Blends Dyeing

Blends Dyeing John Shore Formerly of BTTG-Shirley and ICI (now BASF), Manchester, UK

1998 Society of Dyers and Colourists iii

Copyright © 1998 Society of Dyers and Colourists. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means without the prior permission of the copyright owners. Published by the Society of Dyers and Colourists, PO Box 244, Perkin House, 82 Grattan Road, Bradford, West Yorkshire BD1 2JB, England, on behalf of the Dyers’ Company Publications Trust. This book was produced under the auspices of the Dyers’ Company Publications Trust. The Trust was instituted by the Worshipful Company of Dyers of the City of London in 1971 to encourage the publication of textbooks and other aids to learning in the science and technology of colour and coloration and related fields. The Society of Dyers and Colourists acts as trustee to the fund, its Textbooks Committee being the Trust’s technical subcommittee. Typeset by the Society of Dyers and Colourists and printed by H Charlesworth & Co. Ltd, Huddersfield, UK.

ISBN 0 901956 74 0 iv

Contents Preface

CHAPTER 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7

3.7

Classification of fibre types and their blends

21

Dynamic competition between fibre types in the dyeing of blends

26

Introduction 26 The distribution of acid dyes on nylon/wool blends 29 The distribution of acid dyes on nylon/polyurethane blends 35 The cross-staining of wool by disperse dyes 36 The cross-staining of wool by basic dyes 41 The transfer of disperse dyes during thermofixation of polyester/cellulosic blends 44 References 45

CHAPTER 4 4.1 4.2 4.3 4.4

1

Classification of fibre types in terms of dyeability 21 Colour distribution attainable on binary blends 22 References 25

CHAPTER 3 3.1 3.2 3.3 3.4 3.5 3.6

Why blending is necessary

Blending from the dyer’s viewpoint 1 The composition of blend fabrics 2 The relative importance of individual blends 3 Reasons for the development of fibre blends 5 Colour effects achieved by blending 10 Sighting colours for identification purposes 19 References 20

CHAPTER 2 2.1 2.2 2.3

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Minimising incompatibility between dyes from different classes

Interaction between disperse dyes and reactive dyes 46 Interaction between disperse or vat dyes and basic dyes 47 Interaction between anionic dyes and basic dyes 48 References 52 v

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CHAPTER 5 5.1 5.2 5.3 5.4 5.5 5.6

7.4 7.5 7.6 7.7 7.8

77

Wool/acrylic and other AB blends

86

Dyeing of wool/acrylic blends 86 Dyeing of nylon/acrylic blends 90 Blends of acid-dyeable and basic-dyeable acrylic variants 91 Blends of modacrylic and acrylic fibres 93 Blends of amide fibres with modacrylic or acid-dyeable acrylic variants 94 Blends of basic-dyeable polyester with wool or nylon 96 Dyeing methods and dye selection for AB blends 99 References 99

CHAPTER 8 8.1 8.2 8.3 8.4 8.5

Nylon/wool and other AA blends

Dyeing of nylon/wool blends 77 Blends of wool with other acid-dyeable fibres 79 Blends of nylon with other acid-dyeable fibres 82 Dyeing methods and dye selection for AA blends 84 References 85

CHAPTER 7 7.1 7.2 7.3

53

Design of differential-dyeing variant synthetic-polymer yarns 53 Dyeing of acid-dyeable nylon variants 57 Dyeing of acid-dyeable/basic-dyeable nylon variants 61 Design of differential-dyeing cellulosic fabrics 63 Design of differential-dyeing wool keratin derivatives 71 References 76

CHAPTER 6 6.1 6.2 6.3 6.4 6.5

Principles of design and colouring of differential-dyeing blends

Wool/cellulosic and other AC blends

Dyeing of wool/cellulosic blends 100 Exhaust dyeing of nylon/cellulosic blends 108 Continuous dyeing of nylon/cellulosic blends 113 Dyeing methods and dye selection for AC blends 115 References 118 vi

100

CHAPTER 9 9.1 9.2 9.3 9.4 9.5 9.6

Cellulosic/acrylic and other CB blends

119

Exhaust dyeing of cellulosic/acrylic blends 119 Continuous dyeing of cellulosic/acrylic blends 122 Blends of cellulosic fibres with modacrylic or acid-dyeable acrylic variants 124 Blends of basic-dyeable polyester with cotton 126 Dyeing methods and dye selection for CB blends 126 References 128

CHAPTER 10

Cotton/viscose and other CC blends

129

10.1 Properties and performance of cellulosic fibres in their blends 129 10.2 Dyeing behaviour of cellulosic fibres in their blends 133 10.3 Dyeing methods and dye selection for CC blends 136 10.4 References 137

CHAPTER 11

Polyester/wool and other DA blends

138

11.1 Dyeing of polyester/wool blends 138 11.2 Blends of cellulose acetate or triacetate with wool 149 11.3 Dyeing of polyester/nylon blends 152 11.4 Blends of cellulose acetate or triacetate with nylon 154 11.5 Blends of poly(vinyl chloride) fibres with wool or nylon 157 11.6 Dyeing methods and dye selection for DA blends 160 11.7 References 160

CHAPTER 12

Polyester/acrylic and other DB blends

12.1 Dyeing of polyester/acrylic blends 161 12.2 Blends of cellulose acetate or triacetate with acrylic fibres 163 12.3 Dyeing of normal/basic-dyeable polyester blends 165 12.4 Dyeing methods and dye selection for DB blends 168 12.5 References 168

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CHAPTER 13

Polyester/cellulosic and other DC blends

169

13.1 Exhaust dyeing of polyester/cellulosic blends 169 13.2 Continuous dyeing of polyester/cellulosic blends 187 13.3 Blends of cellulose acetate or triacetate with cellulosic fibres 197 13.4 Blends of poly(vinyl chloride) fibres with cellulosic fibres 201 13.5 Dyeing methods and dye selection for DC blends 201 13.6 References 204

CHAPTER 14

Triacetate/polyester and other DD blends

206

14.1 Dyeing properties of disperse-dyeable fibre blends 206 14.2 Dyeing methods and dye selection for DD blends 210 14.3 References 211

CHAPTER 15

Dyeing properties of three-component blends

15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9 15.10 15.11 15.12

Introduction 212 Dyeing of AAA blends 213 Dyeing of AAB blends 215 Dyeing of AAC blends 216 Dyeing of CBA blends 217 Dyeing of DAA blends 217 Dyeing of DAC blends 218 Dyeing of DBA blends 219 Dyeing of DBC blends 220 Dyeing of DDA blends 221 Dyeing of DDC blends 222 Dyeing methods and dye selection for three-component blends 222 15.13 References 225

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Preface This book is an addition to the series on coloration technology issued by the Textbooks Committee of the Society of Dyers and Colourists under the aegis of the Dyers’ Company Publications Trust Management Committee, which administers the fund generously provided by the Worshipful Company of Dyers. Earlier books on dyeing technology in this series, namely The dyeing of synthetic-polymer and acetate fibres (1979), The dyeing of cellulosics fibres (1986) and Wool dyeing (1992), each contained a chapter on the dyeing of those fibre blends most relevant to their respective titles. Inevitably, this approach lacked balance, and material on specific blends was either partially duplicated or, more often, entirely overlooked. When replacements for the 1979 and 1986 books were under consideration in the early 1990s, the decision was taken to produce a separate volume dedicated to the dyeing of fibre blends. This book is the result of that change of approach. Very few books have been devoted solely to this subject. The best known is undoubtedly the ‘classic’ Dyeing of fibre blends (1966), written by Roy Cheetham of Courtaulds. Invaluable in its time, Cheetham’s book was a mine of practical information and detailed recommendations for every conceivable blend. The treatment in this present book is intended to provide only general guidelines in this respect, since a dyer encountering an unfamiliar blend for the first time cannot avoid undertaking preliminary development work. An attempt is made in the first five chapters of this book to express some general principles applicable to the theme. A classification of blends according to the dyeing properties of their component fibres is introduced in Chapter 2. These categories form the respective topics of the remaining ten chapters on dyeing methods. The author is indebted to the referee of this book and to Jim Park for valuable comments and suggestions for improvement of the text. Grateful thanks are due to Paul Dinsdale (the editor of the Society), Gina Walker (copy editing and proof reading) and Sue Petherbridge (typesetting and layout). JOHN SHORE

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CHAPTER 1

Why blending is necessary

1.1 BLENDING FROM THE DYER’S VIEWPOINT The term ‘blending’ is used by the yarn manufacturer to describe specifically the sequence of processes required to convert two or more kinds of staple fibres into a single yarn composed of an intimate mixture of the component fibres. This may be necessary to obtain a uniform yarn from different varieties of the same fibrous polymer, as in the blending of wool qualities differing in origin, or in the blending of two colours of mass-pigmented man-made fibre to give a target hue. Any blend must have acceptable properties for the spinner. Important factors include the relative diameters, staple lengths and extensibilities of the fibres present. A mismatch can create a blend that has lower strength than that of either of the component fibre types. Polyester has an advantage over nylon in blends with cotton in that its initial modulus matches that of cotton more closely. To the dyer, however, the significant type of staple-fibre blend is that in which the components are two different fibrous polymers, each with its own characteristic dyeing properties. The term ‘blend’ has therefore been used more loosely by the dyer to refer to any combination of fibre types, whether they occur as different filaments or staple fibres in the same yarn, or as different yarns assembled in the same fabric or garment. This is the sense in which ‘blend’ is used here, the essential difference between the components being that of dyeing characteristics. Blended-staple yarns occupy a highly important position alongside the major types of homogeneous staple-fibre yarns in the textile industry. Blends of synthetic fibres, notably polyester, with cellulosics are produced in such quantities, for shirtings, dresswear, outerwear, rainwear, workwear and household textiles, that continuous dyeing methods for these blends are as important as for the parent cellulosic fabrics. Polyester/wool blends are particularly useful in suitings, dresswear and outerwear, whilst wool yarns in

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hand-knitting, hosiery, knitwear and carpets have yielded much ground to nylon/ wool blends. Mixed-ply yarns have been incorporated in woven fabrics for many years, often to introduce special effect threads, or in more substantial proportion to confer stretch, bulk or resilience. The contrasting dyeability of the component yarns may give attractive marl effects and prove useful in carpets, knitwear or hand-knitting yarns. In support hosiery and foundation garments elastomeric warp yarns are often covered with nylon filament yarn. Fabrics woven from polyester staple-core/cotton wrap yarns in both warp and weft directions can be successfully desized, bleached, dyed to solid shades and given a durable press finish without difficulty using conventional procedures with only slight modifications. The finished fabrics are soft but exceptionally strong. They are especially useful where high strength, durability, moisture absorbency and easy-care performance are important features [1]. Fabrics constructed from these staple-core yarns and from intimate-blend yarns have been compared before and after durable press finishing. The superior properties of the treated staple-core fabrics are attributed to the consolidation of the stronger but more extensible polyester staple in the core of the yarns [2]. Ingenious methods of combining man-made fibrous polymers in the same extruded filament or bundle of filaments have been developed from time to time but have failed to generate much more than novelty interest. Filaments made from two different polymers fused together within the material are known as bicomponent filaments [3,4]. Multifilament yarns, formed by the intermingling of two types of filament by extrusion from a special spinneret, contain a random distribution of the individual components [5]. 1.2 THE COMPOSITION OF BLEND FABRICS Staple-fibre yarn blends are long-established in woven fabrics and there is an exceedingly wide variety of fabric constructions woven or knitted from two (or more) types of homogeneous yarn. Materials of the latter kind have often been referred to as ‘union fabrics’, but to avoid confusion this term will be avoided here. The broader description ‘blend fabrics’ will be used where necessary to describe all types of construction made from two or more fibrous polymers or variants that differ in dyeing characteristics, including filament unions, blendedstaple fabrics, pile fabrics and carpets. Apparel and domestic textiles are important for such blend fabrics, which may exhibit desirable two-way differences in physical properties and often provide scope for attractive multicolour patterning. The availability of wholly synthetic blend fabrics, such as polyester/acrylic dresswear, polyester/nylon outerwear or

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THE COMPOSITION OF BLEND FABRICS

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nylon/acrylic half-hose, as well as the differential-dyeing variants of these individual fibres, offers considerable scope for striking coloured effects. Pile fabrics play an important part in the upholstery and furnishings market. They often consist of a nylon or cellulosic backing fabric with a resilient pile made from wool or acrylic staple. Cotton pile in nylon support fabric is widely used in lightweight towelling, leisurewear and children’s clothing. The carpet industry is a long-established outlet for fibre blends. Apart from the notable share of nylon/wool blended staple in the traditional woven field, the availability of differential-dyeing nylon has simplified the production of multicoloured designs in tufted carpeting, made by needling the appropriately identified pile yarns into a suitable backing.

1.3 THE RELATIVE IMPORTANCE OF INDIVIDUAL BLENDS It is often difficult to obtain detailed information on the relative demand for different types of fibre blends. Statistics of production or consumption of textile fibres are almost always classified in terms of the total amount of each fibre type, irrespective of whether that fibre is used alone in a garment or other textile, or as a component of blended material. The figures in Tables 1.1 and 1.2 are taken from part of a confidential market research survey for 1985, in which the information was gathered for each market according to whether the amounts of fibres were used alone or in one of several major categories of fibre blends. In order to exclude from consideration those industrial uses of fibres (normally not blended) where coloration is not a possibility, the statistics were limited to those

Table 1.1 Textile fibres available for coloration worldwide

Fibres

Amount (kg × 106)

Proportion (%)

Cotton Polyester/cellulosic blends Nylon (including polyurethane) Polyester Acrylic (including modacrylic) Viscose (including modal, polynosic) Wool (including other animal fibres) All other blends Linen (including other bast fibres) Cellulose acetate and triacetate Silk

11640 4520 3090 2840 2210 2030 1560 1220 370 285 65

39.0 15.2 10.4 9.5 7.4 6.8 5.2 4.1 1.2 1.0 0.2

Total

29830

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WHY BLENDING IS NECESSARY

Table 1.2 Blends of fibres available for coloration worldwide

Blends

Amount (kg × 106)

Proportion (%)

Polyester/cotton Polyester/viscose Miscellaneous blendsa Polyester/wool Wool/acrylic Other synthetic blendsb Other cotton blendsc Nylon/wool

3350 1170 545 410 80 75 65 45

58.4 20.4 9.5 7.1 1.4 1.3 1.1 0.8

Total

5740

a Includes wool/polyurethane, wool/viscose, cellulose acetate/nylon, ... b Includes nylon/acrylic, polyester/nylon, polyester/acrylic, ... c Includes nylon/cotton, cotton/acrylic, cotton/viscose, ...

quantities of each fibre or blend that were available for coloration, i.e. to be dyed, printed or finished as white apparel or household textiles. Several interesting facts emerge from these tables. About 20% of the total fibres in Table 1.1 are constituents of blended materials and about 80% of this total, broken down in Table 1.2, is represented by the polyester/cellulosic sector. As a substrate type, polyester/cellulosic is more significant than any of the three main all-synthetic types and is second only to cotton in importance (Table 1.1). All other cotton or viscose blends are very much less significant than either polyester/cotton or polyester/viscose. Polyester/wool is also a more important blend than either nylon/wool or wool/acrylic, but here the differences in demand are less dramatic. The numerous ‘synthetic blends’ and ‘miscellaneous blends’ making up the remaining 10% of the total in Table 1.2 are individually of minor significance but collectively they have presented a wide variety of problems to those devising satisfactory dyeing procedures for them. The blending and processing of an above-average proportion (i.e. more than 20%) of total fibres in the form of blended materials is characteristic of the relatively complex and sophisticated textile industries found in economically developed or developing countries. In the Asia Pacific region this figure exceeds 40% in some instances (Thailand, Malaysia and Indonesia) and is above average in several others (Australia, Burma, PR China, Hong Kong, Japan, Korea, Philippines, Singapore and Taiwan). South Africa, Canada, USA, Mexico and Brazil are other markets with an above-average proportion of blended fibres. Most European textile industries process 10–20% of total fibres as blends, with above-average values in Germany, Spain, Portugal, Greece, Bulgaria, Rumania

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THE RELATIVE IMPORTANCE OF INDIVIDUAL BLENDS

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and Poland. The overall figure for blends in the UK textile industry is 17.5% of total fibres, but the proportions represented by polyester/wool (42% of UK blends) and nylon/wool (16%) are substantially above average, reflecting the continuing importance of suitings and carpets respectively in the UK industry. 1.4 REASONS FOR THE DEVELOPMENT OF FIBRE BLENDS Several interrelated factors may contribute to the justification for replacing a homogeneous textile material by a blend: (1) Economy: the dilution of an expensive fibre by blending with a cheaper substitute. (2) Durability: the incorporation of a more durable component to extend the useful life of a relatively fragile fibre. (3) Physical properties: a compromise to take advantage of desirable performance characteristics contributed by both fibre components. (4) Colour: the development of novel garment or fabric designs incorporating multicolour effects. (5) Appearance: the attainment of attractive appearance and tactile qualities using combinations of yarns of different lustre, crimp or denier, which still differ in appearance even when dyed uniformly to the same colour. 1.4.1 Balance of economy and physical properties Cellulosic fibres, especially viscose staple, have been used for many years in blending with more expensive wool or synthetic fibres. In such blends the balance of physical properties is at least as important as economic considerations. During the 1930s the cheaper fibres from regenerated cellulose, i.e. viscose and cellulose acetate, as well as regenerated protein fibres helped to compensate for fluctuations in the price of wool by providing blend yarns at more stable prices in periods of high demand for wool. When synthetic staple fibres became available for blending in the 1950s, prices were high and blending with natural or regenerated fibres was a valuable means of establishing outlets for them using existing methods of processing. As the price levels of synthetic fibres fell with the tremendous growth in competition and volume of production that followed, the cost differentials between these blends and the component fibres lost most of their significance. In recent years there has been some movement from 80:20 wool/nylon to 50:50 wool/polypropylene yarns in carpets on price grounds [6,7]. Fibre blending can be regarded as a contribution to fabric engineering. By using fibres that differ in absorbency, fabrics with specific moisture regain values can be created. With fibres that differ in denier, desired stiffness and drape

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qualities can be designed into the fabric. Blends of synthetic fibres with natural fibres offer the most valuable possibilities for combining desirable physical properties, because the two components are so dissimilar. In blends of polyester or acrylic fibres with cotton or viscose the synthetic component provides crease recovery, dimensional stability, tensile strength, abrasion resistance and easy-care properties, whilst the cellulosic fibre contributes moisture absorption, antistatic characteristics and reduced pilling. The antistatic effect is particularly significant: for example, only 10–20% of viscose (or a smaller proportion of metallic filaments) is required to confer antistatic properties on an acrylic fibre. Apparel fabrics, hosiery and carpet yarns combining the durability and elastic recovery of nylon with the warmth, bulk and softness of wool or high-bulk acrylic staple are important examples of a desirable balance of properties. Men’s socks in 100% nylon were heavily promoted in the 1960s for their stretch, easycare properties and durability compared with traditional wool socks. However, these garments had not been designed to meet comfort needs [8] and were soon perceived to be hot and uncomfortable when worn in shoes. It was at this time that coarse-filament nylon blends with wool or cotton began to appear. This development resulted in nylon/wool and nylon/cotton socks that were more comfortable and had the added benefits of dimensional stability with stretch properties, easy-care laundering, attractive appearance and excellent durability. Spun-dyed yarns and differential-dyeing variants were exploited to provide increased colour and design potential. Stretch fabrics for leisurewear are available in a wide range of qualities, often based on a crimped nylon warp with a wool, acrylic or viscose staple weft. The development of durable flame-retardant finishes for conventional synthetic fabrics and their blends has proved difficult and there has been considerable exploitation of the inherent flame resistance and thermal insulation properties of poly(vinyl chloride) fibres, or certain modacrylic copolymers with chloro substitution, in blends with wool for thermal underwear, nightwear garments, children’s clothing and knitwear. Many characteristics of all-wool cloths can be simulated by blending longstaple polyester or acrylic fibres with wool. These blends generally do not possess equivalent suitability for milling because of the absence of any directional friction effect with the smooth synthetic fibres, although small amounts (up to about 20%) of these fibres can accelerate wool shrinkage during milling. Such blends exhibit the valuable features of excellent dimensional stability (often at least equal to shrink-resist wool), abrasion resistance and durable pleat retention. The beneficial effect on crease recovery of blending polyester with wool is illustrated in Table 1.3. The value for the intact synthetic fibre alone is

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REASONS FOR THE DEVELOPMENT OF FIBRE BLENDS

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approximately 1.5 times that for wool, and when blended at a typical 55:45 ratio the crease recovery of the blend fabric is significantly higher (74) than the value anticipated from this composition (68). When dyed with carrier, however, changes in the fine structure of the polyester involving chain folding result in a marked reduction in crease recovery, by about 6% in this instance [9].

Table 1.3 Crease recovery of blended polyester and wool [9] Crease recovery (%)a after dyeing at 105°C Fabric

No carrier

With carrier

100% Wool 55:45 Polyester/wool 100% Polyester

55 74 80

55 68 74

a Data obtained by the CSIRO multiple pleat test

The improvement in dimensional stability that takes place when wool is blended with an ester fibre is demonstrated in Figure 1.1. This records the marked decrease in milling shrinkage observed in worsted blend fabrics as the proportion of cellulose triacetate staple to 48s wool increases. In this instance the shrinkage is halved (or the dimensional stability is doubled) when the proportion of triacetate reaches about 40%. The stabilising effect of the man-made fibre component is more pronounced in the case of coarser wool qualities.

Weft shrinkage/%

50 40 30 20 10

20

40

60

80

Triacetate in blend/%

Figure 1.1 Milling shrinkage of cellulose triacetate/wool

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1.4.2 Development of microfibre variants Microfibres feel pleasant against the skin, combining the easy-care properties of a synthetic fibre with the silky appearance and comfort of a natural fibre. The rate of growth of the market for polyester microfibres (less than 1 dtex per filament) and supermicrofibres (<0.3 dtex fil–1) will depend on their success in penetrating the polyester/cellulosic blend apparel sectors in Europe, the USA and Asia Pacific regions. So far the uses of polyester microfibres have been mainly confined to unblended (mainly filament) materials, such as: (1) fashionable woven outerwear suede and velour fabrics with attractive handle and drape [10]; (2) woven sportswear and skiwear with improved transfer of moisture; (3) polar fleece garments, which provide excellent thermal insulation [11]; (4) tightly woven rainwear fabrics affording effective protection with breathability; (5) warp- and weft-knitted twin-layer microliner fabrics [12]; (6) imitation silks with attractive lustre and drape. In 1992, microfibre variants represented less than 1% of total demand for all forms of polyester but by 2000 AD they are expected to achieve a 10–25% share. In woven fabrics, filament blends of polyester microfibres and viscose are gaining popularity in dresswear and blouses. Polyester microfilaments and cotton are being introduced into knitted sportswear. So far, unfortunately, spinning problems and pilling behaviour have inhibited the potential uses of intimate staple blends of polyester microfibres with cotton [12]. Supermicrofibres have a filament diameter less than one-tenth of that of fibres in conventional filament or staple yarns (2–3 dtex fil–1). In contrast with standard polyester, microfilament yarns of 0.6 denier or less cannot be packageor beam-dyed because the high density of the wet material prevents adequate liquor circulation. Fabrics woven from these yarns are preferably dyed in winches, jets or overflow machines to preserve their bulky characteristics. Approximately twice as much disperse dye is required for microfilaments of 0.3– 1.0 dtex fil–1 compared with conventional polyester of 2–3 dtex fil–1 for the same visual depth. Cost increases of 15–20% are anticipated because of the higher dye concentrations that have to be used to dye standard-depth shades [13]. One factor contributing to this difference is the smaller proportion of microcrystalline material in the microfilament structure. The absorbed dye forms larger aggregates in the amorphous regions and the tinctorial power is correspondingly reduced [11]. The rate of dye absorption is inversely proportional to the square root of the

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REASONS FOR THE DEVELOPMENT OF FIBRE BLENDS

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filament diameter. Thus another important difference between microfibres and conventional polyester is a markedly increased rate of dyeing, which tends to give rise more often to levelling problems. These can arise from: (1) commencing dyeing at too high a temperature; (2) raising the temperature too rapidly above 90°C; (3) incompatibility in mixture recipes because of differences in rates of exhaustion; (4) poor fabric agitation at slow running speeds [14]. Improvements can be made by selecting dye combinations with greater compatibility, starting at a low temperature and heating the dyebath at a slow rate of rise. A hold period at 90–100°C may well be advantageous [13], although diffusion into the fibre only proceeds rapidly at 130°C. Rapid jet or overflow machines are recommended, loaded with relatively short lengths of fabric to facilitate agitation. The exploitation of polyester microfibres blended with viscose in wovens or with cotton in knitgoods will depend to some extent on parallel advances in developing new disperse dyes to meet these more stringent demands in dyeing microfibres. Such dyes must combine outstanding build-up, excellent fastness and minimum cross-staining of the cellulosic component. Application techniques must optimise right-first-time productivity without sacrificing the aesthetic appeal of the microfibre-based fabric constructions [12]. It is essential to select dyes of high fastness to light for microfibre polyester, as the fastness ratings are inferior to those on standard polyester [15]. The standards for fastness to washing and rubbing are also lower than on standard polyester because of the higher concentration of dye required to give the target shade. Clearing of loose dye from the microfibre surfaces is more difficult and reduction clearing is always necessary on these variants. Post-dyeing heat treatments and inappropriate finishing chemicals often enhance these problems. Durable press characteristics of microfibre fabrics are inferior to those woven from standard yarns and crease-resist treatments must be applied carefully to achieve satisfactory results [13]. In 1983 the introduction of Tactel (ICI) nylon heralded a marked revival of interest in nylon apparel. The most notable features of Tactel were enhanced aesthetic and comfort properties. Cotton-look Tactel fabrics became highly popular for skiwear, anoraks, beachwear and track suits, combining the established assets of nylon, i.e. strength, easy-care performance and abrasion resistance, with enhanced handle and attractive appearance. In 1989 Tactel Micro (<1 dtex fil–1) was introduced to yield fabrics that may be modified during

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finishing by sueding or coating to give novel effects. Blends of Tactel Micro with cotton or wool have broadened the variety of constructions being offered to garment designers and consumers. Lightweight, soft, comfortable apparel fabrics with enhanced easy-care performance result from these blends [8]. Microfilament nylon fabrics tend to float in winches or atmospheric jets because they do not readily absorb liquid, but this problem can be obviated by pressure dyeing at 105–110°C. In blends with elastomerics such as Lycra (DUP), Tactel Micro produces a revolutionary combination of handle, comfort, garment fit and shape retention in stretch-knit constructions that are highly suitable for bodywear and aerobic sportswear. 1.5 COLOUR EFFECTS ACHIEVED BY BLENDING There are four major types of coloured effect (Figure 1.2) achieved by dyeing a blend of two fibres: (1) Solid: both fibres are dyed as closely as possible to the same hue, depth and brightness. (2) Reserve: only one fibre is dyed and the other is kept as white as possible. (3) Shadow: the two fibres are dyed to the same hue and brightness but the depth on one fibre is only a fraction of that on the other. (4) Contrast: usually the intention is to achieve the maximum possible contrast in hue at approximately the same depth on both fibres, but sometimes more subtle contrasts are preferred. In either case, optimum brightness on both component fibres enhances the pleasing appearance of the contrast effect. Reserve, shadow and contrast effects are mainly of interest for mixed-ply hand-knitting yarns, fabric woven or knitted from homogeneous yarns, as well as garments or tufted carpets made from differential-dyeing variants. The fact that synthetic fibres tend to absorb dyes less readily than the natural fibres is an advantage in achieving reserve or contrast effects, which are thus less prone to contamination by cross-staining problems. 1.5.1 Solid effects A solid effect (sometimes called a union-dye effect) is most frequently the objective of dyeing a binary blend, since most of those blends developed for reasons of economy, durability or physical properties, especially the blendedstaple yarns, are not intended for use in multicoloured designs. The attainment of a solid effect is most difficult with those blends in which an ester fibre that can only be dyed with disperse dyes is blended with another disperse-dyeable type, i.e. cellulose acetate or any synthetic fibre. If nylon or an acrylic fibre is present,

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COLOUR EFFECTS ACHIEVED BY BLENDING

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Shadow

Applied depth

A

50

Solid or contrast

67

Colour and reserve

50 33

B

Differences between fibre components A and B Solid Contrast Shadow Reserve

Matching hue and depth Matching depth but contrasting hues Matching hue at a ratio of depths Contrast between colour and white

Figure 1.2 Diagrammatic representation of colour effects in blends

there is some scope for adjustment towards solidity by shading with acid dyes or basic dyes respectively, but distribution of the disperse dyes between the fibre components can be controlled to only a limited extent by adjustment of dyeing temperature or (more objectionably) by addition of a carrier. Solid dyeings on blends of cellulose acetate with polyester or acrylic fibres are impracticable because the acetate fibre is damaged under the relatively severe conditions required to achieve reasonable depths on the synthetic component. Much more control of distribution is possible in blends of nylon with wool, polyurethane or cellulosic fibres, because anionic reserving or blocking agents can be added to control the degree of uptake of the anionic dyes by nylon below the saturation limit. Solid effects are not difficult to achieve on other types of binary blend, the easiest situation being found with blends of acid-dyeable variants of nylon or acrylic fibres with basic-dyeable nylon, polyester or acrylic variants, where the relative freedom from cross-staining gives optimum reproducibility of effect and control of shading. 1.5.2 Reserve effects Cross-staining constitutes a serious problem if a reserve effect is required. Staining is more likely to occur in blends of fibres with distinctly different dyeing

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properties. It is especially prevalent in blends of a natural fibre with a synthetic one that can only be dyed with disperse dyes. In these circumstances it is impracticable to dye the latter component without some cross-staining of the natural fibre. It is difficult to obtain a satisfactory reserve of any acid-dyeable fibre and none of the blends with wool will give a reserve effect on the wool component. Reserve effects are impracticable on either component of blends of wool, nylon or polyurethane with one another. Nylon does not give a good reserve in its blends with cellulose acetate, triacetate, polyester or cellulosic fibres. Acrylic fibres cannot be reserved in the presence of the disperse-dyeable ester fibres. In blends of ester fibres with one another, only the less dyeable component can be reserved. This principle also applies to blends of normal and deep-dye variants, where the latter cannot be reserved. Both fibres can be reserved satisfactorily in blends of acrylic or the ester fibres with cellulosics, and in synthetic blends of acid-dyeable and basic-dyeable variants. Fastness standards may be impaired by the cross-staining of one fibre by a class of dyes intended for the other component. Several measures can be considered with a view to minimising the degree of cross-staining in blends where this is a potential problem: (1) selection of dyes with the lowest affinity for the fibre to be reserved, as well as those with the highest affinity for the component that is to be dyed with them; (2) selection of dyeing conditions that favour maximum exhaustion by the component fibre to be dyed and hence the minimum cross-staining of the adjacent fibre; (3) addition to the dyebath of a colourless agent that is preferentially absorbed on the dyeing sites of the fibre to be reserved and is able to act as a resist against subsequent staining by dyes with sorption behaviour similar to that of the agent; (4) a clearing treatment with a detergent to desorb, or a reducing agent to destroy, the dye stain that has been taken up by the adjacent fibre during dyeing. In two-bath sequences it is often advisable to interpose a clearing step after the first dyeing stage in order to remove any stain from the component to be dyed in the second stage. 1.5.3 Shadow effects The shadow effect (cumbersome expressions like two-tone, tone-in-tone, or toneon-tone have also been used) may be regarded as an intermediate stage between solid and reserve effects (see Figure 1.2). The most pleasing shadow effects are

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obtained when the paler depth is between about one-third and one-half the depth of the deeper component. If the two depths are closer together, the effect approaches that of incomplete solidity, whereas if the paler component is too weak it resembles a stained reserve. It is obviously much simpler to obtain shadow effects using one class of dyes – as in blends of cellulosic fibres with one another, with disperse dyes on blends of triacetate with polyester or acetate, or with acid dyes on blends of nylon with wool or polyurethane – than to attempt to achieve a similar effect on those blends requiring different classes of dyes on the component fibres. The most attractive shadow dyeings are seen on the differential-dyeing variants, such as pale-dye/deep-dye nylon or normal/deep-dye polyester, where the appeal of the depth difference is not impaired by distracting differences of surface texture or lustre of the two components. 1.5.4 Contrast effects Contrast effects (also called cross-dye or two-colour effects) represent the primary justification for the development of differential-dyeing yarns and have contributed much to the design of patterned apparel fabrics and tufted carpeting over the years. Colour contrasts cannot be obtained on blends in which the two fibres resemble one another too closely in dyeing properties, as in blends of cellulosic fibres with one another, blends of ester fibres with one another, or blends of wool, nylon or polyurethane with one another. The best contrast effects are shown by fabrics containing acid-dyeable with basic-dyeable synthetic yarns (e.g. nylon/acrylic blends), since the freedom from significant cross-staining under optimum dyeing conditions permits the contrast of complementary pairs i.e. red–green, blue–orange or violet–yellow. In those blends where considerable cross-staining is unavoidable, the sharpness of the contrast is obviously seriously muted. In many instances only partial contrast effects are possible by using a disperse dye on both components and an ionic dye on the more dyeable component, as on normal/deep-dye nylon or the blends of ester fibres with ionic-dyeable fibres. In these circumstances, the hue on the more dyeable fibre is partly determined by that on the other fibre and the resulting contrasts are limited (Table 1.4). It is obviously easier, when selecting dyes for contrast effects, to dye the deeper and/or duller colour on the fibre component that is most prone to cross-staining. This general approach, however, may have to be modified if the two fibres differ greatly in abrasion resistance. For example, the polyester component of a polyester/viscose shirting fabric dyed in contrasting hues should be dyed more heavily, because differential abrasion at the collar and cuffs leaves a higher

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Table 1.4 Limitations of colour contrast on a disperse-dyeable/ionicdyeable blend Colours of individual dyes

Colours on component fibres

Disperse dye

Ionic dye

Disperse-dyeable

Yellow Yellow Red Red Blue Blue

Red Blue Yellow Blue Red Yellow

Yellow Yellow Red Red Blue Blue

Ionic-dyeable Orange Green Orange Violet Violet Green

proportion of polyester on those edges and the abraded areas become much more evident if the polyester is dyed to a paler hue. A particularly striking example of a pronounced contrast effect is provided by the use of Lurex metallic threads in decorative apparel fabrics. Even here the familiar problem of staining by disperse dyes often arises. The degree of staining of silver and gold Lurex threads by typical disperse dyes in the dyeing of polyester/Lurex blends by carrier and high-temperature methods has been tabulated. The stability of Lurex threads to various chemical treatments, and application of a sodium dithionite clearing treatment to minimise disperse dye staining, have been examined [16]. 1.5.5 Colour matching problems and colour measurement of dyed blends The degree of solidity that will prove acceptable to the customer varies according to the end-product. Whilst piece-dyed fabrics woven from blended yarns call for high standards of solidity, carpet yarns may prove less critical, since for certain designs and qualities of carpet a slightly ‘broken’ appearance better simulates that presented by a blend of different qualities of wool. Furthermore, on tufted carpets and pile fabrics the upper surface of the material is uneven in height and the interplay of incident lighting with the effects of differential crushing ensure that the uniformity of appearance presented by a woven fabric can never be approached. Nevertheless, spinning of a staple blend must be carefully controlled. Even a carpet yarn containing 20:80 nylon/wool may give an unlevel appearance in the piece if there are clumps of nylon fibres that have not been thoroughly opened and mixed. The minimum level of each component in a staple blend should be at least 5% to ensure uniformity of mixing. Collaboration between ICS at Newbury and IWS at Ilkley in the 1980s resulted in a programme to enable fibre

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blending to be carried out by computer. This could be used to predict the proportions of various fibre-dyed components to match a target blended sample [17]. A particularly critical substrate from the viewpoint of solidity of shade is polyester/wool, which is complicated by serious staining of the wool by disperse dyes. Another limitation is the need to employ a carrier to achieve satisfactory yield and penetration on the polyester component at a temperature that does not seriously damage the wool. The problems of achieving solidity of hue, brightness and saturation between textured polyester filament yarns and 55:45 polyester/ wool staple yarns in the same fabric have been examined in detail [18], by reference to colorimetric data obtained by dyeing in the presence of various concentrations of carriers based on o-phenylphenol, trichlorobenzene, or mixtures of them. Perhaps the most complex and intractable challenges have been faced by dyers of wool shoddy over the years since synthetic fibres became important. The three sources of raw material for the dyers of reclaimed waste are: (1) collected rags or worn-out garments; (2) cuttings and fents left over from the making-up of garments or household textiles; (3) fibre producer’s waste, which is often uncoloured and does not require sorting. When military uniforms were all-wool materials, outworn uniforms were a prime source of recovered wool. Trade in these used garments declined, however, when polyester blends were adopted and difficulties arose at the stripping and redyeing stage. Special dye selections were devised, particularly from those products not affected by the iron content of the stripped wool arising from the crude equipment in use at the time. There was significant conversion to wool/ viscose unions, often dyed wool way only in fabric form. Category 1 causes most problems as the rags and garments must be sorted by colour and fibre type. They often contain blended materials that only become clearly identifiable after dyeing. Disperse dye staining is much more troublesome when dyeing a 99:1 blend of wool shoddy and polyester than in conventional 55:45 polyester/wool blends [19]. Carrier dyeing of these synthetic fibre impurities is highly inefficient. It is not difficult to achieve acceptable solidity on all-wool shoddy, but reclaimed acrylic material usually contains several different acrylic variants that may differ widely in dyeability. A novel method has been devised to cover synthetic fibre impurities in deepshade dyeings on fabrics made from reclaimed wool [20]. This is to pass the wool

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WHY BLENDING IS NECESSARY

fabric, together with a textured polyester fabric already dyed to a matching shade, through the nip of a transfer printing calendar. The synthetic fibre impurities in the reclaimed fabric are dyed by vapour-phase transfer but the wool fibres remain virtually unstained because the disperse dyes have such high affinity for polyester at the heat transfer temperature. Conventional transfer coloration from a disperse dye transfer paper is uneconomic and carries a greater risk of wool staining. The textured polyester ‘reservoir’ fabric can be used repeatedly in this way with only very slow loss of disperse dye. Shade matching problems are often more difficult to deal with in union fabrics than in fabrics made from intimately blended yarns [21]. Recipes based on difluoropyrimidine reactive dyes were applied to both components of a viscose warp/nylon weft fabric dyed by the one-bath two-stage process. Reflectance data for the dyed fabric and colour difference values between the viscose warp and nylon weft yarns were measured [22]. It was shown that objective matching is possible and that the reproducibility of the matching operation can be improved by careful selection of the dyes used. The development of computer colour matching programmes for the dyeing of blends presents specific difficulties. Dyes that cause minimal cross-staining are preferred for better reproducibility of matching. Preliminary calibration work on the laboratory scale must be carried out at the effective liquor ratio that corresponds to bulk conditions for the blend. All fibre components must be present in the calibration dyeings in order to account for the competitive effects between them. Fibre fragments taken from dyed blends can be used to produce felt-like discs that retain the colour properties of both constituent fibres. These discs are prepared using a new type of press that has been described and illustrated [23]. The discs are easy to prepare and are thus preferred to yarn windings for calibration purposes. Two methods utilising this technique were described for prediction of the initial recipe for the acrylic pile and cotton backing of a cotton/ acrylic velour fabric. Instrumental techniques have been applied to the colour effects obtained on differential-dyeing nylon. Loop-pile carpet and pile yarns dyed with CI Acid Blue 277 on deep-dye and normal variants, and CI Basic Yellow 45 on a basic-dyeable variant, were measured using a colour computer system, a spectrophotometer to measure small areas of the design and a goniophotometric colorimeter [24]. Methods of determining the levelness of wool dyeings and of union dyeings on wool/cotton textiles by digital image analysis have been developed [25]. Levelness values were derived from the standard deviations of the grey scale ratings corresponding to individual histograms of the dyed fabrics. The levelness

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of union dyeings could be assessed by analysing the shapes of the histograms obtained from the wool/cotton dyeings. Correct formulation of the pad liquor is an essential element in the quality assurance of continuous dyeing. Transmission measurement is a viable alternative to control dyeings prepared in the laboratory to check pad liquors in bulk. Practical experience has demonstrated [26] that this method can be used not only for soluble systems such as reactive dyes but also dispersions of disperse or vat dyes, as well as mixtures of soluble dyes with disperse dyes in polyester/ cellulosic dyeing. With extensive automation of the measuring process, high operational reliability and continuous monitoring are feasible. Potential causes of incorrectly set pad liquors include: (1) varying dye deliveries from the supplier; (2) moisture sorption of powder brands; (3) sedimentation of liquid brands; (4) incorrect choice of dye from storage; (5) incorrect weighing; (6) incorrect dissolution or dispersion; (7) inadequate stirring during mixing; (8) soiling during transfer to pad trough; (9) lack of temperature control in storage or padding. Commercial mixture recipes containing (a) disperse dyes with vat, vat leuco ester or reactive dyes, for the dyeing of polyester/cellulosic blends, or (b) disperse dyes with 1:2 metal-complex or acid dyes, for polyester/wool blends, were examined in pad liquors made up in readiness for continuous dyeing. Photometric measurements using analogue, digital and turbidity photometers [27] were used as alternatives to conventional control dyeings to save time and running costs. Calibration curves confirmed that the Beer–Lambert law was sufficiently applicable to these mixtures, yielding adequate reproducibility for on-line control purposes. These approaches virtually eliminate the production of faulty batches attributable to incorrectly set pad liquors, as well as freeing laboratory dyeing effort for other tasks. 1.5.6 Colour effects on three-component blends With the exception of nylon/wool/viscose blends in carpet yarns, ternary blends are seldom dyed in solid shades because of the matching difficulties involved. Three-depth shadow effects are given by acid dyes on pale-dye/normal/deep-dye or normal/deep-dye/ultra-deep nylon (see Chapter 15), but a three-way contrast with primary colours on all three components is most difficult because of the

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WHY BLENDING IS NECESSARY

problems of cross-staining. Multicoloured designs on ternary blends can be based on two-way shadow or contrast effects with white reserve of the third component, or more often, a two-way shadow effect with a contrasting hue on the third fibre. Ternary blends on which disperse, basic and acid dyes can be used will give a limited range of three-way contrasts, but the hues on the ionic-dyeable variants are dependent on the hue of the disperse dye, which cannot be restricted to only one of the component fibres (Table 1.5).

Table 1.5 Limitations of colour contrast on a typical three-component blend Colours of individual dyes

Colours on component fibres

Disperse dye

Basic dye

Acid dye

Dispersedyeable

Basicdyeable

Aciddyeable

Yellow Red Blue

Red Yellow Yellow

Blue Blue Red

Yellow Red Blue

Orange Orange Green

Green Violet Violet

1.5.7 Scintillant effects on staple blends Colour contrast effects on intimate blends of two different fibre types can be accentuated by deliberately modifying the distribution of the constituent fibres within the yarns. In conventionally blended staple yarns sufficient doublings are given to ensure that this distribution is regular and uniform in cross-section. This uniformity is an inherent property of a conventional blend and it results in reproducible spinning and weaving characteristics and reliable in-service performance. Not all natural fibres exist as single or ultimate fibres, however. Flax and other bast fibres occur naturally in bundles of ultimate fibres held together by interstitial material, varying in size from about 10 to 40 ultimates. When these are dry spun the bundles do not break down but remain as groups within the spun yarn. Thus the intimacy of blend found in a conventional polyester/cotton is not found in a typical polyester/linen yarn. The flax bundles are distributed through a matrix of the individual polyester staple fibres (Figure 1.3). This grouping of fibres can be utilised to produce striking colour contrast effects using the dye selectivity of the constituent fibres. The fibre groups can be accentuated by incorporating short fibres into the blend to produce slubs, or by

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Cellulosic fibres Polyester fibres

Conventional fibre blend

Grouped fibre blend

Figure 1.3 Diagrammatic cross-sections of conventional and grouped polyester/cellulosic blend yarns [28]

injecting clumps of fibres into the yarn during spinning [28]. Fabrics made from coarse dry spun yarns exhibit particularly attractive scintillant effects and can be produced with pronounced slub content. It is necessary to have at least 30% of each fibre present to make a significant contribution to the differential colour contrast. Yarns with a short slub character are of great interest because these slubs create focal points of colour that can be utilised by the designer. The most effective results are achieved when the paler or brighter colour is applied to the slub component so that it is highlighted against the darker background. 1.6 SIGHTING COLOURS FOR IDENTIFICATION PURPOSES Sighting colours are especially useful in knitting or weaving plants that handle a wide variety of man-made fibres and blended staple yarns. Selected low-fastness dyes are used to stain the surface of the fibres, but they must be readily and completely removable in scouring before dyeing and finishing. They may become difficult to extract if the ‘sighted’ yarn is steamed or dry heat set before scouring. Specially designed varieties of sighting colours include [29]: (1) an ionic dye complexed with a surfactant of opposite charge; (2) a water-soluble vinyl polymer associated with a dye of opposite charge; (3) a water-soluble starch derivative covalently linked to a reactive dye via the hydroxy groups; (4) a water-soluble dye in which the replaceable hydrogen atoms in the structure (as in OH, NH or CONH groups) are substituted by long polyoxyethylene chains; (5) an analogous polyoxyethylene-substituted disperse dye structure with sufficient hydrophobic character to inhibit penetration into the intermicellar regions of highly swollen cellulosic fibres.

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1.7 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

A P S Sawhney, R J Harper, K Q Robert and.G F Ruppenicker, Text. Res. J., 61 (1991) 393. L B Kimmel, A P S Sawhney, G F Ruppenicker and R J Harper, Text. Chem. Colorist, 26 (Mar 1994) 22. P A Koch, Textilveredlung, 7 (1972) 570; Chemiefasern und Textilind., 29/81 (1979) 431. S Shiomura, Textile Asia, 22 (Sep 1991) 140. P Lennox-Kerr, Text. Horizons, 11 (Jan 1991) 33. P Lennox-Kerr, Text. Horizons, 2 (Nov 1982) 18. S Roberts, Dyer, 178 (June 1993) 10. L Jacques, J.S.D.C., 109 (1993) 315. I B Angliss and J D Leeder, J.S.D.C., 93 (1977) 387. G Jerg and J Baumann, Text. Chem. Colorist, 22 (Dec 1990) 12. A Lallam, J Michalowska, L Schacher and P Viallier, J.S.D.C., 113 (1997) 107. P W Leadbetter and S Dervan, J.S.D.C., 108 (1992) 369. J C Dupeuble, Chemiefasern und Textilind., 40/92 (1990) 986. D Wiegner, Chemiefasern und Textilind., 41/93 (1991) 148. C L Chong, Textile Asia, 25 (Mar 1994) 59. V Walther, Chemiefasern und Textilind., 35/87 (1985) 321. J Park, Wool Record, (Aug 1987) 23. G Römer, Teinture et Apprets, No 145 (Dec 1974) 203. K Barras, Dyer, 153 (13 June 1975) 612. M E Fielding, Dyer, 157 (21 Jan 1977) 68. W Pape, Melliand Textilber., 69 (1988) 737. A Gantsheva and E Kantscher, Textilveredlung, 26 (1991) 116. P Medilek, Melliand Textilber., 75 (1994) 822. K Konno, I Hirai and T Gunji, J. Text. Mach. Soc. Japan, 37 (Apr 1992) 93. J M Cardamone, W C Damert and W N Marmer, AATCC International Conference and Exhibition, (Oct 1994) 246; Text. Chem. Colorist, 27 (Oct 1995) 13. H P Locher and H Firmann, Textilveredlung, 26 (1991) 393. V Reith, Melliand Textilber., 72 (1991) 774. B Hill and G Gray, J.S.D.C., 108 (1992) 419. K Marquardt, Chemiefasern, 24 (1974) 940.

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CHAPTER 2

Classification of fibre types and their blends

2.1 CLASSIFICATION OF FIBRE TYPES IN TERMS OF DYEABILITY The examples mentioned in Chapter 1 have given an indication of the wide variety of fibre blends available and the complex ways in which they may be assembled and dyed as blended yarns or in certain types of fabric, garment or carpet. Before considering the fundamental principles of blends dyeing and the methods devised to colour them, it is useful to classify fibres and their blends in terms of their dyeing characteristics. This is more appropriate in this context then the usual division into natural, regenerated and synthetic fibres. Although more than one class of dyes is important on cellulosic fibres, and disperse dyes are often used in pale depths on all types of synthetic fibre, a simple classification based on the classes of dyes used to obtain fast dyeings in full depths can be made (Table 2.1). This can then be applied to classify binary blends (Table 2.2) and ternary blends (Table 15.1) in a similar way [1,2]. The dyeing characteristics of the component fibres in full depths are particularly important because many of the problems associated with the dyeing of blends are more serious under such conditions. These problems include: (1) the interference with solidity caused by differences in the saturation limits on the component fibres; (2) the greater degree of staining of reserved fibres, often making it necessary to employ a two-bath method; (3) more critical fastness requirements that must be achieved adequately on both fibres; (4) more serious incompatibility of dyes and auxiliaries of opposite charge at higher concentrations. The variations from alphabetical order in the codes for blend categories listed in Table 2.2 are deliberate. When referring to blends of polyester with natural fibres it is customary to name the synthetic fibre first, i.e. polyester/cotton (DC) or even ‘poly/cotton’ is heard far more than cotton/polyester, which is usually 21

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CLASSIFICATION OF FIBRE TYPES AND THEIR BLENDS

Table 2.1 Classification of fibres by dyeing properties

A Fibres (dyed with acid dyes in full depths) Wool and other animal fibres Silk Nylon homopolymer Acid-dyeable nylon variants Polyurethane fibres Acid-dyeable polypropylene Acid-dyeable acrylic and modacrylic fibres

B Fibres (dyed with basic dyes in full depths) Basic-dyeable acrylic and modacrylic fibres Basic-dyeable nylon Basic-dyeable polyester

C Fibres (dyed with cellulosic dyes in full depths) Cotton Viscose Lyocell, modal and polynosic fibres Linen and other bast fibres

D Fibres (dyed with disperse dyes in full depths) Cellulose acetate Cellulose triacetate Polyester homopolymer Deep-dye polyester variants Poly(vinyl chloride) fibres

encountered if cotton-rich blends are being considered. Furthermore, when a fibre type is referred to by an adjectival term, such as ‘acrylic’ or ‘cellulosic’, it is preferable to name this component second, as in nylon/acrylic (AB) or wool/ cellulosic (AC) blends. Where both adjectival terms occur in the same category, i.e. the cellulosic/acrylic (CB) category, this order is preferred for the individual blends, such as cotton/acrylic and viscose/acrylic, rather than their reversals. 2.2 COLOUR DISTRIBUTION ATTAINABLE ON BINARY BLENDS General comments can be made regarding the dyeing characteristics of the major classes of binary blends (Table 2.3). The AA blends, based mainly on nylon and the protein fibres, are particularly important in knitting and carpet yarns. Physical properties usually provide the main justification for developing these blends, which are often blended-staple yarns. Solid dyeings are therefore most important and not too difficult to obtain because preferential uptake by nylon in pale depths can be controlled using reserving agents. These blends are ideally

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Table 2.2 Classification of binary blends

AA blends

CC blends

Wool/silk Wool/mohair Wool/cashmere Wool/angora Nylon/wool Nylon/silk Wool/polyurethane Nylon/polyurethane Wool/acid-dyeable polypropylene Nylon/acid-dyeable polypropylene Normal/deep-dye nylon

Cotton/viscose Cotton/modal fibre Cotton/polynosic fibre Cotton/linen Linen/viscose Linen/modal fibre

AB blends Wool/acrylic fibre Silk/acrylic fibre Nylon/acrylic fibre Polyurethane/acrylic fibre Acid-dyeable polypropylene/acrylic fibre Wool/modacrylic fibre Mohair/modacrylic fibre Nylon/modacrylic fibre Acid-dyeable/basic-dyeable acrylic fibre Modacrylic fibre/acrylic fibre Deep-dye/basic-dyeable nylon Wool/basic-dyeable polyester Nylon/basic-dyeable polyester

AC blends Wool/cotton Silk/cotton Nylon/cotton Polyurethane/cotton Acid-dyeable polypropylene/cotton Wool/viscose Silk/viscose Nylon/viscose Wool/modal fibre Nylon/modal fibre Nylon/linen

CB blends Cotton/acrylic fibre Viscose/acrylic fibre Modal fibre/acrylic fibre Polynosic fibre/acrylic fibre Cotton/modacrylic fibre Viscose/modacrylic fibre

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DA blends Cellulose acetate/wool Cellulose acetate/silk Cellulose acetate/nylon Cellulose triacetate/wool Cellulose triacetate/silk Cellulose triacetate/nylon Polyester/wool Polyester/silk Polyester/nylon Polyester/acid-dyeable polypropylene Poly(vinyl chloride)/wool Poly(vinyl chloride)/nylon

DB blends Cellulose acetate/acrylic fibre Cellulose triacetate/acrylic fibre Polyester/acrylic fibre Cellulose acetate/modacrylic fibre Cellulose triacetate/modacrylic fibre Polyester/modacrylic fibre Normal/basic-dyeable polyester

DC blends Cellulose acetate/cotton Cellulose triacetate/cotton Polyester/cotton Poly(vinyl chloride)/cotton Cellulose acetate/viscose Cellulose triacetate/viscose Polyester/viscose Poly(vinyl chloride)/viscose Cellulose triacetate/modal fibre Polyester/modal fibre Polyester/polynosic fibre Polyester/linen

DD blends Cellulose acetate/triacetate Cellulose acetate/polyester Cellulose triacetate/polyester Normal/deep-dye polyester

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Table 2.3 Colour effects attainable on binary blends Colour effect Blend type (example)

Solid

Reserve

Shadow

Contrast

AA (nylon/wool)

Use of auxiliaries

Neither component

Easily controlled

Not possible

AB (nylon/acrylic)

Easily controlled

Acrylic reserve

Seldom required

Wide range available

AC (nylon/cellulosic)

Easily controlled

Cellulosic reserve

Seldom required

Wide range available

CB (cellulosic/acrylic)

Easily controlled

Either component

Seldom required

Wide range available

CC (cotton/viscose)

Dyeing conditions

Neither component

Viscose deeper

Not possible

DA (polyester/wool)

Dyeing conditions

Polyester reserve

Seldom required

Limited range

DB (polyester/acrylic)

Easily controlled

Polyester reserve

Acrylic deeper

Limited range

DC (polyester/cellulosic)

Easily controlled

Either component

Seldom required

Wide range available

DD (triacetate/polyester)

Dyeing conditions

Polyester reserve

Easily controlled

Not possible

suited to shadow effects because one class of dyes can be used for both components. Contrast and reserve effects are generally impracticable. Optimum reproducibility of bright complementary colour contrasts is achieved on AB blends, containing nylon or a protein fibre with an acrylic or basic-dyeable copolymer. Good reserve of the basic-dyeable fibre, or solid effects, can be obtained if required. Two-bath dyeing methods are generally preferred, because of the need to inhibit precipitation between the classes of dyes of opposite charge. The anionic dyes applicable to both components of an AC blend are, in most cases, fully compatible with one another. This facilitates the exploitation of onebath dyeing methods and gives ample opportunity for controlled shade matching in either solid effects or colour contrasts. There is less interest in shadow or reserve effects on these blends. Colour contrasts and solid effects are readily obtainable on CB blends. Either the cellulosic or the acrylic fibre may be reserved if desired, so this is a versatile

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type of blend from the viewpoint of design. Two-bath methods are preferable, however, because basic dyes are incompatible with all classes of dyes for the cellulosic component. Dye selection and control of dyeing conditions provide reasonable scope for solidity on staple blends of the CC category, but there may have to be some sacrifice of optimum fastness and colour gamut. The CC blends are ideally suited to shadow effects, as in brocade designs for furnishing or curtaining fabrics. As with the AA blends, reserve effects and colour contrasts are not attainable. Blends of an ester fibre with nylon or a protein fibre (DA blends) are mostly blended-staple yarns and solidity is therefore often important. The acid-dyeable component cannot be reserved because of disperse dye staining, but good reserve of the ester fibre is attainable. Colour contrasts are limited because of the dulling effect of the disperse dye on the acid-dyeable fibre. The attainment of contrast or reserve effects by differential dyeing is the main justification for DB blends, which contain an ester fibre with an acrylic or other basic-dyeable copolymer. The basic-dyeable component cannot be reserved satisfactorily, but good reserve of the ester fibre is possible. Contrast effects are limited by cross-staining of disperse dye on the basic-dyeable yarn. The DC blends represent the most important category and solid effects are a primary objective. Disperse dye staining of the cellulosic fibre is much less serious than for the DA and DB types, so either component of a DC blend can be reserved. Colour contrasts are possible but not of much interest in practice. Disperse dyes offer the only possibility for colouring DD blends and this completely eliminates the contrast option. Shadow and reserve effects are particularly appropriate because the ester fibres differ so much in dyeability. Cellulosic acetate absorbs dye much more readily than the triacetate at low temperatures, making it easy to reserve the latter component. The degree of distribution on triacetate/polyester can be controlled using dyeing temperature and carrier additions, but solid effects are difficult to achieve.

2.3 REFERENCES 1. 2.

J Shore in The dyeing of synthetic-polymer and acetate fibres, Ed. D M Nunn (Bradford: SDC, 1979) 419. J R Aspland, Textile dyeing and coloration (North Carolina: AATCC, 1997) 331.

