Degradation and Stabilisation of Aromatic Polyesters

Degradation and Stabilisation of Aromatic Polyesters Stuart Fairgrieve Smithers Rapra Update Degradation and Stabili...

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Degradation and Stabilisation of Aromatic Polyesters

Stuart Fairgrieve

Smithers Rapra Update

Degradation and Stabilisation of Aromatic Polyesters Stuart Fairgrieve

iSmithers – A Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.rapra.net

First Published in 2009 by

iSmithers Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2009, Smithers Rapra

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder.

A catalogue record for this book is available from the British Library.

Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologise if any have been overlooked.

ISBN: 978-1-84735-457-0 (Hardback) 978-1-84735-458-7 (ebook)

Typeset by Argil Services Printed and bound by Lightning Source Inc.

C

ontents

1

2

3

Background .......................................................................... 1 1.1

Historical Development .............................................. 1

1.2

Structure and Morphology ......................................... 9

1.2.1

Introduction ................................................9

1.2.2

PET ...........................................................10

1.2.3

PTT ...........................................................11

1.2.4

PBT ...........................................................12

1.2.5

PCT ...........................................................13

1.2.6

PEN...........................................................14

1.2.7

LCP ...........................................................15

Thermal Degradation ......................................................... 21 2.1

Poly(ethylene terephthalate) (PET) ............................ 21

2.2

Poly(butylene terephthalate) (PBT) ........................... 35

2.3

Poly(trimethylene terephthalate) (PTT) ..................... 42

2.4

Other Poly(alkylene terephthalate)s .......................... 44

2.5

Poly(alkylene naphthalate)s (PAN) ........................... 47

2.6

Poly(alkylene phthalate)s and Poly(alkylene isophthalate)s ...................................... 48

2.7

Poly(p-phenylene alkanedioate)s ............................... 48

2.8

Highly Aromatic Polyesters ...................................... 49

Thermo-Oxidative Degradation ......................................... 65 3.1

Poly(ethylene terephthalate) (PET) ............................ 65

iii

Degradation and Stabilisation of Aromatic Polyesters

4

3.2

Poly(butylene terephthalate) (PBT) ........................... 73

3.3

Poly(trimethylene terephthalate) (PTT) ..................... 77

3.4

Other Aromatic Polyesters ........................................ 80

Photodegradation and Radiation Degradation .................. 85 4.1

Photodegradation and Oxidation of Poly (ethylene terephthalate) (PET)................................... 85

4.2

Photodegradation and Oxidation of Other Poly(alkylene terephthalate)s .................................... 93

4.3

Photodegradation and Oxidation of Poly(alkylene naphthalate)s ...................................... 94

4.4 5

6

7

iv

4.3.1 Formation of a naphthalic acid end group .95 Radiation Degradation ............................................. 96

Chemical Degradation and Recycling ............................... 107 5.1

Hydrolytic Degradation .......................................... 107

5.2

Ester Interchange .................................................... 109

5.3

Aminolysis .............................................................. 110

5.4

Biodegradation ....................................................... 110

5.5

Chemical Recycling ................................................ 112

Thermal and Hydrolytic Stabilisation ............................... 143 6.1

Introduction ........................................................... 143

6.2

Thermal Stabilisation .............................................. 144

6.3

End-capping ........................................................... 153

6.4

Chain Extension ..................................................... 156

6.4.1

Without Additives ...................................156

6.4.2

Chain Extenders ......................................157

Thermo-oxidative Stabilisation......................................... 181 7.1

Introduction ........................................................... 181

7.2

Studies on Antioxidants .......................................... 183

7.3

Potential New Chemistries ...................................... 187

Contents 7.4 8

Patents .................................................................... 188

Stabilisation Against Ultraviolet and Ionising Radiation... 199 8.1

Introduction to Ultraviolet (UV) Stabilisation ......... 199

8.2

UV Screeners and Absorbers ................................... 200

8.3 8.4

8.5

8.2.1

Background .............................................200

8.2.2

Salicyclates ..............................................202

8.2.3

Benzophenones ........................................203

8.2.4

Benzotriazoles. ........................................204

8.2.5

Cinnamates and Related Types ................206

8.2.6

Oxanilides ...............................................209

8.2.7

Cyclic Imino Esters ..................................209

8.2.8

Triazines ..................................................210

8.2.9 Miscellaneous ..........................................211 Excited State Quenching ......................................... 212 Radical Scavengers ................................................. 213 8.4.1

Background .............................................213

8.4.2

HALS ......................................................214

8.4.3 Other Radical Scavengers ........................219 Ionising Radiation Stabilisation .............................. 219

Appendix – Commercial Additive Structures ............................ 241 Abbreviations ........................................................................... 261 Index ........................................................................................ 263

v

Degradation and Stabilisation of Aromatic Polyesters

vi

P

reface

While over the years there have been many books and review articles published on the subject of polymer degradation and stabilisation, most have been rather wide-ranging studies, covering either polymers in general, or a range of polymers and/or additives. Even in cases where speciﬁc polymers or classes of polymers have been covered, the literature contains few specific reviews of the degradation and stabilisation of condensation polymers such as the aromatic polyesters. There appear to be a number of reasons for this. Firstly, there are the factors of tonnage and range of application. The most used polymers, such as the polyoleﬁns and styrenics, tended to attract the most attention simply as a consequence of their ubiquity. Secondly, there is the factor of rate of deterioration. In general, polyoleﬁns and styrenics degrade noticeably faster than, for example, Nylon 6 or poly(ethylene terephthalate) under equivalent conditions of heat or light exposure, so that the need for effective stablisation against such reactions seemed to be more urgent for the former. A third factor, I would tentatively suggest, was the perception that the degradation and stabilisation of condensation polymers was a study that was somehow more ‘difﬁcult’ due to their very different chemistry compared to that of addition polymers such as the polyoleﬁns. Indeed, in some quarters the view has been expressed that the condensation polymers might not, in fact, require stabilisation, or that the reactions undergone by such polymers during degradation did not lend themselves to being suppressed by known stabilisation pathways.

vii

Degradation and Stabilisation of Aromatic Polyesters As we head towards the second decade of the 21st century, I feel that the points raised in the previous paragraph no longer apply to aromatic polyesters, hence the decision to produce this review of the state of the art of degradation and stabilisation of this class of polymer. In terms of tonnage and use, poly(ethylene terephthalate) is these days virtually a commodity plastic, with widespread, large volume, use in food packaging, beverage bottle and ﬁbres. Other aromatic polyesters have increased in volume production over the last decade or so, such as poly(butylene terephthalate) and poly(ethylene naphthalate). There is also the promise of large quantity use of the most recently commercialised member of the family, poly(trimethylene terephthalate). All such polyesters are also being increasingly pushed into more demanding environments in their applications, including greater heat exposure (for example in hot-ﬁll food packaging applications and ovenable containers) and outdoor applications, where long term stability towards sunlight and oxygen are a prerequisite. The need for a full understanding of the degradation mechanisms, and means of suppressing these, are thus now more required than in the past While some controversies remain, the study of aromatic polyester degradation in the last few years has begun to untangle the mechanisms involved, particularly through the use of analytical techniques previously unavailable. The study of stabilisation has also moved on, with examination of why the established commercial stabilisers (largely developed for use in polyoleﬁns and rubbers) are not as effective in aromatic polyesters, and also attempts to develop new stabilisers speciﬁcally for use in such polymers. I have also included in this book information on the chemical recycling of aromatic polyesters, which may be regarded as controlled degradation of these polymers. With the increasing emphasis on conservation of non-renewable resources, the recovery of various useful chemicals from materials which have reached the end of their

viii

Preface useful lives, or are off-speciﬁcation for their intended use, is a process which needs to be considered. Especially useful is the recovery of starting materials which may then be used to manufacture further polyesters, and the study of the means for carrying out such processes, and their industrial implementation, are important topics in the current ‘green’ climate. Finally, I would like to express my appreciation to the publishing team at iSmithers, Shawbury, for their assistance with the compilation of this book. Especial thanks go to Frances Gardiner for commissioning this work, and to Eleanor Carter for assistance, above and beyond the call of duty, with literature searches and the supply of papers. Dr. Stuart Fairgrieve SPF Polymer Consultants Kidlington July 2009

ix

Degradation and Stabilisation of Aromatic Polyesters

x

1

Background

1.1 Historical Development The condensation reaction between carboxylic acid and alcohol to form an ester and water is a fundamental chemical reaction, and has been known at least from when the term ‘organic chemistry’ was coined in the early nineteenth century. From the mid- to latenineteenth century, studies were carried out on the reactions of compounds containing more than one acid group with compounds containing more than one hydroxyl group. These studies produced the ﬁrst polyester resins, and lead to the development in the early twentieth century of various thermoset polymers such as the alkyd resins. By the late 1920s, researchers had started to try to produce useful thermoplastic materials by the reactions of A-, W-hydroxy acids, or of pairings of diacids with diols. The ﬁrst systematic study was carried out by Wallace H. Carothers, initially using standard distillation equipment to drive the reaction [1], then using a molecular still to further shift the equilibrium of the reaction to synthesise products of sufﬁciently high molecular weight to be fabricated into commercial articles such as ﬁbres and ﬁlms [2]. Other investigators found that the reactions could be enhanced by using catalysts such as salts of alkaline earth or heavy metal ions [3]. These studies appear to have been restricted to reactions between aliphatic diacids and aliphatic diols. The products found were, at this time at least, not commercially viable for several reasons, and Carothers, who was interested in all aspects of condensation polymerisation, quickly discovered more interesting materials: aliphatic polyamides. The Nylons were rapidly

1

Degradation and Stabilisation of Aromatic Polyesters developed based on these studies, and Carothers did not return to polyester study before his death in 1937. Attempts had been made to synthesise polyesters based on phthalic acid as the diacid component, but these products were amorphous, had low softening points, and were rapidly attacked by organic solvents and acids and bases. Research into polyesters made by the reaction of terephthalic acid (or esters thereof) with aliphatic diols, led to the discovery of polyesters of high commercial value: poly(alkylene terephthalate)s [4]. This pairing of diols with terephthalic acid eventually led to the most commercially successful aromatic polyesters, but other synthetic pathways were also investigated towards such products in the early days of polyester development. These included the ‘self-condensation’ of hydroxy acids of the structure -HO-R-Ph-CO2H, where R-OH is para to the acid group and R is -(CH2)- or -(CH2)2- [5], and reactions of aliphatic diacids with 1,4-dihydroxybenzene and similar aromatic diols [6, 7]. Also synthesised about the same time were polyesters based on C2-C6 aliphatic diols and any of the isomeric naphthalene dicarboxylic acids [8]. Research was carried out in commercial laboratories into wholly aromatic polyesters, especially those based on the self-condensation of p-hydroxybenzoic acid, to produce materials of higher potential strength and stability [9–11]. It was found that such homopolymers were extremely refractory materials, and could not be processed by normal techniques used for standard thermoplastics. Further work on aromatic polyesters centred on the condensation of aromatic diacids and dihydric phenols [12], and the condensation of non-fused polynuclear aromatic diacids with non-fused polyaromatic phenols [13, 14] which were expected to be less refractory than the prior art aromatic polyesters. Neither approach provided the desired combination of extreme physical properties with processability. Research centred on copolymers of terephthalic acid, isophthalic acid, bisphenols, and p-hydroxybenzoic acid to provide materials with the

2

Background necessary tractability while retaining improved physical properties and thermal resistance [15–17]. Further variations of this approach elicited several co-polyesters which featured the unusual property of forming anisotropic melts [18, 19], and these led to the development of liquid crystal polyesters (LCP) [20]. Even without considering copolymers, i.e., polymers made by condensing two or more different diacids and/or two or more different diols, it is evident that there are hundreds of possibilities for aromatic polyesters. As is usually the case with potentially large families of polymers, combinations of physical properties, ease of processing, potential applications, and economics limit the number of family members that make it into successful commercial production and use. The ﬁrst material to be widely commercialised (and still the most successful family member) was poly(ethylene terephthalate) (PET). This was initially utilised in the ﬁbres and textiles industries, where it still remains a major player; later it was used for the manufacture of ﬁlms, and as an engineering plastic. Its most recent large-scale use is in the production of vast quantities of soft-drinks bottles and packaging applications for other foods and beverages. By the early 1970s, poly(butylene terephthalate) (PBT) gained commercial acceptance as an engineering plastic. While having no really outstanding properties, its balance of properties makes it a useful alternative to more expensive speciality engineering plastics, especially glass-ﬁlled grades. While poly(ethylene naphthalate) (PEN) was known from early research, it became a commercially viable product only in the late 1980s when full-scale production of the required precursor dimethyl ester of 2,6-naphthalenedicarboxylic acid began. PEN is used in similar applications to PET (i.e., ﬁbre, ﬁlm, packaging) where higher temperature resistance and lower gas permeability are required. LCP caused great excitement when they were initially developed, and great things were expected from them. The initial large number of companies producing this class of polyester rapidly dwindled to

3

Degradation and Stabilisation of Aromatic Polyesters a handful, and LCP remain very much high-cost speciality materials. Although some applications have been found in electronics and electrical connectors, they remain to a large extent ‘a solution looking for a problem’. Poly(propylene terephthalate), more often referred to as poly(trimethylene terephthalate) (PTT), was identiﬁed from research into aromatic polyesters, but was not commercialised at the time due to difﬁculty in obtaining pure, low-cost, 1,3-propanediol. Finally introduced into large-scale production in the late 1990s, there are great hopes for commercial application of this polymer, especially as a ﬁbre. In general, it has properties between those of PET and PBT, but has certain unique properties of its own, including superior resilience and wear properties, giving carpets tufted with such ﬁbres physical properties akin to the nylons, and stain resistance similar to PET. Other aromatic polyesters which have been commercialised include poly(1,4-cyclohexylenedimethylene terephthalate) (PCT; used in the production of circuit board components and automotive applications) and the clear, amorphous, polymer poly(1,4-cyclohexylenedimethylene terephthalate-co-isophthalate). The methods and materials used to synthesise most polymers, and especially condensation polymers such as the polyesters, can exert profound effects on the ﬁnal product, including inﬂuencing polymer stability. It is therefore useful to brieﬂy examine the methods used to manufacture aromatic polyesters and to assess the likely effects on the degradation behaviour of the resultant products. Thermoplastic aromatic polyesters need to be produced to a molecular weight of between 12,000 and 60,000 to be useful. The ﬁrst stage is usually esteriﬁcation (diacid and diol) or ester-exchange (diester and diol) to provide the ﬁrst stage product, e.g., a bis(hydroxyalkyl) terephthalate, and potentially some linear oligomeric species. Water or alcohol (usually methanol) is evolved, and removed by fractional distillation. The decision to use the diacid or dimethyl ester in the manufacturing process is usually a matter of balancing questions

4

Background of economics, purity of feedstock, and ease of handling; in fact for poly(alkylene terephthalates), most methods start with the dimethyl ester of terephthalic acid. The earliest work on polyester synthesis used no catalyst or a simple acid catalyst such as p-toluenesulfonic acid, but use of weakly basic metallic salt catalysts is now almost universal. Many salts have been claimed to be useful in this context, but the best known examples are alkaline earth and transition metal acetates, tin compounds and titanium alkoxides [21–23]. Care must be exercised in selecting ester-interchange catalysts because some may cause degradation/ discoloration in the polymer during the subsequent polymerisation reaction [24], especially for PET and PEN. To prevent this occurrence, catalysts are often sequestered/complexed at the end of the esterinterchange phase by addition of phosphorus compounds such as phosphites, phosphates or polyphosphoric acid [25]. Titanium and tin compounds operate as catalysts for ester-interchange and polymerisation reactions, and in general do not require such procedures. The second (polymerisation) stage is usually achieved in an autoclave, ﬁtted with a stirrer system of suitable power to deal with the viscous melt, under high vacuum and at a temperature above the melting point of the target polymer. During this stage, preventing oxidative degradation of the melt is important, and is achieved by blanketing the vessel with an inert gas. During the polycondenation stage, linear oligomers and the bishydroxyalkyl terephthalate esters undergo a succession of ester-interchange reactions, eliminating the diol which is removed, again under high vacuum, and the molecular weight of the polymer is gradually built up to a suitable level. A catalyst is required for the polymerisation stage of the synthesis: tin and titanium compounds are suitable catalysts for both stages of the reaction but, for the speciﬁc case of PET, antimony trioxide (Sb2O3) is a favoured polymerisation catalyst [26]. This catalyst becomes active only at the higher temperature associated with the polymerisation stage of the reaction, and can be added at the beginning of the

5

Degradation and Stabilisation of Aromatic Polyesters ester-interchange reaction. Residual antimony compounds can lead to discoloration of PET due to the presence of Sb3+ ions, and their potential further reduction to Sb0 metal. PET can become more grey or green as a result of this reduction of Sb3+ to Sb0. The ﬁnely divided elemental form of antimony can cause Rayleigh scattering of incident light, giving a green tinge to the hot polymer, whereas the grey discoloration is associated with the black coloration of elemental Sb. Paradoxically, this effect may be more pronounced if phosphorus-based stabilisers are added to PET [27]. Titanium catalysts are even more of a problem with PET, leading to a distinct yellow discoloration of the polymer [28], possibly due to reactions with unsaturated chain ends (cf the intense yellow colour of titanium complexes with dienes and aromatic systems). Ge catalysts also suffer from this problem in PET, but to a markedly lesser degree. Such discoloration does not appear to be a problem for PBT and PTT. Discoloration of the ﬁnal polymer may also result from the manner in which the starting materials have been made. If transition metal catalysts have been used in the manufacture, for example, of terephthalic acid or its ester, residues remaining may subsequently have an effect on the polymer made using such materials. If a polyester is manufactured in the standard melt-condensation manner, a small amount of cyclic oligomer(s) is formed, which is in equilibrium with the polymer [29–32]. This can be extracted, but this is not an economic process, and in any case the equilibrium will be re-established when the polymer is re-melted for fabrication into ﬁnished products. The presence of such oligomers, usually at about 1.5 wt%, is not a major problem in most cases, but they can exude to the surface of a polyester product. In the case of ﬁbres, with their very high surface-to-volume ratio, this can interfere with dyeing operations, for example. Even for moulded articles the presence of oligomers at the surface can give trouble in operations which require clean surfaces (e.g., electroless metal plating).

6

Background Because of the reversible nature of the polycondensation reaction, and the high temperature used in the manufacture of polyesters, it is sometimes not possible to generate polymers of sufﬁciently high molecular weight directly from melt polycondensation. Attempts to do so result in uneconomically long reaction times and/or thermal degradation of the polyester. This results in adverse effects on the usefulness of the product. To prepare polyesters of very high molecular weight, a process known as solid-state post-condensation (SSP) is used [33, 34]. Dried polymer chips of moderate molecular weight are heated to a temperature approximately 20 °C below the polymer melting point, either in a high vacuum or in a stream of hot inert gas, in a device which agitates the solid. Before carrying out the procedure, it is necessary to crystallise the polymer to the highest possible extent to prevent sintering together of the chips. By careful annealing, an optimum melting point of the polymer is achieved, allowing the SSP process to be carried out at the maximum possible temperature and with a reasonably short reaction time. In the SSP process, the volatile by-products of the continuing polycondensation reaction escape by vapour diffusion through the solid chip and are rapidly removed from the chip surface instead of the much slower process of diffusion through a bulk melt. During the polymerisation reaction, various by-products may be formed. In the case of PET there are two main by-products, both of which can result in problems with the use and stability of the polyester. Diethylene glycol (DEG) units can be generated in the polyester chain by dehydration of 2-hydroxyethyl ester chain ends to form an ether link. This process cannot be entirely prevented, but can be minimised only by careful control of the parameters of the polymerisation process. DEG units depress the melting point of PET. They also have an adverse effect on polymer crystallinity, reducing the strength of ﬁbres and oriented ﬁlms, and increasing the susceptibility of the polyester towards chemical attack and aqueous hydrolysis [35].

7

Degradation and Stabilisation of Aromatic Polyesters The other signiﬁcant by-product generated during PET manufacture is acetaldehyde, which is produced by thermal degradation processes taking place alongside the polymerisation. Random O-CH2 scission of ester units leaves a vinyl ester end and a carboxyl chain end. The vinyl ester reacts with a polymer end group to form a new polymer link and expels acetaldehyde, the tautomer of the actual leaving group, vinyl alcohol [24]. Although this by-product is highly volatile, its presence even at very low levels (approximately 3 ppm) is sufﬁcient to cause tainting of beverages and foodstuffs packaged in PET. PET bottles are therefore SSP-treated immediately before stretch blow-moulding to keep this contamination to an absolute minimum. Other reactions associated with acetaldehyde formation lead to further problems in the thermal degradation processes of PET (see Section 2.1). PTT acts in a similar manner to PET, forming bis-3-hydroxypropyl ether groups in the polymer chain, and volatile by-products. The volatile product in this case is allyl alcohol, and the more problematical acrolein [36]. In the case of PBT, at least in the ﬁrst stage of thermal degradation during polymerisation, the principal volatile material is tetrahydrofuran, formed by cyclisation of 1,4-butanediol or by internal cyclisation of C4 ester units [37]. This by-product is largely harmless because it is non-reactive under polymerisation conditions and, being highly volatile, is quickly removed. Polyesters with higher aromatic content such as LCP are made via an alternative route. Because they are phenolic esters, they cannot be made by direct ester exchange between a diphenol and a lower dialkyl ester due to unfavourable reactivities. The usual method is reverse ester exchange or acidolysis reaction [38] where the phenolic hydroxyl groups are acylated with a lower aliphatic acid anhydride, and this ester is heated with an aromatic dicarboxylic acid, with or without catalyst. Many of these polymers are derived from hydroxyacids, and their acetates readily undergo self-condensation in the melt, stoichiometric balance being inherent to the reaction [39, 40].

8

Background In some cases it is possible to directly polycondense acid and phenols evolving water at about 300 °C [41]. This process works well when catalysed with compounds of group IV or V metals; tin salts are preferred, especially dialkyl tin dialkanoates or oxides. The process is not really suitable if p-hydroxybenzoic acid is involved because this can undergo decarboxylation at >200 °C. The polymerisation is not affected because the phenol formed volatilises at the high temperatures involved, but the resultant polyester will contain less than the anticipated mole ratio of hydroxybenzoate-derived units. Hydroxynaphthoic acid does not suffer from this problem, and direct esteriﬁcation processes may be used if non-benzoate copolymers are required [42]. The use of essentially all-aromatic starting materials, and the volatilisation of potential organic contaminants at the high polymerisation temperatures encountered in such processes, means that few problems are anticipated with instability caused in this manner, with the possible exception of metal catalyst residues. Studies on the effect of polymerisation-induced foreign species on the utility and/or degradation of such high-aromatic (co)polyesters seem to be lacking.

1.2 Structure and Morphology 1.2.1 Introduction Most aromatic polyesters are semi-crystalline materials; they exhibit crystalline and amorphous regions within one article. The processing of the polyester will have a profound effect on the relative ratios of the two morphologies. There is also the possibility, especially in highly oriented specimens such as biaxially oriented ﬁlms and ﬁbres, that a third morphology may emerge: ‘oriented amorphous’. Polymer morphology will have a direct effect on the degradation and on the potential for stabilisation of the substrate. In most cases,

9

Degradation and Stabilisation of Aromatic Polyesters crystalline regions are denser than the amorphous regions, i.e., the diffusion of impurities, radicals and oxygen will be much faster in amorphous zones than through crystalline regions. This will result in degradation being conﬁned to amorphous regions, at least in the initial stages of degradation or oxidation. It is also known that stabilisers, initially dispersed throughout the polymer melt during processing, will be largely excluded from the growing crystals as the melt cools, and will be further redistributed during later processes such as ﬁbre orientation. This process will, as might be surmised, work in favour of stabilisation of the host polymer because the additives are concentrated in the most vulnerable regions. There is a caveat to this: the concentration of a stabiliser in the amorphous regions of a highly crystallised polyester could theoretically result in saturation and subsequent loss of additive. Though it is outside the scope of this review to fully discuss the morphological complexities of aromatic polyesters, data are provided on the differences in crystallinity and structure of several important aromatic polyesters.

1.2.2 PET The crystal structure of PET is triclinic [43], i.e., none of the cell angles is a right angle, and there is only one repeat unit per unit cell. The cell parameters are: a = 0.456 nm, b = 0.594 nm, c = 1.075 nm, A = 98.5°, B = 118°, and G = 112°. The chain is fully extended (i.e., comparison of c with length calculated from standard bond lengths and angles for the extended trans conformer shows that the unit length is approximately 99% of cell length). The direction of p-disubstitution on the benzene ring makes an angle of approximately 19° with the c-axis, giving a very shallow ‘sawtooth’ appearance. Packing is good, with the benzene rings on adjacent chains virtually eclipsing each other. In general, projections on one molecule ﬁt the hollows on an adjacent one. Early work on morphological changes on tensile testing of PET

10

Background ﬁbres [44] indicated that crystalline strain was small compared with macroscopic overall strain. The conclusion was that crystalline regions possessed a very much higher stiffness than macroscopic stiffness, and that under stress most of the strain could be attributed to distortion of non-crystalline regions.

1.2.3 PTT Two studies [45, 46] on PTT structure were published almost simultaneously. Both determined that the unit cell was triclinic, with the parameters shown in Table 1.1.

Table 1.1 Crystalline structure of poly(trimethylene terephthalate) – Unit cell parameters [45]

[46]

a (Å)

0.464

0.46

b (Å)

0.627

0.62

c (Å)

1.864

1.83

A (°)

98.4

98

B (°)

93.0

90

G (°)

111.1

112

Both sets of authors also concluded that each unit cell contained two repeat units. Comparison of the unit length of the gauche-gauche conformer noted from the diffraction data showed that this is only 76% extended, considerably less than for PET Both sets of authors conﬁrmed that both methylene bonds in the diolderived segment of the chain are in gauche conformation, making the -O-(CH2)3-O- sequence into a helical, shortened conﬁguration. This results in a virtually Z-shaped overall conformation in the a–c plane of the unit cell, making for a considerably deeper sawtooth appearance. In the case of good packing, this provides for the possibility of much

11

Degradation and Stabilisation of Aromatic Polyesters greater steric hindrance to lateral movement of adjacent molecules than would be possible in PET. The crystal lattice of PTT is, as expected from these data, unusually compliant. Recent successful growth of large crystals of PTT [47] has allowed a more precise set of parameters to be obtained. This has conﬁrmed the triclinic nature of the unit cell, which was ambiguous in the light of the value of 90° previously given for B [46]. The new values are: a = 0.453 nm, b = 0.620 nm, c = 1.870 nm, A = 97.6°, B = 93.2°, and G = 110.1°

1.2.4 PBT As with the other polyesters in this series, the unit cell of PBT is triclinic [48], with the parameters: a = 0.48 nm, b = 0.59 nm, c = 1.16 nm, A = 98°, B = 116°, and G = 110°. The unit cell contains one repeat unit. Data show that the methylene section of the chain has the conformation gauche-trans-gauche. Comparison of overall length of this conﬁguration to the unit cell shows that the chain is not fully extended, although not to the same degree as in PTT, being 86% of unit cell length. The chain appears to be folded or ‘crumpled’, rather than the helical conﬁguration noted for PTT. The conﬁguration described above can change, reversibly, to transtrans-trans under low strain. This conformational change also produces, as might be expected, a change in unit cell parameters to: a = 0.47 nm, b = 0.58 nm, c = 1.3 nm, A = 102°, B = 121°, and G = 105°. The angle between the plane of the p-disubstituted phenylene group and the alkylene chain is similar to that of PET, but the sawtooth formed is deeper than PET due to the greater length of the methylene chain. It is nowhere near the deep zigzag of PTT as shown in a–c plane models. Because the conformation angles in the strained form are close to those of PET, it might be expected that this would be the stable form.

12

Background The fact that it is not suggests that each conformation represents an energy minimum, with the relaxed form being the lowest, with the energy barrier between them being sufﬁciently low for thermal activation to transform most material to the unstrained form at lower temperatures. Tension reverses the relative heights of these energy levels. The volume of the unit cell of the shortened, relaxed, form is smaller than that of the other, indicating a more economically packed, lower energy, form.

1.2.5 PCT First manufactured in 1959 [49], PCT was initially developed as a ﬁbre, particularly for carpets, but has more recently been proposed as a ﬁlm-forming or moulding plastic. Replacement of a straight chain diol with cyclohexanedimethanol produces a polymer with a regular structure but a stiffer chain than PET. Because cyclohexanedimethanol can exist as all-trans, all-cis, or mixtures of the two isomers, this can affect the structure and stability of the molecule. For example, the all-trans isomer has a melting point of 315–320 °C, whereas all-cis melts at 260–267 °C. Using mixtures of isomers, a range of melting points can be obtained. The polymer is generally noted as having a distinctly improved thermal and UV stability over PET, which means that it is much more suited to outdoor applications. Investigations of the crystal structure of the ‘homopolymers’ [50, 51] showed that both were triclinic, and exhibited the parameters shown in Table 1.2.

Table 1.2 Crystalline structure of of poly(cyclohexanedimethylene terephthalate) - Unit cell parameters a (Å)

b (Å)

c (Å)

A (°)

B (°)

G (°)

Trans

6.37

6.63

14.2

89

47

114

Trans

6.46

6.65

14.2

89

47

115

Cis

6.02

6.01

13.7

89

53

112

13

Degradation and Stabilisation of Aromatic Polyesters Various copolymers based on this chemistry have also been commercialised, including poly(ethylene-co-1,4-cyclohexylenedimethylene terephthalate) and poly(1,4-cyclohexylenedimethylene terephthalateco-isophthalate). Both of these types are largely amorphous, clear, polymers.

1.2.6 PEN Although known since the 1940s, PEN attained commercial signiﬁcance only in the late 1980s when large-scale production of the precursor dimethyl-2,6-naphthalene dicarboxylate was achieved. This polyester exhibits higher temperature resistance, UV resistance and tensile strength than PET, and is also a superior barrier to water and oxygen. Structurally, the main difference between PEN and PET is replacement of the phenyl ring with the naphthyl unit. This leads to a stiffer chain, so that the melting point and the glass transition temperature are higher in PEN. Although PEN crystallises at a slower rate than PET, crystallisation is similarly enhanced by orientation, and the barrier properties are much superior to PET. This latter factor has meant that PEN has been proposed as a useful packaging material, especially in hot-ﬁll applications. PEN also ﬁnds major applications in ﬁbres [52]. Various copolymers, such as poly(ethylene terephthalate-co-naphthalate) have been produced, as well as other homopolymers in the naphthalate series, especially poly(butylene naphthalate). PEN appears to exist in two crystalline modiﬁcations, both triclinic [53, 54], with the parameters shown in Table 1.3.

Table 1.3 Unit Cell Parameters of Poly(ethylene naphthalate) a

b

c

A o

Alpha form

6.51 Å

5.75 Å

13.2 Å

81

Beta form

9.26 Å

15.6 Å

12.7 Å

122o

14

B 144 96o

G o

100o 123o

Background Analysis of the crystal structure of PEN shows that the chains are arranged in such a way that naphthalene rings in one chain are very closely adjacent to -CH2-CH2- units of a neighbouring chain, much closer than in PET. It is therefore conceivable that some additional interaction could take place between these atoms in crystalline regions of this polyester.

1.2.7 LCP Certain aromatic polyesters, containing stiff segments in the chain, exhibit ordered phases which are similar to those seen in smallmolecule liquid crystalline, or mesophasic, substances. In particular, the rod-like molecules are oriented in shear to such an extent that interchain entanglement is small, and the melts consequently exhibit a lower than expected viscosity. On cooling of the melt, the molecules remain oriented, effectively self-reinforcing the polymer in the direction of ﬂow. In general, LCP are synthesised from monomers that are long, ﬂat and rigid along their major axis. Common examples of such mesogens are terephthalic acid, p-hydroxybenzoic acid, hydroquinone, 2-hydroxy-6-napththoic acid, and 4,4´-bisphenols. The ratios of these components may be varied to ‘ﬁne tune’ the properties of the product. With only such aromatic mesogens in place, the polyesters thus formed are highly insoluble, have high melting points, and exhibit a wide temperature range of mesogenic behaviour. The order exhibited by the LCP may vary (in descending degree of ordering) through crystalline, smectic, nematic and isotropic phases. Smectic LCP exhibit what may be referred to as ‘two-dimensional’ ordering, where the polymer chains are largely parallel to each other, creating further order or layering of said mesogens orthogonal to the polymer chain main axis. In nematic ordering, the chains are largely parallel, but the mesogens no longer exhibit the additional ordering associated with the smectic phase. Note that not all LCP exhibit smectic and nematic behaviour.

15

Degradation and Stabilisation of Aromatic Polyesters Wholly aromatic polyesters, while exhibiting some mesophasic behaviour, can be extremely difﬁcult to process by standard methods, and various means for alleviating such concerns have been used: a) Incorporation of ﬂexible spacer units. b) Copolymerisation of several mesogenic monomers of different sizes to give a random or more irregular structure. c) Introduction of units with lateral substituents to disrupt chain symmetry. d) Synthesis of chains with built-in kinks through the use of unsymmetrically linked aromatic units (‘crankshaft’ approach). The homopolymer of p-hydroxybenzoic acid is a largely crystalline polymer, with a very high degree of order. The polymer chains have been shown [61] to form a double helix where the two chains are in a reversed head-to-tail order. The unit cell dimensions are a = 17.8 and c = 18.4, where c is the chain axis, and where the unit cell contains three repeat units.

References 1.

W.H. Carothers, inventor; DuPont de Nemours & Co., assignee; US 2012267, 1935.

2.

W.H. Carothers, inventor; DuPont de Nemours & Co., assignee; US 2071250, 1937.

3.

C.S. Fuller, inventor; Bell Telephone Laboratories Inc., assignee; US 2249950, 1941.

4.

J.R. Whinﬁeld and J.T. Dickson, inventors; DuPont de Nemours & Co., assignee; US 2465319, 1949.

5.

J.G. Cook, J.T. Dickson and A.R. Lowe, inventors; Imperial Chemical Industries Ltd., assignee; US 2471023, 1949.

16

Background 6.

F.C. Wagner, inventor; DuPont de Nemours & Co., assignee; US 2035578, 1936.

7.

J.G.N. Drewitt and J. Lincoln, inventors; Celanese Corporation, assignee; US 2595343, 1952.

8.

J.G. Cook and H.P.W. Huggill, inventors; Imperial Chemical Industries Ltd., assignee; GB 604073, 1948.

9.

J.R. Caldwell, inventor; Eastman Kodak Co., assignee; US 2600376, 1952.

10. D. Aelony and M.M. Renfrew, inventors; General Mills Inc., assignee; US 2728747, 1955. 11. W.K.T. Gleim, inventor; Universal Oil Products Co., assignee; US 3039994, 1962. 12. S.W. Kantor and F.F. Holub, inventors; General Electric, assignee; US 3160602, 1964. 13. A.J. Conix and U.L. Laridon, inventors; Gevaert PhotoProducten NV, asignee; US 3028364, 1962. 14. A.J. Conix, inventor; Gevaert Photo-Producten NV, assignee; US 3317464, 1967. 15. M.H. Keck, inventor; Goodyear Tire & Rubber Co., assignee; US 3133898, 1964. 16. S.G. Cottis and J. Economy, inventors; no assignee; US 3637595, 1972. 17. H. Inata and K. Shoji, inventors; Teijin Ltd., assignee; US 4064108, 1977. 18. T.G. Pletcher, inventor; DuPont de Nemour & Co., assignee; US 3991013, 1976. 19. J.R. Schaefgen, inventor; DuPont de Nemours & Co., assignee; US 4118372, 1978. 17

Degradation and Stabilisation of Aromatic Polyesters 20. W.J. Jackson, G.G. Gebeau and H.F. Kuhfuss, inventors; Eastman Kodak, assignee; US 4153779, 1979. 21. R.E. Wilfong, Journal of Polymer Science, 1961, 54, 385. 22. Inventor unknown; Imperial Chemical Industries Ltd., assignee; FR 1169659, 1959. 23. W.K. Easley, J.K. Lawson and J.B. Ballentine, inventors; Chemstrand Corporation, assignee; CA 573301, 1959. 24. H. Zimmermann and N.T. Kim, Polymer Engineering and Science, 1980, 20, 10, 680. 25. J.L. Adams, inventor; Eastman Kodak, assignee; US 4501878, 1985. 26. C.M. Fontana, Journal of Polymer Science: Polymer Chemistry Edition, 1968, 6, 8, 2343. 27. S.M. Aharoni, Polymer Engineering and Science, 1998, 38, 7, 1039. 28. J.G. Smith, C.J. Kibler and B.J. Sublett, Journal of Polymer Science: Polymer Chemistry Edition, 1966, 4, 7, 1851. 29. L.H. Peebles, M.W. Huffmann and C.T. Ablett, Journal of Polymer Science: Polymer Chemistry Edition, 1966, 7, 2, 479. 30. I. Luderwald, H. Urrutia, H. Herlinger and P. Hirt, Die Angewandte Makromolekulare Chemie, 1976, 50, 163. 31. G.C. East and A.M. Girshab, Polymer, 1982, 23, 3, 323. 32. H. Zeitler, Melliand Textilberichte, 1985, 2, 132. 33. B. Huang and J.J. Walsh, Polymer, 1998, 39, 26, 6991. 34. Y. Ma, U.S. Agarwal, D.J. Sikkema and P.J. Lemstru, Polymer, 2003, 44, 15, 4085. 18

Background 35. W. McMahon, H.A. Birdsall, G.R. Johnson and C.T. Camilli, Journal of Chemical and Engineering Data, 1959, 4, 1, 57. 36. H.L. Traub, P. Hirt, H. Herlinger and W. Oppermann, Die Angewandte Makromolekulare Chemie, 1995, 230, 179. 37. R.M. Lum, Journal of Polymer Science: Polymer Chemistry Edition, 1979, 17, 1, 203. 38. T. Takekoshi, inventor; General Electric, assignee; US 3549593, 1970. 39. G.W. Calundann, inventor; Celanese Corporation, assignee; US 4161470, 1979. 40. A.J. East and G.W. Calundann, inventors; Celanese Corporation, assignee; US 4431770, 1984. 41. A.J. East, inventor; Celanese Corporation, assignee; US 4393191, 1983. 42. A.J. East, inventor; Celanese Corporation, assignee; US 4421908, 1983. 43. D.I. Bower, An Introduction to Polymer Physics, Cambridge University Press, Cambridge, UK, 2002, p.111. 44. W.J. Dumage and L.E. Contois, Journal of Polymer Science, 1958, 28, 275. 45. S. Poulin-Dandurand, S. Perez, J-F. Revol and F. Brisse, Polymer, 20, 4, 419. 46. I.J. Desborough, I.H. Hall and J.Z. Neisser, Polymer, 1979, 20, 5, 545. 47. R.M. Ho, K.Z. Ke and M. Chen, Macromolecules, 2000, 33, 20 7529. 48. I.H. Hall and M.G. Pass, Polymer, 1976, 17, 9, 807.

19

Degradation and Stabilisation of Aromatic Polyesters 49. C.J. Kibler and J.G. Smith, inventors; Eastman Kodak Co., assignee; US 2901466, 1959. 50. C.A. Boye, Journal of Polymer Science, 1961, 55, 1, 275. 51. B. Remillard and F. Brisse, Polymer, 1982, 23, 13, 1960. 52. P. Chen and R. Kotek, Polymer Reviews, 2008, 48, 2, 392. 53. I. Ouchi, M. Hosei and S. Shimotsuma, Journal of Applied Polymer Science, 1977, 21, 3445. 54. G. Wu, Q. Li and J.A. Cuculo, Polymer, 2000, 41, 22, 8139. 55. S.K. Varshney, Journal of Macromolecular Science Macromolecular Reviews, 1986, C26, 4, 551. 56. M. Cox, Liquid Crystal Polymers, Review Report No.4, Rapra Technology, Shrewsbury, UK, 1987. 57. W.J. Jackson, Molecular Crystals and Liquid Crystals, 1989, 169, 1, 23. 58. D. Coates, Liquid Crystal Polymers - Synthesis, Properties and Applications, Review Report No.118, Rapra Technology, Shrewsbury, UK, 2000. 59. A.M. Donald, A.H. Windle and S. Hanna, Liquid Crystalline Polymers, 2nd Edition, Cambridge University Press, Cambridge, UK, 2006. 60. J.M.G. Cowie and V. Arrighi, Polymers: Chemistry and Physics of Modern Material, 3rd Edition, CRC Press, Boca Raton, FL, USA, 2008. 61. J. Economy, R.S. Storm, V.I. Matkovic, S.G. Cottis and B.E. Nowak, Journal of Polymer Science: Polymer Chemistry Edition, 1976, 14, 9, 2207.

