Biaxial stretching of film
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Pre.indd 1
7/4/11 5:56:28 PM
Related titles: Environmentally compatible food packaging (ISBN 978-1-84569-194-3) Food packaging performs an essential function, but packaging materials can have a negative impact on the environment. This collection reviews bio-based, biodegradable and recycled materials and their current and potential applications for food protection and preservation. The first part of the book focuses on environmentally-compatible food packaging materials. The second part discusses drivers for using alternative packaging materials, such as legislation and consumer preference, environmental assessment of food packaging and food packaging eco-design. Chapters on the applications of environmentallycompatible materials for particular functions, such as active packaging, and in particular product sectors then follow. Advances in polymer processing: From macro- to nano-scales (ISBN 978-1-84569-396-1) Processing techniques are critical to the performance of polymer products, which are used in a wide range of industries. This book provides a comprehensive review of polymer procesing, focusing on recent developments in techniques and materials. Thermosets, thermoplastics, elastomers, foams and nanocomposites are all discussed; multiphase systems are considered from macro to nano scales. Developments in established techniques are reviewed, such as extrusion technologies, injection moulding and blow moulding, in addition to recently developed processing technologies, such as those using supercritical fluids, micromoulding and reactive processing. Finally post-processing techniques are examined, as well as analysis of the moulding process. Packaging technology: fundamentals, materials and processes (ISBN 978-1-84569-665-8) Packaging technology: Fundamentals, materials and processes provides a comprehensive introduction to packaging. The book thoroughly reviews the basics and more advanced concepts in packaging technology. It is based on the degree level Diploma in Packaging Technology course promoted by IOP: The Packaging Society. Written by a team of experienced packaging professionals, the text discusses all major topics in packaging. Part one reviews packaging fundamentals such as the supply chain, packaging functions, legislation and marketing. The materials and package components are discussed in part two. Packaging processes are the focus of the final part, covering design and development, printing, machinery, quality, risk management and cost implications. Details of these books and a complete list of Woodhead’s titles can be obtained by: ∑ visiting our web site at www.woodheadpublishing.com ∑ contacting Customer Services (e-mail:
[email protected]; fax: +44 (0) 1223 832819; tel.: +44 (0) 1223 499140 ext. 130; address: Woodhead Publishing Limited, 80, High Street, Sawston, Cambridge CB22 3HJ, UK) If you would like to receive information on forthcoming titles, please send your address details to: Francis Dodds (address, tel. and fax as above; e-mail: francis.dodds@ woodheadpublishing.com). Please confirm which subject areas you are interested in.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Pre.indd 2
7/4/11 5:56:28 PM
Biaxial stretching of film Principles and applications
Edited by Mark T. DeMeuse
Oxford
Cambridge
Philadelphia
New Delhi
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Pre.indd 3
7/4/11 5:56:28 PM
Published by Woodhead Publishing Limited, 80 High Street, Sawston, Cambridge CB22 3HJ, UK www.woodheadpublishing.com Woodhead Publishing, 1518 Walnut Street, Suite 1100, Philadelphia, PA 19102-3406, USA Woodhead Publishing India Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi – 110002, India www.woodheadpublishingindia.com First published 2011, Woodhead Publishing Limited © Woodhead Publishing Limited, 2011 The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Control Number: 2011932266 ISBN 978-1-84569-675-7 (print) ISBN 978-0-85709-295-3 (online) The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elemental chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by Replika Press Pvt Ltd, India Printed by TJI Digital, Padstow, Cornwall, UK
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Pre.indd 4
7/4/11 5:56:28 PM
Contents
Contributor contact details
Part I Fundamental principles of biaxial stretching 1
ix
1
Fundamentals of biaxial stretching and definitions of terms M. T. DeMeuse, Independent Consultant, USA
1.1 1.2 1.3 1.4 1.5
Introduction Methods of biaxial stretching Recommendations Conclusions References
3 3 11 12 13
2
14
Equipment design and requirements of biaxially stretched films M. T. DeMeuse, Independent Consultant, USA
2.1 2.2 2.3 2.4 2.5
Introduction Double bubble process for biaxial stretching of films Tenter process for production of biaxially oriented films Recommendations References
14 14 17 24 25
3
Laboratory evaluations of biaxially stretched film M. T. DeMeuse, Independent Consultant, USA
27
3.1 3.2 3.3 3.4
Introduction T.M. Long stretcher for laboratory evaluations Karo IV laboratory stretcher from Brückner Literature studies involving laboratory stretching equipment Recommendations References
27 27 29
3.5 3.6
3
30 34 35
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Pre.indd 5
7/4/11 5:56:28 PM
vi
Contents
4
M. T. DeMeuse, Independent Consultant, USA
4.1 4.2 4.3 4.4 4.5 4.6
Introduction Polypropylene Use of polyethylene in biaxial stretching Conclusions Recommendations References
36 36 41 43 44 45
5
Other polymers used for biaxial films M. T. DeMeuse, Independent Consultant, USA
47
5.1 5.2 5.3 5.4 5.5 5.6
Introduction Polyethylene terephthalate (PET) Polyamides in biaxially oriented films Poly(lactic acid) (PLA) in biaxially stretched films Recommendations References
47 47 51 53 56 56
6
Biaxial film structures M. T. DeMeuse, Independent Consultant, USA
59
6.1 6.2 6.3 6.4
Introduction Film structures based on homopolymer polypropylene Recommendations References
59 59 65 65
7
Typical industrial processes for the biaxial orientation of films M. T. DeMeuse, Independent Consultant, USA
67
7.1 7.2
Polyolefins used in biaxial stretched films
36
7.3 7.4 7.5
Introduction Commercial production processes for biaxially oriented films Novel technologies currently being developed Recommendations References
67
8
Post-production processing of biaxially oriented films M. T. DeMeuse, Independent Consultant, USA
76
8.1 8.2 8.3 8.4 8.5
Introduction Surface treatment of films Conclusions Recommendations References
76 76 83 84 84
67 72 74 74
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Pre.indd 6
7/4/11 5:56:28 PM
9
Contents
vii
Strain energy function and stress–strain model for uniaxial and biaxial orientation of poly(ethylene terephthalate) (PET) M. A. Ansari, M. R. Cameron and S. A. Jabarin, University of Toledo, USA
86
9.1 9.2 9.3 9.4 9.5 9.6
Introduction Experimental Stress–strain behavior of poly(ethylene terephthalate) (PET) Modeling of the stress–strain behavior – literature review Development of a stress–strain model References
86 89 90 99 103 113
10
Academic investigations of biaxially stretched films M. T. DeMeuse, Independent Consultant, USA
117
10.1 10.2 10.3 10.4 10.5
Introduction Literature studies of common commodity polymers Biaxial studies of specialty polymers Recommendations References
117 117 120 123 123 125
11
Biaxially stretched polyamide film
T. Barth, Brückner Maschinenbau GmbH & Co. KG, Germany
11.1 11.2 11.3 11.4
Introduction Processing of biaxially oriented polyamide (BOPA) BOPA film properties Sources of further information and advice
Part II Applications of biaxial films 12
12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8
Fresh-cut produce packaging and the use of biaxial stretched films C. F. Forney and E. S. Yaganza, Agriculture and Agri-Food Canada, Canada Introduction Quality factors determining shelf-life Respiration and metabolism Package atmosphere modification Packaging methods and quality maintenance Future trends Sources of further information and advice References
125 126 133 140 141 143
143 144 146 147 151 159 160 160
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Pre.indd 7
7/4/11 5:56:28 PM
viii
Contents
13
E. M. Mount III, EMMOUNT Technologies, USA
13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8
Introduction Basic principles and methods for snack packaging Technologies and techniques Advantages and limitations Applications Future trends Sources of further information and advice References
165 169 179 195 197 198 200 201 204
Biaxial stretched films for use in snack packaging
165
14
Biaxially stretched films for product labeling
B. Hostetter, Formerly of Applied Extrusion Technologies, Inc., USA
14.1 14.2 14.3 14.4 14.5
Introduction Labeling systems and technologies Label applications Label preparation – label design, printing and converting Future trends and new developments in labeling and label films Conclusions References
204 205 215 220
231
14.6 14.7
226 229 229
15
Applications of biaxial stretched films
S. H. Tabatabaei and A. Ajji, Ecole Polytechnique of Montreal, Canada
15.1 15.2 15.3 15.4 15.5
Introduction Biaxial stretching of nanocomposite and multilayer films Conclusions Future trends References
231 234 238 238 239
16
240
Future trends for biaxially oriented films and orienting lines J. Breil, Brückner Maschinenbau GmbH & Co. KG, Germany
16.1 16.2 16.3 16.4 16.5
Introduction Trends for packaging film Trends for technical film Development environment for biaxially oriented film References
240 249 263 267 273
Index
275
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Pre.indd 8
7/4/11 5:56:28 PM
Contributor contact details
(* = main contact)
Editor and chapters 1–8, 10
Chapter 11
Mark T. DeMeuse Independent Consultant 10915 Arvind Oaks Ct. Charlotte NC 28277 USA
T. Barth Brückner Maschinenbau GmbH & Co. KG Königsberger Str. 5-7 83313 Siegsdorf Germany
E-mail:
[email protected]
E-mail:
[email protected]; rita.weisbecker-schehl@ brueckner.de
Chapter 9 Dr M. A. Ansari, Dr M. R. Cameron and Dr S. A. Jabarin* Polymer Institute and Department of Chemical & Environmental Engineering University of Toledo Toledo Ohio 43606 USA E-mail:
[email protected]
Chapter 12 Dr C. F. Forney* and Dr E. S. Yaganza Atlantic Food and Horticulture Research Centre Agriculture and Agri-Food Canada 32 Main Street Kentville Nova Scotia B4N 1J5 Canada E-mail:
[email protected]
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Pre.indd 9
7/4/11 5:56:28 PM
Chapter 13
Chapter 16
Dr E. M. Mount III President of EMMOUNT Technologies 4329 Emerald Hill Circle Canandaigua NY 14424 USA
Dr J. Breil Brückner Maschinenbau GmbH & Co. KG Königsberger Str. 5-7 83313 Siegsdorf Germany E-mail:
[email protected]
E-mail:
[email protected];
[email protected]
Chapter 14 Dr B. Hostetter 110 Gold Hawk Lane Landenberg PA 19350 USA E-mail:
[email protected]
Chapter 15 Dr S. H. Tabatabaei and Dr Abdellah Ajji* National Research Council of Canada CREPEC, Chemical Engineering Department Ecole Polytechnique of Montreal Montreal, QC Canada E-mail:
[email protected]
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Pre.indd 10
7/4/11 5:56:28 PM
1
Fundamentals of biaxial stretching and definitions of terms
M . T . D e M e u s e , Independent Consultant, USA Abstract: This chapter provides definitions of key terms which are involved with the process of biaxial orientation of films. The two common types of biaxial orientation processes which are currently used, simultaneous and sequential orientation, are described in detail and advantages and disadvantages of both processes are discussed. Polymers which are commonly used in the biaxial stretching process are mentioned as well as their common uses. Key words: simultaneous, sequential, polypropylene.
1.1
Introduction
Biaxial stretching of film is a common processing technology for the production of products in numerous applications, including food packaging and labels. There are many unique terms to the process which are not encountered in other polymer processes and a clear definition and understanding of the terminology is required to better understand the details of the events that occur during biaxial stretching. This chapter describes the fundamental aspects of the biaxial film production process and provides definitions of the key terms which are encountered.
1.2
Methods of biaxial stretching
Biaxial stretching of films is the process of forming hot plastic films in crossmachine directions, resulting in a stronger film. Another term commonly used for stretching is orientation. On a molecular level, orientation is the alignment of polymer chains in the film in particular directions. What causes the molecular orientation is a force which acts on the polymer molecules to pull them in a direction and then the molecule is frozen in place, as by quenching from a molten state. Relative levels of orientation can be measured in many different ways, ranging from X-ray diffraction to tensile properties. The simplest ways to measure film orientation are by measuring the shrinkage and tensile properties of the film. These measurements will give relative levels of orientation and can be correlated with the absolute measures of orientation, obtained from techniques such as X-ray. 3 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-01.indd 3
7/1/11 8:52:55 AM
4
Biaxial stretching of film
The biaxial stretching process is performed in two directions within the film. The machine direction (MD) is the direction that the film moves through the machine from start to finish. The transverse direction (TD) is the direction perpendicular to the machine direction. It is usually the same as the width of the film. Film can be biaxially oriented in two different ways, sequentially and simultaneously. In the sequential orientation process, the film is first oriented in the machine direction. This orientation can modify the crystallization features of the polymer. After the machine direction orientation, the film is stretched independently in the transverse direction. Owing to the changes in crystallization induced during the first orientation step, the temperatures used in the transverse orientation are usually higher than in the machine direction. Also, because of the different stretch ratios which are normally used in the two steps, the film physical properties are different in the two directions. In the simultaneous orientation process, on the other hand, the film is stretched at the same time in both directions in a single step. Typically, the stretch ratios in both the machine and transverse directions are equivalent. This leads to the film’s physical properties being quite similar in both directions and more balanced than when the stretching is done in the sequential manner. Depending on the type of property profile that is desired in the final product, preference can be given to either the simultaneous or the sequential stretching approach. There are two primary industrial processes for biaxial stretching of film, the double-bubble method and the tenter method. In the double bubble method, a circular die is used from the extruder to form a thick walled tube of polymer. This is then blown under air pressure, orienting the film in the transverse direction. At the same time, an equal orientation in the machine direction is achieved by adjusting the speed at which the tube is pulled downwards and collapsed. The double bubble method produces film which is balanced in the sense that it has the same mechanical properties in both the machine and transverse directions. The bubble method is an example of simultaneous orientation. In the tenter production method, polymer is extruded as a sheet directly onto a chilled chrome roller. The film is then passed through a stretching unit by rollers moving faster than the rate at which the material is extruded. This orients the film in the machine direction. Film can typically be oriented up to ten times in the machine direction. The film is then fed into a tenter frame for transverse direction orientation. In the tenter, the film is gripped along each edge by clamps that are attached to moving chains. These move outwards to stretch the film in the transverse direction. Typical stretch ratios depend on the polymer being used, but ratios as high as 10–12 times are possible with some polymers.1 The tenter method is an example of sequential orientation. © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-01.indd 4
7/1/11 8:52:55 AM
Fundamentals of biaxial stretching and definitions of terms
5
The initial application of tenter frame technology was not in plastics but in fabric. The tenter frame was used to treat and dry the fabric while maintaining the fabric width by clips which ran on two parallel chains. It was only after the successful implementation of the technology to fabric materials that it was extended to the film stretching process. There are certain benefits to bubble film versus tenter film. Among these are balanced orientation and excellent shrink tightening properties. Bubble film also offers improved film cutting, including die cutting. It provides access to both thin and thick film technology, down to 15 microns and up to 250 microns in thickness. Finally, owing to the high orientation level in the machine direction, bubble films offer high MD stiffness, high MD tensile strength and low MD elongation, important factors for conversion.
1.2.1 Biaxially oriented polypropylene (BOPP) One of the primary polymers which is used in the biaxial film stretching process is polypropylene (PP). Film which is produced from PP in the biaxial orientation process is usually designated BOPP (biaxially oriented polypropylene). BOPP films are used in food packaging, cigarette package overwrap, labels, adhesive tapes and a variety of other applications. A typical BOPP production line consists of the casting sheet extrusion, biaxial orientation, after treatment, rolling, cutting and automatic control. The focus of this discussion will be on the biaxial orientation portion of the line. Other aspects of the production line have been discussed elsewhere (see, for example, Jenkins and Osborn).2 Some of the features of BOPP films are: ∑ high tensile strength that facilitates high-speed conversion; ∑ high gloss and clarity; ∑ good puncture and flex-crack resistance over a wide range of temperatures; ∑ good barrier to water vapor; ∑ resistance to oils and greases; ∑ not affected by moisture and does not wrinkle or shrink with environmental changes. Many of these features are important in the use of the BOPP film in the applications previously mentioned. More specific details of these features and how they impact the uses of the film will be provided in later chapters of this book under the sections on the various applications which are discussed.
1.2.2 The tenter frame biaxial stretching process Very little technical information about the tenter frame biaxial stretching process has been published in the open literature. Typical machine parameters © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-01.indd 5
7/1/11 8:52:55 AM
6
Biaxial stretching of film
that can be adjusted to control molecular orientation are stretch ratios, both MD and TD, strain rates and stretching temperatures. The strain rate is adjusted through the line speed, which varies from tens of feet per minute up to speeds greater than 1000 feet per minute (300 m/min) for commercial production lines. The degree of crystallinity, crystal morphology, and degree of orientation in the stretched film depend on these processing conditions. Both off-line experimental studies of biaxial orientation 3 and on-line techniques4 have been applied to understand the evolution of the film microstructure during the stretching process. An in-depth discussion of these results applied to BOPP film is provided in Chapter 4. In the case of the double-bubble film production process, very little prior research is available in the literature. While it appears very similar to the more traditional melt blowing process, there are differences between the two processes that lead to different film properties. Those differences are: ∑ cooling of the extrudate – the limiting of crystallization and spherulitic growth provides the film superior optical properties ∑ radiation reheating below the polymer melting point – the polymer has the necessary mobility to stretch but immediate recovery of the oriented molecules to the random state is prevented, considerably improving the tensile properties of the films ∑ rapid air cooling of the film from inside and out – freezes the orientation produced in stretching ∑ annealing under tension – offsets residual stresses formed during cooling, preventing shrinkage of the film. Polymers other than PP can be produced into film and stretched biaxially. Among these are polyethylene (PE), polyethylene terephthalate (PET), polyamide (PA), polystyrene (PS) and polylactic acid (PLA). The published literature which is available on the biaxial stretching of these polymers is even more scarce than on PP. Chapter 10 gives a discussion of these other polymers in terms of their stretching features and a summary of the reports that have appeared thus far in the literature, both journal articles and patents. Both transparent and opaque films can be produced by the biaxial stretching process. In the case of transparent films, the only additives which are usually included in the polymer formulation are present to modify the behavior of the film in additional converting operations such as slitting and printing. These would include materials known as slip additives which modify the frictional properties of the film and antilock additives which prevent layers of the film from adhering to each other during film winding operations. In the case of opaque films, on the other hand, specific materials are included in the formulation which contribute to the opacity or whiteness of the final film stretched product. Those additives can be either organic or
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-01.indd 6
7/1/11 8:52:55 AM
Fundamentals of biaxial stretching and definitions of terms
7
inorganic in nature. During the film stretching process, there is a debonding process which occurs at the interface between the stretched polymer and the filler particle (see for example Amon).5 This occurs primarily because of the lack of adhesion between the two materials being stretched. The debonding produces voids in the film. The scattering of light from these voids is what produces the opacity or whiteness in the film. Both transparent and opaque films can be either single layer or multiple layers. When multiple layers are present, the films are produced by a process known as co-extrusion in which the various layers are brought together in the molten state in the die. Often times in the co-extrusion process materials known as tie layers must be used to adhere dissimilar polymers. More detailed discussions of common examples of co-extruded structures will be provided in Chapter 6. The proper processing conditions for both bubble and tenter films are often developed through an approach of trial and error. In such an approach, films made using a certain set of process conditions are tested for some physical property of interest. However, a complete theoretical understanding of the effect of different stretching conditions on the physical properties of the final film is still lacking. Mathematical modeling of the biaxial film stretching process could prove to be very helpful in this regard. One exception to the above statement is the work of Lin et al.6 in which they sought correlations between the oxygen permeability of BOPP films and film processing conditions. These workers were able to vary the crystalline morphology in the final stretched film made on a laboratory stretcher by changing the cooling conditions of the precursor extruded sheet. Surprisingly, there was no consistent relationship observed between the oxygen permeability and the thermal history of the precursor sheet. Biaxial orientation resulted in a decrease in the oxygen permeability compared with unstretched film. The lowest permeability was consistently obtained with the use of the lowest orientation temperature. Further, a lower oxygen permeability was obtained by increasing the film stretch ratio. The authors analyze their results using a two-phase model for polymers and conclude that the reduced oxygen permeability does not correlate with the overall amount of orientation as measured by birefringence, or with the amorphous phase fraction, as determined by density. Instead, the decrease in permeability was attributed to the reduced mobility of amorphous tie molecules. A oneto-one correlation between the oxygen permeability and the intensity of the dynamic mechanical beta relaxation in polypropylene was demonstrated in that work. In another similar study, Orbey and coworkers7 examined the effect of processing parameters such as the temperature of crystallization and the degree of MD orientation on the mechanical, optical and thermal properties of BOPP films. Various physical properties such as tensile strength, yield
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-01.indd 7
7/1/11 8:52:55 AM
8
Biaxial stretching of film
strength and strain at break, among others were measured in the machine and transverse directions. Haze, degree of crystallinity and crystallization temperature were determined and these properties were correlated with the structure of the stretched film. Increasing machine direction orientation lead to a higher level of crystallinity, so the modulus, yield stress and tensile strength increased in both directions. Also, for highly unequal orientation, the strain at break and the initial tear resistance of the film in the MD decreased because the tie chains were highly oriented in the machine direction. The film haze became higher with increasing MD orientation. On the other hand, when the crystallization temperature was increased, the degree of crystallinity became higher and the film haze increased slightly. Young’s modulus, yield stress and tensile strength increased in both directions, owing to the rise in the degree of crystallinity. Since larger crystal superstructures are formed at higher crystallization temperatures, the initial tear resistance was lower in both directions. The yield strain and tear propagation resistance did not change significantly, but the strain at break became higher in both directions. In another study, Lin et al.8 examined the effect of the film processing conditions on the optical transparency of the final product. These workers prepared BOPP films by simultaneous biaxial stretching using a laboratory unit at high strain rates and elevated temperatures. The measured transparency of the films was found to depend on the thermal history of the sheet that was used for the oriented film and also on the temperature at which the orientation was performed. Thus, cooling the sheet more rapidly from the melt and orienting the sheet at a lower temperature resulted in a more transparent film. Surface roughness was determined to be the cause of the loss of transparency. Studies such as these are limited in the literature and the ones that are available tend to be quite specific in their emphasis. General conclusions and observations about the effect of processing conditions on the stretching behavior of films are lacking. Further, the effect of these processing conditions on the final film properties are scarce. This is one area where additional study is necessary. It should be noted that the limited structure–property–process structure alluded to in the previous paragraphs all focused on PP. This is primarily because PP is the polymer which is most frequently used in applications which are related to the biaxial stretching of films. Such studies are even more limited for polymers other than PP. Further discussions of these other polymers are given in Chapter 5.
1.2.3 Equipment used in the biaxial stretching of films There are several types of small-scale laboratory pieces of equipment which have been utilized to investigate the biaxial stretching of films. The most © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-01.indd 8
7/1/11 8:52:55 AM
Fundamentals of biaxial stretching and definitions of terms
9
common is called a T.M. Long stretcher.9 The T. M. Long stretcher can perform the stretching in either a simultaneous or sequential process using a range of stretching temperatures. Also, the stretch ratio can be varied according to the process needs. This stretcher has primarily been used in the literature to compare the laboratory stretching features of various PP resins.10 As such, it is used as a screening tool to establish initial stretching conditions for additional process scale-up investigations on larger equipment. Not only can the T.M. Long stretcher be used to perform screening experiments to evaluate the stretching features of films, but quantitative information about the stretching process can also be obtained. 11 In this way, investigators have measured the stress–strain curves during the stretching and have focused primarily on the yield stress values as a way to compare various resin formulations. There have been correlations attempted between the yield stress values and the features of the formulations12 as a way to design resins with a particular film stretching characteristics. Recently, 13 a company named Inventure Labs in Knoxville, TN, upgraded some of the features of the original T.M. Long stretcher and computerized the stress–strain output feature. This makes analysis of the data much simpler and easier. Inventure Labs also offers larger versions of the original T.M. Long stretcher as a way to understand the scale-up features of the biaxial stretching process. Brückner also offers a laboratory stretching unit called the Karo IV. 14 Ease of sample loading is one of the key features of it compared to other laboratory devices. It also has the capabilities of measuring and recording the stress–strain curves during the actual stretching process itself. As such, it offers the possibility to examine the dynamics of the stretching rather than simply measuring the properties of the final stretched product. More investigations into this area of research are required to better understand the events that occur during biaxial film stretching. One of the big advantages of the Karo IV unit is that it can provide stretching ratios up to 10 ¥ 10 with temperature capabilities of up to 400 °C. Also, there are separate oven zones possible which make the stretching process resemble production conditions that are available in tenter frames used for commercial applications. During the stretching process itself, stretching forces, sample displacement and sample surface temperature can be accurately measured using state of the art measuring equipment, allowing for excellent reproducibility of the experimental process conditions. Such reproducibility has not always been possible in past laboratory investigations.
1.2.4 Laboratory studies of the biaxial stretching of films The largest remaining challenge concerning these laboratory studies is their correlation with larger-scale tenter frame and double bubble process
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-01.indd 9
7/1/11 8:52:55 AM
10
Biaxial stretching of film
equipment. In other words, questions remain as to how the processing parameters defined on laboratory equipment translate into processing schemes on commercial equipment. Owing primarily to the high speeds and strain rates used in present commercial tenter equipment, it is not clear how to precisely use the laboratory experiments to help define commercial processes. This is one area where considerable more work needs to be done. Extremely important in this regard are experiments which are done on pilotline equipment which is smaller than commercial lines but can still use the laboratory experiments as guides. The best way to do this is to establish the initial correlation with a control material and then make comparisons of new resin formulations directly to the control sample. That was the approach pursued by Kimund and Townsend15 who examined a variety of beta-nucleated crystalline PP products and their potential applications in oriented film. They determined the T.M. Long yield stress at a given stretch temperature and stretch ratio and used the hypothesis that a lower T.M. Long yield stress value as well as the shallow slope of the yield stress versus temperature curve is preferred for better film processability. 16 It was shown that the T.M. Long yield stress is significantly lower than that for conventional BOPP film grade materials but higher than for a random copolymer. Based on the information of an earlier study,17 they concluded that beta-nucleated PP provides a wide processing window without sacrificing the properties of oriented film. On the other hand, these workers did not provide any information on the processing and properties of the materials on larger tenter equipment. Such information would have completed the loop in this very interesting property/processing relation study. In a similar study, Kim at Sunoco Chemicals attempted to use the T.M. Long stretcher to establish relationships between polymer characteristics, processability, and properties of biaxially oriented pp film.18 He determined that in order to establish a balance between processability on a tenter frame and the film properties, an optimum level of xylene solubles is preferred. Based on those results, a pp material having a higher crystallinity level and good processability was developed. As a way to examine the events which occur in polymers at a microscopic level, the molecular mobility of amorphous chains can be characterized by their dynamic mechanical relaxation behavior. In pp, main polymer chain motions in the amorphous phase are typically associated with the beta relaxation. It has been shown that biaxial orientation decreases the intensity of the beta relaxation.19 However, a direct correlation of the film physical properties was not established. In a related study20 Kim studied the orientability of films made from blends of pp homopolymers with ethylene/propylene copolymers. He observed that the crystalline state of the material at the stretching temperature determines the measured yield stress. Also, the yield stress decreases with
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-01.indd 10
7/1/11 8:52:55 AM
Fundamentals of biaxial stretching and definitions of terms
11
decreasing density, and, hence, crystallinity of the cast sheet. In that work, the crystallinity is modified in the film formulations by systematically changing the concentration of the ethylene/propylene copolymer in the blend composition. A similar phenomenon was observed by DeMeuse21 when he examined several metallocene-catalyzed PP materials in T.M. Long stretcher experiments. The observation was made that the polymers that had the highest mechanical properties also had the smallest processing window, as defined by the T.M. Long stretcher and pilot line tenter equipment. On the other hand, when the processing window was widened, the physical properties of the oriented film decreased and approached those of standard BOPP film. This is another example of the necessity to balance processability and the film properties. Although the focus of the work was a polymer other than PP, the work of Nevalainen et al.22 on the voiding behavior of a polyester film is also relevant to the present discussion. The data in that study were analyzed to follow the development of voids, molecular orientation and crystallinity. The results demonstrate the importance of the viscoelastic nature of a polymer to void formation. It was shown that the temperature and draw rate are interchangeable parameters in terms of their effect on the development of a void. It was proposed that selection of the correct draw temperature during the first stage of a biaxial film process is the most important factor when a highly voided polyester film is desired. These quoted studies have all focused on an understanding of the relationship between the film processing conditions and the final properties of the oriented film products. As such, they are part of a large class of studies referred to in the polymer literature as structure–processing–property studies. Unfortunately, in general, such studies are lacking in the area of biaxial stretching of films and much work is based on a trial and error approach rather than a firm understanding of the underlying principles.
1.3
Recommendations
As already alluded to, one big area where much work needs to be done is the establishment of structure–property–processing relationships for biaxially oriented films. Much effort to date has relied on trial and error rather than an understanding of the basic principles behind the film stretching process. A better understanding of the correlation between the stretching parameters and the final film properties can lead to the development of new and unique polymer formulations with previously unavailable properties and processing features. As part of that understanding, relationships between the small-scale stretching performance of films on laboratory equipment and their performance on commercial tenter equipment need to be better established.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-01.indd 11
7/1/11 8:52:55 AM
12
Biaxial stretching of film
This is definitely true for films produced from pp and is even more evident in films made from other polymers. The majority of published studies in the open literature have focused on the biaxial film stretching process as it relates to pp. Additional studies are required on different polymers to increase the general understanding and knowledge base. Another aspect of the biaxial stretching process that must be examined in much more detail is the development of mathematical models which can be used to predict the behavior of a particular polymer with certain features under well-defined stretching conditions. The development of such models would be extremely helpful in defining oriented film structures that have a particular set of properties. As such, they would be useful in helping to design specific film structures for particular end-use applications. Such an approach would allow the entire area of biaxial stretching of films to be taken to an entirely new level of development. From an equipment perspective, future developments will be focused on the design and construction of units for specific materials and applications. The realization that standard orientation equipment for high-speed production of film for food packaging applications does not meet the requirements of everyone is gradually becoming accepted. Specialty and niche applications which require the modification of existing designs are increasing and equipment suppliers are responding accordingly. It is expected that this trend will continue as new and unique film structures with presently unforeseen applications continue to be developed. These developments will require further collaborations between material and equipment suppliers so that the most effective utilization of available resources can be achieved. The other area where equipment can continue to advance is the development of on-line testing to monitor the changes which occur during the film stretching operation. Presently, much testing is done off-line and requires the generation of samples at a set of processing conditions and, subsequently, those samples are analyzed. Additional on-line tests could save time in property measurement and the subsequent adjustment of processing conditions to obtain the desired property profile. Because of such time savings, these developments could be a beneficial addition to present equipment.
1.4
Conclusions
In summary, then, there are several areas in the area of biaxial stretching of films that require additional investigation. Many of these areas are related to development of approaches to handle new materials with new processing challenges. Overall, the goal is to define a framework which is applicable to any film structure, regardless of how complex that structure is. That framework needs to involve a relationship between material features and
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-01.indd 12
7/1/11 8:52:55 AM
Fundamentals of biaxial stretching and definitions of terms
13
their processing, leading to a better understanding of the structure–property– processing features of polymers.
1.5
References
1. H.Y. Nie, M.J. Walzak and N.S. McIntyre, Polymer, 41, 2213 (2000). 2. W.A. Jenkins and K.R. Osborn, Plastic Films: Technology and Packaging Applications CRC Press (1992). 3. V. Ratta, G.L. Wilkes, and T.K. Su, Polymer, 42, 9059 (2001). 4. A.J. Bur and S. Roth, Polym. Eng. Sci., 44, 805 (2004). 5. M. Amon, US Patent 6183856 issued on February 6, 2001. 6. Y.J. Lin, P. Dias, H.Y. Chen, A. Hiltner, and E. Baer, Polymer, 49, 2578 (2008). 7. C. Yuksekkalayci, V. Yilmuzer, and N. Orbey, Polym. Eng. Sci., 39, 1216 (1999). 8. Y.J. Lin, P. Dias, S. Chum, A. Hiltner, and E. Baer, Polym. Eng. Sci., 47, 1658 (2007). 9. T. M. Long Company, Polymer Film Stretcher, Florida, Integrated Publishing. Available from: http://www.tpub.com/content/nasa2000/NASA- 2000-tm210294/ NASA-2000-tm2102940161.htm [accessed 21st December 2010]. 10. R.A. Phillips and T. Nguyen, J. Appl. Polym. Sci., 80, 2400 (2001). 11. B. Laroux, T. Elmes and P. Mills, J. Mat. Sci., 27, 1475 (1992). 12. E. Bullock and W.W. Cox, TAPPI J., 79, 221 (1996). 13. Inventure Laboratories, Accupull Biaxial Film Stretcher, Tennessee, Available from: http://accupull.com/ [accessed January 3, 2011]. 14. M. McLeod, J. Plastic Film Sheeting, 22, 275 (2006). 15. S. Kimund and E. Townsend, SPE Proceedings, 59, (2002). 16. S. Kim and M.R. Stephans, US Patent 6733898 issued on May 11, 2004. 17. S. Kim and M. Fujii, US Patent 6596814 issued on July 22, 2003. 18. S. Kim, US Patent 7282539, issued on October 16, 2007. 19. M.B. Elias, R. Machado, and S.V. Canevarolo, J. Therm. Analysis Cal., 59, 203 (2000). 20. S. Kim, J. Plastic Film Sheeting, 21(2), 99 (2005). 21. M. DeMeuse, J. Plastic Film Sheeting, 18, 17 (2002). 22. K. Nevalainen, D.H. MacKerron and J. Kuusipalo, Mater. Chem. Phys., 92, 540 (2005).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-01.indd 13
7/1/11 8:52:56 AM
2
Equipment design and requirements of biaxially stretched films
M . T . D e M e u s e, Independent Consultant, USA Abstract: Various equipment designs for the production of biaxially oriented films are reviewed and the specifics of present designs are highlighted. The equipment is discussed in terms of both present and future material needs. Future equipment needs are mentioned in terms of specialty film applications which are currently being developed. Key words: double bubble, tenter, orientation, MD, TD.
2.1
Introduction
There are several types of equipment which are currently used to evaluate and produce biaxial oriented films. Among these, the most common is a tenter frame, which typically produces material through a sequential orientation process. However, there are other technologies, such as the double bubble process, which are currently practiced commercially. In this chapter, the various equipment designs will be reviewed and the specifics of present designs will be highlighted. The equipment will be discussed in terms of both present and future material needs. Chapter 1 focused on an introduction of the concept of biaxial stretching of film as well as the definition of key terms which are encountered in the process of biaxial stretching. That chapter was intended to serve as the general framework for the subsequent chapters of this book. As such, detailed explanations of concepts and technologies were not provided but instead general information was highlighted. The intent is to go into much more detail in this and subsequent chapters. The present chapter will focus on equipment design used in biaxial stretching of films and what some of the present equipment designs are. Those designs will be discussed in terms of both present and future materials needs.
2.2
Double bubble process for biaxial stretching of films
There are two main types of equipment used for the biaxial stretching of films. These are the double bubble blown film equipment and tenter frame. Both of these processes and the associated equipment have definite advantages 14 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-02.indd 14
7/1/11 8:53:31 AM
Equipment design and requirements of biaxially stretched films
15
and nuances associated with them. Each process and the equipment involved with that process will now be described with a high level of detail. The first process for the biaxial stretching of films that will be discussed is called the double bubble process. A typical schematic for the double bubble process is shown in Fig. 2.1. In that process, a circular die is typically used to form a thick walled tube of polymer from an extruder. This tube of polymer is then inflated with air, which provides orientation to the film in the transverse direction (TD). Simultaneously, a similar level of orientation is achieved in the machine direction (MD) by controlling the speed at which the tube is pulled downwards and collapsed. Owing to similar orientation levels in both the machine and transverse directions of the film, the film is described as balanced because the mechanical properties in both directions are nearly identical. Different stretch or orientation ratios are possible in the double bubble process through the control of the inflation air pressure. Typically, polypropylene, one of the most common polymers for biaxial stretching, is oriented at least seven times in both the machine and transverse directions using the double bubble process. The measured mechanical properties,
Preheater 1st nip rolls Preheater Stretching unit
Collapsing boards 2nd nip rolls
Annealing oven
Extruder Water bath Wind up unit
2.1 Typical schematic of double bubble process.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-02.indd 15
7/1/11 8:53:31 AM
16
Biaxial stretching of film
particularly strength and modulus, are directly related to the level of orientation in the final film and increase with an increase in the orientation level (see for example Peterlin).1 With increasing stretch ratios, both modulus and strength tend to increase, due primarily to the increased molecular orientation of the polymer chains. This orientation is known to lead to an increase in the mechanical properties of fibers as well (e.g. Gordeyev and Nekrasov).2 After the bubble has been collapsed, the film is generally subjected to some additional treatment or post-processing. This can involve processing steps such as annealing, to limit the final shrinkage of the film or surface treatment processes such as corona or flame treatment. Such treatment processes are very useful for providing additional chemical functionalities to the film surface for additional film converting steps, such as printing and lamination. Films which are produced by the double bubble process have certain definite advantages over films produced by other processes. Among these are the balanced orientation already alluded to, as well as excellent shrink properties. They also offer excellent film cutting properties, including die cutting. The double bubble technology provides access to a wide range of film thicknesses, from about 15 up to 250 microns. Finally, due to the high level of orientation in the MD, bubble films provide high MD stiffness, high MD tensile strength and low MD elongation, all of which are important factors for further conversion of the film. This is an issue with film that has been produced by other process technologies, like a tenter frame. As described above, the double bubble film process involves simultaneous orientation of the film in both directions. This means that the film is oriented in both the machine and transverse directions at the same time. This approach is to be contrasted with sequential orientation in which the film is independently stretched first in the machine direction, and, then, subsequently stretched in a separate step in the transverse direction. The sequential orientation process has a tendency to produce film with unbalanced mechanical properties compared to simultaneous orientation in which the physical properties in the machine and transverse directions are quite similar. As already mentioned, this balance in properties can significantly impact additional converting processes of the final film, such as printing and lamination. Typically, as shown in Fig. 2.1, after collapse of the bubble, the oriented film is divided into two sections which are handled independently of each other. At this point in the operation, the film is heat-set or annealed to reduce the shrinkage imparted by the orientation and treated to increase the surface energy. The increase in surface energy which is imparted by the treatment process allows for the adhesion of materials, such as ink for printing applications, to the surface of the film. After the treatment of the
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-02.indd 16
7/1/11 8:53:31 AM
Equipment design and requirements of biaxially stretched films
17
film, the two sections are wound into rolls for subsequent slitting and further conversion. A more detailed description of the entire corona and flame treatment processes, including new alternative technologies, is provided in Chapter 6. In the double bubble process, the level of orientation in the film is controlled by the level of air pressure used during the inflation step. The higher the level of air pressure used, the higher is the draw ratio which is obtained. The highest draw ratio which is possible is dependent on the type of polymer being used and also the temperature used during the orientation step. Polypropylene is the most common polymer which is used and, typically, it is stretched at least seven times in both the machine and transverse directions. Polyamide, more commonly referred to as nylon, is another polymer which is oriented using the double bubble process and, in that case, the orientation ratio which is used is less than is the case for polypropylene, on the order of 3 to 4. High density polyethylene (HDPE) is another polymer which is typically used in the double bubble process and stretch ratios for it are also less than for polypropylene. Lower stretch ratios lead to films which have lower values of the mechanical properties than does the use of higher stretch ratios. Other properties of the final film, such as tear strength and barrier, are also dependent on the stretch ratio which is employed during the film production process. As such, the final properties can be controlled through the use of the appropriate stretch ratios.
2.3
Tenter process for production of biaxially oriented films
The double bubble process, which was just described, for the production of biaxial stretched films is much less common in commercial applications than is the tenter frame approach. For example, in the biaxial oriented polypropylene (BOPP) film production process, almost 85–90% of the overall world’s production takes place by the tenter process. 3 This is related primarily to the very high production rates which are possible with the tenter process compared with the double bubble process. Also, the commercial production film widths are usually higher in the tenter frame method than in the double bubble. At the same time, however, the double bubble process does offer the possibility to produce a series of products with a wide range of thicknesses which is not easily possible with the tenter process. This is particularly significant for thin film products with thicknesses less than 15 microns. A typical tenter process film production line consists of some type of extrusion equipment, some type of MD orientation device, usually rollers of some type, a TD orientation oven which typically consists of at least three different sections, post-treatment and, finally, a film winding section.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-02.indd 17
7/1/11 8:53:32 AM
18
Biaxial stretching of film
In some tenter processes, there is also an unwind section present before either the MD and TD stretching sections. This allows for the orientation of film which has been produced separately from the extrusion process and provides additional flexibility to the type of film structures which can be processed on the tenter equipment. In that case, the extrusion process is done in an off-line operation and is not part of the continuous film production process. Unlike the double bubble process in which the orientation is simultaneous, the tenter process usually involves sequential orientation with MD orientation normally occurring first, followed by TD orientation. Orientation in the MD is normally achieved by passing the film over heated rollers which are operating at different speeds. Typical orientation ratios in the MD are around one to five times and the maximum stretch ratio attainable is dependent on the polymer being used and also the stretching temperature being used. Typically, not only stretching of the film is achieved, but also annealing, to potentially reduce the MD film shrinkage, is utilized in the MD orientation process. As already alluded to, the MD orientation temperature which is used is dependent on the polymer being stretched. Typically, temperatures above the glass transition temperature, at which macroscopic flow of the polymer is observed, are used. Usually with increasing orientation temperature, the stretch ratios which can be obtained without film breakage increase. Since it is not the objective of the present work to discuss in detail the polymer physics associated with the glass transition temperature, a detailed discussion is not provided. The interested reader is referred to several standard texts on polymers for more details.4,5 The orientation process in the MD typically reduces the thickness of the original film in an amount directly proportional to the MD stretching ratio which is used. Thus, an MD stretching ratio of three times can be expected to reduce the film thickness by a factor of about 3. However, caution must be exercised in using this relationship with porous films. In that case, the development of porosity during the stretching process must also be considered. On the other hand, for solid films, the proposed relationship is usually valid and applicable. After the film is oriented in the machine direction, it is directly fed into the TD orientation oven. This oven normally consists of three sections: (1) preheat, (2) stretching, and (3) heat-setting or annealing. The length of each oven section depends upon the speed that the line is operated at. Faster line speeds require longer ovens due to the need for the film to reach the desired temperature. The preheat section of the oven is used to bring the film sample to the desired orientation temperature. Since the film orientation temperature is usually near the melting point of the polymer,6 the set point temperature
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-02.indd 18
7/1/11 8:53:32 AM
Equipment design and requirements of biaxially stretched films
19
used in the preheat zone is usually near the polymer melting temperature. However, owing to heat transfer issues, there may be a substantial difference between the temperature setting in this zone and the actual surface temperature of the film. This is more pronounced at higher line speeds used in commercial production settings because under those conditions the film has less residence time in the oven to equilibrate with its surroundings. In fact, for most commercial BOPP film lines, the temperatures used in the preheat zone are actually higher than the quoted polypropylene melting temperature of about 165 °C.7 As already alluded to, this approach can be used, however, because the film generally does not have sufficient time in the oven to reach the setting temperature which is higher than the polymer melting point. This temperature difference effect becomes less significant with correspondingly slower line speeds and, hence, longer times in the tenter oven. After having been heated to the proper orientation temperature in the preheat section, the film next enters the stretching section of the oven. This is where the actual TD orientation of the film occurs. The temperature setting in this section of the oven is usually either the same as the preheat section or slightly lower. The actual stretch ratio that the film is provided in this section of the oven is largely dependent on which polymer is used. For example, in commercial applications, polypropylene is typically oriented up ten times in the tenter frame and can be oriented up to about twelve times. On the other hand, poly(ethylene terephthalate), PET, is usually oriented much less than that, around three to four times. The maximum orientation which is possible for a particular polymer is also somewhat dependent on the orientation temperature which is used, much the same effect as discussed for the MD orientation. The orientation itself is done by using rails which pull the film outward in a controlled manner. The film is gripped by clips at each edge during the orientation process. As the rails move outward and the film orientation occurs, the film remains gripped in the clips and the clips circulate through the oven in a chain arrangement. The position setting of the rails define the orientation level or stretch ratio which is imparted to the film. The wider that the rails are spread apart, the higher the level of orientation achieved. The rails are adjustable to allow for the various stretch ratios as required. The adjustment can be either manual or computerized. After the stretching section of the oven, the film passes into the annealing or heat-setting section, which is the final section of the tenter oven. The purpose of this section is to stabilize the film from further dimensional changes and shrinkage that may occur during additional processing. This is necessary because the film stretching process produces polymer chains which are far removed from their equilibrium state. There is a natural tendency for the chains to return to their equilibrium condition and that return is
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-02.indd 19
7/1/11 8:53:32 AM
20
Biaxial stretching of film
accompanied with significant dimensional overall changes, or shrinkage, in the film. In order to reduce those changes, the temperature in the annealing section is set at a temperature higher than the temperature used in the orientation step. This step ‘freezes’ in the orientation which was provided in the stretching step in the overall process. This allows for the film to be subsequently heated to progressively high temperatures without the subsequent loss of orientation and the corresponding decrease in mechanical properties. Since the heat-setting temperature is higher than the orientation temperature, the film has a tendency to shrink in the heat-setting section of the oven. Thus, to compensate for that shrinkage in that section of the tenter oven, the rails are normally closer together than in the orientation section. This heat-setting, then, eliminates further shrinkage when the film is heated to high temperatures. In that sense, the film has a ‘memory’ of having been heated to the most recent temperature that it has experienced. Heating it to lower temperatures than the most recent temperature has a minimal effect on the properties. This is the main reason why the setting temperature in the final annealing section of the tenter oven is normally higher than in the other two sections. The desire is to have the films experience as high a temperature as is possible to reduce film shrinkage without losing the orientation effect on the film strength and modulus. After passing through the heat-setting section of the tenter oven, the film exits the oven and usually passes through devices called slitters which remove a portion of the film at the edges. This is normally necessary because the film at the edges is thicker than the desired film thickness because it is not stretched as much as the remainder of the film. In commercial operations, the film which is trimmed from the edges is recycled back into the extrusion process to reduce production costs. The goal is to limit the amount of this recycled material as much as possible. It is after the slitting of the edges that the oriented film is usually treated to increase its surface energy. This is necessary because in the case of some film surfaces, such as polypropylene, they are not polar enough to have sufficient adhesion with other materials, such as printing inks. The surface treatment process has the effect of creating polar chemical groups which can form chemical bonds or interact with other materials. The most common form of surface treatment is corona discharge, but there are other possibilities, such as flame treatment, available. Chapter 6 will discuss these approaches in much more detail. The final step in the film production process using a tenter frame is the winding of the film into a usable roll for further conversion steps. One of the key factors in this step is the maintenance of the proper tension level on the film to reduce and/or eliminate wrinkles or creases in the roll of film as it is being wound on the roll. There are many sources of these creases
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-02.indd 20
7/1/11 8:53:32 AM
Equipment design and requirements of biaxially stretched films
21
and wrinkles but among the most common is a non-uniform thickness distribution in the final product. Such a non-uniformity can be related to the extrusion process or the stretching process itself. Selection of the appropriate temperature profile in the oven can limit these thickness non-uniformities. There are usually on-line thickness indicators, such as beta gauges, which provide an indication of the thickness profile both after the extrusion and stretching steps in the process. Owing to the high speeds at which current tenter lines operate on a commercial basis,8 equipment designs for the takeoff and winding portions of the equipment have needed to be constantly updated. Also, as production requirements have increased, significantly larger rolls of film are being produced at increasingly higher speeds. These requirements have also necessitated a revision in the takeoff equipment needed to deal with these massive rolls of film. Thus far, this has not been a major concern and these needs have been adequately met and satisfied. As there is a desire, however, to constantly increase the speed at which commercial lines operate this need must be constantly monitored and upgraded.
2.3.1 Commercial biaxial orientation equipment There are several suppliers of commercial tenter equipment in the United States and abroad. Foremost among these are Parkinson Technologies, 9 located in Woonsocket, RI (USA) and Brückner, Inc. 10 headquartered in Siegsdorf, Germany. It should be noted that Parkinson Technologies was previously known as Marshall and Williams. Many production lines at various production facilities around the world are products of either Parkinson or Brückner. To date, the largest commercial tenter lines available are 10 meters wide and are present at the facilities of several companies including Applied Extrusion Technologies and others. Operating line speeds in excess of 1000 feet/minute (300 m/min) are very common these days. These lines operating at those speeds are normally used for the production of BOPP for food packaging applications. Films which are produced for specialty-type applications are generally produced at lower speeds. Specialty applications for biaxial stretched films are beginning to appear more frequently. As the need for such applications increases, it will likely be necessary that modifications or adjustments to present equipment designs will be required. Most present equipment designs have been established to take advantage of high line speeds and wide film web applications. Most of those applications are focused around commodity applications, such as food packaging. However, as those markets become increasingly mature, there will be a need to focus on specialty applications which will require the design and construction of equipment very specific to those applications.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-02.indd 21
7/1/11 8:53:32 AM
22
Biaxial stretching of film
This very likely will be one of the next advances in equipment design and development for biaxial stretching of films. To a certain extent, that line of thinking will be driven by the discovery and development of new polymers and materials for unique applications. As these new materials are developed, they will have unique stretching features which are completely different from polypropylene, for which many of the present high speed lines were designed. Owing to those stretching differences, one of two approaches will need to be adopted and incorporated. Either existing equipment will need to be adjusted to deal with new materials or new equipment will need to be designed and constructed. From an economic perspective, the first approach may not be possible depending on the flexibility of the existing equipment. This is because some equipment has been designed with specific stretch ratios and adjustment is not possible. Following up on that line of thinking, most commercial tenter frame equipment currently is used for the production of BOPP film. As already alluded to earlier, typical stretch ratios for BOPP production are on the order of one to five times in the MD and up to ten times in the TD. Not many other polymers can withstand that high level of orientation without having breakage of the film. Thus, in order to be able to evaluate and run these films based on these other polymers on such equipment, the stretch ratios must be adjustable, sometimes over a wide range. This is not as large an issue for the MD stretch ratio as it is for the TD stretch ratio, for which the position of the rails on the tenter frame need to be moved, either manually or automatically. This is because the stretch ratio in the MD can be easily adjusted by changing the speeds of the rolls which perform the stretching. On the other hand, in order to change the stretch ratios in the TD, the rails must be adjusted and to do that manually can be a major physical task. Not only will the stretching portion of tenter equipment need to be appropriately modified to deal with new materials but also the winding equipment will need to be adjusted. This is particularly true as softer or even porous materials are run and evaluated on presently available commercial tenter equipment. This is because films made from such materials can typically not be wound and made into rolls using the same tension levels as used for BOPP film. This would result in an unacceptable level of web breaks and equipment downtime. The low tension levels which are required may necessitate the design of specific equipment for appropriate film handling, or, at least, modifications to existing equipment will be necessary.
2.3.2 Literature studies involving tenter frame equipment Surprisingly, there have been very few reports published in the open literature of the tenter frame biaxial stretching processes. Recently, Wilkes and coworkers discussed the tentering process of HDPE. Their work only
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-02.indd 22
7/1/11 8:53:32 AM
Equipment design and requirements of biaxially stretched films
23
focused on MD stretching and the microstructural characterization of the HDPE film after the MD stretching step.11 Owing to the fact that the overall stretching process was done sequentially, the information that was obtained provided the basis from which the TD stretch was undertaken, but no TD stretching data were actually presented. MD stretching experiments were carried out as a function of stretch ratio and stretch temperature, after which the workers examined the morphological state of the HDPE film using various off-line techniques such as X-ray, atomic force microscopy, transmission electron microscopy, differential scanning calorimetry and refractive index observations. Their study focused on the change in the crystalline morphology from spherulitic lamellae into lamellar stacks that resulted from the MD stretching at temperatures near the polymer melting temperature of 135 °C. The manner in which that change affects the TD properties and stretching was not reported in their work. In another study Bur and Roth12 developed and implemented an on-line, real-time sensor based on fluorescence spectroscopy to monitor both the orientation level and temperature of biaxially stretched polypropylene. These workers showed that the film temperature measured by fluorescence agreed quite well with the adjacent air temperature measured by a thermocouple. Also, they determined that there was significant positional variation in the TD temperature for the particular tenter oven used. In other words, film of varying effective stretch ratio was produced at different TD positions. Despite this fact, however, the film made had less than 3% thickness gauge variation and had no observable visual defects. This exemplifies the hidden non-uniformities that the spectroscopy method can help detect. As limited as the reports are in the literature on the use of tenter equipment to produce film, there are even fewer reports on the use of the double bubble process. This may be related to the fact that a fewer number of polymers are amenable to use in the double bubble process compared to the polymers which can be used in the tenter process. Alternatively, it could be because the tenter process is used much more extensively in commercial applications than is the double bubble process and, hence, has generated more academic interest. One of the few investigations that has extensively examined factors which affect the double bubble process was conducted by Rhee and White13 at the University of Akron. They studied different polyamide resins in the double bubble process. In terms of processability and film structure development, the second air ring temperature was found to be critical. In addition, the crystallization rate of the polymers provided a determining factor for double bubble inflation and orientation ratios which could be achieved. As the crystallization rate increases, the film becomes increasingly unsuitable for double bubble processing. This limits the type of polymers that can be used. Additional specific details of this work are provided in Chapter 10.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-02.indd 23
7/1/11 8:53:32 AM
24
Biaxial stretching of film
In a similar study, Song and White14 studied the double bubble tubular film extrusion and stretching of a polymer known as polyethylene-2,6-naphthalate (PEN). They examined the film formation and structure development of PEN during the extrusion process. They determined that the mechanism of structure evolution in PEN was related to its behavior during the inflation period and compared its behavior with that of other polymers. A final study which is particularly relevant to the relation of the equipment to the production of the final film in the double bubble biaxial film stretching process was performed by Sakauchi et al.15 They studied the relationship between the cooling water temperature in the double bubble process and the stretchability and physical properties of the stretched film and related them to the film superstructure of products made from random propylene/ ethylene copolymers. Their main finding was that the stretchability was not influenced by the cooling water temperature and that the physical properties and film superstructure were slightly changed by the cooling water temperature. However, those changes could be compensated for by adjusting the preheating process temperature so that similar stretching stress values could be obtained as previously noted. These last examples are situations in which the processing conditions of standard equipment were adjusted for the polymer type. Another approach is to design equipment specific to the polymer. Thus far, this latter approach has not been extensively examined by anyone in the literature. The main reason for this is because much development in biaxial stretching of films has been driven by commodity, not specialty, products. As more niche applications are developed, equipment which is designed for particular materials and applications will be necessary. This offers the opportunity for the development of unique biaxial film stretching equipment, both for the double bubble and tenter processes.
2.4
Recommendations
One of the most significant areas in which equipment design in the biaxial stretching areas can move forward is in realizing that not all materials behave in the same way and that equipment will need to be developed for specific applications and materials. This will become increasingly necessary as more niche markets become available for biaxially stretched films. As those specialty niche areas continue to develop, the acceptance of the fact that equipment designed to accommodate more commodity-type applications will need to be modified to penetrate those markets will be essential. Otherwise the opportunity within those areas will not be fully realized and appreciated. Optimization of equipment for specialty uses will be essential. In order for that to occur in an efficient and effective way, there must be a close collaboration between the materials scientists and equipment design
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-02.indd 24
7/1/11 8:53:32 AM
Equipment design and requirements of biaxially stretched films
25
engineers. This collaboration is necessary because each of the groups brings their own area of specialization to the problem and the issues extend across cross-functional lines. No one single expertise is required to address the questions raised in each of the various areas. Instead, cooperation between different disciplines is necessary and an interdisciplinary approach is required. At the same time, it will be necessary to develop experimental protocols to define exactly how to scale-up small laboratory experiments to commercial scale equipment. The most effective way to accomplish this is to have an intermediate step between laboratory experiments and commercial equipment which involves doing film stretching work on a so-called ‘pilot-line’ scale. Often, pilot line equipment has the same operating features as commercial lines except the final film width is narrower and the line speed is slower. Other than that, the pilot line is essentially just a smaller version of a commercial production line. However, the fact that a pilot line operates at a slower speed than commercial production lines cannot be completely ignored in the scale-up process. For example, in the tenter process, the fact that production speeds are generally much higher than pilot line speeds translates into the fact that the temperature set points for the various zones need to be higher in the production equipment. This is related to the fact that the use of higher line speeds means that the film has less residence time in the oven and, hence, less time to achieve the desired temperature. Unfortunately, there does not appear to be a simple mathematical relationship between the operating speed and the necessary set temperatures. Often, proper values are determined by a trial and error approach. A more systematic and scientific approach is desired and should be part of any scale-up procedure. The other area where work needs to be done is in the encouragement and support of more fundamental, academic investigations into understanding and developing the equipment end of the biaxial film stretching process. As already noted earlier in this chapter, there have been surprisingly few studies of the biaxial film process in the open literature, particularly for polymers other than polypropylene. The reason for this is presently unclear, but in order to gain a better understanding of the fundamental details behind the entire process, such studies are critical. Also, mathematical models which describe the entire biaxial film stretching process need to be developed. This will allow for better predictive tools to be utilized and implemented.
2.5
References
1. A. Peterlin, J. Appl. Sci., 6, 490 (1971). 2. S.A. Gordeyev and Y.P. Nekrasov, J. Mat. Sci., 18, 1691 (1999). 3. NIIR Board Handbook on Modern Packaging Industries, Asia Pacific Business Press, Inc. (2008).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-02.indd 25
7/1/11 8:53:32 AM
26
4. 5. 6. 7.
8. 9. 10. 11. 12. 13. 14. 15.
Biaxial stretching of film A. Mishra and V.K. Ahluwalia, Polymer Science: A Textbook, CRC Press (2008). F.W. Billmeyer, Textbook of Polymer Science, Interscience Publishers (1962). J.R. Collier, Ind. Eng. Chem., 61(10), 72 (1969). V. Vittoria, ‘Properties of isotactic polypropylene’, Encyclopedia of Materials Science and Engineering, Vol. 2, M.B. Bever (ed.), MIT Press (1986). W.H. DiNardo and N.Z. Karnavas, ‘Polypropylene Biaxial Oriented Film’, US Patent 6733719, issued May 11, 2004. Parkinson Technologies, Tenter Frame Equipment, Rhode Island, available from: http://www.parkinsontechnologies.com/ [accessed January 3, 2011]. Brückner Inc., LISM Technology, Germany, available from: http://www.brueckner. com/[accessed January 3, 2011]. V Ratta, G.L. Wilkes, and T.K. Su, Polymer, 42, 9059(2001). A.J. Bur and S. Roth, Polym. Eng. Sci., 44, 805 (2004). S. Rhee and J.L. White, Polym. Eng. Sci., 39, 1160 (1999). K. Song and J.L. White, Polym. Eng. Sci., 40, 1122 (2000). K. Sakauchi, T. Takebe, H. Vehura, T. Yamada, Y. Obata, and T. Kanai, J. Polym. Eng., 27, 447 (2007).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-02.indd 26
7/1/11 8:53:32 AM
3
Laboratory evaluations of biaxially stretched film
M . T . D e M e u s e, Independent consultant, USA Abstract: This chapter focuses on laboratory evaluations and primarily discusses the several types of equipment which are used to perform them. Such laboratory evaluations can provide extremely useful insight into the manner that new formulations and film structures will behave on larger equipment, thus saving both time and effort on these larger scale experiments. It is desirable to be able to perform such small scale experiments to gain an understanding of the predicted behavior. Key words: T.M. Long stretcher, Karo IV, stress–strain curve.
3.1
Introduction
Often, prior to performing evaluations of biaxial stretched film products on commercial or semi-commercial sized equipment, it is desirable to have performed a series of small scale laboratory experiments to gain an understanding of the predicted behavior. Such laboratory evaluations can provide useful insight into the manner that new formulations and film structures will behave on larger equipment, thus saving both time and effort on these larger scale experiments. This chapter focuses on laboratory evaluations and primarily discusses the several types of equipment which are used to perform them. The previous chapter focused on large scale equipment used for the biaxial stretching of film and how that equipment operates. Films which are made from various new polymers need to be evaluated on a laboratory scale so that an idea of the stretchability features on large equipment can be properly assessed. This type of laboratory evaluations and the equipment which is used to perform them constitute the main focus of this chapter.
3.2
T.M. Long stretcher for laboratory evaluations
There are two pieces of equipment which have been primarily used in the laboratory evaluation of film stretching. They are the T.M. Long stretcher 1 which was one of the first versions of a laboratory film stretching device and the Karo IV unit from Brückner,2 which is more automated in its operation than is the original T.M. Long stretcher. Both units are extremely 27 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-03.indd 27
7/1/11 8:54:33 AM
28
Biaxial stretching of film
useful for small-scale evaluations of the film stretching characteristics of new film formulations. Since its initial design, a company named Inventure Labs3 has significantly updated the T.M. Long stretcher.4 It has become more automated and computerized than it was initially and, to a large extent, this has made operation easier. Still, even with those upgrades, the operating principles behind the T.M. Long stretcher remain largely unchanged. An explanation of its operating features will now be provided. The T.M. Long stretcher operates by the movement of two bars which are perpendicular to each other, corresponding to the machine and transverse directions of the film. The stretching heads are hydraulically driven and the grips which hold the sample during the stretching process are also driven by hydraulics. The device can orient films in both the monoaxial and biaxial mode at stretching ratios up to at least 5:1. Higher stretch ratios, up to seven times, are also possible with slight equipment modifications. The dimensions of the film samples are originally two inches by two inches (50 ¥ 50 mm). Biaxial stretching is usually performed by stretching in the machine and transverse direction of the film simultaneously, but the device can also be operated to stretch sequentially. The film deformation takes place in a temperature-controlled environment. The T.M. Long stretcher is invaluable for the study of smaller samples based as experimental grades. The small sample size alluded to above allows for the film stretching features of new formulations to be assessed on small quantities of materials. As such, the initial results from the T.M. Long stretcher can be used as an effective guide for larger scale stretching work. Inventure Laboratories has taken the initial design of the T.M. Long stretcher and made changes to it, primarily upgrading the automation. They also supply replacement parts to organizations that have T.M. Long stretchers. One of the upgrades, which the AccuPull unit provides and which the original T.M. Long stretcher did not have, is the ability to generate and save stress–strain curves during the stretching process. Along with the qualitative information which is already available from the T.M. Long stretcher, the addition of the stress–strain capabilities allows for the generation of quantitative information which permits comparison of different polymers and formulations to be easily made. Other published features of the AccuPull unit are improved temperature uniformity and a simple sample loading system.5 Good temperature uniformity becomes very important when the material being stretched is sensitive to small temperature variations in its stretching features. An example of such a material is a narrow molecular weight polypropylene synthesized using a metallocene catalyst.6 In that case, if the temperature is too low, the film does not stretch well and if the stretching temperature is too high, the optical
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-03.indd 28
7/1/11 8:54:33 AM
Laboratory evaluations of biaxially stretched film
29
properties of the final stretched product are unacceptable. Temperature variations of several degrees Celsius will hurt the performance features of the stretched film so in that case the improved temperature uniformity which is offered by the AccuPull unit is highly advantageous. The ease of sample loading is a feature of the AccuPull which should not be overlooked. That feature allows for the stretching and subsequent evaluation of numerous samples in a short period of time. Thus, if there are many formulation or material variations to be stretched, the AccuPull unit allows for the work to be done very quickly and efficiently. This is a feature which has drawn an increased level of interest in the equipment.
