1.2-Polybutadiene, 1 10, 1 1 I 1,4-HD, 843 1,4-polybutadiene. I 10, l 1 1, 1 12, 1 14 1,4-polyisoprene. 110, I l 1 1,S-Naphthalene diisocyanate (NDI), 766 ""Xe-NMR, 228, 229, 235 W N M R , 228,235 2,4-toluene diisocyanate, 766 2-butyl styrene sulfonate isoprene copolymer, 438 2-mcrcaptoimidazole, 575 3A sanitary standard, 385 4,4'-diisocyanatodiphenyImethane (MDI), 765 Abrasion, 334. 370, 427 resistance, 576. 730, 819 ABS. 269, 273, 301, 302. 346 Accelerated storage hardening, 44 Accelerators, 666 Acetone extraction, 49 Acetoxy cures. 652 Acid-coagulated rubber, 40 Acidic proteins, 39 ACM, 269, 301, 302, 507, 5 10. 5 1 I , 659, 664 Acrylated chlorinated rubber, 22 Acrylic-based elastomers. 659 Acrylonitrile content. 787, 788, 791 Activators. 666, 861 Adhesion, 2 17, 2 19, 22 I , 346, 380, 7 16, 7 I7 strength, 426 Adhesives, 341, 584, 675 Adiprenes, 727, 730 AFLAS. 547, 549, 55 I , 553. 559 AGE, 685
Age resisters, 667 Aging, 102, 536, 542, 569, 578, 615. 616. 694, 717. 778 natural rubber, 40 Air permcabilitics, 85 Alcryn, 503. 504 Alkylation curatives. 901 Allophanate, 390 Alloprenc (ICI), Parlon, 63 Alloys. 695 Allylglycidyl ether, 684 Alpcx, 66 Alumina, 740 Amino acids, 31, 35. 39, 40, 56, 57, 58 Aniline point, 802 Anionic (living) initiators, 827 Anionic polymerization. 324, 328, 821. 824. 825 Anisotropy, 243, 246 ratio, 248 Annealing, 364 Antioxidants, 125, 333, 337, 575. 740. 792, 863, 889 Aramids, 242 Arnitel, 354, 367, 370, 374, 379 Aromatic oil. 691 Ash, natural rubber, 37, 40, 44, 45 Aspect ratio, 242, 243. 256 Asphalt blends, 347 ASTM oil, 802 ATBN, 146, 306 Atomic absorption, 843 Atomic force micr~scopylmicrographs.4 19, 420 Autoclave, 776 Auto-coagulated rubber, 40 Automobile exteriors, 346
909
910 Automotive, 674, 772, 810, 902 Automotive application. 382 Autoxidation, 39, 56, 404 Azeotropic mixture, 789 Back pressure, 378 Bagasse, 22, 23 Balata, 29, 62, 65, 488 Banbury. 168, 181, 185, 191, 192, 337, 406, 617, 667, 689, 741, 190 Bases, 861 Basic proteins. 39, 56 BDO. 393 Belting, 902 Belts, 81 1 Bimodal, 35. 36 BIMS. 881, 896. 897, 898, 899 Bin stability, 572 Biodegradable, 55 Biosynthesis. 33. 36, 56, 59 Bis-alkylation, 891 Bis-maleimide-sulfur, 670 Bisphenol, 543, 553 AF, 553, 554 Biuret. 141. 390 Blacks. 528. 533, 544, 573. 740, 748, 756, 757, 790. 792 Blend viscosity. 212, 213 Blending, 485, 497 Blends, 197, 227, 234, 265, 267, 272, 282, 284, 287, 309, 341, 345. 358, 406, 468, 498, 500, 502, 503, 506, 521, 524, 529, 534, 538. 545, 559, 578. 579, 581, 582, 675, 707, 708, 7 14, 7 16, 790, 820, 893 of natural rubber, 95 with thermoplastics, 345 with thermosets, 346 Block. 269 copolymcr, 265, 321, 325 length distributions, 357 Blow molding, 314, 433 Blown film, 345 JSR RB820.484 Bonding, 244. 7 I8 Bond strength, 5 16 Bottom fraction, 39, 56 BR, 267, 269, 275, 290, 301. 302 Brabender. 271, 272, 898 Branched block copolymers, 322 Branchmg, 34, 35, 37. 59, 334, 789, 819, 821, 853, 854, X59 agents, 86 I
Index Breathable film, 43 1 Brominated isobutylenclpmethylstyrenc elastomers, 895 Bromination, 118, 119, 862, 897 Bromo-(isobutylene-co-paramethylstyrene)elastomers, 880 Bromobutyl, 888 Bulk density, 842 Bulk polymerization, 787 Burst strength. 246 Butadiene acrylic acid copolymer, 435 Butadiene rubbers, 268, 8 18 Butanediol- 1.4. 767 Butyl rubber, 877, 879, 885 applications, 885 Cable filling, 345 Calcined clays, 749 Calcium carbonate, 740, 792 Calendering, 246, 314, 406, 408, 409, 668, 776, 890 Cambium, 29. 30 Capillary extrusion, 50 Capillary number, 260 Capillary rheometer, 52. 54, 57, 251, 261 Carbenatlon, 124 Carbene, 123 Carbon black, 183, 209, 210, 213, 214, 217, 219, 220. 221, 230, 337, 407, 61 I , 689, 705, 845, 888 Carbon fiber, 242, 485 Carboxylated elastomers, 434 Carboxylated latices, 583 Carboxylated NBR, 581 Carboxylated rubber, 561 Cast elastomers, 765 Castable liquids, 753 Casting into open molds, 765 Casting machines, 768 Catalysts, 826 Cationic polymerization, 157, 878, 890 Caulks, 676 Cellobond, 60, 70 Cellulose fibers, 241, 244, 246, 251, 253, 255, 257 retnforccment, 243 Cement, 675 Ccntrifugation, 47 Centrifuged latex, hevea rubber, 39 Chain cleavage, 160 Chain entanglement, 33 1 Chain extender. 390. 392, 652, 767 Chain extension, I36
