Identification of Textile Fibers (Woodhead Publishing Series in Textiles)

Identification of textile fibers

The Textile Institute and Woodhead Publishing The Textile Institute is a unique organisation in textiles, clothing and footwear. Incorporated in England by a Royal Charter granted in 1925, the Institute has individual and corporate members in over 90 countries. The aim of the Institute is to facilitate learning, recognise achievement, reward excellence and disseminate information within the global textiles, clothing and footwear industries. Historically, The Textile Institute has published books of interest to its members and the textile industry. To maintain this policy, the Institute has entered into partnership with Woodhead Publishing Limited to ensure that Institute members and the textile industry continue to have access to high calibre titles on textile science and technology. Most Woodhead titles on textiles are now published in collaboration with The Textile Institute. Through this arrangement, the Institute provides an Editorial Board which advises Woodhead on appropriate titles for future publication and suggests possible editors and authors for these books. Each book published under this arrangement carries the Institute’s logo. Woodhead books published in collaboration with The Textile Institute are offered to Textile Institute members at a substantial discount. These books, together with those published by The Textile Institute that are still in print, are offered on the Woodhead website at: www.woodheadpublishing.com. Textile Institute books still in print are also available directly from the Institute’s website at: www.textileinstitutebooks.com. A list of Woodhead books on textile science and technology, most of which have been published in collaboration with The Textile Institute, can be found on pages xv–xx.

Woodhead Publishing in Textiles: Number 84

Identification of textile fibers Edited by Max M. Houck

Cambridge

New Delhi

Published by Woodhead Publishing Limited in association with The Textile Institute Woodhead Publishing Limited, Abington Hall, Granta Park, Great Abington Cambridge CB21 6AH, England www.woodheadpublishing.com Woodhead Publishing India Pvt Ltd, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi-110002, India Published in North America by CRC Press LLC, 6000 Broken Sound Parkway, NW, Suite 300, Boca Raton, FL 33487, USA First published 2009, Woodhead Publishing Limited and CRC Press LLC © Woodhead Publishing Limited, 2009 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 Cataloging in Publication Data A catalog record for this book is available from the Library of Congress. Woodhead Publishing ISBN 978-1-84569-266-7 (book) Woodhead Publishing ISBN 978-1-84569-565-1 (e-book) CRC Press ISBN 978-1-4398-0114-7 CRC Press order number: N10014 The publishers’ 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 publishers ensure that the text paper and cover board used have met acceptable environmental accreditation standards. Project managed by Macfarlane Book Production Services, Dunstable, Bedfordshire, England ([email protected]) Typeset by SNP Best-set Typesetter Ltd., Hong Kong Printed by TJ International Limited, Padstow, Cornwall, England

Contents

Contributor contact details Woodhead Publishing in Textiles

xi xv

Part I: Textile fiber structure and characteristics

1

1

Introduction to textile fiber identification M M Houck, West Virginia University, USA References

3

Ways of identifying textile fibers and materials M M Houck, West Virginia University, USA Introduction Identification and comparison of fibers Classification of fibers Pyrolysis gas chromatography Analysis of fiber colors and dyes Future trends References

6

1.1 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 3

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9

Natural animal textile fibres: structure, characteristics and identification S R Tridico, Australian Federal Police, Australia Introduction Animal fibre growth, structure, composition and properties Types of natural animal fibres Natural animal fibre characteristics Identification of natural animal fibres Future trends Sources of further information and advice Acknowledgements References

5

6 8 9 19 22 22 23

27 27 28 35 38 44 61 62 65 67 v

vi

Contents

4

Synthetic textile fibers: structure, characteristics and identification K Kajiwara, Otsuma Women’s University, Japan and Y Ohta, Toyobo Co. Ltd, Japan Introduction Fundamental characteristics of fibrous materials Common synthetic fibers Crystal structure of synthetic fibers Identification of synthetic fibers References

4.1 4.2 4.3 4.4 4.5 4.6 5

5.1 5.2 5.3 5.4 5.5 5.6 6

6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8

High performance fibers: structure, characteristics and identification Y Ohta, Toyobo Co. Ltd, Japan and K Kajiwara, Otsuma Women’s University, Japan Introduction The primary structure and physical properties of HPFs Identification of high strength and high modulus fiber Alternative methods for analyzing higher-order structure Sources of further information and advice References The use of classification systems and production methods in identifying manufactured textile fibers K L Hatch, The University of Arizona, USA Introduction Polymer origins and fiber classification PLA/polylactide fiber Fiber subclasses Multicomponent fibers Future trends Sources of further information and advice References

68

68 72 84 86 87 87

88

88 88 95 107 109 109

111 111 112 115 119 123 126 127 128

Part II: Methods of fiber identification

131

7

133

7.1 7.2 7.3 7.4

Optical microscopy for textile fibre identification M Wilding, The University of Manchester, UK Introduction Practical and quality control considerations Initial identification based on physical appearance Identification based on properties

133 134 137 139

Contents 7.5 7.6 7.7 7.8

Examples of more advanced microscopic techniques Future trends Sources of further information and advice References

8

The use of spectroscopy for textile fiber identification M M Houck, West Virginia University, USA Introduction: spectroscopy of fibers Categorizing methods by nature of excitation Categorizing methods by measurement process Common methods of spectroscopy References

8.1 8.2 8.3 8.4 8.5 9

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 10

10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 11

Microspectrophotometry for textile fiber color measurement S Walbridge-Jones, Bureau of Alcohol, Tobacco, Firearms and Explosives, USA Introduction An understanding of spectroscopy Microspectrophotometer design Types of microspectroscopy Perception of color: human vs. machine Metamerism Applications of microspectroscopy in fiber analysis Limitations, strengths, and future trends References Alternative and specialised textile fibre identification tests P H Greaves, Microtex, UK Introduction Alternative methods of fibre identification Scanning electron microscopy Further techniques Benefits of scanning electron microscopy compared to a light microscope Quantitative aspects Sources of further information and advice References Analysis of dyes using chromatography S W Lewis, Curtin University of Technology, Australia

vii 147 150 152 156

158 158 159 159 160 163

165

165 166 167 169 172 175 175 178 180

181 181 181 187 195 197 199 200 201 203

viii

Contents

11.1 11.2 11.3 11.4 11.5 11.6 11.7

Introduction Dyes Forensic analysis of dyes Conclusions Sources of further information and advice Acknowledgments References

12

DNA analysis in the identification of animal fibers in textiles P F Hamlyn, BTTG Ltd, UK Introduction Extraction of DNA from animal fibers Development of methods for using DNA analysis to identify animal fibers Effect of fiber processing on DNA analysis and the use of DNA amplification technology Future trends Sources of further information and advice References

12.1 12.2 12.3 12.4 12.5 12.6 12.7

Part III: Applications 13

13.1 13.2 13.3 13.4 13.5 13.6 14 14.1 14.2 14.3 14.4 14.5 14.6 14.7

Identifying plant fibres in textiles: the case of cotton S Gordon, CSIRO Materials Science and Engineering, Australia Introduction Cotton fibre structure and composition Cotton fibre properties Future trends Sources of further information and advice References The forensic identification of textile fibers M M Houck, West Virginia University, USA A forensic mindset Microscopy of fibers Manufactured fiber production and spinning Polarized light microscopy Fluorescence microscopy Conclusions References and further reading

203 204 206 219 220 220 220

224 224 226 227 229 233 235 236 237

239

239 241 245 256 256 256 259 259 261 262 266 271 272 273

Contents 15

15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9 15.10 15.11 15.12 15.13 15.14 15.15 15.16 15.17 15.18 16 16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8

Identifying and analyzing textile damage in the textile industry W D Schindler, University of Applied Sciences Hof, Germany Introduction: importance of and reasons for textile damage analysis in the textile industry Main types, manifestations and causes of textile damage Methods of identifying and analyzing textile damage Damage analysis according to the type of fiber Damage analysis of cellulosics, especially cotton Damage analysis of wool Damage analysis of silk General types of damage to synthetics Analysis of damage to polyester fibers Analysis of damage to nylon fibers Analysis of damage to acrylic fibers Analysis of damage to elastane (spandex) fibers Analysis of damage to polyolefin fibers, especially polypropylene Special types of textile damage and their analysis Sources of further information and advice Conclusions Acknowledgment References

ix

275

275 278 280 292 292 295 303 304 306 309 311 312 317 320 325 326 327 327

The role of fibre identification in textile conservation P Garside, University of Southampton, UK Introduction Analytical techniques Conservation strategies Case studies Future trends Sources of further information and advice Acknowledgements References

335

Index

366

335 337 349 351 356 357 357 358

This book is dedicated to my father, Max W. Houck (1917–2008). ‘Dad, I’m sorry you didn’t get a chance to read this one.’

Contributor contact details

(* = main contact)

Chapters 1, 2, 8 and 14

Chapter 4

M. M. Houck Forensic Science Initiative Forensic Business Research and Development West Virginia University Morgantown West Virginia USA E-mail: [email protected]

Professor K. Kajiwara* Faculty of Home Economics Otsuma Women’s University 12 Sanban-cho Chiyoda-ku Tokyo 102-8357 Japan E-mail: [email protected]

Chapter 3 Silvana R. Tridico Biological Criminalistics Australian Federal Police PO Box 401 Canberra ACT Australia 2601 E-mail: [email protected]

Dr Y. Ohta Toyobo Co. Ltd Toyobo Research Center 1-1 Katata 2-Chome Otsu Shiga 520-0292 Japan E-mail: [email protected]

xi

xii

Contributor contact details

Chapter 5

Chapter 9

Dr Y. Ohta* Toyobo Co. Ltd Toyobo Research Center 1-1 Katata 2-Chome Otsu Shiga 520-0292 Japan E-mail: [email protected]

S. Walbridge-Jones Forensic Chemist-Trace Evidence Bureau of Alcohol, Tobacco, Firearms and Explosives Forensic Science Laboratory 355 N. Wiget Lane Walnut Creek, CA 94598 USA E-mail: [email protected]

Professor K. Kajiwara Faculty of Home Economics Otsuma Women’s University 12 Sanban-cho Chiyoda-ku Tokyo 102-8357 Japan E-mail: [email protected]

Chapter 6 Professor Kathryn L. Hatch Department of Agricultural and Biosystems Engineering The University of Arizona Shantz Building Room 403 1177 E. 4th Street Tucson, AZ 85721-0038 USA E-mail: [email protected]

Chapter 7 Dr M. Wilding School of Materials The University of Manchester Sackville Street Building PO Box 88 Manchester M60 1QD UK E-mail: mike.wilding@manchester. ac.uk

Chapter 10 Dr P. H. Greaves Microtex 7 Newall Carr Road Otley LS21 2AU UK E-mail: [email protected]

Chapter 11 S. W. Lewis Curtin University of Technology GPO Box U1987 Perth Western Australia 6845 Australia E-mail: [email protected]

Chapter 12 Dr P. Hamlyn BTTG Ltd Unit 14 Wheel Forge Way Trafford Park Manchester M17 1EH UK E-mail: [email protected]

Contributor contact details

Chapter 13

Chapter 16

S. Gordon Cotton Research Unit CSIRO Materials Science and Engineering Henry Street Belmont Victoria Australia 3216 E-mail: [email protected]

P. Garside Textile Conservation Centre University of Southampton Winchester Campus Park Avenue Winchester SO23 8DL UK E-mail: [email protected]

Chapter 15 Professor em. Wolfgang D. Schindler Fichtelgebirgsstraße 17 D-95126 Schwarzenbach Germany E-mail: [email protected]

xiii

Woodhead Publishing in Textiles

1

Watson’s textile design and colour Seventh edition Edited by Z. Grosicki

2

Watson’s advanced textile design Edited by Z. Grosicki

3

Weaving Second edition P. R. Lord and M. H. Mohamed

4

Handbook of textile fibres Vol 1: Natural fibres J. Gordon Cook

5

Handbook of textile fibres Vol 2: Man-made fibres J. Gordon Cook

6

Recycling textile and plastic waste Edited by A. R. Horrocks

7

New fibers Second edition T. Hongu and G. O. Phillips

8

Atlas of fibre fracture and damage to textiles Second edition J. W. S. Hearle, B. Lomas and W. D. Cooke

9

Ecotextile ’98 Edited by A. R. Horrocks

10

Physical testing of textiles B. P. Saville

11

Geometric symmetry in patterns and tilings C. E. Horne

12

Handbook of technical textiles Edited by A. R. Horrocks and S. C. Anand

13

Textiles in automotive engineering W. Fung and J. M. Hardcastle xv

xvi

Woodhead Publishing in Textiles

14

Handbook of textile design J. Wilson

15

High-performance fibres Edited by J. W. S. Hearle

16

Knitting technology Third edition D. J. Spencer

17

Medical textiles Edited by S. C. Anand

18

Regenerated cellulose fibres Edited by C. Woodings

19

Silk, mohair, cashmere and other luxury fibres Edited by R. R. Franck

20

Smart fibres, fabrics and clothing Edited by X. M. Tao

21

Yarn texturing technology J. W. S. Hearle, L. Hollick and D. K. Wilson

22

Encyclopedia of textile finishing H-K. Rouette

23

Coated and laminated textiles W. Fung

24

Fancy yarns R. H. Gong and R. M. Wright

25

Wool: Science and technology Edited by W. S. Simpson and G. Crawshaw

26

Dictionary of textile finishing H-K. Rouette

27

Environmental impact of textiles K. Slater

28

Handbook of yarn production P. R. Lord

29

Textile processing with enzymes Edited by A. Cavaco-Paulo and G. Gübitz

30

The China and Hong Kong denim industry Y. Li, L. Yao and K. W. Yeung

Woodhead Publishing in Textiles 31

The World Trade Organization and international denim trading Y. Li, Y. Shen, L. Yao and E. Newton

32

Chemical finishing of textiles W. D. Schindler and P. J. Hauser

33

Clothing appearance and fit J. Fan, W. Yu and L. Hunter

34

Handbook of fibre rope technology H. A. McKenna, J. W. S. Hearle and N. O’Hear

35

Structure and mechanics of woven fabrics J. Hu

36

Synthetic fibres: nylon, polyester, acrylic, polyolefin Edited by J. E. McIntyre

37

Woollen and worsted woven fabric design E. G. Gilligan

38

Analytical electrochemistry in textiles P. Westbroek, G. Priniotakis and P. Kiekens

39

Bast and other plant fibres R. R. Franck

40

Chemical testing of textiles Edited by Q. Fan

41

Design and manufacture of textile composites Edited by A. C. Long

42

Effect of mechanical and physical properties on fabric hand Edited by Hassan M. Behery

43

New millennium fibers T. Hongu, M. Takigami and G. O. Phillips

44

Textiles for protection Edited by R. A. Scott

45

Textiles in sport Edited by R. Shishoo

46

Wearable electronics and photonics Edited by X. M. Tao

47

Biodegradable and sustainable fibres Edited by R. S. Blackburn

xvii

xviii

Woodhead Publishing in Textiles

48

Medical textiles and biomaterials for healthcare Edited by S. C. Anand, M. Miraftab, S. Rajendran and J. F. Kennedy

49

Total colour management in textiles Edited by J. Xin

50

Recycling in textiles Edited by Y. Wang

51

Clothing biosensory engineering Y. Li and A. S. W. Wong

52

Biomechanical engineering of textiles and clothing Edited by Y. Li and D. X-Q. Dai

53

Digital printing of textiles Edited by H. Ujiie

54

Intelligent textiles and clothing Edited by H. Mattila

55

Innovation and technology of women’s intimate apparel W. Yu, J. Fan, S. C. Harlock and S. P. Ng

56

Thermal and moisture transport in fibrous materials Edited by N. Pan and P. Gibson

57

Geosynthetics in civil engineering Edited by R. W. Sarsby

58

Handbook of nonwovens Edited by S. Russell

59

Cotton: Science and technology Edited by S. Gordon and Y-L. Hsieh

60

Ecotextiles Edited by M. Miraftab and A. Horrocks

61

Composite forming technologies Edited by A. C. Long

62

Plasma technology for textiles Edited by R. Shishoo

63

Smart textiles for medicine and healthcare Edited by L. Van Langenhove

64

Sizing in clothing Edited by S. Ashdown

Woodhead Publishing in Textiles 65

Shape memory polymers and textiles J. Hu

66

Environmental aspects of textile dyeing Edited by R. Christie

67

Nanofibers and nanotechnology in textiles Edited by P. Brown and K. Stevens

68

Physical properties of textile fibres Fourth edition W. E. Morton and J. W. S. Hearle

69

Advances in apparel production Edited by C. Fairhurst

70

Advances in fire retardant materials Edited by A. R. Horrocks and D. Price

71

Polyesters and polyamides Edited by B. L. Deopora, R. Alagirusamy, M. Joshi and B. S. Gupta

72

Advances in wool technology Edited by N. A. G. Johnson and I. Russell

73

Military textiles Edited by E. Wilusz

74

3D fibrous assemblies: Properties, applications and modelling of three-dimensional textile structures J. Hu

75 Medical textiles 2007 Edited by J. Kennedy, A. Anand, M. Miraftab and S. Rajendran 76 Fabric testing Edited by J. Hu 77 Biologically inspired textiles Edited by A. Abbott and M. Ellison 78 Friction in textiles Edited by B. S. Gupta 79 Textile advances in the automotive industry Edited by R. Shishoo 80 Structure and mechanics of textile fibre assemblies Edited by P. Schwartz

xix

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81

Engineering textiles: Integrating the design and manufacture of textile products Edited by Y. E. El Mogahzy

82

Polyolefin fibres: industrial and medical applications Edited by S. C. O. Ugbolue

83

Smart clothes and wearable technology Edited by J. McCann and D. Bryson

84

Identification of textile fibres Edited by M. M. Houck

85

Advanced textiles for wound care Edited by S. Rajendran

86

Fatigue failure of textile fibres Edited by M. Miraftab

87

Advances in carpet technology Edited by K. Goswami

1 Introduction to textile fiber identification M M HOUCK, West Virginia University, USA

Abstract: The identification of fibers is critical to a number of industries, including textiles, forensic science, fashion, and design. The actual identification, however, varies with industry and method. Changes in textile technology create a constant need to improve identification methodology. The old methods – despite the increased pace of new technology – are often the best. Microscopy still dominates the field for analytical methods and provides a range of analysis barely possible with any other method. Combined with spectroscopy, microscopy is the quintessential fiber identification tool. Key words: fiber identification, microscopy, methodology, spectroscopy.

Manufacturers use set methods to ensure a quality product fit for purpose. This implies a market-based taxonomy with a company-product orientation, a supply chain of raw and processed materials, and explicit rules on categories. For example, the American Association of Textile Chemists and Colorists (AATCC) lists the following specified methods: • colorfastness to commercial laundering and to domestic washing • flammability of clothing textiles • smoothness of seams in fabrics after repeated home laundering • electrostatic propensity of carpets • wrinkle recovery of fabrics: appearance method • dimensional changes in textiles other than wool. The titles to these methods indicate what is important to define in their products. AATCC lists microscopy as a method to identify fibers but notes that it should be used with caution since manufactured fibers are frequently produced in a number of modifications which alter their appearance.1 The Association also lists ‘reaction to flame’ (Table III) as a test method with the following diagnostics: melts near flame, shrinks from flame, burns in flame, etc. By contrast, ASTM International lists the following in their methods: • • •

flame resistant materials used in camping tentage pile retention of corduroy fabrics elastic properties of textile fibers 3

4 • •

Identification of textile fibers performance specifications for underwear fabrics, woven, men’s and boys’ commercial moisture regains for textile fibers.

While the view is similar to that of AATCC, ASTM International lists infrared spectroscopy as the preferred method for fiber identification: ‘additional physical properties of the fibers such as density, melting point, regain, refractive indices, and birefringence . . . are useful for confirming the identification’ (Volume 7.01, D276). Both organizations treat the samples as bulk for purposes of the possible identifying tests; after all, if you make them, sample size should not be a problem. By comparison, the forensic sciences treat microscopy as the primary method for fiber identification: Microscopic examination provides the quickest, most accurate, and least destructive means of determining the microscopic characteristics and polymer type of textile fibers. Additionally, a point-by-point, side-by-side microscopic comparison provides the most discriminating method of determining if two or more fibers are consistent with originating from the same source.2

Forensic fiber examiners use microscopy first and then other methods, such as infrared spectroscopy, as confirmatory techniques. Why not start with infrared spectroscopy, as do the manufacturers? A company that spins fibers knows what products it makes; therefore, the universe of possible answers is sufficiently limited. For a forensic scientist, however, fibers found at a crime scene could conceivably be from nearly any source and you can’t make assumptions. A rayon fiber and a cotton fiber will both show up as ‘cellulose’ on an infrared spectrum; a microscope easily distinguishes between them. Two nylon 6,6 fibers may be chemically identical but have different diameters, cross-sections, or birefringences. The orientation of the analysis is different (comparison), although the goal is partially the same (identification). Forensic fiber analysis routinely has minimal samples with which to work and this structures what tests can be used. Forensic fiber analysis is also interested in traits of which the manufacturers may not be aware. For example, delustrants, typically titanium dioxide, are added to deluster or dull otherwise bright fibers. The manufacturers add the delustrants during the fiber spinning process at a certain rate to achieve the desired end goals with little regard for the distribution of the granules within the fiber. For a forensic scientist, however, that distribution can be a significant comparator between two otherwise similar samples: one fiber with large, aggregated granules is dissimilar to one with small, evenly-distributed granules, all other factors being the same. The difference could be batch-to-batch variation, plant-to-plant variation, or some other node along the supply chain. Suffice it to say, the fiber manufacturer did not intentionally distribute the delustrant in such a way as to aid the forensic scientist. Forensic scientists, therefore, add additional infor-

Introduction to textile fiber identification

5

Production analytical methods Market taxonomy company-product orientation supply web explicit rules on categories Forensic analytical methods After-market taxonomy end use as used implicit rules on categories

Sharing methodologies Different approaches due to different goals – quality at lowest price for manufacturing, reconstruction and product tracking for forensics

Manufacturing

Forensics

1.1 Analytical methods used by manufacturers and forensic scientists.

mation to the market-based taxonomy with their own forensic taxonomy of products. As the physicist P.W. Bridgman succinctly said, ‘The concept is synonymous with the corresponding set of operations’,3 meaning, the methods you use frame the orientation of your analysis from the start. The analytical schemes used by both manufacturers and forensic scientists have value, although they may have different goals (Fig. 1.1). Some methods are used exclusively by one group, others are shared, while some shared methods have greater or lesser utility for the analyst. This book strives to provide a broader perspective about the methods available for fiber identification to create a fuller toolbox for the fiber analyst, regardless of their scientific orientation.

1.1

References

1. American Association of Textile Chemists and Colorists, AATCC Technical Manual, Research Triangle Park, NC: AATCC, 2008. 2. Scientific Working Group for Materials Analysis, Forensic Fiber Examination Guidelines, 1999, Forensic Science Communications 1(1), online at www.fbi.gov. 3. Bridgman PW. The Logic of Modern Physics. New York, NY: McMillan Publishers, 1928; page 23.

2 Ways of identifying textile fibers and materials M M HOUCK, West Virginia University, USA

Abstract: The identification of fibers is critical to a number of industries, including textiles, forensic science, fashion, and design. The actual identification, however, varies with industry and method. Changes in textile technology create a constant need to improve identification methodology. The old methods – despite the increased pace of new technology – are often the best. Microscopy still dominates the field for analytical methods and provides a range of analysis barely possible with any other method. Combined with spectroscopy, microscopy is the quintessential fiber identification tool. Key words: fiber identification, microscopy, methodology, spectroscopy.

2.1

Introduction

The identification of fibers is an important component to the textile industry, forensic science, fashion designers, and the automotive industry, among others. The process, however common across disparate industries, is conducted very differently in each. A quick look at the methods published by various disciplines demonstrates what characteristics and properties are important (Table 2.1). The American Association of Textile Chemists and Colorists (AATCC) lists microscopy as a method to identify fibers but notes that it must be used with caution on manufactured fibers since they are produced in a variety of modifications which alter the appearance.1 AATCC also lists ‘reaction to flame’ as a test method with the following categories for test reactions: ‘melts near flame’, ‘shrinks from flame’, and ‘burns in flame’. ASTM International, in their volumes on textiles, lists infrared spectroscopy as the preferred method for fiber identification and adds, ‘additional physical properties of the fibers such as density, melting point, regain, refractive indices, and birefringence . . . are useful for confirming the identification’.1 This stands in stark contrast to the methods listed for forensic casework which maximize analysis on minimum sample sizes; most methods center on microscopy which the other groups find ‘useful’. Forensic scientists typically get very little sample with which to work, often one or 6

Ways of identifying textile fibers and materials

7

Table 2.1 Various published methods for textile testing and fiber identification Methods from the American Association of Textile Chemists and Colorists: • Colorfastness to commercial laundering and to domestic washing • Flammability of clothing textiles • Smoothness of seams in fabrics after repeated home laundering • Electrostatic propensity of carpets • Wrinkle recovery of fabrics: appearance method • Dimensional changes in textiles other than wool Methods from ASTM International: • Flame resistant materials used in camping tentage • Pile retention of corduroy fabrics • Elastic properties of textile fibers • Performance specification for underwear fabrics, woven, men’s and boys’ • Commercial moisture regains for textile fibers Methods from Scientific Working Group on Materials Analysis (forensic): • Microscopy of textile fibers, including polarized light and fluorescence microscopy • Thin-layer chromatography of non-reactive dyes in textile fibers • Pyrolysis-gas chromatography of textile fibers • Infrared analysis of textile fibers

two individual fibers, while fiber producers have by comparison nearly unlimited samples. Analytical differences at the application level mask a deeper truth about the textiles being tested. Materials used in commercial manufacturing have to be fit for purpose or else they will not be economically viable as end-use products.2,3 This extends even to natural products, such as wool or silk, which can be thought of as ‘designed’ through domestic breeding programs or feedback from customers which leads to intentional selection of raw materials. If the raw goods are selected for their product-specific desirable characteristics, then how much more so the methods employed to analyze them? To the users of AATCC’s methods, smoothness of seams after laundering is an important trait; customers do not like sewn textiles that pucker and ruin the lines of a garment and will not buy them. In determining if a homicide suspect left fibers from his sweater on the victim’s body, however, smooth seams are not a priority. It is more important to determine the individual fiber’s morphological, optical, and chemical properties as minutely as possible to be able to make as close an association as possible with the textile in question. The two scientists have different goals, so they use different methods and thereby reveal separate analytical realities of the materials.

8

Identification of textile fibers Testing regime 1

Testing regime 2

2.1 Various analytical methods (rectangles) make up analytical regimes (shades) which in turn create product taxonomies. These taxonomies differ depending upon which traits are tested for and with which methods. Often taxonomies do not intersect, like testing for consumer goods versus testing for forensic casework; some methods do overlap between disciplines (dark gray). The methods employed for characterizing a fiber as ‘low moisture regain’ will differ from those that identify it as ‘negative sign of elongation’.

A taxonomy of goods is constructed from such analytical realities (Fig. 2.1). One testing regime will place a particular product in one set or sets while another might reclassify it elsewhere, although not necessarily radically. This is explained in Fig. 2.1.

2.2

Identification and comparison of fibers

The process of fiber analysis can be thought of in two phases – identification and comparison. Although the methods used in these processes may be similar, the goals of each are quite different. Identification is a process of classification4 or placing the fiber into a group or set with shared characteristics. This involves observing the physical and chemical properties of the fiber that help put it into sets with successively smaller memberships. These properties can be observed by a combination of microscopy and chemical analysis. Identification tests are performed prior to comparisons and every effort should be made to conserve fibers for later comparison if the quantity is limited.5,6

Ways of identifying textile fibers and materials

9

Comparison of fibers involves observing any correlation between fibers from a questioned source (from a crime scene, a defective sample, or counterfeit seizure, for example) and fibers from a known source (such as an item of clothing from the suspect, a quality assurance sample, or a known manufacturing source). Although the goal of this analysis is to determine if the two fibers could have come from the same source, each comparison test is performed with an eye towards looking for significant differences between the known and questioned fibers. Recognition of counterfeit fibers or textiles, for example, involves a set of known parameters against which the suspect fibers are tested. If the suspect fibers do not compare favorably with the known parameters, then they are more likely to be fakes. It is only when all of the testing is complete and no significant differences are found that is it possible to conclude that the known and suspected fibers exhibit the same microscopic, optical, and chemical properties and therefore could have come from the same source – that is, they are genuine.

2.3

Classification of fibers

A textile fiber is a unit of matter, either natural or manufactured, that forms the basic element of fabrics and other textile structures. Specifically, a textile fiber is characterized as having a length at least 100 times its diameter and a form that allows it to be spun into a yarn or made into a fabric by various methods. Fibers differ from each other in chemical structure, cross-sectional shape, surface contour, color, as well as length and width.7–9 The diameter of textile fibers is small, generally 0.0004 to 0.002 inch (in.), or 11–51 micrometers (μm). Their length varies from about 7/8 in. or 2.2 centimeters (cm) to many miles. Based on length, fibers are classified as either filament or staple fiber. Filaments are a type of fiber having indefinite or extreme length, such as synthetic fibers which can be made to any length; silk is the only naturally occurring filament. Staple fibers are natural fibers or cut lengths of filament, typically being 1.5 to 8 in (3.75 to 20 cm) in length. The size of natural fibers is usually given as a diameter measurement in micrometers. The size of silk and manufactured fibers is usually given in denier (in the US) or tex (in other countries). Denier and tex are linear measurements based on weight by unit length. The denier is the weight in grams of 9000 meters of the material fibrous. Denier is a direct numbering system in which the lower numbers represent the finer sizes and the higher numbers the larger sizes. Glass fibers are the only manufactured fibers that are not measured by denier. A 1-denier nylon is not equal in size to a 1denier rayon, however, because the fibers differ in density. Tex is equal to the weight in grams of 1000 meters (one kilometer) of the fibrous material.7 Fibers themselves are classified into two major classes: natural and manufactured. A natural fiber is any fiber that exists as such in the natural state,

10

Identification of textile fibers

such as cotton, wool, or silk. Manufactured fibers are made by processing natural or synthetic organic polymers into a fiber-forming substance; they can be classified as cellulosic or synthetic. Cellulosic fibers are either made from regenerated or derivative cellulosic (fibrous) polymers, such as wood or cotton. Synthetic fibers are formed from substances that, at any point in the manufacturing process, are not a fiber; examples are nylon, polyester and saran. No nylon or polyester fibers exist in nature and they are made of chemicals put through reactions to produce the fiber-forming substance.10 The generic names for manufactured and synthetic fibers were established as part of the Textile Fiber Products Identification Act enacted by the US Congress in 1954.

2.3.1 The Textile Fiber Products Identification Act Pursuant to the provisions of section 7(c) of the Textile Fiber Products Identification Act [16 CFR Part 303], the US Federal Trade Commission thereby established the generic names for manufactured fibers, together with their respective definitions, as set forth in this section, and the generic names for manufactured fibers, together with their respective definitions, set forth in International Organization for Standardization (ISO) Standard 2076: 1999(E), ‘Textiles – Man-made fibres – Generic names’. (a)

(b)

(c)

(d)

Acrylic. A manufactured fiber in which the fiber-forming substance is any long chain synthetic polymer composed of at least 85% by weight of acrylonitrile units. Modacrylic. A manufactured fiber in which the fiber-forming substance is any long chain synthetic polymer composed of less than 85% but at least 35% by weight of acrylonitrile units except fibers qualifying under paragraph (j)(2) of this section and fibers qualifying under paragraph (q) of this section. Polyester. A manufactured fiber in which the fiber-forming substance is any long chain synthetic polymer composed of at least 85% by weight of an ester of a substituted aromatic carboxylic acid, including but not restricted to substituted terephthalate units, and para substituted hydroxy-benzoate units. Where the fiber is formed by the interaction of two or more chemically distinct polymers (of which none exceeds 85% by weight), and contains ester groups as the dominant functional unit (at least 85% by weight of the total polymer content of the fiber), and which, if stretched at least 100%, durably and rapidly reverts substantially to its unstretched length when the tension is removed, the term elasterell-p may be used as a generic description of the fiber. Rayon. A manufactured fiber composed of regenerated cellulose, as well as manufactured fibers composed of regenerated cellulose in

Ways of identifying textile fibers and materials

(e)

(f)

(g) (h)

(i)

(j)

(k)

(l)

11

which substituents have replaced not more than 15% of the hydrogens of the hydroxyl groups. Where the fiber is composed of cellulose precipitated from an organic solution in which no substitution of the hydroxyl groups takes place and no chemical intermediates are formed, the term lyocell may be used as a generic description of the fiber. Acetate. A manufactured fiber in which the fiber-forming substance is cellulose acetate. Where not less than 92% of the hydroxyl groups are acetylated, the term triacetate may be used as a generic description of the fiber. Saran. A manufactured fiber in which the fiber-forming substance is any long chain synthetic polymer composed of at least 80% by weight of vinylidene chloride units (—CH2—CCl2—). Azlon. A manufactured fiber in which the fiber-forming substance is composed of any regenerated naturally occurring proteins. Nytril. A manufactured fiber containing at least 85% of a long chain polymer of vinylidene dinitrile (—CH2—C(CN)2—) where the vinylidene dinitrile content is no less than every other unit in the polymer chain. Nylon. A manufactured fiber in which the fiber-forming substance is a long-chain synthetic polyamide in which less than 85% of the amide linkages are attached directly to two aromatic rings. Rubber. A manufactured fiber in which the fiber-forming substance comprises natural or synthetic rubber, including the following categories: (1) A manufactured fiber in which the fiber-forming substance is a hydrocarbon such as natural rubber, polyisoprene, polybutadiene, copolymers of dienes and hydrocarbons, or amorphous (noncrystalline) polyolefins. (2) A manufactured fiber in which the fiber-forming substance is a copolymer of acrylonitrile and a diene (such as butadiene) composed of not more than 50% but at least 10% by weight of acrylonitrile units. The term lastrile may be used as a generic description for fibers falling within this category. (3) A manufactured fiber in which the fiber-forming substance is a polychloroprene or a copolymer of chloroprene in which at least 35% by weight of the fiber-forming substance is composed of chloroprene units. Spandex. A manufactured fiber in which the fiber-forming substance is a long chain synthetic polymer comprised of at least 85% of a segmented polyurethane. Vinal. A manufactured fiber in which the fiber-forming substance is any long chain synthetic polymer composed of at least 50% by weight

12

Identification of textile fibers

of vinyl alcohol units (—CH2—CHOH—), and in which the total of the vinyl alcohol units and any one or more of the various acetal units is at least 85% by weight of the fiber. (m) Olefin. A manufactured fiber in which the fiber-forming substance is any long chain synthetic polymer composed of at least 85% by weight of ethylene, propylene, or other olefin units, except amorphous (noncrystalline) polyolefins qualifying under paragraph (j)(1) of this section. Where the fiber-forming substance is a cross-linked synthetic polymer, with low but significant crystallinity, composed of at least 95% by weight of ethylene and at least one other olefin unit, and the fiber is substantially elastic and heat resistant, the term lastol may be used as a generic description of the fiber. (n) Vinyon. A manufactured fiber in which the fiber-forming substance is any long chain synthetic polymer composed of at least 85% by weight of vinyl chloride units (—CH2—CHCl—). (o) Metallic. A manufactured fiber composed of metal, plastic-coated metal, metal-coated plastic, or a core completely covered by metal. (p) Glass. A manufactured fiber in which the fiber-forming substance is glass. (q) Anidex. A manufactured fiber in which the fiber-forming substance is any long-chain synthetic polymer composed of at least 50% by weight of one or more esters of a monohydric alcohol and acrylic acid, CH2=CH—COOH. (r) Novoloid. A manufactured fiber containing at least 85% by weight of a cross-linked novolac. (s) Aramid. A manufactured fiber in which the fiber-forming substance is a long-chain synthetic polyamide in which at least 85% of the amide linkages are attached directly to two aromatic rings. (t) Sulfar. A manufactured fiber in which the fiber-forming substance is a long-chain synthetic polysulfide in which at least 85% of the sulfide (—S—) linkages are attached directly to two aromatic rings. (u) PBI. A manufactured fiber in which the fiber-forming substance is a long chain aromatic polymer having reoccurring imidazole groups as an integral part of the polymer chain. (v) Elastoester. A manufactured fiber in which the fiber-forming substance is a long-chain synthetic polymer composed of at least 50% by weight of aliphatic polyether and at least 35% by weight of polyester, as defined in paragraph (c) of this section. (w) Melamine. A manufactured fiber in which the fiber-forming substance is a synthetic polymer composed of at least 50% by weight of a crosslinked melamine polymer. (x) Fluoropolymer. A manufactured fiber containing at least 95% of a longchain polymer synthesized from aliphatic fluorocarbon monomers.

Ways of identifying textile fibers and materials (y)

13

PLA. A manufactured fiber in which the fiber-forming substance is composed of at least 85% by weight of lactic acid ester units derived from naturally occurring sugars.

2.3.2 Natural textile fibers A number of important vegetable fibers, such as cotton, jute and sisal, and animal fibers, such as wool, camel and silk, appear as evidence. Vegetable fibers are characterized primarily by microscopy11–13 as are animal fibers, which, except for silk, are hairs.13–15 Vegetable fibers The three major sources for fibers derived from plants are the seed, the stem and the leaf, depending upon which source works best for a particular plant. Plant fibers are found in two principal forms: the technical fiber, used in cordage, sacks, mats, etc., or individual cells, as in fabrics or paper. The examination of technical fibers should include a search for internal structures, such as spiral elements or crystals, and the preparation of a cross-section; additionally, a chemical test for lignin may be done. Technical fibers should be macerated, the fabrics teased apart, and paper re-pulped for the examination of individual cells. The relative thickness of the cell walls and the size, shape and thickness of the lumen, cell length, and the presence, type, and distribution of dislocations should be noted. The direction of twist of the cellulose in the cell wall can also be determined by the dry twist test.6 Other characteristic cells should be noted and compared to reference specimens. The most common plant fibers encountered are cotton, flax, jute, hemp, and sisal.7,8 Bast fibers are those derived from the stems of the plant, leaf fibers from the leaves and seed fibers from in and around the reproductive pod. The basic plant materials are extensively processed before being incorporated into the final product.7,8,11,16 For additional information about plant fiber identification, the interested reader should consult references in Gaudette’s chapter6 and Mauersberger.16 Bast fibers Flax (linen) is derived from the stem of Linum usitatissimum. It has a clockwise twist and may range in diameter from 40 to 80 μm. The ultimates are polygonal in cross-section, with thick walls and small lumina. Microscopically, the fibers have dark dislocations that are roughly perpendicular to the long axis of the fiber. Flax may be ‘cottonized’, a process similar to mercerization, and may present a cottony appearance. Flax is most often found in clothing and household textiles.

14

Identification of textile fibers

Jute (Corchorus capsularis) appears bundled microscopically and may have a yellowish cast. The ultimates are polygonal but angular with mediumsized lumina. It can be distinguished easily from flax by its counterclockwise twist. The dislocations appear as angular X’s or Y’s and may be numerous. Jute is used in cordage, rugs, and hardware cloth among other products. Ramie (Boehmeria nivea) has very long and very wide ultimates, with the width ranging from 25 up to 75 μm. The walls are thick and appear flattened in cross-section. Ramie has frequent, short dislocations and longer transverse striations. In cross-section, radial cracks may be present. Ramie may be used in ropes, sacking as well as some clothing items. With more bundled ultimates, a wider lumen and fewer nodes, hemp (Cannabis sativa) is easy to distinguish from flax. Hemp also has a counterclockwise twist. Cross-sectioning hemp helps in distinguishing it from jute in that hemp’s lumina are rounder and more flattened than jute’s. Hemp may also have a brownish cast to it. The products hemp is most often found in are ropes, bags, and, occasionally, clothing. Leaf fibers Sisal (Agave sisilana) is relatively easy to identify due to its irregular lumen size, acicular crystals, spiral elements and annular vessels. Sisal has a counterclockwise twist. In cross-section, sisal looks somewhat like cut celery. Sisal is used in carpets, ropes, twine and floor mats. Although potentially difficult to distinguish from sisal on a slide mount, abaca (Musa textilis) has many characteristics that help to identify it. Its ultimates have a uniform diameter and a waxy appearance; often it is darker than sisal; also they are polygonal in cross-section and vary in size. Abaca may present spiral elements but often will have stegmata which are visible as small crown-like structures. Abaca, like sisal, has a counter-clockwise twist. Ropes, cordage and floor mats are typical sources of abaca. Seed fibers Cotton is by far the single most common textile fiber and accounts for approximately half of all textile fibers processed each year. Mature cotton has a flat twisted ribbon-like appearance which is easy to identify. Cotton fibers are made up of several spiralling layers around a central lumen. About half of the world’s cotton is grown in the United States. The appearance of cotton can be modified by chemical finishing to produce a desired result. Mercerization, for example, is the process where cotton fibers are soaked in sodium hydroxide and this causes the fibers to untwist and swell.

Ways of identifying textile fibers and materials

15

Kapok fiber is used primarily for life preservers and upholstery padding. The fibers are hollow, producing very buoyant products, but are brittle, which precludes spinning or weaving. Various synthetics, such as polyester and polyurethanes, are the main competition for kapok. Coir (Coco nucifera) comes from the husk of the coconut and, accordingly, is a very dense, stiff fiber easily identified microscopically. On a slide mount, coir appears very dark brown or opaque with very large, coarse ultimates. Coir also has a distinctive cross-sectional shape. Coir is usually found in floor or door mats. Animal fibers Textile fibers derived from animal sources are typically the hairs of mammals, such as sheep’s wool. Animals produce three main types of hairs: vibrissae (whiskers), guard, and fur. Guard hairs are the relatively long, thick hairs which cover the main portion of an animal’s body. Guard hairs are the most useful in identifying and comparing animal hairs. Fur hairs, by contrast, are small, thin hairs that provide bulk and warmth; microscopically, they are not very distinctive and may appear similar between otherwise dissimilar animals. The microscopy of hairs has been covered elsewhere14,17–19 and the interested reader is directed to those references. The importance of silk, produced by the Bombyx silkworm, should not be underestimated, however. Recently, silk, especially spider silk, has been of intense research interest for its tensile strength and other phenomenal physical properties.20–22

2.3.3 Manufactured textile fibers: physical and optical analysis Microscopic analysis The microscope is the primary tool for fiber analysis5,6,23–28 and the applications range from simple stereomicroscopy through higher power optical and polarizing microscopy to scanning electron microscopy. Instrumentation routinely has integrated microscopes to analyze small samples. Despite the power of modern computerized instrumentation, microscopy should always come first.26 Low- and high-power optical microscopy is the most commonly employed of all of the microscopic techniques. It is also the most discriminating method because most textile fibers can be excluded from a known sample by size, shape, color, or some other easily observable microscopic characteristic. Stereomicroscopes, polarized light microscopes, comparison microscopes, and fluorescent light microscopes are all used in the identification of fibers as well as in comparison of known and questioned fibers.

16

Identification of textile fibers 1.7 polyester 0.153 1.65

nparallel

1.6 polyethylene 0.042

1.55 polypropylene 0.03

nylon 0.048 rayon 0.03 modacrylic 0.015 acrylic 0.003

1.5 acetate 0.003 1.45 1.45

1.47

1.49

1.53 1.51 nperpendicular

1.55

1.57

1.59

2.2 Average refractive indices and birefringence (next to fiber type) for manufactured fibers. Birefringence is defined as nparallel minus nperpendicular.

A polarized light microscope is a central tool for the identification and analysis of manufactured fibers. Many characteristics of manufactured fibers can be viewed in non-polarized light, however, and these provide a fast, direct and accurate method for the discrimination of similar fibers. Given proper training and experience, a fiber examiner can identify a fiber’s generic class simply by its microscopic characteristics and optical properties.29,30 Refractive index and birefringence are the two most distinguishing features for the identification of a fiber’s generic class5,30–34 (Fig. 2.2). A comparison light microscope is required to confirm whether the known and the questioned fibers truly present the same microscopic characteristics.5 The cross-section is the shape of an individual fiber when cut at a right angle to its long axis. The shapes of manufactured fibers vary with the desired end result, such as the fiber’s soil hiding ability or a silky or coarse feel to the final fabric.7 Some fiber types tend to stay within certain crosssectional families, for example acrylics tend to appear as bean-shaped fibers and rayon tends to be irregular. The particular cross-section also may be indicative of a fiber’s intended end-use: many carpet fibers have a lobed shape to help hide dirt and create a specific visual texture to the carpet. A physical cross-section should be prepared and numerous approaches have been published for cross-sectioning. The method outlined by Palenik and Fitzsimmons35,36 is simple, inexpensive and conservative of sample. Additional methods, including fiber microtomes, fibers suspended in epoxyfilled pipette tips, and Teflon-coated slides, have been published.5,37,38

Ways of identifying textile fibers and materials

17

r

R

2.3 Modification ratio is defined as R/r.

The modification ratio of a fiber is a geometrical measurement used in the characterization of trilobal fiber cross-sections. The modification ratio is the difference in size between the outside diameter of the fiber and the diameter of the core (Fig. 2.3). Many manufacturers use modification ratios in the descriptions of their fibers for patent purposes.6,39,40 The way a fiber’s diameter is measured is dependent upon its crosssectional shape; there is more than one way to measure the diameter of a non-round fiber. Manufactured fibers can be made in diameters from about 6 μm (so-called microfibers40) up to monofilament fibers used in fishing line, which can be as large as 1 mm.7–9 By comparison, natural fibers vary in diameter from cultivated silk (10–13 μm) to US sheep’s wool (up to 40 μm or more) and human head hairs range from 50–100 μm. A manufactured fiber greater than 40 μm is probably a carpet-type fiber.41 Some manufactured fibers retain air-pockets or voids after production. For example, wet-spun fibers, such as acrylics, may have voids that range in size from submicron up to several microns. Voids are created when pores in the solidifying fiber are filled with a mixture of solvent and non-solvent fluids.9 The size, shape, distribution, and concentration of voids are related to the composition and production methods of the fiber and are an important comparative feature. Inclusions are materials or discontinuities that are placed or occur in fibers. These may be accidental inclusions, such as the draw marks sometimes seen in melt spun fibers or intentional inclusions, such as large clumps of delustrant or anti-static materials.7,10 Delustrants are finely ground particles of materials, such as titanium dioxide, that are put into the fiber as it is made. These particles diffract light passing through the fibers, reducing their luster. Fibers are classified in the textile industry as bright, semi-bright, or dull;10,16 forensic scientists classify fibers as slightly, moderately, or heavily delustered.6 The size, shape, distribution, and concentration of delustrant granules should be noted.

18

Identification of textile fibers

A fiber’s construction is an important indication of its production and end use. Examples are bicomponent fibers (two or more polymer types spun in a sheath/core or bilateral relation), biconstituent fibers (two different polymers spun together from a homogeneous solution), or microfibers. These specialty fibers are distinctive and particular attention should be paid to their construction and composition if they are recovered as evidence.42 A polarizing light microscope will be necessary for determining a fiber’s optical properties. These properties include refractive indices, retardation, birefringence, sign of elongation, and dichroism. There are a number of excellent works that describe the use of polarized light microscopy for the analysis of fibers.26,43 In his chapter on fiber analysis in Maehly and Williams’ Forensic Science Progress, Grieve44 included a thorough discussion of microscopic methods. Gaudette also presented microscopic techniques is his chapter on fiber analysis in Saferstein’s Forensic Science Handbook.6 Schemes of analysis for textile fibers by polarized light are used in training and have been published.28 Another important microscopic technique is thermal microscopy, which can be accomplished conveniently using a commercial hot stage. In this apparatus, a computer-controlled hot stage is mounted on the stage of the microscope. A very small piece of the fiber is then placed on a special slide and inserted into the hot stage. The melting point range can be observed and recorded. This technique can be used to distinguish between certain subclasses of synthetic fibers that differ only in polymer structure,45 such as some nylons and polyesters.46 Table 2.2 contains the softening and melting points for some common fibers. Thermal microscopy can also be used to accurately determine the refractive indices of fragments of fiber. The fiber is mounted in a liquid with a refractive index slightly above that of the fiber and then put in the

Table 2.2 Softening and melting points for some synthetic fibers5–7,9,10,11,16 Softening

Acetate Triacetate Nylon Polyester Olefin Acrylic Modacrylic Saran

Melting

°F

°C

°F

°C

364 482 340 445–490 260–320 473–490 300 285

184 250 171 229–254 127–160 245–254 149 141

500 550 415–509 450–500 275–338 – 370 334

260 288 213–265 250–260 135–170 – 188 168

Ways of identifying textile fibers and materials

19

hot stage. As the temperature of the hot stage is increased, the refractive indices of the liquid and the fiber decrease, but the decrease in the refractive index of the fiber is negligible compared to that of the liquid. When the Becke line disappears, the computer records the temperature. Some programs are capable of storing the standard refractive index and coefficient of temperature, and will thus keep a running track of the refractive index of the liquid as the temperature is raised. Dispersion curves can also be generated this way, if suitable interference filters or a monochromator are available.6,26,30,32,43 At times, it may be necessary to examine the ends or the surface of a fiber or textile at higher power than is available with light microscopy. The scanning electron microscope (SEM) can be used to visualize the surface of fibers and textiles for cross-sectional shape; it is also very useful for characterizing damage.47–50 In assessing the type of damage evident on a textile and evaluating the possible instrument(s) that could have caused it, empirical testing is crucial. Many papers outline various methods of causing and evaluating fabric and fiber damage. The book by Hearle, Lomas and Cooke48 offers a wealth of visual and descriptive data and it should be in the library of any laboratory that performs fabric examinations. Chemical analysis Before the development of modern instrumentation, solubility tests were used widely to characterize fibers. While destructive and not as specific as instrumental tests (such as infrared spectroscopy), solubility tests are easy and quick to perform. A small portion of the questioned fiber is placed under a coverslip and viewed microscopically. A drop of the appropriate solvent is applied with a pipette and the changes are recorded. Schemes for solubility tests have been published and the specific test employed depends upon the resources, materials and protocols of the particular laboratory.6,45,46 There are several instrumental tests for characterization and comparison of synthetic fibers. These include pyrolysis-gas chromatography, infrared spectroscopy, Raman spectroscopy and, less often, mass spectrometry, to determine or confirm the generic polymer class and/or sub-class. Microspectrophotometry in the ultraviolet and visible wavelength ranges is used to characterize the color of dyed or pigmented fibers. And, finally, fluorescence microspectrophotometry and high performance liquid chromatography are employed in the analysis of the dyes used to color fibers or textiles.

2.4

Pyrolysis gas chromatography

Most synthetic fibers are polymers that contain one or two monomers. If such a fiber is heated to high enough temperatures in an inert environment,

20

Identification of textile fibers

the polymers will decompose. If pyrolysis is carried out under controlled conditions in the inlet of a gas chromatograph (GC), the resultant array of peaks will be highly reproducible, although some variation will exist from day to day and among different instruments. This array of peaks is highly specific and sensitive to small differences in the polymer backbone.5 In order to identify a polymer class or subclass, it is necessary to analyze known fibers under the same conditions (consecutively with the questioned fibers, if possible) and compare these results with the results of the questioned fibers. Many forensic laboratories have pyrolysis GC instruments, making it readily available to trained personnel. A noted problem with pyrolysis GC as a forensic analysis is that it is destructive. As with any chromatography technique, shifts in the retention times due to injection techniques or instrument maintenance limits the use of reference chromatograms. The addition of mass spectrometry to identify individual pyrolytes somewhat compensates for this problem.

2.4.1 Infrared spectroscopy Substances absorb infrared (IR) energy characteristic of the atoms and bonds that make up their molecules. Because each substance is made up of different molecules, the wavelengths of infrared radiation are different from all other substances. The absorption of infrared radiation corresponds to the vibrating and rotating of all or parts of the molecules. Infrared spectra can be quite complex and, because of their specificity, allow for the easy identification of a fiber’s generic polymer class and subclass.29,51,52 In modern instruments, energy from an infrared source is directed through the fiber by an interferometer. A computer keeps track of the wavelength and amount of light transmitted through the fiber at each wavelength, the result being a plot of wavenumber (cm−1) against intensity. Infrared spectra are highly reproducible from day to day and from instrument to instrument, making the development of spectral libraries feasible. Numerous polymer libraries are available commercially. Most of the recent work in IR has been devoted to distinguishing subclasses of generic fiber classes.29,42,51–53 The application of Fourier transforms to forensic fiber analysis has been well documented and these instruments are now commonplace in laboratories. Studies have highlighted the usefulness of FT-IR in casework, particularly as a confirmation method for microscopy work and as a technique for finer discrimination of subgeneric classes of fibers, such as acrylics and nylons.54–56 FT-IR currently serves as an important tool in the analytical scheme of fibers for forensic purposes.5 Fiber analysis by FTIR is usually carried out on single fibers. This requires that a microscope be interfaced with the IR instrument. In most instruments, the microscope is mounted in an auxiliary module that is connected

Ways of identifying textile fibers and materials

21

to the FTIR via a light pipe or a set of mirrors. The microscope may have its own detector or may use the one in the main instrument. Another consideration is sample preparation: the fiber may have to be flattened to reduce the effects of spectral distortion due to the diffraction caused by the fiber’s cross-section. There are a number of techniques available for flattening fibers. A metal roller is an effective and easy way to flatten fibers. The fiber is flattened on the smooth portion of a clean glass microscope slide with the roller; the fiber is then carefully transferred to the specimen holder and then to the microscope stage. Sometimes the available fiber evidence is too short or too thin to obtain an FTIR spectrum by conventional means. In such cases, attenuated total reflectance (ATR), also called internal reflection, spectroscopy, may be used. In this technique, the fiber is placed in tight contact with an IR transparent, high refractive index crystal. When IR radiation is passed into the sample and crystal at angles greater than the critical angle, total reflection takes place, and an excellent IR spectrum can be obtained.

2.4.2 Raman spectroscopy Raman spectroscopy is an IR technique different from, but complementary to, traditional IR spectroscopy. It involves the measurement of bond vibrations by a light scattering method. Many infrared modes that are weak or not permitted in IR are very strong in Raman. For example, IR spectroscopy requires that a vibration causes a change in dipole moment in the molecule. Nonpolar bonds have vibrations that do not result in this change and are thus IR inactive but strong in Raman. Raman spectroscopy has been in existence since 1928 but has recently come into its own as a spectroscopic method due to technological advances in monochromators, lasers, filters and CCD devices.57–59 An early paper on the application of Raman spectroscopy characterized natural and synthetic fibres, organic and inorganic in composition. Both methods were used in tandem with a diamond cell used to obtain FTIR spectra.59 Dye or pigment information is also available with some Raman methods.60,61 The largest collaborative forensic study undertaken to date on Raman was conducted by the European Fibres Group (EFG), a working group of fiber experts from across Europe.62 Three dyed fiber samples, two red acrylic and one red wool, were analyzed by six different makes and models of Raman instrumentation from 458 nm to 1064 nm. Blue (488 nm) and green (532 nm) lasers produced the best overall results for spectral quality.63 The authors recommend a Raman instrument that can be tuned over the ranges listed previously to account for luminescence effects. This study also shows that Raman spectroscopy can identify the main dye type present in a color fiber but minor dye(s) may be obscured and difficult to identify.

22

Identification of textile fibers

2.5

Analysis of fiber colors and dyes

The vast majority of fibers used in commercial applications are colored. Fiber color is one of the most important properties in the comparison of fibers and is thus a critical test in the analytical scheme. Synthetic dyes and pigments belong to 29 different chemical categories with more than a dozen different application methods. Even simple dyes might require between eight and ten processes to convert the raw materials into a finished dye. Given that the total annual production of any particular dye might not amount to more than ten tons and that small process batches are becoming the rule in the dyeing industry,64 color becomes a powerful discriminating property. The selection of dyes is based upon many factors that, while not based on the final desired color, nevertheless affect the textile’s appearance.65 Color is particularly significant when the gamut of colors is considered: literally, millions of shades are possible in textiles. When these colors are spread out across the range of garments and carpeting produced in any one year, and ‘multiplied’ by the number of garments and carpets produced in previous years, the importance of color cannot be underestimated. Besides polymer class, color may be the single most discriminating trait the fiber examiner observes. There are a variety of methods for characterizing either the color of and/or the dye(s) in the fibers. They fall into three major categories: visual, chemical, and instrumental. The visual method is the simple observation and comparison of the fiber colors by use of the unaided eye. Visual comparison is easy, fast, accurate and non-destructive. It is a crucial first step in any fiber comparison, as many otherwise similar fibers can be excluded from consideration by color.66 Chemical methods, which include thin-layer chromatography and high performance liquid chromatography, address the make-up of the dyes used to color the fiber. This latter statement is an important distinction: analyzing the color of a fiber is not the same as analyzing the dyes used to color that fiber. Instrumental analyses include microspectrophotometry in the UV and/or visible ranges and, more rarely, spectrophotometric measurement of fluorescence.

2.6

Future trends

Keeping up with fiber technology is all but impossible. Changes in technology create potential new products and applications and, of course, consumer demands for the next big thing drive the entire system: Every market into which the consumer’s fashion sense has insinuated itself is, by that very token, subject to this common, compelling need for unceasing change in the styling of its goods . . . No single style of design, no matter how brilliantly it is conceived, can claim any independent fashion significance at all, nor can it possess more than a fugitive lease on life.67

Ways of identifying textile fibers and materials

23

The fugitive life of textile fibers creates a swirl of technology, fashion, analysis, and interpretation. New applications in medical textiles, micro- and nano-fibers, geotextiles, and specialty fibers, such as Spectra®, broaden and deepen the analytical world of the fiber expert. Yet, as far as analysis goes, the old methods are the best. Microscopy still dominates the field for analytical methods and provides a range of analysis barely possible with any other method. Combined with an analytical bench, such as infrared or Raman, it is the quintessential fiber identification tool. Microscopy should be the first method of choice for any fiber scientist.

2.7

References

1. D-276-87 Standard Test Methods for Identification of Fibers in Textiles, I. ASTM, Editor. 2003, ASTM International: Philadelphia, PA. 2. Molotch, H., Where stuff comes from: How toasters, toilets, cars, computers, and many other things come to be as they are. 2003, New York: Routledge. 3. Norman, D., The psychology of everyday things. 1988, New York: Basic Books. 4. Thornton, J., Ensembles of class characteristics in physical evidence examination. Journal of Forensic Sciences, 1986. 31(2): p. 501–503. 5. SWGMAT, Forensic Fiber Analysis. Forensic Science Communications, 1999. 1(1). 6. Gaudette, B.D., Forensic Fiber Analysis, in Forensic Science Handbook, Volume 3, R. Saferstein, Editor. 1993, Prentice-Hall, Inc.: Englewood Cliffs, NJ. 7. Hatch, K., Textile Science. 1993, St. Paul, MN: West Publishing. 8. Yeager, J. and L. Teter-Justice, Textiles for Residential and Commerical Interiors. 2001, New York: Fairchild Publications. 9. Ziabicki, A., Fundamentals of Fibre Formation. 1976, New York City, N.Y.: John Wiley and Sons, Inc. 10. Kroschwitz, J., Polymers: Fibers and Textiles, A Compendium. 1990, New York: John Wiley & Sons. 11. The Textile Institute, Identification of Textile Materials. 1975, Manchester, UK: The Textile Institute. 12. Catling, D. and J. Grayson, Identification of Vegetable Fibres. 1982, London, U.K.: Chapman & Hall, Ltd. 13. Cook, J., Handbook of Textile Fibers I: Natural Fibers. 1969, Metuchen, NJ: Textile Book Service. 14. Brunner, H., The Identification of Mammalian Hair. 1974, Melbourne: Inkata Press. 15. Palenik, S., Light microscopy of medullary microstructure in hair identification. Microscope 1983. 31: p. 129–137. 16. Mauerberger, H.R., Matthew’s Textile Fibers. 6th ed. 1954, New York City, New York: John Wiley and Sons, Inc. 17. Hicks, J.W., Microscopy of Hairs, Federal Bureau of Investigation. 1977, Washington, D.C.: Federal Bureau of Investigation. 18. Robertson, J., ed. Forensic Examination of Hair. Forensic Science Series, ed. J. Robertson. 1999, Taylor and Francis: Philadelphia, PA.

24

Identification of textile fibers

19. Bisbing, R., Forensic Hair Comparisons, in Forensic Science Handbook, R. Saferstein, Editor. 2002, Prentice-Hall: Englewood Cliffs, NJ. 20. Garrido, M., et al., Active control of spider silk strength: comparison of drag line spun on vertical and horizontal surfaces. Polymer, 2001. 43(4): p. 1537–1540. 21. Hinman, M., J. Jones, and R. Lewis, Synthetic spider silk: A modular fiber. TIBTECH, 2000. 18(September): p. 374–379. 22. Arcidiacono, S., et al., Purification and characterization of recombinant spider silk expressed in Escherichia coli. Applied Microbiology and Biochemistry, 1998. 49(1): p. 31–38. 23. Hinsch, J., The Technology of the Polarized Light Microscope. Fiber Producer, 1983. 11(3): p. 10–20. 24. Locard, E., The analysis of dust traces. The American Journal of Police Science, 1930. I: p. 176–249. 25. Locard, E., Manual of Police Techniques. 3rd ed. 1939, Paris: Payot. 26. McCrone, W.C., L.B. McCrone, and J.G. Delly, Polarized Light Microscopy. 1978, Ann Arbor, MI: Ann Arbor Science. 27. Siegel, J.A. and M.M. Houck, Forensic Textile Fiber Analysis, in Forensic Sciences, C. Wecht, Editor. 2001: Philadelphia, PA. 28. Stoeffler, S.F., A flowchart system for the identification of common synthetic fibers by polarized light microscopy. Journal of Forensic Sciences, 1996. 41: p. 297–299. 29. Tungol, M., A. Montaser, and E. Bartick, Analysis of single polymer fibers by fourier transform infrared microscopy: The results of case studies. Applied Spectroscopy, 1990. 44: p. 1655–1658. 30. Coyle, T., R. Robson, and P. Bauer, Identification of lyocell using dispersion staining. Science and Justice, 2002. 2: p. 75–79. 31. Gorski, A. and W. McCrone, Birefringence of fibers. Microscope 1998. 46: p. 3–16. 32. Heyn, A.N.J., Observations of the birefringence and refractive index of synthetic fibers with special reference to their identification. Textile Research Journal, 1952. 22: p. 513–522. 33. Johri, M.C. and D.P. Jatar, Identification of some synthetic fibers by their birefringence. Journal of Forensic Sciences, 1979. 24: p. 692–697. 34. Thetford, A. and S.C. Simmens, Birefringence phenomena in cylindrical fibres. Journal of Microscopy, 1969. 89: p. 143–150. 35. Palenik, S.J. and C. Fitzsimons, Fiber Cross Sections: Part 1. Microscope, 1990. 38: p. 187–195. 36. Palenik, S.J. and C. Fitzsimons, Fiber Cross Sections: Part 2. Microscope, 1990. 38: p. 313–320. 37. Grieve, M. and Paterson, Preparation of fiber cross sections. Laboratory Practice, 1967. 13: p. 167–171. 38. Grieve, M.C., Identification of polyester fibres in forensic science. Journal of Forensic Sciences, 1977. 22: p. 390–402. 39. Deadman, H.A., The importance of trace evidence, in Trace Evidence Analysis: More Cases from Mute Witnesses, M.M. Houck, Editor. 2003, Academic Press: San Diego, CA. 40. Clayson, N. and K. Wiggins, Microfibers – a forensic perspective. Journal of Forensic Sciences, 1997. 42: p. 4–7.

Ways of identifying textile fibers and materials

25

41. Deadman, H.A., Fiber evidence and the Wayne Williams trial: Part I. FBI Law Enforcement Bulletin, 1984. 53(3): p. 12–20. 42. Grieve, M., J. Dunlop, and T. Kotowski, Bicomponent acrylic fibres – their characterization in the forensic science laboratory. Journal of the Forensic Science Society, 1988. 28: p. 25–34. 43. Carroll, G.R., Forensic Fibre Microscopy, in Forensic Examinations of Fibres, J. Robertson, Editor. 1992, Ellis Horwood: New York City, NY. p. 99– 126. 44. Grieve, M., ed. Fibers and their examination in forensic science. Forensic Science Progress, ed. A.M.a.R.L. Williams. 1990, Springer-Verlag: New York. 45. Hartshorne, A., F. Wild, and N. Babb, The discrimination of cellulose di- and triacetate fibers by solvent test and melting point determination. Journal of the Forensic Science Society, 1991. 31(4): p. 457–461. 46. Grieve, M., The Use of Melting Point and Refractive Index Determinations to Compare Colorless Polyester Fibers. Forensic Science International, 1983. 22: p. 31–48. 47. Choudry, M., The use of scanning electron microscopy for identification of cuts and tears – an observation based on criminal cases. Scanning Microscopy, 1987. 1: p. 119. 48. Hearle, J., B. Lomas, and W. Cooke, Atlas of Fibre Fracture and Damage to Textiles. 1998, Boca Raton, FL: CRC Press. 49. Monahan, D. and H. Harding, Damage to clothing – cuts and tears. Journal of Forensic Sciences, 1990. 35: p. 901–912. 50. Stowell, L., The use of scanning electron microscopy to identify cuts and tears of a nylon fabric. Journal of Forensic Sciences, 1990. 35: p. 947–950. 51. Tungol, M., E. Bartick, and A. Montaser, The Development of a Spectral Data Base for the Identification of Fibers by Infrared Microscopy. Applied Spectroscopy 1990. 44: p. 543–549. 52. Tungol, M., E. Bartick, and A. Montaser, Forensic analysis of acrylic copolymer fibers by infrared microscopy. Applied Spectroscopy, 1993. 47: p. 1665–1658. 53. Bartick, E. and M. Tungol, Infrared microscopy and its forensic applications, in Forensic Science Handbook, R. Saferstein, Editor. 1993, Prentice-Hall: Englewood Cliffs, NJ. 54. Grieve, M.C., Another Look at the Classification of Acrylic Fibres, Using FTIR Microscopy. Science and Justice, 1995. 35: p. 179–190. 55. Grieve, M.C., Forensic Examination of Fibres. Forensic Science Progress, 1990. 4: p. 41–125. 56. Grieve, M. and L. Cabiness, The recognition and classification of modified acrylic fibers. Forensic Science International, 1985. 29: p. 129–146. 57. Greive, M., R. Griffin, and R. Malone, Characteristic dye absorption peaks found in the FTIR spectra of coloured acrylic fibres. Science and Justice 1998. 27–37: p. 27. 58. Jochem, G., Fiber-plastic fusions and related trace material in traffic accident investigation, in Trace Evidence Analysis: More Cases in Mute Witnesses, M.M. Houck, Editor. 2004, Academic Press: San Diego, CA. 59. Lang, P., et al., The Identification of Fibers by Infrared and Raman Microspectrophotometry. Microchemical Journal 1986. 34: p. 319–331.

26

Identification of textile fibers

60. Munro, C., W. Smith, and P. White, Qualitative and semi-quantitative trace analysis of acidic monoazo dyes by surface enhanced resonance Raman scattering. Analyst 1995. 120: p. 993–1003. 61. Thomas, J., et al., A further look at Raman spectroscopy for the forensic examination of fibres. Forensic Science International, 2003. 136: p. 125–136. 62. Wiggins, K., The European Fibres Group (EFG) 1993–2002: Understanding and Improving the Evidential Value of Fibres. Analytical and Bioanalytical Chemistry, 2003. 376: p. 1172–1177. 63. Massonnet, G., et al., Evaluation of Raman Spectroscopy for the Analysis of Coloured Fibres: A Collaborative Study. Journal of Forensic Sciences, 2005. 50: p. 1028–1038. 64. Apsell, P., What Are Dyes? What Is Dyeing?, in AATCC Dyeing Primer. 1981, American Association of Textile Chemists and Colorists: Research Triangle Park, North Carolina. 65. Park, J. and J. Shore, Dye and fibre discoveries of the twentieth century, Parts I and II. Journal of Society of Dyers and Colourists, 1999. 115: p. 157–167. 66. Houck, M.M., Intercomparison of unrelated fiber evidence. Forensic Science International, 2003. 135: p. 146–149. 67. Robinson, D., The Meaning of Fashion, in Inside the Fashion Business, J. Jarnow, B. Judelle, and M. Guerreiro, Editors. 1965, John Wiley & Sons: New York, p. 52.

3 Natural animal textile fibres: structure, characteristics and identification S R TRIDICO, Australian Federal Police, Australia

Abstract: The use of natural animal fibres in textile materials began before recorded history. Animal fibres of the most significant economic value in the textile market today are those made from wool, mohair, Angora rabbit, cashmere, camel, alpaca and cultivated silk. Natural fibres sourced from the pelage of animals exhibit a variety of morphological features which may be used to identify the particular family the hair originated from, which contrasts to the processes involved in the identification of silk. This chapter details the growth, structure and properties of animal fibres which affords each animal fibre type different and unique properties enabling industry to manufacture a plethora of textiles destined for a variety of end-uses. Key words: natural animal textile fibres, growth, structure, composition and properties of animal hairs and silk fibres, morphology of animal hairs and silk fibres, identification of animal hairs and silk fibres, silk production.

3.1

Introduction

The use of natural fibres of animal origin for textile materials began before recorded history. Textile fibres can be classified into two main categories, natural and man-made (see Fig. 3.1). Wool is generally accepted by the textile industry as a term referring to animal fibres originating from sheep; accordingly, this convention is used in the remainder of the chapter. Fibres from wool, mohair, angora, cashmere, camel and alpaca have the most significant economic value in the textile market today. The other significant animal fibre is cultivated silk originating from the silkworm Bombyx mori (B.mori). Accordingly, the following sections will predominantly focus on these seven fibre types. Section 3.2 details the growth, structure and properties of animal fibres. All mammalian hairs grow from follicles embedded in the skin, in contrast to the silks which are extruded from silk moth larvae. The chemical composition of all animal hairs is the same; they are made from the protein keratin, whereas silk fibres consist of the protein fibroin. This difference affords each animal fibre type different and unique properties which result in a plethora of textiles destined for a variety of end-uses. 27

28

Identification of textile fibers Fibre Natural

Animal (protein fibres)

Plant

Mineral

Man-made

Regenerated

Synthetic

Wool sheep Hair

Goat family (Bovidae) (mohair, cashmere)

Camel family (Camelidae) (camel, alpaca, vicuna) Other fur-bearing animals, in particular the rabbit family (Leporidae) (angora) Silk

3.1 Classification of textile fibres.

Sections 3.3 and 3.4 detail the different types of hairs found on the pelage of animals, the types of silk fibres produced and the range of morphological characteristics exhibited by these fibres. Section 3.5 focuses on the manner in which animal hairs may be identified as originating from a particular animal family or species and the contrast to the processes involved in the identification of silk. Sections 3.6 and 3.7 recommend sources of further information and future trends. The most significant future trend is the production of OptimTM fibres created from stretched wool which produces a thinner more luxuriant fibre akin to silk. The impact this fibre will have regarding identification is that the OptimTM fibres microscopically look like silk fibres and as a result may be erroneously identified.

3.2

Animal fibre growth, structure, composition and properties

3.2.1 Hair growth All mammalian hair fibres are of similar structure, chemistry and physical behaviour differing only in fine detail between the species; as Wildman stated,1 ‘it will help the reader to an understanding of the unique structure of animal fibres, their reactions to reagents, and the principles employed in

Natural animal textile fibres

29

their identification if he follows. . . . how they are developed in the skin. . .’ If a piece of skin was to be sectioned at right angles to the skin surface to produce a thin vertical section, a microscopic examination would reveal that the skin consists of two main portions; an ‘underskin’ or dermis and overlying this a thinner ‘outerskin’ or epidermis. These two major components of the skin will retain their separate identities throughout the growth of the animal. When a hair fibre is going to develop, a series of changes begins in the skin which results in the formation of a little plug or follicle. Hair follicles develop in utero as a downgrowth or invagination of the epidermis into the dermis and it is from the bottom of this structure that a new hair fibre starts its growth. The hair follicle is a dynamic organ in which division, differentiation and migration of cells occur in the various tissues of which it is composed. The mature hair fibre contains at least two cell types, the surface layer or cuticle, consisting of flattened overlapping cells, whose free margins point towards the tip of the hair fibre, and the main central cortex, or inner ‘body’, made up of spindle-shaped cortical cells. The hair may also possess a third and central structure consisting of an open meshwork of condensed cells called a medulla (or air space). The main cellular features and processes of a mature, growing hair follicle are illustrated in Fig. 3.2. As the cells of the immature forming hair fibre are pushed upward from the base of the follicle, their central nuclear bodies become reduced in size; whilst this is happening there is deposited in the cell a material which is an intermediate product in the formation of the protein keratin of the mature hair fibre. This keratinisation, or hardening, process of follicular structures proceeds faster and lower down, particularly in the cells forming the outermost layer of the inner root sheath, than it does in the more central layers which form the main ‘body’ of the hair fibre. The presence of this comparatively rigid inner root structure around the soft young hair fibre cells and the direction of growth are important factors in determining the shape of the hair fibre when it later emerges from the skin. Attached to the hair follicle is the arrector pili muscle which upon contraction causes the hair to ‘stand on end’, and to some follicles one or more sebaceous glands. The wax or sebum produced from the sebaceous gland facilitates the movement of the growing hair fibre as it pushes its way through the cells in the follicle and ultimately, through the horny dermis. Thus, the mammalian hair fibre is the product of a delicately adjusted living organism and results from a series of intricate growth structures genetically designed to produce a variety of hairs of finite lengths. The type of mammal and its hereditary or genetic constitution can have an effect on the appearance of these three main parts of the hair fibre which will ultimately dictate the end-use of that fibre.

30

Identification of textile fibers

Epidermis Dermis Mature hair

Arrector pili muscle Sebaceous gland

Medulla

Zone of hardening of hair fibre Disulphide bonding, resorption and dehydration

Inner root sheath Cuticle Cortex Cortical cells Outer root sheath Dermal sheath Basement membrane Follicle bulb Dermal papilla

Keratin gene expression

Cell proliferation and differentiation

3.2 Schematic diagram of a hair follicle showing the various features and major areas of cell proliferation and keratinisation (courtesy of Thomson publishers2).

3.2.2 Silk production Silk is an animal fibre but instead of being grown from a follicle embedded beneath the skin in the form of hair, it is produced by insects during the construction of their webs, cocoons or climbing ropes. Two types of silk fibres are utilised in the textile industry: cultivated silk (produced through the process of sericulture), from the mulberry silkworm B. mori, the mainstay of the silk industry comprising 95% of the world’s silk production; and wild or tussah (tasar) silk mainly produced by various species of the silkworm of the Antheraea genus which live in the wild, feeding mainly on oak leaves. As a result of their varied diets B. mori produces silk filaments which are usually white in colour and highly lustrous in contrast to the tussah silkworms which produce silk filaments ranging green or tan in color and lack the high lustre of the cultivated silk.

Natural animal textile fibres

31

Irrespective of the type of genus of silkworm used to produce cultivated or wild silk, the mode in which the animal produces the fibre is the same. The silkworm larva possesses a pair of modified salivary glands (sericiteries) which produce a clear, viscous, proteinaceous fluid that is extruded through openings or spinnerets on its mouthparts. As this fluid is exposed to the air it hardens; this hardened silk filament is then used by the larva to wrap the fibre around itself in the form of a cocoon.

3.2.3 Animal fibre structure and composition The composition of all the animal fibres are the same in that they are made up amino acid chains joined together through condensation to form the polymeric molecule protein. However, the protein constituting wool and hair fibres (keratin) varies enormously to the protein comprising the silk (fibroin). The major difference being that keratin fibre proteins are highly cross-linked by disulphide bonds, whereas the secreted silk fibroin fibres tend to have no cross-links and a more limited array of less complex amino acids. Irrespective of this significance difference in their composition and structure all fibres of animal origin are all fibres of character; each one exhibiting unique properties which ensures it a position of special significance as a textile fibre. All animal fibres consist exclusively of proteins and, with the exception of silk, constitute the fur or hair of animals. Proteins are nitrogen-containing substances which are essentially chain-like molecules formed by the union of α-amino acids joined together by peptide linkages which retain one terminal amino (NH+) group and one terminal carboxylic acid (CO−) group resulting in the elimination of water (condensation). Although amino acids may have other formulae, those in proteins invariably have a general formula as illustrated in Fig. 3.3. Each amino acid consists of a single carbon atom to which is attached a carboxyl function (–COOH), an amino function (–NH2), a hydrogen atom and a side-chain (R) which defines each particular amino acid and its chemical character. Over 20 amino acids with different side groups (R) are known; the difference between proteins arises from the differences between these side groups attached to the main chain illustrated in Fig. 3.4.

H H2NCCOOH R

3.3 Amino acid structure.

32

Identification of textile fibers O CHCNH R

n

3.4 Protein structure.

Two major classes of natural protein fibres exist and include keratin, found in hair and fur, and fibroin secreted (insect) fibres. In general, the keratin fibres are proteins highly cross-linked by disulfide bonds from the cystine, (–CH2SSCH2–), residues in the protein chain which comprises some 10–15% of wool fibres. Although the exotic fibres alpaca, cashmere, Mohair, angora and camel are chemically similar in their composition to wool, their cystine content in their protein chains differ, which can be up to 24% in some fibres. The keratin fibres tend to have helical, intermittent helical sections within the protein sequence and are extremely complex in structure with the inclusion of a cortical cell matrix surrounded by a cuticle sheath laid on the surface as overlapping scales. The cell matrix, or cortex, of some hair fibres may contain a central cavity or medulla. The keratin fibres tend to be round in cross-section with an irregular crimp along the longitudinal fibre axis. Raw silk, whether cultivated or wild, contains about 75% fibre and 25% of a globular protein called sericin. The sericin is usually left on the silk filaments to protect them from mechanical damage during processing. The silk yarn or fabric is degummed to remove the sericin, resulting in a silk fibre which is essentially pure fibroin. The protein fibroin, as illustrated in Table 3.1, has a markedly different amino acid composition from that of keratin. Fibroin, unlike the keratin fibres, has a more limited array of less complex amino acids; glycine (H–) and alanine (CH3–) constitute in total some 60% of the amino acids comprising this protein. Literature citations regarding glycine, alanine, tyrosine and serine as the major amino acids comprising fibroin accord with the notable of exception of the amino acid cystine. Needles3 and Cook4 report that fibroin lacks the amino acid cystine whereas Kushal and Murugesh5 report that fibroin contains negligible amounts of this amino acid. Thus, with the virtual absence of cross-links and with limited bulky side chains present in the amino acids, fibroin molecules align themselves parallel to each other and hydrogen bond to form a highly crystalline and oriented ‘pleated-sheet’ or ‘beta’ structure. Because of its high cost, silk finds a limited use in textiles; a minor amount of wild or tussah silk is produced for specialty items. The silk fibres comprising the ‘wild’ silks differ from those of cultivated silk in colour and texture; however, the wild silk and

Natural animal textile fibres

33

Table 3.1 The content of the α-amino acid side-groups (R) in wool and silk protein fibres (grams of amino acid per 100 g protein). Data from ‘Textile Fibres, Dyes, Finishes and Process – A Concise Guide’3 with the exception of * value which is taken from ‘Studies on Indian Silk’5 α-Amino acid

Wool keratin

Silk fibroin (cultivated)

Inert Glycine Alanine Valine Leucine Isoleucine Phenylalanine

5–7 3–5 5–6 7–9 3–5 3–5

36–43 29–36 2–4 0–1 0–1 1–2

Acidic Aspartic acid Glutamic acid

6–8 12–17

1–3 1–2

Basic Lysine Arginine Histidine

0–2 8–11 2–4

0–1 0–2 0–1

Hydroxyl Serine Threonine Tyrosine

7–10 6–7 4–7

13–17 1–2 10–13

Miscellaneous Proline Cystine Methionine Tryptophan

5–9 10–15 0–1 1–3

0–1 0.00 / 0.13* 0.0 0–1

cultivated silk fibres are sufficiently similar in chemical composition and structure as to be considered as homogeneous fibre types.

3.2.4 Physical and chemical properties of protein fibres Hair fibres are related to wool in their chemical structure; they all comprise keratin. But they all differ from wool, and from each other, in their physical (and morphological) characteristics; they are of different length and fineness and have different shapes and internal structures. With silk fibres, on the other hand, despite several species of silkworms being used in their production, the construction of their cocoons are sufficiently alike for the silk to be regarded as a fairly homogeneous material. The configuration and orientation of the individual molecular chains within each protein fibre, in conjunction with its the overall shape, will affect the fibre properties. In protein fibres, like other natural fibres, the

34

Identification of textile fibers

orientation of the molecules within the fibre is determined by the biological source during the growth or production, and the maturity process of the fibre. According to Needles3 there are several essential ‘primary’ properties that any polymeric material must possess in order to produce a fibre adequate enough for its intended final product. These properties are fibre length to width ratio, fibre uniformity, fibre strength and flexibility, fibre extension and elasiticity and fibre cohesiveness. However, the polymeric material should also exhibit additional characteristics in order to increase their desirability and value in its intended end uses. Such properties include moisture absorption characteristics, fibre resilience, density, lustre and chemical resistance. Man-made fibres are specifically manufactured in order to meet these essential critera; however, nature ensures that the protein fibres it produces are ‘ready made’ to fulfil these requirements. In relation to fibre lengths, hairs are grown to genetically determined finite lengths and as such, they are regarded as staple fibres and treated as such in the production of wool products; silk fibres, however, are extruded as extremely long continuous filaments and therefore regarded as a filamentous fibre in the textile industry. Protein fibres are generally fibres of moderate strength, resilience and elasticity and at moderate humidities do not build up significant static charge. Wool, like other hair fibres, contains a substantial amount of the amino acid cystine. Cystine residues play a very important role in the stabilisation of the fibre structure due to the cross-linking action of their disulphide bonds, which holds the polymer chains together not only in wool, but also in other animal fibres. The disulphide bonds are responsible for relatively good wet strength of wool. Wool is resistant to attack by acids but is readily attacked by weak bases even at low dilutions and is irreversibly damaged and coloured by dilute oxidising bleaches such as hypochlorite. Reducing agents will cause reductive severance of the disulphide bonds within the wool, evetually causing it to dissolve. However, this property is exploited in the textle industry as under controlled conditions, reducing agents can be used to partially reduce the wool and flat set or set permanent pleats in the wool. Wool, unless chemically treated, is susceptible to attack by several species of moths which are able to to dissolve and digest wool fibres. However, it is reasonably resistant to attack by other biological agents such as mildew. Wool fibres have excellent resiliency and recovery rate from deformation except under high humidity, it is insoluble in all solvents with the exception of those capable of breaking the disulphide cross-links. Wool is a good heat insulator due to its low heat conductivity and bulkiness, which permits the air to become trapped in the fibres comprising the textile constructions.

