Tài liệu Basic principles of textile coloration

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Basic Principles of Textile Coloration Arthur D Broadbent Professor, Université de Sherbrooke, Département de génie chimique, Faculté de génie, Sherbrooke, QC, J1K 2R1, Canada 2001 Society of Dyers and Colourists iii Prelims.p65 3 27/07/01, 10:06 Copyright © 2001 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 Thanet Press Ltd, Kent. ISBN 0 901956 76 7 iv Prelims.p65 4 27/07/01, 10:06 Contents Preface xiii CHAPTER 1 1.1 1.2 1.3 34 Fibrous polymers 50 Synthetic fibres Synthetic fibres 50 Nylon fibres 51 Polyesters 58 Acrylic fibres 65 Other synthetic fibres 68 References 69 v Prelims.p65 20 Polymer structure 34 Molecular organisation in fibres 40 Intermolecular forces 42 Thermal properties of polymers 45 References 49 CHAPTER 4 4.1 4.2 4.3 4.4 4.5 Fibres and textiles: properties and processing Properties of fibres 20 Production and properties of yarns 22 Fabric manufacture 25 Preparation for dyeing 29 Dyeing and finishing 32 References 33 CHAPTER 3 3.1 3.2 3.3 3.4 1 Historical background 1 Modern textiles 10 Colour, dyes and dyeing 13 References 19 CHAPTER 2 2.1 2.2 2.3 2.4 2.5 An introduction to textiles, dyes and dyeing 5 27/07/01, 10:06 CHAPTER 5 5.1 5.2 5.3 5.4 5.5 Introduction 70 Cotton 70 Cellulose 74 Cotton processing 80 Other vegetable fibres 90 References 91 CHAPTER 6 6.1 6.2 6.3 107 Protein fibres 130 Water treatment Water quality for the dyehouse 130 Water hardness 132 Water softening 138 Boiler water 144 Dyehouse effluent and its treatment 146 References 151 vi Prelims.p65 92 Introduction 107 Structure of wool fibres 107 Physical and chemical properties of wool 116 Wool processing 122 Speciality animal fibres 128 References 129 CHAPTER 8 8.1 8.2 8.3 8.4 8.5 Artificially made fibres based on cellulose The first regenerated cellulose fibres 92 Viscose fibre 93 Cellulose acetates 102 References 106 CHAPTER 7 7.1 7.2 7.3 7.4 7.5 70 Natural cellulosic fibres 6 27/07/01, 10:06 CHAPTER 9 9.1 9.2 9.3 9.4 197 Dyeing theory Dyeing equilibria 197 Dyeing kinetics 207 Aggregation of dyes 213 Conclusion 214 References 214 CHAPTER 12 12.1 12.2 12.3 12.4 12.5 12.6 174 An introduction to dyes and dyeing Dyes 174 Dyeing methods 177 Dyebath and fabric preparation 179 Terms used in direct exhaust dyeing 180 Continuous dyeing 190 References 196 CHAPTER 11 11.1 11.2 11.3 11.4 215 Dyeing machinery Basic features of batch dyeing machines 215 Dyeing machines for loose fibre and sliver 216 Machines for dyeing yarn 218 Machines for dyeing fabric 223 Dyeing machines for specific articles 233 Continuous dyeing equipment 234 References 239 vii Prelims.p65 152 Impurities in textile fibres 153 Surface activity of detergents 155 Synthetic surfactants 164 Other applications of surfactants 172 References 173 CHAPTER 10 10.1 10.2 10.3 10.4 10.5 Auxiliary chemicals for wet processing and dyeing 7 27/07/01, 10:06 CHAPTER 13 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 13.9 13.10 13.11 13.12 13.13 General description of acid dyes 240 Classification of acid dyes 241 The application of acid dyes in dyeing wool 243 Mechanism of wool dyeing 248 Problems of dyeing wool level 251 Special wool dyeing processes 255 Mordant dyes for wool 257 Pre-metallised metal-complex dyes 264 Dyeing nylon with acid dyes 268 Dyeing nylon with metallised dyes 280 Light and ozone fading of acid dyed nylon 282 Nylon carpet dyeing 283 Dyeing modified nylons 285 References 286 CHAPTER 14 14.1 14.2 14.3 14.4 14.5 14.6 14.7 Dyeing cellulosic fibres with direct dyes 307 Disperse dyes Introduction to disperse dyes 307 Chemical constitutions of disperse dyes 309 Disperse dye dispersions 310 Fastness properties of disperse dyes 313 Dyeing cellulose acetate fibres 314 Dyeing nylon with disperse dyes 317 Dyeing polyester with disperse dyes 319 Dyeing of other synthetic fibres 330 References 331 viii Prelims.p65 287 Introduction 287 Chemical constitutions of direct dyes 288 Dyeing properties of direct dyes 289 The effects of variations in dyeing conditions 296 The aftertreatment of dyeings with direct dyes 300 Dyeing different types of cellulosic fibres 303 The origins of substantivity for cellulose 304 References 306 CHAPTER 15 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 240 Acid, metallised and mordant dyes 8 27/07/01, 10:06 CHAPTER 16 16.1 16.2 16.3 16.4 16.5 16.6 The development of reactive dyes 332 Reactive dyes for cotton 333 Batch dyeing of cotton with reactive dyes 