Index Chain scission, 38, 54, 120, 686 Chain-transfer, 878 agents, 861 CHDM, 393 Chemical crosslinking effects, 400 Chemical crosslinks, 54 Chemical properties, 882 Chemical relaxation, 54 Chemical resistance, 37 I , 374, 378, 480, 558 Chicle, 29, 56, 59 Chin implants, 639 Chlorinated hydroxylated guayule rubber, 23 Chlorinated natural rubber, 62 Chlorinated polyethylene elastomers, 269, 500, 517, 518, 519 Chlorination, 22, 118, 119, 518, 519, 530, 862 Chlorobutyl rubber, 516, 581, 888 Chlorocarboxylation, 564 Chlorosulfonated polyethylene elastomers, 5 16, 521, 522, 523, 581 Chlorosulfonation. 522 Chlorotrifluoroethylene (CTFE), 538 Cis-1,4 polyisoprene, 4, 29, 50, 59 Cis-trans isomerization, 115 Classification, hevea natural rubber, 4 1 Clay, 740, 749, 792, 889 Clones, 30, 31, 35, 49, 591. Closed cell sponge weather seals, 867 Cluster formation, 462 CM, 521 CO, 524, 528 Coagulation, 46 Coalescence, 20 I , 266 Coating, 22, 344, 380 CO-continuous, 274, 279 phases, 337 Coconut fiber, 245 Cohesive energy density, 203, 672 Cold crystallization, 38, 49 Cold flow, 8 I9 Cold mills, 337 Cold polymerization, 785, 788 Cold-feed extruders, 190 Colloidal stability, 31, 39, 49, 60 Comb-grafted natural rubber, 8 I Commercial latices, hcvea natural rubber, 34 Compaction factor, 178 Compactor, 179 unit, 185 Compatibility, 197, 266 Compatibilizing agents, 23 1 Compatible, 265, 266, 337, 345 Composite, 241, 245, 247, 250, 254,
91 1 Compounding, 549, 573, 666, 689, 739, 758, 777, 792, 793, 888, 898 ingredients, 336 Compression molding, 190, 272, 314, 776 Compression set, 578, 801 Condensation, 429, 613, 654 polymerization, 159, 592 Constant viscosity CV, 35, 41 Construction adhesives, 343 Construction industry, 772 Continuous mixers, 170 Continuous vulcanization, 742 Coolingkycle time, 378 COPE, 367, 370, 382 Copolyester thermoplastic elastomers, 3 12 Copolymerization, 499, 549 Copolymers, 321 Coupling elements, 77 1 Coupling reactions, 326, 327 Covulcanizability, 898 Covulcanization, 94, 517, 710, 714, 896 CPE. 301, 302, 5 I 1 CR, 178, 182, 186, 269, 301, 302 powder, 174, 175 Creamed latex, 47 Creaming, 847 Creep, 39, 54, 55 Critical entanglement spacing, 270 Critical molecular weight, 33 I Critical surface tension, 270 Crosslink, 331, 343, 684, 686 density, 673, 713 Crosslinked polyethylene, 739 Crosslinking, 136, 151, 231, 532, 536, 543, 565, 568, 569, 613, 615, 737, 898 agents. 740 Crumb rubber, 847 Cryogenic grinding, 837, 838 Cryogenically ground rubber, 840 Crystal melting point, 337 Crystalline, 206, 210, 274, 303 polyolefins, 288 Crystallinity, 266, 268, 302, 361, 363, 397, 479, 488, 489, 684, 702, 710, 71 1, 735, 821, 847, 85 1, 864 Crystallites, 702 Crystallization, 206, 218, 221, 231, 234, 274, 52 I , 532, 620 rate, hevea natural rubber, 38, 49, 50, 56, 57, 58 CSM, 521 CTBN, 146 Cuplump, 4 1, 42
912
Curatives, 295, 892 Cure rate, 572, 892 Cure systems, 330, 892 Curing, 794, 817 reaction, 138, 162 Custom mixing, 189 CV. 742 steam, 869 CVJ boots, 381, 383 Cyanaprcne, 410 Cyclatex, 66 Cycle times, 181 Cyclic siloxanes, 607 Cyclite, 66 Cyclization, 1 15, 116, 117 Cyclized natural rubber, 61, 65, 68 Cyclooctene, 698 Damping coefficient, 78 1 DCPD, 853 Deformation resistance, 740 Degradation, 272, 330, 343, 348, 371. 402, 406, 536, 702, 747, 756, 761, 898 Degree. 253 Dehalogenation, 5 19 Dehydrohalogenation, S20 Depolymerization, 92, 1 15 Depolymerized natural rubber, 91, 93, 94 Deproteinizcd, natural rubber, DPNR, 38. 39, 42, 56, 58 Desmodur, 757 Desmopan, 4 10 Devulcanization, 57 I Diamine, S 16, 543 Diblock, 328, 344 Die swell, 251, 331 Dielcctric propcrties. 423, 489 Diene, 817 Differential scanning calorimetry (DSC), 204, 227, 229, 360, 363, 672 Diffusivity, 882 Diisocyanates, 140, 390, 391, 755, 765 Diphenylmethane diisocyanate, 754 Dipped articles, 340, 583 Dispersion fiber, 241, 242, 243, 250, 251, 252, 253, 256, 257, 258, 259, 260, 261, 262 measurement, 176 Dispersive mixing, 206, 207, 208 Distributive mixing, 206, 207, 208 Dithiocarbamate, 792, 901 Domain formation, 445 Domains, 322, 330, 397
Index