20

2

Thermal Degradation

2.1 Poly(ethylene terephthalate) (PET) As the ﬁrst aromatic polyester of high commercial practicality, PET has been studied extensively. Results of the earliest studies, from 1950 until the late 1960s [1–5], provided (on paper at least) a viable explanation of the features of PET thermal degradation under oxygenfree conditions. In ‘pure’ PET, three general structures are present: A

~Ph-(C=O)-O-CH2-CH2-O-(C=O)-Ph~

‘in chain’

B

~Ph-(C=O)-O-CH2-CH2-OH

‘hydroxyl end’

C

~Ph-(C=O)-OH

‘carboxyl end’

From the high polymeric nature of PET, and the known synthetic route for its manufacture (formation of bis(hydroxyethyl terephthalate) followed by condensation of same), it can be deduced that in the as-produced polymer by far the predominant structure will be A. B will be the next highest, although much less than A; and C will constitute only a very low initial level. PET stability towards all forms of degradation is very much dependent on their being a low content of the carboxyl end-group. From these early studies, it was considered that thermal degradation of PET does not involve a radical (homolytic) pathway, at least at the temperatures generally encountered by this polymer. The initial

21

Degradation and Stabilisation of Aromatic Polyesters reaction undergone by the chain was said to be scission into an acid chain end and an unsaturated chain end, the process taking place via a six-membered ring intermediate in which a hydrogen from a carbon B to ester group is transferred to the ester carbonyl. In the solid state such reactions may or may not proceed but, depending on the conformation of the affected segment of the polyester chain, it is assumed that a PET melt exhibits no order and that the necessary positioning of the atoms in the chain will occur through random motions of a freely rotating polymer chain. Reactions may thus be posited to occur as follows: A

‡

~Ph-(C=O)-OH

+

CH2=CH-O-(C=O)-Ph~

B

‡

~Ph-(C=O)-OH

+

CH2=CH-OH

C

‡

no products

It is immediately obvious from the above speculations that degradation via this route will result in a rapid increase in the number of acid end groups present. For unit B, the vinyl alcohol produced is unstable, and most probably will immediately rearrange to acetaldehyde, i.e., CH2=CH-OH

‡

CH3(C=O)-H

This situation may be exacerbated by a possible reaction of the unsaturated chain end produced from A, via an intermediate, to form yet another acid chain end, and acetylene. While the above reactions are taking place, there will be competition in the melt between chain scission and chain building, and esterexchange reactions will also occur [6]. In the case of the new chain ends created in the above reaction schemes, further chain end reactions could occur, e.g., an unsaturated end group acting as ‘acid’ and a hydroxyl end as ‘alcohol’. This

22

Thermal Degradation would result in formation of a new polymer chain, and expulsion of another molecule of vinyl alcohol, which would again rearrange to form acetaldehyde. Similarly, hydrolysis, by any water present, of the unsaturated end would result in formation of an acid chain end and acetaldehyde. Taking all these reactions into consideration, at least as a ﬁrst approximation of degradation, i.e., a combination of chain scission, splitting off of small molecules from such chain ends, hydrolysis and (trans)esteriﬁcation, it may be seen that most reactions taking place result in the creation of acid end groups; also that acetaldehyde is produced as the main small molecule product. The fact that acetaldehyde is so volatile (boiling point (bp) = 21 °C) means that it is easily lost from the bulk of the polymer. If, as is sometimes claimed, acetaldehyde is soluble in the PET melt, an acetalisation reaction may take place between this aldehyde and ethanediol to form 2-methyl1,3-dioxalane and water, or possibly even a chain extending reaction between the aldehyde and two adjacent hydroxyl chain ends. Due to the rapid rearrangement of vinyl alcohol to acetaldehyde, the reactions involving loss of this molecule are essentially irreversible because this rearrangement means, for example, that there is no hydroxyl group available to reform the unsaturated chain end. It has been noted that initial work on the thermal degradation of PET suggested that homolytic (i.e., free radical-based) scission did not have a role, but this was not conﬁrmed and, in any case, certain conditions may exist where such reactions might come into play. At very high temperatures (or under certain conditions in processing machinery where formation of mechanoradicals may occur), homolytic scission of the polyester chain may occur [7–10]. The weakest link in the PET chain would appear to be the sequence carbonyl–oxygen–methylene, which may be expected to homolytically cleave in two possible ways: 1

~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ ~Ph(C=O)O. + .CH2CH2O(C=O)Ph~

23

Degradation and Stabilisation of Aromatic Polyesters 2

~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ ~Ph(C.=O) + .OCH2CH2O(C=O)Ph~

Even in a polymer melt, the most likely next step to the scissions described above will be recombination of the radicals, in effect an immediate reversal of the reaction. Only in the case where a small, highly mobile, radical species such as oxygen is present is recombination likely to be anything but the predominant reaction. Similar types of scission might also occur in the hydroxyl end groups. Considering the myriad potential reactions which might cascade down from these simple homolytic bond scissions, it can be shown that various small molecule products are possible, as well as grafting, chain extension, and crosslinking reactions. Such small molecule species might include CO, CO2, ethylene, ethanol, water, acetaldehyde and ethylene glycol. So far only ‘pure’ PET has been discussed. Ether linkages also appear in PET, brought about by esteriﬁcation reactions involving diethylene glycol, which is formed from ethanediol via a dehydrative coupling reaction. Any diethylene glycol present will preferentially react into the polymer because it has the same reactivity as ethanediol but lower volatility. Such moieties cannot be avoided, and appear to be at an equilibrium level of about 1–3% in as-produced PET. While attempts may be made to remove such species, this is a futile exercise because they quickly return to the equilibrium level on further processing of the polymer due to dehydrative coupling between hydroxyl chain ends. Such units may react via the molecular or homolytic pathways noted above. In the former, this will theoretically give various unsaturated ether and etheralcohol species, which may then go on to give polymeric and/or coloured species. In the case of homolytic scission, virtually the same reaction sequences will take place as for normal PET in-chain structures [10].

24

Thermal Degradation As indicated earlier, the other likely contaminants present in assynthesised PET are metallic catalyst residues. In the case of anaerobic thermal degradation, it is posited [11] that the metal will complex with one carbonyl in the in-chain structure, while the other carbonyl is involved in the six-membered intermediate to chain scission. Because there is only one oxygen atom between this complexed carbonyl and the methylene group from which hydrogen is leaving, the resultant shift in electron densities will favour chain scission. If there is another methylene group intruding between the oxygen and the active methylene, such a catalytic effect is much less likely. Two further points may be made: (i) antimony residues did not appear to catalyse chain scission, whereas titanium residues did (as might be expected); and (ii) while conﬁrming the effects of transition metal ions on the thermal degradation of PET, studies of model compounds did not show the same potential effect in poly(butylene terephthalate) (PBT). Buxbaum [5] proposed the following scheme for the main reactions involved in the thermal degradation of PET: ~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ ~Ph(C=O)OCH=CH2 + HO(C=O)Ph~ ~Ph(C=O)OCH=CH2 ‡ Radical grafting, crosslinking ~Ph(C=O)OCH=CH2 ‡ Vinyl polymer ‡ ~Ph(C=O)OH + Polyene 2 ~Ph(C=O)OH j ~Ph(C=O)O(C=O)Ph~ + CH3(C=O)H CH3(C=O)H ‡ Volatilises or Polyenealdehyde

25

Degradation and Stabilisation of Aromatic Polyesters Gaseous by-products are said to consist of CO, CO2, H2O, CH3(C=O) H, C2H4, 2-methyldioxalane, CH4, and C6H6, with acetaldehyde by far the major product. Cyclic oligomers, especially the trimer, are also formed, although at this stage no concrete evidence of how they were formed was available. Very low concentrations of other non-gaseous products were also detected, including substituted benzoic acids, and biphenyl mono- and di-carboxylic acids. These were present at a total of <0.1 wt% of all by-products. An attempt was not made to fully identify the coloured species forming during thermal degradation, and the author suggested that polyenes and polyenealdehydes could contribute. During the 1970s and 1980s, further attempts were made by various researchers to elucidate the thermal degradation mechanism(s) of PET [12–31]. Their observations largely supported the general premise of the earlier work. Foti and co-workers [26] added one interesting observation. In a comparative study of poly(alkylene phthalate)s and poly(alkylene terephthalate)s, the primary fragmentation products of the former were cyclic oligomers formed by selective ester exchange. The direct pyrolysis mass spectrometry technique used to study the reactions did not provide sufﬁcient clarity of data to deﬁnitely say whether the same was the case for PET and the other terephthalates, but the authors were convinced this was the case, and that the B hydrogen transfer-mediated chain scission processes, along with decarboxylation and hydrolytic ester cleavage, constituted secondary thermal fragmentation of PET. The kinetics of acetaldehyde formation from PET (an important consideration due to the tainting this can produce in foodstuffs and beverages) had been assumed to follow ﬁrst-order kinetics, i.e., to be related to the concentration of in-chain ester linkages in the polymer. Halek [29] noted that a better ﬁt of data in the region of 225–300 °C was obtained if zero-order kinetics were used. This ﬁnding indicates that some other factor is involved in controlling

26

Thermal Degradation the rate of the acetaldehyde-producing reaction. It was postulated that this is due to the difﬁculty of forming the cyclic transition state in the polymer because of the slow chain movements at or near the melting point of the polymer (most studies which suggested ﬁrst-order kinetics were carried out on model compounds, which do not suffer from this restriction). It was also noted that there is a sharp discontinuity at the melting point; while above and below this temperature a zero-order plot gives the best ﬁt to the data, the activation energies and Arrhenius parameters are markedly different. It is speculated that the acetaldehyde is mainly derived from reactions on unsaturated ends, or between such species and acid ends which would form anhydrides. The appearance of such end groups in the starting polymer may result from ‘damage’ during manufacture and prior processing of the polymer. Such kinetics would not ﬁt with a homolytic reaction. Further studies have been made on how acetaldehyde is generated during thermal degradation of PET [32–37]. Villain and co-workers [33] studied this problem with a particular regard to understanding the effect of drying conditions and processing temperature on this phenomenon. Analysis of the evolution of acetaldehyde versus time at various temperatures provided very interesting results. At 260–280 °C, the amount evolved accelerates initially, but reaches a plateau level after a few minutes; whereas at 290–300 °C, the amount evolved increases exponentially. While the authors do not comment on this, it is clear that at the lower temperature the ‘availability’ of acetaldehyde is limited in some way. The most likely explanation is that acetaldehyde comes from reaction of the hydroxyl chain ends under these conditions, and that new ends are not being created. At higher temperatures, chain scission now comes into play, along with the associated reactions of the fragment chain ends, and the amount of acetaldehyde potentially available is essentially limited only by the amount of PET in the sample. Study of PET samples of differing molecular weight demonstrated that the amount of acetaldehyde evolved depends on molecular weight, providing further evidence to support this hypothesis.

27

Degradation and Stabilisation of Aromatic Polyesters Shukla and co-workers [37] took a practical approach to the problem, examining the effect of various processing parameters on the evolution of acetaldehyde during injection moulding of PET. They found that, within the temperature range 280–300 °C, an increase of 10 °C doubled the amount of acetaldehyde evolved. In 1991, McNeill and Bounekhel [38] studied the thermal degradation of PET using thermal volatilisation analysis (TVA), and subsequently made rather controversial assertions based on the results. Ramped heating of PET to 500 °C produced the following product fractions: a) Non-condensable gases – Carbon monoxide, and traces of methane. The latter is probably due to methyl ester chain ends because the PET sample was synthesised using the dimethyl ester of terephthalic acid. b) Condensable gases and volatile liquids – Three separate fractions could be elucidated: (i) carbon dioxide, with traces of ethylene and ketene; (ii) acetaldehyde; and (iii) vinyl benzoate and dioxane, with traces of benzaldehyde, toluene and divinyl terephthalate. c) Cold-ring fractions (i.e., materials condensing in the apparatus just above the heating chamber which did not make it as far as the cold traps) – Two separate bands formed; the upper band showed the presence of terephthalic acid plus short-chain fragments terminated by carboxyl, unsaturated or aldehyde groups; the lower band consisting of starting polymer plus some longer-chain moieties with anhydride linkages therein. Isothermal studies were also undertaken at various temperatures. At 365, 385 and 405 °C, acetaldehyde was observed to be the main component of the degradation. CO2, CO, and methane were formed, along with the other volatile components; the amounts, though small, increasing with run temperature. For the cold ring fractions, only cyclic trimer was noted at 305 °C, but in the higher-temperature runs a range of products was obtained, similar to those observed for

28

Thermal Degradation non-isothermal runs. Carboxl structures appeared at r365 °C, but anhydrides appeared only at the two higher temperatures. In the case of the minor product dioxane, the authors suggest this to be the product of the dimerisation of vinyl alcohol. The most controversial conclusion of these authors, based on the constant appearance of CO2 and CO at all test temperatures, is that the mechanism of degradation is via a homolytic route. They admit that it is virtually impossible to distinguish the homolytic and concerted reaction schemes in terms of many of the products formed. Although not cited by the authors, there may be further evidence for at least some radical reactions taking place through short-chain species with aldehydic end groups and benzaldehyde. A possible cause of the appearance of such species could be hydrogen abstraction by a carbonyl radical formed from homolytic scission of the polymer chain between the carboxyl carbon and the ester oxygen. The trace product ketene may also be present as a result of homolytic scission. The proposed reaction scheme was as follows: ~OCH2CH2O(C=O)Ph(C=O)OCH2CH2O(C=O)Ph(C=O)~ ‡ ~OCH2CH2O(C=O)Ph(C=O)OH {A} + CH2=CHO(C=O)Ph(C=O)~ {B} {A} ‡ {B} + HO(C=O)Ph(C=O)OH {A} ‡ ~(C=O)Ph(C=O)OCH2CH2OH {C} + H(C=O)Ph(C=O)OH {B} ‡ ~OCH2CH2O(C=O)Ph(C=O)H + CH2=CHOH CH2=CHOH ‡ acetaldehyde + 1,4-dioxane {C} ‡ ~(C=O)Ph(C=O)H + HOCH2CH2OH 29

Degradation and Stabilisation of Aromatic Polyesters Also suggested were the following homolytic reactions to produce the observed carbon oxides: ~OCH2CH2O(C=O)Ph(C=O)OCH2CH2O(C=O)Ph(C=O)~ ‡ ~OCH2CH2O(C=O)Ph {D} + {B} + CO2 and ~OCH2CH2O(C=O)Ph(C=O)OCH2CH2O(C=O)Ph(C=O)~ ‡ {D} + {C} + CO {D} ‡ {A} + CH2=CHO(C=O)Ph There has been a great deal of effort expended in investigating the thermal stability of PET in several situations, including tyre yarns [39], magnetic tapes [40], ﬁlms [41], and in the form of ultra-high molecular weight polymer [42]. Also investigated have been the problems of melt reprocessing of PET [43–46] and the deliberate anaerobic pyrolysis of PET to produce other low molecular weight species [47, 48]. In parallel with these more speciﬁc investigations, work has continued to further elucidate the thermal degradation mechanism(s) of PET [49–63]. Popoola [49] deduced from a study of the evolution of dioxane during thermal degradation of PET occurring in the polymer production process that this was due to diethylene glycol units. Andrassy and Mencer [52] studied the changes taking place in molecular-weight distribution during processing, and noted a general narrowing of the same. The importance of this phenomenon in terms of the applicability of PET to various applications was discussed.

30

Thermal Degradation Montaudo and co-workers [53] studied PET thermal degradation by direct pyrolysis mass spectrometry using negative chemical ionisation. Mass spectra were dominated by a series of ions which could be cyclic, or open-chain, and oligomers with unsaturated and carboxyl end groups. Comparison with specially prepared samples of cyclic and open-chain oligomers led the authors to conclude that the species found were cyclic, and that such species are the true primary pyrolysis products, formed via intramolecular exchange (ionic) reactions. These cyclic species are then proposed to further decompose by a B-hydrogen transfer, involving a six-membered intermediate transition state, to generate the open chain species with unsaturated and carboxyl end groups. It is speciﬁcally stated that the species cited by McNeill and Bounekhel [38] were secondary and tertiary breakdown products. This conclusion also suggests that the primary breakdown products cited by Buxbaum [5] and Zimmermann [28] are secondary breakdown products from the thermal degradation of PET. While the intramolecular hydrogen transfer taking place as part of the chain scission process in condensation polymers such as PET is usually referred to as occurring via a six-membered intermediate, Lehrle and Pattenden [54] suggested that this may not be the whole story. Combining theoretical studies with observations of breakdown products, these authors suggest that, while six-membered intermediates are at least one order of magnitude more likely than any other size, others are energetically possible. Of these others, only ﬁve and seven have any signiﬁcant possibility, and the likelihood ratio is 5:1 in favour of the ﬁve-membered transition state. Further studies on the generation of acetaldehyde were carried out by Khemani [55]. Using gas chromatography at constant elevated temperature for extended time periods, he found that acetaldehyde production decreased with time, eventually reaching an asymptotic value. It was proposed that acetaldehyde is generated in three ways: hydroxyl ends, vinyl ends, and during chain scission. The ﬁrst two deplete with time, leaving the last source, which is virtually inexhaustible.

31

Degradation and Stabilisation of Aromatic Polyesters Botelho and co-workers [56] studied the thermal breakdown of ethylene dibenzoate under nitrogen as a model compound for PET. From the evolved products, these authors suggested that the degradation process takes place through hydrogen transfer reactions and homolytic reactions: Ph(C=O)OCH2CH2O(C=O)Ph ‡ cyclic intermediate ‡ Ph(C=O)OH + CH2=CHO(C=O)Ph and Ph(C=O)OCH2CH2O(C=O)Ph ‡ Ph(C=O)O. (I) + .CH2CH2O(C=O)Ph (II) (I) ‡ hydrogen abstraction ‡ Ph(C=O)OH (II) ‡ dimerisation ‡ Ph(C=O)O(CH2)4O(C=O)Ph The authors tentatively suggest that this provides evidence that thermal degradation of PET may itself involve both pathways. Holland and Hay [57] studied the thermal degradation of PET, along with copolymers thereof containing isophthalate groups and/ or diethylene glycol linkages. It was found that diethylene glycol and isophthalate units promote thermal degradation through increased ﬂexibility and more favourable bond angles, respectively. All samples investigated gave a ﬁnal non-volatile residue, which was shown by infrared spectroscopy to consist almost exclusively of interconnected aromatic rings. In terms of speciﬁc reactions earlier in the reaction sequence, it was suggested that ethylene glycol end groups degrade via a ﬁve-membered cyclic intermediate to form an acid end group and vinyl alcohol; the latter rapidly rearranging into acetaldehyde. In the case of a diethylene glycol end group, it is suggested that 1,4-dioxane splits off via what might be best described as a ‘psuedo

32

Thermal Degradation six-membered’ ring transition state, leaving yet another acid chain end. The highly aromatic ‘ash’ left at the end of the degradation was said to form through reaction of benzyne moieties occurring by chain scission of the polyester chain at the bond between the aromatic ring and the carbonyl group. Samperi and co-workers [60] examined the thermal degradation behaviour of PET under nitrogen within the temperature range used in commercial processing of this polymer (270–370 °C) using a combination of matrix-assisted laser desorption ionisation-time of ﬂight (MALDI-TOF) mass spectrometry and nuclear magnetic resonance techniques. From the data obtained it appears that the degradation proceeds through the formation of cyclic oligomers which themselves decompose at higher temperature. Vinyl ester-terminated oligomers could not be detected in MALDI spectra, whereas anhydride-containing oligomers were. The authors were unable, from the data obtained, to state categorically that the anhydride-containing oligomers originated by unimolecular expulsion of acetaldehyde from the PET chain, or by in-situ reaction of vinyl ester-terminated oligomers with carboxyl ended oligomers. The former was proposed as being the more likely reaction. The non-appearance of vinyl ester end groups may be a consequence of the conditions and technique used. To summarise, from the data available, it is not clear whether the thermal degradation of PET proceeds exclusively by heterolytic or homolytic processes, or whether the two processes are both present, perhaps varying in importance with test conditions (e.g., with temperature). The primary reaction appears to be formation of cyclic oligomers via a long-range back-biting process to produce much more mobile species which then follow the general pathway initially proposed by Buxbaum [5]. From this scheme there will be a selection of end groups, initially present in the polymer and created by the B-hydrogen transfer reactions posited. The main species which lead to further reaction and creation of new by-products are as follows:

33

Degradation and Stabilisation of Aromatic Polyesters I)

Hydroxyl end {~Ph(C=O)OCH2CH2OH}

I

‡

~Ph(C=O)OH + [CH2CH2O]

I + ~Ph(C=O)OH

‡

~Ph(C=O)OCH2CH2O(C=O)Ph~ + H2O

I + ~Ph(C=O)O(C=O)Ph~ ‡

~Ph(C=O)OCH2CH2O(C=O)Ph~ + ~Ph(C=O)OH

I + ~Ph(C=O)OCH=CH2 ‡

~Ph(C=O)OCH2CH2O(C=O)Ph~ + CH3(C=O)H

The ‘caged’ moiety [CH2CH2O] may then rearrange to acetalehyde or possibly react with another hydroxyl end to form an ethylene glycol end: I + [CH2CH2O]

‡ ~Ph(C=O)OCH2CH2OCH2CH2OH

The ethylene glycol end may then back-bite to form an acid chain end and eject 1,4-dioxane. II)

Unsaturated end {~Ph(C=O)OCH=CH2}

II

‡

~Ph(C=O)CH2(C=O)H

II

‡

~Ph(C=O)OH + CHyCH

II

‡

~PhCH=CH2 + CO2

II + II ‡ II

34

‡

~Ph(C=O)OCH=CH-CH=CH2 + ~Ph(C=O)H ~Ph(C=O)H + CH2CHOH

Thermal Degradation II + ~Ph(C=O)OH ‡

~Ph(C=O)O(C=O)Ph~ + CH2=CHOH

II + ~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡

~Ph(C=O)OCH=CHO(C=O)Ph~ + ~Ph(C=O)OH + CH2=CH2

With long reaction times and/or high temperatures, chain scissions will lead to the formation of smaller molecules while, conversely, branching and crosslinking reactions will occur, leading to the formation of highly aromatic residues.

2.2 Poly(butylene terephthalate) (PBT) The non-oxidative thermal degradation of PBT has been studied by several researchers, although the literature is not as extensive as that for PET [14, 16, 26, 38, 51, 53, 56, 59, 64–71]. The general structures in PBT are broadly similar to those in PET and have been discussed earlier, with the exception of four methylene groups in the aliphatic portion of the polyester chain. Assuming, as is plausible, that the degradation route undergone by PET will apply to PBT, the chain scission via B-hydrogen transfer through a six-membered cyclic intermediate will proceed as: ~Ph(C=O)OCH2CH2CH2CH2O(C=O)Ph~ ‡ ~Ph(C=O)OH + CH2=CHCH2CH2O(C=O)Ph~ Via any of a number of possible reactions, the unsaturated chain end may now eliminate 3-buten-1-ol (bp = 114 °C). In this situation, a very different scenario would result than in the case of PET, i.e., the alcohol thus-formed is highly unlikely to rearrange (cf vinyl alcohol rearranging to acetaldehyde in PET) and is available for reaction with 35

Degradation and Stabilisation of Aromatic Polyesters the same, or another, acid end group. An alternative reaction would be a further scission of this chain end via the cyclic intermediate, which would appear to be a more favourable reaction in the case of PBT, to produce an acid chain end and 1,3-butadiene (bp = –5 °C): ~Ph(C=O)OCH2CH2CH=CH2 ‡ ~Ph(C=O)OH + CH2=CH-CH=CH2 For a hydroxyl end group, several reactions are possible: 1. Ester exchange or hydrolysis to produce an alternative ester, or acid, and 1,4-butanediol. The diol may then cyclise by dehydration to form tetrahydrofuran (THF). 2. A direct cyclisation reaction of the end group to produce an acid chain end and THF. 3. Reaction via a six-membered cyclic intermediate to give an acid end group and butenol. This alcohol may then react with an acid chain end to form an unsaturated chain end, which then may react further. As to potential impurities involved in PBT, ether groups are a possibility, at least via hydroxyl end dehydration in the melt. The presence of such groups in the as-made polymer, from 1,4-butanediol, is unlikely, due to the greater likelihood of THF formation than formation of the etherdiol. In the case of metal impurities, the discussion of this aspect in the previous section would seem to suggest that, at least by complexation between metal ions and carbonyl groups, there is unlikely to be a deleterious effect of catalyst residues on PBT thermal stability. The fact that use of titanium catalysts for both parts of PBT synthesis is well known and practised, while use of titanium catalyst in PET manufacture results in severe yellowing, would seem to support this hypothesis. An early study of PBT thermal degradation was carried out by researchers at the University of Bologna [64, 67]. They initially 36

Thermal Degradation conﬁrmed that the degradation mechanism proceeds via the sixmembered ring intermediate and that butadiene is the major volatile product. The initial scission to provide the acid and unsaturated chain ends was the rate-determining step, and the scission of butadiene took place at a noticeably higher rate than the initial chain scission. The reason given for this is that there is a favourable resonance between the cyclic intermediate and the contiguous double bond. The authors also make the illuminating statement that ‘The chain ﬁssions of PBT are followed by an evolution of butadiene, while carboxyl end groups increase and unsaturated end groups disappear. This justiﬁes the absence of those secondary reactions which take place in PET, where unsaturated compounds are more stable and acetaldehyde more soluble in the polymer bulk’. The authors also suggest that THF formation (apart from that formed by cyclisation of 1,4-butanediol) is most probably via a cyclic intermediate from hydroxyl chain ends. Using laser microprobe techniques and mass spectrometry, Lum [66] investigated PBT degradation under vacuum. A complex, multistage, decomposition was observed, which included two major reaction pathways. Initial degradation was said to occur via the familiar cyclic transition state to provide acid chain ends and unsaturated ends. The subsequent reactions of the unsaturated chain ends were said to proceed in two ways. Initially, it was proposed that degradation occurred via an ionic process to produce THF: ~Ph(C=O)OCH2CH2CH=CH2 + H+ ‡ ~Ph(C=O)OH+CH2CH2CH=CH2 ~Ph(C=O)OH+CH2CH2CH=CH2 ‡ ~Ph(C+=O) + THF It was also noted that water is present as a degradation product, so this may also be involved in THF evolution:

37

Degradation and Stabilisation of Aromatic Polyesters ~Ph(C=O)OH+CH2CH2CH=CH2 + H2O ‡ ~Ph(C=O)OH2+ + THF ~Ph(C=O)OH2+ - H+ ‡ ~Ph(C=O)OH This is said to be followed by ‘concerted ester pyrolysis’ to produce butadiene and an acid chain end: ~Ph(C=O)OCH2CH2CH=CH2 ‡ ~Ph(C=O)OH + CH2=CH-CH=CH2 Simultaneous decarboxylation reactions were also posited in both mechanisms. Later stages of the decomposition are characterised by evolution of CO and aromatic species such as toluene, benzoic acid and terephthalic acid. It is also suggested that, unlike PET, there are no primary fragments from PBT-containing carbonyl groups, thus no acetalisation reactions are expected. Pyrolysis–gas chromatography studies of various poly(alkylene terephthalate)s (PAT) [16] appear to show very similar fragmentation products occur in all the materials studied (polyesters with 2–6 methylene groups). Various fragments with acid and/or unsaturated chain ends were identiﬁed, and decarboxylation reactions also conﬁrmed. From PBT, the following species were identiﬁed: HO(C=O)Ph(C=O)OCH2CH2CH=CH2

1

CH2=CHCH2CH2O(C=O)Ph(C=O)OCH2CH2CH=CH2

2

HO(C=O)Ph(C=O)OCH2CH2CH2CH2O(C=O)Ph(C=O)OH

3

HO(C=O)Ph(C=O)OH

4

38

Thermal Degradation These may then, if feasible, decarboxylate: 1 ‡ Ph(C=O)OCH2CH2CH=CH2 2 ‡ None 3 ‡ Ph(C=O)OCH2CH2CH2CH2O(C=O)Ph 4 ‡ benzene + biphenyl As with PET, the studies of Foti and co-workers [26] have been interpreted as showing that the primary thermal degradation process, preceding the formation of acid and unsaturated chain ends via scission, leads to the formation of cyclic oligomers. The TVA studies of McNeill and Bounekhel [38] included PBT. Volatile products found in the experiments include: a) Non-condensable gases

CO

b) Condensable gases

CO2*, butadiene*

c) Volatile liquids

THF*, toluene, dihydrofuran, 4-vinylcyclohexane*, 1,4-butanediol*, benzaldehyde

(*denotes an important product) Two cold ring fractions were obtained. The upper fraction contained benzoic acid, terephthaldehydic acid, terephthalic acid*, mono-3butenyl terephthalate*, butylene terephthalate cyclic dimer, and HO(C=O)Ph(C=O)O(CH2)4O(C=O)Ph(C=O)OH*. The lower cold ring fraction consisted of short-chain fragments containing anhydride groups. Under isothermal conditions, 1,3-butadiene and CO2 began to be formed at 305 °C, and signiﬁcant amounts were evolved at higher

39

Degradation and Stabilisation of Aromatic Polyesters temperatures. THF was not produced until 340 °C (cf above comments regarding THF evolution versus butadiene evolution). With regard to the cold ring fraction, PBT is said to produce few cyclic products under isothermal conditions, but produces the same fragments as noted in the non-isothermal study at higher temperatures. As noted in the discussion on TVA analysis of PET, the authors believe that thermal degradation can be adequately explained by a free-radical (homolytic) breakdown mechanism. Montaudo and co-workers [53] provided evidence that PBT and PET form cyclic oligomers as their primary decomposition process via an ionic intramolecular exchange. This is followed by a B-C-H hydrogen transfer to generate a series of open-chain oligomers. This transfer is believed to occur through a six-membered cyclic transition state. The authors note that, while addition of acid to the PBT was found to accelerate degradation, it did not change the reactions or products. As such an addition might be expected to change the nature of the reaction if the same was homolytic in the absence of acid catalyst, they cite this as evidence against the proposals of McNeill and Bounekhel. Botelho and co-workers [56], using butylene dibenzoate as a model compound, proposed the following reaction scheme for thermal degradation under at atmosphere of nitrogen: Ph(C=O)O(CH2)4O(C=O)Ph ‡ intermediate ‡ Ph(C=O)OH + CH2=CHCH2CH2O(C=O)Ph (X) (X) ‡ intermediate ‡ CH2=CH-CH=CH2 + Ph(C=O)OH (X) + Ph(C=O)OH ‡ Ph(C=O)O(CH2)2O(C=O)Ph + CH2=CH2 Samperi and co-workers [70] studied the isothermal degradation of PBT at a series of temperatures coinciding with the standard range of processing temperature for the polyester. Cyclic oligomers were present in the starting polymer and the level of these increased on 40

Thermal Degradation heating at 270 °C and 280 °C. On heating at r290 °C, the level of these species rapidly declined. MALDI spectra indicated cyclic oligomers ranging from trimer up to undecamer. At higher temperatures, cyclic oligomers were replaced with linear oligomers containing two carboxyl chain ends, and new peaks began to appear, indicating the presence of oligomers with unsaturated chain ends. The major difference between PBT and PET thermal degradation appeared to be the absence of anhydride-containing oligomers. The B C-H hydrogen transfer is noted to be very efﬁcient in PBT, resulting in formation of unsaturated chain ends at temperatures around the processing temperature of the polyester. The unsaturated oligomers can readily undergo another B C-H transfer process to yield butadiene. This process competes with the formation of anhydride-containing oligomers. Formation of anhydride-containing oligomers and concommitant appearance of THF was observed at approximately 400 °C. The authors also note that the vinyl group formed as the chain end in PET is electronically conjugated to the adjacent ester group, and a second hydrogen transfer to yield acetylene is unlikely. This, coupled with the ease of expulsion of acetaldehyde, leads to the greater importance of anhydride-containing breakdown products in PET. In summary, it would appear that the basic thermal breakdown process of PBT proceeds as follows: ~(C=O)Ph(C=O)O(CH2)4O(C=O)Ph(C=O)O~ ﬂ‡ cyclic oligomers ~(C=O)Ph(C=O)O(CH2)4O(C=O)Ph(C=O)O~ ‡ ~(C=O)Ph(C=O)OH + CH2=CHCH2CH2O(C=O)Ph(C=O)O~ ~O(C=O)Ph(C=O)OCH2CH2CH=CH2 ‡ ~O(C=O)Ph(C=O)OH + CH2=CHCH=CH2 41

Degradation and Stabilisation of Aromatic Polyesters

2.3 Poly(trimethylene terephthalate) (PTT) Based on the studies of the thermal degradation of PET and PBT mentioned above, it is possible to speculate as to the likely breakdown pathways in PTT. Chain scission via the familiar six-membered ring intermediate would give an acid end group and an unsaturated end group. The rate of the reaction is unlikely to be affected by catalyst residues for the same reason given in the case of PBT. As before, there are two main possible subsequent steps: (i) a further scission via a six-membered intermediate to give an unsaturated low molecular weight species, or (ii) creation of a low molecular weight hydroxyl species via ester exchange or hydrolysis: ~Ph(C=O)OCH2CH=CH2 ‡ ~Ph(C=O)OH + CH2=C=CH2 ~Ph(C=O)OCH2CH=CH2 ‡ ~Ph(C=O)OH + CH2=CHCH2OH The expected species are therefore allene and allyl alcohol. Allene (1,2-propadiene) is an unstable species but, with a bp of –34 °C, might be expected to be rapidly lost from the bulk polymer. It is also readily isomerised to propyne (methylacetylene), which also has a very low bp (–23 °C) and in the presence of water can form CH3C(OH)=CH2 which itself may rapidly rearrange to acetone. In the presence of oxygen and a catalyst such as titanium, allene could fragment to produce one molecule of CO2 and two molecules of formaldehyde. As to the likelihood of allene being produced during thermal breakdown of PTT, this depends on the energetics of removal of a last hydrogen from the B carbon of the unsaturated end group. Allyl alcohol has a bp of 97 °C, and is thus considerably less volatile than any of the above. In the presence of acid catalyst, allyl alcohol can isomerise to propionaldehyde, even though even this acid-catalysed reaction is much slower than the isomerisation of vinyl alcohol to acetaldehyde. There is thus a distict posibility that allyl alcohol can react with any available acid end group to reform the unsaturated

42

Thermal Degradation chain end. It has also been known for such reactions to result in the formation of acrolein and hydrogen. In the presence of oxygen and/ or peroxide, allyl alcohol can expoxidise to form glycerol, or may be oxidised to acrolein or acrylic acid. For hydroxyl end groups, the inference from studies of PBT is that there may again be two possible outcomes, resulting in the creation of allyl alcohol or oxetane (trimethylene oxide). Also, due to the potential presence of carbonylcontaining species (in particular acrolein and propionaldehyde), there exists the possibility of acetalisation reactions similar to those conjectured in PET. If such species were to be formed, their unsaturated nature could result in the creation of polymeric species which might lead to discolouration of the polyester. Early pyrolysis–mass spectroscopy [14] and pyrolysis–gas chromatography [16] studies compared the breakdown of PET, PTT and PBT and, while providing no clear-cut reaction pathways, showed that all three polyesters appeared to thermally degrade via the same mechanism. As alluded to earlier, PTT was initially synthesised at the same time as PET, but was not commercially developed at the time because of the high cost and low purity of the 1,3-propanediol then available. With the discovery of better, more economic ways of manufacturing this starting material, further work on the degradation mechanism proceeded in the 1990s. Academic studies at the Univerity of Stuttgart [72, 73], while not speciﬁcally aimed at studying PTT degradation, provided the ﬁrst clues towards possible mechanisms and by-products. During the polycondensation reactions carried out, allyl alcohol and acrolein featured as volatile by-products. The formation of allyl alcohol was explained to be via the same transesteriﬁcation route as proposed above. Researchers found that allyl alcohol was produced in amounts of approximately 1–2 g per kg of 1,3-propanediol used. They also proposed that the acrolein observed was not simply an impurity in the supplied diol, but was formed by dehydrogenation of allyl alcohol (although no mechnism was suggested). It was also noted that the

43

Degradation and Stabilisation of Aromatic Polyesters laboratory-produced samples contained <0.1% ether linkages, and it was suggested a signiﬁcant effect on physical or chemical properties could not be expected at such low contamination levels. At about the same time as the above, workers at Degussa AG and Zimmer AG were investigating large-scale production of PTT [74]. They noted that small amounts of acrolein and allyl alcohol were produced during polymerisation (approximately 0.2–0.3% of each) and stated that the mechanism of formation of allyl alcohol was identical to the formation of vinyl alcohol from PET. It was also stated that ‘A convincing reaction mechanism for the generation of acrolein has not yet been established.’ The authors noted the level of cyclic oligomers in as-synthesised PTT was between 1.6% and 3.2% (cf PET approximately 1.7%; PBT approximately 1.0%). With the establishment of PTT as a viable commercial polyester, more studies have been carried out on the thermal decomposition of this material [75–82], although a great deal of investigation is required to push the understanding of the processes and products to the same level as for PET or even PBT.

2.4 Other Poly(alkylene terephthalate)s On paper at least, the simplest of the PAT is poly(methylene terephthalate): ~(C=O)Ph(C=O)OCH2O~ There is no such compound as methanediol, so the polymer cannot be made by the usual reaction between diacid and diol, and is instead synthesised from an alkali metal salt of terephthalic acid and dibromomethane. Trials with this polymer [83] showed that it rapidly thermally degraded at the melt processing temperature. It was suggested that this initially involved loss of formaldehyde and formation of anhydride structures, swiftly followed by crosslinking and rapid discoloration.

44

Thermal Degradation Few studies have been made of the thermal degradation of straight-chain PAT with more than four methylene units in the aliphatic chain section. The pyrolysis–gas chromatography studies of Sugimura and Tsuge [16] included poly(pentamethylene terephthalate) and poly(hexamethylene terephthalate) as well as PET and PBT. Results indicated that they all thermally degraded via the same basic mechanism, involving chain scission to acid and unsaturated chain ends by means of six-membered ring intermediates. Poly(decamethylene terephthalate) was included in the TVA studies undertaken by McNeill and Bounekhel [38], and this again showed this polyester to have similar breakdown behaviour to PET and PBT. The major volatile by-product was 1,9-decadiene (as might be expected), along with minor amounts of 1,10-decanediol and 1-decen-10-ol. The major upper cold ring fraction products were terephthalic acid and mono-9-decenyl terephthalate, whereas the lower cold ring fraction consisted of short-chain fragments containing some anhydride groups. As noted in the discussion on PET, these authors suggest a homolytic breakdown mechanism due to the early detection of CO and CO2. The other products are exactly those one would expect from the ring-intermediate chain scission. A few studies have been made on the thermal degradation of poly(1,4cyclohexylenedimethylene terephthalate) [84–86], although these have little to say concerning the likely breakdown mechanism. Assuming the same process is applied here as in straight-chain poly(alkylene terephthalate)s, the process might be deduced as follows [NB: (C6H10) denotes the 1,4-cyclohexyl moiety in the polymer backbone]: ~Ph(C=O)OCH2(C6H10)CH2O(C=O)Ph~ ‡ ~Ph(C=O)OH + CH2=(C6H10)O(C=O)Ph~ (Y) (Y) B scission ‡ ~Ph(C=O)OH + CH2=(C6H10)=CH2 (Y) ester-exchange or hydrolysis ‡ ~Ph(C=O)OH + CH2=(C6H10)CH2OH

45

Degradation and Stabilisation of Aromatic Polyesters Also, an hydroxyl end may also undergo either of these two types of reaction: ~Ph(C=O)OCH2(C6H10)CH2OH ‡ ~Ph(C=O)OH + CH2=(C6H10)CH2OH ~Ph(C=O)OCH2(C6H10)CH2OH ‡ ~Ph(C=O)OH + HOCH2(C6H10)CH2OH It is thus anticipated that the small-molecule breakdown products will be 1,4-bis(methylene)cyclohexane and 4-methylenecyclohexanemethanol. Such by-products have been found in the pyrolysis of the copolymer poly(ethylene-co-1,4-cyclohexylenedimethylene terephthalate) [87]. Although the potential number of poly(alkylene terephthalate) copolymers is vast, only a few appear to have merited interest due to their thermal degradation properties. These include: poly(ethylene-co-trimethylene terephthalate) [88, 89] poly(ethylene-co-butyelene terephthalate) [90, 91] poly(ethylene-co-hexylene terephthalate) [92] poly(ethylene-co-octamethylene terephthalate) [93] poly(ethylene-co-1,4-cyclohexylenedimethylene terephthalate) [87] poly(butylene terephthalate-co-isophthalate) [94] poly(butylene terephthalate-co-adipate) [95] poly(butylene terephthalate-co-succinate) [96] As might be expected, the use of two different alkylene diols in a copolymer largely results in degradation mechanisms which include

46

Thermal Degradation features from both homopolymers. Where things might change is in the possible effects of interactions between the two unsaturated chain ends formed, and potential reactions between said end groups and polymer chain units not available in their respective homopolymers. In the particular instance of poly(ethylene-co-butylene terephthalate) [90], the main volatile products were butadiene, THF, acetaldehyde and ethylene, and the ratio of acetaldehyde to ethylene increased markedly with increase in ethanediol content in the copolymer. It has been suggested [59] that ethylene is produced in PET thermal degradation by an intermolecular reaction between unsaturated end groups and the -CH2-CH2- chain unit; it may be that the greater ﬂexibility and availability of such units in the ‘PBT’ part of the copolymer would result in higher evolution levels of ethylene, and this could account for this observation. In all cases of ethylene–higher alkylene copolyesters, the overall thermal stbility of the copolyester is reduced. Pyrolysis of poly(ethylene-co-1,4-cyclohexylenedimethylene terephthalate) [87] provides the expected breakdown products: acetaldehyde, 1,4-bis(methylene)cyclohexane and 4-methylenecyclohexanemethanol. At the high temperatures used in this study (425–650 °C) other by-products were also observed, including CO2, benzoic acid, ethyl benzoate, benzene, biphenyl and 4-methylbenzaldehyde. The thermal degradation behaviour of copolymers containing terephthalaic acid along with other acids, while dependent to some extent on the type of co-acid, is largely controlled by diol content, following the now familiar pathway for polyester chain scission [94–96].

2.5 Poly(alkylene naphthalate)s (PAN) Speciﬁc studies of the anaerobic thermal degradation of poly(alkylene naphthalate)s have not been done, but it is most likely that these polymers will degrade via similar pathways to their equivalent 47

Degradation and Stabilisation of Aromatic Polyesters poly(alkylene terephthalate). Both series decrease in stability with increasing number of methylene groups in the starting diol, and the naphthalates are more stable than their equivalent terephthalates [97]. The methylene chains are said to be equally ﬂexible in both series, but the two differ in geometric conﬁguration, which will have some effect on rates of degradation [98] but not on mechanism or outcome.

2.6 Poly(alkylene phthalate)s and Poly(alkylene isophthalate)s Both these types of polyester would appear to thermally degrade in the same general manner as their analagous terephthalates. The main difference is noted with poly(alkylene phthalate)s where the later, low molecular weight, degradation products include large amounts of anhydrides and other cyclic species derived from the 1,2 - position of the carboxyl groups on the benzene ring [99]. Poly(ethylene isophthalate) is observed [57] to be less stable than PET, possibly due to the easier back-biting reactions with the 1,3 - conﬁgured aromatic ring structure.

2.7 Poly(p-phenylene alkanedioate)s Almost a ‘mirror image’ of poly(alkylene terephthalate)s (PAT), these polymers are essentially polyesters formed from aliphatic diacids and hydroquinone (1,4-dihydroxybenzene), giving the general structure: ~O-Ph-O-(C=O)(CH2)x(C=O)~ The B hydrogen atoms necessary for the chain scission via cyclic intermediate much cited for the preceeding classes of polyester are absent. It is therefore thought that thermal degradation will occur selectively by ester exchange to produce high yields of cyclic

48

Thermal Degradation oligomers [26]. This is analagous to probable reactions in wholly aromatic polyesters (see Section 2.8). Poly(p-phenylene adipate) produces dimer, trimer and tetramer, whereas poly(p-phenylene succinate) produces dimer and trimer. Further fragmentation leads to the creation of ketene, hydroxyl and unsaturated chain ends, although the precise reaction pathway is not clear from the limited information available.

