The Application of Textiles in Rubber

The Application of Textiles in Rubber developed during the 1940s to become one of the major yarn types used.. to mean any fibre which is produced by man, and so rayon is classified as a

The Application of Textiles in Rubber

David B Wootton

Rapra Technology Limited

Shawbury, Shrewsbury, Shropshire SY4 4NR, United KingdomTelephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118

http://www.rapra.net

First Published in 2001 by

Rapra Technology Limited

Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2001, Rapra Technology Limited

The right of David Wootton to be recognised as the author of this workhas been asserted by him in accordance with sections 77 and 78 of the

Copyright, Designs and Patents Act 1998

All rights reserved Except as permitted under current legislation no part

of this publication may be photocopied, reproduced or distributed in anyform or by any means or stored in a database or retrieval system, without

the prior permission from the copyright holder

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

Typeset by Rapra Technology LimitedPrinted and bound by Polestar Scientifica, Exeter, UK

ISBN: 1-85957-277-4

Preface

Rubber and textiles have been used together, each working with the other to give improvedperformance in a very wide range of applications, since the earliest days of the rubberindustry in the more developed areas of the world

For many years, rubber companies of reasonable size, using textile reinforcement, wouldemploy their own textile technologist working alongside the rubber technologists Overthe last third of the twentieth century, faced with global competition and the need tocontrol and reduce total costs, this luxury has largely disappeared apart from the largestcompanies (particularly the tyre companies) Most organisations now rely on their textilesuppliers to provide technical knowledge and expertise As a result, the textile componentfor many applications is now considered in much the same way as the other raw materials,that is as an existing product, which only requires introducing into the manufacturingprocess, without any special knowledge or understanding, and is supplied against anagreed specification, which was probably drawn up by the textile manufacturer anyway.The aim of this current work is to provide a general background to and a basic awareness

of the technology of textiles, to give the rubber technologists an improved understanding

of the uses, processes and potential problems associated with the use of textiles inrubber products

The most important and by far the largest use of textiles in rubber is in the tyre industry.This area is not covered in this book, as the field covers such a wide range that itwould require a volume on its own In addition, most tyre companies have their owntextile specialists and have developed their own technologies, shrouded in the mysteries

of ‘trade secrets’

The first part of this volume covers the basic technology of the textile fibres and theprocesses used in preparing these ‘ready made’ raw materials for rubber reinforcement.Particular attention is given to various aspects of adhesion, adhesive treatments, theeffects of rubber compounding and processing and the assessment of adhesion

In the second half of the book, the major applications of textiles in rubber are described;the aim here is to illustrate the way that the textile component can be designed andengineered to obtain the optimum reinforcement and performance for each particularapplication These descriptions are not intended to be definitive technological theses on

The Application of Textiles in Rubber

2

the different applications However, they indicate the balance of properties required andhow these can be obtained in the textile component by selection of the fibres used, thephysical form of the reinforcement and the processes and treatments required

Over the years since the earliest days of Hancock, Goodyear and Macintosh, there havebeen many significant breakthroughs and developments, in both textile and rubbertechnologies Originally, there were only cotton and natural rubber, now there are wideranges of both synthetic rubbers and of man-made fibres There have been great advances

in the technologies of vulcanisation and of adhesive treatments; the service requirementshave become more stringent and operating conditions more severe, but these issues havelargely been overcome by improving expertise and knowledge

However, over the last two decades, there has been relatively little advance in the generaltechnologies of textiles or rubbers; most developments have been targeted either at costcontainment or at very high performance (and consequently very high cost) applications,particularly aerospace, with only minor spin-offs for everyday terrestrial applications.Where possible, the general content of the chapters has been kept as simple and practical

as possible but where there is a more theoretical discussion of certain aspects, these havebeen separated into appendices, at the end of the relevant chapters The general discussioncan thus be read without the intrusion of the more theoretical aspects, but these are stillavailable, if desired A glossary of terms has been included to assist the reader

I wish to thank all those at Rapra who have encouraged and assisted me in the preparationand publication of this book, in particular Clair Griffiths and Steve Barnfield, for theirwork in preparing the manuscript for publication