3.3
Karo IV laboratory stretcher from Brückner
The other laboratory film stretching device which has recently attracted a great deal of attention in the open literature is the Karo IV unit from Brückner. 7 Stretch ratios up to 10 times in both the machine and transverse directions are possible with this device and a maximum stretching temperature of 400 °C is attainable. This means that practically any polymer can be evaluated with this equipment. Among the polymers reported by Brückner on its website as having been evaluated are polyamides, polyesters, polyolefins, polystyrene and poly(lactic acid). The wide range of materials investigated is further evidence of the overall flexibility that the Karo IV unit offers. The Karo IV unit, like the AccuPull device from Inventure Labs, offers ease of sample loading as one of its attributes. As with the AccuPull unit, this means that many experimental formulations and variations can be evaluated in a short time. As such, the unit is ideal for exploratory investigations for which the optimum stretching conditions have not been defined and need to be determined. Another advantage of the Karo IV unit is the ability to more directly simulate the tenter production process. This is made possible through the potential inclusion of a second stretching oven in the design of the Karo IV Stretcher. The second oven can be operated completely independently of the first oven. This means that the effect of having already stretched the material in one direction can be studied in detail. This is significant because the orientation which is provided to a film in one direction can significantly affect its subsequent crystallization features and orientation features in a second step.8 This is a point which is often missed when simultaneous orientation is used to simulate the tenter film production process. This omission means that conclusions which are drawn from the laboratory stretching evaluations do not translate directly into larger scale work, sometimes leading to erroneous results.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-03.indd 29
7/1/11 8:54:33 AM
30
3.4
Biaxial stretching of film
Literature studies involving laboratory stretching equipment
There have been evaluations in the open literature which have utilized both T.M. Long stretchers and Brückner stretchers. The work done using the T.M. Long stretcher will be discussed first. Historically, this device was designed and constructed first so the earliest laboratory film stretching evaluations were performed using it. Much of the published work using the T.M. Long stretcher has focused on polypropylene.9,10 For example, Lyondell Basell reports physical properties of film produced from a new grade of polypropylene on samples which have been stretched 7 ¥ 7 on a T.M. Long stretcher.9 In those reported values, they quote a stretching temperature of 150 °C but do not provide information for other temperatures. Also, they do not explain the rationale for using a temperature of 150 °C compared with other possible stretching temperatures. Finally, they do not make a comparison of those reported properties to film properties obtained via a commercial production process using a tenter frame. In another laboratory study involving polypropylene, Bullock and Cox 10 used a T.M. Long stretcher to study bulk process polypropylene resins which generally have higher tacticity and crystalline order than slurry process resins. They presented a correlation between resin tacticity, as measured by polymer solubility in xylene, and film orientability, as measured by the biaxial draw on a T.M. Long stretcher. The laboratory screening tests and process model were used to develop a packaging film with improved thermal dimensional stability. In general, the work presents a new approach to optimize polymer processing conditions for the attainment of superior physical properties of oriented films. In a related study, Kim11 used a T.M. Long stretcher to study the orientability of films made from blends of polypropylene homopolymers with ethylene/propylene copolymers. He showed that it is possible in the blends to use propylene homopolymers having a higher crystallinity than would otherwise be necessary for processing into biaxially oriented polypropylene films. Further, he concluded that the crystalline state of the material at the stretching temperature dictates the T.M. Long yield stress. In addition, the T.M. Long yield stress decreases with increasing stretching temperature and/ or with decreasing density of the cast sheet. The latter is modified in the film formulations by varying the concentration of the ethylene/propylene copolymer in the blend composition. As in the work of Bullock and Cox, this is attributed to an increase in the overall xylene soluble level in the blend compared to a polypropylene homopolymer. There are films made from polymers other than polypropylene which have been investigated using a T.M. Long stretcher. For example, McGonigle
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-03.indd 30
7/1/11 8:54:33 AM
Laboratory evaluations of biaxially stretched film
31
et al.12 used a T.M. Long stretcher to produce biaxially oriented films based on poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN) and copolymers containing PET or PEN moieties. They compared data for materials produced with different biaxial draw ratios. Their main interest was in permeability, diffusion and solubility coefficients. They discovered that the diffusivity of gas molecules is influenced not only by the void size and content, but also by the effects of crystallinity on the movement of the gas through the polymer. The observed behavior for the gas permeation is interpreted as being the result of the interplay between the changes in crystalline content, the polymer chain alignment and the structure of the polymer amorphous state. The T.M. Long stretcher can also be used to stretch films which are based on mixtures or blends of two polymers. Thus, Hwo13 used a T.M. Long stretcher to evaluate films which are made from mixtures of a low molecular weight polybutene polymer mixed with homopolymer polypropylene. The samples were typically stretched 4.4 times in each direction using a stretching temperature of 150 °C. The optimum stretching temperature for films made from the mixtures was lower than that for the polypropylene control film. Also, an improvement in the optical properties in the polypropylene film with the addition of the polybutene was noted. A T.M. Long stretcher has also been used to stretch film of multiple layers, so-called ‘multilayer’ films, and the resultant adhesion between the various layers after stretching has been evaluated.14 Films which contain polyvinyl chloride (PVC) and a polyolefin, such as polypropylene, were stretched using various conditions and film structures. The adhesion between the various film layers remained intact after the stretching process and the films exhibited excellent shrinkage properties. Summarizing the work with a T.M. Long stretcher, then, it has effectively been used in several investigations to screen new resins and film structures. It does a good job of defining qualitative features of the stretching process but, unless upgraded from its initial design, quantitative stretching information is lacking. This lead to the development of laboratory film stretching equipment, such as the Brückner Karo IV, with computer control and automated data acquisition. As with the T.M. Long stretcher, many of the published studies using the Karo IV unit of biaxial stretched films have been focused on the use of polypropylene. For example, Lupke et al.15 used a Brückner stretcher to orient polypropylene cast films successfully in two perpendicular directions at temperatures near the polymer melting point. In order to investigate the structure formation during the deformation, the transverse stretch ratio was varied between 1 and 9. The results of the experimental investigations show that the MDO process changes the initial spherulitic morphology of the cast film into a stacked lamellae morphology by partial melting. This
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-03.indd 31
7/1/11 8:54:33 AM
32
Biaxial stretching of film
stacked lamellae morphology is deformed during the transverse drawing into a fibrillar network. Further deformation orients the fibrils towards the actual draw direction. In another study related to polypropylene, Lin et al.16 produced biaxial oriented polypropylene films by simultaneous and sequential stretching using a Karo IV stretcher. In that work, the films were stretched to various balanced and unbalanced draw ratios. It was found that the density and the crystallinity of the films decreased as the area draw ratio increased. Sequential stretching led to a slightly lower density than simultaneous stretching to the same draw ratio. Also, sequential stretching produced lower orientation in the first stretching direction and higher orientation in the second stretching direction compared to simultaneous stretching. As with the T.M. Long stretcher, the Brückner Karo IV stretcher has also been used to evaluate and define the stretching features of films which are based on novel formulations. For example, Lin et al.16 used the Karo IV unit to biaxially stretch a film which is based on a mixture of a polyether block copolymer and a polyester. In that work, the final stretched film product is a biaxially stretched breathable film which can be used in the fabrication of protective apparel such as medical gowns. The Brückner stretcher was used to help establish the stretching parameters which will optimize the breathability features of the final film. In another study involving the use of a Karo IV Brückner stretcher, Nevalainen et al.18 studied the voiding behavior of a filled polyester film from PET as a function of different ratios of drawing. The data they obtained were analyzed to follow the development of voids, molecular orientation and crystallinity. The results demonstrate the importance of the viscoelastic nature of a polymer to void formation. It was shown that the temperature and the draw rate are interchangeable parameters in terms of their effect on the development of a void. It was also suggested that defining the appropriate draw temperature of the first draw stage of a biaxial film process is the most important factor when a highly voided polyester film is desired. These studies have shown that, much like the T.M. Long stretcher, the Brückner Karo IV lab stretcher can be used to quickly assess the biaxial stretching features of newly developed film formulations. At the same time, however, comparison must always be made to a material for which the stretching features are well known and documented, such as polypropylene in order to produce somewhat qualitative statements such as ‘the material stretches similarly to polypropylene’. As an initial step in the evaluation process, such work is necessary but, at the same time, eventually more quantitative information about the relative stretching features of various film formulations will be required. This is precisely where the ability of these lab stretchers to generate stress–strain curves during the stretching process can have significant
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-03.indd 32
7/1/11 8:54:34 AM
Laboratory evaluations of biaxially stretched film
33
applications. By comparing these curves for different materials as a function of temperature, it will be possible to quantify the differences between different polymers and even the differences between the same base polymer from different suppliers. This approach can also be extended to be used as a type of quality control method to quantitatively compare the stretching features of different batches of the same polymer from the same supplier. This is particularly important if one batch of polymer is proving to be particularly difficult to process, yielding many breaks in the film during the stretching operation. An example of the type of analysis that can be performed is shown in Fig. 3.1. That figure contains the stress–strain curve for a sample of film of homopolymer polypropylene which is biaxially stretched using a temperature of 140 °C and is taken from Billmeyer.19 A temperature of 140 °C is a usual temperature for performing a biaxial stretching experiment for polypropylene. Even though it is below the quoted melting point of polypropylene of 165 °C,20 it is still hot enough for significant softening to occur, allowing for film orientation to high levels. Several features are to be noted from the stress–strain curve shown in Fig. 3.1. The first is that the stress values quickly reach a peak value at very low strain. This peak is called the yield stress of the polymer and gives an indication of the ease in which the polymer can be oriented. The yield stress value itself systematically decreases with increasing temperature until finally when the melting point of the polypropylene is reached, it is essentially zero. As the orientation process continues, the strain increases further and in the case of polypropylene, the stress which is associated with the strain Stress
1
3
2
4
5
Strain
3.1 Stress–strain curve for homopolymer polypropylene stretched at 140 oC. The numbers correspond to the various regions of behavior in the curve.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-03.indd 33
7/1/11 8:54:34 AM
34
Biaxial stretching of film
also increases. This effect is referred to in the polymer literature 19 as ‘strain hardening’ and is not observed in all polymers. Its presence shows that it gets increasingly harder to stretch the polypropylene film to high stretch ratios. Eventually, at stretch ratios typically greater than 7–8 times the film breaks. The key point about the stress–strain curves is that they are different for different grades of polypropylene (see for example Kim).21 The yield stress values as a function of orientation temperature are different for the different types of polymer. Also, the shape of the stress–strain curve can vary depending on which type of polypropylene is used. The behavior which is observed during the stretching of the films can be used as a ‘fingerprint’ to enable the resins and behavior which is outside of the usual characteristics to be highlighted and further investigated. Taking that approach one step further, the stress–strain curves for new materials and formulations as a function of various stretching temperatures can be evaluated. From the observed behavior and a comparison to wellcharacterized polymers such as polypropylene insight into the expected behavior on large scale processing equipment can be gained. This would be useful information to have as it would help to reduce some of the trial and error work on commercial equipment.
3.5
Recommendations
It is recommended that more quantitative information than is generally obtained from laboratory stretching evaluations be the focus of future investigations. Often, those evaluations discuss only the qualitative features of the film stretching, such as the film was stretched on a particular piece of equipment to a certain stretch ratio. With the present capabilities of both the AccuPull stretcher from Inventure Laboratories and the Karo IV stretcher from Brückner to generate stress–strain curves during the stretching operation itself, it is possible to obtain information about the dynamics of the actual stretching process. This information will be extremely useful in comparing different polymers as well as comparing the same polymer from different polymer suppliers. The information that is obtained from the laboratory stretching evaluations will be useful in estimating the behavior of new polymers and formulations on commercial stretching equipment. In order to be able to do that effectively, better models which relate the laboratory results to tenter equipment need to be developed. The starting place for such modeling efforts can be polypropylene for which the most data are available on laboratory stretchers and for which the most experience on commercial tenter equipment has been obtained. In order for any models to be valid, they must accurately reproduce the known results for polypropylene. Having demonstrated that, they can be
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-03.indd 34
7/1/11 8:54:34 AM
Laboratory evaluations of biaxially stretched film
35
extended to new and unique materials. Important in this regard is the fact that not all polymers stretch identically. This means that an accurate model of the film stretching process must include not only the mechanics of the stretching itself but also the details of the polymer being evaluated.
3.6
References
1. Operation Manual of T.M. Long stretcher, T.M. Long Co., Inc., Neshanic Station, NJ. 2. Brückner, Inc. Karo IV Laboratory Stretcher, Germany, available from: http://www. brueckner.com/ [accessed January 3, 2011]. 3. Inventure Laboratories, Accupull Biaxial Film Stretcher, Tennessee, available from: http://accupull.com/ [accessed January 3, 2011]. 4. T.M. Long Company, Polymer Film Stretcher, Florida, Integrated Publishing. available from: http://www.tpub.com/content/nasa2000/NASA-2000- tm210294/NASA-2000tm2102940161.htm [accessed 21st December 2010]. 5. Inventure Laboratories, Inventure Accupull, Tennessee, available from: http:// inventurelabs.com/ [accessed January 3, 2011]. 6. M. DeMeuse, J Plastic Film Sheeting, 18, 17 (2002). 7. Brückner Inc., Karo IV Stretcher, Germany, available from: http://www.brueckner. com/fileadmin/user_upload/downloads/08-Karo.pdf[accessed January 3, 2011]. 8. S.V. Vlasov and G.V. Sagalaev, Mech. Composite Mater., 5(4), 642 (1969). 9. Lyondell Basell, Moplen HP5287, Netherlands, Available from: https:// polymers.lyondellbasell.com/portal/binary/com.vignette.vps.basell.productgrade. ProductGradeFileDisplay [accessed January 3, 2011]. 10. E. Bullock and W.W. Cox, TAPPI J., 79, 221 (1996). 11. S. Kim, J. Plastic Film Sheeting, 21(2), 99 (2005). 12. E.A. McGonigle, J.J. Liggett, R.A. Patrick, S.D. Jenkins, J.H. Daly and D. Hayward, Polymer, 42, 2413 (2001). 13. C. Hwo, European Patent EP 0343943 B1, Publication Date August 17, 1994. 14. G.D. Wofford and W.P. Roberts, US Patent 6214477, April 20, 2001. 15. T. Lupke, S. Dunger, J. Sanze, and H. Radusch, Polymer, 45, 6861 (2004). 16. Y.J. Lin, P. Dius, H.Y. Chen, A. Hiltner, and E. Baer, Polymer, 49, 2578 (2008). 17. L. Schosseler, A. Grosrenaud D.H. Mackerron, and V. Rebizant, ‘Biaxially stretched breathable film, process for making the same and use thereof’, US Patent Application 20090111362. 18. K. Nevlainen, D.H. MacKerron, and J. Kuusipulo, Mater. Chem. Phys., 92, 540(2003). 19. F.W. Billmeyer, Textbook of Polymer Science, Interscience Publishers (1962). 20. V. Vittoria, ‘Properties of isotactic polypropylene’, Encyclopedia of Materials Science and Engineering, Vol. 2, M.B. Bever (ed.), MIT Press (1986). 21. S. Kim, US Patent 7282539, issued on October 16, 2007.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-03.indd 35
7/1/11 8:54:34 AM
4
Polyolefins used in biaxial stretched films
M . T . D e M e u s e, Independent Consultant, USA Abstract: There are several polymers from which biaxially stretched film can be produced. The present chapter specifically discusses polypropylene and polyethylene, together termed polyolefins, which are two of the most commonly used polymers in biaxially stretched film. Film production processes which are commonly used for each of the two polymers will be highlighted. Key words: polypropylene, polyethylene, barrier films.
4.1
Introduction
The majority of the first part of this book has focused on the equipment which is used in biaxial stretching of films, both in laboratory and production applications. This is only one portion of the film stretching operation, the other being the polymers or plastics from which the film is produced. There are several polymers from which biaxial stretched film has been produced and the most significant of those polymers will be discussed in the following several chapters. The present chapter will specifically discuss polyethylene and polypropylene, together termed polyolefins, which are two of the most commonly used polymers in biaxially stretched film. Polypropylene will be discussed first. This is because its use is more prevalent in biaxial stretched films than polyethylene. Polypropylene can be used in both the tenter and double bubble processes described in earlier chapters while polyethylene is used primarily in the double bubble process.1 It will be the intent of the discussion to focus on the properties of the polymers that pertain most directly to their behavior in film stretching operations and not discuss all of the chemical and physical features. The interested reader is referred to several polymer texts2,3 for more details.
4.2
Polypropylene
The generic chemical structure of polypropylene is shown in Fig. 4.1. Polypropylene can exist in three forms, differing in the relative position of the methyl group on the chemical backbone. In the isotactic form, all of the methyl groups are on the same side. In the syndiotactic configuration, the methyl groups lie alternately above and below the plane. A random sequence of positions for the methyl groups occurs in the atactic configuration. The 36 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-04.indd 36
7/1/11 8:55:03 AM
Polyolefins used in biaxial stretched films [ CH2
37
CH ]n CH3
4.1 Chemical structure of polypropylene.
overall crystallinity of the isotactic form of polypropylene is the highest and is the form which is of most importance in biaxial stretched film applications. In fact, most commercially available polypropylene is isotactic. Polypropylene has a melting point of about 160 °C, as determined by differential scanning calorimetry (DSC). The melt flow rate (MFR), or melt flow index (MFI) are commonly used measures of polypropylene’s molecular weight. The lower the value of the MFR, the higher the molecular weight of the polymer. The MFR value helps to determine how easily the melted raw material will flow during processing. Also, the MFR has a direct effect on the orientability of film produced from polypropylene, with film made from lower MFR polymer typically being more difficult to orient than similar film made from higher MFR material. Typical MFR values of polypropylene and polymers which are used in biaxial stretching applications are in the 1–5 dl/g range, much lower than the MFRs of polymers which are developed and used in injection molding and fiber spinning uses. The density of polypropylene is 0.905 g/cm3,4 making it the lightest major plastic. Typical crystallinity values which can be easily achieved are on the order of 60%. Owing to this relatively high crystallinity level, articles made from the polymer have high strength, stiffness and hardness. As will be discussed later, these properties can be further impacted by the biaxial stretching process. One of the properties that makes polypropylene attractive for many applications involving biaxial stretching, particularly food packaging uses, is its moisture resistance. This is a property which is typical of many hydrocarbon polymers. Polyolefin polymers possess some of the lowest moisture vapor transmission rate (MVTR) values of any polymers with typical values of less than 1.0 g/cm3/24 h being measured (see for example Jenkins and Osborn).5 Further, these values decrease even more when film made from polyolefins is biaxially stretched. This is one of the features of biaxial stretched polypropylene film that makes it extremely useful in food packaging applications. Along with the absolute value of the molecular weight, another parameter which affects the processability of polypropylene in biaxial stretching applications is the molecular weight distribution, or MWD. Typical polypropylene homopolymers which are synthesized using traditional Ziegler–Natta catalysts have polydispersity values, a measure of MWD, of about 8–12.6 Recently, polypropylene materials with much lower polydispersity value, approaching 3, have been synthesized using
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-04.indd 37
7/1/11 8:55:03 AM
38
Biaxial stretching of film
metallocene-type catalysts. As shown by DeMeuse,7 films made from these narrow molecular weight distribution polymers can be made with improved physical properties compared with traditional polypropylene films, but the temperature processing window for orienting them is very small. This result implies that the MWD of the polymer is an important parameter for controlling its processability, with broader molecular weight polymers being easier to process and orient. Much of the open published literature on biaxial stretching of film has involved investigations of polypropylene. This is no doubt due to the fact that biaxial oriented polypropylene (BOPP) film is one of the major components in many packaging applications and the packaging industry currently has sales of approximately $6 billion.8 Owing to the size of this industry, there has been considerable interest in understanding the details of the process leading to the production of film. Kim9 concluded that the crystalline state of the polypropylene controls the stretching features of the products as measured in laboratory evaluations. Further, he found that the ability to orient film becomes easier with increasing stretching temperature and/or with an decreased density of the cast sheet. Since the density of the cast polymer sheet is directly related to the crystallinity level,10 this result implies that sheets with higher crystallinity are more difficult to orient. Kim systematically varied the crystallinity in the cast sheet by blending polypropylene homopolymers with ethylene/propylene copolymers. Using that approach, he was able to use polypropylene homopolymers of higher crystallinity than normally used in biaxial stretching applications. By varying the amount of the ethylene/propylene copolymer in the blend, he was able to modify the overall crystallinity as desired. A study of a similar nature was performed by Bullock and Cox11 who used a laboratory stretching unit to study the orientability of bulk process polypropylene resins having higher tacticity and crystalline order than traditional slurry process resins. A correlation was presented between the resin tacticity, as measured by the polymer solubility in xylene, and the film orientability, as defined by the yield stress during biaxial stretching on the laboratory unit. The screening tests and subsequent process model were used to define the features of a packaging film with improved thermal dimensional stability compared with other oriented polypropylene films. These two studies are examples of laboratory work that has been done to establish correlations between the structure of polypropylene and its performance in biaxial stretching applications. Unfortunately, such studies are very limited in the open literature. The reason for those limited studies is presently unknown but could be related to the fact that such work requires an interdisciplinary team of engineers and scientists with widely diverse backgrounds. There have also been published reports of the behavior of polypropylene
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-04.indd 38
7/1/11 8:55:03 AM
Polyolefins used in biaxial stretched films
39
in oriented film on larger scale equipment. Diez et al.12 studied the production of biaxial oriented polypropylene films in its different steps to determine the crystalline morphology of the films. A wide variety of the tests, including dynamic mechanical analysis (DMA), DSC and wide angle X-ray diffraction (WAXD) were carried out for the cast film, the produced film in the machine direction orienter unit (MDO film) and the BOPP film. The results which were obtained suggest that the various stretching steps lead to strong alignment of the polymer crystals, producing material oriented in the stretching direction. Interestingly, those observations can be verified by WAXD, DMA and tensile testing, but DSC does not detect the changes. A similar study was performed by Lupke et al.13 in which polypropylene cast films were oriented successively in two perpendicular directions near the polymer melting point as a way to investigate the structure formation during the orientation process. The stretching ratio in the transverse direction was varied between 1 and 9 and X-ray texture analysis, small angle X-ray scattering and atomic force microscopy techniques were used to characterize the structure of the films. In addition, both the melting behavior and the mechanical properties were investigated. The experimental results show that stretching in the machine direction (MD) transforms the initial spherulitic crystalline morphology of the cast film into a stacked lamellar type morphology due to partial melting. This stacked lamellar morphology is deformed during the transverse direction (TD) stretching step into a fibrillar network through crystal slip processes. Additional stretching beyond that point orients fibrils in the actual draw direction. The structural changes which are observed and reported correlate with both the melting behavior and mechanical/thermomechanical properties of the films. Rettenberger et al.14 studied the effect of temperature on the stress–strain behavior during stretching of polypropylene. They found a ductile behavior with a corresponding yield point, neck propagation, and strain hardening up to orientation temperatures of 155 °C. At higher temperatures than that, instead of yielding, the observed deformation was a quasi-rubber-like. Similar behavior was also reported when the cast films were stretched at high strain rates of over 750 mm/s. These workers also observed that the non-homogeneity of the deformation process was reduced with increased strain rate. Studies have also been performed on the properties of biaxial stretched film, with and without additional additives. For example, Masuda and Ohkura15 examined a novel ternary polymer blend containing long-chain branched polypropylene (LCB-PP), conventional polypropylene and a hydrogenated polydicyclopentadiene (hDCPD) material. It was found that the addition of the LCB-PP improves the MD stretchability of the BOPP film compared with the PP/hDCPD blend without the LCB-PP. Depending on the content
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-04.indd 39
7/1/11 8:55:03 AM
40
Biaxial stretching of film
of LCB-PP in the blend, the ternary blend could be stretched up to an MD stretching ratio of 12 without film breakage whereas the conventional BOPP film could be stretched only 6 times in the MD. The resulting final film has a dimensional stability approximately equivalent to that of the conventional one despite the use of higher stretch ratios. Further, due primarily to the presence of the hDCPD in the ternary blend, there is an improvement in the moisture barrier property as well. There has been significant patent activity around the moisture barrier improvement possible with the use of additives in biaxial oriented polypropylene film. One of the earliest mentions of this type of film is US Patent 4921749 assigned to Exxon.16 That patent discloses a film based on a mixture of from 70 to 97 wt% of a polyolefin, for example polypropylene, and 3 to 30 wt% of lower molecular weight, e.g. hydrogenated petroleum resin. Specifically, an example is provided of a biaxial oriented film which contains 20 wt% of a hydrogenated resin in the base layer which possesses barrier properties about twice as good as the same film without the addition of the hydrogenated resin. There have been follow-up studies to that initial work. For example, US Patent 550028217 discusses the production of oriented film made from a mixture of high crystallinity polypropylene (HCPP) and a polyterpene resin which explicitly improves the moisture barrier features of the film. The HCPP has an intermolecular stereoregularity of greater than 93% as determined by IR spectroscopy. The polyterpene component is added to the HCPP resin at levels less than 10 wt%. It is proposed in the patent that the addition of the terpene polymer increases the extent of amorphous orientation, thereby restricting diffusion of water molecules, in the orientation step, particularly the tentering process. In similar work, in US Patent Application 20080286547 assigned to Exxon Mobil Chemical Company,18 biaxial oriented polypropylene films are disclosed which are based on mixtures of polypropylene, a nucleating agent and a hydrocarbon resin. The hydrocarbon resin is present in amounts sufficient to lower the moisture permeability in comparison to the moisture permeability in the absence of either or both the nucleating agent and the hydrocarbon resin. Typical weight percentages are 3–10 wt% with amounts up to 30 wt% being quoted. Rather than using hydrocarbon resin alone to improve the moisture barrier features of polypropylene film, Toray19 has recently submitted a patent application which involves the use of crystalline Fischer–Tropsch waxes in possible combinations with hydrocarbon resin. The polypropylene resin used in that invention is classified as highly crystalline, due to its high isotactic level. The Fischer–Trospch wax is used in an amount of 2–20 wt% and the hydrocarbon resin is present at loadings up to 10 wt%. Along with providing an improvement in the moisture barrier properties of the film,
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-04.indd 40
7/1/11 8:55:03 AM
Polyolefins used in biaxial stretched films
41
the hydrocarbon resin is also quoted as aiding in the orientation stretching of the highly crystalline polypropylene to reduce unstretched or uneven stretch marks or film breaks. It is possible to decrease not only the moisture vapor transmission rate of traditional Ziegler–Natta polypropylene polymers but also that of metallocene-catalyzed polypropylenes. Thus, US Patent 616559920 shows that the addition of small amounts of hydrocarbon resin to polypropylene made using metallocene catalysts reduced the moisture vapor transmission rate of biaxially oriented film.20 This reduction was in addition to the reduction observed when comparing the moisture barrier properties of film made from the metallocene polypropylene to film made from a Ziegler–Natta polypropylene. In addition to the moisture barrier effect, the hydrocarbon resin also acted as a processing aid, improving the film optical properties, such as haze, gloss and clarity. Another additive which has been shown to significantly lower the moisture vapor transmission rate of polypropylene films is a highly crystalline Fischer–Tropsch wax. For example, it has been shown21 that amounts of less than 5 wt% of such waxes can lower the moisture vapor transmission rate of BOPP films by a factor of 4–5 times. It is hypothesized that the mechanism by which these waxes work is to phase separate from the polypropylene and migrate to the film surface. Once at the film surface, due to their crystallinity level of greater than 90%, they restrict the passage of water molecules. At the very low loading levels used, these waxes do not significantly negatively impact the film optical properties.
4.3
Use of polyethylene in biaxial stretching
Another polyolefin which has been studied in biaxial film stretching operations is polyethylene (PE). However, when reference is made to polyethylene, more details about the type of polyethylene being used must be specified. For example, there are different types of polyethylene, including low density and high density polyethylene. Obviously, the difference between these two species is the density of the polymer which is produced. However, in the context of the present discussion concerning biaxial stretching of films, it is important to note that the different types of polyethylene behave differently in the stretching operation. Wilkes and coworkers discussed the tentering process of high density polyethylene (HDPE), focusing on MD stretching and the microstructure characterization of the HDPE film after the MD stretching step.22 MD stretching experiments were carried out as a function of stretch ratio and stretch temperature, after which the morphological state of the HDPE film was examined using X-ray, atomic force microscopy, transmission electron microscopy, DSC and refractive index observations. The study examined
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-04.indd 41
7/1/11 8:55:04 AM
42
Biaxial stretching of film
the change in the crystalline morphology from spherulitic lamellae into lamellar stacks that resulted from the MD stretching at temperatures near the polymer melting temperature. Since the stretching was done sequentially, the information that was obtained from the work provided the basis from which the TD stretching was undertaken, but no actual TD stretching data were presented. Ajji et al.23 investigated the biaxial stretchability, structure developed, molecular orientation and shrinkage of linear low density octene copolymers (LDPEs) biaxially stretched using a Brückner laboratory stretcher. Seven different resins having different molecular features were used in that study. The effects of stretching temperature and rate on the stretchability were assessed. The results indicate that the high molecular weight tail and comonomer content play important roles in the orientation of the resins. Higher molecular weight tails, molecular weight distribution, and resin content eluting above 90 °C, as measured by the temperature rising electron fractionation (TREF) technique, tend to increase orientation. In another study of linear LDPE films,24 biaxial oriented samples were produced using the double bubble process with different machine direction orientation levels and the same transverse direction blow-up ratio. The mechanical behavior of the films was characterized in terms of the tensile strength and tear resistance. The results indicate that the machine direction ultimate tensile strength increases and the TD ultimate strength decreases with MD stretching ratio. Tear propagation resistance remained constant in the TD and decreased in the MD as the draw ratio was increased. The film morphology was a typical biaxial lamellar structure for all of the samples with different lamellar dimensions. There has also been significant patent activity in the area of biaxial stretching of polyethylene films. For example, in US 6168826, issued to Mobil,25 a process is disclosed for preparing biaxial oriented polyethylene films with improved optics and sealability properties. Specifically, the described process involves stretching a multilayer base sheet with in the MD, extrusion coating the base sheet with either LDPE or linear LDPE and, then, stretching the base sheet in the TD. The properties of the final film are such that an alternative to blown HDPE films is said to be provided at significantly thinner film thicknesses. In European Patent Application EP 0876250 assigned to Exxon,26 films are provided which are based on ethylene-based polymers made using a metallocene catalyst system. It is shown that biaxial orientation dramatically improves clarity, dart drop impact, puncture resistance and shrink properties. Film strength is increased by 300% over an unstretched film made from the same ethylene polymers. The final films are useful as shrink wrap and overwrap and also for packaging applications such as snacks and cereals. Porous biaxially oriented polyethylene films have also been disclosed in
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-04.indd 42
7/1/11 8:55:04 AM
Polyolefins used in biaxial stretched films
43
the patent literature. In US 6828013 to Exxon Mobil,27 a porous film which is based on HDPE is described. The film is biaxially oriented and has a water wettable surface consisting of silicone glycol in the pore surface. The pores are created in the HDPE through the use of a cavitating agent, like calcium carbonate. The final application of the films is in inkjet applications, due to the hydrophilic nature of the films. In US Patent Application 20050042397 to 3M Corporation, 28 films are described which are based on the use of mixtures of a semicrystalline polymer, including both HDPE and LDPE an ink-absorbing polymer and an inorganic filler. The films are biaxially oriented and pores are produced. The voids or pores are produced due to both poor stress transfer between the two immiscible polymers and debonding from the inorganic particulates. The biaxial orientation conditions are chosen so that the voids at the film surface have an average diameter less than or equal to the average size of the applied ink droplets. The resultant films are claimed to have applications as graphic films for advertising and promotional displays. From these discussions, it can be seen that there is much interest in the production of biaxial oriented films from different types of polyethylene, including both high density and low density types. However, as with the reported work on polypropylene, the majority of the studies have focused primarily on formulation development with a lower level of work having been devoted to an understanding of the structure/processing/property relationships for these materials. There continue to be opportunities available in that general area of research. The general theme needs to be the production of unique structures through the combination of novel materials and unique processing scenarios. It is through that combination that truly new and original biaxial oriented films will be developed.
4.4
Conclusions
In general, the biaxial film stretching process allows for an improvement in film mechanical properties, like strength and modulus, and that effect is clearly exploited with the use of polyolefins, such as polypropylene and polyethylene. Particularly in the case of polypropylene, significant increases in the mechanical properties with increasing orientation level have been observed. Also, improvements in the film barrier properties, specifically moisture vapor transmission rate, are observed with orientation. This is particularly important in food packaging applications, as requirements become increasingly challenging. Both clear and opaque film structures have been reported using polypropylene and polyethylene as the base polymer and the resultant films have a range of diverse applications, ranging from snack packaging to labels. Later chapters will highlight some of those specific applications
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-04.indd 43
7/1/11 8:55:04 AM
44
Biaxial stretching of film
and the markets that they serve. It has been the objective of this chapter to address the science and technological aspects of biaxial stretched films produced from polypropylene and polyethylene rather than to focus on the uses of those films.
4.5
Recommendations
There is still much to be done in the technology area. As alluded to earlier, much of the published work in the polyolefin area has focused on formulations and not how those formulations can be tailored and optimized to yield unique products. There is a continuing need to develop structure/ processing/property relations for materials, including polyolefins, so that structures with a particular set of properties can be produced. The most efficient way to perform those studies is through laboratory evaluations of the stretching features of formulations. Such laboratory studies can be used to initially assess the potential of newly developed polymers and additives along with their processability features. That entire package of information can, then, be used as a starting place for larger scale work on pilot tenter equipment. It should be emphasized, however, that the laboratory studies can be used only as a starting place for the tenter evaluations. Owing primarily to the different strain rates used in the two different processes and also differences between batch and continuous processing, there will not be a direct correlation between the laboratory studies and the pilot line work. Because of this it will still be necessary to do some empirical work on the tenter equipment to completely optimize the stretching conditions. Unfortunately, there are a limited number of such studies in the published open literature. This chapter has summarized many of the published articles which pertain to polyolefins, particularly polypropylene and polyethylene. Even with these common and popular polymers there have been very few reports that develop the structure/processing/property relations already discussed. It is recommended that this is an area in which additional studies need to be done. Overall, the science of the biaxial film stretching process needs to be further developed. Much of the present knowledge can be best described as an art rather than a science. Since polyolefins are readily available, they offer an excellent possibility to further the understanding of the film stretching process and its effects on the final film properties. The development of the knowledge which is obtained from these kinds of study will greatly facilitate the evaluation of new polymers and formulations. This will help eliminate much of the trial and error work which is presently done on these new formulations. As such, time savings will be realized in the production of films from new materials.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-04.indd 44
7/1/11 8:55:04 AM
Polyolefins used in biaxial stretched films
4.6
45
References
1. NIIR Board, ‘Handbook on Modern Packaging Industries’, Asia Pacific Business Press, Inc. (2008). 2. A. Mishra and V.K. Ahluwalia, ‘Polymer Science : A Textbook’, CRC Press (2008). 3. F.W. Billmeyer, ‘Textbook of Polymer Science’, Interscience Publishers (1962). 4. V. Vittoria, ‘Properties of isotactic polypropylene’, Encyclopedia of Materials Science and Engineering, Vol. 2, M.B. Bever (ed.), MIT Press (1986). 5. W.A. Jenkins and K.R. Osborn, Plastic Films: Technology and Packaging Applications, CRC Press. 6. J.M. Dealy and K.F. Wissbrun (1990), Melt Rheology and Its Role in Plastics Processing, Van Nostrand Reinhold, New York. 7. M. DeMeuse, J. Plastic Film Sheeting, 18, 17 (2002). 8. M.A. Cliff, P.M.A. Toivonen, C.F. Forney, P.Y. Liu and W. Lu, Postharvest Biol. Technol., 58(3), 254 (2010). 9. S. Kim, J. Plastic Film Sheeting, 21(2), 99 (2005). 10. C. Yuksekkalayci, U. Yilmazer, and N. Orbey, Polym. Eng. Sci., 39, 1216 (1999). 11. E. Bullock and W.W. Cox, TAPPI J., 79, 221 (1996). 12. F.J. Diez, C. lvarion, J. Lopez, C. Ramirez, M.J. Abad, J. Cano, S. Garcia-Garabal, and L. Barral, J. Thermal Analy. Calorimetry, 81, 21 (2005). 13. Th. Lüpke, S. Dunger, J. Sänze, and H.J. Radusch, Polymer, 45, 6861 (2004). 14. S. Rettenberger, L. Capt, H. Manstedt, K. Stopperka, and J. Sanze, Rheol. Acta, 41, 332 (2002). 15. J. Masuda and M. Ohkura, J. Appl. Polym. Sci., 106(6), 4031 (2007). 16. US Patent 4921749, ‘Sealable films’, assigned to Exxon Chemical Patents, May 1, 1990. 17. US Patent 5500282, ‘High moisture barrier opp films containing high crystallinity polypropylene and terpene polymer’, assigned to Mobil Oil Corporation, March 19, 1996. 18. US Patent Application 20080286547, ‘Polypropylene films with enhanced moisture barrier properties, process for making and compositions thereof’, assigned to Exxon Mobil Chemical Company. 19. US Patent Application 20080205800, ‘Transparent biaxially oriented polypropylene films with low moisture vapor and oxygen transmission rate’, assigned to Toray Plastics (America) Inc. 20. US Patent 6165599, ‘Biaxially oriented film prepared from metallocene catalyzed polypropylene’, assigned to Applied Extrusion Technologies. 21. US Patent 6033514, ‘Biaxially-oriented polypropylene films’, assigned to QPF LLC. 22. V. Ratta, G.L. Wilkes, and T.K. Su, Polymer, 42, 9059 (2001). 23. A. Ajji, J. Auger, J. Huang, and L. Kale, Polym. Eng. Sci., 44, 252 (2004). 24. A.L. Bobovitch, R. Tkach, A. Ajji, S. Elkown, Y. Nir, Y. Unigowski, and E.M. Gutman, J. Appl. Poly. Sci., 100(5), 3545 (2006). 25. US Patent 6168826, ‘Biaxially oriented polyethylene film with improved optics and sealability properties’, assigned to Mobil Oil Corporation, January 2, 2001. 26. European Patent Application EP0876250, ‘Biaxially oriented polyethylene films’, assigned to Exxon, August 14, 1998.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-04.indd 45
7/1/11 8:55:04 AM
46
Biaxial stretching of film
27. US Patent 6828013, ‘Porous biaxial oriented high density polyethylene film with hydrophilic properties’, assigned to Exxon Mobil Oil Corporation, December 7, 2004. 28. U.S. Patent Application 2005042397, ‘Biaxially-oriented ink receptive medium’, assigned to 3M Corporation, February 24, 2005.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-04.indd 46
7/1/11 8:55:04 AM
5
Other polymers used for biaxial films
M . T . D e M e u s e, Independent Consultant, USA Abstract: Polymers other than polyolefins which are commonly used in the production of biaxial stretched films are the topic of this chapter. Specifically, polyamides, polyesters and poly (lactic acid) are the primary polymers which are discussed. Typical processing scenarios for each of these are provided and advantages of the films produced are mentioned. Key words: polyester, polyamide, poly (lactic acid).
5.1
Introduction
Chapter 4 focused on a description of polyethylene and polypropylene, the two most common polymers used for the production of biaxially oriented films. Many food packaging applications have been developed around the use of those two materials. However, even though the polyolefins, as represented by polyethylene and polypropylene, are the most commonly used polymers in oriented film applications, there are other polymers which are also used. Some of those other polymers are the primary focus of the present chapter.
5.2
Polyethylene terephthalate (PET)
The first polymer to be discussed will be polyethylene terephthalate (PET). Its chemical structure is shown in Fig. 5.1. It belongs to the class of polymers known as the thermoplastic polymer resin of the polyester family and has many applications in fibers, containers and injection molding as well as biaxial oriented films. Biaxially oriented PET film (BOPET) is manufactured with a film of the molten polymer being extruded onto a chill roll, which quenches it into the amorphous state. Biaxial orientation, then, occurs through a drawing process which can be either simultaneous or sequential. The most common way of
p
O
O
C
C
O
CH2
CH2
O
n
5.1 Chemical structure of polyethylene terephthalate (PET).
47 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-05.indd 47
7/1/11 8:59:58 AM
48
Biaxial stretching of film
producing BOPET is the sequential process. Draw ratios are typically about 3 to 4 in both the machine (MD) and transverse (TD) directions. Once the drawing operation is complete, the film is heat-set under tension in the stretching oven at temperatures which are typically above 200 °C. The orientation is responsible for the high strength and stiffness of oriented PET film, which has a typical modulus of about 4 GPa. Also, due to the orientation, many crystal nuclei are formed that remain smaller than the wavelength of visible light. As a result, BOPET film has excellent clarity. The final BOPET film is used for its high tensile strength, chemical and dimensional stability, transparency, reflectivity, gas and aroma barrier properties and electrical properties. A variety of companies manufacture BOPET and other polyester films. The most well-known trade names are Mylar, Melinex and Hostaphan. There are numerous applications which have been developed for BOPET film. Among these are flexible packaging and food contact applications such as lidding for fresh or frozen ready meals. Also, owing to its excellent electrical properties it has uses in electronics such as being the carrier for flexible printed circuits. Finally, an emerging and developing application for BOPET film is graphic arts in which engineering plans and architectural drawings are often plotted on sheets of BOPET film.
5.2.1 Literature studies of PET Bernes et al.1 studied BOPET films using thermally stimulated current (TSC) spectroscopy in order to define the effects of biaxial orientation on the molecular behavior of the polymer. In biaxial oriented film, the peak corresponding to the glass transition temperature (Tg) shifts to 100 °C from a temperature of 82 °C in amorphous PET. The shift in the Tg peak is interpreted as being due to a stiffening of the amorphous chains under stretching. In related work, Rao et al.2 investigated the relationship between the microstructure and toughness of biaxial stretched polyester films. Optically transparent sheets were prepared by simultaneous biaxial orientation of melt-cast sheets which were stretched near the Tg. PET film with different crystalline morphologies was produced by constrained high temperature annealing of the biaxially oriented films. Using mechanical testing, synchotron small angle X-ray scattering, wide angle X-ray diffraction techniques and differential scanning calorimetry, the toughness, degree of crystallinity and crystalline morphology/molecular ordering were studied. The results indicate that the toughness of the film is determined by the interconnectivity of the crystalline phase within the amorphous phase and is greatly influenced by the degree of crystallinity and the underlying crystalline morphology. An earlier study by Gohil3 examined the effect of biaxial film processing
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-05.indd 48
7/1/11 8:59:58 AM
Other polymers used for biaxial films
49
parameters on the orientation of the phenyl ring shown in Fig. 5.1 of the PET structure. In that work, it was shown that the total phenyl ring orientation factor reflects the nature of the amorphous and crystalline phases as well as the morphological changes taking place due to variations in processing conditions. Thus, that orientation factor can be used to offer a useful predictive tool for oxygen permeability of biaxially oriented PET films prepared under a variety of processing conditions. Furthermore, the cause of the nonlinear relation between oxygen permeability and percent crystallinity is discussed in detail. Another study involving an examination of the oxygen permeability of oriented PET film was performed by Perkins.4 In that article, the influence of molecular weight and annealing temperature on the crystallinity and subsequent resistance to oxygen permeability was evaluated for BOPET films. Within the range investigated, the molecular weight affected the level of crystallinity developed at a given temperature, but had very little influence on oxygen permeability. The annealing temperature more directly influenced permeability than did the absolute level of crystallinity. In a recent article,5 blends of polyester with other high barrier polymers, such as polyamide, were examined as a way to improve the oxygen barrier properties of biaxially oriented PET films. The effects of morphology on the oxygen gas permeability and processability were analyzed using a combination of characterization techniques. The general conclusion of the work is that stretching enhances the barrier properties of the polyester/ polyamide blends.
5.2.2 Patent activity for PET As would be expected from the list of many applications for BOPET films, there has been much patent activity in that area also. Since BOPET film was first developed in the mid-1950s, originally by DuPont and Imperial Chemical Industries (ICI), many of the early patents are at least fifty years old. Rather than attempting to discuss all of the BOPET patents, it is the present intent to provide an overview of the patents in this area. One of the earliest US patents which is related specifically to the biaxial orientation of PET films is US 3432591 issued to DuPont.6 That patent provides a film structure with improved toughness and durability over prior art films and also a greater retention of the toughness over a broad temperature range. The film structure has a well-defined range of orientation and crystallinity. Those properties are obtained through an extension of the film operability into regions of orientation which were achieved with previous structures. After the initial patent activity by DuPont and ICI there were many patents issued to companies in the area of BOPET film. Several of the subsequent
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-05.indd 49
7/1/11 8:59:59 AM
50
Biaxial stretching of film
patents7,8 dealt with developing improved processes for heat-setting the polyester film to provide improved dimensional stability to the film. Much of that development work was focused on the production of film for use as the basis for magnetic recording tape. As such, it is very desirable to have a film product which has low shrinkage at elevated temperatures, i.e. one which exhibits good thermal dimensional stability. Another patent which discussed the thermal treatment conditions of BOPET film as being important is US 5534215 assigned to SKC Limited.9 In that patent, the inventors discuss the fact that film which is produced by prior art approaches has a poor leveling property due to residual stresses applied during the thermal treatment process. This poor leveling property is noted to causes issues with the general appearance of the film as well as curling of the film when laminated to other materials. In order to resolve those problems, the inventors define and develop specific thermal treatment conditions for the polyester film. They do that specifically by using thermal treatment temperatures with a well-defined mathematical relationship to the polyester melting temperature. There has also been much activity in the patent literature concerning the use and inclusion of various additives, including inorganic particles, in biaxially oriented polyester film. Many of these films are used as magnetic recording media, for which the polyester films are required to have uniform surfaces. The use of inert particles to make the film surface appropriately rough improves the abrasion resistance and the running features of the polyester film. An example of this approach is provided in US Patent 5580652, assigned to SKC Limited.10 In that patent, work is described which involves the addition of a particulate slip agent which is based on aluminum hydroxide. That material was chosen largely for its affinity to PET, unlike calcium carbonate, which, owing to its low affinity to the polyester, can lead to the formation of voids during the draw processing of the film. Such voids can lead to abrasion of the film surface, separation of the particles and scratching of the film. As with polypropylene and polyethylene, there has been interest in the production of voided films based on PET. One of the initial patents 11 in that area claims a process for the production of a biaxially oriented matte surface film from polyethylene terephalate in which the film also contains incompressible particles like calcium carbonate or silicon dioxide at a concentration of from 1 to 25 weight%. Specific stretching temperatures and stretch ratios are defined for the production of the opaque, matte-finish film product. Other polymers, such as polyolefins, have been used as voiding agents in polyester films.12 In that particular work, homopolymers or copolymers of ethylene and propylene are used as additives with the polyester. The
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-05.indd 50
7/1/11 8:59:59 AM
Other polymers used for biaxial films
51
polymeric additive is incompatible with the polyester component of the film and exists in the form of discrete globules dispersed throughout the film. The opacity of the film is produced by a voiding process which occurs between the polymeric additive and the polyester when the film is stretched. It is required that an intimate bond not develop between the polyester and polymeric additive in order for this approach to work effectively. That idea has been extended in a recent US Patent Application 13 in which films are produced from mixtures of a polyester resin, particles of another polymer which is not miscible with the polyester resin, inorganic particles, and a whitening agent. The produced films in that invention consist of a single layer. The void formation is facilitated by the immiscible polymer particles together with the inorganic particles. In order for the approach to be effective it is important that the immiscible polymer has a heat deformation temperature higher than the drawing temperature of the film by at least 10 oC.
5.3
Polyamides in biaxially oriented films
A second polymer which has been extensively examined in biaxially stretched film applications is polyamide (PA). Generically, polyamides are also referred to as nylons and contain recurring amide units. Characteristically, polyamides are very resistant to wear and abrasion, have good mechanical properties even at elevated temperatures, have low permeability to gases and have good chemical resistance. Within the general class of polyamides, there are several specific chemical structures which have been the focus of biaxial film studies. Among those, one of the most common polymers is designated Nylon 6. Its chemical structure is shown in Fig. 5.2. Also shown in the figure is polymer which is O N H n
Nylon 6
O
H N O
N H
Nylon 6, 6
n
5.2 Chemical structure of Nylon 6 and Nylon 6,6.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-05.indd 51
7/1/11 8:59:59 AM
52
Biaxial stretching of film
related to Nylon 6, known as Nylon 6,6. The Tg of Nylon 6 is about 50 °C and its melting temperature is about 220 °C. Investigations on crystal structure, orientation and mechanical properties of biaxially stretched Nylon 6 films were carried out using differential scanning calorimetry, wide angle X-ray diffraction, birefringence and tensile testing by Rhee and White.14 They developed a pseudo-orthorhombic unit cell model in order to calculate biaxial orientation factors of the crystalline phase. The mechanical properties of the films were successfully correlated with out-of-plane birefringence. Workers have also examined the oxygen barrier of Nylon 6 films and the effect than moisture plays on the oxygen transmission rate (OTR) values. 15 Contrary to popular opinion, it was found that there is no deterioration in OTR values up to 80% relative humidity (RH) for biaxially oriented Nylon 6 films. Prompted by this unexpected result, the role of morphological parameters in controlling the OTR values was revisited. It was demonstrated that the contribution of orientation, after correcting for induced crystallinity, to the OTR of Nylon 6 products is insignificant. The main highlight of the manuscript is that it quantifies the impact of moisture and processing related parameters on the OTR of Nylon 6. Primarily since biaxially oriented nylon films provide excellent barrier properties, good growth is predicted in markets, particularly in China. Biaxially oriented polyamide (BOPA) film is presently growing at an annual rate of 5–6% in Asia. Brückner, DMT and Mitsubishi are the three major equipment suppliers for the production of BOPA film. Three processes, sequential stretching, simultaneous stretching and blown film are presently commercially used to produce this product. The sequential tenter process accounts for greater than 50% of present production capacity. As would be expected, there has been significant patent activity in the area of biaxially oriented nylon films. One of the earliest patents in this area is US 4522867 to Phillips Petroleum Co.16 That patent focuses on a biaxial stretched amorphous polyamide film or sheet. The polymer resin is extruded into film or sheet at a temperature of about 250 to about 350 °C under an inert atmosphere to minimize oxidation. The biaxial orientation process is carried out about 5 to about 20 °C above the polymer Tg. Both tentering and bubble-blowing are discussed as effective means for providing biaxial orientation to the film. Specific attention is paid to the tubular film process in US Patent 5094799 to Idemitsu Petrochemical.17 That patent relates to a process for the production of a biaxial oriented nylon single-layered or multi-layered film. Processing parameters were defined which allowed for the continuous production of nylon film via the tubular film process. In particular, stretching ratios were defined which allowed for the satisfactory production of the final oriented film.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-05.indd 52
7/1/11 8:59:59 AM
Other polymers used for biaxial films
53
In EP 0826731A1,18 a tough biaxial stretched polyamide film is described which is based on a mixture of glass fiber and a blend of two different polyamide resins. The resulting film has an excellent matte surface appearance and little surface gloss. A food packaging bag made of this film is also disclosed which shows little deformation when it is subjected to a heat treatment step with food packaged inside the bag. US Patent 6685871 to Honeywell International19 describes a biaxially stretched polyamide film which has superior strength and resistance to defects such as pinholes and cracks. The invention provides for toughened BOPA through the use of a blend of a polyamide homopolymer with a functionalized polyolefin terpolymer. The resultant biaxial oriented films are well suited for making packaging materials for various food products.
5.4
Poly(lactic acid) (PLA) in biaxially stretched films
The final polymer that will be discussed in this chapter is poly(lactic acid) (PLA). Its chemical structure is shown in Fig. 5.3. It is a biodegradable, thermoplastic, aliphatic polyester derived from renewable sources, such as corn starch or sugar canes. It has been of commercial interest only in recent years, primarily in light of its biodegradability. Since PLA is a relatively new polymer to the biaxial orientation field, publications and patents which discuss it are relatively new. However, studies are beginning to appear that discuss the orientation features of films made from the polymer. For example, Yu et al.20 discuss the effect of annealing and orientation on the microstructures and mechanical properties of PLA. Stretching at temperatures above Tg produced simultaneous crystallization and polymer chain relaxation, which resulted in increases in both the film modulus and toughness. Ou and Cakmak 21 examined the structural evolution during both simultaneous and sequential biaxial stretching of PLA films from cast amorphous precursors. Simultaneous biaxial stretching leads to films with in-plane isotropy and poor crystalline order. In the first step of sequential biaxial stretching, oriented crystallization slowly develops while transverse isotropy is maintained. Further, the application of transverse stretching
O O n
5.3 Chemical structure of poly(lactic acid) (PLA).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-05.indd 53
7/1/11 8:59:59 AM
54
Biaxial stretching of film
to a film possessing semicrystalline structure gradually destroys the crystalline structure produced in the MD during this realignment while establishing a second population of oriented but poorly ordered crystallites in the TD. This destruction is caused primarily by a splaying action under transverse stretching, as evidenced by the decrease of crystallite sizes in the MD. Lehermeier et al.22 examined the permeation of nitrogen, carbon dioxide and methane gases in oriented films made from PLA. They found that polymer chain branching and small changes in the stereochemical content have no effect on the permeation properties. Crystallinity was found to dominate the permeation properties in the biaxially oriented film. The separation factor for a carbon dioxide/methane mixed gas system suggests that continued studies as a separation medium for this film are highly warranted. As a way to investigate the molecular features of oriented PLA film, Shinyama and Fujita23 studied the conduction current and dielectric properties of a biaxially oriented PLA film sample. They found the volume resistivity was larger than that of polyethylene and polypropylene and the relative permittivity was intermediate between that of polyesters and polyolefins. Further, the dielectric loss tangent was larger than for polyethylene and polypropylene but smaller than that of polyester. There has also been significant recent patent activity in the area of biaxially oriented PLA films. Much of that effort has focused on the development of commercial processes for the production of a final product. There is a desire to use standard tenter equipment presently used for biaxially oriented polypropylene (BOPP) for the production of oriented PLA films. However, typically PLA cast films can only be oriented about 3 times in the machine direction and approximately 3.5 times in the transverse direction. Thus, work has been done to increase the stretch ratios of PLA products to make them run more like BOPP products. In that sense, PLA products presently run more like BOPET films and there has also been work done to produce them on modified PET lines. For example, in US Patent Application 20090148715 assigned to Toray Plastics24 novel formulations are developed that exhibit significantly improved ability to stretch in the TD in a biaxial orientation process. The films include specific processing aids as a minor component, which enables the film to be oriented in the TD at much higher rates than previously achieved while maintaining good productivity. This allows for the opportunity of producing biaxially oriented polylactic acids on BOPP film manufacturing assets without incurring permanent modifications to such assets. In addition, US Patent 712896925 describes a film which contains a minority component of a thermoplastic polyolefin such as polypropylene or polyethylene. Such films are stretchable in the transverse direction up to about 6 times. However, the use of the polyolefin additives such as
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-05.indd 54
7/1/11 8:59:59 AM
Other polymers used for biaxial films
55
polypropylene or polyethylene causes incompatibilities with the polylactic acid polymer, resulting in a hazy film appearance. US Patent Application 200900311544 to Toray Plastics26 describes a biaxial oriented film based on a blend of polylactic acid and an inorganic antilock particle. The film is biaxially oriented using a low transverse orientation temperature to impart a degree of cavitation around the particles. This allows for the production of a matte or opaque appearance. The combination of processing and inorganic cavitating agent results in a consistent and uniformly fine matte or opaque film previously not obtained. US Patent Application 2010004090427 to Toray Plastics addresses a novel formulation with improved barrier properties particularly for moisture vapor transmission rate, after metallizing. Technology is described for a sputtering method that deposits a thin layer of a metal primer such as copper onto the PLA metal receiving layer prior to the vacuum deposition of aluminum. This pre-treatment process using the metal primer improved the metal adhesion of a subsequent metal layer and also improved the gas barrier of the metallized PLA film compared with an aluminum metallized PLA film without this pre-treatment. Particularly improved are the barrier features to moisture vapor. There are presently several commercial manufacturers of biaxially oriented PLA film. Among the most prominent of these is Natureworks which produces a product designated 4032D film.28 Typical orientation values which are quoted in the company literature are 3.5 times in the MD and 5 times in the TD. The film is described as having excellent optics, good machinability, and excellent twist and dead fold properties. Additional properties which are noted include barrier to flavor and grease and oil resistance. SKC has also developed a film called SKYWEL PLA film, claiming it has many possible applications for the product such as envelope window film, twist film, carton window film, snack packaging and general packaging. The film is touted as being metallizable and having excellent adhesion to coatings. SKC offers both standard PLA film grades as well as heat sealable and shrinkable PLA films. The standard PLA film is comparable to PET and OPP film in tensile strength and elongation and is superior to oriented polystyrene in all properties. Biaxially oriented film for food packaging is a major application of PLA because of its excellent barrier to flavor constituents and its heat sealability.29 In terms of a parameter called seal initiation temperature (SIT), which is the lowest temperature at which a seal to another structure forms, PLA films have SIT values in the 80 °C range. This value is lower than for most polyolefin-based films, and as such offers a potential advantage for the PLA-based materials. All the polymers discussed in this chapter, including polyesters, polyamides and poly (lactic acids), show distinct processing features which are different
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-05.indd 55
7/1/11 8:59:59 AM
56
Biaxial stretching of film
from each other and also significantly different from polyolefins, such as polypropylene and polyethylene. Thus, in order to potentially realize the most benefit from the properties provided from these films, a complete optimization of the processing schemes for these polymers is necessary. Work is still ongoing in this area as new applications develop. Also, there may be possibilities for the design of unique processing equipment for these non-polyolefin polymers. Much of the existing biaxial film stretching equipment has been designed with polyolefins, particularly polypropylene, in mind. There is no fundamental reason to assume that equipment design is also the best one for the polymers discussed in this chapter. As such, opportunities may exist for machine producers and suppliers.
5.5
Recommendations
As new applications continue to emerge for biaxially oriented films, new polymers must naturally be investigated to meet the needs of those applications. Often, those new polymers will not be processable under similar conditions as are well-known and well-established materials, such as polypropylene. In those cases, biaxial stretching profiles which are unique to optimize the properties of the new materials need to be developed. Work into the further development of films based on polymers such as PLA needs to continue. Compared with other materials, that polymer offers barrier property advantages, particularly in terms of flavor and aroma. In addition, it offers the possibility of recycling films which are based on its use. This is an important area which continues to gather a great deal of attention. In order to effectively accomplish that, it is recommended that additives and plasticizers continue to be explored for PLA which will make its films easier to orient. One of the present issues with PLA films in the biaxial orientation process is their stiffness, which is a contributing factor to the relatively low orientation levels which are possible. The introduction of additives to PLA formulations which will allow for the attainment of higher orientation levels should increase the applications in which the final oriented film can be used.
5.6
References
1. A. Bernes, D. Chatain, C. Lacabanne, and G. Lorentz, Electrical Insulation, 1990, Conference Proceedings of the 1990 IEEE International Symposium; 3–6 June 1990, 457–460. 2. Y. Rao, J. Greener, C.A. Avila-Orta, B.S. Hsiao, and T. Blanton, Polymer, 49(10), 2507 (2008).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-05.indd 56
7/1/11 8:59:59 AM
Other polymers used for biaxial films
57
3. R. Gohil, J. Appl. Polym. Sci., 48(9), 1635 (1993). 4. W. Perkins, Polym. Bull., 10(4), 1988 (1983). 5. I. Ozen, G. Bozoklu, C. Dalgoida, O. Yucel, E. Unsai, M. Cakmak, and Y. Mencelogl, Eur. Polym. J., 46(2), 226 (2010). 6. US Patent 3432591, ‘Biaxially oriented heat set film of high molecular weight polyethylene terephalate’, C.J. Heffelfinger, assigned to DuPont (March 11, 1969). 7. US Patent 3461199, ‘Process for improving dimensional stability of tensilized polyester film’, D. Campbell, assigned to DuPont (August 12, 1969). 8. US Patent 4042569, ‘Heat setting process for polyester film’, R.G. Bell, E. Gillyns and O. McDaniel, assigned to DuPont (August 16, 1977). 9. US Patent 5534215, ‘Process for the preparation of biaxially oriented polyester film’, C. Song, S. Kim and Y. Lee, assigned to SKC Limited (July 3, 1996). 10. US Patent 5580652, ‘Biaxially oriented polyester film containing an aluminum hydrate’, assigned to SKC Limited (December 3, 1996). 11. US Patent 3154461, ‘Matter-finish polymeric film and method of forming the same’, assigned to 3M Corp. (October 27, 1964). 12. US Patent 4187113, ‘Voided films of polyester with polyolefin particles’, assigned to Imperial Chemical Industries (February 5, 1980). 13. US Patent Application 200904215, ‘White, porous single-layer polyester film and method for preparing same’, assigned to SKC Company (February 12, 2009). 14. S. Rhee and J.L. White, Polymer, 43(22), 5903 (2002). 15. Y.P. Khanna, E.D. Day, M.L. Tsai, and G. Vaidyanathan, J. Plastic Film Sheeting, 13(3), 197 (1997). 16. US Patent 4522867, ‘Biaxially oriented polyamide film’, W.H. Hill and J.O. Reed, assigned to Phillips Petroleum Co. (June 11, 1985). 17. US Patent 5094799, ‘Process for producing biaxially oriented nylon film’, M. Takashige, Y. Ohki. T. Hayashi, K. Utsuki and M. Fujimoto, assigned to Idemitsu Petrochemical Co. (March 10, 1992). 18. EP0826731A1, ‘Polyamide resin composition, use thereof and biaxially stretched film’, H. Uarbe, K. Suguira and K. Watanabe, assigned to Mitsubishi Eng. Plastics Corp. (March 4, 1998). 19. US Patent 6685871, ‘Toughened biaxially oriented film’, J. Moulton, B. Blissett, G. Bardzak, A. Crochunis, A. Majestic,. S. Porter and T. Staskowski, assigned to Honeywell International (Feb. 3, 2004). 20. L. Yu, H. Liu, F. Xie, L. chen, and X. Li, Polym. Eng. Sci., 48(4), 634 (2008). 21. X. Ou and M. Cakmak, Polymer, 49(24), 5344 (2008). 22. H.J. Lehermeier, J.R. Dorgan, and J.D. Way, J. Membrane Sci., 190(2), 243 (2001). 23. K. Shinyama and S. Fujita, Properties and Applications of Dielectric Materials, 2003. Proceedings of the 7th International Conference on, June 2003, Vol. 2, p. 707–710. 24. US Patent Application 20090148715, ‘Process to produce biaxially oriented polylactic acid film at high transverse orientation rates’, (June 11, 2009), assigned to Toray Plastics. 25. US Patent 7128969, ‘Method for the production of biologically degradable packagings made from biaxially-drawn films’, (October 31, 2006), assigned to Trespaphan GmBH.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-05.indd 57
7/1/11 9:00:00 AM
58
Biaxial stretching of film
26. US Patent Application 20090311544, ‘Method to produce matte and opaque biaxially oriented polylactic acid film’, assigned to Toray Plastics. 27. US Patent Application 20100004904, ‘Biaxially oriented polylactic acid film with high barrier’, assigned to Toray Plastics. 28. NatureWorks, LLC, 2011, PLA oriented film, Minnesota, available from: http://www. natureworksllc.com/product-and-applications/ingeo-biopolymer/technical-resources/. [accessed January 3, 2011]. 29. P.B. Smith, M.A. Leugers, S.-H. Kang, X.-Z. Yang, and S.L. Hsu, Macromol. Symp., 175(1), 81 (2001).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-05.indd 58
7/1/11 9:00:00 AM
6
Biaxial film structures
M . T . D e M e u s e, Independent Consultant, USA Abstract: This chapter focuses specifically on the actual film structures which are produced by various biaxial film orientation processes. The main focus of the discussion is based on structures that contain polypropylene. However, as appropriate, information on other film structures is also provided. Key words: five-layer films, copolymers, tie layer, metallization.
6.1
Introduction
This chapter will focus specifically on the actual typical film structures which are produced by the biaxial film orientation process. Since the majority of structures are based on polypropylene, that will be the main focus of the discussion. However, as appropriate, discussions of other film structures will be provided.
6.2
Film structures based on homopolymer polypropylene
The most usual commercially produced biaxially oriented film structure involves the use of homopolymer polypropylene as the central layer in the film with two additional, thin film layers attached to the central layer to provide required functionality to the film. The desired three-layer film is produced in a coextrusion process in which the polymers are melted separately and extruded together to produce the final film. This coextrusion process can be performed via either a blow film process or an extrusion casting process, with the latter approach being more common. Typically, the main central layer is much thicker than the outer, functional layers, making up at least 75% of the total film thickness. The homopolymer polypropylene material which is used as the material of choice for the core layer typically has a melt flow rate (MFR) in the range of 1–10 degree/min. (See ASTM procedure D 1238 for more details MFR tests.) There are numerous commercially available polypropylene materials which fall into this category of materials. Some of the suppliers of such polymers include AtoFina, ExxonMobil, Lyondell Basell and Borealis. The outer layers of the three-layer films provide added functionality to 59 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-06.indd 59
7/1/11 9:00:24 AM
60
Biaxial stretching of film
the structure, including attributes such as sealability and printability. As such, the presence of the outer layers in the film structures often provides the film with increased utility in the final structure. For example, often the three-layer film will be laminated to another film and it is these outer layers which allow for the lamination process to occur effectively. In another common scenario, the outer film layers will be treated, either through a flame or corona treatment process, which imparts chemical functionality to the film surface. This chemical functionality is extremely important to the printing operation for these films. It is the chemical functionality which is imparted by the treatment process which allows for the adhering of ink to the film surface. Often, the polymers which are used in these film outer layers are copolymers of ethylene and propylene, with relatively low contents of ethylene. Such copolymers do not require the use of tie layer materials to have acceptable adhesion between the various layers. Owing to the low level of ethylene in the copolymers, there is compatibility between the outer film layers and the film core layer. This is often not the case when other polymers are used as the outer layers and the necessity for tie layers makes the film structure more complex, requiring additional layers to ensure adequate adhesion. Such structures will be discussed later in this chapter. The chemical composition of the ethylene/propylene copolymer which is used as the material of choice for the outer layer is highly dependent on the application that it serves. When printing is the desired application, copolymers which contain less than 2 weight% ethylene are commonly used as the polymer. On the other hand, when sealing of the film structure is desired, copolymers with up to about 6–8 wt% ethylene are commonly used. This is because the melting point of the ethylene/propylene copolymers is a function of the ethylene content and decreases with increasing ethylene content.1 Thus, for applications in which very low sealing temperatures are desired, copolymers with higher amounts of ethylene are utilized. A similar effect can also be obtained by the use of copolymers which are synthesized using metallocene catalysts. For example, it has been shown that copolymers synthesized by the metallocene process generally have lower melting points than copolymers of the same composition which have been produced using traditional Ziegler–Natta catalysts.2 Typically, the thickness of the outer layers of three layer films is in the order of 5 gauge of less. Thus, if the total film thickness is, for example, 100 gauge the core layer, which is usually produced from homopolymer polypropylene, is about 90–95 gauge thick and each outer layer is 2–5 gauge thick. This is primarily done so that the main features of the film are provided by the core layer with additional functionalities provided by the two outer layers. The situation in terms of the film structure becomes more complicated if
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-06.indd 60
7/1/11 9:00:24 AM
Biaxial film structures
61
polymers other than polyolefins are used in the outer layers. This is mainly because such polymers are polar compared with the non-polar polyolefins. Because of this difference, there is generally poor adhesion between polyolefins and these polar polymers. In order to improve this adhesion, materials termed ‘tie layers’ are added to the film structure as separate layers. The inclusion of these tie layer materials increases the required number of layers from three to five. A typical five-layer generic film structure is shown in Fig. 6.1. In these types of structure, the thickness of the tie layers is usually less than one gauge. Also, it should be noted that often the polar polymer is not used on both sides of the film, but only one. The other outer layer, in this case, may be a sealable or printable material, as previously described.
