Colorants and Auxiliaries: Organic Chemistry and Application Properties: Colorants v. 1

Colorants and auxiliaries ORGANIC CHEMISTRY AND APPLICATION PROPERTIES Second Edition

Volume 1 – Colorants Edited by John Shore Formerly of BTTG/Shirley and ICI Dyes (now DyStar), Manchester, UK

2002 Society of Dyers and Colourists

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Copyright © 2002 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. Typeset by the Society of Dyers and Colourists and printed by Hobbs The Printers, Hampshire, UK.

ISBN 0 901956 77 5

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Contributors John Shore Formerly of BTTG/Shirley and ICI Dyes (now DyStar), Manchester, UK David Patterson Formerly senior lecturer, Department of Colour Chemistry and Dyeing, University of Leeds, UK Geoff Hallas Formerly senior lecturer, Department of Colour Chemistry and Dyeing, University of Leeds, UK

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Contents Preface

ix

CHAPTER 1

Classification and general properties of colorants

1.1 1.2 1.3 1.4 1.5 1.6 1.7

Introduction 1 Development of colorant classification systems 2 Colour Index classification 4 Chemical classes of colorants 5 Colour and chemical structure 14 Application ranges of dyes and pigments 18 Colorants and the environment 33 References 42

CHAPTER 2

Organic and inorganic pigments; solvent dyes

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13

Pigments 45 Dyes converted into pigments 48 Azo pigments 53 Phthalocyanine pigments 67 Quinacridone pigments 71 Isoindolinone pigments 73 Dioxazine pigments 73 Diketopyrrolopyrrole pigments 73 Fluorescent pigments 74 Inorganic pigments 75 How pigments act as colorants 82 Solvent dyes 86 Conclusion 86 References 87 Bibliography 88

CHAPTER 3

Dye structure and application properties

3.1 3.2 3.3

Dye characteristics and chemical structure 89 Dyeability of fibres in relation to dye structure 116 Application properties and chemical structure 134 References 176

6

45

89

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CHAPTER 4

Chemistry of azo colorants

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14

Introduction 180 Mechanism of diazotisation and coupling 180 Diazo components and diazotisation methods 182 Preparation and use of coupling components 186 Structure of azo dyes 193 Preparation and importance of naphthalene intermediates 196 Schematic representation of coupling 204 Sulphonated azo dyes 204 Unsulponated monoazo dyes 211 Basic azo dyes 218 Azoic diazo and coupling components 220 Stabilised diazonium salts and azoic compositions 223 Azo pigments produced by final coupling 225 Implications of new technology in diazotisation and coupling 227 References 228

CHAPTER 5

Chemistry and properties of metal-complex and mordant dyes

5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8

Introduction 231 Fundamental concepts 233 Electronic structure of transition-metal ions 235 Structural characteristics necessary for complex formation 240 Preparation of metal-complex colorants 248 Isomerism in metal-complex dyes 260 Stability of metal-complex dyes 261 Chromium-related problems in the mordant dyeing of wool 268 References 277

CHAPTER 6

Chemistry of anthraquinonoid, polycyclic and miscellaneous colorants

6.1 6.2 6.3 6.4 6.5 6.6

180

Anthraquinone acid, disperse, basic and reactive dyes 280 Polycyclic vat dyes 294 Indigoid and thioindigoid dyes 316 Sulphur and thiazole dyes 321 Diarylmethane and triarylmethane dyes 327 Miscellaneous colorants 344 References 353

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231

280

CHAPTER 7

Chemistry of reactive dyes

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10

Introduction 356 Reactive systems 358 Monofunctional systems 361 Bifunctional systems 385 Chromogens in reactive dyes 400 Stability of dye–fibre bonds 410 Reactive dyes on wool 415 Reactive dyes on silk 420 Reactive dyes on nylon 424 Novel reactive dyeing processes 426 References 440

356

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Preface to Volume 1 This Second Edition of a textbook first published in 1990 forms part of a series on colour and coloration technology initiated 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 trust fund generously provided by the Worshipful Company of Dyers. The initial objective of this series of books has been to establish a coherent body of explanatory information on the principles and application technology of relevance for students preparing to take the Associateship examinations of the Society. This particular book has been directed specifically to the subject areas covered by Section A of Paper B: the organic chemistry and application of dyes and pigments and of the auxiliaries used with them in textile coloration processes. However, many qualified chemists and colourists interested in the properties of colorants and their auxiliaries have found the First Edition useful as a work of reference. For several reasons it has been convenient to divide the material into two separate volumes: 1. Colorants, 2. Auxiliaries. Although fluorescent brighteners share some features in common with colorants, they have been treated as auxiliary products in this book. This first volume of the book is concentrated on the chemical characteristics of dyes and pigments, with emphasis on attempts to interpret their colouring and fastness properties in terms of the essential structural features of colorant molecules. This Second Edition has been extensively updated and greater attention has also been given to factors associated with the potential impact of colorants and their metabolites on the environment. All chapters have been affected by these changes, but the concluding chapter on reactive dyes contains more new material than the others. Rationalisation of the global dyemaking industry during the 1990s means that many of the traditional commercial names of dyes and pigments have disappeared. For this reason Part 2 of the Colorants Index has been eliminated and colorants have been specified almost always by their CI Generic Names. The fundamental value of the unique Colour Index International to colorant makers and users is recognised worldwide. Chapters 4 and 7 in the First Edition were written by Vivian Stead and Chapter 5 by Frank Jones. Sadly, Frank died in 1989 and Vivian in 1996, but my co-authors and myself would like to record our tribute for the major contributions to this volume by our former friends and colleagues. We have tried to preserve their original style intact during the necessary updating process. Our grateful thanks are due to John Holmes and Catherine Whitehouse for their patient copy editing and to the publications staff of the Society, especially Carol Davies, who have prepared all the material in this new edition for publication. JOHN SHORE

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Chapters in Volume 2 Chapter 8

Functions and properties of dyeing and printing auxiliaries

Chapter 9

The chemistry and properties of surfactants

Chapter 10

Classification of dyeing and printing auxiliaries by function

Chapter 11

Fluorescent brightening agents

Chapter 12

Auxiliaries associated with main dye classes

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CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS

1

CHAPTER 1

Classification and general properties of colorants John Shore

1.1 INTRODUCTION It is important to distinguish clearly between dyes, pigments and colorants. Such terms are sometimes incorrectly used in various major scientific languages, as though they were synonymous [1]. All dyes and pigments are colorants: when present on a substrate they selectively modify the reflection or transmission of incident light. During application to a substrate, a dye either dissolves or passes through a state in which its crystal structure is destroyed. It is retained in the substrate by adsorption, solvation, or by ionic, coordinate or covalent bonding. A pigment, on the other hand, is insoluble in and unaffected by the substrate in which it is incorporated. These inherent characteristics mean that dyes and pigments have quite different toxicological and environmental profiles [1]. Synthetic dyes and pigments have been available to the colorant user since the midnineteenth century. The important naturally occurring substrates of pre-industrial societies (cotton, linen, silk, wool, leather, paper, wood) share certain similarities, since they are all essentially saccharidic or peptide polymers. They could thus be coloured using a relatively short range of dyes and pigments, also of natural origin. An early objective of planned research on synthetic dyes, therefore, was to replace the leading natural extracts (alizarin and indigo) by their synthetic equivalents. Simultaneously with this diligent and ultimately successful effort, other chemists were discovering totally new chromogens unknown in nature: azine, triarylmethane and others from arylamine oxidation, azo colorants from the diazo reaction, and eventually azo–metal complexes and phthalocyanines. Building on success with indigo and anthraquinone derivatives, the systematic approach led on to related but new chromogens with outstanding properties: vat dyes and novel pigments. Linked to this research by a common interest in certain versatile intermediates and a similar urge to extend the limited range of natural substrates, a new breed of organic chemist, the polymer specialist, was vigorously developing novel regenerated and synthetic fibres, plastomers, elastomers and resins. Most of these differed markedly in structure and properties from natural polysaccharides or polypeptides. Particularly in the mid-twentieth century, urgent demands arose for special new colorants and application techniques designed to colour these substrates. Disperse dyes for ester fibres, modified basic dyes for acrylic fibres and pigments for the mass coloration of fibres and plastics are typical examples of the response of the colour chemist. Natural fibres also gained from this broad wave of research: reactive dyes for cellulosic and protein fibres, and fluorescent brighteners for undyed textiles, paper and detergent formulations were discoveries stemming essentially from this exceptionally active period.

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CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS

In the closing decades of the twentieth century, the emergence of an unknown substrate became a rare event. The rate of introduction of radically new colorants, auxiliary products and processes fell markedly. An increasingly adverse balance arose between the escalating costs of the research effort and of much more stringent hazard testing, as against the diminishing value of marginal technical or economic improvements to existing ranges of colorants on standard substrates. Many of the pathways of colorant research have turned away from conventional outlets for dyes and pigments towards more esoteric applications [2–7]. Although colorants of these types are unlikely to match the traditional textile dyes in terms of total sales value, their unit prices and profit margins can often be exceptionally high. Many specialised applications of colorants are related to the way in which they absorb and emit light. The ability of a dye molecule to absorb depends critically on its orientation with respect to the electrical vector of the incident light, i.e. the polarisation of absorption. In recent years this has become of practical significance in the field of liquid crystal displays [8]. Colorants exhibiting high absorption of infrared light have found many diverse applications, ranging from solar energy traps to laser absorbers in electro-optical devices [9,10]. Dye lasers are based on dyes that fluoresce with high quantum efficiency. They must show good photostability and be marketed in a state of high purity, thus commanding a high unit price. Fluorescent dyes are also used in biochemical and medical analysis where extremely low detection limits are required. Polymeric colorants have been developed as potential food colourings [11], since chemicals of relative molecular mass greater than about 20 000 cannot be absorbed into the gastro-intestinal tract. Such colorants should pose no toxicological problems as food additives. The chemical or photochemical activity of dyes forms the basis of many of their innovative uses. Indicator systems and lactone colour formers exploit reversible colour changes. Thermochromism is applied in novelty inks, temperature sensors and imaging technology. Photosensitising cyanine dyes are used to transfer absorbed light energy to silver halides in photography. Certain dyes are effective sensitisers of free-radical reactions, thereby initiating the crosslinking or photodegradation of polymers on exposure to light. Photochromic colorants have been employed in light monitors, reversible sun screens, optical data recording and novelty surface coatings. 1.2 DEVELOPMENT OF COLORANT CLASSIFICATION SYSTEMS A major objective of this chapter is to outline the principal system by which colorants are classified, namely the widely accepted Colour Index classification. After tracing the developments from which this system has evolved [12,13], the distribution of existing dyes and pigments among the various classes listed therein will be introduced. Each of these classes will be discussed in turn, illustrated by structural formulae. The earliest comprehensive alphabetical listing [14] of synthetic products used in the coloration industry was published in 1870. The beginnings of systematic classification based on chemical structure, with subgrouping according to hue, were first seen in the 1880s. A typical presentation of this period [15] listed about 100 ‘coal-tar dyes’ in hue order. It is interesting that 50% of them were acid dyes and 20% basic dyes, about 40% being placed in the ‘red’ category. Undoubtedly the most successful of these early systems were the famous ‘Farbstofftabellen’ of Gustav Schultz, which ran through seven editions between 1888 and

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DEVELOPMENT OF COLORANT CLASSIFICATION SYSTEMS

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1932. The number of chemical entries rose from about 280 to nearly 1500 over these years. The later editions of this work pioneered many of the features eventually adopted in the Colour Index. The Society of Dyers and Colourists embarked on the First Edition of the Colour Index in 1921 as a series of monthly issues that were first offered as a bound volume in 1924. There were over 1200 entries for synthetic colorants, as well as sections on natural dyes and inorganic pigments. Updating was discontinued in 1928, so that by 1945 the need for a Second Edition had become urgent. Much detailed information on the products of German manufacturers became available following the Second World War. Collaboration with the American Association of Textile Chemists and Colorists resulted in the four-volume Second Edition published in 1956–58. This contained about 3600 colorants differing in constitution and an especially useful innovation was the separate listing of commercial names (31,500) under equivalent CI generic names (4600 entries). This edition and the completely revised five-volume Third Edition (1971) established the Colour Index as the leading reference work for the classification of colorants, fully justifying the cognomen International belatedly added in 1987. The fourth revision (1992) of the Third Edition consisted of nine volumes. The original data on technical properties (Volumes 1–3) and chemical constitution (Volume 4) was supplemented (Volumes 6–9) at roughly five-year intervals. The latest revision of the Colour Index has become an electronically searchable database available on CD-ROM as well as the traditional book form, providing improved functionality and better value for money. Chemical constitutions, indexes of commercial names and lists of manufacturers have been computerised for ease of reference and search purposes. The commercial listing function Volume 5 was detached in 1997 to form a new annual publication, the SDC Resource File. The aim of this novel concept was to provide colorant users with the latest comprehensive information on relevant products and services. This is provided by suppliers to the colour-using industries and coordinated by the SDC through its Colour Index organisation [13]. In 1998 a new edition covering pigments and solvent dyes designed explicitly for the pigment industry was published [16], the technical and scientific content of the material being upgraded [17]. This divergence is a response to certain problems that have arisen, particularly in relation to commercial product listings. As non-traditional suppliers based in low-cost countries have taken a greater share of world trade in colorants, the attitude of established European and Japanese producers towards disclosure of information has changed. When such companies have already expended substantial resources on research, development and hazard testing to launch a new product, they are understandably reluctant to surrender commercially sensitive data into the public domain and thus give their competitors a head start. Colorant users rely on the equivalence of CI generic names of commodity products as a basis of comparison between suppliers, but the long-established dyemakers are wary of this equivalence for novel products because it offers low-cost competitors an easy entry into traditional markets [13]. The Colour Index has become a standard reference for customs and importing authorities in many countries. Health and safety inspectorates have used CI designations in dealing with colorant manufacturers notifying hazard testing data for their products. As with some other European Union initiatives, administration of legislation governing the notification of commercial chemicals for hazard control purposes has generated problems for suppliers,

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CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS

users and enforcing authorities. Distinguishing between ‘existing’ (i.e. notified in 1981) and ‘new’ products is not as easy as it sounds. Organic colorants present special difficulties because of their complex structures and chemical nomenclature, variations of counter-ions with the same ionic dye, obsolete or confusing identities of ‘existing’ products, and, not least, the ubiquitous marketing of mixed colorants to match specific colours or technical properties [18]. 1.3 COLOUR INDEX CLASSIFICATION Most organic colorants in the Colour Index, including many of those not assigned a specific chemical constitution number, are placed in one of 25 structural classes according to their chemical type (where this is known). The largest class, azo colorants, is subdivided into four sections depending on the number of azo groups in the molecule. Metal-complex azo colorants are designated separately by description but are classified together with their unmetallised analogues in the same generic class. Excluding the colorant precursors, such as azoic components and oxidation bases, as well as the sulphur dyes of indeterminate constitution, almost two-thirds of all the organic colorants listed in the Colour Index belong to this class, one-sixth of them being metal complexes. The next largest chemical class is the anthraquinones (15% of the total), followed by triarylmethanes (3%) and phthalocyanines (2%). No other individual chemical class accounts for more than 1% of the Colour Index entries. The distribution of each chemical type between the major application groups of colorants is far from uniform (Table 1.1). Stilbene and thiazole dyes are almost invariably direct dyes, also containing one or more azo groups. Acridines and methines are usually basic dyes,

Table 1.1 Percentage distribution of each chemical class between major application ranges Distribution between application ranges (%) Chemical class

Acid

Unmetallised azo Metal-complex azo Thiazole Stilbene Anthraquinone Indigoid Quinophthalone Aminoketone Phthalocyanine Formazan Methine Nitro, nitroso Triarylmethane Xanthene Acridine Azine Oxazine Thiazine

20 65

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Basic 5

12

12

6

10 12

5 13

2

25

3

4 17

6

2 9

20

40 40

5 15 2 30 11 14 70 31 35 33 39

4

Direct Disperse Mordant Pigment Reactive Solvent Vat

4 71 2 22 16 92 39 22 55

30 10 95 98

8

1

23 48 1

8 4

3 43 30

9

2 24 9

1 5 5 2

2

3 9

10

4 17

2

40 10

10 8 15

30 3

5 12 12 38 4 19 10

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36 81

25

CHEMICAL CLASSES OF COLORANTS

5

whereas nitro, aminoketone and quinophthalone derivatives are often disperse dyes. Metalcomplex azo and formazan types are mainly acid dyes but phthalocyanines are important for reactive dyes. Indigoids are predominantly vat dyes but anthraquinones retain considerable significance as acid, disperse or vat dyes. The data in this table are given in percentages because the actual numbers of dyes recorded gradually increase as new products are added. They relate to all those dyes listed where the chemical class is known, including products no longer in commercial use. There are nineteen generic name groups (application ranges) listed in the Colour Index. CI Acid dyes constitute the largest range, with about 55% of them still in commercial use. Next come CI Direct dyes (40% still active) and CI Disperse dyes (60% active). CI Reactive dyes (75% active), CI Basic dyes, CI Solvent dyes and CI Pigments (all 60% active) continue to progress, but CI Vat dyes (45% active) and CI Mordant dyes (33% active) are in decline [19]. CI Sulphur dyes can be regarded as a distinct chemical class as well as an application range, although a few vat dyes are manufactured in a similar way. Essentially CI Food dyes and CI Leather dyes are selections from the larger ranges of textile dyes. Several of these application types are represented in the CI Natural dyes and pigments category. The groupings according to generic name also include colorant precursors (CI Azoic Components and CI Developers, CI Ingrain dyes, CI Oxidation Bases) and uncoloured products (CI Fluorescent Brighteners and CI Reducing Agents) associated with textile coloration. Chemists concerned with innovative applications for specialised colorants have highlighted the need for an independent directory covering all such products and uses that do not fit easily into the above textile-oriented categories [5]. Quite often these research projects signal a need for a specific combination of relatively unorthodox properties, such as fluorescence, infrared absorption or photosensitivity combined with solubility in an unusual solvent. Often this has involved a time-consuming search through the hardcopy Colour Index and other more specialised catalogues. Now that a searchable CD-ROM version has become available, together with the annual SDC Resource File in which to locate suitable suppliers, it has become easier for less conventional demands on the database to be adequately met [13]. 1.4 CHEMICAL CLASSES OF COLORANTS A brief description of each class of colorant is given below, in order to show how they contribute to the overall distribution outlined in Table 1.1. For further details the reader is referred to the later Chapters 2 to 7. Many of the lesser-known chemical classes are more fully described in Chapter 6. The order of discussion of the chemical classes included here differs somewhat from that in the Colour Index. Thiazole dyes are dealt with immediately after the stilbene class because both of these, like the polyazo types, contribute notably to the range of direct dyes. The anthraquinone, indigoid, quinacridone, quinophthalone, benzodifuranone and aminoketone classes form another series with certain structural similarities and important applications in vat or disperse dyes and pigments. Phthalocyanine and formazan are stable metal-complex chromogens. The remaining seven categories included are already less important and still declining in commercial significance. The arylmethane, xanthene, acridine, azine, oxazine and thiazine chromogens share a limited degree of resemblance in structural terms.

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CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS

1.4.1 Azo colorants The presence of one or more azo (–N=N–) groups, usually associated with auxochromic groups (–OH or –NH–), is the characteristic feature of this class. Hydroxyazo dyes exhibit benzenoid–quinonoid tautomerism with the corresponding ketohydrazones [4,20,21]. At least half of all commercial azo colorants belong to the monoazo subclass, that is, they have only one azo group per molecule. This proportion is considerably higher among the metalcomplex azo dyes. Direct dyes represent the only application range where monoazo compounds are relatively unimportant; disazo and trisazo dyes are preferred in order to confer higher substantivity for cellulose. The numerous ways in which diazo and coupling components can be used to assemble azo colorants for many purposes are discussed in Chapter 4. Yellow azo chromogens are occasionally linked to blue anthraquinone or phthalocyanine structures in order to produce bright green colorants. 1.4.2 Thiazole dyes The characteristic chromogen of this class is the thiazole ring itself, normally forming part of a 2-phenylbenzothiazole grouping. Most are yellow direct dyes of the azophenylthiazole (1.1) type, but a minority are simple basic dyes with an alkylated thiazolinium group (1.2), such as Thioflavine TCN (CI Basic Yellow 1) shown. The thiazole ring enhances substantivity for cellulose and thus has been incorporated into certain anthraquinonoid and sulphurised vat dyes. Several important blue basic dyes are 2-phenylazo derivatives of 6alkoxybenzothiazolinium compounds (1.3). A typical red disperse dye for cellulose acetate in this class is the 6-methoxybenzothiazole CI Disperse Red 58 (1.4). S N

CH3

N

S N(CH3)2

N 1.1

N+

_

CH3 X

RO

1.2

S N N+

N

CI Basic Yellow 1

NR2

_

R X

1.3 H3CO

N

S

N(CH2CH2OH)2

N N

1.4

CI Disperse Red 58

1.4.3 Stilbene dyes and fluorescent brighteners Stilbene dyes are mixtures of indeterminate constitution resembling polyazo direct dyes in their application properties. They result from the alkaline self-condensation of 4nitrotoluene-2-sulphonic acid (1.5) or its initial condensation product (1.6; X = NO2),

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either alone or with various arylamines. The characteristic chromogens are azo- or azoxystilbene groupings (1.7). As with sulphur dyes, the CI generic names of stilbene dyes refer not to specific chemical entities but to mixtures of related compounds with closely similar dyeing and fastness properties. Almost all of them are yellow to brown direct dyes for cellulosic fibres and leather. SO3H O2N

SO3H

CH3

CH

X

CH

X

HO3S

1.5

1.6

(O)

CH

CH

N

N 1.7

Approximately 75% of fluorescent brighteners belong to the stilbene class. These are almost invariably derived from 4,4′-diaminostilbene-2,2′-disulphonic acid (1.6; X = NH2), often condensed with cyanuric chloride to take advantage of the further contribution of the s-triazine rings to substantivity for cellulose. 1.4.4 Anthraquinone colorants Strictly speaking, the characteristic chromogen of these should be anthraquinone (1.8) itself, but the term, ‘anthraquinonoid’, is frequently extended, in the Colour Index as elsewhere, to include other polycyclic quinone structures. These are often synthesised from anthraquinone derivatives and most of them, including dibenzopyrenequinone (CI Vat Yellow 4), pyranthrone (CI Vat Orange 9), isoviolanthrone (CI Vat Violet 10) and violanthrone (CI Vat Blue 20), are strongly coloured even in the absence of auxochromes. Indanthrone (1.9; CI Vat Blue 4), the first polycyclic vat dye to be discovered, resulted from an unsuccessful attempt to link two anthraquinone nuclei via an indigoid chromogen. Polycyclic pigments are dealt with in Chapter 2 and the many derivatives of anthraquinone applicable as acid, basic, disperse, mordant, reactive and vat dyes are discussed in Chapter 6. O

NH

O

O

O

1.8

Anthraquinone

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O

HN

O 1.9 Indanthrone

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CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS

1.4.5 Indigoid colorants This substantial class of vat dyes and pigments has declined markedly in importance relative to the anthraquinone derivatives. The still pre-eminent representative is indigo (1.10; CI Vat Blue 1), the dye most consistently in demand of all time. Originally obtained from natural sources, indigo was probably the decisive impetus for the early development of the synthetic dye industry [22]. In indigo and thioindigo (1.11; CI Vat Red 41) the chromogenic system is symmetrical. These dyes can exist in both cis and trans forms; the latter is the more stable form that predominates in the solid state. Unsymmetrical indigoid dyes are also known, in which the two halves of the molecule united by the central C=C bond differ in substitution pattern, heteroatom or orientation of the hetero ring, as in the monobrominated indolethianaphthene analogue (1.12). O

O H N

S

N H

S O

O

1.11

1.10

Indigo

Thioindigo O Br

S N H

O 1.12

1.4.6 Quinacridone pigments This chromogen (1.13) is somewhat reminiscent of the indigoid and anthraquinone types but it has not yielded useful vat dyes. Bluish red pigments of the quinacridone class are especially important in violet and magenta colours or for deep reds in admixture with inorganic cadmium scarlets. O

H N

N H

O 1.13

Quinacridone

1.4.7 Benzodifuranone dyes The discovery in 1979 of the benzodifuranone chromogen (1.14) and its exploitation in red disperse dyes for polyester fibres [23,24] emerged from ICI research towards new chromogens of high colour value, brightness and substantivity to overcome the relative weakness of anthraquinones and dullness of monoazo alternatives in the red disperse dye area. A striking improvement in build-up properties was found by introducing asymmetry

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into the dye molecule, especially where the substituents R are different. Benzodifuranone derivatives are unlikely to yield useful water-soluble dyes for cotton or wool, since the lactone rings in the chromogen are readily hydrolysed. In fact this property is utilised to advantage when these disperse dyes are applied to polyester, the dyeings produced being readily cleared with alkali [25]. O O R

R O

1.14

O

1.4.8 Quinophthalone dyes The name of this structural class (‘quinoline’) in the Colour Index is not ideal because quinoline derivatives feature in other related classes, such as the methine basic dyes with a quinolinium cationic group. The class is more precisely associated with quinophthalone (1.15), the characteristic chromogen derived by condensation of quinoline derivatives with phthalic anhydride. This small class of yellow compounds contributes to the disperse, acid, basic and solvent ranges of dyes. O

N H O

1.15

Quinophthalone

1.4.9 Aminoketone and hydroxyketone dyes This small group of hydroxyquinone (1.16), arylaminoquinone (1.17) and aminophthalimide (1.18) derivatives has contributed a few members of some application ranges, mainly yellow disperse dyes and reddish brown vat dyes, but there are superior alternatives available from the major chemical classes. HO

HO

OH

C

CH3

O

O

O

1.16 NH

N

NH

O O

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1.18

1.17

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CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS

1.4.10 Phthalocyanine colorants Substituted derivatives of metal-free phthalocyanine (CI Pigment Blue 16) and a series of metal complexes, notably copper phthalocyanine (1.19; M = Cu; CI Pigment Blue 15), contribute brilliant blue and green colours to several application ranges [26]. Pigments and reactive dyes are especially dependent on this chemical class, but examples exist in all the important ranges except disperse dyes. These colorants are discussed in more detail in sections 2.4, 5.4.3 and 7.5.10. More recently, owing to their complex molecular structure and high electron-transfer ability, phthalocyanine derivatives are being used increasingly in non-coloration applications such as catalysis, optical recording, photoconductive materials, photodynamic therapy and chemical sensors [27].

N

N N N

M

N

N N

N

1.19

1.4.11 Formazan dyes This small class of blue copper-complex dyes has made a significant contribution to the acid and reactive ranges in recent years (sections 5.4.2, 5.4.3 and 7.5.8). The essential chromogen is the bicyclic 1:1 chelated grouping illustrated (1.20). Trivalent metals such as chromium, nickel or cobalt will give tetracyclic 1:2 complexes with a central metal atom, analogous to conventional 1:2 metal-complex azo dyes. O Cu N

N

N

N

1.20

1.4.12 Cyanine colorants Dyes in this category, classified as methines (1.21) and polymethines (1.22) in the Colour Index, characteristically contain a conjugated system through one or more methine (–CH=) groups terminating with heterocyclic atoms, usually nitrogen as in the quinoline (1.21) and

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CHEMICAL CLASSES OF COLORANTS

trimethylindoline (1.22) types [4]. The most important function of cyanine colorants is as sensitisers in photography. One or more methine groups may be replaced by nitrogen atoms, as in the azacyanines. Many methine compounds intended for textiles are yellow or red basic dyes, but uncharged yellow methine disperse dyes (1.23) and azomethine chromiumcomplex solvent-soluble dyes (1.24) are also significant.

H3C CH

N

CH3

CH3

R

CH

N+

N CH

CH

N+

R X–

H3C

CH3 X–

CH3 1.22

1.21

Cr O R2N

CH

C

O

R

CH

CN

N

1.24

1.23

1.4.13 Nitro and nitroso colorants Nitro dyes exhibit benzenoid-quinonoid tautomerism (1.25) and their colour is attributed mainly to the o-quinonoid form, since this can be stabilised by hydrogen bonding. The tautomeric o-nitrosonaphthols (1.26) readily form chelate complexes with metals. A few yellow nitro disperse dyes, including CI Disperse Yellow 1 (1.25), and brown acid dyes remain of significance. The remaining nitro and nitroso colorants, such as (1.26) and its 1:3 iron (II) complex (1.27), are no longer of commercial interest. O

O O

N

N

O H

H O2N

O2N

N

N

OH

OH

1.25

CI Disperse Yellow 1

N

O

O H

N

O

NaO3S

H O

NaO3S 1.26

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CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS –

SO3Na

O

Na+

N O

O

Fe

N O

O O N

NaO3S 1.27 SO3Na CI Acid Green 1

1.4.14 Diphenylmethane and triarylmethane colorants Although treated as separate classes in the Colour Index, these structural types are closely related and the few diphenylmethane dyes such as auramine (1.28; CI Basic Yellow 2) are now of little practical interest. Commercial usage of the triarylmethane dyes and pigments has also declined considerably in favour of the major chemical classes. They were formerly noteworthy contributors to the acid, basic, mordant and solvent ranges, primarily in the violet, blue and green sectors. Numerous structural examples are recorded in the Colour Index. The terminal groupings can be amine/quinonimine, as in auramine and crystal violet (1.29; CI Basic Violet 3), hydroxy/quinone, or both. The aryl nuclei are not always benzenoid (section 6.5). _ + N(CH3)2 X

(CH3)2N

C N(CH3)2

(CH3)2N

C _ + NH2 X Auramine

1.28

N(CH3)2

1.29

Crystal violet

1.4.15 Xanthene colorants Structurally related to the triarylmethanes is the xanthene chromogen (1.30), in which two of the aryl nuclei are linked by an oxygen atom to form a pyrone ring. Similar terminal groupings (amino, hydroxy, or both) are usually present. Xanthene dyes have mainly

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13

contributed red members to the acid, basic, mordant and solvent ranges, but there has been slow decline in their commercial importance. _ + NR2 X

O

1.30

1.4.16 Acridine dyes Acridine derivatives, such as acriflavine (1.31), can be regarded as relatives of the diphenylmethane class in which the two benzene nuclei are linked by nitrogen to form a pyridine ring. Their insignificance nowadays resembles that of their relatives, but they were formerly useful mainly as orange or yellow basic dyes [28]. CH3 H2N

N

+ _ NH2 X

1.31 Acriflavine

1.4.17 Azine, oxazine and thiazine colorants These three classes are treated separately in the Colour Index but it is useful to compare them in view of their structural similarity. Their chromogenic groupings differ only in the bridging link of the central pyrazine (1.32), oxazine (1.33) or thiazine (1.34) ring. Azine dyes from a wide variety of structural subclasses (quinoxalines, eurhodines, safranines, aposafranines, indulines, nigrosines) are illustrated in the Colour Index. Their commercial importance, mainly as red to blue basic dyes, blue acid dyes and blue or black solvent-soluble dyes, has declined markedly. Thiazine dyes, such as methylene blue (1.34; CI Basic Blue 9), were never of much real significance. Only the oxazine class has retained its standing, not only for long established products such as CI Basic Blue 3 (1.33), but also as bright blue members of the direct and reactive dye ranges containing the triphenodioxazine chromogenic system (1.35) [29]. Bluish violet pigments of exceptionally high tinctorial strength have also been derived from this chromogen.

R2N

_ + NR2 X

N

(CH3CH2)2N

O

_ + N(CH2CH3)2 X

N 1.33

N 1.32

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CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS

(CH3)2N

_ + N(CH3)2 X

S

O

N

N

O

N 1.35

1.34 CI Basic Blue 9

Triphenodioxazine

1.4.18 Indamine, indophenol and lactone dyes These three chemical classes listed in the Colour Index are no longer of any practical significance. They are mentioned here only for their resemblance to quinonoid and ketone dyes already discussed. The chromogens are quinonimine (1.36), with amino or hydroxy auxochromes respectively in the indamines and indophenols, and a lactone ring (1.37) with a hydroxy auxochrome.

N

N

O

C

1.36

O 1.37

1.5 COLOUR AND CHEMICAL STRUCTURE Before moving on to a description of the application ranges of dyes and pigments, it is appropriate to trace briefly the developments in understanding of the relationship between colour and chemical constitution. This subject has been reviewed most thoroughly elsewhere [30–33] and the intention here is only to outline the basic principles so that the reader can appreciate the need for such a variety of structural types of colorant. The requirements of colour and application are often in conflict and this forms a major part of the subject matter in succeeding chapters. The first general theory relating depth of colour to molecular structure was made by Witt (1876), who recognised that all dyes then known contained aryl rings bearing unsaturated groups, such as =C=O, –N=O or –N=N–, which he termed ‘chromophores’. Intense colour could be developed from such a ‘chromogenic’ grouping by attaching weakly basic substituents such as –OH or –NH– groups, called ‘auxochromes’, to the aryl ring. Subsequent developments recognised the particularly strong chromophoric effect of charged centres, as in the triarylmethane dyes that lack conventional uncharged chromophores. Although no longer tenable, the quinonoid theory of Armstrong (1888) helped to explain the intense colour of these and related basic dyes so prevalent at that time. The influence of multiple auxochromic substitution on colour was examined by Kauffmann (1904), who showed that the deepest (most bathochromic) colour was obtained with auxochromes in the 2,5-positions relative to the chromophore, as in the dihydroxyazobenzene anions 1.38 (orange) and 1.39 (blue). Improved understanding of the interaction of visible and ultraviolet radiation with organic structures aroused interest in the tautomeric capabilities of dye molecules such as benzenoid-quinonoid tautomerism in azo (1.40), anthraquinone (1.41) and triarylmethane (1.42) systems. According to Watson (1913), if a dye could show a quinonoid structure in all

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_

_ O

O _ O

15

N

N

N _ O

1.38

N

1.39

of its possible tautomeric forms, then it would be deeply coloured, however small its molecular size. For example, the blue indamine (1.43) may be contrasted with the yellow 4,4′-diaminoazobenzene (1.44), even though the latter has the greater degree of conjugation. Watson and Meek (1915) suggested that the oscillation between tautomeric forms corresponded to the reversal of double and single bonds along the conjugated chain of the molecule: the longer the chain, the slower the period of vibration. This accorded qualitatively with the relation between conjugation and absorption. O–

H O

H

N

O

O

N N

N O–

1.40

C

C

N

1.41

O

+ N

1.42

HN

N

NH2

1.43

H2N

N

N

NH2

1.44

Mathematical elucidation of the principles of quantum mechanics in the 1920s, leading to the concept of molecular orbitals and a much more precise understanding of the nature of chemical bonding in the 1930s, provided the first opportunity for chemists to develop qualitative predictive methods of relating colour to chemical constitution. The equations involved are so complicated, however, that approximation methods had to be employed, particularly for calculating transition energies in organic molecules. Three distinct theoretical approaches were adopted: the valence bond, the free electron and the molecular orbital methods. Valence bond theory was initially the most widely accepted approach, probably because it depended on familiar concepts of mesomeric effects in conjugated systems. The theory assumed that the true wave function for the mesomeric state of a molecule is a linear sum of those of the contributing canonical forms. The technique was never successful for quantitative calculation of the absorption spectra of dyes, however, because of the difficulties encountered when introducing the numerous canonical structures necessary for computational precision.

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CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS

The free electron (FEMO) theory had its origins in work on the conduction electrons of metals in the 1940s, when several workers independently recognised the close analogy between these and the delocalised π-electrons of polyene dyes. The method was extended to many other classes of dyes, notably by Kuhn in the 1950s, but it has not found general acceptance for spectroscopic calculations, since it lacks adaptability by simple parameter adjustment. The molecular orbital (MO) approach developed more slowly but was ultimately much more successful than the other approaches. Dewar (1950) was able to predict absorption maxima for various types of cyanine dyes in excellent agreement with experiment. A major advance was made in 1953 when a self-consistent molecular orbital method specifically taking into account antisymmetrisation and electron repulsion effects was developed by Pariser, Parr and Pople. The PPP-MO method established itself over the next two decades as the most useful and versatile technique for colour prediction, especially when the microelectronics revolution provided facilities for overcoming the complexity of the necessary calculations. Limitations of space preclude mention of more than a handful of recent examples where the PPP-MO method has been employed to predict the absorption properties of dyes from various classes. Simulation of the change in molecular geometry on excitation by iteration within the framework of the PPP-MO procedure has been used to calculate the fluorescence maxima of dyes with satisfactory accuracy to permit reliable predictions [34]. A technique for predicting absorption bandwidths has been devised, based on the linear relationship between the fluorescence Stokes shift of a dye and the absorption half-bandwidth. Theoretical Stokes shifts were computed using PPP-MO parameters for the various types of bands encountered in dye spectra. The predictive value of the method was tested on dyes from various chemical classes. The correlation between calculated and experimental bandwidths was good enough to predict brightness as well as colour [35]. Modification of the parameters for protonated N atoms enabled the visible spectral shift attributable to formation of the azonium cation (1.45) from N,N-diethylaminoazobenzene to be predicted reliably. Using these new parameters, halochromism could also be quantified in this way for more complex bis-azonium ions with insulated chromogens [36].

N

N

N(CH2CH3)2

HX

N

N

1.45

+ NH(CH2CH3)2 _ X

A striking feature of disperse dye development in recent decades has been the steady growth in bathochromic azo blue dyes to replace the tinctorially weaker and more costly anthraquinone blues. One approach is represented by heavily nuclei-substituted derivatives of N,N-disubstituted 4-aminoazobenzenes, in which electron donor groups (e.g. 2acylamino-5-alkoxy) are introduced into the aniline coupler residue and acceptor groups (acetyl, cyano or nitro) into the 2,4,6-positions of the diazo component. A PPP-MO study of the mobility of substituent configurations in such systems demonstrated that coplanarity of the two aryl rings could only be maintained if at least one of the 2,6-substituents was cyano. Thus much commercial research effort was directed towards these more bathochromic ocyano-substituted dyes.

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17

A more important trend, however, has been the development of intensely bathochromic monoazo blues by replacing either one of the aryl rings in the aminoazobenzene chromogen by a heterocyclic ring, such as thiazole or thiophene. Study of a series of thiazolyl- and thienylazo dyes of this kind using the PPP-MO method and its CNDO/S refinement showed that their highly bathochromic intensity is not attributable to the 3d atomic orbitals on the sulphur atom of the heterocyclic ring [37]. New parameters for S-containing heterocyclic systems enabled good agreement between experimental and calculated spectral features to be obtained for a series of monoazo dyes derived from N-phenylpyrrolidine [38]. Spectral data for dyes prepared by coupling diazonium salts with 2-thioalkyl-4,6-diaminopyrimidine and with 3-cyano-1,4-dimethyl-6-hydroxypyrid-2-one indicated that the yellow to orange pyrimidine dyes exist in the azo tautomeric form (1.46), whereas the brighter yellow pyridone dyes exist as ketohydrazones (1.47). The spectral behaviour was predicted well by PPP-MO calculations, although steric effects intervened when a substituent in the diazo component was ortho to the azo group of a diaminopyrimidine dye [39]. H3C

H2N

CN

N Ar

N

N

N

R

S

Ar

N H2N

O N

N H

O

CH3

1.46

1.47

The peak wavelengths of some arylated diaminoanthrarufin disperse dyes calculated by the PPP-MO route agreed well with the observed values. The 1-arylamino-3-aryl-5-amino derivatives (1.48; X = Ar, Y = H) showed better dichroism and improved solubility in liquid crystals than their 1-amino-5-arylamino analogues (1.48; X = H, Y = Ar). The tris-arylated derivatives (1.48; X = Y = Ar) also had satisfactory dichroism and solubility. The presence of 1-arylamino and 3-aryl substituents in these deeply coloured anthraquinone dyes resulted in favourable orientation in liquid crystals, producing the desired properties [40]. A range of metal-free phthalocyanine colorants and their metal complexes were prepared and their spectral characteristics in solution were predicted by PPP-MO calculation. The influence of the central metal atom, substituents on the aryl rings and the type of solvent was examined. The nature of the electronic excitation processes was interpreted in terms of the calculated changes in π-electron charge density. Good agreement with experimental absorption maxima was obtained, except in the case of a tetranitro derivative [41]. X HO

O

HN

Ar NH

O

OH

Y 1.48

One of the most noteworthy consequences of the refinement of these quantitative treatments of colour-structure relationships has been the stimulus it has given to the search

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CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS

for radically new chromogenic systems. The benzodifuranone chromogen (1.14) is a particularly interesting example. One of the remarkable properties of this system is the sensitivity of the absorption band to the strength of the electron-donating groups R. Thus the unsubstituted chromogen (1.14; R = H) absorbs at 460 nm, whereas the dimethoxy derivative (1.14; R = OCH3) absorbs at 540 nm [23]. Another novel chromogen of high bathochromic intensity is the dihydrobenzothiophene1,1-dioxide nucleus with an arylmethine group in the 2-position and a dicyanovinyl residue in the 3-position (1.49). This provides brilliant blues with molar absorption coefficients in the 70 000 region. Analogous dyes with carbonyl in place of the sulphonyl group are less intense, raising speculation about other applications of the sulphone acceptor group as part of a heterocyclic ring system. NC

CN

CH S O

O

1.49

1.6 APPLICATION RANGES OF DYES AND PIGMENTS As mentioned earlier, the nineteen application categories forming the respective series of CI generic names are not restricted to colorants. In the following discussion, however, the colorant ranges are emphasised because this is where the major variations in chemical class are found. Thus it is necessary to depart from the alphabetical order of listing in the Colour Index. The application ranges of most interest for dyeing cellulosic fibres (vat, sulphur, reactive and direct dyes) are discussed first, followed by those for synthetic fibres and wool (disperse, basic, acid and mordant dyes) and finally the non-textile colorants (leather dyes, food dyes, solvent dyes and pigments). The colorant precursors and uncoloured products listed form more homogeneous groups that are best discussed where relevant in the appropriate chapters of Volumes 1 and 2. There is much variety of chemical structure among fluorescent brighteners, but the characteristic classes represented are mostly different from those in colorants. 1.6.1 Vat dyes A vat dye is a water-insoluble colorant containing two or more keto groups. It can thus be brought into aqueous solution by a reduction process (vatting), which converts the vat dye into its alkali-soluble enolic (leuco) form. As the soluble sodium enolate the leuco vat dye has substantivity for cellulose. The application of vat dyes to cellulosic fibres (virtually the only fibre type on which their outstanding fastness properties can be exploited) thus proceeds in four stages: (a) reduction and dissolution, (b) absorption by cellulose, (c) reoxidation and (d) association of the vat dye molecules within the fibre (Scheme 1.1).

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APPLICATION RANGES OF DYES AND PIGMENTS

O

O

O

O

19

O

dyebath

(a)

cellulose

O

(b)

O

(c)

O

(d) O

Scheme 1.1

n

O

Sodium dithionite is the traditional reducing agent used for vat dyeing but concern about environmental problems has aroused interest in less damaging alternatives, such as thiourea dioxide and the biodegradable and sulphur-free hydroxyacetone (section 12.9.1). Recycling is worthy of consideration for vat dyebaths because the redox process is reversible and only the dye absorbed by the cellulosic fibres is oxidised at a later stage of processing [42]. One of the dyehouses in North Carolina, USA, has been successfully recycling indigo dyebaths for many years [43]. Vat dyes are used to colour both components in pale depths on polyester/cellulosic fibre blends [44] but coloration of the polyester component in this case is more closely analogous to disperse dyeing (section 1.6.5). Anthraquinone disperse dyes resemble those vat dyes that are substituted anthraquinone derivatives and in both instances it is exclusively the virtually water-insoluble keto form that is absorbed by the polyester fibre. Approximately 80% of all vat dyes belong to the anthraquinonoid class (see Table 1.2) and this certainly includes the leading products in all sectors of the colour gamut. Indigo and its derivatives as well as the sulphurised vat types contribute mainly to the navy blue sector, whereas thioindigoid and indolethianaphthene derivatives occupy the red to brown region. In this and succeeding tables, the figures again relate to all dyes of known chemical type listed and are given in percentage terms to permit easier comparison between the distributions in different hue sectors. The total number of dyes in each of the primary sectors (yellow, red or blue) is, of course, almost always larger than those in the non-primary sectors. The vat dyes section of the Colour Index incorporates a subgroup called solubilised vat dyes. These are sodium salts of sulphuric acid esters of the parent leuco vat dyes, such as CI Solubilised Vat Blue 6 (1.50). In contrast to the leuco compounds, the vat leuco esters dissolve readily in water at neutral pH. They have relatively low substantivity for cellulose and thus have been used mainly in continuous dyeing and printing. In the presence of an oxidant in mineral acid solution (sodium nitrite and sulphuric acid, for example) the leuco ester is rapidly decomposed and the insoluble vat dye regenerated. Thus application of a vat leuco ester represents a simpler (but more costly and less versatile) alternative to conventional dyeing methods via the alkaline leuco compound.

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CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS

Table 1.2 Percentage distribution of chemical classes in vat dye hue sectors Distribution in hue sector (%) Chemical class

Yellow Orange Red

Anthraquinonoid 94 Indigoid Thioindigoid Indole-thianaphthene Aminoketone 3 Sulphurised vat 3 Miscellaneous

92

65 11 15 2 5

4

4

2

Violet Blue

Green

Brown

Black

67

90

83 2 4 2 9

90

14 14

5

62 17 5

5 5

11 5

% of all vat dyes 79 5 4 4 3 3 2

10

NaO3SO Cl

N NaO3SO

OSO3Na

N

Cl OSO3Na

1.50

CI Solubilised Vat Blue 6

1.6.2 Sulphur dyes These resemble the vat dyes in certain ways, although they are of indeterminate constitution and usually mixtures of different chemical species (section 6.4). The characteristic disulphide group (D-S-S-D in Scheme 1.2) is always present in the insoluble form of a sulphur dye, which is brought into aqueous solution by reduction to the alkali-soluble (leuco) form (D-S –). The soluble sodium thiolate form of the leuco sulphur dye has substantivity for cellulose. Thus the application of sulphur dyes to cellulosic fibres is a threestage process (Scheme 1.2) broadly similar to that already outlined for vat dyes.

D

S

S

D

D

S dyebath

(a)

D

S

D

S

S

D

cellulose (b)

(c)

Scheme 1.2

In the Colour Index the sulphur dyes are classified into four subgroups. Conventional insoluble disulphide brands, taking the generic CI Sulphur dye designation, are converted to the leuco compound by adding sodium sulphide to a boiling aqueous dispersion of the parent

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21

dye. CI Leuco Sulphur dyes are marketed as concentrated solutions (or dispersions) of the pre-reduced dye with a small excess of the reducing agent. They can be added directly to a dyebath prepared with additional reducing agent and are particularly convenient for continuous dyeing. CI Solubilised Sulphur dyes are sodium salts of thiosulphato derivatives of the parent dyes. Like the CI Solubilised Vat dyes they are highly soluble in water but low in substantivity for cellulose. Addition of alkali and reducing agent converts them to the leuco sulphur form. The Colour Index refers to a fourth subgroup, the CI Condense Sulphur dyes, but these are only of historical interest. They were water-soluble alkyl- or arylthiosulphato derivatives, mostly of phthalocyanine dyes. In the presence of an alkaline reducing agent (sodium sulphide or polysulphide) they underwent a polycendensation reaction to give an insoluble coloured polymer within the fibre, the chromogenic groupings being linked together via disulphide bridges as in conventional sulphur dyeings. Improvements in dye synthesis, formulation and application in recent years have led to modern ranges of sulphur dyes with notable advantages: freedom from heavy metals, AOX or carcinogenic arylamines, high exhaustion and insolubility after reoxidation. Developments in application methods include exhaust dyeing under a nitrogen atmosphere, indirect electrochemical reduction and pad–steam dyeing with fixation in a flash ager [45]. Sulphur dye effluent from traditional dyeing systems contains sulphides and thiosulphates as well as the residual unfixed dyes. The discharge of sulphide liquors to drain is not normally permissible because of the toxicity of hydrogen sulphide vapour that would be released under acidic conditions. Corrosion of the sewerage system and damage to the treatment works would also occur. Stringent standards are required for consent and sulphide waste is normally treated by oxidation or precipitation separately from other effluent streams [46]. Sulphur-free biodegradable products such as glucose or hydroxyacetone have been evaluated as alternative reducing agents for sulphur dyes but they are more costly and less versatile than the traditional sulphide or hydrosulphide.

1.6.3 Reactive dyes These dyes are capable of reacting chemically with a substrate under suitable application conditions to form a covalent dye-substrate bond. The characteristic structural feature is thus the possession of one or more reactive groupings of various kinds (section 7.2). Almost always these may be categorised as either: (a) an activated unsaturated vinyl group that reacts with cellulose by addition to the double bond, or (b) an activated halogeno substituent (C1 or F) that undergoes nucleo-philic substitution by a cellulosate anion (section 7.3.1). A marked trend in recent years has been the growth in availability and variety of ranges of dyes containing two or more reactive groups per molecule (section 7.4). Although more costly to manufacture, such structures react with greater efficiency, so that less dye is lost by hydrolysis. The introduction of such a specific means of ensuring dye fixation freed the colour chemist from many of the structural limitations implicit in the design of most other ranges of dyes, so that virtually any chemical class may be employed as a chromogen [47]. In practice, however, the reactive dye chemist has relied heavily on the azo chromogen (see Table 1.3),

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CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS

Table 1.3 Percentage distribution of chemical classes in reactive dye hue sectors Distribution in hue sector (%) Chemical class

Yellow Orange Red

Violet Blue

Green Brown Black

Unmetallised azo Metal-complex azo Anthraquinone Phthalocyanine Miscellaneous

97 2

63 32 5

16 5 37 42

90 10

1

90 9

1

20 17 34 27 2

57 43

42 55 3

% of all reactive dyes 66 15 10 8 1

usually unmetallised but sometimes as metal complexes in the duller hues, such as navy, brown and black. Anthraquinone, phthalocyanine, formazan and triphenodioxazine chromogens have made notable contributions in blue and turquoise hues but in all other hue sectors the azo dyes account for more than 95% of reactive dye structures. Several instances of respiratory sensitisation resulting from exposure to reactive dyes have been reported. If an individual becomes sensitised to reactive dyes, it is essential that any future contact with these dyes or other respiratory allergens be avoided. Subsequent exposures may cause anaphylactic shock and can progress into convulsions, coma and death. As yet, there is no animal test that can be used to predict reliably the potential of a reactive dye to cause respiratory sensitisation [1]. Reactive groups in dye molecules that fail to react with the substrate are hydrolysed during dyeing or discharge of the residual dyebath. Recycling is therefore not a viable option in the case of reactive dyeings [42]. Bioelimination (the sorptive removal of dyes during biological treatment of effluent) is also ineffective for reactive dyes, which show little adsorption in this way. This behaviour is independent of the degree of sulphonation or the ease of hydrolysis of the reactive dye molecules. The use of cationic flocculants for removal of reactive dye hydrolysates is of considerable interest [48]. By far the most important application segment for reactive dyes is the dyeing and printing of cellulosic fibres. Their impact in wool dyeing has been less dramatic but the exceptionally high wet fastness requirements of machine-washable knitwear, for example, do justify the use of reactive dyes, in spite of their higher cost relative to conventional dyes for wool. Reactive dyes are suitable for dyeing glove leather, silk and similar materials where resistance to water and cleaning aids is essential. 1.6.4 Direct dyes These are defined as anionic dyes with substantivity for cellulosic fibres applied from an aqueous dyebath containing an electrolyte. The forces that operate between a direct dye and cellulose include hydrogen bonding, dipolar forces and non-specific hydrophobic interaction, depending on the chemical structure and polarity of the dye. Apparently multiple attachments are important, since linearity and coplanarity of molecular structure seem to be desirable features (section 3.2.1). The sorption process is reversible and numerous attempts have been made to minimise desorption by suitable aftertreatments (section 10.9.5). The two most significant non-textile outlets for direct dyes are the batchwise dyeing of leather and the continuous coloration of paper.

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23

In every hue sector, at least 70% of direct dyes are unmetallised azo structures (see Table 1.4). In contrast to all other application ranges, the great majority of them are disazo or polyazo types, the former predominating in the brighter yellow to blue sectors and the latter in the duller greens, browns, greys and blacks. The copper-complex direct dyes are mostly duller violets, navies and blacks derived from the disezo and polyazo subclasses. Stilbene and thiazole dyes, the only non-azo contributors in the yellow to red sector, bear certain similarities to the disazo and polyazo structures (sections 1.4.1 to 1.4.3). The dioxazine and phthalocyanine chromogens are represented in bright blue direct dyes of high light fastness. Table 1.4 Percentage distribution of chemical classes in direct dye hue sectors Distribution in hue sector (%) Chemical class

Yellow Orange Red Violet Blue

Green Brown

Black

Monoazo Disazo Polyazo Copper-complex azo Stilbene Thiazole Dioxazine Phthalocyanine

4 58 8

7 53 16

17 13

23 1

8 21 64 4 1

23 68 8 1

14 71 10 3 2

4 83 3 9

1

52 31 12

3 2

3 22 67 1 7

2

% of all direct dyes 5 49 33 5 5 1 1 1

As there is no chemical change of direct dyes during orthodox application, their exhaust dyebaths are eminently suitable for recycling. Membrane processes have been used successfully to remove direct dyes from dyehouse effluents. There are possible cost savings associated with reuse of the electrolyte, depending on the rejection properties of the membrane [42]. High adsorption of direct dyes occurs during biological treatment of dyehouse waste liquors containing them. This effect is not dependent on the degree of sulphonation of the direct dye molecules [48]. Oxidation with ozone or precipitation using cationic flocculants are effective ways of eliminating direct dye residues. 1.6.5 Disperse dyes These dyes have affinity for one or, usually, more types of hydrophobic fibre and they are normally applied by exhaustion from fine aqueous dispersion. Although pure disperse dyes have extremely low solubility in cold water, such dyes nevertheless do dissolve to a limited extent in aqueous surfactant solutions at typical dyeing temperatures. The fibre is believed to sorb dye from this dilute aqueous solution phase, which is continuously replenished by rapid dissolution of particles from suspension. Alternatively, hydrophobic fibres can absorb disperse dyes from the vapour phase. This mechanism is the basis of many continuous dyeing and printing methods of application of these dyes. The requirements and limitations of disperse dyes on cellulose acetate, triacetate, polyester, nylon and other synthetic fibres will be discussed more fully in Chapter 3. Similar products have been employed in the surface coloration of certain thermoplastics, including cellulose acetate, poly(methyl methacrylate) and polystyrene.

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CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS

Table 1.5 Percentage distribution of chemical classes in disperse dye hue sectors Distribution in hue sector (%) Chemical class

Yellow Orange Red Violet

Blue

Green Brown

Black

Azo Anthraquinone Nitro Aminoketone Methine Quinophthalone Miscellaneous

48 6 16 8 14 4 4

27 72

30 65

100

1

5

92 2 3 2

73 25

1

1

1

47 53

100

% of all disperse dyes 59 32 3 2 2 1 1

Yellow disperse dyes are known from a wide range of chemical classes (see Table 1.5) but azo dyes derived from heterocyclic coupling components are particularly important for economic reasons. The monoazo subclass dominates throughout the disperse dye range and there are no metal-complex disperse dyes designed for the ester fibres, although they do exist as a subgroup of ‘acid dyes’ for wool. Anthraquinone derivatives traditionally dominated the bright red to bright green series but here again cost considerations have favoured the introduction of new monoazo dyes, often with heterocyclic amines as diazo components, for the dyeing of polyester fibres [49]. Browns and blacks on polyester and acetate fibres have always been provided almost entirely by azo disperse dyes. The disperse range contains relatively few homogeneous green, brown or black dyes, however. Many of the commercial navy, green, brown and black products are in fact mixtures of two or more components (section 3.2.3). Disperse dyes from the monoazo and anthraquinone classes have been implicated in cases of contact dermatitis. Circumstances common to such cases appear to be heavy depths of these dyes on nylon rather than polyester and occurring in articles of clothing that are in direct contact with the skin, often in areas that are likely to become moistened by perspiration. Hosiery, socks, blouses and close-fitting athletic or fashion wear, such as velvet leggings, are representative of the types of garment where this problem has arisen [1]. There is no chemical change of disperse dyes during orthodox application, so their exhaust dyebaths are suitable for recycling. Such dyes were successfully removed from an effluent stream by a microfiltration membrane module on an industrial scale [50]. In this trial the permeate was reused but not the dyes, although other work has demonstrated that recycling of disperse dyes is possible [51]. Moderate to high adsorption of disperse dyes takes place during biological treatment of dyehouse effluent. Although dispersible but insoluble in watercourses, disperse dyes released to the aquatic environment have shown no evidence of bioaccumulation in fish [48]. 1.6.6 Basic dyes Controversy has arisen at times [52] regarding the apparent synonymity of the terms ‘basic dye’ and ‘cationic dye’. The Society of Dyers and Colourists defines a basic dye as ‘characterised by its substantivity for the acidic types of acrylic fibres and for tanninmordanted cotton’, whereas a cationic dye is defined as one ‘that dissociates in aqueous

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solution to give a positively charged coloured ion’. Since the second of these describes a chemical feature rather than an application property, the term ‘basic dyes’ is adopted in this book for this range, as in the Colour Index. This category includes those weakly basic dyes (1.51) that are essentially uncharged under neutral or alkaline conditions and only form a cation by protonation in acidic solution, such as chrysoidine (CI Basic Orange 2). The powerful dye-fibre forces responsible for the excellent exhaustion and high wet fastness of basic dyes on acrylic fibres are believed to include electrostatic attraction to anionic sites in the fibre and non-specific hydrophobic interaction. H2N

H2N N

_ + NH3 X

N

N

N

NH2 + HX

1.51 Chrysoidine

Many brilliantly coloured and tinctorially strong basic dyes for silk and tannin-mordanted cotton were developed in the early decades of the synthetic dye industry. Most of these belonged to the acridine, azine, oxazine, triarylmethane, xanthene and related chemical classes; their molecules are usually characterised by one delocalised positive charge. Thus in crystal violet (1.29) the cationic charge is shared between the three equivalent methylated p-amino nitrogen atoms. A few of these ‘traditional basic’ dyes are still of some interest in the dyeing of acrylic fibres, notably as components of cheap mixture navies and blacks, but many ‘modified basic’ dyes were introduced from the 1950s onwards for acrylic and modacrylic fibres, as well as for basic-dyeable variants of nylon and polyester [44]. Most of these products are azo or anthraquinone types, often with a localised quaternary ammonium group isolated from the chromogen by a saturated alkyl chain, as in CI Basic Red 18 (1.52). Such products often exhibit higher light fastness than the traditional delocalised types. Improved azomethine, methine and polymethine basic dyes of good light fastness are also available. In contrast to the more specialised traditional classes, the azo and methine dyes have contributed to the basic dye range across the entire spectrum of hues (see Table 1.6) and now account for a clear majority of all basic dyes listed in the Colour Index. Basic dyes do not undergo chemical change during dyeing, but the proportion remaining in the exhausted dyebath is low (typically 2–3%) and scarcely justifies isolation for reuse. Recycling of the process water, however, may allow recovery of the inorganic salts and other auxiliary chemicals present [42]. There is normally a high degree of sorptive removal of residual basic dyes during biological treatment of effluent. This is important, because basic dyes tend to exhibit toxicity to aquatic organisms. In a survey of 3000 dyes in common use, 98% of Cl CH2CH3 O2N

N

N

N

_ + CH2CH2N(CH3)3 X

1.52 CI Basic Red 18

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Table 1.6 Percentage distribution of chemical classes in basic dye hue sectors Distribution in hue sector (%) Chemical class

Yellow Orange Red

Violet Blue

Green Brown

Black

Azo Methine Triarylmethane Acridine Anthraquinone Azine Oxazine Xanthene Miscellaneous

33 41

51 13

55 23 5

38 21 18

34 2 23

12 10 56

60

12

36 6

5 13

15 5 11

11

5

14

10

100

20 20 22

% of all basic dyes 43 17 11 7 5 5 3 3 6

them showed low toxicity to fish (LC50 > 1 mg/l). In 27 instances, however, 16 of them basic dyes and 10 of these triphenylmethane types, the LC50 was of the order of 0.05 mg/l [53]. A study by the American Dye Manufacturers’ Institute (ADMI) of the effects of 56 dyes on the growth of green algae, 15 of them inhibited algal growth and 13 of these were basic dyes. In screening tests designed to determine whether dyes have an adverse effect on waste water bacteria and hence on the operation of effluent treatment plants, only 18 of the 202 dyes examined gave an LC50 value less than 100 mg/l and all of these were basic dyes [53]. 1.6.7 Acid dyes These are defined as anionic dyes characterised by substantivity for protein fibres. Levelling acid dyes and 1:1 metal-complex dyes are normally applied to wool from strongly acidic solutions, but the 1:2 metal complexes and many milling acid dyes have considerable substantivity even from neutral dyebaths (section 3.2.2). Wool, silk and nylon contain basic groups and the uptake of levelling acid dyes by nylon at acidic pH can usually be related to the amine end group content of the fibre. Under neutral dyeing conditions, however, nonspecific hydrophobic interaction makes a considerable contribution, reinforcing the electrostatic bonding between such fibres and acid dyes of higher wet fastness. Azo acid dyes represent the biggest single segment in the Colour Index, about two-fifths of them being metal-complex types (see Table 1.7). Monoazo dyes are supreme in all hue sectors of the azo acid range except the unmetallised browns and blacks, where disazo structures are more numerous. Anthraquinone and triarylmethane acid dyes traditionally provided most of the brilliant violet to green members of this range but demand for them declined, especially the triarylmethane types, as bright azo alternatives were developed. The significance of nitro yellows and browns, xanthene reds and violets and the phthalocyanine blues has always been marginal in the acid dye range. The first acid dye, Orange I (1.53; CI Acid Orange 20), was discovered in 1876. All but a handful of the acid dyes developed since then were evaluated initially with wool dyeing in mind. In terms of adaptability to the coloration of other substrates, however, acid dyes have proved pre-eminent. This is the main reason for their number and variety. As well as the dyeing and printing of nylon and protein fibres, acid dyes are important for the coloration of leather, paper, jute, wood and anodised aluminium. Most of the permitted dyes for food and

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Table 1.7 Percentage distribution of chemical classes in acid dye hue sectors Distribution in hue sector (%) Chemical class

Yellow Orange Red Violet Blue

Green Brown Black

Unmetallised azo Metal-complex azo Anthraquinone Triarylmethane Xanthene Azine Phthalocyanine Nitro Miscellaneous

65 31

55 42

64 29 2

17 21 36 16

15 39 22 16

1

1 1

4 1

4 5

1

1

7

1 2

22 44 18 12 3 1

79 13 3

47 46 3

1

1

5 3

% of all acid dyes 48 31 10 5 2 1 1 1 1

H NaO3S

N N

O

1.53 CI Acid Orange 20

cosmetics were originally marketed long ago for the dyeing of wool. Some solvent-soluble dyes are formed by co-precipitation of acid dyes and basic dyes. Concentrated monoazo acid dyes, often unmetallised bright yellows and reds, can be precipitated as their water-insoluble metal salts for use as pigments (section 2.2.1). Acid dyes are highly suitable for reuse because they do not undergo chemical change during dyeing. Removal of acid dyes from dyehouse effluent has been achieved by membrane processing on an industrial scale. The process water and auxiliary chemicals are also suitable for recycling to the dyebath [42]. The sorptive removal of acid dyes during biological treatment of effluent varies with their degree of sulphonation. Levelling acid dyes of high solubility exhibit low sorption, whereas the more hydrophobic neutral-dyeing dyes are bioeliminated to a much greater extent [48]. As in the case of reactive dyes, flocculation with cationic agents is an effective method of removing the highly sulphonated acid dyes. Electrochemical coagulation of acid dyes is a viable alternative, with values of 55–85% colour removal from carpet dyeing waste waters being reported [54]. Reuse of the permeate from a reverse-osmosis membrane process on dyebath effluent containing premetallised acid dyes has been achieved [50]. 1.6.8 Mordant dyes The somewhat ambiguous term ‘mordant dye’ is defined as a dye that is fixed with a ‘mordant’. A mordant is itself defined as ‘a substance, usually a metallic compound, applied to a substrate to form a complex with a dye which is retained by the substrate more firmly than the dye itself’. Unfortunately, the mordant dyes category in the Colour Index follows

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the more restricted convention of excluding both chelatable direct dyes capable of complexing with copper salts and those traditional basic dyes that were formerly applied to tannin-mordanted cotton, both of which subgroups conform to the above less than precise definition. A more logical generic name for those chelatable anionic dyes capable of being fixed to wool by aftertreatment with a dichromate would be the more popular term ‘chrome dyes’ or ‘chrome mordant dyes’, but the designation CI Mordant dye is well entrenched. Since the range has been steadily declining for many years, the ambiguity is of decreasing significance too. In all hue sectors except violet and blue, where triarylmethane, anthraquinone and oxazine dyes make notable contributions, 80% or more of chrome dyes are of the o,o′-dihydroxymonoazo type (1.54; CI Mordant Violet 5). Although most chrome dyes are only of interest for wool dyeing, a minority are applicable to most of those other substrates where acid dyes are found useful. O

H H

O

N N NaO3S 1.54 CI Mordant Violet 5

The complex formed when a mordant dyeing is aftertreated in a dichromate solution is retained by the wool in preference to the unmetallised mordant dye, which may desorb to some extent during the treatment. The latter is rather unstable in an oxidising solution and quinonoid by-products are often formed. If the chromium complex of the dye is formed from the desorbed dye in solution, this will further complicate the composition of the aftertreatment liquor. Thus reuse of mordant dyeing and aftertreatment baths is not an option. Furthermore, 100% rejection of dichromate ions would be required if the permeate of a membrane process treating the effluent was to be recycled [42]. 1.6.9 Leather dyes The application range designated by this generic name in the Colour Index incorporates those acid, direct and mordant dyes with substantivity for leather and satisfactory fastness on that substrate [55]. It is a commercially important sector, the number of products listed being exceeded only by the complete acid or direct dye ranges. As expected from the sources of this selection, about 85% of leather dyes are azo compounds (35% disazo, 30% monoazo, 20% metal-complex monoazo) and the remainder are mainly yellow to orange stilbene dyes and anthraquinone or triarylmethane types in the violet to green sectors. There is growing concern over the potential risks to human health and the environment arising from leather goods. Dye manufacturers and tanneries are concerned about effluent, air pollution, containers and packaging [56]. In the light of the relatively important contribution to leather dyeing of azo dyes that can yield hazardous arylamines on reduction, careful guidance on the selection of dyes for leather is essential, with emphasis on procurement from reliable sources and the utilisation of liquid formulations to minimise

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effluent treatment [57]. Based on theoretical considerations of dye structure and application properties, a range of compatible dyes that give deep colours of good fastness on leather has been developed [58]. For the reasons already outlined, only selected dyebath wastes from leather dyeing will be suitable for recycling (providing mordant dyes are absent). Bioelimination of the disazo direct dyes and monoazo metal-complex dyes will be relatively high. Flocculation using cationic precipitants should be an effective way of dealing with the residual dyes remaining in leather dyeing effluents. It is also necessary to consider methods for the disposal of leather goods at the end of their useful life. Dyed leather in itself, however, is not regarded as posing a hazard to humans, nor is it considered harmful to the environment [56]. 1.6.10 Food dyes Increasing legislation over the last fifty years has been imposed to control additives such as colorants that might enter foodstuffs directly or by migration from food containers or packages. Similar criteria cover the composition of drugs and cosmetics, but the cosmetics regulations tend to be less stringent in terms of chronic toxicity because skin represents an effective barrier to many chemicals. Freedom from toxicity is clearly the primary consideration for dyes used in foods, drugs and cosmetics, followed by high solubility and chemical stability in the appropriate medium of incorporation. The possibility that they may affect consumers adversely has caused growing concern [59,60]. Legislation over many years has increasingly restricted the usage of synthetic colorants to certain permitted products that have shown no harmful effects when tested rigorously. There is as yet no agreed international list of permitted food colours. Thus a food dye that is permitted in one country may be considered unacceptable in another. The synthetic food colorants permitted in the European Union are listed in Table 1.8 [60]. All were originally introduced as acid dyes for wool many years ago. Furthermore, more than thirty colorants of natural origin are permitted in most countries. The natural carotenoid dyes are of outstanding importance for colouring edible fats and oils. These yellow to red methine dye structures occur in many families of plants and animals, including vegetables, berries,

Table 1.8 Synthetic colorants permitted for use in food in the European Union (EU)

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EU No

Colorant

CI Food

CI Acid

Chemical class

E102 E104 E110 E122 E123 E124 E127 E131 E132 E142 E151

Tartrazine Quinoline Yellow Sunset Yellow Carmoisine Amaranth Ponceau 4R Erythrosine Patent Blue V Indigo Carmine Green S Black PN

Yellow 4 Yellow 13 Yellow 3 Red 3 Red 9 Red 7 Red 14 Blue 5 Blue 1 Green 4 Black 1

Yellow 23 Yellow 3

Monoazo Quinophthalone Monoazo Monoazo Monoazo Monoazo Xanthene Triarylmethane Indigoid Triarylmethane Disazo

29

Red 14 Red 27 Red 18 Red 51 Blue 1 Blue 74 Green 50

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fungi, insects, bacteria, feathers and egg yolks [61]. In general, the regulations governing the use of permitted additives such as those added directly in food manufacture are applied similarly in different member countries of the EU. There is greater variation, however, with regard to the indirect contamination of food by transfer of additives from packaging materials. 1.6.11 Solvent dyes Solvent dyes are characterised by their solubility in one or more organic solvents. Numerous media are of interest in practice, the most significant being alcohols, ethers, esters, ketones, chlorinated solvents, hydrocarbons, oils, fats and waxes. The many practical applications in which the specific solubilities of these dyes are exploited have been outlined elsewhere (section 2.12). More than half of the solvent dyes listed in the Colour Index are unsulphonated azo compounds (27% monoazo, 18% monoazo metal-complex, 9% disazo) and a further 7% are complexes formed by co-precipitation of acid dyes with basic dyes, mostly yellows and reds. Unsulphonated anthraquinones lead the field in the bright violet to green sectors (17%). Xanthene reds (5%) and triarylmethane blues (5%) are the other noteworthy segments represented, together with a miscellany (12%) drawn from almost all other chemical classes. Some simple yellow solvent dyes used widely in the past, such as 4aminoazobenzene (CI Solvent Yellow 1), are now known to be carcinogenic. 1.6.12 Pigments In contrast to solvent dyes, pigments are substances in particulate form. They are essentially insoluble in the media into which they are incorporated, and are mechanically dispersed therein in order to modify the colour and/or light-scattering properties of such media. These characteristics of low solubility in water and organic solvents, which are obligatory for the satisfactory technical performance of pigments, also mean that their bioavailability is unusually low. This is certainly the prime reason for the remarkably low toxicity of organic pigments [1]. In many cases, the solubilities in n-octanol and water are so low that the experimental determination of solubility poses serious difficulties, so that calculation procedures are adopted in assessing the toxicological hazards of pigments [62]. The complete range of pigments may be conveniently divided into three main groups on the basis of their chemical constitution and mode of preparation for use: (a) Organic pigments (approximately 60% of the total number of products) are concentrated nonionic colorants from various chemical classes, (b) Water-soluble dyes (approximately 20%) that are rendered insoluble by various techniques of precipitation, (c) Inorganic pigments (approximately 20%) are coloured insoluble materials from inorganic sources. Monoazo and disazo compounds form by far the largest subgroup among the organic pigments, mainly occupying the yellow-orange-brown-red sectors of the colour gamut. This class may be subdivided further into metal-free structures with inherently negligible solubility in water and those containing potentially ionisable groups that must be converted to their heavy-metal salts (section 2.3). Anthraquinonoid pigments are analogous to vat

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dyes in their keto forms (section 1.6.1) and they provide mostly red-violet-blue colours of high light fastness. Further chromogens represented include isoindolinones (yellow-orange), quinacridones (red-violet) and phthalocyanines (blue-green). Conventional basic dyes (section 1.6.6) can be rendered insoluble by precipitation using phosphomolybdic or phosphotungstic acids, or alternatively copper(II) hexacyanoferrate. These complexes exhibit higher light fastness than the parent basic dyes on a traditional tannate mordant. Concentrated anionic monoazo dyes (mainly yellows, oranges and especially reds) can be precipitated as their insoluble metal salts, typically those of barium, aluminium or calcium. These processes of converting soluble dyes into pigments by precipitation, often in the presence of an inert substrate such as the so-called alumina hydrate, are now of declining importance. The inorganic group of pigments represents the naturally occurring coloured minerals that have been widely used throughout recorded history to colour ceramics, glass and many other artefacts. This class includes coloured metal salts such as lead chromate (CI Pigment Yellow 34), complex salts such as Prussian blue (CI Pigment Blue 27) and ultramarine (CI Pigment Blue 29), metal oxides such as titanium dioxide (CI Pigment White 6) and free elements such as carbon black (CI Pigment Black 6). 1.6.13 Azoic components and compositions These are the only ranges of precursor products in the Colour Index that are still commercially significant. Azoic dyes have a close formal relationship to those monoazo pigments derived from BON acid or from acetoacetanilides (section 2.3.1) and some are chemically identical with them, although they are used in a totally different way. Azoic components are applied to produce insoluble azo dyes within the textile substrate, which is almost always cotton. Corresponding azoic components for the dyeing of cellulose acetate, triacetate and polyester fibres were once commercially important, but are now obsolete because of environmental hazards and the time-consuming application procedure. In dyeing with azoic components, an insoluble azo compound is produced within the fibre by the coupling of an azoic diazo component (a diazotised arylamine) with an azoic coupling component (usually an arylide of BON acid or a related naphthol). Azoic dyes provide only relatively dull violets and navy blues, costly greens, and browns or blacks of limited versatility, but are tinctorially strong, bright and economical in the orange to red sectors. Some of the diazotisable amines of former importance in this range, such as 4-chloro-2methylaniline (CI Azoic Diazo Component 11) or 3,3′-dimethoxybenzidine (CI Azoic Diazo Component 48) are now known to be carcinogenic. The two types of component form separate generic series in the Colour Index. There is also a third series, designated CI Azoic compositions. Now obsolescent, these prepared mixtures of an azoic coupling component and a stabilised diazo component were intended mainly for textile printing. The stabilisation prevented coupling until the printed fabric was steamed under appropriate conditions to regenerate the active diazonium salt and promote formation of the azoic dye. When dyeing with azoic components it is difficult to prevent some desorption of coupling component into the developing bath containing the diazo salt. The latter is also inherently unstable, releasing nitrogen to leave the phenolic analogue of the original arylamine. Thus recycling is not a realistic option for residual azoic dyebaths because of their complex composition [42].

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1.6.14 Developers This short range of simple coupling components, comprising phenols, naphthols and 1phenyl-3-methylpyrazol-5-one, is seldom used nowadays. Developers were once used as aftertreatments to enhance the wet fastness of certain direct dyes for cellulosic fibres or disperse dyes for cellulose acetate. Each of these dyes contained a primary arylamine grouping that was capable of diazotisation and coupling to the developer molecule. This application technique was prolonged, however, and the improvement in fastness performance was disappointingly slight. The usage of developers in this way has fallen to negligible levels. 1.6.15 Oxidation bases Like azoic components, these are precursors of a distinct chemical class of colorant as well as an application category, but they are too restricted in attainable hues to be described as a ‘range’. This cheap but unpleasant method of continuous coloration, once popular for cotton printing in visually attractive illuminated resist styles under aniline black or paramine brown colours, has been superseded by environmentally more acceptable and controllable alternative processes. Oxidation dyeing was simply the in situ formation of a polymeric black or dark brown azine colorant of indeterminate constitution by acidic oxidation of a simple arylamine such as aniline or p-phenylenediamine on the fibre, using a chlorate or dichromate as oxidant and a copper salt catalyst. Although no longer significant in textile dyeing or printing, coloration techniques of a similar kind have remained the primary method of colouring human hair, feathers and natural furs. 1.6.16 Ingrain dyes This is another somewhat ambiguous category in the context of the Colour Index classification, but fortunately it is now merely of historical interest. An ‘ingrain’ dyeing process is defined as the formation of a colorant ‘in situ in the substrate by the development and coupling of one or more intermediate compounds’. This imprecise description clearly embraces azoic dyeing and the application of oxidation bases, but the generic term ‘CI Ingrain dye’ is limited to a third small group of now obsolete colorant precursors, again too restricted in hue to be regarded as a ‘range’. They were introduced in the late 1940s for the textile printing of cellulosic fabrics under the trade names Alcian (ICI) and Phthalogen (BAY). Both types resulted in the insolubilisation of copper phthalocyanine or related pigments within the fibre, although the respective application techniques differed considerably. Reactive phthalocyanine dyes in the 1960s superseded these early approaches to the attainment of fast bright blues and turquoises on cellulosic fabrics [63]. 1.6.17 Fluorescent brightening agents These essentially colourless compounds have been defined as substances ‘that, when added to an uncoloured or a coloured substrate, increase the reflectance of the substrate in the visible region by converting UV radiation into visible light and so increase the whiteness or brightness of the substrate’. The air of uncertainty in the final clause of this definition

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reflects the general indecision among practitioners in this field about whether to call them ‘fluorescent brightening agents’ (or FBAs), as in the Colour Index, ‘fluorescent whiteners’ (FWs) or ‘optical whiteners’ (OWs). Since their commercial introduction during the 1940s as components of proprietary detergents and laundry preparations, these products have found extensive usage in the whitening of paper and textile materials. Disperse FBAs are available for whitening hydrophobic fibres and solvent-soluble FBAs impart fluorescence to oils, paints, varnishes and waxes. Approximately 75% of commercially established FBAs are stilbene derivatives with inherent substantivity for paper and cellulosic textiles, but the remainder come from about twenty different chemical classes. These include aminocoumarins (6%), naphthalimides (3%), pyrazoles and pyrazolines (each about 2%), acenaphthenes, benzidine sulphones, stilbene-naphthotriazoles, thiazoles and xanthenes (each about 1%). FBAs of these and other chemical types are discussed in detail in Chapter 11 of Volume 2. 1.6.18 Reducing agents As noted earlier, the specific inclusion in the Colour Index of a group of textile auxiliaries under this generic name seems anomalous, if not aberrant. The justification claimed is that they are indispensable for the application of vat dyes and in discharge printing or stripping processes. A similar level of interdependency could be claimed for certain other classes of auxiliaries, as is discussed particularly in Chapters 10 and 12 of Volume 2. One has only to visualise textile printing without thickeners or emulsifying agents to realise this. In future plans for further development of the Colour Index, however, there seems to be no scope for extending coverage to other groups of auxiliary products used in colorant application processes [l3]. 1.7 COLORANTS AND THE ENVIRONMENT Health and safety were absent from the list of priorities in the early decades of the synthetic dyes industry. Practical experience in the primitive working conditions of the time [64] no doubt made workers aware of the more obvious dangers, such as corrosive acids, flammable solvents and potentially explosive nitro compounds. Accidents must have occurred frequently, reminding victims and supervisors alike of the penalties suffered if hazardous chemicals were handled carelessly. More insidious, however, were the potential health risks resulting from longer-term exposure to certain organic and inorganic chemicals. Operatives were expected to provide and wash their own working clothes. These must have become heavily contaminated with chemical stains, putting the worker’s family also at risk of exposure to hazardous vapours [65]. It was not until the 1940s that the connection was made between the high incidence of bladder cancer in employees of dye-making and dye-using firms and their exposure to certain arylamines, following epidemiological studies of individuals who had spent their working lives in these industries. Compounds now recognised as highly carcinogenic, notably 2-naphthylamine and benzidine, had been widely used as dye intermediates since the 1880s. Even if such causal links had been suspected over the intervening decades, it is difficult to see how they could have been demonstrated in any other way. Following a period of unprecedented growth and optimism in the chemical industries

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worldwide during the 1950s and 1960s, a sociopolitical reaction set in during and after the oil crises of the 1970s. Public demand, often fuelled by media misinformation and speculation [60,66], became insistent that the manufacture and use of industrial chemicals should be more closely regulated and monitored. A series of tragic incidents occurred over this period, involving cyclohexane at Flixborough (UK), dioxins at Seveso (Italy), mercury compounds at Minamata (Japan) and methyl isocyanate at Bhopal (India). These were followed in 1986 by the Chernobyl (USSR) nuclear explosion that polluted much of Northern Europe and the Schweizerhalle (Switzerland) fire that polluted the Rhine basin in the heart of Europe, reinforcing this trend towards a political climate of reform and control. 1.7.1 Risk evaluation and prevention Indispensable to the management of risk by reduction or avoidance is a knowledge of the risk and the controlling factors determining its magnitude. Risk in this context is a function of the potentially harmful effects arising from inherent toxicological properties of the chemical and the extent of its bioavailability to the organism exposed. Risk is also a function of degree of exposure and the probability of its occurrence. Obviously, the risk of experiencing harmful effects can be lowered by limiting the degree of exposure and this approach affords a means of improving safety [67]. Industry has a substantial interest in helping the risk assessment approach to work. Failure to do so will encourage regulation based on hazard considerations alone. There is already a tendency in favour of the precautionary principle and the introduction of various ‘black-lists’. Such discriminatory actions undermine the agreed basis for chemicals control and warrant an unreserved rejection by the chemical industry [68]. Thus two components, exposure and hazard, must be evaluated together in determining the level of risk posed by a given colorant or other chemical. Risk management may therefore be regarded as a series of interdependent steps: (1) Exposure assessment (2) Hazard assessment (3) Risk evaluation (4) Risk prevention The process of risk evaluation for personnel working with dyes and textile chemicals has been discussed in detail [69]. The more extensive the database covering toxicological, physical, chemical and application properties of the product, the easier it is to assess the risks involved. Although exposure levels are just as important as hazard potential for the risk assessment, the quality of the exposure data is often the weak point in the data available. Consequently, in many instances it is not possible to be fully confident of the reliability of the risk assessment, which may tend to be in error on the side of overestimation. Particular attention should be given to improving aspects of exposure assessment (occupational exposure, consumer exposure, environmental release) [70]. The widespread use of colorants creates a great diversity of exposure situations. The most serious exposure potential exists for operatives in colorant manufacture and those employees of dye user firms engaged in weighing and dispensing dyes. Dust particles less than 7 µm in size can gain access into the lungs and pose the greatest problem. It is not feasible to market all colorants in liquid form

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and considerable efforts have been devoted to the development of low-dusting solid forms. The monitoring of amine excretion in the urine of individuals exposed to arylamines, or dyes expected to be metabolised in the body to such amines, offers a possibility of checking the adequacy of safety precautions [67]. The conventional classification of organic colorants into broad chemical classes or application ranges is of limited help in hazard assessment. It is not possible to generalise about the toxicological properties of entire groups like these. Biological activity can vary dramatically in spite of close structural relationships. Nevertheless, an observed toxic effect can often be attributed to a specific structural feature within a narrow subclass of colorants, or to a specific metabolite produced from them as in the formation of benzidine from its parent disazo dyes. Organic colorants generally exhibit relatively low acute toxicity. This is especially true of organic pigments because of their extremely low bioavailability. Of major concern, however, are the potential carcinogenic and allergic effects. As a basis for the determination of risk it must be assumed that the colorants are properly handled and applied. It is not appropriate to estimate risk primarily on the basis of exposure values obtained under improper working conditions, or where appropriate plant and equipment are not available. Ensuring satisfactory operating conditions and training of operatives to handle products correctly is essential nowadays for technological success as well as for health and safety requirements. In this way, exposure levels can be kept below the threshold of unacceptable risk. It is reasonable to accept that for practical purposes levels of exposure exist below which the risk becomes trivial [67]. The various measures to reduce risk are an integral part of risk management. A state of ‘zero risk’ cannot be reached, but efforts to maintain exposure levels below the threshold of unacceptability must be unremitting, in order to increase the margin of safety. An essential prerequisite for effective risk control is the provision of readily accessible hazard information on computer disc, in safety data sheets and on warning labels. It is prudent to minimise exposure to all chemicals through good working practice. Respiratory protection by approved equipment must be worn wherever dusts or aerosols are being generated or disturbed. A constructive approach to reducing risk is the replacement of hazardous products by safer ones. This cannot be achieved quickly in most instances, because of the complex profile of technical and economic requirements that governs selection of a colorant for a specific purpose [71]. Over recent years there has been an extraordinary growth of ecolabelling schemes worldwide, but notably in Europe. The term ‘ecolabel’ has been defined as an EU scheme to promote products with reduced environmental impact during their life cycle [72]. These schemes build upon increasing public awareness of environmental issues, offering an opportunity to increase and sustain consumer interest in reducing the environmental impact, as well as opening up new marketing opportunities. The chemical industry’s stance has been supportive of these objectives but somewhat sceptical and concerned at the multitude of different schemes [68,70]. As a basis for evaluating individual schemes, the Ecological and Toxicological Association of the Dyestuffs Manufacturing Industry (ETAD) concluded that a satisfactory scheme should meet the following criteria: (1) Objective criteria must be provided (2) The approach should be risk-based, not hazard-based (3) Consumer goods should be specified, rather than the colorants used to colour them

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(4) Dye selection should only be necessary if there is real environmental benefit (5) European or international schemes are preferable, rather than a proliferation of national ecolabels. The main deficiency of existing schemes is that they focus on the exclusion of certain hazardous dyes but do not take into account that it is the conditions of manufacture and application that are the most important determinants of environmental impact [68,70]. 1.7.2 Sensitisation, toxicity and carcinogenicity Allergic contact dermatitis or skin sensitisation by dyes or other chemicals manifests itself as a persistent irritating rash. The presence of certain dye stains on the skin has been known to accelerate the reddening effect of sunlight exposure (erythema). The causation of skin sensitisation by dyes, both in animal tests and in exposed workers or the general public, has been reviewed [73,74]. Cases of occupational skin sensitisation attributable to dyes are uncommon [75], even in those workplaces where inadequate handling precautions have been taken. Occasionally, reports of organic pigments causing skin sensitisation have arisen but such cases appear to be due to the presence of soluble impurities. Instances of contact dermatitis linked to the wearing of close-fitting garments often seem to be associated with nylon dyed with certain disperse dyes of low fastness in full depths (section 1.6.5). A list of nine such dyes that may be sensitising has been drawn up: CI Disperse Yellow 3, Orange 3, 37 and 76, Red 1 and Blue 1, 35, 106 and 124. Garments containing any of these dyes should carry a hazard warning label and for contact clothing such as hosiery they should not be used. The possible mechanism of sensitisation in the case of azo dyes is thought to be the production of the quinonimine derivative by reduction and oxidation [75]. Sensitisation of the respiratory tract by inhaling dust particles from various chemicals has frequently been reported in industry. It is likely to result in symptoms of respiratory disease or distress when the sensitised individual is exposed to a specific allergen. Respiratory allergy is the clinical manifestation of this state, with bronchial asthma or allergic rhinitis (resembling hay fever) constituting typical disease symptoms. It is believed that relatively high exposure levels are important in the induction phase. There is also evidence that a predisposition to respiratory allergy may be caused by genetic or other factors. Instances of severe sensitisation to the dust from reactive dyes have been reported [76]. These prompted the UK Health and Safety Executive to initiate a study involving about 440 workers in 51 dyehouses who were in contact with reactive dye powders. About 15% of them showed work-related respiratory or nasal symptoms. In 21 individuals their allergic reactions could be attributed to contact with one or more specific reactive dyes [77]. Reactive dyes are able to react with amino, hydroxy and thiol groups in proteins. Such a reaction seems to be the initial step of the sensitisation process. The reactive dye may react with human serum albumin (HSA) to form a dye-HSA conjugate, which behaves as an antigen. This in turn gives rise to specific antibodies and these, through the release of mediators such as histamine, produce the allergic symptoms [67,77]. Acute toxicity refers to effects that occur within a brief time after a short-term exposure, such as a simple oral administration. The generally low acute oral toxicity of colorants is well-established [78–81]. This is normally expressed in terms of the LD 50 value, a

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statistically derived dose that is expected to cause death in 50% of treated animals (typically rats) when administered over a prescribed period in the test. In 1974 ETAD began a programme to generate a systematic toxicological database. More than 80% of commercial dyes have an LD50 value (rat, oral) greater than 5000 mg/kg. In response to an EEC Council Directive of 1979 (6th amendment) regarding the labelling of dangerous substances, ETAD in 1986 decided to publish a list of twelve colorants that have been classified as toxic on the basis of their acute peroral LD50 values [82]. These varied within the range 25 mg/kg (CI Basic Red 12) to 205 mg/kg (CI Basic Blue 81) and the list included six basic dyes, three azoic diazo components, two acid dyes and one direct dye. Although such data provide an essential basis for advice on safe handling procedures, long-established experience indicates that dyes, and even more so organic pigments, present few acute toxicological risks providing good working practices are followed. In the debate about the toxic effects of dyes and chemicals, there is no doubt that carcinogenic effects are perceived by the general public as the most threatening. Chemicals remain a focus for this concern in spite of the weight of evidence that they make only a minor contribution to the incidence of cancer [60,67,83]. The generally accepted estimate of cancer causation, based on mortality statistics, indicates that only 4% of all cancer deaths are attributable to occupational exposure. Another 2% are considered to arise from environmental causes and 1% from other forms of exposure to industrial products. As by far the largest chemical class, it is perhaps not surprising that azo dyes have attracted most attention with regard to carcinogenicity. Some structure-carcinogenicity trends for azo dyes and their metabolites have been discussed with a view to the prediction of dye carcinogenicity [84,85]. If an azo dye is carcinogenic and is relatively stable in the hydroxyazo tautomeric form, the dye itself is likely to be the active carcinogen. In contrast, those dyes that exist predominantly in the ketohydrazone form are more readily reduced to metabolites. In this case, the pro-carcinogen is likely to be an arylamine and the ultimate carcinogenic potential can then be deduced from the availability of a suitable active site on the metabolite. Azo pigments, because of their extreme insolubility and low bioavailability, are unlikely to be metabolised even if they exist preferentially in the hydrazone form [84]. A large majority of water-soluble azo dyes do not form carcinogenic arylamines when reductively cleaved. In most cases, the reduction products are arylaminesulphonic acids, which have little or no carcinogenic potential. Epidemiological studies first alerted the colorants industries to the causal links between certain manufacturing operations and an increased risk of bladder cancer among workers [1,60,86]. Regulations were passed in the 1960s that placed a virtual ban on the importation and use of certain dye intermediates, such as benzidine and 2-naphthylamine, and certain processes, including auramine (1.28) manufacture [87]. Most responsible colorant manufacturers in Europe, Japan and the USA ceased production of benzidine-based dyes in the early 1970s due to inability to ensure their safe handling in dyehouses. However, owing to the attractive economic and technical merits of such dyes on leather and cellulosic fibres, manufacture continued in other parts of the world (for example, Latin America, India and the Asia Pacific region). The risk to workers in manufacturing and dyeing plants in these areas is of concern because of the poor conditions of occupational hygiene that often exist [1].

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1.7.3 Colorants in waste waters The high visibility of water-soluble dyes released to the environment ensures that only extremely low concentrations in watercourses would not be noticed. A typical visibility limit in a river would be about 0.1 to 1 mg/l, but this varies with the colour, illumination and degree of clarity of the water. The human eye can detect a reactive dye concentration as low as 0.005 mg/l in pure water, particularly in the red to violet hue sector [88]. There is considerable debate, however, about what level of environmental hazard is represented per se by colour in effluent. The view has been expressed that dyestuffs should not be regarded as water pollutants because at concentrations of the same order of magnitude as these visibility limits their harmful effects are negligible [89]. Nevertheless, even though this colour problem is mainly if not entirely an aesthetic one, the fact is that the general public will not tolerate coloured amenity water and the problem therefore has to be addressed and rectified [90,91,92]. The main consideration regarding the environmental impact of dye residues is concerned with toxicity to aquatic organisms. This is normally expressed in terms of the LC50 value, which represents the concentration of the substance under test that is required to kill 50% of the organisms exposed. With the exception of a small minority (about 2%, mainly basic dyes), organic dyes generally show only low toxicity to fish and other organisms such as Daphnia magna [48]. Bioaccumulation is also important, defined as the factor F = Ca/Ce, where Ca is the concentration of the pollutant in the fish species and Ce that for the general environment [93]. The partition coefficient (P) of the colorant in an n-octanol/water mixture is used as an indicator of bioaccumulation. If P is less than 1000 it can be predicted that F in fish will be less than 100, at which level no problems are foreseen. Water-soluble dyes do not bioaccumulate and even those disperse dyes and pigments that give P values above 1000 still show no evidence of bioaccumulation in fish [48]. Algae are an important part of the aquatic ecosystem, with algal photosynthesis a critical source of oxygen. The adverse effects of dyes in inhibiting the growth of green algae do not parallel the effects on fish, so that no conclusions about the one can be drawn from the other. Nevertheless, those basic dyes that yield low LC50 values in fish toxicity tests also tend to inhibit algal growth at concentrations as low as 1 mg/l. It is also true that a majority of basic dyes have an inhibitory effect on waste water bacteria, having LC50 values of less than 100 mg/l, so that they tend to have a deleterious effect on the operation of effluent treatment plants (section 1.6.6). In view of the good to excellent fastness of most colorants, it is not surprising that they are not readily biodegradable. Biodegradability may be defined as the degree of decomposition of an organic contaminant after biological treatment under specified conditions [94]. With the brief retention times normally prevailing in effluent plants, there is practically no evidence of biodegradation of colorants under aerobic conditions [53]. Bioelimination includes removal of the colorant by adsorption on the biomass as well as that undergoing biochemical decomposition. A large majority of dyes are adsorbed by the biomass to the extent of 40–80%. High adsorption occurs with basic dyes, direct dyes, disperse dyes and most of the premetallised and milling acid dyes [48,95,96]. The only dye types that are not substantially adsorbed during biological treatment are the highly soluble multisulphonated levelling acid dyes and virtually all reactive dyes, which share similar characteristics.

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1.7.4 Hazardous arylamines Anaerobic conditions, such as apply when digesting sewage sludge or when residual colorants are present in river sediments, favour biodegradative reactions. Under these circumstances the biodegradation of dye chromogens is a primary cause of colour removal. Dyes are quite readily absorbed by sludge, suspended solids or sedimental matter. With azo colorants these conditions render the azo group susceptible to reductive cleavage, giving rise to arylamines as breakdown products [97,98]. There is concern that arylamine metabolites formed under such anaerobic conditions could be desorbed later into the aquatic aerobic environment and thus represent a hazard. The arylamines, however, are generally susceptible to aerobic degradation. Aniline and its monosubstituted derivatives, such as anisidines, phenetidines and toluidines, are readily biodegraded. Diaminobiphenyls, namely benzidine, dianisidine, dichlorobenzidine and tolidine, are more resistant but still inherently biodegradable [99]. A wider selection of arylamine metabolites from azo dyes, including arylaminesulphonic acids, gave broadly similar results [100]. The voluntary cessation of manufacture of benzidine and the dyes derived from it by major dyemakers in the early 1970s created a series of research targets to find replacements with corresponding technical properties. Alternative non-mutagenic diamines were sought and found to yield dyes exhibiting satisfactory performance [101]. Unfortunately, these were almost always substantially less cost-effective than the analogous benzidine-based products they were intended to replace and which were still commercially available from nontraditional suppliers. Following the emergence in the early l990s of conclusive evidence of animal carcinogenicity from CI Acid Red 114 (1.55) derived from o-tolidine and CI Direct Blue 15 (1.56) derived from o-dianisidine, several dye manufacturers ceased production of these and other dyes made from these two diamines. H3C

CH3 H

N O H3C

O

N N

N

SO2 NaO3S 1.55 SO3Na

CI Acid Red 114

NH2

H3CO O

OCH3

H

NaO3S

H2N H

N

O SO3Na

N

N

N NaO3S

SO3Na CI Direct Blue 15

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The German government issued in July 1994 an amended regulation concerning consumer goods involved in direct body contact, including clothing, shoes and bedlinen. This banned the use of azo colorants that could be reduced to give any of twenty specified arylamines. These amines have been classified by the German MAK commission as substances that have been unequivocally proved to be carcinogenic. This regulation embraces the concept that an azo dye that can cleave to yield a carcinogenic arylamine is itself a carcinogen [70]. It is estimated [1] that about 150 commercial azo colorants are carcinogenic according to this definition. Since the German ban, ecolabelling schemes within the EU have adopted the same list of twenty arylamines and added two more that have been classified by the EU as Category 2 carcinogens [102]. A full list of these specified arylamines is given in Chapter 4 (Table 4.1). Since the implications of this unilateral ban by the German authorities began to be realised, it seems to have developed into a prime example of how not to enact legislation in such a commercially and technically complex area. There was an immediate and substantial impact on certain sectors of industry, notably textiles and leather. The globalisation of trade in the l990s ensured that there were many repercussions both inside and outside the EU. No risk analysis had been carried out and there was no consultation with interested parties outside Germany before the ban, in spite of notification procedures required by EU regulations [103]. The Netherlands later enacted a similar ban and several other EU member states [70] are intending to introduce regulations concerning consumer goods involved in direct body contact. Although the 1994 regulation specified consumer goods, suppliers were asked by German retailers to guarantee that all materials supplied were free from the banned dyes. Thus companies higher up the supply chain became involved, if their goods containing such dyes were to be imported into Germany. A major stumbling-block was the lack of an official list of dyes to be banned [104]. Member firms of ETAD quickly undertook to inform their customers and other interested parties if they supplied any of the azo dyes implicated in this legislation [105]. This still left dye users and others uncertain about supplies from elsewhere, especially from colorant merchants who do not manufacture the products that they sell. Until 1997, there was no officially recognised test method [106] to isolate and identify the specified amines from textiles or leather. Analysis was sometimes undertaken by organisations lacking the necessary skills or expertise. It was not surprising, therefore, that spurious results (false positives) were sometimes obtained [103,105]. The early tests carried out with alkaline dithionite at temperatures above 70 °C could give rise to banned amines by reactions other than azo reduction, including reduction of nitro groups [107], hydrolysis of amides and desulphonation of water-soluble amines [108]. In order to minimise the misleading results obtained under these conditions, methods of reduction using buffered dithionite at 70 °C or lower temperatures were developed [108,109]. 1.7.5 Halogenated colorants During the l990s, several environmental agencies and activist groups have argued that the banning of chlorine and all chlorinated organic chemicals will be necessary to protect the environment. The impact of such a ban would be immense, particularly for those organic dyes and pigments that are predominantly dependent on chlorine-containing intermediates leading

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to their manufacture. Approximately 40% of all organic pigments contain chlorine in the pigment itself, although this corresponds to only 0.02% of total chlorine usage. Interestingly, organofluorine compounds do not fall into the AOX classification since the fluoride ion liberated as soluble silver fluoride according to the test protocol is not detected [25]. In the EU and Japan, controls on the discharge of absorbable organohalogen compounds are becoming increasingly severe. Insect-proofing agents for wool, trichlorobenzene carriers for polyester dyeing and reactive dyes of the chloroheterocyclic types certainly fall into this category [65,110]. It seems likely that reactive dyes containing vinylsulphone or fluoroheterocyclic groups will become increasingly important [25]. Many direct, disperse and vat dyes also contain chloro-substituted aryl nuclei [111]. It is to be hoped that rational evaluation of the available evidence will convince regulatory authorities that the mere presence of an inert chloro substituent in a molecule does not mean that it will pose an environmental risk [1]. 1.7.6 Heavy-metal contaminants Heavy metals are widely used as catalysts in the manufacture of anthraquinonoid dyes. Mercury is used when sulphonating anthraquinones and copper when reacting arylamines with bromoanthraquinones. Much effort has been devoted to minimising the trace metal content of such colorants and in effluents from dyemaking plants. Metal salts are used as reactants in dye synthesis, particularly in the ranges of premetallised acid, direct or reactive dyes, which usually contain copper, chromium, nickel or cobalt. These structures are described in detail in Chapter 5, where the implications in terms of environmental problems are also discussed. Certain basic dyes and stabilised azoic diazo components (Fast Salts) are marketed in the form of tetrachlorozincate complex salts. The environmental impact of the heavy metal salts used in dye application processes is dealt with in Volume 2. The toxic effects of trace metals towards animals or aquatic life are highly dependent on the physical and chemical form of the contaminant [112]. For example, dissolved copper(II) or chromium(III) ions are highly toxic, whereas the same atoms coordinated within stable organic ligands such as dye molecules are not harmful. Unfortunately this is not widely acknowledged in setting limits for consent conditions, where the total metal content is often specified rather than the forms in which it is present. The permitted levels for trace metals in dyehouse effluents vary from one country to another and even between different areas in the same country [65,94,113]. Improved dyeing methods that have been developed to minimise release of residual chromium when applying chrome dyes to wool are outlined elsewhere (section 5.8.2). When restrictions on the contamination of effluent by chromium residues were imposed in the 1970s, the initial reaction in the wool dyeing industry was to predict the rapid demise of chrome dyes. This forecast decline did not materialise because of the outstanding fastness of chrome dyeings and the efforts made to minimise effluent pollution [25,65]. In 1996 member firms of the GuT carpet ecolabelling scheme in the EU introduced a voluntary ban against the use of metal-complex dyes on certain nylon floorcoverings. Extension of this ban to all carpets made from nylon, wool or their blends has been predicted [104]. Ecolabelling schemes covering apparel and household textiles must also take account of the presence of premetallised dyes because of the obvious risk that extraction into perspiration or saliva can take place from dyeings of inadequate wet fastness [114].

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1.7.7 Natural and synthetic dyes In Germany during the 1970s there was a growing demand by supporters of the Green movement for greater use of natural dyes of vegetable origin to dye natural fibres such as wool, silk and cotton. This trend has been taken up enthusiastically in tropical countries with climates suitable for growing such dye-yielding plants [115,116], including woad [117], lac dye [118], lichens [119] and even tea plants [120,121]. Research has been carried out with a view to minimising the amounts of mordanting chemicals necessary to apply natural dyes [122–124], in order to offset criticism that such processes would be as polluting as the use of premetallised synthetic dyes. Contamination of effluent streams with residual heavy metals from mordanting [125,126] is by no means the only drawback of a return to colouring methods that prevailed before the discovery of synthetic dyes [127,128]. Natural dyes are tinctorially weaker and duller, are often difficult to dye level and show inferior fastness to light and washing compared with their synthetic counterparts. Although applicable to natural fibres with the aid of mordant chemicals, important synthetic polymers such as polyester or acrylic fibres cannot be dyed using these products [126,129]. Calculations show that about 400 kg of cultivated dye plants are required to dye to the same depth as given by 1 kg of synthetic dye on cotton or wool, at a cost ratio of about 100:1. Furthermore, if the worldwide consumption of dyed cotton were coloured with natural vegetable dyes rather than synthetic ones, approximately 30% of the world’s agricultural land would be needed for the cultivation [125,127]. This is more than 13 times the area currently in use to grow the cotton and does not take into account what would be required if the other textile fibres, paper and leather were also to be coloured in the same way. The extraction of natural dyes from animal sources is just as wasteful of resources, timeconsuming and by no means environmentally friendly. To obtain 1 kg of cochineal scarlet requires the harvesting of 150 000 insects reared on cactus plants. The living insects are swept off the leaves into bowls or cloths and executed by immersion in steam or hot water, then dried by long exposure to sunlight. If production of the classical dye Tyrian purple were to be restored on a large scale, isolation of only 1 kg of this colorant would demand the slaughter of about 10 million specimens of a Mediterranean mollusc (Murex brandaris – now rare). Vast quantities of these discarded shells that develop an obnoxious odour when exposed to the sun would become unsightly spoilheaps, extremely offensive to the coastal environment [104,127].

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B J McCarthy and B C Burdett, Rev. Prog. Coloration, 28 (1998) 61. F Klaschka, Melliand Textilber., 75 (1994) 193. K L Hatch, Text. Chem. Colorist, 30 (Mar 1998) 22. V A Shenai, Colourage, 45 (Oct/Nov 1998) 62. J M Wattie, J.S.D.C., 103 (1987) 304. Clinical and immunological investigations of respiratory disease in workers using reactive dyes (London: HSE, Aug 1986). E A Clarke and R Anliker, The handbook of environmental chemistry, Vol 3A, Ed O Hutzinger (Berlin: SpringerVerlag, 1980) 181. E A Clarke and R Anliker, Rev. Prog. Coloration, 14 (1984) 84. R Anliker in Toxic hazard assessment of chemicals, Ed M L Richardson (London: Royal Society of Chemistry, 1986) 116. V A Shenai, Toxicity of dyes and intermediates, (Bombay: Mirachem Industries, 1996). R Anliker et al., J.S.D.C., 104 (1988) 223. J H Duffus, Cancer and workplace chemicals, (Leeds: H & H Scientific Consultants, 1995). P Gregory, Dyes and Pigments, 7, No 1 (1986) 45. K Hunger, Chimia, 48 (1994) 520. C E Searle, Chem. Brit., 22 (1986) 211. Carcinogenic Substances Regulations 1967 (London: HMSO, 1967). J H Pierce, J.S.D.C., 110 (1994) 131. W Beckmann and U Sewekow, Textil Praxis, 46 (1991) 346. P Cooper, J.S.D.C., 109 (1993) 97. Colour in dyehouse effluent, Ed P Cooper (Bradford: SDC, 1995). Environmental chemistry of dyes and pigments, Eds A Reife and H S Freeman (New York: Wiley-Interscience, 1996) F Moriarty, Chem. Ind., (1986) 737. J Park and J Shore, J.S.D.C., 100 (1984) 383. H R Hitz, W Huber and R H Reed, J.S.D.C., 94 (1978) 71. A J Greaves, D A S Phillips and J A Taylor, J.S.D.C., 115 (1999) 363. K Wührmann, K Mechsner and T Kappeler, Eur. J. Appl. Microbiol. Biotechnol., 9 (1980) 325. V A Shenai, Colourage, 44 (Dec 1997) 41. D Brown and P Laboureur, Chemosphere, 12 (1983) 405. D Brown and B Hamburger, Chemosphere, 16 (1987) 1539. V G Yadav, Colourage, 45 (Jan 1998) 53. P A Turner, J.S.D.C., 111 (1995) 53. K T W Alexander, J.S.D.C., 112 (1996) 341. B C Burdett, Dyer, 180 (Sep 1995) 16. ETAD, J.S.D.C., 111 (1995) 97. K Hübner, E Schmelz and V Rossbach, Melliand Textilber., 78 (1997) 720. C T Page and J Fennen, J. Soc. Leather Tech. Chem., 82 (1998) 75. W B Achwal, Colourage, 44 (May 1997) 29. F Planelles et al, J. Soc. Leather Tech. Chem., 82 (1998) 45. B M Müller, Rev. Prog. Coloration, 22 (1992) 14. W D Kermer, Melliand Textilber., 69 (1988) 586. G M P Morrison, G E Batley and T M Florence, Chem. Ind., (1989) 791. W D Kermer and I Steenken-Richter, Melliand Textilber., 76 (1995) 433. U Sewakow, Text. Chem. Colorist, 28 (1996) 21. S I Ali, J.S.D.C., 109 (1993) 13. P M Chan, C W M Yuan and K W Yeung, Text. Asia, 29 (May 1998) 59. M Ryder, Wool Record, 157 (May 1998) 41. Eco-Fab, Colourage, 45 (May 1998) 79. K D Casselman, Wool Record, 157 (Oct 1998) 57. P M Chan, C W M Yuan and K W Yeung, Text. Asia, 28 (Oct 1997) 58. H T Deo and B K Desai, J.S.D.C, 115 (1999) 224 G Dalby, J.S.D.C., 109 (1993) 9. U Sewekow, Melliand Textilber., 76 (1995) 330. H Bürger, Melliand Textilber., 76 (1995) 910. B Glover, J.S.D.C., 114 (1998) 4. V A Shenai, Colourage, 45 (Jan 1998) 19. B Glover and J H Pierce, J.S.D.C., 109 (1993) 5. B Glover, Text. Chem. Colorist, 27 (Apr 1995) 17. W B Achwal, Colourage, 45 (Jan 1998) 45.

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CHAPTER 2

Organic and inorganic pigments; solvent dyes David Patterson

2.1 PIGMENTS 2.1.1 General introduction – what are pigments? The word ‘colorant’ is defined in dictionaries as ‘a substance used for colouring’ and it can therefore be used to describe both dyes and pigments. Since this chapter is concerned only with pigments it is necessary to find a criterion for deciding whether a given colorant should be treated as a pigment or a dye. In practice this is not as simple as it might seem. One widely used criterion is that of the solubility of the colorant in the material which it is being used to colour. If it is insoluble, it is a pigment; if soluble, it is a dye. The criterion of solubility is insufficient in itself, however. The same colorant can sometimes be a dye and, in other applications, a pigment. Vat dyes are an example of this. They were originally developed as dyes for textiles, but later were prepared in a different physical form for use as pigments. Indeed, it may be argued that since vat dyes are waterinsoluble and to get them to diffuse into fibres they must first be reduced, they should not be considered as dyes at all, but as pigments. Again, the aqueous solubility of many disperse dyes is so low that various devices have to be used to induce them to diffuse into polyester fibres. From all of this, it is fair to say that there is no infallible simple criterion for deciding whether a colorant is a dye or a pigment. It is always necessary to take into account the actual use to which the colorant is being put. A further, and most important, difference between pigments and dyes is that pigments are used as colorants in the physical form in which they are manufactured. Here, physical form means both the crystal structure and the particle size distribution of the pigment. The physical form of dyes is becoming of increasing importance as methods of handling them become more automated. In most dyeing processes, however, the dyes are first dissolved in water and their physical form is thereby destroyed. Thus physical form is not generally of such overriding importance as it is in the case of pigments. Because of the requirements of insolubility in water, in organic solvents and in the medium that it is being used to colour, the application processes for using pigments are quite different from those for dyes. Coloration with pigments is essentially a process of dispersion of solid particles of the pigment in a semi-solid medium. Entire technologies are involved in each of the three largest applications of pigments, which are as colorants in paints, plastics and printing inks [1]. Each of these media is composed mainly of organic constituents and the broad general rule that ‘like dissolves in like’ gives rise to considerable technical problems in some cases, particularly where entirely covalent organic pigments are being used. However, since some coloured pigments are

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inorganic, as are all white pigments, many problems of migration and colour bleeding can be solved by changing from organic to inorganic pigments. The chemical nature of most organic pigments is closely similar to that of the synthetic dyes that have been discovered during the past 150 years. In fact, with the exception of the phthalocyanines, almost every chemical class of pigments has been developed first for dyestuff use. There are some signs that this may not continue to be the case, with the development of some new organic pigments for specialised uses. Finally, passing mention must be made of the two most important organic pigments in our world, both natural products. These are chlorophyll and haemoglobin, which are absolutely vital in the strict meaning of the word, but only chlorophyll has found a commercial use as a colorant in food preparation. 2.1.2 The background history of pigments The discoveries of archaeologists indicate that the first pigments used by mankind, mainly to decorate both himself and his possessions, were the earth pigments. These are widely distributed throughout the world, are highly resistant to decomposition by heat, light and weathering, and indeed without these properties would not have survived through the centuries. Earth pigments were probably first recognised simply because their colour stood out when hard lumps of rock were examined. Such rocks were broken up and the desirable coloured bits picked out. The coloured pigments were then ground to a fine powder and blown onto the painting surface using a hollow tube, or mixed with fatty materials to form a kind of crude paint that was applied with the fingers or a reed. The prehistoric cave paintings found in parts of Spain and France were made in this way. Examples of such earth pigments are the bright red pigment vermilion (mercury sulphide), the yellow orpiment (arsenic trisulphide), the green malachite (basic copper carbonate) and the blue lapis lazuli (natural ultramarine). There are many natural sources of white pigments such as chalk and kaolin, while black pigments could be obtained as charcoal from incompletely burnt wood and as soot in smoke from burning oils. The Colour Index gives more examples, as do Kearton [2] and Skelton [3]. For many centuries, pigments have been derived from the colouring matters found occurring naturally in many plants and even in some animals. Examples are the red pigment madder and the blue indigo, both extracted from plants, cochineal and lac lake both from insects, the much-prized Tyrian purple derived from certain shellfish, logwood extracts ranging in colour from red to brown or black, depending on the precipitant, and finally sepia obtained from cuttlefish [4]. The methods used for making pigments from these and other natural dyes were more like recipes than scientific procedures and were probably derived from the work of alchemists and herbalists. The former spent their lives trying to prepare gold by dissolving all kinds of cheap substances in acids and then re-precipitating them (hence their discovery of many precipitants), while the latter sought to extract compounds of medicinal value from plants and some of their extracts must have included natural dyes. Among the precipitants employed were tannic acid, tartar emetic, rosin soaps, fatty acid (stearic, oleic) soaps, sulphonated oils (Turkey red oil), ‘earth lakes’ (mixed natural silicates), phosphates, casein and arsenious acid. The fastness properties of these pigments

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were relatively poor, both to water and to light. Furthermore, the colour of the pigments made by these methods largely depended on the particular precipitant or combination of precipitants used, a fact also exploited by medieval dyers to extend the range of colours that they could produce from the very limited number of natural dyes available to them. Many of the methods for making these pigments [5] are of historical interest only, except for a few which are mentioned later. 2.1.3 More recent developments The industrial revolution brought about in the 19th century the rise of the chemical industry, which is such an important part of the world economy today. The discovery in the 1850s of the first synthetic dye by William Perkin and of the diazo reaction by Peter Griess started the search for other synthetic colorants, leading eventually to their almost complete market dominance. Many of what are now the largest international chemical companies began their activities in this area, later expanding into plastics and pharmaceuticals. For over a hundred years research aimed at synthesising new organic colorants was actively pursued. Patents were filed and new products appeared on the market. Thousands of new compounds were prepared and tested so that tens of new colorants, generally dyes, could be introduced commercially. Some of these were also tested for suitability as pigments, but few had all the properties required. Only the organic phthalocyanine and the inorganic titanium dioxide pigments have gone into large-scale production directly as pigments without being first used as dyes. Certain commercial developments in the last twenty years or so have had important effects on the pattern of pigments production. One is the rapid growth of textile production in the Asia Pacific region and another is the development of automated dyeing methods, coupled with instrumental methods of colour measurement and of computerised colour recipe prediction. Colorimetric prediction is equally applicable to pigment applications and this has directed attention to the possibility of rationalising the ranges of dyes and pigments needed. In theory, it should always be possible to match any shade by mixing three or four suitable colorants. This is probably one of the reasons for the reduction in the number of pigments commercially available. For example, the number of different yellow pigments allocated Colour Index numbers and listed in the 1982 edition of Pigments and Solvent Dyes was 166, but this had fallen to 121 in the 1997 edition. Of these, no less than 26 are inorganic, including 8 new entries. These are mixed oxides of various metals including titanium, zinc and vanadium intended for ceramic coloration and are mentioned later. Other factors, such as the increasing use of pigments in textile printing, to such an extent that pigments now comprise nearly half of the colorants used for this purpose, means that the textile industry is an important market sector for pigments. The geographical shift of the textile industry has resulted in extensive reorganisation of some of the largest international companies that in the past have dominated the manufacture of colorants and some longestablished commercial names have disappeared as a result. Within the pigments industry itself, there has been a growing realisation that the physical state of pigments is of the greatest importance to pigment users. Most suppliers now offer several products based on the same chemical structure and the physical form in which it is supplied is chosen to suit the intended application. Thus, the 1997 edition of Pigments and

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Solvent Dyes shows for each product, not only the application but also in which of eight different physical forms it is supplied. These forms are: powder, presscake, granule, chip or flake, liquid dispersion, paste, master batch, flush colour. Not all pigments are offered in all these physical forms, since for a particular application this may be neither possible nor necessary. To assist users, manufacturers frequently give a separate commercial name to each form in which they supply the pigment. With the exception of the powder form, this means that they will generally contain auxiliaries such as dispersing agents, chosen to assist in the process of incorporating them into the particular medium for which they are intended. The increase in colour printing of newspapers and periodicals, in colour printing on various plastics for the packaging of foods sold in supermarkets, as well as the introduction of colour printers for use with personal computers, have all presented many technical problems in formulating coloured inks for these purposes. In Pigments and Solvent Dyes, CI Pigment Yellow 1, which has been in commercial production for a century, now has no less than 96 entries of commercial products based on it. This illustrates its widespread use in printing as well as in other applications, but also is a symptom of the tendency to contract the range of chemically different pigments in production. Other recent developments, including fluorescent pigments, polymeric pigments and the problems which have arisen concerning health hazards from the use of benzidine-based intermediates in making certain pigments, are mentioned later in this chapter. It should also be mentioned that the structural formulae of the azo pigments discussed in this chapter now take account of the investigations by Whitaker [6] into the crystal structures of these pigments. These have shown the importance of hydrogen bonding between the pigment molecules in the solid state. The molecular formulae and the chemical synthesis stages of the preparation of these pigments remain unchanged. 2.2 DYES CONVERTED INTO PIGMENTS 2.2.1 Dyes converted by precipitation on substrates – ‘lakes’ With the introduction of synthetic dyes, attempts were made to prepare pigments from them by methods based on those that had been used with the natural dyes. Many soluble azo dyes can be rendered insoluble by precipitating them as the salts of heavy metals in the presence of so-called alumina hydrate. This method will be treated in detail later and here only the making of pigments from acid dyes and basic dyes will be mentioned. The use of alumina hydrate as a basic substrate for making pigments from dyes is now, however, of diminishing commercial importance. Alumina hydrate is made by slowly adding a 10% solution of sodium carbonate to a wellstirred 10% solution of aluminium sulphate, thus producing a white gelatinous precipitate of aluminium hydroxide. This is colloidal in nature and even prolonged washing does not remove all the sulphate ions adsorbed on it, but by careful control a consistent product with a high absorption power for precipitated dyes can be made. The precipitants used were antimony, barium and aluminium salts in the presence of rosin. In the USA alone, some 400 tons p.a. of methyl violet (CI Basic Violet 3) were precipitated by these methods for use on copying paper and in typewriter ribbons. The advent of computers with printers and word processing facilities has made such products obsolescent.