David B Wootton

Contents

Preface 1

1 Historical Background 3

Introduction 3

1.1 The Textile Industry 3

1.2 The Rubber Industry 6

1.3 Textile and Rubber Composites 10

References 13

2 Production and Properties of Textile Yarns 15

Introduction 15

2.1 Production Methods for Textile Fibres 15

2.1.1 Cotton 15

2.1.2 Rayon 21

2.1.3 Nylon 24

2.1.4 Polyester 26

2.1.5 Aramid 28

2.2 General Characteristics of Textile Fibres 30

2.2.1 Cotton 30

2.2.2 Rayon 32

2.2.3 Nylon 33

2.2.4 Polyester 34

2.2.5 Aramid 35

2.3 General Physical Properties of Textile Fibres 36

2.3.1 Cotton 36

The Application of Textiles in Rubber

ii

2.3.2 Rayon 38

2.3.3 Nylon 39

2.3.4 Polyester 40

2.3.5 Aramid 40

References 40

3 Yarn and Cord Processes 41

Introduction 41

3.1 Yarn Preparation Methods 41

3.1.1 Twisting 42

3.1.2 Texturing 49

3.2 Warp Preparation 52

3.2.1 Direct Warping 53

3.2.2 Sectional Warping 54

3.3 Sizing 57

4 Fabric Formation and Design of Fabrics 59

Introduction 59

4.1 Fabric Formation 59

4.1.1 Weaving 59

4.1.2 Knitting 64

4.1.3 Non-Woven Fabrics 68

4.2 The Design of Woven Fabrics 70

4.2.1 Physical Property Requirements 70

4.2.2 Selection of Fibre Type 71

4.2.3 Selection of Fabric Construction 74

5 Heat-Setting and Adhesive Treatments 83

Introduction 83

5.1 Heat-Setting Machinery 83

Contents

5.2 Heat-Setting 90

5.3 Adhesive Treatment 94

5.3.1 Cotton 94

5.3.2 Rayon 95

5.3.3 Nylon 98

5.3.4 Polyester 99

5.3.5 Aramid 101

5.4 The In Situ Bonding System 102

5.5 Mechanisms of Adhesion 103

5.6 Environmental Factors Affecting Adhesion 107

Appendix V Interfacial Compatibility 109

References 112

6 Basic Rubber Compounding and Composite Assembly 113

6.1 Compounding 113

6.1.1 Polymers 113

6.1.2 Curing Systems 114

6.1.3 Fillers 116

6.1.4 Antidegradants 117

6.1.5 Other Compounding Ingredients 117

6.2 Processing 117

6.3 Composite Assembly 118

6.3.1 Calendering 118

6.3.2 Coating 124

References 127

7 Assessment of Adhesion 129

Introduction 129

7.1 Cord Tests 129

The Application of Textiles in Rubber

iv

7.1.1 Pull-Out Tests 130

7.1.2 Cord Peel Test 130

7.2 Fabric Test Methods 133

7.3 Testing and Interpretation of Results 138

7.4 Adhesion Tests for Lightweight Fabrics and Coatings 140

7.5 Peeling by Dead-Weight Loading 142

7.6 Direct Tension Testing of Adhesion 143

7.7 Adhesion and Fatigue Testing 145

7.8 Assessment of Penetration into the Textile Structure 146

Appendix VII: The Physics of Peeling 148

References 153

8 Conveyor Belting 155

Introduction 155

8.1 Belt Construction and Operation 160

8.1.1 Carcase 160

8.1.2 Insulation 161

8.1.3 Covers 162

8.2 Belt Design 165

8.2.1 Plied Belting 167

8.2.2 Single-Ply and Solid-Woven Belting 171

8.2.3 Steel Cord Belting 172

8.3 Belting Manufacture 172

8.3.1 Belt Building 173

8.3.2 Pressing and Curing 173

8.3.3 Belt Joining 178

8.4 Belt Testing 182

8.4.1 Tensile Strength and Elongation 182

Contents

8.4.2 Gauge 183

8.4.3 Adhesion 183

8.4.4 Abrasion 183

8.4.5 Troughability 183

8.4.6 Fire Resistance 183

References 184

9 Hose 187

Introduction 187

9.1 Hose Manufacture 188

9.1.1 Braiding 188

9.1.2 Spiralling 190

9.1.3 Wrapped Hose 191

9.1.4 Knitted Hose 192

9.1.5 Oil Suction and Discharge Hose 192

9.1.6 Circular Woven Hose 193

Appendix IX 195

i Neutral Angle 195

ii Bursting Pressure 196

10 Power Transmission Belts 199

Introduction 199

10.1 Main Types of Power Transmission Belts 200

10.1.1 V-Belts 200

10.1.2 Timing Belts 203

10.1.3 Flat Belting 203

10.1.4 Cut-Length Belting 205

10.2 Manufacture of Power Transmission Belting 206

10.2.1 Manufacture of V-Belts 206

10.2.2 Manufacture of Timing Belts 209

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vi

10.3 Effect of the Textile Reinforcement on Belt Performance 209

References 212

11 Applications of Coated Fabrics 213

Introduction 213

11.1 Inflatable Structures 214

11.1.1 Inflatable Boats 214

11.1.2 Oil Booms 218

11.1.3 Inflatable Dams 219

11.1.4 Inflatable Buildings 220

11.1.5 Dunnage Bags 221

11.2 Non-Inflated Structures 222

11.2.1 Reservoir and Pond Liners 222

11.2.2 Flexible Storage Tanks 223

11.2.3 Supported Building Structures 223

References 224

12 Miscellaneous Applications of Textiles in Rubber 225

Introduction 225

12.1 Hovercraft Skirts 225

12.1.1 Types of Skirt 226

12.2 Air Brake Chamber Diaphragms 229

12.3 Snowmobile Tracks 230

References 231

Abbreviations and Acronyms 233

Glossary 234

Index 239

Introduction

The modern world relies to a great extent, on textile/polymer composites, the majority

of which are rubber/textile compositions In fact, it is difficult to imagine the functioning

of modern everyday life without the use of such products It is only necessary to considerthe need for transport systems (relying on textile/rubber tyres), materials handling systems(relying on textile/rubber conveyor belting) and mechanical drive systems (using rubber/textile drive belts) to see the important role played by such materials

Whereas textiles have been produced and used for many thousands of years, it was onlysome 500 years ago that rubber was introduced to Europe and really only in the last twohundred years that textiles and rubber have been used together in this region Since then,however, there has been very great development in the design and use of these materials.Within the last 75 years, there has been a great move away from natural materials (naturalrubber and cotton) to synthetic products, both as regards the fibres and the polymersused, resulting in a very wide diversity of engineered composites, to meet many andvaried performance requirements

1.1 The Textile Industry

The origin of the textile industry is lost in the past Fine cotton fabrics have been found

in India, dating from some 6-7000 years ago, and fine and delicate linen fabrics havebeen found from two to three thousand years ago, at the height of the Egyptiancivilisations More recent archaeological excavations, among some of Europe’s oldestStone Age sites, have found imprints of textile structures, dating back some 25,000 years,but in the humid conditions obtaining in these more northerly areas, all traces of theactual textiles have long disappeared, unlike those from the dry areas of India and Egypt.Until more recent times, the spinning of the yarns and the weaving of the fabrics weregenerally undertaken by small groups of people, working together – often as a familygroup However, during the Roman occupation of England, the Romans established a

The Application of Textiles in Rubber

‘factory’ at Winchester, for the production, on a larger scale, of warm woollen blankets,

to help reduce the impact of the British weather on the soldiers from southern Europe

In the family context, it generally fell to the female side to undertake the spinning,while the weaving was the domain of the men Spinning was originally done using thedistaff to hold the unspun fibres, which were then teased out using the fingers andtwisted into the final yarn on the spindle In the 1530s, in Brunswick, a ‘spinningwheel’ was invented, with the wheel driven by a foot pedal, giving better control anduniformity to the yarns produced Often, great skill was developed, as shown by therecords of a woman in Norwich, who spun one pound of combed wool into a singleyarn measuring 168,000 yards, and from the same weight of cotton, spun a yarn of203,000 yards In today’s measures this is equivalent to a cotton count of 240, orapproximately 25 decitex Cotton count is the number of hanks of 840 yards (768metres) giving a total weight of 1 lb (453.6 g) A Tex is a measurement of the lineardensity of a yarn or cord, being the weight in grams of a 1,000 m length; a decitex isthe weight in grams of a 10,000 m length

By the eighteenth century, small co-operatives were being formed for the production oftextiles, but it was really only with the mechanisation of spinning and weaving duringthe Industrial Revolution, that mass production started

Up to this time, both spinning and weaving were essentially hand operations looms were operated by one person, passing the weft (the transverse threads) by hand,and performing all the other stages of weaving manually (see Chapter 4 for a description

Hand-of the weaving process) In 1733, John Kay invented the ‘flying shuttle’, which enabled amuch faster method for inserting the weft into the fabric at the loom and greatly increasedthe productivity of the weavers

Until the advent of the flying shuttle, the limiting factor in the production chain forfabrics was the output of the individual weaver, but this now changed and with the morerapid use of the yarns, their production became the limiting factor in the total process In

1764, this was partly resolved by the invention, by James Hargreaves, of the ‘SpinningJenny’, which was developed further by Sir Richard Arkwright, with his water spinningframe, in 1769, and then in 1779, by Samuel Crompton, with his ‘spinning mule’.Alongside these developments in spinning, similar changes were taking place in the weavingfield, with the invention of the power loom by Edmund Cartwright, in 1785

With this increase in mechanisation of the whole industry, it was logical to bring theproduction together, rather than keeping it widely spread throughout the homes of theproducers Accordingly, factories were established The first of such was in Doncaster in

Historical Background

1787, with many power looms powered by one large steam engine Unfortunately, thiswas not a financial success, and the mill only operated for about 3 or 4 years

Meanwhile, other mills were being established, in Glasgow, Dumbarton and Manchester

A large mill was erected at Knott Mill, Manchester, although this burnt down after onlyabout 18 months The first really successful mill was opened in Glasgow in 1801.However, this industrialisation was not to everyone’s liking; many individuals were losingtheir livelihoods to the mass production starting to come from the increasing number ofmills This led to a backlash from the general public, resulting in the Luddite Riots in1811-12, when bands of masked people under the leadership of ‘King Ludd’ attackedthe new factories, smashing all the machinery therein It was only after very harshsuppression, resulting in the hanging or deportation of convicted Luddites in 1813, thatthis destruction was virtually stopped However, there were still some outbreaks of similaractions in 1816, during the depression following the end of the British war with France,and this intermittent action only finally stopped when general prosperity increased again

in the 1820s

Following this, the textile industry expanded considerably, particularly in the areas wherethe raw materials were readily available For example, the woollen mills in East Anglia,where there was good grazing for the sheep, and in West Yorkshire and Eastern Lancashire,where either coal was available for powering the new steam engines, or where fast flowingstreams existed to provide the energy source for water-powered mills (particularly incentral Lancashire) The main woollen textile production developed in Yorkshire, as itwas easier and cheaper to transport the raw wool there, than to carry the large quantities

of coal required to power the mills to the wool growing areas In Lancashire, with theports of Liverpool and Manchester close by for the importation of cotton from America,the cotton industry grew and flourished However, in the 1860s, due to the AmericanCivil War, the supply of cotton from America dried up and caused great hardship amongthe cotton towns of south and east Lancashire