Natural animal textile fibres

35

Wool and other hair fibres are unique amongst natural fibres for their possession of overlapping cuticular scales present on the outermost surface of the fibre, their position akin to overlapping tiles on a roof. This characteristic scaly outer surface is vital in the process of felting; a process which is unique to wool and other animal fibres. Felting is the consolidation of these fibrous materials by the application of heat, moisture and mechanical action, causing the scales on the wool and hair fibres to interlock and mat together. The fabric shrinks and undergoes characteristic changes in its structure. The fabric becomes thicker and the fibres are matted into closely packed masses. The outline and character of the yarn pattern in the fibre becomes indistinct and the fabric loses much of its elasticity; the surface of the fabric is covered by fibres, and its appearance is altered. The outer scales on the wool or hair fibre are aligned such that their edges point towards the tip end of the fibre. Felting appears to most amenable with wool or hair fibres which bear prominent cuticular scales, probably due to the fact that the felting treatment tends to bend the scale edges and fibres into loops, which during the mechanical action with repeated fabric compression, causes the loops and scales to ‘travel’ and become interlocked and entangled; unlike bonded fabrics, felts do not require adhesive for their production. Felted fabrics are used in the hat industry, apparel and drapery, in industry for insulation, packing and polishing materials and felt padding is used in apparel and furniture. Cashmere fibres are almost identical to wool in relation to their chemical composition; however, owing to fineness and better wetting properties, cashmere fibres are more susceptible to chemical damage, especially with respect to alkalis. Silk, due to the virtual absence of cystine, is not as resistant to acids as wool, but is more resistant to alkalis. Silk is very resistant to organic solvents but soluble in hydrogen bond breaking solvents such as cupammonium hydroxide. Unlike other natural fibres silk is more resistant to biological attack. Strong oxidising agents such as hypochlorite will cause silk to rapidly discolour and dissolve, whereas reducing agents have negligible effect except under extreme conditions. Silk fibres are relatively stiff and show good to excellent resiliency and recovery from deformation depending on the temperature and humidity conditions. These fibres exhibit favourable heat-insulating properties but owing to their moderate electrical resistivity, tend to build up static charge.

3.3

Types of natural animal fibres

3.3.1 Mammalian hairs The pelage, or coat, of animals comprises various hair types which are illustrated in Fig. 3.5. Close examination of the pelage will reveal that some

36

Identification of textile fibers

(a)

(b)

(c)

3.5 Examples of hair types which may be found on the pelage of animals (a) over hair, (b) guard hair and (c) under hairs (courtesy of Dr Hans Brunner).

sparsely distributed hairs are distinctly longer than the hairs comprising the bulk of the coat; these longer hairs are called overhairs. The larger or coarser hairs forming the bulk of the pelage are termed guard hairs. These hairs generally exhibit a variety of sizes in one pelage, ranging from the coarse and long to those that cannot be distinguished from the underhairs. The guard hairs may be of uniform diameter along the hair shaft, tapering to a tip. However, some guard hairs are specialised into a type described as shield hairs. In shield hairs the distal (tip region) of the hair is noticeably wider and flattened, forming a shield. The underhairs are shorter and much finer than the overhairs and guard hairs, these hairs are usually found close to the body and serve to insulate the animal. In general, underhairs are wavy and retain a uniform diameter along the length of the hair with the exception of the tip which tapers to a point. The classification of hair types, as outlined above, predominantly relies upon the appreciation of the profile or general outline of the hair, e.g. straight or wavy. Some examples of the types of profiles which may be seen on the pelage of animals are illustrated in Fig. 3.6.

Natural animal textile fibres

(a)

(b)

37

(c) (d)

3.6 Examples of hair profiles (a and b) guard hairs (c) under hair, (d) magnified view of a constriction in an under hair (courtesy of Dr Hans Brunner).

Carpets Bedding Upholstery Blankets Woollen fabrics Worsted fabrics 17–24

25–27

28–30

31–33

34–40

41+

Fibre diameter in microns

3.7 Wool fibres and their uses in the textile industry based on their diameters (modified from FAO Agricultural Bulletin 1227).

The coarseness or fineness of the animal hairs determines their end use in the textile industry and as such animal hair fibres need to be graded according to their fibre diameter. For example, the major end uses of wool are apparel products, bedding and carpets. Figure 3.7 illustrates how the fibre diameters determine the end product, with the coarser fibres being used

38

Identification of textile fibers

for carpets and the finer fibres being used for apparel fabrics which need to be softer against the skin. In general wool fibres coarser than 21 microns in diameter cannot normally be processed into yarns destined to produce lighter, softer fabrics that are both functional as well as aesthetically pleasing.

3.3.2 Cultivated and wild silk fibres Silkworm is a common name for the silk-producing larvae several species of moths; however, the mulberry silkworm B. mori is the most common moth used in the commercial production of silk. B. mori feeds exclusively on the leaves of the mulberry tree and has flourished only where conditions are suitable for large numbers of leaf-bearing mulberry trees. However, this moth has been cultivated over many centuries and is no longer found in the wild, and today is totally dependent on humans for its existence and as such the silk it produces is known as cultivated silk. Unlike silk produced from silkworms living in the wild, cultivated silk is harvested from a cocoon as a continuous silk filament approximately 1000 m in length. This is achieved by killing the pupa prior to its emergence as a moth during which the pupa secretes an alkali which dissolves the cocoon threads thereby ruining the silk. Silk is a continuous filament around each cocoon and is freed by softening the cocoon in water, locating the free end and harvesting the silk thread. Wild or tussah silk, on the other hand, is produced by silkworms which live in an environment free to feed on a variety of leaves and complete their life cycles, with the pupa contained in the cocoon being allowed to live and emerge as a moth. As a consequence of this ‘wild’ existence the integrity of the single silk filament produced by some species of larvae to build its cocoon is broken, resulting in silk which consists of numerous strands. This silk is generally coarser than the cultivated silk because the wild silk consists of numerous strands, rather than a single seamless one, and is also variously coloured due to the uncontrolled diet of the insects.

3.4

Natural animal fibre characteristics

3.4.1 Animal hairs Morphological characteristics exhibited by animal hairs are usually examined using the optical microscope, which is excellent for examining the interior of the hair shaft; however, the exterior of the hair, i.e. the cuticle, is best examined using a scanning electron microscope (SEM) as the resolution of the image is far higher than that attainable with the optical microscope, resulting in a much more detailed image. The use of these two

Natural animal textile fibres

39

Scale

Cuticle Medulla Cortex

3.8 Diagrammatic representation of the major structural components which may be exhibited by hairs (courtesy of Dr Hans Brunner).

microscopes in the examination of animal hair is detailed in the identification section of the chapter and unless otherwise stated, any references to microscopy will be in relation to the use of the optical microscope. As seen in preceding sections, animal hair fibres have a unique structure consisting of the outermost scale cuticle, an inner cortex and, in some hair fibres a central medulla, as diagrammatically illustrated in Fig. 3.8. All mammalian hairs bear morphological characteristics typical to the family of the particular species. These morphological characteristics may be seen on the outside of the hair shaft as cuticular scale patterns, inside the cortex as medullary patterns or at the root end which, in some animals, bears a characteristic shape; however, for the majority of animal fibres used in the textile industry the root will be absent, with exception of the coarse kemp fibres, which may be found in some fabrics; these fibres are medullated and exhibit a brush-like root. The works of Wildman1 and Brunner and Coman6 are considered as seminal works and standard references in relation to the idenification of animal hairs based on their classification of morphological features and characteristics on the basis of their microscopical appearances. Wildman, and Brunner and Coman classified the hair characteristics on the basis of their cuticular scale patterns and medullae; Brunner and Coman further classified hair characteristics on the basis of their cross-sectional shapes. As illustrated in Fig. 3.8 the cuticle or outer layer of the hair shaft comprises a single layer of overlapping cells, arranged like tiles on a roof, with the free edges pointing towards the tip. Unlike human hairs, animal hairs exhibit a variety of cuticular scale arrangements to form distinct patterns. In relation to the cuticular scale patterns exhibited by animal hairs, Brunner and

40

Identification of textile fibers Form of scale margins

(a) Smooth

(b) Crenate

(c) Rippled (d) Scalloped (e) Dentate

Distance between scale margins

(f) Distant

(g) Near

(h) Close

Scale patterns

(i) Simple coronal

(o) Regular wave

(j) Diamond petal

(p) Irregular wave

(k) Narrow diamond petal

(l) Broad petal

(m) Regular mosaic

(n) Flattened irregular mosaic

(q) Single chevron

(r) Double chevron

(s) Streaked

(t) Transitional

3.9 Cuticular scale patterns as classified by Brunner and Coman (courtesy of Dr Hans Brunner).

Coman divided the classification into three criteria, each based on the following main features: terms which describe the form of the scale margin, terms which describe the distance between the external scale margins and terms which are descriptive of the general scale patterns as illustrated in Fig. 3.9. As depicted in Figs 3.10 and 3.11 Wildman similarly classified the cuticular scale patterns with the exception that the dentate scale margin, coronal

Natural animal textile fibres

Mosaic (regular)

Interrupted regular wave

Single chevron (a form of regular wave)

Mosaic (irregular)

Mosaic (irregular wave)

Double chevron

41

Simple regular wave

Wave (medium depth)

Streaked wave (a variety of interrupted wave)

3.10 Cuticular scale patterns as classified by Wildman (courtesy of BTTG Ltd).

and transitional scale patterns are not represented. Owing to the differences in the terminologies of these two main works it is recommended to use one reference or the other when identifying animal textile fibres in order that consistency of terms and descriptors is maintained and to provide the source used in the identification process.

42

Identification of textile fibers

Irregular petal (a form of interrupted irregular wave)

Lanceolate (a form of fine pectinate and also of regular wave)

Coarse pectinate (a form of regular wave)

Diamond petal

Narrow diamond petal

3.11 Cuticular scale patterns as classified by Wildman (courtesy of BTTG Ltd).

Brunner and Coman defined four major structural groups of animal hair medullae: unbroken, broken, ladder and miscellaneous. They further subdivided each of these groups into a total of 12 distinct types as depicted in Fig. 3.12. The top half of each type details the structure seen in a hair in which the medulla is filled with air; the lower half illustrates animal hairs which have been treated in order to facilitate the observation of the medulla. The medulla consists of shrunken cells, the spaces between these shrunken cells are usually filled with air. Under the microscope these appear as obvious black, opaque structures which can obscure the structure or pattern of the medulla. If these hairs are treated in such a way as to allow mounting medium to enter the cortex and infiltrate these air spaces, viewing the medulla shape and form is facilitated. Wildman, on the other hand, classified the medulla types into the following four broad categories: (a) unbroken (wide) lattice, (b) and (c) simple

Natural animal textile fibres

(a) Narrow medulla lattice

(g)

(b)

(c)

Wide medulla lattice

Narrow aeriform lattice

(h)

Fragmental

Uniserial ladder

(i) Multiserial ladder

(d) Wide aeriform lattice

43

(e)

(f)

Simple

Interrupted

(j)

(k)

(l)

Globular

Stellate

Intruding

3.12 Medulla types as classified by Brunner and Coman (courtesy of Dr Hans Brunner).

unbroken, (d) interrupted and (e) fragmental as illustrated in Fig. 3.13. The following medulla types by Brunner and Coman are illustrated in Fig. 3.12: narrow medulla lattice, uniserial ladder, multiserial ladder and globular, stellate and intruding being absent. The latter three comprise the ‘miscellaneous’ major structural group being found in animals such as seals, wombats and platypus. The different classifications of medulla types and scale patterns may reflect the different aims of each of the works. Wildman’s work details morphological characteristics of animal fibres of importance in the textile industry; whereas Brunner and Coman deal with the morphological characteristics of a variety of mammalian hairs for use by animal ecologists and in the examination of animal hairs found as contaminants in food. Brunner and Coman noted that in addition to the differences in medullae types and cuticular scale patterns exhibited by animal hairs, significant

44

Identification of textile fibers

(a)

(b)

(c)

(d)

(e)

3.13 Medulla types as classified by Wildman (courtesy of BTTG Ltd).

differences are also apparent in the cross-sectional shapes of animal hairs. In Fig. 3.14, Brunner and Coman depict the most common cross-sectional shapes enountered in animal hairs, the dark, central features representing the medulla.

3.4.2 Silk fibres Silk, although produced by an animal and a protein-based fibre, is not generally regarded as a true animal fibre since it comprises fibroin and not keratin, nor does it grow from a follicle embedded beneath the skin but is extruded from modified salivary glands from a larva. As such it does not bear any of the morphological characteristics exhibited by the true keratin animal fibres. Under the microscope silk has the appearance of a glass-like filament of uniform diameter which may bear striations along its length. In the raw state the colours of the fibres may reveal if the fibre has been produced as cultivated silk or as the product of wild silk; cultivated silk, once degummed, has a high natural lustre and sheen white in colour. Wild silks vary in colours such as, but limited to white, cream, green, brown, and amber. The variety of colours is attributable to the variety of leaves consumed by the various wild silk moth species.

3.5

Identification of natural animal fibres

3.5.1 Keratin-based animal fibres All animal hairs bear morphological characteristics and features which not only allow differentiation between them but also their identification to a species or family level. Animal hairs used in the textile industry, despite

Natural animal textile fibres

Circular medium size medulla

45

Circular large medulla

Oval large medulla

Oval medium size medulla

Oval medulla absent

Eye-shaped

Oblong large medulla

Oblong medium size medulla

Cigar-shaped

Concavo-convex divided medulla

Concavo-convex bilobed medulla

Concavo-convex large medulla

Reniform

Dumb-bell shaped

3.14 Cross-sectional shapes as classified by Brunner and Coman (courtesy of Dr Hans Brunner).

being processed and dyed, may retain sufficient characteristics to determine the possible source. The identification of animal hair fibres begins with the determination that the hair is animal not human in origin. This is generally easy to achieve and Table 3.2 details the morphological characteristics human and animal hairs show, which enable their differentiation. Clearly, in animal hairs used to manufacture textile products, the banding characteristic may not be commonly seen. Once a hair has been identified as animal in origin, a further more detailed macroscopic examination is performed to determine a number of

46

Identification of textile fibers

Table 3.2 Characteristics which may be used to differentiate between human and animal hairs Feature

Human hair

Animal hair

Colour

Relatively consistent along hair shaft

Medulla

Less than 1/3 of the width of the hair shaft Amorphous, mostly not continuous when present

Pigment distribution Cuticular scales

Even, slightly more towards the cuticle Imbricate, similar along the shaft from root (proximal end) to tip (distal end) Usually bulbous (club shaped) and indistinct

Often showing abrupt, profound colour changes known as banding Usually greater than 1/3 of the width of the hair shaft Continuous, often varying in appearance along the shaft, defined structure Central or denser towards the medulla A variety of patterns often showing variation in structure from root to tip Variety of shapes and forms, usually distinct

Root

features such as the profile of the hair, e.g. the length and appearance of the hair, i.e. straight or wavy, and determine whether the hair is likely to be a guard hair or an underhair. A preliminary examination of the animal fibre with a stereomicroscope (up to 100× magnification) may reveal the width and gross morphology of the medulla characteristic to a particular hair type and possible animal or origin, e.g. a hair with a brush-like root and an obvious wide unbroken medulla would strongly indicate the presence of a kemp fibre. In general the largest of the guard hairs (primary guard hairs) are of paramount importance in the identification of animal hairs, for it is these hairs which generally exhibit the most diagnostically useful characteristics. The underhairs are generally of little diagnostic value in determining the identification of animal fibres. The identification of animal hair fibres predominantly relies upon the morphological features present inside the cortex, such as the medulla and on the outside of the hair from the cuticular scale patterns, as the size and shape of these scales and their pattern of arrangement around the hair are useful criteria for identification purposes. In relation to observing the cuticular scale patterns present on animal fibres for identification purposes, a number of methods can be employed, each with their own merits and limitations. The scale patterns may be visualised by mounting the hairs in a semipermanent mounting medium with a refractive index (RI) lower than that of keratin (RI 1.55) which facilitates the viewing of the external scale patterns but slightly masks the internal features. Figure 3.15 illustrates the efficacy of three semi-permanent mounting media each with varying RIs.

Natural animal textile fibres

(a)

(b)

47

(c)

3.15 Photomicrographs of the same woollen fibre mounted in cedar oil RI 1.513 (a), liquid paraffin RI 1.470 (b) and glycerine and water RI 1.403 (c) (courtesy of BTTG Ltd).

The hair will need to be dried and/or cleaned of the semi-permanent medium following the examination. Scale casts may also be made by coating a cover slip with a medium such as clear nail polish, laying the hair in the polish, once the polish is dry, the hair is removed, the cover slip inverted and placed on a microscope slide; cuticle scale cast, embedded in the nail polish, can be viewed under a compound microscope. This method does not require the hair to be dried or cleaned and the cast is a permanent record. Examination of the external features of animal hair fibres may be achieved with the use of a scanning electron microscope (SEM). The SEM uses secondary electrons to ‘view’ the surface characteristics of a fibre, the surface of which is usually coated with a thin layer of gold to assist the speedy display of the scanned surface. The major advantage of the SEM, over the optical microscope, is its very high resolution (down to 2 nanometres) and the relatively large depth of field. This enables the complete surface of a fibre to be seen in high detail thus enabling a better discrimination of the

48

Identification of textile fibers

surface characteristics. However, unlike the optical microscope, no internal features are visible and due to the thin gold coating, the hair cannot be examined further. If the textile of origin is unknown, determining the width or diameter of the animal fibre may assist in determining its textile provenance, e.g. a coarse wool fibre greater than 40 microns may indicate that it originated from carpet; or if a garment is a blend of wool and cashmere, determining the diameters of the fibres may assist in their differentiation and identification. Although Brunner and Coman regard the cross-sectional shape of a hair as ‘undoubtedly the most single important criterion used in . . . hair identification’ it is a destructive technique and as such should be used with caution. Wildman does not illustrate cross-sectional shapes of animal hairs; he does, however, discuss their value and use for animal hair identification and for studies on the micro-structure of the hairs. Seta8 makes the following comments in relation to the use of cross-sectioning of hairs regarding their identification: The variable shape of the shaft gives a clue to the identification of species. . . . Many hair examiners have adopted the cross-sectional shape for characterising hairs. . . . For this examination some investigators used the longitudinal mount without preparing a cross section. . . . This may be justifiable from the following points: 1. The consumption of the . . . hair would be minimal 2. The cross-sectional shape is variable from hair to hair and from point to point on the same hair. 3. The cross-sectional shape does not have as much validity as has been thought. 4. The production of suitable cross-sections depends completely on the experience and ability of the examiner.

The preceding sections detail a plethora of medullae types and the cuticular scale patterns which may be seen on the animal fibre. The medulla types and scale patterns not only vary from hairs of different species but these characteristics may vary along the length of the same hair fibre and between hairs comprising the pelage of the one animal. The widest point of the hair fibre provides the most diagnostic medulla type. Tables 3.3 and 3.4 illustrate the most significant, general features and characteristics present in fine and coarse animal fibres for identification purposes. Wool, like all other fibres of animal origin, consists of a cuticle of scales, a cortex and in some instances a medulla. Very fine fibres, for example those produced by merino sheep, have no detectable medullae but consist of cuticle and cortex as depicted in Fig. 3.16. The images of representative types of wool, illustrated in Figs 3.17 and 3.18 show that there is a variation in thickness not only between the fibres, i.e. inter-species variation but also

None Circular to elliptical 15–24 μm

Medulla Cross-section

Simple broken Circular to oval

Irregular waved mosaic

Mohair

Uniserial ladder Oval to rectangular

Single or double chevron

Angora rabbit Waved mosaic near to distant scale margins Not observed Circular to oval

Camel Coronal, distant scale margins smooth Non medullated Circular to oval

Cashmere

Varies Almost circular

Irregular waved mosaic

Alpaca

Fibre diameter

Cross-section

Medulla

Cuticular scale pattern

30–36 μm

Up to 40+ μm

Fragmental or unbroken to wide unbroken lattice Circular to oval

Irregular mosaic and simple waved pattern

Mohair (Angora goat)

Regular mosaic and irregular mosaic; smooth, near margins Wide lattice Simple unbroken narrow or fragmented narrow Round to elliptical

Wool

Dumb-bell ovoid TBA

Wide unbroken multi-serial ladder

Double chevron

Angora rabbit

Up to 120 μm

Circular to oval

Irregular waved mosaic; near and smooth margins Simple unbroken (fine lattice)

Camel

mfd 80–86 μm

Simple broken or unbroken medulla (medium diameter) Circular to oval

Irregular waved mosaic; near margins

Cashmere goat

40–60 μm

Varies

Irregular waved mosaic; smooth; near margins Varies

Alpaca

Table 3.4 Morphological features present on coarse wool, mohair, angora rabbit, camel, cashmere and alpaca animal fibres

Fibre diameter

Simple, coronal

Cuticular scale pattern

Wool

Table 3.3 Morphological features present on the finer wool, mohair, angora rabbit, camel, cashmere and alpaca animal fibres

50

Identification of textile fibers

3.16 Photographs showing the scale structure of fine woollen fibres (courtesy of BTTG Ltd).

3.17 Photographs showing the structure of coarse woollen fibres (courtesy of BTTG Ltd).

3.18 Photographs showing the structure of coarse woollen fibres (courtesy of BTTG Ltd).

Natural animal textile fibres

(a)

51

(b)

3.19 Diagrammatic representations illustrating the extremes of uniformity and irregularity of woollen fibre diameters. (a) Crosssectional shapes of fine, high quality woollen fibres; (b) crosssectional shapes of the coarser, lower quality woollen fibres (courtesy of BTTG Ltd).

along the length of each fibre. The inter-species variation is mainly due to genetic or inherited characteristics, i.e. the intra-species variation is generally due to nutritional feasts and famines during the period of fibre growth. The lower quality and coarser wool fibres tend to become medullated with the wide lattice type occuring in the very coarse fibres, including the kemps; this medullation is commonly seen in carpet wool fibres. The simple unbroken medium to narrow type of medulla is seen in many fibres from longwools and cross bred wools. The fragmental type of medulla often occurs in wool fibres, but it is often significantly smaller in relation to the rest of the fibre. Fibre diameters of wool fibres can vary from uniform to irregular fibre diameters, as illustrated in Fig. 3.19. The higher qualities of wool fibres exhibit a smaller mean fibre diameter but also less variation in the fibre thickness; the lower quality and coarser fibres have an increased variation in fibre diameter. The coarser wool fibres as well as being medullate, exhibit a regular mosaic-type scale pattern illustrated in Fig. 3.20, which may alternate with short lengths of irregular waved pattern; in contrast to the fine wool fibres which exhibit the same scale pattern type irrespective of the breed of sheep they originated from (see Figs 3.21 and 3.22). Alpaca is from the fleece of the alpaca Lama pacos which belongs to the llama family, so that alpaca fibres and llama fibres have many morphological features and characteristics in common which may be seen in the preceding figures. Alpaca fibres produced, range in diameter from 24–26 μm. These fine alpaca fibres bear scales which are smooth-margined

52

Identification of textile fibers

3.20 Cuticular scale patterns which may be exhibited by coarser woollen fibres, all of which are of the regular mosaic pattern with the exception of (d) which shows the regular mosaic pattern merging into an irregular waved mosaic form (courtesy of BTTG Ltd).

Natural animal textile fibres

53

3.21 Scale pattern exhibited by fine woollen fibres all of which have the same type of scale pattern (irregular-waved mosaic), with scale margins which are smooth and distant (courtesy of BTTG Ltd).

18 μm

3.22 SEM image of a scale pattern of a merino fibre (courtesy of CSIRO Textile and Fibre Technology).

54

Identification of textile fibers

25 μm

3.23 SEM image of the scale pattern of an alpaca fibre (courtesy of CSIRO Textile and Fibre Technology).

as illustrated in Fig. 3.23. The coarser fibres (fibres 50–60 μm or over) have scales which form an interrupted irregular wave pattern as illustrated in Fig. 3.24. Regarding alpaca fibre cross-sections, these animals produce a spectacular array of shapes and forms which can be seen in Figs 3.25 and 3.26 and are characteristic of not only the alpaca but also the llama. The pelage of the angora rabbit, like that of other animals, has two major fibre types, the outer guard hair and the shorter fur or underhair. In the textile industry, angora rabbit fibres may be used alone or blended with wool or nylon. The angora rabbit fibre bears a medulla which is characteristic of the lagomorph family to which it belongs (which includes ‘domestic’ rabbits and hares). The coarser angora rabbit hairs have a wide, unbroken, mulitserial ladder medulla; the finer hairs, in general, bear a uniserial ladder medulla. The cuticle shows a single or double chevron scale pattern as seen in Fig. 3.27. The dumb-bell cross-sectional shape is typical for rabbit fibres as seen in Fig. 3.28. Mohair comes from the angora goat Capra hircus aegragus; these fibres are very regular in thickness along their lengths and have smooth outlines, which cause the scale margins to be difficult to detect in profile. The outlines of mohair are, in this respect, sharply distinct from those of wool fibres with

Natural animal textile fibres

55

3.24 Scale pattern found on a coarse alpaca fibre (courtesy of BTTG Ltd).

which they may be mixed. The cuticular scale pattern of coarse mohair is illustrated in Fig. 3.29. Cashmere originates from the cashmere goat, Capra hircus laniger; the outstanding characteristic of the very fine cashmere fibres is that the majority of scale margins are distant, as illustrated in Figs 3.30 and 3.31. This characteristic of distant smooth margined scales, together with the even fibre outline and thickness makes them easily recoginisable by the experienced examiner. In contrast, very coarse cashmere fibres in the basal half of the hair exhibit irregular waved mosaic with near and crenate rippled margins as seen in Fig. 3.32. Like cashmere only the soft underhair (or underwool) or down hair of the camel Camelus bactrianas is used in the production of yarn. For the

56

Identification of textile fibers

Fibres of fine to medium thickness Outline of fibre section almost circular, outline of medulla section almost circular and relatively narrow. Type A

Medium to coarse fibres (first type) (45–50m diameter) Outline of fibre section approaching circularity, outline of medulla section almost circular and relatively narrow. Type B

Medium to coarse fibres (second type) Outline of fibre section ovoid, medulla elongated in section and in a direction along the major axis of the section. Type C

Rather coarse fibres Outline of fibre section ovoid to angular. Medulla section characteristically dumb-bell shaped in outline. This type is frequently seen in the coarser grades of brown, white and black alpaca. Type D

Coarse to very coarse fibres A coarse fibre whose fibre section outline is approximately triangular, but with two of the sides almost equal to each other in length, i.e. almost the shape of an isosceles triangle. Found in samples of white alpaca. Medulla section appoximately T-shaped. Type E

3.25 Excerpt from ‘The Microscopy of Animal Textile Fibres’1 showing the various cross-sectional shapes exhibited by alpaca fibres (courtesy of BTTG Ltd).

Natural animal textile fibres

57

3.26 Photomicrographs showing the cross-sectional shapes exhibited by alpaca fibres (courtesy of BTTG Ltd).

camel, the colour of the hairs collected or harvested range from reddish to light brown with variants from brown to grey (white hairs may occur but these are extremely rare). The camel fibres possess certain features which help in their identification. With the use of low power microscopy camel hairs, unlike wool fibres, are seen to be very regular in outline or profile and to exhibit a uniform diameter along their lengths; the cuticular scale edges project so very slightly from the hair shaft that its profile appears

58

Identification of textile fibers

3.27 Scale pattern characteristic of rabbit fibres (courtesy of Dr Hans Brunner).

3.28 Photomicrograph depicting cross-sectional shapes exhibited by rabbit fibres (courtesy of BTTG Ltd).

Natural animal textile fibres

59

3.29 Photomicrographs depicting scale patterns present on mohair fibres (courtesy of BTTG Ltd).

almost a straight line (see Figs 3.33 and 3.34). This particular feature is extremely useful in distinguishing camel fibres from wool fibres with which they may be mixed or blended. According to Wildman1 the medulla of the coarse camel hair fibre is of the unbroken type and is quite narrow in relation to the diameter of the fibre. This tendency to have a relatively narrow medulla is characteristic of the most coarse camel fibres and is a useful characteristic for identification.

3.5.2 Non-keratin silk fibres In contrast to the animal hair fibres, the identification of silk is generally easy to achieve. Degummed silk filaments are smooth-surfaced and semi transparent. Kushal and Murugesh5 found that the mulberry and nonmulberry silks exhibit an entirely different cross- and longitudinal sectional shapes and varieties which are illustrated in Figs 3.35, 3.36 and 3.37; the mulberry silks show a more or less triangular cross-section and a smooth surface, which markedly differ from the the non-mulberry (wild silk) varieties. Until recent times, cultivated silk was easily distinguished from all other fibres by its narrow diameter, but the advent of microfibres has changed this. The identification of silk must now be approached with care as nylon

60

Identification of textile fibers

3.30 Photomicrograph showing the smooth distant scale margins present on cashmere (goat) fibres (courtesy of BTTG Ltd).

Natural animal textile fibres

61

10 μm

3.31 SEM image of the scale pattern of a cashmere fibre (courtesy of CSIRO Textile and Fibre Technology).

microfibres and silk can be confused because of the similarities in their diameters and their infrared spectra. Silk, however, is normally less regular in appearance along its length than a microfibre. An easy way to view this irregularity is between crossed polars using the interference colours in the same way that one views a topographic map. The most definitive difference, if difficulties are encountered, is to place a short segment or cross-section from the fibre in question in the hot stage. Nylon will melt while silk will not. Colour is the principal point of comparison once it has been established with certainty that the fibre is silk and what type of silk it is. Fluorescence microscopy may provide additional features based on any fluorescence of the dyes.

3.6

Future trends

The most significant breakthrough in wool technology since the development of shrink-resistant technology in the 1960s is touted to be the OptimTM fibres created by the Textile and Fibre Technology Division of the Commonwealth Industrial Research Organisation (CSIRO) in Australia. Woollen fibres are stretched and then set, these ‘parent’ woollen fibres are then processed which causes them to rearrange themselves, resulting in fibres which possess a structure akin to that found in silk fibres. A wool fibre with a diameter of 19 microns will be reduced to a fibre with a diameter of

62

Identification of textile fibers

3.32 Photomicrograph showing the scale pattern on a coarse cashmere fibre (courtesy of BTTG Ltd).

15–16 microns. OptimTM fine is smoother than wools as in the stretching process become elongated along the wool fibre which gives the fibre a smoother appearance and feel; the cross-sectional also changes from round to oval found in untreated wools to triangular which resemble those of silk fibres as illustrated in Figs 3.38 and 3.39.

3.7

Sources of further information and advice

Australia and New Zealand represent the major contributors of wool used in the textile industry. The following internet sites may be accessed for further

Natural animal textile fibres

63

3.33 Photomicrographs showing the cuticular scale arrangement on a camel fibre (courtesy of BTTG Ltd).

10 μm

3.34 SEM image of a camel fibre (courtesy of CSIRO Textile and Fibre Technology).

64

Identification of textile fibers

3.35 SEM images showing the various longitudinal scale patterns exhibited by mulberry silk fibre and wild silk fibres (courtesy of Wiley Publishers).

3.36 SEM image showing cross-sectional shapes of mulberry silk fibres (courtesy of Wiley Publishers).

Natural animal textile fibres

65

3.37 SEM image showing the cross-sectional appearance of wild silk (tussah/tasar) fibres (courtesy of Wiley Publishers).

information: CSIRO at http://www.csiro.au and the Wool Research Organisation of New Zealand at http://www.woolresearch.com. There are several publications which deal with the identification of animal fibres. The publication produced by Wildman1 is a very comprehensive and significant reference guide solely in relation to animal textile fibres featuring numerous photographs depicting the various morphological characertistics exhibited by many animal hairs used in the textile industry in the 1950s and describes various methodologies employed for their examination; despite being published over 50 years ago the principles of the examination of animal textile fibres and the basis of their identification apply today. In 1978 H.M. Appleyard9 produced a ‘Guide to the Identification of Animal Fibres’ which was a concise version of ‘The Microscopy of Animal Textile Fibres’ by A.B. Wildman;8 the purpose of the publication being to act more as a laboratory manual to assist practitioners who may not require all the details given in ‘The Microscopy of Animal Textile Fibres’ in relation to laboratory techniques. The book describes the morphological features exhibited by 49 different animal species, which includes those described by Wildman.

3.8

Acknowledgements

I am indebted to the following people for their invaluable assistance: Ms Tahnee Dewhurst (VPFSC document section) for her patience in the production of images, Ms Tracey Archer (VPFSC Librarian) for her tenacity and skills in obtaining reprints, my dear friend Dr Hans Brunner for his

66

Identification of textile fibers

Merino wool

36 μm

(a)

Optim wool

36 μm

(b) 3.38 SEM images of the cross-sectional shapes of unprocessed wool fibres (a) and the processed OptimTM wool fibres (b) (courtesy of CSIRO Textile and Fibre Technology).

Natural animal textile fibres

Silk

67

18 μm

3.39 SEM image of the cross-sectional shapes of mulberry silk fibres (courtesy of CSIRO Textile and Fibre Technology).

‘carte-blanche’ approach to my republication requests. I would also like to thank Ms Heather Forward and Margaret Pate (CSIRO-Textile and Fibre Technology Division, Melbourne), Lyndon Arnold (RMIT Melbourne) and Dr Matthew Fleet (SARDI, South Australia) who were generous in devoting their time and efforts in assisting a person whom they had never met.

3.9

References

1. Wildman A B (1954), The Microscopy of Animal Textile Fibres, Leeds, Wool Industries Research Association (WIRA). 2. Harding H and Rogers G (1999), ‘Physiology and Growth of Human Hair’, in Robertson J (Ed), Forensic Examination of Hair, London, Taylor and Francis, 6. 3. Needles H L (1986), Textile Fibres, Dyes, Finishes, and Processes A Concise Guide, New Jersey, Noyes. 4. Cook J G (1984), Handbook of Textile Fibres Natural Fibres, Durham, Merrow. 5. Kushal S and Murugesh B K (2004),‘Studies on Indian Silk. I. Macrocharacterization and Analysis of Amino Acid Composition’, Journal of Applied Polymer Science, 92, 1080–1097. 6. Brunner H and Coman K (1974), The Identification of Mammalian Hairs, Melbourne, Inkata. 7. Petrie O J (1995), ‘Harvesting of textile animal fibres’. Food and Agriculture Organisation (FAO) of the United Nations, Bulletin 122. 8. Seta S, Sato H and Miyuke B (1988), ‘Forensic Hair Investigation’, Forensic Science in Progress, 2. 9. Appleyard H M (1978), Guide to the identification of Animal Fibres, Leeds, Wool Industries Research Association (WIRA).

4 Synthetic textile fibers: structure, characteristics and identification K KAJIWARA, Otsuma Women’s University, Japan and Y OHTA, Toyobo Co. Ltd, Japan

Abstract: Various types of conventional synthetic fibers are reviewed in terms of their structure, mechanical and physical properties. The identification method for each fiber is also briefly described for practical convenience. Key words: synthetic fibers, chemical fibers, polyester fiber, polyamide fiber, polyacryl fiber.

4.1

Introduction

By 1930, most scientists were convinced that polymers were, in fact, covalently linked macromolecules. Although a polymer has a high molecular weight, its primary structure is not particular complicated and is composed of multiple simple units (monomers). Synthetic polymers are widely used in fibers and plastics, which can be seem in clothes, molded parts and tyres, and nowadays we cannot imagine our life without them. However, originally the polymer industry utilized mainly natural polymeric materials including wood, leather, resins, fibers and rubber. Polymeric materials support our food, clothing and housing needs, and are indispensable even in advanced technology as demonstrated by optical fiber, carbon fiber, polymer battery, artificial kidney, and ion-exchange membrane. The polymers used for non-structural materials are often referred to as functional polymers. The synthetic polymer industry was already in existence in the early 1930s when nylon was invented. The production of nylon 66 began in 1938 (Carothers, Du Pont) and nylon 6 in 1942 (Schlack, IG, Germany). The chemical formulae are written as: H − [ HN ( CH 2 )m ⋅ NHCO (CH 2 )n CO]x − OH (nylon m, n + 2 ) or, H − [ HN ( CH 2 )n CO]x − OH (nylon n + 1) Nylon is melt-spun to make a filament which is said to be thinner than a spider’s web but stronger than steel. Polyvinylalcohol (PVA) was first synthesized in 1924 in Germany, but its industrial production had to wait until 68

Synthetic textile fibers

69

a PVA that was stable in boiling water was developed in 1950 by Sakurada using thermal treatment and partial formylation. Whinfiled and Dickson developed polyester fiber in 1940. Although Carothers was the first to attempt to synthesize polyester using various combinations of polyol and aliphatic polybasic acid, he failed to achieve a high melting point and switched his strategy to the combination of polyol and polyamine. Whinfield and Dickson (Calico Printers) employed aromatic polybasic acid (terephtharic acid = 1,4-diformylbenzene) to achieve a high melting temperature. The commercial production of polyester fiber started in 1948 in the USA (Du Pont) and in 1952 in the UK (ICI). Nowadays, polyester is the most widely produced synthetic fiber. Acrylic (PAN) fiber is made from the copolymer of the main component acrylonitrile (CH2=CHCN over 50%) with the addition of acrylic acid ester (CH2=CH ⋅ COOR), vinyl acetate (CH2=CHOCOCH3), acrylamide (CH2=CH-CONH2), and methacrylic acid ester (CH2=C(CH3)COOR) to improve dyeability, solubility and transparency. PAN fiber is suitable for knitting and blanket material, and is a precursor for carbon fiber, which is produced in two steps: PAN fiber is heat-treated with air at 250°C, and then carbonized into graphite at a higher temperature without oxygen. These are the three major synthetic fibers, in contrast to the three major natural fibers: cellulose, wool and silk. The fiber-forming polymers possess common molecular characteristics such as a large intermolecular interaction through polar groups, a symmetric structure for high crystallinity, an appropriate range of melting temperatures and glass transition temperatures, a high molecular weight and a good drawability. Table 4.1 compares the performance of the most important fibers. Here the tensile strength denotes the weight (g) to break 1 d fiber. Denier (d) corresponds to the weight (g) of a fiber that is 9000 m in length. A silk filament is about 1 denier. The SI unit (Le Système International d’Unités, the basic units of which are summarized in Table 4.1) for tensile strength and modulus is given by GPa (1 GPa = 1010 dyne/cm3 = 1.0197 × 104 kg/cm2), which is related to g/d as: g d=

11.3 × GPa density ( g cm 3 )

The tensile strength required for apparel applications is at most a few g/d. For example, a conventional nylon is 5 g/d, while steel wire is 3.5 g/d. However, the strength per cross-section is given as 50 kg/mm2 for nylon and 248 kg/mm2 of steel because of the density difference. The elongation is defined as Elongation (%) =

Δ × 100 

3.8∼5.3

2.6∼4.4 3.7∼4.4

2.2∼4.4

4.2∼5.7

4.4∼5.7

3.5∼5.7

Polyester (PET)

Polyester (PBT) Polyester (PTT)

Polyacrylonitrile

Nylon 6

Nylon 66

Vinylon (PVA)

2.8∼4.6

4.0∼5.3

3.7∼5.2

1.8∼4.0

2.6∼4.4 3.7∼4.4

3.8∼5.3

1.14

1.26∼1.30

12∼26

1.14

1.14∼1.17

1.31 1.34

1.38

Specific gravity

25∼38

28∼45

25∼50

20∼40 20∼40

20∼32

dry

dry

wet

Elongation (%)

Tensile strength (cN/dtex)

Table 4.1 Fiber performance

53∼79

26∼46

18∼40

34∼75

18∼35 23

79∼141

Young’s modulus (cN/dtex)

5.0

4.5

4.5

2.0

0.4 0.4

0.4

Moisture regain (%)

Hot pyridine, phenol, cresol, conc. formic acid

Phenol, mcresol, conc. formic acid

N,N′-dimethylformamide Dimethylsulfoxide Phenols

m-cresol (hot), o-chlorophenol (hot), nitro benzene (hot), dimethyleforamide (hot)

Specific solvent

Vat dye, metal complex dye, sulphur dye, direct dye, pigment

Cationic dye, disperse dye Acid dye, metal complex dye, disperse dye, reactive dye Acid dye, metal complex dye, disperse dye, reactive dye

Disperse dye, cationic dye for basicdyeable polyester Disperse dye, pigment

Dyeing agent

DuPont (Nylon®) <1939> Nihon Gosei Sen-I <1946>

I.G. (PerlonL®) <1942>

Teijin <1979> Asahi Chemical (Solotex®) <1999> DuPont (Orlon®) <1950>

ICI (Terylene®) <1955>

Company (trade name)

4.0∼6.6

4.4∼7.9

0.5∼1.1

1.5∼2.0

1.1∼1.2

2.6∼3.5

0.9∼1.5

2.6∼4.3

Polypropylene

Polyethylene

Polyurethane

Viscose rayon

Triacetate

Silk

Wool

Cotton

2.9∼5.6

0.7∼1.4

1.9∼2.5

0.6∼0.8

0.7∼1.1

0.5∼1.1

4.4∼7.9

4.0∼6.6

3∼7

25∼35

15∼25

25∼35

18∼24

450∼800

8∼35

30∼60

1.54

1.32

1.33

1.30

1.50∼1.52

1.0∼1.3

0.94∼0.96

0.91

60∼82

10∼22

44∼88

26∼40

57∼75

35∼106

8.5

15

11.0

3.5

11.0

1.0

0

0

Copper ammonium

Methylene chloride, glacial acetic acid Copper ammonium

Tetra chloroethane, carbon tetrachloride, cyclohexane, monochloro benzene, xylene, tetralin, toluene Pyridine (hot), phenol, phenols Copper ammonium, copper ethylene diamine

Reactive dye, direct dye, vat dye, naphthol dye, sulphur dye Acid dye, metal complex dye, reactive dye Reactive dye, direct dye, vat dye, naphthol dye

Reactive dye, direct dye, vat dye, naphtol dye, sulphur dye, pigment Disperse dye, acid dye

Pigment

Pigment, disperse dye

I.G. (PerlonU®) <1941>

Montecatini <1959> Courtaulds (Courlene®) <1952>

72

Identification of textile fibers

Δ艎 corresponds to the elongated length. Most fibers have an elongation of over 10%. Young’s modulus (E) is an intrinsic parameter of the materials, defined as the ratio of the stress per unit cross-section (S) to the strain (ε). E=S ε A high Young’s modulus indicates a hard material.

4.2

Fundamental characteristics of fibrous materials

Fiber is a general term for materials which are characterized by a long and thin shape. Fibrous materials may be organic, inorganic or metal, but should possess a small cross-sectional diameter in comparison with the length. Because of this characteristic, fibrous materials possess flexibility as well as high strength. Fibrous materials are a fundamental part of our lives, not only as the materials for clothes to protect us from the environment, but also as part of our bodies, which are made of fibrous materials that sustain flexibility and strength. Commercially available fibrous materials can be classified into two categories: natural fibers and chemical fibers. Natural fibers include plant fibers (such as cotton, hemp and pineapple fiber), animal fibers (such as wool, mohair and silk), and mineral fibers (such as asbestos). Chemical fibers include regenerated fibers (such as rayon), semi-synthetic fibers (such as acetate) and synthetic fibers (such as the organic fibers of nylon, polyester and acrylonitrile, and the inorganic ones of glass fiber, metal fiber and carbon fiber). The commercial success of nylon ignited the development of synthetic fibers. In the early days of the polymer industry, most R&D effort was targeted at finding new synthetic fibers. Tables 4.2 and 4.3 summarize the molecular structure and thermal properties of polymers for fibrous materials. New polymers were created, and now polymers are applied in various fields such as fibers, plastics and elastomers. The application of fibrous materials is not limited to apparel. Non-apparel (industrial) use of fibers can be found in ropes, fishing nets and composites. Industrial synthetic fibers include (i) the fibers that were first developed for apparel but were unsuitable and so were converted for industrial use (e.g., PVA, PP), (ii) apparel fibers applied to industrial applications (PET, nylon, PAN), and (iii) the fibers specially developed for industrial use (hightenacity/high-modulus fibers). The performance of fibers depends heavily on spinning, drawing and further processing. For example, polyester for apparel should possess easy dyeability and a good hand, while polyester for tyre cords requires a high tenacity/modulus, toughness for repeated deformation and thermal stability. The fiber production technology has been

Synthetic textile fibers

73

Table 4.2 Molecular structure of polymers for fibrous materials Polymer [Polyolefin/vinyl polymers] Polyethylene (PE) Polytetrafluoroethylene (PTEE) Polypropylene (PP)

Molecular structure

CH2 -CH2 n CF2 -CF 2 n CH2 -CH n CH3

Polyvinyl alcohol (PVA)

CH2 -CH n OH

Polyvinyl chloride (PVC)

CH2 -CH n Cl

Polyacrylonitrile (PAN)

CH2 -CH n CN

Poly-4-methyl-1pentene (P4M1P)

CH2 -CH CH2

n

CH CH3 CH3

[Nylon/aliphatic polyamide] Nylon 6

=

(CH2)5 -C-NH O

Nylon 66 Nylon 46

NH

O

C

O

O

NH- C

C

O

=

=

NH- C

n

=

NH

O

=

NH

=

NH-(CH2)6-NH-C-(CH2)4-C =

O

O

n

PPTA

C =

NH- C

n

=

[Aramid/ aromatic polyamide] Poly-mphenyleneiso-phthalamide (PMIA) Poly-p-phenylene terephthalamide (Kevlar; PPTA) Aramid copolymer (Technora)

n

O

O

n

n n=0.5

74

Identification of textile fibers

Table 4.2 Continued Polymer

=

=

O

O

=

O-(CH2)3 -O- C

C =

O

O

O-(CH2)4 -O- C

C

C n

n

=

= O

n

O O

=

Poly-ε-caprolactone

O- (CH2)2 -O- C

O

-C-O-(CH2)2-O

=

[Polyester] Polyethylene terephthalate (PET) Polytrimethylene terephthalate (PPT) Polybutylene terephthalate (PBT) Polyethylene naphthalate (PEN)

Molecular structure

C

n

=

O -(CH2)5-C

n

O

Polylactic acid

CH3

O

=

O

C

C n

H

[Polyarylate (aromatic polyester)] Vectran(X:Y=7:3)

CH3

O

=

Poly([R]-3hydroxybutyrate); P(3HB)

O

C

CH2

C n

H

O

O

C =

C =

O

O

X

O

C

O

= O

[Other heterocyclic polymer] Poly-p-phenylene benz-bisoxazole (PBO) Poly-p-phenylene benz-bisthiazole (PBT) Poly(benz imidazole)(PBI) Poly(phenylene sulfide)(PPS)

O

O

N

N

X

Y

n

N

S

S

N

n

NH

H N

N

N

S n

n

C

C =

O

Y

=

Econol

O

O

Z

Synthetic textile fibers

75

Table 4.2 Continued Polymer

Molecular structure

Polysulfone (PSF)

CH3 SO2

O

C

O n

CH3

Poly(ether sulfone)(PES)

SO2

O n

Poly(ether ether ketone)(PEEK)

O

O

C =

n

O

[Polymide] Polymide (P84; PI)

CH3 CO

N CO

CO

Ar

Ar : n

CH2 O C

=

=

O C

C

C O

N

O

N

[Cellulose] Cellulose

n

=

= O

[Amorphous polymer] Polymethyl methacrylate (PMMA)

CO

N

or

Poly(promellitic imide) (Capton; PPI)

CO

H

OH

OH H H H O CH2OH

H

H O

CH2OH O H OH H H

O H n

OH

CH3 CH2

C C= O O CH3

Polycarbonate (PC) O

n CH3 C

O

C =

CH3

O

n

developed to a high degree of sophistication, controlling the fiber and fabric characteristics to match the specified design requirements. A single filament is composed of fibrils (an assembly of polymers), and a basic unit of fibrils is known as a microfibril, which is dimensionally similar for all fibers as shown for three natural fibers (Table 4.4). Synthetic fibers also possess a similar structural element. Figure 4.1 shows the high-order structural model of a synthetic fiber proposed by Peterlin.1 Here the

76

Identification of textile fibers

Table 4.3 Thermal properties of polymers for fibrous materials Polymer

Tg (°C)

Tm (°C)

T 0m (°C)*

Heat of fusion (cal/g)

PE PTFE PP (isotactic) PVA PVC PAN P4MP1 POM Nylon 6 Nylon 66 Nylon 46 PMIA PPTA

−36 −73 −3 85 −19 105 29 −82 40 76 82 (R.H. 0%) 270∼280 260∼270

125∼135 330 165∼173 Not known 200∼240 Not known 230∼240 179 215∼220 250∼260 290∼319 370 No melting point (560)** No melting point (>500)** 255∼265 272 60 270 (transition to liquid crystal) No melting point (630∼650)** No melting point (630∼650)** 280∼290 (>440)** 334∼345 373 No melting point (459)**

141.1 332 187.5 265 272.8 – – – 260 301

70.0 19.6 49.4 38.6 42.1 – 33.7 45∼80 54.9 61.0

280 337 –

33.5 24.5 – 4.8

Technora PET PEN PCL Vectran

69∼81 115 −60 150

PBO

(300∼350)

PBT

(300∼350)

PPS PEEK PES PBI

85∼90 143 162 –

* Equilibrium melting point temperature, which corresponds to the melting temperature of an ideal crystal free from the surface energy. ** The value in brackets denotes the temperature at thermal degradation.

microfibrils of ca. 10–20 nm in diameter combine in parallel to form fibrils of 0.1 μm in diameter, and then the fibrils combine into a filament. Hess and Kiessing2 (Fig. 4.2) proposed the fringed micelle model to explain schematically the structure of microfibrils of synthetic fibers. Here the microfibril is composed of a crystalline region (where polymer chains are oriented in the fiber axis direction) and an amorphous region (where polymer chains are less oriented), and a single polymer chain penetrates through the two regions. However, polymer chains are found to fold to

Synthetic textile fibers Table 4.4 Wide-angle X-ray diffraction pattern from various fibers Fiber

Crystallographic data

Cotton (Cellulose I)

Monoclinic a = 0.835 nm, b = 1.034 nm, c = 0.79 nm a = 90°, b = 84°, g = 90° rc = 1.592 g/cm3 (Z) 1 (101), 2 (101¯), 3 (002), 4 (021)

Silk

Viscose rayon (Cellulose II)

Polyethylene

Polypropylene

Polyacrylonitrile (PAN)

Monoclinic a = 0.965 nm, b = 1.040 nm, c = 0.695 nm a = 90°, b = 90°, g = 62.40° rc = 1.45 g/cm3 1 (100), 2 (002), 3 (201), 4 (112) Monoclinic a = 0.814 nm, b = 1.034 nm, c = 0.914 nm a = 90°, b = 62°, g = 90° rc = 1.583 g/cm3 (Z) 1 (101), 2 (101¯), 3 (002), 4 (021)

X-ray diffraction pattern 4 1 2

4 2

1

3

4 1 2 3

Orthorhombic a = 0.740 nm, b = 0.493 nm, c = 0.253 nm a = 90°, b = 90°, g = 90° rc = 1.00 g/cm3 (Z) 1 (110), 2 (200), 3 (210), 4 (020), 5 (310), 6 (011), 7 (111), 8 (201), 9 (211)

Monoclinic a = 0.665 nm, b = 2.096 nm, c = 0.650 nm a = 90°, b = 99°20′, g = 90° rc = 0.936 g/cm3, ra = 0.85 g/cm3 (H) 1 (110), 2 (040), 3 (130), −− 4 (131) (041), 5 (022) (112) Orthorhombic a = 2.10 nm, b = 1.19 nm, c = 0.504 nm a = 90°, b = 90°, g = 90° rc = 1.592 g/cm3 (Z) 1 (400) (200), 2 (620), 3 (040)

3

6 7

8 9

1 23 4 5

5 4 1 23

1

23

77

78

Identification of textile fibers

Table 4.4 Continued Fiber

Crystallographic data

Polyvinylalcohol (PVA)

Monoclinic a = 0.718 nm, b = 0.252 nm, c = 0.551 nm a = 90°, b = 91°42′, g = 90° rc = 1.35 g/cm3, ra = 1.29 g/cm3 (Z) − 1 (100), 2 (001), 3 (101), 4 (101), 5 (200), 6 (301), 7 (202), 8 (110), 9 (111) − (111)

Nylon 6 (α type)

Nylon 66

Poly-L-lactic acid (PLA)

Carbon fiber

X-ray diffraction pattern 8

45

1 23

3 12

Orthorhombic a = 1.06 nm, b = 0.61 nm, c = 2.88 nm a = 90°, b = 90°, g = 90° rc = 1.592 g/cm3 (H) 1 (011), 2 (110) (200), 3 (211), 4 (120), 5 (103), 6 (014), 7 (113), 8 (016), 9 (116), 10 (216)

Hexagonal a = 0.2462 nm, b = 0.2462 nm, c = 0.6707 nm a = 90°, b = 90°, g = 120° 1 (002), 2 (100)

67

1 23

Monoclinic a = 0.956 nm, b = 1.724 nm, c = 0.801 nm a = 90°, b = 67°30′, g = 90° rc = 123 g/cm3 (Z) 1 (200), 2 (002), 3 (202) Triclinic a = 0.49 nm, b = 0.54 nm, c = 1.72 nm a = 48°30′, b = 77°, g = 63°30′ rc = 1.24 g/cm3, ra = 1.09 g/cm3 (Z) 1 (100), 2 (010) (10), 3 (002)

9

8 5

6 7 1

2

1

9 10

2

3

4

Synthetic textile fibers

79

Table 4.4 Continued Fiber

Crystallographic data

Poly(ethylene terephthalate) (PET)

Triclinic a = 4.56 nm, b = 5.94 nm, c = 10.75 nm a = 98°30′, b = 118°, g = 112° rc = 1.455 g/cm3, ra = 1.335 g/cm3 (Z) − 1 (010), 2 (110), 3 (100), − − 4 (011), 5 (111), 6 (011), − − − 7 (111), 8 (112), 9 (103), − 10 (013), 11 (003)

Poly(trimethylene terephthalate) (PTT, 3GT)

Triclinic a = 0.459 nm, b = 0.621 nm, c = 1.831 nm a = 98°, b = 90°, g = 111.7° rc = 1.428 g/cm3, ra = 1.314 g/cm3 (H) − 1 (002), 2 (010), 3 (012), 4 (012), 5 (101), 6 (102) − − − (112), 7 (113), 8 (113), − − 9 (104), 10 (114)

X-ray diffraction pattern 11 9 10 8 6

5

4

7 3

1 2

3

1

4 6 5

2

8

7

9 10

Poly(butylene terephthalate) (PBT, 4GT)

Poly(ethylene naphthalate) (PEN)

Triclinic a = 0.483 nm, b = 0.59 nm, c = 1.159 nm a = 99.7°, b = 15.2°, g = 110.8° rc = 1.396 g/cm3, ra = 1.281 g/cm3 − 1 (010), 2 (110), 3 (100), − − − 4 (120), 5 (011), 6 (101), − 7 (011), 8 (111), 9 (101), 10 (001) Triclinic a = 0.651 nm, b = 0.597 nm, c = 1.32 nm a = 81.33°, b = 144°, g = 100° rc = 1.407 g/cm3, ra = 1.328 g/cm3 − 1 (010), 2 (100), 3 (110)

10 11

56 1

1 2 3

789 23 4

80

Identification of textile fibers Fibril

Fibril (~0.1 mm)

Fibril end (defect) Microfibril (~10 nm)

4.1 High-order structural model of a synthetic fiber.

L

4.2 Fringed micelle model.

L

Synthetic textile fibers

81

(c)

(d)

(a)

(b)

(a) Amorphous

(b) Folded crystal

(c) Extended chain crystal

(d) Fringed micelle

4.3 Various phases of polymer chains.

ca. 10 nm in length in a single crystal of polyethylene,3 and the folded crystal structure is now considered to be the basic structure of a crystalline polymer rather than the fringed micelle. The condensed phase of polymer is composed of folded chains as illustrated in Fig. 4.3.4 Drawing is an essential process for fiber formation. Folded microfibrils contain tie molecules linking the folded crystal (indicated by arrow A in Fig. 4.4); the tie molecules designated as B in Fig. 4.4 connect the folded microfibrils. When the fiber is drawn, the folded parts deform and align in the drawing direction (schematically shown in Fig. 4.5). The polymer chains tying the crystal regions form the amorphous phase with the chain ends. The mesophase (the intermediate phase between the crystalline and amorphous phases) may exist, but no definite proof can be given. As yet, no synthetic fiber possesses as highly organized and complex a structures as that of natural fibers. The fibrils of natural fibers have a more sophisticated structure, as illustrated in Figs 4.6, 4.7 and 4.8, which show images of the microfibril structures of cotton, wool and silk respectively. Cotton fiber is made of cellulose and has a hollow centre (lumen). A single filament is twisted 80 to 120 times. The thin primary cell membrane (ca. 0.1 μm in thickness) covers the secondary cell membrane (ca. 4 μm in thickness), which has a hollow centre (lumen) for transferring water and nutritious substances (Fig. 4.6). A cotton fiber is naturally twisted, and therefore is elastic and has a good texture. The air trapped in the lumen functions as an insulator. Hemp is also a cellulose fiber, but its surface

82

Identification of textile fibers Microfibril

B

B

A

A

A

4.4 Folded microfibrils.

A

L A A

A

L1 L Before drawing

Natural drawing

4.5 Schematic representation of drawing.

structure is different from that of cotton. The surface of hemp fiber is composed of fibrils arranged in parallel, so that the fiber is hard and straight (not twisted). Wool is an animal fiber (protein) produced in cells. The main protein component is keratin, and the fiber has a sophisticated high-order structure as shown in Fig. 4.7. The hydrophilic main fiber component (cortex) is

Synthetic textile fibers

83

Secondary cell membrane (thickness 4 μm)

Primary cell membrane (0.1 μm)

Lumen Outer skin

Winding (thickness ca. 1 μm)

Cellulose net

4.6 Structure of cotton fiber. Epi-cuticle Exo-cuticle Low-s protein

Nucleic residue

Endo-cuticle Cuticle

High-s protein

Left-handed coil-coil bundle Right-handed α-helix

Amorphous matrix

Cell membrane complex

Microfibril (0.3 μm in diameter)

Microfibril (80 Å in diameter)

Cortex

Cuticle Vapor Vapor Water droplet

Ortho Para Cortex

Water droplet

Vapor

4.7 Structure of wool fiber.

84

Identification of textile fibers

(100 Å) (1 μm) (ca. l d)

Average 2.8 d

4.8 Structure of silk fiber.

covered with a hydrophobic cuticle (scales). Since the cortex has a bilateral structure, where the ortho-cortex and para-cortex are stacked together, wool is crimped. Raw silk has a characteristic structure composed of two types of protein: fibroin and sericin (Fig. 4.8). Sericin glues two fibroin filaments together, but dissolves in hot water. Silk is customarily used as an example of a fibroin filament, and its characteristic lustre and scroop are the result of a particular triangular cross-section with a lateral slit.

4.3

Common synthetic fibers

4.3.1 Nylon Nylon is a common name for aliphatic polyamide, and is classified according to its number of constituent aliphatic carbons. However, nylon is conventionally referred to as polyamide fiber. Nylon 66 and Nylon 6 are the most common nylons, which are produced by polycondensation of diamine (hexamethylene diamine) and dicarbooxylic acid (adipic acid), and by polycondensation of ω-aminocarboxylic acid or ring-opening polymerization of lactam (ε-caprolactam), respectively. Nylon 66 and Nylon 6 possess similar characteristics except for their melting points (see Table 4.3), and are used for both apparel and industrial (non-apparel) products because of their lightness, appropriate elasticity and elastic recovery, good heat set property and dyeability. Other commercially available nylons include Nylon 610 (poly(hexamethylene sebacamide), mp 210°C), Nylon 11 (poly(11-aminoundecanoic

Synthetic textile fibers

85

acid), mp 186°C) and Nylon 12 (poly(12-aminododecanoic acid lactam), mp 177°C), but these nylons are used mainly for industrial applications. Nylon 4 (poly-2-pyrolidone) has similar moisture-absorbing characteristics to cotton, but is not yet used commercially because of its severe water-free polymerization condition.

4.3.2 Polyester Polyethylene terephthalate (PET), which is the most successful synthetic fiber, is composed of ester links of aliphatic (ethylene diol) and aromatic (terephthalic acid) groups. The aromatic group is rigid and is considered as a long virtual chemical bond, so that the extended chain assumes a large zigzag form, resulting in a lower intrinsic Young’s modulus (ca. 110 GPa) of the crystalline region in the molecular axis direction in comparison with the more extended polyethylene chain (ca. 280 GPa).5 When the aliphatic carbon number increases, the 3D structure of molecular chains then changes and the Young’s modulus decreases. Polytrimethylene terephthalate (PTT or 3GT) (see Table 4.2) and polytetramethylene terephthalate (PBT or 4GT) possess 3 methylene and 4 methylene groups, respectively. PTT and PBT exhibit a slightly lower Young’s modulus (23 cN/dtex) than Nylon 66. Polytrimethylene terephthalate (PTT) fiber is produced by melt-spinning of the polycondensate of terephthalic acid and 1,3-propane diol. PTT has excellent stretching characteristics and its elastic recovery (88% at 20% elongation) is one of the best among synthetic fibers. Aliphatic polyester fiber is in general biodegradable, although its melting point is low and its application is limited. Polylactic acid (PLA) is produced by melt-spinning the polycondensate of lactic acid, obtained by fermentation of corn starch. Another example of aliphatic polyester is poly(3hydroxybutyrate) (P(3HB)) produced by marine bacteria. The melting point of these aliphatic polyesters is low, between 170 and 175°C. The tensile strength of polylactic acid fiber could be as high as 4–5 cN/dtex, the Young’s modulus is about half that of PET and the glass transition temperature is 57°C, while the tensile strength of P(3HB) can reach 10 cN/dtex but its glass transition temperature is lower (4°C).

4.3.3 Polyacrylonitrile Polyacrylonitrile is atactic, so that large cyanic groups stick out randomly from the main chain and prevent it from having a close packing density. Thus the crystallinity of polyacrylonitrile fiber is low. Since the dyeability of pure polyacrylonitrile fiber is poor, methyl acrylic acid or methyl methacrylic acid is copolymerized into acrylonitrile in commercial fibers to improve it.

86

Identification of textile fibers

4.3.4 Polyolefin Polyethylene and polypropylene are two major polyolefin fibers that are available on the market. Polyethylene has a simple chemical structure consisting only of methyl groups. The most stable conformation is an all-trans planar zigzag structure, and its intrinsic Young’s modulus in the molecular axis direction is calculated as 280 GPa. However, this simple chemical structure results in thermal internal rotation around the C—C bond, even at lower temperatures, which disturbs the all-trans planar zigzag conformation, so that the polyethylene fiber is soft and has a low Young’s modulus. If the all-trans planar zigzag conformation could be maintained, the thermal conductance in the chain axis direction would be as good as that of a diamond.6 Polypropylene has a methyl side group on each ethylene unit. Methyl side groups may attach on the same side (isotactic) or the opposite side (syndiotactic) of the main backbone chain when it is stretched on the plane parallel to ethylene backbone units. The methyl side groups should appear on every ethylene repeat unit (head-tail structure), but in some cases the methyl groups are found on two adjacent units (head-head structure) or no methyl groups are found on two adjacent units (tail-tail structure). As a consequence, the characteristics of polypropylene fiber depend on its stereo regularity (isotactic or syndiotactic) and the head-head/tail-tail to the head-tail ratio. At present, commercial polypropylene fiber mainly consists of isotactic polypropylene. The polypropylene chain assumes a helical conformation composed of trans-gauche alternate unit arrangements due to the steric hindrance of bulky methyl side groups. Since the methyl groups attach directly to the main chain, the steric repulsion between methyl groups is strong enough to prevent the trans-gauche transformation. The helical structure of the polypropylene’s main chain behaves like a spring and suppresses the intrinsic Young’s modulus, which is much lower than that of polyethylene and nylon. The Young’s modulus of polyethylene and nylon is relatively high because of the planar zigzag main chain structure.

4.4

Crystal structure of synthetic fibers

The crystal structures of synthetic fibers are well documented. Table 4.4 summarizes some crystal structures and corresponding schematic wideangle X-ray diffraction patterns. The crystal system (monoclinic, triclinic, orthorhombic or hexagonal), the lattice constants (three axial lengths a, b and c, and three axial angles a, b and g), the density of the crystal region (rc) and the amorphous region (ra), the chain conformation (Z: a planar zigzag conformation, H: a helical conformation), and the Miller indices are

Synthetic textile fibers

87

Table 4.5 Infrared spectroscopic absorbance bands from synthetic fibers Fiber

Characteristic absorbance bands (cm−1)

PET Nylon 6 Nylon 66 Polyacrylonitrile Polyethylene Polypropylene Polyvinyl alcohol Cotton Wool Silk (fibroin) Rayon Triacetate

1730–1410 1340 1250 1120 1100 1020–870 730 3300 3050 2950 2850 1630 1530 1450 1250 680 570 3300–2950 2850 1630 1530 1470 1270 930 680 570 2950 2250 1730 1450 1360 1220 1160 1060 540 2900 2850 1470 1460 1370 740 720 2970–2940 2850 1450 1370 1160 990 970 840 3400–2950 1430–1400 1090–1050 1020 850 790 3450–3250 2900 1630 1430 1370 1100–970 550 3400–3250 2900 1720–1600 1500 1220 3300 2950 1710–1630 1530–1500 1440 1220 610 540 3450–3250 2900 1650 1430–1370 1060–970 890 3500 2950 1750 1430 1370 1230 1040 900

shown, together with the schematic diffraction pattern for each synthetic (and natural) fiber. Synthetic fibers can be identified by comparing the Xray diffraction patterns.

4.5

Identification of synthetic fibers

Synthetic fibers can be identified by a combination of various tests including microscopic observation, specific gravity measurement, infrared spectroscopic analysis, the burning test, the coloration test and the dissolving test. Table 4.5 summarizes the characteristic absorbance bands for synthetic fibers to facilitate their identification. Other characteristics of the respective fibers can be seen in Tables 4.1 to 4.4.

4.6

References

1. A. Peterlin, J. Polymer Sci., C9, 61 (1965); J. Polymer Sci., A-2, 7, 1151 (1967). 2. K. Hess, H. Kiessing, Naturwissenschaft., 31, 171 (1943); Z. Physik. Chem., A193, 196 (1944). 3. A. Keller, Phil. Mag., 1171 (1957). 4. B. Wunderlich, Ber. Bunsenges., 74, 772 (1970). 5. K. Tashiro, Prog. Polym. Sci., 18, 377 (1993). 6. A. Yamanaka, T. Kashima, Sen’I Gakkaishi, 56, 128 (2000).

5 High performance fibers: structure, characteristics and identification Y OHTA, Toyobo Co. Ltd, Japan and K KA JIWARA, Otsuma Women’s University, Japan

Abstract: Structural and mechanical properties of various high strength and high modulus fibers (HPFs) can be characterized mainly by their high tensile strength, which is at least twice that of conventional fibers. The detailed methods to identify each HPF are also described. Key words: high strength and high modulus fiber, aramid fibers, ultra high molecular weight polyethylene fiber, spinning, super fibers.

5.1

Introduction

A high strength and high modulus fiber is identified by a tensile strength of over 2 GPa, and is often referred to as a high performance fiber (HPF) or super fiber. Inorganic fibers such as carbon fiber are included in the extended definition of HPF, but this chapter will focus on HPFs based on organic polymers. Organic HPFs are produced not by conventional melt spinning, but by gel spinning and liquid crystal spinning (considered as a special type of solution spinning), originally developed for ultra-high molecular weight polyethylene (UHMW-PE) fiber and aramid fiber, respectively. These fibers have been developed for various industrial uses including civil engineering, the aerospace industry, the automobile industry and sports goods. This chapter focuses mainly on organic HPFs, and reviews their structures and physical properties.

5.2

The primary structure and physical properties of HPFs [1,2]

5.2.1 Classification of HPFs Table 5.1 and Fig. 5.1 summarize the mechanical and physical properties of organic HPFs together with their primary structure. These HPFs are commercially available or will soon appear on the market. HPFs are conventionally classified according to their chain rigidity, because the rigidity of the chain determines the spinning method and thermal properties such as 88

Flexible chain

Polyketone PVA

UHMW-PE

Polybenzazole

K2

Zylon AS Zylon HM M5 DyneemaSK60 DyneemaSK71 Spectra900 Spectra1000

1.39 1.47

Polyarylate

Technora Vectran

HPF Rigid chain

1.14 1.38 1.43 1.45 1.44 1.45 1.47 1.44 1.45

– – Kevlar 29 Kevlar 49 Kevlar 119 Kevlar 129 Kevlar 149 Twaron Twaron HM

PA6 PET p-Aramid

Conventional

1.54 1.56 1.70 0.97 0.97 0.97 0.97

(g/cm3)

Density

Fiber

Polymer

Type

37 37 23 26–32 >35 26 30 16–20 14–18

25 22

8 8 20 20 21 23 16 21 21

(cN/dtex)

5.8 5.8 3.9 2.6–3.2 >3.5 2.6 3.0

3.4 3.2

0.9 1.1 2.9 2.9 3.1 3.4 2.3 3.0 3.0

(GPa)

Strength

1150 1720 1940 880–1230 >1230 1235 1765 192–360

520 494

35 106 485 838 379 688 970 500 720

(cN/dtex)

180 270 330 88–123 >123 120 171

71 91

4 14 70 135 55 99 143 72 105

(GPa)

Modulus

3.0–5.0 3.0–5.0 3.5 2.7

3.5 2.5

4.6

10–20 10–30 3.6 2.8 4.4 3.3 1.5 3.5 2.7

(%)

Elongation

Table 5.1 Mechanical and physical properties of HPFs in comparison with those of conventional fibers

650 ↑ 530 145 ∼155 146

500 327–331

223 265 560 ↑ ↑ ↑ ↑ >500 >500

(°C)

Melting/ decomposition temp.

68 ↑ >50 18

25 28

29 29

29

(–)

LOI

90

Identification of textile fibers N

N

O

O

N H

n

Zylon®

O

O

C

C

N H

n

Kevlar®, Twaron®

O C N H

O C

N H

N H

O N C H

O

C

n

O

Technora®

H

H O

C O n

Vectran®

C

O C O m

H

H C OH

C n

Polyvinyl alcohol

H

m

H C H

n

Dyneema®, Spectra®

5.1 Chemical structures of HPFs.

heat resistance and flame resistance. UHMW-PE and aramid polymers represent HPFs made from a flexible polymer and a rigid polymer, respectively, as shown in Table 5.1. Generally, HPFs made from rigid polymers have superior heat resistance and flame resistance compared with HPFs made from flexible polymers. The details of the manufacturing process for each fiber will not be discussed, but it depends strongly on the chain rigidity. Gel spinning [3] is applied mainly to flexible chain polymers, while HPFs from rigid chain polymers are produced by liquid crystal spinning [4]. More details of the respective spinning processes can be found in the reviews [1, 2].

5.2.2 The primary structure and physical properties of HPFs made from rigid chain polymers Aramid fiber The pioneer of HPF was aramid fiber, developed in the 1970s by DuPont. Aramid is a general term applied to the polycondensates composed of aromatic dicarbooxylic acid and aromatic diamine. Numerous combinations of carboxylic acid and diamine can be used to produce aramids, including the AB-type polycondensates made from monomers containing both carboxylic acid and diamine groups in a molecule. The commercially available aramids such as Kevlar® and Twaron® are poly(p-phenylene terephthalamide) (PPTA), which is a polycondensate of terephthalic acid and p-

High performance fibers

91

Tensile modulus (GPa)

400

CF(HM)

300

Zylon-HM CF(Reg.) Zylon-AS Steel

200

Spectra1000

CF(HT)

Kevlar 149 100 Dyneema SK60 PET

Vectran Kevlar 29

0 0.0

2.0 4.0 6.0 Tensile strength (GPa)

8.0

5.2 Mechanical properties of HPFs. 80 Limiting oxygen index (LOI)

Zylon-HM 60 PPS 40

PBI PI p-Aramid m-Aramid

20 PET 0

0

200 400 600 800 Melting or decomposition temp. (°C)

5.3 Heat resistance and anti-flammability properties of HPFs.

phenylene diamine as shown in Fig. 5.1. The aramid fiber is at least twice as strong in terms of tensile strength as conventional fibers (Fig. 5.2), and is thus referred to as a super fiber. Aramid fiber also exhibits excellent creep resistance, and has better mechanical properties in terms of fatigue resistance for bending and impact resistance/absorption as it has a higher elongation at break (3–4%) than carbon fiber. These characteristic features of Kevlar® have been used in bullet-proof vests. Since Kevlar® is composed of extremely rigid molecular chains, the melting point of the crystalline part is higher than its decomposition temperature. Thus, besides its outstanding mechanical properties, Kevlar® (p-Aramid) exhibits high heat resistance (the decomposition temperature is 560°C) as shown in Fig. 5.3. It is highly

92

Identification of textile fibers

flame-resistant and is therefore applied to flame-retardant garments and insulators where high heat resistance is required. It is chemically stable and resistant to organic solvents and alkaline solutions, although it is less resistant to acid, such as sulfuric acid, which is used as a spinning solvent. Kevlar® is less resistant to UV light and other outdoor exposure than conventional polyester, and care should be taken to shield it sufficiently from the light by applying a coating or coloring to increase its useful life. Kevlar® is hygroscopic (4–6%), and may be hydrolyzed causing mechanical deterioration; care should therefore be taken to store and use Kevlar® in relatively dry conditions. In recent years, the production process has been much modified to improve fatigue resistance and tenacity, and a variety of aramid fibers have been produced with additional functions including dyeability and crimp. Copolymer-type aramid fiber Since polymer-composed aramid fiber is highly cohesive and rigid, it cannot be melt-spun and is generally dissolved into strong acid, such as sulfuric acid, for solution spinning (liquid crystal spinning). Strong acid restricts the process technologically, and many attempts have been made to solubilize aramid in an organic solvent by copolymerization with flexible or less linear monomers. The Teijin Co. has succeeded in developing and commercializing an aramid fiber called Technora®. that is soluble in an organic solvent. Technora® is a random copolymer composed of terephthaloyl dichloride (TPC), p-phenylenediamine (PPDA) and 3,4′-diaminodiphenyl ether (3,4′DAPE) in the ratios 100 (TPC) : 50 (PPDA) : 50 (3,4’-DAPE) as shown in Fig. 5.1. Unlike PPTA fiber, Technora® this copolymer dissolves into an organic solvent and can be drawn after being spun [5,6]. The less linear molecular structure of Technora® results in a lower tensile modulus than that of PPTA fiber, but its tensile strength is higher due to its improved molecular orientation and crystallinity from drawing. Since Technora® is composed of less rigid molecular chains, its durability, including fatigue resistance, is superior to that of PPTA fiber. The heat resistance and other characteristics of Technora® are similar to those of other aramid fibers. Wholly aromatic polyester fiber HPF can also be produced from wholly aromatic polyester made up of ester linkages. An ester linkage is relatively flexible for internal rotation, so that wholly aromatic polyester can be molecular-designed to have a melting point lower than the decomposition temperature by introducing suitable copolymeric units. Wholly aromatic polyester forms a thermotropic liquid crystal phase when it melts, and can be melt-spun without any solvent. In

High performance fibers

93

order to improve the tensile strength, the molecular weight should be increased by solid phase polymerization at a higher temperature after being spun. Vectran® is one of the wholly aromatic polyester fibers that are commercially available (see Table 5.1). Vectran® is a copolymer made up of 70% ABA (p-hydroxy-benzoic acid) and 30% ANA (6-hydroxy-2naphthoic acid) [7]. Its fatigue resistance is high, and it is applied to ropes and nets used in contact with water. The high strength monofilament of Vectran® has been developed by giving it a sheath-core structure and used as printing screen application. Heterocyclic polymer fiber Heterocyclic polymers were intensively investigated in the 1990s for high heat resistance, mostly by the US Air Force Research Institutes. These polymers have a more rigid chain structure than aramid fibers. Poly(pphenylene-benzo-bisoxazole) (PBO) has been developed from that research into a high performance fiber, which is now commercially available as Zylon®. The PBO fiber has an extremely rigid and linear chain structure, and its molecular cross-section is small. As a result, as-spun PBO fiber (Zylon®-AS) is approximately twice as strong as aramid fiber in terms of tensile strength and modulus. By heat-treating as-spun PBO fiber (high performance PBO fiber; Zylon®-HM), its modulus increases further as shown in Fig. 5.2. Zylon®-HM is a high performance organic fiber, which has a higher tensile strength and modulus than conventional carbon fiber in GPa units (for the same cross-sectional basis). The decomposition temperature of PBO is approximately 100°C higher than that of PPTA fiber, and PBO exhibits an excellent flame-resistance, with its LOI (limiting oxygen index) being 68%, the highest value among commercially available organic polymer materials. With these characteristics, PBO fiber is referred to as a second-generation super fiber [8, 9]. The chemical characteristics of PBO fiber are similar to those of PTTA fiber. PBO fiber exhibits a resistance to organic solvents and alkaline solutions at room temperature, but is less resistant to strong acid. Under some conditions, PBO fiber deteriorates more quickly from UV and other light than aramid fiber. Although the moisture absorption (official moisture regain 2–3%) of PBO fiber is lower than that of PTTA fiber, adequate care should be taken for its storage and use, since its strength will drop considerably when stored in a hightemperature and high-humidity environment for an extended period. Another heterocyclic polymer, poly{2,6-diimidazo [4,5-b:4′,5′-e] pyridinylene-1,4 (2,5-dihydroxy) phenylene} (PIPD:‘M5’) (see Table 5.1) is being developed as an ‘M5’ fiber for commercial use, where a benzene ring is replaced by a pyridine ring [10]. ‘M5’ fiber is supposed to possess similar mechanical and thermal characteristics to a PBO fiber. As a result

94

Identification of textile fibers

of intermolecular hydrogen bonds from hydroxyl groups introduced onto the benzene rings, ‘M5’ fiber has a high compression strength (approximately 1 GPa), an improvement which make it about twice as strong as aramid fiber.

5.2.3 The primary structure and physical properties of HPFs made from flexible polymer Ultra-high molecular weight polyethylene fiber HPFs made from rigid polymers were developed between the 1970s and 1980s. In the late 1980s, the flexible polymer ultra-high molecular weight polyethylene (UHMW-PE) was used to produce HPFs by an innovative spinning process known as gel spinning, and now HPFs made from UHMWPE are commercially available as Dyneema® and Spectra®. The details of gel spinning are discussed in the review [3]. UHMW-PE fiber is extremely light (its specific gravity is less than 1), while its tensile strength and modulus are high enough to be a HPF. Since polyethylene is flexible, UHMW-PE fiber has high impact strength as well as high abrasion and fatigue resistance. Polyethylene is also chemically stable over a wide pH range and has excellent weather resistance. However, polyethylene swells in organic solvent at high temperatures. The melting point of high strength polyethylene is increased to 147°C from its equilibrium melting point of 141°C due to extended chain crystallization by a high drawing ratio, but for normal use, the temperature should be kept below 100°C. Polyethylene is completely hydrophobic, and increases its strength at lower temperatures. These characteristics are utilized for marine ropes, tag ropes, fishing lines and other products used in watery environments or at extremely low temperatures, e.g. bobbins for superconducting coils (the size change caused by temperature could be reduced by utilizing a negative thermal expansion coefficient characteristic) [11]. Other flexible polymers The commercial success of UHMW-PE fiber stimulated the application of gel spinning to ultra-high molecular weight polyvinyl alcohol (PVA), polyacrylnitril (PAN) and polyethylene terephthalate (PET). By controlling the coagulating process with a non-aqueous solvent as a coagulant, 2 GPa fiber is produced from PVA. Gel spinning has been intensively applied to produce HPF from the copolymer of polyolefin and carbon monoxide (polyolefin keton) in recent years. This fiber was first developed by Akzo Nobel Co. [12], but its commercialization was abandoned. It is only recently that Asahi Kasei Co. has restarted the project to develop super fiber from polyolefin ketone.

High performance fibers

95

5.2.4 Other high strength and high modulus fiber Professor Kikutani’s group has been developing HPF from commodity polymers including PET as part of a national project. By controlling the chain entanglements in a melt state, they have succeeded in improving the tenacity of PET up to 1.7 GPa [13]. Among natural polymers, cellulose resembles PET with respect to molecular structure and usage amount in the world as both cellulose and PET are composed of semi-rigid chain molecules and are commonly used for various applications in large quantities. The theoretical strength and modulus calculated from the molecular structure predict that cellulose could satisfy the requirements of HPF fiber. In fact, 2 GPa cellulose fiber has been obtained by liquid crystal spinning from its polyphosphoric acid solution, but no commercial product is available as yet [14]. Spider silk extracted from goats’ milk proved to have a high tenacity and high modulus; genetic engineering was used to splice a spider gene into a goat’s milk gland [15]. The spider silk that was obtained exhibited a high elongation at break, unlike other HPFs.

5.3

Identification of high strength and high modulus fiber

5.3.1 Mechanical and thermal characteristics HPF is markedly different from other fibers in its tensile strength and modulus. As shown in Table 5.1, the tensile strength of HPF should be over 2 GPa. Care should be taken when measuring the tensile strength, because there will probably be slippage at the jaw of the tension tester due to the high tension at the measurement point, and the low friction coefficient of the fiber surface resulting from the extended chain structure. Since the distribution of strength among single filaments affects the strength of the total assembly, this should be taken into account in evaluating the tensile strength. The procedure for the evaluation of the tensile strength of super fiber is reviewed in detail by Hagege et al. [16]. The heat resistance of HPFs is evaluated in terms of melting temperature and thermal decomposition temperature by a conventional DSC or TGA, or by the microscopic observation of fiber at the heating condition with the hot plate stage equipment. HPF is generally classified into two types: HPF composed of rigid chain polymer and HPF composed of flexible chain polymer. UHMW-PE fiber represents a HPF composed of flexible chain polymer, and Fig. 5.4 shows its DSC endothermic peak pattern at melting point [17]. UHMW-PE fiber has a melting point around 145–150°C. However, its apparent melting point shifts, dependent mainly on the fiber’s restraint conditions during the DSC measurement. In some cases,

96

Identification of textile fibers . Q

Chopped (1 mm) Fixed (in epoxy) T (°C) 120

100

140

160

5.4 DSC chart for UHMW-PE fiber [3]. 100 In air Residual weight (%)

80 60

Zylon-AS Zylon-HM

40

Aramid Aramid-HM

20

Copoly-Aramid 0

0

100 200 300 400 500 600 700 Temperature (°C)

5.5 TGA charts for HPFs.

conventional polyethylene fiber (with tensile strength less than 1 GPa) shows a melting point slightly lower than that of HPF, so that molecular weight measurement and primary structure analysis would have to be performed in order to identify polyethylene HPF. Rigid polymer exhibits no DSC endothermic peak from crystal melting, but its thermal decomposition temperature can be detected from the weight loss by TGA. Figure 5.5 shows the weight loss curve observed by TGA for various HPFs in air. Most heatresistant HPFs have a thermal decomposition temperature above 500°C as shown in Fig. 5.5. The HPF could be identified from the absolute value of

High performance fibers

97

the thermal decomposition temperature listed in Table 5.1. However, exact identification should be performed by analysis of the primary structure.