6.2.1 Tie layer materials used in film structures The tie layer materials which are used in these applications are often based on polypropylene itself which has been appropriately chemically functionalized. The rationale for the use of such materials is that the polypropylene portion of the polymer will be compatible with the polypropylene homopolymer core material and the functionalized portion of the polymers will interact with the polar polymer outer layer. Typical chemical functionalities which are often provided to the polypropylene include maleic anhydride and acrylic acid. One commercially available tie layer material is called Plexar and is available from Lyondell Basell.3 Plexar tie layers are touted as adhering well to ethylene vinyl alcohol (EVOH), nylon and polypropylene and offer orientation, toughness and heat resistance. Plexar is currently offered as several grades with different melt flow rates for various applications, including film. Another typical tie layer material is called Bynel, which is commercially available from DuPont.4 Particularly relevant to polypropylene-based films is the DuPont Bynel series 5000 resins which are anhydride-modified
Polar polymer Tie layer Homopolymer polypropylene Tie layer Polar polymer
6.1 Typical five-layer film structure.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-06.indd 61
7/1/11 9:00:24 AM
62
Biaxial stretching of film
polypropylene resins. They are available in pellet form for use in conventional extrusion and coextrusion equipment and processes typical for polypropylene resins. It should also be noted that there are other Bynel series resins which are available based on polyethylene and ethylene copolymers, including ethylene/acetate materials. There are several examples in the patent literature of the use of these types of tie layers in the production of oriented films. US Patent 4627657 5 describes film constructions which are useful in the manufacture of flexible packaging materials. The general film structure is based on the used of biaxially oriented polypropylene (BOPP) as the base layer with a top layer being a copolymer of ethylene and an unsaturated carboxylic acid and the bottom layer preferably being a copolymer of ethylene and an unsaturated carboxylic acid. In this case, the BOPP is stated to give the film structure sufficient strength and toughness which is lacking in metal foils and paper films. On the other hand, the copolymer outer layers are touted as providing excellent adhesion to metals, such as aluminum. One of the reasons why this example is cited here is because it represents a clear explanation of the use of both the core and the outer layers in the film. The patent clearly describes that the function of the homopolymer polypropylene core layer is to provide toughness and stiffness to the film structure. On the other hand, the function of the outer layers is to provide necessary functionality to the film. In this case, that functionality is the ability to adhere to other substrates, such as metals like aluminum. This is an example of the general direction that biaxially oriented film structures are heading. As more diverse final applications become available, there will be a continuing need to have different functionalities within a single film structure. Those different functionalities can be included with different layers but, often, the different layers do not adhere well. Thus, the need for tie layers to produce the final product. It should be noted that presently often these needed different functionalities are provided through the use of two different films which are subsequently laminated together. While that approach does indeed often provide the identical end result in terms of properties for the final structure, the addition of a lamination step to the production process adds cost and complexity. If the required layers can be included in a single film, those issues can be avoided. Another example of the type of structure in which the core layer provides a very definite function in the film is described in WO/2002/098655. 6 In that patent, five layer films are described in which the central layer consists of a liquid crystalline polymer (LCP), which unlike most LCPs can be biaxially oriented. In this case, the LCP is primarily used because of its excellent barrier properties to both oxygen and moisture vapor. It also provides stiffness to the film. The described five-layer structures have a
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-06.indd 62
7/1/11 9:00:24 AM
Biaxial film structures
63
semi crystalline polymer, such as polyethylene or polypropylene, as each outer layer. In order to improve the adhesion between the LCP core layer and the outer polyolefin layers, the use of tie layers which are modified copolymers or terpolymers, is described. There has also been much work done in the development of five-layer shrink films. For example,7 OMNIA produces and sells strong coextruded biaxially oriented five-layer films with good shrinkage, sealing and tear resistance. The films are made with 100% polyolefin formulations and are available in a range of thicknesses. The entire area of metallized films, primarily for use in food packaging application, is often focused on the use of five-layer films. In these types of structure, the core layer is often polypropylene and the outer film layers are produced from polar polymers which provide good adhesion to the aluminum metal. Since there is often poor adhesion between the non-polar polypropylene and the polar polymer outer layers, it is necessary to include tie layers. It should be noted that the addition of the aluminum metal layer to the base film increases the number of layers in the final product. An example of this type of structure is described in US Patent 5153074 issued to Mobil Oil.8 That patent discloses a high barrier film which has been metallized. The technology is based on a multi-layer film based on polypropylene which has at least one surface which is a layer of EVOH copolymer and an aluminum layer is directly deposited on the EVOH surface layer. The presence of the EVOH copolymer provides excellent oxygen barrier properties to the film. That concept was extended to include a surface layer of a lactic acid polymer in US Patent 6844077.9 Disclosed in that patent are five-layer films which have a polyolefin core layer, two tie layers, each of which is a functionalized polymer, and at least one surface layer which is a lactic acid polymer. Preferably, the metal-receiving layer in this case is a homopolymer of lactic acid but it can also be a copolymer of lactic acid and another hydroxycarboxylic acid. The issue of improving the adhesion between the metal and polymers is addressed in US Patent 5206051.10 In that patent, it is noted that approaches such as corona discharge or plasma treatment, application of very thin adhesion promoting solutions, use of dispersions or emulsions prior to, or after the orientation process, and chemical etching of the film surface have provided some improvement for metal adhesion. However, it was also noted that the prior art processes involved great complexity and increased cost. Further, crazing or cracking of the metal layer during lamination is noted as an issue. In order to overcome those concerns, the inventors developed an approach based on the production of blends of polypropylene with an acid terpolymer. This homogeneous blend is then processed as a single layer or coextruded
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-06.indd 63
7/1/11 9:00:24 AM
64
Biaxial stretching of film
as one of multiple layers, with the condition that the blend constitutes the surface to be metallized. A wide range of acid terpolymer compositions and addition levels are discussed which effectively enhance the adhesion of the metal which is deposited on the film surface.
6.2.2 Coated films Many of the same ideas apply to the production of coated biaxially stretched films as are used to make metallized films. The general concept is often to provide the base film with some additional functionality, chemical or otherwise, not present in the base product itself. Many of the same issues encountered in metallized products exist in coated products as well. An example of a commercially available coated BOPP product is offered by Exxon Mobil under the tradename Bicor 84 AOH.11 It is a two-side coated film designed for use in high oxygen barrier laminations. It is designed to be used as the outer film web in gas flush applications for dry products. The film is touted as providing outstanding flavor and aroma barrier as well. The coated surface, which is poly(vinyl alcohol) (PVOH), is receptive to water-based or solvent-based inks and adhesives. Another polymer which is often used to coat BOPP film products is poly(vinylidene chloride) (PVDC). Triton offers a product called CAPP-05 which is coated on one side with acrylic and the other side is coated with PVDC.12 The product is offered in a range of thicknesses from about 20 to 50 microns. The films provide excellent oxygen and moisture barriers, due in large part to the PVDC coating. They are also touted as having outstanding optical properties and being printable on both sides. A method for producing a coated biaxially oriented film is described in US Patent Application 20090197022 to Exxon Mobil Chemical Company.13 A sequential orientation process with an in-line coating method is described. The method involves orienting a base film in one direction to provide an uniaxially oriented film, coating a portion of the uniaxially oriented film with a polyolefin dispersion, and, then, orienting the coated uniaxially oriented film in a second direction. The finished products are suitable for use in consumer packaging applications. Transparent inorganic coating materials, such as silicon dioxide, have been deposited on oriented polypropylene films to enhance film barrier features.14 This was accomplished through a plasma deposition process. The silicon dioxide coatings were successfully printed, adhesive and extrusion laminated and made into finished packages. These steps are critical in the commercialization of these barrier technologies into the flexible packaging market. This same approach was used with a variety of polymer films, including polyesters and polyolefins by Amberg-Schwab et al.15 Using a barrier coating
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-06.indd 64
7/1/11 9:00:24 AM
Biaxial film structures
65
material which is described as an inorganic–organic polymer, high barrier properties with respect to the permeation of oxygen, water vapor and various organic compounds were provided. The hybrid polymers were produced via the sol–gel technique. The properties of the produced multilayer structures are said to be preserved even under conditions of high mechanical and thermal stress and storage in humid conditions.
6.3
Recommendations
As an increasing number of new applications continue to develop for biaxially oriented films, unique film structures will be required. These novel structures will need to combine polymers and materials with very different chemical structures. This will further necessitate that multilayer films, of five or seven, or even more, layers be produced. These films will have as several of the layers tie materials which adhere to the various dissimilar polymers. The use and development of these tie layer materials will become increasingly important as the number of layers in the films increases. Another approach to provide increased functionality to films is to produce a base film with fewer layers and, subsequently coat another polymer or material onto the product. Since such coating materials do not undergo the same stretching conditions as the base film, many different materials can be used in this approach. Owing to this fact, the described coating technology offers the possibility to provide chemical functionalities not possible by other means. As such, unique films can be produced and this is an area where additional work is warranted. Of course, the use of coatings may require the use of tie layer type materials to have adequate adhesion of the coating to the base film. This is related to the same issue involved with the development of multi layer films, in general. That is, one of the big concerns with these types of film is developing ways to improve the adhesion between chemically different structures. Owing to this need in all of these unique films, improving and measuring this adhesion is an area where additional work is necessary.
6.4
References
1. Z. Du, J. Xu, X. Wang, and Z. Fan, Polym. Bull., 58(5–6), 903 (2007). 2. Ma. Joaquina Caballero, I. Suarez, B. Coto, R. Van Grieken and B. Monrabai, Macromol. Symposia, 257(1), 122 (2007). 3. Lyondell Basell, Plexar Tie Layer Resins, Netherlands, available from: http:// www.lyondellbasell.com/Products/ByCategory/polymers/type/Polyethylene/ SpecialtyPolyethylene/TieLayerResins/Guidelines_for_Plexar_Tie_layer_Resins. htm [accessed January 3, 2011]. 4. DuPont, Bynel Resins, Delaware, available from: http://www2.dupont.com/Bynel/ en_US/ [accessed January 3, 2011].
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-06.indd 65
7/1/11 9:00:24 AM
66
Biaxial stretching of film
5. US Patent 4629657, ‘Biaxially oriented polypropylene film construction for special lamination’, S. Gulati and E. Enderle (December 16, 1986). 6. WO/2002/098655, ‘Biaxially oriented film comprising a layer of liquid crystalline polymers’, assigned to Trespaphan GmBH (December 12, 2002). 7. Omnia, Five-layer Films, California, available from http://www.omniausa.com/ [accessed January 3, 2011]. 8. US Patent 5153074, ‘Metallized film combination’, assigned to Mobil Oil Corporation (October 6, 1992). 9. US Patent 6844077, ‘High barrier metallized film with mirror-like appearance’, assigned to Exxon Mobil Oil Corporation (January 18, 2005). 10. US Patent 5206051, ‘Metallized polypropylene film and process for manufacture’, assigned to Curwood, Inc. (April 27, 1993). 11. Exxon Mobil, AOH film, Texas, available from: http://www.matweb.com/search/ datasheettext.aspx./ [accessed January 3, 2011]. 12. Triton International, Coated Film, Pennsylvania, available from: http: //www.tritonint. com/datasheet/CAPP-05.pdf [accessed January 3, 2011]. 13. US Patent Application 20090197022, ‘Coated biaxially oriented film via in-line coating process’, assigned to Exxon Mobil Chemical Company. 14. J.T. Felts, ‘Transparent barrier coatings updates: flexible substrates’, Society of Vacuum Coaters, 36th Annual Technical Conference Proceedings, April, 25–30 1993. 15. S. Amberg-Schwab, M. Hoffmann, H. Bader, and M. Gessler, J. Sol–Gel Sci. Technol., 13, (1–3), 141 (1998).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-06.indd 66
7/1/11 9:00:24 AM
7
Typical industrial processes for the biaxial orientation of films
M . T . D e M e u s e, Independent Consultant, USA Abstract: Details are provided of the production processes which are currently practiced commercially in the general area of biaxial orientation of film. This chapter will specifically focus on the industrial, commercial aspects of the processes and the issues which are involved with them. In addition, information is provided on the companies that utilize the various production processes. Key words: sequential, simultaneous, double bubble.
7.1
Introduction
This chapter will provide details of the production processes which are currently practiced commercially in the area of biaxial orientation of films. While other previous chapters have already mentioned some of these details, this chapter will specifically focus on the industrial, commercial aspects of the processes and the advantages and issues associated with them. As such, much of the knowledge which is provided will be of significant practical importance to those interested in large scale production of biaxially oriented films. In addition to a discussion of the various industrial processes, information will also be provided on the companies that utilize the various approaches in their commercial operations.
7.2
Commercial production processes for biaxially oriented films
There are three different techniques currently commercially practiced to produce biaxially oriented films. They are the sequential tenter frame process, the simultaneous tenter frame orientation and the double bubble process, which is also a simultaneous film stretching method. For different film types, the different stretching processes are preferred. For example, biaxially oriented polypropylene (BOPP) and biaxially oriented polyethylene terephthalate (BOPET) are mainly produced by the sequential process, as previously described. On the other hand, for biaxially oriented polyamide (BOPA) all three of the approaches are used by various organizations in commercial production operations. 67 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-07.indd 67
7/1/11 9:00:53 AM
68
Biaxial stretching of film
Briefly reviewing each of these processes, in the sequential stretching process, the first step is a stretching of an extruded cast sheet in the machine direction (MD) between a pair of rolls which operate at very high stretching speeds. As a second step, in the transverse direction (TD), the film is gripped by a fixed clip system and stretched over the film width by use of a track rail. Owing primarily to reasons of product quality and processability, the stretching ratios in the MD as well as the TD can only be varied between relatively narrow limits. Typical stretching ratios for BOPP, the most commonly stretched film product, are 5 in the MD and 9 in the TD. Stretching ratios below those limits typically result in large film thickness variations while ratios above those values lead to numerous web breaks and, hence, reduced productivity. By comparison, in the simultaneous tenter frame process, the MD and TD stretching is done at moderate stretching speeds, with a maximum of 300%/second, but at the same time. This is done through a continuous extension of the distances between the clips in the machine direction during the simultaneous transverse track rail stretching. Owing, in general, to a low flexibility in stretching ratios as well as high mechanical efforts and low line speeds, the long known and well-established simultaneous pantograph or spindle-systems are seldom currently used in commercial processes.
7.2.1 Linear inverse space-mapping (LISM) technology These disadvantages have been overcome1 by the use of a simultaneous tenter frame linear inverse space-mapping (LISM) technology, introduced by Brückner, in which all of the clips can be separately driven by linear motors. Thus, in comparison to the sequential stretching process, the utilizable range of MD and TD stretching ratios is significantly increased. For example, even MD stretching ratios up to 10 are possible, which results in a significant enhancement of the film mechanical properties, e.g. modulus, in the MD. This is because the higher the MD stretching ratio, the higher is the mechanical property value (see for example Samuels2). This can lead to products with unique property profiles not attainable using other production methods. An example of another advantage which is provided by the LISM technology is described in by Breil et al.3 In that work, it is shown that simultaneous orientation at low stretching temperatures and high stretching speeds subdues crystallization of ethylene vinyl alcohol (EVOH) copolymers enough to enable the defect-free stretching of thin EVOH coextrusion layers. Such is not the case with the sequential stretching process in which EVOH copolymers with low ethylene content are used. In that case, crystallization causes a deterioration of the stretchability, resulting in so-called ‘net
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-07.indd 68
7/1/11 9:00:53 AM
Typical industrial processes for the biaxial orientation of films
69
structures’ and, hence, inferior film optical properties. This is believed to be due to MD orientation-induced crystallization and the formation of microfibril structures, which result in voids and other optical defects upon further transverse stretching. These observations have allowed for the production of biaxially oriented coextruded film structures using EVOH grades which contain 24 and 27% of ethylene without optical defects. Since it is well established that the oxygen transmission rate (OTR) values of EVOH copolymers decrease with decreasing ethylene content,4 the use of these copolymers containing low amounts of ethylene allows for the production of final oriented film structures with very low OTR values. Through a combination of this technology with the use of polyolefins, such as polypropylene and polyethylene, in coextruded structures, it is possible to produce final film structures that possess both extremely good oxygen and moisture barrier features. Those structures offer excellent solutions for many food packaging applications. The effective orientation of EVOH copolymers is an excellent example of using the unique features that commercial, industrial biaxial orientation film production equipment provides to produce value-added specialty BOPP-type films. This is becoming increasingly important to many BOPP producers as the profit margins for standard biaxially oriented films, like BOPP coex products or BOPP tapes, have decreased in recent years due mainly to high resin prices and overcapacity. As a result, two of the market trends which are demanding new packaging solutions are cost reduction and convenience.
7.2.2 Double bubble commercial production process The double bubble process can also be used to produce relatively unique film structures not easily possible by standard tenter frame techniques. This process results in balanced film properties by simultaneous stretching of a cooled and reheated bubble. Thus, film properties are similar in both orientation directions, an effect which is not realizable with standard tenter frame technologies. However, in contrast to the usual tenter frame processes, there are certain disadvantages regarding film product quality and reproducibility associated with the double bubble process. These include the difficult control of temperatures and other process parameters of the open process. These issues can lead to wider thickness variations than are usually observed with tenter frame processing.. In addition, the final output rate for the double-bubble process is relatively low compared to tenter processes, typically on the order of 500 kg/h. As of 2008, the worldwide capacity for biaxially oriented films amounted to about 12.5 million tons/year. The biggest share, with 65%, came from BOPP, followed by BOPET with 26%. As mentioned earlier in this chapter,
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-07.indd 69
7/1/11 9:00:53 AM
70
Biaxial stretching of film
the main approach used for the production of those products is the sequential tenter frame technology, and so the discussion of some of the features of commercial industrial lines will begin with that technology. A brief discussion of some of the companies who own and operate those lines will also be provided, when appropriate.
7.2.3 Worldwide producers of biaxially oriented films One of the largest tenter lines in the world is operated by Applied Extrusion Technologies (AET) Films at their manufacturing facility in Terre Haute, Indiana. At that facility,5 there are six tenter lines, ranging from 5.5 to 10 meters in width. It is not unusual for the line operating speed of the larger tenter line to reach speeds in excess of 1000 feet/min (5 m/s). The Terre Haute site of AET Films is one of the largest manufacturing facilities in North America. Owing to the very high operating line speeds which are utilized on these commercial production tenter frames, temperature profiles in the stretching operation must be adjusted accordingly. For example, even though the published melting point of polypropylene is about 165 °C,6 the set point temperatures in the tenter oven are often higher than this, in the 180 °C range. This is done because, owing to the very high speed, the film never really experiences the set point temperature but it does need to get hot enough to allow for adequate orientation. This is not necessarily the case for slower line speeds at which the set point temperatures can be set much closer to the actual desired orientation temperature. As a side note, AET Films also operates lines for the double bubble process at its facility in Terre Haute. As already noted, those lines do not operate at the same high line speeds as the large commercial tenter lines, and so the output from those lines is considerably less than that of the tenter lines. AET Films is the only major supplier of BOPP film that operates both tenter and double bubble production equipment. Another company that has one extensive work in the area of BOPP films is Exxon Mobil, which has primarily focused its efforts on films made by the sequential tenter frame process. Also, metallized and coated films have been described in various of Exxon’s patent documents. The films are produced in various thickness ranges as well as clear and opaque structures. Those films are produced typically on 8 meter tenter lines with both three- and five-layer coextrusion dies. Exxon Mobil has plants in many portions of the world including USA, Canada, Belgium, Italy and the Netherlands. Another large supplier of BOPP film is Treofan, which is headquartered in Raunheim, Germany.7 It has 15 production lines which produce 200 000 tons of product at five manufacturing facilities, in several different continents. Treofan touts its specialty films as being designed to meet specific customer
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-07.indd 70
7/1/11 9:00:53 AM
Typical industrial processes for the biaxial orientation of films
71
needs such as in-mold labels, capacitor applications and tobacco films. Its range of BOPP products is divided into four segments which consist of packaging, labeling, tobacco and technical films. In the technical films sector, the main products are dielectrics for use in film capacitor applications, but it also produces specially tailored films for other applications as well. The products of Treofan are presently marketed in more than 90 countries. The next BOPP supplier to be discussed will be Vifan, which presently is part of the Vibac group of companies. Vifan has production plants in Italy, Montreal, Canada and the USA. The combined worldwide production capacity of the facilities is 150 000 metric tons/year with 100 000 Mt/year coming from the European facilities and 50 000 Mt/year from the plants in North America. Vifan produces a number of products in the areas of transparent, metallized, pearlized and solid white films. Those products serve a variety of markets and applications which include snack packaging, fresh-cut produce packaging and labels. Subsequent book chapters will discuss the details of each of those applications in more depth and detail. According to an article in Plastics News,8 world demand for BOPP films will grow by an average of 5.7% per year until the year 2013. It quotes a report from PCI Films Consulting Ltd. as providing that market growth information. One of the most significant points mentioned in the article is that the expected growth will be driven primarily by the Asia Pacific markets and that the emerging markets of China and India could well lead the growth. On the other hand, demand in more mature markets, such as Europe and North America, is predicted to grow much more slowly. The main reason why this information is particularly pertinent to the present discussion and analysis is because the large BOPP producers already mentioned have large commercial production assets, like 8 and 10 meter lines, in markets with slowing growth rates. They have recently found that strategy to not be particularly effective to utilize capacity and, also, they have been quite slow to invest in the regions which are showing the largest levels of growth. This has led to the development of smaller companies with smaller production lines and somewhat specialized products being produced. The report from PCI Films continues by stating that companies in the biaxially oriented film market will in the future have to adopt one of two completely different strategies. They must be either a low cost commodity film producer or a specialty film producer. To be in the middle, as many large and medium-sized producers currently are, will make the generation of profits very much more difficult. That situation means that the biaxial orientation film industry is ready for the introduction of novel types of equipment and processes that can yield specialty-type film products. As already suggested, much of that effort will be prompted by developments in areas of high growth, such as Asia. There may need to be modifications to the thought process that larger and
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-07.indd 71
7/1/11 9:00:54 AM
72
Biaxial stretching of film
faster commodity lines are the only way to proceed in the area of biaxially oriented films.
7.3
Novel technologies currently being developed
An interesting technology to produced films with unique properties is discussed by Parkinson Technologies on its website9 where the use is highlighted of a patented compression roll drawing (CRD) process which involves the simultaneous compression and machine direction orientation (MDO) of extruded plastic material. Parkinson Technologies is the worldwide exclusive licensee of this technology. The described CRD technology allows for the production of thicker oriented films. Also, it permits for an adjustment of the orientation level to match the needs of the product. Further, improved material properties can be obtained. These include film optical properties such as clarity and haze, low temperature impact, tensile strength and modulus. In a collaborative effort with Dow Chemical Company, Parkinson Technologies conducted a series of trials which were designed to illustrate the advantages which are possible using the CRD technology for orienting polypropylene sheet material. In that work, materials which were provided by Dow were processed at Parkinson’s facility in Woonsocket, Rhode Island. Many of the conclusions discussed above concerning improvements in film properties using the CRD approach were obtained from the results of that study. This is just one additional example of an effort to adopt commodity-type assets used in BOPP film production to produce specialty-type products. As the trends described above continue, there will be more approaches in that direction. There is a general movement toward having smaller production lines which can be adapted to running different products rather than just a single commodity film. Nowhere is that trend more prevalent than in southeast Asia, especially India, where growth has been stimulated by increased availability of film. 10 New players such as Dubai-based Taghleef Industries and others are emerging in India and China. For example, Taghleef has invested in new low-cost equipment to emerge as one of the world’s largest BOPP film producers. This is part of a shift in balance of power away from traditional suppliers of BOPP film. Much of this is being driven by specialty films which require unique processing and equipment for applications like tapes/adhesives and industrial products. As one moves away from the BOPP industry, the need for specialized processes and equipment has already begun to be realized. For example, in the case of BOPET film, the PET is extruded onto a chill roll, which
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-07.indd 72
7/1/11 9:00:54 AM
Typical industrial processes for the biaxial orientation of films
73
quenches it into the amorphous state.11 It is then biaxially oriented by drawing, usually by the sequential process. Typical draw ratios are around 3 to 4 in each direction, much less than the stretch ratios which are used to produced BOPP. Another unique feature of the BOPET film production process are the relatively high temperatures which are used for the heat-setting step in the production process. Typically, once the biaxial drawing is completed, the film is heat-set or crystallized in the tenter oven at temperatures above 200 °C. The heat setting step prevents the film from shrinking back to its original unstretched shape and locks in the molecular orientation in the film. Usual line speeds and final web widths for BOPET film are different than for BOPP film production. For example, typical final web widths up to about 5 meters are quoted for BOPET production plants.12 In the same article, production rates up to about 800 feet/min (4 m/s) are quoted. This is a slower production speed than BOPP production lines, which typically will operate at speeds in excess of 1000 feet/min (5 m/s). A similar situation is true of BOPA film lines. On the BASF website, for example,13 it is stated that BOPA film may be manufactured from Ultramid B resins by rapid cooling of the film and subsequent simultaneous or sequential stretching in the machine and transverse direction. Cast (tenter frame) or blown (double bubble) technology may be applied. Stretch ratios between 2.7 and 3.2 in both the MD and TD are quoted as being usual. On the same data table as the stretch ratios, a typical line production speed of about 450 feet/min (2.3 m/s) is quoted for the production of BOPA film. It can be seen that this is even slower than for BOPET. Further, the quoted stretch ratios are less than for BOPET and considerably less than for BOPP. It should also be noted that biaxially oriented poly(lactic acid) (BOPLA) film is oriented to similar stretch ratios or even less than is BOPET. Also, typical production speeds are similar to BOPET values in magnitude. In fact, much BOPLA production was first done on equipment initially designed for BOPET production. Similar stretching ratios of the two polymers was one of the primary reasons for this. This trend toward lower stretch ratios and possible slower line speeds will continue with the further movement toward the development specialty film products. Since such products command a higher price in the marketplace than do commodity films, extremely fast line speeds are not necessarily required for substantial profits to be realized. Also, as different and unique polymers are investigated, it may be necessary to use smaller stretch ratios to obtain the maximum yield of film with a limited number of web breaks. This general trend will surely lead to the design of novel stretching equipment as well as the development of unique biaxial orientation processes.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-07.indd 73
7/1/11 9:00:54 AM
74
7.4
Biaxial stretching of film
Recommendations
As the trend in biaxial oriented films continues to move away from commodity products toward more specialty-type materials, unique equipment and processes will need to be developed to manufacture the final films. No longer will production lines operating at the highest speed possible be the normal mode of operation but lines which operate at slower speeds to accommodate specialty polymers will become more frequent. Also, the final web width of product which is being produced by these specialty production lines may be smaller than the 10 meter BOPP lines currently available, due primarily to the smaller stretch ratios which are generally possible with the specialty polymers. However, that is not to say that high speed biaxially oriented film lines which are focused on commodity polymers will cease to exist. Instead, those lines will need to be adaptable to novel processing opportunities which take advantage of the high throughput possibilities. In that sense, some type of post-processing, such as film coating, should be considered as a way to make unique products. Some type of unique extrusion capability to produce novel biaxially oriented films is another approach. However, overall the theme of producing specialty films through either unique materials or processes or both will continue to be prevalent in the years ahead.
7.5
References
1. J. Breil, ‘LISM-linear motor simultaneous stretching technology’, CMM International Conference, Chicago, USA, April 16, 1997. 2. R.J. Samuels, Structured Polymer Properties, Wiley (1974). 3. J. Breil, R. Lund, and M. Wolf, ‘Biaxially oriented BOPP barrier films with thin EVOH layers’, European Metallizers Association, Spring Meeting, March 9–10, 2006, London. 4. Eval Americas, Texas, Gas barrier properties of resins, available from: http://www. evalca.com/ [accessed January 3, 2011]. 5. Applied Extrusion Technologies, Indiana, Biaxial Film Equipment, available from: http://www.syndecor.com/aboutus.html [accessed January 3, 2011]. 6. E.P. Moore, Polypropylene Handbook. Polymerization, Characterization, Properties, Processing, Applications, Hanser Publishers: (1996). 7. Treofan, Germany, Tenter Frames, available from: http://www.treofan.com [accessed January 3, 2011]. 8. Plastics News, 2010, New York, BOPP film, available from: http://www. plastics news.com [accessed January 3, 2011]. 9. Parkinson Technologies, 2011, Rhode Island, Compression Roll Drawing, Available from: http://www.parkinsontechnologies.com/ [accessed January 3, 2011]. 10. Plastemart, 2009, New York, BOPP film supply, available from http://www.plastemart. com/Plastic-Technicle-Article [accessed January 3, 2011]. 11. S. Hashemi and Y. Xu, J. Mater. Sci., 42(15), 6197 (2007).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-07.indd 74
7/1/11 9:00:54 AM
Typical industrial processes for the biaxial orientation of films
75
12. Available from: http://www.fuzing.com/BOPET-film-production [accessed January 3, 2011]. 13. BASF, 2011, Germany, Ultramid B Resins, available from: http://www.basf.com/ group/pressrelease/P-10-299/ [accessed January 3, 2011].
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-07.indd 75
7/1/11 9:00:54 AM
8
Post-production processing of biaxially oriented films
M . T . D e M e u s e, Independent Consultant, USA Abstract: After films are produced, they are often subjected to various post-processing steps to make them more utilizable in the final proposed application. This post-processing can include surface treatments of various types. The post-processing steps are the main focus of this chapter. Key words: surface treatment, coated films, metallizing.
8.1
Introduction
Thus far, the main focus of the chapters in this book has been on the production of biaxially oriented films and the polymers and materials that they are made from. Often, after the films themselves have been produced, they are subjected to various post-processing steps which make them more usable; such post-processing steps are the main focus here.
8.2
Surface treatment of films
The first process to be discussed in detail will be surface treatment of the films. Surface treatment of the film, particularly films which are based on polypropylene, provides chemical functionality to the non-polar, inert material. This is often necessary because the adhesion between biaxially oriented polypropylene (BOPP) and most materials is poor, due primarily to the chemical inertness and the smooth surface, which prevent chemical and mechanical bonding. Several methods have previously been developed to introduce chemical functionality for adhesive bonding, including photochemical and chemical modification, surface grafting and glow or corona discharge treatment. 1–4 Of these various approaches, corona discharge treatment in air, because of its low cost, is often the preferred method. After being treated by corona discharge, the polymer surface is effectively oxidized to produce various kinds of polar groups containing oxygen, such as ester, ether, ketone, hyrdoperoxy, epoxy, carboxylic acid, etc.5 This surface treatment process improves the bonding characteristics of the film by raising its surface energy. A picture of a typical corona system for film treatment is given in US 76 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-08.indd 76
7/1/11 9:01:34 AM
Post-production processing of biaxially oriented films
77
Patent 6894279.6 The corona discharge equipment consists of a high frequency power generator, a high voltage transformer, a stationary electrode and a treater ground roll. Standard utility electrical power is converted into higher frequency power which is then supplied to the treater station. The treater station applies this power through ceramic or metal electrodes over an air gap onto the surface of the film. All films provide a better bonding surface when they are treated at the time that they are produced. This is commonly referred to as ‘pre-treatment.’ However, the effects of corona treatment diminish over time. Therefore, many surfaces often require a second ‘bump’ treatment at the time that they are converted to generate adequate bonding with printing inks, coatings and adhesives. One of the first steps involved in using corona treater systems is to determine the required ‘dyne level’ which is needed for the substrate which is being converted. The ‘dyne level’ is a measure of the surface energy and is a commonly used parameter in the converting industry. 7 It reflects the surface wettability – the higher the dyne level, the better the wettability/ adhesion. It is normally measured either through contact angle measurements or by using dyne inks or pens. An alternative to corona discharge as a way to treat film surfaces is flame treatment.8 It was initially developed in the 1950s as a way to improve the surface adhesion of polyolefin films. Flame treatment typically creates fixed levels of oxidized species on the surface of the films, along with the formation of hydroxyl, carboxyl and carbonyl functionalities. Treatment or oxidation depths vary by substrate, as does the generation of low molecular weight organic material at the surface. Surface exposure to flame treatment directly modifies electron distributions of polyolefin molecules, resulting in polarization at the film surface up to several nanometers. In the flame treatment process, a moving film is exposed to a gas-fired flame at high enough temperatures to create a plasma of free oxygen and nitrogen, electrons and ions. The plasma reacts chemically with the film surface, which adds polar functional groups and increases surface energy. Flame treatment can be beneficial in several ways. It works very well in the removal of annealing oils from foil to promote coating or lamination. The oxygen-rich portion of the flame, known as the secondary zone, promotes oxidation in a very similar way that the corona does to plastic substrates. The real benefit of flame lies in the intensity of the plasma which enables higher treatment levels at faster speeds with no backside treatment. Additionally, flame treatment is not limited by the thickness of the material. Flame treatment definitely has a place in the printing market, but its popularity is presently limited by the additional cost and complexity of its operation compared with corona treatment. Currently, only a limited number of applications justify the additional cost associated with flame treatment.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-08.indd 77
7/1/11 9:01:34 AM
78
Biaxial stretching of film
Today, flexographic printing speeds have increased to the point where flame treaters are required. The other type of surface treatment which has generated considerable interest is plasma or ‘modified atmospheric’ treating. It is very similar to traditional corona treatment with the difference that gases are injected into the corona discharge to modify the reaction with the film substrate. Some materials are less reactive to a traditional corona treatment and require this special treatment. In addition, semi-conductive gases such as helium can be used to help lower the operating voltage at the corona discharge to meet other application requirements. It is evident that atmospheric-pressure treating is very similar to corona treatment but there are a few differences between the two processes. Both systems use one or more high voltage electrodes which positively charge the surrounding blown air ion particles. However, in atmospheric plasma systems, the rate at which oxygen molecules bond to a material’s molecules ends develops up to 100 ¥ more. From this increase of oxygen, a higher ion bombardment occurs. This usually results in stronger material bonding features and increased reception for inks and coatings. Atmospheric plasma treatment technology also eliminates the possibility of treatment on a materials non-treated side; also known as backside treatment. One of the big issues with the widespread implementation of plasma treatment, as with flame treatment, is the increased cost compared with corona treatment. In order to justify the expense associated with plasma treatment, the attributes of the final film product must be somewhat unusual. That is the main reason, why until now, plasma treatment has been used primarily for the production of specialty film products.
8.2.1 Literature studies of surface treatment processes There have been investigations in the open literature of the various treatment processes, particularly as they pertain to biaxially oriented polypropylene (BOPP). For example, Zenkiewicz9 studied the effect of corona treatment energy level on the surface energy of BOPP film. In the range of corona treatment energy up to 1.2 kJ/m2, a rapid increase in the surface energy with the treatment energy is observed. Above that value, the surface free energy rises relatively slowly. Also, the extent of oxidation of the surface layer is in direct proportion to the energy of the corona treatment. In another study related to corona treatment, Guimond et al.10 compared air corona treatment of BOPP with nitrogen atmospheric glow discharge (APGD). In that work, it was shown that the nitrogen APGD treatment leads to a higher surface energy than air corona treatment and leads to the formation of mainly amine, amide, and hydroxyl functional groups at the polypropylene surface. Further, for both treatment types, the increased
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-08.indd 78
7/1/11 9:01:34 AM
Post-production processing of biaxially oriented films
79
surface energy is found to decay in a similar manner with increasing time after treatment. Strobel et al.11 compared corona treated and flame treated polypropylene films with the goal of providing insight into the mechanism of the two processes. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to characterize treated biaxially oriented polypropylene film. While both processes oxidize the polypropylene surface, corona treatment leads to the formation of water-soluble low molecular weight oxidized materials while flame treatment does not. Computer modeling results indicate that the ratio of oxygen to hydroxyl is much higher in a corona discharge than in a flame. Chain scission and the formation of low molecular weight oxidized materials are associated with reactions involving O atoms. The higher ratios of O to OH in a corona are more conducive to low molecular weight oxidized production. Surface oxidized PP exhibits considerable thermodynamic contact angle hysteresis that is primarily caused by microscopic chemical heterogeneity. This article is an example from the open literature which points out that there are differences in the surfaces which are created using the various treatment processes. A much earlier article by Podhajny12 had provided a summary of studies on the corona treatment process itself. Chemical functional groups which are generated by the corona discharge on films were identified and their effect on film wettability and adhesion was discussed. A similar study of the chemical groups provided by flame treatment was given by Sutherland et al.13 In that study, about 30% of the incorporated oxygen on flame-treated PP surfaces was found to be present as hydroxyl groups. Also observed was a reorientation or migration of surface functional groups that had been incorporated during the flame treatment process. Further, scanning electron microscopy (SEM) showed definite changes in surface topography induced by intense flame treatment. Other substrates than BOPP have also been studied in comparing flame and corona treatment. NatureWorks LLC has published a technical bulletin14 on the effects of both types of treatment on biaxially oriented polylactic acid (BOPLA). The reported study was performed with the stated purpose of obtaining the best equipment settings to obtain the maximum increase in surface energy. In addition, samples of the BOPLA films were stored and tested over time to determine how long the film retains its high surface energy level. In that study, it was shown that a 1:1 ratio of fuel to oxygen in flame treatment leads to the highest surface energy level (> 70 dynes/cm) for the BOPLA film. On the other hand, most packaging films used for flexible packaging achieve the highest surface energy when treated with a 0.5% excess oxygen in the fuel mixture. When the film line speed increases, the surface energy drops when using a fixed burner output and burner gap.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-08.indd 79
7/1/11 9:01:34 AM
80
Biaxial stretching of film
Therefore, the burner output must be increased as the line speed increases to maintain a high surface energy and also the burner gap must be increased to prevent the film from distorting from the heat of the flame. In the corona treatment portion of this study, both BOPLA film that had been corona treated during manufacturing and BOPLA film that had no previous treatment were evaluated for discharge energy versus the obtained surface energy and the length of time in days that the film retained the surface energy. Very little watt density needs to be applied to the treated BOPLA film to effectively ‘bump’ treat the surface energy to a level of 48 dynes or higher. Unlike BOPP film previously discussed, BOPLA film that has not been corona treated during film manufacturing can be effectively treated at a later date. However, untreated BOPLA film does require more watt density to effectively increase the surface energy when compared to a ‘bump’ treatment, but does not require an unreasonable level of watt density to be effective. As already alluded to earlier in this chapter, one of the main reasons for treating polymers, but most noticeably polyolefins, is to improve the adhesion with inks and other coatings. It should be noted that within the broad range of the term coating is also included the concept of metallizing films. Also, it needs to be understood that the treatment level which is required is quite specific to each application. For the printing process, the problem is being further compounded by the general current industry to move away from solvent-based inks towards water-based inks or UV curable inks. In addition to this, the treatment level of film substrates tends to change with time. Also, additives such as slip additives tend to migrate to the surface of the film as the film ages, having the overall effect of masking the treatment level. In addition to slip additives, physical handling of the film as well as storage temperature can affect the treatment level. All of these factors must be considered when deciding what type of treatment procedure as well as what level of treatment is necessary to achieve the desired result.
8.2.2 Metallized films and their production One of the main reasons, in the packaging industry, for treating film surfaces is to be able to apply a layer of metal to it. Metallized materials are produced by melting and vaporizing a metal, usually aluminum, in a vacuum while passing the film web around a chilled roll and over the point of vaporization. The vaporized molecules, then, collect on the cool film web, thus providing the film with a metallic finish. Such metallized films often are used because they provide both good water and oxygen resistance. A picture of a direct metallizing process is shown on the Vaccuplast website.15 Vacuum metallizing is a batch process in which the film to be
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-08.indd 80
7/1/11 9:01:34 AM
Post-production processing of biaxially oriented films
81
metallized is loaded onto the unwind zone of the vacuum chamber, threaded around the cooling rollers and onto the rewind roll. The vacuum chamber is sealed and evacuated. Aluminum wire is then resistance heated in boron nitride boats to a temperature of about 1500 °C and evaporates and condenses onto the film as it passes over the cooling roll. Typically, the following metallizing parameters are used: ∑ ∑ ∑ ∑ ∑
The film roll length is commonly up to 24 000 meters. 99.9999% of the air is removed during the metallization process. The evaporation temperature is about 1500 °C. The deposited metal layer is on the order of 0.03–0.05 microns thick. Usual films which are vacuum metallized include – polyester, – polypropylene, – polyethylene, – polystyrene, – Cellulose.
In general, metallized polyester films are metallized to achieve certain desired properties like a metallic appearance, making it resistant to gases, and less diffusive with respect to aromas and flavors. The other advantageous properties of the metallized polyester film are that they can be shrunk with the application of heat, can be molded into different forms as required, are printable, sealable and capable of lamination and extrusion. This combination of properties allows the metallized polyester films to have many applications for packaging food items, as they resist outside gases while at the same time retaining the aroma and flavor of the packaged food. Metallized polypropylene films are also used in many packaging applications. Some of the advantages that they provide include low permeability to water vapor, brilliant appearance, low density and low cost. However, a major issue which is still prevalent is the relatively poor metal to polymer bond. Also, the ability to successfully laminate two or more films by extrusion techniques is difficult due to the tendency of the metallized layer to crack or craze. This results in poor appearance and property deficiencies. Unfortunately, there are very few articles in the open literature on metallized films. One of the articles which does address the topic is Vassiliadiand and Tarantili,16 where the effect of the energy of a corona unit applied for retreatment of metallized BOPP films before lamination was studied using a variety of techniques. Increased surface roughness and polarity due to the presence of oxygen groups were detected, and these changes became more pronounced with an increase in corona treatment intensity. Also, the number and size of spots on the film surface increase with corona treatment. This is possibly due to additives, such as processing aids, which were incorporated into the polymer. Also, significant effects in the film physical properties, such
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-08.indd 81
7/1/11 9:01:34 AM
82
Biaxial stretching of film
as strength and modulus, were observed during the film aging process. Another general article on metallized BOPP film was written by Marra. 17 In that article, it is pointed out that the barrier properties to gas and water vapor correlate with opacity, which depends on the degree of coverage obtained by the metallization process. Minor defects, such as scratches, will generally represent only a small percentage of the total coverage and have a small effect on the barrier properties. However, the metallized film is easily coated with trace amounts of any low energy organic material with which it makes contact. Due to that fact, for assurance of consistent wetting and bonding, it is recommended that BOPP surfaces should be cleaned inline. There has also been significant patent activity in the general area of metallized films. For example, US Patent 4345005 to Mobil Oil Corporation18 discusses coextruded BOPP films which have a polypropylene core layer, with an ethylene–propylene copolymers as at least one film outer layer. The outer layer contains no slip agent. The avoidance of the use of a slip agent acts to provide enhanced adhesion of the film to the metal coating layer. In US Patent 4604322 to Hercules Incorporated19 it is pointed out that films that contain no slip agent have high coefficient of friction (COF) values and blocking of the film upon winding results due to the lack of the slip agent. On the other hand, as noted above, the use of conventional concentrations of slip agents in the film destroys the adhesion of the metal coating to the film. In order to achieve the correct balance of metal adhesion and slip properties, US Patent 4604322 recommends the use of slip agents in nonconventional concentrations in the film core. The issue of improving the metal adhesion to the film substrate is addressed in a different way in US Patent 5206051 to Curwood, Inc. 20 In that patent, the film-to-metal bond is improved through the use of a blend of polypropylene with an acid terpolymer. A wide range of terpolymer compositions and addition levels are disclosed to effectively enhance the adhesion of the metal. The homogeneous blend must constitute the surface to be metallized. US Patent 6190760 to Toray Industries 21 discusses yet a different approach to improve the metal-to-film adhesion in a metallized film. That patent discloses the use of polypropylene resins as the surface layer to be metallized with low crystal fusion heat values. In order to achieve those low values, isotactic polypropylene resins with a well-defined structure are proposed. Through the use of these resins, it is possible to obtain a BOPP film with excellent stiffness and also have high adhesion between the metal layer and the base material of the BOPP film. Finally, in US Patent Application 2007029268222 are described multi layer BOPP films with a metal adhesion layer of blends of polypropylene homopolymer with either amorphous poly-alpha-olefins or ethylene–propylene
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-08.indd 82
7/1/11 9:01:35 AM
Post-production processing of biaxially oriented films
83
elastomers. Films which are produced with those type of metal receiving layer formulations provide high metal adhesion, a bright shiny reflective metal appearance, resistance to crazing in extrusion lamination, and excellent gas and moisture vapor barrier properties. The blend metal receiving layer is appropriately treated using either flame or corona in this invention. It is apparent from the previous discussion that the adhesion of the metal layer to the base film substrate is of primary importance for the proper functioning of the final metallized product. Treatment of the film surface prior to the metallization is one common approach to improve the adhesion. Treatment is also an important factor for the use of coatings other than aluminum metal as well.
8.2.3 Coated films As an example of coated films, WO/2000/00295623 describes a process for improving the adhesion of a water-borne, inorganic barrier coating to polyolefin substrates. The method involves corona treatment of the substrate, applying an acrylic primer layer and a barrier coating composition. Examples of the inorganic water-borne coating which can be used are sodium polysilicate, potassium polysilicate and lithium polysilicate. The same situation is also true of using many organic coatings, such as Saran, onto the surface of polypropylene base film. For example, in US Patent 3923693,24 it is mentioned that in order to achieve a substantial bond between a polypropylene base film and a Saran coating, it has been found beneficial to treat the polypropylene film surface to be coated according to any of a number of conventional surface oxidation techniques. Patents are quoted in which corona and flame treatment are both used in this regard. In general, a similar situation is true of most coatings to be applied to the surface of polyolefin films. The primary reason for this need is because polyolefins are chemically non-polar and inert in nature and, thus, adhesion to most other materials is very poor. In order to improve that adhesion, chemical functionality is imparted to the film surface. That chemical functionality is usually provided through one of the processes described in this chapter.
8.3
Conclusions
As has already been noted throughout this chapter, surface treatment of films is one of the most important post-production processes currently practiced. Whether it is performed by corona, flame or some other means, such treatment is often necessary to impart chemical functionality to films, particularly polyolefin films. This functionality is required to provide adequate adhesion of additional layers, such as aluminum, which can extend the applications to which the final film can prove useful.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-08.indd 83
7/1/11 9:01:35 AM
84
8.4
Biaxial stretching of film
Recommendations
Since the treatment process is such an essential part of these applications, it is imperative that a better fundamental understanding of its effects continue to be developed. Issues such as how different treatment parameters, such as energy and time, affect the generation of different levels of the various chemical groups need to be addressed. Further, correlations between the levels of the various chemical groups and the actual adhesion between the base film and the aluminum layer need to be further explored. It is possible that to have the highest level of adhesion that certain functional groups in specific amounts are desired. In addition to those fundamental studies, more work should be done to define the optimum polymer, and its characteristics, to serve as the metal receiving layer. For the majority of commercially available BOPP film at this time, the metal receiving layer is typically some type of propylenebased copolymer. However, as noted earlier in this chapter, blends of polar polymers with polypropylene have also been used, as have polar polymers alone. Each of these variations has certain advantages and disadvantages to their use. Also, each of the different polymers and polymer combinations respond differently to treatment and, hence, would be expected to have different adhesion characteristics to applied metal layers. General schemes for optimizing these features need to be developed so that certain trial and error aspects can be reduced and a firm scientific basis for further developments can be established.
8.5
References
1. J.A. Lanauze and D.L. Myers, J. Appl. Polym. Sci., 40, 595 (1990). 2. J.F. Carley and P.T. Kitze, Polym. Eng. Sci., 20, 330 (1980). 3. L.J. Gerenser, J.F. Elman, M.G. Mason, and J.M. Pochan, Polymer, 26, 1162 (1985). 4. C.K. Kim and A.I. Goring, J. Appl. Polym. Sci., 15, 1357 (1971). 5. D. Briggs, Polymer, 25, 1379 (1984). 6. US Patent 6894279 ‘Narrow web corona treater’, assigned to Illinois Tool Works, Inc. (May 17, 2005). 7. Dyne Technology, United Kingdom, Dyne Level, Available from: http://www. dynetechnology.co.uk [accessed January 3, 2011]. 8. Enercon Industries, Inc., United Kingdom, Surface Treatment, available from http:// www.enerconindustries.com/ [accessed January 3, 2011]. 9. M. Zenkiewicz, J. Adhesion, 77(1), 25 (2001). 10. S. Guimond, I. Radu, G. Czeremuszkin, D. Carlsson, and M. Wertheimer, Plasmas and Polymers, 7(1), 71 (2002). 11. M. Strobel, V. Jones, C.S. Lyons, M. Ulsh, M.J. Kushner, R. Doraland, and M.C. Branch, Plasmas and Polymers, 8(1), 61 (2003). 12. R.M. Podhajny, J Plastic Film Sheeting, 4(3), 177 (1988).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-08.indd 84
7/1/11 9:01:35 AM
Post-production processing of biaxially oriented films
85
13. I. Sutherland, E. Sheng, D.M. Brewis, and R.J. Heath, J. Adhesion, 44(1 & 2), 17 (1994). 14. Nature Works PLA Film Technical Bulletin, NatureWorks, LLC website. 15. Vaccuplast, India, Metallized Films, Available from: http://vaccuplast.com/[accessed January 3, 2011]. 16. E. Vassiliadiand and P.A. Tarantili, J. Appl. Polym. Sci., 105(4), 1713 (2007). 17. J.V. Marra, J. Plastic Film Sheeting, 4(1), 27 (1988). 18. US Patent 4345005, ‘Oriented polypropylene film substrate and method of manufacture’, assigned to Mobil Oil Corporation (8/17/1992). 19. US Patent 4604322, ‘Metallizable polypropylene film’ assigned to Hercules Incorporated (August 5, 1986). 20. US Patent 5206051, ‘Metallized polypropylene film and process for manufacture’, assigned to Curwood, Inc. (April 27, 1993). 21. US Patent 6190760, ‘Biaxially oriented polypropylene film to be metallized, a metallized biaxially oriented polypropylene film and a laminate formed by using it’, assigned to Toray Industries (Feb. 20, 2001). 22. US Patent Application 2007029682, ‘Metallized bixaxially oriented polypropylene film with high metal adhesion’, assigned to Toray Plastics (December 20, 2007). 23. WO/2000/02956, ‘Method of coating pre-primed polyolefin films’, assigned to Hoechst Trespaphan GmBH (1/20/2000). 24. US Patent 3923693, ‘Laminated packaging film having low vapor and gas permeability’ assigned to Continental Can Company, Inc. (January 13, 1976).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-08.indd 85
7/1/11 9:01:35 AM
9
Strain energy function and stress–strain model for uniaxial and biaxial orientation of poly(ethylene terephthalate) (PET)
M . A . A n s a r i, M . R . C am e r o n and S . A . J a b a r i n, University of Toledo, USA Abstract: Poly(ethylene terephthalate) has wide commercial application because of its excellent end-use properties. To achieve these properties, the polymer must be stretched within the rubbery region to an extent which results in strain hardening. The region for and extent of strain hardening depends upon such processing conditions as temperature, strain rate, extent of strain and mode of stretching. Stretching experiments on film samples produced data which was used to develop an empirical stress–strain model. Such a model can be used to predict the processing behavior of sheet or stretch blow molded bottles during manufacture. Use of this model and stress–strain modeling approaches developed by other researchers are discussed. Key words: poly(ethylene terephthalate), stress–strain model, strain hardening, stretch blow molding, orientation, film stretching.
9.1
Introduction
9.1.1 Processing of poly(ethylene terephthalate) (PET) Poly(ethylene terephthalate) (PET) has wide commercial application because of its excellent end-use properties such as high mechanical strength, optical clarity, surface gloss, high barrier properties and recyclability. It is used in applications for fibers (tire cord), film (magnetic recording tape and photographic film) and packaging (bottles and film). To achieve enhanced properties, commercial processes involving PET are carried out in the orientation range for the polymer. This lies between the glass transition temperature (Tg) of the polymer and the temperature at which crystallization commences. Drawing (or stretching) the film in this temperature range results in segments of the molecules becoming aligned. This molecular orientation is conducive to strain-induced crystallization. Both the orientation and strain-induced crystallization lead to property improvements over those of the amorphous material. Oriented PET film can have a tensile modulus increase of about five times greater than unoriented film. Barrier properties such as the oxygen permeability can decrease by half (Jabarin, 1984; Natu et al., 2005). 86 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 86
7/1/11 9:06:14 AM
Strain energy function and stress–strain model
87
Manufacture of fibers involves drawing in one direction. This is termed uniaxial stretching. The most common commercial biaxial film process for PET utilizes drawing in the machine direction followed by stretch in the transverse direction (Salem, 1999). Heat setting the resultant film imparts dimensional stability. Simultaneous biaxial stretching is another method. Typically for film the drawing is done in the range from 80 to 95 °C. Beverage containers are made by a stretch blow molding process. First a preform is made via injection molding. This is followed by heating the preform to the orientation range. Blow air is introduced into the preform expanding it into a bottle. During the expansion the preform undergoes biaxial stretching. This can be simultaneous stretching or sequential where a stretch rod mechanically elongates the preform axially before the blow air stretches it radially. Typical stretch ratios are four to five times in the radial direction and 2.5 times in the axial direction. This results in planar extension ratios of 10–12 or more. There are two different methods of carrying out the stretch blow molding process, single stage and two stage (Miller, 1980; Whelan and Goff, 1985; Weissman, 1988; Rees, 1994; Jog, 1995). In the two-stage process, the injection molded preform is cooled down to room temperature. Blow molding takes place on a separate unit where the preform is reheated from room temperature to the orientation temperature followed by stretch blow molding. In the single stage process, injection molding and blow molding take place on the same machine. The preform is injection molded at one station, passed to a temperature conditioning station, indexed to a blow station where the bottle is formed and finally moved to a take out station. Regardless of the choice of blow molding process, the main criteria for making a good bottle is determining the correct preform design. Unfortunately all too often preform design is more of an art than a science. The use of computer simulation to aid in preform design should eliminate a lot of trial and error. What is needed is a model of the stress–strain behavior characterizing the blowing process. Data for model development comes from experiments on stretching PET film. This will lead to a constitutive model describing the material behavior. Mathematical models exist for both films and fiber production. For instance, for fiber production, process models have been developed for the steady-state heat transfer, velocity and stress profiles (Kase and Matsuo, 1965, 1967; Petri, 1979; Denn, 1980). Bottle production, on the other hand, involves a more complicated orientation process and therefore presents other challenges, especially in regards to preform design. The proper material model should take into account the effects of orientation and strain-induced crystallization on the stress–strain curve. The onset and magnitude of these effects are determined by processing parameters such as temperature, strain
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 87
7/1/11 9:06:14 AM
88
Biaxial stretching of film
rates and ultimate strains. The empirical model presented here characterizes the material behavior for a range of processing conditions.
9.1.2 The nature of the stress–strain curve for PET Typical stress–strain behavior for stretching PET film in the rubbery state is shown in Fig. 9.1. This type of curve for PET is similar to that obtained earlier by Thompson (1959) and later by Jabarin (1991), in which the stress rises rapidly at first up to a yield point. With further elongation, the curve shows a flat region (the strain softening region) where the stress stays relatively constant while large deformations occur. Following strain softening, PET can show a region of strain hardening where the stress increases rapidly. This is a characteristic not shown to this extent by other polymers such as poly(vinyl chloride) and polypropylene. This results from chain alignment which gives a high degree of orientation in the deformation direction, leading to strain-induced crystallization. This property of PET differentiates it from other polymers and can be utilized to a great advantage in the industry as was described earlier by Bonnebat et al. (1981) and later by other scholars (Miller, 1980; Erwin et al., 1983; Cakmak et al., 1989). The point at which onset of strain hardening occurs is known as the strain hardening point. This strain hardening point is a function of orientation conditions. Chandran and Jabarin (1993a) characterized a method of obtaining it from the stress–strain curve and discussed in detail its dependence on orientation conditions (Chandran and Jabarin, 1993b,c).
2000
Stress s, psi
1500
1000 Strain hardening region
500 Yield point Strain softening region
Strain hardening point
0 1
2
3 Extension ratio, l
4
5
9.1 A typical stress–strain curve for PET film.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 88
7/1/11 9:06:14 AM
Strain energy function and stress–strain model
89
The stretching of PET films starts with the uncoiling of the entangled polymer chains. Chain alignment occurs simultaneously with stress relaxation. If the chain alignment overcomes the stress relaxation process of the polymer chains we obtain an oriented specimen. This oriented specimen upon further orientation can lead to strain-induced crystallization, a process favored by entropic reduction. Chain disentanglement, alignment, stress relaxation and strain-induced crystallization are all dependent upon stretching conditions. Consequently, so is PET stress–strain behavior. Strain-induced crystallization has been studied in detail with respect to temperature, strain rate and mode of orientation for PET (Heffelfinger and Lippert, 1971; de Vries et al., 1977; Bonnebat et al., 1981; Matsuo et al., 1982; Jabarin, 1984, 1991). Jabarin and Lofgren (1986) investigated orientation of PET and its relation to the processing parameters in great length and found it a highly strain rate and temperature-dependent process. To develop the current model for the stress–strain behavior of bottle grade PET, a series of biaxial stretching experiments was carried out. These are best carried out using film samples where each experimental variable can be investigated individually.
9.2
Experimental
9.2.1 Material and properties The material used was a homopolymer of PET supplied by Eastman Chemical Co. The material as received was in the form of extruded amorphous sheets of thickness 0.01 inches (0.25 mm). The intrinsic velocity (IV) of the sheets was 0.8 dl/g (Mn = 28 000), its density 1.3378 g/cm3 and melting point 251 °C. The temperature range for orientation was found using differential scanning calorimetry which for our samples showed an onset of crystallization at about 128 °C and a Tg of 78 °C. Since PET orientation is carried out between its Tg and the temperature at which onset of thermal crystallization occurs, a temperature range of 80–110 °C was selected for experimental studies. To explore the strain rate (or extension rate) effect, the range from 5 to 200%/s (or 0.05–2.0 s–1) was selected. The upper limit is based on equipment limitations.
9.2.2 Stretching of samples Biaxial stretching of the sheet samples was carried out using a biaxial extensiometer built by T.M. Long Co., Inc. The Long extensional tester (LET) has the following capabilities: ∑ adjustable stretching mode (simultaneous vs. sequential);
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 89
7/1/11 9:06:14 AM
90
Biaxial stretching of film
∑ adjustable temperature; ∑ adjustable strain rate or stretching speed. Samples 2 inches square (50 mm2) are held in the machine by a set of grips along each of the four edges. The sample and gripping mechanism are contained within an insulated, heated chamber where the temperature is controlled. An equilibration time of two minutes is allowed for the sample to reach the set temperature. The maximum possible extension ratio in either direction is about 7 but in practice, it is limited to less than this. Maximum strain rates possible with this instrument are about 200% strain per second (%/s). The strain rates can be set independently for either axes as can the final extension. In addition, stretching along both axes can be carried out at the same time or we can stretch first along one axis to the full extent of the test and then along the second. Stains (e) referred to are engineering strains. Similarly the stress (s) is the engineering stress (based on the initial sample dimensions). Extension ratios (l1 or l2) refer to the final dimension in one of the two stretching directions divided by the original dimension. For sequential stretching, the sample is initially stretched in only one direction a given amount, the first extension ratio (l1). This is followed by stretching in the second direction an amount denoted by the second extension ratio (l2). It is the stresses and strains in second direction which are of interest. During simultaneous biaxial stretching, the sheet sample is stretched in both directions at the same time. The strain rates of both directions are equal. However the final strains will usually be different. For example, for a 2 ¥ 4 stretch ratio the sample is stretched biaxially until the first stretch direction reaches an extension ratio of 2. Stretching in this direction halts while the sample continues to stretch in the second direction up to its final stretch ratio. Computer acquisition of stress versus strain data is carried out, but only for one axis. For convenience, this is the second direction. There are two special cases for simultaneous biaxial stretching. The first is termed constrained uniaxial stretching where one sample dimension is held constant (l1 = 1.0) while stretching occurs in the other dimension. The second special case is where the extension ratios in both directions are equal (equibiaxial stretching).
9.3
Stress–strain behavior of poly(ethylene terephthalate) (PET)
9.3.1 A typical stress–strain curve A typical stress–strain curve for PET close to the glass transition temperature looks like curve B in Fig. 9.2. As the sample is stretched the stress © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 90
7/1/11 9:06:14 AM
Strain energy function and stress–strain model 3000
91
Extension @ 80 vs s @80 °C Extension @ 90 vs s @90 °C
2500
Stress, s, psi
2000
1500 B
1000
A
500
0 1.0
1.5
2.0 2.5 3.0 Extension ratio (l = L1/L0)
3.5
4.0
9.2 Stress–strain curves for PET film specimens stretched in the simultaneous equibiaxial mode at an extension rate of 50%/s and at 80 and 90 °C.
rises quickly at first up to the yield point and shows a sharp peak when stretched at temperatures close to Tg. This yield stress value decreases as the temperature of stretching increases and at some ‘critical temperature’ (in this case between 80 and 90 °C), the sharp location of the yield point ceases to exist. At temperatures above the Tg the behavior is similar to curve A of the figure. Besides temperature, the location of the sharp peak is also dependent upon strain (or extension) rate. Even at 80 °C, the appearance of the sharp peak can be avoided if the specimen is stretched at a very slow speed. After the yield point, upon further elongation there is no appreciable change in stress values until at some high extension ratio the stress level starts increasing again due to the effects of strain-induced crystallization. This takes place in the sample due to a high degree of orientation. The point at which the stress rises rapidly has been termed the strain hardening point or strain hardening parameter. Jabarin has defined a systematic way of graphically obtaining the value for this parameter from the stress–strain curves (Chandran and Jabarin, 1993a).
9.3.2 Effect of temperature on stress–strain data Figure 9.3 shows the stress–strain curves for PET sheets stretched at an extension rate of 50%/s for temperatures in the range of 90–105 °C. This is
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 91
7/1/11 9:06:15 AM
92
Biaxial stretching of film 2500
@ 90 °C @ 95 °C @ 100 °C @ 105 °C
Stress, s, psi
2000
1500
1000
500
0
1
2
3 Extension ratio, l
4
5
9.3 Effect of temperature on stress vs. strain for constrained uniaxial stretching at an extension rate = 50%/s.
for constrained uniaxial stretching where the amount of the first extension is equal to 1.0. Higher temperatures result in lower stresses at comparable strains. Stretching at lower temperatures also results in greater strain hardening than stretching at higher temperatures. This can be seen by comparing the 90 °C data with that from 105 °C. Similar effects of temperature can be seen at other extension rates for simultaneous biaxial stretching in general. The decrease in stress levels with the increase in temperature can be attributed to the viscoelastic nature of PET. As the temperature increases, the viscosity of the polymer decreases, resulting in lower stresses. Secondly, at higher temperature, the chains of the polymer are freer to move owing to higher thermal energy and therefore have a greater tendency to relax. This causes a net decrease in the degree of orientation as some of the oriented chains relax back to the random state. To offset the higher relaxation at higher temperature, the given sample has to be stretched more in order to achieve the same degree of orientation. That is why we see a shift of the strain hardening region towards higher extension ratios as the temperature of stretching is increased.
9.3.3 Effect of extension rate (or stretch speed) on stress–strain data The overall effect of an increase in extension rate on the stress–strain curve is to some extent opposite to that observed on increasing the stretch © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 92
7/1/11 9:06:15 AM
Strain energy function and stress–strain model
93
temperature. The stress-strain curves of a sample stretched at higher speed lie above those stretched at lower speeds at the given temperature. Increasing extension rate decreases the time available for the molecular chains to relax, lowering the amount of stress relaxation. As the stress relaxation is less at higher extension rates, the comparative net degree of orientation and the amount of strain-induced crystallization increase. This results in higher stress levels and appearance of stronger strain hardening region compared with those at lower extension rates. These effects are shown in Fig. 9.4 for PET samples stretched at 100 °C for extension rates of 5, 20, 50 and 200%/s. We observe that the effect of extension rate is more pronounced in the strain hardening region of the stress–strain curve than on the yield point or the strain softening region.
9.3.4 Simultaneous biaxial extension Figure 9.5 shows a comparison of stress–strain curves for specimens stretched simultaneously at 100 °C and an extension rate of 50%/s in a simultaneous biaxial mode to various limiting extension ratios in the first stretch direction (first extension). The sample is stretched simultaneously in the both directions up to the amount of the first extension, after which the stretching is stopped in the first direction while continued in the second direction. Note that the 2500
@ @ @ @
Stress, s, psi
2000
5%/s 20%/s 50%/s 200%/s
1500
1000
500
0 1
2
3 4 Extension ratio, l
5
6
9.4 Effect of extension rate on the stress–strain behavior at 100 °C for constrained uniaxial stretching.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 93
7/1/11 9:06:15 AM
94
Biaxial stretching of film 2000
1st 1st 1st 1st
1800 1600
Extension Extension Extension Extension
= = = =
1.0 2.0 3.0 4.0
Stress, psi
1400 1200 1000 800 600 400 200 0
1
2
3 Extension ratio, l
4
5
9.5 Effect of the extent of the first extension on the stress–strain behavior at 100 °C and an extension rate = 50%/s for simultaneous biaxial extension.
strain hardening point shifts towards lower values of extension ratio with the increase in the amount of the first extension. This can be explained in terms of planar extension which is the product of the extension ratios in the first and the second directions. As the amount of extension in first direction increases so does the amount of planar extension, and as we compare the stress levels at some extension in the second direction we see that the stress levels are higher. This is because the amount of planar extension has increased with the increase in the first extension which results in more orientation in the chains of the stretched sample. This in turn makes the polymer more rigid due to more strain-induced crystallization and as a result the strain hardening point occurs earlier for samples stretched with higher amounts of the first extension. Similar trends were observed for specimens stretched at 90 and 105 °C. Also for samples stretched at 5, 20, and 200%/s similar trends were observed, i.e., the stress levels at the same extension rate and temperature for samples with higher limiting extension in the first direction were higher than for samples with smaller first extension values.
9.3.5 Sequential biaxial extension In the case of sequential biaxial orientation, the sample is first stretched in one direction up to a limiting value (the first extension) followed by
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 94
7/1/11 9:06:15 AM
Strain energy function and stress–strain model
95
stretching in the second direction. Stress–strain data is shown only in the second direction. Figure 9.6 shows the stress–strain curves for samples stretched in the sequential mode at 90 °C and at an extension rate of 50%/s when stretched to various limiting extension ratios of 1.0, 2.0, 2.25 ... 4.0 in the first direction. The stress–strain curve of a sample stretched to a higher limiting extension ratio lies above that of a sample stretched to lower limiting extension ratio in the first direction. At higher values of the first extension, the stress–strain curve changes its shape from concave upward to convex upward. This shows that for a particular temperature and extension rate there is a critical value of the limiting extension ratio (first extension) above which samples stretched at higher limiting extension ratios show convex upward shape. Thus for the case of sequential biaxial orientation at conditions of 50%/s extension rate and 90 °C orientation temperature, this critical value for the limiting extension ratio in the first direction is about 2.75, above which stress–strain curves show convex upward trend. All these cases show that the nature of the stress–strain curve is highly dependent upon the orientation conditions of temperature, extension rate, mode of extension and the amount of extension. Since the actual extension rates during the stretch blow molding of a preform are much higher than the extension rates we can carry out in the extensiometer, the superposition
2500
Stress, s, psi
2000
1500
1000 Ist Ist Ist Ist Ist Ist
500
0 1
2
3 Extension ratio, l
4
Extension Extension Extension Extension Extension Extension
= = = = = =
2.0 2.25 2.75 3.25 3.75 4.0
5
9.6 Stress vs. strain as a function of the first extension at 90 °C and extension rate = 50%/s for sequential biaxial stretching.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 95
7/1/11 9:06:15 AM
96
Biaxial stretching of film
principle was used to obtain stress–strain curves at higher rates, as discussed in the following section.
9.3.6 Superposition to higher extension rates Actual extension rates in the bottle blowing process are much higher then can be obtained on our experimental film stretching machine. Time–temperature superposition was therefore used to extrapolate the obtained stress–strain data to higher extension rates. Time–temperature superposition has been traditionally used in creep, stress relaxation and aging experiments where the desired timescale is too long for experimentation (Ferry, 1970). The method involves shifting curves either horizontally or vertically to give a single master curve covering a large range of data. In a typical application of superposition, experimental data taken at different temperatures are shifted along a logarithmic timescale to obtain a single master curve covering a large scale of time. The amount of horizontal shift along the time axis is called the shift factor. If the Tg is chosen as the reference temperature, the shift factor for most amorphous polymers is given by the Williams–landel–Ferry (WLF) equation (Williams et al., 1955). The shift factor can be related to the ratio of the relaxation time at temperature T to the relaxation time at the reference temperature. Ibar (1979a,b, 1984) has investigated in detail the use of superposition to analyze the tensile deformation behavior of amorphous uncrosslinked copolymers of styrene and acrylonitrile. In his work he described a ‘double shift’ procedure to obtain a single master curve from stress–strain data at different temperatures. Gordon et al. (1994) have shown that the true stress versus draw ratio curves of PET can be superimposed in the strain hardening region and concluded that this is evidence that PET behavior could be described by the deformation of a molecular network. In this work, PET film stress–strain data at three different temperatures (90, 100, 110 °C) and extension speeds (20, 50, 200%/s) were used to extrapolate to higher extension rates of 400, 600 and 800%/s. Superposition was used to generate stress–strain curves which are more characteristic of PET behavior under commercial blow molding conditions. These master curves were then used to extrapolate to the results shown in Fig. 9.7 for 100 °C. For more detail, see Ansari (1998). From these extrapolations we concluded that at 90 °C the higher strain rates have little effect on the stress–strain behavior. At temperatures close to Tg, much less relaxation of the polymer chains occurs during the time of stretching. At higher temperatures such as 110 °C, the effect of extension rate shows a significant increase in the strain hardening region on going from 200 to 600%/s. We find that at higher extension rates the weak strain hardening characteristics at 110 °C are replaced by a strong strain hardening
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 96
7/1/11 9:06:15 AM
Strain energy function and stress–strain model 2500
@ @ @ @ @ @
2250 2000
Stress, s, psi
1750
97
20%/s 50%/s 200%/s sp400%/s sp600%/s sp800%/s
1500 1250 1000 750 500 250 0 1
2
3 Extension ratio, l
4
5
9.7 Using superposition to extrapolate the stress–strain behavior at 100 °C for strain rates of 400, 600 and 800%/s for equibiaxial extension.
region. Extrapolation of stress–strain curves at 100 °C, as shown in Fig. 9.7, show intermediate results, with some increase in the strength of the strain hardening region at higher extension rates above 200%/s. It also seems that at 400%/s the stress levels almost reach their maximum values, and that even at higher temperatures such as 110 °C, there is no appreciable increase in stress values on going above extension rates of 400 %/s.