2.8 Highly Aromatic Polyesters Early studies of the thermal degradation of so-called ‘polyarylates’ were covered by Neiman [100] and Ehlers and co-workers [101]. Since then, several highly aromatic and speciﬁcally liquid crystalline (mesogenic) polyesters have been examined in terms of their anaerobic thermal degradation characteristics. These include homopolymers of hydroxybenzoic acids [102–105]; copolymers of hydroxybenzoic acid with hydroxynaphthoic acid [105–108]; polymers which are essentially copolymers of hydroxybenzoic acid and alkyene terephthalates [107–118]; copolymers of hydroxybenzoic acid with other aromatic polyesters [119–122]; phenolic and bisphenolic terephthalates [123–127]; poly(oxynaphthoate)s [128]; and liquid crystal polyesters (LCP) containing unsaturated acids as part of a copolyester chain [129–131]. Stucturally, the simplest of the wholly aromatic polyesters are those based on hydroxybenzoic acids, with poly(p-oxybenzoate) being the most commonly encountered: ~(C=O)PhO~ Initial studies of poly(p-oxybenzoate) by Jellinek and Fujiwara [102] were carried out under a vacuum at 505–565 °C. The main degradation products were CO, CO2, phenol, and a higher molecular weight fragment tentatively identiﬁed as p-hydroxyphenylbenzoate. In studies on poly(m-oxybenzoate), Foti and co-workers [103] noted that the primary degradation products were cyclic oligomers. It 49

Degradation and Stabilisation of Aromatic Polyesters may be that the same is the case with the para-polymer, but that the conformation of this species is such that the formed cyclic oligomers are of greater molecular weight, and cannot be isolated before they undergo secondary scission reactions. The authors proposed the following provisional outline of the breakdown process of poly(poxybenzoate): ~Ph(C=O)OPh(C=O)OPh(C=O)OPh(C=O)OPh(C=O)O~ ‡ ~Ph + CO + HOPh(C=O)OPh + CO2 + Ph(C=O)H + PhOH + H(C=O)O~ All the above products result from the scission of the various backbone bonds followed by hydrogen abtraction. Due to the lack of aliphatic bonds and the high temperatures involved in the breakdown processes, it is probable that the mechanism is via homolytic bond scission. Hummel and co-workers [105], using pyrolysis with a combination of mass spectrometry and Fourier transform infrared (FT-IR) spectroscopy identiﬁed many degradation products. CO2 and CO showed release maxima at 520 ° and 550 °C, respectively, whereas the following materials exhibited evolution maxima as noted: 487 °C

‘monomer’ (oxybenzoyl), oligomers

542 °C

phenol

572 °C

diphenyl ether

The oligomers featured phenoxy and/or benzoyl end groups. The remaining residue appeared to include diarylketone, 9-ﬂuorenone, and phenolic structures. Taking into account minor products, overall more than two dozen compounds were provisionally identiﬁed.

50

Thermal Degradation Investigations of the thermal degradation of copolymers of p-hydroxybenzoic acid with 2,6-hydroxynaphthoic acid [105–108], and of the naphthoic homopolymer [128] showed similar results, indicating complex random chain scission processes. LCP consisting essentially of copolymers of p-hydroxybenzoic acid and PET are commercial materials, and were studied by Saikrasun and Wongkalasin [118]. As might be expected, the degradation products found were a mixture of those that might be forecast from the homopolymers alone, i.e., acetaldehyde, ethylene, CO and phenol; these also being accompanied by complex random scission products resulting from high-temperature degradation of the two types of segment and interreactions between the same. Similar results were obtained in studies of poly(p-oxybenzoate-cobiphenylene terephthalate) [119] and poly(p-oxybenzoate-co-ethylene naphthalate) [122]. Moving away from (co)polymers based around hydroxybenzoic acids, which feature essentially ‘head-to-tail’ structures, some studies have been carried out on terephthalate polyesters synthesised using diols based on aromatic species. LCP based on poly(alkyl-4,4´-diphenoxy terephthalate)s were investigated by Campoy and co-workers [124]: ~OPhO(CH2)xOPhO(C=O)Ph(C=O)~ These are polymerised from terephthalic acid and the appropriate 4,4´-dihydroxy-A, W-diphenoxyalkanes. The authors synthesised various polymers, with x = 5, 6, 7 and 10; most studies were carried out on poly(hexamethylene-4,4´-diphenoxy terephthalate). A combination of thermogravimetric and FT-IR analysis suggested the reaction pathway shown below, resulting in fragments with phenolic chain ends and unsaturated chain ends. Although this is essentially a B-hydrogen transfer, it is not the same as that noted in the dicussions on the poly(alkylene terephthalate)s. It is much more likely to be a free-radical process, perhaps involving a caged radical

51

Degradation and Stabilisation of Aromatic Polyesters pair, with the incipient Ph-O. radical abstracting the B-hydrogen, thus being converted to a phenolic group and generating the unsaturated chain end: ~OPhOCH2CH2(CH2)4OPhO(C=O)Ph(C=O)~ ‡ ~OPhOH + CH2=CH(CH2)4OPhO(C=O)Ph(C=O)~ The unsaturated chain end may then further react to produce: CH2=CH(CH2)2CH=CH2 + HOPhO(C=O)Ph(C=O)~ Results also indicate that a second, possibly overlapping, degradation stage will be scission of the Ph(C=O)-OPh bond in the mesogenic section of the chain. This may then lead to the appearance of aldehydic and further phenolic species via hydrogen abstraction. In their studies of various terephthalate polyesters by TVA, Bounekhel and McNeill also investigated diphenylene and various phenylene terephthalates [127]. ~PhPhO(C=O)Ph(C=O)O~ ~PhO(C=O)Ph(C=O)O~ The latter with the diol being catechol (1,2), resorcinol (1,3) or hydroquinone (1,4). The only non-condensible product formed in all cases was CO. Condensible products were as follows: a) Diphenylene: CO2, benzene, phenol; with traces of biphenyl, 4-phenylphenol, benzaldehyde. b) 1,4: CO2, benzene, phenol, benzoquinone; with traces of benzaldehyde

52

Thermal Degradation c) 1,3: CO2, benzene, phenol, biphenyl; with traces of benzaldehyde and 3-hydroxyphenylbezoate. d) 1,2: CO2; with traces of phenol, catechol, benzene, benzaldehyde Solid materials captured as a cold ring fraction, i.e., products which condense in the upper part of the apparatus and do not reach the traps included to catch more volatile materials, were: a) Diphenylene: Terephthalic acid, p,p´-dihydroxybiphenyl, oligomers, with possible traces of 4-hydroxybiphenylbenzoate, di(4-hydroxybiphenyl)terephthalate. b) 1,4: Terephthalic acid, hydroquinone, 4-hydroxyphenylbenzoate, terephthaldehydic acid, benzoic acid, mono- and di(4hydroxyphenyl)terephthalate, short-chain fragments. c) 1,3: Terephthalic acid, resorcinol, benzoic acid, terephthaldehydic acid, mono- and di(3-hydroxyphenylterephthalates). d) 1,2: cyclic dimer. While all the above may be deﬁned as thermally stable polymers, poly(1,2-phenylene terephthalate) is by far the least stable, and shows a different degradation mechanism. It is especially noticeable that, while all the others leave a carbonaceous residue of approximately 30–40%, that from 1,2 is almost as low as 10%. This difference is proposed to be due to the ease of formation of the cyclic dimer brought about by conﬁguration of the 1,2 polymer. In general, a homolytic reaction is proposed, i.e., chain scission followed by hydrogen abstraction. Only aromatic hydrogens are available, so this leads to formation of large percentages of char by the end of the thermal degradation process.

53

Degradation and Stabilisation of Aromatic Polyesters

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Degradation and Stabilisation of Aromatic Polyesters 130. S.A. Mule, R.S. Ghadage, N.E. Jacob, C.R. Rajan and S. Ponrathnam, Journal of Applied Polymer Science, 2005, 97, 3, 784. 131. F. Bertini and V.V. Zuev, Polymer Degradation and Stability, 2006, 91, 12, 3214.

64

3

Thermo-Oxidative Degradation

3.1 Poly(ethylene terephthalate) (PET) The mechanism developed originally by Bolland and Gee [1] to explain the thermal oxidation of rubbers and polyoleﬁns has been successfully applied to the same situation in various polymers. A generalised form of the reaction sequence may be set out as follows: Chain initiation PH ‡ P. + H.

(3.1)

PH + O2 ‡ P. + HOO.

(3.2)

Impurities ‡ free radicals

(3.3)

Chain propogation P. + O2 ‡ POO.

(3.4)

POO. + PH ‡ POOH + P.

(3.5)

Chain branching POOH ‡ PO. + HO.

(3.6a)

POOH + PH ‡ PO. + P. + H2O

(3.6b)

65

Degradation and Stabilisation of Aromatic Polyesters 2 POOH ‡ PO. + POO. + H2O

(3.7)

PO. + PH ‡

POH + P.

(3.8)

HO. + PH ‡ H2O + P.

(3.9)

Chain termination 2 POO. ‡ POOP + O2

(3.10a)

2 POO. ‡ 2 PO. + O2

(3.10b)

2 POO. ‡ inactive products + O2

(3.11)

POO. + P. ‡ POOP

(3.12)

2 P. ‡ PP

(3.13)

As a rule, commercial polymers of all types contain catalyst residues and externally introduced functional groups (e.g., hydroperoxides) from manufacturing and processing which can sensitise the matrix to thermal oxidation. The exact nature of the initiating reactions (3.1) to (3.3) is not, even now, fully understood. It is generally assumed that primary radicals are formed through the action of heat and mechanical stress. In the case of PET, mechanical stress and the labile nature of the hydrogens A to the ester groups (and where such structures feature as ‘impurities’ A to ether links) towards direct abstraction by oxygen (reaction (3.2)) may provide the most likely point of initiation. Reaction (3.4) is ﬁxation of an oxygen molecule onto an alkyl radical. It is generally a very fast reaction if the concentration of oxygen in the polymer is moderate-to-high; this rapidly transforms alkyl radicals into peroxy radicals. It is essentially reaction (3.5) that will determine the rate of oxidation of the polymer, the rate being a function of the bond strength of the C-H bond broken and the stability of the alkyl

66

Thermo-Oxidative Degradation radical formed. These factors, along with the reaction temperature, determine the kinetic chain length, i.e., the mean number of oxidative cycles (reactions (3.4 and (3.5)) before termination occurs. The chain-breaking reactions consist of the monomolecular reaction (3.6a), the pseudo-monomolecular reaction (3.6b) and the bimolecular reaction (3.7). They show the decomposition of hydroperoxide groups. The pure thermal decomposition of hydroperoxide involves high activation energies, especially the monomolecular reaction (3.6a). In the case of molten PET, the temperature is more than sufﬁcient to bring such reactions into play. Decomposition of hydroperoxide is efﬁciently catalysed by various metal ions which may be present in the polymer as catalyst residues, or as part of additive packages. This is especially the case with metal ions that exhibit more than one stable oxidation state. Such catalysed reactions are equivalent to reaction (3.7) but are much faster. The radicals formed in the initiation, propagation and chainbranching steps can not only ﬁx oxygen and abstract hydrogen, they may also be subject to monomolecular decomposition processes. This type of reaction leads to chain scission and, as a consequence, to a decrease in the molecular weight of the polymer. The chain termination reactions (3.10)–(3.13) are bimolecular reactions leading to the destruction of two radicals each. As a rule, in the presence of a sufﬁcient amount of oxygen, only reactions (3.10) and (3.11), involving peroxy radicals, need be considered. Termination reactions (3.12) and (3.13) become increasingly important with reduction in the amount of oxygen available. Bimolecular reactions such as (3.10a), (3.12) and (3.13) give rise to branching and crosslinking. This can give rise to increases in molecular weight and possible gel formation. Studies of the thermal oxidation of PET have been carried out by various authors over several years [2–20]. An early overall oxidation scheme for PET was compiled by Buxbaum [3]. From his own, and earlier observations, he deduced that the

67

Degradation and Stabilisation of Aromatic Polyesters hydrogens most vulnerable to abstraction, and hence to initiation of free radical processes, were those A to the ester group in the polyester backbone: ~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ ~Ph(C=O)OC.HCH2O(C=O)Ph~ In the presence of oxygen, the alkyl radical formed will rapidly react with oxygen to form a peroxy radical: ~Ph(C=O)OC.HCH2O(C=O)Ph~ + O2 ‡ ~Ph(C=O)OC(OO.)HCH2O(C=O)Ph~ The peroxy radical will then abstract hydrogen from the same or another polymer chain to form a hydroperoxide: ~Ph(C=O)OC(OO.)HCH2O(C=O)Ph~ + RH ‡ ~Ph(C=O)OC(OOH)HCH2O(C=O)Ph~ + R. The next stage posited is scission of the hydroperoxide which, in theory, can take place in one of two ways: O-O bond scission to form an alkoxy macroradical and HO., or C-O bond scission to form an alkyl macroradical and HOO.: ~Ph(C=O)OC(OOH)HCH2O(C=O)Ph~ ‡ ~Ph(C=O)OC(O.)HCH2O(C=O)Ph~ + HO. Or ~Ph(C=O)OC(OOH)HCH2O(C=O)Ph~ ‡ ~Ph(C=O)OC.HCH2O(C=O)Ph~ + HOO.

68

Thermo-Oxidative Degradation The macroalkoxy radical may then abstract a hydrogen to form a hydroxyl group, and this species can then undergo chain scission via a cyclic intermediate to form an acid chain end and an aldehyde chain end: ~Ph(C=O)OC(O.)HCH2O(C=O)Ph~ + RH ‡ ~Ph(C=O)OC(OH)HCH2O(C=O)Ph~ + R. then ~Ph(C=O)OC(OH)HCH2O(C=O)Ph~ ‡ ~Ph(C=O)OH + H(C=O)CH2O(C=O)Ph~ The aldehydic chain end may then be oxidised to a carboxylic acid. The alkyl macroradical may also undergo chain scission, allegedly via a homolytic route: ~Ph(C=O)OC.HCH2O(C=O)Ph~ ‡ ~Ph(C=O)O. + CH2=CHO(C=O)Ph~ and ~Ph(C=O)O. + RH ‡ ~Ph(C=O)OH + R. From this description of the primary oxidation processes in PET and the related chain scissions, we see that the main difference between aerobic and anaerobic degradation is the appearance of aldehyde groups as chain ends, in addition to the previously encountered acid and unsaturated chain ends. Looking at one of the likely following stages of reaction – the formation of low molecular weight fragments from chain ends – each

69

Degradation and Stabilisation of Aromatic Polyesters chain end may now react via three general reactions: concerted ester pyrolysis (CEP), transesteriﬁcation/hydrolysis (TH) and oxidation (OX). The last reaction may take several forms. Likely reactions that may occur (in theory) are described in the following paragraphs. Chain ends available for further reaction include: ~Ph(C=O)OCH2CH2OH

Hydroxyl

~Ph(C=O)OH

Acid

~Ph(C=O)OCH=CH2

Unsaturated

~Ph(C=O)OCH2(C=O)H

Aldehyde

Likely evolved species may be: a) Hydroxyl TH

HOCH2CH2OH; 1,2-ethanediol

This can further react by ‘dimerisation’ to form 1,4-dioxane, or can form 2-methyl-1,3-dioxolane. The latter can also possibly break down to two molecules of acetaldehyde. OX (via alkoxy radical) H(C=O)CH2OH; ‘hydroxyacetaldehyde’ This structure is most probably unstable, and may rearrange into two molecules of formaldehyde. A less likely reaction would be further oxidation to form glycolic acid. OX (via alkyl radical)

70

CH2=CHOH; vinyl alcohol

Thermo-Oxidative Degradation As already noted in previous discussions, vinyl alcohol rapidly isomerises to acetaldehyde. The latter can be oxidised to acetic acid under certain conditions. CEP

CH2=CHOH; vinyl alcohol

b) Acid Acid ends are created via most of the reactions undergone by other chain end types to split off small molecules. Transition states, particularly via potential homolytic pathways, may undergo further scission to produce gaseous products such as CO and CO2. This can occur during ongoing reaction of non-acid chain ends, and by reaction of the existing acid chain ends. Several reactions may then occur on the phenyl radicals formed, including hydrogen abstraction (aromatic chain end), reaction with hydroxyl radicals (phenolic chain end), and reaction with another alkyl radical (chain extension or crosslinking). c) Unsaturated TH

CH2=CHOH; vinyl alcohol

CEP

HCyCH; acetylene

Not energetically favoured because the required B-hydrogen is attached to an unsaturated group. OX (via alkoxy radical)

CH2=C=O; ketene

Unstable compound, which may further react. OX (via alkyl radical)

HCyCH; acetylene

Overall, the oxidation of unsaturated chain ends in PET requires abstraction of a hydrogen from an unsaturated group. This reaction is not favourable, especially given the high concentration of more labile hydrogens available. 71

Degradation and Stabilisation of Aromatic Polyesters d) Aldehyde TH

HOCH2(C=O)H; ‘hydroxyacetaldehyde’

CEP

CH2=C=O; ketene

This process would require transfer of an aldehydic hydrogen in a cyclic transition state, a process which is almost certainly less likely even than abstraction of hydrogen from an unsaturated site. If the aldehydic chain end is oxidised in situ to an acid, then it cannot take part in this type of reaction because a B hydrogen is not available for transfer. OX (via alkoxy radical)

H(C=O)(C=O)H; glyoxal

Could be further oxidised to glyoxilic acid or oxalic acid. OX (via alkyl radical)

CH2=C=O; ketene

Overall, taking into account the theoretical considerations stated above and available laboratory data, acetaldehyde remains the major volatile product from PET during thermal degradation/oxidation. Of the proposed products mentioned above, we cannot be certain which one is important: few have been identiﬁed in studies to date. Many of the initially derived small-molecule species may undergo further reaction, or reaction with other species in or on the polymer. From a practical standpoint, the other problem with oxidative degradation of PET besides evolution of tainting species such as acetaldehyde is polymer discoloration. As already noted, Buxbaum [3] suggested that discoloration could be due to unsaturated polymeric species such as vinyls and aldol derivatives, at least in the case of thermal degradation. An extensive study of the yellowing of PET under thermal and thermo-oxidative conditions was carred out by Edge and co-workers [11]. They concluded that coloured species were due to hydroxylation of the terephthalate ring, followed

72

Thermo-Oxidative Degradation by formation of unsaturated ester species, and to the formation of quinonoid-type structures. Both this study and later work by Botelho and co-workers [13] showed that, unlike polyoleﬁns, the thermo-oxidative degradation of PET involves non-oxidative thermal degradation processes, especially in the early stages. The overall thermo-oxidative process in PET is therefore extremely complex. Recent studies [19, 20] using state-of-the-art mass spectroscopic techniques have provided further evidence for the role of hydroxylated terephthalate fragments in PET discoloration. Oxidative degradation of PET can also result in crosslinking and gel formation [4–6].

3.2 Poly(butylene terephthalate) (PBT) Few studies have been carried out speciﬁcally on the thermo-oxidative degradation of PBT [12, 13, 21], and it is generally considered that the oxidation pathway is very similar to that described in the section on PET. Taking the same theoretical approach to the likely low molecular weight products, the following processes are the most probable from the available end groups: ~Ph(C=O)OCH2CH2CH2CH2OH ~Ph(C=O)OH ~Ph(C=O)OCH2CH2CH=CH2 ~Ph(C=O)OCH2CH2CH2(C=O)H

73

Degradation and Stabilisation of Aromatic Polyesters 1. Hydroxyl TH ‡ HOCH2CH2CH2CH2OH

1,4-butanediol

As evidenced by laboratory studies, this product may then relatively easily cyclise to form tetrahydrofuran (THF). Some workers have also suggested that THF could be formed directly from the hydroxyl chain end via a cyclic intermediate. CEP ‡ CH2=CHCH2CH2OH

3-buten-1-ol

Unlike the situation in PET, there is no likely rearrangement to form an aldehyde in this case. Potentially, this species might react with an acid end group to produce an ‘extra’ unsaturated chain end. The likelihood of this will depend on the volatility of the unsaturated alcohol (boiling point (bp) = 112 °C) and its miscibility/solubility in the polymer. OX ‡ H(C=O)CH2CH2CH2OH

4-hydroxybutanal

Oxidation of the hydroxyl end group could alternatively form 3-buten-1-ol. It is also possible for the hydroxyaldehyde to further oxidise to form 4-hydroxybutyric acid. 2. Acid As for PET 3. Unsaturated TH

74

‡ HOCH2CH2CH=CH2

3-buten-1-ol

CEP ‡

CH2=CH-CH=CH2

1,3-butadiene

OX ‡

H(C=O)CH2CH=CH2

3-butenal

Thermo-Oxidative Degradation Alternatively, the last reaction may produce additional 1,3-butadiene. The unsaturated aldehyde, if formed, could further oxidise to vinylacetic acid. 4. Aldehyde TH

‡ HOCH2CH2CH2(C=O)H

CEP ‡ CH2=CHCH2(C=O)H OX

‡ H(C=O)CH2CH2(C=O)H

4-hydroxybutanal 3-butenal butane-1,4-dial

Alternatively, the last reaction could produce additional 3-butenal. The dialdehyde, if formed, could further oxidise to form succinic acid. From investigations of non-oxidative degradation, the main products posited from PBT are 1,3-butadiene, THF, 1,4-butanediol and 4-vinylcyclohexane (dimer of butadiene), as well as carbon oxides and water. Potential minor products, as suggested in this theoretical examination, in some way resemble those of PET, with hydroxy acids, hydroxy aldehydes, dialdehydes and diacids. Unlike PET, the above examination reveals the possibility of the formation of unsaturated species such as acids, alcohols and aldehydes which are not possible with PET. One can regard ketene as an unsaturated aldehyde, and acetaldehyde can be derived from an unstable unsaturated alcohol. As it is known that a cyclic ether (THF) may be formed during degradation and from the 1,4-butanediol by-product, the 1,4-diacid and hydroxyacids noted above could therefore also cyclise, leading to formation of succinic anhydride and G-butyrolactone. Further studies are required to investigate if any of these possible products occur in the thermo-oxidation of PBT. A study of the oxidation of butylene benzoate as a model compound for PBT was undertaken by Botelho and co-workers [13] and several products identified by gas chromatography-mass spectrometry

75

Degradation and Stabilisation of Aromatic Polyesters (GC-MS). Initial reactions involved abstraction of a hydrogen from a methylene group alpha to an ester group. This was followed by oxidation, formation of hydroperoxide, and homolytic scission of the hydroperoxide to form an alkoxy radical. This can be summarised as: Ph(C=O)O(CH2)4O(C=O)Ph ‡ Ph(C=O)OC(O.)H(CH2)3O(C=O)Ph The formed alkoxy radical could then react in three ways: a) H-abstraction ‡ Ph(C=O)OC(OH)H(CH2)3O(C=O)Ph {I}. b) O-C bond scission ‡ Ph(C=O)O. + H(C=O)(CH2)3O(C=O)Ph. The carboxy radical will then abstract a further hydrogen to form Ph(C=O)OH {II}. The aldehydic fragment then having aldehydic H abstracted there from and passing through an oxidation/ hydroperoxidation/alkoxy radical formation/H-abstraction cycle to form Ph(C=O)O(CH2)3(C=O)OH {III}; or through a series of reactions starting with loss of carbon dioxide to eventually form benzoic acid {II}. c) C-C bond scission ‡ Ph(C=O)O(C=O)H {IV} + . CH2(CH2)2O(C=O)Ph. The alkyl radical fragment may then go through a series of reactions to again form benzoic acid {II}. Species with numbers in parentheses are those identiﬁed by the authors. Studies on the effect of the level of acid end groups present on the thermo-oxidative stability of PBT [12] noted that increased carboxyl content lowered stability; this behaviour matches that of PET.

76

Thermo-Oxidative Degradation

3.3 Poly(trimethylene terephthalate) (PTT) Very little has been published in thesis form [22] or in the literature [23, 24] relating to the thermo-oxidation of PTT. A theoretical consideration of the likely small molecule by-products from thermal degradation and oxidation may provide useful information. Available end groups are: ~Ph(C=O)OCH2CH2CH2OH ~Ph(C=O)OH ~Ph(C=O)OCH2CH=CH2 ~Ph(C=O)OCH2CH2(C=O)H a) Hydroxyl TH

‡ HOCH2CH2CH2OH

CEP ‡ CH2=CHCH2OH

1,3-propanediol allyl alcohol

Under these circumstances, PTT would be expected to behave more like PBT than PET. Allyl alcohol is relatively stable, and there does not appear to be a likely driving force for it to rearrange to an aldehyde. There is potential for this species to form an ester with an acid chain end, thus forming an additional unsaturated chain end. OX ‡ H(C=O)CH2CH2OH

3-hydroxypropanal

Alternatively, oxidation could lead to formation of additional allyl alcohol. The aldehyde could further oxidise to 3-hydroxypropanoic acid.

77

Degradation and Stabilisation of Aromatic Polyesters b) Acid As for PET c) Unsaturated TH ‡ HOCH2CH=CH2

allyl alcohol

CEP ‡ CH2=C=CH2

allene

In this case we have, unlike PBT and more like PET, the requirement for abstraction of a hydrogen from an unsaturated carbon during chain scission via the proposed 6-membered cyclic intermediate. It is difﬁcult to be certain whether this product will form. The potential for resonance stabilisation of the intermediate is difﬁcult to assess; the allyl radical and allyl ions show some resonance characteristics, but it is also known that in the ﬁnal compound the two double bonds are orthogonal, i.e., there is no interaction between them. Allene is extremely volatile (bp = –34 °C), but it is also quite reactive with water and oxygen and can, under certain circumstances, form a resonance structure with methyl acetylene: CH2=C=CH2 + H2O ‡ CH2C(OH)=CH2 ‡ (CH3)2C=O

acetone

CH2=C=CH2 + O2 ‡ 2H2C=O formaldehyde + CO2 CH2=C=CH2 ﬂ‡ CH3CyCH OX ‡ CH2=CH(C=O)H

acrolein

Alternatively, oxidation of this end group could lead to the formation of more allene. The potential formation of acrolein is interesting because

78

Thermo-Oxidative Degradation this has been noted in the literature as a by-product during PTT synthesis [22]. Potentially it could also further oxidise to acrylic acid. d) Aldehyde TH ‡ HOCH2CH2(C=O)H

3-hydroxypropanal

CEP ‡ CH2=CH(C=O)H

acrolein

An alternative route to acrolein is possible. If aldehyde ends are present in a PTT sample (possibly by prior oxidation during manufacture or processing), then further oxidation is not required to produce acrolein. OX ‡ H(C=O)CH2(C=O)H

propanedial

Alternatively, acrolein may also be formed by oxidation of the aldehyde chain end. Propanedial, if formed, could potentially further oxidise as far as malonic acid. Studies by Wang and co-workers [23] indicated a three-step process for the degradation of PTT in air: Step 1: Degradation of chains into smaller fragments by an initial end-chain scission. Step 2: Oxidation of small fragments into volatile products. Step 3: Decomposition of initially stable structures formed in 1 and 2, probably to form crosslinked residues. In a study on PTT recycling, Ramiro and co-workers [24] noted that the structure of the polymer did not change despite lowering of molecular weight. This suggests that chain-end scission is probably the correct ﬁrst step. They noticed that, despite the lack of ‘new’ structures, the polymer did yellow considerably. This was a paradoxical observation unless the colour is attributable to small fragments or polymerised unsaturated species.

79

Degradation and Stabilisation of Aromatic Polyesters

3.4 Other Aromatic Polyesters Botelho and co-workers [25] made a comparative study of the thermo-oxidative degradation of poly(ethylene naphthalate) (PEN) and poly(butylene naphthalate) (PBN) The mechanism of the degradation of model compounds for these two polymers was similar in many ways to that noted for the terephthalate equivalents [13]. GC-MS analysis of the oxidation of ethylene dinaphthalate revealed the following products (where Np denotes a naphthalene ring system): Np(C=O)OH Np(C=O)OCH2CH3 Np(C=O)OCH2(C=O)H Np(C=O)O(C=O)CH2O(C=O)Np whereas butylene dinaphthalate gave: Np(C=O)OH Np(C=O)O(CH2)3CH3 Np(C=O)O(CH2)3(C=O)H Np(C=O)O(C=O)(CH2)3O(C=O)Np CO and CO2 were not detected, unlike with the terephthalate equivalents. The thermo-oxidation of PEN and PBN leads to yellowing, with the latter discolouring faster than the former. For both polymers, yellowing is related to oxygen uptake, indicating that this discoloration is due to

80

Thermo-Oxidative Degradation oxidative reaction(s). Compared with their terephthalate equivalents, these polymers discolour much more quickly, although the rate of oxidation appears to be lower. This may be accounted for by the greater extinction coefﬁcients likely with oxidised derivatives of the naphthalene ring system. Carboxylic acid end groups and anhydrides were detected in oxidised samples of these polymers, so it is proposed that the oxidation mechanism is very similar to that of the model compounds. The oxidation stability of poly(butylene isophthalate-co-terephthalate) copolyesters has been shown to decrease steadily with isophthalate unit content [26]. As expected, GC-MS analyses show that oxidation takes place mainly in the butylene units, through the same mechanism as before. Small-molecule products of a copolyester containing 25 mol% isophthalate included THF, butyrolactone, 3-buten-2-one, 2-propenal, and various other cyclised and carbonyl fragments, along with acetic acid. As has been observed for most polyesters, thermal and thermo-oxidative reactions occur simultaneously, and the lower stability of butylene-isophthalate units is most probably responsible for the lower overall stability of copolymers containing this structure, even under the oxidation conditions used. Studies have also been carried out on the oxidation of copolymers of terephthalic acid and 1,4-butanediol with aliphatic diacids such as adipic acid [27] and succinic acid [28]. No speciﬁc information was revealed on the details of the reaction mechanism. It was assumed that the butylene unit would remain the point of initial oxidative attack. In the case of poly(4-hydroxybenzoate) [29], thermo-oxidative degradation occurs only at very high temperature and even then only slowly. From the data available, it would appear that oxidation occurs at chain ends, and that reduction in molecular weight is brought about essentially by a depolymerisation process. This conjecture is backed by the fact that, even at 75% weight loss, the chemical composition of the remaining polymer is virtually unchanged.

81

Degradation and Stabilisation of Aromatic Polyesters A few studies have been made on polyarylates (i.e., copolymers of hydroxybenzoic acid with other ester-forming co-condensates), some of which provided information on oxidative degradation [30–32]. As might be expected, in the case of a hydroxybenzoate-co-ethylene terephthalate, the two parts of the copolymer degrade separately: the PET ﬁrst via the usual pathways, and the hydroxybenzoate unit later by a ‘depolymerisation’ route with the formation of phenol and carbon dioxide.

References 1.

J.L. Bolland and G. Gee, Transactions of the Faraday Society, 1946, 42, 236.

2.

I. Gomory, O. Mejnek and J. Stimel, Journal of Polymer Science, 1962, 59, 1, 71.

3.

L.H. Buxbaum, Angewandte Chemie International Edition, 1968, 7, 3, 182.

4.

K. Yoda, A. Tsuboi, M. Wada and R. Yamadera, Journal of Applied Polymer Science, 1970, 14, 9, 2357.

5.

D.L. Nealy and L.J. Adams, Journal of Polymer Science: Polymer Chemistry Edition, 1971, 9, 7, 2963.

6.

P.A. Spanninger, Journal of Polymer Science: Polymer Chemistry Edition, 1974, 12, 4, 709.

7.

J.D. Cooney, M. Day and D.M. Wiles, Journal of Applied Polymer Science, 1984, 29, 3, 911.

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J. Friedrich, I. Loeschke, H. Frommelt, H.D. Reiner, H. Zimmermann and P. Lutgen, Polymer Degradation and Stability, 1991, 31, 1, 97.

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K.S. Seo and J.D. Cloyd, Journal of Applied Polymer Science, 1991, 42, 3, 845.

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Thermo-Oxidative Degradation 10. F. Villain, J. Coudane and M. Vert, Polymer Degradation and Stability, 1994, 43, 3, 431. 11. M. Edge, R. Wiles, N.S. Allen, W.A. McDonald and S.V. Mortlock, Polymer Degradation and Stability, 1996, 53, 2, 141. 12. D.N. Biriakis and G.P. Karayannidis, Polymer Degradation and Stability, 1999, 63, 2, 213. 13. G. Botelho, A. Queiros, S. Liberal and P. Gijsman, Polymer Degradation and Stability, 2001, 74, 1, 39. 14. M. Dzieciol and J. Trzezczynski, Journal of Applied Polymer Science, 2001, 81, 12, 3064. 15. E.V. Kalugina, T.N. Novotorseva and M.B. Adreeva, International Polymer Science and Technology, 2002, 29, 4, T51. 16. P.K. Fearon, S.W. Biggar and N.C. Billingham, Journal of Thermal Analysis and Calorimetry, 2004, 76, 1, 75. 17. Y.N. Gupta, A. Chakraborty, G.D. Pandy and D.K. Setna, Journal of Applied Polymer Science, 2004, 92, 3, 1737. 18. S.V. Levchik and E.D. Weil, Polymers for Advanced Technologies, 2004, 15, 12, 691. 19. C.F.L. Ciolacu, N.R. Choudhury and N.K. Dutta, Polymer Degradation and Stability, 2006, 91, 4, 875. 20. C.F.L. Ciolacu, N.R. Choudhury, N.K. Dutta and N.H. Voelcker, Macromolecules, 2006, 39, 23, 7872. 21. A. Massa, A. Ecettri, S. Contessa, V. Bugatti, S. Concilio and P. Ianelli, Journal of Applied Polymer Science, 2007, 104, 5, 3071.

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Degradation and Stabilisation of Aromatic Polyesters 22. H.L. Traub, Synthesis and Textile Chemical Properties of Polytrimethyleneterephthalate, University of Stuttgart, Stuttgart, Germany, 1994. [PhD Thesis] 23. X. Wang, X. Li and D. Yan, Polymer Degradation and Stability, 2000, 69, 3, 361. 24. J. Ramiro, J.I. Eguiazábal and J. Nazábal, Journal of Applied Polymer Science, 2002, 86, 11, 2775. 25. G. Botelho, A. Quieros and P. Gijsman, Polymer Degradation and Stability, 2000, 70, 2, 299. 26. M.S. Chen, S.J. Chang, R.S. Chang, S.M. Chen and H.B. Hsai, Polymer Degradation and Stability, 1989, 23, 3, 239. 27. S. Chang, M. Chen, R.S. Chang, S. Chen and H. Tsai, Journal of Applied Polymer Science, 1990, 39, 2, 225. 28. F. Li, X. Xu, Q. Li, Y. Li, H. Zhang, J. Yu and A. Cao, Polymer Degradation and Stability, 2006, 91, 8, 1685. 29. J. Economy, R.S. Storm, V.I. Matkovic, S.G. Cittis and B.E. Nowak, Journal of Polymer Science: Polymer Chemistry Edition, 1976, 14, 1, 2207. 30. M.C. Gupta and A.K.S. Vishnu, Journal of Materials Science, 1984, 19, 2, 347. 31. X. Li and M. Huang, Polymer Degradation and Stability, 1999, 64, 1, 81. 32. S. Saikrasun and O. Wongkalasin, Polymer Degradation and Stability, 2005, 88, 2, 300.

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4

Photodegradation and Radiation Degradation

4.1 Photodegradation and Oxidation of Poly(ethylene terephthalate) (PET) For a photochemical reaction to occur there first must be a photophysical event. That is, before a light-induced reaction of a polymer occurs it must absorb energy from incident light. In the case of PET, the polymer itself includes a large concentration of potential light-absorbing species (chromophores) in the form of ester carbonyl groups and phenyl rings. Polypropylene (PP) would appear to have no chromophores in its structure. In the case of PP, any light-absorbing capability will be due to impurity groups in the chain (e.g., carbonyls, hydroperoxides) and to impurities in the polymer matrix (e.g., metal ions from catalyst residues or from other sources). It might therefore be expected that PP would be inherently more photostable than PET, but the reverse is true. The reason for this apparent paradox can be found through examination of the differing quantum yields, or efficiencies, of the primary photochemical reactions. In the case of hydroperoxide groups and aliphatic carbonyls in PP, the absorption of light at, for example 300 nm, is very low, but the efﬁciency of chain-breaking reactions is very high, especially in the case of the hydroperoxides. The absorption of light by aromatic carbonyls in PET at 300 nm is almost total, but the efﬁciency of such absorption in leading to chain scission and radical formation is, to all intents and purposes, very low. The result is that whereas PET can absorb considerably more energy directly from sunlight (or other light sources) than PP, the number of events which lead to chain scission and/or radical formation is small.

85

Degradation and Stabilisation of Aromatic Polyesters Initial investigations of the effects of light on PET were carried out between about 1950 and 1970 [1–8]. These initial experiments showed that the polyester rapidly yellows and embrittles, evolves gases such as CO and CO2, and reduces in solubility after exposure to monochromatic light at wavelengths between 254 nm and 313 nm. Probable reaction schemes are as follows: ~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ ~Ph. {I} + CO + .OCH2CH2O(C=O)Ph~ {II} I + II ‡

~PhOCH2CH2O(C=O)Ph~

and/or ~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ ~Ph. + CO2 + .CH2CH2O(C=O)Ph~ {III} I + III ‡ ~PhCH2CH2O(C=O)Ph~ Hydrogen abstraction from neighbouring molecules could also occur, leading to various radical species which can then crosslink. The ﬁrst major systematic study of PET photochemical degradation was begun by Day and Wiles in the early 1970s [9–15]. Samples were irradiated using carbon-arc and xenon-arc light sources, and their degradation followed by means of tensile properties, intrinsic viscosities, infrared (IR) absorption and ﬂuorescence emission. From the data obtained, the authors postulated that there were three potential primary photolytic reactions for PET [13]: 1) ~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ ~Ph(C=O)O. + .CH2CH2O(C=O)Ph~

86

Photodegradation and Radiation Degradation In this case the carboxyl radical may then go on to abstract a hydrogen to form an acid chain end, or split off CO2 and additionally form a phenyl radical chain end. 2) ~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ ~Ph(C.=O) + .OCH2CH2O(C=O)Ph~ This is the well-known Norrish I reaction. The carbonyl radical can abstract hydrogen to form an aldehydic chain end, or split off CO to form a phenyl radical chain end. 3) ~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ ~Ph(C=O)OH + CH2=CHO(C=O)Ph~ This is the Norrish II reaction, occurring via a six-membered ring intermediate, and is a non-radical process. We are considering a reaction in a solid polymer. Due to the low mobility of radical chain ends created by the chain scissions described previously (at least in the initial stages of the degradation when the molecular weight of the polymer is high), these species will very probably reform the polymer chain via cage recombination, or possibly form other species as noted in the earlier proposed reaction schemes. This would not be the case for the Norrish II reaction, which makes this reaction likely to be the major cause of chain scission under non-oxidative photolysis conditions. Photo-oxidation of PET will proceed, via formation and scission of hydroperoxides, in a similar manner to that described for thermal oxidation in Chapter 3 [12]. The main difference in this case will be (especially under outdoor exposure) the more ready availability of oxygen, at least on the surfaces of substrates. Diffusion of oxygen into and through the polymer must be considered. The rate of oxygen diffusion can be markedly affected by order in a polymer ﬁbre or ﬁlm (be it crystalline or ordered amorphous). It is generally considered that oxidation is largely limited to fully amorphous regions of the polymer matrix. 87

Degradation and Stabilisation of Aromatic Polyesters The primary photolytic reactions postulated are not themselves self-propagating, but provide the variety of radical species involved in subsequent oxidation and chain scission reactions characteristic of PET. The extent of oxidation reaction will depend in part on the balance of scission reactions in the initial process. Once established, the autoxidation cycle has a very long kinetic chain length, and will result in a rapid build-up of hydroperoxide in the polyester. As well as reactions leading to chain scission, other reactions can occur. These are mainly associated with phenyl radicals and hydroxyl radicals. These include the formation of ﬂuorescent species by addition of hydroxyls to the phenyl ring, and crosslinking. In nonoxidative conditions, crosslinking and discoloration predominate, whereas under oxidative conditions chain scissions and build-up of ﬂuorescent species result. The likely physical effects of photodegradation have been summarised [11, 14]. Ultraviolet (UV) degradation of PET in the absence of oxygen results in relatively little change in mechanical properties, whereas in the presence of oxygen rapid deterioration in elongation at break, and a less marked reduction in tensile strength, are observed. This appears to be due to suppression of cage recombination of radicals by oxygen, along with an increased degree of chain scission resulting from scission of alkoxy radicals generated during the autoxidation cycle. From IR spectroscopic data, it is known that degradation takes place almost entirely in the surface layer of a PET substrate (approximately 1–2 microns). The strong absorption of short wavelengths of light by PET means that this thin layer will absorb most of the damaging radiation; only longer wavelengths (which are weakly absorbed and of lower energy) will reach the bulk of the sample. The useful mechanical properties of all polymers depend on the chain length of the polymer backbone. This length allows extensive entanglement, and folding leads to the formation of crystallites interconnected by amorphous regions, which may themselves,

88

Photodegradation and Radiation Degradation particularly in oriented products such as ﬁbres or ﬁlms, contain regions of greater or lesser order. Backbone cleavage will result in weakening of polymer structure, and hence a reduction in physical strength. This is a particular problem because, as indicated earlier, scissions will occur mainly in the more oxygen-permeable amorphous regions. PET with an initial molecular weight of 20000 can exhibit a decrease in molecular weight of as much as one-third after prolonged exposure to UV radiation. The extent of backbone scission is even more startling when it is remembered that degradation occurs only in the surface layer, so that molecular weight in this region could be reduced almost to oligomeric levels. This surface oxidation and associated backbone scission can be expected to result in drastic reduction in surface toughness and resistance to stress. PET shows no surface changes (except in colour at high levels of photo-oxidation), and only a small increase in density. Cracking is not expected in the unstressed state during degradation because inter- and intra-molecular bonding forces in the polymer will resist surface contraction. Backbone scission in the surface region will rapidly become so extensive that the ﬁlm or ﬁbre cannot tolerate even a small extension without this photo-oxidised layer cracking at right angles to the applied stress. The formation of surface cracks in PET under low stress thus accounts for the rapid drop in tensile properties after prolonged exposure to UV radiation. PET is a semi-crystalline polymer and, on application of increasing stress to degraded materials the cracks, initially produced at low stress in the degraded surface layer, propagate into the remainder of the largely unaffected polymer. The nature of PET morphology, especially in oriented ﬁbres and ﬁlms, precludes sufﬁcient plastic deformation at the crack tips, even in this non-degraded material, to dissipate the energy. With the basics of UV degradation of PET established by Day and Wiles, to the limit of techniques available in the 1970s, further research has been carried out up to the present day to provide more detailed insights into the behaviour of this polymer under photolysis and photo-oxidation [16–42].