5.3.2 Analysis of chemical (primary) structure The most authentic way of identifying a HPF is the determination of the primary structure by various chemical analyses. The chemical analyses include infrared (IR) spectroscopy, Raman spectroscopy and highresolution nuclear magnetic resonance (NMR) for examining the chemical species and linkage modes, and intrinsic viscosity measurement and GPC for determining the molecular weight. The identification of a HPF could be assisted by measuring the specific gravity, thermal decomposition temperature and solubility in various solvents. The chemical analyses and corresponding sample preparations of HPFs are performed in the same way as for conventional synthetic fibers. However, many HPFs do not dissolve in ordinary solvents, and thus the applicability of conventional chemical analyses will be restricted. For example, UHMWPE dissolves only into organic solvents of relatively low polarity including decalin, tetralin and liquid paraffin at higher temperatures. PPTA and PBO polymers dissolve only in strong acid such as sulfuric acid and methanesulfonic acid, respectively. Therefore, fewer reports are available for the NMR and GPC measurements on those polymers. IR spectroscopy could be applied generally to the analysis of HPF primary structures. The ATR (attenuated total reflection) method affords a simple measurement for HPFs. The characteristic IR absorption is summarized in Table 5.2 for each HPF. For example, the blending ratio of meta-aramid and Table 5.2 The typical IR absorption values for HPFs Polymer

Fiber

Producer

IR absorption (cm−1)

p-Aramid

Kevlar® Twaron®

DuPont Teijin Twaron

m-Aramid Polyarylate

Nomex® Cornex® Vectran®

DuPont Teijin Kuraray

Polybenzazole

Zylon®

TOYOBO

M5

Magellan

Dyneema® Spectra®

TOYOBO/DSM Honeywell

3500–3200, 1660, 1545, 1515, 1405, 1320, 1265, 1115, 1020, 830 3500–3200, 1660, 1610, 1540, 1490, 1420, 1310, 1240, 780 3050–3100, 1930, 1740, 1600, 1260, 1160, 1050, 1010, 890, 760 3010–3120, 2830–2990, 1630, 1410, 1365, 1115, 1055, 1010, 850, 700 3000–3600, 1570, 1500, 1360, 1280, 1200, 960, 720–870 2920, 2850, 1470, 720

UHMW-PE

98

Identification of textile fibers

para-aramid can be determined from the intensity ratio at absorption peaks [18]. However, the mode of monomer sequence or the molecular weight cannot be determined easily from IR spectroscopy. The determination of the monomer sequence by NMR is reported for Technola [19]. GPC is suitable for molecular weight determination, but cannot be applied in practice for aramid or PBO because of their corrosive solvents, sulfuric acid or methanesulfonic acid. The molecular weight and branching degree can be evaluated for UHMW-PE fiber by applying high temperature GPC. UHMW-PE has a high molecular weight, over 500 000 dl/g, while the molecular weight of conventional polyethylene is less than 100 000 dl/g. The intrinsic viscosity is the simplest criterion for the molecular weight. It can be measured in the same manner for HPFs as for conventional polymers, although the type of solvent that can be used is limited. Care should be taken in the solvent preparation because, for example, the viscosity of PBO solution will vary greatly with the water content in the solvent and/or the addition of salt [20].

5.3.3 Analysis of higher-order structure Wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) HPFs are basically composed of crystalline polymer, and many reports have been published on the details of the respective crystalline structures, which enable the type of HPF to be identified. Wide-angle X-ray diffraction (WAXD) is one of the main tools for the identification of the crystalline structures. Figure 5.6 shows the WAXD pattern and the corresponding crystal structure for UHMW-PE fiber (a, b) [21–23], PPTA fiber (c, d) [24, 25], PIPD fiber (e, f) [26] and PBO fiber (g, h) [27, 28], respectively. The crystal lattice constants of the respective fibers are summarized in Table 5.3. Fewer reports are found for the crystal structure analysis of copolymeric HPF including Vectran® and Technola® because of their structural ambiguity [29–31]. The WAXD pattern yields strong spot reflections basically in the equatorial plane for any HPF as shown in Fig. 5.6, suggesting a high degree of crystallinity and a higher crystal orientation in the fiber axis. This high degree of crystallinity and higher crystal orientation characterize the high-order structure of HPFs. UHMW-PE fiber exhibits extremely strong spot-like scattering intensity in the equatorial direction, reflecting its high degree of crystallinity (over 90%) and higher crystal orientation. The crystal structure is characterized by an orthorhombic crystal as generally observed in conventional isothermal polyethylene crystal prepared at normal temperature. In fewer cases,

High performance fibers

99

(a) ¼

HIV

HIII HIII′ HII

¼

¼

¼

¼

HIV′

HI HI′

HII′

¼ HIV′ b

¼

¼

¼

¼

HIII′ HII′

HII HI HIV

HI′ a

HIII (b)

5.6 (a) WAXD profile for UHMW-PE fiber [21]; (b) crystalline structure for UHMW-PE fiber [23]; (c) WAXD profile for PPTA fiber [24]; (d) crystalline structure for PPTA fiber [24]; (e) WAXD profile for PIPD fiber [26]; (f) crystalline structure for PIPD fiber [24]; (g) WAXD profiles for PBO fiber: as-spun fiber (left side) and heat treated fiber (right side) [9]; (h) crystalline structure of PBO fiber [27].

100

Identification of textile fibers

(c)

b

a

C

M

D

b

Projection parallel to the c-axis

Projection parallel to the a-axis (d)

(e)

5.6 Continued

High performance fibers

c

b a

(f)

AS: As-spun PBO fiber

HM: Heat treated PBO fiber (g)

5.6 Continued

101

102

Identification of textile fibers

(h)

5.6 Continued

Table 5.3 Crystalline structure of HPF fibers HPF

PPTA Kevlar®

UHMW-PE* Spectra®900

PBO As-spun

PIPD Heat treated

Reference Type A (nm) B (nm) C (nm) a (degrees) b (degrees) g (degrees) Chain number in single unit cell Crystal (g/cm3) density

24 Monoclinic 0.780 0.519 1.29 90 90 90 2

22 Orthorhombic 0.7422 0.4944 0.2550 90 90 90 2

27 Monoclinic 1.120 0.354 1.205 90 90 101.3 2

26 Monoclinic 1.260 0.348 1.201 90.0 108.6 90.0 2

1.5

0.974

1.64

1.77

* setting angle = 44.7°

UHMW-PE fiber indicates the existence of a small amount of monoclinic crystal, which might result from drawing in the production process. The crystal structure of PTTA fiber is reported as monoclinic as shown in Fig. 5.6 (d). Figures 5.6 (f) and (h) show the proposed crystal structures for PIPD fiber and PBO fiber, respectively. Here PPTA fiber and PIPD fiber form intermolecular hydrogen bonds, whereas PBO fiber has no such intermolecular hydrogen bond. Traditionally, the crystal structure was determined by trial and error by adjusting the calculated diffraction pattern from a model crystal structure to the observed X-ray diffraction pattern. In recent years, computational chemistry has developed to such an extent that the details of a crystal structure can be discussed in terms of the positional probability of each atom composing the crystal by potential energy calculations [32].

High performance fibers

103

Since a rigid chain polymer has less freedom for constituent molecular chains than polyethylene, a small distortion occurring in the crystal can only be released by axial shift. In consequence, the ordering deteriorates gradually in the molecular chain direction. The crystal structures summarized above are those of an ideal structure within a short distance order, but include in reality the molecular chain distortion and/or the non-unique structural states in the molecular chain direction. When the chain distortion progresses further, the chain disorder develops into an amorphous state. In this context, the amorphous state of rigid polymers differs from the amorphous state composed of random coils in flexible polymers with respect to the structure and molecular motion. Although the amorphous structure is different, there is a difference in electron density between the amorphous and crystal phase, so that the space distribution of the amorphous region can be analyzed by SAXS. A common feature in the SAXS profile from HPF is that no clear long period structure (normally observed for commodity fibers) is observed in the fiber axis direction. A ‘pleats structure’ is proposed for PPTA fiber from the SAXS profile and observation by a transmission electronic microscope, where the c axis is inclined 0° and 10° alternately in the period of 150 nm to 250 nm with respect to the fiber axis as shown in Fig. 5.7 [33, 34]. UHMW-PE fiber exhibits a strong streak characteristic to the fully extended chain structure in the equatorial direction [21], but no such clear long period pattern is observed in the meridian direction as with conventional polyethylene fiber (Fig. 5.8). This SAXS profile is thought to be the result of the structural characteristics of UHMW-PE fiber which assumes an extended chain structure, resulting in large crystalline

5.7 Pleat structure proposed for PPTA fiber [33].

104

Identification of textile fibers

5.8 SAXS profile for UHMW-PE fiber [21].

sizes in the fiber axis direction, and is also made of an almost 100% crystalline region, resulting in the disappearance of the electron density contrast between the crystalline and amorphous region. A four-point interference long period pattern is observed for PBO fiber (especially PBO HM fiber) as shown in Fig. 5.9 [9]. Computational chemistry indicates that the crystalline region of PBO fiber is mainly constituted of crystal domains as shown in Fig. 5.10, characterized by the inclined structure of liquid crystal domain units formed during the spinning process [35]. The average image for a higher-order structure of fiber is composed of the ordinary crystalline structure with superimposed inhomogeneity in the radial direction of a monofilament. The crystal structures of PPTA fiber, UHMWPE fiber and PBO fiber are analyzed in accordance with this image. Here the analysis is performed hierarchically over a wide range of dimensions from nano to sub-micron and micron size. Thus the identification of fibers could be done from the analysis of an overall structure as fiber possesses a hierarchical structure as demonstrated by the representative examples below. Optical microscopy and electron microscopy Figure 5.11 shows the optical microscopic photographs under (a) a bright field condition and (b) a crossed-Nicols condition (with crossed polarizers

AS fiber

HM fiber

5.9 SAXS profiles (contour plot) for PBO fiber: as-spun fiber (top), heat treated fiber (bottom) [9].

5.10 Microstructure model proposed for heat-treated PBO fiber [35].

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20 μm Dyneema® SK60

Spectra® 1000

Kevlar® 29

Technola®

Vectran® (a)

Zylon® AS

A

20 μm Dyneema® SK60

Spectra® 1000

Kevlar® 29

Technora®

Vectran® (b)

Zylon® AS

P

5.11 (a) Micrograph for HSFs under bright field condition; (b) micrograph for HSFs under crossed-Nicols condition.

placed at 45° with respect to the fiber axis, respectively) of the monofilaments of UHMW-PE fiber (Dyneema® SK60 and Spectra® 1000), PPTA fiber (Kevlar® 29), PBO fiber (Zylon® AS), Vectran® and Technora®. All the fibers exhibit strong brightness under a crossed-Nicols condition, and confirm the high orientation of molecular chains (crystal) in the fiber axis direction. The high orientation of molecular chains is one of the common features of HPF. HPF commonly assumes a so-called skin-core structure; that is, a different structure at the surface or in the middle part of fiber.

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5.12 SEM observation of the surface of UHMW-PE fiber [21].

When we observe the photograph of Kevlar carefully, horizontal lines can be observed, corresponding to a ‘pleat structure’. However, information obtained from optical microscopy is, in general, not enough to distinguish individual HPFs. The resolution of the optical microscope is limited to the sub-micron order of size, and so the electron microscope is useful for analysis at the nano level. The surface of an HPF is as smooth as conventional synthetic fibers, but streaks were observed on the surface of UHMW-PE fiber (Dyneema® SK60) by a scanning electron microscope (SEM) as seen in Fig. 5.12. Analysis of the microcrystal structure inside fiber can effectively be done by observation through a transmission electron microscope. The micro-structural analyses of internal monofilaments are reported for PPTA fiber using detailed observations by optical and electron microscopes [36, 37]. Figure 5.13 shows another example of PBO fiber, where the crystal lattice was directly observed by an electron microscope [9].

5.4

Alternative methods for analyzing higher-order structure

High-resolution solid-state NMR has been applied to HPF in order to understand the correlation between the higher-order structure and molecular motion. Since solid-state NMR affords only limited information, a few applications are found where, for example, the higher-order structure of UHMW-PE fiber is discussed in terms of the crystal phase, the amorphous phase and the intermediate phase from the difference in the relaxation times. The molecular chain motion in the crystal phase is discussed in detail

108

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Microvoid Surface

The a-axis of crystal is radially oriented in a fiber. Cross-section

Void-free region < 0.2 μm

Longitudinal section

Microfibril

Microvoid

PBO molecules are highly oriented in the microfibril. (orientation factor > 0.95)

5.13 Microstructure model proposed for PBO fiber [9].

using the results of solid-state NMR. However, solid-state NMR should be regarded as a complementary tool for the identification of a higher-order structure [38, 39]. Young et al. apply Raman spectroscopy extensively. A specific Raman shift was observed from the fiber under tension, and the amount of this shift enabled the microscopic stress exerted on a specific bond to be estimated. Although it would be difficult to fully identify the chemical structure of a fiber, the higher-order structure can be analyzed with respect to the mechanical characteristics [40–42]. A strong beam source from synchrotron radiation has been available for a few years. Here an incident beam is focused at the micron level, and is being used for the analysis of the microscopic structure, at the monofilament surface and inside it, by WAXD and/or SAXS [43].

High performance fibers

5.5

109

Sources of further information and advice

Many reviews and monographs are available on the production, structural characteristics and applications of high performance fiber (HPF). The monographs edited by Bunsell et al. provide a good introduction to reinforcing fibers for composite materials [44]. Descriptions of the production of various HPFs have been compiled by several authors including Ward [1, 2]. The pioneer of HPF, aramid fiber, is reviewed by Yang [4], who has been engaged in the development of PPTA fiber from the beginning [1, 2]. Although the function of HPF is specific, common methods are applied for the characterization and identification of HPF, and no preference for any particular method is given in this review. The characterization of molecular oriented materials is reviewed by Ward [45]. The application and development trends for HPF can be found in the monograph by Hongu [46].

5.6

References

[1] ‘Advanced Fiber Spinning Technology’ (Ed. T. Nakajima), Woodhead, 1994. [2] ‘Developments in Oriented Polymers-2’ (Ed. I. M. Ward), Elsevier, 1987. [3] H. Yasuda, K. Ban and Y. Ohta, ‘Advanced Fiber Spinning Technology’ (Ed. T. Nakajima), Woodhead, 1994, p.172; J. Lemstra, R. Kirschbaum, T. Ohta and H. Yasuda, ibid, p.39. [4] H. H. Yang and S. R. Allen, ‘Kevlar Aramid Fiber’, John Wiley & Sons, 1993. [5] S. Ozawa, Polym. J., 19, 119 (1987). [6] H. Matsuda, T. Asakura and Y. Nakagawa, Macromolecules, 36, 6160 (2003). [7] J. Nakagawa, ‘Advanced Fiber Spinning Technology’ (Ed. T. Nakajima), Woodhead, 1994, p.160. [8] K. Yabuki, High Strength High Modulus Fibers in Progress in Textiles: Science & Technology, Vol. 2 Fibers (Ed. V. K. Kothari), p.615, IAFL Publications, New Delhi (2000); K. Yabuki, Look Japan, Aug.1995, p.24. [9] T. Kitagawa, H. Murase and K. Yabuki, J. Polym. Sci., Polym. Phys. Ed., 36, 39 (1998). [10] D. J. Sikkema, Polymer, 39(24), 5981 (1998); M. Lammers et al., Polymer, 39(24), 5999 (1998). [11] T. Kashima, A. Yamanaka, E. S. Yoneda, S. Nishijima and T. Okada, Adv. Cryog. Eng., 41, 441 (1996); A. Yamanaka, T. Kashima, K. Hosoyama, IEEE. Trans. Appl. Superconductivity, 11, 4061 (2001). [12] B. J. Lommerts et al., J. Polym. Sci., Polym. Phys. Ed., 31, 1319 (1993); C. C. Copel et al., ANTEC, Vol.2, 1800 (1997). [13] T. Kikutani, PPS-22 Proceedings 22nd Annual Meeting of the Polymer Processing Society, p.144 (2006). [14] S. J. Eichhorn, R. J. Young, R. J. Davies and C. Riekel, Polymer, 44, 5901 (2003). [15] Nexia Biotechnologies Inc., http://www.nexiabiotech.com [16] R. Hagege and A. R. Bunsell, ‘Fiber Reinforcements for Composite Materials’ (Ed. A. R. Bunsell), Elsevier, 1988, p.479. [17] P. J. Lemstra, R. Kirschbaum, T. Ohta and H. Yasuda, ‘Developments in Oriented Polymers-2’ (Ed. I. M. Ward), Elsevier, 1987, p.39.

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[18] M. Nanbu, Jpn. Res. Assn. Text. End-Uses, 37, 357 (1996). [19] H. Matsuda, T. Asakura and Y. Nakagawa, Macromolecules, 36, 6160 (2003). [20] D. B. Roitman, R. A. Wessling and J. McAlister, Macromolecules, 26, 5174 (1993). [21] A. W. Saraf, P. Desai and A. S. Abhiraman, J. Appl. Polym. Sci., Appl. Polym. Symp. 47, 67 (1991). [22] Y. Fu, W. Chen, M. Pyda, D. Londono, B. Annis, A. Boller, A. Habenschuss, J. Cheng and B. Wunderlich, J. Macromol. Sci.–Phys., B35(1), 37 (1996). [23] H. Tadokoro, ‘Structure of Crystalline Polymer’, John Wiley & Sons, New York (1979). [24] M. G. Northolt and J. J. van Aartsen, J. Polym. Sci., Polym. Letters Ed., 11, 333 (1973); M. G. Northolt, Euro. Polym. J., 10, 799 (1974). [25] S. J. Krause, D. L. Vezie and W. W. Adams, Polym. Commun., 30, 10 (1989). [26] E. A. Klop and M. Lammers, Polymer, 39(24), 5987 (1998). [27] A. V. Fratini and W. W. Adams, Am. Cryst. Assoc. Abs., 13, 72 (1985); A. V. Fratini, P. G. Lenhert, T. J. Resch and W. W. Adams, ‘The Materials Science and Engineering of Rigid-Rod Polymers’ (Ed. W. W. Adams et al.), Materials Research Society, 1990; Material Research Society Symposia Proceedings 134, 465 (1984). [28] D. C. Martin and E. L. Thomas, Macromolecules, 24, 2450 (1991). [29] J. Blackwell, R. A. Cageao and A. Biswas, Macromolecules, 20, 667 (1987). [30] K. Tashiro, Y. Nakata, T. Ii, M. Kobayashi, Y. Chatani and H. Tadokoro, Sen’i Gakkaishi, 43(12), 627 (1987). [31] T-M. Wu and J. Blackwell, Macromolecules, 29, 5621 (1996). [32] K. Tashiro, J. Yoshino, T. Kitagawa, H. Murase and K. Yabuki, Macromolecules, 31(16), 5430 (1998). [33] M. G. Dobb, D. J. Johnson and B. P. Saville, J. Polym. Sci., Polym. Phys. Ed., 15, 2201 (1977). [34] H. H. Yang and S. R. Allen, ‘Kevlar Aramid Fiber’, John Wiley & Sons, 1993. [35] S. Ran, C. Burger, D. Fang, D. Cookson, K. Yabuki, Y. Teramoto, P. M. Cunniff, P. J. Viccaro, B. S. Hsiao, B. Chu, NSLS Activity Report 2001, Science Highlight, 2-147. [36] R. Hagege, M. Jarrin and M. Sotton, J. Microscopy, 115, 65 (1979). [37] M. Panar, P. Avakian, C. Blume, K. H. Gardner, T. D. Gierke and H. H. Yang, J. Polym. Sci., Polym. Phys. Ed., 21, 1955 (1983). [38] K. Kuwabara and F. Horii, Macromolecules, 32, 5600 (1999). [39] S. Bourbigot, X. Flambard and B. Revel, Euro. Polym. J., 38, 1645 (2002). [40] W. F. Wong and R. J. Young, J. Mater, Sci., 29, 510 (1994); ibid, 520 (1994). [41] K. V. Prasad and D. T. Grubb, J. Polym. Sci., Polym. Phys. Ed., 27, 381 (1989). [42] Y. Takahashi, J. Polym. Sci., Polym. Phys. Ed., 39, 1791 (2001). [43] C. Riekel, A. Cedola, F. Heidelbach, and K. Wagner, Macromolecules, 30(4), 1033 (1997). [44] ‘Fiber Reinforcements for Composite Material’ (Ed. A. R. Bunsell), Elsevier, 1988. [45] ‘Structure and Properties of Oriented Polymers’ 2nd edn, (Ed. I. M. Ward), Chapman & Hall, London, 1997. [46] ‘New Fibers’ (Ed. T. Hongu and G. O. Phillips), Ellis Horwood, New York, 1990.

6 The use of classification systems and production methods in identifying manufactured textile fibers K L HATCH, The University of Arizona, USA

Abstract: This chapter explains the connection between polymer origins and fiber classification and the connection/distinction between fiber classes and fiber subclasses. Then, this chapter focuses on the recently approved generic fiber class called PLA or polylactide fiber, the recently approved subclasses of fibers including lyocell, elasterell-p (also known as elastomultiester), lastol, and then various types of multicomponent fibers. The final topic of the chapter is future trends in fiber manufacture. Readers are referred to a series of books which detail the synthesis of polymers used to create fibers and the production of fibers from those polymers. Key words: PLA, polylactide, lyocell, elasterell-p, elastomultiester, lastol, multicomponent fiber.

6.1

Introduction

Every day in laboratories around the world, polymer and textile fiber scientists are at work seeking ways to modify existing polymers and textile fiber manufacturing processes to enhance fiber performance, reduce manufacturing costs, and create more environmentally friendly processes. These scientists are also developing new polymers to form into entirely new textile fibers which require new methods of manufacture. While much of what is discovered and/or underlies commercial production is probably not reported in the public domain, an incredible amount of outstanding information on polymer synthesis and manufactured fiber production is in the public domain. A continuous procession of books and monographs have been published in recent years which include those by Lewin and Preston (1985), Mukhopadhyay (1993), Hongu and Phillips (1990 and 1997), Hatch (1993 and 2006), Klein (1994), Nakajima (1994), Masson (1995), Lewin and Pearce (1998), Hearle (2001), Woodings (2001), McIntrye (2004), Blackburn (2005), and Hongu et al. (2005). Countless articles, a few of which are cited in this chapter, have appeared in scientific journals. Myriads of internet sites provide information about textile polymers and textile fiber production including the most recent developments. 111

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Because there is so much information available on the manufacture of manufactured textile fibers, no attempt is made in this chapter to summarize that information. Rather, the approach is to discuss several fundamental concepts, describe the manufacture of recently approved generic class and subclasses of fiber, and types of multicomponent fibers. Then, future trends and further sources of information and advice follow.

6.2

Polymer origins and fiber classification

As is well known, the fundamental unit of all textile fibers, whether natural or manufactured, is a polymer. Polymers in natural fibers are biosynthesized. Polymers in manufactured fibers may be (a) extracted from living plants or animals that biosynthesized the polymeric material or (b) manufactured (synthesized) from monomers obtained from petroleum or from living plants and animals. Based on this fundamental difference in the origin of polymer, manufactured fibers are listed in two tables. Table 6.1 lists fibers manufactured from naturally occurring polymers which may be cellulosic, protein, or other polymer chemistries. Table 6.2 lists fibers manufactured from lab-synthesized polymers which may be synthesized using monomers from petroleum or using monomers extracted from living plants and animals. Where confusion seems to occur is the tendency to call some manufactured fibers ‘natural fibers’; specifically, these are fibers made with naturallyoccurring polymers (fibers listed in Table 6.1) as well as fibers whose monomers were extracted from plants and animals (fibers listed in the righthand column of Table 6.2). The argument for calling these fibers ‘natural’ is because the polymers or monomers for polymer creation come from a natural source. However, none of the fibers listed in Tables 6.1 and 6.2 are natural fibers because they do not exist as such in the natural state, as is the case for cotton, flax, hemp, wool, mohair, and asbestos fibers. Rather all fibers listed in Tables 6.1 and 6.2 are derived by a process of manufacture from a substance(s) which, at any point in the manufacturing process, was not a fiber. A second point of some confusion centers on the terms generic class and generic subclass. Part of the confusion probably arises because the United States Federal Trade Commission (FTC) which oversees the Textile Fiber Products Identification Act (TFPIA, (http://www.ftc.gov/os/statutes/textile/ rr-textl.htm) places the name and definition for a generic subclass within the appropriate generic class definition and never uses the term ‘subclass’ in the definition. In contrast, in Europe the International Bureau for the Standardization of Man-Made Fibres (BISFA, www.bisfa.org) only lists and defines generic classes of fibers. For example in the TFPIA, polyester fibers are:

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Table 6.1 Fibers manufactured from naturally occurring polymers Cellulose polymer fibers (polymers obtained from cotton fiber, trees, bamboo stalks, other plants)

Protein polymer fibers (polymers extracted from milk (casein), peanuts, soybeans, etc.)

Fibers from other polymers (obtained from sources given below)

Rayon/Viscose, Modal, Cupro Lyocell (subclass) Acetate Triacetate (subclass)

Azlon

Alginate (polymer from seaweed) Rubber (polymer from Brazilian rubber tree and guayule-desert shrub and potentially from genetically engineered sunflowers) Chitosan (polysaccharide polymer from shellfish), not a generic class

Table 6.2 Fibers manufactured from lab-synthesized polymers Monomers for polymer synthesis from petroleum

Monomers for polymer synthesis from living plants and animals

Polyester Elasterell-p (sub)/ Elastomultiester Nylon/Polyamide Olefin Lastol (subclass) Acrylic Modacrylic (subclass) Spandex/Elastane Rubber Lastrile (subclass) Anidex Aramid Elastoester

Carbohydrate sources

Protein sources

PLA/polylactide

Synthetic silk, not yet a generic class

Fluorocarbon/ Fluorofiber Sulfar PBI/polyimide Novoloid Nytril Vinal Vinyon Saran

manufactured fiber(s) in which the fiber-forming substance is any long-chain polymer of at least 85% by weight of an ester of a substituted aromatic carboxylic acid, including but not restricted to substituted terephthalate units [formula omitted here] and parasubstituted hydroxybenzoate units [formula omitted here]. Where the fiber is formed by the interaction of two or more chemically distinct polymers (of which none exceeds 85% by weight), and

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contains ester groups as the dominant functional unit (at least 85% by weight of the total polymer content of the fiber), and which, if stretched at least 100%, durably and rapidly reverts substantially to its un-stretched length when the tension is removed, the term elasterell-p may be used as a generic description of the fiber.

Elasterell-p is the subclass even though not distinctly called that in the definition. In contrast, BISFA lists and defines polyester and separately lists and defines elasterell-p (actually elastomultiester). The definition for elastomultiester is: fiber formed by the interaction of two or more distinct macromolecules in two or more distinct phases (of which none exceed 85% by mass) which contains ester groups as dominant functional unit (at least 85%) and which after suitable treatment when stretched to one and a half times its original length and released recovers rapidly and substantially to its original length. www.bisfa.org/booklets/Terminology_2006.pdf

To further emphasize the important distinction between generic class and subclass as used in the TFPIA and in this chapter, note these statements taken from the TFPIA: A generic class is ‘a grouping of fibers having similar chemical compositions or specific chemical characteristics’. A generic fiber subclass is created when ‘a manufactured fiber (1) has the same general chemical composition as an established generic fiber category; (2) has distinctive properties of importance to the general public as a result of a new method of manufacture or substantially differentiated physical characteristics, such as fiber structure; and (3) the distinctive feature(s) make the fiber suitable for uses for which other fibers under the established generic name would not be suited, or would be significantly less well suited. (http://www. ftc.gov/os/2002/11/16cfrpart303amend.htm) The bottom line is that the FTC and BISFA have approved the ‘same’ fiber names to appear on textile product labels. Most of those names appear in Tables 6.1 and 6.2 except for the inorganic fibers: metallic, glass, carbon, and metal. Subclass names are identified by including ‘subclass’ after the name. Where the TFPIA name and BISFA name differ, the TFPIA name is given first and the BISFA name follows a slash. For clarity, the term fiber variant (fiber type, specialty fiber) needs to be mentioned, as it has close ties to generic subclass. Variant refers to fibers resulting from a change in the polymer or to the basic spinning process, which are designed to improve one or more fiber properties. Modifications include a) altering fiber fine structure by altering polymer length and or orientation and cystallinity within the fiber, b) changing the cross-sectional shape of the fiber, c) incorporating a chemical between polymer chains, d) incorporating a small proportion of a monomer in the polymer chains, and e) grafting and incorporating bulky reactive groups along polymer chains.

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Table 6.3 Production of viscose rayon fiber variants Description of change

Variant name

Inclusion of new chemical prior to or in spinning solution a. Titanium dioxide Delustered rayon UV protective rayon b. Optical whitening compound Optically whitened rayon c. Dye molecules Producer-dyed rayon Acid-dyeable rayon d. Protein or polymers containing –NH2 groups e. Flame retardants Flame-resistant rayon f. Water-holding polymers (such as sodium Super-absorbent rayon polyacrylate or sodium carboxy methyl cellulose) g. Nano-scale-engineered silver-bearing Anti-odor/anti-microbial nano-particles Change in spin bath conditions a. Increase of zinc sulfate concentration in spin bath b. Increase of spin bath temperature c.

Reduction of acid concentration and an increase of temperature in the spin bath

Multiple changes a. Use of a higher grade of cellulose, elimination of ageing and ripening, lowering of chemical concentrations in spin bath to preserve polymer chain length 1

High-tenacity rayon Crimped high-performance (crimped HWM rayon1) Self-crimped rayon

High wet-modulus (high performance) rayon

In BISFA, HWM rayon is called Modal.

Standard practices for identifying a fiber variant/type in the consumer marketplace are to (a) place a trademarked name before the generic name of the fiber; for example, Qiana® nylon and (b) use a descriptive adjective before the generic class name; for example, delustered rayon fiber and static-free nylon fiber. Usually, fiber manufacturers have a numbering system to identify fiber variants they are selling to yarn and fabric manufacturers. Tables 6.3–6.6 provide the names of common fiber variants and a description of modifications made for rayon, acrylic, and melt-spun fibers. Note that Table 6.4 outlines fiber variants within the lyocell subclass.

6.3

PLA/polylactide fiber

PLA/polylactide fibers are ‘manufactured fiber(s) in which the fiber-forming substance is composed of any long-chain synthetic polymer at least 85% by weight of lactic acid ester units derived from naturally occurring sugars’

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Table 6.4 Production of solvent-spun rayon fiber variants Description of change

Variant name

Multiple

Men’s wear fiber

Additive to spinning solution

Non-fibrillating fiber High-fibrillating fiber Delustered/UV-light scattering fiber (with TiO2) Skin-healthy fiber (with seaweed powder)

During fabric finishing

Peach-skin surface Smooth (clean) surface retention Resin-treated Plasma surface treated

Table 6.5 Production of melt-spun fiber variants Description of change

Variant name

Polymer modification Introduction of side-groups

Dyeable olefin

Spin solution alteration Polymerize to specified length/degree Add substitutents Additive to melt Titanium dioxide Anti-static compounds Optical whitener Titanium dioxide Nanoscale-engineered silverbearing nanoparticles Change in spinneret shape Trilobal, flat/ribbon, square, multichanneled, etc. Post draw parameters (speed, etc.)

High and low tenacity Hydrophilicity increase Delustered UV-reflecting (protective) Anti-static Optically whitened UV-resistant Anti-odor/anti-microbial Greater luster, improved resilience, enhanced moisture transport, etc. (http://www.fitfibers.com/cross_ sections.htm) Staple and filament lengths (texturing) Reduction of fiber size (denier)

(FTC-TFPIA). This generic classification of fibers is the most recently approved generic class in the United States and Europe. The PLA generic classification is the first classification for a manufactured synthetic fiber whose origin is a renewable natural resource – corn, and more precisely corn kernels. The sequence of events involved in the manufacture of PLA polymers is: harvesting corn, removing the kernels from the cob, fermenting the kernels to extract dextrose, converting dextrose to lactic acid, forming dimers of lactic acid called lactide, and then

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Table 6.6 Production of acrylic fiber variants Description of change During polymerization Those with 85% or more acrylonitrile composition. Streams of two different acrylonitrile polymer solutions merge to form one fiber. See last section of this chapter for further detail. Fibers were originally this until the introduction of basic dyes which became the standard placing aciddyeable fibers as the specialty (variant) fiber. Use of only acrylonitrile polymer, the formation of higher molecular weight polymers than found in ‘textile-grade’ applications, and higher draw ratio during spinning of the fiber. Copolymers of acrylonitrile with small amounts of monomers such as carboxylic acids or vinyl bromide and alteration in spinning. Additions to the polymer solution Inclusion of an optical whitener in the spinning solution. Addition of colored pigment to the polymer solution prior to fiber spinning. Addition of titanium dioxide as a delustrant to the polymer solution prior to fiber spinning. Contact of acrylic fiber with a concentrated solution of cationic dyes solution during many stages of wet-spinning except to dried fiber. Other methods Incorporation of stable voids in the fiber structure to hold water. Manufacturers use different processing procedures to obtain this end result. Production of a fiber with a corrugated surface, a structure that allows loose fiber to move out of the yarn before it entangles itself with other fibers, thus not producing a pill (an entangled mass of fiber) on a fabric’s surface.

Variant name Flame-retardant Bicomponent

Acid-dyed fibers – the result of the ‘original’ production of acrylic fibers, now not often produced

Reinforcing (asbestos replacement)

Carbon fiber precursor

Optically whitened Pigmented

Delustered

Producer dyed

Moisture absorbent

Pill-resistant

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polymerizing the lactides to form the polylactic acid (PLA) polymer. Formation of PLA textile fibers then involves the extrusion of a melt of PLA polymers through a spinneret. PLA polymers can also be used to coat paper, make films and injection molded plastics, among other useful products. Cargill-Dow operates the world’s first industrial-scale PLA production manufacturing facility (plant) in Blair, Nebraska, a facility having the capability of producing 300 million tons of fiber annually. The following claims have been made about PLA fibers: 1. The properties of PLA fiber are unique so new fabric performance results. 2. Woven and knit apparel fabrics; bedding products including fiber-fill for pillows, comforters, and pillow-top mattresses; carpeting including recyclable carpeting, disposable products such as diapers and wipes, and simulated suede and leather can be made. 3. Eco-efficient and environmentally friendly textile products result because: a. The fundamental (starting) chemical in PLA fibers is obtained from an annually renewable natural resource (corn) rather than from a diminishing supply of oil which takes millions of years to replenish/ renew. b. All chemicals used in processing PLA are recognized as environmentally safe. c. The process of producing PLA polymer uses 30–50% less fossil fuel than usually required to produce oil-based manufactured synthetic polymers (such as polyester and nylon and olefin). d. Making PLA polymer generates about half the greenhouse gas emissions as making petroleum-based polymers. e. PLA fibers are biodegradable (other manufactured synthetic fibers are not) so PLA fibers can be composted. f. The products (chemicals) from composting can be used to grow more corn, beets, rice etc. for possible future conversion to PLA. g. PLA fiber can be recycled (ground up, melted) and formed into new fiber or film. h. PLA fiber is highly sustainable because it decomposes to CO2 and H2O by natural processes. ‘PLA fiber offers superior sustainability and lower environmental impact than any other non-cellulosic synthetic fiber, and is possibility superior to some natural fibers.’ 4. Making PLA fiber creates a new market for corn (the most common starting material) and therefore more income for farmers who produce the corn. 5. PLA fabrics can be offered at the same price per yard as polyester fabrics because the specific gravity of PLA is only 1.25 which is less than

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polyester. Because fibers are sold by the pound and converted into fabrics sold by length or area, it is possible to sell PLA fabrics at the same price as polyester fabrics even when PLA fiber is priced 12% higher. PLA fiber is being used in the following products: a)

Woven and knit fabrics for apparel including socks, shirts, and sports clothing (activewear). b) Home furnishing products particularly fiberfill for pillows, bed comforters, mattress pads, fiber beds (similar to feather beds), and pillowtop mattresses. c) Synthetic suede and leathers produced with splitable segmented fiber production (see variants section of this chapter). d) Recyclable carpeting because both the face fiber and backing can be made of PLA. e) Disposable products such as diapers and wipes. While all PLA fibers are composed of polylactic acid polymers, fibers for various uses will be composed of PLA polymers differing in spatial configuration. This results because lactic acid, the monomer, exists as L-lactic acid and D-lactic acid. The most common form is L-lactic acid. Following polymerization, when the predominant lactide in the polymer chains is L, the internal structure of the fiber is highly crystalline. When polymer chains are composed of 15% or more of D-lactide units, then the internal fiber structure may be described as amorphous. It is also possible to control the sequencing of D-lactide and L-lactide units along a polymer chain. Currently, most PLA fiber is available as a smooth rod (round crosssection) fiber but is also available in a cross section that enhances its wicking ability and is present in several multicomponent fibers. For further details about PLA fiber refer to Lunt (2000), Lunt and Bone (2001), Yang and Huda (2003), Gupta et al. (2006), and Hatch (2006).

6.4

Fiber subclasses

Prior to 1995, the fiber subclasses in the TFPIA were triacetate (subclass of acetate) and lastrile (a subclass of rubber). Since 1995, three subclasses have been approved: lyocell as a subclass of rayon on 15 April 1996, elasterell-p as a subclass of polyester on 7 November 2002, and lastol as a subclass of olefin on 17 February 2003. The Federal Trade Commission is currently considering a petition to create triexta as a subclass of polyester. A rich source of information about the chemistry and properties of new fibers is contained in the petition documents submitted by fiber manufacturers to the FTC. These documents can be found on the FTC’s homepage (www.ftc. gov) along with all documents generated during the approval process.

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6.4.1 Lyocell fiber Rayon fibers are ‘manufactured fiber(s) composed of [100%] regenerated cellulose, as well as manufactured fiber of regenerated cellulose in which substituents have replaced not more than 15% of the hydrogens of the hydroxyl groups. Where the fiber is composed of cellulose precipitated from an organic solution in which no substitution of the hydroxyl groups takes place and no chemical intermediates are formed, the term lyocell may be used as a generic description of the fiber.’ (FTC-TFPIA) Fibers which are supposed to be marketed as lyocell are often marketed (incorrectly) as Tencel®, the trademark of the producer. Like viscose rayon fibers, lyocell fibers are 100% cellulose but the average polymer length is higher in lyocell fibers than in viscose rayon fibers. Lyocell fibers are manufactured using the solvent spinning manufacturing process, a process with beginnings in the 1970s. The source of cellulose for lyocell fibers is wood pulp from trees grown especially for this purpose on managed tree farms. This quality wood pulp is mixed with amine oxide (N-methylmorpholine-N-oxide) in water. The mixture is then passed to a continuous dissolving unit where a clear, viscous solution is formed. The solution is then extruded through a spinneret into a dilute aqueous solution of amine oxide, which precipitates the cellulose as fiber. After washing and drying, the fiber is ready for finishing processes that are tailored to develop performance required in various end-uses. The diluted amine oxide from washing is purified and after removal of excess water is reused. The materials used in the process are environmentally clean, and recycling of the solvent is an integral part of the process. Waste products are minimal and non-hazardous. Virtually all of the solvent is reclaimed and reused making the process environmentally friendly. The ‘fundamental’ process is used to make fiber for women’s garments. This process has been altered in a number of ways to produce a menswear fiber, and also altered to produce a number of other variants shown in Table 6.4. Lyocell fiber may also be modified to change fiber surface appearance and properties during fabric finishing. Selected references about the production and properties of lyocell fiber include Bullio (1992), Cole (1992), Kumar (1994), Watkins (1995), Nechwatal et al. (1996), Hongu and Phillips (1997), Keesee (1998), Kumar and Harden (1999), Karypidis et al. (2001), Taylor et al. (2001), and Hatch (2006).

6.4.2 Lastol and elasterell-p/elastomultiester Lastol and elasterell-p, are considered together because they are both elastomeric fibers (a classification based on common distinctive property); those fibers that extend noticeably when a tensile force is applied and

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recover quickly and almost completely to their original length when that tensile force is released. Fibers of the spandex, rubber (natural and synthetic), and anidex (no longer produced) classes are also elastomeric fibers. Lastol fiber was developed by Dow Fiber Solutions of the Dow Chemical Companies to compete with spandex fiber primarily in the manufacture of easy-care comfort-stretch fabrics/garments but also to compete where power stretch is desired. The statement added to the end of the olefin fiber class definition in the FTC/TFPIA is as follows: ‘When the fiber-forming substance is a cross-linked synthetic polymer, with low but significant crystallinity, composed of at least 95 percent by weight of ethylene and at least one other olefin unit, and the fiber is substantially elastic and heat resistant, the term lastol may be used as a generic description of the fiber.’ Other names associated with lastol fiber include: (a) CEF, an acronym for Crosslinked Elastic Fiber; (b) XLATM Freedom Fiber, a Dow Fiber Solutions tradename; and (c) DCC-0001, a temporary name assigned by the FTC while the Dow petition was under review. The structure of lastol fiber differs from the structure of conventional olefin fibers because: • • •

the polymers in lastol are short branched, not linear as in other olefin fibers, the polymers are crosslinked, and there is a low but significant crystallinity of 12–16% in lastol which is in contrast to conventional olefin fiber with a crystallinity of >50 and to rubber fiber which has no crystalline structure.

Further, the polymer in lastol is a co-polymer, not a homopolymer as in conventional olefin fibers. Most interesting is that each lastol polymer is essentially composed of ethylene monomers but a co-monomer is inserted into the ethylene chain on occasion. Scientists know that substantially every lastol polymer in a fiber has the same ethylene to co-monomer ratio and that each polymer within the lastol fiber has about the same molecular weight. This magic of polymer formation is achieved by homogeneous or single site catalyst systems known as a metallocene or a constrained geometry catalyst system. This is even more complex than the Ziegler-Natta method used to make other olefin polymers. Excellent information about the properties of these fibers is available online: http://www.ftc.gov/os/ statutes/textile/info/petition_dowsubclass.pdf and www.dowxla.com. Elasterell-p fiber’s definition is ‘where the fiber is formed by the interaction of two or more chemically distinct polymers (of which none exceeds 85% by weight), and contains ester groups as the dominant functional unit (at least 85% by weight of the total polymer content of the fiber), and which, if stretched at least 100%, durably and rapidly reverts substantially to its

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un-stretched length when the tension is removed, the term elasterell-p may be used as a generic description of the fiber.’ Each elasterell-p fiber is a side-by-side bi-component fiber with a distinctive ‘snowman’ cross sectional shape. The polymers on one side of each fiber are ‘regular’ commercial polyester polymers and the polymers forming the other side differ by one methylene unit. (Note: it is not known whether this means one more or one less unit). When heated, the fiber becomes helical due to differential shrinkage of the two polyester polymers forming the fiber. The primary market for elasterell-p fiber is comfort-stretch apparel, apparel that fits close to the skin and therefore needs to elongate (stretch) to allow initial movement and to recover as movement is reversed to retain garment shape (dimension/fit). Elasterell-p fabrics compete with fabrics containing spandex fibers and with polyester stretch-textured fabrics. A current manufacturer of elasterell-p is Invista. Elasterell-p was known as T-400 during its development and initial sale and assigned the name DP0002 elastic fiber during FTC petition review. For further information, an excellent online source is www.ftc.gov/os/statutes/textile/fedreg/020215comments.pdf

6.4.3 Potential ‘triexta’ In 2006, the FTC was petitioned to allow fiber content labels of carpeting to indicate the percentage of fiber in the product made with PPT polyester to be given as triexta rather than as PPT polyester or triexta polyester. The request was to add the following definition to the end of the polyester fiber definition: ‘and where specifically the glycol used to form the ester consists of at least ninety mole percent 1,3-propanediol, the term triexta may be used as a generic description of the fiber.’ In this particular situation, the focus of the argument for triexta to be a new subclass is on showing that carpeting made from PPT has distinctively better performance than carpeting made with other polyester fibers. And that being allowed to label the product in a manner that consumers would not relate the performance of the carpet to the inferior performance of polyester carpeting was essential. To follow the approval process for triexta fiber, begin with reading the petition: www.ftc.gov/os/statutes/textile/info/PTT001_Petition.pdf. Some history of the development of PPT polymers and PPT fibers provides insight into the reason that PPT polyester was not previously used for carpeting. Although a process was developed and patented in the late 1940s to synthesize PPT polymers, textile fibers were not commercially produced for a long time even though the fibers had good physical and chemical properties and there was potential for their use in textile products was that

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PPT polymers and fibers were just too expensive to produce. Two starting chemicals (monomers) are needed: PDO (1,3-propanediol) and PTA (purified terephthalic acid) with PDO being the expensive monomer. Shell Chemical started to commercially produce PDO in the 1960s but stopped in the early 1970s because of cost and quality issues. In the early 1990s, Shell developed an innovative way to produce PDO economically. Later, it would be discovered how to produce PDO biochemically with chemicals from corn. The next step was to work on methods to make PPT fiber and study PPT fiber properties. After Shell acquired a PET manufacturing facility from Goodyear in 1992, it had a facility to not only produce PET but PPT as well. In other facilities Shell owned, Shell began to manufacture PPT fiber. Shell then partnered with the largest carpet manufacturer in the world and with one of the world’s biggest producers of textured yarns. In May 1995, Shell Chemical introduced PPT fiber into the marketplace with the tradename ‘CorterraTM’.

6.5

Multicomponent fibers

A multicomponent fiber (conjugate fiber) is one that contains more than one type of polymer (ASTM D4466-02). Most multicomponent fibers are bicomponent, ‘a fiber consisting of two polymers which are chemically different or physically different or both’ (ASTM, 2006). A tricomponent fiber is one ‘consisting of three polymers which are chemically different, physically different, or any combination of such differences’ (ASTM, 2006). The manufacture of a multicomponent fiber is usually accomplished by extruding the two or three types of polymers simultaneously through the same spinneret orifice. When a fiber is composed of polymers are from different polymer classes such as polyester and polyamide or polyester and polypropylene, it may be called bicomponent-bigeneric (preferably) or biconstitutent (a deprecated term). Multicomponent fibers are further described by the physical placement (arrangement) of the polymers within the fiber. Fibers composed ‘of two or more polymers at least two of which have a continuous longitudinal external surface’ are called lateral (preferably) or side-by-side. A fiber ‘consisting of a continuous envelope which encases a continuous, internal region’ is called sheath-core (preferably). A fiber ‘in which one or more polymeric fibrous materials is dispersed in another’ is called a matrix fiber (preferably) or islands-in-the-sea (a deprecated term). Three excellent sources of detailed information about the formation and uses of these fibers are Jeffries (1971), Placek (1971), and Hersh (1985), and this URL: www.eng.utk.edu/mse/ pages/Textiles/bicomponent%20fibers.htm.

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Four major reasons for the manufacture of multicomponent fibers are to (a) produce fibers having three-dimensional crimp, (b) make binder fibers, (c) make fabrics composed of micro-fibers, and (d) make suede-like fabrics. These processes are now described briefly.

6.5.1 Fibers having three-dimensional crimp These manufactured fibers have either a lateral arrangement of two components with different water sensitivities or have a sheath-core arrangement of two components with different melting points. Bicomponent acrylic fibers are the primary example of 3-D fibers in the marketplace today having a lateral arrangement of two components with different water sensitivities. These fibers are modeled after wool fiber, a natural fiber which has three-dimensional crimp because the protein polymers on one side of its cortex region (ortho side) differ from the protein polymers on the para side of the cortex region. The polymers in the cortex region have different sensitivities to water. When the fiber is wet, the fiber straightens. As the fiber dries, the fiber crimps placing the ortho cortex on the inside of the crimp provided the fiber is free from constraint. While not all 3-dimensionally crimped acrylic fibers are manufactured using the same components consider the manufacture of the 3-D crimped acrylic fibers successfully sold under duPont’s trademarks SayelleTM and WintukTM. These fibers were manufactured by dry spinning of polyacrylonitrile (PAN) polymer and a sulfonate copolymer through a spinneret. Although the spinneret holes were circular, the forces acting on each polymer in gellation produced mushroom-shaped filaments comprising a ‘cap’ of PAN and a ‘stem’ of the copolymer. Crimp was developed during dyeing and drying operations. The sulfonate copolymer by virtue of its many ionic sulfonate groups is hydrophilic and shrinks as the water is removed, whereas the PAN component of the fiber is stable. This difference in length creates the spiral crimp with the copolymer portion being on the inside of the spiral. The mechanical forces creating the crimp are of low magnitude which means that the yarn or garment made with the fiber must be relatively free of restraint for full crimp development in the fibers. When a garment containing this type of bicomponent acrylic is laundered – the hydrophilic portion elongates, straightening out the crimp, then on tumble drying, the crimp redevelops. An example of a 3-dimensionally crimped fiber having a sheath-core arrangement of components with different melting points is a 100% PLA fiber. Because the PLA polymer can be polymerized with control over the content and arrangement of the three stereoisomers, polymers with varying melt temperatures can be made. In fact, polymers can be made with melt temperatures ranging from about 120oC/248oF and 175oC/347oF. When

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polymers with different melt temperatures are extruded in a sheath-core configuration, with the core being the lower melt temperature component, 3D crimped fibers result. Self-crimping fibers are straight as they are produced but after drawing and heat-setting and heating the fiber above its heat-set temperature causes the fiber to shrink. Because the two polymers shrink at different rates, the fiber curls into a helical shape, providing bulk.

6.5.2 Binder fibers in nonwovens Binder fibers are the most efficient way to impart strength to a nonwoven fabric. Binder fibers are bicomponent fibers with a sheath of one polymer and a core of another polymer. The binder fibers’ sheath melts at a relatively low temperature, and the core melts at a higher temperature. Nonwoven fabrics made with binder fibers can be bonded together simply by heating the fabric to melt the sheath but not the core of the binder fiber. Upon cooling, the molten sheath freezes, gluing the other fibers together and producing a strong fabric. Binder fibers may be composed entirely of PLA polymers, of polyethylene and PET polyester polymers, or of coPET and PET polymers.

6.5.3 Fabrics composed of microfibers Microfibers are those that have a denier (the weight in grams of 9000 meters of fiber) less than one which makes them as fine as cultivated silk fiber. While making microfibers to subsequently spin into yarns and make into fabric is not impossible, it is a challenge due to the fragility of the fibers. Therefore, the usual method for making microfiber fabrics is to extrude fine polyester (or nylon) filaments into a thin stream of polystyrene. Each filament fiber – part styrene and part filaments in a matrix arrangement – is of standard denier. These filament fibers, gathered into a filament yarn, are woven or knitted to form a fabric. The next step is to pass the fabric through a bath of solvent to remove the polystyrene, thus liberating the nylon or polyester fine filaments. A major drawback is that organic solvents have to be used to remove the polystyrene, a process that creates environmental and flammability concerns. Replacement of the poly-styrene with watersoluble polymers has been tried but it is expensive and filaments become tacky when moisture is absorbed from aqueous fiber finishes or from the air. The development of PLA polymers provided another method of making microfiber fabrics that are 100% nylon or 100% polypropylene (PP) but not 100% polyester microfiber fabrics. Here, nylon or polypropylene microfilaments are extruded into a stream of PLA polymer making a fiber having

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the matrix arrangement. The PLA polymer can be easily removed with three minutes exposure in a bath of hot, 3% caustic soda, conditions similar to those used in commercial bleaching and scouring operations. Fabric composed of 0.6 DPF fibers can be simply and cost effectively made starting with a 3-denier per-filament fiber. The reason that 100% PET polyester microfiber fabrics cannot be made in this manner is because PET will hydrolyze under the conditions of hydrolyzing the PLA.

6.5.4 Making simulated suede and leather fabrics Traditionally, simulated leather and suede fabrics, a type of nonwoven fabric, are made from bicomponent, (3-denier filament) fibers containing alternating segments of PET-polyester and nylon. The filaments are carded into a loose web. Hydro-entanglement, a process of using water jets to cause the fibers to entangle themselves into a coherent structure and to cause each fiber to split into 0.2 denier filaments (micro-fibers), follows. After fabric finishing, the nonwoven fabric’s surface has the feel of natural suede. The white fabric is then dyed twice, once with dyes that will color the nylon micro-fibers and then with dyes that will color the PET-polyester micro-fibers. A newer way to produce simulated leather and suede fabrics involves making a bicomponent fiber with alternating segments of PLA and PET polymers. These two polymers easily split apart during hydro-entanglement and the fabric can be dyed in one dye bath because both are dyeable with disperse dyes. This means greater efficiency and improved uniformly of the dyeing.

6.6

Future trends

Future trends for textile fibers are numerous. Two excellent lengthy descriptions of future trends can be found in New Fibers 2nd edition (Hongu and Phillips, 1997) in a chapter titled ‘Fibers for the next millennium’ and in a 2005 book titled New Millennium Fibers (Hongu et al. 2005). The authors classify textile fiber developments as ‘new frontier fibers’, ‘superfibers’, and ‘high function fibers’. They also include chapters that summarize fiber developments for specific applications: a) expected advancements to enhance human heath and comfort, b) specific advancement in fibers for medical healthcare, and describe possible advancement using nanotechnology to create a variety of nano-fibers. Interestingly, the authors also write about fiber for health and nutrition, a topic that diverges from textile fiber. Another trend to be noted is that of greater utilization of chemicals from agricultural products for the synthesis of polymers. Certainly, the extraction of lactic acid from corn and other agricultural products to produce PLA

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polymers and the production of PLA fiber is a major example of the beginning of this trend. Another example is the extraction of latex from plants, such as sunflowers and guayule, to product rubber fiber and other rubber products (Wood, 2002). Further, greater utilization of polymers that exist in naturally-occurring abundant materials is anticipated. A recent example is the extraction of cellulose from bamboo and the use of this cellulose polymer to produce ‘rayon from bamboo’. Likewise, crab and shrimp shells, are an abundant source of chitin, a polysaccharide which can be made into fibers, fabrics, sutures, and nonwoven fabrics. Research continues to improve production and find additional uses. Not to be forgotten is the interest in and research activity related to genetically engineering polymers for fibers. Currently, there is considerable interest in genetically engineering dragline spider silk because here is a fiber that has high strength (is stronger per unit weight than steel), is light-inweight, has high extensibility, has high water absorption, and is biodegradability. The properties of spider silk make it suitable for everything from surgical sutures to body armor (Mahish and Laddha, 2005). However, harvesting spider silk from spider webs is not practical. Genetic engineering is being used to make spider silk as chemical synthesis is not viable at the present time. Mahish and Laddha (2005) describe several ways that scientists have approached the synthesis of dragline spider silk including the most recent and promising way which is the insertion of silk genes into goats and cows to produce silk protein in their milk. A method of spinning spider silk from solution on a large scale has not been developed. Some problems yet to be solved include: the spider spinning dope is about 50% protein, making the dope too viscous, and the silk is not soluble in water.

6.7

Sources of further information and advice

Sources of information about the development of new polymers for fibers and methods for spinning fibers are abundant. These include online and print scientific journals and scientists at universities with polymer and fiber development departments, at fiber manufacturers, and at major textile fiber related associations. Specific examples follow.

6.7.1 Major scientific journals Journal of Applied Polymer Science (http://www3.interscience.wiley.com/ cgi-bin/jhome/30035), Journal of Engineered Fibers & Fabrics (www.jeffjournal.org), Chemical Fibers International http://www.chemical-fibers.com/), Textile Research Journal (http://trj.sagepub.com/),

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AATCC Review (formerly Textile Chemist and Colorist and American Dyestuff Reporter and Textile Chemist and Colorist) (http://www.aatcc. org/magazine/aatccReview.cfm).

6.7.2 Organizations that focus on polymer and textile fiber production The American Manufactured Fiber Association (http://www.fibersource. com/afma/afma.htm) Numerous Global Manufactured Fiber Associations listed at the following URL: http://www.fibersource.com/f-tutor/assoc.htm

6.7.3 United States universities with polymer and fiber development programs Polymer and Fiber Engineering program at Auburn University (http://www. eng.auburn.edu/txen/) School of Materials Science and Engineering at Clemson University (http:// mse.clemson.edu/index.htm) School of Polymer, Textile and Fiber Engineering at Georgia Tech University (http://www.tfe.gatech.edu/) Textile Engineering, Chemistry and Science program at North Carolina State University http://www.tx.ncsu.edu/departments/tecs/index.html

6.7.4 Book publishers Marcel Dekker (www.dekker.com) Woodhead Publishing (www.woodheadpublishing.com)

6.8

References

ASTM, ‘Standard Terminology for Multicomponent Textile Fibers,’ ASTM Standard D4466-02, Annual Book of ASTM Standards, Vol 7.02, ASTM International, Conshohocken PA, 2005. Blackburn R S (2005), Biodegradable and sustainable fibres, Cambridge, Woodhead Publishing Ltd. Bullio P G (1992), ‘The fiber of tomorrow: special characteristics place Tencel, a new cellulosic fiber, in a class of its own’, Fiber World, Sept 16–18. Cole D J (1992), ‘A new cellulosic fibre – Tencel’, in Mukhopadhyay S K, Advances in Fibre Science, Cambridge, The Textile Institute, 25–44. Federal Trade Commission, Textile Fiber Product Identification Act, Code of Federal Regulations, Title 16, Section 303.7, 2006. http://www.ftc.gov/os/statutes/textile/rrtextl.htm

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Gupta B, Revagade N, Anjum N, Atthoff B, Hilborn J (2006) Preparation of poly(lactic acid) fiber by dry-jet-wet spinning. II. Effect of process parameters on fiber properties; J Applied Polymer Science; 93, 3774–80. Hearle J W S, High Performance Fibres, Woodhead Publishing Limited, Cambridge England, 2001. Hatch K L, Textile Science, West Publishing Co. Minneapolis MN, 1993. Hatch K L, Textile Science, Tailored Text Custom Publishing, Apex NC, 2006. Hersh S P (1985), ‘Polyblend fibers’, in Lewin M and Preston J, Handbook of Fiber Science and Technology Volume III Part A, New York and Basel, Marcel Dekker, Inc, 2–50. Hongu T, Takigami M, Phillips G O, New Millennium Fibers, Woodhead Publishing Limited, Cambridge England, 2005. Hongu T and Phillips G O, New Fibers, 1st ed., Woodhead Publishing Limited, 1990. Hongu T and Phillips G O, New Fibers, 2nd ed., Woodhead Publishing Limited, 1997. Chapter 8 Cellulosic Fibers, 191–208. Jeffries R (1971), Bicomponent Fibres, London, Merrow Publishing Co. Ltd. Karypidis M, Wilding M A, Carr C M (2001), ‘The effect of crosslinking agents and reactive dyes on the fibrillation of lyocell’, AATCC Review, 1(7), 40–4. Keesee S H (1998), ‘Troubleshooting in wet processing: Acetate, rayon/lyocell and spandex blends’, Textile Chemist Colorist, 30(6), 10–11. Klein W (1994), Man-made Fibres and Their Processing, Cambridge, Woodhead Publishing Limited. Kumar A (1994), Lepola M, Purtell C. ‘Enzymatic finishing of man-made cellulosic fabrics’, Textile Chemist Colorist, 26(10), 25–7. Kumar A, Harden A (1999). ‘Cellulase enzymes in wet processing of lyocell and its blends’, Textile Chemist Colorist/Am Dyestuff Reporter, 1(1), 37–45. Lewin M and Pearce E M (1998) Fiber Chemistry (2nd edition), New York, Basel, and Hong Kong, Marcel Dekker Inc. Lewin M and Preston J (1985) Handbook of Fiber Science and Technology Volume III Part A, New York and Basel, Marcel Dekker, Inc. Lunt J ‘(2000), Polylactic acid polymers for fibers and nonwovens’, Inter Fibers J, June, 48–52. Lunt J, Bone J (2001), Properties and dyeability of fibers and fabrics produced from polylactide (PLA) polymers’. AATCC Review, 1(9), 20–3. Mahish S, Ladda S K (2005), ‘Spider silk: the miracle material’, AATCC Review, 5(1), 14–16. Masson J C (1995), Acrylic Fiber Technology and Applications, New York, Basel, and Hong Kong, Marcel Dekker. McIntyre J E (2004), Synthetic Fibres: Nylon, polyester, acrylic, polyolefin, Woodhead Publishing Limited, Cambridge England. Mukhopadhyay S K (1993), ‘High performance fibers’, Textile Progress, 25 3/4, 1–85. Nakajima T (1994), Advanced Fiber Spinning Technology, Cambridge, Woodhead Publishing Limited. Nechwatal A, Nicolai M, Mieck K-P (1996), ‘Crosslinking reactions of spun-wet NMMO fibers and their influence on fibrillability’, Textile Chemist Colorist, 28(5), 24–7.

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Placek C (1971), Multicomponent fibers, Park Ridge NJ, Noyes Data Corporation. Taylor J N, Bradbury M J, Morehouse S (2001), ‘Dyeing Tencel and Tencel A100 with poly-functional reactive dyes’, AATCC Review 1(9), 21–4. Watkins P (1995), Tencel: The mystery explained’, Apparel International, 36(1), 21–4. Wood M (2002), Sunflower Rubber?, Agricultural Research, 50(6), 22. Woodings C R (2001), Regenerated Cellulose Fibres, Cambridge, Woodhead Publishing Inc. Yang Y, Huda S (2003), ‘Comparison of disperse dye exhaustion, color yield, and colorfastness between polylactide and poly(ethylene terephthalate)’, J Applied Polymer Science, 90(12), 3285–90.

7 Optical microscopy for textile fibre identification M WILDING, The University of Manchester, UK

Abstract: This chapter examines some of the most important methods in optical microscopy for the identification of fibres, briefly discussing the underlying optical phenomena on which they are based and indicating areas where each might be advantageously applied. In addition, it considers some of the practical issues involved in preparing fibre specimens for microscopy, and straightforward procedures that can greatly simplify the process of making an unambiguous identification. A list of resources and further reading is provided. Key words: Optical microscopy of fibres, refractive index and birefringence in fibre identification, fluorescence microscopy in fibre identification, confocal microscopy in fibre identification, specimen preparation for fibre microscopy.

7.1

Introduction

It may be said with some justification that the simplest and most obvious way to identify a given fibre is to look at it. In this regard it is useful to recall the Textile Institute’s definition of a fibre: ‘. . . a unit of matter characterised by length, fineness and a high ratio of length to thickness’. This property of fineness (many fibres being so delicate as to be barely visible to the naked eye) makes meaningful examination impossible in most cases without the aid of magnification, and therefore some form of microscope. However, there is a wide diversity of instruments available, based on a variety of different principles; and each type will have its own particular strengths and weaknesses so that it may not always be easy to decide on the best technique to use in any specific case. This chapter examines some of the most important methods, briefly discussing the underlying optical phenomena on which they are based and indicating areas where each might be advantageously applied. Of potentially equal value, it also considers straightforward procedures that can greatly simplify the business of making an unambiguous identification. There are several excellent reference sources: I have found the book by Greaves and Saville (1995) to be particularly valuable, and would certainly recommend it as a suitable starting point for readers intending to pursue any of the experimental techniques described here. 133

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7.2

Practical and quality control considerations

Before examining specific microscopic methods that can be used in relation to fibre identification, it is appropriate to consider a number of general points associated with the nature of fibrous materials themselves, sampling methods and specimen preparation. In addition, it should be realised that any microscopic method being contemplated is likely to be covered by at least one internationally recognised standard. It is therefore important to ensure that, wherever possible, the relevant prescribed procedure is followed. Sources of information concerning the major standards organisations can be found in Section 7.7. Whilst the operation of a given type of microscope will demand specific procedures, there are nevertheless some general principles that should be adhered to, as well as points to consider, examples of which will now be discussed briefly.

7.2.1 Destructive versus non-destructive examination A question which should be asked at an early stage in the investigation is: does it matter whether the test is destructive? In forensic studies, for example, it is not uncommon for the amount of material available to be minute; and this may be required in its original form at some future date – either as evidence or to enable further testing should this become necessary. The answer to this question will therefore have a bearing on what procedures and techniques may be suitable. If the subject must be preserved, then it is most likely that it will be the sole (or nearly-so) individual available for inspection. Thus, those methods which, for example, involve immersing the specimen in a refractive-index liquid will probably be ruled out because once the fibre has been contaminated in this way, it will be virtually impossible to recover it in its original pristine state. Similarly, methods which call for physical sectioning, melting, etc., will obviously have to be avoided. On the other hand, sampling is clearly not a relevant issue in such cases, which does at least simplify matters somewhat.

7.2.2 Sampling If a relatively large quantity (say 100 individuals or more) is available, then a wider range of techniques becomes possible. However, it should be appreciated that fibres, particularly those of natural origin, can be considerably more variable in character than many other common materials. This variability is not limited to obvious features such as dimensions, but extends to physical (e.g., mechanical, electrical and thermal), and in some cases

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chemical, properties also. For this reason it will normally be necessary to examine a representative sample of fibres, rather than a single specimen. How many individuals should be included, and the sampling procedure adopted to ensure the sample really is representative then become important additional considerations. In this regard, an important factor is the supposed (or known) source of the fibre(s) to be identified. In many cases this will be a textile of some description. Textile fibres can exist either in ‘raw’ form (e.g., bale cotton, fluff, lint and dust) or as part of a manufactured structure such as a roving, sliver, yarn, knitted or woven fabric, or finished article. Moreover, a textile fibre’s origin could be an article of clothing or some other domestic item (carpet, curtain, upholstery, etc.), but equally it might be an industrial product such as a nonwoven filter or vehicle tyre cord. Each such structure will have its own distinctive characteristics, irrespective of the fibre type(s) it contains, and these will tend to dictate the sampling procedure that should be followed. A further complication is that very many textile products contain not one, but a mixture (blend) of different fibre types. This intensifies the need to employ the proper sampling method to ensure that these differing components will be correctly represented. It is not the intention to describe detailed sampling methods here. An account of the main procedures in use for preparing samples for microscopy from loose fibre, yarns, fabric and made-up articles can be found in Chapter 2 of Greaves and Saville (1995). Additionally, most standard texts that deal with the physical testing of textile materials also cover issues relating to sampling; for example, the books by Booth (1983) and Saville (1999).

7.2.3 The conditions of temperature and humidity The variability of textile fibres has already been referred to. Of equal importance is the fact that most of them are, to a greater or lesser degree, sensitive to temperature and humidity. With only very few exceptions, fibres (whether strictly ‘textile’ or not) are composed of carbon-based polymers. Some of these (e.g. polyesters, polyamides and polyolefins) are thermoplastic, which means they soften as the temperature rises, but other properties can also be affected. Moreover, the temperature need not be particularly high for discernible effects to occur: in some cases even raising the temperature from 20 °C to 30 °C will produce measurable changes in physical properties. Many fibres (wool, cotton and silk being notable examples) are in addition hydrophilic. Thus, variations in the surrounding humidity can have a similarly dramatic effect. Since it is the physical properties that are likely to be used in the identification process, it is vital to maintain the temperature and humidity of the testing laboratory as close as possible to the

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appropriate internationally agreed standard (e.g., 20 ± 2 °C; 65 ± 2% relative humidity). Testing the specimen in the prescribed laboratory atmosphere is not sufficient in itself, however. Fibres tend to absorb (and desorb) moisture relatively slowly. Thus, if a subject is transferred from one environment to another, it may take a significant time to reach equilibrium. This necessitates a procedure called ‘conditioning’, in which, typically, the specimen is left in the controlled atmosphere for a period of up to 24 hours prior to testing or examining.

7.2.4 Specimen preparation A vital component of practical microscopy is, of course, preparation of the specimen itself. Space does not allow a detailed account of all the various sample-mounting procedures that may be employed, but it is important to highlight some basic principles. There are essentially two main aspects from which a fibre specimen can be viewed: (a) longitudinally (i.e. side-on); or (b) transversely (i.e. in cross-section). Except when making very superficial examinations (using, for instance, a stereo zoom microscope), the specimen will generally be mounted between a glass slide and cover slip. A few drops of an appropriate mounting liquid should be included in order to ensure good optical contact, thus improving image clarity. Common examples are xylene, liquid paraffin (‘nujol’), cedarwood oil, ‘Permount’ (a mixture of piccolyte: a B-pinene polymer; and toluenene polymer) and phytohistol. Moreover, the refractive index of the liquid must be known if quantitative information is required. It is also very important to note that many refractive-index liquids are not only toxic in the conventional sense, but also carcinogenic. Whatever mounting method is used, caution must be taken not to damage the specimen in the process, as even the tiny forces associated with normal handling can cause permanent physical changes. Tweezers and mounting needles will probably be used to assist in preparation, but care still needs to be exercised. Longitudinal specimens The specimen (either a single length of fibre or, for example, a small tuft) is usually placed on the microscope slide using tweezers. In the case of a tuft, this should be carefully teased out with a mounting needle so that as far as possible the entire specimen is a single fibre in thickness. If a liquid is to be used it can be added at this stage using a dropper. It may be necessary to tease the specimen again with the needle to ensure thorough wetting. If the fibres should absorb some of the liquid, then extra drops can be added

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as appropriate, but be sure not to use more than is needed to fill the space between cover slip and slide. The cover slip is then gently applied, using the tip of the needle to lower it down in the manner of closing a lid. The liquid should spread evenly under the cover slip, but if any air bubbles appear, they can sometimes be removed by pressing down very gently onto the top surface with the needle, and easing it slightly from side to side. Transverse (cross-sectional) specimens Preparing transverse sections does require some skill, but is not particularly difficult to do. If sufficient fibre is available it can be achieved using the ‘plate method’, in which a bundle of fibres is pulled, using strong thread, through a small hole in a metal plate. The bundle must be thick enough so that it is a snug fit in the hole. The protruding fibres on both surfaces are then sliced off flush with the plate, using a razor blade or scalpel. Alternatively a microtome, such as the Hardy microtome, can be used. Palenik and Fitzsimmons (1990) describe a relatively simple method for preparing cross-sections. In their procedure the fibre to be examined is sandwiched between two thin films of polyethylene, the whole being mounted on a glass slide with a cover slip, and placed on a microscope hotplate. The composite specimen is heated just sufficiently to melt the polyethylene, and light pressure is exerted on the cover slip to ensure effective embedding of the fibre in the molten polymer. The sandwich is then allowed to cool, after which the cover slip is removed. Aided by a stereo zoom microscope, thin slices are taken using a razor blade. These must be thin enough to lie on a microscope slide with the original fibre axis vertical. Polyethylene is chosen because it does not adhere to glass. It should be pointed out, though, that its melting temperature is around 145 °C, which to some extent limits the range of fibres for which the technique is applicable. An alternative embedding medium such as epoxy resin may be considered where the specimen is particularly temperature-sensitive, although this requires a different procedure. For further guidance on preparing specimens, see, for example, Greaves and Saville (1995) (Chapters 2 and 5).

7.3

Initial identification based on physical appearance

7.3.1 Stereo zoom and simple light microscopy Considering the definition of a fibre given in the introduction, one might be forgiven for thinking that all fibres are of generally similar appearance – i.e., long thin cylinders. Most synthetic (i.e. artificial) fibres do admittedly

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tend to have this geometry, but it is certainly not the case for the vast majority of natural fibres or, for that matter, many of the regenerated fibres. These classes of fibre encompass considerable diversity of form, and usually display clearly distinguishable features. For this reason it is often best to begin with a straightforward examination using a simple instrument such as a stereo zoom or a conventional light microscope, which will enable these characteristics to be identified very quickly. Ideally, the microscope should be equipped with a vernier-type graticule to facilitate measurement of apparent thickness, etc. A small sample of the material should be prepared so that short lengths (say a few mm) of individual fibres will be observable. This can be placed with very little fuss on to a microscope slide, using tweezers; a few drops of a suitable mounting liquid, such as liquid paraffin, can be added before the cover-slip is lowered. This should give good optical definition, although it may not be necessary. The appearance of the fibres is then noted, and possibly a tentative identification made. For instance, wool and other ‘hair’ fibres nearly always display cuticular scales on their surface. These overlap in the manner of roof tiles. It might not be possible on first examination to state with confidence that one has a particular breed of wool – or indeed wool from a sheep at all – but it would generally be obvious that the subject belongs to the broader category of hair fibres. At the very least, if it has overlapping scales then its animal origin is essentially confirmed. This example brings to mind the Holmesian Maxim (Conan Doyle, 1890): ‘When you have eliminated the impossible, whatever remains, however improbable, must be the truth.’ Adopting this principle, it is often easier to proceed on the basis of what a fibre is not, rather than what it is. Thus, if it does not have overlapping scales then we can deduce that it probably is not wool. (One does need to exercise caution, however, since it is possible for wool to be chemically treated so as to remove the scales; additionally, continued wear may have rendered the fibre surface smooth.) Many other natural, and man-made, fibres can be identified (or eliminated) in this way, by seeking out their most distinctive features – convolutions in the case of cotton, striations in viscose rayon, and so on. Moreover, in some instances this quick method of initial identification can also be applied to the visually plainer artificial fibres; polyesters, polyamides (nylons) and so on. Whilst these are most commonly manufactured to be cylindrical, this is not by any means always the case. Melt-spun synthetic fibres, in particular, are often produced with non-circular cross-sections: e.g., elliptical, triangular, trilobal and ‘propeller’; again, these features are relatively easy to observe. However, it may be necessary to prepare crosssection samples (as distinct from the longitudinal ones assumed above). The

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transparency/opacity of the fibres may be another factor to be noted, as well as any colouration. Even in the case of cylindrical fibres, all is not lost as regards making a useful initial examination: there are many different detailed specifications that may be encountered, and which can therefore readily be used as identifiers. These include the diameter, and whether or not the fibre is hollow; if it is, then a further factor will be the relative thickness of the fibre wall. The reason why such features are so useful is that, whilst textile fibres as a whole have a reputation for being variable, this judgement is based mainly on the natural fibres. For most synthetics the extrusion and other manufacturing conditions are generally well-controlled so that one would not expect large variations in dimensions within the same batch. Therefore if, for example, the diameter of a synthetic fibre under observation is significantly different from that of a reference sample, the Holmesian Maxim would suggest that it is not from the same batch, irrespective of whether it is the same fibre type. The first stage in the identification process, then, would normally be to attempt to place the subject into its correct group within the overall fibre classification scheme: natural-animal; natural-vegetable; man-made (regenerated); synthetic. Greaves and Saville (1995; p. 9) present a flow-chart describing a useful procedure whereby this might be accomplished.

7.4

Identification based on properties

Although the ‘quick look-see’ approach is very useful in making the initial examination, it will inevitably be necessary to seek more detailed clues as to a fibre’s exact identity. An obvious case in point is where a specimen has been narrowed down to being synthetic, based on its physical appearance under the microscope, but where the exact polymer type remains unknown. We may not be able to tell, for example, if it is polyester, polyamide, polyolefin or acrylic, all of which can appear the same in a simple microscopic observation. The key here is to utilise certain physical fibre properties that are related to the specific chemical composition and which can be observed using microscopic techniques. Among the most important of these are: refractive index, usually considered together with its related quantity, birefringence; melting (or otherwise) behaviour; and solubility.

7.4.1 Refractive index and birefringence Before discussing how fibres might be identified on the basis of their refractive index, it is important to appreciate that they are seldom optically isotropic (or indeed isotropic in any respect). This arises ultimately because

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the chemical repeat, or monomer, groups that make up the constituent polymers of most fibres are themselves anisotropic. The anisotropic nature of the fibre as a whole results from a greater or lesser degree of orientation within its structure. This may occur naturally, as in the helical fibrils of cotton and the aligned cortical cells of wool; alternatively, it may be a result of deliberate stretching or other processes applied during manufacture in order to improve performance properties. (This is routinely the case for melt-spun synthetics such as nylons and polyesters.) It can even be induced inadvertently during use. Simple polarised light microscopy As a result of the structural orientation referred to above, the vast majority of fibres possess a property known as ‘birefringence’, implying, in practical terms, that their longitudinal and transverse refractive indices differ. A birefringent specimen will rotate the plane of polarisation of planepolarised light passing through it. Useful information can often be gained quite quickly from such subjects by using a relatively simple polarising microscope. This is in essence just a standard microscope to which has been added one polarising filter (the ‘polariser’) between the light source and specimen, and a second (the ‘analyser’) after the specimen. Both of these may be capable of being rotated, but the analyser certainly must be. The polariser ensures that the light incident on the specimen is plane-polarised. In use, the analyser is set to transmit only light which is polarised at 90 ° to that of the incident beam. This arrangement is generally referred to as ‘crossed polars’. Thus, in the absence of a specimen no light would reach the eyepiece and the entire field would appear dark. If a birefringent fibre is placed on the stage, however, it will rotate the plane of polarisation of the incoming light by an amount related both to its birefringence and to its orientation with respect to the electric field vector. Light leaving the specimen which is polarised at any angle other than 90 ° to the analyser will be partially transmitted, giving rise to an image. Figure 7.1 illustrates this schematically. For simplicity the lenses and other standard optics of the microscope have been omitted. Thus, if the sample stage is rotated through 360 °, the intensity of the image will be observed to change. In particular, there will be four symmetrically spaced positions of maximum brightness and four for which the image is essentially completely dark. ‘Polarisation colours’ will also usually be seen, varying in hue and intensity not only with the degree of rotation, but also across the fibre. Moreover, the specific colours and patterns of variation will differ from one fibre type to another. This can provide a very useful means for making a tentative identification.

Optical microscopy for textile fibre identification Light transmitted

No light transmitted

141

Some light transmitted

Analyser

Uncrossed polars

Crossed polars

Polariser

Unpolarised light Without specimen

With birefringent fibre

7.1 Schematic illustration of the effect of rotation of polarisation plane by a birefringent fibre.

Polarised light microscopy is most commonly performed in the transmission mode, as implied in the above, but for opaque specimens it may be possible to gain similar information by using the reflection mode, where the subject is illuminated from above. Optical anisotropy in more detail The approach just described is rather limited owing to its being purely qualitative. For a more conclusive identification it will generally be necessary to make measurements, either of the birefringence or the individual refractive indices of the fibre, for which more sophisticated instrumentation is needed. It is also important to examine optical anisotropy in rather more detail. Possibly the most significant advance in understanding of the optical anisotropy of fibres was that provided by Ward (1962), who devised a model, referred to as the ‘aggregate theory’, to account for the observed optical (and, indeed, mechanical) behaviour of partially crystalline fibrous polymers. In essence, the solid material was imagined to comprise a large collection of identical anisotropic, orientable units. The model was successfully applied to several different polymeric fibre types (Pinnock and Ward, 1964, 1966, Hadley et al., 1964, 1969). The full analysis is rather complicated, but it is appropriate to highlight several key aspects here. The molecular property that controls light-refraction is called ‘polarisability’. This is a second-rank tensor quantity, which therefore has (formally) nine components. A typical polymer monomer unit will have only six independent components, however, which can be further reduced to three principal values by choosing a particular set of orthogonal spatial axes

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Identification of textile fibers

referred to as ‘principal axes’. In nearly all fibre-forming polymers, one of these axes will be closely parallel to the main chain axis, and the polarisability in this direction is usually significantly greater than in the two transverse directions, since it corresponds to the direction of covalent bonding. The two transverse components will generally also differ from one another, but to a much smaller degree. Polarisability being an extensive property (indeed, having the dimension of volume), it is essentially additive. This means that in larger assemblies, its magnitude will be in proportion to the number of ‘molecules’ contained. Based on knowledge of the unit cell size and shape, details of the chemical bonding (including secondary interactions such as H-bonding) and packing within and between chains, it is possible to determine the mean polarisability components for structural units occurring within the fibre. Thus, an elementary ‘crystallite’ or fibril, say, will typically have three different principal polarisability components. To a reasonable approximation, these can be related to three corresponding orthogonal refractive indices via the Lorenz-Lorenz equation, shown here in an inverted form: ni =

1 + 2α i V 1 + αi V

In the above, ni (i = 1, 2 or 3) is any one of the three refractive indices and αi is the corresponding principal component of polarisability. The subscript, i, refers also to the electric-field direction (polarisation direction) of the incident light. V is the molar volume occupied by the chosen ‘unit-cell’. Figure 7.2 represents an elementary crystal, in which the principal axes have been set up in such a manner that the long- (i.e., molecular chain, in most cases) axis is labelled ‘3’, with the two transverse directions being ‘1’ and ‘2’, respectively. If a fibre could be assumed to consist entirely of a random collection of identical anisotropic units of the kind described – thereby being isotropic overall – it would have effectively only one refractive index, which may be called niso, numerically equal to the average of the three individual values: niso =

1 (n1 + n2 + n3 ) 3

[7.1]

The problem is how to deal with the departure from isotropy encountered in real fibres. Most commonly, and certainly for the majority of manmade and synthetic fibres, it can be assumed that the distribution of orientation is such that the chain axes of the elementary units are preferentially inclined towards the fibre axis direction, but with no preferred orientation transversely. The symmetry associated with this type of orientation is referred to as ‘cylindrical’ or ‘uniaxial’. Owing to this, most fibres display

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3 Crystal or fibril Refraction controlled by n3 Refraction controlled by n3

Refraction controlled by n1 2

Refraction controlled by n2 1

Electric vector of plane-polarised light

7.2 A crystalline fibril with three principal polarisability components has three corresponding refractive indices.

only two distinct refractive indices at the macroscopic level, conventionally denoted by: n// for light polarised parallel to the fibre axis; and n⊥ for light polarised perpendicularly to the fibre axis. (Technically, the fibre still has three principal refractive indices, but two of them are identical, and equal to n⊥.) The ‘birefringence’ (Δn) of a fibre is defined as the difference between the measured values of its two independent refractive indices: Δn = n/ / − n⊥

[7.2]

The birefringence of a given fibre may be positive, approximately zero or – exceptionally – negative. In any event, the magnitude of Δn is a valuable indicator of the degree of orientation within a fibre’s structure. Although there are microscopic methods whereby the birefringence can be estimated directly, the most reliable techniques tend to be based on measurements of the two individual refractive indices. The measurement of n// and n⊥ normally entails the use of a polarising microscope, by means of which a fibre sample can be illuminated, in turn, with light which is polarised parallel to, and at right angles to its longitudinal axis. One method frequently used is to immerse each of a set of nominally-identical samples in a different liquid of known refractive index. The larger the difference between the refractive index of the liquid and that of the sample, the higher will be the definition and contrast of the image formed. In principle, a perfect match would render the sample invisible. In practice, this is very unlikely to be achieved; instead, the refractive index of the liquid giving the closest match would generally be taken as that of the fibre. If a suitable set of liquids is available it should be possible to interpolate for better precision. Clues as to whether a particular sample has a higher or lower refractive index can be gained by using techniques such as the Becke test: the

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Identification of textile fibers

‘Becke’ line is a bright fringe which appears around the edges of a slightlyout-of-focus image. As the objective lens is moved away from the subject the line appears to move towards the medium with the higher refractive index. From the above discussion, it is apparent that if one is primarily interested in identifying the polymeric type of a fibre based on its refracting power, then orientation will complicate matters: there are no unique values either of birefringence or of the individual refractive indices that can be assigned to a given type. However, suppose for the present that the simple fibre-structure model discussed above is applicable. It may be appreciated that whereas the individual refractive indices and the birefringence will in general all change with differing degrees of orientation, the average refractive index should remain reasonably constant – and equal to niso – provided that orientation is the only variable feature of the fibre. Owing to the uniaxial symmetry generally present, the practical definition of niso is slightly different from that given earlier; viz: niso =

1 ( n/ / + 2n⊥ ) 3

[7.3]

The range of isotropic refractive index values presented by textile and other fibres as a whole is not great, but significant nonetheless. It is at any rate sufficiently wide to allow a degree of differentiation between the major fibre types. Figure 7.3 presents typical values of isotropic index for a number of fibres. It is important to note that the values shown are only typical, because even the isotropic refractive index is in fact a non-unique property: the simple model previously discussed ignored several important aspects of fibre structure which need to be addressed. For example, there will usually be an amorphous, or at least poorly-ordered, phase present in addition to the so-called ‘crystalline’ phase. This disordered material will also refract, and the observed value of niso will therefore depend on the relative abundance of the crystalline and non-crystalline material. Because the molecular packing density is greater in the crystalline than in the amorphous regions, it follows that raising the crystallinity will result in increased values of n// and n⊥, and hence of niso. A further complication arises because the amorphous material may itself be oriented independently of the ordered phase, and thus contribute towards the birefringence. Moreover, in many fibres the crystalline phase, in particular, will be inhomogeneous: it is very likely to comprise a distribution of objects of varying states of crystalline perfection, size and shape. Notwithstanding the above complications, however, with caution a tentative identification (or elimination) can often be made. For example, if the refractive index is greater than 1.5 the fibre in question is probably not

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1.650 1.630 Isotropic refractive index

1.610 1.590 1.570 1.550 1.530 1.510 1.490 1.470

Fl

ax W oo Po l ly es Sil k te r( PE T)

se

lu el C

C

el

lu

lo

lo

se

tri

ac et a Po dia te ly ce pr ta op te yl en Ac e St Mo ryl an d ic da ac rd ryl vi ic C sco hl or se of N ibre yl on 66 N yl on C 6 ot to n

1.450

7.3 Typical values of (isotropic) refractive index for fibres; calculated from published data (Greaves and Saville, 1995).

acetate; and if it is less than 1.57 then polyester is an unlikely candidate. On the other hand, it is clear (Fig. 7.3) that there are groups of fibres for which the values of refractive index lie very close to one another; but often, one or more of these may be recognised on the basis of its visual appearance. For instance, Nylon 66, Nylon 6 and cotton have similar refractive indices, but cotton has convolutions whereas nylon does not. Similarly, it is unlikely that acrylics and viscose would be confused owing to the longitudinal striations on viscose fibres. Admittedly, distinguishing between fibres such as Nylon 66 and Nylon 6 can be less straightforward: it will generally necessitate the measurement of some other property, such as the melting point. There are several ways in which refractive index and birefringence can be estimated. For example, in appropriate cases the individual values of n// and n⊥ can be determined via the ‘double-immersion’ technique using an interference microscope. With care, this method can be very accurate. However, it is not straightforward, and since it requires a reasonable supply of the material to be investigated, it is not really suitable where a single fibre specimen is all that is available. More often – and certainly more conveniently – fibre identification on the basis of birefringence is carried out using a ‘quartz wedge compensator’, the commonest of which is undoubtedly the Berek. This is, in essence, a device for measuring the optical retar-

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Identification of textile fibers

dation produced by a birefringent specimen, based on a tiltable calcite plate. The technique entails a polarising microscope capable of incorporating the compensator. The fibre is mounted in the usual way, using a refractive-index liquid, and viewed under crossed polars. The illumination must be of known wavelength, and so either a monochromatic light source or a suitable filter must be employed. The stage is first rotated until the fibre image appears dark. It is then further rotated by 45 ° (clockwise, say). The compensator is inserted and the tilt angle adjusted to bring the zero-order (dark) interference fringe to the centre of the image. The stage is then rotated by 90 ° in the opposite sense and the measurement repeated. This procedure has the effect of eliminating any ‘off-set’ error. The retardation is determined from the mean of the two angle values, using calibration tables provided with the compensator. In order to convert the retardation into birefringence, the optical path-length through the specimen must be known. Hence the fibre thickness must be measured. A more detailed description of the compensator method can be found at, for example: http://www.olympusmicro.com/ primer/techniques/polarized/berekcompensator.html. Hamza and co-workers (1992) also give a detailed account of the determination of the refractive indices and birefringence of fibres using a microinterferometric technique.

