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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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.
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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
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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
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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
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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)
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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
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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
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