DOP, 529 DOS, 519 Double pass mixing, 255 Draw ratio. 247 Dry blending, 338 Dry ruber content (DRC), 30, 47, 48 Dry-blended compound, 339 DSC, 398,497, 702,703, 704, 85 1 Dump mixing, 169 DVNR, 99 Dynamic crosslinking, 97, 492 Dynamic mechanical propcrties, 283, 42 I , 458, 508, 577, 691, 693, 716, 780, Dynamic relaxation, 672 Dynamic shear modulus, 267, 286 Dynamic viscosity, 849, 854, 855, 860 Dynamic vulcanization, 102, 273, 281, 289, 293, 508 E 1 mechanism, 5 17 E2 mechanism, 517 Ear implant, 640 Ebonites, 93, 94, 7 11 Ecdel, 354, 367, 369, 731 ECO, 524, 528, 683 Economics, powdered rubber systems, 188 EEC food approval, 384 Efficient vulcanization, 17, 18, 21, 83 Ejection, 377 Elastic fibers, 695 Elasticity, 265 Elastollan, 410 Elastomer segments, 328 Electrical properties, 466, 482, 626 Elcctron, 743 bcam, 736, 738, 743, 744, 904 microscopy, 31, 59, 202, 249, 275, 330, 358, 450 Electron spin resonance spectroscopy, 450 Elitel, 354 Elvax, 435, 474 Emulsion, 53 1, 787 polymerization, 434, 562, 826 ENB, 853 End uses, guayule, 21 ENE reactions, 120 Energy dissipation, 2 13 savings. 182 Engage, 497 ENPCAF-modified NR, 61, 76 ENR, 83, 90 Entanglemcnt, 271, 303, 328, 709
913
Index Environmcntal resitance, 333 Environmcntal stress cracking, 747 Environmental stress-crack resistance. 489 Environment-friendly,29 Enzymes, 39, 59r EPDM, 242, 251, 252, 269. 273. 274, 275, 281, 283, 289, 290, 291, 301, 302, 336, 436, 460, 5 I I , 582, 777, 845, 846, 859, 865 polyolefin blends, 288 polypropylene TPV, 309 EPDMPP, 3 I 1 EPDM-polyolefin TPV, 290 EPDM-polypropylene, 202, 3 15, 3 16 Epichlorohydrin elastomer, 5 16, 524, 525 nylon TPV, 310, 31 I rubber, 579 EPM, 706 Epoxidation,119 Epoxidized NR, 6 1, 82. 1 I3 Epoxidized SBS, 120 Epoxy crosslinks, 570 Epoxy cure sites, 670 EPR, 336 Estane, 410, 41 1 Ester plasticizers, 69 I Esterification, 570 Ethylene, 736, 85 I Ethylene-acrylic rubber-nylon TPV, 309 Ethylene glycol, 355 Ethylene methacrylic acid copolymer, 435 Ethylenc methacrylic ionomers, 447 Ethylenc-octenc copolymer, 497 Ethylene propylcne rubber, 500 Ethylene-styrene interpolymers, 499 Ethylenc vinyl acetate. 269, 488, 495 EVA, 301, 302, 324, 489 blends, 495 Exothermic, 266 Extender oils, 848 Extensional flow, 245 Extruder, I78 barrel, 379 mixing, 337 Extrusion, 15, 178, 246, 314, 379, 408, 429, 668, 748, 776, 890 aids, 575 die, 245 rheometer, 399 Exxpro, 878 EYPEL, 596 Ezcure, 666
Farrel bridge MVX mixing, I80 Fatigue, 370 FDA food approval, 385 FEMA specification, 12 Fenolac, 66 Fiber, 361 orientation, 241, 245, 247. 248, 249 rubber bonding, 249 suspensions, 246 Field latex, 34, 37, 43, 47 Fill factor, 226, 253, 255, 261, 262 Filler, 336, 337, 520, 524, 528, 533, 537, 544, 573, 656, 667. 740, 748 Film, 340, 429, 497 blowing, 408 Fire rctardants, 337 Flame rctardancy, 526 Flammability, 188, 343 Flcx resistance. 334 Flexibility, 735, 801. 802 Flexible automotive, 339 Floatation process, 9, 10 Flour filler, 243 Flow curve, JSR RB820, 483 Flow induced coalescence, 200 Flow orientation, 245 Fluid resistance. 523, 527, 532, 536, 687, 688 Fluorescent light exposure, 850 Fluoroelastomers, 539 Fluorosiliconc elastomers, 534, 535. 537 FMQ, 630 Foaming, 3 14 Foams, 736 Footwear, 586 Formwork mats, 773 Fourier transform infrared spectroscopy ( R I R ) , 852, 854 Free energy, 266 Free radical polymerization, I54 Friction, 331, 334 Fuel resistance, 510, 526, 801, 802, 804, 812 Fume extraction, 38 1 Functional polybutadiene, 147 Fungus cracks, 405 Furanized NR, 82 FVMQ, 609, 630 FZ, 593,596 Gaflcx, 354 Gamma radiation, 405, 736, 738 Gas permeability, 427, 805 Gas permeation, 761 Gas-phase polymerization, 828, 858
914