7.4.2 Melting behaviour Observation of the melting behaviour is another approach that can be taken in the fibre-identification exercise. This may be done using either a standard or polarising light microscope fitted with a hot-stage incorporating the means to control and measure the specimen’s temperature to within a few degrees Celcius. Greaves and Saville (1995; p. 16) give more detail regarding the techniques used. Not all fibre types melt, and this in itself can be a valuable means of eliminating candidates. Those that do generally belong to the category of thermoplastic fibres – most commonly encountered as the melt-spun synthetics – which includes polyolefins (polyalkene), polyamides, polyesters, acetates and vinyl fibres, along with certain of their copolymers and other variants. Figure 7.4 shows some typical values.

7.4.3 Solubility The extent to which a fibre dissolves in various solvents offers a further means of identification, and a suitable procedure for this is described by Greaves and Saville (1995; pp 12–16). As these authors emphasise, however, safety (of both operator and equipment) is of paramount importance, and must be the first consideration when this technique is being contemplated.

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147

350

Melting point/ °C

300

Lower Upper

250

200

150

Po

ly Po eth ly yle Po pro ne p ly (la ylen ct e ic ac N id) yl on C 1 hl or 1 of C i br el e lu lo Ny se lo n di ac 6 et a Po Ny te lo l y C el est n 6 6 er lu lo se (PE T tri ac ) et at e

100

7.4 Typical values of melting temperature for fibres; data for poly(lactic acid) from Malmgren and co-workers (2006); all other data from Greaves and Saville (1995).

There, are of course, many characteristics of fibres additional to those discussed, that can be useful in effecting their identification, but space does not permit further discussion. Reference, to the resources listed, especially Greaves and Saville (1995) is again recommended.

7.5

Examples of more advanced microscopic techniques

The most straightforward methods of identification tend to be those using the conventional types of light microscope already referred to, such as stereo zoom, polarising, and interference instruments. However, the information these provide can be limited, and it is appropriate to consider, albeit briefly, several more sophisticated, recent developments.

7.5.1 Fluorescence microscopy Certain organic materials display the phenomenon of ‘fluorescence’. This is a process in which light of one characteristic wavelength (and hence colour) is emitted following excitation of the molecule by light of another, shorter, wavelength. It is quite commonly observed in biological systems, where it can give rise to a very reliable means of identification. The same can hold

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Identification of textile fibers

true for textile fibres. It may be that the constituent polymer itself contains one or more fluorescent species or that certain additives within the fibre (including dyes and pigments) fluoresce. A fluorescence microscope makes us of this emitted light, from which an image of the fibre, or part thereof, is formed. The wavelength of the light stimulating fluorescence varies from one molecular species to another, as does the fluorescence itself. Thus, a given species will fluoresce with a characteristic colour, provided light of the appropriate wavelength is used to stimulate it. In the most basic system, the illumination source is a bright1 white light; a set of filters and a dichroic beam-splitter is then used to ensure that only the excitation wavelength reaches the specimen, and that only the expected fluorescence colour reaches the observer (or detector). Figure 7.5 is a simple representation of

Eyepiece

Fluorescence Barrier filter

Dichroic beam-splitter

Excitation filter

Objective lens

Sample

7.5 The principal components of a simple fluorescence microscope. (Based on a diagram appearing at http://www.seas.upenn.edu/ ~confocal/epi-fluor.html.) 1

The fluorescence is invariably very weak compared to the light stimulating it, which means a particularly bright source is generally needed.

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a fluorescence microscope. Some of the more advanced types of instrument employ a tuneable laser as the illumination source. A single fibre specimen, then, may yield a series of images which differ in colour and in form, corresponding to the various fluorescent species it contains, and the wavelengths selected for its illumination. In some experimental studies, specimens are prepared that have been selectively stained with a fluorescent dye in order to highlight specific features within the structure. For example, Thomson et al. (2007) used the technique in a study of the fibre–fibre interface regions in spruce cellulose. Fluorescence microscopy is also rapidly emerging as a crucial tool in the medical and biological sciences. Palomero et al. (2006), for instance, have used it to investigate free-radical generation in muscle fibres.

7.5.2 Confocal microscopy Ideally, an optical microscope image should be formed exclusively from light originating from the plane within the specimen upon which the objective is focussed. Inevitably, however, stray light arising from out-of-focus regions will also reach the eye (or, more generally, detector). This produces blurring of the image and, in consequence, degradation of the information obtained. It can be particularly severe in fluorescence microscopy, where the whole specimen may be contributing to the fluorescent effect. Confocal microscopy was developed in order to eliminate this problem. The basic principle upon which it is based dates back to an invention by Minsky (1988). In essence, a pinhole is used to block out the unwanted light. Figure 7.6 illustrates very simplistically how this is achieved. Suppose the microscope objective is focussed on the point within the specimen marked F in the diagram. A pinhole is located where the real image is expected, in front of the ocular (point F¢). Provided the pinhole is not too small, all the light originating from F will thus be transmitted, and so an image will be observed. Other points within the specimen also potentially produce images, though. Consider one of these, marked P. This

Screen with pinhole Sample P

P´ F

F´

7.6 The essential principle of confocal microscopy; light from the point of focus enters the detector via the pinhole. Light from elsewhere in the specimen is largely rejected.

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Identification of textile fibers

happens to lie beyond F, and so will produce a real image lying somewhere behind F¢ (shown as point P¢). Normally, most of the light from P¢ would also enter the eyepiece, thus blurring the intended image, but the screen effectively prevents it from being transmitted. The same would be true, to a greater or lesser extent, for all other points within the specimen. This is all well and good, but there is a problem: not only will light from points lying behind or in front of F be rejected, but also that from any point within the same plane as F. That is, the screen blocks light originating from everywhere except F, which means the observable image comprises a single point (actually an Airy disk) rather than a 2-dimensional picture. In order to produce a pictorial image of the entire plane containing F, it is necessary to arrange for the incident light to scan across the specimen in two perpendicular in-plane directions. The image has then to be built up gradually from the information obtained for each point in the scan. It is not generally observed directly; instead, a CCD (charge-coupled device) detector is used, and a bitmapped pixel image displayed on a monitor. Currently, the most important practical confocal instrument is the ‘laserscanning fluorescence microscope’, a very informative description of which may be found at http://www.physics.emory.edu/~weeks/confocal/. As with fluorescence microscopy more generally, the exploitation of the confocal technique is currently increasing very rapidly. A significant feature of this method is the ability, for suitable subjects, to build up a 3-dimensional picture by superimposing images generated from successive planes within the specimen (a process referred to as ‘virtual sectioning’): Albrechtova and coworkers (2007), for example, used this approach to study the effects of acid rain on fibres of Norway spruce; and Kubinova et al. (2004) present a useful review of this field of image analysis.

7.6

Future trends

Nowadays, a wide range of sophisticated analytical methods other than optical (light) microscopy is available for use in fibre identification; atomic force microscopy, electron microscopy, vibrational spectroscopy and so on. Nevertheless, there is no doubt that imaging methods using light in one form or another will remain crucial. This final section describes briefly some of the more recent developments taking place.

7.6.1 Multiphoton fluorescence microscopy This is a relatively new technique for imaging based on the simultaneous absorption by a fluorophore of two (in almost all practical instruments) photons and subsequent emission of a single, shorter-wavelength, photon.

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The energy of the emitted photon is approximately equal to the sum of the energies of the exciting photons, which means that comparatively low energy excitation can be employed (typically within the infrared region of the spectrum). This is a considerable advantage, particularly where live biological specimens are being examined, because it greatly reduces the risk of cell damage, or photodegradation. In practical instruments the exciting photons are provided by two separate lasers of either equal or unequal wavelengths. The beams from these lasers, which are sometimes tunable, are combined at the point of focus within the sample. Because the probability of simultaneous absorption of two photons is so small, it tends to occur only where the beams meet, and where the intensity is highest, which is at this focal spot. This effectively eliminates unwanted fluorescence in much the same way that the pinhole does in a confocal microscope, so that these instruments also lend themselves to virtual sectioning. Although multiphoton microscopy was developed specifically for the bio-sciences, it is likely that it will become more widely exploited, and should prove valuable in relation to fibre identification. Many forensic subjects, for example, are vulnerable to radiation damage when viewed under high-intensity visible light. An exhaustive review of two-photon fluorescence microscopy for biological studies is given by Diaspro and co-workers (2005)

7.6.2 Overcoming the classical resolution limit The classical (Abbe) diffraction limit for a light microscope is determined by the wavelength of the illumination and the angle subtended by the objective lens. In the best practical case this results in a resolution, for white light, of approximately 200 nm. It is possible to reduce this to some extent by, for example, using confocal methods, and by employing various modifications to the microscope optics. Even then, though, 100 nm is about the best that can be expected. However, recent developments have taken place, making use of the nonlinear properties of fluorescent dyes, which promise resolutions down to as little as 10 nm. Successful use of such techniques, which include so-called stimulated emission depletion (STED), is reported in publications by Willig et al. (2006a, 2006b).

7.6.3 Optical coherence tomography Originally reported in 1991 (Huang et al., 1991), optical coherence tomography (OCT) is a non-invasive technique for through-specimen imaging based on interferometry. Because of its ability to produce high-resolution three-dimensional images at penetration depths of the order of millimetres, it has become increasingly exploited in the biomedical field, especially

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ophthalmic optics (Ko et al., 2001). Significant recent advances in OCT instrumentation have taken place through the introduction of broad-band light sources, including high-output LEDs and femtosecond pulsed lasers, and it is likely that OCT will prove to be valuable in relation to fibre studies.

7.6.4 Other developments Other areas for development include methods of compensating for variations in fibre thickness and refractive index, as well as the dispersion effects often associated with thicker specimens. See, for instance: http://www.hhmi.org/janelia/pdf/july_workshop.pdf.

7.7

Sources of further information and advice

7.7.1 Books Booth, J. E. (1983) Principles of Textile Testing, London, NewnesButterworth. ISBN: 0408014873. Diaspro, A. (Ed.) (2001) Confocal and Two-Photon Microscopy: Foundations, Applications and Advances, New York, Wiley-Liss. ISBN-10: 0471409200. ISBN-13: 978-0471409205. Greaves, P. H. & Saville, B. P. (1995) Microscopy of Textile Fibres, Oxford, U.K., Bios Scientific Publishers. ISBN: 1 872748 24 4. Heath, J. P. (2005) Dictionary of Microscopy, Wiley. ISBN-10: 0470011998. ISBN-13: 978-0470011997. Matsumoto, B. (Ed.) (2002) Cell Biological Applications of Confocal Microscopy (2nd Edn.), Academic Press. ISBN-10: 0125804458. ISBN-13: 978-0125804455. Murphy, D. B. (2001) Fundamentals of Light Microscopy and Electronic Imaging, Wiley-Liss. ISBN-10: 047125391X. ISBN-13: 978-0471253914. Paddock, S. W. (Ed.) (1999) Confocal Microscopy Methods and Protocols, Humana Press. ISBN-10: 0896035263. ISBN-13: 978-0896035263. Pawley, J. B. (Ed.) (1995) Handbook of Biological Confocal Microscopy (2nd edn.), Springer. ISBN: 0306448262. Rochow, T. G. and Tucker, P. A. (1994) Introduction to Microscopy by Means of Light, Electrons, X Rays, Or Acoustics, Springer. ISBN: 0306446847. Rost, F. W. D. (1991) Quantitative Fluorescence Microscopy, Cambridge Universtiy Press. ISBN-13: 9780521394222. ISBN-10: 0521394228. Saville, B. P. (1999) Physical Testing of Textiles, Cambridge, Woodhead, with The Textile Institute. ISBN: 1855733676. Online version also available at: http://www.knovel.com/knovel2/Toc.jsp?BookID=925&VerticalID=0.

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7.7.2 Electronic resources http://micro.magnet.fsu.edu/primer/photomicrography; accessed 21 August, 2007 http://www.fz-juelich.de/inb/inb-1/Two-Photon_Microscopy/; accessed 22 August, 2007 http://www.futureimage.com/; accessed 24 August, 2007 http://www.hhmi.org/janelia/pdf/july_workshop.pdf; accessed 24 August, 2007 http://www.lightlabimaging.com/oct.html; accessed 24 August, 2007. http://www.loci.wisc.edu/multiphoton/fastmp.html; accessed 24 August, 2007 http://www.loci.wisc.edu/multiphoton/mp.html; accessed 24 August, 2007 http://www.microscopyu.com/articles/polarized/polarizedintro.html; accessed 23 August, 2007 http://www.physics.emory.edu/~weeks/confocal/; accessed 17 August, 2007 http://www.rsc.org/chemistryworld/Issues/2007/March/TheMillionDollarMi croscope.asp; accessed 22 August, 2007 http://www.seas.upenn.edu/~confocal/epi-fluor.html; accessed 17 August, 2007 http://www.stjapan.de; accessed 21 August, 2007

7.7.3 Suppliers of microscopes and accessories Brunel Microscopes Ltd Unit 2, Vincients Road Bumpers Farm Industrial Estate Chippenham Wiltshire SN14 6NQ UK Helpline: 0044 (0)1249 462655 Fax: 0044 (0)1249 445156 http://www.brunelmicroscopes.co.uk/; accessed 24 August, 2007 Carl Zeiss (UK) Ltd. Head Office 15–20 Woodfield Road Welwyn Garden City Hertfordshire AL7 1JQ UK Phone: +44 1707 871200 Fax: +44 1707 330237 http://www.zeiss.co.uk/; accessed 25 August, 2007.

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GX Optical http://www.gxoptical.com; accessed 21 August, 2007 Division of GT Vision Ltd (UK, Europe, Africa, Asia, Australiasia): Hazel Stub Depot Camps Road Haverhill Suffolk CB9 9AF UK Tel: (from UK) 01440 714737 (outside UK) +44 1440 714737 Fax: (from UK) 01440 709421 (outside UK) +44 1440 709421 Division of GT Vision LLC (USA, Canada, S and Central America): 10205 Easterday Court Hagerstown, MD 21742 USA Tel: +1 240 235 4118 Fax: +1 240 235 4120 Leica Microsystems International Headquarters: Leica Microsystems GmbH Ernst-Leitz-Strasse 17–37 35578 Wetzlar Phone +49 6441 29-0 Fax +49 6441 29-2590 http://www.leica-microsystems.com/ Microscope Systems Scotland 11 Strathblane Road Glasgow G62 8DL UK Phone: +44 (0) 141 563 9696 Fax: +44 (0) 141 563 9696 Email: [email protected] http://www.microscopesales.co.uk; accessed 25 August, 2007. Olympus http://www.microscopy.olympus.eu; accessed 25 August, 2007. Prior Scientific Toll Free: 800-877-2234 http://www.prior.com/; accessed 25 August, 2007.

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Selectscience.net http://www.selectscience.net/; accessed 25 August, 2007.

7.7.4 Supporting organisations The American Microscopical Society, Inc. http://www.amicros.org/; accessed 25 August, 2007. ASTM International (formerly The American Society for Testing and Materials (ASTM)) 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959 USA http://www.astm.org/cgi-bin/SoftCart.exe/index.shtml?E~mystore; accessed 24 August, 2007. The Australian Microscopy and Microanalysis Society http://www.microscopy.org.au/; accessed 25 August, 2007. British Standards Online http://www.bsonline.bsi-global.com/server/index.jsp; accessed 24 August, 2007 The European Microscopy Society (EMS) http://www.eurmicsoc.org/; accessed 25 August, 2007. The International Organization for Standardization (ISO) 1, ch. de la Voie-Creuse, Case postale 56 CH-1211 Geneva 20 Switzerland Telephone +41 22 749 01 11; Fax +41 22 733 34 30 http://www.iso.org; accessed 25 August, 2007. Olympus Microscopy Resource Center http://www.olympusmicro.com/primer/techniques/polarized/ berekcompensator.html; accessed 17 August, 2007 The Royal Microscopical Society 37/38 St Clements Oxford OX4 1AJ UK Tel +44 (0)1865 248 768 or +44 (0)1865 254760

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Fax +44 (0)1865 791 237 http://www.rms.org.uk; accessed 25 August, 2007. The Textile Institute 1st Floor, St James’s Buildings, Oxford Street, Manchester M1 6FQ UK Tel: +44(0)161 237 1188 Fax: +44(0)161 236 1991 http://www.texi.org; accessed 25 August, 2007. Directories of international societies and of suppliers may also be available at: http://www.mwrn.com/default.aspx; accessed 25 August, 2007.

7.8

References

Albrechtova, J., Janacek, J., Lhotakova, Z., Radochova, B. and Kubinova, L. (2007) Novel efficient methods for measuring mesophyll anatomical characteristics from fresh thick sections using stereology and confocal microscopy: application on acid rain-treated Norway spruce needles. Journal of Experimental Botany, 58, 1451–1461. Booth, J. (1983) Principles of textile testing, London, Newnes-Butterworth. Conan Doyle, A. (1890) The Sign of Four. Diaspro, A., Chirico, G. and Collini, M. (2005) Two-photon fluorescence excitation and related techniques in biological microscopy. Quarterly Reviews of Biophysics, 38, 97–166. Greaves, P. H. and Saville, B. P. (1995) Microscopy of Textile Fibres, Oxford, U.K., Bios Scientific Publishers. Hadley, D. W., Pinnock, P. R. and Ward, I. M. (1964) Mechanical Anisotropy in Oriented Polymers. Polymer, 5, 384–385. Hadley, D. W., Pinnock, P. R. and Ward, I. M. (1969) Anisotropy in Oriented Fibres from Synthetic Polymers. Journal of Materials Science, 4, 152–&. Hamza,A.A., Sokkar,T. Z. N. and Ramadan,W.A. (1992) On the Microinterferometric Determination of Refractive Indices and Birefringnce of Fibres. Pure and Applied Optics, 1, 321–336. Huang, D., Swanson, E. A., Lin, C. P., Schuman, J. S., Stinson, W. G., Chang, W., Hee, M. R., Flotte, T., Gregory, K., Puliafito, C. A. and Fujimoto, J. G. (1991) Optical Coherence Tomography. Science, 254, 1178–1181. Ko, T. H., Ghanta, R. K., Hartl, I., Drexler, W. and Fujimoto, J. G. (2001) Ultra-high resolution optical coherence tomography for quantitative topographic mapping of retinal and intraretinal architectural morphology. Investigative Ophthalmology & Visual Science, 42, S793–S793. Kubinova, L., Janacek, J., Karen, P., Radochova, B., Difato, F. and Krekule, I. (2004) Confocal stereology and image analysis: Methods for estimating geometrical

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characteristics of cells and tissues from three-dimensional conflocal images. Physiological Research, 53, S47–S55. Malmgren, T., Mays, J. and Pyda, M. (2006) Characterization of poly (lactic acid) by size exclusion chromatography, differential refractometry, light scattering andthermal analysis. Journal of Thermal Analysis and Calorimetry, 83, 35–40. Minsky, M. (1988) Memoir on Inventing the Confocal Scanning Microscope. Scanning, 10, 128–138. Palanik, S. and Fitzsimmons, C. (1990) Fiber cross-sections: Part II. A simple method for sectioning single fibers. The Microscope, 38, 313–320. Palomero, J., Pye, D., Kabayo, T. and Jackson, M. J. (2006) In situ detection and measurement of intracellular ROS and nitric oxide generation in isolated mature skeletal muscle fibres by real-time fluorescence microscopy. Free Radical Research, 40, S77–S77. Pinnock, P. R. and Ward, I. M. (1964) Mechanical and Optical Anisotropy in Polyethylene Terephthalate Fibres. British Journal of Applied Physics, 15, 1559–&. Pinnock, P. R. and Ward, I. M. (1966) Mechanical and Optical Anisotropy in Polypropylene Fibres. British Journal of Applied Physics, 17, 575–&. Saville, B. (1999) Physical Testing of Textiles, Cambridge, Woodhead with The Textile Institute. Thomson, C. I., Lowe, R. M. and Ragauskas, A. J. (2007) Imaging cellulose fibre interfaces with fluorescence microscopy and resonance energy transfer. Carbohydrate Polymers, 69, 799–804. Ward, I. M. (1962) Optical and Mechanical Anisotropy in Crystalline Polymers. Proceedings of the Physical Society of London, 80, 1176–&. Willig, K. I., Keller, J., Bossi, M. and Hell, S. W. (2006a) STED microscopy resolves nanoparticle assemblies. New Journal of Physics, 8. Willig, K. I., Kellner, R. R., Medda, R., Hein, B., Jakobs, S. and Hell, S. W. (2006b) Nanoscale resolution in GFP-based microscopy. Nature Methods, 3, 721–723.

8 The use of spectroscopy for textile fiber identification M M HOUCK, West Virginia University, USA

Abstract: The use of spectroscopy for fiber analysis is widespread and ranges from simple identification of polymer type(s) to structural information. Colorants used on textiles, be they dyes or pigments, are also the subject of spectroscopic analysis. Of necessity, the spectroscopy of fibers and related materials is a broad and complicated topic. This chapter will be limited, therefore, to basic concepts, applications, and future improvements in some of the spectroscopic analysis of textile fibers. Key words: fibers, spectroscopy, identification.

8.1

Introduction: spectroscopy of fibers

The use of spectroscopy for fiber analysis is widespread and ranges from simple identification of polymer type(s) to structural information. Colorants used on textiles, be they dyes or pigments, are also the subject of spectroscopic analysis. Of necessity, the spectroscopy of fibers and related materials is a broad and complicated topic. This chapter will be limited, therefore, to basic concepts, applications, and future improvements in some of the spectroscopic analysis of textile fibers. A spectroscope is an optical instrument for producing spectral lines and measuring their wavelengths and intensities, originally producing colored bands relevant to the elements of the sample captured on photographic film. This spectrum shows narrow (also called sharp) lines distributed in energy across the visible spectrum. For example, sodium emits two close lines in the yellow region of the spectrum, while cadmium emits a strong red and a strong green line. Spectroscopy is the study of any measurement produced by a spectroscope; a plot of the response as a function of wavelength or frequency (more common) is referred to as a spectrum. Spectrometers and spectrometry are often conflated with spectroscopes and spectroscopy. Thus:1 Spectroscopy: The study of physical systems by the electromagnetic radiation with which they interact or that they produce. Spectrometry: The measurement of such radiations as a means of obtaining information about the systems and their components. In certain types of 158

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optical spectroscopy, the radiation originates from an external source and is modified by the system, whereas in other types, the radiation originates within the system itself. Spectroscope: A device which enables visual observation and evaluation of optical spectra (usually confined to the visible spectral region). Spectrometer: A general term for describing a combination of spectral apparatus with one or more detectors to measure the intensity of one or more spectral bands. Typically, practitioners recognize the practical, if nominal, differences between methods and instruments.

8.2

Categorizing methods by nature of excitation

The method of spectroscopy applied in identifying or analyzing fibers depends on the physical quantity to be measured, usually an intensity either of energy absorbed or produced. Optical spectroscopy involves interactions of matter with electromagnetic radiation or light. Ultraviolet-visible spectroscopy is an example Electron spectroscopy involves interactions with electron beams, such as those produced by an electron microscope, either scanning or transmission. Mass spectroscopy (more typically called mass spectrometry) involves the interaction of charged species with magnetic or electric fields. The spectrum produced has the mass m as the variable, but the measurement is essentially one of the kinetic energy of the particle.

8.3

Categorizing methods by measurement process

Spectroscopic methods are differentiated as either atomic or molecular based on which structure they measure; additionally, they can be further classified on their interaction with the sample. Absorption spectroscopy measures in the range of the electromagnetic spectrum where a substance absorbs; this includes atomic absorption and other molecular techniques, such as the infrared region (infrared spectroscopy) and the radio wave region (nuclear magnetic resonance or NMR). Emission spectroscopy, by contrast, exploits the range in which a substance radiates or emits. Given that, it is a prerequisite that the substance first absorbs energy and this can be from a number of sources but each determines the title of the subsequent emission (fluorescence spectroscopy, for example). Scattering spectroscopy measures the light scattered by the sample at specific wavelengths, incident angles, or polarization angles. Raman spectroscopy is one application of scattering spectroscopy.

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8.4

Common methods of spectroscopy

Fluorescence spectroscopy excites a sample with high energy photons and the sample then emits lower energy photons. The species being examined will have a low energy state and an excited state of higher energy. Within each of these electronic states are various vibrational states. Photons of high frequency have higher energy than those of lower frequency light. When the photons are absorbed by the molecules in the sample, the molecule gains or emits the energy of the photon and the photon carries some of the energy of the molecule away. As molecules fall into any of various vibrational levels in the ground state, the emitted photons will have different energies that are indicative of the species in the material. Analyzing the different frequencies and relative intensities of light emitted in the fluorescence spectrum determines the structure of the sample. Atomic absorption spectroscopy (AA) vaporizes the sample in a flame or graphite furnace. The temperature of the flame is low enough that the flame itself does not excite sample atoms from their ground state. Instead, lamps exposed to the flame at various wavelengths for each type of analyte of interest excite the sample. The amount of light absorbed after it passes the flame is proportional to the analyte’s concentration in the sample. Among other applications, AA has been used to analyze metal ions in fibers.2–4 Atomic emission spectroscopy (AES) also uses flame excitation but at a higher temperature than atomic absorption spectroscopy. The analyte atoms are directly excited by the flame and, thus, no elemental lamps are needed to shine into the flame. This high-temperature atomization provides sufficient energy to promote the atoms into high energy levels. The emission lines in the spectra are narrow because the transitions occur between distinct energy levels. AES excites all the atoms in a sample simultaneously; therefore, they can be detected simultaneously and this is a major advantage of AES compared to atomic-absorption; high resolution is required for this type of analysis as the spectra of multi-elemental samples can be very crowded and complicated. Plasma emission spectroscopy is a more modern version of this method. AES has been used in textile conservation,5 analysis of antimicrobial agents in textiles,6 and the treatment of textile effluents,7 among other topics. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) is a method of emission spectroscopy that excites atoms and ions with a plasma, causing it to emit electromagnetic radiation at wavelengths characteristic of a particular element. Intensity of the emission is proportional to the element’s concentration in the sample. ICP-AES may be referred to as Inductively Coupled Plasma Optical Emission Spectrometry (ICP-

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OES). ICP-AES has been applied to the analysis of metals in textiles for health and safety,8 forensic science,9 and the kinetics of dye chemistry,10 among other analyses. Laser induced breakdown spectroscopy (LIBS), also called laser-induced plasma spectrometry (LIPS), is a type of atomic emission spectroscopy which uses a high energy laser pulse to excite the source. The main advantage of LIBS is that it can analyze a sample regardless whether it is solid, liquid, or gas. At sufficiently high temperatures, all elements will emit light; therefore, LIBS can detect any element, depending on the power of the laser and the sensitivity of the detector. Given this flexibility, LIBS has been applied to, among many other areas, art and colorants,11,12 characterization of textile materials,13 and more specifically, nanofibers and electrospinning.14

8.4.1 Visible spectroscopy A thorough discussion of visible spectroscopy is given elsewhere in this book and so will not be repeated here. Ultraviolet and visible range spectroscopy is useful in the analysis of colorants in textiles and is routinely used in the forensic analysis and comparison of fiber-related evidence.

8.4.2 Infrared spectroscopy Infrared spectroscopy offers much to the textile chemist: minor sample preparation, fast analysis, small sample size, and detailed information about the different types of bonds present in the sample. The infrared portion of the electromagnetic spectrum is divided into three regions; the near-, the mid-, and the far-infrared, based on their position relative to the visible spectrum. The three regions are not distinctly divided (by exact molecular or electromagnetic properties) and have different spectroscopic utilities: • • •

the far-infrared (400–10 cm−1) has low energy and is used for rotational spectroscopy, the mid-infrared (4000–400 cm−1) is used to analyze the fundamental vibration and rotational-vibrational structures, and the near-IR (14 000–4000 cm−1) is used for overtone or harmonic vibrations.

Infrared spectroscopy exploits the fact that molecules rotate or vibrate at specific frequencies depending on their discrete energy levels. The shape of the molecular energy surfaces, the masses of the composite atoms, and the associated vibrational couplings determine the frequencies at which the molecule will move. IR active vibrational modes in a molecule are associated with changes in the permanent dipole. The resonant frequencies are

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related to the strength of the bond and the mass of the atoms at either end of it. This relationship allows for the bond type to be discerned through its IR active components. The complexity of the molecule and its bonds determine the types of ‘movement’ it can make when excited in an IR region. Simple diatomic molecules have only one bond, for example, which may stretch; more complex molecules more bonds, some of which may be conjugated. The complexity leads to infrared absorptions at specific, characteristic frequencies related to chemical groups or classes. The IR spectrum of a sample is collected by passing a beam of infrared light through the sample over a range of energies within the IR region; either a monochromatic beam (changing wavelength over time) or a Fourier transform15 instrument (FTIR; measuring all wavelengths at once) is used. The transmitted signal indicates how much energy was absorbed at each wavelength. The spectrum can be displayed as either transmittance or absorbance data. Because of its multiple advantages, virtually all modern infrared spectrometers are FTIR instruments. Spectra may also be obtained by a variety of alternative IR techniques. Other techniques include micro internal reflection spectroscopy (MIR), which differs from attenuated total reflectance (ATR) in that the infrared radiation is dependent upon the amount of sample in contact with surface of the prism.16 IR spectroscopy works well in conjunction with a microscope allowing for the analysis of single textile fibers. The fibers must be flattened in preparation for analysis. Because the flattening is destructive of morphology, the minimum length of fiber necessary for the analysis should be used (as a typical IR microscope is optimized for a 100 μm spot size, analytical performance will not necessarily be improved with the use of fibers greater than 100 μm in length). The flattened fiber may be mounted across an aperture, on an IR window, or between IR windows. Common IR window materials used for this purpose include but are not limited to KBr, CsI, BaF2, ZnSe, and diamond. Consensus guides for fiber sample preparation and analysis by FTIR are available.17

8.4.3 Raman spectroscopy Much like infrared spectroscopy, Raman spectroscopy analyzes vibrational and rotational modes of molecules. It does so through the so-called Raman effect, which occurs when laser light impinges upon a molecule and interacts with the electron cloud of the bonds of that molecule. The molecule is excited from the ground state to a virtual energy state and then relaxes into a vibrational excited state. This relaxation generates Stokes Raman scattering; molecules in an already elevated vibrational energy state generate anti-Stokes Raman scattering. A change in molecular polarizability is

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required for the molecule to demonstrate a Raman effect. The amount of polarizability change determines the intensity of the effect. Raman spectroscopy lends itself to microscopic analysis for the following reasons: • specimens do not need to be fixed or sectioned, • a very small sample volume (<1 μm in diameter) can be analyzed, • raman spectroscopy identifies the species present, and • water does not interfere significantly. Thus, Raman spectroscopy is suitable for the microscopic examination of a wide variety of materials, such as minerals, polymers, ceramics, and proteins. Raman microscopy generally uses near-infrared lasers, which reduce damage to the sample because of their relatively low power.

8.5

References

1. IUPAC Compendium of Chemical Terminology (2nd Edition)(the ‘Gold Book’) compiled by Alan D. McNaught and Andrew Wilkinson, Blackwell Science, Oxford, 1997, all on page 387. 2. Masri, M. and F. Reuter (1974). ‘Interaction of wool with metal cations.’ Textile Research Journal 44(4): 298–300. 3. Izumi, S., Y. Shimizu, et al. (2002). ‘Absorption Behavior of Metal Ions on Chitin/ Cellulose Composite Fibers with Chemical Modification by EDTA.’ Textile Research Journal 72(6): 515–519. 4. Bozzi, A., T. Yuranova, et al. (2004). ‘Self-cleaning of wool-polyamide and polyester textiles by TiO2-rutile modification under daylight irradiation at ambient temperature.’ Journal of Photochemistry and Photobiology A: Chemistry 172(1): 27–34. 5. Hardin, I. and F. Duffield (1992). ‘Microanalysis of Persian textiles.’ Iranian Studies 25(1–2): 43–59. 6. Nakashima, H. and T. Ooshima (2007). ‘Analysis of Inorganic Antimicrobial Agents in Antimicrobial Products: Evaluation of a Screening Method by X-ray Fluorescence Spectrometry and the Measurement of Metals by Inductively Coupled Plasma Atomic Emission Spectroscopy.’ Journal of Health Science 53(4): 423–429. 7. Fersi, C., L. Gzara, et al. (2005). ‘Treatment of textile effluents by membrane technologies.’ Desalination 185(1–3): 399–409. 8. Rezic, I. and I. Steffan (2006). ‘ICP-OES determination of metals present in textile materials.’ Microchemical Journal 85(1): 46–51. 9. Bisbing, R. (2006). ‘A trace evidence primer for microscopists.’ Microscopy and Microanalysis 12: 12–14. 10. Dordevic, D., J. Cerkovnik, et al. (2006). ‘The comparison of the kinetics of hydrolysis of some reactive dyes before and after purification.’ Fibres and Textiles in Eastern Europe 14(2): 85–88. 11. Oujja, M., A. Vila, et al. (2005). ‘Identification of inks and structural characterization of contemporary artistic prints by laser-induced breakdown spectroscopy.’ Spectrochimica Acta Part B: Atomic Spectroscopy 60(7–8): 1140–1148.

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12. Clark, R. (2002). ‘Pigment identification by spectroscopic means: an arts/science interface.’ Comptes Rendus Chimie 5(1): 7–20. 13. Andrews, D. (2007). Optical Techniques for the Analysis and Characterization of Chemicals and Materials. The Optics Encyclopedia. T. Brown, K. Creath, H. Kogelnik et al. New York, John Wiley and Sons. 14. Ayutsede, J., M. Gandhi, et al. (2006). ‘Carbon Nanotube Reinforced Bombyx mori Silk Nanofibers by the Electrospinning Process.’ Biomacromolecules 7: 208–214. 15. A Fourier transform transforms one function into another, which is called the frequency domain representation of the original function, often a time-domain. For more information, the interested reader is directed to R. N. Bracewell, The Fourier Transform and Its Applications, 3rd ed., Boston, McGraw Hill, 2000. 16. Bartick, E. G., Tungol, M. W. and Reffner, J. A. A new approach to forensic analysis with infrared microscopy: Internal reflection spectroscopy, Analytica Chimica Acta (1994) 288:35–42. 17. Scientific Working Group for Materials Analysis (1999). ‘Forensic Fiber Analysis.’ Forensic Science Communications 1(1); available online at www.fbi.gov.

9 Microspectrophotometry for textile fiber color measurement S WALBRIDGE-JONES, Bureau of Alcohol, Tobacco, Firearms and Explosives, USA

Abstract: In a scientific setting when an optical property such as color needs to be measured with some standard of accuracy, the science of spectroscopy must be applied. This chapter discusses how and why spectroscopy is the method of choice for color measurement. When sample sizes are reduced, Microspectroscopy is utilized and is the main method discussed. Microspectroscopy is a non-destructive technique used to measure the chemical properties of microscopic samples. The non-destructive analysis by this technique makes it the primary application for textile fiber color measurement. The strengths and limitations of microspectrophotometry and future trends in textile analysis are discussed. Key words: microspectrophotometry (MSP), dyed textile fibers, spectroscopy of color, metameric fibers, color matching, forensic analysis of dyed textile fibers.

9.1

Introduction

Color is an integral part of our daily lives. The color choices we make reflect our inner moods and feelings. We are subjective when choosing the color of our automobiles to the colors we paint our homes. Human perception of color will always be subjective; it is not measurable. The human brain ‘perceives’ color; but it cannot measure color. The ocular sensitivity of humans is limited to only the visible region of the electromagnetic spectrum: red to violet with maximum ideal ocular sensitivity in the green region. Therefore, in a scientific setting when an optical property such as color needs to be measured with some standard of accuracy, the science of spectroscopy must be applied. This chapter will discuss how and why spectroscopy is the method of choice for color measurement. When sample sizes are reduced, Microspectroscopy is utilized and will be the main method discussed. Microspectroscopy is a non-destructive technique used to measure the chemical properties of microscopic samples. The non-destructive analysis by this technique makes it the primary application for textile fiber color measurement. The strengths and limitations of microspectrophotometry and future trends in textile analysis will also be discussed. 165

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White light

Red Green Blue

9.1 Dispersing prism.

9.2

An understanding of spectroscopy

The electromagnetic spectrum ranges, in increasing wavelength, from x-rays to radio waves. The word spectrum as defined by Merriam-Webster is a ‘continuum of color formed when a beam of white light is dispersed (as by passage through a prism) so that its component wavelengths are arranged in order.’ The branch of science called spectroscopy studies how the wavelength of light changes after it interacts with a sample. The prism in Fig. 9.1 is the ‘sample’ for this demonstration. The resultant color spectrum provides information about the sample. Hence the information gained here is that the ‘sample’ (a prism) has the ability to disperse white light into its respective wavelengths (each wavelength has its own color). For a given chemical compound, every light and matter interaction is unique. Spectroscopy is a branch of science that allows us to examine this light/matter interaction. This interaction is measured by a spectrophotometer, an instrument which measures the intensity (or wavelength) of light after interaction with a sample. The result of this interaction, displayed as a spectrum, provides information about the sample on a molecular level. A spectrophotometer has the ability to analyze a sample in the ultraviolet (UV), visible (Vis), and near infrared region (NIR) spectral regions. This capability makes a spectrophotometer a principal tool for studying the structures of molecules and their environment. In contrast, human perception of the ‘light-matter interaction’ is a perception and is only accomplished in the visible region of the spectrum. The differences between human color perception and instrumental color measurement will be discussed later. The theory of spectroscopy is based on quantum mechanics. Energy in matter and light is quantized. Atoms and molecules have discrete energy levels known as quantum levels. Translated, atoms and molecules may exist only in certain allowed energy states. Spectroscopy as previously stated is the study of light and matter interactions or the study of the transitions between

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the allowed energy states of atoms and molecules. When energy is transferred between quantum levels radiation is either emitted or absorbed. Transitions can involve atoms or molecules and the choice of the energy of light (light source) controls which transitions take place. Thus, it is possible in the application of spectroscopy to manipulate a sample by exciting it with a known source of energy and measuring the expected energy transition. A majority of molecular transitions occur in the UV-Vis region. The UV-Vis region is a part of the electromagnetic spectrum where interactions between light and matter can be observed. Dyes which produce the colors in our favorite shirts, pants, carpets and automobiles are substances that are easily excitable and absorb light in the visible region. Some dyes and pigments also absorb light in the UV and NIR range. The chemistry behind color requires that for absorption of radiation (light) in the visible region to occur some groups of conjugated double bonds must be present in the dye molecule. Conjugated bonds increase the likelihood of excitation, a requirement for spectroscopy. Molecules containing systems of conjugated bonds which absorb specific wavelengths of light are called chromophores. Auxochromes are any side groups that when attached to a chromophore change the wavelength and intensity of absorption. Dye compounds are made from the combination of a chromophore with an auxochrome attached to a benzene ring or rings. The chromophore provides the color of the dye compound and the auxochrome affects the strength of the dye. UV-visible-NIR spectra of dyes can provide information about the chromophore itself, the chemistry that the chromophore molecules may have undergone, and even the effects that temperature and substrates may have on the chromophore. Because the majority of the textile environment is dyed or pigmented, the usefulness of spectroscopy prevails when the molecular structure of these dyes and pigments are studied. Although hand-held spectrophotometers exist, an accurate and precise analysis of a dye and/or pigment can best be evaluated on single fibers with the help of a microspectrophotometer. Microspectroscopy is the study of light and matter interactions in microscopic samples and a microspectrophotometer combines the microscope’s ability to magnify small samples with the capabilities of a spectrophotometer. Microspectrophotometry is commonly used in forensic science laboratories as a quick and non-destructive means for examining colored microscopic items such as fibers, paint and ink. Microspectrophotometry is also a suitable method for dye examination and quality control of dye production in the textile science field.

9.3

Microspectrophotometer design

A microspectrophotometer is a combination of a microscope with a spectrophotometer. A basic understanding of the microscope’s optics is

168

Identification of textile fibers Dispersing element

*

Light source

Focusing element

Focusing element Sample

CCD array

*Entrance slit

9.2 How a spectrophotometer works (diagram provided by Paul Martin, CRAIC Technologies and reprinted with permission).

necessary to utilize a microspectrophotometer. A microscope is a complex system that uses lenses and mirrors to form a magnified image of a sample. The light ray paths, lenses and diaphragms used in the microscope’s system are all interrelated and require optimal ‘tuning’ for effective illumination. A good reference for understanding proper illumination and set-up for a microscope is necessary, as the techniques are not intuitive.1 The function of the microscope in a microspectrophotometer system is to transmit light energy while magnifying a sample. The spectrophotometer of a microspectrophotometer system is the device which measures the change in light intensity versus wavelength after the light has interacted with the sample. A spectrophotometer can be used to measure the light that is transmitted through a sample, the light that is reflected off a sample, or the light that was emitted from a sample. The basic design of a spectrophotometer is shown in Fig. 9.2. A light source, typically a xenon lamp, is focused on a sample. The sample will absorb some wavelengths of light while transmitting others. The transmitted wavelengths are separated by a monochromator into spectral bands which in turn create a spectrum. The wavelengths of light separated by the monochromator are actually photons of light. Thus, a device is needed in order to detect these photons and convert them into an electrical signal. Now with a general idea of how each individual component works independently, the combined system, the microspectrophotometer can be discussed. Figure 9.32 depicts a microspectrophotometer. A lamp in the microscope unit emits white light which is focused, via a condenser lens, on the sample which is sitting on the sample stage. Transmitted light rays which are not absorbed, reflected, scattered, or diffracted by the sample/system are collected by the microscope objective. The objective lens focuses the light onto the entrance aperture of the spectrophotometer. Most of the light is reflected by a mirrored aperture into a digital imaging system which allows you to 1

For example, McCrone, W., Delly, J. and McCrone, L. (1978) Polarized Light Microscopy, Ann Arbor Science Publishers, Ann Arbor, MI is a standard reference in this area. 2 Courtesy of Paul Martin, CRAIC Technologies.

Microspectrophotometry for textile fiber color measurement

Holographic grating

High resolution digital imaging

169

TE cooled CCD detector

Mirrored aperture

UV-visible-NIR microscope objective Sample

Condenser

UV-visible-NIR light source

9.3 Basic design of a transmission microspectrophotometer (diagram provided by Paul Martin, CRAIC Technologies and reprinted with permission).

visualize the point in which spectra is collected. The light, which is not reflected, passes through the spectrophotometer aperture and is separated by an optical grating into wavelength components. A charged coupled device (CCD) is the detector that measures the intensity of these wavelengths and converts them into an electrical signal. A computer serves in data processing and recording.

9.4

Types of microspectroscopy

As mentioned previously a spectrophotometer can be used to measure the light that is transmitted through a sample, the light that is reflected off a sample, or the light that is emitted from a sample. A microspectrophotometer is capable of measuring the transmittance, reflectance and/or fluores-

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Identification of textile fibers

cence in the ultraviolet-visible (UV-vis) range of microscopic samples such as individual fibers. Transmission microspectroscopy is the method of choice for analyzing individual colored fibers. The basics of this method were explained above and can be referenced in Fig. 9.3. A dark scan, a reference scan, and a sample scan must all be collected. A dark scan collects the background ‘noise’ of the system. A reference scan collects the mounting or support materials of the sample plus the system. A sample scan is of the sample plus mounting or support materials. The data is analyzed by calculating the ratio of the sample scan minus the dark scan to the reference scan minus the dark scan. The resultant data, in the form of an absorbance spectrum, represents the sample only with all system artifacts ratioed out. Transmittance microspectroscopy measures how much light was transmitted through the sample or, more accurately, how much light was absorbed by the sample. Figure 9.4 represents reflectance microspectroscopy. Reflectance microspectroscopy can be utilized for opaque samples such as paint chips. The

Holographic grating

High resolution digital imaging

UV-visible-NIR light source

TE cooled CCD detector

Mirrored aperture

Beamsplitter

UV-visible-NIR microscope objective Sample

9.4 Basic design of a reflectance microspectrophotometer (diagram provided by Paul Martin, CRAIC Technologies and reprinted with permission).

Microspectrophotometry for textile fiber color measurement

Holographic grating

171

TE cooled CCD detector

Mirrored aperture

High resolution digital imaging

Barrier filter UV-visible-NIR light source

Dichroic filter Excitation filter

UV-visible-NIR microscope objective Sample

9.5 Basic design of a fluorescence microspectrophotometer (diagram provided by Paul Martin, CRAIC Technologies and reprinted with permission).

lighting technique is different for an opaque sample than a transparent sample. In this process the light is reflected off the sample rather than transmitted through it; however, the sampling procedure is similar. A reference material with similar reflectance properties to the sample material is required to ensure that the system’s artifacts are correctly ratioed out of the final result. The result is a reflectance spectrum representing the ‘color’ reflected from the sample’s surface which is the same ‘color’ we see when macroscopically examining the sample in our environment. Fluorescence microspectroscopy provides information about lightabsorbing specimens. Fluorescent samples will absorb light energy at one wavelength and then emit energy usually at a longer wavelength. No reference scan is taken during this technique and thus there is no correction to a standard. A dark scan is subtracted from the sample scan with the resulting spectrum being measured in counts. Figure 9.5 represents the basic setup for microfluorometry. Optical brighteners are highly fluorescent and

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Identification of textile fibers

often added to textile fibers. During a textile fiber examination, fluorescent samples are often visually evaluated first by incident fluorescence. When fluorescent fibers are observed under the microscope, an examiner will most likely continue the fiber comparison using fluorescence microspectroscopy. Reflectance microspectroscopy is seldom used for the examination of colored textile fibers. Transmission microspectroscopy is the most common method of choice when analyzing and comparing colored textile fibers.

9.5

Perception of color: human vs. machine

The textile industry and consumers alike are concerned with product appearance. When an individual buys a black business suit, it is anticipated that the product, both pants and jacket, will be the same color black. A rack of suits may have shades which appear slightly different, however, somewhere in the production process, this ‘batch’ of suits passed a color test within a certain tolerance and is now being sold as a group of ‘black’ suits from the vendor. In the forensic laboratory, if two of these suits were to be compared by microspectrophotometry the analyst might find that the fibers which make up the two visually ‘identical’ suits are not dyed with an analytically indistinguishable set of dyes. Two people may observe these suits and describe them all as being ‘black’. Unlike a spectrophotometer, humans lack the ability to specifically measure the outcome of the interaction of light and an object (matter). What humans can do though is experience the interaction between light and an object. The color detectors in the eye are the cones. Individual cones are sensitive in three defined frequencies of light: red (‘L’, maximum 564 nm), green (‘M’, maximum 534), and blue (‘S’, maximum 420 nm). The brain via the optic nerve processes the eye’s detection of the three primary colors and the result is the perception of color (what color we see). The rods in the eyes are sensitive in dim light and thus help in the ability to detect grays. Figure 9.6 illustrates the human eye color response and also includes the absorption spectra for the rod cells ‘R’. If the cones and rods were at 100% functionality in all humans, all people would perceive color in an identical manner. Human color vision, however, is a direct expression of the wavelengths of light absorbed by each eye [1]. The eye-brain system translates the absorbed wavelengths into a color. For example, when looking at a red apple, the maximum wavelength perceived is red, as the skin of the apple absorbs the majority of the rest of the spectrum except red. Not every person may describe the red apple as being the same red. One reason is the influence of environment: The brightness and color of the foreground in which an object is being observed will influence the observer’s interpretation of the brightness and color of the object being observed. For a visual textile color comparison, it is optimal that the comparison be carried out

Microspectrophotometry for textile fiber color measurement 498

Normalised absorbance

420

173

534 564

100

50

S

R

M

L

0 400 Violet

Cyan

Blue

500 Green

600 Yellow

700 Red

Wavelength (nm)

9.6 Human color response.

9.7 Color matching light box.

under a defined illumination and that the physical conditions of the environment are identical and reproducible. These requirements have pushed the textile industry to use color matching boxes that are lined in neutral gray and use a defined illuminant. Figure 9.7 is a photograph of a color matching booth. A second reason why color recognition is subjective is that the eye-brain response and recognition of color can be influenced by such factors as fatigue, mood, and appetite. Thus, the perception of color not only varies between observers but can also vary for the same observer.

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Identification of textile fibers

Communicating about color is a subjective experience. As such, it often results in errors in textile quality control when two people communicate about one or more colors. Various systems for defining and measuring color have been developed. One is the use of color atlases, such as the Munsell Book of Color. Albert Munsell, an artist, recognized that color is a three dimensional concept having distinct properties of hue, value, and chroma [1]. Hue is the color name used to describe a specific wavelength of light [1]. There are six primary hues: red, orange, yellow, green, blue, and violet. The light and dark properties of a color are defined as their value [1]. A color’s hue can be changed in value by adding white or black. Chroma is the intensity of a color [1]: bright colors have a high intensity or what could be described as deepness in color. White, black, and gray are achromatic colors because they are colors with no brightness (that is, no intensity). Munsell developed a three-dimensional system for communicating about color by using printed chips arranged by hue, chroma, and value. These color chips in the form of either a color tree or color book can be used to reduce variance in color communication. The chance of asking two individuals to describe the color ‘red apple’ with exact concurrence is unlikely; using physical samples in a color ordering system with specific designations (2YR/3) is helpful but this still can fall short in accuracy of color communication – the sample may fade over time and the perception of the color chip can vary, depending on the observer and the lighting conditions. A Munsell Book of Color is only one example of how color can be defined and communicated. Mathematical methods of describing color have been developed by the CIE: Commission Internationale de L’Eclairage (International Commission on Illumination). One of these methods, CIE L*a*b* (CIELAB), is the most complete color model used conventionally to describe all the colors visible to the human eye. A full discussion of this model, and color systems in general, are beyond the scope of this book.3 The three basic coordinates represent the lightness of the color (L*, where an L* of zero yields black and an L* of 100 indicates white), its position between red/magenta and green (a*, where negative values indicate green and positive values indicate magenta), and its position between yellow and blue (b*, negative values indicate blue and positive values indicate yellow). The L*a*b* color model is meant to be a device-independent model as a reference; because of this, it is important to remember that the visual representations of the entire range of colors in this model cannot be accurate. Since the L*a*b* model is a three-dimensional model, it can only be represented properly in a three-dimensional space.

3

For a fuller discussion, see, for example, Kuehni, R. (2001) ‘Color space and its divisions,’ Color Research & Application 26(3):209–222.

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The environment and any color vision deficiencies of the observer affect visual color comparison regardless of whether the sample is whole textile items or single fibers. Microspectrophotometry is an approach to defining and measuring color which is more accurate in fiber color comparison. Microspectrophotometry has many advantages over visual comparison of colored fibers. Microspectrophotometry has the ability to control the illumination and environment of the samples. The instrument can detect small intensity and wavelength variations. In addition, the instrument has the ability to detect differences between two colored samples outside the visible range (approx. 400–700 nm).

9.6

Metamerism

Color matching is essential in the textile industry but even with effective communication along the design and production process relating to a product’s color, a visual match between two colored samples does not imply that the two samples were dyed with identical dyes. Textile materials are dyed in batches that vary in size and dye content. Common practice in dye houses is to ‘top up’ a dye bath when the volume has reduced [2]. This often means adding a different set of dyes to still achieve the same original requested color. Textile objects that appear the same colors under the same conditions of light but have different spectral curves and can appear not to match under a different lighting condition are called metameric pairs [3]. Metamerism is often resolved on larger textile items by using different light sources when color comparing. However, metamerism in smaller items such as textile fibers is difficult to resolve because a microscope, which is needed for observation of these small fibers, has only a single illumination source. Metameric textile fibers will therefore appear similar macroscopically and microscopically but can be distinguished by Microspectroscopy. In a fiber analysis scheme, the ability to recognize metamerism in fibers is imperative. Therefore, Microspectroscopy is an essential technique to be used in fiber comparisons, especially when comparing metameric fibers.

9.7

Applications of microspectroscopy in fiber analysis

9.7.1 Sample preparation Considerations for sample preparation are dependent upon the type of microspectroscopy used; regardless of the method, a well-prepared sample will reduce the uncertainty in spectral data. It is recommended that a stereomicroscope be used when choosing fibers for sample preparation. When selecting known fibers for comparison, a representative sampling

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Identification of textile fibers

based on available sample size is necessary. The sample population chosen should include all apparent color and shade variations. Factors such as: laundering, extent of wear, bleaching, and thermal, biological and/or mechanical degradation can cause artifacts in the spectral data and should be considered. The absorption spectrum of colored fibers is dependent on the dyestuff as well as the fiber. The absorption spectrum of natural fibers will exhibit more intra-sample variation compared to man made fibers. Based on the factors mentioned the number of fibers selected for examination is not easily standardized and may vary. For transmission analysis, a representative sample of fibers is placed on a clean glass microscope slide. The fibers should be teased apart, straightened and flattened on the slide reducing the amount of fiber twist. A drop or two of mounting media is added to the slide and the fibers are covered with a cover slip. The same number of questioned and known fibers should be prepared separately, if possible, but as above. Sample preparation for fluorescence microspectroscopy requires that the sample be mounted in a non fluorescing media such as glycerin. Considerations should be made of any fluorescent coating on the glass slides used with quartz slides being a good substitute for eliminating this problem. Reflectance microspectroscopy does not require the use of mounting media.

9.7.2 Data collection The initial steps for setting up a microspectrophotometer are identical for transmission, reflectance and fluorescence work. The first step is to turn on the instrument and allow it to thermally stabilize. Typically this takes at least 30 minutes but a user should follow their instrument manufacturers’ recommendations. Proper illumination and set up of the microscope should be performed before use. The condenser aperture setting and objective lens selection are based on the microscope and type of analysis respectively. For transmission microspectrophotometry with single fibers, a 20× objective is common. Prior to data collection, an instrument calibration is performed. Instrument calibration should be performed before daily use. Each individual laboratory may have additional quality control requirements that will also need to be followed. Instrument calibration for a microspectrophotometer involves checking wavelength and photometric accuracy. Wavelength accuracy is checked by using holmium oxide and didymium filter glasses. These glasses can be certified by the manufacturer with National Institute of Standards and Technology (NIST) traceable standards. Using NIST traceable standards is recommended for forensic analysis where instrument calibration data may be scrutinized in a court of law. Photometric accuracy is checked using a set of neutral density filters. Usually sets contain three calibrated filters at 10% (1.1A), 20% (0.78A),

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177

and 30% (0.56A). These filters are used to check spectral linearity and are also certified with NIST traceable standards. Additional calibration may be required in a laboratory setting depending on the application of the microspectroscopy analysis. For example, if tristimulus values X, Y, Z and chromaticity coordinates x, y, z are critical, then using a set of transparent filters for the blue, green and red part of the spectrum would calibrate for colorimetry work [2]. The procedure for any of the above calibrations is as follows. After the instrument has optimized and a reference slide is in focus, a dark scan is taken. The light source can be blocked by placing a black piece of plastic over the field diaphragm. Unblock the light source and collect a reference scan. Run the wavelength standards followed by the photometric standards. Compare data to the NIST standards and/or previous instrument calibration data to determine if the instrument is functioning properly. Once the microspectrophotometer has stabilized and the wavelength and photometric calibrations have been verified; collecting transmission microspectroscopy data from colored fibers is a quick, non-destructive method. Mounted fibers are placed on the stage, brought into best focus and Koehler illumination is set. A dark scan is taken following the directions above. A reference location, just off the sample in an area containing only slide, mounting media and coverslip, is chosen for the reference scan. Following the reference scan, focus on an area of interest on the sample and collect spectra. The resultant spectrum will be an ‘absolute’ spectrum of the fiber’s chemical components (dyes).

9.7.3 Evaluating microspectroscopy data The collection of absorbance spectra is straightforward but the same can not always be said for the data evaluation. When comparing questioned and known fiber samples the evaluator must ask themselves, what determines a significant difference? Or where is the quality assurance threshold for this colored textile? Where should the tolerance level be set for color differences? Several approaches in evaluating spectral data can be taken in an effort to answer these types of questions. A general suggested approach for a fiber comparison is to begin by collecting a number of spectra of the known fibers and evaluating variance within the known. If little or no variance is observed then the analyst should collect a number of spectra of the questioned fibers. If variance is observed in the question fibers then the next step would be to analyze more known fibers to evaluate the range of variation observed in the question fibers. Fiber spectra can be compared from the absorbance and/or transmittance plots. When dealing with single colored fibers, comparison of absorbance plots is visually easier and common in a forensic fiber comparison. In a quality assurance textile situation, individual questioned fiber spectra may be compared to a larger set of target (known)

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Identification of textile fibers

spectra. Another approach would be to calculate the standard deviation of the sample sets. This type of mathematical function is likely to be part of the software package that accompanies the microspectrophotometer. In any spectral comparison, the overall shape of the spectral curve is a good place to start. A difference in the overall shape between spectra of what are presumed to be the absorbance patterns of ‘identically’ dyed fibers most often can be considered a significant difference. This is especially true when the absorbance patterns are completely different. An evaluator of such spectra need not be an expert in the spectroscopy field to understand a basic pattern matching difference. The slopes of the absorbance curves are another comparison point as well as any shoulders or points of inflection present/absent in the spectra. Differences in the slope of an absorbance curve, for example, may or may not be considered a difference or may be calculated as falling within the set tolerance for that fiber and that dye. Artifacts in the data caused by such factors as thermal, chemical, biological and mechanical degradation of the fibers, will weigh into a color comparison and evaluation. Relative intensities of absorbance spectra can vary but this variance is not always an indication of a color difference. The uneven dye uptake typical of cotton fibers is an excellent example of this. A good knowledge of textile fibers and dyeing will assist the evaluator in the interpretation of microspectroscopy data. Determination of the ‘threshold,’ ‘tolerance,’ and/or ‘significant difference’ between two colored textiles analyzed by microspectrophotometry is fluid and based on the application. In forensic science laboratories, textile fibers are often evaluated using a variety of instruments such as polarized light microscopy, Fourier transform infrared spectrometry, fluorescence microscopy and microspectrophotometry, to determine any differences between known and questioned fibers.

9.8

Limitations, strengths, and future trends

For a meaningful comparison/analysis of fiber color there is a need for a careful examination of the sampling population. Factors to consider when choosing a fiber sampling scheme were discussed in the sample preparation section of this text. One problem mentioned was the influence the chemical make-up of colored fibers can have on the absorption spectrum. Spectra can vary between colored fibers from the same source. This influence seldom exhibits itself in man-made fibers such as polyesters and polyamides. These types of fibers are more chemically homogeneous, resulting in limited intrasample variation reflected in the spectra. However natural fibers, such as cotton and wool, are not chemically homogeneous but instead are composed of many different chemical components, many of which are unevenly distributed throughout the fiber. The result of adding a dye to a fiber with

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179

a chemically unbalanced make-up is color variation. This type of intrasample variation in natural fibers is quite abundant, making forensic fiber comparisons difficult. When color comparing natural fibers, a complete and careful sampling of questioned and known can help in reducing the inhomogeneity of the resultant spectra. How color is applied and absorbed along the length of the fiber is an important comparison characteristic in both natural and man-made fibers but more so in man-made fibers. Often several dyes are used to give a fabric a desired color. Color can be applied to the surface of a fabric; a common procedure for printed fabrics. Yarns can be dyed and then spun to make a fabric. Individual fibers can be colored prior to being spun into yarns. Variations such as these can result in dye defects and dye uptake variability. Dye uptake variability was discussed above in relation to natural fibers but may also be found in man-made textiles that are dyed after being constructed especially in tight woven fabrics. The cross-sectional shape of the fibers can also influence a color assessment especially during a quality control assessment of a particular fabric dye using a microspectrophotometer. For example, consider two sample sets of fibers, identically dyed and analyzed by microspectrophotometry. The first fiber sample set are round blue (nylon) fibers while the second fiber sample set turn out to be tri-lobal blue (nylon) fibers. The absorbance curves for each fiber set analyzed could differ depending on the QC definition of ‘difference.’ In this example any possible variations in the absorbance curves between the two fiber sets would most likely be caused by the difference in thickness of the trilobal fiber versus the round fiber. However, the result from this situation could cause the analyst, unknowing of the fact identical dyes were used, confusion in thinking that a different blue dye was used during the second dye batch for the set of nylon fibers or that a problem exists with that particular dye. Even processing techniques such as draw ratio can affect the resulting dyed color.4 A fiber’s thickness can limit the use of microspectrophotometry. When analyzing smaller diameter fibers, such as micro fibers, by microspectrophotometry, the light focused on these fibers has less surface area to interact with. This causes much of the light to scatter resulting in weak spectral curves. Thus, the absorbance data for colored micro fibers will not provide the same information about the chemical dye(s) typical with larger diameter colored fibers. Despite some limitations in the application of microspectroscopy to fiber color, this technique has many benefits when compared to visual color 4 For example, see Maillo, J. et al. (2008) ‘The relationship between dyeing behavior and fine structure in polyamide 6 fibers – the influence of drawing ratio,’ American Association of Textile Chemists and Colorists Review, March: 39–43.

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assessment. The instrumental measurements are absolute, objective, and repeatable. Color is measured at each wavelength along an extended range versus human vision which ‘perceives’ color all at once and in the visible region only. Measurement methods can be standardized with a microspectrophotometer. In addition, the results from this technique are quantitative which allows for statistical treatment of the data. The resulting data is also linear data which is simple to visually assess. The spectrophotometers currently being used for color measurement range from bench top to handheld models. However, these instruments are limited to larger sample sizes. Loose fibers are difficult to measure with repeatability and there is always the threat of the fibers falling in to the sampling port and interfering with measurement [4]. Using a microspectrophotometer would relieve this difficulty. As mentioned early in this chapter, color and color design is an emotional choice. Modern consumers are more aware of how products look and equate good visual design with product quality. In turn higher aesthetic standards are set driving competition among industry to produce the ‘best quality product’. Companies have to consider the potential devastation a color difference can make by the time it is repeated in a product. An example of this often presents itself in the carpeting industry. The number one consumer complaint in the carpeting industry is color inconsistency [5]. Consumer complaints such as this can cause high monetary loss for the carpeting manufacturer. Utilizing microspectrophotometry to assist in a problem such as dye absorbance in carpet fibers could only be beneficial. Research in this arena is beginning to emerge in the textile industry. In the forensic world, microspectrophotometry is a critical aspect to a color analysis and can add enormous significance to a fiber comparison.

9.9

References

1. http://daphne.palomar.edu/design/color.html. 2. Adolf F and Dunlop J (1999), ‘Microspectrophotometry/Colour Measurement’, in Robertson J and Grieve M, Forensic Examination of Fibres, London, Taylor & Francis, 251–289. 3. Eyring M (2002), ‘Visible Microscopical Spectrophotometry In The Forensic Sciences,’ in Saferstein R, Forensic Science Handbook Volume 1, New Jersey, Prentice Hall, 322–387. 4. http://www.datacolor.com. 5. Connelly R L (1997), Colorant formation for the textile industry. In: Color Technology in the Textile Industry. American Association of Textile Chemists and Colorists, Research Triangle Park, North Carolina, pp. 91–96.

10 Alternative and specialised textile fibre identification tests P H GREAVES, Microtex, UK

Abstract: The less frequently used methods of fibre identification are summarised, from elementary burning, staining and solubility tests, to density, melting point and refractive index determinations. More advanced instrumental techniques are explained: differential scanning calorimetry (DSC) and thermal analysis, thermogravimetric analysis (TGA), pyrolysis and briefly transmission electron microscopy (TEM). Scanning electron microscopy (SEM) is considered in some detail. Principles, specimen preparation, factors affecting the image and information obtained are considered. Advanced scanning electron microscopy techniques are noted, and the optimal instrument choice between the scanning electron and light microscopes compared. The principles of quantitative analyses of fibre mixtures are introduced, for blends of both chemically equivalent and chemically different fibres. Key words: fibre identification, alternative identification techniques, scanning electron microscopy (SEM) of textiles, specimen preparation, quantitative analysis of fibre mixtures.

10.1

Introduction

The two most frequently used methods of fibre identification are arguably those of light microscopy and infrared spectroscopy – or a combination of the two. There are, however, other techniques available, and these range from simple physical and chemical tests, to complex instrumental methods. The purpose of the chapter is to provide an outline of the principles of some of the more important of these alternative methods of identification, and then to describe in more detail the theory and application of one of most vital means of obtaining detailed images of fibres – that of scanning electron microscopy.

10.2

Alternative methods of fibre identification

10.2.1 Burning tests Originally considered to be the first step towards classifying a fibre type, the increased use of fibre blends and fire retardant fibres has led to the 181

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Identification of textile fibers

burning test being of only minor value today. A further disadvantage is that the test is destructive and requires a considerable amount of material to be available. The burning test relies on the fact that the chemical composition of a fibre will largely determine its behaviour when exposed to a flame. The nature of contraction, smell, combustion and ash of a fibre will normally provide an indication of its generic type – classic examples being animal fibres, cellulosic fibres, acetates and other thermoplastic synthetics. Burning tests are still used in industry, where, for example, the composition of an unlabelled bale of raw material may be in question. One of the most distinctive odours produced by burning a sample of fibre is that of nylon, where the smell is traditionally described as being like that of ‘burning celery’. It should be noted that burning tests remain subjective, and are essentially non-reproducible. There is the added hazard that toxic fumes may be generated in such tests (e.g., with acrylic fibres).

10.2.2 Staining tests The different chemical composition and reactivity of different fibre types causes them to have particular affinities for specific dye or stain types. This has led to the development of mixtures of dyes which are specifically prepared as fibre identification stains, and which produce known colours for individual fibre types. A further refinement is the compilation of stain mixtures for distinguishing variants of the same fibre type, e.g. acrylics, of which Meldrum’s Stain (Meldrum, 1961) is an example. The chief limitation of staining tests is that they can only be used on undyed material, and chemical damage may also affect the nature of staining – though in certain cases this can be used as a diagnostic feature when attempting to classify types of fibre damage (Park and Shore, 1982). Nevertheless staining tests can be very useful – particularly if combined with light microscopical examination.

10.2.3 Chemical solubility An indication of the chemical composition of a fibre can be readily gained by establishing in which solvent it dissolves. This can be achieved by immersing fibres in a prescribed series of solvents, usually of ascending chemical reactivity, until the first is found in which it dissolves. Confirmatory tests may then be carried out using solvents of known behaviour for the initially identified fibre type. A fresh specimen must be used for each solubility determination, to avoid any risk of cumulative damage effects.

Alternative and specialised textile fibre identification tests

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Table 10.1 Examples of some commonly applied solvents in solubility tests Fibre type

Soluble in

Nylon Acrylic

80% Formic acid Boiling dimethyl formamide Viscose/Modal/Lyocell Cuprammonium hydroxide Polyester Dichloroacetic acid Polyolefins

Boiling xylene

Insoluble in Xylene 80% Formic acid Xylene, 80% formic acid Xylene, 80% formic acid, dimethyl formamide Cuprammonium hydroxide, 80% formic acid

Solubility tests may be carried out in small tubes on a macro scale, or more usefully while the specimen is under observation on the stage of a microscope (ideally kept for just such tests). There are two advantages of the microscopical method: •

as the fibres are under observation during the test, mixtures of fibres may be identified and • fractions of millilitres of solvent are used, thus minimising safety risks. Tables providing examples of solvents for particular fibre types, and supporting confirmatory solvent tests are available (The Textile Institute, 1985; Greaves and Saville, 1995). Examples of some commonly applied solvents in solubility tests are shown in Table 10.1.

10.2.4 Melting point determination The thermoplastic nature of many chemical (‘synthetic’) fibres causes them to soften and shrink on the application of heat. The temperature at which these changes occur can provide a useful diagnostic feature of the fibre type. Even where true melting does not take place, the test can serve to eliminate certain fibre types. Instruments specifically designed for determining the melting point of fibres or plastics are available, or the test may be carried out under the light microscope using a specially adapted hot stage. As with the solubility tests this latter method permits positive recognition of different component fibres in a blend. Most fibre science books will have tables of melting point values. Some examples are shown in Table 10.2. Note that the measured value of a fibre’s melting point may vary according to the apparatus and conditions used, and the dimensions (e.g. denier) of the fibre. Calibration tests should be conducted.

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Identification of textile fibers Table 10.2 Melting points of some manufactured fibres Fibre type

Melting point (°C)

Acrylic

Does not melt but decomposes with discolouration Does not melt 250–255 290–300 185–210 Does not melt but decomposes with discolouration 210–216 252–260 250–260 133 160–165

Aramid Cellulose diacetate Cellulose triacetate Chlorofibre Modacrylic Nylon 6 Nylon 6.6 Polyester Polyethylene Polypropylene

10.2.5 Refractive index measurements The molecular structure of textile fibres makes them optically anisotropic, i.e. they have two refractive indices – one for light polarised along the fibre’s axis and one for light polarised across it. The fibres are thus said to be birefringent. For colourless (undyed) fibres, the refractive index for each direction may be found by simply mounting the fibre in different liquids of known refractive index until the two are found which causes the fibre to become invisible for light polarised along the fibre axis, and for light polarised across it (normally referred to as ‘parallel’ and ‘perpendicular’). The difference between these values is the birefringence of the fibre, and constitutes a distinct identification feature. While the most accurate method of determining the refractive index of a fibre is to use monochromatic light and an interference microscope, it is quite possible to achieve satisfactory results with white light and a rotatable polarising filter attached to an ordinary light microscope. The accuracy with which polarised light microscopy may be used to identify fibres has been documented by Carroll (1992), while detailed descriptions of identification by FTIR microscopy may be found in Tungol et al. (1990).

10.2.6 Density measurements The polymers of which fibres are made have different densities, and this can be used as a diagnostic test. Values may range from below 1.0 g/cm3 for polyolefins to over 2.5 g/cm3 for glass fibres.

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In its simplest form, a density gradient column is made which consists of a vessel of liquid having a near linear density increase from the top to the bottom of the column. Ethanol/water mixtures, or varying strengths of salt solutions are examples of possible liquids. It is vital that any medium does not cause sorption or swelling to the fibre under test as this will result in a change of density. Fibres of known density are placed in the column and allowed to reach their stable positions in the liquid, i.e. where the density of the fibre matches that of the liquid. The test fibre is introduced into the system and the point at which the sample stops sinking is taken as the point of equivalent density. Variants of the density gradient technique are titrimetric methods, where the volume of liquid adjusting the density is metered through a burette and the ‘end point’ of no movement of the sample is recorded, centrifugally accelerated determinations and kinetic (rate of fall) measurements. With all density based determinations, accurate control of temperature is critical.

10.2.7 Differential scanning calorimetry and differential thermal analysis Chemical and physical changes induced into a fibre polymer by the application of heat result in changes in enthalpy and entropy. Differential scanning calorimetry (DSC) monitors the difference in the relative heat flow into the sample compared with that into a blank (inert) system as the basic heat source experienced by both is linearly raised. Changes in the flow of heat will occur at those temperatures at which the sample undergoes either a physical or chemical change. DSC is primarily a quantitative tool in the form of a calorimeter. This permits the size of the enthalpy and entropy changes to be positively identified. The temperatures at which they arise are less reliably pinpointed. In contrast to DSC, differential thermal analysis (DTA) monitors the temperature differences between the sample and the inert system as the enthalpy and entropy changes that accompany physical and chemical transformations of matter take place as both are linearly heated. This provides accurate values for the temperature of such induced changes; however, only limited quantitative information is available. Examples of heat-induced changes in fibrous polymers are second order or glass transition (Tg), desorption of moisture, crystallisation, fusion, chemical reactions and irreversible decomposition processes. A recently obtained DSC curve for a particular type of polyester is shown in Fig. 10.1.

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Identification of textile fibers Sample: Weak green Size: 6.3250 mg DSC Method: Ramp Comment: N2 Purge 200 cm3 / minute –0.4 168.77 °C

Flie: C:\TA\Data\DSC\Data.052 Operator: PHIL G Run Date: 5-Aug-02 12:28

Heat flow (W/g)

–0.6

–0.8

–1.0

–1.2

252.86 °C

–1.4 0 Exo Up

50

100

150

200

250

300

350

Temperature (°C) Universal V2.6D TA Instruments

10.1 DSC curve for printed polyester fibre from upholstery fabric.

10.2.8 Thermogravimetric analysis Thermogravimetry involves the monitoring of the sample mass on a thermal balance, as the temperature is linearly raised. The sample mass– temperature curves enable the occurrence of all thermally induced mass changes to be quantitatively recorded. These changes may be due to drying, chemical reaction, chemical degradation or volatilisation. The degree of mass change and the temperature of occurrence can be positively identified and are characteristic of the heated sample. This method should allow all types of fibre – whether or not thermoplastic – to be definitively characterised.

10.2.9 Pyrolysis The flash pyrolysis of a fibre in an inert atmosphere, either in a micro-oven or by electrically heating a thin coil of wire on which the sample is mounted, produces a breakdown of volatile components that may be analysed by gas chromatography. If the conditions of pyrolysis are accurately controlled then the volatile products of the reaction may be reproducibly obtained and a characteristic pyrolysis chromatogram or ‘pyrogram’ may be used as a fingerprint to aid in the identification of the fibre sample. Key areas of importance in pyrolysis gas chromatography are the rate or speed of the pyrolysis, the carrier gas which serves as the inert atmosphere

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to transfer the volatile pyrolysis products on to the chromatography column and the medium of the column allowing separation of the components. The theory and practice of gas chromatography has been described in detail (Harris and Habgood, 1966).

10.2.10 Transmission electron microscopy (TEM) Although not generally used for generic identification of fibre type, the vastly superior resolution of the TEM compared with that of the light microscope can be employed to provide information about the fine interior structure of a material examined in thin section. The instrument may be considered loosely analogous to the projection light microscope, in that the illuminating and image forming radiation is passed though the material via condensing lenses. The emergent beam, having interacted with the specimen is then enlarged by magnifying lenses and projected onto a screen or means of image recording. By using electrons instead light, the magnification and resolution of the instrument can be increased many thousands of times over that of the light microscope. Transmission electron microscopy is able to resolve distances comparable with inter-atomic spacings – though to do so the specimen must necessarily be very thin – less than 100 nm. Apart from the great expense of the instrument itself and the very specialised and precisely controlled specimen preparation techniques which are required to use the TEM (Chescoe and Goodhew, 1984), a disadvantage when compared with the light microscope is the need to operate under high vacuum. It is also normally only possible to study a small part of a thin layer of the material in question. Fibre investigations using the TEM have traditionally concentrated on resolving the structure of crystalline regions in the material by studying their electron diffraction patterns (Johnson, 1965; Dobb, 1970) and it is not as such used for straightforward fibre identification (Sikorski, 1975). An exception to this is in distinguishing between different asbestos fibre types (Park and Shore, 1982), where the extreme fineness of the fibrils, and their specific diffraction patterns are suitable for analysis in the TEM.

10.3

Scanning electron microscopy

If the TEM can be considered roughly analogous to the projection light microscope, the closest approximation to the scanning electron microscope (SEM) is usually said to be the reflected light stereo microscope. The SEM is the instrument whose images of surfaces are so frequently seen as dramatic ‘3-D’ features of stunning quality in the semi-technical press, ‘serious’

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television news and scientific advertising features. In actuality, the closest optical analogue to the SEM is probably the confocal laser scanning reflected light microscope – though limited numbers of textile scientists are wholly familiar with the technology of this instrument.

10.3.1 Principle of operation of the SEM In order to understand how the SEM forms images of such magnification and clarity, it is first necessary to appreciate some fundamental concepts. The limiting factor of resolving power – wavelength The maximum useful magnification attainable with a light microscope is governed firstly by the numerical aperture (NA) of the objective lens. The second and most restrictive factor controlling resolution is that of the wavelength of the imaging radiation, i.e. light. For a shorter wavelength, the distance resolvable between two points becomes smaller. For light this wavelength is generally between the range of 400–700 nm. It is generally accepted that these constraints limit the maximum useful magnification of the light microscope to around ×1000–×1200. This translates to an approximate distance of around 0.3 microns (1 micron = 10−6 metre). Further magnification may be produced, but contains no additional detail and is said to be ‘empty’ magnification. Electrons are small, negatively charged particles and whose wavelengths are orders of magnitude shorter than those of light. The performance of the light microscope may be increased vastly by using electrons as the imaging radiation. The wavelength of a free electron is given by De Broglie’s equation:

λ=

h mv

where h = Planck’s constant m = mass v = velocity For a microscope operating at (say) 10 000 volts (10 kV), this will produce an electron wavelength in the order of 12 pm – several thousand times shorter than that of light. The NA of the system can thus be relatively small. Interactions of electrons with matter An electron incident on a specimen surface may undergo a number of reactions.

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If the material is very thin, the electron may pass through unaffected. This happens to a large degree in the TEM and may necessitate methods of contrast enhancement. The electron may displace another electron from an outer orbital of an atom, which is emitted by the specimen. These are called secondary electrons and have a low energy compared with the incident electron beam. Their emission is highly dependent on surface topography. If the electron is elastically scattered by the atomic nucleus, a small number may be deflected back by a high angle. These are called backscattered primaries. The majority of elastically scattered electrons however will proceed to spread out and strike further atoms within the specimen without losing much energy in the process. Some of these electrons will find their way back to the surface of the material where they may be collected. The emission of these backscattered electrons is highly dependent on atomic number. When the secondary electron is displaced from the atom, the resulting energy change and replacement transfer from a higher energy orbital produces a characteristic X-ray photon. These are of specific wavelength and energy to the atom from which they have been generated. The incident electron beam may produce light, and this is used to form the cathodoluminescence image. A sometimes troublesome effect of the electron beam is the generation of heat, which can lead to the melting or decomposition of thermoplastic or delicate specimens. The effect can be controlled to a large extent by coating (see page 193) or by specially cooled stages or specimen holders. Other interactions may also occur, such as transmission of electrons, atom displacement, diffraction and current generation. For the analysis of fibres, however, it is the secondary electrons which are most useful for topographical image formation. The backscattered electrons and characteristic X-rays are also important. Typical electron–specimen interactions are shown in Fig. 10.2.

The mechanics of SEM operation The key to the functioning of the SEM is synchronisation. In essence, a fine electron probe, of typically 10–20 kV, is scanned across the surface of the specimen in a raster fashion: I___________________ ___________________I I___________________ ___________________I I___________________

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Identification of textile fibers Electron beam

Secondary electrons Backscattered primaries X-Rays } 10 nm Escape depth for secondaries } 1 μm

10.2 Schematic diagram of some of the possible interactions of electrons with a specimen (e.g., a fibre).

Electrons may be generated from a heated tungsten filament, a lanthanum hexaboride tip, or a field emission gun. The signal generated by this is used to modulate the intensity of a simultaneously scanning beam in either a cathode ray tube or on an image recording medium. Thus when the probe encounters a feature which produces a strong signal (for example, a ‘peak’ generating an increase in secondary electron emission), the intensity of the corresponding CRT spot is increased. Magnification in the SEM is achieved by making the scanned area of the specimen small in relation to the scanned area of the CRT or image recording medium. The scanning of the two synchronised beams may thus be likened to a pantograph of they type used to produce enlarged drawings. A 1 mm2 area of the specimen scanned and synchronised with the modulated beam scanning a 10 cm × 10 cm CRT screen will produce a magnification of ×10 000 for example. Unless there are special circumstances, the whole system must be operated under high vacuum, to avoid deflection of the beam, contamination effects and loss of both incident and generated signal strength.