89

Degradation and Stabilisation of Aromatic Polyesters Savchuk and Neverov [20] studied the photo-oxidation of oriented ﬁlms of PET. They found that, while the number of chain scissions was higher in UV-irradiated oriented ﬁlms, and hence a greater degree of molecular weight loss, the same ﬁlms exhibited improved retention of physical properties over non-oriented ﬁlms treated in a similar manner. This effect was deduced as being due to the high degree of order in oriented ﬁlms. This put strain on chains, making them more susceptible to scission, but also increased the strength of the overall sample. Studies carried out by Gardette and co-workers [26] provided further data on the reactions undergone by PET under UV irradiation. They conﬁrmed the probable initial radical scission reactions undergone by the PET chain, where the three possible pairs of radicals formed from the initial structure ~O(C=O)Ph(C=O)OCH2CH2~ were: (1)

~O(C=O)Ph(C=O)O. + .CH2CH2~

(2)

~O(C=O)Ph(C.=O) + .OCH2CH2~

(3)

~O(C=O)Ph. + (.C=O)OCH2CH2~

Each pair may then undergo cage recombination, or the individual radical may then undergo oxidation, formation of further (possibly coloured) products or may abstract hydrogen atoms from other polymer chains. A less likely, but possible, series of reactions would involve reaction between radicals from different pairs. The authors also proposed that hydrogen abstraction by these initial radicals could lead to species which themselves may oxidise and undergo further reactions. Two pathways are proposed, depending on whether the hydrogen abstraction has taken place in the alkylene or aromatic sections of the PET chain. In the case of formation of a radical on the phenyl group, the following reactions may take place (structures in square brackets are attached to the phenyl ring, but do not form part of the polymer chain): 90

Photodegradation and Radiation Degradation ~O(C=O)Ph.(C=O)OCH2CH2~ + O2 +RH ‡ ~O(C=O)Ph[OOH](C=O)OCH2CH2~ The phenyl hydroperoxide may then undergo the familiar O-O bond scission, followed by hydrogen abstraction, to form a hydroxyl group: ~O(C=O)Ph[OH](C=O)OCH2CH2~ This species may then undergo the same reaction sequence to form a dihydroxy derivative: ~O(C=O)Ph[OH]2(C=O)OCH2CH2~ A radical formed in the alkylene segment of the chain can react with oxygen and abstract a hydrogen to form a hydroperoxy species: ~O(C=O)Ph(C=O)OCH(OOH)CH2~ Further reaction of the previously species is said to lead to the formation of: ~O(C=O)Ph(C=O)O(C=O)CH2~ A short review of photo-oxidation of a range of polymers by Faucitano and co-workers [30] summarised the understanding at the time of PET photolysis as a splitting of C-O bonds in the ester groups, with formation of acyl and carboxyl radicals, which themselves can lose carbon oxides to produce phenyl or alkyl radicals, or abstract hydrogen to produce aldehydes and carboxylic acids. When oxygen is present, the authors state that the autoxidation chain reaction will lead to formation of anhydrides and aldehydes, and to hydroxysubstituted phenyl species. Studies of UV irradiation of stacks of PET ﬁlms by Wang and coworkers [32] conﬁrmed that the major cause of loss of strength in

91

Degradation and Stabilisation of Aromatic Polyesters the polymer is reduction in molecular weight of the surface layers brought about by UV-induced chain scission. The same authors also examined the photodegradation behaviour of single thin ﬁlms [34] and noted that the degradation appeared to proceed in two stages: a rapid initial degradation, followed by a slower steady degradation after a period of a few hours. This was attributed to ‘weak links’ in the polymer, a concept which has been noted for other polymers besides PET. Identiﬁcation of the nature of these weak links was not attempted, but this phenomenon could be associated with impurities in the polymer such as hydroperoxides, peroxides, or ether linkages in the backbone from diethylene glycol units. Alternatively it may be an experimental artefact from strain induced in the polymer during preparation of the sample thin ﬁlms. Subsequent studies on coextruded multilayer ﬁlms [42], provided similar results to those obtained with ﬁlm stacks. Hrdlovic [35] differentiated between two further reaction routes for the initially formed radicals in PET photolysis. These may recombine (before or after loss of carbon oxides) or may separately abstract hydrogen from the polymer. The former may lead to various recombination products, for example benzophenones, which may photolyse to produce coloured species, possibly accounting for the discoloration of PET under non-oxidative conditions. The latter leads to various end groups such as acids, aldehydes and alcohols. A useful summary of the understanding of PET photolysis mechanisms in the ﬁrst decade of the twenty-ﬁrst century is provided by Fagerberg and Clauberg [38]. Fechine and co-workers [36] studied the effect of UV irradiation on the structure of oriented ﬁlms of PET, noting that chain scission, while not immediately changing the crystallinity of the sample, could lead to increased crystallinity if the ﬁlms were heated because long chains in the amorphous regions which had been broken could now rearrange into a crystalline phase.

92

Photodegradation and Radiation Degradation It is suggested by Fraisse and co-workers [40] that measurement of oxygen uptake provides a useful tool for the detailed examination of PET photo-oxidation. Most studies on PET photodegradation have concentrated on the effects of sunlight (or sunlight-simulating light sources) and near-UV exposure on the polymer, but some studies have been carried out on the inﬂuence of far-UV wavelengths (<200 nm) on PET deterioration [31, 39]. In terms of mechanism and conﬁnement of major damage to the surface layers of samples, these experiments revealed little difference from those carried out at longer wavelengths.

4.2 Photodegradation and Oxidation of Other Poly(alkylene terephthalate)s Several studies have been conducted over the last twenty years on the effect of UV irradiation on poly(butylene terephthalate) (PBT) [26, 43–56]. In general, the reaction schemes of photolysis and photooxidation are virtually identical to those of PET, except that there is some controversy about if the Norrish II scission, via a six-membered ring intermediate, is signiﬁcant in PBT. Despite its increasing commercial importance, studies on the UV degradation of poly(trimethylene terephthalate) (PTT) are lacking. It is currently assumed that the mechanisms of photolysis and photo-oxidation of PTT will be broadly similar to those previously discussed for PET. Studies have been carried out on the UV degradation of poly(ethyleneco-1,4-cyclohexanedimethylene terephthalate) [35, 38, 56]. The ‘PET’ sections of the chain will undergo the various initiation, abstraction and oxidation processes already considered, but a tertiary hydrogen on the cyclohexane ring of the ‘poly(1,4-cyclohexanedimethylene terephthalate)’ units means that formation of tertiary alkyl radicals and subsequent autoxidation at this site will predominate over formation of secondary alkyl radicals on methylene units or elsewhere

93

Degradation and Stabilisation of Aromatic Polyesters in the cyclohexane ring. This leads to a complex series of reactions involving opening of the cyclohexane ring and formation of various chain ends and small-molecule products.

4.3 Photodegradation and Oxidation of Poly(alkylene naphthalate)s Few studies have been undertaken on the photodegradation of poly(ethylene naphthalate) (PEN) [57–60], PEN-PET copolymers [61] and poly(butylene naphthalate) (PBN) [62]. Ouchi and co-workers [57, 60] demonstrated that, while PET ﬁlms become slightly discoloured upon exposure to light of 310 nm and were subsequently subject to severe chain scission, PEN ﬁlms were coloured on exposure to 382 nm light and underwent crosslinking and chain scission. Because of the coloration, photodegradation was conﬁned to the surface of PEN and the effect did not propagate into the interior of the ﬁlm. At around the same time as the original work by these authors, investigations by Allen and McKellar [58] appeared to show that the major primary photochemical reaction taking place in the UV photolysis of PEN was scission at the naphthyl-carbon bond in the backbone structure: ~O(C=O)Np(C=O)OCH2CH2~ ‡ ~O(C=O)Np. + (.C=O)OCH2CH2~ It was some time before these studies were superseded by a more comprehensive study of PEN photolysis and photo-oxidation by Scheirs and Gardette [59]. Photolysis mechanisms were postulated as shown next: 1)

~O(C=O)NpOCH2CH2~ ‡ ~O(C=O)Np. + (.C=O)OCH2CH2~

2 ~O(C=O)Np. ‡ ~O(C=O)Np-Np(C=O)O~ 94

Photodegradation and Radiation Degradation Crosslinking and chain extension resulted in gel formation and increased colour due to extended conjugation of directly bonded naphthalene systems. 2)

~O(C=O)Np(C=O)OCH2CH2~ ‡ ~O(C=)Np(C=O)O. + .CH2CH2~

~O(C=O)Np(C=O)O. + RH ‡ ~O(C=O)Np(C=O)OH

4.3.1 Formation of a naphthalic acid end group Proposed photo-oxidation schemes are similar to those noted for PET, with initial hydrogen abstraction from a methylene group followed by oxidation and hydroperoxide formation. As with PET, the hydroperoxide formed may split homolytically, leading to B scission and ultimate formation of aliphatic aldehyde and naphthalic acid chain ends, or a caged reaction may occur leading to formation of an in-chain anhydride structure. PEN yellows and crosslinks rapidly under photolysis, but due to the high absorption coefﬁcient of the chromophoric unit in the polymer this effect is restricted to the very outer layers of an exposed ﬁlm or ﬁbre. This outer layer provides a protective barrier against further degradation of the bulk of a sample. Despite the greater propensity for PEN to form coloured species and anhydride groups, the rate of chain scission (and hence loss of physical properties) is higher for PET than for PEN. Care must therefore be taken in selection of criteria for deciding which polymer is more photostable than the other. These authors make no reference to the formation of hydroxylated naphthalene species analogous to those seen with PET. The mechanisms of photolysis and photo-oxidation of PBN [62] appear to be broadly similar to those of PEN, with small variations that might be expected in the light of previous investigations of PBT reactions.

95

Degradation and Stabilisation of Aromatic Polyesters

4.4 Radiation Degradation Polymers can, under certain circumstances and in several industrial processes, be exposed to high-energy radiation such as X-rays, G-rays and electron beams; less commonly they may also be exposed to particles such as protons, neutrons and A-particles. Such interactions can be highly damaging, and take place over extremely short timescales. The photo-initiation of free-radical producing processes by UV light is extremely slow compared with initiating processes induced by high-energy radiation. This is because photo-initiation is dependent on weakly absorbing chromophores in or on the polymer, whereas high-energy photons or particles are absorbed by the entire polymer structure. Polymer irradiation can produce crosslinking, backbone scission, hydrogen evolution, and various other reactions. A generalised sequence of events for the process may be set out as follows: P + G ‡ P+ + e-

electron ejection

nP + e- ‡ nP+ + (n+1)e-

secondary electron ejection

P+ + e- ‡ P*

excited state formation

P* ‡ P. + P.

C-C bond scission

P* ‡ P. + H.

C-H bond scission

Initial interaction of each G-ray photon with the polymer yields fast electrons (similar to those involved in direct electron irradiation) which in turn cause subsequent ejection of secondary electrons of lower energy. The spread of damage from an initial G-ray photon strike can be extensive, with a radius of several microns. Under normal conditions of temperature and pressure, ion-electron recombination is rapid, resulting in the formation of excited states. These excited states can dissipate their energy via bond scission.

96

Photodegradation and Radiation Degradation In the case of bond scission caused through absorption of high-energy radiation, C-H bond scission appears to be favoured over C-C bond scission, even though the former are stronger than the latter [63]. This phenomenon has been attributed to energy migration along the backbone by an exciton mechanism, minimising energy localisation in speciﬁc C-C bonds, whereas energy deposited in C-H bonds cannot migrate. Alternatively it may be that the relative strengths of C-C and C-H bonds are different in the highly excited states formed under absorption of high-energy radiation. Secondary reactions undergone by the free radicals formed in the processes described previously will be similar to those noted for thermolysis and photolysis. The effect of high-energy radiation on the breakdown of aromatic polyesters has been studied, including the effect of electrons [64-66], protons [67, 68] and G-rays [69-80]. In general, these materials are less susceptible to radiation damage than, for example, polyamides and polyoleﬁns. Bell and Pezdirtz [73] studied the effects of changing the aromatic and aliphatic units in polyesters, comparing the radiation degradation of PET, PEN, poly(ethylene trans-1,4-cyclohexanedicarboxylate), PBT and poly(decamethylene terephthalate). Complete removal of the aromatic character resulted in severe de-protection, as evidenced by rapid loss of molecular weight, followed by extensive crosslinking to form an insoluble gel. Replacement of terephthalate with naphthalate resulted in a slow, steady, crosslinking process which, under the experimental conditions used, was insufﬁciently severe to compromise the useful properties of the polymer. PEN was shown to be considerably more resistant to radiation damage than other polyesters. Increasing the length of the aliphatic portion led to a diminished rate of chain scission, but increased crosslinking. Low-temperature electron spin resonance studies of radiation degradation of PET have, to some extent, failed to clarify the fundamentals of the reaction. They produced evidence for positively

97

Degradation and Stabilisation of Aromatic Polyesters and negatively charged ionic species [74] and cyclohexadienyl-type radicals [76]. Using a combination of thermal analysis, viscosity measurements and positron annihilation lifetime spectroscopy, Buttafava and co-workers [77] noted that PET decreases in molecular weight and increases in crystallinity when G-irradiated. A comparative study of the gaseous products formed during G-irradiation of PBT, PET and PEN was carried out by NavarroGonzalez and co-workers [78]. The total gases evolved were equal to 0.1, 0.07, and 0.02 molecules/100 eV, respectively. This demonstrated the different degrees of radiation stability of the three polyesters. All produced H2, CO, CO2 and CH4 (the latter in very low quantities). PBT also produced ethane, propane, propene and n-butene; PET also produced a trace of ethane; no other hydrocarbons were detected from PEN. The differences between photolysis of PET and degradation by highenergy radiation can be summarised as follows: a) Photolysis: Primary reaction products are ~Ph. , CO, and ~Ph(C=O)OCH2CH2O.; few carboxyl groups and no hydrogen are formed; crosslinking occurs. b) G-rays: Primary reaction products are H . and ~Ph(C=O) OC.HCH2O(C=O)~; hydrogen and carboxyl groups are formed; no gelation occurs up to dosage of 2000–5000 Mrad. The results of experiments with electrons [64] were, as expected, different to those using G-rays because electrons are known to cause a temperature increase in PET, and to disrupt crystallinity by affecting the steric conformation of the chain. Gel formation was not observed at up to 175 °C for doses up to 2000 Mrad; rapid crosslinking occurred even at low doses at >200 °C.

98

Photodegradation and Radiation Degradation A general reaction scheme may be postulated as: ~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ ~Ph(C=O)OC.HCH2O(C=O)Ph~ + H. 2H. ‡ H2 ~Ph(C=O)OC.HCH2O(C=O)Ph~ ‡ ~Ph(C=O)OCH=CH2 + .O(C=O)Ph~ ~Ph(C=O)O. + ~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ ~PH(C=O)OH + ~Ph(C=O)OC.HCH2O(C=O)Ph~ ~Ph(C=O)O. ‡ ~Ph. +

CO2

~Ph(C=O)OCH=CH2 + ~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ chain branching 2 ~Ph(C=O)OC.HCH2O(C=O)Ph~ ‡ cross linking ~Ph(C=O)OCH=CH2 ‡ coloured species Several more complex species may also result from addition of H. to the polymer, especially on the aromatic rings. The presence of oxygen will lead to an autoxidation cycle to be established. The relative importance of chain scission versus crosslinking in PET will be very dependent on conditions, e.g., humidity, oxygen levels, crystallinity, dosage, dosage rate and temperature.

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Degradation and Stabilisation of Aromatic Polyesters 51. P. Gijsman, G. Meijers and G. Vitarelli, Polymer Degradation and Stability, 1999, 65, 6, 433. 52. P. Hrdlovic, Polymer News, 2000, 25, 8, 261. 53. N. Manabe and Y. Yokota, Polymer Degradation and Stability, 2000, 69, 2, 183. 54. J.E. Pickett, Polymer Preprints, 2001, 42, 1, 424. 55. L.R. Khashkhozheva, T.A. Borukaev and N.I. Mashukov, International Polymer Science and Technology, 2003, 30, 11, T34. 56. N.S. Allen, G. Rivale, M. Edge, I. Roberts and D.R. Fagerburg, Polymer Degradation and Stability, 2000, 67, 2, 325. 57. I. Ouchi, M. Hosoi and F. Matsumoto, Journal of Applied Polymer Science, 1976, 20, 7, 1983. 58. N.S. Allen and J.F. McKellar, Journal of Applied Polymer Science, 1978, 22, 8, 2085. 59. J. Scheirs and J.L. Gardette, Polymer Degradation and Stability, 1997, 56, 3, 339. 60. I. Ouchi, I. Nakai, M. Ono and S. Kimura, Journal of Applied Polymer Science, 2007, 105, 1, 114. 61. P.R. Cheung, C.W. Roberts and K.B. Wagener, Journal of Applied Polymer Science, 1979, 24, 8, 1809. 62. J. Scheirs and J.L. Gardette, Polymer Degradation and Stability, 1997, 56, 3, 351. 63. D.J. Carlsson in Atmospheric Oxidation and Antioxidants, Volume II, Ed. G. Scott, Elsevier, Amsterdam, Netherlands, 1993, p.498.

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Photodegradation and Radiation Degradation 64. N.A. Slovokhotova, G.K. Sadovskaya and V.A. Kargin, Journal of Polymer Science, 1962, 58, 1293. 65. A. Hegazy El-Sayed, T. Sagusa, M. Nishii and T. Seguchi, Polymer, 1992, 33, 14, 2904. 66. M. Zenkiewicz, Journal of Adhesion Science and Technology, 2004, 18, 5, 607. 67. T. Sagusa, S. Kawanishi, T. Seguchi and I. Kohno, Polymer, 1989, 30, 11, 2054. 68. G. Peng, D. Yang, H. Liu and S. He, Journal of Applied Polymer Science, 2008, 107, 6, 3625. 69. S.D. Barrow, D.T. Turner, G.F. Pezdirtz and G.D. Sands, Journal of Polymer Science: Polymer Chemistry Edition, 1966, 4, 3, 613. 70. D.T. Turner, G.F. Perzdirtz and G.D. Sands, Journal of Polymer Science: Polymer Chemistry Edition, 1966, 4, 1, 252. 71. D. Campbell, L.K. Monteith and D.T. Turner, Journal of Polymer Science: Polymer Chemistry Edition, 1970, 8, 9, 2703. 72. M. Pietrzak and J. Kroh, European Polymer Journal, 1972, 8, 2, 237. 73. V.L. Bell and G.F. Pezdirtz, Journal of Polymer Science: Polymer Chemistry Edition, 1983, 21, 11, 3083. 74. A. Torikai and K. Fueki, Polymer Degradation and Stability, 1985, 13, 2, 183. 75. A. Hegazy El-Sayed, T. Sagusa, M. Nishii and T. Seguchi, Polymer, 1992, 33, 14, 2897.

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Degradation and Stabilisation of Aromatic Polyesters 76. A.Y. Orlov and V.I. Feldman, Polymer, 1997, 38, 15, 3927. 77. A. Buttafava, G. Consolati, L. DiLandro and M. Mariani, Polymer, 2002, 43, 26, 7477. 78. R. Navarro-Gonzalez, D. Likhatchev and R. Aliev, Polymer Bulletin, 2003, 50, 1-2, 77. 79. A. Buttafava, G. Consolati, M. Mariani, F. Quasso and U. Ravasio, Polymer Degradation and Stability, 2005, 89, 1, 133. 80. U. Ravasio, G. Consolati, A. Faucitano, M. Mariani and F. Quasso, European Polymer Journal, 2007, 43, 6, 2550.

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5

Chemical Degradation and Recycling

5.1 Hydrolytic Degradation Simple esters such as ethyl acetate can be considered to be created by the elimination of water from an alcohol and an organic acid, and the reaction as being fully reversible. In reality, the reaction between acetic acid and ethanol is a complex bimolecular process, and in the absence of a catalyst this occurs very slowly, i.e., ethanol is too weak a nucleophile to add readily to the carbonyl double bond of acetic acid. If a strong acid is present as a catalyst it should protonate the acetic acid to yield a carbonium ion, which is sufﬁciently electrophilic to react with the ethanol molecule. Obviously, the addition of the acid catalyst will only serve to increase the rate at which equilibrium between ester/water on the one hand and acid/alcohol on the other is reached; to drive the reaction one will need to remove water from the system or start with a large excess of alcohol. The reaction may be reversed by use of the same (or different) acid catalyst and excess water, but in most organic chemistry approaches to this it is preferred to utilise base catalysis to hydrolyse an ester. The last step in the base hydrolysis of an ester is proton transfer from the carboxylic acid molecule to the alkoxide ion. This reaction is virtually irreversible. From a practical standpoint, base hydrolysis is a more useful process as, with for example sodium hydroxide, the acid is now in its salt form. This means that the products can be separated much more easily.

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Degradation and Stabilisation of Aromatic Polyesters These features of the chemistry of esters need to be taken into consideration when discussing similar reactions in polyesters. Many articles have been published on the hydrolytic degradation of poly(ethylene terephthalate) (PET) under acidic conditions [1–20] and basic conditions [21–30], but far fewer on similar degradation studies on poly(butylene terephthalate) (PBT) [31, 32], poly(trimethylene terephthalate) (PTT) [33] and poly(ethylene naphthalate) (PEN) [34, 35]. Under ambient temperature and pressure, PET can be attacked by dilute bases, but is relatively inert to dilute acids and to water. PET is more susceptible to hydrolytic degradation at elevated temperatures. At temperatures between 100 °C and 120 °C, it has been suggested that hydrolysis of the ester linkage occurs at a rate several orders of magnitude higher than its thermal breakdown rate [2]. The moisture content of PET undergoing melt processing must be kept below about 0.02% to avoid rapid loss of molecular weight. The equilibrium water content of PET is between 0.3% and 0.35%, so strict drying regimens are required when processing this polymer. The degradation of PET has been found [4, 7] to occur in two distinct kinetic stages: an initial fast rate via hydrolysis, and a later slow rate attributed to thermooxidative cleavage. The rate of hydrolysis is directly proportional to the concentration of water in the polymer. As well as monitoring the water content of the polymer, care needs to be exercised in choosing additives. For example, pigments with water of hydration can result in damage to the host polymer. The inﬂuence of proton concentration on hydrolysis rate is evident in the hydrolysis of PET in hydrochloric acid [1]. The rate does not respond to increases in acid concentration <3 M, but increases rapidly as concentration is increased above this value. The explanation for this appears to lie with the low dielectric constant of PET, which means that the acid in the polymer is considerably less ionised than in aqueous solution. Similar observations have been made in experiments carried out in sulfuric acid [16] and nitric acid [13, 17]

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Chemical Degradation and Recycling Organic acids can also inﬂuence PET hydrolysis, and considering the acid end groups in the polymer is important. Studies by Zimmerman and Kim [3] found PET hydrolysis to be an autocatalytic reaction, suggesting that low carboxyl group content is crucial to the processing stability of the polymer. Hydrolytic degradation can affect, and be affected by, PET structure. For example, oriented fibres are less susceptible to hydrolysis than non-oriented ﬁbres. This is said to be due to changes in the crystallinity and orientation [5, 6] and to alterations in dielectric constant brought about by changes in the cis/trans ratio of extended molecules [2]. Acid-catalysed hydrolysis of PET is facilitated by carboxyl end groups in the polymer, but this is not the case with alkaline-catalysed hydrolysis. The reaction is therefore largely conﬁned to the outer layers of a substrate under alkaline attack, although the irreversibility of the reaction and likely removal of products from the reaction site means that prolonged exposure can result in the gradual ‘eating away’ of the polymer. This phenomenon has been used to good effect by the ﬁbre industry, where alkaline treatments have been used to produce aesthetically useful effects on fabric surfaces [22, 23]. PTT and PBT appear to be more resistant to hydrolysis than PET, as might be expected. PEN is also more hydrolytically stable than PET.

5.2 Ester Interchange PET can be degraded by alcohols and carboxylic acids via esterinterchange reactions [36]. Similarly, low molecular weight esters can degrade PET via transesteriﬁcation [37]. Interactions within a polyester melt involving transesteriﬁcation can result in changes in molecular weight distribution, whereas transesteriﬁcation of PET with other polyesters can result in drastic changes [38]. This type of reaction has been used to convert PET to PBT [39], and to form

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Degradation and Stabilisation of Aromatic Polyesters copolymers of PET and poly(lactic acid) [40]. Similar interchanges are also possible between polyesters and amino compounds [41]

5.3 Aminolysis Polyesters can be attacked by amines, which they can come into contact with when these are used, for example, as additives in other polymers or as components of lubricating oils and adhesives. Attack is usually in the amorphous regions of the polymer [42] but may extend into crystalline domains if exposure is prolonged. Examination of the mechanism of the reaction [43–46] suggests a bimolecular substitution reaction, with protonation of the ester followed by nuleophilic attack by the amine. Polyester ﬁbres have been shown to swell and undergo surface cracking on exposure to amine vapours [47]. Studies of PET tyre cords [48] have highlighted the problem of amine additives in the rubber migrating into the polyester.

5.4 Biodegradation Natural polymers such as cellulose and starch biodegrade easily. Most commercial synthetic polymers are resistant to biodegradation apparently because micro-organisms cannot assimilate them directly. Biological attack on polymers generally occurs at sites in the polymer containing oxygenated groups such as carbonyls. If these sites are sterically accessible to microbes, then the polymer may be subject to biodegradation. This potential form of degradation is essentially a chemical attack because the microbes secrete chemical enzymes which promote/ catalyse hydrolysis, acidolysis or aminolysis of the backbone bonds in the polymer. With their high levels of carbonyl (ester) linkages, it might be expected that aromatic polyesters would be attacked by micro-organisms (as is the case with many aliphatic polyesters). Due to the close-packed nature of the chains in, for example, PET

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Chemical Degradation and Recycling brought about by interactions between aromatic rings on adjacent chains, organisms are largely unable to attack the vulnerable ester groups. At temperatures where the chain ordering is loosened or broken, micro-organisms are highly unlikely to survive. In the case of PET (or any polymer) which has already been oxidised, situations may exist in which these damaged substrates could be attacked and degraded further by micro-organisms. Various studies have been carried out on the effect of micro-organisms, and their associated enzymes, on PET and other aromatic polyesters [49-53]. In general, poly(alkylene terepthalate)s are resistant to biodegradation, unless under specific conditions such as high temperature, ﬁnely divided samples, or extra available nutrients. Smith and co-workers [50] noted that, in in vitro experiments, PET underwent very mild degradation under the action of esterase and papain, but that trypsin or chymotrypsin had no effect. Lefevre and co-workers [52] examined the degradation of PET and PEN, along with their component monomers and low molecular weight model compounds, using soil bacteria used in industrial composting. Six strains were identiﬁed which could assimilate terephthalic acid, dimethyl terephthalate and ethylene glycol terephthalate, but only two could do the same with similar fragments of PEN, and none appeared capable of assimilating the polymers. In a study of the mechanism of polyester degradation by enzymes [53], it was noted that the main factor controlling biodegradation is mobility of the polymer chains rather than the chemical structure of monomer components. Rate of degradation is thus correlated to the difference between the melting point and the temperature at which degradation is taking place, especially when the difference is <30 °C. Thus, with poly(alkylene terephthalate)s, it is unlikely that biodegradation will occur except under carefully controlled conditions and over long time scales. Despite this observation, there have been claims that speciﬁc types or mixtures of bacteria/enzymes can be utilised as a means of degrading PET. Research into this area has been boosted by environmental considerations, and is being actively pursued as a means of dealing with waste polyesters.

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Degradation and Stabilisation of Aromatic Polyesters Among the attempts being made are the use of hydrolases from the actinomycete Thermobiﬁda fusca [54, 55], hydrolytic enzymes such as lipases [56], and micro-organisms of the genus Rhizobium [57]. With the twin driving forces of usefulness and eventual biodegradation, many studies have been made on the biodegradation of copolyesters containing aromatic and aliphatic segments to develop ‘greener’ materials [58–79].

5.5 Chemical Recycling Polymer recycling has grown from a fringe activity in the third quarter of the twentieth century largely conﬁned to in-house re-use of offspec material by manufacturers to a multimillion-dollar industry. This change has been spurred on by technical developments and by changes in public attitude (often backed by legislation). In the case of condensation polymers such as aromatic polyesters, there are several ways of recovering starting materials from scrap polymer (post-industrial and post-consumer) rather than the polymer itself. It is therefore possible to isolate purer materials than would be the case with attempting to recover a single polyester, and even contaminated and degraded scrap polymer can be utilised. Various techniques are known for the chemical breakdown of poly(alkylene terephthalate)s to feedstock materials, and these can be described under four main headings. Some less well-known and newer approaches have also been claimed, and these will be discussed later in this section. The four main categories are detailed below. 1. Methanolysis: Treatment of PET with methanol, under high pressure at around 200 °C, results in depolymerisation to dimethyl terephthalate (DMT) and ethylene glycol (EG). DMT is then puriﬁed by distillation and crystallisation to give a highquality intermediate which can be used to manufacture fresh PET. Once reﬁned, EG may also be used in PET production, amongst

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Chemical Degradation and Recycling other applications. Efﬁcient ﬁltration of the mix after reaction is required to remove impurities, including catalysts. 2. Glycolysis: Reaction of scrap PET with excess EG under pressure at approximately 200 °C reverses the polymerisation process to give bis(hydroxyethyl)terephthalate (BHET) and some oligomers. The BHET formed is puriﬁed by melt ﬁltration under pressure to remove physical impurities, and treated with carbon to remove chemical impurities. In most glycolysis processes known, not all contaminants can be removed, and some degradation and yellowing of the product results. Normally, such a process is best carried out adjacent to a PET polymerisation plant, where the BHET can be used at the appropriate stage in the overall polymerisation process. In some situations, the BHET is further broken down to terephthalic acid (TA) and EG. 3. Hydrolysis: Hydrolysis of PET with water, usually with acid or base catalyst, results in direct production of TA and EG. The process can be slow and produces lower yields than with other methods, and has been commercially less applied than approaches (1) and (2). 4. Saponification: Direct reaction of PET with alkali metal hydroxides is used to produce EG and, for example, disodium terephthalate (DNaTA). DNaTA may be recovered as-is or may be reacted with sulfuric acid to produce TA and sodium sulphate. If a market for the sodium sulfate is available, this can contribute to the process economics; if it is simply a waste product, it will detract from the economics because the cost of disposal will need to be included in overall process costs. Many commercial processes can utilise combinations of two or more of the above basic approaches. Several general reviews on the depolymerisation recycling of PET have been published [80–86], as well as many academic studies of a range of individual techniques as potential means to chemically

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Degradation and Stabilisation of Aromatic Polyesters recycle the aromatic polyesters. The literature may be summarised as shown below. a) Alcoholysis: Studies have been carried out on the effect of surface area on the methanolysis of powdered PET [87], and on the use of aluminium isopropoxide as a catalyst in the same reaction [88]. Keen interest has been shown in the use of supercritical methanol as a means to effect rapid methanolysis of waste aromatic polyesters, including PET [89, 90], PEN [91], PBT [92, 93] and PTT [94, 95]. Ethanolysis of PET by supercritical ethanol has also been studied [96]. b) Glycolysis: With its potential for obtaining a product which can then be directly re-polymerised in (partial) replacement for virgin feedstock, a great deal of effort has been expended on the study of glycolysis of aromatic polyester wastes [97–117]. These include glycolysis in the melt [99]; the use of various catalysts such as zinc salts [101–103], titanium compounds [110] and various non-transition metal salts [112]. Pardal and Tersac have investigated the effect of using various glycols in the reaction [114, 115] and diethylene glycol [116, 117]. Guclu and co-workers used simultaneous glycolysis and hydrolysis of PET to produce high yields of mono-2-hydroxyethyl terephthalate [118]. A ‘two-step, one pot’ reaction of glycolysis and aminolysis has been shown to produce useful chemicals from PET [119, 120] and from PEN and PBT [121]. c) Hydrolysis: This reaction has also been extensively studied under various conditions. Base-catalysed hydrolysis of PET has been carried out in solution/suspension [122–132] and in the melt [133]; studies have also been undertaken of hydrolysis followed by oxidation to produce TA and oxalic acid [134]. Acid-catalysed hydrolysis has been carried out using sulfuric acid [135–138] and nitric acid [139], and some investigations have been carried out on the use of Lewis acid catalysts such as zinc salts [140]. The use of neutral water has been investigated under various conditions, including elevated temperature and pressure [141–148]; using

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Chemical Degradation and Recycling supercritical water [149]; under ﬂuidised bed conditions [150]; and under microwave irradiation [151]. d) Saponiﬁcation: A few studies have been undertaken on this approach towards aromatic polyester depolymerisation [152– 154]. e) Miscellaneous: The use of amines to break down scrap PET into useful chemicals has been the subject of recent investigations [155, 156]. In addition to the academic literature on chemical recycling of aromatic polyesters, especially PET, there are many patents dealing with this topic. These cover four basic techniques, combinations of same, and some novel approaches. To make a survey of this area less unwieldy, the six apparent main players will be described ﬁrst, then the others discussed by the category of chemistry used in the particular process. 1. Eastman Eastman have patented a range of techniques for recovering monomers from aromatic polyesters. Initial intellectual property dealt with generalised schemes for methanolysis catalysed by zinc acetate, tin salts or titanium tetraisopropoxide [157, 158]. A major effort was made to develop a process which may be described as high-pressure methanolysis, in which superheated methanol is passed through a solution of scrap PET in oligomers of the same material [159–166]. Various reﬁnements have also been made to the basic process, including addition of trace amounts of base to the reaction to prevent formation of dioxin [167], and recovery of additional aliquots of DMT from the EG stream and puriﬁcation of the latter product [168–171]. In related intellectual property, a process is described in which the polyester is depolymerised to component ester monomers and halfesters, mixed with additional virgin monomers, and used to produce a low molecular weight polyester. This may then be further reacted

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Degradation and Stabilisation of Aromatic Polyesters to produce polyesters with suitable molecular weights for various applications [172–174]. This approach is thought to eliminate the need to separate and purify monomers such as DMT and TA. A speciﬁc apparatus for carrying out this procedure is also claimed. Amorphous (co)polyesters were chemically recycled by methanolysis or glycolysis of their solutions in N-methylpyrrolidone or dimethylsulfoxide [175]. A high temperature and pressure hydrolysis breakdown of PET has also been patented by Eastman which is said to be environmentally friendly but capable of providing conversion yields close to those associated with the usually more efﬁcient methanolysis process [176]. Further intellectual property deals with claimed glycolysis processes [177, 178]; reaction of PET with ammonium hydroxide (‘ammoniolysis’) to form diammonium terephthalate, which is easily converted to TA [179]; and a process whereby PET is reacted with acetic acid or a salt thereof to form TA and ethylene glycol diacetate [180]. Eastman also have extant intellectual property on a process speciﬁcally for the recovery of naphthalene dicarboxylic acid from poly(alkylene naphthalate)s [181]. 2. DuPont DuPont was granted an early patent [182] on glycolysis of PET, although the invention is limited to the recovery of BHET and its immediate mixing with virgin BHET for polymerisation into fresh PET. Other intellectual property covers glycolysis of PET [183], methanolysis [184–187], hydrolysis of PEN [188], and a multistage process in which a poly(alkylene terephthalate) is ﬁrst subjected to glycolysis then two hydrolysis stages to recover a maximised yield of TA [189]

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Chemical Degradation and Recycling 3. Hoechst Intellectual property assigned to Hoechst covers hydrolysis [190– 192], methanolysis [193, 194] and a method of purifying recovered EG fractions via solvent extraction in xylene [195]. The hydrolysis process is carried out in large volumes of water at high pressure and temperature, typically 4.2 MPa and 250 °C, with residence times of 2–3 hours. The methanolysis process differs from other known versions of this approach in that the polyester is ﬁrst reacted (preferably in the melt) with a transesteriﬁcation catalyst and that methanolysis is carried out on the resulting breakdown products. This is said to allow the second stage to be carried out under milder conditions. 4. Teijin This Japanese concern claims a process [196–199] in which PET waste is ﬁrst glycolised under fairly mild conditions (0.1–0.5 MPa/175 °C), and the ﬂoating residue separated and heavy residue ﬁltered off. The remaining solution is distilled to recover EG, and the leftover material subjected to methanolysis. Further EG and DMTA are recovered. The residual liquid is further distilled to recover methanol and a ﬁnal portion of EG. From the available information this process appears to be chemically and economically efﬁcient. 5. Synergistics Industries Intellectual property assigned to this company covers an alcoholysis process using higher alcohols aimed at the production of higher esters of TA for use as plasticisers from scrap PET rather than the straight recovery of monomers or other PET precursors [200–202] 6. Amoco Amoco generally takes a hydrolysis depolymerisation approach, the inventive nature of the intellectual property being conﬁned to the means by which components are recovered. This may be done by recovering TA only and oxidising other components with oxygen and

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Degradation and Stabilisation of Aromatic Polyesters a catalyst [203]; vapourisation of all recovered components and their recovery from said vapour phase [204]; or ﬂash crystallisation at set low pressures to recover products sequentially [205]. Various other inventors and industrial concerns have intellectual property relating to the chemical recycling of aromatic polyesters, and these are described below under the heading of the main chemistry employed. a. Methanolysis Vereinigte Glanstoff Fabriken AG were granted early patents on the methanolysis of polyester melts [206, 207]. The polymer is mixed with a transesteriﬁcation catalyst or sprayed therewith in ﬁnely divided molten droplets, and treated with superheated methanol in a sealed chamber. Societa Italiana Resine SpA carry out the process in the presence of acid catalysts [208]. Rhone-Poulenc use magnesium methylate as a catalyst [209]. The process claimed by Cudmore has ﬁnely divided PET slurried in aqueous methanol depolymerised at high temperature and pressure, with DMT recovery by ﬂash crystallisation [210]. b. Glycolysis Toray Industries mixed molten BHET and EG with scrap PET and depolymerised the mix to BHET at 215 °C [211]. Zimmer claimed a process in which PET is glycolised with 1,4-butanediol, and the mass then converted to PBT [212]. BASF claimed a titanium saltcatalysed glycolysis which also includes a cyclic carbonate, which is allegedly incorporated to prevent accumulation of impurities [213]. Karl Fischer GmbH described a process in which wet polyester waste is melted, resulting in partial hydrolysis, and a diol is added to this to glycolise the polymer. This is then re-polymerised to polyester in a continuous process [214]. Another process for the glycolytic conversion of scrap PET to pristine PBT was claimed by Petrecycle of Australia [215]. Most recently, Invista patented a process for glycolysis of coloured PET scrap in which the colours are removed by a combination of activated charcoal and solvent extraction to

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Chemical Degradation and Recycling produce BHET which can be used directly to manufacture colourless PET [216]. c. Hydrolysis Puszaszeri depolymerised PET via acid-catalysed hydrolysis, and claimed to recover 98% of available TA [217]. GE created macrocyclic polyester oligomers via tin or titanium salt catalysed hydrolysis [218]. Shell used base-catalysed hydrolysis, followed by oxidation of impurities to insoluble products which could then be ﬁltered off prior to isolation of ﬁnal products [219]. Sirek and Jirousek proposed a process in which PET is crystallised and crushed to a ﬁne powder, hydrolysed with steam, then ammonium hydroxide added. The TA is recovered by acid treatment of the diammonium salt, and EG recovered by distillation of the liquid component of the reaction mix [220]. d. Saponiﬁcation Michigan Technological University used ammonium hydroxide to directly form the diammonium salt of TA [221]. Schwartz used sodium hydroxide in solution, which was heated to depolymerise and distil off EG, the residue being mixed with water, ﬁltered and acidiﬁed to recover TA [222, 223]. Institut Francais de Petrole also used sodium hydroxide, but in this case in the melt, preferably in an extruder [224]. Smuda used bicarbonates rather than hydroxides [225], as has also featured in patents from Tsukishima Kikai [226, 227]. Tredi used alkali metal hydroxides in a process in which pure salt is recovered [228]. Broccatelli mixed scrap PET bottles along with metal salt such as sodium carbonate in a grinder, followed by dissolution of salts formed in this crude reaction [229, 230]. e. Miscellaneous It has been claimed that PET can be depolymerised directly with stoichiometric amounts of sulfuric acid [231] or with glacial acetic acid [232].

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Degradation and Stabilisation of Aromatic Polyesters Alternative depolymerisation methods for polyesters using biochemical methods are considered by some to potentially offer low-cost, low-energy routes towards the chemical recycling of such polymers. The main task is to identify, or produce by genetic modiﬁcation, safe micro-organisms or enzymes that will speciﬁcally target ester cleavage rather than total breakdown of the polymer to carbon dioxide and water [56, 57]. Biotechnology Forschung [233] appear to be near to this goal, claiming an ester-cleaving enzyme derived from Thermonospora fusca. Commercial exploitation of any of the methods described above will rely more on economic considerations than technical constraints. As with most polymer recycling processes, chemical recycling of aromatic polyesters has suffered over the years from severe ﬂuctuations in economics, resulting in a ‘stop–start’ effect on commercial enterprises, with some technically successful plants having to be mothballed (or even abandoned) as circumstances have changed. Tracking down who is doing what at present is difﬁcult. Combined with various takeovers, name changes and swapping of product portfolios, this makes the situation extremely complex. The information collated in this section is thus likely to be subject to change in the short and long term. Available information, in alphabetical order by company, is presented, along with web addresses (if known). Asahi: Carry out chemical recycling of PET bottles and post-industrial ﬁbre waste. A methanolysis process termed Ecosensor is used to produce DMTA and EG, and is claimed to provide high-purity products which are used in ﬁbre applications (‘bottle to ﬂeece’ system). No intellectual property has been found directly assigned to this company, which suggests that the process may be licensed from another concern. The website is www.ashai-kasei.co.jp. DuPont: PET is dissolved in DMTA, contaminants are removed and then converted to DMTA and EG by methanolysis; this process is called Petretec. A plant for carrying out this process was based

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Chemical Degradation and Recycling at Cape Fear, NC, USA. The process had to be subsidised because customers expected the price of products to be lower than those of their virgin equivalents (a common problem for plastics recyclers), so the process became economically unsustainable. The plant was dismantled in 2000 after a drop in price of virgin PET and problems with high scrap collection costs. The process, and associated intellectual property, is currently for sale/license on the technology exchange website www.yet2.com. Eastman: In the late 1990s, Eastman had a chemical recycling process called Optisys, presumably based on their own intellectual property, ready to enter pilot trial at Kingsport, TN, USA. It would appear that this did not go ahead at all or that the trial was insufﬁciently successful to lead to full-scale production. EvCo Research Incorporated: This organisation is based in Atlanta, GA, USA, with a proprietary glycolysis process for PET recycling. Recovered materials appear to be used in-house for the manufacture of coatings, binders and adhesives, rather than in the re-polymerisation of PET. The website is www.evco-research.com. Invista: This company includes the polyesters businesses originally associated with Hoechst-Celanese and later with KoSa, and parts of the DuPont product portfolio. Some chemical recycling of PET appears to be carried out, but for the manufacture of polyols for use in the synthesis of polyurethanes. The website is www.invista.com. M&G Polymers: This company is part of Gruppo Messi & Ghisolﬁ, an Italian family-owned company which is a major player in aromatic polyesters, having also acquired the Shell polyester portfolio in 2000. They are thought to have a PET recycling plant at Point Pleasant, WV, USA. This does not use Shell technology, but a process based on Renew technology, licensed from the Australian company Petrecycle Pty. The process includes glycolysis and hydrolysis. M&G Polymers may also have carried out joint development work on the process with Petrecycle Pty in the early 2000s. The website is www. gruppomg.com.