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DYNAMIC COMPETITION BETWEEN FIBRE TYPES IN THE DYEING OF BLENDS

CHAPTER 3

Dynamic competition between fibre types in the dyeing of blends

3.1 INTRODUCTION Dyeing systems in which more than one fibre type is present have been almost completely neglected by those researching the mechanisms of dyeing processes. This is not surprising, because the complications that arise when attempting to predict quantitatively the uptake of a single dye by one fibre type are considerable. Theories of dyeing established on the basis of careful measurements of one such dye at various depths under a variety of dyeing conditions can seldom be transferred intact to other members of the same range of dyes, especially if these are distinctly different in structure from the first dye chosen. Further difficulties arise with the binary or ternary combinations of dyes that are used routinely in practice to achieve the wide gamut of colours with which design colourists and dyers must work. Two dyes undergoing absorption by the fibre simultaneously rarely reproduce the dyeing rate curves that they give alone. There is almost invariably an interaction between them, often (but not always) resulting in a slower rate of uptake and a lower equilibrium exhaustion for each of the dyes in combination, compared with the corresponding values when applied individually. An entirely different dimension is introduced when two fibres are present in the same dyebath. A single dye may be distributed between them according to a complex relationship that is determined by the differences in dyeability characteristics between the two substrates, the dyeing conditions and the applied depth of the dye. An unequal distribution that may be found in an early stage of the process, arising from differing rates of uptake by the competing substrates, may later undergo a levelling effect as dye is desorbed from the initially more heavily dyed fibre and is taken up by the other fibre type for which the dye has inherently higher affinity. When two different substrates are present in the same dyebath, initial kinetic

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INTRODUCTION

27

conditions and later the energetic characteristics of the system will dictate which of the substrates a given dye will tend to favour. More dye will ultimately be absorbed by the substrate to which it is more substantive. This favoured substrate will require less energy for the sorption process because the dye–fibre forces of interaction are stronger. When a combination of two or three dyes is applied to two different substrates, however, it does not always follow that the dyes present will all be absorbed preferentially by the same fibre component to the same extent. It is certainly possible in practice to choose a set of dyeing conditions in which a trichromatic combination of dyes with similar dyeing properties will yield a matching hue on both substrates at equal depth (a solid effect) or at markedly different depths (a shadow effect). To arrive at such ‘ideal’ combinations of dyes and dyeing conditions, however, implies considerable preliminary laboratory work, especially when other important factors, such as fastness demands and non-metameric matching, must be taken into account [1]. This task can be facilitated by deriving characteristic thermodynamic parameters of the dyeing system from the sorption behaviour of the individual dyes on the separate homogeneous substrates. It is claimed that these parameters allow computation of the sorption behaviour of the dyes in combination on more than one substrate simultaneously [2]. The substantivity ratio (Df /Ds) for each dye on each substrate depends on temperature, pH, liquor ratio, concentrations of dye and electrolyte, and particularly in this case on the other dyes and substrates involved in the competitive sorption process. According to the Gouy-Chapman theory, the substantivity ratio is related to the electrolyte concentration or ionic strength and the total amount of ionic charge imparted to the substrate by the absorption of dye ions. This factor encapsulates the mutual restraining effects of the dyes on one another. The three characteristic constants in the Gouy-Chapman equation are as follows: A0 is a function of the standard affinity and thus the substantivity ratio under the relevant conditions. A1 is proportional to the charge density and is a measure of the overall charge on the substrate. A2 is related to the specific surface of the substrate accessible for dye sorption. The simplest dyeing system that may be considered as representative of the dyeing of a blend is that in which one dye is distributed between two different fibres that have broadly similar dyeing properties, so that the same dye will give an economic colour yield on both. Even if solidity of colour between them is the

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DYNAMIC COMPETITION BETWEEN FIBRE TYPES IN THE DYEING OF BLENDS

objective, there is no guarantee that the dyeing conditions to achieve this at the target depth will ensure satisfactory penetration and fastness on both substrates. It is often easier to adopt this approach when a shadow effect is desired, because any inherent difference in dyeability between the substrates can be exploited in adjusting dye uptake levels towards the target difference in depth. The serious limitations of this simple dyeing system become clear when reserve and colour contrast effects are considered. By definition it is usually extraordinarily difficult to suppress the dyeability of the less dyeable component to zero in order to reserve it as white or to dye it in another colour, which means introducing a dye from another dyeing class. Another deceptively simple approach to the dyeing of a binary blend of fibres is to dye each in turn with an appropriate class of dyes in two completely separate dyeing processes, using the optimum conditions of application in each case just as if they were entirely separate substrates. At first sight, this appears an ideal way in which to dye for solidity, reserve, shadow or contrast effects at will. In practice, however, technical limitations do arise here too. The wet fastness characteristics of the dyeing achieved in the first process must be such that no significant desorption from it occurs during the second one. In other words, the fastness to cross-dyeing must be excellent. Thus the order in which the two processes are carried out is important and the dyeing that requires the higher dyeing temperature is normally applied first. The two-bath process is also less than ideal unless both classes of dyes are free from cross-staining problems, i.e. a class selected to dye one of the fibre types should not cause significant staining of the other fibre present. In a majority of instances, however, cross-staining of one or both fibre types must be taken into account and a clearing process introduced after one or both of the dyeing processes. Thus the so-called ‘two-bath’ process may require several baths for completion. It is easy to see why this technically straightforward possibility turns out to be the least attractive from an economic viewpoint. Between these fairly obvious extremes of the single-class and the two-bath methods there are two other compromises that offer greater flexibility than the former and economic savings over the latter. These are the simultaneous onebath and the one-bath two-stage methods. For clarification the four possibilities are summarised in Table 3.1. These will be referred to frequently in later chapters and it is convenient to use the four hyphenated abbreviations listed in the first column to distinguish between them. In general, the progressive increase in cost in moving down this list is compensated by a wider choice of suitable dyes and greater freedom from the practical problems discussed in terms of typical examples in the remainder of this chapter. Another general rule is that the greater

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INTRODUCTION

29

Table 3.1 Summary of general dyeing methods for binary blends Method

Dyebaths

Dye classes

Stages

Single-class One-bath Two-stage Two-bath

One One One Two

One Two simultaneously Two in sequence Two

One One Two Two

the depth of shade required, the more likely is it necessary to move to a method lower down this list. 3.2 THE DISTRIBUTION OF ACID DYES ON NYLON/WOOL BLENDS Nylon/wool blends are often dyed with a single class of anionic dyes, which may be levelling acid, milling acid or metal-complex types. Ensuring solidity of shade is normally the main requirement and this calls for careful selection of combinations of dyes with similar rates of dyeing and build-up characteristics. Partitioning of the dyes between the two fibres can be influenced by many factors, including dye structure, applied depth, dyebath pH, blend ratio and the quality of the component fibres. Wool and nylon contain both basic and acidic groups, amongst which by far the most important are amino and carboxyl groups respectively. Just like the parent amino acids from which all proteins are derived, both of these polymeric amides show zwitterionic characteristics at pH values close to the isoelectric point, i.e. the pH at which the fibre contains equal numbers of protonated basic and ionised acidic groups. As the pH decreases below this point, the carboxylate anions are progressively neutralised by the adsorption of protons and the fibre acquires a net positive charge (Scheme 3.1). +

H3N

[fibre]

COO– + H+

+

H3N

[fibre]

COOH

Scheme 3.1

Conversely, as the pH rises above the isoelectric point, the fibre becomes negatively charged as a result of deprotonation of the amino groups by adsorption of hydroxide ions or other simple anions (Scheme 3.2). +

H3N

[fibre]

COO– + –OH

H2N

[fibre]

COO–

+ H2O

Scheme 3.2

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DYNAMIC COMPETITION BETWEEN FIBRE TYPES IN THE DYEING OF BLENDS

The rate of dyeing of anionic dyes on nylon is much more rapid than on wool, particularly at 60–80°C and low applied depths, so that pale dyeings on nylon/ wool show a marked preferential dyeing of the nylon. Even at 1% applied depth, for example, the uptake by nylon is much more rapid than by wool and the difference in rate is greater for dyes with a lower degree of sulphonation. Figures 3.1 and 3.2 are rate-of-dyeing curves for disulphonated and tetrasulphonated dyes (Figure 3.3) respectively on these two fibres, at 93°C and an initial pH of 4.2 in aqueous acetic acid [3]. As dyeing proceeds, the pH rises towards 5 as a result of sorption of acetic acid, particularly by wool.

100

Exhaustion/%

80 60 40 Nylon

20

Wool

20

40

60

80

100

Dyeing time/min

Figure 3.1 Rate of dyeing of CI Acid Red 1 [3]

100

Exhaustion/%

80 60 40 20

10

20

40

60

80

100

Dyeing time/min

Figure 3.2 Rate of dyeing of CI Acid Red 41 [3] (for key see Figure 3.1)

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THE DISTRIBUTION OF ACID DYES ON NYLON/WOOL BLENDS

COCH3

H O

HN

NaO3S

H O N

31

SO3Na

N N

SO3Na N

NaO3S SO3Na

NaO3S CI Acid Red 1

CI Acid Red 41

Figure 3.3 Disulphonated and tetrasulphonated acid dyes

The origin of the markedly different rates of dyeing lies in the differences in hydrophobic character between the two dyes and between the two fibres. The nylon polymer is much more hydrophobic than wool and so it attracts the more hydrophobic of the two dyes preferentially. Thus the disulphonated dye shows the most rapid rate of absorption on nylon, with a time of half dyeing (time for 50% of the equilibrium exhaustion) of only about 2 minutes. The tetrasulphonated dye is relatively hydrophilic and is therefore absorbed more slowly by either fibre, showing the slowest rate of dyeing on wool, with a time of half dyeing of approximately 3 hours. Although nylon absorbs acid dyes more readily than does wool, partition between the two fibres is not constant at all depths since the saturation concentration on nylon is much lower than that on wool. The saturation limit on wool is not approached at the applied depth necessary to saturate the amine end group content of the nylon. In pale depths both the initial uptake and the ultimate exhaustion are higher on nylon. In full depths, on the other hand, the initial strike still occurs on nylon but eventually the wool becomes more heavily dyed because it has a much higher saturation uptake. At some intermediate depth depending on dyeing conditions, there is a point at which both fibres are dyed to the same depth even though the nylon reaches this equilibrium position more quickly. This critical depth is specific for the dye and is much higher for a monosulphonated dye than for a disulphonated analogue, since a disulphonated dye requires about twice as many amine end groups for a given tinctorial yield (Figure 3.4). Blocking effects may occur if monosulphonates and disulphonates are applied together, resulting in a heavier depth on the wool that is closer in hue to the disulphonated dye. Above the critical depth the nylon becomes progressively more difficult to dye with levelling acid dyes and the distribution on the nylon/wool blend increasingly favours the wool. The blocking of disulphonates by analogous monosulphonated dyes may be

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DYNAMIC COMPETITION BETWEEN FIBRE TYPES IN THE DYEING OF BLENDS

NH CO +

NH3

H –O

3S

O

N

CH2

N

(CH2)6

CH2 CH2 CH2

NH

CO

CO

NH

CI Acid Red 88

CO H

+

NH3 –O3S (CH2)6

N N

NH

O

NH (CH2)3 CH2 CH2

CO

CH2 CI Acid Red 13

SO3– NH3 +

Figure 3.4 Electrostatic and hydrophobic bonding of acid dyes on nylon

exploited by applying mixtures of selected mono- and disulphonated pairs of similar hue, varying the proportions according to applied depth in order to minimise the disparity in depth on the two fibre components. The preferred monosulphonated dyes are mainly monoazo yellows and reds with anthraquinone blues. The disulphonates have lower saturation values on nylon and so they can only be used for pale and medium depths. They are virtually all yellow to red monoazo and disazo types, or violet to green anthraquinone derivatives. Given the widely varying qualities of the two fibre types, as well as possible variations in the blend proportions, it is not surprising that specific dyeing conditions for solidity cannot be laid down, only guideline starting points for initial experimentation [4]. Anionic agents capable of controlling the uptake of anionic dyes by the nylon component below the critical depth are of two general types: (1) levelling or blocking agents that are preferentially absorbed by nylon and act as a partial reserve for levelling acid dyes; (2) retarding agents of higher relative molecular mass capable of controlling the distribution of dyes of higher wet fastness on nylon/wool blends.

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THE DISTRIBUTION OF ACID DYES ON NYLON/WOOL BLENDS

33

Typical examples of the moderately substantive agents used to control the uptake of levelling acid dyes are shown in Figure 3.5. They compete with these dyes for the basic groups and facilitate the approach to an equilibrium distribution that does not alter even on prolonged boiling, unless further additions of agent are made to shift the equilibrium more in favour of the wool. Such products improve the coverage of any dye-affinity variation in the nylon but have insufficient affinity to exert a blocking effect with premetallised or milling acid dyes. A novel agent developed recently, however, permits either equalisation or a total reserve to be obtained even with metal-complex or milling dyes [5]. R R

SO3Na SO3Na

Sodium alkylbenzene sulphonate

R OSO3Na

Sodium alkylnaphthalene sulphonate

CH3(CH2)5 CH

CH2CH CH(CH2)7COONa

OSO3Na Sodium alkanol sulphate

Disodium sulphoricinoleate

Figure 3.5 Levelling or blocking agents for nylon/wool dyeing (R = long-chain alkyl)

Retarding agents that control the initial uptake of these dyes by the nylon component have higher affinity than the typical levelling agents defined in Figure 3.5 Many of these are also used as syntan aftertreating agents for dyed nylon. These belong to a distinct group of condensates of formaldehyde with certain sulphonated phenols, thiophenols or naphthylaminesulphonic acids [6]. The rapid sorption of syntans by nylon is mainly attributed to electrostatic bonding between negatively charged sulpho groups in the syntan and the protonated amino groups in the fibre. Hydrogen bonding between uncharged polar groups and hydrophobic interaction between nonpolar moieties in the syntan and the nylon also contribute to the mechanism [4]. Maximum retarding effect is found when the syntan molecules are retained close to the fibre surface, since any treatment leading to diffusion of the syntan into the fibre interior tends to lower its effectiveness. When used with levelling acid dyes, this type of agent does not prevent migration from wool to nylon on prolonged boiling. With metal-complex and milling acid dyes, however, the initial distribution is preserved on boiling because these dyes show only limited

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DYNAMIC COMPETITION BETWEEN FIBRE TYPES IN THE DYEING OF BLENDS

migration. It is not normally necessary to employ levelling or retarding agents on the rare occasions when chrome dyes are preferred, since these give quite good solidity on nylon/wool blends at the heavy depths where they may offer an advantage. Some of the syntans darken on exposure to light and this significantly lowers the light fastness of the dyeings. Considerable attention has been given in recent years to devising suitable procedures for dyeing wool at lower temperatures and near the isoelectric point (pH 4–5) in order to avoid the damage that inevitably occurs when dyeing at the boil. The low-temperature dyeing of wool offers: (1) optimum handle and durability; (2) improved carding, spinning and weaving performance; (3) brighter shades because of the lower degree of yellowing of the wool; (4) shorter dyeing cycles, higher productivity and lower process costs; (5) a better-quality end-product. Problems associated with low-temperature dyeing include: (1) slower diffusion into the interior of the wool fibres, resulting in inadequate wet fastness; (2) inadequate exhaustion and poor reproducibility. The epicuticle of wool is the main barrier to penetration and this is mainly responsible for the dyeing problems. These problems can be overcome by damaging the fibre scales in a chlorination process before dyeing, but this lowers wool quality too. The best results are achieved using monosulphonated 1:2 metal-complex and milling acid dyes. Shade partition between wool and nylon in blend dyeing is dependent on dyeing temperature. As already discussed, the tendency at the boil is for the nylon to absorb dye more quickly than wool and to dye more deeply below the critical depth. This is easily corrected by adding an anionic agent to retard uptake by the nylon and achieve solidity of shade over a range of depths. At lower temperatures (e.g. 80°C) the wool is generally dyed more heavily than the nylon even without an anionic agent present. As dyeing proceeds there is some migration from wool to nylon until the final partition is reached, but this is a different equilibrium at the lower temperature than in conventional dyeing at the boil. In general, partition favours the wool and solidity is difficult to attain. This difficulty can be overcome, however, using a specific equalising agent in place of the conventional anionic agent normally employed when dyeing at the boil [7]. When dyes of high substantivity are applied below the boil, surface dyeing of the wool occurs to give dyeings of inadequate wet fastness.

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THE DISTRIBUTION OF ACID DYES ON NYLON/WOOL BLENDS

35

Premetallised and milling acid dyes (relative molecular mass 700–1000) demand more energy input for good fibre penetration. There is a close relationship between dye substantivity, exhaustion and diffusion into the fibre and this is mainly determined by the relative molecular mass and degree of sulphonation of the dye. Addition of an equalising agent shifts the partition in favour of the nylon at the lower temperatures, allowing solidity to be achieved at any temperature in the 80–100°C range, depending on the amount of auxiliary required at a given depth of shade. The optimum dyeing temperature is recipe-specific and a temperature selector system has been developed for use with the specific equalising agent. In pale depths (below ca. 0.6% total dye) the conventional syntan-type retarder may be used at a relatively low concentration [8].

3.3 THE DISTRIBUTION OF ACID DYES ON NYLON/POL YURETHANE BLENDS The polyurethane elastomeric fibres vary in their ability to absorb anionic dyes, depending on their content of basic groups, but in general the equilibrium distribution on a nylon/polyurethane blend tends to favour the nylon component. The higher rate of dyeing of the elastomeric fibre at lower temperatures, however, results in preferential dyeing of that component in the early stage. There are two main methods of controlling this distribution, both of them depending on the use of dyeing auxiliaries. Anionic agents, such as sodium alkanesulphonates, sulphated castor oil, disodium dinaphthylmethanedisulphonate or selected syntans, can be added to promote dye uptake by the polyurethane at equilibrium by becoming absorbed by the nylon and restraining subsequent migration of dye from the polyurethane to the nylon. This method is more effective when dyeing with 1:2 metal-complex or milling acid dyes of the monosulphonate type with good levelling properties. The amount of agent required decreases progressively as the applied depth of the dyeing increases. Cationic agents (Figure 3.6) will form labile ionic complexes with typical anionic dyes. This type of complex is absorbed more slowly and tends to favour the polyurethane component more than does the parent dye. Acid dyes of the disulphonate type with only moderate levelling properties can be readily controlled on nylon/polyurethane materials by this method. The concentration of cationic agent required increases with applied depth and the degree of sulphonation of the dyes. A nonionic dispersing agent of the alkanol polyoxyethylene class is necessary to solubilise the dye–agent complex (Figure 3.7).

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36

DYNAMIC COMPETITION BETWEEN FIBRE TYPES IN THE DYEING OF BLENDS

CH3 H33C16

+

H33C16

N

Cetylpyridinium chloride

N+ CH3 CH3

Cl –

CH3 H35C17

N + CH2 CH3

Br –

Cetyltrimethylammonium bromide

Cl –

Stearyldimethylbenzylammonium chloride

Figure 3.6 Cationic complexing agents for nylon/polyurethane dyeing

H CH3 +

O3S

O

R

(OCH2CH 2)x OH

N N

R N CH3

Alkanol polyoxyethylene

CH3 CH3

SO3– CH3

CI Acid Red 13 as bis-alkyltrimethylammonium complex

+

N R

R

(OCH2CH2)x

OH

CH3

Figure 3.7 Schematic representation of solubilised dye–agent complex (R = long-chain alkyl)

3.4 THE CROSS-STAINING OF WOOL BY DISPERSE DYES Wool keratin is the most sensitive of textile fibres towards staining by dyes of all types because it is a natural protein containing many different functional groups. The staining of wool by disperse dyes is a serious problem in dyeing blends with any of the ester fibres. Cellulose acetate suffers a loss in lustre if it is treated at the boil, as is normally necessary to dye the wool component during the second stage of a two-stage or two-bath sequence. Migration from the acetate to the wool increases with dyeing time and temperature of the wool-dyeing stage. Migration from cellulose triacetate to wool is slower under these conditions but treatment at 105°C or at the boil with carrier, in order to attain full depth and penetration on the triacetate, usually results in serious staining of the wool. Polyester/wool presents the most difficult problem and studies have revealed numerous factors that may affect the degree of staining observed in practice [9]. The stain on the wool is dull in hue and exhibits poor fastness to light and wet treatments. Staining implies a loss of colour yield on the polyester and makes shade matching more difficult because individual dyes differ in their propensity to staining. Bulky, low-twist wool becomes stained more easily then fine, hightwist yarns. Wool quality is another factor influencing the degree of staining.

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THE CROSS-STAINING OF WOOL BY DISPERSE DYES

37

Dye structure (molecular size, number, type and distribution of polar groups) has less influence on the initial level of staining than dyebath conditions and the quality of the dye dispersion. The mechanism of wool staining involves: (1) hydrogen bonding between proton donor groups (e.g. amino, amido, hydroxy) in the disperse dye molecules and in wool keratin; (2) interaction by means of secondary (dipolar and van der Waals) forces between the dye molecules and wool; (3) physical sorption of aggregated particles of disperse dyes on the scaly surface of the wool fibre. The preferred disperse dyes for these blends have low intrinsic saturation values on wool, low tendency to aggregate at the dyeing temperature, rapid rates of diffusion into polyester and high equilibrium exhaustion. Mechanical retention in yarn crevices may play a part in the initial deposition, since particle size and stability of the dye dispersion are important. Staining tends to decrease with the concentration and anionic charge of the stabilising agents present in the dye dispersion, but the magnitude of the effect is specific to the types of dye and agent. In a recent investigation of the kinetics of polyester/wool dyeing and the wool staining problem, polyester and wool fabrics in the weight ratio 55:45 were used to allow the disperse dye uptake by both components to be determined independently [10]. Dyeings with CI Disperse Blue 185 (Figure 3.8) and CI Acid Red 211 were carried out at the isoelectric point (pH 4.5) and wool damage was assessed by measuring tensile properties and alkali solubility. Both disperse and acid dyes are absorbed by wool far more quickly and easily than the disperse dyes are taken up by the polyester at relatively low temperatures, because the rates of diffusion in wool are so much more rapid. After only 30 minutes at 110°C, therefore, the amount of disperse dye that has stained the wool is more than twice that absorbed by the polyester (Figure 3.9).

O2N

O

NH2

CH3 HO

O

NHCH CH2CH3

Figure 3.8 CI Disperse Blue 185

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38

DYNAMIC COMPETITION BETWEEN FIBRE TYPES IN THE DYEING OF BLENDS

Dye uptake/%

4

3

2

1 Wool Polyester

15

30

60

90

120

Dyeing time/min at 110 oC

Figure 3.9 Rate of uptake of disperse dye by polyester and wool [10]

As dyeing proceeds, desorption from the wool occurs at a rate determined by the slow diffusion of the disperse dye into the polyester. The distribution coefficient for a disperse dye between polyester and wool is controlled by the difference in affinity values of the dye for the two substrates. After about an hour at 110°C, an equilibrium partition is approached but the degree of staining of the wool is not much less than after the first 15 minutes. As the disperse dye diffuses slowly into the polyester, the wool stain slowly desorbs at a rate that keeps the low concentration of disperse dye in the dyebath roughly constant. After a dyeing time of 30 minutes, wool staining is considerable at all dyeing temperatures up to about 115°C (Figure 3.10). However, above around 120°C the diffusion of the disperse dye into the interior of the polyester fibre is sufficiently rapid to give much lower staining of the wool. The pH dependence of wool staining is not critical under the mildly acidic conditions preferred for dyeing both components of this blend (Figure 3.11). At pH values below the isoelectric point, wool staining is at a minimum. As the pH increases towards the alkaline side, so does the degree of wool staining. When tested in the presence of the premetallised CI Acid Red 211, the staining of wool by the disperse dye was markedly greater [11]. This was attributed to possible hindering of disperse dye desorption from the wool by the presence of the metal-complex dye. Since anionic dyes of this kind interact strongly with wool keratin they desorb again very slowly and incompletely. Sequestering agents such as ethylenediaminetetra-acetic acid (EDTA) and citric acid are useful to

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THE CROSS-STAINING OF WOOL BY DISPERSE DYES

39

4

Dye uptake/%

3

2

1

30

60

90

120

Dyeing temperature/oC for 30 min

Figure 3.10 Temperature dependence of disperse dye uptake [10] (for key see Figure 3.9)

Dye uptake/%

4

3

2 2

3

4

5

6

7

8

Dyebath pH for 60 min at 110oC

Figure 3.11 pH dependence of disperse dye uptake [10] (for key see Figure 3.9)

minimise wool staining, particularly the former. It is claimed that such agents interact with certain disperse dyes (e.g. the 1-amino-4-s-butylamino system in CI Disperse Blue 185) to hinder diffusion into wool (Figure 3.12). These agents have

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40

DYNAMIC COMPETITION BETWEEN FIBRE TYPES IN THE DYEING OF BLENDS

low affinity for wool and formation of the dye–agent complex favours retention of the disperse dye in the dyebath, allowing the polyester to absorb more dye as it is released from the complex at higher temperatures and improving the ultimate exhaustion.

O2N

O

+

–OOC

NH3

CH2 CH OH

–OOC

CH2

CH3 HO

O

NH2CH +

CH2CH3 Figure 3.12 Interaction between CI Disperse Blue 185 and citric acid

When dyeing wool-rich blends, the problems of wool staining and the attainment of solidity at equilibrium are greatly aggravated. Uptake of disperse dyes by the polyester in full depths can be 25% less on a 20:80 polyester/wool blend than on a 50:50 blend [12]. If a 50:50 blend is dyed at a liquor ratio of 10:1, then the individual components are each actually at 20:1 with respect to the dyebath. When the blend ratio is 20:80, however, the polyester is being dyed at 50:1, making exhaustion far more difficult. To compound this, the liquor ratio for the wool is only 12.5:1 and absorption of the disperse dyes by wool is favoured preferentially. Less dye remains in the bath for dyeing the polyester directly and this component becomes even more dependent on disperse dye transfer from the wool [12]. Wool progressively loses its strength at elevated dyeing temperatures, especially about 110°C, although at this temperature the damage is not severe for dyeing times of 60 minutes or less. In the isoelectric region (pH 4–5) the keratin structure is reinforced by electrostatic linkages between protonated amino groups and carboxylate-containing amino acid residues. Hydrolysis of peptide and disulphide bonds is also at a minimum under these conditions. The loss in strength increases rapidly at pH values above 5 and temperatures above 110°C. The sensitivity of wool to degradation makes the use of a carrier essential to dye full depths on the polyester at a relatively low temperature. Carrier addition lowers the degree of staining at a given temperature and also tends to lower the temperature of maximum staining. Carriers are able to minimise wool staining by accelerating the rate of uptake of disperse dyes by polyester. It is believed that the sorption of carrier molecules weakens the attractive forces between polyester segments, making them more

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THE CROSS-STAINING OF WOOL BY DISPERSE DYES

41

mobile at lower temperatures than when the carrier is absent, thus changing the internal structure of the fibre and lowering the glass-transition temperature. Hence carriers accelerate dye diffusion within polyester and so indirectly increase the rate of transfer from wool to polyester. The chemical type of carrier exerts only a marginal influence, the degree of staining tending to increase slightly in the series: aryl esters < chlorobenzenes < aryl ethers < phenylphenols. Dyebath conditions are more important. Unfortunately, carriers are harmful to the health of operatives and to the environment. The wool staining problem is an important criterion in the decision whether to dye polyester/wool sequentially in the one-bath mode or by the two-bath method with an intermediate clear to remove the disperse dye stain from the wool. In the two-stage sequence, the wool is cleared after dyeing by scouring with a nonionic detergent at pH 4–5 and 50–70°C. This causes no significant damage to the wool but is only moderately effective and this process is only suitable for pale and medium depths because of fastness limitations. Nevertheless, it offers shorter processing times and higher productivity at lower cost. During the wool dyeing stage, disperse dye can migrate from the surface of the dyed polyester and cause back-staining of the wool [13]. There is no obvious relationship between the degree of back-staining and the heat fastness class of the disperse dyes. Full depths are usually dyed by a two-bath method. In the absence of the anionic dyes the blend can be given an intermediate reduction clear after applying the disperse dyes to the polyester. This is more effective than nonionic scouring, especially for azo disperse dyes, but the wool suffers some loss in strength and elasticity. When dyeing wool-rich blends, the two-bath sequence gives better shade partition than the one-bath method, especially when the polyester component is dyed first [12]. Two-bath dyeing allows a wider choice of disperse dyes, since the wool staining is less important provided the intermediate clearing is adequate and the wool is dyed at a lower temperature than the polyester. Certain anthraquinone-based disperse dyes, however, are reduced but not destroyed by the reducing conditions. The degradation products may still discolour the wool, especially after reoxidising back to the quinone form during the wool dyeing stage. In heavy depths the two-bath process gives improved fastness to rubbing and perspiration [14]. These advantages are particularly significant for the dyeing of deep navy and black shades on polyester/wool [15]. 3.5 THE CROSS-STAINING OF WOOL BY BASIC DYES Although ultimately less serious than staining by disperse dyes, the uptake of basic dyes by wool in the initial stage of the one-bath dyeing process for wool/ acrylic blends with milling acid and basic dyes can be troublesome. Above the

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DYNAMIC COMPETITION BETWEEN FIBRE TYPES IN THE DYEING OF BLENDS

second-order transition temperature, however, dyeing of the acrylic component can take place. The thermodynamic affinity of a basic dye for the acrylic fibre is much higher than for wool. By the time that the dyebath reaches the boiling temperature, most of the basic dye initially taken up by the wool has been transferred to the acrylic component. This transfer proceeds during treatment of the fibre blend at the boil even after exhaustion of the basic dye from the dyebath is virtually complete. The degree of initial staining of the wool by the basic dye varies with the type of acrylic fibre present in the blend. This variation becomes particularly important when the applied depth approaches saturation of the dyeing sites in the acrylic fibre. Cationic dyeing auxiliaries have been widely used in wool dyeing, ranging in ionic character from the weakly basic alkylamine polyoxyethylene types to the much more strongly basic fatty alkyl quaternary ammonium salts. These products are used as levelling agents and it is not surprising that the cationic retarders necessary when dyeing acrylic fibres with basic dyes behave in a similar manner when added to the wool/acrylic dyeing system. The pronounced retarding effect of a typical cationic retarder on the rate of dyeing of wool with a milling acid dye of the disulphonated anthraquinone type (Figure 3.13) is illustrated in Figure 3.14. The time of half dyeing is increased from about 22 to 33 minutes by addition of the agent [16]. Dyeing rate curves for this milling acid dye and a typical monoazo basic dye (Figure 3.13) on a 50:50 wool/Orlon (DUP) blend in the presence (Figure 3.15) and absence (Figure 3.16) of the cationic retarder demonstrate that the anionic NaO3S O

HN

O

HN

NaO3S

CH2CH2CH2CH3

CH2CH2CH2CH3 Cl

CI Acid Green 27

CH2CH3 O2N

N N

N

CH3 CH2CH2 N CH3 +

CI Basic Red 18

CH3 X–

Figure 3.13 Structures of typical milling acid and basic dyes

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THE CROSS-STAINING OF WOOL BY BASIC DYES

43

100

Exhaustion/%

80 60

40 1% Dye, no retarder

20

1% Dye, 1% retarder

10

20

30

40

50

60

90

100

100

Dyeing time/min

60

70

80 Temperature/oC

Figure 3.14 Rates of dyeing of CI Acid Green 27 on wool [16]

dye is highly sensitive to the agent of opposite charge. Under these conditions the agent increases the time of half dyeing from about 17 to 32 minutes for the acid dye but only about 26 to 32 minutes for the basic dye. These results indicate that in the initial stage of a wool/acrylic dyeing the cationic retarder is either absorbed by the wool or forms with the acid dye a labile complex that has lower affinity for wool than the parent dye. Either effect will significantly decrease the rate of dyeing of the acid dye. It is also evident that 100

CI Acid Green 27 CI Basic Red 18

Exhaustion/%

80 60

40 20

10

20

25

30

35

40

45

50

100

100

100

Dyeing time/min

66

84

92

100

100

Temperature/oC

Figure 3.15 Rates of dyeing of wool/Orlon with 1% retarder [16]

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DYNAMIC COMPETITION BETWEEN FIBRE TYPES IN THE DYEING OF BLENDS

100

Exhaustion/%

80 60

40 20

5

10

15

20

25

30

35

92

100

100

Dyeing time/min

58

66

75

84

Temperature/oC

Figure 3.16 Rates of dyeing of wool/Orlon with no retarder [16] (for key see Figure 3.15)

ca. 20% exhaustion of the basic dye occurs at temperatures below 80°C (i.e. below the glass-transition temperature of the acrylic fibre) and this dye must be absorbed by the wool. This effect is irrespective of whether the cationic retarder is present or not, implying that complex formation between the acid dye and the retarder is the most probable explanation of the mechanism [16].

3.6 THE TRANSFER OF DISPERSE DYES DURING THERMOFIXATION OF POLYESTER/CELLULOSIC BLENDS The pad–thermofix dyeing of polyester/cellulosic fabrics is one of the few systems of dyeing of blends that has been subjected to theoretical study. During the early stages of padding and drying, much of the dye applied becomes deposited on the relatively more absorbent cellulosic component. Several possible mechanisms of transfer of the disperse dyes from cellulose to polyester during the thermofixation stage have been proposed, but it is now widely accepted that the transfer proceeds through the vapour phase [17]. The extent to which this transfer takes place depends on the time and temperature of thermofixation. As heating of the polyester/cellulosic fabric is continued, the total amount of disperse dye available for colouring the polyester decreases as a result of volatilisation into the atmosphere inside the heating chamber and deposition on the inner surfaces of the latter. Some disperse dyes of low relative molecular mass (Mr) and relatively high volatility may suffer oxidative decomposition,

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THE TRANSFER OF DISPERSE DYES DURING THERMOFIXATION

45

particularly if they contain sensitive substituents such as primary amino groups. It follows that a critical combination of fixation time and temperature gives optimum yield of a specific dye, when the supply of dye from the reservoir provided by the cellulosic component is sufficiently depleted for these progressive losses to begin to favour desorption rather than adsorption of vapour at the polyester surface. Low-energy disperse dyes (approx. Mr <300) suffer relatively serious losses under the conditions required for optimum transfer and fixation on the polyester. Maximum transfer for these dyes is found at 200–210°C. For most dyes of intermediate energy (approx. Mr 300–400) a temperature of 210–220°C is needed. Good fixation of high-energy dyes (approx. Mr >400) often requires a temperature of 220–230°C, but optimum transfer is limited by the onset of thermal degradation and yellowing of the cellulosic fibres, or even some softening of the polyester.

3.7 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

J Park and J Shore, Rev. Prog. Coloration, 12 (1982) 1. O Annen, H Gerber and B Seuthe, J.S.D.C., 108 (1992) 215. B C Burdett, C C Cook and J C Guthrie, J.S.D.C., 93 (1977) 55. T M Baldwinson in Colorants and auxiliaries, Vol. 2 Ed. J Shore (Bradford: SDC, 1990) 568. Anon, Dyer. 177 (Apr 1992) 31. C C Cook, Rev. Prog. Coloration, 12 (1982) 73. A F Doran, Dyer, 176 (Aug 1991) 49. A F Doran, J.S.D.C., 109 (1993) 15. R E Lacey, V S Salvin and W A Schoeneberg, Am. Dyestuff Rep., 50 (1951) 978. J Wang and H Asnes, J.S.D.C., 107 (1991) 274. J Wang and H Asnes, J.S.D.C., 107 (1991) 314. A F Doran, unpublished work. K Türschmann and K H Röstermundt, Z. Ges. Textilind., 71 (1969) 326. K H Röstermundt, Deutscher Färber Kalender, 80 (1976) 247. W T Sherrill, Text. Chem. Colorist, 10 (1978) 210. D R Lemin, J.S.D.C., 91 (1975) 168. C J Bent, T D Flynn and H H Sumner, J.S.D.C., 85 (1969) 606.

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MINIMISING INCOMPATIBILITY BETWEEN DYES FROM DIFFERENT CLASSES

CHAPTER 4

Minimising incompatibility between dyes from different classes

4.1 INTERACTION BETWEEN DISPERSE DYES AND REACTIVE DYES The risk of interaction between these two dye classes is a significant factor limiting the selection of suitable dyes for the one-bath pad–thermofix dyeing of polyester/cellulosic blends. These problems may arise as a result of covalent reaction between reactive dyes and certain disperse dyes, interaction between reactive dyes and dispersing agents, or instability of the dye dispersion system under the alkaline conditions of padding. Interaction leads to losses of yield on both fibre components and may result in gelling or settling of the pad liquor. High reactivity dyes are unsuitable because of chemical reaction in many cases. For example, the monochlorotriazine dye formed by reaction (Scheme 4.1) between CI Reactive Red 11 and the phenolic group in CI Disperse Yellow 1 was isolated from a pad liquor containing 8 g l–1 sodium bicarbonate and identified by chromatographic analysis [1]. Further evidence for chemical reaction between monochlorotriazine reactive dyes and primary amino or phenolic groups in disperse dyes has been presented [2]. The reaction products formed tend to be unstable and are readily decomposed by alkaline hydrolysis. Since these problems of chemical reaction in the alkaline pad liquors necessary for one-bath application are closely connected with structural features of the individual dyes, most of them can be avoided. Where possible, disperse dyes that do not contain nucleophilic primary amino or phenolic groups should be selected. Low-reactivity ranges of reactive dyes should be chosen in preference to high-reactivity types, so the selection of compatible disperse dyes that can be used becomes much wider. Control of the pH of the pad liquor is a most important technique to minimise problems of chemical reaction between dyes from the two classes. Thus the mixture of disperse and reactive dyes can be padded from a neutral bath containing migration inhibitor and sodium m-nitrobenzenesulphonate to prevent

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INTERACTION BETWEEN DISPERSE DYES AND REACTIVE DYES

Cl CI Reactive Red 11

N Cl

N

CI Disperse Yellow 1

N O2N

NH SO3Na H O N N

+ SO3Na

HO

NH

NO2

NaHCO3

NaO3S Cl

O2N N

N

O

NH

NO2

N NH SO3Na H O N N

SO3Na

+ NaCl + CO2 + H2O

NaO3S Scheme 4.1

reductive degradation of certain azo reactive dyes. After drying and thermofixation to ensure diffusion of the disperse dyes into the polyester fibres, the fabric is padded in an alkaline brine bath to minimise desorption of the reactive dyes, steamed to achieve fixation of these dyes on the cellulosic component, rinsed cold and soaped at the boil. This two-stage method has few restrictions attributable to dye interaction and the choice of suitable dyes is much greater than in the simple pad–dry–thermofix application of both classes.

4.2 INTERACTION BETWEEN DISPERSE OR VAT DYES AND BASIC DYES In the one-bath dyeing of a typical DB blend (see Chapter 2), i.e. a blend of one of the ester fibres with an acrylic fibre, it is most important to minimise the risks of incompatibility between a cationic species, such as one of the basic dyes or a cationic retarder used with them for dyeing the acrylic fibre, and an anionic moiety that has an important function in dyeing the ester fibre, such as a disperse dye stabiliser or a carrier emulsifying agent (Figure 4.1). There are three aspects to minimising the risks of incompatibility in the onebath dyeing of DB blends of the ester/acrylic type. An anionic retarder can be used to control uptake of the basic dyes by the acrylic fibre, but this tends to be

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MINIMISING INCOMPATIBILITY BETWEEN DYES FROM DIFFERENT CLASSES

Cl CH2CH3 O2 N

N N

CH3

N

CH2CH2 N CH3 +

CH3 CI Basic Red 18

H33C16

+

N

–O

3SO(CH2CH2O)x

R

Alkanol polyoxyethylene sulphate (carrier emulsifier)

–O S 3

+

SO3–

Cetylpyridinium (retarder)

N C16H33 Cetylpyridinium (retarder)

CH2 Dinaphthylmethanedisulphonate (dispersant) Figure 4.1 Possible interactions in dyeing a DB blend (R = long-chain alkyl)

less effective than a cationic retarder and more prone to restrain the ultimate exhaustion of the basic dyes. It is possible to select a carrier formulation containing a nonionic emulsifier instead of a conventional anionic carrier type. Anionic dispersing agents are best avoided but it is not normally possible to exclude the anionic stabilising agents that are already present in disperse dye formulations as marketed. More severe limitations apply to the simultaneous application of vat and basic dyes to cellulosic/acrylic CB blends. The dispersing agents present in vat dye formulations are incompatible with the basic dyes and cationic retarders normally used for dyeing acrylic fibres. A more serious problem, however, is that almost all basic dyes show chemical instability in even moderately alkaline conditions. The highly alkaline reducing conditions of a vat dyebath are even more extreme. One-bath methods generally are thus excluded and only a twobath sequence can be considered.

4.3 INTERACTION BETWEEN ANIONIC DYES AND BASIC DYES The range of bright colour contrasts is much wider on AB blends than on all other types of binary blend because the fibres carry opposite charges and ionic dyes are much more selective than disperse dyes. The opposite charges carried by the dyes, however, lead to incompatibility in one-bath dyeing. There is a strong

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INTERACTION BETWEEN ANIONIC DYES AND BASIC DYES

49

tendency for water-soluble anionic dyes and basic dyes to interact (Figure 4.2). It is highly likely that precipitation of the complex formed would occur even when applied in pale depths. Even where this does not occur the interaction seriously interferes with reproducibility and causes increased cross-staining. CI Basic Yellow 21 H3C

CH3 HC CH N N+ CH3

H3C –O S 3

CI Acid Blue 27 O

O

CH3

HN

NHCH3

Figure 4.2 Interaction between typical basic and acid dyes

Addition of ca. 1% of an alkanol polyoxyethylene surfactant is sufficient to inhibit co-precipitation of basic dyes and milling acid dyes when used at concentrations up to 1/1 standard depth in a one-bath exhaust dyeing process for wool/acrylic blends, for example [3]. With many combinations of basic dyes and the relatively hydrophilic levelling acid dyes, it is possible to go to much heavier depths with no problems. In practice, however, it is advisable, for dyeings above 1/1 standard depth, to check that a specific combination of dyes will be suitable for one-bath application. Direct and reactive dyes present similar problems when applied with basic dyes in one-bath methods for the dyeing of cellulosic/acrylic CB blends. A further problem in the case of reactive dyes is that an alkaline fixation stage must be given to fix them on the cellulosic fibre. Almost all basic dyes are chemically unstable under alkaline conditions. The best and most reliable method of dealing with these problems of interaction is to dye each fibre component separately. However, it has been possible to develop viable one-bath methods of applying acid/basic combinations to AB blends and direct/basic combinations to CB blends. These involve the addition of anti-precipitants to overcome the problems of dye interaction. Many proprietary products are marketed specifically as anti-precipitants for use in

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MINIMISING INCOMPATIBILITY BETWEEN DYES FROM DIFFERENT CLASSES

blend dyeing (Table 4.1) but some levelling agents used with acid dyes on wool or nylon, e.g. fatty amine polyoxyethylenes, will also function effectively in this role, either alone or with the addition of a nonionic dispersing agent (Figure 4.3). Nonionic agents are by far the most common anti-precipitants and their solubilising power by disaggregation is well known. Table 4.1 Chemical types of anti-precipitant [4,5]

Type

Composition

OCH2CH2 units per molecule

Nonionic

Fatty alcohol or alkylphenol polyoxyethylenes

≥20

Block copolymers of ethylene oxide and propylene oxide

≥40

Weakly cationic

Coco, oleyl, soya or tallow fatty amine polyoxyethylenes

≥20

Weakly anionic

Fatty alcohol or alkylphenol polyoxyethylene sulphates

≥3

Weakly anionic

Fatty alcohol or alkylphenol polyoxyethylene phosphates

4–12

Weakly anionic

Alkali metal, amine or alkanolamine salts of fatty alcohol or alkylphenol polyoxyethylene acetates

≥5

Alkyl chloride or dialkyl sulphate salts of quaternary ammonium polyoxyethylene sulphates or phosphates

≥15

Nonionic

Amphoteric

Numerous nonionic and weakly cationic surfactants with at least 20 oxyethylene units per molecule are available, as well as block copolymers of ethylene oxide and propylene oxide. Suitable hydrophobic groups include fatty alcohols, alkylphenols, alkylamines and alkanolamines. Weakly anionic polyoxyethylene sulphates and phosphates with polyoxyethylene chains approximately 10 units in length are also widely available [4]. Mixed formulations often function better than single products and many proprietary products are empirically balanced mixtures [5]. In dye liquors containing substantial concentrations of both anionic and basic dyes, purely nonionic agents may be inadequate to effect complete solubilisation

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INTERACTION BETWEEN ANIONIC DYES AND BASIC DYES

51

CI Acid Blue 27 O

NHCH3 R

(OCH2CH2)x OH

Alkanol polyoxyethylene HN

O

–O

CH3

3S

(CH2CH2O)x H +

R = long-chain alkyl

R NH (CH2CH2O)x H Alkylamine polyoxyethylene

Figure 4.3 Schematic representation of solubilised acid dye–agent complex

of the complex. If an anionic surfactant is added, this will tend to disrupt the dye–dye complex to form a basic dye–agent complex that responds more readily to addition of the nonionic stabiliser (Figure 4.4). If strongly basic dyes are present, however, the complexes formed by this method may be too stable, leading to restraining of the basic dyes. Azo and anthraquinone derivatives with a localised charge tend to show this behaviour. Surface deposition may result in poor fastness to rubbing and a tendency for the basic dye–agent complex to stain the acid-dyeable fibre. CI Basic Yellow 21 H3C

CH3 HC CH N N+ CH3

H3C

–O SO(CH CH O) 3 2 2 x

R = long-chain alkyl

R

Alkanol polyoxyethylene sulphate

R (OCH2CH2)x OH Alkanol polyoxyethylene

o

Figure 4.4 Schematic representation of solubilised basic dye–agent complex

Mixtures of another type that may be useful for minimising the cross-staining of fibres by complexes of basic and acid dyes contain a weakly cationic alkylamine polyoxyethylene with a weakly anionic alkanol polyoxyethylene sulphate or phosphate. In many systems of interacting anionic and cationic

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MINIMISING INCOMPATIBILITY BETWEEN DYES FROM DIFFERENT CLASSES

surfactants an excess of either one may have a solubilising effect by disaggregating the complex formed between them. The hybrid surfactants generally characterised as weakly cationic or weakly anionic are particularly useful in this respect, since the oxyethylene chains inhibit ionisation and exert a solubilising and stabilising action on the complex entities present. Care should be taken to rinse off all traces of anti-precipitants after dyeing to avoid potential problems later. For example, residual nonionic surfactants can interfere with the syntan aftertreatment of dyed nylon and its blends.

4.4 REFERENCES 1. 2. 3. 4. 5.

B Taylor and J Shore, unpublished work. M Duscheva, L Jankov and K Dimov, Melliand Textilber., 56 (1975) 147. D R Lemin, J.S.D.C., 91 (1975) 168. H D Pratt, Am. Dyestuff Rep., 68 (Sep 1979) 39. T M Baldwinson in Colorants and auxiliaries, Vol. 2, Ed. J Shore (Bradford: SDC, 1990) 568.

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CHAPTER 5

Principles of design and colouring of differential-dyeing blends

5.1 DESIGN OF DIFFERENTIAL-DYEING VARIANT SYNTHETICPOLYMER YARNS The development of novel fabric constructions by combining variants of the same man-made fibre differing in lustre, denier or cross-sectional shape presents no serious problems for the dyer. The introduction in the 1970s of acid-dyeable and basic-dyeable variants of the conventional synthetic fibres, with excellent resistance to staining by dyes of opposite charge to those for which they show affinity, greatly enhanced the variety of multicoloured effects that could be achieved [1]. After initial popularity, the use of differential-dyeing yarns declined somewhat. However, in recent years there has been a revival of interest following the development of sophisticated tufting machine controllers with the ability to produce a wide range of patterned effects, e.g. the shifting needle bar technique controlled by computer, and this trend is continuing [2]. The advantages of differential dyeing of fabrics, compared with the longestablished alternative of knitting or weaving with coloured yarns, include the avoidance of stockholding of dyed yarn, easier adaptation to minor changes in fashion and quicker delivery of finished goods. These advantages proved particularly relevant in tufted carpets, yielding an attractive range of bold or subdued colour combinations in versatile designs. As indicated later, however, there are restrictions of colour gamut in certain styles and careful attention to stringent conditions of processing is necessary to ensure reproducible effects. The use of differential-dyeing nylon yarns has made an impact on the styling of automotive upholstery in multicolour patterns, ranging from tonals to bold saturated colours. Colour with a white reserve is obtained with nylon and polyester and shadow effects with the acid-dyeable nylon variants. Basic-dyeable nylon or polyester cannot be included, however, because basic dyes do not meet the unusually high light fastness standards specified by the car manufacturers [3].

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PRINCIPLES OF DESIGN AND COLOURING OF DIFFERENTIAL-DYEING BLENDS

Four types of nylon yarn with dyeing properties different from those of the parent homopolymer were introduced in the 1970s for weaving, warp-knitting and weft-knitting, as well as carpets. Tufted carpeting, however, was the sector where this development was fully exploited. The variant yarns introduced then were as follows: (1) Ultra-deep: acid-dyeable variant with a higher concentration (>80 milliequivalents per kg polymer) of basic groups than the deep-dye variant (2) Deep-dye: acid-dyeable variant with a higher concentration (60–70 milliequivalents per kg) of basic groups than the normal fibre (3) Normal fibre: poly(hexamethylene adipamide) containing 35–45 milliequivalents per kg basic groups (4) Pale-dye: acid-dyeable variant with a lower concentration (15–20 milliequivalents per kg) of basic groups than the normal fibre (5) Basic-dyeable: variant containing acidic groups to provide affinity for basic dyes. The amine end group content of nylon 6.6 can be increased by adding excess diamine to the polymer salt or by lowering the Mr of the polymer by limiting the amount of water liberated during the melt polycondensation reaction between hexamethylenediamine and adipic acid [4]. The deep-dye and ultra-deep variants, however, were obtained by the inclusion of a small proportion of a primary aliphatic diamine containing a tertiary amino group, e.g. N-(2-aminoethyl)- or N,N-bis(2-aminoethyl)piperazine, in place of some of the hexamethylenediamine used in manufacture of the normal nylon 6.6 polymer (Figure 5.1).

CH2CH2 CONH(CH2)6NHCO(CH2)4CONHCH2CH2 N

N CH2CH2NHCO CH2CH2

Major component

Minor component

Figure 5.1 Segment of deep-dye nylon 6.6 variant

This change in structure provided new basic sites to increase the equilibrium uptake of anionic dyes at a given pH. The pale-dye variant (sometimes called ‘low-dyeing’ or ‘light-dyeing’) was made by reacting a proportion of the amine end groups in the normal polymer with a suitable blocking reagent such as γ-butyrolactone (Scheme 5.1).

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DESIGN OF DIFFERENTIAL-DYEING VARIANT SYNTHETIC-POLYMER YARNS

CONH(CH2)6NH2

N-terminal group

O

+

55

CONH(CH2)6NHCO(CH2)3OH

O

Blocked N-terminal group

γ-Butyrolactone Scheme 5.1

Basic-dyeable (or acid-dye-resist) nylon 6.6 was produced by replacing some of the adipic acid used in the polymerisation by a suitable tribasic acid, such as 5sulphoisophthalic acid (Figure 5.2). All these structural changes decrease the degree of crystallinity of the nylon, so that the rates of diffusion of dyes into (and out of) the variant fibres are more rapid than in the homopolymer. SO3H NHCO(CH2)4CONH(CH2)6NHCO Major component

Minor component CONH(CH2)6NHCO

Figure 5.2 Segment of basic-dyeable nylon 6.6 variant

Similar multicolour effects can be derived from a single variant yarn, preferably a deep-dye type, using readily available reactive chemicals. Acid-dye-resist agents, e.g. condensates of the dihydroxyarylsulphone-formaldehyde-aminoarylsulphonate type, offer the simplest approach for temporarily neutralising the basic groups of an acid-dyeable variant yarn. Permanent acid-dye-resist effects can be obtained by reaction of amino end groups with an active halogen derivative, such as 4-(dichlorotriazinylamino)benzenesulphonic acid (Figure 5.3). Cl N N

NH

SO3H

N Cl Figure 5.3 Typical acid-dye-resist agent

Deep-dye modifications of poly(ethylene terephthalate) were developed by varying the proportions of ethylene glycol and dimethyl terephthalate and including other diols, such as propylene glycol [5], n-butylene glycol [6] or diethylene glycol [7], or an aliphatic dicarboxylic acid, e.g. adipic acid (Figure 5.4), to lower the glass-transition temperature by increasing the extent of

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PRINCIPLES OF DESIGN AND COLOURING OF DIFFERENTIAL-DYEING BLENDS

COO CH2CH2

COO CH2CH2

OOC

OOC

OOC

COO CH2CH2CH2CH2 OOC

COO CH2CH2OCH2CH2

OOC

COO CH2CH2 OOC CH2CH2CH2CH2 COO

Major component

Minor component

Figure 5.4 Segments of deep-dye polyester variants

amorphous regions in the polymer. Some of these copolymer fibres were designed for dyeing in medium or full depths at the boil without addition of a carrier. Unfortunately, there are problems associated with excessive soiling of carrier-free polyester variants under domestic washing conditions [8]. Basic-dyeable polyester copolymers, in which some of the terephthalic acid units are replaced by a tribasic acid, e.g. 5-sulphoisophthalic acid (Figure 5.5), have proved useful in differential-dyeing blends, but anionic-dyeable polyester fibres failed to progress beyond the development scale. The basic-dyeable variants have a more accessible structure than normal polyester, lower strength but a reduced tendency to pilling in blends with wool. They are readily dyeable with disperse dyes below 120°C in the absence of a carrier. The use of basic dyes, however, results in less staining of wool compared with disperse dyes.

SO3H OOC

COO

CH2CH2

OOC COO CH2CH2

Major component

OOC

Minor component

Figure 5.5 Segment of basic-dyeable polyester variant

Conventional acrylic fibres are readily dyed with basic dyes at the boil because they are copolymers of acrylonitrile with up to 15% of an inert comonomer, such as an acrylate or methacrylate ester, to make the structure more amorphous and lower the glass-transition temperature. The anionic sites include end groups arising from residues of the polymerisation catalyst (e.g. a persulphate or benzoyl peroxide) as well as carboxylate groups introduced in the form of an acidic

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DESIGN OF DIFFERENTIAL-DYEING VARIANT SYNTHETIC-POLYMER YARNS

57

comonomer. Acid-dyeable acrylic fibres are also copolymers containing basic comonomer units, such as vinyl-pyridine, acrylamide or methacrylamide (Figure 5.6). Acid-dyeable CONH2 CH2CH CN Major component

Basic-dyeable COOCH3

CH2CH CH2CH

CH2CH N

n

Minor components

CN

CH2CH CH2CH n

Major component

COOH Minor components

Figure 5.6 Segments of acrylic-fibre copolymers

Conventional acrylic fibres can also be rendered dyeable with acid dyes by treatment with hydroxylamine sulphate (HONH3)2SO4 (a) alone at pH 4 and 120°C, (b) in the presence of sodium tripolyphosphate, glycerol and sodium alginate at pH 6, or (c) in the presence of benzyl alcohol as a plasticiser at 80°C and pH 5. Substantivity for acid dyes is conferred by the introduction of basic sidechains into the polymer structure [9]. 5.2 DYEING OF ACID-DYEABLE NYLON VARIANTS It has been shown that many of the practical features of the differential dyeing of acid-dyeable nylon variant yarns are consistent with a simple theoretical model. This is based on the assumption that the electrical phenomena determining the sorption at equilibrium by the individual substrates in competition with one another conform to a Donnan-type membrane equilibrium [10]. Attempts have been made to combine this model with colour match prediction calculations for finite dyebath conditions with a view to improving performance in practical situations. The dyer, however, does not normally have access to sufficient prior details about substrate variability that would be necessary to adopt a quantitative approach of this kind. Blends of normal or pale-dye nylon with the deep-dye variant yarn are ideally suitable for shadow effects. The distribution of an acid dye between the components of these blends depends considerably on the dyeing conditions and the molecular structure of the dye, especially the degree of sulphonation. At a given pH in the neutral region, dyebath exhaustion of a series of dyes differing only in degree of sulphonation decreases as the number of sulpho groups in the molecule increases. These differences in substantivity become more marked as

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PRINCIPLES OF DESIGN AND COLOURING OF DIFFERENTIAL-DYEING BLENDS

the content of basic groups in the fibre increases, so that the deep-dye variant is more sensitive than the normal fibre to differences in degree of sulphonation within the series of dyes. Consequently, more highly sulphonated dyes give better differentiation between the variant yarns at a given pH than the less-sulphonated dyes with higher neutral-dyeing affinity. The reason for this sensitivity is the variation with pH of the partition of individual dyes between the fibre variants. The relative sensitivity to pH of acid dyes of increasing polarity (Figure 5.7), and therefore of increasing contrast effect, is shown in Figure 5.8. A carpet made from 50:50 pale-dye/deep-dye nylon was dyed at various pH values and the degree of contrast between the two fibre variants expressed as a ratio:

Contrast ratio =

Dye uptake by deep - dye nylon Dye uptake by pale - dye nylon

(5.1)

As the pH decreases so the contrast ratio approaches unity (i.e. solidity between the variants). The more polar the dyes used to obtain high contrast at pH 6.5 – the usual pH for differential-dyeing – the more marked the variations in contrast ratio if the dyebath pH is not closely controlled [11]. If the pH varies from batch to batch it will cause the partition to vary accordingly. This fluctuation will become apparent either as a strength difference between the dyeings of the two fibre variants or, less acceptably, a hue difference in a mixture recipe. The dyebath pH preferred for a given blend of acid-dyeable nylon variants is determined mainly by the most dyeable component. It is necessary to control the pH carefully using a suitable buffer, preferably a mixture of sodium dihydrogen phosphate and disodium hydrogen phosphate, to ensure optimum reproducibility of shadow and contrast effects. If the desired degree of contrast can be achieved with dyes of high neutral affinity, then these should be selected in the interest of good reproducibility. Table 5.1 shows the percentage change in contrast that a pH variation of 6.5 ±0.3 would produce for the three dyes compared in Figure 5.7 [11]. As indicated in Table 5.1 Variation of contrast with pH and dye affinity [11]

CI Acid

Neutral affinity

Contrast type

Contrast change (%)

Blue 25 Blue 41 Blue 175

Very high High Moderate

Low Medium High

4 11 20

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DYEING OF ACID-DYEABLE NYLON VARIANTS

O

59

NH2 SO3Na

SO3Na H2C O

HN O

HN

O

HN

CI Acid Blue 25 O

NH2 SO3Na

H2C O

HN SO3Na N

CI Acid Blue 41

O

CH3 CI Acid Blue 175 CH3

Figure 5.7 Acid dyes of increasing polarity (25 < 41 < 175)

CI Acid Blue 175

5

CI Acid Blue 41 CI Acid Blue 25

Contrast ratio

4

3

2

1 5

6

7

pH

Figure 5.8 Effect of pH on contrast ratio [11]

Figure 5.8, the higher the dyebath pH the greater the differentiation between the variant yarns and the better the reservation of the less dyeable component. However, other factors must be taken into account in deciding the optimum

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PRINCIPLES OF DESIGN AND COLOURING OF DIFFERENTIAL-DYEING BLENDS

dyebath pH. Total exhaustion begins to decrease at higher pH values and the degree of differentiation attainable is limited by the need to achieve economic exhaustion on the more dyeable variant. If subdued shadow effects are required, dyes of lower sulphonation are preferred and are applied at a carefully controlled pH in the 5–6 region. For sharper differentiation, more highly sulphonated dye can be used at pH 6–7. There are three approaches to selection of dyes for binary blends of aciddyeable nylon. They differ only in the dyes used and the colour effect obtained, since the same dyeing conditions can be employed in all cases: (1) Shadow effects using acid dyes giving differentiation between the variant yarns. Selected direct dyes can also be used but these are more sensitive to any physical irregularities in the variant yarns. (2) A limited degree of colour contrast using appropriate combinations of monosulphonated and disulphonated acid dyes. It is advisable to evaluate whether the mixtures selected have satisfactory fastness to light on the deepdye yarn. (3) A wider but still limited degree of contrast using highly sulphonated acid dyes with selected disperse dyes that give the same depth on both components of the blend. The hue on the deep-dye nylon is dependent on that of the disperse dye (section 1.5.4) and the wet fastness of these dyeings is generally lower than that of contrast dyeings produced by method (2). The acid dyes showing only moderate differentiation are mainly monosulphonated monoazo or anthraquinone dyes, whereas those dyes showing sharper differentiation are almost all disulphonated monoazo, disazo or anthraquinone types. The direct dyes recommended for method (1) are almost all disazo disulphonates, although a few disazo tetrasulphonates can be used where sharper differentiation is required. The disperse dyes preferred for solidity on these blends in method (3) are mainly low-energy monoazo or anthraquinone types. The distribution of acid dyes between acid-dyeable variant yarns is affected significantly by anionic levelling agents, such as alkylarylsulphonates and sulphated oils (Figure 3.5). These agents are preferentially absorbed by the deepdye variant, so that the degree of differentiation is markedly reduced. Weakly cationic levelling agents for nylon, however, such as long-chain alkylamine polyoxyethylenes, provide control of the rate of dyeing of acid-dyeable nylon blends without suppressing the differentiation, as they operate by complexing with the anionic dyes. For similar reasons, a nonionic dispersing agent should be used in preference to anionic alternatives when using disperse and acid dyes for

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contrast effects by method (3). Acid-dyeable nylon blends may be aftertreated with a syntan to improve wet fastness, but if disperse dyes are present the treatment temperature should not exceed 70°C or desorption may occur and the degree of improvement in fastness is only marginal. In the case of differential-dyeing tufted carpets or mats made with a jute backing, it is particularly important to avoid too low a dyebath pH because the lignin impurities in the jute tend to become transferred to the nylon pile, especially the deeper-dyeing variants. This yellow-brown staining tends to dull the brighter hues and to lower the light fastness because it discolours further on exposure to light. Staining at pH 6–8 is usually only slight and confined to the deep-dye or ultra-deep yarns, which are often dyed to a full depth. Much of the lignin and identification sighting colours on the variant yarns can be removed by alkaline scouring at 80°C before dyeing. When dyeing unusually bright or pale colours it may be necessary to scour-bleach with alkaline dithionite at 70°C, followed by sodium perborate and a nonionic detergent at the same temperature.