On account of this, and with the great strides being made in chemistry, research wasbegun to try to find ways of making artificial yarns and fibres The first successful artificialyarn was the Chardonnet ‘artificial silk’, a cellulosic fibre regenerated from spunnitrocellulose Further developments lead to the cuprammonium process and then to theviscose process for the production of another cellulosic, rayon This latter viscose wasfully commercialised by Courtaulds in 1904, although it was not widely used in rubberreinforcement until the 1920s, with the development of the balloon tyre

Research continued into fibre-forming polymers, but the next new fully synthetic yarnwas not discovered until the 1930s, when Wallace Hume Carothers, working for DuPont,discovered and developed nylon This was first commercialised in 1938 and was widely

The Application of Textiles in Rubber

developed during the 1940s to become one of the major yarn types used Continuingresearch led to the discovery of polyester in 1941, and over the ensuing decades, polyolefinfibres (although because of their low melting/softening temperatures, these are not used

as reinforcing fibres in rubbers) and aramids

As the chemical industry greatly increased the types of yarns available for textile applications,

so the machinery used in the industry was being developed Whereas the basic principles ofspinning and weaving have not significantly changed over the millennia, the speed andefficiency of the equipment used for this has been vastly been improved In weaving, themajor changes have been related to the method of weft insertion; the conventional shuttlehas been replaced by rapiers, air and water jets, giving far higher speeds of weft insertion.Other methods of fabric formation have similarly been developed, such as the high speedknitting machines and methods for producing fabric webs known as ‘non-wovens’

1.2 The Rubber Industry

Whereas the basic properties of rubber, or caoutchouc as it was then called, were known

to the natives of South America, the first reports of it in Western Europe were given byChristopher Columbus in 1492 and then more detailed accounts were given by Gonzalo

Fernandez d’Ovideo y Valdas, in his Universal History of the Indies [1], in which he

describes the game of ‘batos’ as like a game of balls, ‘But played differently and the ballsare of other material than those used by Christians’

Later, Juan de Torquemada [2] describes the use of elastic balls from the sap of the Ulaqahiltree, which juice was also used for painting on linen fabrics, to protect the wearer from therain; water did not penetrate but the sun’s rays ‘had an evil effect on the coating’

In 1731 the French government sent the geographer Charles Marie de La Condamine toSouth America on a geographical expedition In 1736 he sent back to France a report tothe Paris Academy, together with several rolls of crude rubber and a description of theproducts fabricated from it by the people of the Amazon Valley In this report, he statedthat the resin (caoutchouc) from the Hévé tree was used, in the province of Quito, tocover linen material, which was then used like oilcloth Fresnau, an engineer, later reportedmore fully on this use and suggested other possible applications, such as waterproofsails, divers’ hoses and bags for keeping food, etc He also commented, however, thatsuch goods could only be produced in those areas where the trees grew, as the juice driesvery quickly and looses its fluidity

During the 18th century, small quantities of rubber were sent to Europe and foundsome limited applications For example, in 1770, Joseph Priestly drew attention to the

Historical Background

fact that small pieces of caoutchouc could be used for rubbing out pencil marks Since

1775, small pieces have been available in stationers’ shops for this purpose, called

‘India Rubbers’, by which name this material has been known ever since, in Englishspeaking countries

More important uses were found for this material, however, and in spite of the comments

by Fresnau some fifty years earlier, one of these earlier applications was for coatingfabrics, to render it waterproof, where the ‘loss of fluidity’ was overcome by solution ofthe rubber in turpentine; this was the subject of one of the earliest patents for the use ofrubber, granted to Samuel Peal in 1796 [3]

All the rubber available at this time, was, of course, wild rubber, gathered from the rainforests of Central and Southern America This rubber was mainly in the form of ‘bottles’,from the wooden formers on which the latex was dried and smoked, or roughly spherical

‘negro-heads’, consisting of many small lumps of dried rubber stuck together Originally,products could only be made by cutting the rubber from these rough blocks or by dissolving

it in a suitable solvent, such as turpentine, and spreading it onto fabric or some similarsubstrate However, in 1820 Thomas Hancock [4] noted that on heating, rubber becamesoft and plastic; also on kneading it in a dough mixer, without solvent, it would becomesoft and more easily worked Accordingly, he designed a machine to enable the roughlumps and offcuts to be worked together into a soft mass This could then be pressedinto a heated mould to give a regular and uniform block of rubber, which was mucheasier to handle and work with

From these prepared blocks, sheets of various sizes and thicknesses were cut for manyapplications; one of these was for use as pads between the railway lines and the sleepers,

to reduce vibration More complicated mouldings were also made and textiles were plied

up with thinly cut sheets or coated for solution One of the best known names in thislatter context was that of Charles Macintosh, who patented many applications e.g., [5]for proofing fabrics In 1823 he established a factory in Glasgow and then later moved

to Manchester, building his plant in Cambridge Street, which site is still used for rubbermanufacture and coating

Many other uses were found for rubber; by 1825, hoses were being built on mandrels,with reinforcement of two or more plies of fabric, and with wire spiralling for suctionhose In 1826, rubber insulated cables were use by Baron Schilling, for detonatingexplosives in mines; drive belts made from layers of fabric bonded together with rubberwere used by Isambard Kingdom Brunel to drive the machines used in sinking the ThamesTunnel Inflatables of many kinds were produced from coated fabrics Hancock [4] incollaboration with Macintosh produced air beds and pillows, such as were used by KingGeorge IV on his deathbed Large floating pontoons, for floating bridges, were produced

The Application of Textiles in Rubber

and tested to the satisfaction of the Duke of Wellington Throughout this period,waterproof cloaks were worn by the passengers on the stagecoaches

All these products, however, had severe service limitations They would soften and becomesticky in warm weather or would harden and become brittle in the cold Much work wasdone to overcome these problems and, independently, Charles Goodyear in the USA andHancock in England, discovered the effects of sulphur in vulcanising rubber

This discovery of vulcanisation gave a great boost to the rubber industry The properties,and especially the service life, of all the rubber articles were vastly improved and newoutlets and applications were continually being found In 1845 a Scotsman, RobertWilliam Thomson, invented the pneumatic tyre [6] However, this was designed for usewith steam road engines, which were not in favour with the Government of the time, itwas not developed further until the advent of the bicycle and motor car, when it was re-invented by John Boyd Dunlop [7, 8] Between these times, the solid tyre sufficed andindeed was given royal approval, in 1846, by Queen Victoria

This was all accomplished with supplies of wild rubber In 1836, the consumption ofrubber in Western Europe was some 65 tonnes per annum As the industry grew, so didconsumption, reaching 2,250 tonnes in 1853 and 15,600 tonnes by 1887 At this time,

rubber from sources other than the Hevea brasiliensis, such as Ficus elasticus and the shrub Guyale, was being imported into Europe.