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Degradation and Stabilisation of Aromatic Polyesters Oxid: This US company produces polyester polyols from waste PET for use in polyurethane manufacture. The website is www.oxid. net. Petrecycle Pty: This is an Australian concern. They are the originators and intellectual property holder of the Petrecycle glycolysis/ hydrolysis process. The website is www.petrecycle.com. RecoPET: This is a French/Belgian company. The process of the same name was initially developed by a consortium of European companies which, at various times, included Polyphenix France, Tredi, and Institut Francais du Petrole. The process is a multistage saponiﬁcation said to be inexpensive because it uses low-cost reagents, requires relatively simple processing equipment, and does not involve excessive temperatures or pressures. The website is www.petcore.org. Recypet: This is a Swiss recycler using the HybridUnPET process from the United Resource Recovery Corporation. The website is www.cleanaway-pet.com. SKP Kunstoff Aufbereitung GmbH: This company ran, in 2002, on behalf of the German government agency Deutsche Gessellschaft fur Kunstoff Recycling, a 6000 tonnes per year bottle recycling facility in Rostock, Germany. It utilised the HybridUnPET process. The current status of this company is not known. Techniques Batiment Industrie: This is a French company recycling PET into polyester polyols for the polyurethanes industry. Teijin: This is the largest and most active Japanese chemical recycler of PET. The process ﬁrst uses methanolysis to produce DMTA then hydrolysis to convert this to TA. In the early 2000s, Teijin underwent major expansion in capacity, although the current situation is unclear. PET re-polymerised from recovered feedstock is sold under the Ecopet brand, which is said to include bottle-grade resins. They produce a range of ‘recycle-friendly’ products such as laboratory and factory wear, waterproofs, and interlocked cushioning ﬁbres, all of which are 100% polyester. Fibres may also be used in bottle-to-ﬂeece

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Chemical Degradation and Recycling applications, and may be blended with other ﬁbres for apparel use. The website is www.teijin.co.jp. Tredi: This company was initially involved with the RecoPET process, but later developed their own saponiﬁcation-based method. The process was, in 2002, ‘on hold’ pending favourable economic circumstances, but the current situation is unknown. United Resource Recovery Corporation: This outfit were the originators of the UnPET and HybridUnPET processes. The former appears to be a saponiﬁcation process. The latter is less clear, but appears to at least include an additional grinding stage to increase the efﬁciency of the overall process. The company is currently mainly involved in licensing out these processes to other recyclers. The website is www.urrc.net.

References 1.

D.A.S. Ravens, Polymer, 1960, 1, 2, 375.

2.

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Chemical Degradation and Recycling 202. L.A. DuPont and V.P. Gupta, inventors; Synergistics Industries Ltd., assignee; US5319128, 1994. 203. J.L. Broeker, J.A. Macek, A.B. Mossman, B.L. Rosen and T.M. Bartos, inventors; Amoco Corporation, assignee; US5414113, 1995. 204. F. Johnson, D.L. Sikkenga, K. Danawala and B.L. Rosen, inventors; Amoco Corporation, assignee; US5473102, 1995. 205. T.M. Bartos, B.L. Rosen and J.I. Rosenﬁeld, inventors; Amoco Corporation, assignee; US5502247, 1996. 206. E. Heisenberg, E. Siggel and R. Lutz, Vereinigte Glanzstoff Fabriken AG, US3037050, 1962. 207. R. Lutz, G. Wick and C. Neuhaus, inventors; Vereinigte Glanzstoff Fabriken AG, assignee; US3321510, 1967. 208. F. Ligorati, G. Aglietti and V. Nova, inventors; Societa Italiana Resine SpA, assignee; US3776945, 1973. 209. J. Delattre, R. Raynaud and C. Thomas, inventors; RhonePoulenc Textile, assignee; US4163860, 1979. 210. W.J.G. Cudmore, inventor; no assignee; US4578502, 1986. 211. A. Fujita, M. Sato and M. Murakami, inventors; Toray Industries, assignee; US4609680, 1986. 212. M. Kyber, W. Schmidt and U. Schollar, inventors; Zimmer AG, assignee; US5266601, 1993. 213. P.C. Kierkus and K.K. You, inventors; BASF Corporation, assignee; US5763692, 1998. 214. L. Gerking, R. Hagen and D. Taurat, inventors; Karl Fischer Industrieanlagan GmbH, assignee; US6162837, 2000.

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Degradation and Stabilisation of Aromatic Polyesters 215. S.M. West, inventor; Petrecycle Pty. Ltd., assignee; US6518322, 2003. 216. B.L. Smith, P. Nagar and G. Shaw, inventors; Invista North America SARL, assignee; US7192988, 2007. 217. S.F. Pusztaszeri, inventor; no assignee; US4355175, 1982. 218. D.J. Brunelle, G. Kailasam, J.A. Serth-Guzzo and P.R. Wilson, inventors; General Electric Co., assignee; US5668186, 1997. 219. K.L. Rollick, inventor; Shell International Research, assignee; WO9510499, 1995. 220. M. Sirek and J. Jirousek, inventors; no assignee; US6649792, 2003. 221. R.A. Lamparter, B.A. Barna and D.R. Johnsrud, inventors; Michigan Technological University, assignee; US4542239, 1985. 222. J.A. Schwartz, inventor; Partek Inc., assignee; US5395858, 1995. 223. J.A. Schwartz, inventor; United Resource Recovery Corporation, assignee; US5580905, 1996. 224. J. Benzaria, F. Dawans, B. Durif-Varambon and J. Gaillard, inventors; Institut Francais du Petrole, assignee; US5545746, 1996. 225. H. Smuda, inventor; no assignee; US6239310, 2001. 226. J. Yuzaki, K. Sakano, N. Funakoshi and K. Tanaka, inventors; Tsukishima Kikai Co. Ltd., assignee; US6580005, 2003. 227. J. Yuzaki, K. Sakano, N. Funakoshi and K. Tanaka, inventors; Tsukishima Kikai Co. Ltd., assignee; US7173150, 2007. 140

Chemical Degradation and Recycling 228. J. Thauront, L. Bonnamich and G. Freyesse, inventors; Tredi, assignee; US6657077, 2003. 229. M. Broccatelli, inventor; no assignee; US6670503, 2003. 230. M. Broccatelli, inventor; no assignee; US6720448, 2004. 231. G.E. Brown and R.C. O’Brien, inventors; Safetech Inc., assignee; US3952053, 1976. 232. H. Al Ghatta, inventor; Sinco Recherche SpA, assignee; US6562877, 2003. 233. W. Deckwer, R. Muller, I. Kleeberg and J. Van Den Heuvel, inventors; Gesellschaft fur Biotechnologische Forschung GmbH, assignee; EP1218519, 2007.

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6

Thermal and Hydrolytic Stabilisation

6.1 Introduction Aromatic polyesters, in particular poly(ethylene terephthalate) (PET), are subject to thermal degradation processes during polycondensation, melt processing and, to a lesser extent in service, in situations where oxygen is absent or subject to limited availability. As described in Chapter 2, heating of aromatic polyesters to high temperatures in the absence of oxygen can lead several consequences, as described below. a) Chain scission via a concerted non-radical process involving (mainly) a six-membered ring intermediate, resulting in increased levels of acid end groups, and in formation of unsaturated end groups. It is also thought by some authors that the polymers can undergo longer-range back-biting reactions to split off cyclic oligomers, which could be regarded to be the primary degradation products. b) Homolytic scission, i.e., formation of radical chain ends. The exact extent to which this type of reaction has a role in thermal degradation of aromatic polyesters is controversial. It is most likely that this type of reaction would provide only a signiﬁcant contribution in melt processing conditions where excessive temperature and high mechanical shear were involved. c) Chain lengthening or chain branching by reaction of existing chain ends with each other, reaction of existing chain ends

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Degradation and Stabilisation of Aromatic Polyesters with other chain-end species created during degradation, or transesteriﬁcation reactions within or between chains. d) Hydrolysis of polymer by water trapped in the melt process. e) Production of small, volatile molecules, especially relevant in PET where acetaldehyde is produced which can render products unﬁt for some applications. Most purely thermal or hydrolytic damage will take place during melt processing of an aromatic polyester, so this chapter will concentrate on the means of preventing degradation and, due to the nature of these materials as condensation polymers, on ways and means of ‘repairing’ degraded materials.

6.2 Thermal Stabilisation Thermal stabilisation of aromatic polyester melts is a goal which, on paper at least, appears to be very difﬁcult to achieve. If, as proposed by many researchers, that the main route of breakdown of the polyester chains is via concerted reaction via a cyclic intermediate, little may be achieved by using standard melt-stabilising additives. The non-radical nature of the reaction, and its likely high rate in a molten polymer under high temperature and shear conditions, provides no point of attack for conventional stabilisers. However, the potential for metal salt impurities such as catalyst residues to accelerate the chain scissions has been alluded to, and could provide a target for stabilising additives. As with virtually all organic polymers, some oxidative damage may have occurred in the polyester before the melt process under consideration. There may be hydroperoxides present which would be extremely useful to target, and to decompose to inert products by use of a melt stabiliser. If there is a contribution to degradation by homolytic scission, there is also the possibility of increasing stability by means of an additive

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Thermal and Hydrolytic Stabilisation capable of reacting with alkyl radicals, although efﬁcient species of this type are rare. It was discovered early in the development of aromatic polyesters that addition of alkyl phosphite esters or alkyl-aryl phosphite esters to the melt resulted in signiﬁcant improvements to the process stability of the polymers [1]. Since then, process/thermal stabilisers based on derivatives of phosphorus-based acids have remained the main additives used commercially for this purpose. Many additives of this type have been patented over the years, along with combinations of these with co-additives. Examples include: UÊ *Ã«>ÌiÊ ÀÊ «Ã«ÌiÊ >``i`Ê >ÌÊ ÌiÊ «ÞiÀÃ>ÌÊ ÃÌ>}i]Ê speciﬁcally to deactivate catalyst residues [2] UÊ /«Ã«>ÌiÃ]ÊiÃ«iV>ÞÊÌÀ«iÞÌ«Ã«>ÌiÊQÎR UÊ -`ÕÊ«ÞÀ«Ã«>ÌiÊQ{R UÊ /ÀÃ>Þ>>Þ®«Ã«iÊQxR UÊ Þ`>ÝÞ«Ã«ÞÊvÀ>ÌiÊQÈR UÊ *Ã«ÀÕÃÊÌÀÃVÞ>>ÌiÊÀÊ«Ã«ÀÞÊÌÀÃVÞ>>ÌiÊQÇR UÊ ÊÀÊ`«iÞ«Ã«VÊ>V`ÊQnR UÊ *iÞ`«iÝÞ«Ã«iÊQR UÊ -ÕLÃÌÌÕÌi`Ê>ÀÞÊÌ«Ã«ÌiÃÊQ£äR UÊ ÀÊ«Ã«>ÌiÊQ££R UÊ Þ>ÕÊyÕÀ«Ã«>ÌiÃÊQ£ÓR UÊ ÞÊ>`ÉÀÊ>ÀÞÊ«Ã«>ÌiÃÊQ£ÎR UÊ «ÞÝÞiÌÞii®Þ`ÀÝÞiÌÞ«Ã«>ÌiÊQ£{R UÊ *Þ«Ã«ÀVÊ>V`ÊQ£x]Ê£ÈR 145

Degradation and Stabilisation of Aromatic Polyesters UÊ *Ã«>ÌiÊÊVL>ÌÊÜÌÊ>Ê«Þ>`iÊQ£ÇR UÊ ÌÞÓÞ`ÝÞiÌÞA`iÌÞ«Ã«ÃÕVV>ÌiÊQ£nR UÊ *Ã«ÀÕÃÊ >V`Ê iÃÌiÀÃÊ ÜÌÊ >ÌÊ i>ÃÌÊ iÊ ÝÀ>iÊ ÃÕLÃÌÌÕiÌÊ Q£R UÊ iÌ>«Ã«ÀVÊ>V`ÊQÓäR UÊ /À«iÞ«Ã«ÌiÊ>`Ê£]Ói«ÝÞÎ«iÝÞ«À«>iÊQÓ£R UÊ /iÌÀ>n-alkylphosphonium acetate or halide [22] UÊ «iÞ«Ã«ÌiÃÊÀÊ«Ã«>ÌiÃÊQÓÎqÓnR UÊ >ÀLiÌÝÞiÌ>i«Ã«VÊ>V`Ê`iÌÞÊiÃÌiÀÊQÓR UÊ *Ã«ÌiÃÊÊ}iiÀ>]ÊÊÜÞÊ>À>ÌVÊ«ÞiÃÌiÀÃÊQÎäqÎÓR UÊ *Ã«ÌiÊ ÀÊ «Ã«>ÌiÊ >`Ê `i«ÝÞÊ V«Õ`Ê Ê «Þ£]{ cyclohexylenedimethylene terephthalate) [33] UÊ ``ÌÊvÊ«Ã«ÀÕÃL>Ãi`Ê>``ÌÛiÃÊÌÊ«ÞiÃÌiÀÃÊÃÞÌiÃÃi`Ê with high-activity catalysts, with addition taking place immediately before melt processing [34] UÊ *Ã«ÌiÊ ÀÊ «Ã«>ÌiÊ ÌÊ «ÀÛiÊ ÌiÊ ÜÌiiÃÃÊ vÊ poly(trimethylene terephthalate) (PTT) [35] UÊ /À>Þ«Ã«>ViÌ>ÌiÊQÎÈR UÊ iÌ>ÊÃ>ÌÃÊvÊ«Ã«ÀVÊ>V`ÊÊ*//ÊQÎÇR UÊ *Ã«ÀÕÃÊV«Õ`ÃÊÊ}iiÀ>Ê>ÃÊV>Ì>ÞÃÌÊ`i>VÌÛ>ÌÀÃÊ>`Ê >ViÌ>`iÞ`iÊÃÕ««ÀiÃÃiÀÃÊÊ* /Ê«À`ÕVÌÊQÎnq{£R UÊ *Ã«ÀÕÃÊV«Õ`ÃÊÊ}iiÀ>Ê>ÃÊ>VÀiÊÃÕ««ÀiÃÃiÀÃÊÊ*//Ê [42, 43]

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Thermal and Hydrolytic Stabilisation The probable key to the continued success of the phosphorus-based additives in aromatic polyesters is their ability to take part in various processes beneﬁcial to the non-oxidative heat stability of their host polymers. They are known hydroperoxide decomposers, and thus could safely destroy such species present in the polyester. They are, for the same reason, excellent secondary antioxidants, especially if used in conjunction with primary antioxidants such as hindered phenols, in a wide variety of polymers. Their ability to react with catalyst residues and prevent these contributing to degradation reactions of the polymer is also important. They would also appear to be capable vÊÀi>VÌ}ÊÜÌÊÌiÊ«ÞiÃÌiÀÊV>Êi`Ã]Êi>`}ÊÌÊi`V>««}q>`Ê consequent reduction of the amount of volatiles such as acetaldehyde >`Ê>VÀiqÀÊiÛiÊ«ÀÛ`}ÊV>ÊiÝÌiÃ° Several academic studies have been carried out on the use of phosphorus-based thermal stabilisers with aromatic polyesters Q{{qÈ£R°

>}Ê>`ÊVÜÀiÀÃÊQ{ÈRÊÃÌÕ`i`ÊÌiÊivviVÌÊvÊ>``}ÊÃÌ>LÃiÀÃÊ to the polymerisation process of PET synthesised from terephthalic acid using antimony catalysts. Unlike processes starting with dimethyl terephthalate, which use different catalysts, this catalyst system is not inhibited by stabilisers. The authors used triphenyl phosphate, trimethyl phosphate, diethyl-3,5-di-t-butyl-4-hydroxybenzylphosphonate (Irganox 1222; Ciba) and pentaerythritol tetrakis(3,5-di-t-butyl4-hydroxyphenylpropionate) (IrganoxÊ £ä£äÆÊ L>®°Ê /À«iÞÊ phosphate and trimethyl phosphate greatly reduced the carboxyl content of the PET produced, and appeared to also lower diethylene glycol content. The best balance of low carboxyl and high thermal stability was found using triphenyl phosphate at bä°ä{ÊÜÌ¯° iiÀÊ Q{Ç]Ê xäRÊ ÛiÃÌ}>Ìi`Ê LÃÓ]{`t-butylphenyl) pentaerythritoldiphosphite (Ultranox Ê ÈÓÈÆÊ À}7>ÀiÀ®Ê >ÃÊ >Ê process stabiliser in PET. The author stated that the stabilising effect was mainly due to trapping of catalyst residues, and that the additive could also deactivate hydroperoxides. One problem to be overcome with the use of any additive in PET is the high processing temperature,

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Degradation and Stabilisation of Aromatic Polyesters which can lead to chemical and/or physical loss of the additive. The >``ÌÛiÊÕÃi`ÊiÀi]ÊÜiÊiÌ}Ê>ÌÊ£ÇäÊc ]ÊÜ>ÃÊÃÌ>LiÊ>`Ê«iÀÃÃÌiÌÊ Ê* /Ê>ÌÊ«ÀViÃÃ}ÊÌi«iÀ>ÌÕÀiÃÊLiÌÜiiÊÓnäÊc Ê>`ÊÎÓäÊc ° >ÀÊ>`ÊVÜÀiÀÃÊQ{RÊÌi`ÊÌ>ÌÊ>À>ÌVÊ«Ã«ÌiÃÊVÕ`Ê induce chain extension in PET (for more on this process, see Section 6.4.2). The chain extension in this case was said to take place via a two-stage reaction: a) Rapid reaction of triphenylphosphite with hydroxyl chain ends, with displacement of a phenoxy group. b) Slow reaction of ‘phosphated’ chain ends with acid chain ends to form an ester bond, which extends the polymer and produces diphenylphosphite. iÌâÊQx£]ÊxÓRÊ`iÃÌÀ>Ìi`ÊÌ>ÌÊÕÃiÊvÊ«Ã«ÌiÃÊ>ÃÊ>``ÌÛiÃÊÊ the reprocessing of post-consumer scrap PET resulted in reduction of discoloration, lowered development of acid end groups, and reduced acetaldehyde formation. Particular additives investigated were tris(2,4di-t-butylphenyl)phosphite, distearylpentaerythritoldiphosphite and bis(2,4-di-t-butylphenyl)pentaerythritoldiphosphite. The use of aryl phosphites for this purpose was also examined by Solera [53] and Pfaendner and co-workers [54] Ê>Ê>ÀÌViÊÊ£nÊiÝ>}ÊiÜÊÌÀi`ÃÊÊÃÌ>LÃ}Ê>``ÌÛiÃ]Ê Solera [55] noted that, while phosphites are effective process stabilisers in PET, they can themselves be subject to hydrolytic degradation. A new phosphorus-based stabiliser with increased hydrolytic stability was suggested in the form of 2,2´,2´´-nitrilo[triethyl-tris(3,3´,5,5´tetra-t-butyl-1,1´-biphenyl-2,2´-diyl)]phosphite (Irgafos 12; Ciba). ÃÌÊ>`ÊVÜÀiÀÃÊQxÇRÊ>ÃÃiÃÃi`Ê>ÊÜ`iÊÀ>}iÊvÊ«Ã«ÌiÃÊvÀÊ their process-stabilising abilities in PET. Additives used were: >®Ê Ó]{]È/Àt-butylphenyl-2-butyl-2-ethylpropanediolphosphite L®Ê ÃÓ]{`t-butylphenyl)pentaerythritoldiphosphite

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Thermal and Hydrolytic Stabilisation c) Phenyldidecylphosphite d) Poly(dipropylene glyco)phenylphosphite e) 4,4´-IsopropylidenediphenolC12-15alcoholphosphite f) Tris(dipropylene glycol)phosphite g) Tris(2,4-di-t-butylphenyl)phosphite ®Ê ÃÓ]{`VÕÞ«iÞ®«iÌ>iÀÞÌÀÌ`«Ã«Ìi 7iÊ¼L½ÊÃÊÜiÊÜÊ>ÃÊ>ÊivviVÌÛiÊ«ÀViÃÃÊÃÌ>LÃiÀ]ÊÌiÊ>ÕÌÀÃÊ found that ‘a’ was best, possibly due to its having similar high activity, but signiﬁcantly improved hydrolysis resistance. It is also said to be more soluble in the polymer than any of the other additives. Another drawback noted for the use of phosphites in PET is the tendency to form grey discoloration in the presence of antimony catalyst residues by reduction of antimony salts to the metal by the phosphite. Once again, compound ‘a’ exhibited the lowest propensity for this problem of all the additives tested. >ÃViÌÊ>`Ê >ÃÊQxnRÊÃÌÕ`i`ÊÌiÊVÀi>ÃiÃÊÊiVÕ>ÀÊÜi}ÌÊ vÕ`ÊÊ>``}Ê«Ã«ÌiÃÊÌÊÀiVÞVi`Ê* /°Ê7iÊÌÀ>ÀÞ«Ã«ÌiÃÊ were said to be effective in this regard, the authors suggested that alkylphosphites were ineffective, and could even act as prodegredants. -i}>iÛÊ>`ÊVÜÀiÀÃÊQÈäRÊvÕ`ÊÌ>ÌÊVÀi>Ãi`ÊÌiÀ>ÊÃÌ>LÌÞÊ could be achieved by synthesising poly(butylene terephthalate) * /®Ê Ê ÌiÊ «ÀiÃiViÊ vÊ >Ê ÃÌ>LÃiÀÊ «>V>}iÊ ÜVÊ VÕ`i`Ê tris(nonylphenyl)phosphite and sodium hypophosphite.

>Û>V>ÌÊ>`ÊVÜÀiÀÃÊQÈ£RÊvÕ`ÊÌ>ÌÊÌiÊ«Ì>ÊV`ÌÃÊ vÀÊV>ÊiÝÌiÃÊÜÌÊÌÀ«iÞ«Ã«ÌiÊÜiÀiÊ£ÊÜÌ¯Ê>``ÌÊ >`Ê>Ê«ÀViÃÃ}ÊÌi«iÀ>ÌÕÀiÊvÊÓÈäÊc ÊÊÊ* /]ÊÜVÊV`ÌÃÊ

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Degradation and Stabilisation of Aromatic Polyesters resulted in maximum chain extension with minimum subsequent degradation. Two important points were discovered: a) Chemical degradation of the extended PET occurred very easily, even during storage, and was considered to be caused by by-products from the reaction between polyester and additive. Acetone extraction of the offending impurities was effective in improving storage life. b) Recycled PET was shown to be much less reactive to triphenylphosphite than virgin polyester. This was most probably due to the high levels of acid end groups in the recycled polymer. The patent literature contains a number of additives which have been said to be effective thermal stabilisers for aromatic polyesters, and which are not phosphorus-based. Early patents include a series of possibilities researched by two companies in particular: Eastman Kodak and FMC Corporation. Eastman Kodak have claimed the use of hydroquinone derivatives, such as pÌÀ«iÞÞ`ÀµÕiÊÀÊx]xALÌÞÞ`ÀµÕiÊQÈÓRÆÊ gentisic acid derivatives such as gentishydrazine or gentisaldehyde QÈÎRÆÊpÞ`ÀÝÞ>âLiâiiÊQÈ{R]ÊÓ]{]xÌÀÞ`ÀÝÞ«iiÃÊvÊv>ÌÌÞÊ >V`ÃÊQÈxRÆÊ>ÊV`iÃ>ÌÊ«À`ÕVÌÊvÊÞ`ÀµÕiÊ>`Ê>ÞÊ>VÊ QÈÈRÆÊ >ÞÊ }>>ÌiÃÊ QÈÇRÆÊ }iÌÃVÊ >V`Ê `iÀÛ>ÌÛiÃÊ Ài>VÌi`Ê ÌÊ ÌiÊ «ÞiÃÌiÀÊV>ÊQÈnRÆÊy>Û>ÌÀiÊQÈRÆÊ>`ÊvÕÀ>iÊ`iÀÛ>ÌÛiÃÊÃÕVÊ as 5-benzylidene-4-methyl-3-pÌÀ«iÞÓx®vÕÀ>iÊQÇäR° FMC Corporation have claimed an even wider range of potential thermal ÃÌ>LÃiÀÃ]Ê VÕ`}Ê «iÞÊ ÃÕvÝ`iÃÊ QÇ£RÆÊ >«>ÌVÊ ÀÌÃV>ÌiÃÊ QÇÓRÆÊ Þ`ÀÝÞV>ÀLÞÌÀ>ÝÞÃ>iÃÊ QÇÎRÆÊ >Liâ«iiÃÊ QÇ{RÆÊ ÃÌi>À>`iÊ QÇxRÆÊ ÃÕLÃÌÌÕÌi`Ê >>iÊ >`Ê >VÃÊ ÃÕVÊ >ÃÊ `«iÞiÌ>iÊ ÀÊ LiâÞ«iÞV>ÀLÊ QÇÈRÆÊ `ÃÕw`iÃÊ ÃÕVÊ >ÃÊ «iÞ`ÃÕw`iÊÀÊiÝÞ`ÃÕw`iÊQÇÇRÆÊÃÕviÃÊÃÕVÊ>ÃÊÊ`iÝÞÃÕviÊ or di-pÌÞÃÕviÊ QÇnRÆÊ ÕÀi>ÃÊ ÀÊ ÌÕÀi>ÃÊ QÇRÆÊ >iÃÊ ÃÕVÊ >ÃÊ didoceylamine or tri-niÝÞ>iÊ QnäRÆÊ >`Ê >À>ÌVÊ ÃVÞ>>ÌiÃÊ Qn£R

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Thermal and Hydrolytic Stabilisation Many other companies have patented additives which they claim to be useful in preventing thermal degradation of polyesters, and are discussed below. /À>ÞÊ `ÕÃÌÀiÃÊ QnÓRÊ ÕÌÃi`Ê >Ê À>}iÊ vÊ ÌÀ}iVÌ>}Ê compounds, especially amides and imides. Preferred additives were benzylphthalimide, terephthaldiamide and phenacetin. They were recommended to be added at the polymerisation stage and incorporated into the polyester chain. ÕÊ *ÌÊ Ê QnÎRÊ `iÃVÀLi`Ê ÌiÊ ÕÃiÊ vÊ Þ`ÀÝÞÃÕLÃÌÌÕÌi`Ê heterocyclic compounds as heat stabilisers, including 2,2,4-trimethylÈÞ`ÀÝÞÇt-butylchroman. i`Ê iV>Ê À«À>ÌÊ Qn{RÊ V>Ê ¼VÌÀ½Ê ÛiÀÊ ÌiÊ carboxylic acid end content (see Section 6.3) of PET by adding xäqxääÊ ««Ê `VÀ`Ó«ÞÀ`Þ>i®V««iÀ®Ê ÀÊ LÃ`Ó piridylamine)copper(II)chloride to the melt after completion of the polycondensation.

i>iÃiÊ À«À>ÌÊQnxRÊV>i`ÊÌÊÜ>ÃÊ«ÃÃLiÊÌÊi>ÌÊÃÌ>LÃiÊ wi`Ê*//ÊÀÊ* /ÊÜÌÊÜÊiÛiÃÊä°Óxq£ÊÜÌ¯®ÊvÊ>Ê«Þ>`iÊÃÕVÊ >ÃÊ ÞÊÈ]Ê ÞÊÈ]ÈÊÀÊ ÞÊÈ]£ä° iiÀ>Ê iVÌÀVÊ À«À>ÌÊQnÈRÊvÕ`ÊÌ>ÌÊÌiÊ>``ÌÊvÊÃ>Ê >ÕÌÃÊvÊ>Ê«ÞLÕÌ>`ii]ÊÜÌÊÛÞÊVÌiÌÊÛiÀÊnä¯ÊÀiÃÕÌi`Ê in substantial melt stabilisation of linear aromatic polyesters such as PET. LiÀÊ`ÕÃÌÀiÃÊVÀ«À>Ìi`ÊQnÇRÊÌi`Ê>Ê>Ài`ÊÀi`ÕVÌÊÊÌiÊ levels of diethylene glycol units in PET when the carboxylic acid salt of a quaternary ammonium base was added to the polymerisation reaction. *Þ«>ÃÌVÃÊ QnnRÊ >``i`Ê Þ`>ÌÊ ÌÊ * /]Ê *//Ê ÀÊ * /Ê vÀÊ ÌiÊ purpose of reducing evolution of terephthalic acid vapour during processing, thus preventing deposits of this material building up in processing equipment.

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Degradation and Stabilisation of Aromatic Polyesters

Ã>ÊQnRÊ>`Ê L>i}ÞÊQäRÊV>ÊÌiÀ>ÊÃÌ>LÃiÀÃÊvÀÊ>À>ÌVÊ polyesters of the dioxasilepin and dioxasilocin type. These are similar in structure to the Ciba-Geigy biphenyl phosphites, with silicon replacing phosphorus.

i>iÃiÊ À«À>ÌÊQ£qÎRÊÃÕ}}iÃÌi`ÊÃiÛiÀ>Ê>``ÌÛiÃÊÃ«iVwV>ÞÊ targeted at thermal stabilisation of wholly aromatic polyesters, including dimercaptothiadiazoles, 2-mercaptobenzothiazoles, and phenylene oxide oligomers. Õ*ÌÊ Q{RÊ LÃiÀÛi`Ê Ì>ÌÊ ÌiÊ >``ÌÊ vÊ £xqÎäääÊ ««Ê >>Ê metal, alkaline earth, magnesium or calcium salts, preferably at the polymerisation stage, resulted in improved heat resistance in liquid crystalline polyesters. Õi>Ì>Ê/ÕÊÊQxqnRÊi>ÌÊÃÌ>LÃi`Ê«ÞiÃÌiÀÃÊÜÌÊÛ>ÀÕÃÊ formulations composed of tannins, or derivatives thereof. <iÀÊ Ê Q]Ê £ääRÊ V>Ê Ì>ÌÊ *//Ê V>Ê LiÊ i>ÌÊ ÃÌ>LÃi`Ê by contacting the polymer with ‘a melt unstable, organic nitrogen-containing stabilising compound’ e.g., 1-aminoethanol, triacetonediamine, 4-aminosalicyclic acid, or ‘a melt stable alcoholate’ such as sorbitol or polyvinyl alcohol.

V> >Ê Q£ä£RÊ `iÃVÀLiÊ >Ê «ÀViÃÃÊ vÀÊ Ài`ÕV}Ê >ViÌ>`iÞ`iÊ evolution from PET by adding a hydrogen supplier, preferably poly(methylhydro)siloxane and a hydrogenation catalyst such as nickel or palladium. The additives are slurried in oil, coated on PET pellets, and the resultant material extruded.

L>Ê-«iV>ÌÞÊ iV>ÃÊQ£äÓRÊÃÕ}}iÃÌÊV®«ÞiÀÃÊvÊÓ«À«iVÊ acid esters as aldehyde-reducing additives, especially poly(glycidyl methacrylate). The above survey of intellectual property on thermal stabilisation of aromatic polyesters shows that a great deal of effort has been expended in trying to ﬁnd suitable additives, but few (if any) of the above non-phosphorus based approaches have been put into commercial practice. 152

Thermal and Hydrolytic Stabilisation As is often the case with patents, little information is provided on any proposed mechanism for the stabilising action of these additives, although it is very likely that many of them will act in some way to control or suppress reactions at chain ends. A more focused discussion of this aspect of polyester stabilisation, along with examples of additives specifically targeted towards this aspect of polyester chemistry, is given in Section 6.3. ,iViÌÊ ÃÌÕ`iÃÊ >ÛiÊ ÃÕ}}iÃÌi`Ê Q£äÎ]Ê £ä{RÊ Ì>Ê «ÞiÌÞiiÊ naphthalate) (PEN) and liquid crystalline polyesters containing carbon nanotubes surface-modiﬁed with carboxylic acid groups not only exhibit increased strength and modulus, but may also exhibit ÃÕLÃÌ>Ì>ÞÊ«ÀÛi`ÊÌiÀ>ÊÃÌ>LÌÞ°Ê"ÌiÀÊÜÀiÀÃÊQ£äxRÊ`iÞÊ this. Further investigations are required to clarify this problem.

6.3 End-capping As discussed in Chapter 2, the chain ends of polyester molecules exert VÃ`iÀ>LiÊyÕiViÊÊÌiÊÌiÀ>ÊÃÌ>LÌÞÊvÊÌiÃiÊ«ÞiÀÃ°ÊÊ particular, carboxylic acid chain ends which are present in the assynthesised polymer and which build up with thermal degradation and hydrolytic reactions, can severely restrict the stability of the polymer. It is therefore unsurprising that a great deal of effort has been put into ﬁnding additives and processes which can control or eliminate such moieties. Additives of this type are known as ‘end-cappers’, although some are also referred to as ‘hydrolytic stabilisers’. An allegedly very useful type of additive for this purpose is a carbodiimide, i.e., a substance of the general formula R-N=C=N-R Q£äÈ£ÓÓR° ÕÊ Q£äÈRÊ Ü>ÃÊ vÊ ÌiÊ «Ê Ì>ÌÊ Ì>ÌÊ vÊ «ÞÀÞÃÃÊ vÊ * /Ê was associated with an ionic process involving an acid-catalysed hydrolysis-type reaction, and that the best approach to stabilising the polymer against damage during processing would be to inhibit this reaction. Using a carbodiimide formulation from Mobay Corporation Q£äq£££R]Ê iÊ vÕ`Ê Ì>ÌÊ >Ê ä°xÊ ÜÌ¯Ê >``ÌÊ Ü>ÃÊ ÃÕvwViÌÊ ÌÊ 153

Degradation and Stabilisation of Aromatic Polyesters produce a useful effect. It was observed that the temperature at which ÌiÊ`i}À>`>ÌÊÀ>ÌiÊÀi>Vi`Ê>Ê>ÝÕÊÜ>ÃÊVÀi>Ãi`ÊLÞÊÓäÊc ]Ê and examination of evolved gases showed that the onset and amount of tetrahydrofuran and carbon dioxide produced were markedly changed. The timing and rate of the later butadiene evolution was not changed signiﬁcantly. It was concluded that carbodiimide affected the initial reaction, but that further stabilisation would be needed once thermal degradation was well established. /ÌiÊQ£äÇRÊÌi`ÊÌ>ÌÊV>ÀL``iÃÊ>ÀiÊ}ÞÊÀi>VÌÛiÊÌÜ>À`ÃÊÌiÊ carboxylic acid end groups of PET. The reaction was said to result in the formation of an intermediate adduct which, after rearrangement, leads to amidation of the acid group and the formation of an isocyanate. One caveat was given: liberation of isocyanates from the melt could be a serious drawback to the use of low molecular weight carbodiimides. It was thus suggested that polymeric carbodiimides would be a better option for this reason. For best results, it was suggested that an excess of the additive should be incorporated so that the acid ends formed during degradation of the polymer matrix could be dealt with. Versions of carbodiimides patented up to the present time include «ÞV>ÀL``iÃÊQ£ä]Ê££Ç]Ê££n]Ê£ÓäR]Ê>ÀÞÊV>ÀL``iÃÊQ££ä ££xR]Ê >VÞVVÊ V>ÀL``iÃÊ Q££ä]Ê £££]Ê ££]Ê £Ó£RÊ >`Ê Li`ÃÊ vÊ carbodiimides and polycarbodiimides [122]. Various additives of this class are commercially available. The use of cyclic organic carbonates, especially ethylene carbonate, as end-cappers/hydrolysis stabilisers in aromatic polyesters has been «>ÌiÌi`Ê Ê Û>ÀÕÃÊ vÀÃÊ LÞÊ >Ì>Ê -Ì>ÀVÊ Q£ÓÎq£ÓxR]Ê i`Ê

iV>ÊQ£ÓÈ]Ê£ÓÇRÊ>`Ê,`>ÊQ£ÓnR° Formulations by National Starch also include antioxidant species, including phosphorus-based [123], hindered phenols [124] or aromatic amines [125]. Allied Chemical use ethylene carbonate along with an alkali metal iodide, whereas Rhodia enhance the reactivity of the ethylene carbonate by including an alkylphosphonium salt as

154

Thermal and Hydrolytic Stabilisation a catalyst. The reaction of ethylene carbonate with the acid chain end would appear to result in the creation of a hydroxyethyl ester, and the loss of carbon dioxide.

«ÝÞÊ}ÀÕ«VÌ>}ÊÃ«iViÃÊ>ÛiÊLiiÊÛiÃÌ}>Ìi`ÊQ£äÇ]Ê£ÓRÊ >`Ê>ÛiÊLiiÊÌiÊÃÕLiVÌÊvÊ«>ÌiÌ}Ê>VÌÛÌÞÊQ£Îäq£{£RÊ>ÃÊi` cappers in aromatic polyesters. End-capping of target carboxylic acid groups by epoxy moieties results in opening of the oxirane ring and formation of a hydroxyalkyl ester. Epoxides are relatively unreactive towards carboxylic acids, and thus catalysts may be required to drive ÌiÊÀi>VÌÊvÀÜ>À`ÊQ£Îä]Ê£ÎÈR Allied Chemical have patented the use of monoepoxides with various substituent groups such as amides or esters [131, 132], imides [133] and aromatic species [134]. Taiwanese researchers used ethylenestyrene copolymers with epoxy substituents [135]. Most recently, a ÕLiÀÊvÊV«>iÃ]ÊVÕ`}Ê -ÊQ£ÎÇ]Ê£{£RÊ>`ÊÌÃÕLÃÊ Q£Înq£{äRÊ ÕÃi`Ê i«Ý`Ãi`Ê >ÌÕÀ>Ê ÃÊ >ÃÊ ÃÌ>LÃiÀÃÊ Ê >À>ÌVÊ polyesters. Ê ÃÞÃÌi>ÌVÊ ÃÌÕ`ÞÊ vÊ V>ÀLÝÞVÊ i`Ê }ÀÕ«Ê V>««}Ê vÊ * /Ê Ê ÃÕ«iÀVÀÌV>ÊV>ÀLÊ`Ý`iÊQ£ÓRÊ>ÃÊV>i`ÊÌÊ«ÀÛ`iÊ>Ê«ÞiÃÌiÀÊ with marked improvements in hydrolytic and process stability. Other end-capping approaches have been tried, including 5-hydroxyisophthalic acid to reduce acetaldehyde emissions [142]; benzoyl lactams or benzoylphthalimides as general-purpose endcappers [143]; 2-oxazolines, preferably with fatty acid substituents to control volatility [144]; and end-capping of PTT with a hindered phenolic acid to reduce acrolein emissions during processing. 6>Ê>`ÊVÜÀiÀÃÊQ£{ÈRÊiÝ>i`Ê>ÊÜ`iÊÛ>ÀiÌÞÊvÊÃ«iViÃÊvÀÊ potential use as hydroxyl reactive end-cappers suitable for inclusion in PET to minimise acetaldehyde contamination of products. The purpose behind the study was to be able to use cheaper grades of PET for beverage bottles rather than the more expensive solid state post-condensation (SSP)-treated grades normally used (see Section 6.4.1®°Ê iÃÌÊ ÀiÃÕÌÃ]Ê ÜÌÊ ÌiÊ >``ÌÛiÃÊ ÃiiVÌi`]Ê ÜiÀiÊ

155

Degradation and Stabilisation of Aromatic Polyesters obtained with 4-aminobenzoic acid, 3,5-dihydroxybenzoic acid and 4-hydroxybenzoic acid. Good results were obtained under laboratory conditions, but additives were signiﬁcantly less effective under processing conditions.

6.4 Chain Extension 6.4.1 Without Additives In the polycondensation reaction undergone by polyesters, a point is reached at which the molecular weight achievable in the reaction kettle will reach a plateau value. Any increase in molecular weight over this value will require implementation of a different approach. In a similar way, polyesters which are to be recycled, whether as in-house or as post-consumer scrap, may require other means to convert them into material suitable for further processing into high added-value products. Attempts have been made to increase the molecular weight of post-consumer PET scrap by extrusion with very high degrees of iÌÊ `i}>ÃÃ}Ê Q£{ÇR]Ê LÕÌÊ ÃÕVÊ >ÌÌi«ÌÃÊ >ÀiÊ }iiÀ>ÞÊ `i`Ê ÌÊ failure by the increased thermal and mechanical stress placed on the polymer by such severe processing, and by the presence of impurities, «>ÀÌVÕ>ÀÞÊÜ>ÌiÀÊQ£{nRÊ>`Ê«ÞÛÞÊVÀ`iÊQ£{R° Known for some time and utilised extensively, especially for the creation of acetaldehyde-free, high molecular weight, polyesters for use in food and drink packaging application, the favoured route towards chain extension without the use of additives is via SSP. The «ÀViÃÃÊ>ÃÊLiiÊÕÃi`ÊvÀÊÛÀ}Ê* /ÊQ£xäq£xÇR]Ê«ÃÌVÃÕiÀÊÃVÀ>«Ê * /ÊQ£xn]Ê£xR]Ê*//ÊQ£ÈäRÊ>`Ê* ÊQ£È£R° Ê--*]ÊÌiÊÀiÃÊÃÊi>Ìi`ÊÌÊ>LÕÌÊÓäÊc ÊLiÜÊÌÃÊiÌ}Ê«Ì]Ê during which the polycondensation continues but at a lower rate than in the melt. Side reactions continue as well, but at a lower rate than the polycondensation reactions.

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Thermal and Hydrolytic Stabilisation The process is used extensively for the production of yarns and ﬁbres; monoﬁlaments and strapping; for recycling of scrap polyesters; in production of PET containers and soft-drink bottles; engineering }À>`iÃÊvÊ* /Ê>`Ê* /ÊvÀÊiiVÌÀV>]Êi}iiÀ}Ê>`Ê>ÕÌÌÛiÊ applications. The process gives satisfactory results but has several disadvantages: a) High equipment costs. b) High energy consumption. V®Ê /iÊVÃÕ}]ÊÜÌÊÌiÃV>iÃÊÀ>}}ÊLiÌÜiiÊ£äÊÕÀÃÊ>`Ê >ÃÊ}Ê>ÃÊxäÊÕÀÃÊÌÊ>ViÛiÊÌiÊÀiµÕÀi`ÊiVÕ>ÀÊÜi}Ì° d) Many versions of the process, especially batch processing in tumbling reactors, can produce ﬁnes which vary in molecular weight from the pellets, and which need to be separated out before melt processing into fabricated articles.