5.3 DYEING OF ACID-DYEABLE/BASIC-DYEABLE NYLON VARIANTS The diffusion kinetics of CI Basic Blues 3 and 9 (Figure 5.9) on a basic-dyeable nylon 6 variant containing 5-sulphoisophthalic acid N-terminal residues (Figure 5.10) demonstrated that diffusion was much more rapid than on the parent homopolymer [12]. Measurements of zeta potentials by the flow-potential method showed that the modified fibres have an unusually high negative charge at the surface, which decisively promotes sorption of the dye cations. The uptake of dye can be regarded as an ion-exchange reaction. The agreement of dye saturation data with the 5-sulphoisophthalic acid content confirmed that these oxazine and thiazine dyes of the delocalised-charge type become bound stoichiometrically to the terminal sulphonated residues in the modified fibre [13]. The scope for bright colours on normal/basic-dyeable blends is narrower than on deep-dye/basic-dyeable blends, because basic dyes stain normal nylon more than the deep-dye variant. It is also more difficult to achieve heavier depths on normal nylon without cross-staining of acid dyes on the basic-dyeable component. For these reasons it is usual to dye normal/basic-dyeable combinations in pale shades below one-third standard depth on both components. Anionic scouring agents should be avoided because they may complex with the basic dyes and increase the basic dye staining of the acid-dyeable nylon variants. After scouring with a nonionic detergent at 70°C, the basic and acid dyes are added separately after an alkanol polyoxyethylene anti-precipitant, sodium thiosulphate and a sequestering agent. A cationic retarder can be used to slow

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PRINCIPLES OF DESIGN AND COLOURING OF DIFFERENTIAL-DYEING BLENDS +

(CH3CH2)2N

N(CH2CH3)2 X–

O N CI Basic Blue 3

+

(CH3)2N

S

N(CH3)2 X–

N CI Basic Blue 9

Figure 5.9 Basic blue dyes of the delocalised-charge type SO3H NHCOCH2CH2CH2CH2CH2NHCO COOH Figure 5.10 Sulpho-containing terminal group in basic-dyeable nylon 6

down the initial strike of basic dyes on the basic-dyeable component at ambient temperature, permitting improved levelling without impairing the ultimate exhaustion [14]. Dyeing proceeds readily at 85°C and pH 6. A phosphate buffer is essential to ensure stability of pH for reproducible results on the normal/basicdyeable blend. When dyeing the deep-dye/basic-dyeable combination, however, pH control with ammonium acetate is satisfactory and more economical. Acid dye staining of the basic-dyeable nylon is cleared by boiling in dilute acetic acid at pH 4–5. The acid dyes recommended for reserve of the basic-dyeable component of a differential-dyeing nylon AB blend are mainly mono- or disulphonated monoazo or anthraquinone types. Levelling acid dyes of this type give better reserve of basic-dyeable nylon than do milling acid dyes or especially 1:2 metal-complex types. Monoazo and anthraquinone basic dyes of the localised-charge type are preferred for reserve of the acid-dyeable component of these blends. The light fastness and wet fastness of basic dyes on basic-dyeable nylon are markedly inferior to those of corresponding dyeings on acrylic fibres, and syntans are only moderately effective in improving wet fastness on this type of nylon. Many anionic dyes on the acid-dyeable nylon variants show much better wet fastness, so it is preferable to dye the basic-dyeable fibre only to no more than a medium

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depth and to select the acid-dyeable component for the heavier depth in the design.

5.4 DESIGN OF DIFFERENTIAL-DYEING CELLULOSIC FABRICS 5.4.1 Aminisation of cellulosic fibres The concept of pretreating cellulosic material with a cationic compound in order to confer affinity for acid dyes has been explored since long before reactive dyes were discovered. In those days the approach was of little practical value because of poor fastness to washing of the cationic agents themselves and the dyeings that could be obtained with them. In the last twenty years or so, however, there has been much activity in this area by applying the principles of reactive dyeing and reactant finishing to the application of these pretreating agents [15–19]. Much of this research has been mainly intended to improve the utilisation of reactive dyes on cotton or viscose, however, rather than the development of colour effects on binary cellulosic blends. Deep-dye viscose can be produced by incorporating additives containing amino groups into the spinning dope before extrusion. Thus aminoethylcellulose, prepared by condensation of 2-chloroethylamine with cellulose (Scheme 5.2) confers enhanced dyeability with direct or acid dyes under mildly acidic dyeing conditions. These primary amino dyeing sites enable reactive dyes to form covalent bonds without requiring an alkaline fixation step after exhaustion.

[cellulose]

[cellulose]

OH + ClCH2CH2NH2

[cellulose]

OCH2CH2NH2 + HO3S

[cellulose]

OCH2CH2NH2 + Cl

[dye]

[dye]

OCH2CH2NH2

[cellulose] [cellulose]

+

OCH2CH2NH3 –O3S

OCH2CH2NH

[dye]

[dye]

Scheme 5.2

Yarns of deep-dye viscose can be knitted or woven into designs with normal viscose yarn. These are suitable for obtaining either of the following: (a) shadow effects with direct or alkali-fixed reactive dyes; (b) normal viscose reserve using acid dyes or neutral-fixed reactive dyes.

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PRINCIPLES OF DESIGN AND COLOURING OF DIFFERENTIAL-DYEING BLENDS

By incorporating an acrylic yarn into the fabric, three-way shadow-contrast or reserve-contrast designs can be produced using basic dyes with these two approaches [20]. Diethylaminoethylated cotton or linen can be prepared in a similar way by reacting the substrate with the tertiary amine 2-chloroethyldiethylamine in the presence of alkali at 95°C. The diethylaminoethyl substituents act as built-in catalysts, capable of initiating fixation of reactive dyes even in the absence of alkali. A more effective approach to the aminisation of cotton fabric involved padding with caustic soda solution, followed by immersion in an acetone solution of epichlorohydrin and triethanolamine. The etherifying agent is believed to be the reactive tertiary amine formed by the initial condensation between the starting materials (Scheme 5.3). 3 H2C CHCH2Cl + (HOCH2CH2)3N O

(H2C CHCH2OCH2CH2)3N O

Scheme 5.3

5.4.2 Monofunctional quaternary ammonium reactants Many of the early attempts to incorporate quaternary nitrogen sites in cellulose depended on the use of epoxy reactants. The first product of this type on the market [21] was Glytac A (Protex). This is readily available from epichlorohydrin and trimethylamine (Scheme 5.4) [22]. CH3 H2C CHCH2Cl + N(CH3)3 O

H2C CHCH2 O

Cl–

N+ CH3 CH3

Scheme 5.4

Reaction with cellulose proceeds via the glycidyl group at alkaline pH. Reactive dyes can be fixed to the modified fibres at neutral pH without salt, conditions that are environmentally attractive. This behaviour has been attributed to predomination of the zwitterionic form of the sidechain after alkaline fixation (Scheme 5.5). Thus the anionic dye is absorbed on the quaternary site and, if reactive, it is attacked by the nucleophilic ionised alcoholic group nearby [23]. A reagent exhibiting some features in common with Glytac is 1,1-dimethyl-3-hydroxyazetidinium chloride (DMAC). It reacts with cellulose by a similar ring-opening etherification. The results from the two quaternised

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DESIGN OF DIFFERENTIAL-DYEING CELLULOSIC FABRICS

CH3 [cellulose]

OCH2CHCH2 OH

Cl–

N+ CH3 CH3

65

OH– CH3 [cellulose]

OCH2CHCH2 O–

N+ CH3 + Cl– + H2O CH3

Scheme 5.5

cellulose ethers are similar, but there is marginally better reactive dye fixation to the DMAC grouping [17]. Certain problems are characteristic of epoxy quaternising agents [15]. Discoloration of the substrate can occur during the alkaline cure necessary for epoxy fixation. Loss of trimethylamine by thermal instability destroys the effectiveness and simultaneously gives rise to objectionable odour in the treated fabric. This difficulty can be only partly overcome by incorporating more bulky substituents at higher cost. Probably the most important limitation of the epoxypropyl agents is their very low substantivity. Not only are they unsuitable for exhaust application but poor dye penetration arises from agent migration at the drying stage of the pad–dry–bake application process. The N-methylolation reaction that has been so important in traditional chemical finishing can be exploited as a first step to the aminisation of cellulose using N-methylolacrylamide. This acrylamidomethylated cellulose reacts readily with ammonia or alkylamines to yield cellulose derivatives containing amino, imino or quaternary groups (Scheme 5.6). Dyeing tests on these derivatives showed generally good colour yields and high fixation, but reactive dyes on those aminised with di- or trimethylamine gave poor fixation. The dye–fibre linkage is labile owing to the strongly electron-withdrawing quaternary group [23].

cell OH + HOCH2NHCOCH CH2

cell OCH2NHCOCH CH2

cell OCH2NHCOCH CH2 + H R

cell OCH2NHCOCH2CH2

R

+

R = NH2, NHCH3, NHCH2CH2OH, N(CH3)2 or N(CH3)3Cl–

Scheme 5.6

Groups of the haloheterocyclic type found in traditional reactive dyes, such as aminochlorotriazine or difluoropyrimidine, have been exploited in aminising agents that also contain mono- or bis-quaternary ammonium groups to boost the uptake of anionic dyes by the aminised substrate. Such agents react more readily with cellulose and show higher thermal stability than the ring-opening types such

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PRINCIPLES OF DESIGN AND COLOURING OF DIFFERENTIAL-DYEING BLENDS

as Glytac or DMAC. Both classes of monofunctional reactive system, however, share the disadvantage of low substantivity for cellulose and must be applied by padding. More complex multifunctional structures were designed for exhaust application [15] and these gave impressive colour yields. There are practical drawbacks to all these agents, however, including lower light fastness [17], hue changes and poor penetration into the fibre [15]. In a differential-dyeing evaluation of aminochlorotriazine agents, colourless cationic and anionic products of this type (Figure 5.11) were applied to cotton yarns [24]. Yarn pretreated with the quaternary ammonium agents showed enhanced uptake of CI Acid Red 13 or CI Direct Blue 10. The degree of differential uptake was dependent on the amount of agent applied, so fabrics exhibiting a range of shadow effects were obtained. Pretreatment with the sulphonaphthylamine agent gave cotton dyeable with CI Basic Yellow 15. Combinations of anionic- and cationic-modified yarns yielded contrast effects with minimal cross-staining. The pretreated cotton samples were sufficiently stable to be converted to viscose or cellulose triacetate without loss of the modified dyeability, providing potential routes to differential-dyeing viscose or triacetate variants. Cl N

N N

HN

CH2CH3 NHCH2CH2 N + R

X– CH2CH3

Cl N HN

N N

NH

SO3Na

Figure 5.11 Cationic and anionic aminochlorotriazine agents

This approach to differential-dyeing is flawed because the costly pretreatment with a sophisticated reactive agent is not justifiable. The fastness of the colour effects to washing with detergent will be inadequate because the dyes are linked to the ionic dyeing sites mainly by electrostatic bonds. In the case of basic dyes on the sulphonaphthylamine sites, low fastness to light is also anticipated. An alternative approach to the aminisation of cellulose involves esterification using chloropropionyl chloride. This chloropropionate ester condenses readily

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DESIGN OF DIFFERENTIAL-DYEING CELLULOSIC FABRICS

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with ammonia or alkylamines at 50°C to yield aminised cellulose derivatives (Scheme 5.7). If the aminisation treatment is carried out at the boil, however, chloropropionic acid is eliminated and the cellulose is regenerated. The ester bond remains intact during reactive dyeing but it is hydrolysed during alkaline soaping at the boil [25]. [cellulose] [cellulose]

OH + ClCOCH2CH2Cl OCOCH2CH2Cl + H R

50°C

[cellulose]

OCOCH2CH2Cl

[cellulose]

OCOCH2CH2 R

+

R = NH2, NHCH3, N(CH3)2 or N(CH3)3Cl–

Scheme 5.7

Sodium or ammonium phosphates can be used to produce chambray or other special colour effects on cotton fabrics. Yarn treated with the phosphorylating agent (Scheme 5.8) is dried, cured and then woven as the weft with an untreated warp. The fabric is then piece dyed with reactive, vat or direct dyes. Only the warps absorb dye because the phosphorylated cellulose resists these conventional dyes for cotton. It can be cross-dyed, however, using 1:1 metal-complex dyes at pH 4. Interesting colour contrast effects are obtained by treating selected areas of a cotton pile fabric with the phosphorylating agent before selective dyeing [26]. These techniques offer the advantage of a cheap and effective modifying reactant but the coordinate bonds linking the metal-complex dyes to the phosphate dyeing sites will show only limited fastness to acidic perspiration tests.

[cellulose]

ONa OH + NaO P O ONa

[cellulose]

ONa O P O + NaOH ONa

Scheme 5.8

5.4.3 Polymeric cationic reactants Many cationic polymers have been applied to cellulosic fibres with a view to enhancing uptake of anionic dyes [18]. It is considerably more difficult in these instances to interpret the precise mechanisms involved, apart from the obvious participation of electrostatic attraction between dye anions and basic groups in the polymer. Recent studies have included the application to cotton [17] of the polyamide-epichlorohydrin resin Hercosett 125 (Hercules), originally marketed as a shrink-resist treatment for wool. Improved dyeability with reactive dyes and

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PRINCIPLES OF DESIGN AND COLOURING OF DIFFERENTIAL-DYEING BLENDS

good wet fastness were obtained, but dullness of hue and impaired light fastness were disadvantages. Incorporation of thiourea into the Hercosett polymer during application was intended to overcome deficiencies when using low-reactivity dyes on Hercosett alone. Thiourea reacts with azetidinium groups in the resin to form isothiouronium groups. These are more strongly nucleophilic and improve the fixation of low-reactivity dyes. Some of the isothiouronium groups decompose during dyeing to yield thiol groups, which form further sites for dye fixation and may also react with remaining azetidinium groups to form thioether crosslinks in the resin (Scheme 5.9).

+

CH2

NH2 CHOH + S

N

NH2

CH2 Cl– +

NH2

N CH2CHCH2 S OH +

Cl–

+

NH2

N CH2CHCH2 S

OH–

NH2

Cl–

OH

NH2 NH2

N CH2CHCH2SH + O NH2

OH

CH2 CHOH + HSCH2CHCH2 N

N CH2 Cl–

OH

N CH2CHCH2 S CH2CHCH2N OH

OH

Scheme 5.9

Differences in colour yield were still observed between dyes of high and low reactivity and therefore ethylenediamine was evaluated as an additive to the Hercosett resin [27]. This reacts readily with azetidinium groups to form primary, secondary and tertiary amino sites for fixation of reactive dyes (Scheme 5.10). Enhanced substantivity, good brightness and excellent fastness to washing were achieved but the light fastness was still impaired in most cases. +

CH2 CHOH + H2NCH2CH2NH2

N CH2

N CH2CHCH2NHCH2CH2NH2 OH

Cl–

N CH2CHCH2NHCH2CH2NHCH2CHCH2 N OH

OH

Scheme 5.10

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Cotton has little inherent substantivity for disperse dyes, but a water-soluble benzoylating agent, sodium benzoylthioglycolate, can be applied by a pad–dry– thermofix process in the presence of alkali (Scheme 5.11) to yield a benzoylated cellulose for which disperse dyes show enhanced substantivity. Dyeings of high colour strength and good fastness to ISO 3 washing are obtained [28]. Disperse dyeings on benzoylated cellulose exhibit much better fastness to washing than corresponding dyeings on cotton esterified using long-chain acylating agents [29]. The ease of application of sodium benzoylthioglycolate is a great advantage over earlier processes based on the Schotten–Baumann reaction of cellulose with benzoyl chloride. This process is of interest for the single-class dyeing of blends of ester fibres with cellulosic fibres using disperse dyes to colour both components. NaOH C S

CH2COONa + HO

[cellulose]

O Sodium benzoylthioglycolate

C O

[cellulose] + HSCH2COONa

O Sodium thioglycolate

Scheme 5.11

Polymer treatments devised to increase the versatility of cotton as a dyeable substrate [30] included: (1) treatment with a disperse-dyeable polymer followed by disperse dyeing; (2) addition of basic or acid dyes to a crosslinking formulation so that the dyes are retained by interaction with the finish; (3) application of reactive or vat dyes with an anionic-dyeable polymer. Bicoloured contrast effects can be produced by selective coating of the two sides of the fabric with different combinations of dye and finish. Disperse and basic dyes exhibit poor light fastness on cotton although the brightness of basic dyes may appear superficially attractive. Reactive dyes give the best balance of properties, since vat dyes sometimes show unlevelness under these conditions. A two-stage dyeing sequence has been proposed for the contrast dyeing of cotton terry towelling using reactive dyes [31]. The first step involves continuous dyeing of the fabric using a polyelectrolyte that promotes coloration of the tips of the terry towelling loops. Adverse drying conditions on hot cans (‘fry drying’) favour dye migration to the tips of the loops. The dried fabric is then padded with a highly alkaline concentrated salt solution to minimise dye desorption and

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PRINCIPLES OF DESIGN AND COLOURING OF DIFFERENTIAL-DYEING BLENDS

steamed to complete alkaline fixation of the dyes. Unfixed dye is removed by rinsing. The second stage consists of uniformly dyeing on a winch or atmospheric jet in a contrasting colour also formulated with reactive dyes in conventional exhaust dyeing.

5.4.4 Introduction of activated sites for nucleophilic dyes In this approach the reactive function is incorporated into the substrate and a reactive dyeing is carried out using a ‘non-reactive’ dye containing nucleophilic groups. A model dye of this kind was prepared by reacting the aminochlorotriazine dye CI Reactive Red 58 with ethylene diamine to form a 2aminoethylaminotriazine derivative (Scheme 5.12). Acrylamidomethylated cellulose prepared by condensation of N-methylolacrylamide with cotton (Scheme 5.6) reacts readily with nucleophilic dyes of this kind, which show no hydrolysis during dyeing. –O

3S

[dye] HN

N N

Cl N

+ H2NCH2CH2NH2 –O

3S

OH–

[dye] HN

N

NHCH2CH2NH2

NHAr

+ HCl N

N NHAr

Scheme 5.12

High fixation is achieved either by exhaustion at pH 10.5 in salt solution or by pad–batch at the same pH for 24 hours [23]. These aminoalkyl dyes show zwitterionic character below pH 8 and this lowers the nucleophilicity of the primary amino group. Nucleophilic dyes containing thiol groups would be more reactive than aminoalkyl analogues but there would be problems of toxicity, odour and a tendency to oxidise to disulphide. Aminoaryl or hydroxyalkyl analogues would be less reactive than the aminoalkyl derivatives. The novel reactant 2,4-dichloro-6-(2′-pyridinoethylamino)-s-triazine (DCPEAT, Figure 5.12) was evaluated [32] as a means of activating cellulose by pad–batch application at pH 8.5 for 24 hours. After a cold water wash, the modified substrate was dyed with aminoalkyl dyes. High fixation was achieved by exhaust dyeing at pH 9 without salt. Nucleophilic dyes can also be fixed on cotton or nylon that has been pretreated [33] with the trifunctional reactant 2chloro-4,6-bis(4′-sulphatoethylsulphonylanilino)-s-triazine (CSESAT, Figure 5.12).

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DESIGN OF DIFFERENTIAL-DYEING CELLULOSIC FABRICS

+

N

N CH2CH2NH N DCPEAT

NaO3SO

CH2CH2SO2

Cl N

Cl N

HN N

CSESAT

71

NH

SO2CH2CH2

OSO3Na

N Cl

Figure 5.12 Bifunctional and trifunctional reactants for cellulose

Developments in this area seem to be leading to daunting complexity rather than elegant simplicity. Certain aspects of this concept inspire unease rather than confidence: (1) The risk of premature hydrolysis leading to impaired fixation has been transferred from the dye to the substrate. (2) The pretreatment with a colourless reactant necessary to activate the substrate must be exceptionally uniform if dye fixation is to be consistent. (3) If penetration of the reactant is poor, ring dyeing will follow and the fastness properties will be adversely affected. (4) Only the aminoalkyl dyes will react with the activated substrate and other classes of dyes for unmodified cellulose may be inapplicable. (5) Cost implications give concern because both dye and substrate must be specially modified before the desired reaction can occur.

5.5 DESIGN OF DIFFERENTIAL-DYEING WOOL KERATIN DERIVATIVES Wool is routinely chlorinated with a dilute acid solution of hypochlorite or with gaseous chlorine to give shrink-resist effects, whereby absorption of acid dyes is appreciably increased. The production of shadow effects on blends of chlorinated and untreated wool depends not only on a selection of multisulphonated dyes but also on dyeing conditions. Dyeing at low temperatures will favour absorption by chlorinated wool, particularly in the case of aggregated milling acid dyes that are less readily absorbed by the unmodified fibre except at higher temperatures. Certain chrome dyes will also produce distinctive colours on blends of chlorinated and unchlorinated wool. Levelling

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PRINCIPLES OF DESIGN AND COLOURING OF DIFFERENTIAL-DYEING BLENDS

acid dyes are unsuitable because of the rapid migration of these dyes even at temperatures below the boil. The differential dyeing of wool can be achieved using several other techniques [34]: (1) localised pretreatment with acid-dye-resist agents of the dichlorotriazinylaminoarylsulphonate type (Figure 5.3) before dyeing the untreated areas with acid dyes; (2) pretreatment with cationic agents such as Glytac A (section 5.4.2) to confer deep-dye characteristics; (3) overdyeing of fabrics containing wool yarns predyed with e.g. the chromium complex of CI Mordant Black 9 (Figure 5.13) to give colour/ black effects; (4) application of mordant dyes to prechromed and unchromed wool yarns, but this is not of much value owing to poor shade reproducibility. OH SO3Na N N O O CrIII O O N N NaO3S OH

Figure 5.13 CI Mordant Black 9 chrome complex

Provided that the nucleophilic sites of reaction within the accessible regions of wool keratin can be blocked, there will be no remaining opportunity for reactive dyes to become fixed. By choosing reactive dyes of high reactivity but relatively low substantivity it should be possible to achieve a high degree of resist effect. The principal nucleophilic groups are primary amino (lysine), secondary imino (histidine) and free thiol groups (cysteine). A reactive dichlorotriazinylaminoarylsulphonate (Figure 5.3) effectively blocks these nucleophilic sites. Acid-dye-resist agents of this kind that introduce extra anionic sidechain groups into wool keratin will confer enhanced basic-dyeable character. Conversely, a reactive agent with a cationic centre, such as Glytac A (section

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DESIGN OF DIFFERENTIAL-DYEING WOOL KERATIN DERIVATIVES

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5.4.2), will act as a basic-dye-resist and confer increased acid-dyeable behaviour. Sandospace R (S) at weight gains of 8% or more resisted all the reactive, mordant and acid dyes tested to 70% resist or higher, except for a monosulphonated acid dye. Uptake of the reactive and acid dyes was increased by prior reaction of the wool with Glytac A, a moderately effective resist treatment with respect to basic dyes (40–80% resist) but hydrophobic dye–fibre forces appeared to predominate over cationic repulsion effects [35]. There is no universally effective reactive resist that will reserve all classes of dyes normally applied to wool. Dyes of high Mr become attached to wool by nonpolar van der Waals forces between the hydrophobic dye anions and hydrophobic sidechains in the wool fibre, their strength being proportional to the area of contact. High wet fastness is often achieved by increasing the Mr with substituents that do not form part of the chromogenic system, such as dodecyl or arylsulphonyl groups. As wool dyes become more hydrophobic they show a greater tendency to aggregate in the dyebath. Aggregation within voids in the keratin structure is probably partly responsible for the poor migration, high wet fastness and excellent light fastness of these dyes. The benzoylation of wool using benzoic anhydride forms benzoylamino residues on lysine sidechains and N-terminal end groups, as well as benzoate ester groups on the hydroxy-containing sidechains of serine and threonine (Scheme 5.13). This destroys most of the sites for reactive dye fixation and also acts as an effective resist for acid dyes with two or more sulphonate groups.

[wool] [wool]

NHCO

NH2 O +

+ H2O

O O

[wool]

OH [wool]

OCO

Scheme 5.13

If a water-soluble bifunctional arylating agent (Scheme 5.14) is applied by exhaustion at pH 6.5 together with conventional disperse dyes, absorption by wool takes place with the agent held initially by electrostatic attraction and hydrophobic interaction as if it were a colourless milling acid dye. Fixation occurs during subsequent treatment for an hour at the boil, the active vinylsulphone group being released by β-elimination and reaction taking place

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PRINCIPLES OF DESIGN AND COLOURING OF DIFFERENTIAL-DYEING BLENDS

Cl N

N

HN

NH

N

Cl

SO2CH2CH2OSO3Na

N

N

Cl + H2N [wool]

HN N

N

NH

+ NaHSO4

N

HN

N

SO2CH

NH SO2CH

CH2 + H2N

CH2

[wool]

NH [wool] N HN

N N

NH SO2CH2CH2NH [wool]

Scheme 5.14

with nucleophilic groups in the fibre. Some reaction also occurs via the less reactive chlorotriazine group. Hydrophobic groupings attached to the wool in this way confer enhanced substantivity for disperse dyes. This approach is of interest for the single-class dyeing of blends of ester fibres with wool using disperse dyes to colour both components [36]. Sulphamic acid reacts with the amino groups of lysine sidechains and the Nterminal end groups in wool to form ammonium sulphamate groups, and with the hydroxy groups of serine and threonine to form ammonium sulphate groups (Scheme 5.15). Wool that has been treated with sulphamic acid and urea in a pad–dry–bake process shows increased uptake of basic dyes. Milling acid dyes that depend mainly on hydrophobic dye–fibre interaction are poorly resisted by sulphamylated wool, but levelling acid dyes absorbed mainly by electrostatic

[wool] NH2 + HO3SNH2 [wool]

OH + HO3SNH2

[wool] NHSO3– [wool] OSO3–

NH4+ NH4+

Scheme 5.15

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DESIGN OF DIFFERENTIAL-DYEING WOOL KERATIN DERIVATIVES

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attraction are strongly resisted. Reactive dyes, almost all of which are highly sulphonated with no hydrophobic alkyl substituents, are also effectively resisted. Sulphamylated wool shows much higher exhaustion of typical basic dyes than acrylic fibres. These contain only about 50–150 milli-equivalents per kg of anionic groups, whereas wool treated with sulphamic acid contains up to ca. 1000 milli-equivalents per kg of acidic dyeing sites [37]. Sulphamylation of wool has a marked effect on the light fastness of basic dyes. The fastness ratings on the modified wool are much higher than on untreated wool (Table 5.2). This brings the ratings of the modified wool close to values obtained for the same dyes on acrylic fibres. This effect is attributed to the beneficial influence of the strongly anionic groups introduced by the sulphamylation reactions. Basic dyes on untreated wool are absorbed because of the presence of the carboxyl groups in aspartic and glutamic acid sidechains and the C-terminal end groups. Unfortunately, the fastness to washing of basic dyes on untreated and sulphamylated wool is extremely poor. The treatment with sulphamic acid makes the wool more hydrophilic. Marginal improvement of the wet fastness is possible by aftertreatment with tannic acid but this causes dulling of the dyeings. Table 5.2 Exhaustion and light fastness of typical basic dyes [37] Exhaustion (%)

Light fastness

2% o.w.f. of CI Basic

Acrylic fibre

Sulphamylated wool

Acrylic fibre

Untreated wool

Sulphamylated wool

Yellow 11 Red 51 Blue 3

71 30 33

99 97 98

6 6 4–5

4 3–4 1

6 6 4

The treatment of merino wool with ethanolamine in aqueous isopropanol to confer enhanced dyeability and a shrink-resist effect was investigated using a factorial design experiment with three variables [38]. These effects of ethanolamine have been attributed to alkaline hydrolysis of the cystine disulphide groups to form dehydroalanine residues that can react with the amine to form β-aminoalanine groups, which are able to interact with anionic dyes. A vinylsulphone reactive dye was used to assess the improvement in dyeability. The optimum conditions found were 1 mol l–1 ethanolamine in 50% aqueous isopropanol at 55°C, which gave the best dyeability and shrink resistance with only 3% weight loss and acceptable whiteness.

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5.6 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.

C Baur, Teinture et Apprets, 144 (Oct 1974) 163. J Park, J.S.D.C., 109 (1993) 133. A Anton and J C Browne, Text. Chem. Colorist, 16 (Sep 1984) 135. A Anton, Text. Chem. Colorist, 13 (Feb 1981) 46. R Gutmann, T Barth and H Herlinger, Chemiefasern und Textilind., 40/92 (1990) 104. T Sato, Chemiefasern und Textilind., 40/92 (1990) 35. M D Teli and N M Prasad, Am. Dyestuff Rep., 80 (June 1991) 18. S M Doughty, Rev. Prog. Coloration, 16 (1986) 25. M M Marie, Am. Dyestuff Rep., 82 (Sep 1993) 86. R McGregor, AATCC Int. Tech. Conf., (Oct 1976) 124. T L Dawson and B P Roberts, J.S.D.C., 95 (1979) 47. H Muller and V Rossbach, Text. Research J., 47 (1977) 44. H Muller, Text. Research J., 47 (1977) 71. M A Herlant, Am. Dyestuff Rep., 81 (June 1992) 15. G E Evans, J Shore and C V Stead, J.S.D.C., 100 (1984) 304. R L Stone and R J Harper, AATCC International Conference and Exhibition, (Oct 1986) 214. D M Lewis and X P Lei, Text. Chem. Colorist, 21 (Oct 1989) 23. J Shore, Rev. Prog. Coloration, 21 (1991) 23. J Shore, Indian J. Fibre Text. Res., (Mar 1996) 1. R Aitken, J.S.D.C., 99 (1983) 150. M Rupin, G Veaute and J Balland, Textilveredlung, 5 (1970) 829. T S Wu and K M Chen, J.S.D.C., 108 (1992) 388. D M Lewis and X P Lei, J.S.D.C., 107 (1991) 102. J A Clipson and G A F Roberts, J.S.D.C., 105 (1989) 158. X P Lei and D M Lewis, Dyes and Pigments, 16 (1991) 273. E J Blanchard, J T Lofton, J S Bruno and G A Gautreaux, Text. Chem. Colorist, 11 (Apr 1979) 76. X P Lei and D M Lewis, J.S.D.C., 106 (1990) 352. D M Lewis and P J Broadbent, J.S.D.C., 113 (1997) 159. E Einsele, H Sadeli and H Herlinger, Melliand Textilber., 63 (1981) 967; 64 (1982) 61. R J Harper, E J Blanchard, J T Lofton, J S Bruno and G A Gautreaux, Text. Chem. Colorist, 6 (Sep 1974) 201. J N Etters, Am. Dyestuff Rep., 83 (Sep 1993) 70. D M Lewis and X P Lei, AATCC International Conference and Exhibition, (Oct 1992) 259. D M Lewis and Y C Ho, AATCC International Conference and Exhibition, (Oct 1994) 419. A C Welham, Am. Dyestuff Rep., 81 (Oct 1992) 15. V A Bell, D M Lewis and M T Pailthorpe, J.S.D.C., 100 (1984) 223. D M Lewis, J.S.D.C., 113 (1997) 193. B A Cameron and M T Pailthorpe, J.S.D.C., 103 (1987) 257. L Coderch, A M Manich, J L Narra and P Erra, J.S.D.C., 107 (1991) 19.

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CHAPTER 6

Nylon/wool and other AA blends

6.1 DYEING OF NYLON/WOOL BLENDS Blends in which both fibres are dyeable with anionic dyes in full depths (AA blends) are usually developed because of a desirable balance of physical characteristics. The attainment of solid colour effects is therefore usually necessary. As soon as nylon was readily available for commercial outlets after the Second World War, it was recognised that the exceptional strength, durability and abrasion resistance made it the ideal fibre for blending with wool. Blended twill fabrics gave the same performance as the all-wool counterpart but at half the weight. Hand-knitting yarns and half-hose blended from 20:80 to 40:60 nylon/wool give a valuable combination of softness and strength. Stretch fabrics woven from a crimped nylon warp and a wool weft are established in winter sportswear, particularly skiwear, and leisure clothing. Woven carpets and high-quality tufted carpets are most important outlets for blended-staple nylon/wool yarns, usually containing about 20% of nylon to confer improved durability and abrasion resistance whilst retaining the absorbency, softness and antistatic qualities of the natural fibre. An interesting anomaly of the 1990s market, however, is a demand for carpets made from blended nylon/ wool but dyed and woven to simulate sisal, a natural cellulosic fibre viewed as a fashionable floor covering by ecology-conscious consumers [1]. The quality of the wool and its base colour will influence the visual effect obtained between the two fibre types when dyed. Even when the depth of colour is the same, the yellowness of the wool can result in the shade appearing duller on that component. Blends containing nylon 6 show higher saturation uptake and critical depth than nylon 6.6 blends and require more retarding or levelling agent. Supplies of the same nylon type from different sources, or even different merges from the same source, may also vary in dyeing properties [2]. Good solidity is generally easier to achieve on staple-fibre blends, such as hand-knitting yarns or carpets, because the blend often contains only 10–20% of nylon and the distribution can be controlled using an appropriate auxiliary. 77

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NYLON/WOOL AND OTHER AA BLENDS

Solidity is a more serious problem when dyeing nylon/wool furnishings or sportswear woven with a crimped nylon warp and a wool weft. Apart from the quality and proportions of wool and nylon present, the distribution of dye between them depends on several factors already discussed (section 3.2), including the constitution and degree of sulphonation of the dyes, applied depth, agent concentration, pH and temperature of dyeing [3]. Like the disulphonated levelling acid dyes, the 1:1 metal-complex dyes have low substantivity for nylon and the critical depth on nylon/wool is relatively low compared with monosulphonated acid dyes and 1:2 metal-complexes. Dyes of the 1:1 metal-complex type and the levelling acid disulphonates are dyed at pH 2–3 with formic or sulphuric acid and salt at the boil. Monosulphonated acid dyes and premetallised 1:2 types dye nylon more readily, so they are applied to nylon/wool at pH 5–6 with ammonium acetate and acetic acid. A near-neutral pH is necessary to control levelness in pale depths, but more acetic acid can be used for full depths. Medium or heavy depths on nylon/wool carpet blends are usually dyed with 1:2 premetallised dyes in order to ensure good fastness to light and shampooing. The monosulphonated type offers the best economic compromise of good partition with moderate usage of retarding agent. Partition is, however, dependent on control of pH, being improved in favour of the wool with decreasing pH and increasing temperature. The control of both of these factors can be ensured using an automatic dosing system [4]. Coverage of physical variations in the nylon is limited but an alkanol polyoxyethylene levelling agent gives some improvement. Heavy, dull colours can be dyed by a two-stage method in which the nylon is first dyed preferentially with 1:2 metal-complexes at pH 6 and the wool is then filled in with selected 1:1 metal-complex types. It is difficult to attain satisfactory solidity on nylon/wool with chrome dyes, which tend to favour wool. Some solid hues can be based on mixtures of selected chrome and 1:2 premetallised types. The chrome dyes giving acceptable solidity on nylon/wool are mainly monosulphonated naphthylazo derivatives of Mr 350–450. They are used only for economical dull orange, red, brown and black dyeings of high wet fastness. At such depths no anionic retarder is required. Chroming of the dye is more difficult on nylon/wool than on nylon alone because the chromium tends to be absorbed preferentially by the wool. Chelation with the dye can be improved by adding a mild reducing agent, such as sodium thiosulphate. Novel black 1:2 metal-complex azo dyes containing chelated iron(III) instead of chromium(III) atoms have been synthesised. Results from in vitro hazard testing reveal that these iron complexes are satisfactory with regard to

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DYEING OF NYLON/WOOL BLENDS

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genotoxicity [5]. The fastness properties of these new dyes on both nylon and wool are comparable with those of their chromium analogues. It is anticipated that they will be strong candidates for nylon/wool dyeing in carpets or furnishings requiring high light fastness [6]. Disperse dyes are rarely used on nylon/wool because their light fastness and wet fastness ratings are low, and the heavy stain on wool has poor fastness to light, rubbing and perspiration. If heavy depths are dyed with disulphonated levelling acid dyes above the critical depth, disperse dyes can be applied with an alkanol polyoxyethylene dispersant from the same bath to fill in the nylon component. The disperse dyes chosen for this shading purpose are mainly lowenergy monoazo or anthraquinone types. Owing to the cross-staining of wool that inevitably occurs, the fastness of the dyeing may be impaired. If dye partition between nylon and wool is unequal, particularly if there is an off-tone shade variation, this can lead to a stripy appearance in plain woven or tufted carpet constructions. Further problems can arise after wear, for if there is a marked tonal difference between the fibre components in a loop-pile construction, the carpet begins to show pronounced local colour changes where the wool fibres become abraded away in worn areas [4]. Unmetallised acid dyes fade more slowly in wool than in nylon and the fading mechanism appears to be different, probably because oxidative degradation in nylon is not inhibited, whereas the reducing environment of the wool fibre has a retarding effect. The differences are also consistent with the formation of dye aggregates in wool, which has a much higher amine end group content [7]. 6.2 BLENDS OF WOOL WITH OTHER ACID-DYEABLE FIBRES Owing to its characteristic lustre and excellent physical and chemical properties, silk has remained the traditional dress fabric for kimonos and saris in Asia. It is becoming increasingly important in other forms of dresswear, either alone or blended with wool, cotton, nylon or polyester. Natural silk is blended in equal proportions with wool to make high-class apparel, contributing lustre and strength. Intimate blends are usually dyed in solid shades, but it is not difficult to achieve shadow effects because of the higher initial rate of dyeing of the silk component, especially at low pH. Silk yarns are also used as effect threads in fine wool dress fabrics. Effect threads are preferably yarn dyed before weaving and the wool is then cross-dyed, making allowance for staining of the silk at this stage. Attempts have been made to dye the wool and reserve the silk, although careful dye selection is necessary and considerable skill is required to formulate a wide range of shades in this way. Any cross-staining cannot be completely

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NYLON/WOOL AND OTHER AA BLENDS

cleared without affecting the shade on the wool. Strongly acidic dyebaths increase the depth on the silk component. Wool is favoured at higher pH values and at pH 5–6 the silk is almost completely reserved. Monosulphonated milling acid dyes of Mr 450–550 give the best reserve under these conditions. If solidity is required on wool/silk blends it is necessary to dye at a low pH below the boil. Under these conditions, however, levelling acid and 1:1 metalcomplex dyes do not give adequate fastness on the wool. It is possible to apply some selected milling acid dyes but level dyeing is difficult. Common salt and a mildly cationic retarder are necessary to control the rate of dyeing. The distribution of direct, acid and metal-complex dyes between wool and silk has been examined by dyeing simulated blends [8]. Blends of natural silk and virgin wool can be dyed at ambient temperature with high-reactivity dyes for the silk and then the wool is filled in with milling acid or 1:2 metal-complex dyes at pH 5–6 and 90°C, to minimise cross-staining of the reactive-dyed silk. The fastness to light, water and alkaline perspiration of reactive dyes from the aminofluorotriazine and α-bromoacrylamide classes is satisfactory and superior to that of copper-complex direct or 1:2 chromiumcomplex acid dyes on silk [9]. Besides wool, the animal fibres of interest include mohair and cashmere from species of goat, alpaca and vicuna from camel species, and angora fur from rabbits. These are relatively scarce and costly but may be blended with wool to increase lustre and give a distinctive appearance. Blends of wool with silk, mohair, cashmere or alpaca are largely subject to the dictates of fashion. Handknitting yarns are luxury items and processing costs tend to be low relative to retail prices, so that more attention can be paid to high quality rather than productivity and materials cost. Typical blends for such yarns include wool/angora, wool/cashmere, wool/mohair and nylon/mohair. Some cashmere is diluted with fine wools for economic reasons but may still carry a cashmere label. The outstanding properties of angora and cashmere in knitwear apparel are well known. These fibres will not withstand prolonged boiling, so reproducible colour matching and first-class levelness are essential [10]. Angora is only processed in blends with wool, sometimes with the addition of a small proportion of nylon to improve the durability. For economic and technical reasons 1:2 metal-complex and milling acid dyes are preferred. Chrome dyes and 1:1 metal-complex types are seldom used because strongly acidic dyebaths may damage the angora. A simple test using CI Acid Red 18 and CI Basic Blue 9 (Figure 6.1) can be used to assess the degree of oxidation damage to the angora. The bluer the staining with this mixture, the greater the degree of damage [11]. In pale dyeings, any nylon present dyes more intensely, whereas in full depths it is

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BLENDS OF WOOL WITH OTHER ACID-DYEABLE FIBRES

81

H O NaO3S

N S

(CH3)2N

N NaO3S CI Acid Red 18

+

N(CH3)2 X–

N SO3Na

CI Basic Blue 9

Figure 6.1 Components of stain test for damage in angora fibres

the wool that gives the deeper shade. Solidity can be achieved using a retarding agent, the addition needed being dependent on the applied depth and blend proportion, particularly the amount and quality of any nylon present. The amount of retarder required varies inversely with dye concentration. Major outlets for wool/mohair blends are worsted outerwear and suitings. Such fabrics may be made from intimate blends for both warp and weft, but they often consist of a mohair warp with a botany wool weft. The enhanced lustre and good wear properties make these blends suitable for lightweight suitings and dresswear. Wool/mohair fabrics may be piece dyed with 1:2 premetallised dyes, or more economically with levelling acid dyes. If the blend is to be used in suitings, it is customary to dye the wool and mohair separately in sliver or top form for subsequent blending. The rate of dyeing and equilibrium exhaustion on mohair fibres are higher than for wool fibres of similar diameter. Visual and instrumental assessment of depth of shade, however, shows little difference between the two fibre types dyed separately with the same dyebath concentration of an acid dye. These measurements support the view that the pronounced surface lustre associated with mohair is responsible for its apparently slightly lower content of absorbed dye when compared with other less-lustrous wools dyed from the same bath [12]. Mohair and wool show a similar tendency to yellow as a result of aqueous oxidation or thermal treatments. Urea-bisulphite solubility data indicate that mohair suffers less modification under mild conditions but this position is gradually reversed with increasing severity of treatment [13]. Thus the loss in weight during aqueous treatment is ultimately greater for mohair than for wool. The dyeing of wool/polyurethane blends has much in common with the dyeing of blends of nylon with either of these fibres. Anionic dyes are used in a similar way with anionic agents (sections 3.2 and 3.3) to control the tendency of milling and 1:2 premetallised dyes to favour the polyurethane component. Chrome and metal-complex dyes generally give better wet fastness on wool/ polyurethane than milling acid dyes, but 1:1 metal-complex and levelling acid

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NYLON/WOOL AND OTHER AA BLENDS

dyes are unsuitable because of degradation of the polyurethane under the strongly acidic dyeing conditions required. Blends of acid-dyeable polypropylene staple with wool are of interest for tufted carpets, upholstery and certain apparel outlets, such as men’s socks. Both fibres are dyeable with anionic dyes and the blend gives a full handle at low fabric weight per unit area, only moderate formation of electrostatic surface charge and a high fibre–fibre frictional coefficient. This leads to softness of handle and a high resistance to pilling. Blends of 50:50 to 20:80 wool/aciddyeable polypropylene are also of growing importance in Axminster carpets [1]. It is difficult to control the distribution of anionic dyes between these fibres owing to their high substantivity but slow rate of diffusion in the synthetic component. At temperatures below about 75°C the polypropylene is only ringdyed. Only at 80°C and above does diffusion into the polymer matrix become appreciable and dyeing of the acid-dyeable fibrillar regions begin. The attainment of satisfactory solidity entails commencing dyeing at 75–80°C and pH 3 to 5, according to applied depth. The initial distribution favours the polypropylene surface but as the temperature approaches the boil the wool becomes heavily dyed. Penetration of the acid-dyeable regions of the synthetic fibre proceeds steadily at the boil and some improvement in solidity is possible at this stage with those dyes showing good migration properties. A weakly cationic levelling agent of the alkylamine polyoxyethylene type is recommended with metal-complex dyes and the level dyeing of milling acid and chrome dyes can be promoted by addition of a nonionic agent of the alkanol polyoxyethylene class. 6.3 BLENDS OF NYLON WITH OTHER ACID-DYEABLE FIBRES Elastomeric polyurethane yarns are mostly of interest to provide stretch properties for characteristic nylon outlets in the knitting industry. Warp-knit swimwear and sportswear, and Raschel-knit foundation garments, underwear, surgical hose and half-hose tops, are important outlets for nylon/polyurethane blends. Although the elastomeric fibre is often covered by the nylon in the relaxed fabric, penetration of the close construction during dyeing is essential because the elastomeric fibre is revealed on stretching the fabric. Careful control of processing tensions, as well as the pH, time and temperature of treatment, is necessary to preserve the optimum strength and elastic properties of polyurethane fibres. Nylon/polyurethane fabrics are normally scoured at 60–70°C with tetrasodium pyrophosphate and an anionic detergent. Disperse dyes diffuse into polyurethane fibres even more readily than into nylon or cellulose acetate, but the wet fastness properties of the dyeings are correspondingly low. Selected dyes give solidity on nylon/polyurethane, but for

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acceptable wet fastness these dyes can only be used for pale depths on hosiery or warp-knit stretch garments. The preferred dyes have good migration properties and are mainly monoazo yellows with anthraquinone reds and blues. Anionic dyes are widely used for solid effects on nylon/polyurethane. The factors influencing partition between these components have been outlined in section 3.3. Shadow effects are of no interest because the pale-dyed polyurethane is revealed only on stretching the fabric. Reserve or contrast effects are impracticable. Acid dyes are most useful for bright full depths and moderate depths of all hues. Levelling acid dyes are absorbed preferentially by the polyurethane at 40–60°C, but migrate in favour of the nylon at higher temperatures. Some milling acid dyes favour the polyurethane considerably and these dyes do not migrate readily to nylon. Metal-complex dyes generally give better wet fastness on polyurethane than most acid dyes but 1:1 metal-complexes are unsuitable because the strongly acidic dyeing conditions required would impair the physical properties of the elastomeric fibre. Duller and heavier depths are usually dyed with 1:2 metalcomplex dyes, but the economy offered by chrome dyes is still preferred in some instances. Chrome blacks give the best solidity and fastness. Poor migration is a problem with 1:2 metal-complex dyes and these dyes are generally more sensitive to dye-affinity variations in the nylon filament yarns, which often form the outer surface of a nylon/polyurethane fabric. Basic complexing agents such as ethylenediaminetetra-acetic acid or hydroxylamine derivatives are used as protecting agents to minimise the acidic degradation of polyurethane when dyeing nylon/polyurethane blends at low pH [14]. Acid dyes readily giving solid effects on nylon/polyurethane are mainly yellow to red monoazo monosulphonates and violet to blue anthraquinone monosulphonates with generally good levelling properties but only moderate wet fastness. The wet fastness of acid dyes on nylon/polyurethane can be improved with syntan aftertreatment. Alternatively, better wet fastness but only moderate migration and coverage properties can be achieved using selected yellow to red monoazo disulphonates, red to blue disazo disulphonates and blue to green disulphonated anthraquinone dyes. The preferred 1:2 metal-complex dyes are monoazo types with no more than one solubilising group in general. Most of the chrome dyes suitable for this blend are monoazo structures. There are more monosulphonates than disulphonates, but some have one or two additional carboxyl groups, one of which usually participates in complexing with the chromium atom. Many of the problems encountered in one-bath dyeing methods for wool/silk blends (section 6.2) are even more critical on nylon/silk. Experimental disperse

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NYLON/WOOL AND OTHER AA BLENDS

reactive dyes of the sulphatoethylsulphone type (Figure 6.2) have been evaluated by applying them to nylon and silk fabrics simultaneously [15]. Maximum uptake occurred at pH 8 on silk and at pH 6–8 on nylon. Optimum solidity between the two fibres was found at pH 6. The ratio of fixation to exhaustion was very high. Exhaustion and fixation increased slowly with dyeing time and temperature, even after 4 hours at 90°C, and the rates of dyeing were similar on the two fibres. Regrettably, no fastness values were recorded for these dyeings. R N

SO2CH2CH2OSO3H

N N

R CH3 or CH2CH2OH

R Figure 6.2 Experimental reactive disperse dyes for nylon/silk blends

6.4 DYEING METHODS AND DYE SELECTION FOR AA BLENDS The preferred dyeing method for all binary blends of acid-dyeable fibres is the application (Table 6.1) of a single class of anionic dyes, usually premetallised 1:2 Table 6.1 Dye selections for AA blends

Blend

Colour effect

Dyeing method

Dye selection

Wool/silk

Solid

Two-stage

Reactive dyes on silk, then milling acid and 1:2 metal-complex dyes on wool at 90°C

Silk reserve

Single-class

Monosulphonated milling acid dyes at pH 5–6

Wool/mohair

Solid

Single-class

Levelling acid or 1:2 metal-complex dyes

Wool/angora

Solid

Single-class

Milling acid and 1:2 metal-complex dyes with anionic retarder

Nylon/wool

Solid

Single-class

Monosulphonated 1:2 metal-complex and acid dyes with anionic retarder

Wool/polyurethane

Solid

Single-class

Milling acid and 1:2 metal-complex dyes with retarder

Nylon/polyurethane

Solid

Single-class

Chrome, 1:2 metal-complex or milling acid dyes with retarder

Wool/acid-dyeable polypropylene

Solid

Single-class

Chrome, 1:2 metal-complex or milling acid dyes at pH 3.5

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or milling acid types, with an anionic retarder to control uptake by the component that is dyed preferentially. This component is wool in its blends with silk or angora, but the other component in wool blends with mohair, nylon, polyurethane or acid-dyeable polypropylene. In nylon/polyurethane blends it is the polyurethane that is preferentially dyed in the early stage. It is possible to reserve silk in its blends with wool or nylon but in general there is virtually no interest in shadow, reserve or contrast effects on AA blends.

6.5 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

S Roberts, Dyer, 178 (Jun 1993) 10. J Park, Carpet Manufacturer Int., (Autumn 1987) 21. D Schwer, H Ritter and K Zesiger, Textilveredlung, 16 (1981) 479. T L Dawson, Rev. Prog. Coloration, 15 (1985) 29. H S Freeman, J Sokolowska-Gajda, A Reife, Z D Claxton and V S Houk, AATCC International Conference and Exhibition, (Oct 1993) 254. J Sokolowska-Gajda, H S Freeman and A Reife, AATCC International Conference and Exhibition, (Oct 1994) 388. H L Needles and I Weatherall, Text. Chem. Colorist, 24 (Dec 1992) 7. J H Qian and Z T Song, Proc. 7th Int. Wool Text. Res. Conf., Tokyo (1985) 249. R Rohrer, Textilveredlung, 20 (1985) 85. J A Galek, Dyer, 163 (22 Feb 1980) 133. F Sakli, M van Parys, R Dubois and J Knott, Melliand Textilber., 69 (1988) 191. M B Roberts and E Gee, SAWTRI Bull., 11 (Sep l977) 32. M B Roberts, SAWTRI Tech. Report, 351 (1977). D Schwer, Textilveredlung, 23 (1988) 296. M Dohmyou, Y Shimizu and M Kimura, J.S.D.C., 106 (1990) 395.