By this time it was obvious that the industry could not survive on wild rubber only In

1876 the British explorer Sir Henry Wickham collected about 70,000 seeds of Hevea

brasiliensis, and, despite a rigid embargo, smuggled them out of Brazil The seeds were

successfully germinated in the hothouses of the Royal Botanical Gardens at Kew inLondon, and were used to establish plantations first in Ceylon in 1888 and then in othertropical regions of the eastern hemisphere During the next decade, plantations weremore widely established in Ceylon and Malaya but significant imports of plantationrubber into Europe were not made until 1901, by which time the consumption of wildrubber had increased to 27,000 tonnes per year The plantations soon proved their worth,and by 1936 over 1,000,000 tonnes of rubber were being produced annually, generallywithin the geographical range of around 1,100 km either side of the Equator

While the production of rubber and its uses were expanding, so the technology wasdeveloping It was quite soon found that the addition of certain metal oxides assisted invulcanisation In 1880, while trying to use ammonia to produce sponge rubber, T Rowleyfound that this vastly increased the rate of vulcanisation [9] Work in this area continuedand in 1906, George Oenslager discovered two much more readily applicable materials,

to accelerate vulcanisation, aniline and thiocarbanilide

Historical Background

Research continued and, in 1912, the use of piperidine was patented [10] to be followed

by the thiuram disulphides, which were also shown, in 1919, to be able to cure rubberwithout the addition of sulphur Then, in 1923, mercaptobenzthiazole, the basis of manymodern accelerators, was discovered

Meanwhile, chemists were also studying the composition of the rubber itself It had beenshown to possess the same empirical formula as isoprene and, in 1860, Charles GrevilleWilliams established that it was in fact a linear polymer of isoprene By the 1890s, it wasshown that isoprene could change, on standing, into a rubbery solid, albeit with ratherdifferent properties from those of natural rubber itself This reaction is now known aspolymerisation The generally poor properties of the spontaneously polymerised isoprenearise from the lack of steric regularity, a problem only overcome some 60 years later.The search for a synthetic rubber continued and was spurred on, in the early 20th century,

by the increasing price of natural rubber and then by the First World War Various dieneswere investigated for their potential for polymerisation The most promising of thesewas dimethyl butadiene and, during the period from 1915 to 1918, Germany was able

to produce some 2,500 tonnes of ‘methyl rubber’ using the sodium polymerisation routestill in use today These early synthetic rubbers left much to be desired in their overallproperties; the use of carbon black for reinforcement was not known in Germany andthe technology of vulcanisation and the use of protective anti-oxidants were in the veryearly stages of development On account of these shortcomings, research into syntheticrubbers was largely allowed to drop

However, Father Julius Nieuwland, of the University of Notre Dame but working for theDuPont Company, discovered polychloroprene in 1930, which was first marketed underthe trade name of ‘Duprene’ but latterly called ‘Neoprene’ This group of synthetic rubbers,

as they became available during the 1930s, largely changed the attitude of the rubberindustry towards synthetics The general properties of these rubbers were quite good butthe ageing and properties, such as the resistance to oils and solvents, were very muchbetter than with the natural rubber

This gave a further boost to research and in 1935, the chemists of IG Farbenindustrie inGermany, developed the ‘Buna’ rubbers, the name being derived from butadiene, one ofthe common monomers, and Na, the chemical symbol for sodium, used as the catalyst.The major types developed were the standard Buna rubbers, copolymers of butadienewith styrene, and the Buna N types, with acrylonitrile as one of the monomers

A further great impetus was given to research by the advent of the Second World War,when supplies of natural rubber from the Far East were completely cut off and the USGovernment launched a crash programme to develop a viable alternative This quickly

The Application of Textiles in Rubber

led to the development of the GR-S rubbers (styrene-butadiene rubber, now known asSBR) and the rapid establishment of large scale production of these polymers

In the 1950s work by Natta and Ziegler on catalysation processes led to the discovery of

novel methods for obtaining highly stereo-regular polymers, including the high

cis-polyisoprene, ‘synthetic natural rubber’, which had been sought since the composition

of natural rubber had been established a century before

Today, there are many synthetic polymers available, ranging from the general purposehydrocarbons with properties largely similar to those of natural rubber; to the specialpurpose types with excellent resistance to ageing, oils and solvents; to highly sophisticated(albeit very expensive) polymers with outstanding resistance to the most hostile ofenvironments, as found in aerospace, marine and oil exploration applications

1.3 Textile and Rubber Composites

From the very first references to rubber in South America, its use with textiles has beennoted This is not very surprising, as from the earliest times, one of the major drawbacks

of textiles was their performance under wet conditions; in the dry, they gave excellentprotection and warmth, but in the wet they soon became saturated and, if anything,made things seem worse Many treatments were tried over the years to overcome thisdeficiency, using coatings of tars, resins and waxes; the most successful of these was thetreatment with natural drying oils, to give the waterproof oilcloths The main disadvantage

of these was the stiffness and brittleness imparted to the fabrics

With rubber, many of these disadvantages virtually disappeared, giving a soft, flexibleand waterproof material (at least at normal ambient temperatures) This is essentiallythe stage that Macintosh and Hancock started Macintosh improved the coatingprocess, with his single and double textures (this latter consisting of two layers offabric adhered together with a thin film of rubber) but it was largely Hancock, withhis imaginative approach, who developed a wide range of applications Apart fromwaterproof coats and cloaks for travellers, he produced waterproof bags Some wereused by Captain Parry on his expedition to the North Pole, who, in his report on thevoyage, refers to saving a bag of cocoa, which fell off an ice-floe during unloading,but ‘…the bag being made of Macintosh’s waterproof canvas, did not suffer theslightest injury I know of no material which, with an equal weight, is equallydurable and waterproof’ [11]

Hancock realised the advantages of combining the strength of the textile with the otherproperties of rubber He produced hose by wrapping successive layers of rubber and

Historical Background

fabric onto a mandrel In spite of fierce opposition from the traditional leather hosemakers, he persuaded Barclays Brewery, in London, to completely re-equip with rubberhose This quickly proved its worth as, being seamless, leakage, which had always been

a problem with the stitched leather hose, was reduced to negligible levels

In the same way that bags could be made waterproof, so also could they be made airtightand a great many applications for inflatable articles were found, covering air cushionsand pillows, air beds and inflatable boats A development from these, using inflatablebags, connected with rubber/fabric air hose, was used for lifting sunken ships The conceptworked well, but in the end, the project failed because of difficulties in attaching thebags to the object to be lifted

In the early days, many of these applications foundered simply because of the poor servicelife of the raw rubber Being unvulcanised, the rubber was susceptible to changes intemperature but the major problem arose from the poor ageing characteristics As allthese applications relied on only thin layers of rubber, they were very susceptible tooxidation and the service life was accordingly very short It is only over the last fewdecades, with the development of effective anti-ageing products, that this has satisfactorilybeen overcome and many of Hancock’s inventions have been ‘rediscovered’ and proved

to be sound concepts

Not all the early products were doomed to failure, however One of the early successeswas in the field of textile machinery One of the processes in the spinning of cotton iscarding (for more detail of this, see Chapter 2) The carding engine is equipped withrollers to which are attached a multitude of fine steel wires; originally these wires werefixed by means of a leather backing, but the variability of the natural product led toconsiderable problems in achieving uniformity when these wired leather strips were woundonto the steel rollers Hancock solved this problem by producing a backing of textilelaminated with rubber: this enabled a very uniform ‘card clothing’ to be provided, withsignificant advantages in consistency and life of the clothing The advantages of thismaterial were rapidly recognised and within a few years, the textile/rubber backed clothinghad completely replaced the original leather version on the cotton cards, and in fact isstill used today

The earlier products were, with the exception of hose, flat composites The next greatdevelopment, however, was the pneumatic tyre The tyre, developed by Dunlop, wasoriginally based on a tube strapped to the wheel by means of rubberised fabric, but soon,the inner tube with a separate outer tyre was evolved The outer tyre was made fromlayers of square woven cotton canvas and rubber, with wire beads to hold it in place onthe rim of the wheel By 1915, however, the canvas was replaced by cord fabrics Thesegave improved properties and performance to the tyres, but the limiting properties were

The Application of Textiles in Rubber

still those of the rubber At this time, carbon black was starting to be used: this effectivelydoubled the life of the tyres, which now lasted up to 4,000 miles Further improvements

in tyre life were achieved by the introduction of the balloon tyre in 1923: this used amuch larger cross-section tyre, operating at considerably lower pressure (200-300 kPa)than the earlier narrow section tyres, which required pressures of up to 700 kPa.These improvements in tyre performance now threw the restrictions on performance back

to the textile component The answer to this was to employ the relatively new artificialfibre, rayon, for the reinforcing plies of fabric But this introduced another problem.This was the first major use of fibres other than cotton Up to now there had been noproblem in adhering the rubber to the textile inserts: the techniques of spreading or frictioninghad resulted in good mechanical adhesion, due to the embedding of the fibre ends of thestaple yarns into the rubber With the continuous filament artificial fibre, there were nofibre ends to embed The search to find some system to give adequate adhesion led to thefirst adhesive dips These were originally based on natural latex and casein, but the caseincomponent was soon replaced with a resorcinol/formaldehyde resin