9.3.7 The use of the strain hardening property for preform design Bonnebat et al. (1981) studied preform blowing by the technique called ‘free blow’ and characterized the results in terms of ‘a natural draw ratio’ to describe the strain hardening point of the PET. Figure 9.8 illustrates the inflation process of a cylindrical preform. The data represent the digitized outline from video of a free-blown preform. It is similar to that described by Bonnebat et al. and confirmed by the studies of Miller (1980) and later by Cakmak et al. (1989). It shows that on introducing pressurized air into a preform preheated to the stretching temperatures, initially a ‘bubble’ formation is observed. The bubble then expands to a point known as the natural draw ratio, where it is stable. This bubble then propagates along the length of the preform giving a free blown container. Further expansion takes place in a nearly uniform manner.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 97
7/1/11 9:06:15 AM
98
Biaxial stretching of film 7
Radial expansion (inches)
5
3
1
–1
–3
–5
–7 0
5
10 Axial extension (inches)
15
20
9.8 Successive stages in the inflation of a cylindrical PET parison.
The same process is observed on stretching films as well, which can be seen easily when stretching is performed at low speeds. When a sample of PET film is stretched, first a formation of a neck region is observed followed by the propagation of the necked area in both directions. Propagation of the necked area is similar to the propagation of the bubble throughout the length of the preform in the case of free blow. When this propagation has been completed final stretching of the film occurs uniformly until break. As a PET preform is inflated due to an internal blow pressure, it expands into the strain hardening region where thin sections stop stretching and thicker sections continue to stretch. This necking and subsequent strain hardening lead to a uniform bottle wall thickness plus enhanced material properties. This natural draw ratio then represents a good target draw ratio for preform/ bottle design or for specifying draw ratios for film. If PET film is extended into the strain hardening region, mechanical and barrier properties will be optimized, at least for the specified temperature and strain rates.
9.3.8 Effect of moisture content In a study to determine the effects of moisture content on the stress–strain behavior of PET, Jabarin and Lofgren (1986) established a direct relationship between the amount of moisture content present in the film and the Tg of the
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 98
7/1/11 9:06:15 AM
Strain energy function and stress–strain model
99
polymer. They showed that in order to obtain the same degree of orientation, the stretching temperature needs to be adjusted accordingly depending upon the amount of moisture content present. They found that the parameter T–Tg (the difference between the stretch temperature and the glass transition temperature) was the controlling factor in maintaining a certain level of orientation. Thus the correlations were developed by taking the factor T–Tg into account. Here Tg is 80 °C for the PET films used in this study.
9.4
Modeling of the stress–strain behavior – literature review
early approaches to the mathematical modeling of the biaxial stretching process of PeT fall into two categories. Many mathematical stress–strain models of interest for polymers followed developments for rubbers. This is because the processing of many polymers is carried out in the ‘rubbery’ state. one approach is based on statistical thermodynamics. The other is a purely phenomenological approach. bonnebat et al. (1981) carried out a fundamental investigation of PeT orientation in relation to bottle making. They measured stress–strain curves for a temperature range of 80–110 °C and found that stretching PET at a temperature close to Tg produced necking, a phenomenon which disappeared when stretching was done at temperatures equal to and greater than 85 °C. They also showed that for both uniaxial and biaxial oriented PeT, the natural draw ratio decreases with increase in molecular weight and with decreasing temperature. This natural draw ratio is defined as the amount of extension at which strain hardening commences. They concluded from their studies that uneven wall thickness distributions in biaxially stretched preforms can be avoided by stretching the preform by an amount greater than the natural draw ratios.
9.4.1
Statistical kinetic model
This approach is based on the concept of the random fluctuations of longchain molecules in an irregular three-dimensional network. The theory was first postulated by Kuhn (1938) and was later amended by James and Guth (1943, 1947), and by Treloar (1958). Following this approach the total entropic contribution from deformation for the network is given by DS = ∑ DSi = –
1 Nk (l12 + l 22 + l 32 – 3) 2
[9.1]
Here N is the total number of chains per unit volume and k is boltzman’s constant. A chain is the segment of a polymer molecule between two crosslinks. Assuming no change in internal energy due to deformation, the
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 99
7/1/11 9:06:15 AM
100
Biaxial stretching of film
work of deformation or the strain energy function W is then given by W =
1 NkT (l12 + l 22 + l 32 – 3) 2
[9.2]
l1, l2, l3, are the extension ratios in the three principal directions. According to this equation, the work of deformation is directly proportional to N, the number of chains per unit volume, which is in turn determined by the crosslinking. The above model, however, gives a poor fit to the PET stress–strain data, particularly in the strain hardening region. it is a very simplistic model and does not account for several effects, including strain-induced crystallization, finite extensibility of the chains and interactions between polymer chains. In order to represent the stress–strain behavior in the strain hardening region, a number of authors have made modifications to this approach. One such model is known as the ‘inverse Langevin approximation’. one of the fundamental problems with the above model is that it is based on Gaussian statistics. This means the probability of having a given end-toend distance for each polymer segment will follow a Gaussian distribution. This type of behavior does not correctly account for the finite extensibility of the polymer segments. A more realistic model assumes the probability distribution of end-to-end distances follows a Langevin distribution (Boyce and Arruda, 2000). The probability distribution function for chain ends is expressed as a series approximation based on the Langevin distribution with the degree of approximation being determined by the number of terms in the series. Although this approach was found to be capable of featuring the steep rise seen at high extensions it is still not very accurate.
9.4.2
Extensions of the strain energy approach
The failure of the simple statistical approach to define the behavior of rubber led many to focus on models based on a phenomenological approach. A number of approaches have exploited a power series type expansion of the strain energy function in terms of the three invariants of the deformation tensor. The invariants I1, I2 and I3 can themselves be expressed in terms of the principal stretch ratios (l1, l2, l3). Here I1 = l12 + l22 + l32, I2 = 1/l12 l22 + I1/l22 l32 + I1/l12 l32, and I3 = l12 l22 l32. Mooney (1940) developed a more practically useful form of strain-energy function, given by Ê 1 ˆ 1 1 W = C1 (l12 + l 22 + l 32 – 3) + C 2 Á + + – 3˜ 2 2 2 l2 l3 Ë l1 ¯
[9.3]
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 100
7/1/11 9:06:15 AM
Strain energy function and stress–strain model
101
Here C1 and C2 are known as Mooney’s constants whose values are found by regression to the experimental data. The Mooney equation is quite simple but is usually good only for l < 2.2. Another simple model similar to Mooney’s equation was given by Gent and Thomas (1958), where the strain energy is given by: ÊI ˆ W = C1 (I1 – 3) + C 01 Á 2 ˜ Ë 3¯
[9.4]
The authors claim that their equation has considerable advantage over the two term Mooney form. Later Rivlin (1948), and Rivlin and Saunders (1951) developed a more general constitutive model defined in terms of the three strain invariants as follows: W =
N
N
i+ j=1
i
 Ciji (I1 – 3)i (I 2 – 3) j +  1 (J – 1 – R)2i Di
[9.5]
where: Cij are the rivlin constants, Di defines the material compressibility, R defines the volumetric expansion related to temperature and J = l1l2l3. rivlin thought that since the strain energy function is unaltered by a change in sign of the l1, corresponding to a rotation of the body through 180°, the form of W must depend only on even powers of li. equation 9.5 is composed of two series. The second series accounts for any volumetric change upon stretching. For rubbers this is not a significant factor and can therefore be dropped. The first term of the series represents the deviatoric component of the stored energy function. Tschoegl (1972) suggested that failure of the Mooney and similar equations arises because of not taking enough number of terms in the expansion of the power series. Afterwards Fried and Johnson (1988) developed a three parameter model and included bulk modulus K into their formulation as W = C1(I1 – 3I31/3) + C2(I2 – 3I32/3) + (K/2)(ln I3)2
[9.6]
George et al. (1987) used the following form in their finite element analysis (FeA) of o-ring seals: W = C1(I1 – 3) + C2(I2 – 3) + C3 ln(I2/3) + C4(I2 – 3)2 + C5(I13/2 –1)2 – (C1 + 2C1 + 2C3/3)(I3 –3)
[9.7]
Here C5 is one-half of the bulk modulus. Kawabata (1981) examined extensively the variation of the partial derivatives of the strain energy functions, ∂W/∂I1 and ∂W/∂I2, for several rubbers over a wide range of extensions. He found the values of the two derivatives to change with both I1 and I2. A study of temperature dependence
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 101
7/1/11 9:06:16 AM
102
Biaxial stretching of film
of these derivatives (Matsuda et al., 1981a,b, 1982) showed that ∂W/∂I1 is nearly proportional to the absolute temperature while ∂W/∂I2, was found to be independent of temperature. on the basis of those results they came up with the following form: W = CT(I1 – 3) + b(I1, I2)
[9.8]
where C is a constant, T the temperature (K), and b(I1, I2) a function dependent only upon I1 and I2 and not upon temperature. The functional form for b was not explicitly established although they determined the relative range of values of 0.32 to 0.46 for b/W for the conditions tested. In a more recent development Kawabata and co workers (1995) added another term to the above given strain energy function to include the effect of carbon black fillers.
9.4.3
Other modeling approaches
ogden (1972, 1986) and ogden and Chadwick (1972) departed from Rivlin’s approach in formulating the strain energy function in two ways. First their formulation is expressed directly in terms of the three principal extension ratios instead of using invariants. second they avoided the restriction of expressing W in terms of even powers of the extension ratios. According to ogden’s formulation: W =
N ∑ m i (l1ai + l 2ai + l 3ai – 3)
i =1
ai
[9.9]
where mi and ai are constants. Usually a two-term or three-term expansion of the above series gives a satisfactory fit to the experimental data. Ogden’s model seems to fit the PET stress–strain behavior better than many of the other models. The Gaussian distribution is a poor descriptor of extended chain statistics. The more successful models for large strains take another approach such as using Langevin chain statistics. In addition to modifying the chain statistics, models based on a network structure have been developed. Some of these are discussed in the review by boyce and Arruda (2000). one of the earliest was the three-chain model whose chains are aligned with the three axes of a unit cell. A four-chain model is based on a tetrahedron structure. While the three-chain model gives good prediction at high strains for uniaxial stretching it fails for biaxial cases. The four-chain model is a little better but difficult to use. The most successful model is the eight-chain model of Arruda and boyce (1993). Here the chains are connected in the center of a unit cell and are aligned along the diagonals. Model characteristics include Langevin chain statistics and affine deformations. Owing to symmetry
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 102
7/1/11 9:06:16 AM
Strain energy function and stress–strain model
103
of the chains, a strain energy function can be conveniently obtained. The model requires only two material properties, the network chain density and the limiting chain extensibility. it gives good prediction for uniaxial, biaxial and shear modes of deformation. Dupaix and boyce (2007) extended the development to PeTG (a copolymer of PeT) and PeT. PeTG shows strain hardening but very little crystallization while PET can have significant strain-induced crystallization when stretched under appropriate conditions. The constitutive model applies at temperatures at and above Tg. it contains two parallel sets of elements, one of which treats the resistance of intermolecular interactions while the other deals with the network resistance due to stretching and orientation. There is also a contribution which accounts for molecular relaxation. The model predicts the strong rate and temperature dependence of many of the stress–strain features for PeT and PeTG such as strain hardening.
9.5
Development of a stress–strain model
it was decided to develop an empirical model to characterize the stress– strain behavior of PeT in the temperature range just above Tg. This simple algebraic model is easily used for initial preform design and has been incorporated into computer simulations of the stretch blow molding process. by simulating the heating and blowing of a preform, predictions of the resultant bottle wall thickness distribution can be made. Iterative use of the simulation can result in an optimum preform design. This model, developed by Ansari (1998), contains four parameters characterizing the shape of the stress–strain curve. A representative stress– strain curve is shown in Fig. 9.9. initially there is an ‘elastic’ response followed by a yield point and a region of relatively constant stress, the strain softening region. The stress in this region is represented by the relation: Ê ˆ s = G Á1 – 1n ˜ Ë l ¯
f rl
[9.10]
Thus G represents the stress level in the plateau region – the yield stress – and the parameter n controls the rate of rise to the yield point. The larger the value of n, the quicker the rise. These two parameters govern the shape of the curve in region i. Here s is the engineering stress while l is the extension ratio. At sufficiently higher extensions where strain hardening occurs, a second term is added to the model. Again, this term is empirical and was chosen because it gives a shape similar to the experimental data.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 103
7/1/11 9:06:16 AM
104
Biaxial stretching of film 2000
Stress, s, psi
1500
S 1000 Region II 500
H
G Region I 0
n 1.0
1.5
2.0
2.5 3.0 Extension ratio, l
3.5
4.0
9.9 Stress–strain plot for PET illustrating the four parameters used to characterize the shape of the curve.
Ê ˆ s = G Á1 – 1n ˜ + 100 ¥ S[(l – H ) tan –1 (l – H ) Ë l ¯ – 1 log e (1 + (l – H )2 ] fo f r l ≥H 2
[9.11]
This new term contains the strain hardening point H (Chandran and Jabarin, 1993a) and the parameter S relating to the slope of the stress–strain curve in the strain hardening region. The initiation of strain hardening and the rate at which stress levels rise in the strain hardening region are a result of both the strain-induced crystallization and the finite extensibility of the chains. These four characteristics were determined for each experimental stress–strain curve. For example, Fig. 9.10 shows the effect of temperature in region i on the stress–strain curve. For the temperatures of 90, 100 and 105 °C, the respective values of G were 270, 175 and 160 psi while those for n were 5.2, 4.7 and 4.33. The four parameters were then related to the stretching conditions such as temperature, moisture content, extension rates and mode of stretching via simple regression models.
9.5.1
Simultaneous biaxial extension
For simultaneous biaxial stretching, the film sample is stretched in both directions at the same time and at the same rate – at least up to the amount
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 104
7/1/11 9:06:16 AM
Strain energy function and stress–strain model 400
Stress, s, psi
Region I
s@90 °C s@100 °C s@105°C fit@90 °C fit@100 °C fit@105 °C
300
200
105
Yield point
Plateau
100
0 1.0
1.5
2.0 Extension ratio, l
2.5
3.0
9.10 The effect of temperature on the yield stress in region I, constrained uniaxial extension and extension rate = 50%/s.
of the first stretch. Then the film is stretched only in the second direction. The stress–strain curve will depend upon the amount of the first extension ratio (l1). For example, in comparing the stress–strain responses for film samples stretch biaxially to 2 ¥ 4, 3 ¥ 4 and 4 ¥ 4, the curves should be the same up to the first extension value of l1 = 2. After this point, stresses for the 3 ¥ 4 and 4 ¥ 4 samples will exhibit higher stresses. For this reason, the amount of the first extension, l1, affects the four parameters used to characterize the shape of the curves. The values for the four shape parameters will be functions of ∑ the amount of the first extension – l1; ∑ the temperature of stretching – T; ∑ the extension rate or rate of stretching – ER. Moisture levels will affect Tg and are taken into account by developing a model in terms of the temperature difference (T – Tg). Relations were developed for the four shape parameters by using regression to fit the experimental data at different conditions of temperature (T), extension rate (ER) and amount of first extension (l1). When temperature is given as °C and extension rate as reciprocal seconds, the relationships for the four shape parameters are shown below for simultaneous biaxial stretching: G = exp(5.87 – 0.037(T – Tg) + 0.04l1 + 0.105 ER + 0.016l1ER)
[9.12]
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 105
7/1/11 9:06:17 AM
106
Biaxial stretching of film
n = exp(1.67 – 0.01(T – Tg) + 0.038l1 + 0.048ER)
S = exp(3.03 – 0.052(T – Tg) + 0.178l1
[9.13]
+ 0.048ER – 0.005l1(T – Tg) + 0.0085(T– Tg)ER)
[9.14]
H = exp(1.117 + 0.023(T – Tg) – 0.097l1
+ 0.073ER – 0.00934(T – Tg)ER)
[9.15]
Figure 9.11 shows the yield stress G as plotted against the amount of first extension at two different temperatures of 90 and 105 °C for extension rates of 20 and 200%/s. It is seen that G increases with an increase in the extension rate while it decreases with an increase in the temperature. Also there is some increase in the values of G with the increase in the amount of the first extension, although the change is not very significant. Two important parameters for design purposes are the strain hardening point and the slope in the strain hardening region. In Fig. 9.12, where the slope S has been plotted against the amount of the first extension at three different temperatures of 90, 100 and 105 °C, we see that although the slope S of the strain hardening region does not change in its value significantly at higher temperatures of 100 and 105 °C, it does increase sharply at 90 °C with G@90 °C – 20%/s G@90 °C – 200%/s G@105 °C – 20%/s G@105 °C – 200%/s
450
Gm@90 °C – 20%/s – model II Gm@90 °C – 200%/s – model II Gm@105 °C – 20%/s – model II Gm@105 °C – 200%/s – model II
Yield stress, G, psi
400 350 300 250 200 150 100 0
1
2 1st extension
3
4
9.11 Yield stress parameter G as a function of the 1st extension at 90 and 105 °C and for extension rates of 20 and 200%/s for simultaneous biaxial stretching. Gm represents the model values from equation 9.15.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 106
7/1/11 9:06:17 AM
Strain energy function and stress–strain model
26
S@90 °C – 50%/s
Sm@90 °C – 50%/s – model II
S@100 °C – 50%/s
Sm@100 °C – 50%/s – model II
S@105 °C – 50%/s
Sm@105 °C – 50%/s – model II
107
24 22 20
Slope S
18 16 14 12 10 8 6 4 0
1
2 3 1st extension
4
5
9.12 Slope S as function of the first extension at 90, 100 and 105 °C and for an extension rate of 50%/s for simultaneous biaxial stretching. Sm represents model values obtained from equation 9.17.
the increase in the amount of first extension, suggesting a sharp upswing and fast increase in the stress values in the strain hardening region. The low values of S at higher temperatures, which means weak strain hardening region, are also an indirect indication of smaller amount of crystallinity at the corresponding values of the first extension. The strain hardening point H follows the opposite trend, i.e., its value decreases with the increase in the amount of the first extension as seen in Fig. 9.13. In other words the value of the natural draw ratio decreases as the amount of first extension increases.
9.5.2 Sequential biaxial extension A similar procedure was adopted to develop correlations for sequential biaxial extension.
G = exp(5.33 –0.0243(T – Tg) –0.917l1 – 0.0534ER
1.5 + 1.09l1.2 1 + 0.012(T – Tg)ER –0.0195(T – Tg) )
[9.16]
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 107
7/1/11 9:06:17 AM
108
Biaxial stretching of film
Strain hardening parameter SHP, H
5 H@90 °C – 50%/s H@105 °C – 50%/s Hm@90 °C – 50%/s – model II Hm@105 °C – 50%/s – model II 4
3
2 0
1
2 3 1st extension
4
5
9.13 Strain hardening point H as a function of the first extension at 90 and 105 °C and for an extension rate of 50%/s for simultaneous biaxial stretching. Hm represents model values obtained from equation 9.18.
n = exp(1.57 – 0.021(T – Tg) – 0.277l1 + 0.021ER
[9.17]
S = exp(2.54 – 0.0418(T – Tg) + 0.76l1
+ 0.36l1.2 1 + 0.026l1ER) + 0.0026ER – 0.252l1.5 1 + 0.00457(T – Tg)ER)
[9.18]
H = exp(0.078 – 0.0333(T – Tg) + 0.56l1 – 0.0866ER
1.5 – 4.31l1.2 1 + 0.0103(T – Tg) )
[9.19]
For sequential stretching G decreases as the stretching temperature decreases. This trend is similar to that found in the case of simultaneous stretching. However, at higher values of the first extension, close to l1 = 2.5, the yield stress values increase sharply. This is in agreement with what we observed in the stress–strain curves. That is, the stress–strain curve changes its shape from concave upward to convex upward at a critical value of the first extension. Above this critical value the values of yield stress are much higher, especially as compared with those of simultaneous biaxial extension for the same planar extension. An important point to make here is the magnitude of the slope S does not reflect the degree of strain hardening, contrary to the case of the simultaneous biaxial extension. It is true that in the sequential biaxial mode, S decreases
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 108
7/1/11 9:06:18 AM
Strain energy function and stress–strain model
109
as the amount of the first extension increases. This can be seen by looking at Fig. 9.17.
9.5.3 Comparison of model behavior with experiment In order to check the validity of the model over the entire range of the available data, the following sets of conditions were selected: ∑ Temperature = 90–105 °C ∑ Extension rate = 20–200%/s ∑ Amount of first extension = 1.0–4.0 Figures 9.14–9.17 show the comparison between the experimental stress–strain curves and those predicted by the model. The values of the four parameters are obtained using the two sets of equations presented in the previous two sections. Experimental values are shown by points whereas the values obtained by the model are shown by the lines. The first three figures are for simultaneous biaxial extension. The first figure illustrates the effect the first extension has on the behavior. The values from the model match very well at the yield point, the flow plateau and the strain hardening region. In Fig. 9.15, the effect of temperature on the stress–strain behavior is shown. Again the values predicted by the model
3000
Ist Ist Ist Ist Ist Ist
2500
Stress, s, psi
2000
Ext. Ext. Ext. Ext. Ext. Ext.
= = = = = =
1.0 2.0 4.0 1.0 2.0 4.0
– – – – – –
exp exp exp model II model II model II
1500
1000
500
0 1
2
3 Extension ratio, l
4
9.14 Stress vs. strain as a function of first extension at 90 °C and an extension rate of 50%/s for simultaneous biaxial stretching – comparison of model to experiment.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 109
7/1/11 9:06:18 AM
110
Biaxial stretching of film 2500
@90 °C – exp @95 °C – exp @100 °C – exp @105 °C – exp 90 °C – model II 95 °C – model II 100 °C – model II 105 °C – model II
Stress, s, psi
2000
1500
1000
500
0 1
2
3 Ext. ratio, l
4
5
9.15 Stress vs. strain as a function of temperature at an extension rate of 50%/s and for a first extension = 4.0 – simultaneous biaxial stretching – comparison between experiment and model.
2500 @20%/s @20%/s @20%/s @20%/s @20%/s @20%/s
Stress, s, psi
2000
– – – – – –
model model model exp. exp. exp.
1500
1000
500
0 1
2
3 Extension ratio, l
4
5
9.16 Stress vs. strain as a function of extension rate at 100 °C and first extension of 2.0 for simultaneous biaxial extension – comparison between experimental and model.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 110
7/1/11 9:06:18 AM
Strain energy function and stress–strain model
111
are in close agreement. Similarly the comparisons in Fig. 9.16 show that the model correctly characterizes the strain rate effect. The final plot, Fig. 9.17, represents sequential biaxial stretching. The stretching temperature and strain rate were held constant at 90 °C and 250%/s. Each curve shown represents a different value of the first extension ratio. Again we find that the results are quite good, although not as accurate as it was in the case of simultaneous biaxial extension.
9.5.4 Predicting temperature rise during stretching Considering the extension process as adiabatic, the temperature rise on stretching the PET films was estimated based on the amount of work done during stretching. This amount of work done can be calculated either using a stress–strain model or graphically from the area under the stress–strain curve. It was found that the temperature rise follows more or less the same pattern as the increase in stress levels do during stretching in a stress–strain 1st 1st 1st 1st 1st 1st
Extension Extension Extension Extension Extension Extension
= = = = = =
2.0 – exp. 2.25 – exp. 2.75 – exp. 3.25 – exp. 3.75 – exp. 4.0 – exp.
1st 1st 1st 1st 1st 1st
Ext Ext Ext Ext Ext Ext
= = = = = =
2.0 – model II 2.25 – model II 2.75 – model II 3.25 – model II 3.75 – model II 4.0 – model II
3000
2500
Stress, s, psi
2000
1500
1000
500
0 1
2
3 Extension ratio, l
4
5
9.17 Stress vs. strain as a function of first extension at 90 °C and an extension rate of 250%/s for sequential biaxial stretching – comparison between experimental and model values.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 111
7/1/11 9:06:19 AM
112
Biaxial stretching of film
curve. The temperature rise was found to be in the range of 10–15 °C for simultaneous biaxial stretching. These values are comparable to the values calculated independently based on the heat of crystallization. Maruhashi and Asada (1996) have also reported experimentally measured increases of 10–15 °C at different extension rates. For sequential biaxial extension the calculated temperature rise was found to be more than that predicted for simultaneous biaxial extension. For instance, for sequential biaxial extension with the amount of the first extension equal to 3.75, the total temperature increase was more than 20 °C. These temperature increases during stretching are quite significant and may affect the analysis of the results.
9.5.5
Comments on the use of the stress–strain model for PET
The parameters G and H characterize the yield stress and the strain hardening point of the stress–strain curve. These two characteristics of the stress–strain curve are of great importance to the design engineer. The yield stress is important in the determination of the minimum air pressure required to initiate the preform blowing process. on the other hand the location of the strain hardening point provides valuable information about the natural draw ratio. This information can then be utilized for a better preform design, which when blown into the shape of the container ensures uniform wall thickness, better material distribution, and good final properties. As an example, consider the expansion of a cylindrical PeT preform initially with radius Ri = 10 mm and wall thickness Wi = 2.5 mm. internal pressure to inflate the preform is 65 psi. The engineering stress in the wall of the preform is given by (see erwin et al., 1983): s =
PRi l = 260l psi Wi
[9.20]
This is the stress due to the internal pressure in a cylinder and is represented in Fig. 9.18 by the dashed line. The cylindrical preform will expand until this stress is balanced by the material resistance to stretching. This material resistance is characterized in this example by the stress–strain curves for PET at 90 and 100 °C for an extension rate of 200%/s. For 100 °C the final extension ratio will be 5.0 if there is no mold constraint. The free blown preform will have a final diameter of 10 cm. However, if the inflation is done at 90 °C, the final diameter will only be 4.3 ¥ 2 cm = 8.6 cm. since both these points are in the strain hardening region, the blown container would be expected to have a uniform material distribution plus optimized mechanical and barrier properties. For this case, preform design would depend upon blowing temperature. Typically one would be given a bottle design and would use this approach
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 112
7/1/11 9:06:19 AM
Strain energy function and stress–strain model
113
1600 1400 Final extension ~ 4.3 1200
Stress, psi
1000 90 °C
800 600
100 °C
400 200 0 0
1
2 3 Extension ratio, l
4
5
9.18 Preliminary preform design based on the cylindrical expansion of PET at temperatures of 90 and 100 °C and for a strain rate of 200%/s.
to determine the preform geometry. For more accurate estimates of preform design, the stress–strain model has been combined into simulations, along with a heating analysis, to allow the user to estimate bottle wall thickness distributions for a given preform design. The designer can then get a good appreciation on how preform design affects both the preform temperature profile and the resultant bottle wall distribution. Yang et al. (2004a,b) have used such an approach using finite element techniques coupled with the Buckley model for the material behavior.
9.6
References
Ansari M A (1998), ‘Strain energy function and stress–strain model for poly(ethylene terephthalate)’, PhD thesis, University of Toledo. Arruda E M and Boyce M C (1993), ‘A three-dimensional constitutive model for the large stretch behavior of rubber-elastic materials’, J. Mech. Phys. Solids, 41(2), 389–412. Bonnebat C, Rouuet G, and de Vries A J (1981), ‘Biaxially oriented poly(ethylene terephthalate) bottles: effects of resin molecular weight on parison stretching behavior’, Polym. Eng. Sci., 21(4), 189–95. Boyce M C and Arruda E M (2000), ‘Constitutive models of rubber elasticity: a review’, Rubber Chem. Technol., 73, 504–23. Cakmak M, White J, and Spruiell J E (1989), ‘Optical properties of simultaneous biaxially stretched poly(ethylene terephthalate) films’, Polym. Eng. Sci., 29(21), 1534–43. Chandran P and Jabarin S A (1993a), ‘Biaxial orientation of poly(ethylene terephthalate. Part I: nature of the stress–strain curves’, Adv. Polym. Technol., 12(2), 119–32.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 113
7/1/11 9:06:19 AM
114
Biaxial stretching of film
Chandran P and Jabarin S A (1993b), ‘Biaxial orientation of poly(ethylene terephthalate. Part II: the strain hardening parameter’, Adv. Polym. Technol., 12(2), 133–51. Chandran P and Jabarin S A (1993c), ‘Biaxial orientation of poly(ethylene terephthalate. Part III: comparative structure and property changes resulting from simultaneous and sequential orientation’, Adv. Polym. Technol., 12(2), 153–65. de Vries A J, Bonnebat C, and Beautemps J (1977), ‘Uni – and biaxial orientation of polymer films and sheets’, J. Polym. Sci., Symp, C, 58, 109–56. Denn M M (1980), ‘Continuous drawing of liquids to form fibers’, Ann. Rev. Fluid Mech., 12, 365–87. Dupaix R B and Boyce M C (2007), ‘Constitutive modeling of the finite strain behavior of amorphous polymers in and above the glass transition’, Mech. Mat., 39, 39–52. Erwin L, Pollock M, and Gonzalez H (1983), ‘Blowing of oriented PET bottles: predictions of free blow size and shape’, Polym. Eng. Sci., 23(15), 826–29. Ferry J D (1970), Viscoelastic Properties of Polymers, 2nd Ed., New York, Wiley. Fried I and Johnson A R (1988), ‘A note on elastic energy density functions for largely deformed compressible rubber solids’, Comput. Meth. Appl. Mech. Eng., 69, 53–64. Gent A N and Thomas A G (1958), ‘Forms for the stored (strain) energy function for vulcanized rubber’, J. Polym. Sci., 28, 625–8. George A F, Strozzi A, and Rich J I (1987), ‘Stress fields in a compressed unconstrained elastomeric O-ring seal and a comparison of computer predictions and experimental results’, Tribol. Int, 20(5), 237–47. Gordon D H, Duckett R A, and Ward I M (1994), ‘A study of uniaxial and constant-width drawing of poly(ethylene terephthalate)’, Polymer, 35(12), 2554–9. Heffelfinger C and Lippert E (1971), ‘X-ray low-angle scattering from oriented poly(ethylene terephthalate) films’, J. Appl. Polym. Sci., 15(11), 2699–731. Ibar J B (1979a), ‘Nonequilibrium tensile deformation of amorphous polymers in the rubbery state: I. Temperature, strain rate, and strain effects’, J. Macromol. Sci. Phys. B, 16(3), 355–75. Ibar J B (1979b), ‘Nonequilibrium tensile deformation of amorphous polymers in the rubbery state. II. Effect on birefringence and elastic strain energy of temperature, strain rate, and strain’, J. Macromol. Sci. Phys., B, 16(4), 551–79. Ibar J B (1984), ‘Nonlinear relaxation behavior of amorphous polymers in the rubbery state (T > Tg): a new analysis of the data according to the ‘double-shift’ procedure’, J. Macromol. Sci. Phys., B, 23(1), 29–63. Jabarin S A (1984), ‘Orientation studies of poly(ethylene terephthalate)’, Polym. Eng. Sci., 24(5), 376–84. Jabarin S A (1991), ‘Orientation of precrystallized poly(ethylene terephthalate)’, Polym. Eng. Sci., 31(14), 1071–8. Jabarin S A and Lofgren E A (1986), ‘Effects of water absorption on physical properties and degree of molecular orientation of poly(ethylene terephthalate)’, Polym. Eng. Sci., 26(9), 620–5. James H M and Guth E (1943), ‘Theory of the elastic properties of rubber’, J. Chem. Phys., 11, 455–81. James H M and Guth E (1947), ‘Theory of the increase in rigidity of rubber during cure’, J. Chem. Phys., 15, 669–83. Jog J P (1995), ‘Crystallization of poly(ethylene terephthalate)’, Rev. Macromol. Chem. Phys., C35(3), 531–53. Kase S and Matsuo T (1965), ‘Melt spinning. I. Fundamental equations on the dynamics of melt spinning’, J. Polym. Sci., A, 3(7), 2541–54.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 114
7/1/11 9:06:19 AM
Strain energy function and stress–strain model
115
Kase S and Matsuo T (1967), ‘Melt spinning. II. Steady-state and transient solutions of fundamental equations compared with experimental results’, J. Appl. Polym. Sci., 11(2), 251–87. Kawabata S (1981), ‘Fracture and mechanical behavior of rubber-like polymers under finite deformation in biaxial stress field’, J. Macromol. Sci. Phys. B, 8(3–4), 605–30. Kawabata S, Yamashita Y, Ooyama H, and Yoshida S (1995), ‘Mechanism of carbon black reinforcement of rubber vulcanizate’, Rubber Chem. Technol., 68(2), 311–29. Kuhn W (1938), ‘Relations between the constitution and elastic properties of highly polymeric compounds’, Kautschuk, 14, 182–6. Maruhashi Y and Asada T (1996), ‘Structure and properties of biaxially stretched poly(ethylene terephthalate) sheets’, Polym. Eng. Sci., 36(4), 483–94. Matsuda M, Kawabata S, and Kawai H (1981a), ‘Experimental survey of the strain energy density function of isoprene rubber vulcanizate’, Macromolecules, 14(1), 154–62. Matsuda M, Kawabata S, and Kawai H (1981b), ‘ Quantitative analysis of the strain energy density function for cis-1,4-polyisoprene rubber vulcanizate’, Macromolecules, 14(6), 1688–92. Matsuda M, Kawabata S, and Kawai H (1982), ‘Dependence of strain energy density function on cross-link density and degree of swelling for cis-1,4-polyisoprene rubber vulcanizates’, Macromolecules, 15(1), 160–5. Matsuo M, Tamada M, Terada T, Sawatari C, and Niwa M (1982), ‘Deformation mechanism of poly(ethylene terephthalate) film under uniaxial stretching’, Macromolecules, 15(4), 988–98. Miller B H (1980), ‘Reheat blow molding of PET bottles’, SPE ANTEC Tech. Papers, vol. 26, 540–2. Mooney M (1940), ‘A theory of large elastic deformation’, J. Appl. Physics, 11, 582–91. Natu A A, Lofgren E A, and Jabarin S A (2005), ‘Effect of morphology on barrier properties of poly(ethyelene terephthalate)’, Polym. Eng. Sci., 45(3), 400–9. Ogden R W (1972), ‘Large deformation isotropic elasticity. Correlation of theory and experiment for incompressible rubberlike solids’, Proc. Royal Soc., A, 326(1567), 565–84. Ogden R W (1986), ‘Recent advances in the phenomenological theory of rubber elasticity’, Rubber Chem. Tech., 59(3), 361–83. Ogden R W and Chadwick P (1972), ‘On the deformation of solid and tubular cylinders of incompressible isotropic elastic material’, J. Mech. Phys. Solids, 20, 77–90. Petri C J (1979), Elongational Flows, London, Pitman. Rees H (1994), Understanding Injection Molding Technology, Cincinnati, Hanser/Gardner Publications Inc. Rivlin R S (1948), ‘Large elastic deformations of isotropic materials. IV. Further developments of the general theory’, Phil. Trans. R. Soc. Lond. A, 241, 379–97. Rivlin R S and Saunders D W (1951), ‘Large elastic deformations of isotropic materials. VII. Experiments on the deformation of rubber’, Philos. Trans. R. Soc. Lond. A, 243, 251–88. Salem D R (1999), ‘Orientation and crystallization in poly(ethylene terephthalate) during drawing at high temperatures and strain rates’, Polym. Eng. Sci., 39(12), 2419–30. Thompson A (1959), ‘Strain-induced crystallization in poly(ethylene terephthalate)’, J. Polym. Sci., 34, 741–60. Treloar L R J (1958), The Physics of Rubber Elasticity, 2nd Ed., Oxford, Oxford University Press.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 115
7/1/11 9:06:19 AM
116
Biaxial stretching of film
Tschoegl N (1972), ‘Constitutive equations for elastomers’, Rubber Chem. Technol., 45, 60–70. Weissman D (1988), ‘On the dynamics of stretch blowing PET’, SPE Antec Tech Papers, 34, 808–10. Whelan A and Goff J P (1985), Developments in Injection Molding, London, Elsevier Science Publishers. Williams M L, Landel R F and Ferry J D (1955), ‘The temperature dependence of relaxation mechanisms in amorphous polymers and other glass-forming liquids’, J. Am. Chem. Soc., 77, 3701–7. Yang Z J, Harkin-Jones E, Menary G H, and Armstrong C G (2004a), ‘A non-isothermal finite element model for injection stretch-blow molding of PET bottles with parametric studies’, Polym. Eng. Sci., 44(7), 1379–90. Yang Z J, Harkin-Jones E, Menary G H, and Armstrong C G (2004b), ‘Coupled temperature-displacement modelling of injection stretch-blow molding of PET bottles using Buckley model’, J. Mat. Proc. Tech., 153–154, 20–7.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-09.indd 116
7/1/11 9:06:19 AM
10
Academic investigations of biaxially stretched films
M . T . D e M e u s e, Independent Consultant, USA Abstract: This chapter will focus specifically on the academic investigations which have been reported on biaxially stretched films and the fundamental knowledge which has been obtained from those studies. Additional fundamental studies are proposed to continue to gain insight into the biaxial film stretching process. Further, continued work on the film production process is stressed due to the effect that it has on the subsequent film stretching features. Key words: polypropylene, stretch ratios.
10.1
Introduction
Previous chapters have discussed various aspects of the production of biaxially stretched films, primarily as it relates to current industrial processes. There have been occasional references to work from academic institutions. However, the majority of those references have been discussed in relation to the relevance to industrial work. This chapter will focus specifically on the academic investigations which have been performed on biaxially stretched films and the fundamental knowledge which has been obtained from these studies. Only articles which have appeared in the open literature will be discussed. Patent documents will not be mentioned in this chapter.
10.2
Literature studies of common commodity polymers
DeVries1,2 was one of the first to study in detail the effects of temperature, draw ratio and annealing on the morphology and properties of sequentially biaxially oriented films based on isotactic polypropylene. In that work, the preparation of films of varying nominal draw ratios was conducted on a continuous basis by means of extrusion of the polymer melt through a slit die, followed by cooling on a chill roll and stretching in the machine direction (MD) and then in the transverse direction (TD). The mechanical and other properties of the series of biaxial oriented films were also reported. In a similar way, Capt et al.3 studied the effect of cast film morphology on deformation behavior, resulting morphology and physical properties of 117 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-10.indd 117
7/1/11 9:07:04 AM
118
Biaxial stretching of film
simultaneously equi-biaxial stretched films. In that work, wide angle X-ray pole figure measurements were used to yield information about crystal texture. The thermal behavior and crystallinity of cast and oriented films were studied by means of differential scanning calorimetry (DSC). Further, wide angle X-ray scattering (WAXS) in reflection was used to follow the evolution of crystal structure and mean crystalllite size. Several general conclusions are drawn from those studies. Simultaneous equi-biaxial stretching of isotactic polypropylene in the partly molten state follows a ductile deformation behavior, i.e. a yield point followed by strain hardening, for the temperature range 140–155 °C. This suggests that spherulites are being transformed into fibrils, as proposed by Peterlin’s model for cold-drawing of semi crystalline polymers.4 On the other hand, as the stretching temperature approaches the polypropylene melting point, a quasi rubber-like deformation behavior is observed and it likely does not follow the same deformation model. Another important conclusion from this study is that different cast film processing conditions will affect the cast film morphology, which in turn will affect the stretching behavior, the morphology, and consequently the properties of the biaxially drawn films. Specifically, the level of yield stress was found to be dependent on the crystallite size of the cast film. Further, it was observed that the average crystallite size present in the biaxially stretched films is significantly affected by the cast film morphology. Nie et al.5 used atomic force microscopy to study the surface morphology of sequentially oriented polypropylene films. The surface was shown to be dominated by a nanometer-scale fiber-like network structure, the configuration of which was found to be determined by the relationship between the machine draw and transverse draw. For film fabricated using MD and TD ratios of 5.2:1 and 9:1, respectively, preferential orientation of fine fibers to the TD was observed. On the other hand, when the MD and TD ratios become similar, no predominant TD fiber direction alignment was observed. Residual effects of the first stretching of the film surface can provide information on the manner in which morphological development of the BOPP occurs. Elias et al.6 studied the effect of biaxial orientation on the morphology of polypropylene films using thermal and dynamic mechanical techniques. Upon biaxially orienting, the folded lamellae crystals or kebabs are the ones to support all the force applied, and when their maximum level of stress slippage is reached they deform and form a new shish structure. These shishes are aligned to the TD and by linking the original shishes in the MD produce a planar or orthogonal net of linked shish structures. The space among the shishes is filled with small and imperfect folded lamellae and preferentially oriented in the MD and TD keeping constant crystallinity throughout.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-10.indd 118
7/1/11 9:07:04 AM
Academic investigations of biaxially stretched films
119
Lin et al.7 produced biaxially oriented polypropylene (Bopp) films by both simultaneous and sequential stretching using various balanced and unbalanced draw ratios. The films that were produced were characterized in terms of density, crystallinity, refractive index, oxygen permeability and dynamic mechanical relaxation behavior. It was found that the density and crystallinity of the BOPP films decreased as the area draw ratio increased. Sequential stretching led to a slightly lower density than simultaneous stretching to the same draw ratio. Further, sequential stretching produced lower orientation in the first stretch direction and higher orientation in the second direction compared with simultaneous stretching. The study verified the general nature of a one-to-one correlation between the oxygen permeability of the films and the mobility of amorphous tie chains, as measured by the intensity of a dynamic mechanical beta relaxation. Finally, the polymer chain mobility in the stretch direction was found to depend on the final stress during stretching. The morphology and structure of biaxially oriented films made from polymers other than polypropylene have also been studied in the open literature. For example, Chang et al.8 studied the morphology of biaxially stretched poly(ethylene terephthalate) (PET) films. In that study, cast PET films were biaxially drawn and subsequently heat-set. All films were drawn 3.5 ¥ at 90 °C in the machine direction and subsequently drawn to various degrees in the transverse direction. The structure of the films, before and after heat treatment, was investigated using X-ray scattering and transmission electron microscopy. The MD drawn material exhibits crystallization in the form of thin fibrillar crystals with chain axes along the MD. TD drawing disrupts this structure and ultimately produces a second crystal population with chain axes along the TD. Subsequent heat treatment produces a fibrillar-to-lamellar transition and a subsequent increase in the degree of crystallization. Greener et al.9 examined the effects of a controlled heat-setting treatment on the properties and microstructure of biaxially stretched polyester films, including PET. Substantial changes in the crystalline fraction, crystallite size and glass transition temperature were observed upon an increase in the heat-set temperature. Also, there is a significant enhancement in dimensional stability which is observed with an increase in the heat-setting temperature. A distinct second melting peak is observed in the vicinity of the heat-set temperature and underlies the dual nature of the morphology of the heat-treated films. The oriented PET films undergo significant molecular realignment on heat setting. A morphological transition was detected at heatsetting temperatures near the primary melting range, causing a qualitative change in the physical response of the film. The morphological models of Schultz and Lee10 and Fischer and Fakirov11 were used to explain some of the observations in terms of a fibrillar to lamellar transformation process.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-10.indd 119
7/1/11 9:07:04 AM
120
Biaxial stretching of film
Along with studies of the morphology of biaxially oriented PET (BOPET) films, there have also been reports related to other properties such as oxygen barrier properties. For example, Perkins12 studied the influence of both molecular weight and annealing temperature on the crystallinity and subsequent resistance to oxygen permeability for PET films which were biaxially oriented. Within the range investigated, molecular weight affected the level of crystallinity developed at a given temperature, but had little influence on oxygen permeability. On the other hand, annealing temperature more directly influenced permeability than did the absolute level of crystallinity. Biaxially oriented films of polyethylene have also been studied in the open literature. For example, Kojima et al.13 examined biaxial oriented films of commercial polyethylenes using a variety of techniques, including transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Discernible differences in morphological features were found for the inner and outer film surfaces that experienced different conditions during the blown film extrusion. The surface roughness of the films was measured and correlated with the film haze parameters. Trends in surface roughness with crystallinity are attributed to surface lamellar-like crystallization. Tsai et al.14 have recently studied the dimensional stability and crystallinity of biaxially oriented poly(lactic acid) (PLA) films. The crystalline morphology of these films can be manipulated by changing certain processing parameters, such as stretch ratio, heat setting temperatures, and hear setting time. It was found that the optical and mechanical properties as well as dimensional stability of the films are governed by their crystallinity and crystalline morphology. Mechanical properties and the dimensional stability of the biaxially oriented PLA films were obtained and correlated with their processing conditions. As an example, shrinkage of less than 2% was achieved for a film sample stretched 300% in both directions at 75 °C and, then, annealed at 160 °C for 30 seconds.
10.3
Biaxial studies of specialty polymers
Rhee and White at the University of Akron have studied the morphology of several types of biaxially oriented polyamide films.15 For example, they investigated the crystal structure, orientation and mechanical properties of polyamide 6 films using DSC, wide angle X-ray diffraction, birefringence and tensile testing. The measurements were taken at three stages: (1) after extrusion casting (2) after film stretching and (3) after annealing. Poorly defined crystals were found in the aged films when the film stretching was done at a low temperature. On the other hand, by increasing the stretching temperature from 40 to 180 °C, the beta unit cell from low temperature stretching becomes similar to the alpha unit cell. In addition, the measured
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-10.indd 120
7/1/11 9:07:04 AM
Academic investigations of biaxially stretched films
121
mechanical properties for the films were correlated with out-of-plane birefringences. In a similar study,16 the same workers provided structural and morphological characterization of biaxially oriented polyamide 12 films using differential scanning calorimetry, wide angle X-ray diffraction, polarized Fourier transform infrared (FT-IR) spectroscopy and SAXS. The wide-angle X-ray diffraction patterns of the oriented films show only a single crystal type regardless of the stretching conditions. Annealing of the films stretched at 115 °C did not result in structural changes of the crystalline unit cell. Kohno and Tamemoto17 studied biaxially oriented Nylon 66 films made via the tubular process. The relationship between the structure resulting from manufacturing conditions and film properties was examined in that work. A key parameter for film manufacturing is quenching of the polymer melt, due to the high rate of crystallization. The mechanical properties of the biaxially oriented Nylon 66 film are found to be determined mainly by the degree of polymerization, the orientation within the film, the hydrogen and molecular bond structure, and the degree of crystallinity. Toughness and high thermal resistance are important properties of the film which is produced. Khanna et al.18 have examined the effect of relative humidity on the oxygen transmission rate (OTR) of Nylon 6 films, including biaxially oriented film. They found that, contrary to common opinion, there is no deterioration of OTR up to 80% relative humidity (RH) for biaxially oriented Nylon 6 films. Further, it was demonstrated that the contribution of orientation, after correcting for induced crystallinity, to the OTR of Nylon 6 is insignificant. The impact of moisture and processing parameters on the OTR of Nylon 6 was quantified. The deformation and stress relaxation behavior of biaxially oriented polystyrene films was studied by Chau and Li.19 They found that when a biaxially oriented polystyrene film was stretched along one direction and subsequently stretched along the perpendicular direction, the resulting film showed enhanced ductility with pronounced yield softening and extended strain hardening. In the initial deformation, two types of shear bands were observed. The bands which were observed at the early stages of yielding did not contribute to the reduction in film thickness. On the other hand, the shear bands developed in the later stages contributed to the thickness reduction. In the cross-deformation process, new shear bands developed that were likely related to the reverse shearing of the existing bands. The enhancement in ductility which is observed can be attributed to a lowering of the internal stress during the cross-deformation. The final polymer which has received some attention in the open literature in biaxially oriented film form is poly(ethylene 2-6-naphthalate) or PEN. Its molecular structure is shown in Fig. 10.1. Chemically, it belongs to
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-10.indd 121
7/1/11 9:07:04 AM
122
Biaxial stretching of film
the same class of polymers, known as polyesters, as does PET, previously discussed. It has many of the same attributes as PET but also possesses greater hydrolytic resistance than PET. Song and White20 provided a study of film formation and structure development of PEN film in double bubble tubular extrusion. The films that were produced were characterized with wide angle X-ray diffraction, DSC and refractometer techniques. The structure and orientation in the films varied substantially with process and film formation history. Films produced by the double bubble process contained only a single crystal type. Further, the mechanism of structure evolution in PEN was determined to be related to its behavior during the bubble inflation period. Hardy et al.21 studied biaxially oriented PEN films of different morphologies using polarized infrared spectroscopy. The different structures were obtained by thermally treating biaxially stretched films. The results show that the amorphous phase in the films becomes significantly disoriented when annealed at a temperature of 260°C despite an increase in the crystallinity. Kim et al.22 took a slightly different approach and examined the influence of biaxial deformation on the development of thermal, optical and mechanical properties of PEN as well as blends of PEN with polyetherimide (PEI). The refractive indices in the normal direction of biaxially stretched films decreased with areal expansion ratio. This is due to the orientation of the naphthalene rings shown in Fig. 10.1 becoming parallel to the film surface as the expansion ratio increases. Blends of PEN with up to 20% PEI reduce the naphthalene ring orientation, thereby changing the deformation behavior of the PEN by eliminating the localized necking behavior which is attributed to this orientation behavior. Wide angle X-ray diffraction studies indicate that biaxially oriented PEN films show bimodal orientation of the polymer chain axis. However, when blended with PEI at 10–20% concentrations, the bimodal orientation is eliminated and the biaxially stretched films exhibit in-plane isotropy. Many of these studies which are reported from the open literature show that the production process of films greatly affect their behavior during the biaxial orientation process. In that sense, the stretching portion of the biaxial film production process should only be regarded as one portion or step of a complicated process. These literature studies have clearly shown that it is possible to affect the film stretching features through the extrusion and O CH2
CH2
O
C
O C
O
n
10.1 Molecular structure of poly(ethylene 2-6-naphthalate) (PEN).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-10.indd 122
7/1/11 9:07:04 AM
Academic investigations of biaxially stretched films
123
production methods. Further, the final film properties will also be affected by the stretching characteristics. So, one of the main lessons to be learned from the quoted literature articles is that the biaxial film production process actually involves several different steps which are all interconnected with each other. The key to optimizing the final film properties is to understand the relationships between the various steps.
10.4
Recommendations
Additional fundamental studies of the biaxial film orientation process need to be performed to continue to gain insight into exactly what is occurring at each step in the process. Unfortunately, there have been a limited number of studies performed thus far and far more additional work is necessary in this regard. It is only through such basic investigations that a clear and complete understanding of the effects of various processing parameters on the final film properties can be obtained. Further, continued work on the effect that different extrusion and production conditions has on the stretchability and orientability of films needs to be done. As already alluded to previously, the stretching step in the biaxial film production process is just one portion of the entire process and, as such, the various steps are interconnected. The relationships and correlations between the various steps need to be further defined and elucidated. Such studies will ultimately lead to the development of films with improved properties beyond what is possible with present marketplace products. There should be continued collaborations between academic and industrial partners in biaxial stretching of films. In such collaborations, the academic partners bring the ability to perform fundamental studies, such as thermal analysis and X-ray diffraction studies, while the industrial partner brings the capability to perform larger scale experiments on equipment like tenter frames, not accessible to academic institutions. Through such collaborations, it should be possible to make faster progress than is possible through separate and individual studies.
10.5
References
1. A. J. DeVries, Pure and Appl. Chem., 53, 1011 (1981). 2. A. J. DeVries, Pure and Appl. Chem., 54, 647 (1982). 3. L. Capt, M. R. Kamal, and H. Munstedt, 13th International Congress on Rheology (2000), Cambridge, UK. 4. A. Peterlin, J. Mater. Sci., 6, 490 (1971). 5. H. Y. Nie, M. J. Walzak and N. S. McIntyre, Polymer, 41(6), 2213 (2000). 6. M. Elias, R. Machado, and S. Canevarolo, J. Thermal Analysis Calorimetry, 59, (1&2), 143 (2000). 7. Y. J. Lin, P. Dias, H. Y. Chen, A. Hiltner, and E. Baer, Polymer, 49(10) 2578.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-10.indd 123
7/1/11 9:07:05 AM
124
Biaxial stretching of film
8. H. Chang, J. M. Schultz, and R. M. Gohil, J. Macromole. Sci., Part B, 32(1), 99 (1993). 9. J. Greener, A. H. Tsou, and J. N. Blanton, Polym. Eng. Sci., 2403 (1999). 10. K. G. Lee, and J. M. Schultz, Polymer, 34, 4455 (1993). 11. E. W. Fischer and S. Fakirov, J. Mater. Sci., 11, 1041 (1976). 12. W. Perkins, Polym. Bull., 19, 397 (1988). 13. M. Kojima, J. H. Magil, J. S. Lin, and S. N. Magonov, Chem. Mater., 9(5), 1145 (1997). 14. C. C. Tsai, R. J. Wu, H. Y. Cheng, S. C. Li, Y. Yu Siao, D. G. Kong and G. W. Yang, Polym. Degradation and Stability, 95(8), 1292 (2010). 15. S. Rhee and J. L. White, Polymer, 43(22), 5903 (2002). 16. S. Rhee and J. L. White, J. Polym. Sci. Part B: Polym. Phys., 40(12) 1189 (2002). 17. M. Kohno and K. Tamemoto, Polym. Eng. Sci., 27(8), 558 (2004). 18. Y. P. Khanna, E. D. Day, M. L. Tsai, and G. Vaidyanathan, J. Plastic Film Sheeting, 13(3) 197 (1997). 19. C. C. Chau and J. C. M. Li, J. Polym. Sci., Part B: Polym. Phys., 47(7) 687 (2003). 20. K. Song and J. L. White, Polym. Eng. Sci., 40, 1122 (2000). 21. L. Hardy, I. Stevenson, A. M. Voice, and I. M. Ward, Polymer, 43(22), 6013 (2002). 22. J. C. Kim, M. Cakmak, and X. Zhou, Polymer, 39(18), 4225 (1998).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-10.indd 124
7/1/11 9:07:05 AM
11
Biaxially stretched polyamide film
T . B a r t h, Brückner Maschinenbau GmbH & Co. KG, Germany Abstract: This chapter gives an overview of biaxially oriented polyamide film. To begin with, the different stretching methods for polyamide film are outlined and finally, the main properties of polyamide film and the typical applications are specified. Also, the main differences between polypropylene and polyester film are described. Key words: BOPA, nylon, polyamide, biaxially oriented, film, properties.
11.1
Introduction
Biaxially oriented polyamide (BOPA) film was introduced in the late 1960s in Japan. Today, the worldwide demand is 260 000 tons per year, which – although not comparable with other biaxially oriented film, such as biaxially oriented polypropylene (BOPP) and biaxially oriented polyethylene terephthalate (BOPET) – shows a high growth rate, especially in the Far East. With outstanding film properties, especially in terms of tear resistance, puncture resistance and barrier against oxygen, BOPA plays an important role, especially in the food packaging industry. In particular newer applications, such as packaging of convenience food, is an important driver for the growth rate of BOPA film within Asia.
11.1.1 Nylon For BOPA film, only polyamide 6 (PA6) is used. PA 6 was developed in 1938 by Dr Paul Schlack at I.G.–Farben in Germany. PA 6 is produced from e-caprolactam via a ring opening polymerization. The reaction principle is shown in Fig. 11.1. The melting point is ~220 °C and the glass transition point is 52 °C. The major PA 6 producers are BASF (Germany), DSM (Netherlands), UBE (Japan), EMS (Switzerland). The total worldwide PA 6 production is approximately 5 million tonnes/year. Only approximately 7% is used in the film business. The majority is processed into synthetic fibres and for injection moulding, i.e. for automotive applications.
125 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 125
7/1/11 9:10:58 AM
126
Biaxial stretching of film O O
NH
H2N
OH
H N
H2N
O OH
O
11.1 Caprolactam polymerisation.
11.1.2 History of biaxially oriented polyamide (BOPA) The first BOPA film was produced by Unitika in Japan in 1968 on a simultaneous tenter frame. Kohjin followed in 1970 using a double bubble technique. The sequential stretching process was introduced by Toyobo in 1976. Over the years, the BOPA market has been dominated by these three global players. Under the brand name of Emblem Film, Unitika gave several joint ventures. Bonyl is the trade name of Kohjin Film and Harden stands for Toyobo. The outline in Fig. 11.2 shows the three different stretching modes possible for BOPA. Prior to 2005, the market share for sequential, simultaneous and double bubble stretching was almost equal. In the BOPA boom between 2003 and 2005, most of the newly installed lines were sequential stretching lines and the market share altered – see figures 11.1 and 11.2.
11.2
Processing of biaxially oriented polyamide (BOPA)
11.2.1 Extrusion Prior to 2005, only single screw extruders were used for the processing of PA 6. In 2005, twin screw technology for the main extruder was introduced, combined with a vacuum system. This resulted in beneficial features, such as ‘direct edge trim recycling’ and less influence of unstead raw material moisture levels. For melt filtration, different approaches are applied, such as disk filters, candle filters and more common screen changers. The filter media is usual sintered fibre mesh with a nominal filter ratio between 40 and 60 mm (400 to 200 mesh). State-of-the-art production lines are equipped with three layer configurations, such as ABA or even ABC layer structure. The main extruder feeds the middle layer, including regenerated material or colour masterbatches. The outside
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 126
7/1/11 9:10:59 AM
Biaxially stretched polyamide film
Sequential stretching MDO/TDO
127
Simultaneous stretching Pentagraph LISIM®
Double bubble
11.2 Biaxial orienting technologies for BOPA (MDO, machine direction orienter; TDO, transverse direction orienter).
layers are supplied by smaller co extruder including the masterbatch, such as anti-block.
11.2.2 Casting and pinning The melt is cast with a single layer flat sheet die; the three different layers coming from the extrusion are laid together in a so-called feed block. An alternative is a real three layer die, where each layer has its own coat hanger in the flat die. A pinning device is necessary in order to press the PA melt out of the extrusion die onto the chill roll, so that it is rapidly cooled in order to suppress the crystallization of the so-formed cast film. Generally, a low degree of crystallization and small and homogeneous spherulites are necessary for a homogeneous stretching process and uniform properties. State of the art is a so-called ‘needle pinning’ system, which is an electrostatic system in order to apply the high voltage using a line of metal needles. The voltage is up to 8 kV, and the current is up to 250 mA. Figure 11.3 shows a typical electrostatic needle pinning configuration. The needles are flexibly fixed, the distances between needle and film can be adjusted, in order to maintain a perfect pinning result over the whole cast film width. The chill roll speed which can be achieved with such configuration is typically up to 65 m/min.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 127
7/1/11 9:10:59 AM
128
Biaxial stretching of film
11.3 Electrostatic needle pinning configuration.
A new development suitable for high speed operation is a special air knife which applies compressed air. Owing to the design, it is possible to pin down the melt in a proper manner. Figure 11.4 shows the so-called high pressure air knife on a 5 m BOPA production line which operates at a production speed > 200 m/min.
11.2.3 Different stretching processes BOPA is produced by means of three different stretching processes; however, each process has specific advantages and disadvantages. The specific features are summarized below. Sequential tenter frame process The design layout of a sequential BOPA stretching lines is very similar and close to the BOPET process (Fig. 11.5). The cast film is reheated over chrome-plated preheating rolls in the machine direction orienter (MDO). The film temperature needed for this longitudinal stretching is approximately 50–55 °C. Over the glass temperature, the film has a tendency to stick on the roller. Stretching is done in one or two stretching gaps, the speed difference of the slow and the fast stretching roll determines the stretching ratio. The typical stretching ratio in longitudinal direction is between 1:2.6 and 1:3.3. After stretching, the film is then again cooled via several cooling rolls.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 128
7/1/11 9:10:59 AM
Biaxially stretched polyamide film
129
11.4 High pressure air knife pinning.
Working width
m
4.2
5.0
Output/hour
kg/h
700
920
Output/year (7500 hours)
tpa
5250
6900
Thickness range
µm
8–30
8–30
Production room length approx.
m
97
102
Production room width approx.
m
18
20
11.5 BOPA sequential line.
In the transverse direction orienter (TDO), the longitudinal stretched film is stretched perpendicular to the first stretching. Similar to BOPP and BOPET, this is done via clips mounted on a chain track system. First © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 129
7/1/11 9:10:59 AM
130
Biaxial stretching of film
of all, the film is heated up to approximately ~80 °C by air nozzles. The stretching temperature can vary in a wide range. Some BOPA producer use low temperature settings between 75 and 110 °C, while others use up to 180 °C. Lower temperature settings are better for the thickness profile and bring higher mechanical properties, whereas higher settings attain more even shrinkage values over the width. A so-called anti-bowing zone between stretching and heat set is also described in patents from Toyobo. The bowing phenomenon of BOPA film is described in detail in Section 11.3. The heat setting of biaxially stretched film is done while the film is still under tension from the chain track system at a temperature of 200 – 220 °C. The timespan to ensure a successful heat set is in between 3 and 6 seconds, depending on the finally required film shrinkage value. To reduce the heat shrinkage in TD direction, the width of chain track system is reduced in order to allow a certain relaxation of the film. After that step, the film is then cooled and fed to the pull roll or take-off stand. As on other stretching lines, thickness measurement is performed, edge trim and – if necessary – corona treatment of the film. Typically, BOPA film is wound up via a two turret winder. Figure 11.6 shows the first 5 m BOPA stretching line worldwide. With an annual output of 7690 tons, the 5 m BOPA lines from Brückner are the most powerful production lines.
11.6 BOPA line from Brückner.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 130
7/1/11 9:10:59 AM
Biaxially stretched polyamide film
131
Simultaneous tenter frame process Lines for simultaneous stretching process are equipped with a scissor chain type, e.g. from Hitachi. The cast film passes a heated water bath in front of the stretching oven. The cast film conditioning ensures an even and homogeneous stretching and is state of the art for all simultaneous production lines. Depending on residence time and water temperature, the total amount of water in the cast film is between 2 and 6%. To reach these values, a diving length up to 60 seconds and a water bath temperature between 40 and 60 °C is necessary. The stretching temperatures are significantly higher than in the sequential process. Typical stretching temperatures are approx.180 °C. The stretching ratio is 3.0 in longitudinal direction and 3.3 in transverse direction; adjustments are not possible. Such stretching ratios have been fully proven over the years. The heat set time and temperatures are comparable with the sequential process settings. The Brückner LISIM® system provides an alternative to the mechanical system (Fig. 11.7). The grippers are not mounted on a mono rail system, but are driven separately by linear motors. The advantage is the flexibility in MD and TD stretching ratios compared with the mechanical system, in particular the possibility of performing MD retardation, similar to the wellknown TD retardation to improve the shrinkage behaviour. The layout is designed to produce 1045 kg/h of 15 mm BOPA film with a high yield of A-Grade film properties. From the stretching oven onwards, the line layout is the same as for sequential lines.
Output/hour
kg/h
1045
Output/year
tpa
7840
Film width
m
5.1
Thickness range
µm
12–30
Prod. speed
m/min
200
Mech. speed
m/min
250
Features ∑ High output ∑ High yield ∑ Better film quality ∑ Higher film value
11.7 BOPA LISIM® line.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 131
7/1/11 9:10:59 AM
132
Biaxial stretching of film
Simultaneous double bubble process In the first step, the first bubble is cast from a round die. In the following step, the bubble is heated by infrared heaters and stretched simultaneously in both directions. The stretching ratio depends on the inner bubble pressure. Normally, the inner pressure of the bubble is around 100 MPa and the stretching ratio is 1:3.0 to 1:3.2. After stretching, the bubble is folded and fed into a monorail chain system. The heat set is similar to that described previously for the sequential tenter frame process.
11.2.4 Winding BOPA film is relatively easy to wind in comparison to BOPP and BOPET film. The reason being the lower line speed and consequently less air entrapment between the layers. BOPA film does not require aging; the film can be wound similar to BOPET. The winding density is typically above 94%. On sequential and simultaneous tenter frame lines, contact winding with a high pressure and a tapered tension is preferred, whereas on double bubble lines, mainly gap winding is applied. The winding is done typically under controlled climatic conditions.
11.2.5 Converting The main challenge during the entire converting chain is the sensitivity to moisture. BOPA film picks up moisture rather fast; depending on air humidity, up to 4%. With the absorption of water, the dimensions of the film change, and subsequently could lead to converting problems. If the air humidity is lower, the BOPA film could also transpire moisture to the environment, which could lead to the same kind of problems. In view of the above, the film needs to be stored under controlled climatic conditions or wrapped into aluminized PE or PP film to prevent water absorption. As a rule, it is recommended to slit, store and convert the film under the same conditions. BOPA film has a very high surface tension; even without surface treatment the dyne-level reaches values of 48 mN/m. With the regular corona treatment, the dyne-level is over 58 mN/m. From this aspect, converting is uncritical. Printing of BOPA film can be done on all kinds of printing machines. Owing to the high surface tension of BOPA film, water-based colour systems can also be used. Normally, seven-colour printing is problem-free. Higher numbers of colours could lead to a mismatching of the final colours. BOPA film can be laminated against cast PP (CPP), polyethylene (PE), and also BOPP and BOPET. The structure can be quite simple, such as applied
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 132
7/1/11 9:10:59 AM
Biaxially stretched polyamide film
133
for cheese or seafood packaging, see Fig. 11.8. Alternatively, a multilayer structure, e.g. for coffee, where several different layers are applied, including BOPA, BOPET and aluminum foil. Obviously, the glue system needs to be specialized for the relevant purpose or combination. Retort pouches are very special and demanding applications for BOPA, whereas the content is sterilized after packaging. The typical conditions therefore are 121 °C/30 min. For that reason, CPP as lamination partner for BOPA is needed first, since standard PE would be damaged during this process. Figure 11.9 shows a possible retort application. In Section 11.3, the special requirements placed on BOPA film for such application is described. Helium designer balloons constitute a special application for metalized BOPA. Compared with BOPP a standard BOPA film can be used for the metallization. Migrating additives, such as anti-static or slip masterbatches are not common in BOPA. A problem can be too high content of ethylenebis-stearoyl amide (EBS). This slip agent processing aid is in almost all available PA 6 grades.