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Other dyes that were extensively precipitated for use in printing inks were erioglaucine (2.1) and eosine (2.2) but their light fastness is too low to be acceptable nowadays. The process was called ‘laking’ and the products were termed ‘lakes’. CH2CH3

H3CH2C

N+ CH2

C

N CH2 NaO3S

SO3Na

SO3–

2.1 CI Acid Blue 9 Br HO

Br O

O

Br

Br COOH

2.2 CI Acid Red 87

2.2.2 Dyes converted by precipitation with complex acids – ‘toners’ In the early years of the 20th century efforts to improve the properties of pigments prepared from basic dyes took a fresh direction: precipitation of the pigments using the mordants that were employed when the same substances were being used as dyes. The light fastness of the resulting pigments remained poor, however. Another approach was much more successful. It had been noted in biochemical work that some amines could be precipitated by treatment with acid in the presence of sodium phosphate and sodium tungstate or molybdate. When these precipitants were tried on basic dyes, pigments were produced with a higher light fastness than that of the parent dyes. This important discovery was patented by the Bayer company in Germany just before the First World War, but the shortage of tungsten and molybdenum resulting from the demand for these metals for use in armoured steel during the war delayed the exploitation of this discovery until peace returned. Between the two world wars it was found that there was no need to isolate the complex acids used as precipitants; they could be used in solution. The use of mixed solutions of phosphotungstic and phosphomolybdic acids was also studied, as well as the effects of changing the pH of the solutions used and of various heat treatments after precipitation on the colour of the pigments produced. The Second World War again resulted in a shortage of tungsten and molybdenum and in an effort to overcome this the so-called ‘copper toners’ were made by precipitating basic dyes with copper(II) hexacyanoferrate. The resulting products were less brilliant in colour

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than their predecessors and lower in strength and light fastness. Despite being cheaper, they found only limited use when supplies of tungsten and molybdenum resumed after the war had ended. At that time, manufacturers typically produced ranges of about fifteen pigments of this chemical type, comprising blues, greens, yellows and reds. Some were made by precipitating mixtures of dyes; for example, blue pigments of increasingly greener hue could be prepared by adding minor but increasing amounts of a yellow dye to a blue dye. Commercially significant differences in technical properties, as well as price, could also be achieved by changing the ratio of tungsten to molybdenum in the complex acids. The chemistry of these complex phosphotungstic (PTA), phosphomolybdic (PMA) and mixed (PTMA) acids was not fully understood until X-ray diffraction methods were used to determine their structures in the early 1950s. This work has shown that the structure of these compounds is built up from WO6 octahedra. By sharing two oxygen atoms a W2O10 unit (which does not exist independently) can be formed. In the phospho-12-tungstic acid the sharing process is continued so that there are four W3O9 (hypothetical) units clustered around the central PO4 group in the form of a tetrahedron. The overall structure of the anion is shown in formula 2.3. Since the phosphorus atom has five positive charges, the overall negative charge on the ion is three, i.e. [P(W3O10)4]3–. The hydrated free acid is thus phospho-12-tungstic acid, i.e. H3[P(W3O10)4].14H2O. There are other complex phosphotungstic acids known with different phosphorus:tungsten ratios, but only the 1:12 series has found use in pigment making. Since tungsten and molybdenum are isomorphous in these compounds, the use of any ratio of these two metals is possible theoretically in preparing the complex acids. In practice, ratios of 85:15 or 60:40 seem to have been found satisfactory for most purposes. Molybdenum is less expensive than tungsten and thus it might seem advantageous to use lower ratios, but the pigments thus produced tend to darken on exposure to light, an undesirable property. The ratio of dye cations to complex anions in the pigments is 4:1. The only examples of pigments of this type currently made are CI Pigment Blue 1, Greens 1 and 2, Violets 1, 2 and 3, and Red 81.

O O

W

O O

O

O

W

O O

W

O

O

O

P

O

W

O

W

O

W

W

O

O O

O

W

O

O

O

O

O

O O

O

O

2.3

W

O

O

O

O O

W

O

O O

O

W

O O

W

O

O

Phospho-12-tungstic acid ion

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2.2.3 Vat dyes converted into pigments Although vat dyes may be regarded as pigments in view of their water insolubility, they were first used as dyes. When prepared for dyeing, their particle size is an important technical factor that influences their rate of reduction (section 3.1.4). Particle size was found to be even more important when attempts were made to use vat dyes as pigments. Commercial success was only achieved when methods for further decreasing and controlling the particle size were found, so that bright and tinctorially strong pigments resulted. The impetus for these developments came mainly from the need for pigments of high light fastness in car finishes, particularly when light-coloured cars became fashionable in the 1950s. The necessary white pigment (titanium dioxide) was available, but to make the desired off-white paints small amounts of coloured pigments had to be mixed with the titanium dioxide. In such formulations many pigments, particularly azo pigments, show much diminished light fastness, from ISO ratings of 7 to ratings of 3 or 4. The light fastness of vat dyes is generally good, so they were tried out for this purpose. In the physical form available at the time the dullness and poor tinctorial strength shown by many vat colours was unacceptable. Work was put in hand to alter the purity, particle size distribution and in some instances the crystal modification to make them more suitable for pigmentary use. Vesce [7] of Harmon Colors and Gaertner [8] of Ciba-Geigy have given accounts of these developments. The chemical classes of vat colours from which suitable pigments have been developed are anthraquinones, perinones, perylenes and thioindigos. Many vat dyes have been tested, but only about a dozen have met the stringent fastness standards demanded. Seven of those found suitable in all respects are shown in Table 2.1 and their chemical structures are given as 2.4 to 2.10. The methods used to convert these vat dyes into a suitable physical form (and in some cases, crystal structure) for use as pigments have been carefully guarded industrial secrets, revealed only in patents. The general principles are clear, however. One method is to reduce the vat dye in the usual manner to bring it into solution and then to re-precipitate it under very carefully controlled conditions. The other is to subject the dye to a fine grinding operation. Whichever approach is used, the aim is to reduce the mean particle size to below 1 µm (1000 nm).

Table 2.1 Typical pigments prepared from vat dyes Structure No

Common name

CI Vat

CI Pigment

2.4 2.5 2.6 2.7 2.8 2.9 2.10

Flavanthrone Yellow Anthanthrone Orange Anthrapyrimidine Yellow Indanthrone Blue Isoviolanthrone Violet Perinone Orange Perylene Maroon

Yellow 1 Orange 3 Yellow 20 Blue 4 Violet 1 Orange 7 Red 23

Yellow 24 Red 168 Yellow 108 Blue 60a Violet 31 Orange 43b Red 179

a α-form only b trans form only

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ORGANIC AND INORGANIC PIGMENTS; SOLVENT DYES

O

O Br

N N Br O

2.4

2.5

CI Pigment Red 168

O CI Pigment Yellow 24

O

N

N

O O N

H O

N

H

N

O

O H

O 2.7

2.6 O

CI Pigment Yellow 108

CI Pigment Blue 60

Dichloro derivative of O

O 2.8

CI Pigment Violet 31

H3C

N

O

N

N

O

N

CI Pigment Orange 43

O

O

N

N

O

2.9

CH3

O 2.10 CI Pigment Red 179

Dry grinding can be carried out in two ways. The first comprises dry grinding in the presence of an inert substance such as salt, which can be removed by aqueous washing when the particle size of the dye has been sufficiently decreased. Alternatively, grinding in the presence of an organic solvent can be used. It is possible that local heating under the intense shearing conditions in the mill causes the dye particles to pass temporarily into solution. On

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release from the local high-temperature zone the solubility limit is exceeded and the dye comes out of solution at a smaller particle size and in some cases with a different crystal structure. Sometimes hybrid processes of grinding in the presence of both salt and an organic solvent have been used. More detailed information has been published about the details of the dry grinding method in relation to phthalocyanine and quinacridone pigments, rather than vats, and this will be mentioned later. The development of effective grinding processes for pigments, where the required mean particle size is much smaller than the wavelength of visible light (400–650 nm), was delayed because visible light microscopy could not be used. The desired size of the particles is below the resolving power of these instruments. The increased resolving power of ultraviolet microscopes was of some help, but still not sufficient for effective monitoring of the comminution processes. Centrifugal and surface area measurements were also useful, but laser light scattering, and in particular scanning electron microscopy, finally enabled the processes to be fully monitored. Although instruments for carrying out this research are expensive, together with X-ray diffraction they enabled both the size and shape of the particles and their crystal structure to be defined. The importance of pigment particles of submicron size is considered later (section 2.11). 2.3 AZO PIGMENTS Azo pigments comprise by far the largest chemical class of compounds from which pigments are made, reflecting the wide range of aromatic amines that can be diazotised and the numerous compounds to which the resulting diazo compounds can be coupled, not all of course yielding pigments. The hues of the azo pigments range from red, yellow and orange to blue, in descending order of numbers of each hue. The diazotisation and coupling reactions were exploited by the firm of Read Holliday in 1880 with the discovery of Vacanceine red, the first of the ‘ice colours’. Cotton pretreated with 2-naphthol was immersed in a solution of diazotised 2-naphthylamine, prepared in the presence of crushed ice. Para red (2.11), however, patented by the same company in 1887, achieved much more success. This was applied to cotton in the same way using 2-naphthol and diazotised p-nitroaniline. Para red was also manufactured and marketed as a useful azo pigment. H O2N

O

N N

2.11

CI Pigment Red 1 Para Red

Since the chemistry of the reactions involved in making both azo dyes and azo pigments is the same, discussion of these details is confined to Chapter 4. Only the structures of azo pigments and the special features of their preparation, largely concerned with preparing the pigments in their optimum physical form appropriate to their end use, are considered here. To do this, a classification of azo pigments is needed, as shown in Figure 2.1.

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Azo pigments

Metal-free

Metal-containing Chelates (group 3)

Water-insoluble (group 1)

Water-soluble (group 2)

Low solubility (group 2a)

High solubility (group 2b)

Figure 2.1 Classification of azo pigments

The primary division separates azo pigments into those that are metal-free and those that contain a metal atom in their structure. This means that those in the former group are entirely covalent in nature while those in the latter are not. An entirely covalent molecule possesses one property that is highly desirable in a pigment: insolubility in water. At the same time, such molecules are to some extent soluble in the organic media and solvents in which these pigments are generally used – paints, printing inks and plastics. The result is that unmetallised azo pigments have a tendency to diffuse in these media, giving rise to colour changes and poor fastness properties. The need to make azo pigments that are metal-containing arises from the requirement to reduce the aqueous solubility of some azo colorants to a sufficiently low level to make them usable as pigments. The problem arises when either the amine or the coupling component (or both) from which the colorant is prepared contain substituents such as hydroxy or acidic groups. Depending on their number and location in the molecule, these can confer some degree of aqueous solubility on the resulting azo compound. The means adopted to reduce this solubility to an acceptable level depends on its degree, causing the pigment to fall into either group 2a or group 2b of the classification shown in Figure 2.1. All the pigments in group 2b are made from azo dyes. Those in group 3, of which there are only very few, are metal chelates. 2.3.1 Metal-free azo pigments Pigments in group 1 made from 2-naphthol Para red (2.11), which has already been mentioned, was the earliest pigment of this type. Toluidine red (CI Pigment Red 3) was first made in 1905. To make it, the amine m-nitro-ptoluidine (called MNPT in the pigments industry) is diazotised and coupled to 2-naphthol, the same coupling component that was used to make Para red. There is probably more published information about the making of CI Pigment Red 3 than any other pigment, since in BIOS report 1661 [9] no less than six detailed works

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processes are described. While all these processes yielded pigments with the same chemical structure, the hue and other properties of the resulting products differed sufficiently for them to be sold under different trade names. Since one of the recurrent problems of the industrial production of azo pigments is to keep the coloration properties of the various batches constant within very close limits, it is useful to see the effects of changes in the process variables. They are set out in Table 2.2 [10]. In order to see which of these many variables had the greatest effect on the pigment properties, Hildreth and Patterson [11] investigated the effects of changing the following variables on the colour of the resulting pigments: (a) concentration of coupling component (b) pH of coupling (c) temperature of coupling (d) effect of the use of dispersing agent in the coupling bath. The effects of the changes in the preparation methods were assessed by measuring the colour of the pigments in draw-downs and the particle size distributions of the powders. It was found that changes in all the listed variables could produce changes of several NBS

Table 2.2 Batch processes for preparation of Toluidine Red (CI Pigment Red 3) Helio Red RBL

Lithol Fast Scarlet GRN

1 2–5 0.59 No

1.25 0 0.71 No

3.3 0 1.1 Yes

1.25 0 0.71 No

0.25

0.35

0.25

No 0.061 soln. 10

Yes (HCl) 0.245 suspn. 8

No 0.082 soln. 10

Yes (HCl)

10 above surface 2.5

25 above surface 1

25 above surface 2.5

36 below surface 3

4. Subsequent treatment Heating after coupling (°C) 50 Washing water (2 hr)

none water

90 none

95 (30 min) water

70 (2.5 hr) water

Drying temp (°C)

50–55

(paste made)

45

50–55

1. Diazotisation m-nitro-p-toluidine suspn. (mol/l) Temperature (°C) Diazo conc. (mol/l) Carbon addition

2. Coupling component Conc. of soln. of 2-naphthol in caustic soda (mol/l) 0.36 2-naphthol, whether precipitated Yes (HCl) 2-naphthol conc. at 0.21 coupling (mol/l) suspn. pH at coupling 5–8 3. Coupling conditions Temperature (°C) Method of adding diazo component Time taken (hours)

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50–55

55

Lithol Fast Scarlet RB

Lithol Fast Scarlet RN

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1.25 0 0.71 No

suspn. –

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ORGANIC AND INORGANIC PIGMENTS; SOLVENT DYES

units of colour difference (one unit being about the commercial limit of acceptability between pigment batches). The tinctorially strong pigments were those with the largest proportion of particles below 500 nm in size and the largest surface area. The need for the strictest possible control of process variables to reduce batch-to-batch variability was clearly shown, but the variables needing the closest control were not the same for all the pigments investigated. Both Para red and CI Pigment Red 3 are prepared using 2-naphthol as the coupling component. Other pigments made from the same coupling component, all discovered before 1910, are compounds 2.12 to 2.14 (Table 2.3). All these pigments, except CI Pigment Orange 5, are made by direct diazotisation of the appropriate amine using sodium nitrite and hydrochloric acid at temperatures around 0 °C, then adding the diazotised amine (the ‘diazo’) to an alkaline solution of the coupling component. There may be problems in carrying out diazotisation because of the hydrophobic nature of many aromatic amines. Two methods of overcoming this are frequently used. Either the hydrochloride of the amine is first formed, by stirring it with concentrated hydrochloric acid, or the particle size of the amine is reduced by dissolving it in concentrated sulphuric acid and then re-precipitating. Both methods aim to increase the rate of diazotisation, which tends to be slow on account of the low temperature and the small surface area for chemical attack, particularly if the amine particles have aggregated because of unfavourable storage conditions or other reasons. Another technical problem in diazotising these hydrophobic amines is how to decide when diazotisation is complete, especially when the diazo component is sparingly soluble

Table 2.3 Typical group 1 azo pigments made from 2naphthol Structure No

Commercial Name

CI Pigment

2.12 2.13 2.14

Fire Red Red Toner Permatone Orange

Red 4 Red 6 Orange 5

NO2

Cl H O2N

H

O

Cl

N

N N

N

2.12

2.13

NO2 H

CI Pigment Red 4

O2N

O

CI Pigment Red 6

N N

2.14 CI Pigment Orange 5

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O

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and therefore appears in the form of a suspension. It is essential that the amine should be completely diazotised, because any unreacted amine will remain in the precipitated pigment with adverse effects on the colour and fastness properties. Diazotisation is complete if a test shows that free nitrous acid is present after sufficient reaction time has elapsed, as indicated by experience. A further test is carried out a few minutes later to confirm that a small excess still remains. The presence of nitrous acid produces an immediate and intense dark blue colour on starch-iodide paper when the diazo solution is spotted onto it. If this colour is not obtained the cause may be lack of acid or of nitrite. The amount of acid is increased if a test with pH paper shows insufficient to be present. Retesting with starch-iodide paper will then indicate whether more sodium nitrite is needed or not. The amine that has to be diazotised to make CI Pigment Orange 5, namely 2,4dinitroaniline, cannot be diazotised by the usual direct method. Nitrosylsulphuric acid (HSO4.NO) is needed and this is so corrosive that lead-lined vessels must be used for handling it. Even though this pigment has an unusually high light fastness in reduction (5–6 at 1/25 International Standard Depth, which is two units higher than most of the others in this group), the high costs of its production have led to a decline in its use. Pigments in group 1 made from acetoacetanilides In the early 1900s several yellow pigments were made by coupling substituted nitroanilines to acetoacetanilides. They are commonly known as the Hansa Yellows (2.15–2.18); those that are greener in hue are identified by the increasing number before the ‘G’ in the suffix to their names (Table 2.4). The G (CI Pigment Yellow 1) and 10G (CI Pigment Yellow 3) pigments are the most widely used, the latter being much greener than the former but having only about one-third the tinctorial strength. The light fastness of all these pigments is 7 in full strength but falls to 4 in 1/25 International Standard Depth, with the exception of CI Pigment Yellow 3 which is rated 6+. This high light fastness in reductions makes CI Pigment Yellow 3, in the words of one of its many manufacturers: ‘an inexpensive pigment with good to very good light fastness’. It is available in many physical forms and finds extensive use in paints and printing inks. Disazo pigments made from substituted benzidines Benzidine is an aromatic diamine that can be tetrazotised and coupled to two molecules of a coupler such as acetoacetanilide. This gives a yellow disazo pigment that can be regarded as a double molecule of the monoazo pigments described above. The possibilities of making a

Table 2.4 Typical group 1 azo pigments made from acetoacetanilides

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Structure No

Commercial Name

CI Pigment

2.15 2.16 2.17 2.18

Hansa Yellow G Hansa Yellow 3G Hansa Yellow 5G Hansa Yellow 10G

Yellow 1 Yellow 6 Yellow 5 Yellow 3

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ORGANIC AND INORGANIC PIGMENTS; SOLVENT DYES

H3C

H3C

O N

H

N

H3C

N H

O N

H

N

Cl

O

N H

O

NO2

NO2 2.15

2.16

CI Pigment Yellow 1

CI Pigment Yellow 6

H3C H3C

O N

O

NO2

H

N

N

N H

O

Cl

H N

N H

NO2

Cl

O

2.17 2.18

CI Pigment Yellow 5

CI Pigment Yellow 3

range of colorants were well researched and both dyes and pigments of this general structure were produced. Of the commercial pigments, most were made from 3,3′-dichlorobenzidine or 3,3′-dimethoxybenzidine (o-dianisidine) but this is not the case with some of the disazo dyes. The hues of these yellow disazo pigments are generally redder than those of the corresponding monoazo pigments made from the same coupling components, but their main advantage is that their tinctorial strength is three or four times greater. This is particularly useful in printing ink applications, where the flow properties of the inks depend on the loading of pigments in the formulations. Disazo yellows have another advantage over monoazo yellows in that they show less blooming in plastics. Blooming is a tendency of pigment molecules to migrate to the surface of the plastic articles that they are used to colour. It can occur in moulding processes and also more slowly when in use. In either case it leads to colour loss and sometimes to contamination of articles coming into contact with the surface. The reason for the better performance of the disazo pigments is that their doubled molecular size leads to slower diffusion in plastics, but the matter is complicated by the presence of organic plasticisers and other additives in some types of plastics. On the other hand, the light fastness of the disazo yellow pigments is usually lower than that of the monoazo types. The most widely used pigment of this type is CI Pigment Yellow 12 (2.19), made by direct tetrazotisation of 3,3′-dichlorobenzidine as the hydrochloride, followed by coupling of one mole of the product to two moles of acetoacetanilide. During the tetrazatisation reaction the green solution becomes yellowish brown. The analogous structure CI Pigment Yellow 13 (2.20) is made by tetrazotising 3,3,’-dichlorobenzidine and coupling to acetoacet-m-xylidide. It has found wide use in printing inks, including water-based formulations, and opaque and transparent types are available.

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H3C Cl

O

H

O

N N

N H

H

N

N

N H

O

O

Cl CH3

2.19 CI Pigment Yellow 12

H3C H3C O

Cl O H3C

H N

N

N

N

H

H

N N H

O

CH3

O

Cl CH3

2.20 CH3

CI Pigment Yellow 13

The influence of mixed coupling on the properties of CI Pigment Yellow 12 has been studied recently [12]. Carboxy- or sulpho- substituted derivatives of acetoacetanilide were evaluated as co-coupling components and analysis revealed that the state of the crystal and the particle size were changed and new diffraction peaks were observed. When these modified pigments were treated with a fatty amine such as stearylamine, the hydrocarbon chains enclosed the anionic groups in the co-coupler so that properties such as flowability, wettability and dispersibility in nonpolar solvents were greatly improved. The undoubted technical merits of these pigments have been overshadowed, however, by the fact that they are simple derivatives of benzidine and therefore possibly carcinogenic. In 1971 the major European companies voluntarily ceased manufacture of dyes based on benzidine. Some of the subsequent results were unforeseen. As part of their consumer protection laws, the German government in 1994 framed some regulations in which the wording was not very precise, in particular on the point about whether the regulations apply to pigments as well as to dyes. There have been some amendments to clear up this point and a 1996 amendment excluded poorly soluble pigments with a relative molecular mass greater than 700. There is now a list (Table 4.1) of 22 different amines (including the diamine used for the preparation of CI Pigment Yellows 12 and 13). If a colorant, on reduction by a standard test method, can be shown as a possible source of any of these amines, then its use is prohibited. As with other apparently simple applications of scientific testing to legal matters (such as vehicle driving with a blood alcohol value above the legal limit) the sensitivity of the test method used and the test errors can give rise to problems. The whole matter is by no means finally settled, but remains important because Germany is both within the European Union and an important market for colorant producers throughout the world. The above restrictions on the usage of colorants yielding the banned amines apply equally to imported supplies.

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ORGANIC AND INORGANIC PIGMENTS; SOLVENT DYES

Pigments made from derivatives of 3-hydroxy-2-naphthoic acid (BON acid) In about 1910, problems were encountered when pigments made using BON acid as the coupling component were reduced with alumina hydrate to make printing inks. The attractive bluish red of the pigments turned brown, according to Zitscher [13]. The difficulty was overcome by using anilides of BON acid instead of the acid itself. Several new pigments were introduced, the so-called Permanent Reds, only one of which, CI Pigment Red 2 (2.21), is currently made. The anilide of BON acid was marketed in 1912 as Naphtol AS (CI Azoic Coupling Component 2) for use in azoic dyeing and printing. In the 1920s the hue range of pigments with this type of chemical structure was further extended by using other Naphtols as coupling components. These pigments have a light fastness of about 7 in full strength. Being rather complex hydrophobic structures they tend to have poor solvent fastness properties; this is a considerable disadvantage in the formulation of paints for vehicles, which are applied by spraying methods and are thinned with solvents. Two pigments discovered at this time and still in production are CI Pigment Brown 1 (2.22) and the blue disazo pigment CI Pigment Blue 25 (2.23), made by coupling tetrazotised o-dianisidine to Naphtol AS.

H3CO Cl

H H

Cl

N

O

H H

O

N

OCH3 N

O

O

N N

N

Cl

Cl 2.21

2.22

CI Pigment Red 2

CI Pigment Brown 1

H3CO

O N

O

N

H

H

N

N

H H

N OCH3

O

N O

2.23 CI Pigment Blue 25

Azo condensation pigments Although metal-free azo pigments have good chemical resistance their completely covalent nature leads to a tendency to bleed in organic solvents and to migration and blooming in

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plastic formulations. It was found that the problems of migration and blooming decreased when the overall size of the molecule was increased. Attention turned to the possibility of doubling the size of the pigment molecules by first condensing one mole of benzidine with two moles of BON acid to form a dinaphthol and then coupling this to two moles of diazotised aromatic amine, as shown in scheme 2.1. Difficulties in the synthesis were soon encountered, because after one molecule of diazotised amine has coupled the solubility of the product in water is so low that it comes out of solution and does not couple with the second molecule of diazotised amine. HO

COOH

+

H2N

NH2

+

HOOC

OH

1. condense

HO

HNOC

CONH

OH

dinaphthol

2.

HO N

to two moles of diazotised amine

couple

CONH

+ N

2

HNOC

N

OH N

N

N

Scheme 2.1

The solution to this difficulty, discovered by Schmid of Ciba in the early 1950s, is to reverse the order of the two steps. The diazotised amine is first coupled with BON acid and then two moles of this intermediate are condensed with one mole of benzidine in the form of its hydrochloride. The condensation is carried out in a high-boiling solvent such as nitrobenzene, when yields of 95% can be attained. These pigments have been called ‘azo condensation pigments’ in view of their method of synthesis.

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ORGANIC AND INORGANIC PIGMENTS; SOLVENT DYES

Gaertner [8] has given some interesting insights into the problems of developing new products in a field of organic synthesis that has been the subject of well-funded and intensive research for nearly one hundred years. He stated that the Ciba company had used their new method to prepare well over 10 000 speculative samples. Those selected for development were given the commercial name of Cromophtals and initially eleven new pigments were marketed. Not all pigments now termed Cromophtals are of this chemical type, however, and allowing for this there are now about ten azo condensation pigments being sold. This illustrates the large expense involved in introducing new technical products and the difficulty of finding a place for them in a very competitive market, particularly as their initial selling price must be fixed at a level high enough to recoup their development costs. New pigments made from intermediates that are already in large-scale production seem to have the best chance of competing with existing products. The small range of azo condensation pigments has become established only because they combine outstanding fastness to light, heat, solvents and chemicals, finding acceptance for use in paints, printing inks and the coloration of plastics and some synthetic fibres. Benzimidazolone pigments A further solution to the problem of reducing the reactivity of the carboxyl group in BON acid, when this compound is used as a coupling component in the preparation of azo pigments, was found in 1964 when the Hoechst company introduced their benzimidazolone pigments. A series of new Naphtol AS derivatives was made by condensing 5aminobenzimidazolone (2.24) with BON acid. Other new coupling components have been made by condensing this compound with acetoacetanilides, which are used as coupling components for the Hansa Yellow pigments mentioned earlier. H N CO N H

H2N 2.24

5-Aminobenzimidazolone

Two examples of benzimidazolone pigments are CI Pigment Yellow 120 (2.25) made by diazotising dimethyl 5-aminoisophthalate and coupling with N-(2-oxobenzimidazol-5yl)acetoacetamide and CI Pigment Brown 25 (2.26) made from diazotised 2,5-dichloroaniline and 2-hydroxy-3-N-(2′-oxobenzimidazol-5′-yl)naphthalamide. Benzimidazolone pigments have given improved migration fastness and lower bleed in paint and PVC formulations, possibly because of hydrogen bonding between the carbonamide groups in adjacent molecules in the crystal lattice of such compounds. Pigments made from pyrazolones The last group of metal-free azo pigments are the pyrazolone compounds, the most commonly used examples being made from the coupling component 1-phenyl-3methylpyrazol-5-one, also used in making azo dyes. The two most important pigments are

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AZO PIGMENTS

H

H3C H3COOC

O N

O N

H

N

N

N H

O

H

H

H3COOC

N

H

Cl H

N 2.25

O

N

O

O

N

H

N

CI Pigment Yellow 120 Cl

2.26 CI Pigment Brown 25

the reddish yellow monoazo Hansa Yellow R (2.27; CI Pigment Yellow 10), in which the amine diazotised is 2,5-dichloroaniline, and the disazo pigment CI Pigment Orange 13 (2.28), made from 3,3′-dichlorobenzidine. The latter pigment comes into the same category as the other disazo pigments mentioned earlier, as regards precautions in its use. In making both of these pigments the coupling component must be dissolved in concentrated caustic soda solution before diluting it prior to the coupling stage itself. Cl H

O

N N

N

N

Cl H3C 2.27 CI Pigment Yellow 10

CH3 Cl O

H

N N

N

N

N

N N

H O

N

Cl CH3

2.28 CI Pigment Orange 13

2.3.2 Metal-containing azo pigments As already mentioned, certain azo colorants have low aqueous solubility but not sufficiently low for them to be usable as pigments. These azo colorants are prepared from amines and coupling components containing hydroxyl or carboxyl groups in their sodium salt form as a result of alkaline coupling conditions.

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ORGANIC AND INORGANIC PIGMENTS; SOLVENT DYES

In many instances the aqueous solubility can be reduced to acceptably low levels by replacing sodium with a heavier metal, most commonly calcium, barium or manganese. These colorants fall into group 2a of the classification of azo pigments shown in Figure 2.1. When the solubility of the sodium-containing azo colorants is higher, so that they can be used and regarded as azo dyes, it may be possible to form heavy-metal salts only in the presence of basic substrates, usually alumina hydrate. These pigments consequently fall into group 2b of the classification. Whether a particular pigment falls into group 2a or 2b depends on the method used to make it in the form of these heavy-metal salts. The hues of the products formed using various heavy-metal precipitants are often so different that they are sold as separate commercial brands. Pigments in group 2a The first pigments in this group appeared about 1900 and were termed the Lithols. Lithol Red (2.29; CI Pigment Red 49) is made by diazotising Tobias acid (2-naphthylamine-1sulphonic acid) and coupling it to 2-naphthol. Diazotised Tobias acid is zwitterionic and therefore sparingly soluble, forming a dispersion rather than a solution. This gives rise to difficulties in deciding when the diazotisation reaction is completed, which can be overcome by the method described in section 2.3.1. The product of coupling is the sodium salt, which is yellowish red with a solubility just low enough for it to be used as a pigment in printing inks. The pigment found wider use when converted to its barium and calcium salts, which are of a bluer hue.

SO3Na N N

H O

2.29 CI Pigment Red 49

Lake Red C (2.30; CI Pigment Red 53), made by diazotising CLT acid (5-amino-2chlorotoluene-4-sulphonic acid) and coupling to 2-naphthol, is a pigment that has also found considerable use in printing inks. The sodium salt gives prints with a strong bronze surface finish, which seems to depend on the presence of three moles of water of crystallisation to each mole of pigment. If these are driven off by strong heating the bronzing effect is lost and cannot be regained. The barium salt found wider pigment applications. Tobias acid diazotised and then coupled to BON acid has been used as the manganese salt (CI Pigment Red 63) to give a bordeaux shade, but two other pigments (2.31; CI Pigment Red 48, and 2.32; CI Pigment Red 57) made from the same coupling component are of much greater current importance. The first is made from 2B acid (4-amino-2-

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CH3

Cl

CH3

Cl

CH3

NaO3S

NaO3S

N

N N

65

N

SO3Na

H

N

O

H

N

H O

O

COONa 2.31 CI Pigment Red 48

COONa

2.30

2.32

CI Pigment Red 53

CI Pigment Red 57

chlorotoluene-5-sulphonic acid) and the second from 4B acid (4-aminotoluene-3-sulphonic acid). They are called 2B and 4B toners, as might be expected, whilst compound 2.32 is also named Lithol Rubine. The 2B toner is converted into strontium, barium, calcium and manganese salts, whilst the 4B toner is used as its calcium and barium salts. The principal use for both pigments was originally in printing inks, particularly in oil-based types. Nowadays, however, numerous specially prepared commercial brands are available with their physical properties adjusted for use in plastics, various types of paints and even, in the case of CI Pigment Red 57, as colours for cosmetics and pharmaceuticals. The specifications of these products, particularly as regards the absence of heavy metals, is exceptionally stringent. Specially designed plant, used only for pigment intended for these purposes, is needed. Rosination Long before such specialised uses of pigments in group 2a were considered, the process of rosination played an important part in their manufacture. In the preparation of all these pigments in the form of their heavy-metal salts, the method of metal exchange is to heat the sodium salt with a solution of the chloride of the required heavy metal. Outstanding technical advantages result if this metal exchange is carried out in the presence of rosin soap. The resulting pigments are more easily dispersed, of greater tinctorial strength and higher brilliance, imparting a glossier appearance to printing inks. Rosination is therefore a most important part of pigment technology. Rosin is a natural product derived, along with turpentine, from the resin collected from certain American pine trees. It is graded by colour and the grade WW (water white) is used in the pigments industry. The acidic components originally contain about 40% L- and Dpimaric acid but during processing these are converted to abietic acid (2.33). Neoabietic, dehydroabietic and dihydroabietic acids are also present, according to Harris [14]. Rosin soap is made by dissolving the rosin in a hot solution of sodium hydroxide maintained at as low a temperature as possible consistent with keeping the soap in solution – it is almost insoluble in cold water. The hot solution, containing also the dissolved heavy metal chloride, is added to a dispersion of the sodium salt of the pigment. The whole is brought to the boil and, at a temperature between 80 and 90 °C, a marked colour change indicates that metal

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H3C

COOH

CH3

H3C

CH 2.33

CH3

Abietic acid

exchange has occurred. The amount of rosin soap used comprises some 15–30% of the mass of the final pigment. Rosin acts as a non-diluting substrate and has the effect of increasing the proportion of particles in the pigment with a size below 250 nm, but there is no evidence of chemical combination. Attempts to fractionate rosin to identify any specially active constituent have been unsuccessful, as have attempts to find better general dispersing agents, although various manufacturers have produced ranges of easily dispersible pigments containing other additives. Preparations of crude CI Pigment Yellow 13 (2.20), the recrystallised material and the rosin-coated pigment were compared recently using solid state NMR spectroscopy and powder X-ray diffraction. The primary effect of rosination with either abietic acid or its derivative staybelite was to increase the crystalline order of the pigment [15]. Pigments in group 2b prepared from azo dyes Many azo dyes prepared as sodium salts can be made into pigments by precipitating them as the salts of heavy metals, but only in the presence of basic substrates. The most commonly used substrate is alumina hydrate, as mentioned in section 2.2.1. The pigment is prepared by running solutions of the dye and a salt of the precipitating metal, generally barium or aluminium, into a suspension of alumina hydrate. An intimate mixture of the precipitated heavy-metal salt and alumina hydrate is formed and filtered off. The colloid chemistry of such systems, along with the technical and colour properties of the resulting pigments, is complex and depends on the dye: alumina hydrate ratio and the pH of the mixture. Weiser and Porter [16] studied the preparation of pigments from the dye Orange II (2.34; CI Acid Orange 7). Many other dyes, including tartrazines and eosines, have been made into pigments in the past for use in printing inks, in which their soft texture and bright hues were much valued. Their light fastness was generally only 1 on the ISO scale and they are now obsolete.

H HO3S

O

N N

2.34 CI Acid Orange 7

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2.4 PHTHALOCYANINE PIGMENTS Phthalocyanine was the first new chromogenic type to be introduced into the field of the colour chemistry of organic compounds concerned with pigments. Before this discovery, all the known organic pigments had been developed either by making dyes insoluble or by synthesising new insoluble azo compounds. This development is therefore interesting both technically and scientifically. It began in 1928 when a blue impurity was found in phthalimide during its manufacture at Scottish Dyes (later part of ICI). The method of preparation being used was to pass ammonia through molten phthalic anhydride in an iron vessel [17]. It is now known that the blue impurity was iron phthalocyanine. This was found to be exceptionally stable and steps were taken to determine its structure. It is of interest that another important blue pigment (ultramarine; section 2.10.2) was also first synthesised as the result of investigating a blue impurity. This was in an industry not directly related to colour making, namely, sodium carbonate manufacture at St Gobain in France. With hindsight it can be guessed that other ways of solving the impurity problem in making phthalimide, such as the use of a ceramic vessel, might well have solved the initial difficulty but would have resulted in missing the discovery of what has turned out to be a most important chromogen. Two other preparations of phthalocyanine had already been reported in the chemical literature but did not lead to the recognition of its usefulness, the first in 1907 [18] and the second in 1927 [19]. Linstead and his co-workers investigated the chemical structure of the blue impurity and its novel structure was finally established by Robertson [20] using X-ray diffraction. Copper phthalocyanine (2.35; CI Pigment Blue 15) first appeared commercially in 1935 under the ICI trade name of Monastral Blue B. 4

5

3

6

N

N N N

Cu

N

N N

N

2.35 Copper phthalocyanine

2.4.1 Manufacture of copper phthalocyanine [21,22] Two processes have been used for making copper phthalocyanine, but neither yields a form suitable for immediate use as a pigment since the physical form and size distribution of the resulting particles are far from the optimum. The product is generally referred to as ‘crude blue’ and requires purification and a sometimes complex series of finishing processes before it is in a form ready for use as a pigment.

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ORGANIC AND INORGANIC PIGMENTS; SOLVENT DYES

The solvent process The reactants are phthalic anhydride, urea and copper(II) chloride, which are heated in a high-boiling aromatic solvent such as 1,2,4-trichlorobenzene, nitrobenzene or mdinitrobenzene in the presence of a catalyst, usually ammonium molybdate. The solvent also acts as a heat-transfer medium. On heating to 120 °C an exothermic reaction begins and this temperature is maintained for about an hour. The temperature is then raised to 160– 180 °C and kept constant for 6–12 hours. During this time ammonia and carbon dioxide are evolved, together with some solvent; the reaction is complete when ammonia evolution ceases. The remaining solvent is then removed by either steam or vacuum distillation. The yield is 90–95%. For many years the solvent process was in almost exclusive use. Dry bake method As the name implies, no solvents are used in this process and the expensive solvent recovery stage is not needed. Instead of phthalic anhydride, the more expensive phthalonitrile is used as reactant. Urea is not necessary as a source of nitrogen but seems to have been used by some manufacturers. The copper needed comes from copper(I) chloride. Continuous processing on moving heated copper belts has been attempted, but there were mechanical difficulties in handling the reactants and products after hard solid masses had formed. This problem was only partly solved by adding neutral inorganic diluents such as sodium sulphate or common salt to improve the fluidity. Interest in the dry bake process was renewed when polychlorobiphenyls (PCBs) were recognised as carcinogens and detected in crude blues produced by the solvent process. Carcinogenic by-products The mechanism of formation of PCBs from the trichlorobenzene solvent involves hydrogen abstraction by the urea present, leading to the formation of trichlorophenyl radicals. Two such radicals can then combine (Scheme 2.2) to form a PCB molecule (2.36). This reaction can be suppressed by adding antioxidants such as hydroquinone or sodium phosphite to yield hydrogen (H•) radicals, which convert (Scheme 2.3) the trichlorophenyl radicals back to the parent substance (2.37). Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl Cl

Cl

+ Cl

Cl 2.36

Scheme 2.2 Cl

Cl

Cl

+

Scheme 2.3

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H Cl

Cl

68

Cl

2.37

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Other high-boiling solvents such as nitrobenzene and 2-nitrotoluene can also be used in the solvent process, but 4-nitrobiphenyl, a listed carcinogen, is produced when nitrobenzene is employed. One way to avoid the formation of substituted biphenyls is to employ a polysubstituted toluene as solvent, the substituents being chlorine or alkyl groups, mainly propyl. These are difficult to prepare but they have been patented by Toyo Inks. Another approach is to use high-boiling aliphatic solvents with minimal aromatic content; boiling points are in the range 150–250 °C. Much higher ratios of solvent to reactants are needed than when aromatic solvents are used, owing to the high viscosity of the system. Yet another method of avoiding PCB formation in the solvent process is to dispense with the need for urea as a reactant by using the more expensive phthalonitrile instead of phthalic anhydride. Isolation of crude blue No method of synthesis produces crude blue in pigmentary form. The solvent (if any) and the impurities must first be removed. Vacuum or steam distillation is used to remove the solvent, the former being generally cheaper. Impurities can then be removed by thorough washing for about two hours at 90–95 °C, first with 10% aqueous sodium hydroxide solution and then with 10% aqueous hydrochloric acid, followed by filtration, pressing, water washing and drying. From the purified crude blue, the copper phthalocyanine blue pigment is isolated in either the α- or β-crystal form. By introducing halogeno substituents into the molecule a series of green pigments can be derived (section 2.4.2). Conversion of crude blue into copper phthalocyanine blue pigments Purified crude blue exists in the most stable crystal form of copper phthalocyanine, the β-form, but the particles are much too large to make a good pigment. The β-form was that prepared by Robertson [20], from which he proved the molecular formula as mentioned earlier. Three other crystal forms of copper phthalocyanine have been reported but only one of them, the αform, is of interest in the pigment industry. Mixtures of α- and β-forms have caused various technical problems because the former is reddish blue and the latter is greenish blue. The αform has a tendency to revert to the more stable β-form in the presence of the organic solvents that are constituents of many paint formulations, with a consequent shade change and sometimes a loss of colour strength. Hence there is a need to prepare solvent-stable α-form pigments. Since β-form crystals also tend to grow and lose strength in the presence of organic solvents, there is a need to render them solvent-stable too. Methods for making both forms solvent-soluble were the subject of many patents and closely guarded industrial secrets, but much of the mystery was cleared up in two papers by Gerstner [23] and Smith and Easton [24] published in 1966, by which time X-ray diffraction, electron microscopy and disc centrifuge particle sizing methods had been brought to bear on the problem. Converting the β-form to the α-form The β-form of crude blue can be converted to the α-form by complete disintegration of the crystals. This can be readily achieved by dissolving them in concentrated sulphuric acid,

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thus forming a solution of single molecules, and then re-precipitating as α-form crystals by running the solution into a large volume of water. Careful control of the flow rate, coupled with stirring and temperature control (since heat is evolved in the process) and perhaps the presence of surfactants, can give tinctorially strong α-form pigments. Other methods, termed ‘acid slurrying’ or ‘permutoid swelling’ involve the use of insufficient sulphuric acid to bring about complete solution. Copper phthalocyanine sulphate crystals are formed; these are then drowned in water as before, forming α-form crystals. Yet a third method of conversion is by prolonged salt grinding in a ball mill in the absence of any solvent. The αform so produced, however, is not solvent-stable. Solvent-stable forms Smith and Easton [24] give the scientific basis of many of the patent claims. Salt grinding of either α- or β-form and treatment with solvent either during or after grinding gives the βform. In the absence of solvent the product is the α-form, irrespective of the starting material. However, mixtures of α- and β-forms can result from salt grinding in the presence of solvent, depending on the grinding efficiency and the time of grinding in the mill. Two stages, first grinding and then solvent treatment, are often easier to control. For example, if the grinding process in the absence of solvent is stopped when about one-third of the material remains as the β-form, exposure to solvent will then cause a rapid reversion of the α-form back to the β-form, giving solvent-stable β-form. Solvent-stable α-form pigment can be made by incorporating some copper monochlorophthalocyanine in the pigment or by carrying out the salt grinding in the presence of compounds that can be adsorbed on the surface of the growing α-form crystals, hindering further growth and stabilising them against reversion to the β-form. 2.4.2 Other phthalocyanine derivatives The discovery of the phthalocyanine chromogen gave rise to much research, particularly in view of its outstanding fastness properties. Many other metallic derivatives have been prepared and many substituents introduced into the phthalocyanine nucleus. Sulphonation has given some soluble dyes, which have even been converted back into pigments by making them insoluble again as barium salts. The only widely produced colorants are a range of green pigments made by halogenating copper phthalocyanine using chlorine either alone or mixed with bromine. There are 16 hydrogen atoms on the benzenoid rings of the phthalocyanine molecule (2.35) that can in theory be replaced by halogens. Those in the eight 3,6-positions can be replaced easily whereas more severe conditions are needed for substitution in the 4,5positions. The products of different manufacturers vary in the chlorine content, 14 or 15 atoms per molecule being usual in pigments under the generic name CI Pigment Green 7. As the number of chlorine atoms increases, the green pigments become increasingly yellow in hue. This trend can be carried still further by using mixtures of chlorine and bromine containing increasing amounts of bromine. The yellowest pigments have twelve bromine and two chlorine atoms per molecule and carry the generic name CI Pigment Green 36. Direct chlorination of copper phthalocyanine can be carried out by passing the gas into a molten eutectic of aluminium chloride and sodium chloride at a temperature of 180–200 °C,

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the copper phthalocyanine content being about 20% of the eutectic by mass. Iron(III) chloride is present as a catalyst [25]. Other methods include: (a) chlorination in trichlorobenzene at 200 °C using 10% antimony trisulphide as a catalyst; (b) chlorination under a pressure of 5–10 atmospheres at a lower temperature of 160– 170 °C, a process in which the release of hydrogen chloride gas is used to control the pressure; and (c) dry chlorination in a fluidised bed. Yet other processes have used sulphuryl chloride (SO2Cl2), thionyl chloride (SOCl2) or sulphur dichloride (SCl2) as chlorinating agents. The crude polychlorinated copper phthalocyanine requires a finishing process to develop the full potential colour strength, but the problems are less than those experienced with copper phthalocyanine itself, as no problems of polymorphism occur. Finishing is carried out by acid pasting of the crude green in oleum or chlorosulphonic acid. The crystallinity, as shown by the sharpness of the X-ray diffraction pattern, can be improved by solvent treatment as there is no tendency for this to cause crystal growth. The light fastness rating (ISO 8) and solvent resistance (generally 5) are both as high as those of phthalocyanine blues, making them exceptionally high-quality pigments. A great deal of research has been carried out to try to find organic pigments of red colour to match phthalocyanines in these two fastness requirements. The many red azo pigments show much inferior light fastness in reductions with titanium dioxide. Pigments made from vat dyes, mentioned earlier, have met the need to some extent, but there is still a gap in the organic colour pigment range for brilliant reds of the highest fastness properties. 2.5 QUINACRIDONE PIGMENTS The synthesis of linear trans-quinacridone (2.38) was reported in 1935 by Liebermann [26] and was cursorily looked at as a red vat colorant but not developed commercially. It was more than twenty years later that the DuPont company introduced these compounds as pigments under the trade name of Cinquasia. Their chemical structures are based on CI Pigment Violet 19 (2.38). As with the phthalocyanines, this compound can exist in several polymorphic forms; in this case there are three, termed α-, β- and γ- forms; only the last two being useful as pigments. The first three pigments were called Cinquasia Red B (γ-form, size above 1000 nm), Cinquasia Red Y (γ-form, size below 1000 nm) and Cinquasia Violet R (βform). Quite apart from the problems of chemical synthesis, it is clear that the application of physico-chemical methods to pigment manufacture, first needed in the development of the O H N

C

N H

C O 2.38

CI Pigment Violet 19

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phthalocyanines, was essential to the successful introduction of these new pigments. X-ray diffraction methods are needed to recognise the crystal forms. The β-form was made from the crude product as synthesised by milling with a large amount of salt in the presence of xylene, whilst preparation of the γ-form required wet milling with dimethylformamide, followed by removal of the solvent and the salt with aqueous acid. Since the Red B and Red Y pigments differed only in their particle size, methods capable of measuring particles less than 1000 nm in diameter were obviously necessary. Such methods were not yet available in 1958 when DuPont first marketed these products. The chemical synthesis of linear trans-quinacridones and their substituted derivatives that have been marketed subsequently is a complicated multi-stage sequence, making such pigments very expensive and sustainable only where high-fastness red pigments are essential, as in the car industry. There are four routes of synthesis, details of which have been given by Pollack [27]. The DuPont company used the succinic acid ester process, in which diethyl succinate is first cyclised to diethyl succinylsuccinate (2.39) in a sodium/alcohol mixture. One mole of the product is next condensed with two moles of aniline under oxidising conditions, forming diethyl 2,5-dianilino-3,6-dihydroterephthalate (2.40). Ring closure at 250 °C gives dihydroquinacridone, from which quinacridone is obtained by oxidising away the two extra hydrogen atoms. O EtOOC

C HC H2C

CH2

H N

H

H

EtOOC

H

H

COOEt

CH C

COOEt

O 2.39 Diethyl succinylsuccinate

N H

2.40

The Sandoz company used the dibromoterephthalic acid method. This acid was made from p-xylene by brominating it to form 2,5-dibromo-p-xylene and then oxidising this to 2,5dibromoterephthalic acid. Reaction of one mole of this acid with two moles of an arylamine in the presence of copper(II) acetate gives 2,5-bis(arylamino)terephthalic acid, which can be ring-closed to a linear quinacridone. Unsymmetrical substitution using two different arylamines is possible. The BASF route started from hydroquinone, which was converted to 2,5dihydroterephthalic acid by a Kolbe-Schmitt reaction. One mole of this acid was treated with two moles of an arylamine, both components being in the form of a suspension in aqueous methanol. This was added to a small amount of a solution of vanadium(III) chloride and sodium chlorate. Gentle heating gave a 95% yield of 2,5-bis(arylamino)benzo1,4-quinone-3,6-dicarboxylic acid. Ring closure to the trans-quinacridonequinone took place in the presence of concentrated sulphuric acid at 60–80 °C. This was then reduced to the required crude pigment by zinc or aluminium powder in caustic soda under pressure,in an aluminium chloride/urea melt or by the use of a sulphuric acid/polyphosphoric acid mixture. The fourth synthesis, developed by the Swiss Lonza company and known as the diketene process, resembles the succinic acid ester process of DuPont in that the central ring of the

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quinacridone molecule is totally synthesised. Acetone vapour is passed over an electrically heated wire to form first ketene, H2C=C=O, which then dimerises to form diketene (2.41). Chlorination in the presence of alcohol yields ethyl 4-chloroacetoacetate, ClCH2COCH2COOEt. This reaction proceeds at low temperature but is exothermic to a considerable extent and can only be successfully carried out on a large scale in special reactors having an extensive surface area. Diketene also reacts with aromatic amines to yield acetoacetanilides, which are the coupling components needed to prepare Hansa Yellows (Table 2.4). H2C

C

CH2

O

C

O

2.41 Diketene

In the manufacture of quinacridone pigments only the first and last of the four routes outlined have been operated commercially. Synthesis is followed by the milling processes necessary to give products with the crystal structure and particle size required for their use as pigments. 2.6 ISOINDOLINONE PIGMENTS Another new chemical class of pigments, called Irgazins, was produced in 1964 by the Geigy company [28]. These products are made by condensing two moles of 4,5,6,7-tetrachloroisoindolin-1-one with one mole of an aromatic diamine. Thus p-phenylenediamine gives the yellow pigment CI Pigment Yellow 110 (2.42), which has high light fastness and heat resistance. The analogous CI Pigment Yellow 109 derived from 2-methylphenylene-1,4-diamine is greener in hue. Both of these pigments are used in plastics coloration, particularly of polyolefins. Cl

N

N

Cl

Cl

Cl NH

HN

Cl

Cl Cl

O

O

Cl

2.42 CI Pigment Yellow 110

2.7 DIOXAZINE PIGMENTS Triphenodioxazine compounds have been used to make brilliant blue direct and reactive dyes for cellulosic fibres, but only one pigment in this chemical class, CI Pigment Violet 23 (2.43), is widely used. It is prepared by condensing 3-amino-9-ethylcarbazole with chloranil in trichlorobenzene [29]. As with many other pigments, manufacturers offer it in many physical forms adapted to the intended end-use. 2.8 DIKETOPYRROLOPYRROLE PIGMENTS This novel brilliant red chromogen was discovered by Iqbal and Cassar of Ciba-Geigy in 1983. It has yielded several useful derivatives with first-class application and fastness

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Cl

H5C2 N

O

N

N

O

N C2H5

Cl 2.43 CI Pigment Violet 23 H N

O C

Cl Cl C O

N H

2.44 CI Pigment Red 254

properties in this region of the colour gamut [30]. A typical example is the dichlorinated compound CI Pigment Red 254 (2.44). 2.9 FLUORESCENT PIGMENTS A colorant of this type is not a pigment in the same sense that has been used so far in this chapter, that is, essentially a single chemical compound that can be used to colour media in which it is not soluble. Fluorescent pigments are based on fluorescent dyes which are soluble in certain resins. A resin coloured in this way is ground to a powder of small particle size and this ‘fluorescent pigment’ is dispersed into various media in the same way as other pigments. The molecular mechanism by which dissolved dye exhibits fluorescence is discussed in section 11.2. The overall result is that the paints, printing inks and plastics into which fluorescent pigments have been incorporated have vivid bright colours which attract the eye. This property is very valuable for safety clothing worn by police, rescue workers and others working in hazardous situations. Their vehicles also become much more visible when parts of them are painted with special finishes containing these colorants. Other uses include advertising and plastic toys. The resins used to make fluorescent pigments are usually toluenesulphonamidemelamine-formaldehyde matrices. The dyes used for this purpose include CI Disperse Yellow 11, Rhodamine 6G (CI Basic Red 1) and Rhodamine B (CI Basic Violet 10). More details of the fluorescent dyes used have been given in a review by Christie [31]. Unfortunately, the fluorescent effect is not directly proportional to the concentration of colorant present, since there is considerable quenching if quite low concentrations are exceeded. The light fastness of the fluorescent pigments is also less than that of many other organic pigments now available, but improvement can be achieved using overlayers containing ultra-violet absorbers. This is an area in which further research will clearly be needed.

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2.10 INORGANIC PIGMENTS Inorganic pigments have a special place in pigment chemistry for several reasons. Some are cultural and historical. The art galleries of the world are filled with oil paintings in which the colours were, until the industrial revolution, produced entirely from mixtures of inorganic pigments obtained from natural sources, the so-called earth pigments. Later some of these were made industrially. Another reason for the importance of inorganic pigments is that there are no white organic pigments. White pigments are essential to provide opacity to the paints and printing inks used on metal, wood, paper, textile fabrics and plastic films. White inorganic pigments are also used to provide opacity to synthetic fibres and the vast numbers of articles made from plastics by moulding and extrusion processes. The term plastics covers a whole range of different polymers, including polyamides, polyesters, ABS (acrylonitrile-butadiene-styrene) copolymers, poly(vinyl chloride) and polyolefins. The uses to which these plastics are put, their different physical and chemical properties, and the technologies used in fabricating them sometimes including the use of quite high temperatures, impose stringent demands on any colorants used in them. Inorganic pigments, both white and coloured, can generally meet these demands more often than organic pigments. There are no general rules which apply and the only way to find out which individual pigments are suitable is by practical trials. In uses where high light and weathering fastness is required, such as in paint for cars, many organic pigments do not meet the required standards. Inorganic pigments are the only ones capable of withstanding the very high temperatures used in the manufacture of glass and in the glazing of pottery after it has been decorated. Carbon black in one of its many forms is the colorant most widely used in printing inks. Vast amounts are also used in formulating rubber for vehicle tyres, where the carbon black also plays an important part in the vulcanisation process when rubber is hardened by reacting it with sulphur during manufacture of the tyres. The primary criteria for carbon black selection in fibre coloration are colour and undertone, but secondary properties must match the final application. Carbon black performance is determined by prime particle size and aggregate structure. Inconsistent dispersion performance may arise by sub-optimal compounding leading to poor dispersion of the carbon black agglomerates, or from lowquality carbon black containing undispersible contaminants. Recent quality improvements include a new carbon black for increasing the lifetime of continuous filters and close control of residual impurities improving carbon quality in black-pigmented polyester film used in food packaging [32]. A recent survey of the technical and environmental aspects of inorganic pigments by Schwarz and Endriss [33] has put the importance of these products into numerical perspective. Of a total worldwide production of some 5M tonnes of pigments, 96% are inorganic. Of this 4.8M tonnes, carbon black and various whites account for 84%, that is just over 4M tonnes. This means that world production of coloured inorganic pigments is approximately 0.8M tonnes. 2.10.1 White inorganic pigments The primary requirement for a compound to be of a white appearance is that it should not absorb any of the radiation in the visible part of the electromagnetic spectrum, that is, of wavelengths between 400 and 650 nm; in practice this is seldom attainable and slightly less

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than 100% transmission can be tolerated, but there must be no marked absorption bands. The additional requirement for a substance to be usable as a white pigment is that it should have as high a refractive index throughout the visible spectrum as possible. A third criterion is that it should be in the form of particles fairly uniform in size, with diameters of the order of 1000 nm. Because pigments are used as dispersions of particles in media of a refractive index of about 1.5, this is the lower limit for their practical use. There are several white inorganic compounds with refractive indices between 1.5 and 1.63 which are called ‘transparent whites’ or ‘extenders’. These include whiting (CaCO3; CI Pigment White 18), china clay (CI Pigment White 19), barytes (BaSO4; CI Pigment White 22), gypsum (CaSO4.2H2O; CI Pigment White 25), talc and silica. These compounds confer very little opacity and are mainly used to thicken the paints into which they are incorporated. White pigments, some of rather variable composition, that have been used in the past include white lead (CI Pigment White 1), refractive index 1.9; zinc oxide (CI Pigment White 4), r.i. 1.9; lithopone (CI Pigment White 5), r.i. 1.84; zinc sulphide (CI Pigment White 7), r.i. 2.3; antimony oxide (CI Pigment White 11), r.i. 2.2. All have been eclipsed by titanium dioxide (CI Pigment White 6). Titanium dioxide As a pigment this is used in two crystalline forms: rutile (r.i. 2.71) and anatase (r.i. 2.51). It is these high refractive indices which, combined with modern manufacturing methods, have ensured its pre-eminence. The ore used for making commercial titanium dioxide is the black ilmenite, nominally FeO.TiO2 but containing small amounts of many impurities, depending on the source. The development of manufacturing methods to prepare pure titanium dioxide in the form of either rutile or anatase took many years. Anatase pigments appeared in the mid 1930s and rutile pigments in the 1940s. One of the problems was to remove the last traces of iron and other metallic impurities which distorted the crystalline lattice, leading to inferior whiteness. There are now two processes in widespread use for making titanium dioxide pigments. In the sulphate process, finely ground ilmenite is digested in sulphuric acid and the iron is reduced and separated as iron(II) sulphate. The titanium(IV) sulphate is hydrolysed by steam to a hydrous oxide, which is thoroughly washed to remove soluble impurities and finally calcined at a temperature of about 1000 °C to give the anatase form of titanium dioxide. In the chloride process, developed in about 1960, the titanium in the ore is converted to titanium(IV) chloride by heating it to 800 °C with chlorine in the presence of carbon, which combines with the released oxygen. The purified chloride is then oxidised to titanium dioxide at 1000 °C and the chlorine formed is recycled. Technical problems arise because the oxidation of titanium(IV) chloride is not sufficiently exothermic to make the reaction self-sustaining but these can be overcome by pre-heating the reactants and by burning carbon monoxide in the reactor to raise the temperature. By careful control of the conditions, it is possible to produce pure rutile particles of a mean size of 200 nm. Titanium dioxide is non-toxic, in marked contrast to pigments such as white lead, and is widely used as a pigment in utensils and on surfaces coming into contact with food in industry, as well as in almost all other uses calling for white pigment. It is not the easiest

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pigment to disperse and, after a long struggle to produce it in pure form, most manufacturers now produce a range of quite heavily coated forms, each prepared to facilitate and maintain dispersion in particular media. Both crystal forms of titanium dioxide are photochemically active, anatase more than rutile. Under ultraviolet light the pigment releases active oxygen, which can tender textile fibres or degrade paint films, causing ‘chalking’. In the dark, atmospheric oxygen regenerates the titanium dioxide and the cycle of degradation recurs. In applications where this is a major problem, rutile-form pigments containing additives such as antimony(III) sulphide are therefore used. Much research had to be done during the development of large-scale processes to prepare titanium dioxide in two different crystal forms and in a high state of purity. The knowledge so gained has been put to good use and led to the production of several new coloured inorganic pigments. These products are based on the discovery that the rutile lattice can absorb certain metal oxides into its structure to give coloured pigments which retain the high fastness properties of pure titanium dioxide. These pigments are discussed in the next section.

2.10.2 Coloured inorganic pigments During the first half of the 20th century, when linseed oil was the primary medium from which paints were made and phenol-formaldehyde resin was the basis of most plastic materials, both paints and plastics were usually coloured using inorganic pigments. The second half of the century saw the introduction of many new types of plastics and the annual world production of them rose above the l00M tonnes mark. Much of this had to be coloured for identification or decorative purposes. The simplest way to do this is to colour the whole of the plastic before extruding or moulding it into the shape of the finished article, rather than to apply a coloured surface coating later. Whilst some of the pigment was no doubt wasted by being buried in the interior, this loss was completely outweighed by the economy of a single-step operation to the required finished article and uniformity of colour between separate batches. The result was a big increase in the demand for pigments which met the necessary fastness standards and withstood the heating involved. Many organic pigments could not meet these requirements and some of them adversely affected the moulding process itself, or the dimensional stability of the finished articles. Inorganic pigments, or mixed organic/ inorganic products, overcame the problems in some instances, despite the less easy dispersibility of the inorganics. The coloured inorganic pigments in current production are generally those that have particular advantages which organic pigments cannot match. Ultramarine This is the synthetic form of the blue mineral lapis lazuli, which was imported from China and India in the Middle Ages. It is characterised by its attractive blue colour, high light fastness (ISO 7–8), excellent resistance to alkalis and all organic solvents, easy dispersibility and non-toxicity, but it has poor resistance to acids. It was first made by Guimet in France in 1830 after having been noticed as an impurity in the glass furnaces at St Gobain. It is now made by calcining a mixture of the ingredients shown in Table 2.5, which are first finely

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ground and formed into bricks. The reaction takes 4–5 days in a sealed muffle furnace at 800 °C, during which time the sodium carbonate reacts with the alumina and silica to form a zeolite, a hydrated silicate of calcium and aluminium. This reacts with the sodium polysulphides formed at the same time to give primary green ultramarine. As the furnace cools, air diffuses into it and the primary green oxidises to form raw blue ultramarine. The oxidation process can be accelerated by blowing in sulphur dioxide. The product at this stage contains 20–25% sodium sulphate, which is leached out with water. The remainder is then ball-milled in the wet state until the size of the particles is in the range 500–6000 nm. The particles are next separated into size fractions by sedimentation or centrifugal methods. The coarsest fraction is used for laundry blues, the intermediate fraction in plastics coloration and the finest in high-grade printing inks.

Table 2.5 Ingredients used in the manufacture of ultramarine Ingredient

Amount (%)

China clay Sodium carbonate Sulphur Silica Rosin

30 32 30 4 4

The formula as deduced from chemical and X-ray structural analysis is Na6Al6Si6O24S4. There is an open framework of SiO4 tetrahedra with shared corners. Some silicon ions are replaced by aluminium ions, however, and to preserve electrical neutrality sodium cations are needed. These are in excess and the overall balance is maintained by the S2–, S22– and S32– ions present. Consequently, chemical formulae containing fractional proportions of elements are possible. The colour of inorganic compounds is associated with charge transfer within the structure. This can be brought about by specific light absorption, so that the variability of ultramarine composition that can be achieved opens up many possibilities for colour changes. Pure starting materials, particularly the absence of iron impurities, are necessary in making consistent pigments. As well as the basic ultramarine with two sulphur atoms in its formula, which is CI Pigment Blue 29, two other pigments based on it are commercially available. These products are CI Pigment Violet 15 and CI Pigment Red 259, the latter having a pink colour. Both pigments are made from ultramarine by oxidation and ion exchange but have a lower colour strength. The refractive index of these pigments is about 1.5, which means that they give a transparent blue when used in paints and clear plastics. Opacity can be increased by adding small amounts of titanium dioxide. The principal failing of ultramarine is its lack of resistance to acid, which can even decompose the pigment if there is sufficient available. Coated grades are made with substantially improved acid resistance.

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Cadmium pigments A range of mixtures containing cadmium sulphide and varying amounts of zinc sulphide or cadmium selenide can be made, the resulting colours being as shown in Table 2.6. The outstanding brilliance of shade of these pigments cannot be matched by any other inorganic pigment. Equally bright organic pigments have lower heat stability than have cadmium pigments, which can withstand temperatures of 400 °C. Other advantages of the cadmium pigments are better solvent fastness and high fastness to migration, particularly in plastics, including ABS, polyolefins and extended PVC. Table 2.6 Colours obtainable with cadmium pigments Colour

Components

CI Pigment

Primrose Yellow Orange Red Maroon

CdS, ZnS CdS CdS, 0.2 CdSe CdS, 0.4 CdSe CdS, 0.7 CdSe

Yellow 35 Yellow 37 Orange 20 Red 108 Red 108

The starting material for making these pigments is cadmium sulphate, which must be free from iron, nickel and copper impurities. Cadmium sulphide is precipitated from the sulphate solution by adding an alkaline solution of pure sodium sulphide under controlled conditions of pH, temperature and addition rate. The yellow product is in the cubic crystal form, which is converted into the required hexagonal form by calcination at 500–600 °C in the absence of air. Primrose pigment containing zinc sulphide is made by adding zinc sulphate to the cadmium sulphate starting material and the zinc sulphide appears in solid solution in the final product. The pigments consisting of mixtures of cadmium sulphide and selenide are made rather differently. An alkaline slurry of cadmium carbonate is made by mixing solutions of cadmium sulphate and sodium carbonate. At the same time selenium is added to a solution of sodium sulphide to form sodium sulphoselenide as a source of S-Se polyanions. This must be kept alkaline to prevent the precipitation of selenium. Mixing of a solution of the correct composition with the cadmium carbonate slurry ensures the precipitation of cadmium sulphoselenide that will yield the desired hue. After washing, the precipitate is calcined to give the pigment. Both cadmium and selenium are poisonous and there are regulations about the limits of free cadmium in the pigment (100 p.p.m.) and on the disposal of waste liquors from manufacturing plants. As well as the uses in plastics already mentioned, these pigments are used in car paints and stoving enamels, and also in leather finishes and rubber. Iron oxide pigments Naturally occurring iron oxide pigments are widely distributed geographically and can be found in a wide range of colours from black to reds and yellows, depending on composition and crystal structure. Ochres of many kinds and from different sources were often used in oil

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paintings by the Old Masters The use of magnetic iron oxides in recording tapes has stimulated their investigation at advanced technological level, but in the field of pigments their use has declined to that of cheap fillers for controlling paint flow properties and of facing colours for building bricks. Iron(II) sulphate is a by-product in many industrial processes, such as the manufacture of titanium dioxide, the pickling of steel sheet before galvanising and the reduction of aromatic nitro compounds to amines using iron catalysts. Conversion of waste iron(II) salts to usable iron oxide pigments, where the quality requirements are not too stringent, is therefore a useful proposition, since it uses up chemicals that would otherwise be regarded as waste products. Yellow iron oxides can be prepared by precipitating iron(II) hydroxide from a solution of iron(II) sulphate by adding sodium hydroxide and then bubbling air through the heated suspension to oxidise it to α-iron(III) oxide hydrate (α-FeOOH or Fe 2O 3.H 2O). An alternative method uses a single step of passing a mixture of air and ammonia into the iron(II) sulphate solution, followed by a heating stage. Seeding of the precipitated dispersion with a previously prepared pigment of the required crystal form helps to control the quality of the final product. Red, brown and black iron oxides are prepared by first heating green iron(II) sulphate crystals to remove six of the molecules of water of crystallisation (leaving FeSO4.H2O) and then calcining the product to the desired form of iron(III) oxide with the evolution of sulphur oxides. Currently produced iron oxide pigments are listed in Table 2.7. Similar pigments occur naturally in many parts of the world, but are of course mixed with other mineral substances. The crystallography of iron oxides is complex because there are two oxides, FeO and Fe2O3, as well as α-FeOOH, which can occur in mixed crystals. Kittel [34] has given a full account of the chemistry and Feitknecht [35] has discussed the theory of the colours of these pigments. Table 2.7 Synthetic iron oxide pigments Colour

Formula

CI Pigment

Yellow Red Brown Black

FeOOH Fe2O3 Fe2O3 & FeO.Fe2O3 mixtures FeO.Fe2O3

Yellow 42 Red 101 Brown 6 Black 11

Lead Chromate pigments This group of pigments includes chrome yellow, chrome orange and molybdate red (Table 2.8). Their colours range from lemon yellow to bluish red. The chrome yellow pigments are pure lead chromates or mixed-phase pigments of lead chromate and lead sulphate. Partial replacement of chromate by sulphate in the mixed-phase crystals causes a gradual reduction of tinting strength and hiding power, but allows production of chrome yellows with a greenish yellow hue [33]. Chrome orange pigments are basic lead chromates that vary in content of PbO and PbSO4. Molybdate red pigments are mixed-phase pigments containing lead chromate, lead sulphate and lead molybdate. Their colour depends on the proportion of molybdate, crystal form and particle size.

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Table 2.8 Lead chromate pigments Common name

Formula

CI Pigment

Chrome yellow Chrome orange Molybdate red

Pb(Cr,S)O4 Pb(Cr,S)O4 Pb(Cr,S,Mo)O4

Yellow 34 Orange 21 Red 104

The presence of lead and hexavalent chromium in these products is of chronic hazard concern and the EU has classified these pigments as harmful substances. Lead is soluble at stomach-acid concentrations and can accumulate in the organism. The results of a high lead intake include inactivation of enzymes and disturbances in the synthesis of haemoglobin. Hexavalent chromium compounds are considered to be carcinogenic. For these reasons the usage of lead chromate pigments has declined considerably in recent years. Complex coloured inorganic pigments There are two types of pigments in this category, rutile and spinel pigments. The first, as mentioned already in section 2.10.1, have been developed as a result of the research done to produce titanium dioxide in pigmentary form. By calcining pure rutile with nickel and antimony oxides at a high temperature, the rutile lattice becomes able to absorb about 3% of nickel(III) oxide and 10% of antimony(V) oxide. The first of these imparts a yellow colour and the second serves to maintain the overall valency of the cations at four. The characteristic properties of nickel and antimony oxides are no longer apparent after incorporation in the rutile lattice. The commercial product is designated CI Pigment Yellow 53. A buff pigment of this type can be made using chromium(III) oxide instead of nickel oxide [36]. In either of these products niobium oxide can be used instead of antimony oxide, giving a pigment of higher colour strength. Brown pigments are produced if manganese(III) oxide is used instead of nickel or chromium oxide in the calcination, an example being CI Pigment Brown 37. Spinels are minerals which have the formula MO.R2O3, where M is a divalent metal such as magnesium and R is a tervalent metal, for example aluminium. Blue pigments are obtained when the metals in the spinel MgO.A12O3 are partially or completely replaced by cobalt. Cobalt Blue is CI Pigment Blue 28 with the formula CoO.Al2O3. The main uses of these pigments are in paints and in plastics, where their fastness properties are excellent. Their high-temperature stability is particularly useful in plastics. Metallic pigments Attractive metallic effects can be obtained by incorporating very thin, small flakes of aluminium, copper or copper/zinc alloys in otherwise transparent surface coatings. The metal flakes act as tiny mirrors within these paints, which are particularly effective on the curved surfaces of cars since the colour changes with the angle from which the surface is viewed. The orientation of the flakes within the paint film also changes the colour seen by the eye, so that careful dispersion of the metal components of the paint is essential.

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Aluminium flakes are listed as CI Pigment Metal 1, whilst copper and copper/zinc alloys are CI Pigment Metal 2. The latter can give the appearance of gold in suitable formulations. The transparency of the surface coatings in which the metallic pigments give their best effect is achieved in formulations which use solvent dyes, rather than coloured pigments, as the principal colorants. However, adequate fastness of the entire coating must be the overiding consideration. 2.11 HOW PIGMENTS ACT AS COLORANTS This chapter has been concerned mainly with the chemistry of making pigments but has also stressed the importance of preparing them in the correct physical form. In the case of those pigments which can be made in more than one crystal form, such as the anatase and rutile forms of titanium dioxide, this may mean that all the pigment should be in one form only. In others, a mixture of polymorphic forms may be required, and it is then necessary to ensure that the desired ratio of these forms is present. With all pigments, the particle size distribution of the pigment as synthesised is the most important single factor in ensuring its optimum potential performance in use, but this potentia1 can only be achieved if the particles are properly dispersed. To understand why particle size is so important it is necessary to look in some detail at the optical behaviour of tiny pigment particles dispersed in a transparent organic medium. The exact nature of this medium depends on whether the end-use is a paint, a printing ink or a plastic. In the case of paints and printing inks, the initial preparations will be in the semi-solid state because solvents are needed both in the process of dispersing the pigment in the paint or ink medium and for application purposes. These solvents dry out after the paint or ink is applied. When making coloured plastic articles, both heating and solvents may be used to aid dispersion in the plastic medium as part of the moulding process. However, from the viewpoint of the optical properties in all of these pigment uses, what is most important is that each of these media has a refractive index close to 1.5. Refractive index is strictly defined as the ratio of the velocity at which light travels in a vacuum to that at which it travels through the medium. For present purposes, it is a sufficiently close approximation to use the velocity at which light travels in air, rather than in a vacuum. Figure 2.2 gives a simplified analysis, based only on the theory of geometric optics and ignoring the wave properties of light, of what happens when a single particle of a lightabsorbing (coloured) pigment, embedded in a transparent medium, is struck by a ray of white light. The size of the particle is many times greater than the wavelength of visible light (400–650 nm) and its refractive index is greater than that of the medium, which is about 1.5. At point A in Figure 2.2, a ray of white light enters the medium and its velocity diminishes, with the result that its path is bent towards the normal at this point. Snell’s law of refraction states that the relation between the angles of incidence (i) and refraction (r) is given by Equation 2.1, where n is the ratio of refractive indices of the medium and air.

n=

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sin i sin r

(2.1)

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White light

i

Air

A

Medium

r

i′ B

Pigment particle

r′ i′′ E

C

Coloured light

r ′′

D

Figure 2.2 Optics of a single coloured pigment particle in a medium when struck by a ray of white light. The light becomes increasingly coloured as it travels from point B to point C

The ray then travels on in the medium and strikes the pigment particle at point B. Here it is again refracted and its path bent towards the normal. The relation between the angles i′ and r′ at point B is again given by Equation 2.1, but this time n is the ratio of refractive indices of the particle and medium, that of the particle being the greater. As the ray passes through the pigment particle some wavelengths of the light are absorbed. Which wavelengths these are and how much of them are absorbed, is determined by the shape of the absorption curve of the particular pigment involved. When the ray reaches the point C it will certainly be coloured to some extent, depending on the distance BC. At point C, one of two things may occur. If the angle of incidence i″ is smaller than the critical angle, the coloured light ray will emerge from the pigment particle and re-enter the medium, following a path CD with the angle r″ greater than i″, as shown in Figure 2.2. If the angle i″ at point C is greater than the critical angle, however, total internal reflection will occur at that point and the ray will not emerge from the particle. The direction of travel of the ray will be effectively reversed and it will start on a path CE back through the particle and then possibly out into the medium and the air and thus to the observer’s eye. Its exact path depends on the particle shape and its orientation in the medium. Which of these two possible paths the ray follows at point C depends on the value of the critical angle, Φc. This is given by Equation 2.2, where np and nm are the refractive indices of the particle and medium, respectively. The important point is that the greater the refractive index of the particle, the greater is the chance of the critical angle being exceeded and the more light will start back towards the eye. A complete and detailed analysis would be very complicated, if not impossible, since in a real case lots of other particles, including those of different pigments, would be present.

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ORGANIC AND INORGANIC PIGMENTS; SOLVENT DYES

sin F c =

np

(2.2)

nm

If total internal reflection does occur at point C, the amount of absorption could be doubled. Too long a path within the particle, or a very high absorption coefficient, could mean that all the light is absorbed and the colouring power of the particle will thus be lost. This demonstrates one reason why the mean particle size of the coloured pigment must be arranged to minimise this possibility. So far, the effect of light scattering has been ignored. Scattering occurs when light rays strike particles of linear size considerably less than the wavelength of visible light. What occurs is shown in Figure 2.3. The intensity of the scattered light is proportional to its incident intensity and also to the 1/λ4 power of its wavelength. Short-wavelength violet or blue light is scattered ten times more than red light. Incidentally, this explains why a clear sky in daylight appears blue, the scattering agent in such a clear sky being the molecules that form the atmosphere. This effect is, however, relatively unimportant in considering the optics of pigment particles dispersed in a medium. The most important points that follow from a consideration of the scattering phenomenon are that white pigments should (a) not significantly absorb visible light, (b) consist of particles of mean size well below the wavelength of visible light, and

Observer

White light

Air

Figure 2.3 Scattering of light produced by a single very small pigment particle

Medium

Pigment particle

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(c) have a refractive index greater than 1.5, this being that of the media in which white pigments are generally used. In practical applications of pigments there will usually be white pigment to confer opacity and one or more coloured pigments to provide the colour. The main effects in this case are indicated in Figure 2.4, which shows the combination of a coloured pigment particle of a size considerably greater than the wavelength of light and a white pigment particle considerably smaller than that wavelength. If the coloured particle is also smaller than this, its colouring power is lost and its effect virtually the same as that of the white pigment particle. In practice most pigment particles produce both scattering and absorption effects.

White light

Coloured light

Observer

Coloured light

Air

Figure 2.4 Combination of a coloured pigment particle and a very small white pigment particle

Medium

Coloured pigment particle

Small white pigment particle

Even though this analysis takes account of both the predictions of geometric optics and of the wave theory of light, there are still further factors to consider. Every refraction is accompanied by some degree of reflection. Coefficients of light absorption depend on the plane of polarisation of the light with respect to crystal planes in the structure and pigments show both birefringence and dichroism, topics which are fully discussed in textbooks of physical optics. In a concentrated dispersion of white and coloured pigments, each present in a range of particle sizes, a detailed analysis of ray directions becomes impossible. It is possible to deduce some general principles, however. These are that white pigments should be of a narrow size distribution, peaking in the region of 200 nm, and that coloured pigments should be of larger sizes, peaking around 500 nm, depending on the value of their absorption coefficients.

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This analytical approach is difficult to apply to individual pigments because physical data relating to refractive index, dispersion curves and the absorption curves in the solid state are not available. A colligative approach, based on the Kubelka-Munk analysis which characterises pigments by only two constants, an absorption and a scattering coefficient, has been applied with considerable success to the computation of the proportions of pigments in mixtures needed to match a given colour. Much of the book Colour physics for industry is devoted to this topic [37]. There are also problems arising from the electrical interactions between different pigments and between pigments and the media in which they are dispersed, particularly when these are liquid paints and inks. This topic is discussed in the textbooks by Parfitt and Apps (amongst others), given in the bibliography at the end of this chapter. 2.12 SOLVENT DYES Textile dyes can be dissolved or solubilised in water to a greater or lesser extent, this being essential for all conventional dyeing processes. Since solubility in water is generally accompanied by insolubility in nonpolar solvents, most textile dyes have this property. Solvent dyes, on the other hand, are soluble in organic solvents but insoluble in water. Solvent dyes are used as colour markers for the many different hydrocarbon fractions produced in oil refineries. Although the concentration of dye is low, the vast volumes involved makes the overall dye usage considerable. Solvent dyes are also used for tinting transparent varnishes and lacquers in the furniture and leather industries, where the surface texture of the finished articles needs to be protected but not obscured. There is also a growing demand for coloured but transparent inks for printing on the ever-increasing amounts of plastic packaging of all kinds used for the food sold in supermarkets. Solvent dyes are used to colour the transparent paints used to exploit the special properties of metallic pigments, as mentioned earlier. Another use for solvent dyes is in the colouring of many plastic articles to supplement the colouring power of pigments, if the use of pigments alone in a particular application is found to have undesirable effects on the physical properties of the plastic. The new Colour Index volume Pigments and Solvent Dyes lists some 350 solvent dyes and gives their chemical structures, unlike earlier editions which named 800 dyes but included few structures. This fall in numbers is not because of any decreased use but rather the general contraction in numbers of all dyes used in the textile industry. Solvent dyes have been introduced not by attempts to synthesise new colorants but by selection and in some cases modification of known disperse dyes to meet the technical requirements. The majority of solvent dyes are azo compounds but among the blue dyes there are anthraquinones. The aqueous solubility of some of the parent sulphonated dyes has been reduced to acceptable levels by formation of their salts with heavy metals or long-chain alkylamines. 2.13 CONCLUSION The diversity of chemical structures involved in pigment chemistry is much less than that of dyes, but extends from organic to inorganic compounds. The technology involved in the use of pigments in a wide range of different media is that of dispersing the pigment in the

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87

medium, a process requiring quite different properties from those needed for dyes. As a result, even a compound that is chemically suitable for pigment use may not be acceptable in practice because it cannot be prepared in a physical form that allows its potential colouring properties to be exploited fully as a stable dispersion. This occurs quite frequently with the wide variety of plastic media now available and requiring to be coloured. Manufacturers of pigments meet these difficulties by selling the same pigment chemical in a series of different forms with additives and coatings chosen to suit the intended end-use. This development has also been necessary because of the increased automation in many colour-using industries. For example, a computer colour match prediction recipe may call for the use of exact amounts of particular pigments and these have to be metered by volume into the machine that shapes the finished article. This simply cannot be done using dry powders, which were at one time the principal products of pigment makers. Dispersions of the component pigments with precise and constant colour strength are essential if large numbers of the finished articles are to be kept within acceptable colour tolerances. The range of pigments commercially available is diminishing, though producers usually make several physical forms of the same pigment, each intended for specified applications. There are several reasons for the fall in numbers of different pigments. As well as the needs of the automated systems mentioned above, there has been a rationalisation of the range of intermediates available from the chemical industry, especially if an intermediate is no longer required for dye making. Certain pigments have been discontinued because they cannot meet the current demands from users for higher fastness standards, particularly to light. Low-volume production may be uneconomic when suitable substitutes are available. The increasing use of instrumental colour match prediction, with the possibility, at least in theory, of matching wide colour ranges using only three or four pigments, as already achieved in colour printing, is yet another factor contributing to contraction of the pigment range. No doubt technical limitations such as dispersion stability will prevent such a drastic change happening in practice. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

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L M Valentine, J. Oil Col. Chem. Assoc., 67 (1984) 157. R Kearton, J.S.D.C., 111 (1995) 368. H Skelton, Rev. Prog. Coloration, 29 (1999) 43. A G Perkin and A E Everest, The natural organic colouring matters, (London: Longmans, 1918). A W C Harrison, The Manufacture of lakes and precipitated pigments, (London: Leonard Hill, 1930). A Whitaker, J.S.D.C., 104 (1988) 294. V C Vesce, Official digest, 28 (1956) 1; 31 (1959) 530. H Gaertner, J. Oil Col. Chem. Assoc., 46 (1963) 13. BIOS Report 1661, (London: HMSO, 1946) 49, 83, 98-100. D Patterson, Bunsengesell. Phys. Chem., 71 (1967) 271. J D Hildreth and D Patterson, J.S.D.C., 79 (1962) 519. S Wang and C Zhou, Dyes and Pigments, 37 (1998) 327; 38 (1998) 185. A Zitscher, J.S.D.C., 70 (1954) 530. J C Harris, J.Amer. Chem. Soc., 70 (1948) 3671. F G Riddell et al, Dyes and Pigments, 35 (1997) 191. H B Weiser and E E Porter, J. Phys. Chem., 31 (1927) 1704. N H Haddock, J.S.D.C., 61 (1945) 68. W Braun and O Tcherniac, Ber. , 40 (1907) 2709. H de Diesbach and E van der Weid, Helv. Chim. Acta, 10 (1927) 886. J Robertson, J.C.S., (1935) 615. BIOS Report 960, (London: HMSO, 1946) 47.

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88 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

ORGANIC AND INORGANIC PIGMENTS; SOLVENT DYES

FIAT Report 1313, Vol 3, 273. W Gerstner, J. Oil Col. Chem. Assoc., 49 (1966) 954. F M Smith and J D Easton, J. Oil Col. Chem. Assoc., 49 (1966) 614. FIAT Report 1313, Vol 3, 285. H Liebermann, Ann., 518 (1935) 245. P Pollack, Chimia, 30 (1976) 357. Geigy, US Pat. 2 973 358 (1961). B L Kaul, Rev. Prog. Coloration, 23 (1993) 19. A Iqbal et al, J. Coatings Technol., 60 (1988) 37. R M Christie, Rev. Prog. Coloration, 23 (1993) 1. M Potter, Chemical Fibers Internat., 48 (Mar 1999) 62. S Schwarz and H Endriss, Rev. Prog. Coloration, 25 (1995) 6. H Kittel, Pigments, (Stuttgart: Wissenschaft Verlag, 1960). W Feitknecht in Pigments, an introduction to their physical chemistry, (London: Elsevier, 1967). D Huguenin and T Chopin, Dyes and Pigments, 37 (1998) 129. Colour physics for industry, 2nd Edn, Ed R McDonald (Bradford: SDC, 1997).

BIBLIOGRAPHY The most extensive compilation of information about pigments is in the volume Pigments and Solvent Dyes (1997), which is part of the Colour Index. For various reasons, there is now less technical information about the chemistry of the synthesis and manufacturing methods for pigments than appeared in the 1982 volume, although much of this was in references to patents.

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CHAPTER 3

Dye structure and application properties John Shore

3.1 DYE CHARACTERISTICS AND CHEMICAL STRUCTURE Virtually all commercial textile dyeing and printing processes take place by the application of a solution or a dispersion of the dyes to the textile material followed by some type of fixation process. The dye solution or dispersion is almost always in an aqueous medium. A major objective of the fixation step is normally to ensure that the coloured textile exhibits satisfactory fastness to subsequent treatment in aqueous wash liquors. In view of the overriding importance of water as a transfer medium in dyeing and printing it seems reasonable to begin with a discussion of the properties of dyes in solution and in dispersion. 3.1.1 The solubility of dyes in water In a pure ice crystal each oxygen atom is surrounded by four hydrogen atoms located in a tetrahedral configuration. Two of them are covalently bonded to the oxygen atom, conveniently forming an angle of 120 degrees. Hydrogen atoms from two other molecules are thus positioned equidistantly on opposite sides of the plane of this H–O–H structure. In steam, on the other hand, each molecule is freely and rapidly in motion, frequently colliding with other molecules but not capable of forming any type of bond with them at such elevated temperatures. The association between water molecules in the liquid state represents an intermediate condition. Molecular motion is slower than in steam and water is much denser. The formation, motion, collision and re-formation of clusters of two or more molecules is a characteristic feature of the liquid state. A hydrogen atom normally consists of a positively charged nucleus (a proton) and a much smaller orbiting electron. The proton is thus virtually unscreened, permitting the two electronegative oxygen atoms in neighbouring water molecules to approach one another more closely than they could in the absence of the intervening hydrogen atom. This reduction in the force of repulsion between the two electronegative atoms is known as a hydrogen bond [1]. This idealised two-dimensional representation of a seven-molecule cluster (3.1) illustrates that some molecules are more firmly held than others. In reality, of course, each cluster is three-dimensional in shape and the individual molecules adopt a more random orientation than indicated here. As the temperature of the liquid increases and the cluster becomes more mobile, the loose network of hydrogen bonds (indicated by dashed lines) holding the molecules together is weakened. In a collision the cluster is more likely to lose one or more fringe members (a) depending on a single hydrogen bond than those held in place by two (b), three (c) or four (d) links. The average cluster size thus decreases as the temperature rises. A higher proportion of free molecules is formed as the boiling point is approached, heralding the incipient evaporation as steam. 89

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H

H

H

Oa

H Ob

H

H

H

Od H

H Oa

H Oc

H

H

H Ob

H Oa

3.1

The dissolution of an inert nonpolar solute molecule in water has been described in terms of the ‘iceberg’ concept, by which transient local clusters of water molecules are stabilised by cohesive association around the nonpolar molecule. The enveloping cluster surrounding the latter contains a higher proportion of multilinked members (b–d) than are present in the freely mobile clusters typical of pure water. Hydrophobic interaction between nonpolar solute molecules arises directly from this ‘iceberg’ effect. Pairs of such stabilised cohesive clusters tend to merge and redistribute, forming a larger and more stable sheath around two or more solute molecules. These molecules then behave as if a binding force exists between them, described as hydrophobic interaction [2]. An analogous process within a clustering aqueous sheath favours preferential interaction between the hydrophobic segments of dissolved solute ions in concentrated solutions of dyes or surfactants. A dissolved ionic solute always has a more stable cluster of water molecules around the hydrophobic portion of the ion. This asymmetrical distribution favours coalescence of solute molecules in concentrated solution to form a dye aggregate or surfactant micelle. Once formed, however, such an associated moiety is further stabilised by the inner sheath of charged ionic groupings oriented outwards towards the aqueous clusters. Many anionic dyes (section 1.6) depend on their sulphonic acid groups for their solubility in water. Dye sulphonic acids have pK values within the range of pH 1–2 and are fully ionised under dyeing conditions as either the free acid or the sodium salt. The mutual electrostatic repulsion between dye sulphonate anions ensures their uniform separation and distribution in dilute aqueous solution. At higher concentrations, however, this repulsion is counterbalanced by mutually attractive forces of various kinds operating at shorter range [3]: (1) hydrogen bonding (2) van der Waals forces (3) hydrophobic or nonpolar bonding. The most important types of hydrogen bond are the hydroxy-ether, hydroxy-amine and imino-amine linkages (Figure 3.1). Thus one of the most significant forces of attraction between dye molecules containing hydroxy groups is identical with that governing mutual interaction in clusters of water molecules. Many water-soluble dyes (section 1.6) contain primary, secondary or tertiary amine groupings, all of which are capable of participating via hydrogen bonding in dye–dye association or dye–water solubilisation mechanisms. The van der Waals forces include dipole–dipole interaction between atoms of unlike polarity in

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neighbouring molecules and less specific p-bonding or dispersion forces between unsaturated groupings and aryl nuclei. Hydrophobic bonding between nonpolar segments of dye ions is particularly significant for acid dyes of high wet fastness solubilised by a single sulphonate group (section 1.6.7) and for basic dyes, which normally contain only one quaternary nitrogen atom (section 1.6.6).

O

H ... O

>

O

H... N

>

N

H... N

Figure 3.1 Types of hydrogen bond in order of relative strength (from the left: hydroxy-ether, hydroxyamine and imino-amine)

The bonding forces involved in hydrophobic interaction can be quite specific to the structure of the dye ions involved. A series of isomeric derivatives of 2-phenylazophenol-4sulphonic acid, each containing a trifluoromethyl substituent, was synthesised recently. The aqueous solubility of these monosulphonated acid dyes was found to be dependent on the location of this specific grouping in the dye molecule [4]. The solubility of an ionic dye in water normally increases with temperature, since the enhanced mobility favours electrostatic repulsion between ions rather than closer approach to form aggregates by means of the short-range attractive forces. Addition of a simple inorganic electrolyte, on the other hand, normally lowers the solubility limit at a given temperature. Such additions enhance the ionic character of the aqueous phase and help to stabilise the structure of dye aggregates by forming an electrical double layer within the sheath of clustered water molecules around them. In contrast, the addition of a water-miscible nonionic solute such as urea or an alkanol to an aqueous solution containing aggregates of dye ions has a disaggregating action at a given temperature and electrolyte concentration. Urea, for example, has a strong tendency to inhibit clustering because of the competitive influence of urea–water hydrogen bonds. Consequently the stabilising ‘iceberg’ cluster surrounding each dye aggregate tends to disintegrate. Dye–urea hydrogen bonds or dipole–dipole attractive forces further disrupt the hydrophobic interaction between pairs of dye ions. Nonionic solutes with a directional polar/nonpolar structural arrangement, such as longchain alkyl polyoxyethylene adducts, destabilise the loose structure of dye aggregates by a somewhat different mechanism. These additives readily form micelles in which the hydrophobic alkyl groups form the core and the oxyethylene chains are hydrogen-bonded within an extensive sheath of water molecules clustered around them. The formation of these large clusters denudes the less firmly held envelopes surrounding the dye aggregates, which become thinner and less stable. The lower dielectric constant of the surfactant solution enhances the electrostatic forces of repulsion between monomeric dye ions. Hydrogen bonding or dipolar interaction between the oxyethylene chains and individual dye ions further weakens the hydrophobic attraction between pairs of dye ions. All aqueous solutions of anionic or cationic dyes have lower surface tension than pure water. Solutions of the free sulphonic acids give lower values than solutions of the sodium salts of anionic dyes. The decrease in surface tension depends more on the degree of sulphonation of the dye than on its relative molecular mass, although alkyl substituents may

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exert a further lowering effect [5]. Thus a relatively hydrophilic tetrasulphonated disazo dye such as CI Direct Blue 1 (3.2) has comparatively little influence on surface tension, whereas a monosulphonated milling acid dye such as CI Acid Yellow 72 (3.3) can exhibit a mild detergency effect [6]. NaO3S

NH2

H3CO O

H2N

OCH3

H

H N

SO3Na

O

N N

N NaO3S

SO3Na

3.2 CI Direct Blue 1 Cl SO3Na H CH3(CH2)11

O

N

N N

N H3C

Cl 3.3

CI Acid Yellow 72

Measurements of the surface tension of aqueous solutions of various sulphonated and unsulphonated phenylazonaphthol dyes showed that the degree of surface activity (that is, the lowering of surface tension) tended to increase progressively with the degree of alkyl substitution in the series of dyes [7]. The surface-active behaviour of such alkylated dye ions ensures that they become more concentrated at the interface between the dyebath and the fibre surface, just as they do at the air–water interface of the dye solution. Foaming of dyebaths can be a serious practical problem with relatively hydrophobic dye structures solubilised by means of a single ionised group. Increasing concern about respiratory sensitisation by dust from dyes in powder form, especially reactive dyes, has generated extensive development work on concentrated solutions and low-dusting cold-dissolving granular forms of water-soluble dyes (section 1.7.1). Difficulties arise in attempting to market dissolved brands at concentrations close to the solubility limit, although these can be overcome by adding miscible solvents or solubilising agents. Hydrolysis during storage is another problem with reactive dyes of inherently high reactivity. Fluorotriazine dyes show maximum stability when buffered to pH 7–8, whereas sulphatoethylsulphone dye solutions require pH 4–5 to minimise premature hydrolysis. Dyes marketed as granules should show neither dusting nor hydrolytic instability, but the rate of dissolution may depend on average granule size and dust can be formed from granules during transportation [8]. Dyes marketed as aqueous solutions or dispersions should exhibit the following characteristics [8,9]: (1) Constant flow properties under normal ambient conditions (2) Complete and rapid homogenisation on stirring after a freeze/thaw cycle

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(3) Ease of dispensing in automatic metering equipment (4) Complete miscibility with water and other liquid products used in dyebath or printpaste formulations (5) No drying-out or deposition of dye or other component on the inside of the container (6) No crystallisation of solid particles from solution (7) No agglomeration or sedimentation of particles in dispersion (8) No microbial growth on storage. Disperse liquid brands of sulphur, vat or disperse dyes, including mixed formulations of disperse and vat dyes in matching hues for the dyeing of polyester/cellulosic blends, have been commercially available for many years. The proportion of reactive dyes marketed as aqueous solutions is expected to increase markedly [10] because of: (1) Optimum safety for operatives (2) Versatility of formulation to suit specific customer needs (3) Rapid cleaning of dispensing equipment when changing the target shade (4) Ease of dispensing via automatic metering devices (5) Unrestricted application with special suitability for continuous dyeing and printing. 3.1.2 Aggregation of dyes in aqueous solution Dyes often exist in aqueous solution as aggregates of several ions or molecules rather than individual moieties. It is unlikely that all component anions in a multisulphonated dye aggregate are in the same state of ionisation. Unlike surfactants, dye molecules in general do not possess structures with directional polar and nonpolar segments. Thus they do not associate as micelles of ordered structure with a well-defined hydrophobic core and a charged hydrophilic sheath. Dye aggregates should be envisaged as relatively amorphous in composition with zones of more or less polar character distributed within them, although there will be a tendency for individual molecules to become oriented with their ionised groupings towards the aqueous phase. Within a structural series, aggregation is usually greater for dyes with a higher ratio of relative molecular mass (Mr) to ionic group content, that is, a higher value for the equivalent mass per sulphonic acid group in the case of sulphonated anionic dyes. Multisulphonated dye anions may be stabilised as dimers in which the two components adopt a coplanar arrangement with the sulpho groups located if possible at opposite ends of the dimer. More hydrophobic structures of higher equivalent mass, especially monosulphonates, can readily form aggregates of larger sizes. Dimers are formed first, growing further by accretion of more dye anions to form lamellar micelles, in which the dye moieties are stacked like cards in a pack [3]. Planar chromogens, especially the sulphonated phthalocyanine dyes, are particularly prone to such stacking. Equilibrium constants for dimerisation (KD) have been determined [3] for several classical dyes from different chemical classes (Table 3.1). Although differing considerably in Mr and structural type, these values are all within the same order of magnitude (logKD = 3–4 at 25 °C). It seems likely that with most dyes the forces governing aggregation are mainly of the van der Waals type, including dipole–dipole and dispersion forces. Unlike hydrogen bonds, these are able to operate through successive layers of stacked molecules. An attempt to obtain equilibrium constants for the dimerisation of four p-substituted phenylazo-1-naphthol-6-

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Table 3.1 Equilibrium constants for dimerisation of dyes [3] CI Generic name

Chemical class

Mr

Log KD

Basic Orange 14 Basic Red 6 Basic Blue 9 Basic Red 1 Mordant Violet 5 Acid Orange 7

Acridine Azine Thiazine Xanthene Monoazo Monoazo

265 422 284 443 344 328

4.02 3.78 3.77 3.36 3.29 3.12

H R

O

N

SO3Na N

Ketohydrazone 3.4

H R

N

O SO3Na

N Hydroxyazo

Scheme 3.1

sulphonic acids (3.4) failed because of the superimposed effects of acid-base, tautomeric (Scheme 3.1) and oligomeric equilibria [11]. Thermodynamic aspects of the aggregation behaviour of three monoazo acid dyes in aqueous solution have been studied by a potentiometric method. An anion-selective membrane electrode permitted the direct derivation of dye monomer concentration in the system. Graphs of e.m.f. against dye concentration were non-linear owing to dye–dye association at the higher concentrations. These deviations enable the dye monomer concentration to be determined at any total dye concentration. Adoption of an appropriate stepwise association model provided a framework for computing the successive multimerisation constants and the respective concentrations of dimers and higher multimers [l2]. Another novel procedure to determine the aggregation constants of water-soluble dyes and the extinction coefficients of the monomeric dye ions has been proposed recently [13]. Most measurements of aggregation number (the average number of ions or molecules per aggregate) have been carried out on solutions of direct dyes, because these aggregate more readily than most of the other classes of water-soluble dyes. Regrettably, aggregation numbers determined for the same direct dyes under the same conditions using different methods of measurement are seldom consistent. Most results indicate, however, that such solutions contain a mixture of aggregates of various sizes in dynamic equilibrium, somewhat analogous to the mixed clusters of molecules in pure water. When individual anions or dimers are absorbed by a cellulosic fibre during dyeing, larger aggregates break down to maintain a similar overall distribution of aggregate sizes. The extent of aggregation of a direct dye increases with increasing concentration of dye or added electrolyte, but decreases as the dyebath temperature is increased. Although many direct dyes are highly aggregated at

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ambient temperature, the degree of aggregation is often negligible under exhaust dyeing conditions at the boil even in the presence of electrolyte. Although quantitative agreement between the aggregation numbers obtained using different techniques of measurement is poor, meaningful conclusions can be drawn from them regarding the relative tendency of different direct dye structures to aggregate. Under given conditions of temperature and electrolyte concentration, CI Direct Orange 26 (3.5) is much more highly aggregated than CI Direct Yellow 12 (3.6). Both are disulphonated disazo dyes, but presumably the two o-ketohydrazone groupings and especially the ureido residue in the orange dye provide much greater scope for intermolecular hydrogen bonding than the etherified phenolic groups in the yellow dye. The sulpho groups on the central stilbene chromogen of Yellow 12 tend to inhibit aggregation. In a detailed study CI Direct Red 81 (3.7; X = H), an important dye that has good migration properties but low affinity and poor fastness to washing, was compared with its p-benzoylamino analogue (3.7; X = NHCOPh). The higher affinity and wet fastness but inferior migration of the latter was found to be associated with an increased tendency to aggregate, presumably due to the greater scope for hydrogen bonding via the amide groupings. O H

O

O

C

N

N N

H

NaO3S

H

N

N N

H

SO3Na

3.5 CI Direct Orange 26

SO3Na CH3CH2O

CH

N N

N N

CH

OCH2CH3

NaO3S

3.6

CI Direct Yellow 12 H N NaO3S

N

O

N

H N

N

C

X

O 3.7

NaO3S

Three disazo analogues derived from naphthionic acid as coupling component are all highly aggregated in solution: CI Direct Red 28 (3.8; X = Y = H), CI Direct Red 2 (3.8; X = CH3, Y = H) and its m-tolidine isomer (3.8; X = H, Y = CH3). The two tolidine derivatives aggregate more readily than does Red 28, although the nonplanar m-tolidine dye is not so highly aggregated as the coplanar o-tolidine structure in Red 2, which is more sensitive to flocculation by salt and several times more substantive to cellulose than its mtolidine isomer. The tetrasulphonated dye CI Direct Blue 1 (3.2) is considerably less aggregated than these disulphonated naphthionic acid derivatives.

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DYE STRUCTURE AND APPLICATION PROPERTIES

X

Y

Y

X NH2

NH2 N

N

N

N

SO3Na

SO3Na

3.8

In a recent investigation the aggregation of CI Direct Red 1 (3.9) in aqueous solution was studied as a function of pH, temperature and dye concentration [14]. At a given temperature and concentration, the high aggregation number at pH 4 confirmed that only the sulphonate group in the γ acid residue at one end of this disazo structure was ionised. At higher pH values the carboxyl group in the salicylic acid residue at the other end of the molecule became ionised, increasing the solubility and lowering the aggregation number. The degree of aggregation decreased with increasing temperature as expected, but this effect of pH on ionisation and disaggregation was evident at temperatures between 25 °C and 60 °C. SO3Na N

HO

HO N

N N

NaOOC 3.9

H2N

CI Direct Red 1

Although acid dyes in general are considerably less aggregated than direct dyes under similar conditions, they show broadly similar trends between structure and aggregation behaviour. Thus an increase in the size of the coplanar aryl nuclei in monoazo acid dyes (for example, replacement of a phenyl by a naphthyl residue) increases the tendency to aggregate. A recent spectrophotometric study of aggregation equilibria in a series of alkylsubstituted azo acid dyes revealed that only dimerisation took place with alkyl chain-lengths up to hexyl. Formation of multimers, however, was observed with octyl or decyl groups. As a general rule the dimerisation equilibrium constant increased progressively with the length of the n-alkyl chain, but the presence of a sec-butyl substituent produced an anomalous decrease in equilibrium constant attributable to steric interference with hydrophobic interaction between dye anions [15]. The aggregation and tautomeric equilibria of CI Acid Red 138 (3.10) have been examined recently. In spite of the length of the dodecyl substituent in the diazo component only monomers and dimers were present [16], presumably because the H acid residue contributes two sulpho groups to solubilise the molecule. Both monomer and dimer species were predominantly in the hydrazone form, as expected, and hydrophobic interaction between the terminal dodecyl chains played an important part in formation of the dimer. An increase in the degree of sulphonation of a typical azo acid dye structure minimises aggregation, especially if the sulpho groups are widely separated from one another and from the azo group. Light-scattering studies demonstrated freedom from aggregation with the

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important monoazo monosulphonate CI Acid Orange 7 (3.11) and many simple monoazo di- or trisulphonates, but typical disulphonated milling dyes such as CI Acid Red 111 (3.12) showed salt-sensitive aggregation. With the disazopyrazolone disulphonate CI Acid Orange 63 (3.13) the aggregation appeared to be virtually independent of the concentrations of dye and electrolyte at low temperatures. Aggregation greatly decreased above about 60 °C, in good agreement with the anomalous dyeing properties of this dye. It may form stable micelles analogous to those of surfactants and this effect is apparently highly sensitive to temperature in the 50–70 °C region. O C

CH3

HN H CH3(CH2)11

H

O

N

SO3Na

NaO3S

O

N N

N 3.10

NaO3S

3.11 CI Acid Orange 7

CI Acid Red 138 CH3 H H3C

SO2 O

O SO3Na

N

N

N

N H3C 3.12

NaO3S

CI Acid Red 111

SO3Na H H3C

N

SO2 O

O

N

N

N N

3.13

N CH3

CI Acid Orange 63

The aggregation behaviour and tautomerism of three o,o′-dihydroxy and one o-hydroxyo′-methoxy monoazo dyes have been studied by UV-visible spectroscopy [17]. Evidence of monomer-dimer equilibria was obtained for all four of these mordant dye structures. Intermolecular hydrogen bonding between the hydroxy groups and hydrophobic interaction between aryl nuclei contribute to the dimerisation effect. The influence of metal ions on the aggregation of CI Reactive Red 2 (3.14) in aqueous solution at various pH values has been examined in detail recently [18]. Sodium ions have a profound effect on these disulphonated dye anions, so that the enhancement of substantivity during the dyeing process is accompanied by aggregation attributable to hydrophobic interaction between the phenyl and s-triazine ring systems. Aggregation is much greater,

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DYE STRUCTURE AND APPLICATION PROPERTIES

however, in the presence of divalent calcium or magnesium ions, which can form intermolecular electrostatic linkages between sulpho groups in different dye anions. This accounts for the adverse influence of hard water on the level dyeing properties of reactive dyes. Cl N N

Cl N

HN H

O

N

SO3Na N 3.14

NaO3S CI Reactive Red 2

The introduction of a cyclic amide system into a monoazo dye molecule considerably enhances its substantivity for cellulose. This was demonstrated recently by evaluating a series of aminochlorotriazine structures derived from 5-aminobenzimidazolone as diazo component, typically the bluish red H acid derivative (3.15). The urea residue in the imidazolone ring is a powerful hydrogen bonding moiety. These dyes exhibited excessive aggregation when dyed at a temperature lower than 95 °C in the presence of a conventional salt concentration but showed markedly higher exhaustion at a relatively lower ionic strength, where the degree of aggregation was similar to analogous dye structures derived from aniline as diazo component. These results indicate that benzimidazolone-type reactive dyes are of potential interest as ‘low-salt’ products [19].