6.4.2 Chain Extenders Following on from the need for increasing the polyester chain length for several important applications, and the discussion undertaken in Section 6.3, there is the possibility of using difunctional species as means of assembling longer chains by reacting each functionality with a different polymer chain. It is also possible to envisage further branched structures through polyfunctional species. Several studies have been carried out on the chain extension of aromatic polyesters through the use of various additive species over ÌiÊ>ÃÌÊÓäqÎäÊÞi>ÀÃÊQ£ÈÓq£äRÆÊ>ÞÊ«>ÌiÌÃÊ>ÛiÊ>ÃÊLiiÊ>««i`Ê vÀÊÊÌÃÊ>Ài>ÊQ££qÓ£äR° The basic concept for a chain extender is a molecule which is polyfunctional (preferably difunctional), and where functionalities

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Degradation and Stabilisation of Aromatic Polyesters can react readily with carboxylic acid and/or hydroxyl chain ends of the host polyester. Selection of a suitable chain extender requires certain properties to be considered: a) The additive should be thermally stable at any required processing temperature. b) The reaction between extender and polymer should take place rapidly but controllably under the conditions of incorporation. c) Reaction should be essentially irreversible under reaction conditions and under likely conditions to which a product made will be subjected. d) Reaction should preferably produce no small-molecule side products. e) If small-molecule side products are produced, these should not induce unwanted side reactions during the processing or in subsequent use, and they should be capable of facile removal from the system by simple processes. f) The new linkage formed should be thermally, photolytically and oxidatively stable; should not interfere with the orientation or crystallisation of the product; and preferably should not contain moieties subject to further unwanted reactions. Early attempts at chain extension took the route of using esters of dicarboxylic acids which had greater reactivity towards the polyester chain ends than simplistic additives such as dimethyl terephthalate Q££]Ê£Î]Ê£{R]ÊLÕÌÊ>ÞÊvÊÌiÊÀiÊÀi>VÌÛiÊÃ«iViÃÊ}>ÛiÊ volatile small-molecule by-products such as phenol, which were `vwVÕÌÊÌÊÀiÛi°ÊÌiÀÊi>ÀÞÊ>ÌÌi«ÌÊQ£ÓRÊÕÃi`Ê`ÃVÞ>>ÌiÃ]Ê but this approach can give undesirable branching, and the new linkage formed was thermally unstable. Later studies used diisocyanates to V>iÝÌi`ÊÀiVÞVi`Ê* /ÊQ£ÇnR°

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Thermal and Hydrolytic Stabilisation -ViÌÃÌÃÊ>ÌÊ/iÊÌ`°]Ê«>ÌiÌi`ÊQ£È]Ê£ÇRÊ>`Ê«ÕLÃi`ÊiÝÌiÃÛiÊ ÃÌÕ`iÃÊÊQ£ÈÓq£ÈÇRÊÌiÊÕÃiÊvÊLÃVÞVVÊÊiÌiÀÃ®Ê>`ÊLÃVÞVVÊ imino esters) as chain extenders for PET.

ÞVÊÊiÌiÀÃÊÃÕVÊ>ÃÊÓ]ÓALÃÓÝ>âi®Ê "<®ÊÀi>VÌÊÜÌÊ carboxylic acid chain ends without production of a leaving group: "<ÊÊÊ³ÊÊÊÓÊH "2H ‡ÊÊH "2CH2CH2NH(C=O)(C=O)NHCH2CH2O2 H Molecular weight distribution and melting point data for treated PET `V>Ìi`ÊÌ>ÌÊ "<ÊÀi>VÌi`ÊÌÊvÀÊi>ÀÊV>iÝÌi`i`Ê«ÞiÀÃÊ without branching. The authors also proposed that, under thermoÝ`>ÌÛiÊ V`ÌÃ]Ê ÕÀi>VÌi`Ê "<Ê Ài>}Ê Ê ÌiÊ «ÞiÀÊ could act as a stabiliser, reacting with carboxylic acid end groups and preventing decrease in molecular weight, even in the solid state. Conversely, under thermal degradation conditions, i.e., during melt processing, the linkage units formed are less stable than polyester units. *ÞiÃÌiÀÊ ÌÀi>Ìi`Ê ÜÌÊ "<Ê >ÃÊ ÜÊ V>ÀLÝÞVÊ >V`Ê VÌiÌ]Ê >ÃÊ might be expected, and considerable improvement in the hydrolytic ÃÌ>LÌÞÊÜ>ÃÊLÃiÀÛi`°ÊÌÊÜ>ÃÊÌi`ÊÌ>ÌÊÕÀi>VÌi`Ê "<Ê>`ÛiÀÃiÞÊ >vviVÌi`ÊÞ`ÀÞÌVÊÃÌ>LÌÞ]Ê>ÌÕ}ÊÌiÊivviVÌÊÜ>ÃÊÞÊ>LÕÌÊ£Ó¯Ê as powerful as that of the carboxylic acid groups.

ÞVVÊÊiÃÌiÀÃÊÃÕVÊ>ÃÊÓ]ÓALÃ{Î]£LiâÝ>â{i®Ê <®Ê chain extend polyesters by reacting with hydroxyl end groups, again with no small-molecule by-products. This chain extended PET has a similar molecular weight distribution and slightly lower melting point, indicating some degree of chain extension. The new linkages >««i>ÀÊÌÊiÝiÀÌÊÌÌiÊivviVÌÊÊÌiÀ>ÊÀÊÞ`ÀÞÌVÊÃÌ>LÌÞ°Ê7iÊ ÕÀi>VÌi`Ê <Ê V>Ê >VÌÊ >ÃÊ >Ê ÌiÀ>Ê ÃÌ>LÃiÀ]Ê ÌÊ >ÃÊ >VÌÃÊ >ÃÊ >Ê hydrolysis promoter, with effectiveness similar to that of carboxylic acid end groups.

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Degradation and Stabilisation of Aromatic Polyesters

>À`Ê>`ÊVÜÀiÀÃÊQ£ÈnRÊÕÌÃi`Ê "<ÊÌÊV>iÝÌi`ÊÀiVÞVi`Ê * /°Ê 7iÊ ÀiÃÕÌÃÊ ÃÜi`Ê ÃiÊ Ì}>ÌÊ vÊ ÌiÀ>Ê >`Ê hydrolytic degradation caused by impurities in the scrap feedstock, any meaningful degree of improvement required excessive levels of additive, which resulted in accumulation of unwanted side reactions. Other additives of this type which have been investigated are Ó]ÓA£]Î«iÞii®LÃÓÝ>âi®ÊQÓäxRÊ>`ÊÓ]ÓA£]{«iÞii® LÃÊÓÝ>âi®ÊQ£ÇxR°Ê/iÊ>ÌÌiÀÊÜ>ÃÊ«>ÀÌVÕ>ÀÞÊÃÕVViÃÃvÕÊ`ÕiÊ ÌÊÌÃÊ}iÀÊÀi>VÌÛÌÞÊÌÜ>À`ÃÊ>V`ÃÊÌ>Ê "<]ÊLÕÌÊÕÃivÕÊVÀi>ÃiÃÊ in molecular weight appear to have required supplementary chain extenders of different chemistries. Another approach which has been taken by many researchers for chain extension of polyesters is the use of di or poly-epoxy V«Õ`Ã°Ê i«Ý`iÃ]ÊÕÃÕ>ÞÊvÊiVÕ>ÀÊÜi}ÌÊLiÌÜiiÊ£ääÊ >`Ê£ÓääÊ>ÛiÊLiiÊÕÃi`]ÊvÌiÊÜÌÊV>Ì>ÞÃÌÃÊÃÕVÊ>ÃÊÌiÌÀ>Ã`ÕÊ iÌÞii`>iÌiÌÀ>>ViÌ>ÌiÊQ£xR]Ê>>ÊiÌ>Ê>`iÃÊQÓääR]ÊÀÊ>>Ê iÌ>ÊLÀ>ÌiÃÊQÓäÇR°ÊN,N´-bis(glycidyl ester)pyromellitimides have LiiÊÛiÃÌ}>Ìi`ÊQ£Çä]Ê£Ç£R]Ê>ÃÊ>ÛiÊ`}ÞV`ÞÌiÌÀ>Þ`À«Ì>>ÌiÊ Q£ÇÓR]Ê`ÊÀÊ«ÞÃÕLÃÌÌÕÌi`Ê`«iÞ«À«>iÊ>`ÊÀi>Ìi`ÊÃÕLÃÌ>ViÃÊ QÓäxR°Ê ÌÊ >ÃÊ >ÃÊ LiiÊ ÃÕ}}iÃÌi`Ê Ì>Ì]Ê vÀÊ i>ÃiÊ vÊ >`}Ê >`Ê addition, such compounds may be combined with a titanium dioxide V>ÀÀiÀÊQÓäR° >ViÊ QÓäÈRÊ ÕÃi`Ê >Ê V>Ê iÝÌi`iÀÉLÀ>ViÀÊ L>Ãi`Ê Ê >Ê >VÌiÊ oligomer with cyclohexane rings at each end. Each cyclohexane has two oxirane attached thereto. This is commercially available as CelloxideÊÓän£° -ÊÊQÓänRÊ«À«Ãi`Ê}ÞV`ÞÊiÌ>VÀÞ>ÌiÊV«ÞiÀÃÊ>ÃÊ>ÊV>Ê extending additive. A particular version of this approach is Joncryl ,Ê {ÎÈn]Ê >Ê ÃÌÞÀii}ÞV`ÞÊ iÌ>VÀÞ>ÌiÊ V«ÞiÀ°Ê /ÃÊ >ÃÊ appears to be the active ingredient in CESA-Extend, a masterbatch for chain extension of recycled polyesters jointly developed by

>À>ÌÊ>`ÊÃÊ*ÞiÀÊQ£n£]Ê£nÈR°

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Thermal and Hydrolytic Stabilisation >À>>L>«ÕÃÊ >`Ê VÜÀiÀÃÊ Q£ÇÎRÊ V>ÀÀi`Ê ÕÌÊ >Ê ÃÌÕ`ÞÊ vÊ PET chain extension using various bis-epoxides. Additives studied were: UÊ LÃÎ]{i«ÝÞVÞViÝÞiÌÞ®>`«>Ìi UÊ Î]{i«ÝÞVÞViÝÞiÌÞÎ]{i«ÝÞVÞViÝÞV>ÀLÝÞ>Ìi UÊ £]{VÞViÝ>i`iÌ>Ê`}ÞV`ÞÊiÌiÀ UÊ £]{LÕÌ>i`Ê`}ÞV`ÞÊiÌiÀ UÊ LÃ«iÊÊ`}ÞV`ÞÊiÌiÀ The main aim of this study was to assess the utility of these relatively cheap, readily available diepoxides. Authors admitted that all were less effective chain extenders than custom-made diepoxides and the bisoxazolines. The cyclic diepoxides appeared more effective than the glycidyl ethers. Low extender concentration and short reaction times favoured the extension by reducing thermal decomposition, and prior drying was necessary to prevent hydrolysis. Resulting polymers showed intrinsic viscosity higher than processed PET, and in some cases equal to that of virgin PET. In addition, carboxylic acid end group levels were reduced to values well below those of virgin PET. Ê >Ê >ÌiÀÊ ÃÌÕ`ÞÊ LÞÊ >Û>>ÀÊ >`Ê 8>ÌÃÊ Q£näR]Ê ÌiÊ >ÕÌÀÃÊ investigated di- and tri-functional additives. The diepoxides used were the commercially oriented additives, diglycidyl ether of bisphenol Ê ÜÌÊ iVÕ>ÀÊ Üi}ÌÊ vÊ >««ÀÝ>ÌiÞÊ ÎxäÊ «Ê nÓn®Ê >`Ê N,N´-bis[3(carbo-2´,3´-epoxypropoxy)phenyl]pyromellitimide. Such additives have long been considered to be suitable chain extenders, with their ability to react with carboxyl and hydroxyl end groups:

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References 1.

°°Ê,ÌÀVÊ>`Ê,°°Ê }i]ÊÛiÌÀÃÆÊ/iÊ,iÃÕÃÊ *À`ÕVÌÃÊ>`Ê iV>Ê °]Ê>ÃÃ}iiÆÊ1-Ó{ÎÇä{È]Ê£{n°

2.

° °ÊLÀÃÊ>`Ê ° °Ê-À}]ÊÛiÌÀÃÆÊ Õ*ÌÊ`iÊ iÕÀÃÊEÊ °]Ê>ÃÃ}iiÆÊ1-ÓÓ£äx£]Ê£Èä°

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° °Ê*ÀiÛÀÃi]ÊÛiÌÀÆÊ/iÊ`Þi>ÀÊ/ÀiÊEÊ,ÕLLiÀÊ °]Ê >ÃÃ}iiÆÊ1-ÎÎää{{ä]Ê£ÈÇ°

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7°°ÊiÀ}iÀÌiÀ]ÊÛiÌÀÆÊÀiÃÌiÊ/ÀiÊEÊ,ÕLLiÀÊ °]Ê >ÃÃ}iiÆÊ1-Î{ÇÎ£]Ê£È°

5.

M.J. Stewart and J.A. Price, inventors; FMC Corporation, >ÃÃ}iiÆÊ1-Î{È£ÎÇ]Ê£Çä°

È°Ê

M.J. Stewart and J.A. Price, inventors; FMC Corporation, >ÃÃ}iiÆÊ1-ÎxÎÎx]Ê£Çä°

Ç°Ê

M.J. Stewart and J.A. Price, inventors; FMC Corporation, >ÃÃ}iiÆÊ1-ÎxÎnä{x]Ê£Çä°

n°Ê

M.J. Stewart and J.A. Price, inventors; FMC Corporation, >ÃÃ}iiÆÊ1-ÎxÇÇÎn£]Ê£Ç£°

°Ê

°Ê*â]Ê°ÊiÃiÊ>`Ê °Ê7iÀiÀ]ÊÛiÌÀÃÆÊ>iÊ]Ê >ÃÃ}iiÆÊ1-ÎÈä££n]Ê£Ç£°

£ä°Ê °Ê*â]Ê°ÊiÃiÊ>`Ê °Ê7iÀiÀ]ÊÛiÌÀÃÆÊ>iÊ]Ê >ÃÃ}iiÆÊ1-ÎÈä££]Ê£Ç£° 11. M.J. Stewart and O.K. Carlsson, inventors; FMC

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Degradation and Stabilisation of Aromatic Polyesters 12. J.A. Price and M.J. Stewart, inventors; FMC Corporation, >ÃÃ}iiÆÊ1-ÎÈÈ£n{Ó]Ê£ÇÓ° 13. A. Piirma, inventor; The Goodyear Tire & Rubber Co., >ÃÃ}iiÆÊ1-ÎÈÇÈÎÎ]Ê£ÇÓ° 14. °°Ê À>ÕÃÌi]ÊÛiÌÀÆÊ i>iÃiÊ À«À>Ì]Ê>ÃÃ}iiÆÊ 1-ÎÇn{xäÇ]Ê£Ç{° 15. °7°Ê9>ÜÃÞ]ÊÛiÌÀÆÊLiÀÊ`ÕÃÌÀiÃÊV°]Ê>ÃÃ}iiÆÊ 1-ÎnÎÓÇÓ]Ê£Ç{° £È°Ê °7°Ê9>ÜÃÞ]ÊÛiÌÀÆÊLiÀÊ`ÕÃÌÀiÃÊV°]Ê>ÃÃ}iiÆÊ 1-Î£ÓnÓ]Ê£Çx° £Ç°Ê °-°Ê>]ÊÛiÌÀÆÊ i>iÃiÊ À«]Ê>ÃÃ}iiÆÊ1-{ään£]Ê £ÇÇ° £n°Ê 7°°Ê"½ Ài]ÊÛiÌÀÆÊ iÀÞÊ`ÕÃÌÀiÃÊV°]Ê>ÃÃ}iiÆÊ 1-{äÈÓnÓ]Ê£ÇÇ° £°Ê °°Ê,>Ü}ÃÊ>`Ê°°Ê ÀÜ]ÊÛiÌÀÃÆÊL>ÞÊ iV>Ê

À«À>Ì]Ê>ÃÃ}iiÆÊ1-{äÈÈÈ£Ç]Ê£Çn° Óä°Ê *° iÀÊ>`Ê,°Ê Ã>V]ÊÛiÌÀÃÆÊ >ÞiÀÊ]Ê>ÃÃ}ii]Ê 1-{££xÎÇ£]Ê£Çn° 21. 7°ÊiÝ>`iÀ]Ê°°Ê >ÕÃ>Ê>`Ê°°ÊÀ>ÃiÀ]ÊÛiÌÀÃÆÊ >ÛiÊLÀiÃÊÌ`°]Ê>ÃÃ}iiÆÊ1-{£ÓÓäÈÎ]Ê£Çn° 22. -° °Ê>â>ÀÕÃ]Ê°Ê >À>Û>ÀÌ]Ê°°Ê,Ü>Ê>`Ê°°Ê Neal, inventors; Allied Chemical Corporation, assignee; 1-{£Ç£{ÓÓ]Ê£Ç° 23. ° °Ê-«Û>]ÊÛiÌÀÆÊ L>i}ÞÊ À«À>Ì]Ê>ÃÃ}iiÆÊ 1-{£È£ÇÇ]Ê£nä° 24. ° °Ê-«Û>]ÊÛiÌÀÆÊ L>i}ÞÊ À«À>Ì]Ê>ÃÃ}iiÆÊ 1-{ÓnnÎ£]Ê£n£°

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Thermal and Hydrolytic Stabilisation 25. ° °Ê-«Û>]Ê°Ê iÝÌiÀÊ>`Ê-° °Ê*>ÃÌÀ]ÊÛiÌÀÃÆÊ L> i}ÞÊ À«À>Ì]Ê>ÃÃ}iiÆÊ1-{Î£nn{x]Ê£nÓ° ÓÈ°Ê ° °Ê-«Û>]ÊÛiÌÀÆÊ L>i}ÞÊ À«À>Ì]Ê>ÃÃ}iiÆÊ 1-{Îx£Çx]Ê£nÓ° ÓÇ°Ê ° °Ê-«Û>]ÊÛiÌÀÆÊ L>i}ÞÊ À«À>Ì]Ê>ÃÃ}iiÆÊ 1-{{ÎÎänÇ]Ê£n{° Ón°Ê ,°Ê,>ÛV>`À>Ê>`Ê° °Ê-«Û>]ÊÛiÌÀÃÆÊ L>i}ÞÊ

À«À>Ì]Ê>ÃÃ}iiÆÊ1-{nänÈ{x]Ê£n° Ó°Ê °ÊÜ>Ê>`Ê°Ê >À>`iÀ]ÊÛiÌÀÃÆÊâ>ÊV°]Ê >ÃÃ}iiÆÊ1-{Óx{ä£n]Ê£n£° Îä°Ê -°°Ê ÌÌÃ]ÊÛiÌÀÆÊ >ÀÌÊ`ÕÃÌÀiÃÊV°]Ê>ÃÃ}iiÆÊ 1-{ÈÎxä{]Ê£nÇ° 31. J.C. Rosenfeld and J.A. Pawlack, inventors; Celanese

À«À>Ì]Ê>ÃÃ}iiÆÊ1-{ÈnäÎÇ£]Ê£nÇ° 32. J.C. Rosenfeld, inventor; Celanese Corporation, assignee; 1-{nÓ££Î]Ê£n° 33. °°ÊVÊ>`Ê,°7°Ê-iÞÕÀ]ÊÛiÌÀÃÆÊ >ÃÌ>Ê

iV>Ê °]Ê>ÃÃ}ii]Ê1-x{ÓnänÈ]Ê£x° 34. °-°Ê VÃ]Ê/° °ÊÀiÊ>`Ê7°°Ê `Ü>À`Ã]ÊÛiÌÀÃÆÊ 7i>ÊV°]Ê>ÃÃ}iiÆÊ1-xnnäxn]Ê£° 35. J. Kato, K. Fujimoto and T. Takahashi, inventors; Asahi

iV>Ê À«À>Ì]Ê>ÃÃ}iiÆÊ1-È{ÓÎn£{]ÊÓääÓ° ÎÈ°Ê -°Ê ÌiÃÃ>Ê>`Ê°Ê ÃL}>]ÊÛiÌÀÃÆÊÌiwLÀiÊ-«]Ê >ÃÃ}iiÆÊ *£ÓÈ{nxÓ]ÊÓääÓ° ÎÇ°Ê H. Nakamura, inventor; Asahi Chemical Corp, assignee; *ÓääÎäÓÈä]ÊÓääÎ°

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Degradation and Stabilisation of Aromatic Polyesters În°Ê °,°Ê+Õi]Ê,°-°ÊÀiÌÀÕÌ]Ê°/°ÊiÀ}>]Ê-°°Ê-Ì>vvÀ`Ê >`Ê<°Ê8>]ÊÛiÌÀÃÆÊ >ÃÌ>Ê iV>Ê °]Ê>ÃÃ}iiÆÊ 1-ÓääÇäÈÈÇÎx]ÊÓääÇ° Î°Ê °/°ÊiÀ}>]Ê,°-°ÊÀiÌÀÕÌ]Ê°6iiÌÌÊ>`Ê °,°Ê iLÀÕ]Ê ÛiÌÀÃÆÊ >ÃÌ>Ê iV>Ê °]Ê>ÃÃ}iiÆÊ1-ÓääÇäÈÈÇ£]Ê ÓääÇ° {ä°Ê M.T. Jernigan, inventor; Eastman Chemical Co., assignee; 1-ÓääÇäÈÈÇ{]ÊÓääÇ° 41. M.T. Jernigan, inventor; Eastman Chemical Co., assignee; 1-Óään£nnÈäÓ]ÊÓään° 42. °,°ÊiÃiÞ]Ê °<°Ê/Ãi]Ê,°°Ê >VLÕÀ]Ê°°Ê >V>Ê >`Ê °Ê-i`i]ÊÛiÌÀÃÆÊ-iÊ"Ê °]Ê>ÃÃ}iiÆÊ1-ÇÎn£ÇnÇ]Ê Óään° 43. °,°ÊiÃiÞ]Ê °<°Ê/Ãi]Ê,°°Ê >VLÕÀ]Ê°°Ê >V>Ê>`Ê

°Ê-i`i]ÊÛiÌÀÃÆÊ-iÊ"Ê °]Ê>ÃÃ}iiÆÊ1-Óään£{ÇÎ]Ê Óään° 44. °Ê*>Ì]Ê*°Ê>>ÀiÃ]Ê °ÊÀÌÕÌ>]Ê°ÊÕiÀÊ>`Ê6°Ê Passalacqua, Polymer]Ê£n£]Ê22]Ê££]Ê£xÈÈ° 45. H. Zimmermann, Plaste und Kautschuk]Ê£n£]Ê28]Ên]Ê{ÎÎ° {È°Ê S. Chang, M. Sheu and N. Chang, Journal of Polymer Science: Polymer Chemistry Edition]Ê£nÓ]Ê20]Ên]ÊÓäxÎ° {Ç°Ê C. Fleenor in Proceedings of the 1st SPE International Conference on Processing and Property Enhancement utilising Modiﬁers and Additives in Polymers]Ê£nx]Ê iÜ>À]Ê NJ, USA, p.211. {n°Ê H. Zimmermann, Developments in Polymer Degradation, £nx] 5]ÊÇ°

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Thermal and Hydrolytic Stabilisation {°Ê -°°Ê>À]Ê ° °ÊÀLiÃ]Ê7° °Ê>`]Ê °°Ê `i>}]Ê°Ê>ÀiÃ]Ê°Ê"½ ÀiÊ>`Ê,° °Ê-i`}ÜV]Ê Journal of Polymer Science: Polymer Chemistry Edition, £nÈ]Ê24]ÊÈ]Ê£Ón£° xä°Ê C. Fleenor in Proceedings of the SPE Regional Technical Conference - Total Systems Approach - Color XXII, iÜ«ÀÌ]Ê,]Ê1-]Ê£nn]Ê«°ÓÎÎ° 51. -° °Ê iÌâÊÊProceedings of an SPE Conference - New Developments in Plastics Recycling, Charlotte, NC, USA, £n]Ê*>«iÀÊ °£Î° 52. -° °Ê iÌâÊÊEmerging Technologies in Plastics Recycling,

`Ã°]Ê° °Ê`ÀiÜÃÊ>`Ê*°°Ê-ÕLÀ>>>]Ê -Ê-Þ«ÃÕÊ -iÀiÃÊ °x£Î]Ê -]Ê7-}Ì]Ê ]Ê1-]Ê££]Ê«°£Î{° 53. P. Solera in Proceedings of an SPE RETEC Conference Weathering Well with Colorants and Additives, Orlando, FL, 1-]Ê£Î]Ê«°£È£° 54. R. Pfaendner, H. Herbst, K. Hoffmann and F. Sitek, Die Angewandte Makromolekulare Chemie]Ê£x]Ê232]Ê£Î° 55. P. Solera, Journal of Vinyl and Additive Technology]Ê£n]Ê4, Î]Ê£Ç° xÈ°Ê °Ê*Ã«Ã]Ê7° °Ê>LViÀÊ>`Ê-°Ê iÃ«ÕÀiÊÊProceedings of a Rapra Technology Conference - Addcon World ‘99,Ê£]Ê Prague, Czechoslovakia, Paper No.22. xÇ°Ê ° °ÊÃÌ]Ê7°Ê ÜÊ>`Ê/°Ê iiÊÊProceedings of an SPE Conference – Antec 2000]Ê"À>`]Ê]Ê1-]ÊÓäää]Ê *>«iÀÊ °xxÇ° xn°Ê °,°Ê >ÃViÌÊ>`Ê°°Ê >Ã]ÊJournal of Polymer Engineering]ÊÓäää]Ê20, 2, 143.

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À«À>Ì]Ê>ÃÃ}iiÆÊ1-ÎxnännÈ]Ê£Ç£° nÓ°Ê /°Ê>Ì]Ê°Ê9>>}]Ê9°ÊV>Ü>]Ê°Ê"ÕÃÕ}]Ê°ÊÀ]Ê T. Tsutsumi and M. Minami, inventors; Toray Industries, >ÃÃ}iiÆÊ1-Î{£äxÇ]Ê£Çä° nÎ°Ê T. Okutsu, N. Tsuji, R. Ohi and T. Kondo, inventors; Fuji *ÌÊÊ °ÊÌ`°]Ê>ÃÃ}iiÆÊ1-În£ÓäÇ]Ê£Ç{° n{°Ê -° °Ê>â>ÀÕÃÊ>`Ê °°Ê iiÌ]ÊÛiÌÀÃÆÊi`Ê iV>Ê

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Degradation and Stabilisation of Aromatic Polyesters nx°Ê J.S. Gall, inventor; Celanese Corporation, assignee; 1-ÎäÎä{Ó]Ê£Çx° nÈ°Ê 7°°°Ê À>Ê>`Ê °*°Ê,iÞ]ÊÛiÌÀÃÆÊiiÀ>Ê iVÌÀVÊ

°]Ê>ÃÃ}iiÆÊ1-ÎÈÎäÈ]Ê£ÇÈ° nÇ°Ê G.R. Ure, inventor; Fiber Industries Inc., assignee; 1-{äÓnÎäÇ]Ê£ÇÇ° nn°Ê T. Kasuga, K. Takahashi and T. Nakashima, inventors; *Þ«>ÃÌVÃÊ °ÊÌ`°]Ê>ÃÃ}iiÆÊ1-{{ÎÈnÇÇ]Ê£n{ n°Ê ° °Ê-«Û>Ê>`Ê-° °Ê*>ÃÌÀ]ÊÛiÌÀÃÆÊ Ã>Ê °ÊÌ`°]Ê >ÃÃ}iiÆÊ1-{xäÎÓ{Î]Ê£nx° ä°Ê ° °Ê-«Û>Ê>`Ê-° °Ê*>ÃÌÀ]ÊÛiÌÀÃÆÊ L>i}ÞÊ

À«À>Ì]Ê>ÃÃ}iiÆÊ1-{xnÓnÇä]Ê£nÈ° £°Ê J.C. Rosenfeld, inventor; Celanese Corporation, assignee; 1-{ÈÇnnÓx]Ê£nÇ° Ó°Ê J.C. Rosenfeld, inventor; Celanese Corporation, assignee; 1-{näÎÓÎÈ]Ê£n° Î°Ê J.C. Rosenfeld, inventor; Celanese Corporation, assignee; 1-{nÓ{nx]Ê£n° {°Ê ° °Ê7>}}iÀ]Ê°,°Ê->ÕiÃÊ>`Ê °°Ê`VV]ÊÛiÌÀÃÆÊ Õ*ÌÊ`iÊ iÕÀÃÊEÊ °ÊÌ`°]Ê>ÃÃ}iiÆÊ1-xÎÇxäÓ]Ê £x° x°Ê S. Kusakaishi and C. Konno, inventors; Munekata Tohoku ]Ê>ÃÃ}iiÆÊ*ÓääÓÓÓÈÈÎ]ÊÓääÓ° È°Ê S. Kusakaishi and C. Konno, inventors; Munekata Tohoku ]Ê>ÃÃ}iiÆÊ*ÓääÓÓÓÈÈx]ÊÓääÓ° Ç°Ê S. Kusakaishi and K. Ohashi, inventors; Munekata Tohoku ]Ê>ÃÃ}iiÆÊ*ÓääÓÓÓÈÈÈ]ÊÓääÓ°

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iV>Ê À«À>Ì]Ê>ÃÃ}iiÆÊ1-{Î£äÈxn]Ê£nÓ° 135. °Ê>]Ê°Ê i]Ê °Ê7Õ]Ê°Ê7>}]Ê-°Ê iÊ>`Ê-°Ê9>}]Ê inventors; Industrial and Technical Research Institute Taiwan, >ÃÃ}iiÆÊ1-È{nÓ£Ó]ÊÓääÓ° £ÎÈ°Ê7° °Ê>ÀÀÃÊ>`Ê°°ÊVi>]ÊÛiÌÀÃÆÊ Õ*ÌÊ`iÊ iÕÀÃÊEÊ °ÊÌ`°]Ê>ÃÃ}iiÆÊ1-ÈÇ{n{È]ÊÓääx°

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Thermal and Hydrolytic Stabilisation £ÇÇ°ÊA. Rodriguez and J. Puiggali, Revista de Plasticos Modernos, Óää£]Ê82, 543, 312. £Çn°Ê °Ê/ÀÀiÃ]Ê°°Ê,LÊ>`Ê °Ê ÕÌiÛ]ÊJournal of Applied Polymer Science]ÊÓää£]Ê79]Ê£ä]Ê£n£È° £Ç°Ê°Ê8>ÌÃ]Ê,°Ê >Û>iÀ]Ê °Ê7>]Ê+°Ê<>}Ê>`Ê-°°Ê iÞÊÊProceedings of the SPE Conference - Antec’02, San À>VÃV]Ê ]Ê1-]ÊÓääÓ]Ê*>«iÀÊ °xÇ{° £nä°Ê,°Ê >Û>>ÀÊ>`Ê°Ê8>ÌÃ]ÊJournal of Applied Polymer Science]ÊÓääÎ]Ê87]Ê{]ÊÈ{Î° £n£°ÊAdditives for Plastics]ÊÓää{]ÊÈ]ÊÎ° £nÓ°Ê°ÊÜ>>]Ê°Ê >ÛiÀÊ>`Ê °ÊiÃÀ]ÊPolymer Engineering and Science]ÊÓää{]Ê44]Ên]Ê£xÇ° £nÎ°Ê ° °Ê<>>ÀÛ]Ê/° °Ê6>ÌÃ>Þ>]Ê-°6°ÊÀi>]Ê/° °Ê Prudskova and T.I. Andreeva, International Polymer Science and Technology]ÊÓää{]Ê31]Ên]Ê/xÇ° £n{°Ê°ÊÜ>>Ê>`Ê °Ê*>Ûi]ÊEuropean Polymer Journal]ÊÓääx]Ê41, Ç]Ê£{xÎ° £nx°Ê° °°Ê >VÕ]Ê °,°Ê Õ`ÕÀÞ]Ê °Ê ÕÌÌ>Ê>`Ê °ÊÃÀ]Ê Macromolecules]ÊÓääÇ]Ê40]ÊÓ]ÊÓÈx° £nÈ°ÊV. Karayan, Plastics Technology]ÊÓääÇ]Ê53]ÊÓ]ÊÇä° £nÇ°Ê*°Ê>ÀÃ]Ê ° °Ê*>«>Ã«ÞÀ`iÃÊ>`Ê,°Ê*v>i`iÀ]ÊJournal of Applied Polymer Science]ÊÓääÇ]Ê104]ÊÎ]Ê£ÈÇ£° £nn°ÊU. Stier and M. Schweizer, Journal of Applied Polymer Science]ÊÓääÇ]Ê106, 1, 425. £n°Ê-°Ê<>}]Ê9°Ê7>}]Ê°Ê/i}Ê>`Ê8°Ê7>}]ÊPolymer Materials Science and Engineering]ÊÓään]Ê24]ÊÓ]Ê££°

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Thermo-oxidative Stabilisation

7.1 Introduction As with all organic polymers, exposure of aromatic polyesters to heat and oxygen can, especially over long time periods, result in degradation of the polymer. This thermo-oxidation manifests as discoloration of materials, loss of physical properties, and complete failure of the substrate. To prevent (or more likely control) such processes it is necessary to incorporate additives which can protect the host polyester against the effects of heat and oxygen: antioxidants. As may be deduced from the discussions undertaken in Chapter 3, the most damaging reactions within the autoxidation cycle posited for polymers are initiation and chain branching, and propagation. These are also the reactions where it may be possible to utilise speciﬁc chemical compounds to interfere with the processes involved. Antioxidants are usually roughly divided into two categories: 1. Primary, or chain-breaking antioxidants interfere with the chain propagation step, i.e., the reactions: P. + O2 ‡ POO. POO. + PH ‡ POOH + P. 2. Secondary, or preventative antioxidants react with the hydroperoxides responsible for chain initiation and chain branching.

181

Degradation and Stabilisation of Aromatic Polyesters The primary antioxidants are normally broken down further into the classes of chain-breaking donor (CB-D) and chain-breaking acceptor (CB-A). CB-D additives interact with peroxy radicals, and are by far the commonest class of antioxidant in general use. They are represented by such additives types as hindered phenols and secondary aromatic amines. CB-A additives interact with alkyl radicals but, due to the rapid oxidation of such radicals, these additives are really useful only under low oxygen availability. CB-A types are represented by aromatic nitro and nitroso compounds, and a few speciality ‘stable’ free radicals. Some transformation products of CB-D antioxidants can also act as CB-A species. Secondary antioxidants, often called peroxide decomposers, also may be further categorised as stoichiometric peroxide decomposers and catalytic peroxide decomposers. The former are represented by, for example, phosphites; the latter by organic sulﬁdes and their transformation products. A classic overview of antioxidants and their modes of action is provided by Scott [1]. In considering antioxidants for aromatic polyesters, it should be remembered that most commercial products were originally developed for use in polyoleﬁns and rubbers. Several points must be taken into account when selecting antioxidants for these polyesters: 1. Stability at higher processing temperatures. 2. Unwanted reactivity with host polymer. Conversely, deliberate reactions can be induced to attach antioxidants to the polymer chains, or to build them into chains. 3. Physical loss from substrate during polycondensation or melt processing, depending on at which stage the antioxidant is incorporated into the host. 4. Compatibility with the host, which will depend on chemical and physical factors.

182

Thermo-oxidative Stabilisation Taking poly(ethylene terephthalate) (PET) as an example of an aromatic polyester, this material is less susceptible to oxidation than many other polymers, such as polypropylene (PP), from a chemical reactivity point of view, and from its relative permeability to oxygen (Table 7.1) [2]

Table 7.1 Oxygen Permeability of Selected Polymers Polymer

Permeability (P × 1010 cm3 s–1 mm cm–2 hg–1; 30 °C)

PET

0.22

High-density polyethylene

10.26

PP

23

Low-density polyethylene

15

Polybutadiene

191

Natural Rubber

233

In 1985, Zimmermann [3] was moved to state that ‘Many patents propose the addition of antioxidants to PET, but it is seldom done in technical practice.’ Be that as it may, there will be situations and applications where an antioxidant may be required to boost the innate stability of aromatic polyesters, and an examination of proposed additives will also allow workers in this ﬁeld to ascertain which adducts have been tried and how successful each category may have been. We will also examine newer additives which, while they may not have been used to date in polyesters, might be candidates for future examination.

7.2 Studies on Antioxidants Only a few articles have been published examining the effects of various antioxidant packages on the thermo-oxidative degradation of aromatic polyesters [3–19]. Angelova and co-workers [4, 5] used differential thermal analysis (DTA) to examine the effects of some antioxidants on PET and model

183

Degradation and Stabilisation of Aromatic Polyesters compounds thereof. They found that a combination of diethyl-3,5di-t-butyl-4-hydroxybenzylphosphonate (Irganox 1222; Ciba) with triphenylphosphine constituted an effective antioxidant package, with a marked synergism between the components. Zimmermann [6] noted that phosphites, while effective thermal stabilisers (see Chapter 6), were also effective antioxidants, especially if used in combination with primary antioxidants such as hindered phenols. Kosinska and co-workers [7] suggested that hindered phenols and secondary aromatic amines could be effective antioxidants in PET. Karayannidis and co-workers investigated the effect of various phosphorus-containing compounds on the thermo-oxidative stability of PET [9]. Additives were incorporated in the reaction kettle after the transesteriﬁcation stage, but before polycondensation. Additives investigated were: phosphoric acid, tributylphosphate, triphenylphosphate, phenylphosphonic acid, phenylphosphinic acid and sodium phenylphosphinate. The most effective antioxidants were tributylphosphate, phenylphosphonic acid and phenylphosphinic acid. The above authors also investigated the effectiveness of various commercial primary antioxidants in PET [11]. The additives assessed were: UÊ {]{A ÃA,A-dimethylbenzyl)diphenylamine (Naugard 445; Uniroyal) UÊ £]Î]x/ÀiÌÞÓ]{]ÈÌÀÃÎ]x`t-butyl-4-hydroxybenzyl) benzene (Ethanox 330; Ethyl Corporation) U

N,NAiÝ>iÌÞiiLÃÎ]x`t-butyl-4-hydroxycinnamide) (Irganox 1098; Ciba)

U

N,NA/ÀiÌÞiiLÃÎ]x`t-butyl-4-hydroxycinnamide) (Irganox 1019; Ciba)

184

Thermo-oxidative Stabilisation Additives were again incorporated in the reaction kettle between the transesteriﬁcation and polycondensation stages. Due to the high vacuum required during polycondensation, a great deal of the most volatile additive (secondary aromatic amine) was lost. Test results, utilising infra-red spectroscopic assessment and oxidation induction times, demonstrated that 445 and 330 gave the best performance at addition levels between 0.01 and 0.03 wt%, although neither provided full protection to the polyester matrix at such low levels. At loadings between 0.5 and 1.0 wt%, which are more normal levels to attain adequate protection, 1098 and a 1:1 blend of 1098 and 1019 gave superior results. The authors also stated that 1019 exhibited some metal chelating ability, possibly due to the shorter alkylene chain length between the two hindered phenolic ends allowing for better metal ion complexation. Ciba researchers [10, 12, 15] studied the problems inherent in ‘restabilising’ post-consumer recyclates, including PET. The need to at least attempt to take into account the degree of degradation, level of residual stabilisers, and type and quality of contamination that might be present was emphasised. Various combinations of hindered phenolics and phosphorus-based secondary stabilisers were noted as being effective antioxidant packages, including speciﬁcally designed commercial combinations such as Recyclostab, proprietary mixtures of stabilisers from Ciba. Speciﬁc antioxidants recommended (in various combinations) were: UÊ * i Ì > i À Þ Ì À Ì Ì i Ì À > Ã Î ] x ] ` t - b u t y l - 4 hydroxyphenylpropionate) (Irganox 1010) UÊÊ ÌÞiiLÃÝÞiÌÞii®LÃÎxt-butyl-4-hydroxyphenyl-mtolyl)propionate) (Iragnox 245) UÊÊ /ÀÃÓ]{`t-butylphenyl)phosphite (Irgafos 168)

185

Degradation and Stabilisation of Aromatic Polyesters Tock [13] suggested that the process stabiliser bis(2,4-di-tbutylphenyl)pentaerythritoldiphosphite (Ultranox 626; GE) was also a moderately effective antioxidant. Feng and Ning [14] recommended the addition of 1,6-hexanediolbis(3(3,5-di-t-butyl-4-hydroxyphenyl)propionate (Irganox 259; Ciba) as an antioxidant for PET. Allen and co-workers [17] investigated commercial antioxidants in poly(ethylene-co-1,4-cyclohexanediemthelyene terephthalate) using the additives: UÊ ÃÓ]{`VÕÞ«iÞ®«iÌ>iÀÞÌÀÌ`«Ã«ÌiÊ>Ý 28; Great Lakes Chemical) UÊ £]Î]x/ÀÃÎ]x`t-butyl-4-hydroxybenzyl)-1,3,5-triazine2,4,6(1H, 3H, 5H)trione (Irganox 3114; Ciba) UÊ £]Î]x/ÀÃ{t-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5triazine-2,4,6(1H, 3H, 5H)trione (Irganox 1790; Ciba) Best thermal antioxidant performance was seen with a combination of 0.2 wt% phosphite and 0.2 wt% of either of the hindered phenols. Russian researchers [18, 19] investigated the efﬁciency of thermal antioxidants in poly(butylene terephthalate) (PBT). Good results were obtained using combinations Irgafos 168 or Ultranox 626 with Irganox 1010 or stearyl-B-(3,5-di-t-butyl-4-hydroxyphenyl) propionate (Irganox 1076; Ciba). From the available studies it would appear that hindered phenols in combination with phosphites, provided both components are sufﬁciently non-volatile and thermally stable, would be the preferred option for the thermo-oxidative stabilisation of aromatic polyesters. While as yet not fully tested, additives from Polnox Corporation [20– 24] provide some variations on the theme of ‘traditional’ antioxidants which could be useful in aromatic polyesters. Utilising what they say

186

Thermo-oxidative Stabilisation is novel chemistry, this concern has produced molecules which contain either bulky constituents to reduce volatility, or which contain two different stabiliser moieties, e.g., both hindered phenol and aromatic amine. Through the use of novel synthetic methods, this concern has produced two series of additives. One contains an active site and one or more bulky side groups, which renders the additive less susceptible to loss for the polymer matrix through volatilisation. The other type contain two different active stabilising groups, e.g., a hindered phenol and an aromatic amine, holding out the possibility of one dealing with rapid initial oxidation and the other operating over a longer time scale. This could potentially provide a greater overall stabilising effect, or could allow lower levels of additives to be used.