10.3.2 Factors affecting the image The most commonly used signal of the SEM in fibre work is the secondary electron, or emissive mode. The generation of secondary electrons is highly dependent on surface topography, and a spike or thin edge will emit con-

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Electron probe

Detector

Specimen

10.3 Effect of topography on secondary electron image.

siderably more electrons than a flat area – which will in turn emit more than a trough or hollow (Fig. 10.3). These lower energy secondaries, primarily from the thin surface layer of the specimen are collected by a detector to which is applied a slight bias to attract the electrons. The low penetrating power of secondary electrons means that their area of origin is comparable to that of the diameter of the electron probe itself, and this provides the highest resolution images of the SEM. Backscattered electrons are those elastically scattered by the atomic nucleus, and thus are highly dependent on the atomic number of the element(s) from which the specimen is composed. These high energy electrons are not easily deflected by charging effects (see Section 10.4), and their high energy means they can originate from deeper in the specimen where the interaction volume is larger. For uncoated specimens it is also possible to use the backscattered electrons to obtain an image based on the atomic number of the specimen – where the contrast is due to different elements with different atomic weights. Purpose-made backscattered detectors collect from a much greater solid angle than the normal collector and thus a better signal to noise ratio is obtained. Because the number of backscattered electrons increases with the accelerating voltage of the probe, it is often advantageous to use a higher kilovoltage than when imaging with secondary electrons. Characteristic X-rays are produced as a result of energy transfers between electron orbitals during the generation of secondary electrons. These X-rays are characteristic of the element from which they have originated, and their energies and/or wavelengths can be used to characterise and quantify the elements present.

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10.3.3 Specimen preparation Cleanliness Unless a fibre is being examined specifically to determine the presence or nature of a surface residue, it is important that the specimen placed in the SEM chamber should be as clean as possible. There are several reasons for this, but the most important is to avoid contamination of the column with any extraneous volatile organic matter that may be degraded by the electron beam, and to ensure that the surface studied is that of interest and not some additional residue which has been acquired inadvertently (e.g., from handling). Samples whose compositions are required to be analysed by backscattered electron or X-ray emissions may be polished, so that surface roughness does not interfere with the elemental information generated. Fixation (chemical, cryofixation) The electron beam, and some preparatory processes may generate heat in insulating specimens. Fibres of low melting point (e.g., polyolefins) or some natural (biological) fibres are particularly prone to damage by this means. Such materials may be stabilised by chemical treatment, where the more vulnerable groups are modified or replaced by molecules of similar size but greater thermal stability. It is vital, however, that any such procedures do not change the inherent nature of the material or fibre to be studied. An alternative to chemical fixation is thermal fixation, i.e. to stabilise the specimen by rapid freezing, so that its structure remains unchanged. Care must be taken to ensure that cooling conditions do not allow the formation of ice crystals in the material, and that the specimen is protected against damage by the beam in the SEM itself. This may be achieved by using a nitrogen cooled stage, and specimens viewed in such ways may often be left uncoated. Mounting Whereas in the light microscope, the choice of mounting medium and cover glass thickness are the two major concerns, specimen mounting in the SEM offers many more options. In general, the fibre or textile sample is fixed to a small metal support called a stub. The sample must be securely attached to this, so that it remains fixed during manipulations in the chamber. There are several media which may be used to hold the specimen to its stub, and these include: •

Double-sided tape (metal based). This is perhaps the most easily used method, though care must be taken to ensure that no air bubbles are

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trapped in mounting. The tape may give an unwanted background X-ray signal, or may crack and shrink during examination. • Carbon adhesive. This has the advantage of being a poor electron emitter, so will not hide the specimen, but can migrate and be absorbed. • Silver paint (‘Dag’). Possibly the most widely used medium, silver paint has excellent conductive properties, dries rapidly and provides a good, bright background to the SEM image. The paint is relatively expensive, however, particularly if frequent thinning of the suspension is needed. Coating The electron beam which is used to obtain the SEM image is composed of negatively charged particles. Unless the specimen to be examined is a conductor (e.g., a metal), it is necessary to render the surface layer of the fibre specimen conductive. This is applied in the form of a coating. There are two reasons why insulating materials must be coated prior to SEM examination: (1) To avoid build-up of negative charge which may be destructive (beam damage) and which may sporadically discharge and deflect the incident beam. The negative charge can also prevent the collection of generated secondary electrons. These unwanted phenomena are collectively known as ‘charging effects’. (2) To provide a higher secondary electron-emitting surface than the base specimen material, so that high energy beams and long scan times are not needed to obtain a clear SE signal of the specimen’s surface topography. The most usual coating materials are metals (e.g., gold, or goldpalladium) or carbon. The latter has the advantage of permitting X-ray analysis. These coatings are applied by sputter, evaporative or thermal deposition means and their control is critical. The balance must be struck between potential damage effects (e.g. thermal – particularly in sputter coaters), rate and thickness of deposition and uniformity of coating (Chapman, 1986). A specimen coated for routing morphological studies may have a coating thickness of 15–30 nm thickness (Postek, 1980), though for finer detail the coating would need to be far thinner. An optimum coating should be continuous from the specimen to the stub, and of uniform thickness. As an alternative to metal or carbon deposition, it is possible to use proprietary sprays which are applied to the specimen to provide a conductive coating. Similar in operation to ‘anti-static’ products, these can be useful

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where the specimen is so delicate that it may not withstand the thermal, mechanical or vacuum condition of the coating unit. Such sprays will not be as effective as metal or carbon, but if used with a low KeV can provide a useful substitute in certain circumstances. Clearly, if the layer of interest lies just beneath the specimen surface, and X-ray analysis is required, coating with metal or any extraneous matter is futile. Many further examples of the principles of correct specimen preparation for the SEM may be derived from texts aimed primarily at the TEM, as their methods of operation and crucial parameters are broadly similar (Goodhew, 1973).

10.3.4 Examination Once the specimen has been prepared in a satisfactory manner, and any special conditions accounted for (e.g., melting point, cleanliness, conductivity, information required, etc.), then it can be placed in the chamber of the instrument, vacuumed down and stabilised for examination. Among factors to consider when attempting to obtain the optimum image are: Accelerating voltage: Interactions of signal strength, penetration of probe, noise, beam damage, spot size. Working distance: Objective lens to specimen. Depth of field vs. resolving power. Spot size: Resolution vs. signal to noise ratio. Charging. Tilt (w.r.t collector): Signal strength vs. geometry. Signal collection: SE topography vs. BE atomic number. Charging. Vacuum: Dehydration, charging, preservation of hydrated areas. Final aperture: Resolution, signal strength, depth of field. Image composition: Ratio of SE to BSE signal. Filtering. Topography vs. atomic number. Contrast. Scan time: Signal strength and resolution vs. charging and beam damage. Corrections for electron lens aberrations: Chromatic, spherical, astigmatism. This list is intended merely to cite examples of parameters which must be adjusted in order to obtain the best quality image for the particular specimen being examined in the SEM. There are many more factors and possible interactions which may be involved, but which are outside the scope of this chapter. It should be apparent, however, that the operation of the SEM is somewhat more complex than for the operation of the equivalent light microscope in Brightfield mode.

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Conversely, many of the above SEM considerations are now automated, while the mathematical wave theories, depictions and consequent practical application techniques of (e.g.) multiaxial indicatrix figures, Nomarski differential interference contrast image manipulation and metrology in the microscopy of reflected and transmitted polarised light and multiple beam interferometry remain somewhat of a specialised art.

10.4

Further techniques

10.4.1 High resolution SEM While the normal range of resolution for an SEM is within the range of around 5–10 nm, the development of field emission guns and high excitation probe-forming ‘immersion’ objective lenses has allowed the development of systems capable of achieving spatial resolutions of less than 1 nm. The small source size and high brightness of the field emission gun is able to produce a probe spot of sub-nanometre size, which with the low energy spread gives less chromatic aberration than a Lab6 source. This probe contains sufficient current for secondary electron imaging, while the immersion type objective lens is also of low aberration and ensures the smallest probe diameter. Additional benefits of this type of lens are the screening out of external magnetic fields and the elimination of relative vibrational motion between the sample and the optic axis. For high resolution it is, of course, necessary that the pixel size of the image is smaller than the area of emitted secondary electrons which are generated within a few nanometres of the point where the probe enters the specimen (Joy, 1989). Modern tungsten filament gun SEMS will have resolutions nearer 3.0 nm at 30 kV if operating under very high vacuum (Hitachi Corporation, 2006).

10.4.2 Environmental SEM (ESEM) Although traditionally the SEM has operated only under high vacuum, which has meant dehydration of water-containing specimens, and the possible introduction of artefacts, recent years have seen the introduction of variable pressure or environmental SEMs. Essentially, the electron column is operated under vacuum as normal, until just above the specimen. Here in a small chamber with aperture to permit access of the beam, it is possible to have a gaseous or water vapour medium. The signals collected include interactions of the beam with the gas – in particular ionising collisions of emitted secondary electrons to both amplify

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the signal and provide a source of dynamic charge control (Stokes, 2006). In general it is possible to image specimens in conditions much closer to their natural environments – and even to see globules of water or oil on some surfaces. The images from these instruments working with pressure will normally never be as of ultimately high resolution as those obtained from a system under high vacuum, but the environmental SEM can be an extremely useful instrument for specimens whose features may be changed by the conditions imposed by vacuum. Typical gas pressures for the ESEM are 102–104. This is lower than atmospheric pressure, but sufficiently high to permit the observation of live insect specimens (Ford and Stokes, 2006). A review of the principles of ESEM has been provided (Thiel, 2004).

10.4.3 X-ray microanalysis As previously explained, one of the results of the interaction of an electron beam with a specimen is the generation of X-rays. A proportion of these X-ray have wavelengths and energies that are characteristic of the element from which they have been emitted. It is thus possible to use the SEM to carry out elemental analysis of specimens. There are two different types of X-ray microanalysis systems – energy dispersive and wavelength dispersive. The essential features of each may be broadly listed as below: Energy dispersive (EDX) • • • • • •

rapid (qualitative analysis) display of entire spectrum in same format high collection efficiency low sensitivity to alignment or topographical effects low cost and relatively compact simultaneous analysis of full X-ray spectrum above a given Z value.

Wavelength dispersive • high resolution (element separation) • analysis can be quantitative • wide range of elements analysed (Be to U) • high sensitivity • the energy dispersive systems are perhaps the most commonly used in the SEM. An EDX spectrum of nylon in an upholstery fabric is shown in Fig. 10.4. The count rate of EDX (or EDS) analysis is an important factor. If the number of X-rays reaching the detector is low (e.g., in thousands per unit

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cps 80 C 60 40 O

20

F

0 0

P 2

Ti 4

6

8

Energy (keV)

10.4 EDX spectrum of nylon fibres in treated upholstery fabric.

time) a long period of analysis is required. This may result in damage to the specimen and an increased level of background counts (noise). Conversely, if the count rate is high (perhaps hundreds of times the frequency of a low count rate), than as a signal is processed by the analogue to digital converter, a discriminator rejects all other pulses and these are not included in the spectrum sent to the multi channel analyser. At count rates of around 2000 per second the converter processes pulses at an acceptable rate and few are rejected. At higher rates more pulses will be rejected and this leads to an unacceptably large proportion of ‘dead time’ – when the processor is closed to additional signals. A high amount of dead time should be avoided as it leads to a bias in the analysis. 5% or below is generally considered a satisfactory amount of dead time.

10.5

Benefits of scanning electron microscopy compared to a light microscope

The main purpose of the SEM is to provide high resolution and high depth of field images of the surfaces of specimens. It can thus operate at much higher useful magnifications than the light microscope, but will not normally show interior detail. One well-known area where the characteristics of the SEM are put to good use is in the identification of animal fibres. Here, the great depth of field and higher magnification can permit the measurement of scale edge thickness. This feature is used in the distinction between cashmere and wool, and in the quantitative analysis of blends of these and other animal fibres (Kusch and Arns, 1986; Wortmann and Arns, 1986; Weidemann et al., 1987). It must be remembered, however, that as interior features are

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Identification of textile fibers

not shown, the instrument must be supplemented by light microscopy where, for example, the nature of the ladder medullae of fur fibres are to be studied. This proviso is acknowledged in the new ISO Standard 17751: Quantitative Analysis of Cashmere, Wool and Fibre Mixtures (International Organisation for Standardisation, 2005). Comparative micrographs using the LM to illustrate the interior features and the SEM to illustrate surfaces of the same samples of fibre have been produced, and form valuable reference works (Sich, 1989; Greaves and Saville, 1995). When the fibre examination is combined with elemental analysis by X-ray microanalysis, the SEM becomes a very powerful tool indeed. It can, for example, show the nature and distribution of an anti-soil fluorocarbon finish on a single fibre or the elemental distribution of contaminants on a sample of fabric. Further areas where the SEM can provide information not easily accessible with light microscopy are in the examination of surface coatings, the distribution of shrink resist polymers (Greaves, 1991) and detailed studies of fibre damage (McCarthy and Greaves, 1988). One fact that must be noted, however, is that the SEM will not show colour (though it is quite possible to add ‘pseudo colour’ to images electronically). The ideal system would be to use light and electron microscopies as complementary techniques – each revealing information not readily accessible to the other, but which together provide the most detailed picture (Langley and Kennedy, 1981). An example of the usefulness of the SEM’s high resolving power and depth of field is shown in Fig. 10.5. This is the surface of a wool

Mag = 10.00 K X

2 μm*

EHT = 3.00 kV WD = 5 mm

Signal A = Inlens Photo No. = 213

Date: 10 May 2006 Time: 16:45:13

10.5 Secondary electron image of treated wool fibre surface.

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fibre, with scale distal edge prominent. The micrograph was taken in order to establish the presence of ‘nanosphere’ anti-staining treatment on the fabric of a school uniform. The fine surface asperities confirm the treatment has been applied. These were not visible with light microscopy. Finally, one field in which the SEM has recently proved invaluable, is in the study of the so-called microfibres. Examinations with the instrument have shown clear images of the constituent parts of both the precursors for the different types, and the morphology of the end product (Hongu and Phillips, 1997).

10.6

Quantitative aspects

Although a full description of the quantitative analysis of fibre blends is outside the scope of this work, it will nevertheless be helpful for the reader to have a basic appreciation of the principles involved, since today the number of products made from mixtures of different fibres is likely to exceed those made from a single fibre type only. The determination of the percentage by weight of different components in a material may be required for authenticating identification, labelling, costing or processing of the material. Broadly, unless the mixture is of clearly separable individual components (e.g., the warp and weft yarns of a fabric) methods of analysis are divided into those where the fibres mixed are chemically the same, and those where they are chemically different.

10.6.1 Mixtures of chemically equivalent fibres Where fibres have the same chemical compositions, but are of different generic types, it is necessary to use physical means to distinguish them. The most commonly applied method of carrying this out is to use a microscopical identification, measurement and counting procedure – either with the light microscope or the SEM. Measurement is necessary because straight numerical counting would not take into account different diameters of fibres and thus not produce an accurate weight percentage. The first application of this technique was developed for the analysis of animal fibre blends (Wildman, 1953) and is still in use today, though in sometimes modified form. The technique permits the determination of proportions in blends such as the commercially valuable mixtures of cashmere and wool, mohair and wool, wool and angora, etc. The procedure can also be adapted for use with blends of vegetable fibres, and mixtures of natural and regenerated cellulosics. Other techniques that may be applied to blends of fibres chemically the same are X-ray diffraction (Chidamberswaran et al., 1987) and analysis by DNA (see Chapter 12).

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Identification of textile fibers

10.6.2 Mixtures of chemically different fibres The traditional means of separating individual components from a mixture of chemically different fibres is to use selective dissolution. In essence, the more soluble component in the blend is removed using an agent which is known not to cause any damage or weight loss to the component(s) which are to remain. As an example, to determine the percentage of cotton in a cotton/polyester blend (as used in shirtings and bed linens, etc.), after pretreatment to remove any non-fibrous matter and accurate determination of the original weight of the material, the cotton would be dissolved from the blend using 75% sulphuric acid. After complete dissolution the polyester residue would be rinsed, dried and weighed, and the proportions of the two components determined – either on a dry weight basis, or, more usually, with the agreed allowances for moisture regain. Strict procedures for the determination of fibre composition by chemical dissolution are laid out in International and National Standard Documents (ISO, 2006; British Standards Institute, 2002; AATCC, 2003). Other methods of analysing chemically different fibres are gaining increased acceptance after continuing development, particularly that of near infrared spectroscopy, infrared spectroscopy and nuclear magnetic resonance spectroscopy. There are no formal standard test procedures for these techniques as yet, however.

10.7

Sources of further information and advice

The Textile Institute International Headquarters First Floor St James’s Buildings Oxford Street Manchester M1 6FQ UK Tel. 44 (0) 161 237 1188 Fax 44 (0) 161 236 1191 Email [email protected] (Publications, Conferences, Committees, Meetings, Consultancy Register, Special Interest Groups) The Royal Microscopical Society 37/38 St Clements Oxford OX4 1AJ UK Tel. 44 (0) 1865 254760 Email [email protected]

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(Publications, Courses, Meetings, Exhibitions, Journals, Liaison with Other Institutes) Society of Dyers and Colourists Perkin House Grattan Road Bradford BD1 2JB UK Tel. 44 (0) 1274 390955 Fax. 44 (0) 1274 392888 Email [email protected] (Publications, Meetings, Lectures, Reference Materials)

10.8

References

American Association of Textile Chemists and Colorists. AATCC Method 20A. AATCC, NC USA (2003). British Standards Institution. BS4407. Quantitative Analysis of Fibre Mixtures B.S.I. London (1988 (2002)). Carroll GR. Forensic Fibre Microscopy in Forensic Examinations of Fibres, J Robertson (Ed.) Ellis Horwood, New York City, NY (1992). Chapman SK. Working with a Scanning Electron Microscope. Lodgemark Press, Kent (1986). Chescoe D and Goodhew PJ. The Operation of the Transmission Electron Microscope. Royal Microscopical Society Handbook. Oxford University Press (1984). Chidamberswaran PK et al. Textile Research Journal (1987) 57, 167. Dobb MG. Journal of the Textile Institute 61, 232 (1970). Ford BJ and Stokes DJ. Bug’s Eye View. In Focus 2006. 3. Royal Microscopical Society, Oxford. Goodhew PJ. Specimen preparations in materials science in Practical Methods in Electron Microscopy. AM Glauert (ed). North Holland Publishing Co., New York and Oxford (1973). Greaves PH and Saville BP. Microscopy of Textile Fibres. Royal Microscopical Society Handbook. Bios Scientific Publishers, Oxford (1995). Greaves PH. Proceedings International Conference on Fibre and Textile Science, Ottawa, Canada. The Fibre Society and The Textile Institute (1991). Harris WE and Habgood HW. Programmed Temperature Gas Chromatography. John Wiley & Sons Inc. (1966). Hitachi High Technologies Corporation. Microscopy and Analysis 115 (2006). Hongu T and Phillips GO. New Fibres (Second edition). Woodhead Publishing Co. Abington, Cambridge (1997). International Organisation for Standardisation. ISO 17751 Final Draft (2005). International Organisation for Standardisation. ISO 1833 Quantitative Analysis of Binary and Ternary Fibre Blends (2006). Johnson DJ. PhD Thesis, University of Leeds (1965).

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Joy DC. High Resolution Scanning Electron Microscopy. Institute of Physics Conference Series No. 98, Chapter 10. IOP Publishing Group, Bristol and New York (1989). Kusch P and Arns W. Schriftenriche des Deutsches Wollforschungsinstitut 87 Deutches Wollforschungsinstitut, Aachen, Germany (1986). Langley KD and Kennedy TA. Textile Research Journal (1981) 51. McCarthy BJ and Greaves PH. Wool Science Review (1988) 65. Meldrum, K. Journal of the Society of Dyers and Colourists (1961) 77, 22. Park J and Shore J. Review of Progress in Colouration (1982) 12, 43. Postek MT et al. Scanning Electron Microscopy – A Student’s Handbook. Postek MT Jr and Land Research Industries Inc. (1980). Sich J. Proceedings 2nd International Conference on Speciality Fibres. DWI Aachen, Germany (1989). Sikorski J. Structural Studies of Mammalian Keratin (in Structures of Fibrous Biopolymers. Colton Papers 26. Editors EDT Atkins and A Keller) (1975). Stokes DJ. Progress in the Study of Biological Specimens using ESEM. In Focus 2006. 2. Royal Microscopical Society, Oxford. The Textile Institute. Guide to the Identification of Textile Materials 7th edition. Manchester, UK (1985). Thiel BL. Image Analysis in Materials Science by Low Vacuum SEM. International Materials Reviews (2004) 49 (2) 109–122. Tungol MW, Monaster A and Bartick EG. Analysis of Single Polymer Fibres by Fourier Transform Infrared Microscopy. The Results of Case Studies. Applied Spectroscopy 44, 543 (1990). Weidemann E et al. SAWTRI Bulletin (1987) 21. Wildman, AB. The Microscopy of Animal Textile Fibres. Wool Industries Research Association, Leeds (1953). Wortmann F and Arns W. Textile Research Journal (1986) 56.

11 Analysis of dyes using chromatography S W LEWIS, Curtin University of Technology, Australia

Abstract: The colour of fibre, present naturally or imparted through use of a dye or pigment, can be extremely discriminating for the forensic scientist. One approach for colour analysis available to the forensic scientist is chromatography and, while partially destructive of the evidence, it can provide a much greater degree of discrimination than physical or optical methods alone. This chapter will give an overview of chromatographic methods applied to the forensic analysis of dyes in fibres focusing on thin-layer chromatography (TLC). Other approaches to instrumental chromatographic analysis of dyes, including high performance liquid chromatography (HPLC) and capillary electrophoresis (CE) will also be discussed. Key words: forensic fibre dye analysis, thin-layer chromatography, extraction, high performance liquid chromatography, capillary electrophoresis.

11.1

Introduction

A key element of a fibre’s functionality is its colour, which can be extremely discriminating for the forensic scientist. This colour may be present naturally or can be imparted through use of a dye or pigment. As described in Chapter 9, microspectrophotometry can provide a good deal of differentiation; however, in a small number of cases, different dyes may produce the same or similar colour (Wiggins et al., 2005). One approach available to the forensic scientist is chromatography and, while partially destructive of the evidence, it can provide a much greater degree of discrimination than physical or optical methods alone. Chromatographic methods require the extraction of the colourant from the fibre followed by analysis. This chapter will give an overview of chromatographic methods applied to the forensic analysis of dyes in fibres, focussing on thin-layer chromatography (TLC) which is the most widely used chromatographic technique applied in this area. Other approaches to instrumental chromatographic analysis of dyes that have been suggested, including high performance liquid chromatography (HPLC) and capillary electrophoresis (CE) will also be discussed. 203

204

Identification of textile fibers

11.2

Dyes

Dyes have been used to alter the appearance of cloth and fabric by changing their colour since ancient times (Garfield, 2000). Dyes can be defined as any substance which has an affinity for a substrate and, on binding to that substrate, alters the way in which the material absorbs light, and hence changes the colour that is subsequently observed. Dying of cloth with a wide variety of vegetable and animal dyes has been carried out for thousands of years. Famous historical dyes include the rare and expensive Royal purple, extracted from certain species of mollusc and used by the Roman emperor and his household; cochineal, the crimson dye obtained from the dried and pulverised bodies of cactus insects; and indigo sourced from the leaves of certain plants (Garfield, 2000). The birth of modern dyestuffs occurred with the discovery of mauveine by William Perkin in 1856 and today synthetic dyes predominate (Gregory, 2000, Garfield, 2000). Colour is due to the preferential absorption of certain wavelengths of light, the observed colour being those wavelengths of light that are reflected. The particular bands of light absorbed, and hence the colour observed, by a particular dyed fabric is due to the chemical structure of the dye. Light is absorbed by a molecule when it has sufficient energy to promote an electron from a ground state orbital, where electrons within a molecule normally reside, to an unoccupied higher energy orbital. Dyes are organic molecules with extensive areas of high electron density due to the presence of double bonds. When the double bonds alternate along the structure the molecule is said to be conjugated. This leads to overlapping π orbitals, which results in decreased energy gaps between ground state and higher energy, unoccupied orbitals. The areas of a molecule that absorb visible light are termed chromophores, the wavelengths of light they absorb will depend on their structure and the presence of other groups, termed auxochromes, which alter the electron density within the conjugated group. For a chemical to act as a dye it must have some affinity for the substrate to which it is being applied. Dyes can be bound either physically, through a variety of different attractive forces such ionic, van der Waals, and hydrogen bonding, or chemically, through covalent bonds (Gregory, 2000). Classification and identification of individual dyes is complicated by the bewildering array of chemical structures, methods of application, and commercial names (Gregory, 2000). One of the best known sources of information on dyes is the Colour Index, which was first published by the Society of Dyers and Colourists in 1925 and is now available in an on-line version (www.colour-index.org, 2007). This utilizes a dual system of numbers and names to classify dyes and pigments, based primarily on the method of application, which are listed in Table 11.1.

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Table 11.1 Dye classes and associated fibres Class

Fibre type

Method of application

Acid

Wool, silk, polyamide, protein, polyacrylonitrile, polypropylene Cotton, viscose

Acidic dyebaths

Azoic

Basic Direct

Disperse

Polyacrylonitrile, modified acrylic, polyester, polyamide Cotton, viscose

Metallized

Polyester, polyacrylonitrile, polyamide, polypropylene, acetate/triacetate Wool, polypropylene

Reactive

Cotton, wool, polyamide

Sulfur

Cotton

Vat

Cotton

Fibre impregnated with coupling component and treated with stabilized diazonium salt Acidic dyebaths Neutral or slightly alkaline dyebath containing additional electrolyte Aqueous dispersion of dye, often applied at high temperatures or in the presence of a carrier Formation of a metal complex, dye, metal (mordant) may be applied before, after or at same time as dye Dye reacts chemically with fibre forming covalent bonds, may be under acid or alkaline conditions Dyes reduced in alkaline medium to produced leuco form of dye which penetrates fibre and then is oxidized back to insoluble form of dye Complex process similar to sulfur dyes

Sources: Gregory (2000), Wiggins (1999)

The following examples illustrate classification of the dyes illustrated in Fig. 11.1. Dye A is CI Vat Green 1, CI Number 59825, indicating that its method of application is via a vat dying process. Dye B is CI Direct Red 45, CI Number 14780, which is applied directly to the fibres. One disadvantage of this classification process is that it does not provide information concerning the chemical structure of the dye, which is essential in developing analytical chemistry procedures for their extraction, separation and identification. In terms of chemical properties the unique identifier for the each dye is the Chemical Abstracts Service (CAS) Registry number, for example for Dye A in Fig. 11.1 this is 128-58-5. This number can be used for searching in on-line databases like Scifinder Scholar.

206

Identification of textile fibers

OCH3 OCH3

SO3Na H3C

OH

S N

O

N

N

SO3Na Dye A

O

Dye B

11.1 Chemical structures of selected dyes.

Due to the variation in chemical structure of fibres, some dyes are more suited for use with certain fibres. For example acid dyes are particularly well suited to the dyeing of wool and nylon due to the presence of the amino groups that are charged under acid conditions thus attracting the dye anion (Wiggins, 1999). The Metropolitan Laboratory of the Forensic Science Service in the UK carried out a study on fibre/dye combinations encountered during a 12 month period from 1993 to 1994 and discovered that while certain combinations were more prevalent, it was clear that many dyes are available for each of the fibre types commonly encountered in forensic examinations (Wiggins, 1999). It should also be noted that most textiles are dyed with multiple dyes, and due to the complexity of the dying process, this occurs via a batch process. This leads to variability in colour between batches (Connelly, 1997, Wiggins et al., 1987). This may be useful to the forensic scientist, but also leads to complications in developing protocols for the analysis of dyes in unknown samples.

11.3

Forensic analysis of dyes

As stated above, fibres are most likely to be dyed with a combination of several different dyes. This combination has the potential to be characteristic of a particular source of fibres. Forensic analysis of dyes is thus generally carried out to compare recovered samples with controls, rather than to absolutely identify a particular dye. The two issues which face the forensic analyst when considering the analysis of fibre dyes are: (i)

the sheer complexity of the problem with the wide variety of dyes and fibres and potential combinations thereof, and (ii) the destructive nature of chromatographic analysis due to the extraction of the dye from the fibre. It is for these reasons that extraction and analysis of dye from fibres is only carried out once the other physical and optical methods of analysis have been exhausted.

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207

11.3.1 Extraction The first step in the analytical process is extraction of the dye/dyes from the fibre. A number of different extraction schemes utilising a variety of solvents have been proposed (Beattie et al., 1979, 1981a, 1981b, Cheng et al., 1991, Hartshorne and Laing, 1984, Home and Dudley, 1981, Laing et al., 1990, 1991, Macrae and Smalldon, 1979, Resua, 1980, West, 1981, Wiggins, 1999). These schemes vary in detail, but have essentially the same goal of using a sequence of extractions, which not only provide an extract of the dye but can also be used to classify the dye. A comprehensive approach is that developed by the Forensic Science Service in the United Kingdom which involves different solvent sequences depending on the identity of the fibre, which had been established by microscopy and/or infrared spectrophotometry (Wiggins, 1999). Subsequent separation and identification of the dyes was by thin layer chromatography (TLC). Details of the various extraction schemes are summarized in Figs 11.2 to 11.7 (Wiggins, 1999). The extraction process itself is very simple. A single fibre is placed in a glass tube (2.5 cm × 1.5 mm i.d.), sealed at one end. Solvent (around 10 μL, it should be sufficient to completely immerse the fibre) is added and the tube is heat sealed prior to incubation in an oven for the requisite time and temperature, as described in the extraction procedure (Wiggins, 1999). If the fibre is very pale, a larger sample than a single fibre is needed, due to sensitivity issues. Control samples are utilized in order to establish dye class and the best extraction procedure prior to analysis of recovered fibres (Wiggins, 1999). Once classification of the dye has occurred via the schemes described in Figs 11.2 to 11.7, extraction for TLC analysis can be carried out as outlined in Tables 11.2 and 11.3. Care must be taken when using this approach to characterise dye classes in unknown samples, with dyes being classified as ‘being equivalent to, or Stage 1: Pyridine/water (4:3) 100 °C 10 min

Good extraction ACID DYE Good extraction METALLIZED DYE

Poor/no extraction Stage 2: 2% aqueous oxalic acid 100 °C 20 min Poor/no extraction REACTIVE DYE

11.2 Extraction and classification of dyes from wool fibres (Wiggins, 1999).

208

Identification of textile fibers Stage 1: Glacial acetic acid 100 °C 20 min Poor/no extraction

Good extraction AZOIC DYE

Stage 2: Pyridine/water (4:3) 100 °C 20 min

Poor/no extraction

Good extraction

Stage 3: Dithionite/polyvinylpyrrolidone* 100 °C 20 min Extract applied to TLC plate

Fibre colour changed No coloured spot/ spot not original fibre colour REACTIVE DYE

DIRECT DYE

Fibre colour unchanged No coloured spot/ spot not original fibre colour INGRAIN DYE

Fibre colour changed coloured spot original fibre colour

Stage 4: New fibre, 10–14% sodium hypochlorite 100 °C 10 min

Fibre colour changed SULFUR DYE

Fibre colour unchanged VAT DYE

*sodium dithionite (80 mg) polyvinylpyrrolidone (30 mg) sodium hydroxide (10%, 450 μL) water (9 mL); use immediately and discard excess

11.3 Extraction and classification of dyes from cotton and viscose fibres (Wiggins, 1999).

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209

Stage 1: Formic acid/water (1:1) 100 °C 20 min Good extraction Stage 2: TLC procedure – methyl acetate eluent

No movement

Movement

Stage 3: TLC procedure – methanol eluent

Sharp line at solvent front ACID DYE

DISPERSE DYE

Little or no movement or smeared BASIC DYE

11.4 Extraction and classification of dyes from polyacrylonitrile fibres (Wiggins, 1999). Stage 1: Chlorobenzene 150 °C 15 min Poor/no extraction

Good extraction DISPERSE DYE

Stage 2: Pyridine/water (4:3) 100 °C 20 min

Good extraction

Stage 3: TLC procedure – methanol eluent

Sharp line at solvent front ACID DYE

Poor/no extraction REACTIVE OR DIAZO DYE

Little or no movement or smeared BASIC DYE

11.5 Extraction and classification of dyes from polyamide fibres (Wiggins, 1999).

210

Identification of textile fibers Stage 1: Chlorobenzene 130 °C 10 min Poor/no extraction

Good extraction DISPERSE DYE

Stage 2: Dimethyl formamide/formic acid (1:1) 100 °C 20 min Good extraction BASIC DYE

11.6 Extraction and classification of dyes from polyester fibres (Wiggins, 1999).

Stage 1: Methyl acetate/water/acetic acid (5:5:1) 100 °C 20 min Poor/no extraction

Good extraction DISPERSE DYE

Stage 2: Pyridine/water (4:3) 100 °C 20 min

Some extraction Stage 3: 2% Aqueous oxalic acid 100 °C 20 min then pyridine/water (4:3) 100 °C 20 min

Improved extraction METALLIZED DYE

No extraction PIGMENT

No improvement ACID DYE

11.7 Extraction and classification of dyes from polypropylene fibres (Wiggins, 1999).

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Table 11.2 Extraction and disruption solutions for fibre dye analysis Solution

Composition and preparation

Pyridine/water Formic acid/water Aqueous oxalic acid

4 : 3 v/v, prepare 100 mL and use until exhausted 1 : 1 v/v, prepare 100 mL and use until exhausted 0.2 g in 100 mL water, use immediately and discard excess 0.5 M acetic acid, prepare 100 mL and use until exhausted Cellulase (Penicillium funiculosum) 1.6 mg/mL in sodium acetate buffer (0.1 M in water adjusted to pH 5 ± 0.2 with glacial acetic acid), prepare 50 mL and discard at end of each week 0.75 M in water, prepare 100 mL and use until exhausted or end of one month 3.0 M in water, prepare 100 mL and use until exhausted or end of one month

Acetic acid Cellulase

Sodium hydroxide for wool Sodium hydroxide for cotton Source: Wiggins (1999)

acting as, a particular dye class’ (Wiggins, 1999). This covers the possibility if new dyes are introduced. This scheme as originally proposed did not account for reactive, vat, ingrain and sulfur dyes which were not considered extractable. Some additional approaches to extraction of reactive dyes from wool and cotton fibres have been suggested. The extracts obtained are not true dye extracts, as the dyes are covalently bound to the fibres (Wiggins, 1999). The fibre structure needs to be disrupted to release a coloured solution. The procedures are rather more involved than the simple extraction procedure outlined above. For wool, fibres are placed in a glass tube with 3 μL sodium hydroxide solution (see Table 11.2 for details), the tube is sealed and then incubated at 45°C for 24 hours with continuous agitation. This is followed by the addition of 2 μL citric acid, mixing and centrifugation at 7000 rpm for 5 minutes. Cotton fibres are placed in a more concentrated sodium hydroxide solution (5 μL, see Table 11.2 for details) at 0°C for 4 hours. The solution is then discarded and the fibres re-suspended in 5 μL acetic acid for 20 seconds. The resulting solution is again discarded and 3 μL cellulase solution added. The tube is re-sealed and incubated at 45°C for 24 hours with continuous agitation. This is followed by the addition of 3 μL methanol, mixing and centrifugation at 7000 rpm for 5 minutes (Wiggins, 1999). Once classification of the dye has occurred via the schemes described in Figs 11. 2 to 11.7, extraction for TLC analysis can be carried out as outlined in Table 11.3.

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Identification of textile fibers

Table 11.3 Choice of extraction solutions for fibre dye analysis Dye class

Fibre type

Extraction solution

Acid

Wool, silk, polyamide, protein, polyacrylonitrile, polypropylene Cotton, viscose Polyacrylonitrile, modified acrylic Polyester, polyamide Cotton, viscose Polyester, polyacrylonitrile, polyamide, polypropylene, acetate, triacetate Wool, polypropylene

Pyridine/water

Azoic Basic Direct Disperse

Metallized

Pyridine/water Formic acid/water Pyridine/water Pyridine/water Pyridine/water

Aqueous oxalic acid then pyridine/water

Source: Wiggins (1999)

11.3.2 Separation After extraction, separation of the coloured components is required, and this has generally been achieved through the use of chromatographic techniques. Chromatography is defined by IUPAC as a ‘physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary (stationary phase) while the other (the mobile phase) moves in a definite direction’ (MacNaught and Wilkinson, 1997). The chromatographic techniques that have been used for forensic fibre dye analysis are thin layer chromatography (TLC)(Beattie et al., 1981b, Home and Dudley, 1981, Laing et al., 1990, Shaw, 1980, Wiggins, 1999, Wiggins et al., 2005) and high-performance liquid chromatography (HPLC) (Griffin and Speers, 1995, 1999, Griffin et al., 1994, Huang et al., 2004, 2005, Laing et al., 1988, Oxspring et al., 1994, Petrick et al., 2006, Speers et al., 1994, Wheals et al., 1985, Yinon and Saar, 1991). More recently a non-chromatographic separation technique, capillary electrophoresis (CE) has also been proposed (Oxspring et al., 1994, Robertson, 1999, Siren and Sulkava, 1995, Tetler et al., 1994, Xu et al., 2001). Thin layer chromatography (TLC) In TLC the stationary phase is present as a thin layer upon an inert rigid support. Izmailov and Schraiber first suggested this approach in the 1930s, but TLC in the form it is recognized today was not established until the 1950s (Robards et al., 1994). Today there is a wide range of pre-made plates with a variety of different stationary phases available to the scientist. A typical TLC separation involves application of a solution of the sample to

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213

be analyzed as a discrete spot or smear upon the chromatographic plate. Once the solvent in which the sample was applied has evaporated, the plate is developed by allowing a mobile phase to move by capillary action through the stationary phase, carrying the components contained within the sample with it. These components are retained to different extents by the stationary phase thus leading to separation. The elution characteristics of compounds can be reported as Rf values, which are a measure of the relative distance travelled by each compound from the origin with respect to the solvent front. TLC is essentially a variant of liquid chromatography or HPLC, with the mobile phases and stationary phases utilised being common to both. There are some key differences between the two approaches. In TLC, unlike HPLC, there is no need for the sample solvent to be compatible with the mobile phase as it is evaporated off when applying the sample to the plate. Similarly there is no requirement for the mobile phase to be compatible with detection as is the case with HPLC. TLC is an ‘open bed’ method, with retention being observable at all stages, unlike in HPLC, where if a component is strongly retained, the only indication will be the lack of a peak. In TLC standards and samples can be run simultaneously on a single plate under identical conditions. A key advantage of TLC is its relative simplicity when compared to other chromatographic techniques. The steps in a TLC analysis have been defined as (Robards et al., 1994): (i) selection of a suitable chromatographic stationary phase (ii) application of the sample (iii) selection of a mobile phase (iv) development (v) visualization and detection. This can also be followed by quantification, however this is not generally utilized in forensic fibre dye analysis (Wiggins, 1999). There are, however, some disadvantages to TLC. Owing to reliance on capillary action for development, flow rate of mobile phase is difficult to control with any precision. Detection limits are also an issue, with HPLC being far better in this regard. In addition there is a certain amount of manual dexterity required to load (or ‘spot’) the samples onto the plate and this can introduce some variability. A range of stationary phases is available for TLC, which can be classified on the basis of their composition and primary mechanism of retention. The most popular phases in common use are silica gel and alumina, the main retention mechanism for both being due to an adsorption process involving affinity of polar and polarisable compounds to the stationary phase (Robards et al., 1994). Non-polar components are not retained and move further along the TLC plate. Aromatic compounds are readily polarizable; hence

214

Identification of textile fibers

this mode of TLC is well suited to the separation of fibre dyes that have extensive aromaticity and many other polarisable functional groups. The stationary phase is laid down as a thin layer (100–2000 μm) thick upon an inert rigid support, such as glass or aluminium (Robards et al., 1994). While in earlier times the scientist had to undertake the laborious and time consuming process of preparing these plates from scratch, there are now many commercial sources of pre-made plates. A specific type of plate, which has been previously been recommended for forensic analysis of dye fibres, is aluminium-backed silica gel 60F 254, with dimensions of 5 × 7.5 cm available from Merck (Wiggins, 1999). It has been recommended that prior to use these silica plates are either stored in desiccator or heated. This step is to ensure that any water that may be present is driven off, as this will alter the activity of the silica stationary phase (Wiggins, 1999). The term activity describes the particular properties of the adsorbent, which give rise to retention of a given substance, it is determined by both the chemical structure of the sorbent and any adsorbed species. Adsorbed water has a great effect on both retention, which is decreased with increasing humidity, and also may alter the selectivity of the separation. The efficacy of heating has been questioned as re-adsorption of water occurs extremely rapidly, with a plate becoming significantly re-equilibrated with ambient humidity within five minutes of removal from a desiccator or other humidity controlled environment (Robards et al., 1994). What is clear is that the plates need to be stored and prepared in a controlled and reproducible manner, preferably in a humidity-controlled environment. Appropriate application of the sample is highly important in obtaining good separations. In principle it is very simple, using a glass capillary or micropipette, the dye extract is applied as spot of approximately 2 mm in diameter, typically about 1 cm from the bottom edge of the plate (Wiggins, 1999). To ensure good results there are a number of practical issues that need to be faced. Care needs to be taken not to damage the adsorbent layer or to produce an excessive spot size. The solvent used to load the sample should be able to wet the stationary phase and it should also be volatile so that it can be driven off prior to development of the plate. This process can be accelerated through the application of heat, either applied using a hair dryer or by having the plate rest upon a hot surface. To increase the amount of dye material on the plate, thus providing a more detectable result, repeated application of sample on the initial spot could be carried out. It is recommended that both questioned samples, known samples and standard dye mixtures are run at the same time, with the questioned sample being bracketed by known samples or standard dyes (Wiggins, 1999). The applied spots should not be too close to each other, or to the edge of the plate. There are a number of different approaches to developing TLC plates, with varying levels of complexity. However the simplest, and most widely

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215

used approach is the ascending linear method (Robards et al., 1994). Once the plate is completely dry, it placed in a pool of eluent contained within a covered chamber. This can be a simple as a beaker with a petri dish cover, or alternatively a commercially available development chamber. Prior to use the eluent needs to stand for a few minutes in the covered chamber to allow the chamber to become saturated with eluent vapour (Robards et al., 1994). The depth of eluent should be such that when the plate is placed in the chamber, the eluent level should be below the applied spots, a distance of at least 0.5 cm has been recommended (Wiggins, 1999). The eluent will then move up the plate by capillary action, this process takes only a few minutes and separation of the components within the applied dye spots will be observed. Development should be carried out until the eluent has travelled around 2 cm beyond the origin where the spots were applied (Wiggins, 1999). Any further than this may lead to the separated spots become diffuse, which makes visualization and interpretation problematic. The eluents used for TLC of fibre dyes will depend on the class of dye being analysed, Table 11.4 summarizes the composition of eluent mixtures and their application to particular dye classes. It has been recommended that a minimum of two eluents should be applied to the analysis of a particular dye extract (Wiggins, 1999). Issues which need to be considered when deciding whether a particular eluent system is suitable for a dye extract include; separation and sharpness of bands, how far the bands have moved from the origin, how close to the separated bands are to the solvent front and the strength (concentration) of the dye extract from the recovered (questioned) fibres (Wiggins, 1999). Interpretation of the developed TLC plates is carried out by comparing the band position and colours of the questioned fibre extracts with control fibre extracts and standard dye mixtures (Table 11.5). This can be done under normal laboratory lighting or the use of ultraviolet light. Rf values can be calculated; however, they are of limited value when comparing between plates due to subtle differences in development that lead to different retention. A recovered fibre may be too pale in colour or too short to enable the dye extract to be concentrated enough to obtain a successful TLC. There is no simple way to predict whether a fibre is going to be long enough; analysis of an equivalent control fibre is therefore highly recommended (Wiggins, 1999). In these circumstances the more sensitive approach of HPLC may yield more successful results. High performance liquid chromatography (HPLC) Since its introduction in the late 1960s HPLC has become perhaps the most widely used analytical separation technique in use today. In HPLC, the

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Identification of textile fibers

Table 11.4 Eluents for TLC analysis of extracted fibre dyes Eluent no. 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Solvents (proportions v/v)*

Fibre type

Dye class

n-butanol, acetone, water, ammonia (5 : 5 : 1 : 2)

Wool or silk

Cotton or viscose Cotton Cotton or viscose Cotton Cotton or viscose Polyester Polyester

Acid or metallized Direct Basic Reactive Acid or metallized Direct Reactive Direct Reactive Azoic Disperse Disperse

Polyester

Disperse

Polyester

Disperse

Polyamide

Acid

Polyamide

Acid

Polyacrylonitrile

Basic

Polyacrylonitrile

Basic

Wool

Reactive

Cotton

Reactive

Cotton

Reactive

Pyridine, amyl alcohol, 10% ammonia (4 : 3 : 3) n-Butanol, ethanol, ammonia, pyridine, water (8 : 3 : 4 : 4 : 3) Methanol, amyl alcohol, water (5 : 5 : 2) Toluene, pyridine (4 : 1) Chloroform, ethyl acetate, ethanol (7 : 2 : 1)** n-Hexane, ethyl acetate, acetone (5 : 4 : 1) Toluene, methanol, acetone (20 : 2 : 1) n-Butanol, acetic acid, water (2 : 1 : 5)*** n-Butanol, ethanol, ammonia, pyridine (4 : 1 : 3 : 2) Chloroform, butanone, acetic acid, formic acid** (8 : 6 : 1 : 1) n-Butanol, acetic acid, water (4 : 1 : 5)*** Propan-1-ol, methanol, water, ammonia (6 : 3 : 1 : 4) n-Butanol, ethanol, ammonia, pyridine, water (8 : 3 : 4 : 4 : 6) n-Butanol, ethanol, ammonia, pyridine, water (6 : 3 : 2 : 6 : 6)

Cotton or viscose Polyacryonitrile Wool Wool or silk

* Ethanol used is 99%, ammonia is 0.880 SG unless otherwise stated, eluents discarded weekly except for ** which should be discarded daily. *** These eluent combinations form an upper and lower phase; use upper phase as eluent. Source: Wiggins (1999)

stationary phase, typically particles in the range 3–5 μm, are contained within a column with dimensions of 3–25 cm in length and 3–5 mm internal diameter. The mobile phase is pumped through the stationary phase at high pressure (up to 6000 psi) with flow rates of 0.1–10 mL/min. Samples are introduced into the system before the separation column using a variety of injection devices which can be manual or automated. A number of different

Analysis of dyes using chromatography

217

Table 11.5 Standard dye mixtures for TLC analysis of fibre dyes Standard dye mixture*

Eluent no.

A

Solway green G (CI acid green 25) Solway blue RNS (CI acid blue 47) Naphthalene fast orange (CI acid orange 10)

1, 2, 3, 4, 9, 10, 12, 13, 14, 15

B

Superacet fast orange (CI disperse orange 3) Superacet fast violet B (CI disperse violet 8) Superacet scarlet 2G (CI disperse orange 1)

5, 7, 8

C

Superacet fast orange (CI disperse orange 3) Superacet fast violet B (CI disperse violet 8)

6

D

Solway green G (CI acid green 25) Superacet fast orange (CI disperse orange 3) Superacet fast violet B (CI disperse violet 8)

11

* Approximately 5 mg of each dye made up to a final volume of 25 mL with pyridine/water 4 : 3. Use until exhausted. Source: Wiggins (1999) Sample injector Mobile phase reservoir

Pump

Column

Detector

Data recorder

11.8 Schematic diagram of the components of an HPLC system.

approaches can be used to detect the eluting analytes; the most common is ultraviolet-visible absorbance spectrophotometry; however, mass spectrometry has also been used in the analysis of fibre dyes. The resulting separation is recorded as a graph, formerly on a chart recorder but now electronically using a computer, which is referred to as a chromatogram. A schematic of a typical HPLC system is presented in Fig. 11.8. There are a number of different forms of HPLC depending on the type of retention mechanism involved. By far the most common is partition

218

Identification of textile fibers

chromatography, which can be further divided into reversed phase chromatography and normal phase chromatography. The stationary phase for partition chromatography is chemically bonded to silica particles. For reversed phase chromatography the bonded phase is relatively non-polar with polar compounds eluting first from the column, while for normal phase chromatography, which utilizes relatively polar stationary phases, non-polar compounds are the first to elute. HPLC is an extremely powerful separation technique that is widely used in all areas of analytical science. A wide range of stationary phases is available to the analyst in combination with an almost limitless range of mobile phase compositions. It is this flexibility that also increases the complexity of the development process for a new analytical separation. HPLC also has high initial set-up costs and significant running costs when compared with TLC. As samples are introduced in solution into an HPLC system, another issue is the potential incompatibility between the extraction solvent and the separation conditions (Griffin and Speers, 1999). HPLC, however, has the advantages of far greater efficiency separations, being able to resolve the components of very complex mixtures, and greater sensitivity. This latter is a particular advantage when analyzing extremely small samples of material. The powerful combination of HPLC coupled with mass spectrometry has been applied to forensic fibre dye analysis (Huang et al., 2004, 2005, Petrick et al., 2006, Yinon and Saar, 1991). This allows the determination of molecular structure information on eluted bands and thus provides an extra dimension of information.

Capillary electrophoresis (CE) The term capillary electrophoresis (CE) is used to describe a group of techniques where separation of the components of a chemical mixture occurs in a narrow bore capillary under the influence of an electric field. In its simplest form, separations in CE are based upon the differential migration of charged species through a fused silica capillary filled with the background electrolyte. Detection is typically by ultraviolet-visible absorbance spectrophotometry, resulting in graph, which looks similar to a chromatographic separation and is often referred to as an electropherogram. A schematic of typical CE instrument is presented in Fig. 11.9. Dedicated capillary electrophoresis instruments, which automate many of the processes in carrying out an analysis, are available. For neutral species separations are made possible by addition of surfactants to the background electrolyte above the critical micelle concentration. This leads to the formation of micelles, which act as a ‘pseudo-stationary’ phase, separation being achieved by interaction between the analytes and the micelles. This hybrid

Analysis of dyes using chromatography Capillary filled with electrolyte

219

Detector Data recorder and instrument control

Buffer reservoirs

Electrodes

High voltage supply

11.9 Schematic diagram of a CE system.

of chromatography and CE is termed micellar electrokinetic chromatography (MEKC). Highly efficient, rapid separations are possible and CE in its various modes is a well-established analytical technique with over 30 000 references to date in a diverse range of application areas. Forensic application of CE was first demonstrated by Weinberger and Lurie and since then has been the subject of a number of reviews and book chapters. What makes CE so attractive for forensic analysis is its exceptional separation power coupled with rapid analysis, minimal sample preparation, low reagent usage and waste production. That being said, there has been little use of this technique for the forensic analysis of fibre dyes, with only a few applications reported in the literature (Oxspring et al., 1994, Robertson, 1999, Siren and Sulkava, 1995, Tetler et al., 1994, Xu et al., 2001). In a similar fashion to HPLC there is the issue of compatibility between the extraction solvent and the separation conditions. Robertson in an early report of the application of CE to the separation of dyes also noted issues with reproducibility and dyes becoming trapped in the capillary (Robertson, 1999). Nevertheless CE has tremendous potential due to its unrivalled separating power and this area is worthy of further research.

11.4

Conclusions

Despite the rise of more modern instrumental methods such as HPLC and CE, TLC still has a key role to play in the forensic analysis of dyes. Its simplicity and significant body of knowledge concerning the separation of dyes by this technique ensure that it remains a key tool. HPLC and CE will also have a place, particularly where sensitivity is an issue. However, more

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research is required, particularly with the latter technique. One area, which appears not to have had much attention in the published literature over the last decade, is extraction of dyes from fibres. Many of the solvents used in the extraction schemes described earlier in this chapter are hazardous and it would be advantageous to replace them with safer alternatives. Approaches to automating the process would also be an aid in terms of increasing throughput, reducing exposure to hazardous solvents and releasing personnel for other less onerous duties. There are indications that work is commencing in this area; however, there have not been any publications on this subject to date.

11.5

Sources of further information and advice

A good overview of the basics of chromatography including TLC and HPLC can be found in Skoog, D.A., Holler, F.J. and Crouch, S.R (2007) Principles of Instrumental Analysis, 6th Edition, Belmont, Thomson Brooks/ Cole. The same volume also provides a review of CE. A more in-depth study of can be found in Robards, K., Haddad, P. R. and Jackson, P. E. (1994) Principles and Practice of Modern Chromatographic Methods, London, Academic Press. Information on dyes and their application can be found in Gregory, P. (2000) Dyes and Dye Intermediates, Kirk-Othmer Encylopedia of Chemical Technology, John Wiley & Sons. The Colour Index can be found on-line at www.colour-index.org. A key resource for forensic dye analysis by TLC, with particular reference to the extraction schemes developed by the Forensic Science Service in the United Kingdom is Wiggins, K. G. (1999) Thin Layer Chromatographic Analysis of Fibre Dyes, in Robertson, J. and Grieve, M. (Eds) Forensic Examination of Fibres, 2nd edn, London, Taylor and Francis.

11.6

Acknowledgments

The author would like to than Dr Susan Bennett (New South Wales Forensic Services Group) for guidance in the early stages of preparation of this chapter and Emma Patton (Curtin University of Technology) for useful comments on the draft manuscript.

11.7

References

Beattie, I. B., Dudley, R. J. & Smalldon, K. W. (1979) The extraction and classification of dyes on single nylon, polyacrylonitrile and polyester fibers. Journal of the Society of Dyers and Colourists, 95, 295–302.

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Beattie, B., Roberts, H. & Dudley, R. J. (1981a) The extraction and classification of dyes from cellulose acetate fibers. Journal of the Forensic Science Society, 21, 233–7. Beattie, I. B., Roberts, H. L. & Dudley, R. J. (1981b) Thin-layer chromatography of dyes extracted from polyester, nylon and poly(acrylonitrile) fibers. Forensic Science International, 17, 57–69. Cheng, J., Wanogho, S., Watson, N. D. & Caddy, B. (1991) The extraction and classification of dyes from cotton fibers using different solvent systems. Journal of the Forensic Science Society, 31, 31–40. Connelly, R. (1997) Colorant formation for the textile industry. Color Technology in the Textile Industry. Research Triangle Park, North Carolina, American Association of Textile Chemists and Colorists. Garfield, S. (2000) Mauve, London, Faber and Faber. Gregory, P. (2000) Dyes and Dye Intermediates. Kirk-Othmer Encylopedia of Chemical Technology. John Wiley & Sons. Griffin, R. M. E. & Speers, S. J. (1995) Use of tapered vials for in situ extraction of fiber dyes for analysis by HPLC. Science & Justice, 35, 203–5. Griffin, R. M. E. & Speers, S. J. (1999) Other Methods of Colour Analysis: High Performance Liquid Chromatography. In Robertson, J. & Grieve, M. (Eds) Forensic Examination of Fibres. London, Taylor and Francis. Griffin, R. M. E., Speers, S. J., Elliott, L., Todd, N., Sogomo, W. & Kee, T. G. (1994) An improved high-performance liquid chromatography system for the analysis of basic dyes in forensic casework. Journal of Chromatography, A, 674, 271–80. Hartshorne, A. W. & Laing, D. K. (1984) The dye classification and discrimination of colored polypropylene fibers. Forensic Science International, 25, 133–41. Home, J. M. & Dudley, R. J. (1981) Thin-layer chromatography of dyes extracted from cellulosic fibers. Forensic Science International, 17, 71–8. Huang, M., Yinon, J. & Sigman, M. E. (2004) Forensic identification of dyes extracted from textile fibers by liquid chromatography mass spectrometry (LC-MS). Journal of Forensic Sciences, 49, 238–49. Huang, M., Russo, R., Fookes, B. G. & Sigman, M. E. (2005) Analysis of fiber dyes by liquid chromatography mass spectrometry (LC-MS) with electrospray ionization: Discriminating between dyes with indistinguishable UV-visible absorption spectra. Journal of Forensic Sciences, 50, 526–34. Laing, D. K., Gill, R., Blacklaws, C. & Bickley, H. M. (1988) Characterization of acid dyes in forensic fiber analysis by high-performance liquid chromatography using narrow-bore columns and diode array detection. Journal of Chromatography, 442, 187–208. Laing, D. K., Hartshorne, A. W. & Bennett, D. C. (1990) Thin layer chromatography of azoic dyes extracted from cotton fibers. Journal of the Forensic Science Society, 30, 309–15. Laing, D. K., Dudley, R. J., Hartshorne, A. W., Home, J. M., Rickard, R. A. & Bennett, D. C. (1991) The extraction and classification of dyes from cotton and viscose fibers. Forensic Science International, 50, 23–35. Macnaught, A. D. & Wilkinson, A. (1997) IUPAC Compendium of Chemical Terminology, Oxford, Blackwell Science.

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Macrae, R. & Smalldon, K. W. (1979) The extraction of dyestuffs from single wool fibers. Journal of Forensic Sciences, 24, 109–16. Oxspring, D. A., O’Kane, E., Marchant, R. & Smyth, W. F. (1994) The separation and determination of reactive textile dyes by capillary electrophoresis and high performance liquid chromatography. Analytical Methods & Instrumentation, 1, 196–202. Petrick, L. M., Wilson, T. A. & Fawcett, W. R. (2006) High-performance liquid chromatography-ultraviolet-visible spectroscopy-electrospray ionization mass spectrometry method for acrylic and polyester forensic fiber dye analysis. Journal of Forensic Sciences, 51, 771–9. Resua, R. (1980) A semi-micro technique for the extraction and comparison of dyes in textile fibers. Journal of Forensic Sciences, 25, 168–73. Robards, K., Haddad, P. R. & Jackson, P. E. (1994) Principles and Practice of Modern Chromatographic Methods, London, Academic Press. Robertson, J. (1999) Other Methods of Colour Analysis: Capillary Electrophoresis. In Robertson, J. & Grieve, M. (Eds.) Forensic Examination of Fibres. London, Taylor and Francis. Shaw, I. C. (1980) Micro-scale thin-layer chromatographic method for the comparison of dyes stripped from wool fibers. Analyst (Cambridge, United Kingdom), 105, 729–30. Siren, H. & Sulkava, R. (1995) Determination of black dyes from cotton and wool fibers by capillary zone electrophoresis with UV detection: application of marker technique. Journal of Chromatography, A, 717, 149–55. Skoog, D.A., Holler, F.J. & Crouch, S.R (2007) Principles of Instrumental Analysis, 6th Edition, Belmont, Thomson Brooks/Cole. Speers, S. J., Little, B. H. & Roy, M. (1994) Separation of acid, basic and dispersed dyes by a single-gradient elution reversed-phase high-performance liquid chromatography system. Journal of Chromatography, A, 674, 263–70. Tetler, L. W., Cooper, P. A. & Carr, C. M. (1994) The application of capillary electrophoresis/mass spectrometry using negative-ion electrospray ionization to areas of importance in the textile industry. Rapid Communications in Mass Spectrometry, 8, 179–82. West, J. C. (1981) Extraction and analysis of disperse dyes on polyester textiles. Journal of Chromatography, 208, 47–54. Wheals, B. B., White, P. C. & Paterson, M. D. (1985) High-performance liquid chromatographic method utilizing single or multi-wavelength detection for the comparison of disperse dyes extracted from polyester fibers. Journal of Chromatography, 350, 205–15. Wiggins, K. G. (1999) Thin Layer Chromatographic Analysis of Fibre Dyes. In Robertson, J. & Grieve, M. (Eds) Forensic Examination of Fibres, 2nd edn, London, Taylor and Francis. Wiggins, K. G., Cook, R. & Turner, Y. J. (1987) Dye Batch Variation in Textile Fibres. Journal of Forensic Sciences, 33, 998–1007. Wiggins, K. G., Holness, J.-A. & March, B. M. (2005) The importance of thin layer chromatography and UV microspectrophotometry in the analysis of reactive dyes released from wool and cotton fibers. Journal of Forensic Sciences, 50, 364–8. www.colour-index.org (2007) Colour Index International. Society of Dyers and Colourists, American Association of Textile Chemists and Colorists.

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Xu, X., Leijenhorst, H., Van Den Hoven, P., De Koeijer, J. A. & Logtenberg, H. (2001) Analysis of single textile fibres by sample-induced isotachophoresis – micellar electrokinetic capillary chromatography. Science & Justice, 41, 93–105. Yinon, J. & Saar, J. (1991) Analysis of dyes extracted from textile fibers by thermospray high-performance liquid chromatography-mass spectrometry. Journal of Chromatography, 586, 73–84.

12 DNA analysis in the identification of animal fibers in textiles P F HAMLYN, BTTG Ltd, UK

Abstract: Animal hair fibers of commercial importance include wool and speciality fibers such as alpaca, mohair and cashmere. Commercially the most important speciality fiber is cashmere. However, cashmere apparel has frequently been found to be adulterated with cheaper fibers. A major problem facing the speciality fiber industry has been the lack of a fast and reliable analytical technique to differentiate between expensive fibers and lower-cost contaminants. This chapter summarizes recent developments in molecular biology allowing the use of DNA analysis for the identification of unknown animal fiber blends and leading to the provision of a commercial service for speciality fiber analysis. Key words: animal hair fibers, cashmere, deoxyribonucleic acid (DNA), DNA analysis, identification of fibers.

12.1

Introduction

Animal hair fibers of commercial importance include wool from various breeds and cross breeds of sheep, and speciality fibers. Speciality animal hair fibers (hereafter referred to as speciality fibers) also known as luxury fibers due to their softness, lustre and scarcity value may be defined as all types of animal hair fibers used in textiles other than sheep’s wool. The principal speciality fibers are alpaca, angora from Angora rabbits, camel hair obtained primarily from the undercoat of the Bactrian or two-humped camel, mohair from the fleece of Angora goats (Capra hircus aegagrus) and cashmere. New types of speciality fibers may occasionally be introduced into the marketplace. For example, cashgora, a fiber derived from crosses between cashmere-bearing goats and Angora (mohair-bearing) goats, is now recognized as a speciality fiber in its own right. Commercially the most important speciality fiber is cashmere, which commands some of the highest prices in the world of textiles. Cashmere is defined as the fine (dehaired) undercoat fibers produced by cashmere goats (Capra hircus laniger). As with most commodities, fiber supply and demand varies with time and socio-economic factors. Increased competition and demand for cashmere over recent years has resulted in increased product contamination. There is a mandatory requirement in many countries to define the fiber content of clothing and related domestic textile materials 224

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225

and breach is a criminal offence. Cashmere, in particular, has frequently been found to be adulterated with much cheaper fibers such as fine wool, angora, yak hair and occasionally mohair. Angora and mohair are speciality fibers in their own right but their lower prices relative to cashmere make them suitable candidates for substitution. Positive identification of animal fibers is important not only in textiles where regulations govern the labeling of fiber content, but also in forensic science and archaeology. A major problem facing the speciality fiber industry has been the lack of a fast and reliable analytical technique, which will differentiate between high-quality, often expensive fibers and lower-cost contaminants. To date, the most important means of identifying animal fibers has been microscopic analysis. Chemically there is little difference between fibers from different animal species. Therefore, physical parameters such as fiber diameter, cuticle scale height and surface characteristics (e.g., scale frequency and scale patterns) have to be assessed. However, there is considerable overlap in parameters such as fiber diameter between different types of fiber due to the range of variation within species and the scale patterns of animal fibers can suffer mechanical damage and chemical erosion during processing. Detailed measurements have to be made on many individual fibers for each sample received for analysis. Even skilled personnel working in this area, however, may have difficulty in positively identifying the components of certain fiber blends. There is one chemical that can be used to differentiate between animal species and even individuals of the same species. Deoxyribonucleic acid (DNA) contains the unique genetic instructions or code for the development and functioning of a living organism offering the potential for the identification of any species through an analysis of its DNA. Some of the most important advances in DNA analysis were made in the 1980s. In 1984 traces of DNA were successfully isolated from a 150-year-old museum specimen of the Quagga, an extinct subspecies of the plains zebra. This ‘ancient’ DNA could not only be extracted but was also sequenced (Higuchi et al., 1984). The invention of DNA or genetic fingerprinting was announced by Sir Alec Jeffreys at the University of Leicester in 1985. Three years later by combining DNA fingerprinting with a new technique called the polymerase chain reaction (PCR) the sensitivity of DNA fingerprinting could be increased to allow the identification of an individual from the minute amounts of DNA that could be extracted from a single hair (Higuchi et al., 1988). Using the PCR technique DNA could be analyzed from the root region of shed, as well as freshly plucked single hairs. A breakthrough in speciality fiber analysis was made when it was demonstrated that DNA was not only present in hair roots but could also be recovered from animal hair shafts (Kalbe et al., 1988). This was an important development since some fibers such as wool are shorn rather than combed.

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Following on from these advances in molecular biology BTTG set out to apply DNA analysis to the identification of unknown fiber blends although initial progress was slow because the techniques were difficult and expensive (Hamlyn et al., 1990). However, the advent of automated procedures for performing PCR and the development of DNA extraction kits led to the provision of a commercial service for speciality fiber analysis (see Section 12.6).

12.2

Extraction of DNA from animal fibers

Animal hair fibers are constructed from cellular material. In the living cells of animals the DNA is contained inside spherical bodies known as nuclei and smaller cellular bodies called mitochondria. The DNA inside nuclei contained within immature hair follicles can be made visible using a technique known as in situ DNA hybridization. As the follicle matures, the nuclei become elongated but the signal does not extend into the more mature parts of the fiber where keratinization has taken place (Fig. 12.1). During keratinization the cells die, but traces of DNA from both nuclei and mitochondria remains trapped inside the fiber. The recalcitrant and hydrophobic (waterproof) nature of fibers protects the DNA from chemical and biological degradation. Various methods have been developed for the recovery of DNA from animal hair fibers. DNA has been extracted from cuticle cells removed from the surface of fibers by washing in an aqueous solution of sodium dodecyl sulphate (Fig. 12.2), from cryogenically milled fibers and from whole fiber snippets treated with a suitable proteolytic enzyme. A disadvantage of

12.1 Direct visualization of nuclei inside a single hair follicle using the technique of in situ hybridization and a total genomic DNA probe (from Broadbent, 1997).

DNA analysis in the identification of animal fibers in textiles

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kb

23

2.3 2.0

0.56

λ Hind III CASHMERE MOHAIR WOOL YAK ANGORA CAMEL

12.2 DNA extracted from the cuticle of speciality fibers and visualized by UV fluorescence after resolution on an agarose gel (from Nelson et al., 1992).

using cuticle cells is that they are more susceptible to damage by processing treatments. Improved DNA extraction procedures and methods of DNA analysis resulted in the requirement for only 20 milligrams of a fiber sample for routine analysis at BTTG whereas originally 20 grams was needed. A standard protocol (reproduced from Hamlyn et al., 2001) is given below: Fibers are chopped into small fragments using clean and sterile scissors and approximately 25 mg weighed into a microtube. Extraction buffer (600 μl) and papain (20 μl) is added to each tube and the reagents mixed and centrifuged at 8000 rpm (MSE minifuge) for a few seconds. The mixture is then incubated at 50°C for up to 48 hours. DNA is extracted from the enzyme-treated fibers using a DNA extraction kit for hard tissues (Nucleon Biosciences). It is important to avoid using the ethanol precipitation step at the end of the manufacturer’s protocol since this causes natural pigments present in the fibers to bind to the extracted DNA making subsequent DNA analysis extremely difficult. Instead the DNA is purified and concentrated using Microcon 100 microconcentrators (Millipore). The extracted and purified DNA should be stored at −20°C until required.

12.3

Development of methods for using DNA analysis to identify animal fibers

12.3.1 Selection of target DNA sequences A prerequisite to the DNA profiling of animal fibers is the identification of short DNA sequences that are unique to each species. It is an advantage if

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these sequences are present as multiple copies, since this increases the sensitivity of the test. Once species-specific DNA sequences have been located (e.g., from databases containing published sequence information), complementary DNA sequences known as oligonucleotides can be constructed, which, under carefully controlled conditions, specifically hybridize to the target DNA molecule giving a positive signal, confirming the presence of a particular fiber type. Oligonucleotides based on the repetitive Satellite II DNA region are available that can be used to distinguish between DNA samples from closely related species such as sheep and goats. Satellite II DNA is present in high concentrations in the nuclei of many higher animals. Evolutionary pressure has produced a series of mutations in this DNA, however, resulting in natural variations between species. Mitochondrial genes such as cytochrome b are also present in high copy numbers (up to 1000 mitochondria are present for each nucleus in an animal cell) and have a faster evolution rate than nuclear genes. These have proved useful in the design of species-specific primers for the analysis of processed fibers (Bayliffe, 1999).

12.3.2 Conventional DNA hybridization analysis Conventional DNA hybridization analysis can be carried out using a simple dot-blot technique. Initial attempts at analysing the DNA recovered from animal hair fibers involved the use of total genomic DNA probes (Kalbe et al., 1988). However, a major limitation of this approach in relation to fiber profiling was the inability to differentiate between DNA recovered from fibers of related genera such as sheep, goats and yak, although more distant species such as rabbit were readily distinguished. Both sheep’s wool and yak hair have been used to adulterate cashmere, so it is important to be able to distinguish between all three fiber types. Therefore, species-specific oligonucleotides were specifically designed for use as probes to distinguish between DNA extracted from raw wool, yak hair and cashmere fibers (Hamlyn et al., 1992; Nelson et al., 1992). The dot-blot technique is illustrated in Fig. 12.3. DNA isolated from fiber samples together with control samples of genomic DNA is denatured by boiling to convert the native double-stranded DNA molecules into single strands and then spotted onto a nylon membrane (Bio-Rad). A singlestranded preparation of the oligonucleotide to be used as the probe is then incubated with the membrane to allow hybridization to take place between the probe and any complementary strands of DNA present in the sample. After free unhybridized probe has been removed by washing the membrane several times, the hybrid double-stranded DNA molecules can be visualized by several different methods according to how the probe has been labeled (i.e., by using radioactive, colorimetric or luminescent labels).

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Sample Isolation of double stranded DNA Immobilization of single stranded DNA onto membrane (dot blotting) Hybridization (binding) with labeled probe Washing to remove unbound probe Visualization of labeled DNA

12.3 Analysis of DNA by the dot-blotting technique.

For example, a sheep-specific oligonucleotide probe can be used to detect the presence of wool but does not give a positive signal with DNA extracted from other types of fiber (Fig. 12.4). Thus the presence or absence of wool in a bale of raw cashmere can be quickly determined.

12.4

Effect of fiber processing on DNA analysis and the use of DNA amplification technology

Although standard hybridization methods can be used for the identification of samples of raw fibers, with processed materials (scoured, bleached or dyed) and finished garments the quality and quantity of DNA present within the fiber is much reduced and it is not possible to use conventional methods of DNA analysis. Only small quantities of DNA can be extracted from processed fibers, typically less than 1 ng DNA per gram of sample. Therefore, in vitro DNA amplification technology known as the polymerase chain reaction (PCR) which allows for the analysis of minute quantities of DNA was developed at BTTG for animal fiber analysis (Hamlyn et al., 1996). DNA is first extracted from the fiber sample and purified then the purified DNA is amplified prior to analysis. The PCR technique was conceived by Kerry Mullis along with other researchers in 1983 while working at the Cetus Corporation in Emeryville, CA. In essence, PCR reproduces what happens when the DNA inside a cell is replicated during cell division and is able to duplicate DNA fragments

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Identification of textile fibers Cashmere Mohair Wool

Yak Angora Camel

Sample 1 Sample 2

Genomic DNA

Goat Sheep Bovine Rabbit Llama

12.4 Hybridization of a sheep-specific oligonucleotide probe to DNA extracted from various animal fibers. A positive signal was only obtained with wool DNA (from Hamlyn et al., 1992).

12.5 Thermal cycling machine used for carrying out the polymerase chain reaction.

many times producing sufficient amplified DNA for subsequent analysis. By using an automated temperature cycling block (Fig. 12.5) this ‘enzymatic copying system’ can be used to produce millions of identical copies of a specified DNA sequence after only 1–2 hours (Fig. 12.6). These copies provide sufficient DNA for further analysis (e.g., sequencing), or can simply

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231

Denaturation of DNA strands at 95°C

Binding of primers at 55°C

After each cycle of heating and cooling the number of copies of the specified DNA sequence is doubled. After 1–2 hours millions of copies have been produced.

Replication of DNA starting from the primers at 72°C

12.6 Amplification of DNA using the polymerase chain reaction. The primers are synthetically prepared oligonucleotides having sequences complementary to the DNA on either side of the segment of DNA to be amplified.

confirm the presence of the target sequence in the sample. Species-specific primers were designed to give different sized amplification products with wool, yak hair and cashmere, and these can be visualized by ultraviolet fluorescence after resolution on an agarose gel containing ethidium bromide (Fig. 12.7). Thus the presence or absence of an adulterant can be quickly determined and this result can later be confirmed by DNA sequencing. The resulting DNA sequence can be compared to published sequences

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No DNA Wool Yak Cashmere

300

200

100bp

12.7 DNA amplification products obtained from wool, yak hair and cashmere using different primer sets (from Hamlyn et al., 1996).

providing courts with evidence of an exact match that leaves no room for doubt. Because of the prodigious sensitivity of PCR it is essential that negative controls are included with the samples and go through the same extraction process. In practice, each test is run as a multiplex containing a second universal set of primers (12S ribosomal DNA) which coamplify a non-specific 403 bp product from all mammalian DNA samples thereby confirming that the extracted DNA is of sufficient quality for analysis and that no inhibitory substances such as pigments are present (Fig. 12.8). Using this approach, DNA has been successfully amplified from samples of dyed material that did not yield detectable levels of DNA using conventional extraction techniques and it has been demonstrated that the PCR technique is capable of amplifying and therefore allowing rapid detection of minor components in mixed DNA samples (Hamlyn et al., 1996, 2001). Although this type of analysis is not quantitative, it is a simple and rapid test providing useful criteria for the confirmation of fraudulent substitution allowing us to state with absolute certainty whether contamination by other fibers is present at any stage of processing from raw material to finished garment. There are alternative strategies for carrying out DNA analysis using the PCR technique. At BTTG the use of species-specific PCR primers allowed the development of a rapid and relatively inexpensive test for commercial application. Using PCR primers designed from conserved regions and a PCR-RFLP technique Subramanian et al. (2005) reported that they could differentiate between cashmere and wool fibers. RFLP or restriction fragment length polymorphism involves cutting the amplified DNA into fragments using suitable enzymes (endonucleases), which only cut the DNA molecule at specific sites (restriction sites). The DNA fragments produced by enzymatic cleavage are then separated according to length by agarose

DNA analysis in the identification of animal fibers in textiles 1

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3

4

5

6

7

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11 12

-- Internal Control (403 bp) -- Camelid DNA (330 bp)

12.8 Analysis of DNA from various fiber samples using Camelid primers1. Lanes 1 & 12: Lane 2: Lane 3: Lane 4: Lane 5: Lane 6: Lane 7: Lane 8: Lane 9: Lane 10: Lane 11:

DNA ladder in multiples of 100 bp2 alpaca sample A alpaca sample B no DNA blank extraction cashmere wool yak hair rabbit angora natural camel hair blonde camel hair

1

The Camelid primers give an amplification product of 330 base pairs. The DNA ladder consists of differently sized DNA fragments having lengths in multiples of 100 base pairs that serve as visual molecular weight standards for use in qualitative agarose gel analysis. The 600 base pair band has increased intensity to serve as a reference point. 2

gel electrophoresis resulting in a unique RFLP profile. Appleby et al. (1997) used degenerate PCR primers for the selective amplification of a polymorphic region of the ribulose-1, 5-bisphosphate carboxylase (RuBisCo) small subunit genes to devise a new DNA-based test for identifying natural products (e.g., fibers, food products and timber) derived from plants. With highly processed samples, where fibers have undergone harsh chemical treatments, it is conceivable that it may not be possible to isolate any intact DNA suitable for amplification and therefore subsequent DNA analysis will not be possible. In our experience this has only been found in a couple of cases which both involved fibers that had been recycled and heavily dyed (Hamlyn, unpublished).

12.5

Future trends

The main limitations of DNA analysis are the inability to carry out quantitative analysis of unknown fiber blends and the lack of species-specific

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primers for some types of fiber. For example, it is currently not possible to distinguish between the different types of goat fiber – cashmere, cashgora and mohair using DNA analysis. In a detailed study on the phylogenetic history of the domestic goat, six keratin loci, both coding and non-coding, were examined in a broad spectrum of contemporaneous goat breeds exhibiting different fiber types. However, no polymorphisms uncovered at the keratin loci appeared to exhibit breed or fiber type specificity (Brown, 2003). In the case of the South American Camelids, although alpaca is the major fiber type available commercially, there is much interest in developing higher value fibers. The South American Camelids consist of two wild species (vicuña and guanaco) and two domestic forms (alpaca and llama). Crosses between all four species produce fertile offspring, although it is unlikely that the wild species interbreed naturally. The wild vicuña was hunted and killed until recent times for its soft, fine coat but is now protected by the Peruvian government. Vicuña fiber, whose color ranges from chestnut to fawn, is the finest and most luxurious of the llama group but is only produced in relatively small quantities. In addition, breeding trials are being carried out to investigate genetic crosses of alpaca and vicuña, the so-called Paco-vicuña. Unfortunately, the genetics of the domestic camelids is very complex. Work by Jane Wheeler (1995) on mummified llamas and alpacas from pre-Spanish conquest times indicate that extensive crossbreeding between llamas and alpacas occurred following the Spanish conquest leading to the loss of the original fine fiber breeds. The result is that today fiber from these animals is very heterogeneous with regard to color, fleece types and fiber diameter. It is therefore likely to be a difficult task to design suitable PCR primers that are specific enough to allow differentiation between DNA isolated from Camelid fibers currently in use or under development. Although DNA sequencing could potentially be employed by making a comparison to published Camelid sequences this may be problematical with fiber blends. If DNA analysis could be carried out on individual intact fiber shafts, this would enable the quantitative determination of the composition of unknown fiber blends. However, investigations at BTTG have indicated that in situ DNA analysis of single fiber shafts is not practical because the DNA is encapsulated inside the waterproof environment of the keratinized cells and attempts to get at this DNA leads to the dissolution of the fibers (Broadbent, 1997). Therefore, extraction-based techniques have to be employed. A key strategy will be to improve the efficiency of recovery of DNA from whole fibers. Proteolytic enzymes such as proteinase K (Sigma) can partially degrade animal fibers but the cells of the cortex remain intact even after several days incubation with the enzyme (Fig. 12.9). A second problem relates to the large number of fibers that will have to be analyzed

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12.9 Degradation of wool after extended incubation with proteinase K. The spiked shaped fragments are released cells of the cortex (from Broadbent, 1997).

to give an accurate result making any test prohibitively costly using manual techniques. However, automated systems have been developed for both DNA extraction and amplification, and chip-based systems will ultimately lead to low cost procedures that can handle large numbers of samples. The latter are starting to have an impact in medical diagnostics and related areas.

12.6

Sources of further information and advice

BTTG can analyze fiber, yarn or fabric samples for goat DNA to confirm the presence or absence of goat fibers (the DNA test cannot distinguish between cashmere, cashgora or mohair), sheep DNA (for sheep’s wool) and yak DNA (for yak hair). The DNA test is qualitative only. We therefore cannot report a percentage figure for the level of each fiber type in blends. The results are reported as the presence or absence of goat DNA, sheep DNA or yak DNA. The presence of the DNA can be used to infer the presence within the product of fiber from the target animal. Samples can also be analyzed for rabbit DNA (for angora) and alpaca (for Camelid fibers). Commercially, the present position at BTTG is that fiber blend analysis is carried out using the projection microscope for quantification (i.e., the different types of fibers in the blend are identified, their diameters measured and used to calculate the percentage by weight of the components of the blend) and DNA analysis is employed to objectively confirm the nature of those fiber types. This joint approach provides an objective speciality fiber

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analysis service that is currently available to our customers. Please contact the author of this article for further details.

12.7

References

Appleby J M, Nelson G, McPherson, M J and Hamlyn P F (1997), ‘PCR amplification of the RuBisCo small subunit genes and their novel application to plant tissue identification’, Heredity, 79(6), 557–563. Bayliffe A I (1999), Speciation of livestock in textile and food industries: application of DNA diagnostics, PhD thesis, University of Leeds, UK. Broadbent E L (1997), The identification and quantification of the constituents in processed animal fibre blends, PhD thesis, University of Leeds UK. Brown R J P (2003), Genetic variation in the domestic goat Capra Hircus, PhD thesis, University of Leeds, UK. Hamlyn P F, Nelson G and McCarthy B J (1990) ‘Applied molecular genetics – new tools for animal fibre identification’, Speciality Fibres: Scientific, Technological and Economical Aspects (A Körner, F J Wortmann, G Wortmann and H Höcker, eds), Schriftenreihe des Deutschen Wollforschungsinstitutes, a.d. Technischen Hochschule Aachen, e.v.,106, pp. 249–258. Hamlyn P F, Nelson G, and McCarthy B J (1992) ‘Wool Fibre Identification using Novel Species-specific DNA Probes’, Journal of the Textile Institute 83(1), 97–103. Hamlyn P F, Ramsbottom S, McCarthy B J and Nelson G (1996), ‘Analysis of speciality fibres using DNA amplification techniques’, Metrology and Identification of Speciality Animal Fibres (J P Laker and F J Wortman, eds), European Fine Fibre Network, Occasional Publication No. 4, Macaulay Land Use Research Institute, Aberdeen, Scotland, pp. 59–68. Hamlyn P F, Nelson G, Asghar N and McCarthy B J (2001) ‘Identification of speciality animal fibres using DNA profiling’, 3rd European Symposium and SUPREME European Seminar Progress in South American camelids research, Göttingen, Wageningen Pers, pp. 117–121. Higuchi R, Bowman B, Freiberger M, Ryder O A and Wilson A C (1984), ‘DNA sequences from the quagga, an extinct member of the horse family’, Nature 312, 282–284. Higuchi R, von Beroldingen C H, Sensabaugh G F and Erlich H A (1988) ‘DNA typing from single hairs’, Nature, 332, 543–546. Kalbe J, Kuropka R, Meyer-Stork S, Sauter S L, Loss P, Henco K, Riesner D, Höcker H and Berndt H (1988), ‘Isolation and characterisation of high-molecular mass DNA from hair shafts’, Biol. Chem. Hoppe-Seyler, 369, 413–416. Nelson G, Hamlyn P F and Holden L (1992) ‘A Species-specific DNA Probe for Goat Fibre Identification’, Textile Research Journal, 62(10), 590–595. Subramanian S, Karthik T and Vijayaraaghavan N N (2005). ‘Single nucleotide polymorphism for animal fibre identification’ J. Biotechnol., 116(2), 153–158. Wheeler J C (1995), ‘Evolution and present situation of the South American Camelidae’, Biol. J. Linnean Society, 54, 271–295.

13 Identifying plant fibres in textiles: the case of cotton S GORDON, CSIRO Materials Science and Engineering, Australia

Abstract: This chapter discusses techniques and test methods for identification of cotton fibre properties and the existence of cotton fibres in textile products. Identification of cotton fibres by their physical, chemical and genetic attributes serves various commercial purposes along the cotton supply chain. In the first part of this chapter the physical attributes of raw cotton, upon which the fibre is initially valued, are described. These have a large effect on cotton’s processing ability and are of particular importance to the spinner and fabric manufacturer. Whilst appreciation of these physical attributes is often overlooked by the final consumer, high value is often attached to perceptions of the fibre’s origin or genetic make-up. In this case, identification of cotton fibre and origin of fibre in retail products is important, but often quite difficult to test. Current and new techniques for identifying cotton fibre in fabric and its origin are discussed in the second part of this chapter. Key words: cotton fibre properties, identification of cotton fibres, cotton fibre origin, genetically modified cotton, quantitative analysis of cotton in textiles.