339 Bifunctional reactive dyes 347 Continuous dyeing processes for cotton 348 Reactive dyes for wool 353 References 357 CHAPTER 17 17.1 17.2 17.3 17.4 17.5 17.6 17.7 17.8 17.9 17.10 17.11 17.12 17.13 17.14 358 Vat dyes Introduction 358 Chemical constitution of quinone vat dyes 358 The reduction of quinone vat dyes 360 The substantivity and dyeing characteristics of vat dyes for cellulosic fibres 366 Dyeing cotton with leuco vat dyes 369 Oxidation and soaping after dyeing 372 Pre-pigmentation dyeing methods 373 Fastness properties of vat dyes 376 Dyeing with indigo and indigoid vat dyes 376 Solubilised vat dyes 378 Sulphur dyes 379 Batch dyeing procedures with sulphur dyes 382 Continuous dyeing with sulphur dyes 386 Environmental concerns 386 References 387 CHAPTER 18 18.1 18.2 18.3 18.4 18.5 332 Reactive dyes 388 Cationic dyes Introduction 388 Chemical structures of cationic dyes 389 Preparation for dyeing acrylic fibres 389 Dyeing acrylic fibres with cationic dyes 391 Dyeing modified polyesters and nylons 397 References 397 ix Prelims.p65 9 27/07/01, 10:06 CHAPTER 19 19.1 19.2 19.3 19.4 19.5 Introduction 398 Azoic dyes 399 Application of azoic dyes 404 Fastness properties of azoid dyeings on cotton 407 Other types of ingrain dye 408 Reference 410 CHAPTER 20 20.1 20.2 20.3 20.4 20.5 20.6 427 Colour measurement Factors influencing colour perception 427 Light sources and illuminants 428 Reflection or transmission of light by an object 431 Human colour vision 436 Characterisation of the CIE standard observers 438 Determination of the tristimulus values of a colour 446 The Munsell colour system 457 Visual uniformity of colour spaces 459 References 464 CHAPTER 22 22.1 22.2 22.3 411 Union dyeing Fibre blends 411 Union dyeing 412 Dyeing cotton/polyester blends 413 Dyeing wool/polyester blends 423 Dyeing cotton/nylon blends 423 Dyeing nylon and polyester variants 425 Reference 426 CHAPTER 21 21.1 21.2 21.3 21.4 21.5 21.6 21.7 21.8 398 Dyes synthesised in the fibre Colour differences and colorant formulation Colour difference equations 465 Shade sorting 473 Colorant formulation 477 References 492 x Prelims.p65 10 27/07/01, 10:06 465 CHAPTER 23 23.1 23.2 23.3 23.4 23.5 23.6 23.7 23.8 23.9 23.10 Introduction 493 Flat screen printing 494 Rotary screen printing 498 Engraved roller printing 502 Printing styles 504 Pigment printing 509 Printing with soluble dyes 512 Transfer printing 515 Carpet printing 518 Thickeners 520 References 526 CHAPTER 24 24.1 24.2 24.3 24.4 24.5 527 Testing of dyes and dyeings Spectrophotometric analysis of dye solutions 527 The evaluation of the colour yield of dyes 530 Fastness properties of dyeings and their assessment 531 Identification of dyes on the fibre 540 Separation of dyes by chromatographic techniques 541 References 549 CHAPTER 25 25.1 25.2 25.3 25.4 25.5 493 Printing 550 Textile finishing Introduction 550 Mechanical finishing methods 551 Thermal finishing processes 553 Chemical finishing of fabrics from cellulosic fibres 554 Other types of finishing chemicals 567 Reference 568 xi Prelims.p65 11 27/07/01, 10:06 Preface Around 1993–94 I became interested in writing a textbook on textile dyeing and related topics along the lines of E R Trotman’s Dyeing and chemical technology of textile fibres, the sixth edition of which was published in 1984. This led to a sabbatical leave in the Department of Colour Chemistry at the University of Leeds during 1994–95 that allowed completion of the planning and some initial writing of this work. My original idea was to produce a book dealing with the basic principles of textile dyeing and related subjects. In teaching these subjects, I had found that the available multi-author books, published mainly by the Society of Dyers and Colourists, were often too advanced for students, and I thought that a single book serving as an introduction to these works might be useful. I remember reading around that time that such an undertaking is partly ego driven. Over the past six years, any ideas of fame or fortune rapidly dissipated. The constant effort required of a single author to produce a 25 chapter book, in addition to full-time professional work, was only sustained because of my love for the subject and of my fascination with how dyeing takes place. The latter was reinforced on reading once again Tom Vickerstaff’s classic book Physical chemistry of dyeing. I began to realise that, despite all the wonderful technology available for textile dyeing, we really understand so very little