11.3
BOPA film properties
BOPA film is semi crystalline and shows excellent mechanical properties, such as puncture resistance, high stiffness and tensile strength. In addition, a BOPA film has good resistances against chemicals, a high aroma and odour barrier. The oxygen transmission rate (OTR) values are significantly lower than BOPET. Typical attainable values for standard 15 mm BOPA film
Packaging system: vacuum pouch
Structure: PE 70 µm BOPA 20 µm
11.8 BOPA film application: smoked salmon.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 133
7/1/11 9:10:59 AM
134
Biaxial stretching of film
Packaging system: stand-up pouch, retort Structure: Wall structure: PEt 12//ALU foil 8//CPP 50 Bottom Structure: PET 12//Alu Foil 8//BOPA 15//CPP 50
BOPA 15 µm Al foil 8 µm CPP 50 µm
11.9 BOPA film application: pet food.
are in the range of 30–50 cm³/m² ¥ day ¥ bar. The value strongly depends on the humidity level. With higher humidity the OTR values of BOPA will be lower. Shrinkage can be controlled similar to BOPET in a range below 2% in MD and TD direction. With these end film properties, BOPA is designated for all different types of food packaging. Negative points of BOPA film are the high water vapour transition rate (WVTR) and moisture absorption value. Table 11.1 gives typical values of BOPA film. The BOPA film produced with different stretching methods was evaluated and compared in the Brückner technology centre. The values are calculated out of three measurement points over the film width. The samples have been derived from major BOPA film producers. The film was stored for 48 hours under controlled climatic conditions and measured afterwards within one day, in order to prevent influences of moisture. While the mechanical properties are similar, the heat shrinkage shows significant differences between the three stretching principles. The BOPA film produced in a double bubble process has the highest heat shrinkage, whereas the sequential produced film shows the lowest values. A different picture is obtained if the shrinkage values of each film are measured over the working width and in each direction. In Fig. 11.10, the so-called polar shrinkage diagrams of a sequentially stretched BOPA film are shown. To obtain the polar diagrams, a 100 ¥ 100 mm film sample was cut out with different angles from the well-known MD and TD direction (0° and 90°). The samples were applied to standard shrinkage test conditions
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 134
7/1/11 9:10:59 AM
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 135
7/1/11 9:10:59 AM
MD TD
MD TD
F/F-T/O F/F-T/O
Elongation at break
Modulus of elasticity
Coefficient of friction (%)
(N/mm2) (N/mm2)
Thermal shrinkage
MD TD
(%) (%)
Gloss
Haze
(N/mm ) (N/mm2)
MD TD
Tensile strength (%) (%)
(µm)
Thickness 2
1.6 1.1
107
1.74
0.48 0.44
4347 3686
132 100
274 296
15
Properties Unit Sequential
Table 11.1 Comparison of film properties
2.5 0.35
108
1.68
0.57 0.6
4458 4118
97 107
245 272
15
Simultaneous (mech.)
1 0.8
100
5.8
0.3 0.28
3741 3956
107 72
193 240
15
Simultaneous (LISIM®)
BMS TT 11 BMS TT 11
ASTM 2457
ASTM 1003
DIN 53375 DIN 53375
ASTM D 882 ASTM D 882
ASTM D 882 ASTM D 882
ASTM D 882 ASTM D 882
DIN 53370
Method
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 136
7/1/11 9:11:00 AM
TD
225°
315°
MD
11.10 Polar shrinkage diagrams of sequentially stretched BOPA film.
MD
225°
0.0
135°
0.5
0.0 TD
0.5 TD
1.0
2.0
1.0
315° 1.5
45°
MD 2.5
Sequential centre
1.5
2.0
MD 2.5
Sequential edge
135°
45°
TD
TD
225°
315°
MD
0.0
0.5
1.0
1.5
2.0
MD 2.5
Sequential edge
135°
45°
TD
Biaxially stretched polyamide film
137
for BOPA, i.e.160 °C for 5 minutes. After heat treatment, the shrinkage of the samples was measured and plot into a polar diagram. The sequential film was produced on a 5 m production line. The sample from the central part of the film shows a low and homogeneous shrinkage behaviour. In all directions, the shrinkage is more or less around 1%. The edge part of the film (film is from the same jumbo roll) shows a completely different shrinkage. The main shrink direction is 45°. During sterilization at 121 °C, the pouch is distorted due to the uneven shrinkage of the BOPA film. The distortion does not influence the function of the packaging, only the appearance is affected. For that reason, only about 60–80% of the film out of the centre can be used for this application. Film produced on a simultaneous stretching line usually can supply the film over the entire working width into that special application. In Fig. 11.11, the polar shrinkage behaviour of the simultaneous produced film is shown. Even if the total shrinkage is a bit higher than on sequential lines, the improved homogeneity of the shrinkage provides the benefit here. The polar shrinkage diagrams for centre and side of the web are similar; the main shrinkage direction is in MD, which leads to fewer problems during the conversion. Compared with simultaneous lines, this uneven shrinkage behaviour over the working width on sequential lines is a disadvantage. The present higher output is attained with the sequential technology. The film from the side is not waste material; it still can be used for various applications. On Brückner’s simultaneous laboratory line, several test runs have been performed to determine the influence of the possible MD retardation to the shrinkage behaviour. The most balanced shrinkage over the width and the relevant stretching settings are displayed in Fig. 11.12. The shrinkage behaviour for BOPA is often discredited in view of the bowing phenomena. Bowings mean the distortion of a straight line in front of the tenter compared to line leaving the oven. The middle of the line will exit the oven later than the edge part. Standard sequential lines show a bowing of 7–10%, whereas simultaneous lines usually show bowing values of only 5–8%. During the above-mentioned trials at the Brückner laboratory line, the Bowing could be reduced to a minimum. The possible MD retardation is an additional degree of freedom to adjust bowing and shrinkage on the end film. It can be expected that the demand for BOPA will continue to grow, especially in Asia, and also that the demand for higher quality film (i.e. shrinkage characteristic) will gain importance in the future. This will have an impact on the range of suitable stretching equipment. Also some trends for the future will be a further downgaging and improvements of the barrier properties.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 137
7/1/11 9:11:00 AM
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 138
7/1/11 9:11:00 AM
TD
225°
315°
45°
TD
TD
315°
MD
11.11 Polar shrinkage diagrams of simultaneous produced film.
MD
0.5 0.0
225°
1.0
0.5 0.0
2.0 1.5
1.0
2.0 1.5
135°
MD
2.5
MD
2.5
Simultaneous centre
Simultaneous edge
135°
45°
TD
TD
225°
315°
MD
0.5 0.0
1.0
2.0 1.5
MD
2.5
Simultaneous edge
135°
45°
TD
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 139
7/1/11 9:11:00 AM
TD
225°
315°
45°
TD
TD
315°
MD
0.0
135°
TD
225°
ratio: 3.25 3.0 (Relax = 8%) 3.4 3.1 (Relax = 9%)
MD
0.5 0.0
1.5
2.0 1.0
TD
315°
0.5
Stretching MD draw ratio max: MD draw ratio out: TD draw ratio max: TD draw ratio out:
225°
45°
2.5
MD
LISIM® edge
1.0
2.0 1.5
11.12 Polar shrinkage diagrams of simultaneous Brückner’s laboratory line.
MD
0.5 0.0
1.0
2.0 1.5
135°
MD
2.5
MD
2.5
LISIM® centre
LISIM® edge
135°
45°
TD
140
Biaxial stretching of film Sequential BOPA line Mechanical simultaneous BOPA line
∑ Bowing 5–8% ∑ Uneven shrinkage ∑ Uneven properties
LISIM®–BOPA line
∑ Bowing <1% ∑ Low and even shrinkage <0.5% ∑ Equal distribution of properties
11.13 Significant reduction of bowing with LISIM® Technology.
11.4
Sources of further information and advice
L Bottenbruch, R Binsack (eds): Polyamide, Kunststoff-Handbuch Band 3/4: Technische Thermoplaste. Hanser, 1998. H-G Elias: Makromoleküle. Bd. 2 – Technologie. 5. Auflage. Hüthig & Wepf Verlag, 1992. J Nentwig: Kunststoff-Folien: Herstellung – Eigenschaften – Anwendung 3. Hanser 2006. L Bottenbruch, R Binsack: Technische Thermoplaste 4. Polyamide, Hanser, 1998. Paper, Film & Folie Converter; Rush to BOPA, http://pffc-online.com/mag/bopa_ rush_0405/ M Takashige et al., Thickness uniformity of double bubble tubular film; Intern. Polymer Processing XIX, 2004.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-11.indd 140
7/1/11 9:11:00 AM
12
Fresh-cut produce packaging and the use of biaxial stretched films
C . F . F o r n e y and E . S . Y a g a n z a, Agriculture and Agri-Food Canada, Canada Abstract: Fresh-cut fruits and vegetables are highly perishable and packaging is a critical component to preserve product quality and shelf-life. Biaxial stretched films are used extensively for the packaging of fresh-cut produce and this chapter discusses the principles underlying proper package design. Gas permeation properties of the package must be balanced with the respiration rate of the fresh-cut product to establish and maintain a favourable atmosphere composition that will preserve product quality. The interaction of fresh-cut produce with the package environment and strategies to preserve quality are discussed. Key words: fresh-cut fruits and vegetables, modified atmosphere packaging, active packaging, microperforations, shelf-life, produce quality and safety.
12.1
Introduction
Fresh-cut produce comprises any fruit or vegetable that has been cut, peeled or physically altered in some manner but remains in a fresh state. Fresh-cut processing adds value to the product by making the utilization and consumption of fresh produce more convenient for the consumer. In North America, fresh-cut produce currently makes up about 15% of the fresh produce market and has grown substantially in the past two decades (Cook, 2007). Fresh-cut produce sales in the United States in 2007 were estimated to be $15.9 billion compared with $6 billion in 1999 (Cook, 2007). Of these sales, about 60% supply the food service industry, while 40% are directly purchased by consumers at retail outlets. The fresh-cut industry expanded rapidly in the 1980s with the marketing of fresh-cut lettuce and is now offering an ever-widening selection of vegetables and fruits. New products are continually being added as new processing technologies are developed to ensure their quality and safety. In the United States, nearly half of retail sales are packaged salads with the remainder being split between other vegetables and fruits (Cook, 2007). Of the other vegetables nearly half are carrots, with fresh-cut/baby carrots now accounting for nearly 80% of all carrots consumed (Lucier and Lin, 2007). Fresh-cut fruits are dominated by watermelon (46%) and mixed fruits (21%) (Cook, 2007). Packaging is critical for the handling and marketing of fresh-cut fruits 143 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 143
7/1/11 12:52:19 PM
144
Biaxial stretching of film
and vegetables. Fresh-cut produce is subject to quicker physiological, biochemical and microbiological deterioration resulting in a shorter market life than whole intact fresh fruits and vegetables (Brecht, 1995). Most fresh-cut fruits and vegetables have a shelf-life of only 8–10 or 12–14 days, respectively (Garrett, 2002). Consequently, fresh-cut products require very adapted combination of packaging and handling practices in order to maintain their freshness and safety standards, while providing convenience for the busy consumers of today. Many fresh-cut products are packaged using biaxial stretched films. The diverse properties of these films provide many possibilities for functional, attractive and convenient packages for fresh-cut fruits and vegetables. A variety of flexible bags and pouches, as well as trays with sealed film lids or over-wraps can incorporate custom permeation properties of these films. In addition, features to preserve product quality and/or add convenience for the consumer can be incorporated into the package design. However, the package design must be appropriate for the product it contains as well as the target market. The package must perform many functions. These include maintaining the quality of the product by providing physical protection and a favourable atmosphere composition. The package also helps to ensure the safety of the product by providing a barrier to contamination and may provide antimicrobial properties. In this context, the advent of biaxial stretched films has facilitated the quality maintenance and value of fresh-cut fruit and vegetable products. In this chapter, we will discuss the importance, application and limitations of biaxial stretched films for the quality preservation of fresh fruits and vegetables.
12.2
Quality factors determining shelf-life
The quality of fresh fruits and vegetables is determined by many factors, including produce appearance, texture, aroma, flavour, nutritional value and perceived safety (Hu and Jiang, 2007). All of these factors are dependent on the initial quality of the product, and are subject to change during processing, handling and marketing.
12.2.1 Sensory quality The sensory quality of fresh-cut fruits and vegetables determines consumer satisfaction and must be maintained during marketing through proper processing and packaging. Appearance is related to the colour, shine, size and shape of the product. Texture relates to whether the produce is turgid, firm, crisp or soft and its appreciation depends on the type of product. Both appearance and texture are important attributes that dictate the attractiveness
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 144
7/1/11 12:52:20 PM
Fresh-cut produce packaging and the use of biaxial stretched films
145
and acceptance by the consumer. They can be affected by maturity, decay, discoloration, dehydration or physical damage, which may occur during processing or marketing. Flavour is the perception of the diverse chemistry of the product that includes sugars, acids, phenolics and a diverse array of volatile compounds. While appearance will often determine an initial purchase, flavour ultimately determines consumer satisfaction and repeat purchases. Flavour attributes are dynamic and can change due to exposure to adverse environments within the package, interaction with packaging materials, or as a response to adverse handling or decay (Forney, 2008).
12.2.2 Nutritional value The nutritional value of fruits and vegetables has received increased attention in recent years and their consumption is widely recognized as an important part of a healthy diet. Fruits and vegetables are important sources of vitamins C, A and B6, thiamine, niacin, minerals and dietary fibre. They are also sources of flavonoids, carotenoids, polyphenols and other phytonutrients known to reduce the risk of various diseases (Kader, 2002). Nutritional value varies greatly among commodities and within cultivars of each commodity. The nutritional value of the product is not readily apparent to the consumer and is not normally accounted for during the assessment of the product shelf-life. However, nutritional value can be lost if the product is not packaged and handled properly, affecting consumer acceptance and company credibility. The postharvest loss of nutrients such as vitamin C and lycopene in a variety of fruits and vegetables can be substantial, but proper temperature management and packaging have been shown to reduce these losses (Rojas-Graü et al., 2009).
12.2.3 Microbiological quality The processing of fresh-cut produce can encourage the growth of microorganisms, (Leistner and Gould, 2002). Cutting exposes tissues to microorganisms and promotes cellular nutrient leakage that favours microbial growth. Cellular damage that occurs during processing also increases the rate of product senescence and reduces its resistance to microbial spoilage (Artés et al., 2007). Microbial growth may cause browning, production of off-odours, loss of texture, and the development of soft rots and decay. Fruit products can be fermented by lactic acid bacteria or yeasts, resulting in the production of acids, alcohol and CO2. Processing also may introduce contamination of human pathogens, which is a safety concern. To minimize microbial spoilage and reduce the risk of contamination, raw materials being processed must be of high quality and free of decay. In addition, processes must be in place to suppress microbial growth, reduce
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 145
7/1/11 12:52:20 PM
146
Biaxial stretching of film
microbial load during processing, and prevent post-processing contamination (Artés and Allende, 2005). Some processing strategies to minimize microbial load and help ensure safety of fresh-cut products were recently reviewed by Artés et al. (2009). They include treating products and equipment with sanitizing agents such as chlorine (NaClO, ClO2), peroxyacetic acid, hydrogen peroxide, citric acid, ascorbic acid, and electrolyzed water. Other treatments that may reduce microbial load include mild heat, ozone, UV–C light, and intense light pulses. However, none of these intervention strategies assures the elimination of pathogens from fresh produce. Therefore, the industry must focus on the prevention of contamination of fresh produce with human pathogens to assure that these products are safe and wholesome for human consumption. The use of packaging films can affect microbial growth and consequent spoilage and health risk in various ways. First, it represents a physical barrier to microbial contamination. Second, establishment of modified atmosphere in the package may inhibit respiration, slow senescence and consequently decrease the potential for microbial development. However, establishment of modified atmospheres in the package combined with cold storage may eliminate competition of mesophilic aerobic microorganisms, favouring psychrotrophic microaerophiles such as Listeria monocytogenes, Samonella sp. and Escherichia coli O157:H7. Hence, the consumer remains at risk of food-borne illness if the produce was initially contaminated.
12.3
Respiration and metabolism
Fruits and vegetables are metabolically active and therefore continue to respire and perform other processes that sustain life. Altering respiration and other metabolic processes may impact product quality. Respiration is the process through which living cells utilize atmospheric oxygen to degrade carbohydrates, fats and other substrates to produce energy and other molecules to maintain tissue health (Brandenburg and Zagory, 2009). End products of respiration include cellular energy, CO2, water and heat; its rate is usually measured through O2 consumption or CO2 production. Under normal aerobic conditions, molecular consumption of O2 is equal to the emission of CO2 resulting in a respiration quotient of 1. The respiration rate is generally used as an indicator of the overall metabolism and produce with high respiration have been associated with short shelf-life (Saltveit, 2004). Respiration rates are dependent on produce type, processing and environmental conditions (Saltveit, 2004). For example, when measured at 20 °C, respiration rates of onions, cranberries and kiwifruit averaged <20 mg CO2 kg–1 h–1 compared with bananas, asparagus and broccoli that had rates of 270–300 mg CO2 kg–1 h–1 (Gross et al., 2004). Postharvest handling may also affect respiration. Physical handling of produce that causes bruising
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 146
7/1/11 12:52:20 PM
Fresh-cut produce packaging and the use of biaxial stretched films
147
or other mechanical damage may result in increased respiration (Kader, 2002). Mechanical stress of cutting increases rates of respiration 1.2- to 7-fold, which depends on the extent and nature of the cutting process (Hu and Jiang, 2007). This increase in respiration may be transient or sustained depending on the commodity. Product temperature also has a strong effect on respiration rate. Reducing storage temperature down to near 0 °C minimizes respiration in most cases (Brandenburg and Zagory, 2009). In general, when temperature increases by 10 °C, respiration rate will increase 2 to 4 times (Saltveit, 2004). Other factors that impact respiration rates include: cultivar differences, physiological state or ripeness of the product, and storage or marketing duration. For example, following harvest, respiration rates of green asparagus and head lettuce held at 0 °C declined about 50% over the first 3 to 5 days of storage (Lipton, 1990; Morris et al., 1955). This variability in rates of respiration presents a challenge when attempting to design passive packages to maintain beneficial package atmospheres. When the O2 concentration in the atmosphere is reduced to values <5% with a concomitant increase of CO2, normal respiration may be suppressed. However, if O2 concentration becomes too low or produce is exposed to an injurious concentration of CO2, anaerobic respiration (fermentation) will occur (Saltveit, 2004). Fermentation can result in the production of ethanol, acetaldehyde, ethyl acetate and other molecules that may result in off-odours or off-flavours or the development of physiological breakdown of the product (Forney, 2008). This can result in the loss of freshness and reduction or termination of shelf-life. Cutting may also alter metabolism as a result of leakage of cellular components, induction of wound healing reactions, and in response to induces stress. Tissue browning, which can be a major defect of freshcut fruits and vegetables, arises from loss of cellular compartmentation between the phenolic compounds and polyphenol oxidase, an enzyme that catalyzes browning reactions. Cutting and exposure to oxygen can lead to carbohydrate degradation, lipid oxidation and increased water loss. Cutting can also stimulate membrane breakdown through the lipoxygenase pathway (Karakurt and Huber, 2003), which can lead to altered aroma production and stress metabolism.
12.4
Package atmosphere modification
Modified atmosphere packaging (MAP) is a primary technology to maintain the quality of fresh-cut produce and extend its shelf-life. The process of respiration of fresh-cut products alters the atmosphere composition within the package, affecting concentrations of O2, CO2, water vapour and other volatile compounds that affect product physiology and quality. Package design can facilitate the establishment and maintenance of beneficial atmospheres.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 147
7/1/11 12:52:20 PM
148
Biaxial stretching of film
The response of the product to altered concentrations of O2, CO2, water vapour and other volatile compounds is variable and will dictate the design of the package (Ben-Yehoshua et al., 2005). Recommendations for optimum concentrations of O2 and CO2 for shelf-life retention of a variety of freshcut fruits and vegetables are listed in Table 12.1. These values demonstrate the diverse response of different commodities to atmosphere composition and provide an initial target atmosphere for MAP design. However, some deviation from these recommendations may be desirable or necessary to accommodate variability in commodity response, preservation of specific quality traits, or limitations in packaging technologies.
Table 12.1 Suggested optimum atmosphere compositions for a variety of fresh-cut fruits and vegetables Commodity
Target atmosphere (kPa)
O2
CO2
Apple, sliced Beets, red, cubed Broccoli, florets Cabbage, shredded Cabbage, Chinese, shredded Cauliflower, florets Cantaloupe, cubed Carrot, shredded Celery, diced Fennel, diced Galega kale, shredded Garlic, peeled Honeydew, cubed Jicama, sticks Kiwifruit, sliced Leek, sliced Lettuce, chopped or shredded Mango, cubed Onions, diced Orange, sliced Peach, sliced Pear, sliced Persimmon, sliced Pineapple, sliced Pomegranate, arils Strawberry, sliced Tomato, slices Watermelon, sliced
<1 5 1–2 5–7 5 2–5 3–5 3–5 5 4–6 1–2 1 2 – 2–4 5 1–3 2–4 2 14–21 1–2 0.5 2 2 1–3 1–2 2 3
– 5 5–10 15 5 2–5 6–15 2–3 4 10–14 15–20 10 10 10 5–10 5 5–6 10 10 7–10 5–12 <10 12 10 5–10 5–10 20 3
Source: Artés et al., 2006, Barth et al., 2004, Beaulieu and Gorny, 2004
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 148
7/1/11 12:52:20 PM
Fresh-cut produce packaging and the use of biaxial stretched films
149
12.4.1 Oxygen Reduced concentrations of O2 can reduce oxidative browning, suppress ethylene perception, slow respiration and metabolism of the product, and inhibit microbial growth and spoilage. Concentrations of O2 below 3 kPa can inhibit browning of cut surfaces by inhibiting the enzyme polyphenyl oxidase that catalyzes the polymerization of phenols, which results in brown pigment formation. This effect has been especially beneficial in cut lettuce and other leafy vegetables, where low O2 atmospheres have substantially extended market life (McDonald et al., 1990). The production and action of ethylene, a plant hormone that stimulates ripening and senescence, can be reduced by O2 concentrations of 0.5–5 kPa (Beaudry, 2007). Similar O2 concentrations can also reduce respiration and general metabolic activity; however, this effect may have limited benefits in extending market life of fresh-cut products. Reduced O2 concentrations also affect microbial growth. Low O2 atmospheres slow the growth of Gram-negative, aerobic spoilage organisms such as Pseudomonas, but may enhance Gram-positive, microaerophilic bacteria such as Lactobacillus or Brothothrix (Labuza et al., 1992). Elevated concentration of O2 (>60 kPa), which can be obtained by flush packages prior to sealing, may also benefit shelf-life by reducing enzymatic browning and inhibiting the growth of some microorganisms (Jacxsens et al., 2001). While low concentrations of O2 in packages may be beneficial to fresh-cut produce market life, injurious levels of O2 must be avoided. If O2 concentrations become too low, anaerobic respiration can be induced. Anaerobic respiration or fermentation results in the production of ethanol as well as other volatile compounds that may result in off-odours and offflavours. In addition, excessively low concentrations of O2 may lead to physiological breakdown of the tissue, which may cause tissue softening and discoloration and facilitate microbial growth. The tolerance and response of produce to excessively low O2 concentration vary among commodities and must be considered when designing a package (Mir and Beaudry, 2004). In fresh-cut lettuce, commercial packages commonly have atmospheres of <1 kPa O2 (Cameron et al., 1995). While these atmospheres are effective in preventing browning, they do induce fermentation of the lettuce. However, the resulting off-odours are relatively mild and are accepted by consumers. In many other commodities, off-odours induced by fermentation are not acceptable. In broccoli, for example, O2 concentrations of 1 to 2 kPa slow yellowing (Bastrash et al., 1993), but if concentrations decrease below 1 kPa, broccoli produces a strong sulphury odour that renders it unmarketable (Forney et al., 1991). Critical levels of O2 that must be maintained to avoid these deleterious effects vary substantially among commodities and must be considered in package design.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 149
7/1/11 12:52:20 PM
150
Biaxial stretching of film
12.4.2 Carbon dioxide Elevated concentrations of CO2 can be effective in inhibiting microbial growth. Concentration >5 kPa are effective in inhibiting the growth of many spoilage bacteria (Hendricks and Hotchkiss, 1997) and concentrations >8 kPa are especially effective in inhibiting the growth of Botrytis, a fungus responsible for the decay of many fresh fruits and vegetables. Carbon dioxide concentrations of 3–5 kPa can reduce chlorophyll degradation in a variety of commodities including cabbage, broccoli, spinach, banana and apple (Beaudry, 2007). Elevated concentrations of CO2 are also effective in inhibiting the action of ethylene, and therefore slow the senescence of fruits and vegetables. However, high concentrations of CO2 may cause injury to fresh fruits and vegetables and their tolerance varies widely (Mir and Beaudry, 2004). Tolerance limits for CO2 range from 2 kPa for lettuce and pears to 25 kPa for strawberries, raspberries and blackberries. However, tolerance varies depending on temperature, duration of exposure and physiology of the product. Depending on the commodity, excessive concentrations of CO2 can cause discoloration and softening, and induce fermentation.
12.4.3 Water vapour Maintaining high concentrations of water vapour in the package can minimize wilting and shrivelling of the product (Ayala-Zavala et al., 2008). In addition, minimizing water loss and the resulting stress can slow senescence and maintain resistance to microbial breakdown (van den Berg, 1981). However, fluctuations in temperature, which commonly occur during marketing, may cause condensation to form. This typically results in the formation of water droplets on film packaging when its temperature drops below the dew point of the package atmosphere. The resulting free water that may accumulate on the product can promote microbial growth and product decay (Grierson and Wardowski, 1978).
12.4.4 Other volatiles In addition to O2, CO2 and water vapour, many other volatile compounds are present in the package atmosphere that may affect product quality. Ethylene is produced by ripening fruit and its production may be induced in both fruits and vegetables by physical stress imparted by fresh-cut processing (Brecht, 1995). Ethylene accumulation in the package may enhance deterioration of the product including chlorophyll loss, texture changes, compositional changes and membrane deterioration. When ethylene action in apple slices was inhibited through treatment with 1-methylcyclopropene (1-MCP), apple slices were firmer and had reduced rates of respiration and ethylene
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 150
7/1/11 12:52:20 PM
Fresh-cut produce packaging and the use of biaxial stretched films
151
production compared to untreated fruit (Perera et al., 2003). Other volatile compounds that may affect product physiology or microbial growth, or contribute to aroma, may interact with the product package. However, the properties of many of these compounds are poorly characterized.
12.5
Packaging methods and quality maintenance
The properties of the package design and its materials contribute to the development of a beneficial atmosphere composition within the package. These properties include the permeability of the packaging film, the film thickness, size of the package including the film area, inclusion of any active components such as gas adsorbing or emitting sachets, and the quantity and physiology of the product it will hold. The respiration rate of the product drives atmosphere modification and, as discussed previously, environmental conditions, degree of processing and natural variability affect respiration rates and therefore the resulting package atmosphere composition.
12.5.1 Gas permeability The permeability of polymer packaging films to O2, CO2, water vapour and other volatiles depends on the polymer composition of the film, the condition under which the film was extruded, the presence of any perforations in the film, and the environmental conditions including temperature and humidity. Solid films A variety of polymer films having a range of gas permeabilities are used for the packaging of fresh-cut produce. These films can consist of individual polymers or combinations such as blended monolayers, coextruded or laminated films (Brandenburg and Zagory, 2009). Polyolefins are the dominant films used for packaging of fresh-cut produce. These films are relatively inexpensive and have high permeabilities to O2 and CO2, but low water vapour permeability (Massey, 2003). Adequate permeability to these respiratory gases is critical to establish and maintain favourable atmosphere composition in produce packages. Gas transmission across a solid film is determined by the classical permeability equation based of Fick’s first law of diffusion as follows:
Jgas = P(A Dpgas/l)
[12.1]
where Jgas is the total flux of gas (cm3/day), P is the permeability of the film (cm3 mm/m2 day atm), A is the area of the film (m2), Dpgas is the partial pressure difference across the film (atm), and l is the thickness of the film (mm). Therefore, the gas transmission across a film increases linearly
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 151
7/1/11 12:52:20 PM
152
Biaxial stretching of film
with increased surface area and/or increased partial pressure difference. The O2 transmission rate (OTR) typically provided for packaging films by manufacturers is expressed in cm3/m2/day (cm3/100 in2/day in US standard units). These rates are determined under standard test conditions of 23 °C and 0% relative humidity (RH). The CO2 transmission rate (CO2TR) through solid films is 2 to 5 times greater than the OTR depending on the film composition (Brandenburg and Zagory, 2009). This means that equilibrium atmospheres in solid film MAP tend to have low concentrations of O2 accompanied by relatively low concentrations of CO2. A concern with using published OTR and CO2TR in package design is the change in rates at different temperatures and RH. Beaudry (2007) described the relationship between temperature and permeability of a solid film using the Arrhenius equation. The energy of activation in this equation describes the response of gas permeation to temperature and varies among different films. The rate of gas permeation decreases as temperature decreases for most films (Beaudry, 2007). For a low-density polyethylene (LDPE) film, this rate was 2.5-fold greater at 15 °C than at 0 °C (Cameron et al., 1994). However, the energy of activation for O2 consumption of fresh-cut fruits and vegetables tend to be greater than that of most films (Beaudry, 2007). Since packaged fresh-cut produce are typically held at temperatures of 0–4 °C, published OTRs are not appropriate for package design. If the energy of activation is known for a film, the OTR for the intended storage temperature can be calculated using the Arrhenius relationship. However, in addition to temperature, RH in the package can also alter gas transmission rates. This effect can vary depending on the chemistry of the polymer and its interaction with water vapour. Since fresh produce generates a nearly saturated RH in the package, OTR can be substantially modified. This may result in further alteration of expected gas permeation rates. Therefore, both OTR and CO2TR of packaging films should be determined at the temperature and RH in which the packaged product will be stored and marketed. Perforations To obtain higher gas permeability in packages for commodities with high respiration rates or those that will not tolerate low O2 atmospheres, perforations may be used. Microperforation (40–200 mm) or macroperforations (>200 mm) can be added to a package to facilitate gas exchange. Microperforations are produced using electrostatic discharge or lasers, while macroperforations are produced using hot or cold needles or punches (Ben-Yehoshua et al., 2005). In addition, micropores can be produced during film production by adding inert inorganic materials such as CaCO3 or SiO2 to the polymer resulting in pores ranging from 0.14 to 1.4 mm in diameter (Mizutani, 1989). The rate of gas movement through perforations is proportional to the
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 152
7/1/11 12:52:20 PM
Fresh-cut produce packaging and the use of biaxial stretched films
153
difference in partial pressures across the film similar to transmission through solid films. However, the rate of gas exchange through pores is much greater than a solid film. Beaudry (2007) estimated that on a per area basis, a perforation permits 4 to 20 million times the flux of O 2 or CO2 compared with a solid 50 mm thick low density polyethylene (LDPE) film and he estimated that the gas exchange through one 100 mm diameter perforation in this film was equivalent to that in 400 cm2 of the film. Therefore, to predict gas exchange through microperforations the holes must be uniform and of known size. However, this is often not the case and commercially produced microperforation can be variable and irregular in size and shape (Allan-Wojtas et al., 2008). Oxygen and carbon dioxide exchange rates through a perforation are approximately 1:1 in contrast to solid films that have ratios of 1:2 to 1:5. This makes the use of perforations preferable when high concentrations of CO2 are beneficial or can be tolerated. In addition, gas transmission through perforations is affected little by temperature. An increase in temperature from 0 to 15 °C will increase the rate of gas flux by only 11% (Beaudry, 2007). The addition of perforations to a package poses the risk of allowing the product to become contaminated by foreign material. It is obvious that the pore diameters (40–200 mm) of microperforated films are substantially higher than the diameter of most pathogenic bacteria (generally <10 mm) and that produce contamination can occur through film perforations. However, in most cases this risk appears to be small, when the produce is handled properly in environments with low contamination. Piergiovanni et al. (2003) reported that small macroperforations, ranging from 220 to 1350 mm in diameter, provided a low risk for contamination of bread by artificial sweat or saliva and that the risk was reduced with reduction of perforation size and frequency. Perforations can be incorporated into all types of films whether they are barrier or permeable. If permeable, the gas transmission properties of the film used with perforations must also be considered in calculating the total permeation properties of the package. If macroperforations or numerous microperforations are used in a package, O2 and CO2 exchange will maintain a near ambient atmosphere in the package, while maintaining a high humidity.
12.5.2 Package design To successfully achieve beneficial MAP, the package must be properly designed, taking into consideration the physiology of the product to be packaged. Respiration of a packaged product will consume O2 within the package and produce CO2 (Fig. 12.1). As the product modifies the atmosphere
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 153
7/1/11 12:52:20 PM
154
Biaxial stretching of film
within the package, the concentration gradients of these gases increase, driving a net flux of gases in and out of the package. If the permeability of the package is adequate, the atmosphere composition within the package will reach a steady-state in which the package atmosphere composition remains constant and the rate of O2 consumption and CO2 production of the product is equal to the permeation rate of these gases through the package. Figure 12.1 shows how steady-state atmospheres develop in both solid film and perforated packages of diced red onions after about 7 to 10 days. The time required to reach a steady-state atmosphere is dependent on the headspace volume of the package and the respiration rate of the product. A steady-state will be reached more rapidly with packages having low headspace volumes and products with high respiration rates. For a steadystate atmosphere to be maintained the respiration rate of the product must remain constant. This process of package atmosphere modification is referred to as passive MAP since it is dependent solely on the respiration of the product and the permeation properties of the package. Several models have been published that describe this relationship of product respiration, package permeability and the environment (Cameron et al., 1989; Emond et al., 1991; Fishman et al., 1996; Fonseca et al., 2002; Tanner et al., 2002). To develop a MAP for fresh-cut produce, a target steady-state package atmosphere must be determined. Target atmosphere compositions must consider both the potential benefits of an optimum atmosphere composition and the risks of developing an injurious composition (Artés et al., 2006;
O2 solid film
Respiratory gas (kPa)
25
CO2 solid film O2 perforated
20
CO2 perforated
15
10
5
0 0
5
10 Days at 4.5 °C
15
20
12.1 Change in atmosphere composition of passive modified atmosphere packages containing red diced onions during storage at 4.5 °C. Packages consisted of a sealed solid film polyethylene bag and a perforated bag having one 0.7 mm perforation.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 154
7/1/11 12:52:20 PM
Fresh-cut produce packaging and the use of biaxial stretched films
155
Forney, 2007). Target atmospheres will vary for each commodity being packaged. Recommended atmospheres (Table 12.1) are a starting point in determining target atmospheres; however, in many cases packaging systems may not be capable of obtaining these atmosphere compositions (Mir and Beaudry, 2004). For example, blueberries tolerate high concentrations of CO2 and concentrations >10 kPa are effective in inhibiting decay, while low O2 concentration provide little benefit and may induce off-flavour (Forney, 2009). Therefore, there is a potential to develop microperforated packages that could develop high CO2 atmospheres while maintaining high concentrations of O2. Low concentrations of O2 (< 1 kPa) are effective in preventing browning of fresh-cut lettuce and fermentation and slight offodours that occurs in these atmospheres have been found to be acceptable to consumers (Cameron et al., 1995). Therefore, cut lettuce is commercially packaged in sold film packages designed to maintain low O 2 atmospheres. High concentrations of CO2 and humidity slow yellowing of broccoli florets, but low O2 can induce the production of extreme off-odours (Forney et al., 1991). Therefore, commercial packaging of broccoli will often use perforations to minimize the risk of low O2 atmospheres. The possible steady-state package atmospheres obtainable with passive MAP are limited by the permeation properties of solid films and perforations. Figure 12.2 illustrates the limitations of steady-state atmospheres that can be obtained using existing solid packaging films and perforations. The atmospheres in the area between the 1:1 perforated line and the 1:5 solid
1:1 (perforated film) 1:2 (solid film) 1:5 (solid film, LDPE)
Carbon dioxide (kPa)
20
15
10
5
0 0
5
10 Oxygen (kPa)
15
20
12.2 Potential theoretical steady-state package atmospheres in packages composed of a perforated film or solid films having typical permeation rates with O2:CO2 ratios of 1:3 to 1:5. Produce respiration is assumed to be aerobic with a respiration quotient near 1.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 155
7/1/11 12:52:20 PM
156
Biaxial stretching of film
film line are theoretically possible through passive MAP using existing film technologies and assuming aerobic respiration is maintained. However, some beneficial atmospheres are not possible through this system. Several additional factors may limit the use of passive MAP on many products. These include the need for a constant and predictable respiration rate, and variable temperatures during transportation and marketing. Variability in product respiration rates is common and may result from cutting, variation among cultivars, product age, interactions with atmosphere composition, biological variation and effects of handling (Fonseca et al., 2002). As discussed earlier, temperature generally has a greater effect on rates of respiration than on package permeability. Therefore when temperature changes there is a shift in the steady-state atmosphere composition (Exama et al., 1993). Changes in temperature are not uncommon during the marketing of freshcut produce. Gorny (2005) reported that the average shelf temperature for fresh-cut produce is around 7 °C, while those held on ice may exceed 10 °C. These temperatures are well above the optimum holding temperature for which the package may be designed.
12.5.3 Active modified atmosphere packaging (MAP) To address some of the limitations of passive MAP, active packaging technologies are being developed. An active package can be defined as a packaging system that has additional features to improve the quality, shelf-life and/or safety of the product (Gavara et al., 2009). Active features may control the concentration of gases such as O2, CO2, water vapour or ethylene, as well as control microbial growth within the package. Methods used to achieve active packaging include gas flushing, the incorporation of active agents within the package or package material, and interactive mechanisms to control the package atmosphere. The latter may respond to environmental changes such as temperature or atmosphere composition, or to physiological changes in the product, such as the evolution of volatile compounds such as ethanol or ethylene. Flushing packages with a desirable gas mixture can achieve rapid establishment of a modified atmosphere. Flushing displaces the air in the package, often following evacuation, with the desired atmosphere composition. Normally the package is designed to maintain this atmosphere once established. Flushing is often used with highly perishable commodities such as cut salad greens, where rapid reduction of oxygen concentration may be critical to maintain quality (Ben-Yehoshua et al., 2005). In addition, other beneficial gasses can be introduced to the package. Supra-concentrations of oxygen (>60 kPa) have some inhibitory effects on microbial growth, while reducing browning and the risk of anaerobic metabolism (Oms-Oliu et al., 2009; Wang, 2006). Flushing with other gasses such as argon or nitrous
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 156
7/1/11 12:52:20 PM
Fresh-cut produce packaging and the use of biaxial stretched films
157
oxide may help to retain firmness and color and reduce browning in cut fruit (Wang, 2006). Devices can be added to the produce package in the form of sachets, pouches or pads. These devices may contain a variety of substances that can absorb or release gases for regulating atmosphere composition and product quality (Brody, 2005; Ozdemir and Floros, 2004). Numerous compounds and formulations can be used to provide controlled rates of adsorption or release of gases depending on the purpose. They can be used to regulate O2 and/or CO2 concentrations in the package, or to remove free water and regulate package humidity. Sachets can also be designed to adsorb physiologically active volatiles such as ethylene or odour-active compounds. In addition, sachets can release volatile antimicrobial or flavour compounds. Some examples of atmosphere modifying devices include the use of sachets containing potassium permanganate or activated carbon and PbCl to remove ethylene from package atmospheres. These systems have been reported to prolong the storage life of kiwifruit, banana, persimmon and avocado fruit, delay softening of fresh-cut kiwifruit and bananas, and delay yellowing of spinach (Abe and Watada, 1991; Sacharow, 1988). Antimicrobial compounds can also be released from sachets to inhibit decay and microbial growth. Toivonen and Lu (2007) described a sachet that generated CO2 and ethanol that was effective in delaying decay of fresh-cut fruit. Other natural volatile compounds such as hexanal, 2-(E)-hexenal, hexyl acetate and some essential oils have also been shown to have antimicrobial properties when released in MAP of fresh-cut produce (Lanciotti et al., 2004). Almenar et al. (2007) describe the design of a device for the slow release of these natural antimicrobial volatiles within a package. Sachets also can be designed to reduce off-odours or enhance aroma. Clay absorbent packaged with fresh diced onions reduced off-odours and maintained fresh aroma of the product (Toivonen, 1997). Sachets also can be designed to release flavour compounds (Ozdemir and Floros, 2004) or flavour precursors (Song et al., 1996) to enhance the aroma of fresh-cut produce. A variety of active substances have been incorporated into packaging films to alter the package environment. Films containing ethylene absorbers such as zeolite or potassium permanganate have been produced (Gavara et al., 2009); however, their capacity to effectively lower package ethylene concentrations for many fresh-cut products may be limited. Another common film additive is antifog agents. The addition of agents such as ethoxylate or monoglycerides to the inner surface of the package can help to prevent fogging of the film and droplet formation (Gavara et al., 2009). A variety of antimicrobial compounds can also be incorporated into films (Ozdemir and Floros, 2004), but their effectiveness in reducing microbial growth may be limited owing to limited surface contact of packaging films with fresh-cut produce (Brandenburg and Zagory, 2009).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 157
7/1/11 12:52:20 PM
158
Biaxial stretching of film
Other features being developed to modulate gas transmission properties of the package include pores that open to increase gas transmission in response to a rise in temperature (Cameron et al., 1995). Similarly, pores may open to vent the package in response to changes in O2, CO2 or ethanol concentrations inside the package. Incorporation of biosensors that respond to physiological changes occurring in the product also could help to regulate package gas exchange in the future.
12.5.4 Flavour interactions Flavour is an important quality parameter that is often not considered when optimizing packaging for fresh-cut produce. Poor flavour is one of the main causes of failure for fresh-cut fruit products in the marketplace (Gorny, 2005). Packaging could contribute to flavour loss through the development of injurious atmospheres or by enhancing flavour loss (Forney, 2008). As mentioned previously, injurious concentrations of O2 and CO2 can induce fermentation and the formation of off-flavours. Even during the short period fresh-cut fruit is exposed to modified atmospheres, atmosphere composition can impact flavour synthesis and retention. When ‘Gala’ apple slices were held in MAP that developed low (3 kPa) or high (16 kPa) O2 concentrations in combination with 6 kPa CO2, slices held in the high O2 had superior fruity flavour than fruit in the low O2 atmosphere (Cliff et al., 2010). This is in contrast to the recommendation for low O2 atmospheres (Table 12.1), which is most likely based on maintaining visual appearance. Greater attention needs to be given to the effects of ‘beneficial’ atmospheres on the retention of product flavour. In addition to atmosphere effects on flavour metabolism, packaging materials may have a direct effect on the retention or loss of flavour of fresh-cut fruits and vegetables. Fresh-cut processing removes natural barriers that serve to retain volatile flavour compounds. When these barriers are removed there is an increased diffusion of volatile compounds into the package atmosphere surrounding the product (Del-Valle et al., 2004). The concentration gradient between the package atmosphere and the product will drive the rate of flavour loss. The chemical and physical properties of the packaging film can reduce or enhance the loss of flavour volatiles. Volatile loss can be enhanced if the film has a high affinity for the volatile, which can result in sorption of the volatile compounds onto the film or permeation through the film (Brody, 2002; Nielsen and Jägerstad, 1994). Polymer materials used for packaging have variable affinities for flavor volatiles (Fayoux et al., 1997a,b). Non-polar polyolefins, including polyethylene and polypropylene, were found to cause the largest losses of d-limonene from orange juice (Fayoux et al., 1997a). Rates of sorption of other compounds decreased with increasing polarity and affinity was greatest with hydrocarbons followed by ketones, esters,
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 158
7/1/11 12:52:20 PM
Fresh-cut produce packaging and the use of biaxial stretched films
159
aldehydes and alcohols (Fayoux et al., 1997b). Sorption also increases with molecular size and branching. Therefore, the differential affinity for different volatile compounds may alter volatile profiles, thus affecting flavour. Low temperatures and high humidities found in MAP may also enhance flavour loss since sorption of volatiles from the gas phase increases with reduced temperature (Paik and Writer, 1995) and high RH (Fayoux et al., 1997b). Permeation of volatiles through the film also may enhance loss (Mount and Wagner, 2001). Permeation rates are dependent on many of the same properties that determine rates of sorption. In addition, as with respiratory gasses, perforations in films can greatly enhance loss of flavour volatiles. Permeation of ethanol through a 60 mm thick polypropylene film increased 186-fold with the addition of three 100 mm pores (Del-Valle et al., 2004). While fresh-cut fruits and vegetables are routinely packaged in various polymer materials, the impact of these materials on flavour quality and retention is not clearly defined. Research is needed to determine the significance of sorption and permeability of different polymer materials on flavour. Understanding these process may lead to improved packaging that will ensure acceptable product flavour.
12.6
Future trends
Packaging technologies for the preservation and marketing of fresh-cut produce have made dramatic advancements in recent years. Improved technologies to regulate the O2, CO2 and water vapour permeation properties of packaging films have aided in the development of MAP for fresh-cut produce that develop more predictable steady-state atmospheres. However, owing to limitations in film properties, optimum package atmosphere compositions may not be obtainable for many products using passive MAP systems. In addition to limitations of packaging materials, variability in product biology and the market environment can impact respiration rates and thus alter target atmosphere composition. To overcome these limitations, further development of MAP technologies is needed. The development of new packaging films with broader ranges of O2, CO2 and water vapour permeation properties would help to accommodate the diverse requirements of the broad spectrum of fruits and vegetables on the market today. Increasing the temperature responsiveness of film gas permeability to match that of produce respiration rates would also aid in the development of more robust packages that could adapt to changing environmental conditions in the marketplace. Improving the size consistency and cost of microperforations would provide additional mechanisms to optimize package atmosphere composition. Further development of active and intelligent features in fresh-cut produce packaging holds promise to reduce risk and provide better assurance of
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 159
7/1/11 12:52:21 PM
160
Biaxial stretching of film
quality and market life. Packaging technologies that respond to changes in the storage environment and/or product physiology in order to maintain optimum package atmospheres hold promise for optimizing product market life. Future innovations in active packaging technologies have the potential to improve the control of microbial development and thus improve product quality and safety. Attention should be given to residuals of antimicrobials in the produce, since this has been a concern in recent years. In addition, package design and active features hold promise to maintain and possibly enhance the flavour quality of many fresh-cut fruits and vegetables. In the near future, multi-feature active packaging systems may be able to track and control several aspects of produce quality, and thus ensure consumer satisfaction.
12.7
Sources of further information and advice
Many resources on the postharvest handling and packaging of whole and fresh-cut fruits and vegetables are available on the internet. The United States Department of Agriculture has published Handbook 66 The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stock (http://www. ba.ars.usda.gov/hb66/), which has extensive information on the optimum storage conditions for a wide range of fresh produce as well as discussion on packaging and other postharvest technologies. The University of California Postharvest Technology Research & Information Center (http:// postharvest.ucdavis.edu/) and Postharvest Fresh at Sydney University, Sydney, Australia (http://www.postharvest.com.au/) both provide produce fact sheets and numerous publications on postharvest handling of fresh produce including modified atmosphere recommendations. Postharvest.biz (http:// www.postharvest.biz/) is an international directory of suppliers, equipment, materials and services that has information on the fresh-cut industry and packaging materials. In addition to these Internet resources, numerous books and technical articles have addressed the packaging and quality of fresh-cut produce, some of which are referenced in this chapter.
12.8
References
Abe K and Watada AE (1991), ‘Ethylene absorbent to maintain quality of lightly processed fruits and vegetables’, J Food Sci, 56, 1589–1592. Allan-Wojtas P, Forney CF, Molys L and Moreau DL (2008), ‘Structure and gas transmission characteristics of microperforations in plastic films’, Packaging Technol Sci, 21, 217–229. Almenar E, Del Valle V, Catala R and Gavara R (2007), ‘Active package for wild strawberry fruit (fragaria vesca l.)’, J Agric Food Chem, 55, 2240–2245. Artés F and Allende A (2005), ‘Minimal fresh processing of vegetables, fruits and juices’, in Sun DW, Emerging technologies in food processing, San Diego, CA, Elsevier Academic Press, 675–715. © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 160
7/1/11 12:52:21 PM
Fresh-cut produce packaging and the use of biaxial stretched films
161
Artés F, Gómez PA and Artés-Hernández F (2006), ‘Modified atmosphere packaging of fruits and vegetables’, Stewart Postharvest Rev, 5:2, 1–13. Artés F, Goméz PA and Artés-Hernández F (2007), ‘Physical, physiological and microbial deterioration of minimally fresh processed fruits and vegetables’, Food Sci Technol Intl, 13, 177–188. Artés F, Gómez P, Aguayo E, Escalona V and Artés-Hernández F (2009), ‘Sustainable sanitation techniques for keeping quality and safety of fresh-cut plant commodities’, Postharvest Biol Technol, 51, 287–296. Ayala-Zavala JF, Del-Toro-Sánchez L, Alvarez-Parrilla E and González-Aguilar GA (2008), ‘High relative humidity in-package of fresh-cut fruits and vegetables: advantage or disadvantage considering microbiological problems and antimicrobial delivering systems?’, J Food Sci, 73, R41–R47. Barth MM, Zhuang H and Saltveit ME (2004), ‘Fresh-cut vegetables’, in Gross KC, Wang CY and Saltveit ME, The commercial storage of fruits, vegetables, and florist and nursery stocks, USDA handbook 66, available from: http://www.ba.ars.usda.gov/ hb66/contents.html [accessed 8 July 2010]. Bastrash S, Makhlouf J, Castaigne F and Willemot C (1993), ‘Optimal controlled atmosphere conditions for storage of broccoli florets’, J Food Sci, 58, 338–341. Beaudry R (2007), ‘MAP as a basis for active packaging’, in Wilson CL, Intelligent and active packaging for fruits and vegetables, Boca Raton, FL, CRC Press, 37–55. Beaulieu JC and Gorny JR (2004), ‘Fresh-cut fruit’, in Gross KC, Wang CY and Saltveit ME, The commercial storage of fruits, vegetables, and florist and nursery stocks, USDA handbook 66, available from: http://www.ba.ars.usda.gov/hb66/contents.html [accessed 8 July 2010]. Ben-Yehoshua S, Beaudry RM, Fishman S, Jayanty S and Mir N (2005), ‘Modified atmosphere packaging and controlled atmosphere storage’, in Ben-Yehoshua S, Environmentally friendly technologies for agricultural produce quality. Baco Raton, FL, Taylor & Francis, 61–112. Brandenburg JS and Zagory D (2009), ‘Modified and controlled atmosphere packaging technology and applications’, in Yahia EM, Modified and controlled atmospheres for the storage, transportation, and packaging of horticultural commodities, Boca Raton, FL, CRC Press, 73–92. Brecht JK (1995), ‘Physiology of lightly processed fruits and vegetables’, HortScience, 30, 18–22. Brody AL (2002), ‘Flavor scalping: quality loss due to packaging’, Food Technol, 56, 124. Brody AL (2005), ‘What’s fresh about fresh-cut’, Food Technol, 59, 74–77. Cameron AC, Boylan-Pett W and Lee J (1989), ‘Design of modified atmosphere packaging systems: modeling oxygen concentrations within sealed packages of tomato fruits’, J Food Sci, 54, 1413–1416. Cameron AC, Beaudry RM, Banks NH and Yelanich MV (1994), ‘Modified-atmosphere packaging of blueberry fruit: modeling respiration and package oxygen partial pressures as a function of temperature’, J Amer Soc Hort Sci, 119, 534–539. Cameron AC, Talasila PC and Joles DW (1995), ‘Predicting film permeability needs for modified-atmosphere packaging of lightly processed fruits and vegetables’, HortScience, 30, 25–34. Cliff MA, Toivonen PMA, Forney CF, Lui P and Lu C (2010), ‘Quality of fresh-cut apple slices stored in solid and micro-perforated film packages having contrasting O 2 headspace atmospheres’, Postharvest Biol Technol, 58, 254–261.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 161
7/1/11 12:52:21 PM
162
Biaxial stretching of film
Cook R (2007), ‘Trends in the marketing of fresh produce and fresh-cut products’, University of California, Davis, Agricultural Marketing Resource Center. available from: http:// www.agmrc.org/media/cms/FreshCut2007TrendsNovUpdate_A2F33DBFFA097.pdf [accessed 7 July 2010]. Del-Valle V, Almenar E, Hernández-Muñoz P, Lagarón JM, Catala R and Gavara R (2004), ‘Volatile organic compound permeation through porous polymeric films for modified atmosphere packaging of foods’, J Sci Food Agric, 84, 937–942. Emond JP, Castaigne F, Toupin CJ and Desilets D (1991), ‘Mathematical modeling of gas exchange in modified atmosphere packaging’, Trans ASAE, 34, 239–245. Exama A, Arul J, Lencki R, Lee L and Toupin C (1993), ‘Suitability of plastic films for modified atmosphere packaging of fruits and vegetables’, J Food Sci, 58, 1365–1370. Fayoux SC, Seuvre AM and Voilley AJ (1997a), ‘Aroma transfers in and through plastic packagings: orange juice and d-limonene. A review. Part I: Orange juice aroma sorption’, Packaging Technol Sci, 10, 69–82. Fayoux SC, Seuvre AM and Voilley AJ (1997b), ‘Aroma transfers in and through plastic packagings: orange juice and d-limonene. A review. Part II: Overall sorption mechanisms and parameters – a literature survey’, Packaging Technol Sci, 10, 145–160. Fishman S, Rodov V and Ben-Yehoshua S (1996), ‘Mathematical model for perforation effect on oxygen and water vapor dynamics in modified-atmosphere packages’, J Food Sci, 61, 956–961. Fonseca SC, Oliveira FAR and Brecht JK (2002), ‘Modelling respiration rate of fresh fruits and vegetables for modified atmosphere packages: a review’, J Food Engin, 52, 99–119. Forney CF (2007), ‘New innovations in the packaging of fresh-cut produce’, Acta Hort, 746, 53–60. Forney CF (2008), ‘Flavour loss during postharvest handling and marketing of fresh-cut produce’, Stewart Postharvest Rev, 3:5, 1–10. Forney CF (2009), ‘Postharvest issues in blueberry and cranberry and methods to improve market-life’, Acta Hort, 810, 785–798. Forney CF, Mattheis JP and Austin RK (1991), ‘Volatile compounds produced by broccoli under anaerobic conditions’, J Agric Food Chem, 39, 2257–2259. Garrett EH (2002), ‘Fresh-cut produce: tracks and trends’, in Lamikanra O, Fresh-cut fruits and vegetables science, technology, and marketing, Boca Raton, FL, CRC Press, 1–10. Gavara R, Catalá R and Hernández-Muñoz P (2009), ‘Extending the shelf-life of fresh-cut produce through active packaging’, Stewart Postharvest Rev, 4:2, 1–5. Gorny JR (2005), ‘Leveraging innovative fresh-cut technologies for competitive advantage’, Acta Hort, 687, 141–147. Grierson W and Wardowski WF (1978), ‘Relative humidity effects on the postharvest life of fruits and vegetables’, HortScience, 15, 576–584. Gross KC, Wang CY and Saltveit ME (2004), ‘Summary of respiration and ethylene production rates’, in Gross KC, Wang CY and Saltveit ME, The commercial storage of fruits, vegetables, and florist and nursery stocks, USDA handbook 66, available from: http://www.ba.ars.usda.gov/hb66/contents.html [accessed 15 June 2010]. Hendricks MT and Hotchkiss JH (1997), ‘Effect of carbon dioxide on the growth of Pseudomonas fluorescens and Listeria monocytogenes in aerobic atmospheres’, J Food Prot, 60, 1548–1552.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 162
7/1/11 12:52:21 PM
Fresh-cut produce packaging and the use of biaxial stretched films
163
Hu W and Jiang Y (2007), ‘Quality attributes and control of fresh-cut produce’, Stewart Postharvest Rev, 2:3, 1–9. Jacxsens L, Devlieghere F, Van der Steen C and Debevere J (2001), ‘Effect of high oxygen modified atmosphere packaging on microbial growth and sensorial qualities of fresh-cut produce’, Int J Food Microbiol, 71, 197–210. Kader AA (2002), ‘Quality parameters of fresh-cut fruit and vegetable products’, in Lamikanra O, Fresh-cut fruits and vegetables: Science, technology, and market, Boca Raton, FL, CRC Press, 11–20. Karakurt Y and Huber DJ (2003), ‘Activities of several membrane and cell-wall hydrolases, ethylene biosynthetic enzymes, and cell wall polyuronide degradation during lowtemperature storage of intact and fresh-cut papaya (Carica papaya) fruit’, Postharvest Biol Technol, 28, 219–229. Labuza TP, Fu B and Taoukis PS (1992), ‘Prediction for shelf life and safety of minimally processed CAP/MAP chilled foods: a review’, J Food Prot, 55, 741–750. Lanciotti R, Gianotti A, Patrignani F, Belletti N, Guerzoni ME and Gardini F (2004), ‘Use of natural aroma compounds to improve shelf-life and safety of minimally processed fruits’, Trends Food Sci Technol, 15, 201–208. Leistner L and Gould G (2002), Hurdle technologies: Combination treatments for food stability, safety and quality, New York, Kluwer Academic/Plenum Publishers. Lipton WJ (1990), ‘Postharvest biology of fresh asparagus’, Hort Rev, 12, 69–155. Lucier G and Lin BH (2007), ‘Factors affecting carrot consumption in the United States’, Outlook Report from the Economic Research Service, VGS319–01. Massey LK (2003), Permeability properties of plastics and elastomers: A guide to packaging and barrier materials, Norwich, NY, Plastic Design Library. McDonald RE, Risse LA and Barmore CR (1990), ‘Bagging chopped lettuce in selected permeability films’, HortScience, 25, 571–673. Mir N and Beaudry RM (2004), ‘Modified atmosphere packaging’, in Gross KC, Wang CY and Saltveit ME, The commercial storage of fruits, vegetables, and florist and nursery stocks, USDA handbook 66, available from: http://www.ba.ars.usda.gov/hb66/ contents.html [accessed 4 June 2010]. Mizutani Y (1989), ‘Micropourous polyethylene sheets’, Ind Eng Chem Res, 32, 221–227. Morris LL, Pratt HK and Tucker CL (1955), ‘Lettuce handling and quality’, Western Grower Shipper, 26(5), 14–16. Mount III EM and Wagner Jr JR (2001), ‘Aroma, oxygen and moisture barrier behavior of coated and vacuum coated OPP films for packaging’, J Plastic Film Sheeting, 17, 221–237. Nielsen T and Jägerstad M (1994), ‘Flavour scalping by food packaging’, Trends Food Sci Technol, 5, 353–356. Oms-Oliu G, Hertog MLATM, Soliva-Fortuny R, Martín-Belloso O and Nicolaï BM (2009), ‘Recent developments in the use of modified atmosphere packaging for freshcut fruits and vegetables’, Stewart Postharvest Rev, 4:3, 1–11. Ozdemir M and Floros JD (2004), ‘Active food packaging technologies’, Crit Rev Food Sci Nutr, 44, 185–193. Paik JS and Writer MS (1995), ‘Prediction of flavor sorption using the Flory–Huggins equation’, J Agric Food Chem, 43, 175–179. Perera CO, Balchin L, Baldwin E, Stanley R and Tian M (2003), ‘Effect of 1-methylcyclopropene on the quality of fresh-cut apple slices’, J Food Sci, 68, 1910–1914.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 163
7/1/11 12:52:21 PM
164
Biaxial stretching of film
Piergiovanni L, Limbo S, Riva M and Fava P (2003), ‘Assessment of the risk of physical contamination of bread packaged in perforated oriented polyproplene films: measurements, procedures and results’, Food Addit Contam, 20, 186–195. Rojas-Graü MA, Oms-Oliu G, Soliva-Fortuny R and Martín-Belloso O (2009), ‘The use of packaging techniques to maintain freshness in fresh-cut fruits and vegetables: a review’, Intl J Food Sci Technol, 44, 875–889. Sacharow S (1988), ‘Freshness enhancers: the control in controlled atmosphere packaging’, Prepared Foods, 157, 121–122. Saltveit ME (2004), ‘Respiratory metabolism’, in Gross KC, Wang CY and Saltveit ME, The commercial storage of fruits, vegetables, and florist and nursery stocks, USDA handbook 66, available from: http://www.ba.ars.usda.gov/hb66/contents.html [accessed 16 August 2010]. Song J, Leepipattanawit R, Deng W and Beaudry RM (1996), ‘Hexanal vapor is a natural, metabolizable fungicide: Inhibition of fungal activity and enhancement of aroma biosynthesis in apple slices’, J Amer Soc Hort Sci, 121, 937–942. Tanner DJ, Cleland AC, Opara LU and Robertson TR (2002), ‘A generalised mathematical modelling methodology for design of horticulture food packages exposed to refrigerated conditions: Part 1, Formulation’, Intl J Refrigeration, 25, 33–42. Toivonen PMA (1997), ‘Quality changes in packaged, diced onions (Allium cepa L.) containing two differnt absorbent materials’, in Gorny JR, CA ‘97 proceedings volume 5: Fresh-cut fruits and vegetables and MAP, Davis, CA, University of California, 1–6. Toivonen PMA and Lu C (2007), ‘An integrated technology including 1-MCP to ensure quality retention and control of microbiology in fresh and fresh-cut fruit products at non-ideal storage temperatures’, Acta Hort, 746, 223–228. van den Berg L (1981), ‘The role of humidity, temperature, and atmospheric composition in maintaining vegetable quality during storage’, in Quality of selected fruits and vegetables of North America, American Chemical Society, 95–107. Wang CY (2006), ‘Biochemical basis of the effects of modified and controlled atmospheres’, Stewart Postharvest Rev, 5:8, 1–4.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-12.indd 164
7/1/11 12:52:21 PM
13
Biaxial stretched films for use in snack packaging
E . M . M o u n t I I I, EMMOUNT Technologies, USA Abstract: This chapter describes the properties and manufacturing principles used for the design and production of oriented films for snack packaging. The manufacture and use of oriented films for flexible snack packaging are described. Key words: oriented films, coextrusion, metallization.
13.1
Introduction
Flexible packaging as we understand today has evolved from the early development of papermaking. Based on this, modern plastics packaging used today have evolved from the science of papermaking and the subsequent conversion of cellulose into cellophane. The development of the wood pulping process in the late 1890s led to the production of corrugated packaging, paper board and various forms of paper. These materials were used to replace wood boxes and are characterized by relatively high strength and stiffness. Cellophane was discovered in the 1892 (Sweeting 1971, p 370) and ultimately developed as a packaging film in France around 1913 but was used for surgical dressings and gas masks by the French army during World War I. Commercial manufacture began around 1920 and quickly spread worldwide; it was dramatically improved by DuPont in 1927 with the addition of waterproof coatings which greatly expanded the packaging application of the cellophane. Cellophane use as packaging was also further expanded with Sylvania Industrial Corporations introduction of a heat sealing layer in 1931 and US sales reached 32 million pounds (14 500 tonnes) in 1932. After the discovery of polyethylene in the late 1930s and its growth after World War II in polyethylene coated paperboard for milk cartons and in 1949 with polyethylene coated cellophane, the basic definition of flexible films for packaging had been set. Flexible packaging machines had been developed specifically for the properties of the coated cellophanes and a large machine base was present for packaging food stuffs. Consequently, when various polymers such as nylon, polyester, polyethylene (PE), polystyrene (PS) and polypropylene (PP) were invented and commercialized in the period of the 1930s to the early 1960s they began to steadily replace cellophane in packaging. Cellophane sales in the 165 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 165
7/1/11 9:12:31 AM
166
Biaxial stretching of film
United States peaked about 1961 as the growth of oriented polyester, and especially PE films began to dominate plastics film production by 1966 (Sweeting, 1968, p 12). Early discussions of what constituted an ideal film started in the early 1950s with a goal that the film would be complete in itself without modification such as by coating; however, owing to the wide range of packaging applications today this goal is not considered practical or desirable. Consequently, the films which evolved to replace cellophane first became simple substitute substrates to carry the functional coatings which had already been developed for cellophane packaging. Later as polymer technology evolved generating a wide range of copolymers, the new resins were used with the base layer polymers as coextruded skins which can add additional functionality such as for printing, laminating, metallization and heat sealable films. However, from this consideration of an ideal film the following list of general film properties were developed (Sweeting, 1968, pp 15–21) ∑ ∑ ∑ ∑
Appearance – haze and gloss – printability Physical properties – dimensional stability – gas transmission – durability – tenacity – tear strength – impact strength – sealability – lubricity (slip) – stiffness – abrasion resistance Chemical properties – moisture resistance (vapor and liquid) – oil and grease resistance – flavor and aroma barrier Economics – film yield (m 2/kg) – thickness range from 0.064 to 0.102 mm (0.25 to 4 mils) – converting speed on automatic machinery – acceptable widths for converting
These desirable properties for the ideal film tend to favor highly oriented polymers over the use of less oriented blown and cast films in many applications. This is primarily due to the beneficial impact of high orientation on the overall film property profile, especially for semicrystalline polymers.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 166
7/1/11 9:12:31 AM
Biaxial stretched films for use in snack packaging
167
What produces the most desirable film for a particular packaging application is a complex combination of appearance, barrier properties, thermal properties, mechanical strength and cost. High orientation is generally developed during the stretching of a polymer between its glass transition temperature (Tg) and its melting point (Tm) as is done with the double bubble and tenter frame processes (Kanai and Campbell, 1999). Completely amorphous polymers, such as PS, polycarbonate and polymethylmethacrylate, are stretched just above the Tg of the polymer in a high viscosity rubbery state and are quickly quenched below Tg to freeze in the molecular orientation. During these stress-induced orientation processes the molecular structure is preferentially aligned in the direction(s) of stretching and any crystalline structure in the polymer is rearranged. It is generally accepted that the deformation of the semicrystalline materials takes place in the amorphous regions of the polymer while the crystalline regions are rotated and aligned preferentially to the direction of stretching (Hoshino, et al., 1962). The crystals serve as mechanical reinforcement and minimize the relaxation of the amorphous phase orientation in place. Compared with highly oriented films, cast and blown film processes produce films with relatively low levels of molecular orientation induced by flow of the melt which is frozen into the film on quenching. Consequently, the films relatively soft and weak in terms of tensile properties and not easily used on high speed packaging machinery without lamination to a stronger, stiffer web, such as paper, cellophane, oriented polyester (OPET) and oriented polypropylene (OPP). Because of the low tensile forces and high elongations possible, cast and blown films are often used as sealant webs in packaging or for use in subsequent orientation processes such as for thermoforming fill and seal applications. Optical properties of blown films are also relatively poor due to the slow quenching inherent to the slow crystallization of the process, which can enhance its tensile properties to some degree. Cast films with higher quenching rates are clearer than but not as stiff as the generally more crystalline blown films. Relative to cast and blown films, highly oriented films have superior optical properties with lower haze and higher gloss values. The lower haze arises from a decrease in light scattering between crystalline and amorphous regions in the film. As the amorphous phase is oriented, its density is increased due to chain alignment and the density difference between the amorphous and crystalline phase is decreased causing lower internal light scattering, improving the film clarity (Mount, 1987). The surface roughness of a film is also diminished during stretching above the strain hardening point (Cakmak and Simhambhatla, 1995), which is typical of commercial orientation processes. Stress-induced orientation also significantly changes the mechanical
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 167
7/1/11 9:12:31 AM
168
Biaxial stretching of film
properties of the polymer relative to the cast and blown films from the same materials. In general a tensile strength in excess of 1500 psi (10.34 MPa) is desired for use as a packaging film (Sweeting, 1968) but the actual acceptable value will be dependent on the control of the automatic packaging equipment it is used with. Early packaging machines were designed for paper and cellophane, which are high modulus, strong, but brittle materials. Cellophane and paper were easily substituted by OPET, which is as strong but more ductile than the paper or cellophane. However, OPP which has replaced the majority of paper and cellophane is much weaker than all of the other materials (Fig. 13.1). In order for OPP to be successful as a cellophane and OPET replacement, it was necessary to modify the packaging machines to accommodate the lower strength and temperature stability of OPP. At first these equipment modifications were performed by applications engineers at the OPP suppliers and later by the packaging machine manufacturers. As time went by and the volume of the OPP films increased existing packaging machines were replaced by newer machines capable of running a majority of the major packaging films. Applications work still continues and is important as packaging structures are changed and new films such as oriented polylactic acid (PLA) films are introduced for packaging applications.