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WOOL/ACRYLIC AND OTHER AB BLENDS

CHAPTER 7

Wool/acrylic and other AB blends

7.1 DYEING OF WOOL/ACRYLIC BLENDS The dimensional stability, strength and abrasion resistance of wool/acrylic blends (usually 50:50 to 20:80) are superior to those of all-wool fabrics. They are of particular interest for garments in which thermal insulation is important, as in woven or knitted sweaters, skirts and outerwear. Blends containing the coarser quality wools are used in blankets and floor coverings. Acrylic fibres dyed in gel form or as loose stock before blending with wool are stable to conventional wool processes, such as milling, decatising and blowing. Acrylic fibres may also be blended with mohair, angora or silk to lower costs and improve physical performance. Wool/acrylic blends may be carbonised to remove vegetable debris from the wool and it is sometimes possible to carbonise after dyeing. Basic dyes showing little or no change of hue when subjected to carbonising with sulphuric acid, followed by prompt neutralisation, are mainly yellow, orange and red methine dyes with selected azo reds and anthraquinone blues. Certain other basic dyes change colour substantially on carbonising, however. Although a subsequent treatment with ammonia at 40°C will fully restore the original hue there is invariably a loss in depth. Worsted-spun wool/acrylic yarns are scoured at pH 5 and 60°C with a nonionic detergent. Woollen-spun yarns are scoured at 30°C with soda ash and an anionic detergent. Blend yarns containing high-bulk acrylic fibres should be fully relaxed before dyeing by autoclave treatment in saturated steam or by immersion in boiling water in the dyeing vessel. Blend fabrics are prepared for dyeing by conventional techniques for all-wool materials but precautions should be taken to allow for the thermoplastic properties of the acrylic fibres. Relaxation to remove inherent tensions is important, particularly if the fabric is to be subsequently steamed or pleated. Blends in the AB category are of particular interest for reserve or colour

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contrast effects because the fibre components are dyeable with ionic dyes of appropriate charge. Good solidity is also important, however, and can be obtained without difficulty when required. Although either component can be reserved, it is often more convenient to reserve the acrylic fibre using anionic dyes. The range of bright colour contrasts is much wider on AB blends than on all other types of binary blend because the fibres carry opposite charges and ionic dyes are much more selective than disperse dyes. The opposite charges carried by the dyes, however, can lead to incompatibility in one-bath dyeing (section 4.3). Disperse dyes are of little interest for the acrylic component of wool/acrylic blends because of inadequate fastness and severe staining of the wool. It is necessary that the dyed material has adequate fastness to pleating, pressing and ironing, particularly in the case of knitwear and jersey fabrics. Combinations of disperse and acid dyes can only be used in pale depths. Because of the relatively slow rate of absorption of disperse dyes by acrylic fibres, these often give rise to considerable staining of wool (section 3.4). There is markedly less cross-staining by basic dyes because these have much higher affinity for acrylic fibres. Dispersion stability is much more important in package or beam dyeing, as is the limited efficiency of clearing treatments. Wool/acrylic blends can be readily dyed to reserve the acrylic fibre. It is less convenient to reserve the wool because of the cross-staining by basic dyes (section 3.5). Solidity of shade is often required in dyeing these blends for dresswear or knitwear and is invariably specified for carpet yarns. Solidity is readily obtained by applying combinations of anionic and basic dyes with an anti-precipitant, either at pH 2–3 for 1:1 metal-complex types or at pH 6–7 for milling acid dyes or 1:2 metal-complexes, with or without sulpho groups. Cationic retarders are not required, except for pastel shades, as the anionic dyes exert a marked retarding effect. Mildly cationic agents of the alkylamine polyoxyethylene type form water-soluble complexes with 1:1 metal-complex dyes under strongly acidic conditions. The alternative method of improving compatibility by complexing the basic dyes with an anionic retarder gives better control of the rate of dyeing of the acrylic component (section 4.3). For pale depths the dyebath is set at 50°C and pH 4–5 with acetic acid, salt, an anionic retarder and an alkanol polyoxyethylene anti-precipitant. The anionic and basic dyes are added separately and then the temperature is raised and held at 90°C to slow down the rate of uptake of the basic dyes. Finally, the wool component is dyed to shade with the anionic dyes at the boil. Better compatibility is found in intermediate depths by commencing at 50°C and pH 6– 7 with salt, nonionic anti-precipitant and neutral-dyeing anionic dyes. After

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raising slowly to 80°C to promote level dyeing of the wool, acetic acid (to give pH 4–5), an anionic retarder and the basic dyes are added and dyeing of the acrylic component completed at the boil. In those instances where adding the basic dyes at 80°C gives levelling problems, particularly when 1:1 metal-complex dyes are being used on wool with an alkylamine polyoxyethylene complexing agent, it may be preferable to set the initial dyebath at 60°C and pH 2–3 (sulphuric acid) with the basic dyes and complexing agent, and to raise the temperature to 80°C before adding the premetallised dyes. Under these conditions the rates of dyeing of the two classes of dyes are more closely synchronised in the final stage at the boil. The zwitterionic character of the 1:1 metal-complexes at low pH confers greater compatibility with basic dyes than the more anionic neutral-dyeing dyes. At one time, wool/acrylic blends were mainly dyed by two-bath methods in medium or heavy depths to avoid cross-staining of wool or dye precipitation problems. Two-stage processes are now usually employed with the basic dyes applied first at the boil, followed by cooling to 60°C, adding the anionic dyes and completing the wool dyeing at the boil. Heavy depths may still be obtained by a two-bath sequence with the basic dyes and a cationic retarder at the boil and pH 5 in the first stage. After an intermediate scour with nonionic detergent at 80–90°C, or with acidified formaldehyde-sulphoxylate at 70–75°C if necessary to clear the basic dye stain, the wool component may be dyed to shade at the boil and pH 6–7 with 1:2 metal-complex or milling acid dyes in the presence of an alkylamine polyoxyethylene levelling agent. The degree of staining of conventional acrylic fibres by reactive, premetallised or milling acid dyes is very slight and does not present a problem in the dyeing of wool/acrylic blends. Because of this, when dyeing union fabrics where light and dark contrasting colours are present in the design, e.g. black/yellow, it is usual to dye the wool to the darker colour where possible [1]. On the other hand, since wool contains amino acid residues with carboxyl-containing sidechains, as well as C-terminal end groups that are ionised under the mildly acidic dyeing conditions usually used for these blends, sites are available for the sorption of basic dyes (Figure 7.1). Thus there will always be some sorption by wool and the extent to which this occurs is of considerable practical interest. At the boil, decomposition products of the wool are produced, particularly at pH 7 or above, that have a reducing action on certain basic dyes, thus necessitating careful dye selection [2]. Basic dyes of the localised-charge monoazo and anthraquinone types are widely used, but cyanine and oxazine types are also important. The staining of wool by basic dyes is particularly troublesome in the early stage of dyeing at

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DYEING OF WOOL/ACRYLIC BLENDS

NH CH CH2COO– CO Aspartic acid residue

89

R CONH

CH

COO–

C-terminal residue

Figure 7.1 Sites in wool for sorption of basic dyes

temperatures below 80°C. As the boil is approached, transfer of basic dyes from wool to the acrylic component proceeds, so that the wool exerts an effective retarding influence on the acrylic dyeing process (section 3.5). This migration and hence the final distribution of the basic dyes depends on many factors, including the presence of surfactants (anti-precipitants and levelling agents), electrolyte, time, temperature and pH. Thus the degree of wool staining is determined by dyebath conditions and is minimised by dyeing for at least one hour at the boil and pH 5. Nonionic auxiliaries promote transfer to the acrylic fibre but cationic products tend to increase staining of the wool. Certain basic dyes of small molecular size exhibit exceptionally good migration properties and are reasonably compatible with anionic dyes, allowing minimal use of nonionic anti-precipitant [3]. These dyes have proved especially suitable for dyeing wool/acrylic blends. IWS fastness requirements must be met for machine-washable performance of shrink-resist garments made from these blends [4]. Dye selection for both components is important. Basic dyes should be used on the acrylic fibre and reactive, premetallised or chrome dyes for the wool. Mordant dyes are usually chosen for black and navy blue on economic grounds, a pH of 3.5 being used for chroming. The use of basic dyes and α-bromoacrylamide or vinylsulphone reactive dyes is a popular one-bath method, as a wide range of bright hues can be obtained with optimum fastness [4,5] and simultaneous yarn bulking and dyeing is possible. Pale dyeings can be produced with mixtures of reactive or selected 1:1 metal-complex dyes and basic dyes at pH 4–5 together with special levelling agents that also function as precipitation inhibitors. In medium and full depths, ammonia treatment after dyeing is recommended to complete the fixation of reactive dyes on the wool and give dyeings of good fastness to perspiration [6]. Problems of interaction with basic dyes are encountered in the one-bath dyeing of these blends in the following order of increasing difficulty: chrome < reactive < 1:1 metal-complex < levelling acid < milling acid < sulphonated 1:2 metal-complex < unsulphonated 1:2 metal-complex. Hence chrome and reactive dyes give the best compatibility with basic dyes and can be used for wool in the

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isoelectric region at pH 4–5. The high degree of exhaustion of chrome dyes leads to virtually complete avoidance of precipitation. With chrome dyes the oxidative chromium compounds present in the aftertreatment bath help to counteract any tendency for the basic dyes to be decomposed by any reductive products formed in wool at the boil. 7.2 DYEING OF NYLON/ACRYLIC BLENDS Nylon/acrylic blends are used mainly for half-hose, knitted sweaters, sportswear and swimwear, blankets, furnishing fabrics and floor coverings. These have a wool-like handle and appearance, but better tensile strength, abrasion resistance and durability. Nylon contributes more to the strength, extensibility, wrinkle recovery and resistance to wear, whilst the acrylic fibre confers softness, bulk and warmth. Blends of acrylic fibres with nylon are more durable but less bulky than wool/acrylic blends. Nylon/acrylic blends (typically 20:80) have been popular in carpets for many years, particularly for Axminster designs [7]. The physical characteristics of Dralon U325(BAY) acrylic fibre in blended carpet yarns have been described. This modified fibre has high substantivity for basic dyes and virtually complete exhaustion of the dyebath can be achieved in a short time at 80–85°C [8]. It is suitable not only for yarn dyeing but also for piece dyeing on a carpet winch or jet machine. Blends containing 20–30% nylon for carpets can be dyed on winches or atmospheric jets with basic and acid dyes in the presence of a cationic retarder, a migration assistant and a nonionic anti-precipitant. Thus velour carpets tufted from 30:70 nylon/Dralon yarn can be dyed by first applying acid dyes to the nylon as the temperature is raised to 70°C, then adding the basic dyes and heating to 80–85°C to dye the Dralon U325 component. Although these carpets have good resistance to pile deformation during dyeing, slow indirect cooling to 60°C is recommended at the end of the dyeing cycle [8]. Nylon/acrylic apparel fabrics are normally scoured with dilute ammonia and an alkylphenol polyoxyethylene detergent at 50–60°C and then dyed by a onebath method. Thus selected levelling acid dyes may be complexed with a weakly cationic alkylamine polyoxyethylene and added to a bath containing Glauber’s salt and acetic acid at pH 4–5 and 40°C before the basic dyes. After the acid dyes have become absorbed by the nylon at 80–85°C, the dyebath is heated to the boil and the acrylic component dyed to shade with the basic dyes. Any cross-staining of nylon by the basic dyes may be cleared using acidified formaldehydesulphoxylate at pH 4–5 and 70°C. An alternative one-bath method is based on a similar temperature/time cycle but the initial dyebath is set at pH 2–3 (sulphuric acid) with the basic dyes, an

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alkanol polyoxyethylene sulphate complexing agent and an alkanol polyoxyethylene anti-precipitant. When the temperature reaches 80°C, selected mono- or disulphonated monoazo 1:1 chromium-complexes are added and heating continued slowly to the boil. Full depths are usually dyed by a more prolonged two-stage process. The acrylic fibre is first dyed with the basic dyes at the boil and pH 4–5. The dyebath is then cooled slowly to 70°C and adjusted to pH 2–3 with sulphuric acid. The 1:1 metal-complex dyes and an alkylamine polyoxyethylene complexing agent are added and the nylon component dyed to shade at the boil. The opposite sequence is preferred for full depths dyed with 1:2 metalcomplex dyes. The nylon is dyed first at the boil with the selected premetallised dyes in dilute ammonia and the alkanol polyoxyethylene sulphate complexing system. After cooling the dyebath slowly to 80°C, the acrylic fibre is dyed conventionally with basic dyes at the boil and pH 4–5. If a two-bath method is preferred for optimum fastness and freedom from any risk of co-precipitation, the basic dyes are applied first at the boil and pH 4–5. After an intermediate rinse, a fresh dyebath is set at pH 6–7 and the nylon is dyed at 80–85°C with neutral-dyeing 1:2 metal-complex or milling acid dyes. Polyurethane/acrylic blends for sweaters and leisurewear are usually made from core-spun yarns and complete solidity is not essential. Basic dye stains show low fastness on the polyurethane component, however, so that it is often necessary to use a two-bath method. Selected basic dyes are applied to the acrylic fibre at the boil from a near-neutral bath and an intermediate clear is given using sodium dithionite, soda ash and a nonionic detergent. The polyurethane is then filled in with selected neutral-dyeing 1:2 metal-complex or milling acid dyes. Some chrome dyes are also suitable on the polyurethane if heavy, dull dyeings are required. 7.3 BLENDS OF ACID-DYEABLE AND BASIC-DYEABLE ACRYLIC VARIANTS Conventional acrylic fibres made from acrylonitrile and up to 15% of an inert comonomer (section 5.1) are readily dyeable with basic dyes at the boil. Aciddyeable acrylic variants contain basic comonomer units that provide sites for sorption of anionic dyes. Blends of acid-dyeable and basic-dyeable acrylic fibres are used in typical wool outlets, such as jersey dresswear, sweaters, hand-knitting yarns, blankets and pile fabrics. Basic-dyeable and acid-dyeable acrylic fibres both absorb basic dyes under neutral conditions, but the uptake of basic dyes by the acid-dyeable variant decreases markedly as the pH falls from 7 to 2. The exhaustion of these dyes on

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basic-dyeable acrylic fibres is almost independent of pH over this range. Basic and acid dyes, therefore, can be applied selectively to the two types of fibre at pH 2 because most basic dyes are stable and reserve the acid-dyeable variant reasonably well under these conditions. Chlorite bleaching of blends containing acid-dyeable acrylic fibres results in increased uptake of basic dyes. It is usual to prescour with an alkanol polyoxyethylene at 60–70°C before dyeing. Blends of acid-dyeable and basic-dyeable acrylic fibres offer interesting possibilities for solid, reserve or colour contrast effects. The basic-dyeable variants can only be dyed with disperse or basic dyes, whereas acid-dyeable types can be dyed with either of these classes as well as direct, levelling acid, premetallised and chrome dyes. The inherent wet fastness of anionic dyes on acid-dyeable acrylic fibres is far superior to that of the same dyes on wool or nylon, or of direct dyes on cellulosic fibres [9]. The 1:2 metal-complex dyes are not used extensively but selected monoazo 1:1 metal-complex types (Mr 450–550) are widely used for deep shades of good fastness to light and wet treatments. These and the levelling acid dyes for brighter shades are applied with sulphuric acid at pH 2–3 to obtain full yield, penetration and fastness. Chrome dyes of the monoazo monosulphonate type (Mr 350–450) can also be used for dark shades of good fastness to light, washing and pleating. Chrome dyes sensitive to low pH should be avoided, as a strongly acidic dyebath is essential for optimum yield. Disperse dyes with good dyeing properties and adequate pleating fastness on acid-dyeable acrylic fibres are the intermediate-energy nitro, monoazo and anthraquinone types (Mr 300–400). Basic dyes exhibit similar fastness properties on both types of acrylic fibre but give higher yields and dye more rapidly on the basic-dyeable fibre, requiring a cationic retarder to control the rate of absorption. Shadow effects are achieved at neutral pH but at pH 2 all of the basic dyes are absorbed by the basic-dyeable variant, leaving the acid-dyeable fibre reserved. Basic dyes stable to strongly acidic conditions and giving effective reservation of the acid-dyeable variant are almost all delocalised-charge types, including yellow, orange and red methines or monoazothiazole derivatives and oxazine blues. Since basic-dyeable acrylic fibres have no affinity for acid dyes it is possible to reserve them in blends with acid-dyeable variants. The wet fastness of acid dyes on these fibres is superior to that on wool but light fastness is sometimes inferior. Levelling acid dyes that offer optimum light fastness and good reservation of the basic-dyeable component are mainly mono- or disulphonated monoazo or anthraquinone derivatives. The recommended method of application is at the boil and pH 2–3 with

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sulphuric acid and Glauber’s salt to obtain adequate penetration and fastness of these dyes. Careful control of temperature rise is necessary because the rate of strike is rapid and the migration properties on these fibres are relatively poor. An alkylamine polyoxyethylene levelling agent helps to decrease the initial rate of dyeing. The dyebath is cooled back to 80°C before shading additions are made. Reserve of the acid-dyeable component is obtained with basic dyes and a suitable anionic retarder at pH 2. Dyes of the methine, cyanine, monoazo, oxazine and anthraquinone types are usually satisfactory. Colour contrasts can be produced on these blends with suitable combinations of basic and levelling acid dyes by a one-bath method, although the most economical effects are those with a deep shade on the acid-dyeable type and a paler depth on the basicdyeable component. Complexing between the basic dyes and a combination of anionic and nonionic anti-precipitants, or between the acid dyes and a weakly cationic alkylamine polyoxyethylene (section 4.3), must be adopted in order to minimise the risk of co-precipitation and each of these measures exerts a retarding influence on the corresponding class of dyes. Specific retarding agents for the basic dyes should be avoided if possible when dyeing solid or contrast effects on this type of blend. Cationic retarders are preferentially absorbed by the basicdyeable fibre and this may impair the development of crimp in high-bulk yarns. Absorption of an anionic retarder by the acid-dyeable variant may cause restraining of the acid dyes in heavier depths. The levelling acid dyes are applied at 80°C and pH 2 with Glauber’s salt and an alkanol polyoxyethylene anti-precipitant. The basic dyes are then added and both components dyed to shade at the boil. Scouring at 70°C with an alkanol polyoxyethylene detergent clears any stain of basic dyes from the acid-dyeable variant. Full depths are dyed by a two-stage method. The acid-dyeable component is first dyed at the boil and pH 2–3 with the levelling acid dyes and salt. After cooling to 60–70°C, the basic dyes, nonionic anti-precipitant and more salt are added and the basic-dyeable fibre is dyed to shade at the boil.

7.4 BLENDS OF MODACRYLIC AND ACRYLIC FIBRES Modacrylic fibres, e.g. Dynel (Union Carbide), that contain less than 85% acrylonitrile with other inert or basic comonomers, form blends with acrylic fibres that are mainly used in traditional wool outlets. Blends of Dynel with an acrylic fibre in pile fabrics and floor coverings are less flammable than the acrylic fibre alone. There is a legal requirement to have at least 15% modacrylic fibre in an acrylic carpet. A blend of 50:50 modacrylic/acrylic fibres may be dyed by a

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one-bath method at the boil with disperse dyes for the modacrylic component and basic dyes for the acrylic fibre. A dependent range of colour contrasts with light fastness 5 can be obtained in this way, although the fastness ratings of the basic dyes on acrylic are approximately 1 to 1.5 units higher than the disperse dyes on the modacrylic fibre. Dynel is delustred during dyeing at temperatures above 80°C but the lustre can be restored by subsequent treatment at a higher temperature, i.e. by drying at 120–130°C or by hydrosetting. For example, after dyeing at 80°C the lustre is restored at 105°C. After dyeing at the boil a treatment at 120°C is required, and if dyed under pressure at 105°C the lustre returns at 130°C. Increased quantities of salt in the dyebath may also be used to maintain lustre in circumstances where the adoption of higher temperatures in drying or hydrosetting is not acceptable. Blends of modacrylic fibres with conventional acrylic fibres may be dyed in solid or shadow effects with basic dyes applied to both components by a twostage method. The acrylic component is dyed preferentially by temperature control in the absence of a retarder, raising slowly to the boil to avoid unlevelness. When the target depth has been reached on the acrylic fibre, the dyebath is cooled to 80°C, a butyl benzoate carrier is added and the modacrylic fibre dyed to shade at the lower temperature. Blends of modacrylic fibres with acid-dyeable acrylic variants provide more scope for reserve and contrast effects. Selected disperse dyes will give satisfactory solidity in pale or medium depths but fastness ratings are barely adequate. Reserve effects or bright colour contrasts in moderate or full depths are obtained by methods similar to those already outlined for acid-dyeable/basic-dyeable acrylic blends (section 7.3). 7.5 BLENDS OF AMIDE FIBRES WITH MODACRYLIC OR ACIDDYEABLE ACRYLIC VARIANTS Further AB blends of minor importance are those containing an amide fibre (wool, silk, mohair or nylon) with a modacrylic or acid-dyeable acrylic variant. Dynel (Union Carbide) is a modacrylic fibre made from acrylonitrile and vinyl chloride. The controlled shrinkage properties of the fibre can be turned to practical use in the manufacture of bulked fabrics and of backing yarns in knitted pile fabrics. Dynel has been blended with amide fibres and used in pile fabrics intended for apparel or furnishings. Blends containing 75:25 Dynel/mohair are scoured and dyed at 80°C, care being taken to avoid fabric distortion. Basic dyes are used on the modacrylic fibre and then milling acid dyes on the mohair. Butyl benzoate is recommended to give adequate colour yields on Dynel at 80°C. Pale and many medium depths can be achieved by a two-stage sequence but for deep shades it is advisable to

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operate a two-bath method with an intermediate clear of the basic dyes from the mohair. The basic dyes must be selected for minimum staining of mohair. Clearing is carried out at 60°C with a mildly acidic solution of zinc formaldehyde-sulphoxylate (Figure 7.2). HO– Zn2+

O S O CH2OH

Figure 7.2 Zinc formaldehyde-sulphoxylate

Possible dyed effects on Dynel/wool include solidity of shade, shadow or contrast effects, normally obtained by one-bath methods. Selected basic dyes are applied to the modacrylic fibre and 1:2 metal-complex or milling acid dyes to the wool. A butyl benzoate carrier for Dynel, a levelling agent and an antiprecipitant are also required. The fabric is first treated with these agents at 40°C and the dissolved anionic dyes added. The temperature is raised to 70°C, the pH lowered to 4–5 (acetic acid) and the basic dyes added gradually before raising the temperature slowly to the boil. Verel (Eastman) is a modacrylic fibre of high flame resistance and a soft handle. It has been blended with wool for use in pile fabrics and floor coverings, in 30:70 blends with wool in knitwear and in 50:50 blends for half-hose. The dyes used include disperse, basic and 1:2 metal-complex dyes. The premetallised dyes give satisfactory fastness to light and washing for apparel or furnishing fabrics. Verel is preferably dyed at 80–90°C, or at 70–80°C in the presence of an organophosphate carrier. If it is dyed at or near the boil, however, loss of lustre and deformation of the fibre take place, resulting in fabric creasing. Verel and wool can be dyed simultaneously with selected 1:2 metal-complex dyes. Wool can also be dyed and Verel reserved using 1:1 metal-complex types applied at a low pH. Fabrics containing 50:50 Dynel/nylon may be dyed in colour contrasts or with reservation of the modacrylic component. Those acid dyes for nylon with least affinity for Dynel, mainly disulphonated premetallised or milling acid dyes of Mr 700–800, are preferred, a levelling agent being recommended with the 1:2 metalcomplex types. Suitable basic dyes for Dynel showing good reserve of nylon are delocalised-charge structures of the azomethine, azothiazole and azotriazole classes. The nylon component of blends with acid-dyeable acrylic fibres may be dyed with selected acid dyes that show good neutral-dyeing affinity to reserve the acrylic variant. Nylon can be almost completely reserved by applying basic dyes in the presence of 3% urea after dissolving with the aid of methanol or a nonionic surfactant. The dyebath pH changes from slightly acidic to slightly

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alkaline during dyeing. Solid or contrast effects are achieved by a one-bath method with selected milling acid and basic dyes in the presence of a nonionic anti-precipitant. In general, acid-dyeable acrylic fibres will readily absorb the dyes usually applied to wool but the basic groups in the acrylic variant are less strongly basic than those in wool and adequate dyebath exhaustion requires appreciably higher concentrations of acid. Under normal wool-dyeing conditions a heavier depth is attained on the wool, giving a shadow effect. Solidity of shade can be achieved, however, by adding a nonionic agent that controls the rate of dyeing of the wool as well as promoting levelness. The most critical factor is the acid concentration needed to achieve solidity in the presence of the retarder. Suitable dyes include selected milling acid or chrome dyes, as well as certain premetallised types. Wool may be dyed and the acid-dyeable acrylic reserved by applying milling acid or 1:2 metal-complex dyes with good neutral-dyeing affinity in the presence of a weakly cationic levelling agent. The best reserve effect is obtained at 90–95°C. Basic dyes can be applied to the acrylic component at low pH if a limited range of colour contrasts is desired.

7.6 BLENDS OF BASIC-DYEABLE POLYESTER WITH WOOL OR NYLON Although more costly than normal polyester, the basic-dyeable variant (section 5.1) can be used in blends with nylon or wool to achieve a wider and more attractive gamut of coloured effects, since the problems of cross-staining of wool or nylon by disperse dyes are absent. One-bath methods using basic and acid dyes are available for bright colour contrasts or reserve styles. Solid or shadow effects can also be obtained without difficulty, but are less often required. Disperse dyes show lower light fastness on the basic-dyeable variant than on SO3H

COO CH2CH2

OOC

COO CH2CH2

OOC

SO3H COO CH2CH2

OH + HOOC

+ HO

CH2CH2

OOC

COOH

Scheme 7.1

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normal polyester and basic dyes on basic-dyeable polyester are significantly less fast to light than on acrylic fibres. A blend of basic-dyeable polyester and nylon can be dyed with excellent contrast by the following one-bath method. After scouring at 60–70°C with soda ash and a nonionic detergent, the dyebath is prepared at 50°C and pH 5 with an alkanol sulphate as a retarder for the basic dyes, an alkanol polyoxyethylene anti-precipitant, Glauber’s salt to inhibit hydrolysis of the basicdyeable variant (Scheme 7.1), and finally the acid dyes and the basic dyes separately. The temperature is raised slowly from 80°C to the boil and the two components dyed simultaneously. Similar methods have been devised for blends of basic-dyeable polyester with wool [10]. A reserve effect on either fibre can be obtained using the above procedure by simply omitting the appropriate class of dyes from the recipe. When using basic dyes only, the reserve of the wool or nylon can be improved by a final treatment with sodium dithionite, ammonia and a nonionic detergent at 50°C. Table 7.1 Dye selections for AB blends Blend

Colour effect

Dyeing method

Dye selection

Wool/acrylic

Acrylic reserve

Single-class

Reactive, metal-complex or milling acid dyes

Solid or contrast

One-bath (pale depths)

Premetallised or milling acid dyes and basic dyes with anti-precipitant

Two-stage (full depths)

Basic dyes with retarder, then 1:2 metal-complex or milling acid dyes

One-bath (machinewashable)

Reactive dyes and migrating basic dyes with levelling/stabilising agents

One-bath (pale depths)

Levelling acid or 1:1 metal-complex dyes and basic dyes with anti-precipitant

Two-stage (full depths)

Basic dyes at pH 4–5, then premetallised dyes at appropriate pH

Nylon/acrylic

Solid or contrast

Polyurethane/ acrylic

Solid

Two-bath

Basic dyes, reduction clear, then 1:2 metal-complex or milling acid dyes

Wool/modacrylic

Solid or contrast

One-bath

Premetallised or milling acid dyes and basic dyes with carrier and anti-precipitant

Mohair/ modacrylic

Solid or contrast

Two-stage

Basic dyes with carrier, then milling acid dyes at pH 6–7

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Table 7.1 Continued Blend

Colour effect

Dyeing method

Dye selection

Nylon/modacrylic

Modacrylic reserve

Single-class

Disulphonated 1:2 metal-complex or milling acid dyes

Contrast

One-bath

Premetallised or milling acid dyes and basic dyes with anti-precipitant

Acid-dyeable reserve

Single-class

Delocalised-charge basic dyes with anionic retarder at pH 2

Basic-dyeable reserve

Single-class

Monoazo 1:1 metal-complex or levelling acid dyes at pH 2–3

Solid or contrast

One-bath (pale depths)

Levelling acid dyes and basic dyes with anti-precipitant at pH 2–3

Two-stage (full depths)

Levelling acid dyes, then basic dyes

Solid

Single-class

Basic dyes at the boil, then at 80°C with carrier

Solid or contrast

One-bath

Disperse dyes and basic dyes

Acrylic reserve

Single-class

Premetallised or milling acid dyes at 90°C

Solid or shadow

Single-class

Selected chrome, metal-complex or milling dyes

Limited contrast

Two-stage

Basic dyes at low pH, then premetallised or milling acid dyes at pH 7

Acrylic reserve

Single-class

Premetallised or milling acid dyes at pH 6–7

Nylon reserve

Single-class

Basic dyes with urea

Solid or contrast

One-bath

Milling acid dyes and basic dyes with anti-precipitant

Nylon reserve

Single-class

Basic dyes with cationic retarder

Polyester reserve

Single-class

Premetallised or milling acid dyes at pH 6–7

Contrast

One-bath

Acid dyes and basic dyes with anti-precipitant

Acid-dyeable/ basic dyeable acrylic

Modacrylic/acrylic

Wool/acid-dyeable acrylic

Nylon/acid-dyeable acrylic

Wool or nylon/ basic-dyeable polyester

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7.7 DYEING METHODS AND DYE SELECTION FOR AB BLENDS This range of versatile blends offers valuable opportunities for bright colour contrast, shadow or reserve effects with relative freedom from cross-staining problems. One-bath methods, normally using basic dyes and acid dyes with a nonionic anti-precipitant, are available in most cases to give solid or contrast effects, although two-stage procedures may be preferred for full depths on the various blends with acrylic variants, the latter component being dyed first (Table 7.1). Excellent white reserve effects are attainable on all the synthetic fibre components in these blends. Reactive dyes are recommended to meet machinewashable standards on wool/acrylic blends.

7.8 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

D R Lemin. J.S.D.C., 91 (1975) 168. W Haertl, Textil Praxis, 44 (1989) 285; Melliand Textilber., 70 (1989) 354; Textilveredlung, 24 (1989) 214. R Parham, Am. Dyestuff Rep., 71 (Sep 1982) 42. H Flensberg and A Laepple, Textilveredlung, 26 (1991) 342. R Hüls, Textilveredlung, 10 (1975) 399. W G Prinzel, Textilveredlung, 18 (1983) 230. T L Dawson, Rev. Prog. Coloration, 15 (1985) 29. R Block and J Honsel, Chemiefasern und Textilind., 34/86 (1984) 345. K Nagawa, Japan Textile News, No. 251 (Oct 1975) 74. J Park and S Davis, J.S.D.C., 89 (1973) 37.

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CHAPTER 8

Wool/cellulosic and other AC blends

8.1 DYEING OF WOOL/CELLULOSIC BLENDS The wool/cotton blend is superior in durability to all-wool fabrics but there is a loss in other desirable characteristics, such as handle, drape, pleat retention and crease recovery. There is a resurgence of interest in blending these two natural fibres throughout the developed world, where such blends in garments traditionally made from cotton are seen as conferring desirability and exclusivity in high-quality dresswear and shirting fabrics [1]. Developments in shrink-resist treatment of the wool component have greatly improved the washability of such materials, which offer value, comfort, versatility and styling. Typical examples of traditional union fabrics with a cotton warp and a woollen weft include blazer cloths, gabardine rainwear, shirtings and pyjamas. The traditional ‘linsey-woolsey’ fabric, closely woven for household or apparel uses, was made from a linen warp to give strength and a wool weft to provide the aesthetic qualities of the construction. Blended worsted yarns containing approximately equal proportions of wool and cotton have been long-established in knitwear, dresswear, underwear, children’s clothing, lightweight shirtings, pyjama cloths and blankets. For washable wear such blends have been stabilised by a gaseous chlorination treatment. The original Viyella shirt fabric was a wool/ cotton blend. The 20:80 wool/cotton yarn is the best for achieving washable apparel without the use of chlorinated wool. Draw blending and intimate blending yield different fabric properties but both give satisfactory dyeing and finishing performance in woven or knitted constructions. Finishing techniques and formulations have been optimised to provide fully washable performance with easy-care and low shrinkage properties [2]. Attractive pile fabrics are made with a cotton backing cloth and a worstedspun pile yarn of either mohair or wool. Mohair gives higher lustre and better crush resistance than wool. Fabrics of this kind have been used to cover toys and as outerwear. Grey cotton is often chosen as a backing cloth unless a white is

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specified, when the fabric is peroxide bleached to minimise any risk of damage to the pile. After scouring on the winch, mohair pile fabrics are dyed in open width. Higher tensile strength and lower shrinkage on washing are positive attributes contributed by viscose to blends with wool. Traditional outlets for wool/viscose blends include lightweight suiting, outerwear, dresswear, knitwear, blankets and floor coverings. These are usually made from intimate blends of the two fibres but outerwear, dresswear and knitwear may include two-fold or fancy yarns for novelty effects. There has never been more than limited interest in blends of silk with viscose, usually encountered in dresswear if at all. Economy-priced blazer cloths have been made with a viscose warp and a wool/viscose blended weft. Pile fabrics for low-cost apparel are sometimes made with a wool pile and a viscose backing cloth. Ramie has distinctive features that do not appear to be fully recognised. The tensile strength is high and an outstanding feature is the high wet strength [3]. Attractive lustre and good abrasion resistance make the fibre applicable in numerous outlets. Blends of 70:30 wool/ramie have been used for dress fabrics. The satisfactory dimensional stability of these blends makes them suitable as shirtings, although the texture is somewhat heavy and firm. Such blends may give rise to ‘prickle-itch’ problems, however, when worn next to the skin. Ramie has dyeing properties similar to those of cotton and these blends are normally piece dyed by a one-bath method using direct and milling acid dyes. Traditionally, solid shades on blended wool/cotton apparel fabrics and wool/ viscose woven carpeting were dyed with prepared mixtures of direct and acid dyes called ‘union dyes’, selected to give matching colours on the two fibres with good fastness to light. Some dyers preferred to formulate their own recipes, however, containing direct dyes selected to dye both components together with acid dyes to adjust the shade on the wool. Self-levelling or temperaturecontrollable direct dyes with a low degree of sulphonation are substantive to both substrates, whereas multisulphonated salt-controllable types often tend to reserve the wool [4]. Dyeing under slightly acidic conditions with ammonium sulphate at 90°C gives optimum partition of the direct dyes. The wet fastness achieved in this way is largely determined by that of the direct dyes on the cellulosic component but some improvement is possible by aftertreatment with a cationic dye-fixing agent [5]. Mohair/cotton pile fabrics are dyed in open width with direct and milling acid dyes from the same bath, or by a two-stage process involving application of levelling acid dyes to the mohair pile from a sulphuric acid bath, followed by dyeing of the cotton backing with suitable direct dyes. The latter method ensures

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a superior handle and appearance of the pile but some acidic hydrolysis of the cotton may occur in the first stage. Blends containing chlorinated or stripped wool, or wool damaged in wet processing, are likely to offer difficulty in union dyeing. Blends of chlorinated or damaged wool with cotton are more difficult to dye level than those containing virgin wool because of more rapid absorption of direct dyes by the wool. It is necessary to add an anionic retarding agent to slow down this absorption. Wet fastness on chlorinated or damaged wool is also inferior to that on the intact substrate. Pale or medium depths in bright shades on cotton and chlorinated wool can be achieved with high-reactivity dyes applied at low temperatures, however, together with an anionic retarder. Solidity of shade is normally required on intimate blends of wool and viscose for dresswear, knitting yarns and carpets, but in knitwear and dresswear the two fibre types are sometimes dyed in contrasting colours. One-bath dyeing with combinations of direct and acid dyes is generally used, although appropriate reactive dyes can be applied to either fibre type and direct or acid dyes used to fill in the other component. Optimum fastness is given by reactive dyes on the viscose component, followed by milling acid or 1:2 metal-complex dyes on the wool. Vat, sulphur and azoic dyes are not considered for these blends because of the strongly alkaline dyeing conditions necessary, which would damage both fibre types. Direct dyes should be selected with good build-up on viscose to minimise cross-staining of the wool. The preferred dyes are mostly disazo tetrasulphonates, particularly those of the symmetrical diarylurea type. Preferred dyes for the wool are disulphonated milling acid dyes of Mr 500–800 and unsulphonated 1:2 metal-complex monoazo types of Mr 850–950. Levelling acid or 1:1 metal-complex dyes should be avoided because the sulphuric acid required for adequate exhaustion would damage the cellulosic fibre. The absorption of direct dyes by wool can be reduced using anionic retarding agents of the syntan type. Dyeing for long periods at the boil or under acidic conditions will result in increased absorption of direct dyes by the wool component. Chemically damaged wool also absorbs direct dyes more quickly. Alkaline conditions favour the absorption of acid dyes by viscose and also tend to damage the wool fibre. The products of wool hydrolysis may cause reductive attack of certain sensitive azo direct and acid dyes in the dyebath. All these undesirable complications can be minimised using ammonium sulphate to maintain an acidic pH. Advantages of dyeing wool/cellulosic blends under mildly acidic conditions include those listed on the following page.

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(1) The method is applicable to a wide range of blend proportions. (2) Better penetration of yarn or fabric is achieved by the freedom to dye at the boil without fibre damage. (3) Higher exhaustion of the dyebath results in better shade reproducibility. (4) Independent shading of the two fibres is facilitated. (5) Contrast effects can be produced by selective dyeing of the two fibres simultaneously with direct and acid dyes. (6) Mildly acidic dyebaths permit selective dyeing of the wool fibre and reserve of the cellulosic component without significant hydrolytic damage of the cellulose. (7) Dyeing wool under acidic conditions results in improved handle and less risk of damage. Pile fabrics dyed in this way are more resilient and lustrous. (8) Satisfactory results may be obtained on blends of carbonised wool with cellulosic fibres that are difficult to dye under neutral conditions. (9) Sensitive azo direct or acid dyes are less likely to be chemically attacked. (10) Rubbing fastness is improved compared with dyeing under neutral conditions. A two-stage process for the exhaust dyeing of wool/viscose blends with direct and acid dyes has been examined by a factorial design method. The influence of seven independent parameters (dyeing time, concentrations of auxiliaries, pH and temperature of the two stages) on colour yield and fastness was evaluated. The optimum conditions were found to be acid dyeing for one hour at pH 4.5 and 100°C with urea but no reserving agent (to inhibit direct dye staining of wool), followed by direct dyeing at pH 7 and 95°C. Significant improvements in fastness to perspiration and wet rubbing were achieved [6]. Exhaust dyeing techniques are well established using two classes of fibrespecific reactive dyes on wool/cotton blends. Pad–batch is also a viable option, though not yet fully adopted in the production environment [7]. Bifunctional aminochlorotriazine-sulphatoethylsulphone dyes (Figure 8.1) exhibit a high degree of fixation on wool under acidic conditions and are particularly suitable for wool/cellulosic blends. Two-stage and two-bath dyeing methods have been devised [8] to give high colour yields with good levelling and excellent fastness on garments, hosiery, knitgoods, yarn and loose stock. The relationships between exhaustion, fixation and dyebath pH have been illustrated and fastness ratings on wool recorded [9]. Using a special selection of difluoropyrimidine reactive dyes (Figure 8.2), wool/cellulosic blends can be dyed with good solidity by a two-stage procedure. The cellulosic component is dyed first at low temperature and high pH, followed

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Cl N SO3Na H O N N

HN

N N

NH SO2CH2CH2OSO3Na

SO3Na

NaO3S

Figure 8.1 Typical bifunctional cellulose-reactive dye

H3CO

SO3Na H O N N NaO3S

NH

N

F N

Cl F

Figure 8.2 Typical difluoropyrimidine cellulose-reactive dye

by adjustment to an acidic pH and an increase in temperature to dye the wool. The pH changes are controlled by a programmable multiproduct injection device capable of providing continuous monitoring of pH [10]. The main advantages of this method are a considerable reduction in dyeing time and improved wool quality on blended goods where high wet fastness is demanded. Vinylsulphone dyes of the cellulose-reactive type (Figure 8.3) can be applied to wool/cotton blend fabrics by the one-bath pad–batch method using sodium silicate and caustic soda. The wool should be prechlorinated to give improved substantivity for these vinylsulphone dyes. After padding at ambient temperature and batching overnight, the alkali is washed out by cold rinsing and the dyeing is soaped and rinsed under neutral or mildly acidic conditions. Advantages of this technique include low energy consumption, effective penetration of thick fabrics, maintenance of wool quality, high wet fastness, good reproducibility and satisfactory yield compared with exhaust dyeing methods [11]. When dyeing wool/cellulosic blends with reactive dyes in one-bath methods, blue dyes derived from bromamine acid (1-amino-2-bromoanthraquinone-4sulphonic acid) can not be selected in most instances because they tend to react preferentially with wool. Nevertheless, azo reactive blues can be absorbed and fixed about equally on both components if applied under appropriate conditions. The lower uptake by cellulose of bromamine acid derivatives is probably

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OCH3 H O N N O2S

SO3Na CH2CH2OSO3Na

Figure 8.3 Typical vinylsulphone cellulose-reactive dye

O

NH2 SO3Na

O

HN

SO3Na

Cl

N N

HN N

NH SO3Na Figure 8.4 CI Reactive Blue 2

attributable to their non-planarity. It has been shown that the triazine ring of CI Reactive Blue 2 (Figure 8.4), and of other dyes containing the triazinylaminoanilinoanthraquinone grouping, is twisted by almost 90 degrees from the plane of the anthraquinone nucleus. All three NH links in such structures cause twisting of the adjacent ring system on both sides [12]. A serious drawback of most reactive dyeing techniques for wool/cellulosic blends is the adverse effect of alkaline fixation treatment on the quality of the dyed wool fibres. The influence of various concentrations of sodium carbonate on degradation of the wool fibres in a wool/cotton blend was estimated in terms of urea-bisulphite solubility. It was demonstrated that an acceptable two-stage exhaust method entails dyeing the wool first from a mildly acidic dyebath and then dyeing the cotton with salt and alkali at a pH of no more than 10, and a temperature of not more than 50°C for a dyeing time of not more than 1 hour [13]. In a later study of this problem, samples of merino slubbing were treated (a) under exhaust dyeing conditions with 2–12 g l–1 sodium carbonate and salt at 40°C, or (b) as recommended for pad–batch dyeing in carbonate/hydroxide mixtures at pH 10–13 and ambient temperature. The wool samples were analysed in detail to assess the degree of damage [14]. Batching treatments for various times (20–50 hours) at ambient temperature in the pH range 10.5–12.5

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were carried out on a wool/cotton fabric. Amino acid analysis after enzymatic hydrolysis of the wool component revealed the detailed effects of the alkaline degradation. Only in the case of cystine was decomposition extensive. A minority of the aspartic and glutamic acid residues showed deamination reactions and certain other amino acid units had undergone racemisation [15]. Dyeing of the wool portion in a wool/cotton blend presents few difficulties and high wet fastness on machine-washable goods can be attained using woolreactive dyes, chrome dyes or, in certain cases, 1:2 metal-complex or milling acid dyes with a suitable aftertreatment. As already noted, however, it is the cotton portion that causes problems. Until the 1980s, all the dye classes that could provide satisfactory fastness on cotton required strongly alkaline conditions. These caused significant damage to the wool and consequently impaired acceptability and performance of the finished garment in use. Those vat dyes that are capable of reduction with relatively low concentrations of caustic soda can be used but even with these dyes decreased abrasion resistance is observed. Certain azoic combinations can also be applied but fastness to rubbing is limited and the processing sequence is complicated. High-reactivity cellulose-reactive dyes are often adopted but these generally require a two-bath or at least a two-stage process, as already discussed. With an appropriate cationic pretreatment for the cotton, however, such as pad–dry–cure application of dimethyloldihydroxyethyleneurea and choline chloride, wool/ cotton blends can be dyed by a one-bath method using selected reactive dyes designed for wool [16,17]. The introduction of Indosol SF(S) reactant-fixable dyes (Figure 8.5) in the 1980s provided the opportunity to use fast dyes for wool (wool-reactive, premetallised or milling acid dyes) with them in a one-bath process. The dyebath is set at pH 6 with a syntan to minimise cross-staining of the wool by the Indosol dyes. The reactant-fixable dyes and 1:2 metal-complex or milling acid dyes are applied simultaneously at the boil in the presence of Glauber’s salt. The recommended aftertreating agent is applied from a fresh bath of Glauber’s salt and soda ash solution at 40°C. This enhances the wet fastness of the anionic dyes on wool as well as the reactant-fixable dyes on cotton [1]. For certain deep shades chrome dyes are preferred for the wool but a two-bath technique is necessary in these cases, the wool being dyed and chromed first at the boil and pH 4. The cotton is then dyed at the boil from a fresh bath containing the reactant-fixable dyes, electrolyte, an alkylamine polyoxy-ethylene levelling agent and a weakly anionic blocking agent to minimise surface staining of the dyed wool by the dyes. If the undyed wool is shrink-resist treated by an oxidative process such as chlorination, the affinity of the wool for all anionic

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OH2

H2NO2S

O

H2O

CuII

CuII

O

O

SO2NH2

O N N

N N NaO3S

107

N

SO3Na

H

Figure 8.5 Typical reactant-fixable direct dye

dyes is increased substantially. In these circumstances, the reactant-fixable dyes can be used for both fibres, with the degree of uptake by wool being controlled by means of a blocking agent of the syntan type. Reactive dyes represent an obvious choice for dyeing wool/linen blends in view of their brightness and high wet fastness. Application of reactive dyes to the linen component can give rise to bleeding of acid dyes from wool, especially when exhaustion and fixation temperatures higher than 40°C have to be used. The alkaline conditions required to fix the reactive dyes on linen may cause damage of the wool fibre as well as dye desorption. Bleeding of the dyed wool can be minimised if a reserving agent of the syntan type is added during the alkaline fixation step of the linen dyeing stage [18]. The dyeing of blends of shrink-resist wool with linen by a one-bath process often leads to differential uptake. Deeper dyeing of the wool component can be inhibited using reactive dichlorotriazinyl-substituted anionic auxiliaries in carefully controlled amounts [19]. Further problems include tendering of the linen by acid and hydrolytic degradation of the wool keratin by alkali. Several reactive dyes were evaluated on wool/linen yarns and fabrics at pH 4.5 (ammonium sulphate and acetic acid), followed by fixation to wool in ammonia solution at pH 8, rinsing and neutralisation with acetic acid. The dyeings were tested for levelness and fastness to rubbing and perspiration. Damage of the wool was assessed by alkali solubility, urea–bisulphite solubility, cysteine content and wet strength [20]. The substantivity of the linen can be enhanced using a suitable cationic pretreating agent. This yields a solid effect with wool-reactive dyes but lowers the light fastness. Blends of silk and cellulosic fibres can be dyed with vinylsulphone reactive dyes (Figure 8.3) using the normal pad–batch conditions devised for all-cellulosic fabrics. The fastness of these dyes on silk is generally good, the light fastness being comparable with that established on cellulosic fibres [21]. Decomposition of silk can occur during dyeing under extreme pH conditions and this mainly

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entails rupture of peptide bonds to form new chain-terminal groups, with progressive lowering of Mr (Scheme 8.1). R1 CONH CH

CONH

CH R2

CONH

R1 CONH CH

COOH + H2N

Scheme 8.1

CH

CONH

R2

To investigate this behaviour, degummed silk was treated in blank dyebaths at various pH values at the boil and tested for strength, extensibility, N-terminal amino group content, viscosity and protein loss. The results confirmed that silk is relatively stable under dyeing conditions between pH 4 and pH 9. This moderate stability, even in the mildly alkaline region, indicates that the dyeing of silk/ cotton blends with high-reactivity dyes is much less damaging to the protein structure than the dyeing of wool/cotton blends in the same way [22]. 8.2 EXHAUST DYEING OF NYLON/CELLULOSIC BLENDS Serious problems were encountered with the early nylon/cotton blends in the 1950s. Blends containing less than 50% nylon were actually weaker than allcotton yarns. Owing to the lower modulus of the nylon, the load on the yarn as it was extended was increasingly borne by the cotton fibres in the blend. This problem was solved by developing nylon with a stress–strain curve closer to that of cotton [23]. Nylon/cellulosic staple blends containing 10–30% nylon with cotton or viscose are used in lightweight suiting and dresswear, leisure shirts and half-hose. Many of these blends, as well as workwear fabrics with a 25:75 nylon/cotton warp and a cotton weft, or 20:80 nylon/viscose carpet yarns, contain relatively minor proportions of nylon and acceptable solid effects are not difficult to achieve. Similar considerations apply to pile fabrics, such as upholstery with a nylon pile in a woven cotton backing, or cotton-pile terry towelling with a weftknit nylon backing for beachwear, children’s clothing or leisure shirts, where slight two-sided differentiation may present no problem in made-up garments or covers. Tactel (ICI) nylon/cotton blends have been strongly promoted in sportswear. Good solidity of hue and depth is more critical in 50:50 blends and in union fabrics, such as nylon warp stretch fabrics containing cotton or nylon/cotton wefts for swimwear and narrow fabrics, crimped nylon warp/viscose weft sportswear or swimwear, nylon/viscose filament dresswear, or cotton warp/nylon weft constructions for uniforms, rainwear and workwear.

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Warp-knitted velvet fabrics with a viscose pile and a nylon backing are useful as furnishing fabrics, outerwear, trimmings and lining fabrics, often on cost grounds. Dyeing and printing are usually completed before raising. Overprinting of these dyed fabrics with metallic pigments is popular [24]. Plush velour and velvet fabrics with a nylon or nylon/wool pile and a cotton backing are encountered in the upholstery sector. Occasionally the pile is made from novelty yarns in which the filaments vary in denier and crystallinity along their length, so that an attractive shadow effect is obtained within each filament. When jet bleaching nylon/cotton blends with hydrogen peroxide at the boil, the amount of peroxide should be decreased according to the proportion of nylon present and complex organic bases are added as protective agents to minimise oxidative damage of the nylon, i.e. deamination of N-terminal end groups. An organic stabiliser for the peroxide, e.g. an aminocarboxylate, aminophosphonate or hydroxycarboxylate, should be present as this has a sequestering action on any Fe(III) or Cu(II) ions present, which may cause catalytic degradation of the cellulose. Peroxide scavengers containing thiosulphite may be used to ensure that there is no residual peroxide in the goods at the end of the bleaching operation [25]. There are various possibilities regarding the choice of dye classes for solid effects on nylon/cellulosic blends. Apart from fastness considerations, the choice of dye system is much influenced by blend construction. Single-class methods are mainly used where the nylon is a minor component, i.e. where only the cellulosic fibre plays a significant part in the surface appearance of the blend fabric, the nylon occupying the interior or the reverse side of the construction. Reserve, shadow and limited contrast effects are practicable on nylon/ cellulosic blends, but seldom encountered in practice. Shadow effects are sometimes required in certain woven upholstery designs, for example. The nylon may be reserved by applying selected direct dyes to the cellulosic fibre at 80– 90°C, with the usual salt addition and a syntan to protect the nylon from crossstaining. Many direct dyes are suitable, but the most important are the salt-controllable disazo or trisazo types with two to four anionic groups per molecule. A smaller range of yellow to red disazo dyes with two solubilising groups and a disulphonated phthalocyanine blue have been recommended for solid effects in pale or medium depths with good levelling characteristics. Solidity is favoured by applying these dyes at pH 5–6 with sodium dihydrogen phosphate as buffer and with limited salt addition at the boil. A wider gamut in good solidity is attainable if disperse and direct dyes are applied by a one-bath method at pH 8 and 70°C. An alkanol polyoxyethylene is recommended as dispersing and levelling agent, together with a syntan to

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minimise uptake of direct dyes by the nylon. Multisulphonated direct dyes of the salt-controllable type are used and the preferred disperse dyes are low-energy types with good levelling properties on nylon. The wet fastness properties of nylon/cellulosic blends dyed in this way are severely limited and the method is restricted to low-quality fabrics. Many suitable neutral-dyeing 1:2 metal-complex and milling acid dyes are available to give good reserve of the cellulosic fibre when applied to the nylon component by conventional methods in the presence of ammonium acetate. Disulphonated disazo and anthraquinone dyes with excellent wet fastness, but only moderate levelling properties, can be used widely. Coverage of dye-affinity variations in the nylon is much less of a problem than on filament nylon fabrics, especially when nylon/cellulosic staple-blends are to be dyed. Most of the premetallised dyes used are monosulphonated monoazo 1:2 chromium complexes. If premetallised or milling acid dyes of this kind are applied with the saltcontrollable direct dyes already described above, solid-effect dyeings of good fastness to light and moderately good wet fastness can be obtained economically on nylon/cellulosic blends. This method is especially important where both fibres make a major contribution to the appearance of the material. It is often useful to include a syntan to inhibit staining of the nylon by the direct dyes. Dyeing commences with the acid dyes, a weakly cationic alkylamine polyoxyethylene retarder if necessary and the minimum amount of syntan, depending on the applied depth and the direct dyes selected. If a cationic levelling agent is found necessary, sufficient of an alkanol polyoxyethylene antiprecipitant should be added to solubilise the dye–retarder complex. The dyebath is buffered to an optimum pH between 5–6 for full depths of milling acid dyes and 7–8 for pale depths of premetallised dyes. The direct dyes are added at about 60°C and the temperature raised to the boil, adding salt to promote exhaustion of the direct dyes by the cellulosic fibre. Bright hues with excellent fastness can be achieved on nylon/cellulosic blends using reactive dyes. Unfortunately, reactive dyes are highly sensitive to the type of nylon present and to dye-affinity variations in filament nylon. Many reactive dyes contain several sulphonic acid groups per molecule and pronounced blocking on the nylon component is observed when attempting to apply a multisulphonated dye in the presence of a less-sulphonated dye of higher affinity. The problems of incompatibility arising from this phenomenon are particularly difficult when dyeing the blend because differences in distribution of individual dyes between the component fibres are accentuated. In spite of these difficulties, methods employing one class of reactive dyes to

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colour both fibres have been established, especially for those fabrics composed mainly of cotton in which the nylon is hidden when the fabric is made into a garment. Weft-knitted terry towelling with a strong nylon filament base and an absorbent cotton pile has proved popular for children’s wear, beachwear and leisure shirts. Bright hues with very good fastness to washing and light are required. This is usually achieved with reactive dyes and metal-complex or milling acid dyes, but a wide variety of reactive dyes can be used satisfactorily on both fibres if the proportion of nylon is not too high. In a typical three-stage method from a single bath, the reactive dyes are first exhausted on to the nylon from weakly acidic solution in the absence of salt. Electrolyte is then added to promote further uptake by the cellulosic fibre and a final alkaline fixation treatment is given. Some control over the distribution between nylon and cellulose is possible by selection of dyebath pH, temperature and electrolyte concentration. Nylon is favoured at low applied depths but the distribution shifts in favour of the cellulosic fibre as the saturation level of nylon is approached. Reactive dyes with good neutral-dyeing properties in the presence of salt can be applied by a simpler two-stage sequence of neutral exhaustion and alkaline fixation, as for 100% cellulosic materials. Most metal-complex reactive dyes, as well as multisulphonated unmetallised types, require pH values of 4 or lower for reasonable uniformity of distribution. This gives some risk of degradation of the cellulosic fibre, especially if it is viscose, and may lead to inefficient utilisation of dye on the cellulosic component by acid hydrolysis of the reactive group. Many 1:2 metal-complex and several milling acid dyes are fast to soda boiling. This means that they can be applied with reactive dyes in the two-stage method, provided no serious interaction occurs. Reactive dyes are applied in the presence of alkali, together with non-reactive metal-complex dyes for the nylon. Free acid is added to give pH 7 and the temperature raised to the boil to fill in the nylon portion. There is only slight staining of the cellulosic fibre under these conditions. It is important to keep the pH slightly alkaline during washing and rinsing to avoid possible reaction of nylon with residual reactive dyes. When selecting reactive and milling acid dyes for this blend, it is more usual to adopt a two-bath method that gives a wider choice of suitable dyes. In this case, an important criterion is that the bond between the reactive dye and cellulose must be stable to acid hydrolysis during dyeing of the nylon component in full depths. Anionic dyes with high neutral-dyeing affinity are therefore preferred and vinylsulphone reactive dyes (Figure 8.3) are generally unsuitable. The facility to attain high wet fastness standards on nylon/cellulosic blends by a one-bath technique at mildly acidic pH is a substantial advantage over the two-

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bath or two-stage procedures based on reactive dyes [1]. Reactant-fixable direct dyes (Figure 8.5) and 1:2 metal-complex or milling acid dyes are applied simultaneously. The bath is set at pH 6 with Glauber’s salt and a syntan to minimise staining of the nylon by the reactant-fixable dyes. The 1:2 metalcomplex or milling acid dyes are added with a weakly cationic retarder and a nonionic anti-precipitant if necessary. The reactant-fixable dyes are then added and dyeing of both fibres is completed at the boil. Kayacelon React (KYK) bis(aminonicotinotriazine) dyes (Figure 8.6) are hightemperature neutral-fixing reactive dyes especially suitable for the one-bath dyeing of nylon/cellulosic blends. By choosing acid dyes that are relatively insensitive to salt it is possible to dye solid shades on nylon/cotton in one bath. The reproducibility of the method depends on buffering the dyebath carefully to pH 6–7 with a phosphate buffer [26]. –OOC

COO– N

N

+

N SO3Na H O N N

HN

+

N N

N

N

HN

NH

N

NH O

SO3Na

NaO3S

NaO3S

NaO3S H N N

SO3Na

Figure 8.6 Typical bis(aminonicotinotriazine) reactive dye

Knitwear made from polyurethane/cotton blends, in which the elastomeric fibre may range from 5% to 20%, has been widely popular in recent years for stretch garments, such as skiwear, sportswear, underwear and leisure clothing. If the content of polyurethane is higher than about 8%, the slitted tubular-knit goods show a strong tendency to roll at the edges, leading to a ‘cigarette’ form that is most difficult to penetrate during dyeing. This problem has been solved by presetting on a stenter at 190°C after slitting and then either carefully ‘bag stitching’ to restore the tubular form, or preferably dyeing in an overflow machine capable of achieving satisfactory penetration and levelness [27]. Blends of polyurethane and cellulosic fibres are dyed by methods similar to those for nylon/cellulosic blends. Monosulphonated monoazo or disulphonated anthraquinone acid dyes, or 1:2 metal-complex monoazo types, are preferred for

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the polyurethane component and practically all direct dyes for cellulose will give low staining of polyurethane fibres at pH 8–8.5. Similar considerations apply to the dyeing of blends of acid-dyeable polypropylene with cellulosic fibres. Selected premetallised or milling acid dyes are applied to the synthetic component at the boil and pH 3–4 in the presence of an anionic retarder. Selected direct dyes are then applied from the same bath after adjusting to 70°C and pH 7–8. A syntan is included to minimise staining of the polypropylene by the direct dyes. 8.3 CONTINUOUS DYEING OF NYLON/CELLULOSIC BLENDS The carpet industry is the most important field in which AC blends have been used in quantities sufficient to justify the development of continuous dyeing processes. Another area for the adoption of continuous methods has been the dyeing of heavy-duty nylon/cotton woven fabrics, which are closely related to the much more extensively used polyester/cotton blends for this outlet. Similar principles of application are relevant for the continuous dyeing of plush and velvet upholstery fabrics made in nylon pile/cotton backing constructions. Continuous dyeing systems have also been developed for nylon warp/cotton weft sportswear materials that have proved exceptionally popular in recent years [28]. Warps in these sportswear fabrics are usually treated with acrylate sizes so that desizing with enzymes is unnecessary. Pretreatment comprises singeing, cold bleaching, alkaline boiling out, washing-off and drying. After dyeing, these goods are often given a hydrophobic finish usually based on a glyoxal-fluorocarbon resin or a silicone polymer. This can contribute to improvements in fastness of anionic dyes to perspiration and water, as well as giving the desired finish. Modified viscose carpets containing only 10–20% nylon can be dyed continuously with direct dyes that give an acceptably solid effect on the two fibres. The preferred dyes are mostly disazo types with two solubilising groups and good levelling properties, but for mode shades the best results are obtained with salt-controllable dyes. Migration of the disazo disulphonates during drying and steaming may lead to colour variations in mixture dyeings. Temperaturecontrollable direct dyes tend to give poor penetration and inferior fastness to rubbing. A wetting and levelling agent of the nonylphenol polyoxyethylene type is added to the pad liquor, with urea if necessary to improve solubility of the direct dyes. Solidity at full depths requires careful control of pH and anionic surfactants should be avoided as these may interfere with uptake of the direct dyes by nylon. An alternative process, recommended for plush or velvet upholstery with a nylon pile and cotton backing, is based on the same selection of acid or metalcomplex dyes used on all-nylon carpets, together with the salt-controllable

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multisulphonated direct dyes that give low staining of nylon. Suitable foaming agents are added to improve levelness and penetration. These avoid undesirable migration and frosting of the nylon pile. Steaming for 5–15 minutes is necessary according to depth applied. Continuous methods that include direct dyes suffer from problems of substantive tailing and differential affinity for the two fibres, as well as limited brightness and inferior fastness to light and wet treatments in many instances. Reactive dyes for the cellulosic component offer advantages in most of these aspects and they are suitable for one-bath application methods in general. Satisfactory fastness in pale depths is given by padding with reactive and disperse dyes, urea and an anionic migration inhibitor, followed by thermofixation at 180–200°C. Low-energy monoazo and anthraquinone disperse dyes are recommended. Slight discoloration and stiffening of the fabric may occur if a high proportion of nylon is present. Batching for two hours after padding may improve fixation of the reactive dyes when viscose is the cellulosic component. A modification of this process can be used to dye full depths with reactive dyes and selected metal-complex or acid dyes. Less urea is required and after thermofixation under alkaline conditions the dyes on nylon are developed fully by an acid shock treatment in dilute formic acid solution at the boil. An alternative one-bath sequence for reactive dyes and unsulphonated 1:2 metalcomplex dyes is: (1) neutral pad–dry–thermofix treatment for fixation of the metal-complex dyes on nylon; (2) padding with caustic soda in near-saturated brine; (3) batching to fix the reactive dyes on the cellulosic fibre. The relative merits of wet steaming without intermediate drying, a pad–steam process with intermediate drying and a pad–batch–steam sequence for the fixation of metal-complex and acid dyes on the nylon component have been evaluated in terms of the resulting wet fastness [28]. Selected reactive or reactantfixable dyes can be used for the cellulosic fibre. Improvements in colour fastness are possible using a syntan to fix the anionic dyes on the nylon and a cationic aftertreatment for the reactant-fixable dyes. Dull dyeings of high fastness to light and wet treatments may be required on nylon/cotton fabrics for workwear or uniforms. Vat and metal-complex dyes are often used in these circumstances, although selected sulphur and milling acid dyes provide more economical recipes where fastness standards permit. Stability of the premetallised and milling acid dyes to reduction and oxidation is an

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essential criterion of selection for application with vat or sulphur dyes. Sulphur dyes can be used for the cotton by a simplified method in which the metalcomplex dyes are applied alone by pad–dry–thermofix and the reduced sulphur dyes included in a subsequent chemical pad–steam stage. A costly two-bath application of 1:2 metal-complex dyes by pad–steam on nylon, followed by a vat pigment pad–dry–chemical pad–steam process for the cotton, may be necessary to achieve maximum fastness. 8.4 DYEING METHODS AND DYE SELECTION FOR AC BLENDS Solid effects are mainly of interest on these blends. They can be readily obtained (Table 8.1) by single-class (reactive dyes) or one-bath methods on blends of wool,

Table 8.1 Dye selections for exhaust dyeing of AC blends Blend

Colour effect

Dyeing method

Dye selection

Wool/cotton

Wool reserve

One-bath

Multisulphonated salt-controllable direct dyes

Solid

One-bath

Acid dyes and low-sulphonated direct dyes

Solid

Single-class

Monofunctional or bifunctional cellulose-reactive dyes

One-bath

Premetallised or milling acid dyes and reactant-fixable dyes

Two-stage

Wool-reactive and then cellulose-reactive dyes

Two-bath

Chrome dyes, then reactant-fixable dyes

Single-class

High-reactivity dyes with anionic retarder

One-bath

Acid dyes and direct dyes with anionic retarder

One-bath

Milling acid dyes and direct dyes

Two-stage

Levelling acid dyes at pH 2–3, then direct dyes

Single-class

Disulphonated milling acid dyes or unsulphonated 1:2 metal-complex dyes

Wool/cotton (machinewashable)

Chlorinated wool/cotton

Mohair/ cotton

Wool/viscose

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Solid

Solid

Viscose reserve

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Table 8.1 Continued Blend

Colour effect

Dyeing method

Dye selection

Wool/viscose

Solid or contrast

One-bath

Milling acid dyes and direct dyes Wool-reactive dyes and direct dyes Milling acid dyes and cellulose-reactive dyes

Wool/linen

Solid

Two-stage

Milling acid dyes, then cellulose-reactive dyes

Wool/ramie

Solid

One-bath

Milling acid dyes and direct dyes

Nylon/ cellulosic

Nylon reserve

Single-class

Salt-controllable direct dyes with syntan

Cellulosic

Single-class Monosulphonated 1:2 metal-complex or disulphonated

Solid

Single-class (pale depths)

Disulphonated disazo and phthalocyanine direct dyes

Solid

Single-class

Reactive dyes at pH 4–5, then salt and alkali

One-bath

Low-energy disperse dyes and salt-controllable direct dyes

reserve milling acid dyes

Nylon/cellulosic

Premetallised or milling acid dyes and salt-controllable direct dyes Premetallised or milling acid dyes and reactant-fixable dyes Selected acid dyes and bis(aminonicotinotriazine) neutral-fixing dyes

Polyurethane/ cotton

Acid-dyeable polypropylene/ cotton

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Two-stage

Reactive dyes with alkali, then selected premetallised and milling acid dyes

Polyurethane reserve

Single-class

Direct dyes at pH 8 with syntan

Solid

One-bath

Premetallised or milling acid dyes and direct dyes at pH 8

Solid

Two-stage

Premetallised or milling acid dyes at pH 4, then direct dyes at pH 8

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chlorinated wool, mohair, nylon or polyurethane with cotton, or blends of wool or nylon with viscose. Two-stage procedures are necessary for wool/linen or aciddyeable polypropylene/cotton blends, the non-cellulosic fibre being dyed first. The higher wet fastness standards provided by reactive dyes with premetallised or milling acid dyes on nylon/cellulosic blends or machine-washable wool/cotton goods also require two-stage or two-bath processes. On wool/cotton the wool is dyed first but on nylon/cotton it is preferable to first dye the cotton. Continuous dyeing methods for solid effects on nylon/cellulosic blends range from low-cost pad–steam dyeing with direct dyes or the pad–thermofix disperse/ reactive process, to much more elaborate and costly two-bath sequences for high all-round fastness performance. Premetallised and milling acid dyes by pad– steam are mostly used to colour the nylon but the class preferred on the cellulosic fibre may be direct, reactant-fixable, reactive, sulphur or vat dyes (Table 8.2).

