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Advances in knitting technology
i © Woodhead Publishing Limited, 2011
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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 that 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 web site at: www.woodheadpublishing.com. The Textile Institute books that are 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 xiii to xviii.
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Woodhead Publishing Series in Textiles: Number 89
Advances in knitting technology Edited by K. F. Au
iii © Woodhead Publishing Limited, 2011
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Published by Woodhead Publishing Limited in association with The Textile Institute Woodhead Publishing Limited, 80 High Street, Sawston, Cambridge CB22 3HJ, UK www.woodheadpublishing.com Woodhead Publishing, 1518 Walnut Street, Suite 1100, Philadelphia, PA 19102-3406, USA Woodhead Publishing India Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi – 110002, India www.woodheadpublishingindia.com First published 2011, Woodhead Publishing Limited © Woodhead Publishing Limited, 2011 The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publisher cannot assume responsibility for the validity of all materials. Neither the authors nor the publisher, 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. ISBN 978-1-84569-372-5 (print) ISBN 978-0-85709-062-1 (online) ISSN 2042-0803 Woodhead Publishing in Textiles (print) ISSN 2042-0811 Woodhead Publishing in Textiles (online) The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elemental chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by RefineCatch Limited, Bungay, Suffolk, UK Printed by TJI Digital, Padstow, Cornwall, UK
iv © Woodhead Publishing Limited, 2011
Contents
Contributor contact details Woodhead Publishing Series in Textiles
xi xiii
Part I Introduction: fundamentals of knitting
1
1
3
Types and suitability of yarns for knitting
E. Mielicka, Textile Research Institute, Poland
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12
Introduction Types of packages Structure of cope Knitting yarns defects Types of knitting yarns: yarns made of natural fibers Types of knitting yarns: yarns made of synthetic fibers Types of knitting yarns: fancy threads Yarns made for special applications Methods of joining the polyurethane yarns in composites Other yarns for special applications Future trends References
3 7 10 13 19 21 26 28 32 34 34 35
2
The physical properties of weft knitted structures
37
B. Cooke, Formerly Senior Lecturer in Knit Design (UMIST),
University of Manchester, UK
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9
Introduction Stretch and recovery properties Recovery properties Dimensional stability Creasing Thickness and compression properties Air permeability Thermal properties Liquid transfer properties
37 38 39 40 41 42 43 43 44 v
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Contents
2.10 2.11 2.12 2.13
Comfort Pilling and abrasion Knitted fabrics with special properties Sources of further information and advice
45 45 46 47
3
Modelling of knitting
48
R. B. Ramgulam, Albany International, Bury, UK
3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9
Introduction Knitted fabric geometry Mechanics of knitted fabric: 2D model Mechanics of plain-weft knitted fabrics: 3D model Knitted fabric mechanics: energy model Knitted fabric pressure on a surface Heat and water vapour diffusion in fabrics References Appendix: initial guess estimates
48 48 51 59 66 72 80 83 84
Part II Advances in knitting
87
4
89
Intelligent yarn delivery systems in weft knitting
R. Kovar, Technical University of Liberec,
Czech Republic
4.1 4.2 4.3 4.4 4.5 4.6 4.7
Introduction Theory of yarn delivery Yarn storage and delivery systems on circular knitting machines Yarn storage and delivery systems on flat knitting machines Future trends Sources of further information and advice References
5
Advances in warp knitted fabric production
89 90 103 107 109 109 109 110
Bharat J. Gajjar, Warp Knits, Delaware, USA
5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8
Introduction Commercial warp knit machines Delaware stitch and modified Delaware stitch tricot fabrics Tricot and Raschel containing spandex Key Raschel fabrics containing spandex Newly developed constructions with spandex Americana and modified Americana tricots Surface interest fabrics
© Woodhead Publishing Limited, 2011
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Contents
vii
5.9 5.10 5.11
Milanese fabrics Conclusion Sources of further information
132 135 135
6
Weft-knitted structures for industrial applications
136
M. de Araujo and R. Fangueiro, University of Minho,
Portugal and H. Hu, Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hong Kong, China
6.1 Introduction 6.2 Current problems and limitations 6.3 Knitting 3D structures using weft-knitting technology 6.4 Directionally oriented structures (DOS) and combined DOS weft-knitted structures 6.5 Weft-knitted multifunctional structures 6.6 Simulating mechanical properties of weft-knitted structures 6.7 Applications 6.8 Future trends 6.9 References
136 138 141
7
171
Advances in circular knitting
148 155 157 163 167 168
D. Semnani, Isfahan University of Technology, Iran
7.1 7.2 7.3 7.4 7.5 7.6 7.7
Introduction Current problems and limitations of circular knitted structures Recent advances in circular knitting Structure and properties of circular knitted fabrics Applications Future trends: smart garments References
171
8
Knitted fabric composites
193
175 177 182 184 189 190
M. Duhovic and D. Bhattacharyya, University of Auckland,
New Zealand
8.1 Introduction 8.2 Types of fibre and yarn used in knitted fabric composites 8.3 Composite preforms 8.4 Knit structures for fabric composites 8.5 Types of matrix materials 8.6 Developments in manufacturing methods for knitted fabric composites 8.7 Mechanical properties
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viii
Contents
8.8 8.9 8.10 8.11
Applications Conclusion Acknowledgements References
9 Quality control in the knitting process and common knitting faults K. F. Au, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong, China 9.1 Importance of quality knitted fabric 9.2 Knitted fabric quality 9.3 Quality control in the knitting process 9.4 Parameters of knitting control 9.5 Relationship between yarn count and machine gauge 9.6 Examples of quality control mechanisms for circular knitting 9.7 Techniques to reduce knitting faults: online data monitoring system 9.8 Knitted defects 9.9 Conclusion 9.10 References
208 209 210 210 213
213 214 215 217 220 220 224 225 231 231
Part III Case studies: advanced knitted products
233
10
235
Women’s apparel: knitted underwear
J. Kar, J. Fan, and W. Yu, Institute of Textiles and Clothing,
Hong Kong Polytechnic University, Hong Kong, China
10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8
Introduction Functional requirements of knitted underwear Performance evaluation of knitted underwear Engineering of knitted underwear fabrics Recent developments in knitted underwear fabrics Properties of commercially knitted underwear fabrics Acknowledgements References
235 235 239 252 255 256 258 258
11
Knitted structures for sound absorption
262
R. M. Monaragala, Ministry of Defence, Sri Lanka
11.1 11.2 11.3 11.4 11.5 11.6
Introduction Acoustic textiles in vehicles Sound absorption of plain knitted structures Engineering advanced knitted fabrics for sound absorption Thick spacer structures Dense spacer structures
© Woodhead Publishing Limited, 2011
262 262 263 275 275 280
Contents
ix
11.7 11.8 11.9 11.10
Conclusion Future trends Sources of further information and advice References
283 284 284 285
12
Weft-knitted structures for moisture management
287
G. B. Delkumburewatte, Open University of Sri Lanka,
Sri Lanka
12.1 12.2 12.3 12.4 12.5 12.6 12.7
Introduction Basics of wetting Wicking and absorption Experimental liquid take-up Future trends Sources of further information and advice References
287 288 291 298 306 306 307
Index
309
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Contributor contact details
(* = main contact)
Editor and Chapter 9
Chapter 3
K. F. Au Institute of Textiles and Clothing The Hong Kong Polytechnic University Hung Hom Hong Kong, China
R. B. Ramgulam Albany International Pilsworth Road Bury BL9 8RS UK E-mail:
[email protected]
E-mail:
[email protected]
Chapter 4
Chapter 1 E. Mielicka Textile Research Institute Piotrkowska 270 90-361 Lodz Poland
R. Kovar Technical University of Liberec Studentska 2 str. 461 17 Liberec Czech Republic E-mail:
[email protected]
E-mail:
[email protected]
Chapter 5
Chapter 2 B. Cooke Formerly Senior Lecturer in Knit Design (UMIST) William Lee Innovation Centre School of Materials University of Manchester M13 9PL UK
B. J. Gajjar 614 Loveville Road Building D-1F Hockessin DE 19707 USA E-mail:
[email protected]
E-mail:
[email protected]
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Contributor contact details
Chapter 6
Chapter 10
M. de Araujo* and R. Fangueiro School of Engineering University of Minho 4800–058 Guimarães Portugal
J. Kar, J. Fan, and W. Yu Institute of Textiles and Clothing The Hong Kong Polytechnic University Hung Hom Hong Kong, China
E-mail:
[email protected]
H. Hu Institute of Textiles and Clothing The Hong Kong Polytechnic University Hong Kong, China
Chapter 7 D. Semnani Isfahan University of Technology Isfahan 84156-8311 Iran
E-mail:
[email protected] [email protected]
Chapter 11 R. M. Monaragala Chief Coordinator Electronics Wing Centre for Research and Development Ministry of Defence 15/5 Baladaksha Mawatha Columbo 03 Sri Lanka
E-mail:
[email protected]
E-mail:
[email protected] [email protected]
Chapter 8
Chapter 12
M. Duhovic* and D. Bhattacharyya Centre for Advanced Composite Materials Building 740 Tamaki Campus Morrin Road Glen Innes Department of Mechanical Engineering University of Auckland New Zealand
G. B. Delkumburewatte Senior Lecturer Head/Department of Textile and Apparel Technology Open University of Sri Lanka P.O. Box 21 Nawala Nugegda Sri Lanka E-mail:
[email protected]
E-mail: m
[email protected] d.bhattacharyya@auckland. ac.nz
© Woodhead Publishing Limited, 2011
Woodhead Publishing Series in Textiles
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Watson’s textile design and colour, Seventh edition Edited by Z. Grosicki Watson’s advanced textile design Edited by Z. Grosicki Weaving, Second edition P. R. Lord and M. H. Mohamed Handbook of textile fibres, Volume 1: Natural fibres J. Gordon Cook Handbook of textile fibres, Volume 2: Man-made fibres J. Gordon Cook Recycling textile and plastic waste Edited by A. R. Horrocks New fibers, Second edition T. Hongu and G. O. Phillips Atlas of fibre fracture and damage to textiles, Second edition J. W. S. Hearle, B. Lomas and W. D. Cooke Ecotextile ’98 Edited by A. R. Horrocks Physical testing of textiles B. P. Saville Geometric symmetry in patterns and tilings C. E. Horne Handbook of technical textiles Edited by A. R. Horrocks and S. C. Anand Textiles in automotive engineering W. Fung and J. M. Hardcastle Handbook of textile design J. Wilson High-performance fibres Edited by J. W. S. Hearle Knitting technology, Third edition D. J. Spencer Medical textiles Edited by S. C. Anand xiii © Woodhead Publishing Limited, 2011
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Chemical testing of textiles Edited by Q. Fan Design and manufacture of textile composites Edited by A. C. Long Effect of mechanical and physical properties on fabric hand Edited by Hassan M. Behery New millennium fibers T. Hongu, M. Takigami and G. O. Phillips Textiles for protection Edited by R. A. Scott Textiles in sport Edited by R. Shishoo Wearable electronics and photonics Edited by X. M. Tao Biodegradable and sustainable fibres Edited by R. S. Blackburn Medical textiles and biomaterials for healthcare Edited by S. C. Anand, M. Miraftab, S. Rajendran and J. F. Kennedy Total colour management in textiles Edited by J. Xin Recycling in textiles Edited by Y. Wang Clothing biosensory engineering Y. Li and A. S. W. Wong Biomechanical engineering of textiles and clothing Edited by Y. Li and D. X-Q. Dai Digital printing of textiles Edited by H. Ujiie Intelligent textiles and clothing Edited by H. Mattila Innovation and technology of women’s intimate apparel W. Yu, J. Fan, S. C. Harlock and S. P. Ng Thermal and moisture transport in fibrous materials Edited by N. Pan and P. Gibson Geosynthetics in civil engineering Edited by R. W. Sarsby Handbook of nonwovens Edited by S. Russell Cotton: Science and technology Edited by S. Gordon and Y.-L. Hsieh Ecotextiles Edited by M. Miraftab and A. R. Horrocks Composite forming technologies Edited by A. C. Long
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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 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. Deopura, 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 and healthcare textiles Edited by S. C. Anand, J. F. Kennedy, 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 textile materials 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 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 © Woodhead Publishing Limited, 2011
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83 Smart clothes and wearable technology Edited by J. McCann and D. Bryson 84 Identification of textile fibres Edited by 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 88 Handbook of textile fibre structure, Volume 1 and Volume 2 Edited by S. J. Eichhorn, J. W. S. Hearle, M. Jaffe and T. Kikutani 89 Advances in knitting technology Edited by K. F. Au 90 Smart textile coatings and laminates Edited by W. C. Smith 91 Handbook of tensile properties of textile and technical fibres Edited by A. R. Bunsell 92 Interior textiles: Design and developments Edited by T. Rowe 93 Textiles for cold weather apparel Edited by J. T. Williams 94 Modelling and predicting textile behaviour Edited by X. Chen 95 Textiles, polymers and composites for buildings Edited by G. Pohl 96 Engineering apparel fabrics and garments J. Fan and L. Hunter 97 Surface modification of textiles Edited by Q. Wei 98 Sustainable textiles Edited by R. S. Blackburn 99 Advances in yarn spinning technology Edited by C. A. Lawrence 100 Handbook of medical textiles Edited by V. T. Bartels 101 Technical textile yarns Edited by R. Alagirusamy and A. Das 102 Applications of nonwovens in technical textiles Edited by R. A. Chapman 103 Colour measurement: Principles, advances and industrial applications Edited by M. L. Gulrajani 104 Textiles for civil engineering Edited by R. Fangueiro © Woodhead Publishing Limited, 2011
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105 New product development in textiles Edited by B. Mills 106 Improving comfort in clothing Edited by G. Song 107 Advances in textile biotechnology Edited by V. A. Nierstrasz and A. Cavaco-Paulo 108 Textiles for hygiene Edited by B. McCarthy 109 Nanofunctional textiles Edited by Y. Li 110 Joining textiles Edited by I. Jones and G. Stylios 111 Soft computing in textiles Edited by A. Majumdar 112 Textile design Edited by A. Briggs-Goode and K. Townsend 113 Biotextiles as medical implants Edited by M. King and B. Gupta 114 Textile thermal bioengineering Edited by Y. Li 115 Woven textile structure B. K. Behera and P. K. Hari 116 Handbook of textile and industrial dyeing. Volume 1: Principles processes and types of dyes Edited by M. Clark 117 Handbook of textile and industrial dyeing. Volume 2: Applications of dyes Edited by M. Clark 118 Handbook of natural fibres, Volume 1: Types, properties and factors affecting breeding and cultivation Edited by R. Kozlowski 119 Handbook of natural fibres, Volume 2: Processing and applications Edited by R. Kozlowski 120 Functional textiles for improved performance, protection and health Edited by N. Pan and G. Sun 121 Computer technology for textiles and apparel Edited by Jinlian Hu 122 Advances in military textiles and personal equipment Edited by E. Sparks 123 Specialist yarn, woven and fabric structure: Developments and applications Edited by R. H. Gong 124 Handbook of sustainable textile production M. Tobler © Woodhead Publishing Limited, 2011
2 The physical properties of weft knitted structures B. COOKE, Formerly Senior Lecturer in Knit Design (UMIST), University of Manchester, UK Abstract: Modern knitted fabrics cover a wide range of structures; therefore this chapter is confined to a generalised summary of weft knitted fabrics. It explains why knitted fabrics stretch more easily than woven fabrics made from similar materials and compares the dimensional ability, creasing and properties of both fabrics. Key words: knitted fabrics, weft, woven, yarn, creasing, properties.
2.1
Introduction
Modern knitted fabrics range from gossamer thin veil-like structures knitted on ultra-fine gauge circular weft knitting machines to immensely strong rigid multi-layer multi-axial structures produced on Raschel warp knitting machines, and include every conceivable combination of properties in between these two extremes. For this reason any attempt to discuss or characterise the physical properties of knitted structures within the confines of a single chapter must be limited to a generalised summary of the commercially significant weft knitted fabrics. A further complication that arises in such a discussion is the extent to which the yarn properties influence the properties and performance of the yarn/structure combination. For example if a comparison is made between a simple plain-weft knitted structure (single jersey) knitted from textured polyester and the same structure knitted on the same machine from PTFE coated stainless steel, then the polyester fabric will be soft, warm and stretchy and the stainless steel fabric will be hard, cold and unyielding and with relatively little stretch – the same knitted structure producing totally different fabrics with radically different properties. For this reason the discussion will be limited to fabrics produced with conventional textile yarns. On a historical timescale knitting is a relatively modern technique. Weaving was developed around 7000 BC in the Middle East, whereas weft knitting was most probably developed in Coptic Egypt in the third to fourth century AD, more than 7000 years later. The early uses of knitted fabrics are not known, but by the ninth century AD weft knitted stockings were in use in the Arab world and by the tenth century two-colour jacquard patterned stockings were relatively common in Egypt. Knitting spread from North Africa to Spain following the Arab conquest of the southern regions and by the twelfth century had spread into northern Spain 37 © Woodhead Publishing Limited, 2011
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Advances in knitting technology
and southern France. The manufacture of knitted products then disseminated north and east through Europe reaching Italy in the 14th century, Britain in the 15th century and Scandinavia in the 16th and 17th centuries. By the time of Elizabeth I of England, knitted products were well established in three main product categories: stockings, gloves and hats; and their success in these markets was due to their physical properties, which could not be replicated by weaving. For each of these products stretch, warmth, softness and conformability were desirable; properties inherent in knitted wool. In contrast warm woven fabrics were thick, hard, stiff and unyielding with very little stretch. During the following three centuries knitted products substantially retained the same markets, only expanding into outerwear in the nineteenth century with the development of jerseys and guernseys and into household textiles with the development of warp knitting and the manufacture of lace and blankets.
2.2
Stretch and recovery properties
In order to understand why knitted fabrics stretch so much more easily than woven fabrics made from similar materials it is necessary to study their basic structure. In simple woven structures the yarns pass through the fabric in an undulating fashion diverting above and below the yarns in the other system (see Plate I between pages 46 and 47). This undulation is called crimp (warp or weft crimp). Consequently when a stress force is applied across the fabric there is only scope for relatively little extension, in the region of 3–5%, before the crimp is removed and the yarns straighten. At this point a much greater force is required to stretch the yarns, usually more than a wearer is able to tolerate, and further stretch beyond this point becomes uncomfortable. In contrast to woven fabrics where the accessible extension is usually less than 10%, knitted fabrics are often able to develop very high extensions, as high as 100%, through totally different deformation mechanisms. The diagram of a model of the simplest plain fabric shows the fabric to be constructed of linear arrays of needle loops linked together horizontally by sinker loops and vertically by interlinking (see Plate II between pages 46 and 47). Horizontal stretch is achieved in the first instance by changes in curvature (flattening) of the base of the sinker loops and the top of the needle loops. This is typically a low modulus deformation. As extension proceeds and the curved yarn segments straighten, sufficient stress eventually develops to exceed the inter-yarn frictional forces at the cross-over points or binding areas (see Plate III between pages 46 and 47) and yarn interchange takes place moving yarn segments from the vertical sides of the loops into the tops and bottoms of the loops. In this way a plain-knit fabric may stretch by 15–20% in the width without any significant yarn extension taking place. In a similar manner extension in the length axis occurs initially by a straightening of the sides of the loops. Then as yarn stress levels rise yarn interchange moves yarn segments from the bottoms of the sinker
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The physical properties of weft knitted structures
39
loops and tops of the needle loops into the vertical sides of the loops, and the loops lengthen. Axial stretch is normally lower than width-wise stretch, typically 5–10%. When the knitted loops are interlinked into rib structures such as 1 × 1 rib, then a further stretch mechanism comes into play (see Plate IV between pages 46 and 47). In order to explain this mechanism it is helpful to examine the top view of a section taken across the wales. This figure shows that the face and reverse loops are joined by inter-stitches that link the face and reverse in a ‘concertina’-like manner. As a result of the bending and torsional forces in the inter-stitches, rib fabrics contract once they leave the needles with the result that the wale density becomes much higher than the needle density on the knitting machine. Typically 1 × 1 rib structures may contract by as much as 50% of their knitted width. When lateral forces are then applied to the retracted structure it is able to extend to its original knitted width before the mechanisms described above come into play. This ‘concertina’ deformation provides very low modulus extension and for this reason rib structures are used at the cuffs, neck and waist of garments where high stretch is essential for ease of fit. Normally 1 × 1 rib and 2 × 2 rib structures provide the greatest stretch and therefore are most frequently used in this way. Whenever horizontal linear yarn elements are incorporated into the structure, for example inlay or floats such as those in jacquard fabrics, then the width stretch is greatly reduced.
2.3
Recovery properties
Once a knitted structure has been stretched in use, for example when the neck of a sweater has been stretched over the wearer’s head, it is desirable for the appearance of the garment that the fabric should contract or recover to its original dimension. The deformation described above that involves a flattening of the curvature of the yarn loops usually recovers quickly and can be considered to be largely elastic. However, the second stage of deformation involving yarn movement against frictional resistance tends to act like a ‘ratchet mechanism’, and is not recovered immediately and may not recover at all without ‘outside’ help, leading to hysteresis in the stress–strain cycle. This fraction of the original extension can be considered as a ‘set’ and can cause a garment to become ‘baggy’. This explains why rib fabrics are so much better than plain fabrics when used for cuffs, necks and waists because they can provide the necessary stretch without the yarn forces rising to a level where yarn interchange against friction takes place. The process of recovery in weft knitted fabric is complicated by the mechanism of relaxation shrinkage and this will be discussed under dimensional stability below. If an even greater level of stretch is desired, for example the top rib of socks, then the ‘concertina’ contraction may be enhanced by incorporating an elastomeric yarn element under tension into the structure in such a way that once the fabric leaves the needles the stress in the elastomer forces the fabric to contract much
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more than normal. The elastomer is typically introduced as an ‘inlay’ element. When the elastomer is incorporated as a purely horizontal element, the main effect is to enhance the width-wise stretch. If the elastomer is incorporated under tension into the loop structure it will cause the fabric to contract in both the width and axial directions and thus enhance the stretch in both directions. A further advantage of using elastomer in knitted fabrics is the improvement in recovery following extension that such elements provide.
2.4
Dimensional stability
Fabrics are considered to have good dimensional stability if they resist permanent deformation during use and during washing and dry-cleaning. When compared with woven structures, most simple knitted fabrics cannot be considered to have good dimensional stability. First they do not recover well from stretching due to the ‘ratchet mechanism’ described above. Second, they are subject to a process known as relaxation shrinkage which commences as soon as the fabric/garment is removed from the knitting machine and does not cease until the product has been washed or dry-cleaned several times. Third, if they are made from untreated animal fibres such as wool or alpaca they may also be liable to felting shrinkage. Relaxation shrinkage leads to progressive changes in the width and axial dimensions of the textile, which usually results in reductions in both dimensions. However, this is not always the case, and fabric that has been excessively stretched in length during finishing and has not been effectively stabilised by heat-setting or consolidation may increase in width during wash and wear. This problem is frequently encountered with cotton T-shirts. Relaxation shrinkage occurs due to two different mechanisms. The majority of common textile polymers are non-Newtonian in their response to stress. That is to say they have a non-linear stress–strain curve and they ‘creep’ (extend) with time under constant load. When the force is removed the creep deformation is not immediately recovered and this deformation then gradually reduces over time. On the way into the knitted fabric the yarn is subjected to quite high tensions and a proportion of the deformation (stretch) that occurs is retained in the yarn after knitting as creep deformation. With time this deformation reduces and the yarn becomes shorter with the consequence that the knitted loops become smaller and the fabric will shrink in the width and length. In the case of hydrophilic polymers immersion in water will accelerate this recovery and steaming will remove some of the creep deformation for both hydrophilic and hydrophobic polymers. The second cause of relaxation shrinkage is the mechanism described above that enables knitted fabric to stretch easily. During most knitting processes the fabric is subject to take-down tension and the needles are generally spaced further apart than the loops will lie in the finished fabric. As a consequence the fabric is stretched in both the width and length and a proportion of the stretch will be in the form of
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the ratchet type deformation that will not recover immediately once the fabric/ garment is removed from the machine. This residual deformation will be stored in the fabric in addition to the yarn creep deformation and together they have the potential to cause quite considerable dimensional changes in the form of relaxation shrinkage as the product moves along the supply chain towards the user, and even after purchase during the first few cycles of use. Unfortunately a third cause of dimensional change can also occur with hydrophilic fibres such as cotton and wool and that is conventional shrinkage caused by contact with water. When yarns are not pre-shrunk they have the potential to reduce in length on wetting and this latent shrinkage is in addition to the two forms of relaxation shrinkage described above. As a consequence of these causes of shrinkage, knitted products often have a reputation for poor dimensional stability. Fortunately solutions do exist. Thermoplastic fabrics can be heat-set to stable dimensions and hydrophilic fabrics can be finished through tension-free processes or through consolidation machinery that forces them to a stable configuration. The main difficulties now arise with garments produced from dyed yarns that are not subjected to wet finishing or tumble drying and thus retain their potential for relaxation shrinkage. Since the advent of Superwash wool and other shrink resistant wool yarns the problem of felting shrinkage has been largely eliminated. However, untreated animal fibres will felt if treated in hot/wet conditions with agitation. The effect of felting is to cause the yarn to retract and thicken and as a consequence the fabric will reduce in width and length and increase in thickness. Tumble drying a wet garment often precipitates felting and can lead to severe irreversible shrinkage.
2.5
Creasing
Most flexible laminar materials are subject to creasing. Creasing occurs when the sheet of material is folded or bent, and the sharper the fold and the longer it is maintained the more permanent the crease will become. Materials such as paper, which are easily deformed into very sharp folds, are subject to serious permanent crease damage. When normal writing or printing paper is folded the crease easily becomes so sharp that it becomes a permanent feature and indeed creasing can be used to convert paper into three-dimensional forms. This permanent creasing is exploited in the Japanese art form origami. The mechanism of creasing is interesting because it involves both time-dependent deformation (creep) as well as compressive yield. Paper and most textiles are made from fibres containing linear polymers. When these fibres are bent into a sharp radius of curvature the molecules in the outer half of the bend suffer extension and if this bend is maintained over time they develop creep deformation. Conversely the molecules on the inner half of the bend are subjected to compressive forces, and as the compressive yield stress is so much lower than the extensive yield stress for any given polymer they can easily suffer compressive yield that is an irreversible failure (collapse) of the molecular structure. In other words the crease becomes permanent.
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In textile materials the extent to which this mechanism will occur depends on the type of polymeric material and the radii of curvature that develop in the fibres as the fabric is folded. In general terms creasing is most severe in thin fabrics where the fibres are rigidly held in place, for example in thin tightly woven cotton poplins, where they are constrained to bend to tight radii during creasing. Knitted fabrics are by their nature less susceptible to creasing than woven fabrics. Generally knitted fabrics are thicker than woven fabrics (see section on fabric thickness below) and the yarns are more mobile in the fabric structure than the yarns in woven fabrics. In addition, knitting yarns are twisted to lower twist factors than weaving yarns and the fibres are less tightly held in the yarn structure. Thick fabrics bend to lower radii of curvature than thin fabrics and the more mobile yarn and fibre arrangement means that yarns can roll or bend in such a way that their bending radii are reduced and permanent deformation is avoided. For all these reasons a knitted fabric will crease less than woven fabric of a similar area density and the creases that do form are more easily removed. It should be noted that wet or steam ironing will remove the appearance of creases from the majority of fabrics both knitted and woven. However, once the fibres in a particular section of yarn have suffered compressive yield they are permanently weakened and creases are much more likely to occur in exactly the same places. This process is particularly evident with cotton denim jeans. Creases develop at the tops of the legs where the legs meet the pelvis and behind the knees and although they appear to vanish after washing and ironing they reappear as soon as they are worn again. The effect is highlighted by the fact that the denim loses colour along the crease line and the permanent creases show as pale lines against the dark blue background.
2.6
Thickness and compression properties
Fabric thickness is an extremely important parameter for a variety of reasons. Fabric thickness is an important variable in determining fabric stiffness and hence the extent to which the fabric will drape and conform. Fabric thickness is the most important variable determining the rate of heat transfer and hence the so-called ‘warmth’ of the fabric. Fabric thickness affects air permeability and moisture absorbency and also has a great influence on the abrasion resistance. For the majority of simple woven fabrics, plain weave, twills and satins, the fabric thickness is very close to the sum of the yarn diameters in the warp and weft. This is because the crimp curvature is small and is offset by the yarn compression that occurs between the warp and weft yarns under the warp and weft crowns. In contrast, when a cross-section through a plain knit is examined, it is apparent that yarn curvature out of the plane of the fabric contributes to the effective fabric thickness such that the thickness is greater than twice the yarn diameter. When rib and double jersey fabric sections are studied, yarn curvature is again present together with the inter-stitch that joins the face and reverse loops,
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and in this case the fabric thickness is significantly greater than four times the yarn diameter. Although it is difficult to compare knitted and woven fabrics on a like-for-like basis because they still tend to occupy different market niches, it is generally a fact that knitting yarns have a much higher diameter than weaving yarns and this together with the factors discussed above means that knitted fabrics are usually much thicker than woven fabrics although exceptions can always be found to such generalisations. If comparable woven and knitted fabrics are compared in terms of their compression properties, knitted fabrics are found to be much more easily compressed than woven fabrics. Weaving yarns are almost always more highly twisted than knitting yarns and are therefore harder and less easily compressed. Most of the fabric thickness is occupied by two yarn diameters one on top of the other and the application of a compressive force almost immediately results in yarn compression. In other words the compressive modulus of the fabric is almost immediately equal to the compressive modulus of the yarns, which themselves are hard twisted. A crosssection through a plain knitted fabric shows that only approximately two-thirds of the fabric thickness is occupied by the two yarn diameters. The resistance to compression is therefore initially due to bending forces in the loop segments rather than the direct compression of the yarns, and when the yarn segments are flattened such that the yarns themselves start to compress the softer more open yarn structure results in a much lower compression modulus. In short, knitted fabrics are much softer and more easily compressed than comparable woven fabrics.
2.7
Air permeability
The diagrams shown in the colour section in the discussions of stretch, thickness and compression do not provide much help in assessing the comparative air permeability of woven and knitted fabrics. Air permeability is linked to fabric coverfactor, and photographic images give a much better impression of cover-factor than diagrams. Cover-factor is defined as the ratio of the area covered by the yarn to the area covered by the fabric. A fabric with zero permeability would therefore have a cover factor of 1. The photographs show that the single jersey fabric has a much lower cover factor than a comparable plain weave and therefore the air permeability would be much higher. Because each knitted loop in effect encloses a ‘hole’ in the fabric the air permeability of knitted fabrics is generally much higher than that of woven fabrics and knitted fabrics are not used where wind resistance is required unless they are coated with an airflow resistant coating such as polyurethane.
2.8
Thermal properties
As the majority of textile fibres have very similar coefficients of thermal conductivity, two main structural factors determine the thermal conductivity of
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fabrics; the thickness, and the extent to which the structure traps stationary air. Knitted fabrics are naturally much thicker than equivalent woven fabrics (see the section on fabric thickness above), and as discussed above the knitted loop encloses a cell or ‘hole’ in the fabric structure that is capable of trapping air under the right conditions. Double knits and ribs have two layers of offset loops and therefore two layers of offset holes. The combination of high thickness and a high volume of trapped air leads to fabrics with low thermal conductivity provided they are protected from wind pressure by windproof outer layers or shell fabrics. The thermal insulation of knitted fabrics can be improved further by using techniques such as plush knitting with micro-fibre yarns. This technique increases the fabric thickness, and subsequent raising and cropping produces fleece fabrics that have outstanding thermal insulation properties together with a higher resistance to ‘wind chill’ than conventional knits, and they have replaced conventional sweaters in the performance sportswear market.
2.9
Liquid transfer properties
Liquid transfer through textiles is usually a two-stage process. Firstly the liquid wets the surface of the fabric and is absorbed into the yarn by capillary attraction and also into the fibre structure in the case of hydrophilic polymers. Secondly the liquid moves through the structure by a combination of diffusion and capillarydriven wicking. In the case of fabrics produced from hydrophilic fibres such as cotton or wool the initial wetting enables liquid molecules to access the surface of the fibres and they then effectively move ‘into solution’ into the polymer structure until the polymer becomes saturated. Liquid then enters the capillary spaces between the fibres and moves or wicks along the yarn structure away from the liquid reservoir. The distance travelled depends on the contact angle between the liquid and the polymer, and the physical dimensions of the capillary spaces. The liquid that enters into solution within the fibres is effectively trapped and prevented from moving away from the source. Naturally hydrophobic polymers prevent liquid solution entering into the fibre and thus prevent the trapping of moisture. With time the liquid that has travelled along the yarn capillaries will evaporate and then diffuse into the air spaces in the fabric. If there is an air pressure gradient across the fabric the liquid molecules in the air spaces will be moved through and out of the fabric. The more permeable the fabric the higher the air flow will be for a given pressure gradient and therefore the greater the rate of liquid transfer. When a comparison is made between woven and knitted fabrics made from the same polymer/fibre/yarn combination then the capillary attraction and wicking and the solution of the liquid into the polymer will naturally be similar. The main difference will be in the air flow through the fabric, and the knitted fabric will be more permeable and hence the rate of liquid transfer will be greater.
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2.10 Comfort Fabric comfort is an extremely complex issue depending as it does on a large number of variables. For example, a fabric that is comfortable in a cool dry environment may be unpleasant to wear in a hot humid climate. In order to have a meaningful discussion it is necessary to specify the following: • environmental conditions including temperature, humidity, wind speed and precipitation; • the activity level of the wearer; • the function of the garment, for example underwear, outerwear, shell, lining, etc.; • the fit of the garment. The perception of comfort takes place through a number of different physiological interactions between the textile and the wearer: • The mechanical interaction between the skin surface and the fibres and yarns. This includes roughness or smoothness as well as pressure, shear and abrasion. • The moisture balance as the wearer sweats or as rain wets the outer layer. • The temperature balance as the body generates heat through exercise or as heat penetrates to the skin surface from the outside. Against this complex background knitted fabrics offer advantages in three main areas. Firstly the stretch properties of knitted fabrics offer better conformability and prevent excess pressure and/or shear developing between the garment and the body surface. This is particularly important for underwear as well as for active sportswear and swimwear to the extent that the overwhelming majority of these goods are made from knitted fabric. Secondly, as already discussed above, knitted fabrics offer considerable advantages over woven fabrics in terms of insulation especially when they are protected from the wind. Thirdly, knitted fabrics perform well when there is a need to transport sweat away from the skin surface. This property enhances the advantage of natural stretch in active sports applications. Conversely knitted fabrics do not provide good protection from wind chill and must be used under a woven shell fabric in such conditions. Similarly they are difficult to waterproof effectively.
2.11 Pilling and abrasion Pilling occurs when repetitive rubbing causes the surface fibres to become entangled and this entanglement is shown as a ball of fibres being drawn out of the yarn structure. The fibre length that is drawn out then rolls into a pill or ball, and the draw-out roll-up process continues and the pill increases in diameter.
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Large numerous pills are unsightly and ruin the appearance of the garment. The key to controlling pilling is the use of spinning and fabric manufacture techniques that prevent fibres being drawn out of the yarn structure and/or the use of fibres that fatigue easily so that the pills that form break off. Fibres are held tightly within the yarn structure when the lateral forces are high due to high twist levels or due to high inter-yarn forces within the fabric. Natural fibres such as cotton or wool tend to fatigue rapidly and allow the pills to break off. Unfortunately the trends in the Industry during the last forty years have led to the production of fabrics that pill extensively. In the search for softer and more comfortable products the twist levels have been progressively reduced. Similarly both woven and knitted fabrics have been made looser. Finally there has been a progressive increase in the use of synthetic fibres such as nylon, acrylic and polyester that do not fatigue quickly and develop into pills that do not break off. All these developments impinge more markedly on knitted fabrics because the inter-loop compressive forces are lower than the warp/weft forces in woven fabrics. The result is that many knitted garments sold by well-known high street retailers pill very badly and lose their smart appearance within only a few days of use. The solution to the problem is simple: increase the twist and knit tighter, but this would lead to the fabric becoming harsher and more expensive.
2.12 Knitted fabrics with special properties For the reasons given in the introduction this description of the properties of knitted fabrics has focused on the most common weft knitted structures manufactured from normal textile fibres and used in apparel end-uses. During the last thirty years the introduction of high-performance yarns such as Kevlar, Nomex and Lycra, the development of industrial products on v-bed machines and the very rapid diversification of the warp knitting machinery sector have led to the production and market acceptance of knitted products with properties that are radically different from those described above. By incorporating elastomers such as Lycra, warp knitted fabrics can stretch by as much as 300% with good recovery properties. By using modified stainless steel yarns weft knitted gloves can be produced that are resistant to band saw damage. The use of Nomex with wool enables weft knits to be produced that are resistant to the very high temperature gasoline fires that occur in motor racing. Polyester microfibre yarns are warp knitted into prosthetic arterial implants. The list of special high performance knitted products is growing by the week and in most cases their properties are different from those described above. However, a thorough knowledge of fabric structure together with an understanding of the physics and mechanics of polymers and yarns enables these special fabrics to be designed and developed in a rational systematic way.
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2.13 Sources of further information and advice Each of the topics covered above is of sufficient complexity to deserve a separate publication and the books listed below expand and advance the discussions summarised in this chapter. Thermal and Moisture Transport in Fibrous Materials, N. Pan and P. Gibson. Woodhead Publishing Series in Textiles No. 56. Wetting and Wicking in Fibrous Materials, A. Patnaik, A. Gosh, R. S. Rengasamy and V. K. Kothari. Textile Progress, 38 (1), Woodhead Publishing. Pilling, J. O. Ukponmwan, A. Mukhopadhyay, K. N. Chatterjee. Textile Progress 28 (3), Woodhead Publishing. Textiles for Protection, R. A. Scott. Woodhead Publishing Series in Textiles No. 44. Textiles in Sport, R. Shishoo. Woodhead Publishing Series in Textiles No. 45. Textiles for Cold Weather Apparel, J. Williams. Woodhead Publishing Series in Textiles No. 93. Biomechanical Engineering of Textiles and Clothing, Y. Li and D. X.-Q. Dai. Woodhead Publishing Series in Textiles No. 52. Smart Textiles for Medicine and Healthcare, L. Van Langenhoven. Woodhead Publishing Series in Textiles No. 63.
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Plate I
Plate II
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Plate III
Plate IV
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Modelling of knitting R. B. RAMGULAM, Albany international, bury, UK Abstract: This chapter discusses the various aspects of mathematical modelling of knitted fabrics, with the emphasis on tensile behaviour. Key words: geometry, energy, pressure, loop, deformation.
3.1
Introduction
Various aspects of mathematical modelling of knitted fabrics are discussed in this chapter. The purely geometrical models of knitted fabrics are very useful to the knitter not only conceptually but in practical terms as well, as they allow decisions on parameters such as stitch length to achieve the required fabric dimensions. Mechanical models of tensile behaviour occupy most of this chapter. Elastica theory is at the core of the force-equilibrium models. An extended summary of an energy method, for which the 3D loop is very appropriate, is described in detail. Knitted garments, on account of their ability to fit closely to the body of the wearer, are naturally suitable as garments for medical purposes. Knowledge of the pressure exerted on the body by the garment is then important. A model that predicts the pressure profile based on membrane mechanics is included in this chapter. Finally a model on heat and moisture transmission in textiles in general is described on account of its importance to comfort.
3.2
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Knitted fabric geometry
The basic principles underlying the geometry of knitted fabrics were laid by Doyle1 and extended by Munden.2 Doyle found that in the case of the dry relaxed plain knit structure, the stitch density is a function primarily of the length of yarn in a loop and is independent of the yarn material, yarn structure and the system used to form the stitches. The foundation of the work is that knitted fabrics, given the opportunity, will take up a stable relaxed configuration guided by the principle of minimum energy. The dimensions of plain-knitted fabrics in such a state, first described numerically by Munden,2 are dependent on the length of yarn knitted into each loop. Munden’s model is based on an assumption by Leaf and Glaskin3 that any homogeneous strip of material bent into a loop will always take up the same configuration, independent of its physical properties. Munden was then able to show that the dimensions of plain-knitted fabrics are uniquely determined by the 48 © Woodhead Publishing Limited, 2011
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length of yarn knitted into each loop. For this to apply, the fabric has to be in its fully-relaxed state, i.e. a configuration of minimum energy. Consider Fig. 3.1 below. The two loops AB and A1B1 are similar in that if the distance of an arbitrary point P from the origin is r then the distance of the corresponding point P1 from the origin is r1 = p * r, where p is a constant. If the loops are described in terms of polar coordinates then the length of loop segment AC is given by: [3.1] and r1 = p * r,
Since Therefore:
Hence the loop length of A1B1 is p * loop length of AB.
3.1 Two similar loops.
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Further we have
But OBx and OB1x are half the wale
spacing of the small and large loop respectively. Hence:
The course spacing, Fig. 3.2, is given by the length Aa. Minimum bending energy consideration suggests that the crossover of loops for a stable configuration occurs when the vertical coordinate of D, which is the narrowest point of loop ACDB, coincides with the vertical coordinate of c, the widest point of loop acdb. Since the two loops are identical in shape and size we have: AL = aN ⇒ Aa = LN. Hence the course spacing is given by the length LN, which is the difference of the vertical coordinates of the widest and narrowest points of the loop. The course spacing is therefore a parameter dependent on the loop shape structure only. Therefore the course spacing for loop A1B1 in Fig. 3.1 is p * Aa. The above analysis shows that course and wale spacing depend on the loop structure only. Hence:
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.
3.2 Course spacing.
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51 [3.4]
[3.5]
[3.6] From these simple equations the weight and dimensions of the fabrics are derived as follows: [3.7]
[3.8]
3.2.1 Cover factor The space in the plane of the fabric occupied by the yarn forming the loop is given by L * d, where L is the length of yarn in the loop and d is the yarn diameter. The area occupied by a single loop is
. It has been shown by Doyle1 for
non-relaxed fabrics and Munden2 for relaxed fabrics that the stitch density is, for plain-knitted fabric, proportional to the loop is
. Therefore the fractional area occupied by .
This relationship ignores the bending of yarn at a right angle to the plane of the fabric and the crossover areas of loops. For practical purposes, given that the yarn the cover factor dimension is expressed in terms of linear density and can be conveniently defined as
. The cover factor affects properties such as
collapse from stretch yarns, pilling and change in fabric area by drying.
3.3
Mechanics of knitted fabric: 2D model
In this section the mechanism of deformation of plain knitted fabric is described, assuming yarns behave like thin elastic rods or elasticas. First the structure of a knitted loop, the unit of the fabric, is explained. The mechanics of thin extensible filaments subject to large deformations is developed followed by a mechanical model of fabric behaviour subjected to external forces. The model is based partly © Woodhead Publishing Limited, 2011
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on the work of Shanahan and Postle4,5 whilst the computational aspects borrow from the approach of Konopasek and Hearle6 as regards elasticas. The following list gives the variables and their meanings for the 2D mechanical model. Table 3.1 Nomenclature Variable Description x, y, z v, w s so ε wx wy M Fv Fw p Ub E E b A d β
Global coordinates. Local coordinates. Arc length measured along centreline of deformed elastica/yarn. Arc length measured along centreline of undeformed elastica/yarn. Strain. Direction cosine of local coordinate, w, with respect to the x-axis. Direction cosine of local coordinate, w, with respect to the y-axis. Moment on small element of elastica/yarn. Internal force component, perpendicular to centreline of elastica/yarn. Internal force component, tangential to centreline of elastica/yarn. Curvature of centreline of elastica/yarn. Bending energy. Tensile modulus of elastica/yarn. Bending modulus of elastica/yarn. Cross-section area of elastica/yarn. Thickness of elastica/yarn. Interlocking angle.
3.3.1 Structure of the knitted loop
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A loop has mirror symmetry about the vertical line going through points D in Fig. 3.3. Further consideration of half the loop ACD indicates that the curve AC can be transformed to the curve CF through a 180° rotation about point C. Hence for modelling purposes it is sufficient to consider a quarter of a loop, AC. Figure 3.3 illustrates two interlacing loops. The course direction is along the horizontal x-axis while the y-axis defines the wale direction. The origin is taken at point B. The forces and moments at point B results from external forces and reaction forces due to interaction of knitted loops and that may include friction and forces applied along the course and/or wale direction. One important feature is the direction of the reaction force at point B, for example. If it has a vertical component there would be a fabric tension along the wale direction that would require an external force to balance it. Such vertical components would therefore be absent in relaxed fabrics or when external forces are applied along the course direction only. Hence in the latter cases reaction force at B runs along the horizontal x-axis/course direction. In Fig. 3.3, Fw and Fv are, respectively, the internal force components along and perpendicular to the tangent to the centreline. The equivalent force-moment equilibrium diagram, with forces and moments acting at B, is shown Fig. 3.4.
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3.3 Two interlacing knitted loops.
3.4 Equivalent force diagram. © Woodhead Publishing Limited, 2011
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The additional moment
for yarn diameter d gives rise to the
difference in initial curvatures at point B for the elasticas BA and BC. Further, concerning path BC the curve is straight at C, i.e. curvature p = 0 at C, and the x-coordinate at C is
. The angle β is the interlocking angle. The latter are
two boundary conditions that are required to find a solution for the relaxed and strained fabric. In the case of external force acting along the wale direction or absence of external forces, the shape of segment BA is a circular arc. The shape of the loop is obtained by assuming that the yarn is an elastica and therefore, under the effect of forces and moments, will satisfy the elastica equations. The yarn is treated as two separate elasticas, BA and BC, and the paths of these parts are obtained separately using the elastica model. The moment acting at B on elastica BA is generally not the same as that at B for elastica BC, i.e. the curvature is not continuous at B. The elastica model for extensible thin filaments is described in the next section followed by its application to knitted loop mechanics.
3.3.2 Planar inextensible elastica: governing equations The equations for inextensible elasticas are derived in terms of a number of first order differential equations, as the latter are more amenable to integration by numerical methods. Figure 3.5 shows a small segment of length ds of a loop with internal forces and moments balanced.
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3.5 Small element of knitted loop.
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From differential geometry: [3.9]
[3.10]
[3.11]
Taking moments about centre of element in Fig. 3.5 gives:
[3.12]
[3.13] Since M = Eb.p where Eb is the bending rigidity, equation 3.13 can be rewritten as follows: [3.14] Owing to equilibrium of forces on the element in Fig. 3.5: cos(d θ) ≈ 1, sin(dθ) ≈ dθ and neglecting second order terms:
[3.15]
[3.16]
[3.17] [3.18]
Finally bending energy, Ub, is given as follows:
[3.19]
3.3.3 Load-deformation behaviour of plain knitted fabrics An increasing load is applied to the knitted fabric in regular small increments, along the course or wale direction, and the deformation simulated. The model is divided into two stages. In the first stage there is no slippage at contact points. During the second stage slippage occurs when the tangential force at the contact
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point exceeds the frictional force. At each step guess estimates of curvature, load, interlocking angle or arc lengths are required. The values of the previous steps provide the estimates. For the first loading step a procedure to compute the guess estimates for curvature, load, and arc lengths is described in the Appendix. Point B in Fig. 3.3 is taken as the origin.
3.3.4 Coursewise deformation No slippage The unknown initial values are curvature pc(0) of elastica BC in Fig. 3.3, reaction load FX at B and interlocking angle β. To simulate the loading process guess estimates are provided for these parameters and Newton–Raphson’s iterative process is used to improve the estimates to a specified accuracy. Loop slippage Slippage occurs when frictional force is exceeded at contact points. The condition for slippage is tan(β ) ≥ Coefficient of Friction. During the slippage phase the interlocking angle remains unchanged and is equal to the value at the start of slippage. The path lengths LBC and LAB in Fig. 3.3 change mainly because of loop deformation. For modelling loop deformation mechanics with yarn slippage, the unknown variables that will have to be estimated prior to using the elastica model are the curvature pc(0), the external force FX acting at B of loop part BC and the length LBC. Initial values for loop segment BA [3.20]
[3.21]
[3.22]
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[3.23] Fcourse is the external force applied along the course direction that coincides with the x-axis direction.
[3.24]
[3.25]
Initial values for loop segment BC
[3.26]
[3.27]
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[3.29] [3.30]
3.3.5 Model: walewise deformation External forces are applied on the loop along the y-axis/wale direction. For modelling purposes the load is increased in small steps and at a given loading the guess estimates for unknown initial parameters are the values evaluated at the preceding step. The external load is applied in the y-direction. The vertical force component at B is half the wale load per loop. The main difference with the previous model for coursewise deformation is that the reaction or interlacing force acting at B will have a vertical component. For the previous model the interlacing force acts along the horizontal, i.e. the x-axis. The angle between the interlacing force and the x-axis is as follows: ,
[3.31]
where FX is the horizontal component of reaction load at B. As in the case for coursewise deformation the no slippage and slippage situations are considered separately. No slippage The unknown initial values are the initial curvature pc(0) and internal load FX on segment BC as well as the interlocking angle β. As in the case for coursewise deformation the model for no slippage is slightly different to the situation when frictional force is overcome and the loops slide past each other. Slippage The condition for slippage is tan(β + θ) ≥ Friction Coefficient. Hence slippage occurs earlier for walewise deformation compared to coursewise elongation. When the loops start to slide the interlocking angle remains fixed while the segments lengths LBC and LBA are treated as unknown initial values just like for coursewise deformation. Initial values for loop segment BA [3.32]
[3.33]
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[3.35] Fwale is the load applied in the wale direction.
[3.36]
[3.37]
As in the case for the relaxed loop, no external force acts at point A; hence the loop part BA will maintain a circular shape. Initial values for loop segment BC
[3.38]
[3.39] [3.40]
[3.41]
[3.42]
3.3.6 Computational aspects
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The models for plain knitted fabric have been reduced to solving the elastica problem. The solution involves a numerical integration of a set of differential equations, given the initial values of the parameters. In the case of both coursewise and walewise deformation with or without slippage, three initial values are unknown. The resulting boundary value problem is solved by the Newton– Raphson’s method. After each run of the integration certain parameters are compared with specified boundary values. The definition of these boundary values is unique to each elastica problem. For knitted loop mechanics the following boundary conditions are defined: • Curvature at point C, in Fig. 3.3, is 0.0. • The x-coordinate of point C in Fig. 3.3 is
.
• Direction cosine at point A, in Fig. 3.3, along the y-axis is 0.0. Given these boundary conditions we can construct three functions: [3.43]
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[3.45]
where A and C are points on the knitted loop defined in Fig. 3.1. Let’s say the three unknown parameters are X1, X2 and X3 where X1 is the unknown initial curvature at point B of segment BC; X2 is the unknown external load at point B of segment BC; X3 is the unknown interlocking angle or length LBC. Using the Newton–Raphson procedure:
[3.46]
The values of ∆X1, ∆X2 and ∆X3 are obtained using the Gaussian elimination procedure. Improved estimates for X1, X2 and X3: Xi = previous estimate Xi + ∆Xi. Figure 3.6 shows the computational results for load applied in the course direction. The load is normalised to the dimensionless value
3.4
.
Mechanics of plain-weft knitted fabrics: 3D model
The previous model assumes that the loop is planar. A more realistic model would consider the 3D nature of the loop geometry. The knitted loop in this case is represented by a three-dimensional elastica, which coincides with the centrelines of the yarns. The equilibrium of the yarn is described by a set of differential equations. The yarn is assumed to be homogeneous, perfectly elastic in bending and torsion, inextensible, undeformed by shear, initially straight, massless and frictionless with a constant circular cross-section. Stress–strain relationships are given by application of the ordinary approximate theory of bending used by Love.7 The treatment of elastica theory, crucial to the force–equilibrium methods, based on the work of Konopasek and Hearle,6 is first described, followed by the 3D knitting model based mainly on the work of Hepworth.8
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3.6 Load-extension of knitted fabrics for different L/d ratios.