Gate location, 376 Gating system, 376 399 Gehman Gel, 6, 14, 34, 35, 36, 55, 58, 59, 99, 795 Gel permeation chromatogrpahy (GPC), 36, 55, 399 Gelation, 34, 35 Geolast, 95 Glass, 374 Glass fiber, 242, 243 Glass transition temperature, 83, 203, 266, 282, 333, 335, 343, 353, 518, 532, 549, 622, 672, 728, 761, 799, 800, 851, 853 Gloves, 340 Glycols, 729 Goldenrod, 2, 29 Gordon-Taylor equation, 360 GPC, 4, 5,700, 704, 729, 846, 847 Grading, hevea natural rubber, 41, 42, 46 Graft, 269, 507 Grafted NR, 61 Grafting, 244 Graphite fibers, 485 Green strength, 35, 54, 57, 312, 847 Green tire, 818 Grinding, 837 Guayule rubber, 1, 2, 22, 23, 29, 36 Gutta percha, 29, 56, 62, 488 Halobutyl elastomers, 517, 886, 887, 891, 892 Halogen reactivity, 5 15 Halogenated butyl rubber, 118 Halogenation, 118, 839, 877, 886 Hard segment, 765 Heat-aging, 898 Heat build-up, 801 Heat resistance, 519, 526, 532, 536, 541, 755, 801, 893 Heat-shrinkable film, 497 Heat stability, 380, 739, 848 Heat welding, 433 Herelor, 684 Hevea hrusiliensis, 1, 2, 29, 55-60 Heveaplus, 7 1, 72 High density polyethylene, 324 Higher fatty acids (HFA), 38, 49, 56, 58 High-pressure process, 735 Hinge finger joint implants, 637 HIPS, 819, 820 HNBR, 785, 787,790, 793, 796 Holding pressure, 378 Hose, 246, 586, 865, 810, 902 Hot dry gas, 869
Index Hot melt, 341, 342, 343 HQEE, 393 HRH, 242, 244, 245 HTBN,146 HTRI, 854 Hybrid, 241 Hycar, 435 PA, 660 Hydin, 683 Hydrated silicas, 889 Hydraulic fluids, 598 Hydroboration,115,126 Hydrocarboxylation,123 Hydrofluorocarbon elastomers, 5 17 Hydroformylation,126 Hydrogen bonding, 395 Hydrogenated epoxidised natural rubber, 90 Hydrogenated natural rubber, 61, 62 Hydrogenated nitrile rubber (HNBR), 785 Hydrogenated polybutadiene, 110 Hydrogenated styrene butadiene rubber, 502 Hydrogenating, 327, 328 Hydrogenation, 91, 109, 113 Hydrohalogenated natural rubber, 61, 63 Hydrohalogenation, I 18, 1 19 Hydrolysis, 402 resistance, 755 stability, 403, 730, 770 Hydroperoxidation, 404 Hydrorubber, 62 Hydrosilation, 124, 6 12 Hydroxyl cure sites, 670 Hydroxyl number, 729 Hydroxylated polybutadienes, 142, 143 Hypalon, 523 Hysteresis, 91, 92, 219, 220, 282, 399, 692, 693, 730, 819, 821 Hytrel, 354, 367, 369 I-B-I, 117 IcBIc,117,118 IIR, 269, 290, 301, 823 Image analysis, 249 Impact resistance, 345, 346, 347, 371 Infrared spectroscopy, 445, 795 Injection mold, 180 Injection molding, 179, 190, 281, 314, 371, 377, 388, 408, 428, 433, 484, 492, 493. 747, 776 Injection pressure, 378 Injection rate, 378 Inner liners, 89 1, 894 Inner tube, 882, 885
Index Insert-molded, 339 INSITE catalyst, 497, 499 Insulation compound, 496 Interaction parameters, 203 Interfacial tension, 269, 283 Intermix, 168 Internal mixers, 169, 183, 272 International rubber quality and packing committee, 43 Interpenetrating, 346 Interphases, 267 Intrinsic viscosity, 399, 796, 882 Iodine value, 795, 802 Ionic clusters, 567 Ionic elastomer, 123 Ionic polymers, 433 Ion-ion interaction, 445 Ionomeric modification, 122 Ionomeric thermoplastic elastomers, 433 IR, 114, 269 Irradiating, 244 I S 0 specification, natural rubber, 47 Isobutylene, 877 -isoprene rubbers, 825 Isocyanate chemistry, 389 Isocyanate crosslinked polyester, 757 Isoelectrical point, 39 Isomerization, 1 17, 118 Isopentenyl pyrophosphate, 3, 33 Isoprene rubbers, 822 Isotactic, 704 Izod impact strength, 98 Jectothane, 41 0 Jute fiber, 244 KOH number, 48, 49 Kopel, 367 L/D ratio, 374 Ladder, 1 17 Latex, 9, 29-31, 37, 39, 42, 47, 56-60, 272, 847 concentrates, 37, 39, 42, 47-49, 57, 58, 60 handling, 48 particles, 29, 31, 39, 49 stability, 40, 49 LCM, 758, 776, 868 LCST, 198, 229, 231 LDPE, 489,497 Leather finishing, 583 Levapren, 496 Lewis acids, 116, 517, 607, 878, 896
915
Light stability, 848 Lipids, 31, 37, 39, 49, 58 Liquld curing medium, 75 Liquid elastomers, 133, 135, 705 Llquid natural rubber (LNR), 61, 91, 94, 790 Liquid silicone rubber, 634 Lithium alkyls, 325 Living polymer, 324 Living polymerization, 8 18 LLDPE, 345, 735 Lomod, 354 Low-density polyethylene, 110 Low-pressure process, 735 Low-temperature properties, 347, S 19 LSR, 606 Lube oils, 348 Lubricants, 598 Lubricating oil, 804 Macrocyclics, 7 10 Macrodiols, 754 Macroglycol, 390, 391 Maleation, 863 Masterbatching, 189, 258 Mastication, 35, 36, 46, 47, SO, 54, 57, 168 MDI, 392 Mechanical goods, 870 Mechanical properties, 45 I , 458, 496, 502, 503, 509, 510, 626, 672, 779 Mechanical stability time (MST), 48, 49, 56, 58 Medical, 675 Melt flow, 332, 509 index, 380, 427 Melt index, EVA, 489 Melt mixing, 272 Melt processable rubber, 503 Melt viscosity, 271, 328, 332 Mercerization, 244 Metal oxide crosslinks, 567 Metal sulfonated EPDM, 435, 454 Metallizing, 38 1 Metallocene catalyst, 826, 862 Metathesis, 697, 698, 775, 819 Methacrylic acid, 563 Methylene-bis-orthochloro aniline, 767 Metton, 705 MFI, 427 MC rubbers, 103 Microbial attack, 40.5 Microbial stability, 770 Microcellular PU elastomers, 772 Microdiols, 755 Microgel, 36, S9