Table 8.2 Dye selections for continuous dyeing of nylon/cellulosic blends

Blend Nylon/ cellulosic

Colour effect

Dyeing method

Dye selection

Solid

Pad–dry–steam

Salt-controllable direct dyes Premetallised or milling acid dyes and salt-controllable direct dyes Premetallised or milling acid dyes and reactant-fixable dyes

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Pad–dry–thermofix

Low-energy disperse dyes and reactive dyes

Pad–dry–thermofix– acid shock

Selected premetallised or milling acid dyes and reactive dyes

Pad–dry–thermofix– pad–batch

Unsulphonated 1:2 metal-complex dyes and reactive dyes

Pad–dry–thermofix– chemical pad–steam

Selected premetallised dyes, then pre-reduced sulphur dyes

Pad–dry–steam, pad–dry–chemical pad–steam

Selected 1:2 metal-complex dyes, then vat dyes

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8.5 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

J A Hook and A C Welham, J.S.D.C., 104 (1988) 329. R L Stone, R H Wang and G P Morton, Text. Chem. Colorist, 18 (Aug 1986) 11. T P Nevell in Cellulosics dyeing, Ed. J Shore (Bradford: SDC, 1995) 6. T L Dawson, Rev. Prog. Coloration, 15 (1985) 29. T M Baldwinson in Colorants and auxiliaries, Vol. 2, Ed. J Shore (Bradford: SDC, 1990) 444. M M Muratova, T Y Rosinskaya, L I Belenkii and L L Vidrevich, Tekstil. Prom., 12 (Dec 1980) 48. N E Houser, AATCC Nat. Tech. Conf., (Oct 1985) 116. K Imada, M Sasakura and T Yoshida, Text. Chem. Colorist, 22 (Nov 1990) 18. A N Lee, Dyer, 178 (Apr 1993) 30. D Hildebrand, Proc. 7th Int. Wool Text. Res. Conf., (Tokyo), Vol. V (1985) 239. H Putze and G Dillmann, Textilveredlung, 15 (1980) 457. M Matsui, U Meyer and H Zollinger, J.S.D.C., 104 (1988) 425. H Zahn, I Steeken and U Altenhofen, Chemiefasern und Textilind., 31/83 (1981) 684. I Steenken, I Souren, U Altenhofen and H Zahn, Textil Praxis, 39 (1984) 1146. U Altenhofen, I Souren and H Zahn, Textilveredlung, 20 (1985) 144. J M Cardamone, AATCC International Conference and Exhibition, (Oct 1994) 7. J M Cardamone, W N Marmer, E J Blanchard, A H Lambert and J Bulan-Brady, Text. Chem. Colorist, 28 (Nov 1996) 19. I Steenken, I Funken and G Blankenburg, Textilveredlung, 21 (1986) 128. J Haarer, H Thomas and H Höcker, Melliand Textilber., 76 (1995) 1003. G Kratz, A Funder, H Thomas and H Höcker, Melliand Textilber., 70 (1989) 128. H Putze, Textil Praxis, 39 (1984) 1051. B Vogt, U Altenhofen and H Zahn, Textilveredlung, 20 (1985) 90. R M Hoffman and R W Peterson, J. Text. Inst., 49 (1958) 418. G Wünsch, Textilveredlung, 24 (1989) 57. D R Wallwork, Textile Technology Internat., (1990) 229. A N Lee, Dyer, 179 (Apr 1994) 29. Brazzoli SpA, Dyer, 179 (Aug 1994) 31. O Annen, F Somm and R Buser, Textilveredlung, 22 (1987) 19.

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CHAPTER 9

Cellulosic/acrylic and other CB blends

9.1 EXHAUST DYEING OF CELLULOSIC/ACRYLIC BLENDS Cellulosic/acrylic blends are the most important of the acrylic fibre blends. The cellulosic fibre contributes economy, moisture regain and antistatic properties. In apparel outlets, the acrylic component is included for heat insulation, crease recovery and abrasion resistance. Important characteristics of cellulosic/acrylic fibre blends in upholstery, pile fabrics and tufted carpets are appearance retention, resilience and wear resistance. Cotton/acrylic blends are widely used in the rapidly expanding sportswear and leisurewear sectors. Fine-spun 20:80 to 50:50 cotton/acrylic yarns are used in lightweight woven suiting, dresswear and sportswear, or knitted underwear, leisurewear and swimwear. Blends of viscose and acrylic fibres are used in skirts, dresswear, linings and hosiery yarns. It may be desirable to insert durable pleats in dresswear garments with the aid of a reactant resin finish. Modacrylic fibres in blends with cotton or viscose (20:80 to 50:50) have been exploited for underwear, hosiery, leisure clothing and dresswear, particularly pleated skirts. High-bulk acrylic and modal fibre long-staple yarns are suitable for dresswear and suitings with a wool-like appearance. Fleece fabrics are made from woollenspun acrylic fibre pile in a cotton backing. Fancy high-bulk acrylic jersey knitgoods containing linen or viscose slubs are of interest for furnishing fabrics as well as apparel. Viscose/acrylic staple yarns have proved effective as high-twist pile for non-crush carpets, as they are significantly cheaper than all-wool or wool/viscose carpets. Package dyeing of cotton/acrylic yarns is important. Careful prescouring of the cotton is essential using trisodium phosphate at 60–70°C and an alkanol polyoxyethylene detergent. This is necessary to remove residual lubricants and, in the case of unrelaxed yarns containing high-bulk acrylic, to avoid fixation of contaminants during relaxation by steaming. Good whiteness is achieved on the cellulosic component by bleaching with silicate-stabilised hydrogen peroxide at 70°C and pH 9–10. Careful control is necessary to avoid yellowing of the acrylic fibre.

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The selection of dyes for cellulosic/acrylic blends is complicated by the fact that disperse dyes show poor build-up and limited fastness on the acrylic component, so that basic dyes must be used in most instances. Problems of incompatibility between these dyes and the various classes of anionic dyes required to meet appropriate fastness demands on the cellulosic fibre (sections 4.2 and 4.3) are therefore characteristic of such blends, in contrast to those of cellulosics with other synthetic fibres. Cross-staining is less of a problem, however, and a good reserve can be obtained on either component, although solid effects are most often required. The choice of best dyeing method is determined by the colour effect and gamut required, as well as the wet fastness specification. Cellulosic/acrylic blends can be readily dyed to reserve the acrylic fibre by pretreating with a syntan at 60– 70°C and then dyeing with salt-controllable direct dyes at 70°C and pH 7–8. It is less convenient to reserve the cellulosic component because staining by basic dyes may be troublesome, particularly with regenerated cellulosic fibres or with cotton that has suffered oxidative damage during scouring or bleaching. The carboxyl groups formed as a result of alkaline oxidation of cellulose provide anionic sites for the sorption of basic dyes. Basic dyes are applied to the acrylic component with a cationic retarder at the boil. The rates of exhaustion of basic dyes of various types on acrylic fibres have been analysed in self shades and combination recipes using high-pressure liquid chromatography [1]. The preferred dyes for reserving the cellulosic component are mainly of the localisedcharge monoazo or anthraquinone types. Any basic dye staining of the cellulosic fibre can be removed using sodium dithionite and a nonionic detergent at 60°C. Solidity or contrast effects can be easily obtained by one-bath methods. The most economical approach for pale depths is one-bath application of disperse and direct dyes. Wet fastness is seldom acceptable at intermediate depths, but the levelling properties of disperse dyes are much superior to those of basic dyes on acrylic fibres. The disperse dyes recommended for pale depths on cellulosic/ acrylic blends are mainly low- and intermediate-energy dyes of Mr 230–350, particularly nitro, monoazo and anthraquinone types. The preferred direct dyes are almost all salt-controllable disazo, polyazo or stilbene types with three or four anionic groups. The blend is dyed at the boil and pH 4–5 in the presence of an anionic dispersing agent. Disperse dyes are partly absorbed by the cellulosic component in the early stage of dyeing, but transfer in favour of the acrylic fibre proceeds as the dyeing temperature approaches the boil. The acrylic component is dyed first, then the dyebath temperature is lowered to 80°C, Glauber’s salt added and the

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cellulosic fibre filled in with the direct dyes. It is not difficult to remove the residual stain from the cellulosic fibre by subsequent scouring. Cellulosic/acrylic blends can be dyed in pale or medium depths by a one-bath process with direct and basic dyes, but there is a risk of co-precipitation of these dyes. Careful selection of basic dyes and the addition of a nonionic antiprecipitant are necessary. Problems of incompatibility can be virtually overcome by the selection of basic dyes of small molecular size and the relatively hydrophilic multisulphonated direct dyes with a low tendency to aggregate in the absence of electrolyte [2]. The cellulosic fibre is dyed first at 80°C and pH 5–6 (acetate–acetic buffer) with the direct dyes, Glauber’s salt and an alkanol polyoxyethylene antiprecipitant. The basic dyes and an anionic retarder are then added and the temperature raised to the boil to approach target depth on the acrylic component. If necessary, more Glauber’s salt may be added to improve exhaustion of the direct dyes. The acidic dyebath is important to avoid aggregation and possible precipitation of the dyes. The direct dyes must show satisfactory exhaustion under these mildly acidic conditions that are preferred for applying basic dyes. The direct dyes tend to retard the uptake of the basic dyes by the acrylic component and may give restraining of final exhaustion in heavy depths. An advantage of dyeing under mildly acidic conditions is to avoid the risk of reductive decomposition of some direct dyes under alkaline conditions at elevated temperatures. Medium or heavy depths may be dyed by a more lengthy procedure that minimises any risk of incompatibility. In the two-stage sequence, the basic dyes are applied with an alkanol polyoxyethylene anti-precipitant at pH 5–6, raising the temperature slowly from 80°C to the boil. When dyeing of the acrylic fibre is complete, the temperature is lowered again to 80°C, the direct dyes and Glauber’s salt are added and dyeing of the cellulosic component is completed at this temperature. A two-bath method is similar, except that the basic dyes are applied at pH 4–5 with a cationic retarder and the direct dyeing is commenced at 40°C and pH 7, after an intermediate clear with a nonionic detergent. Better fastness of the cellulosic fibre is given by reactive, reactant-fixable or vat dyes. Basic dyes and reactive dyes are preferred for bright hues of high wet fastness. Basic dyes would be restrained by the presence of reactive dyes and they tend to be unstable to the conditions of alkaline fixation, so that two-bath methods are normally necessary. Full depths are usually achieved by dyeing the acrylic fibre first with basic dyes. The temperature of the reactive dyeing stage must not exceed the glass-transition temperature of the dyed acrylic fibre in order

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to avoid possible desorption of basic dyes [3]. Data have been given on the stability to cross-dyeing of basic dyes tested over the pH range 2 to 9 [2]. There is a risk of change in hue of basic dyeings during the alkaline fixation and soaping of reactive dyeings on the cellulosic component. The vinylsulphone reactive dyes are stable to cross-dyeing with basic dyes at pH 5. Advantage may be taken of this in a two-stage sequence, in which the alkaline fixation bath for the reactive dyeing is subsequently adjusted to pH 5 with acetic acid in order to dye the acrylic fibre with the basic dyes. A two-stage process with reactant-fixable copper-complex direct dyes and basic dyes is suitable to achieve high wet fastness in pale and medium depths [4]. The reactant-fixable dyes are applied first to the cellulosic component at pH 5–6 (acetate–acetic buffer) and 70°C in the presence of electrolyte. The basic dyes are then added with an alkylamine polyoxyethylene as anti-precipitant and mildly cationic retarder, and the acrylic fibre is dyed at the boil. An appropriate cationic fixing agent is used to aftertreat the dyeings. Advantages of this process include a short dyeing time, good level dyeing behaviour and high standards of reproducibility and fastness performance [5]. When dyeing yarn in package form it is usual to select vat dyes for the cellulosic fibre. Problems of incompatibility are more evident when vat dyes are used with basic dyes. A two-bath sequence is necessary because the anionic dispersing agents in the vat dye formulations would restrain uptake of the basic dyes and promote staining of the cellulose, at the same time causing instability of the vat dye dispersion. If the cellulosic fibre were dyed first, the vat dye would act as a mordant and the basic dyes would stain the strongly anionic fibre surface. This stain would not be removed completely by soaping [2]. Thus it is preferable to dye the acrylic component first, because the basic dyes are fast to cross-dyeing, and the reducing conditions of vat dyeing at 50–60°C help to remove the basic dye stain from the cellulosic fibre. Some vat dyes stain the acrylic fibre significantly and allowance for this must be made in shade matching. The preferred vat dyes for minimum staining of the acrylic fibre are mainly the polycyclic quinones and their halogenated derivatives, including the dibromopyranthrones, indanthrones, dichloro- and dibromoiso-violanthrones, and alkoxyviolanthrones [6]. 9.2 CONTINUOUS DYEING OF CELLULOSIC/ACRYLIC BLENDS These are the only acrylic blends produced in sufficient quantity to justify continuous dyeing. The principles of dye selection in relation to compatibility and fastness requirements are essentially similar to the analogous batchwise methods, but the application techniques have much in common with the

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continuous dyeing of polyester/cellulosic blends. Blends of acrylic fibres with either cotton or viscose can be processed satisfactorily on continuous ranges after singeing, desizing and scouring in open width. For blends containing a high proportion of acrylic fibre with cotton, it is often possible to obtain an acceptably solid appearance and moderate fastness in pale depths by dyeing the acrylic fibre only using disperse dyes applied by pad–dry– thermofix. Higher fastness in pale or medium depths is attainable on these acrylic-rich blends using basic dyes by pad–steam, leaving the cellulosic fibre undyed. The fabric is padded at 70°C with the basic dyes, the liquor being adjusted to pH 5 with citric acid. The fabric is then steamed at 100–103°C, but it may be necessary to give a prolonged steaming with certain basic dyes. If reserve of the cellulosic fibre is desired, this may require a clearing treatment in an anionic detergent solution at 90°C, or in a reduction clearing bath at 70°C in the case of deeper shades. For pale depths of outstanding fastness, selected vat dyes can be applied to give reasonable solidity on both fibres. These are applied by pigment padding from dispersion and drying, followed by chemical padding. Salt (100 g l–1) is added to the alkaline dithionite solution to minimise transfer from the acrylic fibre to the cellulose during subsequent steaming. Halogenated vat dyes of the pyranthrone, indanthrone and isoviolanthrone types, as well as alkoxy derivatives of violanthrone, are especially suitable [6]. Solid dyeings on cotton/acrylic blends can also be obtained using liquid brands of sulphur dyes to colour both fibres. Advantages of this approach over alternative two-stage systems are the lower costs of dyes and chemicals, a shorter and simpler dyeing cycle, satisfactory fastness and improved coverage of immature cotton [7]. On the other hand, of course, sulphur dyes are much more restricted to a gamut of relatively dull hues. Basic and direct dyes may be applied together in the pad–steam process, using an alkanol polyoxyethylene sulphate to complex with the basic dyes and an alkanol polyoxyethylene to disperse the complex in the pad liquor (section 4.3), but there are serious limitations. The depth on the acrylic fibre is limited by the stability of the dye–agent complex and the restraining influence of the anionic agent and the direct dyes. A fixation accelerator is also required to promote yield and penetration of the basic dyes into the acrylic component during steaming. These agents are usually water-insoluble aryl ethers containing halogeno or cyano substituents [8]. A product of this type is emulsified in the pad liquor together with an anti-precipitant system. Selection of the direct dyes is restricted by their limited solubility in the pad liquor at pH 5, slow diffusion into the cellulosic fibre during steaming and only moderate wet fastness.

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Conventional basic dyes are not compatible with vat dye dispersions but selected basic dyes have been marketed in complex form as liquids stabilised with anionic dispersing agents. These are compatible with liquid brands of vat dyes for continuous dyeing. Before dyeing with these complexes, the anionic groups in the acrylic fibre must be converted to the ammonium salt form by pretreatment with an ammonium salt at 80°C. If these basic dye complexes are applied to untreated acrylic fibres with anionic groups in their usual sodium salt form, the complexes do not dissociate and give relatively low yields with poor fastness. The pretreated cellulosic/acrylic fabric is padded with the complexed basic dyes together with selected vat dyes at 50°C and pH 8–9 (phosphate buffer). During thermofixation at 200°C the complexes are transferred from the cellulosic to the acrylic fibre and also dissociate to give the parent basic dyes. These blends show high wet fastness after the residual basic dye stain has been cleared from the cellulosic fibre during conventional chemical pad–steam fixation of the vat dyes. Optimum fastness in full depths on untreated cellulosic/ acrylic blends can be achieved by pad–steam dyeing with conventional basic dyes, followed by the usual pigment pad–dry–chemical pad–steam sequence for vat dyeing of the cellulosic fibre. This relatively costly process gives a wide range of shades of high fastness to light and wet treatments. The application of vat dyes is instrumental in removing any basic dye that may have stained the cellulosic fibre in the first stage. Basic and reactive dyes are generally incompatible at only moderate concentrations in the pad liquor, especially the localised-charge basic dyes with the most highly sulphonated members of the reactive class. Nevertheless, these two classes yield exceptionally bright hues for deep contrast effects of excellent fastness, applied by a two-bath sequence of conventional pad–steam processes. If a resin finish is required for the cellulosic component, the fabric should be treated under conditions that minimise discoloration of the acrylic fibres, particularly with regard to curing temperature. The amount of resin applied should be determined by the cellulosic content of the blend. Fabrics containing more than 50% of acrylic fibre do not normally require a resin finish. 9.3 BLENDS OF CELLULOSIC FIBRES WITH MODACRYLIC OR ACIDDYEABLE ACRYLIC VARIANTS Dynel (Union Carbide) modacrylic fibre is used in 20:80 to 50:50 blends with cotton or viscose for half-hose, underwear and nightwear. The modacrylic component contributes dimensional stability, improved handle and good

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launderability. Woven goods include pile fabrics with a cotton warp and a 70:30 cotton/Dynel weft. Dynel is resistant to mercerising and peroxide bleaching of the cotton. It is usual to dye solid shades on these blends but a wide range of colour contrasts is possible because the dyes preferred for Dynel do not normally stain the cellulosic fibre. Shades of moderate fastness (light 5–6 and mild washing tests) are obtained with disperse and direct dyes by a one-bath method. Higher wet fastness requires vat, sulphur or reactive dyes for the cotton and basic dyes for Dynel but a twostage process is unavoidable. The general approach is to dye the Dynel at the boil and pH 5 with basic dyes and then to fill in the cotton under normal conditions for the cellulosic dyes chosen. Verel modacrylic fibre is blended with cotton for sportswear and underwear, typically as 25:75 blends. Flannel-type fabrics can be made from 75:25 Verel/ viscose yarns. No resin finish is necessary for this modacrylic-rich blend. Solid shades can be obtained on Verel/cellulosic blends using disperse or 1:2 metalcomplex dyes on Verel and direct dyes on the cotton or viscose. Either fibre can be reserved and contrast or shadow effects are also possible if desired. A full range of dyed effects can be produced on blends of acid-dyeable acrylic variants with cellulosic fibres. Several techniques are available to dye the acrylic and reserve the cellulosic component: (1) The acrylic fibre is dyed with basic dyes by the urea method. The dyes should be dissolved in hot water and either methanol or a nonionic surfactant, rather than acetic acid. Dyeing takes place in the presence of 3% urea and the dyebath pH gradually rises from slightly acidic to slightly alkaline during the course of dyeing. The dyed material is rinsed well and given a mild scour with sodium hypochlorite at 40°C to clear the cellulosic fibres. (2) Selected chrome dyes may be applied from an acidic dyebath. It is essential to scour with a nonionic detergent and tetrasodium pyrophosphate after chroming is complete. These dyes have satisfactory fastness to perspiration and steam pleating. (3) The acid-dyeable acrylic fibre is dyed with selected levelling acid dyes from phosphoric acid solution, followed by a neutralising scouring treatment as for chrome dyeings. The light fastness of these dyes may be lower when dyed with phosphoric acid. Preferred dyes include selected monoazo or anthraquinone disulphonates of Mr 400–600. The fastness of levelling acid dyes is generally lower than that of basic or chrome dyes, so this approach is usually confined to bright shades on lower-quality goods.

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It is less convenient to dye the cellulosic fibre and reserve the acid-dyeable acrylic variant because of the affinity of the latter for most classes of dyes. Selected saltcontrollable direct dyes of the multisulphonated type can be applied after the material has been pretreated at 60–70°C with a weakly cationic surfactant. The best reserve of the acid-dyeable acrylic fibre is achieved by dyeing at pH 8 and 70°C. The wet fastness is improved by finishing with a crease-resist resin. If solid shades are required a two-bath method is preferred, first dyeing the acrylic variant with basic or chrome dyes and then filling in the cellulosic fibre with direct dyes at pH 8. 9.4 BLENDS OF BASIC-DYEABLE POLYESTER WITH COTTON Knitted fabrics made from cotton and basic-dyeable polyester fibres can be dyed in contrasting colours using direct and basic dyes applied together with a nonionic anti-precipitant by a one-bath method at 120°C in a jet or overflow machine. Aftertreatment with a cationic fixing agent or a reactant resin is advisable to attain satisfactory wet fastness on the cotton. Improved fastness without resin treatment is possible using reactive and basic dyes. A two-bath method is necessary because of the problems of interaction [9]. The basic dyes may be applied first, followed by aminochlorotriazine dyes under conventional conditions at 80°C. An inverse process is preferred for vinylsulphone dyes and basic dyes, because these reactive dyeings are stable to the mildly acidic conditions necessary for dyeing the basic-dyeable polyester component. 9.5 DYEING METHODS AND DYE SELECTION FOR CB BLENDS Reserve of the basic-dyeable component in these blends can be readily obtained using salt-controllable direct dyes, but it is less convenient to reserve the cellulosic fibre because of potential staining by basic dyes. Solid or contrast effects are attainable in various ways (Table 9.1), according to the fastness performance required and the processing costs that can be tolerated. One-bath methods with direct dyes and disperse or basic dyes offer the simplest approach in pale or medium depths. Various two-stage or two-bath sequences are available using reactive, reactant-fixable, sulphur or vat dyes before or after basic dyeing of the non-cellulosic component. Continuous dyeing methods for solid effects on cellulosic/acrylic blends range from low-cost coloration of the acrylic fibre only, using disperse or basic dyes especially on acrylic-rich materials, to elaborate two-bath sequences entailing double steaming operations. The more costly processes necessary for high all-

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Table 9.1 Dye selections for exhaust dyeing of CB blends Blend

Colour effect

Dyeing method

Dye selection

Cellulosic/ acrylic

Acrylic reserve

Single-class

Syntan pretreatment, then salt-controllable direct dyes

Cellulosic reserve

Single-class

Localised-charge basic dyes

Solid or contrast

One-bath (pale depths)

Direct dyes and disperse dyes at pH 4–5

One-bath

Multisulphonated direct dyes and migrating basic dyes with anti-precipitant

Two-stage

Basic dyes at the boil, then direct dyes at 80°C Reactant-fixable dyes at 70°C, then basic dyes at the boil Vinylsulphone reactive dyes, then basic dyes at pH 5

Two-bath

Basic dyes at the boil, then aminochlorotriazine dyes at 80°C Basic dyes at the boil, then vat dyes at 50°C

Cellulosic/ modacrylic

Cellulosic/aciddyeable acrylic

Solid

One-bath

Direct dyes and disperse dyes

Two-stage or two-bath

Basic dyes at the boil, then vat, sulphur or reactive dyes

Acrylic reserve

Single-class

Multisulphonated salt-controllable direct dyes at pH 8 and 70°C

Cellulosic reserve

Single-class

Basic dyes with 3% urea Chrome dyes at pH 4–5 Levelling acid dyes from phosphoric acid solution

Cotton/basicdyeable polyester

Solid

Two-bath

Basic dyes or chrome dyes, then salt-controllable direct dyes at pH 8

Solid or contrast

One-bath

Direct dyes and basic dyes with anti-precipitant at 120°C

Two-bath

Basic dyes, then aminochlorotriazine reactive dyes at 80°C Vinylsulphone reactive dyes, then basic dyes at pH 5

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Table 9.2 Dye selections for continuous dyeing of cellulosic/acrylic blends Blend

Colour effect

Dyeing method

Dye selection

Cellulosic/ acrylic

Solid

Pad–dry–thermofix (pale depths)

Disperse dyes

Pad–dry–steam

Basic dyes with citric acid Direct dyes and basic dyes with anti-precipitant and fixation accelerator

Pad–dry–chemical pad–steam

Halogenated vat dyes or sulphur dyes

Pad–dry–thermofix– chemical pad–steam

Ammonium salt pretreatment, then vat dye liquids and stabilised basic dyes at pH 8–9

Pad–dry–steam, pad–dry–chemical pad–steam

Conventional basic dyes, then vat dyes

Pad–dry–steam, pad–dry–steam

Conventional basic dyes, then reactive dyes

round fastness require reactive or vat dyes for the cellulosic component and basic dyes for the acrylic fibre (Table 9.2).

9.6 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

M White, F Schlaeppi, N E Houser and J T Larkins, AATCC Nat. Tech. Conf. (Oct 1983) 280. A Laeppli and R Jenny, Textilveredlung, 23 (1988) 248. W Haertl, Textil Praxis, 44 (1989) 285; Melliand Textilber., 70 (1989) 354; Textilveredlung, 24 (1989) 214. J A Hook and A C Welham, J.S.D.C., 104 (1988) 329. Anon, Chemiefasern und Textilind., 34/86 (1984) 752. F R Latham in Cellulosic dyeing, Ed. J Shore (Bradford: SDC 1995) 246. H M Tobin, Am. Dyestuff Rep., 70 (Sep 1981) 32. H Fischer, Textilveredlung, 13 (1978) 449. M A Herlant, Text. Chem. Colorist, 17 (June 1985) 117; Am. Dyestuff Rep., 74 (Sep 1985) 55, (Oct 1985) 37.

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CHAPTER 10

Cotton/viscose and other CC blends

10.1 PROPERTIES AND PERFORMANCE OF CELLULOSIC FIBRES IN THEIR BLENDS Bast fibres consist of bundles of thick-walled cells held together by non-cellulosic material. The ultimate fibres in flax and ramie are about 30 µm in diameter. Flax fibres are about 30 mm in length but those of ramie are unusually long (ca. 150 mm). In the bast bundles the ends of these individual fibres overlap. Flax and ramie are separated into their ultimate fibres before spinning into fine yarns. Separation of flax fibre bundles from the harvested stems is time-consuming. During prolonged ‘retting’ or soaking in water, the effect of bacterial action on the intercellular material loosens the fibres sufficiently for mechanical separation by ‘scutching’ and ‘hackling’. The mechanical decortication of ramie can be achieved without preliminary retting. Bast fibres contain far less cellulose than does cotton (Table 10.1). The intercellular material includes pectins, hemicelluloses and lignins. Much of this is removed when flax or ramie is scoured. Inadequately scoured goods are difficult to dye level because the bast fibre and the non-cellulosic impurities differ in dyeability.

Table 10.1 Composition of typical natural cellulosic fibres [1] Proportion of dry weight (%)

Constituent

Raw flax

Decorticated ramie

Grey cotton

Cellulose Intercellular material Wax Ash Residual material

80.1 10.5 2.6 1.5 5.3

83.3 7.5 0.2 2.1 6.9

94.0 2.5 0.6 1.2 1.7

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Traditional viscose filaments have been produced by extruding a solution of sodium cellulose xanthate dissolved in aqueous sodium hydroxide through a spinneret into a coagulating bath containing sodium sulphate and sulphuric acid. The filaments are stretched mechanically during regeneration and their properties are determined by the concentration of the viscose solution, the rate of coagulation and the rate of stretching. The coagulation process is controlled by temperature and additives such as zinc sulphate or glucose. Regular viscose differs from cotton in being non-fibrillar, having no central lumen and having a much lower degree of polymerisation (DP). Although consisting wholly of cellulose, the skin and core of viscose filaments differ somewhat in supramolecular structure. The relative proportions of skin and core vary according to the conditions of coagulation. The presence of viscose in blends with cotton improves the appearance by imparting more lustre and firmness of handle. The regenerated fibre provides additional absorbency, which is useful in towelling constructions. Apparel uses for cotton/viscose blends include poplin shirts, blouses, dresswear, knitwear, leisure garments, T-shirts, underwear and children’s clothing. These blends offer comfort appeal with good wear and laundering properties. The optimum blend composition for wear resistance is approximately 70:30 cotton/viscose. Terry towelling may be made from 50:50 or 65:35 cotton/viscose blends for greater absorbency than all-cotton cloths. An important traditional use for cotton/viscose unions is in brocade material for curtains and furnishings in which the viscose appears on the surface in the form of floral designs. These constructions often contain both filament and staple yarns. Pile fabrics are sometimes made with a viscose pile in a cotton backing fabric. These are an economical alternative to wool pile/cotton backing fabrics. Hollow viscose fibres, such as Viloft (Courtaulds), have been produced in an attempt to simulate the natural lumen of cotton. Sodium carbonate is incorporated in the spinning dope. When this is extruded into the acidic coagulating bath the carbon dioxide formed inside the filament creates a continuous hollow central channel. Careful control of the conditions of the carbonate decomposition reaction is necessary to obtain a reproducible product [2]. The hollow structure of the fibre imparts high torsional rigidity leading to an attractive handle with higher bulk and fabric cover than regular viscose at the same fabric density. Hollow viscose fibres have a lower density (1.15 g cm–3) and higher water imbibition (130%) than regular viscose (1.52 g cm–3 and 90%), giving good insulation, extra absorbency and comfort. These characteristics make an important contribution to the appeal of cotton/ Viloft and polyester/Viloft blends. These have been exploited successfully in

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knitted underwear, sportswear, leisure garments, dresswear and towelling. In roller towels, for example, a polyester/Viloft warp yarn offers substantially greater strength than traditional cotton yet contributes even higher absorbency to the construction [2]. Compared with cotton, regular viscose suffers from the disadvantage of much lower breaking strength, particularly when wet. This is seldom a problem in apparel but it renders viscose unsuitable for more critical end uses. The term ‘modal’ was introduced to describe all regenerated cellulosic fibres having tenacities in the conditioned state and wet moduli at 5% extension above certain defined values [1]. Modal fibres are made with an above-normal concentration of zinc sulphate in the coagulating bath and ‘modifiers’ (such as dimethylamine or cyclohexylamine) in the spinning dope. These fibres have a higher initial wet modulus than regular viscose but lower than that of the polynosic fibres. Polynosic fibres have been defined as having low wet extension even under alkaline conditions, high knot strength and a higher DP than regular viscose. The distinctive features of their method of production include: (1) a xanthate solution of viscosity higher than that used in the manufacture of regular viscose, achieved using an aged pulp of intrinsically higher DP rather than a more concentrated solution; (2) a coagulating bath of low salt concentration with no modifiers or other additives; (3) a lower temperature of extrusion than regular viscose. Under these conditions, high stretch (up to 300%) can be achieved. Polynosic fibres are highly oriented and have stress–strain curves closely similar to those of cotton, rather than to other regenerated cellulosic fibres. They appear to be fibrillar in structure and are largely unaffected by dilute solutions (up to 8%) of sodium hydroxide, which will dissolve as much as 25% of regular viscose. Polynosic fibres show improved laundering performance and give good yarn strength in blends with cotton but they are generally less useful for blending with polyester fibres [2]. Modal and polynosic fibres are finding increasing application in blends with cotton for knitgoods. Vincel (Courtaulds) in a polynosic fibre that is used alone or in blends with cotton, viscose or polyester staple fibres. A popular blend for apparel fabrics is 50:50 cotton/Vincel. Blends of cotton with modal fibres are particularly important in woven dresswear and lightweight suitings. These modalrich blends (80:20 to 55:45) are usually designed to exploit the lustre, drape and softness of the regenerated fibre, whereas in cotton-rich mixtures the strength, washability and durability of the natural fibre make important contributions.

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The degree of swelling of modal and polynosic fibres alone and in 50:50 blends with cotton was compared with an all-cotton control in solutions of sodium hydroxide at concentrations up to 250 g l–1 and temperatures in the range 20–80°C. At ambient temperature the regenerated fibres were much more swollen than cotton and the 50:50 blends showed the expected intermediate degree of swelling. As the temperature of treatment increased towards 80°C, all fibre types and blends showed enhanced swelling, so that the behaviour of the individual fibres and their blends became more closely similar. Under mercerising conditions of high alkali concentration and low temperature for a short time, the modal fibres and cotton control behaved similarly [3]. The adverse effects of conventional viscose manufacturing plants on the environment has been recognised for many years. Only recently, however, has Courtaulds plc established commercial production of a regenerated cellulosic fibre using a non-aqueous solvent method. Tencel (Courtaulds) is a ‘lyocell’ fibre obtained by continuous dissolution of wood pulp in mesomorphic Nmethylmorpholine-N-oxide (Figure 10.1) and extrusion into a dilute aqueous solution of the amine oxide to precipitate the regenerated fibre [4]. The diluted solvent is then purified and reused at the continuous dissolving stage, so that the process is environmentally innocuous.

CH3

CH2 CH2 O

N CH2 CH2

O

Figure 10.1 Solvent for manufacture of lyocell regenerated cellulosic fibres

Tencel has a bright lustre and a circular cross-section. The tenacity (wet and dry) is markedly higher than that of cotton or any other type of regenerated cellulosic fibre (Table 10.2). The wet tenacity is only about 15% lower than the dry value and is markedly higher than that of cotton. The exceptionally high wet modulus results in very low shrinkage, about 2% in warp and weft [5]. Tencel is fibrillar in structure and resembles cotton even more closely than modal fibres in its behaviour under stress and capacity for absorbing liquid water. Because of the close similarity between the stress– strain curves of Tencel and cotton, it can contribute to the strength of the blended yarn even at low blend levels. An interesting feature of Tencel is that

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Table 10.2 Typical physical properties of cellulosic fibre types [4] Fibre tenacity (cN tex–1) Fibre type

Wet

Dry

Elongation (%)

Viscose Modal Lyocell Cotton

10–15 19–21 34–38 26–30

22–26 34–36 40–42 20–24

20–25 13–15 13–15 7–9

Moisture regain (%)

Water imbibition (%)

13 12.5 11.5 8

90 75 65 50

the conversion of fibre strength to yarn strength is considerably higher than for other cellulosic fibre types, because of the high cohesion between the closely packed fibres of circular cross-section in the yarn. Tencel improves the performance of blends with cotton by enhancing strength, lustre, yarn regularity, spinning and wear performance.

10.2 DYEING BEHAVIOUR OF CELLULOSIC FIBRES IN THEIR BLENDS Linen and its blends with cotton have been used traditionally in fine woven apparel and household textiles, notably tablecloths, napkins, curtains and furnishings. Ramie/cotton blends (60:40 to 50:50) are of interest for woven or knitted leisure clothing. Linen fabrics and blends of linen or ramie with cotton will withstand an alkaline scour at the boil, followed if necessary by a combined peroxide/chlorite bleach. Dyeing in rope form with direct dyes is followed by conventional resin finishing. Treatment with liquid ammonia at 20 m min–1 can be carried out before or after dyeing to enhance the performance of the final finish. It is essential to carefully neutralise any retained ammonia by treatment in rope form with acetic acid solution. Effective finishing is also important to achieve optimum wet fastness of the direct dyes [6]. The traditional growing of flax has been resumed in Saxony since 1993. Fabrics woven from open-end yarns spun from 40% short-staple linen and 60% modal fibres are being produced. Various preparation sequences have been evaluated, including enzymatic desizing in a jet machine and cold pad–batch bleaching, or continuous pad–steam scour-bleach treatment with alkaline peroxide. The preferred dyeing process is exhaust dyeing with reactive dyes, which offer excellent reproducibility, levelness, penetration and fastness performance [7]. Direct dyes usually dye viscose and mercerised cotton preferentially because a greater surface area is available for sorption on these substrates than on

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unmercerised cotton. The order of increasing substantivity on various cellulosic fibres does not differ significantly from dye to dye, except in the case of phthalocyanine blues. These show higher substantivity for cotton than for viscose or modal fibres. This is because their affinity for regenerated cellulose is lower in spite of its greater accessibility [8].The higher dyeability of mercerised cotton is attributable to the lower surface charge on this substrate. This difference is less marked at higher electrolyte concentrations but under these conditions direct dyes show slower migration and inferior levelling. Solidity of shade with direct dyes on cotton/viscose blends varies considerably with dye structure but it can be controlled by adjusting salt concentration and dyeing temperature. The substantivity of direct dyes for cellulose is approximately inversely related to their degree of sulphonation. Direct dyes giving good solidity on cotton/viscose blends tend to be mainly disazo tetrasulphonates, including some copper-complex types. Pale shades present little difficulty when dyed in the absence of salt. Solidity in deeper shades is achieved more readily with little or no salt present at the boil. Where salt must be used for medium and full depths in order to attain economical exhaustion, sometimes solidity can be ensured only by dyeing at a temperature as low at 60°C. The optimum conditions vary from one dye to another and result in reduced penetration and lower wet fastness. Blends of mercerised cotton and viscose, however, will often give good solidity in full depths by dyeing at the boil with only low concentrations of salt (0–5 g l–1) because of the higher dyeability of mercerised cotton. The development of viscose microfibres (section 1.4.2) has enabled colour yields and reflectance values to be obtained with direct and reactive dyes that are close to those on cotton. As a result, the attainment of solid effects on fabrics containing viscose microfibres and cotton is now easier than on conventional cotton/viscose blends [9]. Solidity of shade is normally aimed at in the dyeing of cotton/viscose blends and it is not feasible to attempt reservation of either fibre. Vat or sulphur dyes are often used because it is generally more difficult to achieve solidity with direct or reactive dyes. Brocades and other furnishing fabrics woven into designs formed by raised viscose wefts on a cotton warp ground are not as critical as intimate blends where lack of solidity gives an objectionable skittery appearance. As with direct dyes, the degree of solidity attainable with vat dyes depends on dye selection and conditions of application. Preferred vat dyes include acridones, carbazoles, indanthrone blues and violanthrone blues and greens. If conventional dyeing conditions are applied, regular viscose will invariably be dyed more heavily than the cotton. A decrease in dyeing temperature favours the cotton, so

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cold-dyeing vat dyes are more suitable in general. Pigment padding methods tend to give better solidity than batchwise dyeing on the jig at 30–50°C. The proportions of crystalline material in regenerated cellulosic fibres are about 40% in regular viscose, 50% in modal fibres and 65% in polynosics, compared with 70% in cotton. As crystallinity increases the water imbibition and dyeability decrease accordingly. Thus direct dye uptake under a given set of conditions generally increases in the order: cotton < polynosics < modal fibres < regular viscose. The uptake of selected direct dyes by the hollow viscose fibre Viloft (Courtaulds) and two crimped modal fibres Avril (Avtex Fibers) and Prima (ITT Rayonier) has been compared with cotton and regular viscose as controls (Table 10.3). CI Direct Red 80 is a tetrazo hexasulphonate of unusually high affinity, whereas Blue 218 (a copper-complex disazo tetrasulphonate) and Yellow 106 have only moderate affinity for cellulose. The percentage exhaustion at equilibrium was consistently lower on cotton than on any of the regenerated cellulosic fibres, as expected. The dyeability of Viloft was closely similar to that of regular viscose. Prima was consistently more dyeable than Avril, which was in turn more dyeable than regular or hollow viscose.

Table 10.3 Differences in dyeability between various cellulosic fibres [10] Equilibrium exhaustion (%) of CI Direct Fibre

Yellow 106

Red 80

Blue 218

Prima Avril Viscose Viloft Cotton

80 74 66 66 59

99 92 90 88 84

74 66 66 66 50

Methods of dyeing modal fibres and cotton/modal blends with direct, reactive or vat dyes have been reviewed [11]. The selection of reactive dyes for the production of shadow effects on 50:50 cotton/modal blends was outlined. Exhaust dyeing systems for loose stock, yarn, knitted and woven fabrics were described. Several padding methods on fabrics made from these blends were detailed, including pad–batch or pad–thermofix with reactive dyes, pad–jig or pad–steam with vat dyes, and pad–develop with vat leuco esters.

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The dye affinity of polynosic fibres such as Vincel (Courtaulds) can be enhanced by pretreatment with sodium hydroxide solution. The concentration should not exceed 60 g l–1 or there may be a deterioration in physical properties. Most dyes are absorbed more slowly and migrate less readily on Vincel than on regular viscose. Blends of Vincel with either cotton or viscose can be dyed in solid shades with carefully selected direct, reactive, vat or sulphur dyes, many dyes being most suitable at specific depths of shade. The fastness properties of most of these dyes are the same on Vincel and regular viscose. It is not practicable to dye Vincel and reserve the other cellulosic fibre, although attractive shadow effects can be produced with selected dyes and controlled dyeing conditions. A minority of direct dyes, especially from the self-levelling and temperaturecontrollable classes, will give solid shades in pale depths on cotton/Vincel. Selected high-reactivity dyes can also be used, either by pad–batch or exhaust application. Certain bright reactive dyes appear slightly duller on Vincel than on cotton or viscose. Vat dyes are usually selected from those applicable at 50°C or higher temperatures and are preferably applied by pigment padding to improve penetration and solidity of shade. Selected sulphur dyes can be used in medium and full depths. Any of the dye classes used for cotton can be applied to Tencel (Courtaulds) lyocell fibre. Consistency of Tencel in terms of dyeability is routinely monitored using dyes known to be sensitive to potential variations in this important property [4]. In exhaust dyeing the colour yield on Tencel is similar to that on viscose and greater than that on cotton. Thus care is required when dyeing cotton/Tencel blends because of preferential uptake by Tencel. This makes shadow effects much easier to achieve than solidity in exhaust dyeing. Viscose/ Tencel blends, on the other hand, readily give solid effects. The yield of reactive dyes on Tencel is exceptional by all dyeing methods and especially by printing. Thus alkaline treatment analogous to causticisation of viscose or mercerisation of cotton is not necessary for Tencel. Blends of cotton and Tencel can be mercerised, however. By careful modification of conventional semi-continuous dyeing methods, solid dyeings with reactive dyes can be achieved on 50:50 cotton/Tencel, using either pad–batch application or a sodium metasilicate development technique.

10.3 DYEING METHODS AND DYE SELECTION FOR CC BLENDS Blends of cellulosic fibres with one other are ideally suitable for shadow effects, especially those in which unmercerised cotton is the paler component and a regenerated cellulosic fibre is the deeper one. A wide selection of dyes and dyeing

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conditions yields satisfactory solidity on blends of mercerised cotton or regular viscose with one another or with other regenerated cellulosics. Dye selection and dyeing techniques are more restricted when dyeing solid effects on unmercerised cotton blends (Table 10.4).

Table 10.4 Dye selections for CC blends Blend

Colour effect

Dye selection

Cotton/linen Cotton/ramie

Solid or shadow

Direct dyes or vat dyes

Mercerised cotton/viscose

Solid

Direct dyes at low salt concentration at the boil

Unmercerised cotton/viscose

Solid

Disazo tetrasulphonated direct dyes at 60°C

Cotton/viscose

Solid or shadow

Selected vat dyes at 20–30°C

Cotton/viscose microfibres

Solid or shadow

Direct or reactive dyes

Cotton/modal

Solid or shadow

Direct, reactive or vat dyes

Cotton/polynosic Cotton/lyocell

Solid

Selected self-levelling and temperature-controllable direct dyes Selected vat dyes at 50°C Selected high-reactivity dyes by pad–batch

Viscose/polynosic Viscose/lyocell

Solid or shadow

Direct, reactive or vat dyes

10.4 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

T P Nevell in Cellulosics dyeing, Ed. J Shore (Bradford: SDC, 1995) 6. R Aitken, J.S.D.C., 99 (1983) 150. D Bechter, H Herlinger and E Pelz, Textil Praxis, 41 (1986) 59. J M Taylor and P Mears, J.S.D.C., 107 (1991) 64; Chem. Brit. 30 (1994) 628. I D Holme, Dyer, 178 (Oct 1993) 13. G Kratz and A Funder, Melliand Textilber., 68 (1987) 775. H Hellwich, Melliand Textilber., 78 (1997) 346. O Annen, H Gerber and B Seuthe, Melliand Textilber., 72 (1991) 1015; J.S.D.C., 108 (1992) 215. D Hildebrand and F Stöhr, Melliand Textilber., 73 (1992) 281. V Davis and R R King, J.S.D.C., 100 (1984) 342. W Schaumann, Textilveredlung, 22 (1987) 15.

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POLYESTER/WOOL AND OTHER DA BLENDS

CHAPTER 11

Polyester/wool and other DA blends

11.1 DYEING OF POLYESTER/WOOL BLENDS 11.1.1 Properties and preparation of polyester/wool blends The achievement of a desirable combination of physical properties is usually the main justification for utilising a DA blend. Strength, abrasion resistance, crease recovery and durable pleating characteristics are contributed by the polyester fibre component. Virtually all goods made from polyester/wool blends are intended for outerwear, typically suitings, dresses and skirts. Modified polyester fibres with improved resistance to pilling have been blended with wool in knitted jersey dresswear. This can eliminate the need to singe polyester/wool fabrics, a treatment not usually available in wool processing. Singeing may also introduce dyeability differences. Unlike all-wool jersey, polyester/wool fabrics can often be dyed on the beam and these blends show better dimensional stability on washing. Such blends were developed in the early 1950s and have been established in woven suitings ever since. The important 55:45 polyester/wool blend arose from the realisation that this is the minimum polyester content that allows durable pleating of the blend fabric. Reducing the wool content lowers the aesthetic appeal but decreasing the polyester proportion makes it no longer possible to retain pleated effects after washing. The most important blend in the USA is an 80:20 fabric, composed of a textured filament polyester warp and a 55:45 polyester/wool blended staple weft. In Western Europe another luxurious fabric is a 20:80 blend, containing a 55:45 blended staple warp and a pure wool weft. Smaller market niches exist, e.g. a 40:60 polyester/wool blend for luxury automotive fabrics. This specific outlet puts high demands on both fabric and dye performance. To meet these demands the two fibres are usually dyed separately as loose stock or tops and subsequently blended [1]. In recent years the ‘re-discovery’ of natural fibres has given increased emphasis to the aesthetic appeal of wool-rich blends, rather than the optimum balance of comfort, wear and easy-care properties that is provided by the popular 55:45 138

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blend. In these blends the content of polyester may be as low as 20–30% and the production of solid shades using conventional one-bath dyeing methods can be quite difficult. The polyester component is often dyed much weaker in depth than expected [2]. Polyester fibres will withstand the normal processes used to prepare wool fabrics, such as carbonising and milling. The stentering temperature must not be too high when carbonising polyester/wool fabrics, however, because appreciable damage and yellowing of the polyester component may occur under such conditions. Neutralised fabrics should have a slightly acidic pH to avoid possible damage to the wool [3]. It is usual to give a crabbing treatment to polyester/wool fabrics to minimise creasing during winch or jet scouring and subsequent dyeing. Jet dyeing has a mild milling action on these goods and yields a softer handle compared with the somewhat crisper feel characteristic of beam-dyed fabrics. Careful preparation of the fabric prior to beam dyeing is most important. Preshrinking is necessary to prevent any moiré effect (water marking) that may arise from differential shrinkage on the beam [4]. Presetting at 170–190°C protects against rope creasing or possible shrinkage in beam dyeing. Higher setting temperatures cause yellowing of the wool. Heat setting improves the handle, resilience, crease resistance, dimensional stability, shrink resistance and pilling performance of the goods. It does, however, reduce the dyeability of the polyester component after setting. This may aggravate wool staining. Scouring with an anionic detergent and soda ash eliminates the risk of residual nonionic detergent being carried out into the dyebath and adversely affecting the dispersion stability of disperse dyes. Polyester/wool knitted fabrics may be scoured in the jet machine with ammonia at 40°C before dyeing. Bright and/or pastel shades may require a preliminary mild bleaching treatment. The wool may be given either an oxidative or a reductive bleach, whereas the polyester only requires treatment with a fluorescent brightening agent.

11.1.2 Stages at which dyeing may be carried out The dyer of polyester/wool has three options: (1) Dyeing each fibre type separately as loose stock or tops before blending. (2) Dyeing as an intimately blended yarn. (3) Dyeing as fabric in rope form or open width. Dyeing separately prior to blending allows the choice of dyes of maximum fastness, with level dyeing performance being of secondary importance. The polyester can be dyed at 130°C with high-energy disperse dyes of maximum

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sublimation fastness. Wool as loose stock or tops can be dyed with chrome reactive, 1:2 metal-complex or milling acid dyes. Dyeing in yarn or piece form allows greater flexibility with shorter lead times in production and lower stockholding. The most important factor in polyester/ wool yarn dyeing is shrinkage of the yarn, for which 3–5% is considered acceptable [5]. In contrast to polyester/wool fabrics, yarns blended from these two fibres are not heat set before dyeing. Since disperse dyes show higher affinity for unset polyester, high- or intermediate-energy dyes can be applied to the yarn at 105°C to give higher fastness to sublimation and less staining of the wool. Dyeing at these later stages (especially as fabric) demands better level dyeing behaviour from the dyes. This introduces constraints on the level of fastness attainable from the disperse dyes, which must be low- or intermediate-energy types in piece dyeing. Beam dyeing is particularly suited to flat woven constructions and those fabrics where felting could be a problem, i.e. wool-rich blends. Since the fabric is held stationary and the liquor percolates through it, there is no mechanical action on the cloth to induce felting shrinkage. The beam is less effective on structured fabrics where the jet or overflow machine helps to retain fabric handle and bulk. A specific problem associated with dyeing polyester/wool yarn or piece in enclosed machinery at temperatures above the boil is control of dyebath pH. Chemical changes brought about in wool by heat can cause the pH to rise. It if rises above pH 7 the disperse dye dispersion can become unstable, as well as causing further damage to the wool. Acetate–acetic acid buffer systems are often used for their economy and relative freedom from effluent problems. 11.1.3 Selection of dyes and carriers The dyeing of polyester/wool is almost always directed towards solidity rather than differential effects, because unfortunately the most troublesome of all crossstaining problems is the staining of wool by disperse dyes (section 3.4). This blend is most frequently dyed using disperse dyes for the polyester and milling acid or 1:2 metal-complex dyes for the wool. Matched formulations containing both disperse and anionic dyes are commercially available. Since neutral to slightly acidic conditions are required for both dye classes there are no conflicting pH requirements. The critical parameter is that of temperature, since the top temperature (130°C) that would allow all disperse dyes to be used would also cause unacceptable degradation of the wool. A compromise temperature within the range 95–120°C is adopted and disperse dyes of low- or intermediate-energy classes are selected to perform well at the chosen temperature. The temperature at which the dyeing machine operates is the most important

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factor affecting the length of the dyeing process, the choice and colour yield of the disperse dyes and the amount of carrier required. The two important factors governing disperse dye selection are: (1) the temperature at which the polyester/wool is to be dyed; (2) the degree to which the disperse dye stains the wool. The higher the dyebath temperature, the wider the choice of dyes, particularly for medium or heavy depths of shade. Disperse dyes without carrier have virtually no substantivity for polyester below 85°C but above this point the dyeing rate doubles for each 5°C rise in temperature. The slow rate of diffusion of dyes into the polyester is an indirect cause of the preferential staining of wool below the boil. Even at low temperatures wool is rapidly penetrated by disperse dyes, especially if dyed in the presence of a carrier. Some low-energy azo disperse dyes are taken up by wool extremely rapidly. High-energy dyes do not build up satisfactorily on polyester at the boil even in the presence of a carrier, compared with low-energy dyes with better levelling properties but lower fastness to heat. Only pale depths can be dyed with intermediate-energy dyes at the boil. Medium depths with these dyes require a dyeing temperature of 105°C at least. Good carrier-dyeing properties and low staining or ease of clearing from wool are given by selected intermediate-energy dyes of the nitro, monoazo and especially anthraquinone types. Many anthraquinone dyes are absorbed quickly onto the polyester surface. Those blue and navy dyes that build up satisfactorily at the boil tend to be anthraquinone-based and relatively expensive to manufacture. The inherently more cost-effective azo types will not build up to full depths at the boil, showing inferior levelling and more staining of the wool. To gain the benefit of this cost-effectiveness, it is necessary to dye at 105°C or above. Full navy or black shades are best dyed with intermediate-energy dyes at 110–120°C using a wool protective agent. If maximum colour yield and fastness are essential, high-energy dyes should be applied to the polyester at 130°C before blending with predyed wool. Even if the depth on the polyester is slightly heavier, the blend may still give an appearance of solidity because of the higher lustre of the synthetic fibre. Several factors must be considered in selecting a suitable carrier for polyester/ wool dyeing. These include the types of dyeing equipment available, the degree of staining of the wool and the relation between dye yield and applied concentration of carrier. Carriers exert a plasticising influence on the polyester structure and cause the glass-transition temperature to be lowered, although the

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enhanced swelling that accompanies carrier treatment is not necessarily associated with acceleration of diffusion of the dyes into the fibre. An important aspect of carrier dyeing is the concentration–efficiency profile, i.e. the range of carrier concentrations within which maximum acceleration of dyeing takes place. This occurs when the voids in the polyester fibre become saturated with carrier [1]. Further additions of carrier above this saturation limit merely result in a corresponding increase in carrier concentration in the dyebath. This is accompanied by a restraining action on the disperse dyes present. This is detrimental to colour yield and hence wastes both carrier and dye. It is known that o-phenylphenol tends to accentuate wool staining more than methylnaphthalene or phenolic esters like methyl cresotinate. These are in turn inferior in this respect to diphenyl, trichlorobenzene and the inert esters like butyl benzoate. Diphenyl, o-phenylphenol and trichlorobenzene tend to cause more wool damage and yellowing on exposure to light than do ester carriers. Nevertheless o-phenylphenol has been much used traditionally for polyester/ wool dyeing on ground of cost-effectiveness. Odour has been a problem with methylnaphthalene or methyl salicylate and trichlorobenzene has been long regarded as a serious toxic hazard. Diphenyl has tended to give localised carrier spotting and the esters are inferior in cost-effectiveness. There is relatively little cross-staining of the polyester fibre by anionic dyes for wool. Levelling acid dyes give negligible staining and the reserve of polyester by 1:1 metal-complex or milling acid dyes is good to very good, with 1:2 metalcomplexes and chrome dyes moderate to good in general. Generally speaking, unsulphonated 1:2 metal-complex dyes stain polyester more than their sulphonated analogues and premetallised azo dyes more so than unmetallised azo acid dyes. Anionic dyes begin to dye the wool at 40–50°C and dyebath exhaustion is virtually complete after about 30 minutes at the boil. Dye selection for wool is almost independent of dyeing temperature. Wool dye selection can thus be made on grounds of wet fastness, since the levelling and coverage is adequate under the conditions required to obtain a satisfactory dyeing of the polyester fibre. Neutral dyeing of polyester/wool blends is not recommended for three reasons: (1) Damage of the wool is much less in the isoelectric region (pH 4.5–5) compared with pH 6–8 conditions. (2) The release of wool breakdown products causes the dyebath pH to rise, resulting in instability of the disperse dye dispersion. (3) Certain azo disperse dyes susceptible to reduction are adversely affected by high temperature dyeing, as a result of the progressive hydrolysis of cystine residues in wool.