3.4.1 Elastica theory The objective is to describe the shape of the centreline of an elastica subjected to external forces and moments. The obvious way of describing a 3D-curve is in terms of a moving trihedron (t, n, and b), where t is the tangent to the curve, n the principal normal and b the binormal vector. Any point of the space curve is represented by the position vector r(s), a function of arc length s. The derivatives of the moving trihedron are the Serret–Frenet formulas: [3.47]
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[3.48] [3.49]
D is the Darboux vector and is defined as D = kb + τ t The Serret–Ferret equations define the curvature, k, and torsion τ of a curve, which are both purely geometric quantities. However, what is required for the study of an elastica are measures of curve which relate to both geometry and mechanical properties of the thin rod. A new moving trihedron (u, v and w), based on the Serret–Ferret frame is defined as follows:
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[3.51]
In practical situations the vectors u and v represent the principal directions of the elastica cross-section and vector w is the tangent of the centreline of the elastica. The angle λ varies along the centreline and hence is a function of arc length s. With this change of local coordinates the following differential equations can be derived to express the geometry of a curved elastica in three dimensions in terms of cross-section parameters and twist.
[3.52] [3.53] [3.54]
where: ψ = pu + qv + σ w, analogous to the Darboux vector; p and q are components of curvature in the u and v directions; σ is the twist, relative displacement of two neighbouring cross-sections. Constitutive equations The elastica is assumed to be inextensible, of constant cross-section and unaffected by shear. In that case the elastica shows resistance to bending and torsion according to Love’s ‘Ordinary Approximately Theory’.
[3.55]
[3.56] [3.57]
where:
Mu and Mv are the components of internal moments and A and B are the bending rigidities; Mw is the internal torque and C is the torsional rigidity. Moment and force equilibrium equations The vector form of equations of equilibrium of forces and moments on a small element shown in Fig. 3.7 are as follows:
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3.7 Elastica element.
[3.59]
where: M is the concentrated moment; F is concentrated force.
Within a moving basis, such as the [u, v, w] the equations 3.58 and 3.59 have to be modified as follows to yield:
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[3.60] [3.61]
Combining equations 3.60 and 3.61 with the constitutive equations relate the geometric parameters, curvature and twist to external loads and moments. Hence the complete mathematical model of three-dimensional elastica consists of equations 3.52 to 3.54, equations 3.60 and 3.61 modified by the constitutive model and equation 3.62 below, which relates the local coordinates to the Cartesian viewpoint (x, y and z):
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63 [3.62]
In scalar form the model consists of 18 first-order linear differential equations.
3.4.2 Geometrical aspects of knitted loop Due to the symmetry of the knitted yarn only the quarter of a loop needs to be considered. Figure 3.8 below shows the interlacing of two loops A1B1 and A2B2 at two different angles. In the interlacing region, because the yarn is incompressible, contact occurs at two points, C and D, at which the normals to the yarn surfaces cut the centrelines at points C1, D1, C2 and D2. In the absence of friction these normals are the lines of action of the inter-yarn forces at these points that by symmetry have equal magnitudes P. If the direction cosine of the force at D is ( fx, fy, fz) with line of action C2D1 then the force at C is along C1D2 and its direction cosine is ( fx, –fy, fz). Since YD1 – YC1 = fy*d and YD1 = YD2, from symmetry consideration we therefore have: [3.63] The point E is midway between C and D. From Fig. 3.8, again due to symmetry, we have: .
[3.64]
But
and
. Hence: .
3.8 Interlacing knitted loops.
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[3.65]
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Further: .
[3.66]
Hence: .
[3.67]
In the case of jamming the inter-yarn force Qc is parallel to the z–x plane and goes through the contact point Jc as shown in Fig. 3.9. The direction cosine of the line of action of Qc is say (–ax, 0, –az). XB2 = XJC and ZB2 = ZJC, i.e. symmetrical. The point JC1 lies on the centreline of loop A1B1 along the line of action of Qc. It is obvious from Fig. 3.9 that the distance between JC and JC1 is d/2. [3.68]
Also using equation (3.67) we have: .
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3.9 Jamming of knitted loops. © Woodhead Publishing Limited, 2011
[3.69]
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Therefore: .
[3.70]
Hence: .
[3.71] When loop A1B1 touches the adjacent wale at JW the line of action of force Qw is parallel to the z-axis and meets the yarn centreline at JW1. . By symmetry ZJW – ZA1 = 2*(ZB1 – ZA1). Hence: .
[3.72]
3.4.3 Equilibrium of forces and moments The only force at A1 is the tension parallel to the z-axis and a bending moment perpendicular to the z-axis. This is because the loops are symmetrical and therefore there is no twisting couple and shear stress at A1. Similarly at B1, where the tangent at the centreline is parallel to the z–x plane, there is no bending moment and shear stress parallel to the y-axis. Suppose Tc and Tw are the externally applied loads on the fabric per course and wale respectively. Further, let the inter-yarn force at C and D be P. Then the tension at A1 is Qw + Tc, parallel to the z-axis. Component of tension at B1 in the x-direction is: .
[3.73]
Component of tension at B1 in the z-direction is: .
[3.74]
If half a loop is considered the resultant force in the x-direction is half the external force per wale. This force is balanced by loop inter-forces at C and D and the force Qc at two course-jamming points. Hence: .
[3.75]
3.4.4 Method of solution The differential equations describing a 3D elastica are integrated from A1 to JC1. The coordinates at A1 are taken as the origin. The tangent to the curve at A1 makes
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an angle of π to the z-axis. Not all the initial conditions at A1 are available. The curvature component p and the jamming force Qw are unknowns and so estimates must be provided at this stage. Furthermore, the length L1 of the loop from A1 to JC1 is also an estimate. The second integration is from JC1 to C1. The magnitude of the inter-force Qc is not known and has to be provided as an estimate. The orientation of Qc is perpendicular to the tangent at JC1, which is obtained from the first integration. Again the length, L2, for the integral is an estimate. The third integration is from C1 to D1. The magnitude of inter-force P is obtained from equation 3.75 above. Its orientation is perpendicular to the tangent at C1 obtained from the second integral. The length, L3, of the curve from C1 to D1 is provided as an estimate. The last part of the integration is from D1 to B1. The length of the integration is obtained from the difference of a quarter of the loop length, L, which is given, and the sum of the estimates L1, L2 and L3. To perform the integration in stages from A1 to B1, we need to give estimates for a number of unknown parameters: L1, L2, L3, Qw, Qc, and p; hence the loop calculated is only approximate. To obtain a more accurate loop, sufficient boundary conditions are required to improve the estimates. These boundary conditions are the equations derived from geometric considerations (equations 3.63, 3.65, 3.70 and 3.72) and also the condition that the component of curvature, p, at B1 is zero. These conditions are just enough to solve the boundary value problem to obtain an accurate loop. The general procedure of solving this type of problem is discussed in more detail for the solution of the 2D-loop mechanics problem in the section below. In brief, the procedure involves iterating the four-stage integration. At the end of every iterative cycle the boundary conditions are verified. If they are satisfied within predefined tolerance the iteration is terminated and the final loop A1B1 is accepted. Otherwise the parameters estimated are improved using the standard Newton–Raphson method.
3.5
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Knitted fabric mechanics: energy model
One of the shortcomings of the previous mechanical models concerns the nature of the contact between the yarns. This is assumed to be a single point for the 2D-model and two-point for the 3D-model. In reality the situation is far more complex. The energy model developed by De Jong and Postle9,10 handle this issue far more comprehensively. In general, the energy method attempts to model fabric mechanics by formulating and minimising equations of potential energy in a fabric subjected to external loading. De Jong and Postle9,10 presented a general theory of elastic behaviour of fabrics based on modelling the shape of deforming yarns. The energy model developed in this section is adapted from their work. They assume linear elastic deformation properties of the yarns, and that any fabric structure consisting of those yarns comes to equilibrium in a minimum energy configuration.
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3.5.1 Formulation of Hamiltonian function We have six state variables z1, z2 . . . z6 which describe our system, in this case a knitted loop. These variables, except for z6, are shown in Fig. 3.10. In addition three control variables, m1, m2 and m3 are at our disposal. The state variables satisfy the ordinary differential equations: [3.76]
[3.77]
[3.78] [3.79]
[3.80] . [3.81] The derivatives are with respect to the arc length s, the independent variable.
3.10 Coordinate system.
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We wish to choose function mk(s), k = 1, 2 and 3 in order to minimise the functional: [3.82] where U is the total energy per unit bending rigidity; B is the bending rigidity; G is the torsional rigidity; C is the compression rigidity; r is the measure of distance between two yarns in contact. Given the above definitions of state and control variables the curvature, Κ is as follows: .
[3.83]
The torsion θ is: [3.84] The functional can then be expressed in terms of variables zi and mi as follows:
[3.85]
This function has to be minimised subject to the following constraints: . 3 4 5 6 7 8 9 40 1 2 43X
[3.86] [3.87]
and z(s) are the coordinates of two yarn elements in contact. where It is often convenient and certainly more elegant to introduce the Hamiltonian function H defined as: [3.88] The Lagrange multipliers λi are related to H through the canonical equations. [3.89] [3.90] © Woodhead Publishing Limited, 2011
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The expanded Hamiltonian H is therefore:
[3.91] The terms
are obtained from the state equations. For example, . The other terms are constructed in the same way.
To obtain the Hamiltonian H is differentiated with respect to zi. For example in case of we have: [3.92] But: .
[3.93]
Hence:
[3.94] Therefore: [3.95] Proceeding in a similar fashion we get: [3.96]
[3.97]
[3.98]
[3.99]
[3.100] The above equations (3.95 to 3.100) are known as the costate equations. © Woodhead Publishing Limited, 2011
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3.5.2 Equilibrium equations The functions mk (s), k = 1, 2 and 3 which minimise functional U make the Hamiltonian H stationary for all s: 0 ≤ s ≤ L. H is constant along the optimal path and the constant has value zero when the limit L is not specified according to Pontryagin’s minimum principle []. The functions mk (s) cause U and therefore H to take a minimum value when: .
[3.101]
This condition yields the following relationships: [3.102]
.
[3.103] [3.104]
3.5.3 Method of solution Guess functions for m1, m2 and m3 are used to integrate the state equations and therefore to solve the loop geometry. The shapes of adjacent loops are obtained to determine the distance r between loops. Then the costate equations are integrated to obtain the Lagrange multipliers along the whole path. Minimum energy is achieved if the following condition has been reached:
. If this
condition is not satisfied then the values mi are improved according to:
[3.105]
where: 3 4 5 6 7 8 9 40 1 2 43X
In the case of the knitted loop the integration path is along a quarter of a loop due to the symmetry of the loop. The symmetry condition holds if we neglect yarn twist. In that case five state and five costate equations are required to define the
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quarter loop geometry as m3 and λ6 are equal to zero. The costate variables, though arbitrarily included in the Hamiltonian, do have actual physical meanings. The first three costate variables refer to shear forces while λ6 to λ3 are moments. To integrate the state and the costate equations we need the boundary values so as to compute the integration constants, and we also need to define the compression function g(r). The coordinates at point B on the loop, Fig. 3.11, is defined as the origin so that zi = 0 for i = 1 to 3. At B the component of curvature in the Z1–Z2 plane is zero which sets z4 = 0. Further, the curvature at A in the Z1–Z3 plane is zero which in this case sets z5 = 0. The value of λ1 at A is the external force applied per course in the course direction, i.e. the Z1-axis. The reaction force at A in the Z2 direction is zero so that λ1 is zero at A. Further, at A, λ3 is the transverse force applied to the fabric. At B, the tangent to the curve is the Z1–Z2 plane. The curve in the Z2–Z3 plane at B is stationary. The control variable m1, which defines rotation in the plane containing the tangent and the Z1-axis, is therefore zero at B. This implies from the equilibrium equation that λ4 is zero at B. Similarly at A we have a stationary point, which means that the control variable m2 is zero. Again we can deduce from the equilibrium equation that λ5 is zero at A.
3.11 Projections of loop.
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The authors9,10 have defined the compression energy per unit of compression , where a is the compression index and varies for different rigidity as yarns. The boundary conditions and the compression function definition are sufficient to integrate the state and costate equations numerically for guess estimates of the control functions. The iteration proceeds until the change of the Hamiltonian with respect to the control variables, that is, the energy gradients are zero within the required tolerance over the whole quarter loop.
3.6
Knitted fabric pressure on a surface
Knitted fabrics are increasingly used for a wide variety of applications ranging from composites’ parts to wearable medical devices. Sensors are knitted into the fabric to monitor the heart rate and other physiological parameters. For medical devices the pressure profile of the garment on the wearer affects the quality of the signals being measured. The model developed in this section assumes that a knitted fabric behaves like a membrane, that is, bending is ignored.
3.6.1 Curved membrane mechanics Tensor T is the stress tensor. The force acting on element ds, in length, and with unit normal e, is T.eds as shown in Fig. 3.12. For equilibrium: [3.106]
where: n: unit normal to surface p: pressure Using divergence theorem:
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[3.107]
,
[3.108]
given dS is arbitrary. Assume stress tensors vary only with the coordinates (u, v) of the membrane:
[3.109]
a and b are each either u or v; gu and gv are the curvilinear base vectors. In general:
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3.12 Membrane subjected to pressure and edge forces.
where: is the surface covariant derivative of tensor T; are the components of the surface curvature tensor. [3.111]
where:
[3.112]
Equations in curvilinear coordinates The first tensor equation, as follows:
, can be expanded into a set of four equations
[3.113]
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[3.114]
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[3.115]
[3.116] The second tensor equation Kαβ Tαβ + p = 0 is equivalent to the following:
[3.117]
3.6.2 Differential geometry [3.118]
[3.120]
[3.121]
[3.122]
[3.123] [3.124]
[3.125]
[3.126]
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[3.119]
[3.127]
[3.128]
[3.129]
Christoffel symbols
[3.130]
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[3.132] [3.133] [3.134] [3.135] Surface definition [3.136]
[3.137]
[3.138]
[3.139] where
[3.140]
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HT is the transposition of matrix H:
[3.141]
. Matrix A is a 4 × 4 matrix and is defined as follows:
[3.142]
[3.143]
[3.144]
[3.145]
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[3.146] [3.147]
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3.6.3 Application to knitted fabrics The objective is to compute the pressure distribution, given the fabric’s tensile properties. In this case only the second tensor equation is necessary, i.e. Kαβ Tαβ + p = 0, which is equivalent to: [3.148] If we assume that wales and courses remain orthogonal on the body, i.e. wales follow the u-curves and the courses the v-curves, then shear force can be neglected and the pressure equation is simplified to: [3.149] The tensile components are obtained experimentally. Hence it is convenient to rewrite the above equation in terms of physical components. Physical components of curvature tensor are: [3.150] [3.151] The physical components of the tensile components are: [3.152]
Replacing in the above equation:
[3.153]
[3.154]
3.6.4 Experimental data Knitted fabrics have been tested in tubular form using special grips designed for this purpose. The stress–strain property of a typical fabric in the course direction is shown in Fig. 3.13 below. The stress–strain relationship in the wale direction is also required. To obtain the stress profile for a fabric deformed in a specified manner, a cubic polynomial is fitted to the stress–strain data. This approach is simple and saves on computation time. Suppose the number of experimental data is N, and stress is represented by Y and strain by X, then the following parameters are defined in the set of equations 3.155.
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3.13 Actual stress–strain characteristics of fabric.
[3.155]
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The coefficients, pi, of the cubic polynomial are obtained by solving the following set of linear equations:
[3.156] The stress–strain relation is then: . © Woodhead Publishing Limited, 2011
[3.157]
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3.6.5 Stress–strain and pressure profile A knitted fabric made of elastomeric yarns of known initial width and length, when worn, will conform to the shape of the body if the undeformed dimensions of the garment are less than the body size. The initially flat fabric is deformed to the profile of the wearer’s body, which is represented digitally through 3D-scanning and the post-processing described above. The strain along a given wale or course is constant but because of the complex geometry of the body the strain will vary, in general, between courses and wales. The course strain profile is obtained by computing the length of each cross-section, i.e. the v-curves and comparing with the undeformed length. The wale strain is the difference between the curve length connecting two extreme points along a u-curve on the body and the vertical distance between these points. The metric curvature tensor is computed at the intersection of the u–v curves or coordinate system. The physical components of this tensor as well as the tensile tensor are used to obtain the pressure profile. Pressure is given as: ,
[3.158]
where: is the stress component tangential to the v-constant lines, i.e. the stress in the courses, is the stress component tangential to the u-constant curves, i.e. the stress in the wales. Figure 3.14 shows the pressure of a garment on a body.
3.14 Pressure profile on hemispherical surface.
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3.7
Heat and water vapour diffusion in fabrics
Heat is lost through clothing by a variety of transport mechanisms which interact and occur together. In the absence of water the main heat flow mechanisms are conduction by trapped and thermal radiation. When water is added to the system, by sweating or otherwise, other heat loss mechanisms have to be considered. Heat conduction is of importance if sufficient water is present in the clothing. Evaporation and diffusion of the water vapour is the predominant heat loss mechanism when water is present in the clothing. The numerical model of Farnworth11 is discussed in some detail in this section. However, before describing the model it is important to list the main assumptions: • Regain is proportional to relative humidity. • The three forms of water that is vapour, liquid and absorbed, are in equilibrium.
3.7.1 Theoretical background The basic theory of heat loss through clothing treats heat loss from the skin by radiation and conduction on the one hand and by evaporation on the other as simultaneous but separate phenomena. Hollies and Goldman12 suggested the following equations:
[3.159]
,
[3.160]
where:
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kc is the coefficient of convective heat transfer; ke is the evaporative coefficient; Psk is saturated vapour pressure at skin temperature; Pab is saturated vapour pressure at dry ambient skin temperature; Tsk is mean skin temperature; Tab is dry bulb ambient temperature; A is the surface area of the body. The equations are expressed in term of heat transfer resistance
and
. The total heat loss per unit area QT is then of
moisture transfer resistance the form:
[3.161]
The two forms of heat loss from the skin, conduction and evaporation, are not independent. There is the possibility of condensation within the clothing layer or layers if vapour permeability is low and ambient temperature is low. Evaporation
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can occur, not only from the skin but also from within the clothing, if say sweat has condensed in it. The simple equation for total heat loss is generalised to partial differential equations for heat and vapour flows as functions of time and distance from the skin. For clothing systems made up of more than one layer a differential equation for flows with time will suffice if instead of distance the equations are expressed for a given layer. [3.162]
where
Rci is heat transfer resistance of layer i; Ti is temperature of layer i; Qci is quantity of heat per unit time per unit area liberated by layer i; Ci is heat capacity per unit area of layer i. For flow of vapour a similar equation is as follows: ,
[3.163]
where
for layer i. Pi: Vapour pressure at layer i.
3.7.2 Relationship between pressure and mass and temperature The amount of water (Mi) present in a layer can be in different forms: vapour (Mvi), liquid (Mli) or absorbed by the fibres (Mai). Hence: [3.164] Assuming the vapour behaves like an ideal gas, then using the ideal gas law we have: .
[3.165]
Xi is the thickness of layer i, and fi is the fibre volume. So Xi(1 – fi) is the volume of unit area of the layer. The number of molecules is
as Mw is the mass of one
water molecule. The constant kB is the Boltzmann constant. If the mass of water absorbed is assumed proportional to the relative humidity then: .
[3.166] © Woodhead Publishing Limited, 2011
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On the left-hand side of the equation γi is the proportionality constant, and the relative humidity is the ratio of vapour pressure Pi to the saturation vapour pressure at the temperature Ti of the layer. The right-hand side of the equation is the ratio of mass of water absorbed to mass of layer i. If water is not present in liquid form in the layer then using equations 3.165 and 3.166 we get: [3.167]
Hence:
[3.168]
.
If there is liquid water the vapour pressure Pi = Ps the saturation vapour pressure at Ti. In that case the maximum amount of water present as vapour and absorbed is given as follows: .
[3.169] To summarise, the relationships between vapour pressure on one side and temperature and mass on the other are: when Mi < M imax Pi = Ps when Mi > M imax.
3.7.3 Calculation of heat of condensation 4 5 6 7 8 9 40 1 2 43X
When Mi > M imax then all the water accumulates as liquid by condensation. Then the heat released is given by: [3.170] where H is heat of vaporisation per unit mass. When Mi < M imax then water is partly vapour and partly absorbed. Then:
[3.171]
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Hence: ,
[3.172]
where Ha is the heat of sorption.
3.7.4 Method of solution The first differential equations (equations 3.162 and 3.163) can be expressed in terms of mass, temperature and time. They can be solved numerically a layer at a time. The properties for each layer, heat and vapour resistances, heat capacity and regain are provided together with the initial values of surface and ambient temperatures and mass. There are several methods for determining the resistances of the clothing.11,12 For the initial conditions, the temperature distribution can be calculated by dividing up the total temperature difference between surface and environment proportionately according to the heat resistance of each layer. The vapour pressure is taken as that of ambient air and the masses of water calculated from the temperature and pressure of each layer (equations 3.166 to 3.168).
3.8 1. 2. 3. 4.
5.
6. 7. 8. 9.
10.
11.
References Doyle, P. J. J. 1953. Fundamental aspects of the design of knitted fabrics, Textile Institute, 44, P561. Munden, D. L. 1959. The geometry and dimensional properties of plain-knit fabrics, J. Text. Inst., 59, T448. Leaf, G. A. V. and Glaskin, A. 1955. The geometry of a plain knitted loop. J. Textile Inst., 46, T587, Shanahan, W. J. and Postle, R. A. 1974. Theoretical analysis of the tensile properties of plain-knitted fabrics. Part 1: The load-extension curve fro fabrics extension parallel to the courses. J. Text Inst., 65, 200–212. Shanahan, W. J. and Postle, R. 1974. A theoretical analysis of the tensile properties of plain-knitted fabrics. Part 2: The initial load-extension behaviour for fabric extension parallel to the wales. J. Text Inst., 65, 254–260. Konopasek, M. and Hearle, J. W. S. 1972. Fibre Science and Technology, 5, pp 1–28. Love, A. E. H. 1944. A Treatise on the Mathematical Theory of Elasticity, Dover, New York. Hepworth, B. 1978. The biaxial load-elongation behaviour of a model of plain weft knit. J. Textile Inst., 69, 101–107. de Jong, S. and Postle, R. 1977. An energy analysis of the mechanics of weft-knitted fabrics by means of optimal-control theory: The nature of weft-interlocking in the plain-knitted structure. J. Textile Inst., 68, 307–315. de Jong, S. and Postle, R. 1977. An energy analysis of the mechanics of weft-knitted fabrics by means of optimal-control theory: Relaxed-fabric dimensions and tensile properties of plain-knitted structure. J. Textile Inst., 68, 316–323. Farnworth, B. 1986. Numerical model of the combined diffusion of heat and water vapour through clothing. Textile Research Journal, 56, 653. © Woodhead Publishing Limited, 2011
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12.
Hollies, N. R. S. and Goldman, R. F. 1977. Clothing Comfort: Interaction of thermal ventilation, construction and assessment factors, Michigan, Ann Arbor, Science Publishers Inc., U.S.
3.9
Appendix: initial guess estimates
The analysis in this appendix refers to the 2D mechanical model of knitted fabric described in section 4.2. Using the elastica model a data set is created for an initially vertical inextensible cantilever, which relates initial curvatures, the horizontal load applied and the displacement of the free end. The data are for a normalized beam, i.e. unit length and bending modulus. the direction cosines at the fixed end of the cantilever are given by the interlocking angle β. Table 3a.1 shows a sample of dataset for a small range of curvature values. The data are dimensionless and actual values depend on the bending modulus (Eb) and the length of elastica (L). Actual force is
, curvature , displacement =
.
If we consider the loop section BC, in Fig. 3.3, with B fixed, the x-coordinate of C is
. The dataset is searched to locate the cantilever that meets this boundary
condition most satisfactorily. However, to search the dataset of cantilevers the length L is required and this is not a priori known. Hence the algorithm described in the next section has been developed to obtain the length of BA and BC.
3.9.1 Algorithm to compute lengths of loop sections The length of loop section BC is obtained using the Newton–Raphson’s procedure. 1. A guess estimate of the length BC say LBC, is required to initiate the algorithm. A good estimate is one-eighth of the stitch length. 2. Curvature of loop section BA: ,
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[3a.1] ~ where p~ is the normalized initial curvature at B for elastica BC and F is the load acting at C. both values are obtained through searching the dataset of cantilevers for elasticas of length LAB as described in the previous section. 3. The loop part BA is circular in shape as no external forces are acting on it. Hence its length as given as: [3a.2] 4. Compute value of the function:
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Table 3a.1 Cantilever dataset ˜ ˜ Load,F Curvature, p
Horizontal displacement, x˜
1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2
0.26 0.29 0.32 0.35 0.38 0.40 0.43 0.45 0.47 0.50 0.52
1.05 1.17 1.29 1.42 1.55 1.68 1.83 1.97 2.13 2.29 2.46
5. If J > 0.001, then the new improved estimate of length AB is as follows: [3a.4] where:
[3a.5]
where ∆LBC is a small change in LBC. Steps 2 to 5 are repeated until the required precision is achieved, i.e. J ≤ 0.001.
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4 Intelligent yarn delivery systems in weft knitting R. KOVAR, Technical University of Liberec, Czech Republic Abstract: The chapter describes yarn delivery systems from different points of view. It begins with a theoretical analysis of yarn delivery, i.e. with a description of the main variables influencing changes in yarn speed and tensile stress during feeding. Passive (yarn tensioners) and active systems (yarn feeders) for yarn delivery are described in respect of the quality and stability of knitting production. Yarn feeders controlling yarn length (so-called positive) or yarn tensile stress (so-called negative) are introduced. The available intelligent systems and an overview of the main features of weft knitting and yarn feeding on circular and flat knitting machines are defined. Key words: yarn delivery, tensioner, feeder, friction, disc tensioner, storage feeder, elastan roller, creel, frictional feeder.
4.1
Introduction
Yarn delivery systems for weft knitting are very important parts of knitting machines. The main objectives of these systems are as follows: • To supply the machine with processed textile material, usually yarn, in sufficient quantities, to unwind the yarns from bobbins and to guide the yarns into the knitting mechanism. • To control the quantity of the yarn supply, i.e. the lengths and tensile stresses of the yarns. • To inspect the yarns from different viewpoints and to stop the machine if necessary. The knitting process will be interrupted if a break occurs in the yarn, if the tensile stress is too high or if the yarn becomes too thick in places. An efficient yarn delivery system is necessary for a reliable and stable knitting process. It specifically requires: • Reliability. The process must minimize the defects affecting fabric quality or causing lost time that lower machine utilization. (See Chapter 11.) • Stability. There should be minimal variation in the geometrical parameters of the fabric and its properties. These goals can be ensured only by the stability of the independent parameters of the knitting process, mainly the stability and quality of yarns (including yarn diameter) and by the stability of the lengths of the yarns in knitted elements (stitch lengths, etc.). Other geometrical parameters 89 © Woodhead Publishing Limited, 2011
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To ensure these conditions in available material of lower quality the tensile stress in the processed yarn should be the minimum required for reliable knitting. If the tensile stress is too low, there will be insufficient control of yarn geometry and a danger of snarling.
4.2
Theory of yarn delivery
In general, the yarn delivery system aids the stability of the knitting process by means of yarn length or yarn stress control. The mechanisms controlling these factors can be:3 • passive, without an energy supply and only able to increase tensile stress in yarns (so-called yarn tensioners or breaks); • active, with an energy resource that can either increase or decrease tensile stress in yarns (yarn feeders). Feeders can control and stabilize the supply of yarn length per fabric unit (positive feeding) or the yarn tensile stress (negative feeding). It is important for effective yarn delivery that yarn tensile stress increases in the stitch forming zone of the machine. Variation in speed of yarn consumption can also influence yarn tensile stress. Figure 4.1 shows the optimal situation for the stitch forming zone, in which only one needle (2) forms the stitch from yarn (1) over trick walls (3); (4) is the yarn guide (finger). The speed of the yarn feeding, vy, equals the sum of gradual changes of its segment lengths, l1, l2, l3, which results in Equation 4.1: ,
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4.1 Stitch forming zone on weft-knitting machine.
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where vk is the speed of the knitting (machine working speed) and vn is the speed of the needle motion (vn = vk tan β, β is the actual angle of the stitch cam). Angles γ and δ are variable. Needle speed decreases to zero when the stitch forming is over. This results in the variable yarn speed vy with a frequency of changes corresponding with the frequency of individual stitch formation. The duration of one period is tp. Yarn tensile stress at guide F1 increases due to capstan (Euler’s) friction, when the yarn slips over the needle bed trick walls or sinkers (3), needle hooks (2) and previously knitted elements. It can be calculated approximately in accordance with Equation 4.2: [4.2] The friction coefficient f is variable over a wide range;2 yarn-to-yarn friction coefficient is usually greater than that of yarn-to-metal. The situation deteriorates if several needles form stitches at the same time (see Fig. 4.2). The yarn may easily reach its breaking stress value, which will result in casting off stitches and the formation of a hole in the fabric. A sudden increase in the tensile stress of the yarn can result in damage. It may be caused by a thick place in the yarn (such as a knot, nep, etc.). Yarn breakage can be partly prevented by so-called ‘robbing-back’ – a movement of the yarn in the opposite direction after a needle/needles are released and the relevant stitches shortened by yarn tension. In Fig. 4.2 robbing-back1,4 is shown at speeds v4, v5 and v6 in opposing directions. The disadvantage of robbing-back is a deterioration in the regularity of the fabric because the final shortening of stitches cannot be accurately controlled. Robbing-back can be prevented by the use of a stitch cam with a horizontal part (delay) at the end of the stitch formation zone. The number of simultaneously formed stitches depends on the stitch cam angle β, machine pitch (spacing) p and sinking depth h.
4.2 Yarn robbing-back in knitting.
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4.2.1 Passive yarn delivery systems The majority of passive yarn delivery systems (yarn breaks or tensioners) increase tensile stress by means of friction, which occurs when the yarn moves over a suitable frictional surface such as steel or ceramic. The disadvantage of this system is its sensitivity to irregularity of the yarn, mainly changes in its crosssection and a variable friction coefficient. An example of an additive tensioning system (frictional force Ff is added to original yarn stress F0) in Fig. 4.3a introduces a disc tensioner. Here the yarn (1) is held between two discs (2), which are pressed against each other by a normal force, Fn, which is generated either by a spring (3) or electromagnetically. The output yarn tensile stress can be calculated in accordance with: [4.3] where F0 is input yarn stress and f is friction coefficient. Multiplication by 2 is due to contact of two discs (left and right) with the yarn. A positive property of the disc tensioner is that variations in input yarn stress ∆F0 are not increased after passing the tensioner. (Friction force Ff = 2 Fn · f does not depend on yarn tensile stress F0), and so ∆F = ∆F0. Multiplying systems (in which the original yarn stress F0 is multiplied by a coefficient greater than 1) increase not only the actual tensile stress of the yarn, but also its variations. An example is shown in Fig. 4.3b, where the yarn is wound on the frictional cylinder (4) with an angle of contact α, that can be continuously changed by setting the position of the disc (5). Capstan (Euler’s) Equation 4.4 can be used (but note that the friction coefficient f is variable): [4.4] Variations in yarn stress ∆F increase in a similar manner and so Equation 4.5 can be used:
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4.3 Passive yarn tensile stress control.
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[4.5]
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The disadvantages of yarn slippage on frictional surfaces can be eliminated by using a kinetic tensioner. (See Fig. 4.3c.) Two rotary discs (6) increase yarn stress by break (7), independent of yarn properties. No yarn slippage occurs in this case. The shape of the disc (6) enables a zigzag movement of the yarn which increases the angle of contact. To improve evenness in output yarn tensile stress, compensation tensioners can be used. In this case, the frictional or breaking force Ff is controlled in accordance with yarn stress F. A higher value for F results in a decrease in Ff and vice versa. The yarn stress F can be detected by electronic sensors and the frictional force Ff changed electromagnetically. In this way it is possible partially to stabilize the axial stress of the yarn, although this will not be completely successful because the tensioners cannot decrease yarn tension. Similar systems for stabilization of the stitch length are described in section 4.4. Other techniques of yarn tensioning are seldom used. Pneumatic tensioners5 use an opposing air stream to increase yarn stress (Fig. 4.3d) but have the disadvantages of high cost and noise. Examples of compensation tensioners can be seen in Fig. 4.4. Figure 4.4a shows a ball tensioner, which increases yarn stress by holding the yarn between the steel ball (6) and the thread eye (7). A principle of compensation operates: when the input force F0 on the ball (6) is strong, gravitation is overcome and the clutch is released. The example in Fig. 4.4b consists of two independent parts. Both increase friction by wrapping the yarn (1) over metal or ceramic surfaces. Rings (3) hang on the yarn and their position depends on the tensile force in yarn F0. Increased yarn tension causes a higher positioning of the rings and thus a smaller angle of contact with the lower frictional force. The left mechanism has no compensation ability. The function of yarn breaks is replicated by yarn thread eyes, yarn guides, etc. An undesirable increase in yarn stress may cause problems in the knitting
4.4 Examples of compensation tensioners.
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process (greater frequency of yarn breaks, unevenness in the fabric, etc.) and is affected by the geometry of the yarn path from bobbin to knitting system. For elastomeric yarns, where small changes in tensile stress are connected with great strains (see Fig. 4.6), delivery systems are designed to have as short a yarn path as possible from bobbin to guide and thread eyes are replaced with small rollers.
4.2.2 Feeders with yarn length control (positive feeders) Feeders with yarn length control can be used only when the length of yarn per stitch in the stitch forming zone of the machine is constant (as in plain knitting). Measuring of the yarn improves the evenness of the fabric. For example, the effect of inexact adjustment or abrasion of the stitch cam can be partially eliminated. Greater sinking depth leads to an increase in yarn stress, thus shortening stitch length by an axial elongation of the yarn. There are several types of feeders with yarn length control. The most widely used are described below. Storage positive feeder
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The general advantage of storage feeders is that stored yarn prevents any transfer of changes in yarn stress (even those that may occur suddenly) across the feeder with their consequent influence on fabric quality. The length of stored yarn is usually sufficient for yarn feeding to continue, should the machine stop when an accident occurs (such as a yarn breakage at input to the feeder). Storage feeders will produce an immediate improvement in fabric quality. Yarn (1) is usually stored on a rotary winding reel or drum (2) in the form of certain number of winds or turns (see Fig. 4.5). Individual turns must not be crossed on the reel. As stored yarn is wound up on the upper side and unwound on the lower side, it is necessary to keep all the turns in a stable position by slippage all turns down (arrows). The simplest way to ensure this stability is by applying sufficient tension to the slippage conical part of the reel (3) F0 before the feeder. This is provided by the disc tensioner (4). Another effect of the disc tensioner (4) is to ensure more equal yarn elongation during winding. The feeder can ensure constant yarn length, but cannot recognize to what extent the yarn is axially strained. This is important mainly for high extensible yarns (elastomeric, textured yarns etc.). For example, if an elongation of 200% of the original length occurred, only 50% of the material would be fed through. This effect of the non-linear stress-strain curve is explained in Fig. 4.6a using a typical example for highly extensible yarns. At low axial stress, the change in tensile force ∆F1 has a greatly variable relationship to yarn elongation ∆ε1, whereas in the part of the curve with the greater yarn tensile force, the curve is steeper and the same interval of ∆F2 corresponds to a much
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4.5 Storage feeder with length control (positive).
4.6 The effect of yarn tensile stress on quantity of yarn delivered.
lower change in yarn length ∆ε2. Figure 4.6b documents the changes in fed yarn due to elongation; L0 is the length of the relaxed part of the yarn, L is the the corresponding length of the yarn after extension and Ls is the length of the yarn measured for one stitch. The rotary movement of the storage reel (2) (Fig. 4.5) must be accurately adjusted to the machine motion. This condition can be met by the use of pulley (5) and belt (6). Usually one belt is used to drive several feeders on the circumference
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of the machine. If different lengths of yarn need to be fed, a clutch (9) may be coupled with a pulley (7), driven by a belt (8) at a different speed, vb2. The yarn speed v = v0 corresponds to the peripheral speed of the reel; the output yarn tensile force is not directly controlled and so a yarn breakage sensor is required. The speed v must correspond with the speed of yarn consumption on the relevant cam system. Pulleys (5) and (7) and belts (6) and (8) need to be toothed or perforated to prevent slippage that would cause a change in the length of the fed yarn. Elastan roller In principle, this is an unwinding unit (see example in Fig. 4.7). Yarn (1) is unwound from the package (2) by two shafts (3) that move in the direction of the arrows. When slippage is prevented, the peripheral speed of the shaft and the yarn package is identical and the length of fed yarn well be controlled effectively if the yarn extensions are all in packages with a similar volume. A single unit is usually designed for feeding several yarns (in Fig. 4.7 it supplies bobbins (2), (4), (5) and (6), which may have different diameters). The feeder drive can be connected with a circular knitting machine motion as in the case of positive storage feeders (Fig. 4.5). Belt feeder The belt feeder was popular before storage feeders were developed. The yarn was gripped between the belt and the pulley and the speed of the yarn corresponded to the speed of the belt. The main disadvantages of this mechanism are a lack of stored material and the danger of transferring sudden changes in yarn tension or yarn breaks across the feeder with a detrimental effect on fabric quality.
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4.7 Elastan roller.
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4.2.3 Feeders with yarn tensile stress control (negative feeders) Feeding of constant yarn lengths cannot be used where when the formation of rows requires variable lengths. As an example, a two-colour pattern is introduced with knit and miss back (Fig. 4.8). In this case, each yarn is knitted into three of a maximum of four stitches in areas of corresponding colour on the fabric face, but only into one of four stitches in areas of the complementary colour. This reduces yarn consumption by approximately one-third and for this reason, yarn tensile stress control (negative systems) are mainly used for patterned fabric knitting. Negative storage feeders The solution is similar to that shown in Fig. 4.5, but the yarn (1) is drawn off the reel (2) in the direction of its axis into the thread eye (8) (Fig. 4.9). As the yarn tensile stress would be too low for reliable control, a tensioning ring (7) is used on the feeder. This could be, for example, a ring with nails, slanted in the direction of the yarn balloon rotation. Yarn tension out of the feeder is so well controlled that a breakage sensor is not necessary on this side, but a detector for yarn breakage and high tensile stress should be used at the input side of the feeder. In comparison with positive storage feeders, there are several differences: 1) The speeds of the yarn before and after the feeder, v0 and v, are different. If a yarn break occurs before the feeder, the yarn may still be supplied to the knitting system until the machine is stopped.
4.8 Two-colour pattern with knit and miss back.
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4.9 Principle of negative storage feeder.
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2) The number of yarn turns on the reel may change and must be controlled by using an appropriate sensor. A motor drive (9), independent of machine speed, or a coupled belt drive may be used. If the number of turns is too small, the reel (2) increases the speed of its rotation and vice versa. 3) Differences between input and output speeds will cause false-twist, which can influence the quality of the fabric. This is the reason why some modifications were developed (see Fig. 4.10). Three basic variants are shown here: • Previously described system (Fig. 4.9) with rotary reel (2) causing falsetwist formation. • Fixed reel (3), winding the yarn through the rotary thread eye (4) from the lower side of the reel. All stored yarn turns must be moved up (arrows). • Fixed reel (5), winding the yarn through the rotary thread eye on the disc (6) from the upper side of the reel. All stored yarn turns are moved down (arrows). The design of the feeder is rather complex because the yarn balloon on neither the upper nor lower side permits fixed connection of the reel (5) with the frame. Remote control of the reel position can be ensured magnetically.
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4.10 Basic systems of stored yarn supply.
4.11 Chosen ways of stored and wound yarn slippage (a, b); separated wound yarns (c, d).
Slippage of yarn winds towards the unwinding side (arrows) can be provided in different ways, for example by the conical part of the reel (Fig. 4.9), by the inclined ring (Fig. 4.11a) or by the wheel (Fig. 4.11b), etc. Systems have been developed that enable the separation of individual turns of yarn, for example the use of a set of two or more skewed cylinders instead of one (Fig. 4.11c) or of a set of belts (Fig. 4.11 d). All examples show both yarn length (P: positive) and yarn tensile stress (N: negative) control. Some storage feeders, available for knitting and for weaving machines, use an assembly of two mutually eccentrically mounted cages for the separation of wound yarn. There are two types of these eccentricities. First, mutual translation of the two axes insures that each cage carries the wound yarn only on a certain part of the feeder perimeter. Second, the bias position of the axes ensures that the yarn turns move in an axial direction.
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Frictional feeders In this case there is only a small number of yarn turns (1) (Fig. 4.12), so slippage between the yarns and the reel (drum) (2) is allowed. The reel is driven with peripheral speed vd, which is higher than the speed of the yarn, thus pushing it forward and decreasing its tensile force. The output yarn stress, F, can be counted using capstan (Euler’s) equation with the opposite sign of exponent, Equation 4.6
[4.6]
The advantage of this feeder is that variations in yarn tensile stress are decreased, because of ∆F = ∆F0 · e–α · f, (derived from Equation 4.6). The speed of the yarn supply can be changed over a wide range from zero up to the peripheral speed of the reel and so the system can be used for knitting different patterns or even in collaboration with a yarn striper. In yarn stress control mode, some use can be made of originally positive storage feeders set at a higher speed and with a small number of yarn turns (see subsection 4.2.2). Modification of this feeder for a flat machine will be introduced in section 4.4. Similarly, striper feeders can be used on machines that are equipped with a striper mechanism (exchange of yarn guides), Fig. 4.13. A feeder is equipped, instead of a reel or drum, with a set of discs (3), each controlling a single yarn. Discs are coupled in pairs to enable individual yarn control when two ends are threaded into one guide. Only one pair of yarns (1) is actually fed: in this example, the yarn tension swings the arm (4), thereby increasing the angle of contact. The
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4.12 Principle of frictional feeder.
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4.13 Striper feeder: (a) horizontal view, (b) vertical view.
tension of the non-active yarns (2) is low so the arms (5) and springs (6) decrease the angle of contact of the yarn. Electronic feeders The use of electronic feeders enables quick and efficient regulation of yarn tensile stress. These systems are often based on the principle of storage feeders with
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control of reel rotation according to output yarn stress F, Fig. 4.14. Yarn (1) is wound on the reel (2), which is designed as a lightweight cage with self-cleaning abilities. Input yarn tension F0 is controlled by the disc tensioner (4). Output force F is measured by the sensor (3), which controls the motor drive (5). The motor reaction is so quick that the feeder allows knitting of patterned fabric and can be combined with a striper (yarn guide or finger exchange), which is standard equipment on hosiery machines. Some electronic feeders can be used on circular machines with reciprocated movement (for example in hosiery and body technologies). In this case, the feeder must allow backward yarn movement to compensate for shortening the length of the yarn path on the machine. In Fig. 4.14 this is achieved by the thread eye (6) mounted on a separately controlled arm. When the reel (20) moves backwards, the eye (6) turns in the same direction and keeps the input section of the yarn extended.
4.14 Outline of electronic yarn tensile stress control.
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Yarn storage and delivery systems on circular knitting machines
The specific features influencing yarn delivery on large-diameter circular knitting machines are high productivity, continuous knitting and a great number of simultaneously processed yarns. Some of these machines are equipped with a striper (yarn guide exchange), but only a few enable reciprocated knitting. Small diameter hosiery machines have up to four (or occasionally eight) knitting systems (feeders) and an important feature is the combination of rotary and reciprocal movement of the needle bed (beds). Between these extremes are the middle diameter machines for ‘body’ technologies. Figure 4.15 shows the simplified yarn supply system on a large-diameter circular knitting machine. Yarns (1) are brought from the bobbins (2), passed through the side creel to the feeder (3) and finally to the yarn guide (4). Usually the feeder (3) is equipped with stop-motion sensors for yarn checking. The creel of the knitting machine controls the placement of yarn packages (bobbins) on all machines. Modern large-diameter circular machines use separate side creels, which are able to hold a large number of packages in a vertical position. Floor projection of these creels may differ (oblong, circular, etc.). If there is a long distance between the bobbin and the yarn guide, the yarns may be threaded pneumatically into tubes. The modular design facilitates the changing of the number of bobbins where required. Small-diameter machines with a smaller number of cam systems use either side creels or creels designed as integral to the machine. Modern creels make it possible to use double bobbins. Each pair of creel pins is centred on one thread eye (Fig. 4.16). The yarn of a new bobbin (3) may be linked to the end of the previous length of yarn (1) on bobbin (2) without stopping the machine. Some of the creels are equipped with systems for blowing off dust (fancreel), or with air circulation and filtration (filtercreel). The example in Fig. 4.17 shows the bobbins (2) in six rows, closed in a box with internal air
4.15 Outline of yarn feeding on a circular knitting machine.
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4.16 Double bobbins on creel.
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circulation, provided by fans (4) and tubes (3). A filter (5) clears dust from the air. The creel can be air-conditioned. When the machine is not equipped with a striper, this can be supplied by yarn exchange on the creel; some systems enable the knots to be positioned in the optimal area of the fabric. Yarn length control (positive feeding), when not used for patterned fabric knitting, must enable different yarn lengths to be fed into courses in different structures. As an example, in Milano-rib knit there is one double-faced course (1) and two single-faced (2), (3) courses in the repeated pattern (see Fig. 4.18). As a double-faced course contains twice as many stitches, the yarns must be fed at approximately twice the length per machine revolution. This is the reason why these feeders use several belts, individually adjusted for speed, whilst feeders using yarns of the same length are controlled by one belt. The feeders are usually mounted onto two or three rings around the machine. If a configuration with two belts on each ring is used (Fig. 4.5), yarns can be fed simultaneously at four or six speeds.
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4.17 Cross-section of creel with internal air circuit.
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4.18 Milano-rib knit.
Feeders with yarn tensile stress control may use a belt drive with a clutch or an electronically controlled drive to regulate the number of turns of the yarn. When knitting plain rather than patterned fabric, there is a danger of striping due to uneven adjustment of the feeding of individual yarns. This necessitates the use of instruments for measuring the length of fed yarns and their tensile force. Some types of circular knitting machines enable 3D (spatial) knitting in different types of products. Three possibilities are considered here: 2 3 4 5 6 7 8 9 40 1 2 43X
• Hosiery products, including those for medical use. The number of needles cannot be changed so the 3D shaping of the fabric is achieved by controlling the length of fed yarn. For example, in the production of compressive stockings and similar products, the yarn delivery system is responsible for regulating the length of the inlayed elastomeric yarn so as to produce a compressive effect. • ‘Body’ technologies, often used in the manufacture of ready-to-wear products, use 3D shaping in which the number of needles in the machine bed is constant. Advanced yarn delivery systems are important in this process. • Single-knitting system (feeder) circular machines with rotary cams provide the advantage of both stable creel and needle beds. The yarn feeding system can be very simple. This type of machine is mainly used in the manufacture of technical products and dress accessories.
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Yarn storage and delivery systems on flat knitting machines
Flat knitting machines characteristically offer efficient patterning and 2D and 3D shaping for smaller scale productivity. As flat knitting machines require a smaller quantity of yarn and fewer feeders or cam systems and bobbins, the standard creel may be placed at the upper rear part of the machine (Fig. 4.19). The yarn is fed from the bobbins (1) to the checking device (2) and then to either the left or right side of the machine where yarn tensioners and/or feeders (3) are situated. This prevents the yarn from coming into collision with the carriage (6) bow. After the yarn guide returns from the extreme left or right position, a length of yarn is released. To permit this change, a yarn length compensator (5) is used. This is typically a spring-loaded thread eye, which enables the yarn to return and form a loop. This is not a perfect solution as the tensile force Fy of the yarn fed from this loop is less than that imparted by the tensioner (3). The creel capacity (the number of package pins) will be at least twice the number of yarn guides, as two or more ends are threaded in one guide to form a plaited fabric. A solution, providing two yarns with a loose twist, is introduced in Fig. 4.20a. The yarn from bobbin (1) goes through bobbin (2), and one yarn balloon turn forms one twist on the yarns. Controlling the length of fed yarn is not easy on this type of knitting machine. Difficulties may arise from the interrupted unwinding of yarn from the packages due to reciprocated knitting, or in the case of intarsia machines, by guides and yarn exchange during the knitting. Fortunately, the majority of flat knitting machine products are patterned fabrics, which are not very sensitive to striping.
4.19 Outline of yarn feeding on a flat knitting machine.
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4.20 Doubling the yarn with some twisting (a); outline of frictional feeder (b, c).
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Another problem is consistency of dimensions in plane shaped or ready to wear products. High quality yarn stress control stabilization (negative feeding system) is a partial solution. Flat knitting machines often use frictional feeders (Fig. 4.12), adapted for changed conditions. The principle is shown in Fig. 4.20b, 4.20c (two aspects). One drum (4) controls all the yarns (3), which are fed from one side of the machine. Similar systems are usually used on each side of the machine. Actually, knitting yarns are pressed towards the drum (4), which decreases yarn tensile force in accordance with the equation (4.6), and yarns that are disengaged slip on it; (5) and (6) are yarn length compensators. An advanced solution is to measure the yarn length consumption in the knitted courses with feedback to the stitch cam position DSCS (Digital Stitch Control System).8 The sensor could be a very light pulley driven by wrapped yarn so that the peripheral speed of the yarn and the pulley is the same. Movement of the pulley is monitored, usually optically without any mechanical contact. The machine’s computer is able to calculate the required yarn length per course by summing up the pre-set length in knitted stitches multiplied by the number of knitted stitches, and does the same with tuck stitches and other structure elements. Predicted and actual yarn length may be compared and differences corrected by adjusting the stitch cams. Another similar approach (i-DSCS) is the intelligent active control of fed yarn length.
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Future trends
Trends in the development of yarn delivery systems are relatively stable as the main goals remain constant: • Steady production • High fabric quality • Easy processing of material, even in lower priced yarns with imperfections. However, the technology available for the realization of these goals is changing and developing. The contemporary trend is for the use of electronics or computers and the introduction of new systems of drives and sensors. Electric and hydraulic servo motors are now available, enabling both exact and flexible movements in controlled mechanisms. The position of stitch cams and needle bed racking is already controlled by step motors and these drives can dose (measure) the length of yarn, even in jacquard fabrics, which are composed of knitted elements with different yarn consumption. Such computer controlled yarn delivery systems are flexible and widely used, thus increasing production and lowering costs.
4.6
Sources of further information and advice
Several papers offer theoretical analyses of yarn delivery systems, but the majority of publications prefer descriptions of the design and properties of particular systems. Company prospectuses and brochures (for example see References 7 and 8) are important sources of information and the internet is an increasingly important resource. Unfortunately, there are not many monographs dealing with the technological aspects of industrial knitting. One of the best known6 is concerned only marginally with the problems of yarn supply.
4.7
References
1 Araujo M. D. and Smith G. W. 1989. Spirality of Knitted Fabrics, Text. Res. J., 59, 247–256 and 350–356. 2 Gupta B. S. et al., Friction in Textile Materials, Cambridge, Woodhead Publishing, 2008, ISBN 978-1-85573-920-8. 3 Kovar R., Flat knitting technology, Knitting Technology, 2/2002, 6–69, ISSN 09470972. 4 Kovar R., Structure and Properties of Flat Textiles (in Czech), Technical University of Liberec 2003. ISBN 80-7083-676-8. 5 Paepke H. and Paepke J. 1994. ‘Fadenbremsen und Spannungsausgleicher für Textilmaschinen’, Wirkerei- und Strickerei-technik, 44, 7, 591–593. 6 Spencer D. J. 1989. Knitting Technology, Oxford, Pergamon Press, ISBN 0-08035912-4. 7 Prospectuses of Memminger-IRO company from ITMA 2007 Munich. 8 DSCS (Digital Stitch Control System) 2001. Instruction manual of Shima Seiki Company.