When natural rubber had to be replaced with synthetic, this, of course, applied to theadhesive systems too The SBR latex behaved similarly to natural and gave adequateadhesion to rayon, albeit with some loss of building tack When nylon was introduced, itwas found that these resorcinol/formaldehyde/latex (RFL) dips did not give satisfactoryadhesion Research led to the development of a terpolymer latex, containing vinyl pyridine

as the third monomer, which gave significantly improved adhesion with nylon and rayon.With the introduction of polyester, further adhesion problems arose: the standard RFLsystems did not work The first systems found to give good adhesion to polyester werebased on very active isocyanates from solvent solution, either on their own, to besubsequently treated with RFL, or in a rubber cement, in which case, no further treatmentwas required Solvent systems not being popular, much effort was devoted to the searchfor a satisfactory aqueous based process and this was finally achieved Then, severalyears later, a similar exercise had to be undertaken to find a system suitable for use withthe newly introduced aramid fibres

Similarly, with each new synthetic polymer introduced, special adhesive systems havehad to be developed in order to obtain the optimum performance from the resultanttextile/rubber composite

Thus, over the years, the two technologies, those of rubber and of textiles, have developedside by side Today, composites are available which satisfy the stringent performancerequirements met under such diverse and hostile environments as those of outer space orthe depths of the sea and at extremes of temperature

Manufacture in England, Longmans and Roberts, London, 1857.

10 F Hoffman and K Gottlob, inventors; Bayer Co., assignee; DT 226,619, 1912

11 Capt W.E Parry, Narrative of an Attempt to Reach the North Pole in Boats,

attached to HMS HECLA, in 1827, London, 1828.

to mean any fibre which is produced by man, and so rayon is classified as a synthetic yarn.

In Europe, however, the term synthetic is used only when referring to fibres in which thefibre-forming polymer is not of natural origin Thus in Europe, rayon, based on naturallyoccurring cellulose, is classified as ‘man-made’ or ‘artificial’ but is not considered to be a

‘synthetic’ yarn Rayon, the first of the successful artificial fibres, is chemically very similar

to cotton, but the various processes by which the yarn is produced introduce certaindifferences in properties between the two The nylons (both nylon 6.6 and nylon 6) werethe first of the truly synthetic fibres to be adopted for use by the rubber industry, and offercertain advantages over the cellulosic fibres Polyester, with strength similar to nylon, has

a higher modulus, which renders it more suitable for certain applications The aramids,with considerably higher strength and modulus, are the latest reinforcing yarns to beintroduced The latter are still somewhat limited in their application due mainly to theirrelatively high cost, although on a strength/cost basis, they are comparable with steel wire.Although not strictly textile fibres, glass and steel have found many applications asreinforcements in elastomers Their general physical properties are briefly compared withthe true textiles, in order to cover the complete range of materials in use at the present time

2.1 Production Methods for Textile Fibres

2.1.1 Cotton

Cotton is a natural fibre, consisting of the seed hairs of a range of plant species in the

Mallow family (Genus Gossypium) The plants are grown, mainly as an annual crop, in

many countries around the world between latitudes 40°N and 40°S

The Application of Textiles in Rubber

The seed is usually sown in the spring and by early summer the plants are in flower:within three days, the flowers fall, leaving the small seed-pod or boll The boll, containingthe seeds with the cotton fibres attached, grows and in about three months, bursts Atthis stage, the cotton fibres are wet and tightly crushed together, but they rapidly dry outand form a fluffy ball, ready for picking This was originally all done by hand, butmachinery is now available to do this work Average production these days is something

in excess of 600 kg/ha

On picking, the cotton is still attached to the seeds in the boll and so needs separating.This is done with a machine called a gin Essentially, this consists of a steel comb, withtoothed discs running between the teeth of the comb The disc teeth catch the cottonfibres and pull them through the comb, but the seeds are too large to pass through and soare separated The cotton, known at this stage as lint, is collected, compressed and baledready for shipment to the spinning mills Not all of the cotton is stripped off at this firstpass, so the residue is usually passed through the gin for a second time; on this secondpass, it is only the remaining broken and short fibres that are removed and these, known

as linters, are used mainly for stuffing upholstery or as a source of cellulose for industrialuses, such as the production of rayon

After baling, the cotton is sent to the Cotton Exchanges in various parts of the world, forsale to the spinners At this stage, it is necessary to grade the cotton This grading takesinto account many properties of the fibres, such as general appearance, cleanliness,maturity, etc., but the main characteristic is the staple length, that is the average length

of the individual fibres Broadly speaking, the cotton falls into four main types, known

as Sea Island, Egyptian, American Upland and Indian These designations originallyindicated the areas where the cotton has been grown, but they have now become more of

a type classification rather than an indication of origin, as is shown in Table 2.1.

s e p y t n t o c f o n i a c i i s a l c l a r e n e G 1 2 e l b a T n

i t a n g i

s

e

D F i b r e L e n g t h R a n g e

) m m (

s n i t a c i p A r o j a M

dalsI

a

e

naitp

y

dalpUnacire

m

snitacipalairtsunin

a

i

d

Production and Properties of Textile Yarns

At the spinning mill the cotton goes through various processes to convert it from therough compressed bales to a strong coherent and uniform yarn The various stages are asfollows:

(1) Bale Breaking: the bales are opened and slabs pulled off and fed into the breaking

machine, in which the lumps are subjected to the action of contrarotating rollers,fitted with steel spikes These pull tufts of the cotton off the compressed mass, andthese then pass over various screens, to remove some of the impurities present, such

as twigs, leaves and sand

As cottons of different grades and from different sources are usually blended together,

in order to obtain the desired properties in the final yarn, the blending normallystarts at this stage, by feeding slabs from different bales consecutively

After bale breaking, the cotton, still in fair-sized lumps, passes to the next stage

(2) Opening and Cleaning: the lumps of cotton from the breaker pass through fluted

steel rollers to a beating section, where rotary bladed cylinders beat the lumps, reducethe size of the tufts and at the same time remove still more of the contaminants,which fall through the bottom mesh of the machine

At the output end of the opener, the sheet of loose randomly laid fibres is fed throughnip rollers and wound up into a lap, for feeding to the next stage By this point, thecotton has changed from a hard compressed bale to a soft fibre web, similar to

‘cotton wool’

(3) Carding: a diagram of a cotton card is shown in Figure 2.1 The cotton, in the form

of the lap from the opener, passes through a feed nip and is presented to the in’, which consists of a roller covered with ‘card clothing’ Card clothing comprises aheavy backing made from rubberised fabric, through which angled steel wires pass,

‘taker-as shown in Figure 2.1 (a) ; the angle and length of these wires are of great importance

as they control the efficiency and performance of the card

As the lap approaches the taker-in, the wires take hold of the fibres; as the roller has

a much higher surface speed than the lap feed, the web of cotton becomes considerablyattenuated This web of fibres passes round the taker-in until it reaches the maincylinder, also covered with card clothing, with the wires angled in the same relativedirection As the main cylinder is moving faster than the taker-in, the condition as

shown in Figure 2.1 (b)(i) applies and the fibres are stripped from the taker-in, being

completely transferred to the main cylinder and becoming still more attenuated Asthe fibres are carried round the main cylinder, they reach the point where they meetthe ‘flats’, also covered with clothing These are moving more slowly than the cylinder,

The Application of Textiles in Rubber

but in the same direction and with the wires angled in the opposite direction, as in

Figure 2.1(b)(ii) A carding action occurs; the fibres are divided between the two

surfaces and thereby the tufts are teased out more fully still, until ultimately, allagglomerations of fibres are broken down, giving a web of largely unentangled fibres

As the flats are carried round, they are cleaned so that clear surfaces are continuouslypresented to the cylinder The fibre web continues round the cylinder until it meetsthe ‘doffer’, which rotates faster than the cylinder and strips the web off; the web isthen in turn stripped from this by the ‘doffer comb’, from whence it passes overguide rollers and a funnel shaped guide, which reduces its width to about 25 mm, inwhich form it is coiled into a large can for passing to the next stage In this form, thecontinuous ‘rope’ of cotton is called a ‘roving’

(4) Drafting: the drafting stage in the spinning process performs three essential functions.