7.3 Potential New Chemistries Although they do not appear to have been investigated in aromatic polyesters, there are a few other antioxidant types which might prove useful in this regard. Benzofuranones [25–27], exempliﬁed by the Ciba product HP136 , which is said to be a mixture of 90% 5,7-di-t-butyl-3-(3,4dimethylphenyl)-3H-benzofuran-2-one and 10% 5,7-di-t-butyl-3(2,3-dimethylphenyl)-3H-benzofuran-2-one, are said to be fast-acting radical scavengers, which may effectively trap peroxy and alkyl radicals. Classed as ‘a potent melt processing co-additive’ [25], this is mainly used as a very effective oxidation inhibitor at low levels in combination with hindered phenol and phosphite. The capability of HP-136 in reacting with alkyl radicals is said to boost the stabiliser package performance, and there is some suggestion that this additive can also react with phenoxy radicals, thus regenerating the hindered phenolic antioxidant. Commercial packages include: Irganox HP2215 - 15% HP-136, 28% Irganox 1010, 57% Irgafos 168 Irganox HP2225 - 15% HP-136, 42.5% Irganox 1010, 42.5% Irgafos 168 187

Degradation and Stabilisation of Aromatic Polyesters Classed by Marin and co-workers [27] as a moderate CB-D antioxidant, the use of HP-136 (or the previous packages containing the same) in PET and related polyesters may be worth investigating. Hydroxylamines [25] have the advantage that they are almost completely colourless, unlike the aromatic amines which are coloured, and the hindered phenols which form highly coloured breakdown species. This class of additive is exempliﬁed by N,N-di(hydrogenated tallow)hydroxylamine (Irgastab FS-042; Ciba). This type of additive appears to operate as a radical scavenger and as a peroxide decomposer. Nitrones formed by oxidation of the hydroxylamine are also said to be stabilisers. Patents claim these additives as useful in reducing aldehyde content in polyester [28, 29], but there does not appear to have been any systematic study of their antioxidant capabilities in aromatic polyesters. Vitamin E (A-tocopherol) [30] is another potential candidate for an antioxidant in polyesters, and is available as a polymer additive from Hoffmann-La Roche as Ronotec 201. It has been shown that this additive can react with peroxy and alkyl radicals, and can trap more than one radical. The potential drawbacks may be the volatility and/ or thermal stability of this additive. As well as being excellent UV stabilisers for several polymer applications, there is ample evidence [31] to suggest that hindred amine light stabilisers can also operate as thermal antioxidants. Discussion of these additives will be undertaken in Chapter 8.

7.4 Patents The patent literature relating to antioxidants speciﬁcally for use in aromatic polyesters covers a multitude of additives, but is quite restricted in the class of antioxidant claimed, being largely limited to hindered phenolic derivatives. These include versions with no heteroatoms [32–39], phosphorus-containing [40–47], nitrogen-containing

188

Thermo-oxidative Stabilisation [48, 49], sulfur-containing [50–53] and patents dealing with hindered phenols packaged alongside speciﬁc co-additives [54–63]. Aromatic amines are also represented [64–66], whereas single patents have been found claiming the use of phenothiazines [67], sulﬁdes [68] and quinones [69]. Hindered phenolics containing only carbon, hydrogen and oxygen include: a triphenol reaction product of crotonaldehyde and 3-methyl-6-t-butylphenol (C4H7[C6H2(CH3)(C4H9)OH]3) [32]; di- or tri-phenols where the phenolic rings are attached to each other by a -CH2- bridge, cited as useful in PBT or poly(butylene naphthalate) (PBN) [33]; reactable phenols with a -R1COOR2 moiety in the 4-position, again aimed at PBT or PBN [34]; reactive phenols claimed vÀÊÕÃiÊÊâVV>Ì>ÞÃi`Ê«ÞiÀÃ>ÌÊvÊ* /]Êi°}°]ÊiÌÞÎA]xA `Ã«À«Þ{AÞ`ÀÝÞ®Liâ>ÌiÊ QÎxRÆÊ Ài>VÌÛiÊ «iÃÊ ÜiÀiÊ ÌiÊ stabilising functional group is attached via a -O(CH2)n- bridge to isophthalic acid [36]; hindered phenolic acids or esters speciﬁcally aimed at end-capping of poly(trimethylene terephthalate) (PTT) to reduce acrolein formation and improve oxidative stability [37]; Irganox 1010 claimed to be reacted into the polymer backbone in titanium-catalysed polyesters [38]; and antioxidants formed by combining reactive hindered phenols with polyvinyl alcohol or polyacrylic acid [39]. Phosphorus-containing hindered phenols include: UÊ *Ã«VÊ >V`Ê iÃÌiÀÃÊ ÃÕVÊ >ÃÊ {Þ`ÀÝÞÎ]x`tbutylbenzylphosphonic acid diethyl ester (Irganox 1222; Ciba) [40]; UÊ «iÃÊÃÕVÊ 9H19PhOP[OPh-t-C3H6Ph(t-Butyl)OH]2 [41]; UÊ O-hydroxycarbyl-substituted hydroxyphenylalkylenephosphonic acid esters of polycarbocyclic polydroxy compounds [42]; UÊ «iÃÊÃÞÌiÃÃi`ÊLÞÊÀi>VÌÊvÊ«Ã«ÀÕÃÊ>V`Ê>ÃÌiÀÃÊÜÌÊ pentaerythritol [43];

189

Degradation and Stabilisation of Aromatic Polyesters UÊ -ÌiÀV>ÞÊ `iÀi`Ê ÌÀÃQÞ`ÀÝÞ«iÞÌ®«iÞR«Ã«ÌiÃ]Ê e.g., P[OPh(t-butyl)(methyl)SPh(t-butyl)(methyl)OH]3 (Hostanox OSP-1; Clariant), said to be especially useful in antimony oxide catalysed polyesters as they prevent formation of antimony metal and its associated grey discoloration [44]; UÊ >VÕÊ Ã>ÌÊ vÊ `iÌÞLÃÎ]x`iÌÞ{Þ`ÀÝÞ«iÞ® methyl)phosphonate) (Irganox 1425; Ciba) [45]; UÊ `iÞ`iÊ ÃÕ««ÀiÃÃÊ ÕÃ}Ê >Ê VL>ÌÊ vÊ >Ê «ÞÊ ÃÕVÊ as sorbitol or maltose and either Irganox 1222 or 1425 [46]; and UÊ ÊVL>ÌÊV>ÊiÝÌi`iÀÊ>`Ê>ÌÝ`>ÌÊ«>V>}iÊÜÊ as Irgamod RA20, which is a masterbatch consisting of 75.5% polyester carried resin, 20.5% pyromellitic dianhydride, 2% pentaerythritol and 2% Irganox 1425 [47]. Nitrogen-containing species include triphenols where the stabilising moieties are attached to an isocyanurate ring, e.g., Irganox 3114 and 1790 [48] and other hindered phenol-substituted N-heterocyales such as N-2-(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxy)ethyl succinimide [49]. Sulfur-containing hindered phenolics include diphenols with alkylthio bridges, e.g., 1,6-bis(3,5-di-t-butyl-4-hydroxyphenylthio) hexane [50]; tri- and tetra-(substituted hydroxyphenylthio) alkanes and cycloalkanes, e.g., 1,1,2,2-tetrakis(3,5-di-t-butyl-4hydroxyphenylthio)ethane and 1,1,3-tris (3-t-butyl-5-t-octyl-4hydroxyphenylthio)-3,5,5-trimethylcyclohexane [51, 53]; and di(substituted hydroxyphenylthio)alkanes and cycloalkanes [52]. Patents claiming the use of hindered phenols in aromatic polyesters along with speciﬁc co-additives include: cyclic carbonates and phenols, e.g., ethylene carbonate and 1,3,5-trimethyl-2,4,6tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (Irganox 1330; Ciba) [54]; combinations with oxetane-substituted phosphites [55]; addition at the polymerisation stage of a synergistic mix of

190

Thermo-oxidative Stabilisation triethyl or tributylphosphite and a hindered phenolic phosphonite [56, 57]; combination of any commercial hindered phenol with Ó]ÓA]ÓAAÌÀÌÀiÌÞÌÀÃÎ]ÎA]x]xAÌiÌÀ>tLÕÌÞ£]£AL«iÞÓ]ÓA diylphosphite) (Irgafos 12; Ciba) [58]; addition of primary and secondary antioxidants in reaction kettle before polycondensation, preferably a hindered phenol and a phosphonite, e.g., Irganox 1010 and Sandostab P-EPQ [59, 60]; hindered phenol and phosphite in PTT to reduce acrolein emissions [61]; package consisting of an oxetane-substituted phosphorus-containing hindered phenol and an unsubstituted phosphorus-containing phenol [62]; and poly(cyclohexanedimethylene terephthalate) with an antioxidant package consisting of hindered phenol, phosphite, and a polyamide terpolymer [63]. Patents citing the use of other antioxidants include: combinations of cyclic carbonate and secondary aromatic amine [64]; PBT moulding grades stabilised with diamines such as diphenyl-pphenylenediamine [65]; polymeric diphenylamines made by reacting diphenylamine with dialkylalkenylbenzene or dihydroalkylbenzene [66]; substituted phenothiazines [67]; aliphatic sulﬁdes such as R(OCH2 ,A ,AA-,AAA®n [68]; and combinations of aromatic carbodiimides and quinones [69].

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Degradation and Stabilisation of Aromatic Polyesters 29. P.A. Odorisio, S.M. Andrews, D. Lazzari, D. Simon, R.E. King, M. Stamp, R. Reinicker, M. Tinkl, N. Berthelon, D. Muller and U. Hirt, inventors; Ciba Speciality Chemicals Corporation, assignee; US7022390, 2006. 30. Y. Ohkatsu, T. Kajiyama and Y. Arai, Polymer Degradation and Stability, 2001, 72, 2, 303. 31. K. Schwetlick and W.D. Habicher, Polymer Degradation and Stability, 2002, 78, 1, 35. 32. D. Ranson, inventor; ICI, assignee; US3186962, 1965. 33. S. Kawase, H. Inata and T. Shima, inventors; Teijin Ltd., assignee; US3904578, 1975. 34. S. Kawase, H. Inata and T. Shima, inventors; Teijin Ltd., assignee; US3989664, 1976. 35. F.E. Carevic and A. Labriola, inventors; FMC Corporation, assignee; US4011196, 1977. 36. A.W. White and R.S. Beavers, inventors; Eastman Kodak, assignee; US4910286, 1990. 37. D.R. Kelsey, inventor; Shell Oil Co., assignee; US6242558, 2001. 38. K. Tano, U. Marschall and H. Kliesch, inventors; Mitsubishi Polyester Film GmbH, assignee; US6777099, 2004. 39. A.L. Cholli, A. Dhawan and V. Kumar, inventors; University of Massachusetts Lowell, assignee; US7323511, 2008. 40. H. Gysling and H. Peterli, inventors; JR Geigy AG, assignee; US3376258, 1968. 41. C.E. Gleim and R.B. Spacht, inventors; The Goodyear Tire & Rubber Co., assignee; US3386952, 1968.

194

Thermo-oxidative Stabilisation 42. M. Minagawa, Y. Nakahara and M. Takahashi, inventors; Argus Chemical Corporation, assignee; US4145333, 1979. 43. H. Buysh, R. Binsack and D. Rempel, inventors; Bayer AG, assignee; EP48878, 1982. 44. M. Lu and M. Golder, inventors; Ticona LLC, assignee; US6696510, 2004. 45. R. Tsukamoto, N. Hashimoto and T. Hoshi, inventors; Solotex Corporation, assignee; US2006020103, 2006. 46. D. Simon, D. Lazzari, S.M. Andrews and H. Herbst, inventors; Ciba Speciality Chemicals Corporation, assignee; US7205379, 2007. 47. N. Berthelon and D. Muller, inventors; Ciba Speciality Chemicals Holding AG, assignee; EP1882009, 2008. 48. G. Kletacka and P.D. Smith, inventors; BF Goodrich Co., assignee; US3678047, 1972. 49. G. Kletacka and P.D. Smith, inventors; BF Goodrich Co., assignee; US3862130, 1975. 50. M.H. Keck, R.E. Gloth and J.J. Tazuma, inventors; The Goodyear Tire & Rubber Co., assignee; US4330462, 1982. 51. J.D. Spivak and S.D. Pastor, inventors; Ciba-Geigy Corporation, assignee; US4560799, 1985. 52. J.D. Spivak and S.D. Pastor, inventors; Ciba-Geigy Corporation, assignee; US4611023, 1986. 53. J.D. Spivak and S.D. Pastor, inventors; Ciba-Geigy Corporation, assignee; US4612341, 1986. 54. P.C. Georgoudis, inventor; National Starch, assignee; US3985705, 1976.

195

Degradation and Stabilisation of Aromatic Polyesters 55. D. Freitag, D. Rathmann, U. Hucks, P. Tacke and L. Bottenbruch, inventors; Bayer AG, assignee; EP150497, 1985. 56. J.M. Verheijen and A.M. Marien, inventors; Agfa-Gevaert NV, assignee; EP501545, 1992. 57. J.M. Verheijen and A.M. Marien, inventors; Agfa-Gevaert NV, assignee; US5185426, 1993. 58. A. Schmitter, inventor; Ciba Speciality Chemicals, assignee; US5763512, 1998. 59. X. Huang and L. Dominguez, inventors; Hoechst-Celanese, assignee; US5874515, 1999. 60. X. Huang and L. Dominguez, inventors; Hoechst-Celanese, assignee; US5874517, 1999. 61. D.R. Kelsey, inventor; Shell Oil Co., assignee; US6093786, 2000. 62. M. Bienmuller, K. Idel and P. Friedemann, inventors; Bayer AG, assignee; US2002137823, 2002. 63. C.S. Valentine, H. Zhang and S. Patel, inventors; Voith Paper Patent GmbH, assignee; US7138449, 2006. 64. P.C. Georgoudis, inventor, National Starch, assignee; US3987004, 1976. 65. K. Schlichting, P. Horn and W. Seydl, inventors, BASF AG, assignee; GB1496396, 1977. 66. R.E. Gloth, J.J. Tazuma and M.H. Keck, inventors; The Goodyear Tire & Rubber Co., assignee; US4414348, 1983. 67. C.E. Tholstrup and J.W. Thompson, inventors; Eastman Kodak Co., assignee; US3494886, 1970.

196

Thermo-oxidative Stabilisation 68. J.M. Bohen and J.L. Reilly, inventors; Atochem North America Inc., assignee; US5081169, 1992. 69. V. Ulrich, inventor; Rhein Chemie Rheinau GmbH, assignee; US5130360, 1992.

197

Degradation and Stabilisation of Aromatic Polyesters

198

8

Stabilisation Against Ultraviolet and Ionising Radiation

8.1 Introduction to Ultraviolet (UV) Stabilisation Whereas the use of thermo-oxidative stabilisers can be regarded as largely optional for aromatic polyesters unless particularly harsh conditions are likely to be met in service, the light stabilisation of these polymers is more of a requirement in any situation where an article made thereof is going to be exposed to short-wavelength light, or a combination of this with oxygen. Additives formulated with polymers to improve their photostability must carry out one or more of the following functions effectively to protect their host polymer [1, 2]: a) Screening of detrimental wavelengths of light. b) Quenching of electronically excited states. c) Non-radical decomposition of hydroperoxides. d) Complexing of trace metal ions. e) Interception of photo-oxidation products. f) Radical scavenging. g) Transformation into a species capable of fulﬁlling one or more of the above roles. From what is known of the initial photophysical and photochemical processes involved in polyester photodegradation, it would appear

199

Degradation and Stabilisation of Aromatic Polyesters that option ‘a’ is the most favoured stabilisation route. Option ‘g’, where some form of transformation results in a stabiliser of type ‘a’ is also an option, provided such a transformation is rapid and does not result in the appearance of concomitant pro-degredant species. This is not to say that the other options available for photostabilisation are not worth considering. Most of the other options set out above would be helpful to a greater or lesser extent. Excited state quenching could be useful, especially because it is generally considered that the Norrish Type II chain scission via the excited cyclised transition state occurs via a relatively long-lived triplet state. Peroxide decomposers are helpful in slowing down the autoxidation cycle, whereas catalyst residues or metallic species from other sources (machinery, other additives) are pro-degredant species which it would be useful to suppress. Process stabilisers, such as phosphite (see Chapter 6) can assist with peroxide decomposition and metal ion complexation. Interception of photo-oxidation products and radical scavenging could be achieved by antioxidants (see Chapter 7), although the additives required may not be exactly the same ones used in thermooxidative stabilisation. As was noted in Chapter 4, polyesters such as polyethylene terephthalate (PET) degrade mainly in a thin surface layer under the inﬂuence of light and oxygen. This suggests that photostabilisation in the bulk of an article will not be optimally used; at best, portions of the additive distributed in the interior will act as a reservoir to replace stabiliser lost in the outer regions during any of the chemical or physical processes under way.

8.2 UV Screeners and Absorbers 8.2.1 Background A simple approach to light stabilisation of a substrate is to incorporate therein some substance which will reﬂect or absorb incident light. It would therefore be thought that incorporation of pigments or dyes 200

Stabilisation Against Ultraviolet and Ionising Radiation [3] would be helpful. The prime example of this approach is carbon black, which is well known as an extremely effective UV absorber. Carbon black appears to operate in several ways besides simple screening. Other pigments, ranging from coloured to white, also exhibit some UV screening capability. Speciﬁc to aromatic polyesters, it is claimed that titanium dioxide is useable as a light stabiliser [4]. A study by Kashkhozheva and co-workers [5] indicated that Fe2O3, Al2O3, MgO and CaCO3 exhibited some photostabilising effect in poly(butylene terephthalate) (PBT), whereas Guedri and co-workers [6] noted the applicability of ZnO as a screener for poly(ethylene naphthalate) (PEN). Some authors have deduced that dyes, especially aroylbenzimidazoles, provided they are suitably thermally stable [7, 8], and ﬂuorescence brighteners such as Leucopor EGM [9] can photostabilise PET. Naphthol AS dye intermediates have been patented as UV screeners applicable in aromatic polyesters [10]. A number of classes of additives speciﬁcally for use as UV absorbers in polymer formulations are available including, among the better known ones, aromatic salicylates [11], o-hydroxybenzophenones [12], 2-hydroxyphenylbenzotriazoles [12, 13], derivatives of cinnamic acid and related materials [14, 15], aromatic oxanilides [16], cyclic imino ester derivatives known variously as benzoxazines or benzoxazones [17, 18] and hydroxyphenyltriazines [19]. Phenyl salicylates are one of the oldest UV stabilisers known. They are not particularly good stabilisers, but they rearrange on exposure to UV radiation to form 2-hydroxybenzophenones, which may be regarded as the active species. The problem with these materials is that the conversion is not rapid or, in some cases, ‘clean’, which can lead to poor performance and some discoloration of the host polymer. These additives have been largely superseded by later developments. o-Hydroxybenzophenones carry out their stabilisation function by absorbing UV energy and dissipating this safely by means of a tautomeric equilibrium involving formation of a six-membered ring

201

Degradation and Stabilisation of Aromatic Polyesters excited state, often referred to as ‘keto-enol tautomerism’, which involves the hydroxyl proton transferring to the carbonyl group, then back again. The result of this mechanism of light absorption and energy dissipation is that it leaves the stabiliser chemically unchanged, and thus capable of undergoing this action many times over. 2-Hydroxyphenylbenzotriazoles and the more recent hydroxyphenyl triazines would appear to operate via largely similar mechanisms. Cinnamic acid derivatives are said to operate via a mechanism in which the excited state dissipates energy by rotating about the double bond, which is weakened towards such a rotation in the excited state. There are also many vibrational energy-dissipation possibilities with cinnamates which can allow safe loss of absorbed energy. Once again, the molecule should be capable of undergoing several of these transformations. Aromatic oxanilides may also dissipate absorbed energy by proton transfer, but this is less clear with these materials. There may also be a contribution from a simple screening effect because these additives have very high extinction coefﬁcients in the region of 280–340 nm Cyclic imino esters were originally developed as PET chain extenders (see Section 6.4.2) but the UV-stabilising ability of some members of the family was noted. The mechanism of energy dissipation is not totally clear, but may involve ring opening and closing cycles in the excited state to achieve a similar effect to the excited state tautomerism of other UV absorbers.

8.2.2 Salicyclates Salicylate-type additives are largely unsuitable for use in aromatic polyesters, but some UV-stabilising ability has been shown by polyesters which contain moieties capable of transformation in the same way as the salicylates: the so-called Photo-Fries rearrangement [20–22]. More recently, re-arrangeable polymers such as poly(phenyl acrylate) and poly(p-methylphenyl acrylate) have been proposed 202

Stabilisation Against Ultraviolet and Ionising Radiation as UV stabilisers in PET [23, 24], but appear to require quite high loadings (5–10 wt%) to be effective. Similar rearrangements have been posited for Ardel, a polyarylate made by copolymerising isophthalic acid, terephthalic acid and bisphenol A, which may be used as an additive or coating for other aromatic polyesters [25]

8.2.3 Benzophenones Benzophenone-type UV absorbers have been suggested as viable stabilisers for PET and related polyesters [26–45]. Various additives were claimed for incorporation into a melt processing stage of an aromatic polyester. These include: dihydroxybenzophenones such as 2,2´-dihydroxy-4,4´dimethoxybenzophenone [26]; o-hydroxybibenzoylmethane, especially in poly(cyclohexanedimethylene terephthalate) [27]; bis(2hydroxybenzyl)alkanes, e.g., 1,8-bis(2-hydroxy-5-methylbenzyl) n-octane [28]; a polyester, strictly for use as an additive for other polyesters, consisting of the reaction product of 2,2´,4,4´tetrahydroxybenzophenone and sebacoyl chloride [29]; 2,2´,4,4´tetrahydroxybenzophenone itself [34]; a bis-benzophenone with a bridging group consisting of a -OCH 2-Ph-CH2O- unit [35]; terephthalamide-bis(benzophenone)s [38]; and, most recently, bisbenzophenones with a polyoxyalkylene bridge connecting the two active moieties [45]. It has also been suggested that benzophenones such as 2,2´-dihydroxy4,4´-dimethoxybenzophenone may be incorporated into dyed or non-dyed ﬁbres through the use of a polyhydric alcohol solution of the stabiliser [33]. Later studies [44] noted that this type of UV absorber diffused throughout a ﬁbre cross-section, which was not the case for the other classes investigated. Suggestions have also been made that better overall performance can be achieved by the use of more than one benzophenone [37], or combinations with other classes of stabiliser such as benzotriazoles or cinnamates [36], or in combination with antioxidants [41, 42]. 203

Degradation and Stabilisation of Aromatic Polyesters Due to the reactivity of polyesters, it is claimed that the surface of a ﬁbre or ﬁlm can be rendered UV-resistant by ﬁrst treating it with terephthaloyl chloride, then with a benzophenone with available reactive hydroxyl groups [30]. An alternative approach is to incorporate a benzophenone-type stabiliser into the polymer backbone, usually at low levels (0.1–5 wt%). Hoechst AG [32] claimed to achieve this by creating a stabilising moiety in the backbone by reacting ethylene glycol, terephthalic acid and 4-hydroxy-6-t-butylisophthalic acid, but other workers preferred to use a benzophenone stabiliser with a substituent containing two hydroxyl groups, e.g., 2-hydroxy-4(2,3dihydroxypropoxy)benzophenone, to react with other ingredients in the reaction kettle [39, 40, 43], or 2-hydroxy-4(2,3-epoxypropoxy) benzophenone [31].

8.2.4 Benzotriazoles. 2-Hydroxyphenylbenzotriazole-type UV absorbers in aromatic polyesters feature in a number of studies [44, 46–52], and are claimed in many patents [36, 41, 42, 53–65]. General Electric [53] claimed the use of 2(2´-hydroxy-3´,5´-di-tamylphenyl)benzotriazole (Tinuvin 328; Ciba) for UV stabilisation of ﬁre-retarded PET formulations. Adeka Argus [54] patented alkylidene-bis(benzotriazolylphenols) such as 2,2´-methylene-bis(4methyl-6-benzotriazolylphenol) for use in aromatic polyesters, later commercialised as ADK Stab LA31. Heat-shrinkable polyester ﬁlms [56] have been stabilised with a variety of benzotriazoles, including 2(2´-hydroxy-5´-methylphenyl)benzotriazole (Tinuvin P; Ciba), 2(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol (Tinuvin 234; Ciba) and 2(2´-hydroxy-5´-(1,1,3,3-tetramethylbutyl) phenyl)benzotriazole (Cyasorb UV5411; Cytec). Rhodia Filtec [57] used Tinuvin 234 in pigmented PET formulations, mixing the stabiliser and pigments together before addition to the polymer. Coltro and co-workers [49] studied the leachability of 2-(2´-hydroxy-

204

Stabilisation Against Ultraviolet and Ionising Radiation 3´-t-butyl-5´-methylphenyl)-5-chlorobenzotriazole (Tinuvin 326; Ciba) from PET bottles, although the additive was mainly present to protect the bottle contents form incident light. Ciba [59] patented benzotriazoles with polyoxyalkylene substituents and bisbenzotriazoles with bridging components of the same type. Begley and co-workers [50] studied the migration of Tinuvin 234 into foodstuffs from PET. Great Lakes [51] produce 2(2-hydroxy-3,5di(1,1-dimethylbenzyl)-2H-benzotriazole (Lowilite 234) and 2,2´methylenebis(6-(2H-benzotriazol-2-yl)-4-(1,1,3,3-tetramethylbutyl) phenol) (Lowilite 36), both of which are claimed to feature low volatility from the polyester substrate. Andrews [52] studied the use of Tinuvin 234 and octyl-3-(5-chloro2H-benzotriazol-2-yl)-5-(1,1-dimethyl)-4-hydroxybenzene propionate (Tinuvin 109; Ciba) in PET formulations to determine the best point in the process for addition of the stabilisers. It was found that Tinuvin 234 was rapidly lost if added at the polycondensation stage, but that Tinuvin 109, with its potentially reactive substituent, was retained when added at this stage. Both additives gave good stabilising performance if added at the melt processing stage. DuPont [63] claimed a process for stabilisation of dyed poly(trimethylene terephthalate) fibres by adding a speciality benzotriazole in the dye bath (Cibafast USM; Ciba). The structure of this additive was not revealed, but it most probably features extra substituents in the form of sulfonic acid(s) or derivatives thereof, which would allow the additive to be dispersed or dissolved in the aqueous dye bath. Milliken [64] recently patented benzotriazoles with longchain substituents of the type -CH2CH2(C=O)NR-R-O[(C=O) CH2CH2CH2CH2O]nH. These appear to form the basis of the Clearshield product, used as a polyester coating for the protection of a variety of substrates. Benzotriazoles are also available in the form of masterbatches in poly(ethylene terephthalate-co-isophthalate) [48] for extrusion of multilayer ﬁlms with considerably increased useful outdoor life. 205

Degradation and Stabilisation of Aromatic Polyesters Additive packages containing benzotriazoles along with other co-additives are also known. These include combinations with benzophenones and/or cinnamate-types [36]; with antioxidants [41, 42]; with fatty acid salts of manganese [55]; combinations of polyoxyalkyene-based benzotriazoles and PEN used in PET [61]; and benzotriazole plus poly(isobornyl acrylate) for the protection of polyalkylene naphthalates [62]. Ciba Speciality Chemicals have patented molecules which contain benzotriazole and hindered amine functional groups [60], e.g., 1(2hydroxy-2-methylpropoxy-2,2,6,6-tetramethylpiperidin-4-yl)-3-(5chlorobenzotriazol-2-yl)-5-tbutyl-4-hydroxycinnamate, and have also claimed their efﬁciency in PET, particularly in the form of ﬁbres. As with benzophenones, attempts have been made to incorporate benzotriazole stabilisers into the polyester chain. Additives investigated include dihydroxy-2-(2-hydroxyphenyl)-2H-benzotriazoles [46], tris(hydroxyphenyl)ethane benzotriazole [47] and diol-functionalised species of the type benzotriazole - CH2N(ROH)2 [58]. A study of the diffusion of benzotriazoles into PET ﬁbre [44] demonstrated that they penetrate only into the outer regions, unlike the benzophenones which diffuse throughout the substrate.

8.2.5 Cinnamates and Related Types Cinnamate-type UV absorbers have been patented by Bayer AG [66, 67] and Eastman Kodak/Eastman Chemical [68–81] for incorporation into polyalkylene terephthalates, mainly as reactive species incorporated into the polymer backbone. More recently low molecular weight additives of this type, which may or may not react into the polymer, have been commercialised by Clariant [82] and BASF AG [83–86]. Intellectual property assigned to Bayer covers benzylidene malonates and benzylidene bis-malonates [66], and includes the use of this type of stabiliser in the manufacture of polyesters [67]. It is likely

206

Stabilisation Against Ultraviolet and Ionising Radiation that the Clariant product, [(4-methoxyphenyl)methylene]dimethyl ester of propendioic acid (Sanduvor PR-25) is based on the same chemistry. The development of this type of chemistry built into polyesters was part of an extensive effort undertaken by Eastman over a number of years to develop various grades of self-coloured, UV-brightened and UV-stabilised polyester ﬁbres. Initial polyesters utilised reactive versions of the simplest cinnamate chemistry, such as: ~OCH2CH2O-PhCH=C(CN)-CO2~

[68]

~O2C-R-C(=C(Ph)(CN))-CO2~

[69]

and

while later versions used a variety of different structures to achieve improved reactivity and UV stabilising function: ~NH-Ph-CH=C(CN)~

[72]

~Heterocycle-CH=C(CN)~

[73]

~PhCH2OPh-CH=C(CN)~

[75]

~S-Ph-CH=C(CN)~

[76]

~C2H4O-Ph-CH=C(CN)~

[77]

~Ph-CH=C(CN)-R-Ph-CH=C(CN)~

[78, 80]

Also noted were PET copolymerised with these structures and with naphthalene dicarboxylic acid [79], and PEN itself with built-in cinnamate-type moieties [81]. Sanduvor PR-25 has been assessed as a UV stabiliser in a variety of polymers, including PET [82], where it is said to be photo-graftable to

207

Degradation and Stabilisation of Aromatic Polyesters the substrate, producing a non-staining, highly absorptive, stabilising system. Comparison with benzophenones and benzotriazoles showed this additive to have much lower colour development and considerably higher UV absorption. It was also shown to outperform the newer phenyltriazines in both respects. BASF products based on cinnamate-type chemistry, classed by the manufacturers as cyanoacrylates, include: Ethyl-2-cyano-3,3-diphenylacrylate

(Uvinul 3035)

2-Ethylhexyl-2-cyano-3,3-diphenylacrylate

(Uvinul 3039)

3-Bis((2-cyano-3,3´-diphenylacryloyl)oxy)-2,2-bis((2-cyano3´,3-diphenylacryloyl)oxyl)methylpropane (Uvinul 3030) Comparison of the absorption spectra of these products with those of benzophenones and benzotriazoles shows a marked difference [85]. The diphenylcyanoacrylates have a maximum absorption at 303 nm, whereas benzophenones have two peaks, at 280 nm and 340 nm, and benzotriazoles at 300 nm and 350 nm. The last two types also exhibit an absorption tail up to 400 nm, giving a yellowish tinge in some substrates. The diphenylcyanoacrylates are also said to be insensitive to metal ions, and to be equally effective under different pH conditions. Uvinul 3030, with its tetrameric functionality and low volatility is likely to be useful in thin polyester structures, such as ﬁlms and ﬁbres. This additive has also gained FDA approval for food contact use [86]. Allied Signal Incorporated [87] have patented cinnamamides of the general structure Ph[NH-(C=O)-C(CN)=CPh2]2, and have observed that these are especially effective stabilisers for aromatic polyesters. Clariant have noted that a combination of benzylidene bis-malonates with hindered amine light stabilisers showed excellent synergism in UV-stabilising polyalkylene terephthalates [88].

208

Stabilisation Against Ultraviolet and Ionising Radiation

8.2.6 Oxanilides Oxanilide UV stabilisers of the general form ArNH(C=O)(C=O) NHAr have been known for some time in symmetric [89] and asymmetric [90] forms. Both are available commercially from Clariant as Sanduvor EPU and VSU respectively. Also patented were bis-oxalic acid diamides [91] of basic structure ArNH(C=O)(C=O) NH-R-NH(C=O)(C=O)NHAr. While no studies have been found on the efﬁcacy of these additives in aromatic polyesters, their high thermal stability and the absorption peak at 290 nm with a tail only going as far as 350 nm means that they could be useful stabilisers for these polymers. Attempts have also been made to produce dual-functional stabilisers incorporating oxanilide and other functional groups. This was achieved by attaching a phenylbenzotriazole unit to one or both of the phenyl rings of the oxanilide [92], or replacing one of these phenyl rings with a phenylbenzotriazole [93] or a hindered piperidine group [94]. Later patents to General Electric Company [95, 96] claimed oxanilides which could be used to form polymeric stabilisers or could be added to the polymerisation stage of polycarbonates or polyesters to provide built-in stabilisers. An example of this type of structure is HOPh-R-PhNH(C=O)(C=O)NHPh-R-PhOH.

8.2.7 Cyclic Imino Esters Cyclic imino esters were originally developed, at least in terms of plastics additives, as chain extenders for polyesters (see Section 6.4.2). The same company involved in this research, Teijin, discovered that some of these compounds, most notably 2,2´-p-phenylene-bis(3,1benzoxazin-4-one), were effective UV absorbers [97]. It was also noted by the inventors that if the additive reacted with chain ends in a polyester or polyamide, this could result in a diminution of the UV protective ability of the additive. Careful compounding, with as low

209

Degradation and Stabilisation of Aromatic Polyesters a temperature as was practical and with short melt-processing times, was recommended. 2,2´-p-Phenylene-bis(3,1-benzoxazin-4-one) is commercially available from Cytec as Cyasorb 3638. Stabilisers of this type have been claimed as extremely effective UV absorbers for poly(ethylene-co-1,4-cyclohexanedimethylene terephthalate) [98–100], allowing the use of this material in outdoor applications. Developments in novel synthesis methods have improved the inherent stability of this class of additive, paving the way towards their more effective use in aromatic polyesters [101–106] Other additives claimed, besides the aryl-bisbenzoxazinones, include: oligomeric benzoxazin-4-ones end-capped with a phenyl group containing reactive substituents to allow the molecule to be copolymerised into polyesters, especially wholly aromatic polyesters [107]; water-dispersible versions for improvement of light-fastness of textiles, including those based on polyesters [108]; benzoxazinones with bulky side groups [109]; and isobenzoxazinones such as 2,2´-mphenylene-bis(3,1-benzoxazin-4-one) also having an -OR substituent on the 4-position of the bridging phenyl ring, where R is a long-chain alkyl group such as C18H37 [110].

8.2.8 Triazines Triazine-type UV absorbers have been patented in various forms by a number of companies, including Ciba [111–121], Cytec [122–125], Agfa Gevaert [126] and Asahi Denka [127]. Various stabilisers of this type are also commercially available, for example: 2(2-hydroxy-4-hexyloxyphenyl)-4,6-diphenyltriazine (Tinuvin 1577; Ciba) 2(hydroxy-4-octyloxyphenyl)-4,6-bis(2,4-dimethylphenyl) triazine (Cyasorb UV 1164; Cytec)

210

Stabilisation Against Ultraviolet and Ionising Radiation Versions with diacid, diol and di-ester substituents have been patented [111] which are suitable for reacting into aromatic polyesters, as have stabilisers with suitable substituents to allow dispersion or dissolution in dye baths for impregnation into ﬁbres or textiles [113, 114, 116]. Other related species include those with biphenylene groups on the 4- and 6-positions on the triazine core [119], naphthalene groups on 4 or on both 4 and 6 [120], 1,2,3,4-tetrahydronaphthalene on 4 and 6 [124], and 2-hydroxynaphthalene on the 2-position [125]. Versions with longer side chains than hexyl or octyl have also been synthesised, and it is claimed that these have increased compatibility with their host polymer [121, 127]. UV1164 has been shown to be particularly effective at protecting PEN [126]. It has also been observed that triazine-type UV absorbers form a particularly good synergistic combination with hindered amine light stabilisers in photostabilisation of a range of polymers [122]. Various studies of the effectiveness of these stabilisers in PET have been carried out [128–131]. Wang and co-workers [128], in an investigation into the kinetics of photo-oxidation of PET, noted that a 2,4,6-triphenyl-1,3,5-triazine derivative improved the lifetime of samples by a factor of 4.5 under the irradiation conditions used. Fechine and co-workers [129–131], in a study on the photo-oxidation of biaxially oriented PET ﬁlms observed that Tinuvin 1577 was a superior stabiliser to titanium dioxide or carbon black. It was also noted that this stabiliser was equally effective at protecting surface and inner regions of the sample ﬁlms.

8.2.9 Miscellaneous As research has been carried out into UV absorbers suitable for use in aromatic polyesters, a number of species have been identiﬁed which do not easily ﬁt into any of the categories covered above. Amongst such species are: 4-thiazolidone derivatives such as 5-benzal3-(p-B-hydroxyethylephenyl)-2-(p-B-hydroxyethylphenylamino)4-thiazolidone, a reactive UV absorber added to the polyester

211

Degradation and Stabilisation of Aromatic Polyesters reaction kettle [132]; heterocyclic esters such as 4(1,1,3,3tetramethylbutyl)phenyl-4-chloro-5-phenyloxazole-2-carboxylate, which were reasonable stabilisers but themselves highly coloured [133]; 3-benzalphthalides, which may have operated in a similar manner to the cinnamate-type stabilisers due to their PhCH=CR2 structure [134]; various imide types such as phthalimidobenzothiazoles [135] and aromatic diimides [136], although both were coloured species; and benzoxazolyl stilbenes, which again may operate as UV stabilisers via a similar mechanism to the cinnamates [137]. A more promising new additive type is based on 4-hydroxyquinoline3-carboxylic acid derivatives [138], which feature a very high extinction coefﬁcient in the UV, but little or no self-colour. These are claimed to be highly efﬁcient UV absorbers and thus particularly suitable for fibres and films. Similar additives may have been commercialised for use as UV barriers to protect the contents of plastics packaging. Very recently, BASF AG applied for patents on UV absorbers based on 4-cyano-naphthalene-dicarboximide derivatives, such as 4-cyano-N(2,6-diisopropylphenyl)naphthalene-1,8dicarboximide [139], and pyridindione derivatives such as 1,4dimethyl-5-dimethylaminomethylene-2,6-dioxo-3-cyano-1,2,5,6tetrahydropyridine [140]. It remains to be seen how the patenting process will proceed, and whether a commercial product will result.

8.3 Excited State Quenching While in the past the stabilisation of polymers via long-range or short-range energy transfer leading to quenching of excited states created in a polymer by photolysis was considered an important means of photostabilisation, this no longer appears to be the case [2, 141]. Many stabilisers thought to operate via this mechanism have been shown to owe their efﬁciency to other mechanisms, such as radical scavenging or UV absorption. This is not to say that this mechanism does not feature in the stabilisation of polymers to light. 212

Stabilisation Against Ultraviolet and Ionising Radiation Many stabiliser species operating mainly by other means do show some quenching activity. Researchers at Clemson University [142–145] have shown that inclusion of 0.5–4 mole% of naphthalene dicarboxylate or biphenylene dicarboxylate in a PET synthesis results in polymers with increased UV stability, which they demonstrated was at least partly due to energy transfer processes to the copolymerised units, which could then safely dissipate the energy. Milligan [146] later suggested that such copolymers could be useful in ﬁbres for automotive upholstery applications.

8.4 Radical Scavengers 8.4.1 Background A common feature of degradative processes undergone by polymers exposed to UV radiation is homolytic cleavage of covalent bonds. Thus, regardless of the photoinitiating species or process, or of the precise nature of the subsequent degradation, the polymer thus affected will contain radicals such as alkyl, alkoxy, hydroperoxy and hydroxy, and one or more of these reactive species will be involved in the perpetuation of photodegradation. Photostabilisation of some polymers might therefore be expected to occur via reduction of the numbers or activity of these radical species. Chain breaking – donor antioxidants such as hindered phenols are ineffective under conditions of photo-oxidation due to their rapid consumption in the higher rates of initiation present compared to thermal oxidation, and to photodegredant activity of their derived molecules such as quinones and quinone methides. Although they have limited photo-antioxidant ability when used alone, they can be protected from photolytic destruction by, for example, UV absorbers, and for this reason they can produce good synergistic results with a variety of co-stabilisers.

213

Degradation and Stabilisation of Aromatic Polyesters Of available light stabilisers potentially operating via an antioxidant mechanism, the most interesting are the hindered amine light stabilisers (HALS). Initially developed in Japan, these are largely based on active sites comprising 2,2,6,6-tetraalkylpiperidine [147] and 1,2,2,6,6-pentaalkylpiperidine [148]. Many studies have been made to identify the mechanism(s) by which HALS provide such effective UV stabilisation, and these have been reviewed [149–151]. Most authors agree that the >NH moiety is not, in fact, the stabilising species, but that >NO. stable radicals and >NOR hydroxylamine ethers formed during photo-oxidation are the true antioxidant species. These are constantly regenerated; the nitroxyl scavenging alkyl radicals and the hydroxylamine ethers intercepting hydroperoxy radicals and hydroperoxides. Hydroxylamines may also be formed, which are efﬁcient photostabilisers in their own right. Other factors contributing to the stabilising ability of HALS have been postulated. Carbonyl excited state quenching has been put forward as a possible factor [152], although there is also evidence contradicting such a mechanism [153]. It has also been shown that HALS, with their high basicity, are extremely powerful metal ion-chelating agents [153, 154], which could contribute to the stabilisation of a host polymer.

8.4.2 HALS No systematic study of the use of HALS as UV stabilisers in aromatic polyesters has been found in the literature; very few articles even touch on the subject, and these are limited to PBT [157–159]. Without comparisons with other stabiliser types, it is difﬁcult to assess the true capability of the HALS tested by these authors in PBT. It was noted by Borukaev and co-workers [159] that poly[[6[(1,1,3,3tetramethylbutyl)amino]-s-triazine-2,4-diyl]-[(2,2,6,6-tetramethyl4-piperidyl)imino]-1,6-hexamethylene[(2,2,6,6-tetramethyl-4piperidyl)imino]] (Chimmasorb 944; Ciba) was a better UV stabiliser than the dimethylsuccinate polymer with 4-hydroxy-2,2,6,6tetramethyl-1-piperidineethanol (Tinuvin 622; Ciba).