916 Microstructure, 53 I , 698, 819, 821 Microtomcd, 250 Microwave, 776, 869 Milkweed. 2 Millable gum, 388, 753 Millable polyurethane, 753 Milling, 41, 43, 246, 250, 252 Mills, 406, 668 MIL-R-25988, 535 Mineral fillcrs, 889 Mining applications, carboxylated rubber, 585 Miscible, 204, 266, 267 Miscible blends, 198, 206, 215, 220, 229, 231, 234 Miscible mixture, 228 Mixers, 170, 74 1 Mixing, 168. 250, 252, 520, 524, 528, 533, 537, 544, 667, 739 cycle, 181 efficiency, 256 time, 257 Modification, 563 Modulus, blcnd, 213 Moisture content, 842 Moisturc cure, 136, 738, 742, 745 Mold, 378 dcsign, 375 flow, polyphosphazenc clastomer, 595 shrinkage, 378 Molding, 406, 408, 758 Molecular weight, 5 , 34-36. 39, 58. 59, 328 Mono-carbodiimides, 771 Monogalactosyl diglyceride (MGDG), 38 Monomers, 525, 540, 565, 661 Monosil, 742, 746 Monsanto rheomctcr, 667 Mooncy scorch, 687 Mooney valuc. 702 Mooncy viscometer, 667 Mooncy viscosity, 35, 44, 45, 50. 60, 252, 262, 686, 689, 705, 708, 753, 776, 788, 791, 800 Morphology, 281, 330, 331, 358, 509 Morthanc, 4 10 Morton and Krol line, 86 Mossbauer spectroscopy, 449 m-phcnylene bismalcimidc, 891 MQ, 609, 620 Multifunctional initiation, 326, 327 Multimodal distribution, 3 1 MVX,179,183,193 direct, 185 strip, 185 MWD. 854
Index
Natural mbber, 1. 61, 68, 70, 71, 73, 244, 268, 500, 8 I7 Natural rubber latex, 340 NBAR, 790 NBIR, 790 NBR, 114, 174, 182, 184, 186, 269, 290, 301, 302, 578, 580, 785, 786, 793, 798, 801 nylon, 295, 297, 298, 299, 31 1 TPV, 309 polyolefin, 304 PP thermoplastic vulcanizate, 307, 308 powder,175 PVC blends, 288 NDI, 392 NEM, 790 Ncmatic interaction, 212, 229 Neoprene, 253, 808 Neutron scattcring, 204, 206, 232, 448, 568 Nitration, 126 Nitrile rubber, 69, 84, 244. 785 Nitromercuration,126 NMR, 204, 227. 228, 229, 230, 232. 233, 236, 795. 854 spectrum, 550 Nonrubber in natural rubbcr, 37, 48, 49, 58 Norbornene, 698 Norsorex, 704, 775. 776, 777, 779, 782 Nouvelan, 369 NR, 290, 301. 302 NR-SBR powder, 175, 176 preblcnd,174 Nuclear magnetic resonancc, 204, 227, 447 Number averagc molecular weight, 399 N-vinylpyrrolidone, 863 Nylon, 31 I , 505 ACM swelling, 509, 510 Nylon-6, 243, 507 Nylon-6, 9 (PA), 269, 273 Nylon- 12,4 17 ODR, 686 Oils, 845 alcryn, 506 drilling, 8 13 field applications, carboxylated rubber, 585 industry, 772 palm fibcr, 244 resistancc, 371, 510, 527, 755, 761, 802 swelling, 523 One-shot, 768 Open mill mixing, 338 Opcn-mill mixers, 168 Optical properties, 468
Index Organic fibers, 242 Organotransition metals, 113 Orientation, 246 O-rings, 8 1 1 Oxidation, natural rubber, 33, 34, 38, 56, 59, 882 Oximc cure, 654 Oxonium ion. 724 Ozonc resistance. 578 PA, 30 l , 302. 368 PA80, 75 Packing, 8 1 1 Palc crcpc. 42 Pandex, 410 Paper coating, 583 Paracril, 806, 807 Parallel-model, 285 Paraprene, 4 10 Parcl, 684, 685 Ruthenium argentaturn, I , 29 Particle size and distribution, 31, 843 Partitioncr, 172 PBT, 30 I , 302, 369 PC, 269. 301, 302 PCL, 393 PE, 301, 302 PEA. 417,429 block copolymers, 429 PEBAX, 417 PEEA, 429 Peel strength, 334 Pcllethanc, 410, 412 Pelletizing and chopping, 339 Pclprcne. 354, 367 Perfluoroelastomcrs, 541 Pergut, 63 Permanent set, 497 Permeability, 215, 216, 383, 624, 806, 893 Pharmaccutical closurcs, 895 Phase diagram, 234 Phase morphology, 274, 280, 328 Phase separation, 330 Phase transfer catalysis, 109, 1 19, 123, 126 PHC, 393 Phenol-formaldehyde resins, 66, 69 Phenylcncdiaminc, 792 Phospholipd, 31, 33, 34, 37, 39 Photorcsists, I17 Physical properties, 480, 486, 490, 747, 750, 758 Pibitlex, 354, 367 Pigments, 333, 337