of the fundamentals. I firmly believe that the optimum choice, use, control and adaptation of modern dyeing technology can only be achieved through a sound understanding of basic principles. This book is the fruit of my efforts to provide that understanding. It is designed for readers who have completed studies in chemistry and mathematics up to pre-university level. Because of the wide range of topics included, some subjects only receive superficial coverage. Those that are presented in more detail obviously reflect my personal bias. I am solely responsible for any limitations of content or detail, as well as the invariable errors required by Murphy’s law. At the end of each chapter are a limited number of references. Some of these are cited in the chapter text, the latter ones are usually general reading references. The interested reader will find more detailed information and references in the books published by the Society of Dyers and Colourists and in technical periodicals. In addition, several of the colorant structures shown in the book are identified by their Colour Index Generic Name. It is worth noting that in the Colour Index itself many of these structures appear as sodium salts and not in the free acid forms shown in these pages. xiii Prelims.p65 13 27/07/01, 10:06 My thanks to Prof. David Lewis and the staff of the Department of Colour Chemistry at Leeds for their kind hospitality during my 1994–95 leave. The photographs of fibre cross-sections were kindly provided by Tom Micka of DuPont Fibers (Figure 4.2) and by Doug Tierce of Acordis Fibers (Figure 6.2). The American Association of Textile Chemists and Colorists (AATCC) kindly allowed reproduction of Figure 4.6. I would also like to acknowledge Greentex Inc. (Montréal), Regent Ltd. (Montréal), C A Kennedy Inc. (Montréal), Then GmbH (Germany), Macart Textiles Ltd. (UK) and MCS SpA (Italy) for dyeing machine illustrations. The completion of this book is the result of the dedicated work of the editorial staff of the SDC, in particular Paul Dinsdale and Carol Davies, who all have my sincere gratitude. ARTHUR D BROADBENT xiv Prelims.p65 14 27/07/01, 10:06 1 CHAPTER 1 An introduction to textiles, dyes and dyeing The manufacture of textiles is a major global industry. It provides vast quantities of materials for clothing and furnishings, and for a variety of other end-uses. This book deals specifically with textile coloration. It begins by introducing this subject along with some technical terms and concepts related to dyes, fibres and dyeing. At this stage, mastery of all the new ideas is not necessary. They will be encountered again throughout the book. Several examples of the molecular structures of dyes will be presented in this chapter so that the reader gains some familiarity with the variations in molecular size, shape and ionic character. Do not be intimidated by these. In due course, the relationship between the key features of the molecular structure of a dye and its dyeing properties will be more evident. 1.1 HISTORICAL BACKGROUND 1.1.1 Natural dyes and fibres The production of fabrics and their coloration precedes recorded history. Several cultures had established dyeing technologies before 3000 BC. These ancient artisans transformed the available natural fibres – linen, cotton, wool and silk – into fabrics, at first by hand, and later using simple mechanical devices. Short fibres were first carded or combed, to lay them parallel to one another. Drawing out of a band of combed fibres by pulling, with gradual twisting, produced yarn. Finally, yarns were interlaced to form a woven fabric. The techniques used hardly changed until the Industrial Revolution, when they became fully mechanised. Although finely ground, coloured minerals, dispersed in water, were used in paints over 30 000 years ago, they easily washed off any material coloured with them. Natural dyes were extracted from plant and animal sources with water, sometimes under conditions involving fermentation. Fabric was dyed by soaking it in the aqueous extract and drying. These dyes had only a limited range of dull colours and the dyeings invariably had poor fastness to washing and sunlight. The fastness of a dyeing is a measure of its resistance to fading, or colour change, on 1 chpt1Pages.p65 1 27/07/01, 10:07 2 AN INTRODUCTION TO TEXTILES, DYES AND DYEING exposure to a given agency or treatment. Most natural dyes also lacked substantivity for fibres such as wool and cotton. Substantivity implies