SO3Na

Cl N N

NH N

HN H N

HN C O

O SO3Na

N NH NaO3S

3.15

Owing to experimental difficulties, knowledge of aggregation effects in alkaline dithionite solutions of leuco vat dyes is sporadic [20,21]. Investigations based on absorption spectra have shown that, depending on concentration and temperature, planar polycyclic molecules such as the violanthrone derivatives CI Vat Blues 19, 20 and 22 and the perylene tetracarboxydiimide derivatives CI Vat Reds 23 and 32 are mainly present as monomers or dimers in leuco vat solutions. Violanthrones that do not have a coplanar structure because of the presence of

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DYE CHARACTERISTICS AND CHEMICAL STRUCTURE

alkoxy groups in the 16,17-positions, such as CI Vat Blue 16 and the widely used Greens 1, 2 and 4, are always present as single molecules under vat dyeing conditions. 3.1.3 Decomposition of dyes under reducing conditions The primary objective of an exhaust dyeing process is to transfer as much as possible of the dye in the system from the dyebath into the fibre, provided this can be achieved with the dye uniformly distributed throughout the dyed material at the end of the process. If dyeing is unduly prolonged, either to improve the levelness or to attain an exact colour match to standard, decomposition of a minor proportion of the dye present may occur. The rate of decomposition is normally much more rapid in the dyebath phase than on the fibre and the onset of degradation can sometimes be observed as a change in colour of the exhaust liquor. This effect usually occurs as a hypsochromic shift, such as reddening of a blue or yellowing of a red solution. Many direct dyes show instability of this nature if applied at a temperature above the boil, as in the dyeing of polyester/cellulosic blends, for example. A frequent cause of the problem is reduction of azo linkages. This may be accelerated in the presence of viscose, which has a reducing action under alkaline dyeing conditions. Direct dyes with exposed azo groups free from o-substituents, like the two azo links in CI Direct Yellow 12 (3.6), are particularly prone to reductive decomposition. However, some dyes with o-aminoazo groups, such as CI Direct Red 2 (3.8; X = CH3, Y = H) or Red 28 (3.8; X = Y = H), are also reductionsensitive. In the relatively unstable trisazo structure of CI Direct Green 33 (3.16), the most vulnerable of the three azo groups is the unprotected central linkage, so that the likely initial products of reduction (Scheme 3.2) are two monoazo dyes (3.17 and 3.18), resulting in a hypsochromic reddening and dulling of the blue-green solution. COCH3

H3C OCH2CH3

SO3Na N

N

N

H N

HN O

N

H3C

SO3Na N 3.16

NaO3S NaO3S CI Direct Green 33 COCH3

4H H3C

H

SO3Na NH2

N

HN

OCH2CH3 +

H2N

N

O

N

SO3Na N

H3C 3.17

NaO3S NaO3S

Scheme 3.2

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3.18

100

DYE STRUCTURE AND APPLICATION PROPERTIES

Dyes containing azo links protected by o,o′-substituents, especially o-sulphonate groups, are generally relatively resistant to reductive breakdown during dyeing. Azothiazole dyes such as CI Direct Yellow 29 (3.19) and especially copper-complex structures such as CI Direct Blue 98, in which both azo groups are fully protected as the metal complex (3.20), show excellent stability in high-temperature dyeing conditions [22]. Buffering with ammonium sulphate confers a stabilising influence when dyes of inferior stability have to be used on viscose. Mild oxidants such as sodium m-nitrobenzenesulphonate are occasionally useful and more powerful inorganic oxidants, such as potassium chlorate or dichromate, can be most effective if used with care. Excessive use of these additives, however, can result in oxidative decomposition of azo dyes. SO3Na H3C

SO3Na

S S

CH3

S N

N N

S N

N N

3.19 CI Direct Yellow 29

O

OH2

H2 O

O

O

Cu

NH

Cu

O NH

N

N N

N SO3Na

NaO3S

3.20 CI Direct Blue 98

Direct dyes of the disazo diarylurea type, such as CI Direct Orange 26 (3.5) and CI Direct Red 79 (3.21), are particularly prone to decomposition in high-temperature dyeing because molecular breakdown can occur by hydrolysis as well as by the reductive mechanism under alkaline conditions. Thus the ureido linkage in this red dye may be broken by hydrolysis to give two monoazo dye fragments (3.22), or the azo groups can be reduced to yield a diaminodiphenylurea (3.23) and two molecules of H acid (Scheme 3.3). The azo linkage in some acid dyes is sensitive to the mild reducing action of the cystine and cysteine residues in wool under hot alkaline conditions. CI Acid Blue 113 (3.24) is a commodity dye showing good all-round wet fastness on wool and nylon, but it tends to turn brown during dyeing if exposed to alkaline reducing conditions at high temperatures. The degradation involves reduction to metanilic acid and a brown monoazo dye (3.25), the rate increasing rapidly above pH 7 (Scheme 3.4). Hydrolysis of ester groups can also occur in certain wool dyes. For example, CI Acid Red 1 (3.26) loses its acetyl group (Scheme 3.5) on prolonged dyeing in an acid dyebath, the hue becoming bluer and duller and the dyeing much less fast to light. An interesting study of the relative rates of reduction of N,N-bis(2-chloroethyl)aminoazobenzene and various monosubstituted derivatives (3.27) was initiated because of the ability of such dyes to inhibit the growth of animal tumours [23], even though some

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DYE CHARACTERISTICS AND CHEMICAL STRUCTURE

CH3 SO3Na H 2N

N N

HCOONa + 2 H3CO

HO NaOH

3.22 H3C

SO3Na

CH3

O

NaO3S

SO3Na N

N

N

N

H OCH3

N N

H H3CO

HO

OH

3.21 NaO3S

SO3Na

CI Direct Red 79

H3C

SO3Na

8H

CH3

O

H2N H2N

N

N

H

NH2

H

OCH3

2 HO

H3CO SO3Na

3.23 H acid

Scheme 3.3

NaO3S N

N

N

N

NH SO3Na

3.24 CI Acid Blue 113 4H

NaO3S NH2

+

H 2N

N N

NH SO3Na

3.25

Scheme 3.4

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101

102

DYE STRUCTURE AND APPLICATION PROPERTIES

COCH3 HN H

H2N

O

H

H+

N

SO3Na

O SO3Na

N H2O

N

+ HOOC.CH3

N

3.26 NaO3S

NaO3S

CI Acid Red 1

Scheme 3.5

dialkylamino analogues such as 4-(dimethylamino)azobenzene have long been known to cause liver cancer in rats. A selection from the results is presented in Table 3.2, expressed in terms of the relative rate constant 100 k/k0 where k and k0 are the respective rate constants for the substituted and unsubstituted compounds. Electron-donating substituents (methyl, methoxy) in the 3- and 4-positions were found to retard reduction, whereas electronwithdrawing groups (bromo, chloro, acetyl, carboxy, sulpho) in these positions enhanced the rate. Steric effects tended to predominate, however, when relatively bulky substituents (phenyl, bromo, iodo) were present in the 2-position. Y Z

X N

CH2CH2Cl N

3.27

N CH2CH2Cl

Table 3.2 Rates of reduction of monosubstituted monoazo dyes by tin(II) chloride [23]

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Substituent in structure 3.27

Relative rate constant

X = nitro X = carboxy X = sulpho Z = acetyl Z = carboxy Z = sulpho Z = bromo Z = chloro X = chloro Z = phenyl None Y = methyl X = bromo X = phenyl X = methyl X = iodo Z = methyl Z = methoxy

very high very high 2470 2100 1180 374 254 206 113 101 100 94 80 68 62 39 36 20

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DYE CHARACTERISTICS AND CHEMICAL STRUCTURE

103

The mechanism of the reduction reaction involves the initial protonation of the azo nitrogen further from the N-chloroethyl groups (Scheme 3.6), formation of the quinonoid cationic species (3.28) being responsible for the bathochromic effect observed when dyes of this series are treated with acid. In the case of the 2-carboxy and 2-nitro derivatives of the parent dye, reduction of the azo linkage took place so rapidly that the rate could not be measured. The 2-carboxyphenylazo grouping readily chelates with metal ions and it seems likely that chelation of the tin(II) ion exerts a powerful catalytic effect on the reduction rate. In the 2-nitrophenylazo dye both the nitro and azo groups are potential sites for reduction to occur. In fact the azo linkage was reduced very rapidly but the yield of o-nitroaniline was quantitative, indicating that the nitro group was quite stable under these circumstances [23].

N

CH2CH2Cl N

N CH2CH2Cl

_ N

+ N

N

CH2CH2Cl CH2CH2Cl

H+ H N

+ N

N

CH2CH2Cl 3.28 CH2CH2Cl

Scheme 3.6

Mordant dyes are notoriously troublesome from the viewpoint of colour matching because the hue of the chromium complex usually differs greatly from that of the unmetallised parent dye (section 5.4.1). If other metal ions are present in the treatment bath or on the fibre during chroming, the colour obtained is likely to differ from that of the pure chromium complex. Certain important chrome dyes, including CI Mordant Black 11 (3.29) and Black 17 (3.30), are particularly sensitive to traces of iron or copper. The hue of the black dyeings obtained is redder in the presence of copper and browner with iron contamination. The fastness to light and wet treatments may also prove inferior under these conditions. Even certain 1:2 metal-complex acid dyes show similar effects in the presence of these impurities, OH

OH H NaO3S

H

O NaO3S

N

N N

N

O2N

3.29 CI Mordant Black 11

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O

3.30 CI Mordant Black 17

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DYE STRUCTURE AND APPLICATION PROPERTIES

indicating that the iron or copper ions are able to displace chromium from the desired chromium complex. Sequestering agents of the sodium polyphosphate type (section 10.2) can help to minimise these problems. With some leuco vat dyes, chemical changes may take place in alkaline reducing media at high temperatures [24,25]. Many vat dyes contain benzoylamino substituents to confer higher affinity for cellulosic fibres. Alkaline hydrolysis of such dyes may result from prolonged treatment with loss of the benzoyl groups and a gradual lowering of substantivity, leading to unexpectedly low exhaustion of the dyebath. Thus CI Vat Yellow 3 (3.31) is hydrolysed under these conditions (Scheme 3.7), the hue becoming redder and duller because of the formation of the red 1,5-diaminoanthraquinone, and the dyeing exhibits lower fastness to washing. O H O

C N

3.31 CI Vat Yellow 3 N

O H

C

O

NH2

O NaOH

2

COONa

+

H2N

O

Scheme 3.7

High-temperature vatting or dyeing conditions may cause dehalogenation of vat dyes containing bromo or chloro substituents, resulting in a change of hue towards the parent chromogen. Thus if dichloro (CI Vat Violet 1) or monobromo (Violet 9) derivatives of isoviolanthrone are vatted and dyed on cotton at 90 °C rather than normally at 60 °C, a markedly bluer hue closer to the unsubstituted compound (Violet 10) is obtained. Under similar conditions, dyeings of dibromo (Green 2), dichloro (Blue 17) or trichloro (Blue 18) derivatives of violanthrone do become slightly redder and duller, but the contribution of the parent dye (Blue 20) formed is small because these halogenated structures are more stable than their isomeric analogues. Chlorinated acridone derivatives such as Red 39 and Violet 14 may also suffer dechlorination. Monochloro (Blue 14) or dichloro (Blue 6) derivatives of indanthrone lose chlorine even when dyed above 50 °C and this involves a deterioration in fastness to bleaching. Indanthrone (3.32; CI Vat Blue 4) and its derivatives require the reduction of only two of the four keto groups to form the sparingly soluble disodium leuco compound (Scheme 3.8). Hydrogen bonding between the inner phenolic groups and the neighbouring azine nitrogen atoms stabilises the structure (3.33). Loss of oxygen atoms can occur, however, as a result of over-reduction at temperatures above 60 °C. The tendency for this irreversible reaction to ensue increases with time and temperature of reduction and with the concentrations of

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ONa

O

H

H O

O

N N

N N

O

O H

H 3.32 O

105

3.33 ONa

Indanthrone

Scheme 3.8

ONa

O

N

N

N

N

3.35

3.34 Flavanthrone

O

2H

ONa

ONa

H N N H

3.36 ONa Scheme 3.9

alkali and dithionite. Additions of glucose or sodium nitrite have a stabilising influence on the disodium enolate solution. Over-reduction yields redder and duller dyeings because of the formation of a brown derivative that will not respond to the oxidising agents normally used after vat dyeing. Flavanthrone (3.34; CI Vat Yellow 1) behaves in a similar way (Scheme 3.9). Only two hydrogen atoms are required to yield the blue disodium leuco form (3.35), but this is more stable than the analogous disodium enolate of indanthrone. Nevertheless, over-reduction to the brown flavanthraquinol (3.36) can take place under severe conditions of high alkalinity and temperature. In these circumstances the dyeings can no longer be reoxidised to flavanthrone even using acidified dichromate solution. These irreversible overreduction reactions are not known for any classes of anthraquinonoid vat dyes other than the indanthrone and flavanthrone chromogens.

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DYE STRUCTURE AND APPLICATION PROPERTIES

3.1.4 Rates of reduction and redox potentials of vat dyes The rate of reduction of a vat dye depends partly on the intrinsic chemical properties of the dye and partly on the size and physical form of the dispersed particles undergoing this reaction. The physical factors are much less important than the chemical aspects [26]. The vatting process entails conversion of the insoluble keto form into the soluble sodium enolate (section 1.6.1). The reaction takes place in two stages at ambient temperature. Extremely rapid reduction to the hydroquinone is followed by slower dissolution in the alkaline solution. At higher temperatures, however, the dissolution rate approximates more closely to the rate of reduction. Temperature and dithionite concentration are the important variables and the rate of reduction is much less dependent on dye or alkali concentration. The ease of reduction of a vat dye depends on its chemical structure in relation to the reducing power of the agent selected. In practice this question is seldom critical because the reduction potential developed by a typical reducing agent is low enough to permit the reduction of all vat dyes of practical interest. Indeed, such dyes would never have been exploited commercially if they had not responded effectively to reduction under conventional conditions of application. The times of half-reduction of five typical vat dyes are listed in Table 3.3. Most of the commercially important products give values within the range 25–500 seconds. Few dyes are as slow to reduce as CI Vat Red 1 and even fewer are reduced as quickly as flavanthrone. In fact, when these times were measured the average particle size of the sample of flavanthrone under test was greater than that of the CI Vat Green 1 sample, which was reduced at least ten times more slowly [26]. Table 3.3 Times of half-reduction of typical vat dyes [26] CI Vat

Chemical name

Yellow 1 Blue 17 Orange 9 Green 1 Red 1

Flavanthrone 16,17-Dichloroviolanthrone Pyranthrone 16,17-Dimethoxyviolanthrone 6,6′-Dichloro-4,4′-dimethylthioindigo

Time(s) <5 31 36 50 2880

Vat dye reduction is a reversible reaction leading to the formation of an equilibrium mixture of the oxidised and reduced forms. The relative rates of reduction of vat dyes bear no obvious relation to their intrinsic reduction potentials, which determine the position of the equilibrium reached when the keto form of the vat dye is dissolved under a given set of reducing conditions. The relative stabilities of the reduced and oxidised forms can be estimated from the reduction potential developed at a bright platinum electrode (Table 3.4). An unstable leuco compound tends to revert to the oxidised form by transfer of electrons to the electrode, developing a high negative reduction potential. On the other hand, a dye with a small negative reduction potential is easy to reduce because the leuco form is relatively stable. Few vat dyes are as difficult to reduce as CI Vat Yellow 3 (3.31) in which the amide NH groups form hydrogen bonds with the keto groups, or as easy to reduce as tetrabrominated

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107

Table 3.4 Standard reduction potentials of vat dyes in pyridine [26] CI Vat

Chemical name

Potential(mV)

Yellow 3 Orange 9 Green 1 Yellow 4 Yellow 1 Red 6 Blue 1 Blue 4 Blue 20 Blue 17 Red 1 Blue 5

1,5-Dibenzoylaminoanthraquinone Pyranthrone 16,17-Dimethoxyviolanthrone 1,2,4,5-Dibenzopyrenequinone Flavanthrone 6-Chloro-6′methoxy-4-methylthioindigo Indigo Indanthrone Violanthrone 16,17-Dichloroviolanthrone 6,6′-Dichloro-4,4′-dimethylthioindigo 5,5′,7,7′-Tetrabromoindigo

–490 –270 –250 –240 –230 –220 –220 –220 –220 –210 –190 –70

indigo. Most dyes of commercial interest have reduction potentials within the range –200 to –300 mV. It is interesting to note from Tables 3.3 and 3.4 that CI Vat Yellow 1 is inherently less stable in the reduced form than is CI Vat Red 1, even though the yellow flavanthrone is reduced approximately a thousand times quicker than is the red thioindigo derivative. Although standard reduction potentials have to be measured in anhydrous solvent solution for practical reasons, the results appear to be in general agreement with the qualitative behaviour of vat dyes in aqueous media. Thus indigoid dyes are more stable in the leuco form than are the anthraquinone derivatives in general, but they tend to be reduced more slowly and need a higher temperature for complete reduction. Cyclic voltammetry has been used to determine the standard heterogeneous rate constants and standard Gibbs free energy for the oxidation-reduction of thioindigo (CI Vat Red 41) on cellulose. The substrate plays an active role in these reactions of the dye when they take place during dyeing, including lowering of the activation energy of the redox processes. This technique allows the vat dyeing mechanism to be modelled more closely [27]. The application of indirect electrolysis as a reduction technique in indigo dyeing has been reported recently. The build-up, penetration and final depth attained were similar to those observed in conventional dyeing with alkaline dithionite. Various reversible redox systems were evaluated as potentially suitable for indigo dyeing, particularly an iron(II)triethanolamine complex. The iron(III) form of this complex can be transformed into the iron(II) form by cathodic reduction, thus leading to a regenerable reducing agent. This approach offers the prospect of improved process stability, because the reduction state of the dyebath can be monitored by measuring the reduction potential. By regenerating the mediator system, significant reductions in consumption of chemicals and water can be achieved. The process engineering involved can readily be applied to the commercial production of indigo-dyed denim [28]. 3.1.5 Substantivity and ionisation of indigo Indigo was in short supply during the 1970s and development work at Dan River Inc. (USA) demonstrated that the consumption of indigo in the dyeing of cotton denim yarn could be

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DYE STRUCTURE AND APPLICATION PROPERTIES

markedly reduced using a sodium carbonate/hydroxide buffer rather than caustic soda alone to generate lower dyebath pH values. Dyebaths produced in this way were less stable, however, resulting in a variable pH, more deposition of inadequately dissolved dye and a greater risk of unlevel dyeing. In recent years the effect of dyebath pH on the yield and penetration of indigo dyeings has been studied in greater detail [29]. Measurements of apparent colour yield, expressed in terms of the reflectance absorptivity coefficient of the indigo-dyed yarn, showed this to be a maximum at pH 10.5–11.5 and to decrease progressively with pH, until at pH 13.5 the colour yield was only about one-sixth of that at pH 11 (Figure 3.2). The degree of penetration of the dyeings was also highly dependent on dyebath pH, poorly penetrated ring dyeings being obtained at pH 11 and almost fully penetrated ones at pH 13. This is attributable to a rapid rate of strike onto the fibre surface under conditions of much higher substantivity at the lower pH. It is well-known that the reduction of indigo (3.37) proceeds according to Scheme 3.10, to give firstly the monoionised sodium hydrogen enolate (3.38) and then its conjugate base the disodium enolate (3.39). The relative amounts of these species present is governed by the dyebath pH. By extrapolation from the data for the pK values of the various sulphonated derivatives of indigo, it was shown that approximate pK values of 8.0 and 12.7 respectively can be taken to represent the ionisation equilibria in Scheme 3.10. Using these statistically estimated values, the fractions of reduced indigo present as species 3.34 and 3.35 at a given pH can be calculated [29].

Reflectance absorptivity coefficient

200

150

100

50

10.5

11

12

11.5

12.5

13

13.5

Dyebath pH

Figure 3.2 Effect of dyebath pH on apparent colour yield in a five-dip indigo dyeing process [29] _ Na+O

H

O

H

H

N

N N

N

3.37

3.38 Mono-enolate

Indigo

N N

OH

H

O

H

_ Na+O

_ O

H

3.39 Di-enolate

Scheme 3.10

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DYE CHARACTERISTICS AND CHEMICAL STRUCTURE

109

Sorption studies have revealed that both equilibrium sorption and the ratio of the monoionised to diionised forms of indigo are markedly higher at pH 10.5–11.5, where the highest colour yield is achieved in indigo dyeing [30]. Above this region of pH, the fraction of indigo that exists as the monoionised species decreases rapidly with increasing pH. Above about pH 12.7, a greater proportion of the leuco dye is present as the disodium enolate (3.39). Good linear correlation was obtained (Figure 3.3) between the colour yield data converted to a fractional scale and the fraction of indigo present in the dyebath as the monoionised form over the pH range 10.5–13.5. Improved formulation of buffered alkalis has enabled the degree of ring dyeing obtained in the commercial indigo dyeing of cotton denim yarn to be closely controlled [29].

Fractional colour yield

1.0

0.8

0.6 0.4

0.2

0.2

0.4

0.6

0.8

1.0

Fraction of monoionised indigo present

Figure 3.3 Relation between fractional colour yield and fraction of monoionised indigo [29]

3.1.6 Oxidative decomposition of dyes Oxidising agents have always been important reagents for the dyer and printer, not only for the reoxidation of vat or sulphur dyes but also in bleaching, stripping and discharge treatments. In recent years, interest in the oxidative decomposition of dyes has greatly increased as a means of dealing with the problem of colour in waste-water (section 1.7.3). Catalytic oxidation is probably the simplest approach to decolorisation of residual dyes. Polyp-phenylene-1,3,4-oxadiazole may be used to induce oxidative decomposition under UV irradiation [31]. Azo acid dyes derived from H acid or K acid can be degraded by photoassisted catalysis in an aqueous suspension of titanium dioxide. The reaction followed apparent first-order kinetics, the effect of pH varying with the dye structure [32]. Hydroxyl radicals play an essential role in the oxidative fission of the azo groups. The triplet state lifetimes of the dye species are almost independent of the presence of titanium dioxide, implying that the triplet state does not react with hydroxyl radicals or anionic superoxide free radicals [33]. Ozone treatment has potential as a powerful means of decolorising exhaust dyebaths with at least 95% colour removal for good reproduction of bright shades after recycling [34]. The effect of ozone concentration on the decomposition of reactive dye hydrolysates (eight orange or red monoazo and two blue anthraquinone dyes) was investigated [35]. The ecological properties of ozone-treated N-containing hydrolysates and the influence of

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surfactants on the ozonolysis reaction were evaluated. GLC, HPLC and mass spectrometry are valuable techniques for following the reactions and identifying the products of decomposition [35,36]. Fundamental study of the ozonolysis mechanism has shown that the azostilbene CI Direct Yellow 12 (3.6) yields mainly 4-carboxy-4′-ethoxyazobenzene-3sulphonate (3.40) and hydrogen peroxide, indicating that the stilbene group is more oxidation-sensitive than the azo linkages in spite of the protection afforded by the two osulphonate groups [37]. The stability to ozone of 2-arylazo-1-naphthol-3,6-disulphonate dyes (3.41) was investigated. Electron-withdrawing nitro or bulky perfluoroalkyl substituents in the diazo component conferred increased resistance to ozone attack [38].

SO3Na CH3CH2O

N N 3.40

COONa

H

O

N

SO3Na N

NaO3S

3.41

Commercially viable systems for the decolorisation of spent dyebaths can be based on hydrogen peroxide treatment initiated by UV radiation. A representative selection of six disulphonated monoazo acid dyes and two disazo disulphonated types was exposed for various times in a pilot-scale photochemical reactor of this kind. The controlling parameters were dye structure, pH, peroxide dosage and UV light intensity [39]. In a wider survey of the response of various classes of dyes to the combination of UV radiation and hydrogen peroxide, viable candidates for further in-plant treatment trials were the water-soluble reactive, direct, acid and basic classes. On the other hand, water-insoluble colorants such as disperse and vat dyes did not appear to be viable [40]. The photo-oxidation of several water-soluble monoazo dyes was studied in hydrogen peroxide solution, irradiation being carried out using a low-pressure mercury vapour lamp. The reaction was found to be pseudo-first-order with respect to dye concentration. A labile hydrogen atom in the dye molecule was the primary target of attack and the predominant hydrazone form of the chromogen was more sensitive to oxidation than the azo tautomer [41]. An alternative hydrogen peroxide treatment at temperatures close to 100 °C in the absence of UV irradiation was evaluated using three repreeentative anionic dyes of commercial importance: a monosulphonated anthraquinone milling dye (CI Acid Blue 25), a tetrasulphonated disazo dye (CI Direct Blue 25) and a sulphatoethylsulphone phthalocyanine dye (CI Reactive Blue 21). The effects of pH, temperature and Fe(II) or Fe(III) ions as catalysts were examined and a reaction mechanism proposed [42]. The influence of substituents on the rates of degradation of arylazo reactive dyes based on H acid, caused by the action of hydrogen peroxide in aqueous solution and on cellulose, has been investigated [43]. The results suggested that the oxidative mechanism involves attack of the dissociated form of the o-hydroxyazo grouping by the perhydroxyl radical ion [.OOH]. The mechanism of oxidation of sulphonated amino- and hydroxyarylazo dyes in sodium percarbonate solution at pH 10.6 and various temperatures has also been examined. The initial rate and apparent activation energy of these reactions were determined. The ketohydrazone form of such dyes is more susceptible to attack than the hydroxyazo tautomer [44].

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DYE CHARACTERISTICS AND CHEMICAL STRUCTURE

The anthraquinone derivative CI Acid Blue 45 (3.42) is somewhat sensitive to oxidation (by sodium nitrite for example), especially in the presence of trace metals such as Fe(III) or Cu(II) ions, forming the dull reddish blue tetrahydroxyanthraquinone disulphonate, which exhibits inferior levelling properties. The formation of isatin (3.43) and anthranilic acid (3.44) as oxidative decomposition products from indigo (3.37) during the acid washing of denim with permanganate has been confirmed. Either of these compounds is capable of causing undesirable yellowing and dulling of the indigo dyeing if not removed in subsequent hot water rinsing [45]. HO

O

NH2

O

NH2

NaO3S O N

SO3Na H2N

O

OH

3.43

COOH

H

3.44 Anthranilic acid

Isatin

3.42 CI Acid Blue 45

The decolorisation of disodium 1-phenylazo-2-naphthol-6,8-disulphonate (CI Acid Orange 10) and CI Direct Red 2 (3.8; X = CH3, Y = H) by aqueous sodium hypochlorite was followed by UV spectroscopy. The rates of degradation of both dyes in the initial stage of the reactions conformed to a second-order rate equation [46]. The mechanism of oxidation of a series of orange disodium 2-arylazo-1-naphthol-3,6-disulphonates (3.45) in dilute sodium hypochlorite involved initial attack at the imino nitrogen of the dye in its ketohydrazone tautomeric form by the electrophilic chloronium ion (Cl+). The diazo component is regenerated and the naphthol residue is oxidised to the o-quinonoid analogue (Scheme 3.11). Sulphonic or carboxylic acid groups located ortho to the azo linkage are particularly effective in retarding the rate of azo decomposition [47]. H

HO SO3Na

N

O

N

SO3Na N

N 3.45

NaO3S

NaO3S + N

N

Cl

Cl+

_

Cl O

O +

N

O

SO3Na N

SO3Na O

NaO3S

NaO3S

Scheme 3.11

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DYE STRUCTURE AND APPLICATION PROPERTIES

The effect of ionic strength on the aggregation and degradation rates of the orange dye disodium 1-(2′-methylphenylazo)-2-naphthol-3,6-disulphonate (3.46) in sodium hypochlorite solutions was studied by visible absorptiometry. The slowest rate of decomposition of the dye was observed under conditions in which the degree of aggregation was at a maximum. This confirmed that the dye molecules shielded inside aggregates are oxidised significantly more slowly than the free monomer in solution [47]. When this oxidation reaction was followed in surfactant solutions, the rate of degradation was markedly dependent on the chemical structure of the surfactant molecule [48]. The influence of substituents on the rates of degradation of red arylazo reactive dyes based on H acid, caused by the action of hypochlorite in solution and on cellulose, has been evaluated [49]. At pH values above 5 only the dissociated form of the o-hydroxyazo grouping undergoes attack by the chloronium ion, so that the pKa value of this group becomes a decisive criterion for the fastness to hypochlorite treatment. CH3 H

O

SO3Na

Ar

N

N

X N

N

N

H SO3Na 3.47

3.46 SO3Na Ar

X H H Ar

N

N

N

O SO3Na

N N NaO3S

3.48

The influence on colour fastness to chlorinated water resulting from incorporating a sulpho group into various positions of the arylazo-1-naphthol chromogen in typical orange reactive dyes was examined. Enhanced fastness could be attributed to steric protection of the imino site in the predominant ketohydrazone form of these dyes [50]. In a similar study of orange reactive dyes based on the arylazoarylamine system, it was found particularly effective to incorporate the sulpho group into the 8-position of these 4-arylazo-1naphthylamine dyes (3.47; Ar = aryl, X = reactive group), peri to the imino group bearing the reactive system. In the case of blue disazo reactive dyes derived from twice-coupled H acid (3.48), the two phenylazo groups located in the 2,7-positions make a major contribution to protecting the imino group in the 8-position. In all such imino-substituted acid, direct or reactive azo dyes of these or related types, electrophilic attack on the imino group by the chloronium ion in sodium hypochlorite solution initiates the degradation process, drastically accelerating the subsequent cleavage of the azo linkage [51].

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3.1.7 Dispersion properties and aqueous solubility of disperse dyes An aqueous dispersion of a disperse dye contains an equilibrium distribution of solid dye particles of various sizes. Dyeing takes place from a saturated solution, which is maintained in this state by the presence of undissolved particles of dye. As dyeing proceeds, the smallest insoluble particles dissolve at a rate appropriate to maintain this saturated solution. Only the smallest moieties present, single molecules and dimers, are capable of becoming absorbed by cellulose acetate or polyester fibres. A recent study of three representative CI Disperse dyes, namely the nitrodiphenylamine Yellow 42 (3.49), the monoazo Red 118 (3.50) and the anthraquinone Violet 26 (3.51), demonstrated that aggregation of dye molecules dissolved in aqueous surfactant solutions does not proceed beyond dimerisation. The proportion present as dimers reached a maximum at a surfactant:dye molar ratio of 2:5 for all three dyes, implying the formation of mixed dye-surfactant micelles [52].

O2N

H

Cl

N NH

Cl

SO2

O2N

CH2CH2OH

N N

3.49

Cl

CI Disperse Yellow 42

N CH2CH2OH

3.50 O

NH2

CI Disperse Red 118 O

O O

NH2 3.51

CI Disperse Violet 26

Surfactants are invariably incorporated in the formulation of commercial disperse dye powders, grains or liquid brands. Further additions of surfactants to the dyebath may be made in batchwise or continuous application. The initial purpose of the agents included by the manufacturer is to prevent the finely divided particles of dye from agglomeration during milling or spray drying. They also help to maintain concentrated dispersions in a stable condition during transportation and handling, to minimise agglomeration and settling during storage of liquid brands. In the dyeing process, where the dispersion is much less concentrated, the function of the surfactant addition is to control a process of dynamic change rather than to preserve a static equilibrium for as long as possible. Here the agents play an active part in accelerating the disintegration and dissolution of dispersed particles and thus in increasing the concentration of dissolved dye present in the dyebath. This has the secondary effect of increasing the rate at which dye can be adsorbed onto the fibre surface. During cooling of the dyebath surrounding the dyed material, the presence of a surfactant helps to retard agglomeration, coalescence or crystallisation of dye particles from the saturated solution as it cools.

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DYE STRUCTURE AND APPLICATION PROPERTIES

Coalescence of dyes during cooling is characteristic of their existence in a viscous state at elevated temperatures [53]. Surfactants minimise the redeposition of insolubilised dye onto the surface of the fibres from the cooling bath. Numerous disperse dyes are marketed in a metastable crystalline form that gives significantly higher uptake than the corresponding more stable modification. The molar free enthalpy difference can be used as a criterion of the relative thermodynamic stabilities of two different modifications [53]. Certain dyes can be isolated in several different morphological forms. For example, an azopyrazole yellow disperse dye (3.52) was prepared in five different crystal forms and applied to cellulose acetate fibres. Each form exhibited a different saturation limit, the less stable modifications giving the higher values [54]. In a later investigation three different crystal modifications of each of three commercially important monoazo CI Disperse dyes, namely Yellow 3 (3.53), Yellow 42 (3.49) and Red 54 (3.54), were characterised in terms of melting point, X-ray diffraction pattern and dyeing behaviour on polyester fibres. Different fibre saturation values were found for the various forms of Yellow 42 and Red 54, but the three modifications of Yellow 3 did not differ in this respect [55]. A dispersing agent may influence the aqueous solubility of crystal modifications of the same dye to differing extents, so that the rate of change from a metastable to a more stable form on heating may be modified in the presence of an agent. On the other hand, a metastable form of Yellow 3 showed no tendency to revert to the stable form in the absence of any dispersing agent [56]. HO

NO2 H2N CH3CH2SO2

N

N N

N

CH3CONH

N

N 3.52

CH3 3.53

H3C

CI Disperse Yellow 3

Cl O2N

N

CH2CH2CN N

N CH2CH2OCOCH3

3.54 CI Disperse Red 54

The saturation solubility of a disperse dye in water at a given temperature may be defined as the maximum amount of pure dye that will give a single homogeneous phase when dissolved in a given volume of pure water. The qualification as ‘saturation solubility’ implies that any further addition of dye would remain undissolved as a separate solid phase. Solubilisation of a disperse dye refers to the marked increase in saturation solubility that occurs when a water-soluble solubilising agent is added. If a water-miscible co-solvent such as acetone or ethanol is added to an aqueous suspension of a disperse dye, there is a gradual increase in the amount of dissolved dye that is proportional to the quantity of co-solvent added.

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A disperse dye suspension responds somewhat differently when a surface-active solubilising agent is added. At low concentrations of this type of additive the saturation solubility of the dye remains relatively little changed, but when the critical micelle concentration of the surfactant is reached a sudden marked increase in dye solubility is observed [57]. When micelles of the surfactant begin to form in the solution, these provide a more amenable environment into which the dye molecules or dimers can transfer. Above the critical micelle concentration the increase in solubility of the dye is directly proportional to the concentration of surfactant present [1,56]. The saturation solubility of pure disperse dyes in water is often extremely low at ambient temperature. An early survey of about forty typical dyes for cellulose acetate yielded values within the range 0.1 to 32 mg/l [58]. Raising the temperature to 80 °C results in a considerable increase in saturation solubility, but the solubilising power of surfactants is essential for the satisfactory dyeing of cellulose acetate with the less soluble dyes. Dyes for polyester tend to be more hydrophobic in structural balance. These require considerable solubilisation by agents even at 130 °C in order to give satisfactory dyeing behaviour and freedom from agglomeration or redeposition on cooling. The hydrocarbon groupings in disperse dye molecules represent the most hydrophobic portions and these favour dimerisation and agglomeration in an aqueous environment. Substituents containing electronegative atoms (such as nitrogen, oxygen or sulphur), especially when bearing hydrogen atoms (as in =N–NH–, –NH2, –CONH–, –OH, –SH groups) are able to interact strongly with water to form hydrogen bonds and thus to contribute to solubility in water [1]. Much of the available published data on the aqueous solubility of disperse dyes was accumulated during the 1960s in the course of studies of the mechanism of dyeing cellulose acetate with disperse dyes. Most of the dyes examined were low-energy types for dyeing acetate (Table 3.5). Particular attention was given to monoazo dyes derived from aniline or Table 3.5 Aqueous solubility of disperse dyes of related structure [59-63} Solubility(mg/l)

CI Disperse

Orange 3 Orange 1

Red 19 Red 1 Blue 23 Blue 3 Blue 14 Violet 1 Red 15

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Structure

Temp (°C): 60 Lit. ref.: 59

3.55; X = H, Y = H 3.55; X = NH2, Y = H 3.55; X = NO2, Y = H 3.55; X = NO2, Y = Ph 3.56; X = Ph 3.56; X = 1-naphthyl 3.57; X = Y = CH2CH2OH 3.57; X = CH2CH3, Y = CH2CH2OH 3.57; X = Y = CH2CH3 3.58; X = Y = NHCH2CH2OH 3.58; X = NHCH3, Y = NHCH2CH2OH 3.58; X = Y = NHCH3 3.58; X = Y = NH2 3.58; X = OH, Y = NH2 3.58; X = Y = OH

115

80 60

80 61

141 74 3

90 62

100 63

16.6 1.1

214 7.8 26.4 7.7 ca 0.2 61 30 2 21.7 11.7

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DYE STRUCTURE AND APPLICATION PROPERTIES

X

N

H N

X

N Y

3.55

O2N

N CH2CH2OH

3.56

N

O

X

O

Y

X N

N Y

3.57

CH2CH2OH

N

N

3.58

p-nitroaniline coupled to N,N-disubstituted anilines with ethyl or 2-hydroxyethyl substituents and to 1,4-disubstituted anthraquinones containing potential hydrogen bonding groups (–OH, –NH2, –NHCH3 or –NHCH2CH2OH). In the azo series, unsubstituted aniline as a diazo component gave more soluble dyes than did p-phenylenediamine or p-nitroaniline, in that order. Introduction of an additional aryl ring resulted in a marked decrease in solubility, as shown by comparing CI Disperse Orange 1 with Orange 3, or phenylazo-N,N-bis(2-hydroxyethyl)aniline (3.56; X = Ph) with its 1naphthylazo analogue. Hydroxyethyl groups are readily solvated in water by hydrogen bonding and therefore confer much higher solubility than alkyl substituents in analogous dyes. Examples are given in Table 3.5 from both the monoazo (3.57) and the anthraquinone (3.58) series. The solubility is approximately doubled if two hydroxyethyl groups are introduced instead of one. Aminoanthraquinone dyes are markedly more soluble than their hydroxy analogues. In a recent study of twenty disperse and solvent dyes, data for water solubility, octanol/ water partition coefficient, entropy of fusion and melting point were subjected to regression analysis. Complicating factors such as impurities, polymorphism, tautomerism, polarisation and hydrogen bonding precluded the development of reliable predictions of solubility and partition coefficient. Anthraquinone dyes exhibited much lower entropy of fusion than many of the azo dyes [64,65]. 3.2 DYEABILITY OF FIBRES IN RELATION TO DYE STRUCTURE The chemical forces that operate between dye molecules and fibrous polymers are essentially the same as those governing the clustering of water molecules, dye solubilisation in aqueous media and the aggregation or agglomeration of dyes. In dyeing systems depending on an essentially ionic mechanism, such as the dyeing of acrylic fibres with basic dyes or the application of anionic dyes to nylon, silk or wool, a decisive role is played by electrostatic attraction between the charged dye ions and sites of opposite charge in the substrate. Shorter-range forces make a notable contribution to the affinity of the dye ion but their function is essentially supportive. In all other dyeing systems, however, the charge (if any) on the dye ion or molecule does not enhance affinity. A cellulosic fibre, for example, normally carries a negative charge like that of an approaching dye anion and measures must be taken to minimise the forces of

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117

repulsion arising from this similarity. Under these circumstances the less specific shorterrange forces (hydrogen bonding and van der Waals or dispersion forces) make a much more substantial contribution to dye-fibre bonding. 3.2.1 Dyeing of cellulosic fibres with anionic dyes Within a structural series of direct dyes, the lower the equivalent mass (that is, the ratio of relative molecular mass to sulphonic acid group content) the higher will be the aqueous solubility of the dye. As already noted, the converse of this general trend is that dye molecules of higher equivalent mass tend to aggregate together more readily because of the greater opportunity for hydrophobic interaction. In the presence of a cellulosic substrate, such dyes will tend to escape more readily from the aqueous dyebath by adsorption onto the fibre surface. It is reasonable to assume that the dye molecules will become adsorbed in such a way that as far as possible the hydrophilic sulphonate groups are directed outwards towards the aqueous phase. Just as addition of an electrolyte to a solution of a direct dye tends to lower the electrostatic repulsion between the negatively charged dye ions and promote aggregation, the effect of the presence of salt when dyeing a cellulosic fibre is to overcome the long-range forces of repulsion between the dye anions and the negatively charged fibre surface. The closer approach then allows hydrogen bonding and other short-range attractive forces to operate between the dye molecules and the glucoside units of cellulose. This favourable influence of electrolyte addition is as useful with anionic leuco forms of vat or sulphur dyes as it is with sulphonated direct or reactive dyes. Hydrogen bonding between the hydroxy groups of cellulose and centres of electronegativity (nitrogen, oxygen or sulphur), especially those substituted with hydrogen atoms (as in =N–NH–, –NH2, –CONH–, –OH, –SH groups), is widely acknowledged to contribute to adsorption and retention of the dye molecules. Stereochemical studies have shown that hydrogen bonding is possible for almost all cellulose hydroxy groups in the amorphous regions of the fibre. Measurements of the heat of dyeing confirm that a typical adsorption process entails the formation of at least two hydrogen bonds per molecule of dye adsorbed [66,67]. It is obvious that retention of the adsorbed molecule will be favoured the higher the number of potential dye–fibre bonding points and the more widely they are spaced along the length of the dye molecule. The more hydrogen bonds that a dye can form with the glucosidic polymer, the more readily it can compete with and rupture the fibre–fibre bonds in order to penetrate more deeply into the amorphous structure of the fibre. The recognition that the existence of conjugation in all organic colorants was essential to provide intense colour (section 1.5) and that an increase in the length of the conjugated chain produced a bathochromic shift of the main waveband in the absorption spectrum led to the adoption of related ideas in the context of the theory of substantivity. Many direct dyes with high affinity for cellulose are disazo or trisazo structures in which the azo groups are located in para positions relative to one another, so that the longest conjugated chain through the azo-linked aryl nuclei tends to be as near linear as practicable. The presence of an amide group that is conjugated via a 1,4-phenylene or 2,6-naphthylene residue to the nearest azo group, as in the commercially important CI Direct Red 81 (3.7; X = H), enhances the substantivity of the molecule by lengthening the conjugated chain, a modification that also causes a bathochromic shift.

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DYE STRUCTURE AND APPLICATION PROPERTIES

A methine group oriented para to an azo group through an intervening aryl nucleus boosts substantivity even more than does an amide group in a similar position. This was demonstrated for a series of azostilbene and related dyes (3.59; Ar = Ph, X = CH=CH, CONH or CO) with their thiophene analogues (Table 3.6). Affinity increased in the order of increasing electron mobility (CH=CH > CONH > CO) and was greater for the heterocyclic derivatives than for the corresponding phenyl-substituted dyes [68]. A series of analogues of the most substantive of these dyes (containing both the thiophene ring and the methine linkage) proved to be highly sensitive to the nature of the coupling component at the opposite end of the conjugated chain, however (Table 3.7). The presence of an amino group favours substantivity (naphthionic acid > NW acid, H acid > chromotropic acid) but the extra sulpho group in the three disulphonated couplers affects it adversely. The significance of conjugation as a contributor to the substantivity of dyes for cellulose is not always easy to distinguish from the effect of the degree of linearity of the molecule. Almost all direct dye molecules possess flexible chains of aryl nuclei linked by azo or other unsaturated groups. Such structures can readily adopt a near-linear spatial conformation, as

NH2 Ar

X

CH

N

S

N

H

CH

N Y

N

3.60 SO3Na

3.59

H2N O

O SO3Na

Y1 =

Y2 = SO3Na

NaO3S O

SO3Na

HO O

Y4 = SO3Na

Y3 = NaO3S

SO3Na

Table 3.6 Substantivity and structure of p-substituted monoazo naphthionic acid derivatives [68] Substituents in structure 3.59

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Ar

X

Affinity(kJ/mol)

Thienyl Phenyl Thienyl Phenyl Thienyl Phenyl

CH=CH CH=CH CONH CONH CO CO

–15.8 –14.3 –13.1 –12.0 –8.8 –7.0

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DYEABILITY OF FIBRES IN RELATION TO DYE STRUCTURE

Table 3.7 Substantivity of thienyl monoazo dyes with various coupling components [68] Coupling component

Structure

Affinity(kJ/mol)

Naphthionic acid NW acid H acid Chromotropic acid R acid

3.59; Ar = thienyl, X = CH=CH 3.60; Y = Y1 3.60; Y = Y2 3.60; Y = Y3 3.60; Y = Y4

–15.8 –9.7 –9.4 –7.1 –6.3

far as the angular constraints of the participating bonds will allow. Azo groups attached to central naphthylene residues are frequently oriented in 1,4- or 2,6-positions relative to one another. Typical central diamines that give linear dyes on tetrazotisation include pphenylenediamine, benzidine, p,p′-diaminostilbene, p,p′-diaminodiphenylamine and p,p′diaminodiphenylurea. For similar reasons, 1,4-bis(arylamino)anthraquinone vat dyes are markedly more substantive than their 1,5-disubstituted analogues. H2N H

H

O

N

O SO3Na

N N

N

NaO3S

NaO3S 3.61

SO3Na

H2N

H2N SO3Na

O H H

N

O

N

N

SO3Na

HN H

O

N

O

N

N

SO3Na N

NaO3S

3.62

NaO3S

3.63

The ability to adopt an extended configuration has been recognised for many years to be a desirable feature of substantive dyes. It helps to explain why J acid is such a popular choice as a central component in unsymmetrical disazo dyes. For example, it is much easier for aniline→J acid→H acid (3.61) with the 2,6-naphthylene substitution pattern to adopt a linear conformation than for the similar disazo dyes aniline→γ acid→H acid (3.62) and aniline→H acid→J acid (3.63) with 2,7- and 2,8-disubstitution respectively.

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DYE STRUCTURE AND APPLICATION PROPERTIES

A further refinement of the concept that linear conjugated systems such as aryl–azo–aryl– azo–aryl represented the preferred design for highly substantive molecules was the stipulation that the aryl nuclei must be able to adopt a coplanar conformation [69]. This favours the adsorption of one dye molecule onto another during the formation of laminar aggregates, as well as the multipoint adsorption of a dye molecule onto two or more glucoside units of a cellulose chain via hydrogen bonding sites. There are unequivocal examples that clearly demonstrate the validity of this interpretation. Thus CI Direct Red 2 (3.8; X = CH3, Y = H) and other 3,3′-disubstituted analogues are markedly more substantive than their meta analogues with 2,2′-disubstitution (such as 3.8; X = H, Y = CH3). These substituents adjacent to the central bond in these symmetrical disazo structures inhibit free rotation of the phenylene nuclei and thus prevent the adoption of a coplanar conformation. The scope for intramolecular hydrogen bonding between each azo group and coupling component in symmetrical disazo structures of this kind also contributes significantly to the maintenance of coplanarity and hence to the affinity for cellulose. The number of commercial direct dyes in which coupling has been directed para to the amino or hydroxy substituent in the coupling component is much smaller than those manufactured by ortho coupling. Within the series of symmetrical o-tolidine disazo dyes in which the naphthionic acid of CI Direct Red 2 (3.64; X = SO3Na, Y = H) is replaced in turn by Laurent’s acid, Cleve’s 6 and 7-acids, and finally Peri acid (Table 3.8), there is a marked lowering of affinity for the 6-, 7- and 8-sulpho derivatives compared with the first two members of the series [70]. Naphthylamine-4- and –5-sulphonic acids can only couple ortho to the amino substituent and the derived dyes preferentially adopt an extended coplanar arrangement (3.64) by intramolecular hydrogen bonding of the amino group to the more distant nitrogen atom of the adjacent azo group. In the remaining members of the series, however, coupling readily takes place in the para position (3.65) and the amino groups are no longer able to form intramolecular hydrogen bonds. Free rotation is possible on either side of the azo linkages and the preferred coplanar arrangement is no longer preferentially stabilised.

H

CH3

H3C N

H

H

H

N

N

N

N

N Y

Y X

X

3.64 CH3

H3C N H2N

N

N

N

NH2

Z

Z 3.65 Y

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Y

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Table 3.8 Substantivity of symmetrical disazo dyes derived from naphthylamine monosulphonic acids [70] Coupling component

Structure

Affinity(kJ/mol)

Naphthionic acid Laurent’s acid Cleve’s 6-acid Cleve’s 7-acid Peri acid

3.64; X = SO3Na, Y = H 3.64; X = H, Y = SO3Na 3.65; X = SO3Na, Y = Z = H 3.65; Y = SO3Na, X = Z = H 3.65; Z = SO3Na, X = Y = H

–26.8 –28.0 –22.6 –22.2 –23.4

Vat dye molecules are relatively hydrophobic and are usually capable of assuming a coplanar configuration that favours aggregation in a laminar arrangement as well as the formation of multiple attachments to a segment of the cellulose chain. Hydrogen bonding can be invoked to account for the favourable influence of arylamino or benzoylamino substituents on the affinity of the parent anthraquinonoid nuclei in vat dye structures. Purely carbocyclic quinones such as pyranthrone, violanthrone and dibenzopyrenequinone must depend mainly on coplanarity, hydrophobic interaction and hydrogen bonding between dye keto groups and cellulose hydroxy groups. In any closely related series of vat dyes the affinity does seem to increase with molecular size, but the differences between members of different subclasses emphasise the specific influence of their characteristic structural features, especially nitrogen-containing groupings, on the forces of affinity. A full interpretation of the relationships between direct or vat dye structure and substantivity for cellulose must take into account the contribution of multilayer adsorption of dye molecules within the pore structure of the fibre [71]. The great difference in substantivity between CI Direct Red 28 (3.66) and the monoazo acid dye (3.67) that is the ‘half-size’ analogue of this symmetrical disazo dye may be interpreted in terms of their relative tendencies to form multilayers within the fibre pores as a result of dye–dye aggregation. Saturation adsorption values of these two dyes on viscose fibres at pH 9 and 50 °C corresponded to monolayer coverage areas of approximately 90 and 11 m2/g of internal surface respectively [72]. In view of the smaller molecular area and greater mobility of the ‘half-size’ acid dye, higher uptake than the direct dye would be anticipated if there were only a limited area of internal surface available for true monolayer adsorption. NH2

NH2 N

N

N

N

3.66 SO3Na

CI Direct Red 28

SO3Na

NH2 N N

3.67

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The much higher adsorption of the direct dye at saturation equilibrium can be explained satisfactorily on the basis that both dyes are adsorbed initially on essentially the same internal surface area, but the direct dye has a much greater tendency to aggregate and so build up readily into multilayers. The greater length of the direct dye molecule relative to the acid dye of similar cross-sectional area favours penetration into the intermicellar spaces between the cellulose fibrils, followed by multipoint adsorption onto the glucoside units or another dye molecule already adsorbed. Inorganic electrolytes assist powerfully in promoting these interactive forces leading to multilayer adsorption of direct dyes, since they are present in the intermicellar pore spaces as well as the external phase. Acid dyes aggregate much less readily in solution (section 3.1.2) and are thus less likely to form multilayered aggregates in the fibre pores. The pore structure of cellulosic fibres has a decisive influence on direct dyeing. It can be defined using probe molecules of specific graduated sizes, measurement being by either the stationary non-solute water method or by chromatographic elution on a cellulose column. The distribution of pores can be determined from chromatographic values, revealing the accessibility of the inner voids to dye molecules of different sizes [73]. Pore structure data have been related to the adsorption and diffusion kinetics of the disazo dyes CI Direct Blue 1 (3.2) and Red 81 (3.7; X = H). The specific pore surface can be decisive for equilibrium uptake, whereas the porosity or the amount of free water can be important when interpreting differences in diffusion rate [74]. 3.2.2 Dyeing of amide fibres with anionic dyes Wool, silk and nylon contain both basic and acidic groups, amongst which by far the most important are amino and carboxy groups respectively. Just like the parent amino acids from which protein fibres are derived, all three of these polymeric amides show zwitterionic characteristics at pH values close to the isoelectric point, 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 3.12). 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 3.13). + H3N

[fibre]

_ COO

+ H+

+ H3N

[fibre]

COOH

Scheme 3.12

+ H3N

_ [fibre]

COO

_ + OH

_ H2N

[fibre]

COO

+ H2O

Scheme 3.13

The capacity of nylon or the protein fibres to adsorb simple organic or inorganic acids is closely equivalent to their respective contents of accessible amino groups. More complex dye anions, however, differ in their affinity for these fibres owing to nonpolar bonding between the hydrophobic portions of the dye molecule (alkyl substituents and unsubstituted aryl

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nuclei) and nonpolar side-chain or main-chain segments in the fibrous polymer. The contributions to dye anion affinity of electrostatic attraction to protonated amino groups in the fibre and hydrophobic interaction with nonpolar segments tend to mask any further influence of hydrogen bonding forces, although this form of bonding seems likely to reinforce the effects of the other factors. According to specific adsorption studies on wool, silk and nylon, the tendency of proton donor molecules to form hydrogen bonds with amide groups in these polymers is in the order: ArOH > ArNHCOCH 3 > ArNH2, ArNHR, AlkOH. As a general rule, the lower the dyebath pH the more rapid is the rate of initial adsorption by the amide fibre and the subsequent approach towards equilibrium exhaustion. Consequently, the attainment of a safely controllable rate of dyeing becomes easier the higher the pH at which dyeing begins. Eventually, however, a pH is reached at which the dyebath is no longer exhausted within a reasonable time. Dyeing conditions are usually controlled to give a moderate initial rate of dyeing for optimum levelling, followed by a gradual lowering of pH by acid release to achieve as much exhaustion as possible at the end of the process. Selection of the preferred initial pH and rate of acidification is determined by the affinity of the dyes at neutral pH values. Neutraldyeing affinity is dependent on the structural features and hydrophilic-lipophilic balance of the dye molecule. The degree of sulphonation of an acid dye (section 1.6.7) has a major influence on the profile of exhaustion values reached at various pH values on a specific substrate. At low pH in the absence of electrolyte all acid dyes show high exhaustion, irrespective of their sulpho group content. In neutral solution, on the other hand, multisulphonated dyes generally show much lower exhaustion than dye monosulphonates. Above the isoelectric point the fibre is negatively charged and the forces of electrostatic repulsion increase with the degree of sulphonation. Hydrophobic interaction permits adsorption of a dye monosulphonate onto the fibre surface in such a way that the charged group is directed outwards towards the dyebath phase. The rate of dyeing of acid dyes of relatively small molecular size decreases with increasing degree of sulphonation. Simple monoazo monosulphonate types are rapidly absorbed but readily desorbed again. Thus they migrate easily but show extremely poor fastness to wet treatments. There are two main types of levelling acid dye: (1) Disulphonated dyes of relative molecular mass (Mr) about 400–600 that are somewhat sensitive to dyeability differences in the substrate and show the lowest wet fastness of all dye classes used on wool (2) Monosulphonated dyes of lower Mr (300–500) and slightly higher wet fastness that migrate more readily and cover dyeing faults (such as carbonising damage) more effectively. In an early study of the dyeing of blends of normal and chlorinated wool with a series of naphthionic acid→2-naphthol dyes [75], the trisulphonated CI Acid Red 18 (3.68; X = Y = SO3Na) showed the most marked contrast. Penetration of the hydrophobic epicuticle of normal wool by this hydrophilic dye was negligible in the initial stage of dyeing but chlorinated fibres were rapidly and deeply dyed. The monosulphonated Red 88 (3.68; X = Y = H), however, revealed relatively little contrast in dyeability between the two types of wool, whereas Red 13 (3.68; X = SO3Na, Y = H) gave a degree of contrast intermediate

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between those of the other two dyes. Apparently the relatively hydrophobic 2-naphthol residue in CI Acid Red 88 is almost as effective a means of attachment to the epicuticle of normal wool as the negatively charged naphthionic acid residue is towards the hydrophilic surface of chlorinated wool. H NaO3S

O

N N Y 3.68 X

There are three main subclasses of milling acid dyes: (1) Monosulphonated dyes of Mr 500–600 are somewhat more hydrophobic than the monosulphonated levelling acid dyes. They also migrate and cover well but they are a little inferior to disulphonated milling dyes in wet fastness and thus have sometimes been described as ‘half-milling’ dyes; (2) Disulphonated dyes of Mr 600–900 diffuse much more slowly than typical levelling acid dyes. Thus they exhibit correspondingly higher wet fastness but their migration and coverage properties are inferior; (3) Certain disulphonated milling acid dyes contain higher alkyl groups (such as butyl, octyl, dodecyl) to confer higher neutral dyeing affinity, better coverage of dyeability irregularities and exceptionally good wet fastness. They are sometimes described as ‘super-milling’ dyes to distinguish them from category (2), the largest of the three subclasses. Direct evidence has been provided to demonstrate the contribution of hydrophobic interaction to the substantivity of milling dyes for wool [76]. In a series of monosulphonated phenylazopyrazolone dyes (Table 3.9), substantivity for wool increased with chain length in the alkyl series (B >> D > F) but the increased polarisability of alkenyl groups almost completely negated this effect (B >> E). Cyclisation of the saturated alkyl group produced maximum substantivity (A > B) and formation of an unsaturated aryl ring had a similar effect (C >> E). Finally, a hydroxy substituent almost completely inhibited the hydrophobic interaction attributable to the ethyl group (D > G). It was evident from these results that coulombic interaction and hydrogen bonding played insignificant roles in these equilibria. Table 3.9 Exhaustion of p-substituted phenylazopyrazolone dyes on wool [76]

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Dye

Substituent (R) in 3.69

Exhaustion (%)

A B C D E F G

Cyclohexyl n-Hexyl Phenyl Ethyl Hexatrienyl Hydrogen 2-Hydroxyethyl

85 65 59 17 10 8 3

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SO3Na H R

O

N N

N

N H3C

3.69

Mordant dyes have always been important for the dyeing of wool. Chromium has become the only significant mordant because it yields exceptionally high fastness to light and wet treatments. Aftertreatment with dichromate is the only chroming process of any practical importance on wool. Chromic salts must be used on silk, however, which lacks the reducing power of wool to convert dichromate Cr(VI) anions to chromic Cr(III) cations. Despite the emergence of various ranges of metal-complex acid dyes and reactive dyes, chrome dyes still retain a market share at about one-third of all dyes for wool (Table 3.10). Table 3.10 Estimated proportions of wool dyed worldwide in 1995 with different ranges of dyes [77] Dye range

Proportion (%)

Chrome mordant dyes 1:2 Metal-complex acid dyes Milling acid dyes Levelling acid dyes 1:1 Metal-complex acid dyes Reactive dyes for wool

29 29 20 9 7 6

The practical advantages of chrome dyes include economic build-up to full depths, good level-dyeing properties (Mr only 300–600) and excellent fastness after chromium-complex formation. Unfortunately, the marked change in colour of the dyeing during aftertreatment complicates the colour matching stage. Further drawbacks include the prolonged dyeing procedure, oxidative damage to the wool and the environmental hazards associated with chromium-containing effluent. All dyes requiring the use of chromium compounds have come under increasing pressure recently for ecological reasons. Hexavalent chromium in the form of dichromate salts is causing concern because of its toxic and mutagenic properties requiring special safety measures in handling [77]. The 1:1 metal-complex acid dyes are almost all monosulphonates of Mr 400–500. This gives them dyeing characteristics not unlike those of the monosulphonated levelling acid dyes, with good migration and coverage of damaged wool. In fabric dyeing it is usual to apply the 1:1 metal-complex dyes at pH 2, sacrificing optimum physical properties of the wool in favour of better migration and coverage. The 1:2 metal-complex dyes that were originally developed following the successful adoption of the 1:1 metal-complex types were symmetrical structures free from ionic solubilising groups and some were applied from aqueous dispersion. Most dyes of this type now used on wool or nylon are solubilised by one nonionised but polar group, such as sulphonamide (–SO2NH2), methylsulphonamide (–SO2NHCH3) or methylsulphone (–SO2CH3) on each of the dye ligand groups, which are often dissimilar. These are more expensive to manufacture than the traditional symmetrical structures, however, and share

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most of their drawbacks. They show high neutral-dyeing affinity and very good fastness to light, although they are significantly inferior to chrome dyes in fastness to the most severe wet treatments. Such hydrophobic structures favour coverage of dye-uptake variations in wool but their high affinity can cause rapid initial strike and they diffuse and migrate slowly (Mr 700– 900). Advances were made in developing dyeing methods and auxiliary products (sections 5.8.3 and 12.2) to improve the level-dyeing and coverage properties of sulphonated 1:2 metalcomplex dyes, leading to the widespread adoption of the more economical of the monosulphonates in uses where unsulphonated dyes were previously considered essential. Sulphonated 1:2 metal-complex dyes may be divided into two subclasses: (1) Unsymmetrical monosulphonated dyes of Mr 700–900. The presence of the ionic solubilising group reduces manufacturing costs and minimises the staining of adjacents in severe wet tests, but these dyes are more sensitive to dye-affinity differences than are the unsulphonated 1:2 metal-complex types. (2) Disulphonated dyes of Mr 800–1000, many of them symmetrical in structure. Although often cheaper to manufacture than the unsymmetrical monosulphonates and the unsulphonated types, these slow-diffusing complexes have intrinsically poor levelling and migration properties. Reactive dyes for wool are unmetallised acid dye structures that contain reactive groups capable of forming covalent bonds with nucleophilic sites in wool. Several types of reactive system have proved effective on wool, the most important being α-bromoacrylamide, 5chloro-2,4-difluoropyrimidine and the N-methyltaurine precursor of vinylsulphone. Special amphoteric levelling agents were developed to minimise the inherent skittery dyeing properties of these relatively hydrophilic dyes (section 12.7.2). Inherently more expensive than traditional dyes for wool, reactive dyes also require aftertreatment with ammonia to ensure optimum fixation and wet fastness. Consequently, their usage has been limited (Table 3.10) mainly to bright dyeings on machine-washable wool and other high-quality goods. Their moderate to good levelling and migration before fixation (Mr 500–900, usually two or three sulpho groups), together with excellent wet fastness make them highly suitable for dyeing shrink-resist wool, usually in slubbing or yarn form. If the wool cuticle is damaged by local chemical attack, abrasion or exposure to light, more rapid strike of dye occurs on the exposed cortical cells. At low dyeing temperatures all anionic dyes are taken up preferentially by damaged fibres and fibre tips [78], but at temperatures close to the boil the more polar the dye the stronger is the preferential absorption by damaged fibres [79]. Dyes of low Mr, particularly milling acid and sulphonated 1:2 metal-complex dyes, cover such differences well by subsequent migration away from the tip towards the undamaged root and good coverage can be promoted using cationic auxiliaries. These interact with anionic dyes to form hydrophobic complexes that are less sensitive to differences in fibre dyeability. Chrome dyes reveal a marked contrast between root and tip, since both the mordant and the dye are preferentially absorbed by the damaged tips and the reducing action of the cystine breakdown products resulting from wool damage accelerates the conversion of dichromate Cr(VI) to chromic Cr(III) ions. Most reactive dyes are highly sensitive to dye-affinity variations, especially on unchlorinated wool. Normal nylon 6.6 fibres contain only about 35–45 milliequiv./kg of amine end groups and

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this limits the build-up attainable with the hydrophilic levelling acid dyes. Differential aciddyeable nylon variant yarns can be made by modifying the amino-group content. Deep-dye (60–70 milliequiv./kg) and ultra-deep (>80 milliequiv./kg) variants are obtained by the inclusion of a primary aliphatic diamine containing a tertiary amino group, such as N,Nbis(2-aminoethyl)piperazine, in place of some of the hexamethylenediamine used in manufacture of the normal polymer. Pale-dye nylon (15–20 milliequiv./kg) is made by reacting a proportion of the amine end groups in the normal polymer with a suitable blocking reagent such as γ-butyrolactone. Basic-dyeable nylon is produced by replacing some of the adipic acid used in the polymerisation by a suitable tribasic acid, such as 5sulphoisophthalic acid. The use of acid, basic and disperse dyes to achieve multicoloured effects on differential dyeing nylon has been described in detail elsewhere [80]. The distribution of an acid dye between the components of blends of acid-dyeable nylon variants depends 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 the content of basic groups in the fibre increases, so that the deeper-dyeable variants are 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 neutraldyeing affinity.

3.2.3 Dyeing of ester fibres with disperse dyes Since disperse dyes and ester fibres (cellulose esters and polyester) do not contain any ionic groups, dye-fibre attraction clearly depends on hydrogen bonding, dipole–dipole interaction and dispersion forces. Simple nonionised proton donors, such as phenol, are readily adsorbed by cellulose acetate and this has been taken as evidence of hydrogen bonding between the phenolic hydroxy groups and keto groups in the polymer. Most disperse dye structures contain proton-donating groups such as hydroxy and amino, as well as proton acceptors such as keto, methoxy, azo and dimethylamino groups. Refractometric studies have demonstrated that the α-keto hydrogen atoms in the acetate grouping –OCOCH3 are sufficiently polarisable to take part in hydrogen bonding with proton acceptors [81]. Thus all the polar groups in a disperse dye molecule can contribute to its affinity for an ester fibre and not merely those with proton-donating character. Monolayer experiments have indicated that face-to-face complexing occurs between azo disperse dye molecules and the hexa-acetylcellobiose units of cellulose triacetate, with the molecular segments lying parallel [82,83]. The relatively hydrophobic disperse dye molecule leaves the aqueous dyebath and penetrates between the polymer chains, entering a nonpolar environment where water molecules are unable to follow. New sites for adsorption become accessible as the dye molecule diffuses further into the amorphous regions of the fibre. The more numerous the substituents on the dye molecule that can interact with the polymer segments, the more it is able to break the interchain bonds of the fibre structure. Thus the greater the bonding capacity of a disperse dye molecule, the higher will be its saturation limit in the fibre.

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It is possible to account for the absorption of any disperse dye by cellulose acetate, triacetate or polyester in terms of the combined action of powerful hydrogen bonds and weak dispersion forces. The latter forces (p-bonding) between aryl rings in the dye and terephthalate residues make a more important contribution on polyester, but on polypropylene only weak aliphatic dispersion forces can operate [84]. Provided the dye molecule is planar and can lie flat against the cyclic segments of the ester polymer, all polar groups on the fringes of the rings have the potential to contribute to the overall strength of the dye-fibre multiple bonding. This accounts for the much higher affinity of planar disperse dye molecules compared with nonplanar analogues (Table 3.11). Partition coefficient in this table is defined as the ratio of the concentration (mol/kg) of dye in cellulose acetate to that (mol/l) in water. Table 3.11 Partition coefficient and saturation value of disperse dyes on secondary cellulose acetate [85] Dye structure

Substituent (R)

Partition coeff.

Satn. Value(mol/kg)

3.70 3.71 3.70 3.71

COOCH2CH3 COOCH2CH3 CONHCH2CH2OH CONHCH2CH2OH

998 39 194 2.6

0.218 0.0319 0.135 0.0031

3.70

CO

N

O

216

0.212

3.71

CO

N

O

17

0.018

O

O CH

N =

Ar

Ar

N

Ar CH

O 3.70

R

O

R

3.71 (non-planar)

(planar)

Measurements of aqueous solubility and partition coefficient between cellulose acetate and water were compared for thirty disperse dyes and an approximate inverse relationship was postulated [60]. This can only be valid to a limited extent, however, because the partition ratio also depends on the saturation solubility of the dye in cellulose acetate. This property varies from dye to dye and is not directly related to aqueous solubility. The solubilities of four dyes in a range of solvents were compared with their saturation values on cellulose acetate. Solubilities in benzene showed no significant correlation. With the other solvents the degree of correlation increased in the order: ethanol < ethyl acetate < 20% aqueous diethylene glycol diacetate (CH3COOCH2CH2OCH2CH 2OCOCH3). The lastnamed compound was suggested as a model with polar groups similar to those in cellulose acetate [86].

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Dyes containing hydrophilic N-2-hydroxyethyl groups interact with unsubstituted hydroxy or keto groups in cellulose acetate by hydrogen bonding, whereas dyes with more hydrophobic N-alkyl substituents depend much more on nonpolar bonding. Hence dyes of the latter type have more affinity for cellulose triacetate or polyester than those of the former type, which are more suitable for acetate dyeing. Unsymmetrically disubstituted dyes often give higher absorption on secondary acetate than their symmetrical analogues (Table 3.12), presumably because they can readily participate in both types of dye–fibre bonding mechanism. Table 3.12 Solubility of disubstituted dyes in secondary cellulose acetate [86]

O2N

Dye structure

X

Y

Solubility at 80 °C (g/kg)

3.72 3.72 3.72 3.73 3.73 3.73

H H OH H H CH2OH

H OH OH H CH2OH CH2OH

ca 2.4 18 14 ca 4.6 22 6.6

N N

CH2CH2

X

CH2CH2

Y

O

NHCH2

X

O

NHCH2

Y

N

3.72 3.73

In order to achieve efficient build-up to heavy depths when dyeing cellulose acetate at 80 °C it is customary, particularly for navy blues, to use a mixture of two or more components of similar hue. If these behave independently, each will give its saturation solubility in the fibre. In practice, certain mixtures of dyes with closely related structures are 20–50% less soluble in cellulose acetate than predicted from the sum of their individual solubilities [87]. Dyes of this kind form mixed crystals in which the components are able to replace one another in the crystal lattice. The melting point depends on composition, varying gradually between those of the components, and the mixed crystals exhibit lower solubility than the sum of solubilities of the component dyes [88]. Dyes of dissimilar molecular shape do not form mixed crystals, the melting point curve of the mixture shows a eutectic point and they behave additively in mixtures with respect to solubility in water and in the fibre. Quite small variations in disperse dye structure can markedly modify substantivity for polyester [89]. This is evident from Figure 3.4, where the two blue dye structures differ only in the 3-acylamino substituent of the diethylaniline coupling component. Replacing acetylamino by propionylamino in dye 3.74 increases the colour yield by at least 30% for a 1.5% depth applied to polyester fibre for 45 minutes at 130 °C. An even more striking example is provided in Figure 3.5, illustrating two isomeric greenish blue dyes applied to

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DYE STRUCTURE AND APPLICATION PROPERTIES

polyester under the conditions already specified. In this case, replacement of the N-n-hexyl group in dye 3.75 by N-1-methylpentyl increases the colour yield at 0.5% depth by about 25%. 70 Dye 3.74: R = ethyl

Colour yield (Integ value)

60 50

Dye 3.74: R = methyl

40 30 20 10 0.3

0.6

0.9

1.2

1.5

Dye applied/%

Figure 3.4 Colour yield of acylamino-substituted blue monoazo disperse dyes [89]

40 Colour yield (Integ value)

Dye 3.75: R = n-hexyl

30

Dye 3.75: R = 1-methylpentyl

20

10

0.1

0.2

0.3

0.4

0.5

Dye applied/%

Figure 3.5 Colour yield of N-alkylated greenish blue monoazo disperse dyes [89]

CO R

COCH3 HN

HN CN

CH2CH3 N

O2N

N Br

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N

N

CH2CH3

3.74

130

O2N

NH

N NO2

3.75

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DYEABILITY OF FIBRES IN RELATION TO DYE STRUCTURE

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One of the criteria that determine the substantivity of disperse dyes for ester fibres is that the solubility parameter of the dye must be within one unit (cal/cm3)½ of that of the fibre [90]. Thus the 3-acetylamino dye (3.76; R = CH3) with a solubility parameter(s) of 11.3 has much higher substantivity for polyester (s = 10.8) than has the 3-propionylamino analogue (3.76; R = CH2CH3, s = 13.0). The effects of dyeing auxiliaries on the exhaust dyeing process can also be interpreted by reference to the solubility parameters of fibre, dye and auxiliary [91]. The solubilities of six azo disperse dyes in water and polyester were determined and the thermodynamic parameters influencing solubility were calculated. Water solubility is governed by the mixing enthalpy but this does not play a role in the polyester environment and cannot be predicted satisfactorily from solubility parameters [92]. CO R HN Cl

CH2CH3 N

N CH3SO2

CH2CH3

N

3.76

Derivatives of diaminoanthrarufin (3.77; X = Y = H) and its 1,8-dihydroxy-4,5-diamino isomer (diaminochrysazin) have been among the most widely used anthraquinone dyes for ester fibres. For example, methylation of diaminoanthrarufin gives CI Disperse Blue 26, a mixture of several components. Study of the pure N-alkylated derivatives from the base confirmed that monosubstitution (3.77; X = H, Y = alkyl) gives mid-blue dyes with excellent dyeing properties and acceptable fastness on polyester, but the bis-alkyl dyes (3.77; X = Y = alkyl) are greener and inferior in application properties. Mixtures of the unsubstituted base with alkylated components, as obtained industrially, were especially advantageous for build-up to heavy depths, however [93]. X

3.77

NH

HO

O

O

OH

HN

Y

The effects of disperse dye-substrate interaction on the tensile properties of coloured poly(ethylene terephthalate) film was examined recently [94]. Incorporation of various hydroxy derivatives of anthracene and anthraquinone into the polymer gave indications that aggregation of dye molecules within voids was favoured when their affinity for the substrate was inadequate. Lowering of the glass-transition temperature (Tg) of polymers was investigated recently by incorporating various concentrations of a series of yellow or orange disperse dyes of increasing Mr, namely p-nitroaniline, p-nitrophenylazoaniline (CI Disperse Orange 3), p-nitrophenylazodiphenylamine (CI Disperse Orange 1) and the azopyridone CI Disperse Yellow 231 (3.78). As expected, the Tg values were lowered with increasing concentrations of dye in the polymer and on poly(vinyl butyral) the degree of lowering of Tg

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DYE STRUCTURE AND APPLICATION PROPERTIES

increased in the order of increasing Mr of the dye present. On a copolymer of vinyl chloride and vinyl acetate, however, the degree of lowering of Tg with Yellow 231 was less than with Orange 1 or Orange 3, suggesting that Yellow 231 interacts in different ways with these two polymers [95]. H3C H CH2CH2OOC

CH

O

N

CH2CH3

N N

O

3.78 H3C

CI Disperse Yellow 231

CN

3.2.4 Dyeing of acrylic fibres with basic dyes Most acrylic copolymer fibres contain sulphate and sulphonate groups on the polymer chains, derived from the inorganic redox catalysts (potassium persulphate and sodium bisulphite) that initiate the polymerisation reaction. The adsorption of basic dyes at low pH occurs mainly on these strongly acidic groups [96–99]. Carboxylic groups are sometimes present, however, if benzoyl peroxide has been used as initiator, resulting in dyeing behaviour that varies considerably more with dyebath pH [100]. The concentration of strongly acidic sites for dye adsorption in an acrylic fibre, as determined by sulphur analysis or by non-aqueous titration with a suitable base, agrees extremely well with saturation values derived from Langmuir isotherms defining the uptake of basic dyes [97,101]. This correspondence provides excellent confirmation that the dyeing mechanism is essentially a process of ion exchange. In commercial acrylic fibres the strongly acidic groups are normally stabilised in sodium salt form. If this is transformed into the acid form during preparation, anomalous colour changes (halochromism) may take place in the subsequent dyeing stage. Thus when an acrylic fibre prepared in the free acid form was dyed at 80 °C with CI Basic Orange 33 (3.79; X = Y = H) the dyeing became increasingly much redder than normal, only reverting slowly to the usual orange hue after prolonged dyeing for 1.5 hours or more. A control dyeing on the same fibre prepared as the sodium salt showed no hue change, building up to the normal orange hue. Chloro-substitution in the diazo component inhibited the effect, since with CI Basic Red 18 (3.79; X = Cl, Y = H) and especially CI Basic Brown 30 (3.79; X = Y = Cl) the hue changes were much less marked [102]. This halochromic change is attributed to transient formation of the redder associated complex 3.80 on the acid-treated fibre, followed by reversion to the usual ion-exchange product 3.81 that is formed by conventional dyeing (Scheme 3.14). Addition of a neutral electrolyte such as sodium sulphate suppresses the protonation of the strongly acidic groups in the fibre. X O2N

N

CH2CH3 N

Y

132

+ N

CH2CH2 3.79

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N

_ Cl

CH3 CH3 CH3

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DYEABILITY OF FIBRES IN RELATION TO DYE STRUCTURE

_ + SO3H + [dyeN] Cl

[acrylic]

[acrylic]

Acid form [acrylic]

SO3H

_ Cl

3.80

_ + + Na2SO4 [dyeN] Cl

_ [acrylic]

SO3

+ [dyeN]

+ NaCl + NaHSO4

+ [dyeN]

+ NaCl

3.81

3.80 _ [acrylic]

+ SO3H [dyeN]

133

+ Na+ + [dyeN]

SO3

Sodium salt form

_ Cl

_ [acrylic]

SO3 3.81

Scheme 3.14

The rate of dyeing of basic dyes on acrylic fibres is highly sensitive to temperature. This originates in the response of the segmental mobility of the polymer chains to changes in temperature. At or near the glass-transition temperature (Tg), the rate of diffusion of dye into the polymer structure increases markedly owing to the onset of enhanced mobility of the chain segments. Water acts as a plasticising agent for this process, by lowering the Tg of the acrylic polymer by 30–35 °C [103]. This acceleration of the rate of dyeing as soon as Tg is exceeded, together with the virtual irreversibility of the electrostatic interaction between the dye cation and the anionic dyeing site (3.81) in an otherwise nonpolar local environment, means that careful control of application conditions is essential if level dyeing is to be achieved. Difficulties of incompatibility can arise with mixtures of basic dyes on acrylic fibres because of competition for the limited number of dyeing sites available and the differences between dyes in terms of affinity and rate of diffusion. The rate of uptake of each dye when applied in admixture with another is invariably slower than when the dye is applied alone at the same concentration. Competition effects of this kind can lead to serious practical problems unless the dyes are carefully designed and selected to have similar dyeing characteristics [97,98,104,105]. Dyes with exceptionally low affinity and rapid rates of diffusion have been developed, offering improved migration on acrylic fibres [106]. These dyes have migration properties not unlike those of monosulphonated acid dyes on nylon. With the majority of basic dyes for acrylic fibres, however, levelling by migration is extremely limited because of their high affinity. A slow rate of increase of dyeing temperature (0.5 °C/min or even less) over the range within which the diffusion rate is increasing rapidly gives an improved degree of control. Colourless cationic retarding agents provide temporary competition for the available anionic sites in the early stage of adsorption of the dye. The amount of retarder applied is adjusted according to the proportion of unoccupied sites remaining when the dyeing equilibrium is attained, in order to minimise the risk of undesirable restraining. Since dye uptake is related to the number of unoccupied sites remaining, the temporary blocking action of the retarder effectively slows the rate of dyeing [107].

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DYE STRUCTURE AND APPLICATION PROPERTIES

3.3 APPLICATION PROPERTIES AND CHEMICAL STRUCTURE The choice of dyes for a required colour on a given substrate almost always involves compromise between conflicting requirements. Similarly there is seldom an ‘ideal’ dye structure of a desired hue that embodies all possible attractive features with regard to application and fastness properties. Many bright azo dyes have only moderate fastness to light. The incorporation of a metal atom to form a complex of higher fastness often entails a significant loss in brightness. The gain in aqueous solubility provided by an extra sulpho group in a direct or reactive dye often has to be paid for by a decrease in affinity for cellulose. An increase in the hydrophobic character of an acid dye for wool or nylon may improve the neutral dyeing affinity at the expense of impaired level-dyeing properties. Likewise, the migration behaviour of a disperse dye is often affected adversely by an increase in molecular size intended to confer higher wet fastness on cellulose acetate or improved sublimation fastness on polyester. Further examples of such conflicting requirements will emerge in the remainder of this chapter. 3.3.1 Dyeing kinetics and dye structure The rates of uptake of direct dyes by cellulose vary extremely widely, as indicated by the data in Table 3.13. The time of ‘half-dyeing’ (the time to reach an exhaustion level half of that attained at equilibrium) on viscose for the highly substantive disazo J acid derivative CI Direct Red 23 is approximately 400 times that for the rapidly diffusing azostilbene dye CI Direct Yellow 12 [108]. Coefficients of diffusion for five of these dyes have been measured in cellophane film [109] and these values do account for some of the differences in dyeing rate, although the inverse relationship is only approximate. There is a tendency for smaller molecules to diffuse more quickly, in that structures 3.82 to 3.84 contain only four or five aryl nuclei and one or two sulpho groups, whereas structures 3.88 to 3.90 have six to eight aryl rings and two to four solubilising groups. A much more decisive factor in determining the rates of dyeing of many direct dyes from aqueous salt solutions is their tendency to aggregate (section 3.1.2), since this greatly retards their diffusion into the water-swollen voids of the substrate. Ureido-linked J acid residues

Table 3.13 Times of half-dyeing on viscose [108] and diffusion coefficients in cellophane film [109] for direct dyes at 90 °C

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CI Direct

Structure

Yellow 12 Red 81 Yellow 59 Red 31 Red 2 Blue 1 Red 26 Black 22 Red 23

3.82 3.83 3.84 3.85 3.86 3.87 3.88 3.89 3.90

134

Diffusion coeff. in cellophane film × 1014 (m2/s)

Time of half-dyeing on viscose yarn (min)

150 50

0.26 0.84 1.0 1.7 8.9 15.9 43.8 43.8 100

25.3 7.7 13.3

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APPLICATION PROPERTIES AND CHEMICAL STRUCTURE

SO3Na CH3CH2O

CH

N

N

CH

N

OCH2CH3

N

NaO3S 3.82 CI Direct Yellow 12 H N NaO3S

O

H

N

N

N N

C O

3.83

NaO3S

CI Direct Red 81 SO3Na H3C

S S NH2

N N 3.84 CI Direct Yellow 59

H

O

O

H

NH

N

N

N

N 3.85

NaO3S

CI Direct Red 31

H3C

SO3Na

CH3

NH2

NH2 N

N

N

N 3.86 CI Direct Red 2

SO3Na

NaO3S

NH2

H3CO O

H2N

OCH3 H

H N

O

N SO3Na

3.87 CI Direct Blue 1

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135

SO3Na

N

N NaO3S

SO3Na

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136

DYE STRUCTURE AND APPLICATION PROPERTIES

OCH3

O H

O

O

C

N

N N

N

H

NaO3S

N

SO3Na

N

H

SO3Na

3.88

H2N

H

NH2

CI Direct Red 26 NH2 N

H2N SO3Na

N O

N

H

H N

N

O

N

N

N NaO3S

SO3Na

NaO3S 3.89 CI Direct Black 22

O H

O

O

C

N

N N

H

H

H

N

N

N

N

H

C

CH3

O NaO3S

SO3Na

3.90 CI Direct Red 23

(CI Direct Reds 23 and 26), especially with an acetanilide residue elsewhere in the molecule (as in Red 23), provide opportunities for hydrogen-bond stabilisation of the weaker pbonding attraction between aryl rings in neighbouring component molecules of an aggregate. The kinetic behaviour of anionic dyes on amide fibres tends to be much more closely related to molecular size and hydrophilic-lipophilic balance. Thus within a related series of dye structures it is possible to discern more specific relationships. For example, in the series of p-substituted aniline→R acid dyes (3.91; R = H, methyl, n-butyl or n-dodecyl) the logarithm of the rate of dyeing on wool is inversely proportional to the molecular volume [110]. H

O

SO3Na

R N

3.91 SO3Na

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137

A somewhat similar inverse relationship between dyeing rate and molecular size was invoked to interpret the relative degree of penetration of three anthraquinone blue dyes into nylon fibres after dyeing under various conditions [111]. After only 5–10 minutes at 90 °C, CI Acid Blue 25 (3.92) had penetrated the nylon almost completely, with no evidence of ring dyeing. CI Acid Blue 138, a disulphonated analogue with an additional dodecyl group on the aniline ring (3.93), showed a marked difference in degree of penetration after dyeing for one hour (40% at 90 °C, compared with 100% at 100 °C). The extremely slow-diffusing disulphonated dye CI Acid Blue 175 (3.94), with seven interlinked rings able to form a coplanar arrangement, attained only 40–50% penetration of the substrate even after dyeing for two hours at 100 °C. O

SO3Na

NH2 SO3Na

H2C O

HN

O

HN

O

HN

3.92 CI Acid Blue 25

O

NH2 SO3Na

NaO3S H2C

O

HN

3.93

SO3Na

(CH2)11CH3

CI Acid Blue 138 3.94 CI Acid Blue 175

The relative rates of diffusion of disperse dyes in hydrophobic fibres tend to exhibit an approximately inverse relationship to molecular size, although here again the overall trend is somewhat obscured by specific forces of interaction between hydrogen-bonding substituents in the dye and polymer chain segments. CI Disperse Orange 3 (Mr 242) diffuses about five times more quickly than CI Disperse Blue 24 (Mr 328) in nylon and polyester but only about 1.5 times in cellulose acetate [112]. These differences (Table 3.14) suggest that the orange azo dye interacts more strongly than the blue anthraquinone dye with the acetate fibre, possibly by hydrogen bonding of the terminal primary amino group in Orange 3 with an acetyl group in cellulose acetate. The relative rates of diffusion of CI Disperse Orange 3 on nylon, acetate and polyester indicated in Table 3.14 are reasonably consistent with those measured independently in the early days of polyester dyeing [113], as shown in Table 3.15. These figures, for three disperse

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DYE STRUCTURE AND APPLICATION PROPERTIES

Table 3.14 Diffusion coefficients of disperse dyes in hydrophobic fibres [112] Diffusion coefficient at 100°C × 1015 (m2/s) CI Disperse

Structure

Nylon

Acetate

Polyester

Orange 3 Blue 24

3.95; X = Y = H 3.96; X = NHCH3, Y = NHPh

23.1 4.2

15.1 10.6

1.3 0.3

O2N

N

O

X

O

Y

X N

N Y

3.95

3.96

dyes of almost equal Mr also emphasise a marked increase in diffusion rate on polyester between 85 and 100 °C, that is much less evident on nylon or cellulose acetate. Disperse dyes tend to diffuse more rapidly in polypropylene than in other hydrophobic fibres but to give relatively low saturation values indicative of poor affinity for this fibre. Table 3.16 illustrates these differences for 1-anilinoanthraquinone (3.96; X = NHPh, Y = H) on cellulose acetate and polypropylene [84].

Table 3.15 Relative diffusion coefficients of disperse dyes [113] Relative diffusion coefficient

Rel. mol. mass (Mr)

CI Disperse

Structure

Orange 3 Red 15 Violet 1

3.95; X = Y = H 242 3.96; X = NH2, Y = OH 239 3.96; X = Y = NH2 238

Nylon

Acetate

Polyester

85°C

85°C

85°C

100°C

680 1000 450

460 286 452

1 1 1

48 34 31

Table 3.16 Saturation values and diffusion coefficients of 1-anilinoanthraquinone [84]

chpt3(1).pmd

Dyed at 80°C

Saturation value (g/kg)

Diffusion coeff. × 1014 (m2/s)

Polypropylene Cellulose acetate

3.2 6.0

6.74 1.12

138

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APPLICATION PROPERTIES AND CHEMICAL STRUCTURE

139

There is a roughly inverse relationship for a series of structurally related dyes between the time of half-dyeing and the saturation solubility in an appropriate substrate, as illustrated for several 4-alkylamino derivatives of 1-anilinoanthraquinone on cellulose acetate (Table 3.17). It is interesting that methylamino and 2-hydroxyethylamino substituents confer good solubility in this substrate, but ethylamino groups are even less effective than isobutylamino groups in this respect [114]. Table 3.17 Times of half-dyeing and solubilities of disperse dyes in cellulose acetate at 85°C [114]

Substituent (R) in 3.97 Methyl 2-Hydroxyethyl Isobutyl Ethyl

Time of half-dyeing (min) 30 35

Solubility (g/kg) 8.8 4.2 4.1 1.5

37

O

HN

O

HN R

3.97

The times of half-dyeing on cellulose acetate listed in Table 3.18 give some significant pointers to the relationship between disperse dye structure and dyeing rate on this fibre. The most mobile molecular structure shown is the dinitrodiphenylamine dye CI Disperse Yellow 1, closely followed by the unsubstituted p-nitrophenylazoaniline Orange 3. The introduction of one N-phenyl group into the latter structure (to form Orange 1) has a much more pronounced effect than two ethyl or 2-hydroxyethyl groups (Red 1 or Red 19). In the series of 1,4-diaminoanthraquinone (3.96) derivatives, the unsymmetrically substituted types (Red 15, Violet 4 and Blue 3) are absorbed much more rapidly than is the symmetrical analogue with two methylamino groups (Blue 14). This dye is greatly retarded in comparison with the unmethylated dye (Violet 1). In a similar way, Blue 26:1 containing mono- and dimethylated diaminoanthrarufin is absorbed several times more slowly than Blue 26, a mixture of the monomethyl and unmethylated derivatives. Possibly the presence of methylamino substituents favours aggregation of these anthraquinone dyes. Diffusion coefficients [115] and dyeing rate constants [116] for the same fifteen dyes on nylon are given in Table 3.19, confirming many of the trends already noted above. Yellow 1, Orange 3 and Violet 4 are again the three most rapidly absorbed dyes, in the same order. The diffusion coefficients place the first two dyes in reverse order, however, suggesting that the terminal phenolic group in Yellow 1 interacts more effectively with proton-acceptor sites

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DYE STRUCTURE AND APPLICATION PROPERTIES

Table 3.18 Times of half-dyeing of disperse dyes on cellulose acetate at 85 °C [115]

CI Disperse

Structure

Time of half-dyeing (min)

Yellow 1 Orange 3 Violet 4 Red 15 Blue 3 Violet 1 Red 19 Red 1 Red 11 Blue 26 Orange 1 Red 13 Blue 14 Orange 13 Blue 26:1

3.98 3.95; X = Y = H 3.96; X = NH2, Y = NHCH3 3.96; X = NH2, Y = OH 3.96; X = NHCH3, Y = NHCH2CH2OH 3.96; X = Y = NH2 3.95; X = Y = CH2CH2OH 3.95; X = CH2CH3, Y = CH2CH2OH 3.99 3.100; X = H 3.95; X = H, Y = Ph 3.101 3.96; X = Y = NHCH3 3.102 3.100; X = CH3

0.75 0.8 1.6 2.4 2.5 3.0 3.3 4.0 4.8 6.5 14.0 16.5 16.5 19.0 28.5

O

NH2 OCH3

NO2 NH

O2N

OH

3.98

3.99

CI Disperse Yellow 1

HO

NH2

O

HO

NHCH3

O

CI Disperse Red 11

O

HN

X

+

H2N

NH

X

OH

O

O

OH Cl

3.100 O2N

N

CH2CH3 N

N CH2CH2OH

N

3.101

N

N

N

OH

CI Disperse Red 13

3.102 CI Disperse Orange 13

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141

in nylon than the primary amino terminal group in Orange 3. As before, an N-phenyl substituent (Orange 1) exerts a markedly greater retarding influence than N-ethyl-N-2hydroxyethyl (Red 1) or N,N-bis(2-hydroxyethyl) (Red 19) substitution of the parent structure (Orange 3). Once again in the 1,4-disubstituted anthraquinone series, the rate constants for the unsymmetrical structures (Red 15, Violet 4 and Blue 3) are much higher than that for the symmetrical bis-methylamino analogue (Blue 14). The rates for the two mixed diaminoanthrarufin derivatives, however, are much closer on nylon than on cellulose acetate. The diffusion coefficients of these dyes do differ markedly, perhaps because the unmethylated component of Blue 26 interacts more strongly with proton-acceptor sites in nylon than does the more fully methylated mixture Blue 26:1. Table 3.19 Diffusion coefficients [115] and dyeing rate constants [116] for disperse dyes on nylon at 80 °C

CI Disperse

Structure

Yellow 1 Orange 3 Violet 4 Blue 3 Red 19 Violet 1 Red 15 Red 1 Orange 13 Red 13 Blue 14 Red 11 Blue 26 Blue 26:1 Orange 1

3.98 3.95; X = Y = H 3.96; X = NH2,Y = NHCH3 3.96; X = NHCH3,Y = NHCH2CH2OH 3.95; X = Y = CH2CH2OH 3.96; X = Y = NH2 3.96; X = NH2,Y = OH 3.95; X = CH2CH3,Y = CH2CH2OH 3.102 3.101 3.96; X = Y = NHCH3 3.99 3.100; X = H 3.100; X = CH3 3.95; X = H,Y = Ph

Diffusion coeff. × 1015 (m2/s)

Rate constant

11 23

1820 940 840 570 450 350 280 200 160 130 120 80 80 68 56

11 3 1 11 15 5 17 3

The study of the degree of penetration of blue acid dyes in nylon mentioned earlier also included a further series of measurements for two blue disperse dyes on polyester [111]. A fully penetrated dyeing with CI Disperse Blue 56 (3.103; X = Br) was achieved after 90 minutes at 120 °C, but with the Blue 83 mixture (3.103; X = PhOCOCH3 or PhOCOPh) complete penetration within this time of dyeing could only be attained at 140 °C. In practice, the dyeing of polyester fibres is often restricted to 30–60 minutes at 125–130 °C in order to minimise the release of oligomer into the dyebath or to preserve desirable fabric properties. When applying high-energy dyes of the Blue 83 type these conditions can produce dyeings that are only 30–40% penetrated. H2N

O

OH X

3.103 HO

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O

NH2

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DYE STRUCTURE AND APPLICATION PROPERTIES

Diffusion coefficients for five disperse dyes on polyester are compared in Table 3.20. Four of these structures were included in the corresponding data for the nylon and cellulose acetate systems. As before, the unsymmetrical 1,4-diaminoanthraquinone derivative (CI Disperse Blue 3) diffuses significantly more rapidly than the symmetrical bis-dimethylamino analogue (Blue 14). It is interesting that the disazo dye Orange 13 with a terminal p-hydroxy group shows a higher diffusion coefficient on polyester than the two monoazo dyes with a terminal p-nitro group (Reds 7 and 13). Table 3.20 Diffusion coefficients of disperse dyes on unset polyester at 100 °C [61]

CI Disperse

Structure

Diffusion coeff. × 1014 (m2/s)

Blue 3 Orange 13 Red 7 Blue 14 Red 13

3.96; X = NHCH3, Y = NHCH2CH2OH 3.102 3.104 3.96; X = Y = NHCH3 3.101

1.6 1.4 1.25 1.0 0.9

Cl O2N

CH2CH2OH

N N

N CH2CH2OH

3.104 CI Disperse Red 7

As in the case of disperse dyes on polyester, the coefficients of diffusion of basic dyes in acrylic fibres tend to decrease as molecular size increases. Most basic dyes have a single cationic centre surrounded by a relatively compact hydrophobic system containing three or four aryl rings. Five typical structures are compared in Table 3.21 in terms of their relative diffusion coefficients in an acrylic fibre and in a basic-dyeable polyester variant in the presence and absence of a diphenyl-type carrier [117]. Individual substituents exert only minor effects on the overall rate of diffusion. Thus the N-ethyl-N-2-hydroxyethyl substitution pattern in CI Basic Blue 41 produces only slightly lower diffusion coefficients than the dimethylamino analogue (Blue 54) and even the dimethyltriazole diazo component in Yellow 25 has a minor retarding effect relative to the monomethylthiazole group in Red 29. Table 3.21 Relative diffusion coefficients of basic dyes on basic-dyeable polyester and acrylic fibres at 100 °C [117] Basic-dyeable polyester

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CI Basic

Structure

No carrier

Carrier

Acrylic fibre

Red 29 Blue 54 Blue 41 Yellow 25 Yellow 11

3.105 3.106; X = CH3, Y = CH3 3.106; X = CH2CH3, Y = CH2CH2OH 3.107 3.108

1.1 1.1 1.0 1.0 0.7

1.9 1.8 1.8 1.7 1.6

3.6 3.4 3.1 2.8 2.6

142

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APPLICATION PROPERTIES AND CHEMICAL STRUCTURE

S

X

N

N

H3CO

N _ Cl

+

N

143

N

S

N

N N+ Cl CH3

CH3

Y

_ 3.106

3.105 CI Basic Red 29

H3CO H3C

CH3 N N N

N+ Cl

CH3

N

N

N+ Cl

_

CH3

CH3

CH

NH

HC _

OCH3

3.108 CI Basic Yellow 11

3.107 CI Basic Yellow 25

3.3.2 Dye structure and affinity for the fibre As already noted in relation to kinetic behaviour, minor changes in direct dye structure can have marked effects on dyeing properties. Within the closely related series of J acid disazo dyes in Table 3.22, insertion of an acetylamino group in CI Direct Orange 26 to form Red 23 gives a significant boost to the affinity as well as a bathochromic influence on the colour. Comparing the other two members of this series, an additional aryl ring (Red 26) has a much greater effect on the overall affinity of the molecule than a simple methyl group (Red 24). Table 3.22 Affinity of direct dyes for viscose at 60 °C [67] CI Direct

Substituents in structure 3.109

Affinity(kJ/mol)

OCH3

Red 26

X=

Y=

SO3Na

–23.8

Red 23

X=

Y=

NHCOCH3

–21.3

Orange 26

X=

Y=

OCH3

Red 24

chpt3(1).pmd

X=

143

–20.1 H3C

Y=

SO3Na

–18.4

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DYE STRUCTURE AND APPLICATION PROPERTIES

O H X

O

O

C

N

N N

H

NaO3S

H

N

N

H

SO3Na

3.109

O

H

Y

N

SO3Na

N

X

N

3.110 SO3Na O

NaO3S

H

H N

Y

O

SO3Na

N

N

N

3.111 NaO3S

SO3Na

The important contribution of hydrogen bonding to the substantivity of direct dyes was explored by the synthesis of red (3.110; X = CHO) and violet (3.110; X = PhNHCSNH2) monoazo derivatives of R acid. The existence of hydrogen bonding between the X group and hydroxy groups in cellulose was demonstrated. This research was followed up by a more comprehensive survey of various disazo R acid dyes derived from a range of diamines (3.111; Y = O, NH, CH2, CO, SO2, CH=CH, CONH, NH.CO.NH, NH.CS.NH, CO.NH.NH.CO, NH.CO.CO.NH, NH.CO.CH2.CO.NH, NH.C=NH.NH and CO.NH.C=NH.NH.CO). The significance of such central functional groups with scope for hydrogen bonding or dipolar interaction with hydroxy groups in cellulose was clearly evident [118]. Numerous linear diamines, such as many of those in the survey mentioned above, have been evaluated as potential replacements for benzidine, an inexpensive and highly versatile intermediate that was banned in the 1970s because it posed a carcinogenic threat. Two trisazo dyes have been synthesised recently using 4,4′-diaminodiphenyl thioether instead of the benzidine component of CI Direct Black 38 (3.112; X = Y = NH2) and CI Direct Green 1 (3.112; X = OH, Y = H). These new dyes exhibited higher substantivity and fastness to washing than the two traditional products on cellulosic fibres [119].

H

Y

NH2 N

X

N

N

144

N N

N 3.112

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O

NaO3S

SO3Na

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145

Affinity values of the symmetrical disazo dyes CI Direct Yellow 12 (3.6), Red 2 (3.8; X = CH3, Y = H) and Blue 1 (3.2) on cellophane have been measured recently under hydrostatic pressures up to 600 Mpa [120]. The affinity of Yellow 12 increased slightly but values for the other two dyes decreased considerably with increasing hydrostatic pressure (Table 3.23). The sulpho groups on the central stilbene nuclei of the Yellow 12 molecule tend to inhibit aggregation, whereas Red 2 and Blue 1 aggregate much more readily. The small increase in the affinity observed with Yellow 12 may indicate that isomerisation from the cis to the more stable trans form may occur as the hydrostatic pressure is increased. Table 3.23 Affinity of direct dyes for cellophane at 55 °C [120] Affinity (kJ/mol) Hydrostatic pressure (MPa)

CI Direct Yellow 12

CI Direct Red 2

CI Direct Blue 1

0.1 100 200 400 600

–18.2 –18.2 –18.8 –19.3 –19.8

–20.9 –17.5 –16.2 –14.9 –14.1

–31.0 –28.3 –25.3 –22.7 –21.6

The attraction between cellulose and the leuco form of a simple polycyclic vat dye is believed to be mainly attributable to dispersion forces. Not surprisingly therefore, affinity tends to increase with relative molecular mass in the series shown in Table 3.24. This is not the only factor, however, because the eight-ring pyranthrone (3.113) has a higher affinity than the nine-ring violanthrone isomers (3.114 and 3.115). The configuration of the rings and the location of the keto groups are also significant, since violanthrone is more substantive than isoviolanthrone. Table 3.24 Affinity of polycyclic vat dyes for cotton at 40 °C [115,121] Polycyclic vat dye

Structure

Mr

Affinity (kJ/mol)

Pyranthrone Violanthrone Isoviolanthrone Dibenzopyrenequinone Anthanthrone Benzanthraquinone

3.113 3.114 3.115 3.116 3.117 3.118

406 456 456 332 306 258

–23.2 –22.8 –19.9 –15.7 –12.0 –4.9

O

O 3.113

3.114

Pyranthrone

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O

O

145

Violanthrone

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DYE STRUCTURE AND APPLICATION PROPERTIES

O

O

3.116

3.115 O

O

Dibenzopyrenequinone

Isoviolanthrone

O

O

3.118 3.117

O

O

Benzanthraquinone

Anthanthrone

Hydrogen bonding is believed to strongly influence the substantivity of benzoylaminoanthraquinone vat dyes, as detailed studies have shown (Table 3.25). Introduction of a second benzoylamino group into 1-benzoylaminoanthraquinone approximately doubles the affinity, 1-4-disubstitution having a greater effect than 1,5-disubstitution. The enhanced affinity conferred by the p-R-substituted benzoylamino group increases with the dipole moment of the C–R bond (Table 3.25). Such substituents are almost equally effective in either m- or p-positions, but o-R-substituted analogues are much less substantive because coplanarity of the aryl rings with the plane of the anthraquinone nucleus is inhibited.

Table 3.25 Affinity of benzoylaminoanthraquinone vat dyes on cotton at 40 °C [115] Affinity(kJ/mol) Substituent (R) Dipole moment (C-R)

H

CH3 0.41

CH3O 1.16

Cl 1.56

Structure 3.119

X=Y=R

CONH, Z = H

–15.3

–16.6

–16.0

–19.6

X=Z=R

CONH, Y = H

–12.0

–12.8

–14.5

–15.6

CONH, Y = Z = H

–6.9

–8.2

–9.1

–9.6

X=R

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X

O

Y

O

147

Z

3.119

Early studies of the affinity of acid dyes for wool revealed noteworthy correlations with dye structure. For example, in four pairs of monoazo dyes differing only by replacement of a benzene by a naphthalene nucleus, the affinity increase in each pair was consistently within the range -4.6 to -6.3 kJ/mol. In three related pairs of dyes, an additional sulpho group reduced the affinity by about -4 kJ/mol. In a series of alkylsulphuric acids (ethyl, octyl, dodecyl) and in two series of monoazo dyes containing alkyl chains of increasing length, the increment per methylene group was consistently about -1.66 kJ/mol. A close correlation between affinity and Mr was also obtained for a series of substituted phenylazo-1-naphthol4-sulphonic acid dyes [115]. Two further interesting series of systematic changes in structure have shed light on the relative effects of alkyl and aryl groupings in conferring enhanced affinity for wool. The phenylazo member (3.120; X = Ph) of the series shown in Table 3.26 has an affinity of -16.7 kJ/mol and the introduction of a 4-n-butyl substituent has almost as great an effect as the additional aryl ring when the phenylazo is replaced by a naphthylazo grouping. Likewise, in the three anthraquinone derivatives shown in Table 3.27, replacing the 4-methylamino by an n-butylamino group in structure 3.121 boosts the affinity by about 10%, but the effect of an anilino residue represents almost a 15% increase. H X

O

NH2

O

N

SO3Na

N NaO3S

O 3.120

N H

3.121

X

Table 3.26 Affinity of monoazo acid dyes for wool at 70 °C [122] Substituent (X) in 3.120

Affinity (kJ/mol)

–22.6

CH3(CH2)3

–22.2 –16.7

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DYE STRUCTURE AND APPLICATION PROPERTIES

Table 3.27 Affinity of anthraquinone acid dyes for wool at 50° C and pH 4.6 [123] Substituent (X) in 3.121

Affinity (kJ/mol) –25.1

–(CH2)3CH3 –CH3

–24.2 –22.2

Careful measurements have been made of the affinity of many levelling and milling acid dyes for nylon using various experimental techniques. Radio tracer analysis was used to follow the uptake of a series of 1-naphthylazo-2-naphthol dyes (3.122) on nylon 6.6 film. The highest neutral-dyeing affinity was shown by the monosulphonate CI Acid Red 88 (3.122; X = Y = Z = H), which exhibited the greatest degree of overdyeing in excess of the amine end group content of the nylon. The disulphonate Red 13 (3.122; Y = SO3Na, X=Z=H) and the trisulphonate Red 18 (3.122; X=Y=SO3Na, Z=H) gave progressively lower neutral-dyeing affinity and overdyeing, whilst the tetrasulphonate Red 41 (3.122; X = Y = Z = SO3Na) showed no tendency to overdye [124]. H NaO3S

O

Z

N N X

3.122

Y

In a recent study of the series of acid dyes obtained by diazotisation of p-alkylanilines and coupling with 1- or 2-naphtholsulphonic acids [125], the substantivity, wet fastness and water repellency characteristics of these dyes on nylon were found to increase progressively with alkyl chain length, provided the dyes were dissolved completely in the dyebath. Aggregation to form multimers and ultimately micelle stabilisation complicates the behaviour of alkylated dye monosulphonates of this kind (section 3.1.2). This research was followed up more recently in a detailed evaluation of anthraquinone blues (3.123) prepared by condensation of bromamine acid with a series of p-n-alkylanilines (R = H, butyl, octyl, dodecyl or hexadecyl). Related series of monoazo dyes were synthesised using the same p-nalkylanilines as diazo components with various couplers such as H acid (as in structure 3.124), γ acid, M acid, NW acid or R acid [126]. In some cases (as in 3.124; X = H or COCF2CF2CF3) the effects of inserting a perfluorobutyroylamino substituent in the coupling component were also assessed [127]. Although the perfluoro group and the long-chain alkyl groups adversely affected substantivity for nylon 6, they did enhance wet fastness and water repellency as expected. The perfluorobutyroyl grouping was particularly effective in conferring water repellency [126]. Affinity values have been used to interpret the important practical problem of mutual blocking effects in mixture recipes on nylon [128]. Typical values for some commercially

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APPLICATION PROPERTIES AND CHEMICAL STRUCTURE

O

X

NH2

HN

SO3Na

H R

O SO3Na

N N

O

HN

3.123

3.124

NaO3S R

Table 3.28 Affinity of acid dyes for nylon 6.6 by chloride desorption at 75 °C [128] CI Acid

Structure

Affinity (kJ/mol)

Red 88

3.125; X =

Violet 5

3.126; X = CH 3CONH

Orange 20

3.127

Y=H

NaO3S

Y=

–32.0

SO2

CH3

–27.7 –25.8

CH3

Orange 8

3.125; X =

NaO3S

Y=H

–25.2

Red 18

3.125; X =

NaO3S

Y = SO3Na

–24.6

Orange 7

3.125; X =

NaO3S

Y=H

–24.3

Violet 7 Yellow 17 Red 1 Yellow 23

3.126; X = CH 3CONH 3.128; X = CH3 3.126; X = H 3.128; X = COOH

Y = COCH3 Y = Cl Y = COCH3 Y=H

–22.4 –22.2 –21.4 –20.9

Orange 10

3.125; X =

Y = SO3Na

–20.9

important monoazo acid dyes are given in Table 3.28. CI Acid Orange 7 is the ortho isomer of Orange 20 (3.127). The latter has a significantly higher affinity, probably because hydrogen bonding of the keto group with proton-donor sites in nylon is favoured. Orange 7 and Orange 10 have the same 1-phenylazo-2-naphthol chromogenic system but the additional sulpho group in Orange 10 lowers the affinity by about -3.4 kJ/mol. Similarly, the two extra sulpho groups in Red 18 reduce the affinity by about -7.4 kJ/mol compared with the analogous Red 88, the most substantive of the dyes listed in Table 3.28. Orange 8 differs from Orange 7 only in having an additional methyl substituent ortho to the azo linkage. This only raises the affinity by about -0.9 kJ/mol, but the bathochromic change from phenylazo to naphthylazo in going from Orange 7 to Red 88 gives an increase of approximately -7.7 kJ/ mol, virtually the same magnitude as the lowering effect of two sulpho groups.

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DYE STRUCTURE AND APPLICATION PROPERTIES

H X

Y

O

HN

N

H

N

X

O

N

SO3Na N

Y

3.126

NaO3S

Y

3.125 H NaO3S

N

Y

N

O

SO3Na H

3.127

NaO3S

O

N

CI Acid Orange 20

N N

3.128

N

Y

X

In the series of three N-acylated phenylazo H acid dyes (3.126), the additional pacetylamino group in Violet 7 enhances the affinity above that of Red 1 by about –1.0 kJ/ mol, as well as producing a marked bathochromic shift in hue. Replacing the N-acetylamino group in the H acid residue by N-p-tosylamino (Violet 5) has less effect on hue but raises the affinity by about –5.3 kJ/mol. Somewhat surprisingly, replacing the carboxyl group in the azopyrazolone chromogen (3.128) of Yellow 23 by a methyl group and introducing two chloro substituents into the N-phenyl ring only increases the overall affinity by about –1.3 kJ/mol. The yield of disperse dyes on polyester is limited by their slow rate of diffusion into the fibre rather than by inherently low substantivity. If dyeing times are sufficiently long, saturation values on polyester approach those on cellulose acetate and often exceed those on nylon (Table 3.29; for structures of these dyes see Tables 3.14 and 3.18). Some early measurements of the partition of various aminoanthraquinone derivatives between cellulose acetate and ethanol (Table 3.30) dramatically illustrate the value of primary amino groups in conferring affinity for cellulose acetate. Indeed, the introduction of

Table 3.29 Saturation values of disperse dyes on hydrophobic fibres at 85 °C [113] Saturation value (% o.w.f.)

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CI Disperse

Structure

Acetate

Nylon

Polyester

Yellow 1 Red 15 Violet 1 Orange 3

3.98 3.96; X = NH2, Y = OH 3.96; X = Y = NH2 3.95; X = Y = H

16.0 11.2 8.3 5.1

5.0 4.4 4.9 2.0

7.1 12.0 4.4 4.1

150

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APPLICATION PROPERTIES AND CHEMICAL STRUCTURE

151

Table 3.30 Partition coefficients of aminoanthraquinone derivatives between cellulose acetate and ethanol at 60 °C [129] Anthraquinone derivative

Partition coefficient

1,4,5,8-Tetra-amino 1,5-Diamino 1,8-Diamino 1,4-Diamino 1-Amino-2-methyl 1-Methylamino 1-Amino 1-Amino-4-hydroxy

8.77 6.64 5.73 2.80 2.67 2.38 1.79 1.59

a 4-hydroxy group into 1-aminoanthraquinone actually lowers the affinity for the fibre slightly, perhaps by favouring intermolecular hydrogen bonding and thus greater aggregation in the ethanolic phase. Methylation of the 1-amino group promotes affinity, but introduction of a 2-methyl substituent into 1-aminoanthraquinone has a greater effect. The most impressive increases, however, are caused by further amino substitution. The partition coefficients of the 1,4-, the 1,8- and the 1,5-isomers are respectively about twice, three times and four times that of the monoamino analogue. Further amino substitution has a less marked effect, the 1,4,5,8-tetra-substituted derivative having a partition coefficient about five times that of 1-aminoanthraquinone. 3.3.3 Effect of dye structure on migration and levelling Early investigators of the skittery dyeing of wool concluded that the differences in dyeability between individual fibres are attributable to the variations in extent of degradation that they have undergone in the fleece and during preparation [130,131]. The responses of levelling and milling acid dyes to such differences were studied in detail [132]. With typical levelling acid dyes such as CI Acid Red 1 (3.129; R = H) the proportion of ring-dyed fibres present gradually increases with dyeing temperature up to a maximum at about 70 °C. As dyeing continues above this temperature the content of ring-dyed fibres gradually falls again and uniformly penetrated fibres begin to predominate. Eventually the migration process ensures that a level and fully penetrated dyeing is achieved. COCH3 HN H R

O

N

SO3Na N

NaO3S

3.129

Milling acid dyes behave in a more individual way. The disazo monosulphonate CI Acid Red 151 (3.130) is absorbed extremely rapidly at low temperature, all fibres becoming ringdyed. Little further uptake takes place until the temperature approaches 90 °C. Diffusion

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DYE STRUCTURE AND APPLICATION PROPERTIES

into the fibre then accelerates markedly, the proportion of ring-dyed fibres rapidly falling to zero and soon all fibres present become deeply and uniformly dyed. The disazo disulphonate CI Acid Red 85 (3.131), however, shows virtually no uptake at all below 70 °C and ring dyeing is at a maximum in the range 80–90 °C. Only after boiling for about 30 minutes does the proportion of fully penetrated fibres exceed that of ring-dyed ones. A level dyeing of satisfactory fastness requires at least an hour at the boil. More hydrophobic ‘super-milling’ dyes such as CI Acid Red 138 (3.129; R = n-dodecyl), the p-alkylated analogue of Red 1, give ring dyeings at 70 °C and above. Even after prolonged boiling, it is difficult to attain more than a small proportion of fully penetrated fibres. Such dyes tend to be especially sensitive to skitteriness and to root-tip differences of dyeability in weathered wool. H N NaO3S

O

N

N

N 3.130 CI Acid Red 151

H H3C

N

SO2 O

O

N

N

N

3.131

NaO3S

CI Acid Red 85

SO3Na

As already indicated in section 3.3.2, the introduction of a further sulpho group into an acid dye tends to lower the affinity by about –4 kJ/mol. The results in Table 3.31 for two series of phenylazo-l-naphthol dyes with varying degrees of sulphonation clearly demonstrate that the increase in hydrophilic character brought about in this way significantly impairs their migration behaviour. Thus the monosulphonated levelling acid dye sulphanilic acid→1-naphthol has excellent migration properties, but the pentasulphonated analogue aniline-2,5-disulphonic acid→oxy-Koch acid migrates only with great difficulty. Systematic studies of the relationship between disperse dye structure and levelling properties on the ester fibres have shown that in general levelling tends to decrease as molecular size increases. Thus an inverse linear correlation was found between the molar volume of a series of disperse dyes and their barriness rating on polyester [90]. As molecular size increased, migration became less effective in covering dye affinity variation in a textured polyester fabric prone to show barriness. Molecular size is not the only relevant factor, however, because disperse dyes with significant aqueous solubility show better migration properties than less soluble dyes of similar molecular size. Disperse dyes containing n-alkyl substituents within the range butyl to octadecyl showed only 20-40% exhaustion on polypropylene fabric. Exhaustion and washing fastness increased but levelling decreased with increasing alkyl chain length. Dyeing with 1,4bis(octylamino)anthraquinone in the presence of an ethoxylated octadecanol surfactant,

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APPLICATION PROPERTIES AND CHEMICAL STRUCTURE

Table 3.31 Migration properties of phenylazonaphthol acid dyes on wool [115] Structure (3.132; unspecified X and Y = H)

Basicity

Migration

Y4 = SO3Na X4 = Y4 = SO3Na X2 = X5 = Y4 = SO3Na X4 = SO3Na X4 = Y4 = SO3Na X4 = Y6 = Y8 = SO3Na X4 = Y3 = Y6 = Y8 = SO3Na X2 = X5 = Y3 = Y6 = Y8 = SO3Na

1 2 3 1 2 3 4 5

5 4 3–4 5 4 3–4 3 2

X2

Y8 H

X4

O

N

Y6 N

X5 Y3

Y4

3.132

however, gave 70-90% exhaustion and markedly improved levelling. Exhaustion increased progressively with the degree of ethoxylation of the nonionic levelling agent [133]. An interesting comparison of the migration behaviour of a related series of alkylaminothiazine basic dyes on an acrylic fibre is outlined in Table 3.32. Methylene blue (CI Basic Blue 9), the best known of these, is the tetra-N-methylated derivative with a migration rating of 2–3. Insertion of a nitro group meta to one of the dimethylamino groups slightly impairs migration. As might be expected, the completely unmethylated diaminothiazine structure shows excellent migration, whereas the tetra-N-ethyl analogue of methylene blue migrates only with great difficulty. Surprisingly, removal of one of the Nmethyl groups from the methylene blue structure reduces migration slightly, but the isomeric trimethylated derivative with one of the methyl groups ortho to the unmethylated amino group has a migration rating almost as good as the parent unmethylated diaminothiazine.