917 Pineapple leaf fiber, 245 Pipeline pigs, 772 Plant maintenance costs, 181 Plant process equipment, 771 Plasma treatment, 244 Plasticity retention index, PRI, 35, 40, 44, 45, 56r, 58r Plasticization, 37 I , 407 Plasticized PVC, 243 Plasticizer, 520, 524, 533, 538, 544, 575. 689, 691,692, 756, 788, 793, 889 Plastoprene, 66 Pliofilm, 64 Plioform, 66 Pliolite, 66 PMMA, 273, 290, 30 I , 302 PMQ, 630 PMS, 884, 896 PO, 683, 685 Poisson cffect, 247 Poly PTHF, 727 Poly( a-methyl-styrene), 112 Poly( l-chloro-l-butenylene), 530 Poly(acrylate)s, 662 Poly(butadicne-co-N-vinylpyridine), 113, 114 Poly(butadiene-co-sodiumstyrene sulfonate), 438 Poly(buty1ene terephthalate), 358 Poly(ester-ether) copolymers, 358 Poly(ester-urethane), 406 Poly(ether) diol, 354 Poly(ether-h-ester) thermoplastic elastomers, 359 Poly(ether-ester) copolymers, 363 Poly(ethy1ene ether) diol, 355 Poly(ethy1cnc oxide), 354 Poly(ethy1enc-co-butylcne), 110 Poly(ethy1ene-co-methacrylicacid) ionomer, 439 Poly(ethy1cne-co-propylene), 1 1 I , 1 12 Poly(isobuty1cnc-co-para-mcthylstyrene),880 Poly(methy1 methacrylate) grafted natural rubber, 71 Poly(oxycthy1cne) diol, 354 Poly(oxypropy1ene) diol, 354 Poly(oxytetramethy1ene) diol, 354, 360 Poly(phcny1ene oxide), 345, 346 Poly(propy1ene ether) diol, 355 Poly(propy1ene oxide) elastomer, 683, 684 Poly(styrcnc-co-isoprcne-co-butadiene) terpolymer, 824 Poly(tctramethy1enc ether) diol, 355 Poly(tetramethy1cne oxide), 723 Poly(tetramethy1ene terephthalate), 290 Poly(viny1 chloride) (PVC), 243, 324
918
Polyacrylic rubber, 58 I Polyalkenylenes, 697, 699, 701, 704 based TPEs, 3 12 Polyamides, 346, 417, 429, 507, 582 elastomers, 418 Polyaryloxyphosphazene, 600 Polybutadiene, 114, 579, 819, 820 Polybutylenc. 11 1 Polybutylene terephthalate, 5 1 1 Polycarbodiimides, 771 Polycarbonate (PC), 269, 273 Polychloroprcne, 69, 84, 516, 531, 581 elastomer, 529, 530 Polydichlorophosphazene,592 Polydimethylsiloxane, 606 Polyester, 242, 354, 723, 730, 766 esters, 373 thermoplastic elastomer, 353, 354 Polyether block amide, 417 Polyether block esters, 357, 372, 426 Polyethers, 140, 683, 766 Polyethylene (PE), 1 1 I , 112, 253, 268, 269, 273, 289, 290, 324, 345, 346, 489, 495, 500, 502, 519, 735, 736, 747 elastomers, 5 19 terephthalate (PET), 51 I , 582 Polyfluoroalkoxyphosphazene, 595 Polyisobutylene, 879, 884 Polyisoprenc, 80, 81, 1 12, 1 14, 823 Polymeg, 727 Polymerization, 526, 535, 540, 563, 564, 606, 685, 697 Polymethylmethacrylatc, 269, 273 Polynorbomcne rubber, 775 Polyoctenylcnes, 698, 700, 703, 709, 710 Polyolefin resin, 291 Polyoxytetramethylene, 723 Polypentenamer,114 ionomers, 439 Polyphosphazene elastomers, 591 Polypropylene, 96, 267, 268, 269, 273, 281, 283, 290, 307, 316, 324, 336, 337, 340, 341. 345, 346 compatibilized, 346 Polystyrene (PS), 79, 112. 268, 269, 273, 290, 324, 336, 340, 345, 346 grafted NR, 61 segment content, 328 Polysulfides,135 Polytetrahydrofuran, 723 Polytetramethylene terephthalate, 269, 290 Poly-trans-pcntenamer, 269
Index Polyurethane, 140, 141, 311, 723, 729, 765 elastomers, 387 ionomer, 439 rubber-nylon TPV, 3 I O Postcure, 668, 768 Postpolymerization,123 Powder NBR, 790 Powdered polymer technology, 17 1 Powdered reclaim, 172 Powdered rubber, 167 Powdering, 171 Power consumption, 181 Power law, 262 Power requirements, 379 PP, 114, 273, 274, 301, 302 PPG, 393 Preblcnd, 177 Prepolymer, 768 Pressure-sensitive adhesives, 342 Prevulcanized, 74 Printing, 381 Process aids, 94, 520, 524, 528, 533, 537, 544, 666, 889 Processability, 337, 492, 595, 821, 824, 853 Processing, 9, 14, 35, 41, SO, 52, 54, 58, 169, 31 1, 312, 336, 370, 407, 427, 469, 506, 666, 667, 889, 898 condition, 339, 340 temperature, 380 Proofing,190 Pro-oxidants, 40 Properties alcryn, 504 BIMSlhalobutyl, 881, 887, 897 EPDM, 859 ethylene acrylic acid copolymer, 457 ethylene-styrene interpolymers, 499 ethylene vinyl acetate ionomer, 459 NBWHNBR, 793, 797, 801 norsorex, 777 PTHF, 728 SBS, 330, 339 urethane composition, 145 vestamid, 424 Propylene, 851 oxide, 684 Prorads, 738 Proteins, 7, 31, 34, 35, 37, 39, 40, 41, 46, 49, 55, 58, 59 serum fraction, 39 PS, 301, 302 PTAd, 393 FTMEG, 369