300
Paper
PET 258.5@180%
OPET OPP
250
Stress, MPa
200 Paper 151@3%
150
100
OPP 101@232%
50
0 0
50
100 150 Percent elongation
200
250
13.1 Plot of stress vs. elongation showing comparative properties of oriented PET and OPP packaging films compared to paper. Ultimate stress and % elongation for each are shown.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 168
7/1/11 9:12:32 AM
Biaxial stretched films for use in snack packaging
13.2
169
Basic principles and methods for snack packaging
Snack packaging is an engineering science. The main focus of the science is to determine the key product protections required for packaging any particular product. Said another way, it is necessary to understand how the product fails, so that the packaging material can be engineered to provide the proper types and levels of protection for the required or desired shelflife of the packaged product. Therefore, the property requirements of the packaging material will depend greatly on the definition of the desired shelf-life of the product. Because there is no single material or polymer film which contains all the desired properties at the desired level to obtain the desired shelf-life for all snack products, composite structures have to be engineered and prepared to create a successful snack packaging material for each snack packaging application. While the meaning of ‘desired shelf-life’ will vary among customers, in general it will consist of several primary attributes. First for snacks the product must not become stale from moisture gain or loss. Next it must not become rancid from fat oxidation. Beyond these two primary shelf-life indicators the meaning becomes much more customer specific. For some the shelf-life is reached when the ‘first day flavor’ is lost regardless of no staling or rancidity. In other instances the loss of a specific level of flavor intensity (by scalping or loss by permeation) determines the desired shelflife. Flavor changes by rancidity reactions to generate off-odors and taste or from diminished concentrations of the flavor and aroma components by oxidation or hydrolysis will also define the shelf-life. Having defined the required product protections to achieve the desired shelf-life, the level of packaging properties may be defined (i.e. what is the oxygen barrier required for 120 days of protection). It is then possible to define the packaging material combinations necessary to provide the desired protection to achieve the desired shelf-life. At this point it is possible to estimate the cost of the packaging and determine if it is a desirable or competitive alternative to an existing packaging material or a fit for use design for a new application. Product protections exceeding the levels required for a particular, or desired, shelf-life can be defined as over-packaging and are not desired, especially if they increase the cost or environmental impact of the packaging materials. Under-packaging, where product protection properties do not meet the shelf-life requirements, is also undesirable as it leads to premature failure of the product before consumption and is a net loss of resources to society. Aside from the general list of properties for an ideal film listed above, In general there are five primary product protections which a snack packaging material must supply: © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 169
7/1/11 9:12:32 AM
170
∑ ∑ ∑ ∑ ∑
Biaxial stretching of film
protection from vermin and bacterial contamination; light barrier; moisture barrier; gas (generally oxygen) barrier; chemical resistance (flavor and aroma barrier).
All of these protections are important, but the level of each protection and the primary, or most important, protection will vary depending on the product to be packaged, its failure mechanisms and the desired shelf-life. Protection from vermin and bacteria is primarily a function of the seal integrity and the resistance to pin holing damage of the packaging material. Setting aside the seal integrity for now, this leaves the concept of four primary barrier protections for snack packaging. For example, moisture and oxygen barriers are both important for the packaging of fried snacks and iron machine parts. Oxygen and moisture will cause fried snacks to become rancid and stale while iron machine parts will oxidize (rust). For the oxidation of iron both moisture and oxygen are necessary and either a high oxygen barrier or a high moisture barrier even in the presence of a low moisture or low oxygen barrier respectively will supply protection against oxidation. In comparison, high levels of oxygen barrier are not important for snacks if the moisture barrier is low because the snacks will become stale (fail) before they become rancid. However, if the snacks are protected by a high moisture barrier but a poor oxygen barrier, the fried snacks become rancid before they become stale. There are multiple failure modes for the snacks. Indeed, for many fried snacks it is more important to supply a light barrier than either oxygen or moisture barrier because the UV radiation will cause rancidity faster than the dark exposure of the snacks to oxygen (Gavitt, 1993). The determination of the primary barriers necessary for packaging optimization requires a detailed study of the product failure mechanisms as well as the robustness of the packaging material in the converting and package forming steps (Specht, 1998). Data of this type can be gleaned from shelf-life studies with taste panels, chemical analysis of packaged products and the general literature on food chemistry. Figure 13.2 shows the relative impact of light, moisture and oxygen barrier levels on package shelf-life for potato chips. In the absence of light barrier the shelf-life of chips can be seen to be approximately 9 days. The addition of moisture barrier extends the shelflife to approximately 28 days and the addition of oxygen barrier, with gas flushing, to approximately 63 days. The acceptable level of protection for a 60 day shelf-life for maintaining the flavor and texture of potato chips was determined from a series shelf-life studies for light barrier (optical density), moisture barrier and oxygen barrier (Gavitt, 1993). The results of the studies are detailed in Figs 13.3–13.5.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 170
7/1/11 9:12:32 AM
Biaxial stretched films for use in snack packaging 100
Gas
90
Moisture
80 Shelf-life, days
171
Light
70 60 50 40 30 20 10 0
Light Light + moisture Light + moisture + gas
13.2 Shelf-life of potato chips for several packaging materials with the barrier properties shown, light alone, light + moisture and light + moisture + gas (redrawn from Specht, 1998). Product attribute: flavor 100 Met. films
90
Shelf-life, days
80 70 60 50 40
Minimum shelf-life
30 20 10
Opaque OPP
0 2.5
2.0
1.5 1.0 Optical density
0.5
0
13.3 Plot of shelf-life vs. optical density for all bag sizes overlaid with the performance levels of metallized OPP and opaque OPP showing that metallized films meet the performance requirements (redrawn from Gavitt, 1993).
In the example of plain potato chips, the chemical barrier becomes the fourth most important barrier and is important in controlling flavor tainting from environmental contamination as opposed to flavor loss. In other snack foods, the relative importance of the oxygen and flavor barrier may change.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 171
7/1/11 9:12:32 AM
172
Biaxial stretching of film Product attribute: texture 100
mPET
90 80
mOPP
Shelf-life, days
70 60 50 40
Minimum shelf-life
30
PVDC
20 10 0
OPP 0
0.01
0.02
0.03
0.04 0.05 0.06 0.07 WVTR (g/100 sq-in/day)
0.08
0.09
0.1
13.4 Plot of shelf-life vs. moisture barrier (WVTR g/100 sq-in/day @ 85 oF/80% RH) for potato chips overlaid with the performance levels of metallized PET (mPET), metallized OPP (MOPP) and clear OPP showing that metallized films meet the performance requirements (PVDC = polyvinyl dichloride; 1 sq-in = 645 mm2) (redrawn from Gavitt, 1993).
Product attribute: flavor
100 90 80
mPET
Shelf-life, days
70 60 mOPP
50 40
Minimum shelf-life
30 20 10 0
0
1
2
3
4 5 6 OTR, cm3/100 sq-m/day
7
8
9
10
13.5 Plot of shelf-life vs. oxygen barrier (OTR cm3/100 sq-in/day @ 73 oF/0% RH) for potato chips overlaid with the performance levels of metallized PET, metallized OPP and clear OPP showing that metallized films meet the performance requirements, (redrawn from Gavitt, 1993).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 172
7/1/11 9:12:33 AM
Biaxial stretched films for use in snack packaging
173
This is due to the addition or presence of distinct flavors in the packaged product which could overshadow the or be highlighted by the initial onset of oil rancidity, for example ranch, barbeque, oil and vinegar flavors as opposed to unflavored chips. Flavors and aromas are just specific chemicals and chemical mixtures (Heath, 1981) and can be characterized by their chemical structure and the taste or smell they evoke. They can be due to the natural chemical composition of a product or from degradation reactions taking place. A short list of several specific flavors and aromas (Foster, 1989) are: garlic (allyl sulfide), vinegar (acetic acid), soap (ethyl phenyl acetate), pine (beta pinene), peppermint (menthol) and vanilla (vanillin). Many more examples can be found in the literature. When determining the desired barrier property for a flavor it will be necessary to find or measure the permeability of the aroma or taste chemical. Every packaging material will have its own profile of flavor and aroma barrier based on the chemical nature of the packaging material. Because permeability is the product of solubility and diffusion rate, the chemical compatibility of the flavor with the packaging material will determine the type and extent of diffusion through a package. It is the chemical resistance of the films and coatings to the flavor and aroma chemicals which control the loss of flavor components (Mount and Wagner, 1998). If the flavor chemical is soluble in the packaging material, it will be removed from the product by scalping and/or permeation. If the solubility of the flavor in the packaging material is low then the flavor loss can be significantly reduced. In addition the relative locations of chemically resistant components of the packaging materials can also impact the level and rate of flavor loss. Components which will scalp the flavor from the product but which add desirable features such as durability and sealing should be isolated from the product by an effective barrier to the flavor/aroma. Balancing the position and functions of the packaging materials is an important consideration in the design of the packaging material. An alternative approach to determining barrier requirements for a packaging film can be obtained from knowledge of the chemical failure of a product. For example you have a fresh snack food which you know contains 2% moisture as produced. You determine or find in the literature that when the product contains 4% moisture it is considered unsuitable for consumption or is stale. Knowing the product density and the total area of the packaging you can determine from the product weight what amount of moisture a 2% gain represents. Then from the total area of the package and a desired shelf-life you can readily determine the minimum moisture barrier for the packaging material. As an example, consider 32 grams at 2% moisture packaged in a bag which is (excluding end seal area) 19.5 ¥ 15 cm long and wide which gives
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 173
7/1/11 9:12:33 AM
174
Biaxial stretching of film
for two surfaces an area of 585 cm2 (0.0585 m2). Therefore 2% weight gain equals 0.64 g of moisture and once this amount of moisture permeates into the bag the product is unsuitable. All that is necessary now is to define the desired shelf-life and calculate the minimum moisture barrier. For 100 days the answer is:
Minimum water vapor transmission rate (WVTR) for 100 days = 0.64 g/(0.0585 m2 ¥ 100 days) = 0.109 g/m2/day
Alternatively if you knew the moisture barrier for the film (WVTR of 0.38 g/ m2/day) you could calculate the maximum shelf-life from the weight gain and package size which for a would be
Shelf-life in days = 0.64 g/(0.38 g/m2/day ¥ 0.0585 m2)
= 28.8 days
These simple estimates assume that there is no modification of the atmosphere inside the package, i.e. that the moisture entering the package is absorbed by the product and that the outside environment supplies a constant source of moisture. For a better analysis, beyond the scope of this chapter, some knowledge of the interior and exterior equilibrium moisture conditions should be known as well as the equilibrium absorption characteristics of the product. Some failure data of this sort for both moisture and oxygen can be found in the literature but data are generally supplied by the end user for their particular product and their particular requirements for taste and shelf-life. This is difficult information to find and will generally require a guide to the shelf-life literature (Man and Jones, 2000), a survey of food failure references or the conduct of a shelf-life evaluation. Once the levels of barrier protection are determined, it is necessary to establish the various materials necessary to meet all the mechanical, sealing, packaging machine and barrier requirements of any particular snack package. Table 13.1 lists a development table of required properties with a series of films listed. From Table 13.1 a potential combined packaging structure can be developed from a description of the package requirements relative to the listed film properties. Then the various cells of Table 13.1 would be filled out to indicate which materials meet the application requirements previously defined. Once the table is complete the various polymers necessary to yield an acceptable product, i.e. with all the necessary properties can be defined. Once defined the means of construction of the composite material will need to be defined and then the cost can be determined. Some properties such as mechanical strengths are additive and easily estimated. Barrier properties are also easily estimated as a sum of the resistances of each layer barrier. Barrier data for polymers and films can be
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 174
7/1/11 9:12:33 AM
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 175
7/1/11 9:12:33 AM
Hot tack
Seal integrity
Flex crack Moisture Oxygen Aroma resistance barrier barrier barrier
OPP, oriented polypropylene; OPET, oriented polyethylene terephthalate; OPA, oriented polyamide; OPLA, oriented polylactic acid; PP, polypropylene; LDPE, low density polyethylene; HDPe, high density polyethylene; OPS, oriented polystyrene.
Paper
OPS
HDPE
adhesive
LDPE
sealant
LDPE
Cast PP
OPLA
OPA
OPET
Seal initiation
Tensile Yield Elongation Toughness Clarity Heat sealing strength point
Film type
OPP
Required property
Table 13.1 Properties required for flexible snack packaging
176
Biaxial stretching of film
found in the literature, but often the units must be changed. A useful source of film permeability data is the Plastics Design Library’s Permeability and other Film Properties of plastics and elastomers in (1995). Other properties such as tear resistance, pin holing resistance or bursting properties are not as easily predicted for individual layer properties and must be tested directly or estimated as to their relevance to the packing material being designed. Having established the necessary combination of polymers to be used in a snack packaging structure, it is then possible to establish a manufacturing method for the package. Table 13.2 lists many of the various manufacturing methods available for combining various polymers and films. In general polymers can be broken into several general categories, those which are easily extrudable, those which are not extrudable but can be coated and those which, although difficult to extrude, can be extruded with special skills and equipment. Having selected the polymers for the packaging structure it then becomes possible to define several manufacturing methods. The final choice will depend on the equipment available to a given converter and then the cost of manufacturing the combination can be determined. In general the packaging structure will consist of a combination of multiple layer films which are laminated together. The multiple layer films may be produced as a single layer film but more commonly the films are made by coextrusion (Mount, 2010), solution coating of films (Gutoff and Cohen, 2010), extrusion coating films (Bezigian, 1999) and metallization of films (Bishop and Mount, 2010) to optimize the substrate properties. More complex laminations are then produced by adhesive or extrusion lamination (Bakker, 1986) to complete the production of the packaging material. The use of lamination technologies (Bezigian, 1999) is important as they allow delicate surfaces of the individual films to be protected from abrasion and direct exposure to the environment. This is necessary for coated and metallized films as well as printed films. Many packaging materials consist of an outer film which is a reverse printed film with an inner heat sealing film which contains a thin barrier layer such as aluminum, aluminum oxide, SiO2 or a polymer coating such as saran®, acrylic polymers, polyvinyl alcohol (PVOH), etc. Figure 13.6 shows a cross-section of a typical snack packaging lamination. The reverse printed surface is protected from abrasion during package forming, packing and shipping and shelf display and can take on the outer surface characteristics such as high gloss or a matte surface. Also the outer surface is modified to permit improved performance on the packaging machine and is usually in contact with forming collars and metal transport surfaces of the packaging machine. The inner film contains a barrier layer often added by vacuum deposition such as metallization or an out-of-line coating. Snack packaging initially comprised steel cans and freshness was
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 176
7/1/11 9:12:33 AM
© Woodhead Publishing Limited, 2011
Fluoropolymer
PAN
PS
PVC
Cellulose
Ionomer
PVDC
EVOH
Nylon
Copoly-PET
PET
PP
EVA
LLDPE
LDPE
HDPE
X X X
X
X
X
X
PMMA
HDPE, high density polyethylene; LDPe, Low density polyethylene; LLDPE, linear LDPe; EVA, poly(ethylene covinyl acetate); PP, polypropylene; PET, polyethylene terephthalate; Copoly-PET, PET-based copolymers; EVOH, poly(ethylene vinyl alcohol) copolymer; PVDC, polyvinylidene chloride; lonomer, acid salt of polyethylene acrylic acid polymers; PVC, polyvinyl chloride; PS, polystyrene; PAN, polyacrylonitrile; EMMA, poly(ethylene methyl methacrylate) copolymer; EMA, poly(ethylene methyl acrylate) copolymer; PMMA, poly(methyl methacrylate).
Extrusion casting X X X X X X X X X X X Uniaxially orientable C X X X X X X X X Biaxially orientable X X X X X X X X X X X X Extrusion coatable X X X X X X X X X X X X Extrusion laminatable X X X X X X X X X X Emulsion coating X X X X X X X X Solution coating X X Thermoformable X X X X X X X X X X X X X X X X Injection blow moldable X X X X X Coextrudable X X X X X X X X X X X X X X X X Adhesive laminatable X X X X X X X X X X X X X X X X Thermal laminatable X X X X X X X X X X X Metallizable X X X X X X X X X X X X X X X X X X Printable X X X X X X X X X X X X X X X X X X
Manufacturing method
EMMA
Table 13.2 The manufacturing methods available for combining and converting various polymers and films made from them
EMA
WH Biaxial-DeMeuse-13.indd 177
7/1/11 9:12:33 AM
178
Biaxial stretching of film
Reverse printed clear (or matte) coextruded film
12 to 18 microns
Print layer Adhesive (LDPE or polyurethane)
12 microns LDPE, 1–2 microns if polyurethane
Aluminum deposit
150 nm
Clear or metallized heat sealable coextruded film
18 microns
(a) Outer machining surface: gloss or matte
1 micron
Bulk PP layer of outer film
16 microns
Reverse printed surface
1 micron
Laminating adhesive
12–25 microns of LDPE or thin adhesive layer (polyurethane, saran, etc.)
Barrier surface (coated or metal deposit)
1 microns + coating or vacuum metallized
Bulk PP layer of inner film
15.5–16 microns
Inside sealing surface
1–1.5 microns (b)
13.6 (a) Generalized packaging lamination (PET or OPP substrate) showing the construction and location of print and metallized layers. (b) Typical OPP snack packaging lamination structure showing additional details of the OPP films used in the lamination.
controlled by distribution. The can supplied the primary light and vermin barrier and limited the ingress of oxygen and moisture into the can volume unless opened. Cans were delivered and unconsumed product was picked up approximately twice a week because of the short shelf-life due to staling. As the cans were replaced with flexible packaging, they were always designed to be opaque. Initially the flexible packages were wax coated glassine paper (Koltzenburg, 2000). Then with the introduction of cellophane, polyester and later the inventions of polyolefins and extrusion coating and laminating, packaging materials evolved through a series of structures, such as polyvinyl dichloride (PVDC) coated glassine paper and or cellophane, extrusion coated or laminated with a brown pigmented low density polyethylene (LDPE) and eventually to laminations of metallized OPET and OPP films used today. Oriented PET and OPP compete directly in many of the packaging film markets which are described later in Section 13.5. The basis of the head to head competition in the snack packaging markets is on comparable properties and comparable film yields and cost. Comparable film yields of OPET to OPP are obtained by the use of thinner OPET films which is possible due the higher tensile properties of OPET. There are advantages to clear and
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 178
7/1/11 9:12:33 AM
Biaxial stretched films for use in snack packaging
179
metallized OPP in moisture protection and for clear and metallized OPET in oxygen protection and this can form the basis for selection for specific packaged products. Where barrier protection is of secondary importance then cost may be the determinate or the simple preference for the service between two suppliers or the relative OPET and OPP film sales volume of a converter.
13.3
Technologies and techniques
Packaging films are produced by many techniques and combinations of multiple films which are individually produced. The primary technologies for making films for use in packaging are film casting, film blowing and film orientation. Cast and blown films are sometimes used for sealant layers in laminated snack packaging and are widely used for frozen vegetables and the bundling of items for sale. However, the basic film properties of strength and barrier are significantly enhanced by orientation of the polymers and today the majority of snack packing films is based on biaxially oriented polymers, predominantly based on OPP and OPET. Other oriented polymers such as Nylon 66 and Nylon 6, have some application in packaging but not particularly for snacks. Oriented high density polyethylene (HDPE) and PLA have been recently developed for snack packaging but currently are not as widely used as are the OPP and OPET films. Orientation is the process of stretching of polymers to align the polymer molecules. The molecular alignment results in the enhancement of many of the polymer properties and greatly improves the film’s suitability for snack packaging applications. Without the mechanical strength improvement of the orientation, the polymer films would not have been suitable for use on existing packaging equipment developed for the coated cellophane packaging materials. Cellophane is characterized by a high stiffness, high mechanical strength, low elongation and high temperature resistance. Cast and blown polymer films were simply not able to supply the necessary mechanical properties for use on existing packaging equipment. Orientation of the polymers significantly improved the mechanical properties of OPP and OPET so that they could be used on existing packaging equipment with little modification for OPET and with straightforward modifications for OPP films. While the machine modifications were necessary for the OPP films, the value of the films in terms of moisture resistance and barrier and a significant yield improvement (number of square meters of film per kilogram) relative to cellophane, paper and OPET drove the growth of OPP as a cellophane and glassine paper replacement. OPET and OPP compete today as packaging films on a yield equivalent basis (thin OPET compared to OPP) and use complementary orientation technologies.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 179
7/1/11 9:12:33 AM
180
Biaxial stretching of film
13.3.1 Orientation technology OPET was developed before the invention of polypropylene and is based on a sequential tenter process (Park and Mount, 1987; Kanai and Campbell, 1999). OPP was originally produced using the double bubble process and the sequential tenter process. The parallel manufacturing technology continued until line outputs increased above 500–900 kg/h, at which point the higher cooling capacity of the cast tenter film process permitted significantly higher manufacturing rates which currently range from 4500 kg/h or higher on modern orienting equipment. The double bubble process is shown in Fig. 13.7. The polymer is cast downwards into a water bath over a cooled internal metal mandrel. On exiting the water quench, the tube is pinched shut with rollers forming the bottom of the first bubble. Next the tube enters a heating chamber where it is reheated to the stretching temperature. The second bubble is formed
Die Weir Quenching zone
Water bath
P1
Transport zone
Pinch rolls
Preheat or reheat zone
Radiant heaters
P2 Biaxial orientation zone
Collapsing frame Haul-off nip Slitting knife To winder
To winder
13.7 Schematic diagram of the double bubble orientation process typical of that used for OPP production.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 180
7/1/11 9:12:34 AM
Biaxial stretched films for use in snack packaging
181
between the first and second set of pinch rollers. The second set of pinch rollers are turning at a higher speed than the first which draws the film in the machine direction (MD). Compressed air is injected into the second bubble expanding it to the desired diameter stretching the film in the transverse direction (TD). The compressed air is trapped between the two pinch rollers and is considered by some to act as a stationary mandrel over which the film is stretched. As applied to OPP production the double bubble process is operated at stretch ratios of approximately 6 ¥ 6 and is a simultaneous biaxial orientation process. This produces film with approximately balanced mechanical properties. However, there are limited means to anneal the films and they tend to be less dimensionally stable than tentered films. While this was an initial disadvantage to the films, today it is an advantage in cigarette packaging where the increased shrinkage gives a tighter wrap which is more difficult to achieve with the tentered films. However, in most converting processes the improved dimensional stability of the tentered film is a distinct advantage and double bubble films are sometimes heat stabilized with an annealing process using a tenter frame (Matsugu et al., 1972) or a third bubble where the film is reheated and relaxed without stretching (Reade, 1974)). Today the majority of double and triple bubble lines are used to make five to seven layer heat shrinkable barrier packaging films containing nylon/EVOH barrier layers. The sequential tenter process is shown in Fig. 13.8. It is the principal technology used to manufacture snack packaging films from both polyester and polypropylene. In the sequential process the polymer melt is extruded onto a water cooled casting drum. The melt is forced against the drum using one of several ‘pinning’ processes to improve the quenching rate Stretching oven Winding
Extruding
Casting MD stretching
Preheat TD stretching Annealing
13.8 Schematic diagram of the sequential tenter process typical of that used for biaxial orientation of polyester, polypropylene, polystyrene, nylon and polylactic acid films.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 181
7/1/11 9:12:34 AM
182
Biaxial stretching of film
during the forming of the cast sheet. For OPET films the MD ¥ TD stretch ratios are nominally 3.5 ¥ 3.5 while for OPP they are nominally 5 ¥ 8. Simultaneous tenter processes are available using several approaches ranging from mechanical pantographic clip supports, a variable pitch rotating screw clip drive and linear motor clip drive (LISIM®) technology. The variable pitch screw drive is seldom seen, the pantographic chain system is limited to relatively slow line speeds and the LISIM® linear motor technology is very expensive. In general the simultaneous orientation technologies are not required for acceptable film packaging film properties and offer no exceptional improvement in film properties. Therefore, the simultaneous tenter technologies for both OPET and OPP cannot compete economically or technologically for snack packaging applications with the highly developed sequential tenter frame. However, the simultaneous orientation processes will have generally higher MD tensile properties but lower TD tensiles properties than sequential processes. This is because simultaneous systems are operated with a higher MD and lower TD stretch ratios than sequential systems, i.e. 6 ¥ 6 compared with 5 ¥ 8 respectively or a 36 to 40 times area enlargement. In the stretching process the chains are aligned along the directions of stretching and the relative balance is controlled by the relative stretching ratios. Higher MD ratios give higher MD chain alignment and therefore higher MD tensile strength. Also, increasing the TD stretch ratio to 8 in the sequential orientation redistributes the initial MD alignment of the chains to the TD direction to a greater extent compared with that of the lower TD stretch ratio of 6 for the simultaneous stretching. While a somewhat higher MD tensile property for OPP may be desirable in some applications, such as packaging tape and in the initial considerations in replacing stronger films such as cellophane and OPET, in the long run the higher productivity and lower manufacturing cost of the sequential OPP films are more important. Consequently, there was good reason to modify packaging and converting equipment to successfully handle the slightly lower MD properties of the sequential OPP films and take advantage of the lower film costs as manufacturing rates increased For OPET the pinning process used is predominantly electrostatic pinning (Segransan and Joly, 1981) electrostatic pinning applies an electrostatic charge to the cast melt on the casting drum. The electrostatic charge leak from a tungsten wire held at high voltage, towards the grounded cast roll, is deposited onto the passing melt located near the die exit. This charge creates an electrostatic force on the surface of the melt directed to the grounded cast roll. This is a very effective pinning process excluding air from behind the cast sheet up to approximately 125 m/min casting speed. Above 125 m/min some air may be trapped between the sheet and the roll which can give some visual defects on the cast roll surface of the finished
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 182
7/1/11 9:12:34 AM
Biaxial stretched films for use in snack packaging
183
film. For many of the OPET industrial applications and metallized films this is unacceptable, but could be adequate for some packaging applications. However, OPET manufacturing generally will not accept these pinning defects. Also, electrostatic pinning does not permit the use of a water bath to add extra cast sheet quenching. OPET cast sheets are quenched below the Tg of the OPET and are typically amorphous, generally containing less than 3% crystallinity. For OPP manufacturing the melt is usually pinned onto the casting roll with an air knife and then enters a chilled water bath. The casting roll is immersed in a bath of chilled water to increase overall quenching rate of the cast sheet. An air knife uses high volume, low pressure air flows to create a jet of air which impinges on the melt curtain, forcing it against the cast roll surface. In general, the edges of the sheet are also forced outward and against the casting roll using edge pinners to minimize melt neck in at the die, to prevent water from entering between the casting roll and cast sheet and to give a better edge for clipping at the tenter infeed. In general air knife pinning supplies a lower force than the electrostatic pinner and some air is always trapped between the cast roll surface and the melt. This results in differences in the surface quenching of the sheet leaving a visible pattern of variable surface gloss on the finished film surface (upper surface) called variously ‘water spots’ or ‘leopard spots’ (Fig. 13.9). In some
13.9 Picture of a cast film surface showing the variation of surface quenching due to air trapped between cast roll surface and the cast sheet, which when stretched give surface defects visible in metallized films and termed water spots or leopard spots.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 183
7/1/11 9:12:34 AM
184
Biaxial stretching of film
cases the pattern is very visible and in others not so visible depending on the polymer surface cast against the chill roll surface. For this reason it is generally better to metallize or print the film surface cast against the air or water bath side (bottom surface) of the film. PP is cast above its Tg and the sheet is typically 50–60% crystalline. The machine direction orienter (MDO) for OPET may differ from that for OPP in part because of the lower temperature ranges for stretching OPET and the higher surface quality requirements for many OPET industrial film applications. OPET MDO equipment may tend to have fewer driven rolls, the preheat rolls may be smaller in diameter, will have a higher surface polish and may stretch the film between the tops of two rolls, isolating the stretching point between the rolls with an IR heater, or in an ‘S-wrap’ from roll to roll with a short draw gap (Fig. 13.10). OPP MDOs are characterized by many large diameter, driven preheating rolls, which are each geared to Radiant heaters Draw point
Driven slow nip
Preheat section
Roll to roll draw section
Cooling section
Driven fast nip
Roll to roll MD stretching of PET with stretch point isolated by radiant IR heaters. Only the nips are driven Draw point
Driven slow nip
Preheat section
S-wrap draw section
Cooling section
Driven fast nip
Roll to roll MD stretching of PET with S-wrap stretch point. Only the nips are driven
13.10 Schematic diagrams of the two primary MDO stretching configurations for PET films.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 184
7/1/11 9:12:34 AM
Biaxial stretched films for use in snack packaging
185
slightly increase the roll surface speed between rolls to account for thermal expansion of the cast sheet and then stretch between small diameter rolls with a short ‘S wrap’ draw gap (Fig. 13.11). MD stretching temperatures for PET range from 76 to 95 °C (Cakmak, 1999, p 287) while MD stretching temperatures for OPP range form 120 to 150 °C (Schuhmann et al., 1996) which accounts for the need for longer preheat roll contact areas for OPP relative to the preheat requirements for OPET. Tenter frame technology relies on diverging chain systems traveling through a series of heated oven zones to properly heat the films for stretching and to heat stabilize the finished film (Fig. 13.12). The chains travel in a track (or on a rail) which carry the gripping clips used to hold the sheet edges during TD stretching (Fig. 13.13). There are two basic chain technologies in use to control and stabilize the moving clips; roller bearing and lubricated sliding chain systems. Generally speaking the roller bearing technology is used exclusively for OPET orienters and the roller bearing and sliding systems are used in the production of OPP. Roller bearing systems are more expensive than the sliding systems and will support higher stretching forces generally associated with OPET stretching, also they may run with less lubricating oil which is sometimes important for critical industrial applications typical of OPET. A third clip and track system is the LISIM® system where the track and the clip are constructed with electromagnets and form the sections of a linear motor. Computer control is used to control the relative MD orientation ratios by clip acceleration down the rail system while the TD ratios are set by the final width of the rail relative to the incoming sheet width (Breil, 1998). This is a very flexible but expensive orientation system and has not generally replaced the sequential orientation equipment due to the high capital costs. However, in critical areas of OPP film use, it may have
Preheat section Draw section Annealing section
From casting Nip
Slow rolls
Fast rolls
13.11 Schematic diagrams of the primary MDO stretching configuration for OPP. All rolls are driven.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 185
7/1/11 9:12:35 AM
186
Biaxial stretching of film Buffer zone PET only Endless chain with clips
Infeed
Chain divergence (strength)
Preheat
Film exit Crystallization (PET only) Annealing and/or annealing Cooling
Sheet from MDO
Entrance sprocket Hot air oven Exit sprocket
13.12 Schematic diagram of primary TDO oven zones located within a forced hot air oven of several different temperature zones. The buffer zone is typical of a PET orienter and used to isolate the stretch zone from the high temperature crystallization zone. For OPP there would be no buffer zone and the annealing zone would directly follow the stretch zone. Tenter rails are typically converged (toed in) after the stretch zone to improve TD film stability.
advantages of producing a balanced OPP film and perhaps OPP films with higher overall orientation ratios and better film dimensional stability. This is also true for OPET and other polymers, but for packaging applications the need to minimize manufacturing costs has prevented its widespread application in new orienter construction.
13.3.2 Coextrusion Perhaps the most significant technology for packaging films is that of coextrusion combined with the development of copolymers. In film coextrusion two or more separate melt streams are combined in layers prior to casting the melt from the die (Mount, 2010). There are three primary equipment approaches to coextrusion, combination prior to a single cavity die with a multilayer feedblock, combination of the melts after individual layer spreading in a multicavity die and the combination of a multilayer feedblock with a multicavity die. Currently all three approaches are used for OPP production and each approach can add advantages and limitations
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 186
7/1/11 9:12:35 AM
Biaxial stretched films for use in snack packaging
187
Fig. 1
Fig. 2
13.13 Figure 1 of US Patent 4068356 and Figure 2 of US Patent 5797172 showing modern clip support and tenter chain systems. Figure 1 shows a roller bearing clip support system with a monorail and Figure 2 shows a combined roller bearing and sliding clip support system, also with a monorail. © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 187
7/1/11 9:12:35 AM
188
Biaxial stretching of film
in terms of product design flexibility for possible layer combinations and manufacturing flexibility and stability. Coextrusion has led to a revolution in OPP product design especially for commodity but especially for high value added products such as vacuum metallization, opaque films, advanced heat sealing technologies and label films. Coextrusion technology is more widely used in OPP films than OPET films, perhaps because of the wider range of available copolymers and polyolefins but is growing for OPET applications as well. It was thought (hoped?) that coextrusion would replace out-of-line solution coating for the application of heat sealing and converting properties but this was only partially realized (see coating below). In coextrusion it is common to add one or two separate surface layers to the core homopolymer which enhance the converting and package forming capabilities of the film. Three layer films with copolymer surfaces are common place for packaging and define commodity OPP products. For OPET films two-layer coextruded heat sealable films are relatively common and three-layer films are available but single layer in-line coated OPET films still appear to predominate. This is especially true for older OPET film lines while new OPET construction are more likely to have coextrusion systems installed. This is in part due to the lack of copolyester resins and the relatively widespread use of in-line coating for surface modification in OPET film lines. However, there also appears to be a relative lack of imagination in the OPET film industry compared with the OPP film industry for coextrusion product design modifications. In comparison five-layer coextrusions are now common for OPP film lines and new construction is moving towards seven or more layer OPP films. Commodity three-layer coextruded OPP films can be characterized by Fig. 13.14 which shows the oriented PP core with two coextruded copolymer surfaces. Commodity coextruded films are available for both the inside (heat sealable lamination film) and outside (reverse printable slip film) film of a packaging lamination. For the inner or heat sealing film, there are two surfaces, the heat sealing and the treated (functional or converting) surface. Typically one copolymer is used for heat sealing and can be a moderate Copolymer skin
1 micron
Homopolymer core – slip / antistat modified
9.5 to 48 microns
Copolymer sealant skin
1 to 1.5 microns
13.14 Commodity coex OPP film design showing the three layer coextruded film design and the use of copolymer skins for printing and heat sealing surfaces.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 188
7/1/11 9:12:35 AM
Biaxial stretched films for use in snack packaging
189
ethylene content (EP) copolymer or an ethylene, butene propylene copolymer (EBP copolymer – sometimes called a terpolymer) with a relatively low seal initiation temperature. The use of the EBP copolymers enhances the hot tack of the sealing layer. The treated surface is typically a low to moderate ethylene content EP copolymer for good lamination strength or good metal adhesion for commodity metallized films. Treated copolymers give improved adhesion for ink, evaporated aluminum and coatings relative to homopolymer surfaces and are often designated as an adhesion promoting layer in product literature. For the outside lamination slip films, the treated surfaces of commodity films are generally a low to moderate ethylene content copolymer which gives good ink adhesion for reverse printing or lamination adhesion. Typically, the outer or untreated surface is a slip and antiblock formulated copolymer to give a good coefficient of friction (COF) for packaging machine bag forming and to permit the formation of lap back seals. If fin seals are used then the outer machining surface may be a formulated homopolymer in place of the copolymer. If some surface printing is required for the lamination, such as a tax stamp or bar code, the outermost surface may be a treated copolymer, and they would then be formulated for surface properties needed for outer surfaces of laminations. However, it is in the area of specialty OPP films where coextrusion has had the largest impact on product designs and today many specialty OPP films no longer have a polypropylene based surface. Coextrusion of non-propylene olefins, barrier resins and other condensation polymers are more common. For instance polyethylenes have been used for high barrier metallized films (Migliorini and Mount, 1993) and for in-mold labels (Poirier, 1993) as well as slip and printing enhanced films (Osgood and Therrian, 1989). Complex PE/PP blends are used for matte films. Ethylene vinyl alcohol (EVOH) and PLA surfaces give ultra-high barrier metallized films (Migliorini, 1992; Squier and Lockhart, 2005). Thick linear LDPE (LLDPE) layers give enhanced heat sealing properties (Donovan et al., 1999) and amorphous polyester and amorphous nylon surfaces (Mount and Benedict, 1991; Mount and Migliorini, 1997) have been found to be best for high barrier films based on silica deposition (Wagner and Mount, 1999). Today cavitated films (see below) are routinely based on five-layer coextrusions both to improve film properties and to improve manufacturing efficiencies. Combining several of these innovations may require the use of more than five layers and one or two seven layer coextrusion OPP lines have been sold in recent years. Value added heat sealing films are prepared by coextrusion. The seal initiation temperature (SIT) and hot tack properties are a function of the relative ethylene and butylene contents. Lower seal initiation and better hot tack films are quickly becoming commodity films as improved terpolymers
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 189
7/1/11 9:12:35 AM
190
Biaxial stretching of film
become widespread and lower in cost. Seal initiation temperature is defined as the heat sealing jaw temperature at which a specific level of seal strength is obtained and may be determined by plotting the seal strength versus seal jaw temperature. The desired SIT value is determined by product specification, which should be related to packaging machine operations. The seal jaw configuration (crimp, flat), jaw pressure and dwell time of the jaw closure should be defined and samples may be prepared on an operating packaging machine or a lab-based sealer using ASTM F2029-00. Actual testing of the sealed film strength is generally performed according to ASTM F88-05. Hot tack (ASTM F1921-98(2004) is the measurement of the strength of a heat seal immediately after the seal has been formed and before it cools to ambient temperature. It is important as it is a means of quantifying the impact of dump loading a bag on the hot seal integrity on a vertical form fill and seal machine. Today there are instrumented heat sealing machines designed to measure it. On the packaging machine, the amount of seal opening can be quantified for a particular film with a particular product being drop loaded onto the hot seal as the seal jaws open. Insufficient hot tack causes the seal to open partially or fully when loaded compromising the package integrity. SIT and hot tack combine to determine the ultimate packaging machine speed and the effective seal range in vertical form fill and seal machines (Fig. 13.15). For OPET films, coextrusion has primarily been used to improve clarity by locating the surface modification particles to the surface where they are 600
Opp film
Strength, g/inch
500
SS 15 °C seal range
400 300 200 100
HT
220
240
260 Temperature, °F
280
300
13.15 Plot of heat seal strength (SS) and hot tack (HT) strength vs. sealer jaw temperature for a coextruded OPP film. A heat seal and hot tack strength of 200 g/in and above defines a combined hot tack and seal strength range of 15 °C (30 °F).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 190
7/1/11 9:12:35 AM
Biaxial stretched films for use in snack packaging
191
most effective. This removes light scattering particles from the core where they are not functional and improves film clarity. Coextrusion is also used to produce heat sealable PET films. In this case, amorphous copolymer polyester is coextruded on one surface of the film. The amorphous character of the layer allows it to be heat sealable to itself. This product was first developed in the late 1970s and early 1980s for metallizing grades of OPET films for packaging applications. Owing to the sealing and polymer characteristics of the amorphous layer, it was prone to blocking if not handled properly. Special roll handling procedures with supporting the rolls always from the cores and not the face of the roll and with lifting from the core ends and not with a sling against the film face were put in place.
13.3.3 Cavitation Cavitated films are produced in such a way that the interior film layers are produced with small voids which cause the film to thicken and to decrease in density (Fig. 13.16). The voids are formed during the MD orientation as the core layer is fibrillated. The size, but not the number of the voids is increased and changed in dimensions during the TD stretching. The number of voids, their size and the density of the cavitating particle combine to give the final film density and the film optical properties of visual aspect, opacity and spectral reflectance. The core is typically surrounded by uniform uncavitated film to maintain a smooth surface for converting. The cavitation with clear skins gives the film a pearl-like luster. Depending on the number and size of the voids, the opacity of white films ranges from 20 to 30%. The outer layer of threelayer films or intermediate layers in five-layer films are often pigmented white with TiO2 to control the appearance and spectral characteristics and to make it a flat white (Fig. 13.17). Cavitated films also have a pronounced dead fold property and can be creased since they hold a fold better than uncavitated films. All oriented polymers may be cavitated with a suitable cavitating agent and examples of cavitated PET, PP, HDPE, PS, PLA, etc. are known. There are two basic cavitation technologies; incompatible polymer blends (Ashcraft and Park, 1983) or mineral-based cavitation agents (Schuhmann et al., 1994). Each technology yields a cavitated film with a white appearance, but the different cavitating agent technologies affect the final film properties such as the spectra of the reflected light and the film density achieved. Polymeric cavitating agents, such as polybutylene terephthalate (PBT) or Nylon 66, when compared with CaCO3, yield films with lower densities and subsequently better yields than cavitated films with mineral fillers. This is primarily due to the lower polymer densities (1.3 g/cm3) compared
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 191
7/1/11 9:12:36 AM
192
Biaxial stretching of film
(a)
15 kV
1 kx
20 mm
(b)
13.16 (a) Cavitated OPP with PBT cavitation agent showing fibrillated core and spherical cavitating particles of PBT (from Ashcraft and Park, 1983). (b) Cavitated film with CaCO3 cavitating agent showing the fibrillated core and irregular cavitating particles. Treated surface layer
4–5% homopolymer or copolymer
Pigmented interior layer
8–12% homopolymer + 4% TiO2
Cavitated core
6–10% PBT or nylon alternatively 15–25% CaCO3
Pigmented interior layer
8–12% homopolymer + 4% TiO2
Heat seal layer
4–6% terpolymer sealant
13.17 Typical five-layer product design for white cavitated film.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 192
7/1/11 9:12:36 AM
Biaxial stretched films for use in snack packaging
193
with the CaCO3 density (2.5 g/cm3) and the lower weight percent of added cavitating agent (8% vs. 20–25%). In general mineral filled cores for cavitation use CaCO3 as the cavitating agent and give internal cavitation structures as shown in Fig. 13.16b. Because the concentration of CaCO3 is high it tends to color the film slightly due to visible light absorption. In the case of the PBT cavitation, the lower weight of particles yields a larger number of more uniform diameter particles and therefore reflecting void surfaces. Also the visible light absorption characteristics are good and consequently, in OPP, the PBT cavitated films give a true white light spectrum, unlike CaCO3 which makes them superior for image reproduction (Gula et al., 1999). The refinement of the reflected spectra is further controlled by the addition of white pigments such as TiO2. The voided structure also gives cavitated films lower thermal conductivities than uncavitated films and are therefore usable to improve laminated printing substrates for dye sublimation printing (Cambell et al., 1993).
13.3.4 Coating property improvement (out-of-line and inline coating) Coating films to improve the properties for packaging has a history as long as the use of polymeric packaging films. Early packaging materials were based on wax coated paper and as new substrate materials became available, such as cellophane, oriented OPET and later oriented PP films their suitability for packaging was limited due to their property profiles. The general lack of extrudable polymers also limited the possible ways to modify the substrates. Consequently, the primary means of layering polymers to improve film functionality was limited to solution coating and adhesive lamination. In this environment the addition of moisture and oxygen barrier, heat sealing and improved printing and converting surfaces to cellophane and OPET films was of primary importance. This need to add properties led to the development of coatings for application to flexible substrates, from the available polymers such as nitrocellulose, cellulose acetate and polyvinyl chloride (PVC) and PVDC to name a few. This situation was still in existence as PP was invented and first produced as an oriented film and then the development of acrylic coatings for OPP began. All of the coatings were added out of line after the orientation was complete. Coaters were developed to surface treat and prime for adhesion and then top coat the films at high speeds. Both one side and two side coaters are used to produce packaging films. Out-of-line coating layers were relatively thick, ranging up to several microns in thickness depending on the film property requirements. Coating added the basic properties of heat
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 193
7/1/11 9:12:36 AM
194
Biaxial stretching of film
sealability to the substrates. Next the improvement of moisture and oxygen barrier with PVDC coatings allowed for effective packaging properties and extended shelf-lives over wax paper. Acrylic and nitrocellulose coatings greatly aided the improvement of film surface and antistatic properties for film winding and high speed packaging as well as enhanced heat sealing. In short, solution coatings permitted the development of a viable flexible polymer film packaging industry and defined the performance goals of later film developments such as coextruded films. While the basic functionality of heat sealability and machineability can today be added to many polymer film types by coextrusion, solution coating is still important for several basic coated film properties which are difficult to replace. Chief among them are the addition of oxygen and moisture barriers. PVDC coatings can effectively enhance both the moisture and oxygen barrier of most polymeric packaging films. Only metallization can supply a better combination of these two primary packaging barriers than PVDC. However, PVDC is not a favored polymer due to environmental concerns and is no longer as widely used in film coatings, especially in the more developed countries. The barrier properties of PVDC can be partially replaced but not duplicated as few polymers have both the oxygen and moisture barrier of PVDC. The oxygen barrier of PVDC can be replaced with PVOH coatings but the moisture barrier has not yet found a polymeric substitute. An under-appreciated property of film coatings is the chemical barrier of many of the coatings, such as acrylic and PVDC. The basic chemical resistance of the coatings adds a flavor and aroma barrier to polymer films that is difficult to replace. This is especially true of coextruded polyolefinbased films which have inherently poor flavor and aroma barrier properties. In this instance the use of acrylic and PVDC coatings give enhanced flavor and aroma barrier and if needed are the best means of adding this property. Metallized polyolefin films may not have an inherent flavor and aroma barrier (Mount, 1997; Mount and Wagner, 2001). The converting characteristic of surface coatings is also of importance in packaging film as well as label applications. The basic film polymers are not inherently printable and the addition of coatings significantly improves the wetability of the film surface. They can also improve the film surface smoothness and therefore the gloss and printing characteristics. The ability to formulate the coatings also aids in developing the graphic arts requirements for modern printing technologies as well as for labels. In the polyester film industry, in-line coating has been well developed. In this process a thin solution coating, typically water-based, is added to a film surface during orientation. The coating is added between the MD and TD orientation steps. The coatings are primarily a sub or primer coating to improve adhesion for subsequent processes such a metallization, coating
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 194
7/1/11 9:12:36 AM
Biaxial stretched films for use in snack packaging
195
of photographic emulsion and printing. Coating compositions tend to be proprietary but the application method is well known and developed. This process is more difficult, but not impossible to use with OPP films due to the high temperature of the film between the MD and TD ovens relative to the boiling point of the water-based coatings. OPP coatings for enhanced metallized barrier are known (Chu and Johnson, 1993).
13.3.5 Metallization for high barrier No other film technology supplies the level of gas and moisture barrier for packaging as does vacuum metallization. In addition the metallization adds light barrier which aids in maintaining freshness of processed foods and is the key to long shelf-life packaging (Gavitt, 1993; Mount, 1996). However, the level of gas and moisture barrier obtained is controlled by the surface onto which the metal layer is deposited (Yializis et al., 1999). Of the several technologies used to improve metallized film barriers, it is the coextrusion and orientation in the OPP process of EVOH (Migliorini, 1992) and flame treated HDPE (Mount and Migliorini, 1993) which has resulted in the best metallized packaging films. Other processes such as the inline coating of PVOH/EAA (polyethylene acrylic acid) blends (Chu and Johnson, 1993) and the use of in vacuum chamber plasma treatment of the HDPE surface on OPP (Mount and Wagner, 1999) can also generate high barrier metallized films. But it is the coextruded OPP films described above which combine the best overall gas and moisture barrier properties and cost structure. Figure 13.18 shows a mapping of the metallized moisture and oxygen barrier properties of OPP films with various polymers coextruded as the metallization skin. Figure 13.18 demonstrates that it is the surface layer of the OPP film which controls the ultimate moisture and oxygen barrier properties developed, not the OPP base film. Flavor and aroma barrier may also be impacted (Mount and Wagner, 2001) These same surface chemistry control concepts for improving or tailoring the gas and moisture barrier of metallized OPP films are applicable to other metallizable substrates such as PET, HDPE, PLA and other packaging film substrates.
13.4
Advantages and limitations
Orientation significantly improves the polymer films from which they are produced for packaging applications. The primary advantage of orientation to packaging films is in the improvement in the mechanical and thermal properties which permit the use of the films in high speed converting and packaging machines. The improved tensile yield strength, stiffness and dimensional stability of oriented films give them the mechanical strength
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 195
7/1/11 9:12:36 AM
196
Biaxial stretching of film 10
mNylon/OPP mPET
mPETG/OPP mOPP
WVTR, g/m2/day
1
0.1
mEVOH/OPP
0.01
mHDPE/ OPP
Al foil
0.001 0.001
0.01
0.1
1 TO2, cm3/m2/day
10
100
1000
13.18 Barrier mapping of aluminum foil and metallized polyester film compared with several coextruded OPP films. The metallized polymers are coextruded and stretched in the OPP film process as metallization skins (EVOH, HDPE, PETG and amorphous nylon).
to withstand the stresses of printing, laminating and metallization while maintaining the dimensions and register of printed materials. The improved mechanical properties permit the use of thinner films and thereby minimizes material costs relative to unoriented films. The improved thermal stability possible with oriented films relative to cast and blown films also permits their use in high temperature drying and laminating ovens while maintain the dimensions and register of printed materials. The orientation processes also improve the flatness of the polymer films relative to cast and blown films. This permits higher speed converting and manufacturing due to improved film tracking and winding, and can significantly improve the overall manufacture yields and economics of the final packaging material production. These oriented film mechanical and thermal properties were important in part because of the high strength cellophane materials which they were attempting to replace. Existing converting and packaging materials were developed for the strong and stiff cellophane and PET films and the mechanical properties of cast and blown films were insufficient for substitution on existing machinery. Of primary importance were the MD mechanical properties which maintain the film shape on converting and packaging machines. The relative balance between MD and TD mechanical properties are less important, as seen by the concurrent success of both OPP double bubble (balanced) and sequential tentered (unbalanced) films. In addition orientation significantly improves the optical properties of films in terms of haze and gloss enhancing the use of graphics for product
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 196
7/1/11 9:12:36 AM
Biaxial stretched films for use in snack packaging
197
display and this was important in the early film development again for competitive substitution of existing packaging materials. Currently film clarity and surface gloss, while still important, are not as universally important due to the widespread use of both metallized and white opaque films. In these instances the desire for light barrier or a particular graphic appeal trumps the need for film clarity to display the product. In addition the growth of matte surface films to change the visual aspect of the package makes the requirement for high surface gloss less significant. A property which orientation diminishes is the tear strength of the films. The high orientation inherent to the stretching processes gives films with extremely low and at times directional tear properties. This can be mitigated with lamination structures but required careful handling of film rolls to prevent edge damage. However, this property can also be beneficial as in cigarette and similar packaging where the use of tear strips can direct the film tear and control the opening of a packaging material. However, the low tear strength, while often mentioned, has not significantly diminished the widespread growth of oriented films in many packaging applications. While there are many advantages to the use of oriented films relative to cast and blown films the orientation equipment costs are very high. A typical biaxial film orienter today will cost from 15 to 40 million dollars depending on the scope of the equipment supplied. This serves as a significant barrier to entry for small and medium sized companies. However, owing to the high output levels possible for film production, the relative manufacturing cost per unit weight is not high. However, to start such a high capacity line also requires a relatively large volume of film sales to support the cost of start up and continued operations. The combination of high initial equipment cost with the high initial demand for film sales combines together to create the entry barrier.
13.5
Applications
Oriented films have found many diverse uses since they were first developed and continue to expand into emerging technology areas as they are invented. Also oriented films continue to replace existing materials in products as they get smaller, lighter and less expensive, and demand enhanced properties supplied by the films. Originally they were targeted for replacement of existing packaging materials such as coated cellophane and paper as polymer films were first made. As quality, film properties and availability improved expansion into industrial applications increased. Growth has continued in both the areas of substrate replacement and new market areas such as electronic and electrical, identification and security, imaging and magnetic recording media, photovoltaic applications, construction, healthcare and other emerging fields. As the growth of new industries and the availability of new polymers
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 197
7/1/11 9:12:36 AM
198
Biaxial stretching of film
continue, we can expect the application of oriented films to continue to grow and expand. Many of the films become valuable in applications when they are coated, printed, metallized, laminated or converted in some way. Current market areas for several existing and emerging oriented films are listed in Table 13.3. This is not a complete listing of available films and applications as they continue to grow but rather an attempt to list some of the better known applications.
13.6
Future trends
Oriented films for packaging will continue to evolve in terms of film product design complexity and the material combinations which are becoming possible. The continued evolution of coextruded films and the material and orientation equipment developments such as the LISIM ® technology will permit new layer combinations which can be oriented together. These product design changes will drive the future evolution of the film products. Today three- and five-layer OPP films are common and seven-layer films are commercially possible. The rapid growth of micro- (nano) layer coextrusion technology offers the potential for significant improvement in oriented film gas and aroma barrier, mechanical property improvement and perhaps moisture barrier with layer counts easily exceeding 36 layers in a portion of the structure. Already four-layer heat sealing technologies exist, which give oriented films heat sealing and hermetic sealing properties equivalent to cast sealant films. While the technology is not widely available commercially, it has been defined and is based on the coextrusion and orientation of PP and LLDPE coextrusions (Donovan et al., 1999). This coextruded sealant concept combined with thin formulated sealant cap layers for film machinability placed over thick, low melting compliant sealant layers should revolutionize oriented film sealant performance. If this sealant technology is combined with high barrier metallized film technologies which are surface printed on the metal surface, these structures could perhaps permit the replacement of some film laminations. In particular, the incorporation of orientable nylon and EVOH into OPP products both as surface layers for metallization and as internal layers to enhance gas and aroma barrier will continue to grow. Currently they are available in double and triple bubble processes and as volume grows should be producible on tenter frame machines. Specific incorporation of HDPE, cyclic olefinic copolymers or a new category of polyolefin as microlayer combinations in the core of OPP may permit significant improvements to film moisture barrier. Growth of coextrusion technology for improved product offerings and orientation of PET and nylon films would seem to be a natural opportunity
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 198
7/1/11 9:12:36 AM
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 199
7/1/11 9:12:37 AM
Fluoropolymer & PCTFE
PLA
OPS
X
X X
X X X X
X X X
X
X
PEN
PET, polyethylene terephthalate; OPP, oriented polypropylene; Nylon, polyamide; HDPE, high density polyethylene; LDPe, low density polyethylene; Fluoropolymer, polytetrafluoroethylene; PCTFE, polychlorotrifluoroethylene; PLA, polylactic acid; OPS, Oriented polystyrene; PEN, polyethylene naphthalate.
Flexible packaging Food and snacks X X X X X X X X Retort Liquid bag in box Multiwall bags Balloons X X Graphic arts labels X X X X X Paper replacement X X X X holograms X X Industrial capacitors X X X Release sheets X X X X X Fiberglass panels X Laminating films X X X X X X X Hot stamping foils X Insulation facing X X Electronics and electrical Displays X X Wire and cable X X X Radio frequency identification (RFID) X X Flexible circuits X Photovoltaic Front sheets X X Back sheets X Imaging and recording media Photographic paper X X X X-ray film X Thermal transfer imaging X X
Application PET OPP Nylon HDPE LDPE
Table 13.3 Current market areas for several existing and emerging oriented films
200
Biaxial stretching of film
for innovation and growth. However, it seems that this may require a new generation of OPET manufacturing capability as the current oriented PET and nylon producers and technology appears stuck in the concept of single layers with in-line coated films. At the same time continued growth of the existing oriented packaging films will continue as the general standard of living improves driving film volume growth from expansion of improved packaging materials into new geographical areas as well as continued population growth.
13.7
Sources of further information and advice
Primary sources of information on film orientation and oriented film properties are the patent literature, a growing literature on flexible packaging and the orientation equipment suppliers. There are only a few primary sources of film orientation information in the open literature and much of the literature of polymer films focus on film mechanical and morphological properties as opposed to fabrication structure–property relationships and the impact of coextrusion on oriented film properties. Pilot lines for producing rolls of single or coextruded biaxial films exist at several commercial film manufacturing companies as well as the orientation equipment suppliers. While the pilot facilities of film manufacturers are generally not available for public use, at the time of writing, the use of orientation pilot equipment can be purchased for film production and subsequent testing from Brückner at their R&D Technology center located in Austria, BIAX International in Tiverton, Ontario, Canada and Parkinson Technologies Marshall and Williams Plastics pilot orientation lab, located in Woonsocket, RI. There are also scattered around the world many small stretching pieces of equipment produced by Brückner, T.M. Long and other private manufacturers for the stretching of small single samples of oriented film. These can produce biaxially oriented film samples ranging from 6 ¥ 6 cm to 30 ¥ 30 cm starting from 2.5 ¥ 2.5 cm and 10 ¥ 10 cm square samples. Basic information sources I would recommend starting with are: The Science and Technology of Polymer Films, Vol. I, ed. O. J. Sweeting, Interscience Publishers, New York (1968) The Science and Technology of Polymer Films, Vol. II, ed. O. J. Sweeting, Interscience Publishers, New York (1971) Structured Polymer Properties, R. Samuels, Wiley Interscience Publication, John Wiley & Sons, New York (1974) Film Processing, ed. T. Kanai and G. A. Campbell, Polymer Processing Society: Progress in Polymer Processing Series, ed. W.E. Baker, Hanser/ Gardner Publications Inc., Cincinnati (1999)
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 200
7/1/11 9:12:37 AM
Biaxial stretched films for use in snack packaging
201
Solid Phase Processing of Polymers, ed. I. M. Ward, P. D. Coates, M. M. Dumoulin, Polymer Processing Society: Progress in Polymer Processing Series, ed. K. S. Hyun, Hanser/Gardner Publications Inc., Cincinnati, (2000) Plastics Film Technology, W.R R. Park, Van Nostrand Reinhold Company, New York (1969) Plastic Films For Packaging: Technology, Applications and Process Economics, C. J. Benning, Technomic Publishing Co. Inc., Lancaster PA, (1983) Extrusion Coating Manual 4th Edition, ed. T. Bezigian, TAPPI Press, Atlanta GA (1999) Multilayer Flexible Packaging, ed. J. R. Wagner, PDL Handbook Series, Elsevier, New York (2010) The Wiley Encyclopedia of Packaging Technology, Ed. M. Bakker, John Wiley & Sons, Ney York (1986) The interested student of orientation technologies and their impact on film properties should search the polymer literature for structured polymer property information to better understand the impact of crystal and amorphous orientation on film properties. Primary journals to search would be Polymer Engineering and Science, Macromolecules, Journal of Applied Polymer Science, Journal of Polymer Science, Journal of Applied Physics and related journal topics in several languages such as German, Russian and Japanese.
13.8
References
Ashcraft, C. R. and Park, H. C. (1983), ‘Lustrous satin appearing, opaque film compositions and method of preparing same’, US Patent 4377616. Bakker, M., editor (1986), The Wiley Encyclopedia of Packaging Technology, John Wiley & Sons, New York. Bezigian, T., editor (1999), Extrusion Coating Manual 4th Edition, TAPPI Press, Atlanta GA. Bishop, C. A. and Mount III, E. M. (2010), ‘Vacuum metallizing for flexible packaging’, Multilayer Flexible packaging, ed. J. R. Wagner, PDL Handbook Series, Elsevier, New York, pp 185–202. Breil, J. (1998), ‘S-OPP Film Enhancement by LISIM® Technology’, Specialty Plastics Films ’98 Global Film resins, Markets, Applications, 14th Annual World Conference, October 19–21, Dusseldorf, Germany available from: http://www.brueckner.com/ fileadmin/user_upload/downloads/TZ_3.pdf Cakmak, M. (1999), ‘Influence of processing conditions on structure and physical properties of biaxially stretched engineering thermoplastics’, Film Processing, Ed. T. Kanai and G. A. Campbell, Polymer Processing Society: Progress in Polymer Processing Series, ed. W.E. Baker, Hanser/Gardner Publications Inc., Cincinnati. Cakmak, M. and Simhambhatla, M. (1995), ‘Dynamics of uni and biaxial deformation
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 201
7/1/11 9:12:37 AM
202
Biaxial stretching of film
and its effects on the thickness uniformity and surface roughness of poly(ether ether ketone) films’, Polym. Eng. & Sci., 35(19), 1562–1568. Campbell, B. C., Harrison, D. J., Lee, J. S., Maier, L. K., Mruk, W. A. and Warner, C. L. (1993), ‘Receiving element for use in thermal dye transfer’, US Patent 5244861. Chu, S-C. and Johnson, J. A. (1993), ‘Metallized composite film structure and method’, US Patent 5192620. Donovan, K., Kong, D.C., Liu, L., Sexton, D. F. and Su, T.K. (1999), ‘Multi-layer hermetically sealable film and method of making same’, US Patent 5888648. Foster, R. (1989), ‘A guide to the packaging of taste and smell’, Converting Magazine, (April), 62–68. Gavitt, I. F. (1993), ‘Vacuum coating applications for snack food packaging’, Proceedings of the 36th Annual Technical Conference of the Society of Vacuum Coaters, 254–258. Gula, T. S., Aylward, P. T., Bourdelais, R. P. and Haydock, D. N. (1999), ‘Photographic element with microvoided sheet of opalescent appearance’, US patent 5888681. Gutoff, E. B. and Cohen, E. D. (2010), ‘Water- and solvent-based coating technology’, Multilayer Flexible Packaging, ed. J. R. Wagner, PDL Handbook Series, Elsevier, New York, 163–184. Heath, H. B. (1981), ‘Flavor chemistry’, Source Book of Flavors, AVI Books, Van Nostrand Reinhold, New York, 78–147. Hoshino, S., Powers, J., Legrand, D. G., Kawai, H. and Stein, R. S. (1962), ‘Orientation studies on drawn polyolefins’, J. Poly. Sci., 58, 185–204. Kanai, T. and Campbell, G. A., editors (1999), Film Processing, Polymer Processing Society: Progress in Polymer Processing Series, ed. W.E. Baker, Hanser/Gardner Publications Inc., Cincinnati. Koltzenburg, T. (2000), ‘Snack food packaging: 100 years and counting’, PFFC-online, (Jul 1), 12:00 PM, available from: http://pffc-online.com/mag/paper_snack_food_ packaging/ Man, C. M. D. and Jones, A., editors (2000), Shelf-life Evaluation of Foods, 2nd Edition, Aspen Publishers, Inc., Gaithersburg, MD. Matsuga, T., Notomi, R. and Kanoh, T. (1972), ‘Apparatus for heat-setting biaxially stretched films of thermoplastic material’, US Patent 3678546. Migliorini, R. A. (1992), ‘Metallized film combination’, US Patent 5153074. Migliorini, R. A. and Mount III, E. M. (1999), ‘Multilayer film with metallized surface’, US Patent 5194318. Mount III, E. M. (1987), ‘Optics: principle sources of haze and gloss’, Tappi J., 70(3), 160–166. Mount III, E. M. (1996), ‘Study shows two-barrier film technology enhances flavor and aroma protection’, Snack Professional, V4(5) Nov/Dec, 30–31. Mount III, E. M. (1997), ‘Aroma and moisture barrier properties of plain, coated and metallized OPP films’, Technical Proceedings SME Barrier Technology for the Food Packaging Industry, June 4–5. Mount III, E. M. (2010), ‘Coextrusion equipment for multilayer flat films and sheets’, Multilayer Flexible packaging, ed. J. R. Wagner, PDL Handbook Series, Elsevier, New York, 75–95. Mount, E. M. and Benedict, A. J. (1991), ‘Metallisable heat-sealable, oriented polypropylene film – has layer of co-polyester to improve bonding to metal’, European Patent 444340. Mount III, E. M. and Migliorini, R. A. (1993), ‘Multilayer film with metallized surface’, U.S. Patent 5194318.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 202
7/1/11 9:12:37 AM
Biaxial stretched films for use in snack packaging
203
Mount III, E. M. and Migliorini, R. A. (1997), ‘High barrier film’, US Patent 5591520. Mount III, E. M. and Wagner, J. R. (1998), ‘Aroma, oxygen and moisture barrier behavior of coated and vacuum coated OPP films for packaging’, Technical Proceedings of 1998 Polymers, Laminations & Coatings Conference, San Francisco, Tappi Press, 875. Mount III, El. M. and Wagner, J. R. (1999), ‘Enhanced barrier vacuum metallized films’, US Patent 5981079. Mount III, E. M. and Wagner, J. R. (2001), ‘Aroma, oxygen and moisture barrier behavior of coated and vacuum coated OPP films for packaging’, J. Plastic Film and Sheeting, V17 (July), 221–237. Osgood, Jr. W. R. and Therrian, M. A. (1989), ‘Low coefficient of friction biaxially oriented film’, US Patent 4855187. Park, H. C. and Mount III, E. M. (1987), ‘Films, manufacture’, Encyclopedia of Polymer Science and Engineering, Kroschwitz, J.I. ed., Vol. 7, second edition, John Wiley & Sons, Inc., New York, 88–106. Plastics Design Library (1995), Permeability and other Film Properties of plastics and elastomers, William Andrew Inc., Norwich, NY. Poirier, R. V. (1993), ‘High gloss label face stock’, US Patent 5194324. Reade, G. M. (1974), ‘Heat stabilization of oriented thermoplastic films’, US Patent 3814785. Schuhmann, D. E., Peiffer, H. and Schloegl, G. (1994), ‘Sealable, opaque, biaxially orientated multilayer polypropylene film, process for its production and its use’, US Patent 5326625. Schuhmann, D. E., Wilhelm, A., Murschall, U., Peiffer, H. and Meyer, W. (1996), ‘Opaque, Matte, multilayer polypropylene film, process for the production thereof, and the use thereof’, US Patent 5492757. Segransan, M. and Joly, J-C. (1981), ‘Process and apparatus for the manufacture of films by electrostatic application’, US Patent 4244894. Specht, J. (1998), ‘Metallization: an end-user’s perspective’, 41st Annual Technical Conference, Proceedings, Society of Vacuum Coaters, 440–445. Squier, J. A. H. and Lockhart, M. W. (2005), ‘High barrier metallized film with mirrorlike appearance’, US Patent 6844077. Sweeting, O. J. (1968), The Science and Technology of Polymer Films, Vol. I, Interscience Publishers, New York. Sweeting, O. J. (1971), The Science and Technology of Polymer Films, Vol. II, Interscience Publishers, New York. Wagner, J. R. and Mount III, E. M. (1999), ‘Enhanced barrier vacuum metallized films’, US Patent 5981079. Yializis, A., Mikhael, M. G., Ellwanger, R. E. and Mount III, E. M. (1999), ‘Surface functionalization of polymer films’, 42nd Annual Technical Conference Proceedings Society of Vacuum Coaters, 469–474.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-13.indd 203
7/1/11 9:12:37 AM
14
Biaxially stretched films for product labeling
B . H o s t e t t e r, Formely of Applied Extrusion Technologies, Inc., USA Abstract: This chapter discusses the various labeling processes and plastic label films used to decorate containers, bottles, packages and products. Labels are routinely used to identify and decorate products, with plastic labels as a significant and fast growing segment of the label market. The benefits and limitations of each labeling process and specifically designed plastic label films are described. Biaxial orientation of plastic films is utilized to develop critical and cost-effective film properties for label applications. Key words: plastic films, label, labeling, container decoration, biaxial orientation.