Table 3.32 Migration properties of thiazine basic dyes on acrylic fibre [134]

chpt3(1).pmd

Structure (3.133)

Migration

R=X=Y=Z=H R = Y = CH3, X = Z = H R = X1 = CH3, X2 = Y = Z = H R = X = CH3, Y = Z = H R = X = CH3, Y = H, Z = NO2 R = X = CH2CH3, Y = Z = H

5 4 2 2–3 2 1

153

S

+ X1 N X2

N

Y

R N R

Z

3.133

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DYE STRUCTURE AND APPLICATION PROPERTIES

3.3.4 Dye structure and light fastness The fastness to light of a dyed fibre depends on many factors. These include the inherent stability of the dye chromogen when exposed to photochemical attack and the way in which this interaction is modified by the polymer substrate and the conditions of exposure. General reviews of the mechanisms of fading reactions and the influence of dye structure and aggregation behaviour on these processes are available [135–140]. Kinetic evidence indicates that the light fastness of direct dyes on cotton is markedly dependent on the formation of aggregates within the fibre. This can sometimes lead to apparently anomalous differences in fastness rating for the same dye applied and aftertreated in various ways. Hence the light fastness of direct dyeings is determined more by their state of aggregation than by structural features of the dye chromogen [141–145]. Interesting pointers to the reasons for the limited light fastness of monoazo red and disazo blue reactive dyes on cellulosic fibres, particularly those derived from H acid coupling components, have emerged in recent years. Experimental evidence for free-radical formation has been presented and the important role of radical reactions in the fading mechanisms of reactive dyes has been defined [146]. The environmental effects of atmospheric oxygen on the fading behaviour of aminochlorotriazine reactive dyes on dry cotton fabric have been investigated. Most of the dyes were degraded in two stages, a rapid initial fade followed by a slower and incomplete process. Oxygen accelerated the major fading step for almost all dyes, but a phthalocyanine derivative was more sensitive in a nitrogen atmosphere. In some instances the mechanism of fading in air was initially oxidative but became reductive in the second stage [147]. Accelerated fading in the wet state is characteristic of many azo reactive dyeings. If cellulose dyed with such dyes is exposed to light in aerated water the fading mechanism is an oxidative one. The rate of fading depends on dye structure, pH and oxygen concentration present [148]. Photoreduction can take place on exposure in deaerated water, however, especially in the presence of sodium mandelate (3.134) under a nitrogen atmosphere. Catalytic wet fading of certain azopyrazolone yellow reactive dyes in mixture dyeings with phthalocyanine or triphenodioxazine blues has long been recognised as an important problem of reactive dye selection. The effect is very slight in the dry state and is almost completely suppressed in the absence of oxygen. Considerable protection of the azopyrazolone component is achieved by adding the singlet oxygen quencher 1,4diazabicyclo[2,2,2]octane (3.135). Oxidative destruction of the azopyrazolone chromogen does not rupture the ether bond formed in the dye-cellulose fixation process [148]. Complaints of accelerated or anomalous fading of reactive dyeings often implicate azo dyes derived from H acid and may involve wetting of the affected textile by perspiration during sunlight exposure. The mechanism of fading by light and perspiration simultaneously is not always the same as fading by light alone. In the dry state, dyes of the H acid type generally fade by a reductive mechanism but in the presence of aerated water the CH2CH2 CH

COONa 3.134

Sodium mandelate

154

CH2 CH2

N

CH2 CH2

OH

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N

3.135 DABCO

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155

degradation reaction becomes oxidative. Dyes containing strongly electron-donating substituents tend to fade oxidatively, whereas those with strongly electron-withdrawing groups are more vulnerable to reductive attack. The reductive fading mechanism is greatly enhanced when the exposed dyeing is wetted with acidic or alkaline perspiration [149]. In a recent study of the combined effects of perspiration and light, garments dyed with five typical reactive dyes of various chromogenic types (Table 3.33) were worn by tennis players in bright sunlight. The fastness ratings showed good agreement with results from the Japanese ATTS test on these dyeings. The sensitivity to this problem shown by the commercially predominant disazo CI Reactive Black 5 (3.136) derived from H acid was confirmed, compared with the much more stable anthraquinone derivative CI Reactive Blue 19 (3.137). Table 3.33 Wearer trial and fastness tests on dyeings representing various chromogens [150]

CI Reactive

Chromogen

Wearer trial

Perspiration (acid)/light ATTS test

Blue 19 Blue 221 Blue 28 Blue 222 Black 5

Anthraquinone Copper-formazan Copper-monoazo Azo H acid disazo

4 3 1–2 2–3 2

3–4 3 1 2–3 1

Light Xenon arc 5 5 5 4–5 4–5

SO2CH2CH2

H2N H NaO3SO

CH2CH2SO2

N

N

O

N

SO3Na N

NaO3S 3.136 CI Reactive Black 5 O

NH2 SO3Na

O

HN

SO2CH2CH2

OSO3Na

3.137 CI Reactive Blue 19

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DYE STRUCTURE AND APPLICATION PROPERTIES

The tendency of Black 5 and other widely used azo reactive dyes of acceptable light fastness to give rise to customer complaints when exposed to light in the presence of perspiration was examined further. Dyeings of eight reactive dyes representing a variety of azo chromogens, mostly with a sulphatoethylsulphone reactive system but also including one dichlorotriazine dye and one with a bifunctional system, were subjected to ATTS tests and xenon arc exposure (Table 3.34). Surprisingly, the J acid dye was more resistant to perspiration/light fading than to a conventional light fastness test. The four dyes derived from H acid or K acid faded much more rapidly in the ATTS test, but the azopyrazolone, azonaphthalene and γ acid monoazo chromogens appeared to be unaffected by the presence of perspiration during exposure. Table 3.34 Perspiration and light fastness of various azo reactive dyes [150]

Reactive system

Azo chromogen

Perspiration (acid)/light ATTS test

Sulphatoethylsulphone (SES) Bifunctional (SES/MCT) SES Dichlorotriazine (DCT) SES SES SES SES

Azopyrazolone Azonaphthalene γ acid monoazo J acid monoazo H acid monoazo K acid monoazo H acid Cu-monoazo H acid disazo

4–5 5 4 4–5 2 1 1 1

Light Xenon arc 5 5 4 3–4 4–5 4–5 4–5 4–5

These marked differences in response between different types of azo chromogen have been interpreted [150] in terms of tautomerism of their aminonaphthol structures (Scheme 3.15). When the imino nitrogen is peri to the hydroxy group, as in H acid (3.138) and K acid (3.139) dyes, the reduction-sensitive hydroxyazo form is stabilised by hydrogen bonding with the imino grouping. In J acid (3.140) and γ acid (3.141) derivatives, however, hydrogen bond stabilisation of either tautomer by the imino group cannot take place. Independent studies of the fading behaviour of tautomeric azo dyes have shown that the hydrazone form is more resistant to photoreduction than the azo form, but has a lower stability to photo-oxidation by singlet oxygen [151]. In the monoazo red reactive dyes derived from H acid, the location and nature of the reactive system has a significant influence on fastness to the perspiration/light test (Table 3.35). When the reactive group is attached to the diazo component this leads to lower fastness ratings than when it is linked via an imino group to the coupling component (3.138). The fading behaviour of azoic dyeings on cotton is determined mainly by the chemical characteristics of the diazo and coupling components. Although they show generally high fastness when exposed in full depths under dry conditions, this stability declines sharply with increasing humidity or decreasing applied depth. Electron-withdrawing substituents in the phenylazo grouping, especially trifluoromethyl or nitro, enhance light fastness but electrondonating groups such as o-methyl or o-methoxy usually lower the ratings. Substituents in the anilide nucleus of Naphtol AS combinations, however, influence light fastness in the opposite way. For example, in a series of coupling components used with o-nitroaniline as diazo component (3.142), the highest light fastness was found for R = p-methoxy and the lowest for R = m-nitro [152].

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APPLICATION PROPERTIES AND CHEMICAL STRUCTURE

Y H H X

157

Y

N

H H

O

Ar N

SO3Na

X

N

O

Ar N

N

SO3Na N

3.138 NaO3S

NaO3S H acid dyes

Scheme 3.15

R H H R

H

N

R

O

O NH R

Ar N N

Ar N N

3.140

NaO3S

SO3Na J acid dyes

NaO3S 3.139 K acid dyes

NH H R

R

O

NO2 H

N

R

O

Ar N

O

C

N

N H

N

3.141

NaO3S γ acid dyes

3.142

Table 3.35 Location of reactive system in H acid monoazo red reactive dyes [150] Perspiration/light ATTS test

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Reactive system

Location (3.138)

Acid

Alkali

Dichlorotriazine Monochlorotriazine (MCT) Dichloroquinoxaline Bifunctional (SES/MCT) Sulphatoethylsulphone Sulphatoethylsulphone Bifunctional (SES/MCT)

Y Y Y Y X X X

2 3 2–3 3 2 1–2 1–2

1–2 2–3 2 2–3 1–2 1 1

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DYE STRUCTURE AND APPLICATION PROPERTIES

The fading of azo acid dyes for amide fibres has attracted a great deal of interest. Photochemical oxidation of the commercially important H acid disazo CI Acid Black 1 (3.143) in aqueous solution is accelerated by hydrogen peroxide but retarded by mannitol addition. The reaction rate is increased by incorporating a triplet-sensitising dye, suggesting that the azo groups, particularly the o-hydroxyazo in its hydrazone form, undergo attack by hydroxyl free radicals and triplet-sensitised degradation [153]. Measurements of the fading of a series of monoazo dyes of the phenylazo R acid type on wool demonstrated that this was a reductive process influenced particularly by the nature of ortho substitution in the diazo component;. Anionic groups (sulpho, carboxyl) conferred higher fastness to light but strongly electron-withdrawing nonionised substituents (chloro, nitro) had adverse effects [154]. A similar study of p-substituted phenylazo γ acid dyes revealed that electron-donating groups, such as ethoxy, amino or anilino, markedly lowered the light fastness. NO2

H2N H

N

N

O SO3Na

N N NaO3S

3.143 CI Acid Black 1

The kinetics of fading of the monoazo dyes CI Acid Orange 8 (3.144) and Red 1 (3.145) on exposure to sunlight in aqueous solutions at pH values in the range 2 to 4.5 and on silk fabric were investigated recently [155]. In the presence of silk these dyes were more photoreactive, exerting a protective action against photodegradation of the silk fibroin. As part of a broad chemometric strategy to design novel acid dyes for silk [156], eight phenylazo J acid dyes with o,p-substitution (3.146) were synthesised and their light fastness ratings compared on this fibre (Table 3.36). The results indicated that electron-withdrawing substituents (nitro, benzoyl), or an additional phenyl group in the p-position, enhanced light fastness slightly, whereas benzoate ester groups or an o-phenyl substituent lowered the fastness significantly. Disubstitution by propoxy did not modify the rating from that shown by the parent dye. All these dyes retained the 6-amino group in the coupling component, which tends to impair light fastness. O

C

CH3 H NaO3S

N

O

H

O

N

N

SO3Na N

NaO3S

3.144 CI Acid Orange 8

chpt3(1).pmd

CH3

HN

158

3.145 CI Acid Red 1

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159

Table 3.36 Light fastness of substituted phenylazo J acid dyes on silk [156] Substituents in structure 3.146

Light fastness

X = Y = nitro X = H, Y = benzoyl X = phenyl, Y = nitro X = benzoyl, Y = H X=Y=H X = Y = propoxy X = nitro, Y = phenyl X = Y = benzoate

4 4 4 4 3–4 3–4 2–3 2

Y H

O

N

X

NH2 N

NaO3S

3.146

Aggregates of dye molecules normally fade more slowly than do single molecules distributed uniformly through the substrate. These aspects were examined in the dyebath and on nylon for three monoazo dyes, CI Acid Orange 10, Red 18 and Red 88 (for structures see Table 3.28). Aggregation was found to influence the fastness to light, washing and rubbing [157]. When alkyl groups are introduced into the structures of milling acid dyes to confer higher neutral-dyeing affinity and better wet fastness on wool, relatively short chains (C4–C6) can have a favourable influence on light fastness by promoting aggregation in the fibre [136]. If the chain length is increased (C12–C16), however, the fastness to light decreases markedly, owing to the higher surface activity of such dyes tending to destabilise aggregates in favour of monomolecular distribution [158]. For example, the light fastness on wool of the phenylazopyrazolone levelling acid dye (3.147; X = H) is 6 but that of the longchain alkylated CI Acid Yellow 72 (3.147; X = n-dodecyl) is only 4–5. Cl SO3Na H X

O

N

N N

3.147

N

Cl

H3C

The influence of various dyeing and heat setting parameters on the light fastness of three acid dyes on nylon 66 has been examined recently. The capability of the dyes to quench the photoactive luminescent species in the polyamide matrix was found to be related closely to light fastness rating. Since acid dyes are linked to terminal amino groups through electrostatic bonds, they are able to protect their own chromogens and the polymer chain from photodecomposition through excitation energy transfer via the ionic bonds [159].

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DYE STRUCTURE AND APPLICATION PROPERTIES

The nature of the substrate has little effect on the light fastness of anthraquinone acid dyes on amide fibres. Most substituents hardly affect the slow rate of fading of these structures, but photodealkylation may occur when alkylaminoanthraquinones are exposed to light. The fastness can be improved, however, by incorporating arylamino in place of alkylamino groups [160,161]. The photodegradation in dimethylformamide solution and in nylon 6.6 of the 1:2 chromium-complex CI Acid Orange 60 (3.148) and the important anthraquinone CI Acid Green 25 (3.149) used on nylon automotive carpets was studied. Each of the dyes was found to fade by a reductive mechanism in both media. Dimethylformamide was found to be a suitable model for nylon in characterising photofading [162]. Consequently, the photodecomposition of several 1:2 metal-complex azo dyes was investigated in DMF solutions exposed to daylight or ultraviolet radiation at 300 or 350 nm. The coordinated metal atom and the wavelength of photolysis influenced the degradation reaction significantly. The presence of a ketone sensitiser accelerated dye decomposition, confirming that hydrogen abstraction is responsible for initiation of the fading reaction. Dyes containing cobalt(II) atoms were found to be effective quenchers of singlet oxygen [163]. NaO3S

SO2NH2

CH3

CH3 N

N

O

HN

O

NH

N

N

O O

Cr O O N

N

N

N H3C H2NO2S

NaO3S

CH3

3.148

3.149

CI Acid Orange 60

CI Acid Green 25

The photochemistry of four triphenylmethane acid dyes was studied in poly(vinyl alcohol), methylcellulose and gelatin films. These model systems were chosen with a view to elucidating the complex free-radical reactions taking place in the heterogeneous dyed wool/ water/air system on exposure to UV radiation. The dye fading mechanism seems to involve an excited triplet state of the dye molecule [164]. The rate of fading is governed by: (1) the positions of the sulpho groups in the dye molecule (2) the ability of the substrate or residual solvent to donate electrons or hydrogen atoms to the dye molecule (3) the degree of aggregation of the dye in the substrate (4) the internal physical and chemical structure of the substrate. Catalytic fading of certain mixtures of acid dyes on nylon, wool and their blends is a particularly significant problem of dye selection for carpets [165]. Xenotest ratings as low as 3–4 (much bluer) were found for green shades on nylon containing an azopyrazolone component such as CI Acid Yellow 19 (3.150), which fades much more rapidly in the presence

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161

of the widely used disulphonated 1,4-bis(arylamino)anthraquinones (3.151) such as CI Acid Blue 140, Green 25 (X = H, R = Me), Green 27 (X = H, R = n-Bu) or Green 28 (X = OH, R = n-Bu). Furthermore, commercially important unmetallised azopyrazolones such as Yellow 19, 49 or 79 are catalytically faded by typical 1:2 chromium-complex azopyrazolone dyes from the yellow to violet sectors of the colour gamut, including CI Acid Yellow 59 (3.152; X = H), Brown 384 (3.152; X = SO3Na) and substituted 2-hydroxyphenylazopyrazolone Reds 225, 226, 359 and 399. Cl

NaO3S

R

SO3Na SO3Na O

H N

X

O

HN

X

O

HN

N N

Cl

N H3C

3.150

CH3

CI Acid Yellow 19

N N N

NaO3S

N

R

3.151

O X

Cr

O

O

C

O

O

X

O

C

N

N N

N H3C

3.152

Apart from the catalytic effects between these fairly well-defined subclasses of acid dyes on nylon, that do not occur on wool [165], certain 1:2 metal-complex dyes such as CI Acid Yellow 59 (3.152; X = H) are able to catalyse the fading of various unmetallised milling acid dyes in mixture recipes on either wool or nylon [166]. For example, orange or scarlet shades formulated with Yellow 59 and Red 114 (3.153) can give light fastness as low as 2 (much yellower). Unacceptable off-tone fading can also arise in green mixtures of Yellow 59 with disulphonated 1,4-bis(arylamino)anthraquinones (3.151) that also fade much yellower. Photosensitised degradation of dyed nylon fibres is another practical problem that is characteristic of Yellow 59 and similar 1:2 chromium-complex azopyrazolone dyes. For the same disperse dyes on various hydrophobic fibres the highest light fastness is H3C

CH3 H

H3C

N

SO2 O

O

N

N

N NaO3S 3.153

CI Acid Red 114

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SO3Na

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DYE STRUCTURE AND APPLICATION PROPERTIES

found on polyester, normally followed by cellulose acetate and triacetate, with nylon giving the lowest ratings. Anthraquinone disperse dyes often contain one or two primary or secondary amino groups in the 1,4-positions. Tetra-amino derivatives have lower fastness than mono- or diamino analogues. Light fastness on cellulose acetate or polyester may be enhanced by: (1) a hydroxy group para to an amino group (2) acetylation of a primary amino group (3) replacement of an alkylamino by an arylamino group. Electron-donating groups (amino, methylamino, hydroxy, methoxy) in the 2-position, on the other hand, are extremely undesirable because, unlike similar substituents in the 1,4positions, they are unable to form intramolecular hydrogen bonds with the keto groups of anthraquinone and hence are highly susceptible to photo-oxidation [167]. The photochemical decomposition of typical azo and anthraquinone disperse dyes was found to correlate closely with the wavelength of UV radiation at which the fibre substrate showed maximum photodegradation (230 nm on cellulose acetate, 259 nm on triacetate and 316 nm on polyester). When exposed to UV radiation in ethyl acetate solution, the fading of CI Disperse Red 60 (3.154) occurred most rapidly in the 316 nm region [168]. Application of a UV absorber showing peak absorption in this region conferred some protection to dyeings of CI Disperse Red 73 (3.155) on polyester against dye fading and fibre photodegradation [169]. The primary mechanism of fading of the latter azo dye was a reductive reaction, whereas decomposition of the anthraquinone derivative Red 60 was essentially an oxidative process [168].

O

NH2

CN

O

O2N O

N

CH2CH2 N

OH

N CH2CH2CN

3.155

3.154

CI Disperse Red 73

CI Disperse Red 60

The fading rates of substituted 4-phenylazo-1-naphthylamine dyes on cellulose acetate were consistent with a photo-oxidative mechanism [170], but studies of the fading of phenylazo-2-naphthol dyes on polypropylene were indicative of photoreduction [171,172]. In simple azobenzene derivatives on polyester, an electron-withdrawing 4-nitro or especially 3-nitro group enhances light fastness (Table 3.37). An electron-donating 3′-methoxy substituent in the opposite ring boosts these effects further, but the 2-nitro-3′-methoxy combination yields the same fastness as azobenzene itself. The 2′-methoxy-5′-methyl substitution pattern also reinforces the favourable influence of the 3- or 4-nitro group but markedly lowers the rating of 2-nitroazobenzene. All three nitroazobenzenes are adversely affected by a 2′-hydroxy-5′-methyl arrangement [173].

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Table 3.37 Light fastness of substituted azobenzenes on polyester [173] Azobenzene substituents

Light fastness

3-Nitro-3′-methoxy 3-Nitro-2′-methoxy-5′-methyl 4-Nitro-2′-methoxy-5′-methyl 3-Nitro 4-Nitro-3′-methoxy 4-Nitro 3-Nitro-2′-hydroxy-5′-methyl 2-Nitro None 2-Nitro-3′-methoxy 4-Nitro-2′-hydroxy-5′-methyl 2-Nitro-2′-methoxy-5′-methyl 2-Nitro-2′-hydroxy-5′-methyl

7–8 7–8 7–8 7 7 6–7 6–7 6 6 6 5 4–5 3

O2N

N

H N

N R

3.156

Photofading studies of the simple azobenzene derivatives CI Disperse Orange 1 (3.156; R = Ph) and Orange 3 (3.156; R = H) were carried out in solution and in the solid phase under a variety of conditions. Attempts were made to interpret light fastness ratings on nylon and polyester substrates in terms of behaviour of the dyes in solution in the presence of model compounds representing functional groups in the polymers. Various mechanisms of fading were postulated and potential photostabilisers evaluated [174]. Many important orange, red, brown and blue disperse dyes belong to the substituted pphenylazoaniline subclass (Table 3.38). On nylon, light fastness in the p-nitrophenylazo series (3.157; X = NO2) is marginally improved by electron donation (methyl, methoxy) but markedly lowered by electron withdrawal (chloro and especially cyano or nitro) in the ortho position [175]. On polyester, electron-withdrawing groups oriented ortho (chloro, cyano) or para (nitro, acetyl, methylsulphonyl or diethylaminosulphonyl) to the azo linkage improve the fastness but electron-donating substituents (methyl, methoxy) lower the ratings. The 2,4-dinitrophenylazo arrangement is also highly unfavourable. In the 2-chloro-4nitrophenylazo series (3.158), the presence of N-cyanoethyl or N-acetoxyethyl groups in the coupling component confers much higher light fastness than the N,N-diethyl analogue but an N-2-hydroxyethyl group is slightly unfavourable in this respect. Only a limited range of nitro, azo and anthraquinone disperse dyes exhibit adequate fastness to dry heat, light and weathering for application on polyester automotive fabrics. The structure of CI Disperse Yellow 86 was modified to incorporate UV absorbers of the benzophenone, benzotriazole or oxalanilide types into the dye molecule. The derived dyes showed better fastness properties than the parent unsubstituted dye. Positioning of the photostabilising moiety within the dye molecule had little influence on the light fastness obtained, however. Built-in benzophenone residues were more effective than the other two types [177]. Nevertheless, several further monoazo and nitrodiphenylamine disperse dye

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DYE STRUCTURE AND APPLICATION PROPERTIES

Table 3.38 Light fastness of substituted aminoazobenzene dyes on nylon [175] and polyester [89,176] Light fastness CI Disperse

Structure

Orange 45 Red 54 Red 169 Red 186 Red 73 Red 50

3.158; X = Y = CN 3.158; X = CN, Y = OCOCH3 3.158; X = CN, Y = OCH2CH2CN 3.158; X = Y = OCOCH3 3.157; X = NO2, Y = CN, Z = H 3.158; X = H, Y = CN 3.158; X = H, Y = OCOCH3 3.157; X = NO2, Y = Z = H 3.157; X = NO2, Y = H, Z = CH3 3.157; X = NO2, Y = CH3, Z = H 3.158; X = OH, Y = CN 3.157; X = COCH3, Y = H, Z = CH3 3.157; X = SO2CH3, Y = H, Z = CH3 3.158; X = Y = H 3.157; X = NO2, Y = OCH3, Z = H 3.158; X = H, Y = OH 3.157; X = SO2NEt2, Y = H, Z = CH3 3.157; X = Y = NO2, Z = H 3.157; X = Y = H, Z = CH3 3.157; X = OCH3, Y = H, Z = CH3

Orange 25

Red 56

Red 13

Nylon

1–2 3–4 4 4–5

4

1

Polyester 7 7 6–7 6–7 6–7 6 6 5 5 4–5 4 4 4 3–4 3–4 3 3 2 2 2

Y Z X

CH2CH3

N N

N CH2CH2CN

Cl

3.157

O2N

N N

C

X

CH2CH2

Y

N

O Cl

CH2CH2

3.158

CH2CH3

HN O2N

N

CH2CH2OCOCH3 N

N CH2CH2OCOCH3

3.159 CI Disperse Red 167

structures containing an oxalanilide photostabilising grouping were synthesised and evaluated as potential dyes for automotive trims. The light fastness of CI Disperse Red 167 (3.159) was enhanced by inclusion of an electron-donating o-ethyl group in the diazo component [178].

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165

A similar approach has been adopted in modifying the anthraquinone system represented by CI Disperse Blue 77 (3.160), the aniline residue being replaced by a more elaborate arylamine with UV absorbing characteristics, The modifying intermediates selected included hydroxybenzophenone and chlorobenzotriazole derivatives, as well as a hindered photostabiliser. In general, the improvement in light fastness from this approach is inferior to that given by carefully formulated physical mixtures of the corresponding dye and stabilising agent. The modified dye structures tend to exhaust more slowly and show poor levelling but do offer higher fastness to sublimation [179]. HO

O

O2N

O

OH

HN

3.160 CI Disperse Blue 77

Two commercial disazo disperse dyes of relatively simple structure were selected for a recent study of photolytic mechanisms [180]. Both dyes were found to undergo photoisomerism in dimethyl phthalate solution and in films cast from a mixture of dye and cellulose acetate. Light-induced isomerisation did not occur in polyester film dyed with the two products, however. The prolonged irradiation of CI Disperse Yellow 23 (3.161; X = Y = H) either in solution or in the polymer matrix yielded azobenzene and various monosubstituted azobenzenes. Under similar conditions the important derivative Orange 29 (3.161; X = NO2, Y = OCH3) was degraded to a mixture of p-nitroaniline and partially reduced disubstituted azobenzenes. Y N X

N

N

N

OH

3.161

3.3.5 Dye structure and wet fastness The fastness of a dye to wet treatments, such as washing or perspiration, is a function of kinetic (diffusion) and thermodynamic (affinity) effects. Certain trends can be discerned, but conclusions derived from simple structural changes in closely related structures have only limited applicability. Relationships between wet fastness ratings and molecular structure of direct dyes on cellulosic fibres are somewhat tenuous because of the major obscuring influence of hydrogen bonding and other forces of association between dye anions leading to aggregation within the substrate (sections 3.1.2 and 3.3.4). In general, the wet fastness of disperse dyes on hydrophobic fibres tends to increase with the molecular size of the dye and with the increasingly hydrophobic character of the substrate (acetate < nylon < triacetate < polyester).

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DYE STRUCTURE AND APPLICATION PROPERTIES

Acid dyes are normally applied to wool at acidic pH values but are washed under neutral or mildly alkaline conditions. Forces of affinity between dye and substrate tend to retard desorption. The higher the degree of sulphonation in a series of naphthylazo-2-naphthol dyes (Table 3.39), the higher is the rate of desorption into sodium borate solution at 40 °C. With a series of monosulphonated acid dyes the desorption rate under these conditions was inversely related to the affinity of the dye for wool at pH 3 and 60 °C (Table 3.40). Table 3.39 Basicity and desorption rate of monoazo acid dyes on wool [181] H

Substituents in 3.162

Basicity

Desorption rate (%/min)

R = X = Y = Z = SO3Na R = Y = Z = SO3Na, X = H R = X = H, Y = Z = SO3Na R = SO3Na, X = Y = Z = H

4 3 2 1

16.0 12.1 5.2 1.6

R

O

N Z Y

3.162

Table 3.40 Affinity and desorption rate of monosulphonated acid dyes on wool [181] Desorption rate (%/min) CI Acid

Structure

Affinity (kJ/mol)

40°C

60°C

Orange 7 Yellow 36 Violet 43 Blue 25

3.163 3.164 3.165 3.166

–18.4 –21.7 –27.3 –29.7

6.4 5.4 3.0 2.0

18.5 16.6 9.1 6.0

H NaO3S

O

N

NaO3S N

N

NH

N 3.163

3.164

CI Acid Orange 7

NaO3S

CI Acid Yellow 36

CH3

O

NH2 SO3Na

O

HN O

O

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X

N

HN

3.165

3.166

CI Acid Violet 43

CI Acid Blue 25

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APPLICATION PROPERTIES AND CHEMICAL STRUCTURE

167

The increase in affinity for wool imparted to acid dye structures by the inclusion of longchain alkyl groups has a noteworthy effect on wet fastness. Thus the incorporation of such a group into the levelling acid dye CI Acid Red 1 (3.129; R = H) to form the ‘super-milling’ acid dye Red 138 (3.129; R = n-dodecyl) raises the fastness to washing at 50 °C (effect on pattern) from 2–3 to 4–5. Similar effects for a series of alkylated phenylazo-2-naphthol-6sulphonate dyes are recorded in Table 3.41.

Table 3.41 Structure and wash fastness of monoazo dyes on wool [182]

Y Wash fastness (effect on pattern)

Substituents in 3.167

H

X = (CH2)11CH3, Y = H X = (CH2)3CH3, Y = H X = CH2CH3, Y = H X = H, Y = CH2CH3 X = CH3, Y = H X=Y=H

O

N

X

N

4 3 2 2 1–2 1

3.167 SO3Na

The series of eight phenylazo J acid dyes with o,p-substitution in the diazo component (3.146) synthesised for evaluation on silk was subjected to various wet fastness tests (Table 3.42). The results indicated that fastness to these agencies was lowered slightly by o,pbenzoate ester disubstitution or by a benzoyl group in the p-position. Increases in fastness ratings with dipropoxy, dinitro, o-nitro-p-phenyl or p-nitro-o-phenyl substitution were also slight, but an o-benzoyl grouping was surprisingly effective in this respect. Possibly the keto moiety stabilises this dye in its hydrazone form (3.168) by hydrogen bonding, implying that the tautomeric hydroxyazo form of these dyes has lower affinity for silk than the corresponding ketohydrazone.

Table 3.42 Wet fastness of substituted phenylazo J acid dyes on silk [156] Washing

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Perspiration

Substituents in structure 3.146

40°C

50°C

60°C

Acid

Alkaline

X = benzoyl, Y = H X = Y = propoxy X = Y = nitro X = nitro, Y = phenyl X = phenyl, Y = nitro X=Y=H X = H, Y = benzoyl X = Y = benzoate

5 4 3–4 4 4 3 3–4 2–3

4 2–3 3 2 2 2 3 1–2

3 1 1 1 1 1 1 1

5 5 5 4–5 4–5 5 4 4

5 5 5 5 5 5 4 5

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DYE STRUCTURE AND APPLICATION PROPERTIES

C

O O

H

NH2

N N

3.168

NaO3S

In a further exploration of the relationship between dye structure and wet fastness on silk, four novel monoazo J acid derivatives (3.169; X = X1 to X4), including 3.168 (X = X2) made from 2-aminobenzophenone, were synthesised. Silk was dyed at pH 4 and 85 °C and the dyeings tested for fastness to washing, perspiration and dry cleaning. The highest allround fastness was shown by the 4-aminobenzophenone derivative (X = X4), a structure that resembles the anti-parallel pleated sheet arrangement of polypeptide chains in silk [183]. H X

O

N

NH2 N

C O2N

O NaO3S

= X1

3.169

= X2 NO2 O = X3

C

= X4

Affinity values were determined for an interesting series of five anthraquinone acid dyes [184]. The first four were monosulphonated derivatives that differed only in the length of the n-alkyl R substituent (3.170). The fifth dye (3.171) was composed of two monosulphonated molecules linked at the R position by means of a methylene group. The affinity for nylon increased progressively with relative molecular mass as the length of the nalkyl chain was extended (Table 3.43). The average increment per methylene group up to nbutyl was -2.55 kJ/mol. The affinity of the disulphonated dye could be calculated to a good approximation from the sum of twice that of the non-alkylated control dye plus the increment for the linking methylene group, as in Equation 3.1. 2( -23.8) + ( -2.55) = - 50.15 kJ/mol

(3.1)

The wet fastness of these dyeings increased consistently with the Mr of the dye.

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O

169

NH2 SO3Na

HN

O

R

3.170

O

NH2

H2N SO3Na

O

HN

O

NaO3S

CH2

NH

O

3.171 Table 3.43 Affinity and relative molecular mass of anthraquinone acid dyes on nylon [184] Structure

Mr

Affinity (kJ/mol)

3.170; R = H 3.170; R = methyl 3.170; R = n-butyl 3.170; R = n-dodecyl 3.171

394 408 450 562 800

–23.8 –26.6 –34.0 –45.6 –52.8

3.3.6 Dye structure and heat fastness This fastness requirement is of practical significance only for disperse dyes on hydrophobic fibres. Dyes of low fastness are sublimed from the surface of the heated fibre at a rate dependent on the temperature of treatment. The diffusion coefficient of the dye in the dry polymer controls the rate at which loss of dye from the surface by volatilisation is replenished from the interior of the fibre. Dye decomposition during the heat fastness test can be highly significant in determining the overall rating attainable with a given dye structure [185]. Inverse relationships between the vapour pressure of disperse dyes and their molecular size and polarity have been established [186]. In Table 3.44 the vapour pressure values at 200 °C for three typical disperse dyes of low relative molecular mass are given. A decrease of only 5% in Mr between CI Disperse Red 11 (3.172; X = OCH3, Y = NH2) and Violet 4 (3.172; X = H, Y = NHCH3) produces a tenfold increase in vapour pressure, although a further decrease of 4% to give Orange 3 (3.173) has only a doubling effect. A more striking example appears in Table 3.45. The 12% decrease in Mr from Blue 14 (3.174; X = NHCH3) to Red 9 (3.174; X = H) has profound effects, raising the diffusion coefficient in polyester about sixfold and the vapour pressure almost 700-fold. Both factors have an adverse

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DYE STRUCTURE AND APPLICATION PROPERTIES

Table 3.44 Vapour pressure and relative molecular mass of disperse dyes [187]

CI Disperse

Structure

Mr

Vapour pressure at 200°C × 103 (kPa)

Orange 3 Violet 4 Red 11

3.173 3.172; X = H, Y = NHCH3 3.172; X = OCH3, Y = NH2

242 252 268

18.4 9.67 1.03

Table 3.45 Vapour pressure, diffusion coefficient and structure of disperse dyes [188]

CI Disperse

Structure

Mr

Vapour pressure at 160°C × 105 (cm Hg)

Diffusion coefficient × 1013 (m2/s)

Red 9 Yellow 3 Blue 14

3.174; X = H 3.175 3.174; X = NHCH3

237 269 266

692 13.5 1.01

6.87 2.39 (1.2)

O

NH2 X

O2N

N N

NH2

3.173

3.172

O

Y

O

NHCH3

CI Disperse Orange 3

CH3CO

HO

N

N

H

N 3.175

O

X

3.174

CH3

CI Disperse Yellow 3

influence on heat fastness. The monoazo Yellow 3 (3.175) is almost identical with Blue 14 in Mr but diffuses twice as rapidly and vaporises much more readily, possibly because it has only two aryl nuclei and a linear conformation. The requirements of disperse dyes for the transfer printing of polyester fabrics are almost exactly opposite to those of the dyer. Adequate sublimation during the transfer step can normally be achieved only with dyes of low Mr (<350) that are substantially free from highly polar substituents [189]. For steric reasons, however, there are some exceptions to this general rule. Thus 2,4,6-trisubstituted phenylazo groups, which often feature in useful brown and navy blue members of the aminoazobenzene class, can interfere with the coplanarity of such structures. This may hinder dye-dye and dye-fibre bonding to such an extent that such dyes of Mr exceeding 400 can sometimes be used for transfer printing. The relationships between dye structure and heat fastness for two series of phenylazopyrazolone dyes (Table 3.46) demonstrate the significance of changes in molecular

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171

Table 3.46 Structure and heat fastness of phenylazopyrazolone disperse dyes on polyester [175] Structure

Heat fastness

3.176; X = CH3, Y =

SO2

5

3.176; X = COOCH2CH3, Y = Ph 3.177; X = NH 3.176; X = CH3, Y = Ph 3.177; X = O 3.176; X = CH3, Y = H

3–4 3–4 2–3 2–3 1–2

Cl H O2N

O

N

N N

CH3OOC

X

N

N N

N X

3.176

H

Y

3.177

N H3C

size and polar substituents for this property [175]. In the 2-chloro-4-nitrophenylazo (3.176) series, the tetramethylenesulphone ring enhances heat fastness far more than does a conventional N-phenyl group, even though both changes in molecular size are relatively modest. A 3-carbethoxy substituent in the pyrazolone ring raises heat fastness about one point higher than that of the 3-methyl analogue, a difference roughly equivalent to the insertion of an N-phenyl ring into the 3-methylpyrazolone parent dye. In the 4carbomethoxyphenylazo (3.177) structure, an improvement of this magnitude can be achieved merely by using the pyrazol-5-imine instead of the pyrazol-5-one as the coupling component. Variations in the dialkylamino terminal substitution pattern of typical alkylaminoazobenzene derivatives have a greater effect on heat fastness than have substituents in the aryl nuclei of the diazo and coupling components [138]. The fastness can be significantly improved by incorporating polar groups (acetoxy, acetylamino, cyano) capable of hydrogen bonding or dipole–dipole interaction with polymer segments. The effect on heat fastness of replacing an acetate ester by a benzoate ester group in the coupling component is clearly illustrated by comparing two commercially important yellow brown disperse dyes of the 2,6dichloro-4-nitrophenylazo series (Table 3.47). Dyes of high Mr with numerous substituents or bulky groups almost invariably show higher fastness to sublimation than simpler aminoazobenzene or aminoanthraquinone analogues [182]. An interesting and comprehensive survey of 2,4,6-trisubstituted phenylazo dyes comparing the relative influence of all four possible halogeno substituents explored many such mutual steric and electron-withdrawing effects in the diazo components (Table 3.48). In all three series of structures represented (3.179–3.181), heat fastness of the trisubstituted derivatives increased consistently in the order: 2,6-dihalogeno-4-nitro < 2,4dinitro-6-halogeno < 2-cyano-4-nitro-6-halogeno. As expected, heat fastness increased markedly with the size of the halogen atom and in the halogenated phenylazo-N-

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DYE STRUCTURE AND APPLICATION PROPERTIES

Table 3.47 Structure and heat fastness of esterified N-hydroxyethyl disperse dyes on polyester [89]

CI Disperse

Substituent R in structure 3.178

Heat fastness (30s at 210°C)

Orange 62 Orange 30

O.CO.Ph O.CO.Me

4–5 3–4

Cl O2N

N

CH2CH2CN N

N CH2CH2

Cl

R

3.178

cyanoethyl-N-hydroxyethylanilines (3.179) heat fastness increased in the order: 4-halogeno < 2-halogeno < 2-halogeno-4-nitro. Heat fastness in Tables 3.48, 3.50 and 3.51 is defined as the dry heat treatment temperature at which a 2.5% o.w.f. dyeing on polyester gives a satisfactory rating [190]. Proton-donor groups (amino, hydroxy) in the 2-position of 1,4-disubstituted anthraquinone dyes enhance the fastness to heat because they are readily able to form intermolecular hydrogen bonds with the substrate or other dye molecules. Similar groups in the 1,4-positions normally form intramolecular hydrogen bonds with the keto groups of anthraquinone. The main drawback of electron-donating groups in the β-positions of anthraquinone dyes, however, is their usually adverse effect on light fastness (section 3.3.4).

Table 3.48 Structure and heat fastness of halogenated phenylazo disperse dyes on polyester [190] Sublimation for 2.5% depth (°C)

chpt3(1).pmd

Structure

Iodo

Bromo

Chloro

3.179; X = nitro, Y = halogeno, Z = cyano 3.181; X = nitro, Y = halogeno, Z = cyano 3.180; X = nitro, Y = halogeno, Z = cyano 3.181; X = Z = nitro, Y = halogeno 3.179; X = Z = nitro, Y = halogeno 3.180; X = Z = nitro, Y = halogeno 3.180; X = nitro, Y = Z = halogeno 3.181; X = nitro, Y = Z = halogeno 3.179; X = nitro, Y = Z = halogeno 3.179; X = nitro, Y = halogeno, Z = H 3.179; X = Z = H, Y = halogeno 3.179; X = halogeno, Y = Z = H

210 200 200 200 190 190 190 190 190 180 170 170

210 190 180 180 180 180 180 180 170 170 160 150

210 180 180 170 180 170 170 170 170 170 160 160

172

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Fluoro

170 170 170

160 150 140

APPLICATION PROPERTIES AND CHEMICAL STRUCTURE

Table 3.49 Structure and heat fastness of substituted aminohydroxyanthraquinone disperse dyes on polyester [175] Structure

Heat fastness

3.182; X =

3.183; R = 3.182; X = 3.183; R =

4–5

NHCOCH3

4–5

SO2NHCH2CH2CH2OCH2CH3

SO2

3–4 3–4

Cl

3.183; R = –CH2CH2CH2CH2CH2CH 2OH

3–4

3.182; X =

2–3

OCH3

3.182; X = –CH2CH2COOCH2CH3

2–3

3.183; R =

2–3

SCH3

3.183; R =

2–3

3.182; X =

1–2

3.182; X = H 3.183; R = CH3

1–2 1–2

Table 3.50 Structure and heat fastness of N-substituted aminodihydroxyanthraquinone disperse dyes on polyester [190]

chpt3(1).pmd

Substituent R in structure 3.184

Sublimation for 2.5% depth (°C)

Benzothiazol-2-yl 4-n-Dodecylphenyl 3-Fluoromethylphenyl Morpholin-4-ylpropyl Pyrid-3-ylmethyl Phenyl 2,3-Dihydroxypropyl 2-Hydroxypropyl Morpholin-4-ylethyl Benzyl Cyclohexyl Methoxypropyl Hydroxypropyl Hydroxyethyl Pyran-2-ylmethyl Methoxyethyl Ethyl Methyl Hydrogen

180 180 180 160 160 160 160 160 150 150 150 150 150 150 140 140 140 140 140

173

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173

174

DYE STRUCTURE AND APPLICATION PROPERTIES

Table 3.51 Structure and heat fastness of p-substituted anilinoanthraquinone disperse dyes on polyester [192] Sublimation for 2.5% depth (°C) Anthrarufin derivatives

Chrysazin derivatives

3.187

3.188

3.189

3.190

Substituent R in 3.187–3.190

1-amino5-anilino

1-anilino5-nitro

1-amino8-anilino

1-anilino8-nitro

Benzoylamino Acetylamino Aminosulphonyl Benzoyl Carbethoxy Acetyl Hydroxyethyl Methoxy Ethoxy Methyl Hydrogen

200 200 200 190 210 180 170 170 160 160 160

200 200 200 200 190 190 180 160 170 170 160

210 200 200 200 210 200 160 160 160 160 160

210 210 200 210 190 190 180 180 170 170 170

Y X

CH2CH2OH

N N

N

Y

COCH3

CH2CH2CN

Z 3.179

HN X

CH2CH2OH

N N

Y

N

Z

CH2CH2CN OCH3

3.180 X

N NHCH2CH2OH

N Z

H 3.181

O

X

O

N

O

OH

NH2

O O

R

OH NH

3.182

O

OH

O 3.184

3.183

chpt3(1).pmd

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OH

R

APPLICATION PROPERTIES AND CHEMICAL STRUCTURE

HO

O

NH2

H2N

O

NH2

H2N

O

OH

HO

O

OH

HO

NH

175

O

NH2

O

OH

3.186

3.185

R R

HO

O

O2N

O

3.187

R

HN

NH

R

OH

HO

O

NH2

O

OH

3.189

3.188 O2N

O

HO

O

HN

OH

3.190

Trends indicating the capability of various polar and nonpolar substituents to enhance the low heat fastness of 1-amino-4-hydroxyanthraquinone (3.182; X = H) and its 2-hydroxy derivative (3.183; R = H) are revealed in Table 3.49. In the series of secondary amino derivatives (3.182) a phenyl group offers little or no improvement. An ethyl propionate residue or a p-methoxyphenyl group produces a modest increase in fastness but more polar moieties such as tetramethylenesulphone or p-acetanilide groupings are necessary to achieve high ratings. Methylation of the 2-hydroxy group in structure 3.183 improves the light fastness but not the fastness to sublimation. A phenyl or p-methylthiophenyl group raises heat fastness somewhat, but more marked improvements follow with 6-hydroxy-n-hexyl, pchlorophenyl and the complex alkoxyalkylsulphamoylphenyl grouping shown in Table 3.49. In a related series of 1,2,4-trisubstituted anthraquinone compounds, the effectiveness of various polar and nonpolar substituents to improve on the low heat fastness of 2-amino-1,4dihydroxyanthraquinone (3.184; R = H) was examined (Table 3.50). Short-chain alkyl groups (methyl, ethyl) and even the pyranylmethyl ether are relatively ineffective but hydroxyalkyl, cyclohexyl, benzyl and morpholinylethyl groups show moderate increases. Further improvement is given by phenyl, pyridylmethyl and morpholinylpropyl. Outstandingly effective, however, are the benzothiazolyl, dodecylphenyl and fluoromethylphenyl groupings. The influence of the number and orientation of bromo substituents on heat fastness in two systematic series of multibrominated derivatives of 1,5-diaminoanthrarufin (3.185) and

chpt3(1).pmd

175

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176

DYE STRUCTURE AND APPLICATION PROPERTIES

1,8-diaminochrysazin (3.186) was investigated in detail [191]. More recently, p-substitution of the aryl ring in four series of aminoanilino and anilinonitro analogues has been studied (Table 3.51). Only moderate improvements in heat fastness were afforded by methyl, methoxy or ethoxy substituents. Groupings incorporating more polar moieties, including hydroxyethyl, acetyl, carbethoxy and benzoyl, were much more effective, but the highest ratings were shown by the amido groupings, namely aminosulphonyl, acetylamino and benzoylamino. 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. 39. 40. 41. 42. 43. 44. 45. 46.

chpt3(1).pmd

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K Hamada, M Nishizawa and M Mitsuishi, Dyes and Pigments, 16, No 3 (1991) 165. K Hamada et al., Dyes and Pigments, 36 (1998) 313. K Bredereck and C Schumacher, Dyes and Pigments, 21, No 1 (1993) 23, 45. T Omura, Y Kayane and Y Tezuka, Dyes and Pigments, 20, No 4 (1992) 227. T Omura, Dyes and Pigments, 23 (1993) 179; 24 (1994) 125; 26 (1994) 33. A K Mairal and S B Guha, J.S.D.C., 109 (1993) 120. Zhang Zhi-bai et al., AATCC Internat. Conf.& Exhib. (Oct 1987) 227. W Biedermann, J.S.D.C., 87 (1971) 105; 88 (1972) 329. V A Shenai and M C Sadhu, J. Appl. Polymer Sci., 20 (1976) 3141. A S Ghanekar and F Jones, J.S.D.C., 92 (1976) 445. H Schott, J. Phys. Chem., 70 (1966) 2966. C L Bird, J.S.D.C., 70 (1954) 68. J R Aspland and C L Bird, J.S.D.C., 77 (1961) 9. C L Bird and P Harris, J.S.D.C., 73 (1957) 199. E Merian et al., J.S.D.C., 79 (1963) 505. D Patterson and R P Sheldon, J.S.D.C., 76 (1960) 178. W McDowell and R Weingarten, J.S.D.C., 85 (1969) 589. G L Baughman and E J Weber, Dyes and Pigments, 16, No 4 (1991) 261. M Hou, G L Baughman and T A Perenich, Dyes and Pigments, 16, No 4 (1991) 291. H F Willis et al., Trans. Far. Soc., 41 (1945) 506. W J Marshall and R H Peters, J.S.D.C., 63 (1947) 446. S Fisichella, G Scarlata and M Torre, J.S.D.C., 94 (1978) 521. H H Hodgson, J.S.D.C., 49 (1933) 213. K Nishida, J.S.D.C., 82 (1966) 313. H Bach et al., Angew. Chem., 75 (1963) 407. C H Giles and A S A Hassan, J.S.D.C., 74 (1958) 846. M Grunwald, E Burtscher and O Bobleter, Textilverediung, 27 (1992) 45. K Bredereck, F Bader and U Schmitt, Textilveredlung, 24 (1989) 142. F Townend, J.S.D.C., 61 (1945) 144. J Meybeck and P Galafassi, Appl. Polymer Symposia, 18 (1971) 463. K Hannemann and H Flensberg, Melliand Textilber., 78 (1997) 160. J Hertig and H Scheidegger, Textilveredlung, 18 (1983) 325. J A Bone, Proc. Internat. Wool Text. Res.Conf., Aachen, 10 (1984) 170. J Shore, Blends Dyeing, (Bradford: SDC, 1998). F M Arshid, C H Giles and S K Jain, J.C.S., (1956) 1272. A Cameron, C H Giles and T H MacEwan, J.C.S., (1958) 1224. C H Giles, V G Agnihotri and A S Trivedi, J.S.D.C., 86 (1970) 451. C L Bird and A M Patel, J.S.D.C., 84 (1968) 560. E H Daruwalla, S S Rao and B D Tilak, J.S.D.C., 76 (1960) 418, 680. C L Bird, J.S.D.C., 74 (1958) 688. C L Bird and P Rhyner, J.S.D.C., 77 (1961) 12. K Hoffmann, W McDowell and R Weingarten, J.S.D.C., 84 (1968) 306. J F Dawson, J.S.D.C., 107 (1991) 395. H Gerber, J.S.D.C., 94 (1978) 298. A Urbanik, AATCC Nat. Tech. Conf., (Oct 1981) 258. W Biedermann and A Datyner, Text. Res. J., 61 (1991) 637. A Fisher, C S Lee and A T Peters, J. Chem. Tech. Biotechnol., 32 (1982) 532. Y Muraoka et al., Text. Res. J., 66 (1996) 104. M Mitsuishi et al., J.S.D.C., 112 (1996) 333. J F Laucius, R A Clarke and J A Brooks, Amer. Dyestuff Rep., 44 (1955) 362. O Glenz and W Beckmann, Melliand Textilber., 38 (1957) 296. W Beckmann, J.S.D.C., 77 (1961) 616. S Rosenbaum, Text. Res. J., 33 (1963) 899. K Meldrum and J S Ward, J.S.D.C., 74 (1958) 140. D Balmforth, C A Bowers and T H Guion, J.S.D.C., 80 (1964) 577. M Hoten, J.S.D.C., 112 (1996) 123. S Rosenbaum, J. Polymer Sci., A3 (1965) 1949. U Mayer, W Ender and A Würz, Melliand Textilber., 47 (1966) 653, 772. U Mayer, Amer. Dyestuff Rep., 56 (1967) 869. J A Koller and M Motter, Textilveredlung, 11 (1976) 3.

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178 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133. 134. 135. 136. 137. 138. 139. 140. 141. 142. 143. 144. 145. 146. 147. 148. 149. 150. 151. 152. 153. 154. 155. 156. 157. 158. 159. 160. 161. 162. 163. 164. 165.

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DYE STRUCTURE AND APPLICATION PROPERTIES

S Rosenbaum, Text. Res. J., 34 (1964) 159. J Boulton, J.S.D.C., 54 (1938) 268; 60 (1944) 5. C C Wilcock, J.S.D.C., 63 (1947) 136. C H Giles, A P Montgomery and A H Tolia, Text. Res. J., 32 (1962) 99. T L Dawson and J C Todd, J.S.D.C., 95 (1979) 417. C Renard, Teintex, 36 (1971) 845. E Waters, J.S.D.C., 66 (1950) 609. T Vickerstaff and E Waters, J.S.D.C., 58 (1942) 116. T Vickerstaff, The physical chemistry of dyeing, 2nd Edn (London: Oliver & Boyd, 1954). T Vickerstaff, J.S.D.C., 59 (1943) 92. E Siepmann, Melliand Textilber., 49 (1968) 577. E Riesz, Bull. Sci. Inst. Text. France, 11 (43) (1982) 61; Textilveredlung, 25 (1990) 19. Y C Chao and Y I Pan, Dyes and Pigments, 31 (1996) 253. H Maruyama et al., J.S.D.C., 112 (1996) 53. J A Fowler, A G H Michie and T Vickerstaff, Melliand Textilber., 32 (1951) 296. B G Ferrini, Y Kimwe and H Zollinger, Proc. Internat. Wool Text. Res. Conf., Paris, 3 (1965) 291. S R S Iyer, A S Ghanekar and G S Singh, Symp. on Dye-polymer Interactions, Bombay (1971) 13. D M Lewis, Rev. Prog. Coloration, 28 (1998) 12. Y C Chao and C W Yeh, Dyes and Pigments, 23 (1993) 285. Y C Chao, Colour Science ’98, Vol. 2, Ed. S M Burkinshaw (Leeds: Leeds Univ., 1998) 112. Y C Chao, M J Chang and C H Chung, Dyes and Pigments, 39 (1998) 183. E Atherton, D A Downey and R H Peters, Text. Res. J., 25 (1955) 977. V Kartaschoff and G Farine, Helv. Chim. Acta, 11 (1928) 813. T Barr, F M Rowe and J B Speakman, J.S.D.C., 58 (1942) 52. G L Royer, H E Millson and E I Stearns, Amer. Dyestuff Rep., 32 (1943) 288. W H Watkins, G L Royer and H E Millson, Amer. Dyestuff Rep., 33 (1944) 52. D Fiebig and S Frick, Melliand Textilber., 78 (1997) 604. J T Voltz, Text. Chem. Colorist, 9 (1977) 113. N A Evans and I W Stapleton, J.S.D.C., 89 (1973) 208. P Bentley, J F McKellar and G O Phillips, Rev. Prog. Coloration, 5 (1974) 33. G E Krichevsky et al., Text. Res. J., 45 (1975) 608. G Hallas, J.S.D.C., 95 (1979) 285. R Grecu, M Pieroni and R Carpignano, Dyes and Pigments, 2 (1981) 305. N S Allen, Rev. Prog. Coloration, 17 (1987) 61. C H Giles, J.S.D.C., 73 (1957) 127. G Baxter, C H Giles and W J Lewington, J.S.D.C., 73 (1957) 386. C H Giles et al., J.S.D.C., 88 (1972) 433. C H Giles, D J Walsh and R S Sinclair, J.S.D.C., 93 (1977) 348. C H Giles and E A Strevens, Text.Res. J., 49 (1979) 724. K J Sirbiladze et al., Dyes and Pigments, 19 No 4 (1992) 235. Y Okada, F Fukuoka and Z Morita, Dyes and Pigments, 37 (1998) 47. Y Okada et al., Dyes and Pigments, 12, No 3 (1990) 197; 19, No 1 (1992) 1; 19, No 3 (1992) 203; 20, No 2 (1992) 123; 24 (1994) 1. K Bredereck and C Schumacher, Dyes and Pigments, 23 (1993) 135. K Imada, N Harada and T Takagishi, J.S.D.C., 110 (1994) 231. J Griffiths and C Hawkins, J. Appl. Chem. Biotechnol., 27 (1977) 558; J. Chem. Soc. Perkin II, (1977) 747. J Monheim and R Löwenfeld, Melliand Textilber., 53 (1972) 1134. M T Ball et al., Dyes and Pigments, 19, No 1 (1992) 51. R H Kienle, E I Stearns and P A van der Meulan, J. Phys. Chem., 50 (1946) 363. M R Massafra et al., Dyes and Pigments, 40 (1998) 171. M R de Giorgi et al., J.S.D.C., 109 (1993) 405; Dyes and Pigments, 30 (1996) 79. B Küster, Chemiefasern und Textilind., 37/89 (1987) 1236; Textil Praxis, 43 (1988) 60. J Meybeck, P Ruckstuhl and J M Thumann, Teintex, 25 (1960) 241. N S Allen et al., Dyes and Pigments, 34 (1997) 169. C H Giles and R S Sinclair, J.S.D.C., 88 (1972) 109. D Wegerle, J.S.D.C., 89 (1973) 54. H S Freeman and J Sokolowska-Gajda, Text. Res. J., 60 (1990) 221. J Sokolowska-Gadja, Dyes and Pigments, 36 (1998) 149. D F Duxbury, Dyes and Pigments, 25 (1994) 131, 179. K Dunkerley, J.S.D.C., 108 (1992) 268.

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G Reinert, Melliand Textilber., 69 (1988) 58. N S Allen, P Bentley and J F McKellar, J.S.D.C., 91 (1975) 366. N Katsuda, T Omura and T Takagishi, Dyes and Pigments, 34 (1997) 147; 36 (1998) 231. N Katsuda et al., Dyes and Pigments, 31 (1996) 291; 36 (1998) 193. E Atherton and R H Peters, J.S.D.C., 68 (1952) 64. M Ahmed and V N Mallet, J.S.D.C., 84 (1968) 313. V N Mallet and B T Newbold, J.S.D.C., 90 (1974)4. S V Sunthankar and V Thanumoorthy, Indian J. Chem., 8 (1970) 598. J P Binkley, T M Evans and A R Horrocks, Adv. Col. Sci. Tech., 2 (1999) 114. C Muller, Amer. Dyestuff Rep., 59 (Mar 1970) 37. M F Sartori, J.S.D.C., 83 (1967) 144. M E Mason, J C Posey and H S Freeman, AATCC Internat. Conf.& Exhib. (Oct 1991) 114. H S Freeman, M E Mason and J Lye, Dyes and Pigments, 42 (1999) 53. D O Ukponmwan, Colourage, 45 (Apr 1998) 35. H S Freeman and S A McIntosh, Text. Research J., 64 (1994) 309. C H Nicholls, J.S.D.C., 72 (1956) 479. C H Giles, D G Duff and R S Sinclair, Rev. Prog. Coloration, 12 (1982) 58. M R de Giorgi, Melliand Textilber., 73 (1992) 578. K Grieder, J.S.D.C. 92 (1976) 8. W McDowell, J.S.D.C., 89 (1973) 177. H E Schroeder and S N Boyd, Text. Res. J., 27 (1957) 275. R B Chavan and A K Jain, J.S.D.C., 105 (1989) 73. F Jones and T S M Leung, J.S.D.C., 90 (1974) 286. J Griffiths and F Jones, J.S.D.C., 93 (1977) 176. A T Peters and N O Soboyejo, J.S.D.C., 104 (1988) 486; 105 (1989) 315. A T Peters and B A Tenny, J.S.D.C., 93 (1977) 378. A T Peters and Y C Chao, J.S.D.C., 104 (1988) 435.

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180

CHEMISTRY OF AZO COLORANTS

CHAPTER 4

Chemistry of azo colorants Geoffrey Hallas

4.1 INTRODUCTION Shortly after Perkin had produced the first commercially successful dyestuff, a discovery was made which led to what is now the dominant chemical class of dyestuffs, the azo dyes. This development stemmed from the work of Peter Griess, who in 1858 passed ‘nitrous fumes’ (which correspond to the formula N2O3) into a cold alcoholic solution of 2-amino-4,6dinitrophenol (picramic acid) and isolated a cationic product, the properties of which showed it to be a member of a new class of compounds [1]. Griess extended his investigations to other primary aromatic amines and showed his reaction to be generally applicable. He named the products diazo compounds and the reaction came to be known as the diazotisation reaction. This reaction can be represented most simply by Scheme 4.1, in which HX stands for a strong monobasic acid and Ar is any aromatic or heteroaromatic nucleus. Ar

NH2 + HX + HNO2

Ar

+ N

_ N

X

+ 2H2O

Scheme 4.1

Griess subsequently recognised that the orange compound formed by the reaction of diazobenzene with phenol in neutral solution contained two benzene nuclei joined together via an azo (–N=N–) linkage [2] and again the reaction was found to be general and not confined to phenols. This second reaction became known as the diazo coupling reaction and the products as azo compounds. The wide applicability of this reaction opened the way to a vast multitude of compounds produced in generally high yields in simple plant and led to the azo dyes becoming the largest and most versatile class of dyes. The high reactivity of diazonium salts enables them to enter into a great many reactions other than the azo coupling reaction, but these fall outside the scope of the present chapter. Various other methods can be used for the preparation of azo compounds, although these are of minor importance compared with the diazo coupling reaction and they will only be touched on briefly where appropriate. 4.2 MECHANISM OF DIAZOTISATION AND COUPLING Studies of the mechanism of the diazotisation reaction date back to the end of the nineteenth century and have continued ever since, progressively incorporating the growing knowledge of theoretical chemistry [3,4]. The reaction, which involves nitrosation, has been studied under a wide range of acidities using a variety of amines and nitrosating agents. 180

chpt4(1).pmd

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MECHANISM OF DIAZOTISATION AND COUPLING

181

Numerous effects arising from variations in conditions have been described. In essence, the accepted mechanism is represented by Scheme 4.2, in which X–NO represents one of the many varieties of nitrosating species such as nitrous acid (HO–NO), nitrosyl chloride (Cl– NO), dinitrogen trioxide (O2N–NO), the nitrosyl cation (NO+) or the nitrous acidium ion (H2NO2+) that may be present in solutions containing nitrous acid. According to the conditions employed, either the formation of the nitrosating agent or its initial attack on the amine is the slowest reaction step and therefore determines the overall rate, the subsequent steps in the reaction sequence being rapid. The concentrations of the various possible nitrosating species can be influenced by other ions, such as chloride ions, or compounds, such as thiourea, present in the reaction mixture so that catalytic effects are observed. Ar

NH2 + X

NO

_

+ NH2

Ar

NO X

Ar

NH

NO +

HX

Nitrosamine

Ar

NH

NO

Ar

N

N

HX

OH

Ar

+ N

_ N X

+ H2O

Diazo hydroxide

Scheme 4.2

The diazonium ion is a powerful electrophile that can readily attack compounds having a nucleophilic centre, with the expulsion of a proton, or other entity, from the site of attack [5]. This mode of behaviour leads to the formation of azo compounds by the azo coupling reaction, the compound with the nucleophilic site being termed the coupling component. Suitable coupling components include arylamines, phenols, naphthols and keto–enol compounds. The mechanism with an arylamine coupling component can be represented by Scheme 4.3. In the coupling component illustrated both the o- and p-positions are activated by the dialkylamino group. However, an approach of the diazonium ion to the o-position would cause the electron-donating group to be twisted out of plane and thus to lose its activating influence. For this reason, coupling occurs selectively at the p-position. The situation is somewhat different in non-aqueous media where the counter-ion of the diazonium salt can influence the o-/p-ratio. In dichloromethane, diazonium halides couple to 1-naphthol almost exclusively in the 4-position whereas the corresponding haloacetates couple mainly in the 2-position [6].

Ar

+ N

Ar

N

N

+ N(CH3)2

N(CH3)2

N

H + –H

Ar

N

N

N(CH3)2

Scheme 4.3

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With a phenol, naphthol or keto–enol coupling component the mechanism is given by Scheme 4.4, in which blocking of the p-position forces coupling at the o-position. In certain cases involving the use of feebly reactive diazonium salts, loss of the proton from the transition state (4.1) in Scheme 4.4 may be slow but may often be facilitated by the addition of a tertiary base, such as pyridine, to the coupling mixture [7,8].

Ar

+ N

Ar

N

N

N +

Ar

N

N

H –

O

HO

O

Scheme 4.4

4.1

In these two coupling schemes the arylamine participates in the reaction as the free base, whereas the naphthol coupling component reacts in its ionised form. Thus, whilst raising the pH increases the rate of coupling in both cases, much higher pH values are required to bring about satisfactory coupling with hydroxy compounds than with arylamines. This situation is taken advantage of in the case of certain aminonaphthol coupling components, particularly 8-amino-1-naphthol-3,6-disulphonic acid (4.2; H acid), where one azo group can be selectively directed into the molecule by the amino group at low pH, followed by a second azo group directed by the ionised hydroxy group at higher pH. H2N

OH

low pH (3–5)

higher pH (7–10) HO3S

SO3H 4.2 H acid

4.3 DIAZO COMPONENTS AND DIAZOTISATION METHODS A vast array of arylamines can be converted into the corresponding diazonium salts; from these diazo components and the host of available coupling components an almost limitless range of azo dyes is accessible. It has already been pointed out (section 1.7.4) that various arylamines are hazardous. In July 1994, the German government introduced, unilaterally, stringent regulations banning imports of various textile and clothing products which contain specific arylamine-based azo dyes. Only those colorants which on cleavage of one or more azo groups could form any one of the twenty listed arylamines were affected. These amines (Table 4.1) are classified as proven carcinogens. Two additional arylamines, 2-methoxyaniline and 4-aminoazobenzene, were added subsequently by the EU. The German ordinance restricts the use of only about 5% of azo dyes (about 150 products). Similar regulations have been introduced in the

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Table 4.1 List of specified arylamines Name on list

Alternative name

o-Aminoazotoluene 4-Aminobiphenyl Benzidine p-Chloroaniline 4-Chloro-o-toluidine 3,3′-Dichlorobenzidine 3,3′-Dimethoxybenzidine 3,3′-Dimethylbenzidine 4-Methoxy-m-phenylenediamine 6-Methoxy-m-toluidine 4,4′-Methylene-bis(2-chloroaniline) 4,4′-Methylenedianiline 4,4′-Methylene-o-toluidine 4-Methyl-m-phenylenediamine 2-Naphthylamine 5-Nitro-o-toluidine 4,4′-Oxydianiline 4,4′-Thiodianiline o-Toluidine 2,4,5-Trimethylaniline

2-Amino-4,4′-dimethylazobenzene 4,4′-Diaminobiphenyl 4-Chloroaniline 4-Chloro-2-methylaniline o-Dianisidine o-Tolidine 2,4-Diaminoanisole 2-Methoxy-5-methylaniline 4,4′-Diamino-3,3′-dichlorodiphenylmethane 4,4′-Diaminodiphenylmethane 4,4′-Diamino-3,3′-dimethyldiphenylmethane 2,4-Diaminotoluene 2,Aminonaphthalene 2,Methyl-5-nitroaniline 4,4′-Diaminodiphenyl ether 4,4′-Diaminodiphenyl thioether 2-Methylaniline

Netherlands and in France. By mid 1997, most azo pigments, due to their very low solubility, had been exempted [9]. The legislative requirements have been discussed [10]. A voluntary registration scheme for harmless dyestuffs is available and makes use of the Internet [11]. In 1996 an official German analytical method was published for the detection of banned amines in relation to cotton, viscose, wool and silk; a second method was introduced for leather. The official procedure uses sodium dithionite in a weakly acidic citrate-buffered medium; more aggressive methods are known to produce false positive results where the amine detected is an artefact of the test procedure, resulting from chemical reactions other than azo cleavage. Reactions not involving azo groups can sometimes lead to the formation of forbidden amines [12]. Methods of detection involving thin-layer chromatography, leading to the unequivocal identification of the specified amines, have been devised [13,14]. 2-Naphthylamine has been detected in dyes derived from 1-naphthylamine [15], due to contamination of the intermediate. Benzidine can be formed from 4-nitroaniline after diazotisation by dediazoniation, followed by radical coupling to give 4,4′-dinitrobiphenyl and finally reduction [15]. A variety of diazotisation techniques has been evolved to cope with the variation in physical properties across the range of amines. 4.3.1 Direct method The rudiments of the diazotisation technique consist of treating the amine with at least 2.25 equivalents of a mineral acid (usually hydrochloric acid) and one equivalent of sodium nitrite, in accordance with Scheme 4.5. The slight excess of acid specified is required to ensure that the reaction medium remains acid throughout the diazotisation. In the direct method sodium nitrite is added to a solution of the amine in dilute mineral acid. This

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method is suitable for aniline and its simple derivatives with groups such as alkyl, alkoxy or chloro as substituents in the aromatic ring, simple naphthylamines and benzidine derivatives, many of which are known carcinogens. These compounds can be readily diazotised or tetrazotised by dissolving the amine in dilute hydrochloric acid, cooling the solution to 0–5 °C with ice and adding the requisite amount of sodium nitrite solution as rapidly as the nitrous acid formed from it is used in the reaction. Spotting the solution onto starch–iodide paper is carried out to test for an excess of nitrous acid. On occasions when it is difficult to determine the end point with starch–iodide paper, a dilute acid solution of 4,4′diaminodiphenylmethane-2,2′-sulphone (4.3), which gives a bright blue colour with nitrous acid, can be used [16]. The reaction is complete when a slight excess of nitrous acid has persisted for about 15 minutes. The excess of nitrous acid is then conveniently and rapidly destroyed by the addition of a little sulphamic acid, in accordance with Scheme 4.6, producing innocuous products. The diazonium salt solution is then ready for immediate use in dye synthesis. Ar

NH2

+

+

2HX

NaNO2

Ar

+ N

_ N

X

+

NaX

+

N2

+

H2O

+ 2H2O

Scheme 4.5

H2NSO3H

+

HNO2

H2SO4

Sulphamic acid

Scheme 4.6

H2N

S O

O

NH2

4.3

For the diazotisation reaction to succeed, it is necessary that the amine should be completely converted into the hydrochloride before the addition of sodium nitrite, because any free amine can react with the diazonium salt to form a diazoamino compound (Scheme 4.7). This complication needs to be avoided in the case of amines, such as the dichloroanilines, which do not dissolve easily in dilute hydrochloric acid [17]. With such compounds it is convenient first to dissolve them in a hot acid solution, which is then cooled to 0–5 °C. This procedure ensures the absence of free amine, and even if the hydrochloride precipitates on cooling it readily redissolves as the diazotisation proceeds. Similar precautions are required with the nitroanilines and here an increased amount of mineral acid is advantageous [18].

Ar

NH2

+

Ar

+ N

_ N

X

Ar

NH

N

N

Ar

Scheme 4.7

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DIAZO COMPONENTS AND DIAZOTISATION METHODS

185

Low temperatures are generally necessary for successful diazotisation, except in a few cases where the diazonium salt is exceptionally stable. Temperatures of 50–60 °C are used for the diazotisation of 3-aminodibenzofuran (4.4), for example [l9].

O

NH2

4.4 3-Aminodibenzofuran

4.3.2 Inverse method In this method the order of addition of the reactants is changed. The most general application is to the diazotisation of aniline- and naphthylamine-sulphonic acids, many of which are sparingly soluble in acidic media and form sparingly soluble diazonium salts. Typically, the arylaminesulphonic acid is dissolved as its sodium salt in dilute alkali and sodium nitrite is then added to the solution. The resulting solution is run slowly into a mixture of hydrochloric acid and ice, the diazonium salt being formed almost instantaneously. In a variation of this procedure [19], a small amount of acid is added to a suspension of a sulphonated amine in sodium nitrite solution; the sulphonic acid group on the amine generates nitrous acid and diazotisation proceeds to completion. 4.3.3 Use of nitrosylsulphuric acid Very feebly basic amines cannot usually be diazotised in dilute acid media and in these instances the reaction has to be carried out in a concentrated acid, normally sulphuric acid. The usual technique is first to dissolve dry sodium nitrite in the concentrated acid, when reaction occurs in two stages (Scheme 4.8), resulting in the formation of nitrosylsulphuric acid (4.5). The nitrosyl ion – nitrous acid equilibrium has been evaluated spectroscopically. In 96% sulphuric acid the 15N-n.m.r. signal is characteristic of the free nitrosyl ion [4]. Reaction (2) of Scheme 4.8 is slow at room temperature and it is desirable to heat the mixture to 70 °C in order to attain equilibrium within a reasonable time. After cooling, the amine is added gradually and after a short time the reaction mixture is poured onto ice, giving an aqueous solution of the diazonium salt [20]. This technique is used to diazotise anilines carrying two or more electron-withdrawing substituents, such as 2,4-dinitro- and 2-cyano-4-nitro-aniline, as well as aminoanthraquinones and heteroaromatic amines [4]. Some diazonium salts prepared by this technique, such as those from 6-halogeno-2,4-dinitroanilines, are unstable in water and have to be added directly to the coupling component solution.

(1)

H2SO4

+ NaNO2

NaHSO4

(2)

H2SO4

+ HNO2

+ NO

HNO2

_ HSO4

+

4.5 Nitrosylsulphuric acid

Scheme 4.8

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4.3.4 Other methods Although not used in dye preparation, three other methods of diazotisation deserve passing mention, one being chiefly of historical interest and the other two are sometimes of use in preparative work. The oldest is the method originally used by Griess (section 4.1), who generated the first recognised diazonium salt by passing ‘nitrous gases’ into a cold ethanolic solution of the amine. This reagent is produced by reducing nitric acid with arsenic(III) oxide or starch; it behaves as if it consisted of dinitrogen trioxide. The method can be useful for the preparation of solid diazonium salts, but a better procedure is that due to Knoevenagel [21]. In this second method an alkyl nitrite is added to a solution of the amine and sulphuric acid in ethanol or other non-aqueous solvent. This procedure allows greater control of the reaction than is possible in the Griess method. Finally, in a method originated by Witt [22], which is occasionally useful for the diazotisation of stable amines of high relative molecular mass that pose solubility problems, the amine is dissolved in nitric acid before the addition of sodium metabisulphite. Nitrous acid is produced in the medium according to Scheme 4.9, enabling diazotisation to proceed. Na2S2O5

+

2HNO3

Na2S2O7

+ 2HNO2

Scheme 4.9

Diazonium salts vary greatly in electrophilicity and therefore in reactivity thereby affecting the ease with which they couple. The presence in the aromatic nucleus of electronattracting groups, such as nitro and cyano groups, causes difficulties in the diazotisation step but helps the resulting diazonium salt to couple extremely readily. Electron-donating substituents such as methoxy and acetylamino groups, which facilitate diazotisation, diminish the coupling power. An extreme case is posed by the diazonium salts derived from o- and p-aminophenols and aminonaphthols, which are very feeble couplers, requiring highly alkaline conditions and often the use of a coupling assistant such as pyridine to bring about a satisfactory reaction. This very low level of reactivity is due to the hydroxy substituent interacting structurally with the diazonium group, leading to a mesomeric system in which the limiting forms are represented by Scheme 4.10 [23]. O–

O + N

–

N

Scheme 4.10

+ N N

4.4 PREPARATION AND USE OF COUPLING COMPONENTS As well as being presented with an array of primary amines that can be used for the preparation of azo dyes, the dye chemist has available a multiplicity of coupling components. The coupling components useful in the synthesis of azo dyes cover a range of chemical types which can all be seen to contain a keto–enol system, a hydroxy or an amino group. Since the use of these compounds is largely restricted to the azo dye field, their preparation is in many cases not covered in the common organic chemistry books. In the discussion of these classes, therefore, a brief mention of methods of preparation is given where appropriate.

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4.4.1 Acetoacetarylamides Coupling components of this class have the general structure 4.6 (Scheme 4.11), in which Ar represents an aromatic, usually benzenoid, nucleus that may carry substituents. Initially such compounds were prepared by the reaction of an arylamine with ethyl 3-oxobutanoate (ethyl acetoacetate) but now are more conveniently made by treatment of an arylamine with diketene (Scheme 4.11). The electron-attracting effect of the two carbonyl groups flanking the methylene group in structure 4.6 gives the protons on this latter group an acidic character permitting solubility in alkaline solution, the negative charge produced by the ionisation being spread over the molecule as shown in Scheme 4.12. This resonance provides an electron-rich site on the carbon atom of the methylene group, at which coupling occurs. Coupling of simple diazotised arylamines with acetoacetarylamides produces greenish yellow pigments and dyes of particular value in the direct and azoic dye ranges, under which headings specific structures are mentioned. Related to the acetoacetarylamides is barbituric acid (4.7), which has found similar use as a coupling component in synthetic dyes.

Ar

NH2

+

H5C2OOCCH2COCH3

Ar

NH2

+ O

Ethyl acetoacetate

C

CH2

O

C

CH2

Diketene ArNHCOCH2COCH3 4.6

Scheme 4.11

O ArNHCO

CH

C

_ CH3

ArNHCO

_ CH

O C

CH3

Scheme 4.12

O NH

H

O H

NH O 4.7

Barbituric acid

4.4.2 Pyridones The pyridone coupling components (4.8), which came into use in the 1960s chiefly for the preparation of greenish yellow disperse and reactive dyes, are made by the condensation of an alkylamine with ethyl acetoacetate and ethyl cyanoacetate. Coupling occurs at the position indicated by the arrow in Scheme 4.13.

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CH3 CN

CO H2C

+

C

NH2

CN

C

R H5C2O

Ethyl acetoacetate

CH3

H2C

+

OC2H5

O

H H

O O

N

O

R

Ethyl cyanoacetate

4.8 Scheme 4.13

4.4.3 Pyrazolones The 1-arylpyrazol-5-ones (4.9), prepared by the two-step condensation of an arylhydrazine with ethyl acetoacetate, are the most commonly used coupling components for the synthesis of greenish yellow azo dyes. Coupling occurs at the 4-position of the pyrazolone ring which, as in the case of the acetoacetarylamides discussed above, is activated towards electrophilic attack by the two flanking unsaturated carbon atoms (Scheme 4.14).

H2C

COCH3

H N

+

C

H N

Ar

OC2H5

O

CH3

H

NH2

O

Scheme 4.14

N Ar

4.9

4.4.4 Aminopyrazoles Closely related to the pyrazolones, but less commonly used, are the 5-aminopyrazoles (4.10). Again, these compounds give greenish yellow dyes when coupled at the activated 4-position indicated by the arrow in Scheme 4.15. The aminopyrazoles are prepared by condensation of an arylhydrazine with 3-aminobut-2-enenitrile (diacetonitrile), which is itself produced by dimerisation of ethanenitrile (acetonitrile) over a nickel catalyst. CH3

H 2CH3CN

Ni

NCCH

C

CH3

+ ArNHNH2

H

NH2

N HN

Scheme 4.15

N

4.10

Ar

4.4.5 Phenols When phenol is treated with one equivalent of a diazonium salt at pH 7–8 coupling occurs mainly at the 4-position of the benzene ring, but small amounts of the 2-azo and 2,4-disazo compounds are also formed [24]. With an excess of diazonium salt under neutral conditions the 2,4-disazo derivative is formed [25] and under strongly alkaline conditions the 2,4,6-

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189

trisazo derivative is produced. These factors can lead to impure products so that phenol itself is not often used as a coupling component. 4-Arylazophenols have the disadvantage of being pH-sensitive, ionisation of the hydroxy group under alkaline conditions resulting in a marked change of hue. With 2-arylazophenols, hydrogen bonding between the hydroxy group and the azo linkage (4.11) prevents such ionisation. Because of this effect the most commonly employed simple phenols are compounds such as 4-methylphenol (p-cresol) in which blocking of the 4-position ensures that coupling takes place at the 2-position. With simple diazonium salts p-substituted phenols yield yellow azo compounds. H

O

N N 4.11

Benzene-1,3-diol (resorcinol), in which the two hydroxy groups reinforce one another, is of greater technical importance than phenol, not only because it couples much more strongly but also because the presence of two activating hydroxy groups allows more than one azo group to be introduced into the molecule. The preferred first coupling under neutral conditions takes place at the 4-position [24] and when a further equivalent of diazonium salt is added in the presence of sodium acetate or carbonate a second azo group is introduced into the 2-position. In the presence of sodium hydroxide the second group enters the 6-position. At a sufficiently high pH 2,4,6-trisazo compounds are produced [26–28]. The ability to introduce two different azo groups into resorcinol is exploited in the production of brown dyes mainly for use on leather. The other two dihydric phenols, benzene-1,2-diol (catechol) and benzene-1,4-diol (hydroquinone) are of little interest in this context since they reduce diazonium salts on attempted coupling, being themselves oxidised to the corresponding quinones. 4.4.6 Aniline derivatives Aniline itself couples inefficiently with normal diazonium salts [29], the reaction product of a diazotised amine and aniline often being the diazoamino compound. However, the presence of electron-donating substituents, such as methyl or methoxy groups, in the benzene nucleus makes coupling much easier and allows the introduction of an azo group into the 4-position of compounds such as 3-methylaniline (m-toluidine), 2,5-dimethylaniline (p-xylidine), 3-amino-4-methoxytoluene (cresidine) and 2,5-dimethoxyaniline. In the event of the 4-position being blocked, as in 2,5-dimethoxy-4-methylaniline, the azo group can be introduced into the unoccupied o-position [28]. Like the 4-arylazophenols, these 4arylazoanilines display pH sensitivity, becoming protonated under acid conditions; they are therefore mainly of use as intermediates in the synthesis of disazo dyes. N-Alkylation of primary aromatic amines increases their nucleophilic character, making them couple much more readily, the introduction of the azo group occurring in the 4position. Thus, in contrast to aniline, N-methylaniline couples readily and N,N-dimethylaniline very readily with simple diazonium salts. Diphenylamine also couples in the 4position, but less readily than N-methylaniline.

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The increase in coupling power gained on N-alkylation can be exploited to bring about the coupling of primary arylamines by warming the base with an aqueous solution of one equivalent of sodium bisulphite and one equivalent of formaldehyde. The complex thus formed reacts with the arylamine, giving a clear solution of the methyl-ω-sulphonate derivative (4.12) which couples readily. The methyl-ω-sulphonate group is labile and can be conveniently removed by acid hydrolysis, affording a facile preparation of aminoazobenzenes not accessible by direct coupling (Scheme 4.16). ArNH2 + HOCH2SO3Na

ArNHCH2SO3Na + H2O 4.12

Scheme 4.16

The N,N-dialkylanilines are of paramount importance in the synthesis of disperse dyes, the whole gamut of shades from yellow to greenish blue being obtainable from them by suitable choice of diazo component. If the 4-position in an N,N-dialkylaniline is blocked, as for example in N,N-dimethyl-4-methylaniline, steric hindrance prevents the diazonium ion approaching the o-position and coupling does not take place [30]. As with resorcinol, the 1,3-diamines of the benzene series are useful in the preparation of disazo dyes that can form the basis of brown shades. Generally the 2,4-disazo compound is the product of coupling with two equivalents of diazonium salt, but at higher pH values formation of the 4,6-isomer can be significant [31]. Again, the 1,2- and 1,4-diamines reduce diazonium salts on attempted coupling. 1,3,5-Triaminobenzene has been found to couple three times [32]. 4.4.7 Aminophenols 3-Aminophenol is a useful dyestuff intermediate, capable of coupling in either the 2- or the 4-position. It is often used in preference to resorcinol or benzene-1,3-diamine (mphenylenediamine) as a final coupling component in the preparation of complex dyes containing more than one azo group, typically trisazo direct dyes. Other aminophenols are of little interest in this context. 4.4.8 Naphthols In this discussion of coupling components it is convenient to separate naphthalene compounds from their benzenoid analogues since important differences in behaviour exist between the two classes. Naphthalene, although formally containing two fused benzene rings, only possesses ten π-electrons, has much less than twice the resonance stabilisation energy of benzene and therefore has a lower level of aromaticity. The reason for this situation is related to the fact that in only one of the three limiting canonical forms (4.13a– c) are both rings ‘benzenoid’, the other two forms each containing one ‘benzenoid’ and one ‘quinonoid’ ring. A consequence is that the contribution of 4.13b to the overall structure of naphthalene is greater than that of the other two canonical forms; so that the bonds between adjacent α- and β-positions show a degree of double-bond character. This type of bond stabilisation has an effect on the coupling behaviour of naphthols and naphthylamines, as will be shown later.

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α β

B

Q

B

Q

B

4.13c

4.13b

4.13a

B

The two isomeric naphthols and some of their sulphonic acids are very widely used as coupling components in dye manufacture, generally giving orange hues when coupled with simple diazonium salts. 1-Naphthol (4.14) couples in both the 2- and 4-positions and, as with phenol, it is difficult to make the coupling selective. Highly reactive diazonium salts couple chiefly in the 4-position whilst weakly reactive ones, especially those derived from aminophenols and aminonaphthols, couple mainly in the 2-position. The position of coupling in 1-naphthols is, of course, also influenced by substituents; for example, a sulphonic acid group in the 3-, 4-, or 5-position inhibits 4-coupling and directs the diazonium ion to attack at the 2-position. OH

4.14

With 2-naphthol (4.15) the special characteristics of the structure of naphthalene mentioned above come into play and coupling is directed exclusively into the 1-position. When this position is blocked, as with 1-methyl-2-naphthol, no coupling occurs. The reason for this behaviour lies in the nature of the relevant resonance canonicals (4.15a–c) of the naphtholate ion (Scheme 4.17). In structure 4.15a the unsubstituted ring retains its benzenoid character and is therefore vastly preferred over structure 4.15c, in which the unsubstituted ring is forced to assume a quinonoid configuration. This difference localises the lone pair of electrons and the negative charge on the 1-position and leads to selective attack of the diazonium ion at this site.

OH

4.15 –

O

O

–

O –

4.15b

4.15a

4.15c

Scheme 4.17

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4.4.9 Naphthylamines The aminonaphthalenes couple much more easily than do the amines of the benzene series. This increased reactivity allows azo compounds to be prepared without the need to reinforce the effect of the amino group with additional electron-donating substituents. The resulting dyes have hues similar to those obtained from naphthol coupling components and the parent amines display directive influences similar to those shown by the naphthols in the coupling reaction. Thus with one equivalent of a diazonium salt 1-naphthylamine couples largely in the 4-position [33]. With two equivalents of diazonium salt the 2,4-disazo compound can be obtained [34,35]. 2-Naphthylamine, like 2-naphthol, couples exclusively in the 1-position [31] and derivatives with this position blocked by non-labile substituents fail to couple. When treated with powerful diazonium salts, 2-naphthylamines with labile substituents in the 1position, such as 2-naphthylamine-1-sulphonic acid, couple by introducing the azo group into the 1-position with concomitant expulsion of the labile substituent. In the distant past, 2-naphthylamine was an important intermediate for the synthesis of dyestuffs, but its recognition as a potent carcinogen resulted in its total withdrawal by reputable dye manufacturers. Stringent precautions also need to be taken when using 1naphthylamine to ensure that the material is not contaminated with the 2-isomer. The naphthylaminesulphonic acids, however, remain amongst the most important dyestuff intermediates, finding wide-ranging applications as both diazo and coupling components. 4-Substituted 3-aminoacetanilides have been examined as replacements for 1naphthylamine in monoazo dyes [36]. 3-Amino-4-methoxyacetanilide can be used as an effective substitute for 1-naphthylamine in the synthesis of disazo dyes [37]. 4.4.10 Aminonaphthols There are 14 isomers carrying both an amino group and a hydroxy group attached to a naphthalene nucleus, but only those in which the two groups are located in different rings are of use as coupling components. In these cases, each group exerts the major directive influence within its own ring, so that at pH 7 or below coupling occurs in the aminosubstituted ring, whereas at alkaline pH coupling in the hydroxylated ring becomes important. Often there is still a choice of possible coupling sites, leading to mixtures of products. Very few of the aminonaphthol isomers have found use in dye manufacture, their value being determined by ease of manufacture, toxicological considerations and the properties of the derived dyes. Of far greater importance are the sulphonated aminonaphthols, in which the versatility conferred by the presence of both an amino and a hydroxy group makes them among the most important group of azo intermediates. Three are outstanding, namely, 6-amino-1-naphthol-3sulphonic acid (4.16; J acid), 7-amino-1-naphthol-3-sulphonic acid (4.17; γ acid) and H acid (4.2), which couple under the relevant conditions of pH at the arrowed positions. OH alk

acid 4.16 J acid

H2N

SO3H

OH alk H2N

4.17 γ acid

acid

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193

Particularly important dyes are produced when the N-acylated derivatives are coupled, bright orange hues resulting from the use of N-acyl J acid and bright reds from N-acyl γ acid and N-acyl H acid derivatives. Coupling of γ acid under acid conditions leads to red monoazo dyes that cannot be further coupled and have exceptional fastness to light. These characteristics result from hydrogen bonding between both the amino and the hydroxy groups and the azo linkage (4.18), thus preventing ionisation of the hydroxy group which is essential for coupling to occur and also protecting the azo linkage by involving both lone pairs of electrons in coordination. Ar N H

H N

O

N H SO3H 4.18

Other important dye classes are formed by coupling aminonaphtholsulphonic acids with two equivalents of diazonium salt, giving a disazo dyestuff. In the case of H acid it is essential that the acid coupling stage is carried out first, the products being navy blue in colour. With J acid, which again gives dark blue disazo dyes, the initial coupling should preferably be carried out under alkaline conditions. 4.5 STRUCTURE OF AZO DYES 4.5.1 Stereoisomerism The simplest azo compound, azobenzene, exists as a mixture (Scheme 4.18) of a stable trans (4.19) and an unstable cis (4.20) form [38,39]. Formation of the cis isomer is induced by exposure to light, the quantum yield of the process depending upon the wavelength of the light employed [40]. The proportion of cis isomer can be appreciable in an equilibrium mixture. Thus a concentration of 24% of this unstable form builds up within a few hours when an acetic acid solution of azobenzene is exposed to sunlight in shallow white trays. Reversion to the trans form occurs readily on heating and is catalysed by a variety of substances that can function as electron donors or acceptors [41]. The two isomers display different properties and can be separated by exploiting differences in their distribution between immiscible solvents or by chromatography on alumina. The molecule of the trans form is very nearly planar, whereas in the cis isomer the

N

N

N

N

4.19

4.20

trans

cis

Scheme 4.18

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benzene rings are inclined at an angle of about 56 degrees to one another [42,43]. The trans isomer forms monoclinic crystals, has no dipole moment and melts at 68 °C whereas the cis isomer forms orthorhombic bipyramidal crystals, has a dipole moment of 3.0 debye and melts at 71.4 °C. The two isomers also differ in hue. Some substituted azobenzenes display similar stereoisomerism but these are mainly simple structures, increasing molecular complexity causing steric factors to control the configuration exclusively in the trans form. Some simple azobenzene derivatives serve as disperse dyes, but in these the difference in hue between the cis and trans forms gives rise to the phenomenon of photochromism, manifested as a change in hue on exposure to light; this property is an undesirable feature of such dyes. The thermal cis–trans isomerisation of 4-diethylamino-3′-nitroazobenzene has been studied in various solvents. The conversion appears to involve an inversion mechanism passing through an sphybridised linear transition state [44]. 4.5.2 Tautomerism Whilst azo compounds prepared from diazonium salts and phenolic or keto–enol coupling components are often depicted in the hydroxyazo form (4.11), an alternative tautomeric structure can be drawn for such compounds (Scheme 4.19). This ketohydrazone tautomer (4.21) can, in cases where the azo and hydroxy groups are located on adjacent carbon atoms, exhibit hydrogen bonding between the two groups as shown. Similar pairs of structures, but without hydrogen bonding, can be drawn for p-hydroxyazo compounds. H N

O

H N

N 4.11

O

N 4.21

Scheme 4.19

The actual structure of any particular azo compound will depend on the relative energy levels of the two tautomers which can be drawn for that compound. Where one form has a much lower energy level than the other, then the lower-energy form will predominate; a tautomeric mixture will result when the energy levels of the two forms are similar. In the case of azo compounds derived from keto–enol coupling components (acetoacetarylamides, pyridones, pyrazolones and aminopyrazoles) the ketohydrazone form has a much lower energy level than the hydroxyazo alternative and represents the true structure of these compounds. Physical measurements support this conclusion. For example, spectroscopic evidence shows that azo compounds derived from acetoacetarylamides have structure 4.22 [45]. This finding is confirmed by n.m.r. evidence, which indicates that pyrazolone-based azo compounds have the structure 4.23 [46,47]. Substituted phenylazopyridones favour the ketohydrazone form in solution [48]. The most appropriate representation of tautomeric colorants has been discussed [49]. The converse of this situation is presented by diazotised arylamines coupled to phenols, where assumption of a ketohydrazone structure (4.21) would necessitate loss of the aromatic character of the phenolic ring. Consequently, these compounds exist solely in the hydroxyazo form.

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H

H Ar

O

N

Ar

N N

O

N

N N

N

H O H3C

195

R

4.23

4.22

Molar extinction coefficient/l (mol cm)–1 × 10–4

The phenylazonaphthols present a particularly interesting series. Despite the partial loss of aromaticity in the hydrazone structures that can be drawn for 1-phenylazo-2-naphthol, 2phenylazo-1-naphthol and 4-phenylazo-1-naphthol, these structures are not much higher in energy than the azo forms [50], so that there should be no significant preference for one form over the other. When the visible absorption curves of 4-phenylazo-1-naphthol are measured in a variety of solvents, they all pass through a common point (isosbestic point) as in Figure 4.1 [51,52]. 1-Phenylazo-2-naphthol shows similar behaviour. This finding is clear evidence that in solutions of these compounds both tautomers are present. The solvent effect on the azo–hydrazone equilibrium of 4-phenylazo-1-naphthol has been modelled using ab initio quantum-chemical calculations. The hydrazone form is more stable in water and in methylene chloride, whereas methanol and iso-octane stabilise the azo form, The calculated results were in good agreement with the experimental data in these solvents. Similar studies of 1-phenylazo-2-naphthol and 2-phenylazo-1-naphthol provided confirmation. Substituent effects in the phenyl ring were rationalised in terms of the HOMO–LUMO orbital diagrams of both tautomeric forms [53]. Tautomerism both in the solid state and in solution is confirmed by infrared spectroscopic measurements on all three compounds [54,55]. The highest content of ketohydrazone form within the three isomeric phenylazonaphthols occurs, in solution, with 2-phenylazo-1naphthol [56]. A 15N-n.m.r. study of some azo dyes derived from H acid and related intermediates has confirmed the dominance of the ketohydrazone tautomer [57]. Similar findings have been obtained using high-field 1H- and 13C-n.m.r. spectroscopy [58].

Pyridine Methanol Acetic acid

2

1

350

400

450

500

550

Wavelength/nm

Figure 4.1 Effect of solvent on 4-phenylazo-1-naphthol spectrum

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A totally different picture is presented by 3-phenylazo-2-naphthol. This unusual isomer cannot be prepared by a normal coupling procedure but has been obtained by reaction of 3amino-2-naphthol with thionyl chloride to give the N-sulphinylamine (4.24), condensation of which with N-phenylhydroxylamine yields the desired product [59]. Here, assumption of a ketohydrazone form would entail loss of aromatic character in both rings of the naphthalene nucleus and the energetic unfavourability of this situation ensures that the compound exists solely in the hydroxyazo form. In the case of phenylazonaphthylamines, the aminoazo tautomer is very much favoured relative to the iminohydrazone form [57,60]. OH

N

S

O

4.24

4.6 PREPARATION AND IMPORTANCE OF NAPHTHALENE INTERMEDIATES 4.6.1 Importance of naphthalene When purely benzenoid intermediates are employed in the preparation of azo dyes, the products exist solely in the hydroxyazo form since the energy level of this tautomer is significantly lower than that of the ketohydrazone form. The position changes when a naphthalene intermediate is used, for here the two tautomers are of comparable energy and so coexist. The ketohydrazone form generally possesses a higher extinction coefficient than does the hydroxyazo alternative. The practical consequence is that the former has a greater tinctorial strength – in crude terms, more colour value per unit mass. The participation of the ketohydrazone form in naphthalene-based dyes allows them to gain this strength advantage which, together with the ready availability of naphthalene and its convenient processing to yield ideally substituted derivatives, has made this hydrocarbon of great importance as a primary starting material in the manufacture of intermediates for the dyestuffs industry. Naphthalene intermediates [61] are always built up by substitution reactions starting from the cheap and plentiful hydrocarbon using, in the main, only seven basic reactions. Most of these reactions are generally familiar from benzene chemistry but with some modification, since naphthalene has two different possible positions of substitution. These positions are often designated α and β, the four α-positions being ortho and the four β-positions meta to the nearest carbon atom of the central bond. A further modifying influence is the lower level of aromaticity of naphthalene compared with benzene, leading to increased reactivity. The main reactions used in the manufacture of dyestuff intermediates from naphthalene are described in sections 4.6.2 to 4.6.8. 4.6.2 Sulphonation Treatment of naphthalene with concentrated sulphuric acid [62] results in substitution at one of the α-positions (4.25) if the temperature is below about 60 °C, but the reaction is

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reversible (Scheme 4.20) and at higher temperatures rearrangement occurs, involving desulphonation and resulphonation, yielding the thermodynamically stable β-isomer (4.26). Under more forcing conditions, such as the use of higher operating temperatures or of oleum (sulphur trioxide dissolved in anhydrous sulphuric acid), more than one sulphonic acid group can be introduced. This approach can lead to mixtures of isomers and, in practice, the reaction conditions must be carefully controlled if the desired isomer is to be produced in maximum yield. SO3H < 60 °C

SO3H

> 60 °C

4.25

4.26

Scheme 4.20

4.6.3 Nitration Nitration [63], unlike sulphonation, is not reversible and results very largely in αsubstitution, yielding 1-nitronaphthalene (4.27). NO2

4.27

4.6.4 Reduction Reduction of a nitro compound to the corresponding naphthylamine proceeds exactly as in the benzene series. A batch process using iron and hydrochloric acid is traditional but has been somewhat superseded by catalytic hydrogenation. 4.6.5 Replacement of a sulphonic acid group by a hydroxy group This displacement is accomplished by heating the sulphonated derivative at a high temperature with sodium or potassium hydroxide [64]. Typical is the preparation of 2naphthol (4.15) from naphthalene-2-sulphonic acid (Scheme 4.21). Displacement of the sulphonic acid group occurs more readily when it is located at the α- rather then at the βposition, the former requiring a fusion temperature of about 200 °C and the latter one of about 250 °C. This difference in reactivity can be exploited to prepare naphtholsulphonic acids by fusion of suitable naphthalenedisulphonic acids. SO3H

OH NaOH/250 °C

Scheme 4.21

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4.15

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CHEMISTRY OF AZO COLORANTS

4.6.6 Bucherer reaction This reaction [65–67] is only rarely encountered in the benzene series but is extremely useful for appropriate derivatives of naphthalene, where the mechanism of the reaction has been investigated extensively. The reaction allows a hydroxy group to be exchanged for an amino group or vice versa. When a hydroxy group is to be converted into an amino group, the naphthol is heated under pressure with ammonium bisulphite (often produced in situ by introduction of ammonia liquor and sulphur dioxide into a sealed autoclave) at a temperature of 100–150 °C; the naphthol is thereby converted into the corresponding naphthylamine. The mechanism of the reaction is outlined in Scheme 4.22. O

O HO

NH3

NH2

NH4HSO3 SO3NH4 SO3Na

SO3Na 4.28 Keto tautomer

SO3NH4 SO3Na

4.29

4.30 – H2O

NH2

NH

NH H+

SO3Na 4.33

Scheme 4.22

SO3NH4 SO3Na

SO3Na 4.32 Imino tautomer

4.31

In the initial step the naphthol reacts in its keto tautomeric form (4.28), adding ammonium bisulphite across the isolated double bond in the sulphonated ring. Nucleophilic attack by ammonia on the tetralonesulphonic acid (4.29) is followed by a proton shift from nitrogen to oxygen, producing an intermediate (4.30) which then loses water to give the imine (4.31). After cooling and discharging the autoclave, the reaction mixture is acidified and boiled. The added bisulphite is thereby eliminated from the molecule and rearrangement of the imine tautomer (4.32) so produced completes the conversion into the naphthylamine (4.33). The reverse of this reaction, which allows a naphthylamine to be converted into the corresponding naphthol, is similarly carried out by heating the amine with sodium bisulphite and finally acidifying the mixture; the mechanism is outlined in Scheme 4.23. The Bucherer reaction can also be used in the preparation of substituted naphthylamines. Carrying out the reaction on a naphthol but employing an alkylamine in place of ammonia results in the production of the corresponding N-alkylnaphthylamine. Alternatively, a

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naphthylamine can be used as starting material, whereupon the excess of alkylamine employed results in the amino group being displaced by the alkylamino group. Of greater importance is the preparation of N-ary1naphthylamines by heating a naphthol (or a naphthylamine) with sodium bisulphite and an arylamine, whereupon the hydroxy (or amino) group in the starting material is exchanged for the arylamino group. In the above reaction mechanisms it is noteworthy that the sulphonic acid group introduced has been shown to enter at the 3-position and not the 4-position as previously postulated. A consequence of this situation is that an attempted Bucherer reaction on a naphthol (or a naphthylamine) carrying a sulphonic acid group located meta to the hydroxy (or amino) group would require a second sulphonic acid group to be introduced at this position. Since it is impossible to locate two sulphonic acid groups on the same carbon atom, these compounds cannot undergo the transformation. NH2

NH

O H2O

NaHSO3 SO3Na SO3Na

SO3Na

SO3Na SO3Na

4.33

H+

OH

Scheme 4.23

SO3Na

4.6.7 Arylation In some cases where it is difficult to carry out the Bucherer reaction successfully, it is easier to prepare N-arylnaphthylamines by heating together a naphthylamine and an arylamine. In particular, this reaction is useful in the preparation (Scheme 4.24) of 1phenylaminonaphthalene-8-sulphonic acid (4.34; N-Phenyl Peri acid) and its N-4methylphenyl analogue (Tolyl Peri acid), both of which intermediates are valuable components for the production of navy blue dyes.

H HO3S

NH2

NH2

HO3S

N

200°C 4.34

Scheme 4.24

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CHEMISTRY OF AZO COLORANTS

4.6.8 Kolbe-Schmitt reaction This carboxylation reaction is of importance mainly for the manufacture from 2-naphthol of β-oxynaphthoic acid (BON) acid, the parent acid of sodium 3-hydroxy-2-naphthoate (4.35). The carboxylation is achieved by heating the sodium salt of 2-naphthol under pressure in an atmosphere of carbon dioxide (Scheme 4.25). Under these conditions an equilibrium mixture of the naphthol and its carboxylated derivative is established, the mixture containing about 30% of the carboxylic acid. The desired product is extracted from the reaction mixture and the unchanged 2-naphthol is recycled. ONa

OH

CO2

COONa 4.35

Scheme 4.25

4.6.9 Preparation of specific intermediates The above reactions can be used to prepare a variety of multi-purpose intermediates that are of great value in dyestuffs manufacture. Many of these intermediates were prepared and found to be of value in dye manufacture before the orientation of their substituents was established. This early lack of basic knowledge resulted in these intermediates being given trivial names that have persisted in the dyestuffs industry. In the preparation of naphthalene intermediates the reactions must be employed in the correct sequence to achieve a desired orientation in the final product. A further crucial consideration can be the need to avoid steps that would result in the formation of carcinogenic materials. These points are illustrated in the following examples. Naphtholsulphonic acids and γ acid The preparation of 2-naphthol by high-temperature sulphonation of naphthalene followed by alkali fusion of the resulting naphthalene-2-sulphonic acid has been mentioned previously. Further sulphonation of 2-naphthol yields several useful naphtholsulphonic acids and conditions can be chosen to make one or other of these compounds the main product. The initial product is the unstable 2-naphthol-1-sulphonic acid, which readily rearranges to 2-naphthol-6-sulphonic acid (4.36; Schaeffer’s acid). Further sulphonation leads to 2naphthol-6,8-disulphonic acid (4.37; G acid) at low temperature and 2-naphthol-3,6disulphonic acid (4.38; R acid) at higher temperature.

SO3H

OH

HO3S

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HO3S

4.36

4.37

Schaeffer’s acid

G acid

200

OH

HO3S

SO3H 4.38 R acid

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Fusion of 2-naphthol-6,8-disulphonic acid with sodium hydroxide yields naphthalene-1,7diol-3-sulphonic acid (4.39), the more readily displaced α-sulphonic acid group being replaced. When this compound is subjected to a Bucherer reaction only the 7-hydroxy group is exchanged for an amino group, yielding γ acid (4.40), as in Scheme 4.26. This selectivity arises because the 1-hydroxy group is unable to form the intermediate addition product necessary for the Bucherer reaction to proceed, the sulphonic acid group being located in the meta position. OH

OH

NH2

OH

HO3S

HO3S 4.39

4.40 γ acid

Scheme 4.26

H acid H acid (4.2) is possibly the most important single naphthalene-based intermediate. The preparation of this intermediate starts with a high-temperature sulphonation of naphthalene using 65% oleum (anhydrous sulphuric acid in which 65% by mass of sulphur trioxide has been dissolved) to give mainly naphthalene-1,3,6-trisulphonic acid, the nitration product from which is purified by selective isolation. Reduction of the nitro group followed by hydrolysis of the 1-sulphonic acid substituent by heating with sodium hydroxide solution at 180 °C completes the process (Scheme 4.27). Use of a higher temperature (280 °C) at the final stage results in more extensive hydrolysis, with the amino group also being replaced by a hydroxy group, leading to the formation of naphthalene-1,8-diol-3,6-disulphonic acid (chromotropic acid).

HO3S

HO3S

NO2

HNO3

H2SO4/SO3 HO3S

SO3H

HO3S

SO3H H2/Pd

OH

NH2

HO3S

NH2

NaOH HO3S

SO3H

HO3S

4.2 H acid

Koch’s acid

Scheme 4.27

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J acid Of particular interest is the chain of reactions leading to J acid (4.43), since several of the compounds that feature at intermediate stages in the chain are themselves useful as dyestuff intermediates. The starting point for this chain is 2-naphthol which, in early syntheses, was converted by means of a Bucherer reaction into 2-naphthylamine, this then being sulphonated to yield 2-naphthylamine-5,7-disulphonic acid (4.42; Amido J acid). The recognition that 2-naphthylamine is a potent carcinogen caused this route to be abandoned. In the method of preparation now in use (Scheme 4.28), a sulphonic acid group is introduced into the 1-position of the naphthalene nucleus and is carried through the early stages, so that 2-naphthylamine-1-sulphonic acid (4.41; Tobias acid) rather than the unsulphonated amine figures in the preparation. SO3H

SO3H OH

OH

NH2

4.41 Tobias acid SO3H HO3S

NH2

HO3S

NH2

SO3H

OH

4.43

4.42

J acid

Amido J acid

HO3S

NH2

SO3H

Scheme 4.28

After sulphonation to 2-naphthylamine-1,5,7-trisulphonic acid, the labile 1-sulphonic acid substituent, which has now served its purpose, is eliminated by diluting the sulphonation mixture and heating. Fusion of the resulting disulphonic acid (4.42) with sodium hydroxide replaces the more labile 5-sulphonic acid group by a hydroxy group, forming J acid. 2-Naphthylamine-5,7-disulphonic acid and 2-naphthylamine-1-sulphonic acid, which are intermediate products in Scheme 4.28, as well as 2-naphthylamine-1,5-disulphonic acid (obtained by careful low-temperature sulphonation of Tobias acid), are all used in the synthesis of azo dyes. 1-Naphthylaminesulphonic acids Nitration of naphthalene-1-sulphonic acid produces two isomeric nitronaphthalenes that have very similar solubilities. It is convenient to reduce the mixture without separation, giving a mixture of 1-naphthylamine-8-sulphonic acid (4.44; Peri acid) and 1naphthylamine-5-sulphonic acid (4.45; Laurent’s acid), as in Scheme 4.29. These two

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203

compounds can be readily separated since the former has a much lower solubility. The isomers also differ markedly in their value as intermediates, Peri acid being a very useful dye component whilst Laurent’s acid finds little use. In all the above sequences, single isomers are produced by careful control of the reaction conditions combined with purification by selective isolation at various points in the synthesis. Occasionally two isomers are produced which give dyestuffs that have very similar properties; in these cases it is often quite acceptable and economically beneficial not to separate the individual components but to use the total mixture in dye preparation. An example is the mixture of 1-naphthylamine-6- and 7-sulphonic acids (4.46; mixed Cleve’s acids), which arises by nitration and reduction of naphthalene-2-sulphonic acid (Scheme 4.30). NO2

NH2

SO3H

SO3H

4.44 Peri acid

SO3H +

HNO3

+

H2

SO3H

SO3H

NH2

NO2

4.45 Laurent’s acid

Scheme 4.29

NH2

NO2

SO3H

SO3H

+ SO3H

+ H2

HNO3

SO3H

SO3H

NO2

NH2

4.46 Cleve’s acids

Scheme 4.30

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4.7 SCHEMATIC REPRESENTATION OF COUPLING The simple process of diazotising an amine and joining the resulting diazonium salt with a coupling component produces a monoazo dye, that is, a dye containing a single azo group. Such dyes are often represented as A→E dyes, A standing for the amine, E the end coupling component and the arrow symbol meaning ‘diazotised and coupled with’. The certainty with which diazotisation and coupling proceed, together with the wide variety of components available, allows more complex dye structures to be built up by combining two or more diazotisation and coupling sequences. A convenient shorthand way of classifying these more complex products has gained common acceptance. This shorthand, known as Winther symbols, uses arrows combined with the following capital letters to indicate the nature of the components: – A: an amine that is diazotised – E: an end coupling component – D: a diamine that is tetrazotised (both amino groups are diazotised) – M: an amine that is first coupled with a diazotised amine and then is itself diazotised – Z: a coupling component that can couple twice. Using this nomenclature, disazo dyes (dyes containing two azo groups) fall into one or other of the three classes represented thus:

E1←D→E2

A1→Z←A2

A→M→E

4.8 SULPHONATED AZO DYES In the majority of dyeing processes the dye is transferred from aqueous solution onto the fibre, so that adequate aqueous solubility is a desirable property in a dye. The sulphonic acid group provides the cheapest way of solubilising a dyestuff and does not generally cause any diminution in the light fastness of the chromogen to which it is attached. Consequently, dyes containing sulphonic acid groups find use in the dyeing of cellulosic fibres (section 3.2.1) and amide fibres (section 3.2.2). In the ranges of dyes available for the dyeing of these fibres, sulphonated monoazo structures provide many of the dyes in the yellow to red sector as well as a few of the more bathochromic hues. As indicated previously, the coupling component is of prime importance in determining the hue, with the diazo component merely modifying it, although when the diazo component contains powerful electronwithdrawing or -donating substituents this modification can be profound. Pyridone and pyrazolone coupling components are generally used in yellow dyes and naphthol, naphthylamine, aminonaphthol or acylaminonaphthol coupling components to cover the orange to red region. Disazo dyes span the spectrum from yellow to greenish blue and find use mainly on cellulosic fibres but also on wool and silk. Trisazo and polyazo dyes are restricted to duller hues on cellulosic fibres. The actual structures chosen vary according to the particular fibre as well as the end use to which the dyed material will be put. It is appropriate to divide the discussion of the structures of these dyes according to the fibres on which they are used.

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4.8.1 Acid dyes for wool Simple monoazo dyes containing one or two sulphonic acid groups are used to dye wool, the dye being retained on the fibre by electrostatic bonding. Pyrazolones, naphthols, naphthylamines and acylaminonaphthols are commonly used coupling components. These ‘levelling acid’ dyes are typified by tartrazine (4.47; CI Acid Yellow 23) and CI Acid Orange 20 (4.48). These simple dyes are markedly hydrophilic in character and this necessitates them being applied to the substrate from a strongly acidic dyebath. Under these conditions the dyes readily penetrate the wool fibres and distribute themselves evenly, but their highly hydrophilic character makes them easily desorbed and thus limits their fastness to washing. A higher level of wash fastness can be achieved by using disazo dyes of larger molecular size, usually carrying two sulphonic acid groups. These ‘milling acid’ dyes are typified by CI Acid Yellow 42 (4.49), CI Acid Red 151 (4.50) and CI Acid Blue 116 (4.51). 4-Amino-2′nitrodiphenylamine has been used as the A component in disazo dyes based on structure 4.51 [68]. SO3H H HO3S

HO3S

O

N N

N

OH

N N

N

4.48

4.47

CI Acid Orange 20

HOOC

CI Acid Yellow 23

CH3 SO3H N

N

N

H N

O

O

N

H

N N

N

4.49

HO3S

CI Acid Yellow 42

H3C

HO3S

N

H N

O

N N

4.50 CI Acid Red 151

SO3H H N

N

N N

N

4.51 SO3H

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Much superior wet fastness can be achieved by incorporating a highly hydrophobic weighting group into the dye molecule [69]. Non-polar bonding between this group and hydrophobic side chains in wool imparts neutral-dyeing affinity to these ‘super-milling’ dyes (section 3.2.2). A typical example is CI Acid Red 138 (4.52). Confirmation of the ketohydrazone structure has been obtained by spectroscopic and PPP-MO techniques [70]. CH3(CH2)11 H N

H O

N

COCH3

N

HO3S

SO3H 4.52 CI Acid Red 138

Of particular interest are the light-fast red dyes obtained by coupling onto γ acid under acid conditions. In this latter type, an example of which is CI Acid Red 32 (4.53), both the amino group and the hydroxy group are hydrogen bonded to the azo linkage [71]. This double hydrogen bonding prevents the tautomerism that is normally a feature of naphthalene-based azo dyes by locking the dye in the hydroxyazo form, leading both to the red hue and to the high level of light fastness. Strongly electron-withdrawing or -donating substituents in the diazo component can have a profound effect on the hue, producing a large bathochromic shift (section 1.4). The use of diazo components of the naphthalene series produces a similar but less marked effect. These effects are exploited in CI Acid Blue 92 (4.54), prepared by coupling diazotised O-tolylsulphonyl H acid with N-Phenyl Peri acid and hydrolysis of the product. Here the proximity of the peri-sulphonic acid group to the secondary amino group prevents any marked pH sensitivity, removing the need to stabilise the hue by acylation. Again the hydroxy group on the diazo component is hydrogen bonded onto the azo linkage, resulting in excellent light fastness.