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In the dyeing of these blends an optimum pH 5–5.5 should be maintained when using sensitive azo disperse dyes and, if necessary, a reduction inhibitor such as formaldehyde should be present in the dyebath [6]. Neutral-dyeing 1:2 metal-complex and milling acid dyes are preferred for polyester/wool because 1:1 metal-complex and levelling acid dyes require strongly acidic dyebaths where disperse dye staining is severe. Chrome dyes are given an oxidative aftertreatment that can damage the wool and change the hue of the dyed material [7]. A one-bath method for polyester/wool yarn using disperse and chrome dyes allows full reduction clearing to eliminate loose disperse dye without causing a significant shade change or loss of depth of the wool dyeing [8]. Premetallised and milling acid dyes for wool tend to exhaust too rapidly at pH 4.5–5, especially the unsulphonated types and pale or medium depths of monosulphonated 1:2 metal-complex dyes, resulting in unlevel dyeings. Disperse dye staining is also more severe. For these reasons the dyebath is normally buffered at pH 5.0–5.5, just above the ‘ideal’ isoelectric zone, necessitating careful control of treatment time. Most milling acid dyes show satisfactory exhaustion and levelling after 10 minutes at 110°C in this region of pH. Thus premetallised dyes, supplemented by selected milling acid dyes in bright shades, are widely used. The choice between unsulphonated, monosulphonated or disulphonated 1:2 metal-complex types is related to dyebath conditions, coverage and levelling requirements, as well as the target level of wet fastness in subsequent tests. Unsulphonated premetallised dyes applied from a weakly acidic bath show better levelling properties than sulphonated types but are more expensive and lower in wet fastness, particularly in respect of staining of adjacent nylon, a common lining material in polyester/wool garments. Monosulphonated 1:2 metal-complex dyes have good coverage and levelling properties at the high temperatures necessary for polyester/wool dyeing as well as good wet fastness and better cost-effectiveness than unsulphonated analogues. They are usually applied within the range of pH 5–6, according to depth of shade. Disulphonated 1:2 premetallised dyes show more limited coverage, although this is still adequate at high temperature. The wet fastness is the best of the three types and these dyes are usually the most cost-effective. Disulphonated dyes are typically used for full depths and are best applied at pH 5. The choice between the one-bath and two-bath methods with disperse and neutral-dyeing acid dyes depends on applied depth and target fastness requirements. The one-bath method is more economical and gives satisfactory fastness properties in pale or medium depths. The two-bath sequence gives more reproducible solidity or brighter contrast effects as well as optimum fastness in

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full depths. In the one-bath process the disperse and acid dyes are applied together at pH 5–6 with a dinaphthylmethanedisulphonate dispersing agent and the selected type and concentration of carrier. The disperse dye stain is cleared from the wool with a nonionic detergent at 60–70°C. Low-energy disperse dyes of relatively low affinity for polyester tend to cause more staining in the one-bath process than high-energy dyes, particularly at longer liquor ratios. In the two-bath method the polyester component is dyed conventionally with the disperse dyes alone under the conditions specified above. Surface deposition on the polyester and the stain on the wool are removed by reduction clearing at 45–50°C with sodium dithionite, ammonia and nonionic detergent. The wool is then cross-dyed at the boil with 1:2 metal-complex or milling acid dyes from an ammonium acetate–acetic acid bath containing an alkylamine polyoxyethylene levelling agent. The disperse dyes selected for the two-bath method should show minimum transfer from polyester to wool under these conditions. Dyes of high fastness to sublimation do not transfer so readily as those of moderate fastness. Quite recently, a series of new benzothienylazo disperse dyes (Figure 11.1) has been evaluated [9]. These are capable of dyeing both components of a polyester/ wool blend to yield satisfactory exhaustion and fastness (Table 11.1). Dyeing was carried out at pH 4.5 and the boil in the presence of methyl salicylate as carrier. The wool was invariably dyed more deeply than the polyester, presumably because all the coupling components selected were phenolic types with some anionic character. Substituted aniline-type couplers are almost always used in the synthesis of conventional monoazo disperse dyes. In view of the limited range of relatively dull hues obtained, this approach is not of much practical value so far unless dyes of brighter hue, especially blues, can be discovered to augment them. X N S

N R

Figure 11.1 Benzothienylazo dyes for single-class polyester/wool dyeing

11.1.4 Wool damage and wool protective agents The dyeing of polyester/wool blends presents difficulties of severe damage of wool at the top temperature normally used to dye polyester with high-energy dyes. The range of disperse dyes performing adequately in carrier dyeing at 105°C is limited. Some carriers are toxic (e.g. trichlorobenzene) and all carriers are harmful in the working environment. There are considerable advantages to

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Table 11.1 Hue and fastness properties of benzothienylazo dyes [9] Light fastness

Wash fastness

Substituent (X)

Coupler (R)

Hue on polyester/wool

P

P/W W

P

P/W W

CN CN CN CN CN COOEt COOEt COOEt COOEt COOEt

Resorcinol Naphthalene-2,3-diol Naphthalene-2,7-diol 8-Hydroxyquinoline 2-Naphthol Resorcinol Naphthalene-2,3-diol Naphthalene-2,7-diol 8-Hydroxyquinoline 4-Chloro-1-naphthol

Yellowish orange Reddish brown Reddish orange Reddish violet Reddish orange Reddish brown Reddish violet Reddish violet Reddish brown Reddish violet

7 7 6 7 7 6 6 6 7 7

6 7 7 6 7 7 6 7 7 7

4 4 5 4 4 4 4 5 4 4

5 5 4 5 4 5 4 5 5 5

6 6 7 7 7 6 7 6 6 6

5 5 5 5 5 5 5 5 5 4

P = polyester W = wool P/W = polyester/wool

be gained by dyeing at 110°C or above, in terms of colour yield, levelling, fastness and shorter processing cycles. Against this must be balanced the physical and chemical degradation of the wool keratin when treated under these conditions. Aqueous hydrolysis results in breakage of electrostatic linkages between oppositely charged sidechains. Dyeing wool at the boil and pH 2–4 results mainly in hydrolysis of peptide bonds (depolymerisation). At pH 5 and above the main effect is hydrolysis of the disulphide bonds in cystine units (breaking of crosslinks). Damage is considerable at 120°C and any pH. These chemical changes result in lower strength and abrasion resistance, accompanied by increased elongation, alkali solubility and yellowing. The extent of damage to the wool depends on dyebath temperature, pH and treatment time. The degree of yellowing of the wool is greater at 120°C than at 110°C but the effect of yellowing is less critical when dyeing in dark shades. For these dyeings the higher temperature favours higher exhaustion of intermediate-energy disperse dyes. Such problems are especially acute with ultra-lightweight dresswear made from polyester microfibres blended with exceptionally fine wool. In order to minimise wool damage, it is necessary to add a reagent that will chemically modify the molecular structure of the wool keratin, stabilising certain reactive sidechains and forming new crosslinks that replace those lost during hydrolysis. For many years, formaldehyde was the only reagent to confer effective protection in this way (Scheme 11.1). The crosslinking action of

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CH2S

[wool]

SCH2

H2O [wool]

Cystine (CH2)4

[wool]

CH2SH + HSCH2

[wool]

Cysteine (CYS)

[wool]

CH2SCH2SCH2

[wool]

CYS CYS link

HCHO (CH2)4

HCHO

NH2

Lysine (LYS)

[wool]

[wool]

LYS NHCH2OH

[wool]

(CH2)4NHCH2NH(CH2)4 LYS

Methylollysine

[wool]

LYS link CH2OH

2HCHO

[wool] CH2

[wool] CH2

OH

Tyrosine (TYR)

OH

Dimethyloltyrosine

CH2OH

LYS

[wool] CH2

CH2OH N H

[wool] CH2

Tryptophan (TRY)

CH2NH(CH2)4 TYR

2HCHO [wool]

OH

CH2

[wool]

[wool]

LYS link

CH2

LYS HOCH2

HOCH2 N

N

CH2OH

CH2NH(CH2)4

Dimethyloltryptophan

TRY

[wool]

LYS link

Scheme 11.1

formaldehyde causes embrittlement of wool, slightly reducing the elongation at break and markedly lowering the urea-bisulphite solubility. When dyeing in the presence of 5% o.w.f. formaldehyde (30% solution), the treatment time should not exceed 60–90 minutes at 110°C or 30–40 minutes at 120°C. Several further precautions are necessary if formaldehyde is added: (1) Careful dye selection is important because certain dyes undergo a colour change in the presence of formaldehyde at 110–120°C. (2) Treatment should be within the isoelectric region for wool (pH 4.5–5) and the maximum permitted dyeing time should not be exceeded.

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(3) Ammonium salts for pH control should be replaced by an acetate–acetic acid or mixed phosphate buffer, because ammonia reacts with formaldehyde to form hexamethylene tetramine. (4) Formaldehyde vapour is environmentally hazardous and this increasingly restricts the circumstances in which it can be used. This trend favours the replacement of formaldehyde by precursor compounds of the N-methylolamide class. Disadvantages of this process include residual odour, variations in fabric strength and harshness of handle, especially with beam-dyed fabrics [10]. The leading formaldehyde precursor used for this purpose is dimethylolethyleneurea (DMEU). This functions as a wool protective agent by slow decomposition to release formaldehyde into the dyebath. There is little release of the active species +CH2OH at pH 4–6 and 70°C but at 110–120°C DMEU is almost completely dissociated into ethyleneurea and formaldehyde (Scheme 11.2).

HOH2C N

C

N CH2OH

2H+

NH + 2 +CH2OH

HN C

O

O

DMEU

EU

Scheme 11.2

HO HOH2C N

HO

OH

C

N CH2OH

2H+

OH

HN C

NH + 2 +CH2OH

O

O

DMDHEU

DHEU

Scheme 11.3

The addition of dimethyloldihydroxyethyleneurea (DMDHEU) when dyeing polyester/wool permits treatment for up to one hour at 120°C, resulting in improved colour yields on the polyester component without detrimental effect on the physical properties and structure of the wool. By a similar mechanism (Scheme 11.3), DMDHEU acts as a stabiliser for the wool in pressure dyeing and accelerates the rate of dyeing of the polyester fibre without adversely affecting the equilibrium exhaustion of the acid dyes [11]. Reactive dyes also exert a protective effect on wool keratin. They are believed to minimise damage by reacting with the cysteine residues formed by cystine

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hydrolysis (Scheme 11.1). This concept has been exploited to develop colourless protective agents that imitate the behaviour of reactive dyes [12]. These more sophisticated products are likely to be less cost-effective than formaldehyde precursors but they do avoid the hazards of exposure to formaldehyde vapour. In a recent investigation of this problem, botany wool fabrics were treated with solutions of potential protective agents at 120°C or 130°C and pH 4. Wool damage was assessed in terms of tear strength and wet bursting strength. The addition of either potassium bromate (KBrO3), to counteract the reductive hydrolysis of cystine disulphide crosslinks, or sodium hydrogen maleate (HOOC–CH=CH–COONa), to react with the thiol groups of the cysteine formed, improved the strength retention. Both products caused stiffening of the fabric but the handle of the bromate-treated wool was superior to that treated with sodium hydrogen maleate [13]. Dimethylolethyleneurea and selected vinylsulphone and α−bromoacrylamide reactive dyes gave some protection but the wool damage was still substantial at 130°C. 11.1.5 Future prospects for polyester/wool blends The availability of deep-dye and basic-dyeable polyester yarns (section 5.1) has widened the range of possibilities of fabric design in piece-dyed polyester/wool. Deep-dye polyester staple fibres blended with wool can be dyed without the cost or pollution problems associated with carriers and pressure-dyeing equipment is not essential. The melting point and initial modulus of deep-dye polyester, however, are lower than the normal fibre. Trevira 350 (HOE) is a low-pill staple polyester of high dyeability but lower tensile strength and resistance to abrasion than the standard homopolymer. Pretreatment of Trevira 350/wool blends entails an emulsified solvent scour to remove oil stains, followed by drying and heat setting. The dyebath is set at pH 5–6 with a levelling agent and a crease lubricant. A sequestering agent is not normally required. The wool component is often dyed with 1:2 metal-complex dyes. Dyeing is generally carried out on an overflow machine at 105°C. Under these conditions the colour yield on Trevira 350 is 5–10% higher than on standard polyester [14]. Carriers have been widely recognised to be injurious to the health of operatives working with them. Carrier vapours are hazardous and pollute the environment where they are used. Residual traces present in the dyed fabric may be released on subsequent heat treatment. These products are being increasingly restricted from use in sophisticated markets. Polyester/wool fabrics may still be dyed with carriers in those developing countries where environmental laws are less rigorous than in highly developed economies. This is typical of a general trend in which harmful products or processes that have been abandoned in

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developed countries for environmental reasons may still be undertaken in lowwage or less-regulated industries. Several possible options exist to minimise the future use of carriers by polyester/wool dyers. These include: (1) Improved carriers with no significant environmental problems may be developed, such as ethylenediaminetetramethylphosphonic acid (EDTMP). The surprising effectiveness of this compound (Figure 11.2) in enhancing the uptake of disperse dyes by polyester fibres [15] has not yet been demonstrated on a commercial scale. (2) Improved wool protective agents may be developed for use in dyebaths at 120°C. The existing products intended for this purpose are not impressively effective (section 11.1.4). (3) Improved deep-dye polyester fibres may be developed for dyeing in blends with wool at 105°C or lower temperatures. The existing variants of this type are rather costly and show other disadvantages (section 5.1). (4) Producer-coloured or stock-dyed polyester may be blended with scoured wool that may be cross-dyed later in yarn or piece form. This approach is less versatile than those listed above. (5) Producer-coloured or stock-dyed polyester may be blended with wool already predyed as loose stock or tops. This is even less versatile but there are no restrictions on dye selection and this approach yields a product of maximum fastness. O HO P

O H2C

HO H2C

P OH OH

N CH2CH2 N

O HO P

CH2

O CH2

P OH OH

HO EDTMP

Figure 11.2 Potential carrier to enhance disperse dye uptake

11.2 BLENDS OF CELLULOSE ACETATE OR TRIACETATE WITH WOOL Worsted-spun blends of cellulose acetate/wool are cheap and attractive for handknitting yarns. The full handle, elastic recovery and resilience of knitted wool are not much affected by inclusion of 30–40% cellulose acetate fibre. The acetate component of the blend contributes improvements in crease recovery and dimensional stability during washing. Blends of 70:30 wool/acetate became important for a time in woven carpet constructions. Although more expensive

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than viscose, cellulose acetate offered better resilience and coverage compared with wool/viscose blends [16]. Apparel end uses such as blouses, skirts and suitings are made from blends of wool with cellulose acetate or triacetate. Triacetate/wool is used for dresswear and leisure clothing, but this blend is less important than acetate/wool or polyester/wool. Triacetate/wool fabrics have good crease recovery and dimensional stability (Figure 1.1), but wool is degraded by the ‘S finish’ usually given to 100% triacetate fabrics. This is a surface saponification in 3% sodium hydroxide solution at 80–90°C, often given as an antistatic and antisoiling treatment. It is difficult to set triacetate/wool fabrics without damaging the wool. Durable pleating can be introduced if the triacetate content exceeds 60%, however, and triacetate/wool is more resistant than acetate/wool to the boiling conditions necessary to dye the wool satisfactorily. Cellulose acetate/wool fabrics will not withstand carbonising. The fabrics are easily damaged and scouring or milling processes must be carried out with minimum mechanical friction under mild conditions of alkalinity and temperature. A suitable sequence is: (1) crabbing at pH 5–6 and 80–85°C with an alkylphenol polyoxyethylene; (2) scouring at 40–50°C with ammonia and a nonionic detergent. The staining of wool by disperse dyes increases with decreasing pH and becomes particularly serious if the saturation limit of the cellulose acetate is exceeded. The acid conditions at the boil required to apply 1:1 metal-complex or levelling acid dyes to wool would damage cellulose acetate and cause more severe disperse dye staining of the wool. Pale and medium depths are dyed at 85–90°C or below to avoid loss of lustre by the acetate fibres. Full depths must be dyed at the boil to achieve optimum fastness of the anionic dyes on the wool, but the decreased lustre of the acetate component is less obvious in full depths. Disperse dyes are absorbed mainly by the acetate component below the boil, but migration in favour of the wool proceeds when the boil is reached. On prolonged boiling some migration back to the cellulose acetate may occur if the saturation limit has not been exceeded, but this is usually accompanied by damage to the acetate fibre. Disperse dyes for cellulose acetate giving minimum staining of wool are mainly low-energy (Mr 220–300) monoazo, nitro or quinoline yellows and oranges, together with intermediate-energy (Mr 300–380) monoazo reds and anthraquinone violets, blues and greens. Knitting yarns normally require a one-bath method with disperse and acid dyes to give either solidity of shade or contrast effects, or with acid dyes alone to give reservation of the acetate. Colour contrast effects are often a feature of twofold acetate/wool yarns and these can usually be achieved by dyeing the fibres

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simultaneously. The dyes for wool can be milling acid or 1:2 metal-complex types, according to brightness and wet fastness required. Neutral-dyeing milling acid dyes of Mr 550–850, mainly disulphonates of disazo, anthraquinone, xanthene or triarylmethane chromogens are used for acetate reserve effects, or for shadow and contrast effects in conjunction with disperse dyes. The disperse and milling acid dyes are applied at pH 5–6 from an ammonium acetate–acetic acid bath with an alkanol polyoxyethylene dispersing agent. Neutral-dyeing 1:2 metal-complex dyes are applied in a similar way at pH 6–7 (ammonium acetate) with a weakly cationic alkylamine polyoxyethylene levelling agent. The surface staining of the wool can be cleared by scouring with a nonionic detergent at 40–50°C. If necessary, sodium dithionite may be added for more effective clearing, especially if azo disperse dyes are present. Brighter contrast effects and better fastness in full depths are achieved by dyeing the cellulose acetate first, giving an intermediate clear and then dyeing the wool at pH 6–7 in a fresh bath. The disperse dyes on the acetate must be selected to withstand these cross-dyeing conditions without migrating to the wool. The tendency of cellulose acetate to delustre in aqueous treatments at temperatures above 85°C has always been a problem for the dyers of blends in which the other fibre gives optimum dyeability under these conditions. This has been overcome by the introduction in 1987 of Xtol (Courtaulds) fibre. This can be dyed at the boil without delustring. The physical properties of Xtol are identical with those of conventional cellulose acetate fibres. Thus it retains the soft handle, rich lustre and comfort properties of traditional acetate apparel and conventional scouring and finishing processes can still be used. The stability at temperatures up to the boil allows a much wider selection of disperse dyes to be used on Xtol. The restriction of conventional acetate to 85°C meant that only disperse dyes that exhausted well at that temperature, i.e. the low-energy types with only moderate to poor wet fastness, could be used. Many of the higher-energy dyes developed for polyester dyeing can be used on Xtol and these dyeings show excellent fastness to washing at 50°C [17]. Intimate blends of wool and cellulose triacetate are usually spun on the worsted system and then piece-dyed for solidity, contrast or reserve effects. The cross-staining of wool with disperse dyes is more pronounced in these blends than in wool/acetate blends, because the dyeing rate on triacetate is so much slower. Carriers will accelerate this rate but their use is deprecated on environmental grounds. Before dyeing, triacetate/wool fabrics are scoured at 50–60°C with ammonia and an anionic detergent. Triacetate is more resistant than the secondary acetate to wool dyeing conditions, but to ensure preferential dyeing of triacetate by the disperse dyes it is essential to

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dye this blend at 105°C, or at the boil with a carrier. These conditions cause more severe disperse dye staining than in cellulose acetate/wool blends at 85–90°C. Cross-staining of the wool can be minimised by including a nonionic dispersant in the dyebath. Recommended disperse dyes of the intermediate-energy class (Mr 300–400) are mainly nitrodiphenylamine yellows, monoazo reds and anthraquinone blues. In a one-bath method with selected disperse and milling acid dyes in pale or medium depths, both classes of dyes are applied together at the boil and pH 6–7 (ammonium acetate), with an anionic dispersing agent and butyl benzoate or diethyl phthalate as carrier. A two-bath procedure entails dyeing the triacetate first at 105°C and the wool later in a fresh bath at the boil. The latter sequence is preferred for full depths where 1:2 metal-complex dyes may be required and wool staining is a particularly serious problem. Migration in favour of the triacetate increases with temperature, dyeing time, pH and concentration of carrier, so that relatively severe dyeing conditions are preferred for optimum yield. The disperse dye stain is cleared from the wool using an anionic detergent at 60–70°C. It may be necessary to add sodium dithionite and ammonia but the physical properties of the wool may suffer. The two-bath method for full depths on triacetate/wool is easier than on acetate/wool because there is less tendency for disperse dyes to transfer from triacetate during cross-dyeing of the wool at the boil.

11.3 DYEING OF POLYESTER/NYLON BLENDS Polyester/nylon blends exhibit exceptional strength and durability in robust outerwear and protective clothing. Half-hose is an important outlet for polyester/ nylon as staple yarns or filament unions. The two fibres may be intimately blended in warp or weft yarns, or may be woven or knitted as separate yarns in the form of designs that are usually coloured in distinctive hues. It is customary to dye the intimate blends in solid shades. Polyester became an important carpet fibre in the USA around 1970 because of the popularity at that time of heavyweight shag-pile constructions. The attractive appearance of polyester yarns was retained longer than that of nylon because of their superior heat set retention. However, when polyester was used later in lighter-weight semi-shag and saxony constructions it was found to have inadequate abrasion performance and resistance to pile deformation. The abrasion resistance of nylon is about three times that of polyester [18]. Blends of 50:50 polyester/nylon were used for lighter-weight carpets in the USA but an 80:20 polyester/nylon blend was introduced in the UK for the so-called ‘splush’

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carpeting, mainly for bedrooms and bathrooms, and this has remained popular for many years [16]. Most of the problems encountered with polyester/nylon carpet blends arise with stock-dyed yarns at the heat-setting stage. During autoclave or continuous steam setting, some low-energy disperse dyes migrate into the spinning lubricant on the fibre surface and give a yarn with inferior fastness to rubbing. Highenergy disperse dyes, if dyed for too short a time at top temperature, yield ring dyeings that give rise to unacceptable colour changes during setting as a result of further dye diffusion within the fibre. The affinity of disperse dyes for polyester is higher than for nylon, but the rate of diffusion in nylon is much more rapid than in polyester. Most disperse dyes, therefore, dye the nylon component of a polyester/nylon blend more heavily in the absence of a carrier at the boil, but the polyester is favoured at higher temperatures or when at dyeing at the boil with a carrier. Carriers of the aryl ester or trichlorobenzene types have been preferred to o-phenylphenol or diphenyl because they give a more satisfactory partition and relatively low residual odour. The use of carriers, however, is seldom acceptable nowadays because of their adverse environmental impact. The principle of producing solid shades of moderate fastness on polyester/ nylon using disperse dyes alone is determined essentially by dye selection and dyeing temperature. The most rapidly diffusing dyes are likely to give the best results. Nylon absorbs almost all of the disperse dye present at 60°C but as the temperature is raised the rate of transfer to the polyester can be controlled by the rate of temperature rise. In high-temperature dyeing at 120°C or under carrierdyeing conditions at the boil most of the nitro and aminoketone (yellow), monoazo (yellow to red) and anthraquinone (red to blue) disperse dyes colour polyester more readily, but some disazo orange and aminonaphthoquinone blue dyes still favour the nylon. Shadow effects can be produced without difficulty but the best reserve is with polyester, using acid dyes on the nylon. It is difficult to dye polyester with complete reserve of the nylon. Many acid dyes give an excellent reserve of polyester when applied at pH 4 and 90–95°C with the usual levelling agents for nylon dyeing. Neutral-dyeing 1:2 metal-complex dyes can be applied in a similar way at pH 7–8. Multisulphonated reactive dyes at pH 3–4 give an outstanding reserve of the polyester, although both of these classes of dyes are rather more sensitive to dye-affinity variations in the nylon. The whiteness of the polyester can be enhanced by applying a disperse-type fluorescent brightening agent at the boil after dyeing and aftertreatment of the nylon.

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Patterned contrast effects are produced by cross-dyeing half-hose knitted from nylon and polyester yarns. In the two-stage method, the darker colour is dyed on the nylon with milling acid or 1:2 metal-complex dyes and the lighter colour is then dyed on the polyester. Disperse dyes selected for their relatively low affinity for nylon are mainly intermediate-energy monoazo or anthraquinone types of Mr 300–380. The nylon is dyed with the anionic dyes at 70°C from an ammonium acetate–acetic acid bath, then the disperse dyes are added, the temperature raised and dyeing continued at 120°C to achieve the target depth on the polyester. Where a deep colour is required on the polyester, it is necessary to use a twobath process. More attractive independent contrasts are possible in this way by dyeing the polyester first at 120°C. The disperse dye stain is desorbed from the nylon in an intermediate clear with a nonionic detergent or destructively stripped with an alkaline dithionite treatment at 70°C. The anionic dyes are applied from a fresh bath at the same temperature, pH control being necessary to facilitate exhaustion. Even so, the contrast is difficult to control owing to some transfer of disperse dyes from the polyester during the second stage of the process [19]. A syntan aftertreatment is given finally to improve wet fastness on the nylon. 11.4 BLENDS OF CELLULOSE ACETATE OR TRIACETATE WITH NYLON High tensile strength and abrasion resistance are important contributions made by nylon to these blends. In woven or knitted dresswear, linings and undergarments, the cellulose ester fibre usually predominates and blends ranging from 85:15 to 65:35 are typical. Cellulose acetate/nylon has been used in staple blends for hand-knitting yarns or as nylon warp/acetate weft woven filament dresswear. Special effects are attainable using ply yarns, e.g. a crimped nylon yarn twisted with a bright cellulose acetate filament yarn. Triacetate/nylon fabrics often exhibit better elastic properties and abrasion resistance. These blends are suitable for hosiery, knitwear and sportswear. Warp-knitted velvet fabrics with a cellulose acetate or triacetate pile and a nylon backing are useful as furnishing fabrics, outerwear, trimmings and lining fabrics, particularly on cost grounds. Overprinting of dyed fabrics with metallic pigments is popular. Dyeing and printing are completed prior to raising [20]. Cellulose acetate/nylon blends cannot be preset in steam or hot air because of the sensitivity of the acetate fibre to heat, but the dyeing process exerts a hydrosetting action to stabilise the material. Intimate blends of these fibres for knitting yarns are dyed to a wide range of shades, mainly in pale depths. Selected disperse dyes can be applied at 80°C with a nonionic dispersing agent to give

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shadow effects. Owing to low wash fastness this method is only suitable for pale or medium depths [19]. Whilst it is theoretically possible to use disperse dyes to colour both components of a 50:50 blend, in practice it is found that most of the dye is absorbed by the acetate component, especially at neutral or slightly alkaline pH. Acceptable solidity on 80:20 blends is given by simple monoazo or 1,4-disubstituted anthraquinone dyes. The light fastness of many disperse dyes varies appreciably between nylon and cellulose acetate. Triacetate/nylon fabrics are usually prescoured at 70°C with a nonionic detergent, stenter set at 210°C and then given an S finish. Solid dyeings can be achieved using disperse dyes at the boil with an anionic dispersing agent and, if necessary, addition of a butyl benzoate or diethyl phthalate carrier. Some dyes show better solidity in the presence of a carrier, whereas others with lower substantivity for nylon give a more solid effect without carrier. The concentration of carrier can be adjusted to shift the balance of partition towards either fibre when carrier dyeing at the boil. High-temperature dyeing at 120°C offers greater latitude in dye selection, improved exhaustion and penetration of both fibres [21]. The best solidity is given by certain low-energy monoazo and anthraquinone dyes. Several orange and brown monoazo and disazo dyes of higher fastness tend to favour the nylon component too much. If problems of solidity arise due to preferential dyeing of the nylon at high temperature, this can be improved by pretreating the blend at 50–60°C with a carrier suitable for triacetate. The use of a carrier should be regarded as a last resort, however, because of the adverse environmental impact of these products. Solidity is not easy to achieve on cellulose acetate/nylon or triacetate/nylon because most disperse dyes show a marked bathochromic shift on nylon compared with their respective hues on the cellulose acetate or triacetate component: yellows appear redder, reds bluer, and blues greener on nylon. This effect makes staple blends look skittery (lacking solidity) if a substantial proportion of nylon is present. The difference in hue may be attributable to the preferential absorption of the more bathochromic ionised species (Scheme 11.4) of such dyes by the amine end groups in nylon. Both fibre components are capable of hydrophobic interaction and hydrogen bonding with either charged or uncharged forms of these dyes (Figure 11.3). The colour difference is of little interest for contrast effects on filament unions because the hue on the nylon is usually duller than that on the acetate or triacetate fibre. A good reserve of cellulose acetate or triacetate is obtained using 1:2 metal-

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NO2 O2N

NO2

NH

OH–

OH

H+

O2N

O–

NH

Mid-yellow

Reddish yellow

CI Disperse Yellow 1

O

OH

O

O–

O

NH2

OH– H+

O

NH2

Red

Violet

CI Disperse Red 15

Scheme 11.4 Reddish yellow NO2 NO2 O2N O2 N

NH

OH

NH

O–

(CH2)6

NO2 O

O C H2C Mid-yellow

+

NH3 NH

O2N

NH

CH3

HC O HC CH

OH

O C (CH2)4

Mid-yellow

Acetate fibre

O C NH Nylon fibre

Figure 11.3 Dye–fibre bonding of CI Disperse Yellow 1 on acetate and nylon

complex or milling acid dyes on the nylon component at 80°C (acetate/nylon) or 120°C (triacetate/nylon) and pH 5–6 with ammonium acetate–acetic acid. Dependent colour contrasts with the nylon heavier in depth are given by disperse dyes selected for minimum staining of nylon and neutral-dyeing acid dyes to fill in the nylon component. When anionic dyes are used in these contrast dyeings, careful pH control is necessary for reproducible results. The preferred milling acid dyes are mainly disazo disulphonates (yellow to red) and monosulphonated (violet to blue) and disulphonated (blue to green) anthraquinone dyes with relatively high wet fastness performance.

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BLENDS OF CELLULOSE ACETATE OR TRIACETATE WITH NYLON

157

The two classes of dyes are compatible but a two-stage sequence may give better control of solidity than does a one-bath process. A carrier may be necessary to assist dyeing of the triacetate. Moderate or full depths of disperse and anionic dyes are afterscoured at 50–60°C with a nonionic detergent, or if necessary with sodium dithionite as a reduction clearing agent. Thorough scouring is essential where good wet fastness is important, because the disperse dyes have only low fastness on the nylon component. 11.5 BLENDS OF POLY(VINYL CHLORIDE) FIBRES WITH WOOL OR NYLON Much of the poly(vinyl chloride) or PVC fibre used for textile applications is found in DA blends with wool or nylon. PVC fibre is an economical component of staple blends and exhibits the unusual properties of inherent flame resistance and a high degree of shrinkage when heated above 60°C. Although the shrinkage phenomenon limits the conditions of processing to some extent, it does enable unique effects to be produced in speciality fabrics. It is essential to ensure that the component fibres are thoroughly blended, as the PVC fibre may contract at the boil to 50% of its original length. During processing of the blend the PVC fibres tend to become concentrated in the interior of the yarn, so that dyeing of the PVC component may be unnecessary if it is present only in a small proportion. Solid pale or medium depths, shadow effects and PVC reserve are all possible in blends with wool or nylon. Staple PVC fibres blended with wool at the 10–20% level contribute strength and bulk to the yarn for hand-knitting or as a weft in blanket manufacture. If latent-shrinkage properties are required without resorting to milling, 20–25% of the PVC fibre can be incorporated and the fabric shrunk by scouring or stenter drying at 80–90°C. Owing to the heavyweight characteristics obtained, these blends are of more interest for outerwear or uniforms rather than dressgoods or light suitings. Latent-shrinkage fibres are usually dyed as staple or combed tops at 50–60°C with a trichlorobenzene carrier before blending with dyed wool. The preshrunk PVC fibres can be dyed at 90–95°C as a blend with undyed wool. Selected lowenergy monoazo and anthraquinone disperse dyes give minimum cross-staining of wool when applied with acid dyes in a one-bath method. Full depths may be dyed by a two-bath sequence: (1) PVC fibre dyed with disperse dyes at 90–95°C; (2) wool stain cleared using nonionic detergent and ammonia at 50–60°C; (3) wool dyed at the boil with premetallised or chrome dyes selected for minimum staining of PVC fibre.

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Knitted interlock made from a 70:30 blend of nylon and PVC fibre for washable apparel is relaxed in boiling water, desized, dyed and stenter dried. Such fabrics are processed in open width throughout under minimum tension to allow optimum shrinkage, giving a felt-like material that can be raised to give a sueded appearance and handle. The PVC fibre migrates inwards to such an extent that often only the nylon needs to be dyed. Selected disperse dyes will give a similar depth on nylon and PVC fibre, but light fastness on the PVC component is often a problem. Low-energy nitro, monoazo and 1,4-disubstituted anthraquinone dyes give the best solidity in general. Most of the other disperse dyes favour nylon or give different tones on the two fibres, nylon again showing the more bathochromic hue. These blends are dyed at the boil with a nonionic dispersing agent and a trichlorobenzene carrier if necessary to promote uptake by the PVC fibre. Acid dyes can be used to fill in the nylon. Full depths should be reduction cleared after dyeing using sodium dithionite, soda ash and a nonionic dispersing agent.

Table 11.2 Dye selections for dyeing of DA blends

Blend Polyester/wool

Colour effect

Dyeing method

Dye selection

Polyester reserve

Single-class

1:1 metal-complex or milling acid dyes at appropriate pH

Solid

One-bath

Intermediate-energy disperse dyes and monosulphonated 1:2 metal-complex dyes at 105°C with carrier High-energy disperse dyes and disulphonated 1:2 metal-complex dyes at 120°C with wool protective agent

Acetate/wool

Triacetate/ wool

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Two-bath

Selected disperse dyes at 120°C, then 1:2 metal-complex or milling acid dyes at the boil

Acetate reserve

Single-class

Disulphonated milling acid dyes

Solid or contrast

One-bath

Selected disperse dyes and neutral-dyeing acid dyes at pH 6

Two-bath (full depths)

Selected disperse dyes at 80°C, then 1:2 metal-complex or milling acid dyes

One-bath

Intermediate-energy disperse dyes and milling acid dyes at the boil with ester carrier

Solid

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159

Table 11.2 Continued Colour effect

Dyeing method

Dye selection

Triacetate/ wool

Solid

Two-bath (full depths)

Intermediate-energy disperse dyes at 105°C, then 1:2 metal-complex dyes at the boil

Polyester/ nylon

Polyester reserve

Single-class

Acid dyes at pH 4 and 90°C

Solid or shadow

Single-class

Low-energy disperse dyes at 110–120°C

Solid or contrast

Two-stage

Darker hue with neutral-dyeing acid dyes at 70°C, then intermediate-energy disperse dyes at 120°C

Two-bath (full depths)

Intermediate-energy disperse dyes at 120°C, then anionic dyes at 70°C with syntan aftertreatment

Acetate reserve

Single-class

Premetallised or milling acid dyes at 80°C

Solid or shadow

Single-class (pale depths)

Low-energy disperse dyes at 80°C

Solid or contrast

Two-stage

Selected disperse dyes, then milling acid dyes at 80°C

Triacetate reserve

Single-class

Premetallised or milling acid dyes at 120°C

Solid

Single-class (pale depths)

Low-energy disperse dyes at 120°C

Two-stage

Selected disperse dyes, then milling acid dyes at 120°C

PVC reserve

Single-class

Premetallised acid dyes at the boil

Solid

One-bath (pale depths)

Low-energy disperse dyes and acid dyes at 90–95°C

Two-bath (full depths)

Disperse dyes at 90–95°C, then premetallised or chrome dyes at the boil

PVC reserve

Single-class

Milling acid dyes at the boil

Solid

Single-class (pale depths)

Low-energy disperse dyes at the boil

One-bath

Selected disperse dyes and milling acid dyes at the boil

Blend

Acetate/nylon

Triacetate/ nylon

PVC/wool

PVC/nylon

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11.6 DYEING METHODS AND DYE SELECTION FOR DA BLENDS In these blends a good reserve can invariably be achieved on the ester fibre or PVC fibre using acid dyes at the appropriate pH and temperature for the aciddyeable component. Intermediate-energy disperse dyes and neutral-dyeing acid dyes are compatible in one-bath processes for solid shades on blends of the ester fibres with wool, although full depths require a two-bath sequence with an intermediate clear to remove the disperse dye staining from the wool. Solidity with low-energy disperse dyes in pale depths is possible on the other blends in the DA category (Table 11.2) but more elaborate two-stage or two-bath procedures are necessary for medium or full depths, the ester fibre or PVC fibre being dyed first.

11.7 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

S M Doughty, Rev. Prog. Coloration, 16 (1986) 25. A F Doran, unpublished work. M Brenner and H Zahn, Melliand Textilber., 64 (1983) 845. H H Konrad and K Türschmann, Textile Praxis, 33 (1978) 932. K H Röstermundt, Deutscher Färber Kalender, 80 (1976) 247. P Rube and D Wegerle, Melliand Textilber., 57 (1976) 496. T Balchin, Am. Dyestuff Rep., 72 (Sep 1983) 27. M Drewniak, Am. Dyestuff Rep., 68 (Jun 1979) 45. T H Afifi and A Z Sayed, J.S.D.C., 113 (1997) 256. G Römer, Textilveredlung, 14 (1979) 332. E D Katcher and M P Neznakomova, Textil Praxis, 37 (1982) 637. D M Lewis, Rev. Prog. Coloration, 19 (1989) 49. C M Carr, J.S.D.C., 108 (1992) 531. K H Röstermundt, Textil Praxis, 47 (1992) 649. Y Riad, S M Hamza, H M El-Nahas and A A El-Bardan, J.S.D.C., 106 (1990) 25. T L Dawson, Rev. Prog. Coloration, 15 (1985) 29. J M Taylor and P Mears, J.S.D.C., 107 (1991) 64. T L Dawson and B P Roberts, J.S.D.C., 93 (1977) 83. H W Partridge, Rev. Prog. Coloration, 6 (1975) 56. G Wünsch, Textilveredlung, 24 (1989) 57. P L Moriarty, Text. Chem. Colorist, 14 (Aug 1982) 148.

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CHAPTER 12

Polyester/acrylic and other DB blends

12.1 DYEING OF POLYESTER/ACRYLIC BLENDS Cotton-spun blends of polyester and acrylic fibres (often 85:15 to 70:30), or polyester fabrics containing acrylic effect yarns, are important in woven or knitted upholstery and furnishings, outerwear, easy-care suiting, dressgoods, sportswear and leisure clothing. As the major component, polyester confers strength, easy-care properties, dimensional stability and high whiteness. Acrylic fibre as the minor component contributes a soft, natural handle, high bulk and cover, improved comfort in wear and versatility in coloration. Polyester/acrylic fibre blends are usually piece-dyed and can be readily processed in garment form, owing to their excellent shape retention, crease recovery and abrasion resistance. Blending of polyester with modacrylic fibres is usually intended to take advantage of the inherent flame resistance of the modacrylic component. The relatively low abrasion resistance of the latter is greatly compensated by the presence of polyester in the blend [1]. Polyester/acrylic fabrics are scoured at pH 4 with a nonionic detergent and heat set at 185–190°C before dyeing. Pale or medium depths of selected disperse dyes can be dyed on the polyester with satisfactory reserve of the acrylic fibre using a methylnaphthalene carrier at the boil. Yellow to orange aminoketone or disazo dyes give the best reserve, but selected low-energy red to blue anthraquinone dyes are also acceptable under these conditions. An excellent reserve of the polyester component is given at the boil and pH 5 with selected basic dyes and a cationic retarder, e.g. cetyltrimethylammonium bromide. The same method can be employed for solid, shadow or contrast effects by adding an alkanol polyoxyethylene as anti-precipitant, appropriate disperse dyes and a carrier formulated with a nonionic emulsifying system. Anionic dispersing agents are best avoided because they may either complex with the cationic retarder and increase staining of the acrylic fibre by the disperse dyes, or complex with the basic dyes and cause unacceptable restraining. Anionic emulsifying

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POLYESTER/ACRYLIC AND OTHER DB BLENDS

systems for the carrier would also show these defects and may exhibit lower emulsion stability in the presence of the cationic dyes or retarder. Maximum brightness in contrast effects may be limited because of staining of the acrylic fibre by the disperse dyes. The adverse effect of disperse dye staining on the light fastness of basic dyes on the acrylic component has been quantified [2]. The disperse dye stain should be cleared by treatment at 60°C with sodium dithionite, ammonia and a nonionic detergent. An alternative two-stage approach is to use an anionic retarder for the basic dyes. In the first stage the surface of the polyester is ring-dyed with disperse dyes at pH 7–8 (sodium acetate) and 80°C in a compatible system containing an anionic retarder, a conventional anionic emulsion of the carrier and a nonionic anti-precipitant. The pH is then adjusted to 5 with acetic acid, the basic dyes added and the dyeing temperature increased to the boil to dye the acrylic fibre and promote diffusion of the disperse dyes into the interior of the polyester. A final scour at 60°C with a nonionic detergent, ammonia and dithionite completes the procedure. Kayacryl ED (KYK) basic dyes contain as counter-ion an arylsulphonate that acts as an anionic retarder and migrating agent. This facilitates compatibility with conventional anionic formulations of disperse dyes and auxiliaries. No antiprecipitant is required and the fastness properties are equal to those of conventional basic dyes. Superior levelling is shown without the customary use of a cationic retarder. There is less staining of equipment and savings of time, labour, water, energy and auxiliaries [3]. The Kayacryl ED dyes migrate much more readily than conventional basic dyes and show good stability over a wide range of pH. These one-bath or two-bath dyeing methods on winch, jet or package dyeing machines offer reproducible contrast effects at low cost [4]. In general, it is preferable for the acrylic fibre to be dyed more deeply if reserve, shadow or contrast effects at full depths are required. Solid or contrast dyeings in full depths should be dyed by a two-bath method for optimum fastness and control of colour matching. The polyester component is dyed first because a basic dyeing on the acrylic fibre may show slight bleeding later in the disperse dyebath, especially if treated at high temperature. For optimum solidity, the polyester should be dyed slightly heavier than the target shade to allow for slight transfer to the acrylic fibre in the second stage. Anionic dispersing agents (and an anionic carrier emulsion if necessary) can be used in the first bath and a cationic retarder selected for the basic dyes without restriction. An intermediate scour with a nonionic detergent at 80°C is necessary to clear the disperse dye stain and to remove any residual carrier present.

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BLENDS OF CELLULOSE ACETATE OR TRIACETATE WITH ACRYLIC FIBRES

163

12.2 BLENDS OF CELLULOSE ACETATE OR TRIACETATE WITH ACRYLIC FIBRES Staple 50:50 or 60:40 blends of cellulose acetate or triacetate with acrylic fibres are used in jersey dresswear, high-bulk sweaters, hand-knitting yarns, sportswear, tropical suitings and woven furnishing fabrics. The cellulose ester fibre contributes a smooth, silky handle, lustrous appearance and good dimensional stability, giving an attractive combination of properties with the fuller handle, easy-care performance and heat insulation contributed by the acrylic component. Reserve and contrast effects are of relatively low interest on cellulose acetate/ acrylic fabrics because they are usually made from blended-staple yarns and cross-staining is a problem. Solid effects in pale depths can be obtained with disperse dyes applied at 75–80°C and shaded if necessary with basic dyes for the acrylic component, although this fibre is only ring-dyed under these conditions. Higher temperatures are best avoided because the lustre of the acetate fibre would be impaired. Full depths are achieved by a compromise two-bath method. The acrylic fibre is dyed for the minimum time at the boil and pH 4–5 (acetic acid) with the basic dyes and a cationic retarder. The dyebath is then cooled to 80°C and run to drain. The disperse dyes are then applied at 70–80°C and pH 6 from a fresh bath containing a nonionic dispersing agent to achieve the target depth on the acetate fibre. This method does not give optimum fastness in heavy depths on the acrylic fibre but the degree of damage to the cellulose acetate is just about tolerable. Much improved control of the dyeing of cellulose acetate/acrylic blends has become possible with the introduction by Courtaulds in 1987 of Xtol cellulose acetate fibre, which can be dyed at the boil without delustring. Fancy mixed-ply yarns containing Xtol and Courtelle (Courtaulds) can be dyed in a wide range of solid, shadow or contrast effects using selected disperse and basic dyes in a onebath method. Reserve effects are problematical, particularly alongside deep shades, but acrylic reserve is probably the best choice. Although acrylic fibres are readily dyeable with low-energy disperse dyes, the high-energy dyes recommended for Xtol give little or no cross-staining of the acrylic fibre in 50:50 blends with Xtol. Basic dyes for the acrylic fibre need more care in selection because they tend to stain most fibres including Xtol [5]. It is possible to achieve minimal cross-staining of Xtol by careful selection, however. The highest possible dyeing temperature should be used and the dyeing time should be sufficient to allow migration of the basic dyes from Xtol to Courtelle. Reduction clearing after dyeing may assist in reducing the ultimate staining of the acrylic fibre.

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Triacetate/acrylic blends can be dyed to solid shades, shadow (with the acrylic as the heavier depth), contrast and triacetate reserve. It is not possible to dye the triacetate with reserve of the acrylic fibre. A good reserve of the ester fibre in a triacetate/acrylic fabric is obtained by dyeing with basic dyes selected for minimum staining of triacetate using a nonionic dispersing agent. Any stain on the triacetate is cleared with slightly acidified hypochlorite at ambient temperature, followed by a sodium bisulphite rinse. Solid and contrast effects are achieved by a one-bath method using selected disperse and basic dyes. At temperatures below the boil the basic dyes tend to stain the triacetate, but migration to the acrylic fibre proceeds at the boil. Dyes in which the charge is delocalised (yellow to red methines and cyanines, oxazine blues and triarylmethane greens) migrate more readily than dyes with a localised charge (azo and anthraquinone derivatives). When dyeing colour contrasts, the acrylic component should preferably be dyed to a deeper colour than the triacetate in order to minimise undesirable cross-staining of the acrylic fibre by the disperse dyes. Satisfactory one-bath dyeing is achieved at the boil and pH 5–7 (phosphate buffer) with sodium N-methyloleylaminoethanesulphonate as a weakly anionic retarding agent (Figure 12.1). The basic dyes and disperse dyes are added separately in that order, followed by an aryl ester carrier in an anionic emulsion. Full depths should be scoured at 60°C with a nonionic detergent to remove the residual basic dye stain from the triacetate.

CH3(CH2)7CH

CH(CH2)7CO N CH2CH2SO3– Na+ H3C

Figure 12.1 Anionic retarder for basic dyes

Solid or contrast effects can be produced on blends of cellulose acetate or triacetate with acid-dyeable acrylic variants by two-stage methods using acid or chrome dyes for the acrylic fibre first, then neutralising and applying selected disperse dyes to the ester fibre at 70°C (acetate) or at the boil with an aryl ester carrier (triacetate). Dyes requiring only moderately acidic conditions should be selected for the acrylic variant. The acrylic fibre can be reserved by applying disperse dyes that have good affinity for the ester fibre at the appropriate dyeing temperature. The ester fibre is reserved by applying levelling acid dyes at a

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165

moderately acidic pH, preferably using phosphoric acid. Chrome dyes may be used for higher fastness but metal-complex types tend to stain the ester fibres. 12.3 DYEING OF NORMAL/BASIC-DYEABLE POLYESTER BLENDS Polyester copolymer variant yarns of the basic-dyeable type (section 5.1) are used in combination with the homopolymer for reserve, shadow and contrast effects. The basic-dyeable copolymer is more accessible than the homopolymer and in most instances shows higher yields and rates of dyeing with disperse dyes, although some anthraquinone derivatives do not behave in this way. Basicdyeable polyester exhibits lower tensile strength than the homopolymer and much lower abrasion resistance. It is more readily degraded by acid or alkali than the homopolymer, so that it should be dyed at pH 5–6 and a temperature no higher than 120°C. Glauber’s salt addition minimises degradation within these limits. Caustic soda should be avoided and reduction clearing with ammonia and dithionite should not be carried out above 60°C. Careful scouring with a nonionic detergent and soda ash at 60–70°C to remove spinning oils is important. Presetting at 165°C is also recommended. Satisfactory reserve of the homopolymer is achieved at the boil and pH 5 using basic dyes, Glauber’s salt and a nonionic carrier emulsion. Aryl ester carriers promote optimum reserve of the normal polyester when using basic dyes only. The preferred basic dyes for this blend are mainly methine, cyanine, monoazo or oxazine types. Practically all of them except certain orange to red monoazo dyes reserve normal polyester well. The residual stain is cleared from the homopolymer at 50°C using sodium dithionite, ammonia, a nonionic detergent and Glauber’s salt to protect the basic-dyeable polyester from hydrolysis. Pyrazoline fluorescent brightening agents cannot be used in reserve effects on the homopolymer because they tend to promote damage of the basic-dyeable variant. Compounds based on benzoxazolyl- and benzofuranyl-benzimidazole derivatives (Figure 12.2) are preferred because they are resistant to chlorite bleaching [6]. Shadow effects with disperse dyes on these blends show the sharpest differentiation using slow-diffusing high-energy dyes from a long liquor at about 80°C, if necessary with a low concentration of an ester carrier, but the normal polyester is badly ring-dyed under these conditions. The best approach to solidity is to use rapid-diffusing low-energy dyes at the boil with a diphenyl carrier, or at 120°C with an ester carrier to assist levelling. Some subtle contrasts are possible with selected low- and high-energy disperse dyes in mixtures, but the scope for selection is extremely limited.

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N R1

N

R2

Substituted 1,3-diphenylpyrazolines

CH3 N R1

O

N N

R2

CH3 Substituted benzoxazolyl-benzimidazoles

CH3 N R1

O

N

R2

CH3 Substituted benzofuranyl-benzimidazoles

Figure 12.2 Structural types of fluorescent brightening agents

The most widely used disperse dyes for contrast effects with basic dyes are mainly quinoline and anthraquinone derivatives. Cationic retarders may be used especially in pale depths, but they tend to be less effective than on acrylic fibres because the rate of diffusion of the dye is already much slower on basic-dyeable polyester and restraining may be excessive in full depths. Weakly cationic retarders of the alkylamine polyoxyethylene type promote levelling in the high temperature method at 120°C. Some useful azo and anthraquinone disperse dyes show light fastness at least one blue-scale rating lower on basic-dyeable polyester compared with the homopolymer. All basic dyes show lower light fastness on basic-dyeable polyester than on acrylic fibres and this limits their selection for pile fabrics, upholstery and carpets. In most cases, the monoazo types with a localised charge tend to show better fastness to light than do dyes with a delocalised charge on the molecule. Anionic scouring or dispersing agents and carriers formulated with anionic emulsifiers should be avoided. Diphenyl and aryl ester carriers in nonionic emulsifying systems are preferred for their compatibility with basic dyes, ease of removal from the fibre and minimal influence on light fastness. The disperse and basic dyes are added separately to a solution of an alkanol polyoxyethylene antiprecipitant before the nonionic carrier emulsion. The ester carriers accelerate the rate of uptake of basic as well as disperse dyes and they give better migration

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167

Table 12.1 Dye selections for DB blends Blend

Colour effect

Dyeing method

Dye selection

Polyester/ acrylic

Acrylic reserve

Single-class

Low-energy disperse dyes with methylnaphthalene carrier

Polyester reserve

Single-class

Basic dyes with cationic retarder

Solid or contrast

One-bath

Disperse dyes and basic dyes with nonionic carrier system and anti-precipitant

Two-stage

Disperse dyes at 80°C, then basic dyes at the boil with anionic retarder and carrier

Two-bath (full depths)

Disperse dyes at 120°C, then basic dyes with cationic retarder

Single-class (pale depths)

Disperse dyes at 80°C

Two-bath (full depths)

Basic dyes briefly at the boil with cationic retarder, then disperse dyes at 80°C

Acetate/ acrylic

Solid

Xtol/ Courtelle (Courtaulds)

Solid or contrast

One-bath

High-energy disperse dyes and selected basic dyes at the boil with anti-precipitant

Triacetate/ acrylic

Triacetate reserve

Single-class

Basic dyes with nonionic dispersing agent

Solid or contrast

One-bath

Disperse dyes and delocalisedcharge basic dyes at the boil with anionic retarder and carrier

Acetate reserve

Single-class

Chrome or levelling acid dyes at pH 4

Solid or contrast

Two-stage

Chrome or levelling acid dyes, then disperse dyes at 80°C

Triacetate reserve

Single-class

Chrome or levelling acid dyes at pH 4

Solid or contrast

Two-stage

Chrome or levelling acid dyes, then disperse dyes at the boil with aryl ester carrier

Homopolymer reserve

Single-class

Delocalised-charge basic dyes and nonionic aryl ester carrier emulsion at the boil

Shadow

Single-class

High-energy disperse dyes at 80°C

Acetate/ acid-dyeable acrylic

Triacetate/ acid-dyeable acrylic

Normal/ basic-dyeable polyester

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Table 12.1 Continued Blend

Colour effect

Dyeing method

Dye selection

Normal/ basic-dyeable polyester

Solid

Single-class

Low-energy disperse dyes at 120°C with aryl ester carrier

Contrast

One-bath

Disperse dyes and basic dyes at 120°C with nonionic aryl ester carrier and anti-precipitant

under high temperature conditions. Diphenyl gives the most economical yields at the boil and tends to favour the disperse dyes, so that this type of carrier is often preferred for contrast effects in full depths. Where possible, however, it is preferable to dye at high temperature and to use carriers sparingly, if at all. Full depths are cleared with dithionite and ammonia as already described.

12.4 DYEING METHODS AND DYE SELECTION FOR DB BLENDS The disperse-dyeable component of DB blends containing polyester or cellulose triacetate can be readily reserved. Solid effects with disperse dyes only can be achieved in pale depths on acetate/acrylic and normal/basic-dyeable polyester blends, but better control of shade matching is possible using basic and disperse dyes. One-bath methods for solid or contrast effects are available for polyester or triacetate with acrylic fibres, as well as normal/basic-dyeable polyester. Twostage or two-bath sequences become necessary for full depths on the blends containing conventional or acid-dyeable acrylic variants (Table 12.1).

12.5 REFERENCES 1. 2. 3. 4. 5. 6.

I R Hardin, Man-made fibres: their origin and development, Eds. R B Seymour and R S Porter (London: Elsevier, 1993). J Jeths, Chemiefasern und Textilind., 25/77 (1975) 356. R Parkham, Am. Dyestuff Rep., 82 (Sep 1993) 79. F T Wallenberger, Text. Research J., 50 (1980) 289. J M Taylor and P Mears, J.S.D.C., 107 (1991) 64. T Martini, Chemiefasern und Textilind., 38/90 (1988) 827.

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CHAPTER 13

Polyester/cellulosic and other DC blends

13.1 EXHAUST DYEING OF POLYESTER/CELLULOSIC BLENDS 13.1.1 Properties and preparation of polyester/cellulosic blends Blends of ester fibres with cotton or viscose are produced in greater quantity than the corresponding blends with wool (section 1.3). Factors contributing to this situation have been the relative ease of processing, effective clearing and versatility of application, leading to a wide range of dyed and finished effects. Without question the exploitation of polyester/cellulosic blends represents the most successful compromise between the contrasting physical properties of synthetic and natural fibres. Polyester/cellulosic yarns are used in sewing threads and slub effects for apparel. Woven polyester/cellulosic fabrics are important in shirting, sheeting, dressgoods, outerwear and workwear. Woven staple 67:33 polyester/cotton and 50:50 polyester/viscose blends in numerous constructions form the well-established basis of this field and many of these fabrics are produced in sufficient quantities to justify continuous dyeing. Polyester/cellulosic knitgoods include fleece knits, interlocks and jerseys, sportswear, T-shirts and dresswear. Knitted fabrics are less appropriate for continuous dyeing owing to their lower dimensional stability but the development of atmospheric jet machines has made it possible to dye these fabrics satisfactorily. Presetting can often be avoided if there is no risk of creasing in the jet dyeing machine and the brief dyeing cycle, short liquor ratio, high degree of turbulence and vigorous washing conditions make this technique highly efficient. As the most important fibre blend, ranging in characteristics from lightweight poplin shirting to heavy drill workwear fabric, polyester/cotton is sufficiently familiar to require little description. Polyester/linen blends are noteworthy as a luxury alternative to polyester/cotton in high-quality fashion goods, tableware and bedlinen. Care must be taken not to mar the characteristic nature of the linen texture.