14.1
Introduction
A label is defined as a printed or decorated substrate that is applied to a container, package or product to communicate product identification and product information and to convey an overall decorative and graphical display. Labels are prepared independent of the container, package or product and are applied either during or after the container, package or product is manufactured. Labels are produced as a printed substrate either in roll or sheet form and contain a broad range of information and graphics design. Labels are used extensively to communicate product information including product identification, product ingredients, health information, regulatory information, instructions, promotional information and bar codes for product tracking and inventory management. Labels are also used to provide products with unique graphic designs, product images, coloration, decoration and point of purchase advertisement. A typical label design is informational, graphical and multicolored. Labels are used as an alternative to directly printing or decorating containers or packages based on a label’s ability to provide enhanced features of flexibility, quality, efficiency and cost. Biaxially stretched plastic films are often the choice for a label substrate due to their low cost, ease of producing a wide variety of label types, high quality graphics, compatibility with labeling systems and end-use performance attributes, and are globally available from numerous plastic film producers. Biaxially stretched plastic films that are utilized in label 204 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 204
7/1/11 9:18:08 AM
Biaxially stretched films for product labeling
205
applications include polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), polyethylene terephthalate glycol (PET-G), polystyrene (PS), polyvinyl chloride (PVC), synthetic paper (syn. paper) and polylactic acid (PLA), a recently introduced bio-plastic. Plastic film label substrates are available as clear, matte, white or metallized in appearance. Labels may have all or part of the substrate covered by print or graphics and may also provide special effects such as holograms, tamper evidence and smart packaging capability. The global label market is estimated to be 42 billion square meters in 2011 and growing to 49 billion square meters and $105 billion by 2013. Of the total label market, 55% are pressure-sensitive labels and 30% of all labels will be plastic labels in 2011 (Freedonia Group, 2007 and 2009).
14.2
Labeling systems and technologies
Labeling systems are used to produce a specific label and labeled product or container based on the design and contour of the container and the label’s graphic, regulatory and end-use requirements. This typically begins with consideration for the final product’s label requirements with the labeling process selected as the best approach to deliver this result. For example, labeled products can contain one or more labels covering part or the entire container and can be located at various positions along or around the container. Labeling may be conducted during or after the product or container has been manufactured. Graphics designs may be basic (black and white) or of high quality with photographic-like images, often used as a marketing tool for product identification and point of purchase recognition. Bottles or containers may be composed of plastic, glass, ceramic or metal. Non-container labeled products may be of any type or shape and composed of any type of solid or rigid material. In some applications, a product may utilize more than one label type and labeling process. The container, label and labeling process must be designed with the requirements and limitations of each component to achieve a compatible labeling system. The cost of product labeling (applied label cost) must include the cost for the substrate, printing, adhesive and the process of labeling. Labeling systems are classified as (1) self-adhesive (the adhesive is a component of the label), (2) glue applied (adhesive is applied during the labeling process), (3) sleeves (a cylindrical label sleeve is formed before labeling, no adhesive is applied to the container) and (4) tags (no adhesive or container, the ‘label’ or tag is the product). Glue applied labeling systems include roll-fed, roll-fed shrink and cut and stack. Self-adhesive labeling systems include pressure sensitive, in-mold and thermal transfer. Sleeve
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 205
7/1/11 9:18:08 AM
206
Biaxial stretching of film
labeling systems include shrink sleeve and stretch sleeve. Tag labeling systems include thermal transfer and direct thermal labeling. Each is described below along with the attributes and parameters of each process.
14.2.1 Roll-fed labeling system Labels are delivered from a roll (reel) of continuous labels and cut to an individual full diameter wrap or wrap around (360°) label as the initial step of the labeling process. After the label is cut from the roll, a narrow vertical strip of hot melt adhesive is applied to the leading and trailing edge, on the reverse (back) side of the label. A flat surface on the container is required for the application and placement of the label where the roll fed labeling process uses the container as a mandrel. Rotation of the container, after attachment of the leading edge of the label, results in the label wrapping around the container until a 360° or full wrap is achieved. The length of the label is slightly longer than the diameter of the container and the trailing edge is overlaid onto the leading edge and held intact by the strip of adhesive. Half wrap or 180° labels are also possible with roll fed labeling. A description of the roll-fed labeling process is shown in Fig. 14.1. Roll fed labeling is a high volume, high speed and efficient labeling process and has the most cost-effective total cost of labeling for plastic films. ∑ Label design and parameters: labels are printed as rolls using a monoweb film (single ply) or lamination (2-ply film) structure. The label width is equal to the height of the label and the label length is in the direction of the roll. Exact vertical placement of the label on a flat surface or panel of the container is required. The overall label shape is rectangular. Labels may be clear white, or metallized. ∑ Plastic film substrates: PP, PET. ∑ Applications: plastic and glass bottles and containers, metal cans, any cylindrical container with a straight (flat panel). Most applications are full wrap (360°) labels. ∑ Benefits: high speed, high volume, low applied cost, thin/low stiffness labels. ∑ Deficiencies: only 360 or 180 label, requires straight wall/flat panel section for location of the label on the container.
14.2.2 Roll-fed shrink labeling system Labels are delivered from a roll (reel) of continuous labels and cut to an individual full diameter wrap or wrap around (360°) label. Labels are cut from the roll and a narrow vertical strip of hot melt adhesive is applied to the leading and trailing edge on the reverse (back) side of the label. The © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 206
7/1/11 9:18:08 AM
Biaxially stretched films for product labeling
207
14.1 Roll-fed labeling process. A feeder roll continually pulls the label from the reel to a cutting unit that cuts the label length to the proper dimension. Two narrow strips of hot melt adhesive are applied by a heated glue roller to the leading and trailing label edges. The leading edge of the label is transferred and attached to the rotating container. The rotating container continues to wrap the label around the container until the trailing edge with adhesive is wrapped on top of the leading edge, resulting in a labeled container (Contiroll – HS. Sources: www. krones.com and Buckle and Leykamm, 2001).
label location on the container is a non-flat or contoured section (requiring shrinkage of the label) and at least one area, within the label location, that is a straight or flat panel. A flat surface on the container is required for the application and placement of the label where the roll fed labeling process uses the container as a mandrel. Rotation of the container, after attachment of the leading edge of the label, results in the label wrapping around the container until a 360° or full wrap is achieved. The length of the label is slightly longer than the diameter of the container and the trailing edge is overlaid onto the leading edge and held intact by the strip of adhesive. After the label is applied, the labeled container is exposed to elevated temperature by means of hot air or steam and the label shrinks tightly around the contour of the container. The adhesive must have sufficient strength after labeling to withstand the shrink forces of the label at the label shrink temperature required to achieve a tightly fitted label to the container contour. Specially designed hot melt adhesives and UV cured (thermoset)
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 207
7/1/11 9:18:08 AM
208
Biaxial stretching of film
hot melt adhesives have been used to create an acceptable overlap seam. Historically, solvent (tetrahydrofuran, THF) has been used to create the overlap seam with foamed PS labels for glass beverage bottles. The roll fed labeling process is shown in Fig. 14.1. ∑ Label design and parameters: labels are roll printed as mono-web (single ply) or as a lamination (2-ply film) structure. The roll width is equal to the height of the label. Exact vertical placement of the label on a non-flat or contoured surface or panel of the container is required and with the incorporation of a straight or flat panel area. The film is designed to shrink in the machine direction (MD) only and to have <5% shrinkage in the transverse direction (TD). The MD direction is equivalent to the circumference of the container and the TD direction is the height of the label. The overall label shape is rectangular. Labels may be clear white or metallized. ∑ Plastic film substrates: PP, PET-G, PS. ∑ Applications: plastic and glass bottles and containers, metal cans, any cylindrical container with a non-straight or contoured label panel. Shrinkage equivalent to dimensional container change up to 65%. ∑ Benefits: high speed, high volume, medium applied label cost, thin to medium/low to medium stiffness labels. ∑ Deficiencies: only 360° (full wrap) label, requires non-straight or contoured panel section and a straight wall area for location of the label on the container.
14.2.3 Cut and stack labeling system Pre-cut (die cut) labels are delivered from a stack as a full wrap (360°) label. A hot melt adhesive is applied as a vertical strip to the leading and trailing edge on the reverse (back) side of the label. The length of the label is slightly longer than the diameter of the container and the trailing edge is overlaid onto the leading edge and held intact by the strip of adhesive. Half wrap or 180° labels are also possible with cut and stack labeling (see Fig. 14.2). Spot plastic labels are not commonly available for cut and stack labeling. Spot labeling technology requires cold glue adhesives that cover 100% of the label reverse surface and are compatible only with paper labels. Several plastic spot label substrate and labeling systems are in development. The cut and stack labeling process is shown in Fig. 14.2. Cut and stack plastic labels are best suited for high volume and high speed applications. Cut and stack plastic film labeling is the second most effective in achieving a low applied cost of labeling. ∑ Label designs and parameters: cut and stack labels may be roll or sheet
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 208
7/1/11 9:18:08 AM
Biaxially stretched films for product labeling
209
14.2 Cut and stack labeling process – hot melt. The cut and stack labeler has two independent hot melt stations. The first station applies a vertical strip of adhesive to the container. The rotating container passes a magazine of pre-cut labels and the line of adhesive on the container contacts the leading edge of the label and pulls it from the magazine. Simultaneously, the second adhesive station applies a vertical strip of adhesive to the trailing edge of the label. Continued rotation of the container results in a fully wrapped labeled container (Canmatic Sources: www.krones.com and Buckle and Leykamm, 2001).
printed. Roll printed labels may be a mono-web film or lamination (2ply) structure. The label width is equal to the height of the label. Sheet printed labels are surface printed, mono-web films. Printed rolls or sheets are sheeted (if rolls) and die cut to individual labels and combined, one on top of the next to form a stack of labels. Exact vertical placement of the label on a flat surface or panel of the container is required. Overall label shape is rectangular. Labels may be clear, white or metallized. ∑ Applications: plastic and glass bottles and containers, metal cans, any cylindrical container with a straight (flat panel) surface. ∑ Plastic film substrates: PP, synthetic paper. ∑ Benefits: high speed, high volume, die cut labels. ∑ Deficiencies: only 180° and 360° labels, thicker/higher stiffness labels (compared with roll-fed). Spot plastic labels are not readily available for cut and stack labeling. Spot cut and stack labeling technology requires an adhesive with 100% coverage of the label surface (reverse or back side). Currently, only cold glue (aqueous) adhesives, that are only compatible with paper labels, are effective.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 209
7/1/11 9:18:08 AM
210
Biaxial stretching of film
14.2.4 Pressure-sensitive labeling system Die cut labels are delivered from a roll that is a multilayer structure composed of the label, adhesive and release liner. The individual label and adhesive are separated from the release liner and applied to the bottle, container or product. The release liner is disposed of after label application. Labels may be machine or manually applied. The label is applied in a predetermined location on the container for machine applied applications. The pressuresensitive labeling process is shown in Fig. 14.3. ∑ Label designs and parameters: labels are surface printed on a roll of the multilayer structure composed of the label substrate, adhesive and release liner. The adhesive has 100% coverage of the reverse side of the label. The roll of the printed multilayer structure is individually die-cut for each label to the exact size and shape of the final label. Labels are rectangular, rounded or spot (patch) labels of any shape and dimension or 180° or 360° wrap-around labels.
14.3 Pressure-sensitive labeling process. The label applicator for pressure sensitive labels removes the label with a wedge from the release liner and the label is transferred to the container. The location of the label on the container is precisely controlled and after placement on the container is pressed smoothly into place to insure good adhesive contact and label transfer (Autocol, Sources: www.krones.com and Buckle and Leykamm, 2001).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 210
7/1/11 9:18:08 AM
Biaxially stretched films for product labeling
211
∑ Applications: plastic and glass bottles and containers, metal cans, any cylindrical container with a straight (flat panel), squeezable plastic containers, non-uniform containers with a flat panel. Pressure-sensitive labels allow for exact placement on a container, product or signage. ∑ Plastic film substrates: PP, PE, PET, PS, PVC. ∑ Benefits: maximum flexibility for label type, geometry, location, multiple labels, substrate selection, labeling of non-uniform containers and products. Adhesive coverage of 100% provides excellent adhesion to the container and long-term durability. Pressure-sensitive clear labels are used to achieve a ‘no label look’ (a labeled container that imitates a direct printed container, where the clear, non-printed label is transparent). ∑ Deficiencies: expensive, 100% adhesive coverage, disposal of release liner.
14.2.5 In-mold labeling system Die cut labels are applied to a blow molded or injection molded bottle or container or injection molded or thermoformed product (such as a lid or tray) during the process of blow molding, injection molding or thermoforming. No external adhesive is required as the adhesive capability is part of the substrate and label design. Label location is exactly determined as part of the mold design and may be a panel or cavity in the container and may be slightly curved or concave. ∑ Label designs and parameters: labels are roll or sheet printed. Roll printed labels may be mono-web or a lamination (2-ply) structure. Sheet printed labels are mono-web films. Printed rolls and sheets are sheeted (rolls) and die cut to individual labels and combined, one on top of the next, to form a stack of labels. The adhesive is inherent in the substrate and label and activated during the molding process by temperature and pressure. The adhesive may be incorporated into the label substrate by a coextrusion, extrusion coating or a coating process. ∑ Applications: blow molded, injection molded or thermoformed PE or PP containers and products. ∑ Plastic film substrates: PP, PS, synthetic paper. ∑ Benefits: labeling while producing the container or product. Spot and custom shaped labels, conforms to curvature or a panel of the container, unique container shapes, potential for light weighting of container, no label look. No additional cost or process for labeling or adhesive. ∑ Deficiencies: Only pre-decorated containers, only spot labels.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 211
7/1/11 9:18:09 AM
212
Biaxial stretching of film
14.2.6 Shrink sleeve labeling system Labels are delivered as pre-printed and pre-formed cylindrical sleeves from a stack or roll of sleeves and individually applied around the outer circumference of the container. Once in place, the label is shrunk tight to the contour of the container by application of heat by hot air or steam. Label shrinkage, equivalent to the container dimensional change, ranges from 10 to 75%. Labels may be applied to a part of or the entire container. A label that covers the cap or lid of a container may include a perforation that would allow for a tamper evident feature, where the label above the perforation may be removed to allow access to the container’s cap or lid. The shrink sleeve labeling process is shown in Fig. 14.4. ∑ Label designs and parameters: labels are pre-printed on a roll of monoweb plastic film in the MD. Label sleeves are produced from the preprinted roll that is converted by a vertical forming process into a sleeve
(a)
(b)
(c)
14.4 Sleeve labeling process. (a) The sleeve of labels is unwound from a reel and then opened and placed over a mandrel. A cutting head cuts the sleeve to the exact label length. (b) The sleeve is then pulled over the container by an applicator fork. (c) A clamp is then applied to hold the sleeve in place on the container as the applicator fork is removed. For stretch sleeves, the clamp is removed producing a labeled container. For shrink applications, the labeled container is passed through a hot air or steam tunnel which activates the film shrinkage and produces a label that is tightly wrapped to the contour of the container (Sleevematic AF Sources: www.krones.com and Buckle and Leykamm, 2001).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 212
7/1/11 9:18:09 AM
Biaxially stretched films for product labeling
∑ ∑ ∑ ∑
213
with the long seam sealed with solvent or adhesive. The diameter of the sleeve is slightly larger than the diameter of container. Graphics must incorporate consideration for shrinkage and distortion of the label on the container, often up to 65%. The film is designed to shrink in the TD only and to have <5% shrinkage in the MD. The TD direction is in the circumference of the container and the MD direction is the height of the label. Applications: highly contoured containers and bottles, label may cover all (full body) or part of the container. Plastic film substrates: PVC, PET-G, PS. Benefits: allows for tight fit of label from 0 to 75% on all or part of container. Tamper evident capability. Deficiencies: expensive, additional label preparation step for sleeve production, graphic design needs to incorporate shrinkage, slow labeling speed.
14.2.7 Shrink stretch labeling system Roll of pre-formed label sleeves are cut to a single label, stretched and placed around the circumference of a container. The tension of the stretched label is released and the label relaxes around the container to form a tight fit to the contour of the container. The dimensional change of the container can vary from 0 to 30% and can be applied to a straight contour or ribbed containers. ∑ Label designs and parameters: labels are pre-printed on a roll of mono-web plastic film in the MD. Label sleeves are produced from the pre-printed roll that is converted by a vertical forming process into a sleeve with the long seam sealed with a hot wire. The diameter of the sleeve is slightly smaller than the diameter of the container. Graphics must incorporate consideration for shrinkage and distortion of the label on the container up to 30%. PE is used as the film and is designed to stretch in the TD which is the circumference of the container. The MD direction is the height of the label. ∑ Applications: contoured, ribbed or straight wall containers and bottles. The label may cover all or part of the body of container. ∑ Plastic film substrates: PE. ∑ Benefits: allows for tight fit of label on all or part of container, low cost PE substrate, no adhesive, low applied label cost. ∑ Deficiencies: additional step of sleeve production, slow labeling speed.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 213
7/1/11 9:18:09 AM
214
Biaxial stretching of film
14.2.8 Thermal transfer labeling system Thermal transfer labels may be pressure-sensitive labels, tags or signs that are printed by means of a thermally activated transfer of ink (resin) or wax from an external ribbon onto the surface of the film. Thermal transfer labels can be printed at the time of use and with specifically designated information. Most thermal transfer applications are small sized labels or tags for small volume applications. ∑ Label designs and parameters: the thermal transfer label is designed for the label surface to be compatible and receptive to the designated print system. The film surface has a compatible surface functionally or coating for good ink adhesion. Plastic films are often selected for special use, low volume applications with specific end-use requirements. End-use requirements include tear resistance, outdoor weathering, and water and solvent resistance. ∑ Applications: pressure-sensitive labels, tags. ∑ Plastic film substrates: PP, PET, PVC, synthetic paper. ∑ Benefits: on demand printing, unique and specific information, low volume applications often for single label or tag applications. Durable and multicolored inks, portable print system. ∑ Deficiencies: Expensive, low volume applications, requires thermal transfer ink and thermal transfer print system, heavy ink application.
14.2.9 Direct thermal labeling system Direct thermal labels are pressure-sensitive labels, tags and signs that are printed by means of a thermally activated coating on the surface of the film. Thermal transfer labels are typically printed at the time of use and with specifically designated information. ∑ Label designs and parameters: the direct thermal label is designed for the label surface to be compatible and receptive with the direct thermal print system. The film surface contains a surface functionally or coating that reacts with heat applied from the printer to form a dark or black color at the location of applied heat, resulting in a printed label or tag. Plastic films are often selected for special use, low volume applications with specific end-use requirements. End-use requirements include tear resistance, outdoor weathering, and water and solvent resistance. ∑ Applications: pressure-sensitive labels, tags. ∑ Plastic film substrates: PP, PET or synthetic paper. ∑ Benefits: on demand printing, unique and specific information, low volume applications often for single label or tag applications, portable print and label system.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 214
7/1/11 9:18:09 AM
Biaxially stretched films for product labeling
215
∑ Deficiencies: expensive, low volume applications, requires direct thermal coating and direct thermal print system. Only one print color, typically black, on a white substrate, low graphics definition. Print is not durable or weather resistant.
14.3
Label applications
Labels are frequently utilized for bottles and containers for food, beverage, household, pharmaceutical, health and beauty, medical, automotive and industrial products. Labels are also used as a tag or sign in a broad range of non-container applications and products. When a label is applied to a bottle or container, the label can be applied as one or more labels on a single container and may utilize different label types, label substrates, sizes and a variety of graphic appearances. The label, container and labeling process combine to deliver the required appearance and properties for the final labeled product. For non-container applications, the tag or label is either applied as a pressure-sensitive label with the ability to locate the label on any flat surface of the product or for tags, the label itself is the product and requires no additional container or product application.
14.3.1 Label requirements by application Label properties are predominantly a function of the substrate selected, where the substrate properties are a significant contributor to label properties. Substrate selection is based on the label design, labeling process and enduse label requirements and must be compatible with the labeling system that provides the lowest total applied label cost. The total applied label cost includes the cost of the label substrate (on a yield or coverage basis), label preparation, adhesive and labeling speed and efficiency. Oriented plastic films used in labeling are produced from film forming, thermoplastic resins including PP, PE, polyethylene terephthalate or polyester (PET), polyethylene terephthalate glycol or polyester copolymer (PET-G), PS, PVC, synthetic paper and PLA. Plastic films are often in competition with or a replacement for paper as a substrate of choice for label applications. Many labeling applications and processes have historically utilized paper labels. As a consequence, plastic substrates are often designed to incorporate critical labeling performance properties typical of paper. Compared with paper, plastic-based labels offer many additional attributes such as enhanced graphics, moisture resistance, tear and abrasion resistance, transparent labels and squeezable labels. Label substrates are selected based on the labeling system that delivers the lowest applied label cost and achieves all performance requirements.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 215
7/1/11 9:18:09 AM
216
Biaxial stretching of film
Based on plastic film costs, a simplistic calculation of substrate cost is the combination of resin cost/lb (kg) ¥ resin density ¥ film flex stiffness/film yield, with the flex stiffness being a critical factor for labeling performance. Using this analysis, the cost-effectiveness of plastic label films is rated as: PP < PET < PE < synthetic paper < PVC ~ PS < PET-G < PLA. PP films are widely used for non-durable applications and are the most economical for applications not requiring high levels of shrinkage or for elevated temperature or outdoor exposure. PET films are used for durable applications such as outdoor exposure or elevated temperature. PVC, PET-G and PS-based films are used for high shrink applications. These films are selected for applications requiring >15% shrinkage. Film shrinkage is unidirectional, with maximum shrinkage between 35 and 75%. PE films are used for stretch sleeve applications. PE, PP or PE/PP blends in synthetic paper are used for in-mold and PP and synthetic paper are used for thermal transfer and direct thermal applications. Roll-fed labels are rectangular labels applied to bottles or containers that have a flat paneled surface. Roll fed labeling is used for high volume applications such as PET beverage bottles where the low total applied label cost is a significant factor. For smaller volume and more durable and higher valued applications such as aerosol and metal cans, the label design may use metallized or metallized holographic labels to achieve a high quality graphical appearance. Certain applications, such as ribbed metal cans, utilize higher thickness (stiffness) labels that result in coverage of the slight nonflat undulations in the container surface. PP films are the dominant plastic film used in roll fed labeling based on labeling and end-use performance, a wide range of film types, and favorable economics. The most critical feature for roll fed label substrates is efficient, high speed labeling performance. Key film properties are: minimum label stiffness equivalent to ~ 40 microns for voided opaque films or 30 microns for clear films, good machine direction tensile modulus, low and controlled outside controlled coefficient of friction (COF), label cutting and surface functionality for hot melt adhesion. Roll fed labels may utilize a single web or lamination label design. For both approaches, the final label properties are equivalent. Roll fed label films are offered in a wide range of thicknesses from 12 to 50 microns and are available as clear, matte, white/opaque, metallized and metallized holographic films. The white/opaque films are most commonly a three to five-layer coextruded structure with a voided core design. This voided core film design provides cost-effective film stiffness and opacity. PE films are not utilized for roll fed labeling based on the inherent lower flex stiffness and MD tensile modulus. Both properties are insufficient to achieve effective label printing and high speed roll fed labeling. Glass bottles and containers typically do not use roll-fed labels as glass
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 216
7/1/11 9:18:09 AM
Biaxially stretched films for product labeling
217
containers are not uniformly round and contain small concave and convex areas. Roll-fed labels do not tightly wrap these non-uniform containers. For acceptable labeled glass container appearance, a shrink label or a 100% adhesive covered label is required. PP roll fed labels are the largest volume segment of plastic labels, primarily due to their popular use in carbonated soft drink, water and beverage PET bottles. Roll-fed shrink applications utilize roll-fed labeling technology and that also incorporates the ability for the label to shrink tightly around a contoured section of the container. All performance criteria apply for roll fed labeling substrates with the additional requirement of enhanced machine direction shrinkage in the substrate and label. Roll fed shrink applications typically have a significant part of the container as a flat or straight panel with sections in the middle, top or bottom of the container being contour and requiring shrinkage of the label to achieve a tightly wrapped label. For roll fed shrink applications, the contoured part of the container requires a substrate with shrinkage between 10 and 65% in the MD and less than 5% in the TD, at a temperature appropriate for label shrinkage. This is typically 10 to 20 °C below the maximum film shrinkage temperature. For a labeled container, the machine direction is the direction of the diameter of the container and is in the roll direction of wind for labels. Biaxially oriented PP films are available for shrink requirements up to 20%. These are produced by a post-MD stretching (tensilization) of a biaxial oriented film or by using a simultaneous biaxial orientation tenter process, commercially available from Brückner Maschinebau and known as simultaneous stretching LISIM ® technology (www.brueckner.com). Uniax oriented films produced from a blend of PE and PS resins with MD shrinkage up to 25% and polyolefin-based films with MD shrinkage up to 50% have been recently introduced by Polyphane under the FIT trade name (www.polyphane.com). PVC, PET-G and PS uniaxial MD shrink films are available with shrinkage up to 65%, but are not in abundant use. Foamed opaque PS MD shrink films have been used for glass bottles. These labels traditionally used a solvent (THF) to create the overlap seal, with some newer applications utilizing hot melt adhesives. Cut and stack labels are selected for applications similar to roll-fed labeling with bottles and containers that have a flat paneled surface. For plastic labels, label designs are 180° or 360° rectangular labels and utilize a hot melt adhesive. Labels are pre-cut to size and applied to the labeler from a stack of labels loaded in to a magazine. Due to the cut and stack dispensing process and the relatively large dimension of a 180° or 360° label, cut and stack labels require a higher level of flex stiffness than roll fed labels for efficient labeling performance. To convert cut and stack labels, the printing process may be either roll printed or sheet printed. With roll printed processes, the substrate stiffness
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 217
7/1/11 9:18:09 AM
218
Biaxial stretching of film
is 25–50% greater than for roll fed labeling. For sheet printed labels, the substrate stiffness is between 50 and 100% greater than for roll fed labels. For both processes, the substrates must incorporate attributes of surface roughness, antistatic character and low COF to accommodate a sheet to sheet sliding mechanism between top to bottom substrate surfaces. For sheet printed applications, the print surface must also be designed to accommodate the printing inks that are unique to the sheet printing process. These inks have high solids content and are oxidative or UV cured. Cut and stack labels historically have been paper based and are dominated by spot label applications. The cut and stack labeling process has been developed for aqueous-based (cold glue) adhesives – suitable for paper where the adhesion is created as the paper absorbs the moisture from the adhesive. As plastic films are not water absorbent and have little permeability, the aqueous-based adhesive drying method is not effective. Several development activities, aimed at creating non-aqueous based adhesives or water absorbent plastic films have been developed. None has achieved significant commercial success at this time. The inherent increased flex stiffness (thickness) required for cut and stack sheet printed plastic labels make them a more expensive substrate than roll fed labels on a per label basis. For this reason, roll printed systems have been recently developed (1) which roll print and at the end of, or out of line, sheet and cut into labels or (2) in which the roll is fed into an inline sheeting process that sheet prints and cuts into labels. These roll to sheet print processes have been successful in reducing the label stiffness requirements and inherent label cost for plastic label films. Cut and stack plastic label films are mostly PP films or synthetic paper. PP films are available in clear, white and metallized varieties. All opaque PP films are three- to five-layer voided opaque structures. All sheet fed substrates are mono-web and most roll to sheet films are mono-web. A small percentage use roll printing with out-of-line sheeting that may utilize a mono-web or lamination film approach. Pressure-sensitive or self-adhesive labels consist of a multi component structure that is a combination of the label substrate (facestock), adhesive and a release liner. Upon application, the label and adhesive are applied to the container or product and the release liner is separated and disposed. Pressure-sensitive labels are utilized in a wide variety of applications from larger volume labeled containers to hand applied labels for a broad spectrum of applications and products. The primary requirements of a pressure-sensitive label substrate are: sufficient stiffness and MD modulus to withstand release from the liner, a print surface receptive to a wide variety of print processes and ink types (flexographic, gravure, UV cured and screen printing), and incorporation of end-use service requirements. Pressure-sensitive labels are used in the broadest variety of applications
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 218
7/1/11 9:18:09 AM
Biaxially stretched films for product labeling
219
and include the most significant end-use performance criteria. Large volume applications are applied with labeling machines for plastic and glass bottles and containers. Low volume applications are used in automotive, electronic, medical, marine and household applications. Many of these have specific use environments that include UV and solvent resistance, sterilization, abrasion resistance, and to squeeze and bend with a flexible container. For this reason, the substrate is often selected with these end-use requirements in mind. Almost all film substrates are used in pressure-sensitive label applications including PE, PP, synthetic paper, PET, PS and PVC. Most pressure-sensitive labels are roll surface printed onto the multicomponent structure. The print surface must be receptive to a wide variety of ink systems and end use applications. Many pressure label substrates have a special ink receptive functionality and are top coated with an acrylic or urethane-based coating. After printing, the labels are die cut to the final label size and shape. Pressure-sensitive labels are the largest label application, although many use paper-based substrates. Pressure-sensitive labels are the second largest application of plastic labels behind roll fed labeling. In-mold labels are applied to blow molded, injection molded or thermoformed containers and products during the container or product molding process. In-mold labeled containers and products are most often composed of PE or PP resins. The in-mold label surface that is in contact with the container incorporates 100% coverage heat and pressure-activated adhesive that bonds to the container during the molding process. The label substrate must conform to the curvature of the container. The label must also withstand the heat of the molding process and not display any dimensional shrinkage, surface mottling or visual defects. In-mold labels must have sufficient stiffness to remain upright when loaded into the mold and are often required to hold a static charge as part of this process. Shrink sleeves are used for high shrink applications ranging from 20 to 75%. A pre-printed roll is formed into a tube or sleeve prior to labeling. The sleeve labeling process applies an individual label sleeve onto the container in the designed location and the label is shrunk tight to the container by hot air or steam. The substrate for this application requires a TD shrinkage up to 75% and <5% MD shrinkage. The TD is in the direction of the circumference of the container and the MD is in the direction of the height of the container. Stretch sleeves are pre-formed sleeve labels where an individual sleeve is stretched to a greater diameter than the container and the label is then applied to the designed location on the container. The labeler than releases the stretch tension and the label relaxes to a tight fit around the container. A pre-printed roll is formed into a tube or sleeve prior to labeling. The overlap
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 219
7/1/11 9:18:09 AM
220
Biaxial stretching of film
long seam is typically formed by a hot wire that creates localized melting and fusion of the PE substrate to form a long seam seal. The container may be a flat or straight wall area or the container may have concave or convex contours or ribs. The stretch sleeve label fits tightly to the contour of containers with up to 30% dimensional contour. Thermal transfer labels are pressure-sensitive labels or tags or they may be individual tags, signs or displays where the printing is conducted on demand after the label or tag has been produced. These films are designed with a coating or functional surface designed to be receptive to the thermally transferred inks. The print system is designed to transfer a resin or wax-based ink to the label substrate upon heat activation from the printer. Thermal transfer print systems are one color (black) or multicolored, typically with a heavy ink deposition. Thermal transfer substrates and inks are often selected for durable applications requiring weather resistance, solvent resistance, thermal stability, and tear and abrasion resistance. PP, PET and synthetic paper substrates are used for thermal transfer labeling. Direct thermal labels are pressure-sensitive labels or tags or they may be individual tags where the printing is conducted on demand after the label has been produced. These films are designed with a coated or functionalized surface designed to react with the exposure of heat applied from the thermal transfer labeler to thermally ‘transfer’ the print information to the label. Direct thermal labels are typically one color (black) on a white substrate and often contain bar code information. Most direct thermal printing is nondurable and is potentially damaged with exposure heat, sunlight, moisture and abrasion. PP and synthetic paper substrates are used for direct thermal labeling. Table 14.1 is an overview of labeled container parameters and the relationship to labeling processes and label film substrates.
14.4
Label preparation – label design, printing and converting
Labels are produced by either roll or sheet printing of the plastic film with the application of graphics, coloration and information to create the label. Printed label films may also be coated with an over lacquer or topcoat or laminated with a clear over laminate film. The topcoat or over laminate film impart specific surface properties to the label including surface gloss or matte appearance, abrasion resistance, COF, and surface functionality for adhesion and post-labeling printing. Labels are most often printed in multiple lanes if rolls or an array of 10 to 40 labels if sheets. Before labeling, rolls are slit into a single lane of labels or if sheets, die-cut into individual labels and assembled in a stack. In addition to product information and graphics, labels often include a bar code
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 220
7/1/11 9:18:09 AM
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 221
7/1/11 9:18:09 AM
Cut & stack
Pressure In-mold sensitive
Roll-fed shrink
Shrink sleeve
Stretch sleeve
Thermal transfer
Straight X X X X X X Contour X X X Hide rib/seam X X X Show rib X X X 360° X X X X X X X 180° X X X X Spot/multiple X X X X White/metal X X X X X X X Clear X X X X X X X No label look X X X Plastic X X X X X X X X Paper X X X X X
Roll-fed
Table 14.1 Labeling processes and labeled container requirements
X X
X X
X
Direct thermal
222
Biaxial stretching of film
for product inventory identification and tracking and for some applications a label indexing mark (eye spot) to be used as an optical indication of label length used in the labeling process.
14.4.1 Roll-fed label design Roll fed labels are printed on any type of roll press either as surface (or reverse print) mono-web labels or as a 2-ply lamination structure. For mono-web labels, the converter typically applies a surface top coat over the printed surface to achieve the required gloss, COF, surface functionality and abrasion properties. For lamination-based labels, the required label properties are designed into the over-laminate films and the print is situated between the two lamination films. The lamination is bound by a thin adhesive, applied after printing. A contrasting printed eye spot is required, indicating the length of each label to provide for optical detection during the labeling cutting process. During the roll fed labeling process, a roll of labels is cut into individual labels that are wrapped around the container circumference with a vertical stripe of hot melt adhesive applied to the leading and trailing edge of the label. Roll fed label films may be clear, clear matte, white or metallized and contain special features such as a holographic image. Critical label properties include flex stiffness, MD modulus, COF, antistatic, surface functionality, and easily cut during the labeling process. For white or fully printed labels, film whiteness and opacity are also important attributes and often sufficient as a substitute for printed white ink.
14.4.2 Roll-fed shrink label design Roll fed shrink label requirements and label production are the same as for roll fed labels with the additional requirement that the plastic film and label must have MD shrinkage between 20 and 65% and TD shrinkage less than 5%.
14.4.3 Cut and stack label design Cut and stack labels are predominantly sheet printed with surface printing of a mono-web plastic film. In recent years, roll to sheet presses have been developed where a roll of film is sheeted either before or after printing. In the case of sheeting after printing, it is possible to use a lamination label structure, although this approach is not routinely used. For all print methods, a sheet of printed labels is die cut into individual labels that are collected as a stack, with one label layered on top of another. A typical stack of labels
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 222
7/1/11 9:18:09 AM
Biaxially stretched films for product labeling
223
contains several thousand labels. The die cut label is the exact label size and shape as applied to the container. The label is rectangular in shape and the length is slightly greater than the diameter of the container at the location of the label. Cut and stack plastic labels are either 180° or 360° of the target container diameter. This limitation is based on the design of cut and stack labelers that use hot melt adhesives for label application and are applied to the leading and trailing edge of the label. Plastic spot labels are not possible due to the cut and stack labeling technology that requires 100% adhesive coverage of the label, which is not technically feasible with hot melt adhesives. Cut and stack label films may be clear, white or metallized. Critical label properties include flex stiffness, COF, antistatic, surface roughness, surface functionality and easily cut during the die cut process. For white or fully printed labels, film whiteness and opacity are also important attributes and sufficient as a substitute for printed white ink.
14.4.4 Pressure-sensitive label design For pressure-sensitive labels, the substrate is a label film (often called the facestock) in combination with 100% adhesive layer situated between the label substrate and a release substrate (liner). Labels are roll surface printed onto the facestock, adhesive and release liner combination. Printed labels typically use an over-lacquer or topcoat applied to the print surface to provide the required label surface gloss, print and graphics protection and abrasion resistance. After printing, labels are individually die cut, on the roll with the adhesive and release liner, to produce label designs that may be a uniform rectangular shape or have rounded edges or other non-uniform shapes. The die cut process is well controlled to die cut only the label and adhesive and not the release liner. Pressure-sensitive labels are used in the broadest variety of applications where the properties of the plastic film are critical to meet the label’s end-use and graphic requirements. The pressure-sensitive adhesive is also specifically selected to meet the target end-use requirements. Pressure-sensitive labels are printed by a variety of techniques including flexographic, gravure, UV and screen printing. In some applications, lamination structures are created where a clear over-laminate film is applied in place of the over-lacquer after printing. Development activities have explored the potential to print and create the pressure-sensitive label sandwich in one process. This has achieved limited success and is applicable only to high volume applications. Pressure-sensitive label films may be clear, white or metallized. Critical label properties include flex stiffness surface functionality and ease of cutting during the die cut process. For many applications, the film print surface
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 223
7/1/11 9:18:09 AM
224
Biaxial stretching of film
is required to have a topcoat or highly functionalized surface to achieve outstanding print adhesion and abrasion resistance and to have special print capabilities such as direct thermal and thermal transfer. For durable applications, the film and label are required to withstand critical end-use performance requirements including outdoor weather resistance, solvent resistance and abrasion resistance. For white or fully printed labels, film whiteness and opacity are also important attributes and often sufficient as a substitute for printed white ink.
14.4.5 In-mold label design In-mold labels are surface printed with an over-lacquer or topcoat in a roll or sheet print process. The adhesive system is designed into the film as a coextruded or extrusion coated structure or is applied as a coating before, during or after the printing process. Labels are subsequently die cut to the desired size and shape. Labels are collected as a stack for the in-mold labeling process. The in-mold label surface that is in contact with the container incorporates 100% coverage heat activated adhesive that bonds to the container during the molding process. The label substrate must conform to the curvature of the container. The label must also withstand the heat of the molding process and not result with any dimensional shrinkage, surface mottling or visual defects. In-mold labels must have sufficient stiffness to remain upright when loaded into the mold and are often required to hold a static charge as part of this process. In-mold labels are mostly white or opaque and are produced from either PP or synthetic paper substrates. In mold label films may be clear or white. Critical label properties include flex stiffness, the ability to hold a static charge, surface roughness, surface functionality and ease of cutting during the die cut process. For white or fully printed labels, film whiteness and opacity are also important attributes and often sufficient as a substitute for printed white ink. It is desirable for the composition of the in-mold label substrate to be compatible with the container composition as defective labeled containers are routinely used as reclaim in the molding process – requiring the label films to be PE, PP or combinations of PE and PP resins.
14.4.6 Shrink sleeve label design Shrink sleeve labels are prepared by reverse printing clear films in a roll print process. A single roll of labels is used to create a cylindrical sleeve by utilizing a vertical form and seal process around a mandrel to form a cylindrical sleeve. A continuous sleeve is created with the long seam (open end of the cylinder) sealed with a solvent that is ‘compatible’
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 224
7/1/11 9:18:10 AM
Biaxially stretched films for product labeling
225
with the substrate or a high solid content adhesive. For solvent sealing, the plastic film and solvent are selected so the film is highly soluble in the solvent – typically THF for PVC and PS films. The solvent is applied as a thin line onto one edge of the label and the other edge is aligned to overlap onto the solvent area. The solvent is rapidly dried to produce a fusion seal of the film. On application to the container, sleeves are cut into individual labels. Since the shrinkage of the label is often up to 75%, the graphics and print must incorporate this dimensional change into the design process. Plastic films for this application are required to have the ability to shrink up to 65% in the TD direction with less than 5% shrinkage in the MD direction. The films must also allow for solvent seam formation. Plastic films that meet these criteria include PVC, PS and PET-G. These films are typically clear and ~50 microns in thickness. Critical label properties include film shrinkage, flex stiffness and surface functionality.
14.4.7 Stretch sleeve label design Stretch sleeve plastic films are either surface or reverse roll printed on clear or white PE films. The sleeves are formed from a single roll of labels in a vertical form and seal process, with the long seam seal created by contact with a hot wire that melts the PE film at the seam resulting in a fusion seal. The PE sleeve diameter is slightly smaller than the diameter of the container to create a tight fit. Stretch sleeves are mostly located on a flat, non-contour part of the container. The stretch sleeve label fits tightly to the contour of containers with up to ~30% dimensional contour. Substrates for this application are typically PE films, either clear or white. Critical label properties include film stretch capability (stretch at low tension and relax to original shape with no distortion) and surface functionality.
14.4.8 Thermal transfer label design Thermal transfer labels are produced from PP, PET or synthetic paper substrates. The print surface requires a coating or functional surface that is compatible with the thermal transfer ink or ribbon to allow an effective thermally activated ink transfer that results in acceptable print definition and adhesion. Thermal transfer labels are either used as tags or signs or are pressure-sensitive labels or tags. The pressure-sensitive plastic film and adhesive are selected based on the end-use application. Typical substrates are PP or PET clear or white opaque films or synthetic paper substrates. Substrate thicknesses range from 50 to 350 microns and are often required to be tear and abrasion resistant. Critical label properties include flex stiffness, surface functionality and ease of cutting during the
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 225
7/1/11 9:18:10 AM
226
Biaxial stretching of film
die cut process. Film whiteness and opacity are also important attributes and often sufficient as a substitute for printed white ink.
14.4.9 Direct thermal label design Direct thermal labels are designed with a coated or functionalized surface that reacts with heat from the thermal transfer print process. The label print surface is activated with exposure to heat to thermally ‘transfer’ the print information. Direct thermal substrates are either PP white opaque films or synthetic paper and typically range from 50 to 350 microns. Direct thermal labels and tags are often required to be tear and abrasion resistant. Critical label properties include flex stiffness, surface functionality and ease of cutting during the die cut process. Film whiteness and opacity are also important attributes and are often sufficient as a substitute for printed white ink. Table 14.2 provides a summary of film substrates and their availability and attributes for each labeling process.
14.5
Future trends and new developments in labeling and label films
Plastic label films are a cost-effective and versatile platform for innovative and highly functional label design. Highlighted below are a few examples of newly introduced plastic label films that are a key contributor to the use of plastic films for label applications. Numerous promotional and point of purchase label designs have been utilized as part of marketing and product identification programs. These labels create unique and high profile product and point of purchase identity that is an integral component of the product’s marketing campaign. Typical examples incorporate removable or clip-out coupons, magic window designs (where a clear window is uncovered as a beverage product is consumed) and unique metallized holographic images incorporated into the label’s graphic design. Cut and stack plastic spot labels have been the target of several development activities in an attempt to utilize cut and stack labeling as a more cost-effective approach to more expensive pressure-sensitive labels. In particular, the large glass beer bottle market, which has historically used paper spot labels, is a lucrative opportunity for plastic to replace paper labels. Clear pressure-sensitive labels have been used in recent years as a lower cost alternative to an applied ceramic label (ACL) where a permanent ink is applied directly to the glass bottle – resulting in a no label look. Clear pressure-sensitive labels have been utilized to imitate this no label look, at substantial cost savings and environment benefit compared with an ACL decorated bottle. Although less expensive than ACL, pressure-sensitive
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 226
7/1/11 9:18:10 AM
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 227
7/1/11 9:18:10 AM
12–50 15–50 40–75 30–75 50–75 40–60 40–75 40–300 40–300
C = clear, W = white, M = metallized.
C,W,M C,W,M C,W,M C,W,M W,M C C,W C,W W
Roll fed Roll fed shrink Cut and stack Pressure sensitive In-mold Shrink sleeve Stretch sleeve Thermal transfer Direct thermal
PP PP, PS, PET-G PP, syn. paper PP, PE, PE PP, PE PVC, PET-G, PS, PE PP, syn. paper PP, syn. Paper
Thickness range, microns
Labeling process Substrates Types
Table 14.2 Overview of plastic label films
<10, MD/TD 10–65, MD, <5 TD <10, MD/TD <10, MD/TD <10, MD/TD 10–75, TD, <5 MD <10 MD/TD <10, MD/TD <10, MD/TD
Shrinkage, percent
Low–medium Medium–high Medium High Medium–high High Low Medium–high Medium–high
Relative cost
228
Biaxial stretching of film
plastic labels are significantly more expensive than the paper labels they replaced. The opportunity for lower cost cut and stack plastic labels would provide an opportunity to replace pressure sensitive labels and due to the lower cost differential, replace additional paper labels for current cut and stack applications. Recent developments in plastic film cut and stack spot labeling include (1) AET’s ToppCure system, which is a labeling system with an in-line energy cured (UV), low viscosity adhesive that is converted to a pressure sensitive adhesive during the labeling process (Hill and McNutt, 2003), (2) Exxon Mobil’s Lithor PP films that absorb water from aqueous based adhesives and (3) Arjobex’s Polyart Wet Glue, a cavitated HDPE film with a water absorbing coating designed for cut and stack cold glue labeling systems (www.polyart.com). PE Labelers has introduced their Roll Adhesleeve system for roll fed labeling where the hot melt application is eliminated and replaced with converter applied adhesive. This system utilizes a specially selected pressure-sensitive type adhesive, but does not require a release liner. The PE Roll Adhesleeve labeler is designed to cut and label at competitive roll fed labeling speeds and efficiencies. One major advantage is the elimination of hot melt application and related maintenance issues (Thomas, 2010). Polysack Plastic Industries, Ltd has introduced Polyphane Fit roll fed shrink label films with shrink levels from 15 to 50%. These films are PE/PS blends and polyolefin based films that are machine direction uniax oriented and designed to operate with traditional roll fed labeling and adhesive systems (www.polyphane.com). Roll fed insulating label films have been introduced by DuPont under the brand name of Cool2Go. The Cool2Go films are designed as a 360° roll fed label that incorporates a PET-based low density, insulating film and a printable PET over laminate film. The lamination structure is designed to keep drinks cold twice as long as conventional labels (Benim et al., 2006). Radio frequency identification (RFID) technologies have been incorporated into tags and pressure-sensitive labels for various applications related to product tracking and identification. Currently, most of the applications are at the case and pallet level for supply chain management and are used as a more sophisticated replacement for the existing bar code system. The standard RFID technology utilizes a chip and antenna placed on a PET film. Future growth opportunities are expected to penetrate additional industries and applications and to be utilized for individual products. The long-term opportunities are in the area of printed electronics where the RFID system contains a memory, battery and uses organic based conductive inks., (Genuario, 2008a). Beyond RFID functionality, smart labels (as part of the RFID category or as a separate segment) are defined broadly as a label that provides additional
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 228
7/1/11 9:18:10 AM
Biaxially stretched films for product labeling
229
information, in interaction with some external event or process, related to the product that it is attached. Current applications include sensors that detect spoilage in meat and poultry products (SensorQ by Food Quality Sensor International), temperature sensors (PakSense), freeze sensors (Istrip by CCL Labels) and pharmaceutical labels used to track the supply chain status utilizing electronic records. Based on these initial offerings, it is easy to imagine future technologies attached or integrated into all types of labels for use in smart packaging applications (Genuario, 2008b). Merck KgaA recently announced a new smart label project titled Polytos aimed at smart labels based on printed organic circuits and built-in sensors. Target applications are sensors and memory devices for temperature, humidity and light exposure (Carter, 2009). Sustainability has emerged as a critical design and product attribute to be considered in all packaging activities. This includes aspects of recycling, material reduction and use of bio-plastics. Significant government and supply chain initiatives are underway that are expected to create a major focus on environmental parameters such as the container and label’s carbon footprint, life-cycle assessment and use of renewable resources.
14.6
Conclusions
Product labeling is a critical and effective approach to product identification and decoration. Label growth will continue as directly related to world gross domestic product (GDP) growth and industrialization and also as labels and labeled products continue to provide innovative and valued solutions. Overall, paper labels are expected to have relative low growth and plastic labels are expected to capture an increasing share of the label market. This is based on continued cost improvement and the numerous innovations and versatility for plastic labels. Biaxial orientation of plastic films is a costeffective approach to provide critical label attributes to plastic films. Special effects and performance features either graphic or functional will provide growth opportunities for applications with holographic images, RFID, security and tamper evident features, temperature sensitive inks and smart labels. As always, lower applied label costs are always welcomed through lower cost substrates and higher speed and more efficient labeling.
14.7
References
Benim T, Chamberlin S, Chambers J, Cosentino S, Hunderup P, Lee R, and Procaccini S (2006), Insulating label stock, US Patent 7070841 Buckle J and Leykamm D (2001), The Manual of Labeling Technology: Basics and Practice in Successful Product Dressing, Regensburg, Germany, Volker Kronseder. Carter P (2009), Polytos project launched by the ‘Forum Organic Electronics’ excellence cluster, News Release June 29, 2009, www.merck.de
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 229
7/1/11 9:18:10 AM
230
Biaxial stretching of film
The Freedonia Group, (2007 and June 2009), World Labels #2219 and World Labels #2499, www.freedoniagroup.com Genuario L (2008a), RFID Label Market, Label and Narrow Web, 13/2, 46–50 Genuario L (2008b), Smart Labels, Label and Narrow Web, 13/2, 52–55 Hill W and McNutt T (2003), Labeling method employing radiation curable adhesives, US Patent 6514373 Thomas A (2010), Hot melt under pressure, Labels & Labeling, January, 28–31
Labeler manufacturers B&H Labeling Systems: www.bhlabeling.com Barry-Wehmiller – Accra ply, Trine: www.barry-wehmiller.com Carmichael Scotland LTD: www.carmichaelscotland.co.uk Krones AG: www.krones.com P.E. Labellers: www.pe-us.com, www.pelabellers.it Sacmi Labeling: www.sacmilabelling.com Sidel: www.sidel.com
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-14.indd 230
7/1/11 9:18:10 AM
15
Applications of biaxial stretched films
S . H . T a b a t a b a e i and A . A j j i, Ecole Polytechnique of Montreal, Canada Abstract: More recently, as increasing pressure is directed to increase the shelf-life of foods and to reduce the rejection and defect rate in metallized high barrier films, attention is focusing on the orientation of nanocomposite and multilayer films. In this chapter, structural deformation of biaxial oriented nanocomposite and multilayer films are presented. In addition, various processes for making biaxial oriented structures are discussed. Key words: nanocomposite, biaxially oriented, multilayer films, barrier.
15.1
Introduction
Orientation of polymers enhances many of their properties, particularly mechanical, impact, puncture, barrier and optical. Biaxial orientation has the added advantage of allowing this enhancement in both machine and transverse directions, avoiding any weakness in the properties of the transverse direction (TD) or in tear of the machine direction (MD). Biaxial orientation is particularly important in films, where it allows the production of thinner films having superior mechanical, optical and barrier properties and, if required, the ability to shrink when reheated.1,2 For example, shrink and barrier properties are particularly important in meat packaging whereas good mechanical and optical properties and no shrinkage are desired in the abuse layer of multilayer films as well as box overwrap applications. The production of oriented flat films from thermoplastic materials represents a large segment of the polymer industry. Because of its low density (900 kg/m3), relatively high temperature resistance, mechanical properties and cost, polypropylene (PP) is one of most used in packaging applications, particularly when it is processed into very thin biaxially oriented films (12–25 mm). In addition to food packaging, biaxially oriented PP is used as battery separators. In fact, porous membranes from PP can be obtained using the stretching process if the initial crystalline structure of PP is lamellar with certain orientation.3–5 This orientation can be performed either using a machine orientation process or a biaxial orientation process. Many other polymers are used in the biaxial orientation processes; they include polyethylene (PE), polystyrene (PS), polyamide (PA) and polyester. Most of these are used in food packaging such as snacks (chips, biscuits, 231 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-15.indd 231
7/1/11 9:18:46 AM
232
Biaxial stretching of film
etc.) but also in electronic materials packaging and more recently as synthetic paper (particularly from filled polyolefins). More recently, as increasing pressure is directed to increase the shelf-life of foods and to reduce the rejection and defect rate in metalized high barrier films; attention is focusing in orientation of nanocomposite and multilayer films. In fact, nanocomposites are known to possess improved mechanical and barrier properties. Nanocomposites containing small amounts of nanoparticles (1–5%) have been shown to yield large improvements in barrier properties as well as in physical properties such as tensile strength, tensile modulus and heat distortion temperature.7–10 A doubling of tensile modulus and strength without a sacrifice in impact resistance has been achieved for nylon 6/ clay nanocomposites containing as little as 2 wt% of clays. Even without orientation, improvements in barrier already allowed significant gains in performance such as for soldier ration packaging.11 Ethylene vinyl alcohol (EVOH) nanocomposite cast film had 57% improvement in oxygen barrier properties at 0% relative humidity in comparison to the pure EVOH cast film.11 Upon biaxial stretching, for example for polyethylene terephthalate (PET), exfoliation was further improved and the concentration of thinner tactoids in the matrix increased due to stretching, as a result, mechanical and barrier properties improved.12 In a recent patent application, it was also reported that a significant improvement in strength was obtained for a biaxially oriented nanocomposite film.13 Finally, orientation of multilayer films of polyamide and polyethylene (PA/tie/PE) was also reported recently, but focused mostly on structural aspects and no performance results were reported.14 The most widely used biaxial orientation processes for films are tubular film blowing and cast film biaxial orientation (or tentering). These two processes are illustrated in Figs 15.1 and 15.2, respectively. Other processes such as blow molding, compression molding and thermoforming involve also biaxial orientation. The double bubble film blowing process is illustrated in Fig. 15.1. The molten polymer extruded tube is first cooled in a water bath and then taken to a second stage, the blowing stage, where it is reheated and oriented by blowing while stretched in the MD and TD. Although stretching occurs simultaneously in MD and TD, the force for each direction is controlled separately.15 In fact, the MD force is applied by speed difference between the two sets of nip rolls that hold the tube. The TD force is controlled through the air pressure introduced into the bubble.15 It should be mentioned that the material goes through drastic temperature changes. The temperature at this stage is very important as will be discussed later. The double bubble film blowing process is used for the manufacturing of shrinkable film and also film tubes for the food packaging. The double bubble tubular technique has been commercially used to make biaxially oriented films of PE, PA-6, PP and PET.16
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-15.indd 232
7/1/11 9:18:46 AM
Applications of biaxial stretched films
233
Take up roll
Extruder
Air ring
Die
Heating Cooling bath
Take up roll
To annealing and winding
15.1 Tubular film extrusion.
Longitudinal stretching
Transverse stretching
Extruder
Stabilization
Chill roll
15.2 Film tentering process.
The other important process for biaxial orientation is film tentering, which is illustrated in Fig. 15.2. An extruder delivers the film to the die, with an optional gear pump for optimum control of the overall flow rate. The die is usually of the coat-hanger configuration, with adjustable lips which allows the flow rate distribution at die exit to be evenly distributed. The molten film is stretched on a short distance in air and quenched on a
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-15.indd 233
7/1/11 9:18:46 AM
234
Biaxial stretching of film
chill roll which rotates at a constant speed. This primary film is stretched again in the longitudinal and transverse directions successively followed by annealing and wind-up. The shorter the cooling time and the lower the film temperature achieved in cooling, the better the optical properties of the film. The orientation can be done sequentially or simultaneously in the MD and TD. However, clear differences were observed between the two.6 The properties (transparency, modulus and strength) were lower for the sequential process. In fact, relaxation processes in the material cause a reduction in properties, since the film is stretched first in the direction of extrusion and can therefore partially relax while it is being stretched in the other direction.6 Thus in order to obtain the same properties as for the simultaneous process, the first stretch ratio should be higher and the temperature lowered after this stretching to compensate for and minimize relaxation. Typical data for the extruded film and the chill roll are: molten film 0.5–2 m wide and 0.3–3 mm thick, temperature of the molten film of 250–300 °C, chill roll diameter of 0.5–2.5 m and peripheral speed of 20–60 m/min. The draw ratio in the first direction can be up to 10 and up to 5 in the second direction, such is the case for biaxially oriented PP for example.
15.2
Biaxial stretching of nanocomposite and multilayer films
As mentioned earlier, biaxial drawing of nanocomposite films leads to a significant enhancement in their ultimate properties. Before explaining the effect of nanoparticles on the properties, some basic description concerning the nanofillers is required and is provided below. It is well established that the effective dispersion of anisotropic particles with high aspect ratios such as short fibers, plates and whiskers within a continuous polymer matrix, in combination with adequate interfacial adhesion between the filler and polymer, can account for substantially improved reinforcement of the polymer matrix.17 Layered silica-based polymer nanocomposites have attracted considerable technological and scientific interest in recent years, because they have shown dramatic enhancements in the physical, thermal, mechanical and barrier properties of polymers even with relatively low loading of silicate.18–20 These properties are attributed to the layered silicate and confinement of the polymeric matrix at the nano-scale. 21 If the silicate tactoids delaminated completely, an exfoliated nanocomposite is obtained. By contrast, if there is only an increase in the interlayer distance, the nanocomposite is termed intercalated. Figure 15.3 shows intercalation and/or exfoliation of the nanoclays in PP, PE, PA and PET.12, 22–24 An increased exfoliation of the tactoids results in the formation of a large interface between the soft polymer phase and
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-15.indd 234
7/1/11 9:18:47 AM
Applications of biaxial stretched films (a)
235
(b)
100 nm
200 nm
(d)
(c)
50 nm
500 nm
15.3 Transmission electron microscopy (TEM) micrographs for nanoclay composite films of (a) PP/PP-g-MA (30 wt%)/Cloisite 20A (5 wt%),6 (b) PE/ HDPE-g-MA (20 wt%)/MMT DMTB (18.4 wt%),7 (c) PA11/Cloisite 30B (10 wt%),8 and (d) PET/Somasif MAE (2 wt%).9
hard nanoclay platelets. It should be mentioned that nanoparticles can be dispersed much better in polar and amorphous polymers than in apolar and crystalline ones from the fact that there is a good interaction between the clay platelets and polar groups as well as the crystals prevent the diffusion of the particles during their growth.25 For instance, the nanoclays are dispersed better in PA as a polar polymer matrix than in PP and PE as apolar matrices with a high level of crystallinity. From the literature, 26 various parameters affect the development of nanocomposites; the method for production of nanocomposites, the type of resin, the type and content of nanoclay, and chemical modifications used in producing the nanoclay. However, as pointed out previously, biaxial stretching of polymeric films such PP, PE, polyester and nylon can drastically affect their properties. Since the clay platelets are impermeable to the gas diffusion and have much larger modulus compared with the polymer chains, therefore, it is believed
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-15.indd 235
7/1/11 9:18:47 AM
236
Biaxial stretching of film
that uniaxial and biaxial stretching of nanocomposite precursor films will improve the mechanical and barrier properties, which is advantageous in packaging applications where the optimum properties are obtained for a good dispersion and exfoliation of nanoparticles. Moreover, recent studies have shown that the incorporation of the clay tactoids influences drastically the crystallite orientation of biaxially drawn samples.27 Rajeev et al.12 studied the effect of biaxial stretching and stretch ratio on the exfoliation of PET nanocomposites and their results showed that stretching improved the exfoliation. In addition, drawing further increased the concentration of longer tactoids possibly due to the slippage of the platelets, as depicted in Fig. 15.4. These also confirmed the finding of other researchers28 who showed that biaxial stretching significantly affected the state of dispersion in PET. As stated before, there are two main industrial processes for the production of films: film blowing and cast film extrusion. It is well known that thickness variation in blown films are considerably greater than in cast films. For the preparation of multilayer packaging films, obtaining a film with good thickness uniformity is strongly recommended since any nonuniformity causes variation in oxygen permeability across the produced films. Coextrusion of different polymers into multilayer structure is a conventional way for combining properties of various components to obtain optimum mechanical, optical and gas barrier properties.29 In fact, in a cast coextrusion system, molten polymers from two or more extruders are delivered in a combination point where the melts are stacked and then delivered to a
(a)
(b)
500 nm
500 nm
15.4 TEM surface micrographs of PET nanoclay composite films; (a) unstretched sheet and (b) biaxially stretched sheet at stretch ratio of 3.9
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-15.indd 236
7/1/11 9:18:47 AM
Applications of biaxial stretched films
237
die.29 In general, the cast coextrusion system consists of an adapter, a feedblock and a die. The adaptor is the bridge between the extrusion and coextrusion systems. The adaptor is designed to collect various melt flows to the feedblock and die. The adaptor needs to be designed such that it allows future addition of layers to an existing coextrusion system without the need to replace the melt pipes, filters, melt pumps, etc.29 The feedblock is used to combine the multiple polymers entering from the adapter into the flat die.29 Inside the die, different factors influence the layers integrity including flow channel geometry, melt elasticity as well as shear and elongational viscosities. Figure 15.5(a) shows the adaptor and polymer flow paths for coextrusion of up to seven layers and Figure 15.5(b) illustrates the flow diagram of a coextrusion feedblock where three extruders are utilized to produce a trilayer structure. Since biaxial stretching is largely employed for improving tear and puncture resistance as well as barrier properties, hence applying biaxial drawing on the multilayer films could be of great interests to many industries particularly packaging applications. For instance, PE/tie/PA multilayer film is used for flexible packaging of many products where PE is the sealant layer, which has the advantages of high stiffness and barrier to moisture and PA is the abuse layer, which brings the advantages of tear and puncture resistance as well as barrier to oxygen. In most cases, the abuse layer (i.e. PA) is a biaxial stretched film, which is laminated to the sealant layer (i.e. PE) in a separate process. This additional processing step could make the film production costly compared with biaxial drawing of multilayer coextruded films. In fact, the properties of PE/tie/PA multilayer film can be enhanced drastically if biaxially stretched under the predetermined conditions of draw
1
2 3
(a)
(b)
15.5 Schematic representation of (a) adaptor with removable low selector showing the polymer flow paths (courtesy of Cloeren, Inc.) and (b) flow diagram of a coextrusion feedblock where three extruders are used to produce a three-layer structure.14
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-15.indd 237
7/1/11 9:18:47 AM
238
Biaxial stretching of film
ratio, drawing speed, drawing times (a film can be stretched many times), drawing temperature and heat setting. Recent studies on the structural evolution of biaxially drawn PE/tie/ PA multilayer film have shown that the occurrence of plastic instabilities in the PA layer in the multilayer was strongly reduced with respect to the PA monolayer film. Additionally, the maximum biaxial drawability of PA was considerably improved with increasing PE layer thickness. It should be mentioned that there is yet an actual challenge for applying biaxial drawing on multilayer structures, since the various components display different behaviors and do not necessary have the same optimum processing parameters.
15.3
Conclusions
Double bubble and cast biaxial orientation film processes are the most widely used biaxial orientation processes. In the cast biaxial drawing process, the orientation can be done sequentially or simultaneously in the MD and TD. However, due to the relaxation of polymer chains, the properties including transparency, mechanical and barrier are lower for the sequential process than for the simultaneous process. Biaxial drawing of nanocomposite films yields a significant enhancement in their final properties. Uniaxial and biaxial stretching of nanocomposite precursor films improve the barrier and mechanical properties, because the clay platelets are impermeable to the gas diffusion and have much larger modulus than the polymer chains. Biaxial stretching of multilayer structures can be employed for improving tear and puncture resistances as well as barrier properties. Biaxial drawing of multilayer coextruded films can be more cost effective than the lamination of the biaxial stretched films with the sealant in a two step process. However, much less information is available on these two later applications and many challenges remain to be solved for them in the future.
15.4
Future trends
As mentioned above, biaxial stretching on nanocomposites and multilayer films is still in its early stages and very few studies have been performed. We believe that future developments are in these two areas because of the potential gains in properties improvements and costs. The challenges are, however, significant. For the nanocomposites, exfoliation of the nanoparticles and optimum adhesion are a requirement in order to insure both property improvements (barrier and mechanical) and stretch ability. For multilayer films, reduction of processing steps in packaging (thus the costs) and property improvements are both the drivers for biaxial stretching of multilayer films. The challenges are in finding the optimal combination
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-15.indd 238
7/1/11 9:18:47 AM
Applications of biaxial stretched films
239
of materials that can allow both gains in properties and stretch ability (such as is the case in PE and PA).