13.1

Introduction

For spinners who buy their raw cotton from growers, or more usually cotton merchants, identification of the physical properties in the cotton they buy is essential for optimising processing efficiency and end-quality in the spinning process. Fibre quality specification for the spinner represents one aspect of fibre identification; where quality, and usually the origin of cotton is specified, classed and checked on delivery as part of the sales contract between the merchant and spinner. The importance of this specification is based on the high cost of cotton in terms of mill operating costs and the adverse consequences of having cotton outside specification in the manufacturing process. Accordingly, in this part of the supply chain where there is high risk, there is good information available to the spinner about the cotton they buy. However, this information on the quality and origin largely disappears by the time the finished cotton product appears in a retail outlet. While many surveys show consumers prefer to wear clothing made of cotton, and that consumers think fibre content is an important factor when choosing garments (Cotton Incorporated, 2006), this awareness does not 239

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extend to a detailed appreciation of the origin and physical properties of the fibre in the product they choose. Consumer preference is by-and-large driven by price and the consumer’s own perception of quality, which depend more on emotive attributes such as design, colour and style. Technical quality often comes a distant second in the quality consideration stakes. However, it can be said high quality fibre is generally converted into high quality fabric and garments, and that poor quality fibre is generally, although not always, converted into lower quality fabric and garments. The connection between origin, quality and price comes to the fore in situations when the price of the product is determined by the ‘type’ of cotton used. For example, products labelled as being made from ‘Egyptian’ cotton suggest to the purchaser they should expect their purchase to be made from high quality extra long staple (ELS) Egyptian grown Gossypium barbadense cotton, which is not always the case. In this type of situation the market requires the fibre in the garment to be identified, or the percentage of a particular fibre in the garment to be measured, in order to bring assurance to the licensing of the product Trademark particularly between the retail, wholesale and manufacturing segments. The tests for origin and presence are more forensic-like in their scope and represent the other end of fibre identification in the cotton supply chain. In this chapter the importance of identifying cotton fibres in terms of their quality, for the spinner, and their traceability from origin or quality in finished goods for the consumer, are discussed. A brief introduction of the cotton supply chain and cotton market is given, before the structure and physical properties linked to the identification of cotton, for the two most widely cultivated species, i.e. Gossypium hirsutum known as Upland cotton, which constitutes around 94% of world production, and G. barbadense, referred to widely as Egyptian or Pima type cotton and which makes up 3–4% of world production, are discussed. Many of the techniques used to identify and measure cotton fibres are applied widely to other fibres and these have been discussed in earlier chapters. In this chapter, test methods specific to the measurement of cotton fibre quality are discussed along with the most often used methods to identify cotton fibres from yarn or textile samples of unknown origin, including analysis of remnant DNA extracted from cotton fibres, which has recently been reported. The chapter concludes with a brief discussion on future trends in the identification of cotton fibre.

13.1.1 Cotton supply chain The significance of cotton in the world textile market is evident by its majority share among fibres for traditional textile goods such as apparel and home furnishings. In this market cotton is used in more than 60% of the

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241

Grower (cotton/module) → Ginner (bale) → Merchant (bale lots) → Spinner (yarn) → Knitter/Weaver (fabric) → Dyeing/Finishing (dyed finished fabric) → Design/Cutting/Sewing (product) → Wholesale (product distribution)→ Retail (product sale)

13.1 The cotton supply chain and the points at which fibre specification or identification testing usually occurs.

staple yarn (Oerlikon, 2007), the intermediate that is knitted or woven into fabric for traditional textiles. The dominance of cotton in this market is in part due to the economics of production, distribution and manufacture but also the structural and physical properties of the fibre. Figure 13.1 illustrates the cotton supply chain for traditional textile products. The importance of fibre to fabric engineering from the perspective of the manufacturer is discussed in comprehensive detail in papers by El Mogahzy (1992a, 1992b). For this discussion it is sufficient to say the specification of fibre quality during manufacture is paramount for achieving good processing performance and high quality levels of end-product at the lowest cost possible. Whilst the bulk of world cotton is sold into traditional textile markets, there is a range of non-textile end-uses where cotton fibres are used. These include dissolving fibres for pulp from which cellulose intermediates and synthetic fibres and casings can be made, the production of felts for cushions, pads, automotive upholstery and furniture upholstery, the production of absorbent medical grade cotton for cotton balls and swabs and fibre pulp for specialised papers including fine writing paper, filter paper, currency, sanitary products and battery separators. In many of these products cotton fibres are used as a pure source of cellulose, e.g. as pulp for specialised papers, and as such is no longer recognisable in its fibre form, although its cellulose structure might still be recognisable. In other products such as wadding in upholstery and mattresses, felts and pads the fibre used is often sourced as a by-product of other cotton fibre conversion processes, e.g. short fibres or cotton noil from the combing process before spinning, or sourced from recycled (shredded) cotton fabric waste. In these situations, whilst the fibre is recognisable as cotton, the original physical quality of the fibre is no longer obvious.

13.2

Cotton fibre structure and composition

Cotton is a unicellular fibre that grows from the epidermis cells on the surface of cotton seeds. From a structural perspective the cotton fibre is one

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Identification of textile fibers

of the largest and simplest plant cells; a singularly discrete, elongated cell that has no junctions or inter-cellular boundaries that compromise the physical and chemical processing of other vegetable fibres. Figure 13.2 shows G. hirsutum fibres elongating from the seed epidermis at 2 to 3 days post anthesis (dpa). Fibre growth is continuous and extends for 50 to 60 dpa

(a)

(b)

13.2 (a) Gossypium hirsutum seed (ovule) at 2 to 3 days post anthesis and (b) new fibres elongating from seed epidermis (CSIRO Plant Industry).

Identifying plant fibres in textiles: the case of cotton

243

in two nominally separate growth phases. Initially from flowering, the cell elongates for 20 to 30 dpa. The realised fibre length during this period is dependent upon the plant’s genetics and the environment, in particular on plant water stress. There is some question as to whether fibre fineness or more correctly the perimeter or biological fineness (Ramey, ca 1982), changes during this phase. Investigators in the past have assumed it remains constant according to the genetics of the variety or species. More recent thinking proposes the perimeter might also expand during this phase. The elongation phase is followed by a cell wall thickening phase where the secondary cell wall of the fibre is laid down in a series of concentric growth rings or lamellae that reflect diurnal temperature fluctuations during development (Kerr, 1937). At maturation the cotton fibre remains hollow as a result of the remnant protoplasm, which is referred to as the lumen; the dimensions of which become complementary to measurements of the fibre’s cell wall dimensions, which in turn have an important influence on fibre processing ability. The fully developed cotton fibre consists of a waxy cuticle enveloping the fibre, a cell wall that is differentiated into primary (outer) and secondary (inner) layers, and the lumen. Some regard the cuticle and primary wall layers as a single layer; however, others (Maxwell et al., 2003) have revealed the primary wall is distinct from the waxy cuticle. The wax layer surrounding the outside of the cell wall is essential for the efficient processing of cotton fibre into spun yarn. It provides a lubricating layer that reduces fibre-tometal friction and therefore fibre breakage during mechanical processing. The downside is this layer also acts as an impermeable barrier to the entry of water and dye molecules into the fibre. The chemical composition of the wax is complex and contains a number of lipid classes including wax alkanes, fatty acids, fatty alcohols, plant steroids and mono, di and triglycerides (Fargher and Probert, 1924, Fargher and Higginbotham, 1924, Hornoff and Richter 1964). There appears to be no significant inter-species differences in lipid content, although Amin and Truter (1972) found the waxes extracted from G. barbadense fibre contained a lower proportion of hydrocarbon. For successful dyeing, this barrier and particularly the wax alkanes must be removed by scouring and/or bleaching. The fibre cell wall itself represents one of the purest sources of cellulose. The cellulose content of raw cotton ranges between 86 and 96% by dry weight depending on its maturity and the method of determination (Mauersberger, 1954). There is little inter-species or inter-varietal difference in cotton cellulose content. The other main but smaller components of cotton excluding water, are pectins (0.7–1.2%), proteins (1.1–1.9%), waxes (0.4 – 1.0%) and minerals measured as ash content (0.7–1.6%). After scouring and bleaching, which removes most of these, dried cotton converts to a purity of nearly 99% cellulose. In comparison, bast fibres such as flax, jute,

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Identification of textile fibers

ramie, kenaf and cannabis, from the stems of these plants are about three-quarters cellulose. Wood fibre including coniferous, deciduous and eucalyptus, contain 40–55% cellulose, with other plant parts and species containing much less cellulose. The cellulose ‘monomer’ in cotton fibres, i.e. a β-1,4-D(+)-glucopyranose molecule linked by 1,4-glucodic bonds, is polymerised to form the highest molecular weight compound of all plant fibres with a degree of polymerisation between 3500 and 10 000 units (Liang and Marchessault, 1959, Weiss, 1972). The spatial arrangement of cellulose fibrils and larger aggregates determines the morphology and properties of a fibre and in maturing cotton, the arrangement of these fibrils differs, depending on the state of fibre development. The fibrils in the primary wall are not well aligned and have a crystalline index of 30% in contrast with 70% for mature fibres measured at 42 dpa (Bolyston and Hebert, 1995). In mature fibres the secondary wall comprises about 94% of the fibre material (Goynes et al., 1995) and thus dominates the mechanical and physical chemical properties of the fibre. An impression of the overall structure of the cotton fibre described by Jefferies et al. (1969) is shown in Fig. 13.3. In the secondary wall cellulose fibrils are laid down in helical layers on the inside of the fibre at the protoplasm boundary, with helix angles changing from around 35° in the outer layers to 20° in the inner S3 layers.

Lumen S3 20° S2 20–30°

Reversal S1 20–35°

Primary wall

Pectin Fats

Waxes

13.3 Representation of cotton fibre structure by Jefferies et al. (1969).

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245

13.4 Cross-sections of cotton fibres embedded in methyl-butyl methacrylate resin showing cross-section shape, cell wall and lumen (CSIRO Textile and Fibre Technology).

When a cotton boll opens, the fibres are long, cylindrical and turgid cells containing living protoplasm. With opening there is a loss of water and the fibres dehydrate and become flattened and twisted. The twists that arise are referred to as convolutions and are dependent principally upon wall thickness, which in turn is dependent upon variety, fibre maturity and drying conditions. The cross-section of the cotton fibre after dehydration is generally kidney-shaped, although the shapes range from near circular in mature fibres to a flattened shape in immature fibres. The kidney shape is an inherent phenomenon due to zones of different density in the secondary wall layers (Kassenbeck, 1970). Figure 13.4 shows cross-sections of cotton fibres embedded in methyl-butyl methacrylate resin featuring the variations in shape around the typical kidney-shaped cross-section.

13.3

Cotton fibre properties

The structure of cotton cellulose in the fibre cell wall is comparatively uniform across species and varieties and this is significant in considering the range of physical and chemical processing properties in cotton fibres. The structural uniformity in mature fibres across varieties and to a certain extent species means the physical and chemical differences between cotton of

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Identification of textile fibers

different origins are quite small in comparison with differences within other natural fibres. The consequences of this natural uniformity with regards to processing include the ability to continuously rather than batch process along the cotton supply chain, the relative ease in blending cotton of different origins and a requirement for accurate and precise fibre testing methods. The main physical properties measured and specified in cotton are length (measured today as the upper half mean length (UHML) (Woo, 1968)), bundle strength and fineness (measured as the Micronaire value), and these alone can explain up to 93% of the variation in yarn strength (Bell and Gilham, 1989), an important measure of yarn quality. Other fibre properties important in processing include short fibre content, i.e. percentage of fibres less than 12.5 mm, maturity (the degree of cell wall thickening), elongation and trash content. These physical properties are expressions of the particular cotton variety and the conditions under which it was grown. The cotton grower’s selection of variety and the decisions with respect to nutrients, pests, water, harvesting and ginning all affect the final quality of fibre produced. In the future, these decisions are likely to be more complex particularly as interactions between these inputs, and new variables such as genetically modified varieties that produce enhanced fibre quality come to light. Table 13.1 lists the differences in physical dimensions and properties between cotton species, and between flax, ramie and polyester. Whilst the broad structure of cellulose in mature cotton fibres is similar, differences in terms of cellulose fibril spiral angle have been measured between different species of cotton (Meredith, 1951). These differences are reflected to some degree in different stress and strain and birefringent (optical) properties of the fibre. Colour differences between species are explained by the existence of pigments, pectins, waxes and proteins in the cuticle and primary layer of the cell wall, and are removed by the scouring and bleaching processes applied to all cotton. Differences in dye uptake are largely dependent on the maturity of the cotton sample whatever the species, although fibre fineness also plays a role in dyed appearance. Immature fibre, that is fibre with little or no fibre wall thickening, is associated with non-uniform dyeing of fabrics (Pal and Esteve, 1959, Smith, 1991) and increased relative wax content (Gordon et al., 2002). Mature fibre with more cellulose and therefore binding sites for dye molecules is able to hold more dye and will appear darker dyed against immature fibre. Fibre maturity also affects the lustre of the fibre.

13.3.1 Measuring differences in processing ability For the three short-staple spinning systems used by the cotton textile industry today, i.e. ring, rotor and air-jet spinning, the fibre properties listed in

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247

Table 13.1 Typical fibre properties for cotton of different species, variety and origin Species/Variety

Gossypium hirsutum US CA1 US MS1 Australia2 China3

UHML* (mm)

29 28 29 28

Diameter (μm)

Linear density (mtex)

Bundle strength (cN/tex)

Birefringency n1 − n2**

13–14.7 14.2–16.8 13–14.7 14–16

160–190 180–220 175–215 180–210

>33 >28 >30 >28

0.046 0.044

G. barbadense Giza var. ELS4 Giza var. LS4 Indian ELS US Pima1

36 32 >32 35

11.5–13 12–14 11.5–13 12–14

125–140 155–170 140–165 155–170

>43 >35 – >40

0.049

G. arboreum Bengalense var.

<20

>20

>300

18.5

0.043

Linum usitatissimum Flax (linen)5

>27***

8–25***

22–36

0.060

Boehmeria nivea Ramie Polyester

>60*** var

28–35*** var

up to 75

0.160

50–250

* UHML = upper half mean length, which is nominally equivalent to staple length ** n1 = axial refractive index and n2 = transverse refractive index *** dependent on degree of retting, decortication and hackling 1. http://www.ams.usda.gov/cotton/mncs/ 2. http://www.austcottonshippers.com.au/ 3. http://www.egyptgizacotton.com/ 4. Gordon et al., 2004 5. Sampaio et al., 2005 Table 13.2 Major fibre parameters in short-staple spinning systems in order of importance to productivity and quality Importance rank

Ring

Rotor (open-end)

Air-jet (inc. MVS)

1 2 3 4

Length Strength Fineness

Strength Fineness Length Trash

Length Trash Fineness Strength

Table 13.2 are considered especially important. In combination, the listed parameters describe fibres that are sufficiently flexible to accommodate the continuous rearrangement of fibers during drafting in spinning; have a high length to diameter ratio to permit flexibility, effective consolidation and

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Identification of textile fibers

inter-fibre coherence; and have surface properties that allow smooth drafting. Until 50 years ago a sample of cotton taken from the ginned bale was assessed for its processing ability against physical cotton standards by a human cotton classer. Comparison with physical standards, whether those of the United States Department of Agriculture (USDA) Agricultural Marketing Service (AMS), to which there are over 20 signatory cotton associations from around the world, or other national country standards, is still the predominant method for measuring the value of cotton fibre. There is currently active organisation from within the international cotton industry to promote objective test measurement of important fibre parameters. It is estimated that between 30 and 40% of the world cotton crop today is classed using objective instrument test methods (Qaud, 2008). Subjective classing by hand and eye with the help of physical reference standards has been the predominant method of grading cotton fibre quality since cotton trading began. Establishment of formal cotton classification standards occurred in the United States after the 1907 International Cotton Congress, at a time when the commercial trade of raw cotton and fabrics made from cotton yarns reached significant volumes. Even at that time it was recognised that cotton destined for the manufacture of textiles for household and clothing products, demanded the development of measurements for predicting yarn and fabric attributes. The first cotton classification standards for fibre colour and length grades were established in 1909 by the USDA. The standards for colour were at first based on physical samples that exhibited a range of colour. Cotton fibre length was judged by a human cotton classer using a manual technique that involved pulling fibres away from small bundles into a spread of fibres from which the fibre length, or staple length as it is still known, could be determined. The staple length and the UHML are regarded as being close but not identical measurements of the length of the longest fibres in cotton. A Universal Cotton Standards Agreement was established in 1923 between the USDA and 23 other cotton associations from 21 countries. The USDA Universal Cotton Standards now cover strength, length, uniformity index, Micronaire, colour grade and procedures used to achieve agreement. Other nations have developed or are in the process of developing their own official cotton standards and descriptions, e.g. Chinese cotton grade is classed according China’s Cotton Colour Characterisation Chart that describes grades with similar relativity to the USDA Colour Grades. China is also currently developing quality standards for strength, fineness, maturity and uniformity applicable to high volume instrument (HVI) testing (Butterworth and Xinping, 2004). A range of objective physical test methods now exist to class cotton and determine its value for the grower and spinner. The instruments used to

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249

13.5 High volume instrument lines used to class cotton objectively (Australian Classing Services).

conduct these tests were developed in the early twentieth century and initially involved the use of microscopes, weighing scales and comb sorters to literally measure fibre dimensions. Demand for quicker objective test methods by growers seeking more transparent classing results, and spinners demanding more information to optimise quality and productivity in their mills led to the development of the HVI lines used today (Fig. 13.5). The instruments in these lines measure length, length uniformity (index), strength, extension, the Micronaire value, colour and trash content. Whilst test instruments in these high volume lines are nominally calibrated with direct, first principal reference methods, the fibre properties are measured indirectly via strain gauges, light meters, air-flow meters, pressure gauges, digital scanners and capacitors. The indirect methods introduce some questions about the cause of effects observed; however, the accuracy and precision particularly for length and strength parameters are generally well accepted. Questions about the accuracy of objective measurement of fineness by the Micronaire instrument (Lord, 1956, Lord and Heap, 1988) and short fibre content (Heap, 2004 and Robert et al., 2005) are well documented. The colour of cotton lint has always played a major part in assessing and marketing fibre value, although for nearly all base grade export cotton, e.g. USDA 31-3 or Xinjiang 129, traded each year colour is largely irrelevant as cottons of USDA Classing Grade 31 and higher show very little difference between each other in terms of processing ability.

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Identification of textile fibers

Table 13.3 HVI properties for export cottons into South East Asian spinning mill laydowns sampled during 2004 Origin

UHML (mm)

UNI (%)

SFC (%)

STR (g/tex)

EXT (%)

COL (Rd)

YEL (+b)

MIC (μg/in)

USA SJV USA TX USA CA Australia China W. Africa

29.0 28.6 28.1 28.7 28.5 28.0

82.3 81.9 81.8 82.1 82.7 82.0

9.3 10.3 10.5 10.1 8.1 9.0

31.3 29.7 28.0 29.3 28.1 27.9

7.2 7.4 7.35 7.6 7.5 7.4

78.2 77.0 78.0 78.3 80.1 76.1

9.2 7.8 9.0 8.6 9.0 9.1

4.22 4.18 4.25 4.30 4.05 3.90

Today, a HVI line, which incorporates instruments for length and length uniformity, fineness, bundle strength and extension and colour, measures 825 samples in a 7-hour and 20-minute shift (Ghorashi, 2006). Since 1980 there has also been the development of low volume test instruments to measure other important fibre properties such as stickiness, maturity, and the distribution of fibre properties such as length and trash. Table 13.3 lists the HVI properties measured on export-grade cottons destined for the same laydowns in South East Asian spinning mills during 2004 (Gordon et al., 2004). Notable is the consistent average quality between growths of different origin.

13.3.2 Identification of cotton fibre origin in textiles After fabric manufacture, cotton as an apparel or home furnishing product strongly retains its identity, although this identity is not usually linked to the specific country of origin nor, as described earlier, to its specific quality. The exception to this is where a particular growth brands and controls the supply chain through to the finished garment using a licensing system, e.g. Supima®, the promotional organisation of the American Pima cotton growers. Without the onus of this type of system cotton is usually blended with a range of growths on the basis of price, quality and availability. Classification and authentication of cotton geographic origin is therefore important to brand owners and to governments that regulate international cotton trade. The development of methods and in particular the potential of genetic-based methods to identify the cotton fibre content of finished textiles would represent a significant opportunity for license holders to control their brand and for governments to improve their ability to enforce compliance with trade agreements between nations. Even comparisons based on physical fibre properties do not provide a clear-cut distinction of long, fine G. barbadense fibre from ‘lesser’ quality

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13.6 Comb-sorter staple array of ginned cotton (CSIRO Textile and Fibre Technology).

G. barbadense or Upland cotton. Fibre from yarn pulled from a nominated cotton product and untwisted to extract enough fibres (>60 mg) can be used to build a comb sorter array (American Society for Testing and Materials (ASTM) Standard Test Method D1440, 2007), which provides some distinction of cotton type on the basis of length characteristics. However, this approach is unable to clarify whether or not a fibre is from a particular origin, only that it is likely to be of a particular quality and hence species. Figure 13.6 shows a comb sorter array for an ELS type cotton prior to combing. The mean of the longest fibres is the most telling in this test as this length measurement corresponds with the length values observed in Table 13.1. Using this approach, high quality ELS cotton, with very long staple length, can be distinguished with reasonable confidence from lesser quality Pima and longer staple Upland cottons. Calculation of the short fibre content in the array also allows comments to be drawn about whether or not the fibre has been carded or combed. The diameters of fibres drawn from the same specimen can also be measured and the values compared with the expected ranges listed in Table 13.1 for various varieties and species. Figure 13.7 shows a compilation of comb sorter (effective length) and diameter test results measured on fibre specimens from the pile yarns of bathing towels labelled as being ‘Egyptian’ in origin. The graph shows the cut-off points for Upland and Egyptian-type (G. barbadense) cottons on the basis of the expected length and diameter ranges. The effective length is on average 4 to 5 mm less than the comb sorter equivalent of the UHML. Another identification issue relates to the current and future production of genetically modified (GM) cotton, e.g. Roundup Ready® and ‘Bt’ cotton. To date nine countries have commercialised GM cotton, and around 36% of the world cotton area in 2006/07 was planted to GM varieties (Townsend, 2007). The realisation of new cotton varieties via GM with unique fibre

Identification of textile fibers

Effective length (mm)

252

35 33 31 29 27 25 23 21 19 17 15 10

11

12

13 14 15 16 17 Diameter (microns) G. barbadense

18

19

20

G. hirsutum

13.7 The relationship between comb sorter (length) and diameter test results for fibre specimens drawn from towel pile yarn.

quality characteristics, e.g. fibre with improved extensibility or moisture absorption abilities, whilst not likely in the next ten years, would mark a dramatic and irrevocable change along the cotton supply chain. There are currently no trade barriers for fibre from GM cotton, largely because of the difficulty to date of measuring the difference between non-GM and GM fibre, but also because cotton fibre from GM plants has shown no adverse quality or health effects to the manufacturer or consumer. Issues also exist with the identification and certification of organic cotton, which according to US organic and other international standards do not allow the use of biotech (read GM) cotton (Wakelyn and Chaudhry, 2007). Identification of remnant DNA in fibre cell protoplasm using the Polymerase Chain Reaction (PCR) is thought of as the optimal solution in the authentication of cotton’s origin (species). With the development of GM cotton, consolidation in the number of seed cotton production companies and the increase in market value of growths with a labelled origin, the stakes in fibre and seed cotton authentication have grown. To this end a number of biotech companies have sought to be able to provide a test for genetic authenticity. Extraction of identifiable DNA from seed and other plant matter, e.g. leaves, has been achievable since techniques for DNA extraction and purification first appeared. However, until recently no one company or research group had reported being able to successfully extract and amplify the DNA from mature cotton fibre to enable identification. In fact extraction efficiency of DNA material from fibre specimens using the widely used cetyltrimethylammonium bromide (CTAB) extraction method (Rogers and

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Bendlich, 1994) with subsequent DNA precipitation, relied on cotton fibres being raw, i.e. not washed (read scoured) or wet treated in any way, and there being seed material contained within the specimen. However, according to a recent press release (Applied DNA Sciences Inc., January 2008), Applied DNA Sciences Inc., a company that specialises in “DNA-based security solutions”, has developed a new extraction protocol that improved DNA isolation ‘one thousand-fold’ over previous methods. The extract of cotton (fibre) cell nuclear and chloroplast genomes could be assayed to identify the difference between Pima (G. barbadense) and Upland cotton. The company had been sponsored by Supima® growers to develop an authentication method to identify and confirm the Supima® cotton content of branded apparel and home furnishing products. Measurement of the method’s sensitivity in detecting DNA from processed fibre, and its ability to distinguish between varieties of the same species were not detailed in the press release.

13.3.3 Identification tests for cotton fibre in textiles For situations where the presence of cotton in a fabric or garment needs to be identified, there are a range of standard qualitative test methods that use techniques and measures that will have been described in earlier chapters. These standards or their equivalents in other countries are used to identify fibres under various product identification, labelling and trade regulations throughout the world. For example, the Federal Trade Commission (FTC) in the USA identifies generic fibre names described in legislature but relies upon the American Association of Textile Chemists and Colourists (AATCC) Test Method for identifying those fibres. The standards are also used by yarn and fabric buyers to check and accept purchases. The most widely used standards in this regard are the AATCC Test Method 20, which describes a range of physical, chemical and microscopical tests for identifying commercially used textile fibres and the ASTM Standard Test Method D276, which contains similar techniques as well as descriptions for the use of infrared spectroscopic techniques. It is noted that where the infrared spectrum indicates native (raw) cellulose, then it is desirable to resort to microscopical analysis to elucidate the type of fibre. Under a light microscope cotton fibres are recognised by the presence of the lumen and convolutions, i.e. twists along the length of the fibre. Gossypium barbadense varieties contain fewer convolutions than G. hirsutum varieties although the difference is small and variable (Meredith, 1951). Other features of convolutions, e.g. the convolution angle, show no difference between species, although a statistical albeit speculative relationship

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13.8 Cotton fibres under polarised light microscopy showing reversals marked with arrows at reversal boundary.

exists between the convolution angle and the angle of cellulose fibril orientation (Duckett and Cheng, 1972). Another unique feature of cotton fibres are the reversals in the direction of the spiral (fibril) structure or helix along the length of the fibre. At the point of reversal, the fibrils for a short interval lie parallel with the fibre axis. They are clearly highlighted as changes in transmitted interference colours under polarised light microscopy when using a first order red compensator plate, e.g. as per the ASTM Standard Test Method D1442. Figure 13.8 is a photo showing the reversals in mature and immature cotton under a polarised light microscope. Cross-sectional analysis also reveals the unique ‘kidney’ cross-sectional shape of the cotton fibre (see Fig. 13.4). Microscopical analysis is also useful in elucidating various treatments that have been applied to cotton fibre. For the most part the resolution of the light microscope (up to ×400 magnification) is suitable for identifying most treatments and conditions associated with cotton fibres. For example, microscopical examination easily reveals whether fibre has been mercerised in concentrated sodium hydroxide (18–25% NaOH). Mercerised cotton appears smoother along its length because the swelling effect of the process makes the convolutions less easy to observe, although the reversals are still apparent under polarised light microscopy. The cross-sections of mercerised cotton fibres are more circular, which results in a more uniform reflecting

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surface and increased fibre lustre. The lumen is typically smaller in a mercerised fibre. Examination of cavitomic cotton fibres, i.e. fibre damaged by microorganisms such as cellulolytic bacteria and fungi, under a light microscope can reveal fungal hyphae and fractures in the surface of the fibre. Chiefly, cavitomic cotton is greyer and creates dye uptake problems in processing because of the damage to cotton cellulose. Swelling cavitomic fibres in concentrated NaOH causes differential swelling at fracture points along the length of the fibre for identification purposes. Likewise the presence of shrink-proofing and permanent crease resins can be seen under a light microscope. For greater detail scanning electron microscopy can be used to observe treatments and changes to the cotton fibre form.

13.3.4 Quantitative analysis of cotton in textiles Quantitative tests for determining the cotton fibre blend composition of mixtures of fibres are described in AATCC Test Method 20A and ASTM Standard Test Method D629. Both also describe procedures for the estimation of the amount of moisture and non-fibrous materials in textiles. The use of these tests follows the textile labelling laws in each country. For example, according to the USA FTC each fibre type in a fabric must be labelled by percentage in the order of preponderance by weight, as long as it is greater than 5% of the total fabric weight. Percentages of fibres at less than 5% of the fabric (garment) can be labelled at the discretion of the manufacturer or brand owner. Fibres included as decorative or technical aspects, e.g. elastic ribbon or tie cords, of the garment do not need to be specified. The particular test used depends on the fibre blend, the intimacy of the fibre blend and the type of non-fibrous material to be measured. For cotton, the situation is often that the fibre blend, usually with polyester but also with rayon, spandex or wool in an intimate blend, needs to be tested to check fibre proportions. Substitution of cotton by other fibres and viceversa is common particularly for polyester blends where the price of one regularly varies along a world price parity line against the other. Because cotton, unlike nearly all other fibres, does not undergo any chemical processing until fabric finishing, interest in the content of non-fibrous material in cotton is unusual. The analytical procedure for the quantitative analysis of cotton blend textiles typically involves dissolving the nominated or suspected fibre in the intimate blend using a solvent specific for that fibre. Cotton cellulose is difficult to dissolve and there are a limited range of solvents. As a result hydrolysis in a strong mineral acid, e.g. 70% sulphuric (H2SO4) acid is more common. After dissolving or disintegrating the nominated fibre, the residual

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fibre component is washed and dry-weighted to determine the percentage of fibre in the blend. Blends of cotton with rayon fibres, which are extruded from amorphous cellulose that is more reactive and easily hydrolysed, are subject to a less concentrated mineral acid solution (59.5%) that ‘dissolves’ the rayon and leaves the cotton fibres.

13.4

Future trends

Specification of fibre quality in industrial countries, e.g. the USA, will occur in the gin rather than at classing laboratories. According to Ghorashi (2006), the future fibre testing system will be implemented in the gin. The testing will be fully automated and installed inline with the process flow of the gin. The system of the future will have remote monitoring, calibration and will measure all pertinent fibre qualities a multiplicity of times. Cotton identification in the future on the basis of recovered genetic information extracted from the raw and processed fibre is contingent upon satisfactory DNA-extraction procedures and assays sensitive enough to reveal differences in the extracted genetic material. It remains to be seen whether the genetic variation between varieties of the same species is large enough to be a measurable point of differentiation. Determining the point of origin or place of production will remain unlikely without the application of a DNA-based, electro or nano-based labelling procedure.

13.5

Sources of further information and advice

Identification of Textile Fibres, The Textile Institute, Manchester, England, 7th Edition (1972). Steadman, R. G., Cotton Testing, Textile Progress, 27(1), 66 pp (1997). Gordon, S. G. and Hsieh, Y. L. (eds), Cotton Science and Technology, Woodhead Publishing in Textiles, Cambridge, 547 pp (2007).

13.6

References

AATCC Test Method 20-1995, Fiber analysis: Qualitative, 38–61 (1997). Amin, S. A. and Truter, E. V., Cotton lipids: A preliminary survey, Journal of the Science of Food and Agriculture, 23, 39–44 (1972). Applied DNA Sciences Inc., Applied DNA Sciences Receives Milestone Payments from Supima Company to Exhibit at Supima Trade Show January 22–24, 2008, press release published on January 10th 2008. ASTM D276-00a Standard test methods for identification of fibers in textiles. ASTM D629-99 Standard test methods for quantitative analysis of textiles. ASTM D1440-07 Standard test method for length and length distribution of cotton fibers (Array Method).

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ASTM D1442-06 Standard test method for maturity of cotton fibers (Sodium Hydroxide Swelling and Polarized Light Procedures). Bell, T. M. and Gilham, E. M., The World of Cotton, ContiCotton EMR, Washington DC, 395–403 (1989). Bolyston, E. K. and Hebert, J. J., The primary wall of cotton fibers, Textile Research Journal, 65, 429–431 (1995). Butterworth, J. and Xinping, W., Cotton and Products: Update on China’s Cotton Classification Reform, USDA Foreign Agricultural Service, Gain Report No. CH4039 (December 2004). Duckett, K. E. and Cheng, C. C., The detection of cotton fiber convolutions by the reflection of light, Textile Research Journal, 42, 263–270 (1972). El Mogahzy, Y. E., Optimising cotton blend cost with respect to quality using HVI fiber properties and linear programming, Part I: Fundamentals and advanced techniquies of linear programming, Textile Research Journal, 62, 1–8 (1992a). El Mogahzy, Y. E., Optimising cotton blend cost with respect to quality using HVI fiber properties and linear programming, Part II: Combined effects of fiber properties and variability constraints, Textile Research Journal, 62, 108–114 (1992b). Fargher, R. G. and Probert, M. E., Alcohols Present in the Wax of American Cotton, Journal of the Textile Institute, 15, 337–346T (1924). Fargher, R. G. and Higginbotham, L., Constituents of Wax from Egyptian Sakellarides Cotton, Journal of the Textile Institute, 419, 419–433T (1924). Ghorashi, H., The Universal Transition from Manual to Instrument Cotton Classing, Report to ITMF HVI Working Group, Bremen (2006). Gordon, S. G., Evans, D., Church J., Petersen, P., Thom, S. L. and Woodhead, A., A Survey of Cotton Wax Contents in Australian Cotton, Report to the Australian CRDC, 34 pp (November 2002). Gordon, S. G., Van Der Sluijs, M. H. J. and Prins, M. W., Quality Issues for Australian Cotton from a Mill Perspective, Report to the Australian Cotton Industry, Australian Cotton CRC (pub.), 54 pp (July 2004). Goynes, W. R., Ingber, B. F. and Triplett, B. A., Cotton fiber secondary wall development – time versus thickeness, Textile Research Journal, 65, 400–408 (1995). Heap, A. S., Relative Short Fibre Content, Presentation to ITMF Length Working Group, Bremen (2004). Hornoff, G. V. and Richter, H., Chemical composition of cotton fibres originating from various areas, Fasterforsch. Textiletech., 15, 165–177 (1964). Jefferies, R., Jones, D. M., Roberts, J. G., Selby, K., Simmens, S. C. and Warwicker, J. O., Current ideas on the structure of cotton, Cellulose Chemistry and Technology, 3, 255–274 (1969). Kassenbeck, P., Bilateral structure of cotton fibers as revealed by enzymatic degredation, Textile Research Journal, 40, 330–334 (1970). Kerr, T., Cotton Hair Growth Rings: Structure, Protoplasma, 27, 229–241 (1937). Liang, C. Y. and Marchessault, R. H., Infrared spectra of crystalline polysaccharides. I. Hydrogen bonds in native celluloses, Journal of Polymer Science, 37, 385–395 (1959). Lord, E., Air through plugs of textile fibres, Part II. The Micronaire Test for Cotton, Journal of the Textile Institute, 47, T17–T47 (1956).

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Lord, E. and Heap, S. A., The Origin and Assessment of Cotton Fibre Maturity, International Institute for Cotton (pub.), 40 pp (1988). Mauersberger, H. R. (ed), Matthew’s Textile Fibers, 6th Edition, John Wiley, New York (1954). Maxwell, J. M., Gordon, S. G. and Huson, M. G., Internal structure of mature and immature cotton fibers revealed by scanning probe microscopy’, Textile Research Journal, 73, 1005–1012 (2003). Meredith, R., Cotton fiber tensile strength and x-ray orientation, Journal of the Textile Institute, 42, T291–T299 (1951). Oerlikon, The Fiber Year 2006/07, A World Survey on Textile and Nonwovens Industry, Oerlikon (pub.), Issue 7 (May 2007). Pal, P. N. and Esteve, R. M., A study of the relationship between dye absorption and cototn fiber properties at equilibrium, Textile Research Journal, 29, 811–815 (1959). Qaud, M., (Chair of ITMF HVI Working Group), personal communication (April 2008). Ramey, H. H., The Meaning and Assessment of Cotton Fibre Fineness, International Institute for Cotton (pub.), 40 pp (ca 1982). Robert, K. Q., Dunn, M. C., Cui, X. L. and Price, J. B., Method for determining broken fibre content in ring yarn, Proceedings of the Beltwide Cotton Conferences, New Orleans (2005). Rogers, S. O. and Bendlich, A. L., in Molecular Biology Manual, Gelvin, S. B. and Schilperoort, A. R. (eds), Kluwer, Dodrecht, 2, 1–8 (1994). Sampaio, S., Bishop, D. and Jinsong, S. S., ‘Physical and chemical properties of flax fibres from stand-retted crops desiccated at different stages of maturity’, Industrial Crops and Products, 21, Issue 3, 275–284 (May 2005). Smith, B., A review of the relationship of cotton maturity and dyeability, Textile Research Journal, 61, 137–145 (1991). Cotton Incorporated, Textile Consumer, Global Consumer Apparel Shopping Trends, 39 (Fall 2006) found at http://www.cottoninc.com/TextileConsumer/ TextileConsumerVolume39/. Townsend, T., in Cotton: Science and Technology, Gordon, S. G. and Hsieh, Y. L. (eds), Woodhead Publishing in Textiles, Cambridge, 425–456 (2007). Wakelyn, P. J. and Chaudhry, M. R., in Cotton: Science and Technology, Gordon, S. G. and Hsieh, Y. L. (eds), Woodhead Publishing in Textiles, Cambridge, 130–174 (2007). Weiss, A. H., Conversion of solid waste to liquid fuel Textile Research Journal, 42(9), 526–533 (1972). Woo, J. L., An appraisal of the length measures used for cotton fibres, Journal of the Textile Institute, 59, 557–572 (1968).

14 The forensic identification of textile fibers M M HOUCK, West Virginia University, USA

Abstract: The professional perspective of the analyst shapes which characteristics are important for identification of the materials of interest. In the case of fibers and forensic science, the mindset is focused on limited sample size, murky origins, and potentially uncertain provenance. Microscopy, therefore, becomes the primary method of choice for forensic fiber identification. Key words: forensic, microscopy, investigations.

14.1

A forensic mindset

The forensic mindset began long ago and is intimately tied to the medical mindset of diagnosis. Three names routinely repeat in the historical literature on this topic: Giovanni Morelli, Sigmund Freud, and Arthur Conan Doyle (Ginzburg, 1979; Vakkari, 2001). All of their notions of what might loosely be called ‘detection’ hinge upon the identification and recognition of seemingly common or insignificant bits of information that lead to the questions at issue. Interestingly, all three were doctors or had studied medicine. Morelli created a ruckus in the art world by publishing a critique of Italian paintings in Munich (under a pseudonym, Ivan Lermolieff) that chastised the status quo for misidentifying the artist of several paintings. Morelli based his method on details normally overlooked by other critics, such as how hands or ears were shaped and rendered (Vakkari, 2001); he felt that in these details, the artists were not being mindful of their style and let their individuality shine through. Morelli made hundreds of reattributions of authorship and was judged as correct in over half of them. Freud had read Morelli’s work (as Lermolieff) had commented: It seems to me that his method of inquiry is closely related to the technique of psychoanalysis. It, too, is accustomed to divine secrets and concealed things from unconsidered or unnoticed details, from the rubbish-heap, as it were, of our observations (as cited in Wind, 1963).

Freud later acknowledged that Morelli had an influence on his method of psychoanalysis. Doyle’s connection to this triad is conceptual in that he was also a medical doctor and learned the art and science of diagnosis in medical school. One of Doyle’s instructors, in fact, Joseph Bell, who was particularly adept at using minor details to accurately diagnose disease 259

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conditions, was an influence for the character of Sherlock Holmes (Ginzburg, 1979). Things, then, are heavily coded as to their origins (who is the author of this painting, what is the source of this mental aberration, who committed this crime?) and it is up to the investigator to discern the subtle indicators which reveal the hidden information. The forensic mindset, therefore, comes from the appreciation that minor details (Holmes’ trifles) are the signs pointing to the de-coding of the material’s origins. The mindset of a method is inherent in the approach it takes and the observations it makes. As Henry Marcuse said, the theory is consistent in the corresponding set of operations: If length is measured, then length is important for some reason integral to the material to be analyzed. As well, whether the measurement is in millimeters or inches, kilometers or miles is important. The approach taken with fiber identification reflects what is important to those doing the testing. The American Association of Textile Chemists and Colorists methods manual (AATCC, 2007) lists among its standard analyses the following: • colorfastness to commercial laundering and to domestic washing • flammability of clothing textiles • smoothness of seams in fabrics after repeated home laundering • electrostatic propensity of carpets • wrinkle recovery of fabrics: appearance method • dimensional changes in textiles other than wool. The AATCC technical manual lists microscopy as useful for identification of fibers but ‘[i]t must be used with caution on man-made fibers since they are frequently produced in a number of modifications which alter the . . . appearance.’ (page 20). The Technical Manual also lists ‘reaction to flame’ (Table III) as a test method with the categories for results as: melts near flame, shrinks from flame, and burns in flame, among others. Another example of professional orientation to analysis comes from ASTM, International, Volume 7 lists the following as methods for fiber identification: • • • • •

flame resistant materials used in camping tentage pile retention of corduroy fabrics elastic properties of textile fibers performance specifications for underwear fabrics, woven, men’s and boys’ commercial moisture regains for textile fibers.

ASTM lists infrared spectroscopy as the ‘preferred method’ for fiber identification, noting that: ‘additional physical properties of the fibers such as density, melting point, regain, refractive indices, and birefringence . . . are useful for confirming the identification’ (ASTM D276). Most of the tests

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relate to physical properties but treat fibers as a bulk material with certain performance characteristics. Compare these approaches with that taken by forensic scientists. Rather than starting from the premise of knowing what the fiber is and determining its properties, a forensic scientist has no real idea of what the fiber might be and cares not a bit about its pile retention or ability to regain water. The fiber presented to a forensic microscopist could be anything – almost literally – and the scientist must approach it with an open mind. Therefore, microscopy becomes the method of choice, not just useful or cautionary, for the forensic scientist (Heyn 1952; Longetti and Roche, 1958; McCrone 1982; Rouen and Reeve, 1970; Stoeffler, 1996). Typically, forensic scientists do not receive much material with which to work; one or two fibers may be all there is for evidence. A conservative perspective and preservation of sample is a central concern. Other methods apply and supplement or enhance the analysis, such as infrared spectroscopy or fluorescence microscopy, but the central approach for the forensic identification comes from discerning the subtle microscopic characteristics of fibers.

14.2

Microscopy of fibers

Manufactured fibers differ physically in their shape, size, internal properties and appearance. Some of the microscopic characteristics of certain fibers may indicate a polymer class or a particular end use. The term manufactured fibers refers to various families of fibers produced from fiber-forming substances, which may be synthesized polymers, modified or transformed natural polymers or glass. Synthetic fibers, by contrast, are those manufactured fibers which are synthesized from chemical compounds (for example, nylon or polyester). Therefore, all synthetic fibers are manufactured, but not all manufactured fibers are synthetic. Because fibers begin as unorganized masses of monomers and end up as organized linear materials, the polymers in the fibers become oriented mostly parallel to the longitudinal axis of the fiber. Orientation, then, refers to the degree of parallelism of the polymers in a fiber. When a majority of the polymers are aligned with the fiber axis, the fiber is described as highly oriented. Crystallinity is the degree to which a fiber consists of crystalline regions, where the polymers are in a tightly packed spatial arrangement, rather than amorphous regions, where the polymers are randomly arranged and loosely structured. All fibers have both types of regions; the degree to which a fiber is crystalline or amorphous, however, affects its physical properties and end-uses. Natural fibers are internally structured that precludes useful examination under polarized light.

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Although they are often correlated, it is important to remember that crystallinity and orientation are two separate entities; for example, PET fibers slowly drawn at low temperatures result in highly oriented but amorphous fibers. The degree of fiber orientation depends on the draw ratio, drawing conditions (wet or dry) and the composition of the spinning dope: crystallinity is a temperature-dependent phenomenon.

14.3

Manufactured fiber production and spinning

The bonding together of monomers to form polymers is called polymerization. The reactions that build polymers for synthetic manufactured fibers occur by either a condensation (step-growth) or an addition (chain-growth) mechanism. In a condensation reaction, each bond that occurs involves the release of water or some other simple substance, such as ethylene glycol in polyester production. Condensation reactions yield a product in which the repeating unit has fewer atoms than the monomer or monomers. In an addition reaction, the resulting polymer has sub-units which have molecular formulae identical to those of the monomer. Combination occurs by rearrangement of the combined monomer units. The molecular weight of the polymer is the sum of the molecular weight of all of the monomers in the chain. Synthetic fibers are formed by extruding the fiber-forming substance, called spinning dope, through a hole or holes in a shower head-like device called a spinneret; this process is called spinning. The spinning dope is created by the rendering of solid monomeric material into a liquid or semiliquid form by a solvent or heat. The four major methods of fiber spinning are dry, wet, melt and gel. Dry spinning extrudes the spinning dope into a heated chamber to remove the solvent leaving the solid filament behind. Acetate, acrylic, modacrylic, and triacetate are examples of dry-spun fibers. Wet spinning extrudes the spinning dope into a liquid coagulating medium where the polymer is regenerated. Acrylic, modacrylic, and rayon are examples of wet-spun fibers. Melt spinning differs in that the spinning dope is melted and extruded into air or other gas, or into a liquid, where it cools and solidifies. Nylon, polyester, olefin are examples of melt-spun fibers. Gel spinning is a process in which the primary mechanism of solidfication is the gelling of the polymer solution by cooling to form a gel filament consisting of precipitated polymer and solvent. The solvent is then washed off in a bath. Gel-spun fibers have a high tensile strength and modulus. After the fibers are spun, they may go through a number of steps before they are ready for construction into yarns or shipment as fibers. Fibers are typically drawn to increase their length, strength, and form; this has an effect on their optical properties. Also, they may be treated with chemicals to yield

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a desired property, such as increased apparel comfort or stain-resistance. Fibers may be crimped to alter their physical form. Microscopic properties, such as cross-section and diameter, are important characteristics for the initial comparison of fibers. Very often, the physical traits of a fiber may suggest whether it is an acrylic or rayon fiber, from a garment or carpet or intended for household or industrial use. The examination of the optical properties of manufactured fibers can yield a tremendous amount of information about their chemistry, production, end-use and environment. Careful measurements and analysis of these properties is a crucial step in the identification and later comparison of textile fibers. Optical properties, such as refractive index, birefringence, and color, are those traits that relate to a fiber’s structure or treatment revealed through observation. Some of these characteristics aid in the identification of the generic polymer class of manufactured fibers. Others, such as color, are critical discriminators of fibers that have been dyed or chemically finished. A visual and analytical assessment of fiber color must be part of every fiber comparison. The fluorescence of fibers and their dyes is another useful point of comparison. Thermal properties relate to the softening and melting temperatures for manufactured fibers and the changes the fiber exhibits when heated. Not all manufactured fibers are thermoplastic, or capable of melting, and so it is important to observe the fibers as they undergo increasing temperature. This is done by fitting a special thermal stage to a microscope, which gradually heats up the fiber while the microscopists observes the changes. The range of temperatures within which a fiber is altered should be recorded. Table 14.1 gives ranges of melting temperatures for the more common manufactured fibers. Based upon a fiber’s polymer composition, it will react differently to various instrumental methods, such as Fourier transform-infrared spectroscopy (FT-IR) or pyrolysis-gas chromatography (P-GC), and chemicals, such as acids or bases. These reactions yield information about the fiber’s molecular structure and composition. A polarized light microscope is the primary tool for the identification and analysis of manufactured fibers. Many characteristics of manufactured fibers can be viewed in non-polarized light, however, and these provide a fast, direct and accurate method for the discrimination of similar fibers. A comparison light microscope is required to confirm whether the known and the questioned fibers truly present the same microscopic characteristics. The cross-section is the shape of an individual fiber when cut at right angles to its axis. Shapes for manufactured fibers varies with the desired end result, such as the fiber’s soil hiding ability or a silky or coarse feel to the final fabric. Some fiber types tend to stay within certain cross-sectional families; for example, bean-shaped fibers tend to be acrylics and rayon tends

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Identification of textile fibers Table 14.1 Melting temperatures for some fiber types1 Fiber type

Temperature (°C)

Acetate Acrylic Aramid Modacrylic Nylon 6 6,12 6,6 Olefin polyethylene polypropylene Polyester (PET) Rayon Saran Spandex Triacetate Vinal

224–280 Does not melt Does not melt 204–2252 213 217–227 254–267 122–135 152–173 256–268 Does not melt 167–184 231 260 200–260

1 From Carroll, 1992. 2 Some members of this class do not melt.

14.1 Cross-sections of various fibres.

to be irregular (Fig. 14.1). The particular cross-section may also be indicative of a fiber’s intended end-use: many carpet fibers have a lobed shape to help hide dirt and create a specific visual texture to the carpet. A fiber’s cross-sectional shape can be gleaned from a visual inspection by focusing through the fiber: as the focal plane moves through the fiber, the viewer can see changes that relate to its physical shape. If the cross-sectional shape is difficult to discern from an optical cross-section, or it appears that the cross-

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section may help identify or distinguish the fiber, then a physical crosssection should be prepared. Numerous approaches have been published for cross-sectioning but the method outlined by Palenik and Fitzsimmons (1990) is simple, inexpensive, and conservative of sample. The modification ratio of a fiber is a geometrical measurement used in the characterization of non-round fiber cross-sections. The modification ratio is the difference in size between the outside diameter of the fiber and the diameter of the core. Many manufacturers use modification ratios in the descriptions of their fibers for patent purposes. This characteristic may assist the examiner in providing information to fiber manufacturers during product queries. The way a fiber’s diameter is measured is dependent upon its crosssectional shape; there is more than one way to measure the diameter of a non-round fiber. Manufactured fibers can be made in diameters from about 6 μm (so-called microfibers) up to a size limited only by the width of the spinneret holes. By comparison, natural fibers vary in diameter from cultivated silk (10–13 μm) to US sheep’s wool (up to 40 μm or more) and human head hairs range from 50–100 μm. A manufactured fiber greater than 40 μm is probably a carpet fiber. Some manufactured fibers retain air-pockets or voids after production. For example, wet-spun fibers, such as acetates, may have voids that range in size from submicron up to several microns. Voids are created when pores in the solidifying fiber are filled with a mixure of solvent and non-solvent fluids (Frushour and Knorr, 1998). It can be possible to distinguish between wet-spun and dry-spun acrylics by the size and number of voids (Masson, 1995). The size, shape, distribution, and concentration of voids is related to the composition and production methods of the fiber and is an important comparative feature. Inclusions are materials or discontinuities that are placed or occur in fibers. These may be accidental inclusions, such as the draw marks sometimes seen in melt spun fibers, or intentional inclusions, such as large clumps of delustrant or anti-static materials. Delustrants are finely ground particles of materials, such as titanium dioxide, that are introduced into the spinning dope. These particles help to diffract light passing through the fibers and reduce their luster. Fibers can be classified as bright, semi-bright and dull, although other categories may be denoted, such as slightly, moderately and heavily delustered. The size, shape, distribution, and concentration of delustrants should be noted. A fiber’s construction is an important indication of its production and end-use. Examples are bicomponent fibers (two or more polymer types spun in a sheah/core or bilateral relation), biconstituent fibers (two different polymers spun together from a homogeneous dope), or microfibers (fibers with a denier of less than 1.0). Fibers may be constructed for specific

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traits. These specialty fibers are distinct and particular attention should be paid to their construction and composition.

14.4

Polarized light microscopy

Polarized light microscopy is an easy and quick non-destructive way to determine the generic polymer class of manufactured and synthetic textile fibers. Beyond the immediate characteristics used to discriminate between polymer types, the examination of fibers in polarized light provides valuable information about the production and finishing of the fiber after spinning.

14.4.1 Light and fibers Light is composed of electromagnetic waves and a change in velocity is associated with the polarization (positive or negative charge) that occurs under the influence of an electric field. The outer electrons of molecules, which are taking part in covalent bonds, are thus affected. It is therefore possible to assign a polarization (or bias) to each chemical bond. The polarization will also vary with the direction of the electric field. The optical properties of fibers depends on the formation of oriented molecular aggregates in the fiber when it is spun; these aggregates are called micelles. The configuration of the polymer(s) involved, as well as the individual properties of the micelles, also plays a part. The orientation and crystallinity of the micelles determines how light will be affected as it passes through the fiber. By viewing a fiber with polarized light, the internal structure of the fiber, and, thus, its polymer make-up, may be deduced.

14.4.2 Refractive index Fibers vary in shape but are almost always thicker in the center than near the edges. Thus they act as crude lenses, either concentrating or dispersing the light that passes through them. This alteration of light can be calculated as the ratio of the speed of light in a vacuum to the speed of light in the fiber (or any other medium). This ratio is called the refractive index. If a fiber has a higher refractive index than the medium in which it is mounted, it acts as a converging lens, concentrating light within the fiber. If the fiber has a lower refractive index than the mounting medium, it acts as a ‘diverging’ lens, and the light rays diverge from the fiber. In most fibers, the light rays only slightly converge or diverge and thus appear as a thin bright line, called the Becke line, after the French mineralogist Ferdinand Becke who first described it in 1893, at the interface between the fiber and the mounting medium. While observing the fiber, the working distance on the microscope is increased (the stage is moved down). If the fiber has the higher refractive index, the Becke line moves toward the fiber

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as the working distance is increased. If the mounting medium has the higher index, the Becke line moves toward the medium (away from the fiber) as the working distance is increased. If fibers are mounted in Permount, which has a refractive index of 1.52, then a fiber can be described as being greater than, equal to or less than 1.52. Manufactured and synthetic textile fibers have two optical axes, one parallel to the long axis of the fiber and one perpendicular to the long axis (this could be considered the ‘short’ axis). Internally, manufactured and synthetic fibers have crystalline and amorphous regions and these are more or less oriented to the long axis of the fiber. This orientation creates a difference between the speed of light passing through the long axis of the fiber and the light passing through the ‘short’ axis. Light passing along one axis is impeded more than light passing through the other axis because of the greater electrical polarization of the molecules in that direction. Thus, manufactured and synthetic fibers are said to be anisotropic, meaning that light is affected differently in the two directions. Because of this difference, two distinct refractive indices are created. These refractive indices are called n|| (pronounced, ‘n parallel’), for the refractive index when the long axis is parallel to the orientation of the polarizing filter, and n⊥ (pronounced, ‘n perpendicular’), for the refractive index when the long axis is perpendicular to the polarizing filter’s orientation (Table 14.2).

Table 14.2 Refractive indices of manufactured and synthetic textile fibers Fiber type

n||

n⊥

n|| − n⊥

Acetate Dicel

1.478 1.476

1.473 1.473

0.005 0.003

Triacetate Tricel Arnel

1.469 1.469

1.469 1.468

0 0.001

Acrylic Acrilan 36 Orlon Acrilan

1.511 1.51 1.52

1.514 1.512 1.525

−0.003 −0.002 −0.005

Modacrylic Dynel Teklan SEF Verel

1.535 1.52 >1.52 1.535

1.533 1.516 >1.52 1.539

0.002 0.004 −[low] −0.004

Vinyon Fibravyl Rhovyl Vinyon HH

1.54 1.541 1.528

1.53 1.536 1.524

0.01 0.005 0.004

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Identification of textile fibers Table 14.2 Continued Fiber type

n||

n⊥

n|| − n⊥

Rayon Viscose (regular) Viscose (regular) Viscose (high tenacity) Vincel (high wet modulus rayon) Fortisan Fortisan 36 Cuprammonium Tencel

1.542 1.545 1.544 1.551 1.547 1.551 1.553 1.57

1.52 1.525 1.505 1.513 1.523 1.52 1.519 1.52

0.022 0.02 0.039 0.038 0.024 0.031 0.034 0.05

Olefin Courlene (PP) Polypropylene SWP (PE) Courlene X3 (PE) Polyethylene

1.53 1.52 1.544 1.574 1.556

1.496 1.492 1.514 1.522 1.512

0.034 0.028 0.03 0.052 0.044

Nylon Enkalon (6) ICI nylon (6,6) Qiana Rilsan (11) Nylon 6 Nylon 6,6 Nylon 11

1.575 1.578 1.546 1.553 1.568 1.582 1.55

1.526 1.522 1.511 1.507 1.515 1.519 1.51

0.049 0.056 0.035 0.046 0.053 0.063 0.04

Silk (degummed)

1.57

1.52

0.05

Aramid Nomex Kevlar

1.8 2.35

1.664 1.641

0.136 0.709

Polyester Vycron Terylene Fortrel/Dacron Dacron Kodel Kodel II

1.713 1.706 1.72 1.7 1.632 1.642

1.53 1.546 1.535 1.535 1.534 1.54

0.183 0.16 0.185 0.165 0.098 0.102

Spandex Lycra/Vyrene

1.561

1.56

0.001

Others Vicara (Azlon) Teflon Calcium alginate Saran Novoloid Kynol (drawn) Kynol (undrawn) Polyacrylostyrene Darvan (Nytril) Polycarbonate

1.538 1.38 1.524 1.61 1.5−1.7 1.658 1.649+ 1.56 1.464 1.626

1.536 1.34 1.52 1.61 1.5−1.7 1.636 1.649 1.572 1.464+ 1.566

0.002 0.04 0.004 0 0 0.022 <0.001 −0.012 0 0.06

Source: AATCC, 1996; ASTM, 1996; McCrone et al., 1979; Perry, 1985; Rouen and Reeve, 1970.

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The refractive indices of a fiber can be measured directly by placing the fiber in a series of liquids of specific refractive indices until the refractive indices of the fiber and liquid are the same. At this point, the fiber ‘disappears’ because it and the liquid are now isotropic, meaning that light is traveling at the same speed through both the fiber and the liquid. This method is not always practical in the forensic examination of fibers; a relative refractive index assessment can be made instead. The refractive index of the mounting medium must be known and of a value very near 1.5 (for example, Permount = 1.525), as this provides easy discrimination between polymer types. If n|| is greater than n⊥, that is, n|| − n⊥ would equal a positive number, the fiber is said to have a positive sign of elongation. This means that light is traveling faster along the long axis of the fiber than it is along the short axis. If n|| is less than n⊥, the fiber is said to have a negative sign of elongation. Here, light is traveling faster along the short axis than it is along the long axis, and n|| − n⊥ would equal a negative number. To determine the sign of elongation, it is necessary to add another polarizing filter.

14.4.3 Crossing polarized filters By inserting an additional polarizing filter, which has an orientation perpendicular to the first, between the eyepiece and the objective, any natural light is effectively prevented from traveling to our eye. Since the polarized light undergoes no change in its original direction of vibration, it is stopped at the second filter. This second filter is called the analyzer for reasons which will become clear later. A fiber placed parallel to either orientation would appear black because none of the light vibrations could meet the criteria of both filters. These positions are called extinction points. If we place a fiber at 45° to the polarizer and the analyzer, some light is allowed to pass through, and we see a bright fiber on a black background. This is because the fiber is anisotropic and has two RIs, therefore, the light is traveling through the fiber at two different speeds causing the light to be resolved into two mutually perpendicular planes of vibration. These speeds are determined by the distance the light has to travel (for example, the fiber’s diameter) and the orderliness of the fiber’s molecular construction (orientation and crystallinity). Since the light waves are in different planes, they cannot interfere with each other. As the light passes through the fiber, it is bent by the RIs of the fiber and becomes out-of-phase. This means that the light waves are now free to interfere with each other. When these out-of-phase waves of light strike the analyzer, it diffracts them into various colors depending on the wavelengths being added and subtracted through interference; therefore, they are called interference colors. The colors produced are indicative of the fiber’s polymer type and organization.

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If a fiber shows interference colors, it has a positive sign of elongation. For fibers that show a grey color, we must move the scale up or down to determine which direction the colors go. By placing a 550 nm wave plate between the fiber and our eye, we move the fiber’s value up by 550 nm in either the yellow/orange range (−) or the blue range (+), depending on where it started. The fiber must be in the additive position for this procedure. The plate retards the light by an additional 550 nm, so slow + slow = higher order color (add) or slow + fast = lower order color (subtract). When the two rays are recombined in the analyzer, this retardation causes, to a greater or less degree, destructive interference for certain wavelengths of light. The remaining wavelengths combine to give the interference colors we see when we cross the polars. Interference colors vary in hue with the retardation according to a characteristic sequence of colors known as Newton’s series. The Newton’s series is not a spectrum; it is divided into orders, with the end of each order marked by a red-violet color representing one full wavelength of retardation. Each order equates to 550 nm, so first order colors fall at 550 nm, second order colors at 1100, and so on. The colors become less intense as the orders increase. A printed chart of the Newton series of colors is called the Michel-Levy interference color chart. The chart shows the colors produced by the interference of light waves, relating thickness with wavelength and retardation. Retardation increases linearly with both thickness and birefringence. The difference in the speed of light along the two rays is called the retardation or path difference, because of the difference in lengths of the paths the rays traverse. The amount of retardation depends on the difference in velocities (determined by the RIs of the fiber) and the thickness of the material (how much fiber the ray had to travel through). This relation is expressed by: R = T ( n − n⊥ ) , where R is the retardation, T the thickness of the fiber in nanometers, and (n|| − n⊥) the birefringence. Interference colors allow an examiner to characterize fibers based upon their optical properties, which relate to their chemistry and production. One the more distinctive traits of a fiber is its birefringence. Because the velocity of the two polarized beams differs, they suffer unequal refraction during passage through the fiber, which is said to doubly refracting or birefringent. The birefringence of a fiber can be determined with the polarizing microscope by examining the fiber between crossed polars. The characteristic birefringence of a given substance is the numerical difference between the maximum and minimum refractive indices, that is (n|| − n⊥) for uniaxial substances. Birefringence will be greatest when the polymers in the fiber

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are lined up parallel to the longitudinal axis of the fiber and will be zero if they are randomly organized. To determine the birefringence, we must find the path difference between n|| and n⊥. To do this, the slow ray must be compensated for by artificially increasing the thickness of the fiber. With this compensation, either a wedge or a tilting disk, the thickness is increased until the fast ray is as slow as the slow ray. At this point, the middle of the fiber turns black, indicating that the fiber is isotropic at that point. The influence of the fiber upon the polarized light is therefore reversed in a controlled, measurable manner. It is important to measure the thickness of the fiber at the same point where the retardation is measured; that is, along the optical path your eye takes through the fiber.

14.5

Fluorescence microscopy

Many dyes used to color textiles have fluorescent components to them. These components, called fluorophores, and their response to certain wavelengths of light can be useful in comparing textile fibers. Not all textile dyes fluoresce but fluorescence comparisons should be performed regardless: If the questioned and known fibers both fail to fluoresce, that is another point of meaningful comparison. Fluorescence occurs when a substance is excited by specific wavelengths of light. A light of relatively short wavelength illuminates a substance and the substance absorbs and/or converts (into heat, for example) a certain, small part of the light. Most of the light which is not absorbed by the substance is re-emitted, which is called fluorescence. The fluorescent light has lost some of its energy and its wavelength will be longer than that of the source light. Certain dye combinations may produce fluorescence of a particular intensity and color, both of which should be noted during the examination. Fibers dyed with similar dyes should exhibit the same fluorescence characteristics, unless the fiber and/or dye(s) have been degraded by UV exposure, bleaching or some other similar means. It is important to consider these factors when collecting known samples. In a fluorescence microscope, the specimen is illuminated with light of a short wavelength, for example ultraviolet or blue, produced by placing a filter, called an excitation filter, between the light source and the sample. A series of wavelength filter are useful to cover the range of fluorescence seen in textile dyes. The resulting fluorescent signal, however, is weak compared to the strength of the illuminating light. Therefore, the excitation light must be filtered out by a filter placed between the specimen and the eye which is a barrier to all light except that emitted by the specimen. This barrier

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filter ought to be opaque to the excitation wavelength and transparent to the longer wavelengths to transmit the fluorescence. The fluorescent fiber is therefore bright against a dark, preferably black, background.

14.6

Conclusions

Identifying fibers in a forensic setting requires a knowledge of fiber manufacture, materials, and methods but rarely an application of standard textile industry methods. The goals, the mindset, and the materials are all different. In quality assurance work or in quality control problem solving, a forensic mindset may work better than that of the production laboratory because the questions are similar – what is this unknown, where did it come from, why is it here? Methods may be unique to one industry, to another, or they may overlap (Fig. 14.2) but that all depends on the goals and materials under study. Although forensic scientists routinely turn to the fiber manufacturing industry for information, it may be that the fiber manufacturers may benefit from forensic techniques and methods.

Manufacturing analytical methods

Forensic analytical methods Aftermarket taxonomy

Market taxonomy • company-product orientation

• end use

• supply web

• implicit rules on categories.

• explicit rules on categories.

• as used

Some sharing of methodologies, protocols Different approaches due to different goals – quality at lowest price for manufacturing, reconstruction and product tracking for forensics Analogous to experimental vs. historical sciences

14.2 Comparison of manufacturing and forensic methods of identifying fibres.

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14.7

273

References and further reading

AATCC (1996) Technical Manual, Research Triangle Park, NC: American Association of Textile Chemists and Colorists. AATCC (2007) Technical Manual, Research Triangle Park, NC: American Association of Textile Chemists and Colorists. ASTM, D 276-87 (1996) Standard Test Methods for Identification of Fibers in Textiles, Philadelphia, PA: ASTM. Carroll, G (1992) Forensic fibre microscopy. In: Forensic Examinations of Fibres. (Ed: Robertson, James) Ellis Horwood, New York, 99–126. Delly, JG (1973) Microscopy’s color key. Ind. Res. 15 (11, October), 44–50. Frushour, B and Knorr, R (1998) Acrylic Fibers, in M Lewin and EM Pearce (eds) Handbook of Fiber Chemistry, 2nd edn. Marcel Dekker, Inc.: New York. Ginzburg, C. (1979) ‘Clues: Roots of a Scientific Paradigm’, Theory and Society 7 (3), 273–288. Heyn, A (1952) Observations of the birefringence and refractive index of synthetic fibers with special reference to their identification. Text. Res. J. 22, 513–522. Hinsch, J (1983) The technology of the polarized light microscope. Fiber Producer 11 (3, June), 10–20 (6 pages). Johri, M and Jatar, D (1979) Identification of some synthetic fibers by their birefringence. J. Forensic Sci. 24, 692–697. Longhetti, A and Roche, G (1958) Microscopic identification of man-made fibers from the criminalistics point of view. J. Forensic Sci. 3, 303–329. Masson, J (ed) (1995) Acrylic Fiber Technology and Applications, Marcel Dekker, Inc.: New York. McCrone, W (1982) Microanalytical tools and techniques for the characterization, comparison and identification of particulate (trace) evidence. Microscope 30, 103–117. McCrone, W, McCrone, L and Delly, J (Eds) (1978) Polarized Light Microscopy. Ann Arbor Science, Ann Arbor, Michigan. 251 pages. McCrone, W, Delly, JG and Palenik, SJ (1979) The Particle Atlas, Chicago, IL: McCrone Associates. Palenik, S and Fitzsimons, C (1990) Fiber cross sections: Part I and Part II, Microscope 38, pp. 187–195 and 313–320. Perry, DR (ed.) (1985) Identification of Textile Materials, Seventh Edition, Manchester, UK: The Textile Institute. Raheel, M (1996) Modern Textile Characterization Methods. Marcel Dekker, Inc.: New York. Robinson, P and Bradbury, S (1992) Qualitative Polarized-Light Microscopy. Oxford University Press, Royal Microscopical Society, Oxford, UK. Rost, F (1992) Florescence microscopy, Volume I, New York City, NY: Cambridge University Press. Rouen, R and Reeve, V (1970) A comparison and evaluation of techniques for identification of synthetic fibers. J. Forensic Sci. 15, 410–432. Smith, B and Block, I (1982) Textiles in Perspective. Prentice-Hall, Inc.: Englewood Cliffs, NJ. Stoeffler, S (1996) A flowchart system for the identification of common synthetic fibers by polarized light microscopy. J. Forensic Sci. 41, 297–299.

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Thetford, A and Simmens, S (1969) Birefringence phenomena in cylindrical fibres. J. Microscopy 89, 143–150. Vakkari, J (2001) ‘Giovanni Morelli’s “Scientific” Method of attribution and its reinterpretations from the 1960s until the 1990s,’ Konsthistorisk Tidskrift 70(1–2), 46–54. Wind, E (1963) Art and Anarchy, Faber and Faber: London. Ziabicki, A (1976) Fundamentals of Fibre Formation. John Wiley and Sons: New York.

15 Identifying and analyzing textile damage in the textile industry W D SCHINDLER, University of Applied Sciences Hof, Germany

Abstract: Textile damage identification is in many respects a special application of fiber analysis. This chapter begins with an overview on the complex subject of textile damage in the textile industry. It then discusses the favored methods of textile damage identification and analysis, first in general and then according to the type of fiber. Key words: identification and analysis of damage to textiles; mechanical, chemical, biological and heat (thermal) fiber damage identification; main types, manifestations and causes of textile damage; methods of identifying textile damage, especially textile microscopy and infrared spectroscopy (IR, IRS); unwanted deposits on textiles, especially stains.

15.1

Introduction: importance of and reasons for textile damage analysis in the textile industry

The identification of fibers is an essential part of textile damage analysis. Without the knowledge of the type of fiber most demanding damage examinations, identifications and interpretations would not be possible. Only a few relatively simple tests, such as strength and abrasion loss or some uncomplicated faults of the production of yarns and fabrics may be discussed without the knowledge of the involved type of fibers and their corresponding specific physical and chemical behavior. Textile damage analysis is, like forensic investigations, a very interesting application of fiber identification. Beyond that, damage analysis is in some aspect a continuation, deepening and further development of the fiber identification. The first and main step of fiber analysis is the identification of the standard type of fiber. The second step could be the further investigation of the fiber subtype, e.g. fiber modification or, in special cases such as acrylic or elastane fibers, even finding out the producer (by the type of comonomers, additives and spinning process). The third step or level of fiber analysis could be the analysis of changes of well-known fibers by production and usage, the damage analysis. This chapter intends to give an overview on the broad subject of textile damage and the usual methods of textile damage identification and analysis. The concept of this chapter does not attempt an exact description 275

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of experimental details, but the references will be a help for those who are interested in more information. The analysis of damage to textiles is a fascinating special area of testing of textiles. It has significant practical relevance, considerable charm but also many difficulties. Determining the exact cause of damage can often be a real challenge. Those who carry out damage analysis need wide-ranging knowledge and some experience but also intuition and the ability to reason and weigh up evidence like a detective. Further requirements for successful analysis of damage to textiles and the knowledge and skills which successful analysts should have are: •

understanding of textile technology, including textile physics and textile chemistry • extensive knowledge of textile fibers, their conversion to yarns and fabrics, dyeing and finishing, making-up processes and typical usage • familiarity with the most important methods used in damage analysis, such as microscopy, chromatography, IR spectroscopy and thermal analysis • ability to get peripheral information, starting with the conditions used in producing, dyeing and finishing the damaged textile, including the processes and machines used, further processing stages, storage, transport and, where appropriate, usage. Analysis of damage to textiles is not usually an exact science, although it does use scientific methods. In many cases several different tests are necessary. Their results can sometimes be contradictory. These then have to be evaluated and weighed up against each other very critically, whereby comparison samples, experience with similar cases and information about the circumstances of the damage can be useful. In many ways this process is similar to a court trial when only circumstantial evidence is available but fortunately in the laboratory the damage can often be imitated and the evidence thus verified.

15.1.1 Practical importance of textile damage assessment and reasons for the analysis of causes of damage Textiles can become damaged during their production as well as during distribution and usage. Experience shows that consumers are less likely to make complaints than are those involved in the chain of textile production. For example, textile manufacturers often dispute with textile dyers and finishers about who might be responsible for faults, whereby this can usually be clarified quickly by means of a surface imprint as described in Section 15.3.8. As well as the question of who is responsible and therefore who has to bear the costs, it is naturally also of interest to know how the damage

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can be repaired and also how it can be avoided in future. In the case of large lots or continuous production it is often important to find out very quickly what the cause is in order to stop producing the fault as soon as possible. In this respect damage analysis plays an essential role in quality assurance. Lack of quality and subsequent complaints should not be underestimated as an image problem that they present for the supplier. Experts for damage analysis are thus often employed by well-known companies. In spite of modern process control, optimization and quality control faults cannot be avoided completely. They occur at all stages of textile production including storage and transportation. The experts for damage analysis at fiber, dyestuff and auxiliary agent producers, as well as those at testing and research institutes, still have their hands full in dealing with many cases of damage where the costs caused by the fault can be quite high. Some disputes about damage are settled by courts but more usually the customer and producer agree on a settlement. Depending on the importance of the business relationship, fair dealing and price discounts play an important role here. Occasionally unjustified complaints are also made in the hope of obtaining just these benefits. In order to remain in charge of investigations of complaints most of the West European producers of fibers, dyestuffs and auxiliary agents have their own testing laboratories with damage analysis experts. The alternative, namely to let testing institutes carry out this work, is usually rejected because, amongst other reasons, the companies would have to reveal too much detailed knowledge and also because it often takes too long. Some producers of textile auxiliary agents choose another, risky path. They take care of complaints simply by giving price reductions without carrying out their own laboratory investigations. Most textile companies do not have the personnel or equipment required for the clarification of complicated faults. On the other hand they do have the best insight into peripheral information required for damage analysis. However, when they hand over the fault to an institute or, more usually, to their suppliers of fibers, dyestuffs or auxiliaries for investigation they often only pass on part of this information. Analysis of the fault is then often regarded as technical customer service in order to promote and cement customer relationships. The intensity of these external analyses of damage then sometimes reflects that of the business relationship. Although there are no figures available on the economic importance of textile damage analysis, it can be roughly estimated from the expenses incurred by producers of fibers, dyestuffs and auxiliary agents in dealing with complaints and analyzing damage. These expenses for personnel and equipment are quite high, even though some decades ago they were even higher. The author knows of at least two dozen such laboratories in Germanspeaking countries, in industry and in testing and research institutes. If it is assumed that on average two or three employees with a corresponding

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annual budget of about 300 000 to 400 000 euros per laboratory are involved, this would amount to a total of up to 10 million euros just for Germany, Switzerland and Austria.

15.2

Main types, manifestations and causes of textile damage

Faults are defects which lower the value and usefulness of goods. Faults can be defined as any undesirable kind of deviation from prescribed requirements. Damage is the disadvantage arising from faults. Thus damage analysis is a wider-ranging term than analysis of faults. It is damage which leads to complaints and, as a rule, to demands to repair the damage or compensate for it. Damage analysis is an important part of quality control and contributes to quality assurance.

15.2.1 Main types of damage Mahall divided his book Quality Assessment of Textiles – Damage Detection by Microscopy1 entirely on the basis of types of damage, namely in chapters on: • • • • • • • •

chemical damage mechanical damage thermal and thermo-mechanical damage to synthetics streaks and bars in textile fabrics due to yarn differences and technological reasons causes of the formation of tight threads and their effects defects caused by deposits and encrustations on the fiber material other defects in the quality of textiles microbiological damage to fibers.

There should be added: • •

biological damage, especially to natural fibers and last but not least light damage.

Some explanations and examples are described in Section 15.4, as well as some possibilities of identifying these types of damage. Special types of damage, such as deposits (especially stains), streaks, barriness and biological damage are discussed in Section 15.5.

15.2.2 Manifestations of damage The manifestation of damage to textiles can vary widely. It ranges from obvious, for example, visible or easily recognizable defects to hidden ones,

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for example defects that are hard to detect or those that can only be detected later. It is the latter type which is generally the reason for giving guarantees. Perception and description of the fault are the first steps in damage analysis. Damage can be perceived visually, macro- and/or microscopically, often only with a specific type of illumination, such as reflected, oblique or transmitted light, or with a specific light source, for example UV or polarized light. Some faults are detected by other senses, usually in the form of a handle assessment, or they can be registered by measurements. Examples of the latter are color measurement, tensile and abrasion strength, extensibility, shrinkage and fastness properties. These technical properties can be supplemented by thermophysiological comfort properties and care requirements, where significant deviations from the agreed or specified values can be claimed as faults. Damage is manifested most commonly as stains followed by streaks and barriness. Other types of damage manifestation, such as differences in hue or depth of shade, unevenness of dyeing, lack of strength, deposits, abrasion and holes, occur with less frequency.2 It is important here to describe the fault as exactly as possible. All the typical characteristics and peculiarities, their frequency and possible regularity have to be noted and also whether they can be localized to individual fiber or thread systems. This makes it easier or even at all possible to determine the cause of the damage later.

15.2.3 Damage causes There are mechanical, thermal, chemical and biological causes of damage to textiles. They may be attributed to different causal agents, in particular to textile manufacturers, dyers and finishers, garment producers, distributors or consumers. Although there are cross-relations between all the above-named causes and causal agents, it is usual, for example, to attribute mechanical damage to the textile manufacturer and chemical damage to the dyer and finisher. In contrast to the types of damage manifestation, there is no especially typical cause of damage. In the statistical investigation mentioned above, based on 550 cases of damage, there were 81 different causes of damage where more than 10 cases were registered. Their proportion ranged from two per cent to a maximum of six per cent.2 Other sources also report a similarly wide range of damage causes with a relatively low percentage in each case.3 This enormous range of causes is typical for damage to textiles and makes its analysis considerably difficult. This statement is also applicable to the causes of the most frequent manifestation of damage, namely stains. Statistical analysis of 258 cases of damage

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with stains resulted in 29 types of cause each represented in more than seven cases.2 The most common types of cause gave frequencies in the range from seven to eight per cent. These are, for example, stains caused by mechanical and chemical influences, dyestuffs, grease or oil, silicone and dead or immature cotton.

15.3

Methods of identifying and analyzing textile damage

The methods of textile damage analysis are mostly similar or even identical to those of fiber identification: microscopy, spectroscopy and chromatography are very important for both applications. Each of these powerful methods is described in a separate chapter of this book. Therefore here only methods or aspects of methods that are of specific interest for textile damage analysis will be mentioned.

15.3.1 Chemical and physical assessment of textile damage Of the many possibilities for investigating damaged textiles, two main groups can be ascertained: chemical and physical testing. Corresponding to this, two types of textile laboratories are often to be found, namely chemical and physical testing laboratories. Each offers different advantages for damage investigation. Physical testing does not require any chemicals and often gives results which are easily interpretable and may allow a direct quality assessment such as ‘just acceptable’, ‘second choice’ or ‘reject’. On the other hand, with chemical testing, the cause of the damage can be ascertained, which is what is actually meant by analysis of damage. But this advantage often has to be paid for in the form of higher costs for personnel and equipment.

15.3.2 Procedure for textile damage analysis Unfortunately there are no hard and fast rules on how to proceed with textile damage analysis. The variety of cases and causes is too great for this. Nevertheless, some companies and institutes use their own preprinted forms with long lists of tests for this purpose. This has the advantage that none of the rare tests is overlooked but also the disadvantage of inflexibility and unnecessary work with certain types of damage. Such preprinted forms may be of help to less experienced testers but experienced testers tend to have their own specific procedures, depending on the case, and are often guided by their intuition. As a general rule preliminary tests are made, followed by more painstaking specific tests.

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The usual steps taken in an investigation are illustrated here using the example of stain analysis: 1. Manifestation of the damage with a description of the type, distribution and possible regularity of the stains. 2. Microscopy with increasing magnification, beginning with a magnifying glass or stereomicroscope and progressing to 300- to 1000-fold magnification. Use of different types of illumination and contrast, such as reflected, oblique and transmitted light, UV, polarization or fluorescence. 3. Preliminary tests such as solubility and staining tests. 4. Isolation of the substance causing the stains (for example, by extraction in a Soxhlet apparatus) and concentration of the extraction residue. It is recommended here to cut out the stained areas from the specimen and to extract an equivalent quantity of unstained material for comparison. If the stain is in the form of insoluble deposits, an attempt can be made to dissolve the fiber material and thus isolate the stain substance. 5. Comparison and identification, usually by means of thin layer chromatography and/or IR spectroscopy. Comparison is made with the blank sample (extract from unstained areas) and with authentic substances which could have caused the stain. If the stain cannot be extracted, IR spectra from stained and unstained areas can be compared and the spectra subtracted in order to identify the stain substance. The limits of detection of typical stain substances in IR spectra and a comparison of IR methods have been given in the literature.4,5 6. Reproduction, if possible, of the damage in order to verify the findings, for example with authentic stain substances on the same textile material, using conditions as close as possible to those used with the damaged sample. 7. Further verification, for example, if possible, by means of consultation with the persons concerned in the stage of production suspected of causing the damage. It is important here to consider alternatives and to test the plausibility of the findings critically. 8. Summary of the findings, discussion of the results. If the cause has not been clearly identified, the results should be formulated carefully and alternatives mentioned. Documentation including photographs and possibly samples. 9. If possible, hints should be given on how to avoid such stain formation in future and also on how to remove the stains (suitable solvents and procedures). The last steps in this stain analysis demonstrate that comparative samples and information from peripheral areas of the case can be particularly useful

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aids in damage analysis. An archive with similar cases of damage can also be very useful here. If the archive is very extensive, retrieval of information on all damage cases according to different criteria should be possible, such as type of textile and fiber, damage manifestation, cause of damage, method and procedure leading to clarification and, if available, the source and client for the analysis. In many cases, but unfortunately not in all, about halfway through these steps, or at least after the preliminary tests, the cause of the damage may be suspected and this may lead to a working hypothesis. This hypothesis then has to be either unequivocally verified or rejected on the basis of further tests. It is often hard to give up a hypothesis and look for a better one. Self-criticism and experience can be helpful here as well as literature searches and discussions with interested colleagues or other experts.

15.3.3 Preliminary examination The subject of preliminary examination is as varied as a later part of this section, namely miscellaneous methods. There may also be no clear-cut separation between the two. Preliminary examination is composed of simple tests, carried out in a short time and with little effort, which give the first clues for damage analysis. It is usual to begin with an exact visual examination, if possible in comparison to an undamaged sample. Notice should be made of any peculiarities in appearance. Sometimes abraded and raised areas, holes, thin places and pressure marks can be easily recognized without optical aids. With the use of a magnifying glass they can be seen more clearly and in more detail. The same is true for many visible deposits of foreign matter such as silicates, calcium and magnesium salts (for example, oxalate, phosphate), polyester oligomers and mildew spots. As a next step, easily determinable differences between the damaged sample and undamaged comparison samples can be sought, for example handle assessments, wetting behavior (TEGEWA drop test6) or pH value. The latter can be determined with moist pH paper or more accurately by adding a drop of liquid indicator or with a flat-bottomed pH electrode. The microscopic detection of acid residues by the formation of methyl orange crystals is described in Section 15.4.2. Simple tests of mechanical strength (stretching between the thumbs), crocking fastness tests (dry and wet) and, where appropriate, a test of wet fastness, for example in cold and then heated dimethylformamide, also belong in this group. After marking the damaged area, woven fabrics can be separated into warp and weft threads and knitted fabrics unravelled in order to investigate the isolated threads more thoroughly. Do the threads from the damaged

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area show differences in diameter, twist level or yarn composition? Are stray fibers discernible? Fiber identification Determination of the fiber type, including checking of the stated fiber type, is one of the most important preliminary tests. If no IR spectrometer is available, standard fibers can be most readily identified using the characteristic reactions according to Stratmann.7 For modified fibers or highperformance and specialty fibers the more complicated classification on the basis of solubility groups and their subdivisions7,8 or other methods of analysis9 can be used. Further preliminary tests Sometimes it can be appropriate to carry out preliminary tests for readily detectable elements or compounds, for example the Beilstein test for organically bound chlorine (green coloration of the flame when heating a copper wire with a small fiber sample in the non-luminous region of a gas flame). In this way fibers containing vinyl chloride can be easily recognized. A further example is the detection of iron in the combustion residue of a textile sample or directly on the fiber material. Hints on the presence of silicone deposits are given by the adhesive strip test (lowering of the adhesion due to silicone) and the foam test (marked formation of foam when the textile sample is shaken with chlorinated hydrocarbons). Detection by means of IR spectroscopy is mentioned in Section 15.3.6.