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Advances in warp knitted fabric production Bharat J. GAJJAR, Warp Knits, Delaware, USA Abstract: The chapter begins with a discussion of new structures with ‘woven like’ aesthetics and fabric without stretch. Delaware stitch and modified Delaware stitch fabrics, which offer woven like aesthetics with stretch-free fabric, are defined. Commercial Tricot and Raschel fabrics containing spandex are explained, and a newly developed construction with spandex fabrics that have unique aesthetics is illustrated. The chapter reviews Americana and Modified Americana Tricot, with their unique fabric aesthetics. One of the unique aesthetics is crêpe, which has an unusual visual as well as tactile aesthetics. The chapter also shows knitting of the spun yarn at higher speeds. The final section concerns Milanese fabrics, which have a unique structure where both yarns travel in opposite directions, imparting a unique visual aesthetic. Key words: definitions of warp knitting, warp knitting machines, ‘woven like’ structures, two types of laid in stitch, mattress fabric, spandex warp knits, new structures with spandex, ‘crêpe like’ warp knits, nylon.
5.1
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Introduction
A warp knit fabric has a structure made of several warp threads or warps that form similar loops in a course. The different stitches are knitted by changing the sideways motion or shogging movement of the guide bars during knitting. There are three basic types of fabric – woven, weft knit, and warp knit. Woven and warp knit fabrics need a warp to form a fabric and a weft knit needs an end to form a fabric. A woven fabric is formed by interlacing warp ends with filling or weft ends to form courses. Only one end is needed to form a course in weft knit, but many ends are needed to form a course in a warp knit. In the United States, many technicians call weft knit fabrics ‘circular knits,’ while warp knit fabrics are called ‘flat knits’. In reality, both weft and warp knit fabrics can be knitted on circular as well as flat machines. However, most weft knit fabrics are knitted on circular machines, and most warp knit fabrics are made on flat machines.
5.1.1 Cross-sections of woven and weft knit fabrics To understand warp knit fabrics, cross-sections of woven and weft knit fabrics with the same weight (about 3.5 oz/yd2) should be compared. Warp knit yarn manufacturing ensures that these key fabrics cost about the same. Figure 5.1 shows 110 © Woodhead Publishing Limited, 2011
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5.1 Cross-sections of woven, weft and warp knits.
cross-sections of woven, weft, and warp knit fabrics weighing 3.5 oz/yd2 and having the same thickness and costing the same. In a cross-section, a woven fabric has two ends, a weft knit has three ends, and a warp knit has four ends. But how can they all cost about the same? This is because in woven fabrics two ends cross, in a weft knit one stitch has three ends in a cross-section, and in warp knit there is one stitch with two loops having four ends in a cross-section. In these fabrics 80 denier yarn is the least expensive to produce, 60 denier yarn is a little more expensive, and 40 denier yarn is even more expensive. A basic weaving loom is the slowest in forming a fabric, weft knit weaving is a little faster than a basic weaving loom and warp knit production is the fastest. If all these factors are combined, the end result is that they all cost about the same.
5.1.2 Wale and course A vertical column of loops knitted by a single needle is called a wale and a horizontal row of wales is called a course. A Jersey structure with a loop and lap diagram is shown on page 112 (Fig. 5.2) with a view of the float side of the fabric (the loop side is on the other side).
5.1.3 Face and back The warp and weft knit face is the loop side and the other side is the float or back side (see the figure above). It is important to be able to identify the face and the back of woven, weft knit and warp knit fabrics. In a woven fabric, the face and back are identical. In a warp knit, either the loop side or the float side could be used as the face of the garment. In 100% nylon or polyester Jersey, the loop side is used as the face of the garment because it has a more attractive appearance. When front bar//
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5.2 Loop and lap diagram of the Jersey stitch.
back bar, nylon//spandex Jersey stitch is made, the float side is used as the face of the garment. Spandex contracts and the fabric eliminates the horizontal lines. When a warp knit fabric is put on a table with the technical face or loop side of the fabric facing up, it is called technically upright. The courses are horizontal and the wales are vertical. The top course is knitted last whereas the bottom course is knitted first. If you cut Jersey fabric two wales wide as shown in Fig. 5.2 you will cut the front bar yarn and the back bar yarn will be in your hand.
5.1.4 Curling propensity 1 2 3 4 5 6 7 8 9 40 1 2 43X
Most warp knit fabrics tend to curl, including the most important type known as Jersey stitch (in the USA) or Locknit stitch (in the UK). If they receive appropriate heat treatment, synthetic warp knit fabrics do not curl. In dyeing, finishing, cutting and sewing garments, it helps to know the face and back of the fabric and its curling propensity. When a greige nylon Jersey stitch fabric is put on a table technically upright (having the loop side up), the top and bottom edges of the fabric will curl upwards or towards the loop side or technical face. However, the side edges will curl under the fabric towards the float or technical backside of the fabric. If nylon Jersey stitch fabric is heat set it will not curl, but if that fabric is laid on the table technically upright and it is pulled sideways on the top edge of the fabric, the fabric will curl towards the loop side. There are some warp knit structures that will not curl in the greige state.
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Commercial warp knit machines
5.2.1 Comparing sales of warp knits with woven and weft knits The amount of total fabric produced worldwide per year is highest for woven fabrics, second highest for weft knit fabrics, and lowest for warp knit fabrics. The major reason is that spun yarn cannot be incorporated in warp knit at a high speed.
5.2.2 Types of warp knit machine There are four basic types of warp knit machine. They are Tricot, Raschel, Simplex and Milanese. The Tricot and Raschel machines can be either one-bed or two-bed machines. There are three types of needle used in warp knitting. They are shown in the diagram (Fig. 5.3) below. Warp knit machines require a warp to knit. The four basic types of warp knit machine produce six different warp knit machines with various types of needle as shown in Table 5.1. Tricot machines with spring-bearded and compound needles are lighter weight machines and run very fast whereas Raschel machines with latch needles are very heavy and run slowly. Tricot machines with compound needles are faster as the needles are a more recent discovery. Raschel machines with latch needles are slow whereas newer Raschel machines with compound needles are faster but not as fast as Tricot. It is important at this point to compare Tricot with Raschel machines to understand their function properly (see Table 5.2).
5.3 Three types of needle for warp knitting.
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Table 5.1 Types of warp knitting machine Type of machine
Number of beds Type of needles used
Tricot, flat Raschel, flat Simplex, flat Raschel, flat Milanese, flat Milanese, circular
One One Two Two One One
Spring-bearded needles, compound needles Latch needles, compound needles Spring-bearded needles Latch needles Spring-bearded needles Spring-bearded needles
Table 5.2 Tricot versus Raschel machines
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Tricot
Raschel
Spring-bearded needles
Latch needles
Thinner compound needles Heavier compound needles One needle bar One or two needle bars One sinker bar No sinker bar One presser bar (also cut presser No presser bar needed bar) 1–24 guide bars (usually 1–48 guide bars 2 + 04 bars) Width up to 260 inches (168 and 84 Width up to 240 inches (160 inches common) inches common) Gauge 14–36 needles/inch (28 and Gauge 8–64 (needles per 2 inches) 32 gauge common) Speeds up to 1 800 spm (stitches Up to 1 000 spm per min) for spring bearded; 2 500 spm for compound needles Fabric angle 90° 150° angle Lower take-up tension Higher take-up tension Use mostly filament yarn/ Uses filament, textured and spun yarns compound needle could knit spun yarn Links move one course Links move ½ course Knits up to 240 denier hard yarn Knits up to 1 500 denier hard yarn or filament yarn Knits up to 140 denier spandex yarn Knits up to 1 000 denier spandex yarn Less versatile in styling and designing More versatile for fancy pattern work Small and large mills Mostly small mills including garage operators Gauge is needles/inch Gauge is needle/2 inches
5.2.3 Basic commercial warp knit fabrics Among all of the machines, Tricot machines are the most popular in production. Within the Tricot fabric, the king of all stitch construction is Jersey stitch construction.
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The Jersey stitch can be knitted from spun yarn, textured yarn, filament yarn, and spandex yarns. The spun fabric yarn fabrics are not commercially feasible, as they cannot be knitted at high speed. Commercial fabrics are made from filament yarn and spandex yarns only. Within that, filament yarn is the most used, as it is utilized in women’s undergarments. If there is one stitch that dominates the warp knit industry, it is the Jersey stitch. This two-bar warp knit structure is known by many names. It is called Jersey in the USA, Locknit in the United Kingdom and Chaemeuse in Europe. In common usage, Jersey means a knitted fabric. Jersey fabric is a warp knit which is knitted on any single needle bed warp knitting machine (Tricot or Raschel). This fabric is knitted with two fully-threaded guide bars, where the front bar is knitting a three needle float (Silk Float) using closed stitches (2-3, 1-0 or 1-0, 2-3) and the back bar is knitting a two needle float (Cotton Float) also using closed stitches (1-0, 1-2 or 1-2, 1-0). Both the bars, while knitting, are shogging independently in opposite directions. Long float (LF) Jersey or satin, a modified Jersey fabric, is the same as conventional Jersey except that the front bar shoggs four needles instead of three. In the super float Jersey, the front bar shoggs five needles and in the short float Jersey it shoggs two needles. It is very important to remember that in a Jersey stitch construction, the back bar yarn is always sandwiched into the front bar yarn and when one touches the hand of the fabric on both sides, one feels only the front bar yarn. If the front bar contains white yarn and the back bar contains black yarn, the Jersey fabric will look almost white; a little of the back bar yarn can be seen between the wales and the fabric will look slightly colored.
5.2.4 Knitting and finishing Jersey fabric is knitted at about 1 000 stitches per minute (spm) on older machines and at about 2 000 spm on newer, compound needle Tricot machines. Faster machines are being developed. Good filament yarns are knitted on 168-inch wide machines, knitting at about 1 000 to 2 000 racks per end out, which is considered an excellent knitting performance. There are three types of finishing sequence that are used in finishing Jersey and modified Jersey fabrics. They are: • Heat-set, scour and dye. This sequence is preferred when nylon is used by heat-setting first and the final yield of a fabric is obtained. However, the disadvantage of this sequence is that it fixes the stains and dirt that are picked up during knitting. • Scour, heat-set and dye. This sequence is preferred for polyester. It eliminates the impurities in the fabric, offers greater bulk and contains a bleaching step that removes yellowing caused during heat-setting, but this finishing sequence is expensive as it requires pin tentering twice. © Woodhead Publishing Limited, 2011
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Table 5.3 Jersey and modified Jersey stitches Fabric No. Stitch
Front bar// back bar Aesthetics
1. 2.
Jersey Satin or Long Float Jersey Slipper Satin or
2-3, 1-0//1-0, 1-2 3-4, 1-0//1-0, 1-2
Widely used; classic Tricot fabric Heavier/bulkier than Jersey
4–5, 1-0//1-0, 1-2
1-2, 1-0//1-0, 1-2
11.
Double Tricot or Short Float Jersey Open Jersey Del Jersey Mixed Jersey Open Long Float Jersey Open Super Long Float Jersey Mixed Long Float Jersey Long Float Del Jersey
Heavier/bulkier than Super Long Float Jersey Long Float Jersey Lighter than Jersey
12.
Laid-in Jersey
With all above stitches
3.
4. 5. 6. 7. 8. 9. 10.
3-2, 0-1//0-1, 2-1 3-2, 0-1//1-0, 1-2 3-2, 1-0//1-0, 2-1 4-3, 0-1//0-1, 2-1 5-4, 0-1//0-1, 2-1 3-4, 0-1//0-1, 1-2 4-3, 0-1//1-0, 1-2
Higher luster than Jersey Less distortion than Jersey Drier hand than Jersey Higher luster than Long Float Jersey Higher luster than Super Long Float Jersey Drier hand than Jersey Higher recovery than Long Float Jersey Lower stretch; heavier/bulkier than Jersey
• Scour, heat-set and dye. This sequence is preferred for polyester. This finishing sequence is the most economical as it combines scouring and dyeing. However, there are two problems associated with it. One is that it sometimes imparts rope marks, a defect in a fabric. The other is that heat yellowing affects the final color of the fabric. 1 2 3 4 5 6 7 8 9 40 1 2 43X
Plain Jersey is the most important structure. However, if you would like to look into modified Jerseys, see Table 5.3 above.
5.3
Delaware stitch and modified Delaware stitch Tricot fabrics
5.3.1 Introduction Delaware stitch is a very popular warp knit structure. It has a ‘woven like’ appearance, low stretch, and is used in men’s and women’s outer garments. It is extensively used in women’s lingerie and has captured 50% of the mattress fabric market using filament polyester yarn. Delaware stitch was developed by the author, and DuPont DeNemours and Co. introduced it with the T472 nylon yarn and used it extensively in Qiana® fabric. This fabric has been in high demand
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Table 5.4 Delaware and modified Delaware stitches Fabric No. Stitch name 1. 2. 3.
4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Front bar//back bar
Delaware stitch (DS) 2-3, 1-0//1-0, 0-1 Open Delaware 3-2, 0-1//0-1, 1-0 stitch 2-3, 0-1//1-0, 0-1 Mixed Delaware stitch or 3-2, 1-0//0-1, 1-0 Long Float (LF) 3-4, 1-0//1-0, 0-1 Delaware stitch Open LF Delaware 4-3, 0-1//0-1, 1-0 Mixed LF Delaware Short Float (SF) Delaware Satin Delaware Atlas Delaware Double Needle Delaware Double Needle SF Delaware Double Needle LF Delaware Laid-in Delawarea
4-3, 1-0//0-1, 1-0 1-2, 1-0//1-0, 0-1
Similar aesthetics Woven More luster than DS Drier hand than DS
Woven satin Little more luster than LFDS Drier than number 5 Less luster than DS
4-5, 1-0//1-0, 0-1 High luster 0-1, 2-3/5-4, 3-2//1-0, 0-1 New aesthetics 4-2, 0-2//1-0, 0-1 Heavier than DS 3-1, 0-2//1-0, 0-1 5-3, 0-2//1-0, 0-1 With all above stitches
Lighter weight than number 8 Heavier weight than number 8 Higher drape and heavier than DS
Note: aCould be knitted with the middle bar having one needle (0-0, 1-1), two needle (0-0, 2-2), three needle (0-0, 3-3), four needle (0-0, 4-4) or five needle (0-0, 5-5) Laid-in stitches, e.g. three needle Laid-in Delaware stitch 2-3, 1-0//0-0, 3-3//1-0, 0-1.
over the past three decades and the Delaware stitch warp knit structure is currently used around the world in many applications. Delaware stitch offers a warp knit fabric produced with the front bar knitting a three-needle float stitch (2-3, 1-0) and the back bar knitting a chain stitch (1-0, 0-1). Long Float Delaware stitch fabrics use a four needle float in the front bar and the Short Float Delaware stitch fabrics use a two needle float in the front bar. The Delaware stitch fabrics also use open stitches. Other modified Delaware stitches are in Table 5.4 above.
5.3.2 Advantages of Delaware stitch The Delaware stitch-type fabrics, compared to the Jersey stitch constructions, offer woven like low stretch, greater liveliness and new aesthetics including satintype high luster. They are about 10% lighter at a given stitch density, meaning lower cost, wider fabric width and lower fabric weight. They also offer greater fabric stability, higher air permeability through the open knit structure and greater rigidity.
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In a Jersey stitch fabric, only the loop side is used as the face of a garment and not the float side because the float threads of the Jersey stitch are not parallel and do not have a uniform look. On the other hand, the float threads of a Delaware stitch fabric are parallel, making its float side attractive; consequently, it can be used as the face of a garment. However, the float side is preferred and most widely used. In a Jersey-type of stitch construction, the back bar is always sandwiched in between the front bar yarns, so that the back bar yarn cannot be seen. Unlike the Jersey stitch, in the Delaware stitch fabric the front bar yarn appears on the float or back of the fabric, and the front and back bar yarns (about 50/50) appear on the loop side or the front of the fabric. In other words, if a black yarn is used on the front bar and white yarn is used on the back bar, in a Jersey stitch fabric the black color will be on both sides, whereas in a Delaware stitch fabric, the loop side will be predominantly white with a little black color and the float side will be completely black. Thus, it will be a two-colored fabric (see Table 5.5.)
5.3.3 Common quality problems There are three basic concerns or fabric defects in Delaware stitch fabrics. These are: an uneven fuzz area created during knitting, wale shifting or grouping that develop during knitting and/or finishing, and pickiness or stitch distortion of a fabric. Wale shifting or wale grouping is a typical problem in Delaware stitch fabrics, which is imparted by certain conditions and caused by an unstable knit structure. This problem is peculiar to the Delaware stitch structure. A wale shift-free Delaware stitch fabric was first developed in the Textile Research Laboratory of DuPont deNemours and Company. Wale shifting is defined as a fabric defect where some areas of the fabric encounter uneven shifting and grouping. When it occurs in a finished fabric, the fabric is unacceptable. Make sure wale shifting is avoided. Table 5.5 Some examples of Delaware stitch fabrics (32 gauge Tricot)
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Finished
Runners Yarn Quality FB//BB Weight Count Fabric FB//BB Stitch (inch) (inch) (oz/yd2) 1. 20-10 nylon// 2-3, 1-0// same 1-0, 0-1 2. 40-12 nylon// 3-2, 0-1// 20-10 nylon 0-1, 1-0 3. 40-12 nylon// 4-3, 0-1// 20-10 nylon 0-1, 1-0 4. 40-12 nylon// 4-3, 0-1// same 0-1, 1-0
Thick- ness BS Bulk (inch) (cm3/g)
6
50//30
1.3
38 × 82 0.010
5.76
7
57//32
2.2
36 × 76 0.017
5.79
6
70//33
2.7
38 × 82 0.018
4.99
7
75//33½ 3.4
40 × 84 0.026
5.73
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A number of ways in which to stabilize or to obtain a wale shift-free Delaware stitch fabric are the following. Tighter fabrics are better than the open fabrics. The more courses per inch the better the stability will be. Heavier yarn denier and/or finer machine gauge is better than a lighter yarn denier and coarser machine gauge. Higher friction in the yarns such as mixed shrinkage yarn is better than smoother, regular yarns. Textured or spun yarn on only one of the bars will offer wale shift-free fabrics. A longer float in a fabric offers better stability than the shorter float. Relaxed heat setting (7 to 10% overfeed) and drying offers wale shift-free fabrics. Widthwise stretching of the fabric during heat setting or drying causes wale shifting. Laid-in stitches offer wale shift-free fabrics. Double needle fabrics offer wale shift-free fabrics. Two needle float or short float fabrics do not encounter wale shifting, but the appearance of this fabric is not as attractive. Atlas stitch offers better shift resistance fabric. Note that the double needle and laid-in stitches offer wale shift-free fabrics, but also heavier fabrics, less liveliness, and increased stiffness than the Delaware stitch fabrics.
5.3.4 Two types of laid-in Delaware stitch There are two basic types of laid-in Delaware stitch fabrics. The first, type A, is the laid-in stitch in the middle bar, for example 2-0, 1-0//0-0, 3-3//1-0, 0-1 and the other, type B, is the laid-in stitch on the back bar, for example 2-3, 1-0//1-0, 0-1//0-0, 2-2. The laid-in Delaware stitch fabric, having yarn layout with spandex in the middle bar (T472/spandex/T472), makes an attractive fabric. Heavyweight outerwear (60 denier) Delaware stitch fabrics have been developed. Heavyweight Delaware stitch fabrics do not encounter wale shifting.
5.3.5 Mattress fabric In 1990 it was reported that in the USA 50% of mattress fabrics are woven and 50% are of 70-34//40-14 polyester Delaware stitch fabrics. Many mattresses use long float Delaware stitch fabric (3-4, 1-0//1-0, 0-1).
5.4
Tricot and Raschel containing spandex
5.4.1 Introduction After filament, the most important fabrics are Tricot Raschel material with spandex yarn. This unique fabric, with its new visual and tactile quality and new dimension in fabric properties (high stretch, power, recovery and aesthetics) made it possible to use Tricot fabrics in new areas, such as swimwear, bras, panties, leotards, sportswear and many such similar uses, and now it has even been introduced into medical applications. Spandex yarn requires special handling in
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warping, knitting and finishing; therefore, fabric manufacturers should make sure they provide their consumers with fabrics that have proper stretch, recovery and dimensional stability, and garment manufacturers should provide for the fit and comfort of the garment. The word ‘spandex’ is a generic term in the USA and in many other countries. In continental Europe, the generic term is ‘elastane.’
5.4.2 Yarn used in Tricot The main commercial spandex yarns are Lycra®, Dorlastan®, Glospan®, Fujibo®, and Roica®. Typical spandex-containing Tricot mostly uses 40 denier yarn with about 3 denier per filament (dpf) untextured nylon yarn on the front bar and 40 denier spandex yarn on the back bar. Only bare (uncovered) spandex yarn is used in Tricot knitting. The yarn deniers used are 20, 40, 55, and 70 denier (22, 44, 61, and 78 dtex). Spandex yarn must be used in combination with hard yarn or filament yarn. The spandex yarn is used on the back bar and the hard yarn is used on the front or top bar of the Tricot machine. In this way the spandex yarn is sandwiched into the front bar yarn. Thus, the hard yarn protects the spandex yarn and it does not come into contact with the human skin during wear. Yarn suppliers offer the spandex yarn on spun tubes, rewound tubes and on beams (42 inch and 21 inch). In Tricot knitting, only the 164 inch or 84 inch wide Tricot machines are used, mostly with a 28 and 32 gauge and sometimes a 36 gauge (gauge = needles per inch).
5.4.3 Stitches used in Tricot fabrics containing spandex In the Tricot warp knit industry there are two basic stitch constructions that dominate the trade. As mentioned earlier, the ‘king’ of all stitches is the Jersey stitch. The second most popular is the long float Jersey stitch. 1 2 3 4 5 6 7 8 9 40 1 2 43X
5.4.4 Nylon/Spandex (Lycra) versus 100% Nylon Jersey It is interesting to note Nylon//Spandex F//BB and 100% Nylon Jersey are quite unique and different. (DuPont Lycra is also known as spandex.) The comparison is shown in Table 5.6.
5.5
Key Raschel fabrics containing spandex
5.5.1 Introduction Warp knit Raschel fabrics containing spandex yarn are a very important segment of the warp knit industry. In the USA, Tricot consumes about 11% of spandex,
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Table 5.6 Warp knit Jersey Tricot with and without Lycra®
Nylon//Lycra®
100% nylon (Jersey)
Yarn: Front bar Back bar Stitch: Jersey Quality (inch) Runners (inch) Yarn content Machine width (inch) Gauge (npi)
40-13 T865B nylon 40d Lycra® T-126 FB 2-3, 1–0 BB 1-0, 1-2 7½ 58//24 79.5% nylon 20.5% Lycra® 164 28
40-13 T865AB nylon 40-13 T865 AB nylon FB 2-3, 1-0 BB 1-0, 1-2 7 58//43 100% nylon 164 28
78 5.2
110 2.2
66 × 100 0.029 4.19
55 × 50 0.011 2.90
140 × 130
10 × 70
57 none
– –
0.13 × 0.34
–
4.5/5.0 4.0/5.0 High
3.9/5.0 3.2/5.0 Low
Finished fabrics Width (inch) Weight (oz/yd2) Count W × C (inch) Thickness (inch) Bulk (cm3/g) Hand Stretch % Length × width Wet sag recovery Length direction (%) Seam pull-out Power (lb/inch) Instron – 12 lb (unload at 50%) length × width RTPT-fuzz and pill 30 120 Edge curling
RTPT = random tumble pill tester
Raschel 22%, weft knits 6–10%, woven 2%, and the remaining spandex is used in other types of fabrics. Spandex-containing Raschel is unique. It offers three basic fabric properties not available from any other fiber except rubber, but rubber yarns cannot be spun with either very light denier or high power. The three basic fabric properties are high stretch, recovery, and power.
5.5.2 Knitting machines To understand warp knit Raschel machines, it will be helpful to compare them with warp knit Tricot machines. The following are the 12 basic differences.
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• Raschel machines are heavier and produce heavier fabrics. • Older Raschel machines have latch needles with knitting speeds of 800–900 spm whereas the older Tricot machines have spring bearded needles with knitting speeds of 800 to 2 000 spm. • New Raschel and Tricot machines use compound needles, with Raschel machines knitting more than 1 300 spm and Tricot machines knitting more than 2 000 spm. • Raschel machines use two 50-inch beams for 100-inch wide machines, or four 50-inch beams for 200-inch wide machines, six 21-inch or three 42-inch beams for 130-inch wide machines. Tricot machines use four 42-inch beams for 168-inch wide machines. • In Raschel machines the gauge of the machines is measured in needles per two inches and common gauges are 64 gauge (32 npi) and 56 gauge (28 npi). On the other hand, in Tricot, gauge equals needles per inch, and common gauges are 28, 32, and 36 (40 gauge machines have been developed but are not used widely). • The fabric angle to the warp on Raschel machines is 160° take-up whereas in Tricot machines the angle is 90° take-up. This is the reason why Raschel stitch constructions such as heavy Gentlissimo and Power Net cannot be knitted on Tricot machines. • On Raschel machines, fabric take-up tension is high, whereas on Tricot machines take-up tension is lower. • Raschel machines with latch and compound needles do not need a presser bar. Since the needles are heavier and bigger, Raschel machines can use spun yarn or heavier spandex yarn, whereas the Tricot machines with spring bearded needles need a presser bar, and since needles are smaller, knitting of spun yarn is more critical. • Raschel machines knit up to 1 500 denier yarn, whereas Tricot machines knit up to 240 denier hard yarn (for spandex 1 000 versus 140 denier respectively). • In Raschel machines, the links move half a course, whereas in Tricot machines the links move one course. Because of this, lap notation of Raschel is written 2-0, 2-4 for the back bar Jersey stitch. The same lap notation for Tricot is written 1-0, 1-2. • Raschel machines are more versatile and are used for fancy pattern work, whereas Tricot machines are less versatile in styling and designing. • Raschel mills are smaller whereas Tricot mills are larger and more likely to be integrated mills.
5.5.3 Stitches used in Raschel fabrics containing spandex There are hundreds of stitch constructions available, and many stitches are used in the Raschel industry, but within spandex-containing Raschel fabrics only four stitch constructions are used. They are Gentlissimo (1st bar 2-0, 2-2, 4-6, 2-2, 2nd
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bar 2-2, 2-0, 2-2, 2-4, 3rd bar 2-2, 0-0, 2-2, 4-4) (in Europe it is known as Sleeknit), Techsheen (1st bar 2-0, 0-2, 2-4, 4-2, 2nd bar 2-2, 0-0, 4-4, 0-0), Power Net (1st bar 4-2, 2-4, 2-0, 2-4, 4-2, 4-6, 2nd bar 2-4, 4-2, 4-6, 4-2, 2-4, 2-0, 3rd bar 2-2, 0-0, 2-2, 0-0, 2-2, 0-0, 4th bar 0-0, 2-2, 0-0, 2-2, 0-0, 2-2) and Jersey (Locknit). When nylon//spandex Tricot Jersey (2-3, 1-0//1-0, 1-2) fabrics became very successful, the industry started looking for a suitable new structure for nylon// spandex Raschel. Jersey stitch could be produced on Raschel machines, but the industry required two major improvements: • more power in the fabric; and • lower cost fabric, using heavier denier spandex yarn. These improvements were obtained when Gentlissimo stitch for Raschel was developed. It replaced 40 denier spandex yarn and knit stitch of the Jersey stitch with 140 denier spandex yarn and laid-in stitch.
5.5.4 Face or back The loop side of warp knit fabric is called the technical face and the float side of the fabric is called the technical back. The technical face of 100 percent nylon Jersey fabric is used as the face of the garment. But in all the spandex-containing Tricot and Raschel fabrics, the technical back of the fabric is used as the face of the garment.
5.5.5 Summary of stitches, yarns and end-uses Usually nylon yarns are used on the front (top or first bar) and middle (or second) bar, and spandex yarns are used on the back or the third bar in Raschel knit fabrics.
5.5.6 Fabric testing The key fabric tests in evaluating spandex-containing Raschel fabrics are: (1) weight, (2) construction (wales x courses), (3) yield (percent elastomer), (4) width, (5) tensile properties (power/stretch/set), (6) shrinkage and growth, (7) flex life (spandex pull out and slippage), (8) whiteness retention, (9) heat-set efficiency, (10) molding performance, (11) dyeability and fastness, (12) uniformity and (13) chlorine resistance or durability. Every type of garment has its own needs that have to be satisfied. For example, swimwear and panty fabrics need more stretch and to be lightweight whereas foundation garments need more power and to be a heavier weight.
5.5.7 Quality control Spandex is an expensive yarn compared to nylon, and a very sensitive yarn for a mill to work with. In addition, the mill has to be set to a very high yarn tension. Therefore,
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it is very important to have the proper warping, knitting and finishing conditions, and once the mill has defined the conditions they have to be implemented without much variation. As new machines are computerized, they are a great asset in quality control. When a mill is developing quality fabrics it should avoid the following fabric problems: streaks/bands, poor dimensional stability (not meeting the required growth and shrinkage), moiré, excessive fabric edge curling, dye spots, spandex degradation, dye blotches, rope marks, shading and unevenness, poor wet fastness (causing fabric to lose its color when wet), fabric creases and cracks, width and weight variations, uneven power and stretch properties, and excessive bagginess. To maintain good fabric quality, Raschel machines should be kept clean and the atmosphere in the knitting area should be maintained at 70–75°F (21–24°C) and 50–65% relative humidity (RH).
5.5.8 Guidelines Swimwear and panty markets are very important for the spandex-containing Tricot fabrics. DuPont deNemours & Co. has established its own guideline for the nylon//Lycra® containing warp knit fabrics. These are the minimum required properties to satisfy the customers; however, for nylon//Lycra® Jersey swimwear and underwear, 18–20% Lycra® is recommended. The guidelines are outlined in Table 5.7 below.
5.6
Newly developed constructions with spandex
5.6.1 Introduction These are newer constructions and they have limited use. They will expand in due course and as new end uses are developed. Table 5.7 DuPont’s guidelines for nylon//Lycra®-containing warp knit fabrics
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Test parameter
Warp knit minimum acceptable test value
Swimwear and leotard guidelines • Wet sag recovery • Elongation (wale direction) • Seam pull-out • Lycra® content
≥ 40% ≥ 100% Zero pull-out 0.6 oz/yd2
Panty guidelines • Elongation (length × width) • Power (lb/inch) 45% elongation for return curve (3lb) • Seam pull-out 600 cycles • Lycra®
40 × 30% 0.1 minimum Zero pull-out or pull back ≥10%
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5.6.2 Del-Atlas stitch Del-Atlas stitch is not a Jersey stitch. Jersey stitch is a two-course repeat whereas all Atlas stitches are at least a three-course repeat, But in this study, four-course Atlas stitch was selected. Atlas stitch can be knitted using only one bar, but DelAtlas must have two bars. Del-Atlas has at least a four-course repeat on the front bar and at least a two-course repeat on the back bar. Note that the front bar with nylon yarn has only open stitches and the back bar with spandex yarn has only closed stitches. The improved chlorine of the Del-Atlas durability was confirmed in a break test (failure is Lycra® spandex break). The test also showed greater resistance to power loss. One obvious reason is that this knit structure allows the spandex yarn to be free and does not cut at a crossover point such as Jersey does.
5.6.3 Kuper or twill stitch In the front bar the double needle stitch goes from left to right and goes two needle stitches in the same direction. It then goes two needle floats to the right . . . at the same time, the back bar yarn knits one needle stitch and goes two needle floats in the opposite direction (to the left). In the USA, this stitch is known as the Kuper stitch, but in Europe it is known as the twill stitch. This fabric is a swim- and sportswear-type fabric. This stitch offers a heavier fabric compared to Jersey and Gentlissimo without increasing stitch density, and it also offers a wider fabric. Kuper stitch offers 90-inch fabric, whereas Gentlissimo offers 60-inch fabric. In reality, the Kuper stitch is a double needle long float stitch. There are two kinds of double needle stitch: open and closed. Open stitch fabric is more commonly used.
5.6.4 Double needle Delaware stitch The front bar knits double needle open or closed stitches and the back bar knits spandex using the chain or pillar stitch. This fabric offers a suitable fabric for women’s outerwear. It has only length directional stretch and could be used where one-directional stretch is needed, as in a shoe upper or in medical uses.
5.6.5 Laid-in Delaware stitch Three or four needle float stitch is knitted in the front bar, laid-in stitch in the middle bar and chain stitch on the back bar. This is used in women’s outerwear fabrics.
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5.6.6 Laid-in Jersey stitch The front and middle bar knit Jersey stitch and the back bar knits laid-in stitch. The laid-in yarn could use 1-1, 0-0 or otherwise two or more needle float stitch, e.g. 2-2, 0-0, or 3-3, 0-0, which imparts more weight as well as more stretch. This fabric will have a very small percentage of spandex yarn, as the spandex uses one needle laid-in.
5.7
Americana and modified Americana Tricots
5.7.1 Introduction The Americana stitch and its family of fabric structures were developed by using a basic knit-lay technique similar to that found in weft knit interlock structures. In Americana stitch structure knitting, two fully threaded warps or bars create one bar consisting of only one stitch with one loop per needle. In contrast, knitting two bar Jersey fabric produces one stitch with two loops per needle. Americana stitches are disclosed in the author’s patent Method of Warp Knitting, US Patent 4 802 346, 7 February 1989, assignee E.I. DuPont deNemours & Co., and patent Method of Warp Knitting, US Patent 5 029 457, 11 December 1991, assignee E.I. DuPont de Nemours & Co. The DuPont Co. freely gave this technology to the warp knit industry as a good-will gesture. In 1990, DuPont & Co. introduced Americana stitch technology to selected warp knit customers in the US. Included were a sample book and the technology to develop textured and spun filaments, and Lycra® spandex fabrics created with Americana stitch constructions. Americana stitch, with the knit-lay concepts, provides knitters with a family of stitch construction fabrics.
5.7.2 Definition of Americana stitch 1 2 3 4 5 6 7 8 9 40 1 2 43X
The Americana stitch employs two fully threaded warps or bars to knit a one bar fabric. By using knit-lay on the front bar and lay-knit on the back bar, each needle produces one stitch with one loop. Using more than two fully or partially threaded bars produces fabric with a two-course repeat pattern of alternating first and second bar yarns. The fabric is prepared by interlocking the first and second yarns using a combination of Knit and Laid-in stitches in an alternating fashion. In course number one, the front bar knits and the back lays in. In course number two, the front bar lays in and the back bar knits. The fabric construction is as follows: Americana stitch: nylon//spandex Tricot, 28 gauge, FB and/or MB 40-13 T860 bright nylon//BB 40 denier Lycra® (see Table 5.8).
5.7.3 Two basic types of Americana stitch The two bar Americana structure produces two basic types of stitch. The Americana stitch is produced when bars go in opposite directions, and crêpe © Woodhead Publishing Limited, 2011
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Table 5.8 Stitch construction Fabric Front bar Middle bar Back bar no.
Quality (inch)
Runners FB//MB//BB
A B C D
7½ 6½ 6½ 7½
37½//37½//27 35//35//3 37//37//26 58//24
1-0, 3-3 1-0, 3-3 2-3, 0-0 2-3, 1-0
3-3, 1-0 3-3, 1-0 3-3, 1-0 –
1-0, 1-2 0-0, 3-3 1-0, 1-2 1-0, 1-2
Americana stitch is produced when both bars go in the same direction. See Fig. 5.4 below.
5.7.4 Americana knit structure Americana imparts unique fabric structures. A commercial Jersey stitch fabric, for example, uses two fully threaded warps that offer back bar yarn sandwiched between the front bar yarns. The counterpart Americana stitch fabric puts all front bar yarn on the float or technical back of the fabric, and front and back bar yarns alternatively on the loop side or technical face of the fabric.
5.7.5 Splitting problem Commercial one bar fabrics (e.g. 2-3, 1-0) have a splitting problem, which makes them unusable. Fabrics made with a similar stitch construction have the same splitting problem. Weft knit fabrics such as nylon stockings will run, but will not split. The two sides will stay together. Because of this splitting problem, one bar fabrics are not commercially acceptable. Instead, two bar fabrics are commonly used in developing warp knit fabrics. The two bar Americana stitch can produce a stable one bar fabric providing 100% filament smooth yarn (such as 30-10 nylon) or textured yarn and is tightly knitted.
5.7.6 Americana stitch containing spandex Commercial nylon//spandex Jersey fabrics are ideal for women’s swimwear and sportswear and many other applications. There are two basic Americana stitch
5.4 Two types of basic Americana stitch.
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fabrics that contain spandex, namely Americana and crêpe Americana. Moreover, there are two ways of developing spandex-containing fabrics. Americana stitch with nylon on front bar and middle bar, and spandex on the back bar, creates a three bar fabric. Spandex can also be used on the middle bar with nylon on the front and back bars.
5.7.7 Advantages of Americana stitch Americana stitch offers many advantages: a family of new knit structures and new visual and tactile fabric aesthetics; lighter weight, lower cost fabrics with lower cover and high air permeability; newer, less expensive surface interest fabrics; new tools for styling and designing; better fabric uniformity compared with Jersey; new spun-containing fabrics with improved knitting performance and the ability to blend two different yarns in a fabric without adding any weight (e.g. nylon and polyester.)
5.7.8 Family of Americana stitches Americana stitch is a basic stitch construction that has led to a family of new stitch constructions. Table 5.9 identifies seventeen basic Americana stitch constructions. Knit stitch (e.g. 1-2, 1-0 or 1-0, 1-2) could be used on the front or back of Americana stitches. Laid-in stitch (e.g. 0-0, 3-3) could also be incorporated on the back bar. All the basic Americana stitches can be made with the third bar in front of the back bar with 1-0, 1-2 or 1-2, 1-0 or similar knit stitch constructions (see Table 5.9). As mentioned, the Americana stitch offers two basic structures: Americana stitch, which has a regular flat appearance, and crêpe Americana, which, as its name suggests, looks crêpey and is textured.
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5.7.9 Modified Americana stitch Modified Americana stitches are identified as three bar Americana, four bar Americana, and five bar Americana, etc. A three bar Americana stitch requires three bars that knit a three-course repetition. In the course, the front bar knits and the middle and back bars lay in. In the second course, the middle bar knits and the front and back bars lay in. In the third course, the back bar knits and the front and middle bars lay in. Due to this, the three-course repetition with three fully threaded bars will have only one loop per stitch. Four bar Americana stitch has four courses in a repeat and four bars per repeat. In the first course, the front or first bar will knit, in the second course the second bar will knit, in the third course the third bar will knit and in the fourth course only, the back or the fourth bar will knit. All the other bars will lay in. Figure 5.5 is a lap diagram of the three bar modified Americana stitch.
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Table 5.9 Basic Americana stitches
Stitch notations
Fabric Stitch name No.
Front bar
Back bar
1 Americana 3-3, 1-0 1-0, 3-3 2 Satin or Long Float Americana 4-4, 1-0 1-0, 4-4 3 Slipper Satin or Super Long Float Americana 5-5, 1-0 1-0, 5-5 4 Short Float Americana 2-2, 1-0 1-0, 2-2 5 Mixed Float Americana 4-4, 1-0 1-0, 2-2 6 Open Americana 3-3, 0-1 0-1, 3-3 7 Open Long Float Americana 4-4, 0-1 0-1, 4-4 8 Mixed Americana 3-3, 1-0 0-1, 3-3 9 Crêpe Americana 3-3, 1-0 2-3, 0-0 10 Long Float Crêpe Americana 4-4, 1-0 3-4, 0-0 11 Double Needle Americana 2-0, 4-4 4-4, 2-0 12 Open Double Needle Americana 0-2, 4-4 4-4, 0-2 13 Open Triple Needle Americana 0-3, 5-5 5-5, 0-3 14 Atlas Americana 0-1, 1-1, 2-3, 4-4, 5-4 5-5, 4-3, 3-3, 2-1, 0-1 15 Crêpe Atlas Americana 0-1, 1-1, 2-3, 4-4, 5-4 0-0, 1-2, 2-2, 3-4, 5-5 16 3 Bars Americana FB//MB//BB 2-3, 2-2, 1-0, 1-1//1-1, 1-2, 1-1, 1-0//1-0, 1-1, 1-2, 1-1 17 4 Course Americana FB//BB 1-1, 1-0, 2-2, 2-3//2-1, 3-3, 1-2, 0-0
5.5 Lap diagram of modified Americana stitch.
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Nylon can be used on the first three bars of the above structure and spandex on the fourth bar knitting regular 1-2, 1-0.
5.8
Surface interest fabrics
5.8.1 Crêpe Tricot The dictionary defines crêpe fabric as a lightweight fabric characterized by a crinkled surface obtained by the use of (1) hard-twist filling yarns (women’s wear fabrics), (2) chemical treatment, (3) crêpe weaves or (4) embossing (calendaring). Woven crêpe is made by high twist yarn, but high twist yarn cannot be used in warp knit production, so the crêpe structure has to be developed by other methods. Some textured yarns do offer crêpe-like aesthetics; however, if a textured 40-13 nylon is used, and a Jersey stitch Tricot knitted, it will not have a crêpe aesthetic. The Tricot industry has had a great need for economically feasible surface interest and/or crêpe aesthetics in lightweight lingerie and/or for women’s outerwear applications.
5.8.2 Ten key elements for developing good crêpe in Tricot
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• Evenly and randomly distorted stitches. • Crêpe surface with randomly uniform pebbles. • Crimps per inch (frequency) coinciding with the stitch length, e.g. match crimp/inch with knit quality and runner length (loop needs to be distorted). • Crêpe depth and uniformity of crêpe surface controlled by preferred knit construction (3-4, 1-0//1-0, 1-2 a Long Float Jersey stitch) and knit quality (7 to 9 inches), which depends on yarn shrinkage. • Loop side or technical face of the fabric used as the face. • Stability of the crêpe surface. • Knowledge of yarn’s heat-setting temperature (6 versus 66 nylon yarn). • Low tension at width frame of pin tenter is required. • Low water set in beam pressure setting is required to maintain fabric crimp. • For many fabrics, tumble steam is required for optimum crêpe.
5.8.3 Crepeset® Crepeset® is a crimped nylon monofil produced in a patented toothed-wheel crimp process. The Crepeset® fabric is a soft nylon crêpe Tricot fabric, suitable for lingerie. It is supple with a subtle crêpe appearance. The crimped 30-1 yarn has a low residual crimp force, which requires tumble steam for good crêpe. These yarns are not antistatic. The Crepeset® fabric uses 67% regular yarn and 33% Crepeset® yarn. Crepeset® fabric is produced as follows:
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• FB 30-10 nylon • BB 30-1 Crepeset® yarn • Preferred stitch: Long float Jersey Which is: • FB 3-4, 1-0 • BB 1-0, 1-2
5.8.4 Knit-de-knit crêpe Knit-de-knit crêpe yarn offers very attractive crêpe, but these fabrics have uniformity problems. This fabric cannot be heat-set and therefore also has shrinkage problems. All knit-de-knit fabrics must be tumbled to achieve optimum crêpe. The synthetic yarn 30-1 is knitted and de-knitted. This 30-1 monofilament yarn is used on the back bar whereas 30-10 yarn is used on the front bar and knitted with Long Float Jersey stitch (FB 3-4, 1-0//BB 1-0, 1-2). At present, Knit-de-knit yarn is more expensive than gear crimp yarn.
5.8.5 Textured or surface interest warp knits There are many different types of crêpe structure available but the widely used structures are described above. There are two basic weight fabrics: first, lighter weight Tricot fabrics consisting of 20, 30 and 40 denier textured nylon or polyester yarns, and second, heavier fabrics prepared from 60, 70, or 80 denier textured yarns knitted on a Tricot or Raschel machine. Mills usually buy textured yarns from converters and prepare the beams in their own mills. There are three basic types of warp knit fabric. They are: 1) 100% textured FB textured yarns BB textured yarns 2) Bar-on-bar FB textured yarns BB regular hard yarns or filament yarn 3) Bar-on-bar (second type) FB regular hard yarns BB textured yarns For swimwear, the ideal fabric is nylon//spandex Jersey, but it has limited wear life, owing to the low chlorine durability of the spandex yarn in swimming pools. Swimmers prefer swimsuits made from 100% textured nylon fabrics, because they last longer. However, they are not as aesthetically attractive as nylon// spandex fabrics.
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Lower-stretch fabrics using the bar-on-bar textured nylon//nylon Jersey are used in women’s dress wear and these fabrics, along with bar-on-bar nylon// textured nylon fabrics, can be used for lingerie applications.
5.8.6 Simplex machines These are two bed flat warp knit machines with two needle bars containing back to back spring-bearded needles. The machines have two guide bars, which makes them much slower than Tricot machines. So it costs more and it also produces a double faced fabric that is thicker. It is used mainly in hand gloves. It is available in widths of 84 inches, 93 inches, 138 inches, and 168 inches, while gauges range from 28 to 32 needles per inch in each needle bed.
5.9
Milanese fabrics
5.9.1 Introduction In the warp knit industry, Milanese fabrics are in a class of their own. They are high quality and expensive fabrics. The basic structure of the Milanese fabric cannot be produced on the conventional Tricot or Raschel warp knit machines. There are very few Milanese machines in the USA, but in Europe there are more. The main reason Milanese fabrics are used is because they offer better uniformity to non-uniform yarns such as silk and cotton than the counterpart Tricot or Raschel fabrics. Milanese machines can be flat or circular.
5.9.2 Milanese structure
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In order to understand Milanese fabrics, the workings of the Tricot machine should be explained. In Tricot, a warp end could knit on the first needle course and on the second course; the end could knit on the same needle or a second or third needle or up to the eighth needle. After knitting one repeat of the stitch, the machine keeps repeating the same stitch. However, it must be understood that a warp end must go back and forth between the eight needles, mostly between the first and the third needles. In Milanese, one end of the warp knit knits one needle making the first course. In the second course, it knits the needle to its right, in the third course it knits the third needle, and the end continually keeps going to the right. If the front bar goes in the right direction, the back bar keeps knitting in the left direction. In weft knit, there are no warp ends, only one end that knits a course. In Tricot, one could knit an open loop or closed loop but Milanese can knit only open stitches. In Fig. 5.6, the Tricot structure is shown with a closed loop diagram and Milanese is shown with the open loop diagram. Unlike Tricot, Milanese has very limited design and stitch variation possibilities.
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5.6 Loop diagram of basic Milanese.
If a silk or cotton yarn that is not uniform is knitted, the non-uniformity in the yarn in Tricot will show going in the wale or length direction and the non-uniformity of the weft Jersey will show going in the course or width direction. However, on Milanese machines the non-uniformity will be diffused as the yarn keeps going sideways at an angle and the second or back bar yarn will go at an angle in the opposite direction.
5.9.3 Types of Milanese machine There are three basic types of Milanese machine: continental flat bed with spring bearded needles and two guide bars; English flat bed, which is similar to the continental machine, except that in place of the two guide bars it has a special mechanical device to control warp ends; and the circular Milanese machine with a latch needle. The continental and English flat bed Milanese machines produce cotton lap and silk lap stitches, whereas the circular Milanese machine produces only the cotton lap stitch. In a flat bed machine, the bars move in opposite directions. Thus, any single warp end from the front bar will travel across the width of the machine from left to right and on reaching the last needle or selvage it will be transferred to the back bar. At the same time, the warp thread on the back bar will start traveling in the opposite direction, now from right to left, moving at the same speed and going to the other selvage. After reaching the left selvage, the warp
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thread will be transferred back again to the original position completing a full repeat. In a circular Milanese machine, two bars move in the opposite direction at a continuous speed. It is important to note that, like Tricot and Raschel, the circular Milanese machine will not mix front and back bar yarns. Since a flat bed Milanese machine transfers the front bar warp ends to the back bar and thus mixes the two bars, it does not produce the commonly found sandwich-like structure (back bar yarn between the front bar yarn or the bar-on-bar fabrics) of the warp knit fabrics. As mentioned earlier, Milanese machines produce only two basic types of stitch. One stitch is called the Open Cotton Lap (two needle float), where each warp end knits every consecutive needle (0-1, 2-1//2-1, 0-1). The other stitch is called Open Silk Lap (three needle float), in which each warp end knits every other needle or alternate needles only (0-1, 3-2//3-2, 0-1.) Therefore it moves two needle spaces at each course, passing under one needle and over the next. Spun yarns with staple fibers use cotton lap, whereas the silk lap is believed to be for continuous filament yarn – mostly silk yarns. Continental Milanese The continental flat bed Milanese machines are equipped with spring-bearded needles and two guide bars. These machines are built quite similarly to the Tricot and Raschel machines, except for the guide bars and the warp beam setting. As the warp traverses and transfers the warp end from one bar to the other, stationary beams are not used; instead, a movable set of small sectional beams are mounted on the top of the machine. The guide bars are also modified to suit the moving beams. The guides are in units of six, move sideways and have a combined swinging motion, but the needles stay stationary. A section guide is transferred from the front bar to the back bar at one end, and at the other end it is transferred from the back bar to the front bar.
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English Milanese English Milanese is a more complex machine than the continental Milanese. Instead of the guide bars, the English flat bed Milanese machine has a special warp control device that contains still points, traverse points, picker points, thread bar and so on. The remaining knitting elements are, however, similar to the continental Milanese machine. Circular Milanese Circular Milanese is a circular machine that uses latch needles and therefore it is quite a simple machine. Two sets of 12-inch beams rotate in opposite directions and feed the warp ends to the needles mounted on a vertical cylinder. The needles have a vertical motion combined with a rocking motion. Unlike the flat bed
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Milanese (continental and English), the circular Milanese machine offers only cotton lap stitch owing to the restricted beam movement. Milanese machines can use only open lap stitch due to the machine design. It is believed that better fabric quality partly results from the open lap stitch. All the Milanese machines in the United States are 168 inches wide and have a 28 gauge (28 npi.) These machines use a special type of 6-inch small spool (56 total spools per machine). Hence, a fully threaded spool or beam has a total of 168 ends per spool. The fringes of these spools are 2¼ inches high and it takes seven to eight hours to set or thread the machine. The Milanese machine knits 64 spm at the mill (8 racks/hr or 48 inch/hr), whereas a Tricot machine knits 2 000 spm (250 racks/ hr or >2000 inch/hr). It is important to note that all Tricot and Milanese fabrics are produced from 20-22 and 30-32 denier only. As silk is not a uniform yarn, the denier of yarn count varies from 20 to 22 denier; hence yarn count is referred to as 20-22. Heavier silk Tricot fabrics of 40 denier or more are neither economical nor aesthetically attractive. The selection of a 20-22 or 30-32 denier fabric depends upon the style and the market trend.
5.10 Conclusion Several commercial fabrics were discussed. The fabrics that were developed and patented by the author when he was working at the DuPont deNemours & Company, were released by DuPont for trade use. Additional information regarding the fabrics discussed, including information on knitting a cotton spun yarn, Modified Americana stitches, can be found in Warp Knit Fabrics Technologies by Professor Bharat J. Gajjar. Any questions about this chapter can be directed to
[email protected].
5.11 Sources of further information Reference material, further explanations and other related information for this chapter can be found in Professor Gajjar’s book Warp Knit Fabrics Technologies. Also, visit www.WarpKnitsOfDE.com. Professor Bharat J. Gajjar is a graduate of Philadelphia University, PA and worked for DuPont deNemours and Company for 35 years, has 20 patents to his name, most related to warp knitting, and he worked as a professor at Philadelphia University Graduate School teaching advanced warp knitting.
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7 Advances in circular knitting D. SEMNANI, Isfahan University of Technology, Iran Abstract: Circular knitting technologies are the most productive methods in the knitting industry. Recently, renowned machinery companies have been developing modern circular knitting machines with a high efficiency production rate and the ability to create special properties in the produced fabrics. Seamless, ultrafine gauge and pile or sliver fabrics are suitable linings for conventional knitted fabrics. Circular knitting technology will lead the textile industry in producing special fabrics once the circular knitting machines are able to produce these fabrics at a high speed and of suitable quality, enabling competitive production of special and industrial fabrics for automotive and medical applications and elegant clothing. Key words: circular knitting, hosiery, jacquard, automotive fabric, technical textile, medical textile, knitted fabric, quality garments.