Index PTMO, 393 PTMT, 273 PTPR, 301, 302 Pulp fibers, 244 Pultrusion, 242 PVC, 243, 520, 580 PVMQ, 609,620,630 Pyrolysis gas chromatogrpahy, 795 PZ, 593 Q-thane, 410 Quaternary phosphonium sulfonated EPDM, 437,470,47 1 Quebrachitol, 40, S8r, 59r Q values, 548 Radial (S-B),,x copolymers, 33 1 Radiation crosslinking, 738, 743 Raman spectroscopy, 450 Reactive butadienc, 142 Reactive plasticizers, 870 Reactivity ratios, 325 Rebound resilicnce, 770 Rcciprocating screw, 427 Reclaiming, 837 Rccycled rubber, 843 Recycling, 101, 835 methods, 837 Regrind, 380 Reinforcement, 133, 152, 241, 242, 610 Reinforcing agent, 406 Relaxation models. 236 Reprocessibility, 495, 497 Reprocessing, thermoplastic natural rubber, 101 Resilience, 712, 819 Resin, 335, 336, 340, 341, 342, 344 modified natural rubber, 66 Resistance to chemicals, 761 Retarders, 667 RFS steelcord carcass, 720 Rheological behavior 1,2 polybutadiene, 480 Rheological properties, 21 1, 618, 854, 859 Rheology, 213, 231, 251, 257, 312, 464 Rhodium catalysts, 113 Ring opening polymerization, 160, 592, 607, 723 Riteflex, 354, 367 Road crack sealants, 348 Road surfacc, 348 Rocket propellant binders, 676 Roll compounds, carboxylated rubber, 585 Roller covers. 77 1 Rollers, 77 1
919 Rolling resistance, 88, 821 Roof mastics, 676 Roof sheeting, 870 Roofing, 348 Room-temperature vulcanization, 629 Rotational casting, 765 Rotational molding, 746 RPA-2000, 855 RTV, 606,612, 615 Rubher, 346,426 particle, 30, 31, 39, 47, 49, 57, 58 smoked sheet, RSS, 42, 43 specifications, guayule, 12 Rubberhylon, 31 1 Rubbone, 92 Rucothane, 410 Runners and gates, 376 Russian dandelion, 29 SAE J, 200 81 1 Salt bath, 776, 869 SAN, 269, 273, 290, 301, 302 Sandwich, 257 Santoprene, 95 Santoweb, 262 Sarlink, 95 S-B, 326 SBR, 242, 269, 301, 302, 823 S-B-S, 324 Scorch, 170, 182 control, 57 1 safety, 512, 892 Scrap tires, 835, 836 Screens, 772 Screw design, 379 Screw extruder, 437 Screw plasticization, 371 Screw speed, 378 Sealants, 137, 344, 676 Seals, 81 1, 869 O-rings, 585 S-EB-S, 324, 368 Sec, 819 Seeding, 169 Segment compatibility, 341 Scgmental relaxation, 230 Segmented copolymers, 753 Segmcnted poly(ester-ether) copolymers, 358, 360 Self-crosslinking, 789 Self-curing, 295, 298, 300 Semicontinuous mixers, 170 S-EP-S, 324, 327
920 Sequential extraction, 9, I O Sequential polymerization, 325, 326, 327 Series-model, 285 Serum, 39, 47,49, 56 Service temperature, 331, 347 Shaft seals, 676 Shear flow, 245 Shear resistance, 334, 335 Sheet molding compounds (SMC), 346 Shoe, 339 Short fiber, 241 Shrinkage, EVA, 492 Shrink-wrap film, 736 S-I diblock copolymers, 326 SIBR, 823, 824 Silane, 500, 501 Silanol-silylmethoxy condensation, 654 Silica, 537, 61 1, 63 I , 655, 740, 756, 757, 792 Silicone, 138, 507 rubber, 244, 346, 605, 632, 649 sealant (RTV), 646 Silk, 242 Simultancous extraction, 9, 11 Sioplas, 742, 745 S-I-S, 324 Sisal fiber, 243 Skypel, 367 Slurry phase polymerization, 856 Slurry process, 858, 883 Small-angle light scattering (SALS), 358, 362 Small angle neutron scattering, 201, 205 Small angle x-ray scattering, 397 Soap-sulfur, 669 Soft segment, 765 Softeners, 667 Softening point, 347 Solubility, 806 and chemical resistance, 357 parameters, 34I , 342 Solution behaviour, 342 Solution blending, 272 Solution grinding, 837, 839 Solution polymerization, 787, 826 Solvent affinity 551 Solvent resistance, 335, 337, 343 Sound deadening, 339 applications, 337 Spandex, 730 Specific interactions, 266 Specifications, hevea natural rubber, 43, 47 Spherulite models, 362 Spinning machines, 771 Sports track surfacing, 772
Index Spreading, 190 Squeegee blade, 77 1 SSBR, 114 Stability, 371 Stabilizers, 789 Standard Malaysian Rubber (SMR), 41, 42 Star-branched butyl, 880 Steel, 505 Stiffness, 242, 247, 248, 256, 330 modulus, 489 Stoneware pipes, 773 Storage, 668 hardening, 35, 36, 39, 56-58 stability, S7 1 Strain crystallization, 84 Stress relaxation, 242, 244, 401, 402, S78 Stress softening, 333 Stress-strain properties, 152, 267, 400, 422 Structurc EPDM, 865 Itevea natural rubber, 29, 33, 34, 50, 55-59 NBRMNBR, 793,795 Styrene, 7 1 acrylonitrile, 863 butadiene rubbers, 823 Styrenic triblock, 3 12 Sulfenyl chloride, 123 Sulfonated ethylene propylene dicnc tcrpolymer, 435, 470,471 Sulfonation, 437 Sulfur, 901 Superior processing rubber, 61, 74, 75 Surface energy, 623 mismatch, 270 Surface treatment, 839 Surgical placement, 637 Surlyn, 435, 473 Suspension polymerization, 787, 826 Swelling, 242, 247, 248, 496, 5 IO, 5 19, 7 17, 770 Syndiotactic, 704 poly( l-butene), 1I O Synthetic cis-l ,4polyisoprene, 29, 46, 49 T50 test, 176, 177, 193 Table edging, 773 Tack, 121, 122, 217, 218 Tackifiers, 575 resins, 889, 899 Tackiness, 43 TAIC, 553 Talc, 740, 792, 889 Tangent delta, 849, 854, 856