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The primary shortcoming of woven polyester/cotton blends in the late 1950s was their inability to create fabrics that would retain creases once they had been made into garments. The breakthrough was the development of the durable press finish that allowed deferred curing of the garment. This was constructed and the creases set in before complete reaction of the finish with the cotton component of the blend. This procedure resulted in a garment that would retain its creases through a prolonged series of wash–wear cycles. Initially, the process was applied to all-cotton fabrics in the early 1960s but severe problems occurred because of the adverse effect of the durable press finish on strength and abrasion resistance. When it was later adopted for polyester/cotton blends, however, the durability and crease resistance of the polyester made an impressive contribution to the finish. Improvements were also made to the finish formulations, to blending of the fibres in the yarn and to the garment pressing methods. Polyester/viscose is an essential blend for apparel, replacing polyester to a great extent over the last decade. The superior comfort of this blend over the synthetic fibre alone is without question and it is this, together with the capability to accept chemical finishes that produce fabric qualities unrecognisable from the starting material, that has led to the outstanding popularity of this blend in apparel [1]. Viscose fibres can be chemically crimped in manufacture by selecting regeneration conditions after extrusion that give filaments with an asymmetric cross-section, such as less acid in the spinning bath and a higher bath temperature. The asymmetry causes the filaments to form continuous helical curves that impart the crimp characteristics. Sarille (Courtaulds) is a crimped viscose fibre that has been particularly successful with polyester, including 65:35 Sarille-rich blends for dressgoods [2]. Blends of polyester with regular or crimped viscose, modal or polynosic fibres are important in such outlets as lightweight tropical suiting, fashionwear, raincoats, leisure clothing and sportswear. Blends with viscose, and especially modal or polynosic fibres, are more suitable than cotton blends for knitgoods in view of their higher lustre and softness. Polyester/polynosic fibre blends have particularly good dimensional stability for use in tubular-knitted fabrics and garments [3]. Polyester/Vincel blends with polyester as the main component are of interest for rainwear, and for lightweight lawn constructions in summer dresswear and blouses. A valuable advantage for polyester in blends with cotton is its outstanding resistance to the rather severe preparation necessary for raw cotton before dyeing. Thorough preparation of polyester/cellulosic woven fabrics is the essential prelude to successful dyeing, especially if a continuous method is to be used. Any subsequent unlevelness or stains are almost always related to

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inadequate preparation. About 70% of typical dyeing faults are attributable to poor penetration [4]. The nature of the size used on warp yarns of woven goods should be identified so that it can be effectively removed. Rapid identification of sizes is now much more sophisticated than it was ten years ago [1]. Weavers often change size formulations without prior notification and this can lead to serious dyeing problems. Size removal outside the dyeing machine is greatly preferred to minimise machine occupancy and avoid build-up of residual contaminants. Neutral pH washing is the best for water-soluble sizes that respond well to vigorous treatment, but complete removal may require enzyme desizing before alkaline scouring. Hot- or cold-active enzymes are available for batchwise or continuous desizing. Cotton is usually the minor component in a polyester/cotton blend so that alkaline scouring, peroxide bleaching and mercerising usually provide adequate preparation. Most polyester/cellulosic fabrics may be winch scoured in anionic detergent and soda ash at 70–80°C. Cold pad–batch peroxide bleaching offers a low-cost alternative to batchwise processes to increase the production rate and quality of polyester/cotton knitgoods [5]. Cold mercerising after bleaching improves the absorbency, lustre and dimensional stability of polyester/cotton and improves the colour yield of vat or reactive dyeings. It is not recommended for viscose or modal fibre blends, however. The dyeability of the cellulosic component in these blends can be enhanced by cold causticising followed by thorough rinsing. Crease recovery, dimensional stability and pilling resistance are all improved by stenter setting of polyester/cellulosic fabrics. This is normally carried out at 180–200°C on a stenter either before or after dyeing. The higher temperature is particularly relevant for closely woven poplins or similar constructions destined for continuous dyeing. Heat setting before dyeing reduces the tendency to crease if subsequently dyed in rope form. Dyeability variations arising from differences in thermal history of the polyester may also be less evident. Sensitivity to these differences is greater in batchwise dyeing. If heat setting is carried out before preparation, care must be taken to ensure that there are no oil stains or other contaminants that might become set into the material and prove more difficult to remove later [6]. Heat setting after dyeing removes minor creases introduced in rope dyeing and also stabilises the fabric structure at its finished width. Singeing of both sides of the fabric is essential for polyester/cellulosic staple blends, but this treatment should be carried out after batchwise dyeing. The microscopic beads of fused polymer formed on the tips of the projecting polyester fibres take up dye more readily than the intact fibres in the body of the

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fabric. Because of their amorphous character these produce an unacceptable skittery or speckled appearance, especially after batchwise dyeing. Cropping to sheer off the protruding fibre ends is an alternative that avoids this problem. 13.1.2 Disperse dyeing of the polyester component Polyester/cellulosic yarns are mostly dyed as cones or cheeses, or on beams, under high-temperature conditions. Fabrics are generally dyed on a beam, in a jet or overflow machine, or on a continuous range. In circulating-liquor machines the initial dye uptake should be uniform throughout the load to ensure that the final dyeing is level. A rapid rate of dyeing and a slow rate of circulation of the dye liquor may both contribute to unlevel dyeing problems. To ensure uniform uptake the increment of exhaustion for each cycle of liquor through the machine should be not greater than about 2% exhaustion per cycle. The drains of such machines should be designed so that it is safe to release the exhausted liquor while still under pressure but this technique is effectively limited to circulatingliquor machines. Beam dyeing is unsuitable for polyester/cellulosic fabrics of very low permeability, constructions with a sculptured surface and knitgoods that are often difficult to wind evenly with suitable tension on to a beam. Jet dyeing machines can cause crushing or creasing of fabrics with a sensitive surface, such as velvet or corduroy. After jet dyeing it is desirable to cool the dyebath slowly to about 80°C in order to avoid the risk of creasing of the dyed material. Certain lightweight cloths may also give difficulty in the jet machine owing to the excessive length of an economical machine load. The completely relaxed and tensionless dyeing conditions in a jet or overflow machine allow relaxation, bulking and shrinkage that is physically prevented in the beam machine. Thus the softer, bulkier handle and subdued lustre provided by jet dyeing may be preferred for polyester/viscose knitgoods, for example, whereas the firmer, flatter handle and stronger reflections from beam-dyed goods are usually favoured for polyester/linen and many polyester/cotton fabrics [6]. Many of the comments on wool staining by disperse dyes (section 3.4) are applicable to the staining of cellulosic fibres, but these are less sensitive than wool to the relatively severe conditions required to clear the stain. Staining is aggravated by low pH and poor dispersion stability, as well as marginally by nonionic agents and carrier chemicals. The disperse dye stain has low fastness to light and wet treatments. On polyester/cellulosics the degree of staining is less than on wool, prolonged boiling favours migration to polyester without severe damage to the cellulose and the stain can be reduction cleared safely.

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Cotton staining in the dyebath at 130°C with those dyes having low substantivity for cotton is usually negligible. After the hold period at top temperature, residual dye remains in solution at 130°C and if the dyebath is drained by blowing out under pressure, the deposition of loose dye on the fibre surfaces is minimised. On the other hand, if the dyebath is cooled slowly the dissolved dye will re-precipitate and undesirable deposition will occur. For the highest fastness the unfixed disperse dyes on the polyester surface and the stain on the cellulosic component should be removed by a thorough soaping with detergent or by a suitable reduction clearing. In many cases the clearing treatment can be incorporated in the subsequent dyeing of the cellulosic fibre, as in the reduction, reoxidation and soaping of vat or sulphur dyes. Where reactive dyes are used and the highest fastness standard is required it may be necessary to give a reduction clearing treatment separately before the reactive dyes are applied. Disperse dyes tend to stain the lignin component of linen, so that reduction clearing of the stain is more important than with cotton [6]. Vat or reactive dyes are normally chosen for linen and it is dyed in open width to avoid creasing. Clearing can be combined with the reduction stage of vat dyeing on this blend. Disperse dyes of all relevant chemical types have been used in the batchwise dyeing of polyester/cellulosic blends but the trend has been in favour of the highenergy dyes, especially where durable press finishes are to be applied. Not only have the levels of sublimation fastness in chemical finishing and end-use requirements become more severe, but there have been improvements in the design and operation of jet and overflow machines to process these blends. Highenergy dyes show optimum yield and levelling properties under these conditions. Low-energy dyes are now only of marginal interest for excellent levelling in pale depths. The affinity of disperse dyes for cellulose is a factor of practical significance in selecting suitable dyes for either batchwise or continuous methods. In continuous dyeing the selection has to take into account the rapidity of clearing of the stained cellulosic component and the sensitivity of the staining to variables arising during thermofixation [7]. Light fastness on polyester is usually adequate and after dyeing the dyes are protected from chemical attack at moderate temperatures by the hydrophobic nature and relative impermeability of the fibre. Nevertheless, some dyes are sensitive to alkaline conditions or to the presence of heavy metal ions at relatively low concentrations. Thus disperse dyes are normally applied under slightly acidic conditions (pH 5) and a sequestering agent is normally used with those dyes known to be sensitive to trace metals.

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The rate of exhaustion of a disperse dye by polyester is controlled by the rate at which the temperature is raised. At some temperature between 80°C and 120°C the dyeing rate for that dye reaches a maximum. The temperature range over which the dyeing rate is at this maximum is known as the ‘critical dyeing temperature’ (CDT). Slow-diffusing high-energy dyes have a high CDT, whereas more rapidly-diffusing dyes have a lower CDT. Specific values of CDT depend on the rate of temperature rise, dye concentration, liquor flow rate, liquor ratio and the substrate to be dyed. Rapid-dyeing procedures depend on adding the disperse dyes at a temperature just below the CDT and then raising the temperature slowly in the vicinity of the CDT to ensure that the exhaustion rate that just permits level dyeing is not exceeded. The temperature is then raised from just above the CDT to the top dyeing temperature at the maximum rate. In polyester/cellulosic dyeing the need for levelling agents with disperse dyes is less critical than when dyeing polyester alone. In the early stage of a batchwise dyeing, or in the first padding of a continuous process, the cellulosic fibres absorb a substantial proportion of the disperse dyes applied. These subsequently migrate to the polyester as the top dyeing temperature or the thermofixation step is reached. The cellulosic component is thus, in a sense, acting as a retarding or levelling agent for the disperse dyes [8]. Nevertheless, many dyers still prefer to add a levelling agent, especially for pale depths. The efficiency of ethoxylated nonionic surfactants used as levelling agents during accelerated heating in the early stage of exhaust dyeing of the polyester component was studied with a trichromatic combination of low-energy disperse dyes. Criteria for the selection of nonionic levelling agents include costeffectiveness, ease of handling, effect on dye yield, minimal foaming and ease of removal from the substrate by rinsing [9]. Oligo-soaps are polyethylene glycol fatty acid esters of high Mr and general formula: RCOO(CH2CH2O)nCH2CH2OH. These agents provide excellent dye dispersing and solubilising characteristics. By forming a complex molecular matrix with the polyester surface during dyeing, these compounds give excellent dye levelling under a wide variety of conditions [10]. Their low-foaming properties eliminate the need for mixtures of nonionic levelling agents, dispersing agents and defoamers in jet dyeing systems. Fastness to sublimation of disperse dyes is not a criterion for selecting those that yield optimum wet fastness performance in finished polyester/ cellulosic fabrics. Those found suitable in this respect must show minimum tendency to thermomigration or desorption in aqueous media after heat treatment at temperatures above 140°C. They should also have minimum

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substantivity for adjacent fibres, notably nylon, in wet fastness tests. Selection of dyes that show minimum staining of the cellulosic fibres is also advisable. Unsatisfactory wash fastness of disperse dyes on the polyester fibre, particularly with regard to staining of adjacent nylon, may arise from aftertreatments such as soaping at the boil. These are normally necessary when dyeing the cellulosic fibre with vat, sulphur, reactive or azoic dyes. Even if a reduction clear is given after the boiling soap treatment, unfixed disperse dye may still be present on the polyester surface. This problem can be minimised by selecting disperse dyes that do not migrate readily to the fibre surface when the fabric is soaped at the boil. An alternative approach is to use reactant-fixable direct dyes on the cellulosic component that do not require a boiling soap treatment [11]. Many disperse dyes show lower light fastness on polyester microfibres (section 1.4.2) compared with standard polyester, so this becomes a further factor in dye selection [12]. The relatively higher amounts of disperse dyes needed on microfibres have a significant influence on build-up. Small differences in build-up between dyes on conventional polyester are exaggerated on polyester microfibres. The high applied concentrations needed to achieve full depths on microfibres play a critical part in the resultant limited wash fastness after poststentering. Traditionally acceptable dyes for conventional recipes can show unacceptable results when applied to microfibres. Major dyemakers have put considerable research effort into developing a new generation of disperse dyes designed to optimise wash fastness and minimise the cross-staining of the cellulosic fibre [13]. If the Dispersol XF (BASF) dyes, for example, are compared with conventional monoazo and anthraquinone disperse dyes, the differences in wash fastness and crossstaining performance are magnified when tested on polyester microfibres. Diester-containing azo disperse dyes and certain azothiophene blues (Figure 13.1) that are capable of being rendered soluble by a mild alkaline

COCH3 HN

NO2

CH2CH2COOCH3 Ar1

N N

N CH2CH2COOCH3

O 2N

S

N N Ar2

Figure 13.1 Azo disperse dye structures capable of solubilisation by alkali (Ar = aryl)

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aftertreatment offer considerable benefits when dyeing polyester microfibre/ cellulosic blends. These include: (1) minimal cross-staining of the cellulosic fibre; (2) minimal processing times because the alkaline fixation stage for reactive dyes clears the disperse dye stain; (3) avoidance of a reduction clear with dithionite; (4) achieving good wash fastness standards after post-stentering. 13.1.3 Direct dyeing of the cellulosic component Several possible batchwise dyeing methods are available for polyester/cellulosic blends, based on the use of disperse dyes and the various classes of dyes for the cellulosic fibre. The choice between these possibilities depends mainly on requirements of hue, depth and fastness properties. Direct dyes are popular due to their economy, compatibility, robustness and adequate fastness in pale depths. Their major weakness is their poor wet fastness in depths above about halfstandard depth [1]. The wet fastness of direct dyeings can be enhanced substantially, however, when finishing with a durable press reactant together with a cationic fixing agent [14]. Most direct dyes give a good reserve of polyester. An economical one-bath bleaching and direct dyeing system for achieving an excellent polyester reserve effect entails treating the scoured polyester/cotton fabric with sodium carbonate, hydrogen peroxide, silicate stabiliser and a hexametaphosphate sequestrant at pH 10 and the boil to bleach the cotton in the presence of selected peroxidestable direct dyes [15]. After cooling to 85°C, salt is added and the temperature raised to 95°C to achieve full exhaustion of the direct dyes. Solidity on blended staple yarns is usually the colour effect required. Disperse and direct dyes can be applied in a cheap and simple one-bath process, but the relatively low fastness levels are inadequate for many polyester/cellulosic outlets. Disperse/direct combinations are thus mostly used at the cheaper end of the market. Because of the limited wet fastness of direct dyes, staining of the cellulosic fibre by disperse dyes is less important than with other combinations. The direct dyebath serves as a soaping bath to give a mild clearing of the disperse dyeing. A practical advantage that direct dyeing offers over reactive dyeing of these blends is the markedly lower concentration of electrolyte necessary. The salt concentration (10–15 g l–1) is rarely sufficient to adversely affect the dispersion stability of the disperse dyes, although instability occasionally arises in shortliquor (package or beam) equipment. Similarly, neither the dispersing agents nor any levelling agent required would normally interfere with the direct dyeing

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process. On the other hand, many direct dyes are adversely affected at the high temperature (120–130°C) and acidic pH (4.5–5.5) usually preferred for disperse dyeing. It is necessary to ensure that the direct dyes are sufficiently soluble and chemically stable under these conditions. The influence of pH and additions of electrolyte, carrier, levelling and sequestering agents on the stability and dyeing behaviour of direct dyes selected for the one-bath method has been tabulated [16]. The vulnerability of most direct dyes to alkaline reducing conditions precludes conventional reduction clearing of the dyed polyester component [8]. General recommendations have been given for the package dyeing of polyester/cotton yarns with direct and disperse dyes by the one-bath method, either at high temperature [17] or using 1–2% of a mixture of diphenyl and trichlorobenzene as carrier [18]. Disperse/direct recipes are important for blends of polyester with viscose or other regenerated cellulosic variants for suiting materials, where fast-to-light direct dyes can be used without the need for high wash fastness. Aftertreatment and resin finishing of disperse/direct dyeings can give materials of acceptable if still limited wet fastness. In the one-bath method with disperse and direct dyes, the polyester fibre is dyed at pH 6 with a disodium dinaphthylmethanedisulphonate dispersing agent at 130°C on the beam or in a jet machine. The dyebath is then cooled to 90°C, salt is added and dyeing continued until the cellulosic fibre reaches the target depth. Aftertreatment with a cationic fixing agent is given where appropriate to improve the fastness properties of the direct dyes. These are selected for stability in the high-temperature dyebath and are mainly self-levelling or salt-controllable disazo multisulphonated dyes. A wider choice of direct dyes can be used in the two-bath method with intermediate reduction clearing. This alternative gives moderately good fastness in pale or medium depths, provided the fabric is given a durable resin finish. 13.1.4 Reactant-fixable direct dyes for the cellulosic component Reactant-fixable dyes are those copper-complex direct dyes (Figure 8.5) that are suitable for aftertreatment with certain Indosol (Clariant) cationic fixing agents, yielding exceptionally good fastness to washing. Not all direct dyes respond in acceptable ways to these treatments. In general, bright unmetallised direct dyes show poor light fastness after treatments with Indosol agents. Selected unmetallised reactive dyes have been used to provide bright hues that supplement the limited shade gamut of the reactant-fixable range [11]. Substantial savings in processing time, labour costs, chemicals, water and effluent treatment are claimed for reactant-fixable dyes, compared with vat or reactive alternatives. These savings are even more significant when dyeing

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cellulosic blends than in the dyeing of all-cotton or all-viscose materials [19]. The reasons for this arise from the fact that the dyeing of these blends can be technically complex and time-consuming. In most cases the Indosol system allows for one-bath dyeing, leading to specific savings over other conventional cellulosic dyeing methods. Reduction clearing or soaping at the boil can be avoided, leading to further reductions in time, energy and water consumption. The soaping and rinsing of reactive dyeings are much more time-consuming than cationic aftertreatment [1]. The cationic nature of Indosol E-50 (Clariant) is multifunctional and one molecule of fixing agent can interact electrostatically with several sulphonate groups in the same or different Indosol dye molecules. Furthermore, chelating groups in the fixing agent are able to form coordination bonds with the copper atoms in the dye molecules. These two features result in the formation of a much more stable complex between dye and agent than when simple cationic fixing agents are used to fix conventional direct dyes by electrostatic bonding only. The fastness level achieved corresponds to that of domestic laundering at 50°C. Indosol E-50 is particularly suitable for treatment of knitted underwear, sportswear and hosiery. In the case of Indosol EF (Clariant), the electrostatic and coordination bonds formed as described above are further reinforced because this agent is capable of reacting with a hydroxy group in cellulose to form a covalent ether bond, similar to that produced in a typical reactive dyeing. This additional fixation leads to high levels of colour fastness at all applied depths, e.g. laundering at 60°C. Although Indosol EF is applied by exhaustion at 40°C rather than the 60°C used for Indosol E-50, a further alkaline treatment with caustic soda, or alternatively a high-temperature cure at 150°C, is necessary to initiate the reaction of Indosol EF with cellulose. Most woven cellulosic blend fabrics require at least a moderate degree of crease recovery to exhibit satisfactory easy-care properties. Indosol CR (Clariant) completely replaces the cellulose reactant resins that confer easycare properties by crosslinking. It can only be applied by a pad–dry–bake sequence. A cationic fixing agent of the Indosol E-50 type is reacted with a conventional reactant of the dimethyloldihydroxyethyleneurea type to give Indosol CR. Formation of the dye–Indosol E-50 complex occurs initially, but on curing the reactant crosslinks the cellulose segments and forms a threedimensional matrix in which the dye–agent complex is held permanently. Thus exceptionally high wash fastness is achieved, allowing full depths to yield satisfactory fastness to washing tests at the boil. Microfibre blends of polyester and viscose have been dyed successfully with disperse and reactantfixable dyes.

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The use of selected disperse dyes for the polyester allows for rapid rates of temperature rise and short duration at 130°C, minimising possible decomposition of the copper-complex reactant-fixable dyes. This process has clear advantages: (1) One-bath operation simplifies control and improves productivity [20]. (2) In-to-out times are reduced to between 3.5 hours (Indosol CR) and 4.5 hours (Indosol E-50/EF). (3) Only 5–15 g l–1 Glauber’s salt is required, compared with 20–100 g l–1 electrolyte in reactive dyeing. (4) The levelling power of the reactant-fixable dyes is excellent at 130°C. (5) Shading additions are easy to make before Indosol aftertreatment. The most obvious drawback of this approach is the restricted gamut in bright shades. A copper-specific sequestering agent Plexophor SFI (Clariant) must be used to protect certain disperse blue dyes that are sensitive to copper ions when dyeing in fully-flooded jet machines. Ethylenediaminetetra-acetic acid is too powerful, causing demetallisation of the reactant-fixable dyes, whereas phosphates have little ability to sequester copper [21]. Shading additions to the aftertreated dyeing can be difficult because of the highly cationic surface charge. Addition of anionic dyes results in rapid strike and hence unlevelness. An anionic surfactant must be added before any shading with the reactant-fixable dyes. Two popular routes to black dyeings on polyester/viscose blends are based on either CI Direct Black 22 (Figure 13.2) or the reactant-fixable system. Black 22 after resin treatment shows exceptional fastness on viscose and is a classic dye of the ortho, para primary amino or hydroxy type where the molecules of dye can condense with free formaldehyde or the N-methylol groups of the reactant. A problem with Black 22 is the necessity for alkali to maintain solubility in dyeing and this affects the stability of the disperse dyes adversely. A two-stage procedure is necessary, dyeing the polyester at pH 5 and 130°C and then cooling and adjusting to an alkaline pH to dye the viscose. The use of reactant-fixable dyes avoids the need for this pH swing [1]. In a more recent development, Clariant have introduced the Optisal/Optifix system, supported by a computer program. This consists of a range of bright, metal-free direct dyes with high exhaustion values and low salt content. These features result in minimal contamination of waste waters. The stability of these dyes at 130°C allows them to be used in a one-bath method with disperse dyes on polyester/cellulosic blends. Fixation of the Optisal dyes to cellulose occurs during an alkaline aftertreatment with a cationic reactant, Optifix F Liquid

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H2N

NH2 NH2 N

H2N

N

SO3Na

N N H O

O H N

N N

N NaO3S SO3Na

NaO3S

Figure 13.2 CI Direct Black 22

(Clariant), which does not release formaldehyde. These dyeings will meet domestic laundering requirements up to 50°C [22]. 13.1.5 Reactive dyeing of the cellulosic component Reactive dyeing is by far the most important technique adopted for exhaust dyeing of the cellulosic portion of these blends. Reactive dyes can be used for a full gamut of hues at virtually all depths. When dyeing a polyester-rich blend, such as 70:30 for example, with reactive dyes at a typical liquor ratio of 15:1, the effective liquor:cellulose ratio is 15:0.3 or 50:1. Hence it is essential to select for batchwise dyeing reactive dyes that give high fixation at a long liquor ratio. The use of reactive dyes of low substantivity for the exhaust dyeing of such blends is inefficient, expensive and difficult to reproduce. Reactive dyes are almost completely free from cross-staining of the polyester. The only exceptions are the phthalocyanine-based blues and greens that may give slight staining in some instances. It is with this class of dyes on these particular blends, however, that the total dyeing time can be the most prolonged. The original processing systems for these blends usually consisted of: (1) conventional high-temperature application of the disperse dyes; (2) reduction clearing to remove any disperse dye staining from the cellulosic fibre; (3) conventional reactive dyeing in a fresh bath at the pH, temperature and electrolyte concentration appropriate for the reactive system selected; (4) rinsing and soaping at the boil as usual. Total load-to-unload times could be as long as 12 hours and even then additional shading corrections might be required. Detailed cost comparisons have shown that the two-bath method with disperse and reactive dyes costs in labour, energy,

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dyes and chemicals about three times as much as the one-bath alternative with disperse and direct dyes [23]. Traditionally, this two-bath sequence with intermediate clearing was considered essential for optimum fastness of solid dyeings or maximum brightness of contrast effects in full depths. There was virtually no risk of interaction between the disperse and reactive dyes. The selection of disperse dyes was unrestricted and common salt could be used freely in the reactive dyeing bath. Preparation of blends of microfibre polyester with viscose for sportswear by scouring, drying and heat setting should be followed by a two-bath dyeing sequence. The polyester is dyed first on the jet at 130°C. This is followed by a reduction clear, and then the viscose is dyed with vinylsulphone dyes [12]. Options have been presented for rationalising dyeing sequences on polyester/ cotton compared with the traditional bleach, polyester dyeing, reduction clear and reactive dyeing. Novel concepts for relocating the bleaching step were considered and the savings attainable from rationalisation demonstrated [24]. During the 1980s, reactive dye manufacturers optimised exhaust dyeing techniques for these blends with a view to reducing overall dyeing times and limiting the number of soaping and rinsing steps. Developments were mainly aimed at reducing the time and energy requirements without affecting the fastness of the dyed goods. When dyeing in combination with conventional alkali-fixing reactive dyes, some compromise in processing is required. Ideally, alkali-sensitive disperse dyes should be fully absorbed by the polyester before alkali is added to the dyebath. Likewise, the disperse dyes should not be introduced until the reactive dyes are fully fixed and the dyebath neutralised, otherwise lower or erratic yields may result [6]. The particular process chosen also depends on the type and utilisation of the available machinery. Opportunities for automation, short load-to-unload times, low energy requirements and savings of water and chemicals are important [25]. Most of these developments followed either of two distinct approaches: (1) Two-stage processes in which the polyester was dyed first and then alkali added at a later stage to induce fixation of the reactive dyes, usually from the low-reactivity ranges. (2) The ‘reverse’ two-stage or two-bath processes, in which the cellulosic fibre was dyed first and then after an intermediate rinse the polyester was hightemperature dyed. This sequence was particularly suitable for vinylsulphone dyes. Although dyeing times can be reduced by up to 4 hours using these techniques, there still remains the serious disadvantage of the high concentrations (50–100

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g␣ l–1) of electrolytes required. This interferes with dispersion stability by suppressing the essential ionising mechanism of the anionic polyelectrolyte dispersing agents through a common-ion effect, thus decreasing the charge on the dispersed particles and promoting agglomeration. The interference is variable, frequently affecting one manufacturer’s formulation of a particular dye more than another’s, or even varying between successive batches of the same formulation. Dispersion stability depends on many parameters such as batch quality of the dispersing agents and the physical form of the disperse dye, not merely particle size distribution but also crystal form. This problem may be alleviated by the judicious use of additional dispersing agents, of which the ethoxylated phosphate type is particularly effective [8]. Another problem in disperse/reactive systems is the difficulty of reconciling the different pH requirements of the two classes of dyes. Conventional reactive dyes need alkaline conditions (pH 10–12) for fixation. Although a few disperse dyes will tolerate such alkalinity during dyeing, many containing vulnerable heterocyclic rings or hydrolysable ester groups (Figure 13.1) must be dyed below pH 6. Nevertheless, practically all disperse dyeings will withstand quite severe alkalinity once the dyes have thoroughly penetrated the polyester fibre. In the conventional two-stage approach, the initial disperse dyeing at high temperature ensures uniform wetting out of the cellulosic fibres. Disperse dye staining is normally removed during subsequent alkaline fixation of the lowreactivity dyes at 80–100°C, but disperse dyes with ester groups that can be hydrolysed at pH 11 and 80°C are preferred. Selection of the disperse dyes is limited, however, to those with good dispersion stability in the presence of electrolyte. For this reason, the use of Glauber’s salt rather than common salt is essential. The reactive groups in the low-reactivity dyes are sufficiently stable to withstand the conditions of high-temperature dyeing of the polyester at pH 5–6 (phosphate buffer). High-reactivity dyes are generally unsuitable because of the risk of interaction with the disperse dyes (section 4.1). A mild oxidising agent such as sodium m-nitrobenzenesulphonate is added to inhibit reduction of azo reactive dyes at 130°C, especially when dyeing polyester/ viscose blends. This agent is less effective for azo disperse dyes or under acidic conditions. The reducing action of viscose can be attributed to: (1) high aldehyde end group content arising from oxidative damage in bleaching; (2) residual sulphur in the form of sodium sulphide or carbon disulphide from xanthate regeneration in manufacture. Sulphide levels as low as 10 mg l–1 in enclosed systems can attack some azo direct or reactive dyes [1].

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In a more versatile alternative technique, the disperse and reactive dyes are applied together at high temperature and pH 4.5–6. Salt, if added at the start, would promote exhaustion of the reactive dyes but could then exert its maximum deleterious effect on the dispersion stability. After exhausting the disperse dyes at high temperature, the dyebath is cooled to 80°C. This is the best point at which to add the salt, since the residual concentration of disperse dyes is too low for them to be adversely affected. After the lowreactivity dyes have been exhausted by the cellulosic fibre, alkali (usually sodium carbonate) is added to bring about fixation. Good batch-to-batch reproducibility of hue is related to the relative sensitivity of such systems to process variables as demonstrated in the BASF process for the rapid dyeing of these blends, which has been applied successfully to microfibre polyester/ viscose [25]. As with direct dyeings, reduction clearing of the disperse dyeing must be avoided because of the sensitivity of azo reactive dyes to reduction. Some clearing takes place, however, during the alkaline fixation and soaping to desorb unfixed and hydrolysed reactive dyes. In the Sumitomo RPD-Supra two-stage method, disperse dyes are applied to the polyester in the first stage and Sumifix Supra (NSK) bifunctional reactive dyes (Figure 8.1) are applied to the cellulosic fibre in the second stage. These aminochlorotriazine-sulphatoethylsulphone dyes exhibit high substantivity and reactivity, resulting in excellent fixation. They level well and show low sensitivity to variations in time and temperature, giving highly reproducible dyeings [26]. The levelling of reactive dyes is closely related to substantivity, migration and rate of fixation. In the case of Sumifix Supra structures, chromogens have been selected from a wide range of chemical classes and optimised in substantivity and reactivity to ensure a high degree of levelness. The reaction rate can be controlled mainly by dyebath pH and temperature. Owing to the difference in reactivity between the two functional groups, optimum dyeing conditions cover a relatively wide range of conditions [27,28]. With the high-reactivity ranges of reactive dyes, it is necessary to complete fixation on the cellulosic fibre before applying the disperse dyes to the polyester. After dyeing with the high-reactivity dyes at the appropriate pH, temperature and salt concentration in the presence of sodium m-nitrobenzenesulphonate, followed by the relevant alkaline fixation conditions, the pH is adjusted to 6–6.5 with acetic acid. The disperse dyes and disodium dinaphthylmethanedisulphonate as dispersing agent are added and the polyester is dyed to the target depth at 130°C. High-temperature dyeing is a highly efficient alternative to soaping at the boil

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as a means of desorbing the unfixed reactive dyes at a rapid rate of diffusion in the absence of electrolyte. Shade reproducibility depends on the stability of the dye–fibre bonds in the reactive-dyed cellulosic fibre to the high-temperature polyester dyeing stage. Any final shading of the cellulosic fibre requires a fresh bath to be set after clearing of the disperse dyeing. This two-stage method is particularly appropriate for vinylsulphone reactive dyes because the dye-fibre bond formed by the nucleophilic addition mechanism [29] is more resistant to acid hydrolysis than that formed by most other types of reactive dyes. Thus vinylsulphone dyeings are not impaired by the mildly acidic dyebath preferred for applying disperse dyes to the polyester fibre [30]. Soaping after application of the vinylsulphone dyes is unnecessary because any unfixed dye is readily removed in the final wash-off after the disperse dyeing stage. This reverse approach is useful for piece dyeing on a jet machine, but is less suitable for beam dyeing or package dyeing of yarn than the sequence in which the polyester is dyed first, owing to greater aggregation of the unsorbed disperse dyes in the high concentration of electrolyte present in the dyebath. Advantages of the Then Airflow aerodynamic jet dyeing machine for this process are said to be optimum fabric quality, freedom from creasing, no foaming, a high degree of reproducibility and ease of cleaning of the machine after use [31]. Exhaust dyeing of the cellulosic fibre with Levafix E (DyStar) dichloroquinoxaline and Levafix E-A (DyStar) difluoropyrimidine dyes proceeds with salt at 40–60°C, followed by fixation at pH 9.5–11.5. Dyeing of the polyester with disperse dyes is completed at pH 4–6 and 130°C. The principles and practice of automatic metering of the chemicals have been detailed with special reference to Levametering (DyStar) systems [32]. This ensures optimum fixation conditions for both components of the blend, giving excellent reproducibility, level dyeing and high colour yields. Full automation of the dyeing process eliminates the dead time that occurs with manual control. The time savings enhance productivity and lower the costs of labour and energy. Kayacelon React (KYK) reactive dyes contain two aminonicotinotriazine reactive groups per molecule (Figure 8.6). They readily react with cellulose, eliminating the nicotino group to form a covalent bond that is identical with that given by analogous dyes of the well-established bis(aminochlorotriazine) type. The substitution reaction occurs with nonionised hydroxy groups in cellulose under neutral dyeing conditions at high temperature, although if necessary the reaction can be induced at lower temperatures under mildly alkaline conditions [27,33]. A one-bath process for dyeing polyester/cotton blends with disperse dyes and Kayacelon React dyes entails application at pH 7 and 130°C with salt and a nonionic levelling agent. After dyeing, the fabric is soaped at the boil and rinsed

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as in conventional reactive dyeing methods. The use of a fixing agent is not recommended for pale depths, but a product of the long-chain alkylammonium type is used for medium-depth dyeings and a polyamine auxiliary used for full depths. 13.1.6 Water-insoluble colorants for polyester/cellulosic blends Insoluble vat and sulphur dyes often show moderate to severe cross-staining of the polyester component in these blends. In the case of vat dyes, this can sometimes be exploited in using them to colour both fibres simultaneously. Some of the benzamidoanthraquinone vat dyes of fairly low Mr dye polyester more readily than the cellulosic fibre at 130°C and can be used for solid effects on the two fibres in pale depths. Certain olive green, khaki, brown and black dyes of relatively complex structure give duller and more bathochromic hues on the cellulosic fibre compared with the polyester. Many polycyclic vat dyes give a good reserve of polyester fibres when applied by the usual alkaline leuco methods. These are also useful when dyeing solid or contrast effects with disperse dyes for the polyester. They include the halogenated derivatives of anthanthrone or pyranthrone, the indanthrone and violanthrone blues and greens, as well as most of the acridone and carbazole types. The colour gamut is restricted in the bright orange, red and violet sectors, however. Two-stage dyeing with disperse and vat dyes is a valuable method of achieving excellent fastness to light and washing at all depths. The disperse and vat dyes may be applied at pH 5–6 with an anionic dispersing agent. After dyeing the polyester at 130°C, the temperature is quickly lowered to 85°C and the vat dyes are reduced with alkaline dithionite. The cellulosic fibre is dyed to shade at an appropriate temperature within the range 20–60°C, followed by reoxidation at 50°C and soaping at the boil. Staining of the polyester fibre by the vat dyes may complicate shade matching. The vat dyeing conditions in the second stage act as a reduction clearing bath for the disperse dyeing. Selection of the vat dyes is restricted by problems of instability in hightemperature dyeing. This difficulty can be overcome by introducing the vat pigment dispersion after cooling to 85°C, but before adding the reducing system. Owing to the rapid consumption of sodium dithionite, vat dyes are applied more easily in machines completely filled with liquor, i.e. beams and fully-flooded jet machines, rather than in only partly flooded jets or overflow machines. Only selected vat dyes are sufficiently stable in the reduced form for addition at 85°C, including flavanthrone yellow, the indanthrone and violanthrone blues and greens, as well as several of the carbazole types. In the case of the indanthrone

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blues, carbazole browns and olive greens, sodium nitrite should be added to prevent over-reduction. Well known drawbacks of vat dyeing processes include the difficulty of maintaining satisfactory reducing conditions and the need to avoid over- or under-reduction. These problems can lead to unlevelness, poor reproducibility of shade and, in some cases, poor fastness to rubbing. Sodium dithionite is especially unattractive from the environmental viewpoint. Ecologically innocuous methods for reducing vat dyes currently under consideration include: (1) electrochemical reduction using a mediator; (2) organic reducing agents such as hydroxyacetone; (3) iron pentacarbonyl or other Fe(II) complexes. Iron complexes with ligands derived from triethanolamine or gluconic acid have been investigated with selected Indanthren (BASF) dyes applied by exhaust dyeing. Iron salt requirements, various molar ratios of Fe(II) salt to gluconic acid, the influence of caustic soda concentration and the presence of Fe(III) ions, levelling behaviour and over-reduction were investigated [34]. Two-bath methods of applying disperse dyes followed by vat or sulphur dyes usually present no serious limitations. The reducing conditions in the second dyebath are often sufficient to clear the surface deposition of disperse dyes without an intermediate clear. Alkaline dithionite should be used to solubilise the sulphur dyes because sodium sulphide may damage the polyester fibre. The low cost of sulphur dyes makes disperse/sulphur dye recipes useful for dull, heavy depths, but there are obvious limitations of shade. They are usually satisfactory when the wash fastness requirements are less stringent than the typical standards for disperse/vat dyeings. With certain heavy shades it may be necessary to introduce an intermediate clearing step. Sulphur black dyeing is a rather laborious procedure but it does produce dyeings of excellent bloom and wash fastness. Either pre-reduced or solubilised sulphur dyes may be used. The pre-reduced approach is more economical but it does cause more staining of the dyeing vessel. Pigment coloration of bleached polyester/cotton garments, e.g. knitted leisure shirts, can be carried out after treating them with a suitable cationic binder. Exhaust coloration with the dispersion of organic pigments and a compatible anionic dispersing agent is followed by stonewashing with a surfactant, dry heat fixation of the binder and finally a heat setting treatment to stabilise the fabric. Colour yield, levelness of coloration and fastness to rubbing are all potential problems [35]. Attention must be paid to the time and temperature of pretreatment, binder concentration, pH and the effects of stonewashing on

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fastness to rubbing, yield and levelness. Single-class application to fibre blends is a major advantage claimed for this process.

13.2 CONTINUOUS DYEING OF POLYESTER/CELLULOSIC BLENDS 13.2.1 Disperse dyeing of the polyester component The continuous dyeing of woven polyester/cellulosic blend fabrics is a most important sector of the textile dyeing industry. Statistics for the USA have shown that more than half of the polyester fibre processed is destined for dyed polyester/ cellulosic fabrics, more than half of which are dyed con-tinuously by pad– thermofix processes. Continuous processing of long runs to a given colour ensures consistently high yields and reproducible uniformity at a much more economical price per metre than batchwise methods. The length of run to each colour normally exceeds 5000 m, so a typical order in several colours will take several hours to process through each stage. The trend, however, is for shorter runs. Dyeing and heat setting of woven fabrics can be achieved simultaneously without the risk of rope creasing. Crease formation, however, can still arise in continuous processing and it is responsible for more seconds quality than any other fault. Thorough preparation is especially important before the continuous dyeing of polyester/cotton. Each stage can be carried out continuously, i.e. singeing, enzyme desizing, alkaline peroxide pad–steam scour-bleach, mercerising and heat setting before dyeing. Typical fabrics prepared in this way include shirting, light suiting, rainwear, workwear, military and civilian uniforms. This preparation sequence merely removes the surface impurities from the polyester fibres, which are not significantly penetrated by the alkaline peroxide liquor applied to the cotton. Mercerising enhances the colour yield, lustre and dimensional stability of the fabric, as well as its appearance by improving the coverage of immature cotton neps. The crystalline structure of the cotton cellulose is modified [36], swelling the fibres and eliminating their convolutions, but the polyester is not materially affected. Heat setting is obligatory to minimise subsequent dimensional changes at the high temper-atures reached in thermosol dyeing [37]. Polyester/cellulosic fabrics should be padded evenly with the dye dispersion and a nonionic wetting agent at pH 5–6. A migration inhibitor of the anionic polyelectrolyte type, e.g. sodium alginate, polyacrylamide, poly(vinyl alcohol) or carboxymethylcellulose, is normally included and this gels during drying to ultimately form a solid film in which much of the dye is

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entrapped. Migration inhibitors of the polyacrylamide type do not interfere with dye transfer in thermofixation as much as does alginate [6]. Liquid brands of disperse dyes are preferred to solid forms, as they are less prone to migrate during drying and they give less staining of the cellulosic fibres. Higher colour yields are attainable and the pad liquors are easier to prepare in the large quantities normally required for continuous dyeing. On the other hand, liquid dyes can settle on long storage and thus require thorough agitation before weighing. When stored in partly used containers they may vary in strength owing to loss of water or deposition of solids at the air/ liquid interface. Prior to thermofixation the disperse dyes are present in the interstices of the fabric in a highly aggregated state and are embedded, together with the dispersants and wetting agent, in this solidified matrix of polyelectrolyte. During drying and thermofixation the smaller aggregates release disperse dye molecules that migrate to the fibre surfaces. Further disaggregation proceeds and there is a build-up of dye near these surfaces [38]. Diffusion into the interior of the polyester fibres can only take place when the disperse dyes are in the monomolecular form and the temperature has reached that of the thermofixation chamber. A vertical infrared predrying zone minimises the tendency towards streakiness or two-sided effects if insufficient migration inhibitor has been used. This treatment should reduce the moisture content of the fabric from 50–60% after padding to 25–30%, so that drying can be completed in a hotflue machine at 100–120°C or on modulated drying cylinders (scaled from 80°C to 140°C) without significant migration [39]. Problems encountered during padding and intermediate drying include the induction period while the fabric is being heated up and staining of the cellulosic fibres by the disperse dyes, which is greater than in batchwise dyeing. These factors are both dependent on the type of drying unit, the fabric construction and the composition of the pad liquor. The use of optimised dispersing systems facilitates the transfer of disperse dyes from the surface of the cellulosic fibres to the polyester during intermediate drying to give the highest attainable yield on the polyester [40]. Migration during intermediate drying can cause many faults that are difficult to rectify. It is particularly troublesome in the continuous dyeing of knitted fabrics containing polyester/cellulosic yarns. Migration may occur as a result of high liquor retention after padding, a slow initial rate of drying or a high residual moisture content after the predrying step [41]. Faults arising from dye migration include:

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(1) (2) (3) (4)

Ending: the drying rate and visual depth vary along the length of a run. Streakiness: variable rates of drying over the fabric surface. Frosting: dye migration from the interior to the surface of the fabric. Two-sidedness: one surface of the fabric is hotter than the other during drying, giving a visible difference in depth with the hotter side deeper. (5) Dark selvedges: these regions hotter than the fabric centre. (6) Light selvedges: these regions cooler than the fabric centre. (7) Poor penetration: disturbance of the weave reveals undyed regions at the crossovers and inside the threads. With the exception of creasing, shading across the width of the fabric is the biggest single cause of customer rejects [39]. Several tests are available to assess migration, namely the ‘sandwich’, ‘watch glass’, ‘glass plate’, ‘hot air’ and ‘fold’ tests [42]. Parameters that influence migration are dye class and constitution, physical form of the dyes, dyebath additives and preparation of the substrate. In general, disperse dyes in liquid form perform better in this respect than those formulated as grains or powders. Quality control problems are more likely when handling liquid brands, however, because of the risk of sedimentation on storage. Differences in migration behaviour between ‘equivalent’ disperse dyes with the same CI generic name may arise because different formulating agents are present. Even when the fabric is evenly and fully penetrated during padding, the final dyeing may exhibit poor penetration because of migration or inadequate thermofixation. During drying, water evaporates from the fabric surface and is replaced by capillary flow of dye liquor from the interior. The dyed material will show inferior fastness to washing and wear points of the garments, such as collars, cuffs and elbows, will tend to develop into lightly coloured folds or patches [6]. In a study of the behaviour of disperse dyes padded on to a 70:30 polyester/ cotton blend, the dye applied was found to be distributed with only 30% of the total amount on the polyester and the remainder on the cotton. Thus the concentration of disperse dye before thermofixation was more than five times greater on the cotton than on the polyester. During thermofixation most of the dye was found to be transferred from cotton to polyester by volatilisation into the vapour phase [43]. Factors contributing to staining of the cotton include the chemical nature and concentration of migration inhibitor present, the dyebath pH and the chemical structure of the dyes used [44]. At unusually high temperatures, or if the treatment time is unduly prolonged, dye is lost by volatilisation and contamination of the interior surfaces of the thermofixation

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unit. Subsequent dyeings may be spoiled by volatilisation and redeposition of this condensed dye on the new material. Optimum fixation conditions depend on the design and efficiency of the thermofixation equipment as well as the depth and sublimation fastness of the dyeing, but the treatment temperature is usually within the range 205–220°C. Although high-energy disperse dyes require somewhat critical conditions for optimum transfer and fixation, they are the most widely used dyes for pad– thermofix methods, especially in full depths. The optimum temperature of thermofixation is that at which the maximum amount of dye is transferred to the polyester without significant loss by volatilisation or contamination. Treatment at this temperature ensures optimum colour yield and fastness, high reproducibility, low sensitivity to temperature variations and colour stability during subsequent durable finishing. Too low a temperature of thermofixation or time of treatment may prevent complete transfer of dye from cotton to polyester, resulting in excessive staining of the cotton and poor yield on the polyester. Listing arising from differential temperature distribution may be found after thermosol treatment. Contact with the base-plates on a thermosol pin stenter may cause off-shade selvedge marking [39]. The effective time of thermofixation at a given temperature is not the same as the nominal duration of the treatment. Air is a poor heat-transfer medium and it may take 45–60 seconds before the fabric temperature is within a degree or two of the air temperature in a conventional hot-air thermofixation unit. The time taken also depends critically on the fabric weight and construction. The heavier the fabric, the longer it takes to attain equilibrium temperature. It is important that the material has been dried uniformly before entering the thermofixation unit, otherwise the initial heating stage will be used to evaporate this residual moisture rather than raising the cloth temperature to its maximum. This will result in uneven and inadequate dye fixation [6]. Reduction clearing after the thermosol treatment is desirable to attain optimum fastness on the polyester and to clear any disperse dye stain from the cellulosic fibres. Reduction treatment must be avoided, however, if the cotton has already been dyed. When vat dyes are applied the reducing conditions necessary are often sufficient to clear the unfixed disperse dyes effectively, so that a separate clearing treatment is unnecessary. Those disperse dyes that contain alkalihydrolysable groupings, or certain heterocyclic diazo components such as the nitrothiophene blues (Figure 13.1) can be cleared by alkaline washing rather than reduction clearing. Alkaline washing is substantially cheaper than reduction clearing. It is applicable after dyeing with reactive dyes and it may be combined with the washing-off of unfixed reactive dyes.

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Many dyed polyester/cellulosic fabrics are subjected to fairly severe heat treatments during subsequent durable finishing and garment manufacture. These processes may induce further migration of low- or intermediate-energy disperse dyes from the polyester to the cellulosic fibres or the resin film, resulting in possible colour changes or inferior fastness to rubbing and wet treatments. The selection of high-energy disperse dyes helps to minimise these problems. The severity of the thermomigration effect depends on several factors: (1) constitution and applied depth of the dyes used; (2) heat treatment history of the polyester fibres; (3) degree of penetration of the polyester after thermofixation; (4) degree of staining of the cellulosic fibres; (5) temperature and duration of the heat treatment that causes the thermomigration; (6) presence of other contaminants on the surface of the fibre components. Important counter measures [45,46] that can be taken include: (1) careful dye selection to avoid dyes prone to thermomigration; (2) adequate thermofixation conditions to ensure optimum penetration of the polyester fibres; (3) thorough post-clearing to minimise residual staining of the cellulosic fibres; (4) lowering the curing temperature in subsequent resin finishing; (5) minimal application of softeners and antistatic agents at the finishing stage.

13.2.2 Water-soluble dyes for the cellulosic component The economically attractive one-bath batchwise process with disperse and direct dyes cannot be adapted readily to continuous dyeing. Direct dyes exhibit only limited solubility and are highly aggregated at padding concentrations. Their high substantivity leads to shade matching problems because of rapid depletion from the pad liquor (tailing). Steaming times would have to be prolonged because direct dyes diffuse only slowly into the cellulosic fibres. The possibility of pad–thermofix application of the disperse dyes, reduction clearing and then batchwise dyeing with direct dyes is of no interest because of high processing cost to achieve only moderate fastness. Reactive and vat dyes are the main alternatives for dyeing the cellulosic component. Reactive dyes give an excellent reserve of the polyester. Reserve of the cellulosic component is much less desirable, especially in polyester/cotton workwear fabrics or polyester/viscose blends in general, because the cellulosic fibre is preferentially abraded during wear. One-bath or two-bath processes

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based on disperse and reactive dyes offer exceptional brightness and very good fastness up to medium depths, especially for polyester/viscose dresswear or shirting. The simplest method of reserving the polyester fibre is to pad with highreactivity dyes, sodium bicarbonate and salt to minimise migration, followed by drying at 90°C. Urea is usually added to improve solubility during padding and to enhance the colour yield. For maximum fixation in full depths or under adverse drying conditions, a brief steaming treatment may be given before soaping at the boil. In an alternative process, low-reactivity dyes are applied with soda ash and urea, dried and reacted by thermofixation at 160–200°C. Improved results are obtained from either method on polyester/ viscose blends if the padded fabric is batched and stored for 1–2 hours before drying. Better storage stability of the pad liquor is ensured if the fabric is padded with the dyes in neutral solution and dried, then padded again in caustic soda and salt solution, steamed, rinsed cold and soaped at the boil. Selected disperse and reactive dyes can be fixed simultaneously by the simple and economical one-bath pad–dry–thermofix process. Dye selection is critical because of the risk of interaction between disperse and reactive dyes under alkaline conditions (section 4.1). Suitable disperse and high-reactivity dyes are padded with urea, sodium bicarbonate and migration inhibitor. Sodium m-nitrobenzenesulphonate is added to prevent reduction of certain azo reactive dyes, particularly on polyester/viscose fabrics. After drying, the fabric is thermofixed at 200–220°C and soaped at the boil. Urea concentrations greater than about 50 g l–1 induce excessive staining of the cellulosic fibres under these conditions, resulting in unsatisfactory fastness and inferior yield on the polyester. Another disadvantage of urea is that it decomposes above 135°C (Scheme 13.1) and the ammonia and cyanic acid formed are objectionable. Although more costly, dicyandiamide (cyanoguanidine) and dicyanoguanidine (Figure 13.3) are preferable to urea, because they are thermally stable and do not give rise to toxic by-products [47]. A more versatile selection of dyes is possible in two-stage methods using pad– dry–thermofix application of disperse dyes to the polyester, followed by pad– steam, alkali–pad or alkali–shock treatment to fix the reactive dyes on the cellulosic fibre. Less productive, but giving the widest freedom in terms of dye selection is the semi-continuous two-bath sequence of conventional pad–dry– thermofix to apply the disperse dyes and then cold pad–batch dyeing with the reactive dyes. High-reactivity dyes offer higher productivity than low-reactivity types because of the short batching times required [48].

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NH2

193

NH2

H2N C O

HN C OH

N C OH + NH3

Urea

Cyanic acid

Scheme 13.1

NH2 HN C

NH CN HN C

NH CN Dicyandiamide

NH CN Dicyanoguanidine

Figure 13.3 Alternatives to urea in thermosol dyeing

The advantages of semi-continuous dyeing methods for these blends include: (1) suitability for lengths to a colour that are insufficient to justify fully continuous processing; (2) optimum use of readily available equipment; (3) higher productivity and better uniformity at the padding stage than by batchwise dyeing; (4) continuity of colour over long runs compared with production of several discrete batchwise dyeings; (5) suitability for contrast effects with versatile control of shade matching. Useful alternatives to the above semi-continuous process are the pad–batch– beam and pad–batch–jet sequences, which are more productive with highreactivity dyes and especially suitable for vinylsulphone dyes. The blend fabric is padded with the reactive dyes and the usual amounts of alkali and salt and then batched for 2–24 hours according to dye reactivity. It is then transferred to a high-temperature beam or jet machine, rinsed and soaped at the boil. After addition of the disperse dyes, dispersing agent and acetic acid to pH 6, the polyester component is dyed at 130°C in the usual way. These processes offer the advantages of optimum yields of the reactive dyes with minimum occupation of the available pressure-dyeing equipment. This approach is suitable for tubular-knit polyester/cellulosic fabrics by dyeing the cellulosic component first using a padding unit (e.g. BeauTech, Calator or Jawatex) designed specifically for pad–batch processing of knitted fabrics. The polyester component is then jet-dyed with disperse dyes. Significant savings over traditional exhaust processes for knitgoods are claimed [49]. These semicontinuous techniques are particularly versatile for contrast effects since one run

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on the pad–batch equipment can be divided into a series of batches for beam or jet dyeing to different hues in the second stage.

13.2.3 Water-insoluble dyes for polyester/cellulosic blends Selected members of the vat dye range will dye both components of a polyester/ cellulosic blend in most shades up to medium depth. At full depths the cellulosic fibre is dyed more deeply and a skittery effect is evident. The target shade should be matched with a single dye or binary mixture if possible. Trichromatic combinations of dyes that differ widely in hue should be avoided. The best solidity is given by selected indigoid and thioindigoid dyes but these have limited fastness. Good solidity and better fastness are provided in the yellow to orange sector by polycyclic quinone derivatives. Suitable dyes are padded at 30–40°C with a wetting agent of the sulphoricinoleate type and a migration inhibitor. After drying and thermofixation at 200–220°C to promote diffusion of the vat dyes into the polyester, the fabric is cooled in air and the dyes on the cellulosic component reduced by padding in alkaline dithionite. Because of the previously high temperature of the thermofixation stage, special attention must be given to ensure that the fabric is adequately cooled before chemical padding and that the pad liquor temperature is kept below 30°C. After steaming, the dyeing is reoxidised and soaped at the boil as usual. The same process sequence is used to apply mixtures of selected disperse and vat dyes at the initial padding stage. Matched mixtures of selected disperse and vat dyes have been commercially available for many years. The critical factor is the stability of the disperse dyes to reducing systems, once they have been fixed on the polyester. The chemical pad–steam stage is effective in clearing the disperse dye stain from the cellulosic fibres as well as reducing the vat dyes present. Only in unusually heavy depths is it necessary to apply the vat dyes from a separate bath. The most severe requirements of fastness to rubbing, washing, chlorine and dry heat in all depths for uniforms or workwear are met with selected high-energy disperse dyes and vat dyes [50]. The disperse dyes are selected for stability under the reducing conditions of the pad–steam stage and the vat dyes should give only limited staining of the polyester under thermofixation conditions. The higher the treatment temperature is above 190°C, the greater is the degree of staining. Minimal staining is shown by the indanthrone blues and many of the acridone and carbazole types. Satisfactory results can be achieved with dyes giving an intermediate degree of

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staining, if disperse dyes are used to adjust the hue on the polyester. Vat dyes of this kind include violanthrone and isoviolanthrone derivatives, as well as most of the benzamidoanthraquinones, oxazoles and thiazoles [51]. Vat leuco ester dyes will give a solid effect in pale depths on the two components of a polyester/cellulosic blend. These dyes are padded at 60–80°C with soda ash, sodium nitrite and a wetting agent. The dyeing on the cellulosic fibre is developed on a jig or a continuous washing range by immersion of the fabric in dilute sulphuric acid at 20–40°C, neutralisation with soda ash at 40°C and soaping at the boil. In this process the vat leuco esters dye the cellulose and initially stain the surface of the polyester. Final rinsing, drying and heat setting at 200–210°C achieves optimum fastness and yield of the vat dyes on the polyester. The fabric must be completely free from alkali at the thermofixation stage to avoid discoloration of the cellulosic fibre. The oxidised forms of these dyes diffuse into the polyester during thermofixation, which is an essential part of the dyeing process as it develops and stabilises the colour. A hydrosetting treatment on the beam at 130°C may be given to achieve penetration of the polyester if no thermofixation equipment is available. In applications requiring high fastness in pale depths this method of dyeing both fibre components simultaneously is elegant and easy to use. The only drawbacks are that there is only a limited possibility of adjusting the proportions of dye between the two fibres and the products are costly. In pale depths both of these disadvantages are of minor significance. The solubilised vat leuco esters can also be applied in combination with disperse dyes. The blend fabric is padded at pH 5–6 with, in addition to the usual migration inhibitor, a mixture of anionic and nonionic auxiliaries. Drying and thermofixation to dye the polyester with the disperse dyes initiates preliminary decomposition of the vat leuco esters. These are then fully fixed by padding in the usual nitrite and sulphuric acid, then developing on a jig or washing range [8]. Most disperse dyes in dispersion are unable to withstand the reducing systems required for sulphur dyeing, but once they have been absorbed within the polyester fibre they are quite stable. Disperse dyes are applied together with dispersed or solubilised sulphur dyes by padding at pH 4.5–5.5 and drying, followed by thermofixation at 190–210°C. Padding with alkaline dithionite, steaming, reoxidation to fix the sulphur dyes and soaping at the boil completes the process. Sodium sulphide or hydrosulphide reduction is avoided because it has an adverse effect on the polyester fibres.

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An alternative process involves padding with disperse dyes, solubilised sulphur dyes and the reducing agent thiourea dioxide at pH 5–6, followed by drying and thermo-fixation to fix both classes of dyes. In this case, however, the thiourea dioxide restricts the choice of disperse dyes since some azo dyes are attacked by it [8]. Dyeing problems of various kinds may be associated with chemical pad–steam development of vat or sulphur dyes [52]. Inadequate yield may result from incorrect formulation or subsequent decomposition of the chemical pad liquor. Listing at the selvedges, foaming, shade variation or condensation spotting can all arise in the steaming stage necessary to fix the dyes on the cellulosic component. To obtain full development of shade, it is absolutely essential that there should be no air in the steamer. Ineffective washing to remove chemical residues, insufficient oxidising chemicals to reoxidise the leuco forms of the dyes and deposition of unfixed dye on the surface of the fibres may lead to fastness problems after final rinsing. Reoxidation of indanthrone blues above pH 9 may result in over-oxidation, leading to greener and duller shades. Azoic combinations still offer possibilities for colouring the cellulosic component in certain bright shade areas. In red shades, for example, this approach is more economical and gives higher fastness to chlorine than reactive dyeing. Continuing interest in these products is attributable to the availability of stabilised liquid brands for ease of handling, low costs of machinery and labour, high productivity and flexibility of operation. Computer programs are available on disc for IBMcompatible systems. These supply complete recipes including precise application procedures that ensure simple and reliable application in the dyehouse [53]. In a low-cost one-bath process, the fabric is padded at 40°C with a stabilised azoic diazo component (Fast Salt), an azoic coupler (Naphtol) and a matching mixture of disperse dyes for the polyester, together with an acid donor (sodium monochloroacetate) and urea or dicyandiamide to optimise coupling [54]. After drying at 130°C the fabric is thermofixed at 210°C, rinsed and soaped at the boil. A versatile semi-continuous sequence entails conventional pad–dry–thermofix dyeing of the polyester with disperse dyes, followed by padding at 50°C with an azoic Fast Salt and a Naphtol dissolved in an alkaline solution containing methylated spirit (formaldehyde must be excluded). After impregnation the fabric is batched for 1–2 hours, developed in acetic acid solution and soaped at the boil to remove unreacted azoic components [55].