7.1
Introduction
Until now, circular knitting machines have been designed and manufactured for mass production of knitted fabrics. The special properties of knitted fabrics, especially fine fabrics made by the circular knitting process, makes these types of fabric suitable for application in clothing, industrial textiles, medical and orthopaedic garments, automotive textiles, hosiery, agro and geo textiles, etc. The most important areas for discussion in circular knitting technology are increasing production efficiency and improving fabric quality as well as new trends in quality clothing, medical applications, electronic garments, fine fabrics, etc. Famous manufacturing companies have pursued developments in circular knitting machines in order to extend into new markets. Textile specialists in the knitting industry should be aware that tubular and seamless fabrics are highly suitable for various applications not only in textiles but also in medical, electronic, agriculture, civil and other fields.
7.1.1 Principles and classification of circular knitting machines There are many types of circular knitting machine that produce long lengths of tubular fabric manufactured for specific end uses. Single jersey machines are equipped with a single ‘cylinder’ of needles that produces plain fabrics, about 30 inches in diameter. Wool production on single jersey machines tends to be limited 171 © Woodhead Publishing Limited, 2011
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to 20 gauge or coarser, as these gauges can use two-fold wool yarns. The cylinder system of single jersey machines is demonstrated in Fig. 7.1. Another inherent feature of woollen single jersey fabrics is that the fabric edges tend to curl inwards. This is not a problem whilst the fabric is in tubular form but once cut open can create difficulties if the fabric is not finished correctly. Terry loop machines are the basis for fleece fabrics that are produced by knitting two yarns into the same stitch, one ground yarn and one loop yarn. These protruding loops are then brushed or raised during finishing, creating a fleece fabric. Sliver knitting machines are single jersey machines that have been adapted to trap a sliver of staple fibre into the knit structure. Double jersey machines (Fig. 7.2) are single jersey machines with a ‘dial’ that houses an extra set of needles positioned horizontally adjacent to the vertical cylinder needles. This extra set of needles allows the production of fabrics that are twice as thick as single jersey fabrics. Typical examples include interlock-based structures for underwear/base layer garments and 1 × 1 rib fabrics for leggings and outerwear products. Much finer yarns can be used, as single yarns do not present a problem for double jersey knitted fabrics. The technical parameter is fundamental to the classification of knitting machines. The gauge is the spacing of the needles, and refers to the number of needles per inch. This unit of measure is indicated with a capital E. The circular machines now available from different manufacturers are offered in a vast range of gauge sizes. For example, flat bed machines are available in gauge sizes from E3 to E18, and large-diameter circular machines from E4 to
7.1 Single jersey machine.
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7.2 Double jersey machine.
E36. The vast range of gauges meets all knitting needs. Obviously, the most common models are those with middle gauge sizes. This parameter describes the size of the working area. On circular machines, the width is the operating length of beds as measured from the first to the last groove, and is normally expressed in centimetres. On circular machines, the width is the bed diameter measured in inches. The diameter is measured on two opposite needles. Large-diameter circular machines can have a width of 60 inches; however, the most common width is 30 inches. Medium-diameter circular machines feature a width of about 15 inches, and the small-diameter models are about 3 inches in width. In knitting machine technology, the basic system is the set of mechanical components that move the needles and allow the formation of the loop. The output rate of a machine is determined by the number of systems it incorporates, as every system corresponds to a lifting or lowering movement of the needles, and therefore, to the formation of a course. The system motions are called cams or triangles (lifting or lowering according to the resulting movement of the needles). The systems of flat bed machines are arranged on a machine component called the carriage. The carriage slides forward and backward on the bed in a reciprocating motion. The machine models currently available on the market feature between one and eight systems distributed and combined in various ways (number of carriages and number of systems per carriage). Circular machines rotate in a single direction, and the various systems are distributed along the bed circumference. By increasing the diameter of the
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machine, it is then possible to increase the number of systems and therefore the number of courses inserted per each revolution. Today, large-diameter circular machines are available with a number of diameters and systems per inch. For example, simple constructions such as the jersey stitch can have up to 180 systems; however, the number of systems incorporated on large-diameter circular machines normally ranges from 42 to 84. The yarn fed to the needles in order to form the fabric must be conveyed along a predetermined path from the spool to the knitting zone. The various motions along this path guide the yarn (thread guides), adjust the yarn tension (yarn tensing devices), and check for eventual yarn breaks. The yarn is taken down from the spool arranged on a special holder, called a creel (if placed beside the machine), or a rack (if placed above it). The yarn is then guided into the knitting zone through the thread guide, which is typically a small plate with a steel eyelet for holding the yarn. In order to obtain particular designs such as intarsia and vanisé effects, the machines are equipped with special thread guides.
7.1.2 Hosiery knitting technology
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For centuries, the production of hosiery was the main concern of the knitting industry. The prototype machines for warp, circular, flat and fully-fashioned knitting were conceived for knitting hosiery; however, hosiery production is centred almost exclusively on the use of small-diameter circular machines. The term ‘hosiery’ is used for clothes that mainly cover the lower extremities: legs and feet. There are fine products made of multifilament yarns on knitting machines with 24 to 40 needles per 25.4 mm, such as fine women’s stockings and tights, and coarse products made of spun yarns on knitting machines with 5 to 24 needles per 25.4 mm, such as socks, knee socks and coarse pantyhose. Ladies’ fine-gauge seamless fabrics are knitted in a plain structure on single cylinder machines with holding-down sinkers. Men’s, ladies’ and children’s socks with a rib or purl structure are knitted on double-cylinder machines with a reciprocated heel and toe that are closed by linking. Either an anklet or an overthe-calf length stocking can be produced on a typical machine specification with 4-inch diameter and 168 needles. Currently, most seamless hosiery products are manufactured on circular knitting machines of small diameter, mostly between E3.5 and E5.0 or needle pitches between 76.2 and 147 mm. Sports and casual socks in a plain base structure are now usually knitted on single-cylinder machines with holding-down sinkers. More formal simple rib socks may be knitted on cylinder and dual rib machines termed ‘true-rib’ machines. Figure 7.3 presents the dial system and knitting elements of true-rib machines.
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7.3 The true-rib machine in hosiery technology: 1 – needle, 2 – fabric, 3 – needle cam and 4 – sinker cam.
7.2
Current problems and limitations of circular knitted structures
7.2.1 Problems in high-speed circular knitting machines The most important problems in high-speed knitting machines are classified as the limitation of friction occurrence in knitting elements and tension control in the yarn feeding system. Some research has been done about the influence of different parameters and factors among yarn, needles and knitting elements during the knitting operations. Important factors in the contact between yarn and knitting elements during the knitting process are friction, flexural rigidity, the mechanical properties of the yarn and the velocity of knitting elements and yarn in the knitting zone. Amoton’s law for friction occurrence in the dynamic conditions of loop formation during the knitting process states that the yarn tension increases on the yarn supply side of the stitch cam and the lowest tension occurs on the yarn on the other side of the knitting cam. This matters because it is easier to ‘rob back’ yarn from already-formed loops than to pull new yarn from the yarn package, owing to the fact that the tension is higher on this side. The tension on the yarn during knitting is influenced by the number and the angles of yarn wrap between yarn and machine elements, and the fact that robbing back can reduce tension in the yarn. Robbing back is the term for the yarn that comes from the already-formed loops back into the knitting zone, because the yarn tension is lower on this side and higher on the yarn package side. An increase of input tension makes the position of maximum knitting tension move towards the yarn supply side, and with lower input tension the point of maximum knitting tension lies closer to the knitting point. These factors and the fact that many parameters cooperate and influence each other between yarn and machine elements
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make knitting a rather complex process. When it comes to producing knitted structures of yarns or monofilament fibres with high stiffness such as carbon, aramid or polyester mono-filament, parameters such as friction and the flexural rigidity of the yarn are of considerable importance for the knit-ability of the structure. However, a significant problem is that some of these stiff yarns are almost inextensible, which causes tension peaks with breakage of the yarn or single filament in the yarn bundle, especially in high speed knitting operations.
7.2.2 Limitation of pattern in jacquard circular machines The creation of patterns in jacquard machines is limited to a number of knit elements: the yarn feeder system, the needle selection system and the dial needle bar gauge. Mini-jack circular knitting machines are suited to the production of jacquard fabrics. Some models are equipped with selection systems that have 39 levels, 37 of which are for jacquard selection and two for set selections, but more selection elements are limited by the diameter of the dial. The selection is accomplished through a plug-in cartridge or PVC cards, which can be easily changed and programmed separately. Some firms also offer single-bed full jacquard models with electronic needle-by-needle selection to carry out jacquard and operated motifs with virtually unlimited pattern repeats. Some manufacturers have developed a single-bed circular machine for the production of striped jersey fabrics, in versions with 44 four-colour electronic stripe pattern motions, a model for open-worked jacquard fabrics with a diameter of 36 inches and gauge 20–22, as well as a vast range of fully electronic single-bed models for plain or striped three- or four-colour jersey fabrics. These manufacturers also offer a special single-bed machine for jacquard samples, introduced at the ‘Itma ‘99’ fair, with a diameter of 4 inches, two systems and two six-colour electronic stripe pattern motions, with needle-by-needle selection to produce the prototypes of jacquard patterns with enormous time and yarn savings, as it avoids the repeated setting up of a full-size production machine for the various samples. Developed models can be improved by adding the needle-by-needle selection system for the jacquard pattern application if the selection system can work with high-speed machines.
7.2.3 Production limits of seamless knitting machines The most important limitation of seamless knitting machines in circular form is the poor flexibility of these types of machine for producing fabrics in different diameters. In the commonly used method of apparel making in flat fabrics the cutting operation is important. In seamless technology the fabric cannot be cut, and therefore the various diameters should be prepared in the knitting process. This applies to different types of seamless machine of varying cylinder diameters. The short, thin fabrics are knitted in small diameter cylinders and therefore the
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production rate is lower. This leads to increasing costs and may be a serious problem for seamless, circular knitting technology. Seamless fabric is knitted in simple structures and there is no advanced jacquard machine to prepare a mixed pattern fabric for this purpose. Seamless circular knitting machines have lately been manufactured to apply a fine gauge as well. These machines are suitable if the benefit of selling price of fabric is justified by the cost of production at a limited rate of production.
7.3
Recent advances in circular knitting
7.3.1 Santoni seamless knitting technology Seamless garment knitting can be achieved either on the circular knitting machine or flat (V-bed) knitting machine. However, seamless circular knitting machines differ from seamless flat knitting machines in that seamless circular machines create only a single tubular type of garment such as those produced on Santoni machines. Seamless knitting machines can create more than one tube and join the tubes together on a machine. The complete garments knitted on circular machines may also only need a minimal cutting operation. In addition, seamless circular machines require different diameters to make major changes in garment size, whereas seamless flat machines can adjust to different garment sizes on the same machine. Consequently, seamless knitting on circular machines is not true seamless knitting. It should be mentioned that knitting on V-bed seamless machines produces truly seamless garments since they do not require any cutting or sewing. In recent years, Santoni has developed a four-feed single-jersey electronic circular machine, which enables the creation of a shaped garment by reciprocal movement. To get a higher quality knitted garment, it is crucial to control the manufacturing functions. It is critical for designers and manufacturers to communicate effectively in order to create successful new products in the knitting industry. Designers complain that the designs that they specified are not accurately created, while technicians are of the opinion that the designers do not understand the technical problems in knitting feasibility. It has been proposed that one way to overcome the communication problem between designers and technicians would be the use of intelligent CAD systems. The CAD system gives designers and manufacturers the opportunity to specify and evaluate their design more precisely without requiring great time investment and technical expertise. Diverse computercontrolled systems including CAD/CAM controlled machines have been developed to facilitate communication. Companies have been offered new types of CAD system, which use two different monitors including a technical window and a design window for designers and manufacturers, who require different information for the same design (Fig. 7.4 and Fig. 7.5). © Woodhead Publishing Limited, 2011
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7.4 The seamless circular knitting system with CAD/CAM.
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7.5 Automatic seamless circular knitting elements.
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The technical window shows the developing design in the form of running yarn notations and technical data, while the design window presents the design as a virtual simulation. It is expected to minimise misunderstanding between designers and technicians in the knitting industry.
7.3.2 Ultra fine gauge knitting machines Fine knitted fabrics known as ‘second skin’ are produced on fine gauge circular knitting machines. Cotton, polyester and viscose yarns of 90 to 120 Ne are applied to produce fine circular knitted fabrics. The appearance of these fabrics is similar to woven fabrics but they are more flexible. Groz-Bekert E68 (Fig. 7.6), Mirandsai MV4-3.2 II E60, Havertex BSM2100 E62, and other models are well known, and can produce ultra fine gauge machines. New models by Carl Mayer (MV4-3.2 single and IG 3.2 QC double) are ultra fine with gauge E96. Speeds up to 1.3 m/s in these types of machine (with 3.2 feeders per inch) have been reached. High-grade circular knitting cylinders provide the ideal complement to the company’s premium range of needles and system parts to create the perfect knitting system from a single reliable supplier. It is only with the guarantee of a consistently high standard of component quality and outstanding durability that circular knitting machines are able to reach their full potential for high-performance operation on the factory floor. Ensuring the pinpoint precision of individual elements in existing machines not only simplifies the workflow but also improves capacity utilisation in production. Fine gauge automotive fabrics, known as woven-like fabrics, are produced on fine gauge double cylinders in Pai Lung ultra fine gauge machines. Pile technology
7.6 Ultra fine gauge elements of a circular knitting machine.
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is applied to these machines, where the fine gauge of loop makers is adjusted near to the sinkers.
7.3.3 Loop transfer technology in circular knitting machines
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Stitch transfer is an important operation, as a higher number of transfer modes means more possibilities of making structured designs and shaped fabrics in which the stitch is transferred from one cylinder to the other or within the same cylinder. All the electronic flat bed machines manufactured in Italy offer the double stitch transfer option, i.e. from one cylinder to the other and vice versa, independently of the carriage direction. In addition, some manufacturers have equipped their machines with an extra cylinder arranged above the machine cylinder and provided with special points to receive stitches from and transfer stitches onto other needles with great freedom of movement. In addition to facilitating the knitting of fabrics with braids or embroideries that require stitch transfers, the third bed also allows significant advantages in the production of shaped fabrics, as the narrowing stitches are transferred onto this cylinder and not onto the dial, diminishing the straining and improving the process conditions. An Italian manufacturer introduced some models including an extra bed with individually selectable points, which operate independently on the front and back bed needles for the lateral transfer of stitches. The production of high-quality knitted fabrics – i.e. of homogeneous appearance as a result of a smooth knitting process and the absence of holes and barring – essentially depends on the application of certain technical solutions that are now adopted by all Italian manufacturers, sometimes with mechanical variants (whether patented or not). As for the type of needle used, Italian machines incorporate latch needles, which operate according to the drowned butt principle. This kind of needle remains in an idle position with its heel completely drowned in the needle bed groove without being involved in the action of cams, and retains the loop, which in this case is not subject to strain. Figures 7.7 and 7.8 demonstrate novel loop transfer elements of these circular knitting machines.
7.3.4 Pile and sliver insertion mechanism in circular knitting A special sliver knitting process locks individual fibres directly into a lightweight knit backing, allowing each fibre to stand upright, free from the backing, to form the soft pile on the face of the fabric. This makes comfort pile fabrics softer, warmer, more drapeable and more resilient than fabrics made from yarns. Each fabric originates from premium loose fibres. These fibres include high-tech microfibre acrylics, polyesters and mod-acrylics specially developed for fabric, along with natural fibres such as wool.
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7.7 Loop transfer tool.
7.8 Needle with spring for high speed loop transferring.
Each blend is chosen for its specific end result. By engineering the fibre mix, an incredibly wide range of colours, density, weight, patterning, texture and performance features can be produced in comfort knit pile fabrics. The selected fibres are solution-dyed to ensure consistent, rich, deep colours. The fibres are then blown together in an air chamber, similar to down, blending their colours and fibre types in a predetermined mix. The mixed fibres are then sent through a series of special carding machines that comb the fibres, aligning them parallel to one another. They are then gathered into a soft rope called ‘roving’ or ‘sliver’ – hence the term sliver knitting. The slivers are then fed into sliver inserted fabric state-of-the-art electronic knitting machines as presented in Fig. 7.9 and 7.10. The machine’s fine gauge needles, rotating in a circular motion, pick up fibres from each sliver in a predetermined sequence, locking them directly into soft but strong polyester knit backing. Secured at one end, the length of the fibres remain upright and perpendicular to the backing, allowing them to maintain their resilience, softness, breathability, comfort and light weight. After knitting,
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7.9 Sliver feeding system by using feeding rollers.
7.10 Combing tool of the sliver feeding system.
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the pile fabric is sheared to the desired height. It is then put through a series of technical finishing processes specially developed by this kind of knitting process to control the surface texture and special characteristics of the final fabric.
7.4
Structure and properties of circular knitted fabrics
7.4.1 Tubular knitted fabrics A variety of elastic fabrics is used for making pressure garments for burn rehabilitation. Such fabrics generally contain elastane, which has the disadvantage of exerting a visco-elastic response to an applied load. If such a fabric is under tension over a period of time, some of the stress in it will be relieved, with a consequent reduction in the skin-and-garment interfacial pressure. A study by Cheng et al. (1983) demonstrated that there is a gradual decline in skin-and-garment interfacial pressure when patients wear the pressure garments over a period of time. Views of doctors, therapists and patients support
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their findings: slackening is bound to occur in pressure garments when patients wear them for a long time because the tension of the fabrics has a limited lifespan. Elastic fabrics have different elastic properties lengthwise and widthwise. The rate of tension decay of each elastic fabric depends mainly on there properties, as well as the amount and direction of stretch applied to the fabric. For clinical treatment, it is important to maintain the pressure on the affected area, within a specified range according to instructions from doctors or therapists. The deterioration of tension in the elastic fabrics affects the clinical effectiveness of pressure garments. Tension decay in elastic fabrics means the skin-and-garment interfacial pressure will deteriorate gradually. Elastic fabric can provide the required range of skinand-garment interfacial pressure to the patient during wear. These days, however, pantyhose purchases are down, and although there are many types and kinds of pantyhose, their appearance and sheerness have yet to be satisfactory for women’s needs. The aesthetic properties of pantyhose have long been of great concern to women, and even more so in recent years. Requirements for handling, performance and comfort have been published by Fujimoto et al. (1989) and Harada et al. (1982, 1995).
7.4.2 Influence of structure parameters and relaxation methods on dimensional stability Knitting has always been considered an art. When done by hand, the final product was completely dependent on the weaver’s art, skill and craftsmanship. Even now that 400 years have passed since the first mechanised knitting industry, this value has not changed; the weaver’s art, experience and professionalism can still help create a final product with dimensions and specifications beyond the standard numerical, mechanical and control techniques. At first glance, it seems that such variables create problems in measurements, but this is not the case. All the variables are measurable and controllable, and the most important problem is the precise measurement of dimensional and physical properties of the knitted fabric. One of the most important problems for the knitting industry is the instability of fabrics after knitting, and particularly after their first laundering, which provokes the consumer’s dissatisfaction and may lead to a decrease in demand for the industry. It is to the advantage of the industry that these problems be addressed effectively. This means that there should be criteria for the stability of dimensions in weft knitted fabrics as presented in fundamental research by Fletcher and Roberts (1952), Shinn (1955), Leaf and Glaskin (1955) and Munden (1959). These criteria are expressed as the ‘Ks’ of the intended fabric, and are equal to the density of the stitch multiplied by the square of the stitch length. In the studies carried out in this field, attempts have been made to formulate a geometric model suitable for the stitch that is at complete relaxation so that one can find theoretical
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‘Ks’ for the fabric. This model theory can be applied as a criterion for determining dimensional stability of fabrics (Knapton et al., 1975–1979). Further studies have suggested a three-dimensional structural model for a stitch at complete relaxation, in which according to the calculations, ‘Ks’ for cotton fabric are equal to 26.7765. For a number of weft knitted fabrics at complete rest, attempts have been made to carry out treatments on the fabric at complete relaxation with its ‘Ks’ as close to 26.7765 as possible by actually using chemical relaxants (Semnani et al., 2003). Milano rib fabrics can be used commercially as sportswear, warm outerwear, etc., because they are compact structures with horizontal openings inside the structures, allowing them to retain trapped air and prevent the transfer of heat or cold from one side to the other. Milano rib structures can also be used functionally as technical textiles, for example, by inserting wire, electrical cables, water pipes, yarns, etc., in their horizontal openings. They are also suitable for absorbing impact. The dimensional properties of a fabric are researched for two reasons: to plan a piece of fabric before knitting and to obtain a dimensionally stable fabric. Both reasons are important for fabrics such as Milano ribs, which can be used commercially and functionally. Plain knit and 1 × 1 rib structures are relatively tight fabrics and are subject to jamming. Milano rib fabrics are simple extensions of these two fabrics in that they loosen both the plain and the 1 × 1 rib structures. This applies to cardigan and interlock fabrics, too. These fabrics have a compact structure similar to that of Milano. The relaxation process is especially important for these fabrics in non-equivalent structures of knits. Lately, ultrasonic and mechanical–chemical finishing methods named ‘full relaxation’ have been used in the finishing industry after the knitting process.
7.5
Applications
7.5.1 Seamless knitted garments 2 3 4 5 6 7 8 9 40 1 2 43X
The knitting industry has recently undergone some major advances in the development of seamless whole-garment knitting. One of the main factors for exploring and pursuing the ability to produce seamless whole garments was the ‘labour trap’ (Millington, 2000). For a number of years the garment industry had found it difficult to compete on product price with countries that have a plethora of low-wage labour available. In addition, the more labour required to complete the garment, the more vulnerable the garment producer becomes. Interestingly, the breakthrough of whole-garment knitting has been achieved without a single new advancement in the fundamental knitting process adopted by William Lee when he invented the hand-stocking frame in 1589. The basic principles of knitting remain the same. Today, there are knitting machines that can produce either shaped panels or seamless whole garments. Knitted shaped panels can be sewn together, thereby
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eliminating the cutting process. The cutting and sewing process is entirely eliminated by utilising knitting machines that produce seamless whole garments. Several manufacturers of knitting machines are now able to produce whole seamless garments. This is accomplished by utilising multi-needle/transfer knitting elements (Millington, 2000). The machine starts by knitting three separate tubes – two sleeves and a bodice. The three pieces are then joined under the shoulder, creating one large tube. The number of active needles is then gradually reduced, so the tube diameter decreases up to the neckline. The collar is then knitted and the garment is finished off (Stoll, 1998). There are a number of advantages to full-body knitting besides the elimination of the cutting and sewing process. Other advantages include the elimination of uncomfortable seams at the shoulders and underarms and the reduction of fabric waste generated during the cutting process. The full-body knitting machines are capable of knitting numerous shapes and various structures that possess different physical and aesthetic characteristics. There are many potential applications for seamless shaped multidimensional fabrics. Seamless, shaped knitted fabrics can be used in products designed for the industrial, aerospace, automotive, medical and protective clothing industries. Unfortunately, there are some disadvantages in the manufacture of shaped knits. The installation of machines capable of knitting full-body garments has proceeded with some caution. Countries that have a large population willing to work for low wages can produce huge quantities at very low prices. The capital cost of production time alone on a full-body garment machine can still exceed the price that a cut and sewn knitted garment could be purchased for in China. There are also design limitations to the full-body knitting machines; designs cannot be applied to cylinders during knitting. Furthermore, the type of yarns that can be used are limited. In addition, although fabric waste is eliminated in full-body knitting by eliminating the cutting process, waste can be generated through faults. In some cases knitted constructions do not have appropriate physical properties and are therefore not suited for certain end uses. For example, in medical applications, specifically for implantable arterial prostheses, performance characteristics vary according to the demands of the location in which they are used. Knitted prostheses are used because they are easy to suture and possess acceptable thrombogenicity (the ability to cause blood to clot). These particular characteristics make them exceptionally suitable for replacement or bypass of the abdominal aorta and iliac arteries. However, knitted vascular grafts generally have higher porosity and lower strength than woven grafts. In addition, weftknitted structures have a tendency to unravel, particularly when cut at an angle, and have inferior dimensional stability.
7.5.2 Electro textiles Some examples of electro textiles are body conformable antennae for integrating radio equipment into clothing; power and data transmission – a personal area
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network; flexible photovoltaics integrated into textile fabrics; the capacity for physiological status monitoring to monitor hydration and nutritional status as well as the more conventional heart monitoring; quality footwear that can also convert and conserve energy; and, of course, phase change materials for warming and cooling of the individual. Taping a battery and an LED to a magnet was easy in T-shirts or other knitted apparels. It was fun, and consequently embraced wholeheartedly by all. The Throwie was soon expanded and modified, morphing into massive multi-LED on structures, clip-on clothes lights, or mega magnet monsters. Theoretically, it might have been difficult to explain the process to a diverse audience, but once the technology was clear, comprehension and creativity flowed.
7.5.3 Automotive textiles
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A prime requirement of any textile vehicle seat or door panel base fabric is the ability to apply both visual and aesthetic quality to the product. This may seem somewhat self-evident but many fabrics have been developed in attempts to perfect these properties. Base fabrics should be capable of design and colour variations on short development time scales. One such product is circular fabrics (DNBR), which is in many ways an ideal product with a pile surface, high production rates, low cost, some stretch for ease of engineering, etc., but it lacks the ability to have large surface patterns applied easily and efficiently, without dramatically affecting cost. The result of this in recent years has been that this technology has lost out to more ‘design friendly’ processes such as circular-knitted products. Similar comments could be made of loop-raised tricot fabrics, where fabric aesthetics rather than surface pattern predominate. New ways of applying design to fabric such as ink-jet printing, however, mean that such fabrics could again come to the fore as print substrates, and could compete favourably with jacquard technologies, which are at the moment dominant in the production of figured fabrics. Seat cover fabric must always appear non-creased, and for this reason it is usually laminated to polyurethane foam, with a thickness varying from about 2 to 10 mm. In addition it must resist soiling and be easily cleanable without ever being put into a washing machine. Lamination to polyurethane foam also imparts a soft touch to the fabric, and deep and attractive sew lines are formed in the production process. To help the seat cover slide along the sewing machine surface during manufacture, and to assist sliding when the made-up cover is pulled over the seat structure, a scrim fabric is laminated to the other side of the polyurethane foam. The scrim also helps to control the stretch properties of the seat cover, especially when knitted fabrics are used. Thus, the cover ‘fabric’ is usually in the form of a triple laminate for seats, but when used for door casings the ‘fabric’ (laminate) is used without the scrim. At present the most important technical requirements of a car seat covering fabric are cost, UV degradation resistance,
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light fastness, abrasion resistance and soil resistance. The latter is a natural property of polyester, which can be improved by a fluorocarbon after-treatment if necessary. Once this has been decided, effort should be diverted to the pile yarns that form the aesthetic surface of the fabric, as these can be of an infinite variety with a much greater choice available than for, say, the circular-knitted pile cloths. Figure 7.11 demonstrates the portions of circular knitted fabric that have been used in automotive textiles from 2000 to 2009.
7.5.4 Orthopaedic applications The medical sector is probably the one with the strongest influence on technical textiles, due to its wide application in medical centres, hospitals and first aid centres. Tubular fabrics with elastic weft insertion optimise the compressive effect and are much used in post-operating treatments on articulations in the knee, elbow and ankle. Also, they are frequently used in muscular recuperation processes and orthopaedic treatments (Fig. 7.12). Tubular fabrics of rib texture are produced on weft circular machines, with a mixture of polyester yarns and polyamide recovered elastic yarn of 90 denier. One of the most visible examples are the elastic meshes for use in bandages, compresses and the subjects for post-operating implants. Meshes produced on warp circular machines, with a mixture of polyester yarns and polyamide, also exhibit similar elasticity. The advantages of these fabrics over the traditional fabrics are as follows: they allow transpiration of the skin avoiding allergies; they are easy to apply and adaptable to any part of the body; and they enable more oxygenation of the wound. Knitted cords in polyester/polyamide with a lycra core are used in surgery masks thanks to the adaptability and comfort of the material. The rigid bandages for application in hospitals and first aid centres are supplied in rolls, with the laterals
7.11 The portions of circular knitted fabric in automotive textiles.
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7.12 Tubular fabrics with elastic weft insertion for orthopaedic application.
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knitted, avoiding stray yarns, which can produce infections. The crochet fabric allows higher and immaculate adaptation to the wounds. Bandages are produced with cotton weft and warp. The semi-elastic bandages, thanks to the composition of their polyester warping and/or weft yarns/polyamide texture, give some elongation (elasticity) which allows their use to protect wounds and provide compression. The use of warping rubber yarns lends elasticity to bandages (Fig. 7.13), in many cases up to 200%, increasing their compression properties and making them ideal for muscular contusions and orthopaedic applications. The structure and fibre used in this kind of bandage allow its use in plastering at first aid and emergencies, due to its strong compressive capacity. The structural composition of the fabric in this kind of bandages allows its use in surgical procedures in order to protect fabrics and to be used as prostheses. Tubular fabrics are used for skin protection against plasters, preventing the contact of the skin with the plaster. Another application is its use as protective and subjection bandage. Tubular fabrics are produced on weft circular machines and come in a range of diameters from 1 to 10 inches covering all necessary applications. Rib tubular fabrics of small diameter are commonly used in cuffs, but due to their compressive effect they are also used for the cuffs of sleeves for surgery
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uniforms by optimising their capacity for closing and preventing the dispersion of micro-organisms in surgical procedures (Fig. 7.14). Rib tubular fabrics are produced in most weft circular machines.
7.6
Future trends: smart garments
Smart knitted fabrics can be made due to advances in many technologies coupled with advances in textiles and structures. At a glance, the production of elegant
7.13 Elastic bandages.
7.14 Tubular knitted fabrics in surgery uniforms.
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circular knitted materials such as seamless fabric is due to advances in biotechnology, information technology, microelectronics, wearable computers, nano-coated and micro electromechanical devices. In many cases the purpose of these systems is to provide both military and civilian personnel engaged in highrisk applications with the most effective survivability technologies. Some new applications of knitted garments in circular form are being developed, such as the introduction of conformal antennas to the fabric body for integrating radio equipment into clothing, power and data transmission devices in the knitted structure, photovoltaic integrated into fabrics, smart footwear, quality knitted coating in home applications such as carpets or covers, energy-converting and protective tubular fabrics, which generate electricity from the thermal energy in people’s movements, and the application of phase changing materials for heating and cooling of the individual (used in double face knitted fabrics, or spacer fabrics that are knitted in circular knitting machines). The specific features of circular knitted fabrics such as flexibility, seamless structure and contactability make the circular knitting industry the most improved quality textile sector of the future.
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References
Addington, Michelle and Schodek, Daniel (2005). Smart Materials and Technologies for Architecture and Design Professions. Oxford: Elsevier. Cheng, J. C. Y., Evans, J. H., Leung, K. S., Clarke, J. A., Choy, I. T. C. and Leung, P. C., Pressure therapy in the treatment of post-bum hypertrophic scar; A critical look into its usefulness and fallacies by pressure monitoring, Burns, 10, 154–163 (1983). Double knit electronic circular knitting machine, Santoni Company, http://santoni.com/ en-macchine-item.asp. Double-Jersey Machines: Overvie, Mayer & Cie GMBH (2007), http://www.mayercie.de/ en/produkte/43_269.htm. Exclusivity in ultra-fine fabrics. On the circular knitting machine, Mayer & Cie GMBH (2007), http://www.mayercie.de/en/produkte/43_623.htm. Fletcher, H. M. and Roberts H. S. (1952). The geometry of plain and rib knit cotton fabrics and its relation to shrinkage in laundering. Textile Research Journal, 22, pp. 84–88. Harada, T. and Fusaka, K. (1982), Wear feeling of pantyhose, Japan Research Association for Textile End-Uses, 23, 135. Inamura, A., Nakanishi, M., and Niwa, M., Relationship between wearing comfort and physical properties of girdles, Japan Research Association for Textile End-Uses, 36, 109 (1995). Knapton, J. J. F. (1979). Wet-relaxed dimensions of plain-knitted fabrics. Journal of the Textile Institute, 70 (9), 410–411. Knapton, J. J. F. Truter, E. V. and Aziz, A. K. M. (1975). The geometry, dimensional properties, and stabilization of cotton plain-jersey structure. Journal of the Textile Institute, 66 (12), 413–419. Knitting International (2004a). Shima offers new print option. 111 (1314), 21. Knitting International (2004b). Lonati buys Sangiacomo, 111 (1317), 19. Knitting International (2004c). World equipment buyers’ guide 2004, 111 (1318), 45–75. Knitting International (2004d). Hosiery after-care specialists, 111 (1320), 30–31.
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Knitting International (2004e). Stoll launches flexible all-rounder, 111 (1321), 15. Knitting International (2005). Flexibility is key for cortese, 112 (1328), 42. Leaf, G. A. V. and Glaskin, A. (1955). Geometry of plain knitted loop. Journal of the Textile Institute, 46, 587–605. Millington, John. (2000) Complete garment manufacture. Textile Asia, July, 32–37. Munden, D. L. (1959). The geometry and dimensional properties of plain-knit fabrics. Journal of Textile Institute, 50, 448–471. Santoni (2006a). Retrieved January 11, 2006, from http://www.santoni.com. Santoni (2006b). Seamless Apparel Symposium. Greensboro, North Carolina, April 4. Semnani, D. Latifi, M. Hamzeh and S. Jeddi A. A. A. (2003). A new aspect of geometrical and physical principles applicable to the estimation of textile structures: and ideal model for the plain-knitted loop. Journal of the Textile Institute, 94 (1), 202–211. Shinn, W.E. (1955). An engineering approach to jersey fabric construction. Textile Research Journal, 2, 270–277. Single jersey electronic circular knitting machine, Santoni Company, http://santoni.com/ en-macchine-item.asp. Single-Jersey Machines: Overvie, Mayer & Cie GMBH (2007), http://www.mayercie.de/ en/produkte/43_70.htm. Smart textiles, http://www.smartextiles.co.uk/_f_1_1.htm (November 1, 2007.) Softswitch Electronic Fabrics-Applications. (2001). Retrieved July 23, 2001: http://www. softswitch.co.uk. Spencer, D. (2001). Knitting Technology, a Comprehensive Handbook and Practical Guide. Stoll, Thomas (1998) New knitting technologies on the flat knitting machine. International Textile Bulletin, May, 61–66.
Further reading Banerjee, P. K. and Alaiban, T. S. (1988), Geometry and dimensional properties of plain loops made of rotor spun cotton yarns, Part III: Spirality of the wale line. Textile Res. J. (558), 287–290. Baurley, S., (2004). Interactive and experiential design in smart textile products and applications. Pers Ubiquit Comput, 8–15. Brackenbury, T. (1992). Knitted Clothing Technology. Oxford, Boston: Blackwell Scientific Publications. Bremmer, N. (2005). Circular knitted striped fabrics. Knitting International, 112 (1324). Fujimoto, T. (1989), The evaluation of performance of women’s pantyhose, Part I: Characteristics of deformations in the wearing test and the size effects, Japan Research Association for Textile End-Uses, 30, 80. Fung, Walter and Hardcastle, Mike (2001). Textiles in Automotive Engineering. Cambrige: Woodhead Publishing. Groz-beckert, INFO KN newsletters numbers 2, 4, 7, 8, 9 and 10, now available at http:// www.grozbeckert.com/sprache_english.asp. Harada, T. (1995), Comfort of clothing and sense measurement. Japan Research Association for Textile End-Uses, 36, 24. Hum, A. P. (2001). Fabric Area Network – A new wireless communications infrastructure to enable ubiquitous networking and sensing on intelligent clothing. Computer Networks 35(ER4), 391–399.
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Hunter, B. (2004a). Complete garments – evolution or revolution? (Part I). Knitting International, 111 (1319), 18–21. Hunter, B. (2004b). Complete garments – evolution or revolution? (Part II). Knitting International, 111 (1320), 22–23. Hunter, B. (2004c). Complete garments – evolution or revolution? (Part III). Knitting International, 111 (1321), 20–22. Issacs, M. (2005). Seamless: Eliminating stitches – more than a buzzword. AATCC Review, 5 (11), 16–19. Knit Americas (2001). Seamless sweaters here to stay. Winter, 22–23. Knit Americas (2005). What next for seamless body size clothing? Fall, 20. Linda, G. and Leggett, N. (1990). The Machine Knitter’s Dictionary, London: Batsford. Needle Detector NW from Memminger-IRO, reference 335.905.001.07 DSNL/5, official website http://www.memminger-iro.de/. Onal, L. (2003). The relation of seam-free garments and mass customization. International Textile Bulletin, 49 (3), 44. Paradiso, R. (2006), Seamless H-health systems. Knitting International, P25–26. Powell, N. B. (2003). Italian Textile Technology. Textile World, 153 (6). 40–41. Push, T., I. Wunsh, R. Seifert and P. Offermann (1997), Fine structure of yarn tension on large-diameter circular knitting machines, Melliand textilberichte, 1–2, 52–55. Rajkhowa, I. (2000). Wear your PC. Computers Today, October 31, 90–92. Roberts, S. (2000). Intelligent Garments – Fact or Fiction? Just-Style Features, May 11. Santoni. (2004). Know-How: Santoni. Brescia, Italy: Gruppo Lonati. Schoeser, M. (2003). World Textiles: A Concise History. New York: Thames & Hudson World of Art. Semnani, D. and M. Sheikhzadeh (2007), Online control of knitted fabric quality: Loop length control. International Journal of Electrical Computer and Systems Engineering, 1 (4), 213–218. Semnani, D., K. Matin, M. Sheikhzadehand and M. Latifi (2008), Automation of stitch length cams in high speed flat knitting machine: Online control system and numerical modeling method. International Journal of Applied Engineering Research, 3 (6), 763–772. Shanahan, W. J. and Postle, R. (1973), Jamming of knitted structure. Textile Research Journal, 43, 532–538. Siewiorek, D. (1999). Wearable computing comes of age. Computer, 32 (5), 82–84. Spencer, D. (1989). Knitting Technology (2nd ed.). Cambridge: Woodhead Publishing. Vigo, T. L. and Bruno, J. S. (1988), Properties of knits containing cross-linked PEG. Book chapter/publication, International Conference & Exhibition, AATCC, 83–173.
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8 Knitted fabric composites M. DUHOVIC and D. BHATTACHARYYA, University of Auckland, New Zealand Abstract: This chapter discusses the role of knitted fabric structures as reinforcing elements in polymer composite materials. The types of fibres, yarns and knit structures used to produce knitted fabric composites are presented along with the techniques used to combine them with different types of polymer matrices. An insight into some of the commercially available textile composite precursor (prepreg) technologies is given together with the manufacturing methods, mechanical properties and applications associated with these materials. Key words: textile composite materials (TCMs), high performance fibres, prepregs, multiaxial warp knits (MWKs), sheet forming.
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Introduction
8.1.1 Brief introduction to polymer composites A composite can be defined as a combination of two or more dissimilar materials whose combined properties are superior to those of the individual constituents. Considering the synergistic effects that are achievable by some of today’s composite materials, it is not uncommon to hear expressions amongst composite circles such as 1 + 1 = 11. While expressions like these may disgruntle mathematicians, they remain one of the principal advantages pertaining to the use of composite materials today. It is no mystery that engineers have long used nature as a source of inspiration and examples of composite materials are to be found in mammals, plants as well as geological formations (Chou and Ko, 1989). Wood, a natural composite containing cellulose fibres and lignin (a natural polymer), is one of the most common materials used in the construction industry today. Even man-made composites have existed for many years with the use of clay plus grass to form bricks and other building materials as the classic example (Chou and Ko, 1989). However, it was not until the 1950s that a feasible synthetic composite, fibre reinforced plastic (FRP), emerged, which was able to replace wood and metal in many applications. As research progressed, manufacturing processes and materials were developed to meet the demands of high tech applications such as the aircraft/aerospace and defence industries. The opportunity was there to use FRPs for commercial applications but the cost of producing them was still too high. Continued progress in manufacturing techniques over the past ten years, including the evolution of 193 © Woodhead Publishing Limited, 2011
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more structured types of fibre reinforcements using textile technologies, has allowed FRPs to become a prime candidate for consumer products such as sports equipment, appliances, electronic and corrosion resistant equipment and most of all transportation (Astrom, 1997, p. 103; Ko et al., 1999). Overall there is a wide range of current applications for reinforced thermoplastic composite materials, in particular, that benefit from textile reinforcements.
8.1.2 The role of knitted fabrics in polymer composites Over the past decade, research with regards to the understanding of the forming characteristics of fibre-reinforced thermoplastic composite materials has steadily progressed. New polymer materials are constantly being developed to provide more favourable forming as well as final mechanical property characteristics. On the other hand, fibre reinforcements seem to have followed a similar trend ranging from the development of simple and random to highly structured configurations. This trend has led to research in the area of so-called textile composite materials. Textile composite materials (TCMs) constitute a form of fibre-reinforced polymer, where the reinforcing fibres are structured in such a way as to provide favourable forming characteristics while preserving the fibre continuity needed for final component strength. TCMs represent an important development as far as composite preform materials are concerned since they allow for much easier handling of raw materials. However, the greatest potential of these materials must lie in the fact that the technology to produce the complicated reinforcing structures economically has existed for a number of years in the apparel industry. Now the challenge lies in being able to combine this technology reliably and economically with composites processing and in being able to develop a scientific understanding of these materials that will allow reasonable predictions of both the forming behaviour and end product’s overall mechanical property characteristics. One of the current problems associated with FRPs seems to be the trade-off between final component strength and processability. FRPs for structural applications usually consist of long continuous inextensible reinforcing fibres in the loading directions. This is very favourable for the final component but poses significant problems during the forming stages in many manufacturing processes where virtually inextensible fibres restrict the movements required to form the part. TCMs partially relieve this restriction by allowing for fibre movement through certain modes of compliance present in the geometric structure of the textile reinforcement. Processability, stiffness, strength, interlaminar fracture toughness and material cost also play an integral role in determining the relative advantages and disadvantages of certain FRPs. Figure 8.1 presents a general comparison of these
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8.1 Overview and comparison of the main existing reinforcement types and their composite properties (Houtte et al., 1998).
properties for the main existing types of FRP materials available today (Houtte et al., 1998). Positioned roughly in the centre of Fig. 8.1, knitted fabrics generally possess stiffnesses and strengths that are lower than those of woven or braided fabrics but are higher compared to those of randomly oriented continuous or short fibre mats. Their geometry, which allows a considerable amount of intermingling, results in an interlaminar fracture toughness far superior to that of any two-dimensional weave or braid. As far as processability of complex shapes is concerned, this is where knitted fabrics show their biggest advantage, allowing large strains and shear angles without the occurrence of wrinkling. Although randomly oriented continuous and short fibre mats may seem to have an upper hand with regard to processability, their composites are often susceptible to tearing, resulting in weak spots and inhomogeneous fibre content, problems which do not arise readily in knitted fabrics. The degree of isotropy is a favourable characteristic, a material property only bettered by randomly oriented continuous and short fibre mats. Finally, given that knitted fabric composites are still in the developmental stage, raw material and subsequent composite production cost are an unknown factor for now. However, industrial knitting is a technologically advanced production technique that has gone through many years of refinement in the garment industry, making the initial cost image for knitted fabric composites very favourable. It is not suggested that knitted fabric composites provide the single solution for FRPs but they fit neatly into the broad family of FRP composite materials. Certainly for applications requiring moderate stiffness and strength and excellent processability, they will be the obvious choice.
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Types of fibre and yarn used in knitted fabric composites
8.2.1 High performance fibres To qualify for primary and secondary load bearing applications, knitted fabric composites must be made from high modulus and high strength fibres. Amongst the most common types of commercially available fibre used are glass, carbon, aramid and steel along with some selected natural fibres (see Table 8.1) (Miravete, 1999; Bhattacharyya and Fakirov, 2007, p. 34). With each type of fibre comes a different set of favourable and unfavourable properties. However, availability and cost are usually the determining factors for their use. At around US$2.00 per kg, and considering their relatively good performance compared to other materials, E-glass fibres remain one of the most extensively used reinforcement fibres in industry today. It is interesting to note that the cost of the commingled continuous E-glass and polypropylene yarn composite preform Twintex® is only slightly more expensive than glass fibre yarn on its own. Polymer fibres Although polymers are usually used as the matrix element of the composite, there are several high performance polymers that provide mechanical properties adequate for use as reinforcing elements. Aramids, liquid crystalline polymers and ultrahigh-molecular-weight polyethylene all have sufficient strengths and Table 8.1 High performance fibre materials for knitted fabric composites (Miravete, 1999; Bhattacharyya and Fakirov, 2007)
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Material Density Failure stress (MPa) (g/cm3)
Failure strain (%)
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E-Glass S-Glass Aramid (Kevlar 49) High StrengthCarbon High ModulusCarbon Steel Twintex® Vf 60%/75% Flax Sisal Jute
3.5–4.8 4.0–5.4 2.5–2.7 1.5–2.2 0.6–1.4 20 – 2.7–3.2 2.0–2.5 1.5–1.8
73 88 133 230–300 345–590 205 – 27.6 9.4–22.0 26.5
2.58 2.48 1.45 1.76–1.80 1.83–1.90 7.9 – 1.5 1.5 1.3
2400–3450 3100–4590 3500–3600 3300–6370 2600–4700 275Y/430UTS – 345–1035 511–635 393–773
2.00 11.00 24.00 16–24 100–200 27–35 2.93/3.04 3.50–7.20 10.70 1.00–3.00
Note: costs are approximate average values for fibres in yarn form collected from various sources on the internet as well as direct communications with suppliers. *For natural fibres cost is dependent on yarn size and refinement, e.g. for flax 20 lea = US$7.20/kg, 4 lea = US$3.50/kg.
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stiffnesses for use as reinforcement fibres. These high-performance organic fibres are used extensively in military applications ranging from body armour, helmets, and safety shoe toe caps to protective armour in ground vehicles (Bond-Laminates, 2007). One example of an organic polymer fibre with extremely high mechanical properties is produced by Akzo Nobel in the Netherlands (Northbolt et al., 2002). The M5® fibre is the registered trade name for poly{2,6-diimidazo[4,5-b:4´,5´-E] pyridinylene-1,4-(2,5-dihydroxy)phenylene} or PIPD which has been engineered over a 10-year period. The team of scientists led by Doetze Sikkema have achieved modulus values greater than 300 GPa and tensile strengths greater than 5 GPa (Northbolt et al., 2002). Its high damage-tolerance makes it suitable for knitting and composites made of M5® are reported to show good ductility and impact strength (Northbolt et al., 2002). Micro- and nano-fibrillar fibres Using another technique, even low strength polymers can be processed together in such a way as to produce high strength yarns for use in composite materials. Micro- and nano-fibrillar reinforced composites are prepared by extruding and drawing continuous filaments of immiscible polymer blends of different melting temperatures. By processing the material at the melt temperature of the lower melting point polymer a highly desirable method of in-situ fibre formation is created, reversing the effects of immiscibility and producing blends with improved mechanical performance, in some cases on their own comparable to glass fibre reinforced plastics (Van Gheluwe et al., 1988). Commonly used materials are combinations of polyethylene (PE) and polyethylene terephalate (PET) or polypropylene (PP) and PET amongst several other combinations, the properties of which have been extensively studied by Fakirov et al. (2008) and many others (Shields et al., 2008; Yang et al., 2003; Sarkissova et al., 2004). The continuous filaments can be assembled into yarn and used to produce knitted fabric composite preform materials. Natural fibres Due to the lower and less consistent mechanical properties of natural fibres, not many qualify as worthy candidates for use as reinforcing materials. Among the strongest are ramie, jute, sisal and flax. Certain varieties of flax have been reported in the literature as having ultimate tensile strengths of up to 1500MPa; however, even this range varies from 345–1035MPa to 800–1500MPa (Bhattacharyya and Fakirov, 2007, p. 336). Large variations also appear with regard to the Young’s Modulus, 27.6 GPa and 60–80 GPa (Bhattacharyya and Fakirov, 2007, p. 621). Nevertheless, by offering the best mechanical properties compared to other natural fibres, they can produce composites with mechanical characteristics comparable with that of E-glass fibres (Duhovic and Bhattacharyya, 2007). If the specific
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properties and eco-friendliness of these fibres are taken into account, they become very attractive alternatives.
8.2.2 Yarn types Most high performance fibres are produced by extrusion and drawing, resulting in continuous filaments and subsequently continuous filament yarns. This is more favourable in composite applications since they produce materials with higher stiffness and strength in the desired applications compared to short fibre yarn assemblies. In the textile industry, hybrid yarns are used to incorporate different properties into garments and to minimise the cost. For composite applications, hybrid yarns are also used to optimise costs, mechanical properties and in some cases, even the processability. In one type of thermoplastic composite preform, commingled hybrid yarns are used to promote easier fibre wetting and improve consolidation quality.
8.3
Composite preforms
Perhaps the biggest factor contributing to the increasing use of FRPs has been the development of their composite preforms. This is true for thermoplastic composites in particular, where advantages such as safe processing environment and the potential for high volume production have been offset by relatively difficult-toachieve impregnation quality. Composite preforms eliminate some of the processing difficulties with regard to fibre impregnation quality and allow for quicker and more manageable processing at the end user level. For thermoplastic composites with their high melt viscosities, in the range of 100–10 000 Nsm2, compared to 0.1 to 10 Nsm2 for thermosetting composites, ensuring good impregnation quality becomes an even greater challenge.
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8.3.1 Textile preforms for composites The specific assemblage or arrangement of continuous (or discontinuous) fibrous materials into a form, which becomes the reinforcement for a composite, is known as the textile composite preform (Chou and Ko, 1989). Of course, there literally exists an infinite number of configurations for these preforms ranging from simple to complex 3D geometric fibre orientations. The architecture used is in many cases tailored best to fit the application, which explains why many of these materials are not produced commercially. For knitted structures, it is a common practice to obtain the preform material in a more unrefined form such as continuous filament yarn or roving, and to produce the fabric using industrial knitting machines obtained from specialist textile manufacturers. The development of textile preforms, knitted structures in particular, have attracted much interest, mainly for two reasons: i) their ability to preserve fibre
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continuity for strength while allowing for increased forming flexibility and ii) the fact that processing techniques known for many years from the textile industry can be readily applied in high volume production and at low cost. Preforms such as bidirectional woven mats provide a low degree of forming flexibility but high final strength. Stapled or chopped fibres provide a forming flexibility close to that of the thermoplastic alone; however, the final strength of the component is compromised. Knitted fabric preforms attempt to bridge the gap between forming flexibility and strength while offering a greater in-plane shear resistance than either of the aforementioned (Chou and Ko, 1989). One main disadvantage associated with textile preforms is that they commonly rely on impregnation subsequent to shaping and depend on the end user rather than the specialist manufacturer to ensure impregnation quality.
8.3.2 Textile composite prepregs Composite ‘prepregs’ (pre-impregnated composite fibres) reduce the risk of poor impregnation quality by ensuring that the correct amount of each constituent material is already present and interacting. The development of thermoplastic composite prepregs began in the late 1980s with materials consisting of unidirectional or woven high-strength fibres neatly impregnated into thermoplastic matrices. In the 1990s, commingled technologies using E-glass polypropylene/ polyethylene terephthalate became available in the form of rovings and fabrics. Twintex® roving manufactured by Saint-Gobain Vetrotex (Chambéry, France) (bought by Owens Corning as of 1 February 2007) is one example of a commonly available thermoplastic composite preform. It is a roving consisting of commingled continuous thermoplastic and glass fibre yarns, which are suitable for knitting. Other techniques used to combine thermoplastic matrix and reinforcement include film stacking (Bond-Laminates’ Tepex® and TenCate’s Cetex®), and powder coating reinforcement yarns (Hexcel Composites’ Towflex®), see Fig. 8.2. However, for these materials, woven rather than knitted reinforcement structures are more common. Figure 8.3 shows a selection of currently available thermoplastic composite preform and prepreg materials, four of which use textile composite technology.
8.2 Methods for combining thermoplastic matrices with reinforcement fibres.
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8.3 A range of currently available thermoplastic composite preform/ prepreg materials.