The first of these is the parallelisation of the individual fibres, which up to now havebeen laid in a more or less random manner Secondly, it enables further blending ofthe different fibre types to take place Thirdly, it can thin down the rovings to a muchfiner form, with a slight twist inserted, to give sufficient strength for the final spinning

Figure 2.1 Cotton carding

Flats

LAP IN

Feed nip

in

Taker-Main cylinder

Doffer

Comb

ROVING OUT

(a) CARD CLOTHING

Wires Textile/

rubber backing

(b) CARDING ACTIONS Position Motion Action (i)

(ii)

stripping carding

Production and Properties of Textile Yarns

Drafting is performed by passing the roving, from the card or from a previous drafting,between pairs of rollers, each succeeding pair rotating faster than the previous, sothat the fibres are pulled out in a lengthwise direction This action not only helps toalign the fibres but also enables the size of the fibre bundle to be reduced Generally,

in the first passes through the drafting frames, it is desirable to maintain the thickness

of the roving; in order to do this, it is necessary to feed in several rovings at the sametime, so that when they have been pulled out, the final thickness of the one outputroving is the same as the individual ones originally fed in; in this way, additionalblending occurs In the final stage, it is necessary to reduce the thickness to suit thefeed requirement of the spinning frame; in this case, the roving is allowed to becomethinner, but in order to have sufficient strength to be fed through rollers, it is necessary

to introduce a slight twist into the yarn bundle, which is now referred to as a sliver.Whereas in the intermediate stages of drafting, the rovings are wound into largecans, in this final stage, the sliver is wound onto a supporting tube, for presentation

to the spinning frame

(5) Spinning: in the final spinning stage, the sliver is reduced still further in size and the

required level of twist is inserted In selecting the twist to be used there are variousfactors to be considered The higher the twist level, the more firmly held together arethe individual fibres However, a high twist level gives a much harder and stiffer yarnand the ultimate tensile strength of the yarn is reduced It is therefore usually necessary

to compromise to some extent on the level of twist used It is also necessary to decidewhich way to twist the yarn: the standard designation of twist direction refers to it as

S or Z It can be seen that the central sections of the two letters slope in opposite

directions; the designation relates to the direction of the twist of the yarn, when heldvertically The fibres either slope from top left to bottom right, as in the central bar

of the S or in the opposite direction as in the central bar of the Z; this is illustrated in

Figure 2.2.

A schematic representation of a ring spinning spindle is given in Figure 2.3 The sliver

from the drafting frame is fed between three pairs of drafting rollers, to reduce the size

to that required, and fed through a guide eye From here, it passes down, through the

‘traveller’ and then to the tube on which it is to be wound up The traveller consists of

a metal or plastic C-shaped piece, which is clipped onto the ring; the traveller is notdriven, but the spindle on which the tube is mounted is rotated at high speed (up toabout 12,000 rpm.) This rotation pulls the traveller around and in so doing, for eachrevolution of the traveller, one turn of twist is inserted into the yarn At the same time,

of course, the yarn is being fed forward and wound up onto the tube on the spindle; byadjustment of the rate at which the yarn is fed forward and the speed of rotation of thespindle, so the level of twist inserted into the yarn can be controlled

The Application of Textiles in Rubber

Figure 2.2 Disignation of twist direction

Figure 2.3 Ring spinning

Figure 2.3 Ring spinning

Ring

Traveller

Guide eye

Drafting rollers

Sliver IN

Yarn

Tube

Driven spindle Rotation

Rotation

(pulled

by yarn)

Spun yarn

Production and Properties of Textile Yarns

An alternative method for the spinning of staple fibres, which has been developed overrecent years, is the ‘rotor’ or ‘open-end’ method In this system, the ring and spindleare replaced by a rapidly rotating hollow cylinder; the drawn-down sliver is introducedinto one end of this rotor and is thrown by centrifugal force onto the surface of therotor, where it is carried round for several revolutions, before being withdrawn fromthe other end and wound up onto a bobbin In passing through the rotor, a slight twist

is imparted to the yarn, although generally less than on the ring system, but some ofthe longer protruding fibre ends are picked up by the rotor wall and wrapped roundthe bundle of fibres, thus binding them together into a coherent yarn

With the rotor system, the bulk of the fibres are not compressed together to the sameextent as on ring spinning with the higher level of twist, and so a bulkier yarn isobtained The binding together of the fibres is not as firm with the rotor system, sointer-fibre slippage can occur more readily, which results in a somewhat weaker yarn.However, the productivity and economics of open-end spinning are such, that it hasbecome widely adopted, and, except where strength is of paramount importance,open-end yarn is perfectly acceptable for most applications

2.1.2 Rayon

Rayon is a man-made fibre, based on regenerated cellulose The raw materials used areeither cotton linters, as mentioned above, or, more usually, wood pulp, both of whichhave a very high cellulose content A schematic of the chemistry of the viscose rayon

process is given in Figure 2.4 As can be seen, the basic structure of the cellulose is

essentially unchanged and the various stages are primarily to solubilise and regeneratethe cellulose However, during this process there is some degradation of the polymer,giving a significantly lower molecular weight (the regenerated cellulose molecule containsapproximately 200-300 repeating units, compared with some 2,000 units in the originalraw material) Also, in the spinning of the fibres, the regularity of orientation of themolecules in rayon is very much less than in the naturally laid down structure of cotton,

so that, although continuous filament rayon yarns are stronger than spun cotton (see

Table 2.3 later in this chapter), the individual fibre or ‘hair’ strength of cotton, at around

50 cN/Tex, is greater than that of rayon

In production, the wood pulp is first steeped in and then boiled with caustic soda, to givesoda cellulose One side effect of this stage is that much of the non-cellulose content ofthe raw pulp dissolves in the caustic soda and can be washed out, so that the filtered andpressed sheet consists of essentially pure soda cellulose In the next stage, this sheet iscrumbed and treated with carbon disulphide, with which it reacts to give sodium cellulosexanthate This is then dissolved in dilute caustic soda to give the spinning solution

The Application of Textiles in Rubber

Figure 2.4 Viscose rayon synthesis

Initially, this solution is quite viscous, but on standing the viscosity falls due to oxidativescission of the cellulose chains On further standing, partial hydrolysis of the xanthate tocellulose results in a rise in viscosity The solution is allowed to ‘ripen’ to the requiredviscosity, when it is considered ready for spinning

At the spinning stage, the solution is filtered and pumped through spinnerets (usuallymade of platinum or some other highly corrosion resistant material) into the coagulantbath This process of extruding a solution of the polymer into a bath which causescoagulation of the polymer, is known as a wet spinning process

The coagulant bath for the standard rayons consists of approximately 10% sulphuricacid, with addition of sodium and zinc sulphates and a small amount of glucose.These additives are used to retard the coagulation of the outside of the spun filaments,

to allow the centre to coagulate more rapidly, so that the whole thickness of the fibre

CHO CH

NaOH Boiling

CHO CH

CHO CH

CHO CH

Regenerated cellulose

Production and Properties of Textile Yarns

is coagulated at the same time The main effect of this retardation of the outer layers

is to allow a much greater orientation of the cellulose molecules in the skin layercompared with the core As the core dries it contracts, which causes the skin towrinkle up giving the characteristic many lobed cross-section of the lower tenacityrayons (Tenacity is a measure of the strength of a yarn, quoted as strength per unitlinear density, e.g., cN/Tex.)