some attraction of the dye for the fibre, so that the dye in the solution gradually becomes depleted as it is absorbed by the fibres. The poor substantivity and fastness properties of natural dyes often improved if the fabric was first treated with a solution containing a salt of, for example, iron, copper or tin. The conditions used favoured combination of the metal ions with the particular fibre, or their precipitation inside it. These metal salts were called mordants. When the pre-mordanted fabric was soaked in a bath of a suitable natural dye, the dye penetrated into the fibres and reacted with the metal ions present. This reaction decreased the water solubility of the dye so the colour was less likely to bleed out on washing. The word ‘mordant’ originated from the French verb mordre meaning ‘to bite’. In Chapter 13, we shall see that the idea of the dye biting the mordant, to form a stable dye–metal complex, is a useful description. In modern dyeing procedures, the dye reacts with the mordant in the fibre in a separate process after dyeing, or the metal is incorporated into the dyestuff during its manufacture. A few natural dyes gave better quality dyeings of cotton or wool, but involved long and difficult processes. For example, the colorant extracted from madder root, from the plant Rubia tinctorium, dyed cotton pre-mordanted with aluminium and calcium salts to give the famous Turkey Red. Using an iron mordant, the same colorant gave a purplish-black. Indigo, extracted from leaves of the plant Indigofera tinctoria, and Tyrian Purple from Mediterranean sea snails of the genera Murex and Purpura, are waterinsoluble pigments called vat dyes. These do not require mordants. During the time of the Roman Empire, wool cloth dyed with Tyrian Purple was so highly prized that only the ruling class wore garments made with it. For dyeing with Indigo, a water-soluble, reduced form of the dye was first obtained by extraction and fermentation. The process became known as vatting, from the name of the vessels used – hence the term ‘vat dye’. The soluble, reduced form of the dye is called a leuco derivative. Leuco Indigo has substantivity for wool and cotton fibres. After dyeing, air oxidation of the pale yellow leuco dye, absorbed in the fibres, regenerates the dark blue, insoluble pigment trapped inside them. Because of this, the fastness to washing is very good in comparison to most natural dyes. Scheme 1.1 outlines the essential steps in vat dyeing. chpt1Pages.p65 2 27/07/01, 10:07 HISTORICAL BACKGROUND Insoluble vat dye pigment in aqueous suspension reduction Soluble leuco compound in solution Leuco compound absorbed in the fibres oxidation 3 Insoluble vat dye pigment held in the fibres Scheme 1.1 1.1.2 The development of synthetic dyes and fibres In 1856, William H Perkin reacted aniline with acidic potassium dichromate solution in an attempt to prepare the anti-malarial drug quinine. From the dark, tarry reaction mixture, he isolated a purple, water-soluble compound that dyed both wool and silk directly when immersed in its solution. No mordant was required. Perkin established a factory for the large-scale production of aniline and for the manufacture of this dye, later called Mauveine. He not only discovered the first major synthetic dye, but founded the modern chemical industry. Mauveine (proposed structure 1, Figure 1.1) is a cationic dye since each of its molecules has a positive ionic charge. The methyl groups in the structure of Mauveine arose from the use of aniline contaminated with toluidenes (aminotoluenes). Such cationic dyes are often called basic dyes since many, like Mauveine, have free amino groups capable of salt formation with acids. H3C N CH3 HN N NH2 CH3 1 Figure 1.1 Proposed structure of Mauveine Mauveine has some substantivity for wool and silk. Such protein fibres contain both amino and carboxylic acid groups. In a neutral dyebath, the amino groups (NH2) in the wool are neutral but the carboxylic acid groups (CO2H) dissociate giving negatively charged carboxylate anions (CO2–), associated with positively charged sodium cations (Na+). Under these conditions, dyeing with a cationic dye chpt1Pages.p65 3 27/07/01, 10:07 4 AN INTRODUCTION TO TEXTILES, DYES AND DYEING (Dye+) involves a process of cation exchange in which the more substantive dye cation replaces the sodium ion associated with the carboxylate group in the wool or silk (Scheme 1.2). H2N Wool _ CO2 Na+ + Dye+(aq) _ H2N Wool CO2 Dye+ + Na+(aq) Scheme 1.2 Perkin