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Twintex® can be classified as a textile composite preform, since no bonding between the constituent materials has taken place. Towflex® is a textile composite prepreg since the powder coating has partially established the bond between the fibre and the matrix. Plytron® can also be classified as a composite prepreg, but it is not considered a textile. Tepex® and Cetex® are woven fabric composite prepreg sheet materials produced by using film stacking and double-belt-press methods. Although the focus in this section has been on thermoplastic impregnation techniques, moulding using thermosetting matrices is far more common and established in industry. In processes such as resin transfer moulding (RTM), structural reaction injection moulding (SRIM) and resin film infusion (RFI), the injected low viscosity thermosetting resin flows into and around the textile structure made from the high performance fibres. The material is then cured at room temperature (or elevated temperatures) for several hours to set the resin and form the composite material. Partially cured mouldable thermoset prepreg materials are also common and are available in woven and yarn form, but are not suitable for knitting due to their tackiness.
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Knit structures for fabric composites
Plain warp- and weft-knitted structures are not commonly used for composite applications due to their inherent anisotropy in the wale and course directions. This causes the fabric preform to roll up on itself making handling and manufacturing more difficult. This problem is solved by using weft-knit structures such as the 1 × 1 rib and milano rib, which exhibit balanced properties because of their through-thickness symmetry. However, the highly curved fibre architecture, or crimp, present in these and any knitted structure, means that composites produced using these structures exhibit relatively poor mechanical performance. Characteristics of high conformability and low strength make them ideally suited to producing semi-structural complexly shaped components. To help increase mechanical performance, insert yarns can be placed between the planes of loops in either the warp or weft direction. The technique can be used for both warp- and weft-knitted fabrics which allow the insert yarns to remain perfectly straight, giving a greater yarn to fabric translational strength. This results in an increase in the composite stiffness and strength along the insert direction. Warp- and weft-knitted fabrics with inlay yarns are termed unidirectional knitted fabrics and the incorporation of insert yarns in two directions creates biaxial knitted fabrics.
8.4.1 Multiaxial warp knits Multiaxial Warp Knit (MWK) fabric is a further development of this idea by utilising layers of insertion yarns for the in-plane reinforcement and warp stitch yarns for the through-thickness reinforcement. They consist of one or more parallel layers of yarns held together by a warp knit loop system. Theoretically, as many layers as preferred can be used but typical commercially available machines only allow four layers (Du and Ko, 1996). The purpose of the knit loops is to hold the layers of unidirectional yarns together, but it has also been proven to be the key to increasing the damage tolerance of the material (Zhou et al., 2005). These types of knitted structure are termed non-crimp structures and can be produced in a single knitting process (Du and Ko, 1996). They are particularly suitable for thin to medium thickness parts. The combination of the warp-knitted structure and non-crimp yarns means they have the ability to conform to complex shapes as well as the potential to meet the demands of primary load bearing applications. MWKs have evolved through structural modifications of warp-knitted fabrics and are predominantly fabrics with inlay yarns in the warp (90°), wale (0°) and bias (± θ°) directions. Warp, weft and bias yarns are held together by a chain or tricot stitch through the thickness of the fabric (Du and Ko, 1996). Layers of 0° need to be placed somewhere other than the top or bottom layer to ensure structural integrity. The amount of fibre and the orientation of the inlay yarns can be
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controlled, which is advantageous for preform engineering. As a result, the insert yarns are made from a much higher linear density yarn than the stitch yarns, since they form the load-bearing component of the fabric structure (Du and Ko, 1996). Figure 8.4 shows the configuration of the chain and tricot MWK structures. An improved MWK structure for composite reinforcement, developed by replacing the tricot stitch with a ‘double loop pillar stitch’, has been carried out by Zhou et al. (2005) The modification has yielded improvements of up to 7% in breaking strength (Zhou et al., 2005). The tribology or wear resistance of MWK has also been investigated and has been shown to perform better than biaxial nonwoven structures (Mathew et al., 2007). With respect to composite mechanical properties, MWKs provide the most practical solution for the use of knitted structures in composite materials.
8.5
Types of matrix materials
Matrix materials for knitted fabric composites follow the same criteria that apply to composite materials in general. Important characteristics are fibre-to-matrix compatibility in terms of bonding, mechanical properties, thermal properties, cost and most importantly, processability. The two broad categories of polymers used are thermosets and thermoplastics.
8.5.1 Thermoplastic materials Thermoplastic materials are popular due to their user-friendly safe processing, and they can be melted, shaped and cooled into a dimensionally stable part in a matter of seconds. Processing temperatures start at around 120 °C for polyethylene but can be as high as 345 °C for high-performance engineering thermoplastics such as poly(ether ether ketone) (PEEK). Due to the high viscosity of thermoplastics,
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8.4 Chain and tricot stitched multilayered MWK structures.
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more emphasis is placed on textile processing techniques to prepare the precursor materials for composite component manufacturing. Techniques such as commingling and powder coating, discussed in Section 8.3.2, account for high viscosity by minimising flow paths between the reinforcing fibres.
8.5.2 Thermoset materials The nature of thermoset materials involves chemical reactions with toxic substances that form cross-links upon curing. Thermoset polymers dominate in structural applications because of their high stiffness, strength and shape stability. Due to their low viscosity (which can be up to 100 000 times lower than those of some thermoplastics) they have little trouble flowing in between the network of fibre loops created by the knitting process. However, a major shortcoming of these materials is their limited recyclability and non-biodegradability.
8.5.3 Biodegradable materials Over the past ten to fifteen years a large number of biobased polymers have been developed and are now beginning to emerge as commercially viable replacements for commonly used consumer product polymers. However, to date, biopolymers have struggled to be price competitive with commodity polymers such as PE, PP and polystyrene. Relatively low yield rates and demand have kept manufacturing rates low and hence prices high. This has limited the use of biopolymers to niche products, such as bioresorbable implants and sutures in medical applications. However, increasing environmental awareness, improved yields and manufacturing processes, as well as new legislations have seen an increasing focus on the use of biopolymers for applications such as polymer bags, food packaging, nappies and cosmetic containers. Recently Sainsbury’s, a leading UK supermarket, announced that it would use biodegradable packaging for its house-brand foods and carrier bags. Cargill-Dow LLC, the company that produces the NatureWorks® range of PLA (Poly (lactic acid)) offers its high quality PLA product in various grades suitable for injection moulding, extrusion/thermoforming, film and blow moulding. In addition to price, the degree of biodegradability in polymers often has a detrimental effect on performance characteristics such as strength, flexibility and brittleness. Therefore, both price and performance have been key drivers in the move towards biopolymer-composites. Poly-β-hydroxyalkanoates (PHAs), PLAs and starch are commercially prevalent natural polymers that have been the focus of research into biopolymer composites. Table 8.2 presents some of the current commercially available PHAs, PLAs and starches (including blends) along with other synthetically produced biodegradable polymers and their manufacturers (Haugaard et al., 2001; Mohanty et al., 2000; Plackett and Vazquez, 2004; Johnson et al., 2003).
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Table 8.2 Biodegradable polymer materials currently available in the market (Haugaard et al., 2000; Mohanty et al., 2000; Plackett and Vazquez, 2004; Johnson et al., 2003) Polymer type
Manufacturer
Product name
Polyhydroxyalkanoates Metabolix, Inc. Biopol™, PHA Procter and Gamble Nodax™ Biomer Biomer™ Biomatera Inc. PHB Industrial S/A Cellulose acetate Courtaulds Mazzucchelli Bioceta Polylactides Cargill-Dow LLC NatureWorks™ Galactic SA Galactic Birmingham Polymers, Inc. Lactel, Absorbable Boehringer Ingelheim Resomer® Mitsubishi Plastics, Inc Ecoloju Mitsui LACEA PURAC Purasorb® Shimadzu Corporation Lacty Hycail Biomer Starches and starch blends VTT Chemical Technology Cohpol™ BIOTECH Gmbh Bioplast®, Bioflex®, Biopur® Novamont Mater-Bi Avebe Paragon Earth Shell Starch-based composite Bioplastic (Michigan) Envar™ Hayashibara Pullulan National Starch Eco-Foam® Groen Granulaat Ecoplast Rodenburg Biopolymers Solanyl Starch Tech RenEW, ST1, ST2, ST3 Supol Supol Vegemat Vegemat® Biop Biopar Biochemical Labs Other biodegradable polymers based wholly on synthetics Copolyester BASF Ecoflex Eastmen Chemical Eastar Bio Union Carbide Tone polymer Solvay CAPA Showa Bionolle Polycaprolactone Highpolymer Bayer BAK Polybutylene succinate Bayer MHP 9029 Bayer Degranil VPSP42002 Polyesteramide Fortum Polyesterurethane Dupont Biomax Polyester co-polymer Polylactic acid Polyester
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For biopolymer materials such as PLA, applications in apparel, bedding, furniture, carpet and personal care products are already on the market, which shows that there are no limitations to textile processing. The next logical step is the development of these materials for composite applications.
8.6
Developments in manufacturing methods for knitted fabric composites
8.6.1 Manufacturing methods Sheet forming Composite component forming from solid composite sheets is perhaps the most commonly used method of production for thermoplastic composite parts (Bhattacharyya, 1997). It is also possible to use thermoset composite materials in the case of short fibre reinforcement, or uncured or partially cured materials such as those prepared in sheet moulding compound (SMC). Common sheet forming techniques are variations on a theme involving matching rigid moulds, a single mould/rubber punch or single mould/flexible membrane plus pressure (air or hydraulic) and vacuum pressure. In the case of knitted fabric composites, not all of these forming techniques are suitable due to the material’s geometrical characteristics. Out-of-plane undulations resulting from knit-loop formation give rise to high compaction ratios (10:1) in order to achieve an adequately consolidated composite component of reasonable thickness. If layers of the material are stacked in the same orientation, then nesting helps to alleviate this problem; however, for complete knit geometries (unlike MWKs discussed in Section 4.1), it is difficult to achieve fibre volume fractions greater than 60%. If the material is prepared as a sheet or plate prior to component forming then it tends to reinflate upon reheating due to the bending stresses present in the stiff reinforcing fibres. This can also pose a problem with regard to surface finish unless rigid tooling (mould) is used. Stretch forming To help avoid wrinkling during the forming process, knitted fabric preform sheets can be stretch-formed. By clamping the perimeter of the sheets, the material is restricted from flange draw-in and stretched into the shape of the mould. The process makes excellent use of the strain transmission properties of knit geometries, and highly stretched areas inherently result in aligned fibres giving higher stiffness and strength. Draping Perhaps the most suitable forming method for knitted composite materials is the draping of flat sheets over a male or female mould. Although this is quite a
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labour-intensive process, wrinkling can be avoided manually and the shape of the component is determined before closing the mould. Integral knitting Integral knitting is where the near-net shape of the final composite component is manufactured on the knitting machine and subsequently used as the preform for the component to be produced. The added advantages are low material wastage and labour costs (Savci and Curiskis, 1997). With present day knitting machinery such as Shima-Seiki’s ‘wholegarment’ flat bed knitting machines, it is possible to produce gaugeless (loops of any size) and seamless three-dimensional preforms directly by specifying the geometry using KnitCAD data. The manufacturing process is illustrated in Fig. 8.5; the only post forming manufacturing procedure that may be required is part trimming – however, this can also be incorporated into the tooling.
8.6.2 Influences of forming method Knitted fabric composite components are best produced by the draping or stretching of a flat 2D preform, or the draping of a 3D shape as is shown in Fig. 8.5. One of the questions that has arisen during the study of knitted composites is: does stretching actually improve the mechanical properties? In the work performed by Putnoki et al. (1999), composite specimens made from weft knitted
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8.5 Forming of 2D and 3D commingled knitted fabric thermoplastic composite preforms.
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1 × 1 rib glass fibre and poly(ethylene terephthalate) Twintex® commingled yarn, were stretched in the wale direction to various degrees before consolidation. Their results showed that the mechanical properties in the stretching direction were enhanced, while in the transverse direction the properties were reduced. Therefore, the work highlighted that the material would exhibit an increase in anisotropy due to the stretching and contracting of the wale and course directions. Another interesting observation was that the transverse impact toughness remained constant since an increase in strength was accompanied by a reduced ability for deformation. Overall, it was shown that rather than improving the mechanical properties of the material, stretching during forming provides a means of redirecting strength and stiffness in the directions that may be required by the component during manufacture. Since their inception, research on commingled thermoplastic composites has become popular with researchers concentrating on optimising forming parameters such as forming rate, temperature, pressure and consolidation time to minimise void content, cycle time and to maximise quality (Long et al., 2001; Bernet et al., 2001). With commingled yarn, there are again two options for forming the composite: i) the compaction of the commingled fabric to the required thickness and subsequent heating/consolidation within the mould or ii) external heating and transfer into a cold tool for forming and consolidation. Of course, the second option is more practical these days as quick in-mould heating techniques are currently unavailable. However, technologies such as radio frequency heating such as that used in the plywood industry could change that. In the work by Long et al. (2001) it has been found that heating commingled preform with no application of pressure can result in migration of the constituents, increasing the potential for void formation. The causes of the migration were described as being induced by the shrinkage of polypropylene upon heating, which tends to coalesce within and between the yarns. It is stated that the problem of migration can also be caused by differences in the constituent fibre sizes. In the case of fabrics, this effect coupled with the yarn crossover forces bring about the migration of the smaller diameter fibres towards the contact point. Therefore, an ideal process might be one where the yarn contains equally sized fibres and migration during consolidation is controlled by precompaction.
8.7
Mechanical properties
Of all the textile geometries, knitted fabrics are the most difficult to study due to their complex geometric structures. The shape of knitted structures is determined by their geometric parameters along with the elastic forces (determined by the material properties of the fibres) attempting to unravel the yarn back to its original unstressed state. In other words, the shape of a knitted loop is not based purely on geometric parameters but is a force-determined geometry (Hearle, 1969). For knitted fabric composites, the mechanical properties of the composite are
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influenced by the ‘frozen state’ geometric structure of the reinforcement. During the forming stages, the deformation behaviour of the fabric changes the knit geometry in order to allow shaping of the part. Once the composite material has cooled or cured, the geometry of the reinforcement is locked in place. Therefore, in order to obtain the true geometry of the knitted fabric composite reinforcement, consideration of the deformations that take place during fabric formation is necessary before considering the deformation during composite forming. With these ideas in mind, Duhovic and Bhattacharyya created a numerical simulation capable of generating tensile specimens of any type of weft-knitted structure (Duhovic and Bhattacharyya, 2006). The model could also be modified to cover warp-knitted structures. The simulation manufactured and predicted the mechanics of a 1 × 1 rib fabric specimen by numerically knitting a five-needle specimen, then subjecting it to a tensile test. Once the validity of the model was demonstrated, it was used to investigate the deformation mechanisms of the particular knit structure in terms of energy contributions. The model has implications for the study of the many aspects of knitted fabric composites including fabric permeability and forming property prediction. It can also be used to study the mechanics of the many types of knitted fabrics quantatively. A selection of images from the work is shown in Figure 8.6.
8.8
Applications
8.8.1 Automotive, aerospace
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Potential automotive applications for knitted fabric composites include bumper bars, floor panels and door members (Bannister, 2004). In fact, Van Vuure and Ko (2003) describe the extensive use of knitted fabric composites (MWKs and nearnet shaped knits) for an all-composite electric vehicle in development by Solectria Inc. In this project MWKs are used in the top shell and floor pan of the vehicle, while the wheel wells make use of near net shaped knitted preforms. Other examples in the automotive industry include body panels and interior components for train structures (Miravete, 1999). In the aerospace sector, net-shaped knitted composites have been used to manufacture components such as rudder-tip fairings (structures that reduce drag) for passenger aircraft, helicopter door-track pockets and midsized aircraft radomes (Chou and Ko, 1989).
8.8.2 Consumer items, sports equipment Helmets for a multitude of applications, boot toecap protectors, shin pads and other forms of body armour, and medical prostheses are all examples of consumer level applications for knitted fabric composites. Non-rigid applications must also not be forgotten: these include gloves and other types of protective garments.
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8.6 Simulating the behaviour of knitted fabric composites (Duhovic and Bhattacharyya, 2006).
8.9
Conclusion
Knitted fabric composites are one of the many types of textile structural composite used in the composites industry. They fit neatly into the broad family of FRP structures where medium strength, stiffness and excellent processability are required. The high performance fibres used for these materials range from E-glass and carbon aramid to high strength polymer fibres, and even some selected high strength natural fibres. These fibres are knitted into the preform fabric for the composite, and a variety of thermoset, thermoplastic and even biodegradable polymer materials can be used as the matrix. Preimpregnated materials are also available, although knitted
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structures are not as common as weaves. The most commonly used knit structure is the multiaxial warp knit because of its ability to meet the demands of primary load bearing applications. Forming methods for knitted fabric composites include sheet forming and draping. In the case of sheet forming, stretching can be applied to obtain desired mechanical properties, while for draping, integral shapes can be produced to minimise material wastage and eliminate cutting. Due to their complex geometric structures the mechanical properties of knitted fabric composites are not easily predicted. However, simulations to generate and examine the behaviour of these structures have been carried out. Knitted fabric composites have found applications in automotive components as well as consumer level items such as helmets, toecap protectors and medical prostheses.
8.10 Acknowledgements The authors would like to thank the Foundation for Research Science and Technology (FRST) New Zealand, for their financial support.
8.11 References
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Astrom, B. T. (1997), Manufacturing of Polymer Composites, London, UK, Chapman & Hall. Bannister, M. K. (2004), Development and application of advanced textile composites, Proc. Instn Mech. Engrs: J. Materials: Design and Applications, 218(L), 253–260. doi:10.1243/1464420041579535. Bernet, N., Michaud, V., Bourban, P.-E. and Manson J.-A. E. (2001), Commingled yarn composites for rapid processing of complex shapes, Composites Part A: Applied Science and Manufacturing, 32(11), 1613–1626. doi:10.1016/S1359-835X(00)00180-9. Bhattacharyya, D. (ed.) (1997), Composite Sheet Forming. Composite Materials Series, Vol. 11, Amsterdam, The Netherlands, Elsevier. Bhattacharyya, D. and Fakirov, S. (2007), Handbook of Engineering Biopolymers, Blends and Composites, Munich, Germany, Hanser Verlag. Bond-Laminates GmbH, http://www.bond-laminates.de/en/, (accessed 13.12.07). Chou, T.-W. and Ko, F. K. (eds) (1989), Textile Structural Composites, Composite Materials Series, Vol. 3, New York, Elsevier. Du, G. W. and Ko, F. K. (1996), Analysis of multiaxial warp-knit preforms for composite reinforcement, Composites Science and Technology, 56 (3), 253–260. doi:10.1016/ 0266-3538(95)00108-5. Duhovic, M. and Bhattacharyya, D. (2006), Simulating the deformation mechanisms of knitted fabric composites, Composites – Part A: Applied Science and Manufacturing, 37 (11), 1897–1915. doi:10.1016/j.compositesa.2005.12.029. Duhovic, M. and Bhattacharyya, D. (2007), Commingled flax-polypropylene flax-polylactic acid knitted fabric composites and their performance evaluation, ASC2007, 22nd Annual Technical Conference, September 17–19, UW Campus in Seattle, WA. Fakirov, S., Bhattacharyya, D. and Shields, R. J. (2008), Nanofibril reinforced composites from polymer blends, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 313–314, 2–8. doi:10.1016/j.colsurfa.2007.05.038.
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Haugaard, V. K., Udsen, A. M., Mortensen, G., Hoegh, L., Petersen, K. and Monahan, F. (2001), Food biopackaging. In Weber, C. J., ed. Biobased Packaging Materials for the Food Industry, Status and Perspectives, KVL, Department of Dairy and Food Science, Frederiksberg, 2001, pp. 45–84. Hearle, J. W. S. (1969), Structural Mechanics of Fibers, Yarns and Fabrics, Vol. 1, USA, John Wiley & Son. Houtte, P. V., Verpoest, I. and Gommers, B. (1998), Analysis of knitted fabric reinforced composites: part I. Fibre orientation distribution, Composites – Part A: Applied Science and Manufacturing, 29, 1579–1588. doi:10.1016/S1359-835X(98)00095-5. Johnson, R. M., Mwaikambo, L. Y. and Tucker, N. (2003), Biopolymers, in Rapra Review Report, 158. Ko, F. K., Van Vuure, A. W. and Balonis, R. J. (1999), Textile preforming for complex shape structural composites, 44th International SAMPE Symposium and Exhibition, Long Beach, CA. Long, A. C., Wilks, C. E. and Rudd, C. D. (2001), Experimental characterisation of the consolidation of a commingled glass/polypropylene composite, Composites Science and Technology, 61 (11), 1591–1603. doi:10.1016/S0266-3538(01)00059-8. Mathew, M. T., Padaki, N. V., Rocha, L. A., Gomes, J. R., Alagirusamy, R., Deopura, B. L. and Fangueiro, R. (2007), Tribological properties of the directionally oriented warp knit GFRP composites, Wear 263 (7–12 SPEC. ISS.), 930–938. doi:10.1016/j. wear.2006.12.001. Miravete, A. (ed.) (1999), 3-D Textile Reinforcements in Composite Materials, Cambridge, UK, Woodhead Publishing Limited. Mohanty, A. K., Misra, M. and Hinrichsen, G. (2000), Biofibres, biodegradable polymers and biocomposites: An overview, Macromolecular Materials and Engineering, 276/277, 1–24. doi:10.1002/(SICI)1439-2054(20000301)276:1<1::AID-MAME1>3.0.CO;2-W. Northbolt, M. G., Sikkema, D. J., Zegers, H. C. and Klop, E. A. (2002), PIPD, A new highmodulus and high-strength polymer fibre with exceptional fire protection, Fire and Materials, 26, 169–172. doi:10.1002/fam.793. Plackett, D. and Vazquez, A. (2004), Natural polymer sources, in Green Composites, Polymer Composites and the Environment, Baillie C A (ed), Cambridge, England, Woodhead Publishing. Putnoki, I., Moos, E. and Karger-Kocsis, J. (1999), Mechanical performance of stretched knitted glass fibre reinforced poly(ethylene terephthalate) composites produced from commingled yarn, Plastics, Rubber and Composites, 28 (1), 40–46. doi:10.1179/146580199322913313. Sarkissova, M., Harrats, C., Groeninckx, G. and Thomas, S. (2004), Design and characterisation of microfibrillar reinforced composite materials based on PET/PA12 blends, Composites Part A: Applied Science and Manufacturing, 35 (4), 489–499. doi:10.1016/j.compositesa.2003.09.025. Savci, S. and Curiskis, J. I. (1997), Weft-knitted glass-fibre preforms for composite materials, Eleventh International Conference on Composite Materials, Gold Coast, Australia, 338–347. Shields, R. J., Bhattacharyya, D. and Fakirov, S. (2008), Fibrillar polymer-polymer composites: morphology, properties and applications, Journal of Materials Science, 43 (20), 6758–6770. doi: 10.1007/s10853-008-2693-z. Van Gheluwe, P., Favis, B. D. and Chalifoux, J.-P. (1988), Morphological and mechanical properties of extruded polypropylene/nylon-6 blends, Journal of Materials Science, 23(11), 3,910–3,920. doi:10.1007/BF01106813.
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Van Vuure, A. W., Ko, F. K. and Beevers, C. (2003), Net shape knitting for complex composite preforms, Textile Research Journal, 73 (1), 1–10. doi:10.1177/004051750307300101. Yang, M.-B., Rui, H., Wei, Y. and Li, Z.-M. (2003), Morphology and properties of poly(ethylene terephthalate)/polyethylene in-situ microfiber-contained blend, Cailiao Yanjiu Xuebao/Chinese Journal of Materials Research, 17 (6), 621–629. Zhou, R., Hu, H., Chen, N., and Feng, X. (2005), An Improved MWK Structure for Composite Reinforcement, Textile Research Journal, 75 (4), 342–345. doi:10.1177/0040517505054731.
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9 Quality control in the knitting process and common knitting faults K. F. AU, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong, China Abstract: This chapter outlines the major parameters for the quality control of knitted fabrics. The important variables include loop length and stitch density. Discussion is focused on the high-speed production circular knitting to control knitting process variables such as input tension, take-down tension, and raw material management. An online monitoring approach is the latest system developed to prevent the formation of defects during knitting. The chapter concludes with a description of common knitting faults in circular knitting. Key words: quality control, knitting parameters, process variables, circular knitting, knitting defects.
9.1
Importance of quality knitted fabric
Knitted fabric is gaining popularity in the textile and clothing industry. The demand for knitted fabrics is increasing: consumers today are looking for comfort, fashion and style, which results in ever-changing demands on the apparel market. The advantage of knitted fabrics is that they are able to meet consumer demand for such properties as a softer feel, good draping quality and wrinkle recovery. Knitted fabric is therefore an ideal material for manufacturing sportswear, intimate garments and casual wear as it allows for stretch and free body movement. Owing to the ever-increasing demand for quality products, a high standard is important for the knitting industry. It is a common practice nowadays for customers to demand good quality products at a competitive price. Therefore, knitting manufacturers have to maintain their place in the competitive textile and clothing market by improving quality and maximizing productivity. The high production speed in circular machine knitting may generate fabric faults or defects that are regarded as a trade-off against fabric quality. Fabric defects are an undesirable aspect of the knitting process and may seriously affect the overall fabric quality. For this purpose, specific quality control measures should be taken to ensure high standards for knitted fabrics, and a timely fault detection system becomes more important than ever. In general, a properly functioning system of quality control makes a major contribution to the following two aspects of industry: • Saving time and money. When a defect occurs, the knitting machine has to be stopped to correct the fault, resulting in lost time, which is uneconomical in 213 © Woodhead Publishing Limited, 2011
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the overall production process. In addition, it may increase the subsequent remedial costs of the fabric finishing and garment manufacturing process as a result of defective fabric. • Reducing customer discontent. The knitted fabric may be rejected by customers if quality requirements are not met, which will have an adverse effect on the company’s reputation. In past decades, knitted fabric quality was maintained and achieved by manual inspection. Once a significant amount of knitted fabric had been produced, the fabric roll was removed from the knitting machine and then sent to an inspection frame. However, when using this method of inspection, faults occurring during the production process were often discovered too late, and most of them were irreversible. Another option, a final quality check of the finished knitted fabric, was and is not economically viable as whole fabric lots are rejected once excessive defects are observed. As a matter of fact, an effective monitoring of the knitting process is required. Its function is to avoid or detect the fabric faults as well as to locate the defect and its causes as soon as possible in attempts to cut down the undesirable return of goods and avoiding productivity and quality losses. The optimal solution is an online monitoring system to prevent the occurrence of defects in production or to change process parameters automatically when a defect is spotted, consequently improving quality and minimizing the production costs (Catarino et al., 2004b; Lek-Uthai, 1999). This chapter outlines and discusses the properties and characteristics of circular knitted fabrics that are subject to quality control procedures, as well as the online monitoring quality control system. Lastly, the most common defects in knitted fabrics are highlighted, with a discussion of their causes and the precautions that can be taken against them.
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Knitted fabric quality
Before discussing quality control in the knitting process, it is necessary to clearly define the term ‘fabric quality’. The general trade interpretation of this term is the ‘evenness’ in the following four properties: • Weight per unit area. The mass per unit area of fabric is measured to determine the consistency of the fabric weight of the sampled knitted fabric. The weight deviations of circular knitted fabric should not exceed ± 5% from the stated weight. • Courses per centimetre and wale spacing. Courses and wales per centimetre are measured by placing a centimetre glass on the fabric, and counting the number of courses and wales contained within the area. • Handles. The feel of the knitted fabric to the hand, including the softness or stiffness of the fabric.
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• Elasticity. Each knitted fabric has its own unique elastic property, which is measured by bursting strength testing methods. These four properties are, to a certain extent, interrelated. Knitted fabrics are considered to have identical qualities only when the characteristics listed are the same or vary slightly within an acceptable level of accord. In general, knitted fabric quality is usually defined as a number of loops in a square of prescribed dimensions (known as loop density) and represented by the number of loops per square inch or loops per square centimetre. Loop density is the most important element in defining knitted fabric quality and is directly related to fabric appearance, weight per unit area, dimensional stability, fabric weight, tightness factor, drape and many other factors.
9.3
Quality control in the knitting process
The varied and diverse causes of fabric defects can generally be summarized in one word – inconsistency. Two main causes of inconsistency that lead to fabric defects are raw material management before the knitting process and variation in parameters during the knitting process. Therefore, the best way to improve the quality of knitted fabric is to monitor the knitting parameters and the knitting conditions.
9.3.1 Quality control before the knitting process: raw material management Quality yarn is a prerequisite for faultless knitwear production. To ensure the raw materials are of good quality and in good condition, the following points must be checked before the knitting process: • Yarn appearance. The appearance of the yarn directly affects the appearance of the fabric after the knitting process. Therefore only the better quality yarns should be used to knit quality fabric. Several factors influence the appearance of the yarn, including cleanliness, fluffy texture and colour. A yarn can be labelled as good in appearance when it is free from impurities, contains a reasonable amount of projecting strands and has the minimum level of spinning defects such as short or long yarn slubs. • Yarn count. Yarn count or linear density is used to express the mass per unit length or length per unit mass of a yarn. It has a direct influence on the weight and dimensional stability of the knitted fabric. The selection of yarn with a proper yarn count is essential in determining the knitted fabric quality, since only the correct yarn count gives optimal knitting performance for a specific machine gauge and structure. The relationship between yarn count and machine gauge will be discussed in Section 9.5. • Yarn evenness. This refers to the yarn irregularity and non-conformity, which directly affects the knitted fabric quality and the knitting performance. Yarn
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evenness is expressed by mean deviations (U %) or commonly by the coefficients of variation (CV %). • Yarn elasticity. An elastic yarn is easier to knit and results in fewer knitting faults such as drop stitches, holes and bad selvedges. In a general case, wool yarns perform better than cotton yarns in knitting due to the higher elastic property of wool. • Yarn twist. The direction of yarn twist plays a decisive role in knitted fabric quality. The yarn twist should be in the same direction, either S or Z in knitting the same fabric roll. In addition, the amount of twist has a significant influence on yarn torque. Excessive or improper yarn twist causes distortion of the finished knitted fabric, i.e. skewed fabric. • Yarn friction. The coefficient of yarn friction should be set as low as possible in the knitting process. The higher the yarn friction, the higher the knitting tension will be. When the knitting tension is greater than the yarn strength, the yarn will break or cause a fabric fault. In practice, the yarn friction can be reduced by adding lubricants. A good waxed yarn can reduce the coefficient of friction by nearly 50%. All the above raw material controls are important in quality control carried out before the knitting process. Periodic checks and controls of raw materials are essential to ensure the consistency of the input yarns during the rest of the knitting process. The characteristics of some yarns are linked to the type of fibre used in their production. For instance, fibre diameter in wool, the presence of seed contamination in cotton yarn, residual bulking in acrylic yarn, and crimp rigidity in textured polyester and polyamide yarns, etc., all play an influential role in determining the fabric quality before the knitting process.
9.3.2 Quality control during the knitting process
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In order to produce faultless knitted fabric, precision in the settings of the knitting machine is essential during the whole knitting process. The optimal setting of a knitting machine should be balanced in the following parameters: • Yarn tension before and after the yarn feeder. The yarn tension should be set at the minimum prior to the yarn feeder or with direct feeding (without yarn feeder). • Fabric take-up tension should be set as low as possible. • Drawing-in of yarn at the cylinder and the dial. In knitting, a larger distance between cylinder and dial gives a greater chance of obtaining a loosely knitted fabric. • Height of the dial. The tightest setting should be set between cylinder and dial to ensure the fabric can freely pass through without being torn. As well as the precise settings of the knitting machine, the machine should be kept clean in order to reduce the potential for knitting faults. Faults resulting from poor cleaning can be due to the following: © Woodhead Publishing Limited, 2011
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• dirt, lint and/or yarn fragments in the camming system, tricks, needles, or sinkers; • variation in oil content for machine and needle track lubrication; • worn needles, which generally produce length-direction streaks; • worn cylinder and/or dial.
9.4
Parameters of knitting control
During the knitting process, particular attention needs to be paid to some parameters in order to maintain and control the fabric quality. This section aims to discuss some of the important parameters of knitting control.
9.4.1 Loop length Loop length is defined as the amount of yarn used to form one unit loop.
The loop length is the absolute quantity of any knitted fabric and is directly related to loop density. In general terms, the loop size increases while the loop density decreases (Brackenbury, 1992). In equation expression, their relationships are represented by
where S is loop density, l is loop length and K is a constant. The loop length is an important quality control factor in the production of knitted fabric. It has influential impacts on stitch density, fabric weight, panel length, tightness, fabric width and dimensional stability (Brackenbury, 1992). In an experiment by Yue (1993), three ends of 2/48 Nm (125 tex) worsted yarns are knitted into a plain jersey fabric with a 7-gauge knitting machine with three different stitch settings. The fabric parameters obtained after wet relaxation are illustrated in Table 9.1 below. From the experiment results, it can be seen that parameters such as stitch density, tightness factor, weight and thickness are all inversely proportional to the loop length of knitted fabric. The results indicate that variations in loop length between one finished garment and another can produce size variation, whilst loop length variations within a structure (particularly when using continuous filament yarns) can produce horizontal barring and impair the appearance of the fabric (Spencer, 1983, 2001). As a result, it is vital to keep the variations of loop length to a minimum so that loop length is maintained uniformly and consistently throughout the knitting process. The only effective and reliable way to ensure the consistent loop length is by means of a positive yarn feeding system. © Woodhead Publishing Limited, 2011
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Large
Medium
Small
Loop length (cm) Tightness factor Density ‘S’ (wpi × cpi) Weight (g/m2) Thickness (mm)
1.12 9.9 10 × 13 S = 130 356 2
1.04 10.75 11 × 75 S = 165 363 2.02
0.882 12.6 12 × 17.5 S = 210 382 2.07
9.4.2 Positive yarn feeding Positive yarn feeding is a system often fitted on circular knitting machines to positively drive the yarn at a fixed rate relative to the surface speed of the needle cylinder. It is currently being considered as a standard quality control installation in all modern circular knitting machines. The main function of this system is to regulate the yarn knitting tension to a desired value, by enabling a predetermined length of yarn to be fed positively and consistently to all the needles for each revolution of the machine cylinder. The predetermined length of yarn is commonly referred to as course length; that is the length of yarn per needle or stitch multiple by the number of needles knitting per revolution in the cylinder or cylinder and dial. The positive feeders aim to control the fabric quality by making the course length align with the desired yarn delivery speed. When some courses are widely out of specification and differ from one another, horizontal bars (including barré) will be produced, resulting in an unacceptable quality of knitted fabric.
9.4.3 Knitting tension
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It is generally understood that loop length on knitted fabric is inversely proportional to the knitting tension. Therefore, a uniform loop length can only be produced on a knitting machine with regular knitting tension. By carefully controlling the knitting tension, the variations in loop length can be minimized and the quality of knitted fabric improved. Some parameters affecting knitting tension include yarn-unwinding tension, package diameter and package density, which are discussed below: • Yarn unwinding method. The yarn knitting tension changes during the yarn unwinding process as the yarn is pulled out from the top package layer by layer. This creates a higher unwinding tension at the bottom of the cone, while a lower tension occurs at the shoulder level. • Package size refers to the package diameter. The package diameter changes constantly from full size to empty when unwinding the yarn. The unwinding tension changes upon the package diameter. The unwinding tension on a full size package is much lower than that on those with a small package size or on empty ones.
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• Package density. Package density has a proportional relationship to yarn unwinding tension as well as to the knitting tension. When unwinding yarns from a high-density package, higher yarn frictions between coils are produced, resulting in higher unwinding tension.
9.4.4 Tightness factor Tightness factor indicates the relative tension of a plain-weft knitted structure. It is defined as the ratio of the area covered by the yarn in one loop to the area occupied by that loop. In simplified formula expression tightness factor (K)
in SI units,
where Tex is the unit of yarn count and L is the stitch length in millimetres. The tightness factor of a knitted fabric is the function of the stitch length for a constant yarn count. When a yarn that is finer than usual is knitted into a fabric, the resultant tightness factor increases as the yarn count will directly influence the tightness factor. In plain fabric knitted from worsted yarn, the value of K ranges between 1.4 and 1.5. The value of K in Imperial Units is , where N refers to the worsted count and l represents the loop length in inches.
9.4.5 Yarn input tension Yarn input tension (YIT) is used to tune the feeding of the yam into the knitting zone. The optimal YIT ranges from 2 to 4 grams. An excessive value in YIT results in yarn breaks and machine downtime, both of which are uneconomical. YIT can be used as a means of process control, so that defects can be prevented or quickly detected. The variation of YIT is an ideal indicator to reflect the formation of a loop (Catarino et al., 2002). An exceedingly high yarn tension can arise from improper threading up, dirt and fluff in the yarn path, tilted cones, poorly wound cones or incorrectly set tensioners.
9.4.6 Yarn length per stitch The length of yarn in one stitch is another important factor which permanently affects the quality of a knitted fabric. The yarn length per stitch determines the dimensions and stitch density of the fabric. It is therefore essential to keep the variations of the loop dimension to a minimum.
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9.4.7 Take-down tension It is a well-known fact that the take-down tension can materially affect the dimensions of knitted fabric and so the fabric quality will deteriorate. An excessively high take-down tension would induce undesirable stretching in the length direction of fabric, resulting in a narrower fabric with a lower value of courses per unit length. If the knitted fabric were even temporarily stretched, it would take a longer time for it to return to completely relaxed dimensions. Therefore, it is important to control the take-down in a proper way in order to produce quality knitted fabric with consistent fabric width.
9.5
Relationship between yarn count and machine gauge
In circular knitting machines, yarn count primarily depends on the needle pitch and thus the machine gauge (Lyer et al., 1995). As the diameter of a yarn is proportional to its yarn count (direct system), a relationship exists between the range of optimum counts of yarn that may be knitted on a particular machine, and the gauge of the machine. Machine gauge thus plays an influential role in the choice of yarn count and can have an effect on fabric properties such as weight and appearance. Therefore, it is important to obtain an optimal balance of yarn count and machine gauge in order to ensure the best knitting performance for a specific machine gauge and structure, with high machine efficiency and minimum fabric fault rate. However, there is no concrete formula suitable for calculating the yarn count of a machine gauge. This is because a range of yarn counts can be used on the same knitting machine gauge, and the ‘knittability’ also depends on the knitted structure, the desired fabric appearance and the fabric properties. For a particular machine gauge, a range of yarn counts can be knitted with different loop lengths. The best way to find out the range is by experimenting. The following (Table 9.2 to Table 9.7) illustrates the knittable values of the average yarn counts used for different machine gauges and fabric types. 2 3 4 5 6 7 8 9 40 1 2 43X
9.6
Examples of quality control mechanisms for circular knitting
9.6.1 STARFISH – Engineered knitted program for cotton circular knits The name STARFISH is contracted from the phrase ‘START as you mean to FINISH’. STARFISH is a computer program that resembles a simulator. It models the influence of the major variables in the production and processing of circular knitted cotton fabrics and calculates their effects on the final properties of the finished fabric. Using STARFISH, the most appropriate combination of yarn
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Quality control in the knitting process and common knitting faults Table 9.2 Yarn count and machine gauge for single jersey fabric Machine gauge
Yarn count
Needles/inch
Ne
dtex
14 8.5/1–14.0/1 200 × 2–235 × 1 15 10.5/1–16.5/1 150 × 2–200 × 1 16 12.0/1–19.0/1 250 × 1–167 × 1 18 14.0/1–23.5/1 200 × 1–150 × 1 20 18.0/1–26.0/1 167 × 1–122 × 1 22 21.5/1–29.5/1 150 × 1–110 × 1 24 23.5/1–35.5/1 140 × 1–100 × 1 26 42.0/1–41.5/1 122 × 1–84 × 1 28 29.5/1–47.5/1 110 × 1–76 × 1 30 35.5/1–59.0/1 100 × 1–67 × 1 32 41.5/1–71.0/1 84 × 1–55 × 1
Table 9.3 Yarn count and machine gauge for fleecy fabric Machine gauge
Yarn count
Needles/inch
Ne
12 2.5/1–9.5/1 14 3.5/1–12.0/1 15 4.7/1–14.0/1 16 6.0/1–16.5/1 18 7.0/1–18.0/1 20 8.5/1–20.0/1 22 10.5/1–23.5/1 24 14.0/1–26.0/1 26 16.5/1–29.5/1 28 19.0/1–35.5/1 30 21.5/1–41.5/1 32 23.5/1–47.5/1
dtex 720 × 2–622 × 1 620 × 2–500 × 1 500 × 2–420 × 1 833 × 1–360 × 1 660 × 1–300 × 1 500 × 1–280 × 1 360 × 1–200 × 1 300 × 1–167 × 1 250 × 1–150 × 1 200 × 1–122 × 1 150 × 1–110 × 1 122 × 1–84 × 1
Table 9.4 Yarn count and machine gauge for fine rib fabric Machine gauge
Yarn count
Needles/inch
Ne
14 15 16 18 20 22 24
16.5/1–23.5/1 235 × 1–150 × 1 20.0/1–29.5/1 200 × 1–122 × 1 23.5/1–35.5/1 167 × 1–100 × 1 29.5/1–47.5/1 150 × 1–90 × 1 41.5/1–53.0/1 122 × 1–76 × 1 47.5/1–59.0/1 100 × 1–67 × 1 53.0/1–71.0/1 84 × 1–55 × 1
dtex
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Machine gauge
Yarn count
Needles/inch
Ne
14 15 16 18 20 22 24 26 28 30 32
12.0/1–16.5/1 235 × 1–167 × 1 14.0/1–19.2/1 220 × 1–150 × 1 16.5/1–21.5/1 200 × 1–133 × 1 21.5/1–23.5/1 167 × 1–110 × 1 23.5/1–29.5/1 150 × 1–100 × 1 28.5/1–35.5/1 133 × 1–100 × 1 33.0/1–41.5/1 122 × 1–90 × 1 35.5/1–47.5/1 110 × 1–84 × 1 41.5/1–53.0/1 100 × 1–76 × 1 47.5/1–59.0/1 90 × 1–67 × 1 53.0/1–71.0/1 76 × 1–50 × 1
dtex
Table 9.6 Yarn count and machine gauge for jacquard fabric Machine gauge
Yarn count
Needles/inch
Ne
dtex
14 13.0/1–18.0/1 235 × 1–200 × 1 15 14.0/1–19.0/1 220 × 1–167 × 1 16 16.5/1–21.5/1 200 × 1–150 × 1 18 18.0/1–23.5/1 167 × 1–122 × 1 20 21.5/1–26.0/1 150 × 1–110 × 1 22 23.5/1–28.5/1 122 × 1–100 × 1 24 26.0/1–33.0/1 100 × 1–84 × 1 26 84 × 1–78 × 1 28 78 × 1–67 × 1 30 67 × 1–50 × 1
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Table 9.7 Mean yarn counts (in dtex) for some fibre materials in relation to machine gauge Fibre
Machine gauge (needles/inch)
10
12
14
15
16
18
20
22–24 28–32 40–42
Wool Cotton Polyester filament Polyamide filament Acrylic yarn
640 – 320 400 300
500 355 280 350 235
420 300 235 250 200
300 280 190 150 200
300 235 140 150 200
250 220 140 125 167
200 194 140 125 150
190 160 150 125 122 95 100 76 125 –
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count, stitch length and type of knitting machine necessary to deliver the desired combination of properties in the finished fabric can be calculated in an efficient and accurate way, without using production time or materials to excess. STARFISH helps knitting manufacturers to rapidly develop new fabric qualities or optimize existing qualities in an effective way without recourse to expensive trial and error sampling. In addition, it also helps to optimize the development process and make direct savings in development time and costs. It further helps to optimize the process management and production control procedures, in order to improve product quality and consistency.
9.6.2 Mayer and Cie MCTmatic Quality Monitoring System The MCTmatic system uses computer-controlled adjustment and a processor controlled braking system installed on the knitting machine. It is a monitoring system for setting and altering the yarn delivery and tensioning. The MCTmatic system allows the motors to be set for feed wheel, central stitch adjustment and fabric take-down. Figure 9.1 shows the Mayer & Cie MCTmatic Quality Monitoring System with the setting or motors for feed wheels. The MCTmatic system is very useful in ensuring knitted fabric quality throughout the production process. When non-conformity is detected, the knitting machine will stop and the knitted faults will be indicated on the MCTmatic display panel as shown in Fig. 9.2.
9.1 Setting of motors for feed wheel (Mayer and Cie).
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9.2 MCTmatic display panel.
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Techniques to reduce knitting faults: online data monitoring system
Knitting faults can be categorized into horizontal and vertical components (Araujo et al., 1999). The first category is mainly due to yarn inconsistencies and inappropriate raw material management. The second category usually results from inappropriate knitting conditions, especially incorrect machine settings and maintenance, poor monitoring of the machine performance and improper yarn delivery. In order to eliminate or reduce knitting faults, manufacturers endeavour to set up a standard quality control method with the aid of fault detecting devices. Examples include a needle detector to find closed latches for rising needles, and a yarn breakage detector to show up broken yarn during production. These traditional devices can detect and count the number of yarn faults and help to prevent them occurring and to improve product quality. However, a better way to improve the quality of knitted fabric is to monitor the important knitting parameters and process interference in real-time and to apply counter measures when deviations are identified.
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The online monitoring system is based on the principle of measuring the important knitting parameters in real-time process and comparing these data with predetermined set values for a particular product quality (Lek-Uthai, 1999). In general, the knitting parameters cover all the production processes such as yarn tension, yarn speed, yarn evenness, etc., as well as the continuous performance of the knitting machine, from yarn delivery through to the knitted fabric produced. The online monitoring system enables the knitting machine to detect knitting faults in real-time and provides the user with a set of informative parameters related to yarn consumption and production. The monitoring system offers the following parameters during operations: yarn input tension (YIT), speed of knitting machine (m/s), yarn delivery speed (m/min), yarn consumption per course, fabric production in kg, tightness factor (K) and loop length. It should be realized that yarn input tension (YIT) is a valuable source of information in the knitting process since it directly reflects the influence of different mechanisms along the yarn path, which affects the production of knitted fabric and the overall performance of the knitting machine (Catarino et al., 2004a). YIT can be measured by a set of sensors. The measurement system is composed of a force sensor, which is installed close to the feeding zone and encoders and an optical sensor. Next, the monitoring software is connected and primarily used for analysing the waveforms of the YIT, such as the MonitorKnit. The resulting waveform is analysed by a signal processing technique, which produces a signal when a knitting fault is formed. Therefore, by inspecting and comparing waveforms resulting from normal and abnormal knitting, fabric faults and malfunctioning of the knitting machines can be quantified and identified in an accurate way, which constitutes a major step in reducing repair time. The other online monitoring system is that of fabric image acquisition. Samples of different knitted defects are acquired by image-capture equipment. In image processing, the sensed image (e.g. by a video camera) is translated into a digital image (i.e. a two-dimensional array of numbers or grey levels) by an analogue-todigital converter. The digital image can then be analysed by using imageprocessing techniques to reflect the knitting faults and defects. The defects can be analysed and identified by the image processing algorithms and filters. For example, a drop stitch run can be detected by applying a low-pass filter to the image. A low-pass filter effectively averages out areas in the image to highlight regions of different light intensities (e.g. a hole or normal fabric). In other words, the small areas of light between stitches would be discounted, but the larger areas of light caused by a hole would remain, and so the hole is easily identified (Convery et al., 1994).
9.8
Knitted defects
The imperfections of knitted fabric may be due to faulty yarns, malfunctioning knitting machine parts or poor finishing. The defects in knitting construction are
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considered here in terms of appearance and nature. The major defects in knitted fabric can be divided into (1) bands and streaks and (2) stitch defects.
9.8.1 Bands and streaks Barré effect: A barré effect has the appearance of a stripe with shaded edges. It is a continuous visual barred or striped pattern parallel to the yarn direction (Fig. 9.3). One of the causes of the barré effect is that of physical, optical or dye differences in the yarns or geometric differences in the fabric structure, acting either singly or in combination. Another reason for this effect is inconsistency in the yarn formation, such as variations in carding, running different types of spindle tapes on ring spinning frame, or mixing yarns of different counts or different spinning systems. The third reason for barré effect is due to inefficient fabric formation. It includes wrong stitch length at a feed, incorrect tension at a feed and variation in fabric take-up from loose to tight and uneven cylinder height needles. Barré is caused by inconsistencies in fibre properties, yarn characteristics, knitting parameters and processing. In order to prevent barré effect on knitted fabric, it is necessary to maintain consistency throughout each phase of textile production (Bailey, 2002). suggested ways to prevent barré are as follows: • Stock yarns should be properly and carefully labelled to avoid mix-ups. Fugitive tints can be useful for accurate yarn segregation. • Inventory should be controlled on a First In/First Out basis. • All equipment should be properly maintained and periodically checked. • check for barré by sample dyeing before beginning full-scale production.
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9.3 Knitting fault: barré effect.
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Skewing: Skew can be defined as a fabric condition occurring when the knitted wales and courses are angularly displaced from the ideal perpendicular angle (Fig. 9.4). The skewing effect is seen as a line or design running at a slight angle across the cloth. It is an inherent defect mainly caused by yarn twist parameters. Firstly, the high yarn twist levels result in yarns that exhibit high inherent torsion energy as a result of their great tendency to untwist. The yarn exhibits significant snarling effect, high liveliness and consequently, poor fabric dimensional stability (Badr et al., 2008). Second, the effect of fabric skewness is caused by yarn twist direction. Yarn twist direction depends on the direction of machine rotation (Cotton Incorporated, 2002). For machines rotating in a counterclockwise direction, yarns made using Z twist direction yielded fabric of lower spirality than those made using S direction. Air jet spun yarn made using S twist direction yielded higher fabric spirality than that made on Z direction. In principle, the skew
(a)
(b)
(c)
(d)
9.4 Knitting fault: fabric skew. (a) Ideal course/wale loop alignment. (b) Wale skew. (c) Right-hand course skew. (d) Left-hand course skew (9.4a–d from Badr et al., 2008).
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caused by one set of yarn is countered by that of the other set. The effect of twist direction can also be compensated for by yarns that are doubled or plied for knitting. Doubled yarns are expected to yield lower fabric spirality than single yarns. Figure 9.4 illustrates a comparison between a normal fabric and a skewed fabric in both wale and course direction. Bowing: A bow effect is observed when the course line form an arc across the width of knitted fabric. It is defined as an excessive curvature of the courses in a knitted fabric that may or may not extend over the full width. Bowing is the distortion caused by a faulty take-up mechanism on the knitting machine. It can also be caused by incorrect feeding during the finishing process. Streak or stop mark: A straight horizontal streak or stop mark in the knitted fabric is due to a difference in tension in the yarns, caused by the machine being stopped and then restarted. Needle line: Needle line is a vertical creak that is different from the adjacent normal wales (Fig. 9.5). This is caused by needle movement due to a tight fit in its slot or a defective sinker. It can also be caused by a misaligned or broken needle, which will produce distorted stitches.
9.5 Needle line.
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9.8.2 Stitch defects Dropped stitch: This is a lost stitch caused either by the yarn carrier not having been set properly or the stitch having been knitted too loosely (Fig. 9.6). This may be attributed to improper setting of the yarn feeder or insufficient yarn tension. To solve the dropped stitch problem, re-adjust the yarn feeder or increase the yarn tension. Cloth press off: This defect results from broken yarn coming away from the knitting needles during knitting. A serious press off can be a big section or the entire tube of circular knitted fabric coming off the knitting needles (Fig. 9.7). Press off often occurs accidentally with yarn breakage(s). One preventive measure that can be taken in order to eliminate or reduce the defect is to maintain a smooth yarn path from the cone to the knitting needles. In double jersey knitting, the problem or the extent of the defect can be reduced with alternate needles at alternate feeds. Cockled or puckered: The knitted fabric appears wavy when spread flat. This is difficult to detect during visual inspection on an inspection machine with fabric under roller tension. It is usually due to uneven stitches, stitch distortion, uneven yarn relaxation or shrinkage. Crack or hole: Large holes could be caused by weak places in the yarn, resulting in the yarn breaking during loop formation. Small holes are often the result of a
9.6 Knitting fault: dropped stitches.
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broken yarn before (or after) a knot or splice, since the yarn end with the knot sits tightly in the last stitch. Tucking (bird’s eye): This appears as a small aperture occurring occasionally in a wale (Fig. 9.8). It is generally caused by unintentional tucking from a malfunctioning needle, with two small, distorted stitches, side by side. Another reason is incorrect dial settings. If the dial is set too high, the dial needles do not support the fabric, which then pulls the fabric up. Tucking is also caused by incorrect feeding during finishing.