By modification of the coagulant bath composition, increasing the time of coagulationand stretching the coagulating filaments, it is possible to increase the ratio of skin tocore This gives an increase in strength and modulus and a reduction in elongation,compared with the standard process fibre This method is used to produce the highertenacity industrial grades of rayon

This can be taken even further, resulting in a fibre, which consists essentially of all skinand no core In these fibres, strength is even higher and elongation lower, but the mostsignificant effect is on the wet strength of the fibre With the standard and high tenacityyarns, the wet strength is as much as 50% lower than that when dry The ‘all skin’yarns, produced by slower coagulation and higher stretch on spinning, are classified as

‘polynosic’ yarns and possess much higher wet strength, losing only around 15%-20%

of their dry strength on wetting

Another effect of the increase in the skin content of the higher tenacity and polynosicyarns is to reduce the relative shrinkage of the core on drying; this in turn reduces thewrinkling of the skin so that these yarns have a more regular and smoother surfacethan the standard yarns

After spinning, the yarn is washed and dried and then wound up onto packages suitablefor supply to the converters

Much development has gone into the engineering of the viscose rayon process, so thatwhat was originally a batch process, giving only moderate control of the uniformity ofthe yarn, has now become a high speed continuous process, with very stringent control

of every stage, giving a very consistent product

The majority of all rayon for polymer reinforcement is used as continuous filament,but there is still some use of spun staple rayon, where the main property required isbulk rather than strength The production of the staple fibre is the same as for thecontinuous filament up to the final winding, where for staple, many ends are takentogether to give a ‘tow’ of many thousand decitex This is passed through a machinewhich chops the filaments into the required short lengths, of the desired staple length.This staple is then spun in much the same way as described above for cotton

The Application of Textiles in Rubber

2.1.3 Nylon

Nylon is the generic name for the linear aliphatic polyamides Chemically, they are related

to the naturally occurring proteins, including silk and wool The major difference betweenthe natural products and the synthetics lies in the relative position of the amide groups.The natural products are derived from α-amino carboxylic acids The polyamide nylon 6

is derived from ε-amino caproic acid (caprolactam) which contains 6 carbon atoms,hence giving the designation nylon 6 Nylon 6.6 is obtained from the polycondensation

of hexamethylene diamine and adipic acid, each monomer containing six carbon atoms

so giving the designation of nylon 6.6

2.1.3.1 Nylon 6.6

The original route for synthesis of nylon 6.6 started with benzene as shown in Figure 2.5(a).

The benzene is catalytically reduced to cyclohexane, which on oxidation yields cyclohexanol.This is then dehydrogenated to give cyclohexanone (which also serves as the intermediatefor nylon 6) On oxidation with nitric acid, the ring opens to give adipic acid Here theroute splits, part of the acid passing directly to the nylon salt formation and the otherportion being used for the production of the other monomer, hexamethylene diamine Forthis, adipic acid is converted, by reaction with ammonia, to the acid amide, which ondehydration and subsequent hydrogenation, yields the diamine

The two monomers are dissolved in methanol and react together to give the nylon salt,which crystallises out of solution The salt is then dissolved in water and, on acidification

of this solution, polymerises to nylon 6.6; polymerisation is usually controlled to give amolecular weight of between 12,000 and 20,000

An alternative route, starting from butadiene, has been developed (Figure 2.5(b)) The

butadiene is chlorinated to give dichlorobutadiene, which is then reacted with hydrogencyanide On reduction of this, adiponitrile is obtained, which can either be reduced further

to hexamethylene diamine, or be hydrolysed by alkali to adipic acid, which two monomersare then processed as before

The polymer is washed and dried, to prepare it for spinning As nylon is thermoplastic, a meltspinning technique is used The polymer is melted and forced through the fine holes of aspinneret; on cooling, the fibre is formed On emerging from the spinneret, the polymer starts

to solidify immediately; at this stage, the filaments are pulled away and stretched by betweenfour to six times their original length This drawing stage brings about considerable orientationand alignment of the polymer molecules, resulting in the formation of crystallites, whichsignificantly affect the final properties of the yarn By control and adjustment of the degree ofstretch at this stage, and by selection of the molecular weight distribution, it is possible to varyconsiderably the main properties of the yarn, such as strength, modulus and thermal shrinkage

Production and Properties of Textile Yarns

Figure 2.5 Nylon synthesis

(c) The caprolactam process for nylon 6

Cyclohexanone

O

Cyclohexanone oxime

O2 (HNO3)

NH3

Adipic acid Adipamide

- H2Cu Catalyst

O2Catalyst

H2Catalyst

Benzene Cyclohexane Cyclohexanol

OH (a) The benzene route

Cyclohexanone O

Nylon 6.6

The Application of Textiles in Rubber

2.1.3.2 Nylon 6

Nylon 6 is a homopolymer of caprolactam The caprolactam is obtained from

cyclohexanone, as shown in Figure 2.5(c), by reaction with hydroxylamine to yield

cyclohexanone oxime On treatment with sulphuric acid this undergoes a Beckmanntransformation to caprolactam This monomer is then heated, with approximately 10%

of its weight of water, which causes the ring structure to open and the reactive groups tointeract to yield the nylon 6 polymer

The polymer is washed with warm water to remove any unreacted monomer, and dried,after which it is melt spun and drawn in the same manner as for nylon 6.6

In addition to the production of multifilament yarns, using many very fine holes in thespinneret, monofilaments are also produced, in diameters ranging from around 0.10 mm

up to 2.5 mm These monofilaments are used in the production of industrial fabrics,particularly for filtration fabrics (usually covering the range from 0.10 up to 0.25 mmdiameter, corresponding to a decitex range of 100 to 1000) The heavier diameters areused for stringing tennis and squash rackets, etc

direct oxidation of p-xylene (Figure 2.6(b)) There are two methods for the preparation

of the polymer In Europe, the more widely used route is by ester interchange via dimethyl

terephthalate (Figure 2.6(c)(i)), but in the USA, the direct esterification of the acid with ethylene glycol is the more favoured method (Figure 2.6(c)(ii)).

After polymerisation, the polymer passes through melt spinning and drawing stages, asfor the nylons

A considerable proportion of the polyester produced is used as spun, but because ofproblems with adhesion, there is also a requirement for pretreated yarns For these, atthe spinning stage, the filaments are treated with various materials to modify the surface

of the polyester Although many materials have been patented for this application, themost popular are epoxy derivatives The epoxy is applied, from aqueous solution oremulsion, at the spinnerets, and subsequent heat treatment brings about the modification

to the fibre surface, so that adequate adhesion can be obtained with standard RFL dips,

Production and Properties of Textile Yarns

Figure 2.6 Polyester synthesis

(a) Ethylene glycol process

O 2

COOH

COOH Terephthalic acid (c) Polymerisation (i) Ester interchange method

COOH

COOH Terephthalic acid

Methanol

COO.CH3

COO.CH3Di-methyl terephthalate

Ethylene glycol

CO

CO

CH2O

CH2

O

n Polyethylene terephthalate

COOH

COOH Terephthalic acid

CH2OH

CH2OH

+Ethylene Ethylene oxide Ethylene glycol

The Application of Textiles in Rubber

as used for nylon Yarns which have been treated in this way are variously referred to aspretreated, pre-activated, adhesive primed or rubber receptive Often incorporated withthis treatment and its included heat treatment, is the relaxation of the yarns to reduce thenormal shrinkage level of polyester, of around 10%-12%, to values between 2% and4%, which offers advantages in some subsequent processes

2.1.5 Aramid

The aramids are aromatic polyamides Although they are closely related to the nylons(the aliphatic polyamides), the substitution of the aliphatic carbon backbone by aromaticgroups brings about considerable changes in the properties of the resultant fibres.The first fibre of this class to be developed was Nomex from DuPont This yarn is ofonly medium tenacity, but is non-flammable and widely used for the production offireproof clothing, etc

The newly introduced very high strength yarns, under the trade names of Kevlar (from

DuPont) and Twaron (from Akzo Nobel Fibres) are fibres of poly-p-phenylene

terephthalamide The route for the production of the polymer is shown in Figure 2.7.