even developed a method for dyeing cotton with Mauveine using tannic acid as a mordant. This polycarboxylic acid was precipitated inside the cotton fibres as a tin salt. The mordanted cotton, immersed in a solution of Mauveine, absorbed the cationic dye (positively charged), which combined with the anionic carboxylate groups of the tannic acid (negatively charged) inside the fibres. Perkin’s achievements are all the more impressive when we consider the limited scientific information available in 1856. This was a period of heated debate over Dalton’s atomic theory; the formation of organic compounds was still believed to require a living organism, and Kekulé had not yet proposed the hexagonal structure of benzene (1865). Two years after the isolation of Mauveine, Peter Greiss discovered the diazotisation reaction of primary aromatic amines, which produces diazonium ions, and later, in 1864, their coupling reaction with phenols or aromatic amines to give azo compounds. Primary aromatic amines such as aniline (C6H5NH2) are often diazotised by treatment with sodium nitrite (NaNO2) in acidic aqueous solution at temperatures around 0–5 °C (Scheme 1.3). The diazonium cation produced (C6H5N2+) will couple with a phenol in alkaline solution (in a similar way to the reaction shown in Figure 1.2), or with an aromatic amine in weakly acidic solution, to form an azo compound. This coupling reaction is an electrophilic aromatic substitution, like nitration or chlorination, with the diazonium ion as the electrophile. Today, over half of all commercial dyes contain the azo group (–N=N–) and many thousands of azo compounds are known. Diazotisation and coupling are therefore two very significant reactions. C6H5 NH2 + NaNO2 + 2HCl C6H5 N2+ Cl _ + NaCl + 2H2O Scheme 1.3 Each molecule of the azo dye Orange II (Figure 1.2) has an anionic sulphonate group and will dye wool in the presence of an acid. It is therefore classified as an chpt1Pages.p65 4 27/07/01, 10:07 HISTORICAL BACKGROUND 5 acid dye. In acidic solution, both the amino and carboxylate groups in wool bond with protons, becoming cationic (NH3+) and neutral (CO2H), respectively. Under these conditions, the wool absorbs anionic dyes (Dye–), such as Orange II, by a process of anion exchange (Scheme 1.4). O3S O3S Orange II N NH3 Step 1 Diazotisation NaNO2/HCl 0–5 °C Step 2 Coupling pH 9–10 O3S OH N OH + N N Figure 1.2 Formation of the azo dye Orange II by diazotisation and coupling _ Na+ O2C Wool HO2C Wool NH3+Cl NH2 + 2HCl _ _ + Dye (aq) _ HO2C Wool NH3+Cl HO2C Wool _ _ NH3+Dye + Cl (aq) + NaCl Scheme 1.4 Many of the first synthetic dyes were cationic dyes like Mauveine (1). These had brilliant colours, but poor fastness to washing, and particularly to light. Their use on cotton still required pre-mordanting with tannic acid. Congo Red (2, Figure 1.3), first prepared in 1884, was one of the first synthetic dyes that would dye cotton directly, without a mordant. This is also an anionic azo dye, but, unlike Orange II, its more extended molecular structure imparts substantivity for cotton. Dyeings on cotton with Congo Red only had poor fastness to washing, but the socalled direct cotton dyes that followed were better in this respect. NH2 NH2 N N N O3S N 2 SO3 Figure 1.3 Congo Red chpt1Pages.p65 5 27/07/01, 10:07 6 AN INTRODUCTION TO TEXTILES, DYES AND DYEING Synthetic Indigo was first prepared in 1880 and produced commercially in 1897. Indigo is a vat dye applied to both wool and cotton according to Scheme 1.1. The water-insoluble, blue pigment gives a pale yellow, water-soluble leuco form on reduction (Figure 1.4). Indigo, one of the oldest colorants, is widely used for dyeing cotton yarn for blue jeans. It was not until the discovery of Indanthrone in 1901, however, that other synthetic vat dyes of outstanding fastness to washing and light became available. Precipitation of a water-insoluble pigment inside a fibre is still one of the important ways of producing a dyeing with good fastness to washing. O H N Alkaline reduction N H O Oxidation O Indigo (insoluble) N H H N N O Leuco Indigo (soluble) Figure 1.4 Reversible reduction and oxidation for Indigo The first fibre-reactive dyes did not appear until 1956. Under alkaline conditions, these dyes react with the ionised hydroxyl groups in cotton cellulose forming a covalent bond with the fibre (Figure 1.5). Cellulose is the name of the chemical constituting cotton. It is a polymer of glucose and therefore a Cl SO3 N N OH HN N N N = Dye Cl O3S SO3 