9.7 Press off.
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9.8 Tuck loop.
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Float: A float is defined as a length of yarn extending unbound over a number of wales with which it should be intermeshed. This is caused by a missed stitch, which can be due either to the failure of one or more needles to rise to catch the yarn, or to the yarn prematurely casting off from the knitting elements.
9.9
Conclusion
The important basis of good quality control in knitting rests with loop consistency, i.e. in achieving equal loop length and producing regular knitted structures. This applies to different systems of knitting. Particular attention has to be paid in high speed, multi-feeder circular knitting, in order to produce fabrics with a constant loop size. Modern technology and developments have been able to facilitate the installation of positive and storage feeding devices to achieve this purpose. In summary, a good quality control system should start with the selection of good quality yarn materials and the proper maintenance of machine parts. Monitoring and recording of fabric faults and the checking and immediate rectification of quality problems are also crucial to the production of fault free knitted fabrics.
9.10 References Araujo, M. D., Catarino, A. and Hong, H. (1999). Process control for total quality in circular knitting. AUTEX Research Journal, vol. 1, no. 1, pp. 21–29. Badr, A. A., Auburn, A., El-Helw, E., El-Hawary, I., Mito, A. B., Elmogahzy, Y., Farag, R. and Auburn, A. (2008). Dimensional stability of cotton fabric with emphasis on spirality: Between the theory and the practice. Beltwide Cotton Conferences, January 8–11, 2008, pp. 1527–1540. Bailey, D. L. (2002). Barré: Methods to prevent barré in knitted fabric. Paper presented at the 15th EPS Conference Memphis, TN, June 10–12, 2002. Brackenbury, T. (1992). Knitted Clothing Technology. Oxford: Blackwell Scientific Publications. Catarino, A., Rocha, A. M. and Monteiro, J. (2002). Monitoring knitting process through yarn input tension: New developments. IECON 28th Annual Conference of the IEEE, vol. 3, pp. 2022–2027. Catarino, A., Monteiro, J. L.and Soares, F. (2004a). Technique for unveiling faults during knitting production. IEEE International Conference on Industrial Technology, 4–7 May, 2004, vol. 1, 389–394. Catarino, A., Rocha, A., Monteiro, J. L. and Soares, F. (2004b). A system for knitting process monitoring and fault detection on weft circular knitting machines. Paper presented at the World Textile Conference – Fourth AUTEX Conference, Roubaix, June 22–24, 2004. Convery, S., Lunney, T., Hashim, A. and McGinnity, M. (1994). Automated fabric inspection. International Journal of Clothing Science and Technology, vol. 6, no. 5, pp. 15–19. Cotton Incorporated. (2002). Knit fabrics and the reduction of torque-technical bulletin. TRI 2002. Lek-Uthai, J. (1999). Quality assurance circular in knitting part 1: Theoretical analysis. Thammasat International Journal of Science Technology, vol. 4, no. 1, pp. 72–81.
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Lyer, C., Mammel, B. and Schäch, W. (1995). Circular Knitting (2nd ed.). Bamberg: Meisenbach. Spencer, D. J. (1983). Knitting Technology. Oxford: Pergamon Press. Spencer, D. J. (2001). Knitting technology: A comprehensive handbook and practical guide. Cambridge: Woodhead Publishing. Yue, K. H. (1993). Quality Control in Knitting (Vol. 10). Institute of Textiles and Clothing, Hong Kong: Hong Kong Polytechnic.
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10 Women’s apparel: knitted underwear J. KAR, J. FAN and W. YU, Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hong Kong, China Abstract: This chapter considers the functional requirements of knitted underwear fabrics, describes the relevant standards and test methods for the performance evaluation of such fabrics, and reviews the recent developments in fabric engineering and product innovation. Key words: comfort, sewability, durability, colourfastness, performance evaluation.
10.1 Introduction Underwear is a type of apparel worn next to the skin for reasons of hygiene and comfort.1 It should provide comfort for the wearer, possess good sewability, retain its appearance during wear, be durable and have easy-care properties.2,3 This chapter considers the functional requirements of knitted underwear fabrics, describes the relevant standards and test methods for the performance evaluation of such fabrics, and reviews the recent developments in fabric engineering and product innovation. It serves to provide a reference for the product development and evaluation of knitted underwear fabrics.
10.2 Functional requirements of knitted underwear 10.2.1 Comfort Comfort is a primary requirement of clothing, which can be categorized into aesthetic comfort, thermal comfort, moisture comfort, tactile comfort and pressure comfort.4 Aesthetic comfort is the subjective perception of clothing by visual sensation,5 which is influenced by colour, style, garment fitting, fashion compatibility, fabric construction and finish.6,7 Thermal comfort is primarily related to the efficiency of heat dissipation from a clothed human body6 and is viewed as the ‘neither too hot nor too cold’ feeling of the wearer.8 The body is in a state of comfort when the core temperature of the body is maintained at 37°C and the average skin temperature is approximately 33°C without the presence of sweat. One of the primary functions of underwear is to act as a buffer against environmental changes in order to maintain a thermal balance between the heat generated by the body and the heat lost to the environment while allowing the skin to remain free of liquid.6 235 © Woodhead Publishing Limited, 2011
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Moisture comfort is dependent on the sensation of dampness which is recommended as a sensitive tool to evaluate the thermal function of garments to balance the dampness of skin and clothing.9 Although the human skin has no humidity receptors, it can sense dampness, which determines the degree of comfort or discomfort.10 This has been confirmed by Hollies,11 who found that the sensation of loss in comfort occurred when perspiration was present. When more than 50–65% of the body surface is wet, it feels uncomfortable.12 Tactile comfort is related to the frictional interaction between clothing material and the human body,6 where physical/mechanical properties (surface structure, weight per unit area, thickness, bulk, compressibility, flexure, shear, elongation and frictional properties) of the fabric worn next to skin are thought to influence an individual’s assessment of tactile comfort.13,14 Some of the terms that have been used to describe the tactile sensations are clingy, sticky, scratchy, prickly, soft, stiff, heavy, light and hard.6 Tactile discomfort may be caused by allergy, clinging to the skin, tickling, prickling, abrasion of the skin and coolness.7 The finishes, dyes, softening agents, washing powder used in laundering, the structure and construction of the fibres and fabrics contribute to tactile discomfort. For example, if a fabric is hairy and rough to the touch, and tends to shed fibres, it may cause tickling and irritation, especially when the skin is damp with perspiration.15 Ruckman and Green16 also confirmed that skin irritation could be caused by breakage of the fibres and the fabric remaining wet during perspiration. Fabric hand is a generic term for the tactile sensations associated with fabrics that influence consumer preferences.17 To specify the fabric hand of underwear, Chen18 reported that the best hands for undershirts were rated as softest, slickest, smoothest, thinnest, lightest and coolest. Pressure comfort has already been discussed in detail in Chapter 7. Ishtiaque19 suggested certain comfort requirements for general clothing, which are also relevant to the comfort requirements of underwear. These are listed in Table 10.1.
Table 10.1 Functional requirements of clothing19 • Maintains a comfortable microclimate in terms of temperature and humidity in the skin sensory zone • Good moisture absorption and water vapour transmission • Absence of unpleasant odour such as perspiration • Compatibility with the skin • Good extensibility without restricting mobility • Good fit stability • Low intrinsic weight (not impairing physical performance) • Substantially water-repellent and dirt-repellent
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10.2.2 Sewability Underwear should be manufactured to a consistent quality free from defects. One of the major potential sewability problems of knitted underwear fabrics is sewing damage (needle holes). This is a problem particularly for tighter, denser and lightweight knitted underwear fabrics. There are generally two types of sewing damage derived from frictional forces in the fabrics, namely mechanical damage and needle heating damage. Mechanical damage is the cutting or breakage of yarns in fabric caused by penetration of the needle during sewing. Needle heating damage is the fusing and melting of synthetic yarns in the fabric caused by the high needle temperature arising from the friction between the needle and the fabric. The faults may be noticed only during wear or after being washed when the damaged holes are enlarged as a result of yarn laddering owing to the stresses during wear and laundering.20 Sewing damage is related to the choice of needle (in terms of size, needle length and point shape) and sewing speed. Needle size determines the extent of the deformation of the knitted loops within the fabric, and directly influences the stresses and strains imposed on the yarns. Moreover, shortpoint needles interact with the fabrics more violently than long-point needles, and tend to produce more yarn breakages. The use of bulged-eye needles, in which the diameter of the needle at the eye is enlarged with respect to the diameter of the shaft, can contribute to reducing the sewing temperature effectively (about 15–30 °C). Furthermore, lower sewing speeds are effective in controlling the overheating problems of needles. At the same time, the number of yarn breakages is relatively reduced.20 Therefore, finer needle size,21 bulged-eye needles20 and lower sewing speed22 can reduce sewing damage. On the other hand, the ease of deflection depends not only on the needle, but also on the ease of yarn movement, which is related to yarn friction. With increasing yarn friction, the level of each penetration force value will be greater, since the increased yarn friction will lead to a higher value of tension in the yarn around the needle as it is pulled from the adjacent loops.23 It is also known that the condition under which sewing takes place can also cause sewing damage. It was reported that the lower moisture content in winter of about 8% in cotton fabrics make them brittle and thus susceptible to sewing damage.24 Therefore knitted fabrics should not be sewn in an over-dry state. Applying appropriate lubricants to the fabric can lower the frictional forces in the fabric to ease needle penetration. Heat generation and mechanical strains can also be reduced.20 This can significantly improve sewing performance by the number of yarn breakages being reduced. Cooling attachments can also decrease needle heating. A cooling air jet can be used to increase the convective heat losses from the needle and reduce its temperature. With the vapour spray, a coolant of light oil or similar substance is atomized and sprayed over the needle and onto the fabric so that the coolant can absorb heat.20
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10.2.3 Appearance and appearance retention The presence of pilling and discoloration are major problems related to cotton underwear. Ukponmwan25 defined pilling as a fabric-surface fault in which pills of entangled fibres cling to the cloth surface, giving an unsightly appearance to the garment and reducing its serviceable life.26 The broken loose fibres develop into fluffy agglomerations anchored to the fabric surface and are called pills. Generally, pills are produced by attrition with different parts of the garment, by rubbing against the same fabric or other objects, or by other mechanical actions such as several cycles of laundering and drying, or during wear and cleaning.26–28 As a result, pilling causes an unattractive appearance, feel and texture, and limited service life of the garment.27 Pilling is a serious problem for knitted underwear. Because of its knitted loop construction, a greater yarn surface area is exposed, making such fabrics more abrasive and uncomfortable to the wearer.25 Colourfastness is also an important requirement for underwear. Colourfastness is defined as the resistance of a fabric to change in any of its colour characteristics and to the transfer of its colorant(s) to adjacent materials during end-use.29 According to ASTM D415430 and ASTM D4156,31 the colourfastness requirements of underwear under different end-use conditions are listed in Table 10.2.
10.2.4 Durability The durability of knitted underwear is commonly characterized by bursting strength and abrasion resistance,32,33 which are important attributes for the aesthetics and functional performance of underwear during use.32 ASTM D415430 and ASTM D415631 state that the bursting strength of knitted underwear should exceed 222 N (50 lbf) for a durable garment based on the ball-burst testing method of ASTM D3786, where the test sample is cut into 375 mm (15 in.) along the selvedge.
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For repeated laundering of knitted underwear, shrinkage is potentially the most serious problem. Shrinkage is more likely to be present in the course direction Table 10.2 Colourfastness requirements of knitted underwear fabrics Colourfastness characteristic
Requirements
Laundering, shade change Dry cleaning, shade change Perspiration, shade change Light (40 AATCC FU) (xenon-arc)
Class 4a min Class 4a min Class 4a min Class 4a min
Note: aAATCC Gray Scale for Colour Change
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compared to the wale direction in weft-knitted fabrics following several laundering cycles, as fabrics tend to be stretched more in the course direction during manufacture. Laundering causes the release of these internal strains such that there may be a large increase in the course density and less change in the wale density, as reported in previous research.26,34 Abrasion damage is another problem during daily use. It was reported that as much as 50% of the abrasion damage in some fabrics could be attributed to laundering.35 Much of the laundry abrasion damage is in the form of extensive peeling of the cuticle, primary and secondary wall of the cotton fibre, in the form of fibrils, fibril bundles and sheaths. Fibre encrustation is caused by laundering with sodium carbonate-based detergent used with different degrees of water hardness and phosphate-based detergent used with very hard water (284 ppm).34 In addition to laundering, drying underwear with a household tumble dryer is another factor that causes severe fibre damage, especially for wet fabric dried at high temperature. For cotton, dried at high temperature for five cycles, serious abrasion damage, changes in fibre morphology and fibre breakage have been observed under the microscope. Although the damage was reduced when the wet fabric was tumbled without heat after five cycles, there was also substantial damage to the fibre walls. Buisson36 recommended that the fabric should be placed in the dryer after the set temperature was reached to reduce abrasion damage. Moreover, reducing the time to reach the set temperature by controlling heat energy or airflow may also potentially minimize tumble-drying damage.
10.3 Performance evaluation of knitted underwear 10.3.1 Thermal properties There are many different testing methods and types of apparatus used for measuring the thermal conductivity, thermal transmittance, thermal resistance or insulation and warm/cool character of fabrics. BS 4745 and KES-FB7 are two commonly used testing methods for measuring thermal properties of the fabrics. Thermo Labo II KES-FB7 This instrument (as shown in Fig. 10.1) is used to evaluate thermal conductivity and insulation in dry and wet conditions (simulating perspiration or no perspiration) of fabrics, and the warm/cool feeling when the fabric is briefly in direct contact with the skin. For measuring thermal conductivity, water at 20 °C circulates inside the water box, and the BT-plate and the guard plate in the BT-Box are pre-set to 30 °C and 30.3 °C. The heat loss from the BT-box through the test specimen to the water box in watts is recorded by a digital panel meter. The thermal conductivity in watts/cm °C can be calculated by k = (W · D)/A · ∆To
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10.1 (a) and (b) Apparatus of Thermal Labo II.
where: D = the thickness of samples; A = area of heat plate of BT-box (25 cm2); ∆To = temperature difference between BT-box and water box (10 °C); W = the reading on the digital panel meter, which is the heat consumption of the BT-box.
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On the other hand, the warm/cool feeling test is used to determine the initial contact feeling of the fabric. The T-box and water box are used in this measurement where the BT-box at 30 °C supplies heat to the T-box until they have the same temperature, with the water box temperature set to 20 °C. The warm/cool feeling is represented by a q-max value which is the heat current required per unit area to maintain the condition of a 10 °C temperature difference recorded on the digital panel meter. There are four methods for measuring thermal insulation: • Dry contact method – the thermal insulation of the fabric is measured with the fabric directly in contact with the BT-plate. • Dry space method – the thermal insulation of the fabric is measured under the condition that there is a constant distance between the fabric and the BT-plate. • Wet contact method – the thermal insulation of the fabric is measured under the condition that the fabric is in direct contact with a wet filter paper on the BT-plate.
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• Wet space method – the thermal insulation of the fabric is measured under the condition that a constant distance between the fabric and wet filter paper is present on the BT-plate. BS 4745:1990/ISO 5085-1:1989 method for determination of thermal resistance of textiles This testing standard (BS 4745:1990/ISO 5085-1: 1989)37 specifies a method for ‘the determination of the resistance of fabrics, fabric assemblies or fibre aggregates in sheet form to the transmission of heat through them in the “steady state” condition’. The apparatus is shown diagrammatically in Figs 10.2 and 10.3. This
10.2 Diagram of apparatus.37
10.3 Diagram of cabinet.37 All dimensions in millimetres and approximate.
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standard includes two testing methods: a two-plate method and a single-plate method. The two-plate method is used to measure the thermal resistance of the material with the test specimen between a hot plate and a cold plate. This is similar to the thermal conductivity testing method used in the KES-FB7 apparatus. For the single-plate method, the test specimen is just laid on the hot plate and the outer side is exposed to the ambient air. The single-plate method is more suitable for measuring the thermal properties of underwear because it allows more air circulation, so that it simulates the wearing condition. The thermal conductivity of the fabric is calculated by the thickness divided by its thermal resistance, which is the ratio of the temperature difference between the two faces of the fabric to the heat flow rate per unit area normal to the faces.
10.3.2 Moisture permeability Two testing methods, the desiccant method and the water method, are specified in ASTM E96-9038 to determine the water vapour transmission of sheet materials of a thickness not exceeding 32 mm. According to the wearing conditions of underwear, the moisture properties of fabrics may be evaluated by the water method. A test specimen is attached to cover the dish or cup and wax is used to seal the fabric to prevent edge leakage, which is shown in Fig. 10.4. The change in mass of water is used to calculate the water transmission rate.
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(6 mm)) Depth as convenient (not less than (17 mm) for ‘Water’ method)
10.4 Water vapour transmission tester.38
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10.3.3 Liquid transport properties Wetting ASTM D572539 defines a test method to measure the contact angle of water in contact with a flat specimen of fabric under specific test conditions (Fig. 10.5). A drop of a specified volume of water is applied to a fabric surface using a liquid delivery system. The rate of change of the contact angle is recorded by a video camera and is used to determine the water absorbency of the fabric. Wicking Harnett and Mehta40 have described two methods to measure the wicking properties of fabrics. They are the longitudinal wicking strip test and the transverse wicking plate test. For the longitudinal wicking strip test, a strip of fabric is suspended vertically with its lower edge immersed in a reservoir of distilled water, as shown in Fig. 10.6. It is recommended to add a dye to the water in order to track the movement of the water more easily. The measured height of rise in a given time is used to indicate the wickability of the test fabric. The water wicking performance is highly dependent on the fabric structure and thickness and it is difficult to compare the wicking performance of fabrics with extreme thicknesses or structures.
10.5 Contact angle tester.
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10.6 Longitudinal wicking strip test.
The transverse wicking plate test is used to determine water transmission according to fabric thickness, that is, perpendicular to the plane of the fabric. It simulates the mechanism of liquid perspiration moving from the skin through the fabric. The test fabric is placed between a weight and sintered glass plate as shown in Fig. 10.7. The horizontal sintered glass plate is kept moist by a water supply of which the height can be adjusted so as to keep the water level precisely at the upper surface of the plate. The fabric will draw water from the glass plate at a rate that depends on its wickability. Given the diameter of the capillary tube, the recorded data is used to calculate the mass transfer rate of water into the fabric.
10.3.4 Fabric low-stress mechanical properties The handle and tactile comfort of knitted underwear are strongly related to the fabric’s low stress mechanical properties. The Kawabata evaluation system (KES-F) is a set of sophisticated instruments for characterizing the fabric’s 1 2 3 4 5 6 7 8 9 40 1 2 43X
10.7 Transverse wicking plate test.
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low-stress mechanical properties, which include tensile, shear, bending, compression and surface properties.41 The specimens are cut into 20 × 20 cm2 samples and conditioned at 21°C and 65% RH for at least 24 hours before taking the measurements. The instruments comprising the KESF system are shown in Figs. 10.8–11.
10.8 KES-F1 shear and tensile tester.
10.9 KES-F2 bending tester.
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10.10 KES-F3 surface tester.
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10.11 KES-F4 compression tester.
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10.3.5 Fabric bursting strength As has been previously mentioned, the durability of underwear fabrics is very much related to their bursting strength. ASTM D378742 defines a method that may be used to measure the bursting strength of knitted fabrics using a ball-burst strength tester (Fig. 10.12). The instrument consists of a polished steel ball that has a diameter of 25.4 ± 0.005 mm. The conditioned fabric specimen is placed tension-free in the ring clamp of the device. The polished steel ball is then pushed through the specimen until it ruptures. The bursting strength is determined as the force applied to the ball at the instant of fabric rupture.
10.3.6 L & M sewability test This test measures the needle penetration force to predict the sewability of the fabric. The apparatus is called the L & M Sewability Tester. The fabric is fed forward by rollers beneath a needle that penetrates it. It can operate at a speed of 20 penetrations per minute, which means a test of 100 penetrations takes no longer than five minutes. The peak force of penetration is indicated on the meter and
10.12 Ball burst attachment.42
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registered on a pen recorder. Penetration values that exceed a critical threshold value can be registered on the ‘high reading’ counter.23
10.3.7 Dimensional stability and skewness stability AATCC TM 15043 describes a method to measure the dimensional change after laundering. Locations are marked as shown in Fig. 10.13 by using a plastic or metal tape graduated in millimetres after laundering. This testing method can be used to measure the shrinkage or extensibility at different positions of undershirts after laundering. For measuring the skewing stability of under-shirt fabrics, AATCC TM 17945 provides two methods to mark the positions on the garment or fabrics before laundering, as shown in Figs 10.14 and 10.15. There are three options to calculate the skewness changes in undershirt fabrics. For method 1, the percentage change in skewness to the nearest 0.1% can be calculated by the following two options: • Option 1 (Fig. 10.16): percentage change in skewness = 100 × [2 (AC – BD)/ (AC + BD)]. • Option 2 (Fig. 10.17): percentage change in skewness = 100 × [(AA' + DD' )/ (AB + CD)]. For method 2, change in skewness can be measured by calculating option 3. • Option 3 (Fig. 10.18): percentage change in skewness = 100 (AA' /AB).
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10.13 Dimensional change marking location.44
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10.14 Method 1 of AATCC TM179 skewness stability testing method: square marking.45
10.15 Method 2 of AATCC TM179 skewness stability testing method: inverted marking.45
10.16 Diagonal lines for option 1.
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10.17 Offset marks for option 2.
10.18 Offset marks for option 3.
Tables 10.3, 10.4 and 10.5 summarize alternative washing and drying conditions and settings for the dimensional stability and skewness stability measurements.
10.3.8 Colourfastness to water
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In the AATCC TM 107 method29 for measuring the colourfastness of knitted underwear materials, 60 × 60 mm ± 2mm specimens are immersed in freshly boiled distilled water or deionized water from an ion-exchange device at room temperature with occasional agitation to ensure thorough wetting throughtout (approximately 15 minutes is generally required for average fabrics). Then, the Table 10.3 Alternative washing and drying conditions45 Machine cycle
Washing temperatures
Drying procedures
1. Normal/Cotton sturdy (ii) 27 ± 3°C (80 ± 5°F) (a) Tumble: 2. Delicate (iii) 41 ± 3°C (105 ± 5°F) (i) Cotton sturdy 3. Permanent press (iv) 49 ± 3°C (120 ± 5°F) (ii) Delicate (v) 60 ± 3°C (140 ± 5°F) (iii) Permanent press (b) Line (c) Drip (d) Screen
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Table 10.4 Washing machine setting conditions without load45 Cycle1
X
Y
Z
Water level Agitator speed Washing time Spin speed Final spin cycle
18 ± 1 gal 179 ± 2 spm2 12 min 645 ± 15 rpm3 6 min
18 ± 1 gal 119 ± 2 spm 8 min 645 ± 15 rpm 6 min
18 ± 1 gal 119 ± 2 spm 10 min 430 ± 15 rpm 4 min
Notes: 1 Cycle
names vary with machine model, ‘X’ generally corresponds to ‘heavy duty’, ‘Y’ generally corresponds to ‘delicate’, ‘Z’ generally corresponds to ‘permanent press’. 2 spm = strokes per minute. 3 rpm = revolutions per minute.
Table 10.5 Tumble dry conditions45 Designation
Cycle Maximum exhaust stack temperature with loaded dryer
a Normal or permanent press b Delicate, synthetic, low Cool down time Normal and Delicate Permanent press All
67 ± 6°C (154 ± 10°F) after 1983 (65 ± 6°C (150 ± 10°F) before 1983 < 62°C (144°F) after 1983 (< 60°C (140°F) before 1983) 5 min 10 min 10 min after 1983
test specimen is removed from the solution and passed between squeeze rolls to remove excess liquid when the wet weight of the test specimen is more than three times its dry weight. The test specimen is then placed between glass or plastic plates and pressed under a perspiration tester with 4.5 kg pressure. After heating in an oven at 38 ± 1°C for 18 hours, the specimen is dried by hanging in an airy space at room temperature. The colour change of the undershirt fabric is rated subjectively for colour change using a grey scale.
10.3.9 Wearer trials Wearer trials are the ultimate test for the performance of knitted underwear, although the process tends to be expensive and time consuming and the results tend to be less reproducible and consistent.15 Wearer trials are especially necessary for assessing the subject sensations of the wearers, for example, comfort sensation. Wearers are often asked to judge the comfort of the garments after carrying out a series of instructed activities. This method has been used for evaluating moisture,
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thermal, tactile and aesthetic comfort.11,15,16,46–48 Wearer trials can also be designed to obtain some objective sensory measurements under different wearing conditions,48 which are relevant to the behaviour of the knitted underwear. For example, sensors such as copper-constantan thermocouples may be attached to the wearers to measure skin temperature during a wearer trial.9,49
10.4 Engineering of knitted underwear fabrics Knitted underwear fabrics can be better engineered with improved understanding of the effects of fabric composition, yarn properties and fabric structure. Nevertheless, since such effects are far from fully understood, the following discussion serves only to contribute towards the knowledge base for the engineering of knitted underwear fabrics.
10.4.1 Fabric composition
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Cotton is the most common material used to make underwear. It was found that cotton is associated with both physical and psychological comfort, and is viewed as youthful, honest, pure and dependable.50 From the Australian perspective, cotton was seen to be close to the ideal material for making sports shirts – the only disadvantage of cotton is that it is crushable.51 Boslet50 has also stated that it was difficult to find any fibre matching the advantages of cotton. Furthermore, cotton could be more likely to cause skin irritation. On the other hand, other knitted underwear materials, such as nylon and polyester were regarded as artificial, low quality, unfashionable, clammy, sweaty, clingy, synthetic and causing itchiness.50,51 In recent years, the scene has totally changed. A number of studies have shown that by using appropriate yarn and fabric structures, clothes made from synthetic fibres can be as comfortable to wear as those made from natural fibres, especially the newly developed polyester fabrics.16,52–56 Many researchers7,16,57,58 stated that 100% cotton, or cotton-rich blends, were more comfortable underwear materials as these were more effective to absorb water vapour and perspiration than synthetic fibres. According to ASTM D1909,59 the moisture regain values for specified fibres are shown in Table 10.6. Cotton is a vegetable fibre that consists mainly of natural (plant) cellulose with a thin coating of wax. During finishing, this wax coating will be removed, so the cotton fibre can absorb moisture effectively and allow it to evaporate easily. However, fibres with a higher ability to absorb moisture can increase the weight of the garment when it is worn during exercise, and cause discomfort after cooling down.58 On the other hand, composition is a contributory factor to fabric shrinkage. Quaynor et al.60 conducted research to investigate the shrinkage of different knitted fabrics including polyester and cotton. It was found that cotton knitted fabric seems to undergo some progressive shrinkage. Polyester, another common
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Table 10.6 Moisture regaining by different kinds of fibre (percentage) Fibre regain
Percentage
Cotton, dyed yarn Cotton, mercerised yarn Nylon Polyester
8.0 8.5 4.5 0.4
fabric type used for making underwear, shows better performance in preventing shrinkage. It can be explained by its low moisture regaining property, which is about 0.4%. Polyester knitted fabrics also do not become swollen in water and therefore have a high resistance to deformation. It has also been found that fabrics knitted from blended yarns (50% cotton/50% polyester) had a better dimensional stability compared to the fabrics from 100% cotton ring and open-end yarns.26 In addition, fabric composition is directly related to its appearance after laundering. Bresee et al.61 conducted research to evaluate the pilling problems of six different kinds of single knitted underwear fabrics made from 100% cotton, 50/50 polyester–cotton and 60/40 polyester–cotton. The three samples were bleached and the rest were both bleached and treated with a durable press finish. The unworn, unabraded and unlaundered fabrics were pill free, and laundering caused pilling and affected pill grades of 100% cotton fabrics more than in the polyester blended fabrics. They found that the pills formed on the cotton fabrics were more easily removed during laundering than the pills anchored by the higher tenacity polyester fibres. Fabric composition is also very much related to durability. It was reported that cotton/polyester fabrics possess greater strength than the allcotton fabrics.62
10.4.2 Effect of yarn characteristics Yarn type and structure affect the durability of underwear fabrics. McKinney and Broome62 found that fabrics made from open-end spun yarns were less resistant to both abrasion and bursting than those made of the comparable ring-spun yarns. Conversely, ring-spun yarn is more resistant to pilling than open-end spun yarn.28 Other research showed that fabric constructed from air-jet-spun yarn was the most pill-resistant and a fabric constructed from rotor-spun yarn was the least pillresistant.25 Those spinning methods that control the fibres, such that the finer fibres tend to stay in the centre of the yarn and the coarser fibres remain at the outside, produce yarns with a lower tendency to pill. Conversely, spinning systems that produce yarns in which the longer fibres tend to stay in the centre of the yarn and the shorter fibres at the outside produce yarns with a higher pilling tendency.63 The propensity for pilling is also related to the yarn twist. The higher the twist in
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the yarn, the less the tendency to pill because the twist compacts the yarn and reduces the number of protruding fibres that cause pilling. Consequently, double yarn gives less pilling than single yarn.25
10.4.3 Fabric thickness Fabric thickness is one of the most important factors determining thermal comfort.6 It was found that fabric thickness had a direct effect on thermal transmittance, where the thicker the material, the lower the thermal transmittance.64 Dorkin and Beever8 also stated that the thermal resistance through individual layers of dry fabrics was primarily dependent upon their thickness and was approximately two togs per 1 cm thickness varying from about 0.05 for cotton poplin to about 1 tog for a heavy overcoat. This value would be lower if the wind was present to cause more air penetration and higher natural convective heat loss.
10.4.4 Fabric structure
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Fabric handle is highly related to fabric structure. Chen et al.18 compared four different structures of weft knit fabrics, and found that single jersey fabric was softer, lighter and had a richer hand than the other single knit fabrics. Using the Kawabata evaluation system (KES-F), they found that single jersey fabric was physically lighter and thinner and required less energy to bend, compress and shear than the others. Knitted structure also affects some degree of dimensional deformation.60 Slackly knitted fabrics have a higher tendency to shrink more, attaining complete relaxation at raised temperatures, than tight knits. Comparing weft knitted fabrics with different combinations of knit, tuck and miss stitches, Anand et al.32 found that fabrics containing miss stitches pull the wales closer together, so they had higher relaxation shrinkage in the width direction than the plain single-jersey structure. Fabrics containing 50% miss stitches had higher relaxation shrinkage than a fabric with only 25% miss stitches. With respect to cotton knitted fabrics, the International Cotton Technology Institute in Manchester developed a software program, Starfish, to predict the dimensional behaviour of the fabric based on the knitting parameters (the size and type of the yarn, the stitch length, the size of the knitting machine), the finishing process and the nominal finished dimensions. The name Starfish is derived from ‘Start as You Mean to Finish’. The Starfish program can be used to predict the shrinkage of cotton rib, single jersey, interlock and pique fabrics. Starfish predicts shrinkage mathematically and is based on the following three logical foundations: • determining a particular state of relaxation for cotton knits that is stable and reproducible; the reference state on which all measurements will be made and all calculations based; © Woodhead Publishing Limited, 2011
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• developing a comprehensive database of measurements made on a systematic series of cotton fabrics that have been manufactured and processed under close quality control but nevertheless on a commercial scale and under commercially realistic conditions; • developing suitable mathematical models for the reference that link the knitting and finishing parameters to the dimensions of the relaxed, finished fabrics in a simple and reliable way.65,66 Fabric structure also affects durability. It was reported32 that plain knitted fabric, one of the popular knitted structures used for making underwear, had the worst abrasion resistance. It may be improved by knitting the structure to high area densities. Anand et al.32 found that there was a linear relationship between the stitch density and the bursting strength. The higher the stitch density, the higher was the bursting strength of the fabric. In their fully relaxed state, knitted fabrics with miss stitches in their structure had higher stitch densities that, again, were linearly related to its abrasion resistance. Single jersey fabric containing tuck stitches had lower bursting strengths than fabrics with miss stitches. The construction of a fabric also directly determines its susceptibility to pilling. Pilling problems are often associated with a loosely knitted fabric when continually worn or cleaned as there are more fibres anchored loosely on the fabric surface than on a tightly knitted fabric.25 Fabric structure is also an important factor affecting the comfort properties. Fabrics with more pores or bigger sizes of pore, potentially allow more air movement through the fabric which results in a cooler feeling for the wearer.6 Conversely, the tighter the fabrics, the smaller the spaces and the lower the air permeability. So the tightness and area density of fabrics are important considerations when designing underwear.
10.5 Recent developments in knitted underwear fabrics Underwear is traditionally made from cotton in single jersey or interlock knitted structures. However, in recent years, new fabrics have been developed using engineered fibres and special constructions to achieve improved wicking properties, quick drying, lighter weights, improved durability and easy care.
10.5.1 Akwatek® polyester fabric Akwatek® polyester fabric is one of the performance fabrics that, it is claimed, can transport moisture and assist thermoregulation based on an electrochemical principle. Furthermore, it is also claimed that the chemicals cannot be removed by repeated laundering. The Akwatek® technology modifies the polyester fibre surface at the nano-particle level. With chemical treatment, Akwatek® modifies the chemistry of polyster and releases hydrophilic groups at the molecular level. © Woodhead Publishing Limited, 2011
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The modified polyester has an active surface layer with anionic end groups that transport water molecules and release them to the atmosphere before they can form into liquid water. Consequently, it is claimed that Akwatek® polyester fabric can enhance wearing comfort properties.67,68
10.5.2 Coolmax® fabric Coolmax® is another functional fabric that, it is claimed, can keep the wearer cool and comfortable in any situation. Four channel fibres in Coolmax® fabric can rapidly transport moisture and heat to the outer surface, which makes it a quick drying and breathable fabric.69,70
10.5.3 Nike® Sphere Cool fabric Nike® has developed many different functional materials for making undershirts and sportswear. Nike® Sphere Cool71 is one of their innovative technologies to increase heat loss in order to enhance air circulation. It is claimed that the mesh structure accelerates the evaporation of perspiration, so that the wearer feels cooler and more comfortable. Good moisture absorbency by the inner layer is also claimed to improve the thermal comfort of the wearer (Fig. 10.19).
10.5.4 Nike® Dri-Fit Nike® Dri-Fit71 is a popular inner layer fabric which is claimed to carry the perspiration rapidly from the skin to the outside of a T-shirt, where it then evaporates. It is proposed that it should be worn next to the skin to keep the body dry.
10.6 Properties of commercially knitted underwear For the comparison of commercially knitted underwear fabrics, it is useful to establish reference values of the different properties of these fabrics. In our present 2 3 4 5 6 7 8 9 40 1 2 43X
10.19 Nike® Sphere Cool fabric structure.71
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study, we have tested nine different types of commercial single jersey knitted fabrics and eight different types of commercial interlock knitted fabrics in terms of thickness, mass per unit area, air permeability, thermal conductivity, q-max (warm/cool contact feeling), contact angle and time to full water absorption. The 10%, 20%, 50%, 70% and 90% percentile values are listed in Tables 10.7 and 10.8, respectively.
Table 10.7 Reference values of the properties of single jersey knitted fabrics Percentile Thickness Mass per Air (mm) unit area perme- ability (g/m2)
Thermal Q-max conduc- tivity
Contact angle
WVTR
90% 70% 50% 30% 10%
0.697562 0.64433 0.60753 0.570731 0.517498
123.9107 88.63974 64.25714 39.87455 4.603544
0.055963 0.052057 0.049358 0.046658 0.042753
0.780475 0.713567 0.667314 0.621061 0.554153
259.8803 228.7349 207.2043 185.6737 154.5283
160.3983 117.0496 87.08286 57.11616 13.76738
0.127604 0.119713 0.114257 0.108802 0.10091
Notes: Air permeability was measured according to ASTM D737-96. Thermal conductivity and Q-max were measured by Thermal Labo II KES-FB7, mentioned in section 10.3.1. Contact angle was measured by ASTM D5727, mentioned in section 10.3.3. Water vapour transmission rate (WVTR) was measured by ASTM E96-90, mentioned in section 10.3.2.
Table 10.8 Reference values of the properties of interlock knitted fabrics Percentile Thickness Mass Air (mm) per unit perme- area angle ability (g/m2)
Thermal Q-max conduc- tivity
Contact WVTR angle
90% 70% 50% 30% 10%
0.712801 0.64456 0.597386 0.550211 0.481971
119.2669 80.62629 53.91429 27.20229 –11.4384
1.085118 0.926454 0.816771 0.707088 0.548425
239.0271 203.6248 179.1514 154.6781 119.2758
228.332 192.5585 167.8286 143.0986 107.3252
0.120217 0.111862 0.106086 0.10031 0.091954
0.055893 0.052128 0.049524 0.046921 0.043155
Notes: Air permeability was measured according to ASTM D737-96. Thermal conductivity and Q-max were measured by Thermal Labo II KES-FB7 mentioned in section 10.3.1. Contact angle was measured by ASTM D5727, mentioned in section 10.3.3. Water vapour transmission rate (WVTR) was measured by ASTM E96-90, mentioned in section 10.3.2.
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10.7 Acknowledgements The authors thank the Innovation and Technology Commission of Hong Kong SAR for funding the project (ITS/028/03).
10.8 References 1. Farnworth, B. and Dolhan, P. A. ‘Heat and water transport through cotton and polypropylene underwear’, Text. Res. J., 1985 55 (10) 627–630. 2. ‘Humidity transport in functional underwear’, World Sports Activewear, 1998 4 (4) 27–31. 3. Gate, L. ‘Thermal underwear: a warm future’, Knitting International, 1990 97 (1155): March, 56–59. 4. Li, Y. ‘The science of clothing comfort’, J. Text. Inst., 2001 31 (1–2). 5. Hatch, K. L. ‘Textile science’, West Publishing Company, New York, NY, USA, 1993. 6. Cheng, K. P. S. and Cheung, Y. K., ‘Comfort in clothing’, Textile Asia, 1994 25 (2) 48–52. 7. Smith, J. ‘Comfort of clothing’, Textiles, 1986 15 (1) 23–27. 8. Gagge A. P., Stolwijk, J. A. J. and Hardy, J. D. ‘Comfort and thermal sensations and associated physiological responses at various ambient temperatures’, Environmental Research, 1969 2, 209–229. 9. Nielsen, R. and Endrusick, T. L. ‘Sensations of temperature and humidity during alternative work/rest and the influence of underwear knit structure’, Ergonomics, 1990 33(2) 221–234. 10. Winslow, C. E. A., Herrington, L. P. and Gagge, A. P. ‘Relations between atmospheric conditions, physiological reactions and sensations of pleasantness’, American Journal of Hygiene, 1937 26 103–115. 11. Hollies, N. R. S. ‘Visual and tactile perceptions of textile quality’, J. Text. Inst., 1989 80 (1) 1–18. 12. Gagge, A. P., Stolwijk, J. A. J. and Hardy, J. D. ‘Comfort and thermal sensations and associated physiological responses at various ambient temperatures’, Environmental Research, 1969 2 209–229. 13. Hennrich, L., Seidel, A. and Rieder, O. ‘Hand testing of knitted fabrics’, Knitting Technology, 1999 21(6) 34–36. 14. Bogaty, H., Hollies, N. R. S. and Harris, M. ‘Some thermal properties of fabrics, Part I: The effect of fiber arrangement’, Text. Res. J., 1957 27 445–449. 15. Sawbridge, M. ‘Comfort of clothing’, New Home Economics, 1989 35(9) 5–7. 16. Ruckman, J. E. and Gree, J. A. ‘Comfort of shirts for distance runners’, Journal of Clothing Tech. & Mgt., 1996 13 (1) 1–25. 17. Grover, G. M. A. and Spivak, S. M. ‘A screening technique for fabric handle’, J. Textile Inst., 1993 84 (3) 486–494. 18. Chen, P. L., Barker, R. L., Smith, G. W. and Scruggs, B. ‘Handle of weft knit fabrics’, Textile Res. J., 1992 62 (4) 200–211. 19. Ishtiaque, S. M., ‘Engineering comfort’, Asian Textile Journal, 2001 10 (11) 36–39. 20. Tyler, D. J. ‘Guide to damage-free sewing’, Hatranote, 1976 (42) 1–23. 21. ‘The making-up of knitted fabrics for outerwear’, WST Knitting Technic., 1980 2 (4) 32–34.
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22. Poppenwimmer, K. ‘Sewing damage and its prevention’, International Textile Bulletin – Dyeing/Printing/Finishing, 1986 32 (4) 4–6. 23. Leeming, C. A. and Munden, D. L., ‘Investigations into the factors affecting the penetration force of a sewing needle in a knitted fabric and its relationship with fabric sewability’, Clothing Research Journal, 1978 6 (3) 91–118. 24. Poppenwimmer, K., ‘Sewing damage to knits: how finishers can help prevent it’, American Dyestuff Report, 1981 70 (4) 24–29. 25. Ukponmwan, J. O. ‘Pilling’, Textile Progress, 1998 28(3) 1–55. 26. Candan, C. and Onal, L., ‘Dimensional, pilling, and abrasion properties of weft knitsmade from open-end and ring spun yarns’, Textile Res. J., 2002 72 (2) 164–169. 27. Goswami, B. C., Duckett, K. E. and Vigo, T. L. ‘Torsional fatigue and the initiation mechanism of pilling’, 1980 50 (8) 481–485. 28. Pedersen, G. L., Screws, G. A. and Cedroni, D. M. ‘Biopolishing of cellulosic fabrics’, Melliand Taxtilberichte, 1993 74 (12) E419–420. 29. American Association of Textile Chemists and Colorists (AATCC, 2002), Colorfastness to Water, Test Method: TM 107–2002. 30. American Standards for Testing and Materials (ASTM, 2001), Standard Performance Specification for Men’s and Boy’s Knitted and Woven Beachwear and Sports Shirt Fabrics, Designation: D4154-01, West Conshohocken, PA: ASTM. 31. American Standards for Testing and Materials (ASTM, 2001), Standard Performance Specification for Women’s and Girls’ Knitted Sportswear Fabrics, Designation: D4156-01, West Conshohocken, PA: ASTM. 32. Anand, S. C. and Yanmaz, Y. ‘Some properties of single-jersey weft knitted structures’, Melliand Textilberichte, 2000 81 (3) E43–46. 33. Iredale, J. A. and Samadi, I. ‘Factors affecting the performance of heavy weft-knit fabrics’, Knitting World, 1976 5 20–23. 34. Raheel, M. and Lien, M. D. ‘The use of scanning electron microscopy for studying abrasion phenomena in laundered fabric’, Textile Chemist and Colorist, 1985 17 (5) 101–104. 35. Handu, J. L., Screenivas, K. and Rangonthan, S. R. Text. Res. J., 1967 37 (11) 997. 36. Buisson, Y. L., Rajasekaran, K. and French, A. D. ‘Qualitative and quantitative evaluation of cotton fabric damage by tumble drying’, Textile Res. J., 2000 70 (8) 739–743. 37. International Organization for Standardization (ISO 5085-1), Textiles – Determination of Thermal Resistance – Part 1: Low Thermal Resistance, first edition, 1989. 38. American Standards for Testing and Materials (ASTM, 2000), Standard Test Methods for Water Vapour Transmission of Materials, Designation: E96. 39. American Standards for Testing and Materials (ASTM, 1999), Standard Test Method for Surface Wettability and Absorbency of Sheeted Materials Using an Automated Contact Angle Tester, Designation: D5725-99, West Conshohocken, PA: ASTM. 40. Harnett, P. R. and Mehta, P. N. ‘A Survey and Comparison of Laboratory Test Methods for Measuring Wicking’, Textile Research Journal, 1984 54 (7) 471–478. 41. Kawabata, S., Postle, R. and Niwa, M., eds, ‘Objective specification of fabric quality, mechanical properties and performance’, Textile Machinery Society of Japan, Osaka, 1982. 42. American Standards for Testing and Materials (ASTM, 2001), Standard Test Method for Bursting Strength of Textiles – Constant-Rate-of-Traverse (CRT) Ball Burst Test, Designation: D3787-01, West Conshohocken, PA: ASTM. 43. American Association of Textile Chemists and Colorists (AATCC, 2003), Dimensional Changes of Garments after Home Laundering, Test Method: TM 150–2003.
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44. American Standards for Testing and Materials (ASTM, 2004), Standard practice for the evaluation of machine washable t-shirts, Designation: D6321-98, West Conshohocken, PA: ASTM. 45. American Association of Textile Chemists and Colorists (AATCC, 2004), Skewness Change in Fabric and Garment Twist Resulting from Automatic Home Laundering, Test Method: TM 179–2004. 46. Brand, R. H. ‘Measurement of fabric aesthetics: analysis of aesthetic components’, Textile Res. J., 1964 34 791–804. 47. Cardello, A. V., Winterhalter, C. and Schutz, H. G. ‘Predicting the handle and comfort of military clothing fabrics from sensory and instrumental data: development and application of new psychophysical methods’, Textile Research Journal, 2003 73 (3) 221–237. 48. Li, Y., Holcombe, B. V. and Apcar, F. ‘Moisture buffering behavior of hygroscopic fabric during wear’, Textile Res. J., 1992 62 (11) 619–627. 49. Sugai, K. and Nakajima, T. ‘Simultaneous comparison of clothing microclimates on right and left half bodies’, Sen I Gakkaishi, 1988 44 (4) 204–211. 50. Boslet, R. J. ‘Why cotton sells’, Textile Asia, 1989 20 (12) 161. 51. Byrne, M. S., Gardner, A. P. W. and Fritz, A. M. ‘Fibre types and end-uses: a perceptual study’, J. Text. Inst., 1993 84 (2) 275–288. 52. Chapman, R., ‘Weaving yarns ‘80’, Text. Month, 1980 March, 31–45. 53. Godehn, D. J. ‘Golden-touch polyester – a new dimension in polyester yarn’, Lenzinger Ber., 1979 47 (5) 151–158. 54. Laing, R. M. ‘Clothing, textiles and human performance: a critical review of the effect of human performance of clothing and textiles’, Textile Institute, 2002 32 (2) 104–122. 55. Korner, W., Blankenstein, G., Dorsch, P. and Reinehr, U. ‘Dunova, an absorbent synthetic fibre for high wear comfort’, Chemiefasern/Textilindustrie, 1979, 29/81 (6) 452–462, E62–65. 56. Vokac, Z. ‘Physiological responses and thermal, humidity, and comfort sensations in wear trials with cotton and polypropylene vests’, Text. Res. J., 1976 46 (1) 30–38. 57. Fuzek, J. F. ‘Some factors affecting the comfort assessment of knit knitted underwear’, Ind. Eng. Chem. Prod. Res. Dev., 1981 20 (2) 254–259. 58. Ghali, K., Ghaddar, N., Harathani, J. and Jones, B. ‘Experimental and numerical investigation of the effect of phase change materials on clothing during periodic ventilation’, Textile Res. J., 2004 74(3) 205–214. 59. American Standards for Testing and Materials (ASTM, 2004), Standard Table of Commercial Moisture Regains for Textile Fibres, Designation: D1909-04, West Conshohocken, PA: ASTM. 60. Quaynor, L., Takahashi, M. and Nakajima, M. ‘Effects of laundering on the surface properties and dimensional stability of plain knitted fabrics’, Textile Res. J., 2000 70 (1) 28–35. 61. Bresee, R. R., Annis, P. A. and Warnock, M. M. ‘Comparing actual fabric wear with laboratory abrasion and laundering’, Textile Chemist and Colorist, 1994 26 (1) 17–23. 62. McKinney, M. and Broome, E. R. ‘The effects of laundering on the performance of open-end and ring-spun yarns in jersey knit fabrics’, Text. Res. J., 1977 3 155–162. 63. Sridharan, V. ‘Ways to eliminate pilling’, Man-made Textiles in India, 1982 25(9) 445–447. 64. Vigo, T. L., Hassenboehler, C. B. and Wyatt, N. E. ‘Surface temperature measurements for estimating relative heat flow through textiles’, Textile Res. J., 1982 July 451–456.
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65. Friedenberg, E. D. ‘Starfish program helps Flynt hit shrinkage targets in cotton knits’, Knitting Times, 1993 62(12) 24–25. 66. Heap, S. A., Greenwood, P. F., Leah, R. D., Eaton, J. T., Stevens, J. C. and Keher, P. ‘Prediction of finished weight and shrinkage of cotton knits – the Starfish Project, Part I: Introduction and general overview’, Textiles Research Journal, 1983 53 (2) 109–119. 67. http://www.krystalartfield.com/Akwatek/Akwatek.html. 68. http://www.fabriclink.com/pk/akwadyne/home.html. 69. http://www.dupont.com/coolmax/why_coolmax.html. 70. http://www.fabriclink.com/pk/Coolmax/. 71. http://www.nike.com.cn/product/apparel/product.htm.
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Knitted structures for sound absorption R. M. MONARAGALA, Ministry of Defence, Sri Lanka Abstract: Textiles have the potential to be used in reducing the interior noise of an automobile as they can provide passive sound absorption in upholstery, headliners and other interior parts. Nonwovens have been used for this purpose and a fair amount of research has been done on their sound absorbency properties. However, they have an inferior aesthetic appearance and drapability compared with woven and knitted structures, which can provide a 3D seamless fabric and a pleasing appearance. Knitted fabrics can be further developed as dense and thick fabrics using spacer structures. These fabrics provide promising results on sound absorbency. Key words: interior noise reduction, textiles, Active Noise Control, knitted fabrics, automobile industry, spacer knitted fabric, noise absorption, construction industry, aerospace industry.
11.1 Introduction
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The automobile industry is the largest user of technical textiles, with about 20 kg in each of the 45 million or so cars made every year worldwide. Car interiors have become more important in recent years; people spend more time in their cars, commuting longer distances to work; they have more leisure time and a higher disposable income for days out to visit places of interest, friends and relations, as well as for trips to the supermarket, out of town shopping centres, and sporting venues. For those in business, the car can be a place of work, drivers being able to communicate with colleagues and customers by mobile phone when at rest. The car in fact has become an office, a living room and a motorised shopping trolley. From the point of view of the original equipment manufacturers (OEMs), changing the car interior design is an economical way to revamp a model that is not selling well. Consumer researchers in the USA believe that the car interior will soon become a focal point of brand recognition. Textile design and colour will inevitably be an essential tool in creating these distinctive interiors (Fung and Hardcastle, 2001), and sophisticated technical textiles may well be used to gain a reduction of automotive interior noise. This in turn will make the journey of the occupants more comfortable and may also have a significant impact on the automotive market.
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passenger and driver, making a long journey quite uncomfortable. Noise also contributes to driver fatigue, which is a major cause of road accidents; thus, reducing noise should make a significant contribution to road safety (Fung and Hardcastle, 2001). Two techniques are used to solve this problem, known as active and passive methods. Active methods use the phenomenon of wave interference (Kuo and Morgan, 1996, Nelson and Elliot, 1992) and can reduce noise in the 100 to 600 Hz region. Passive methods use acoustic materials to dampen the noise and may be utilised to reduce noise in the frequencies beyond 600 Hz (Kuo and Morgan, 1996). As textiles comprise a major part of the automotive interior and provide a low cost environmentally friendly material, it would be prudent to use them for noise reduction wherever possible. Research has been conducted on non-woven fibre webs in terms of their noise absorption properties (Shoshani and Yakubov, 2000a, 2000b, 2001). Commercial acoustic products have also been fabricated that are composed of non-woven fabrics; however, despite their promising noise absorption properties, it is difficult to produce a textured surface that gives an aesthetically pleasing appearance, and they are usually draped with a woven textile. Some work has been done on the noise absorption properties of woven structures (Shoshani and Rosenhouse, 1990, 1991) but this was based only on empirical investigation. The headliner, carpets, seats, door panels and other interior parts absorb much of the interior noise by passive means and it would be interesting to consider the use of knitted fabrics in all these areas. This chapter will investigate the noise absorption mechanism of knitted fabrics.