15.3.4 Microscopy Microscopy is certainly the most important method for damage analysis of textiles. Without microscopy and its supporting techniques the elucidation of most cases of damage would not be possible. The extensive literature on this topic also demonstrates the great importance of textile microscopy for textile damage analysis.1,7–26 A number of well-known textile microscopists who also worked intensely on elucidation of faults deserve mention here: A. Herzog (Vienna, Dresden), H. Reumuth (Jena, Mannheim, Karlsruhe), P.-A. Koch (Dresden, Zürich, Krefeld), M. Stratmann (Krefeld), N. Bigler (Basle), K. Mahall and I. Goebel (Düsseldorf) and G. Schmidt (Ludwigshafen). Many microscopic methods are used in the textile industry to investigate raw materials, for product development and analysis of competitors’ samples, to check production and control effects and quality. Textile microscopy is indispensable in dealing with complaints and analyzing damage as well as

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in avoiding faults and repudiating unjustified claims. A short survey on applications of stereo, compound and fluorescence microscopy, including cross-section and grinding techniques for textile damage analysis is described in Chapter 8 of Fan’s book Chemical Testing of Textiles.27

15.3.5 Chromatography, preferably thin layer chromatography With textile damage analysis the possibility of identifying the separated substances by comparing them with authentic samples is often as important as the separation itself. This identification is successful when the separation behavior in one, or preferably more, separation systems is the same and when additional findings such as the same staining or reaction behavior show that the substances are identical. A prerequisite for such chromatographic identification is that the identity of the substance is already suspected so that the relevant substances can be chromatographed at the same time for comparison. An even more fundamental prerequisite is that the substances to be analyzed are soluble in the mobile phase. Naturally it is also important that a suitable separation system is known or can be developed. Of the many chromatographic methods used in analysis the one preferably used in textile damage analysis is thin layer chromatography (TLC). The reason for this is that TLC delivers results quickly, simply and cheaply, with usually sufficient accuracy for elucidation of damage cases. Dyestuffs, optical brighteners, soluble textile auxiliaries and fiber finishes are especially suitable for TLC. Many pigments are also sufficiently soluble. Crosslinked finishes, fibers, coatings and other polymers are unsuitable. For the analysis of these insoluble or polymeric substances the more costly pyrolysis gas chromatography (P-GC) and variations of high performance liquid chromatography (HPLC) or, often simpler in the case of almost homogeneous polymers, infrared spectroscopy can be used. TLC is particularly useful for analyzing cases of damage with soluble stains or with oily or greasy soiling. Further examples of interest are TLC of carriers for polyester28,29 and polyester oligomers.30,31 Many low molecular weight textile auxiliary agents can be analyzed with TLC and this can be useful in elucidating certain cases of damage. Known examples are stains caused by carriers or softeners. Surfactants can be separated using TLC and identified by comparison with authentic surfactant.32,33 When mixtures of surfactants with different ionic forms are present it is recommended to separate them first with an ion exchanger. Ethoxylated products can be separated by TLC into the individual species up to an ethoxylation degree of 20 to 25. This results in an impressive row of spots like a pearl necklace

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and the size of the spots gives a rough idea of the distribution of the degrees of ethoxylation.34–36

15.3.6 Infrared spectroscopy Infrared spectroscopy (IRS) is often a useful supplement to TLC especially in the analysis of insoluble or macromolecular substances. However, with mixtures of substances the superimposed IR spectra are frequently so complex that they can hardly be interpreted. A previous separation, including that with TLC, is very useful for IR investigations. IRS is a particularly powerful method for damage analysis. With this method fibers, coatings and other deposits, textile auxiliaries and substances causing stains can be identified. Chemical damage to fibers can also be detected by means of specific structural changes. In damage analysis the composition of mostly liquid extraction residues is of particular interest. As well as qualitative IRS, quantitative applications are also available, where the determination of the concentration of dissolved substances, the blend ratio in fiber mixtures or estimation of the comonomer content in copolymers is possible. On account of its high energy flow and favorable signal to noise ratio modern Fourier Transform IRS (FT-IRS) enables the use of very low energy analysis methods such as IR microscopy37 and many interesting reflection methods including directed and diffuse reflection as well as attenuated total reflection (ATR). Namely single reflection diamond ATR is of particular interest. In the literature5,38,39 the advantages and disadvantages of the different IR sampling and measuring techniques suitable for textile applications, especially for damage analysis, can be found. The following examples serve to give an idea of the many possibilities for the use of IRS in investigating damage to textiles. The identification of fibers, including unwanted foreign fibers, of coatings, stains and other fiber deposits are the applications which spring most easily to mind. The IR spectra of polyester oligomers are almost identical with those of polyester fibers. The oligomers can be investigated directly with diffuse reflection or the diamond ATR method. Silicone stains still occur quite often and with the aid of IRS they can be readily and fairly sensitively detected by means of the Si-CH3 rocking band at 800 cm−1, several intensive Si-O-Si bands in the range from 1020 to 1120 cm−1 and Si-CH3 bending vibrations at 1260 cm−1. Table 15.1 lists the suitability of these bands for the detection of silicones depending on their superimposition with fiber bands.4 Other types of stains such as paraffins, sizes, softeners and carriers are somewhat more difficult to identify with IRS.4 On account of the intensive F-C main bands at 1200 and 1150 cm−1 fluorocarbon finishes can be detected by IRS at application levels of

286

Identification of textile fibers Table 15.1 Suitability of IR bands for identifying silicone stains, depending on the type of fibre4 Wave number (cm−1)

Cotton

Viscose

Wool

Nylon 6.6

Polyester

770–800 1020–1120 1260 2965

++ – + 0 (+)

++ – + 0

++ + 0 (+) –

++ + – –

0 (+) – – 0 (+)

Explanation of symbols: ++ indicates very good suitability (single, non-overlapping bands) + good suitability. 0 means that because of superimposition silicone can only be detected by the markedly higher intensity of the bands. – means that no increase in intensity of the superimposed bands is recognizable.

fluorocarbon polymer from about 0.3 to 1.2%, depending on the overlapping with fiber bands.5 When dealing with complaints or analyzing competitors’ samples concerning bonded nonwovens, the composition of the binder is of interest as well as the type of fiber. The binder can also be determined with IRS.40

15.3.7 Thermal analysis Thermal analysis (TA) is the comprehensive name for a group of analytical methods in which physical or chemical properties of a sample are measured as a function of temperature and time. TA has been indispensable in polymer technology and research for a long time. In the area of textiles it has increased in importance due to the fast-growing market segment of technical textiles, for example in quality control and in analysis of products, competitors’ samples and damage. In the other textile market segments, namely apparel and household textiles, TA is especially important in damage analysis. A database research using the keywords textile and TA (including the methods DSC, TGA and TMA, which are explained below) showed a marked difference in their relevance for textiles. By far the most important is differential scanning calorimetry (DSC), followed by thermogravimetric analysis (TGA) and finally thermomechanical analysis (TMA). Determination of melting point The determination of melting point as used for fiber identification can itself be included under TA, because here the phase change is determined as a

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function of temperature. It can be carried out very easily using a Kofler hot bench.41 The melting point or melting range can be determined in a few minutes with an accuracy of about ±2°. This is usually sufficient for fiber identification. With intimate fiber blends a melting point or hot stage microscope with polarized light is necessary. The melting behavior of all the types of fibers can be observed consecutively. Even if a fiber does not melt when heated up to 300 °C, for example, this can still be a useful piece of evidence for its identification. Differential scanning calorimetry With DSC not only the temperature range for melting (Tm) or for decomposition (Td) and, during cooling, that of crystallization (Tc) can be determined but also the corresponding enthalpies (heat of fusion Hm, heat of decomposition Hd and heat of crystallization Hc). Furthermore the characteristic temperature for the amorphous areas, the glass temperature (Tg), and the so-called effective temperature or middle endotherm peak temperature (MEPT) can be determined. By comparison of the measured heat of fusion with the theoretical value the purity or content can be determined. This is particularly relevant for fiber recycling. For damage analysis of textiles made from polyester the MEPT is especially interesting because it gives insight into the thermal prehistory of the fibers.42–44 The MEPT is the maximum temperature of a small endothermic peak between the small endothermic stage of glass transition and the large endothermic melting peak. The position of this MEPT peak is variable and depends, besides other facts, on the temperature of thermal pretreatments (Tp). Its size depends on the intensity, and thus mainly on the duration of the thermal treatment and also on the tension, for example during setting. The measured MEPT usually lies several degrees above the temperature of a preceding thermal treatment (MEPT > Tp). It gives useful information for damage analysis. For example, it is possible to determine from this temperature whether polyester goods were dyed at the boil (with carrier), under high temperature (HT) conditions or using the thermosol process. Conclusions about setting temperature are also possible, in particular differences in setting conditions can be determined exactly. Unfortunately, this thermal memory also has notable restrictions. It can be superimposed by mechanical influences; for example, differences in tension also affect the MEPT. The way in which the heat was applied (steam, hot air, conducted heat) also causes significant differences. Finally, a strong thermal influence should not have taken place subsequently as this extinguishes the memory for the weaker influence. The model of Jeziorny45 for explaining the phenomenon of MEPT helps to make these factors understandable. According to this model, small

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increasingly ordered areas, the so-called micro-crystals, are formed under increased heat input from initially unordered structures on the surface of the crystalline areas of the fiber. These then melt during the DSC measurement. If the sample is cooled after the first DSC measurement and then subjected to a second run, a MEPT peak is no longer found. Comparison of the first and second run thus makes it easier to find and interpret this thermal event. If the DSC instrument is sufficiently sensitive, a MEPT peak can also be found with nylon fibers. Its allocation to thermal pretreatments is, however, more difficult than with polyester because, among other reasons, the rate of crystallization is higher with nylon. DSC is also useful for characterizing bicomponent fibers, film-forming finishes and coatings. For example, with polysiloxane a very low glass temperature (about −120 °C) is characteristic, followed by a crystallization peak (at about −100 °C) and a large melting peak (−40 °C). As a matter of interest the enthalpies obtained by integration of these peak areas enable the crystalline ratio to be calculated: (Hm − Hc):Hm. This calculation is also possible with polyester, especially with granules, on account of their higher amorphous content. Thermogravimetric analysis With TGA the change in weight of the samples on heating is determined (usually possible up to 1000 °C). In a nitrogen atmosphere the decomposition of the sample can thus be studied, in air the ability to be oxidized is additionally determined. In this way fibers modified to be flame-resistant can be distinguished from standard fibers. In fiber composites, for example fiber-reinforced rubber, it is thus possible to determine the proportions of the components with relatively little effort: moisture and softeners in the first stage of weight loss up to about 220 °C, then the fiber and rubber components up to 500 °C and finally after changing from a nitrogen atmosphere to air the carbon used as a filling burns and above 700 °C the non-burnable inorganic filling remains. The first derivative of the weight loss curve, the derivative TG (DTG), enables a more exact determination. By coupling TGA with a mass spectrometer or a FT-IR spectrometer the decomposition products can be analyzed. Because of the higher costs such methods are only used in exceptional cases for textile damage analysis. Thermomechanical analysis TMA investigates the changes in the dimensions of a sample as a function of the temperature, for example shrinkage or extension of fibers.46 It is easier to work here with filaments than with staple fibers. Fiber composites and other materials are also analyzed with dynamic loading. This dynamic-

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mechanical analysis (DMA) enables, for example, the glass temperature of elastomers to be determined exactly. But in textile damage analysis TMA is seldom used.

15.3.8 Further methods There are a large number and variety of methods which can be used for damage analysis of textiles and these methods can naturally also be combined. It is economic restraint which most affects the imagination of the damage analyst. In other words, any method can be considered for damage analysis if it is or could be useful, does not cost too much and does not take too long. In addition to the important methods of damage analysis described above three further methods will be briefly described here. Techniques for surface imprints Imprint techniques have been a proven and important method in damage analysis of textiles for a long period of time. It is often advantageous not to investigate the original object under the microscope but rather the negative imprint of its surface: •

In an imprint it is often possible to see if a fault in a colored textile was caused during textile manufacture or during dyeing and finishing. Spinning faults, such as use of different fiber counts or differences in yarn twist, and faults in fabric production can be seen in the imprint as well as in the original (and in the same location). On the other hand, faults arising from dyeing or printing are eliminated in the imprint. • The imprint is transparent and the color of the sample does not interfere. Thus with dark-dyed wool fibers the cuticle scales can only be easily recognized in the imprint. The same applies to abraded places and other types of mechanical damage to the surface of dark dyeings. • Since the surface imprint is very thin (about 0.02 mm) the depth of focus is usually much better than in direct microscopy of the uneven, threedimensional textile surface and possible fiber lustre and transparency do not interfere. In direct microscopy with reflected light the image is usually not sharp because the fiber interior and the underside of the fiber also reflect light. • Since the transparent imprints are examined in transmitted light, it is not necessary to have a microscope with reflected light. In addition, the original sample remains unchanged. There are two different widely used imprint methods in damage analysis, namely imprints on gelatine-coated plates47 and on thermoplastic films,

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Identification of textile fibers

Table 15.2 Comparison of the advantages of the most important surface imprint methods Imprint with gelatine-coated plates

Imprint with thermoplastic films

No thermal influence on the sample No special equipment necessary

No swelling of hydrophilic fibers With a commercial instrument49 larger areas (approximately 20 × 30 cm) can also be tested Detection of grease, oil or wax deposits possible due to diffusion into and dulling of the film

No false indication of structural differences, arising from diffusion of grease, oil or wax deposits (possible effect on films)

usually polypropylene or polystyrene.1,48 In Table 15.2 the most important advantages of these complementary imprint methods27 are compared. Extraction methods A typical textile laboratory is characterized by several Soxhlet extractors standing in the fume cupboard. That is to say, the extraction of textile samples is a routine or standard procedure. During extraction, substances soluble in organic solvents or water are removed from the textile, then, as a rule, concentrated by distillation, and the extraction residue is analyzed qualitatively and/or quantitatively. Examples of extracted substances are stains, fiber spin finishes, lubricants, residual grease in wool, residues of surfactants and other chemicals such as acids, bases or thickeners, soluble finishes, dyes and optical brighteners, pesticides and other biocides, carriers, heavy metals, salts and formaldehyde. Stepwise extraction using solvents of increasing polarity (for example, first hexane, then methylene chloride, then absolute alcohol and finally water) can give a first indication of the nature of the extracted substances. Different extraction methods and apparatus can be used. They are especially useful for damage analysis. Some of the procedures are standardized.50 Mini versions of the Soxhlet extractor, are preferred for very small samples such as stains. As alternatives to the Soxhlet extractor, automated apparatus have been developed. The Morapex rapid extractor51,52 enables the test sample to be extracted non-destructively in a very short time with either water or organic solvents. Average degree of polymerization of fibers Many types of damage, including chemical, thermal, photolytical, biological and some types of mechanical damage, are based on degradation of

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the polymer chains in the fiber. Thus determination of the average degree of polymerization (DP) gives a direct scale for assessing the extend of such damage but not for its cause. The time and cost of determining DP is, however, so great that, whenever possible, simpler but less accurate methods are preferred. Examples of these are loss of tensile strength and abrasion resistance or the pinhead reaction with cotton. An advantage of DP determination is that it allows quantitative estimation of the damage.53,54 Of the many methods known for determining DP, measurement of viscosity according to Staudinger is the one preferred in damage analysis because it can be carried out in any textile laboratory. The viscosity is measured indirectly via the times taken for the polymer solution and the solvent to run through an Ubbelohde or Ostwald viscosimeter. A prerequisite is that a suitable solvent is available for the fiber and that the corresponding constants for the calculation are available. The solvent must not damage the fibers and it should be easy to handle. Sometimes, however, healthdamaging m-cresol has to be used (polyester, nylon). Schefer55 has listed solvents and viscosity constants for 16 undamaged fiber types. In order to give an idea of the other methods which are used for damage analysis1,56 the following examples are listed: • • • •

detection of metals, such as Fe, Cu, Ca, Mg, and nonmetals, such as N, P, S, Cl, F, which can help to elucidate the damage swelling and solubility tests, especially, but not only, with natural fibers determination of concentration, for example by means of titration, gravimetry or colorimetry staining tests which mark, for example, setting differences, oil and grease deposits or fungi. They generally have the disadvantage that the samples have to be undyed or only lightly dyed. It is time-consuming if the original dyeing first has to be removed in order for staining with test dyes to be carried out and there is also the danger that additional damage may occur during stripping of the dyeing.

Choice of the most suitable method is made more difficult if there is very little sample material available. An ideal method should be highly sensitive, reproducible and give clear-cut results. A common combination of methods, especially when analyzing stains, begins with microscopy, followed by concentration after extraction. The extract from an undamaged area serves for comparison. For identification the preferred methods are TLC (if authentic samples are available for comparison) and IR spectroscopy. Reference spectra are also very useful here but it is possible to identify the substances causing damage by direct interpretation of the spectra.

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Identification of textile fibers

15.4

Damage analysis according to the type of fiber

The extensive subject of damage analysis of textiles can be divided into typical cases of damage depending on the stage of processing or the technology of yarn and fabric production such as: • • • • •

filaments, threads and yarns57,58 woven fabrics59 knitted fabrics60–63 nonwovens textile composites, coated fabrics.

An additional group of damages occurs during cleaning operations, such as washing64 and dry cleaning. Although this division may be useful for many typical types of faults, a division according to the type of fiber appears even more suitable. In this way the types of damage typical for a particular fiber can be meaningfully grouped. For example, cellulosic, protein and synthetic fibers each have their own characteristic strengths and weaknesses, which are enlightening when analyzing damage to them. In the following section the most important methods of damage analysis will be mentioned for the respective types of fiber.

15.5

Damage analysis of cellulosics, especially cotton

As opposed to wool, cellulosic fibers are relatively stable to alkali but sensitive to acid. In addition, cellulosic fibers can be damaged by strong oxidizing agents, excessively high temperatures and microorganisms. The extent of this damage (the damage factor) can, among other means, be assessed by viscosimetric determination of the degree of polymerization. A much easier method is the following reaction.

15.5.1 ‘Pinhead’ reaction with cotton65,66 This rapid microscopic test indicates chemical damage to cotton and enables the degree of damage to be roughly estimated. The cotton fiber to be tested is cut with very sharp scissors or a razor blade to snippets of about 1 mm length and embedded with 15% sodium hydroxide on a glass slide. After covering the sample with a cover slip and leaving for 2 to 3 minutes, the formation of ‘pinheads’ at the cut ends is evaluated. In Table 15.3 the appearance of the ‘pinheads’ is described corresponding to different stages of damage and approximate ranges of the degree of polymerization.17,65 If the pinhead reaction demonstrates chemical damage the next point of interest is to determine the exact cause. The tests described below for damage due to chemicals or fungi are applicable to all cellulosic fibers.

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Table 15.3 ‘Pinhead’ reaction and damage to cotton17,65 Type

Formation of pinheads

DP range*

1

Undamaged

>1500

2

Clearly damaged

3

Heavily damaged

4

Very heavily damaged

5

Extremely damaged

Well-rounded pinheads on about ¾ of all fiber snippets Mostly flat protuberances with some semi-rounded pinheads Cut ends mostly flat with some flat protuberances All cut ends smooth with varying width of lumen, fibers partially convoluted Surface notches, longitudinal and transverse splits, fibrillation and marked deformation

1600–1000 1100–700 800–400 <400

* DP = Average degree of polymerization

15.5.2 Detection of acid damage with Fehling’s solution Since cellulosic fibers are sensitive to acids, they can be easily damaged by the acid catalysts used in easy-care, silicone, fluorocarbon and flameretardant finishes as well as by drops of concentrated acid or faulty dyeing of cellulose/wool blends. Carbonization of wool is based on this sensitivity of cellulose to acid. Acid damage is also known to occur in sulphur dyeings (cleavage of sulphuric acid during storage) and by contact between cellulose textiles with easy-care finishes and chlorinated bathing water or detergents containing chlorine bleach (chloroamines are formed, which decompose to hydrochloric acid and oxygen). The 1,4-glucosidic bonds in the cellulose chains are cleaved hydrolytically by acids. Cellulose damaged in this way, so-called hydrocellulose, is characterized by the shorter length of the molecules and thus a higher concentration of end groups. One end of the chain has a ring-forming hemiacetal structure in equilibrium with the open-chain aldehyde form. This chemical test for damage with Fehling’s solution is based on the higher concentration of aldehyde groups after acid cleavage. The formation of a red color due to precipitated copper (I) oxide indicates acid damage. In principle during this redox reaction the aldehyde groups of the cellulose chains are oxidized to carboxylic acid groups and the copper is reduced from copper (II) to copper (I). It is important here to compare with acid-damaged reference samples and undamaged goods, since the latter also show a slight red coloration. The aldehyde groups may already have been partially oxidized by atmospheric oxygen. Only a marked red coloration is a sure sign of acid damage. In cases of doubt this test should be repeated. The following reverse conclusion may

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Identification of textile fibers

also be useful here: if the pinhead reaction indicates chemical damage and the Oxycarmine test described below is negative there is a high probability that acid damage has occurred. The detection of aldehyde groups with ammoniacal silver solution, as recommended in the literature,56,67 is dangerous because the reagent can explode after standing for longer periods.

15.5.3 Detection of oxidative damage with Oxycarmine Cellulosic fibers are often oxidatively damaged when the most commonly used bleaching agent, namely hydrogen peroxide, decomposes catalytically. This unwanted decomposition to aggressive radicals is catalyzed especially by heavy metals. Traces of iron, for example (abraded metal, rust from water or steam pipes), are sufficient to cause severe catalytic damage during peroxide bleaching. The damage is manifested as a marked loss of strength or even formation of holes. Naturally, it is difficult to detect traces of iron around the edge of these holes. Oxidatively damaged cellulose, known as oxycellulose, is characterized by polymer chain degradation and an increased concentration of carboxylic acid groups. These are the basis for the chemical detection of oxidative damage. They bind basic dyes via salt links, whereas these dyes do not stain undamaged cellulose. A well-known example is the staining test with Methylene Blue.68 Staining with the test dye Oxycarmine69 is said to be four times as sensitive as with Methylene Blue66 and is specific for oxycellulose. The test dye has to be freshly prepared from two storeable components and can only be used for one day. The dyeing conditions are simple and explained in the product information. It is also important here to have comparative stainings on undamaged and oxidatively damaged goods in order to interpret the blue coloration correctly.

15.5.4 Damage by microorganisms Cellulosic fibres are more often damaged by fungi than by bacteria. The microscopic detection of fungi is based on the staining with Lactophenol/ Water Blue (see Section 15.5.3, detection of biological damage to wool). Storage of moist fabric overnight in a warm environment can be sufficient to cause fungal attack, leading to variously colored mildew patches, which are difficult to remove.

15.5.5 Faulty quality in cotton, in particular neps from immature and dead cotton Differences in fiber quality in one lot of cotton can lead to unlevel fabric appearance. These differences cause higher costs in dyeing and finishing and

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represent a difficult challenge for the dyer. They are also often the reason for customer complaints. Immature and dead cotton as well as the so-called hard or red cotton (with high concentrations of calcium or iron compounds) are not themselves typical forms of damage but are a frequent form of faulty quality with similar negative effects. There are many publications on the subject of neps and their detection, for example Furter and Frey70 and on the determination of the degree of maturity of cotton.71 Here, only the principle of the so-called red/green test will be mentioned, since it is easy to carry out in any textile laboratory1,56,72 and it gives a quick idea of the proportion of immature cotton. It does not correlate well with other tests of maturity.73–78 On the other hand, cotton samples which show the same degree of maturity after the usual maturity test with the air-flow method often show subsequent differences in depth of dyeing,74 thus underlining the importance of this dyeing test. The red/green test was introduced by Goldthwait, Smith and Barnett and is thus also known as the GSB test.72 It was developed for raw cotton. However, Denter et al. recommend previous alkaline scouring.74 If the red/ green test is used on yarns and fabrics neps become more clearly visible as small green spots. Using a mixture of a red direct dye with small molecules and a green direct dye about twice as large the mature, thick-walled cotton fibers are dyed mostly red and the immature, thin-walled fibers, but also very fine mature fibers, are dyed mostly green. Commercially available replacement dyes for those originally used are listed in the literature.27 It is of interest for damage analysis that the red/green test can also be used for the detection of differences in cotton, caused by alkali treatment or mercerization.79,80

15.6

Damage analysis of wool

15.6.1 Wool chemistry and types of damage The chemistry of protein fibers, especially that of wool, is more complex than that of most other types of fiber. The amide groups which link the repeat units are also to be found in nylon fibers. Amides can be cleaved hydrolytically by strong acids and bases. However, the inter- and intramolecular bonds in wool are particularly numerous. There are hydrogen and disulfide bonds, salt links and hydrophobic attraction. All of these bonds are important for understanding chemical damage to wool and the possibilities for its detection. Wool is particularly sensitive to alkali, but is also damaged by strong acids, reducing agents and oxidizing agents. Indications of wool damage First indications of possible damage to wool are a harsh handle or yellowing, both, where possible, in comparison to a wool sample before the potential

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Identification of textile fibers

damage. Heavy felting and change of shade in dyed fabrics are also signs of possible damage. Microscopic indications are usually more unequivocal, since the characteristic scale structure of wool fibers not only enables their identification and characterization but also analysis of damage to them. Changes in the scale structure can be seen particularly well in surface imprints (see Section 15.3.8). It is important here that several samples (at least 10, in critical cases up to 100) taken from different parts of the fabric are examined in order to gain a representative impression. Even in highquality wool fabrics some individual damaged fibers can be found. Easily recognizable signs of damage under the microscope are: • • • •

• • • •

abraded scale edges or other types of scale damage (mechanical damage, kinks, rupture) projecting, bulky scales (for example, after damage by acids, alkalis, chlorine or light) longitudinal striations in the interior of the fiber (damage due to alkali or bleaching) pore-like openings, especially visible in embedding agents with a similar refractive index to wool (nD 1.55, for example Canada balsam), caused, for example, by too heavy chlorination81 fibrillar delamination of the wool hairs with release of the spindle cells (heavy chemical or enzymatic damage) frayed fiber ends (brush ends, damage by tearing, possibly indicating shoddy wool) root ends and skin remnants (possibly indicating slipe wool) crescent-shaped bites (insect attack).

15.6.2 Staining tests Many dyestuffs have been described in the literature as staining tests for wool damage. Examples are: • • • • • • •

Methylene Blue/C.I. Basic Blue 9356 (0.1% aqueous solution, 3 minutes at room temperature), Neocarmin W1 and Neocarmin MS69 (5 minutes at room temperature), Cotton Blue/Water Blue B or Telon Blue AGLF(DyStar)/ Lactophenol,1,56 Indigocarmin (cold saturated solution and 40 ml/l sulphuric acid),82 Sirius Light Grey GG/C.I. Direct Black 77 and Sirius Light Red 4BL/C.I. Direct Red 79 (0.5% solutions, 3 minutes at the boil), Rhodamine B/C.I. Basic Violet 1068,81 (1% aqueous solution, 1 minute at the boil), Congo Red/C.I. Direct Red 28.83

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It is recommended to separate the sample into the individual fibers before the staining test and to treat these for 10 minutes in a solution of about 0.5% wetting agent. After staining the sample should be thoroughly rinsed. A common factor with these staining tests is that damaged wool usually takes up more dye. Most types of damage destabilize the wool structure so that deeper dyeings take place than with undamaged wool. Crosslinking wool protecting agents can reduce the amount of dye uptake. These staining tests should only be regarded as a rough indication of wool damage, compared with the methods described below. Furthermore, they are restricted to undyed or only slightly dyed wool.

15.6.3 The Pauly reaction as an important indication of damage The diazo reaction according to Pauly1,56 is especially useful in damage analysis of wool. It shows up mechanical, chemical and biological damage and demonstrates destruction or peeling off of the cuticle (scale layer). The Pauly reaction is based on the fact that the cuticle layer contains considerably fewer aromatic amino acids than the cortex layer lying beneath it. These aromatic amino acids, especially tyrosine, react with the Pauly reagent (diazotized sulphanilic acid) to form a red monoazo dyestuff. The intensity of the coloration thus obtained, ranging from yellow-brown via orangebrown to red-brown, corresponds as a rule to the degree of damage. A more detailed description is given in recent literature.27 By counting fibers under the microscope the Pauly reaction can be evaluated semi-quantitatively.56,81 However, Mahall1 has reported that after very heavy alkaline damage the color intensity of the Pauly reaction can decrease. For this reason also this test dyeing should always be combined with a microscopic examination. The Pauly reaction can also be carried out with dyed wool. However, the red coloration is difficult to see on deeply dyed wool, for example at the edges of the fibers.1 The Pauly reaction also works with other animal hairs. In addition it shows up hair roots and skin particles, which can sometimes be present in wool yarns.

15.6.4 The Krais, Markert and Viertel reaction for detecting acid or alkali damage Whereas the Pauly reaction is a useful indicator for every type of damage to the cuticle layers of wool the swelling reaction with ammoniacal potassium hydroxide according to Krais, Markert and Viertel (KMV reaction) enables the cause of the damage to be determined, namely whether damage due to acid or alkali is present.1,56,84,85 A further advantage of this swelling reaction is that it can also be carried out on dyed fibers. For example, fibers dyed by

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Table 15.4 Time until commencement of swelling of wool with KMV reagent56,81,84 Time until commencement of KMV reaction

Conclusions about type and extent of damage

1 to 2 minutes 2 to 6 minutes 6 to 10 minutes 10 to 12 minutes

Very heavy acid damage Acid damage Treated with acid but scarcely damaged Normal undamaged wool, depending on fineness and origin Possibly treated with alkali but without noticeable damage Treated with alkali, in most cases also alkaline damage

15 to 30 minutes More than 30 minutes

the Pauly reaction can be used here (after drying) so that this analysis of damage can be concentrated on the more heavily damaged fibers. Procedure for the KMV swelling reaction: the wool fibers to be tested are laid on a microscope slide, a few drops of the KMV reagent are added and the fibers observed under the microscope at intermediate magnification. The time is noted from the addition of the reagent to the appearance of the first small, hemispherical blisters on the fiber surface. The evaluation of the findings is given in Table 15.4. The irregular small protuberances slowly become larger until they finally break up. With coarse wool fibers the time taken for the appearance of the first blisters is longer than that for fine fibers. Again this illustrates that a comparison with undamaged wool and wool with defined damage is a useful aid to interpretation. In particular, the comparison of samples before and after a potentially damaging process enables a more exact explanation of the damage. When acid and alkali damage have taken place sequentially the results of the KMV reaction are not unequivocal. Long swelling times can occur without alkaline damage if the wool has been treated with aldehydes or other crosslinking agents (as fiber protecting agents). This qualitative analysis of acid and alkaline damage is complemented by quantitative tests as described in a sub-section below. Short swelling times in the KMV reaction are a sensitive and discriminating indication of acid damage. The longer swelling times after alkaline damage provide less discerning evidence. A further indication of alkaline damage is the particularly heavy staining with Lactophenol/Water Blue, but this is not specific, since other chemical, biological and mechanical forms of damage also cause deeper staining. Rolling up of the fiber ends to crosier forms or even more pronounced ringlet forms is, on the other hand, alone typical of alkali-damaged wool.1

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15.6.5 Detection of alkali and acid residues, including formation of methyl orange crystals A simple indication of possible damage can be obtained by measuring the pH value of the wool (liquid indicator, flat-ended pH electrode) or that of the aqueous extract, according to IWTO-2 or DIN 54275 and 54276, for example. The determination of the alkali content (IWTO-21) and acid content (IWTO-3, DIN 54280) are standardized methods.86 In cases where only small amounts of sample are available for damage analysis, microscopic investigations with methyl orange can be useful as an alternative to these methods of direct pH measurement. A few fibers on a microscope slide are covered with a cover slip, 0.1% aqueous methyl orange solution is then added from the side. In the presence of acid thin orange-colored needles are rapidly formed at the fiber surface. They consist of the free acid of methyl orange, which at pH values of 4 or below is hardly soluble in water.17,21 This elegant method of detecting acid residues is also applicable to all other types of fiber.

15.6.6 Detection of damage due to heat and light After longer periods of exposure to heat wool textiles can discolor above temperatures of about 120 °C. The allowable temperature for rapid pressing is about 160–170 °C.87 At higher temperatures and longer periods of time wool yellows. In strong light wool is at first bleached (photo bleaching) and then yellows (photo yellowing). This effect is intensified by additional thermal stress, for example with wool textiles in automobiles. UV absorbers lessen this damage. Alkali residues increase the extent of yellowing by heat. Further indications of this type of damage are markedly lowered peracetic acid/ammonia and urea/bisulphite solubility as well as lower cystine content. Alkali solubility and Methylene Blue uptake are increased slightly by dry heat treatments and markedly by damage due to light and moist heat.88 Peracetic acid/ammonia solubility enables a differentiation between damage due to dry heat (markedly reduced solubility) and due to light (increased solubility). Additionally this is said to be a good indicator for crosslinked wool (reduced solubility). The tryptophan content can also be used as an indicator for photodegradation.89,90 These quantitative methods are mentioned in a following sub-section.

15.6.7 Detection of biological damage to wool Bacterial damage to animal fibers is described more often than fungal damage.1,81,91 Dyed fabric damaged by bacteria, for instance, often has light spots or streaks which cannot be repaired. Typical observations on the

300

Identification of textile fibers

damaged fibers are intensified striations and splitting up into cortex cells. The fact that only the orthocortex has been attacked and the paracortex remains is characteristic.1 In order to differentiate from acid damage, in which the cortex cells can also be exposed, the KMV reaction, which is very sensitive to acid damage, is recommended. The extent of damage by bacteria can be assessed with the Pauly reaction. Bacteria can be stained with Lactophenol/Water Blue (as can fungi) and also with carbolfuchsin.91 However, the bacteria are often washed off during dyeing or scouring so that only the changes in the fiber, as mentioned above, remain as evidence. Mahall has described in detail the procedure and the problems of detecting bacterial damage.1 The same is true for fungal damage. Wool can also be damaged by insects. For example, the larvae of the clothes moth (Tineola biselliella), the furniture carpet beetle (Anthrenus flavipes) and the black carpet beetle (Attagenus) feed on keratin. Treatment with moth- and beetleproofing agents can largely prevent such damage. The damage caused by insects to non-treated wool can be easily recognized under the microscope.63 If fibers with crescent-shaped bites are found at the edge of holes, the holes can be said to originate from the furniture carpet beetle, which sometimes also leaves behind typical hairs. According to Peter, moth larvae usually bite off the end of the fiber and leave behind in the vicinity of the fiber end typical traces of a transparent, voluminous substance, which exudes from the mouth of the larva when feeding in the vicinity of the fiber end.17 Moth bites are sometimes also found on cellulosic and synthetic fibers, the fiber substance then being excreted without being digested. Excrement is further evidence of moth damage.

15.6.8 Quantitative analysis of wool damage The quantitative methods mentioned here enable the degree of wool damage to be estimated and sometimes also the type of damage. The principle of solubility tests is to determine the weight loss after an exactly defined treatment and to compare this with the weight loss of undamaged wool. The weight loss is always calculated on the basis of the dry weight of the fibers and given in percent. As a rule, the greater the difference to the weight loss of undamaged wool the more extensive is the damage. By analogy, this also applies to the methods where the content of certain amino acids (cystine, cysteine, cysteic acid, lanthionine) or the amount of dye uptake (for example, Methylene Blue) is determined. Here also the difference to the reference value for undamaged wool is used to evaluate the damage. Table 15.5 lists the most important quantitative methods for wool damage analysis. More complicated methods are only used for wool damage analysis in exceptional cases. They often involve the chromatographic separation of

Principle

Treatment with 0.1 N NaOH at 65 °C, 60 minutes

Treatment with a urea/ bisulfite solution at 65°C, 60 minutes

Oxidation of cystine with peracetic acid and treatment with 0.2 N ammonia

Method

Alkali solubility IWTO-4 (1) ASTM D-1283 (2) BS 3568 (3) SNV 195 587 (4) DIN 54281 (5)

Urea/bisulphite solubility IWTO-11 DIN 54279

Peracetic acid/ ammonia solubility SNV 195 586

Decrease after treatment with alkali, crosslinking agents or dry heat

Increase after treatment with acids or oxidizing agents; marked decrease after treatment with alkali or crosslinking agents

Increase after treatment with acids or oxidizing agents, also after steaming above 100 °C

Main conclusions

Table 15.5 Important quantitative methods for wool damage analysis88

90 to 86%

40 to 50%

12 to 17%

Value for undamaged wool

60 to 40%

about 5% or 80 to 90%, respectively

50 to 80%

Value with very heavy damage

Total hydrolysis, electrophoretic separation and colorimetry or FTIR preferably with ATR

Staining with Methylene Blue and colorimetry of the residual liquor

Cysteic acid IWTO-23 DIN 54286 and IR spectroscopy

Methylene Blue-uptake SNV 195 588

International Wool Textile Organisation American Society for Testing and Materials British Standards Institution Schweizerische Normen-Vereinigung Deutsches Institut für Normung Eidgenössische Materialprüfanstalt

Hydrolysis of wool with sulfuric acid, reduction of cystine with sulfite, colorimetric determination of the blue color formed with phosphotungstic acid

Cystine and cysteine content IWTO-15 EMPA-Methode (6)

(1) (2) (3) (4) (5) (6)

Principle

Method

Table 15.5 Continued

Global parameter, increase with almost all types of chemical damage to wool

Increase after heavy oxidation, for example with peroxide or chlorine

Decrease in cystine content after treatment with alkali, peroxide, reducing agents (not with acids), increase in cysteine content after reduction

Main conclusions

3 to 5 mmol / 100 g

0.2 to 0.5%

Cystine 11 to 12.5% Cysteine 0.3 to 0.4%

Value for undamaged wool

25 to 35 mmol / 100 g

3 to 5%

Cystine 6 to 8% Cysteine 1 to 2%

Value with very heavy damage

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the total hydrolysate of wool samples, which is available in automated form. Schefer92 names some exceptions to the rule that the results of quantitative wool damage analysis correlate well with mechanical textile tests, for example after crosslinking reactions.

15.6.9 Cumulative wool damage Damage to wool is often due to a combination of several causes, whereby according to Doehner and Reumuth81 the effects can be developed, added, multiplied or even increase exponentially. When more than one chemical acts on wool, as is usually the case in dyeing and finishing, damage can occur which would have been harmless if each of these chemicals had acted alone. This effect is known as cumulative damage. For example, intensive peroxide bleaching and dyeing in strong acid conditions can each be carried out alone in such a way that the performance of the wool is not lessened and other undesirable effects are also avoided. If, however, these treatments are carried out under the same conditions in sequence the wool suffers considerable damage. Sanger93 explained this by the catalytic effect of the cysteic acid formed during oxidation on the acid hydrolysis of the neighboring amide group. The proton from the cysteic acid is transferred via a hydrogenbond-stabilized six-membered ring structure to the amide nitrogen, which makes this amide group easier to hydrolyze.

15.7

Damage analysis of silk

Reports on faults with silk fabrics are more common than would be expected from their share of the world fiber market (about 0.1%). One reason for this is that silk is particularly delicate. Mechanical treatment, especially in the wet state (dyeing and finishing as well as household laundering), causes irreparably abraded areas, the so-called blanched places. Defibrillation and splitting off of the silk filaments causes incident light to be scattered diffusely. The silk luster is no longer present, the fabric appears dull and lighter. This is particularly noticeable with dark dyeings. The abraded fibers can be identified in direct microscopy and even better in surface imprints. The same is true for silk lousiness (exfoliation), which is not actually a case of damage but does represent a quality problem. Silk lousiness is the term for pills formed from fine fibrils which become separated from the filaments of the silk. They are laid bare during degumming and during further processing they are twisted together into pills.94 Again they represent an aesthetic problem especially with dyed fabrics. Mahall and Goebel have reported that the Pauly reagent as used in wool damage analysis can also detect mechanical and chemical damage to silk, although the silk fiber does not have a cuticle layer but only a thin fibroin

304

Identification of textile fibers

sheath.1,94 With silk the Pauly reaction thus has to be carried out for a short time and with cooling (1 to 2 minutes, ice-cooled). To help verify the results it is recommended to carry out comparisons on undamaged silk and on silk with defined damage. In addition, before this analysis is made the Bombyx mori silk has to be carefully checked to see if it is completely degummed, since residues of sericin are stained red similarly to damaged fibroin. Undamaged Bombyx mori silk is stained yellow. Degummed tussah silk is stained yellow-brown by the Pauly reagent and bleached tussah silk light orange, thereby indicating a slight degree of bleaching damage. Similar staining tests have been described for the detection of sericin and for controlling the degumming process.27,95 In all these staining tests the fibroin is colorless after rinsing and the sericin is stained red. Checking of the stained samples under the microscope is in all cases useful for verifying the findings. Since weighted silk is more easily damaged mechanically and the detection of weighting is an important aspect of quality control, this subject will also be mentioned briefly. Weighting with minerals, usually with tinphosphate-silicate, results in a light-colored skeleton residue of ash after burning. At higher levels of weighting the structure of the yarn or fabric is retained in the ash. Agster56 has described conventional chemical detection reactions for weighting. Silk can also be weighted by grafting-polymerization with methacrylamide,96 which can be detected by conventional chemical analysis or by IR spectroscopy.97

15.8

General types of damage to synthetics

As opposed to the natural fibers only a few simple test methods are known for damage analysis of synthetic fibers. On the other hand, the standard synthetic fibers are also generally much less heavily damaged by acids, alkalis and microorganisms than are cellulosic and protein fibers. But there are also typical faults and types of damage with synthetic fibers, as is briefly described in this section.

15.8.1 Thermal damage Thermal damage is one of the most frequent causes of complaints about synthetic fibers, especially if they have a relatively low melting point. It causes, amongst other things, hardening of handle, yellowing, loss of strength, uneven fabric appearance (light reflection) and dyeing behavior (spots, streaks and other types of uneveness such as warp splashes). Thermal damage can occur at many stages of processing. Examples are texturizing, setting, singeing, pressing and sewing. During texturizing the originally circular fiber cross-sections are usually flattened to polygons. When setting is

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at too high a temperature or for too long, the yarns are flattened at the interlacing points. During singeing of staple fiber blends with cellulosics or wool protruding synthetic fibers can melt to form small balls, which cause a hard handle and which dye more deeply in exhaust processes (small dark spots, deeper dyeing being caused by the high amorphous content and the decrease in relative surface area) and after continuous dyeing are lighter than the undamaged fibers (the diffusion time is too short for the melt balls).1 Pressing at too high a temperature causes flattening and bonding of thermally sensitive synthetic fibers. Thermal damage also occurs through friction, impact, striking, cutting or punching out during textile production and garment manufacture. Mahall has shown many typical examples of this.1 The most important methods of investigation for thermal damage are microscopy in longitudinal view (also with surface imprints) and on crosssections, dyeing tests and thermal analysis. Buchanan and Hardegree described the influence of heat and tension (for example, during drawing, texturizing and occasionally dyeing) on faults in yarns made of polyester, nylon 6.6 and polypropylene.98

15.8.2 Damage by light According to their chemical constitution and stabilization, synthetic fibers show differing degrees of sensitivity to light, for example aliphatic and especially aromatic polyamides are more sensitive and polyacrylonitrile fibers less so. Apart from some technical fibers, synthetic fibers are usually delustred with titanium dioxide. This catalyses photolytic degradation (photolysis), which can be recognized by an apparent coarsening of the grains of the delustrant. The delustrant pigments appear larger because they are surrounded by a sphere of degraded fiber substance with a different refractive index.21,22 Damage by light can usually be detected by fiber-specific reactions and also by non-specific effects such as yellowing, loss of strength and decrease in the average degree of polymerization.

15.8.3 Mechanical damage Mechanical damage to synthetic fibers can be just as easily detected under the microscope as with natural fibers. In the case of thermoplastic fibers mechanical damage is often accompanied by thermal damage. This causes deformations which can be clearly seen in the longitudinal view and in fiber cross-sections. The change in reflection of light in the damaged area interferes with the uniform appearance of the fabric. Hearle studied the structure of ruptured fibers (polyester, nylon, acrylic) under the light microscope and with electron microscopy and also damage caused by abrasion and torsional stress, the samples mainly coming from damage in use.99

306

Identification of textile fibers

Differences in drawing ratio, fineness or texturizing of synthetic fibers are often the reason for streaks and barriness in fabrics made from them. They can be identified by marking the threads, stripping off of the dye and redyeing, in which case they reappear at the marked places. These faults can usually be recognized under the microscope (surface imprints), differences in drawing ratio can be seen particularly well in polarized light.100

15.8.4 Chemical damage The chemical weak spots and the corresponding types of damage vary greatly with synthetic fibers depending on their structure. A general difference in chemical stability exists between fibers formed by polymerization and polycondensation. At extreme pH values polycondensate fibres are hydrolytically degraded, for example by cleavage of ester or amide bonds between the constitutive elements. The carbon backbone chain of the polymerisate fibers is stable to hydrolysis but sensitive side groups such as the nitrile groups of acrylic fibers can be hydrolyzed.

15.8.5 Microfibers Microfibers are especially sensitive to light damage, owing to their large specific surface. Light damage is intensified by the catalytic effect of some dyes and their light degradation products. Also a much higher dyestuff concentration is needed for microfibers, compared to the same shade on normal fibers. Fastness problems can also arise from residual size that is usually not completely washed off from the microfibers.

15.9

Analysis of damage to polyester fibers

Polyester fibers (polyethylene terephthalate, PET) are chemically relatively stable, damage due to acid or alkali is therefore rare. At extreme pH values, however, hydrolytic degradation occurs, for example as used in the alkaline titre reduction of polyester textiles. Thermal damage is more common, caused by setting, pressing, pleating or singeing fiber blends containing PET or thermally bonding nonwovens at too high temperatures. Another form of thermal damage is the thermal deformation due to heat of friction, which can occur during sewing and cutting or punching out. Mahall has described many examples of this.1 The damage due to excessive heat and mechanical effects, for example during primary spinning,101 secondary spinning102 or tension during setting, primarily causes structural differences in the PET fibers which lead to barry dyeings and shade differences across the fabric. As a test dyeing for detecting structural differences in PET C.I. Disperse Blue 79 (for example, Dianix Navy Blue NNG, Foron Navy S-2GL or

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Table 15.6 Validity of test methods for changes in structure of PET due to heat and tension106 Test method

Effect of heat

Effect of tension

Equipment costs

Effort

Dye uptake during HT dyeing* Residual shrinkage** (hot air) Tensile strength Elongation at break Work at 5–10% elongation Elongation at 200–300 cN Residual extension Critical solution time* (phenol) Density (graduated column107) Differential scanning calorimetry (DSC) Thermomechanical analysis** (TMA) X-ray (long period)

–

–

large

large

+

++

low

low

– – – – – –

– + ++ ++ ++ –

large large large low low low

low low low low low large

++

–

medium

medium

++

–

large

low

+

++

large

low

+

–

large

large

Effects of heat and tension: * simultaneously, ** separately. Validity: − none, + good, ++ very good.

Ostacet Navy Blue 2 GLS) has been recommended, for example for 30 minutes at 130 °C.103 Another test dyeing uses a mixture of a blue dye which shows up structural differences very markedly and a yellow dye which does not have this property.104 Differences in draw ratio in PET fibers can be analyzed by microscopic determination of their specific birefringence, whereby higher tension results in a higher birefringence whereas the influence of temperature varies.105 Section 15.3.7 described how the thermal prehistory of PET can be analyzed with the aid of the so-called effective temperature or middle endotherm peak temperature (MEPT) determined by differential scanning calorimetry. The term ‘effective temperature’ was introduced by Berndt and Heidemann.106 In this publication a review of many other testing methods for analyzing structural differences in PET is given, including an evaluation of how well they respond to the influence of heat and tension. Table 15.6 shows that the methods of thermal analysis are fairly well suited for this purpose. Photolytically damaged polyester fibers can be recognized under the microscope by means of the apparent coarsening of delustrant grains,108 as

308

Identification of textile fibers

described in Section 15.4.4, and by the fluorescence in UV light of the degradation products oxy- and 2,5-dihydroxy terephthalic acid.109,110 This fluorescence microscopy enables a differentiation between photo- and thermal damage, but only if optical brighteners do not superimpose on the weak fluorescence of the terephthalic acid derivatives.

15.9.1 Polyester oligomers as a cause of damage PET oligomers are low-molecular-weight side-products of the polycondensation reaction with two to about ten repeat units. These linear and cyclic oligoesters are possibly also formed during the melt spinning process. The total content of oligomers is 1 to 3%; 70 to 90% of this amount occurs in the form of the cyclic trimer cyclo-tris-ethylene glycol terephthalic acid ester, c(G-T)3. A large percentage of the cyclic trimer diffuses onto the fiber surface under high temperature (HT) dyeing conditions and from there into the dyebath. c(G-T)3 is practically insoluble in water up to 100 °C and at 100 to 140 °C only to the extent of 1 to 5 mg/l. Thus during dyeing and in particular during cooling down of the dyebath it can precipitate and deposit on the goods being dyed or on the interior surfaces of the dyeing vessel. The problems which thus arise are particularly great when dyeing loose fibers or yarns. These problems include dust formation, poor running properties, difficult spinnability and increased wear on thread guides and needles. With piece dyeings the oligomer deposits can cause light spots, especially due to a filtration effect on the HT beam dyeing machine (possible perforation imprints on the fabric roll),111 but less so with knitted goods in jet dyeing machines. In addition, the fabric then lacks brilliance, a cloudy greyness gives an uneven fabric appearance. Oligomers which adhere to the fiber surface particularly interfere (surface oligomers, as opposed to core oligomers). They are often mixed with spinning oils and other auxiliaries or with dyes and can be difficult to remove. PET oligomers still repeatedly cause complaints. The first indication of damage due to PET oligomers is often the appearance of the fault: light grey deposits, which are partially easy to remove mechanically and form dust. In addition, when an isolated sample is shaken in isopropanol a marble-like suspension with mother-of-pearl lustre is formed.112 Detection methods for PET oligomers, especially for c(G-T)3: • • •

IR spectra resemble that of the PET fiber; TCL detection, especially by comparing with authentic samples; melting range 305 to 327 °C, according to their purity (marked difference to PET fiber dust, which melts at 250 to 255 °C), possible confirmation by means of mixed melting points: a mixture of approximately equal parts of sample and authentic cyclotrimer should not show any marked depression of the melting point;

Identifying and analyzing textile damage in the textile industry •

309

microscopic detection by recrystallization on the microscope slide (dissolve in dichloromethane, c(G-T)3 crystallizes out after concentration by evaporation of the solvent112 or formation of crystals after melting the sample on the microscope slide.1 The crystals thus formed are usually hexagonal, sometimes also in the form of needles or star-shaped and they appear golden yellow to multicolored in polarized light.112

15.10 Analysis of damage to nylon fibers In terms of quantity the standard nylon fibers were overtaken by polyester fibers around 1975. The reason that the latter fibers are now by far the most important group of synthetic fibers has also to do with the fact that nylon 6 and 6,6 fibers are more sensitive to light and less stable to hydrolysis than polyester fibers. Nylon fibers also have a greater tendency to yellowing in heat, nylon 6 in particular being less stable to heat than PET. On the other hand, the abrasion resistance and bending strength of nylon fibers is very high. Hearle has described the microscopic analysis of fatigue appearance in nylon fibers.99 During extreme conditions of use and also during dyeing and finishing oxidative damage can occur, often intensified by heat and/or light. UV radiation damages aliphatic polyamides and especially the aramids. With nylon 6 and 6,6, acid damage and, less common, alkaline damage are usually the result of inappropriate dyeing and finishing treatments. At extreme pH values their amide bonds are cleaved hydrolytically. Comparison of the sensitivity to damage of the standard nylon fibers shows that nylon 6 is more sensitive to hydrolysis and heat whereas nylon 6,6 is more easily damaged by oxidation.

15.10.1 Detection of acid damage According to Bubser and Modlich113 acid damage to nylon fibers can be detected in a simple way by a staining test with Rhodamine B (C.I. Basic Violet 10, available from Aldrich, Fluka, Sigma). Depending on the degree of acid damage, staining of the sample ranges from colorless with weak damage via pale pink to dark red with heavy damage (Table 15.7). Nylon fibers damaged by heat, light or oxidation are not stained in this test.

15.10.2 Detection of thermal damage When treatments such as setting, pressing, pleating or singeing of nylon fibers are carried out at too high temperatures, and also during thermal bonding of nonwovens or during cutting and sewing, fiber deformation, fiber bonding and formation of melt balls at the fiber ends can occur, all of

310

Identification of textile fibers Table 15.7 Extent of damage and staining of acid-damaged nylon fibers with Rhodamine B113 Extent of acid damage

Staining

Loss of strength

No damage Weak damage Medium damage Strong damage Very strong damage

Colorless Pale pink Pink Red Deep red

0% About About About About

5% 15% 30% 70%

which can be easily recognized under the microscope. The melt balls usually dye more deeply114 and give rise to a hard handle. Setting differences in carpet yarns giving streakly dyeings can be rapidly detected by spectroscopy in the near-infrared region.160

15.10.3 Detection of damage by light Despite photostabilization during fiber production and, in some cases, additionally during finishing, damage by light (especially UV light) is fairly common with nylon fibers. The complex photolysis reactions are accelerated by delustrants as well as by contaminants within and external to the fiber and, among many other things, also by nitrous gases.115–117 These reactions cause yellowing and loss of strength and can be recognized under the microscope by the apparent coarsening of the delustrant grains.

15.10.4 Detection of damage by oxidation With a ninhydrin staining reaction a darker color than normal is obtained after oxidative damage.118 Using modified zinc chloride solutions (Frotté reagent I and II) nylon fibers can be distinguished and identified by the so-called Frotté reaction according to Koch19 (crenellated, finely structured transverse folds and cracks on the fiber surface, which gradually change to coarser structures and then dissolve). With solutions of zinc chloride, potassium iodide and iodine (zinc chloride-iodine reagent), modified according to Bubser and Modlich, the Frotté reaction only occurs after oxidative damage, but not after damage by acid, heat or light.113 This is described in more detail in Table 15.8 and is shown in Table 15.9 in a review together with other methods for detecting damage to nylon fibers. The conclusions which can be drawn from these microscopic methods are unfortunately often not exact enough and can only serve as a rough guide,66 probably also because in practice different types of damage are superimposed, for example light and heat or light and oxidation (photooxidation) or even all three causes of damage together.

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Table 15.8 Damage to nylon fibers and Frotté reaction with modified zinc chloride-iodine reagent according to Bubser and Modlich113 Type of damage

Loss of strength

Swelling behavior after 3 minutes at room temperature

None (gray fabric) Weak acid damage Medium acid damage Strong acid damage

0% About 5% About 15% About 35%

Very strong acid damage Heat set Overset

About 70% About 15% About 30%

Thermal damage Damage by light

About 60% About 40%

Oxidative damage

About 80%

Marked Frotté reaction Marked Frotté reaction Moderate Frotté reaction Marked flattening of the crenellations No Frotté reaction Marked Frotté reaction Marked flattening of the crenellations No Frotté reaction No Frotté reaction, dulling of the unswollen part Marked Frotté reaction

Table 15.9 Review of detection methods for damage to nylon fibers113,114,118 Damage due to

Acid

Heat

Light

Oxidation

Rhodamine B staining Swelling in modified zinc chloride-iodine reagent Ninhydrin color reaction Embedding in olive oil Staining with acid dye

+ –

– –

– –

– +

darker

lighter normal

lighter dull

darker than normal darker than normal

15.11 Analysis of damage to acrylic fibers Acrylic fibers (polyacrylonitrile, PAN) for the home furnishings and garment sector are one of the fiber types with the greatest range of variation in their commercial types, similarly to the elastane fibers. Two possibilities for variation in the manufacture of acrylic fibers are characteristic for this: the type and amount of comonomers added to make the structure more flexible and to enable dyeing with basic dyes and secondly the possibility for wet or dry spinning with different solvents. Additional special features are dope dyeing and gel dyeing. The resulting variety of types is a cause of many cases of damage due to mistaken types and dyeing faults. PAN fibers have good resistance to acid up to medium concentrations and somewhat poorer resistance to alkalis. Their biological resistance is good. PAN fibers are very resistant to nonpolar organic solvents, oxidizing agents and weathering. Their abrasion resistance is low because of a ten-

312

Identification of textile fibers

dency to fibrillation.119 Resistance to heat is also relatively low, above 150 °C they show yellowing, and, depending on the type of fiber, from 200–250 °C an exothermic cyclization takes place (discoloration from yellow to brown and then black, due to naphthyridine structures). This short review shows that with PAN fibers one can expect damage from heat, abrasion, polar solvents and effects of strong alkalis to be predominant. In addition, damage is also caused by too great or uneven shrinkage, the reason for which can be found in fiber manufacture, textile production or dyeing and finishing.

15.11.1 Thermal damage The influence of cold or hot stretching in combination with heat setting has been investigated with many physical and chemical methods, which are also suitable for corresponding damage analysis (amongst others porosity, density, strength, extensibility, differential thermal analysis, molecular weight, electron microscopy).120 By measuring the critical solubility time in common solvents (dimethylformamide, dimethylsulphoxide, dimethylacetamide) Gacén and Arias121 developed a simple test which shows differences in fiber structure and shrinkage behavior sensitively and accurately (the less the fibers have been shrunk the shorter the solubility time). This method was also used for development and endurance testing of technical textiles, including those made of PAN fibres.122

15.12 Analysis of damage to elastane (spandex) fibers Elastic fibers are usually elastane fibers, which according to ISO 2076 are made up of at least 85% by weight of segmented polyurethanes. In the USA they have the generic name spandex. Although the share of elastane fibers on the world fiber market is still less than 1% they have achieved increasingly greater importance in the last decades. In the meantime about half of all clothing fabrics contain elastane fibers and around 85% of elastane fibers are used for clothing. This increasing distribution is the first reason for the frequency of complaints relating to textiles containing elastane. The second reason is that they can be overstrained during production, dyeing, finishing and in use. This already suggests the third reason, namely the relatively high sensitivity of elastane fibers to certain types of damage. They are particularly susceptible to thermomechanical damage (by heat and tension) and are also attacked and degraded by hydrolysis (acids and alkalis) and photolysis (especially by UV light). When the conditions are not too aggressive they are stable to oxidizing and reducing agents, except for the heavy damage caused by chlorine. The urethane link, which binds the segments of the elastane repeat units, can be described as half ester and half amide of a carboxylic acid. The sta-

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bility of polyurethanes is thus similar to that of polyamides and polyesters. The latter are more stable because of their high crystallinity and compactness, which is reflected in their density (elastane 1.2 to 1.3 and PET 1.38 g/ cm³). As with all elastic substances elastane fibres are crosslinked at wide intervals. They consist of so-called soft and hard segments. The latter are the fixed points in the network, they determine elasticity (recovering force), setting and heat behavior. The hard segments are highly crystalline and contain urethane and, in some cases, additional urea groups, both groups form strong intermolecular hydrogen bonds (crosslinking via secondary valency). The soft segments are the flexible structures with little order, located between the fixed points, and they are responsible for the extensibility. With molecular weights in the range from 1000 to 3000 they are relatively large123 and consist mainly of aliphatic polyethers (such as polytetrahydrofuran) or aliphatic polyesters. The former are more stable to hydrolysis whereas the latter are more resistant to oxidation (for example, chlorine or photooxidation). The hard segments contain mainly aromatic structures which, as with the aramids, increase cohesion between the chains because of their mutual attraction. On the other hand, because they absorb UV light strongly, they contribute greatly to damage by light. This review is intended to show the main structural weaknesses in elastane fibers. They are the fiber class with the greatest variation of types. This can lead to mistaken identity and thus cause faults if the specific properties of each type are not given sufficient attention during processing.

15.12.1 Mechanical damage Elastic multifilament yarns are less damaged during sewing than monofils because usually only a few of the primary filaments are damaged by the needle and the remainder is sufficient to hold the yarn together. Although the extensibilty at break of elastane fibers is 400 to 800% mechanical damage can occur with overstretching. This does not necessarily mean that a tear occurs, large residual extension (set) and lower recovery forces can be reasons for complaint caused by overstretching. Drastic damage includes thread breaks due to too high mechanical stress during knitting or weaving. A tear in the elastane core of a covered elastic yarn is referred to as a core break. It occurs particularly often with knitted fabrics; with woven fabrics loss of elasticity is the greater problem in practice.124 This type of damage is easy to recognize visually. Mechanical overstraining and fatigue (ageing) can be analyzed by mechanical testing methods, such as stress/strain behavior and hysteresis curves.123 From their experience with many complaints concerning elastane Gähr and Lehr recommend that conclusions about the cause of damage should not be made from the hysteresis behavior of an elastic thread isolated from the fabric.124 They justify this with the extension

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limit of 300% in the test and with the fact that stresses which occur before the actual damage takes place can have a large effect on the hysteresis behavior.

15.12.2 Thermal damage Depending on the type of elastane fiber noticeable thermal damage can occur when the fibers are heated above about 170 °C. Mainly yellowing accompanied by loss of strength and elasticity occurs. Under the microscope deformed elastomer fibers can then frequently be seen and others which have taken on the contours of the covering component or adhere to it.124 The temperature range for softening of elastane fibers lies between 170 and 230 °C, depending on the type, and for melting and commencement of degradation between 230 and 290 °C.123 Water lowers the bonding in the hard segments (secondary valence bonding by means of hydrogen bonds); hydrothermal treatments therefore cause more damage than dry heat at the same temperature. Additional tension also increases the thermal damage considerably (thermomechanical damage). Especially if undamaged material is available for comparison, damage by heat can be investigated by means of the stress/strain behavior mentioned above or with thermomechanical methods of analysis: •

An elastane yarn set under excessive extension has a stress/strain curve lying to the ‘left’ of the curve for undamaged yarn, it has lower tenacity and much lower extensibility.124 • By determination of the hot breaking time, this is the time taken for an elastomer thread stretched by 100% to break at 193.5 °C. • By determination of the heat distortion temperature (HDT); this is the temperature at which an elastomer thread under a pretension of 0.2 mN/tex and heated at a rate of 20 K/minute reaches an extension of 0.25%.123 The HDT differs greatly depending on the type of elastane fiber and often lies between 170 and 190 °C, with melt-spun fibers it can be lower. In order to avoid damage the setting temperature should not noticeably exceed the HDT of the respective elastane fiber type.124,125

15.12.3 Damage by light (photolysis) In spite of the incorporation of stabilizers, elastane fibers are frequently damaged by light, especially when it has a high UV component (for example, when wearing sport or bathing textiles outdoors). Photolysis causes discoloration as well as loss of strength and elasticity or even fiber breakage. The UV stabilizers can be partially washed out during dyeing and finishing.123

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Photolysis can be accelerated by oils and skin creams as well as sebaceous oils. The latter effect was demonstrated by Küster and Herlinger with model substances (squalene and linoleic acid methyl ester) on the basis of the decrease in relative viscosity of the damaged elastane fibers dissolved in dimethylacetamide. Perspiration, on the other hand, is said not to accelerate photolysis.126

15.12.4 Chemical damage Acids: elastane fibers are stable to dilute mineral and organic acids at room temperature. At higher concentrations and higher temperature damage occurs including dissolution. The structural relationship to nylon fibers becomes apparent here, as it also does in respect to dyeing behavior. Alkalis: at room temperature, elastane fibers are astonishingly resistant to alkalis. According to Hueber127 it is possible to causticize and mercerize cotton/elastane blends at low temperature. At the boil damage occurs with more than 2 g/l of soda. Reducing and oxidizing agents: under the usual conditions of dyeing and finishing elastane fibers are relatively resistant. The limits for peroxide bleaching in blends with cotton are said by Naroska128 to lie at pH 11 and 100 °C. The resistance to chlorinated water in swimming pools is said to be good,123 on the other hand cases of damage have been reported here. Chlorine bleach causes heavy damage as does ozone. Exhaust gases, especially nitrous gases (NOx) cause damage by yellowing and loss of strength. Yellowing can also occur when nitrous gases from stenters directly heated with gas react with spinning oils, fiber lubricants and knitting oils used with elastane fibers, if these have not been completely removed by scouring.129 Oils and fats such as mineral oil, paraffin, wax, unsaturated fatty acids (spinning oils125), cosmetic oils and sun protection agents are absorbed by elastane fibers and can lead to loss of strength and elasticity due to loosening of the fiber structure. Some of these products also accelerate photolysis.126 If dry-cleaning is carried out carefully with the usual solvents such as perchloroethylene or benzene, no damage occurs to elastane, except when stabilizers to light are extracted.123 Highly polar organic solvents such as dimethylformamide, dimethylacetamide, cyclohexanone, butyrolactone and phenols damage elastane fibers due to swelling or even dissolution.

15.12.5 Further types of damage, sources of faults and their detection Many of the types of damage already mentioned are worsened in combination. Known examples are photooxidation and thermomechanical damage.

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Table 15.10 Test methods for damage to elastane fibers123–125,130 Method

Comments

Stress/strain behavior Also as a simple manual method for determining elongation at break Hysteresis or tensile-elastic behavior

For all types of damage, especially for analyzing mechanical and thermal damage. Flat form of the curve shows loss of elasticity, decrease in tensile strength and elongation at break. Loss of elasticity and residual extension (set) with almost all types of damage. During textile production modified to such an extent that this method is not recommended for analyzing damage in dyeing and finishing or in use.124,125 Testing of the temperature dependence of the extension and shrinkage behavior at low (TMA) or oscillating (DMA) yarn tension: glass temperature, elasticity and other parameters for exact differentiation of elastomeric fibers. The HDT is the temperature at which a pretensioned elastane thread at a defined rate of heating reaches a certain extension (usually around 200°C). Suitable for determining heat setting conditions and for testing thermal damage. The time taken for an elastane thread stretched by 100% to break at 193.5°C (usually >20 seconds), especially suitable for analysis of thermomechanical damage. Quantitative analysis of chain degradation and degree of damage possible. Used for analysis of photochemical damage,126 also suitable for analyzing other degradation reactions, for example with acids, alkalis, chlorine, exhaust gases or heat.

Thermomechanical analysis (TMA) and dynamic mechanical analysis (DMA) Heat distortion temperature (HDT)

Hot breaking time

Relative viscosity and average degree of polymerization

Resistance to ageing also results from a mixture of many types of influence, for example mechanical, thermal and chemical influences such as atmospheric oxidation and the detergents and cleaning agents commonly used in laundering. As already described for wool, elastane fibers also appear to suffer from cumulative damage. For example, wet processing treatments during dyeing and finishing which normally have tolerable effects lead to noticeable fiber damage after intensive presetting.125 Dyeing of polyester/ elastane blends, often also with a wool component, is relatively problematical. Elastane fibers are damaged above 115 °C (as is also wool) causing softening of the elastane and loss of elasticity.124 The alternative, namely to dye with carriers at lower temperatures, is also difficult because many carriers can cause swelling of elastane fibers and reduce thereby their elasticity. During heat setting of piece goods the elastane fiber component is irreversibly damaged by too high temperatures and tension together with

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excessive duration (stretching without recovery).124 Recommendations for finishing treatments for elastic textiles have been published, for example by Naroska128 and Hueber.127 A frequent cause of faults are silicone stains on textiles containing elastane. During primary spinning elastane fibers require 2–6% of spinning oils,123 this being 6 to 8 times more than on other yarns. These oils contain a large percentage of silicone oil, which in a normal pre-scour (without special detergents) is only partially removed. This is particularly a problem with cotton/elastane blends because cotton also retains a large amount of silicone. The silicone residues assist the thermomigration of dyes, which can lead to poor crocking fastness and the dyes can deposit as stains on the fabric. Silicone stains are often first noticed after coloration and are difficult to remove. Their detection is described in Section 15.3.6. Table 15.10 gives a review of the methods commonly used for analysis of damage to elastane fibers. They usually require a relatively large effort or high costs. There is a disproportional relation between the many possibilities for damage to elastane fibers and the small number of simple detection methods known and suitable for this purpose.

15.13 Analysis of damage to polyolefin fibers, especially polypropylene Of the two polyolefin fibers polypropylene (PP) and polyethylene (PE) polypropylene has by far the greater importance. PP is the second most important man-made fiber type after polyester and continues to grow at a remarkable rate. PP fibers are relatively cheap. According to Schmenk et al.131 they are very resistant to acids, alkalis and organic solvents at room temperature. Damage can be caused by oxidizing substances, such as chlorine bleach and concentrated nitric acid at higher temperatures, as well as hydrocarbons and chlorinated hydrocarbons above 100 °C (swelling and dissolution). Their abrasion resistance is high. Tensile strength and extensibility can be varied within a wide range during fiber production. A great disadvantage of polyolefin fibers, which is particularly noticeable with PE fibers, is the large amount of deformation under stress, so-called creep. This is also the reason for their low degree of elastic recovery after compression, an important property with carpets. Further weaknesses are their low resistance to heat and light, especially UV light, which can lead to loss of strength. Heat stabilizers can increase the temperature for long-period thermal resistance from the usual 80 °C up to as high as 125 °C. Stabilizers to light and UV enable PP textiles to be used outdoors. Unmodified PP fibers cannot be dyed with the usual dyeing methods. Dope dyeing with pigments gives high fastness but is only economical for large quantities. For analysis of damage

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it is important to know that PP fibers exist in many further modifications, such as: •

• •

those based on Ziegler-Natta catalysts (ZN-PP) or on metallocene catalysts (mPP), the latter having a more uniform chain length and being more highly isotactic with a melting point about 15 °C lower; as microfibres and hollow fibers; antimicrobial, flame-resistant or antistatic modifications.

Recently elastic polyolefin fibers have also been introduced (generic name: lastol), they are crosslinked and stable at temperatures up to 220 °C and above.132 High-tenacity polyethylene fibers, with ultra high molecular weight (UHMW-PE, Dyneema) have been available for some time; they are produced by a gel-spinning process at high dilution. This review serves to illustrate that PP fibers are fairly commonly damaged by heat and light, by mechanical overstraining and long periods of strain and by oxidation, including photooxidation. PE fibers are even more sensitive to heat than PP fibers, but are less damaged in principle by light and oxidation. This last point is difficult to generalize because their behavior is strongly dependent on the type and amount of added stabilizers to light and oxidation.

15.13.1 Mechanical damage to polyolefin fibers Polyolefin fibers have relatively good abrasion resistance. Their tensile strength and extensibility can be varied over a wide range by means of the chain length and draw ratio. An extreme example is high-tenacity UHMWpolyethylene Dyneema, which is used, amongst other things, for bulletproof vests. Mechanical damage to polyolefine fibers can occur at many stages during processing and also in use. Sewing damage to PP knitted goods has been investigated by Wang et al.133 Mechanical damage during needling of PP nonwovens has been described by Qian and Chu, who investigated the dynamic creep behavior and ageing of PP geotextiles as a function of the type of bonding of the web.134 Residual extension and deformation after longer periods of strain are also typical types of mechanical damage to polyolefin fibers. Mechanical damage can be detected by the usual physical testing methods and under the microscope, preferably in the form of surface film imprints.

15.13.2 Thermal and thermomechanical damage to polyolefin fibers The low temperatures for the softening ranges of PP (150 to 155 °C) and especially for PE (105 to 120 °C) are the reason for many cases of damage

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caused by excessive heat, often combined with mechanical stress, as can occur in heat setting, sewing or pressing. According to Chidambaram et al.135 the loss of strength in PP fibers during thermal bonding of nonwovens is due more to the temperature than to the mechanical strain. With the aid of the methods for thermal analysis (DSC, TGA, TMA) polyolefin fibers and their modifications can be easily identified. The melting range enables, for example, differentiation between LD-PE and HD-PE as well as ZN-PP and mPP. By comparing the measured value for the latent heat of melting with the theoretical value the purity of raw and recycled material can be determined. The degree of crystallinity can also be evaluated in this way. The stability to oxidation of PE fibers can be determined with isothermal DSC at 200 °C by measuring the time (oxidation induction time OIT) until commencement of oxidation (onset of the exothermal reaction). Buchanan and Hardegree investigated the influence of spinning conditions on the shrinkage behavior of PP fibers by means of TMA.98 With PE a thermal memory has also been noted, an effect which is particularly interesting for the analysis of damage. Thermal treatments cause a so-called melting gap, usually just before the DSC melting curve reaches its maximum. This effect has been explained by the fact that during thermal treatments the amorphous areas of the fiber form crystallites with a sufficiently high melting point. After complete melting of the fiber this thermal prehistory is erased, so that a comparison of the curves for the first and second run of the DSC can improve the validity of the interpretation. Thermal degradation of PP fibers can be analyzed quantitatively by viscosimetric determination of the chain length in decalin at 135 °C. More common is the determination of the melt flow index MFI, which can be carried out in an automated form.131 The fact that marked damage is possible by thermolysis is demonstrated by the decrease of the chain length of PP granulate or PP chips to about half their initial value during melt spinning of PP fibres.136 The molecular weight of PP fibers is given as 150 000– 600 000, but usually 200 000–300 000;131 this corresponds to an average degree of polymerization of 3600–14 300, or usually 4700–8300.

15.13.3 Damage by light and oxidation to polyolefin fibers, including photooxidation The relatively high sensitivity of PP fibers to light and oxidation arises, amongst other reasons, from the fact that radical intermediate products are energetically favored. The methyl groups on the tertiary carbon atom are weak electron donors. They thus stabilize free electrons on the tertiary C atoms. Cleavage by radicals of the C-H bonds of the tertiary C atoms is thus favored. The necessary activation energy can be supplied by heat (thermolysis), light (photolysis) or by reaction with free radicals. In the presence

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of oxygen peroxide radicals are formed at the tertiary C atoms in the chain. These react with other tertiary C-H groups, forming hydroperoxides and new PP radicals. The hydroperoxides decompose with chain cleavage, whereby carbonyl and alkene structures are formed.137 Heat stabilizers are radical catchers, for example sterically hindered phenols or phenol-free compounds. Phosphites, for example, which reduce the peroxides, are used as antioxidants. Hindered amine stabilizers, HALS, can be used as UV stabilizers.131 The above-mentioned types of damage are manifested by chain degradation and yellowing, accompanied by brittleness and loss of strength. They can be detected and analyzed by the appropriate methods described in the preceding sections. However, loss of fiber strength often does not correlate with viscosity or chain length. Martin explains this by the supposition that with damage by light the amorphous areas are preferentially degraded.138 Pezelj et al.139 have investigated damage to PP fibers by ozone and light, whereby low concentrations of ozone sufficed to cause brittleness, loss of strength and increase in hydroperoxide content. Unfortunately there are not enough simple methods for the detection of damage to polyolefin fibers. On the other hand, there are also not so many cases of complaint, based on damage to PP and PE fibers, which are difficult to analyze, although these fibers are increasingly present in all three segments of the textile market.

15.14 Special types of textile damage and their analysis The selection of special types of damage causes described here is restricted to the investigation of deposits on fibers, especially stains, the detection of the causes of streaks and barriness, and to biological damage.

15.14.1 Analysis of unwanted deposits on textiles, especially stains Since these deposits are usually not distributed evenly on the fibers and textile fabrics they often consist of more or less large stains, spots or streaks. They are one of the most frequent causes of damage. Deposits of lime or PET oligomers show up as greyness on white fabric or light-colored structures on dyed fabric. The identification of PET oligomers is described in Section 15.4. Lime is soluble in acid and can be washed off with, for example, acetic acid or sequestering agents. Calcium ions can be detected by precipitating them as oxalate.56 Mahall has described a simple microscopic detection method for lime.1

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Detection of oil, grease, paraffin and wax deposits These hydrophobic substances can often be marked and detected by staining with oil dyes.1 Nowadays are available Sudan Red 7B and C.I. Solvent Red 27 as Oil Red O, both from Aldrich and Fluka. Oily deposits can generally be distinctly seen on account of this coloration. Even more sensitive than these staining tests for detecting grease and oil contamination is an imprint on thermoplastic films (see Section 15.3). During production of the imprint hydrophobic deposits diffuse into the film and can usually be easily recognized by the local cloudiness they thus cause. The natural waxes of cotton do not interfere here because they are evenly distributed. Spots caused by pigments or disperse dyes are also transferred onto the film imprints and are then easier to investigate microscopically.1 In addition, grease, oil, waxes and paraffins can be detected by IR spectroscopy, either by a direct comparison of spectra from the stain and from unstained areas, with the possibility of subsequent subtraction of spectra, or by spectroscopy of the extraction residue (after extraction and concentration). In the latter case it is also recommended to compare extracts from a stained area and from a similarly sized area without stains. Long alkyl chains are characteristic for these compounds, which can thus be identified, for example, by the intensive C-H bands at about nearly 3000 cm−1. Apart from these stretching bands intensive deformation bands at about 1500 cm−1 and a weaker band at 720 cm−1 are also found, the last one being characteristic for a chain structure with more than three methylene groups. The extracted fats and oils can also be analyzed more exactly by thin-layer chromatography. An indication for oil deposits is given by fluorescence in UV light,140 on the fabric (if it has not been optically brightened) as well as in the extraction residue and on the TLC plate.

Detection of unwanted film-like deposits This kind of deposit interferes by causing, for example, a harsh handle, dye reservation or other optical effects, and also chalky streaks when the fabric is scratched. Typical causes are size residues, printing paste thickeners which have not been washed off or unevenly distributed finishing agents. They can usually be identified with the aid of film imprints, since they often show up in the form of flat cakes or crumbly deposits.1 Film-like deposits usually cause a somewhat blurry appearance of the surface imprint, for example blurred scale structures in wool. Size residues can be detected by color reactions on the fabric or in an extract (combined with precipitation reactions).141 An advantage of the imprint method here is that the textile fabric can be investigated without being separated into individual fibers, which

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would destroy film-like deposits. However, deposits can sometimes be better analyzed by means of staining tests and the preparation of crosssections of yarn or fabric. Mahall1 investigated fiber adhesion and size residue (including distribution of size and over-sizing) in this way. Detection of other deposits in the form of stains The reasons for the occurrence of stains and their chemical compositions are numerous. This complicates their analysis. A typical method of approach is explained in Section 15.3.2. Löffel28 has described the comparative selective extraction of stains with subsequent identification, preferentially with TLC and IRS. In Section 15.3.6 information is given on the identification of silicone stains and fluorocarbon deposits using IRS. The detection of silicones with fluorescence microscopy is described in the literature.142 Schindler et al.4 have published a comprehensive review of the relevant literature and of types and causes of stains formed during production, dyeing and finishing of textiles (see Table 15.11). In this review the fiber-dependent limits of detection by IRS of stains caused by mineral oil and paraffin, sizes based on polyacrylate, fabric softeners and polyester carriers are described. Stains which arise during textile usage are often easier to analyze because the circumstances of their occurrence is mostly known or is fairly easy to determine.144 Illing-Günther and Hanus have described a stain analysis with microspectrophotometry.25 As well as the most common form of stain, namely that caused by deposits of foreign substances, there are two further kinds. One occurs due to localized effects of chemicals which modify the fibers in such a way that they reflect light or take up dye differently, for example splashes of caustic soda on cellulose. In addition, residues of the chemical which caused the stain can sometimes be detected directly on the fabric or in an extract. With the third type of stain as a result of mechanical influences the local reflection of light is modified in such a way that a manifestation of damage in the form of a stain occurs. This can best be detected under the microscope, for example by starting with a stereomicroscope and different types of illumination. Identification of the substances which caused the stain is usually the prerequisite to determining who is responsible, who carries the blame and how best to repair the damage. Optimal removal of the staining substance requires knowledge of the type of fiber involved in order to avoid further damage to the textile during stain removal. As a rule small individual stains are removed by stain removing agents. With larger or more frequent stains dry cleaning or scouring, depending on the type of stain, is carried out with possible addition of surfactants, sequestering agents, enzymes, acids or bases.

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Table 15.11 Types of stains and the processing stage where they occur4 Occurrence

Type of stain and cause

Production of yarns and fabrics

Oil and paraffin stains (usually wet paraffinizing), often together with abraded metal, which darkens the stain Small spots due to fly and clumps of foreign fibers Residues of size (usually widely distributed on the warp, blurry warp streaks) Residues of sizing auxiliaries such as paraffins, oils, waxes, fats, softeners and smoothing agents Preserving agents (often inhibit enzymes) Silicone stains from antifoaming agents Acid and alkali stains Antifoam stains based on mineral oils or silicones, sometimes also containing silicic acid Stains caused by PET carriers and oligomers Precipitation of auxiliaries with opposite charges Lime and phosphate deposits Spots due to undissolved or precipitated dye Stain-like lighter dyeing due to air bubbles in wound packages Stains due to drips of water or chemicals (change in dye affinity) Silicone stains (often darker, from softening, stretch or hydrophobic finishes and antifoaming agents) Softeners, hydrophobic agents, flame retardants and other finishing agents which have precipitated due to faulty treatment conditions, usually colorless and uncommon In addition to soiling, stains due to yellowing, caused by antioxidants in plastic films and cartons together with nitrous oxides in the air (combustion engines) and cationic substances143

Pretreatment

Dyeing

Finishing

Storage and transport

15.14.2 Detection of the causes of streaks and barriness in woven and knitted fabrics Streaks and bars are second only to stains as one of the most common manifestations of damage. They occur in numerous forms,145 for example: • • • •

parallel or oblique to the warp or weft direction with a repeat pattern or irregularly in bands or bars running along short or long sections of thread or across differing numbers of wales or courses.

The cause of the fault can usually be clarified here with the aid of a microscope and film imprint. The causes are as numerous as the forms the

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faults take. This is illustrated by the 24 relevant examples in Mahall’s book1 and the 10 examples in Goebel’s publication on the formation of streaks.145 As a rule streaks and bars are caused by faults in textile production. Examples for this are: • • • •

mistaken material, usually use of the wrong yarn differences in yarn count, yarn bulk, yarn twist, thread tension, plying, pile opening, hairiness, inhomogeneous blends faults during texturizing or mercerizing with pile fabrics more deeply incorporated tuft rows or differences in needling.

Faults arising from dyeing and finishing are also known: • •

•

wet abrasion and other types of mechanical damage in jet dyeing machines plaiting-down faults in cotton pretreatment: squashed fibers, notches, cracks and splits in the fibers which occur when the goods, swollen with alkali, are packed down too densely greasy deposits and resinated mineral oil, which have a carrier effect on polyester, leading to deeper dyeing.