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Stabilisation Against Ultraviolet and Ionising Radiation A potential problem with HALS in aromatic polyesters is the very high basicity of these amines, which may result in reaction with the ester linkages and loss of molecular weight. There are indications that this may be a factor with the use of some HALS in ﬁbre-grade polyesters [160]. Taking the point of view of HALS, there is also the possibility that the additive might react with acid end groups, which may have an adverse effect on additive efﬁciency. It is clear from a consideration of the antioxidant mechanism of HALS that formation of a full-blown ionic salt of the additive would severely compromise its ability to form the active species involved in stabilising the host polymer. A study of the effects of acid exposure on HALS [161] demonstrated that the stabilising efﬁciency was destroyed only if additives were exposed to very strong acids such as hydrogen halides or nitric acid. The stabilising function was not signiﬁcantly lowered in the cases of either CO2/water or formic acid. The patent literature on HALS is voluminous, and some patents include a reference to the potential use of their claimed species in polyesters, although it is clear from reading the embodiment that other polymers are the true targets. Many fewer patents deal with HALS which are speciﬁcally claimed as being effective stabilisers for polyesters. A selection of patents in the first category include: piperidine spirooxirane derivatives [162]; low molecular weight polyesters with in-backbone or pendant piperidine substituents [163]; oligomeric esteramides with pendant hindered amine groups [164]; polyoxamate additives prepared by reacting, for example, N,N´-bis(ethoxyoxoacetyl)-N,N´-bis(2,2,6,6-tetramethyl-4piperidyl)-1,6-hexanediamine with 1,4-butanediol [165]; 6(1-hydro2,2,6,6-tetraalkylpiperine-4-oxy)dibenzodioxaphosphepins and dioxaphosphocins [166]; hindered amine-substituted dihydropyridines, such as 2,6-dimethyl-3,5-bis[(1,2,2,6,6-pentamethyl-4-piperidyl) oxycarbonyl)-1,4-dihydropyridine [167]; and polymeric species based on reactions between reactive HALS and diols, diamines or

215

Degradation and Stabilisation of Aromatic Polyesters aminoalcohols [168]. Also noted were additives containing HALS and hydroxylamine functionalities [169], such as N-benzyl-N(2,2,6,6tetramethylpiperidin-4-yl)hydroxylamine. Additives of this dualfunctional type have been claimed to be process and UV stabilisers in a range of polymers. A number of patents have been applied for which utilise existing HALS, speciﬁcally in aromatic polyesters, under speciﬁed circumstances. Chamassorb 944 may be used in polyester ﬁbres provided the melt temperature is kept as low as possible and residence time is minimised [170]. Polyester yarns have been stabilised through the use of a combination of a benzotriazole and the speciﬁc HALS 1-[2[3-(3,5-di-t-butyl-4-hydroxyphenyl)propinyloxy]ethyl]-4-[3-(3,5-dit-butyl-4-hydroxyphenyl)propinyloxy]-2,2,6,6-tetramethylpiperidine [171]. Polyesters of high molecular weight can be produced using a combination of a bisoxazoline chain extender and bis(2,2,6,6tetramethyl-4-piperidinyl)sebacate (Tinuvin 770; Ciba) [172]. It has been claimed that aromatic-aliphatic polyester ﬁbres beneﬁt from the addition of HALS [173]. In antimony-catalysed polycondensation of PET, addition of Tinuvin 622 to the reaction kettle is claimed not only to produce a polymer capable of being spun into UV stable ﬁbres, but also of superior spinnability [174]. PET with improved weather resistance may be achieved by an additive package of 2(5methyl-2-hydroxyphenyl)benzotriazole and a HALS of structure HPCH2CH2(C=O)OCH2CH2PipO(C=O)CH2CH2HP [175], where HP is a hindered phenol and Pip is a tetraalkylpiperidine group. Pigmented polyester ﬁbres may be stabilised with mixtures of HALS and UV absorbers [176], while injection-mouldable compositions beneﬁt from a triple package of UV absorber, antioxidant and HALS [177]. 4-benzoyloxy-2,2,6,6-tetramethylpiperidine has been proposed as a useful UV stabiliser for polyesters [178]. PBT with superior UV stability can be made by reacting appropriate end groups with HALS substituted with -OH, -CO2H or -CO2R groups [179]. A particular stabiliser, bis(2,2,6,6-tetramethylpiperidyl)isophthalamide (Nylostab S-EED; Clariant), while mainly used in polyamides, is also claimed to provide UV stabilisation to polyesters, along with improved processing, and better initial and weathered physical properties

216

Stabilisation Against Ultraviolet and Ionising Radiation [180]. Single or multilayer polyester films are advantageously stabilised with a combination of a HALS, preferably the reaction product of butanedioic acid with 4-hydroxy-2,2,6,6-tetramethyl-1piperidineethanol, and a triazine UV absorber such as Tinuvin 1577 [181]. Cationic dyeable (i.e., sulfonated) PET may be UV stabilised with HALS, particularly tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl)1,2,3,4-butanetetracarboxylate (ADKStab LA57; Adeka Argus) [182]. HALS may also be combined with amide-containing aldehyde scavengers such as anthranilamide or 1,6-bis(2-aminobenzamidoyl) hexane in aromatic polyesters [183]. 2,2,6,6-Tetraalkyl- and 1,2,2,6,6-pentaalkyl piperidine-based HALS which have been claimed speciﬁcally as stabilisers for aromatic polyesters include: alkylamine derivatives such as pentakis(2,2,6,6tetramethyl-4-piperidinyl)diethylenetriamine-N,N,N´, N´´, N´´pentaacetate [184]; unsymmetrical siloxanes with HALS moieties at one end and a reactive group on the other, e.g., -Si(CH2)3OH group capable of reacting with acid chain ends in the polymer [185]; and HALS diepoxides, which chain-extend and UV-stabilise polyesters and polyamides [186]. Combinations of ‘standard’ HALS types with other stabilisers suitable for use in aromatic polyesters include packages containing hindered phenol, sulfur-based antioxidant and HALS [187]; packages of triazine UV absorbers and tetraalkylpiperidines in engineering polyesters [122]; non-reactive siloxane-based HALS with hindered phenol and phosphite [188]; and bismalonate UV absorbers with HALS, which exhibit very good synergism in stabilising engineering polyesters [88]. There have also been patented stabilisers which contain two functionalities within one molecule, which functionalities are a HALS and a UV absorber. These include oxanilides with one phenyl group replaced with 2,2,6,6-tetramethylpiperidine [94], and additive molecules containing both HALS and a benzotriazole functionality [60, 65].

217

Degradation and Stabilisation of Aromatic Polyesters With the potential problem of reaction between hindered amines and polyester, work has been ongoing on identifying means of getting the activity of a HALS into a polyester whilst minimising potential problems. It was found that hydroxylamine ethers could be synthesised which, while being much weaker bases than the parent amines, remain effective UV stabilisers. Ciba-Geigy Corporation [189] developed such materials in the early 1990s and they are now commercially available, e.g., di(1-octyloxy-2,2,6,6-tetramethylpiperidin-4-yl) sebacate (Tinuvin 123; Ciba). Although the original patent does suggest the use of such additives in polyesters, the main target was poly(vinyl chloride), a matrix much more vulnerable to degradation by highly basic additives. Asahi Denka have also applied for patent protection on similar additives [190] based on carbonate structures such as (HE-O(C=O)O)nR, where HE is a hydroxylamine ether group, and n is 1 to 4. Articles describing these ‘NOR’ stabilisers have also been published, describing their use in non-polyester agricultural ﬁlms [191] and in engineering plastics applications [192], including a commercial product Tinuvin NOR 371 (Ciba). Another newer version of HALS is exempliﬁed by hydroxy-substituted N-alkoxy hindered amines, i.e., molecules with at least one active moiety of the structure >N-OR-(OH)n. A very wide range of such additives has been covered in a family of patents assigned to Ciba Speciality Chemicals [193–198], and claims for applications include aromatic polyesters. These additives are said to have as good as or better UV stabilising ability and antioxidant properties as the earlier HALS. The hydroxyl group(s) is said to impart additional advantages not possible with the NOR types, e.g., antistatic attributes, and better pigment dispersion in polar polymers. A HALS-based additive system has been developed by Clariant based on ‘salt-like’ reaction products of HALS with phosphorus-containing organic acids [199] or carboxylic acids [200]. Despite their salt-like character, these additives are said to stabilise in the normal HALS manner, and additionally provide improved processing, mechanical properties and appearance. They are speciﬁcally aimed at polyesters and polyamides. An example of one of these compounds is the

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Stabilisation Against Ultraviolet and Ionising Radiation reaction product of Nylostab S-EED and diphenylphosphinic acid. The salt-like character also means that these additives may be handled as aqueous dispersions or solutions. Already there have been patents published on the use of such salts in polyesters, in combination with UV absorbers [201].

8.4.3 Other Radical Scavengers Few radical scavengers besides the HALS have been suggested as UV stabilisers for aromatic polyesters. Some time ago, it was noted that certain nitro compounds could be used as UV stabilisers for polyesters, but closer reading of the documents showed this to be limited to certain sulfonated copolyesters [202]. Benzofuran-2-ones have been suggested as UV stabilisers with a good chain breaking – acceptor action, i.e., capable of scavenging alkyl radicals [203, 204, 156], but these are more efﬁcacious as thermal antioxidants and process stabilisers, especially when used in conjunction with hindered phenolic antioxidants.

8.5 Ionising Radiation Stabilisation All hydrocarbon polymers will suffer from severe degradative damage if exposed to high doses and/or long time periods of ionising radiation such as gamma rays. The problem of stabilising such substrates against this powerful radiation is that additives themselves will be liable to damage from radiation. Indeed, an early study into the effect of standard antioxidants on the gamma irradiation of Nylon 6 [205] showed them to have little or no effect on the degradation or oxidation processes. Hindered amines have been noted to have some stabilising effect on the gamma-ray degradation of polymers [206], particularly polypropylene [207], although it was observed that HALS were damaged by the radiation, with the resulting ringopening reactions rendering these additives inactive through time. Certain HALS with active sites based on N-(substituted)-1-(piperazin2-one) structures have been claimed as useful radiation stabilisers in

219

Degradation and Stabilisation of Aromatic Polyesters polypropylene [208]. More recently, stabilisation packages consisting of HALS, phosphite or phosphonate, and either hydroxylamine or nitrone [209] or benzofuranone [210], have been patented for use in polyoleﬁns. These systems are used largely to get away from the use of hindered phenols, whose breakdown products under irradiation would be highly coloured. Jipa and co-workers [211] noted that hindered phenols such as Irganox 1330 (Ciba) could be used in conjunction with pyrene to stabilise LLDPE against gamma-rays. Later studies by Balasa and co-workers [212] showed that other fused ring additives such as perylene and benzathracene could protect polyethylene. A polymer closer in structure to aromatic polyesters which has been the subject of much investigation into potential gamma-ray stabilisers is polycarbonate. Miles Incorporated and Bayer AG have claimed a wide variety of additives to be suitable for this task, including (all optionally alongside polyalkylene ether oligomers): aromatic disulﬁdes [213], aromatic sulfonic acid esters [214], halogenated aromatic acid derivatives [215], disulﬁde-aliphatic carbonate copolymers [216], dialkyl or dicycloalkyl mono- or poly-sulﬁdes [217], brominated phthalic anhydrides [218], oxirane-substituted phosphides [219], sulfonamides [220], aromatic, fused ring, sulfonamides [221], sulfur and nitrogen-containing heterocycles [222], dicyclohexyl phthalate [223], compounds of structure R-S(=O)2-(CHR)n-S-R [224] and 2-phenyl-1,3-dioxolane derivatives [225]. Exactly how these additives achieve their effect is unclear, and no speciﬁc recommendations exist within this intellectual property suggesting their use in aromatic polyesters. More generally applicable gamma-ray stabilisers have been claimed in the form of phosphorus-containing pentaerythritol derivatives, either alone [226] or in conjunction with a benzyl compound of the structure Ar-CHR-O-CHR-Ar [227]. Because the capture of secondary electrons produced by the strike of the gamma-ray photons and their safe deactivation is the most likely means of at least partially ameliorating the potential degradation,

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Stabilisation Against Ultraviolet and Ionising Radiation it is not surprising that highly aromatic species, with their ability to ‘soak-up’ electrons have been investigated. Another additive which might have the potential for capturing secondary electrons is a (semi) conductive polymer, and this approach has been the subject of some studies, including the use of polyaniline in styrene-butadiene rubber [228] and nanoﬁbres of the same in poly(methyl methacrylate) [229], poly(p-sulfanilamide) in poly(methyl methacrylate) [230] and polypyrrole in low-density polyethylene [231]. The polyalkylene terephthalates and polyalkylene naphthalates are, to some extent, self-protecting due to the presence of the aromatic rings, with the latter being noticeably more stable towards gamma irradiation than the former. Recently, Klein and co-workers [232– 234] studied conjugated small molecules as protective species to prevent radiation damage (and the associated increased conductivity) in PET ﬁlms. Such species, with electron-withdrawing substituents such as nitro or cyano, were reasonably effective in this role, with nitro-substituted ﬂuorenones being the most useful.

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227

Degradation and Stabilisation of Aromatic Polyesters 81. K.L. Carmen, M.E. Rogers, A.S. Jones, M.A. Weaver, inventors; Eastman Chemical Co., assignee; US6001952, 1999. 82. L.M. Sherman, Plastics Technology, 1999, 45, 5, 37. 83. A. Aumueller, M. Holderbaum, W. Goetze, J. Krockenberger and H. Trauth, inventors; BASF AG, assignee; US5821292, 1998. 84. M. Holderbaum, A. Aumueller, H. Trauth, G. Voit, K. Sperling and A. Krause, inventors; BASF AG, assignee; US5821380, 1998. 85. H. Trauth and J. Krockenberger in Proceedings of a Rapra Technology Conference - Addcon World ’99, Prague, Czechoslovakia, 1999, Paper No.20. 86. Additives for Polymers, 2003, July, 9. 87. K.A. Horn, R.B. Heath and D.B. Schwind, inventors; Allied Signal Inc., assignee; US5888481, 1999. 88. C. Krohnke, J.R. Webster, E. Gronmaier and L. Avar, inventors; Clariant Finance (BVI) Ltd., assignee; US7332535, 2008. 89. H. Biland, M. Duennerberger and C. Luethi, inventors; CibaGeigy AG, assignee; US3906033, 1975. 90. L. Avar, E. Kalt and H. Reinshagen, inventors; Sandoz Ltd., assignee; US4485034, 1984. 91. C. Luethi, H. Biland, M. Duennerberger, inventors; CibaGeigy AG, assignee; US3683020, 1972. 92. L. Avar, inventor; Sandoz Ltd., assignee; US4859725, 1989. 93. R.E. MacLeay, J.P. Kmiec and D.L. Stein, inventors; Elf Atochem North America Inc., assignee; EP593936, 1994. 228

Stabilisation Against Ultraviolet and Ionising Radiation 94. L. Avar, inventor; Sandoz Ltd., assignee; US4916175, 1990. 95. P.J.M. Likibi, inventor; General Electric Co., assignee; US5698732, 1997. 96. P.J.M. Likibi, inventor; General Electric Co., assignee; US5847196, 1998. 97. N. Okumura, T. Ichihashi, S. Matsumura and H. Inata, inventors; Teijin Ltd., assignee; US4446262, 1984. 98. D.R. Fagerberg and M.E. Donelson, inventors; Eastman Chemical Co., assignee; US5480926, 1996. 99. D.R. Fagerberg and M.E. Donelson, inventors; Eastman Chemical Co., assignee; US5558912, 1996. 100. D.R. Fagerberg and M.E. Donelson, Journal of Vinyl and Additive Technology, 1997, 3, 2, 179. 101. A.K. Sarka, inventor; Cytec Technology Corporation, assignee; US6774232, 2004. 102. A.K. Sarkar, inventor; Temsa International Inc., assignee; US2009030119, 2009. 103. T. Osaki and T. Ichihashi, inventors; Takemoto Yushi KK, Teijin Dupont Films Japan Ltd., assignees; US6809133, 2004. 104. T. Osaki and T. Ichihashi, inventors; Takemoto Yushi KK, Teijin Dupont Films Japan Ltd, assignees; US6908998, 2005. 105. T. Osaki and T. Ichihashi, inventors; Teijin Dupont Films Japan Ltd., assignee; US7057037, 2006. 106. T. Osaki and T. Ichihashi, inventors; Takemoto Yushi KK, Teijin Dupont Films Japan Ltd., assignees; US7160936, 2007. 107. J.P. Shepherd, inventor; Hoechst Celanese Corporation, assignee; US5264539, 1993.

229

Degradation and Stabilisation of Aromatic Polyesters 108. J. Mura, inventor; Sandoz Ltd., assignee; US5560852, 1996. 109. R. Ravichandran, S.D. Pastor, D. Judd, J.E. Babiarz, A.B. Naughton, M.G. Wood, A.D. Debellis, R. Xiang, R.E. Detlefsen and J. Suhadolnik, inventors; Ciba Speciality Chemicals Holding AG, assignee; US7244776, 2007. 110. C. Kroehnke, inventor; Clariant International Ltd., assignee; US2007219343, 2007. 111. J. Birbaum, J. Rody, M. Slongo and A. Valet, inventors; CibaGeigy Corporation, assignee; US5189084, 1993. 112. A. Schmitter, K. Burdeska, M. Slongo and J. Birbaum, inventors; Ciba-Geigy Corporation, assignee; US5288778, 1994. 113. K. Burdeska, G. Reinert and D. Reinehr, inventors; CibaGeigy Corporation, assignee; US5298030, 1994. 114. K. Burdeska, G. Reinert and D. Reinehr, inventors; CibaGeigy Corporation, assignee; US5387683, 1995. 115. J. Birbaum, J. Kaschig, D. Reinehr, M. Rembold, A. Schmitter, H. Luther, B. Hertzog and D. Haglin, inventors; Ciba-Geigy Corporation, assignee; US5591850, 1997. 116. D. Reinehr, G. Reinert and M. Rembold, inventors; CibaGeigy Corporation, assignee; US5649980, 1997. 117. J. Birbaum, J. Kaschig, D. Reinehr, M. Rembold, A. Schmitter, H. Luther, B. Herzog and D. Haglin, inventors; Ciba Speciality Chemicals Corporation, assignee; US5760111, 1998. 118. J. Kaschig, D. Reinehr and M. Rembold, inventors; Ciba Speciality Chemicals Corporation, assignee; US5942564, 1999.

230

Stabilisation Against Ultraviolet and Ionising Radiation 119. P. Hayoz, A. Valet and V.V. Toan, inventors; Ciba Speciality Chemicals Corporation, assignee; US6013704, 2000. 120. V.V. Toan, G. Metzger, T. Schaefer, S. Biry, C. Bulliard, D. Reinehr and P. Michaells, inventors; Ciba Speciality Chemicals Corporation, assignee; US7087753, 2006. 121. D. Lazzari, D.C. Leppard, M. Sala, M. Vitali, G. Zagnoni and G. Ferri, inventors; Ciba Speciality Chemicals Corporation, assignee; US7294714, 2007. 122. P.S. Waterman, inventor; Cytec Technology Corporation, assignee; US5714530, 1998. 123. P.S. Sheldon, inventor; Cytec Technology Corporation, assignee; US5760228, 1998. 124. R.B. Gupta, D.J. Jackiela and R.G. Fischer, inventors; Cytec Technology Corporation, assignee; US6384113, 2002. 125. R.B. Gupta, H. Singh and R.C. Cappadonna, inventors; Cytec Technology Corporation, assignee; US68000757, 2004. 126. M. August, J. Hagemann and B. Weber, inventors; Agfa Gevaert BV, assignee; JP10279785, 1998. 127. Y. Negishi, E. Tobita and T. Agabe, inventors; Asahi Denka Kogyo KK, assignee; US7125920, 2006. 128. W. Wang, A. Taniguchi, M. Fukuhara and T. Okada, Journal of Applied Polymer Science, 1999, 74, 2, 306. 129. G.J.M. Fechine, M.S. Rabello and R.M. Souto-Maior, Polymer Degradation and Stability, 2002, 75, 1, 153. 130. G.J.M. Fechine, R.M. Souto-Maior and M.S. Rabello, Journal of Materials Science, 2002, 37, 23, 4979. 131. G.J.M. Fechine, M.S. Rabello, R.M. Souto-Maior and L.H. Catalini, Polymer, 2004, 45, 7, 2303. 231

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232

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Degradation and Stabilisation of Aromatic Polyesters 157. M.H. Tabankia and J.L. Gardette, Polymer Degradation and Stability, 1986, 14, 4, 351. 158. E.V. Kalagina, V.A. Tochin, D.V. Gvozdev, T.N. Vakhtinskaya and T.I. Andreeva, International Polymer Science and Technology, 2004, 31, 8, T50. 159. T.A. Borukaev, L.R. Khashkhozheva and A.K. Mikitaev, International Polymer Science and Technology, 2008, 35, 2, T21. 160. S. Shipes in Proceedings of the INDA International Nonwovens Technical Conference, Atlanta, GA, USA, 2002, Paper No.22. 161. C. Zhang, D.J. Carlsson and D.M. Wiles, Journal of Polymer Science: Polymer Letters Edition, 1986, 24, 4, 453. 162. K. Murayama, S. Morimura, T. Yoshioka, K. Matsui, T. Kurumada, N. Ohta and I. Watanabe, inventors; Sankyo Co., assignee; US3692778, 1972. 163. J. Rody and M. Rasberger, inventors; Ciba-Geigy Corporation, assignee; US4233412, 1980. 164. R.A.E. Winter, R.F. Malherbe and F.T. Fu, inventors; CibaGeigy Corporation, assignee; US4439565, 1984. 165. G. Cantatore and V. Borzatta, inventors; Ciba-Geigy Corporation, assignee; US4751281, 1988. 166. R. Ravichandran and J.D. Spivak, inventors; Ciba-Geigy Corporation, assignee; US4808645, 1989. 167. L. Carette, M. Gay, S. Lavault and G. Mur, inventors; RhonePoulenc Chemie, assignee; US5124456, 1992. 168. T.P. Sassi and R.B. Gupta, inventors; Cytec Technology Corporation, assignee; US6492521, 2002.

234

Stabilisation Against Ultraviolet and Ionising Radiation 169. J. Suhadolnik, R. Ravichandran, V. Borzetta and G. Vignali, inventors; Ciba-Geigy Corporation, assignee; US5552464, 1996. 170. T. Tatebayashi, J. Kuwata and T. Okada, inventors; Toray Industries, assignee; JP61225315, 1986. 171. K. Sato and N. Takeuchi, inventors; Kuraray Co., assignee; JP62276018, 1987. 172. M. Oshida, inventor; Teijin Ltd., assignee; JP63039921, 1988. 173. T. Sakano, J. Kuwata and T. Okada, inventors; Toray Industries, assignee; JP63085111, 1988. 174. M. Oshida, inventor; Teijin Ltd, assignee; JP63179925, 1988. 175. T. Iwakiri, G. Shimaoka and K. Okazaki, inventors; Mitsubishi Gas Chemical Co., assignee; JP07216206, 1995. 176. P. Slera, R. Reinicker, F. Babler, D.W. Horsey, J.S. Puglisi, K. Schumann and J. Suhadolnik, inventors; Ciba Speciality Chemicals Holding Inc., assignee; EP704560, 1995. 177. B.M. Mulholland, inventor; Hoechst Celanese Corporation, assignee; EP953595, 1999. 178. M. Nakamura and K. Ono, inventors; Ube Industries, assignee; JP08048860, 1996. 179. M. Kishishita, M. Saito and O. Kidai, inventors; Mitsubishi Chemical Corporation, assignee; JP08120066, 1996. 180. J.R. Webster, inventor; Clariant Finance (BVI) Ltd., assignee; US5965261, 1999. 181. S.A. Johnson, D.J. McGurran, T.R. Bailey and J.W. Frank, inventors; 3M Innovative Properties Co., assignee; US6613819, 2003. 235

Degradation and Stabilisation of Aromatic Polyesters 182. T. Nakamura, inventor; Teijin Fibers Ltd., assignee; JP2007084974, 2007. 183. F. Zeng and M. Frost, inventors; Colormatrix Europe Ltd., assignee; US2008241450, 2008. 184. Y. Takahashi, Y. Muegawa, H. Yamamoto, T. Kaneoya, H. Okamura, S. Yachigo and T. Ishii, inventors; Sumitomo Chemical Co. Ltd., assignee; US4719037, 1988. 185. H. Friedrich, I. Jansen and K. Ruehlmann, inventors; no assignee; DE4216923, 1993. 186. R. Pfaendner, A. Steinmann, H. Herbst and K. Hoffmann, inventors; Ciba Speciality Chemicals Holding AG, assignee; US6028129, 2000. 187. T. Fujii, T. Ishii, S. Yachigo, T. Kaneoya, Y. Takahashi, Y. Muegawa, H. Okamura and E. Okino, inventors; Sumitomo Chemical Co. Ltd., assignee; US5049604, 1991. 188. W. Budzinsky, G. Kirsch, H. Naumann and T. Wehrmeister, inventors; Tocona GmbH, assignee; DE19820123, 1999. 189. F.P. Cortolano, R. Seltzer and A.R. Patel, inventors; CibaGeigy Corporation, assignee; US5004770, 1991. 190. Y. Negishi, T. Ayabe and E. Tobita, inventors; Asahi Denka KK, assignee; WO2005082852, 2005. 191. P. Solera, Journal of Vinyl and Additive Technology, 1998, 4, 3, 197. 192. J. Markarian, Plastics Additives and Compounding, 2007, 9, 2, 32. 193. J.P. Galbo, G.A. Capocci, N.N. Cliff, R.E. Detlefsen, M.P. DiFazio, R. Ravichandran, P. Solera and C. Bulliard, inventors; Ciba Speciality Chemicals Holding AG, assignee; US6376584, 2002. 236

Stabilisation Against Ultraviolet and Ionising Radiation 194. J.P. Galbo, G.A. Capocci, N.N. Cliff, R.E. Detlefsen, M.P. DiFazio, R. Ravichandran and P. Solera, inventors; Ciba Speciality Chemicals Corporation, assignee; US6391949, 2002. 195. J.P. Galbo, G.A. Capocci, N.N. Cliff, R.E. Detlefsen, M.P. DiFazio, R. Ravichandran and P. Solera, inventors; Ciba Speciality Chemicals Corporation, US6403681, 2002. 196. J.P. Galbo, G.A. Capocci, N.N. Cliff, R.E. Detlefsen, M.P. DiFazio, R. Ravichandran and P. Solera, inventors; Ciba Speciality Chemicals Holding AG, assignee; US6420463, 2002. 197. J.P. Galbo, G.A. Capocci, N.N. Cliff, R.E. Detlefsen, M.P. DiFazio, R. Ravichandran, P. Solera and C. Bulliard, inventors, Ciba Speciality Chemicals Corporation, assignee; US6586507, 2003. 198. J.P. Galbo, G.A. Capocci, N.N. Cliff, R.E. Detlefsen, M.P. DiFazio, R. Ravichandran and P. Solera, inventors; Ciba Speciality Chemicals Corporation, assignee; US6638997, 2003. 199. P. Staniek, inventor; Clariant International Ltd., assignee; US2006079610, 2006. 200. P. Staniek, inventor; Clariant International Ltd., assignee; US2006217467, 2006. 201. J.C. Pearson, D.S. McWilliams, G. Irick and M.A. Weaver, inventors, Eastman Chemical Co., assignee; US7491760, 2009. 202. G.C. Newland and J.W. Tamblyn, inventors; Eastman Kodak Co., assignee; US3247162, 1966. 203. P. Nesvadba and S. Evans, inventors; Ciba Speciality Chemicals Holding AG, assignee; US5693829, 1997.

237

Degradation and Stabilisation of Aromatic Polyesters 204. P. Nesvadba and S. Evans, inventors; Ciba-Geigy Corporation, assignee; US5614572, 1997. 205. W. Szymanski and B. Rymian, Angewandte Makromolekulare Chemie, 1981, 99, 1, 85. 206. E. Stengrevics and K.D. Cooper, Plastics Compounding, 1989, 12, 6, 69. 207. S. Falicki, D.J. Gosciniak, J.M. Cooke and D.J. Carlsson, Polymer Degradation and Stability, 1994, 43, 1, 1. 208. G. Kletecka, J.T. Lai and P. Son, inventors; BF Goodrich Co., assignee; US4797438, 1989. 209. R.E. King, inventor, Ciba Speciality Chemicals Corporation, assignee; US6664317, 2003. 210. R.E. King, inventor; Ciba Speciality Chemicals Corporation, assignee; US6872764, 2005. 211. S. Jipa, M. Nishimoto, H. Otsuki and Z. Osawa, Polymer Degradation and Stability, 1996, 54, 1, 99. 212. N. Balasa, T. Setnescu, S. Jipa, R. Setnescu and I. Mihelcea, Materiale Plastice, 2007, 44, 4, 349. 213. D.G. Powell and C.E. Lundy, inventors; Miles Inc., assignee; US5214078, 1993. 214. U. Grigo, J. Kirsch, K. Idel and C.E. Lundy, inventors; Bayer AG, Miles Inc., assignees; US5274009, 1993. 215. D.G. Powell and S. Krishnan, inventors; Miles Inc., assignee; US5280050, 1994. 216. A.D. Meltzer, H. Pielartzik and R. Archey, inventors; Bayer AG, assignee; US5453457, 1995.

238

Stabilisation Against Ultraviolet and Ionising Radiation 217. R. Archey, C.E. Lundy, A.D. Meltzer, H. Pielartzik, G. Fennhoff, R. Hufen, K. Kircher, R. Schubert and R. Weider, inventors; Bayer AG, assignee; US5464893, 1995. 218. C. Lundy, U. Grigo, A. Sommer, K. Horn, K. Sommer and A. Becker, inventors; Bayer AG, assignee; US5476893, 1995. 219. J.P. Mason, inventor; Bayer AG, assignee; US5491179, 1996. 220. G. Fennhoff, R. Hufen, K. Kircher and W. Ebert, inventors; Bayer AG, assignee; US5612398, 1997. 221. W. Ebert, R. Hufen, R. Schubert and G. Fennhoff, inventors; Bayer AG, assignee; US5684062, 1997. 222. W. Ebert, R. Hufen, R. Schubert and G. Fennhoff, inventora; Bayer AG, assignee; US5773491, 1998. 223. J.Y.J. Chung, Journal of Applied and Medical Polymers, 1998, 2, 1, 19. 224. W. Ebert, R. Hufen, H. Pantke and K. Berg, inventors; Bayer AG, assignee; US5852070, 1998. 225. D.H. Bolton, S. Krishnan, D.M. Derikart and J.B. Johnson, inventors; Bayer AG, assignee; US6197853, 2001. 226. J.A. Mahood, inventor; General Electric Co., assignee; US5559167, 1996. 227. W. Funakoshi, T. Kanda, F. Kondo and K. Sasaki, inventors; Teijin Ltd., assignee; US6485657, 2002. 228. M.N. Ismail, M.S. Ibrahim and A.M. Abd El-Ghaffar, Polymer Degradation and Stability, 1998, 62, 2, 337. 229. P.L.B. Araujo, R.F.S. Santos and E.S. Araujo, Express Polymer Letters, 2007, 1, 6, 385.

239

Degradation and Stabilisation of Aromatic Polyesters 230. S.M. Sayyah, A.B. Khaliel and H.M. Abd El-Salam, Journal of Applied Polymer Science, 2007, 106, 2, 1294. 231. T. Zaharescu and S. Jipa, E-Polymers, 2008, No.167, 1. 232. R.J. Klein, J.L. Schroeder, S.M. Cole, M.E. Belcher, P.J. Cole and J.L. Lenhart, Polymer, 2008, 49, 11, 2632. 233. R.J. Klein, S.M. Cole, M.E. Belcher, J.L. Schroeder, P.J. Cole and J.L. Lenhart, Polymer, 2008, 49, 25, 5541. 234. R.J. Klein, S.M. Cole, M.E. Belcher, J.L. Schroeder, P.J. Cole and J.L. Lenhart, Polymer, 2008, 49, 25, 5549.

240

A

ppendix – Commercial Additive Structures

The structures provided here are of commercial additives associated in the literature with use in aromatic polyesters, and referred to in the preceding text. However, it should be noted that neither the author or the publisher will accept any responsibility, actual or implied, for any loss, damage, injury or legal action resulting from the use of any of the additives in any formulation or process. Compounders are urged to contact their suppliers and discuss with them the applicability of any additive in a particular formulation or process, and to follow all the health and safety data provided by said suppliers.

ADKStab LA31

241

Degradation and Stabilisation of Aromatic Polyesters

ADKStab LA57

Alkanox 28

242

Appendix – Commercial Additive Structures

Chimmasorb 944

Cyasorb UV1164

243

Degradation and Stabilisation of Aromatic Polyesters

Cyasorb UV5411

Cyasorb 3638

Epon 828

Hostanox OSP-1

244

Appendix – Commercial Additive Structures

HP-136

Irgafos 12

Irgafos 168

245

Degradation and Stabilisation of Aromatic Polyesters

Irganox 245

Irganox 259

Irganox 1010

246

Appendix – Commercial Additive Structures

Irganox 1019

Irganox 1076

Irganox 1098

247

Degradation and Stabilisation of Aromatic Polyesters

Irganox 1222

Irganox 1330

248

Appendix – Commercial Additive Structures

Irganox 1425

Irganox 1790

249

Degradation and Stabilisation of Aromatic Polyesters

Irganox 3114

Irgastab FS-042

Joncryl ADR 4368

250

Appendix – Commercial Additive Structures

Leucopor EGM

Lowilite 36

Naugard 445

251

Degradation and Stabilisation of Aromatic Polyesters

Nylostab S-EED

Ronotec 201

Sandostab P-EPQ

252

Appendix – Commercial Additive Structures

Sanduvor PR-25

Sanduvor EPU

Sanduvor VSU

253

Degradation and Stabilisation of Aromatic Polyesters

Stabilizer 7000

Tinuvin P

Tinuvin 109

254

Appendix – Commercial Additive Structures

Tinuvin 123

Tinuvin 213

Tinuvin 234

255

Degradation and Stabilisation of Aromatic Polyesters

Tinuvin 320

Tinuvin 326

Tinuvin 328

256

Appendix – Commercial Additive Structures

Tinuvin NOR 371

Tinuvin 622

Tinuvin 770

257

Degradation and Stabilisation of Aromatic Polyesters

Tinuvin 1130

Tinuvin 1577

Ultranox 626

258

Appendix – Commercial Additive Structures

Uvinul 3030

Uvinul 3035

Uvinul 3039

259

Degradation and Stabilisation of Aromatic Polyesters

260

A

bbreviations

BHET

Bis(hydroxyethyl)terephthalate

BNZ

2,2´-Bis(4H-3,1-benzoxazin-4-one)

BOZ

2,2´-Bis(2-oxazoline)

bp

Boiling point

CB-A

Chain-breaking acceptor

CB-D

Chain-breaking donor

CEP

Concerted ester pyrolysis

cf

Compare

DEG

Diethylene glycol

DMT

Dimethyl terephthalate

DNaTA

Disodium terephthalate

DTA

Differential thermal analysis

EG

Ethylene glycol

FT-IR

Fourier-transform infrared spectroscopy

GC-MS

Gas chromatography-mass spectrometry

HALS

Hindered amine light stabilisers

HE

Hydroxylamine ether group,

HP

Hindered phenol

IR

Infrared

LCP

Liquid crystal polyesters(s)

MALDI-TOF

Matrix-assisted laser desorption ionisation-time of ﬂight

OX

Oxidation

261

Degradation and Stabilisation of Aromatic Polyesters PAT

Poly(alkylene terephthalate)s

PBN

Poly(butylene naphthalate)

PBT

Poly(butylene terephthalate)

PCT

Poly(1,4-cyclohexylenedimethylene terephthalate)

PEN

Poly(ethylene naphthalate)

PET

Poly(ethylene terephthalate)

PIP

Tetraalkylpiperidine group

PP

Polypropylene

ppm

Parts per million

PTT

Poly(trimethylene terephthalate)

SSP

Solid-state post-condensation

TA

Terephthalic acid

TH

Transesteriﬁcation/hydrolysis

THF

Tetrahydrofuran

TVA

Thermal volatilisation analysis

UV

Ultraviolet

262

I

ndex

A Alcoholysis 114 Aliphatic polyamides 1 Aminolysis 110 Anaerobic thermal degradation 25 Antioxidants 183 Aromatic oxanilides 202 Aromatic polyesters 2, 4, 9-10, 16, 21, 49, 80, 97, 110, 112, 114115, 118, 143, 144, 147, 150, 152, 154, 181-183, 186, 187, 188, 190, 202-203, 208-211, 214-215, 219-220 depolymerisation 115

B Benzophenone 203-204, 206, 208 Biodegradation 110 Bis(hydroxyethyl)terephthalate 113, 116, 118-119

C Carbon-arc light source 86 Chemical degradation 107 Cinnamates 203 Copolymerisation 16 Cyclic imino esters 202, 209 Cyclic oligomers 26, 33, 39, 40-41, 44, 49, 143

263

Degradation and Stabilisation of Aromatic Polyesters

D Depolymerisation 81-82, 112, 120 Diethylene glycol 7 Differential thermal analysis 183 Dimerisation 32 Dimethyl terephthalate 112, 120, 122 Diphenyl cyanoacrylates 208 Disodiumterephthalate 113

E End-capping 153, 155 Ester pyrolysis, concerted 70 Ethanolysis 114 Ethylene glycol 112

F Fourier transform infrared spectroscopy 50, 51 Free-radical breakdown mechanism 40

G Gas chromatography 31 Gas chromatography-mass spectrometry 75, 80-81 Gel formation 73, 95, 98 Glycolysis 113-114, 116, 118, 121

H Hindered amine light stabilisers 214-218, 220 Hydantoin 151 Hydrogen transfer, B C-H 41 Hydrolysis 23, 36, 107, 113-114, 117, 119, 121, 144, 149, 161 depolymerisation 117 Hydrolytic degradation 107, 148 Hydrolytic stabilisation 143 p-Hydroxybenzoic acid 2 Hydroxylamines 188

264

Index

I Infrared spectroscopy 32 Injection moulding 28

M Matrix-assisted laser desorption ionisation-time of ﬂight mass spectrometry 33, 41 Melt processing 143, 146, 157, 182 Methanolysis 116-118

N Norrish I reaction 87 Norrish II reaction 87

O Oxanilide ultraviolet stabilisers 209 Oxidation 70

P Peroxide decomposers 182 Photodegradation 85, 88 Photo-Fries rearrangement 202 Photolysis 89, 94-95, 97-98 Photo-oxidation 87, 89-90, 91, 94-95, 213 Photostabilisation 200, 211-213 Poly(alkylene terephthalate) 44, 47, 48, 93, 94 Polyamides 97 Polyaniline 221 Polyarylates 49, 82, 203 Poly(butylene naphthalate) 80, 95 Poly(butylene terephthalate) 3-4, 6, 8, 12, 35, 36-37, 38-45, 47, 73, 75-77, 78, 93, 95, 98, 108-109, 118, 149, 151, 153, 155, 157, 186, 189, 201 Polycondensation 143, 156, 182, 185 melt 7

265

Degradation and Stabilisation of Aromatic Polyesters Poly(1,4-cyclohexylenedimethylene terephthalate) 4, 13 Polyester, liquid crystal 3-4, 15, 51 Polyester photodegradation 199 Polyester stabilisation 153 Polyester yarns 216 Poly(ethylene naphthalate) 3, 5, 14, 15, 80, 94, 95, 97, 98, 108109, 111, 116, 153, 156, 201, 207 Poly(ethylene terephthalate) 3-8, 10-15, 21-28, 30, 31, 33, 35-38, 40-42, 44-45, 47, 51, 65, 66, 67, 69, 71-78, 85-90, 92, 93, 95, 97, 98, 99, 108, 110-116, 118-122, 143, 146-152, 154, 156-161, 183-184, 185, 189, 200, 202, 204-207, 211, 213, 217 hydrolysis 109 morphology 89 photodegradation 93 photolysis 91-92 photo-oxidation 93 recycling 121 Polymer irradiation 96 Polymerisation 5, 9, 44, 151, 190 Polymer matrix 85, 87, 154, 187 Polymer morphology 9 Polymer recycling 112 Polyoleﬁns 73, 97 Poly(trimethylene terephthalate) 4, 6, 8, 11, 12, 42-44, 77, 79, 108, 109, 146, 151, 155-156, 189, 191 Polyurethanes industry 122 Positron annihilation lifetime spectroscopy 98 Pyrolysis 31, 46, 47, 50, 153 Pyrolysis - mass spectrometry technique 26 direct 31 Pyrolysis-gas chromatography 38, 43

R Radiation degradation 85, 96 Radical scavengers 213, 129 Recycling 107

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Index

S Salicylates 202 Saponiﬁcation 113, 115, 119, 122-123 Scission, homolytic 143 Self-condensation 2 Solid-state post-condensation 7

T Tautomerism, keto-enol 202 Tetrahydrofuran 36-37, 40-41, 74, 81 Thermal analysis 98 Thermal degradation 21, 73, 143, 153 Thermal stabilisation 143, 144, 152 Thermal volatilisation analysis 28 Thermogravimetric 51 Thermolysis 97 Thermo-oxidative degradation 65, 80 Thermo-oxidative process 73 Thermo-oxidative stabilisation 181, 199 Thermoset polymers 1 Transesteriﬁcation 184, 185 Transesteriﬁcation/hydrolysis 70 Triazines 210

U Ultraviolet absorber 200-203, 206, 210-213, 217 Ultraviolet barriers 212 Ultraviolet degradation 88 Ultraviolet stabilisation 199 Ultraviolet stabiliser 201, 203, 207, 214, 216, 218, 219 Ultraviolet screeners 200-201

V Viscosity, measurements of 98 Volatilisation 187

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Degradation and Stabilisation of Aromatic Polyesters

X Xenon-arc light source 86

268

Published by iSmithers, 2009

This book provides a comprehensive survey of the degradation and stabilisation processes specific to aromatic polyesters, including thermal, thermo-oxidative, chemical, light and radiation degradation and stabilisation. Current knowledge of all these aspects is discussed and analysed, and some suggestions made as to further studies which might advance the subject. Materials covered include well-known polyesters such as poly(ethylene terephthalate) and poly(butylene terephthalate), through the less wellknown poly(alkylene naphthalate)s and liquid crystalline polyesters, to ‘new’ substances such as poly(trimethylene terephthalate). Also covered are the various means of chemically recycling aromatic polyesters into their starting materials and/or other useful chemical feedstock, including current research into improvements in chemistry and economics of such processes, and information on commercial enterprises carrying out such recycling. With over 1000 references to papers and patents, this book provides both a highly detailed source of information on the degradation and stabilisation of aromatic polyesters in itself, and a useful starting point for further study of this topic both by academic and industrial workers in this field. Those researching or manufacturing aromatic polyester formulations for use in fibres, films, packaging, automotive applications and engineering applications will find much to interest them here.

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