11.3 Sound absorption of plain knitted structures 11.3.1 Model of a plain knitted fabric as a porous material The unit cell of a plain knitted structure is the stitch, which is created by intermeshing yarn loops. The stitches are organised in rows (courses) and columns (wales); W and C represent the wales spacing and course spacing: see Fig. 11.1, where it can be seen that the void space of a unit cell may be approximated as a cylinder of which the circumference is bounded by the yarns forming the loop. As there is a uniform distribution of stitches forming the fabric, there exists a uniform array of circular cylinders in a unit area, whereby a plain knitted fabric can be modelled as a layer with identical cylindrical pores perpendicular to its surface.
11.3.2 Analytical prediction of the sound absorption of a plain knitted fabric Acoustic impedance of a unit cell The air inside each circular cylindrical pore may be considered as a fluid layer of finite thickness l, which is also the thickness of the fabric. Further, consider the
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11.1 Technical face image of a plain knitted structure (Dias and Monaragala, 2006).
fabric placed on a rigid impervious wall such as a metal frame with a onedimensional plane sound wave incident normal to the surface of the fluid as shown in Fig. 11.2 where A is a point on the surface of the fabric, B is a point inside the void of the unit cell near the fabric surface and C is a point on the fabric near the rigid wall. The acoustic impedance at B is given (by Zwikker and Kosten, 1949, Allard, 1993) as: 2 3 4 5 6 7 8 9 40 1 2 43X
[11.1]
where Zc is the characteristic impedance of the fluid layer (which in this case is the air inside the unit cell), and β is the wave number of the fluid. The characteristic impedance and the wave number are complex quantities, which are given by the following relationships (Allard, 1993):
[11.2]
[11.3]
where ρ and K denote the effective density and the bulk modulus of the air in the cylindrical pores respectively, and ω is the angular frequency of the air column. Zwikker and Kosten (1949) have modelled the air flow inside a circular cylinder of radius R as a laminar flow, and have derived relationships for ρ and K, by
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11.2 Sound absorption by a pore formed by the void of a unit cell, which is backed by a rigid wall (Dias and Monaragala, 2006).
treating the thermal exchange effects between the air and the walls of the cylinder, and the viscous effects of the laminar flow of air in the cylinder, as two separate issues. The derived formulae are given below,
[11.4] [11.5] and: ,
[11.6]
where ρ0 is the density of air, κ the thermal conductivity of air, B is the square root of the Prandtl number (Allard, 1993), j0 is the Bessel function of zero order, J1 the Bessel function of first order, P0 the atmospheric pressure, and η the viscosity of air.
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Acoustic impedance and sound absorption of the plain knitted fabric When sound energy is incident normal to the surface (Fig. 11.2), the flow of air is from the surface of the fabric to the pore. Thus the acoustic impedance at the surface of the fabric at point A can be given as (Allard, 1993), ,
[11.7]
where φ is the porosity of the material, that is, the ratio of the air volume (Va) to the total volume of the material (VT). The Noise Absorption Coefficient (NAC) of the fabric can be found by the use of the following relationship (Zwikker and Kosten, 1949, Allard, 1993):
[11.8]
The NAC gives the amount of energy from the incident sound wave absorbed by the knitted fabric; thus, to predict the sound absorbed by the knitted fabric, the pore radius of the void formed by the unit cell and the porosity of the fabric must be determined.
11.3.3 Pore radius Considering the void in a unit cell as a perfect circle of radius R, the pore radius can be taken as (Dias and Monaragala, 2006):
[11.9]
where l is the length of yarn in one loop (cm), d is the diameter of the yarn (cm), and s is the number of stitches per cm2 of fabric or stitch density defined as s = number of courses per cm (c) ⋅ number of wales per cm (w). Pierce (1947) defined the stitch (loop) length (l) in cm as: 1 2 3 4 5 6 7 8 9 40 1 2 43X
[11.10]
Thus, the pore radius of a single stitch of a plain knitted fabric can be calculated by using the stitch density, yarn diameter and stitch length of the fabric. Porosity Guidoin et al. (1987) defined porosity as the ratio of void space or volume of pores within the boundaries of a solid material to the total volume. The porosity or void fraction of a solid material is usually expressed as a percentage and is given as follows (Dias and Monaragala, 2006):
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where M is the mass per unit area of the fabric, t is the thickness of the fabric and ρm is the density of the fibres in the fabric. Thus, to obtain the porosity of a knitted fabric, the fibre density, the mass per unit area and the thickness of the material must be measured.
11.3.4 Measurement of noise absorption coefficient (NAC) The sound absorbency of the fabrics was measured with a two-microphone impedance tube according to the ISO 10534-2 standard. The system is shown in Fig. 11.3. The white noise signal required by the impedance tube is generated by Labview software in the PC, which is fed to the tube via a National Instruments M6259 Data Acquisition device (DAQ). The A and B microphone signals are fed to the PC via the M6259 DAQ device. The ACUPRO sound absorption software in the PC calculates the Noise Absorption Coefficient (NAC) from 100 to 4000 Hz (Dias and Monaragala, 2006). As described in the ISO 10534-2 standard and the impedance tube instructions, the NAC measurements were carried out on three identical samples taken from different regions of the fabric under test and their calculated average.
11.3 Sound absorption by a pore formed by the void of a unit cell, which is backed by a rigid wall (Dias and Monaragala, 2006).
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11.3.5 Plain knitted fabric samples The mathematical prediction was validated by Dias and Monaragala (2006). For this purpose plain knitted structures were produced from PE monofilament yarn with a density of 1.37 g/cm3 and yarn diameter of 0.2 mm (with a yarn count of 430 dtex). The specifications of the plain structure samples are given in Table 11.1. The fabric thickness was measured by using a thickness tester with a pressure of 100 kPa. The fabric samples were conditioned at atmospheric pressure, a temperature of 20 °C, and relative humidity of 63% for 48 hours. The pore area of a stitch and the stitch length were measured by means of a Projectina optical microscope, and PIA 4000 digital image analysing software. The stitch length was determined by measuring the length of yarn in 10 stitches. In practice, the voids in the interstices of a plain knitted fabric are not uniform when seen under a microscope, and there was a variation of 10% in the measured pore area. Therefore, the pore area was obtained by averaging 50 readings to get a more accurate value. The pore radius of the fabric was calculated using Equation 11.9. The porosity of a given fabric was obtained by first measuring the weights (in grams) of three square samples each having an area of 100 cm2. The weights were then averaged and divided by 100 to obtain the mass per unit area of the fabric in grams per cm2. This value and the measured thickness for the fabric were used in Equation 11.11 to obtain the porosity of the fabric. The results are given in Table 11.2. Table 11.1 Plain knitted structure sample details (Dias and Monaragala, 2006) Sample Courses Wales Calculated Measured % Error Measured Fabric number per cm per cm stitch stitch in stitch pore area thickness (mm) length (cm) length (cm) length data (mm2) A1 A2 A3
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10.8 10.23 4.92
8.8 4.52 4.33
0.441 0.559 0.78
0.461 0.538 0.856
4.5 3.8 9.7
0.147 0.427 1.437
0.6 0.6 0.6
Table 11.2 Pore radius and porosity data of the fabric samples (Dias and Monaragala, 2006) Sample number
Pore radius from calculated data (cm)
Pore radius % Error from measured data (cm)
Mass of 1 cm2 Porosity of fabric (g)
A1 A2 A3
0.019 0.041 0.070
0.022 15.789 0.037 9.846 0.068 2.967
0.031 0.018 0.013
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11.3.6 Simplification of mathematical analysis based on fabric data The calculated pore radius values from Table 11.2 are used in Equation 11.6. Figure 11.4 illustrates the variation of the dimensionless parameter ψ of Equation 11.6 with respect to frequency. Figure 11.4 indicates that the parameter ψ is greater than 1 for the fabric pore sizes given in Table 11.2, for the considered spectrum. As ψ ≥ 1, the following approximations,
and
, can be used in Equations 11.4
and 11.5. Therefore the effective density and bulk modulus of the air inside the pores in the plain knitted fabric can be simplified further (Allard, 1993, Zwikker and Kosten, 1949) to:
[11.12]
and: .
[11.13]
11.4 The variation of the dimensionless parameter ψ with the pore radius and frequency (Dias and Monaragala, 2006).
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These two simplified equations are then used in Equation 11.1 to obtain the acoustic impedance of the air inside the unit cell near the surface of the fabric. This simplified value is then used with Equations 11.7 and 11.8 to obtain the Noise Absorption Coefficient of the plain knitted fabric.
11.3.7 Study of the parameters of a plain knitted structure affecting its noise absorbency properties The following values have been used in Equations 11.6, 11.12 and 11.13. The values are based on the normal atmospheric conditions of 18 °C (temperature) and 1.1033 Pa (pressure) (Allard, 1993).
κ = 1.4 B2 = 0.71 P0 = 1.0132 · 105 Pa
ρ0 = 1.213kgm–3 η = 1.84 · 10–5poiseuille It can be seen from Tables 11.1 and 11.2 that a smaller stitch size results in a reduced pore size and porosity; there will be more yarn in a unit area and the fabric becomes denser as discussed by Guidoin et al. (1987). The next sections investigate the effect of pore size and thickness of a plain knitted fabric on the Noise Absorbent Coefficient. For this purpose the pore radius, porosity and fabric thickness data given in Table 11.2 is used in the analytical prediction described in Section 11.3.2. From these results it can be determined whether or not a thicker fabric with reduced porosity will yield higher noise absorbency. The analytical predictions are then validated by measuring the NAC of the same fabric samples, with the use of the sound absorbency measurement system described in Section 11.3.6. 2 3 4 5 6 7 8 9 40 1 2 43X
11.3.8 Different stitch sizes but same thickness The predicted data and the experimental results from the work done by Dias and Monaragala (2006) are shown in Fig. 11.5, which indicate that as the stitch sizes become smaller, the NAC increases. The impedance tube has a minimum threshold in measuring the NAC accurately; thus, as the measured structures are poor sound absorbers, there is a difference between the predicted and experimental values. The impedance tube acts as a resonant sound absorber and its sound absorption effect predominates. It may be noted that as the fabric sample is a poor sound absorber the experimental data gives resonance peaks at 3000 Hz, possibly due to a very minute layer of air between the fabric and the sample holder of the tube acting as
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11.5 NAC for plain knitted structures with the same thickness but different pore sizes (Dias and Monaragala, 2006).
a resonant cavity, with the pores in the fabric acting as a thin micro perforated panel (MPP). The Noise Absorption Coefficient of the fabric only increases beyond 1900 Hz; again this may be due to the fabric being a poor sound absorber. However, the profiles of the experimental and analytical data show that there is a gradual increase of the NAC with the progression of the spectrum from a low to a high frequency. To obtain a more accurate result the same experiment was repeated with increased fabric thicknesses. For this purpose four laminated layers of A1 and A3 fabrics were assembled and designated as A1(4) and A3(4) respectively, the total thickness being 25 mm (Fig. 11.6). The pores between successive layers were aligned as far as possible. The predicted and experimental data is shown in Fig. 11.7, the NAC improving with this increased thickness. The NAC increases for reduced pore size and porosity. As the sound absorption of the structure improved, there was a reasonable agreement between the predicted and the measured data with the NAC increasing from 1 000 Hz. There is very little sound absorption lower than 1 000 Hz, and the resonance at 3 000 Hz is also reduced. This may be because the thick fabric changes the properties of the resonant cavities.
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11.6 Simulated cross-sectional representation of the knitted fabric samples used for the test (Dias and Monaragala, 2006). (a) Four layers of A1 fabric designated as A1(4); (b) Four layers of A3 fabric designated as A3(4).
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11.3.9 The effect of thickness The NAC variation has been predicted in a similar manner by Dias and Monaragala (2006) for different thicknesses but for the same pore radius and porosity. The fabric sample A1 was chosen as it has the best noise absorption performance as seen in the preceding sections. The predicted and experimental data for three different thicknesses is given in Fig. 11.8. The fabric A1(4) is composed of four layers and A1(5) is composed of five layers of A1 fabric (Fig. 11.9 to 11.11). The pores of successive layers were assumed to be in line for the mathematical prediction. It can be seen from both the analytical prediction and the experimental results that as the thickness of the structure increases, there is an increase in NAC. Moreover, the experimental results show that the NAC gradually increases with frequency from 1 000 Hz. As before, when the thickness of the fabric is increased, the sound absorption of the fabric improves and there is reasonable agreement
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11.7 NAC of two knitted fabrics with different pore sizes with each fabric sample consisting of four layers giving a total thickness of 2.5 mm (Dias and Monaragala, 2006).
11.8 NAC data for different thicknesses of plain structure A1 (Dias and Monaragala, 2006).
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11.9 Simulated cross-sectional representation of a single layer of the A1 fabric with a thickness of 0.6 mm, designated as A1(1) (Dias and Monaragala, 2006).
11.10 Simulated cross-sectional representation of four layers of the A1 fabric with a total 2.5 mm thickness, designated as A1(4) (Dias and Monaragala, 2006).
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11.11 Simulated cross-sectional representation of five layers of the A1 fabric with a total 3.1 mm thickness, designated as A1(5) (Dias, 2006).
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between the experimental and predicted data. Furthermore, the resonance at 3 000 Hz of the experimental data is again reduced. It was observed that there is only a slight effect on the lower frequency noise levels. Therefore, apart from considering active noise control, it would be interesting to investigate the sound absorption properties in this region of composite knitted structures compared to foam or non-woven fibre webs.
11.3.10 Analysis Knitted structures with smaller pore sizes and a reduced porosity have good noise absorption. Thus a thicker and denser knitted fabric should have better sound absorbent properties. The analytical model is in reasonable agreement with the experimental data. The NAC values of the experimental data correlate well with the predicted values when the fabric thickness is increased. The differences could be because: • the analytical model considers the pores in the fabric to be a uniform array of cylinders, but in practice they are not uniform and are not true cylinders; • the accuracy of the measurements in the impedance tube at this low NAC level is poor. However, it is evident from the predicted and experimental data that the knitted structures when placed against an impervious solid backing become effective sound absorbers when the frequencies are above 1 000 Hz and would be suitable for reducing the higher frequency noise levels from wind and road noise in a vehicle.
11.4 Engineering advanced knitted fabrics for sound absorption The sound absorption of a single layer of a plain knitted fabric is poor, thus to increase its noise absorption properties the structure has to be made thicker and denser (i.e. the stitch size must be smaller); even then noise absorption is only achieved at the extreme end of the 100 to 4 000 Hz range. However, other knitted structures such as rib, interlock, textured and ripple structures will make the fabric thicker and improve the noise absorption at the higher end of the spectrum. Because basic knitted structures alone may not provide sufficient sound absorption for use in automobile interiors, the sound absorbency of two advanced knitted fabrics known as thick spacer and dense spacer structures will be discussed.
11.5 Thick spacer structures Thick spacer structures can be used in the headliner, parcel shelf, firewall and the underside of the hood of the automobile, to reduce interior noise. Although porous sound absorbers comprising polyurethane and foam are used for this purpose (Diehl,
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1998), they tend to be expensive, environmentally unsound, bulky and heavy (Kuo and Morgan, 1996). Thick spacer structures are lighter and less expensive, and their thickness remains unaltered by moisture and compression, unlike porous sound absorbing materials, including both woven and non-woven structures. Thick spacer structures can be manufactured with ease to suit the design and brand required by the OEM by using modern knitting machines and knitting techniques. The limitations of thick spacer knitted structures are that they are less flexible than conventional woven and knitted structures and have less drapability, and. as they operate in the form of a wide band resonant sound absorber, they can be used only in an airtight cavity to achieve optimum performance. Hence they may not be suitable for use in the vehicle upholstery. A spacer structure consists of two layers of knitted structure, interconnected but spaced apart by the use of a monofilament yarn (Fig. 11.12). The front and back fabric layers are plain knitted structures knitted using double covered elastomeric yarn. Because of the nylon cover of the inner elastomeric yarn and the elastic nature of the core yarn, the void in a unit cell of a plain knitted structure is essentially closed, i.e. the stitches in the structure are in the jamming state (Fig. 11.13), thus, the front and back layers of the spacer may be considered as a fabric sheet, without any pores. However, the locations where the monofilament yarn is tuck looped into the plain knitted layers create a uniform pattern of micropores as shown in Fig. 11.14. Hence the front and back faces of a spacer structure made of double covered elastomeric yarn can be considered as a fabric sheet with a uniform array of micropores (Dias et al., 2007c).
11.5.1 Spacer structure as a MPP with an air cavity In order to model the thick spacer structure as an Micro Porous Panel (MPP) the space between the front and back layers of the spacer structure is considered as a
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11.12 Knitted spacer fabric structure (Dias, 2007b).
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11.13 Plain knitted structure made from elastomeric yarn, viewed by an electron microscope (Dias et al., 2007c).
11.14 Spacer fabric with plain knitted front layer made of double covered elastomeric yarn (Dias et al., 2007c).
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void layer without taking into consideration the volume of the monofilament yarn joining the two layers. Thus the void is essentially that of a rectangular cubic, and volume calculation is straightforward. However if the volume of the monofilament yarns connecting the front and back layers of the structure is considered, the volume of the void (i.e. the volume of rectangular cubic-volume of the monofilament yarns) would be complicated hence the yarn volume is neglected. This point has also been indicated in the respective reference of this part of the chapter.
11.5.2 Plain knitted and two-yarn Jacquard knitted spacer structures with a lattice structure of diagonal pores The front and back faces of three spacer structures (A, B and C) were knitted using double covered elastomeric yarn with a yarn count of 972 dtex. The structure was knitted on a 7 gauge Shima Seiki 122 S knitting machine. The front and back layers of the plain knitted spacer structures were interconnected with 0.2 mm diameter polyester 430 dtex monofilament yarn (A and B) and 0.2 mm diameter polyester 380 dtex monofilament yarn (C). The yarn path notations of the spacer structures A, C and B are given in Fig. 11.16 and 11.17 respectively. The front and back layers of a plain Jacquard spacer structure were created as a two yarn Jacquard knitted structure. The front and back faces of this structure were interconnected with a 0.2 mm diameter 380 dtex polyester monofilament yarn. The structure was designated as structure D and its yarn path notation is shown in Fig. 11.18. Structure E has been knitted according to the yarn path notation given in Fig. 11.18: this structure has a. 0.2 mm diameter 430 dtex polyester monofilament interconnecting yarn. The yarn path notation of structure F is given in Fig. 11.19. The yarns used in this structure were the same as in structure E. The micropores required for absorption are formed when the monofilament yarn is tuck looped on to the plain knitted layer of the structure. Two consecutive tuck loops on a face of the structure are offset by one needle position of the
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11.15 Spacer structure in a metal enclosure (Dias et al., 2007c). © Woodhead Publishing Limited, 2011
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11.16 Yarn path notation of fabric A and C (Dias et al., 2007c).
11.17 Yarn path notation of fabric B (Dias et al., 2007c).
machine. Figure 11.16 has been used to reason out the appearance of micro pores of the fabric due to tuck loops to be diagonal as seen in Fig. 11.4. More details of this point are available in the relevant reference (thesis). The thickness between the front and back surface can be increased by adjusting the distance between two consecutive tuck loops of the monofilament yarn on the front and back faces of the structure (Fig. 11.16).
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11.18 Yarn path notation of fabric D and E (Dias et al., 2007c).
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11.19 Yarn path notation of fabric F (Dias et al., 2007c).
11.6 Dense spacer structures Dense spacer structures have a thickness in the region of 1 cm and possess reasonable sound absorption properties; they are light weight, flexible, can be knitted with any design to suit the OEM and automotive brand requirements, and
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can be used in the automotive upholstery, parcel shelf, headliner and the door panels, to provide both comfort and noise reduction to the automotive interior. The structure is superior to other textile structures used in automobiles in terms of flexibility and ease of manufacture; although its drapability is lower than that of a simple knitted fabric. The dense spacer structure consists of top and bottom plain knitted layers. These two layers are interconnected with a mesh of yarn and can be represented as a tight mesh of yarn sandwiched between two plain knitted layers. The interconnecting mesh of yarn is oriented at an angle between the top and bottom layers (Figs. 11.20, 11.21). The pictorial view of a dense spacer structure is shown in Fig. 11.21. Figure 11.21 shows that the dense spacer structure can be fabricated as a single layer of thick fabric as compared to laminated layers of plain knitted fabrics and their derivatives.
11.6.1 Application of the structures From the sound absorbency data given in Figs. 11.22 and 11.23 (Dias et al., 2007d) it can be observed that structure TS3 has the optimum sound absorbency. However, this structure has the highest density apart from TS5. For a single layer
11.20 Structural representation of the cross section of a dense spacer structure (Dias et al., 2007d).
11.21 Pictorial view of a dense spacer structure (Dias et al., 2007d).
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11.22 Sound absorbency of group A (Dias et al., 2007d).
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of this structure its sound absorbency is greater than 0.5 beyond 2 000 Hz. Consequently, this structure would be suitable for dampening the higher frequencies of automotive interior noise. Noise at this range (‘wind noise’) occurs when travelling at high speeds (especially when the door seals are poor) (Peng and Morrey, 1998). The interior of buses has significant high frequency components beyond 2 000 Hz as was found by a study by Brocklehurst (1992) (due to wind noise). Thus it has been suggested that TS3 can be used for interior noise reduction (high frequency) in city buses. The structure TS5 is suitable for reducing automotive interior noise below 2 000 Hz. Several layers of this structure may significantly dampen frequencies below 1 000 Hz. Dense spacer structures can be knitted with a greater density resulting in better sound absorbency than TS3. This would improve the sound absorbency from 1 000 to 2 000 Hz. Several layers of TS3 structure can be layered together to improve sound absorbency in this range. Moreover, a combination of structure TS3 with thick spacer structures (Dias et al., 2007b) may reduce noise intensity below 1 000 Hz. However, to effectively reduce any engine related noise in the range below 100 Hz, a fabric speaker-based active noise control system should be investigated (Dias et al., 2007a).
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11.23 Sound absorbency of group B (Dias et al., 2007d).
The advantage of using these structures is that they can be seamlessly integrated with automotive upholstery by means of the currently available advanced knitting technology in any design to suit the OEM brand. Furthermore, they do not deteriorate through contact with moisture or alter their sound absorbency, as may be the case with conventional porous sound absorbers.
11.7 Conclusion It was proved that the sound absorbency of these structures increases with both airflow resistivity and thickness. Their porosity is more or less inversely proportional to their airflow resistivity; therefore, their sound absorbency increases with decrease in porosity and increased density. However, the effect of density is more predominant in terms of sound absorbency than thickness. The structures can be made denser by having more rows of the interconnecting yarn between plain knitted courses of the front and back beds. The structure is a suitable alternative to utilising several layers of plain knitted fabrics for achieving better sound absorbency, and may be used efficiently in automobile upholstery to reduce interior noise. The sound absorbency of the dense spacer structure is effective only from 2 000 Hz onwards when its Noise Absorption Coefficient (NAC) is greater than
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50%. Therefore future work on these structures will be directed at the improvement of their sound absorbency in the region lower than 2 000 Hz.
11.8 Future trends Textiles can be used for Active Noise Control (ANC) to reduce automotive interior noise from 100 to 600 Hz. For this purpose the structure may have to be designed as an actuator, which can produce acoustic pressure in the range 100 to 600 Hz. Initial work has been done by Dias et al. (2007a, b). In their work knitted elastomeric fabrics were laminated with polyvinyl fluoride (PVDF) strips and formed on to an arch. These strips vibrate due to the piezoelectric effect when an electric force stimulates them. Acoustic responses have been analysed of PVDF strips laminated with rib and interlock structures. It was reported that the resonance region of the acoustic response where maximum Sound Propagation Level (SPL) from the fabric is emitted depends upon: • the radius of curvature of the arch • the density of the base fabric.
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However, the SPL at the frequency range of interest for ANC is low as this region is in the ascending portion of the acoustic response of the fabric, but this small SPL of the arch may be sufficient to produce the desired quiet zone (Kuo and Morgan, 1996, and McDonald et al., 1991; experiments have shown that this region is relatively large in comparison to the human ear spacing for frequencies less than 200 Hz) in an Active Noise Control system, if the arch is placed in close proximity to the ears of both driver and passengers. This may be done with the aid of the headrests on each seat. Textiles may be actuated with the use of micro motors to vibrate as a woofer. Moreover, Electro Active Polymers (EAP) (Bar-Cohen, 2001) and especially Ionic Polymer Metal composites (IPMC) (Calvert, 2001) may be integrated with textiles to actuate them. However, at present these materials are available only as strips. The stretchability of the knitted structure may be used as a force amplifier to enhance the limited force generated by these materials.
11.9 Sources of further information and advice Further information with regard to the design of advanced knitted fabrics for acoustics and their sound absorbency is available from the William Lee Innovation Centre knowledge bank. Moreover, further information regarding the knitting techniques of the spacer fabrics discussed in this chapter can be obtained from the editor, Dr Tilak Dias of the University of Manchester. Detailed information on the mathematical analysis and experimental data on the sound absorbency tests discussed in this Chapter can be obtained from the PhD thesis entitled, ‘Study of
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developing advanced knitted structures for automotive applications, 2007’, compiled by the author, available from the University of Manchester Main Library.
11.10 References Allard J. F. (1993), Propagation of Sound in Porous Media, Modelling Sound Absorbing Materials, Elsevier Applied Science, London and New York. Bar-Cohen Y. (2001), Electroactive Polymer (EAP) Actuators as Artificial Muscles – Reality, Potential and Challenges (ed. Bar-Cohen Y.), International Society for Optical Engineering (SPIE), Bellingham, Washington, 4–44. Brocklehurst P. (1992), Bus’92 The Expanding Role of Buses towards the Twenty-First Century, Mechanical Engineering Publications Limited, London. Calvert P. (2001), Electroactive Polymer (EAP) Actuators as Artificial Muscles – Reality, Potential and Challenges (ed. Bar-Cohen Y.), International Society for Optical Engineering (SPIE), Bellingham, Washington, 133. Dhiel, G. M. (1998), Noise measurement and control, in Myer, K. (Ed.), Mechanical Engineers’ Handbook, Materials and Mechanical Design, John Wiley and Sons, New York. Dias T. and Monaragala R. (2006), Sound absorption in knitted structures for interior noise reduction in automobiles, Measurement Science and Technology, 17, 2499–2505. Dias T., Monaragala R. M. and Soleimani M. (2007a), Acoustic response of curved active PVDF-paper/fabric speaker for Active Noise Control of automotive interior noise, Measurement Science and Technology, 18, 1521–1532. Dias T., Monaragala R. M. and Soleimani M. (2007b), Analysis of active PVDF-paper/ fabric speaker for Active Noise Control, Journal of Information and Computing, 2 (2), 137–144. Dias T., Monaragala R. M. and Lay E. (2007c), Analysis of thick spacer fabrics to reduce automobile interior noise, Measurement Science and Technology, 18, 1979–1991. Dias T., Monaragala R. M. and Lay E. (2007d), Analysis of sound absorption of tuck spacer fabrics to reduce automotive noise-accepted for publication, Measurement Science and Technology, 18, 2657–2666. Fung W. and Hardcastle M. (2001), Textiles in Automotive Engineering, Woodhead Publishing, Cambridge, UK. Guidoin R., King M., Marceau D. and Cardou A. (1987), Textile arterial prostheses: is water permeability equivalent to porosity? Journal of Biomedical Materials Research, 21, 65–67. Kuo S. M. and Morgan D. R. (1996), Active Noise Control Systems: Algorithms and DSP Implementations, John Wiley and Sons, Chichester. McDonald A. M., Elliot S. J. and Stokes M. A. (1991), Active Noise and Vibration Control within the Automobile, Proceedings of International Symposium on Active Control of Sound and Vibration, 147–156. Nelson P. A. and Elliot S. J. (1992), Active Control of Sound, Academic Press, London, UK. Peng C. and Morrey D. (1998), An investigation into the effect of door seals on noise generated in the passenger compartment. European Conference for vehicle noise and vibration, Professional Engineering Publishing Limited, London, UK. Pierce F. T. (1947), Geometrical principles applicable to the design of functional fabrics, Textile Research Journal, 17, 144–147.
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Shoshani Y. and Rosenhouse G. (1990), Noise absorption by woven fabrics, Applied Acoustics, 30, 321–333. Shoshani Y. (1991), Noise absorption by combinations of woven and nonwoven fabrics, Journal of Textile Institute, 82, 500–503. Shoshani Y. and Yakubov Y. (2000a), Generalization of Zwikker and Kosten Theory for sound absorption in porous materials with variable parameters, Journal of Computational Acoustics, 8, 415–441. Shoshani Y. and Yakubov Y. (2000b), Numerical assessment of maximal absorption coefficients for non-woven fiber webs, Applied Acoustics, 59, 77–87. Shoshani Y. and Yakubov Y. (2001), Use of nonwovens of variable porosity as noise control elements, Nonwovens Research Journal, 10, 23–28. Zwikker C. and Kosten C. W. (1949), Sound Absorbing Materials, Elsevier Publishing Company.
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12 Weft-knitted structures for moisture management G. B. DELKUMBUREWATTE, Open University of Sri Lanka, Sri Lanka Abstract: The absorbency and wicking properties of knitted structures contribute to the wearing comfort of a garment. The hydrophobic or hydrophilic nature of a material depends mainly on the chemical groups present in it. However, absorbency and the wicking property in a textile structure depend on its capillary radius and the viscosity of water. The contact angle and the surface tension between the material and the water also contribute to absorbency and the wicking property. Water absorbency by a knitted spacer structure can be established by measuring the absorbency rate, total absorbency, and the time taken to saturate the structure using a gravimetric absorbency tester. A theoretical model can be developed to estimate water take-up by the structure considering its geometrical parameters. Key words: moisture management, hydrophilic, hydrophobic, wetting, contact angle, wicking, absorbency, capillary, liquid take-up, spacer fabrics, model.
12.1 Introduction With the advancement of technology, moisture management, absorbency and liquid transport in textile structures are becoming more important in clothing, medical and technical applications. In the case of clothing, the absorbency properties of knitted structures are very important in the design of garments that both remove perspiration from the skin and provide tactile and sensorial comfort for the wearer. Generally the removal of moisture also helps to reduce heat stress while evaporation provides a major source of cooling to the body. Because evaporation is not possible in an entirely enclosed protective clothing environment such as CBRN (Chemical, Biological, Radiological or Nuclear) or firefighter clothing, it is important to remove perspiration from the skin in order to maintain tactile and sensorial comfort for the wearer. In the area of technical textiles for water transport, the water retention, absorbency and drying properties of the textiles need to be considered before use; in particular, filtration and drainage are two major functions of geo-textiles where absorbency and liquid take-up are of great importance. In the case of medical textiles, control of the moisture or liquid in the structure is necessary as the skin is very sensitive to wet conditions.
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The importance of moisture and liquid management in textile structures has resulted in better textile fibres and structures being developed and the behaviour of water in contact with them being studied. In this chapter we are going to consider the essential theoretical aspects of the hydrophilic and hydrophobic characteristics of materials, the wetting of solid surfaces by liquids, the contact angle and surface tension at the liquid/solid interface, and the wicking and absorbency of liquid into porous structures. The engineering of knitted structures for better absorbency characteristics is also discussed, as well as the results of experimental investigations of absorbency characteristics and theoretical study.
12.2 Basics of wetting 12.2.1 Hydrophilic and hydrophobic nature of fibrous materials
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Water molecules are bound to fibre molecules in different ways. Some are bound without having freedom to move, whereas in other fibres the water molecules are loosely bound and are able to move. The way water molecules attach to fibre molecules depends on the chemical structure of the fibre molecules and it is well known that natural fibres absorb more water than chemical fibres. However, manmade regenerated fibres absorb more water than chemical fibres. This behaviour can be explained by studying the chemical and physical structure of the fibres. Water molecules can be present on the fibre surface or inside the fibres by penetrating the pores of the fibres. However, many of the water molecules are attached to the surface of the fibre. If the water molecules are attracted to the molecules of a fibre, this condition is known as wetting. Some fibre molecules have chemical groups with properties that attract water molecules, making hydrogen bonds between them. The chemical groups that attract water are known as hydrophilic groups; chemical groups that repel water molecules are known as hydrophobic groups. According to Morton and Hearle (Morton and Hearle, 1997), animal fibres, vegetable fibres, and man-made fibres based on natural materials include hydrophilic groups and as a result attract water. For example, cellulose molecules in vegetable fibres contain hydroxyl groups (–OH), which form hydrogen bonds with water molecules. Cellulose fibres such as cotton flax and jute contain many hydroxyl groups and therefore they absorb water. Protein fibres such as wool have amide groups (-NH-) in their main polymer chain and these groups are capable of attracting water molecules to make hydrogen bonds. Wool fibres also contain other water attracting groups such as –OH, -NH 3+, -COO –, -CO.NH 2 in their side chain (Morton and Hearle, 1997) and can absorb considerable amounts of water. Although silk is a protein fibre, it contains only a few active groups that attract water and therefore silk is less absorbent.
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Synthetic fibres have low water absorbency due to the chemical groups in the main polymer chain and in the side chains. For example, the polyamide fibres Nylon 6.6 and Nylon 6 contain only one amide (-NH-) group for every six carbon atoms in the main polymer chain. Also, polyester fibres such as polyethylene terapthalate, contain groups such as -CH 2-, -CO.O-, and benzene rings in the polymer chain, which are not hydrophilic. The main polymer chain of polyethylene, polypropylene and other vinyl polymer fibres is composed of –CH 2-. Polypropylene has an additional -CH 3 group whereas other vinyl polymers have a substitution of –Cl or -O.CO.CH 3 for some of the hydrogen atoms. Acrylic polymer fibres such as PAN (polyacrylonitrile) absorb slightly more moisture than the vinyl polymer fibres because they contain CN groups and other groups from their minor constituents. Polyvinyl alcohol (PVA) fibres contain hydroxyl groups and therefore absorb more moisture than vinyl fibres (Morton and Hearle, 1997). The water molecules bound directly to the hydrophilic groups of the fibre molecules have less freedom to move; this water is known as directly bound water, and does not show the physical properties of free water or bulk water (Berlin, 1981). Two characteristics of bound water are that it cannot be active as a solvent for other compounds, and that it will not freeze at low temperatures (Labuza and Busk, 1979). However, there is no exact definition for bound water as these characteristics vary in different situations due to the complexities of the binding forces involved. In addition, water molecules are attracted by other weaker hydrophilic groups or by the water molecules that are already bound to the fibre and will produce further layers on top of the already bound water molecules. The moisture absorption theory based on these two types of water attached to fibres was first introduced in 1929 by Pierce (Pierce, 1929). Figure 12.1 illustrates the direct and indirect bound water molecules on a fibre surface. After the absorption of water, fibres tend to change in volume, shape and other physical properties. If the fibre contains non-crystalline or amorphous regions, then the water molecules penetrate the non-crystalline regions causing limited swelling. The fibrils in the dry cotton fibre are bonded together; when fractures occur across the fibre due to complex stresses this bonding leads to early breakdown of fibrils resulting in lower fibre strength. When the fibres are wet, the
12.1 Direct and indirect attached water (Morton and Hearle, 1997).
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fibrils are free to move and can relieve stresses leading to higher fibre strength. The relative movement of fibrils modifies the extension behaviour of wet cotton fibre (Morton and Hearle, 1997). In the case of Tencel, the cellulose crystals are parallel to the longitudinal direction of the fibre, which contributes to high tensile strength in the dry state. A structure of parallel cellulose crystals leads to a tendency for a high degree of fibrillation during abrasion in the wet state (Mukhopadhyay, 1992).
12.2.2 Wetting, contact angle and surface tension Wetting is a term used to describe the affinity of a liquid to a solid or to the displacement of a solid–vapour interface with a solid–liquid interface. Therefore the wetting of a fibre can be referred to as the replacement of a fibre–air interface with a new fibre–liquid interface (Kissa, 1996). The terms complete wetting, partial wetting and non-wetting describe the intensity of the affinity of a liquid to a solid. The degree of wetting is measured in terms of the contact angle, which is the angle formed by a drop of liquid placed on a solid surface (or a liquid–vapour interface) measured from the side of the liquid. Fig. 12.2 shows two different contact angles, high and low, from two different liquids placed on a solid. If a liquid spreads over the surface of a solid it is said to ‘wet’ the solid and if it does not form a contact angle, as shown in Fig. 12.2c, the state is known as complete wetting (de Gennes, 1985). If a liquid is unable to spread and forms a bead on the surface of the solid as shown in Fig. 12.2b it can be termed as ‘non-wetting’. A low contact angle, between 0° and 90°, means the liquid wets the solid surface but it is termed partial wetting. A high contact angle, between 90° and 180°, means the liquid does not wet the solid surface (Kissa, 1996). The wetting characteristics, ‘wetting’, ‘non-wetting’ and ‘partial wetting’, depend on the cohesive energy of the solid and the liquid. In terms of the cohesive energy, a solid can be grouped into two groups, namely ‘hard solid’ and ‘molecular crystals’ (de Gennes, 1998). Hard solids have covalent, ionic or metallic bonds, very high-energy surfaces and surface tensions higher than 500erg/cm2. Weak molecular crystals have low energy surfaces with surface tensions around 50erg/cm2 and are bonded with van der Waals (VW) forces or hydrogen bonds. A molecular liquid such as water
12.2 Low and high contact angles.
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achieves complete wetting with surfaces of a hard solid, which have higher energy surfaces, but achieves only partial wetting with weaker molecular crystals with low energy surfaces. There is a change in the contact angle depending on the surface tension of the solid and the cohesive force of the liquid. The relationship between contact angle and the interfacial tensions at a solid liquid boundary is given by the Young– Dupré equation when the forces are in equilibrium (Miller, 1985) as:
γ SV – γ SL – γ LV cos θ = 0
[12.1]
where γ SV, γ SL and γ LV denote interfacial tension between solid–vapour, solid– liquid and liquid–vapour respectively and θ is the equilibrium contact angle. Table 12.1 shows the critical surface tension of some solid polymers, which a re used to extrude man-made filaments used for making textile structures. It also shows that nylon (polyamide) has a higher surface tension than some other polymers: Table 12.1 Critical surface tension γc of some solid polymers (de Gennes, 1998) Solid γ c (mN/m)
Nylon 46
PVC 39
PE 31
PVF 28
PTFE 18
In general the chemical structure of both the solid and liquid affect the contact angle and the wetting behaviour of the solid surface by the liquid (de Gennes, 1998).
12.3 Wicking and absorption 12.3.1 Wicking and capillary action Wicking is defined as the spontaneous flow of a liquid into a fibrous structure, and is driven by capillary forces in the absence of any external forces. The capillary forces are caused by wetting, and therefore wicking can be also explained as spontaneous wetting in a capillary system (Kissa, 1996). A fibrous assembly, such as a knitted structure, is a porous medium considered to be packed with parallel capillaries. The capillary force driving the liquid into the capillary is a function of the surface tension of the liquid-gas interface, the contact angle and the size of the capillary opening. The driving force for the capillary action can be expressed by: capillary force = 2 π r γ LG cosθ,
[12.2]
where r is the radius of the capillary opening, γ LG is the interfacial tension between liquid and solid and θ is the contact angle. When a liquid comes into contact with a porous structure, it will be driven along the capillaries due to the capillary forces. In effect, liquids ‘wick’ into porous structures and wicking of liquid into a fibre assembly is one aspect of absorption.
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However, absorbency is defined as the ability of a textile structure to absorb water, the first aspect of which is the wetting ability of the structure, and the second being the wicking due to capillary action. Liquid absorption into a porous textile structure is dependent on the properties of the liquid such as viscosity, density and surface tension, fibre surface wetting characteristics and the geometric parameters of the fibre assembly such as thickness of the structure, porosity and pore size or capillary radii. Most of the theoretical studies of wetting, wicking and absorption are based on the assumption that textile structures are bundles of capillaries packed together. Assuming that these capillaries are ideal tubes of uniform dimension, a simple capillary tube flow model can be applied; accordingly, the basis of absorbency is the wetting and wicking dynamic along the capillaries. The capillary force is due to surface tension, and wicking takes place along the capillary until equilibrium is reached. However, depending on the contact angle, two different types of meniscus are formed in the capillary, as shown in Fig. 12.3. In the case of contact angles of less than 90°, a concave meniscus is formed and for contact angles of more than 90° a convex meniscus is formed.
12.3.2 Wicking dynamics The surface tension of the liquid causes a pressure difference (∆P) across the liquid, giving it a curved concave shape, and creating a driving force to the liquid. This pressure difference (∆P) is given by the Laplace Equation (de Gennes, 1985) (Miller and Tyomkin, 1984):
where:
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12.3 Formation of the meniscus shape within the capillary.
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As the liquid in the capillary rises, an opposing pressure head (∆Ph) is created by the weight of the risen liquid, which is a function of the density of the liquid (δ), gravitational force (g), and the height of liquid rise. [12.4] Therefore, the actual pressure gradient (∆P) lessens with the increase of water level in terms of: [12.5] Liquid flow into a porous structure is dependent on several factors such as the wettability of the fibre, the physical capillary radius, and the density and viscosity of the liquid. The volumetric liquid flow through a textile structure can be determined by employing Hagen–Poiseuille’s law for laminar flow (Yoo and Barker, 2004): [12.6] where r is the radius of the capillary, ∆P is the net pressure gradient, L is the length of liquid rise and η is the viscosity of the liquid. Combining Equations 12.5 and 12.6: [12.7] Lucas and Washburn developed an equation based on Hagen–Poiseuille’s equation, considering dV = dLπ r2 for linear flow rate (dL/dt) in equilibrium (Washburn, 1921): [12.8] Taking into account the influence of gravity due to risen liquid, the flow rate becomes: [12.9] After integration, the Lucas–Washburn Equation 12.8, which is the equation without consideration of the gravitational force, can be written as: [12.10] which can be simplified to: where the rate constant K is given by:
[12.11]
[12.12]
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The experimental work of Washburn (1921), also showed that the penetration of liquid into a cylindrical capillary is proportional to the square root of time, a relationship known as Lucas–Washburn kinetics. However, Laughlin suggested that the time exponent is less than 0.5 (Laughlin and Davies, 1961) and modified the equation to: L = C t k
[12.13]
where k is the time exponent and C is a constant. According to Equation 12.11, the height of the water will rise continuously with time but in actual practice the water column will cease to rise after a certain period, when the surface tension is equal to the weight of the water column. From Equation 12.13 we have: ln(L) = ln(C) + Kln(t)
[12.14]
Laughlin plotted this and obtained a straight line. In the above equations, assumptions are made that the contact angle is constant with the rise of the water level, that the moment of inertia of moving water is negligible, and that the effect of gravity is minimal. These assumptions provide the basis of controversy over the Washburn equation. However, experimental results show that the contact angle varies with the increase of the water level (Fisher, 1979; Joos et al., 1990), the influence of the moment of inertia (Jeje, 1979), and the effect of gravity (Zhong et al., 2002). However, in experiments with various surfactant solutions, Hodgson and Berg found that such liquids obey the Washburn theory despite the limitations mentioned above (Hodgson and Berg, 1988). Consequently, Washburn wicking kinetics is still widely accepted. Several researchers (Good and Lin, 1976; Joos et al., 1990; Marmur, 1997) tried to accommodate the effect of gravity into the Lucas–Washburn theory. The theory developed by Landau gives the rate of liquid rise after considering gravity and the angle of capillary to the vertical as: 1 2 3 4 5 6 7 8 9 40 1 2 43X
[12.15] where r is capillary radius, γ is surface tension, ζ is liquid density, g is gravity, β is the angle between the capillary tube axis and the vertical direction, µ is the viscosity of the liquid, and h is the distance travelled by the liquid measured from the reservoir along the tube axis (Landau and Lifshitz, 1988). However, he made the decision that Equation 12.15 is a non-linear ordinary differential equation and can be solved only by ignoring the parameter
. This means
ignoring the gravitational force for horizontal liquid penetration or assuming that r is very small. The equation after integration for the distance travelled by liquid
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as a function of time then becomes h =
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, which is the same as the
Lucas–Washburn equation [12.11]. Joos et al. investigated experimental changes in contact angle and proved that they are dynamic and decrease as the liquid rises (Joos et al., 1990). They proved that the dynamic contact angle (cosθd) changes with the surface tension (σ), viscosity (η) and the velocity of the meniscus (υ) as below: [12.16] where θ 0 is the static advancing contact angle. Marmur and Cohen (1997) studied the kinetics of vertical liquid penetration into a capillary which initially contains a fluid of negligible viscosity (usually gas) by considering gravity, and gives the equation for the liquid rise (h) with the time (t) as: At = –Bh – ln (1 – Bh) where A =
and B =
[12.17] , where ρ is the density difference
between the liquid and the gas, σ is the surface tension, µ is the viscosity, cos θ is the contact angle and g is gravity (Marmur and Cohen, 1997). Equation 12.7 shows that the rate of liquid uptake depends upon the surface tension, density and viscosity of the liquid, the capillary radius, and the contact angle. For a given liquid, the density, viscosity and surface tension are constant and the contact angle (θ) and pore radius (r) are variables. A theoretical model based on Hagen–Poiseulle’s law was created to define the liquid take-up into spacer structures by Delkumburewatte, which takes into account the capillaries formed by the filament spacer yarn. The consideration of gravitational force and an assumption of the angle of the capillaries to the vertical are two important points in this liquid transport model. Figure 12.4 shows a simple yarn path diagram of a weft knitted spacer fabric. The assumptions made in this model are that the capillary lies at an angle of ϕ to the normal, the capillaries have an average radius of r, the fabric thickness is b, and the length of liquid travel with time t is ℓ(t). The angle between fabric surface and the direction of the tuck yarn is ϕ as shown in Fig. 12.5 (Delkumburewatte, 2007). The Lucas–Washburn equation for the above capillary system is: [12.18] where r is capillary radius, γ is surface tension, ρw is liquid density, g is gravity, φ is the angle between the capillary tube axis and the horizontal direction, η is the viscosity of the liquid, and ℓ(t) is the distance travelled by the liquid measured from the reservoir along the capillary axis with time.
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12.4 Yarn path of a spacer fabric.
12.5 Schematic diagram of a capillary formed by a spacer yarn in the structure.
[12.19]
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12.3.3 Improvement of wicking by additives It is common practice to reduce the surface tension of a liquid by adding surfactants to increase the rate of liquid uptake by the fabric. However, according to the Lucas–Washburn equation, [12.8], any reduction of the surface tension of liquid (γ ) would result in the reduction of wicking. This contradiction is due to the fact that adding surfactants not only reduces the surface tension of the liquid but also the contact angle, thereby increasing cosθ and thus the liquid uptake. Therefore, it is important to consider both γ and cosθ together, where γ cosθ is known as the wettability function (Yoo and Barker, 2004).
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A hydrophilic finish to the fabric is one of the most effective ways to improve wicking. In a case where the contact angle is more than 90° there would be no liquid uptake; using a surfactant would lower the contact angle considerably and there would be a net positive effect on wicking. However, when the contact angle is already less than 90°, using a surfactant may cause the wettability to either increase or decrease depending upon the wettability function (γ cosθ ). If the original contact angle is already below 50° a further reduction in the contact angle due to the surfactant will only give a very small increase to the value of cosθ. However, the value of surface tension would decrease by a bigger factor, giving a net negative effect to the wettability function (γ cosθ) (Yoo and Barker, 2004).
12.3.4 Liquid take-up The theoretical model for liquid take-up considering gravity was further developed, and the height (h) of liquid is given below (Saeed, 2006): [12.21]
The equation for volume (v) of liquid is given as;
[12.22]
where v is the volume of water taken up by one pore, r is the pore radius, L is the maximum liquid rise, h is the liquid rise, θ0 is the initial contact angle, g is gravity (980 cm/sec2). The other constants for water are: density (δ ) = 1.0 g/cm2, viscosity (η) = 0.1 dynes.sec/cm2, surface tension (γ ) = 72.8 dynes/cm. The total mass of liquid that can enter the available pores of the fabric can be given by the following equation: [12.23] where A is the fabric area, mg is the fabric weight, φ is the porosity of the fabric, ρl is the density of the liquid and ρy is the density of the yarn. The liquid absorbency rate can be given as a function of time as below: [12.24] The mass of liquid take-up by a capillary with the time t was calculated using the ‘liquid travelled’ equation (12.20) as below (Delkumburewatte 2007);
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where r is the capillary radius, ρw is the liquid density, mw is the weight of the water and ℓ(t) is the distance travelled by the liquid measured from the reservoir along the capillary axis with time. Liquid take-up by fabric as a percentage was given by the following equation:
[12.26]
where φ is the angle between the capillary tube axis and the horizontal direction, Tex is the count of the spacer yarn, 170 is the number of filaments, the crimp relates to the spacer yarn, η is the viscosity of the liquid, and ℓ(t) is the distance travelled by the liquid measured from the reservoir along the capillary axis with time. The above wicking and absorbency theories were developed on the basis of the Lucas–Washburn theory, which expresses the rate at which a liquid is drawn into a circular tube via capillary action, assuming that the textile structures are a bundle of capillaries. In reality, the situation is more complex and therefore Zhong et al. suggested using the Monte Carlo simulation to describe the wetting and wicking phenomenon (Zhong et al., 2002). In the Monte Carlo simulation based on the 3-D Ising model, Zhong et al. described the wetting process as changing the spaces between fibres from a gasdominant state to a liquid-dominant state (Zhong et al., 2001). The difference in energy between the two states causes the liquid to replace the gas until the surface tension is balanced by gravity. They noted that the travelling rate of liquid is higher in the area where the packing density is higher, and also that the width of the liquid column decreases with height owing to the balance of surface tension and gravity. This model can be used for other liquids and fibres after modification. The two-dimensional Ising’s model was used by Zhong et al. (2002) to investigate the fluid flow through fibrous structures in-plane and under pressure. In this model the frictional energy loss in the flow path was considered and it was noted that it would increase with the increase of fibre volume fraction. However, in this twodimensional model they assumed that the fibre distribution and the thickness are even throughout, and suggested that a three-dimensional model is needed to represent the real spatial distribution of fibrous structure in order to reduce the differences between the experimental results and the simulation model.
12.4 Experimental liquid take-up 12.4.1 Gravimetric absorbency testing system (GATS) The transverse wicking test based on the Gravimetric Absorbency Testing System (GATS) is a method used to evaluate the absorbency of porous structures. This system has the advantage of observing the spontaneous transplanar liquid take-up by a fibrous assembly dynamically, and can simultaneously obtain the absorption rate and the capacity of the specimen by observing the real-time absorption © Woodhead Publishing Limited, 2011
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12.6 Allasso Absorbency Analyser.
weight per second (Kim et al., 2003). The Allasso Absorbency Analyser (AAA) is one of the recently developed test instruments based on GATS. Figure 12.6 is a photo of the analyser showing important parts of the instrument that permit recording of the absorbency rate, total absorbency, and the time taken to saturate the structure, using the software available. To evaluate the comfort property of fabrics in terms of moisture removal, the absorbency test uses distilled water; this is better than tap water to represent perspiration for testing purposes, according to the conclusion of Simile (Simile, 2004) after carrying out absorbency tests using distilled water, tap water, and salt water containing 2.5 grams of salt per litre, which is the average NaCl content in perspiration. When the software of Allasso Absorbency Analyser is used to record the weight of water absorbed by the fabric, it records a function of time and gives a curve as well as a data sheet. However, there is some inaccuracy in the absorbency rate in this method due to irregular contact between the fabric sample and the acrylic plate (Saeed, 2006). Total absorbency is also not possible in the case of smaller fabric samples due to the rising of the water at the boundaries caused by surface tension (Delkumburewatte, 2007).