15.5
References
1. Lin YJ, Dias P, Chen HY, Chum S, Hiltner A, Baer E. Polymer 2008; 49: 2578– 2586. 2. Tabatabaei SH, Carreau PJ, Ajji A. Polymer 2009; 50: 3981–3989. 3. Tabatabaei SH, Carreau PJ, Ajji A. Polymer 2009; 50: 4228–4240. 4. Sadeghi F, Ajji A, Carreau P. J Polym Sci Polym Phys 2008; 46: 148–157. 5. Tabatabaei SH, Carreau PJ, Ajji A. J Membr Sci 2009; 345: 148–159. 6. Nordmeier K, Menges Gs. Adv Polym Tech 1986; 6: 59–64. 7. Vaia RA, Giannelis EP. MRS Bull 2001; 26: 391–401. 8. Lucciarini JM, Ratto J, Koene BE, Powell B. SPE ANTEC Proceedings, Dallas, TX, 2001. 9. Marchant D, Jayaraman K. Ind Eng Res 2002; 41: 6402–6408. 10. LeBaron PC, Wang Z, Pinnavaia TJ. Appl Clay Sci 1999; 15: 11–29. 11. Froio D, Lucciarini J, Thellen C, Ratto JA. SPE ANTEC, Chicago, IL, 2009. 12. Rajeev RS, Harkin-Jones E, Soon K, McNally T, Menary G, Armstrong CG, Martin PJ. Eur Polym J 2009; 45: 332–340. 13. US Patent Application 2009/0226711: Biaxially oriented nanocomposite film, method of manufacture, and articles thereof. 14. Sallem-Idrissi N, Miri V, Elkoun S, Krawczak P, Lacrampe MF, Lefebvre JM, Seguela R. Polymer 2009; 50: 5812–5823. 15. Song K, White JL. Polym Eng Sci 2004; 40: 1122–1131. 16. Bobovitch AL, Tkach R, Ajji A, Elkoun S, Nir Y, Unigovski Y, Gutman EM. J Appl Polym Sci 2006; 100: 3545–3553. 17. Katz HS, Milewski JV. Handbook of filler for plastics, Van Nostrand Reinhold, New York, 1999. 18. Kojima Y, Usuki A, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, Kamigaito O. J Mater Res 1993; 8: 1185–1189. 19. Jiang L, Wei K. J Appl Phys 2002; 92: 6219–6223. 20. Wang S, Hu Y, Qu Z, Wang Z, Chen Z, Fan W. Mater Lett 2003; 57: 2675–2678. 21. Pereira de Abreu DA, Paseiro Losada P, Angulo I, Cruz JM. Eur Polym J 2007; 43: 2229–2243. 22. Lertwimolnun W, Vergnes B. Polymer 2005; 46: 3462–3471. 23. Jacquelot E, Espuche E, Gerard JF, Duchet J, Mazabraud P. J Polym Sci Part B: Polym Phys 2006; 44: 431–440. 24. Luo ZP, Koo JH. Polymer 2008; 49: 1841–1852. 25. Hwang WR, Peters GWM, Hulsen MA, Meijer HEH. Macromolecules 2006; 39: 8389–8398. 26. Kaynak C, Tasan C. Eur Polym J 2006; 42: 1908–1921. 27. Tabatabaei SH, Ajji A. Polymer Engineering and Science, 2010, in press. 28. Joel S, Fenouillot F, Rousseau A, Masenelli-Varlot K, Gauthier C, Briois JF. Macromolecules 2007; 40: 3229–3237. 29. Wagner JR. Multilayer Flexible Packaging, Elsevier, Kidlington, 2010.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-15.indd 239
7/1/11 9:18:48 AM
16
Future trends for biaxially oriented films and orienting lines
J . B r e i l , Brückner Maschinenbau GmbH & Co. KG, Germany Abstract: In order to derive future trends for biaxial oriented films, the influences of global mega-trends on different fields of application are discussed, including biospheric, economical, social, political and technological factors. Trends for the packaging markets for the most important film types – biaxially oriented polypropylene (BOPP), biaxially oriented polyethylene terephthalate (BOPET), biaxially oriented polyamide (BOPA), biaxially oriented polystyrene (BOPS), biaxially oriented polyethylene (BOPE) and biaxially oriented polylactic acid (BOPLA) – are introduced. For technical films the trends of flat panel displays, photovoltaic, solar panels and battery separator films are highlighted. New film types have been developed on a versatile pilot line, which can be used to provide services. Finally a development environment is presented, demonstrating how basics as well as application-oriented developments for biaxially oriented films can be implemented. Key words: biaxially oriented films, biaxially oriented polypropylene (BOPP), biaxially oriented polyethylene terephthalate (BOPET), biaxially oriented polyamide (BOPA), biaxially oriented polyethylene (BOPE), biaxially oriented polylactic acid (BOPLA), biaxially oriented polystyrene (BOPS), biaxially oriented cyclo-olefin copolymer (BOCOC), biaxially oriented polyvinylidene fluoride (BOPVDF), biaxially oriented polyethylene naphthalate (BOPEN), sleeves, battery separator film.
16.1
Introduction
In the last chapter of this book, attention is given to various global trends in order to derive how these are influencing future trends for biaxially oriented film. The question is raised, however, as to what extent can marginally reliable statements about our future can be made? One possibility is to review the trends of the past and carry on the idea a bit further to assess the future growth rates. This evaluation can be further refined by taking market developments on a regional basis into consideration and then adding substitution effects which significantly influence on market shares for biaxially oriented film. Such substitutions can either have a cumulative impact on the branch – see how cellophane was massively replaced by biaxially oriented polypropylene (BOPP) in the 1970s – or can take a completely different direction as in the 1990s, namely the replacement of biaxially oriented 240 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 240
7/4/11 9:33:55 AM
Future trends for biaxially oriented films and orienting lines
241
polyethylene terephthalate (BOPET) based magnetic tapes by digital storage media, such as CDs, DVDs and Blu-ray disks. Another more differentiated approach, however, is to analyse certain future factors and mega trends1 and then respond to the question of what influence do these factors have on biaxially oriented film. This is further explained in the following examples.
16.1.1 Future biospheric factors The biosphere is the global ecosystem which is significantly influenced by the human population in certain regions, causing well-known effects such as forest dieback, drinking water shortage and climate change (Fig. 16.1).
Global temperatures CO2 (ice cores) CO2 (Mauna Loa)
390 CO2 concentration (ppmv)
380 370
58.1 57.9
360
57.7
350
57.5
340
57.3
330 320
57.1
310
56.9
300
56.7
290
56.5
Year
2010
2000
1990
1980
1970
1960
1950
1940
1930
1920
1910
1900
1890
1880
280
Global average temperature – °F
Global average temperature and carbon dioxide concentrations, 1880–2004
ad
16.1 Global warming.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 241
7/4/11 9:33:56 AM
242
Biaxial stretching of film
The latter has recently led to heated discussions about the atmosphere’s CO2 exposure and the subsequent systematic and binding measures to reduce CO2 . There will be an influence on the entire packaging industry in future and also on biaxially stretched film. In certain countries, such as France, details must already be declared on packaging about the carbon footprint. The industry, sooner or later, must further consider the analysis of the carbon footprints and the possibility of influencing it namely by reduction of greenhouse gases. As an example the carbon footprint of BOPP film (see Fig. 16.2) shows that by far the dominant portion is the raw material input. 2 In view of the fact that a packaging task always involves the availability of a certain amount of packaging material, it is only logical to further increase the raw material efficiency by the application of thinner film. Basically, this speaks up for the use of biaxially oriented film, as a maximum packaging effect is attained with minimum material input. However, the effect can be improved even further by means of down-gaging. Increasingly, energy consumption is becoming a focal point in film manufacture. A further potential for biaxially oriented film – in terms of the carbon footprint aspect – arises by a possible substitution of aluminum foil either with a metallized high barrier film or with film having as many characteristic properties as possible (such as barrier and sealability). This is implemented during the manufacturing process by means of coextrusion and biaxial orienting – rather than complex and multi-stage converting steps – and attains comparable characteristic properties for the packaging purpose – Section 16.2 outlines further typical examples.
16.1.2 Future economic factors In our closely linked economic world, in addition to regional effects, the global aspects must also be looked into. Owing to the economic boom in Asia, many production facilities for both technical film and also packaging film have been installed. Reports show that China, with a population of 1.3 billion, has a strong urbanization effect (see Fig. 16.3), combined with a constant rise in purchasing power.3 Changes to foodstuff distribution are under way, resulting in a steady growth for flexible plastic film of over 11% per annum. Meanwhile, the worldwide largest production capacities for BOPP, BOPET and biaxially oriented polyamide (BOPA) film have been installed in China. However, also other countries with a high population, such as India, show a double-digit growth rates in the packaging sector. In contrast, the markets in the developed Western countries are saturated, so there is no significant volume growth. However, technology-motivated research and development for technical and packaging film is taking place, leading to sophisticated packaging solutions with some added value.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 242
7/4/11 9:33:56 AM
78.7 kg CO2/1000 m2
Product carbon footprint 20 mm BOPP – Film 100% 90% 80%
kg CO2e /fU
70% 60%
Transport activities BO-process electrical
50%
BO-process thermal Raw material
40% 30% 20% 10% 0%
16.2 CO2 footprint of 20 μm BOPP Film (CO2e = equivalent CO2; fU = functional unit).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 243
7/4/11 9:33:56 AM
244
Biaxial stretching of film
90 000
50 Pop (’000)
80 000
% Urban
70 000 60 000 50 000 26 26
40 000 30 000
28 27 27
29 29 30
32
33
35
36
38
39
41
42
43
44
45
40 35 30
24 18
45
25
19
20
20 000
15
10 000
10 5
0
2007
2006
2005
2004
2003
2002
2001
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
1989
1985
1980
1978
0
16.3 Urbanization in China: 15 million urban consumers added each year (source: National statistics Bureau).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 244
7/4/11 9:33:57 AM
Future trends for biaxially oriented films and orienting lines
245
16.1.3 Future social factors Social factors have a certain relevance to the packaging industry. For example there is a general trend towards smaller households in the most countries (see Fig. 16.4), which subsequently result in an increased demand for smaller packaging units. This in turn leads to a higher value-related Average numbers per household Japan Korea, Rep. Hong Kong Taiwan
2.7 2.6 2.7
2003 2006
3.3 3.3 3.2 3.3 3.2 3.5 3.5
Singapore China
3.5
Thailand
3.6
Indonesia
3.8 3.8 3.8
Malaysia Vietnam
3.3
4.4 4.4
4.7 4.8 4.9 5.0
Philippines
5.4 5.3
India
China: one child policy
16.4 Trends to smaller households (source: The Economist).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 245
7/4/11 9:33:57 AM
246
Biaxial stretching of film
proportion of the packaging in relation to that of the packed goods and an expected increasing demand for barrier properties, i.e. primarily water vapor, oxygen and aroma barriers of packaging film. (Section 16.2 outlines a few examples.) There is also a growing proportion of the elderly population who have other consumer requirements: for example, convenience orientation has the objective of preparing everything promptly and conveniently from home. This has led to a demand for ‘easy opening’ and ‘recloseable’ foodstuff packaging.
16.1.4 Future political factors Political factors for future trends can arise when the market environment is influenced by legal provisions, tax regulations or subvention programs. Today, extensive programs are already underway in many countries with the objective of being independent of oil. This has an effect primarily on the energy supply and technologies for individuals’ mobility. The trend towards an increasing share of alternative energy sources has triggered a boom in the photovoltaic (PV) industry. In turn, this has led to an increased demand for BOPET thick film, which is used for the back sheet of this solar module in a thickness range of 50 to 330 mm. In many countries, there is a funding program for the development and market introduction of electromobility, with subsidies as an additional buying incentive for the purchase of electric vehicles. 4 The long-term trend will show that the growth of electic and hybrid driven cars will be strong (Fig. 16.5) with the effect that in 20 years most cars will be powered by electricity. In hybrid drives and also pure electric vehicles lithium ion batteries are the preferred energy stores. Since these require battery separator films, increasing demand for that product can be expected in the forthcoming years. Almost all battery separator films are based on biaxial orienting technology due to the generation and adjustment of the required porosity for these membranes. A larger market for this film type is expected, with high growth rates reaching over 25% per annum.
16.1.5 Future technological factors Many product groups on the market are also influenced by technological developments or – even more drastically – by technological milestones. As such, within just a few years, a completely new range of biaxially oriented film products have emerged on the market that did not exist before. Optical film for flat screens is a typical example and has created a considerable additional market for BOPET thick film in the range of 188–400 mm and also for other oriented films (cyclo-olefin copolymer
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 246
7/4/11 9:33:57 AM
Future trends for biaxially oriented films and orienting lines 140
120
247
Fuel cell Electric vehicle Hybrid electric vehicle Diesel Gasoline
Vehicle sales p.a.
100
80
60
40
20
0 2000
2005
2010
2015
2020
2025
2030
16.5 Forecast electric vehicle sales p.a. until 2030.
(COC), polycarbonate (PC), cellulose triacetate (TAC)). In view of the fact that cathode ray tube (CRT) screens have been replaced to a large extent by flat screens liquid crystal display (LCD), plasma, organic light-emitting diode (OLED ), this trend will continue worldwide over the coming years
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 247
7/4/11 9:33:58 AM
248
Biaxial stretching of film
(Fig. 16.6) and will lead to a corresponding demand for these film types. A similar progressive growth is to be expected in the flexible electronic sector. Huge potential is anticipated for electronic applications manufactured by the particularly cost-effective ‘roll to roll’ process. These include flexible solar cells, E-paper, flexible displays, flexible conductors and flexible laminar lighting. The technological challenges involve the development of the corresponding printing technologies along with the demand for conducting layers and high geometric resolution in the manufacture of flexible substrates with higher dimension and thermal stability as well as specific surface characteristics. Biaxially oriented PET, polyethylene naphthalate (PEN), polyether ether ketone (PEEK) or polyphenylene sulfide (PPS) film superbly fulfills such requirements. The required surface characteristics (roughness, printability) are attained by means of the in-line process, such as coextrusion, in-line coating as well as the downstream treatment process (corona, flame and plasma treatment). Technological trends for biaxial film, however, are not only tagged by new applications but also by innovative orienting technologies. In particular, further development on the simultaneous stretching technology by linear motors (LISIM®) deserves to be mentioned. This involves simultaneous stretching in machine direction (MD) and transverse direction (TD), whereas the stretching ratios and profiles can be adjusted in a broad range as required to precisely attain the required film characteristics (Fig. 16.7). This technology has proved its worth not only on a pilot line scale but for many years has also resulted – on a production scale – in new film characteristics that are unattainable with conventional sequential technology. According to the stretching curves, the mechanical and shrink properties can be Thin film transistor LCD
Flexible CRT Plasma display pane 1980
1990 CRT era
2000
OLED
2010
3D
2020
Flat panel display era
2030
2040
Flexible display era
16.6 Trend for display technologies.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 248
7/4/11 9:33:58 AM
Future trends for biaxially oriented films and orienting lines
249
Enhanced mechanical properties
Customized shrink properties
Superior barrier properties Aroma H 2O O2
High sealing properties
16.7 Simultaneous stretching with LISIM® technology.
adjusted selectively – either isotropic or anisotropic mode, extremely high (e.g. E-module) or extremely low (e.g. thermal shrinkage). Furthermore, in simultaneous mode, various resins can be jointly stretched by means of combined process latitude (temperature, stretching ratios) which is not existent in sequential stretching technology. The merit of various resins (PP, PA, ethylene vinyl alcohol (EVOH), etc.) can therefore be cost-effectively combined with each other by means of coextrusion and joint stretching. Finally, it is to be emphasized that the simultaneous process involves contact-free stretching, since the film is gripped merely at the edges, thus during stretching the end film has no roll contact. This has the advantage that the surfaces are not prone to damage, which is of great importance for optical films as they have to be manufactured completely scratch-free.
16.2
Trends for packaging film
For a further examination of the trends for biaxially oriented film in various application fields, it seems to make sense to differentiate between packaging and technical applications, as the growth and development of these markets depend on various parameters. The development of the packaging market is dependent on the volume growth in countries with a high population and changes to the distribution chains for foodstuffs and non-food. This trend is particularly apparent in China in view of the ever-increasing proportion of the population stratum with a higher standard of living and has led to higher and consistent growth rates for BOPP, BOPET and BOPA. Meanwhile, China has the worldwide largest production capacity for these film types.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 249
7/4/11 9:33:59 AM
250
Biaxial stretching of film
The years 2008/2009 were hit by a slowdown in the global economy caused by the financial turmoil. Unlike the consumer industry, this crisis did not affect the food production industry and the food packaging sector. Similarly in the market for biaxially oriented film the demand for packaging film continued to rise, although the demand for technical film showed a clear slump.
16.2.1 Biaxially oriented polypropylene (BOPP) In the packaging sector in particular, BOPP is an ideal material since it ensures and combines certain benefits, such as excellent overall properties, reasonable price and good yield. In fact, BOPP constitutes approximately two-thirds of all biaxially oriented films. Worldwide growth rates are on average 5% pa, which means by the year 2020 a production capacity of 10 million tons can be expected. These figures are based on the so-called name plate capacity (NPC), i.e. the sum of the production capacity of all BOPP lines installed (Fig. 16.8). The effective production is around 80% of this figure, as the lines are not always 100% available and running at full capacity and since the output capacity is less for the production of thinner film. In the past, there has been a trend towards wider and faster production lines (Fig. 16.9). Presently 525 m/min can be regarded as the state-of-the-art production speed. In order to attain higher line speeds, further development is required regarding pinning technology, water removal of cast film, machine direction orientation (MDO) stretching section, chain track system and winding technology. Leading line manufacturers are working on these
10 000 000
BOPP China BOPP world (without China)
9 000 000 Tons per year (tpa)
8 000 000 7 000 000 6 000 000 5 000 000 4 000 000 3 000 000 2 000 000
2010
2009
2008
2007
2006
2005
2004
2003
2002
2001
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
0
1990
1 000 000
Year
16.8 Trend of world capacity – BOPP (name plate capacity).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 250
7/4/11 9:33:59 AM
Future trends for biaxially oriented films and orienting lines
251
600 Available line speed
Line speed (m/min)
500
400
n d i ed ren spe t in ne Ma PP li BO
300
200
100
0 10
1960
1970
1980
1990
2000
Available line widths
Line width (m)
8 n d i th ren wid t e in in Ma PP l BO
6
4
2
0
1960
1970
1980
1990
200
16.9 Trends in BOPP production lines: width and speed of biaxial orienting lines.
topics.5 Working widths of 8.7 m (‘net film on winder’) is standard, but there is also a trend to 10.4 m width (Fig. 16.10). In general there are different requirements for commodity films which need to be manufactured with highest possible productivity and speciality films which generate an added value by special application characteristics (Fig. 16.11). This is realized by various coextrusion possibilities with five
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 251
7/4/11 9:34:00 AM
252
Biaxial stretching of film
16.10 BOPP winder –10 m net film width. Commodity films
Speciality films Multi-layer structures Barrier films Replacements (paper, PVC) Shrink films Low sealing temperature Down-gaging
High volume Low margin
• Low specific investment cost • Low production cost • High output (speed, width)
• High uptime • High level of automation • Easy handling
• • • •
High flexibility Multilayer coextrusion Short change-over time Multi-product design
• High safety standards • High reliability • Low maintenance cost
16.11 Requirements for BOPP production lines.
or seven layers and corresponding extruders in order to ensure highest flexibility for various layer configurations. The trend towards down-gaging has often been cited in the past; however, in practice there are several limitations. It is possible to reduce the thickness of BOPP film in combination with higher strength without restricting the packaging requirement or having a negative effect on the packaging process.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 252
7/4/11 9:34:00 AM
Future trends for biaxially oriented films and orienting lines
253
However, this is hardly ever applied, as with a reduction of thickness, the touch and feel effects are impaired and are unacceptable for marketing reasons. In future down-gaging will probably play a more important role when it comes to legal requirements in terms of proof and/or limitation of the carbon footprint. Consequently, the principle ‘to fulfill maximum packaging requirements with minimum material’ will gain higher priority. A further important topic for BOPP film for packaging applications is the barrier function, as the barrier against water vapor (water vapor transmission rate, WVTR) is inherently good but for oxygen (oxygen transmission rate, OTR), is quite poor. An improvement can basically be attained by three different methods: (i) by means of coextrusion with a suitable material, such as EVOH, (ii) by generating a barrier via an offline coating process, for example with polyvinyl dichloride (PVDC), or (iii) by applying vacuum coatings. The latter is the common metallizing process with aluminium, or in case of transparent vacuum layers with SiOx or Al2O3. Despite the fact that these technologies have been known for a long time, in the recent past, certain developments argue in favor of a broader application: First of all, resin development has come up with new alternatives, such as for example EVOH types, which combine good barrier properties with favorable stretchability. Meanwhile, this is also attained commercially for seven-layer BOPP film with OTR values of 10 cm3/m2 day bar (Fig. 16.12). Also the vacuum process has introduced new approaches – already on a production scale. An innovative process implements plasma technology on vacuum coating equipment – transparent AlOx layers are deposited on the film attaining barrier properties similar to those in metallized film. Also for metallized film further optimization of the barrier properties (WVTR and OTR) is possible by using coextruded structures with special skin-layers. An enhancement by a factor of 2 for the OTR value compared with a standard metallized BOPP film can be attained by the application of special copolymers. Another possibility is the use of thin barrier layers with good adhesive strength in the skin layer (e.g. EVOH or PA).6 Such structures lead to an improvement of the barrier properties up to factor 400 – after metallization – compared to standard metallized BOPP film (Fig.16.13). This ultra-high barrier (UHB) film is suitable for many packaging fields as an alternative to laminate structured aluminum film. In view of the good barrier properties and the economic and environmental aspects, such film is gaining interest. A 20 mm UHB BOPP film is shown whereas, compared with a 7 mm aluminum film, the CO2 footprint is minimized by 70% (Fig. 16.14). Such aspects will certainly play a major role in future given the present environmental debates. Apart from an increase of the barrier properties, further property enhancements are constantly being developed. At this stage, just a few criteria shall be outlined which can be improved upon (compared with
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 253
7/4/11 9:34:00 AM
254
Biaxial stretching of film
Highlights • • •
Exceptional gas and aroma barrier OTR: 11.6 (cm3/m2 bar) WVTR: 5.3 (g/m2 day) Excellent optics Good stretchability
Material
OTR cm3 m2 dbar
Gage
PP + additive
1.5 mm
PP
9.2 mm
Tie-layer
1.2 mm
Barrier layer
4.2 mm
Tie-layer
1.2 mm
PP
9.2 mm
PP + additive
1.5 mm
Total
28 mm
Metallized or coated Commercial films New development
1000 100
Standard metallized
10
High barrier metallized SiOx coated film
Transparent high barrier
1 0.1
16.12 Specialty BOPP films: seven-layer transparent high barrier.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 254
7/4/11 9:34:01 AM
Future trends for biaxially oriented films and orienting lines
255
Highlights • • •
Exceptional gas and aroma barrier OTR: 0.33 (cm3/m2 dbar) WVTR: 0.45 (g/m2 day) Excellent optics Excellent metal adhesion
Material
Gauge
Metallized surface High surface energy polymer
0.5 mm
Adhesive layer
1.5 mm
PP core layer
OTR cm3
PP
1.5 mm
Copolymer
0.7 mm
Total
20 mm
Metallized of coated Commercial films New development
m2 dbar 1000 100
Standard metallized
10
High barrier met. SiOx coated film
1 0.1
UHB film metallized
16.13 Specialty BOPP films: ultra high barrier (UHB).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 255
7/4/11 9:34:01 AM
256
Biaxial stretching of film
250
Reduction ~ 75%
kg CO2e / 1000 m2
200 Aluminum foil UHB
150
Reduction by:
100
• Substitution of the aluminum foil
50
0
16.14 Film comparison of CO2 Footprint: 20 μm UHB film vs. 7 μm aluminum foil.
a standard BOPP film), depending on the applicability lead to enhanced application properties and result in higher film prices: ∑ Higher seam strength Æ inside-printed wrappers ∑ Lower sealing temperature Æ high-speed wrapping process ∑ Specific shrinkage properties Æ shrink labels, cigarette film ∑ Low density/higher yield Æ pearlized wrappers, synthetic paper ∑ High transparency Æ general and special cigarette wrappers ∑ Higher haze Æ laminates with matt surfaces ∑ Improved opacity Æ pearlized wrappers ∑ Increased strength/E-module Æ general Down-gaging ∑ Antifog properties Æ wrappers for high moisture content These property enhancements can partly be attained by the use of improved resins, and also by adapted manufacturing processes which eventually need an adapted line configuration. As such, the further development of BOPP film is prompted by activities performed by raw material suppliers, film producers, converters and line manufacturers alike. Special will continue to originate largely from the developed economic regions, whereas the volume growth of standard film will mainly dominate in certain countries, such as China.
16.2.2 Biaxially oriented polyethylene terephthalate (BOPET) In the past, varying market tendencies have taken place for biaxially oriented polyester film. The downturn of magnetic storage media, such as
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 256
7/4/11 9:34:01 AM
Future trends for biaxially oriented films and orienting lines
257
magnetic tape film for audio and video application, floppy discs, computer tapes, etc. – all comprising BOPET film as base material – was mainly offset by an increase in packaging applications (Fig. 16.15 – NPC refers to all BOPET applications, i.e. packaging and technical applications). The increase of BOPET in the packaging sector will continue, particularly in the Asian region, whereas 12 mm film will dominate the market. Line size and output capacity have constantly been further developed to meet such demand. Standard line with a net film width of 8.7 m and a production speed of 430 m/min yields an output capacity of 3750 kg/h = 28 000 tpa. Further increases of line speed are under intensive development. Production speeds of 500 m/m and more are meanwhile considered feasible. The final speed of sequential stretching lines is derived from pinning speed multiplied by the machine orientation ratio. In two-gap orientation technology, the machine orientation ratio of 4.5–4.6 is achieved. In order to attain a production speed of 500 m/min, a pinning speed of approximately 110 m/min must be reached. The pinning speed depends on the used resins (melt conductivity) and is achieved with an appropriate pinning technology. Apart from productivity which is determined by line output and availability, production costs play an important role. Particularly, energy consumption is a main criterion. Research and development (R&D) work is continually carried out by line manufacturers to reduce energy consumption. A few successful measures feature: ∑ ∑ ∑ ∑
direct drive technology; heat recovery; twin screw extrusion with direct feeding of edge trim waste; direct to film technology from polycondensation plant. 6 000 000
BOPET China BOPET world (without China)
Tons per year (tpa)
5 000 000 4 000 000 3 000 000 2 000 000
2010
2009
2008
2007
2006
2005
2004
2003
2002
2001
2000
1998
1997
1996
1995
1994
1993
1992
1991
0
1990
1 000 000
Year
16.15 Trend of world capacity – BOPET (name plate capacity).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 257
7/4/11 9:34:02 AM
258
Biaxial stretching of film
Meltdistribution system Polycondensation plant
Six high performance BOPET – lines Output 4000 kg/h/line
16.16 DFC – direct to film concept for BOPET production.
The latter entails that a polycondensation plant directly feeds several BOPET orientation lines, without an extrusion system. This is particularly efficient, since melting in extruder does not take place, and thus energy input is minimized. Although this technology is not new and has already been in use for many years in the USA and Europe, it is a new trend in China and this concept is being implemented for four to eight high performance lines for BOPET packaging film (Fig. 16.16) with the target for cost-effective production. This concept is feasable, since production concentrates to a large extent on 12 mm packaging film; therefore frequent production changeovers are not necessary.
16.2.3 Biaxially oriented polyamide (BOPA) BOPA film is rather considered as special film in the packaging market and, in terms of volume, cannot be compared to BOPP and BOPET, although a strong growth is also being registered in this market segment. In this case, however, volume growth has not been smooth and steady. It is rather characterized by alternating periods of rapid growth and no growth (Fig. 16.17) due to the heavy investment periods of some Chinese film manufacturers. In times of film shortage and high margins, many new production lines have been installed almost at the same time, leading to overcapacity and consequentially a drop in margins. In turn, a longer phase of reluctance towards new investments has been the outcome and will remain until the demand growth rises to the equivalent of the present production capacities. Today’s sequential film stretching lines feature working width 5 m, production speed 180 m/min and output 920 kg/h (= 7000 tpa). The typical
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 258
7/4/11 9:34:02 AM
Future trends for biaxially oriented films and orienting lines
259
250 000 BOPA China BOPA world (without China)
Tons per year (tpa)
200 000 150 000 100 000 50 000
2010
2009
2008
2007
2006
2005
2004
2003
2002
2001
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
0
Year
16.17 Trend of world capacity – BOPA (name plate capacity).
stretching ratios are about 3 ¥ 3. Existing simultaneous film stretching lines have widths of max. 4.2 m and speeds up to 150 m/min. Such restrictions are attributable to the high mechanical complexity and maintenance intensity of Pentagraph technology. In future, lines with the simultaneous LISIM “ technology could gain importance, as not only can overall performance data of sequential lines be outranked but also at the same time considerably improved product properties are attainable. This is playing an important part for BOPA, as quality demands are constantly increasing. In particular, shrink characteristic is an important quality criterion, since in subsequent film processing, almost all BOPA film is laminated with polyethylene (PE) film and a lower isotropic thermal shrinkage is required to avoid warping and curling. Mainly for this reason, simultaneous orientation process will maintain or even gain a higher market share.
16.2.4 Biaxially oriented polystyrene sheet (BOPS) BOPS film is mainly produced in a thickness range between 100 and 800 mm for thermoforming applications. It is becoming evident that this product is being pushed out by other substitutes. In the USA, particularly PET and PP cast films are gaining market shares for this application. Other products made of oriented polystyrene (PS) film, such as shrink labels (sleeves), however, record higher growth rates. Such products can also be manufactured on modified BOPS lines, whereas orientation occurs in the TD, rather than in the MD, which is either hardly ever applied or just with very small stretching ratios. It is essential to apply the right material recipes for PS and styrene butadiene copolymer (SBC) and optimal process control in order to set the
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 259
7/4/11 9:34:03 AM
260
Biaxial stretching of film
Requirements
• • • • •
High shrinkage TD > 75% Low MD shrink High surface gloss and clarity High stiffness Low density
Density of shrink label films
Density (kg/dm3)
2.0
1.37
1.30
0.0
1.02
0.95
1.0
PP
PS-SBC
PVC
Pet-G
Shrink characteristics 80 PET-G
Shrinkage (%)
70 60
PVC
50 40
PS-SBC
30
PP
20 10 0 50
55
60
65
70 75 80 Temperature (°C)
85
90
95
100
16.18 Sleeve: TD oriented PS/SBC.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 260
7/4/11 9:34:03 AM
Future trends for biaxially oriented films and orienting lines
261
required shrink properties. This necessitates that shrinkage in the TD needs to be as low as possible at <60 °C and as high as possible at >95 °C (at least 60%). On the other hand, OPS/SBC sleeve film competes with PVC and glycol-modified PET (PET-G) shrink labels, whereas compared with PVC, they have a higher image (PVC is increasingly under pressure due to the debates on environment) and – compared to PET-G – a price advantage. Also the low density of close to 1 g/cm³ is an advantage in terms of yield and also recycling issues (Fig. 16.18). In view of the fact that shrink label film will record a worldwide growth and resin suppliers continue to come up with optimized raw materials for this purpose, it can be assumed that the future perspectives also for OPS/SBC are good.
16.2.5 Biaxially oriented polyethylene film (BOPE) Biaxially oriented BOPE film is mainly for shrink applications and is to a large extent produced on double bubble lines. Shrink film can be classified into various fields of application: ∑ Collation film (bundle shrink film) – Application: shrink films for beverages and other collective packaging – Production: special blown film line (low blow-up ratio, BUR) – Characteristics: preferably MD shrinkable films ∑ Premium shrink film: – Application: shrink film for display packages – Production: double bubble line, tenter, monoaxial – Characteristics: good optics, stiffness, toughness and sealability, high shrink ∑ Barrier shrink film: – Application: food packaging – Production: multilayer double bubble technology – Characteristics: as Premium Shrink Film, High Oxygen Barrier The market trends show an extremely positive ranking in all three fields of application.
16.2.6 Biaxially oriented polylactic acid (BOPLA) Packaging film made of bioplastics is becoming more and more attractive, also adopted for marketing purposes, i.e. green image. Various aspects are under discussion. Besides the issue of biodegradability, the aspect of renewable resources is also under consideration of the packaging industry. Among oriented films from bioplastics, BOPLA plays a vital role, as the raw material PLA is produced in a commercial scale and due to the stretching © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 261
7/4/11 9:34:03 AM
262
Biaxial stretching of film
Bags for bread and other bakeries Packaging for fresh food – agricultural products (high WVTR works like anti-fog and can enhance shelf-life)
Packaging for cheese and butter (deadfold retention)
Bags for cheese and salami
(enables ripening – longer shelf-life)
Shrink sleeve film and high modulus label films
Temperature (°C)
16.19 BOPLA film applications. 300 260 220 180 140 100 60 20
Extrusion
Casting unit
Machine directory orientation
Transverse direction orientation
Pull roll
Winder
16.20 BOPLA process typical temperatures.
process a sufficiently special profile of properties for various packaging applications is generated (Fig. 16.19). Compared with BOPP, BOPLA film has a considerably higher (by factor 30) water vapor permeability, and therefore although unsuitable for many packaging applications it is, however, beneficial in some cases. For example, bread and fresh vegetables, salad and fruit benefit from a highly controlled water vapor permeability and preservability. In addition, BOPLA wrappings are characterized by an excellent optic and stiffness. The latter
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 262
7/4/11 9:34:03 AM
Future trends for biaxially oriented films and orienting lines
263
results in a crackle effect, giving the impression of freshness of wrapping content. A further outstanding characteristic of BOPLA, is its good deadfold characteristic, making it predestined for twist wrap applications (e.g. candy wrap). The market for BOPLA is still rather small, but it will grow in future, particularly if prices for oil-based raw materials continue to rise. PLA availability is increased by many supply sources and providing that new applications/processes are developed. The extrusion and orientation process is quite similar to BOPP and BOPET (Fig. 16.20) which makes it feasible to convert older BOPP or BOPET lines.
16.3
Trends for technical film
There are many technical applications for biaxially oriented films, such as flat panel displays, solar cells, capacitors, membranes and battery separator films. Compared with packaging film, the market for biaxially oriented technical film shows a considerably smaller market volume. Technical film has a higher finishing characteristic and is therefore more expensive and is influenced by ever-changing development trends. Furthermore since technical film is more susceptible to economic cycles than packaging film, the market is more crisis-prone. The most important market for biaxially oriented technical film is outlined as follows.
16.3.1 Flat panel displays The sales volume of flat panel displays such as LCD monitors has steadily increased over the past 30 years. Whereas in the 1970s and 1980s, the broad application of LCDs was limited to digital clocks, pocket calculators and various other small-size display applications, a continual improvement of LCD technology has allowed many other fields of application to arise – such as TVs, notebooks and desktop monitor applications. Nowadays more than 95% of desktop monitors and 70% of TV monitors feature LCD technology. Sales volumes of TFT-LCDs >9.1≤ (230 mm) in February 2010 have almost reached 46 million units, in the year 2009 over 527 million units were shipped. In the next few years, sales volume for LCDs are expected to increase primarily in the field of screen sizes over 30≤ (760 mm) because it is assumed that CRT technology will entirely be replaced by LCD technology. In modern LCDs, optical film – such as compensation film, polarizer, diffusion film and brightness enhancement film, substrate and protection film – is an essential component (Fig. 16.21). The materials which are used for these layers are: PET, COC, cyclo olefin polymer (COP), polymethyl methacrylate (PMMA), PC, polyvinyl amide (PVA) and TAC.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 263
7/4/11 9:34:04 AM
264
Biaxial stretching of film Surface treatment film Front polarizer film Front retardation film Adhesive Glass substrate
Adhesive Rear retardation film Rear polarizer film Brightness enhancement film
Backlight
16.21 Flat panel display: layer structure of LCD.
The intrinsic contrast and viewing angle properties of advanced technologies, like vertical alignment, have been further improved so that the demand for sophisticated optical films has been increased. Compensation films are designed to compensate light leakage in LCDs, which is caused by crossed polarizers and the liquid crystal layer at oblique viewing angles. Light leakage under an oblique viewing angle results in color shifts and a reduction of contrast ratio and thus is eliminated as far as possible. Other trends with high future potential are 3D-displays, flexible displays and organic LED (Fig. 16.22), which will create an additional demand for high performance oriented films, especially BOPET films. Also during the production of flat panel displays there is a need for oriented films with excellent optical properties in order to protect the panels during the production steps and allow also a dedicated quality control.
16.3.2 Photovoltaic (PV) solar panels The global trend for the reduction of CO2 Emissions has given a major push to the solar cell industry. Driven by government subsidies and the development of the fossil energy cost there was a global growth rate of more than 25% per year during the last 10 years, in some countries (i.e. China) even more than 40%. The most common systems are crystalline silicon PV modules with advantages in power efficiency, but limitations in cost efficiency. New technologies such as thin film solar cells have been realized with lower production cost, but compromising the efficiency factor. For the crystalline silicon PV modules there is a need for backsheets that can reflect sunlight, protect (against UV light, humidity), achieve electrical isolation and be bondable to. These functions are most effectively achieved
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 264
7/4/11 9:34:04 AM
Future trends for biaxially oriented films and orienting lines
265
Large flat panel display • LCD technology • High contrast • Large viewing angle
3D-Display
• • • •
Medicine Design, CAD Advertizing Games
Flexible displays
Organic LEDs
• E-paper • Mobile communication
• Mobile communication • Digital cameras • Television
16.22 Flat panel display: New Technologies/Trends.
Glass EVA Silicon solar cells EVA Backsheet i.e. BOPVF/BOPET/BOPVF
16.23 PV solar module: layer structure (EVA - ethylene vinyl acetate).
with a laminate of BOPET and biaxially oriented polyvinylfluoride (BOPVF) under the aspect of durability up to 25 years of outdoor exposure (Fig. 16.23). The typical thickness range of the BOPET film for this application is 50–330 mm and for the BOPVF 25–38 mm, which is laminated on both sides of the BOPET. Owing to the expected continuing growth of the PV business the global demand for thick BOPET film will be increasing with at least 30% per year (Fig. 16.24).
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 265
7/4/11 9:34:04 AM
266
Biaxial stretching of film
120 000
Demand (tpa)
100 000 80 000 60 000 40 000 20 000 0 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013
16.24 BOPET film demand for PV applications.
16.3.3 Battery separator film The long term provision of environmently friendly individual mobility is one of the most challenging duty of the automotive industry. Hybrid and electric vehicles are the most promising technologies for the future. Since the batteries determine the bottleneck in terms of driving range/cost ratio this will be the key technology in the car industry. Lithium ion batteries have the best potential for a widespread use as the required performance can be obtained with existing technology. For the future it will be necessary to improve the technical features (energy density, reliability, lifetime and safety aspects) as well as reduce the cost significantly (Fig. 16.25). One contribution for the attainment of this goal is the battery separator film which is used in all lithium ion batteries and represents about 20% of the material cost. The most common material which is used today for this application is a membrane, based on biaxially oriented polymer (i.e. high density polyethylene(HDPE) or ultra-high molecular weight polyethylene (UHMWPE)). As the current technologies (‘wet’ or ‘dry’ process) are complicated and quite limited in terms of output and yield there is a demand for more efficient production facilities. The market volume for this type of film is about 500 ¥ 106 m² today, mainly used for lithium ion batteries in computer and communication applications. But this will change drastically, as the demand for electric vehicle grows as expected in the next 20 years (Fig. 16.26). Additional potential for lithium ion batteries is for stationary energy storage, which is becoming more important due to the growth of alternative energy sources as wind and solar, as these produce power an intermittently.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 266
7/4/11 9:34:04 AM
Future trends for biaxially oriented films and orienting lines Bottle necks
Requirements
• Safety concerns • Expensive technology • Limited lifetime
Separator
• • • • •
267
Safety requirements Cost reduction Reliability Energy density Lifetime
Cathode lead Cathode cover Safety vent film Positive temperature coefficient Gasket (safety device) Insulator
Anode Center pin container Cathode Anode lead
Anode
16.25 Electric mobility: ‘bottlenecks’ and requirements. 7000
Demand (m2 ¥ 106)
6000 5000 4000 3000 2000 1000 0 2009
2012
2015
2018
2021
2024
2027
2030
16.26 Forecast demand of battery separator film for lithium ion batteries.
16.4
Development environment for biaxially oriented film
In view of the variety of biaxially oriented film types and the dynamic market environment, a permanent R&D facility is necessary for all film producers. As most manufacturers of biaxially oriented film do not have their own infrastructure or pilot line testing facilities, the question is how can tests be performed for the development process for new film types? In
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 267
7/4/11 9:34:04 AM
268
Biaxial stretching of film
several cases, existing production lines are used to test recipe variations or modified process setting. This entails that a certain timespan is needed and valuable production time comes to a stand still. Sometimes, even older, smaller lines are retrofitted, for example by implementing multilayer dies or additional components in order to be able to produce new film structures. In any case, this procedure is time-consuming and cost-intensive, as a high resin consumption has be taken into account for this development procedure. This situation can be improved with a pilot line. The extrusion and orientation process is performed on a smaller scale and new developments are possible with minimal raw material and without interference to running production. In order to meet the demands of the oriented film industry, Brückner Maschinenbau GmbH & Co. KG, Germany, operates a technology centre which is designed to be multifunctional and flexible. Almost all relevant structures and film types can be produced in a pilot scale. A three-step course of R&D action is performed in order to derive result data and obtain information required for the design layout of production lines for this newly developed film (Fig. 16.27). The first step comprises a batch processed stretching procedure on a laboratory stretching frame, which is designed in order to simulate the continuous production process (Fig. 16.28). The oriented samples are characterized by representative film properties and can be analysed in the laboratory for mechanical, optical, shrink and barrier properties. The process data, such as temperature, stretching ratio and speed, can be transferred to a continuous process of a film stretching line. During the stretching process the stress–strain data are monitored by force Procedures
Production line
• Evaluation of process window • Optimization of film properties • Measurement of stretching forces • Simulation with finite element method • Up-scaling to production scale
Pilot line
Lab stretcher Customer benefits Basic R&D for film stretching Development of new film types Development of new components Production of sample rolls for pre-marketing
16.27 Upscaling procedure for stretching processes.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 268
7/4/11 9:34:05 AM
Future trends for biaxially oriented films and orienting lines
• • • •
269
Stretching ratio up to 10 ¥ 10 Heating capability up to 400 °C Up to three heating modules Evaluation of stretching forces
16.28 Laboratory stretching machine.
transducers, which is the precondition for modeling the process in a finite element method (FEM) model (Fig. 16.29). Additional important information is derived from the tests on the pilot line (e.g. thickness tolerances, production stability) which allows also the professional design layout of a production line for any new film product. Brückner Maschinenbau has a pilot line in operation which implements MD-, TD-, sequential and the simultaneous stretching process (Fig. 16.30). In combination with a flexible extrusion system for PP, PET, PA, PS, PLA, PEN, PMMA, polyvinylidene fluoride (PVDF) and other polymers, it allows nearly all possible film stretching trials. Also multilayer structures with coextrusion and multilayer give more features for high end applications (Fig. 16.31). Furthermore a in-line coater is available in order to apply thin film coatings for primer, antiblock, antistatic, release, UV protection, barrier, optical enhancements and other functions. Film orienting is realized by using a multigap-stretching MDO followed by a TDO or using a simultaneous tenter which is based on the LISIM® Technology. This technology allows to adjust stretching ratios, stretching curves, relaxation curves and process temperatures in a most flexible way in order to meet the required film properties of new developed films (Fig. 16.32). A wide range of film types have been developed and tested in the past years on the pilot line: ∑ BOPP – low seal initiation temperature (SIT) film – high sealing strength film
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 269
7/4/11 9:34:05 AM
270
Biaxial stretching of film Lab stretcher
Lab extrusion
Lab stretcher profiles
Stress–strain curve MD- TDprofiles
Resin
Stretching force calculation FEManalysis
16.29 Basic evaluations on laboratory stretcher.
All stretching modes MD
TD
MD/TD
LISIM
Processing of all major polymers
PP PA
PVDF COC
PLA PET PS
PE PEEK
PMMA PC
PEN
16.30 Pilot line for all stretching modes.
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 270
7/4/11 9:34:06 AM
Future trends for biaxially oriented films and orienting lines
TDO/LISIM®
Extrusion er) rud ext er) Co d A ( xtru der) E B ( extru der) Co tru C ( Coex uder) D ( oextr ) C er G ( xtrud r) e E B ( xtrud e o C A(
Pull roll
271
Winder
Features
B1-twin screw extruder B2-single screw extruder A,C,D-coextruder
• • •
Gravimetric dosing four components Extrusion • 2 twin screw • 4 single screw Die • Multiflow channel coathanger type • 1-/ 3- / 5- / 7-layer
16.31 Pilot line features: multilayer extrusion.
Extrusion
TDO / LISIM®
Casting
Pull roll Winder
Features • • • •
Linear motor driven clips Full flexibility in MD and TD • Stretching ratios/curves • Relaxation ratios/curves Fast product change High accurate oven temperatures
16.32 Line features: TDO/LISIM®.
– – – – –
MD – shrink label film synthetic paper foamed film shrink cigarette film five-layer UHB film
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 271
7/4/11 9:34:07 AM
272
∑ ∑ ∑ ∑ ∑
Biaxial stretching of film
– seven-layer transparent barrier film – capacitor film BOPET – MD – tensilized film – ultrathin film (1 mm) – thick film (400 mm) – inline coated film BOPE – barrier shrink film – breathable film – low shrink film BOPA – low bowing film – low shrink film – high barrier film Sleeve – monoaxially oriented polystyrol (MOPS)/SBC – monoaxially oriented polyester (MOPE) – monoaxially oriented polylactic acid (MOPLA) – monoaxially oriented polyethylene (MOPE)/COC – MD-shrink film Others – BOPLA – BOPS – BOCOC – BOPVDF – BOPEN – BOPEEK – battery separator film
Based on these experiences this platform offers the best conditions for all types of future developments of oriented films. Some of those have been transferred to the production scale, using the basic stretching data from the pilot line for a dedicated line layout of a production line.
16.5
References
1. Anon, Position Paper – Market Trends and Developments, World Packaging Organisation, April 17, 2008 2. Binder M., Einfluss des Produktionsprozesses auf den Product Carbon Footprint von biaxial verstreckten Kunststofffolien, Diploma Thesis, University of applied science Augsburg, August 2010 3. Sehti A., Chen S., China Megatrends,TNS – Survey Report, February 2009 4. 27th International Battery Seminar & Exhibit, Fort Lauderdale, March 15–18, 2010
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 272
7/4/11 9:34:07 AM
Future trends for biaxially oriented films and orienting lines
273
5. Breil J., Oriented film technology, in Multilayer Oriented Films, John R. Wagner (ed), Elsevier, 2010 6. Wolf M., Breil J., Lund R., Development of new BOPP Barrier Films by Coextrusion and Simultaneous Biaxial Orientation, TAPPI Conference in Portsmouth, USA (2008)
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse-16.indd 273
7/4/11 9:34:07 AM
WH Biaxial-DeMeuse-16.indd 274
7/4/11 9:34:07 AM
Index
AccuPull, 28–9, 34 acrylic, 194 active package, 156 air knife, 128 Applied Extrusion Technologies, 21, 70 ASTM D 1238, 59 ASTM F88-05, 190 ASTM F1921-98(2004), 190 ASTM F2029-00, 190 atmospheric glow discharge (APGD), 78–9 BASF, 125 battery separator film, 266–7, 267 electric mobility: ‘bottlenecks’ and requirements, 267 forecast demand for lithium ion batteries, 267 biaxial film structures, 59–65 based on homopolymer polypropylene, 59–65 coated films, 64–5 five layer film structure, 61 tie layer materials used, 61–4 recommendations, 65 biaxial oriented polypropylene, 17 biaxial stretched films applications, 231–9 film tentering process, 233 tubular film extrusion, 233 fresh-cut produce packaging and use, 143–60 future trends, 159–60 package atmosphere modification, 147–51 packaging methods and quality maintenance, 151–9
quality factors determining shelf-life, 144–6 respiration and metabolism, 146–7 future trends, 238–9 nanocomposite and multilayer films biaxial stretching, 234–8 adaptor and coextrusion feedblock, 237 nanoclay composite films micrographs, 235 PET nanoclay composite films surface micrographs, 236 snack packaging, 165–200 advantages and limitations, 195–7 applications, 197–8, 199 basic principles and methods, 169–79 future trends, 198, 200 technologies and techniques, 179–95, 196 biaxial stretching fundamentals and definitions of terms, 3–13 methods, 3–11 biaxially oriented polypropylene, 5 equipment used, 8–9 laboratory studies, 9–11 tenter frame biaxial stretching process, 5–8 recommendations, 11–12 biaxially oriented films future trends, 240–73 development environment for biaxially oriented film, 267–72 trends for packaging film, 249–63
275 © Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Index.indd 275
7/4/11 9:28:51 AM
276
Index
trends for technical film, 263–7 biaxially oriented polyamide, 52, 73, 242, 258–9 world capacity trend, 259 biaxially oriented polyethylene film, 261 biaxially oriented polyethylene terephthalate, 47–8, 72–3, 120, 125, 256–8 DFC – direct to film concept for production, 258 world capacity trend, 257 biaxially oriented polylactic acid, 73, 79–80, 261–2 film applications, 261 process typical temperatures, 262 biaxially oriented polypropylene, 5, 54, 62, 72–3, 78, 119, 125, 250–6 film comparison of CO2 footprint, 256 requirements for production lines, 252 specialty films: seven-layer transparent high barrier, 254 specialty films: ultra high barrier, 255 trends in production lines, 251 winder –10 m net film width, 252 world capacity trend, 250 biaxially oriented polystyrene, 259–61 sleeve: TD oriented PS/SBC, 260 biaxially stretched films academic investigations, 117–23 biaxial studies of specialty polymers, 120–3 common commodity polymers, 117–20 recommendations, 123 commercial production processes, 67–72 double bubble commercial production process, 69–70 linear inverse space-mapping technology, 68–9 worldwide producers, 70–2 equipment design and requirements, 14–25 double bubble process, 14–17 recommendations, 24–5 schematic of double bubble process, 15 tenter process, 17–24, 23–4 industrial processes, 67–74
novel technologies being developed, 72–3 recommendations, 74 laboratory evaluations, 27–35 Karo IV laboratory stretcher from Brückner, 29 literature studies, 30–4 recommendations, 34–5 stress–strain curve for homopolymer polypropylene, 33 T.M. Long stretcher, 27–9 other polymers used, 47–56 chemical structure of PET, 47 polyamides, 51–3 polyethylene terephthalate, 47–51 polylactic acid, 53–6 recommendations, 56 polyolefins, 36–44 chemical structure of polypropylene, 37 polyethylene, 41–3 polypropylene, 36–41 recommendations, 44 post-production processing, 76–84 recommendations, 84 product labelling, 204–29 future trends and new developments in labelling and label films, 226, 228–9 label applications, 215–20, 221 label preparation – label design, printing and converting, 220, 222–6 labelling systems and technologies, 205–15 surface treatments, 76–83 coated films, 83 literature studies, 78–80 metallised films and their production, 80–3 biaxially stretched polyamide, 125–39 film application pet food, 134 smoked salmon, 133 film properties, 133–9 bowing reduction with LISIM Technology, 139 comparison, 135 polar shrinkage, 139
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Index.indd 276
7/4/11 9:28:51 AM
Index history, 126 nylon, 125–6 caprolactam polymerisation, 126 polar shrinkage diagrams sequentially stretched film, 136 simultaneous produced films, 138 processing, 126–33 biaxial orienting technologies, 127 casting and pinning, 127–8 converting, 132–3 electrostatic needle pinning configuration, 128 extrusion, 126–7 high pressure air knife pinning, 129 winding, 132 stretching processes, 128–32 BOPA line from Brüeckner, 130 LISIM line, 131 sequential line, 129 sequential tenter frame process, 128–30 simultaneous double bubble process, 132 simultaneous tenter frame process, 131 Bicor 84 AOH, 64 biosphere, 241 Bonyl, 126 BOPA see biaxially oriented polyamide BOPE see biaxially oriented polyethylene film BOPET see biaxially oriented polyethylene terephthalate BOPLA see biaxially oriented polylactic acid BOPP see biaxially oriented polypropylene BOPS see biaxially oriented polystyrene Brückner, 130, 131, 134, 137 Brückner Inc., 21 Brückner Maschinenbau GmbH & Co., 268–9 Bynel, 61 Bynel 5000 series, 61 CAPP-05, 64 carbon dioxide, 150 cast film biaxial orientation, 232
277
cast film extrusion, 236 cathode ray tube, 247 cavitation, 191–3 basic technologies, 191, 193 incompatible polymer blends, 191 mineral-based cavitation agents, 191, 193 cavitated OPP with PBT cavitation agent, 192 five-layer product design for white cavitated film, 192 cellophane, 165, 178, 179 cellulose triacetate (TAC), 247 chemical structure, 53 coated films, 83 COC see cyclo-olefin copolymer coefficient of friction, 216 coextrusion, 7, 59, 186, 188–91 commodity coex OPP film design, 188 heat seal and hot tack strength vs sealer jaw temperature for coextruded OPP film, 190 COF see coefficient of friction compression roll drawing (CRD), 72 constrained uniaxial stretching, 90 corona discharge treatment, 76–7, 78–9 CRT see cathode ray tube cut and stack labelling, 208–9, 222–3 design, 222–3 system, 208–9 process, 209 cyclo-olefin copolymer, 246–7 direct edge trim recycling, 126 direct thermal labelling, 214–15, 226, 227 design, 226, 227 plastic label films overview, 227 system, 214–15 double bubble process, 4, 14–17 commercial production, 69–70 literature studies, 23 schematic, 15 Dow Chemical Company, 72 dry process, 266 DSM, 125 dyne level, 77
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Index.indd 277
7/4/11 9:28:51 AM
278
Index
Emblem Film, 126 EMS, 125 engineering strains, 90 engineering stress, 90 EP 0826731A1, 53 equibiaxial stretching, 90 ethylene, 150–1 ethylene, butene propylene copolymer (EBP copolymer), 189 ethylene vinyl alcohol, 189, 232, 249 European Patent Application EP 0876250, 42 EVOH see ethylene vinyl alcohol extension ratios, 90 extrusion, 268 Exxon Mobil, 70 facestock, 223 feedblock, 237 fermentation, 147 Fick’s first law of diffusion, 151 film blowing, 179, 236 film casting, 179 film orientation, 179 film tentering, 232 illustration, 233 Fischer–Trospch wax, 40, 41 flame treatment, 77–8, 79 flat panel displays, 263–5 LCD layer structure, 263–4 new technologies/trends, 264–5 flavour, 145, 158–9 4032D film, 55 free blow, 97 fresh-cut processing, 143, 158 fresh-cut produce package atmosphere modification, 147–51 carbon dioxide, 150 optimum atmosphere compositions, 148 other volatiles, 150–1 oxygen, 149 water vapour, 150 packaging and use of biaxial stretched films, 143–60 future trends, 159–60 respiration and metabolism, 146–7
packaging methods and quality maintenance, 151–9 active modified atmosphere packaging, 156–8 atmosphere composition changes of passive modified atmosphere packages, 154 flavour interactions, 158–9 gas permeability, 151–3 package design, 153–6 theoretical steady-state package atmospheres, 155 quality factors determining shelf-life, 144–6 microbiological quality, 145–6 nutritional value, 145 sensory quality, 144–5 fruits and vegetables see fresh-cut produce future trends biaxially oriented films and orienting lines, 240–73 biospheric factors, 241–2, 243 CO2 footprint of BOPP Film, 243 global warming, 241 development environment for biaxially oriented film, 267–72 basic evaluations on laboratory stretcher, 270 laboratory stretching machine, 269 line features: TDO/LISIM, 271 pilot line features: multilayer extrusion, 271 pilot line for all stretching modes, 270 upscaling procedure for stretching processes, 268 economic factors, 242, 244 urbanisation in China, 244 political factors, 246, 247 forecast electric vehicle sales p.a. until 2030, 247 social factors, 245–6 trends to smaller households, 245 technological factors, 246–9 simultaneous stretching with LISIM technology, 249 trend for display technologies, 248
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Index.indd 278
7/4/11 9:28:51 AM
Index trends for packaging film, 249–62 biaxially oriented polyamide, 258–9 biaxially oriented polyester film, 261 biaxially oriented polyethylene terephthalate, 256–8 biaxially oriented polylactic acid, 261–2 biaxially oriented polypropylene, 250–6 biaxially oriented polystyrene sheet, 259–61 trends for technical film, 263–7 battery separator film, 266–7 flat panel displays, 263–4 photovoltaic solar panels, 264–6 gas permeability polymer packaging films, 151–3 perforations, 152–3 solid films, 151–2 good coefficient of friction (COF), 189 Harden, 126 HDPE see high density polyethylene high crystallinity polypropylene (HCPP), 40 high density polyethylene, 17, 41–2, 179, 266 Hostaphan, 48 hot tack, 189, 190 hydrocarbon resin, 40–1 in-mold labelling, 211, 224 design, 224 system, 211 Inventure Labs, 9, 28 inverse Langevin approximation, 100 Karo IV, 9, 29, 34 Kohjin Film, 126 label applications, 215–20, 221 labelling processes and labelled container requirements, 221 biaxially stretched films, 204–29 future trends and new
279
developments in labelling and label films, 226, 228–9 defined, 204 labelling systems and technologies, 205–15 preparation - design, printing and converting, 220, 222–6 labelling systems classifications, 205–6 glue applied, 205 self-adhesive, 205 sleeves, 205–6 tags, 205, 206 and technologies, 205–15 lamination technologies, 176 Langevin chain statistics, 102 Langevin distribution, 100 LCD see liquid crystal display LDPE see low density polyethylene linear inverse space-mapping (LISM) technology, 68–9 linear motor simultaneous stretching technology, 189, 217, 248 liquid crystal display, 247 liquid crystalline polymer (LCP), 62 LISIM, 131, 139 LISIM technology see linear motor simultaneous stretching technology literature studies, 30–4 Long extensional tester (LET), 89 low density polyethylene, 42, 152, 153, 178 machine direction, 4, 181 machine direction orientation, 250 machine direction orienter (MDO), 128, 184 macroperforations, 152 Marshall and Williams see Parkinson Technologies MDO see machine direction orientation Melinex, 48 melt filtration, 126 metallised films and their production, 80–3 microperforations, 152 modified atmosphere packaging (MAP), 147–51 active packaging, 156–8
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Index.indd 279
7/4/11 9:28:51 AM
280
Index
package design, 153–6 passive packaging, 154 atmosphere composition changes, 154 modified atmospheric treatment, 78 molecular weight distribution (MWD), 37–8 Mooney’s constants, 101 ‘multilayer’ films, 31 Mylar, 48 name plate capacity (NPC), 250 natural draw ratio, 97 needle pinning, 127–8 net structures, 68–9 nitrocellulose coatings, 194 nylon, 17, 125–6 Nylon 6, 51–2, 121 Nylon 6,6, 51 Nylon 66, 121 OMINA, 63 OPET see oriented polyester OPP see oriented polyethylene terephthalate organic light-emitting diode (OLED ), 247 orientation, 3, 179 orientation process, 268 orientation technology, 180–6, 187 cast film surface, 183 double bubble orientation process used for OPP production, 180 primary MDO stretching configuration for OPP, 185 primary MDO stretching configurations for PET films, 184 primary TDO oven zones, 186 sequential tenter process used for biaxial orientation, 181 US Patent 4068356 and 5797172 showing modern clip support and tenter chain systems, 187 oriented polyester, 167 oriented polyethylene terephthalate, 167 oxygen, 149 oxygen transmission rate (OTR), 121, 152, 253
PA see polyamide packaging use of biaxial stretched films for fresh-cut produce, 143–60 future trends, 159–60 package atmosphere modification, 147–51 packaging methods and quality maintenance, 151–9 quality factors determining shelflife, 144–6 respiration and metabolism, 146–7 packaging films, 146, 151–3 Parkinson Technologies, 21, 72 PE see polyethylene Pentagraph technology, 259 perforations, 152–3 PET see polyethylene terephthalate Peterlin’s model, 118 PETG, 103 photovoltaic solar panels, 264–6 film demand for PV applications, 266 solar module: layer structure, 265 ‘pilot-line’ scale, 25 PLA see polylactic acid planar extension, 94 plasma treatment, 78 Plexar, 61 polar shrinkage, 134, 136, 137, 138, 139 polyamide, 51–3, 121, 231 see also nylon Nylon 6 and Nylon 6,6 chemical structures, 51 patent activity, 52–3 polyamide 6, 125 polyether ether ketone (PEEK), 248 polyethylene, 41–3, 120, 231, 248 poly(ethylene 2-6-naphthalate) (PEN), 121–2 molecular structure, 122 polyethylene terephthalate, 47–51, 119, 232 chemical structure, 47 experimental, 89–90 material and properties, 89 stretching of samples, 89–90 literature studies, 48–9 nature of stress–strain curve, 88–9 PET film, 88
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Index.indd 280
7/4/11 9:28:52 AM
Index
patent activity, 49–51 strain energy function and stress– strain model for uniaxial and biaxial orientation, 86–113 processing, 86–8 stress vs. strain as a function of extension rate, 110 as a function of first extension at 90°C and an extension rate of 50%/s, 109 as a function of first extension at 90°C and an extension rate of 250%/s, 111 as a function of temperature, 110 stress–strain behaviour, 90–9 effect of extent of first extension, 94 effect of moisture content, 98–9 extension rate effect, 93 extension rate on stress–strain data, 92–3 extrapolation of stress–strain curves, 97 inflation of cylindrical PET parison, 98 sequential biaxial extension, 94–6 simultaneous biaxial extension, 93–4 strain hardening property for preform design, 97–8 stress vs. strain as function of first extension, 95 stress–strain curve, 90–1 stress–strain curves for PET film specimens, 91 superposition to higher extension rates, 96–7 temperature on stress vs. strain, 92 temperature on stress–strain data, 91–2 stress–strain model development, 103–13 comments on use, 112–13 comparison of model behaviour with experiment, 109–11 parameters characterising shape of curve, 104 predicting temperature rise during stretching, 111–12
281
preliminary form design, 113 sequential biaxial extension, 107–9 simultaneous biaxial extension, 104–7 slope S as function of first extension, 107 strain hardening point, 108 temperature on yield stress, 105 yield stress parameter, 106 stress–strain modelling, 99–103 other approaches, 102–3 statistical kinetic model, 99–100 strain energy approach extensions, 100–2 polylactic acid, 53–6, 120, 168 polymer packaging films gas permeability, 151–3 perforations, 152–3 solid films, 151–2 polymeric cavitating agents, 191 polyolefins, 151 used in biaxially stretched films, 36–44 polyphenylene sulphide (PPS), 248 polypropylene, 17, 36–41, 118, 231 biaxial film structures, 59–65 coated films, 64–5 five layer film structure, 61 tie layer materials used, 61–4 chemical structure, 37 polyterpene, 40 polyvinyl alcohol, 64, 176 polyvinyl dichloride, 178 polyvinylidene chloride, 64 pressure-sensitive labelling, 210–11, 223–4 design, 223–4 system, 210–11 process, 210 pretreatment, 77 product labelling applications, 215–20, 221 biaxially stretched films, 204–29 future trends and new developments in labelling and label films, 226, 228–9 preparation – design, printing and converting, 220, 222–6
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Index.indd 281
7/4/11 9:28:52 AM
282
Index
cut and stack label design, 222–3 direct thermal label design, 226, 227 in-mold label design, 224 pressure-sensitive label design, 223–4 Roll-fed label design, 222 Roll-fed shrink label design, 222 shrink sleeve label design, 224–5 stretch sleeve label design, 225 thermal transfer label design, 225–6 requirements by application, 215–20, 221 labelling processes and labeled container requirements, 221 systems and technologies, 205–15 cut and stack labelling system, 208–9 direct thermal labelling system, 214–15 in-mold labelling system, 211 pressure-sensitive labelling system, 210–11 Roll-fed labelling system, 206, 207 Roll-fed shrink label system, 206–8 shrink sleeve labelling system, 212–13 shrink stretch labelling system, 213 thermal transfer labelling system, 214 PVDC see polyvinyl dichloride PVOH see polyvinyl alcohol research and development, 257 respiration, 146–7 RI, 21 Roll-fed labelling, 206, 207, 222 design, 222 system, 206, 207 process, 207 Roll-fed shrink labelling, 206–8, 222 design, 222 system, 206–8 Saran, 83 SBC see styrene butadiene copolymer
seal initiation temperature (SIT), 55, 189 sensory quality, 144–5 sequential biaxial extension, 94–6, 107–9 sequential stretching, 63, 119 shelf-life, 169 shift factor, 96 shrink sleeve labelling, 212–13, 224–5 design, 224–5 system, 212–13 process, 212 shrink stretch labelling system, 213 silica-based polymer nanocomposites, 234 silicon dioxide, 64 simultaneous biaxial extension, 93–4, 104–7 simultaneous biaxial stretching, 87, 90 simultaneous tenter frame process, 63 SKYWEL PLA film, 55 slip additives, 6 slitters, 20 snack packaging advantages and limitations, 195–7 applications, 197–8, 199 current market areas for several existing and emerging oriented films, 199 basic principles and methods, 169–79 generalised and typical packaging lamination, 178 manufacturing methods available for combining and converting various polymers and films, 177 properties required for flexible snack packaging, 175 shelf-life vs optical density for all bag sizes, 171 biaxial stretched films, 165–200 stress vs elongation showing packaging films properties compared to paper, 168 future trends, 198, 200 metallisation for high barrier, 195, 196 aluminium foil and polyester film barrier mapping vs coextruded OPP films, 196
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Index.indd 282
7/4/11 9:28:52 AM
Index potato chips shelf-life for several packaging materials, 171 shelf-life vs moisture barrier, 172 shelf-life vs oxygen barrier, 172 technologies and techniques, 179–95, 196 cavitation, 191–3 coating property improvement (out-of-line and inline coating), 193–5 coextrusion, 186–91 orientation technology, 180–6 solid films, 151–2 strain energy function, 100 and stress–strain model for uniaxial and biaxial PET, 86–113 experimental, 89–90 nature of PET stress–strain curve, 88–9 PET processing, 86–8 PET stress–strain behaviour, 90–9 stress–strain behaviour modelling, 99–103 stress–strain model development, 103–13 ‘strain hardening,’ 34 strain hardening point, 88, 104 stress-induced orientation, 167–8 stress–strain model and strain energy function for uniaxial and biaxial PET, 86–113 experimental, 89–90 nature of PET stress–strain curve, 88–9 PET processing, 86–8 PET stress–strain behaviour, 90–9 stress–strain behaviour modelling, 99–103 stress–strain model development, 103–13 stretch blow moulding, 87 stretch sleeve label design, 225 styrene butadiene copolymer, 259 Taghleef Industries, 72 tenter process, 4, 17–24, 23–4 commercial biaxial orientation equipment, 21–2
283
literature studies, 22–3 terpolymer, 189 tetrahydro furan, 208 thermal transfer labelling, 214, 225–6 design, 225–6 system, 214 thermoplastic polymer resin see polyethylene terephthalate THF see tetrahydro furan tie layers, 61 time–temperature superposition, 96 tissue browning, 147 T.M. Long stretcher, 9, 27–9 Toyobo, 126, 130 transverse direction, 4, 181 transverse direction orienter (TDO), 129–30 Treofan, 70–1 tubular film blowing, 232 illustration, 233 tubular film process, 52 UBE, 125 ultra-high barrier (UHB) film, 253 ultra-high molecular weight polyethylene (UHMWPE), 266 Ultramid B resins, 73 uniaxial stretching, 87 Unitika, 126 US Patent 3432591, 49 US Patent 3923693, 83 US Patent 4345005, 82 US Patent 4522867, 52 US Patent 4604322, 82 US Patent 4627657, 62 US Patent 4921749, 40 US Patent 5094799, 52 US Patent 5153074, 63 US Patent 5206051, 63, 82 US Patent 5500282, 40 US Patent 5534215, 50 US Patent 5580652, 50 US Patent 6165599, 41 US Patent 6168826, 42 US Patent 6190760, 82 US Patent 6685871, 53 US Patent 6828013, 43 US Patent 6844077, 63 US Patent 7128969, 54–5
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Index.indd 283
7/4/11 9:28:52 AM
284
Index
US Patent Application US Patent Application 82–3 US Patent Application US Patent Application US Patent Application US Patent Application US Patent Application
20050042397, 43 20070292682, 20080286547, 40 20090148715, 54 20090197022, 64 20100040904, 55 200900311544, 55
vacuum metallising, 80–1 vermin, 170 Vifan, 71
water vapour, 150 water vapour transmission rate (WVTR), 174, 253 wet process, 266 WO/2000/002956, 83 WO/2002/098655, 62 yield stress, 33 Ziegler–Natta polypropylene, 41
© Woodhead Publishing Limited, 2011
WH Biaxial-DeMeuse Index.indd 284
7/4/11 9:28:52 AM