NHCOCH3

HO3S O

CH2CH3

H

SO2N H O

N

N N

H

N

N

HO3S

H HO3S

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N H SO3H

4.53

4.54

CI Acid Red 32

CI Acid Blue 92

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4.8.2 Acid dyes for nylon The synthetic polyamide fibre nylon can be dyed with either acid dyes or disperse dyes, the former giving better fastness and the latter better levelling properties. The types of acid dye used are broadly similar to those for wool, that is, monosulphonated levelling acid dyes with relative molecular mass (Mr) in the 300–500 region and disulphonated milling acid dyes with Mr of 600–900. Dyes of small molecular size are often deficient in wet fastness and those of Mr higher than about 800 tend to give unlevel dyeings. Within these crude guidelines certain structural features exert specific effects; for instance, hydrogen bonding substituents such as hydroxy and acylamino groups boost fastness to washing but may impair level dyeing behaviour. Monoazo dyes containing a single sulphonic acid group conveniently fall into the levelling acid group. A typical structure is CI Acid Red 266 (4.55). Monosulphonated levelling acid dyes are mainly yellow or red in colour, whilst the larger molecular size that can be tolerated with a disulphonated milling acid dye allows the use of disazo compounds, such as CI Acid Blue 118 (4.56), to cover more bathochromic hues such as navy blue. Cl

SO3H

CF3 H O

N N

H N

H

N N

N

N

N H HO3S

4.56 SO3H

4.55

CH3

CI Acid Blue 118

CI Acid Red 266

4.8.3 Direct dyes Certain water-soluble dyes are directly adsorbed onto cotton that has not been pretreated with a mordant (section 3.2.1). The first dye in which this phenomenon was observed was Congo red (4.57; CI Direct Red 28), discovered in 1884 by Böttiger. The extreme pH sensitivity of this dye now restricts its use to that of an indicator, but it deserves mention as the forerunner of the direct dye class. H2N HO3S

N N

N SO3H

N NH2

4.57 CI Direct Red 28

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CHEMISTRY OF AZO COLORANTS

If the solubility of dyes of this type is kept to an acceptable minimum, adequate wash fastness can be achieved. This low solubility requirement is met by the use of the minimum number of solubilising groups consistent with the need to prepare an aqueous dyebath. Hydrogen bond formation with the fibre depends upon having numerous groups containing electron-rich atoms, such as oxygen and nitrogen, positioned at intervals along an extended planar molecule. Suitable features conferring substantivity include amino and acylamino groupings, hetero atoms in aromatic rings and the azo group itself. The simplest monoazo dyes fail to meet these requirements, but by choosing intermediates known to confer substantivity and by building up the molecule to provide the necessary length and coplanarity (section 3.2.1), direct dyes can be produced from this class. Thus the highly substantive character of the benzothiazole nucleus is exploited in CI Direct Yellow 8 (4.58), as is the alignment of the azo, ureido and acylamino groups in the substituted J acid coupling component of CI Direct Red 65 (4.59). H3C SO3H

O

H3C

N

S

H

N N

N H

O

4.58

CH3

CI Direct Yellow 8

H N

O

N

NHCOCH3

O HO3S

N

N

H

H

4.59 CI Direct Red 65

The required long planar shape is more readily supplied by simple disazo structures such as CI Direct Blue 1 (4.60). Copper complexes of such disazo compounds are important and are dealt with elsewhere (section 5.5.3). A widely used method of producing A→Z←A type disazo dyes relies on treating J acid with phosgene (COCl2) to give the bis-coupling component carbonyl J acid (4.61). H HO3S

OCH3 N

H N

O HO3S

N

H

H

H

N

O

N

H

N H3CO

SO3H 4.60 CI Direct Blue 1

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OH

209

OH O

HO3S

N

N

H

H

SO3H

4.61 Carbonyl J acid

Disazo dyes can also be prepared by phosgenating suitable amino monoazo dyes. The phosgenation technique for building up direct dye molecules by joining together sub-units generally produces symmetrical structures. However, the use of cyanuric chloride (4.62) allows different sub-units to be joined together and this intermediate has been employed to link non-substantive anthraquinone structures to substantive azo moieties in green direct dyes, such as CI Direct Green 28 (4.63). Cl N N

Cl N

Cl

OH 4.62 Cyanuric chloride

COOH O

NH2 SO3H

N

N

H N O

N H

N N HO3S

N N

H

N H

4.63 CI Direct Green 28

It is in the direct dye class that the more complex polyazo dyes come into their own, and trisazo structures such as CI Direct Blue 78 (4.64), CI Direct Brown 222 (4.65) and CI Direct Black 38 (4.66) are classic examples of this type of dye. The last-named dye is now known to be carcinogenic [72]. Generally, the A→M1→M2→E type (such as 4.64) afford reasonably bright blue shades whilst dyes prepared according to other sequences, such as the E←D→Z←A type (examples 4.65 and 4.66) yield drab shades. The last of this trio highlights an area of concern in direct dye chemistry. Many dyes of this type relied on the extremely useful intermediate 4,4′-diaminobiphenyl (benzidine). In the tetrazonium salt prepared from this diamine, the two diazonium groups are conjugated and strongly activate one another. This activation makes the first coupling proceed very readily, but after completion the second diazonium group is no longer subject to the

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SO3H H N

N N

O

H

N

N

N N

HO3S HO3S 4.64 SO3H

SO3H

CI Direct Blue 78

H2N

N

H2N

CH3

N

H3C N

HOOC N HO

N

N 4.65

CH3

CI Direct Brown 222 HO

N

N

H2N SO3H N

NH2 N H2N

N

N

HO3S 4.66 CI Direct Black 38

powerful activating influence of a conjugated diazonium group and it couples far more sluggishly. The two diazonium groups can thus be coupled selectively in a stepwise manner to produce a host of useful products. In the preparation of CI Direct Black 38, for example, one diazonium grouping in tetrazotised benzidine undergoes coupling at low pH with H acid and diazotised aniline is then added. On raising the pH, the benzenediazonium chloride couples next to the hydroxy group on the H acid unit. Finally, m-phenylenediamine is added and the second benzidinediazonium grouping couples slowly to produce the third azo linkage. The withdrawal of the notorious carcinogen benzidine by major dyestuff manufacturers in the 1970s led to an extensive search for replacements for these dyes. One product that has emerged from this effort is CI Direct Black 166 (4.67), in which 4,4′diaminobenzanilide is used as a benzidine replacement. Sulphonated diaminobenzanilides have also been examined [73]. Other alternative diamines have been reported [74,75]. Interesting examples include 5,5′-diamino-2,2′-bipyridyl [76] and 3,8-diaminophenanthridone [77], a benzidine analogue in which the biphenyl rings are linked by an amide group. The genotoxicity of azo dyes has been reviewed [78].

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N

HO

211

N

H2N N H2N

SO3H

H

NH2

N

N N

N O

HO3S 4.67 CI Direct Black 166

Tetrakisazo dyes (dyes containing four azo groups) are less common but some do find use, these products often arising from the phosgenation of an amino disazo dye. Direct dyes have only modest fastness to washing, which may be improved by aftertreatments such as metal-complex formation (section 5.5.3) or by diazotisation of the dye on the fibre and further coupling of the diazonium salt with an insoluble coupling component (section 1.6.14). In addition to their use on cotton and viscose, direct dyes are important in the dyeing of leather. The cheapest members of this class are also used in the coloration of paper, since for this purpose fastness properties are largely irrelevant and price is allimportant. 4.9 UNSULPHONATED MONOAZO DYES 4.9.1 Solvent-soluble dyes These dyes, already described in section 2.12, require adequate solubility for the coloration of various organic solvents and must be cheap [79]. The simplest and least polar dyes of this class are used for the coloration of petrol and ball-pen inks, with more polar types being used in lacquers, stains and varnishes. Some products of lower solubility are used in mass coloration. These unexacting requirements make the simplest unsulphonated azo structures, often monoazo types, quite acceptable [80]. Typical of the least polar members of this class are CI Solvent Yellow 2 (4.68), CI Solvent Orange 1 (4.69) and CI Solvent Red 17 (4.70). Simple azo structures carrying sulphonamide, sulphone or carboxylate ester groups are used where a somewhat more polar, less soluble dye is needed. Simple disazo compounds (4-aminoazobenzene→2-naphthol, for example) are used as red solvent dyes. Probably the only structural feature worthy of note in this class is the occasional adoption of structures carrying long alkyl chains to enhance solubility, as in the case of the disazo dye CI Solvent Yellow 107 (4.71). N

N(CH3)2

N

N

OH

N 4.68 CI Solvent Yellow 2

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HO

CI Solvent Orange 1

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CHEMISTRY OF AZO COLORANTS

H3C

C9H19

H

H3C

O

N

OH

N

N

N

N HO

N

OCH3

CH3 4.71

H19C9

4.70

CI Solvent Yellow 107

CI Solvent Red 17

4.9.2 Disperse dyes The introduction of the man-made fibre cellulose acetate in the early 1920s posed a new problem for the dyestuff chemist, an initial solution to which was supplied by Green and Saunders in 1923 with the launch of the Ionamine range [81,82]. These dyes carried, as sole solubilising group, an ω-methanesulphonate grouping, which was incorporated into the molecule by coupling onto an arylamine ω-methanesulphonate. During the dyeing process this grouping was hydrolysed, leaving a fine dispersion of a water-insoluble dye as the dyebath. This type of dye, known as a disperse dye [83,84], is now used for the dyeing of cellulose acetate, triacetate and polyester fibres as well as offering an alternative to acid dyes for the dyeing of nylon. In order to facilitate satisfactory dye uptake, the molecular size of a disperse dye must be kept small; monoazo structures are therefore exceptionally important, particularly in the coloration of polyester and cellulose triacetate. In the yellow shade area, molecular size generally poses no problem and the various available coupling components can all be used without making the molecule too large. A very simple example of the type of structure employed using a phenolic coupling component is CI Disperse Yellow 3 (4.72). This dye is known to cause skin sensitisation when on nylon [85] and can also provoke an allergic reaction [86]. CH3 H

N CH3CONH

H3C

O

N

N

N

HO 4.72 CI Disperse Yellow 3

H N N

NO2 H3C

4.73 CI Disperse Yellow 8

Heterocyclic coupling components are widely used in the disperse dye field for the production of yellow dyes. Numerous conventional dyes are based on simple pyrazolones, often combined with an o-nitroaniline diazo component, the o-nitro group being particularly favourable in ensuring good light fastness. CI Disperse Yellow 8 (4.73), which uses a very simple pyrazolone coupling component, is an example. A very noticeable feature has been the use of a wider range of heterocyclic coupling components than in water-soluble azo dyes. An early example of this trend was CI Disperse Yellow 5 (4.74), which used 4-hydroxy-1-methylcarbostyril as coupling component. It was in

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213

this field that the pyridones made their first impact. A typical example is CI Disperse Yellow 211 (4.75). These compounds are now the most important yellow-producing coupling components, giving exceptionally bright greenish yellow hues. Their success has stimulated the investigation of other compounds, with the particular aim of extending the range of hues available from such simple coupling components. Thus, derivatives of 2,6-diaminopyridine with simple diazo components yield orange dyes, such as 4.76. Cl H

CH3

O

N

N

CN

N N

H

NO2

O2N

O

N O

4.74

CH3

N

O

C2H5

4.75 CI Disperse Yellow 211

CI Disperse Yellow 5

H2N

CN

N O2N

NHC2H5

N

N H2N 4.76

Cl

With the more bathochromic hues, the restriction posed by the requirement for small molecular size can only be overcome by using ω-substituted N,N-dialkylaniline coupling components and allying the powerful dialkylamino auxochrome with electron-attracting groups in the diazo component. This arrangement, with electrons being donated from one end of the molecule and attracted to the other, produces highly polar structures, with hues varying from yellow to red to blue as the polarity increases. Examples are CI Disperse Red 72 (4.77) and CI Disperse Blue 183 (4.78). In this latter dye the polarity is further increased by incorporating an amide group in the coupling component to reinforce the electron-donating effect of the dialkylamino group. CH2CH2CN N O2N

N CH2CH2COOCH3

N 4.77

CN

CN

CI Disperse Red 72

N O2N

N(C2H5)2

N H

N

Br

COC2H5 4.78

CI Disperse Blue 183

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The coupling component in CI Disperse Red 72 is a complex tertiary amine, carrying cyano and methoxycarbonyl groups on the alkyl chains. Such a coupling component is prepared by reaction of the primary amine with acrylonitrile (H2C=CHCN) and methyl acrylate (H2C=CHCOOCH3), the amino group adding across the activated double bond. The reason for incorporating these substituents is to enhance the fastness properties of the dye. Thus, the use of a cyanoethyl substituent in preference to a simple ethyl group in the tertiary amine coupling component generally improves fastness to light and sublimation. In addition to benzenoid diazo components, diazotised heterocyclic amines in which the amino group is attached to a nitrogen- or sulphur-containing ring figure prominently in the preparation of disperse dyes [87,88], since these can produce marked bathochromic shifts. The most commonly used of these are the 6-substituted 2-aminobenzothiazoles, prepared by the reaction of a suitable arylamine with bromine and potassium thiocyanate (Scheme 4.31). Intermediates of this type, such as the 6-nitro derivative (4.79), are the source of red dyes, as in CI Disperse Red 145 (4.80). It has been found that dichloroacetic acid is an effective solvent for the diazotisation of 2-amino-6-nitrobenzothiazole [89]. Subsequent coupling reactions can be carried out in the same solvent system. Monoazo disperse dyes have also been synthesised from other isomeric nitro derivatives of 2-aminobenzothiazole [90]. Various dichloronitro derivatives of this amine can be used to generate reddish blue dyes for polyester [91]. NH2

NH2

KSCN/Br2

NH2 O2N

O2N

N S

O2N

SCN

4.79

Scheme 4.31 C2H5 N

N

N

N

CH2CH2CN

S

O2N

4.80 CI Disperse Red 145

The use of heterocyclic diazo components is particularly important in the preparation of blue disperse dyes. It is often difficult to prepare, diazotise and/or couple the highly electronegatively substituted diazo components of the benzene series needed to achieve blue hues, and feebly basic aminoheterocycles offer a convenient alternative. The small molecular size of these heterocyclic amines, compared with that of heavily substituted anilines, is another factor favouring their use. One of the earliest diazo components of this type to be introduced was 2-amino-5nitrothiazole (4.81), prepared by condensation of thiourea with chloroacetaldehyde and nitration of the resultant 2-aminothiazole (Scheme 4.32). This component yields bright dischargeable blues, such as CI Disperse Blue 82 (4.82), which have outstanding build-up, very high extinction coefficients and good fastness to burnt gas fumes. Use of diazo component 4.81 with coupling component 4.83 yields a greenish blue dye.

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UNSULPHONATED MONOAZO DYES

N

N CHO H2C

NH2

+

Cl

C

NH2

NH2 NH2

S

O2N

S Chloroacetaldehyde

S

4.81

Thiourea

Scheme 4.32

OCH3

C2H5 N

N

N

N O2N

S

N(CH2CH2OH)2

CH2CHCH2OH H3C

OH

H

4.82

N

4.83

COCH3

CI Disperse Blue 82

Other examples include 3-aminobenzisothiazoles, especially the 5-nitro-derivative (4.84)[92] (synthesised as shown in Scheme 4.33) and electronegatively substituted 2aminothiophenes (Scheme 4.34). Both of these amines, when diazotised, are capable of giving blues with arylamine coupling components. The non-benzenoid intermediate 4.84 has a strong bathochromic effect, as in CI Disperse Blue 148 (4.85). Diazotised 5-acetyl-2amino-3-nitrothiophene (4.86) gives a greenish blue dye when coupled with compound 4.83 [93]. The corresponding dye from the diacetyl derivative of 4.83 and 2-amino-3,5dinitrothiophene is green on polyester. An extensive range of thiophene-based monoazo disperse dyes has been examined [94].

NH2

O2N

NH2

H2S

CN

O2N

C

N

H2O2 S

S O2N

NH2

4.84

Scheme 4.33 NO2 C2H5 N

N

CH2CH2COOCH3

N

N S

4.85 CI Disperse Blue 148

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CHEMISTRY OF AZO COLORANTS

NO2 CH3COCl S

HNO3

Cl

CH3CO

AlCl3

S

Cl

CH3CO

Scheme 4.34

Cl

NH4OH

NO2

CH3CO

S

NH2

S 4.86

Whilst monoazo dyes of small molecular size dominate the disperse dye field, the use of high-temperature dyeing and printing methods for the coloration of polyester has permitted some relaxation in the necessity for limiting molecular size. This flexibility has allowed the use of monoazo dyes such as CI Disperse Red 220 (4.87), employing a typical azoic coupling component, particularly as disperse dyes in processes for the dyeing of polyester/cotton blends. A novel development in the dyeing of such blends has been the introduction of disperse dyes such as CI Disperse Red 167 (4.88), in which the tertiary amine coupling component contains ester groupings. These groups can be hydrolysed in an alkaline clearing bath to facilitate removal of loose dye at the end of the dyeing process, thus minimising staining of the cellulosic portion of the blend.

H H Cl

O

N

N

OC2H5 O

N O2N

N

H

CH3

N COC2H5

Cl

4.87

N(CH2CH2OCOCH3)2

N

4.88

CI Disperse Red 220

CI Disperse Red 167

The widespread adoption of high-temperature dyeing methods has also allowed the use of simple disazo structures, such as CI Disperse Orange 13 (4.89) and CI Disperse Orange 29 (4.90), as economic dyes giving chiefly yellow and orange hues. The latter dye is known to exist in the syn conformation in the crystal [95]; the unsubstituted parent dye prefers the anti conformation. A few monoazo and disazo disperse dyes have absorption bands in the near infrared [96]. Considerable research effort has been devoted to developing a new generation of disperse dyes designed to optimise fastness to washing and minimise cross-staining of the cellulosic component of polyester/cellulosic blends [97,98]. Diester-containing monoazo disperse dye structures (4.91) that yield a dicarboxylic acid on hydrolysis and certain thienylazo blues

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217

(4.92) that are capable of being rendered soluble by a mild alkaline aftertreatment offer considerable benefits when dyeing polyester/cellulosic blends, including: (1) minimal cross-staining of the cellulosic fibre (2) minimal processing time because the alkaline fixation for reactive dyes clears the disperse dye stain (3) avoidance of a reduction clear with dithionite (4) good washing fastness performance after heat setting during finishing.

N

N N

OH

N

O2N

N

N N

4.89

OH

N

CI Disperse Orange 13 OCH3

4.90 CI Disperse Orange 29

O C

CH3

HN O2N

N

CH2CH2COOCH3 N

N O

CH2CH2COOCH3

NO2

4.91

C

CH3

HN O2N

N S

CH2CH3 N

N CH2CH3

4.92

An investigation of more than twenty blue monoazo structures, mostly derived from 2amino-3-carboxymethyl-5-nitrothiophene (4.93; X = COOCH3) or its 2-amino-3,5-dinitro analogue (4.93; X = NO2) and some containing hydrolysable ester groupings in the coupling component, was reported recently [99]. Depending on the dye structure, applied depth and conditions of heat setting, alkali clearing of these dyeings was as effective as a reduction clear in most instances. Comparisons between dyes containing either acetoxy (OCOCH3) or carboxymethyl (COOCH3) ester groups demonstrated that the lability of the thienyl ring system contributes more to alkali clearing efficiency than the nature of these hydrolysable esters. X

O2N

S

NH2

4.93

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CHEMISTRY OF AZO COLORANTS

A different approach to the incorporation of a hydrolysable grouping is to synthesise monoazo derivatives of 4-aminophthalimide (4.94; R = Me or Et). This diazo component undergoes ring opening under relatively mild alkaline conditions (Scheme 4.35) to give a water-soluble structure containing a 3,4-dicarboxyphenyl grouping (4.95) without affecting the azo group present [100]. Therefore these dyes display good alkali-clearing properties, thus avoiding the need for a reduction clearing step that can result in the liberation of carcinogenic amines. O

N

C HN

R N

N

C O

R

Na2CO3 H2O

4.94 O C CO2 + NH3 +

N

NaO

R N

NaO

C

N R

O Scheme 4.35

4.95

4.10 BASIC AZO DYES This class of dyes [101] includes some old-established products such as chrysoidine (4.96; CI Basic Orange 2) that have been of importance in the past due to their bright, strong, economical colours. Nowadays the main area of application of basic azo dyes is on acrylic fibres. The more advanced dyes developed for this purpose differ from the older ones in that the positive charge is supplied by a quaternary ammonium group rather than merely by a protonated amine. In the dyeing of acrylic fibres problems arise because the levelling properties of the adsorbed dye are poor, since the electrostatic attraction of a basic dye to the acidic centres in the fibre provides extremely strong binding, resulting in excellent fastness properties. + NH3

N

Cl

–

N H2N

4.96 CI Basic Orange 2

The two main classes of basic dyes vary in the way in which the basic centre is built into the molecule. The use of pendant quaternary ammonium groups, where the charge is insulated from the chromogen, allows disperse dyes to be given a cationic character. Alternatively, the cationic charge is delocalised within the chromogen. Typically, CI Basic Red 24 (4.97) has a structure reminiscent of a disperse dye, except that a quaternary ammonium group is carried on the pendant alkyl chain in the coupling component. This coupling component is prepared by reaction of N-ethylaniline with ethylene oxide followed by conversion of the resulting β-hydroxyethyl derivative into the β-

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219

C2H5 N O2N

N

N CN

+ CH2CH2N(CH3)3 CH3OSO3–

4.97

CH3

CI Basic Red 24

N

+ (CH3)3NCH2C Cl

–

N

N

CH2CH2CN

O 4.98 CI Basic Orange 24

chloroethyl compound, which is then treated with trimethylamine. By contrast, in CI Basic Orange 24 (4.98) the basic site is located in the diazo component. Basic dyes in which the positive charge is supplied by a quaternised heterocyclic ring that forms part of the chromogenic system often have particularly bright, strong hues. The intermediates used here are again typical disperse dye components, quaternisation being carried out after coupling by treating the dye with an alkylating agent such as dimethyl sulphate. Using typical tertiary amine coupling components, bright blue dyes, such as CI Basic Blue 41 (4.99), are readily obtained. C2H5 H3CO

N

S

N CH2CH2OH

N

CH3OSO3–

N + CH3

4.99 CI Basic Blue 41

Considerable difficulties are often experienced in the preparation of basic dyes from quaternised heterocyclic diazo components. An alternative technique is to use an oxidative coupling procedure, in which a mixture of a hydrazone and a coupling component is treated with a mild oxidising agent, such as a hexacyanoferrate(III) [102]; an azo compound is then produced as shown in Scheme 4.36 for the synthesis of CI Basic Red 30 (4.100) [103]. Quaternisation of heterocyclic derivatives can lead to the formation of mixtures of isomers, as in the case of CI Basic Red 22 (4.101). As well as the 2,4-dimethyl derivative shown, the product contains about 15% of the 1,4-dimethyl isomer [104].

NH2 N

N

–4e +

N CH3

NH2

NH2

N –3H+

N + X

–

CH3

4.100

CI Basic Red 30

Scheme 4.36

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CHEMISTRY OF AZO COLORANTS

CH3 N

N

N(C2H5)2

N N –

CH3OSO3

N + CH3

4.101 CI Basic Red 22

It is convenient to include in this class certain tautomeric structures that can exist either in the azo form or in the alternative hydrazone form. These dyes are the diazatrimethinecyanines, which can be viewed as being derived from the trimethinecyanines (R–CH=CH–CH=R) by replacement of two of the CH units by nitrogen atoms (R–N=N– CH=R). Such dyes are important in achieving yellow and red shades and they are often most conveniently prepared by an oxidative coupling procedure using coupling components peculiar to basic dyes. CI Basic Red 29 (4.102) and CI Basic Yellow 24 (4.103) are typical of this group. H3C

CH3

H3C

N S

N

N

Cl

N

N N

S

N + CH3 –

CH

CH3

N

4.102 CI Basic Red 29

– CH3OSO3

N + CH3

4.103 CI Basic Yellow 24

4.11 AZOIC DIAZO AND COUPLING COMPONENTS In 1880 a process for the coloration of cotton by initial impregnation with an alkaline solution of 2-naphthol and subsequent treatment with a solution of a diazonium salt was patented by Thomas and Robert Holliday. In this process a water-insoluble monoazo dye of good fastness to washing was formed within the fibre pores. The dyer’s need to diazotise the arylamine component at low temperature led to these dyes becoming known as ‘ice colours’ and the product obtained using diazotised 4-nitroaniline became known as Para Red. Over the next thirty years minor improvements in the process were introduced, but a major step forward was made in 1912 when Griesheim-Elektron introduced a new type of coupling component, 3-hydroxy-N-phenylnaphthalene-2-carboxamide (4.104; 3-hydroxy-2-naphthoic anilide), under the trade name Naphtol AS (CI Azoic Coupling Component 2). The guiding principle behind this advance was that the new intermediate was substantive towards cellulose and therefore was better held in place on the fibre prior to and during the treatment with a diazonium salt. Furthermore, the increased substantivity and diminished solubility of the resulting azo combination gave a marked improvement in the level of wash fastness obtained. The term ‘azoic colours’ was introduced in the late 1920s [105].

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221

O H O N H 4.104 CI Azoic Coupling Component 2

Naphtol AS is best prepared by heating together β-oxynaphthoic (BON) acid, aniline and phosphorus trichloride in an inert solvent, the initially formed intermediate (4.105) reacting further with the naphthoic acid to give the desired anilide. A range of similar products followed, with a variety of arylamines being used in place of aniline. A selection of these compounds is given in Table 4.2. An attempt has been made recently to correlate the chemical structure of azoic coupling components with their substantivity for cotton cellulose. Theoretically computed equations were derived to relate substantivity values to the van der Waals’ surface and to molecular volume and hydrophobicity of the sorbed molecule. The results provide support for the hypothesis of microcrystalline multilayered micelles available for sorption of these compounds [106].

P N

N H

4.105 Table 4.2 Azoic coupling components Naphtol

A.C.C.

Chemical name

AS–D AS–ITR AS–OL AS–PH AS–TR

18 12 20 14 8

3-Hydroxy-2-naphthoic 3-Hydroxy-2-naphthoic 3-Hydroxy-2-naphthoic 3-Hydroxy-2-naphthoic 3-Hydroxy-2-naphthoic

A.C.C.

2′-methylanilide 5′-chloro-2′,4′-dimethoxyanilide 2′-methoxyanilide 2′-ethoxyanilide 4′-chloro-2′-methylanilide

CI Azoic Coupling Component

Ranges of diazotisable amines were supplied, designed to go with these coupling components and known as Fast Bases, despite being marketed as the phenylammonium chlorides (hydrochloride salts). Whilst the hues obtainable at first were limited to the orange to red region of the colour gamut, the later introduction of amines heavily substituted with electron-donating groups allowed extension into the violet to blue region. Examples of Fast Bases are given in Table 4.3; the last two entries in this table have been classified by ETAD as toxic compounds [107].

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Table 4.3 Fast bases Fast base

A.D.C.

Chemical name

Blue BB Orange GC Orange RD Red ITR Red KB Red RC Red RL Red TR Scarlet GG Violet B

20 2 49 42 32 10 34 11 3 41

4-Benzoylamino-2,5-diethoxyaniline hydrochloride 3-Chloroaniline hydrochloride 2-Chloro-5-trifluoromethylaniline hydrochloride 2-Methoxyaniline-5-sulphondiethylamide hydrochloride 5-Chloro-2-methylaniline hydrochloride 5-Chloro-2-methoxyaniline hydrochloride 2-Methyl-4-nitroaniline hydrochloride 4-Chloro-2-methylaniline hydrochloride 2,5-Dichloroaniline hydrochloride 4-Benzoylamino-2-methoxy-5-methylaniline hydrochloride

A.D.C.

CI Azoic Diazo Component

Extension into other hues was not possible using anilides of BON acid, and, to meet this requirement, coupling components of differing structure but similar levels of substantivity were introduced (4.106 to 4.109; the arrows indicate coupling positions). Yellow dyeings were obtained by using the bis-acetoacetarylamide derived from 3,3′-dimethylbenzidine, marketed as Naphtol AS-G (4.106; CI Azoic Coupling Component 5). Browns were supplied by using the 2′-methylanilide, Naphtol AS-BR (4.107; X = CH3, Y = H; CI Azoic Coupling Component 3) and the 4′-chloroanilide, Naphtol AS-LB (4.107; X = H, Y = Cl; CI Azoic Coupling Component 15) of 2-hydroxycarbazole-1-carboxylic acid. Blacks resulted from the 4″-methoxy-2″-methylanilide of 3′-hydroxy-1,2-benzocarbazole-2′-carboxylic acid, Naphtol AS-SR (4.108; CI Azoic Coupling Component 25). Duller greens stemmed from the 2′-methylanilide of 3-hydroxyanthracene-2-carboxylic acid, Naphtol AS-GR (4.109; CI Azoic Coupling Component 36), whilst bright greens were eventually provided by Naphtol AS-FGGR (CI Azoic Coupling Component 108), the reaction product of copper or nickel phthalocyanine with an aminoarylpyrazolone, the constitution of which has not been disclosed [108]. H3C

O

O

H N

H3C

N

H

CH3

O

H O

O

H

CH3

N

4.106 CI Azoic Coupling Component 5

H

N

X

O

H3C H

4.107 N

N H

OCH3

CI Azoic Coupling Component 3

O O

H 4.108

CI Azoic Coupling Component 25

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STABILISED DIAZONIUM SALTS AND AZOIC COMPOSITIONS

223

O H CH3 N H 4.109 CI Azoic Coupling Component 36

O

O

O

CH3

C

CH3

C

N

CH2CH2

O

C

CH3

C

CH3

N O

4.110

Azoic coupling can be achieved on silk where free amino groups have been introduced into the fibroin structure by condensation with 4-aminobenzaldehyde. The diazotised material can then be treated with an azoic coupling component [109]. As recommended laundering temperatures have tended to fall in recent years, a bleach consisting of sodium perborate activated by addition of tetra-acetylethylenediamine (4.110; TAED) has become an important component of household detergent formulations. This system is effective at temperatures as low as 40–50 °C. A recent study of the effects of TAED-activated peroxy bleaching on the colour fastness of azoic dyeings has demonstrated that the sensitivity of these products can be related to their chemical structure. Electrondonating substituents in the diazo component enhance resistance to oxidative attack under these conditions, as do the size and complexity of substituents present in the coupling component [110]. 4.12 STABILISED DIAZONIUM SALTS AND AZOIC COMPOSITIONS 4.12.1 Fast Salts As the range of components available for use in the azoic dyeing process expanded, research was simultaneously targeted on improvements designed to make the process more attractive to the commercial dyer. The necessity for the dyer to diazotise the Fast Base was removed with the introduction of stabilised diazonium salts [111], known as Fast Salts. Stabilisation was achieved by a judicious selection of the counter-ion to the diazonium cation; various anions have found use in commercial Fast Salts and some examples are listed in Table 4.4. Particularly effective is the diazonium tetrachlorozincate, which can be readily prepared by adding an excess of zinc chloride solution to a solution of the diazonium salt. The precipitated complex diazonium salt is usually admixed with an inert diluent, which enhances its stability, and in use the dyer only needs to dissolve the powder in water to prepare the necessary diazonium salt solution. 4.12.2 Rapid Fast colours In addition to these ‘active’ stabilised diazonium salts that give a diazonium salt solution immediately on dissolving in water, there are certain derivatives of diazonium compounds from which the diazonium salt can be readily regenerated. One group of such derivatives is the class of compounds known as anti-diazotates [112]. Diazotates are formed when the pH

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CHEMISTRY OF AZO COLORANTS

Table 4.4 Stabilised diazonium salts Fast salt

A.D.C.

Stabilising anion and parent base

Black K Blue B Blue BB Bordeaux GP Orange GR Red AL Red 3GL Red ITR Scarlet GG Scarlet R

38 48 20 1 6 36 9 42 3 13

Tetrachlorozincate 4-Amino-2,5-dimethoxy-4′-nitroazobenzene 3,3′-dimethoxybenzidine (o-dianisidine) 4-Benzoylamino-2,5-diethoxyaniline 4-Methoxy-2-nitroaniline 2-Nitroaniline 1-Aminoanthraquinone 4-Chloro-2-nitroaniline 2-Methoxyaniline-5-sulphondiethylamide 2,5-Dichloroaniline 2-Methoxy-5-nitroaniline

Orange GC Red GG

2 37

Tetrafluoroborate 3-Chloroaniline 4-Nitroaniline

Red B Red TR

5 11

Naphthalene-1,5-disulphonate 2-Methoxy-4-nitroaniline 4-Chloro-2-Methylaniline

Variamine Blue B

35

Chloride 4-Amino-4′-methoxydiphenylamine

A.D.C.

CI Azoic Diazo Component

of a diazonium salt solution is raised to between 9 and 12 with an alkali metal hydroxide. The syn-diazotates initially formed are somewhat unstable and isomerise into the stable antidiazotate form. The evidence overwhelmingly points to the two forms being stereoisomers (4.111 and 4.112, respectively). Stable mixtures of anti-diazotates and Naphtols were marketed as Rapid Fast colours for printing onto fabric with development of the azoic dye by steaming. The antidiazosulphonates (4.113) [113], which were prepared by treatment of a diazonium salt with sodium sulphite and which regenerate the diazonium ion on treatment with an oxidising agent, found similar use. Both ranges are now of only historical interest. O–

Ar N

Ar

Ar

N

N

N

N

N

O– 4.111

4.112

syn-diazotate

anti-diazotate

O

SO3Na

4.113

4.12.3 Rapidogens and Neutrogenes Superior ‘passive’ stabilised diazo compounds are afforded by the diazoamino compounds (triazenes) that arise by reaction of diazonium salts with a variety of secondary amines [114]. Typically, sarcosine (CH3NHCH2COOH), which gives products based on structure 4.114, as well as N-methyltaurine (CH3NHCH2CH2SO3H) and N-methylaniline-4-sulphonic acid,

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225

have been used for this purpose. Again these were marketed in admixture with a coupler for application to cotton and colour development by acid steaming, under the tradename Rapidogen in the 1930s [114]. A disadvantage was the corrosive effect of the acid (usually acetic or formic) on the equipment, but this problem was eventually overcome with the Neutrogene range [115], which used an N-substituted anthranilic acid, such as 2carboxyphenylglycine (4.115), as the secondary amine required for triazene formation. These products, which have been copied by other manufacturers under various tradenames, are designed specifically for printing applications. Azoic dyeing applications were catered for in 1967 when Hoechst introduced the Azanil salts, a limited range of mixtures of substantive triazenes and Naphtols that can be absorbed onto the fabric and the colour developed by acidification of the dyebath. COOH

CH3 N Ar

N

N

NHCH2COOH

CH2COOH

4.115

4.114

4.13 AZO PIGMENTS PRODUCED BY FINAL COUPLING Insoluble azo pigments produced by a final coupling stage are in the main made from conventional azoic dye intermediates and find wide use as general-purpose pigments where the highest fastness is not essential. For yellow pigments acetoacetarylamide coupling components are the most commonly employed, but the monoazo combinations derived from these, such as CI Pigment Yellow 1 (4.116), are generally of poor fastness to heat and solvents. Superior properties are shown by disazo pigments of the E1←D→E2 type, which are often based on the use of 3,3′-dichlorobenzidine as tetrazo component. A typical disazo pigment is CI Pigment Yellow 170 (4.117). Colour and constitution relationships in azoacetoacetanilide pigments have been examined [116]. Pigments of this type can be identified and analysed by spectroscopic and chemical methods [117]. H3C O H

N H3C

N

N H

4.116

O

CI Pigment Yellow 1

NO2

H3CO

H3C O

Cl O

H

N

N N

H

H N

N H

N

O

Cl

O CH3

4.117 CI Pigment Yellow 170

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CHEMISTRY OF AZO COLORANTS

For orange colours, simple 2-naphthol derivatives are the most commonly used coupling components as, for instance, in 2,4-dinitroaniline→2-naphthol (4.118; CI Pigment Orange 5). As in the yellow series, superior disazo pigments can be prepared using 3,3′dichlorobenzidine as tetrazo component with derivatives of 1-phenyl-3-methylpyrazol-5-one as couplers. NO2

H O2N

O

N N

4.118 CI Pigment Orange 5

In the red to violet sector Naphtol AS and its derivatives are widely used. Simple monoazo combinations such as 2,5-dichloroaniline and Naphtol AS serve where high heat and solvent fastness are not essential, but for better properties polar features, such as amide linkages, are incorporated. CI Pigment Red 245 (4.119), CI Pigment Red 188 (4.120) and CI Pigment Violet 50 (4.121) are typical of this group. H3CO H3CO H

O H

H2NOC

H H

N

O

N O

N

O

O

N

H

N

Cl

N

N

O

OCH3 4.120

4.119

CI Pigment Red 188

Cl

CI Pigment Red 245

OCH3

H N

H H

O

N O

N O

N H3C 4.121 CI Pigment Violet 50

In the blue region, conventional azoic combinations are used as pigments and the adoption of o-dianisidine as tetrazo component allows the disazo type to be produced, such as CI Pigment Blue 25 (4.122).

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227

H3CO

O N

O

H

H

N H

N

N

H

N

O

OCH3

N O

4.122 CI Pigment Blue 25

The objective in preparing a pigment is to produce a highly insoluble organic compound and in order to approach this ideal it is necessary to resort to intermediates that themselves have only very limited solubility. These inevitable difficulties in preparation limit the fastness achieved from a final coupling process and have led to the development of alternative procedures for the production of high-grade azo pigments; these alternatives are discussed elsewhere (section 2.3). The wholly organic azo compounds discussed above have the soft texture and high tinctorial strength that are desired pigmentary properties. Somewhat inferior in these properties are the ‘lakes’ of sulphonated azo compounds, which are also widely used as pigments. These ‘lakes’ are insoluble salts, usually calcium or barium salts of sulphonated arylamines linked to cheap coupling components, which rely on the polar character of the ionic bond to provide their low solubility. Examples are CI Pigment Red 57:1, the calcium lake of compound 4.123, and CI Pigment Red 53:1, the barium lake of compound 4.124. Organic pigments have been reviewed [118]. X-ray powder diffraction data of many organic colorants have been collected and discussed [119]. CH3

Cl CH3

SO3Na

NaO3S

N N

H O

N N

H O

4.124 CI Pigment Red 53

COONa 4.123 CI Pigment Red 57

4.14 IMPLICATIONS OF NEW TECHNOLOGY IN DIAZOTISATION AND COUPLING The manufacture of azo dyestuffs has been for many years a batch operation, individual batches of dye being made in multipurpose plant, isolated by filtration techniques, stovedried and standardised. The traditional processes [120] are now steadily being replaced [121,122] by more efficient production methods that offer greater cost-effectivenes and

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improved working conditions. Although most azo dye production is still carried out by batch operation in vastly improved plants, more dedicated continuous units for the manufacture of high-volume products are being introduced. In these plants the diazotisation and coupling steps are carried out continuously by computer-controlled metering of the intermediates and reactants into a tube, flow rates being chosen to allow the reaction time to be less than the dwell time of the reaction mixture in the tube. The product is still usually isolated by filtration but, even here, automatic discharging has removed much of the heavy physical effort and dirty conditions previously endured. Spray drying of pre-standardised pastes with careful attention to particle form provides much improved products for the user. The supply of dyes as concentrated solutions allows the automatic metering of dyestuffs by the user, making automation easier and allowing greater control over dyeing conditions. These improvements in manufacturing technique, together with the availability of a much wider variety of analytical methods, serve to ensure that the user is supplied with a high-quality product at an acceptable price. 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.

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P Griess, Ann., 106 (1858) 123; 113 (1860) 201; Compt. Rend., 49 (1859) 77. P Griess, Ann., 137 (1866) 85; Ber., 9 (1876) 627. H Zollinger, Color chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 7.2. P Rys in Physico-chemical principles of color chemistry, Eds A T Peters and H S Freeman (London: Blackie, 1996) Chapter 1.2. W Bradley, J.S.D.C., 75 (1959) 289. K Bredereck, B Gülec, B Helfrich and S.Karaca, Dyes and Pigments, 9 (1988) 153. H Zollinger, Color chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 7.3. V Chmatal and Z J Allan, Coll. Czech. Chem. Commun., 30 (1965) 1205. ETAD Information Notice No 6 (July 1997). A Geisberger, J.S.D.C., 113 (1997) 197. Y S Szeto and G Taylor, J.S.D.C.,113 (1997) 103. U Sewekov and A Westerkamp, Melliand Textilber., 78 (1997) 56. B Küster and U Wahl, Textilveredlung, 32 (1997) 121. K Hübner, E Schmele and V Rossbach, Melliand Textilber., 78 (1997) 720. S W Oh, M N Kang, C W Cho and M W Lee, Dyes and Pigments, 33 (1997) 119. O Stein, Ber., 27 (1894) 2806. E Noelting and E Kopp, Ber., 38 (1905) 3506. C Schwalbe, Z.Farb.Text.Ind., 4 (1905) 433. K H Saunders and R L M Allen, Aromatic diazo compounds, 3rd Edn (London: Edward Arnold, 1985) Chapter 1.2. P Rys in Physico-chemical principles of color chemistry, Eds A T Peters and H S Freeman (London: Blackie, 1996) Chapter 1.3. E Knoevenagel, Ber., 23 (1890) 2994; 28 (1895) 2048. O N Witt, Ber., 42 (1909) 2953. V V Ershov, G A Nikiforov and C R H I de Jonge, Quinone diazides, (Amsterdam: Elsevier, 1981). T S Gore and K Venkataraman, Proc. Indian Acad. Sci., 34A (1951) 368. H H Hodgson and D E Nicholson, J.C.S., (1943) 379. W R Orndorff and B J Ray, Ber., 40 (1907) 3211. F Muhlert, Z.Angew.Chem., 21 (1908) 2611. K H Saunders and R L M Allen, Aromatic diazo compounds, 3rd Edn (London: Edward Arnold, 1985) Chapter 6.2. H V Kidd, J. Org. Chem., 2 (1937) 206. W Schwarin and O Kaljanov, Ber., 41 (1908) 2056. F Muzik and Z J Allan, Chem. Listy, 49 (1955) 212. F Muzik and Z J Allan, Chem. Listy, 46 (1952) 774. E R Ward, D B Pearson and P R Wells, J.S.D.C., 75 (1959) 484. C Krohn, Ber., 21 (1888) 3241. V Chmatal and Z J Allan, Coll. Czech. Chem. Commun., 27 (1962) 1835. N Sekar, J.S.D.C., 111 (1995) 390; 112 (1996) 204.

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37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93.

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N Sekar, J.S.D.C., 112 (1996) 242. G S Hartley, Nature, 140 (1937) 281. G S Hartley, J.C.S., (1938) 633. G Zimmerman, L Y Chow and U J Paik, J. Amer. Chem. Soc., 80 (1958) 3528. D Schulte-Frohlinde, Ann., 612 (1957) 131. D J W Bullock, C W N Cumper and A I Vogel, J.C.S., (1965) 5316. D L Beveridge and H H Jaffe, J. Amer. Chem. Soc., 88 (1966) 1948. B Marcandalli, P L Beltrame, E Dubini-Paglia and A Seves, Dyes and Pigments, 11 (1989) 179. Y Yagi, Bull. Soc. Chim. Japan, 36 (1963) 487. R Jones, A J Ryan, S Sternhill and S E Wright, Tetrahedron, 19 (1963) 1497. S Toda, Nippon Kagaku Zasshi, 80 (1959) 402. Q Peng, M Li, K Gao and L Cheng, Dyes and Pigments, 18 (1992) 271. G Hallas and A H M Renfrew, J.S.D.C., 112 (1996) 207. E Fischer and Y F Frei, J.C.S., (1959) 3159. P F Gordon and P Gregory, Organic chemistry in colour, (Berlin: Springer-Verlag, 1983) 100. A Burawoy, A G Salem and A R Thompson, J.C.S., (1952) 4793. L Antonov et al., Dyes and Pigments, 38 (1998) 157; 40 (1998) 163. K J Morgan, J.C.S., (1961) 2151. D Hadzi, J.C.S., (1956) 2143. L Antonov and S Stoyanov, Dyes and Pigments, 28 (1995) 31. K Yamamoto, K Nakai and T Kawaguchi, Dyes and Pigments, 11 (1989) 173. C Szantay, Z Csepregi, P Aranyosi, I Rusznak, L Toke and A Vig, Magn. Res. Chem., 35 (1997) 306. H E Fierz-David, L Blangey and E Merian, Helv. Chim. Acta, 34 (1951) 34, 846. J Kelemen, Dyes and Pigments, 2 (1981) 73; J Kelemen, G Kormany and G Rihs, Dyes and Pigments, 3 (1982) 249; J Kelemen, S Moss, H Sauter and T Winkler, Dyes and Pigments, 3 (1982) 27. N Donaldson, The chemistry and technology of naphthalene compounds, (London: Edward Arnold, 1958). N N Vorozhtsov, The chemistry of synthetic dyes, Vol 3, Ed K Venkataraman (New York: Academic Press, 1970) 93. N N Vorozhtsov, The chemistry of synthetic dyes, Vol 3,Ed K Venkataraman (New York: Academic Press, 1970) 110. N N Vorozhtsov, The chemistry of synthetic dyes, Vol 3, Ed K Venkataraman (New York: Academic Press, 1970) 146. H T Bucherer, J.Prakt.Chim., (2), 69 (1904) 49. N L Drake, Organic Reactions, 1 (1942) 105. H Seeboth, Angew. Chem. Internat., 6 (1967) 307. J Kraska and K Blus, Dyes and Pigments, 31 (1996) 97. J Meybeck and P Galafassi, Appl. Polymer Symp., 18 (1971) 463. S Stoyanov, T Iijima, T Stoyanova and L Antonov, Dyes and Pigments, 27 (1995) 237. D L Ross and E Reissner, J. Org. Chem., 31 (1966) 2571. C T Helmes, Amer. Dyestuff Rep., 83 (Aug 1994) 40. W Czajkowski, Dyes and Pigments, 17 (1991) 297. J Shore, Rev. Prog. Coloration, 21 (1991) 23. Y C Chao and S S Yang, Dyes and Pigments, 29 (1995) 131. F Calogero, H S Freeman, J F Esaney, W M Whaley and B J Dabney, Dyes and Pigments, 8 (1987) 431. J Szadowski, Dyes and Pigments, 14 (1990) 217. H S Freeman, D Hinks and J F Esaney in Physico-chemical principles of color chemistry, Eds A T Peters and H S Freeman (London: Blackie, 1996) Chapter 7. Chi-Kang Dien, The chemistry of synthetic dyes, Vol 8, Ed K Venkataraman (New York: Academic Press, 1978) 81. Colour Index International, Pigments and Solvent Dyes (Bradford: SDC, 1997) 198. A G Green and K H Saunders, J.S.D.C., 39 (1923) 10. A G Green and K H Saunders, J.S.D.C., 40 (1924) 138. J F Dawson, J.S.D.C., 107 (1991) 395. S Abeta and K Imada, Rev. Prog. Coloration, 20 (1990) 19. K D Wozniak, A Keil and D Müller, Textil Praxis, 45 (1990) 965. P Elsner, Textilveredlung, 29 (1994) 98. N Sekar, Colourage, 45 (Dec 1998) 37. A D Towns, Dyes and Pigments, 42 (1999) 3. J Sokolowska-Gajda and H S Freeman, Dyes and Pigments, 20 (1992) 137. A T Peters, S S Yang and E Chisowa, Dyes and Pigments, 28 (1995) 151. A T Peters and S S Yang, Dyes and Pigments, 30 (1996) 291. H G Wippel, Melliand Textilber., 50 (1969) 1090. G Hallas in Developments in the chemistry and technology of organic dyes, Ed J Griffiths (Oxford: Blackwell Scientific Publications, 1984) 31.

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94. G Hallas and A D Towns, Dyes and Pigments, 31 (1996) 273; 32 (1996) 135; 33 (1997) 205; 33 (1997) 215; 33 (1997) 319; 34 (1997) 133; 35 (1997) 45; 35 (1997) 219. 95. H S Freeman, S A McIntosh and P Singh, Dyes and Pigments, 35 (1997) 11. 96. M Matsuoka in Analytical chemistry of synthetic colorants, Eds A T Peters and H S Freeman (London: Blackie, 1995) Chapter 3. 97. P W Leadbetter and A T Leaver, Rev. Prog. Coloration, 19 (1989) 33. 98. P W Leadbetter and S Dervan, J.S.D.C., 108 (1992) 369. 99. J H Choi, S H Hong and A D Towns, J.S.D.C., 115 (1999) 32. 100. J S Koh and J P Kim, J.S.D.C., 114 (1998) 121, Dyes and Pigments, 37 (1998) 265. 101. R Raue, Rev. Prog. Coloration, 14 (1984) 187. 102. S Hünig, Angew. Chem. Internat., 7 (1968) 335. 103. P F Gordon and P Gregory, Organic chemistry in colour (Berlin: Springer-Verlag, 1983) 87. 104. D Brierley, P Gregory and B Parton, J. Chem. Research, 5 (1980) 174. 105. E B Higgins, J.S.D.C., 43 (1927) 213. 106. C Daescu and D Hadaruga, Dyes and Pigments, 40 (1998) 235. 107. R Anliker, G Dürig, D Steinle and E J Moriconi, J.S.D.C., 104 (1988) 223. 108. F Gund, J.S.D.C., 76 (1970) 151. 109. S Ifrim, Amer. Dyestuff Rep., 84 (Mar 1995) 38. 110. J Wang, AATCC Internat. Conf. & Exhib. (Oct 1998) 144. 111. K H Saunders and R L M Allen, Aromatic diazo compounds, 3rd Edn (London: Edward Arnold, 1985) Chapter 3.5. 112. K H Saunders and R L M Allen, Aromatic diazo compounds, 3rd Edn (London: Edward Arnold, 1985) Chapter 5.1. 113. K H Saunders and R L M Allen, Aromatic diazo compounds, 3rd Edn (London: Edward Arnold, 1985) Chapter 5.3. 114. K H Saunders and R L M Allen, Aromatic diazo compounds, 3rd Edn (London: Edward Arnold, 1985) Chapter 7.1. 115. B Jomain, J.S.D.C., 69 (1953) 661. 116. R M Christie and P N Standring in The design and synthesis of organic dyes and pigments, Eds A T Peters and H S Freeman (London: Elsevier Applied Science, 1991) Chapter 11. 117. C Nicolaou and M Da Rocha in Analytical chemistry of synthetic colorants, Eds A T Peters and H S Freeman (London: Blackie, 1995) Chapter 8. 118. W S Czajkowski in Modern colorants: Synthesis and structure, Eds A T Peters and H S Freeman (London: Blackie, 1995) Chapter 3. 119. A Whitaker in Analytical chemistry of synthetic colorants, Eds A T Peters and H S Freeman (London: Blackie, 1995) Chapter 1. 120. H E Fierz-David and L Blangey, Fundamental processes of dye chemistry, (New York: Interscience, 1949). 121. G Booth, Rev. Prog. Coloration, 8 (1977) 1; 14 (1984) 114. 122. G Booth, The manufacture of organic colorants and intermediates, (Bradford: SDC, 1988).

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CHAPTER 5

Chemistry and properties of metal-complex and mordant dyes John Shore

5.1 INTRODUCTION The complexing of chelatable organic compounds with transition-metal atoms is widely exploited in catalytic reactions, biological systems, metal recovery and sequestration of trace metals. Such complexes are important in many non-textile applications, including solvent dyes of high fastness, charge-control agents in photocopying, dye developers in instant colour photography and specific reagents in colorimetric analytical chemistry. This chapter is mainly concerned with the interaction between organic chromogenic systems and chromium or copper (or occasionally iron, cobalt or nickel) to give acid dyes for amide fibres and direct or reactive dyes for cellulosic fibres [1–3]. It has been recognised for centuries that certain natural dyes, including alizarin, kermes, cochineal and fustic, now known to contain o-dihydroxy phenolic or anthraquinonoid residues in their structures, can be fixed on natural fibres using oxides or salts of transition metals as mordants. Although mordanted wool dyed with alizarin showed excellent fastness, reproducibility of shade was difficult to achieve because of the variable composition of the raw materials available. The famous Turkey red, in which alizarin was applied to aluminiummordanted wool in the presence of calcium salts, formed a metallised complex the nature of which remains in considerable doubt. The research of Werner at Zürich in the early l900s on the theory of coordination valency [4] established the concept of a metal-complex dye and was the key to understanding of the mordanting process. He concluded that a metal ion was characterised by two types of valency, which he called primary and secondary. The primary valency satisfied the chemical equivalence (the number of positive charges on the metal cation) and the secondary valency took part in coordination bonding. We now refer to the primary valency as the oxidation number or state of the central metal atom and the secondary valency as the number of associated ligand atoms required to complete the coordination number. It must be borne in mind that the concept of bond angles, such as the tetrahedral configuration of the C-H bonds in methane, was then unknown and knowledge of the mode of attachment of a ligand to the central metal atom was extremely vague. Synthetic alizarin (5.1), CI Mordant Red 5 (5.2) and CI Mordant Orange 1 (5.3), the first azo dye capable of forming a metal complex, in this case via the salicylic acid residue, are examples of the simple mordant dyes in widespread use at the time when Werner propounded his theory. The first metal-complex dyes to be prepared in substance, rather than within the fibre, were discovered by Bohn of BASF in 1912 by treating hydroxyanthraquinonesulphonic acids with a warm solution of a chromium(III) salt. In the

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same year chromium complexes of mordant azo dyes were also patented. However, it was not until Ciba in 1915 proposed applying these sulphonated 1:1 chromium-dye complexes to wool from a sulphuric acid solution at low pH that economic exhaustion values could be attained. CI Acid Yellow 104 is a typical example of these mono- or disulphonated azo dye complexes still in current use. O

OH OH

OH

HO N N

O

NaO3S

5.2

5.1 Alizarin

OH

CI Mordant Red 5

COOH O2N

N N

OH

5.3 CI Mordant Orange 1 H3CH2CO C

O H3C

N

SO2

H

NaO3S

N N

N

O Cr

O

N

O O

O

O

N N

O2N

H

N

N

CH3 O

O

Cr

O

N O2S

NO2

C SO3Na

OCH2CH3 5.4

5.5

CI Acid Black 58

CI Acid Green 12

The first water-soluble symmetrical 1:2 metal-complex dye, which contained no sulpho groups and was solubilised by a polar methylsulphonyl group on each organic ligand, was described by Schetty of Geigy for application to wool under neutral dyeing conditions [5]. This dye was CI Acid Black 58 (5.4), still an important basis for grey shades in wool dyeing. If sulphonated intermediates were used to form symmetrical disulphonated 1:2 metal-

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233

complex analogues, such as CI Acid Green 12 (5.5), unlevel dyeing remained a problem because of the low pH necessary to achieve satisfactory exhaustion. This difficulty was gradually overcome by the use of nonionic or weakly cationic levelling agents, but the most important factor was the introduction of a range of polar but nonionised solubilising groups [l,6]. These included alkylsulphony1 (RSO2–), as in structure 5.4, monoand dialkylsulphonamides (R2NSO2–) and cyclic sulphones. Typical sulphonamido-substituted dyes include CI Acid Orange 60 (5.6; R = H) and 87 (5.6; R = CH3). Insoluble 1:2 metalcomplex dyes devoid of polar solubilising groups are also available, such as CI Acid Black 63 (5.7). They are mainly of interest for the dyeing of nylon from aqueous dispersion. R HN

CH3

SO2 N

N

N

N

O

O O

Cr

O

R

NO2

N

N R

O

O N

N N

O

Cr

O N

O2N

N

N

O2S H3C

NH

5.7

R 5.6

CI Acid Black 63

5.2 FUNDAMENTAL CONCEPTS Metal-complex formation entails the interaction of one or more organic ligands with a multivalent metal cation. This brings about certain fundamental changes in the characteristics of the components of the complex. 5.2.1 Ligand systems A ligand system is any charged or uncharged polar entity or atomic component of a substituent group containing an excess electron or electron pair. The nature of the bond between the ligand and the metal atom depends on the ligand acting as an electron-pair donor and the metal as an electron-pair acceptor. The size of the atom and the electron distribution in the outermost shell of a transition element of the first series are especially conducive to the formation of metal complexes. The donation of an electron pair from ligand atom to metal gives rise to a covalent or sigma bond, in molecular orbital terminology. It is often convenient to distinguish between conventional covalent bonds arising from the sharing of electrons, denoted by a straight line, and coordinated sigma bonds arising by electron-pair donation from a ligand system, denoted by an arrow. However, there is no real difference between them once the bonds are formed.

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

In the synthesis of metal-complex dyes the organic ligand may form part of either a basic or an acidic functional group. A basic ligand carries a lone pair of electrons that may interact with the metal ion. Examples include –NH2 (amino), =NH (imino), =N– (in azo or azomethine), =O (in carbonyl) or –S–(in thioether). An acidic ligand, such as –OH (from carboxylic, phenolic or enolic groups), –SH (from thiophenolic or thioenolic groups) or –NH– (from amino or imino groups), loses a proton during metallisation to give a formal negative charge. In the case of a ligand molecule such as ammonia or water, only one pair of electrons is utilised in forming a coordinated sigma bond with the metal. Ligand systems of this type are said to be unidentate and are found in many metal-complex dye structures, satisfying sites around the central metal atom not already sterically occupied by coordinated dye ligands. Bi- or tridentate ligand systems form respectively two or three coordinated or covalent bonds with the central atom. Dye molecules used as intermediates in synthesising metal complexes have at least two ligand donor groups arranged so that it is geometrically possible to coordinate at two or more positions in the coordination shell of a single metal ion. Examples found throughout this chapter contain from two to six ligand groups bound to the one transition-metal ion. Where two groups originating in the same bidentate molecule combine with the same metal ion, a heteroatomic ring is formed. Ring formation between the metal M and a salicylic acid residue (in a dye structure such as 5.3) containing two donor groups is shown in Scheme 5.1. This process was first described by Morgan and Drew [7] as chelation, which profoundly affects the chemical and physical properties of both the ligand system and the metal atom. Although the formation of a single heterocyclic ring imparts some stability to the complex, this is not generally sufficient to be directly useful in dyeing. Scheme 5.1 is a reversible reaction, so that demetallisation can take place to an extent that depends on the severity of the conditions. The metal chelate may be thermodynamically stable yet kinetically labile. The chromogenic ligand system may be replaced by other electron-pair donors, such as water molecules. This sensitivity to hydrolysis is shown by certain complexes of o-nitrosonaphthols, such as the 1:3 iron(II) complex (5.8) of 1-nitroso-2-naphthol-6-sulphonic acid, although this dye is still of some interest for the dyeing of wool. On the other hand, multidentate ligands form numerous complexes with copper, chromium or cobalt that contain two condensed heteroatomic ring systems. These are much more stable, both thermodynamically and kinetically, undergoing ligand exchange reactions extremely slowly. 5.2.2 Coordination number The number of ligand systems associated with a metal atom in a complex is known as the coordination number, often abbreviated to CN. For transition metals in their lower oxidation states (1+, 2+ or 3+) the CN is usually 4 or 6. The metals used in complex formation with

O

Ar N

N

OH + OH

O

Ar

C M2+

N

N

C O M O

Scheme 5.1

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+

2H+

ELECTRONIC STRUCTURE OF TRANSITION-METAL IONS

235

SO3Na

N N

O

NaO3S

O

O Na+

Fe O

O O N

SO3Na 5.8 CI Acid Green 1

dye molecules fall into this category. In less common complexes the CN can range from 2 to 10. This value depends on the size and geometry of the ligand system relative to the size of the metal ion. The larger the central cation, the greater the number and variety of ligand systems that can be accommodated. The charge on the cation also influences the CN. Metals in a higher oxidation state (such as 6+ or 7+) generally have low CN values, since the removal of six or seven electrons reduces the ionic size and so the accommodation of many ligand systems simultaneously around the ion becomes sterically impossible. 5.3 ELECTRONIC STRUCTURE OF TRANSITION-METAL IONS The structures of metal-complex dyes, which must exhibit a high degree of stability during synthesis and application, is limited to certain elements in the first transition series, notably copper, chromium, iron, cobalt and nickel. The remaining members of the transition series form relatively unstable chelated complexes. The following description of the influence of electronic structure, however, is applicable to all members of the series. The principal characteristic of the transition elements is an incomplete electronic subshell that confers specific properties on the metal concerned. Ligand systems may participate in coordination not only by electron donation to the 3d levels in the first transition series but also by donation to incomplete outer 4s and 4p shells. Figure 5.1 shows that the differences in orbital energy levels between the 4s, 4p and 3d orbitals are much smaller than, for example, the difference between the inner 2s and 2p levels. Consequently, transitions between the 4s, 4p and 3d levels can easily take place and coordination is readily achieved. The manner in which ligand groups are oriented in surrounding the central metal atom is determined by the number and energy levels of the electrons in the incomplete subshells.

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

4p 3d

4s 3p 3s

Increasing energy

2p 2s

1s

s

p

d Levels

Figure 5.1 Relative energy levels up to atomic number 36

The first transition series of elements with atomic number from 21 to 29 (scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper) is built up by the addition of electrons to the outer 3d shell, which can accommodate a total of ten electrons in five subshells of equal energy, each capable of containing two electrons. The influence of the electrical fields originating from the ligand atoms as they approach the electrons of these outermost 3d, 4s and 4p shells is described by ligand field theory. On approach, hybridisation between available ligand electrons and metal electrons takes place as bonds are formed. It is this and the strength of the ligand field that determines the final arrangement of the ligand systems around the central metal atom. For trivalent elements of CN6, such as chromium(III) or cobalt(III), the most stable spatial arrangement of six identical ligand groups is that of an octahedron with four of the groups situated in equatorial quadrants and the other two directly above and below the central metal atom, as shown in Figure 5.2(a). It is possible to obtain a distorted octahedral or tetragonal configuration by an increase in the distance of separation of the ligand groups in the pole positions of the octahedron. In the extreme, when the two pole positions are ligand-free, this leads to a square planar arrangement. For divalent elements of CN4, such as copper(II), there are two possibilities: a square planar distribution on the equatorial plane, as in Figure 5.2(b), or the tetrahedral configuration shown in Figure 5.2(c). In early objections to the electrostatic theory of bonding in metal complexes it was argued that the symmetrical arrangement of four ligands about a spherical central atom was best explained in terms of a tetrahedral structure. It was not appreciated that in fact the d shell electrons of the metal atom are symmetrical, so that stable square planar arrangements are possible.

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237

(b) (c)

(a)

Figure 5.2 Ligand arrangements about a central nucleus

5.3.1 Octahedral arrangement As described above, in each transition-metal atom there are five d orbitals, each of which can accommodate two electrons. These orbitals are arranged spatially in two groups, one acting between the main x-, y- and z-axes of the metal ion and the other acting along these axes. In an octahedral weak ligand field environment, the influence of the six electronegative ligand atoms is to cause the ligand electrons to avoid those regions where the charge density associated with the d electrons is greatest. This charge density distribution is shown in Figure 5.3. Each line of closest approach is therefore along a line of minimum charge repulsion. For the dxy, dxz and dyz orbitals, this is along the main x-, y- and z-axes. The dx2–y2 and dz2 orbitals, on the other hand, have maximum charge densities along these main axes, Hence they conflict with the line of approach of the ligand atoms and are destabilised. The five orbitals are thus split into two groups of different energy: an upper doublet referred to as eg and a lower triplet level known as t2g. The distribution of the d electrons between these various orbitals is determined by the differences in energy level between them. The energy difference is directly proportional to the electric field strength of the ligand systems taking part in complex formation. Since the transition-metal ions do not exist in isolation, they must be evaluated in association with counter-ions or solvents such as water or other ligand systems present. In isolation the electron distribution in the trivalent chromium(III) ion consists of three unpaired electrons in the d shell, as indicated in line (a) of Table 5.1. In line (b) the six electron pairs donated to the central chromium atom by oxygen atoms of water molecules give rise to sp3d2 hybridisation. This is characteristic of an octahedral structure. A similar situation arises with the trivalent cobalt(III) complex in line (e), where each of the three t2g levels is doubly occupied by an electron pair from each cyano ligand. With the divalent chromium(II) ion there are two distinct possibilities, shown in lines (c) and (d). With such an isolated ion having four electrons in the d shell the fourth electron may occupy either an eg level (c) or a t2g level (d), depending on whether the energy required to cause spin pairing is less or greater than the difference between these energy

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

x

x

y

x

y

y

z

z

z

dxy

dxz

dyz

x

x

y

y z dx2–y2

z d z2

Figure 5.3 Atomic d orbitals in transition elements

levels. These alternatives result in a high-spin (c) or low-spin (d) complex respectively, the field strength of the ligand systems determining which is actually formed. The unpaired electrons in the chromium(II) ion are unstable, so divalent chromium cannot be utilised in preparing dye complexes. Invariably a d2 electron is lost and oxidation to chromium(III) ensues, with concurrent reduction of the ligand chromogenic system [8]. The oxidation state and electron distribution in the 3d shell at least partly accounts for the fact that most metal-complex dyes of value for wool or nylon dyeing are based on chromium(III) or cobalt(III) ions. The presence of unpaired electrons in a complex can be detected by the study of its magnetic properties. A single spinning electron has an associated magnetic moment. When an organometallic complex is weighed in a magnetic field any unpaired electrons contribute to an apparent increase in mass compared with that observed in the absence of the field. The magnitude of attraction of the complex to a magnet is a measure of the content of unpaired electrons. Such a complex is described as paramagnetic. Where there is no detectable difference in mass, the complex contains no unpaired electrons and is said to be diamagnetic. Thus these results are of value in determining the electronic distribution around the central metal atom. 5.3.2 Tetrahedral arrangement Depending on the ligand field strength and the number of ligand systems that can be accommodated, hybridisations other than octahedral are possible. The tetracyanocuprate ion in line (f) of Table 5.1 has a tetrahedral configuration arising from sp3 hybridisation, as

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239

Table 5.1 Electron distribution in some transition-metal ions (1s, 2s, 2p, 3s and 3p shells are complete in all cases) Ion

(a)

3d

4s

4p

Cr3+

sp 3d2

(b) Cr(H 2O)63+

(c)

Cr2+

high spin

(d) Cr2+

low spin

(e)

Co(CN)63–

sp 3d2

(f)

Cu(CN)43–

sp 3

(g) Cu(NH3)2+

sp 2d

denotes electron-pair donation to metal ion

in the case of bonding to a saturated carbon atom. The copper(I) ion has a completely filled d shell and electron-pair donation from the four cyano ligands occurs in the 4s and 4p levels. In a tetrahedral arrangement, the eg orbitals are more stable than the t2g orbitals because they are situated at the maximum distance from the influence of the four ligand systems. Conversely, the t2g orbitals become destabilised. The ligands are best visualised as occupying alternate corners of a cube, the main axes of which represent those of the metal ion, as shown in Figure 5.2(c). The energy difference between eg and t2g orbitals in a tetrahedral structure is approximately half that for the octahedron. This effect favours formation of an octahedral rather than a tetrahedral configuration. 5.3.3 Square planar arrangement The square planar configuration illustrated in Figure 5.2(b) can be considered as an octahedral structure in which any opposing pair of ligands on a common axis are completely removed from the sphere of influence of the central metal atom. If two such ligands are moved outwards only slightly, the result is a distorted octagonal or tetragonal structure.

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

5.4 STRUCTURAL CHARACTERISTICS NECESSARY FOR COMPLEX FORMATION 5.4.1 Bidentate ligand systems In a bidentate ligand system, three molecules of a dye containing either a terminal salicylic acid unit (as in 5.2) or an o-nitrosonaphthol residue are able to chelate simultaneously with a trivalent metal ion of CN6, such as chromium(III) or iron(III), to form a 1:3 metal–dye complex (as in 5.8). Historically, the most important bidentate ligand system was alizarin (5.1). It has been suggested that both hydroxy groups and the keto group in the peri position are all involved with the metal atom in the chelation mechanism. Most metal-complex dyes of commercial significance belong to the azo class. In practice, tridentate ligand systems incorporating the azo group and both o,o′-substituents are necessary to achieve adequate stability and fastness. Each nitrogen atom in an azo (–N=N–) or azomethine (–CH=N–) grouping carries a lone electron pair that can participate in coordination. There is no evidence, however, that the azo group in unsubstituted azobenzene, even the cis isomer, can yield a stable complex with any first-series transition metal. On the other hand, an o-aminoazo or o-hydroxyazo grouping can act as a bidentate ligand system with the azo group participating in coordination. Such 1:2 complexes containing divalent cobalt, nickel or copper were synthesised from o-substituted azobenzenes (5.9; M = Co, Ni or Cu, X = O or NH) and the analogous azomethine derivatives (5.10), demonstrating that the βnitrogen of the azo linkage normally acts as the donor atom [9].

N

CH N

X

N X

M

M

X N

X N

N

5.9

CH

5.10

In general, metal complexes formed from bidentate azo chromogens are little used as dyes but do find important applications as pigments (section 2.3.2). Rare exceptions exist, however, such as the nickel(II) complex of p-nitroaniline→BON acid (5.11). This has been used for bordeaux prints of high light fastness on cotton fabrics. Two possible modes of bidentate attachment to the nickel atom can be envisaged (Scheme 5.2). Metallisation of terminal coupling components Salicylic acid has been found useful as a terminal coupling component in direct or mordant dyes to provide a bidentate site for metallisation. The free acid is also capable of complexing and ammonium chromisalicylate (5.12) is used as an intermediate in the synthesis of metal– dye complexes. CI Mordant Orange 1 (5.3) and Yellow 5 (5.13) are examples of dyes

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STRUCTURAL CHARACTERISTICS NECESSARY FOR COMPLEX FORMATION

HO O2N

COOH

H

N

O2N

N

O

COOH

N N

5.11 Ni (II)

Ni (II)

H2O

Ni O2N

OH2

H2O

OH2

Ni

O

COOH

O

H

N

O2N

N

O C

O

N N

Scheme 5.2

_ NH3 O O

C + NH4.3H2O

Cr C O

O

HOOC

O HO

N N

NH3

COOH 5.13

5.12

CI Mordant Yellow 5

Ammonium chromisalicylate

HO HOOC SO2 H2N N N NaO3S HO 5.14

SO3Na

CI Mordant Red 26

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O

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

OSO3Na

N +

N

NaO3SO

5.15

5.15

Cu (II) 2Na+

O

2 Na2SO3 +

N

Cu N

O

Scheme 5.3

containing terminal salicylic acid residues. Since both ligands in these residues are electronically associated with the azo chromogen, changes in hue on metallisation can be dramatic. Difficulties in shade matching are particularly evident when afterchroming such dyeings. This effect may be minimised by introducing an insulating group, such as methylene, sulphonyl or sulphonamido, between the salicylic acid residue and the chromogen. CI Mordant Red 26 (5.14) is a typical example. Another terminal bidentate ligand that has been exploited occasionally in bright disazo direct dyes is the sulphated 8-hydroxyquinoline residue (5.15). On aftercoppering, fastness to light and wet tests is enhanced by hydrolysis catalysed by the copper(II) ion and formation of a bidentate 1:2 complex (Scheme 5.3). Apparently, electron withdrawal by sulphur facilitates removal of the sulphite grouping and approach of the copper(II) cation [10]. The o-nitrosonaphthol ligand system also belongs to the terminal bidentate category (as in structure 5.8). If the sulpho group in each ligand is replaced by a nonionised polar solubilising group, such as methylsulphonyl (as in 5.4) or sulphonamido (as in 5.6), the overall negative charge on the 1:3 metal–dye complex is reduced from -4 to -1, so that dyeing can take place from neutral or weakly acidic dyebaths. Similarly, green to olive hues can be obtained on wool or nylon using 1:3 iron(II) complexes of 6-hydroxy-7-nitrosoindazoles. The amphoteric nature of the three heterocyclic rings in such complexes accounts for their solubility. Protonation of the N atom in each indazole ring (Scheme 5.4) depends on the pH, so that the charge on the complex can vary from -1 for the unprotonated form (5.16) to +2 for the fully protonated form (5.17). The use of iron(III) salts in their synthesis initiates partial oxidation of the hydroxynitroso-indazole with consequent reduction to iron(II), which then interacts to form the 1:3 metal–dye complex [11]. 5.4.2 Tridentate ligand systems Many of the premetallised 1:1 chromium-dye complexes introduced from 1915 onwards for the one-bath dyeing of wool at low pH were o,o′-dihydroxyazo compounds containing chelated chromium(III) ions. Typical examples still in use today include CI Acid Orange 74 (5.18) and other azopyrazolones ranging in hue from yellow to bluish red, as well as

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STRUCTURAL CHARACTERISTICS NECESSARY FOR COMPLEX FORMATION – N

NH

2+ HN

N N

O

NH

NH

NH

O

N

O

NH

O

N

N

O

Fe

O

O

Fe

O

+ _ 3HO 3H

O

O

243

N

O

O N

HN

HN

5.16

N

5.17

HN

Scheme 5.4 OH2

H2O NaO3S

O

OH2

Cr

O

OH2

H2O NaO3S

O

OH2

Cr

O

N

N N

N

N

N

O2N H3C

C

CH3

C

NH

C

O2N

O 5.19

5.18 CI Acid Orange 74

CI Acid Yellow 99

azoacetoacetanilides such as CI Acid Yellow 99 (5.19). For some time after their introduction these dyes were formulated as bidentate systems, utilising only one of the hydroxy groups in conjunction with the azo group. This misconception may have arisen by analogy with true bidentate ligands such as salicylate complexes [7] and the o-substituted azo and azomethine analogues [9]. Only in 1939 were these 1:1 chromium complexes of o,o′-dihydroxyazo dyes unequivocally confirmed to be tridentate structures, in which both hydroxy groups and the azo linkage are coordinated with the metal atom. The remaining three sites are taken up by unidentate ligands such as water molecules [12]. The tridentate azo systems have proved to be the most important of all possible multidentate chromogenic ligand systems available for metal-complex formation. Those capable of coordination with metals of CN6 (trivalent chromium or cobalt) or CN4 (divalent copper) are mainly limited to o-hydroxy-o′-substituted diaryl azo or azomethine derivatives (5.20; X = OH, NH2, or COOH, Y = N or CH). The more important dyes for wool or nylon are chromium complexes in which X is usually a hydroxy group but may be an amino, carboxyl or other ligand group from which a proton is lost during complex formation. Coordination of these systems leads to a bicyclic ring system with the N→M bond shared. If the o-substituent X is hydroxy or amino that ring is five-membered (5.21), but if it is carboxyl both annelated rings are six-membered (5.22). This difference has an important bearing on the physical properties of these complexes (section 5.6.1). Annelated-ring formation enhances the stability of the resultant metal–dye complex to an extent that is fully adequate for conventional dyeing conditions. Single-ring bidentate systems, on the other hand, seldom exhibit this degree of stability.

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

O

X

HN HO

C

M O

5

Y

N

O 6 N

N

5.20

M

N

6

N

O 6

5.22

5.21

1:1 Metal–dye complexes As already described, these are tridentate structures in which the three remaining sites on the chromium(III) ion are occupied by colourless monodentate ligands (as in 5.18). These are quite stable at low pH values, whereas similar cobalt complexes are generally unstable. However, 1:1 cobalt-dye complexes have been obtained by heating a tridentate dye with a cobalt(II) salt in the presence of excess ammonia, the monodentate ligand groups being three ammonia molecules. The reaction involved oxidation of the cobalt(II) ion to cobalt(III) prior to complex formation [13]. Even so, the tesulting complexes were insufficiently stable for use as dyes. Stable 1:1 cobalt-dye complexes suitable for dyeing can be prepared, however, by replacing the colourless monodentate ligands by a tridentate molecule such as diethylenetriamine (as in 5.23). By varying the number of sulpho groups in the chromogenic ligand from zero to three, the net charge on the complex varies from +1 to –2. _ H2C

CH2

H2N O

NH Co

CH2 CH2 NH2

O K

+

N N SO3

O2N O3S 5.23

The presence of sulphonic acid groups in 1:1 chromium or cobalt complexes enhances solubility in water and permits electrostatic bonding to protonated amino groups in wool (section 3.2.2). Strongly acidic conditions are necessary to achieve high substantivity and some degradation of the wool is unavoidable. The relatively low Mr (400–500) of these 1:1 premetallised dyes ensures good level dyeing properties under these conditions. The presence of ionised sulpho groups in these complexes can give rise to anomalous conclusions when trying to elucidate structure from elemental analysis data, since simple metal salts may also be formed between transition-metal cations and these anionic sulpho groups. 1:2 Metal–dye complexes The unfilled sites in 1:1 metal–dye complexes of chromium(III) or cobalt(III) ions can be

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STRUCTURAL CHARACTERISTICS NECESSARY FOR COMPLEX FORMATION

245

occupied by colourless ligands (three mono-, mono-/bi- or one tridentate). Such a triad of sites can generally be replaced by a second tridentate chromogen, as shown by Drew and Fairbairn [12]. This results in a more stable configuration consisting of two bicyclic ring systems as in the general formula 5.24 (M = Co or Cr, X = N or CH). Colour, solubility, dyeing and fastness properties can be controlled by selection from a wide range of sulphonated or unsulphonated azo or azomethine tridentate ligand systems. X 6

N 5

O

O

M

O

O

5 6

N

H2N

CH3

SO2

X

N

O N

X

C C

H H3C

N N

N

5.24

O

N N

C

O

Co

O O

O O

Co

O

N N

O C

N N

CH3 H

C C

X

N

H

O2S

N

NH2 H3C

C O

O

5.26 CI Acid Brown 240

5.25

Symmetrical 1:2 cobalt complexes of phenylazoacetoacetanilides, such as CI Acid Yellow 119 (5.25; X = NO2) or Yellow 151 (5.25; X = SO2NH2), are important in the yellow to orange sector. Many orange and red dyes of this class are symmetrical 1:2 chromium complexes of phenylazopyrazolones, like CI Acid Orange 60 (5.6; R = H). Browns are often unsymmetrical chromium or cobalt types with azopyrazolone and azonaphthol ligands, CI Acid Brown 240 (5.26) being a good example. Dyes in the violet, blue, green and black sectors may be symmetrical or unsymmetrical but often contain two arylazonaphthol ligands, as in C.I.Acid Black 58 (5.4). The reaction of two moles of a tridentate chromogen (A) with one equivalent of the metal M leads exclusively to the symmetrical AMA structure. Reaction with two different chromogenic ligands A and B simultaneously yields a mixture of structures: AMA, AMB and BMB. Such mixtures are commercially important, as they are less expensive to produce than the specific unsymmetrical product AMB, which can be obtained in a pure form only by the reaction of a pure 1:1 complex (AM) with the other ligand (B). Monosulphonated unsymmetrical chromium complexes with two negative charges per molecule are particularly important because they have good neutral-dyeing affinity for wool or nylon.

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

2 Ar

_ + N N Cl

+

Ar

CH3NO2

N

N

CH

N

N

Ar

NO2 Scheme 5.5

C

R N

N

N

N

Ar

Ar

N

Ar

N

N

N

N

O

M

N

N

O

N

N

N C

C

N

N

N

Cu

M Ar N

N

O

C

N

N C R

5.28

5.27

5.29

Tridentate formazans Formazans may be synthesised by the selective coupling under alkaline conditions of two identical or different diazonium cations with an activated methylene group in a suitable coupling component such as acetone or nitromethane, as shown in Scheme 5.5. By coordination with both of the two azo groupings, divalent metals such as copper, nickel or cobalt will give monocyclic 1:2 metal complexes (5.27) of relatively low stability [14]. Formazans containing an o-hydroxy or o-carboxy substituent in one of the phenylazo groupings can be used to give intensely coloured neutral bicyclic 1:1 metal–dye complexes [15]. Thus the red tridentate ligand shown becomes violet on metallisation with copper(II) ions. On completing the CN4 by coordination with an aryl monodentate ligand such as pyridine (5.28), a further bathochromic shift to reddish blue is observed. With trivalent metals of CN6, tridentate formazans give 1:2 complexes with two bicyclic ring systems (5.29). 5.4.3 Quadridentate ligand systems Quadridentate formazans If both of the phenylazo groupings in the formazan chromogen contain an o-hydroxy or ocarboxy substituent, coordination with a divalent metal atom such as copper yields a highly stable annelated tricyclic ring system (5.30). Only a 1:1 metal:dye ratio is possible in this case. For trivalent metals of CN6 the coordination sphere is completed using a colourless ligand molecule such as pyridine, ammonia or water. Solubility and dyeing properties may be varied by the introduction of a sulpho group or an uncharged solubilising group such as sulphonamido. In a recent evaluation of a series of quadridentate formazan complexes with

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247

copper(II) of the 5.30 type, the colours of the products obtained from a given formazan were found to vary according to the method of synthesis [16]. Similar blue formazan–copper complexes that are sufficiently solubilised by the introduction of sulphonic acid substituents in the phenyl rings have become established as important members of most ranges of reactive dyes. In general, they are redder than the turquoise blue copper phthalocyanines, less costly than the anthraquinone blues and exhibit higher fastness to light than the twice-coupled H acid blues. Many of them have the general structure represented by 5.31. Those that react with cellulose by nucleophilic addition mostly have the substituents X = vinylaulphone or precursor, Y = sulpho, whereas those reacting by a nucleophilic substitution mechanism have X = sulpho, Y = NH–Z, where Z is the haloheterocyclic reactive system.

O

5

M 6

N

N

6

N

O

C

O

N C

5.30

O

Y

O

O

C

SO3Na

Cu X

N

N

N

N C

5.31

Phthalocyanine metal-complex colorants Copper phthalocyanine is another important representative of a quadridentate ligand system. It is predominant as a turquoise blue pigment of outstanding light fastness and its halogenated derivatives are important green pigments (section 2.4). The copper phthalocyanine chromogen is a tetracyclic annelated ring system of exceptionally high kinetic and thermal stability. It can be sublimed without decomposition in vacuo at temperatures as high as 550 °C, although the solid form may undergo polymorphic change at 250 °C under atmospheric conditions. Many other transition metals and rare-earth elements are able to form 1:1 complexes with phthalocyanine but the brilliant colour and outstanding stability of the copper complex ensure its predominance. A series of 3- and 4-hydroxylated phthalocyanine complexes of tri- or tetravalent metals (A1, Cr, Ge, In and Sn) has been prepared recently. The stabilities of these complexes to thermal oxidation in air have been compared and the effects of the position of the hydroxy group on the electronic spectra in various solvents were studied [17]. Copper phthalocyanine derivatives are well established as turquoise blue direct and reactive dyes for cellulosic fibres. Chlorosulphonation at the 3-position, followed by hydrolysis, yields sulphonated direct dyes such as CI Direct Blue 86 (5.32; X = H) and Blue 87 (5.32; X = SO3Na). Solubility and dyeing properties can be varied by introducing four chlorosulphonyl groups, some of which are hydrolysed and some converted to sulphonamide by reaction with ammonia or alkylamines. This approach is also the main route to reactive dyes of the copper phthalocyanine type. The reactive system Z is linked to a 3-sulphonyl site

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

by means of an aryldiamine, as in structure 5.33 where Y = H or SO3Na, and the dyeing characteristics varied by the nature of X, which may be NH2 or ONa. Nickel phthalocyanine is the basis of some brilliant green reactive dyes, into which a similar range of 4-sulphonyl substituents can be introduced. HN

Y

SO2

NaO3S

HN

X

N

SO2

N

N

N N

Cu

Z

N N

N

N

N N

N

X

Cu

N

N N

SO3Na

N

O2S

X

O2S X 5.33

5.32

In an interesting study, phthalocyanine complexes containing four anthraquinone nuclei (5.34) were synthesised and evaluated as potential vat dyes and pigments [18]. Anthraquinone-1,2-dicarbonitrile or the corresponding dicarboxylic anhydride was reacted with a transition-metal salt, namely vanadium, chromium, iron, cobalt, nickel, copper, tin, platinum or lead (Scheme 5.6). Substituted analogues were also prepared from amino, chloro or nitro derivatives of anthraquinone-1,2-dicarboxylic anhydride. 5.5 PREPARATION OF METAL-COMPLEX COLORANTS There are numerous ways in which a metal atom can be incorporated into a dye molecule to form a metal complex. Most attention has been given to complexes of chromium, cobalt and copper. There are essential differences in the conditions of preparation of these compounds. 5.5.1 Chromium complexes Table 5.1 in section 5.3.1 reveals that the trivalent hexa-aquo chromium complex cation possesses three low-energy t2g orbitals, each singly occupied. Consequently, the ligand field stabilisation energy is considerable. Since this energy has to be overcome, replacement of the coordinated water molecules by other ligand groups is a slow process. The most successful practical methods are designed to take this into account. The replacement reactions may be represented generally by a two-stage process. Introduction of the first tridentate monoazo dye ligand D2– is the rate-determining step (Scheme 5.7). Where a typical o,o′-dihydroxyazo dye ligand is used in excess with a hydrated chromium(III) salt under alkaline conditions (pH >9) favouring 1:2 metal–dye complex formation, none of the 1:1 complex remains. This indicates that the presence of one

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249

O O

O

CN

C

O C

CN 4

or

O +

4

Metal (M) salt

O

O

O

O

O N

N N

O N

M

N O

N N

N O

O

O 5.34

Scheme 5.6

D2–

+

slow

[Cr(H2O)6]3+

[D.Cr(H2O)3]+

+

3H2O

Scheme 5.7

[D.Cr(H2O)3]+

+

D2–

rapid

[D.Cr.D]–

+

3H2O

HCl

Scheme 5.8

chelated dye ligand favours attachment of the other and the forward reaction in Scheme 5.8 is more rapid than that in Scheme 5.7. In order to obtain a 1:1 complex in high yield, therefore, it is necessary first to synthesise the 1:2 complex in alkaline medium and then to treat this product under strongly acidic conditions. The reverse reaction in Scheme 5.8 then yields the desired 1:1 complex. The chromium salt used can be the acetate, chloride or sulphate, the reactions being carried out in aqueous media at the boil. Various chromium-complex dyes were prepared recently by reacting ammonium chromium sulphate with a series of chelatable o,o′-dihydroxyazopyridone structures (Scheme 5.9). Elemental analyses corresponded to a 1:2 metal–dye ligand ratio (5.35). The

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES O

OH H 2

O

H2N

R

C

CH3

N

N N

O

+

[Cr(NH3)6]2 [SO4]3

O

N N

N C

H3C O

NH2

R

O O

Cr

O

O

R N

N N

Scheme 5.9

5.35

O H3C

C

NH2

O

substituted azopyridone chromogens are more stable in the ketohydrazone form. The dyeability and fastness properties of these unmetallised yellow mordant dyes and their derived chromium complexes were evaluated for the dyeing of wool and polyester/wool blends [l9]. If the metallisable dye is insoluble in water, a miscible solvent such as ethanol or ethylene glycol may be added. Polar solvents such as formamide or molten urea have sometimes been preferred. It is likely that such solvents will preferentially displace water molecules and coordinate with the chromium(III) ion as the first step in the reaction. If colourless organic chelates of chromium, such as those derived from oxalic or tartaric acid, are used instead of or in addition to hydrated chromium(III) salts, the difficulty of replacing the strongly coordinated water molecules in the first stage of the reaction is eliminated. In this way the initial reaction can be carried out at high pH without contamination by the precipitation of chromium hydroxide. Use of the complex ammonium chromisalicylate (5.12) in this connection should also be noted (section 5.4.1). 5.5.2 Cobalt complexes Like trivalent chromium the hexa-aquo cobalt(III) cation contains three low-energy t2g orbitals, this time each doubly occupied. In general, trivalent cobalt complexes are easier to prepare than their chromium analogues and often exhibit significantly higher fastness to light. Chromium complexes normally absorb at longer wavelengths than the corresponding cobalt compounds, however. This bathochromic shift is evident from comparisons between, for example, either CI Acid Violet 128 (5.36; M = Co, X = NO2) and Blue 335 (5.36; M = Cr, X = H), or Brown 403 (5.37; M = Co) and Black 172 (5.37; M = Cr). Direct use of cobalt(III) salts in the synthesis of metal–dye complexes is generally avoided because the cobalt(III) ion is strongly oxidising and slow to react with dye ligands. Most 1:2 cobalt-dye complexes are prepared by the reaction between the chelatable dye and a cobalt(II) salt at a relatively high pH. The dominant product is invariably the diamagnetic cobalt(III) complex, oxidation of the cobalt(II) ion taking place by reduction of some of the azo dye molecules present. This can be avoided, however, by adding a less stable oxidant. Unlike the corresponding 1:1 chromium complexes, the stability of 1:1 cobalt-dye complexes is inadequate for them to be obtained in acceptable yield at low pH. However, advantage can be taken of the strong affinity for cobalt ions of ammonia or amines as

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251

NO2

Cl N

N

N

O

O

O

M

O

M

O

O NaO3S

SO3Na

N

O

O

N

NaO3S

N

N N

X

O2N

5.37

5.36

electron donors. Reaction of the cobalt(II) ion with a tridentate dye molecule in an excess of aqueous ammonia yields a 1:1 complex in which the coordination sphere is completed by three ammonia molecules. This again is diamagnetic, indicating the absence of unpaired electrons necessary to achieve an sp3d 2 octahedral structure. Although 1:1 cobalt-dye complexes have little significance as potential premetallised acid dyes for wool or nylon, they are widely used as intermediates in the synthesis of unsymmetrical 1:2 cobalt-dye complexes, especially when the asymmetry arises from the presence of a sulphonic acid group in one of the chelatable dye ligands. The dyeing and fastness properties of chromium and cobalt symmetrical 1:2 complexes (5.38) of the mordant dye 2-amino-4-methylphenol→2-naphthol were compared recently on nylon 6 and wool fabrics [20]. The dyebath exhaustion and fastness performance were consistently higher on nylon than on wool. The chromium-complex dyeings were slightly more bathochromic, especially on wool, than those of the cobalt complex but the latter showed marginally higher fastness to washing on wool. Fastness to wet rubbing on wool favoured the chromium complex, however (Table 5.2).

CH3 N N O O

M

O

O N N H3C 5.38

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

Table 5.2 Dyeing and fastness properties of 1:2 metal-complex dyes on nylon 6 and wool [20] Fastness properties Exhaustion (%) Dye Complex 5.38

Nylon 6 (80 °C)

M = Cr M = Co M = Cr M = Co

Washing

Rubbing

Wool (90 °C)

λmax (nm) on Fibre

Light (C arc)

E

C

W

Dry

Wet

88 88

569 565 572 551

>5 >5 5 5

5 5 3–4 4

5 5 4 5

5 5 4 4–5

5 5 4–5 4–5

5 5 4 3

93 93

E Effect on pattern C Cotton stain W Wool stain

Chromium and cobalt complexes of azoic dyes Complexes of chromium(III) and cobalt(III) ions with insoluble azoic dyes, such as that (5.39) prepared by coupling Naphtol AS (CI Azoic Coupling Component 2) with diazotised picramic acid (Scheme 5.10), were synthesised recently for use as negative charge control agents in photocopying [21–23]. The aggregation behaviour and tautomerism of structure 5.39 as the free acid (X = H) and the ionised phenolate (X = Na) were studied by UV/visible spectrophotometry in polar solvents. Monomer–dimer equilibria were detected in dimethylformamide and in dimethylsulphoxide. Analysis of the deconvoluted absorption bands attributed the spectral changes to a distribution in favour of dimerised aggregates in the ketohydrazone form, with the monomeric species being more stable in the azo form (5.39). The coupling of Naphtol AS or its phenyl-substituted derivatives with diazonium salts from variously substituted anilines in aqueous alkaline solution (section 4.11) gave incomplete reactions and impure products in some instances, probably because these coupling components have inadequate solubility in aqueous media. Pure dyes in ca. 90% yields were obtained by reaction in dimethylformamide in the presence of sodium acetate. Metallisation of these o,o′-dihydroxyazo ligands with sodium chromium salicylate or a cobalt(II) salt gave metal-complex dyes in 80–100% yields [22]. Specific structural isomers of these complexes were identified by i.r., n.m.r., Raman and UV/visible spectroscopy [23].

O O2N

O

X

HO

_ + N N Cl +

C

N

O2N

O

O

X HO

H

O2N 5.39

Scheme 5.10

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N H

N N

O2N

C

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PREPARATION OF METAL-COMPLEX COLORANTS

253

Mixed chromium/cobalt complexes in reactive dyes An important subclass of black reactive dyes for the dyeing of cellulosic fibres consists of symmetrical 1:2 metal–dye complexes in which the metal content is a deliberate mixture of chromium(III) and cobalt(III) ions, usually rich in the chromium component. The two most important structural types are the nitrophenylazo H acid complex 5.40 and the nitronaphthylazo J acid complex 5.41. In both cases, each reactive system Z is linked to the NH site in each coupling component. SO3Na NO2 N N

NaO3S O NH

O Z

M

Z O

O

HN

NO2

SO3Na

N N

SO3Na

O2N NaO3S 5.40

N Z

N

NH

SO3Na

O O

M

O

O NH

N

NaO3S

Z

N NaO3S O2N 5.41

5.5.3 Copper complexes The only copper complexes of tridentate azo compounds are 1:1 structures, since copper(II) has a CN of 4. They can be prepared by the reaction of the azo compound with a copper(II) salt in an aqueous medium at 60 °C. The major application for copper-complex azo dyes is as direct or reactive dyes for the dyeing of cellulosic fibres. They are seldom developed for use on wool or nylon, although various orange and red 1:1 copper-complex azopyrazolones (5.42) were synthesised recently and evaluated on these fibres by application from a weakly acidic dyebath [24]. Copper-complex azo direct dyes Almost all of these depend on the availability of two o,o′-dihydroxyazo ligands in the unmetallised direct dye molecule, although these may be present as the o-methoxy-o′hydroxyazo precursor system. In the manufacture of these dyes it is often easier to synthesise

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

OH2 O

Cu O N

N

N

N

H

N

SO2

H3C 5.42

the latter grouping because the o-methoxyarylamines are diazotised and coupled more efficiently than the corresponding o-hydroxyarylamines. Metallisation of the chromogen can be achieved using cuprammonium sulphate in the presence of an alkanolamine such as diethanolamine. During this reaction the methoxy groups present become demethylated. It is probable that coordination occurs between the copper ion and the electron-rich oxygen of the methoxy group. Simultaneously or subsequently the methyl group is lost as a carbonium ion that reacts immediately with hydroxide ion to form methanol, as in Scheme 5.11. Removal of the methyl group is probably assisted by coordination of the methoxy oxygen. OH2

H3C O

OH2

Cu

O

O

Cu O

_

N

HO N

N N

+

CH3OH

Scheme 5.11

Many of the premetallised direct dyes are symmetrical structures in the form of bis-1:1 complexes with two copper(II) ions per disazo dye molecule. Scheme 5.12 illustrates conversion of the important unmetallised royal blue CI Direct Blue 15 (5.43), derived from tetrazotised dianisidine coupled with two moles of H acid, to its much greener coppercomplex Blue 218 (5.44) with demethylation of the methoxy groups as described above. Important symmetrical red disazo structures of high light fastness, such as CI Direct Red 83 (5.45), contain two J acid residues linked via their imino groups. Unsymmetrical disazo blues derived from dianisidine often contain a J acid residue as one ligand and a different coupler as the other, such as Oxy Koch acid in CI Direct Blue 77 (5.46), for example. Copper-complex azo reactive dyes In contrast to direct dyes, metal-complex azo reactive dyes are almost always monoazo chromogens coordinated to one copper(II) ion per molecule. The important structural types include phenylazo J acid reds (5.47), phenylazo H acid violets (5.48) and naphthylazo H acid blues (5.49), where Z represents the reactive system attached through the imino group in the coupling component. Less often the reactive system is located on the diazo component, as in CI Reactive Violet 5 (5.50) and analogous red to blue members of various ranges.

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PREPARATION OF METAL-COMPLEX COLORANTS

H3C

CH3 NH2

H2N

O

O O

H

NaO3S

255

O

H N

SO3Na

N

N

N

SO3Na

NaO3S

5.43 CI Direct Blue 15

_ 2 Cu(II) + 2 HO

2 CH3OH H2O

OH2 +

NH2

Cu

O

H2N

Cu

O

O

O

NaO3S

N

SO3Na

N

N

N

SO3Na

NaO3S

5.44 CI Direct Blue 218

Scheme 5.12

OH2 O

H2O O

Cu O

Cu

C N

N N

O

O N

N N

H H

NaO3S

SO3Na NaO3S

SO3Na

5.45 CI Direct Red 83

H2O

NaO SO2

OH2 Cu

O

O

Cu

O

O

NaO3S

N

SO3Na

NH

N

N

N

5.46

NaO3S

CI Direct Blue 77

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

OH2 O

Cu OH2

O NH

N

Z

Z O

N

HN

Cu O

NaO3S

SO3Na

N

NaO3S

N 5.47

NaO3S NaO3S

NaO

OH2

5.48

Z

SO2 O

HN

Cu O

SO3Na

N

OH2

N

COCH3

NaO3S

O

Cu

NaO3S

O NaO3SOCH2CH2SO2

5.49

HN SO3Na

N N

5.50

NaO3S

CI Reactive Violet 5

Oxidative coppering of monohydroxyazo ligands This is an alternative method of introducing copper into an o-hydroxyazo dye structure. The azo compound is treated with a copper(II) salt and an oxidant in an aqueous medium at 40– 70 °C and pH 4.5–7.0. Sodium peroxide, sodium perborate, hydrogen peroxide or other salts of peroxy acids may be used as oxidants, the function of which is to introduce a second hydroxy group in the o′-position [25]. This process is reminiscent of earlier work on CI Acid Red 14 (5.51; X = H), an o-hydroxyazo dye that will not react with a chromium(III) salt to form a 1:1 complex but will do so by oxidation with an acidified dichromate solution. This oxidation product was later found to be identical with that obtained by conventional reaction of CI Mordant Black 3 (5.51; X = OH) with a chromium(III) salt [7]. Replacement of labile o-halogeno substituents The halogen atom in an o-halogeno-o′-hydroxyazo compound may be replaced by a hydroxy group under mildly alkaline conditions, provided that the halogeno substituent is activated by the presence of electron-withdrawing groups (acetyl, cyano, nitro) in the o- and/or ppositions [26]. The mechanism is believed to involve formation of an intermediate complex (5.52; R = electron-withdrawing substituent) of low stability in which chlorine is coordinated with the copper atom [27]. This facilitates attack by hydroxide ion at the

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PREPARATION OF METAL-COMPLEX COLORANTS

257

X H NaO3S

O

N N SO3Na 5.51 OH2

R

R

Cl

Cl

Cu O

HO Cu(II)

N

N N

N R

R

5.52 _

OH2 R

O

HO

Cu O N N

R Scheme 5.13

carbon atom to which the chlorine is attached, leading to the formation of a conventional bicyclic 1:1 complex, as in Scheme 5.13. 5.5.4 Iron complexes With the exception of the 1:3 iron complexes of bidentate ligand systems, for example the onitrosonaphthols (such as 5.8 in section 5.2.1), there has been little interest shown in iron(III) complexes of azo dye ligands as textile dyes until quite recently. Difficulties arising from the presence of chromium residues in effluents from factories involved in the manufacture or use of premetallised dyes have stimulated research on complexes of other transition metals, notably iron(III) which is unlikely to give rise to significant effluent problems because of a much higher permitted level in effluent than that for chromium. Certain black iron-complex dyes are already claimed to be strong candidates for use on wool or nylon in carpets, furnishings and automotive fabrics where high fastness to light is essential [28]. The same naphthylazo-2-naphthol ligand grouping is present in the 1:1 complex CI Acid Black 52 (5.53; M = Cr) and its symmetrical 1:2 analogue Black 172 (5.54; M = Cr), which are both widely used in the dyeing of wool. The corresponding 1:1 and 1:2 complexes of trivalent iron (M = Fe) have been synthesised and their properties compared with the

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

existing chromium blacks. Excellent light fastness was found but shade differences were observed that would prevent direct replacement in existing recipe formulations [29]. In a further evaluation of this kind, the symmetrical 1:2 iron complexes (5.55; M = Fe) analogous to CI Acid Violet 83 (5.55; M = Cr, X = H) and Black 99 (5.55; M = Cr, X = NHCOCH3) were synthesised by reacting the metal-free ligands with iron(III) sulphate. Black dyeings on wool and nylon based on these iron complexes exhibited ratings of fastness to light and washing that were similar to those of control dyeings using the conventional chromium-complex dyes [30]. This work has been extended to the synthesis and evaluation [31,32] of various 1:2 iron(III) complexes of tridentate formazan ligands containing two bicyclic ring systems (as 5.29 in section 5.4.2). In an interesting study of the photodegradation of dyes of this kind, their light fastness ratings on nylon and their rates of photofading in dimethylformamide solution were determined. The light fastness of the three unmetallised formazans (X = H, Cl or NO2) was improved by metallisation, the 1:2 cobalt(III) complexes (5.56; M = Co) NO2 OH2 H2O O

M

OH2

N

O NaO3S

SO3Na

N

N

O N

O

M

O

O NaO3S

O2N

N N 5.54

5.53

O2N

H2N

X

SO2 N

C

N O

H2NO2S

N

N

N

X

O

M

O

N

O

O

M

O

N N

X

N

O2S

N

NH2

X

N C

5.55

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5.56

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SO2NH2

259

PREPARATION OF METAL-COMPLEX COLORANTS

giving consistently higher ratings than the 1:2 iron(III) analogues (5.56; M = Fe). These cobalt formazans, unlike their iron counterparts, were found to be effective quenchers of singlet oxygen. The presence of an electron-withdrawing substituent in the o-hydroxyphenylazo grouping also enhanced photostability (H < Cl < NO2), the highest light fastness being shown by the nitro-substituted cobalt complex [33]. More recently, attention has turned to the aftertreatment of commercially available mordant dyes on wool with iron(II) and iron(III) salts as a potential source reduction approach to eliminating chromium ions from dyebath effluent [34]. The anticipated improvements in fastness performance were achieved. The structures of the conventional 1:2 iron-dye complexes formed on the wool fibres were characterised by negative-ion fastatom bombardment spectroscopy and HPLC analysis [35]. Symmetrical premetallised 1:2 metal–dye complexes of unsulphonated monoazo structures with aluminium (5.57) or trivalent iron (5.58) have been patented recently for use as solvent dyes [36]. They contain a polar methoxypropylaminosulphone grouping in each diazo component and are marketed as alkylamine salts. It remains to be seen, however, whether a full colour gamut of bright aluminium and iron complex dyes can be discovered with light fastness performance equivalent to that of currently available chromium and cobalt complex dyes. _ O CH3OC3H6NHSO2

C

N N

H3C

Al

C

H3C

C

CH3

CH3 CH2

H2C

CH SH OH

N SO2NHC3H6OCH3

N

C

C

H3C

CH3

O O

C

(H3CO)n

(OCH3)n

C C

O O

+ NH2

NH

O 5.57 _

CH3OC3H6NHSO2

N N O O

Fe

+ H3NCH2

O O SO2NHC3H6OCH3

5.58

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CH CH2CH2CH2CH3

N N

CH2CH3

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

5.6 ISOMERISM IN METAL-COMPLEX DYES 5.6.1 Stereoisomerism The three-dimensional character of metal-complex structures allows the possibility that they can form stereoisomers. Thus all structures represented in this chapter should be regarded as a convenient simplification. As already noted, metals of CN4 can participate in either tetrahedral or planar arrangements (section 5.3). Two symmetrical and identical bidentate ligands surrounding a metal atom in a tetrahedral configuration is an entirely unambiguous structure. A pair of unsymmetrical bidentate systems, however, can exist as enantiomers possessing non-superimposable mirror images and exhibiting optical isomerism. Likewise, equatorial planar arrangements of unsymmetrical bidentate entities, such as copper 8hydroxyquinolate (5.59), exist in cis and trans forms (Scheme 5.14). Copper salicylate 1:2 complexes (5.60) exhibit only the trans planar configuration. Apparently the cis isomer is too unstable to be isolated in this instance.

O N

O

O Cu

N

5.59

N

Cu

N

O cis

trans Copper 8-hydroxyquinolate

Scheme 5.14

O C O O Cu O O C O 5.60 Copper salicylate

In those 1:2 complexes of major commercial importance as premetallised dyes containing two unsymmetrical tridentate ligands coordinated to a trivalent metal of CN6 such as chromium(III) or cobalt(III), the possible number of stereoisomers becomes much greater. The requirement that the tridentate dye molecules retain a high degree of coplanarity in forming the complex, however, does restrict this number. As early as 1939 it was concluded [12] that in symmetrical 1:2 complexes of o,o′-dihydroxyazo dyes with chromium(III) the major planes of the two dye molecules are arranged about the central atom in a mutually perpendicular fashion. This spatial structure later became known as the Drew–Pfitzner model or a meridial complex [6]. Such an arrangement is unambiguous and these complexes do not exhibit isomerism. In 1941 two different isomers of the 1:2 chromium complex of an o-carboxy-o′hydroxyazo dye were isolated. It was concluded that the three donor ligand atoms of each

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STABILITY OF METAL-COMPLEX DYES

261

dye molecule were situated at the apices of an equilateral triangle forming the face of a regular octahedron [37]. During the early 1960s the existence of such isomers was elegantly confirmed in an extensive series of papers [38,39]. In meridial complexes derived from o,o′dihydroxyazo dyes the angle subtended by the three coordinating groups is a right angle and is best fitted by the Drew–Pfitzner model. On metallisation such dye ligands form a 5,6′membered bicyclic ring system (5.24 in section 5.4.2). In contrast, o-carboxy-o′-hydroxyazo ligands form a 6,6′-membered ring system (5.22), with an angle of 60 degrees between the three coordinating groups. Such a configuration can be accommodated more fittingly at the corners of the equilateral faces of the octahedron. This so-called Pfeiffer–Schetty ‘sandwich’ model or facial complex exhibits a minimum of three isomeric forms, each existing as an enantiomeric mirror-image pair. Further studies were extended to o-amino-o′-hydroxyazo ligands and their azomethine analogues, as well as o-carboxy- and o-hydroxyformazan complexes [40], fully confirming that all diarylazo chromogens yielding 5,6′-membered annelated rings adopt the meridial configuration, whereas those forming a 6,6′-membered ring system give isomers of the facial type. An elegant confirmation of the latter arrangement has been obtained by X-ray crystallography [41]. Facial stereoisomers may differ in solubility or absorption spectra and can be separated by chromatographic techniques. The activation energy of conversion from one form to another is low, however, so that a solution of one isomer rapidly transforms into an equilibrium mixture containing several components. Attempts to demonstrate differences in dyeing properties between facial isomers are difficult to assess. Nevertheless, facial chromium isomers of o-carboxyazopyrazolone dyes have been claimed to show significantly inferior wet fastness but marginally higher light fastness on wool than analogous meridial structures [42]. 5.6.2 Nα/Nβ isomerism A different type of isomerism in metal-complex azo dyes, originally defined by Zollinger [43], has become known as Nα/Nβ isomerism. It is found in 1:1 complexes of copper(II) or nickel(II), as well as 1:2 complexes of chromium(III) and cobalt(III) ions, provided that the o,o′-dihydroxyazo ligands are unsymmetrical. There are two isomers (α and β) of the bicyclic 1:1 complexes but the doubly bicyclic 1:2 complex structures can form three, namely the α,α (5.61), α,β (5.62) and β,β (5.63) configurations. If group R is alkyl, differences between the proton magnetic resonance signals associated with the alkyl group protons are dependent on the relative positions of the R groups and the N atoms that coordinate with the central metal atom M. From these data and corresponding results for azomethine analogues in which the coordinating N atoms can be identified unambiguously, the proportions of the three isomers present in a mixture can be calculated [44].

5.7 STABILITY OF METAL-COMPLEX DYES 5.7.1 Factors governing the stability of metal complexes The formation of any metal complex is a reversible reaction and at equilibrium the complex is always partially dissociated into its ligand (L) and metal ion (M) components (Scheme 5.15). The thermodynamic stability constant (K) is a measure of the extent of this

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

N

N R

R

N

N O

O

O

O M

M

O

O O

R

O

N

N N

R

N

5.61

5.62

N

R

N O O M O O N N

R

5.63

dissociation. The simplest general definition of K is as the ratio of the factor of the concentrations of the end-products to that of the starting materials, as in Equation 5.1 where the square brackets denote concentration terms such as mol/l. In dilute solution dissolved solutes show an approximation to ideal behaviour, but at higher concentrations activity coefficients should also be included. If the complex L–M is thermodynamically stable, only traces of the components L and M remain at equilibrium and K1:1 has a high value.

K1:1 =

[L1 - M] [L1 ][M]

(5.1)

K1 L1 + M

Scheme 5.15

L1

M

Formation of a 1:1 dye–ligand complex, as in Scheme 5.15, is the simplest case. Further reaction to give a 1:2 complex L–M–L (Scheme 5.16) involves a second equilibrium constant K2 related to the concentrations of the 1:1 and 1:2 complexes present at equilibrium (Equation 5.2). If the two ligands are different, the stability constant of the 1:2 complex (K1:2) is the product of K1 and K2 (Equation 5.3). In the special but widely

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prevalent case of a symmetrical 1:2 complex, the K1:2 value is related to the product of the metal ion concentration and the square of the ligand concentration (Equation 5.4). In practice, premetallised 1:2 dyes have K values of 1015 or greater.