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13.3 BLENDS OF CELLULOSE ACETATE OR TRIACETATE WITH CELLULOSIC FIBRES When cellulose acetate yarns were introduced in the 1920s, they created considerable interest because of their potential for cross-dyeing effects with cellulosic yarns. In blends with viscose, the acetate fibres confer a considerable improvement in crease recovery. Cellulose acetate is used in cotton-spun blends for a range of apparel end uses, mainly dresswear. Cellulose acetate/viscose woven fabrics made from intimate staple blends and conventional viscose filament constructions with acetate filament effect threads were formerly popular for crepe dresswear, gabardines, tropical suitings, leisure shirts, underwear and children’s wear. Two-fold yarns for colour contrast effects, and viscose filament warps with acetate or acetate/ viscose staple wefts, may also be encountered. Acetate/viscose fabrics are still significant for tailored ladieswear and ‘shot silk’ linings. Acetate/cotton brocades have been used for many years in floral patterned curtains and upholstery. When cellulose triacetate was first introduced in the 1950s, it was recognised as a cheaper alternative to polyester with strength and wearing properties intermediate between those of secondary cellulose acetate and polyester. It is quicker-drying and more stable to boiling dyebaths than the secondary acetate. It has better pill resistance than polyester, but is inferior to polyester in abrasion resistance and dimensional stability. Triacetate/cellulosic blends are mainly of interest for dresswear, suiting and skirts, many of the garments being designed with durable pleated effects. The triacetate component contributes easy-care properties, durable pleating and crease recovery. Triacetate/viscose, more important than blends with cotton, is used for children’s clothing, leisure shirts, pleated dresswear and lightweight suiting. Triacetate/polynosic blends have also proved interesting for woven and knitted ladieswear fabrics. A resin finish is required if the proportion of viscose or polynosic fibre exceeds 30%. Intact cellulose acetate shows only slight staining by dyes for cellulose, but acetate/cotton must be bleached carefully under mildly acidic conditions because partial saponification of the ester groups greatly increases subsequent staining of the acetate component. If it is necessary to bleach cellulose acetate/viscose fabrics for reserve effects or pale bright dyeings, this can be carried out with silicatestabilised hydrogen peroxide and an anionic detergent at 70°C. Normally it is

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only necessary to singe, open-width relax in boiling water and then scour with anionic detergent and ammonia at 60–70°C. Reserve of viscose using selected disperse dyes for the acetate may be difficult. The effect depends on the origin of the viscose and the efficiency of desizing and peroxide bleaching, as well as the dyeing conditions. It is important to desize thoroughly, since any residual size in the warp yarns will accentuate staining by disperse dyes and thus require a more severe clearing treatment after dyeing. Viscose staining also depends on dyeing time and temperature, liquor ratio and the selection of anionic dispersing agents. The preferred disperse dyes that give optimum reserve of viscose are mainly low-energy dyes (Mr 230–300), including dinitrodiphenylamine (yellow), nitroaniline monoazo (yellow to red) or 1,4disubstituted anthraquinone (red to blue) types. The stain on the viscose can usually be cleared by alkaline reduction at ambient temperature. Sodium dithionite in trisodium orthophosphate solution is satisfactory with most azo disperse dyes. However, an oxidative clear with sodium hypochlorite at a mildly alkaline pH, followed by an antichlor treatment with sodium bisulphite, is often more effective for anthraquinone disperse dyes. Either type of clearing treatment must be carried out under ambient conditions to avoid decomposition of the disperse dyes on the cellulose acetate. Where both types of disperse dye chromogen are present, it may be necessary to give both clearing treatments in sequence with the reduction clear first. The essential mechanisms are azo fission and leuco-anthraquinonoid solubilisation in the reductive process and destruction of the anthraquinone chromogen by oxidative attack [8]. If a neutral scouring and bleaching sequence is given to avoid saponification of the cellulose acetate, this component can be reserved using selected saltcontrollable direct dyes of the multisulphonated type with salt at 80°C. Solid effects on acetate/viscose are traditionally obtained by a one-bath method at 75– 80°C and pH 6–7 with the disperse and direct dyes, salt and disodium dinaphthylmethanedisulphonate as the dispersing agent. Care is required because some copper-complex direct dyes are able to modify the hue of certain disperse dyeings and the salt addition may lower the dispersion stability of some disperse dyes. Acetate/viscose dresswear or suiting fabrics are often dyed in colour contrasts, or either fibre may be reserved. These blends have been particularly important also in ‘shot silk’ contrast effects on filament viscose warp/acetate weft lining fabrics, where the component yarns are dyed in complementary contrasts such as red–green or blue–gold. Optimum contrast in full depths requires a two-bath

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sequence in which the cellulose acetate is dyed first, an intermediate clear at ambient temperature is given and then the cellulosic fibre is shaded to target with direct dyes. Either dithionite reduction or hypochlorite oxidation may be employed as necessary with these high-contrast effects in which the acetate fibre is usually dyed to the heavier depth. It is easier to obtain clear bright shades on the cellulosic fibre in these instances by clearing the disperse dye stain before applying the direct dyes. Acetate/viscose blend fabrics for apparel are dyed on the winch or jig according to fabric construction. Winch dyeing is avoided if there is likely to be serious rope creasing. Dresswear or leisure shirting is usually plain woven from 50:50 acetate/viscose staple blend yarns, or contains an acetate filament warp and staple viscose weft that can be dyed in contrasting colours. The dyed fabric is normally resin-treated to achieve the required dimensional stability and wet fastness. Acetate/cotton curtaining and furnishing fabrics are dyed with disperse and direct dyes selected from those with adequate light fastness for these outlets. It has also been possible to produce solid effects on these blends using selected vat dyes applied by pigment padding in the presence of disodium dinaphthylmethanedisulphonate as dispersing agent. After drying, the fabric is developed at the boil in a solution of sodium sulphite and sodium formaldehydesulphoxylate, reoxidised and soaped. Woven fabrics prone to creasing are developed on the jig and knitted constructions on the winch. Traditionally, however, there has been little point in using dyes of high fastness on the cellulosic component of an acetate/cellulosic blend because the wet fastness attainable on the acetate using low-energy disperse dyes was relatively poor. Furthermore, the acetate fibre would be delustred by the boiling soap aftertreatment needed to develop optimum fastness with reactive dyes on the cellulosic component. The introduction of Xtol (Courtaulds) acetate fibre in 1987 enabled all unfixed reactive dyes to be completely removed from the cellulosic fibre at the boil without delustring of the acetate fibre [56]. Together with the higher wet fastness attainable with high-energy disperse dyes on Xtol, much improved performance can be achieved compared with what was previously possible on such blends. Solid shades are most common on triacetate/cellulosic blends but contrast effects and reservation of either fibre type are practicable. Most triacetate blend fabrics are dyed on the winch or jet and adequate preparation is essential. Fabrics for reserve effects or pale dyeings are enzyme desized and peroxide bleached. Triacetate is more resistant to saponification under alkaline conditions than the

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secondary acetate, and triacetate/cotton fabrics may be cold mercerised. They should not be S finished (section 11.2) or boiled with alkali, however, as this aggravates cross-staining of the triacetate by direct dyes. Selected disperse dyes will give a satisfactory reserve of the cellulosic fibre. These can be applied at the boil and pH 5–6 with an ester carrier and disodium dinaphthylmethanedisulphonate as dispersing agent. Triacetate absorbs disperse dyes more slowly than acetate and this increases the likelihood of cross-staining of the cellulosic fibre. Adequate clearing should be attainable using an anionic detergent at 70°C. Carrier addition can be avoided by jet dyeing at 120°C but it may then be necessary to reduction clear the disperse dye stain using sodium dithionite at 40°C. The preferred disperse dyes for reserving the cellulosic fibre and showing good stability to subsequent cross-dyeing with direct dyes and salt are mainly intermediate-energy dyes (Mr 300–400) with adequate fastness to pleating, particularly monoazo and disazo (yellow to orange), chloronitroaniline monoazo (red) and tri- or tetra-substituted anthraquinone (red to blue) types. Undamaged cellulose triacetate can be reserved with selected multi-sulphonated self-levelling and salt-controllable direct dyes, particularly stilbene derivatives and disazo tetrasulphonates, including copper-complex types and symmetrical derivatives of diarylurea middle components. It is usual to give a cationic aftertreatment to improve wet fastness and avoid marking-off onto the undyed triacetate component. Resin treatment is then necessary to enhance crease recovery, dimensional stability and colour fastness. Vat dyes are unsuitable for triacetate reserve effects because the triacetate fibre is partially saponified and cross-stained during vat dyeing. Solid and contrast effects are readily obtained by a two-stage process based on the method already described for cellulosic fibre reserve with disperse dyes. Direct and disperse dyes are added initially and after dyeing of the triacetate the dyebath is cooled to 60°C, salt is added and dyeing continued at 80–90°C to target depth on the cellulosic fibre. Full depths show better fastness or brighter contrast effects from a two-bath sequence with an intermediate clear of the disperse dye stain using alkaline dithionite at 40–50°C. Better wet fastness is achieved on triacetate/cellulosic blends using disperse and vat dyes applied by a two-bath method. Vat dyes withstand the conditions of disperse dye application better than do direct dyes and therefore the cellulosic fibre is dyed first in this case. Selected cold-dyeing vat dyes are applied at pH 9 and 45°C. Some staining of the triacetate occurs as a result of partial saponification, but the stain has good fastness and the triacetate can be filled in

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with disperse dyes from a fresh bath. The preferred vat dyes [51] include benzamidoanthraquinones, heterocyclic anthraquinone derivatives (acridones, oxazoles and thiazoles) and halogenated polycyclic quinones (anthanthrone, dibenzopyrenedione and indanthrone). Bright dyeings with good wet fastness can be obtained from a three-stage sequence with disperse dyes applied by the conventional ester carrier process, then exhaustion of the reactive dyes on to the cellulosic fibre with salt and finally an alkaline fixation stage, cold rinsing and soaping at the boil. A limited range of specific shades can be achieved on triacetate/viscose blend fabrics using selected azoic diazo and coupling components. Solidity and levelness were claimed to be acceptable and fastness to various agencies, including wet and dry rubbing, was satisfactory [57]. 13.4 BLENDS OF POLY(VINYL CHLORIDE) FIBRES WITH CELLULOSIC FIBRES Jersey fabrics in this category are made by the controlled shrinkage, dyeing, stenter drying and raising of interlock knitted from staple blends of poly(vinyl chloride) or PVC fibres with viscose or modal fibres. These constructions are suitable for swimwear, sportswear, outerwear and upholstery fabrics. Woven cotton or modal fibre blends with PVC fibre are also of interest, including a 70:30 cotton/PVC staple blend for corduroy constructions and a cotton warp/ PVC filament weft fabric for car upholstery. Processing is usually in open width to minimise creasing. After desizing and scouring with caustic soda (cotton blends) or soda ash (viscose or modal blends) and an anionic detergent, these fabrics can be dyed by a one-bath method using disperse and direct dyes at 80–95°C with dispersing agent and salt. Better wet fastness is achieved by applying disperse and vat dyes in a two-bath sequence. The conventional vat dyebath in the second stage clears the disperse dye stain from the cellulosic fibre.

13.5 DYEING METHODS AND DYE SELECTION FOR DC BLENDS Reserve or contrast effects are seldom in demand on polyester/cellulosic fabrics, but on blends of cellulosics with cellulose acetate or triacetate, especially on acetate/viscose fabrics, they do make a significant contribution. The simple onebath process with disperse and direct dyes is important on cellulosic blends with the cellulose ester fibres or PVC fibre, but is only used on low-quality goods in the polyester/cellulosic sector. Two-stage techniques with disperse and reactive

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Table 13.1 Dye selections for exhaust dyeing of DC blends

Blend

Colour effect

Dyeing method

Dye selection

Polyester/ cellulosic

Polyester reserve

Single-class

Direct dyes stable to peroxide bleach; selected polycyclic vat dyes

Solid

Single-class (pale depths)

Benzamidoanthraquinone vat dyes of relatively low M r

One-bath

Disperse dyes and disazo multisulphonated direct dyes at pH 6 Disperse dyes and reactant-fixable dyes with copper-specific sequesterant Disperse dyes and nicotinotriazine reactive dyes at 130°C

Two-stage

Disperse dyes at 130°C, then low-reactivity dyes at 80–95°C High-reactivity dyes at low temperature, then disperse dyes at 130°C Disperse dyes at 130°C, then selected vat dyes at 20–60°C

Two-bath (full depths)

Disperse dyes at 130°C, then sulphur dyes with dithionite

Acetate reserve

Single-class

Salt-controllable multisulphonated direct dyes at 80°C

Cellulosic reserve

Single-class

Low-energy disperse dyes at 80°C

Solid

One-bath

Low-energy disperse dyes and salt-controllable direct dyes at pH 6–7 and 80°C

Contrast

Two-bath

Disperse dyes at 80°C, reductive or oxidative clear at 20°C, then direct dyes at 80°C

Xtol/ cellulosic

Solid or contrast

Two-stage

High-energy disperse dyes at the boil, then reactive dyes at appropriate temperature

Triacetate/ cellulosic

Triacetate reserve

Single-class

Selected multisulphonated direct dyes with cationic aftertreatment

Cellulosic reserve

Single-class

Intermediate-energy disperse dyes at 120°C

Solid or contrast

Two-stage

Disperse dyes at 120°C, then direct dyes at 90°C

Acetate/ cellulosic

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Table 13.1 Continued

Blend

Colour effect

Dyeing method

Dye selection

Triacetate/ cellulosic

Solid or contrast

Two-bath

Selected vat dyes at 45°C, then intermediate-energy disperse dyes at 120°C

PVC/ cellulosic

Solid

One-bath

Low-energy disperse dyes and direct dyes at 95°C

Two-bath

Disperse dyes at 95°C, then cold-dyeing vat dyes

Table 13.2 Dye selections for continuous dyeing of polyester/cellulosic blends

Blend

Colour effect

Dyeing method

Dye selection

Polyester/ cellulosic

Polyester reserve

Pad–dry

High-reactivity dyes with sodium bicarbonate and urea

Pad–dry– thermofix

Low-reactivity dyes with sodium bicarbonate and urea

Pad–dry– thermofix

Selected disperse dyes and high-reactivity dyes with sodium bicarbonate and urea

Solid

Disperse dyes and stabilised azoic components with acid donor and hydrotropic agent Pad–dry–thermofix– pad–batch

Disperse dyes, then high-reactivity dyes

Pad–batch–beam or –jet

Vinylsulphone reactive dyes, then disperse dyes at 130°C

Pad–jig develop– dry–thermofix

Vat leuco esters for both fibres (pale depths)

Pad–dry–thermofix– pad–steam

Disperse dyes, then low-reactivity dyes Selected vat dyes for both fibres (pale or medium depths) High-energy disperse dyes, then selected vat dyes High-energy disperse dyes, then dispersed or solubilised sulphur dyes

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dyes represent the most important exhaust dyeing approach on polyester/ cellulosic or Xtol (Courtaulds) acetate/cellulosic blends. Two-stage or two-bath processes with disperse and vat dyes offer high standards of all-round fastness performance on blends of polyester, cellulose triacetate or PVC fibre with the various natural or regenerated cellulosic fibres (Table 13.1). Excellent polyester reserve effects are attainable on polyester/cellulosic fabrics using highly productive pad–dry or pad–dry–thermofix application of reactive dyes. Solidity on the two fibre components up to medium depths is attainable using vat dyes or their leuco esters alone, or a simple pad–dry–thermofix process with selected disperse and reactive dyes. Higher all-round fastness and a wider selection of dyes are offered by various two-stage sequences involving thermofixation of the disperse dyes on polyester and then chemical pad–steam fixation of vat, sulphur or reactive dyes. Compromise semi-continuous methods such as pad–dry–thermofix–pad–batch or pad–batch–beam (or –jet) have also been found useful for disperse and reactive dyes (Table 13.2).

13.6 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

J A Hook, J.S.D.C., 108 (1992) 367. R Aitken, J.S.D.C., 99 (1983) 150. G Henze, H Bille, W Thonig and G Schmidt, Textile Asia, 5 (Jul 1974) 44. W Prager and M J Blom, Text. Chem. Colorist, 11 (Jan 1979) 11. W S Hickman and R J Chisholm, Australian Text., 13 (Mar–Apr 1993) 24. W J Marshall in The dyeing of cellulosic fibres, Ed. C Preston (Bradford: SDC, 1986) 320. R T Norris, Textilveredlung, 12 (1977) 258. T M Baldwinson in Colorants and auxiliaries, Vol. 2, Ed. J Shore (Bradford: SDC, 1990) 568. F Schlaeppi, R D Wagner and J L McNeill, Text. Chem. Colorist, 14 (1982) 257. K Miyata, AATCC International Conference and Exhibition, (Oct 1992) 121. H Tiefenbacher, Textil Praxis, 37 (1982) 812; Chemiefasern und Textilind., 35/87 (1985) 797. K H Röstermundt, Textil Praxis, 46 (1991) 56. P W Leadbetter and S Dervan, J.S.D.C., 108 (1992) 369. M A Herlant, Text. Chem. Colorist, 17 (Jun 1985) 117; Am. Dyestuff Rep., 74 (Sep 1985) 55; (Oct 1985) 37. N E Houser, J C Martin and M White, Am. Dyestuff Rep., 70 (Sep 1981) 19. N E Houser and M White, AATCC Nat. Tech. Conf., (Oct 1976) 105. T D Fulmer, America’s Textiles Internat., 18 (Jan 1989) 54. T A Waldrop, Am. Dyestuff Rep., 75 (Sep 1986) 22. D Monney, Dyer, (Apr 1994) 32. U Kreig, Australian Text., 10 (Jan 1990) 38. J A Hook and A C Welham, J.S.D.C., 104 (1988) 329. Anon, Dyer, (Jul 1995) 14. Y Sato, Am. Dyestuff Rep., 72 (Sep 1983) 30. R W Chalk and N E Houser, Text. Chem. Colorist, 20 (Nov 1988) 17.

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REFERENCES

25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57.

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I D Menzies and P W Leadbetter, AATCC Nat. Tech. Conf., (Oct 1985) 108. S Abeta and K Imada, Am. Dyestuff Rep., 74 (May 1985) 25 A N Lee, Dyer, (Apr 1994) 29. S Abeta and K Imada, AATCC Nat. Tech. Conf., (Oct 1985) 112. C V Stead in Colorants and auxiliaries, Vol. l, Ed. J Shore (Bradford: SDC, 1990) 324. G H Kenyon, Am. Dyestuff Rep., 68 (Mar 1979) 19. H U von der Eltz and R Adrion, Textil Praxis, 45 (1990) 1023. D Hildebrand, B Renziehausen and D Hellmann, Melliand Textilber., 69 (1988) 895. N Morimura and M Ojima, Am. Dyestuff Rep., 74 (Feb 1985) 28. B Semet and G E Grüninger, Melliand Textilber., 76 (1995) 161. C L Chong, S Q Li and K W Yeung, Am. Dyestuff Rep., 81 (May 1992) 17. T P Nevell in Cellulosics dyeing, Ed. J Shore (Bradford: SDC, 1995) 16. J Pashley, J.S.D.C., 109 (1993) 379. H U von der Eltz and J Müller, Internat. Text. Bull., No. 2 (1978) 1. H D Moorhouse, Rev. Prog. Coloration, 26 (1996) 20. F Somm, Textilveredlung, 23 (1988) 257. F Somm, C Oschatz and H Lehmann, Teintex, 42 (Mar 1977) 125. F Somm and R Buser, Textilveredlung, 19 (1984) 359. C J Bent, T D Flynn and H H Sumner, J.S.D.C., 85 (1969) 606. W Shimizu and J W Rucker, Am. Dyestuff Rep., 84 (Apr 1995) 32. P Richter, AATCC Nat. Tech. Conf., (Oct 1983) 255. H U von der Eltz and R Kuhn, Melliand Textilber., 67 (1986) 336. W Marschner and D Hildebrand, Chemiefasern und Textilind., 31/83 (1981) 153. R Buser, M Capponi and F Somm, Textilveredlung, 12 (1979) 106. G Harding, Australasian Text., 6 (Jan–Feb 1986) 22. C Otte, Textilveredlung, 18 (1983) 269. F R Latham in Cellulosics dyeing, Ed. J Shore (Bradford: SDC, 1995) 247. L R Smith and O E Melton, Text. Chem. Colorist, 14 (May 1982) 113. P Frey, Textil Praxis, 48 (1993) 521. H Kaiser, Chemiefasern und Textilind., 26/78 (1976) 316. P Frey, Text. J. of Australia, 49 (Aug 1974) 36. J M Taylor and P Mears, J.S.D.C., 107 (1991) 64. L A Kovkin and N A Tikhomirova, Intensif. tekhnol. otdelki i modif. tekstil. materialov, (1984) 53.

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CHAPTER 14

Triacetate/polyester and other DD blends

14.1 DYEING PROPERTIES OF DISPERSE-DYEABLE FIBRE BLENDS Cellulose acetate, triacetate and polyester differ greatly in dyeability at a given temperature. The low-energy dye 1,4-diaminoanthraquinone (CI Disperse Violet 1) yields a 50% exhaustion value at 40°C on acetate, 70°C on triacetate and over 100°C on polyester. The dyeing rate is much more rapid on cellulose acetate, so that it is virtually impossible to achieve solid effects on either acetate/triacetate or acetate/polyester blends. Shadow and reserve effects have been of some interest on mixed-ply yarns. Triacetate/polyester offers more scope for achieving solidity and this blend has proved moderately useful in dresswear fabrics woven from filament yarns. Shadow effects and a limited degree of reserve of polyester with appropriate selections of dye and carrier are also possible. Triacetate/polyester is therefore by far the most versatile of the DD blends. It is usual to scour, stenter set and S finish as for 100% triacetate fabrics (section 11.2). The presetting treatment markedly lowers the dyeability of the triacetate component without greatly affecting the polyester, so that solid effects become easier to obtain. During dyeing the triacetate is dyed preferentially in the early stage, but migration from triacetate to polyester occurs later as the dyes diffuse slowly into the polyester. Eventually a solid effect is achieved, the time depending on the recipe and depth of shade. It is easier to attain acceptable solidity at short liquor ratios and in pale depths. At longer liquor ratios preferential absorption by the triacetate component becomes more obvious and it is progressively more difficult to transfer the dyes to the polyester [1]. Cellulose triacetate and polyester are therefore sufficiently close in dyeing properties to give good solidity with selected dyes under controlled dyeing conditions. There are four possible methods of obtaining good solidity on triacetate/ polyester blends by batchwise dyeing: (1) Monoazo, disazo or anthraquinone disperse dyes of intermediate or high fastness to sublimation (Mr 300–450) are the most suitable for this purpose in the conventional high-temperature process at 120°C.

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(2) A two-stage method, dyeing first at 130°C to achieve satisfactory yield and penetration on the polyester fibre, followed by a period at the boil without carrier to bring the triacetate fibre to the same depth. (3) Selected dyes will give good solidity in the presence of an aryl ester or diphenyl carrier at the boil. Anthraquinone dyes of intermediate sublimation fastness are particularly appropriate. Few azo dyes give adequate solidity under these conditions. (4) In an alternative carrier-dyeing method, selected dyes favouring polyester at the boil are used together with others that favour triacetate. This results in complicated recipes with up to six component dyes. These blends are almost always dyed at high temperature nowadays. The selection of intermediate- to high-energy dyes applied at 120°C is the preferred approach. The two-stage method is time-consuming but provides better control from the viewpoint of shade matching. Carrier-dyeing methods are much more unattractive because of the adverse effect of these products on the working environment. Combinations of dyes that are partially selective for dyeing the respective fibres do give more control of colour matching to achieve a solid effect, however. In high-temperature dyeing small quantities of carrier are still sometimes added to shift the balance in favour of the polyester. Ester-based carriers, such as methyl salicylate or butyl benzoate, are particularly effective [2]. Phenolic carriers lower the light fastness, especially on the triacetate component, and are difficult to remove without appreciable stripping of dye from the triacetate. In addition to its toxicity, trichlorobenzene causes moderate swelling of the triacetate fibres and is best avoided. The dyes favouring polyester under carrier-dyeing conditions at the boil are the high-energy azo and anthraquinone types. Low-energy monoazo and nitro dyes generally favour the triacetate fibres. If those dyes showing a preference for triacetate are applied alone at 90°C with an ester carrier, shadow and polyester reserve effects can be readily obtained. Shadow effects represent the only possibility on cellulose acetate/triacetate blends for dresswear. Solidity, reserve and contrast effects are all excluded. The acetate is dyed more deeply and there may be differences in hue between the component fibres when applying combination recipes in mode shades. Dyeing at 60°C favours the acetate fibre to the greatest extent and as the dyeing temperature is increased the difference in depth decreases, although even at 95°C solidity is not generally achieved and the acetate fibre is delustred above 85–

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90°C. The introduction in 1987 of Xtol (Courtaulds) acetate fibre [3] has offered the possibility of dyeing acetate/triacetate blends at the boil to achieve nearsolidity without delustring but this possibility does not widen the colouring potential of these blends very much. Simple low-energy yellow, orange and red dyes of the substituted p-nitrophenylazoaniline type give the best approach to solidity. Ester carriers do promote uptake of dyes by the triacetate component, but these agents cause unacceptable swelling of the acetate fibres. Solidity is virtually impossible to attain on polyester/acetate blends because the rate of dyeing is so much higher on the acetate. If a reserve of the polyester is required the acetate can be dyed at 60–70°C with a nonionic dispersing agent using mainly monoazo yellow to red dyes of relatively high energy (Mr 330–450) containing N-hydroxyethyl or N-acetoxyethyl substituents in the coupling amine, as well as violets and blues of the 1,4-disubstituted anthraquinone type. Shadow effects on this blend require higher dyeing temperatures and longer times. Unfortunately, such severe conditions tend to degrade conventional acetate fibres badly, leading to delustring, fibre swelling and loss in tensile strength of the blend fabric. Much better shadow dyeings are obtained on blends of Xtol (Courtaulds) acetate fibre with polyester, since high-energy dyes can be effectively absorbed by both components without delustring of the Xtol fibres. It is sometimes possible to use combinations of slow- and rapid-diffusing disperse dyes in the presence of a carrier to increase the rate of dyeing on the polyester. It is particularly important, however, to check the effect of the carrier on the tensile strength of the acetate fibres. Rapid-diffusing dyes are low-energy (Mr 230–270) monoazo and anthraquinone types. The slowdiffusing dyes are similar in structural types but higher in energy (Mr 270–350). Carrier-dyeing methods, however, are becoming increasingly objectionable for ecological reasons. Shadow effects with low- and intermediate-energy disperse dyes are readily controlled on fabrics woven from normal and deep-dye polyester yarns (section 5.1). Dyes with similar rates of exhaustion on these two fibre variants should be selected for mode shades [4]. The rate of temperature rise should be slowed in the critical region and an alkanol polyoxyethylene retarder used to minimise unlevelness. Good liquor circulation or vigorous agitation of the fabric are necessary. Unlevel dyeings can be corrected by treatment with an ester carrier and a dispersing agent at pH 5–6 and the boil. The light fastness of disperse dyes on deep-dye polyester variants designed for dyeing at the boil without carrier is inferior to that on normal polyester. This deficiency is attributed to the presence of alkylene ether groups in the main chain

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of the copolymer (section 5.1). The addition of stabilisers to the dyebath has been examined with a view to overcoming this drawback [5]. An antioxidant Irganox 1010 (Ciba) and an ultraviolet absorber Tinuvin 328 (Ciba) of the hindered phenol type were used (Figure 14.1).

OH tBu

tBu

O

tBu

tAm

tBu

CH2

N

O H2C C CH2 O

HO

N

CH2 tBu

OH

N

OH

tAm

tBu

O

Ultraviolet absorber tAm = tertiary amyl

tBu

tBu OH

Antioxidant tBu = tertiary butyl

Figure 14.1 Stabilising agents for disperse dyes on deep-dye polyester

CI Disperse Red 60

O

CI Disperse Red 82

NH2

CN O

CH2CH2OCOCH3 O2N

N N

N CH2CH2OCOCH3

O

OH Dyes of high light fastness

Red

O

N

CH3

Orange

CH3 CH3 O2N

N N

N CH3

O Dyes of low light fastness

Figure 14.2 Disperse dyes evaluated with stabilising agents

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Two disperse dyes of high light fastness and two of inferior fastness (Figure 14.2) were applied at 130°C in the presence of these additives to polyester fabrics knitted from the two variant yarns. Improvements in xenon-arc light fastness ratings of 0.5 to 1 point were obtained with each additive. The ultraviolet absorber was marginally more effective than the antioxidant and in some cases a synergistic effect was observed using them in combination.

14.2 DYEING METHODS AND DYE SELECTION FOR DD BLENDS Among these blends a reserve effect can only be achieved on polyester in its blends with cellulose acetate or triacetate. There are several methods for attaining good solidity on triacetate/polyester blends (Table 14.1) but this is the only DD blend on which this effect is practicable. The two methods that require

Table 14.1 Disperse dye selections for DD blends Blend

Colour effect

Dyeing method

Dye selection

Triacetate/ polyester

Polyester reserve or shadow

One-stage

Low-energy monoazo and nitro dyes at 90°C with ester carrier

Solid

One-stage

Intermediate- or high-energy dyes at 120°C Intermediate-energy anthraquinone dyes with ester carrier at the boil High-energy dyes for polyester and low-energy dyes for triacetate with ester carrier

Two-stage

Polyester dyes at 130°C, then triacetate dyes at the boil

Acetate/ triacetate

Shadow

One-stage

Low-energy dyes at 60–80°C

Polyester/ acetate

Polyester reserve

One-stage

High-energy monoazo and low-energy anthraquinone dyes at 60–70°C

Polyester/Xtol (Courtaulds)

Shadow

One-stage

High energy dyes at the boil without carrier

Normal/ deep-dye polyester

Shadow

One-stage

Low- and intermediate-energy dyes at 120°C

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the use of a carrier to achieve this are of little or no interest nowadays. All DD blends will readily yield shadow effects, of course, this being the inevitable result of the widely different rates of dyeing between these disperse-dyeable fibres.

14.3 REFERENCES 1. 2. 3. 4. 5.

V R Adomas, R R Zhyamaitaitene and V V Brazauskas, Issled. svoistv. syrya i pererab. ego v shelkov. prom-sti., M (1984 ) 141. T M Baldwinson in Colorants and auxiliaries, Vol. 2, Ed. J Shore (Bradford: SDC, 1990) 568. J M Taylor and P Mears, J.S.D.C., 107 (1991) 64. J Hürter, Melliand Textilber., 63 (1982) 296. B Küster and H Herlinger, Textil Praxis, 40 (1985) 406.

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DYEING PROPERTIES OF THREE-COMPONENT BLENDS

CHAPTER 15

Dyeing properties of three-component blends

15.1 INTRODUCTION Extension of the classification scheme in Chapter 2 to the case of threecomponent blends follows logically to give categories such as those listed in Table 15.1. Clearly, there are twenty possible combinations of three from four fibre types that could be assembled but in practice not all of them are useful. The ten categories included in Table 15.1 are those in which actual three-component blends have been encountered. It is often advantageous to approach each of these blends as a one-off set of requirements. Nevertheless, the general principles adopted in dealing with the binary blends offer the best basis for developing a coherent dyeing method for a three-way blend. The selection of fibre blend components and their proportions is as critical for three-component blends as it is for the more familiar binary blends, but the addition of a third component offers no guarantee of producing a fabric with enhanced characteristics overall. Apparel fabrics for blazers, outerwear, flannels and velours, traditionally manufactured with a cotton warp and a wool weft, may be made with a blended wool/viscose or nylon/viscose weft, for example, on economic grounds. Four-component blends are occasionally encountered, too, since dresswear or other items of apparel may be woven from dissimilar blended staple yarns in both warp and weft. An interesting account has appeared of pushing back the frontiers of what is achievable in multicolour piece dyeing for the woven suitings sector by the Gibbs Burge dyehouse in Australia [1]. Regular production was established of fourcolour designs on multifibre fabrics, sometimes including six man-made fibres (acrylic, modacrylic, nylon, viscose, regular and modified polyester). Numerous fancy yarns (marls, mélanges, slub yarns, bouclés, donegals) were incorporated in design styling. Initially these were included as stripe effects, but then gradually introduced into more complex constructions to extend the versatility of appearance, colour and texture effects. At the outset, limitations were anticipated in fastness to light, washing, perspiration and rubbing, cross-staining, differential abrasion, differential 212

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INTRODUCTION

Table 15.1 Classification of ternary blends

AAA blends Nylon/wool/polyurethane Pale-dye/normal/deep-dye nylon Normal/deep-dye/ultra-deep nylon AAB Blends Nylon/wool/acrylic fibre Polyurethane/wool/basic-dyeable polyester Nylon/modacrylic fibre/acrylic fibre Nylon/acid-dyeable acrylic fibre/basicdyeable acrylic fibre Modacrylic fibre/acid-dyeable acrylic fibre/basic-dyeable acrylic fibre Normal/deep-dye/basic-dyeable nylon AAC blends Nylon/wool/cotton Nylon/wool/viscose Nylon/polyurethane/cotton Normal nylon/deep-dye nylon/cotton Normal nylon/deep-dye nylon/viscose CBA blends Cotton/acrylic fibre/nylon Cotton/basic-dyeable acrylic fibre/aciddyeable acrylic fibre Cotton/modacrylic fibre/acrylic fibre DAA blends Cellulose acetate/nylon/wool Cellulose triacetate/nylon/wool Polyester/nylon/wool Polyester/polyurethane/wool

DAC blends Cellulose acetate/nylon/cotton Cellulose triacetate/nylon/cotton Polyester/nylon/cotton Polyester/polyurethane/cotton Cellulose acetate/wool/viscose Cellulose acetate/wool/linen Cellulose acetate/nylon/viscose Cellulose triacetate/nylon/viscose Polyester/nylon/viscose DBA blends Polyester/acrylic fibre/wool Normal polyester/basic-dyeable polyester/wool Normal polyester/basic-dyeable polyester/nylon Poly(vinyl chloride)/acrylic fibre/wool Poly(vinyl chloride)/acrylic fibre/nylon DBC blends Polyester/acrylic fibre/cotton Polyester/acrylic fibre/viscose Normal polyester/basic-dyeable polyester/cotton Normal polyester/basic-dyeable polyester/viscose DDA blends Cellulose acetate/polyester/nylon Cellulose acetate/poly(vinyl chloride)/wool DDC blends Cellulose triacetate/polyester/cotton Cellulose triacetate/polyester/viscose

shrinkage and cockling. These proved to be less critical than expected, providing careful compromises in dye selection and dyeing conditions were reached. In general, most customers were prepared to accept wider tolerances in order to obtain novel effects with greater intrinsic appeal [1].

15.2 DYEING OF AAA BLENDS Ternary AAA blends of acid-dyeable nylon variants, e.g. pale-dye/normal/deepdye or normal/deep-dye/ultra-deep, are less versatile than the binary blends (section 5.2) because the range of coloured effects attainable is limited mainly to

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three-way shadow effects. The cost of manufacture is similar to that for AAB nylon blends containing two acid-dyeable variants with a basic-dyeable yarn. These offer considerably wider possibilities for contrast and reserve effects. Control of pH is most important for reproducible results on ternary acid-dyeable blends. A relatively slight change in total exhaustion may significantly alter the distribution between the variant yarns. Acid dyes for these blends differ in selectivity according to structural type. The differentiation within the range pH 4–7 tends to increase with degree of sulphonation. Levelling acid dyes with three or four sulpho groups show selective behaviour even in pale depths, but monosulphonates give marked differences only in full depths applied at pH 7–8. Premetallised dyes and the more hydrophobic ‘supermilling’ acid dyes with high wet fastness properties show such poor differentiation that they are of little value for these blends. Three-way shadow effects are given by a range of monoazo or monosulphonated anthraquinone dyes. The normal nylon component can be reserved (if necessary at a slightly higher pH) using a similar series of dyes that are preferentially absorbed by the deep-dye and ultra-deep variants. These are mainly mono- or disulphonated anthraquinone dyes. Many disulphonates and trisulphonates, especially monoazo and disazo types, will give a satisfactory reserve of the normal and deep-dye yarns. These dyes are selectively absorbed by the ultra-deep variant under neutral conditions where hydrophobic dye–fibre bonding predominates. Four approaches to dye selection are available to exploit the colouring possibilities with these groups of dyes: (1) A short but versatile range of hues for three-way shadow effects contains a red and a green that allow reserve of the normal nylon at a higher pH. (2) The same range can be used in conjunction with more highly sulphonated types that dye the ultra-deep variant selectively. Such selections yield a shadow effect on the normal and deep-dye yarns with a much deeper contrasting (but dependent) hue on the ultra-deep nylon. (3) Selected dyes that reserve only normal nylon can be used with the more highly sulphonated types to obtain a dependent contrast on the more dyeable variants with the normal nylon reserved. (4) Alternatively, selected disperse dyes can also be added to give a three-way dependent colour contrast. The ultra-deep yarn is invariably dyed very heavily to a dull hue in these effects. It absorbs all three dye types and the disperse dyes often show some preference for the ultra-deep component. This limits the attractiveness of the coloured design and the wet fastness that can be achieved.

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Polyurethane fibres are sometimes used in nylon/wool fabrics to provide enhanced stretch properties. Fast dyeings on this ternary blend are best obtained with 1:2 metal-complex dyes. Coverage of physical variations in the nylon component is less serious than for nylon/polyurethane blends (section 6.3). As in the application of these dyes to nylon/wool (section 6.1), a syntan is added to retard uptake by the nylon component and an alkanol polyoxy-ethylene is used to improve levelling.

15.3 DYEING OF AAB BLENDS Core-spun polyurethane yarns are sometimes used to confer better stretch characteristics in polyester/wool fabrics. The use of disperse dyes for the polyester often results in low fastness of the dyed polyurethane to spotting with chlorinated solvents. If basic-dyeable polyester is used in blends with wool and polyurethane, however, this problem no longer arises since ionic dyes can be applied to all three fibres. The dyebath is set with an alkanol polyoxyethylene as anti-precipitant, an anionic retarder for the basic dyes, an aryl ester carrier formulated with a nonionic emulsifier, acetic acid and the basic dyes. These dyes are exhausted on to the basic-dyeable polyester at the boil. The dyebath is then cooled to 70°C and the two acid-dyeable components dyed simultaneously using suitable neutral-dyeing anionic dyes with an anionic agent to control the distribution between the wool and polyurethane. Knitted blends of basic-dyeable and acid-dyeable acrylic yarns are sometimes physically strengthened by including nylon. The one-bath method with selected basic and acid dyes at pH 4–5 already described for nylon/acrylic blends (section 7.2) will give attractive contrasts between nylon and the basic-dyeable acrylic fibre while reserving the acid-dyeable acrylic fibre. At lower pH, the acid dyes will give a shadow effect on the two acid-dyeable components. Under nearneutral conditions, the basic dyes will give a similar effect on the two acrylic variants. Blends of normal, deep-dye and basic-dyeable nylon provide a wider range of colouring possibilities than either the ternary AAA combinations or the binary AB blends. There are still limitations imposed by cross-staining, however. The dyeing method is essentially that already described for normal/ basic-dyeable nylon, using a phosphate buffer at pH 6 (section 5.3). The coloured effects are varied by dye selection. Certain monosulphonated acid dyes will give a shadow effect on the acid-dyeable components and reserve the basic-dyeable yarn. If basic dyes of the localised-charge type are added, a

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shadow and contrast effect is obtained. A dependent contrast with reserve of the basic-dyeable nylon is given by selected mono- and disulphonated acid dyes in combination (section 5.2). If the disulphonated acid dyes and basic dyes are used together, the normal nylon can be reserved with an independent contrast on the other two fibres, but there are limitations of depth. Finally, a restricted gamut of dependent three-colour contrasts is possible using both types of acid dyes together with basic dyes.

15.4 DYEING OF AAC BLENDS Traditionally, the most important AAC blend was the nylon/wool/viscose combination for carpet yarns but this is now seldom encountered. Blends of the AAC type are sometimes found in decorative uniforms or blazer-cloth designs, examples being a cotton or nylon/viscose warp with a nylon/wool weft or a nylon staple warp and a wool/viscose weft. All of these may be readily dyed by a one-bath method under neutral conditions using selected metal-complex or acid dyes as discussed for nylon/wool (section 6.1), together with salt-controllable direct dyes mainly of the disazo or trisazo tetrasulphonate type. Salt addition promotes exhaustion of the direct dyes by the cellulosic component and a syntan is required to minimise uptake of these dyes by nylon. Nylon/polyurethane/cotton blends containing 10–20% of the elastomeric fibre are important in knitted underwear. Stretch corduroy fabrics containing these three components can be dyed semi-continuously by first dyeing the cotton with reactive dyes by cold pad–batch, sulphur dyes by pad–steam, or vat leuco esters by a pad–develop method [2]. The two acid-dyeable fibres are then filled in by exhaust dyeing with 1:2 metal-complex or milling acid dyes in the presence of an appropriate auxiliary agent to control the distribution between the nylon and polyurethane components (section 3.3). Normal and deep-dye nylon yarns have been used in blends with cotton for upholstery fabrics. These can be dyed with selected direct and acid dyes as already discussed for nylon/cellulosic blends (section 8.2). The preferred acid dyes are those mono- and disulphonated types that show good reserve of the cotton as well as satisfactory differentiation of the nylon variant yarns. Suitable direct dyes are mainly of the salt-controllable type with up to four sulphonate groups. These give acceptable reserve of the nylon variant yarns when applied in the presence of a syntan. In this way a restricted gamut of shadow with contrast effects is possible, but the wet fastness properties are limited.

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15.5 DYEING OF CBA BLENDS Carpets and rugs are sometimes made with an acrylic/modacrylic pile, one of the objectives being to use the modacrylic fibre to reduce the risk of flammability. Such a blend of 60:40 acid-dyeable acrylic/Dynel (Union Carbide) may be used as the pile on a cotton backing. The dyeing procedure is to use a one-bath method, selecting basic dyes for the Dynel, 1:2 metal-complex dyes for the aciddyeable acrylic variant and salt-controllable direct dyes for the cotton. The use of an anti-precipitant system and careful dye selection are essential. A butyl benzoate carrier favours absorption of the basic dyes by Dynel and the dyebath should be maintained at pH 6 with ammonium sulphate. Wet fastness of the cotton backing can be improved by a conventional treatment with a cationic fixing agent. Carpets made from basic-dyeable and acid-dyeable acrylic fibres on a jute or cotton backing fabric, however, are more difficult to dye in deep contrasting hues. The need to dye at pH 2 to minimise basic dye staining of the acid-dyeable component (section 7.3) degrades the cellulosic fibre backing. Staining of the acid-dyeable fibre by lignin impurities from the jute (section 5.2) may also occur. For these reasons it is preferable to employ more subtle muted contrasts on these constructions.

15.6 DYEING OF DAA BLENDS So-called ‘booster’ blends of low-pill polyester staple with long-staple wool for jersey-knitted suitings, outerwear and casual wear contain 10% nylon staple to provide a favourable balance of pilling resistance and increased durability in suitings [3]. These offer an interesting challenge to the attainment of acceptable solidity. In a one-bath method for pale or medium depths, all three fibres are dyed simultaneously using disperse and 1:2 metal-complex or milling acid dyes, followed by soaping to remove disperse dye staining from the wool and from the surface of the polyester. The polyester, and to some extent the nylon, can be dyed first with disperse dyes and carrier at the boil or under pressure at 105°C in a two-bath method. After reduction clearing, the wool and nylon components are dyed in a fresh bath with the selected anionic dyes. Attainment of optimum solidity on this blend requires: (1) selection of disperse dyes that favour polyester rather than nylon (section 11.3); (2) selection of 1:2 metal-complex and milling acid dyes showing a suitable partition between wool and nylon at the target depth (section 6.1);

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(3) the use of a syntan as a retarding agent on the nylon when dyeing in pale depths. Nylon may be used to improve the tensile strength and abrasion resistance of cellulose ester/wool blends, as in 33:33:33 cellulose acetate/wool/nylon or 65:30:5 triacetate/wool/nylon. Two-bath dyeing methods are preferred for these blends. Disperse dyes in the low- to intermediate-energy range (Mr 220–380) are selected for cellulose acetate to give minimum staining of wool at 80°C (section 11.2). Preferred disperse dyes for triacetate are the intermediateenergy types (Mr 300–400), including nitrodiphenylamine yellows, monoazo reds and anthraquinone violets and blues. These are applied at the boil in the presence of an ester carrier. In both instances the disperse dye stain is cleared from the wool using a nonionic detergent at 50°C. The nylon and wool components are filled in with neutral-dyeing 1:2 metal-complex or milling acid dyes using appropriate auxiliaries (section 3.2). Chrome dyes are also suitable for deep shades. Fabrics of this type that are triacetate-rich can be heat set at 200°C after dyeing to enhance dimensional stability and improve the wet fastness of the disperse dyes. Core-spun polyurethane yarns may be included in the weft direction in woven polyester/wool fabrics to give improved stretch properties in skiwear and sportswear. The polyurethane fibre readily absorbs most disperse dyes and this may result in low fastness to solvent spotting. Disperse dyes for use on these blends, therefore, are selected for low solubility in dry cleaning solvents. Twostage or two-bath methods are necessary, applying these disperse dyes with a carrier at the boil followed by 1:2 metal-complex or milling acid dyes for the wool. These methods are analogous to the corresponding processes in the absence of polyurethane (section 11.1). Carriers of the aryl ester or trichlorobenzene type are preferred, however, because o-phenylphenol may damage the polyurethane fibre.

15.7 DYEING OF DAC BLENDS Solidity is difficult to achieve on polyester/nylon/cotton blends. It is often advisable to leave the cotton undyed or pastel dyed because of problems of colour matching or differential abrasion of the cotton (section 1.5.4). The synthetic components can then be dyed in solid or preferably contrasting hues by the methods devised for polyester/nylon blends (section 11.3). Such considerations do not apply to the dyeing of those polyester/cotton or polyester/viscose outerwear fabrics containing 5–10% of core-spun polyurethane

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weft yarns to impart enhanced stretch to the construction. However, disperse dyes used on these blends should be selected for low solubility in chlorinated organic solvents because of their limited fastness on the polyurethane component. Conventional exhaust methods of applying these dyes, together with direct dyes or followed by reactive dyes for the cellulosic fibre, are analogous to the corresponding processes in the absence of polyurethane (section 13.1). Cellulose acetate/wool/viscose blends are normally dyed by a two-bath method. The acetate is dyed first at 80°C and the stain on the other two components is reduction cleared with dithionite and ammonia. Direct and neutral-dyeing acid dyes are then applied from a fresh bath in the presence of salt. Shirting and pleated dresswear woven from 60:20:20 or 55:15:30 triacetate/ nylon/viscose blends may be dyed by a one-bath method based on the selection of disperse dyes for solidity on the triacetate and nylon compo-nents (section 11.4). Direct dyes of the self-levelling and salt-controllable disazo multisulphonate types are used to fill in the viscose. A problem of unlevel dyeing was encountered on a union fabric woven from a linen warp and a triacetate/nylon blended-staple weft. It was found that the most critical stage of processing was steaming, which could lead to irregular stripy faults in the vat-dyed linen warps. A factorial design was used to establish the optimum conditions for steam fixation in order to minimise the fault [4].

15.8 DYEING OF DBA BLENDS Fabrics containing these three types of yarn are generally intended for threecolour contrast effects or two-colour contrast with the disperse-dyeable fibre reserved. Solidity is not often required, but polyester/wool/acrylic blended-staple yarns for overcoats or suitings are dyed in solid shades by a two-bath method [5]. Bright contrasts in complementary hues can be obtained if only the two ionic-dyeable fibres are dyed. If disperse dyes are used as well they tend to dye (or stain) all three fibre types to some extent, giving a dependent hue (Table 1.5) on the basic-dyeable fibre and a duller hue or clearing problems on the acid-dyeable component, especially when this is wool. Many disperse dyes will give three-way shadow effects on blends of nylon with normal and basic-dyeable polyester, the distribution being controlled by dyeing temperature and addition of carrier (section 12.3), but the low wet fastness on the nylon is a limiting factor. Better fastness is given by two-way shadow with reserve of the acid-dyeable component of this type of blend using a carrier and selected disperse dyes to reserve nylon or wool, typically low-energy anthraquinone dyes (Mr 220–300). Better control of the nylon

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reserve and shadow or dependent contrast effects on the two polyester variants is provided by the method already described using disperse and basic dyes (section 12.3). Anthraquinone basic dyes are generally unsuitable, the preferred types being mainly methine, monoazo, triarylmethane, xanthene and oxazine. All of these shadow or reserve effects based on disperse dyes are more successful when the acid-dyeable fibre is nylon rather than wool, which is too easily stained by disperse dyes and difficult to clear satisfactorily (section 3.4). The method already described (section 7.6) for contrast effects on basic-dyeable polyester and nylon or wool using basic and acid dyes with an alkanol polyoxyethylene anti-precipitant can be used on these blends to reserve the normal polyester fibre. Selected premetallised, milling acid or reactive dyes are used on the acid-dyeable fibre. Dependent three-colour contrasts (Table 1.5) on blends of nylon or wool with normal and basic-dyeable polyester are obtained by a two-stage method at pH 5–6 (ammonium acetate–acetic acid). The acid-dyeable component is dyed with acid dyes at 75°C in the presence of an alkanol polyoxyethylene dispersing agent as anti-precipitant and Glauber’s salt to protect the basicdyeable variant from hydrolysis. The basic dyes are then added, followed by the disperse dyes and a diphenyl or aryl ester carrier formulated with a nonionic emulsifying system. The target shades on the normal and basicdyeable polyester are achieved at the boil. Possible incompatibility between some basic dyes and anionic dispersing agents in the disperse dye formulations can be minimised by adding the disperse dyes along with the acid dyes at the lower temperature. However, this does increase the disperse dye staining of the acid-dyeable fibre.

15.9 DYEING OF DBC BLENDS A troublesome characteristic of dyed contrast effects on polyester/cellulosic blends is for the cellulosic component to be partially lost by differential abrasion on exposed edges of the garment. Resin treatments, particularly the severe curing conditions necessary for durable press processes, accentuate the tendency for this to occur. The fault is especially obvious when the polyester is less heavily dyed, so that the abraded areas have a frosted appearance. An improvement in this respect is shown by blends of normal and basic-dyeable polyester with a cellulosic fibre. If the polyester homopolymer is dyed to the palest depth, the basic-dyeable copolymer to the heaviest depth and the cellulosic component to

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an intermediate level, preferential abrasion of the cellulosic fibre does not alter the apparent balance of the design too drastically. Similar considerations apply to blends of polyester and acrylic fibres with cotton or viscose. Three-colour designs of limited fastness on these DBC blends are obtainable by a simple one-bath method. Selected intermediate-energy disperse dyes and salt-controllable direct dyes are applied at pH 4 and 70°C in the presence of an alkanol polyoxyethylene anti-precipitant, as well as Glauber’s salt to protect the basic-dyeable polyester and to promote exhaustion of the direct dyes on to the cellulosic fibre. The basic dyes and a suitable carrier formulated with a nonionic emulsifier are added and the basic-dyeable polyester is dyed to shade at the boil. A brighter gamut of contrasts with much better wet fastness on the cellulosic fibre is offered by a two-stage method. Selected disperse and reactive dyes are added with Glauber’s salt and an alkanol polyoxyethylene anti-precipitant. The reactive dyes are exhausted on to the cellulosic fibre at 60°C and then fixed at an appropriate alkaline pH and temperature. The pH is adjusted to 5 with acetic acid, basic dyes and a nonionic carrier emulsion are added and the dyeing of the polyester variant yarns is completed at the boil. A three-fibre blend of interest in household textiles has a 65:35 polyester/ cotton warp and an 80:20 acrylic/cotton weft. Multicoloured or reserve effects can be produced but if all three fibre types are to be dyed to a solid shade a threebath process, applying disperse, vat and basic dyes in that order, may be necessary. The cost of this sequence can only be justified to achieve exceptional fastness demands. If the vat dyes can be replaced by direct dyes, a more economical two-bath sequence of disperse and direct dyes followed by basic dyes is practicable at a lower level of fastness.

15.10 DYEING OF DDA BLENDS The attainment of acceptable solidity and fastness is often difficult on these blends and cross-staining makes multicoloured effects of little interest. For example, the components of a 25:50:25 cellulose acetate/polyester/nylon dresswear fabric can be dyed acceptably solid up to medium depth with a limited selection of low-energy disperse dyes applied at 80–90°C with carrier. Adequate yield and fastness on the polyester, however, can only be attained under conditions that damage the cellulose acetate. Similarly, a blend of 67:18:15 wool/PVC fibre/acetate formerly used in lightweight uniforms was most difficult to dye in piece form because of the physical and chemical sensitivities of the component fibres. Both the wool and PVC fibres are prone

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to shrinkage, but not necessarily to comparable extents, and the cellulose acetate tends to become delustred at temperatures normally preferred for dyeing wool. Both wool and cellulose acetate may suffer hydrolytic damage if treated under alkaline conditions. Wool is badly stained by the disperse dyes that must be used to dye the other two components. 15.11 DYEING OF DDC BLENDS Dresswear fabrics made from 65:15:20 or 30:40:30 cellulose triacetate/ polyester/viscose blends can be dyed by a one-bath method. This depends on the selection of disperse dyes for optimum solidity on triacetate/polyester (section 14.1) and direct dyes that give the best reserve of unsaponified triacetate without suffering significant decomposition in the hightemperature dyeing process at 120°C (section 13.3). The preferred disperse dyes are monoazo or disazo intermediate-energy types (Mr 300–380) and the most suitable direct dyes are of the self-levelling and salt-controllable disazo multisulphonate types. An addition of sodium m-nitrobenzenesulphonate assists in minimising the risk of slight decomposition of the direct dyes at high temperature. 15.12 DYEING METHODS AND DYE SELECTION FOR THREECOMPONENT BLENDS It is difficult to summarise the numerous and varied methods that have to be adopted for the exhaust dyeing of blends containing three different fibre types (Table 15.2). Solid effects are usually what is required on those blends (AAA, AAB, AAC and DAA) that contain any two of the major acid-dyeable fibres (wool, nylon or polyurethane). The same limitation is true for blends that contain two ester fibres (DDA or DDC) and the DAC blends of an ester fibre and an amide fibre with a cellulosic fibre. The full versatility of colouring possibilities, i.e. three-way shadow, shadow/reserve, contrast/reserve, shadow/contrast and three-way contrast effects, is only offered by those blends that contain at least two dyeability variants of the same synthetic fibre. These can be the differential-dyeing nylon variants in AAA, AAB or AAC blends, acid-dyeable/basic-dyeable acrylic fibres (AAB blends) or normal/basic-dyeable polyester (DBA or DBC blends).

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DYEING METHODS AND DYE SELECTION FOR THREE-COMPONENT BLENDS

223

Table 15.2 Dye selections for three-component blends Blend

Composition

Colour effect

Dye selection

AAA

Pale-dye/normal/ deep-dye nylon

Three-way shadow

Monoazo or anthraquinone levelling acid dyes

Normal/deep-dye/ ultra-deep nylon

Reserve (n)/ shadow

Monosulphonated or disulphonated acid dyes

Shadow/ contrast

Disulphonated and multisulphonated acid dyes

Reserve (n)/ contrast

Monosulphonated and multisulphonated acid dyes

Three-way contrast

Monosulphonated and multisulphonated acid dyes with disperse dyes

Nylon/wool/ polyurethane

Solid

1:2 metal-complex dyes

Polyurethane/wool/ basic-dyeable polyester

Solid

Basic dyes with anionic retarder, then neutral-dyeing acid dyes

Nylon/acid-dyeable acrylic/basicdyeable acrylic

Reserve (a-d)/ contrast

Acid dyes and basic dyes at pH 4–5

Shadow/ contrast

Acid dyes and basic dyes at pH 2 (acid shadow) or pH 7 (basic shadow)

Shadow/ reserve (b-d)

Selected monosulphonated acid dyes

Shadow/ contrast

Monosulphonated acid dyes with localised-charge basic dyes

Contrast/ reserve (b-d)

Selected monosulphonated and disulphonated acid dyes

Reserve (n)/ contrast

Disulphonated acid dyes with localisedcharge basic dyes

Nylon/wool/ cellulosic

Solid

Selected 1:2 metal-complex or milling acid dyes and salt-controllable direct dyes with syntan

Nylon/polyurethane/ cotton

Solid

Reactive dyes by pad–batch, then 1:2 metal-complex or milling acid dyes

Normal nylon/ deep-dye nylon/ cotton

Shadow/ contrast

Monosulphonated and disulphonated acid dyes and salt-controllable direct dyes with syntan

CBA

Cotton/modacrylic/ acrylic

Solid or contrast

Salt-controllable direct dyes, basic dyes and 1:2 metal-complex dyes with aryl ester carrier and anti-precipitant

DAA

Polyester/nylon/ wool

Solid

Disperse dyes and 1:2 metal-complex or milling acid dyes with syntan

AAB

Normal/deep-dye/ basic-dyeable nylon

AAC

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DYEING PROPERTIES OF THREE-COMPONENT BLENDS

Table 15.2 Continued Blend

Composition

Colour effect

Dye selection

DAA

Acetate/nylon/wool

Solid

Low- to intermediate-energy disperse dyes at 80°C, then 1:2 metal-complex or milling acid dyes with syntan

Triacetate/nylon/ wool

Solid

Intermediate-energy disperse dyes with carrier at the boil, then 1:2 metal-complex or milling acid dyes with syntan

Polyester/wool/ polyurethane

Solid

Disperse dyes with carrier, then 1:2 metal-complex or milling acid dyes

Polyester/nylon/ cotton

Contrast/ reserve (c)

Neutral-dyeing acid dyes at 70°C, then intermediate-energy disperse dyes at 120°C

Polyester/ polyurethane/ cellulosic

Solid

Disperse dyes and disazo multisulphonated direct dyes

Acetate/wool/ viscose

Solid

Low-energy disperse dyes at 80°C, then direct dyes and neutral-dyeing acid dyes

Triacetate/nylon/ viscose

Solid

Low-energy disperse dyes and disazo multisulphonated direct dyes at 120°C

Normal polyester/ basic-dyeable polyester/nylon

Three-way shadow

Selected disperse dyes with carrier

Shadow/ reserve (n)

Low energy disperse dyes with carrier

Contrast/ reserve (n)

Selected disperse dyes and basic dyes at 120°C

Reserve (p)/ contrast

Basic dyes and neutral-dyeing acid dyes with anti-precipitant

Three-way contrast

Acid dyes at 75°C, then basic dyes and disperse dyes with carrier and anti-precipitant

Three-way contrast

Intermediate-energy disperse dyes, basic dyes and salt-controllable direct dyes with carrier and anti-precipitant

DAC

DBA

DBC

Normal polyester/ basic-dyeable polyester/ cellulosic

Disperse dyes and reactive dyes with anti-precipitant, then basic dyes with nonionic carrier at the boil DDA

Acetate/polyester/ nylon

Solid (pale depths)

Selected low-energy disperse dyes at 80°C with carrier

DDC

Triacetate/ polyester/viscose

Solid

Intermediate-energy disperse dyes and disazo multisulphonated direct dyes at 120°C

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REFERENCES

225

15.13 REFERENCES 1. 2. 3. 4. 5.

A Lester, Australian Text., 2 (May–June 1982) 18. F Somm, Textilveredlung, 15 (1980) 7. J B Timmis, Dyer, 153 (4 Apr 1975) 363. L N Nazarenko, R D Efremov and L V Danileika, Kiev tekhnol. I legkoi prom.-sti., Kiev (1988) 4. M V Karve, Indian Text. J., 87 (1977) 123.

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