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purposes, such as towels, wiping cloths, mopping cloths and diapers have been produced with higher porosities. Structures with low knit and woven density, low fineness, higher thickness and a higher pore size are desirable for a higher absorption capacity. A structure with a higher density and lower fineness will have a higher absorption rate (Kim et al., 2003). A water absorbency test (GATS) was carried out on knitted structures made from a range of blends of Tencel and polyester by Firgo (Firgo et al., 2006). They concluded that 100% polyester single jersey structures have an absorbency of less than 1g/g, while blended polyester single jersey structures with 30% Tencel absorb 5g/g. They further concluded that double jersey fabrics made with a single jersey 100% polyester inner layer and a single jersey 70% polyester and 30% Tencel outer layer, absorb more water. Also, for the blended fabrics water spreads over the surface more quickly, there is lower wet cling behaviour, and they dry more quickly. The wet cling index of 100% polyester is about 20 and the wet cling index of polyester with 30% Tencel is 12 (Heinrich et al., 2006). Those knitted structures and non-woven fabrics with higher porosities absorb more water. However, the water take-up percentage and the water take-up per unit area change with the thickness and the bulkiness of the fabrics. Table 12.2 shows that the knitted fabrics have exceptionally high absorbency per square metre, which is the most important factor in case of perspiration management. Liquid take-up rate depends on several factors such as the fibre type, the pore size, and the properties of the liquid (Saeed, 2006). The total absorbency capacity is less influenced by the absorbency characteristic of the fibres; more important is the porosity or bulkiness of the knitted structures. (Delkumburewatte, 2007). Table 12.3 shows the total absorbency and water retention of different double jersey knitted samples with Tencel and Viscose as the spacer yarn. It shows the water take-up and the water retention ability as grams of water per square metre and as a percentage of fabric weight. The composition of
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Table 12.2 Water take-up by different structures (Delkumburewatte, 2007) Fabric sample
Water take-up (%)
Water take-up (g/m2)
Knitted rib – with polyester (PET) Knitted rib – with polypropylene (PP) Woven – Cotton Non–woven – Viscose Double jersey – Lycra outer layer, Tencel inlay & polyester spacer Double jersey – outer layer polyester, tuck inlay Tencel Double jersey – outer layer polyester, tuck inlay Viscose PET spacer fabric with 8 spacer yarns
860 1020 385 1170
1475 1535 406 602
340
760
510
3300
620 1250
3800 13000
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Table 12.3 Absorbency of double jersey knitted structures (Delkumburewatte, 2007) No. of tuck in lay yarns
Courses Wales
Stitch density
Per/cm
Per/cm
Per/cm2
1-Tencel 2-Tencel 4-Tencel 1-Viscose 2-Viscose 4-Viscose
4.9 4.9 4.13 5.28 5.28 4.3
8.8 8.8 10 8.4 8.8 10
43.12 43.12 41.3 44.35 46.46 43
Water takeup
Water retention
g/g
g/m2
g/g
5.10 4.90 4.53 6.20 5.50 6.05
3371 3326 3877 3817 3814 4837
2.90 2.75 2.80 3.25 2.97 2.90
g/m2 1923 1850 2387 1996 1890 2305
the different fibres with Tencel or Viscose in the polyester structures were 17:88, 35:88 and 70:88 for 1 inlay, 2 inlay, and 4 inlay yarns respectively. Absorbency test results with a higher number of spacer yarns and different fibre types show that the fibre contacting the water surface has an influence on absorbency rate. Figure 12.7 shows the absorbency rate with the variation in number of spacer yarns and with different faces, in which the single jersey layer facing the acrylic plate is made with different yarns, polypropylene (PP), polyester (pe) and Shakespeare monofilament (sh). The results show that the absorbency rate and total absorbency of structures with six and seven spacer yarns within a repeat are better compared to structures with eight and twelve spacer yarns within a repeat. They also show that the absorbency rate is better when a polyester or nylon monofilament, rather than polypropylene, is in contact with the water surface.
12.7 Absorbency of a spacer with a different fibre surface in contact with water.
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12.4.3 Engineered knitted spacer structures for management of perspiration Porosities and capillary radii of the structures For the experimental investigation of absorbency, different types of spacer knitted fabrics similar to the structure given in Fig. 12.4 were produced on a 7 gauge Shima Seiki machine. For the purpose of achieving different porosities, the capillary radii and angle of capillaries to the horizontal surface, were changed by varying the number of spacer yarns between two single jersey courses and by varying the number of spaces between two consecutive tucks. Figure 12.5 shows an example of one spacer fabric, which has seven spacer yarns in-between two single jersey courses and seven spaces in-between two consecutive tucks. Table 12.4 shows the fabric specification of different polyester spacer structures that were used in the experimental and theoretical work. It also shows the capillary radius calculated using the model and the porosity based on the fabric sample weight, thickness and area, which is 50.26 cm2. The capillary radii given in Table 12.1 are calculated using the equation (Delkumburewatte, 2007):
[12.27] where b is fabric thickness, w is wales per cm, c is courses per cm, S is the number of needle spaces between two tuck needles, ‘rows’ is the number of spacer yarns within one repeat, T is the count of the spacer yarn and ρf is fibre density. Table 12.4 Fabric specification, capillary radius and porosity of some spacer samples
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Sample Spaces Thickness Weight Wales Courses Stitches Cap. radii Porosity mm grams per cm per cm per cm2 µ m Sp-21 Sp-22 Sp-23 Sp-24 Sp-25 Sp-26 Sp-27 Sp-7 Sp-8 Sp-10 Sp-11
9 13.20 10 13.35 6 9.55 7 10.20 8 10.40 9 10.15 10 10.45 12 10.30 8 11.10 6 9.10 7 10.30
5.40 5.35 4.20 4.69 5.09 5.15 5.40 6.60 5.42 4.52 4.95
5.00 4.60 4.62 4.81 4.81 4.63 4.44 5.00 4.40 4.40 4.06
6.67 6.67 6.60 6.29 6.29 5.93 6.30 5.60 6.75 7.00 7.56
33.35 30.68 30.50 30.30 30.30 27.50 28.00 28.00 29.70 30.20 30.70
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60.94 55.63 59.09 56.69 53.20 51.64 48.14 48.86 63.30 58.12 56.66
0.960 0.954 0.960 0.957 0.953 0.952 0.944 0.931 0.959 0.957 0.958
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A vertical absorbency test was carried out for all eleven fabric samples using the absorbency tester, and the results were recorded. Absorbency rate and total absorbency were compared with the fabric parameters and the theoretical absorbency. Experimental results Figure 12.8 shows the experimental absorbency as grams of water per 100 grams of fabric whereas Fig. 12.9 shows the absorbency in grams of water per 50 cm2. In the case of clothing material it is important to know the absorbency capacity per square unit of area as well as the percentage. Figure 12.8 shows that the fabric samples with higher porosities have higher total absorbency. For example, sample Sp-21 with a porosity of 0.960 and sample Sp-8 with a porosity of 0.959 have a higher total absorbency of about 1200%. Similarly, fabric samples with lower porosities, Sp-7 with a porosity of 0.931 and Sp-27 with a porosity of 0.944, have a lower total absorbency of about 900%. The total absorbency of the other fabric samples follows a similar sequence according to their porosity variation as given in Table 12.4. Figure 12.8 shows that the fabric samples Sp-21, Sp-22, Sp-26 and Sp-8 have a higher and consistent absorbency rate from the beginning to saturation compared to the other structures in the table. This also shows that the absorbency rate, between saturation and 400% absorbency, is similar for most of the structures. However, the fabric samples with lower capillary radii and higher capillary angle (sin ϕ) to the horizon show lower absorbency rates. Figure 12.9 shows that the absorbency per unit area of fabric samples follows a similar pattern in terms of percentage absorbency. However, the total absorbency per unit area seems to vary with the thickness of the fabric; the thicker the fabric the higher the total absorbency per unit area.
12.8 Experimental absorbency rates of spacer fabrics given in Table 12.1 as percentage.
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12.9 Absorbency of spacer fabrics given in Table 12.1 (grams per 50 cm2).
The authors observed that fabric sample Sp-7 had the lowest percentage absorbency but a higher absorbency per unit area compared to other samples. Sp-7 is also comparatively high in fabric thickness. Theoretical liquid absorbency of spacer structures
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First the capillary radii were calculated for all the fabric samples considering the number of filaments and the volume of the filaments in a cuboid (Delkumburewatte, 2007) and given in Table 12.4. It shows the capillary radii are between 48 µ m and 63 µ m. For pore size smaller than 10–7m (0.1 µ m), we have to consider only the surface tension, otherwise the gravitational force has to be considered in predicting the fluid flow in vertical capillaries (Pan and Zun, 2006). The liquid transport along the capillaries was calculated and plotted against time using the liquid transport model, equation 12.21. Constant values for the properties of water were taken as density (ρw) = 1.0g/ cm3, viscosity (η) = 0.1 dynes.sec/cm2, surface tension (γ ) = 72.8 dynes/cm and gravity (g) = 980 cm/sec2. The average contact angle is taken as 75°, which is the average of the minimum (60°) and maximum (90°) (Lehocky and Mracek, 2006), so the value of cos θ for PE is taken as 0.2588. The liquid take-up rate was calculated using Equation 12.26. The graphs in Figure 12.10 show the theoretical liquid take-up rate for different fabrics that we have already experimentally investigated. It also shows that the total absorbency varies from 800% to 1 500% and that the absorbency rate of some structures is very high compared to other structures. Figure 12.11 shows the comparison of theoretical and experimental absorbency of five selected spacer fabrics. T and E indicate the theoretical (continuous) and
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12.10 Theoretical absorbency rates of the spacer fabric samples.
12.11 Theoretical and experimental absorbency of selected spacer structures.
experimental (dotted) absorbency of selected fabric samples Sp-24, Sp-25, Sp-26, Sp-8 and Sp-11 respectively. It shows that the theoretical and experimental total absorbency is almost the same for the given structures. When the experimental values are compared with the theoretical liquid take-up rate, the pattern is almost the same, although there are some discrepancies. In the initial stages the theoretical absorbency rate is higher than the experimental, because we have taken a constant average contact angle, 75° (cos75 = 0.2588), which is lower than the actual dynamic contact angle. At the beginning the
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dynamic contact angle is closer to 90° (Heinrich et al., 2006) and thus gives cos 90 closer to 0. So the value of ℓ(t) in Equation 12.20 will be lower until it reaches the dynamic contact angle of 75°. Thereafter the dynamic contact angle will be lower than 75° resulting in higher values for ℓ(t) in Equation 12.20. So the theoretical absorbency rate will be lower at the beginning than the given theoretical values until the dynamic contact angle is 75°. Similarly, after the dynamic contact angle reaches 75° the actual theoretical absorbency rate must be higher than the given theoretical values until saturation. Figure 12.11 also shows that the time taken for saturation in both theoretical and experimental varies between 6 and 10 minutes for the given structures. The theoretical time taken for saturation is higher than the experimental time taken due to the same explanation given as for the absorbency rate after the dynamic contact angle of 75°. The models developed to predict the absorbency in a knitted spacer structure can be used directly to predict total absorbency in knitted spacer structures made with textured monofilament yarn. The model can be also used to predict the absorbency rate and the time taken for saturation. However, if the dynamic contact angle is considered in the equations, the shape of the absorbency curve and saturation time can be predicted more accurately.
12.5 Future trends Textile structures can be produced to transport water in geo-textiles, agro-textiles and clothing materials under various conditions. Special knitted structures can be produced as liquid accumulators for the purpose of medical and technical textiles. Capillary effects and the gravitational force can be used to ‘pump’ ground water for agricultural purposes to the surface step by step, if properly engineered.
12.6 Sources of further information and advice 1 2 3 4 5 6 7 8 9 40 1 2 43X
Physical Properties of Textile Fibres (Morton and Hearle, 1997) provides extensive information on the behaviour of fibres when in contact with water. It also explains the change of physical properties and characteristics with the absorption of water, and the likelihood of water molecules attaching to the fibres based on the chemical structure. There is also moisture regain depending on atmospheric conditions. Human Thermal Environments (Parsons, 2006) explains the importance of moisture management properties of clothing materials to give the comport properties. It also explains the influence of water vapour transport properties of clothing materials on cooling. Thermal and Moisture Transport in Fibrous Materials (edited by Pan and Gibson, 2006) gives the various theories developed by textile scientists, engineers and chemists to describe behaviour of water when it comes into contact with fibrous materials.
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12.7 References Berlin, E. (1981), Hydration of milk proteins. In L.B. Rockland and G.F. Stewart (eds). Water Activity: Influences of Food Quality. New York, Academic Press, 467. de Gennes, P. G. (1985), Wetting – statics and dynamics. Reviews of Modern Physics, 57 (3), 177–863. de Gennes, P. G. (1998), ‘The dynamics of reactive wetting on solid surfaces.’ Physica A: Statistical Mechanics and its Applications, 249 (1–4), 196–205. Delkumburewatte, G. B. (2007), An investigation into the development of a cooling knitted structure to manage heat and sweat. Second Year report submitted (PhD), School of Materials, University of Manchester. Firgo, H., Schuster, K. C., Suchomel, F., Johann Männer, J., Burrow, T. and Abu-Rous, M. (2006), The functional properties of Tencel® – A current update, Lenzinger Berichte, 85, 22–30, Textile Innovation, Lenzing AG, Austria. Fisher, L. R. (1979), An experimental study of the Washburn equation for liquid flow in very fine capillaries. Journal of Colloid and Interface Science, 69 (3), 486–492. Good, R. J. and Lin, N. J. (1976), Rate of penetration of a fluid in to Pores Body 2, verification of generalisation Washburn equation, for organic-liquid in glass capillary, Journal of Colloid and Interface Science, 54 (1), 52–58. Heinrich, F., Friedrich, S. and Tom, B. (2006), High performance sportswear, TRB Textile Innovation, Lenzing AG, Austria. Hodgson, K. T. and Berg, J. C. (1988), The effect of surfactant on wicking flow in fiber networks, Journal of Colloid and Interface Science, January, 121 (1), 22–31. Jeje, A. A. (1979), ‘Rates of spontaneous movement of water in capillary tubes’, Journal of Colloid and Interface Science, 69 (3), 420–429. Joos, P., Remoortere, P. V. and Bracke, M. (1990), The kinetics of wetting in a capillary, Journal of Colloid and Interface Science, 136 (1), 189–197. Kim, S. H., Lee, J. H., Lim, D. Y. and Jeon, H. Y. (2003), Dependence of sorption properties of fibrous assemblies on their fabrication and material characteristics. Textile Research Journal, 73 (5), 455–460. Kissa, E. (1996), Wetting and wicking. Textile Research Journal, 66 (10) 660–668. Labuza, T. P. and Busk, C. G. (1979), An analysis of the water binding in gels. J. Food Sci., 44:379. Landau, L. D. and Lifshitz, E. M. (1988), Theoretical Physics: Hydrodynamics, Moscow, Nauka. Laughlin, R. D. and Davies, J. E. (1961), Some aspects of capillary absorption in fibrous textile wicking. Textile Research Journal, 31 (10), 904–910. Lehocky, M. and Mracek, A. (2006), Improvement of dye absorption on synthetic polyester fibres by low temperature plasma. Pre-treatment Institute of Physics and Materials Engineering, Faculty of Technology, Tomas Bata University in Zlin, Czech Republic. Marmur, A. (1992), Penetration and displacement in capillary systems of limited size. Advances in Colloid and Interface Science, (39), 13–33. Marmur, A. and Cohen, R. D. (1997), Characterization of porous media by the kinetics of liquid penetration: The vertical capillary model, Journal of Colloid and Interface Science, 189 (2), 299–304. Miller, B. and Tyomkin, I. (1984), Spontaneous transplanar uptake of liquids by fabrics, Textile Research Journal, 54 (11), 706–712. Miller, B. (1985), ‘Experimental aspects of fiber wetting and liquid movement between fibers’, in P. K. Chatterjee, Absorbency, Elsevier Science Publishers B.V., pp. 121–124.
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Morton, E. W. and Hearle J. W. S. (1997), Physical Properties of Textile Fibres. Manchester, UK, Textile Institute. Mukhopadhyay, S. K. (1992), Advances in Fibre Science, Manchester, UK, Textile Institute. Pan, N. and Zun, Z. (2006), Essentials of psychrometry and capillary hydrostatics, in N. Pan and P. Gibson, eds., (2006) Thermal and Moisture Transport in Fibrous Materials, Textile Institute, Woodhead Publishing, [v391]1). Parsons, K. C. (2003), Human Thermal Environments, Taylor & Francis, New York. Pierce, T. F. (1929), A two-phase theory of the absorption of water vapor by cotton cellulose. Journal of the Textile Institute 20: 133T. Saeed, U. (2006), The study of liquid transport behaviour of structures knitted with monofilament yarns, Thesis submitted for the degree of Master of Philosophy, School of Materials, University of Manchester. Simile, C. B. (2004), Critical evaluation of wicking in performance fabric, MSc Thesis, School of Polymer, Textile, and Fiber Engineering, Georgia Institute of Technology. Washburn, E. W. (1921), The dynamics of capillary flow, Physical Review, 27 (3), 273–283. Yoo, Shunjung and Barker, Roger L. (2004), Moisture management properties of heatresistant workwear fabrics, Textile Research Journal, 74 (11), 995–1000. Zhong, W., Ding, X and Tang, Z. L. (2001), Modelling and analysing liquid wetting in fibrous assemblies. Textile Research Journal, 71 (9): p. 762–766. Zhong, W., Ding, X. and Tang, Z. L. (2002), Analysis of fluid flow through fibrous structures. Textile Research Journal, 17 (8), 682–686.
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Index
AATCC TM 150, 248 AATCC TM 179, 248 AATCC TM 107 method, 250 abrasion, 45–6 damage, 239 absorbency, 292 absorption, 291–8 acoustic impedance, 263–5, 266 Active Noise Control, 284 ACUPRO sound absorption software, 267 additive tensioning system, 92 aesthetic comfort, 235 air permeability, 43 Akwatek polyester fabric, 255–6 Allasso Absorbency Analyser, 299 Americana stitches, 126–30 advantages, 128 containing spandex, 127–8 definition, 126 family, 128 list of basic Americana stitches, 129 and modified Americana tricots, 126–30 knit structure, 127 splitting problem, 127 modified stitches, 128–30 lap diagram, 129 stitch construction, 127 two basic types, 126–7 ANC see Active Noise Control ASTM D 1909, 252 ASTM D 2256-88, 159 ASTM D 3787, 247 ASTM D 4154, 238 ASTM D 4156, 238 ASTM D 5725, 243 ASTM E 96-90, 242 Atlas stitch, 119 automotive textiles, 186–7 circular knitted fabric, 187 ball-burst strength tester, 247 bandages, 188 barré effect, 226 bast fibres, 20 BES 4745:1990/ISO-1:1989 method, 241–2 apparatus diagram, 241 cabinet diagram, 241 biodegradable materials, 203–5
biopolymers, 203 Body technologies, 106 Boltzmann constant, 81 bound water, 289 bowing, 228 BS 4745, 239 CAD systems see computer-aided design systems capstan equation, 92, 100 capstan friction, 91 cardigan fabric, 184 car seat covers, 145–6 illustration, 146 cellulosic fibres, 25–6, 288 Cetex, 199, 200 Chaemeuse see knitted fabrics chitin yarn, 30 Christoffel symbols, 74–5 circular knits see weft knit fabrics circular knitted fabrics, 186 in automotive textiles, 187 structure and properties, 182–4 structure parameters and relaxation methods, 183–6 tubular knitted fabrics, 182–3 circular knitted structures current problems and limitations, 175–7 high-speed circular knitting machines, 175–6 jacquard circular machines limitation of pattern, 176 less-seems knitting machines production limits, 176–7 circular knitting, 171–90, 220, 223–4 applications, 184–9, 187–9 automotive textiles, 186–7 elastic bandages, 189 electro textiles, 185–6 orthopaedic applications, 187–9 seamless knitted garments, 184–5 tubular fabrics with elastic weft insertion, 188 tubular knitted fabrics in surgery uniforms, 189 circular knitted structures current problems and limitations, 175–7 high-speed circular knitting machines, 175–6
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Index
jacquard circular machines limitation of pattern, 176 less-seems knitting machines production limits, 176–7 fabrics structure and properties, 182–4 structure parameters and relaxation methods, 183–6 tubular knitted fabrics, 182–3 hosiery knitting technology, 174–5 true-rib machine, 175 machines, 141, 171 loop transfer technology, 180 principles and classification, 171–4 ultra fine gauge elements, 179 principles and classification of machines, 171–4 double jersey machine, 173 single jersey machine, 172 recent advances, 177–82 automatic seamless circular knitting elements, 178 circular knitting machine ultra fine gauge elements, 179 combing tool of sliver feeding system, 182 loop transfer technology, 180 loop transfer tool, 181 needle with spring for high speed loop transferring, 181 pile and sliver insertion mechanism, 180–2 Santoni seamless knitting technology, 177–9 seamless circular knitting system, 178 sliver feeding system by using feeding rollers, 182 ultra fine gauge knitting machines, 179–80 smart garments, 189–90 Cleerspan, 31 cloth press off, 229 cockled, 229 colourfastness, 238 comfort, 45 compact yarns, 19 compensation tensioners, 93 composite preforms, 198–200 textile composite prepregs, 199–200 textile preforms, 198–9 composite prepregs, 199–201 computer-aided design systems, 145, 177 concertina deformation, 39 contact angle, 290, 296 Coolmax fabric, 256 coolmax fibres, 23 cotton, 252 yarns, 19 course, 111 course length, 218 coursewise deformation, 56 cover factor, 51 definition, 43 crack, 229–30 creasing, 41–2 creel, 103 crêpe fabric, 130 Crepeset, 130–1 crimp, 38 crosswound package, 11 cubic polynomial, 77, 78
Cupro fibres, 30–1 cylinder winding process, 8–10 types, 8–9 Darboux vector, 60, 61 Del-Atlas stitch, 125 Delaware stitch advantages, 117–18 and modified Delaware stitch tricot fabrics, 116–19 common quality problems, 118–19 laid-in types, 119 mattress fabric, 119 other modified Delaware stitches, 117 some examples of Delaware stitch fabrics, 118 Digital Stitch Control System, 108 directionally oriented structures, 149–54, 155 bi-axial, 150–2 fixed by jersey, 151 with interlacing of weft and warp yarns, 152 knitting on a circular machine, 151 multi-layer structure, 152 combined DOS-3D-shaped structures, 153, 154 3D-shaped structures with bi-axial reinforcement yarns, 154 combined DOS-3D-spacer structures, 153–5 illustration, 155 mono-axial, 149–50 based on rib, 149 1 × 3 fleece structure, 150 load-extension curves, 150 multi-axial structures, 153 directly bound water, 289 disc tensioner, 94 3D Ising model, 298 Dorlastan, 31, 120 DOS see directionally oriented structures double jersey machines, 172, 173 double needle Delaware stitch, 125 draping, 205–6 dropped stitch, 229 dry contact method, 240 dry space method, 240 DSCS see Digital Stitch Control System 3D structures 3D theoretical form and 2D pattern car seat cover, 146 helmet form, 146 spherical form, 145 knitting, 147, 148 with loop transfer, 148 without loop transfer, 147 produced by circular weft knitting technology, 141–2 spacer structure with tuck loop stitches, 142 tubular weft knitted fabrics for medical applications, 142 spacer structures, 146–8 with different connecting forms, 148 with fabric connecting layers, 147 techniques for shaped hollow structures production, 143–8
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Index car seat covers, 145–6 spherical and helmet forms, 145 tubular forms, 144 weft knitting technology, 141–8 produced by computerised flatbed knitting technology, 143
EAP see Electro Active Polymers E-glass fibres, 198 elastane see spandex elastan roller, 96 elastica model, 54 elastica theory, 60–3, 157–60 elastic bandages, 189 elastic fabric, 183 elastic yarn, 4 elastomers, 40, 46 Electro Active Polymers, 284 electromagnetic radiation, 34 electro textiles, 185–6 EMI see electromagnetic radiation Euler’s equation see capstan equation Euler’s friction see capstan friction fabric bursting strength, 247 fabric hand, 236 fabric thickness, 42–3 fancy threads, 26–8 FEA see Finite Element Analysis fibre reinforced plastic, 193, 194, 197 filament yarns, 4, 115 film stacking, 199 fine gauge automotive fabrics, 179–80 fine knitted fabrics, 179 Finite Element Analysis, 161–3 Finite Element Method, 163 flatbed knitting machines, 143 flat bed machines, 172–3 flat knits see warp knit fabrics flat knitting machines, 107 flat knitting technology, 164, 165, 166 flat thread, 27 flax fibres, 20 float, 231 fluffy yarn, 4 frictional feeders, 100–1 principle, 100 frotte threads, 28 FRP see fibre reinforced plastic Fujibo, 120 GATS see Gravimetric Absorbency Testing System Gaussian elimination procedure, 59 geotextile, 166–7 Glospan, 31, 120 Gravimetric Absorbency Testing System, 298–9, 300 Groz-Bekert E68, 179 Hagen-Poiseuille’s equation, 293 Hagen-Poiseuille’s law, 293, 295 Hamiltonian formulation, 67–9 Hamiltonian function, 68 Havertex BSM2100 E62, 179 helmet forms, 145 hemp fibres, 20
311
high performance fibres, 196–8 materials for knitted fabric composites, 196 micro- and nanofibrillar fibres, 197 natural fibres, 197–8 polymer fibres, 196–7 high-speed circular knitting machines, 175–6 hole, 229–30 hollow core fibre, 23 Hook’s law, 137 hosiery, 174 hosiery knitting technology, 174–5 true-rib machine, 175 hosiery products, 106 Hounsfield Hl0KS universal tester, 159 hydrophilic fabrics, 41 hydrophilic groups, 288 hydrophobic groups, 288 i-DSCS, 108 impedance tube, 270 industrial applications weft knitted structures, 136–67 applications, 163–7 current problems and limitations, 138–40 directionally oriented structures and combined DOS, 149–55 future trends, 167 knitting 3D structures using weft knitting technology, 141–8 simulating mechanical properties, 157–63 weft knitted multifunctional structures, 155–6 industrial knitting, 195 integral knitting, 206 interlock fabric, 184 Ionic Polymer Metal composites, 284 ISO 10534-2, 267 Italian machines, 180 jacquard circular machines, 176 jacquard knitted spacer structures, 278–80 jersey see knitted fabric jersey stitch fabric, 112, 115, 118 Kawabata evaluation system, 246, 254 KES-FB7, 239 apparatus, 242 KES-F2 bending tester, 245 KES-F4 compression tester, 246 KES-F1 shear and tensile tester, 245 KES-F3 surface tester, 246 KDK thread see Knitt de Knitt thread KES-F see Kawabata evaluation system Kevlar, 46 kinetic tensioner, 93 KnitCAD data, 206 Knitt de Knitt thread, 27 knitted defects, 225–31 knitted fabric composites, 193–210 applications, 208–9 automotive, aerospace, 208 consumer items, sports equipment, 208 composite preforms, 198–200 textile composite prepregs, 199–200 textile preforms, 198–9 thermoplastic composite preform/prepreg materials, 200
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Index
thermoplastic matrices with reinforcement fibres, 199 fibres and yarn types, 196–8 reinforcement types and their composite properties, 195 yarn types, 198 high performance fibres, 196–8 natural fibres, 197–8 knit structures for fabric composites, 201–2 knitted fabrics in polymer composites, 194–5 manufacturing methods, 205–6 draping, 205–6 integral knitting, 206 sheet forming, 205 stretch forming, 205 manufacturing methods development, 205–7 2D and 3D commingled knitted fabric thermoplastic composite preforms, 206 forming method influences, 206–7 matrix materials, 202–5 biodegradable materials, 203–5 biodegradable polymer materials currently available, 204 thermoplastic materials, 202–3 thermoset materials, 203 mechanical properties, 207–8, 209 behaviour simulation, 209 polymer composites, 193–4 knitted fabric quality, 214–15 knitted fabrics, 115 see also weft knitted structures dense spacer structures, 280–3 cross section structural representation, 281 group A sound absorbency, 282 group B sound absorbency, 283 pictorial view, 281 structures application, 281–3 2D model mechanics, 51–9 computational aspects, 58–9 coursewise deformation, 56 knitted loop structure, 52–4 load-extension for different L/d ratios, 60 plain knitted fabrics load-deformation behaviour, 55–6 planar inextensible elastica, 54–5 walewise deformation, 57–8 3D model plain weft-knitted fabrics mechanics, 59–66 equilibrium of forces and moments, 65 knitted loop geometrical aspects, 63–5 method of solution, 65–6 elastica theory, 60–3 constitutive equations, 61 elastica element, 62 moment and force equilibrium equations, 61–3 energy model, 66–72 co-ordinate system, 67 equilibrium equations, 70 Hamiltonian formulation, 67–9 loop projections, 71 method of solution, 70–2 future trends, 284 geometry, 48–51 course spacing, 50 cover factor, 51 two similar loops, 49
heat and water vapour diffusion in fabrics, 80–3 calculation of heat of condensation, 82–3 method of solution, 83 pressure and mass and temperature relationship, 81–2 theoretical background, 80–1 importance of quality, 213–14 mathematical modelling, 48–83 NAC, 267 data for different thickness of plain structure A1, 273 measurement, 267 plain knitted structures with same thickness but different pore sizes, 271 sound absorption by a pore formed, 267 two knitted fabrics with different pore sizes, 273 plain knitted fabric, 263–6, 268 analytical prediction, 263–6 model as a porous material, 263, 264 samples, 268 plain knitted structures, 263–75 analysis, 275 different stitch sizes but same thickness, 270–1, 272 dimensionless parameter variation with pore radius and frequency, 269 mathematical analysis simplification based on fabric data, 269–70 NAC measurement, 267 parameters affecting noise absorbency properties, 270 pore radius, 266–7 technical face image, 264 thickness effect, 272–5 pressure on a surface, 72–9 applications to knitted fabrics, 77 curved membrane mechanics, 72–4 differential geometry, 74–6 equations in curvilinear co-ordinates, 73–4 experimental data, 77–8 fabric actual stress-strain characteristic, 78 membrane subjected to pressure and edge forces, 73 pressure profile on hemispherical surface, 79 stress-strain and pressure profile, 79 simulated cross sectional representation A1 fabric with 2.5mm thickness, 274 A1 fabric with 3.1mm thickness, 274 knitted fabric samples, 272 single layer of A1 fabric with 0.6 mm thickness, 274 sound absorption, 262–84 acoustic textiles in vehicles, 262–3 engineering advanced knitted fabrics, 275 spacer structure as MPP with air cavity, 276–8 spacer structure as MPP with an air cavity metal enclosure, 278 thick spacer structures, 275–80 knitted spacer fabric structure, 276 plain knitted and two-yarn Jacquard with diagonal pores lattice structure, 278–80
© Woodhead Publishing Limited, 2011
Index
plain knitted structure made from elastomeric yarn, 277 spacer fabric with plain knitted front layer, 277 yarn path notation fabric A and C, 279 fabric B, 279 fabric D and E, 280 fabric F, 280 knitted loop geometrical aspects, 63–5 interlacing knitted loops, 63 jamming of knitted loops, 64 structure, 52–4 equivalent force diagram, 53 small element, 54 two interlacing knitted loops, 52 knitted underwear, 235–57 clothing functional requirements, 238 dimensional stability and skewness stability, 248–50, 251 alternative washing and drying conditions, 250 diagonal lines for option 1, 249 dimensional change marking location, 248 inverted marking, 249 offset marks, 250 square marking, 249 tumble dry conditions, 251 washing machine setting conditions without load, 251 fabrics engineering, 252–5 effect of yarn characteristics, 253–4 fabric composition, 252–3 fabric structure, 254–5 fabric thickness, 254 functional requirements, 235–9 aftercare, 238–9 appearance and appearance retention, 238 colourfastness, 238 comfort, 235–6 durability, 238 sewability, 237 interlock knitted fabrics reference values, 257 moisture regain of different kinds of fibre, 253 performance evaluation, 239–52 ball burst attachment, 247 colourfastness to water, 250–1 contact angle tester, 243 fabric bursting strength, 247 fabric low-stress mechanical properties, 244–6 liquid transport properties, 243–4 L&M sewability test, 247–8 longitudinal wicking strip test, 244 moisture permeability, 242 transverse wicking plate test, 244 water vapour transmission tester, 242 wearer trials, 251–2 properties of commercial fabrics, 256–7 recent developments, 255–6 Akwatek polyester fabric, 255–6 Coolmax fabric, 256 Nike Dri-fit, 256 Nike Sphere Cool fabric, 256 structure of Nike Sphere Cool fabric, 256
313
single jersey knitted fabrics reference values, 257 thermal properties, 239–42 BES 4745:1990/ISO-1:1989 method, 241–2 Thermo Labo II apparatus, 240 Thermo Labo II KES-FB7, 239–41 knitting, 183 defects, 13–19 future trends, 34–5 polyurethane yarns joining methods in composites, 32–4 preparation for knitting process, 6–7 qualitative factors, 4–6 special applications, 28–32, 34 structure of cope, 10–13 types and definition, 3–4 types of knitting yarns, 19–28 types of packages, 7–10 yarn types and suitability, 3–35 knitting faults from poor cleaning of knitting machine, 216–17 reduction techniques, 224–5 knitting machines, 173, 184–5 faults from poor cleaning, 216–17 optimal setting, 216 knitting process knitted defects, 225–31 bands and streaks, 226–8 barré effect, 226 dropped stitches, 229 fabric skew, 227 needle line, 228 press off, 230 stitch defects, 229–31 tuck loop, 230 parameters of knitting control, 217–20 knitting tension, 218–19 loop length, 217–18 positive yarn feeding, 218 take-down tension, 220 tightness factor, 219 yarn input tension, 219 yarn length per stitch, 219 quality control, 215–17 before knitting process, 215–16 during knitting process, 216–17 quality control and common faults, 213–31 importance of quality knitted fabric, 213–14 jersey fabrics fabric geometry, 218 knitted fabric quality, 214–15 techniques to reduce knitting faults, 224–5 quality control mechanism for circular knitting, 220, 223–4 Mayer and Cie MCTmatic Quality Monitoring System, 223–4 STARFISH, 220, 223 yarn count and machine gauge, 220, 221–2 fine rib fabric, 221 fleecy fabric, 221 interlock fabric, 222 jacquard fabric, 222 mean yarn counts for some fibre materials in relation to machine gauge, 222 single jersey fabric, 221
© Woodhead Publishing Limited, 2011
1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 20 1 2 3 4 5 6 7 8 9 30 1 2 3 4 5 6 7 8 9 40 1 2 43X
314 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 20 1 2 3 4 5 6 7 8 9 30 1 2 3 4 5 6 7 8 9 40 1 2 43X
Index
knitting tension, 218–19 package density, 219 package size, 218 yarn unwinding method, 218 Kuper stitch, 125 Labview software, 267 laid-in Delaware stitch, 125 laid-in Jersey stitch, 126 LAPIS, 139 Laplace equation, 292 less-seems knitting machines, 176–7 Linel, 31 liquid travelled equation, 297 L&M sewability test, 247–8 Locknit see knitted fabric Long Float Delaware stitch fabrics, 117 Long Float Jersey, 115 loop density, 215, 217 loop length, 217–18 loop-raised tricot fabrics, 186 loop transfer technology, 180 loop transfer tool, 181 Lucas-Washburn equation, 293, 294, 295, 296 Lucas-Washburn kinetics, 294 Lucas-Washburn theory, 294, 298 Lycra, 31, 46, 120, 124, 126 vs 100% nylon jersey, 120, 121 machine gauge, 220 magic silver yarns, 29 mathematical modelling knitted fabrics, 48–83 2D model mechanics, 51–9 3D model plain weft-knitted fabrics mechanics, 59–66 energy model, 66–72 geometry, 48–51 heat and water vapour diffusion in fabrics, 80–3 pressure on a surface, 72–9 Mayer and Cie MCTmatic Quality Monitoring System, 223–4 MCTmatic display panel, 224 setting of motors for feed wheel, 223 mechanical damage, 237 Meryl Nexten yarns, 25 Meryl yarn, 25 M5 fibre, 197 microfibrillar fibres, 197 micro perforated panel, 271, 276 Milanese fabrics, 132 machine types, 133–5 circular, 134 continental flat bed, 134 English flat bed, 134 structure, 132 loop diagram, 133 Milano rib fabrics, 184 mini-jack circular knitting machines, 176 Mirandsai MV4-3.2 II E60, 179 moisture absorption theory, 289 moisture comfort, 236 moisture management basics of wetting, 288–91 experimental liquid take-up, 298–306 future trends, 306
weft knitted structures, 287–306 wicking and absorption, 291–8 MonitorKnit, 227 Monte Carlo simulation, 298 MPP see micro perforated panel multiaxial warp knits, 201–2 chain and tricot stitched multilayered structures, 202 multiplying tensioning systems, 92 MWK see multiaxial warp knits NAC see Noise Absorption Coefficient nanofibrillar fibres, 197 National Instruments M6259 Data Acquisition device, 267 natural fibres, 19–21, 197–8 NatureWorks, 203 needle heating damage, 237 needle line, 228 negative systems see yarn tensile stress control Newton-Raphson’s method, 56, 58, 59, 66 Nexen, 24 Nike Dri-fit, 256 Nike Sphere Cool fabric, 256 NMMO technology, 25 Noise Absorption Coefficient, 266, 267, 270, 271, 283 measurement, 267 Nomex, 46 non-wetting, 290 non-woven fibre webs, 263 OEM see original equipment manufacturer on-line monitoring system, 224–5 Ordinary Approximately Theory, 61 organic fibres, 19 origami, 41 original equipment manufacturer, 262 Outlast, 31 package core, 8 paper, 41 parallel package, 11 PEEK see poly(ether ether ketone) PET see polyethylene terephthalate PHA see poly-b-hydroxyalkanoates PIA 4000 digital image analysing software, 268 pile technology, 179–80 pilling, 45–6, 238 PIPD, 197 PLA see poly (lactic acid) plain knitted fabric analytical prediction, 263–6 acoustic impedance and sound absorption, 266 sound absorption by a pore, 265 unit cell acoustic impedance, 263–5 model as a porous material, 263, 264 technical face image, 264 pore radius, 266–7 samples pore radius and porosity data, 268 structure sample details, 268 plush knitting, 44 Plytron, 200 pneumatic tensioners, 93
© Woodhead Publishing Limited, 2011
Index
poly{2,6-diimidazo[4,5-b:4´,5´-E]pyridinylene1,4-(2,5-dihydroxy)phenylene} see PIPD polyamide yarn, 24–5 poly-b-hydroxyalkanoates, 203 polyester, 46, 115, 116, 165, 253, 301 microfibre yarns, 46 staple fibre, 24 polyester yarns, 22–4 poly(ether ether ketone), 202 polyethylene, 197, 202 polyethylene terephthalate, 197 poly (lactic acid), 203 polymer fibres, 196–7 polypropylene, 197, 207 polyurethane yarns, 32–4 polyvinyl fluoride strips, 284 porosity, 266–7 positive feeding see yarn length control positive yarn feeding, 218 powder coating, 199 pre-impregnated composite fibres see composite prepregs pressure comfort, 236 pressure garments, 182–3 Projectina optical microscope, 268 protein fibres, 288 puckered, 229 PVDF strips see polyvinyl fluoride strips Qiana fabric, 116 quality definitions, 5 measurement methods, 5 Raschel fabrics containing spandex, 120–4 DuPont’s guidelines for nylon//Lycra with warp knit fabrics, 124 fabric testing, 123 face or back, 123 guidelines, 124 knitting machines, 121–2 quality control, 123–4 stitches, yarns, and end-uses, 123 stitches used, 122–3 and tricot containing spandex, 119–20 Raschel machines, 113, 114, 123, 124 ratchet mechanism, 39, 40 relaxation shrinkage, 40 resin film infusion, 200 resin transfer moulding, 164, 200 RFI see resin film infusion rib tubular fabrics, 188–9 robbing back, 91, 175 Roica, 31, 120 roving, 181 RTM see resin transfer moulding Santoni seamless knitting technology, 177–9 sea cell fibres, 29 seamless knitted garments, 184–5 seamless shaped knitted fabrics, 185 seat cover fabric, 186–7 seaweed, 26 second skin fabrics, 179 Serret-Ferret frame, 60 Serret-Frenet formulas, 60
315
7-gauge Shima Seiki machine, 302 sewing damage, 237 Shakespeare monofilament, 301 sheet forming, 205 sheet moulding compound, 205 Shima Seiki 122 S knitting machine., 278 Shingosen fibres, 22 Short Float Jersey, 115 silk fibres, 21 single jersey machines, 171–2 cylinder system, 172 single knitting system, 106 skewing, 227 skin-and-garment interfacial pressure, 182–3 slippage, 56, 57 sliver, 181 sliver feeding system, 182 sliver knitting, 180, 181 smart garments, 189–90 smart knitted fabrics, 189 SMC see sheet moulding compound sound absorption knitted fabrics, 262–84 acoustic textiles in vehicles, 262–3 dense spacer structures, 280–3 engineering advanced knitted fabrics, 275 future trends, 284 plain knitted structures, 263–75 thick spacer structures, 275–80 Sound Propagation Level, 284 spacer fabrics, 166 spandex, 119–26 key Raschel fabrics, 120–4 newly developed constructions, 124–6 Del-Atlas stitch, 125 double needle Delaware stitch, 125 Kuper or twill stitch, 125 laid-in Delaware stitch, 125 laid-in Jersey stitch, 126 tricot and Raschel, 119–20 spherical forms, 145 SPL see Sound Propagation Level Split Stitch technique, 140 SRIM see structural reaction injection moulding staple yarns, 4 STARFISH, 220, 223, 254–5 steaming process, 9 stitch defects, 229–31 stitch transfer, 180 stop mark, 228 storage feeders negative, 97–9 principle, 98 stored and wound yarn slippage and separated wound yarns, 99 stored yarn supply basic systems, 99 two-colour pattern with knit and miss back, 97 positive, 94–6 with length control, 95 yarn tensile stress effect on quantity of yarn delivered, 95 streaks, 226–8 stress-strain relationships, 59, 77 stress tensor, 72 stretch forming, 205 structural reaction injection moulding, 200
© Woodhead Publishing Limited, 2011
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316 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 20 1 2 3 4 5 6 7 8 9 30 1 2 3 4 5 6 7 8 9 40 1 2 43X
Index
suede effect, 22 Super Float Jersey, 115 Superwash wool, 41 surface interest fabrics, 130–2 Crepeset, 130–1 crêpe tricot, 130 key elements for developing good crepe in tricot, 130 knit-de-knit crêpe, 131 simplex machines, 132 texture or surface interest warp knits, 131–2 surface tension, 290–1 surfactants, 296, 297 surgery uniforms, 189 synthetic fibres, 21–6, 289 synthetic yarns, 21 T-400, 23 T-403, 23 Tactel yarn, 24–5 Tactel Duo, 25 Tactel Inspira, 25 Tactel Strata, 24–5 tactile comfort, 236 take-down tension, 220 TCM see textile composite materials technical back, 123 technical face, 123 teflon-type fibres, 25 Tepex, 199, 200 textile composite materials, 194 textile composite prepregs, 199–200 textile preforms, 198–9 Textile Research Laboratory, 118 textiles, 262–3 active methods, 263 passive methods, 263 thermal comfort, 235 thermoplastic composite preforms, 200 2D and 3D commingled knitted fabric, 206 thermoplastic composite prepregs, 199 materials, 200 thermoplastic fabrics, 41 thermoplastic materials, 202–3 thermoset materials, 203 thermoset polymers, 203 tightness factor, 219 Towflex, 199, 200 transverse wicking plate test, 244 tricot fabric, 119–20 Tricot machines, 113, 114 true-rib machines, 174, 175 truss element, 162 tubular fabrics, 187 tubular forms circular cross-section, 144 rectangular cross-section, 144 tubular knitted fabrics, 182–3 in surgery uniforms, 189 tucking, 230 twill stitch, 125 Twintex, 199, 200, 207 ultra fine gauge knitting machines, 179–80 van der Waals forces, 290–1 Viloft Excel fibre, 26
Viloft Oryginal fibre, 26 void fraction see porosity wale shift-free Delaware stitch fabric, 118, 119 wale shifting, 118 walewise deformation, 57–8 warping rubber yarns, 188 warp knit fabrics advances in production, 110–35 curling propensity, 112 face and back, 111–12 Jersey stitch loop and lap diagram, 112 wale and course, 111, 112 Americana and modified Americana tricots, 126–30 basic types, 131 commercial warp knit machines, 113–16 Delaware stitch and modified Delaware stitch tricot fabrics, 116–19 key Raschel fabrics containing spandex, 120–4 Milanese fabrics, 132 newly developed constructions with spandex, 124–6 surface interest fabrics, 130–2 Tricot and Raschel containing spandex, 119–20, 121 nylon/spandex vs 100% nylon jersey, 120, 121 stitches used in tricot fabrics containing spandex, 120 yarn used in tricot, 120 and woven fabrics cross-sections, 110–11 woven, weft and warp knits, 111 warp knit machines, 7 commercial, 113–16 basic commercial warp knit fabrics, 114–15 warp knits sales vs woven and weft knits, 113 knitting and finishing, 115–16 Jersey and modified Jersey stitches, 116 types, 113–14 basic types of machines, 114 needles for warp knitting, 113 Tricot vs Raschel machines, 114 Washburn wicking kinetics, 294 water absorbency test, 300 weft knit fabrics, 110, 183 weft knitted structures, 185 applications, 163–7 automotive, 164–6 composite reinforcements, 164 3D-shaped weft-knitted reinforced tube connexion, 165 geotextile, 166–7 gloves, 166 home textiles, 166 safety and protection, 167 technical gloves, 166 basics of wetting, 288–91 direct and indirect attached water, 289 hydrophilic and hydrophobic nature of fibrous materials, 288–90 low and high contact angles, 290 solid polymers critical surface tension, 291 wetting, contact angle and surface tension, 290–1
© Woodhead Publishing Limited, 2011
Index current problems and limitations, 138–40 directionally oriented structures, 149–54, 155 bi-axial DOS structures, 150–2 combined DOS-3D-shaped structures, 153 combined DOS-3D-spacer structures, 153–4, 153–5 mono-axial DOS structures, 149–50 multi-axial DOS structures, 153 3D structures using weft knitting technology, 141–8 produced by circular weft knitting technology, 141–2 produced by computerised flatbed knitting technology, 143 techniques for shaped hollow structures production, 143–8 engineered knitted spacer structures for sweat management, 302–6 experimental results, 303–4 porosities and capillary radii, 302–3 theoretical and experimental absorbency, 305 theoretical liquid absorbency, 304–6 experimental liquid take-up, 298–306 Allasso Absorbency Analyser, 299 double jersey knitted structures absorbency, 301 fabric characteristics influence, 299–301 gravimetric absorbency testing system, 298–9 spacer absorbency with different fibre surface in contact with water, 301 water take-up by different structures, 300 future trends, 167, 306 industrial applications, 136–67 load-extension characteristic curve, 137 structure designing for particular properties, 138 model based on elastica theory, 157–60 calculation results for initial state, 160 extension in walewise direction, 160 forces and moment applied on a loop quarter, 158 loop structure and repeating elements, 158 parameters used for calculations, 159 model based on finite element analyses, 161–3 load extension curves for the biaxial elongation, 163 plain weft knitted fabric structure, 161 representative unit, 161 truss element parameters, 163 moisture management, 287–306 multifunctional structures, 155–6 high performance multifunctional summer vest, 156 homogeneous effect or layered structure, 156 localised effect or patchwork structure, 156 physical properties, 37–46 air permeability, 43 comfort, 45 creasing, 41–2 dimensional stability, 40–1
317
knitted fabrics with special properties, 46 liquid transfer properties, 44 pilling and abrasion, 45–6 recovery properties, 39–40 stretch and recovery properties, 38–9 thermal properties, 43–4 thickness and compression properties, 42–3 simulating mechanical properties, 157–63 spacer fabrics absorbency, 304 experimental absorbency rates, 303 samples fabric specification, capillary radius and porosity, 302 theoretical absorbency rates, 305 wicking and absorption, 291–8 capillary formed by a spacer yarn in the structure, 296 improvement by additives, 296–7 liquid take-up, 297–8 meniscus shape formation within the capillary, 292 spacer fabric yarn path, 296 wicking and capillary action, 291–2 wicking dynamics, 292–6 weft knitting yarn delivery systems, 89–109 future trends, 109 main objectives, 89 storage on circular knitting machines, 103–6 storage on flat knitting machines, 107–8 theory, 90–102 wet contact method, 240 wet space method, 241 wetting, 243, 288–91, 290–1 wicking, 243 and absorption, 291–8 liquid take-up, 297–8 capillary action, 291–2 meniscus shape formation within the capillary, 292 defined, 291 dynamics, 292–6 capillary formed by a spacer yarn in the structure, 296 spacer fabric yarn path, 296 improvement by additives, 296–7 winding angle, 11 winding frames, 7 wind noise, 282 wool, 20 wool fibres, 288 woollen single jersey fabrics, 172 woven-like fabrics, 179 yarn delivery systems future trends, 109 main objectives, 89 negative feeders with yarn tensile stress control, 97–102 electronic feeders, 101–2 electronic yarn tensile stress control outline, 102 frictional feeders, 100–1 storage feeders, 97–9 stored and wound yarn slippage and separated wound yarns, 99
© Woodhead Publishing Limited, 2011
1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 20 1 2 3 4 5 6 7 8 9 30 1 2 3 4 5 6 7 8 9 40 1 2 43X
318 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 20 1 2 3 4 5 6 7 8 9 30 1 2 3 4 5 6 7 8 9 40 1 2 43X
Index
stored yarn supply basic systems, 99 striper feeder, 101 two-colour pattern with knit and miss back, 97 passive, 92–4 compensation tensioners, 93 yarn tensile stress control, 92 positive feeders with yarn length control, 94–6 belt feeder, 96 elastan roller, 96 storage feeder, 94–6 storage feeder with length control, 95 yarn tensile stress effect on quantity of yarn delivered, 95 requirements, 89–90 storage on circular knitting machines, 103–6 creel cross-section with internal air circuit, 105 double bobbins on creel, 104 Milano-rib knit, 106 yarn feeding outline, 103 storage on flat knitting machines, 107–8 doubling the yarn with some twisting, 108 yarn feeding outline, 107 theory, 90–102 stitch forming zone on weft knitting machine, 90 yarn robbing back in knitting, 91 weft knitting, 89–109 yarn input tension, 219, 225 yarn length control, 104 yarns see also specific yarns count and machine gauge, 220 future trends, 34–5 knitting defects, 13–19 cotton, cotton-similar and blend knitting yarn defects, 16–19 wool, wool-similar and blend knitting yarn defects, 13–16 length per stitch, 219
packaging, 7–10 cylinder winding process, 8–10 polyamide yarn and its modification, 24–5 polyurethane yarns joining methods in composites, 32–4 yarns produced using the braiding technique, 32–3 yarns produced using the covering by spinning method, 33 yarns produced using the pneumatic looping technique, 33–4 yarns produced using the twisting method, 33 qualitative factors, 4–6 technological, 6 useful, 6 raw material management, 215–16 appearance, 215 count, 215 elasticity, 216 evenness, 215–16 friction, 216 twist, 216 special applications, 28–32, 34 antibacterial fibres, 28–31 elastomeric yarns, 31–2 electroconductive yarn, 34 high modulus yarn, 34 microcapsules, 31 structure of cope, 10–13 one-thread package formation, 11 thread rolls arrangement on the bobbin surface, 12 winding net structure, 12–13 suitability for knitting, 3–35 preparation for knitting process, 6–7 types, 19–28 fancy threads, 26–8 natural fibres, 19–21 synthetic fibres, 21–6 yarn tensile stress control, 97 Young-Dupré equation, 291 Young’s modulus, 137
© Woodhead Publishing Limited, 2011