K2 =

[L1 - M - L2 ] [L1 - M][L 2 ]

(5.2)

K2 M

L1

+ L2

L1

M

L2

Scheme 5.16

K1:2 = K1K2 =

symm. K1:2 =

[L1 - M - L2 ] [L1 ][L2 ] [M]

(5.3)

[L - M - L]

(5.4)

[L]2[M]

As already discussed (section 5.4.2), factors influencing the stability of metal-complex structures include the size and number of annelated ring systems formed on metallisation. Chelates with five- and six-membered rings are more stable than complexes formed from terminal salicylic acid residues that contain only a single coordinated ring. Tridentate ligands yielding 6,7-annelated ring systems, such as those formed by o,o′-dicarboxyazo groupings, show stabilities closer to those of bidentate ligand systems. Metal-complex stability is also related to the basic strength of the ligand entity. For a series of 1:2 complexes of the bidentate naphthylazophenol ligand (5.64) with copper(II) ion, the acidic dissociation constants (pKa) are linearly related to the stability constants (log K1:2), the more acidic groups forming the less stable complexes. Thus where X = NO2 in structure 5.64 then pKa = 8.1 and log K1:2 = 17.2, and where X = OCH3 then pKa = 8.5 X N SO3Na

N O Cu O NaO3S

N N X 5.64

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

and log K1:2 = 19.2. This relationship is only valid, however, for a series of strictly related ligand groups and is dependent on the nature of the solvent [45]. In a similar investigation of the tautomeric tridentate ligand 2′-hydroxyphenylazo-2naphthol (5.65 in Scheme 5.17), the first and second acidic dissociation constants (pKa) related to the two hydroxy groups in the parent structure (X = H) were found to be 11.0 and 13.75 respectively. On introduction of an electron-withdrawing substituent (X) the first dissociation constant decreased from 11.0 to 10.55 (X = Cl) or 7.67 (X = NO2). The stability constants (log K1:1) of the derived 1:1 complexes were dependent on the metal ion introduced [46], being particularly high for nickel(II) at 19.6 and copper(II) at 23.3. OH

OH HO

H

N

O

N N

N 5.65

X

Hydroxyazo

X

Ketohydrazone

Scheme 5.17

The nature of the donor atom in a coordinating group plays a significant part in the resultant stability of the metal complex. As already mentioned (section 5.5.2), stable 1:1 cobalt(III) complexes can be obtained when the coordination sphere is completed by three nitrogen atoms. Iron(III) atoms tend to form more stable complexes with oxygen donor atoms, as in the 1:3 o-nitrosonaphthol structures. Chromium(III) complexes, on the other hand, show equal stability with either nitrogen or oxygen donors, which helps to explain their ubiquitous versatility. A series of o-substituted-o′-hydroxyazo ligands was derived from the same pyrazolone coupling component, where the o-substituent was methoxy, methylthio or dimethylamino. It was shown that even though the acidic dissociation constants of the methoxy- and methylthio-substituted ligands were practically identical, those metal complexes containing sulphur donors exhibited greater stability constants than those containing oxygen donors [47]. 5.7.2 Instability of metal-complex dyes during textile processing The sensitivity of premetallised acid dyes to the presence of traces of other metal ions during dyeing and finishing is attributable to demetallisation of the complex, leading to marked changes in colour, dyeing properties and fastness to light or other agencies. The chromiumcomplex CI Acid Brown 360, of interest specifically for the dyeing of leather, is sensitive to the presence of copper(II) and nickel(II) ions even at ambient temperature [48]. Ultrafiltration can be applied to distinguish between the chelated metal colorants and free copper, chromium or nickel ions in textile effluents. Chromium transfer through the membrane is the least efficient [49]. Proposed ecological criteria for the EU ecolabel with respect to metal-complex dye effluents are ≤50 mg/kg Cr and ≤75 mg/kg Cu or Ni [50]. It is well known that transition-metal ions such as copper(II) or iron(III) will catalyse the

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decomposition of hydrogen peroxide in solution to form excessive concentrations of perhydroxide (HOO–) anions. If colour-woven cotton fabrics containing undyed yarns as well as yarns dyed with copper-complex reactive dyes, for example, are subjected to a postbleaching treatment with alkaline peroxide, copper ions desorbed from the dyed material may catalyse oxidative degradation of the cotton. Addition of salts of ethylenediaminetetraacetic acid (EDTA) to the bleach liquor is an effective means of sequestering these released ions (5.66) and hence suppressing the damage and discoloration of the substrate [51]. O H2O

O

O Cu

Cu

NCH2CH2N

O

H2O

OH2

O O O

OH2

O 5.66

An appropriate ion-specific electrode was found to provide a convenient, precise and relatively inexpensive method for potentiometry of copper(II) ion in copper-complex azo or formazan dyes. Copper(II) ion in copper phthalocyanine dyes can be quantified after anion exchange. Twelve commercial premetallised dyes evaluated using this technique contained copper(II) ion concentrations in the range 0.007 to 0.2%. Thus many copper-complex direct or reactive dyes are likely to contribute low but possibly significant amounts of ionic copper to textile dyeing effluents [52]. The presence of residual unbound transition-metal ions on a dyed substrate is a potential health hazard. Various eco standards quote maximum permissible residual metal levels. These values are a measure of the amount of free metal ions extracted by a perspiration solution [53]. Histidine (5.67) is an essential amino acid that is naturally present as a component of perspiration. It is recognised to play a part in the desorption of metal-complex dyes in perspiration fastness problems and in the fading of such chromogens by the combined effects of perspiration and sunlight. The absorption of histidine by cellophane film from aqueous solution was measured as a function of time of immersion at various pH values. On addition of histidine to an aqueous solution of a copper-complex azo reactive dye, copper-histidine coordination bonds were formed and the stability constants of the species present were determined [54]. Variations of absorption spectra with pH that accompanied coordination of histidine with copper-complex azo dyes in solution were attributable to replacement of the dihydroxyazo dye molecule by the histidine ligand [55]. On immersion of cellophane films dyed with four copper-complex azo reactive dyes in aqueous histidine, the absorption spectra of the dyeings changed as a result of abstraction of COOH CH2CH NH2 N

NH 5.67 Histidine

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copper ions by the amino acid. The coordination of histidine to a copper-complex dye appears to be promoted by hydrogen bonding of the amino acid to cellulose hydroxy groups. Shade variations and lowering of light fastness were observed following the immersion of cotton fabrics dyed with various premetallised reactive dyes in a histidine solution [56,57]. 5.7.3 Decolorisation of waste liquors containing metal-complex dyes The problem of residual colorants in waste-waters is a subject of increasing concern (section 1.7.3) and various techniques are exploited to treat coloured effluents. Destructive decolorisation using powerful oxidising agents such as ozone is one important approach. The reactivities with this reagent of disodium 1-(2′-hydroxyphenylazo)-2-naphthol-3,6disulphonate and its 1:1 complex (5.68) with copper(II) were compared with those of several classical dye structures: Alizarin (CI Mordant Red 11), Chrysophenine (CI Direct Yellow 12), Indigo carmine (CI Acid Blue 74), Malachite green (CI Basic Green 4) and Orange II (CI Acid Orange 7). The relative ease of oxidation increased in the order: Malachite green < Alizarin < Cu-complex 5.68 < Orange II < Chrysophenine < Unmetallised 5.68 < Indigo carmine. Ozonation of the unsulphonated analogue of the copper-complex 5.68 yielded phenol, 2-naphthol and phthalic anhydride [58]. OH2 O

Cu O

SO3Na

N N

5.68 SO3Na

An alternative approach to decolorising waste dyebaths of particular relevance to anionic metal-complex dyes is by means of ion-pair extraction using long-chain amines. The metalcomplex dyes can be recovered from the organic phase by extraction with caustic soda and then reused in dyeing. The amines that result after dye recovery are practically colourless and may be recycled. Waste water from a cotton dyeing containing the hydrolysed form of the copper phthalocyanine CI Reactive Blue 41 was shaken at pH 3 with a secondary alkylamine and a mixture of aliphatic hydrocarbons. The dye was almost totally removed and the copper content of the liquor reduced from 30 mg/l to 0.1 mg/l. In similar experiments [59] on wool dyeing effluents (a) the concentration of CI Acid Red 214 (5.69) was lowered from 204 to 10 mg/l and the chromium content from 5.0 to 0.1 mg/l, and (b) a solution of 25 mg/l CI Acid Red 362 (5.70) became practically colourless and the AOX from this halogenated dye fell from 1.6 to 0.5 mg/l. An evaluation of the macrocyclic ligand cucurbituril in powder form as a precipitant for direct dyes varying in molecular size, including CI Direct Red 79 (disazo), Blue 71 (trisazo) and Red 80 (tetrakisazo), established the influence of pH and ligand concentration on adsorption [60]. The cucurbituril molecule contains six acetylenediurein units linked in

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CH3 Cl N

OH2

H2O O2N

O

OH2

Cr

N

N

N O

O

Cl NaO3S

N

O

Cr

O

SO3Na

Cl O

N N

NaO3S

N

N N

HO 5.69

N

Cl H3C

CI Acid Red 214 5.70 CI Acid Red 362

annular form via methylene groups between pairs of nitrogen atoms in adjacent units. Most of the copper in effluents from direct dyeing is in the form of copper-complex dyes. Thus selective removal of the dye from the waste liquor by treatment with cucurbituril simultaneously reduces the heavy metal content present. An investigation of this complexing agent to remove the copper phthalocyanines CI Reactive Blues 41 and 116, the copper-complex disazo Blue 120 and the chromium-complexes CI Acid Yellow 120, Reds 214 (5.69), 296 and 362 (5.70), Violet 58 and Black 194 from dyehouse effluents gave highly promising results [61]. Quite recently, the removal of the symmetrical 1:2 chromium complex of an azomethine ligand (5.71) from waste-water using a chromium-tolerant bacterial culture was investigated. At low dye concentration initial biosorption was insignificant but about 50% of dye was subsequently decolorised by bacterial degradative activity. At high dye concentration, however, an initial biosorption of about 20% led to only 27% decolorisation overall. The concentration of chromium in the solution followed a similar pattern [62]. The properties of the corresponding manganese(II) complex for dyeing wool and nylon 6.6 have been assessed [63]. Interest has also been shown in the analogous azomethine-2-naphthol ligand (5.72) and its complexes with copper(II), nickel(II) and cobalt(III) ions, which have been evaluated for the dyeing of wool under controlled conditions [64].

+

O

CH N Cr

O N

N

N

N

O

_

N CH2

ClO4 5.72

CH

5.71

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

5.8 CHROMIUM-RELATED PROBLEMS IN THE MORDANT DYEING OF WOOL There are problems of definition with the term ‘mordant dye’ (section 1.6.8) and it is often more precise to refer to those chelatable dyes, mostly o,o′-dihydroxyazo ligands, that are applied to wool at low pH and fixed by dichromate aftertreatment as chrome dyes. Nevertheless, mordant dyeing is a convenient way to describe this two-stage process that has become the focus of substantial development work in recent years because of increasing concern about the environmental hazards associated with residual chromium in dyehouse effluent. Virtually all of the chrome dyes that remain of major commercial importance are simple monoazo structures, such as CI Mordant Violet 5 (5.65; X = SO3Na). These products are easy to manufacture from low-cost intermediates, readily soluble and build up well to heavy depths. Owing to their relatively small molecular size (Mr 300–600), they show good leveldyeing properties when applied to wool at pH 4 and the boil. Glauber’s salt is an effective levelling agent for chrome dyes but it impairs the efficiency of the afterchroming stage. The unique combination of level dyeing behaviour and outstanding wet fastness offered by chrome dyes made them increasingly important in the 1970s, when shrink-resist wool knitwear suitable for treatment in household washing machines was launched [65]. Levelling agents of the weakly cationic type, often based on ethoxylated alkylamines, are satisfactory for use with chrome dyes. The effect of cationic surfactants with and without phenyl rings on the spectral properties of chrome dyes of the o,o′-dihydroxyazo series has been investigated recently. There was evidence of dye–agent interaction by means of a combination of electrostatic forces and hydrophobic bonding. The specific orientation of the associated molecules was strongly influenced by opportunities for stacking of the planar aryl rings in dye and agent structures. Stacking interactions were promoted by low electron density in the aryl rings [66]. The growing reliance of the dyeing industry on rapid instrumental colour matching has tended to be a handicap for users of chrome dyes. A marked change in hue normally accompanies conversion of the mordant dye to its chromium complex during aftertreatment, although matching difficulties can be overcome by careful control of conditions [67]. More serious problems include the prolonged dyeing procedure, oxidative damage to the wool by dichromate treatment, as well as growing awareness of the environmental hazards associated with chromium compounds, especially the hexavalent chromium form [68]. Although 10 mg per day of chromium(III) in food is normal for good health, it is important to concede that chromium(VI) is highly toxic to mankind and aquatic life [69]. Typical limiting values in the UK regarding permissible amounts of chromium for discharge to effluent are 0.2–0.5 mg/l as the more potent chromium(VI) dichromate anion and 2–4 mg/l as the chromium(III) cation. Proposed ecological criteria for the EU ecolabel with respect to chroming baths are 0.5 mg/l for chromium(VI) and 5 mg/l for chromium(III) ions [50]. Legislation covering the release of chromium-containing effluents is becoming increasingly strict, especially in Germany, the UK and the USA [70]. Draft regulations indicate that no more than 0.1 mg/l total chromium will be tolerated in future [69]. Dichromate anions are readily absorbed under acidic conditions by wool that has been dyed with chrome dyes. The chromium(VI) on the fibre is then gradually reduced by the cystine residues in wool keratin to chromium(III) cations, which react with the dye ligands to form a stable complex. In this way the cystine disulphide bonds are destroyed, resulting in oxidative degradation of the wool fibres [71].

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It has been demonstrated recently that the dichromate oxidation of wool cystine proceeds as shown in Scheme 5.18. The cystine crosslinks (5.73) are cleaved to give dehydroalanine (5.74) and perthiocysteine (5.75) residues, the latter being readily oxidised by the dichromate ions to yield cysteine-S-sulphonate (Bunte salt) residues (5.76). CO CH

CH2

S

S

CH2

NH

CO

CO

CH

C

NH

NH

CO CH2

+

H

S

S

CH2

CH NH

5.73

5.75

Cystine

Perthiocysteine

5.74 Dehydroalanine Cr2O72

O HO

S

_

CO S

O

CH2

CH NH

5.76 Cysteine-S-sulphonate

Scheme 5.18

5.8.1 Controlling factors in the afterchroming process By far the most widely used chroming agent is sodium dichromate, although the potassium salt has occasionally been preferred. The dichromate may be applied before the dye (mordant method), simultaneously with the dyeing process (metachrome method) or as an aftertreatment (afterchrome method), but only the afterchrome process remains of practical significance. The theoretical aspects of chroming have been reviewed [72,73] and the mechanism of the reactions may be represented as follows: (1) Dichromate anions are absorbed and interact with protonated amino groups in wool (Scheme 5.19), the sorption proceeding most rapidly below pH 3.5. (2) The absorbed chromium(VI) is gradually reduced to chromium(III) as a result of participation in the oxidative decomposition of cystine crosslinks as represented by Scheme 5.18. (3) The chromium(III) cations then combine with the carboxylate groups in the wool fibre (Scheme 5.20). (4) The dye ligand interacts with free or complexed chromium(III) to form 1:1 and 1:2 chromium-dye complexes (Scheme 5.21), mainly the more stable 1:2 complex. These coloured complexes are bound to the wool primarily through van der Waals and electrostatic forces. Any excess chromium(III) will remain linked to carboxylate sites in the wool. Originally the amounts of dichromate used in the traditional afterchrome process varied between about 25 and 50% of the total amount of chrome dyes present, with the lower and upper limits set at 0.25% and 2.5% of the mass of wool. These quantities were well in excess

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

+ 2H3N

HOOC

[wool]

[wool]

COOH

+ NH3

Cr2O72–

+

Cr2O72–

+ H3N

[wool]

COOH

Scheme 5.19

O

+ H3N

[wool]

+ Cr3+

C

+ H3N

O [wool]

+ Cr2+ + H

C

OH

O

Scheme 5.20

+ H3N

O [wool]

2+

C

Cr

+ H3N

O [wool]

OH

O +

+

+ Cr

O

OH H

+ H+

C

O

O N

N

N

N

Scheme 5.21

of the stoichiometric amount required even for formation of the 1:1 complex. This approach resulted in excess dichromate remaining in the aftertreatment bath for discharge to effluent, as well as on the wool fibre where it contributed to further oxidative degradation. Users of chrome dyes are increasingly concerned with dyeing under mild conditions at pH 4–5 and 85 °C, followed by chroming at pH 3.5–3.8 and 90 °C to minimise wool damage [73,74]. Chroming in a fresh bath tends to give lower residual chromium content but does increase the processing costs. Hercosett-treated wool generally requires more dichromate than does untreated wool, since some of the dichromate is absorbed by the cationic polymer layer [75]. Every effort should be made to exhaust the dyebath as much as possible, because any residual mordant dye will complex with chromium(III) ions in the dye liquor. Apart from complicating effluent treatment, this raises the possibility of lower fastness resulting from deposition on the surface of the wool. Methods that give lower residual chromium tend to produce unlevel chroming, but this can be effectively countered using appropriate levelling agents. The amphoteric agents originally developed for the reactive dyeing of wool are particularly effective but nonionic alkylarylethoxylates can also be used. Water free from

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traces of iron or copper is essential, since sequestering agents such as EDTA (5.66) or polyphosphates must be avoided. An aftertreatment in ammonia solution (pH 8.5) for 20 minutes at 80 °C, followed by acidification to pH 5, helps the development of optimum fastness, especially on shrink-resist wool. Concern regarding exposure to chromium is not just related to effluent discharge. It is obvious that residual unbound chromium present on the fibre is also a potential hazard. The Oeko-Tex ecolabel specifies 1 ppm total chromium or cobalt on babywear and 2 ppm chromium or 4 ppm cobalt on other garments [76]. These figures represent the amount of free metal extracted by a standard perspiration solution. In general, typical 1:2 metalcomplex dyeings will satisfy these requirements in full depths, chrome dyeings only to medium depths and premetallised 1:1 complexes only in pale-depth dyeings [65].

5.8.2 Improved mordant dyeing methods to minimise residual chromium In one low-chrome method the wool is initially dyed at the boil to facilitate maximum penetration and levelling. The temperature is then lowered to 75 °C, since maximum exhaustion of chrome dyes takes place below the boil. After adjusting to pH 3.5–3.8 by addition of formic acid, the near-stoichiometric quantity of dichromate as recommended by the dye supplier is added and the temperature again raised to the boil. About 7.5% o.w.f. sodium sulphate is added after chroming for 10–15 minutes and boiling is continued for a further 30 minutes. Sodium sulphate gradually displaces chromium from the wool carboxylate sites, making the chromium(III) cations more readily available for interaction with the dye ligands so that less dichromate is needed [73]. An alternative method relies on a constant temperature of 92 °C for dyeing and chroming, thus minimising damage to the wool and dispensing with the need to lower the dyebath temperature before commencing the chroming stage [68]. After dyeing at 92 °C and exhausting the dyebath with a small amount of formic acid, an addition of dichromate is made according to Equation 5.5, where C% is the amount of dichromate required for a dyeing at D% depth. C = 0.2 + 0.15D

(5.5)

Chroming is allowed to continue at a relatively mildly acidic pH for about 10 minutes to improve the uniformity of chroming with this reduced quantity of dichromate. The pH is then lowered to 3.5–3.8 to ensure maximum utilisation of the chromium and treatment is continued at 92 °C for 45 minutes. There are several redox methods of chroming that depend on the addition of a reducing agent to the chroming bath after about 10 minutes at or near the boil. The precise mechanism is not fully understood, but clearly the increased rate and extent of reduction of hexavalent to trivalent chromium plays a crucial part. The first technique of this type used 1–3% o.w.f. lactic acid, which was found to be the most effective of the α-hydroxymonocarboxylic acids evaluated to assist the rapid conversion of chromium(VI) anions to chromium(III) cations [77]. The method should be used in conjunction with reduced amounts of dichromate if the advantages to be gained from a lack of residual chromium fixed in the fibre are to be attained. Lactic acid is only effective at high concentrations, resulting in higher processing costs and an increased total oxygen demand in the effluent.

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

Sodium thiosulphate was later proposed [78] instead of lactic acid. This enabled many chrome dyes to be effectively aftertreated at a temperature as low as 80 °C with consequent advantages in respect of improved levelling and decreased fibre damage. Some dyes are inadequately chromed at 80 °C, however, so a minimum temperature of 90 °C was later recommended [75]. When sodium thiosulphate is present in the chroming bath, the concentration of Bunte salt residues (5.76 in Scheme 5.18) is considerably increased. The thiosulphate nucleophile (5.77) will readily add to the activated double bond in dehydroalanine (5.74), as shown in Scheme 5.22. The protective effect of thiosulphate ions in preventing some of the wool damage is attributable to their reducing properties, so that they compete with perthiocysteine (5.75) in the overall reduction of chromium(VI) to chromium(III) ions [72]. In a modification of this redox approach, a glucose-based proprietary product Lyocol CR (Clariant) is recommended instead of sodium thiosulphate. This agent also accelerates the conversion of Cr(VI) to Cr(III) and forms a complex with the chromium(III) cations that is then absorbed by the wool [79]. Sulphamic acid has been evaluated as a replacement for formic acid in controlling the optimum pH of chroming. If the process is initiated at a pH lower than 3.5 before the dichromate is added and then allowed to gradually become less acidic when reduction of most of the dichromate has taken place, the optimum result for both Cr(VI) and Cr(III) in the effluent should be achieved. Sulphamic acid slowly hydrolyses under these conditions (Scheme 5.23) to give ammonium hydrogen sulphate. It is also able to react [80] with primary amino and hydroxy groups in wool keratin, releasing ammonia (Scheme 5.24). All these reactions will contribute to an increase in pH of the chroming bath. There are no significant differences in wet fastness of chrome dyeings or in the damage caused to the wool by the sulphamic acid process compared with the conventional afterchrome method using formic acid [81].

CO C

O CH2

+ Na

S

NH

ONa

O

CO CH

O CH2

S

NH

S

ONa

+

NaOH

O

5.77

5.76

Sodium thiosulphate

Cysteine-S-sulphonate

5.74 Dehydroalanine

S

H2O

Scheme 5.22

H2NSO3H + H2O

Scheme 5.23

HO

[wool]

NH2

+ 2 H2NSO3H

NH4HSO4

HO3SO

[wool]

NHSO3H

Scheme 5.24

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CHROMIUM-RELATED PROBLEMS IN THE MORDANT DYEING OF WOOL

273

Rare earth cations form complexes of low stability with mordant dyes and can be used to control the chroming process. Mixtures of rare earth chlorides are available at low prices (ca. US$1 per kg in China) and are abundant in some countries. Neodymium and cerium ions have an outer electron distribution similar to that of trivalent chromium. If mordantdyed wool is treated with such a mixture 20 minutes before the dichromate addition in a conventional chroming sequence, coordination complexes of the rare earth elements are temporarily formed. Ion-exchange reactions with chromium(III) cations allow time for the latter to migrate throughout the wool substrate and level chroming is achieved in spite of the low level of dichromate addition. Although the rare earth complexes are significantly different in colour, the ultimate hue of the chromium-complex dyeing is normal. The colour fastness properties are not adversely affected and wool damage is slightly reduced. The environmental impact of the ca. 60 mg/l rare earth cations in the effluent has yet to be evaluated [82]. An interesting new approach has been evaluated to avoid the environmental hazards and oxidative degradation of wool associated with these conventional and modified dichromate aftertreatments. Chromium(III) salts such as chromium fluoride have been used with mordant dyes in steam fixation processes of printing or continuous dyeing but seldom for the aftertreatment of exhaust dyeings. It has now been shown that the chromium lactate 1:2 complex (5.78) formed by reacting chromium(III) chloride with lactic acid (Scheme 5.25) is substantive to wool at pH 3–4. It is capable of adequately chroming a wide variety of chrome dyes to give satisfactory wet fastness, less fibre damage and effluents of low toxicity [83]. More recently, 1:2 complexes of the chromium(III) ion with 5-sulphosalicylic [84,85], salicylic and citric acids have been compared with the lactate complex [85]. Like the latter, the citrate (5.79) and 5-sulphosalicylate (5.80; X = SO3H) complexes are readily watersoluble but the unsubstituted salicylate (5.80; X = H) is not. If formic acid is added, however, the mixed formate–salicylate complex (5.81) shows moderate solubility. The pH value for maximum uptake of the complexes varied slightly but was always within the range of pH 2–4. Compared with conventional dichromate treatment, the lactate complex sometimes gave inferior wet fastness. Fastness ratings equal to conventional aftertreatment were given by the two salicylate mordant complexes (5.80) without the drawbacks characteristic of the dichromate process [85]. 5.8.3 Improved products and processes to compete with chrome dyes In spite of their long-recognised disadvantages, or as a result of the substantial efforts to overcome them, chrome dyes still represent about 30% of total dye consumption in wool dyeing (Table 3.10 in section 3.2.2). Premetallised acid dyes and reactive dyes for wool account for a further 40%, so it is not surprising that much attention has been given to _ H3C CrCl3.6H2O + 2 CH3

CH OH

COOH

O C

OH2 O Cr

C

C O

O

O

OH2 Scheme 5.25

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5.78

273

O C

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CH3

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

_

X _ HOOCH2C HOOCH2C

OH2 O O

C

O

Cr

C O

O OH2

OH2 O

O C

Cr

C

C O

O C

CH2COOH

O

CH2COOH

O

O OH2 X

5.79

_ OH2 O

5.80

O

O

C

H

O OH2

C

H

Cr C O

O

O

5.81

improving the performance of these ranges, which have their own characteristic limitations. There has been considerable activity in the selection of compatible members from different but related classes of wool dyes, such novel selections often being given a distinctive brand name [67]. Carbonised wool can be dyed uniformly with 1:1 metal-complex dyes in 8% sulphuric acid solution without prior neutralisation. These dyes show excellent penetration of tightly woven fabrics and better wet fastness than levelling acid dyes in full depths [86]. Their major disadvantage is the need to apply them at pH 2, causing damage to the wool and requiring a subsequent neutralisation step. The Neolan Plus system was introduced in 1988, consisting of eight Neolan P (Ciba) modified 1:1 metal-complex dyes containing fluorosilicate anions. They are applied with formic acid at pH 3.5–4 using Albegal Plus (Ciba), a synergistic mixture of quaternary and esterified alkylamine ethoxylates and fluorosilicates [87]. High dyebath exhaustion ensures shade reproducibility not attainable hitherto, as well as low residual chromium in the dyebath. Wool quality is preserved by the above-normal dyebath pH and post-neutralisation is not necessary. An improved BASF process for conventional 1:1 metal-complex dyes utilises sulphamic acid, commencing at pH 2–2.5 and gradually increasing to pH 3–3.5 (Schemes 5.23 and 5.24). Together with a novel levelling agent, these conditions promote good exhaustion and dye migration, improved wool quality and low values for residual chromium [70]. The Lanaset (Ciba) system introduced in 1983 is based on a compatible range of fifteen milling acid and 1:2 metal-complex dyes. For minimum damage to the wool, they are applied in the isoelectric region (pH 4.5–5) in the presence of Albegal SET (Ciba), an amphoteric levelling agent [88]. The required pH is maintained with formic acid for loose stock and tops. A dyebath accelerant, Miralan T (Ciba), is mainly of interest on these goods. Full exhaustion is attained in 10–20 minutes at the boil, resulting in increased reproducibility and improved wool quality. Dye liquors can be reused where a full-volume reserve tank is available, giving energy savings and lower volumes of effluent [74]. Glauber’s salt is

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275

recommended for yarn and piece dyeing, with an acetate/acetic acid buffer system for improved control of pH. Although adopted primarily for the dyeing of loose stock, slubbing and yarn, the Lanaset system can be applied with care to the winch dyeing of woollens and the jet or overflow dyeing of worsteds. The versatility of this system is reflected in its suitability for blends of wool with other fibres [89]. The ideal conditions for the dyeing of wool/acrylic blends are at pH 4-5 and Albegal SET inhibits the risk of co-precipitation between Lanaset anionic dyes and basic dyes. Lanaset dyes are quite stable when dyeing polyester/wool at pH 4-5 and 115-120°C using Irgasol HTW (Ciba) as wool protectant. These dyes are also suitable for dyeing wool in its blends with silk, nylon or cellulosic fibres. The new Neolan A (Ciba) range of eight optimised combinations of metal-free acid dyes has been developed specially for the dyeing of wool and wool/nylon blends. Five members of the range can be further combined in trichromatic recipes, enabling wool dyers to achieve 80-90% of fashion shades [90]. Brilliant hues and excellent migration properties are obtained in hank and piece dyeing. Selected products can be used in dyeing wool yarns for carpets. Neolan A dyes are applied at pH 5–6 to give exhaustion values exceeding 95% and a uniform migration rate of about 30%. Albegal NA (Ciba) is a levelling agent developed specially for use with these dyes [91]. Although reactive dyes account for only about 5% of total dye usage on wool, no other class can achieve such brilliant hues of high fastness to light and wet treatments. Chrome dyes are mainly used for navy blue and black dyeings, where reactive dyes in general are particularly costly by comparison. Mill trials have been carried out to compare the fastness of a reactive black dyeing based on CI Reactive Black 5 (5.82) with a dyeing of the chrome black CI Mordant Black 9 (5.83) that gives the highest wet fastness. In the most demanding route of dyeing loose wool and assembling into knitted garments that are given an oxidative shrink-resist treatment followed by resin application, the reactive dyeing met all processing and end-use fastness requirements [65]. Chrome-dyed wool is brittle and has a characteristically harsh handle. This is attributed to oxidative damage and the excessive crosslinking imparted during afterchroming. Reactive dyes, however, offer fibre-protective effects during dyeing by: (1) inhibiting thiol-disulphide interchanging and thus restricting the degree of permanent set during dyeing (2) minimising the hydrolytic peptide bond breakdown that is responsible for impairing the dry tensile strength of wool fibres. In 1996 Ciba introduced a new reactive black, Lanasol Black PV, that is an attractive alternative to established chrome blacks, even as regards price. It has been designed to give tinctorial strength, shade metamerism and light fastness that are close to those of the afterchromed complex of Eriochrome Black PV (5.83). Only in the case of fastness to extreme processes such as potting and cross-dyeing with acetic acid is the target standard not entirely achieved. To attain optimum exhaustion and good level dyeing behaviour, 2% o.w.f. Albegal B (Ciba) must be used and aftertreatment with ammonia is necessary to ensure the highest wet fastness ratings [92]. The new range of Lanasol CE (Ciba) metal-free reactive dyes for wool has been developed to offer the option of replacing chrome dyes in full depths by cost-effective alternatives. These five dyes (yellow, red, blue, navy and black) are based on the α-

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

SO2CH2CH2

H2N

CH2CH2SO2

N

O

H NaO3SO

N

OSO3Na

SO3Na

N N

5.82 NaO3S

CI Reactive Black 5

OH H

O

N N OH

NaO3S 5.83 CI Mordant Black 9

bromoacrylamide reactive system and can be shaded with existing Lanasol brands using the standard application methods for these dyes [93]. Lanasol Navy CE is particularly versatile, being suitable for the exhaust dyeing of untreated, chlorinated or machine-washable wool, as well as continuous dyeing, vigoreux printing and coloured discharge styles and in silk dyeing [90]. Excellent fastness to light and wet treatments with outstanding fixation, levelling and reproducibility are claimed for Lanasol CE dyes, which fulfil the requirements of the Oeko-Tex 100 ecolabel [76]. The influence of transition-metal salts, mordant dyes and premetallised acid dyes on the photodegradation of wool has been investigated [94]. Certain yellow dyes that absorb UV radiation in the 320–400 nm region will inhibit phototendering but most other dyes accelerate the process. The uranyl(II) ion (UO2) also absorbs in this region and was found to be an effective UV screen. Treatment with aluminium or iron(III) hydroxide brought about a light-induced strengthening of wool fabric, but a harsher handle developed on prolonged exposure. Treatment of wool with alizarin-3-sulphonate (5.84; CI Mordant Red 3) resulted in some light-induced increase of breaking strength but on aluminium-treated wool it was considerably more effective and durable for at least 6000 h irradiation. Wool pretreated with a uranyl salt followed by alizarin-3-sulphonate dyeing was phototendered extremely rapidly, in contrast to the protective effects when either component was applied separately. The mordant dye 1-nitroso-2-naphthol-6-sulphonate (5.85) on aluminiummordanted wool imparted protection from strength loss for 3000 h but accelerated photodegradation on untreated wool. The 1:2 chromium-complex dyes CI Acid Yellow 59 (5.86) and Black 58 (5.4) did not significantly modify the rate of phototendering of wool, with or without an aluminium mordant.

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REFERENCES

O

277

OH O

OH

N

HO

SO3Na O CI Mordant Red 3

SO3Na

CH3

5.84

5.85

N

N

N

N O

O C

O

O C

Cr

O O

N

N N

N H3C

5.86 CI Acid Yellow 59

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.

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G Schetty, J.S.D.C., 71 (1955) 705. R Price in Chemistry of synthetic dyes, Ed K Venkataraman, Vol 3 (New York: Academic Press, 1970) 303. F Beffa and G Back, Rev. Prog. Coloration, 14 (1984) 33. A Werner, Ber., 41 (1908) 1062. G Schetty, Textilrundschau, 5 (1950) 399. H Pfitzner, Melliand Textilber., 35 (1954) 649. G T Morgan and H D K Drew, J.C.S., 117 (1920) 1456. R Price, Chimia, 28 (1974) 221. H D K Drew and J K Landquist, J.C.S., 140 (1939) 292. R W Hay and J A G Edmonds, Chem. Commun. (1967) 969. R F M Sureau, Chimia, 19 (1965) 254. H D K Drew and R E Fairbairn, J.C.S., 141 (1939) 823. P A Mack and R Price, Ind. Chim. Belge Suppl. (1967) 31. L Hunter and L B Roberts, J.C.S., 143 (1941) 820. R Wizinger, Chimia (suppl.), 22 (1968) 82. Z Yu-zhen and L Dong-zhi, Dyes and Pigments, 29 (1995) 57. V E Maizlish et al., Zhur. Obshch. khim., 67 (1997) 846. S Hussamy, AATCC Internat. Conf. & Exhib. (Oct 1991) 102. I J Wang, Y J Hsu and J H Tian, Dyes and Pigments, 16, No 2 (1991) 83. H Kocaokutgen, E Erdem and I E Gümrükcüoglu, J.S.D.C., 114 (1998) 93. B R Hsieh, D Desilets and P M Kazmaier, Dyes and Pigments, 14, No 3 (1990) 165. B R Hsieh, Dyes and Pigments, 14, No 4 (1990) 287. B R Hsieh, R K Crandall and B A Weinstein, Dyes and Pigments, 17, No 2 (1991) 141. K Blus, Dyes and Pigments, 25 (1994) 15. H Pfitzner and H Baumann, Angew. Chem., 70 (1958) 232. B I Stepanov in Recent progress in the chemistry of natural and synthetic colouring matters’, Eds T S Gore et al. (New York: Academic Press, 1962) 451. R Price in Chemistry of synthetic dyes, Ed K Venkataraman, Vol 3 (New York: Academic Press, 1970) 348.

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278 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. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87.

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CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

J Sokolowska-Gajda, H S Freeman and A Reife, Text. Res. J., 64 (1994) 388. M Ryder, Wool Record, 151 (Nov 1992) 5. H S Freeman, L D Claxton and V S Houk, Text. Chem. Colorist, 27 (Feb 1995) 13. H S Freeman et al., AATCC Internat. Conf. & Exhib., (Oct 1995) 397. J Sokolowska-Gajda, H S Freeman and A Reife, Dyes and Pigments, 30 (1996) 1. J Sokolowska-Gajda, J.S.D.C., 112 (1996) 364. H A Bardole, H S Freeman and A Reife, Text. Research J., 68 (1998) 141. W Czajkowski and M Szymczyk, Dyes and Pigments, 37 (1998) 197. H S Freeman and J Sokolowska-Gajda, Rev. Prog Coloration, 29 (1999) 8. P Pfeiffer and S Saure, Ber., 74 (1941) 935. G Schetty and W Kuster, Helv. Chim. Acta, 44 (1961) 2193. G Schetty, Helv. Chim. Acta, 45 (1962) 1095, 1473; 46 (1963) 1132; 47 (1964) 921. L Beffa et al., Helv. Chim. Acta, 46 (1963) 1369. H Jaggi, Helv. Chim. Acta, 51 (1967) 580. G Schetty, Textilveredlung, 1 (1961) 3. H Zollinger, Chemie der azofarbstoffe, (Basel: Verlag Birkhäuser, 1958) 238. G Schetty and E Steiner, Helv. Chim. Acta, 57 (1974) 2149. A Johnson et al., de Tex, 21 (1962) 453. F A Snavely, W C Fernelius and B E Douglas, J.S.D.C., 73( 1957) 491. F A Snavely, B D Krecker and C G Clark, J. Amer. Chem. Soc., 81 (1959) 2337. W R Dyson and M A Knight, J. Amer. Leather Chem. Assoc., 86 (1991) 14. G L Baughman, H A Boyter and W G O’Neal, AATCC Internat. Conf. & Exhib. (Oct 1997) 314. L Benisek, Wool Record, 158 (Apr 1999) 42. Y Imabayashi, J. Soc. Fibre Sci. Technol. Japan (1995) 39. J H Kim and G L Baughman, AATCC Internat. Conf. & Exhib. (Oct 1998) 582. K Parton, J.S.D.C., 114 (1998) 8. Y Okada et al., Dyes and Pigments, 24 (1994) 99. Y Okada, T Kawanishi and Z Morita, Dyes and Pigments, 27 (1995) 271. Y Okada, M Asano and Z Morita, Dyes and Pigments, 31 (1996) 53; IFATCC Cong., Vienna (Jun 1996) 360. Y Okada et al., Dyes and Pigments, 33 (1997) 239; 38 (1998) 19. M Matsui et al., Bull. Chem. Soc. Japan, 64, No 19 (1991) 2961. I Steenken-Richter and W D Kermer, J.S.D.C., 108 (1992) 182. H J Buschmann, D Raderer and E Schollmeyer, Textilveredlung, 26 (1991) 157. H J Buschmann and E Schollmeyer, Textilveredlung, 28 (1993) 182. H Matanic et al., J.S.D.C., 112 (1996) 158. Z Grabaric et al., J.S.D.C., 109 (1993) 199. N Koprivanac et al., Dyes and Pigments, 22, No 1 (1993) 1. K Parton, Colour Science ’98, Vol. 2, Ed. S M Burkinshaw (Leeds: Leeds Univ., 1999) 120. H R Sagaster and G Robisch, Dyes and Pigments, 13, No 3 (1990) 187. J A Bone, J Shore and J Park, J.S.D.C., 104 (1988) 12. K Schaffner and W Mosimann, Textilveredlung, 14 (1979) 12. D M Lewis, J.S.D.C., 113 (1997) 193. P A Duffield, R R D Holt and J R Smith, Melliand Textilber., 72 (1991) 938. F R Hartley, J.S.D.C., 85 (1969) 66. D M Lewis and G Yan, J.S.D.C., 109 (1993) 193. G Meier, J.S.D.C., 95 (1979) 252. A Hoyes, Wool Record, 151 (Apr 1992) 49. P A Duffield and K H Hoppen, Melliand Textilber., 68 (1987) 195. Oeko-Tex, Zürich Edn. (Feb 1997) 15. L Benisek, J.S.D.C., 94 (1978) 101. P Spinacci and N C Gaccio, IFATCC Cong., (Budapest: June, 1981). A C Welham, J.S.D.C., 102 (1986) 126. B A Cameron and M T Pailthorpe, Text. Research J., 57 (1987) 619. J Xing and M T Pailthorpe, J.S.D.C., 108 (1992) 17. J Xing and M T Pailthorpe, J.S.D.C., 108 (1992) 265. D M Lewis and G Yan, J.S.D.C., 109 (1993) 13; 110 (1994) 281. J Xing and M T Pailthorpe, Text. Research J., 65 (1995) 70. D M Lewis and G Yan, J.S.D.C., 111 (1995) 316. L C De Meulemeester, I Hammers and W Mosimann, Melliand Textilber., 71 (1990) 898. W Mosimann, Amer. Dyestuff Rep., 80 (Mar 1991) 26.

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88. 89. 90. 91. 92. 93. 94.

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H Flensberg, W Mosimann and H Salathe, Melliand Textilber., 65 (1984) 472. H Flensberg and K Hannemann, Wool Record, 156 (Oct 1997) 42. P Runser, Wool Record, 157 (Oct 1998) 25. K Hannemann and P Runser, Melliand Textilber., 80 (1999) 278. K Hannemann and H Flensberg, Melliand Textilber., 78 (1997) 160. K Hannemann and P Runser, Wool Record, 157 (1998) 53. I J Miller and G J Smith, J.S.D.C., 111 (1995) 103.

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280

CHEMISTRY OF ANTHRAQUINONOID, POLYCYCLIC AND MISCELLANEOUS COLORANTS

CHAPTER 6

Chemistry of anthraquinonoid, polycyclic and miscellaneous colorants Geoffrey Hallas

6.1 ANTHRAQUINONE ACID, DISPERSE, BASIC AND REACTIVE DYES It was pointed out in Chapter 1 that, after the azo class, anthraquinone derivatives form the next most important group of organic colorants listed in the Colour Index. The major application groups are vat dyes, disperse dyes and acid dyes (Table 1.1). Of the various anthracenedione isomers, only the 9,10-compound is used for the synthesis of dyes; it is usually referred to simply as anthraquinone (6.1). The parent compound is pale yellow in colour, having a weak absorption band in the visible region (n→π* transition). The presence of one or more electron-donating substituents leads to significant bathochromic effects so that relatively simple derivatives are of commercial importance as dyes. The colour of such compounds, which usually contain amino or hydroxy groups, can be attributed to the existence of a charge-transfer absorption band [1]. From a historical viewpoint, the advent of synthetic anthraquinone dyes can be traced to the elucidation of the structure of the important naturally occurring compound 1,2dihydroxyanthraquinone (6.2; alizarin) by Graebe and Liebermann in 1868 [2]. Natural alizarin is the colouring matter of madder and was the source of the then valuable mordant dye Turkey red (CI Mordant Red 11), an aluminium–calcium complex. In the following year Caro and Perkin independently discovered a commercially viable route to alizarin from anthraquinone via the 2-sulphonic acid (6.3) Treatment of the sodium salt of anthraquinone-2-sulphonic acid (called ‘silver salt’, from its appearance) with aqueous sodium hydroxide under pressure at an elevated temperature in the presence of an oxidising agent gives the disodium salt of alizarin as the main product (Scheme 6.1). Perkin soon devised a more efficient and cheaper route that involved chlorination of anthracene to give 9,10-dichloroanthracene, which was then sulphonated, oxidised and fused with sodium hydroxide to yield alizarin [3]. 6.1.1 Methods of synthesis Anthraquinone itself is traditionally available from the anthracene of coal tar by oxidation, often with chromic acid or nitric acid; a more modern alternative method is that of air oxidation using vanadium(V) oxide as catalyst. Anthraquinone is also produced in the reaction of benzene with benzene-1,2-dicarboxylic anhydride (6.4; phthalic anhydride) using a Lewis acid catalyst, typically aluminium chloride. This Friedel–Crafts acylation gives obenzoylbenzoic acid (6.5) which undergoes cyclodehydration when heated in concentrated sulphuric acid (Scheme 6.2). Phthalic anhydride is readily available from naphthalene or from 1,2-dimethylbenzene (o-xylene) by catalytic air oxidation. 280

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ANTHRAQUINONE ACID, DISPERSE, BASIC AND REACTIVE DYES

O

O

α

1

8

2

7

281

β

SO3H

H2SO4

3

6 4

5

O

O NaOH

6.1 Anthraquinone

O

6.3

OH OH

O 6.2 Alizarin

Scheme 6.1

O

O

H2SO4 O

6.1 AlCl3

Anthraquinone

COOH

O 6.5

6.4 Phthalic anhydride

Scheme 6.2

OH

O

O

OH

O

OH

H2SO4 O

6.4

O

+

H3BO3 Cl

6.6 Scheme 6.3

Quinazarin

Although anthraquinone is the starting point for the preparation of many derivatives, involving substitution and replacement reactions, certain compounds are obtained directly by varying the components in the above synthesis. Thus, for example, replacement of benzene with methylbenzene (toluene) leads to the formation of 2-methylanthraquinone. A particularly important variation on the phthalic anhydride route is the synthesis of 1,4dihydroxyanthraquinone (6.6; quinizarin) using 4-chlorophenol with sulphuric acid and boric acid as catalyst (Scheme 6.3). The absence of aluminium chloride permits hydrolysis of the chloro substituent to take place.

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CHEMISTRY OF ANTHRAQUINONOID, POLYCYCLIC AND MISCELLANEOUS COLORANTS

Electrophilic substitution at the anthraquinone ring system is difficult due to deactivation (electron withdrawal) by the carbonyl groups. Although the 1-position in anthraquinone is rather more susceptible to electrophilic attack than is the 2-position, as indicated by πelectron localisation energies [4], direct sulphonation with oleum produces the 2-sulphonic acid (6.3). The severity of the reaction conditions ensures that the thermodynamically favoured 2-isomer, which is not subject to steric hindrance from an adjacent carbonyl group, is formed. However, the more synthetically useful 1-isomer (6.7) can be obtained by sulphonation of anthraquinone in the presence of a mercury(II) salt (Scheme 6.4). It appears that mercuration first takes place at the 1-position followed by displacement. Some disulphonation occurs, leading to the formation of the 2,6- and 2,7- or the 1,5- and 1,8disulphonic acids, respectively. Separation of the various compounds can be achieved without too much difficulty. Sulphonation of anthraquinone derivatives is also of some importance. O

O

SO3H

H2SO4 Hg(II) O 6.1

O 6.7

Scheme 6.4

Anthraquinone-1-sulphonic acid is the traditional precursor of 1-aminoanthraquinone (6.8), the most important anthraquinone intermediate. Since it is expensive to eliminate mercury(II) ions from waste water, an alternative route via 1-nitroanthraquinone has been investigated. Nitration of anthraquinone gives, as well as the desired 1-nitro derivative, significant amounts of the 2-isomer together with 1,5- and 1,8-dinitroanthraquinones. Nevertheless, chemists at Sumitomo in Japan have optimised the nitration procedure with respect to both yield and purity of the 1-nitro compound. In particular, nitration is stopped when 80% of the anthraquinone has been substituted [5]. Nitration of anthraquinone derivatives is also of some significance. Direct halogenation is normally carried out only on derivatives of anthraquinone. The importance of chloro or bromo derivatives lies mainly in the displacement of the halogen by nucleophiles, especially amines. Simple derivatives, such as 1-chloroanthraquinone (6.9), are available from the appropriate sulphonic acid by replacement (Scheme 6.5). 1-Amino-4bromoanthraquinone-2-sulphonic acid (6.10; bromamine acid) is a very useful intermediate in the synthesis of dyes derived from 1,4-diaminoanthraquinone. Bromamine acid is obtained from 1-aminoanthraquinone (6.8) by sulphonation to give the 2-sulphonic acid (6.11), which is then brominated (Scheme 6.6). Direct bromination of 1aminoanthraquinone affords the 2,4-dibromo derivative. Hydroxyanthraquinones are also susceptible to halogenation. In the case of alizarin (6.2), for example, the 1-hydroxy group is involved in intramolecular hydrogen bonding with the adjacent carbonyl group so that bromination takes place at the 3-position (6.12), which is both ortho to the 2-hydroxy group and meta to the other carbonyl group (Scheme 6.7).

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ANTHRAQUINONE ACID, DISPERSE, BASIC AND REACTIVE DYES

O

NH2

O

283

SO3H

NH3

O

O

6.8

6.7

HCl O

Cl

NaClO3

O Scheme 6.5

6.9

O

NH2

O SO3H

NH2 SO3H

Br2

O

O

6.11

6.10 Bromamine acid

Scheme 6.6 H O

Br

H

O

O OH

O OH

Br2

Br

Scheme 6.7

O

O

6.2

6.12

Although some hydroxyanthraquinones, such as quinizarin (6.6), are available by direct synthesis, most hydroxy derivatives are produced by nucleophilic displacement of sulphonic acid groups or halogen atoms. Thus 1,5-dihydroxyanthraquinone (6.13; anthrarufin) and the 1,8-isomer (6.14; chrysazin) are obtained by alkaline hydrolysis of the corresponding disulphonic acids using calcium hydroxide or dilute sodium hydroxide. Under such relatively mild conditions additional hydroxy groups are not introduced. In the case of anthraquinone2-sulphonic acid (6.3), as pointed out earlier, hydroxylation as well as displacement can take place to produce alizarin (6.2). From a mechanistic point of view it seems likely that the reaction first involves hydroxylation to produce a hydroquinone (6.15), which is then oxidised to 1-hydroxyanthraquinone-2-sulphonic acid (6.16) before nucleophilic displacement of the sulphonic acid group takes place (Scheme 6.8).

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CHEMISTRY OF ANTHRAQUINONOID, POLYCYCLIC AND MISCELLANEOUS COLORANTS

O

OH

OH

OH

O

O

O

6.13

6.14

Anthrarufin

Chrysazin _ O

_ OH

O

OH

H

OH SO3Na

SO3Na

O

O

O

_ O

OH SO3Na

OH SO3Na

oxidation

6.2 Alizarin O

Scheme 6.8

_ O

6.15

6.16

Polyhydroxyanthraquinone derivatives can be obtained by means of the Bohn–Schmidt reaction, in which hydroxyanthraquinones containing at least one α-hydroxy group react with fuming sulphuric acid; thus, for example, alizarin can be converted into 1,2,5trihydroxyanthraquinone (6.17). The presence of boric acid leads to the formation of mixed anhydrides with sulphuric acid and to cyclic intermediates. The substitution mechanisms are complex but it has been established [6] that sulphur trioxide can interact with the 1-hydroxy group and the adjacent carbonyl group to form esters containing six-membered rings, exemplified by structure 6.18. The sulphuric acid esters are readily hydrolysed to hydroxy compounds during work-up. Sulphonation and subsequent desulphonation can also take place. The Bohn–Schmidt procedure is also applicable to aminoanthraquinones. Thus 1,4diaminoanthraquinone (6.19) can be converted into 1,4-diamino-5,8-dihydroxyanthraquinone (6.20). Nucleophilic displacement of halogen by amines is an important method of introducing amino groups into the anthraquinone ring system. In the Ullmann reaction the displacement is catalysed by metallic copper or by copper ions so that relatively mild conditions can be used. Mechanistic studies suggest that copper(I) ions exert a catalytic effect via complex formation. Derivatives of 1,4-diaminoanthraquinone are of considerable industrial significance. Many compounds are prepared from the reduced form of quinizarin (6.6).

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O

OH

285

OSO3H

HO3SO B

OH

O

O

OSO3H OH

O

H

6.17

O

SO3H

O S

O O

O

6.18

O

NH2 OH

O

NH2

OH

O

NH2

NH2

6.19 CI Disperse Violet 1

6.20

Leucoquinizarin can be represented by various tautomers such as structures 6.21 and 6.22; however, 1H- and 13C-n.m.r. have shown the dominant species to be the 2,3-dihydro tautomer (6.22) [7]. Alkylamines react with leucoquinizarin in a stepwise manner to give the corresponding 1,4-dialkylamino derivative (6.23) after air oxidation (Scheme 6.9). The less nucleophilic arylamines do not react unless boric acid is present as a catalyst. The complex thus formed (6.24) is more susceptible to nucleophilic attack at the 1- and 4positions than is 1eucoquinizarin. Unsymmetrical 1,4-diaminoanthraquinones can be synthesised by using two different amines; the corresponding symmetrical dyes are also formed to a certain extent. 6.1.2 Acid, disperse, basic and reactive dyes Anthraquinone dyes tend to predominate in the violet, blue and green hue sectors. Although they have the advantages of brightness and chemical stability, they are more expensive to manufacture than are azo dyes, which are also tinctorially stronger. In general, 1-substituted derivatives of anthraquinone are more bathochromic than the corresponding 2-substituted isomers, in accordance with PPP-MO calculations [1]. Intramolecular hydrogen bonding is not possible between the carbonyl group and a 2-

6.21

OH

OH

OH

O

OH

OH

OH

O

Leucoquinizarin

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CHEMISTRY OF ANTHRAQUINONOID, POLYCYCLIC AND MISCELLANEOUS COLORANTS

OH

O

OH

NHR

OH

O

RNH2 – H2O 6.22

OH

O

RNH2 – H2O

O

NHR

O

NHR

air

NHR

O

NHR

NHR

OH

NHR

6.23

Scheme 6.9

HO

OH B

H

O

O

O

O

O

6.24

B HO

O

OH

N

CH3

O 6.25

OH

substituent and, in the 2-substituted series, the bathochromic shift increases with electrondonor power in the sequence: OH < OCH3 < NHCOCH3 < NH2 < NHCH3 < N(CH3)2. An enhanced bathochromic effect is observed when a 1-substituent, such as hydroxy or methylamino (6.25), is able to hydrogen-bond with the adjacent carbonyl group and thereby assist the conjugation of the donor lone pair of electrons with the anthraquinone ring system; for example, a methylamino group is a more effective donor than is a dimethylamino group in the 1-substituted series. Moreover, the latter group is unable to conjugate fully due to steric hindrance between the bulky substituent and the adjacent carbonyl group, which

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287

decreases the bathochromic effect and reduces the tinctorial strength (intensity). Steric interaction is minimised by rotation of the dimethylamino group out of the plane of the anthraquinone ring system (6.26). Some illustrative spectral data are given in Table 6.1 [8]. Significant crowding effects can also arise from the presence of ortho groups, as in 1,2disubstituted anthraquinones. Table 6.1 Spectral data for some monosubstituted anthraquinones in methanol [8] 1-position

2-position

Substituent

λmax (nm)

εmax

λmax (nm) εmax

OH OCH3 NHCOCH3 NH2 NHCH3 NHC6H5 N(CH3)2

402 378 400 475 503 500 503

5500 5200 5600 6300 7100 7250 4900

368 363 367 440 462 467 472

3900 3950 4200 4500 5700 7100 5900

Table 6.2 Spectral data for some disubstituted anthraquinones in methanol [8] Substituents

λmax (nm)

εmax

1,4-diOH 1,5-diOH 1,8-diOH 1,4-diNH2 1,5-diNH2 1,8-diNH2

470 425 430 550, 590 487 507

17 10 10 15 12 10

000 000 960 850, 15 850 600 000

Anthraquinone itself can be regarded approximately as consisting of two ortho-interlinked benzoyl groups [9]. Thus, the introduction of a second group into the unsubstituted ring of a monosubstituted anthraquinone effectively doubles the extinction coefficient (compare εmax values for α-hydroxy and α-amino substitution in Tables 6.1 and 6.2). However, the presence of two donor groups in the 1,4-positions gives rise to a pronounced increase in intensity, together with a significant bathochromic shift (Table 6.2) [8]. These enhanced effects have been widely exploited in commercial dyes. The order of bathochromicity for amino-substituted anthraquinones is: 2 < 1 < 1,5 < 1,8 < 1,2 < 1,4 < 1,4,5,8. This sequence can be explained by VB theory [9] as well as by PPP-MO calculations [1], but only the latter approach can account for variations in tinctorial strength. The twin absorption peaks of 1,4-diaminoanthraquinones are thought to be caused by vibrational fine structure associated with one electronic transition in the visible region. 1,4-Bis(alkylamino)anthraquinones are generally blue. The introduction of arylamino substituents into the 1,4-

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positions gives rise to an additional absorption band in the 400 nm region that imparts a yellow component to the dominant blue colour. A good example is CI Acid Green 25 (6.27), which in aqueous solution absorbs at 410, 608 and 646 nm [10]. HO3S O

NH

CH3

O

NH

CH3

6.27

HO3S CI Acid Green 25

Acid dyes In 1871 Graebe and Liebermann discovered that alizarin (6.2) could be applied to wool by mordant dyeing after sulphonation to produce the 3-sulphonic acid (6.28). This dye is still listed in the latest revision of the Colour Index as a commercial product [11]. Although many sulphonated polyhydroxyanthraquinones have been examined, few remain in current use. Another, and more important, classic dye that continues in commercial use as an acid dye is CI Acid Blue 45 (6.29). This dye was discovered in 1897 by Schmidt and can be made from anthrarufin (6.13) by disulphonation, subsequent dinitration and reduction. The dye gives an attractive blue on wool with good all-round fastness properties. O

OH

OH OH

O

NH2

HO3S

SO3H

SO3H

NH2

O

O

OH

6.28

6.29

CI Mordant Red 3

CI Acid Blue 45

Many blue acid dyes have been derived from bromamine acid (6.10) by nucleophilic displacement of the halogen atom using arylamines of varying complexity in the presence of a copper catalyst. Simple examples include CI Acid Blue 25 (6.30; R = H) and the analogous cyclohexyl derivative CI Acid Blue 62 (6.31). Reaction with p-butylaniline gives CI Acid Blue 230 (6.30; R = butyl), which possesses good washing fastness owing to hydrophobic interaction with the substrate. Somewhat more complex structures are illustrated by CI Acid Blue 40 (6.32) and CI Acid Blue 129 (6.33). CI Acid Blue 40 is greener than the parent dye (6.30; R = H) due to the electronic effect of the acetylamino substituent.

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O

NH2

O

NH2

SO3H

SO3H

NH

O

289

R

NH

O 6.31

CI Acid Blue 62

6.30

O

O

NH2

NH2 SO3H

SO3H

CH3 NH

O

O

NHCOCH 3

HN

6.33

6.32

CI Acid Blue 129

CI Acid Blue 40

H3C

CH3

The progressive introduction of methyl groups into the pendant phenyl ring of CI Acid Blue 25 (6.30; R = H) leads to an increase in dye uptake and to improved wet fastness properties on wool [12]. Steric crowding in the case of CI Acid Blue 129 (6.33) reduces the conjugation of the 4-substituent with the remainder of the system and results in a reddish blue hue. The aliphatic cyclohexyl ring in CI Acid Blue 62 (6.31) has a similar effect on the hue. Use of the intermediates 1-amino-2,4-dibromoanthraquinone and 1-amino-4-bromo-2methylanthraquinone leads to similar dyes, exemplified by CI Acid Blue 78 (6.34; X = Br) and CI Acid Blue 47 (6.34; X = CH3) respectively; in the production of these two dyes, sulphonation is the final step. O

NH2 X

O

NH

CH3

HO3S 6.34

Milling dyes with very good wet fastness are obtained by the reaction of one mole of a diamine with two moles of bromamine acid, as in the case of CI Acid Blue 127 (6.35). This dye is suitable for dyeing wool bright blue from a neutral or weakly acid bath.

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O

NH2 SO3H

NH2

O

NH

O

HO3S

CH3 NH

O

C CH3 6.35 CI Acid Blue 127

Condensation of one mole of leucoquinizarin (6.22) with two moles of an arylamine in the presence of boric acid, followed by oxidation and sulphonation, is the route to some symmetrically substituted acid dyes; these are blue to green according to the structure of the amine. The classical, but still important, dye CI Acid Green 25 (6.27) has already been mentioned. A related dye of commercial significance is CI Acid Blue 80 (6.36), which can be obtained either from leucoquinizarin or from 1,4-dichloroanthraquinone. Unsymmetrically substituted 1,4-diaminoanthraquinone derivatives bearing only one sulphonic acid group are better for the level dyeing of nylon than are their disulphonated analogues. SO3H CH3

H3C

O

HN CH3

CH3 O

6.36 CI Acid Blue 60

HN

H3C

CH3 SO3H

A suitable example is CI Acid Blue 27 (6.37; X = NHCH3), which is made from 4bromo-1-methylaminoanthraquinone by nucleophilic displacement followed by sulphonation. Some commercial acid dyes based on 1,4-diaminoanthraquinone contain substituents in both the 2- and the 3-positions. Thus, for example, CI Acid Violet 41 (6.38) is produced by the condensation of 1,4-diamino-2,3-dichloroanthraquinone with phenol in the presence of sodium sulphite and manganese dioxide. Aminohydroxyanthraquinone derivatives are available from leucoquinizarin by heating with an appropriate arylamine and boric acid in aqueous ethanol, followed by oxidation. The use of p-toluidine and subsequent sulphonation gives CI Acid Violet 43 (6.37; X = OH). The photodegradation of CI Acid Green 25 (6.27) on nylon proceeds by hydrogen atom abstraction to produce the leuco compound [13].

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O

O

X

291

NH2 O

SO3H O

NH

O

CH3

NH2

6.38

HO3S

CI Acid Violet 41

6.37

Disperse dyes Anthraquinone dyes are second only to azo dyes in importance as disperse dyes and are predominant in the red, violet, blue and blue-green sectors [14]. Because anthraquinone disperse dyes are relatively expensive to manufacture, successful attempts were made to replace some of them with technically equivalent and more economical products [15]. The replacement process has been most successful in the red region using, for example, heterocyclic azo dyes and novel chromogens. The brilliance of the anthraquinones with their narrow spectral absorption bands is difficult to attain with other structures, however, as is their high light fastness and chemical stability. The development of anthraquinone disperse dyes is included in a review by Dawson [16]. Dyes for cellulose acetate are relatively simple molecules, typified by CI Disperse Red 15 (6.39; X = OH), CI Disperse Violet 4 (6.39; X = NHCH3) and CI Disperse Blue 3 (6.40), the last-named being manufactured from leucoquinizarin and the appropriate amines. The unsymmetrically substituted product inevitably contains significant amounts of the related symmetrical compounds. The widely used CI Disperse Blue 3 is known to cause skin sensitisation when on nylon [17] and can also provoke an allergic reaction [18]. Bright red 2-alkoxy-1-amino-4-hydroxyanthraquinones, such as CI Disperse Red 4 (6.41), can be obtained from 1-amino-2,4-dibromoanthraquinone by hydrolysis to give 1-amino-2-bromo4-hydroxyanthraquinone (CI Disperse Violet 17), which is then condensed with the appropriate alcohol. O

NH2

O

O

NHCH3

NH2 OCH3

O

X

O

6.39

NHCH2CH2OH

O

OH

6.40

6.41

CI Disperse Blue 3

CI Disperse Red 4

Suitable disperse dyes for polyester require good sublimation fastness and generally contain additional or more hydrophobic substituents compared with acetate dyes. Thus, for example, CI Disperse Red 60 (6.42) is important for the dyeing of polyester fabrics but has only moderate sublimation fastness. It is, however, the most important red dye for transfer

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printing. Improved sublimation fastness can be achieved if substituents are introduced into the phenoxy ring. Exceptional fastness is achieved by chlorosulphonation of CI Disperse Red 60 followed by condensation with 3-methoxypropylamine to give the brilliant red dye 6.43 [16]. O

NH2 O O

O

NH2 SO2NH(CH2)3OCH3

O

OH

6.42 CI Disperse Red 60 O

OH 6.43

Monoarylation of 1-amino-4-hydroxyanthraquinone (6.39; X = OH) results in violet dyes such as CI Disperse Violet 27 (6.44; R = H) and the bluish violet CI Disperse Blue 72 (6.44; R = CH3); the latter dye is also important as CI Solvent Violet 13. Chlorination of 1,4-diaminoanthraquinone with sulphuryl chloride gives the 2,3-dichloro derivative (CI Disperse Violet 28), which on condensation with phenol yields CI Disperse Violet 26 (6.45). Monoaryl or dialkyl derivatives of 1,4-diaminoanthraquinone (6.19; CI Disperse Violet 1) are blue. Typical examples include CI Disperse Blue 19 (6.46) and CI Disperse Blue 23 (6.47). O

O

NH

NH2

R

O

O O

O

OH

6.45 CI Disperse Violet 26

6.44

O

NH

O

NHCH2CH2OH

O

NH2

O

NHCH2CH2OH

6.47

6.46 CI Disperse Blue 19

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292

CI Disperse Blue 23

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Many commercially important blue dyes are derived from tetra-α-substituted anthraquinones, especially the diamino derivatives of anthrarufin (6.13) and chrysazin (6.14). Examples include CI Disperse Blue 26 (6.48) and CI Disperse Blue 56 (6.49). Progressive methylation of 1,8-diamino-4,5-dihydroxy- and 1,5-diamino-4,8-dihydroxyanthraquinone leads to a series of reddish-blue to greenish-blue dyes [19]. The presence of N,N-dimethylamino substituents results in marked hypsochromic shifts in the visible absorption maxima due to steric hindrance (compare 6.26). The presence of amino groups is often associated with inferior fastness to burnt gas fumes. This drawback can be overcome by the use of dyes such as CI Disperse Blue 27 (6.50), which, although relatively expensive, has very good all-round fastness properties on both cellulose acetate and polyester. This dye can be obtained from chrysazin (6.14) by nitration and subsequent condensation with p-(2hydroxyethyl)aniline. Bright blue dyes for polyester can be manufactured from the wellknown acid dye CI Acid Blue 45 (6.29) by the C-arylation of its boric ester with, for example, a mixture of phenol and anisole, followed by removal of the sulphonic acid groups. This procedure gives CI Disperse Blue 73 (6.51; R = H or CH 3). 1,4,5,8-Tetraaminoanthraquinone (CI Disperse Blue 1) is a cellulose acetate dye of long standing, as is the product of partial methylation, CI Disperse Blue 31. The parent dye is now known to be carcinogenic [20]. OH

O

NHCH3

OH

O

NH2

Br

H3CHN

O

OH

6.48

O

NH

OH

O

OH

O

OH

6.49

CI Disperse Blue 26

NO2

NH2

CH2CH2OH

6.50 CI Disperse Blue 27

CI Disperse Blue 56

OR OH

O

NH2

NH2

O

OH 6.51

Few heterocyclic anthraquinone analogues are of established importance as disperse dyes. An interesting exception is the system 6.52, which provides bright turquoise blue dyes of very good light fastness on polyester [21]. A route to CI Disperse Blue 60 (6.52; R = CH2CH2CH2OCH3) involves hydrolysis of 1,4-diamino-2,3-dicyanoanthraquinone followed by alkylation of the resulting imide; several closely related products can be obtained by varying the alkyl group.

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Not many green anthraquinone disperse dyes are available commercially. One example is CI Disperse Green 6:1 (6.53), which has wider use as CI Solvent Green 3 and is the precursor of CI Acid Green 25 (6.27).

O

NH2

NH2

NH

CH3

O

NH

CH3

O

N

O

O

R

O 6.53 CI Disperse Green 6:1

6.52

Basic dyes Some anthraquinone dyes with pendant cationic groups are used commercially on acrylic and modacrylic fibres [22]. Only two disclosed structures are included in the latest revision of the Colour Index, the dyes being the reddish blue CI Basic Blue 47 (6.54) and the greenish blue CI Basic Blue 22 (6.55). O

O

NH2

_ + CH2NH(CH3)2 X

NH

6.54

O

NHCH3

O

_ + NHCH2CH2CH2N(CH3)3 X

6.55 CI Basic Blue 22

CI Basic Blue 47

Reactive dyes The chemistry of reactive dyes is discussed in detail in Chapter 7. However, it is appropriate to point out in this section that the anthraquinone system provides several commercially valuable bright blue reactive dyes. Bromamine acid (6.10) is the key intermediate for the synthesis of the vinylsulphone dye CI Reactive Blue 19 (6.56), which finds application in dyeing and printing. The dichlorotriazinyl dye CI Reactive Blue 4 (6.57) is derived from the same intermediate and is used in protein purification [23]. CI Reactive Blue 6 (6.58), obtained from the reaction of 1,4-diaminoanthraquinone with epichlorohydrin, was the only blue dye in the Procinyl (ICI) range for nylon. Anthraquinone blues have been replaced to some extent by derivatives of the intrinsically bright triphenodioxazine chromogen [24]. 6.2 POLYCYCLIC VAT DYES Vat dyes are insoluble in water and are applied to cellulosic fibres, usually with sodium dithionite under alkaline conditions, by a vatting process involving reduction to produce a

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O

NH2

O

295

NH2

SO3H

SO3H Cl N

O

NH

SO2CH2CH2OSO3H

O

NH

N

NH N

6.57

6.56

Cl

SO3H

CI Reactive Blue 4

CI Reactive Blue 19 OH O

NHCH2CHCH2Cl

O

NHCH2CHCH2Cl OH

6.58 CI Reactive Blue 6

water-soluble leuco form which possesses affinity for the substrate. Air or chemical oxidation then results in regeneration of the insoluble form of the colorant trapped within the fibre structure. Vat dyes based on anthraquinone and related polycyclic systems generally exhibit outstanding fastness properties. Most vat dyes contain two carbonyl groups linked by a conjugated (quinonoid) system (Scheme 6.10). They can be applied as the water-soluble disulphuric esters of the leuco forms; these have low affinity for cellulose and can be obtained by esterifying the leuco compound with chlorosulphonic acid. Reduction and esterification may be carried out without isolating the leuco compound using pyridine and iron together with chlorosulphonic acid [25]. O

OH

ONa Na2S2O4 / NaOH

2H+

air O

OH

ONa OSO3H 2ClSO3H

OSO3H

Scheme 6.10

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Anthraquinonoid systems are the most important vat dyes, providing hues covering the whole of the visible spectrum. Nevertheless, the main commercial products are found within the blue, green, brown and black sectors. Some of the relatively simple yellow and orange dyes cause phototendering of the dyed substrate, bringing about adverse structural changes on exposure to light. Most of the currently available vat dyes have been known for many years and very few new dyes have been marketed during the last thirty years [26]. Recent research efforts have concentrated on improved methods of manufacture and more economical finishing of traditional vat dyes. The general stability of the polycyclic vat dyes permits many of them to be used as pigments [27]. Several types of polycyclic vat colorants provide structures suitable for use on cellulosic fibres for infrared camouflage [28]. 6.2.1 Anthraquinones In 1909 Deinet observed that the product of N-benzoylation of 1-aminoanthraquinone could be applied as a vat dye. Although the leuco form of 1-aminoanthraquinone has no affinity for cotton, the corresponding benzoyl derivative does have a moderate affinity. Acylaminoanthraquinones are structurally the simplest of the vat dyes, and are used for yellow, orange, red and violet hues. Numerous acylaminoanthraquinones have been examined for use as vat dyes but only a few have survived competition from more costeffective types. One example is CI Vat Yellow 26 (6.59), which is made from 1aminoanthraquinone and isophthaloyl chloride. Another yellow dye of commercial importance is CI Vat Yellow 33 (6.60), which is made by reducing 4′-nitrobiphenyl-4carboxylic acid with glucose and sodium carbonate. The resulting azo compound is acylated with thionyl chloride before condensing with 1-aminoanthraquinone. It is of significance that the azo bridge remains intact during the vatting process; in this dye the azo group is unable to tautomerise into a hydrazone and consequently resists reduction [29]. O O

O

HN

O

C

C

6.59

NH

O

O

CI Vat Yellow 26

O

O

C O

N

N

C

HN

NH

O

6.60 CI Vat Yellow 33 O

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297

Derivatives of 1,4- and 1,5-diaminoanthraquinone provide some important examples in this class. Relatively simple dyes include CI Vat Yellow 3 (6.61; X = H) and CI Vat Violet 15 (6.61; X = OH). O

C

X

O

HN

NH

O

X

C

O 6.61

It is generally found that dyes based on the 1,4-diaminoanthraquinone system are more bathochromic than isomers derived from 1,5-diamino-anthraquinone. This difference can be related to the relative stabilities of the dipolar excited states of the dyes. Qualitatively, the 1,4-disubstituted derivatives are aromatic (6.62) and therefore at a lower energy, whereas the 1,5-disubstituted isomers (6.63) are not. Consequently, the energy difference between the ground state of the molecule and its first excited state is smaller for 1,4diaminoanthraquinones than for the 1,5-isomers. For example, the 1,4-isomer of CI Vat Yellow 3 (6.61; X = H) is CI Vat Red 42, although this dye is no longer in commercial use. A more complex example is CI Vat Red 21 (6.64), which contains amino groups in the acyl ring systems. _ O

+ NH2

O_

NH2 +

_ O

H2N +

6.62

O

+ NH2

O_

6.63

NH2

NH2 CONH

NHCO

O

O

O

O

6.64 CI Vat Red 21

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6.2.2 Indanthrones In 1901 Bohn discovered the first vat dyes based on anthraquinone, a landmark in the history of synthetic dyes. In an attempt to prepare an anthracene analogue of indigo (6.113), Bohn applied the Heumann synthesis to 2-aminoanthraquinone by fusing the condensation product of this amine and chloroacetic acid with potassium hydroxide. The brilliant blue product was named indanthrene, a name coined from those of indigo and anthracene. Bohn showed that the same product resulted from the alkali fusion of 2-aminoanthraquinone, so the dye could not be an analogue of indigo. The new vat dye was marketed by BASF as Indanthren Blue R (CI Vat Blue 4) and was subsequently given the chemical name indanthrone (6.65). The structure was proposed on the basis of elemental analysis (C28H14O4N2), suggesting the loss of four atoms of hydrogen from two molecules of 2aminoanthraquinone), the absence of azo and of primary amino groups and the exceptional stability. This was confirmed by synthesis: for example, Bohn obtained indanthrone by heating 1-amino-2-bromoanthraquinone in nitrobenzene solution under Ullmann conditions. O 17

2

H O

O

1

16

3

N

4

13

N

12

O

O

H

N N

O

7

11 10

8 O

6.65 CI Vat Blue 4

O

6.66

Indanthrone has low fastness to bleaching because the corresponding yellowish green azine (6.66) is formed. Its importance at the time of its discovery lay in the dearth of fast dyes for cotton. Indanthrone is also used as a pigment (CI Pigment Blue 60). Very careful control of the fusion conditions is necessary in order to minimise the formation of by-products. Alkali fusion at approximately 220 °C gives mainly the blue alkali salt of leuco indanthrone; air oxidation is then required to produce the dye. Fusion at lower temperatures results in the formation of alizarin (6.2), whereas reaction at a higher temperature (above 300 °C) affords significant amounts of a yellow dye as well as the blue indanthrone. The yellow dye was first marketed by BASF as Flavanthren (CI Vat Yellow 1) and was subsequently given the chemical name flavanthrone (6.67). Today flavanthrone is used mainly as a pigment (CI Pigment Yellow 24). Flavanthrone is readily reduced to a blue leuco form; hydrogen abstraction to generate the leuco form can also take place on cellulose, leading to fibre damage. The leuco compound is believed, on the basis of spectroscopic evidence, to have the structure 6.68 [30]. The yellow to blue colour change is made use of in testing for the presence of a reducing agent under alkaline conditions. The structure of flavanthrone was confirmed by an Ullmann synthesis using the Schiff base (6.69) obtained from 2-amino-1-chloroanthraquinone, which gave a bianthraquinonyl precursor (6.70), as in Scheme 6.11. Hydrolysis of compound 6.70 gives the corresponding

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diamine, which subsequently undergoes cyclodehydration to flavanthrone. In a related industrial synthesis, the diphthalimide of 2-amino-1-chloroanthraquinone is converted into the corresponding bianthraquinonyl, which undergoes hydrolysis and ring closure in dilute sodium hydroxide. O

OH

N

N N

N

6.67 O

CI Vat Yellow 1

6.68

OH O

O

Cl N

CH

2

CH

N O

O N

CH

6.69

O

O

Scheme 6.11

6.70

The formation of indanthrone and flavanthrone, as well as alizarin, during the alkali fusion of 2-aminoanthraquinone can be explained mechanistically on the basis of the initial loss of a proton. The resulting anionic species can be represented as a resonance hybrid and is also tautomeric (Scheme 6.12). Primary 1-hydroxylation of 2-aminoanthraquinone is probably the first step in the formation of the alizarin by-product (compare Scheme 6.8). Such an attack may initiate the formation of flavanthrone [31]. It is also possible to envisage the formation of all three species by a radical mechanism [32]. The normal blue vat dye obtained from indanthrone corresponds to a dihydroanthraquinone (compare flavanthrone) and yields a sparingly soluble disodium salt. It seems likely that strong hydrogen bonding between the inner hydroxy groups and the azine nitrogen atoms accounts for the ready formation of the planar molecule 6.71. The infrared spectrum of the disodium salt indicates the absence of carbonyl groups. Treatment of the salt with dimethyl sulphate produces the corresponding dimethyl diether. Halogenated indanthrones provide improved resistance to hypochlorite treatments and many derivatives have been examined. Direct halogenation of indanthrone leads to substitution at the 7-, 8-, 16- and 17-positions. The commercially important 7,16dichloroindanthrone (6.72; CI Vat Blue 6) can be synthesised by a rather lengthy route from phthalic anhydride and chlorobenzene (Scheme 6.13). Polycyclic vat dyes of this type also find use as electrophotographic photoreceptors.

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CHEMISTRY OF ANTHRAQUINONOID, POLYCYCLIC AND MISCELLANEOUS COLORANTS O

O NH2

NH

_ + OH – H2O _

O

O resonance

O

O

_ NH2

H

_ NH

tautomerism

O

O

2-amino AQ 220 °C

2-amino AQ 300 °C O

O

_

_

H2N

O

H

O

H NH2

O

O

_

N

H

NH2

O

O

_ + OH – H2O

+ OH – H2O

O

O

_H H2N

O

_

O

N NH2

O

O

NH2

O

_

_

O oxidation

oxidation

O O

H2N

H

O

O

O

N NH2

NH2

O

O

O cyclodehydration

repeat sequence

6.67

6.65

Flavanthrone

Indanthrone

Scheme 6.12

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O

ONa

Cl H

H O

N

O N

N

O

N

O H

H

Cl ONa

O

6.71

6.72 CI Vat Blue 6

O Cl O

+

CO2H

Cl

AlCl3

O

O

Phthalic anhydride HNO3 / H2SO4

CO2H

CO2H

Cl

Cl

Fe / HCl NH2

NO2

O

O

H2SO4 O

O Cl

NH2 O

Cl

Br2

NH2

NO2 O

Br Ullmann reaction

6.72

Scheme 6.13

CI Vat Blue 6

6.2.3 Benzanthrone derivatives Benzanthrone (6.73) is the source of various commercially important violet, blue and green vat dyes. This tetracyclic system can be prepared from a mixture of anthraquinone and propane-1,2,3-triol (glycerol) by heating with iron powder in concentrated sulphuric acid. The reaction involves reduction of anthraquinone to anthrone (6.74) followed by condensation (Scheme 6.14) with propenal (acrolein), the latter compound being generated

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CHEMISTRY OF ANTHRAQUINONOID, POLYCYCLIC AND MISCELLANEOUS COLORANTS 2 1

H

H

3

11

CH2

CHCHO

10

4

9

5 8

6

O

O

6.74

6.73

Anthrone

Benzanthrone

Scheme 6.14

from glycerol by dehydration. The structure of benzanthrone was confirmed by Scholl who prepared it by cyclising 1-benzoylnaphthalene with aluminium chloride. The benzanthrone system is susceptible to both electrophilic and nucleophilic attack. The most reactive sites towards electrophiles are the 3- and 9-positions, which can be compared with the 4,4′-positions in biphenyl. The 9-position is somewhat deactivated by the carbonyl group, however. Thus, for example, monobromination takes place at the 3-position and further substitution gives 3,9-dibromobenzanthrone. Nitration and benzoylation similarly give rise to the 3-substituted product. The 3-position is in fact peri-hindered (compare naphthalene) so that sulphonation yields the 9-sulphonic acid. Electron withdrawal by the carbonyl group activates the 4- and 6-positions towards nucleophilic attack: for example, hydroxylation occurs at these sites. In 1904 Bally obtained a bluish violet solid by alkali fusion of benzanthrone at approximately 220 °C. Two isomeric compounds were isolated by vatting the reaction mixture and filtering off a sparingly soluble sodium salt. Oxidation of the filtrate gave a blue vat dye, violanthrone (6.75; CI Vat Blue 20), as the main component. The less soluble residue similarly afforded a violet product, isoviolanthrone (6.76; CI Vat Violet 10). The formation of isoviolanthrone can be suppressed by carrying out the fusion in a solvent such as naphthalene or a polyethylene glycol in the presence of sodium acetate and sodium nitrite. Dyes of this type are often referred to as dibenzanthrones. 2

O 17

3

1 18

4

17

3

14

15 6

13

5

12 6 11

7 12

4

13

O

14

8

7 8

10 O

9 O

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16

11

1

16

302

6.75

6.76

CI Vat Blue 20

CI Vat Violet 10

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It was established in 1929 by Lüttringhaus and Neresheimer that 4,4′-bibenzanthronyl (6.77) is an intermediate in the formation of violanthrone. Thus, compound 6.77 results when an Ullmann reaction is carried out on 4-chlorobenzanthrone; the same product is obtained when benzanthrone reacts under relatively mild conditions (approximately 110 °C) with a mixture of potassium hydroxide and potassium acetate in 2-methylpropan-1-ol (isobutanol). Alkali fusion at a higher temperature then converts 4,4′-bibenzanthronyl into violanthrone. Use of aluminium chloride also leads to ring closure (Scholl reaction). Mechanistically it is likely that the resonance-stabilised anion resulting from deprotonation at the 4-position of benzanthrone couples with non-ionised benzanthrone. Linkage across the 3- and 3′-positions under more strongly alkaline conditions may also involve loss of a proton, coupling and oxidation (Scheme 6.15). Chlorination of benzanthrone followed by an Ullmann reaction produces 3,3′-bibenzanthronyl (6.78), which readily undergoes 4,4′-coupling under mild conditions. 3,3′-Bibenzanthronyl is available from benzanthrone directly by treatment with manganese dioxide in sulphuric acid. Isoviolanthrone (6.76) was first made by heating 3-chlorobenzanthrone with ethanolic potassium hydroxide at 150 °C (Scheme 6.15). The reaction probably involves deprotonation at the 4-position to form a carbanion which then displaces the chloro substituent in another molecule of 3-chlorobenzanthrone. Repeating the sequence in the intermediate 3,4′-bibenzanthronyl derivative then gives isoviolanthrone. In an improved, but mechanistically obscure, method of synthesis 3-chlorobenzanthrone is treated with sodium sulphide to produce 3,3′-bibenzanthronyl sulphide; this compound is converted into isoviolanthrone on refluxing with potassium hydroxide in isobutanol. Isoviolanthrone itself is no longer used as a vat dye but some halogenated derivatives are commercially available. For example, dichlorination of isoviolanthrone in nitrobenzene gives CI Vat Violet 1, which is also useful as a pigment (CI Pigment Violet 31). The greenish blue 6,15-dimethoxy derivative (CI Vat Blue 26) is also of some importance. Nitration of violanthrone gives CI Vat Green 9, the main component of which is the 16nitro derivative. Vatting gives the corresponding amino compound, which gives a bluish black hue when oxidised on the fibre. Several halogenated derivatives, such as the trichloro derivative CI Vat Blue 18, are of industrial significance. Of special importance is the outstanding bright green vat dye originally marketed by Scottish Dyes as Caledon Jade Green (6.79; CI Vat Green 1), discovered in 1920 by Davies, Fraser-Thomson and Thomas. Oxidation of violanthrone with manganese dioxide and sulphuric acid gives 16,17-dihydroxyviolanthrone. Methylation of this diol yields the dimethoxy derivative (6.79). This dye has excellent fastness to light, alkali and chlorine. Its constitution was confirmed by a lengthy synthesis from 2-methoxybenzanthrone. An alternative method of manufacture involves the oxidation of 4,4′-bibenzanthronyl (6.77) with manganese dioxide and sulphuric acid to give violanthrone-16,17-dione (6.80), which on reduction with sodium bisulphite gives an excellent yield of the 16,17-diol. Treatment of CI Vat Green 1 with bromine in oleum gives the 3,12-dibromo derivative (CI Vat Green 2). The substitution pattern was confirmed by the n.m.r. spectrum of the dimethyl diether of the leuco compound. Many other alkyl derivatives of 16,17-dihydroxyviolanthrone have been examined but few remain of commercial significance. Of some interest is the navy blue vat dye CI Vat Blue 16, in which the 16- and 17-positions are connected by an ethylenedioxy bridge (6.81). Violanthrone and isoviolanthrone are effectively cis and trans isomers. The important cis compound absorbs more intensely and at a longer wavelength than the trans isomer (Table

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_

H + OH

– H2O O

_ O

6.73 Benzanthrone

resonance

_ +

O

O

KOH / KOAc 110 °C

6.73

H

_ O O oxidation

O

O

6.77 4,4′-Bibenzanthronyl (1) KOH / KOAc or AlCl3 220 °C (2) oxidation

6.76 Isoviolanthrone

6.75 Violanthrone

KOH / EtOH 150 °C

KOH / KOAc 110 °C

Cl

Ullmann reaction O

O

O

6.78 3,3′-Bibenzanthronyl

Scheme 6.15

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H3C

CH3 O

O

12

3

O

O 6.79

O

305

O

O

H2C

CI Vat Green 1

O

CH2 O

O

O

6.80

O 6.81

Violanthrone-16,17-dione

CI Vat Blue 16

6.3). These differences are found to be general for this type of vat dye and can be accounted for theoretically by the PPP-MO approach [33]. It is of interest to compare the absorption maxima of violanthrone and some of its derivatives in Table 6.3. Steric crowding at an essential single bond [34] normally results in a hypsochromic shift of the absorption band as shown by the 16,17-dimethyl derivative and, to a lesser extent, by the 16-methoxy compound. In contrast, however, the blue 16,17-bridged violanthrone (6.81) exhibits a bathochromic effect that is somewhat greater in the green dimethoxy analogue. Since the steric requirements of the two methoxy groups are not significantly less than those of the equivalent methyl groups, it must be concluded that this bathochromic shift is electronic in origin. A possible explanation is the formation of a stabilised excited state (6.82) in the case of the dimethoxy dye in which some attraction exists between the adjacent oxygen atoms; any steric clash can be minimised by rotation of the methoxy groups away from the plane of the ring system. A pronounced bathochromic effect is also exhibited by the 7,8-dimethoxy isomer, in which the substituents are similarly situated, but not by the 3,12-dimethoxy compound [32]. It is likely that the constraint imposed by the puckered eight-membered ring in the 16,17-bridged dye reduces the interaction between the oxygen lone-pair orbitals and the π-orbitals of the polycyclic system. Table 6.3 Spectral data for some dibenzanthrones in dimethylformamide CI Vat

Compound

λmax (nm)

εmax

Violet 10 Blue 20

Isoviolanthrone (6.76) Violanthrone (6.75) 16-Methoxyviolanthrone 16,17-Dimethoxyviolanthrone 16,17-Bridged violanthrone 16,17-Dimethylviolanthrone

588 600 598 636 618 575

41 60 38 41

Green 1 Blue 16

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H3C

+ O O

CH3

_ O

O 6.82

6.2.4 Carbazole derivatives Diphthaloylcarbazole derivatives constitute an important group of vat dyes, possessing good all-round fastness properties and providing hues ranging from yellow to black. The precursors of the carbazoles are the anthrimides, in particular the 1,1′dianthraquinonylamines (1,1′-dianthrimides). These intermediates were originally used as vat dyes themselves, but very few give bright colours and the vatting process also brings about reduction to 1-aminoanthraquinone. Only one dianthrimide is currently active in the Colour Index [11]; this grey vat dye is CI Vat Black 28 (6.83). 1,1′-Dianthrimides are easily prepared by heating a 1-aminoanthraquinone with a 1-chloroanthraquinone in nitrobenzene or another solvent in the presence of sodium carbonate and a copper catalyst. O C O

HN

O

NH

O

NH

O

C O 6.83 CI Vat Black 28

In 1910 Mieg found that 1,1′-dianthrimides undergo carbazolisation (ring closure) when heated with potassium hydroxide at approximately 220 °C or with aluminium chloride at a somewhat lower temperature (Scholl reaction). The simplest dye of this type (6.84; CI Vat Yellow 28) is no longer manufactured. Cyclisation under strongly basic conditions is probably initiated by deprotonation (Scheme 6.16); an oxidation step is necessary after ring closure. The characteristic structure of this type of vat dye, which is stabilised by intramolecular hydrogen bonding (6.84), was established by an alternative synthesis based on the Graebe

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_ O

O N

O

O N

– H2O

H O

_ + OH

O

O

H

O

H

O N

_ O

O

O

N

oxidation

H O

O

O

H

O

6.84 CI Vat Yellow 28 Scheme 6.16

route to carbazole and involving an Ullmann reaction with 2-chloro-1-nitroanthraquinone, followed by reduction and cyclisation. The parent dye (6.84), which has a phototendering action on cellulose, is now obsolete but several relatively simple benzoylamino derivatives are of commercial importance. Thus CI Vat Orange 15 (6.85; R1 = R4 = NHCOPh, R2 = R3 = H) is a 1,5-diaminoanthraquinone type whereas the more bathochromic olive vat dye CI Vat Black 27 (6.85; R1 = R4 = H, R2 = R3 = NHCOPh) is a 1,4-diaminoanthraquinone derivative. The unsymmetrically substituted isomer (6.85; R1 = R3 = H, R2 = R4 = NHCOPh) is a useful brown vat dye (CI Vat Brown 3). R3

R2 O

O N

R1 O

H

R4 O

6.85

The presence of two benzoylamino groups in the dianthrimide system permits carbazole ring closure to take place relatively easily in concentrated sulphuric acid at approximately 30 °C. It is likely that the benzoylamino groups assist protonation at the adjacent carbonyl group and thereby aid the subsequent cyclisation process. The final oxidation step can take place with

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sulphuric acid alone. It is possible that the cyclisation occurs by a radical mechanism [32]. Various more complex dyes of industrial significance contain more than one carbazole system. For example, CI Vat Orange 11 (6.86) contains two carbazole components and a central unit derived from 1,5-diaminoanthraquinone. This dye is prepared by carbazolisation of the trianthrimide produced when two moles of 1-chloroanthraquinone react with one mole of 1,5-diaminoanthraquinone. The equivalent isomeric dye obtained from 1,4-diaminoanthraquinone is reddish brown (6.87; CI Vat Brown 1). The interesting, symmetricallysubstituted tetracarbazole dye CI Vat Green 8 (6.88) was first synthesised in 1911 by Hepp from 1,4,5,8-tetrachloroanthraquinone. Not surprisingly, the product (C70H28N4Ol0; relative molecular mass 1084) is of very low solubility. The structure was confirmed in 1957 by Jayaraman, who found no evidence of uncyclised anthrimides in the UV spectrum of the dye solution in concentrated sulphuric acid [32].

O

O

O

N

H O

O H

N O

O

6.86 N

CI Vat Orange 11 O

O N H

O

O

6.87 CI Vat Brown 1

O

O

O O

N

N

O

O

H H N

H

O

H

O

N

O

O 6.88 CI Vat Green 8

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6.2.5 Acridone derivatives The first anthraquinone vat dye containing an acridone ring system was synthesised in 1909 by Ullmann. The parent compound (6.89) can be made by condensation of 1chloroanthraquinone with anthranilic acid in the presence of copper, followed by cyclodehydration in concentrated sulphuric acid (Scheme 6.17).

H O

Cl

O

NH2

N COOH

COOH Cu +

O

O

H2SO4 11 12

10

H O

O

N

9

N

1

OH

O

2 7

3

6.89

4

6

O

O Scheme 6.17

Alternatively, the synthesis may begin by condensing aniline with the 1-chloro-2-carboxy intermediate. Acridone vat dyes of this type have excellent light fastness but only moderate resistance to alkali due to the keto-enol equilibrium. It is interesting that this pentacyclic dye is approximately 30 nm more bathochromic than the closely related tetracyclic 1-amino2-benzoylanthraquinone. The parent acridone is not useful as a vat dye, although some halogenated derivatives are commercially important. Chlorination of dye 6.89 with sulphuryl chloride gives the reddish violet CI Vat Violet 14, which consists mainly of the 6,10,12-trichloro compound. The colour of acridone derivatives is very sensitive to the number and position of substituents; for example, the 6-chloro derivative is violet, whereas the 9,12-dichloro compound is red and its 9,11-isomer is orange. The presence of a substituted amino group at the 6-position leads to blue dyes such as CI Vat Blue 21 (6.90). This dye is obtained from bromamine acid (6.10) and 4-trifluoromethylanthranilic acid, followed by ring closure, during which desulphonation takes place, and finally benzoylation. The relatively simple bisacridone CI Vat Violet 13 (6.91) is the original dye made in 1909 by Ullmann from 1,5dichloroanthraquinone; this product is still of industrial importance.

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CF3

H O

N

H O

N

O O

O

N

O H

6.90 O

N H

CI Vat Blue 21

6.91 O

CI Vat Violet 13

Various complex carbazole-acridone dyes have been exploited to obtain brown, khaki, grey and olive hues. The fused carbazole-acridone CI Vat Brown 55 (6.92), which possesses excellent all-round fastness properties, is currently available. This compound can be obtained from the 6,10,12-trichloro derivative (CI Vat Violet 14) by condensation with two equivalents of 1-amino-5-benzoylaminoanthraquinone, followed by carbazolisation. The important olive green dye CI Vat Green 3 (6.93; R = H), which has outstanding light fastness, can be regarded as an acridone analogue. This dye is readily available by condensing 3-bromobenzanthrone with 1-aminoanthraquinone under Ullmann conditions followed by ring closure with potassium hydroxide. A useful derivative (6.93; R = 1-anthraquinonylamino), prepared in a similar manner from 3,9-dibromobenzanthrone, is a brownish grey dye (CI Vat Black 25). O H O

N

Cl

C N

O H

H O

N

H O

O

H

R

O

N

N

O

O

O

6.93 O H N C O

6.92 CI Vat Brown 55

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6.2.6 Miscellaneous polycyclic quinones The carbocyclic analogue of flavanthrone was first prepared in 1905 by Scholl. Chlorination of 2-methylanthraquinone with sulphuryl chloride yields the 1-chloro derivative, which under Ullmann conditions gives 2,2′-dimethyl-1,1′-bianthraquinonyl (6.94). Treatment of the latter compound with ethanolic potassium hydroxide produces pyranthrone (6.95; CI Vat Orange 9). This cyclisation reaction (Scheme 6.18) was the first example of its kind and probably proceeds via carbanion formation. As well as being an anthrone derivative, pyranthrone is a fused benzanthrone and also contains a central pyrene ring system. Pyranthrone, a powerful phototenderer, gives rise to brilliant orange hues of good fastness properties. Bromination of pyranthrone in chlorosulphonic acid gives mainly the reddish orange 4,12-dibromo derivative, CI Vat Orange 2. An alternative route to pyranthrone, involving baking 1,6-dibenzoylpyrene (6.96) with aluminium chloride, was also devised by Scholl (Scheme 6.18). The Scholl reaction is a key step in the synthesis of several polycyclic quinones; the cyclisation of 1-benzoylnaphthalene to give benzanthrone (6.73) has already been mentioned. The mechanism of this cyclodehydrogenation reaction may involve an initial protonation step, if traces of water are present, or complexation with aluminium chloride. Electrophilic substitution is thereby O O 1 H3C O

O CH3

6 O

O

6.96

6.94 KOH

AlCl3

O 15

1 2

14

3

13 4

12 5 11 10

6 9

7

O

6.95 CI Vat Orange 9

Scheme 6.18

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H

H

+

+

HO +

H

OH

OH

– H+

H oxidation – 2H

Scheme 6.19

HN

OH

6.73 Benzanthrone

O

NH2

CO

N2

COOH

COOH

HNO2

KOH 2

2

2

6.98 Naphthostyril

Cu

OH

O

O 11

C

1 2

10

AlCl3 or 3 9

H2SO4

C

4

8 7

5

HO

O

O

6.97 Anthanthrone Scheme 6.20

promoted and is then followed by an oxidation step (Scheme 6.19). It has been suggested that radical cations are formed as intermediates [35]. The anthanthrone system (6.97) was discovered in 1912 by Kalb. In the latter stages the synthesis of anthanthrone involves hydrolysis of naphthostyril (6.98), diazotisation and biaryl formation under Gattermann conditions to give 1,1′-binaphthyl-8,8′-dicarboxylic acid before cyclodehydration (Scheme 6.20). The leuco derivative of anthanthrone itself has little affinity for cellulose and the oxidation product has low tinctorial power. However, bromination of anthanthrone or of the intermediate dicarboxylic acid leads to the bright reddish orange 4,10-dibromo derivative CI Vat Orange 3, which possesses good general

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fastness properties and finds use as a pigment, CI Pigment Red 168. This derivative is used as a charge generation material in commercial photocopiers and laser printers [36]. Treatment of one mole of 4,10-dibromoanthanthrone with two moles of 1-amino-4benzoylaminoanthraquinone gives the bluish grey dye CI Vat Black 29. O Cl

O 13

1

12

2

11 3

10

4

9

5 8

6

Cl

O O

6.99 CI Vat Yellow 4

6.100 CI Vat Brown 45

There are several isomeric dibenzopyrenequinones, of which only CI Vat Yellow 4 (6.99) is of commercial importance. This compound is synthesised from 1,5-dibenzoylnaphthalene using the Scholl cyclisation procedure; the same product can be obtained in a similar manner from 3-benzoylbenzanthrone. Its dibromo derivative CI Vat Orange 1, prepared by direct bromination, possesses improved fastness to light, washing and bleaching. The parent dye is now known to be carcinogenic [20]. The reddish brown dye CI Vat Brown 45 (6.100) is a dichloro derivative of the unusual acedianthrone system. This dye is prepared by condensing two moles of 2-chloroanthrone with one mole of glyoxal followed by oxidative cyclisation, which probably takes place in a stepwise manner [31]. The 1,9-heterocyclic derivative flavanthrone (6.67) has already been described. Several related heterocyclic compounds are of importance as vat dyes. The 1,9-pyrimidanthrone derivative 6.101 is of commercial significance both as a vat dye (CI Vat Yellow 20) and also as a pigment (CI Pigment Yellow 108). The pyrimidine ring system, which prevents phototendering on cellulose, can be obtained from the appropriately substituted 1aminoanthraquinone with dimethylformamide and either thionyl chloride (SOCl2) or phosgene (COCl2). 1,9-Pyrazolanthrones, like the anthrapyrimidines, can be regarded as heterocyclic analogues of benzanthrone. The parent tautomeric pyrazolanthrone system (6.102) is available from 1-aminoanthraquinone by diazotisation, reduction to the hydrazine with sodium bisulphite and ring closure in sulphuric acid. As with benzanthrone, dimerisation takes place on fusion with alkali. Improved resistance to alkali is achieved by alkylation. Thus the useful bluish red dye CI Vat Red 13 (6.103) is obtained by ethylation of the parent dimer. Several other valuable dyes also contain a pyrazolanthrone system, such as the navy blue dye CI Vat Blue 25 (6.104), which is prepared by condensing 3bromobenzanthrone with pyrazolanthrone followed by treatment with alkali, and the grey dye CI Vat Black 8 (6.105), which is made in a similar manner from 3,9dibromobenzanthrone, pyrazolanthrone and 1-aminoanthraquinone.

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N

N CONH O

O

O

H N

H

N

N

N

6.101 CI Vat Yellow 20

O

H5C2 N

6.102

O

O

Pyrazolanthrone

N

N

N

N

N C2H5

O 6.103

O

O

CI Vat Red 13

6.104 CI Vat Blue 25

O

N

H N

N

O

O

O

O

6.105

NH2

O

NH

O

N

CI Vat Black 8

S O NH2

O

O

N

C O

O

6.107

O 6.106

O

CI Vat Blue 31

CI Vat Red 10

Heterocyclic ring systems are also used to connect two anthraquinone groups. Typical examples include CI Vat Red 10 (6.106), which is an oxazole derivative obtained from 2amino-3-hydroxyanthraquinone and the appropriate acyl chloride, the similar thiazole derivative CI Vat Blue 31 (6.107) and the oxadiazole derivative CI Vat Blue 64 (6.108).

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O

NH2

N

N

NH2

O

N

O

315

O

O

N

O

O

H

H

H3C 6.108

O

O

N

N

O

O

CI Vat Blue 64

CH3

6.109 CI Vat Red 23

R

O

O

R

N N

N NH

N

N

NH

N

N N

NaOOCCH2HN

NaO3S

O

O

SO3Na

NHCH2COONa

6.110

O

N N

N N

O

N

N

N

N

O

O

6.111

6.112

CI Vat Orange 7

CI Vat Red 15

Although not anthraquinonoid, some related vat dyes derived from perylene-3,4,9,10tetracarboxylic acid and from naphthalene-1,4,5,8-tetracarboxylic acid are worthy of mention. Thus, for example, when N-methylnaphthalimide is fused with alkali the resulting dimer is CI Vat Red 23 (6.109). This product is also useful as a pigment (CI Pigment Red 179). Dyes of this type also find an application in electrophotographic recording processes [36]. Water-soluble dyes of this class (6.110) have been patented recently by Clariant for ink-jet printing [37]. Treatment of naphthalene-1,4,5,8-tetracarboxylic anhydride with ophenylenediamine gives rise to two isomeric diimidazoles, each of which is useful as a vat dye and as a pigment. The products are CI Pigment Orange 43 (6.111) and CI Pigment Red 194 (6.112). The two isomers can be seperated chromatographically or by treatment with ethanolic potassium hydroxide. Dichroic pigments of this type are also useful in liquid crystal display devices.

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6.3 INDIGOID AND THIOINDIGOID DYES The natural blue dye indigo (6.113; CI Vat Blue 1) has been known since antiquity; there are several plants from which indigo can be isolated. Of particular importance is the genus Indigofera, which can be grown in India, China and other tropical or subtropical regions [38]. Aqueous extraction of the indigo plant yields indican, the colourless glucoside of 3hydroxyindole (6.114; indoxyl). Removal of the sugar residue in a fermentation process and air oxidation of the resulting indoxyl gives indigo. As well as the essential colouring matter, indigotin (6.113), natural indigo contains varying amounts of the isomeric by-product indirubin (6.115) and other compounds. In Europe the woad plant (Isatis tinctoria) was cultivated to provide indigo of lower purity in the guise of woad.

4

O

H

1′ N

3 2

5 1 N

6 7

3′ O

6.113

O

6′

2′

H

OH

7′

5′

N

4′

H

N 6.114

H

Indoxyl

O

Indigotin

N

N

H

H

O

6.115 Indirubin

The constitution of indigotin was elucidated by Adolf von Baeyer between 1865 and 1883, on the basis of degradation and synthesis. It was already known that distillation of indigo gave aniline (anil is the Portuguese word for indigo). Among other conversions, von Baeyer showed that indigo could be reduced to indole and oxidised to isatin (6.116). The cis isomer (Z configuration) was proposed by von Baeyer for indigo. The correct E configuration (trans isomer) was eventually established in 1926 by Posner, using X-ray crystallography [39]. The indigo molecule is almost planar in the solid state and forms a hydrogen-bonded lattice in which each molecule is linked to four others. The observed bond lengths suggest some contribution to the ground state of the molecule from charge-separated structures such as the dipolar species 6.117. Not surprisingly, indigo has a high melting point (390–392 °C) and low solubility. Infrared studies have shown the existence of intermolecular hydrogen bonding in the parent compound and in many derivatives, but intramolecular hydrogen bonding (6.113) cannot be discounted. Unlike thioindigo, the trans isomer cannot readily be converted into the cis form although derivatives of the cis isomer are known, such as compound 6.118. The colour of indigo depends dramatically upon its physical state and environment; for example, the vapour is red but the colour on the fibre is blue. The marked solvatochromism of indigo (Table 6.4) is attributable mainly to hydrogen bonding. A progressive bathochromic shift of the visible absorption band is observed as the solvent polarity

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_ O

O

H N

O

O

N

N

O

O

317

+

O N

N

H

H

O

6.116

6.117

Isatin

6.118 O

H

H3C

H

O

N

N

H3C

CH3

N

N H

H

O

O

CH3

6.120

6.119

Table 6.4 Solvatochromism of indigo Solvent

λmax (nm)

Vapour Carbon tetrachloride Xylene Ethanol Dimethyl sulphoxide Solid

540 588 591 606 642 660

(dielectric constant) increases. N,N′-Dimethylindigo and thioindigo, in which hydrogen bonding is absent, exhibit only small solvatochromic effects. The isolated chromogen in indigo vapour, like that of thioindigo (λmax 543 nm in DMF), is red. Klessinger and Lüttke have shown that the structural unit responsible for the red colour is the cross-conjugated H-shaped chromophore 6.119. The secondary role of the benzene rings in indigo has been established by the synthesis of simple analogues such as compound 6.120 [40]. PPP-MO calculations support the idea of an H chromogen [41]. Despite the relatively minor role of the benzene rings in the indigo molecule, the type and position of substituents in the chromogen are important for determining the colour of a derivative. In general, electron-donating groups at the 5,5′- and 7,7′-positions bring about bathochromic shifts, whereas electron-withdrawing substituents at these sites cause hypsochromic shifts. Qualitatively, a donor substituent that is para or ortho to the donor NH group will stabilise the first excited state whereas an acceptor group has a destabilising effect. The reverse situation obtains at 4,4′- and 6,6′-positions that are ortho or para to the acceptor carbonyl group. The VB explanation is supported by PPP-MO calculations [39]. Indigo is still of considerable commercial importance, especially in warp dyeing for woven cotton denim, and finds use as a pigment, CI Pigment Blue 66. Light fastness of indigo dyeings is only moderate but it fades on tone to a pale blue. Photofading of indigo probably

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involves attack by singlet oxygen at the central double bond [42]. Indigoid vat dyes have gradually declined in importance relative to anthraquinone derivatives. 6.3.1 Methods of synthesis The synthesis of indigo was much more difficult than that of alizarin (6.2) [43]. In 1865 von Baeyer first attempted to obtain indigo by reductive dimerisation of isatin (6.116); he finally achieved a seven-step synthesis from phenylglycine via isatin in 1878. Many syntheses have been developed subsequently for indigo, but very few of these have achieved industrial importance. In 1890 Heumann obtained a low yield of indoxyl (6.114) by fusing phenylglycine with potassium hydroxide at 300 °C. Indoxyl, once formed, is readily oxidised by atmospheric oxygen to give indigo. An improved yield was obtained at a lower fusion temperature (200 °C) from 2-carboxyphenylglycine, starting from anthranilic acid (Scheme 6.21). The initial cyclisation is like a Dieckmann condensation [44]. This route was adopted by BASF when synthetic indigo was first marketed in 1897. The advent of synthetic indigo soon led to a rapid decline in demand for the relatively impure natural product. At the time the route to anthranilic acid was difficult and relatively expensive [45]. In 1901 Pfleger found that it is possible to cyclise phenylglycine efficiently by adding sodamide at 190 °C to a low-melting eutectic mixture of sodium and potassium hydroxides. This discovery enabled aniline to be used as a cheaper starting material. Pfleger’s route is still used today, except that the required phenylglycine is produced via the N-cyanomethyl derivative obtained by the condensation of aniline with sodium formaldehyde–bisulphite and sodium cyanide; alkaline hydrolysis then gives sodium phenylglycinate. Derivatives of indigo can be made by synthesis or by direct substitution. Sulphonation of indigo using oleum gives mainly the 5,5′-disulphonic acid (6.121) together with the 5,7′isomer. This colorant is of interest as a food dye (CI Food Blue 1) and as an acid dye (CI Acid Blue 74); the aluminium salt is a pigment (CI Pigment Blue 63). Several halogenated dyes have been marketed, exemplified by CI Vat Blue 5 (6.122), which is obtained from indigo by bromination in an organic solvent such as nitrobenzene. The natural dye Tyrian purple (CI Natural Violet 1) is 6,6′-dibromoindigo [46].

COOH

NH2

ClCH2COONa Na2CO3

COONa

NHCH2COONa

NaOH ONa air 6.113 Indigo

COONa N H

Scheme 6.21

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O

H

HO3S

O

N

Br

N

Br

N

SO3H

H

H

319

N

O

Br

H

Br

6.121

O

6.122

CI Food Blue 1

CI Vat Blue 5

The sulphur analogue of indigo, thioindigo (6.123), was first synthesised in 1905 by Friedländer from anthranilic acid, using a reaction sequence similar to the earlier Heumann synthesis of indigo (Scheme 6.21); the amino group was first converted into a thiol by diazotisation, followed by treatment with sodium disulphide and reduction of the intermediate diphenyl disulphide. Reaction of the thiol with chloroacetic acid and subsequent cyclisation gives thioindoxyl, which is then oxidised, usually with sulphur. The final oxidation step, as in the case of indigo, may proceed by a hydride transfer mechanism or by a free-radical coupling reaction [44]. Thioindigo (CI Vat Red 41) is a bright red dye that is still available commercially. Unlike indigo, it is photochromic; on exposure to light the trans stereoisomer (λ max 546 nm in chloroform) is converted into the more hypsochromic cis compound (λmax 490 nm in chloroform) giving a marked change of colour from violet to yellowish red. O S S O 6.123 Thioindigo

Substituted thioindigoid dyes are usually obtained via the appropriate benzenethiol in a Heumann-type synthesis. The final cyclisation of the phenylthioglycolic acid derivative can often be achieved in concentrated sulphuric acid or by using chlorosulphonic acid. Several routes make use of the Herz reaction (Scheme 6.22), in which a substituted aniline is converted into the corresponding o-aminothiophenol by reaction with sulphur monochloride followed by hydrolysis of the intermediate dithiazolium salt [47]. After reaction between the thiol and chloroacetic acid, the amino group is converted into a nitrile group by a Sandmeyer reaction. Hydrolysis of the nitrile leads to the formation of the required thioindoxyl derivative. NH2

N

S2Cl2 R

R

+ S Cl

_

S

R

Scheme 6.22

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NH2

H2O

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The colour of the thioindigo chromogen is more sensitive to the effects of substituents than that of indigo. Several derivatives of thioindigo are of commercial significance. The presence of donor substituents para to the carbonyl groups leads to a pronounced hypsochromic shift, as in the case of CI Vat Orange 5 (6.124). O OC2H5

S S

H5C2O

O 6.124 CI Vat Orange 5

Colorants containing chloro and methyl substituents are of importance, particularly as pigments [27]. Thus, CI Vat Red 1 (6.125) also finds use as a pigment, CI Pigment Red 181. Tetrachlorothioindigo (6.126) is widely used as a reddish violet pigment (CI Pigment Red 88). A naphthalene analogue of thioindigo (6.127; CI Vat Brown 5) can be prepared from naphthalene-2-thiol and is used for textile printing. A few unsymmetrically-substituted thioindigo dyes are also produced; CI Vat Red 2 (6.128) is an example. CH3

Cl

O

S

Cl

S

S

S

Cl

Cl

O

O 6.125

O

Cl

CH3

6.126 CI Pigment Red 88

CI Vat Red 1

O

CH3

Cl

O

S S

Cl

S

Cl

S

O

Cl O

6.127

6.128

CI Vat Brown 5

CI Vat Red 2

CH3

Dull tertiary hues are available from mixed indigoid-thioindigoid dyes, such as CI Vat Brown 42 (6.129) and CI Vat Black 1 (6.130). Hybrid dyes of this kind are relatively difficult to prepare. In one method of synthesis a suitable indoxyl or thioindoxyl reacts with 4nitroso-N,N-dimethylaniline to form an anil at the 2-position (keto tautomer). Further condensation between the anil and a different thioindoxyl or indoxyl gives the unsymmetrical dye.

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O

321

Cl

Cl

O

S Br

S

N Cl

N

O

H

H

6.129

O

6.130

CI Vat Brown 42

CI Vat Black 1

6.3.2 Application Vat dyes in general have the disadvantage that reduction to a water-soluble form is necessary before application to cellulosic textiles. Traditionally sodium dithionite in alkaline solution serves as the reducing agent. An alternative sometimes used in vat printing is thiourea dioxide, which is converted into formamidine sulphinate in alkaline solution (section 12.9.1). Indigo and its derivatives are readily reduced under weakly alkaline conditions to the water-soluble leuco form. The disodium salt Indigo white (6.131) is classified in the Colour Index as CI Reduced Vat Blue 1 and is the form in which it is absorbed by cellulosic fibres. Acidification of Indigo white gives the more stable acid leuco form. The related leuco disulphuric ester (6.132; CI Solubilised Vat Blue 1) was mainly used for wool dyeing. As mentioned in section 6.2, reduction and esterification can be carried out without isolating the leuco compound by using pyridine and iron together with chlorosulphonic acid. Indirect electrolysis can also be used as a reduction technique in indigo dyeing [48]. ONa H

HO3SO

N

N

N

N H

H

H

ONa

6.131 Indigo white

OSO3H

6.132 CI Solubilised Vat Blue 1

Nearly all the commercially produced indigoid and thioindigoid vat dyes have been marketed in their solubilised forms. Thus, for example, CI Vat Blue 5 is also available as CI Solubilised Vat Blue 5. In addition, several vat dyes have been sold only as their solubilised derivatives. Usage of these expensive derivatives has declined in recent years. 6.4 SULPHUR AND THIAZOLE DYES Although many types of dye contain sulphur other than in sulphonic acid groups, sulphur dyes are usually considered to be those dyes that are best applied from a sodium sulphide dyebath. Like vat dyes, sulphur dyes are water-insoluble before and after application to cellulosic fibres. Disulphide linkages in the dye molecules are readily reduced by sodium

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sulphide to give alkali-soluble thiol groups. The more environmentally friendly thiourea dioxide can be used as a substitute for sodium sulphide [49]. The resulting leuco dye has substantivity for cellulose. Subsequent oxidation within the fibre by exposure to air and oxidising agents such as hydrogen peroxide leads to the regeneration of di- and polysulphide linkages between aromatic ring systems. The dyeings generally possess good wet fastness and satisfactory light fastness. The fastness to bleaching of sulphur dyes is poor and the dyes are often sensitive to acid attack with the evolution of hydrogen sulphide. Nevertheless they are relatively cheap to produce and offer a full range of hues from yellow to black, albeit rather dull colours. The blues and blacks are of greatest importance, especially CI Sulphur Black 1, which is manufactured on a very large scale [50]. Environmental legislation imposes increasing constraints on the use of dyes. Sulphur dye effluents contain, in particular, harmful sulphides which cannot be discharged into sewerage treatment systems directly. The preferred method of removing sulphides is by air oxidation using efficient aerators, known as helixors, to produce thiosulphates [51]. The first commercial sulphur dye was discovered accidentally in 1873 by Croissant and Bretonnière who heated lignin-containing organic waste, such as sawdust, with sodium polysulphide at about 300 °C; the product was sold under the name Cachou de Laval [52]. Even today an equivalent dye (CI Sulphur Brown 1) is derived from lignin sulphonate, which is readily available from waste liquors from wood pulp manufacture. The real pioneer of sulphur dyes was Vidal, the first chemist to obtain dyes of this type from specific organic compounds. In particular, Sulphur Black T (CI Sulphur Black 1) was made from 2,4dinitrophenol in 1899. At the turn of the century many of the intermediates available were subjected to sulphurisation (thionation), that is, treatment with sulphur, sodium sulphide or sodium polysulphide to introduce sulphur linkages.

6.4.1 Methods of formation Two basic methods are used for the manufacture of sulphur dyes. In one, the starting materials are baked either with sulphur alone or with sulphur and sodium sulphide at a temperature between 180 and 350 °C. Alternatively the intermediates are heated under reflux in aqueous or alcoholic sodium polysulphide; this process may also be carried out under pressure at temperatures up to about 130 °C. Following sulphurisation the dye is precipitated by means of air or chemical oxidation, acidification or a combination of these methods. The sulphurisation process results in the evolution of hydrogen sulphide, which is usually absorbed in aqueous sodium hydroxide for use elsewhere – in the reduction of nitro compounds, for example. The commercially available sulphur dyes can be divided into three categories. The conventional CI Sulphur dyes are water-insoluble, polymeric products, which can also be produced in a finely dispersed form [50]. CI Leuco Sulphur dyes are obtained by treating sulphur dyes with, typically, an alkaline mixture of sodium sulphide and sodium hydrosulphide. These dyes are suitable for direct application and are available in concentrated solutions (liquid dyes). Liquid dyes are also produced directly from the thionation mixture without isolation; powder or granular products can be made by drying a slurry of the dye together with a reducing agent. CI Solubilised Sulphur dyes, which are thiosulphonic acid derivatives of sulphur dyes, are obtained by the action of sodium sulphite or bisulphite on the parent dye. These products are highly water-soluble but possess only low

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323

substantivity for cotton until they have been converted into the leuco form with sodium sulphide. After application an oxidative treatment converts the thiol groups into disulphide bonds, leading to dimeric or polymeric dyes [53]. The thiosulphate dyes (ArSSO3Na) belong to a group of compounds known as Bunte salts [54]. In the Colour Index both conventional sulphur dyes and their leuco counterparts are allocated the same CI constitution number; a different number is given to the related solubilised version. Thus, for example, CI Sulphur Black 1 and CI Leuco Sulphur Black 1 have the reference CI 53185 whereas CI Solubilised Sulphur Black 1 appears under CI 53186. Because of the complexity of the final products, sulphur dyes are classified according to the chemical structure of the organic starting material that predominates in the manufacturing process. Typical intermediates include aromatic amines, with or without nitro and phenolic groups, and diphenylamine derivatives. 6.4.2 Structural features The dry baking procedure generally results in yellow, orange or brown sulphur dyes, many of which are known to contain benzothiazole groups. For example, CI Sulphur Yellow 4 can be obtained from a melt of 2-(4′-aminophenyl)-6-methylbenzothiazole (6.133; dehydrothio-ptoluidine) 4,4′-diaminobiphenyl (6.134; benzidine) and sulphur. The thiazole intermediate is obtained when p-toluidine is heated with sulphur. Further sulphurisation gives the dithiazole derivative primuline base, sulphonation of which led to the discovery of primuline (CI Direct Yellow 59) by Green in 1887. Primuline base is now known to contain three related components having one, two or three benzothiazole units, respectively [55]. Benzidine (a carcinogen) is no longer used by European manufacturers but sulphur dyes derived from this intermediate are still available commercially in other parts of the world. The structure of CI Sulphur Yellow 4 was investigated in 1948 by Zerweck et al. [56]. These workers degraded the dye in a potassium hydroxide melt and obtained various o-aminothiophenols, which were condensed with chloroacetic acid to produce identifiable lactams. From the ratio of lactams obtained, it was deduced that CI Sulphur Yellow 4 was a mixture of four main components, exemplified by structure 6.135; these were all synthesised and gave, on admixture, a product that was effectively identical with the commercial dye. N

H2N

NH2

NH2

S

H3C

6.134

6.133

Benzidine N H2N S

S S

N

NH2 S

6.135

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A similar study of the dye CI Sulphur Orange 1, obtained by heating 2,4-diaminotoluene with sulphur, led to the isolation of compound 6.136 after reaction of the alkali melt of the dye with chloroacetic acid. It was concluded that the commercial dye is a polybenzothiazole. The final hue of the product depends on the temperature and the duration of the sulphurisation; the use of a smaller proportion of sulphur produces CI Sulphur Brown 10. A similar product (CI Sulphur Brown 8) is obtained when 2,4-dinitrotoluene is baked with sodium polysulphide. A brown sulphur dye of high fastness to light (CI Sulphur Brown 52) results when the polycyclic hydrocarbon decacyclene (6.137) is baked with sulphur at 350 °C [57].