OH + HO Cell O + H2O O + Dye Cl Cell O Dye + Cl Cell Cell Cl Figure 1.5 The molecular structure of a simple reactive dye (Dye–Cl) and its reaction with the hydroxyl group in cotton (Cell–OH) chpt1Pages.p65 6 27/07/01, 10:07 HISTORICAL BACKGROUND 7 polyalcohol. It is conveniently represented by the short formula Cell–OH. The strong bond between the reactive dye and the cellulose ensures good fastness to washing and the simple chemical structures of the dyes often result in bright colours. Dyes with simple molecular structures can often be prepared with a minimum of contaminating isomers and by-products that tend to dull the colour. Inducing a chemical reaction between a fibre and an absorbed dye molecule is another significant way of producing dyeings of good washing fastness. Reactive dyes have become one of the most important types of dye for dyeing cotton and some types are valuable for wool dyeing. Synthetic dyes, obtained from coal tar and petroleum chemicals, have totally replaced natural dyes. It would be quite impossible to meet even a small fraction of today’s market requirements for colour using only naturally occurring dyes, although a few are still used to colour foods and cosmetics. Since the earliest days of the synthetic dyestuff industry, there has been a constant demand for dyes with brighter colours, and with better fastness properties, for an increasing range of fibre types. Of the many thousands of known synthetic dyes, only a few thousand are manufactured today. They represent the market-driven selection of those with the required performance. Before the twentieth century, textiles were made exclusively from natural fibres such as cotton or wool. The first artificially made fibre of regenerated cellulose was Chardonnet’s artificial silk, first produced in 1884. This was manufactured from cellulose nitrate (Cell–O–NO2), obtained by esterification of cellulose with nitric acid (Scheme 1.5). Forcing an ethanol-diethyl ether solution of cellulose nitrate through tiny holes in a metal plate, and then rapidly evaporating the volatile solvents in warm air, produced very fine, solid filaments of this material. This is the extrusion process. It is a key step in the production of all artificially made fibres. Because cellulose nitrate is highly flammable, the filaments were then treated to hydrolyse it back into cellulose. Later, better processes were found for the preparation of cellulose solutions, their extrusion, and the solidification of the cellulose. Modern fibres of regenerated cellulose are called viscoses. They have some properties similar to those of cotton and can be dyed with the same types of dye. As for most alcohols, the hydroxyl groups of cellulose can also be esterified with Cell + OH HONO2 Cell O NO2 + H2O Scheme 1.5 chpt1Pages.p65 7 27/07/01, 10:07 8 AN INTRODUCTION TO TEXTILES, DYES AND DYEING acetic anhydride to produce cellulose acetates (Scheme 1.6). In 1921, a cellulose acetate fibre was produced with about 80% of the cellulose hydroxyl groups acetylated. This cellulose acetate gave silky, lustrous filaments on extrusion of its acetone solution followed by immediate evaporation of the solvent. These filaments were quite different from cotton or viscose. In particular, they were relatively hydrophobic (water-repelling), whereas cotton and viscose are hydrophilic (water-attracting). Initially, cellulose acetate proved difficult to dye satisfactorily with existing ionic dyes. Effective dyeing occurred, however, using a fine aqueous dispersion of non-ionic, relatively insoluble, hydrophobic dyes. This type of dye is called a disperse dye, of which (3) is an example (Figure 1.6). We now know that such dyes are soluble in the hydrophobic cellulose acetate and dyeing occurs by the fibres continually extracting the small amount of dye dissolved in the water. Dye dissolving from the surface of the fine particles in suspension constantly replenishes the dye in solution. As we shall see later, disperse dyes are suitable for dyeing almost all types of artificially made fibre by the same mechanism (Figure 1.7). Cell OH + (CH3CO)2O Cell O COCH3 + CH3CO2H Scheme 1.6 O NHCH3 O NHCH3 3 Figure 1.6 Disperse Blue 14 Water Dye particles in suspension Individual dye molecules in solution Fibre Diffusion of dye into the fibre Molecules of dye in the fibre surface Figure 1.7 The mechanism of dyeing a synthetic fibre with a disperse dye chpt1Pages.p65 8 27/07/01, 10:07
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