15.14.3 Detection of biological damage As well as damage to wool by the larvae of clothes moths and carpet beetles, microbiological damage to fibers is of interest here. This damage is usually caused by fungi and, less commonly, by bacteria. Bacterial damage to wool is known to occur, it causes fiber degradation and an unpleasant odor. Bacteria often live in symbiosis with fungi on fibers. Both types of microorganism can feed on natural fibers and many types of textile auxiliary based on natural substances, for example sizes, spinning oils, fabric softeners, starching agents and stiffening agents, printing paste thickeners and other types of digestible agents. Synthetic fibers are not completely resistant to microorganisms, for example elastane fibers and polyurethane coatings can be damaged by them. Humidity, warmth and time favor microbial damage. It leads to loss of strength and occasionally to mildew stains, unpleasant handle, odors and loss of color. Microbial damage frequently occurs after lengthy transport of goods packed when damp or containing size, or when damp fabric is stored overnight or over a warm weekend in a textile dyeing and finishing mill. Antimicrobial treatments can prevent such damage but it still occurs repeatedly in practice. Musty-smelling mildew stains are often a first indication of fungal attack. They occur particularly frequently on cellulosic textiles and their color varies depending on the type of fungus from black to olive green, reddish

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brown to orange and yellowish brown. According to Nopitsch146,147 actual detection of the fungus is best made under the microscope by staining with Lactophenol Blue reagent. The Cotton Blue dye which gave this reaction its name is no longer available. Mahall1 recommends the still available substitute Telon Blue AGLF (DyStar) and in addition he mentions a 0.5% solution of Methylene Blue as a possible alternative. In this book1 there are many well-illustrated practical examples of fungal damage and also three examples of bacterial attack on wool. Bacterial damage to wool is also favored by warmth, humidity and time. A neutral to weakly basic environment supports bacterial growth, low pH values inhibit it. Level souring-off of the fabric is the simplest method of protection against bacteria when wool has to be stored moist for longer periods of time. With wool not only the fungi and bacteria cultures are stained with the Lactophenol Blue reagent but also the damaged areas of the wool are clearly and specifically more deeply stained. This is helpful in damage analysis when the actual microorganisms have been washed out during scouring. In bacterial damage, longitudinal striations first appear on the wool fiber and the spindle-shaped cells of the orthocortex are then laid bare. This results in a characteristic appearance for bacterial damage.1 Only after further, more extensive damage are the spindle cells of the paracortex laid bare, since these contain a greater concentration of stabilizing disulphide links. Since wool is also fibrillated by acid damage, it is recommended to differentiate from bacterial damage by carrying out the KMV reaction with ammoniacal potassium hydroxide (see Section 15.4). Macroscopically, stains caused by bacterial attack appear lighter because the fibrillation leads to a greater scattering of incident light.

15.15 Sources of further information and advice A very useful book, in which many typical cases of damage are described and illustrated, has been published by Mahall.1 In this book he has summarized his years of experience in damage analysis and his many publications in such a way that readers receive valuable stimulation for their own work. Further books which may be of assistance in damage analysis include those from Hearle et al.,10 Agster,56 Stratmann,7 Greaves and Saville,11 the Textile Institute12 and Fan.27 Chapter 8 of Fan’s book gives much more detailed information on the subject of this chapter here, including experimental details. Most of the articles on damage analysis published in journals are not recent.13–15,88,148–154 This is also true of company brochures on this topic.16,17,155 In the selection mentioned here, it is the last article which is cited if it is part of a series of articles. Citations for previous articles in the series can be obtained there.

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TESS, an expert system for textile damage analysis Another particularity of textile damage analysis is the expert system TESS. This abbreviation stands for ‘Textiles Experten-System für Schadensfälle’ (Textile Expert System for Cases of Damage). It will be briefly described here because this gives an idea of the complexity and problems of damage analysis on textiles. TESS was developed from about 1993 by the Eidgenössische Materialprüfungs- und Forschungsanstalt (EMPA) in St Gallen, Switzerland, in close cooperation with a dozen project partners and has been in industrial use since 1998.156–159 TESS is a Windows-based diagnostic system for all stages of textile production including dyeing and finishing. The knowledge gained from numerous experts was continuously structured and implemented in a knowledge base. This consists of a network of about 2000 nodes. The initial nodes of the network are five simplified manifestations of damage (stains, streaks, holes, surface differences and differences in handle). These are further subdivided according to size, direction, frequency, color and position of the fault. For an exact determination of the cause of the damage further investigations are requested, for example observation of the fault in reflected and transmitted light and possibly UV light, surface film imprint, determination of whether the fault runs parallel to the threads, determination of count and fastness or extraction. In the form of a dialogue TESS suggests stepwise further tests, delineates the area of possible causes and, if successful, names the cause of the fault and ways to repair it and avoid it in future. Further advantages of TESS are that it supports and relieves experts during damage analysis, and that it is especially useful in training new staff. It is always available, it considers very many possibilities and notes the steps taken (transparent logic). It serves to preserve the specialized knowledge of experts who retire and sometimes enables shorter diagnosis times and earlier recognition of the cause of faults. Disadvantages of TESS are that until now it has mainly been successful with faults arising from textile production and it appears to be limited in its suitability for the numerous types of damage connected with textile dyeing and finishing. In spite of the large amount of work invested in its development much more experience has to be included. Since this continuing effort appeared to be too time-consuming and expensive, further work on this difficult project ceased in 2002. In the long run TESS was not able to cope with the enormous variety of cases of damage in textile dyeing and finishing, the complexity of the manifestations of damage and, in particular, their causes. On the other hand, this emphasizes the importance and illustrates the performance of experienced damage analysts.

15.16 Conclusions The previous section again shows the great variety and complexity of damage analysis on textiles in general and textile-chemical damage analysis

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in particular. These challenges correspond to the demands made on damage analysts in terms of broadly based, thorough knowledge, great experience and the right combination of logical and intuitive approaches depending on the problem at hand. In addition, these experts require many kinds of information, not only concerning the particular case of damage but also on approaches and solutions to similar cases. Private collections of cases of damage, study of the literature and exchange of ideas with colleagues are always helpful. Useful ideas (sometimes generated just by aside comments), that help in interpreting one’s own work can arise while reading the many published cases of damage in practice. Well-known experts have described in these publications their often very individual approaches based on their particular experience in analyzing textile damage. Occasionally they are honest enough to confess that in spite of much effort a case could not be solved under the given circumstances.

15.17 Acknowledgment The author wishes to thank his colleague, Professor Elizabeth Finnimore, for her careful translation.

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97 Schindler W and Wiesel U, ‘Nachweis der Methacrylamid-Erschwerung von Seide’, Melliand Textilberichte, 1999, 80, 546–548. 98 Buchanan D R and Hardegree G L, ‘Thermal stress analysis of textile yarns’, Textile Research Journal, 1977, 47, 732–740. 99 Hearle J W S, ‘Fibre fracture and textile durability – part 2’, Textiles Magazine, 1999, 28, 15–21. 100 Bigler N, ‘Ein Beitrag zur Erkennung von Fehlern in Flachzwirngarnen’, Textilveredlung, 1966, 1, 152–156. 101 Nanal S Y, ‘Problems occuring in manufacture of polyester staple fibre’, Journal of the Textile Association, 2001, 62, 49–52. 102 Topf W, ‘Fehler bei der Verarbeitung von Chemiefasern’, Textil Praxis International, 1983, 38, 641–644. 103 Zimmermann R, DyStar, personal communication, 2003. 104 Schaich B, ‘Bestimmung der Fixierunterschiede von Polyestergewebe, vorzugsweise durch Polarisationsmikroskopie’, Undergraduate Thesis, University of Applied Sciences at Münchberg, 1993. 105 Nettelnstroht K, ‘Schadensuntersuchung im Textilbetrieb mit dem Polarisationsmikrokop’, Melliand Textilberichte, 1983, 64, 918–921. 106 Berndt H-J and Heidemann G, ‘Fehlerursachen und Erkennungsmethoden von Farbstreifigkeit in Polyester-Material’, Deutscher Färber-Kalender, 1972, 76, 408–479. 107 Wiley R E, ‘Setting up a density gradient laboratory’, Plastics Technology, March 1962, and Österreichische Chemiker-Zeitung, 1965, 66, 65–72. 108 Weber R,‘Lichtmikroskopische und elektronenmikroskopische Untersuchungen an lichtgeschädigten Polyesterfasern’, Textilveredlung, 1970, 5, 703–708. 109 Valk G, Kehren M-L and Daamen I, ‘Über die Photooxidation von Polyäthyle nglykolterephthalat-Fasern’, Angewandte Makromolekulare Chemie, 1970, 13, 97–107. 110 Day M and Wiles D M, ‘Photochemical Degradation of Poly(ethylene terephthalate). III. Determination of Decomposition Products and Reaction Mechanism’, Journal of Applied Polymer Science, 1972, 16, 203–215. 111 Nettelnstroth K, ‘Mikroskopische Fehlererkennung an Textilien’, Textil Praxis International, 1983, 38, 572–574, 668–671. 112 Anonymous (probably Höhn W), Oligomere – Phänomen, Analyse und Minimierung von PES-Olgomeren, Firmenschrift Dr. Th. Böhme, Geretsried, 1997. 113 Bubser W and Modlich H, ‘Erkennung und Unterscheidung von Schäden an Polyamidfasern (Perlon und Nylon)’, Textil Praxis International, 1959, 14, 1041– 1043 and 1152–1158. 114 Goebel I, Cognis/Henkel, personal communication, 1994. 115 Kratzsch H and Hendrix H, ‘Zur Unterscheidung von Licht- und Säureschäden bei Polyamidfasern’, Melliand Textilberichte, 1964, 45, 1129–1133. 116 Reinert G, ‘Zur Photostabilität der Polyamidfaser’, Melliand Textilberichte, 1988, 69, 58–64. 117 Bever M, Breiner U, Conzelmann G and von Bernstorff B-S, ‘Protection of polyamide against light’, Chemical Fibers International, 2000, 50, 176–178. 118 Küppers H., ‘Textiluntersuchung – nicht aufwendig’, Textilveredlung, 1974, 9, 67–71. 119 Gries T, Rixe C, Steffens M and Cremer C, ‘Faserstofftabellen nach P-A Koch: Polyacrylfasern’, Melliand Textilberichte, 2002, 83, 795–816.

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16 The role of fibre identification in textile conservation P GARSIDE, University of Southampton, UK

Abstract: Accurate identification of fibres is vital to textile conservators. Such knowledge, in conjunction with an understanding of the properties and usage of textile materials, will inform conservation, display and storage strategies. It may further help to annotate biographical detail concerning the origins of the textile and related history. Microscopy has traditionally afforded the principal means of identifying historic fibres, alongside simple chemical and physical tests. However, these latter techniques are destructive and can require relatively large samples, considerations which are particularly problematic when dealing with fragile and valuable cultural artefacts. Consequently, they are gradually being superseded by instrumental analytical methods which are noninvasive or require just microsamples, such as spectroscopy and chromatography, and advanced microscopic techniques often combined with sophisticated computational analysis. Potentially these approaches can reveal even more about the constituents and may offer clues as to their state of preservation. While the intricate construction and multimedia nature of artefacts often make these investigations particularly challenging, and aesthetic and ethical considerations add another complicating dimension, nonetheless successful material characterisation is essential to the well being and continued enjoyment of our textile heritage. Key words: fibre identification, textile conservation, fibre microscopy, fibre spectroscopy.

16.1

Introduction

The ability to accurately identify fibres is vital to textile conservators, and will influence the way in which an artefact is both understood and treated. Knowing the composition of an object will inform conservation strategies, enabling aspects of particular concern to be highlighted, and to determine not only those treatments that are suitable for the item but also – and perhaps more importantly – allowing those that are particularly inappropriate to be avoided. This knowledge will also inform handling, display and storage decisions. Furthermore, the identity of the fibres and other components in an artefact will help to confirm its provenance, and will bring to light areas of alteration or earlier conservation work; similarly, the information can be used to distinguish original pieces from more modern reproductions. 335

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Traditionally microscopy, alongside chemical and physical tests (such as stains specific to certain fibre types, or burn and solubility tests), have been used in textile conservation, as these require little equipment or training and are generally inexpensive, although they do have many limitations. The value of microscopy means that it is unlikely to be displaced, but the other techniques are now being superseded by analytical methods such as spectroscopy, chromatography and X-ray analysis, which provide more detailed information with less intervention. An important factor that will influence the choice and scope of investigative techniques is the availability of resources. Although modern analytical instruments are available to the larger museums and institutions, many conservators will not have access to equipment more sophisticated than a simple microscope, which will necessarily limit the range and detail of information that can be determined. The analyses are further constrained by the considerations of cost, time and training. In order to use any of these analytical methods effectively, and to be able to accurately interpret the results, it is necessary to have more than an understanding of the techniques themselves – an appreciation of the history of textiles, of the changing uses of materials over time, and of their chemistry and composition is also important. This knowledge will necessarily influence the choice of conservation treatments – of the best methods of cleaning, washing, reshaping, consolidating and supporting as object – and will help to ensure that none of these processes cause further damage. In this regard it is also important to understand the difference between conservation and restoration – although the terms are open to interpretation, in general conservation refers to the process of limiting the effects of past damage and ensuring that future deterioration is prevented as far as possible, with the minimum of intervention, whereas restoration involves returning an object to its original (and often functional) state. Expressed in these terms, the two processes can be considered as the extremes of a continuum, and both are influenced by the requirements to display objects, to stabilise them for long-term storage, and the availability of suitable techniques, materials and equipment. In textile conservation, fibre identification may be complicated by ethical considerations that are not necessarily encountered in other fields: is it possible to take samples from an object without damaging it, either physically or aesthetically, or jeopardising its long term stability? Is the information obtained from the analysis likely to be of sufficient value to justify the interference? Is it acceptable to take samples at all, even if doing so will not cause damage? These concerns have led to an increasing interest in the potential for non-invasive, non-sampling methods of fibre characterisation. This chapter will describe those techniques which are of particular use to textile conservators, and demonstrate, with reference to specific case studies,

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the ways in which this knowledge can then be used to inform conservation strategies.

16.2

Analytical techniques

16.2.1 Sampling As noted above, the ethics of sampling are a significant concern in textile conservation. Ideally, no samples would be taken at all, and all analyses would be carried out in situ, using non-invasive techniques. Obviously this is impractical in many cases, of course; in these situations, a compromise must be sought, balancing the benefits that can be derived from analytical sampling and the extent to which this will inform conservation strategies, with the necessity of intervening to the minimal possible extent. Where samples must be taken, they should preferably be removed as fibres or fragments of yarn which are already detached from the bulk object due to pre-existing damage. If this is not practical then it may be possible to sample from seams, hems or internal areas which will not affect the aesthetic properties of the item. Sampling from obvious, undamaged regions should always be avoided.

16.2.2 Microscopy Microscopy is usually the initial method of characterising the fibres and other components that comprise an artefact.1–4 An initial, relatively low magnification survey of an object can readily be carried out with stereomicroscopy, which can highlight areas of particular concern and interest, which will also give an indication of the general composition and weave structure, and does not require samples to be taken (Fig. 16.1). A more detailed examination of particular components can then be performed with techniques such as transmission and reflectance microscopy, if it is ethically acceptable for samples to be taken. Light microscopy allows the identification of fibres based on their distinctive morphologies (Fig. 16.2)1–13 and is of particular value in identifying natural fibres as many of these materials have well defined, characteristic structures, such as the surface scales of wool, the convolutions of cotton and the nodes of flax and hemp. A detailed knowledge of the structures of these fibres allows more subtle differentiations to be made based on associated structures (so called ‘guide elements’) which may be visible – the species, and sometimes the breed of the animal from which wool or hair has been taken can be determined; superficially similar bast fibres can be distinguished; wild and domestic silks can be separated. A comparison of these general features can readily be made, noting characteristics such as the fibre shape and structure, physical dimensions

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16.1 The use of a stereomicroscope on an articulated arm to carry out an initial examination of an object.

Plant

Silk

Wool

10 μm

Synthetic

16.2 Typical plant (jute), wool, silk and synthetic fibres, as observed via transmitted light microscopy.

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and indicators of overall condition, including the presence of dyes, dirt or degradation. Longitudinal samples are easily prepared by laying fibres flat on a slide. Transverse specimens typically require the sample to be set in resin, or other similar suitable medium, before sectioning to reveal the desired aspect. The interpretation of these examinations relies either on the experience of the user, or access to a library of suitable reference materials or micrographs. Various techniques can be used to gain more information. By ensuring that the mounting medium is of the same refractive index as the exterior of the fibre, thus rendering it more-or-less transparent, it may also be possible to examine internal structures in greater detail.1 Ashing can be used to look for characteristic mineral residues, crystals and other inorganic particles, often found in natural fibres, and is achieved by removing the organic components through pyrolysis in a furnace.4 Maceration may also be used in the preparation of plant fibres, employing a solution of acetic acid and hydrogen peroxide to break down intercellular matrix that holds fibre bundles together to yield the individual cells.4,9 Surface structures may be revealed more clearly by the use of fibre casts, forming an impression of the fibre surface using wax or a suitable thermoplastic polymer;1,11 this is particularly useful in the study of animal fibres (wool and hair) in which dyeing has obscured the pattern of surface scales, but is also applicable to any fibre with distinct surface features. Light microscopy may be refined by using polarised light;1,4,9,14–17 at its simplest this can be used as a contrast technique, highlighting the various structures of a fibre (Fig. 16.3), but it can also reveal considerably more about the crystallinity and composition of the sample. Fibres tend to be

10 μm

Transmitted light

10 μm

Polarised light

16.3 Flax fibre viewed via conventional and polarised transmitted light microscopy.

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optically anisotropic, due to the strong alignment of the component polymers with the fibre axis, and so possess two principal refractive indices, one parallel to the fibre axis (n||) and the other perpendicular to it (n⊥), which are characteristic of the fibre type. These indices may be determined in a variety of ways, perhaps most simply by employing a range of mounting media of known refractive index, illuminating the sample either parallel or perpendicular to the axis with polarised light, and determining the medium in which the fibre becomes invisible; this can be facilitated by the Becke line test, which indicates whether a given medium has a refractive index higher or lower than that of the sample.1 Another refinement, fluorescence microscopy, is of particular use in the identification of chemical modifications to the fibre, including dyes, brightening agents and the like, and of the effects of ageing;1,3,9,14,18 fibre blends are particularly easy to identify by this technique. The addition of fluorescent dyes (fluorochromes) to the sample can also aid the investigation of fibres, particularly when specific moieties or structures within the fibre are of interest. Problems with characterisation by microscopy can arise from several sources, particularly variations in the production, treatment and processing of the fibres, including factors such as spinning, weaving, bleaching and dyeing, alongside the degradation and other damage that accumulates over time. All of these factors can reduce or alter the potential information which can be derived from the fibre, often to the point where the sample can only be identified in general terms, such as ‘bast’ (thus including flax, hemp, ramie, and others). Synthetic fibres are often problematic to distinguish via conventional microscopy, as many morphological features such as crosssectional shape, diameter, etc., are indicative but not uniquely characteristic; however, polarised light microscopy generally offers a reliable method of identifying these materials, due to their particular optical properties.19–24

16.2.3 Electron microscopy and X-ray microanalysis Scanning electron microscopy (SEM) allows the fibre surface to be studied in great detail (Fig. 16.4), and is often combined with energy dispersive Xray spectroscopy (EDS or EDX), which can give an indication of elemental composition and distribution.1,3,6,7,16,25–29 Electron microscopy is broadly analogous to light microscopy, with an electron beam replacing the light source and electromagnets taking the role of the optical elements; information is derived from the electrons or radiation emitted from the sample as it interacts with the beam, and captured by appropriate detectors. In terms of micrography, the two most useful techniques are secondary electron and backscattered electron imaging. The former relies on low energy secondary electron, generated through the influence of the primary electron beam on

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1 μm

16.4 Electron micrograph of a flax fibre, exhibiting defibrillation and surface damage.

the sample, and emitted from the surface layer; it provides a high resolution (∼5 nm) method of observing the surface topology of the specimen. The latter technique utilises high energy primary electrons which are scattered through atomic interactions. As these electrons can escape from greater depths within the sample, the resultant images are of lower resolution (∼100 nm) and contain less information about surface morphology; however, as more massive elements are more efficient scatterers, regions of higher average atomic mass appear brighter in the resultant images, and the technique can also indicate the boundaries of crystalline domains. The incident electrons may also result in the production of X-rays, which are of energies characteristic of the element from which they are generated – EDS exploits these emissions to provide an elemental analysis of the specimen.1,16,25,30,31 However, the detection of low mass elements (such as those that comprise the majority of organic fibres: C, N, O) is poor; therefore this technique is principally of use in assessing those components in which heavier elements are present (for example: in dyes, mordants and pigments; in the salts of tin, iron, lead and other metals used for silk weighting; in metal threads; in the metals, glasses and ceramics of buttons and other fittings; in soiling, dirt, rust stains and the like). The data produced by this technique is analogous to that derived from X-ray fluorescence (XRF) experiments, and can be used to characterise the same kinds of materials. In terms of conservation, SEM is particularly useful for looking for subtle signs of damage in textiles, such as the initial stages of defibrillation

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5 μm

‘Blistered’ corrosion surface

200 μm 5 μm

16.5 A metal thread, examined by light and electron microscopy.

of plant fibres, or the microfractures and plasticiser migration observed in synthetic materials; the morphology of fracture surfaces can also prove informative.27,28 In combination with EDS, the technique can provide invaluable information in the investigation of metal threads (an important component of many high quality historical textiles), as it can be used to investigate not only those morphological features that indicate construction, use and deterioration, but also to investigate the composition of the metal components and the nature and extent of any corrosion products (Fig. 16.5).3,32–39 The technique does possess a number of disadvantages, however.1,16 The first of these is that SEM will only provide surface information about a sample – internal structures, such as the hollow lumen of plant fibres, cannot be seen. A greater problem arises from the conditions associated with the equipment: the experiment is carried out under vacuum, which can result in dehydration, dimensional changes and damage in the sample; damage can also be caused by charging effects and heating by the electron beam. This latter problem can be avoided by coating the specimen with a suitable conductive material, usually carbon or gold, but this means that the sample cannot be recovered in its original state, which may have ethical considerations in its own right and will also limit the range of subsequent experiments that can be carried out on the specimen, an important concern when it is necessary to take the smallest possible sample and derived the maximum information from it.

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Some of these problems can be avoided by using low vacuum or environmental scanning electron microscopy (ESEM);40,41 the lower vacuum does not lead to the same degree of desiccation, and cations, generated by collisions between the electron beam and gas in the chamber, serve to dissipate charge build-up. The main drawback of the technique is that resolution is generally lower than in a conventional SEM system, though recent developments are improving the performance of these machines.

16.2.4 Chemical and physical tests These tests have a long tradition in textile conservation as they require little in the way of equipment and are generally easy to carry out.3,8,11,13,42 They rely on the characteristic chemical and physical properties of fibres in order to identify them, or to highlight damage or the effects of processing. However, they have a number of significant disadvantages, as a result of which they are gradually being replaced by other, more modern techniques: in general, these tests are destructive or will at least significantly modify the sample, a problem which is exacerbated by the requirement for relatively large specimens. Furthermore many of the tests are specific to certain fibre types, so different protocols are required for materials of synthetic, plant and animal origins; as a result, the less is known about the sample initially, the greater the number and range of tests required, which can become extremely time consuming and, of course, significantly increases the amount of material required. All of these tests are complicated by the presence of fibre blends, and are unlikely to be optimised to deal with uncommon fibre types; treatments (including dyes, fire retardants and brightening agents) or other additional components will also influence the results of these experiments, as will degradation. A particularly wide range of chemical tests exist for the identification of plant fibres.4,9,43 The phloroglucinol test selectively stains lignin, and can thus be used to distinguish fibres on the basis of their characteristic lignin content. Zinc chloroiodide (the Herzberg reagent) has the effects both of highlighting structures within fibre cells and differentially staining cellulose, lignin and lignified cellulose. Cuprammonium hydroxide (Schweitzer’s reagent) causes the swelling and dissolution of cellulose; as the content and distribution of cellulose varies between species, the pattern and rate of this process is characteristic. Damage to these fibres can also be assessed with staining tests, highlighting features that may otherwise not be apparent:11,13 Fehling’s solution precipitates red copper(I) oxide in regions of acid damage. The Clibbens and Geake test causes damaged regions to darken. The Turnbull Blue test stains regions of acidic or oxidative damage dark blue. The Congo Red test dyes the interior of the fibre more strongly than the exterior, thus revealing areas where the cell wall has swollen or split.

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Some commercial analytical stains are also available, such as the Shirlastain range, which rely on dye mixtures designed to stain different fibre types specific colours;11,13 generally each stain is designed for a certain category of fibre – for example, distinguishing between individual cellulosic fibres, or determining the general origin of a sample (synthetic, animal or plant). Problems can arise from the qualitative interpretation of the resultant colour, and this colour will of course also be influenced by dyes and surface treatment as well as discolouration due to age or damage. Solubility tests can be used to systemically categorise a fibre sample based on its chemical nature, using readily available reagents,3,8,11 but require large specimen sizes and will not distinguish chemically similar materials such as plant fibres (primarily composed of cellulose) or wools and hairs (composed of keratin proteins). The physical properties of a fibre can also be investigated: burn tests rely on the differing ways in which fibres undergo pyrolysis when subjected to a flame.3,5,11,13 Various factors are taken into consideration, including whether or not the specimen melts, whether it continues burning when removed from the flame, the resultant smell, the final appearance of the material, etc. Buoyancy or specific gravity tests assess the characteristic density of a material.10,13 The twist test, particular to plant fibres, exploits the helical winding of the cellulose in the cell wall, which causes the fibres to characteristically twist clockwise or anticlockwise when allowed to dry after wetting;5,8–10,13 the information provided by the test is limited, but can differentiate between fibres that are morphologically difficult to distinguish, such as flax and hemp.

16.2.5 Spectroscopy A more specific approach to fibre identification can be taken using spectroscopic methods, particularly vibrational spectroscopy. These techniques rely on the stimulation and observation of bond vibrations at characteristic frequencies, and thus identifying specific chemical moieties in the sample; it can be used not only to distinguish fibres, but also to assess their state of degradation and to confirm processing methods, dye treatments, silk weighting and the like.44–54 Identification is generally achieved either by correlating the bands arising from specific constituents with the known composition of materials, or, more commonly, by comparing the spectrum of an unknown material with a suitable spectral library, to identify the fibre by its characteristic ‘signature’ (Fig. 16.6). Spectroscopic identification is of particular use in the case of those specimens that have undergone surface degradation or wear to the point where examination by microscopy provides little or no useful information (a situation often encountered with archaeological samples).44,51,55–59

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Cotton

Intensity

Silk

Polyester

4000

3500

3000

2500

2000

1500

1000

Wavenumber/cm–1

16.6 Characteristic FT-IR spectra of three common textile fibres.

The most widely available spectroscopic technique, Fourier-transform infrared (FT-IR) spectroscopy, can be used both to characterise fibres and to investigate their condition. Materials with broadly different chemistries, such as synthetic fibres, wools, silks and plant fibres, can be readily distinguished; the subtle chemical differences between the more closely related fibres can then be exploited to differentiate these materials – for example, it has been shown that it is possible to identify plant fibres on the basis of their lignin content,43 exploiting the same properties that are used in the phloroglucinol test; similarly, the sub-classes of, for example, nylon (Nylon 6, Nylon 66, Nylon 12 and so on) can be distinguished. Band assignments may be established for particular fibre polymers, such as cellulose60–62 and silk,63–65 and degradation can be assessed, either through the loss of identifiable chemical components and the accumulation of distinctive degradation products (such as carbonyl containing species produced by oxidative processes), or by changes in microstructure deduced from spectroscopically derived crystallinity indices.29,48,65–67 The technique is also of use in the identification of dyestuffs and pigments, as well as other treatments such as optical brightening agents or flame retardants.44–46 Traditionally, FT-IR spectroscopy required a relatively large sample that would be ground to a fine powder before being made up as either a KBr disc or a mull in an appropriate mineral oil; these processes require large sample sizes and, obviously, are destructive, so are not particularly suited

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Identification of textile fibers

to the requirements of textile conservation. Attenuated total reflectance (ATR) spectroscopy provides a more appropriate sampling method, requiring only a small fibre sample (often a millimetre or so of a single yarn);68–70 spectra are derived from the radiation that interacts with the specimen at the interface with a crystal window (often diamond or germanium), and the nature of this interaction means that the data is obtained from the outer layer of the material (the sampling depth is of the order of the wavelength of the radiation, so usually a few microns). Another suitable method is provided by microspectroscopy,1,16,52,71–73 in which a spectrometer is attached to a set of specially designed microscope optics, which allows spectra to be recorded from samples as small as a few tens of microns in diameter. This offers many advantages, particularly where specimens are limited in size, or are found with other materials (such as contaminants, other fibres in the blend, surface films, etc.) as spectra can be recorded from discrete components of the sample. Problems may be encountered due to the generally low power of these systems resulting in low signal to noise ratios and, particularly in the case of single fibres, the roughly cylindrical nature of the specimen can cause lensing effects which will also diminish the quality of the data. As an alternative to the mid-infrared region exploited by conventional FT-IR spectroscopy, near-infrared spectroscopy may be employed instead,74,75 and has been shown to be useable with proteinaceous,76–82 cellulosic54,83 and synthetic84–89 fibres, as well as identifying blends90–92 and processing treatments.93 This technique has a variety of advantages, perhaps the foremost of which, for the purposes of textile conservation, is its ability to be readily used with fibre optic probes (opaque to mid-infrared radiation); this enables spectra to be readily recorded in situ, with the minimum of disturbance to the object in question (Fig. 16.7). In addition spectral accumulation is generally rapid, and under the right conditions spectra can be recorded through glass or an air-gap (a benefit when dealing with framed objects). The main drawback with the method lies in the nature of the spectra themselves – the bands observed in the data are combinations and overtones of those seen in mid-infrared spectra, and are generally associated with vibrations involving hydrogen; this complicates spectra to the point that band assignments are often impossible and at best ambiguous. Dark or reflective surfaces can also prove problematic. Therefore the technique is best used either by comparison of spectra to a library of suitable references, or in conjunction with chemometric analyses that allow subtleties in the data to be drawn out through statistical approaches.74,94–97 Another vibrational technique, Raman spectroscopy, has also proven to be of value in conservation, as it is typically non-destructive, requires little sample preparation, is of use for both organic and inorganic materials and has a good spatial resolution.98–101 It has been employed to differentiate

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16.7 The use of an optical probe with a near-infrared spectrometer to carry out a non-invasive analysis.

untreated plant fibres (ramie, jute, flax, cotton, kapok, sisal and coir) on the basis of peak ratios derived from the associated C-H stretches and glycosidic C-O-C stretches,55 and has shown that the degradation of these materials may be monitored.102 Similar studies have also investigated silk fibres from various sources and at various stages of processing,64,103–105 including spider silk,106 and wool, subjected to treatments such as bleaching and to physical changes induced by stretching.56,107 However, the Raman technique is not routine for many conservation science laboratories and, in any case, luminescence can prove problematic, particularly with historic materials. The relatively high power of the lasers used in Raman spectrometers also potentially leads to the risk of localised burning to the sample. It is also possible to gain an understanding of the microstructure and crystallinity of fibres through spectroscopic techniques; the simplest approach involves defining crystallinity indices based on peak ratios, a technique that has successfully been used with materials such as cellulosic fibres62,67,108,109 and silk.110,111 A more sophisticated method of investigating these properties may be achieved by introducing an infrared polariser into the system; bonds are only observed when they are aligned with the electric vector of the polarised radiation, and the intensity of bands in the resultant spectra reflect the long range ordering, if any, of the various components of the fibre. This has been successfully used to examine the helical winding of

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Identification of textile fibers

cellulose in the cell walls of plant fibres,112,113 distinguishing otherwise similar fibres on the basis of the unique angle and sense of wind, and to assess the links between crystallite orientation and physical integrity in silk fibres.114

16.2.6 Dye analysis Dye analysis can provide a valuable means of assessing the provenance of a fibre. Dyes may be broadly categorised as either natural or synthetic, the latter being first developed in the mid 19th century. Natural dyes were often highly specific to particular locations, or available to a wider market only after trade routes were established, which may give a useful indication of the history or a textile. Synthetic dyes were frequently only used for a handful of years before being superseded by dyestuffs that were either technically more advanced (for example, easier to apply, possessed of better light or wash fastness, or less damaging to the fabric) or more suited to the demands of changing fashions; as these substances are often recorded in detailed patent applications, it may be possible to pin-point their dates of likely use with some accuracy. By understanding the nature of a dye it is possible not only to gain a better appreciation of the origins of a textile, but also how the material has been processed in the past and how it may respond to conservation treatments. For example, some dyes will change colour or solubility as pH varies, in which case the acidity of any wash solution must be carefully controlled to ensure that the dyestuff will not alter or bleed; similarly the metal mordants may exchange with other soluble ions in a wash solution, again potentially leading to the loss or alteration of a dye. Dye analysis has traditionally been carried out by chromatographic techniques, a specialised procedure that requires experience, dedicated equipment and a substantial reference library, but which can provide very accurate results.115–117 Attempts have also been made to use FT-IR, Raman and UVvisible spectroscopic techniques, with varying degrees of success; Raman spectroscopy in particular has proven useful to this end.44,118–120 In some situations, a precise identification of a dye is not necessary. For example, if a support fabric is to be dyed to match an object, to ensure that it is as unobtrusive as possible, the colour of the textile is important rather than the precise formulation of the dye; in these cases, a simpler and more easily determined colour-space measurement may be appropriate, using a colorimeter.

16.2.7 Additional components ‘Textile’ artefacts, such as clothing, upholstery, banners and embroideries, are also likely to contain many other non-textile components, including: buttons, zips and other fastening; decorations, such as beads and sequins;

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paints, lacquers and varnishes; and support materials in the form of foam, stuffing or frames. It is often as important to identify these materials as it is the fibres of the fabric, both to achieve a greater understanding of the history of an object, and to inform conservation treatments. Many of the techniques already discussed are also appropriate to these additional components – FT-IR spectroscopy, for example, is ideally suited to the examination of organic materials, and EDS or XRF can provide valuable information about the composition of inorganic aspects.

16.2.8 Analysis and bulk properties Another important consideration is to be able to gain an understanding of the physical and chemical condition of an artefact, as this will not only influence the choice of conservation methods to be used, but will also help to indicate how the object may behave over time, thus suggesting what handling, display and storage strategies might be the most appropriate. At its simplest, this may be done by looking for visible signs of deterioration via microscopy, or for chemical changes associated with degradative processes, such as oxidation or hydrolysis, through spectroscopic methods. A more systematic approach, and one on which a range of recent research has concentrated, involves drawing correlations between spectroscopic or chromatographic signatures, and measurable physical properties, such as tenacity or elasticity. This not only provides a means of more fully understanding the links between the chemical and microstructural characteristics of a fibre and the bulk behaviour of a fabric into which it is incorporated, but also potentially offers a non-invasive or microsampling method of determining the large-scale condition of an object destined for conservation; additionally it may lead to in situ analyses to determine which objects in a collection are most at risk and in particular need of urgent intervention, or if a particular display strategy is, for example, liable to place an object under undue strain. To this end, surrogate materials are often employed; these are either naturally or artificially aged specimens that mimic the properties of the object itself and can be used in a sacrificial manner to determine how the item is likely to respond to various conservation strategies or display conditions. In order to choose or prepare these materials, it is necessary to have a good understanding not only of the composition of the object, but also its state of deterioration and of the chemistry of the components so that the mechanisms that have lead to its present state might be appreciated.

16.3

Conservation strategies

The principal aim of conservation is to ameliorate the effects of preexisting damage and prevent future deterioration, with the minimum of

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intervention, often with the requirement the object is in a suitable state for display or further investigation; conservation does not generally involve replacing missing areas of fabric, renewing faded or fugitive dyes, restoring lost function or otherwise modifying the item.3,121–123 Above all else, these treatments are intended to cause no further damage to the object and, as far as possible, to be reversible, thus ensuring that they will not interfere with any future interventions; historically not all conservation strategies have abided by these principles. The techniques involved in conservation include the following. Cleaning, to remove dirt, soiling, stains, dye bleeding or other material such as mould spores. It may be desirable to remove other substances as well, such as adhesives which have spread, leading to disparate parts of an object being bound together and not only resulting in the loss of original form but also introducing damaging stresses. Cleaning may simply involve the removal of surface dirt, through low-powered suction, or for more robust fabrics, gentle mechanical action with a suitable brush or cloth, or it may use an aqueous or organic solvent wash; in this latter case it is necessary to have an understanding of the nature of the various components, in order to choose a wash solution of appropriate solvency and pH that will remove the unwanted dirt, but that will not react with or cause the dissolution of any aspect of the object, and similarly will not cause bleeding or colour changes in dyes. As with all aspects of conservation, it is preferable to err on the side of caution and leave soiling rather than risk damaging the underlying object by removing it. It should also be borne in mind that in some cases soiling is intrinsic to the history of the artefact, and it is not appropriate to remove it. Reshaping is important as many objects are encountered in a heavily creased or crumpled state, which not only means that their original form is lost, but may also place it under undue stress. Humidification is often involved in this process, as water acts as a plasticiser for most natural and some synthetic fibres, restoring flexibility to these materials and allowing them to be reshaped with minimal risk of causing damage. However, some materials do not respond well to humidification (heavily weighted, ‘shattered’ silk, for example), so correct identification of materials is important. Support involves affixing a fragile or fragmentary fabric to a suitable inert backing or overlying material (often a silk, nylon or polyester net, dyed to ensure that it is as unobtrusive as possible), using either a suitable, reversible adhesive or careful stitching. This supports the item, thus minimising further physical damage. Cosmetic in-fills may also be used, to make areas of damage less obvious or to aid the intelligibility of an incomplete design or section of text; in this case, the intent is to not to make the object appear undamaged (and the infill should be recognisable as such on close examina-

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tion), but to give a general impression of the missing area, to better to understand the item as a whole. Pest control is an important consideration in those artefacts that may harbour mould spores or insect eggs, capable of remaining dormant on a fabric for extended periods; to some extent cleaning will remove these contaminants, but other measures might also be necessary. The most common procedures are freezing and anoxia, and again a knowledge of the composition of the object is important – on freezing some materials may be taken below their glass transition temperature and thus be at risk of physical damage on subsequent handling, whilst anoxia can induce reactions, particularly colour changes in certain classes of dye. Pesticides are to be avoided as they may interact unfavourably with the artefact and are likely to leave residues. Mounting is usually the final stage of conservation, and provides the object with appropriate structure to minimise stresses whilst, usually, allowing it to be presented in a manner suitable for display. The structures should be purpose built for the artefact in question, and a well designed support may minimise or avoid the need for interventive conservation. In all cases it is important to ensure that the materials used are compatible with those of the object itself and will not interact in a deleterious manner.

16.4

Case studies

16.4.1 George Mallory In 1924 George Mallory, along with Andrew Irvine, led an expedition to climb Everest – if they achieved their goal, they would have made the first successful ascent of the mountain. Mallory and Irvine were last seen on the afternoon of 8 June, at a height of almost 28 000 feet, disappearing into the mist. Mallory’s body was found in 1999, and it became evident that he had died after a severe fall; a DNA sample was recovered, to confirm his identity, along with samples of clothing and equipment, before the body was buried on the mountain (Fig. 16.8). An analysis of the clothing was carried out at the Textile Conservation Centre, on behalf of the Mountain Heritage Trust, in order to determine the style and structure of the garments, as well as the nature of the component fibres and the presence, or otherwise, of weatherproofing treatments or the like. The composition of the jacket, trousers and three different shirts were of particular interest, and were examined by a combination of light and electron microscopy and FT-IR spectroscopy to determine the characteristic morphologies and chemistries of the specimens; solvent extractions on small sections of yarn were also carried to look for evidence of weatherresistant treatments. The jacket and trousers were made of cotton, which

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FRONT TCC 2689.4

16.8 Front panel from Mallory’s jacket.

appeared to be treated with some form of oil-based weatherproofing; each of the shirts was woven from a different material – wool, domesticated silk and tussah (wild) silk. Microscopy also allowed the dimensions, ply and spin of the yarns to be established, along with the weave structures. These results informed decisions that were made about the most appropriate way to store the items, and highlighted those aspects that may be in particular need of future intervention. Subsequently this information, along with the details of construction of the garments, also allowed replicas to be made and tested by the University of Leeds Performance Clothing Research Centre. This revealed that for the kind of climb Mallory intended – a rapid ascent and descent – the performance of the clothing was comparable to that of modern equipment. This not only provided valuable information about the history of extreme weather clothing, but also helped refute the suggestion that Mallory may have died on Everest because his choice of clothing was not suitable for the task.

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17 m

head

foot 24 m

16.9 The fore topsail of the HMS Victory.

16.4.2 The Victory sail The only surviving sail from the HMS Victory, Nelson’s flagship at the Battle of Trafalgar in 1804, is held by the Royal Navy at the Portsmouth Historic Dockyard (Fig. 16.9). The Navy wished to display the sail to mark the bicentenary of the battle, and commissioned the Textile Conservation Centre both to work on the item and also to suggest the most appropriate method of display.124–126 This work was supported by the Society for Nautical Research and the Ministry of Defence. The sail itself was a substantial object (24 × 16 m in size, and weighing nearly half a tonne), and had been extensively damaged during the battle, with numerous holes from cannon and musket fire and a long tear caused by a falling mast. Analysis of the materials, by light and electron microscopy and infrared spectroscopy, showed that the sail itself was constructed from linen, whilst the rope that bound its edge was of hemp; in addition to the obvious damage it was also apparent that the fabric had suffered from more subtle chemical deterioration over time along with extensive mould growth in certain areas.

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16.10 Working on the Victory sail.

The conservation itself involved surface cleaning using low-powered vacuum suction and gentle mechanical action to remove debris and mould spores (Fig. 16.10). The presence of these spores represented a potential health hazard to the conservators, so breathing masks were worn. The size of the object also presented problems, as it was necessary to walk and sit on the sail, so the weight of the conservators and equipment was distributed on sheets of rigid PlastozoteTM; to further protect the sail, it was raised on a well-ventilated platform, to ensure free circulation of air. The sail’s size also meant that it could not be fully laid out in the space available, so was partially rolled on a specially constructed inflatable boom; to move the sail this boom was used in conjunction with a heavy canvas sling. Throughout the cleaning programme, spot tests were carried out to ensure the effectiveness of the process. In parallel to this, work was carried out to determine the most appropriate method of displaying the sail; a suggested means of doing this was to hang the sail from a yard, as it would have originally been employed on the ship, and it was necessary to determine if this would subject the item to damaging stresses. Permission was given to remove yarn samples from regions of pre-existing damage, along with one small piece of loose sailcloth. By characterising the nature and condition of these specimens, it was possible to prepare artificially aged surrogate materials using a modern linen sailcloth. The original specimens, along with the surrogates were then subjected to mechanical testing to determine their physical properties; this not only gave an indication of the condition of the sail itself, but also confirmed that the surrogates were in a sufficiently similar state to the original materials that they could be used to assess the bulk properties of the object, tests that could not accurately be carried out on single yarns, nor on the sail itself

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for fear of causing permanent damage. The samples were also examined by Raman spectroscopy, which suggested a potential link between certain spectral changes and the observed deterioration of the material.102 These analyses revealed that the tenacities of the individual yarns were roughly uniform across the sail, and that although as a whole it still had enough residual strength to support its own weight, to allow it to do so would risk permanent deformation. Therefore the recommendation was made that the sail was displayed either flat or at a shallow angle, on a suitable solid mount, to minimise undue and prolonged stresses.

16.4.3 ‘Parachute silk’ slip A variety of artefacts in the Hampshire County Museums and Archives Service collection were investigated by staff from the Textile Conservation Centre.127 The analyses were carried out as part of a pilot study to assess the value of near infrared spectroscopy as an in situ analytical technique. Amongst the items studied was a slip dating from the 1940s and believed to be constructed from parachute silk. During the Second World War and the years that followed, rationing and the diversion of goods to the war effort limited the availability of many commonplace and luxury items, including fabrics. As a result materials were often adapted from other sources to fill this gap; in particular, silk from discarded or surplus parachutes was often employed to make wedding gowns and under-garments. The slip in question had been donated to the collection as such. However, the spectra recorded from the garment, when compared with a comprehensive library of known materials, actually revealed the fabric to be nylon (and probably Nylon 6) rather than silk. This investigation highlights the benefits of a rapid analytical technique that is non-invasive and can be carried out without removing the object from the collection. The information will allow the garment to be stored and displayed in a manner best suited to the long-term stability of its component materials, and its history and origins are more fully understood.

16.4.4 Freddie Mercury A pair of red faux leather trousers were received by the TCC to be made safe for storage and occasional display;128 these trousers had originally belonged to Freddie Mercury, the lead singer of Queen, and had been purchased at auction by the client (Fig. 16.11). NIR spectroscopy showed that the fabric consisted of a thin layer of polyester polyurethane, bonded to a brushed cotton base fabric. The trousers were in generally good condition, but the polyurethane layer had started to become cracked and was peeling from the underlayer; in addition it was sticky, and had begun to

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5 mm

16.11 Freddie Mercury’s faux leather trousers.

adhere to itself in areas where the fabric was folded. In addition the metal fittings – a zip and rivets – were heavily corroded. The deterioration of the polyurethane is likely to be due to a combination of oxidation and plasticiser migration, whilst the damage to the metal components probably arises from the action of both volatile acidic degradation products of the polyurethane and sweat. Although some work has been done on consolidating materials of this sort,129 it was felt that with the current state of knowledge the best interim measure was to internally support the trousers with custom made forms to prevent further creasing and delamination, and to lacquer the rivets to protect them from further corrosion.

16.5

Future trends

It is likely that modern analytical techniques, such as spectroscopy, chromatography and advanced microscopic analysis, will become more widely used in future, gradually replacing traditional methods, particularly the destructive and often unreliable analyses like stain and burn tests. The trend will be driven by the greater availability of these analytical approaches, the reduction in cost and complexity, and a wider appreciation of the benefits of these methods in the conservation community. It is likely that there will be a

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greater emphasis on microsampling or non-invasive techniques, especially those which allow in situ examinations to be carried out, thus obviating the need to remove objects from collections or disturb displays, reducing the risk of causing damage through handling. Some techniques can potentially further limit the need for intervention – NIR spectroscopy, for example, can be used through glass, theoretically allowing items such as framed banners or textile art to be assessed without interfering with the setting. A further probable development will be the drive to derive the maximum possible information from an analysis, not only identifying the component materials, but also using the investigation to determine the chemical and physical condition of the sample and the manner in which this reflects the properties of the object as a whole. This will be achieved either through direct correlation of analytical signatures with measurable physical properties, or through methods such as chemometrics, which employ multivariate analysis techniques to derive statistical correlations between sets of data; these approaches are potentially very powerful, and are able to exploit subtleties in data too small to be readily observed through conventional assessments. However, there will always be the call for simple, inexpensive methods that can readily be employed by a conservator working alone and without the support of a fully equipped laboratory.

16.6

Sources of further information and advice

Chemical Principles of Textile Conservation (A. Tímár-Balászy and D. Eastop; Butterworth-Heinemann, Oxford, UK, 1998) offers a good general grounding in the role of chemistry and the use of analytical techniques in textile conservation, and provides many useful case studies. As an introduction to the study of plant fibres, particularly by microscopic techniques, Identification of Vegetable Fibres (D.M. Catling and J.E. Grayson; Archetype Publications, London, UK, 1998) is invaluable, and the Handbook of Fiber Chemistry (M. Lewin and E.M. Pearce, eds; Marcel Dekker, Inc., New York, USA, 1998) provides detailed information about the majority of commonly encountered fibres and textile materials. Conservation Science: Heritage Materials (E. May and M. Jones, eds; RSC Publications, Cambridge, UK, 2006) is another valuable resource, highlighting the value of scientific and technical understanding in the wider field of conservation, dealing with a broad range of different materials. The National Parks Service (USA) is currently constructing an online Microscope Slide Fiber Reference Database.

16.7

Acknowledgements

The author would like to thank his colleagues at the TCC, who carried out much of the research presented in the case studies and whose support and

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encouragement has been invaluable, and in particular Nell Hoare (Director of the TCC) for permission to publish; Peter Goodwin (keeper and curator of HMS Victory) and the commanding officer of HMS Victory for permission to publish the research on the sail, and the Society for Nautical Research for supporting the work through the ‘Save the Victory’ fund; Mary Rose and the Mountain Heritage Trust for permission to publish the results of the investigations on the Mallory clothing; Hampshire County Museums and Archives Service for permission to publish the work on the ‘parachute silk’ slip; J. Mitchell for permission to publish the work on Freddie Mercury’s trousers; and the AHRC, which supported much of the author’s research through his position as a Research Fellow at the AHRC Research Centre for Textile Conservation and Textile Studies; images were reproduced by permission of the Textile Conservation Centre, University of Southampton.

16.8

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104 S. Zheng, G. Li, W. Yao & T. Yu; ‘Raman Spectroscopic Investigation of the Denaturation Process of Silk Fibroin’; Applied Spectroscopy; 43(7); 1269–1272; 1989. 105 M. Sonoyama, M. Miyazawa, G Katagiri & H. Ishida; ‘Dynamic FT-IR Spectroscopic Studies of Silk Fibroin Films’; Applied Spectroscopy; 51(4); 545– 547; 1997. 106 D.B. Gillespie, C. Viney & P. Yager; ‘Raman Spectroscopic Analysis of the Secondary Structure of Spider Silk Fiber’; in: ‘Silk Polymers: Materials Science and Biotechnology (ACS Symposium Series 544)’; American Chemical Society; 155–167; 1994. 107 J.S. Church, G.L. Corino & A.L. Woodhead; ‘The Effects of Stretching on Wool Fibre as Monitored by FT-Raman Spectroscopy’; Journal of Molecular Structure; 440; 15–23; 1998. 108 A.K. Bledzki & J. Gassan; ‘Composites Reinforced with Cellulose-Based Fibres’; Progress in Polymer Science; 24; 221–274; 1999. 109 P. Garside & P. Wyeth; ‘Monitoring the Deterioration of Historic Textiles: Developing Appropriate Micromethodology’; in: J.H. Townsend, K. Eremin and A. Adriaens (eds); ‘Conservation Science 2002’; Archetype Publications Ltd, London, UK; 171–176; 2003. 110 X. Hu, D. Kaplan & P. Cebe; ‘Determining Beta-Sheet Crystallinity in Fibrous Proteins by Thermal Analysis and Infrared Spectroscopy’; Macromolecules; 39; 6161–6170; 2006. 111 J. Shao, J. Zheng, J. Liu & C.M. Carr; ‘Fourier Transform Raman and Fourier Transform Infrared Spectroscopy Studies of Silk Fibroin’; Journal of Applied Polymer Science; 96(6); 1999–2004; 2005. 112 P. Garside & P. Wyeth; ‘Identification of Cellulosic Fibres by FTIR Spectroscopy: Differentiation of Flax and Hemp by Polarized ATR FTIR’; Studies in Conservation; 51(3); 205–211; 2006. 113 P. Garside & P. Wyeth; ‘The Use of Polarised Spectroscopy as a Tool for Examining the Microstructure of Cellulosic Textile Fibres’; Applied Spectroscopy; 61(5); 523–529; 2007. 114 P. Garside, S. Lahlil & P. Wyeth; ‘Characterization of Historic Silk by Polarized Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy for Informed Conservation’; Applied Spectroscopy; 59(10); 1242–1247; 2005. 115 J. Wouters; ‘High Performance Liquid Chromatography of Anthraquinones: Analysis of Plant and Insect Extracts and Dyed Textiles’; Studies in Conservation; 30; 119–128; 1985. 116 J. Wouters & A.Verhecken; ‘The Coccid Insect Dyes: HPLC and Computerized Diode Array Analysis of Dyed Yarns’; Studies in Conservation; 34; 189–200; 1989. 117 J. Wouters & A.Verhecken; ‘HPLC of Blue and Purple Indigoid Natural Dyes’; Journal of the Society of Dyers and Colourists; 7; 266–269; 1991. 118 K. Wang & Y-S. Li; ‘Surfactant-Aided Surface-Enhanced Resonance Raman Spectroscopic Study of Some Anionic Dye Molecules’; Journal of Raman Spectroscopy; 27(5); 385–389; 1996. 119 I.T. Shadi, B.Z. Chowdhry, M.J. Snowden, R. Withnall; ‘Semi-quantitative analysis of alizarin and purpurin by surface-enhanced resonance Raman spectroscopy (SERRS) using silver colloids’; Journal of Raman Spectroscopy; 35(8–9); 800–807; 2004.

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120 S.E. Bell, E.S.O. Bourguignon, A.C. Dennis, J.A. Fields, J.J. McGarvey & K.R. Seddon; ‘Identification of Dyes on Ancient Chinese Paper Samples Using the Subtracted Shifted Raman Spectroscopy Method’; Analytical Chemistry; 72; 234–239; 2000. 121 K. Finch & G. Putnam; ‘The Care and Preservation of Textiles’; Batsford; London, UK; 1985. 122 S. Landi; ‘The Textile Conservators Manual’ (2nd Edition); ButterworthHeinemann; Oxford, UK; 1992. 123 J. Robinson & T. Pardoe; ‘An Illustrated Guide to the Care of Costume and Textile Collections’; Museums & Galleries Commission; 2000. 124 K. Gill & P. Garside; “The HMS Victory Fore-Topsail’; in: F. Nuttgens & M. Jordan (eds); ‘ “Big Issues”: Forum of the UKIC Textiles Section, 2005’; ICON; 36–46; 2006. 125 P. Garside & P. Wyeth; ‘Assessing the Physical State of The Fore-Topsail of the HMS Victory’; in: R. Janaway & P. Wyeth (eds); ‘Scientific Analysis of Ancient and Historic Textiles: Informing Preservation, Display and Interpretation’; Archetype Publications; London, UK; 118–125; 2005. 126 P. Garside & P. Wyeth; ‘Textiles’; in: E. May & M. Jones (eds); ‘Conservation Science: Heritage Materials’; RSC Publications; Cambridge, UK; 56–91; 2006. 127 E. Richardson, G. Martin & P. Wyeth; ‘Collecting a Near Infrared Spectral Database Of Modern Textiles for Use of On-Site Characterization’, in: M. Hayward (ed); ‘Postprints of the 3rd Annual Conference of the AHRC Research Centre for Textile Conservation and Textile Studies “Textiles and Text – Reestablishing the Link between Archival and Object-based Research” ’; Archetype Publications; London, UK; 257–263, 2007. 128 D. Lovett; ‘Another One Bites the Dust? A Case Study of Freddie Mercury’s Imitation Leather Trousers’; from: ‘Plastics – Looking at the Future & Learning from the Past’; Victoria and Albert Museum; 2007. 129 T. Bechthold; ‘Wet look in 1960s furniture design: degradation of polyurethanecoated textile carrier substrates’; in: C. Rogerson & P. Garside (eds); ‘The Future of the 20th Century: Collecting, Interpreting and Conserving Modern Materials’; Archetype Publications; London, UK; 128–133; 2006.

Index

abaca 14 absorption spectroscopy 159 acetate 11 acid damage cotton 293–4 elastane (spandex) 315 nylon 309, 310, 311 wool 297–9 acid dyes 205, 206 acrylic (polyacrylonitrile or PAN) 10, 69, 70, 77, 85 damage analysis 311–12 extraction and classification of dye from 209 fibre variants 117 addition reactions 262 additional components 348–9 adhesive strip test 283 adulterants 224–5, 231–3 after-market taxonomy 4–5, 272 aggregate theory 141–6 agricultural product chemicals 126–7 aliphatic polyester 85 alkali damage elastane (spandex) 315 wool 297–9 alpaca 49, 224, 234 identification 51–4, 55, 56, 57 alternative identification techniques 181–7 American Association of Textile Chemists and Colorists (AATCC) 3, 4, 6, 7, 253, 255, 260 amino acids 31, 32, 33 angora rabbit 49, 54, 58, 224, 225 anidex 12, 121 animal fibres 15, 27–67 characteristics 38–44 DNA analysis see DNA analysis future trends 61–2, 66, 67 identification 44–61, 62, 63, 64, 65 physical and chemical properties 33–5 SEM 38–9, 47–8, 197–8

366

structure and composition 31–3 types of 35–8 see also animal hairs; silk animal hairs 15, 27, 28 characteristics 38–44, 45 growth 28–30 identification 44–59, 60, 61, 62, 63 types of 35–8 anoxia 351 appearance 137–9 Applied DNA Sciences Inc. 253 aramid 12, 90–2 arrector pili muscle 29, 30 ascending linear method 214–15 ashing 339 ASTM, International 3–4, 6, 7, 253, 255, 260–1 atomic absorption spectroscopy (AA) 160 atomic emission spectroscopy (AES) 160 attenuated total reflectance (ATR) spectroscopy 21, 162, 285, 346 automated DNA analysis 235 automated testing of cotton 256 auxochromes 167, 204 average degree of polymerisation 290–1 azlon 11 backscattered electrons 189, 190, 191, 340–1 bacterial damage 324–5 bamboo 127 bars/barriness 278, 323–4 bast fibres 13–14, 243–4 Becke line test 143–4, 266–7, 340 Beilstein test 283 Bell, Joseph 259–60 Berek quartz wedge compensator 145–6 bicomponent fibres 18, 123, 124, 265 biconstituent fibres 18, 265 binder fibres 125 biological damage 278 wool 299–300, 324–5

Index birefringence 16, 139–46, 184, 247, 267–8, 269, 270–1 bulk properties 349 bundle strength 246, 247 burning tests 3, 6, 181–2, 260, 344 calibration 176–7 camel hair 49, 55–9, 63, 224 camelids 28, 234 capillary electrophoresis (CE) 218–20 carbon adhesive 193 carbon coating 193 carbon fibre 78 carpeting 180 cashgora 224 cashmere 35, 49, 55, 60, 61, 62, 224–5 cathodoluminescence 189 cavitomic cotton 255 cell wall 243, 244 cellulose 95 content in cotton 243–4 cellulosic fibres 10 damage analysis 292–5 see also cotton chain rigidity 88–90 characteristic X-rays 189, 190, 191 Chemical Abstracts Service (CAS) Registry number 205 chemical analysis 19–22 conservation 343–4 damage 280 HPFs 97–8 see also under individual techniques chemical damage 278, 306 elastane (spandex) 315 see also acid damage; alkali damage; oxidative damage chemical fibres 72 see also synthetic fibres chemical fixation 192 China Cotton Colour Characterisation Chart 248 chitin 127 chlorine 283 chroma 174 chromatogram 217 chromatography 22, 203–23 CE 218–20 damage analysis 284–5 extraction of dyes 207–12, 220 HPLC 22, 213, 215–18, 219–20 TLC 22, 212–15, 216, 217, 219, 284–5 chromophores 167, 204 CIELAB colour model 174 classification of fibres 9–19, 112–23 generic classes 10–13, 112–14, 115–19 manufactured fibres 15–19 natural fibres 13–15 polymer origins and 112–15, 116, 117

367

subclasses 112–14, 119–23 Textile Fiber Products Identification Act 10–13, 112–14 variants 114–15, 116, 117 classification standards for cotton 248 cleaning (in conservation) 350, 354 cleanliness 192 clothing Freddie Mercury’s trousers 355–6 George Mallory’s 351–2 ‘parachute silk’ slip 355 coating, conductive 193–4, 342 cochineal 204 cocoon 38 coir 15 colour analysis of 22 dyes see chromatography; dyes human vs machine perception 172–5 microspectrophotometry see microspectrophotometry colour atlases 174 Colour Index 204–5 colour matching light boxes 173 comb sorter array 251, 252 commercial hot stage 18–19 comparison 4, 8–9 compensator 145–6 complaints 277 condensation reactions 262 conductive coating 193–4, 342 confocal microscopy 149–50 conjugate (multicomponent) fibres 123–6 conjugated double bonds 167, 204 conservation 335–65 analytical techniques 337–49 additional components 348–9 bulk properties 349 chemical and physical tests 343–4 dye analysis 348 electron microscopy and X-ray microanalysis 340–3 optical microscopy 337–40 sampling 337 spectroscopy 344–8 case studies 351–6 Freddie Mercury’s trousers 355–6 George Mallory’s clothing 351–2 HMS Victory sail 353–5 ‘parachute silk’ slip 355 future trends 356–7 strategies 349–51 construction, fibre 18, 123–4, 265–6 conventional DNA hybridisation analysis 228–9, 230 convolution angle 253–4 convolutions 245, 253 copolymer-type aramid fibre 92 Corterra 123

368

Index

cortex 29, 30, 39 cosmetic in-fills 350–1 cotton 14, 71, 77, 81, 83, 239–58 damage analysis 255, 292–5 extraction and classification of dye from 208, 211 fibre properties 245–56 differences in processing ability 246–50 identification of fibre origin 250–3 identification tests for cotton fibre in textiles 253–5 quantitative analysis 255–6 future trends 256 non-textile applications 241 structure and composition 241–5 supply chain 240–1 crimp, three-dimensional 124–5 cross-sectional shapes animal hairs 43–4, 45, 48, 49 alpaca 54, 56, 57 cotton 245 manufactured fibres 16, 263–5 microspectrophotometry 179 cross-sectional specimens 137 crossed polars 140, 141, 269–71 cryofixation 192 crystal structure HPFs 98–104, 105 synthetic fibres 77–9, 86–7 crystallinity 261–2 indices 347 cultivated silk 30, 38 cumulative damage 303, 316 cuticle 29, 30, 39, 243 extraction of DNA from cuticle cells 226–7 cuticular scale patterns 39–42, 46–7, 49, 138 wool 51, 52 cylindrical symmetry 142–3 cystine 34 damage analysis 19, 275–334 according to type of fibre 292 biological damage 278, 299–300, 324–5 causes of damage 279–80 importance and reasons for 275–8 main types of damage 278 manifestations of damage 278–9 methods 280–91 average degree of polymerisation 290–1 chemical and physical assessment 280 chromatography 284–5 extraction methods 290 IR spectroscopy 285–6 microscopy 283–4 preliminary examination 282–3 procedure 280–2

surface imprint techniques 289–90 thermal analysis 286–9 natural fibres 292–304 cotton 255, 292–5 silk 303–4 wool 295–303, 324–5 streaks and barriness 278, 323–4 synthetics 304–20 acrylic 311–12 elastane (spandex) 312–16, 317 general types of damage 304–6 nylon 309–11 polyester 306–9 polyolefin 316–20 TESS expert system 326 unwanted deposits and stains 278, 279–80, 281–2, 320–3 dark scan 170, 171, 177 De Broglie’s equation 188 dead time 197 decomposition temperature 89, 95–7 decorations 348–9 degree of polymerisation, average 290–1 delustrants 4, 17, 265 denier 9, 69 density, linear 9, 69, 184–5, 246, 247 density gradient column 185 deposits, unwanted 278, 320–3 destructive examination 134 detection 259–60 diameter, fibre 17, 139, 265 animal hairs 48, 49 cotton 247 microspectrophotometry 179–80 differential scanning calorimetry (DSC) 185–6, 287–8 differential thermal analysis (DTA) 185 dispersing prism 166 disulphide bonds 34 DNA amplification technology 229–33 DNA analysis 224–36 conventional DNA hybridisation analysis 228–9, 230 cotton 252–3, 256 effect of fibre processing 229–33 extraction of DNA from animal hairs 226–7 future trends 233–5 selection of target DNA sequences 227–8 dot-blot technique 228–9, 230 double-immersion technique 145 double-sided tape 192–3 Doyle, Arthur Conan 259–60 drawing 81, 82 dry spinning 262 dyes 22, 70–1, 167, 204 analysis and conservation 348 chemical structures 205–6 chromatography see chromatography

Index classification 204–5 extraction 207–12, 220 forensic analysis 206 uptake variability 178–9, 246 Dyneema 89, 90, 94 see also ultra-high molecular weight polyethylene (UHMW-PE) dynamic-mechanical analysis (DMA) 288–9 effective temperature (MEPT) 287–8, 307 Egyptian cotton 240, 251, 252 elastane see spandex elasterell-p (elastomultiester) 114, 119, 120–2 elastoester 12 electromagnetic spectrum 166 electron microscopy conservation 340–3 HPFs 107, 108 SEM see scanning electron microscopy TEM 187 electron spectroscopy 159 electrons 188 interaction with matter 188–9, 190 electropherogram 218 elongation 69–72 sign of 269, 270 eluents for TLC 215, 216 emission spectroscopy 159 energy-dispersive X-ray microanalysis (EDX) 196–7, 340, 341–2 energy states 166–7 energy transitions 167 environmental SEM (ESEM) 195–6, 343 European Fibres Group (EFG) 21 extinction points 269 extra long staple (ELS) cotton 240, 251 extraction DNA from animal fibres 226–7 dyes 207–12, 220 methods and damage analysis 290 eye, and colour perception 172–3 fasteners 348–9 Fehling’s solution 293–4 felting 35 fibre casts 339 fibre variants 114–15, 116, 117 fibrils 75–6, 80, 244 fibroin 27, 31, 32, 33, 84 field emission guns 195 filament 9 film-like deposits 321–2 filter glasses 176 flax 13 flexible chain-based HPFs 88–90 primary structure and physical properties 94

369

fluorescence microscopy 147–9, 171–2, 271–2, 340 fluorescence spectroscopy 160–1 fluoropolymer 12 foam test 283 folded crystal structure 81 follicle 29, 30 forensic analysis 4–5, 6, 7, 259–74 dyes 206 fluorescence microscopy 271–2 forensic mindset 259–61 manufactured fibre production and spinning 262–6 microscopy 261–72 polarised light microscopy 266–71 Fourier transform infrared (FTIR) spectroscopy 20–1, 162, 285, 345–6 freezing 351 Freud, Sigmund 259 fringed micelle model 76–81 Frotté reaction 310, 311 fungal damage 324–5 fur hairs 15 gel spinning 262 gelatine-coated plate imprints 289–90 generic fibre classes 10–13, 112–14 PLA/polylactide 115–19 generic subclasses 112–14, 119–23 genetic engineering 127 genetically modified (GM) cotton 251–2 glass fibre 12 goat fibres 28, 234 see also cashmere; mohair GPC 98 grease deposits 315, 321 guard hairs 15, 36, 46 hairs animal see animal hairs human 45, 46 heat distortion temperature (HDT) 314 heat stabilisers 320 hemp 14, 81–2 heterocyclic polymer fibre 93–4 high performance fibres (HPFs) 88–110, 318 classification 88–90 identification 95–108 analysis of higher-order structure 98–108 analysis of primary structure 97–8 mechanical and thermal characteristics 95–7 primary structure and physical properties 90–5 high performance liquid chromatography (HPLC) 22, 213, 215–18, 219–20 high resolution SEM 195

370

Index

high volume instrument (HVI) testing 248–50 Holmesian Maxim 138, 139 hot stage 18–19 hue 174 human hair 45, 46 humidification 350 humidity 135–6 hybridisation conventional DNA hybridisation analysis 228–9, 230 in situ DNA hybridisation 226 hydrocellulose 293 hydro-entanglement 126 hydrolysis 255–6 identification, and comparison 4, 8–9 immersion type objective lens 195 imprint techniques 289–90 in situ DNA hybridisation 226 inclusions 17, 265 indigo 204 inductively coupled plasma atomic emission spectroscopy (ICP-AES) 160–1 infrared polariser 347–8 infrared (IR) spectroscopy 4, 6, 20–1, 161–2, 181, 260 absorbance bands from synthetic fibres 87 damage analysis 285–6 FTIR 20–1, 162, 285, 345–6 HPFs 97–8 NIR 346, 347, 357 insect damage 300, 351 instrument calibration 176–7 interference colours 269–70 internal reflection spectroscopy (ATR) 21, 162, 285, 346 International Bureau for the Standardization of Man-Made Fibres (BISFA) 112, 114 iron 283 Irvine, Andrew 351 isotropic refractive index 142, 144–5 Jeziorny model for MEPT 287–8 jute 14 kapok 15 keratin 27, 31, 32, 33 see also animal hairs keratinisation 226 Kevlar 89, 90–2 knitted fabrics 323–4 Krais, Markert and Viertel (KMV) reaction 297–8 lab-synthesised polymers 112, 113 lactic acid 119

laser induced breakdown spectroscopy (LIBS) 161 laser-scanning fluorescence microscope 150 lastol 12, 120–2, 318 lastrile 119 lateral (side-by-side) fibres 123 leaf fibres 14 leather, simulated 126, 355–6 licensing system 250 light damage 278, 305 elastane (spandex) 314–15 nylon 310, 311 polyolefin fibres 319–20 wool 299 linear density 9, 69, 184–5, 246, 247 llamas 234 longitudinal specimens 136–7 Lorenz-Lorenz equation 142 lumen 243, 244 lyocell 115, 116, 119, 120 M5 (PIPD) 89, 93–4, 98–102 maceration 339 Mallory, George 351–2 manufactured fibres 9–10, 15–21, 27, 28, 111–30, 261–2 chemical analysis 19–22 fibre subclasses 112–14, 119–23 future trends 126–7 generic classes 10–13, 112–14 polylactide fibre 115–19 instrumental tests 19–21 microscopic analysis 15–19 multicomponent fibres 123–6 polymer origins and fibre classification 112–15, 116, 117 production and spinning 262–6 refractive index 267–8 see also synthetic fibres manufacturers’ analytical methods 3–5, 272 market taxonomy 4–5, 272 mass spectroscopy/spectrometry 159, 217, 218 matrix fibres (islands-in-the-sea fibres) 123 mauveine 204 mechanical damage 278, 305–6 elastane (spandex) fibres 313–14 polyester fibres 306, 307 polyolefin fibres 318 mechanical properties HPFs 89, 95–7 synthetic fibres 69–72 medulla 29, 30, 39 medullae types 42–3, 44, 48, 49 melamine 12 Meldrum’s Stain 182 melt spinning 262 fibre variants 116 melting points 18, 183–4, 286–7

Index HPFs 89, 95–7 manufactured fibres 263, 264 microscopy 146, 147 mercerised cotton 254–5 Mercury, Freddie 355–6 metal coating 193 metal roller 21 metallic fibres 12 metamerism 175 methyl orange crystals 299 micellar electrokinetic chromatography (MEKC) 218–19 micelles 266 Michel-Levy interference colour chart 270 micro internal reflection (MIR) spectroscopy 162 microbiological damage 278, 294 wool 299–300, 324–5 microfibres 18, 199, 265, 306 microfibre fabrics 125–6 microfibrils 75–81 microscopy 3, 4, 6, 23, 260 animal fibres 38–9, 46–8, 61, 225 conservation 336, 337–43 damage analysis 283–4 forensic analysis 261–72 HPFs 104–7 manufactured fibres 15–19 mixtures of chemically equivalent fibres 199 see also under individual techniques microspectrophotometry 19, 22, 165–80 human vs machine colour perception 172–5 limitations and strengths 178–80 metamerism 175 microspectrophotometer design 167–9 microspectroscopy 165, 167, 346 applications in fibre analysis 175–8 data collection 176–7 data evaluation 177–8 sample preparation 175–6 types of 169–72 middle endotherm peak temperature (MEPT) 287–8, 307 mitochondria 226 mitochondrial genes 228 mixtures of fibres 199–200 chemically different fibres 200 chemically equivalent fibres 199 cotton blends 255–6 modacrylic 10 modification ratio 17, 265 mohair 49, 54–5, 59, 224, 225 moisture regain 70–1 Morapex rapid extractor 290 Morelli, Giovanni 259 mounting (in conservation) 351

371

mounting media 46–7, 136, 339 SEM 192–3 multicomponent (conjugate) fibres 123–6 multiphoton fluorescence microscopy 150–1 natural fibres 9–10, 13–15, 72, 81–4, 112 damage analysis 292–304 microspectrophotometry 178–9 see also animal fibres; plant fibres; and under individual names naturally occurring polymers 112, 113, 127 near-infrared (NIR) spectroscopy 346, 347, 357 neps 294–5 neutral density filters 176–7 Newton’s series 270 nitrous gases 315 non-destructive examination 134 nonwoven fabrics 125 normal phase chromatography 218 novoloid 12 nylon 11, 68, 70, 72, 78, 84–5 damage analysis 309–11 distinguishing silk from nylon microfibres 59–61 extraction and classification of dye from 209 nytril 11 oil deposits 315, 321 olefin see polyethylene; polyolefin; polypropylene oligonucleotides 228 optical anisotropy 139–46, 184 aggregate theory 141–6 optical coherence tomography (OCT) 151–2 optical microscopy 133–57, 181 advanced techniques 147–50 animal fibres 38–9, 46–7, 61, 225 conservation 337–40 cotton 253–5 future trends 150–2 HPFs 104–7 identification based on properties 139–47 melting behaviour 146, 147 refractive index and birefringence 139–46 solubility 146 manufactured fibres 15–19 overcoming the classical resolution limit 151 polarised light microscopy see polarised light microscopy practical and quality control considerations 134–7 destructive vs non-destructive examination 134 sampling 134–5

372

Index

specimen preparation 136–7 temperature and humidity conditions 135–6 SEM compared to 197–9 stereo zoom and simple light microscopy 137–9 optical spectroscopy 159 Optim fibres 28, 61–2, 66 organic HPFs see high performance fibres orientation 261, 262 origin (of cotton) 240, 250–3 overhairs 36 oxidative damage cotton 294 elastane (spandex) 315 nylon 310, 311 polyolefin fibres 319–20 oxycarmine test 294 ‘parachute silk’ slip 355 paraffin deposits 315, 321 partition chromatography 217–18 path difference (retardation) 270, 271 Pauly reaction 297 PBI 12 PDO 123 pelage 35–6 pest control 351 pH value 282 photolysis 314–15 photometric accuracy 176–7 photooxidation 319–20 physical properties 16–18, 263–6 animal fibres 33–5 cotton 246, 247 HPFs 89, 95–7 synthetic fibres 69–72 physical testing conservation 344 damage 280 ‘pinhead’ reaction 292–3 PIPD (M5) 89, 93–4, 98–102 plant fibres 13–15 see also under individual names pleat structure 103, 107 polarisability 141–2 polarised light microscopy 18, 139–46, 184, 339–40 crossed polars 140, 141, 269–71 forensic analysis 266–71 poly-3-hydroxybutyrate (P(3HB)) 85 polyacrylonitrile (PAN) see acrylic polyamide see nylon polybutylene terephthalate (PBT) 79, 85 polyester 10, 69, 70, 85, 112–14 damage analysis 306–9 extraction and classification of dye from 210 see also polyethylene terephthalate

polyester oligomers (PET oligomers) 308–9 polyethylene 71, 77, 86 UHMW-PE 89, 90, 94, 95, 96, 98–102, 103–4, 107 polyethylene terephthalate (PET) 79, 85 damage analysis 306–9 HPF from 95 polylactide (PLA) 13, 78, 85, 115–19 microfibre fabrics 125–6 polymer chains 76–81 polymerase chain reaction (PCR) 225, 229–33, 252 polymers molecular structure of polymers for fibres 72, 73–5 polymer origins and fibre classification 112–15, 116, 117 thermal properties 72, 76 see also high performance fibres; manufactured fibers; synthetic fibres polyolefin 12, 86 damage analysis 316–20 see also polyethylene; polypropylene polyolefin keton 89, 94 polyphenylene-benzo-bisoxazole (PBO) 89, 90, 93, 98–102, 104, 105, 107, 108 polypropylene 71, 77, 86 damage analysis 316–20 extraction and classification of dye from 210 polytrimethylene terephthalate (PTT) 79, 85 polyurethane 71, 355–6 polyvinylalcohol (PVA) 68–9, 70, 78 PPT 122–3 PPTA 98–102, 103, 104 preliminary examination 282–3 pressing 305 price 240 primary wall 243, 244 principal axes 141–2 processing DNA analysis and processed fibres 229–33 processing ability of cotton 246–50 product taxonomies 7–8 protein fibres see animal fibres proteinase K 234, 235 proteins 31–2 pupa 38 pyrolysis gas chromatography (GC) 19–20, 186–7 quagga 225 quality control 277 quality faults 294–5 quantitative analysis cotton in textiles 255–6

Index mixtures of fibres 199–200 wool damage 300–3 quantum levels 166–7 quartz wedge compensator 145–6 rabbit, angora 49, 54, 58, 224, 225 Raman spectroscopy 21, 108, 162–3, 346–7 ramie 14 rayon 10–11, 116 ‘reaction to flame’ 3, 6, 181–2, 260, 344 reactive dyes 205 extraction 211 red/green test (GSB test) 295 reference scan 170, 177 reflectance microspectroscopy 170–1, 172 refractive index 16, 18–19, 139–46, 184, 340 forensic analysis 266–9 reshaping 350 resolution optical microscopy 151 SEM 188, 195 restoration 336 restriction fragment length polymorphism (RFLP) 232–3 retardation (path difference) 270, 271 reversals (cotton) 244, 254 reversed phase chromatography 218 rigid chain-based HPFs 88–90 primary structure and physical properties 90–4 royal purple dye 204 rubber 11, 121 sail, HMS Victory 353–5 sample scan 170, 171, 177 sampling conservation and 337 microspectroscopy 175–6 optical microscopy 134–5 saran 11 Satellite II DNA 228 Sayelle 124 scale casts 47 scanning electron microscopy (SEM) 19, 187–99 animal fibres 38–9, 47–8, 197–8 benefits compared to optical microscopy 197–9 conservation 340–2 ESEM 195–6, 343 examination 194–5 factors affecting the image 190–1 further techniques 195–7 mechanics of operation 189–90 principle of operation 188–90 specimen preparation 192–4 scattering spectroscopy 159 Scientific Working Group on Materials Analysis 7

373

sebaceous gland 29, 30 secondary electrons 189, 190–1, 340–1 secondary wall 243, 244 seed fibres 14–15 selective dissolution 200 sericin 32, 84 setting 304–5 sheath-core fibres 123 Shell Chemical 123 shield hairs 36 sign of elongation 269, 270 silica stationary phase plates 214 silicone stains/deposits 283, 285, 286, 316 silk 15, 27, 28, 67, 71, 77 characteristics 44 cultivated and wild silk 30, 38 damage analysis 303–4 identification 59–61, 64, 65 physical and chemical properties 33, 35 production 30–1 structure and composition 32–3, 81, 84 silk lousiness 303 silkworms 38 silver paint 193 simulated leather/suede 126, 355–6 singeing 305 sisal 14 slip, ‘parachute silk’ 355 small-angle X-ray scattering (SAXS) 103–4, 105 softening points 18 solid-state nuclear magnetic resonance (NMR) 107–8 solubility tests 19, 146, 182–3, 344 solvent spinning 120 solvent-spun rayon fibre variants 116 solvents 70–1, 183 Soxhlet extractor 290 spandex 11, 121 damage analysis 312–16, 317 speciality fibres 224–5 see also DNA analysis species-specific DNA sequences 227–8 specific gravity 70–1 specimen preparation optical microscopy 136–7 SEM 192–4 Spectra 89, 90, 94 see also ultra-high molecular weight polyethylene (UHMW-PE) spectral comparisons 177–8 spectrometer 159 spectrometry 158–9 spectrophotometer 166, 168 spectroscope 159 spectroscopy 158–64, 166–7 categorising methods 159 by measurement process 159 by nature of excitation 159

374

Index

conservation 344–8 infrared see infrared (IR) spectroscopy Raman 21, 108, 162–3, 346–7 visible 161 see also microspectrophotometry; microspectroscopy spider silk 15, 95, 127 spinning 90, 262–6 cotton 246–8 staining tests 182, 291 conservation 343–4 wool damage 296–7 stains 278, 279–80 analysis of 281–2, 320–3 standard dye mixtures 215, 217 staple fibres 9 stationary phase 213–14, 215–16 stereo zoom microscopy 136–8 stereomicroscopy 46, 337, 338 stimulated emission depletion (STED) 151 streaks 278, 323–4 subclasses, fibre 112–14, 119–23 suede, simulated 126 sulfar 12 Supima cotton 250, 253 supply chain, cotton 240–1 support (in conservation) 350 surface imprint techniques 289–90 surface treatments cotton 254–5 SEM and examination of 198–9 surrogate materials 349, 354 synthetic fibres 10, 68–87, 261, 262 crystal structure 77–9, 86–7 damage analysis 304–20 fundamental characteristics 72–84 identification 87 performance 69–72 refractive index 267–8 see also manufactured fibres; and under individual names taxonomies, product 7–8 technical fibres 13 Technora 89, 90, 92 temperature decomposition temperature 89, 95–7 heat distortion temperature 314 melting points see melting points MEPT 287–8, 307 optical microscopy conditions 135–6 tensile strength 69, 70–1 HPFs 89, 91, 95 TESS expert system 326 testing institutes 277 tex 9 textile damage see damage analysis Textile Fiber Products Identification Act (TFPIA) 10–13, 112–14

texturising 304 thermal analysis 286–9 thermal cycling machine 230 thermal damage 278, 304–5 acrylic 312 elastane (spandex) 314 nylon 309–10, 311 polyester 306, 307 polyolefin 318–19 wool 299 thermal microscopy 18–19 thermogravimetric analysis (TGA) 186, 288 thermomechanical analysis (TMA) 288–9 thermomechanical damage 278, 318–19 thermoplastic film imprints 289–90 thin layer chromatography (TLC) 22, 212–15, 216, 217, 219 damage analysis 284–5 three-dimensional crimp 124–5 tight threads 278 titrimetric methods 185 transmission electron microscopy (TEM) 187 transmission microspectroscopy 169, 170, 172, 177 transverse (cross-sectional) specimens 137 triacetate 71, 119 tricomponent fibres 123 triexta 119, 122–3 trousers, Freddie Mercury’s 355–6 tussah (wild) silk 30, 38 Twaron 89, 90 twist test 344 ultra-high molecular weight polyethylene (UHMW-PE) 89, 90, 94, 95, 96, 98–102, 103–4, 107 underhairs 36 uniaxial symmetry 142–3 Universal Cotton Standards Agreement 248 Upland cotton 251, 252 UV stabilisers 320 upper half mean length (UHML) 246, 247 value (colour) 174 variants, fibre 114–15, 116, 117 Vectran 89, 90, 93 Victory sail 353–5 vicuña 234 vinal 11–12 vinyon 12 virtual sectioning 150 viscose 208 viscose rayon 71, 77 fibre variants 115 viscosity 98, 291 visible-range spectroscopy 161 see also microspectrophotometry; microspectroscopy

Index visual colour/dye comparison 22 visual examination 282 voids (air pockets) 17, 265 wavelength (of electron) 188 wavelength accuracy 176 wavelength dispersive X-ray microanalysis 196 wax deposits 315, 321 weighted silk 304 wet spinning 262 wholly aromatic polyester fibre 89, 90, 92–3 wide-angle X-ray diffraction (WAXD) 98–102 wild (tussah) silk 30, 38 Wintuk 124 wool 34–5, 71, 81, 82–4 applications 37–8 damage analysis 295–303

375

biological damage 299–300, 324–5 cumulative damage 303 indications of damage 295–6 quantitative analysis 300–3 extraction and classification of dye from 207, 211 identification 48–51, 52, 53 woven fabrics 323–4 X-ray diffraction 77–9, 86 X-ray microanalysis 196–7, 340, 341–2 X-rays, characteristic 189, 190, 191 Young’s modulus 70–1, 72 Zylon 89, 90, 93 see also polyphenylene-benzo-bisoxazole (PBO)