H H2N

N

H3C

S

O

6.136 6.137 Decacyclene

The use of intermediates capable of forming quinonimine ring systems leads to the synthesis of red, blue, green and black sulphur dyes on treatment with aqueous or alcoholic sodium polysulphide. Thus, for example, condensation of N,N-dimethyl-p-phenylenediamine with phenol gives a diphenylamine intermediate (6.138), which on sulphurisation in alcoholic sodium polysulphide first yields an indoaniline oxidation product (6.139). This quinonimine in turn undergoes thionation to form a phenothiazone chromogen (6.140). Further reaction leads to the synthesis of CI Sulphur Blue 9. A similar sequence can be envisaged using indophenol derivatives. For example, the simple indophenol 6.141, which is made by oxidising a mixture of 4-aminophenol and phenol with sodium hypochlorite, gives CI Sulphur Blue 14 on sulphurisation with aqueous sodium polysulphide. Similar blue dyes are available from related intermediates. For example, condensation of diphenylamine with 4-nitrosophenol in sulphuric acid gives compound 6.142; this intermediate reacts with aqueous sodium polysulphide to form CI Sulphur Blue 13. The incorporation of a naphthalene ring system enables green dyes to be produced. The important dye CI Sulphur Green 3, for example, is made by heating compound 6.143 with aqueous sodium polysulphide under reflux in the presence of copper(II) sulphate. CI Sulphur Green 14 is based on copper phthalocyanine [50]. (CH3)2N

OH

(CH3)2N

O

N H 6.138

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N

N

(CH3)2N

325

S

HO

O

O 6.141

6.140

H N

N

N

OH

N

O

H

H

HO3S

6.142

6.143

Sulphurisation of 4-hydroxydiphenylamine in a glycol solvent gives CI Sulphur Red 10. A useful red dye (CI Sulphur Red 6) is prepared by heating 3-amino-6-hydroxy-2methylphenazine (6.144) with aqueous sodium polysulphide. The phenazine derivative can be made by oxidising the condensation product of 2,4-diaminotoluene and 4-aminophenol. The substituted phenothiazone 6.145 gives CI Sulphur Red 5 on treatment with aqueous sodium polysulphide in the presence of glycerol. Zerweck et al. [56] showed that the chlorine atoms in compound 6.145 could be displaced stepwise by mercapto groups. The resulting polymeric dye was found to be essentially the same as that generated by sulphurisation of the parent 4-hydroxy-4′-methyldiphenylamine. It can therefore be deduced that sulphur dyes of this type contain structural units similar to that shown in structure 6.146, which possesses a central thianthrene ring system isosteric with indanthrone (6.65). Only one violet sulphur dye is commercially available: CI Sulphur Violet 1, obtained from the substituted indoaniline 6.147. Several black sulphur dyes are currently in use, especially

Cl

HO

N

CH3

N

NH2

H3C

6.144

N

Cl

S

O

6.145 S O

S

S

N S

N

H3C

O

S S

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CH3

6.146

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CI Sulphur Black 1, which is produced from the relatively simple intermediate 2,4dinitrophenol and aqueous sodium polysulphide. A similar product (CI Sulphur Black 2) is obtained from a mixture of 2,4-dinitrophenol and either picric acid (6.148; X = NO2) or picramic acid (6.148; X = NH2). A black dye possessing superior fastness to chlorine when on the fibre (CI Sulphur Black 11) can be made from the naphthalene intermediate 6.149 by heating it in a solution of sodium polysulphide in butanol. An equivalent reaction using the carbazole intermediate 6.150 gives rise to the reddish blue CI Vat Blue 43 (Hydron blue). This important compound, which also possesses superior fastness properties, is classified as a sulphurised vat dye because it is normally applied from an alkaline sodium dithionite bath. Interestingly, inclusion of copper(II) sulphate in the sulphurisation of intermediate 6.150 leads to the formation of the bluish black CI Sulphur Black 4. The many sulphur dyes synthesised via quinonimine intermediates are polymeric products containing numerous disulphide crosslinkages that can be broken by reduction in aqueous sodium sulphide; thioether groups survive the reduction process. The smaller thiolatecontaining molecules formed are substantive to cellulose. Although the actual structures of such dyes are complex, their essential features can be illustrated in an idealised model (Scheme 6.23), in which X = S, NH or O, and R indicates substituents or annelated rings. NH2 OH

N O2N

H2N

X

O NHCOCH3 NO2

6.148

6.147 H

H

N

N

OH

N

OH

H 6.149

6.150

S O

X R

S

SNa

N

N

S

HO

X

S

N

reduction

R

R oxidation X

O S

H N

S

X

OH

R

Scheme 6.23

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326

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H

DIARYLMETHANE AND TRIARYLMETHANE DYES

327

6.4.3 Sulphurised vat dyes and thiazole derivatives CI Vat Blue 43 and the equivalent dye derived from N-ethylcarbazole (CI Vat Blue 42) are sometimes referred to as sulphurised vat dyes. This term is also applied to some anthraquinonoid thiazole derivatives, such as CI Vat Yellow 2 (6.151). This compound can be made by the reaction of 2,6-diamino-1,5-dichloroanthraquinone with benzaldehyde and sulphur. Another anthraquinonoid vat dye containing a thiazole ring system (6.107) has already been mentioned. The thiazole ring system is found in many types of dye. Thiazole-containing sulphur dyes and primuline were considered in section 6.4.2. Quaternised dehydrothio-p-toluidine 6.133 is available as CI Basic Yellow 1 (6.152). Other derivatives of this intermediate are used as direct dyes, such as CI Direct Yellow 8 (4.58). The benzothiazole ring appears in various azo disperse dyes [14], quaternisation of which gives useful cationic dyes, an important example being CI Basic Blue 41 (4.99). Another example containing a quaternised thiazole ring is CI Basic Red 29 (4.102).

O

S N

H3C

S N +Cl

N

N(CH3)2

_

CH3 S

6.152

O

CI Basic Yellow 1 6.151 CI Vat Yellow 2

6.5 DIARYLMETHANE AND TRIARYLMETHANE DYES Derivatives of triphenylmethane were among the earliest synthetic colorants, and are still in demand where bright, intense colours are needed without the necessity for outstanding fastness to light and chemical reagents. Basic dyes of this type, as well as other cationic dyes, are suitable for dyeing conventional acrylic fibres, on which they show better fastness properties than on natural fibres. The photodegradation of triphenylmethane dyes has been reviewed [42]. Traditionally dyes in this category are referred to as triarylmethane dyes; carbocyclic ring systems other than benzene, particularly naphthalene, and various heterocyclic systems are also encountered. Few diarylmethane dyes are of commercial significance, although some heterocyclic derivatives and analogues are of interest. Di- and tri-arylmethane dyes are mesomeric cations (charge-resonance systems) in which the positive charge is localised mainly on the terminal nitrogen atoms, as in the case of Michler’s Hydrol Blue (Scheme 6.24). One of the resonance canonical forms (6.153) is a carbonium ion, so that dyes of this kind are sometimes referred to as di- or tri-arylcarbonium ions [58]. Since the central carbon atom in these systems is sp2 hybridised, Zollinger suggests that the terms di- and triarylmethine dyes are more appropriate [59]; this class of dyes is structurally similar to the polymethine dyes.

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+ N(CH3)2

(CH3)2N

+ (CH3)2N

N(CH3)2

H

H

(CH3)2N

N(CH3)2 + H

6.153 Michler’s Hydrol Blue

Scheme 6.24 (CH3)2N

COCl2

(CH3)2N

COCl

N(CH3)2

ZnCl2 (CH3)2N

N(CH3)2

NH4Cl

(CH3)2N

N(CH3)2

ZnCl2 _ +NH2Cl

6.154 CI Basic Yellow 2

O

6.155 Michler’s ketone

Scheme 6.25

Michler’s Hydrol Blue absorbs at 607.5 nm (ε = 147 500) in 98% acetic acid [60]. This bright blue dye and related diphenylmethane derivatives have poor fastness properties and are not of commercial importance. Attachment of an amino group to the central carbon atom of Michler’s Hydrol Blue gives the yellow dye Auramine (6.154; CI Basic Yellow 2), which absorbs at 434 nm in ethanol and is used in the dyeing of paper and in other applications, although the dye is a known carcinogen. The electron-donating amino group produces a very marked hypsochromic shift. Auramine is somewhat more stable than Michler’s Hydrol Blue but, being a ketimine, it is hydrolysed in boiling water to form Michler’s ketone (6.155) and ammonia. The dye was first obtained in 1883 by Kern and Caro who heated Michler’s ketone with ammonium chloride and zinc chloride. The synthesis first requires the formation of the important ketone intermediate by a Friedel– Crafts acylation (Scheme 6.25). An improved route was devised in 1889 by Sandmeyer,

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involving the corresponding thioketone (6.157). Condensation of N,N-dimethylaniline with formaldehyde gives 4,4′-bis(dimethylamino)diphenylmethane (6.156), which is then heated with sulphur and ammonium chloride, with sodium chloride as diluent, in an atmosphere of ammonia (Scheme 6.26). The resulting Auramine base (6.158) gives Auramine when treated with hydrochloric acid. The ethyl analogue of Auramine is CI Basic Yellow 37.

(CH3)2N 2

+ HCHO

H+

3

3′

(CH3)2N

2′

N(CH3)2

2

H 5′

5 6

6′

H

6.156

S

N(CH3)2

(CH3)2N

(CH3)2N

N(CH3)2

NH3

NH

6.158

S

6.157

HCl

6.154 CI Basic Yellow 2

Scheme 6.26

Reduction of Michler’s ketone gives Michler’s hydrol (6.159), which forms Michler’s Hydrol Blue in the presence of acid (Scheme 6.27). Michler’s hydrol is produced industrially by the oxidation of the diphenylmethane precursor (6.156); further oxidation to give Michler’s ketone takes place readily.

(CH3)2N

N(CH3)2 H

OH 6.159

+ H

(CH3)2N

N(CH3)2 H

OH2 +

Michler’s hydrol

6.153 Michler’s Hydrol Blue

Scheme 6.27

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The more important triphenylmethane dyes probably date from 1834, when Runge obtained red compounds of unknown constitution from crude phenol and from crude aniline. Shortly after Perkin’s discovery of mauveine (section 6.6.1) in 1856 Natanson obtained the triphenylmethane dye magenta from crude aniline by oxidation. Although many related dyes were made empirically over the following two decades, it was not until 1880 that Fischer was able to establish structural relationships within this group of dyes. The essential triphenylmethane ring system was identified by its conversion (Scheme 6.28) into 4,4′,4″-triaminotriphenylmethane (6.160), the source of pararosaniline (6.161; CI Basic Red 9). NH2

H2N

NH2

H2N

H

OH oxidation

Dye (carbinol) base

6.160 Leuco base NH2

NH2 _

reduction

HO HCl _ + NH2Cl

H2N

6.161 Dye (colour salt) NH2

Scheme 6.28

Nearly all commercial triarylmethane dyes contain two or three electron-donor substituents at para sites. In general, dyes containing two p-amino groups (the malachite green series) are green or greenish blue, whereas those with three p-amino groups (the crystal violet series) are reddish to bluish violet. Of lesser importance are monoamino structures containing alkoxy groups, although these orange or red dyes have very good light fastness on acrylic and modacrylic fibres [22]. 6.5.1 Methods of synthesis In the synthesis of the triarylmethane skeleton the central carbon atom is provided either by a simple compound, such as formaldehyde or phosgene, or as part of an aromatic

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intermediate. Depending on the oxidation state of the component used, the resulting dye is obtained directly or as the leuco base. The various reactions involved are essentially electrophilic aromatic substitutions in which, usually, the para position is activated by a powerful electron-donor substituent such as a dialkylamino group. The leuco base of the dye is produced by condensation of one mole of an aromatic aldehyde with two moles of an aromatic amine having an unsubstituted para position (the aldehyde synthesis) in the presence of a mineral acid or a Friedel–Crafts catalyst. Schiff bases are obtained with primary aromatic amines devoid of crowding substituents adjacent to the amino group. The aldehyde method is illustrated by the classical synthesis of malachite green (6.162; CI Basic Green 4), first carried out in 1877 by Fischer (Scheme 6.29). The resulting leuco base is readily oxidised to the dye base and thence to the colour salt. For many years lead(IV) oxide has been used as the oxidising agent but more recently, for environmental reasons, lead-free processes have been used. Of particular interest is oxidation by chloranil (tetrachloro-p-benzoquinone) or by air in the presence of catalysts. Mechanistically it is likely that the conversion involves hydride abstraction in the case of chloranil, so that the dye is formed directly from the leuco base [61]. The chemistry of leuco triarylmethanes has been reviewed [62]. Condensation of a diphenylmethanol derivative, such as Michler’s hydrol (6.159), with a reactive aryl component under acid conditions (the hydrol synthesis) also provides a leuco base. The dye 6.163 (CI Acid Green 50) is made by reacting Michler’s hydrol with R acid (2-naphthol-3,6-disulphonic acid) and oxidising the resulting leuco compound (Scheme 6.30).

CHO

2

N(CH3)2

(CH3)2N

(CH3)2N

H

H+

+

Leuco base

oxidation

+ N(CH3)2 _ Cl

(CH3)2N

(CH3)2N

N(CH3)2 OH

HCl

Dye base 6.162 Colour salt

Scheme 6.29

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(CH3)2N

N(CH3)2 H

(CH3)2N

N(CH3)2 H OH OH HO3S

SO3H

+ OH oxidation HO3S

SO3H R acid + N(CH3)2

(CH3)2N

HO _ SO3

HO3S 6.163 CI Acid Green 50

Scheme 6.30

(CH3)2N

N(CH3)2 + N(CH3)2 _ Cl

(CH3)2N O

6.155 ZnCl2 or

+ POCl3

N(CH3)2 N(CH3)2 6.164 CI Basic Violet 3

Scheme 6.31

Michler’s ketone (6.155) and related carbonyl compounds can be used to obtain colour salts directly by reaction with a reactive intermediate such as an aromatic amine (the ketone synthesis), as in the case of crystal violet (6.164; CI Basic Violet 3) shown in Scheme 6.31. A few triphenylmethane dyes are still obtained by empirical methods first used in the early processes of dye manufacture. For example, in the production of magenta (CI Basic Violet 14), a mixture of aniline, toluidines and nitrotoluenes is heated with zinc and iron(II)

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chlorides for several hours. It is likely that the reaction involves initially the oxidation of a methyl group to an aldehyde group. Condensation to form triphenylmethane derivatives, not surprisingly, leads to mixtures. It is now known that magenta is carcinogenic. The main components of magenta are rosaniline (6.165) and pararosaniline (6.161). The yellowish brown by-product chrysaniline (6.166; CI Basic Orange 15), an acridine dye, is present in the mother liquors of magenta manufacture. In the manufacture of methyl violet (CI Basic Violet 1), air oxidation of N,N-dimethylaniline in the presence of a copper(II) salt gives a mixture of the more highly methylated pararosanilines. H

+ NH2

H2N

+ NH2 _ X

N _

Cl

6.165 CH3

Rosaniline

6.166 CI Basic Orange 15

NH2

NH2

In the laboratory pure dye bases can be prepared by the reaction of diaryl ketones, such as Michler’s ketone (6.155), with appropriate Grignard reagents or organolithium compounds. These tertiary alcohols are also available from related reactions between aryl esters and organometallic reagents. A new method of synthesis has been reported which involves displacement of benzotriazole in diarylbenzothiazolylmethanes by aryl Grignard reagents [63]. The basic dyes are usually marketed as chlorides, oxalates or zinc chloride double salts; in the case of malachite green (6.162), the last-named derivative has the formula 3[Dye]Cl.2ZnCl2.2H2O. Acid dyes of this type are often isolated as the sodium salt. 6.5.2 Acid, basic and mordant dyes Alkylation of aminotriphenylmethane dyes has a bathochromic effect, which is even more pronounced on arylation, as illustrated in Table 6.5.

Table 6.5 Spectral data for some triphenylmethane dyes in 98% acetic acid

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Trivial name

CI Basic

Crystal violet Ethyl violet

Violet 3 Violet 4

Malachite green Brilliant green

Green 4 Green 1

333

Structure

λmax (nm)

6.167; R = H 6.167; R = CH3 6.167; R = C2H5 6.168; R = H 6.168; R = CH3 6.168; R = C2H5

538 589 592.5 570 621 629.5

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+ NR2 _ X

R2N

x + NR2 _ X

R2N

6.167

NR2

+ N(CH3)2 _ Cl

(CH3)2N

6.168 5′

(CH3)2N

5′′ 6′′

6′ 3′

Cl

3′′

6

2

o

5

3

m

4

6.170

p

CI Basic Blue 1

10

+ N(CH3)2 _ Cl

2′′

2′

6.169

Molar extinction coefficient/l (mol cm)–1 × 10–4

y

CI Basic Green 4

x-band

8

6

4 y-band

2

400

500

600

700

Wavelength/nm

Figure 6.1 Absorption spectrum of malachite green

Unlike Michler’s Hydrol Blue (6.153) and crystal violet (6.167; R = CH3), malachite green (6.168; R = CH3) shows two absorption bands in the visible region of the electromagnetic spectrum (Figure 6.1). In 98% acetic acid the long-wavelength absorption band (x-band) of

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malachite green, which is associated with polarisation along the x-axis, appears at 621 nm (ε = 104 000) whereas the less significant y-band, which is polarised along the y-axis, is found at 427.5 nm (ε = 20 000) [64]; the green colour of the dye arises from these two components. Consequently, the presence of an ortho substituent in the phenyl ring of malachite green gives rise to a significant crowding effect, which decreases the importance of the y-band by partial deconjugation; at the same time the x-band is enhanced. Dyes of this type are therefore blue; for example, the 2-chloro derivative (6.169; CI Basic Blue 1) of malachite green (6.170) displays absorption bands at 635 nm (ε = 121 000) and 415.5 nm (ε = 13 000) [64]. As shown by X-ray crystallography, triphenylmethane dyes are unable to adopt a planar conformation and thus the three aromatic rings are twisted out of the molecular plane by approximately 30 degrees. For example, the molecule of pararosaniline perchlorate is shaped like a three-bladed propeller [65]. Steric effects are therefore significant for o-substituted analogues of malachite green but not for those with only m- or p-substitution (6.170). In accordance with theory [66], crowding substituents adjacent to the central carbon atom in triarylmethane dyes give rise to bathochromic shifts of the long-wavelength absorption band. The x-band in malachite green arises from an NBMO→π* transition, so that 3- and 4substituents affect the energy of the excited state only and bring about spectral shifts of the first absorption band which vary linearly with the appropriate Hammett substituent constants. Thus, electron-withdrawing groups cause bathochromic shifts of the x-band whereas donor substituents cause hypsochromic shifts (Table 6.6) [64,67]. The y-band arises from a π→π* transition [68] so that substituent effects are less predictable. As the donor strength of the 4-substituent increases, however, the y-band moves bathochromically and eventually coalesces with the x-band – at 589 nm in the case of crystal violet (6.164), which possesses two NBMOs that are necessarily degenerate [69].

Table 6.6 Spectral data for some derivatives of malachite green x-band

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y-band

Substituent

λmax (nm)

εmax

λmax (nm) εmax

None 3-Cl 4-Cl 3-CF3 4-CF3 3-CN 4-CN 3-NO2 4-NO2 3-CH3 4-CH3 3-OCH3 4-OCH3 4-C6H5 4-N(CH3)2

621 630 627.5 634 637 637 643 637.5 645 618.5 616.5 622.5 608 626 589

104 000 103 000 104 000 106 000 104 000 90 000 88 000 87 000 83 000 106 000 106 000 107 000 106 000 100 000 117 000

427.5 426 433 424 424 426 429 425 425 433 437.5 435 465 457 589

335

20 000 17 000 22 000 16 000 15 000 15 000 16 000 14 000 17 000 22 000 25 000 18 000 34 000 30 000 117 000

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Acid dyes The presence of at least two sulphonic acid groups in the triarylmethane ring system permits the derivatives to be applied as acid dyes. Although sulphonic acid groups are often present in the intermediates used, acid dyes can also be obtained by direct sulphonation of the basic dye itself or at the leuco stage. Typically, the aldehyde synthesis is used to make the 2,4-disulphonic acid (6.171; CI Acid Blue 1) of Brilliant Green (6.168; R = C2H5) from N,N-diethylaniline and benzaldehyde2,4-disulphonic acid. The closely related dye CI Acid Blue 3 (6.172) is obtained by sulphonating the 3-hydroxy derivative of Brilliant Green (CI Basic Green 1) at the leuco base stage. After oxidation the dye is isolated as the calcium salt, which also finds use as a food colorant (CI Food Blue 5). Direct sulphonation of the colour salt is used to prepare CI Acid Blue 93 from diphenylamine blue (6.173; CI Solvent Blue 23). The latter dye is made by phenylation of pararosaniline (6.161) with an excess of aniline in the presence of benzoic acid at about 180 °C. The main product of sulphonation is the trisulphonic acid. CI Acid Green 50 (6.163) has already been mentioned as an example of the hydrol synthesis. The very similar CI Acid Green 16 (6.174) is obtained by condensing Michler’s hydrol with naphthalene-2,7-disulphonic acid followed by oxidation. + N(C2H5)2

(C2H5)2N

+ N(C2H5)2

(C2H5)2N

_ SO3

_ SO3 SO3Na

6.171 CI Acid Blue 1

HO _ SO3

1 2

Ca2+

6.172 CI Acid Blue 3

+ NH

NH

_ Cl

+ N(CH3)2

(CH3)2N

HN 6.173 CI Solvent Blue 23

_ SO3

HO3S 6.174 CI Acid Green 16

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Several acid dyes are derived from N-sulphobenzyl-substituted amines. Thus in the production of CI Acid Violet 17 (6.175), N-ethyl-N-(3-sulphobenzyl)aniline is condensed with formaldehyde to produce the substituted diphenylmethane, which is then oxidised to the hydrol and condensed with N,N-diethylaniline. Further oxidation of the leuco base gives the dye. The presence of an o-methyl group in the diethylaminophenyl ring gives a bright blue dye (CI Acid Blue 15). Condensation of benzaldehyde-2-sulphonic acid with the same intermediate followed by oxidation of the leuco base gives CI Acid Blue 9, which is used in drop-on-demand ink-jet printing [36]. This dye is also a food colorant (CI Food Blue 2) and the barium salt is used as a pigment (CI Pigment Blue 24). The corresponding 2-chloro derivative is CI Acid Green 9. The nucleophilic displacement of suitable para substituents in analogues of malachite green is used as a means of synthesis of some important acid dyes. For example, CI Acid Blue 83 (6.176; R = H) is made by the aldehyde method, using 4-chlorobenzaldehyde and N-ethyl-N-(3-sulphobenzyl)aniline. The resulting leuco base is oxidised to the colour salt,

HO3S

CH2

_ SO3

H2C

N

N

+

H5C2

C2H5

6.175 N(C2H5)2

CI Acid Violet 17

HO3S

CH2

_ SO3

H2C

N

N

H5C2

+ C2H5

R

R

NH

6.176

OC2H5

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which is then condensed with p-phenetidine to give the required product. The sterically hindered dimethyl derivative of this dye, CI Acid Blue 90 (6.176; R = CH3) is also of interest. It is synthesised by first condensing benzaidehyde with N-benzyl-N-ethyl-mtoluidine to give a leuco base, which is then trisulphonated; after oxidation, the 4-sulphonic acid group is displaced by reaction with p-phenetidine. The incorporation of an indole ring system often leads to an improvement in the light fastness. A suitable example is CI Acid Blue 123 (6.177), which is derived from 4,4′dichlorobenzophenone. Condensation with 1-methyl-2-phenylindole in the presence of phosphorus oxychloride produces the triarylmethane ring system. Replacement of the chlorine atoms with p-phenetidine, followed by sulphonation, gives the dye.

H5C2O

+ NH

NH

OC2H5 _ SO3

HO3S

N

6.177 CI Acid Blue 123

CH3

A study of some triphenylmethane acid dyes on model polymer systems has revealed the operation of a complex fading mechanism which probably involves excited triplet-state dye molecules [70]. Basic dyes Several examples of typical triarylmethane dyes have already been mentioned, in particular, pararosaniline (6.161), malachite green (6.162) its o-chloro derivative (6.169), crystal violet (6.164), rosaniline (6.165) and diphenylamine blue (6.173). CI Basic Green 1 (6.168; R = C2H5), the ethyl analogue of malachite green, is prepared by the aldehyde route and is isolated as the sulphate. The ethyl analogue of crystal violet is CI Basic Violet 4 (6.167; R = C2H5) and is obtained by the ketone route. In general, the presence of substituents adjacent to terminal dialkylamino groups leads to partial deconjugation and to complex spectral shifts [71]. Steric effects are minimised, however, in the case of primary and secondary p-amino groups. Thus rosaniline (6.165) contains one o-toluidine residue and three such groupings are present in new magenta (6.178; CI Basic Violet 2), which is effectively obtained by the hydrol route, involving the reaction of two moles of o-toluidine with one of formaldehyde, oxidation to the hydrol, condensation of this intermediate with o-toluidine and oxidation of the leuco base. Methyl groups are also sited in two rings of CI Basic Blue 5 (6.179), made by the aldehyde route using 2-chlorobenzaldehyde and N-ethyl-o-toluidine. The leuco base of the sterically hindered 2′,2″-dimethyl derivative of malachite green (6.170) is used as a charge transport material in electrophotography [36]. Various derivatives of naphthyldiphenylmethane are of value as blue dyes; the naphthyl substituent brings about bathochromic shifts by extending the conjugation and also by a

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steric effect [72]. The Victoria Blues have been used commercially for many years. Typical examples include CI Basic Blue 26 (6.180), which was discovered in 1883 by Caro and Kern. The dye can be prepared by the ketone route using Michler’s ketone (6.155) and 1phenylaminonaphthalene. A similar condensation between the ethyl analogue of Michler’s ketone and 1-ethylaminonaphthalene gives CI Basic Blue 7 (6.181), now classified as toxic by the Ecological and Toxicological Association of the Dyestuffs Manufacturing Industry (ETAD) [73]. H2N

_ + NH2Cl

H3C

CH3

_ + NHC2H5Cl

H5C2NH

H3C

CH3 Cl 6.179

CH3 NH2

6.178

CI Basic Blue 5

CI Basic Violet 2

+ N(CH3)2 _ Cl

(CH3)2N

+ N(C2H5)2 _ Cl

(C2H5)2N

6.180 6.181

CI Basic Blue 26

CI Basic Blue 7

HN

NHC2H5

The phosphotungstomolybdic acid salts (PTMA lakes) of many basic dyes are used as violet, blue and green pigments [27]. Examples include methyl violet (CI Pigment Violet 3), Victoria Pure Blue BO (CI Pigment Blue 1) and Brilliant Green (CI Pigment Green 1). The copper hexacyanoferrate(II) complex of methyl violet is CI Pigment Violet 27. The free dye bases of various triarylmethane derivatives are used as solvent dyes. Illustrative examples include methyl violet (CI Solvent Violet 8), Victoria Blue B (CI Solvent Blue 4) and malachite green (CI Solvent Green 1). Certain derivatives of basic triarylmethane dyes are used in pressure-sensitive papers. Carbonless copy papers that make use of the so-called crystal violet lactone (6.182) are of particular importance. The reverse side of the top sheet is coated with microcapsules containing a solution of the colourless lactone (colour former) in a non-polar solvent. The bottom sheet is coated with an acidic clay or an acidic phenolic resin. Application of pressure ruptures the minute capsules and interaction between the released lactone and the lower acidic surface leads to instant dye formation (structure 6.183) at the point of contact (Scheme 6.32). The resulting colour gradually fades; slower-acting colour formers that give rise to longer-lasting images, such as benzoylated leuco methylene blue, are therefore usually

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CHEMISTRY OF ANTHRAQUINONOID, POLYCYCLIC AND MISCELLANEOUS COLORANTS

(CH3)2N

N(CH3)2

+ N(CH3)2

(CH3)2N

H+

O

_ COO

C O (CH3)2N 6.182

6.183 N(CH3)2

Crystal violet lactone Scheme 6.32

HO

HO

OH H

O

oxidation 6.184

Neutral form

Leuco base H

+ _ OH

+ OH

HO

_ O

O

6.186

6.185

Benzaurine

Scheme 6.33

incorporated into the capsules. The synthesis and properties of phthalide-type colour formers have been reviewed [74]. Mordant dyes Hydroxytriphenylmethane derivatives give rise to anionic charge-resonance systems that are isoconjugate with the corresponding amino-substituted dyes. Thus, for example, the oxonol analogue of malachite green is benzaurine (Scheme 6.33). The quinonoid neutral form (6.184), which is pale yellow, produces a violet anion (6.185) on the addition of alkali; in

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DIARYLMETHANE AND TRIARYLMETHANE DYES

COOH

COOH

HO

COOH

O

H3C

HO

CH3 Cl

COOH O

CH3

H3C

Cl

6.188

6.187

CI Mordant Violet 1

N(C2H5)2

CI Mordant Blue 1

strongly acidic solution the neutral form is protonated to give a cationic charge-resonance system (6.186) which is also violet [75]. The analogous trihydroxytriphenylmethane system is aurine. Technically important dyes are salicylic acid derivatives that function as chrome mordant dyes for wool. Thus CI Mordant Blue 1 (6.187) is made by the aldehyde synthesis from 2,6dichlorobenzaldehyde and 2-hydroxy-3-methylbenzoic (o-cresotinic) acid in concentrated sulphuric acid. Oxidation of the leuco base is achieved by the addition of sodium nitrite. On wool the product, which is isolated as the sodium salt, is a dull maroon colour, changing to a bright blue on treatment with a chromium salt. Some dyes of this type, such as CI Mordant Violet 1 (6.188), also contain a basic group. This compound is also prepared by the aldehyde route. The acid–base indicator phenolphthalein is a derivative of benzaurine (Scheme 6.34). Condensation of phthalic anhydride with phenol generates the colourless lactone form

HO

_ O

_

OH

O _ OH

O

O O

O

6.189 Phenolphthalein

_

_ O

_ O

O

O

_

OH

OH _ COO

COO

6.191

6.190

Scheme 6.34

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Br

Br

HO

Br

O SO2

CH3

CH3 OH

HO

Br

Br

OH

O

Br

SO2

6.192

6.193

Bromophenol blue

Bromocresol purple

(6.189), which ionises in alkaline solution and undergoes ring opening to produce the red dianion (6.190) within the pH range 8.3–10.0. The charge-resonance system is destroyed in an excess of alkali by the formation of the colourless triphenylmethanol derivative (6.191). Several derivatives of phenolphthalein are also used as indicators. The related phenolsulphonephthaleins are particularly versatile as acid-base indicators, covering a wide range of pH. Typical examples include bromophenol blue (6.192) (pH range 3.0–4.6) and bromocresol purple (6.193) (pH range 5.2–6.8). 6.5.3 Xanthene dyes The most important heterocyclic analogues of the di- and triarylmethane dyes are xanthene derivatives containing an oxygen bridge. These dyes produce red, pink and greenish yellow hues that are strongly fluorescent [76]. The oxygen bridge confers greater rigidity on the system, leading to fluorescence and, in accordance with PMO theory [77], the electron donation results in hypsochromic shifts of the long-wavelength absorption band relative to that of the parent dyes. The xanthene analogue of Michler’s Hydrol Blue (6.153) is Pyronine G (6.194), which is used as a red biological stain. Condensation of 3-dimethylaminophenol with formaldehyde followed by dehydration in concentrated sulphuric acid gives the leuco base. Dyes of commercial significance contain three aromatic rings. The bright reddish violet dye Rhodamine B (6.195; CI Basic Violet 10), discovered in 1887 by Cérésole, is a good example. Condensation of 3-diethylaminophenol with phthalic anhydride yields the dye. The free dye base is used as a solvent dye (CI Solvent Red 49) and the phosphotungstomolybdic acid lake finds use as a pigment (CI Pigment Violet 1); the ethyl ester of Rhodamine B is CI Basic Violet 11. A similar reaction between 3-ethylamino-pcresol and phthalic anhydride gives a redder product on esterification (6.196; CI Basic Red 1). The PTMA lake is a valuable pink pigment (CI Pigment Red 81), as is the copper hexacyanoferrate(II) complex (CI Pigment Red 169). Several xanthene derivatives are applied as acid dyes. For example, condensation of 3diethylaminophenol with benzaldehyde-2,4-disulphonic acid, followed by cyclisation and oxidation, gives Acid Rhodamine B (6.197; CI Acid Red 52). This dye is used in drop-ondemand ink-jet printing [36]. Fluorescein (6.198; X = H; CI Acid Yellow 73) is the xanthene analogue of phenolphthalein (6.190) and gives rise to an anionic charge-resonance system. This was the

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+ N(CH3)2 _ Cl

O

(CH3)2N

+ N(C2H5)2 _ Cl

O

(C2H5)2N

343

6.194

COOH 6.195

Pyronine G

CI Basic Violet 10

_ + NHC2H5Cl

O

H5C2NH

H3C

+ N(C2H5)2

O

(C2H5)2N

CH3

_ SO3

COOC2H5

6.196

SO3Na

CI Basic Red 1

6.197 CI Acid Red 52 Br X NaO

O

O

NaO

X

Br O

O

Br X

X

Br Cl

COONa

Cl

Cl

COONa

6.198

Cl

6.199 CI Acid Red 192

first xanthene dye, originally made in 1871 by von Baeyer from phthalic anhydride and resorcinol. This dye has a strong green fluorescence in solution even at very great dilution and is widely used as a marker. Various halogenated derivatives are of interest, especially the bright red tetrabromo compound eosine (6.198; X = Br; CI Acid Red 87), and the equivalent tetraiodo compound erythrosine (6.198; X = I; CI Acid Red 51). Eosine is widely used for colouring paper, inks and crayons; CI Pigment Red 90:1 is the aluminium salt. Purified erythrosine is a permitted food colorant (CI Food Red 14); the aluminium salt is also used for this purpose (CI Food Red 14:1) and as a pigment (CI Pigment Red 172). Condensation of tetrachlorophthalic anhydride with resorcinol, followed by tetrabromination of the product, gives the bright pink Phloxine B (6.199; CI Acid Red 92); the free acid is used as a solvent dye (CI Solvent Red 48) and the aluminium salt is a pigment (CI Pigment Red 174).

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CHEMISTRY OF ANTHRAQUINONOID, POLYCYCLIC AND MISCELLANEOUS COLORANTS

O

+ N

O

N (C2H5)2N

CH3

_ SO3

NH

O

6.201

O

Rhodamine 101 SO3H

6.200

CH3

H (CH3)2N

+ N(CH3)2 _ Cl

N

H2N

N

6.202

6.203

CI Basic Orange 14

Acriflavine

_ + NH2 Cl

Xanthene dyes are used as colour formers. These so-called fluorans usually contain amino groups sited para and meta to the central carbon atom. Such a substitution pattern gives rise to broad absorption bands and leads to almost black colour production; the lactone 6.200 is a typical example. This xanthene derivative finds use in direct thermal printing [36]. The chemistry of fluoran leuco dyes has been reviewed [78]. Various xanthene derivatives, including fluorescein, are also used as laser dyes to cover the spectral region from 500 to 700 nm. A modern example is the julolidine-based dye Rhodamine 101 (6.201), which absorbs at 576 nm and lases at 648 nm [79]. 6.5.4 Acridine dyes The presence of a nitrogen bridge in di- and tri-arylmethane dyes gives rise to the less significant acridine dyes; the parent heterocyclic ring system is so-called because of its irritant action in the nose and throat. The acridine analogue of Michler’s Hydrol Blue is acridine orange (6.202; CI Basic Orange 14). This dye is made by nitration of 4,4′bis(dimethylamino)diphenylmethane (6.156), reduction of the resulting dinitro derivative and oxidation of the cyclised diamino intermediate. The antiseptic acriflavine (6.203) is obtained by condensation of m-phenylenediamine with glycerol and oxalic acid, followed by methylation of the product. The triphenylmethane acridine analogue chrysaniline (6.166) has already been mentioned as a by-product in the synthesis of magenta. 6.6 MISCELLANEOUS COLORANTS 6.6.1 Azines, oxazines and thiazines Heterocyclic analogues of diarylmethane dyes in which the central carbon atom has been

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MISCELLANEOUS COLORANTS

replaced by nitrogen are of some importance. Such a replacement leads to significant bathochromic shifts of the long-wavelength absorption band in accordance with PMO theory [77]. Thus Bindschedler’s Green (6.204), the aza analogue of Michler’s Hydrol Blue, absorbs at 725 nm. This indamine dye is only of historical interest but a related indoaniline derivative is used as a solvent dye (6.205; CI Solvent Blue 22). Of more importance are the azines, oxazines and thiazines which contain a heteroatom bridge.

+ N(CH3)2 _ Cl

(CH3)2N

(CH3)2N

O

N

N 6.205

6.204

CI Solvent Blue 22

Bindschedler’s Green

Azine dyes The first synthetic dye, mauveine, belongs to this group. The previously accepted structure for Perkin’s dye (CI 50245) has been shown to be incorrect [80]. Chromatography of an authentic sample of mauveine yielded two major components (6.206 and 6.207) which were subjected to spectroscopic analysis. Few dyes of this type remain of industrial significance, an exception being Safranine T (6.208; CI Basic Red 2), discovered in 1859 by Greville Williams. The oxidative sequence leading to the formation of Safranine T (Scheme 6.35) was only elucidated in recent times [81]. The 1:1 mixture of o-toluidine and 2-methyl-1,4diaminobenzene required for the first oxidative step can be obtained by reductive cleavage of the monoazo dye formed by coupling diazotised o-toluidine with o-toluidine. Further oxidation of the intermediate diphenylamine derivative gives an electrophilic quinonediimine (indamine), which then reacts with aniline. Oxidative cyclisation produces a dihydroazine that yields the dye in a final oxidative step. Aniline black (CI Oxidation Base 1) is a complex polymeric phenazine that can be produced on cotton fabric by impregnation with aniline hydrochloride and suitable inorganic oxidants, such as sodium chlorate, ammonium vanadate and copper hexacyanoferrate(II). Aniline black is also made directly for use as a pigment (CI Pigment Black 1).

_ H2N

N

H3C

N

+

X NH

CH3

6.206

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CHEMISTRY OF ANTHRAQUINONOID, POLYCYCLIC AND MISCELLANEOUS COLORANTS

_ H2N

N

H3C

N

X

+

NH

CH3

H2N

N

_ + NH2Cl

H3C

N

CH3

CH3

6.208

6.207

NH2

NH2 CH3

H2N

CH3

NH2

oxidation + H3C

NH

CH3

NH2 oxidation

H2N

NH

NH2

H3C

NH

CH3

NH2

+ H2N

H3C

NH2

N

CH3

oxidation

H2N

NH

+ NH2

H3C

N

CH3

oxidative cyclisation

H2N

N

NH2

H3C

N

CH3

H oxidation

6.208 CI Basic Red 2

Scheme 6.35

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MISCELLANEOUS COLORANTS

347

Oxazine dyes Bindschedler’s Green (6.204) can be made by condensing p-nitroso-N,N-dimethylaniline with N,N-dimethylaniline. A similar condensation with 2-naphthol gives Meldola’s Blue (6.209; CI Basic Blue 6), the first oxazine dye, discovered in 1879. The symmetrical CI Basic Blue 3 (6.210) is of more commercial significance. It is synthesised by nitrosation of N,Ndiethyl-m-anisidine followed by condensation with N,N-diethyl-m-aminophenol, and is used for dyeing acrylic fibres. This dye is now classified by ETAD as toxic [73]. The dioxazine ring system is the source of some valuable violet pigments, such as CI Pigment Violet 23 (6.211). This colorant is obtained by condensing 3-amino-9ethylcarbazole with chloranil. Sulphonation of the pigment gives the dye CI Direct Blue 108. Triphenodioxazines have recently been the source of some blue reactive dyes [24]. Examples are known of symmetrical bifunctional structures (6.212; NHRNH = alkylenediamine, Z = haloheterocyclic system) and unsymmetrical monofunctional types such as 6.213 [37]. + N(CH3)2 _ Cl

O

+ N(C2H5)2 _ Cl

O

(C2H5)2N

N

N 6.209

6.210

CI Basic Blue 6

CI Basic Blue 3

C2H5

Cl

N

O

N

N

O

N

Cl

C2H5

6.211 CI Pigment Violet 23

SO3Na

Cl

ZHNRHN SO3Na

N

O

O

N

NHRNHZ

Cl 6.212

Cl

H2N

N

O

O

N

NHCO

SO2CH

Cl 6.213

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CHEMISTRY OF ANTHRAQUINONOID, POLYCYCLIC AND MISCELLANEOUS COLORANTS

Thiazine dyes The most important thiazine dye is methylene blue (6.214; CI Basic Blue 9), which is also widely used as a biological stain and as a redox indicator. Its synthesis (Scheme 6.36) involves the oxidative thiosulphonation of p-amino-N,N-dimethylaniline. Condensation of the product with N,N-dimethylaniline then gives an intermediate indamine that subsequently undergoes oxidative ring closure. Nitration of methylene blue produces methylene green (6.215; CI Basic Green 5), which shows better light fastness on acrylic fibres. The N-benzoyl derivative of leuco methylene blue is used as a colour former in carbonless copy papers (section 6.5.2).

S

(CH3)2N

NO2

+ N(CH3)2 _ Cl

+ N(CH3)2 _ Cl

S

(CH3)2N

N

N

6.214

6.215 CI Basic Green 5

CI Basic Blue 9

N(CH3)2

N(CH3)2 oxidation +

Na2S2O3 SSO3H NH2

NH2

N(CH3)2

Na2Cr2O7

_ SO3 oxidative 6.214

(CH3)2N

S

+ N(CH3)2

cyclisation

CI Basic Blue 9

N

Scheme 6.36

6.6.2 Polymethine dyes Cyanine dyes fall within the more general category of polymethine dyes, in which a chain of methine groups is terminated with a donor group and an acceptor group respectively [82]. The first cyanine dye was made in 1856 by Greville Williams. Thus the blue chargeresonance system 6.216 was produced when oxidative coupling took place between N-

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MISCELLANEOUS COLORANTS

pentylquinolinium iodide and the corresponding 4-methyl derivative. Dyes of this kind were later found to have colour-sensitising properties in photographic films and hundreds of related compounds have been synthesised, making use of a great variety of heterocyclic terminal groups [83]. Long-chain cyanines absorb beyond the visible region and are useful as infrared sensitisers in photography. Few cyanine colorants have found use as textile dyes, however, owing to inferior fastness properties. Astra Phloxine (6.217; CI Basic Red 12) is an example of a carbocyanine (trimethine) dye derived from 1,3,3-trimethylindoline. This product is now classified by ETAD as toxic [73]. Structurally unsymmetrical dyes of this type are exemplified by CI Basic Orange 21 (6.218), which is obtained by condensing Fischer’s aldehyde (6.219) with 2-methylindole. The use of polymethine cyanine dyes in photography and dye lasers has been reviewed [84]. _ H3C

H11C5

N

+ N C5H11

CH

+ N

CH3 CH

CH

CH

N

I–

Cl

H3C

H3C

CH3

CH3 6.217

6.216

CI Basic Red 12

H3C

CH3

N+

CH _ Cl

H3C

CH

CH3

N CH

H

H3C

CH3

6.219

6.218

CHO

N

Fischer’s aldehyde

CH3

CI Basic Orange 21

Of some importance as textile dyes are aza analogues of polymethine (cyanine) dyes. Azacarbocyanines result when Fischer’s aldehyde is heated with primary aromatic amines. Thus CI Basic Yellow 11 (6.220) is obtained when Fischer’s aldehyde is condensed with 2,4dimethoxyaniline. The equivalent reaction with 2-methylindoline gives CI Basic Yellow 21 (6.221), which has superior light fastness but has been classified by ETAD as toxic [73]. The tinctorially strong golden yellow diazacarbocyanine dye CI Basic Yellow 28 (6.222) is prepared by coupling diazotised p-anisidine with Fischer’s base (6.223), followed by quaternisation with dimethyl sulphate. Some triazacarbocyanine dyes are also used commercially. H3C

H3C

CH3

N+

CH _ Cl

CH

NH

CH3

OCH3

N+ H3CO

CH3

N CH3

6.221 CI Basic Yellow 21

CI Basic Yellow 11

349

CH

Cl CH3

6.220

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CHEMISTRY OF ANTHRAQUINONOID, POLYCYCLIC AND MISCELLANEOUS COLORANTS

H3C

CH3 CH

N

N+ _

N

H3C

OCH3

CH3

CH3 CH2

CH3

CH3OSO3

N

6.222

6.223

CH3

CI Basic Yellow 28

Fischer’s base

Hemicyanine (styryl) dyes are readily obtained by heating Fischer’s base with an appropriate aldehyde. Typical products include CI Basic Red 14 (6.224) and CI Basic Violet 16 (6.225). The latter dye has been classified by ETAD as toxic [73]. H3C

CH3 CH _ N+ Cl

CH2CH2CN CH

N CH3

CH3 6.224 H3C

CI Basic Red 14

CH3

N+ Cl CH3

CH _

CH

N(C2H5)2

6.225 CI Basic Violet 16

Cationic monoazo dyes can be classified as diazahemicyanines; examples of such dyes are considered in section 4.10. Uncharged styryl (methine) disperse dyes were originally introduced to provide greenish yellow colours on cellulose acetate fibres. One such dye still in use is CI Disperse Yellow 31 (6.226), which is made by condensing 4-(N-butyl-N-chloroethylamino)benzaldehyde with ethyl cyanoacetate. Suitable compounds for polyester usually contain the electron-accepting dicyanovinyl group, introduced with the aid of malononitrile. An increased molecular size leads to improved fastness to sublimation, as in the case of CI Disperse Yellow 99 (6.227). A novel polymethine-type structure of great interest is present in CI Disperse Blue 354 (6.228), which is claimed to be the most brilliant blue disperse dye currently available [85].

ClCH2CH2

CN N

CH

CH3CH2CH2CH2

C COOC2H5

6.226 CI Disperse Yellow 31

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NC

CN

CH2CH2OCO(CH2)4COOCH2CH2 C

CH

NC

CH

N

N

C CN

H5C2

C2H5

351

H3C

CH3 6.227 CI Disperse Yellow 99

O

CN NC

(H13C6)2N

C

N

CH

H

O2S 6.228

O

6.229 Quinophthalone

CI Disperse Blue 354 OH O

R

N H

O

6.230

Quinophthalone (6.229) and its derivatives [86] also fall into the methine category, although they appear in the Colour Index under quinoline colouring matters. The parent compound was discovered in 1882 by Jacobsen, who condensed 2-methylquinoline (quinaldine) with phthalic anhydride. The product, quinoline yellow, is used as a solvent dye (CI Solvent Yellow 33). The light fastness is improved by the presence of a hydroxy group in the quinoline ring system. Derivatives of this type provide greenish yellow disperse dyes for polyester. The moderate sublimation fastness of CI Disperse Yellow 54 (6.230; R = H) is improved by the introduction of an adjacent bromine atom in CI Disperse Yellow 64 (6.230; R = Br). The chromium complexes of some azomethine derivatives are used as solvent dyes. CI Solvent Yellow 32 (6.231) is an example, obtained by condensing one mole of salicylaldehyde with the appropriate amine, followed by treatment with one equivalent of chromium.

O

HO

OH

O

SO3H CH3(CH2)2O

CH

N NO2

6.231 CI Solvent Yellow 32

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6.232

O

CI Disperse Red 356

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CHEMISTRY OF ANTHRAQUINONOID, POLYCYCLIC AND MISCELLANEOUS COLORANTS

A new cross-conjugated methine-type chromogen was introduced in 1984. The dye CI Disperse Red 356 (6.232) exemplifies this system, which contains two α,ω-donor-acceptor dienone segments. The development of such benzodifuranone disperse dyes has been described [87]. 6.6.3 Nitro and nitroso dyes Yellow to brown compounds are obtained when one or more nitro groups are conjugated with electron-donor substituents such as hydroxy or, of more importance, amino groups. Such dyes are relatively cheap. Picric acid, 2,4,6-trinitrophenol, can be regarded as the oldest synthetic dye; it was produced in 1771 by Woulfe by treating indigo with nitric acid. The hydroxy-nitro dye Naphthol Yellow S (6.233; CI Acid Yellow 1) was discovered in 1879 by Caro and is still manufactured. It is produced by sulphonation of 1-naphthol to give 1-naphthol-2,4,7-trisulphonic acid, followed by replacement of the 2- and 4-sulpho groups in nitric acid medium. Nucleophilic substitution of 1-chloro-2,4-dinitrobenzene with 4aminodiphenylamine-2-sulphonic acid gives CI Acid Orange 3 (6.234). OH HO3S

NO2 NO2

O2N

NO2

6.233

NH

NH

SO3H

6.234 CI Acid Orange 3

CI Acid Yellow 1

Of particular importance are disperse dyes based on o-nitrodiphenylamine. The light fastness of such compounds is better than that of their p-substituted isomers, owing to intramolecular hydrogen bonding. The o-nitro derivatives are more bathochromic than the p-nitro analogues but the latter possess greater tinctorial strength. Dyes such as CI Disperse Yellow 1 (6.235; X = OH) and CI Disperse Yellow 9 (6.235; X = NH2) are examples of typical products used on cellulose acetate. Such dyes generally have lower substantivity for polyester, but this can be improved by minor structural changes. Thus CI Disperse Yellow 42 (6.236; X = C6H5) has higher substantivity for polyester than has the parent dye CI Disperse Yellow 33 (6.236; X = H) [21]. The inclusion of a photostabiliser grouping in the terminal phenyl ring of CI Disperse Yellow 42 (6.236; X = C6H5) leads to improved light fastness [88]. The incorporation of an azo group into the diphenylamine system improves sublimation fastness and colour strength; for example, CI Disperse Yellow 9 (6.235; X = NH2) is the source of CI Disperse Yellow 70 (6.237). O2N

NO2 NH

O2N

NH

X

6.235

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SO2NH

X

REFERENCES

353

NO2 O2N

NH

N

N

OH

6.237 CI Disperse Yellow 70 NO2 HO3S

H3C NH

N

N

OH

6.238 CI Acid Yellow 199

The introduction of sulphonic acid groups into dyes of this kind provides acid dyes suitable for nylon, such as CI Acid Yellow 199 (6.238). The so-called nitroso dyes are metal complexes of 1-nitroso-2-naphthol. In the parent system a tautomeric equilibrium (Scheme 6.37) exists between hydroxynitroso (6.239) and quinoneoxime (6.240) forms. Only the green iron(II) complexes, which have good light fastness, have been exploited commercially. The unsubstituted iron(II) complex is CI Pigment Green 8. The corresponding 6-sulphonic acid complex, Naphthol Green B (CI Acid Green 1), was discovered in 1885 by Hoffmann. O

O N

H

N

O

O

6.239

H

6.240

Scheme 6.37

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

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Y Kogo, H Kikuchi, M Matsuoka and T Kitao, J.S.D.C., 96 (1980) 475. C Graebe and C Liebermann, Ber., 1 (1868) 49, 104. M R Fox, Dye-makers of Great Britain 1856–1976, (Manchester: ICI, 1987) 97. M Titz and M Nepras, Coll. Czech. Chem. Commun., 37 (1972) 2674. S Abeta and K Imada, Rev. Prog. Coloration, 20 (1990) 19. J Winkler and W Jenny, Helv. Chim. Acta, 48 (1965) 119. M Kikuchi, T Yamagishi and M Hida, Dyes and Pigments, 2 (1981) 143. M Nepras, J Fabian, M Titz and B Gas, Coll. Czech. Chem. Commun., 37 (1982) 2569. P F Gordon and P Gregory, Organic chemistry in colour, (Berlin: Springer-Verlag, 1983) Chapter 4.6.

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354 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. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62.

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C F H Allen, C V Wilson and G F Frame, J. Org. Chem., 7 (1942) 169. Colour Index International, 3rd Edn (4th revision) Vol 9 (Bradford: SDC, 1992). Y Zhang and W Hou, Dyes and Pigments, 30 (1996) 283. H S Freeman and J Sokolowska-Gajda, Text. Res. J., 60 (1990) 221. G Hallas in Developments in the chemistry and technology of organic dyes, Ed. J Griffiths (Oxford: Blackwell Scientific Publications, 1984) 31. O Annen, R Egli, R Hasler, B Henzi, H Jakob and P Matzinger, Rev. Prog. Coloration, 17 (1987) 72. J F Dawson, Rev. Prog. Coloration, 14 (1984) 90. K D Wozniak, A Keil and D Müller, Textil Praxis, 45 (1990) 965. P Elsner, Textilveredlung, 29 (1994) 98. A T Peters and X Ma, J.S.D.C., 108 (1992) 244. C T Helmes, Amer. Dyestuff Rep., 83 (Aug 1994) 40. J F Dawson, J.S.D.C., 99 (1983) 183. R Raue, Rev. Prog. Coloration, 14 (1984) 187. S B McLoughlin and C R Lowe, Rev. Prog. Coloration, 18 (1988) 17. A H M Renfrew, Rev. Prog. Coloration, 15 (1985) 15. H Zollinger, Color chemistry, 2nd Edn, (Weinheim: VCH, 1991) Chapter 8.11. U Baumgarte, Rev. Prog. Coloration, 17 (1987) 29. W Herbst and K Hunger, Industrial organic pigments, 2nd Edn (Weinheim: Wiley-VCH, 1997). S M Burkinshaw, G Hallas and A D Towns, Rev. Prog. Coloration, 26 (1996) 47. M Schellenberg and R Steinmetz, Helv. Chim. Acta, 52 (1969) 431. J Aoki, Bull. Chem. Soc. Japan, 41 (1968) 1017. H Zollinger, Color chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 8.9. K Venkataraman and V N Iyer in The chemistry of synthetic dyes, Vol 5, Ed. K Venkataraman ( New York: Academic Press, 1971) Chapter 3. P F Gordon and P Gregory, Organic chemistry in colour (Berlin: Springer-Verlag, 1983) Chapter 5.2. J Griffiths, Colour and constitution of organic molecules (New York: Academic Press, 1976) Chapter 4.6. G A Clowes, J.C.S., C (1968) 2519. P Gregory, Rev. Prog. Coloration, 23 (1993) 1. H S Freeman and J Sokolowska-Gajda, Rev. Prog. Coloration, 29 (1999) 8. K Venkataraman, The chemistry of synthetic dyes, Vol 2 (New York: Academic Press, 1952) 1003. P F Gordon and P Gregory, Organic chemistry in colour (Berlin: Springer-Verlag, 1983) Chapter 5.3. H Zollinger, Color chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 2.5. M Klessinger, Dyes and Pigments, 3 (1982) 235. N Kuramoto in Physico-chemical principles of colour chemistry, Eds. A T Peters and H S Freeman (London: Blackie, 1996) Chapter 6.4. J Shore, Textilveredlung, 12 (1986) 207. H Zollinger, Color chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 8.3. P F Gordon and P Gregory, Organic chemistry in colour (Berlin: Springer-Verlag, 1983) Chapter 1.3. K McLaren, The colour science of dyes and pigments, 2nd Edn (Bristol: Adam Hilger, 1986) Chapter 1.2. S Geyer and R Meyer, Chem. Abs., 95 (1981) 203782r. T Bechtold, E Burtscher, G Kühnel and O Bobleter, J.S.D.C., 113 (1997) 135. W Czajkowski and J Misztal, Dyes and Pigments, 26 (1994) 77. R A Guest and W E Wood, Rev. Prog. Coloration, 19 (1989) 63. J Robinson Ltd, J.S.D.C., 111 (1995) 172. C Heid, K Holoubek and R Klein, Melliand Textilber., 54 (1973) 1314. H Zollinger, Color chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 9.1. C D Weston in The chemistry of synthetic dyes, Vol 7, Ed. K Venkataraman (New York: Academic Press, 1974) Chapter 2. E Riesz, Textilveredlung, 26 (1991) 403. W Zerweck, H Ritter and M Schubert, Angew. Chem., 60 (1948) 141. D G Orton in The chemistry of synthetic dyes, Vol 7, Ed. K Venkataraman (New York: Academic Press, 1974) Chapter 1. P Rys and H Zollinger, Fundamentals of the chemistry and application of dyes (New York: Wiley-Interscience, 1972) Chapter 8. H Zollinger, Color chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 4.1. C C Barker, G Hallas and A Stamp, J.C.S. (1960) 3790. N Nishimura and T Motoyama, Bull. Chem. Soc. Japan, 57 (1984) 1. R Muthyala,and X Lan in Chemistry and application of leuco dyes, Ed. R Muthyala (New York: Plenum, 1997) Chapter 5.

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63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88.

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R Muthyala, A R Katritzky and X Lan, Dyes and Pigments, 25 (1994) 303. C C Barker, M H Bride, G Hallas and A Stamp, J.C.S. (1961) 1285. L L Koh and K Erika, Acta Cryst., B, 27 (1971) 1405. M J S Dewar in Steric effects in conjugated systems, Ed. G W Gray (London: Butterworths, 1958) 46. A S Ferguson and G Hallas, J.S.D.C., 87 (1971) 187. A C Hopkinson and P A H Wyatt, J.C.S., B (1970) 530. J Griffiths, Colour and constitution of organic molecules (New York: Academic Press, 1976) Chapter 9.3. D F Duxbury, Dyes and Pigments, 25 (1994) 179. G Hallas, J.S.D.C., 83 (1967) 368. G Hallas and D R Waring, J.C.S., B (1970) 979. R Anliker, G Dürig, D Steinle and E J Moriconi, J.S.D.C., 104 (1988) 223. R Muthyala and X Lan in Chemistry and application of leuco dyes, Ed. R Muthyala (New York: Plenum, 1997) Chapter 4. J Griffiths, Colour and constitution of organic molecules (New York: Academic Press, 1976) Chapter 9.4. R M Christie, Rev. Prog. Coloration, 23 (1993) 1. M J S Dewar in Recent advances in the chemistry of colouring matters, (London: Chemical Society, 1956) 79. R Muthyala and X Lan in Chemistry and application of leuco dyes, Ed. R Muthyala (New York: Plenum, 1997) Chapter 6. P F Gordon and P Gregory in Developments in the chemistry and technology of organic dyes, Ed. J Griffiths (Oxford: Blackwell Scientific Publications, 1984) 66. O Meth-Cohn and M Smith, J.C.S., Perkin I, (1994) 5. J F Corbett, J.S.D.C., 88 (1972) 438. H Zollinger, Colour chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 3.1. L G S Brooker in The theory of the photographic process, Ed. T H James (New York: Macmillan, 1971) 205. Z Zhu, Dyes and Pigments, 27 (1995) 77. J Hu, P Skrabal and H Zollinger, Dyes and Pigments, 8 (1987) 189. F Kehrer, P Niklaus and B K Manukian, Helv. Chim. Acta, 50 (1967) 2200. C W Greenhalgh, J L Carey, N Hall and D F Newton, J.S.D.C., 110, (1994) 178. H S Freeman and J C Posey, Dyes and Pigments, 20 (1992) 171.

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CHAPTER 7

Chemistry of reactive dyes John Shore

7.1 INTRODUCTION The concept of attaching a coloured molecule to cellulose by means of a chemical bond is at least a century old, but until the 1950s a commercially viable technique for achieving dyeings of high wet fastness in this way remained elusive. The viscose process devised for the manufacture of regenerated cellulosic fibres involved the conversion of alkali-treated cotton to the soluble sodium cellulose xanthate by exposure to carbon disulphide vapour. While working on this process in the 1890s, Cross and Bevan described the synthesis of a coloured polymer by benzoylation of alkali-treated cellulose, nitration of the benzoate ester, reduction to the aminobenzoate and finally diazotisation and coupling [1]. During the following half-century, much fundamental work on the structure of cellulosic fibres was undertaken and numerous esters and ethers of cellulose were prepared, occasionally involving the attachment of coloured sidechain substituents. Although such necessarily complex and esoteric reactions confirmed the formation of covalent bonds between typical chromogenic systems and the hydroxy groups in cellulose, they remained essentially of academic interest only [2,3]. Surprisingly, few attempts were made to adapt reaction conditions or to develop appropriate reagents that would allow such derivatives to be formed under typical dyehouse conditions. Drawbacks of the treatments applied in this period included their multi-stage complexity and the use of costly and hazardous solvent media. Degradative attack of the cellulosic fibres by the vigorous reagents or reaction conditions necessary, or sensitivity of the colorant–fibre linkage to hydrolytic attack during subsequent handling or storage of the coloured product, were further problems that prevented exploitation in practical dyeing systems. The first commercially available dye capable of covalent reaction with a textile fibre is believed to be Supramine Orange R (CI Acid Orange 30). This was introduced by I G Farbenindustrie in the 1930s for the dyeing of wool. It contained a chloroacetylamino substituent from which the labile chlorine atom can be readily displaced under conventional weakly acidic dyeing conditions at the boil to form a dye–fibre bond (Scheme 7.1). It does not seem to have been realised at the time that the high fastness to washing shown by this acid dye was partly attributable to reaction with nucleophilic groups in wool. In 1952 Hoechst marketed two Remalan vinylsulphone dyes that were capable of reacting with wool. These were applied under near-neutral conditions and functioned by nucleophilic addition across the activated double bond of the vinylsulphone group. The chemistry that had been elucidated in the development of these novel dyes provided a springboard for Hoechst to respond quickly with the first range of Remazol dyes when the possibility of dye– fibre reaction was finally achieved on cellulosic fibres.

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INTRODUCTION

NH [dye]

+

NHCOCH2Cl

H2N(CH2)4

NH [dye]

CH

NHCOCH2NH(CH2)4

CO

Chloroacetyl dye

357

CH CO

Lysine in wool

Dye-fibre bond

Scheme 7.1 Cl N Cl

N N HN

SO3Na O

H

SO3Na

N N

Cl

aniline

N

NaO3S 7.1 CI Reactive Red 1

NH

N N HN

SO3Na H

O SO3Na

N N NaO3S

Scheme 7.2

7.2 CI Reactive Red 3

Stephen and Rattee at ICI were also evaluating speculative reactive dyes for wool in the early 1950s. These products included a series containing the dichloro-s-triazine reactive system [4]. Recognising that compounds with activated chloro substituents can be induced to react with cellulose under alkaline conditions, Rattee decided to investigate this by immersing alkali-treated cotton in solutions of these new dyes [5]. Various refinements of this process were necessary including the addition of salt to enhance substantivity and lowering of the pH to minimise hydrolysis of the highly reactive dichlorotriazine system, before a trichromatic set of three Procion (ICI) dyes could be marketed in 1956. Exploitation of the dichlorotriazine dyes soon led to parallel development of the much less reactive aminochloro-s-triazine derivatives, which ultimately became the most successful of all reactive dye systems. Aminochlorotriazine dyes (such as 7.2) are readily prepared by a substitution reaction (Scheme 7.2) at 30–40 °C between an arylamine and the dichlorotriazine precursor (7.1 in this instance). Dyes of the aminochlorotriazine type [6] were launched simultaneously as Cibacron (Ciba) and Procion H (ICI) brands shortly after the dichlorotriazine dyes had been introduced. More stable pad liquors could be formulated

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using these less reactive dyes. The range of reactivities offered by the two classes of dyes in combination with various alkalis greatly extended the scope of novel dyeing methods for them [7]. The growth of reactive dyes over the intervening years has been steady rather than spectacular [8]. It was originally thought that they would replace most of the other classes of dyes for cellulosic fibres, except for vat dyes in the most demanding sectors, and eventually dominate the field. This did not in fact occur and the traditional demands for the more economical direct and sulphur dyes on woven cotton goods remained largely unchanged. One reason for this was the need for complete removal of unfixed or hydrolysed dyes in order to achieve satisfactory wet fastness. As much as 50% of the total cost of a reactive dyeing process must be attributed to washing-off and treatment of the resulting effluent, a limitation that has prevented reactive dyes from attaining the level of success originally predicted for them [2].

7.2 REACTIVE SYSTEMS The characteristic features of a typical reactive dye molecule include: (1) the chromogen, contributing the colour and much of the substantivity for the fibre; (2) the reactive system, enabling the dye to form a covalent bond with the fibre and often also contributing some substantivity; (3) a bridging group that links the reactive system to the chromogen and often exerts important influences on reactivity, stability and substantivity; (4) one or more solubilising groups, usually sulphonic acid substituents on the aryl rings of the chromogen; (5) in some instances, as with the important aminochlorotriazine system, a colourless arylamino or other residue attached to the reactive grouping that also modifies solubility and substantivity. All the important chemical classes of chromogen have been included in reactive dye structures. The sulphatoethylsulphone precursor of the vinylsulphone reactive group contributes significantly to the aqueous solubility of dyes of this type. The nature of the bridging group, especially in dyes of the haloheterocyclic type, greatly influences the reactivity and other dyeing characteristics of such dyes [9]. The structure of the reactive grouping and substituents attached to it is decisive with regard to the chemical stability of the dye–fibre bond that is formed. Numerous factors have to be taken into account in designing reactive dyes of commercial interest [10]. Some of the most important are: (1) Economy – any reactive system selected for a range of dyes must enable them to be produced at acceptable cost. (2) Availability – the system selected must be free from patent restrictions, health hazards or other limitations to exploitation. (3) Versatility – it must be possible to attach the reactive system to a variety of dye chromogenic groupings in manufacture. (4) Storage stability – dyes containing the reactive system must be stable to storage under ambient conditions worldwide.

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(5) Efficiency – the manufacturing yield must be economically viable and the dye fixation must be high under conventional conditions of application. (6) Bond stability – the dye–fibre bonds must be reasonably stable to a range of relatively severe fastness tests. Only a relatively few reactive systems (Table 7.1) have met these requirements sufficiently well to become commercially established in a significant segment of the market for reactive dyes. In addition to these important types, several others have been marketed [3,11] as alternative ranges that have failed to maintain a foothold in the marketplace, or as individual members of established ranges where they show reactivity characteristics similar to one of the more important systems. Many of these systems of relatively minor significance are listed in Table 7.2. Interesting intermediates and reactions were involved in synthesis of some of the systems included in Table 7.2. A carbonamido bridging group was often used to attach the reactive system to the chromogen. This could be a weak point, in that it provided another site at which hydrolytic attack could rupture the dye–fibre bond. The carbonyl group in the highly reactive dichloropyrimidine-5-carbonamide system served to further activate the chloro substituents in the heterocyclic ring as well as providing the means of attachment to the chromogen. The route to this system started from urea and formylenemalonic ester, which were condensed to give 2,4-dihydroxypyrimidine-5-carboxylic acid. This was reacted with phosphorus oxychloride to yield 2,4-dichloropyrimidine-5-carbonyl chloride, which was then condensed with a suitable amino group in the dye chromogen. Isomeric with the dichloropyrimidine was the 3,6-dichloropyridazine system, also linked to the chromogen through a carbonamido bridge. Analogous to the dichloropyridazine was the 1,4-dichlorophthalazine-6-carbonamide system. This was synthesised from trimellitic acid (benzene-1,2,4-tricarboxylic acid) by condensation with hydrazine and treatment of the resulting 1,4-dihydroxyphthalazine-6-carboxylic acid with phosphorus oxychloride and phosphorus pentachloride. Both sulphur and nitrogen were used as activating atoms in the 2-chlorobenzothiazole reactive system [12]. The intermediate 2-chlorobenzothiazole was chlorosulphonated in the 6-position and the derived 6-sulphonyl chloride was condensed with the amino group of the chromogen.

Table 7.1 Important reactive systems for cellulosic dyeing Monofunctional

Homobifunctional

Dichlorotriazine

Bis(aminochlorotriazine) Bis(aminofluorotriazine) Bis(aminonicotinotriazine) Bis(sulphatoethylsulphone)

Aminochlorotriazine Aminofluorotriazine Trichloropyrimidine Chlorodifluoropyrimidine

Heterobifunctional

Chlorofluoromethylpyrimidine Dichloroquinoxaline Sulphatoethylsulphone Sulphatoethylsulphonamide

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Aminochlorotriazine-sulphatoethylsulphone Aminofluorortriazine-sulphatoethylsulphone Difluoropyrimidine-sulphatoethylsulphone

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CHEMISTRY OF REACTIVE DYES

Table 7.2 Reactive systems of minor or historical significance only

Cl [dye] – HNOC N N

2,4-Dichloropyrimidine-5-carbonamide Cl

Cl [dye] – HNOC N

3,6-Dichloropyridazine-4-carbonamide

N Cl Cl [dye] – HNOC N

1,4-Dichlorophthalazine-6-carbonamide

N Cl

[dye] – HNO2S

S Cl

2-Chlorobenzothiazole-6-sulphonamide

N N [dye] – HNOCH2CH2C

N

Cl

3′-(4,5-Dichloropyridaz-3-on-2-yl)propionamide O

Cl

[dye]–NHCOCH=CH2 [dye]–NHCOCH2CH2–OSO3Na [dye]–SO2CH=CHCl [dye]–SO2NHCH2CH2Cl [dye]–SO2NHCH2CH2–OSO3Na

Acrylamide 3-Sulphatopropionamide 2-Chlorovinylsulphone 2-Chloroethylsulphamoyl 2-Sulphatoethylsulphamoyl

A different type of activating arrangement was evident in the dichloropyridazone system. Here the reactive 4-chloro substituent is not activated directly by the two nitrogen atoms in this quinonoid heterocyclic ring but by the C=C–C=O grouping, which is a vinylogue of a carboxylic acid chloride [13]. The starting materials for this system were dichloromaleic acid, made by chlorination of 2-butyne-1,4-diol, and hydrazinopropionitrile from acrylonitrile and hydrazine. These were cyclised to give dichloropyridazonyl propionitrile, which was then hydrolysed and converted to the acid chloride using thionyl chloride. The acrylamide group and its precursor the 3-sulphatopropionamide system (Scheme 7.3) both show only feeble reactivity with cellulose under conventional alkaline dyeing conditions [14] and this has greatly limited their usefulness in spite of their economic attractiveness. Similar considerations apply to the 2-chloroethyl- and 2-sulphatoethylsulphamoyl systems, which are both precursors of the three-membered aziridine (cyclic ethyleneimine) ring system (Scheme 7.4). This unsaturated group undergoes a nucleophilic addition reaction [15] with alkali-treated cellulose (Scheme 7.5). The important vinylsulphone group and its various precursor systems are much more reactive and these provided the only really successful ranges of dyes that react by the nucleophilic addition mechanism.

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NaOH [dye]

NHCOCH2CH2

OSO3Na

[dye]

3-Sulphatopropionamide

CH2 + Na2SO4

NHCOCH

+ H2O

Acrylamide

Scheme 7.3

NaOH [dye]

SO2NHCH2CH2Cl

[dye]

2-Chloroethylsulphamoyl

SO2

CH2

N

CH2 NaOH

Aziridine

[dye]

SO2

OSO3Na

2-Sulphatoethylsulphamoyl

Scheme 7.4

[dye]

SO2NHCH2CH2

N

CH2

+ [cellulose]

OH

CH2

Aziridine

[dye]

SO2NHCH2CH2

O

[cellulose]

Dye-fibre bond

Scheme 7.5

7.3 MONOFUNCTIONAL SYSTEMS The monofunctional reactive systems of outstanding importance contain only one possible reactive centre, such as the halogeno substituent in the aminohalotriazine dyes, or the activated terminal carbon atom in the vinylsulphone system. In others there are two equivalent replaceable halogeno substituents, as in the dichloroquinoxaline or dichlorotriazine heterocyclic ring systems. When one of these halogen atoms is displaced by reaction or hydrolysis, the reactivity of the remaining halogeno substituent is greatly inhibited by the presence of the new hydroxy or cellulosyl substituent. The influence of the reactive system itself on the substantivity of the dye containing it was recognised in the early years of the development of reactive dyes [16]. The dichlorotriazine grouping and even more so the aminohalotriazine or dichloroquinoxaline systems considerably enhance the overall substantivity of the dye molecule. The pyrimidinebased structures, with one less heterocyclic nitrogen atom than the corresponding triazine derivatives, contribute much less to the total substantivity of the dye. The non-aromatic, reactive systems, notably the 2-sulphatoethylsulphone group, modify the substantive effect of the chromogen only slightly or not at all. This can offer a significant practical advantage, since a low overall substantivity that remains unaffected by the reactive system greatly facilitates removal of the unfixed dye on washing-off, a property of especial value for printing or continuous dyeing. The s-triazine ring is unique amongst the six-membered nitrogen heterocycles in possessing three electronegative atoms ideally placed to provide the necessary activation of the halogen atoms attached to the adjacent carbon atoms. Calculations of charge

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distribution over these various heterocyclic rings [17] show that the largest possible charge attained in the series is found on the carbon atoms of the s-triazine ring (Figure 7.1). As a result, none of the dyes derived from the alternative chloro-substituted heterocyclic systems are capable of showing reactivity as high as that of the dichloro-s-triazine derivatives. –0.007

+0.042

N

+0.133

+0.071 N N

N –0.072

–0.031 N

–0.083

–0.109

Pyridazine

Pyrazine

+0.064

N

+0.101

–0.144

Pyridine

N

N

+0.202

–0.178

N N

+0.23

–0.20

Pyrimidine

s-Triazine

Figure 7.1 Levels of reactivity of various heterocyclic rings

Not all reactive systems are sufficiently versatile to be used successfully with a representative selection of chromogens covering the entire colour gamut. In some cases, attempts to achieve a complete range in this way based on the use of only one type of reactive group have resulted in lack of compatibility because of the dominant influences of different chromogens on reactivity and substantivity. An alternative approach designed to overcome such difficulties has been to decide beforehand on a target profile of moderate reactivity and high substantivity. The range is then built up accordingly, using various combinations of reactive system and chromogen that will yield the desired profile. When focusing in this way on gaps in a range of reactive dyes, this approach to research encourages the development of new chromogens as well as novel reactive systems [18]. 7.3.1 Dichloro-s-triazine dyes The key intermediate for these dyes is cyanuric chloride (7.3), in which all three chloro substituents are attached to equivalent carbon atoms with fractional positive charges induced by the neighbouring negative nitrogen atoms. Polarisation of the carbon–chlorine bonds makes the chloro substituents labile and readily susceptible to stepwise nucleophilic substitution. If the attacking nucleophile is a water-soluble amine, the reaction can be conveniently carried out with the cyanuric chloride suspended in an agitated aqueous solution of the amine at a temperature close to 5 °C. Maintaining the pH close to neutrality by addition of alkali as required allows the first of the chlorine atoms to be smoothly displaced (Scheme 7.6), yielding the aminodichlorotriazine product (7.4). The optimum conditions for preparing various 6-alkyl-, 6-aryl- and 6-heteroaryl-substituted derivatives of 2,4-dichloro-s-triazine have been defined recently [19]. Selecting a sulphonated dye molecule containing an amino group as the nucleophile leads directly to a dichlorotriazine dye. In certain cases a suitable intermediate may be condensed with cyanuric chloride and then the chromogenic grouping is synthesised from this reaction product. Both of these routes are illustrated in a simple way for CI Reactive Red 1 (7.1) in Scheme 7.7. In these dyes the electronic effects responsible for the lability of the chloro substituents are muted by feedback of electrons from the electron-donating imino bridging

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MONOFUNCTIONAL SYSTEMS

Cl

Cl

N R

NH2

+ Cl

Cl

N

N

N

N

R

Cl 7.3 Cyanuric chloride

NH2

N Cl NaOH

Transient species

Cl N R

NH

+ NaCl + H2O

N N Cl 7.4

Scheme 7.6

Cl N N H2N

Cl

HO

N SO3Na

+

Cl N

N

HN

NaOH Cl

HO

N

SO3Na

Cl NaO3S

Cyanuric chloride

H acid

NaO3S

Coupling component _ SO3 N +

Diazo component

N

Cl N

H2N

SO3Na H

N

O SO3Na

N

N

Cl +

Cl

N

HN

SO3Na

N

N

H Cl

Cl N

NaOH O

SO3Na

N N

NaO3S

Cyanuric chloride Dye base

NaO3S

7.1

CI Reactive Red 1

Scheme 7.7

link, making the dye quite stable in neutral solution. This highly reactive system is susceptible to attack by hydroxide ions on the alkaline side and subject to autocatalytic hydrolysis on the acid side. To guard against decomposition in these ways a buffer is added to the dye solution to ensure stability during isolation and further buffer is incorporated into the isolated paste before drying.

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The dichlorotriazine dyes are so reactive that they can be readily fixed to cellulosic materials by pad-batch dyeing at ambient temperature or by exhaust methods at 30–40 °C. This means that relatively small chromogens are preferred to ensure adequate mobility of dye on the fibre during the exhaustion stage. This requirement makes these dyes eminently suitable for bright dyeings but less satisfactory for deep tertiary hues, since the larger-size chromogens used for this purpose often fail to give acceptable performance by lowtemperature application. A weakness with certain dichlorotriazine dyes, particularly red monoazo derivatives of H acid such as CI Reactive Red 1 (7.1), is that under conditions of low pH the dye–fibre bond is susceptible to acid-catalysed hydrolysis leading to deficiencies in fastness to washing or acid perspiration. Ionisation of the hydroxy groups in cellulose is essential for the nucleophilic substitution reaction to take place. At neutral pH virtually no nucleophilic ionised groups are present and dye–fibre reaction does not occur. When satisfactory exhaustion of the reactive dye has taken place, alkali is added to raise the pH to 10–11, causing adequate ionisation of the cellulose hydroxy groups. The attacking nucleophile (:X–) can be either a cellulosate anion or a hydroxide ion (Scheme 7.8), the former resulting in fixation to the fibre and the latter in hydrolysis of the reactive dye. The fact that the cellulosic substrate competes effectively with water for the reactive dye can be attributed to three features of the reactive dye/ cellulosic fibre system: (1) the concentration of dye in the fibre phase is greater than that in the solution phase (2) the greater nucleophilicity of the cellulosate anion compared with hydroxide (3) the high selectivity of the electrophilic reactive group Because the pKa of the cellulosic substrate is lower than that of water, reaction with the fibre according to Scheme 7.8 is favoured [20]. When partial hydrolysis occurs as in Scheme 7.8 to form the 2-chloro-4-hydroxy species (7.5), the dye does not have a further chance to achieve fixation via the remaining chlorine atom. Under the alkaline conditions of fixation, ionisation of the acidic 4-hydroxy substituent leads to a massive feedback of electronegativity into the triazine ring, causing total deactivation of the remaining 2-chloro substituent. The remaining chlorine atom in

Cl _ N

Cl N [dye]

NH

_ N

+

X

[dye]

X

NH

N N

N

Cl

Cl Dichlorotriazine dye

Transient species

X N [dye]

NH

_ N + Cl

N

Scheme 7.8

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365

OH N [dye]

NH

N N Cl

7.5

the still active but fixed species (7.6), on the other hand, may be subsequently hydrolysed (Scheme 7.9) to give an inactive fixed species (7.7). Under more extreme conditions of pH or temperature, a crosslinked species (7.8) can be formed linking two neighbouring cellulose chains in an amorphous region of the fibre [21,22]. O

[cellulose]

N [dye]

NH

N N

7.6

Cl

Active fixed dye

O

[cellulose]

O

N [dye]

NH

N

[dye]

NH

N

N N

OH

7.7

[cellulose]

N

Hydrolysed fixed dye

7.8

O

[cellulose]

Crosslinked dye-fibre bond

Scheme 7.9

7.3.2 Monochloro-s-triazine dyes Controlled reaction of dichlorotriazine dyes with either amines or alcohols at 30–40 °C leads to two further classes of monofunctional dyes, the 2-amino-4-chloro- and 2-alkoxy-4chloro-s-triazines respectively. The latter are more reactive than the former but less reactive than the parent dichlorotriazine types. They are now essentially of historical interest, the 2isopropoxy-4-chloro system forming the basis of the Cibacron Pront (Ciba) range for printing. The bulky isopropoxy group was chosen in order to disrupt the planarity of the substituted triazine system and thus favour removal of unfixed dye at the washing-off stage. The mechanism of reaction of methyl-α-D-glucoside (7.9), as a soluble model for cellulose, with a model Cibacron Pront dye in homogeneous solution was examined recently. It was demonstrated that reaction occurs only with the hydroxy groups attached at the C4 and C6 positions, the ratio of reactivities being 12:1 in favour of the primary alcoholic site [23]. In another recent study, the relative rates of hydrolysis of a series of orange N-methyl J acid dyes of the 2-alkoxy-4-chloro-s-triazine type (7.10; R = methyl, ethyl or isopropyl)

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6

CH 5

HO

NaO3S

CH2OH

H

O

CH3

O

N

N

CH 4

1 CH

3

OCH3

N

2

CH

CH

OH

OH

N

Cl

N NaO3S

SO3Na

7.9

N O

R

7.10

Methyl-α-D-glucoside

Cl

Cl N [dye]

NH

kOR N

OH

N [dye]

NH

N

kCl [dye]

N

NH

N

N OH

N N

O

R

O

R

Scheme 7.10

were compared. Surprisingly, the hydroxide ion displaced the methoxy group about 1.5 times more rapidly than the chloro substituent (Scheme 7.10; kOR > kCl). Increase in the size and electron-donating capacity of the alkoxy group resulted in a decreasing propensity for substitution, so that displacement of methoxide ion proceeded about twelve times quicker than that of isopropoxide ion [24]. More energetic conditions of fixation, typically 80 °C and pH 11 for exhaust application, are necessary to achieve efficient fixation of 2-amino-4-chloro-s-triazine dyes on cellulosic fibres. Early studies of the relationships between structure and substantivity of aminochlorotriazine dyes revealed that the NH bridging groups linking the chromogen and the uncoloured 2-arylamino substituent to the heterocyclic ring exert marked effects on the solubility and dyeing properties of these dyes [2]. Replacement of such an NH imino link by an N-methylimino group lowers the substantivity by inhibiting hydrogen bonding to cellulose hydroxy groups. The use of a sulphonated arylamine to form the uncoloured substituent in the 2-position of a monochlorotriazine system is helpful in enhancing dye solubility and migration behaviour. The relationship between the distortion angle caused by steric hindrance and the reactivity of aminochlorotriazine dyes has been demonstrated in a kinetic study of the hydrolysis of model compounds. Bulky substituents in the ortho position with respect to the imino bridge in 2-anilino derivatives cause sectional distortion of coplanarity between the phenyl and triazine rings, leading to an increase in reactivity owing to partial impediment of p-π conjugation [25]. 7.3.3 Monofluoro-s-triazine dyes Fluorine and chlorine are the only important choices for the labile substituents on a heterocyclic reactive system but numerous other leaving groups have been patented, mostly as substituent on an s-triazine ring. These include sulpho, cyano, thiocyanato, azido and trichloromethyl [26], as well as more elaborate groupings (Figure 7.2). Such derivatives are

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MONOFUNCTIONAL SYSTEMS

O2N

NaO3S

O

SO3Na

O2N

O

S

SO3Na

CH2CH3

S

S N

C

S

COOCH2CH3 CH

N CH2CH3

S

367

NO2

OCH3 P

COOCH2CH3

OCH3

O

Figure 7.2 Alternative leaving groups for heterocyclic systems F N N

NH N

HN

NaO3S H

O SO3Na

N N

SO3Na

NaO3S

7.11

Cl

F

N Cl

N N

N

+ 3 KF

F

N

+ 3 KCl

N Cl

Cyanuric chloride

F Cyanuric fluoride

Scheme 7.11

almost always made by replacing a chlorine atom in a chloro-s-triazine dye by reaction with the sodium salt of the appropriate nucleophile. Thus they are invariably more costly than the parent chloro compound and offer no obvious benefits, so they have not been exploited commercially. A fluorine atom is used as the leaving group in the Cibacron F (Ciba) range of 2-amino4-fluoro-s-triazine dyes, as exemplified by the red monoazo H acid dye 7.11. The essential highly reactive intermediate cyanuric fluoride required for their manufacture is made from cyanuric chloride by an exchange reaction with potassium fluoride (Scheme 7.11). The greater electronegativity of fluorine compared with chlorine results in a markedly higher

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level of reactivity for these dyes than for their 2-amino-4-chloro analogues. The compact character of the fluorine atom, resulting from the close binding of the nine-electron sphere around the atomic nucleus, favours reaction with dye bases carrying aliphatic amino groups. This need has prompted the synthesis and exploitation of novel dye intermediates bearing such substituents, such as 5-aminomethyl-l-naphthylamine-2-sulphonic acid (7.12). This is prepared (Scheme 7.12) by condensing 1-acetylaminonaphthalene-2-sulphonic acid with Nmethylolphthalimide (Tscherniac–Einhorn reaction), followed by hydrolysis of the resulting 5-substituted derivative. CH3COHN

O

SO3H N

CH2OH

+

O

CH3COHN SO3H O CH2

N

O

CH3COOH

+ NH2 SO3H

COOH + COOH H2N

Scheme 7.12

CH2

7.12

Considerable work was carried out in the 1980s by Mitsubishi on the preparation and evaluation of disperse dyes containing the highly reactive 2-alkoxy-4-fluoro-s-triazine system, an example being the blue dicyanonitrophenylazo structure 7.13 [27]. These products were designed specifically for the dyeing of both component fibres in polyester/cellulosic blends. Although marketed commercially, these novel dyes have not gained widespread acceptance [8]. The absence of water-solubilising groups allowed these molecules to enter the polyester fibre but the high relative molecular mass (490 in the case of 7.13) implies slow diffusion and poor migration properties. The combination of high reactivity and lack of water solubility seems likely to favour fixation near the cellulosic fibre surface with poor penetration into the interior. In the case of structures like 7.13, there may be a possibility of some self-deactivation

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of the reactive system by temporary quaternisation between the tertiary amino group in one molecule and the fluorotriazine centre in another (Scheme 7.13). F N N

OCH3 N

CN

HN CH2CH3

O2N

N

N

N CH2CH3

CN 7.13 F CH2CH3

N

CH2CH3

N

+

N CH2CH3

N N

N

N N

CH2CH3 + CH2CH3

Scheme 7.13

N

F– OCH3

H2O

OH

N

CH2CH3

OCH3

N + HF N

OCH3

7.3.4 Trichloropyrimidine dyes The 1,3-diazine arrangement in the pyrimidine ring provides less activation of chloro substituents than the s-triazine system (Figure 7.1). Fixation to a cellulosic fibre by exhaust dyeing methods requires treatment at the boil rather than the 80 °C found most suitable for aminochlorotriazine dyes, but the dye–fibre linkage through a diazine ring is more stable than that containing a triazine nucleus [16]. The intermediate required for these dyes, tetrachloropyrimidine (7.15), is prepared by chlorination of barbituric acid (7.14) and treatment of the resulting 5-chloro derivative with phosphorus oxychloride and dimethylaniline as catalyst (Scheme 7.14). Two isomeric aminotrichloropyrimidine structures are possible with the amino substituent in either the 2- or the 4-position. A study of the initial reaction between tetrachloropyrimidine and various arylamines demonstrated that nucleophilic substitution occurs mainly by displacement of the 4-chloro substituent [28], as illustrated for the red monoazo H acid dye CI Reactive Red 17 (7.16). Preferential substitution at the 4-position is much less pronounced in the reaction of 2,4,6-trichloropyrimidine with arylamines, so that the dichloropyrimidine dyes formed in this

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CHEMISTRY OF REACTIVE DYES

O

H

OH

N

N CH

O H

CH

HO

N

N 7.14

O

OH

Barbituric acid

Cl

OH

N

N

Cl

Cl

HO

Cl

N

N Cl

OH

7.15 Tetrachloropyrimidine Scheme 7.14

Cl N Cl

Cl N HN

SO3Na H

O SO3Na

N N NaO3S

7.16 CI Reactive Red 17

way contain a mixture of 2,6- and 4,6-dichloro isomers. These dyes are even less reactive than the trichloropyrimidine dyes but are correspondingly more resistant to acidic or alkaline hydrolysis. The 5-chloro substituent in the trichloropyrimidine reactive system is much less activated by the nitrogen atoms in the heterocyclic ring and is thus not normally capable of hydrolysis or reaction with the fibre. Dichloropyrimidine dyes containing a 5-methyl substituent are less reactive but more stable than their trichloropyrimidine analogues [16]. Dyes prepared with an electron-withdrawing group in the 5-position, such as 5-cyano or 5-nitro, show enhanced reactivity of the 2,6-dichloro substituents but somewhat lower stability of the dye– fibre bond. An example of a 5-cyano-2,4-dichloropyrimidine dye is CI Reactive Red 219, which is compatible in reactivity and dyeing behaviour with dichlorotriazine dyes. The intermediate required for dyes of this type is 5-cyano-2,4,6-trichloropyrimidine (7.17), prepared from barbituric acid (7.14). Reaction of this with sodium cyanate and hydrochloric acid yields the 5-carbonamido derivative and this is treated with phosphorus oxychloride and dimethylaniline as catalyst, resulting in simultaneous dehydration of the carbonamido group and replacement of the hydroxy groups to give the desired product (Scheme 7.15).

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MONOFUNCTIONAL SYSTEMS

O

H

OH

N

N CH

O

CH

HO

N H

371

N 7.14

O

OH

Barbituric acid

OH

Cl N

N Cl

HO

CN

CONH2 N

N

OH

Cl 7.17 Scheme 7.15

7.3.5 Chlorodifluoropyrimidine dyes Another important route to more reactive halopyrimidine dyes is to use fluorine rather than chlorine in the reactive centres. As with the fluoro-s-triazine dyes, this results in a markedly higher level of reactivity compared with the corresponding chloro-substituted analogues. Exhaust dyeing temperature for optimal fixation of a 5-chloro-2,4-difluoropyrimidine derivative, such as the red monoazo J acid dye 7.18, is 40–50 °C. The dye–fibre bond formed with cellulose by fixation of these highly reactive products is more stable to acid conditions than that formed by the competing dichlorotriazine dyes but it does tend to undergo oxidative cleavage more readily under the influence of light exposure in the presence of peroxy compounds. The exchange reaction of tetrachloropyrimidine (7.15) with potassium fluoride yields 5-chloro-2,4,6-trifluoropyrimidine (Scheme 7.16). When this is reacted with the dye Cl

SO3Na H CH3O

F

O NH

N

N N

N

F NaO3S

7.18

F

Cl N

N Cl

Cl

+ 3 KF

F

Cl N

N Cl

F

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7.15

Scheme 7.16

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CHEMISTRY OF REACTIVE DYES

F N 2

[dye]

NH2

+

2 F

Cl N F

Cl

F

F N

NH

[dye]

N

+

[dye]

NH

N

Cl N

F

F

7.20

7.19

Scheme 7.17

base, however, a mixture of two isomers is formed (Scheme 7.17), the symmetrical 5-chloro4,6-difluoro arrangement (7.19) and the asymmetrical 5-chloro-2,4-difluoro group (7.20). Recent work at DyStar has led to the development of 4,5-difluoropyrimidine dyes such as the bluish red 7.21 for the dyeing of cellulosic and polyamide fibres [29]. This reactive system is reported to be superior to the conventional 5-chloro-2,4-difluoro arrangement in structure 7.18, for example. Another novel and interesting system is the 2,4,6trifluoropyrimidine grouping (7.22) included by Clariant in dyes designed for cellulosic dyeing [20]. This is environmentally more attractive than the 5-chloro-2,4-difluoro system containing the relatively inert 5-chloro substituent that will contribute to AOX values. Dyes based on structure 7.22 should also be stable to perborate attack during laundering. F

F

N

N

[dye]

F

NH

F N

N HN

SO3Na H

7.22

F

O SO3Na

N N NaO3S

7.21

7.3.6 Chloromethylpyrimidine dyes In the Levafix P (DyStar) range of rapid-fixing reactive dyes for printing, the leaving group was a methylsulphonyl substituent [30]. Preparation of the required reactive intermediate (7.23) involved condensation of S-methylthiourea with ethyl acetoacetate, chlorination of the product, oxidation of the methylthio group and conversion of the hydroxy group to chloro by the action of phosphorus oxychloride (Scheme 7.18). The 4-chloro substituent in structure 7.23 was preferentially eliminated by neutral condensation with the dye base

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MONOFUNCTIONAL SYSTEMS

S H2N

CH3CH2O

CH3

C

C

S NH2

+

O

373

CH3

N HO

N

O

H2C

C

CH3 CH3

SO2CH3

SO2CH3

N

N

Cl

N

HO

N

CH3

Cl

Cl

CH3

7.23 Scheme 7.18 SO2CH3 N [dye]

NH2

+

N

Cl

CH3

Cl 7.23

NaOH

SO2CH3 N H2O + NaCl +

[dye]

NH

N CH3

Cl 7.24

Scheme 7.19

(Scheme 7.19), leading to dyes with a 5-chloro-4-methyl-2-methylsulphonylpyrimidine reactive system (7.24). During reaction with the cellulosic fibre under alkaline conditions, the methylsulphonyl moiety is a particularly effective leaving group and rapid fixation takes place when the reactive system approaches the cellulosate anion. The Levafix P dyes have been replaced by the Levafix PN (DyStar) range, which is based on the 5-chloro-2-fluoro-4-methylpyrimidine system. These two systems are essentially similar, but the Levafix PN dyes have a fluorine atom as the labile entity rather than the methylsulphonyl grouping. The necessary intermediate 5-chloro-2,6-difluoro-4-methylpyrimidine (7.25) can be made from 4-methyl-2,5,6-trichloropyrimidine by an exchange reaction with potassium fluoride (Scheme 7.20). Condensation with the dye base (Scheme 7.21) will give a mixture of isomers, namely, the 5-chloro-6-fluoro-4-methyl (7.26) and 5chloro-2-fluoro-4-methyl (7.27) derivatives.

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CHEMISTRY OF REACTIVE DYES

Cl

F N

N Cl

Cl

F

+ 2 KF

Cl

+ 2 KCl

N

N

CH3

CH3 7.25 Scheme 7.20

F N 2

[dye]

NH2

+ 2 F

Cl N CH3

7.25

F

F N

N NH

[dye]

[dye]

+

N

NH

Cl N

7.27

Cl

CH3

CH3

7.26

Scheme 7.21

O C

N

Cl

N

Cl

HN SO3Na H

O

SO3Na

N N NaO3S

7.28

7.3.7 Dichloroquinoxaline dyes The reactivity of this system is much higher than that of analogous dichloropyrimidine dyes and comparable with aminofluorotriazine or difluoropyrimidine dyes, optimal fixation being achieved by exhaust dyeing at about 50 °C. Thus these three reactive systems are mutually compatible with one another and all three have been used in members of the Levafix (DyStar) range for long-liquor dyeing. The structure of a typical red monoazo H acid dye containing the dichloroquinoxaline system is illustrated (7.28).

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MONOFUNCTIONAL SYSTEMS

375

The key intermediate (7.29) required for reaction with the dye base is manufactured from 3,4-diaminobenzoic acid and oxalic acid [15]. These are condensed to yield 2,3dihydroxyquinoxaline-6-carboxylic acid and the chloro substituents are introduced using phosphorus oxychloride and phosphorus pentachloride (Scheme 7.22). Unlike all other important haloheterocyclic reactive systems, the bridging link between the chromogen and the quinoxaline nucleus is amidic and thus expected to be readily hydrolysed under acidic conditions. The 1,4-diazine ring in the dye–fibre linkage formed by these dyes, like the 1,3diazine ring present after fixation of the pyrimidine systems, tends to undergo oxidative cleavage when exposed to light or heat under peroxidic conditions. In spite of such potentially severe defects, the commercial success of these dyes over a long period indicates that they do not give rise to serious practical problems under normal circumstances [2]. HOOC

NH2

O +

NH2

HOOC

OH

N

OH

N

OH

C C O

Cl

OH

OC

N

Cl

N

Cl

7.29 Scheme 7.22

7.3.8 Sulphatoethylsulphone and -sulphonamide dyes Reactive dyes of the vinylsulphone type (7.31) are normally marketed in the form of the 2sulphatoethylsulphonyl precursor (7.30) in order to enhance the aqueous solubility and storage stability of the dye. In the presence of alkali the precursor group is converted into the active vinylsulphone form (Scheme 7.23), this being necessary for the reaction with the cellulosic fibre [31–33]. The aqueous solubility of commercial sulphatoethylsulphone dyes generally varies according to the number of sulphonate and sulphate ester groups present. Further reactions at 90–100 °C and pH values above 11.5 in the presence of hydroxide or carbonate anions (Scheme 7.24) result in generation of the 2-hydroxyethylsulphone (7.32) and eventually the diethyl ether form (7.33), which can amount to about 10% of the total dye present [34].

O [dye]

S

O

NaOH CH2CH2 7.30

O

OSO3Na

[dye]

S O

CH

CH2

7.31

Scheme 7.23

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+ Na2SO4 + H2O

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CHEMISTRY OF REACTIVE DYES

O [dye]

S O

O CH

CH2

[dye]

+ H2O

CH2CH2

O

7.31

O [dye]

S

OH

7.32

O

S

CH

O

CH2

+

HO

CH2CH2

O [dye]

S

[dye]

O

7.32

7.31

S

O CH2CH2

O

O

CH2CH2

S

[dye]

O

7.33

Scheme 7.24

In a recent investigation of the effect of low-frequency ultrasonic waves on the stability of a 1:1 copper-complex phenylazo H acid dye (7.34), reaction rates were determined for the 1,2-elimination of the sulphato group to form the vinylsulphone (Scheme 7.23) and for the hydrolysis step that leads to the 2-hydroxyethylsulphone form [35]. Elimination of the sulphato moiety is strongly accelerated by the presence of the electron-attracting sulphone group situated in the β-position relative to the leaving group and is the rate-determining step [36]. This permits control of the kinetics of the fixation process and has important implications for dye application and level dyeing. COCH3 O

HN

Cu O

NaO3SO

CH2CH2SO2

SO3Na

N N

7.34

NaO3S

CI Reactive Violet 5

In contrast to the various haloheterocyclic reactive systems already discussed, the vinylsulphone group reacts by a nucleophilic addition mechanism rather than by substitution. The carbon–carbon double bond forming the vinyl moiety is polarised by the presence of the strongly electron-attracting sulphone group. This polarisation confers a partial positive charge on the terminal carbon atom, favouring nucleophilic addition (Scheme 7.25). The attacking nucleophile (:O–X) can be either a cellulosate anion or a hydroxide ion, the former resulting in fixation to the fibre and the latter in hydrolysis to the 2-hydroxyethylsulphone form.

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MONOFUNCTIONAL SYSTEMS

O [dye]

S

O

_ CH

CH2 +

O

X

[dye]

O

S

_ CH

CH2

O

377

X

O Transient species

+ H

O [dye] Scheme 7.25

S

CH2CH2

O

X

O

The substantivity of the reactive vinylsulphone is much higher than that of the sulphatoethylsulphone precursor or the hydroxyethylsulphone hydrolysis product. Typical values for primary exhaustion of these three forms of CI Reactive Red 22 (7.35) on unmercerised cotton in the presence of 50 g/l sodium sulphate at neutral pH are given in Table 7.3. This dye system offers potentially high fixation through the highly substantive reactive form combined with excellent wash-off potential owing to the low substantivity of the hydroxyethylsulphone. The disadvantage of the system is the major difference in substantivity between the precursor form and the vinylsulphone because the rate of secondary exhaustion after alkali addition is more difficult to control [37]. Table 7.3 Relationship between substantivity and nature of the reactive group for monofunctional dyes [37] Reactive group X

Primary exhaustion (%)

–SO2CH2 CH2OSO3Na –SO2CH =CH2 –SO2CH2 CH2OH

20 80 38

X H

O

N N SO3Na

OCH3 7.35 CI Reactive Red 22

The kinetics of homogeneous reaction of several reactive dyes of the vinylsulphone type with methyl-α-D-glucoside (7.9), selected as a soluble model for cellulose, were studied in aqueous dioxan solution. The relative reactivities of the various hydroxy groups in the model compound were compared by n.m.r. spectroscopy and the reaction products were separated by a t.l.c. double-scanning method [38]. The only sites of reaction with the vinylsulphone system were the hydroxy groups located at the C4 and C6 positions [39,40].

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CHEMISTRY OF REACTIVE DYES

Sulphatoethylsulphone dyes are intermediate in reactivity between the high-reactivity heterocyclic systems, such as dichlorotriazine or difluoropyrimidine, and the low-reactivity ranges, such as aminochlorotriazine or trichloropyrimidine. Exhaust dyeing temperatures between 40 and 60 °C may be chosen, depending on pH, since caustic soda is often selected to bring about alkaline hydrolysis of the precursor sulphate ester. The substantivity of many of these dyes is markedly lower than that of typical haloheterocyclic dyes. Not only has the vinylsulphone group, unlike the heterocyclic ring systems, little if any inherent affinity for cellulose, but the terminal sulphato group enhances the aqueous solubility of the precursor form before 1,2-elimination to the vinylsulphone. In contrast to the haloheterocyclic systems, the dye–fibre bonds formed by the vinylsulphone dyes are at their weakest under alkaline conditions [41]. Two of the earliest Remazol dyes to be discovered by Hoechst turned out to be amongst the most successful of all reactive dyes. The four solubilising groups in the precursor form of CI Reactive Black 5 (7.36) confer high solubility but unusually low substantivity. This dye is almost symmetrical in structure and when the sulphate ester groups are lost by 1,2elimination, the substantivity for cellulose is enhanced and the bis(vinylsulphone) structure formed shows highly efficient fixation under alkaline conditions. After fixation the inherently low substantivity of the unfixed bis(hydroxyethylsulphone) dye facilitates washing-off in a region of the colour gamut where this is often notoriously difficult. The extremely attractive bright blue hue combined with excellent light fastness of CI Reactive Blue 19 (7.37) remained unchallenged by competing blue reactive dyes for many years. The aqueous solubility of this structure is inherently low, depending only on the 2sulphonate group after 1,2-elimination of the sulphate ester has taken place. This has led to

SO2CH2CH2

N

H2N O

H NaO3SO

CH2CH2SO2

N SO3Na

N N

7.36

NaO3S

CI Reactive Black 5

O

NH2 SO3Na

O

7.37 CI Reactive Blue 19

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HN SO2CH2CH2

OSO3Na

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OSO3Na

MONOFUNCTIONAL SYSTEMS

379

poor reproducibility and levelling problems, but nevertheless this dye has remained second only to Black 5 in terms of market share amongst reactive dyes. In the various ranges of haloheterocyclic reactive dyes, the reactive system is usually attached to the chromogen via a simple NH imino or methylimino linkage. Preparation of dyes in the vinylsulphone series, however, often requires the synthesis of special arylamine intermediates that contain the reactive system. These are mostly anilines or naphthylamines containing a β-sulphatoethylsulphonyl substituent. The intermediate 4-(β-sulphatoethylsulphonyl)aniline (7.38), used as the diazo component for Black 5 (7.36), is synthesised by the chlorosulphonation of acetanilide, reduction with sodium sulphite and condensation of the sulphinate with 2-chloroethanol to give the 2-hydroxyethyisulphone. Hydrolysis of the acetylamino group and esterification of the hydroxy group with concentrated sulphuric acid yields the desired product (Scheme 7.26). Synthesis of the isomeric 3-(β-sulphatoethylsulphonyl)aniline (7.39), the key intermediate for Blue 19 (7.37), proceeds by chlorosulphonation of nitrobenzene, reduction to the sulphinate, condensation with 2-chloroethanol, reduction of the nitro group and finally formation of the sulphate ester (Scheme 7.27). A level of reactivity similar to the vinylsulphone dyes is offered by the vinylsulphonamide system. The essential intermediate in this instance is carbyl sulphate (7.40), the cyclic anhydride of ethionic acid, which is readily available from ethylene and sulphur trioxide. Reaction with the amino group of an intermediate or the dye base yields the 2-sulphatoethylsulphonamide (7.41) precursor system directly (Scheme 7.28). As with the sulphatoethylsulphone analogues (Schemes 7.23–7.25), in the presence of alkali these dyes generate the active vinylsulphonamide form and this undergoes nucleophilic addition with a cellulosate anion during fixation treatment. Attack by a hydroxide ion produces the deactivated 2hydroxyethylsulphonamide and this can react further with the vinylsulphonamide to give the diethyl ether form. Tertiary sulphonamides (–NRSO2–, where R≠H) are used because their secondary analogues are deactivated under alkaline conditions [36]: NHSO2

NSO2

ClSO3H

CH3CONH

CH3CONH

SO2Cl

Na2SO3

ClCH2CH2OH CH3CONH

SO2CH2CH2OH

CH3CONH

SO2Na

dil. HCl

conc. H2N

H2N

SO2CH2CH2OH

SO2CH2CH2

H2SO4 7.38

Scheme 7.26

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OSO3Na

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CHEMISTRY OF REACTIVE DYES

ClSO3H

Na2SO3 O2N

O2N

O2N SO2Cl

SO2Na ClCH2CH2OH

reduction H2N

O2N SO2CH2CH2OH

SO2CH2CH2OH

conc. H2SO4

H2N SO2CH2CH2

OSO3Na

7.39 Scheme 7.27

O +

2 SO3

CH2

150°C

CH2

O2S

CH2

O

CH2 SO2 7.40

[dye]

CH3 [dye]

NHCH3

N SO2CH2CH2

OSO3H

7.41

Scheme 7.28

7.3.9 Acid-fixing reactive dyes The realisation that the reaction of phosphonic acid derivatives with alcohols to give phosphonate monoesters could be exploited in a reactive dyeing system originated at the Stanford Research Institute in 1973. Four years later the Procion T (ICI) range of dyes containing such groups was launched commercially. They were intended for the continuous dyeing and printing of cellulosic fabrics, especially polyester/cellulosic blends. The unusual conditions of application under mildly acidic conditions (pH 5–6) followed by thermofixation at 200–220 °C were quite different from those of conventional reactive dyes, making them much more compatible with the thermofixation of disperse dyes on the polyester component of the blend. The entire range of Procion T dyes was based on one versatile intermediate, 3aminophenylphosphonic acid (7.42), readily manufactured by nitration of phenylphosphonic acid followed by reduction (Scheme 7.29) In most instances this intermediate became the

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MONOFUNCTIONAL SYSTEMS

H2N

O2N O

O

O

P

OH

P

OH

P

OH

OH 7.42

O

COCH3

O

HN

P H4N

OH

OH

Scheme 7.29

H4N

381

O

O

H

SO3

N

NH4

N 7.43 H4N H2N

O3S

NH

CI Reactive Red 177 H2N

CN

7.44

C

NH

CN

7.45 Dicyandiamide

Cyanamide

diazo component as in structure 7.43, but it could also be attached to a naphthylamine coupler or to a non-azo chromogen through the imino link [8]. These dyes were marketed as aqueous solutions of their ammonium salts, which generated the free phosphonic acid form under thermofixation conditions. An activating agent of the carbodiimide type, such as cyanamide (7.44) or dicyandiamide (7.45), is necessary to bring about fixation to cellulose and this plays a decisive part in the reactions involved. At least two possible mechanisms of fixation have been proposed and it seems likely that either or both may be operative [8]. In the early work with dyes containing the phosphonic acid group [42], formation of the phosphonic anhydride (7.46) was believed to precede the esterification step, with one dye phosphonate moiety being released for further reaction with another molecule of the carbodiimide activator (Scheme 7.30). It had already been shown [43], that monoesters of arylphosphonic acids could be prepared in high yield by reaction with alcohols in the presence of dicyclohexyl carbodiimide, the esterification proceeding via the arylphosphonic anhydride. An alternative mechanism [8] entails reaction of cyanamide (or dicyandiamide) with the dye phosphonate to give an O-acylisourea derivative (7.47). This is able to react directly with cellulose to form dye-fibre bonds, urea being released as the anticipated by-product (Scheme 7.31). In support of this mechanism, it is known that O-acylisourea derivatives of arylcarboxylic acids react readily with alcohols and this constitutes an efficient route for the preparation of carboxylic esters [44]. More recently, the fixation efficiency on cotton of CI Reactive Red 177 (7.43) and its 4carboxyphenylazo analogue in the presence of various carbodiimides (including 7.44 and 7.45) was investigated, as well as homogeneous reactions of selected carboxylic acids with alcohols (including acetylcellulose in acetone). The carboxylated dye reacted more effectively with cotton cellulose in the presence of cyanamide rather than dicyandiamide,

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CHEMISTRY OF REACTIVE DYES

O [dye]

P

ONH4

+

H2N

ONH4

CN

Cyanamide

O 2 NH3

+

H2N

CONH2

+

[dye]

P

O

P

P

[dye]

OH

7.46 Dye anhydride

OH

O [cellulose]

O

OH

Urea

[cellulose]

O

O [dye]

+

[dye]

OH

P

OH

OH

Scheme 7.30

O [dye]

P

O ONH4

+

H2N

CN

[dye]

ONH4

P

NH2 O

NH

C

+ 2 NH3

OH Cyanamide 7.47 [cellulose]

OH

O [cellulose]

O

P OH

Scheme 7.31

[dye]

+

H2N

CONH2 Urea

whereas either type of carbodiimide was fully effective with the dye phosphonate. Loss of carbodiimide by hydrolysis to urea, followed by thermal decomposition to ammonia and carbon dioxide is the most important factor limiting the optimum level of dye fixation [45]. Further work with the same dye (7.43) and carbodiimides (7.44 and 7.45) concentrated on this problem of limited efficiency. Cotton fabric padded with the dye phosphonate solution was aftertreated with the carbodiimide dissolved in various alcoholic solutions to avoid hydrolytic decomposition. Under these conditions cyanamide was much more effective than dicyandiamide. With conventional reactive dyes the efficiency of the dye– fibre reaction is limited by competing hydrolysis of the active dye. Although phosphonated or carboxylated reactive dyes do not hydrolyse, their level of fixation is limited by competing hydrolysis of the carbodiimide activator [46].

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MONOFUNCTIONAL SYSTEMS

383

Although offering significant benefits, the Procion T (ICI) dyes suffered from certain technical drawbacks. These included dye migration during drying, especially on heavy fabrics such as corduroy, and strength losses of cellulosic fabrics during thermofixation under the acidic conditions required. The main reasons for their withdrawal in 1987, however, were economic rather than technical [8]. Interest in acid-fixing reactive dyes has remained active because of their environmentally attractive features (section 1.7). The freedom from competing hydrolytic reactions potentially offers exceptionally high fixation, extreme stability of the dye–fibre bonds and complete suitability of the unfixed dyes for recycling. In contrast to conventional reactive dyes, sensitisation problems arising from reaction with skin proteins are not anticipated. Unlike the haloheterocyclic reactive dyes, there is no risk of release of AOX compounds to waste waters. Heavy metals are not involved in the application of acid-fixing reactive dyes, nor are the electrolytes or alkalis that normally contaminate effluents from conventional reactive dyeing. A homobifunctional dye (7.48; X = NHCH2CH2PO3H 2) containing two alkylphosphonate groups was synthesised by condensing two moles of aminoethylphosphonic acid with the commercially available bis-aminochlorotriazine dye CI Reactive Red 120 (7.48; X = Cl). For comparison, a model dye (7.49) of the Procion T type was prepared by diazotising 3-aminophenylphosphonic acid (7.42) and coupling with R salt (disodium 2-naphthol-3,6disulphonate). After isolation as the free acids, these dyes were converted to their ammonium salts. When applied to cotton by a pad-batch-bake method in the presence of cyanamide (7.44) and ammonium dihydrogen phosphate, the bis-phosphonoethyl dye gave more than 90% fixation, compared with only 54% for the mono-phosphonophenyl model dye [47]. X

X N

N

N NH

NH

N

N

N

HN

SO3Na

NH

O

H

NaO3S O

SO3Na

N

H

NaO3S

N

N

N SO3Na

NaO3S 7.48 O

HO P

O

O

H

HO

O

SO3Na

HO

P

N HO

N

N

HO HO

7.49

P O

CH2

H2C

H2C

CH2CH2

7.50 EDTMP

SO3Na

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P

OH

OH

N

OH CH2

P O

OH

384

CHEMISTRY OF REACTIVE DYES

HOCH2CH2HN N NH

N CH2

COOH

CH

COOH

CH

COOH

CH2

N SO3Na

HN

H

O

N

COOH

SO3Na N

7.51 BTCA

7.52

NaO3S

An interesting variation on this type of pad-bake fixation system is to use the commercial sequestering agent ethylenediaminetetramethylphosphonic acid (7.50) as a crosslinking agent to react with cotton cellulose and with a dye containing nucleophilic hydroxyethyl groups. A suitable bis-hydroxyethyl dye structure (7.48; X = NHCH2CH2OH) of this kind was derived by condensing two moles of ethanolamine with CI Reactive Red 120 (7.48; X = Cl). Dye fixation values exceeding 80% were achieved in this way in the presence of cyanamide as dehydrating catalyst [47]. A similar concept evaluated recently depends on pad-bake application of the commercial crosslinking agent butane-1,2,3,4-tetracarboxylic acid (7.51) capable of esterification reactions with a hydroxyethyl group in the dye molecule and with cellulose OH groups. The hydroxyethyl derivative (7.52) was made by condensing ethanolamine with the chlorotriazine group in the commercial dye CI Reactive Red 3 (7.2). Thermofixation treatment at 170 °C in the presence of sodium hypophosphite (NaH2PO2) as catalyst gave dyeings of high fastness to washing but no fixation values were reported [48]. A tetracarboxylated derivative was prepared recently by reaction of a commercial reactive dye with two molar equivalents of aspartic acid. This novel derivative was evaluated by pad– dry–bake and pad-batch–bake methods under slightly acidic conditions in the presence of cyanamide as activator [49]. An interesting disperse dye containing a novel reactive anhydride system (7.54) was prepared from the parent dye carboxylate (7.53) by reaction with ethyl chloroformate in the presence of a tertiary base (Scheme 7.32). Such dyes will O C (CH3)2N

OH

N

Cl +

N

C

OCH2CH3

+ N(CH2CH3)3

O

7.53

DMF –5 °C

O C (CH3)2N

N

C N

Scheme 7.32

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O OCH2CH3

+ Cl– NH(CH2CH3)3

O

7.54

384

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BIFUNCTIONAL SYSTEMS

385

react with wool at the boil and pH 6–7, liberating ethanol as a by-product of the reaction. Unlike conventional reactive dyes for wool, no aftertreatment with ammonia is necessary to attain optimum fastness [50].