921
Index Tapered diblock copolymers, 321 Tapping, 30, 35, 36, 41, 58 Taraxacum kok-saghyz, 29 Tear, 370, 371 propagation, 770 strength, 576, 819, 824 Tearing, 427 Technically specified rubber. TSR, 41 Teknor apex, 100 Telechelic carboxylated elastomcrs, 438 Telechelic polybutadiene, 435 Telechelic polymers, 133, 153, 154 TEM, 7 15 Tensile propertics, GPO, 687 Tensile strength, 33 I , 628. 770 Tcrethane, 727 Terminally reactive liquid nitriles, 144 Termination rcaction, 878 Tetrabutylphosphonium oxide, 649 Tetrafluoroethylene,547 -propylene rubber, 547 Tetrahydrofuran. 724 Texin, 410, 412 Textile applications, 583 Textile fibers, 242 Textile industry, 77 1 T,, 399 T,, 360, 363, 395 TGA, 795 Thermal analysis, 360 Thermal conductivity, 427 Thermal field flow fractionation, 37, 57, 58 Thermal properties, 467 Thermal stability, 51 1, 526, 536, 541, 556, 625, 759 Thermoforming, 3 14, 433 Thermoplastics, 735 derivatives of NR, 61 elastomer, 1 12, 1 17, 265, 322, 353, 479, 495, 81
epoxidized natural rubber, 101 natural rubber, 94 olefinics, 3 12 1,2-polybutadiene, 479 polyamide, 417 polyurethane, 426 resins, 753 rubbers, 79 vulcunizates, 290. 300, 309, 310, 316 Thermoprene, 66 Thiazoles, 901 Thiokol, 807 Thiurams, 90 I Tire components, 90 1
Tires, 585, 771, 836 Tolylene diisocyanate (TDI), 754 TOR, 702, 705, 706, 708, 709 Torsional analysis, 704 Torsional viscometer, 356 TOSCA, 384 Toys, 345 TPENR, 102,103 TPNR, 9 5 9 7 , 100, 502 TPO, 368 TPU. 367, 368, 387, 389, 390 TPV. 368, 505, 51 1 Trade names, styrcnic block copolymer, 328, 329 Trans- I ,4 polyisoprene, 488 Trans-esterification,5 1 1 Transfer molding, 179, 776 Transmission clectron micrograph, 358 Transmission microscopy, 250 Transmission shaft seals, 645 Transportation, 772 Triads, 1 15 Triallyl cyanurate, 740, 756 Triallyl isocyanurate, 553, 740 Triazine, 543 diones, 121 dithiols, 670 Triblock copolymcrs, 326 Triblock ionomer, 439 Trifluoropropylmethylsiloxanc, 609 Trimethylolpropane trimethacrylate, 740 Trisiloxane, 535 Truck tubeless tires, 894 Tubes, 810 Twin-screw extruders, 272 Two roll mill, 272, 617 UBEPOL VCR, 485 Ucon oils, 742 UCST, 198, 231 UL recognition, 384 Ultraviolet radiation, 404 Upside-down mixing, 169, 252, 257, 262 Urca group, 389 Urcpan, 755,762 Urethane, 389, 754 linkage, 395 US pharmacopoeia class VI, 385 UV degradation, 404 UV resistance, 427 UV stabilizers, 337 VDE0303, 427 Vcnting. 376 cxtruder, 180
922
Index
Vestamid, 417, 420 Vestenamer, 700, 702, 703, 705, 706, 708, 710, 719 Vibrathanes, 727 Vicat softening, 372 Vinyl acetate, 737 Vinyl polybutadienes, 820 Vinyl silanes, 863 Vinylmethylsiloxane, 609 Virtual crosslink, 387, 395, 397, 399 Viscometry, 704 Viscosity, 150, 21 1, 212, 218, 221, 232, 258, 259. 260, 348,426, 509 index improver, 120 Viscous damping, 334 Vitacom DVNR, 100 VLDPE, 75 1 VMQ, 609. 620,630 Volatile fatty acids, (VFA), 46, 48, 49, 58 VTBN, 146 Vulcanizate properties, 20, 556, 687 Vulcanization, 150, 274, 387, 517, 523, 537, 549, 554, 565, 611, 613, 633, 666, 668, 710, 758, 853, 890, 891, 900 rate and state, 16 Vulcanized NBR, 801
Wet grinding, 839 Wet grip, 86, 87, 88 Wet process hydrophobic (WPH) silica, 632 Wettability, 127 Wetting, 270 White sidewalls, 894, 895 Wide angle x-ray diffraction, 358 Wire and cable, 868 Wire insulation, 339
W series, 531 Wallace plasticity, 16 Waterproofing, 348 Waxes 345, 348 Wear 371, 821 characteristics, 755 resistance, 770 Weathering, 370, 801, 845
Z-blade mixers, 9 I , 186 Zeospan, 354 Ziegler-Natta catalysis, 819, 826, 847, 851, 852, 854, 857, 858 Zinc maleated EPDM, 437 Zinc oxide cure, 890 Zinc peroxide, 572 Zwitterionomer, 440
XDI, 392 Xenon NMR, 235 XG, 393 XLPE, 736, 739, 741, 747. 748 applications, 750 XNBR, 575, 580, 789 XP-50, 880 X-ray diffraction, 447 X-ray scattering, 568 XSBR, 566 Yield point, 333, 747 Yield stimulation, 30, 60 Yield strain, 505 Yield stress, 268, 747 Young’s modulus, 247, 508