Aniline is first acetylated and then nitrated (Figure 2.7(a)): the yield of the p-amino

derivative is over 90% (a straight nitration would give approximately equal amounts of

the o- and p-derivatives) Hydrolysis followed by reduction yields p-phenylene diamine.

Terephthalic acid (produced as shown for polyester) is reacted with chlorine to give the

acid chloride (Figure 2.7(b)) This is then reacted with the diamine to give the aramid polymer (Figure 2.7(c)).

The major problem to be overcome in the production of the aramid yarns, was theselection of the right solvent system both for polymerisation and for spinning As aramidsare infusible, they cannot be melt spun The polymerisation is carried out in a mixedsolvent system consisting of an amide with lithium chloride: using this solvent it is notpossible to obtain a high solids solution, which adversely affects the rate at which thefibres can be spun Also, yarns spun from this solvent required considerable after-treatment

in order to develop the optimum fibre properties

Other useful solvents were not readily found, not only due to the properties of the polymeritself but also due to its behaviour in solution, where it exhibits anisotropy and exists in

a liquid crystal phase A suitable solvent was found in concentrated sulphuric acid; thisenabled the necessary solids content for good spinning to be achieved and also gavebetter liquid crystal formation This resulted not only in better spinning performance butalso gave much better properties to the final fibre

Production and Properties of Textile Yarns

Further development of the spinning stage produced even better results The presentprocess utilises a dry jet/wet spinning system [1] The spinning solution is extruded throughthe spinneret just above the coagulant bath (of water or dilute sulphuric acid); this allowsfurther orientation of the polymer in the solution before the polymer starts to coagulate,

Figure 2.7 Aramid synthesis

Terephthalic acid chloride

COCl

COCl

Terephthalic acid chloride

The Application of Textiles in Rubber

due to the streamlined flow induced by the spinneret As a result of this, the as-spunyarns are superior to the original yarns After-treatments are not necessary, although byincluding such treatment, it is possible to produce yarns with even higher modulus Kevlar

49, a special high modulus yarn for fibre reinforced plastics, gives higher strength withlower weight than standard glass reinforced products

The anisotropy of the solutions of aramid and the existence of liquid crystals are indicative

of the very high orientation and association of the polymer molecules, which accountsfor the very high strength and modulus of the yarns This is even more remarkable when

it is considered that the average aramid molecule comprises not more than 100 repeatunits, which degree of polymerisation is very much lower than is the case with thecellulosics, nylons and polyesters

2.2 General Characteristics of Textile Fibres

The main characteristics of the textile fibres are summarised in Table 2.2; greater detail

is given below for the various types, including simple tests for the identification of thedifferent fibres A proprietary blend of dyes is available for fibre identification and testresults are listed in each section (Shirlastain A from Shirley Developments Ltd., Didsbury,Manchester)

2.2.1 Cotton

Cotton is a naturally occurring 100% cellulose fibre It is only encountered in spunstaple form, as a naturally occurring short staple fibre

Water: Cotton swells on immersion in water, but the wet strength of the yarn is up to

20% higher than in the dry state; on drying, the properties revert to the original Onexposure to a standard atmosphere (20 °C and 65% relative humidity) cotton will regainapproximately 8.5% moisture Regain is a measure of moisture content of a yarn orfabric and is expressed as:

p y T e r b i

2 W e t s t r e g t h

) y r d f o

%

e n t s i s e r t a e

t n i o g i e

0

s d i c a o t e n t s i s e R

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The Application of Textiles in Rubber

Heat: Cotton generally has good resistance to heat and is only slightly affected by

temperatures of up to 150 °C, but above this temperature, and on prolonged exposurebetween 100 °C and 150 °C, will progressively lose strength It starts to decompose ataround 230 °C

Cotton will burn readily, but in an oxygen starved atmosphere will char and leave acarbon skeleton (albeit without any significant strength)

Acids: Cotton is quite susceptible to attack by acids It is quickly attacked by hot dilute

or cold concentrated acids Traces of acid, not properly washed out from certain dyeingand finishing processes, can lead to a progressive tenderising of the fibres and consequentloss of strength

Alkalis: Cotton is resistant to alkali, but will swell This forms the basis of the mercerisation

process, where cotton yarns are stretched in fairly concentrated alkali: during this, thefibres swell, but since they are held, this swelling introduces certain re-orientation of themolecular structure, which results in an improved strength and a more glossy appearance

Solvents: Cotton is not affected by the usual hydrocarbon, aromatic or chlorinated

solvents It will dissolve in some mineral acids, e.g., 70% sulphuric acid, but this isusually accompanied by chemical decomposition Cotton will dissolve in a solution ofcuprammonium hydroxide and this solution can be used to measure the viscosity of aknown concentration of cotton and so give a measure of any chemical degradation theyarn has undergone

Miscellaneous: Cotton is subject to microbiological attack, especially mildew.

Identification: Burns readily with a characteristic ‘burnt paper’ smell; gives a purple

colour on cold staining with Shirlastain A

2.2.2 Rayon

Rayon is a regenerated cellulosic yarn and is used as both continuous filament and asspun staple yarn

Water: As with cotton, rayon is swollen on immersion in water, but the wet strength of

rayon is some 40% lower than the dry except for the polynosic yarns when it is around25% lower; again this is reversible on drying, provided the yarn is not allowed to shrink.Standard moisture regain of rayon is 13%

Heat: Rayon is generally resistant to heat up to about 150 °C, but loses strength onprolonged exposure and more rapidly at higher temperatures It starts to decompose ataround 210 °C Rayon burns readily but as with cotton, under certain conditions, willchar and leave a carbon residue

Acids: The susceptibility of rayon to acids is very similar to that of cotton.

Alkalis: The reaction of rayon to alkali is also similar to that of cotton, but rayon will

lose some strength on swelling in concentrated alkali

Solvents: As for cotton; however, the viscosity of a solution of rayon in cuprammonium

hydroxide is much lower than that of a similar concentration of cotton (due to thereduction in molecular weight during the manufacturing process)

Miscellaneous: Rayon is susceptible to microbiological attack, but the absence of the

small amounts of naturally occurring proteins found in cotton, and the presence of traces

of chemicals from the manufacture, render rayon slightly more resistant than cotton

Identification: Burns readily with the characteristic burnt paper smell; with Shirlastain

A, gives a pink colour in the cold and purple on boiling

2.2.3 Nylon

The chemical properties of both nylon 6.6 and nylon 6 are very similar; significantdifferences in behaviour will be specifically mentioned here

Water: Nylon is not significantly affected by immersion in water; there is perhaps a

slight drop in tenacity, but this is fully reversible on drying

The standard regain of nylon is 4.5%

Heat: Nylon is generally quite resistant to heat and is not appreciably affected by

temperatures of up to 180 °C (unless exposed for prolonged periods)

Nylon 6.6 melts at 250 °C and nylon 6 at 225 °C

On burning, nylon tends to melt away from the flame, and burns less readily than cottonand rayon (the Limiting Oxygen Index of nylon is 0.26 so it has a tendency to be self-extinguishing) However, in bulk, when a molten mass is formed it will burn fairly readily

On burning, nylon has a characteristic celery-like odour

Production and Properties of Textile Yarns