7.4 BIFUNCTIONAL SYSTEMS Evidence soon emerged in the 1960s that monofunctional reactive dyes containing two reactive centres, such as the dichlorotriazines, were capable of forming crosslinks between adjacent cellulose chains (7.8). This resulted in cellulosic fibres that had been dyed in this way showing anomalous behaviour in cuprammonium solubility tests [51]. Individual members of the early ranges of reactive dyes, including CI Reactive Black 5 (7.36), contained more than one reactive system but it was not until around 1970 that ICI introduced the Procion Supra (printing) and Procion H-E (dyeing) ranges of high-fixation dyes containing two aminochlorotriazine groups per molecule. If other factors are equal, the use of a reactive dye containing two reactive groups rather than its analogue with only one reactive group per molecule increases the fixation from a typical 60% to approximately 80% on average in exhaust dyeing. In pad–batch processes the corresponding fixation efficiency levels are about 75% and 95% respectively [52]. Bifunctional systems containing two different kinds of reactive group are popular in exhaust dyeing and gaining ground, especially on account of their relative insensitivity of fixation to fluctuations in dyeing temperature [53]. A detailed study of representative bis(aminochlorotriazine) dyes, as well as other potentially crosslinking reactive systems (dichlorotriazine, chloromethoxytriazine, trichloropyrimidine, chlorodifluoropyrimidine and dichloroquinoxaline) provided convincing evidence of the extent of crosslinking that could take place. Crosslinking was non-existent or relatively insignificant for typical pad–batch dyeings at ambient temperature, but thermal fixation by the pad–dry–steam method resulted in much more prevalent crosslinking by dye molecules [54]. 7.4.1 Bis(aminochlorotriazine) dyes Several different approaches have been employed in order to build the bifunctionality concept into known dye chromogens. In the case of bis(dichlorotriazine) dyes, which were not exploited commercially, only the arrangement 7.55 is feasible. The difficulty of introducing sufficient extra sulphonic acid groups into the chromogen to compensate for the presence of these two reactive systems and provide adequate solubility and mobility at the low dyeing temperature necessary for such highly reactive molecules proved insuperable.

Cl

Cl N

N

N NH

[dye]

N

NH

N N

Cl

Cl 7.55

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CHEMISTRY OF REACTIVE DYES

The versatility of the bis(aminochlorotriazine) concept is much greater, with three different arrangements being possible (7.56–7.58; RNH2 = arylamine, –NH–X–NH– = linking diamine). In the Procion Supra (ICI) dyes marketed for textile printing, the relatively more costly 7.56 pattern was preferred for technical reasons. Arrangements 7.57 and 7.58 have been used to design low-reactivity dyes of high substantivity intended primarily for exhaust dyeing. Structures of the 7.57 type are symmetrical about the central linking diamine and most of them contain a simple yellow, orange or red monoazo chromogen at each end of the molecule, CI Reactive Red 120 (7.48; X = Cl) being a typical example. Blue and green bis(aminochlorotriazine) dyes, including those derived from nonazo chromogens as well as the twice-coupled H acid disazo blues (7.59), usually fit the general structure 7.58. Cl N [dye]

NH

N

Cl

N

N NH

X

NH

N N NH

7.56

R Cl

Cl

N

N

N [dye]

N

NH

NH N

N NH Cl

X

NH

Cl N

N NH

N

[dye]

N R

[dye]

NH

7.57 N

N

HN

SO3Na NH

R

NH

7.58

H2N

SO3Na H

N

O SO3Na

N NH

N

N NaO3S

7.59

Unsymmetrical analogues of type 7.57 can be readily prepared using phenylene-1,3diamine-4-sulphonic acid as the central linking unit, because the bulky sulpho grouping protects the 3-amino group by steric hindrance whilst monoacylation takes place at the 1amino position. This approach, however, introduces a generally undesirable extra sulphonic acid group into the structure. Recent research has demonstrated, however, that it is possible to monoacylate phenylene-1,3-diamine itself using one dichlorotriazine dye and to react the

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BIFUNCTIONAL SYSTEMS

product with a different dichlorotriazine dye to yield novel unsymmetrical structures [55]. Thus CI Reactive Red 1 (7.1) was condensed with one mole of the diamine to give the intermediate 7.60. In the second acylation step the analogous dye CI Reactive Red 11 (7.61) was used, resulting in the formation of the unsymmetrical bis(aminochlorotriazine) dye 7.62. Cl N N

NH

Cl

NH2

N

N SO3Na H

N

HN

Cl N

O SO3Na

SO3Na

N

H

N

HN O SO3Na

N NaO3S

N

7.60

NaO3S

Cl

Cl N

N

N NH

NH

N SO3Na H

N N

HN

NH

O

NaO3S O

N

7.61

SO3Na

H

NaO3S

N

N

N 7.62

NaO3S

SO3Na

These bifunctional dye molecules are approximately twice the size of analogous monofunctional structures (compare 7.2 with 7.48). Their high substantivity ensures excellent exhaustion at the preferred dyeing temperature of 80 °C, leading to fixation values of about 70–80%, although in full depths typical members of the range may require salt concentrations as high as 100 g/l for optimal yield. High-temperature application conditions ensure good levelling and the high fixation leads to better utilisation of the dyes applied with less hydrolysis and less coloration of the effluent. Unfortunately, the rate of removal of unfixed dyes at the washing stage is slow owing to the high intrinsic substantivity of these dyes. A range of bis(aminofluorotriazine) dyes has been marketed by Ciba for long-liquor dyeing under the brand name Cibacron LS, which require only a low salt (LS) concentration in the dyebath. The patent literature indicates that the most likely structures (7.63) to meet this requirement contain two high-substantivity chromogens linked through the central unit carrying the fluorotriazine reactive groups [56]. This system offers an environmental

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F

F

N [dye]

NH

N N

N

N

NH

[dye]

N NH

X

HN

7.63

advantage over the chlorotriazine analogues; organofluorine compounds do not fall into the AOX classification because the fluoride ion liberated as soluble silver fluoride according to the test protocol is not detected [57]. Conferring high substantivity to the dye molecule in these various ways can lead to problems when dyeing in full depths, especially when it comes to the soaping-off stage. It would therefore be interesting to develop labile central linking units between the chromogenic groupings that could be cleaved during fixation or soaping. Such dyes would provide intrinsically high substantivity in the primary exhaustion stage to reduce the salt requirement but in the later alkaline fixation stage their molecular size would be halved and the unfixed single chromogens of much lower substantivity would show good soaping-off behaviour [56]. 7.4.2 Bis(aminonicotinotriazine) dyes An aminochlorotriazine dye will react with a tertiary amine possessing a sterically accessible nitrogen atom to form a quaternary ammonium derivative (Scheme 7.33). The positive charge carried by the quaternary nitrogen atom increases the polarisation of the C–N bond that links it to the triazine ring, so making such a compound much more reactive than the parent dye. Within a few years of the introduction of reactive dyes, the utility of tertiary amines as catalysts to assist the fixation of aminochlorotriazine dyes to cellulose was evaluated [58]. Suitable tertiary amines include trimethylamine (7.64), N,Ndimethylhydrazine (7.65), 1,4-diazabicyclo[2,2,2]octane (DABCO, 7.66), pyridine (7.67; R = H) and substituted pyridines such as nicotinamide (7.67; R = CONH2 ) or nicotinic acid (7.67; R = COOH). _ + N(CH3)3 Cl

Cl N [dye]

NH

N N

+

N(CH3)3

[dye]

NH

N

N

N NH

Ar

7.64

NH

Trimethylamine Scheme 7.33

CH2CH2 N

H2N

N(CH3)2

7.65 Dimethylhydrazine

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CH2CH2 CH2CH2 7.66 DABCO

N N R 7.67

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BIFUNCTIONAL SYSTEMS

389

In the case of quaternary derivatives made from the non-planar aliphatic amines 7.64, 7.65 and 7.66, steric strains further destabilise the C–N+ bond so that reaction with cellulose occurs under alkaline conditions at 30 °C, whereas temperatures of about 40–50 °C are required for the pyridinium derivatives 7.67. The quaternisation approach appeared to offer the opportunity to prepare dyes yielding reactivity levels intermediate between those of aminochloro- and dichlorotriazine dyes without loss of the desirable stability of the dye–fibre bond to acidic conditions that is characteristic of aminohalotriazine dyes. Unfortunately, this ideal was not attainable because of the objectionable odours of the tertiary amines liberated by the fixation reaction and the sensitivity of the reactivity behaviour of the quaternised derivatives to the nature of the chromogen attached to the triazine ring, making it difficult to select compatible combinations of dyes. A further difference in behaviour between certain substituted pyridinium dyes and other analogues was revealed when the relative rates of hydrolysis of a series of quaternary derivatives of a monoazo N-methyl J acid dye (7.68) were compared. It was found that where the leaving group X was nicotinamide (7.67; R = CONH2) or nicotinic acid (7.67; R = COOH) the dye was rapidly and unexpectedly converted to the aminotriazine, whereas with pyridine (7.67; R = H), isonicotinic acid (4-carboxypyridine) or other leaving groups the expected hydroxytriazine was produced, although much more slowly [59]. The mechanism of aminotriazine formation apparently involves attack by hydroxide ion at the 2-position, deprotonation by the 3-carboxylate ion, ring-opening by cleavage of the 1,2-bond and finally hydrolysis of the 1,6- bond (Scheme 7.34). The kinetics of reaction of DABCO (7.66) and nicotinic acid (7.67; R = COOH) with the aminochlorotriazine dye CI Reactive Red 3 (7.2) were studied under neutral conditions at temperatures in the range 100–130 °C. Quaternisation by DABCO was much more rapid than by nicotinic acid under these conditions. Neutral exhaust dyeing tests at 130 °C using the bis(aminochlorotriazine) analogue CI Reactive Red 120 (7.48; X = Cl) with the two catalysts confirmed these trends, in that the degree of fixation was greatly increased by DABCO but nicotinic acid showed no appreciable catalytic effect [60]. This difference may be attributable to steric strain of the C–N+ bond in the quaternised triazine structure by the non-planar DABCO substituent. Nicotinic acid (7.67; R = COOH) is a component of the Vitamin B complex that is essential to the mammalian diet, a deficiency causing pellagra. As a dye intermediate it offers relatively low cost and environmental acceptability. When used as a leaving entity it readily dissolves in the dyebath, a significant practical advantage over the tertiary amines with lower solubility and unpleasant odours. This was recognised at ICI in 1979 when the bis(aminonicotinotriazinyl)-substituted triphenodioxazine dye Procion Blue H-EG (CI Reactive Blue 187) was introduced for exhaust dyeing at 80 °C with the other SO3Na H

CH3

O N

N N

NaO3S

N N

NaO3S

CH3 N

N X 7.68

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CHEMISTRY OF REACTIVE DYES

COO

_

_

COO _

N+ N

CH

OH

N

N N

N

N

N

CH

_ O

CH

N

OH

N

COOH

N N

COO

HC _ HC

O

CH

H2O

+

COOH

COO

OH CH

HC

OH

O

N

N NH2 N N

N

N

N

N N

N

N Scheme 7.34

bis(aminochlorotriazine) members of the range. Although this product enjoyed some commercial success it was more reactive than typical Procion H-E dyes, rendering it somewhat incompatible in terms of dyeing behaviour. Indeed, it was evident that fixation to cellulose commenced prior to the addition of alkali to the dyebath [8]. Procion Blue H-EG was eventually superseded by the more conventional bis(aminochlorotriazinyl)-substituted analogue Procion Blue H-EGN (CI Reactive Blue 198). The observation that alkali was not essential for the fixation of nicotinotriazines was exploited by Nippon Kayaku in 1983. A full range of bis(aminonicotinotriazine) dyes was introduced under the Kayacelon React (KYK) brandname, an example being CI Reactive Red 221 (7.48; X = nicotino) [60]. All three possible arrangements of the two reactive systems are represented in the range, these being described as the two-step, one-arm (7.56), linkage (7.57) and two-arm (7.58) types [61]. Most of the commonly encountered chromogens have been patented in this way, including monoazo, disazo, metal-complex azo, copper formazan and copper phthalocyanine [8]. Exhaust dyeing from a neutral bath at 130 °C is recommended, making these dyes particularly suitable for the one-bath dyeing of polyester/cellulosic blends in conjunction with disperse dyes. By operating under these conditions, the diffusion problems anticipated with such large molecules are minimised. In spite of the anomalous ring-opening decomposition of nicotinotriazine compounds under conditions of alkaline hydrolysis (Scheme 7.34), the product of reaction of a bis(aminonicotinotriazine) dye with cellulose is the same as that from the analogous bis(aminochlorotriazine) dye in terms of hue, colour fastness and stability of the dye–fibre bond. If desired, these bis(aminonicotinotriazine) dyes can be applied satisfactorily at 80 °C and pH 11, as was evident for CI Reactive Blue 187. They have slightly higher reactivity

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than vinylsulphone or chlorodifluoropyrimidine dyes, but are less reactive than dichlorotriazine or dichloroquinoxaline systems [62]. Six bis-quaternary derivatives of C.I.Reactive Red 120 (7.48; X = Cl) were synthesised using trimethylamine (7.64), DABCO (7.66), pyridine (7.67; R = H), nicotinamide (7.67; R = CONH2), nicotinic acid (7.67; R = COOH) and isonicotinic acid (4-carboxypyridine) as the tertiary amines, the nicotinic acid derivative (7.48; X = nicotino) representing the control dye CI Reactive Red 221. The conditions required to achieve at least 90% conversion to the bis-quaternary species were 2 hours at 70 °C and pH 6.5 for DABCO and trimethylamine. The less reactive pyridine and its analogues required 2 or more hours at 90 °C for completion of this reaction. In exhaust dyeing tests on cotton, all six dyes showed similar levels of fixation at neutral pH, irrespective of dyeing temperature. At alkaline pH, however, fixation of the nicotinamide derivative declined from about 70% at pH 7 to 40% at pH 9. Values of exhaustion for this dye also decreased steadily as the dyebath temperature rose from 100 to 130 °C. These poor results were attributed to ring-opening decomposition of the pyridine-3-carbonamide system (as in Scheme 7.34) with formation of the aminotriazine under these adverse conditions [63].

7.4.3 Bis(sulphatoethylsulphone) dyes The most commercially successful reactive dye of all, CI Reactive Black 5 (7.36) contains two sulphatoethylsulphone precursor groups that contribute markedly to its initial solubility. When these are hydrolysed in alkali to release the reactive bis(vinylsulphone) form, the considerable increase in substantivity (Table 7.3) leads to highly efficient fixation. Further hydrolysis of the vinylsulphone groups to give the inactive bis(hydroxyethylsulphone) derivative, however, lowers the substantivity and hence contributes to favourable wash-off performance. A dimethoxy analogue of Black 5 has also been marketed but this (7.69) has not been so successful as it is more expensive to make, greener in hue and thus less suitable as a basis for black. Structures of the bis(sulphatoethylsulphone) type have been commercialised in other sectors of the colour gamut, including the bluish red 1:2 metal-complex CI Reactive Red 23 (7.70), anthrquinone blues such as the symmetrical structure 7.71 and phthalocyanine blues similar to 7.72.

SO2CH2CH2

OCH3

H2N H

NaO3SO

N

CH2CH2SO2

N

N

O SO3Na

N H3CO NaO3S 7.69

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CHEMISTRY OF REACTIVE DYES

NaO3S

SO2CH2CH2

OSO3Na

N N NaO3S

O O

Cu

O O

SO3Na

N N NaO3SO

CH2CH2O2S

7.70 CI Reactive Red 23

SO3Na CH3

O

HN

SO2CH2CH2

OSO3Na

SO2CH2CH2

OSO3Na

7.71

O

HN

CH3 SO3Na NaO3SO

H N

CH2CH2O2S

SO2

N

N N N

Cu

N

N N

7.72

N

O2S

N H

SO2CH2CH2

OSO3Na

NaO3S

A highly substantive bluish red bis(sulphatoethylsulphone) structure (7.73) has been patented recently [29]. In a kinetic study using the t.l.c. double-scanning method, the condensation reactions between the bis(aminochlorotriazine) dye CI Reactive Red 120 (7.48; X = Cl) and the two isomeric sulphatoethylsulphone anilines 7.38 and 7.39 to yield the two corresponding bis(sulphatoethylsulphone) isomeric dyes were compared. The rate constant of the reaction between Red 120 and the meta isomer 7.39 was about ten times as large as that for the para isomer 7.38 [64].

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NaO3S

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N

H

O

NaO3S

N

SO3Na

CH2CH2SO2

7.74

NaO3SO

HN

N

Cl

N

O

N

N

N SO3Na

N

H3C

NaO3S

N N

CH2CH2SO2NH

7.73

NaO3SO

CH2CH2SO2CH2CH2Cl

CH2CH2SO2CH2CH2Cl

H2N

SO3Na

N

NaO3S

N

H

SO2 HN

7.75

NH2

N

H N

N N

H

O

H3C

SO3Na

O

N

N

OSO3Na

OSO3Na

HNSO2CH2CH2

SO2CH2CH2

BIFUNCTIONAL SYSTEMS

393

394

CHEMISTRY OF REACTIVE DYES

Not all homobifunctional reactive dyes that react with cellulose by the nucleophilic addition mechanism are marketed as sulphatoethylsulphones. Thus the bluish red structure 7.74 contains two chloroethylsulphone precursor groups attached via a diethylamine residue and an activated chlorotriazine grouping to the H acid coupling component. The azopyrazolone yellow structure 7.75 depends for its reactivity on sulphatoethylsulphonamide precursor groups located separately at the diazo and coupler extremities of the molecule. 7.4.4 Aminochlorotriazine-sulphatoethylsulphone dyes Reaction of a dichloro-s-triazine dye with an anilino intermediate containing a 2sulphatoethylsulphone substituent, such as 7.38 or the more nucleophilic 7.39, is the preferred route to heterobifunctional reactive dyes of the Sumifix Supra (NSK) class introduced by Sumitomo in 1980. Rate constants for the reactions between two vinylsulphonylaniline isomers and a model dichlorotriazine dye have been determined in DMF by the t.l.c. scanning method. As expected, the meta isomer reacted more quickly than para-vinylsulphonylaniline [65]. The Sumifix Supra dyes are capable of reacting with cellulose via either the monochlorotriazine moiety or the vinylsulphone group released by the precursor sulphate ester. A typical structure is that of the monoazo H acid dye shown (7.76) [66]. Notable features are the high substantivity contributed by the triazine bridging nucleus and the capability this gives to link the two reactive centres to a wide variety of chromogens. The marked differences in substantivity between the various forms of monofunctional vinylsulphone dyes (section 7.3.8) recur to a moderated extent in the Sumifix Supra dyes because of the influence of the substantive triazine ring. The scarlet chromogen (7.77) linked via a chlorotriazine unit to the three variants of the vinylsulphone grouping showed similar trends (Table 7.4) to those already seen for those of CI Reactive Red 22 tested under the same conditions (Table 7.3). The rate of secondary exhaustion will be easier to control in this instance because of the lower difference in substantivity between the precursor and the vinylsulphone form [37]. Rate constants and the products formed in the hydrolysis of CI Reactive Red 194 (7.76) at 50 °C and pH values in the 10–12 region were determined by high-pressure liquid chromatography. In addition to the normal hydrolysis of the two reactive systems, the imino link between the triazine and benzene nuclei was also hydrolysed [67]. The heterobifunctional copper formazan dye CI Reactive Blue 221 and two blue anthraquinone monofunctional reactive dyes of the bromamine acid type, namely the aminochlorotriazine Blue 5 and the sulphatoethylsulphone Blue 19, were compared in terms of their sensitivity to Cl N N

NH N SO2CH2CH2

HN

SO3Na H

OSO3Na

O SO3Na

N N NaO3S

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395

Table 7.4 Relationship between substantivity and nature of the reactive group for heterobifunctional dyes [37] Reactive group X

Primary exhaustion (%)

–SO2CH2 CH2OSO3Na –SO2CH =CH2 –SO2CH2 CH2OH

43 82 63

Cl N N

SO3Na H H3CO

HN N

O HN

N N

X NaO3S

7.77

hydrolysis over a wide range of pH (1–12). Three main products of hydrolysis were isolated from Blue 221 by dialysis and thin-layer chromatography. It was confirmed that most of the fixation of Blue 221 to cotton cellulose in exhaust dyeing at 60 °C occurs via the vinylsulphone group [68]. The presence of two reactive groups that differ in reactivity gives dyes that are less sensitive to exhaust dyeing temperature than any of the typical monofunctional reactive systems. They can be applied over a wider range of temperatures (50–80 °C) and reproducibility of hue in mixture recipes is improved. Moreover, they show minimal sensitivity to electrolyte concentration and are less affected by changes in liquor ratio [69]. Low dyeing temperatures favour reaction via the vinylsulphone group, whereas at higher temperatures the contribution of the chlorotriazine system to fixation becomes more important [70]. This was demonstrated by controlled enzymatic degradation of a mechanically milled cotton fabric that had been exhaust dyed at 60 °C, generating the following conclusions after analysis of the products isolated [71]: (1) About 80% of the vinylsulphone groups had reacted with hydroxy groups in cellulose (2) About 50% of the chlorotriazine groups had not reacted and only about half of these had been hydrolysed to the hydroxytriazine (3) A considerable proportion of the dye molecules had formed crosslinks by reacting via both reactive systems. For further studies of this kind a scarlet monoazo dye of this type was synthesised containing three 13C-labelling atoms in the triazine ring. Enzymatic digestion of cotton dyed by four different exhaust methods yielded various dye-sugar derivatives that were estimated quantitatively using 13C-NMR liquid spectroscopy. As well as crosslinking through both groups, monofixation can occur via either group with the other either hydrolysing or remaining intact. Distribution patterns between these five modes of fixation to cellulose were elucidated for all four dyeing methods. In isothermal dyeing at 50, 60 or 80 °C, the proportion of fixation via the chlorotriazine group increased at the expense of vinylsulphone

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CHEMISTRY OF REACTIVE DYES

fixation and crosslinking. Crosslinking could be optimised by neutral exhaustion at 40 °C followed by two-step fixation at 60 °C and 80 °C [72]. The levelling properties of CI Reactive Red 194 (7.76), a twice-coupled H acid blue dye containing the same heterobifunctional system, the bis(sulphatoethylsulphone) dye CI Reactive Black 5 (7.36) and six monofunctional sulphatoethylsulphone dyes were compared recently in considerable detail. The results confirmed that those anthraquinone blue dyes of the bromamine acid type that rely only on the 2-sulphonate group and the precursor sulphatoethylsulphone substituent for aqueous solubility, such as CI Reactive Blue 19 (7.37), are especially prone to unlevel dyeing. These relatively hydrophobic characteristics promote aggregation in salt solution at relatively low dyeing temperatures [73]. Premature loss of the sulphate ester group should be avoided and such dyes should be applied at the lowest salt concentration and highest temperature that are consistent with the attainment of acceptable exhaustion and fixation performance. The incorporation of two different reactive systems and the high substantivity of heterobifunctional dyes favour the achievement of unusually high fixation. Although this leads to better utilisation of the dyes applied, with less of the hydrolysed by-products to colour the effluent, removal of these unfixed dyes at the washing-off stage may present difficulties because of their high intrinsic substantivity [8]. The formation of two different types of dye–fibre bond has beneficial consequences for fastness performance. Heterobifunctional dyes show superior fastness to acid storage compared with dichlorotriazine or dichloroquinoxaline systems and better fastness to peroxide washing than difluoropyrimidine or dichloroquinoxaline dyes [53]. In a detailed investigation, the dye–fibre bond stabilities of exhaust dyeings on cotton of the orthodox heterobifunctional dye CI Reactive Red 194 (7.76) and two monofunctional control dyes were compared. Both were analogues of Red 194, one having a hydroxytriazine group instead of the normal chlorotriazine and the other a hydroxyethylsulphone group instead of the normal sulphate ester. As expected, in acidic buffer solutions the orthodox dye 7.76 showed higher stability than the hydroxytriazine-vinyisulphone analogue, which was itself more stable than the chlorotriazine-hydroxyethylsulphone dye. Under alkaline conditions the hydroxytriazine-vinylsulphone dye was much less stable than the two dyes with an active chlorotriazine group [74]. Two unorthodox heteromultifunctional dyes were also included in this investigation, neither being of commercial interest. The anthraquinone blue dye 7.78 containing the same heterobifunctional reactive system as Red 194 exhibited surprisingly low fixation. This was attributed to the non-planar conformation of bromamine acid derivatives of this kind [74]. All three imino groups linking together the phenyl-triazine-phenyl-anthraquinone series of nuclei in this structure have a twisting effect on the aryl systems on both sides of each NH link. Thus the triazine ring is twisted through about 90 degrees relative to the plane of the anthraquinone chromogen. Another multifunctional dye that gave disappointingly low fixation was the symmetrical structure 7.79, even though there were no less than five reactive systems linked together with two red monoazo H acid chromogens. Apparently this exceptionally large dye molecule is absorbed so gradually and diffuses so slowly that adequate fixation cannot be attained within an acceptable dyeing time [74]. Another comprehensive evaluation of cotton dyeing parameters has been carried out for eighteen isomeric monoazo H acid red dyes containing aminochlorotriazine and

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O

397

NH2 SO3Na 7.78 Cl

O

HN

N NH

N N

NH SO2CH2CH2

SO3Na

Cl

Cl N HN

N

N N

NH

NH

SO3Na O2S

N N

CH2 CH2

OSO3Na

N

NH

NaO3S

H O

H N O N

N HN

NaO3S

N

SO2 CH2

NH N

N

SO3Na

OSO3Na

CH2 OSO3Na

Cl SO3Na

NaO3S

7.79

sulphatoethylsulphone (Z) reactive groups [66]. Representative results for the affinity parameter, rate of hydrolysis and fixation yield are presented in Table 7.5. Three commercially available dyes are represented in the series (CI Reactive Reds 194, 198 and 227). Substitution in the o-position usually lowers the affinity parameter by sterically hindering the coplanarity of the aryl nuclei. Thus the two dyes with ortho substituents in both rings have the lowest affinity values. Surprisingly, however, the two dyes (Reds 194 and 227) with an o-sulpho group in ring A and the reactive system in the m- or p-position of ring B show the highest affinity values. Dyes with the sulphatoethylsulphone group in the m-position of ring B are the most reactive, especially Red 194, which hydrolyses about ten times more quickly than those dyes with an ortho reactive group in this ring. Two of the commercial products (Reds 194 and 227) achieved the highest fixation values. The combination of a m- or p-sulpho group in ring A and the reactive group in the o-position of ring B leads to poor affinity and the lowest reactivity, resulting in fixation yields less than half those of Reds 194 and 227. In general, dyes like these four with both reactive systems attached via the imino link in the coupling component show more extremes of dyeing behaviour than those in which the sulphatoethylsulphone group is in the diazo component (ring A).

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Table 7.5 Affinity parameter, rate of hydrolysis and fixation yield [66] Substituent in Ring A SO3Na

Ring B Z

o o o m m m p p p

o m p o m p o m p

CI Reactive

Red 194 Red 227

Affinity parameter

Rate of hydrolysis (10–2/min)

Fixation yield (%)

0.29 0.62 0.64 0.33 0.50 0.50 0.35 0.52 0.51

1.9 15.0 4.8 1.1 7.4 2.1 1.3 8.3 2.5

28.4 41.8 44.8 18.7 37.1 37.8 20.6 34.7 39.1

Affinity parameter

Rate of hydrolysis (10–2/min)

Fixation yield (%)

Substituent in Ring A Z

Ring B SO3Na

o m p o m p o m p

o o o m m m p p p

CI Reactive

0.27 0.36 0.36 0.46 0.58 0.54 0.46 0.54 0.51

Red 198

4.9 4.6

28.3 32.4

5.4 4.3

34.5 37.9

3.4 3.0

36.3 38.9

Z Sulphatoethylsulphone

Cl Ring B

N N Ring A m

o

N o

HN H

p

HN

m

O SO3Na

N

p

N NaO3S

7.80

7.4.5 Aminofluorotriazine-sulphatoethylsulphone dyes In 1988 Ciba launched the Cibacron C range of bifunctional reactive dyes. They contain a new aliphatic vinylsulphone system and either a monofluorotriazine bridging group or an arylvinylsulphone function [75]. Owing to the small size of the fluorine atom, the difluorotriazine precursor reacts more smoothly with an alkylamine carrying the sulphato-

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399

ethylsulphone system such as 7.81 than with an arylamine intermediate such as 7.38 or 7.39. The Cibacron C dyes are designed mainly for pad applications and are characterised by low to moderate affinity, good build-up, ease of washing-off and high fixation (often >90%). Their good stability under padding conditions, high solubility, efficiency of reaction and outstanding fixation make them especially suitable for the pad–batch process [76]. The presence of the vinylsulphonylalkyl group assists solubility and most of these dyes form stable liquids without the addition of urea [36].

H2NCH2CH2SO2CH2CH2OSO3Na 7.81

The stability of the dye–fibre bonds in these dyeings is high to both acid and alkali, compared with monofunctional halotriazine analogues, because of the major contribution of the vinylsulphone function to the fixation mechanism. The fluorotriazine group confers much higher stability to alkali than is shown by monofunctional sulphatoethylsulphone dyes [77]. A characteristic feature of the Sumifix Supra (NSK) heterobifunctional system is the major difference in reactivity between the aminochlorotriazine moiety and the much more reactive vinylsulphone group. There are some practical conditions, notably in pad–batch application, that do not allow full advantage to be taken of both types of reactive group present. The combination of aminofluorotriazine and sulphatoethylsulphone in the Cibacron C (Ciba) synchronised bifunctional system, both groups offering effective fixation under virtually the same conditions, exploits the concept of bifunctionality more effectively. These factors have been demonstrated elegantly in a detailed evaluation by pad–batch dyeing of cotton with three commercially important copper formazan reactive dyes: (1) Cibacron Blue F-R (Ciba; CI Reactive Blue 182) with a monofunctional aminofluorotriazine group. (2) Sumifix Supra Blue BRF (NSK; CI Reactive Blue 221) with a heterobifunctional aminochlorotriazine-sulphatoethylsulphone system. (3) Cibacron Blue C-R (Ciba) with a synchronised bifunctional aminofluorotriazinesulphatoethylsulphone system. Under the relatively mild conditions of pad–batch fixation within 6 hours batching time, the heterobifunctional dye (Blue 221) behaved as if it were a monofunctional sulphatoethylsulphone dye and only the synchronised Cibacron C system showed truly bifunctional performance [78]. Interfibrillar crosslinking induced by a colourless cellulose reactant early in the wet processing sequence can prevent further fibrillation of lyocell fibres. Certain bifunctional reactive dyes exert a similar effect but specific molecular characteristics must be present. These include steric orientation and separation of the reactive groups, degree of reactivity, size of chromogen, molecular flexibility and diffusion properties. Cibacron LS bis(aminofluorotriazine) dyes applied by exhaust dyeing and Cibacron C aminofluorotriazine-sulpatoethylsulphone dyes in the cold pad-batch process largely fulfil these requirements and thus provide control of undesirable post-fibrillation of lyocell during finishing and laundering [79].

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CHEMISTRY OF REACTIVE DYES

7.5 CHROMOGENS IN REACTIVE DYES Providing there are enough sulpho groups to ensure adequate solubility in water, the only essential feature of a chromogen needed to build it into a reactive dye molecule of the haloheterocyclic type is a primary or secondary amino group to which the heterocyclic system can be attached. This also applies to the various ranges of bifunctional dyes described in section 7.4, since they all contain this same type of amino-s-triazine grouping attached to the chromogen. Only in the case of monofunctional sulphatoethylsulphone dyes is the selection of chromogens more limited by the various ways in which conventional intermediates for these dyes, such as 7.38 and 7.39, can form an integral part of the chromogenic grouping. These intermediates have wide applicability in azo dye structures, since they are conveniently used as diazo components with a variety of orthodox couplers. The para isomer (7.38) is less nucleophilic than the meta-substituted one (7.39), which is therefore more versatile. Most ranges of reactive dyes contain examples of monoazo, disazo, metal-complex azo, anthraquinone and phthalocyanine chromogens, with copper formazan or triphenodioxazine blues sometimes also present. Within each sector of the colour gamut it is often possible to find the same specific chromogen being used in various ranges, with individual monofunctional dyes differing only in the nature of the reactive system. These trends are often dictated by the need to consider the ready accessibility of key intermediates. In research centred around the design of novel dye structures, computer-based techniques of molecular modelling and statistical experimental design to reveal structure–property relationships are now common practice [80].

7.5.1 Greenish yellow chromogens These dyes are invariably monoazo compounds with the reactive system attached to the diazo component, owing to the ready availability of monosulphonated phenylenediamine intermediates. Pyrazolone couplers are most commonly used, as in structure 7.82 (where Z is the reactive grouping), and this is particularly the case for greenish yellow vinylsulphone dyes. Catalytic wet fading by phthalocyanine or triphenodioxazine blues is a characteristic weakness of azopyrazolone yellows (section 3.3.4). Pyridones (7.83), barbituric acid (7.84) and acetoacetarylide (7.85; Ar = aryl) coupling components are also represented in this sector, with the same type of diazo component to carry the reactive function.

Cl SO3Na

SO3Na

SO3Na

O

H

N N

Cl

H3C

O

N

NH

Z

NH H3C 7.83

7.82

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N

N Z

O

H

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CHROMOGENS IN REACTIVE DYES

401

SO3Na O

H Z

NH

N

N

N 7.84

SO3Na

OH

N

O

H Z NH

N

HO

N 7.85

C

CH3

C

NH

C Ar

SO3Na

O

CONH2 SO3H

HN

_ N

N Cl NH2

+ HO3S

SO3H

7.86

7.87

kd 4 NaOH kc 4 NaOH

SO3Na OH + N2 + NaCl + 3 H2O NaO3S

CONH2

SO3Na

HN SO3Na N

NH2

N

+ NaCl + 4 H2O NaO3S

SO3Na Cyanuric chloride,

then ammonia CONH2 Cl

HN SO3Na

N N

NH

N

N N

NaO3S

NH2

SO3Na

7.88 CI Reactive Orange 12

Scheme 7.35

7.5.2 Reddish yellow chromogens Azo structures covering this sector of the colour gamut are prepared from di- or trisulphonated naphthylamine diazo components and p-coupling anilines, such as 3aminophenylurea as in structure 7.88, 3-aminoacetanilide or cresidine (3-amino-4methoxytoluene), with the reactive system attached to the terminal amino group. The kinetics of the azo coupling reaction (kc) to form the Orange 12 chromogen from diazotised 2-naphthylamine-3,6,8-trisulphonic acid (7.86) and 3-aminophenylurea (7.87), as well as the rate of decomposition (k d) of the diazonium salt (Scheme 7.35), were measured

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CHEMISTRY OF REACTIVE DYES

SO3Na

SO3Na COCH3 HN

NaO3S N

SO3Na N

NH

Z HN

NaO3S

SO3Na

COCH3

N N

trans

NH

Z

cis

Scheme 7.36 SO3Na H

O NH

N

Z

N NaO3S 7.89

potentiometrically [81]. Stepwise condensations with cyanuric chloride and then with ammonia yield the aminochlorotriazine dye (7.88). A well-recognised practical problem with arylazoaniline golden yellow dyes of this kind is photochromism. This slight change in hue is attributed to a reversible, photochemically induced transformation from the more stable trans to the less stable, less linear cis isomer (Scheme 7.36). It occurs very quickly on exposure to light but the thermal reversion from cis to trans in the dark is a much slower process. Greenish yellow chromogens (7.82–7.85) and orange reactive dyes of the J acid type (e.g. 7.89) do not exhibit this phenomenon. These structures are all derived from o-hydroxyazo coupling components that exist predominantly in the ketohydrazone form, so that rotation around the N–N axis is possible and cis–trans isomerism cannot occur [82]. 7.5.3 Orange chromogens Dye bases for these dyes are obtained from a sulphonated arylamine as the diazo component and J acid as the coupler. Yellowish orange dyes are given by mono- or disulphonated anilines, as in structure 7.89, whereas reddish orange hues result from di- or trisulphonated naphthylamines, structure 7.92 being a typical example. If N-methyl J acid is used, as in this instance, the high substantivity that is characteristic of J acid dyes becomes somewhat lower, making it easier to wash-off the unfixed dyes after fixation. In a recent investigation [83], the kinetics of the coupling reaction between diazotised 2-naphthylamine-1,5-disulphonic acid (7.90) and the dichlorotriazinyl derivative of N-methyl J acid (7.91) that yields the Orange 4 chromogen (Scheme 7.37) were examined in detail. In orange dyes of the haloheterocyclic type, the reactive system is invariably attached via the nitrogen of the J acid coupler. In vinylsulphone dyes, on the other hand, it is normally more convenient to use as diazo component an intermediate such as 7.38 or 7.39 bearing the precursor grouping together with an N-acetylated derivative of J acid or γ acid as coupler, structure 7.93 being typical.

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OH

HO3S

403

Cl

_ + N N Cl

N

N

+ NaO3S

N

N

SO3H

Cl

CH3

7.91

7.90 3 NaOH Cl N NaO3S

N

O

H

N

N N

N

Cl

CH3

NaO3S SO3Na

+ NaCl + 3 H2O

7.92 CI Reactive Orange 4

Scheme 7.37

NaO3SO

NHCOCH 3

CH2CH2O2S O

H N N 7.93

NaO3S

CI Reactive Orange 7

SO3Na H CH3O

O NH

N

Z

N NaO3S 7.94

NaO3SO

CH2CH2O2S H

O

N N

7.95 CI Reactive Red 22

OCH3

SO3Na

7.5.4 Scarlet chromogens Dyes in this hue sector are also derived from J acid, N-methyl J acid or γ acid but the diazo component is usually a sulphonated 2- or 4-anisidine, as a methoxy substituent has a bathochromic influence. The highly substantive structure 7.94 is found in various haloheterocyclic (Z) dyes. As in the orange region, vinylsulphone dyes have the precursor grouping located on the diazo arylamine, as exemplified by structure 7.95.

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CHEMISTRY OF REACTIVE DYES

7.5.5 Red chromogens Bluish red reactive dyes are almost totally dominated by H acid as the indispensable coupling component. Various mono- or disulphonated anilines or naphthylamines are suitable diazo components, but the outstandingly important one is orthanilic acid (7.96). As with orange and scarlet dyes, haloheterocyclic (Z) reactive systems are linked via the imino group of the H acid residue but sulphatoethylsulphone substituents are found in the diazo component with N-acetyl H acid as the typical coupler. A characteristic problem associated with reactive dyes derived from H acid is their accelerated fading under the simultaneous influence of perspiration and light (section 3.3.4). Z HN

SO3Na O

H

SO3Na

N N NaO3S

7.96

7.5.6 Rubine chromogens Bordeaux and rubine hues are given by 1:1 copper-complex monoazo structures made from a diazotised aminophenolsulphonic acid and J acid or γ acid, with the haloheterocyclic group (Z) on the coupling component (7.97). As usual in the vinylsulphone series it is more convenient to use a precursor-substituted aminophenol with N-acetyl J acid as coupler. In the red–violet–blue zone of the colour gamut the higher light fastness shown by copper complexes compared with their unmetallised analogues is particularly desirable in spite of the sacrifice of some brightness. O

Cu O

NaO3S

NH

N

Z

N 7.97

NaO3S

7.5.7 Violet chromogens These very much resemble the corresponding rubine dyes but have H acid rather than J acid as the coupling component. This does mean that there is some risk of accelerated fading as a result of the combined effects of perspiration and light. The histidine component of perspiration is able to abstract the copper from some metal-complex azo dyes (section 5.7.2) and the demetallised H acid structure will then be vulnerable to this problem (section 3.3.4). Nevertheless, these copper-complex violet dyes are much less sensitive than the unmetallised bright bluish reds in this respect. Examples of typical structures include those with a haloheterocyclic (Z) group on the H acid residue (7.98) and those such as CI Reactive Violet 5 (7.34) with a sulphatoethylsulphone group in the diazo component.

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405

Z O

HN

Cu O

SO3Na

N N NaO3S NaO3S

7.98

7.5.8 Dull blue chromogens These are reddish blue 1:1 copper-complex monoazo dyes derived from a 2-naphthylamineor 2-aminonaphtholsulphonate as diazo component and another aminonaphtholsulphonate as coupler. Often such dyes are more easily prepared using a 2-naphthylaminesulphonate and oxidatively coppering the resulting monoazo dye (section 5.5.3). In orthodox structures the imino link of H acid carries the reactive system (7.99), but in other instances the naphthylamine diazo component provides the site of attachment of a haloheterocyclic (7.100) or sulphatoethylsulphone (7.101) grouping. Z

SO3Na O

HN

Cu O

SO3Na

N N

O

H2N

Cu

SO3Na

O

NaO3S NaO3S

NaO3S

7.99

N N SO3Na

Z

NaO3SO

7.100

NH

CH2CH2 COCH3

SO2 O

HN

Cu O

NaO3S

SO3Na

N N

7.101

NaO3S

7.5.9 Bright blue chromogens Two different chemical classes contribute to this sector. Initially it was entirely dominated by anthraquinone dyes typified by structure 7.102. The dye bases for attachment of haloheterocyclic (Z) systems are prepared by condensing bromamine acid (7.103) with various phenylenediamines. The outstandingly successful CI Reactive Blue 19 (7.37) is the

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CHEMISTRY OF REACTIVE DYES

condensation product of bromamine acid with the precursor arylamine 7.39. More recently, bright blue symmetrical structures of the triphenodioxazine class (7.104; –NH–R–NH– = alkylenediamine, Z = haloheterocyclic system) have been developed. These are tinctorially much more intense than anthraquinone chromogens and thus offer economic advantages. The marketing of a bis(aminonicotinotriazine) dye of this class (CI Reactive Blue 187) had an important influence on the subsequent development and success of the Kayacelon React (KYK) range of dyes containing this bis-quaternary system (section 7.4.2). O

NH2 SO3Na

O

NH2 SO3Na

O

HN O

7.102

NH

Br

Z 7.103

SO3Na

Bromamine acid

7.5.10 Turquoise chromogens This colour makes an important contribution to the appeal of reactive dyeings and prints. It is totally dominated by dyes derived from copper phthalocyanine (section 5.4.3). A notable early example is structure 7.105, made from copper phthalocyanine by chlorosulphonation of all four 3-positions, partial reaction with phenylene-1,3-diamine-4-sulphonic acid and ammonia, hydrolysis of the remaining chlorosulphonyl groups, then finally introduction of the reactive system using cyanuric chloride and ammonia. Such products are inhomogeneous mixtures of slightly different components, the overall balance of substituents totalling four per molecule as shown in formula 7.105. Solubility and dyeing properties of these dyes can be modified by varying the proportion of 3-sulphonamide (polar) to 3-sulphonic acid (hydrophilic) groups, as well as selection from a variety of phenylenediamines. An important dye of this class is CI Reactive Blue 21, in which the reactive function is provided by a 3-SO2NHPhSO2CH2CH2OSO3Na grouping.

SO3Na

Cl

Z

HN

R

HN NaO3S

N

O

O

N

NH

R

NH

Cl 7.104

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Z

CHROMOGENS IN REACTIVE DYES

407

NaO3S Cl

SO2NH N NaO3S

N

NH

N N N

Cu

N N NH2

N

N N

N

SO2NH2 7.105

NaO3S

CI Reactive Blue 7

7.5.11 Green chromogens Only relatively few green reactive dyes have been marketed and most of them have been designed by linking up separate blue and yellow chromogens. The most important approach has been to attach a yellow monoazo (Y = N) or stilbene (Y = CH) chromogen to a bromamine acid residue, either directly (7.106) or via a phenylenediamine linking group and an aminohalotriazine system (7.107; X = Cl or F). Another possibility is to incorporate a yellow chromogen together with the reactive grouping of a copper or nickel phthalocyanine, but such dyes have unattractive dyeing properties because of the length of this extended substituent. In a few instances, dull bluish greens can be achieved from twice-coupled H acid structures if the two diazo components are correctly chosen, as in the bis(aminochlorotriazine) dye CI Reactive Green 19. O

O

NH2

NH2 SO3Na

SO3Na

O

O

HN

HN X

SO3Na

7.106

N Y

NaO3S

Y

HN

N

SO3Na

N HN

SO3Na NH Z

7.107

Y

Y

NO2

NaO3S

7.5.12 Brown chromogens Unmetallised disazo dyes of the type A→M→E in Winther symbols (section 4.7) dominate this sector. The A component is usually a disulphonated aniline or naphthylamine. Orthanilic acid or p-xylidine (2,5-dimethylaniline) for yellow browns, or a variety of monosulphonated 1-naphthylamines for redder browns, are selected as the M and E components. The terminal amino group provides the site for attachment of the reactive

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system (Z). Structure 7.108 illustrates a typical reddish brown. Monoazo J acid, γ acid or pyrazolone ligands have been used in unsymmetrical 1:2 cobalt or chromium complexes with a reactive group in each ligand to give brown dyes of high fastness to light and wet treatments. SO3Na N

N N

NH

Z

N SO3Na

NaO3S

NaO3S

7.108

7.5.13 Navy blue chromogens Three major approaches have been followed to provide reactive dyes in this important sector. One category is closely related to the reddish blue monoazo 1:1 copper complexes already described (section 7.5.8). To provide the higher substantivity and deeper intensity for build-up to navy blue shades, a second unmetallised azo grouping is introduced. As with the brown dyes, the A→M→E pattern is adopted for their synthesis. Component A is normally a sulphonated aniline, M an aminophenol or aminocresol and E a sulphonated naphthol or aminonaphthol. The reactive system (Z) is usually, but not invariably, located on the E component and the copper atom always coordinates with an o,o′-dihydroxyazo grouping provided by the M and E components (7.109). This hue sector is dominated, however, by the more economical unmetallised twicecoupled H acid derivatives. In these dyes the amino group in the H acid molecule is not used to attach the reactive system because that would prevent conjugation between the two azo groups. As in structure 7.110, the diazo components are almost always aniline-2,5disulphonic acid and a sulphonated phenylenediamine. Usually the former is attached by acid coupling ortho to the amino group and the latter, which provides the site for the haloheterocyclic (Z) group, by alkaline coupling ortho to the hydroxy group. In monofunctional vinylsulphone dyes of this type, a precursor-bearing intermediate such as 7.38 or 7.39 is introduced by alkaline coupling to the hydroxy side of the H acid residue. In the outstandingly successful CI Reactive Black 5, two such precursor-bearing units are used in the synthesis of this near-symmetrical bifunctional structure (7.36). Following this precedent, competing bifunctional dyes of analogous structure were designed with two phenylene-1,3-diamine-4-sulphonate groupings to accommodate the reactive systems O

SO3Na

Cu Z

O N

N

N

N

CH3

N NaO3S

H3C NaO3S 7.109

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CHROMOGENS IN REACTIVE DYES

NaO3S SO3Na

H2N

H

N

409

SO3Na N

O SO3Na

N N Z

NaO3S

NH

NH

Z

7.110

NaO3S

SO3Na H

H2N

N

N

O SO3Na

N N Z

NH NaO3S

Y

7.111

O O

SO3Na

O C Cu

X

N

N

N

N C 7.112

(7.111). Navy blue reactive dyeings that meet exceptional levels of fastness to light and wet treatments can be achieved (at a price) by turning to copper formazan complexes (7.112). In those dyes with a haloheterocyclic reactive system, this is normally attached through an imino link at position Y and X is a sulpho group. Conversely, in vinylsulphone analogues Y is sulpho and X is the sulphatoethylsulphone precursor. 7.5.14 Black chromogens This is almost a contradiction in terms for reactive dyes. As in the green sector, it is difficult to design a homogeneous reactive dye that will yield deep black dyeings without shading. The economically attractive twice-coupled H acid dyes build up well but even those described as blacks, such as CI Reactive Black 5, are really dark navy blues. Dyes of this kind are readily shaded with low-cost browns of the A→M→E type (section 7.5.12), or with smaller amounts of yellow and red shading components, to give general-purpose black mixtures of considerable versatility. Copper-complex navy blues can be used in a similar way with shading components of high light fastness for more demanding outlets. Mixed cobalt/chromium complexes of the symmetrical 1:2 type in which each ligand contains a low-reactivity system linked to the coupling component (section 5.5.2) have been widely used for continuous dyeing and printing. They offer high light fastness and the

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CHEMISTRY OF REACTIVE DYES

unfixed dye is more readily washed-off because such structures tend to be less substantive to cellulose than the unmetallised disazo dyes designed principally for exhaust dyeing. Homogeneous black chromogens are well represented among direct dyes, where highly substantive trisazo or tetrakisazo structures are often encountered. Quite often these contain primary amino groups as auxochromes and hydrogen bonding sites positioned at each end of the molecule. It would not be difficult to convert the typical trisazo dye CI Direct Black 80, for example, into its bis(aminochlorotriazine) derivative (7.113). This hypothetical structure, however, would possess such a high substantivity and such a slow rate of diffusion that undesirable characteristics of poor levelling, inadequate penetration and inefficient fixation would be likely, together with great difficulty at the washing-off stage. This performance profile is reminiscent of that shown by the pentafunctional red dye (7.79) described in section 7.4.4 [74]. Cl N HN H

O

NH2

N

Cl H

N N

NH

N

N

O

N

N N

N

N N

NaO3S

H2N NaO3S SO3Na

7.113

7.6 STABILITY OF DYE–FIBRE BONDS The chromogens used to synthesise reactive dyes normally exhibit poor wet fastness in the unfixed state. The high wet fastness of reactive dyeings depends almost entirely on the resistance of the dye–fibre bonds to the agencies characteristic of wet fastness tests, including pH, temperature, surfactants and oxidants. When the dye–fibre reaction product is formed during the fixation process (Schemes 7.8 and 7.25), many of the factors responsible for the susceptibility of the reactive system to hydrolysis remain operative and may play a decisive part in determining the wet fastness attainable [10]. Thus when a dichlorotriazine dye reacts with a cellulosate anion, one of the chloro substituents is replaced by an electron-releasing cellulosyl grouping that is less electronegative. The other electronegative chlorine and three nitrogen atoms remain, however, activating the substituents on the heterocyclic ring to hydrolytic attack (Scheme 7.38). This can result in either stabilisation or rupture of the dye–fibre bond. A somewhat different balance exists with the less reactive aminochlorotriazine dyes. Since an arylamino group is more strongly electron-releasing than the cellulosyl grouping, the electronegative influence of the three nitrogen atoms in the ring predominates and only the activated cellulosyl ether bond is subject to hydrolytic attack (Scheme 7.39). For this reason, therefore, such dye–fibre linkages will be marginally less stable under alkaline conditions than those formed by the reaction of cellulose with a dichlorotriazine dye. The marginal differences in dye-fibre bond stability between aminochlorotriazine, bis(aminochlorotriazine) and bis(aminofluorotriazine) dyes were examined by treating cotton dyeings

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OH

+

HO

O

[cellulose]

NH

[cellulose]

N

N [dye]

411

H2O

N N

[dye]

NH

N N

NaOH Cl

Cl

Activated dye-fibre bond

Hydrolysed dye

NaOH O

[cellulose]

N

+ NaCl

N [dye]

NH N

OH Stabilised dye-fibre bond

Scheme 7.38

NH [dye]

NH

NH

[aryl]

N

[dye]

N NaOH

N O Dye-fibre bond

[cellulose]

[aryl]

N

H2 O NH

N N OH

+

HO

[cellulose]

Hydrolysed dye

Scheme 7.39

in buffer solutions (pH 10 or 12) at various temperatures (60, 85 or 98 °C). The results were expressed in terms of the percentage of hydrolysed dye released [84]. If a pyrimidine ring forms the basis of a chloroheterocyclic reactive system, the effects are similar to those for chlorotriazine analogues but the activation by only two ring nitrogen atoms is much less marked (Figure 7.1). Thus pyrimidine dye–fibre linkages are more stable to alkali than either of the corresponding chlorotriazine systems. The rate-determining step for acidic hydrolysis of haloheterocyclic systems is nucleophilic attack of the protonated heterocyclic ring by water molecules. The relative fixation given by various haloheterocyclic dyes on cotton and the stability of their dye–fibre bonds under acidic and alkaline conditions have been compared. Among the triazines, the highest relative fixation (70%) was shown by 2-arylamino and 2-heterylamino derivatives. Among the pyrimidines, the 5-cyano-2,4-dichloro derivatives gave 73% but the 5-chloro-2,4-difluoro derivatives achieved 84%. The latter system also yielded the most stable dye-fibre bonds in acidic as well as alkaline media [85]. A series of quantum mechanical calculations for cellulosic dyeings prepared with analogous dyes representing twelve different reactive systems containing either a pyrimidine or an s-triazine ring provided rate constants for hydrolysis under acidic and alkaline conditions [86]. When fixation to cellulose is achieved by a nucleophilic addition mechanism (Scheme

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CHEMISTRY OF REACTIVE DYES

7.25), an important factor is the reversibility of formation of the dye–fibre bond. Severe alkaline treatments are capable of rupturing this linkage to regenerate the unsaturated reactive system, which is able to react again either with cellulose or with water (Scheme 7.40). The presence of a less electronegative activating group than sulphone, as in the vinylsulphonamide dyes (7.41), increases the stability of the dye–fibre bond but reduces the reactivity of the dye. O [dye]

S

H2O CH2CH2OH

O [dye]

S

CH

CH2

NaOH O

O Hydrolysed dye

Active dye

O [dye]

S

CH2CH2

O

[cellulose]

O

Scheme 7.40

Dye-fibre bond

The kinetics of alkaline hydrolysis of a series of eleven vinylsulphone reactive dyes fixed on cellulose have been investigated at 50 °C and pH 11. Bimodal hydrolytic behaviour was observed under these conditions, the reaction rates being rapid at first but becoming slower as the concentration of fixed dye remaining gradually decreased. These results were attributed to differences in the degree of accessibility of the sites of reaction of the dyes within the fibre structure [87]. In an interesting comparison of dye–fibre bond stabilities over the pH range 3.5–10, dyeings of an aminochlorotriazine-sulphatoethylsulphone bifunctional dye were compared with those of two monofunctional analogues of almost identical structure. Under acidic conditions the bifunctional dyeing showed higher stability than the vinylsulphone dyeing, which in turn was more stable than the monofunctional aminochlorotriazine analogue. At alkaline pH, on the other hand, the aminochlorotriazine analogue and the bifunctional dyeing were virtually identical in stability, both being markedly more stable than the monofunctional vinylsulphone [74]. For similar reasons, bifunctional reactive dyeings of the aminofluorotriazine-sulphatoethylsulphone type show better allround stability to acidic and alkaline conditions than analogous monofunctional dyeings based on dichlorotriazine, aminochlorotriazine, aminofluorotriazine or vinylsulphone systems [77]. Reactive tendering refers to the strength loss of reactive-dyed cellulosic garments that can occur during commercial laundering. The effect is attributed to the inductive electronwithdrawing influence of the dye structure on the dye–fibre bond, which accelerates acid hydrolysis of β-1,4-glycosidic linkages in the cellulose chain [88]. Conversion of sodium sulphonate groups in the dye molecule to the free acid form by prolonged rinsing can also contribute to acid tendering. The hydroxide ion is not the most active nucleophile with which the dye–fibre bonds in reactive dyeings have to contend. Many commercial detergent formulations contain sodium

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perborate or percarbonate that may release peroxy species in washing treatments. The perhydroxide anion (HOO–) is an exceptionally powerful nucleophile capable of attacking certain types of haloheterocyclic reactive system to give unstable products. Studies of the reaction of alkaline peroxide solutions with dissolved reactive dyes have shown that reactive chloro substituents are readily displaced. Heterocyclic dyes containing other leaving groups, such as fluoro- or nicotinotriazines, and systems that fix by nucleophilic addition, e.g. vinylsulphone dyes, generally do not show perhydroxide formation. Surprisingly, however, dyes of the chlorodifluoropyrimidine type readily form a perhydroxide derivative that leads to cumulative damaging effects on dye–fibre bond stability. A comparison between seven different haloheterocyclic systems each attached to the same chromogen (phenylazo H acid) demonstrated several important conclusions [89]: (1) Only dye–fibre linkages that carry on the heterocyclic ring an electronegative substituent that is ortho (or para) to the dye–fibre bond show both peroxidation (Schemes 7.41 to 7.43) and bond breakage (Scheme 7.44). (2) Dye–fibre linkages that carry on the heterocyclic ring an electronegative substituent that is meta to the dye–fibre bond may show peroxidation (Scheme 7.45) but only slight breakage of bonds. (3) Dye–fibre linkages with no electronegative substituents on the heterocyclic ring show neither peroxidation nor bond breakage, e.g. aminochlorotriazine, aminofluorotriazine, bis(aminochlorotriazine) and bis(aminonicotinotriazine) dyes. An amine aftertreatment has been developed recently to protect haloheterocyclic dyes with substituents vulnerable to attack by perborates or other peroxy compounds in detergents. This product displaces such substituents, deactivating the dye–fibre bond system and rendering it resistant to peroxidic attack [90]. The relationship between dye–fibre bonding and light fastness was examined for ten sulphatoethylsulphone reactive dyes on cellulose and it was shown that the stronger the bonding between dye and substrate, the more stable was the dyeing when exposed to light

HO

[cellulose] +

OH

Cl _ N

[dye]

N Cl

NH

N

HO NaOH

[dye]

N

NH

O

[cellulose]

X

X

_ Trichloropyrimidine (X = Cl) or Cyanodichloropyrimidine (X = CN)

NaOH

HOO OH

N [dye]

Scheme 7.41

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NH

O OOH

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CHEMISTRY OF REACTIVE DYES

HO

[cellulose] +

N [dye]

OH

F HO–

N

NH

F

NaOH

N [dye]

N

NH

O

[cellulose]

O

[cellulose]

Cl

Cl Chlorodifluoropyrimidine

NaOH HOO–

OH N [dye]

NH OOH

Scheme 7.42

HO

N

[cellulose] +

[dye]

CO

N

Cl

N

Cl

HO NaOH

[dye]

CO

N

Cl

N

O

[cellulose]

_

Dichloroquinoxaline

NaOH HOO

[dye]

CO

N

OOH

N

O

[cellulose]

Scheme 7.43

OH N

OH free-radical

N

N

N +

[dye]

NH

O OOH

[cellulose]

mechanism

[dye]

NH

HO

[cellulose]

O O

Scheme 7.44

[91]. Light-fading studies on cotton dyeings of five trichloropyrimidine reactive dyes revealed that the rate of photodegradation of the cellulose was influenced by the presence of the dye. Compared with an undyed control, the blue dyeing was degraded more rapidly but the yellow, orange, red and green dyes tested exerted a protective effect [92]. Repeated

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REACTIVE DYES ON WOOL

HO

[cellulose] +

Cl

Cl Y

HO

N

NaOH [dye]

415

NH

N

Cl

Y [dye]

N N

NH

O

[cellulose]

_ NaOH HOO

Dichloropyrimidine (Y = CH) or Dichlorotriazine (Y = N)

OOH Y [dye]

Scheme 7.45

NH2 CH2 CO

CH2CH2CH2CH2NH2

O

CH2

Lysine

[cellulose]

NH CH N

CO

CO

N-terminal glycine

CH

N

H N

NH

NH CH

NH

N

CH2

SH

CO

Histidine

Cysteine

Figure 7.3 Nucleophilic sites in wool keratin for reaction with dyes

perborate oxidation of dyed cotton and viscose yarns demonstrated increased tendering of dyeings containing metal-complex reactive dyes. Viscose yarns were more significantly affected than cotton, possibly owing to the lower crystallinity, greater accessibility and higher carboxy content of viscose fibres [93]. 7.7 REACTIVE DYES ON WOOL Wool keratin contains several different types of nucleophilic site for reaction with dyes [9496]. By far the most important ones in intact wool are the amino groups of the N-terminal aminoacid residues and the sidechain groups of lysine and histidine (Figure 7.3). These are the same groups that provide positively charged sites under acidic conditions for absorption and electrostatic bonding with reactive or unreactive anionic dyes. A reactive dye with a labile halogen atom (X) will undergo a nucleophilic substitution reaction with one of these amino sites in the uncharged state (Scheme 7.46). The thiol groups of cysteine residues formed by the hydrolysis of cystine disulphide groups in wool provide further sites for reaction with reactive dyes (Scheme 7.47). The hydrolytic breakdown of the disulphide bonds of cystine liberates some hydrogen sulphide and this can gradually deactivate a vinylsulphone dye by formation of the

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unreactive thioether derivative (Scheme 7.48) [97]. In a similar way, hydrogen sulphide will displace hydrogen bromide from an α-bromoacrylamide reactive system (Scheme 7.49). The thiirane intermediate initially formed is unstable, decomposing to the less reactive unsubstituted acrylamide with the precipitation of sulphur [98]. It is likely that haloheterocyclic reactive dyes are also readily deactivated by hydrogen sulphide, the halo substituent being replaced by a thiol group. The ability of wool reactive dyes to scavenge hydrogen sulphide explains their fibre protective effect when dyeing wool in medium to full depths; damage is related to the extent of the thiol-disulphide interchange or ‘setting’ reaction, which is promoted by cystine degradation in hot aqueous media [99]. X

[dye]

SO3

H

_

+

NH3

NH2

_

CH

HO

R

CH

X R

[dye]

+ NH3

CO

CO

SO3Na

NH

[dye]

CH

R

SO3Na

+ NH4X

CO

N-terminal amino acid

Scheme 7.46

NH

NH CH

CH2

SH +

X

[dye]

CH

SO3Na NH3

CO

CH2

S

[dye]

SO3Na

CO + NH4X

Scheme 7.47

[dye] SO2

CH

CH2 + H2S

[dye]

SO2CH2CH2SH

Scheme 7.48

[dye]

NHCO

C

CH2 + H2S

[dye]

NHCO

CH

CH2 + HBr S

Br

[dye]

NHCO

CH

CH2

Scheme 7.49

Traditional wool dyeing methods have often involved a rapid unlevel initial strike at low temperature, followed by a prolonged migration treatment at the boil to attain optimum levelness. To fit in with these requirements, ranges of reactive dyes developed for wool needed to react slowly with the fibre and this implied reactive systems with low intrinsic reactivity. One such group that was found to react too slowly for exploitation on cellulosic

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fibres was selected for the Procilan (ICI) dyes, the first full range of reactive dyes to be designed for application to wool. These were unsymmetrical 1:2 chromium complexes of Mr 800–900, derived from different monoazo ligands with one sulpho substituent and one acrylamido reactive group per dye molecule. They were intended for application to wool at the boil, at least an hour being necessary for slow addition of the nucleophilic groups across the activated double bonds of the acrylamide dyes (Scheme 7.50). Like their unreactive analogues (section 3.2.2) these chromium-complex dyes exhibited high neutral-dyeing affinity, but this was a problem from the viewpoint of removal of the unfixed hydroxypropionamide derivative at the washing-off stage. Dullness of hue was another limitation and the range was later extended by adding brighter unmetallised monoazo chromogens of similar Mr with two sulpho groups and an aryloxychlorotriazine reactive system. The technical performance achieved did not justify the relatively high cost of this hybrid range, which was eventually withdrawn. An important contributory factor was the emergence in 1966 of the most successful range of reactive dyes designed for wool [100,101]. These are the Lanasol (Ciba) αbromoacrylamide dyes (7.115), usually marketed in the form of their α,βdibromopropionamide precursors (7.114). In dilute alkaline solution this group readily releases hydrogen bromide to yield the active form (Scheme 7.51). The introduction of acrylamide, α-bromoacrylamide or α,β-dibromopropionamide reactive groups into dye molecules is readily achieved by direct acylation of the dye base [dye]–NH 2 with the appropriate acid chloride Cl–CO–R, where R is –CH=CH2, –CBr=CH2 or –CHBr–CH2Br respectively [36]. A bromine atom attached to a vinyl group is deactivated towards nucleophilic substitution. The sp2 hybridisation of the carbon causes the C–Br bond to shorten and become stronger, whilst the delocalisation of the electrons by resonance causes the activation energy for displacement of the bromine to increase [96]. It is now widely accepted that initial reaction with wool involves addition at the double bond [102]. Ring closure to form a three-membered aziridino ring then occurs by intramolecular nucleophilic substitution and this group finally hydrolyses to a β-substituted α-hydroxypropionamide dye–fibre bond (Scheme 7.52). Recent laser Raman spectroscopic studies have provided support for this mechanism and suggest that cysteine thiol groups are particularly important

N-terminal NaO3S

[dye]

NHCOCH

CH2

NaO3S

amino acid

[dye]

NHCOCH 2CH2

NH CH R

H2O

CO NaO3S

[dye]

[wool]

NHCOCH 2CH2OH

Scheme 7.50

[dye]

NHCO

CH Br

7.114

CH2Br

[dye]

NHCO

C

NaOH 7.115

CH2 + NaBr + H 2O

Br

Scheme 7.51

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CHEMISTRY OF REACTIVE DYES

[dye] NHCO 7.115

C

CH2

[dye]

NHCO

Br +

NH2

R

CH

CH

CH2

Br

NH

R CO

CH

CO

[wool]

[wool]

N-terminal amino acid [dye]

NHCO

_

CH

Br NH3

H2O [dye]

NHCO

CH

CH2

OH

NH

R

CH

[dye]

NHCO

R

CH2 + NH CH

CH

CO

CH2 + NH4Br

N CO

[wool]

R

[wool]

CH

CO

[wool]

Scheme 7.52

sites for reaction, especially with damaged wool [103]. Further detailed studies using model compounds have confirmed that the reaction with primary amino groups resulting in the formation of aziridine rings is important. Reactions of the α-bromoacrylamide grouping with the imidazole nitrogen of N-acetylhistidine and the thiol group of N-acetylcysteine were also observed [104]. Although reactive dyes account for only about 5% of total dye usage on wool, no other class can offer such brilliant hues of high fastness to light and wet treatments. Chrome dyes are mainly used for economical navy blue and black dyeings, where reactive dyes in general are particularly costly by comparison. In 1992 Ciba supplemented the Lanasol range with two new dyes, Navy B and Black R, that occupy the shade areas corresponding to chrome navy and black brands. More recently, Lanasol Black PV has been introduced as strong competition for established chrome blacks, even as regards price. It has been designed to give tinctorial strength, shade, metamerism and light fastness close to those of the afterchromed complex from Eriochrome Black PV (CI Mordant Black 9). An amphoteric levelling agent and ammonia aftertreatment are necessary and the fastness to severe wet tests such as potting and cross-dyeing is not quite as high as chrome dyeings [105]. Few of the haloheterocyclic reactive systems that became established for the dyeing of cellulosic fibres have transferred successfully to wool dyeing conditions. The one exception is the 5-chloro-2,4-difluoropyrimidine system (section 7.3.5). This has been utilised in the Drimalan (Clariant) and Verofix (DyStar) ranges of reactive dyes for wool [106]. These are based on conventional reactive dye chromogens (section 7.5) with two or three sulpho groups per molecule and an M r of 600–800. The dye base usually has an alkylamino substituent (section 7.3.3) for reaction with chlorotrifluoropyrimidine to yield the reactive dye. This is a mixture of two isomers (7.19 and 7.20) and thus gives rise to a variety of different dye–fibre bond structures when fixation to the various nucleophilic sites in wool keratin (Figure 7.3) takes place. The high fixation ratio shown by these dyes has been

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attributed to the continuing activity of partially hydrolysed dye, since the reactivity of the second fluorine atom is only slightly decreased by reaction of the first with an amino group in wool [107]. A novel range of heterobifunctional dyes has been introduced recently under the brand name Realan (DyStar). These products contain a 5-chloro-2,4-difluoropyrimidine system and a sulphatoethylsulphone grouping in the same molecule. They are recommended for the dyeing of wool at a mildly acidic pH and of silk under mildly alkaline conditions [108]. Although the first two reactive dyes intended for fixation on wool contained the vinylsulphone reactive system and were marketed by Hoechst as long ago as 1952, it proved difficult to develop a fully compatible coherent range along these lines. Some of the early sulphatoethylsulphone dyes, including the bifunctional Hostalan Black SB (7.36) and the attractive Remalan Brilliant Blue R (7.37), had become highly successful as Remazol equivalents for cellulosic dyeing (section 7.3.8). Various secondary alkylamines, such as diethylamine or N-methyltaurine, were exploited as leaving groups in other Hostalan dyes (Scheme 7.53). Under mildly acidic conditions at the boil gradual elimination of the amine took place, allowing the dye to level more effectively before the vinylsulphone form could react with the nucleophilic sites in wool [109]. This assortment of precursor types and the usual need to vary the chromogen considerably (section 7.5) gave rise to marked differences in Mr (600–900) even though most members of the range were disulphonated. This rather incompatible range of products was eventually discontinued in the 1980s. CH2CH3 [dye]

SO2CH2CH2

CH3CH2NHCH2CH3

N CH2CH3

+ [dye]

Precursor forms

SO2

CH

Diethylamine

CH2

Vinylsulphone form CH3 [dye]

SO2CH2CH2

N CH2CH2SO3Na

+ CH3NHCH2CH2SO3Na

Scheme 7.53

N-methyltaurine

It is widely accepted that reactive dyes protect wool from hydrolytic damage during hot aqueous dyeing processes at pH 3–7. In the dyeing of wool/cotton blends, the wool component is significantly protected if it is first dyed with appropriate wool-reactive dyes before exposure to the strongly alkaline conditions necessary for satisfactory fixation of reactive dyes on cotton [110]. The successful dyeing of severely damaged carbonised wool with the bifunctional sulphatoethylsulphone dye CI Reactive Black 5 (7.36) was attributed to the ability of this product to crosslink the wool keratin chains and thus to exert a protective effect [111]. In a comparison between various reactive dyes for wool and several commercially available colourless fibre-protecting agents, it was shown that reactive dyes in medium and full depths are significantly more effective. Reaction readily occurs between typical reactive dyes and the cysteine thiol groups (Scheme 7.47) released when the cystine disulphide bonds are hydrolysed under dyeing conditions. Such reactions inhibit thiol-disulphide

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CHEMISTRY OF REACTIVE DYES

interchange reactions and thus significantly interfere with the level of set produced in a boiling dyebath. Reactive dyes also react preferentially with non-keratinous proteins in the intercellular material and the endocuticle, thus minimising their tendency to hydrolyse and partially dissolve in the hot aqueous dyebath [112]. The presence of sulphur-containing aminoacids in keratinous fibres seems to have special significance for the clothes moth, which attacks wool and other animal hairs but not silk or other fibre types. An effective method of mothproofing is to reduce the disulphide bonds to thiol groups using thioglycolic acid at pH 7 and 50 °C, followed by an alkylation reaction under similar conditions with an alkylene dihalide (Scheme 7.54). More recently, the possibility of using a bifunctional reactive dye instead of the alkylene dihalide has been explored [113]. The resistance of wool flannel to damage by clothes moth larvae was markedly improved, but the treatment caused adverse effects on physical properties similar to those found with the alkylene bis-thioether crosslinks in wool. NH

NH CH

CH2

S

S

CH2

HSCH2COOH

CH

CH Thioglycolic acid

CO

CO

NH

NH CH2

SH

+

HS

CH

CH2

CO

CO

Cystine

Cysteine

X

[dye]

X

BrCH2CH2Br

NH CH

CH2

S

CH2CH2

S

CH2

CO

NH

NH

CH

CH

CO

CO

NH CH2

S

[dye]

S

CH2

CH CO

Scheme 7.54

7.8 REACTIVE DYES ON SILK Silk fibroin contains no cystine and the content of lysine and histidine is also low (about 1% in total), but it does contain tyrosine phenolic (13%) and serine alcoholic (16%) sidechains. Since glycine accounts for 44% of the total aminoacid content, an N-terminal glycine residue is reasonably representative of most of the primary amino dyeing sites in silk fibres. Amino acid analysis of hydrolysed reactive-dyed silk indicates that the reaction between fibroin and reactive dyes takes place mainly at the ε-amino group of lysine, the imino group of histidine and the N-terminal amino group of the peptide chain. In an alkaline medium, the hydroxy groups of tyrosine and serine also react [114]. Commercial ranges of reactive dyes have not been marketed specifically for silk dyeing, so the more important types of reactive system developed successfully for wool or cellulosic fibres have been evaluated on silk. Apparel made from silk traditionally required dry

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cleaning to avoid colour loss, so the high wet fastness offered by reactive dyes has been advantageous. Vinylsulphone dyes in particular provide good dischargeable grounds for printing styles and show excellent fastness to perspiration [115]. Theoretically, all reactive dyes can be used for silk dyeing. However, to achieve the best quality of dyed silk, reactive dyes have to satisfy the following requirements [114]: (1) Brilliance of hue: this is especially important on mulberry silk. Many dyeings on tussah silk are much duller and the dyed silk shows a lower colour yield because of inferior exhaustion. (2) High reactivity: silk is damaged in an alkaline medium at high temperature, so reactive dyeing should be carried out in an acidic or neutral dyebath. (3) Good storage stability: the consumption of dyes for the batchwise dyeing of silk is small, so the dyes should be highly stable to storage. Silk can be readily dyed with conventional high-reactivity dyes of the dichlorotriazine, dichloroquinoxaline or difluoropyrimidine classes. Exhaust dyeing at 60–70 °C and pH 5–6 gives satisfactory results, especially if a mildly alkaline aftertreatment is given to enhance fixation. Dichlorotriazine dyes can also be applied by pad–batch dyeing with bicarbonate and a batching time of 4–6 hours. The relatively low reactivity of aminochlorotriazine dyes, however, results in moderate to poor build-up on silk. Tertiary amine catalysts such as DABCO (7.66) can be used to accelerate the dye–fibre reaction and increase the fixation substantially [116], but it is difficult to achieve satisfactory compatibility in mixture dyeings by this method (section 7.4.2). In contrast to cellulosic dyeing with reactive dyes, the fibroin–dye bonds are remarkably stable in aqueous media of pH 4 to 10 [117]. Since there exists only a negligible amount of bond hydrolysis even at high temperature and in a medium of pH 2, the cleavage of the fibroin–dye bond is not a problem in reactive-dyed silk. The stability of these bonds when dyeing with difluoropyrimidine dyes is the highest in both acidic and basic media [118]. Chloroacetyl reactive dyes have occasionally been used for the dyeing of wool but they normally require alkaline fixation for acceptable colour yields. Silk can be damaged by alkaline conditions but research has indicated that certain haloacetyl dyes will give satisfactory performance on silk under mild conditions. Table 7.6 presents selected results from a comparison of mono- and bifunctional dyes containing bromo- or chloroacetyl reactive groups [119]. The exhaustion tended to decrease as the dyebath pH or temperature was increased but the fixation values showed trends in the opposite direction. The two symmetrical bifunctional structures yielded consistently higher exhaustion and fixation than their unsymmetrical monofunctional analogues. An additional bromine atom boosted the exhaustion by about 16%. The most striking difference between the bromoacetyl and chloroacetyl reactive groups was the outstandingly higher reactivity of the bromoacetyl system under mild conditions (pH 7 and 75 °C). Sulphatoethylsulphone dyes have proved highly suitable for silk, yielding brilliant hues of high wet fastness by application at 80 °C and pH 7–8 in the presence of Glauber’s salt, usually followed by an alkaline fixation treatment to ensure optimum fixation [115,120,121]. A new approach to this dyeing system has been explored recently [122], in which the sulphatoethylsulphone dyes were first activated fully by a pretreatment at pH 8 for 5 minutes at the boil (Scheme 7.23). The free vinylsulphone dyes were then applied to silk at 80 °C and pH 4–4.5 in the absence of salt. Under these conditions this highly reactive system

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Table 7.6 Exhaustion and fixation of haloacetyl-substituted disazo dyes on silk [119] Exhaustion (%)

Substituents in structure 7.116

Fixation (%)

X

Y

75 °C pH 7

90 °C pH 7

90 °C pH 11

75 °C pH 7

90 °C pH 7

90 °C pH 11

Br Cl Br Cl

Br Cl H H

94 86 78 83

93 86 76 80

86 94 70 74

94 39 79 15

94 89 76 38

98 98 91 90

X

H2C

O

O

NH

N

C

C O

H

H N

NaO3 S

O

SO2CH

SO3Na N

SO3Na

[dye]

NaO3S

7.116

CH2

NaO3S

[dye]

SO2CH2CH2 NH CH

N-terminal H2O

NaO3S

[dye]

Y

O

N

N

NaO3S

CH2

R

CO

amino acid

[silk]

SO2CH2CH2OH

Scheme 7.55

readily fixes to amino groups in silk by a nucleophilic addition mechanism (Scheme 7.55). The nonionised vinylsulphone groups ensure much higher exhaustion than the sulphatoethylsulphone precursor form, especially in the case of the bifunctional CI Reactive Black 5 (7.36). Avoidance of electrolyte additions is highly beneficial from the environmental viewpoint [122]. The Lanasol (Ciba) dyes marketed for the dyeing of wool, containing α-bromoacrylamide or α,β-dibromopropionamide groups, can also be used for dyeing silk to achieve high fixation in an alkaline medium [118,123]. The percentage fixation of the homobifunctional bis(dibromopropionamide) dye 7.117 is exceptionally high. This dye is almost completely fixed on the silk fibre, whereas the monofunctional analogue 7.118 shows much lower fixation [118]. At pH 9, the dibromopropionamide groups in dye 7.117 rapidly eliminate hydrogen bromide to yield the reactive α-bromoacrylamide form. The high fixation levels achieved on silk by various homobifunctional reactive dyes containing two identical reactive systems per molecule, as well as the increasing success of

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O

423

NH2 SO3Na

O

NHCO

HN O

CH

CH2Br

Br

C NaO3S

N H NHCO 7.117

O

CH

CH2Br

Br

NH2 SO3Na

O

HN O C NaO3S

N H

C

CH2

Br

7.118

heterobifunctional dyes on cellulosic fibres, have prompted a detailed study of CI Reactive Red 194 (7.76) on silk. This monoazo dye contains a sulphatoethylsulphone precursor grouping linked to the H acid coupler through a chlorotriazine bridging group of lower reactivity. Maximum exhaustion and fixation of this bifunctional red dye on silk was achieved by dyeing at 90 °C and a neutral pH in the presence of 60 g/l electrolyte. The isoelectric point of silk fibroin is found at pH 3–4 and the salt addition is necessary to suppress the negative charge on the surface of the fibre. For optimum fixation, however, the pH must be high enough for the nucleophilic amino groups in the fibre to be present almost entirely in their unprotonated form and for the sulphatoethylsulphone precursor groups to yield their vinylsulphone active form. Dyeing under alkaline conditions results in lower exhaustion and fixation, not only because of the high concentration of ionised carboxylate groups in the fibre but also because of increased alkaline hydrolysis of the reactive groups in the dye [124]. All reactive dyes inhibit the dissolution of silk fibroin in appropriate solvents but the effect is much more pronounced with bifunctional dyes because these are capable of forming crosslinks between the polymer chains. Solubility tests are particularly sensitive to the formation of crosslinks and can give an estimate of the degree of crosslinking [125–127]. The solvent selected should be effective in disrupting hydrogen bonding between polymer segments without bringing about scission of covalent bonds. The solubility of silk in a solution of calcium chloride in aqueous ethanol (molar ratio 1:2:8) was determined for dyeings of one monofunctional and three bifunctional reactive dyes at various applied depths. A few typical results are given in Table 7.7. Even at a concentration of 0.05 mol/kg

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Table 7.7 Solubility of silk dyed with various reactive dyes [124]

CI Reactive

Structure

Applied depth (% o.w.f.)

Dye on fibre (mol/kg)

Solubility (%)

Orange 16 Black 5 Red 194 Red 120

7.119 7.36 7.76 7.48; X = Cl

1 2 2 6

0.010 0.012 0.013 0.010

99 60 51 41

NHCOCH3 H NaO3SO

CH2CH2SO2

O

N N

7.119

NaO3S

C.I. Reactive Orange 16

of CI Reactive Orange 16, the solubility of the dyed silk was still 83%, indicating how much greater is the effect of bifunctional crosslinking on this property. The formation of crosslinks in silk fibroin increases the tenacity and resistance to deformation of the fibres, as reflected in the initial modulus and the yield point. This protective effect conferred by fixation of the bifunctional dye CI Reactive Red 194 was not shown by the monofunctional Orange 16, which is unable to form crosslinks. The loss in tenacity of undyed silk that is observed on treatment at 90 °C and pH 7 for 2 hours is attributable to lowering of the degree of polymerisation (DP) by hydrolysis of peptide bonds. The crosslinking action of bifunctional dyes tends to compensate for this loss in DP and provides an intermolecular network that helps to maintain the physical integrity of the fibre structure [124]. 7.9 REACTIVE DYES ON NYLON In spite of their unique status as the only commercial range of reactive dyes designed specifically for the dyeing of nylon, the Procinyl (ICI) disperse reactive dyes were developed and maintained on a shoestring budget compared with conventional reactive dyes that achieved greater success on other fibres. The range never contained more than six members, yet three different reactive systems were represented. The only blue was a 1,4diaminoanthraquinone containing two 3-chloro-2-hydroxypropyl groups, precursors of the glycidyl reactive system that reacted only slowly and inefficiently with N-terminal amino groups in nylon (Scheme 7.56). The only yellow chromogen was a 4-aminoazobenzene derivative bearing an aminochlorotriazine grouping of only low to moderate reactivity. The remaining dyes of moderate light fastness covering the orange to rubine sector were each based on a simple monoazo chromogen with a 2-chloroethylsulphamoyl group, precursor of the relatively more active aziridinyl reactive system (Scheme 7.57). The degree of fixation of these dyes under conventional dyeing conditions at 80–90 °C was rather low. Migration and coverage of dye-affinity variations was good but the wet

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[dye]

NHCH2CHCH2Cl

[dye]

NHCH2CH

425

CH2

NaOH

OH

O Glycidylamino

Chlorohydroxypropylamino

N-terminal amino group

[dye]

NHCH2CHCH2NH

[nylon]

OH Scheme 7.56

[dye]

Dye-fibre bond

SO2NHCH2CH2Cl

Chloroethylsulphamoyl

[dye] NaOH

SO2

N

CH2 CH2

Aziridinylsulphone

N-terminal

[dye]

amino group

SO2NHCH2CH2NH

[nylon]

Dye-fibre bond

Scheme 7.57

fastness was not much better than that of high-energy disperse dyes. Only by alkaline aftertreatment or high-temperature dyeing at 120 °C could fixation be improved, but with some risk of unlevel dyeing if a high pH or temperature was reached too quickly. The dyeing of nylon/cellulosic blends with Procinyl dyes and anionic reactive dyes was more successful, because the alkaline fixation stage necessary for the latter on the cellulosic component also ensured good fixation of the Procinyl dyes on nylon. In general, however, the technical performance of this small range of dyes seldom justified their cost and they were eventually withdrawn. Anionic reactive dyes designed for cellulosic fibres or wool are able to give bright shades of high wet fastness on nylon but their drawbacks are clearly evident. The degree of fixation is limited by the number and accessibility of primary amine end groups in nylon. Below this limit (roughly 1% o.w.f. on normal nylon, higher on deep-dye variants) such dyeings show excellent wet fastness but at greater depths the fastness deteriorates as the proportion of unfixed dye present increases. Most anionic reactive dyes contain two or more sulpho groups per molecule and this can result in blocking effects in mixture recipes containing dyes that vary markedly in affinity for nylon. When high-reactivity dyes are applied in depths up to the limit of available end groups, these dyes show very poor migration properties. Catalytic fading can be a problem in certain combination shades. It has been pointed out [99] that, in theory at least, a monofunctional vinylsulphone dye is capable of reacting twice with a primary amino N-terminal group in nylon (Scheme 7.58). In practice, however, fixation limits for these dyes are not significantly higher than for highreactivity dyes of the haloheterocyclic types. When one multisulphonated reactive dye molecule has reacted, steric hindrance and strong mutual electrostatic repulsion effects inhibit access of a second reactive molecule to the imino group in the dye-fibre linking unit.

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CHEMISTRY OF REACTIVE DYES

This problem has been confirmed recently in a study of the mechanism of covalent reaction between nylon 6.6 and the sulphatoethylsulphone dye CI Reactive Blue 19 (7.37). Acid hydrolysis of the dyed fibre and HPLC analysis of the hydrolysate yielded the 6aminohexylaminoethylsulphonyl derivative of Blue 19. Even when the dyeing procedure was optimised to achieve maximal exhaustion and fixation to the fibre [128], only about 30% of the N-terminal amino groups in the nylon 6.6 were accessible because of mutual blocking effects between these bulky anionic dye molecules. [dye]

SO2

[dye]

SO2

[dye]

SO2

[dye]

CH

CH2 + H2N

CH2CH2

CH

NH

[dye]

[nylon]

CH2

SO2CH2CH2 N

Scheme 7.58

[nylon]

[nylon]

SO2CH2CH2

7.10 NOVEL REACTIVE DYEING PROCESSES Precedents show that attempts to forecast the probability of attainment of truly original techniques of coloration are fraught with difficulty. No dyeing expert of the early 1950s could have predicted the impact that reactive dyes would make within that decade. With a new millennium offering continuing challenges, however, the acknowledged world experts in this field are not deterred by this from speculating how this game will go next [129,130]. What is beyond doubt is that there are still important research targets remaining unfulfilled and that reactive systems still offer sufficient versatility to play a major part in these developments. There has been a noteworthy revival of interest recently in adapting the concept of crosslinking reactants, so overwhelmingly successful in chemical finishing processes, to the more demanding task of attaching coloured molecules to cellulose or other fibres containing nucleophilic groups. Early attempts to achieve this technology transfer failed because they relied too closely on the chemistry of the existing finishing processes. Thus in the Procion Resin process of the early 1960s, a reactive dye of the chlorotriazine type was condensed with an amino sugar, such as N-methy1glucosamine. The carbohydrate residue on the resulting dye molecule participated in crosslinking of the cellulosic fibre using a suitable Nmethylol reactant [131]. The introduction of Calcobond dyes a few years later by American Cyanamid exploited a similar principle but incorporated the N-methylol groups into the dye molecule itself [132]. The labile chloro substituents in dichlorotriazine dyes were converted to amino groups by substitution with ammonia and the resulting melamine residue made cellulose-reactive again by reaction with formaldehyde (Scheme 7.59). A typical member of this range was CI Reactive Red 92 (7.120). A characteristic problem of the Procion Resin process and of the

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427

Calcobond dyes was that the colour fastness to light was adversely affected. The fastness to acid treatment was no better than that of the resin finish, i.e. the dyeings could be stripped by boiling at a pH below 5. NH2

Cl N [dye]

NH

N

2 NH3

[dye]

N

NH

N N

N

NH2

Cl 2 HCHO

NHCH2OH N [dye]

NH

N N

Scheme 7.59

NHCH2OH

HOCH2NH N NHCH2OH

N N HN

SO3Na H

O SO3Na

N N

7.120 NaO3S

CI Reactive Red 92

The most interesting departure from conventional reactive dyeing methods of the 1960s was the Basazol (BASF) approach [133]. Selected unmetallised chromogens and 1:2 chromium complexes containing nucleophilic groups such as primary or secondary amino, arylsulphonamide or unsubstituted pyrazolone residues were used (7.121 and 7.122 are typical examples). These were applied to cellulosic fabrics by padding or printing, together with the non-substantive crosslinking agent 1,3,5-tris(acryloyl)hexahydro-s-triazine (7.123). Above pH 10.5 the sulphonamide group is sufficiently nucleophilic to undergo Michael addition with the acryloyl groups of Fixing Agent P. This trifunctional product is sufficiently reactive and versatile to form dye–fibre links, as well as dye dimers or trimers and crosslinks between cellulose molecules [134]. Although Basazol dyes were not hydrolysable, dimerisation and cellulose crosslinking do represent competitive side reactions (Scheme 7.60). Some acryloyl groups will be hydrolysed (Scheme 7.61). The system was not adaptable to exhaust dyeing and the Basazol products were later withdrawn from the market.

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CHEMISTRY OF REACTIVE DYES

CH3 OCH3

N

O

H

N HN

N

N

OH

N

SO3Na

N O O

N

H2NO2S

Cr

HO

O O

7.121 N

NaO3S

NH N N H3C

7.122 O [dye]

SO2NH2

+

H2 C

HC

O

C

C N

7.123

CH2

+ HO

CH

CH2

O

NaOH

NaOH

SO2NHCH2CH2 CO

O

COCH2CH2

N

[cellulose]

N N

Dye dimer

COCH

CH2

H2 C

HC

CO

N

Fibre crosslink

N [dye]

[cellulose]

N C

[dye]

CH

N

N

SO2NHCH2CH2 CO

O

COCH2CH2

[cellulose]

+

+ COCH

CH2

N N [dye]

N COCH2CH2

SO2NHCH2CH2CO

Scheme 7.60

O

[cellulose]

C

CH2CH2OH

Dye-fibre bond

H2O N

C

CH

N

CH2 NaOH

O

O

Scheme 7.61

CH3CH2 N CH3CH2

Cl–

CH2 CH OH CH2

7.124

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NaO3S

HN

N N

7.125

Cl N

Cl

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NOVEL REACTIVE DYEING PROCESSES

429

A relatively uncontrolled attempt to enhance the colour yield and fastness to washing of selected direct dyes involved addition to the dyebath of a colourless reactant that was intended to form covalent attachments between these dyes and hydroxy groups in cellulose [135]. Most of the dyes selected contained at least two primary amino groups in aminonaphthol residues. The reactants evaluated were cyanuric chloride (7.3), N,N-diethyl3-hydroxyazetidinium chloride (7.124) and 2,4-dichloro-6-(4′-sulphoanilino)-s-triazine (7.125), all of which will indeed react readily with primary arylamines. Some of the dyes selected, however, contained only diphenylurea or salicylic acid residues and these are most unlikely to react efficiently under dyeing conditions. In a later development [136,137], cyanuric chloride was proposed as an aftertreatment for cotton already dyed with direct dyes containing amino groups. This approach appears even less likely to succeed than in situ addition to the dyebath. Serious hazards are associated with the handling of cyanuric chloride under these conditions. This highly reactive compound is a primary skin irritant and is known to cause severe allergic reactions in certain individuals. Dye–agent reaction will be inefficient because of hydrolytic deactivation. Uptake of cyanuric chloride (or its hydrolysis products) by the dyed cotton will be poor. A much more controlled approach to the concept involves incorporating the reactive function into the substrate and carrying out the dyeing with a non-hydrolysable dye containing a nucleophilic group [138]. Various nucleophilic groups come into consideration, but the aliphatic amino group gives the best results in practice as it is more reactive than arylamino, phenolic or aliphatic hydroxy groups. In terms of nucleophilicity, the aliphatic thiol group is even more reactive, but thiol compounds are often malodorous and tend to oxidise to their disulphide derivatives [52]. An aminoalkyl derivative of the monoazo H acid dye CI Reactive Red 58 was prepared by reaction with ethylenediamine (Scheme 7.62). Acrylamidomethylated cellulose prepared by the pad–bake condensation of Nmethylolacrylamide with cotton (Scheme 7.63) reacts readily with nucleophilic dyes of this kind. High fixation is achieved either by exhaustion at pH 10.5 in 80 g/l salt solution or by pad–batch treatment at the same pH for 24 hours (Scheme 7.64). These aminoalkyl dyes show zwitterionic characteristics below pH 8 and this lowers the nucleophilicity of the primary amino group (Scheme 7.65). Practically no dye is removed on alkaline soaping of these dyeings at the boil, compared with ca. 30% desorption of unfixed dye after application of the dichlorotriazine analogue CI Reactive Red 58 to untreated cotton. The dyeing of activated cotton with the aminoalkylated derivative thus offers significant savings, since only a minimal rinse would be adequate and soaping at the boil could be omitted [52]. A simple yet rather drastic method of introducing reactive sites into the cellulose macromolecule is to use periodate as a relatively specific oxidant. This ring-opening reaction converts the vic-diol grouping in the 2,3-positions of some of the glycoside rings into a dialdehyde (Scheme 7.66). The aldehyde groups can provide sites for fixation of dye molecules containing nucleophilic aminoalkyl groups, such as a symmetrical bis(aminoalkyl) derivative of a bis(aminochlorotriazine) triphenodioxazine blue reactive dye [139]. The azomethine groups formed are sensitive to hydrolysis unless reduced to the secondary amine. The highly reactive compound disodium 4,4′-bis(dichlorotriazinylamino) stilbene-2,2′disulphonate (7.126; known as DAST or T-DAS), an important intermediate in the manufacture of stilbene-type fluorescent brightening agents (section 11.6.1), has been evaluated recently as a crosslinking agent for the fixation of dyes with nucleophilic groups. Compound 7.126 was applied to cotton simultaneously with the hydrolysed form of CI

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CHEMISTRY OF REACTIVE DYES

NaO3S NaO3S [dye]

HN

NaO3S

N N

Cl + H2NCH2CH2NH2 N

NH

Ar NaOH

NaO3S NaO3S [dye]

N

HN

NaO3S

N

NHCH2CH2NH2 N + NaCl + H2O

NH

Scheme 7.62

[cellulose]

OH + HOCH2NHCOCH

Ar

ZnCl2 CH2

[cellulose]

O

150°C

CH2NHCOCH

CH2

Scheme 7.63

NaO3S NaO3S [dye]

HN

NaO3S

NHCH2CH2NH2

N N

+ H2C

HC

CONHCH2

O

[cellulose]

N NH

Ar

NaO3S NaO3S [dye] NaO3S

Scheme 7.64

HN

N N

NHCH2CH2NHCH2CH2CONHCH2

O

[cellulose]

N NH

Ar

Reactive Black 5 (7.127) by an exhaust process and optimal conditions for fixation were determined [140]. The substantivity of this agent for cellulose is not ideal, as it requires a high salt concentration to ensure a fixation level of 80%. To achieve 85% fixation of dye 7.127 by this method, an addition of 50 g/l sodium sulphate was necessary [29]. The novel cationic reactant 2,4-dichloro-6-(2′-pyridinoethylamino)-s-triazine (7.128) was evaluated as a means of activating cellulose by pad–batch application at pH 8.5 for 24 hours. This agent contains two reactive chlorine atoms but under these mild conditions only

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NaO3S NaO3S [dye]

HN

NaO3S

431

_ + NHCH2CH2NH3X

N N

N NH

Ar

NaO3S NaO3S [dye]

HN

NaO3S

NHCH2CH2NH2

N N

N NH

Scheme 7.65

Ar

CH2OH HC

CH2OH

O

HC

+ HX

CH

periodate

HC

O

HC

CH

HC

CH

HC

HO

OH

O

CH O 2 H2NCH2CH2[dye]

CH2OH HC

CH2OH

O CH

HC H2C

CH2

HN

NH

[dye]H2CH2C

4H

HC

CH

HC N

CH2CH2[dye]

O

HC

[dye]H2CH2C

CH N CH2CH2[dye]

Scheme 7.66

one of them reacts with a hydroxy group in cellulose (Scheme 7.67). The electron-donating cellulosyl substituent deactivates the triazine ring, ensuring that the second chlorine atom remains unreacted. Aliphatic amino groups are more powerful nucleophiles than cellulose hydroxy groups, so that an efficient substitution reaction is still possible during subsequent dyeing with an aminoalkylated dye (Scheme 7.68). After a cold water wash, dyeings were carried out on the modified cotton fabric without salt at pH 9, using the monofunctional aminoalkylated derivative of CI Reactive Red 58 and a bifunctional analogue (7.129) synthesised by reacting one mole of CI Reactive Red 120 (7.48; X = Cl) with two moles of ethylenediamine. Good substantivity and fixation were achieved on cotton treated with 5.4% o.w.f. of the cationic reactant, particularly using the bifunctional nucleophilic dye [141]. Nucleophilic aminoalkyl dyes of the types described above can be fixed on cotton that has

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CHEMISTRY OF REACTIVE DYES

SO3Na

Cl

Cl

N NH

N

N

HC

N Cl

NH

CH

N N

7.126

Cl

NaO3S

SO2CH2CH2OH

H2N H HOCH2CH2SO2

N

N

O SO3Na

N N 7.127

NaO3S

+ N CH2CH2HN _ Cl

N

Cl

N

N

7.128 Cl

pH 8.5

+ CH2CH2HN N _ Cl

Scheme 7.67

cellulose

N N

O

[cellulose]

N Cl

been pretreated with the trifunctional anionic reactant 2-chloro-4,6-bis(4′-sulphatoethylsulphonylanilino)-s-triazine (7.130), which has been given the codename XLC [142]. After conversion to the active bis-vinylsulphone form (XLC–VS), crosslinking of the cellulose is possible under conditions that do not excessively hydrolyse the chlorotriazine function. As in the case of the cationic pretreating agent 7.128, this provides a suitable site for attachment of the aminoalkyl dye (Scheme 7.69).

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NOVEL REACTIVE DYEING PROCESSES

H2NCH2CH2HN

NHCH2CH2NH2 NH

HN

HN

SO3Na

NH

NaO3S O

O

H

433

SO3Na

N

H

NaO3S

N

N

N

NaO3S

SO3Na

7.129

+ N

CH2CH2HN

N

[cellulose]

O

N

N

+

Cl

O3S O3S [dye]

N

HN

O3S

N

NHCH2CH2NH2 N

NH

+ N

Ar

CH2CH2HN

N N

O

[cellulose]

N

O3S O3S [dye] O3S

HN

N N

N NH

Scheme 7.68

NHCH2CH2HN

Ar

If the size of this extended bridge grouping containing five ring systems (including Ar) is compared with that needed by the original vinylsulphone dyes (SO2CH2CH2O in Scheme 7.25) to link the dye chromogen to cellulose, one must ask whether the gain in fixation is justifiable. Some of the active vinylsulphone and chloro substituents in reactant XLC–VS will be lost by hydrolysis (as in Scheme 7.70). This versatile water-soluble reactant has been evaluated in wool and nylon dyeing. The nucleophilic aminoalkyl derivatives of orthodox aminochlorotriazine dyes behave like traditional acid dyes on wool owing to their zwitterionic character under neutral-dyeing conditions (Scheme 7.65). Improved wet fastness can be achieved using the reactant XLC

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CHEMISTRY OF REACTIVE DYES

NaO3SO

CH2CH2SO2

HN

N

NH

N Cl

CH

SO2

HN

N N

XLC-VS [cellulose]

[cellulose]

O

NaOH

NH

SO2

CH

CH2

N Cl HO

HN

N N

+

[cellulose]

NH

SO2CH2CH2

NH

SO2CH2CH2

O

[cellulose]

N Cl

NaO3S NaO3S

7.130 XLC

OH

CH2CH2SO2

OSO3Na

N

NaOH

CH2

SO2CH2CH2

[dye]

N

HN

NaO3S

NHCH2CH2NH2

N

N NH

[cellulose]

O

Ar

CH2CH2SO2

HN

N N

O

[cellulose]

N

NaO3S NaO3S [dye]

HN

NaO3S

NHCH2CH2HN

N N NH

Scheme 7.69

H2C

HC

Dye-fibre bond

N Ar

SO2

[XLC-VS]

SO2

CH

CH2

Cl 3 H2O

HOCH2CH2SO2 Scheme 7.70

NaOH

[XLC-VS]

SO2CH2CH2OH

OH

[143]. The effect of this additive on the conventional dyeing of wool with the anthraquinone sulphatoethylsulphone dye CI Reactive Blue 19 (7.37) was investigated.

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435

Aminoacid analysis and solubility tests for crosslinking revealed that XLC participated in various reactions leading to crosslinked wool [144]. In further work on nylon [145], this trifunctional reactant was applied simultaneously with various nucleophilic dyes of the aminoalkyl type (Scheme 7.71). As in the case of the Basazol system on cellulose (Scheme 7.60), the intended formation of covalent dye–fibre linkages has to compete with side reactions, such as partial hydrolysis (Scheme 7.70), di- or trimerisation that may lead to less than optimum fastness, or substrate crosslinking that may adversely influence desirable fibre characteristics. NaO3S NaO3S

NHCH2CH2NH2

[dye]

N NaO3SO

CH2CH2SO2

H2N

Cl

+

NaO3S

[nylon]

+ N

NH

N

HN

SO2CH2CH2

OSO3Na

7.130 NaOH

NaOH

XLC

NaO3S NaO3S

[dye]

NHCH2CH2NHCH2CH2SO2

NH

Dye dimer

N

N

NaO3S NaO3S NaO3S

[dye]

Cl N

NHCH2CH2NHCH2CH2SO2

NH +

NaO3S

NH

SO2CH2CH2NH

[nylon]

N Cl

N

Fibre crosslink

N NH

SO2CH2CH2NH

[nylon]

SO2CH2CH2NH

[nylon]

+

NaO3S NaO3S

[dye]

NaO3S

NHCH2CH2NHCH2CH2SO2

Dye-fibre bond

HN

N N

NH N

Cl

Scheme 7.71

More controlled and efficient fixation is possible when the reactant is applied as a pretreating agent [146]. If nylon given such a pretreatment is subsequently dyed with the conventional chlorotriazine dye CI Reactive Red 3 (7.2), the substantivity and fixation of the latter are markedly lowered because the anionic XLC residues have reacted with Nterminal amino groups in the fibre. Treatment of the modified nylon with ammonia, however, restores some degree of dyeability. Opposite effects are observed if CI Reactive Red 3 is reacted with ethylenediamine to form an aminoalkyl derivative (7.131). This nucleophilic dye exhibits a high degree of fixation only on the modified nylon that has been pretreated with XLC.

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CHEMISTRY OF REACTIVE DYES

H2NCH2CH2NH N NH

N N HN

SO3Na O

H

SO3Na

N N NaO3S

+ R3N

[dye]

SO2CH

CH2

7.131

+ R3N

+ NH3

[dye]

SO2CH2CH2NH2

Scheme 7.72

_ COO

+ R3N [dye]

SO2CH2CH2NHCH2CH2SO2

HN

N

NH

SO2CH2CH2NH [nylon]

[nylon]

N

N

7.132

C-terminal group

Dye-fibre bond

N-terminal group

Cl

In more recent work by the same authors [147], a monofunctional aminoalkyl dye showed high exhaustion on nylon under mildly acidic conditions. Aftertreatment of this dyeing with XLC–VS, the active bis-vinylsulphone form of the precursor 7.130, resulted in a significant degree of covalent bonding of the dye to nylon. This approach of using a crosslinking reactant to fix a nucleophilic dye showing orthodox dyeing behaviour on nylon was also applied to a specially prepared cationic dye. This contained a vinylsulphonyl group and was converted to a nucleophilic aminoethylsulphonyl derivative by reaction with ammonia (Scheme 7.72). This was readily absorbed on nylon C-terminal sites at pH values above 10. The anionic crosslinking agent XLC was subsequently applied to provide bridging links between these dye molecules and N-terminal amino groups in nylon. The weak link in this system (7.132) is the electrostatic bond between the cationic dye and the C-terminal carboxylate group. Any dye di- or trimers formed with the same trifunctional XLC molecule will be held to the nylon only by virtue of this electrostatic attraction and will be deficient in wet fastness. Partial premature hydrolysis (Scheme 7.70) of the crosslinking agent that reacts with single dye molecules may detract from the colour yield or wet fastness properties. Interest in Fixing Agent P (7.123) has been revived in the context of nucleophilic aminoalkyl dyes of the 7.129 and 7.131 types. Nylon pretreated with this symmetrical trifunctional reactant contains residual acryloyl sites that will undergo an addition reaction with nucleophilic dye molecules [145]. Although in theory each N-terminal amino group in the fibre can give rise to two acryloyl sites (Scheme 7.73), crosslinking between two N-

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437

O H2C

HC

C N N

CO

CH

CH2

H2N

+

[nylon]

N H2C

HC

C

7.123 O O H2C HC

C N N

COCH2CH2NH

[nylon]

N H2C HC

C O

NaO3S 2 NaO3S

+ NHCH2CH2NH2

[dye]

NaO3S

NaO3S NaO3S

[dye]

O NHCH2CH2NHCH2CH2

C

NaO3S

N

NaO3S

N

N NaO3S

[dye]

NaO3S

NHCH2CH2NHCH2CH2

COCH2CH2NH

[nylon]

C O

Scheme 7.73

terminal groups, with or without subsequent dye fixation, will increase the proportion of trifunctional reactant required. Partial premature hydrolysis (Scheme 7.61) will have a similar effect on consumption of the agent. On wool, dyeings of the bis(aminoethyliminotriazine) dye 7.129 and of the corresponding aminoalkyl derivative of CI Reactive Red 58 were carried out at the boil and pH 5 for 60 minutes. A dispersion of the symmetrical reactant 7.123 was then added and dyeing continued for 30 minutes before adjusting to pH 7 to complete the fixation stage in a further 30 minutes. Compared with a control dyeing of the bis(aminochlorotriazine) analogue CI Reactive Red 120 (7.48; X = Cl) for the same total time at the boil and pH 5, the new approach offers deferred fixation and hence improved levelness. The wet fastness performance was significantly enhanced but light fastness was adversely affected [148]. Reactant 7.123 can be prepared by reacting acrylonitrile with s-trioxane (Scheme 7.74) using an acid catalyst in carbon tetrachloride as solvent [147]. Two commercially available bifunctional reactants, N,N′-bis(acryloyl)methylenediamine (7.133) and its water-soluble bis-quaternary precursor (7.134), have been evaluated to achieve dyeings of similar quality at lower cost. The precursor is readily converted to the active reactant at about pH 8 during

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CHEMISTRY OF REACTIVE DYES

CH2 O

acid

O

H2C

+ 3 H2C

CH2

CH

3 H2C

CN

CH

C

CCl4

CH2

O

Acrylonitrile

O

N

s-Trioxane O H2C

HC

O

C

C N

C

CH

CH2

O

+ R3N CH2CH2CONHCH2NHCOCH2CH2 _ X 7.134

pH8

H2C

CH2

N

7.123 Scheme 7.74

CH

N

HC

CONHCH2NHCO

+ NR3 _ X

2 NaOH

CH

CH2

+ 2 NR3 + 2 NaX + 2 H2O

7.133

Scheme 7.75

the dyeing process (Scheme 7.75). Monofunctional (7.131) and bifunctional (7.129) nucleophilic dyes were applied to wool by variants of the process already established using reactant 7.123. The reactivity of agent 7.133 was lower (Scheme 7.76), however, so that fixation was higher when it was applied at the start of the process. The bis-quaternary precursor cannot be used in this way because of its cationic character. Partial hydrolysis of the active reactant (Scheme 7.61) is unavoidable. The light fastness of dyeings using the bisquaternary precursor was marginally lower and the aminoalkyl derivatives of cellulosesubstantive chlorotriazine dyes tended to stain adjacent cotton in tests of fastness to washing and perspiration [149]. The alicyclic tertiary base hexamethylenetetramine (7.135) hydrolyses at the boil to release the simple crosslinking agent formaldehyde (Scheme 7.77). This has been evaluated in wool dyeing by applying the aminoethyl analogue of CI Reactive Red 3 (7.131) and hexamethylenetetramine simultaneously at the boil and pH 5. Covalent dye–fibre fixation did not commence until sufficient formaldehyde had been released at the boil, so the dye was able to migrate during the heating-up phase and good level dyeing was achieved [150]. Recent trends in unorthodox reactive dyeing systems, including Schemes 7.63–7.76 generally employing nucleophilic aminoethyliminotriazine dyes prepared as outlined in

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NOVEL REACTIVE DYEING PROCESSES

439

NaO3S NaO3S

[dye]

NHCH2CH2NH2

+ H2C

HC

CONHCH2NHCO

CH

CH2

+

NaO3S

H2N

[wool]

HS

[wool]

NaO3S NaO3S

[dye]

NHCH2CH2NHCH2CH2CONH HN

NaO3S

Dye dimer

HN

[dye]

NHCH2CH2NHCH2CH2CONH

NaO3S

NH

[wool]

Fibre crosslink

H2C

CH2

NaO3S NaO3S

COCH2CH2

COCH2CH2

S

[wool]

+ NaO3S NaO3S

[dye]

NHCH2CH2NHCH2CH2CONHCH2NHCOCH2CH2

NaO3S

NH

[wool]

Dye-fibre bond

Scheme 7.76

N H2C

CH2

CH2

N CH2 N

H2C

+ 6 H2O

6 HCHO + 4 NH3

N

CH2 7.135

Scheme 7.77

Scheme 7.62 together with a variety of reactive intermediates or crosslinking agents intended to link together the dye and the substrate, raise some important questions. This research has encompassed variants of such systems on cotton, wool and nylon, mainly by exhaust application. Conventional reactive dyes have so far accounted for only about 20% of all dyes used worldwide on cellulosic fibres. Reactive dyes amount to only about 5% of all dyes used on wool and their current usage on nylon is negligible. The special nucleophilic dyes designed for these novel systems are derived from cellulosesubstantive chromogens and below pH 8 their aminoalkyl groups impart a zwitterionic character (Scheme 7.65), especially to the bifunctional type (7.129). Like direct dyes with both amino and sulpho substituents (section 3.1.2), aggregation problems can be foreseen. Although these novel dyes are free from problems of hydrolytic deactivation, the risk of premature hydrolysis leading to impaired fixation has been transferred to the activated fibre (Scheme 7.67) or the crosslinking reactant (7.130) essential for their application. Pretreatment of the fibre with a colourless reactant must be exceptionally reproducible, to ensure that the subsequent exhaustion, levelling and fixation of the dyes remains acceptably consistent. If penetration of the pretreating agent or crosslinking reactant into the fibre is poor, ring dyeing will occur and the colour yield and fastness performance will be adversely affected.

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CHEMISTRY OF REACTIVE DYES

The high substantivity for cellulose of these nucleophilic dyes inevitably ensures that staining of adjacent cotton is unacceptable in wet fastness tests when the novel systems are evaluated on wool or nylon [149]. For the control dyeing representing target behaviour on wool a conventional α-bromoacrylamide dye of similar hue, such as CI Reactive Red 66, should be used. The monochlorotriazine dye from which the aminoethyliminotriazine derivative is prepared is not a valid control [148,149], because such dyes have never given satisfactory results on wool. Recent developments with conventional reactive dyes for wool have highlighted the design of low-cost, dull chromogens to compete more effectively with chrome dyes (section 7.7). It is not easy to see how such targets can be reconciled with these relatively elaborate novel systems. The problems of dyeing nylon with reactive dyes are rather different (section 7.9), although the demand to compete with low-cost, non-reactive alternatives is even greater. Normal nylon contains only a limited number (ca. 40 milliequiv./kg) of accessible primary amine end groups available for covalent fixation of reactive dyes or crosslinking reactants. It is not normally difficult to ensure that virtually all of these react with a conventional monofunctional reactive dye. What is much more challenging is to design a compatible range of dyes to cover the entire colour gamut without unacceptable blocking effects in mixture recipes. In full depths the demand for accessible sites of reaction in normal nylon greatly exceeds their supply. The search for a viable reactive dyeing system in this area will probably have to involve an ultra-deep dyeing variant (>80 milliequiv./kg) as the preferred substrate, although this would be more costly than normal nylon and would exhibit less than ideal physical properties. It will be interesting to see whether such novel reactive dyeing systems will become competitive alongside the long-established orthodox monofunctional ranges (section 7.3) and still-proliferating bifunctional structures (section 7.4) that have made gradual if not spectacular inroads into the textile sectors traditionally dyed with various ranges of nonreactive dyes. Safe handling of any necessary crosslinking reactants in substance or as an active component of the textile substrate will have to be given careful consideration. Each unusual intermediate required in the synthesis of a novel colorant or co-reactant molecule, as well as every possible side reaction or degradation pathway leading from the dyed textile and its associated waste waters, can no longer be introduced without posing searching questions of health and safety (section 1.7). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

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